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8603069
Vutetakis, David George
ELECTROCHEMICAL OXIDATION OF CARBONACEOUS MATERIALS DISPERSED IN MOLTEN CARBONATE
The Ohio State University Ph.D. 1985
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ELECTROCHEMICAL OXIDATION OF CARBONACEOUS
MATERIALS DISPERSED IN MOLTEN CARBONATE
DISSERTATION
Presented in Partial Fulfillment of the Requirements for
the Degree Doctor of Philosophy in the Graduate
School of The Ohio State University
By
David George Vutetakis, B.S.Ch.E., M.S.
*****
The Ohio State University
1985
Dissertation Committee: Approved by
D. R. Skidmore
J. F. Davis Adviser L. S. Fan Department of Chemical Engineering Copyright by
David George Vutetakis
1985 To My Parents
n ACKNOWLEDGEMENTS
First, I want to thank my adviser, Professor Duane Skidmore, for his supervision of the Ph.D. project, his contributions concerning the
experimental work, and his helpful suggestions in preparing this
dissertation. Professor Robert Rapp is thanked for giving technical
guidance in the area of high temperature molten salt electrochemistry.
Dr. Harlan Byker, formerly of Battelle Memorial Institute, deserves
credit for his effort in getting the Ph.D. project underway.
Appreciation is given to Bob Canegali for helping with construction of
the experimental apparatus, to Mike Kukla for assisting with equipment
problems, and to Joe Baling for supplying alumina crucibles and other
refractory items. Bud Farrar, Professor Karlis Svanks, and Professor
Ron Tettenhorst are thanked for performing analytical measurements.
The dissertation was typed by Pat Osborn, and her hard work is grate
fully acknowledged. Financial assistance was graciously provided by
the Department of Chemical Engineering, the Graduate School, Dupont,
and Battelle Memorial Institute, and their support is gratefully
acknowledged.
I am deeply indebted to my wife, Liz, and our children for their
patience and love, which counted more than words can say. Most of all
I would like to thank the Lord God, who gave the motivation to
accomplish what was set before me.
i i i VITA
August 10, 1957 ...... Born in Canton, Ohio
1975...... Born of God through faith in Jesus Christ
1980 ...... B.S.Ch.E., The Ohio State University, Columbus, Ohio
1981 ...... M.S.Ch.E., The Ohio State University, Columbus, Ohio
1980-1984 ...... Part-time Research at Battelle Memorial Institute, Columbus, Ohio
1981-1985 ...... Ph.D. Candidate, Department of Chemical Engineering, The Ohio State University, Columbus, Ohio
PUBLICATIONS
D.O. Vutetakis, "Photoelectrolysis of Acetate Solutions Using Semiconductor Electrodes," M.S. Thesis, The Ohio State University, Columbus, Ohio (1981).
IV FIELDS OF STUDY
Major Field; Chemical Engineering
Thermodynamics - Professor H. C. Hershey
Heat Transfer - Professor T. L. Sweeney
Mass Transfer - Professor C. J. Genakoplis
Momentum Transfer - Professor R. S. Brodkey
Chemical Kinetics - Professor E. R. Haering
Unit Operations - Professor E. E. Smith
Process Control - Professor R. D. Mohler
Coal Processing - Professor D. R. Skidmore
Minor Field: Corrosion and Electrochemistry
Corrosion Engineering - Professors F.H. Beck and D. D. Macdonald
Electrochemistry - Professor T. Kuwana TABLE OF CONTENTS
PAGE
ACKNOWLEDGMENTS...... H i
VITA ...... iv
LIST OF T A B L E S ...... viii
LIST OF F I G U R E S ...... x
CHAPTER I. INTRODUCTION ...... 1
CHAPTER II. LITERATURE REVIEW ...... 3 2.1. Characteristics of Carbonaceous Materials ...... 3 2.2. Characteristics of Molten Alkali Carbonates .... 29 2.3. Previous Work Involving Electrochemical Oxidation of Carbonaceous Materials in Molten Electrolytes . . 76 2.A. Related Literature...... 96
CHAPTER III. EXPERIMENTAL METHODS ...... 100 3.1. Objectives...... 100 3.2. Apparatus ...... 101 3.3. Feedstocks...... Ill 3.4. Procedures...... 114
CHAPTER IV. EXPERIMENTAL RESULTS ...... 119 4.1. Overview of E x p e r i m e n t s ...... 119 4.2. Open Circuit Potential D a t a ...... 123 4.3. Current-Voltage Da t a ...... 129 4.4. Gas Evolution D a t a ...... 169 4.5. Surface Behavior of Gold A n o d e ...... 172
CHAPTER V. DISCUSSION OF RESULTS ...... 174 5.1. Thermodynamic Analysis of OCP D a t a ...... 174 5.2. Interpretation of I-V D a t a ...... 180 5.3. Comparison of Data with Related Investigations . . . 190 5.4. Mechanism of Anodic Process ...... 200
VI CHAPTER VI. CONCLUSIONS AND RECOMMENDATIONS ...... 210 6.1. Conclusions ...... 210 6.2. Recommendations...... 216
CHAPTER VII. S U M M A R Y ...... 219
APPENDICES
A. Analytical Data of Coal and Activated C a r b o n...... 221
B. Thermodynamimc Calculations ...... 227
BIBLIOGRAPHY ...... 231
v n LIST OF TABLES
TABLE PAGE
1. Classification of Coals by R a n k ...... 4
2. Analytical Data of Various American Coals ...... 12
3. Typical Limits of Ash Composition of U.S. Bituminous Coals ...... 13
1*. Comparison of Gasification Rates In C O ^ ...... 18
5. Alkali Carbonate Melting Points ...... 31
6. Densities of Molten Alkali Carbonates ...... 33
7. Contact Angles of Ternary Carbonate Eutetlc with Various Materials at 4 0 0 ° C ...... 3**
8. Viscosities of Molten Alkali Carbonates ...... 35
9. Electrical Conductivities of the Ternary Carbonate E u t e c t i c ...... 37
10. ûG^ of Selected Compounds...... 46
11. Equilibrium Constants for Carbonate Dissociation .... 47
12. Experimental Equilibrium Constants of Carbonate Dissociation ...... 47
13. Summary of Fuel Cell Work Using Carbon Anodes In Molten Electrolytes ...... 78
14. Types of Working Electrodes ...... 101
15. Types of Reference Electrode Sheaths ...... 103
16. Summary of Completed Experiments ...... 120
17. OCP of Various Carbon Types at 700® C ...... 127
viii 18. Effect of Temperature History on OCP of Darco Activated Carbon ...... 127
19. Comparison of the Current Density Achieved Using Different Carbon Types ...... 153
20. Current Densities at Three Sizes of Gold Anodes ...... 154
21. Current Densities at Submerged and Dipping Gold Anodes with Graphite Loading ...... 154
22. Total Gas Evolution Rates and Gas Product Yields .... 170
23. Results of Gas Chromatograph Analysis ...... 171
24. EDAX Analysis of Anode F i l m ...... 173
25. Results of Equilibrium Calculations ...... 175
26. Comparison of Calculated and Experimental OCP Values . . 177
27. Tafel Plot Parameters for Darco Activated Carbon, Bituminous Char and L i g n i t e...... 184
28. Tafel Plot Parameters for Darco Activated Carbon at 600-800“C ...... 184
29. Comparison of OCP Data with Data of Ha u s e r ...... 191
30. Comparison of OCP Data with Data of Arkhipov and S t e p a n o v ...... 192
31. OCP Data of D u b o i s ...... 193
32. OCP Data of Weaver and C o w o r k e r s ...... 195
33. Comparison of I-V Data with Data of Ha u s e r ...... 197
34. Comparison of I-V Data with Data of Arkhipov and Stepanov ...... 197
35. Comparison of I-V Data with Data of Weaver and Coworkers 198
36. Analytical Data for Darco Activated Carbon ...... 218
37. Physical Data for Darco Activated Carbon ...... 219
38. Analytical Data for Bituminous Char ...... 220
39. Analytical Data for North Dakota Lignite ...... 221
40. Analytical Data for Primrose Anthracite ...... 222
ix LIST OF FIGURES
FIGURE PAGE
1. Specific Resistance (Milliohm-cm) versus Temperature for C o k e ...... 7
2. Gas Solubilities in Molten Carbonate ...... 39
3. Solubility of CO in Molten C a r b o n a t e ...... 41
4. Phase Stability Diagram for the System Na-C-0 at 900°C 49
5. Equilibrium Stability Regions of Alkali Metals at 827*C 50
6. Equilibrium Stability Regions of Potassium Metal at Various Temperatures ...... 50
7. Potential - pCO^ Diagram for the System C-CO-COg-O^ in Molten Carbonate at 600®C ...... 57
8. Potential - pCO^ Diagram for Gold in Ternary Carbonate Eutectic at 600 “C ...... 59
9. Diagram of Short's Carbon Fuel C e l l ...... 88
10. Diagram of Reed's Carbon Fuel C e l l ...... 89
11. The SRI Conceptual Carbon Fuel C e l l ...... 90
12. Diagram of Electrochemical C e l l ...... 102
13. Block Diagram of Electronic Instrumentation ...... 108
14. Gas Flow N e t w o r k ...... 109
15. Open Circuit Potential of Darco Activated Carbon versus Temperature...... 124
16. Open Circuit Potential for Darco Activated Carbon Loadings versus Run T i m e ...... 125
X 17. Open Circuit Potential versus Darco Activated Carbon L o a d i n g ...... 126
18. Typical Raw I-V C u r v e s ...... 130
19. I-V Curves Showing the Effect of Run Time Under 3% COg P u r g e ...... 131
20. I-V Curves Showing the Effect of Run Time Under 100% COg P u r g e ...... 132
21. 1-V Curves Showing the Effect Purge Gas Composition . . 133
22. I-V Curves Showing the Effect of Scan R a t e ...... 13^1
23. I-V Curves Showing the Effect of Scan Rate ...... 135
24. I-V Curves Showing the Effect of Stirring Rate .... 136
25. I-V Curves for Darco Activated Carbon at a Series of L o a d i n g s ...... 137
26. I-V Curves for Bituminous Char at a Series of Loadings 138
27. I-V Curves for Lignite at a Series of Loadings .... 139
28. I-V Curves for Spectroscopic Graphite at a Series of L o a d i n g s ...... 140
29. I-V Curves for Anthracite at a Series of Loadings . . . 141
30. I-V Curves Comparing 0% and 1% Carbon Loadings .... 142
31. Current Density versus Darco Carbon Loading at Various Potentials ...... 143
32. Current Density versus Darco Carbon Loading at Various Particle Sizes...... 144
33. I-V Curves Showing theEffect of Cell Temperature .. . 145
34. Arrhenius Plot for Darco Activated Carbon ...... 146
35. Apparent Activation Energy versus Voltage ...... 147
36. I-V Curves Sh^.lng theEffect of Cell Temperature H i s t o r y ...... 148
XI 37. I-V Curves Comparing Gold and Graphite Anodes ...... 1^9
38. I-V Curves Showing the Effect of Added A s h ...... 150
39. I-V Curves Showing the Effect of Added Ash and Carbon 151
40. I-V Curves Comparing Run B-11 with Run C-1 ...... 152
41. Tafel Plots for Darco Activated Carbon ...... 183
42. Illustration of Direct and Indirect Interaction M e c h a n i s m s ...... 202
x n CHAPTER I
INTRODUCTION
Electrochemical conversion of coal has been a long standing goal of academic and industrial science. The higher theoretical energy efficiency compared to thermal conversion processes has provided the major impetus. Over the years, many attempts have been made to util ize coal in a fuel cell, both directly and indirectly, and at both high temperature and low temperature. Despite extensive research, no commercialization has ever been realized, and most investigators have concluded that electrochemical coal conversion is impractical.
In recent years, however, two different lines of approach at electrochemical conversion of coal have emerged. In one approach, coal is made into an electrically conducting, coherent anode which is directly utilized in a high temperature molten carbonate fuel cell.
Although the cell performance is attractive, the carbon utilization efficiency is low due to shedding of the anode. The other approach is known as "coal slurry electrolysis" or "electrochemical coal gasification." In this approach, powdered coal is dispersed in an aqueous electrolyte at low temperature, and acts to reduce the cell voltage for hydrogen evolution compared with conventional water electrolysis. Essentially, the coal serves as an anodic depolarizing agent. The major drawback with coal slurry electrolysis is the low
1 2 reactivity of coal at low temperature.
The approach taken in the present investigation is essentially a marriage of the above two concepts. Basically, the present approach
involves a slurry of carbon particles dispersed in a molten carbonate electrolyte at high temperature. An inert anode serves as the current collector. The use of a coal slurry at high temperature should over come both the low reactivity problem and the anode shedding problem.
The details of the experimental approach and scope are given in
Chapter III.
The ultimate application of this type of system would be a
direct-fired coal fuel cell. However, the study of this system could
have applicability to related areas of research, such as catalytic
coal gasification with alkali carbonates, and molten salt coal
gasification (see Sections 2.1.8 and 2.4.3). Furthermore, the system
to be investigated is attractive from a purely theoretical viewpoint.
Due to the apparent novelty of the present investigation, an
extensive literature review was performed, as presented in Chapter II.
The purpose of the literature review was twofold: 1) to uncover any
previous investigations of a similar nature, and 2) to review related
areas of study that could be useful to this investigation and also in
future investigations. CHAPTER II
LITERATURE REVIEW
2.1. Characteristics of Carbonaceous Materials
2.1.1. Types of Carbonaceous Materials
Carbonaceous materials can generally be defined as materials which contain carbon as the major constituent. Since carbon is a highly abundant element, the types of carbonaceous materials that can be identified are almost innumerable. The scope here is limited to the types utilized in the present work, namely, coal, activated carbon, and graphite.
2.1.1.1. Coal. Coal is a complex agglomeration of organic and inor ganic compounds. Generically, coal embraces a wide range of natural materials from lignite to anthracite. Table 1 presents the ASTM classification of coal by rank (1). The basic scheme of classifica tion is according to fixed carbon and calorific value, calculated on a mineral matter-free basis. The higher rank coals are classified according to fixed carbon on the dry basis and the lower rank coals are classified according to calorific value on a moist basis.
The area of coal science is immense. A few of the many review references are cited here (2-6). Table I Classification of Coals by Rank . Reprinted with permission from the Annual Book of ASTM Standards, Designation 0388-66(1). Copyright ASTM, 1916 Race Street, Philadelphia, 19103.
Fixed Csrboa Limili, Volatile Matter Calorifc Value Limits^ per cm t Limits, per cent Btu per pound (Moist,* (Dry, slisml-Mstlcr- (Dry, MineraPMattcr Miners). Matter- Free B uis) Free Basts) Free Basil) C b ti Group Agsletneratlng Character
EqusI or E rgal or Equal or G reiter L eu Greater Greater T hin Than Than Than Titan
1. Meta anthracite...... 98 2 I. Anthracitic 2. Anthracite...... 92 98 2 8 ... jNonagglcmerating 3. Semianthraeite*...... 86 92 8 14 1. Low volatile bituminous coal...... 78 66 14 22 2. Medium volatile bituminous coal...... 69 78 22 31 II. Bituminous 3. High volatile A bituminous coal...... 69 31 14 06o< Commonly agglom 4. High volatileB bituminous coal...... 13 000' 14 ioioo erating* 5. High volatile C bituminous coal...... j 11 500 13 000 10 500 11 500 Agglomerating
1. Subbituminous A coal...... 10 500 11 600 III. Subbituminous 2. Subbituminous B coal...... 9 500 10 500 3. Subbituminous C coal...... 8 300 9 500 Nonagglomerating 1. Lignite A ...... 6 300 8 300 IV . Lignitic 2. Lignite B ...... ■ • . 6 300
* This classification does not include a Itw coals, principally nonbanded varieties, which have unusual ph>'sieal and chemical properties and which come within the limits of fixed carbon or calorific value of the high volatile bituminous and subbituminous ranks. All of these coals either contain ess than 48 per cent dry, mineral matter free fixed carbon or have more than 16,500 moist, mineral matter free British thermal units per pound. * Moist refers to coal containing its natural inherent moisture but not including visible water on the surface of the coal. * If agglomerating, classify in low-volatile group of the bituminous class. ^ Coals having 09 per cent or more fixed carbon on the dry, mineral matter free basis shall be classified according to fixed carbon, regardless of calorific value. * It is recognized that there may be nonagglomerating varieties in these groups of the bituminous class, and there are notable exceptions in high volatile C bituminous group. 5
2.1.1.2. Activated Carbon. Activated carbon is a porous amorphous carbon, specially prepared to produce a very large surface area rang- 2 ing from 300 to 2000 m /g. It is manufactured from a variety of carbonaceous materials, such as petroleum coke, sawdust, lignite, peat, wood, nut shells, bone char and molasses. The main activation process involves treatment of charred or calcined carbonaceous material with oxidizing gases such as air, steam or carbon dioxide.
Activated carbon is used for adsorbing gases, decolorizing and purify ing liquids, and as a catalyst support. Much additional information can be found in several books (7-10).
2.1.1.3. Graphite. Graphite is a crystalline form of carbon, featur ing a layered structure in which the carbon atoms form a planar array of condensed hexagonal rings. It has a relatively high thermal and electrical conductivity, and is relatively inert to most chemical environments. Although graphite is found naturally, amorphous carbon can be converted to graphite by heating at temperatures of 2200®C or higher. Because of its unique properties, graphite finds many uses in electrochemistry and other fields. References (9,10) cite additional
Information pertaining to graphite.
2.1.2. Physical Properties of Carbonaceous Materials
The physical properties of direct importance to the present investigation are electrical conductivity, porosity, surface area, and density.
2.1.2.1. Electrical Conductivity. Electrical conductivity data for coal can be found in references (11,12), and for industrial carbons and graphite in references (9,10). The electrical conductivity of 6 coal varies widely, depending on the type of coal, moisture content, and temperature. At room temperature, anthracites have specific
“ 5 *1 ""1 conductivities between 1 and 10 ohm cm , and bituminous coals
”*1 0 " * 1 ^ ”*1 — " 1 have values between 10 and 10 ohm cm (11). Thus, coal can generally be classified as a semiconductor. When coal is heated to sufficiently high temperatures (around 1000“C), the variation of conductivity among different coals is much less, generally falling -1 2 -1 -1 between 10 and 10 ohm cm . The narrowed variation in conduc tivity results from the essentially complete removal of volatile matter at high temperature.
The effect of temperature on conductivity for a given coal is illustrated in Figure 1 (13). Two overlapping effects need to be distinguished: (1) the normal increase in conductivity, characteristic of semiconductors, which should be reversible, and (1) a large in crease in conductivity due to changes of the coal composition, which should be irreversible. The samples prepared at lower precarboniza tion temperatures give a steep drop in resistivity as the temperature is increased, with a definite discontinuity corresponding to the precarbonization temperature. The discontinuity marks the onset of further thermal decomposition. As the precarbonization temperature is increased, the discontinuity becomes less pronounced, and the tempera ture coefficient decreases. A large increase in conductivity occurs between the sample precarbonized at 550“C and the one at 700°C. The curves also show a substantial hysteresis, which becomes less as the precarbonization temperature increases. At 800°C, the conductivity cT o>
750'C
log T
Figure 1. Specific Resistance (milliohm-cm) versus Temperature for Coke. The coke samples were prepared from "Anna" coal by precarbonization for 2 hr at the indicated temperatures. Reprinted from Reference (11), p. 100, by permission of the publisher (John Wiley & Sons, Inc.). Original work by Kroger and Dobmaier (13). used by permission of the original publisher (Industrieverlag von Hernhaussen K6). 8
Is fairly reversible, and effect (2) discussed above is essentially negligible. The curves all converge at a temperature around 1000°C, independent of the precarbonization temperature.
Single crystal graphite is highly anisotropic. Its specific
“ 1 “ 1 conductivity is 10 ohm cm parallel to the layer plane, and 0.5
“ 1 “ 1 ohm cm perpendicular to the layer plane (11). Manufactured graphite has an electrical conductivity generally between 600 and 1250 “1 ■*! ohm cm . Corresponding values for manufactured carbon fall between
130 and *100 ohm ^ cm ^ (9).
The electrical conductivity of coal or char can be significantly increased by the addition of inorganic salts, such as alkali carb onates. This effect is discussed further in Section 2.1.8.
2.1.2.2. Porosity. The porosity of coal ranges from 2 to 20$, depending on the specific type (14,15). The porosity generally decreases with increasing rank. The shape and size distribution of the pores vary widely among different coals. The pore sizes can be broadly classified into three categories (12): micropores (diameters less than 20 angstroms), transitional pores (20-200 angstroms) and macropores (greater than 200 angstroms). The porosity can either increase or decrease upon heating.
Manufactured carbon and graphite have porosities ranging from essentially zero to over 50$ (9).
2.1.2.3. Surface Area. Although numerous methods have been used to measure the surface area of coal, the method generally believed to closest approximate the true surface area involves CO^ adsorption
(12). When CO^ is used as an adsorbate, the surface areas of coal 2 generally fall between 100-400 m /g (12,14,15). Low and high rank coals usually have the lowest surface areas.
Carbonization of coal can Increase the surface area several fold for most coals. The surface area depends largely on the heat treat ment temperature. The surface area usually Increases with Increasing heat treatment temperature, passes through a maximum (around 800°C), and then decreases. Gasification by 00^, 0^ and/or steam also results
In an Increase In surface area. The Increase Is most pronounced In the early stages of gasification, and levels off or decreases at relatlvly low fractional conversions. This method of Increasing surface area Is used In the manufacture of activated carbon (7,8). 2 Surface areas of activated carbons are 800-1200 m /g for bltumlnous- 2 derived products and 500-800 m /g for llgnlte-derlved products (9). A 2 value of 40 m /g was cited for graphite (9), but lower or higher values can certainly be expected.
2.1.2.4. Density. Since coal Is a porous material, a distinction needs to be made between "true" density, "apparent" density, and
"particle" density. The true density Is the density of the solid phase In the absence of voldage, and Is obtained by using a displace^ ment fluid that completely fills all pores and does not Interact with the solid matrix (e.g., helium). The apparent density Is usually taken to mean an approximation of the true density, using a displace ment fluid that nearly (but not completely) fulfills the above requirements. The particle density Is the density of the solid In cluding the voldage, and Is measured using a fluid that does not enter the pores (e.g., mercury). A further type of density Is the bulk 10 density, which is the overall density of a load of particles measured in air. Some inconsistency in these terms occurs, such as using apparent in place of particle density, and true in place of apparent.
Reviews of density measurement techniques for coal and reported values can be found in references (11,12).
The true densities of amorphous carbon, graphite and diamond are
1.88, 2.25 and 3.51, respectively (9). The apparent density of coal ranges from 1.3 to 1.7, with the highest values found in high rank coals. The ash content is usually excluded from the density value, and can be subtracted by using an average ash density of 2.7-3.0. The apparent density of coal increases substantially upon heat treatment.
For example, a bright coal (89.7ÎC) increased in density from 1.3 at
500°C, to 1.55 at 700°C, and eventually reached a density of 2.0 around 1300°C (11). Thibaut (16) reported on a variety of cokes having apparent densities (reported as "true") between 1.8-2.1. The corresponding particle densities (reported as "apparent") of these cokes were 0.8-1.0. Mantell (10) cited activated carbon as having apparent densities in the range 1.75-2.1, and particle densities of
0.7-0.9.
2.1.3. Chemical Composition of Coal
The organic matter in coal consists primarily of carbon, hydrogen, oxygen, nitrogen and sulfur, although trace quantities of many other elements are also found. The ultimate analysis represents the elemental composition of coal in terras of these primary components
(C,H,0,N,S), and also includes the ash content. Table 2 gives the 11 ultimate analyses of a variety of American coals (17). As the rank of coal increases, the following trends are observed;
1. Moisture content decreases.
2. Carbon content increases, while oxygen content decreases.
3. Hydrogen content remains fairly constant, but drops substantially
with anthracite.
A. The calorific value increases, levels off, then drops slightly as
the anthracite stage is reached.
In the proximate analysis, coal is analyzed on the basis of moisture content, volatile matter, fixed carbon, ash, and calorific value. The moisture content includes the water that can be removed just above the normal boiling point, even though water is retained in various forms at higher temperatures. The volatile matter represents all volatile products, exclusive of the moisture content, released during heating of coal under specified conditions. The volatile matter generally decreases with increasing coal rank. The fixed carbon is obtained by subtracting the moisture content, volatile matter and ash from the initial sample weight. The ash is the residue derived from the mineral matter after complete combustion of the coal.
It is different in chemical composition and usually is less in weight than the mineral matter originally present in the coal. The major ash constituents of bituminous coals are shown in Table 3 (18). Many other trace elements are found in coal ash, including Au and Pt.
There has been much work aimed at elucidating the molecular structure of coal. Due to the complex nature of coal, this task is exceedingly difficult. Structural information has been obtained by Table 2. Analytical Data of Various American Coals. Adapted from Reference (17).
As Received Ultimate Composition, maf($) Calorific Value Moisture Ash Coal (%) (%) C HN 0 S mmmf(Btu/lb)
Lignite 32.5 5.0 72.38 5.30 1.12 20.53 0.67 8,059
Subbituminous B 20.7 3.9 77.69 5.24 1.60 14.98 0.49 10,472
High-volatile B 13.7 10.3 81 .41 5.64 1.56 7.89 3.50 12,609 Bituminous
High-volatile C 5.4 2.2 81.96 4.98 1.65 10.23 1.18 13.700 Bituminous
High-volatile A 1.7 7.8 82.92 5.91 1.80 6.34 3.03 15,034 Bituminous
Medium-volatile 1.4 1.8 89.47 4.93 1.66 3.49 0.45 15,406 Bituminous
Low-volatile 0.6 7.4 91.36 4.60 1.24 2.07 0.73 15,655 Bituminous
Anthracite 4.2 2.4 94.88 1.83 0.67 1.78 0.84 14,180 13
Table 3. Typical Limits of Ash Composition of U.S. Bituminous
Coals. Adapted from Ode (18).
Constituent Percentage
SiOg 20-60
10-35
5-35 F*2°3 CaO 1-20
MgO 0.3-4
TiOg 0.5-25
NagO+KgO 1-4
SO- 0.1-12 3 Il» subjecting coal to various reactions, such as oxidation by chemical or even electrochemical methods. Numerous models have been proposed to represent the structure of coal, but many aspects are still qualitative. It is generally accepted that coal is composed of aromatic and hydroaromatic layers, with nitrogen and sulfur incor porated into some of the ring structures. Various functional groups appear as cross-links between the layers and as terminal edges of the layers. Oxygen appears predominantly in ether, hydroxl and carbonyl groups, and may also be incorporated in ring structures. Much addi tional information has been previously reviewed (19,20).
2.1.4. Thermal Decomposition of Coal
Coal undergoes a series of complex physical and chemical changes upon heating. Some of these changes are relevant to the present investigation, particularly in relation to drying procedures and the high temperature operation itself. Pertinent aspects from several sources (2-6) will now be summarized. Reviews of more recent work on coal pyrolysis can be found in references (4,5).
Initial heating releases the occluded gases, mainly 00^ and CH^, and continues up to about 100°C. Most of the uncombined water is driven off below 105°C, but this uncombined water is not completely removed until about 300°C. Between 300 and 400“C, the main coal substance begins to decompose, with evolution of light hydrocarbons, oil and tar, together with CO, CO^ and H^O. Above 400®C, evolution of
Hg becomes appreciable. Decomposition of sulfur compounds (FeSg and organic sulfur) occurs between 200 and 300°C, and up to 500°C, with evolution of sulfur, H^S, mercaptans and thio-ethers. Combined oxygen 15 persists to fairly high temperatures (above 700°C), but is virtually eliminated at temperatures above 900°C. The coal decomposition con tinues until about 950°C, and prolonged exposure at this temperature results in a residue of nearly pure carbon (neglecting mineral matter).
The above description holds for a relatively slow temperature progression. Rapid heating rates are known to alter significantly the pyrolysis behavior. For example, during rapid pyrolysis, the threshold of decomposition Is shifted to higher temperatures. Also, higher yields of volatiles and correspondingly smaller coke and char yields are obtained by rapid pyrolysis.
When coal is heated in vacuum, the removal of light molecules
(e.g., HgO, CO, COg, CHy) is naturally enhanced. However, thermal decomposition of the main body of coal does not proceed appreciably under vacuum until such temperatures are reached that would normally initiate decomposition. This behavior is an indication that coal is comprised of large molecules. Above the thermal decomposition threshold (300-it00®C), the action of vacuum results in distillation of the thermal degradation products. Compared with normal pressures, the application of vacuum during thermal decomposition gives a larger weight loss, the main part of the increase being in the tar fraction.
Vacuum distillation has been used to study the structure of coal by analysis of the distilled compounds (21-23 ).
To completely remove volatile matter from coal, temperatures above 900“C are necessary. The ASTM method for determination of volatile matter involves heating for 7 minutes at 950°C (24). 16
However, the volatile matter yield changes little In the range 700-’
950®C, with the major fraction being removed below 700*C (14). Thus, at the typical operating temperature of 700“C used In the present
Investigation, the coal should be essentially devolatlllzed.
One Important question to consider here Is what compounds, originating from the thermal decomposition of coal, might be expected
to be found In the condensed state at 700°C. Most organic compounds boll at temperatures far below 700®C, and would not be expected to remain condensed at 700°C. For example, the highest boiling organic compound Identified from coal tar analysis has been reported to be fulmlnene, with a boiling point of 555“C (25). However, an ap proximate 50? fraction of coal tar bolls above 450°C, and only 5? of
this fraction has been accounted for by Identified components. Heavy residues of coal tar (e.g., pitch) do remain condensed at 700°C.
However, these residues carbonize to form coke at 700°C. Thus, at temperatures around 700®C, It appears that the only stable, condensed product will be solid compounds of high carbon contents. This conclu sion holds only for exposure to gaseous environments; the behavior In a liquid phase at 700°C (I.e., molten carbonate) may be quite dif ferent due to possible llquld-llquld Interaction.
2.1.5. Treatment of Coal with Alkali
Some early Investigations studied the action of alkali on coal, and these were reviewed by Horton et al. (2). The effect of molten caustic may be comparable to the effect of molten carbonate.
Treatment of brown coal with potassium hydroxide gave almost complete conversion to humlc acids (26). Alkali fusion of humlc acids 17 was used to isolate several organic acids, including formic, acetic, oxalic, isophthalic, 2-hydroxybenzoic, and 5-hydroxy isophthalic acids
(27). However, Howard (28) mentions that upon thermal treatment above
300®C, humic acids decompose into a solid residue that is infusible and shows no alkali solubility. Another study found that treatment of bituminous coal with caustic soda (normal to 100 percent) at 250-400°C gave a coke-like residue as the principal product (29).
A recent investigation has been reported dealing with the produc tion of oxalate by reaction of coal and oxygen in concentrated sodium hydroxide solution (30). The work was conducted at 250®C, where the organic acids would remain thermally stable.
2.1.6. Reaction of Carbon and Carbon Dioxide
The reaction of carbon and carbon dioxide proceeds via the
Boudouard reaction:
C + CO2 - 2C0 (2.1.1)
The nature of this reaction is relevant because the carbonaceous materials contact a CO^ atmosphere in the present investigation.
Studies of the Boudouard reaction have been reviewed by Fredersdorff and Elliot (31), Johnson (15) and Wen and Dutta (14).
Reaction (2.1.1) is endothermie (a h '^ - 42.21 kcal/mol at 298°K), and is kinetically sluggish. At temperatures below 1000°C and carbon particle sizes below about 300 microns, the reaction is kinetically controlled, and occurs nearly uniformly throughout the interior of the carbon particles. It has been estimated that at 800°C and 0.1 atm pressure, the rate of the carbon-CO^ reaction is about one third as fast as the carbon-steam reaction, and about 10 as fast as the 18 carbon-oxygen reaction (32). The activation energy of reaction
(2.1.1) ranges from 60 to 90 kcal/mol, depending on the carbon type.
The most widely accepted mechanism of reaction (2.1.1) is;
C + COg + C(0) + CO (2.1.2)
C(0) - CO (2.1.3)
The interesting feature of this mechanism is the formation of the carbon-oxygen surface complex. Desorption of this complex is generally considered as rate determining. Carbon-oxygen complexes are also involved in carbon-oxygen reaction mechanisms.
A comparison of specific gasification rates of various types of carbon was presented by Johnson (15). The specific gasification rate,
W, was defined by:
^ (2.1.4) where x is the carbon conversion fraction (weight of carbon divided by the original weight). At conditions of 1 atm pure CO^ and 900*C, the following W values were given (min ):
Table 4. Comparison of Gasification Rates in CO^.
Adapted from Johnson (15).
Lignite (North Dakota) ...... 0.10
Bituminous Char (Pittsburgh) . . .0.011
Activated Carbon ...... 0.009
Anthracite...... 0.007
Ceylon Graphite ...... 0.0002 19
Although these values should be considered approximate, they do il lustrate the relative reactivities of the various carbons.
A primary reason for variability in rates of the carbon-COg reaction is the catalytic effect of mineral impurities in carbon. For example, it has been reported that addition of 5 wt$ BaCO^ increased the gasification rates of graphite in COg by more than 3 orders of magnitude at 900“C (33). Thus, the actual rate of the carbon-CO^ reaction is influenced strongly by the specific catalytic species present in the coal. Alkali carbonates are also kno.m to have a significant catalaytic effect on the carbon-CO^ reaction and other gasification reactions, and this effect will be discussed in Section
2 . 1 .8.
2.1.7. Surface Chemistry
The surface chemistry of carbon and graphite has been the subject of considerable research (7,8,11,12,34). It is known that the presence of oxygen-containing surface functional groups (surface oxides) plays a major role in determining the specific properties of carbon and graphite, both in liquid phase and in gas phase systems.
The surface oxides directly influence the specific adsorption properties of the carbon surface, and hence the overall reactivity.
They also influence the wettability and electrical properties.
The nature of the surface oxides has been studied by a variety of
techniques, such as acid-base neutralization, spectroscopic techniques
(IR, ESR, NMR), and elecrotrocheraical techniques. The principal
functional groups believed to be present are carboxyl, phenolic 20 hydroxyl, and carbonyl groups, although many others have been suggested.
When carbon is heated, chemisorbed oxygen evolves as CO^. CO and
HgO. This behavior indicates that a thermal breakdown of the adsorbed oxygen layer occurs, rather than a reversible desorption of oxygen.
The complete removal of all bound oxygen requires temperatures on the
order of 1200°C under vacuum. High temperature reaction (greater than
1000“C) with COg can also strip combined oxygen from the carbon sur
face (8).
The surface oxides that remain after exposure to pyrolysis tem
peratures have received some attention (8,12). Based on IR
spectroscopy, a band at 1600 cm persists up to 700-800®C, and is
usually assigned to a carbonyl-type group. However, there is also — 1 evidence that the 1600 cm band may be due solely to the aromatic
carbon skeleton (12).
The role of surface salt complexes in relation to alkali-
catalysed carbon gasification has been recently investigated by a
number of workers. Details are discussed in Section 2.1.8.
2.1.8. Gasification with Alkali Carbonate Catalysts
Catalysis of gas-carbon reactions by alkali metal salts, and in
particular by alkali metal carbonates, has been studied for a long
time. In recent years, a flurry of research has focused on this
topic. Recent reviews have been prepared by Wen (35), McKee (33),
Chen (36) and Datsko (37). A symposium on "Fundamentals of Catalytic
Coal and Char Gasification" was published as a special issue of Fuel
(38), and contains many pertinent articles. Some of the literature 21 regarding alkali carbonate catalysis is applicable to the present investigation, as discussed below.
2.1.8.1. Carbon-Alkali Carbonate Reaction. To understand the catalytic influence of alkali carbonates on carbon-gas reactions, several studies have been made regarding the reduction of alkali carbonates with carbon. This reaction, sometimes known as the Gay-
Lussac reaction, can be written as:
MgCOg + C + 2M + CO + COg (2.1.5) or
MgCOg + 2C + 2M + 3C0 (2.1.6) with reaction (2.1.6) being favored at higher temperatures due to the increasing CO/CO^ equilibrium. Note that reaction (2.1.6) involves reduction of the carbonate-carbon, whereas reaction (2.1.5) does not.
Literature on the formation of alkali metals by carbotherraic reduction of alkali carbonates can be traced back to an IBOB article by Gay-Lussac (39). Hence, the reaction commonly bears this name.
The Deville process (40) represented the first commercial manufacture of sodium, and was based on the Gay-Lussac reaction. A more advanced version of the Deville process was developed by Dow (41), but was not successfully commercialized. The Dow process reacted sodium carbonate and carbon at 1200°C, and relied upon rapid condensation of sodium to prevent recombination with CO (42).
In 1931, Fox and White (43) reported on carbon gasification using a sodium carbonate catalyst. Included was a study of the reaction between sodium carbonate and carbon (graphite). At 1000°C, they measured the Na and CO evolution and found stoichiometric agreement 22 with reaction (2.1.6). However, the experiment was carried out in the presence of air, which promoted complete oxidation of CO to CO^, and therefore could not distinguish between reactions (2.1.5) and (2.1.6).
The reaction was found to proceed at a measurable rate only above
800°C, which roughly correlated with the melting point of the sodium carbonate (858“C).
More recent studies have given evidence for the formation of alkali metal during catalytic gasification of carbon using alkali carbonates. McKee and Chatterji (44) found that heating a 1:1 mixture
(by wt) of NSgCOg and graphite in He gave a rapid weight loss above
900°C, accompanied by the evolution of sodium vapor which sublimed on the cooler parts of the apparatus. The vaporization of alkali metal was also observed in the case of K^CO^, but was less marked with
LigCOg.
Huhn et al. (45) studied the interaction of KgCO^ with a bituminous coal, and concluded that potassium was reduced to the metal at temperatures around 700°C. This conclusion was based on a com parison of various reaction paths with weight loss measurements and isotopic labeling of carbonate-carbon. Direct evidence for the forma tion of metallic potassium at 700°C was not given. However, there was strong evidence that metallic potassium was substantially bounded to carbon at this temperature.
Wood et al. (46,47) detected alkali metal atoms in equilibrium with char-alkali metal-catalyst mixtures at 500-700°C by Knudsen cell mass spectrometry. Because the equilibrium pressure of alkali metal was below the value expected for carbothermic reduction of the salt. 23 it was suggested that dissolved suboxides of alkali metal were present in the catalyst melt.
Dunks (48) found that addition of graphite to molten sodium carbonate at 900°C rapidly shifted the rest potential of a gold electrode from -0.511 to -1.348 volts. This was taken to indicate the formation of sodium metal in the melt. A similar potential was reached after addition of metallic sodium to the melt.
In conclusion, it can be asserted that alkali metal will form via reaction (2.1.5) or (P.1.6) at sufficiently high temperatures, but the
lower temperature limit of alkali metal formation is difficult to pinpoint.
It has also been observed that carbon catalyzes the decomposition
of alkali carbonates. Thus, several investigators have found that mixtures of alkali carbonates and carbon evolve CO^ at substantially lower temperatures than the alkali carbonate alone (49-51). For
example, Mims and Pabst (50) reported that char-KgCO^ mixtures evolve
COg at temperatures as low as 500°C. However, impurities in the char may be partly responsible for the CO^ evolution (36,55). The forma
tion of alkali metal under these low temperature conditions seems unlikely.
2.1.8.2. Catalysis of the Carbon-CO^ Reactions. Of the various reactions catalyzed by alkali carbonates, the carbon-CQ^ reaction is most applicable to the present investigation.
Alkali carbonates generally lower carbon-CO^ gasification tem
peratures by 100-300°C (on an equivalent rate basis). Catalyzed 24 gasification by CO^ first proceeds at a measurable rate as tempera tures are increased to around 700-750°C for graphite (52) and around
600-700°C for coal char (53).
The catalytic effect of alkali carbonates on the carbon-CO^ reaction depends on the alkali cation. Using high purity graphite,
McKee and Chatterji (52) found the catalytic activity to decrease in the following order: Li > Cs > K > Na, and to be correlatable with the melting points of the pure carbonates (Na^CO^ was an exception). In other studies using coal chars (53,54), the order of catalytic ac tivity decreased as Cs > K > Na > Li. Because the catalytic activity of the char showed little dependence on the mineral matter content and available surface area, the difference in catalytic activity between graphite and char was attributed to the residual hydrogen content of the char. A mechanism was proposed involving alkali metal hydrides.
The effect of coal rank on catalytic activity was studied by
McKee et al. (55). For a given catalyst loading, the catalytic effect increased with the rank of the parent coal. Thus, 10 wt % KgCO^ increased the rate of graphite-CO^ reaction by a factor of 4000, but increased the lignite-CO^ rate by only a factor of 3.0 at 700®C.
An interesting finding by McKee (56) was that the rate of weight loss of graphite-alkali carbonate mixtures was greater in He than in
COg. This result was attributed to greater salt dissociation and vaporization in the presence of He. The behavior also indicated a similarity between the catalyzed carbon-CO^ reaction and the Gay-
Lussac reaction. 25
Other studies of the carbon-CO^ reaction catalyzed by alkali carbonates have been recently reported (57,58).
2.1.8.3. Catalytic Mechanism. Although the mechanism of alkali carbonate catalysis is not fully understood, the proposed mechanisms may be applicable to the present investigation. Several reviews cover details of the various mechanistic theories (33,35-37, 59). Salient features will be given here.
Basically, two general types of mechanisms have been proposed to explain the catalytic activity of various catalysts, including alkali carbonates, on carbon gasification reactions. The oxygen-transfer theory, originally proposed by Kroger et al. (60), involves a cyclic oxidation-reduction ofan active form of the catalyst. The electron-
transfer theory, originally proposed by Long and Sykes (61), involves
transfer between the ir-electrons of graphite and vacant orbitals of
the catalyst. In the case of alkali carbonate catalysts, the distinc
tion between these two theories is not clear-cut at times, as revealed below.
McKee (52,59) proposed the following mechanism for the carbon-CO, reaction. For Na^CO^ and K^CO^:
MgCOg + 2C + 2M + 3C0 (2.1.6)
2M + COg MgO + CO (2.1.7)
MgO + COg - MgCOg (2.1.8)
For LigCOg:
LigCOg + C - LigO + 2C0 (2.1.9)
LigO + COg - LigCOg (2.1.10)
McKee (56) also suggested the following possibility: 26 C MgCOg ^ MgO + COg (2.1.11)
MgO + C + 2 M + CO (2.1.12) followed by reactions (2.1.7 - 2.1.8). Steps (2.1.11 - 2.1.12) were also suggested by Huhn et al. (45).
Although evidence for the formation of alkali metal (sodium, potassium) is strong, the subsequent role of the metal is uncertain.
It has been found that the alkali metal can become chemically bound to the carbon. Patrick and Shaw (62) found that a considerable portion
(30 to 74$) of the sodium carbonate initially added was retained by the coke in a strongly bonded form after carbonization to 1000°C.
Huhn et al. (45) added potassium metal to coke and determined that a substantial amount of the potassium was retained as a "K-C" complex, which decomposed above BOO®C.
Various species containing alkali metal have been suggested as active intermediates, such as intercalation compounds (35,63-65), surface salt complexes (49,50,66) and alkali metal suboxides (46,58).
Franke and Meraikib (64) suggested that Na intercalates into the graphite lattice and acts as an electron donor. An electron transfer may promote the following chemisorption and dissociation steps:
COg + e" + COg" (ads) (2.1.13)
COg + e“ - CO + O” (ads) (2.1.14)
Wen (35) proposed the following mechanism involving intercalation compounds:
MgCOg + 2C ^ 2M + 300 (2.1.15)
2M + 2nC - 2 C^M (2.1.16) 27
2 C M + CO- -♦ (2 C M)OCO (2nC)M_0 + 0 (2.1.17) n c: n c (2nC)M20 + COg (2nC)M2C0g - 2nC + MgCO^ (2.1.18)
The stability of intercalation compounds at gasification temperatures has been disputed (33.37,59). Randin (34) and Wen (35) review inter calation compounds of alkali metals in some detail.
Surface salt complexes involving C-O-K structures, such as phenoxide groups, have been postulated by several investigators
(49,50,66). The key chemical entity involves bonding of alkali metal to oxygen-containing anionic groups on the carbon surface. The stability of surface groups such as phenoxide is apparently enhanced by the interaction with the alkali catalyst.
Wood et al. (46) found a significant increase in electrical conductivity of coal char containing 10 wt % KgCO^ after exposure to temperatures around 700°C (from 5 x 10 to approximately 0.1 ohm ^ 1 cm ). Based on this and other experimental evidence, they postulated the presence of non-stoichiometric oxides containing an excess of alkali metal. This suboxide would be present in a dissolved state at gasification temperatures, and would participate in electron transfer processes. A detailed mechanism involving an aromatic array was proposed.
Kapteijn and Moulijn (58) represented the catalytic mechanism as
follows:
*x°y + COg » + CO (2.1.19)
V y M ^ ^ " KxOy + CO (2.1.10)
Based on electrochemical studies of graphite oxidation in molten sodium carbonate. Dunks (48) presented the following sequence: 28
NSgCOg + C •* 2Na + CO^^" + "cf*" (2.1.21)
+ CO + COg (2.1.22)
2Na + COg NagCOg (2.1.23)
NagCOg - NagO + CO (2.1.2%)
NagO + COg - Nag CO (2.1.25)
Overall reaction: C + COg •* 2C0 (2.1.26)
2+ The "C " represented positive centers in the carbon matrix, and were produced by transfer of electrons from carbon to sodium ions in the melt according to reaction (2.1.21).
Wigroams et al. (67) took a combined approach and proposed a model involving three different types of active catalyst species:
1) loosely bonded metal-oxygen complexes
2) intercalated metal-carbon complexes
3) tightly bonded metal-oxygen complexes
Finally, an electrochemical mechanism, including both electron and oxygen-transfer steps, has been proposed by Jalan and Rao (57).
The mechanism relies on the observation of many investigators that the working catalyst forms a liquid phase that wets the carbon surface.
Two alternate schemes were proposed:
Scheme I
Anodic Site: Q . Q ^ ~ + 2C + 3C0 + 2e~ (2.1.27)
Cathodic Site: 2\\ + COg + 2e” -► MgO + CO (2.1.28)
Regeneration: MgO + COg ■* MgCO^ (2.1.29)
Scheme II
Cathodic Site: Z 0 ^ ~ + 2e" - CO + 20^“ (2.1.30)
Anodic Site: C + 0^“ -*■ C - 0 (ads) + 2e" (2.1.31) 29
Desorption: C - 0 (ads) -* CO (2.1.32)
It should be mentioned here that a principal difference exists between alkali carbonate catalysis of carbon gasification and electrochemical oxidation of carbon in molten alkali carbonate in that the alkali carbonate/carbon loading ratio is considerably lower in the catalysis case. Although it is recognized that the catalyst is prob ably molten during gasification, a substantial difference in mechanism may be expected due to the different carbonate/carbon ratio, unless saturation occurs.
2.2. Characteristics of Molten Alkali Metal Carbonates
A considerable amount of material has been published relating to molten alkali metal carbonates. The most complete and up-to-date review on the subject has been compiled by Selman and Maru (68). In this section, the characteristics which pertain to the current inves tigation will be reviewed.
2.2.1. Reasons for Using Molten Alkali Carbonates
Of the various molten salt systems available, molten alkali metal carbonates are the preferred choice as a carbon oxidation medium. The major oxidation product of carbon, namely COg, does not adversely affect the carbonate melt. In fact, a small but finite COg pressure is necessary to prevent decomposition of the carbonate ions. In other molten salts, the anions tend to react with COg to produce carbonate ions, which gradually changes the composition of the melt. For ex ample, with an hydroxide melt, the following reaction is favored,
20H" + COg COgZ" + HgO (2.2.1) 30 so that the hydroxide melt is converted to a carbonate melt.
The invariance of molten carbonate is particularly crucial in fuel cell applications. At the anode, carbonate ions are consumed in the fuel oxidation reaction, but the evolved CO^ can be fed to the cathode to regenerate carbonate ions. In this manner, the total carbonate content remains invariant. The importance of melt in variance has been stressed by several authors (68-70).
Other advantages of molten carbonates include:
(1) Low volatility at high temperature
(2) Relatively low melting point when mixtures are used
(3) Low toxicity
(4) Low cost
(5) Catalytic effect on coal reactivity
A chief disadvantage in the use of molten carbonates is the high corrosivity.
2.2.2. Physical Properties
A very complete compilation of the physical properties of alkali carbonates has been published by the National Bureau of Standards
(71). Pertinent properties from selected sources are discussed below.
2.2.2.1. Melting Points. The melting points of the three common alkali carbonates (Li^CO^, Na^CO^ and KgCO^) and their mixtures are given in Table 5. As illustrated in the table, three binary eutec
tics, one ternary eutectic, a congruently melting compound (LiKCO^)
and a non-congruently melting compound (LiNaCO^) have been experimen
tally identified. The ternary eutectic offers the lowest melting
point. 31
Table 5. Alkali Carbonate Melting Points. Adapted from References (68) and (71).
Composition Type of System molt (wtt) System Melting Point (°C)
LigCO, 100 (100) pure component 723 ± 3
Na^COs 100 (100) pure component 858 ± 1
K^CO, 100 (100) pure component 898 ± 3
LiNaCOg Li-50.3(41.4) incongruently 501 Na-A9.7(58.6) melting compd
LiCOg-NZgCOg Li-52(A6.3) eutectic 501 Na-i»8(53.7)
LiKCOg Li-50(3%.8) congruently 504.5 K-50(65.2) melting compd
LigCO -KgCO Li-42.7(28.5) eutectic 498 K-57.3(71.5)
LigCOg-KgCO, Li-52(46.6) eutectic 488 K-38(53.4)
NagCOg-KgCOg Na-56(49.4) low-melting 710 K-44(50.6) mixture
LigCOg-NagCOg- Li-43.5(32.1) eutectic 397
KgCO, Na-31.5(33.4) K-25.0(34.5) 32
2.2.2.2. Penalty. Densities of various molten alkali carbonates are given in Table 6. Density-temperature correlations can be found in references (68,71). Within the 500->900“C temperature range, the density values generally fall between 1.8 and 2.0 g/cc.
2.2.2.3. Surface tension and contact angle. The surface tensions of
LigCOg, NSgCOg, KgCOg and mixtures thereof are approximately 200 dyne/cm, which is about three times higher than aqueous systems. The surface tension decreases slightly with increasing temperature, and correlations can be found in references (68,71). The surface tension is independent of gas environment as long as sufficient CO^ is present to prevent melt decomposition. The surface tension can be increased by oxide addition (e.g., LigO) and decreased by hydroxide addition
(72,73).
The contact angle reflects the degree of surface wetting of a solid by a liquid. For the ternary carbonate eutectic, the contact angles for various substances are given in Table 7. Complete wetting
(contact angle of 0) occurs on metals in CO^ or helium, but in the contact angle depends on the nobility of the metal. Graphite is not appreciably wetted by molten carbonate in H^, CO^ or He atmosphere.
Increased wettability occurs at higher temperatures (7%). The contact angle on graphite in an 0^ atmosphere has not been reported, but in all probability it would be near zero, since formation of oxy- compounds on the surface usually promotes complete wetting.
2.2.2.4 Viscosity. Viscosities of pure alkali carbonates and the ternary eutectic are shown in Table 8. The viscosities of other carbonate mixtures and temperature correlations can be found in 33
Table 6. Densities of Molten Alkali Carbonates. Adapted from Reference (68).
Density (g/cc) (mol %) 31 III • p • (°C) 500 “C 700 °C 900 “C
LlgCO, 1.834 (723) - - 1.766
NSgCOg 1.971 (858) - - 1.953
KgCO, 1.898 (898) - - 1.897
LigCOg-NagCOg (53-%7) 2.023 (501) 2.023 1.937 1.851
LigCOg-KgCOg (43-57) 2.015 (498) 2.014 1.921 1.829
LigCOg-KgCOg (50-50) 2.006 (505) 2.008 1.917 1.826
LigCOg-KgCOg (62-38) 2.008 (488) 2.002 1.957 1.821
NagCOg-KgCOg (58-42) 2.008 (710) - - 1.924
LigCOg-NagCOg-KgCOg
(43.5-31.5-25) 2.148 (397) 2.092 1.984 1.875 34
Table 7. Contact Angles of Ternary Carbonate Eutectic with Various Materials at 400°C. Adapted from Reference (68). Original work by Moiseev and Stepanov (74).
Material Contact angle (Degrees) co^ He «2
Pt 53 0 0
Pd 51 0 0
Ag 46 0 0
Ni 53 0 0
Fe-Ni-Cr-Al-Ti alloy 8 0 0
Zr 0 - -
NiO - 0 0
Graphite 70 85 50
Boron nitride 90 90 105 35
Table 8. Viscosities of Molten Alkali Carbonates. Calculated from correlations In Reference (71).
Temperature °C Viscosity (cp, ±30$)
(a) LlgCOg
723 (mp) 4.67
800 3.93
850 2.85
(b) NagCOg
848 (mp) 4.55
900 3.00
950 1.89
(c) KgCOg
898 (mp) 3.70
950 2.16
1000 1.34
(d) Ternary eutectic
397 (mp) 13.9
500 4.80
700 1.15
900 0.50 36 references (68,71). Within the temperature ranges indicated, the viscosity values of the pure alkali carbonates are two to five times the value of room temperature water (1 cp). For the ternary eutectic, a wider temperature range is tabulated (397-900°C), and the viscosity values vary from 13.9 to 0.50 cp. However, the original experimental data was measured over a temperature range of 483 to 600°C (75), and extrapolation outside this range should be considered tentative.
2.2.2.5. Diffusivity. The diffusivity of dissolved gases (e.g. CO^,
CO, Hg, Og) in carbonate melts is of the same magnitude as in aqueous -5 2 systems - roughly 10 cm /s. Experimental diffusivity data of dis solved gases is scant and inaccurate, but fair approximations can be obtained using the Wilke-Chang equation (68).
Diffusivities of dissolved ions in carbonate melts have been measured and appear fairly accurate. Self-diffusion coefficients of + + 2“* “5 2 tracer ions (e.g., Na , K , 00^ ) are in the 10 cm /s range.
Diffusivities of a number of different cations (e.g., CO , Pb , Q+ —5 2 Fe ...) are also in the 10 cm /s range. However, diffusivities of halide ions are one to two orders of magnitude lower, and the reason for the lower values is uncertain (68).
2.2.2.6. Thermal Conductivity. Experimental thermal conductivity data is available only for pure LigCO^ and NagCO^, and includes solid as well as liquid values. An empirical correlation has been reported
that can be used for any carbonate mixture, and agrees fairly well with the experimental values. The thermal conductivity of molten carbonates is close to 2 W/m-K, or about 3.5 times greater than the conductivity of water (68). 37
2.2.2.?. Electrical Conductivity. Much experimental data is avail able on the electrical conductivity of molten carbonates. Since molten salts are essentially ionic liquids, the electrical conduc- “1 tivity is quite high - on the order of 1 ohm cm . Electrical conductivity data for the ternary carbonate eutectic are correlated by an Arrhenius-type equation and numerical values are given in Table 9.
The conductivity increases significantly with increasing temperature.
Values for other carbonate compositions are readily available (68,71).
Because of the high electrical conductivity, ohmic overpotential
(IR drop) in free electrolyte systems can ususally be neglected (76).
Table 9. Electrical Conductivities of the Ternary Carbonate Eutectic. Adapted from Reference (71).
Empirical correlation: K - 03.8192 exp[-37l6.7/T]
Temperature (°C) K (ohm ^ cm ^)
500 .68H
600 1.187
700 1.838
800 1.62A
900 3.526 38
2,2.2.8. Solubility of Gases. The maximum current that can be ob tained from an electrode reaction involving a dissolved gas is largely dependent upon the gas solubility. The solubilities of a number of different gases have been measured, but in some cases the data from different investigators are not in agreement. Most of the deviations, however, can be reconciled in terms of physical versus chemical solubility. Some methods of measurement determine only the physical solubility, while other methods determine the combined physical and chemical solubility, which is naturally larger in magnitude. Chemical solubility occurs when the dissolved gas reacts with the melt to form another dissolved substance. For example, CO is believed to dissolve chemically by the following reaction;
CO + COgZ" -► z o ^ ~ + COg (2.2.2)
30 that the chemical solubility of CO is determined by the concentra-
2- 2“ tion of dissolved CO^ . The chemistry of CO^ , however, is not well understood (68).
The solubilities of CO^, H^, CO, and 0^ in various carbonate melts are given in Figure 2, which was originally reported by Broers et al. (77,78), and reproduced by Selman and Maru (68). The solubility is expressed in terms of a Henry’s law constant, H^, and the data is taken to represent combined physical and chemical TEMPERATURE. »C
9 0 0 BOO 700 60 0 3 00 • 4
l - s
"O .00(11 lU) • UjCOj (U4Na)> 53mol»U%CO;+47mol%N@,C^ (U4K)* 30mol% UfCO,* 30mot%Na;CO, (N«+KI ' 60mol% No%C0^40m»l%K2CO, 6 (I ) » 4 3 mol % li%CO,+ 32 m*l % N«%CO, 4 2 3 mol % KjCOi 0.11)
9 ^Oy(U+No) I 0 » (U ) -T 0 6 0» 10 II 1.2 1.3 lO’/T . K"'
Figure 2. Gas Solubilities in Molten Carbonate. Reprinted from Reference (68), p. 211, by permission of the publisher (Plenum Publishing Corp.). Original data from Broers et al. (77,78), reprinted by author's permission. U>VO 40 solubility. With the exception of hydrogen, all gases show increased solubility with increasing temperature.
In Figure 3, the solubility of CO in different carbonate melts is plotted, and in this case the data is taken to represent physical solubility (79). Comparing the CO solubility from Figures 2 and 3, it
is apparent that the chemical solubility increases by one order of magnitude.
2.2.2.9. Solubility of Non-Gases. Relatively little data exists on the solubility of materials such as metals, metal oxides, metal carb onates and other non-oxide compounds in molten carbonate. Solubility data for these materials would be quite pertinent if the dissolved species were electroactive.
In general, the solubility is a function of the temperature, CO^ pressure (or oxide activity), and, if the different oxidation states are Involved, the oxygen pressure (or electrochemical potential). For
low concentrations, the solubility of metal oxides can be calculated
from free energy values by assuming the activity is equivalent to the mole fraction (activity coefficient of unity). This calculation is possible because the dissolution of metal oxides occurs via chemical reaction, as illustrated for FeO in the following equation:
FeO(s) + M2C0g(&) - MgFeOgCt) + ^^(g) (2.2.3)
Quantitative solubility data in molten carbonate are available
for NaAlOg (80,81) NiO (80,82), MgO (83), CaO (84), and AggO (85).
The solubility of FCgOg in molten sodium sulfate has been measured
(86), and may roughly correlate to the solubility in molten carbonate. 1*1
TEMPERATURE.*C 5 850*C 825'C 800-C 775*C 750*C 700*C ixlO I ------1
0 58NO/42K/C03 A 42.7U/57.3K/C03 O 53.3 Li/46.7 No/COj V 43.5 Li/31.5 No/25.0 K/CO; O Lij COj
IX10 E ô Ê
/mole
1.05 lOOO/T.CK)
Figure 3. Solubility of CO in Molten Carbonate. From Reference (79), p. 1657. Reprinted by permission of the publisher, The Electrochemical Society, Inc. 42
Of a qualitative nature, it is known that silica, silicates, chromia and chromâtes are highly soluble in molten carbonates, whereas alkali aluminates are highly insoluble (68,80).
It should also be mentioned that alkali metals in the metallic state exhibit a certain solubility in molten salts. By far, the most data is available for molten halides where the metal solubility can be appreciable (87,88). No data could be found relating to solubility of alkali metals in molten carbonate, however.
2.2.2.10. Vapor Pressure. Molten carbonates do not appear to have a vapor pressure per se; no experimental confirmation of gas phase MgCOg molecules have been reported. Under vacuum, volatilization of sodium carbonate has been attributed to the following reactions (68,89):
Na2C0^(i) Na^OOL) + ^^(g) (2.2.4)
NagO^t) - 2Na(g) 1/2 0^(g) (2.2.5)
Thus, the vapor above a carbonate melt is governed by dissociation reactions, and the vapor presure is determined by the equlibirum constants of the dissociation reactions.
Although the vapor loss from molten carbonates is very low, it becomes quite important in fuel cell applications. Selman and Maru
(68) discuss the mechanism of electrolyte volatilization in some detail.
2.2.3. Chemical Properties
Molten carbonates can undergo a variety of chemical reactions; the important ones are discussed below. U3
2.2.3.1. Carbonate dissociation. The dissociation reaction can be expressed by either of the following equations:
MgCOg 4. MgO + COg (2.2.6)
C0g2" - 0^' + COg (2.2.7)
The latter equation illustrates the fact that the dissociation reac tion can be viewed as an acid-base type equilibrium: 2“ Base -* 0 + acid
This equilibrium is analogous to aqueous systems, in which serves as the acid-base linkage:
Acid + H* + Base
2- 2- A melt high in 0 concentration is termed basic, and low in 0 concentration is termed acidic. As reaction (2.2.7) indicates, the
2- 0 concentration can be minimized by sufficiently great COg pressure.
2.2.3.2. Hydrolysis. In the presence of water, molten carbonate reacts according to:
MgCOg + HgO - 2M0H + COg (2.2.8)
2.2.3.3. Oxide Reactions. In the presence of oxygen, the alkali oxide (MgO) can be oxidized to peroxide (MgOg) and superoxide (MOg):
MgO + 1/2 Og + MgOg (2.2.9)
MgOg + Og + 2M0g (2.2.10)
2.2.3.4. Alkali Metal Reduction. In the presence of a suitable re ducing agent, alkali metal ions can be reduced to the metallic state. As previously mentioned in Section 2.1.8.1, carbothermic reduction by the Gay-Lussac reaction can take either of the following two pathways:
MgCOg + C + 2M + COg + CO (2.2.11) or
MgCOg + 2C + 2M + 3C0 (2.2.12)
The reduced alkali metal can be formed as a liquid, vapor or dissolved phase, depending on the temperature and the properties of the par ticular alkali metal.
2.2.3.5. Reaction with Sulfur Compounds. Depending on the oxidation state of the sulfur, the following reactions of sulfur-’containing gases can occur:
HgS + MgCOg - MgS + COg + HgO (2.2.13)
SOg + MgCOg MgSOg + COg (2.2.14)
SO^ + MgCOg - MgSOy + COg (2.2.15)
In the presence of both sulfur and carbon, sulfide is formed:
2 MgSOg + 3C + 3C0g + 2MgS (2.2.16)
MgSOy + 2C + 2C0g + MgS (2.2.17)
2.2.3.6. Carbide Formation. Carbide formation from carbonate in the presence of carbon can be expressed by the following reaction:
5C + 2MgC0g - 2MgCg + 3C0g (2.2.18) ^5
2.2.4. Thermodynamic Properties
2.2.4.1. Thermochemocal Data. The JANAF Thermochemical Tables (90) provide a good source of internally-consistent high temperature ther modynamic data. In Table 10, the JANAF Tables were used to tabulate the free energies of formation for the chemical compounds pertinent to this investigation. It should be mentioned that the accuracy of the
JANAF data is sometimes questionable, as pointed out by Anderson (91) and Selman and Maru (68).
The free energy values of Table 10 provide a basis for calcula tion of the free energy changes and equilibrium constants for pertinent reactions. For the carbonate dissociation reaction
(reaction (2.2.6)), the equilibrium constants are listed in Table 11 at various temperatures. As the table reveals, the equilibrium ac tivity of oxide is quite low at CO^ pressures near one atmosphere.
Of the various carbonates, pure Li^CO^ has the greatest oxide activity.
Anderson (91) performed an extensive evaluation of the ther modynamics of alkali carbonates, including measurement of the carbonate dissociation equilibria. His data are considered the most accurate to date (68); Table 12 lists his results. Comparing with
Table 11, it can be seen that the K values differ appreciably.
Anderson's values actually represent "apparent" thermodynamic data, since it was necessary to assume a^2- - x^2- to calculate the results from the experimental measurements. However, the deviation from ideality is not considered sufficiently great to account for the large Table 10. û G^. of Selected Compounds. Interpolated from JANAF Tables (90).
Compound ^ f (kcal/mole) 500 °C 600 700 800 900 LigCOgd) 235.886 230.180 22%.626 219.208 213.909 LigO (s) 119.02% 115.688 112.373 109.08% 105.82% LigO (t) 111.36% 108.608 105.925 103.303 100.738 LigOg (1) 111.899 106.706 101.568 96.%95 91 .%90 LlgCg (s) 11.606 11.200 10.817 10.%53 10.110 NagCOg (t) 216.526 210.58% 20%.806 199.17% 192.999 NagO (s) 7%.5%8 71.222 67.9%3 6%.738 60.925 Na^O (t) 68.5%7 66.190 63.915 61.715 58.909 NagOg (s) 82.%%7 77.%36 72.52% 67.667 62.186 NaOg (s) 36.712 33.663 30.703 27.833 2%.717 Na2Cg(s)* -1.62% -0.58%2 0.%55% 1.%95 2.535 NagSOy (&) 256.661 2%7.153 237.039 227.057 216.517 Na^S (t) 80.015 77.807 7%.891 72.012 68.%95 K C0_ (S,) 219.165 213.%18 206.630 199.151 190.1%7 KgO (s) 60.793 57.598 5%.%89 %9.970 %3.8%5 KgO (&)b 59.238 56.69% 5%.278 50.%75 %%.9%5 KgOg (s) 77.227 72.182 67.2%3 60.933 53.056 COg (g) 9%.5%% 9%.585 9%.619 9%.650 9%.675 CO (g) %3.033 %5.168 %7.288 %9.39% 51.%86 HgO (g) %8.989 %7.701 %6.39% %5.071 %3.733
From John (80) Estimated by Anderson (91). 47
Table 11. Equilibrium Constants for Carbonate Dissociation. Calculated from Table 10.
Reaction: M^CO^d) - M^Od) ♦ COg(g)
‘’CO 0 K - Ê - exp[-ûG"/RT]
System "2 ""3 500'C 600 700 800 900
LI O.BltxIo'®)*®’ (1.75x10'^> (3.89x10'^) 4.68x10”^ 3.58x10"**
Nc (7.78x 10"’®) (3.39x10"’^) (4.03x10“” ) (1.90x10‘^) 4.53x10“® -iq K (3.26x10 (2.77x10 '°) d.07x10“’^) (9.86x10“’^) 3.85x10"'° - m -a LlNaK^’’^ 8.54x10 1.75x10“” 1.35x10”® 4.15x10 ° 6.82x10'^ (ternary eutectic)
(a) Parenthesis Indicates subcooled liquid. (b) K calculated assuming mole fraction additivity.
Table 12. Experimental Equilibrium Constants of Carbonate Dissociation. From Data of Anderson (91).
Reaction: M^CO^d) - M^Od) ♦ COg(g)
’’CO^ Xg2-
System 500 "C 600 700 800 900
LI d.67x10"^)^®^ (5.01x10”^) (7.47x10”®) 6.73x10”“ 4,l7x10'^
lia (6.44x10”’'*) d.32xio”” ) (9.06x10” ’°) (2.83x10”®) 4.91x10”^
K (2.23x10” ’^) (9.85x10'’“ ) (1.24x10"” ) (6.37x10”’°) 1.67x10'® -11 -A LlHaK 1.77x10 1.79x10”^ 6.99x10 ° 1.38x10"^ 1.64x10'^ (ternary 1 uteetlc)
(a) Parenthesis Indicates subcooled liquid. K8 large difference between Anderson's values and the values calculated from the JANAF tables.
Anderson (91 ) also measured thermodynamic properties of the hydrolysis reaction (reaction (2.2.8)), and the 0^ - 0 ^ - 0^ reac tion system, and the results have been summarized elsewhere (68).
2.2.4.2. Thermodynamic Diagrams. Free energy data for a large number of carbonates, oxides and carbides from selected sources have been tabulated by John (80), and were used by him to construct phase stability (Pourbalx) diagrams for various chemical systems. The phase stability diagram for the Na-C-0 system Is reproduced In Figure 4.
Other dlagramed-systems Include Nl-Na-C-Q, Cr-Na-C-0, Co-Na-C-0, Fe-
Na-C-0, Sl-Na-Co-Q and Al-Na-C-0 (80). With carbon present In the system, the equilibrium conditions are restricted to the carbon saturation line (C+O^ - COg). Thus, at a given COg pressure, the Og pressure In equilibrium with carbon Is uniquely defined.
McKee (33,5%) has reported various phase stability diagrams for the carbon-alkall carbonate system, two of which are reproduced In
Figures 5 and 6. The diagrams were employed for mechanistic Inter pretations of alkali carbonate catalysis of gas-carbon reactions. It can be seen that the equilibrium pressure of alkali metal Is quite low at CO pressures close to 1 atm.
Bartlett and Johnson (92) constructed Elllngham diagrams for carbonate reduction reactions, but they assumed unit activity of alkali metal oxide, which Is not directly applicable to the present
Investigation. 49
log 0,No,0 0 - 4 -8 + 4 NaO,-solid data 0.2
Na.O.- salld data 0
•OniiO — ---■ • — T ” lO"*otm 0.2 - 4 No.O
-0 .4 8 No.O, No.CO -0.6 (T O' o" -12 No,0 (s ) O'
-16
- 1.0
UJ -20
Pu.«latm - 2 4 - 1 .4
Na (g) Region of Unstable Gases
-16 -12 -8 - 4 0
Figure 4. Phase Stability Diagram for the System Na-C-0 at 900°C. Reprinted from Reference (80), p. 29, by permission of the author. 50
0
M,CO
-2
-3
•4
-5
•6 MjC0j»2C= 2 M ( g U 3 C 0 827*C(II00 Kl
T
Figure 5. Equilibrium Stability Regions of Alkali Metals at 827“C. Reprinted from Reference (33). p.65, by courtesy of Marcel Dekker, Inc.
-400
^ -600
J -800
-1000
0 -400 -600 -800 -1000 Log P, ( k )
Figure 6. Equilibrium Stability Regions of Potassium Metal at Various Temperatures. Legend: A - 1000“C; B - 900°C; C - 800“C; D - 700°C; E - 600®C. Scale is 100 log (atm). Reprinted from D.W. McKee, Fuel, 62, 170 (1983), by permission of the publishers, Butterworth & Co. (Publishers) Ltd. ©. 51
2.2.M.3. Excess Properties. The thermodynamic properties discussed so far apply only to pure component equilibria. In the general case, the thermodynamic properties of mixtures must be considered; not only mixtures of individual carbonates, but mixtures of the carbonates with other species such as oxides, hydroxides, and silicates as well. The
interactions of the various components in a mixture alter the chemical activity compared to that in the pure state.
Anderson (91) critically evaluated excess properties of alkali
carbonates from available phase diagrams and mixing data. Selman and
Maru (68) reviewed the work of Anderson and that of other
investigators. In general, the data available are unreliable due to
experimental complexities. However, after considering the most reli
able data, the general conclusion is that the deviations from ideality
are relatively small, and ideal solutions can usually be assumed.
This assumption indicates that species such as oxides and hydroxides
would have activities coefficients close to one. Selman and Maru (58)
also point out, however, the conflicting result of Stern (93), who
measured very low activity coefficients of oxide in Na^CO^.
Additional work is needed to clarify this apparent anomaly.
The solution behavior of the ternary carbonate eutectic was
discussed by Selman and Maru (68), who mentioned that Na^CO^ behaves
almost ideally (activity coefficient of one), and KgCO^ and Li^CO^
have activity coefficients of less than one half.
John (80) determined activity coefficients of NaNiO^ and NiCO^ in
NagCOg, and the values were close to unity. 52
2.2.5. Electrochemical Properties
2.2.5.1. Reference Electrodes. A general treatment of reference electrodes in molten salts has been given by Laity (94). Selman and
Maru (68), Anderson (91) and Boruka (95) reviewed specific reference electrodes which have been utilized in molten carbonates.
In many respects, the 33% 0^, 67% COg/COg^ /Au reference electrode features the most desirable characteristics, and its use has been favored in many molten carbonate studies. This reference electrode is based on the following half-cell reaction:
2C0 2 (R) + OgCR) + 4e" ■* 2 C Q ^ ~ (R) (2.2.18) where R refers to "reference” gas. Reaction (2.2.18) has been shown
to follow Nernstian behavior accurately over a wide range of tempera
tures and partial pressures (96-100). The half->cell potential
according to reaction (2.2.18) is:
£(00^,0^) - EO^COg.Og) + ^ &n (R)]^[Pq (R)] (2.2.19)
2“ with the CO^ activity taken as unity. To express the potential of
2- another electrode reaction relative to the O^'COg/COg reference, the half-cell reaction must first be written. For the case of carbon
oxidation, the half-cell reaction and half-cell potential are:
BCDgfW) + 4e" + C + 2C0g"2(w) (2.2.20)
£(0,00^) - E°(C,C02) + II %n[Pgo (W)]^ (2.2.21)
2- where W refers to "working" gas and the CO^ activity is again unity.
The overall cell reaction is accordingly:
S C O ^ m 4. c + 2C02(R) + OgiR) (2.2.22) 53 2“ and the cell potential realtive to the 0^, COg/CO^ reference electrode is:
^cell ■ " cell * «F
,o -ûG° where E - —]jp- for the following reaction:
COg + C + Og (2.2.2%)
The C0g,0g reference gas is usually set at0.333 atm Og and 0.667 atm COg because this composition is most noble on the potential scale, and the potential is least sensitive to fluctuations in composition.
Gold is the preferred electrode material due to its high nobility; Pt can corrode in molten carbonate, and it takes longer to equilibrate
initially. The equilibration time for gold is 2-3 hours at 600®C, whereas for platinum it occurs in about 20 hours (97).
2- The 0g,C0g/C0g reference electrode differs from the standard oxygen electrode (SOE). The latter reference is based on the reaction:
2cf" - Og + %e" (2.2.25) with the oxygen pressure at one atmosphere. The SOE is not employed
experimentally in carbonate melts, but serves as a useful ther" modynamic reference reaction.
Another nobel metal/gas electrode that has been utilized to a 2” certain extent is the CO.COg/CO^ electrode, as governed by the
following reaction:
2C0g(R) + 2e“ CO(R) + COg^'fR) (2.2.26)
The half-cell potential is: 54
RT ECCO.COg) - E^tCO.COg) + ^ &n H p (%) (2.2.27)
The standard CO,COg electrode is defined as corresponding to /P^g
» 1 atm, at 1 atm total pressure (P^g - 0.382 atm, Pgg - 0.618 atm).
This reference electrode has been extensively evaluated by Borucka
(95,101), anc Borucka and Sugiyama (100).
The standard CO,COg electrode works quite satisfactorily at temperatures above 650°C. However, below 650°C, a restriction occurs due to the Boudouard reaction, which limits the CO pressures that can be utilized.Another complication arises from the following reaction;
CO + 20H C0g2~ + Hg (2.2.28) which seems to interfere when moisture is not adequately excluded
(102). These factors and the high toxicity of CO have limited ex perimental use of the co.COg reference electrode.
The configuration of the noble metal/gas reference electrodes typically takes the form of a closed->bottom alumina tube which con tains the reference melt. Ionic contact with the outside melt is established by a restricted channel, such as a pinhole, in the alumina tube. In the reference compartment, the electrode wire (e.g., gold) is immersed directly into the melt, and the reference gas flows con tinuously over the melt (or is bubbled through it).
Anderson (91) used an Og, COg/Au reference electrode consisting of a closed-bottom alumina tube which was made "porous" by cutting the bottom of the tube, thus creating a thin diaphragm. The internal re sistance between the test and reference electrodes was 1-10 kilo-ohm. 55
John (80) employed a beta-alumina tube (Na* conductor) as a reference electrode sheath in molten sodium carbonate at 900“C. With
Og.COg and CO,COg gas mixtures, the open-circuit potentials agreed well with thermodynamic values.
Kim and Devereux (103) used a non-porous alumina tube (McDanel
998) as a C0,C0g/Pt reference electrode sheath. The tube resistance was quite high (about 1 megaohm-cm), but satisfactory behavior was found at 1000°C.
Regardless of the type of reference sheath employed, the junction potential for the noble metal/gas reference electrode should be zero.
The melt on both sides of the sheath has essentially identical ionic composition, which eliminates the possibility of a junction potential
(94,104). This assumes that the ionically conducting species in the sheath are the same as in the melt, which is true in most cases.
Reference electrodes of the metal/metal ion type have also been used in molten carbonates, the most notable one being the Danner-Rey
Ag/Ag* electrode (105). These reference electrodes are less desirable due to the presence of liquid junction potentials and dissolution of silica from the porcelain sheaths. The Danner-Rey Ag/Ag* electrode has been measured to be -570 ±20 mV versus 33$ Og, 67$ COg/CO^ /Au at
600°C (105).
2.2.5.2. Potential^pCOg Equilibrium Diagrams. Similar to the phase stability diagrams discussed in Section 2.2.4.2, equilibrium diagrams can be constructed in which the potential is plotted as a function of log PçQ or pCOg (pCOg - -log Pgg ). The potential-pCOg diagrams are analogous to Pourbaix diagrams in aqueous electrolytes, in which the 56 potential is plotted as a function of pH. These diagrams are very useful for defining the regions of stability of the various components. However, the diagrams only define the equilibrium condi tions, and do not indicate which reactions are kinetically favored.
Potential-pCOg diagrams for molten carbonates have been reported by
Pourbaix (106,107) and Ingram and Janz (108).
The potential-pCOg diagram of Pourbaix (106,107) is reproduced in
Figure 7. This diagram considers the species C^CO.COg.Og in equi librium with a carbonate melt. The potential is expressed versus the
2- standard oxygen electrode (0^/0 ,P^ - 1 atm). Lines of constant total pressure (Pq + P^^ + P^q ) are indicated on the diagram. The dashed lines indicate the stability limits of the melt. Above the
Og/COg line, carbonate decomposes into Og and COg at the pressures indicated. At low COg pressures, carbonate reduction forms CO as indicated by the COg/CO dashed line. At higher COg pressures, carb onate is first reduced to carbon. When carbon is present in the carbonate melt, the Boudouard equilibrium will force the E-pCOg condi tions to the carbon saturation line (line a), and the CO pressure will be uniquely determined. However, if the Boudouard equilibrium is not established, the CO pressure can be varied independently of the COg pressure. For example, consider the reaction C + 1/2 Og + CO or the corresponding half-cell reaction C + 0 + CO + 2e . At a fixed COg pressure, the CO pressure will be defined by the Boudouard equilibrium, and this value of CO will put the potential-pCOg condi tion on the carbon saturation line. However, if the Boudouard 57
log (O'-) -7 - 6 -5 -4 “ 3 "2
CO..
-2 -3 CO: > X UJ
CO
CO. cC
-2 0 2 - 3 - 4 - 5 6 log p CO,
Figure 7. Potential - pCO^ Diagram for the System C-CO-CO^-O^ in Molten Carbonate at 600“G. Reprinted from Reference (107), p. 309, by permission of the publisher (Plenum Publishing Corp.). 58 equilibrium is not assumed, lines of constant CO pressure can be plotted independently of the carbon saturation condition. Randin (34) illustrated a potential-pCOg diagram constructed by Pourbaix (106) in which lines of constant CO pressure were plotted.
The potential-pCOg diagrams of Ingram and Janz (108) were con structed mainly to study the corrosion behavior of metals. In their diagrams, the equilibrium of various metal-metal oxide-metal carbonate systems is superimposed on the ternary eutectic domain. The voltage scale is with respect to a CO^/0^ reference electrode (0.667 atm COg,
0.333 atm O^). The diagrams do not generally consider the carbon equilibrium condition; instead the negative voltage limit is set by alkali metal reduction. The potential-pCOg diagram for the gold- ternary eutectic at 600®C is shown in Figure 8. The figure indicates that gold should be completely immune from attack unless very positive potentials are encountered (pr... ;l”e of 0.0 volts at one atm COg).
2.2.5.3» Anodic Reactions. In this section, the .. »dic reactions of
2- carbon, carbonate, oxide, CO, H^, hydrocarbons and CO^ will be discussed.
2.2.5.3.1. Carbon oxidation. The electrochemical oxidation of carbon in molten carbonate favors CO^ at lower temperatures and CO at higher temperatures:
C + 2C0g2" 4. gcOg + 4e~ (2.2.29)
C + C0g2" + CO + COg + 2e" (2.2.30)
2C + C0g2" -*■ 3C0 + 2e" (2.2.31 )
The characteristics of carbon electro-oxidation in molten carbonate and other molten salts are treated separately in Section 2.3. 59
40 lo,(Au'*l
20
Au.O •20
L-ZO 40 «I -40 Au M E T A L
20 UJ -60
-2 -80
-20 -3 20 -10 pCO; -40 -60 -80 log (0%)
Figure 8. Potential - pCOg Diagram for Gold in Ternary Carbonate Eutectic at 600“C. Reprinted with permission from Reference (108), p. 790, Copyright 1965, Permagon Press, Ltd. 60
2,2.5.3.2. Melt Oxidation. The oxidation of a carbonate melt occurs either by discharge of oxide ions or direct discharge of carb onate ions:
0^" - 1/2 Og + 2e“ (2.2.32)
COgZ- ^ COg + 1/2 Og + 2e" (2.2.33)
It should be noted that the equilibrium potentials of these reactions 2“ 2“ serve as the basis for the 0^, CO^/CO^ and Og/O reference electrodes, as discussed in Section 2.2.5.1.
In general, two waves can be observed upon anodic polarization, the first generally ascribed to reaction (2.2.32) and the second to reaction (2.2.33). In acidic melts, reaction (2.2.32) predominates only at low current density owing to the low activity of oxide ions.
In basic melts, reaction (2.2.32) can proceed at much higher current 2 densities (10-100 mA/cm ). As the limiting current density for reac tion (2.2.32) is approached, reaction (2.2.33) becomes dominant and, owing to the large supply of carbonate ions, does not reach a limiting current density.
The polarization behavior of the melt oxidation reaction has been studied using Pt electrodes (109-115), Au electrodes (111,116) Ag electrodes (111,114) and Fe,Co, and Ni electrodes (114). Only Au electrodes are substantially free from corrosion under melt oxidation conditions.
Smirnov et al. (116) studied the oxidation of Li^CO^-Li^O mix tures on gold electrodes at 750“C. The amount of gold dissolution in a melt containing 7.1 mol% LigO corresponded to only 2-4 percent of 61 the current. The evolved gas was 97 to 99$ oxygen at current den-> 2 slties between 0.05 and 1 amp/cm , which indicated that reaction
(2.2.32) was the principal process. However, it seems possible that
reaction (2.2.33) also contributed, with subsequent uptake of the CO^ by:
COg + - COgZ- (2.2.3%) 2 The polarization at low current densities (0.1 to 10 mA/cm ) was very sensitive to traces of oxygen in the initial gas environment. As
the initial oxygen level was decreased, the open circuit potential became more negative (as low as -1.1 volts vs. O^'COg/COg ), and the
initial current level increased substantially. In the potential range between -1.1 and -0.% volts, it was suggested that the polarization is governed by the level of adsorbed oxygen on the gold surface. With
the passage of current, the surface concentration of adsorbed oxygen
increases, and the potential shifts in a positive direction. When the
surface becomes completely covered, oxygen gas evolves.
Of particular interest is the polarization behavior at high 2 current density (greater than 100 mA/cm ). It was found that with alumina present in the melt, passivation of the gold electrode resulted. The passivation was attributed to the formation of a film of alumina on the gold surface, presumably by the following dissociation:
2AIO2" -*• AlgOg + 0^" (2.2.35)
This reaction becomes favored as the oxide ions are depleted in the anode boundary layer. It was also suggested that TiOg and SiOg, which 62 were present as impurities in the AlgO^, may have contributed to the passivating film.
Ozeryanaya et al. (115) investigated the equilibrium potentials and polarization curves of Pt and Pd electrodes in the ternary eutec tic between 600 and 800°C. As found by Smirnov et al. (116), the equilibrium potential was very dependent on trace levels of oxygen in
the gas environment. It was suggested that the equilibrium potential depended on the adsorbed oxygen activity: RT E - const. + In (Og^j) (2.2.36)
The equilibrium potential in a CO^ atmosphere containing very low oxygen levels was found to shift in a positive direction with increas
ing temperature, in agreement with equation (2.2.36).
In this context the work of Borucka (95) on CO^/Au electrodes should be mentioned. After long equilibration periods, the COg/Au electrode assumed a reproducible potential, which showed Nernstian behavior with respect to the CO^ pressure. The potential at one atm
2— COg and 800°C was about -0.22 volts vs. 02'^^2^^^3 ' However, it was
concluded that the CG^/Au electrode was effectively a dilute Og,
COg/Au electrode since traces of oxygen (about 200 ppm) were present
in the COg source.
The polarization curves by Ozeryanaya et al. (115) revealed
passivation when the equilibrium potential was driven to negative
2— values (about -0.8 volts vs. O^'COg/COg ). Passivation was at
tributed to the build-up of an adsorbed oxygen layer, which inhibited
the metal dissolution. The metal dissolution and passivation occurred
negative of the oxygen evolution potential (between -0.8 and -0.2 63 2 volts) and at currents between 1 and 10 mA/cm . At about +0.1 volt, the current rose rapidly, corresponding to the onset of reaction
(2.2.33).
Janz et al. (110,111) measured steady state polarization curves of Pt, Au and Ag electrodes in the ternary eutectic in the temperature range of 500 - 900°C. At lower temperatures, the Tafel plots showed two linear portions, while at higher temperatures, the plots were non-> linear. In the first communication (110), it was suggested that the low current density region was controlled by concentration polariza-’ tion, and the high current density region by activation polarization.
In the later communcation (111), this suggestion was discarded, and both regions were believed to be activation controlled. The limiting step was proposed to be:
MO + + M + Og + 2e” (2.2.37) where MO is a "metal oxide".
Measurement of the gases evolved from melt electrolysis has been performed primarily by Russian investigators (112,113.116-122). The
CO2/O2 ratio depends strongly on the current density and the oxide ion activity in the melt. The CO^/0^ ratio reaches 2 at high current density and low oxide ion activity.
2.2.5.3 .3 . CO Oxidation. Carbon monoxide can be oxidized in molten carbonate according to:
CO + COg^~ + 2C0g + 2e" (2.2.38)
The equilibrium behavior of this reaction has already been discussed
2- in relation to the CO.CO^/CO^ reference electrode. 6H
Reaction (2.2.38) proceeds to a certain extent In molten carb-* onate fuel cells employing mixtures of Hg and CO. However, oxidation of CO Is appreciably slower than that of Hg, and a large fraction of the CO Is Internally shifted by:
CO + HgO + Hg + COg (2.2.39) rather than undergoing direct electrochemical reaction. Studies of molten carbonate fuel cells using only CO as the fuel have been fre quently reported (123-128), but will not be discussed here.
Of a more fundamental nature, the oxidation of CO/CO^ mixtures has been studied on Au electrodes by Borucka (101,129,130), on Au, Pt and N1 electrodes by Vogel et al. (131) and on N1 electrodes by Kim and Devereux (132,133). These Investigations are summarized below.
Borucka (129,130) found two waves for the oxidation of CO at
800°C. The first wave occurred close to equilibrium, and was at tributed to physically dissolved CO. The second wave occurred at 240
2- to 300 mV versus the standard CO^COg/COg electrode, and was at-
2- trlbuted to CO chemically dissolved as the CO^ Ion. The electrodes were fully submerged "flags" and stirring dependence was found at all potentials. At 0.8 atm CO and 0.2 atm CO^, the limiting current 2 densities for the first and second waves were approximately 0.2 mA/cm 2 and 1.0 mA/cm , respectively. The exchange current density for the 2 overall process was about 0.3 mA/cm at 0.382 atm CO.
For the first wave, the following mechanism was proposed:
CO CO* + e" (2.2.40)
CO* + zo^~ ■* Z 0 ~ + COg (2.2.41)
COg" - COg + e" (2.2.42) 65 with reaction (2.2.41) being rate->deterraining. For the second wave and at low the same sequence was proposed, but with CO supplied to the electrode boundary layer by the dissociation,
COg^" 0^“ + CO (2.2.43)
At higher direct electrochemical oxidation of COg^ was proposed to be rate-determining:
COgZ" - COg" + e" (2.2.44)
The study by Vogel et al. (131) employed dipping wire "meniscus” electrodes, in which the wire barely touched the melt surface. The diameters of the wires were not specified which prevents estimation of the current density. The melt temperature was 650°C. The polariza tion curves for different electrode materials showed little difference between Au and Pt, but with a Xi electrode oxidation to NiO was evident.
The polarization curves for Au electrodes had long-term time dependency. After replacing a He/COg mixture with a CO/COg mixture
(0.33 atm CO, 0.67 atm COg), the curves continued to shift upward for at least 50 hours. After these prolonged time periods, the polariza tion curves showed three waves. The first wave, occurring at overpotentials up to 200 mV, was attributed to physically dissolved
CO. The subsequent waves were believed to result from oxidation of products formed by reaction of CO with the melt, as suggested by
Borucka. However, Vogel et al. (131) considered the formation of 2“ COg to be unlikely. They proposed a mechanism which involved forma tion of oxalate ions, either chemically or electrochemically:
CO + C0g2" 4. CgOy2" (2.2.45) 66
2C0 + 2C0g2" 4. CgOyZ- + 2CO2 + 2e“ (2.2.46)
The oxalate can then be oxidized electrochemically by:
CgOyZ" zCOg + 2e" (2.2.47)
One way to test this mechanisms would appear to be bulk addition of an oxalate compound (e.g. Na^C^O^). It should be mentioned that Borucka
(130) excluded the oxalate species based on reaction order evidence.
Kim and Devereux (132,133) developed a computer model to describe
the electrochemical processes at a CO, CO^/Ni electrode. The model
included the reactions proposed by Borucka (129,130), as well as reactions involving Ni dissolution and passivation.
2 .2 .5 .3 .4 . Oxidation. The oxidation reaction in molten
carbonates can be written as:
Hg + COgZ" ^ HgO + COg + 2e" (2.2.48)
With Hg in the melt, the following reactions are introduced:
HgO + COgZ" 4. 20h" + COg (2.2.49)
HgO + o^~ - 20H~ (2.2.50)
COg + Hg + CO + HgO (2.2.51)
CO + 3Hg - CHj^ + HgO (2.2.52)
2C0 + COg + C (2.2.53)
Thus, the gas phase contains a mixture of Hg, HgO, COg, CO and CH^, ** 2*“ 2*** and the melt contains OH in addition to CO^ and 0 . Due to the
formation of CO, reaction (2.2.48) cannot be studied without the
simultaneous consideration of reaction (2.2.38). The formation of
carbon by reaction (2,2.53) can be prevented by minimizing the amount
of CO. Leibhafsky and Cairns (69) described thermodynamic calcula
tions involving these multiple reactions. 67
Vogel and lacovangelo (13%) studied the equilibrium potential of reaction (2.2.%7) at a gold electrode under various feed gas compositions. By considering simultaneous equilibrium of the shift and methane reactions, the experimental data agreed well with ther modynamic values calculated from the following equation:
o RT PCO, E - E° iin — ^ ---- (2.2.5%)
Considerable deviation occurred when the input methane pressure was far from its equilibrium value, and when carbon formation was per mitted during preheating of the gas.
Fundamental kinetic studies of hydrogen oxidation have received relatively little attention (131,135-139). The earlier studies demonstrated that polarization was due primarily to diffusional rather than activation limitations, but no attempts were made to separate the activation and concentration contributions. More recent studies have effectively separated the contributions (138,139).
Arkhipov and Stepanov (135) measured polarization curves at smooth and porous Pt electrodes which were bathed with bubbles of dry hydrogen. The smooth electrode gave a diffusion limited current 2 density of about 5 mA/cm at 700“C. With the porous electrode, the 2 current density was about 50 mA/cm ,
Klevstov, Arkhipov and Stepanov (136) found very similar current levels for partially immersed Ni, Pt and Pd electrodes, after taking into account hydrogen diffusion within the metal. This suggested that activation overvoltage was negligible. 68
Arkhipov, Klevstov and Stepanov (137) studied in more detail the oxidation of hydrogen dissolved in Pd electrodes. The current den-» 2 sities where quite high - 600 to 800 mA/cm at 200-300 mV overpotential.
Vogel et al. (131) studied oxidation at meniscus electrodes using water-gas shift gases of low CO content. The polarization curves were adequately correlated assuming pure diffusion overvoltage.
Limiting currents were about 0.1 mA, but the current density could not be calculated because the area was not specified.
Ang and Sammells (138) were the first to separate effectively the activation and concentration overpotentials. Linear voltage sweep measurements showed definite dependency on sweep rate and agitation, but this did not distinguish between pure diffusion-control and raixed- control. Using a potential step technique, the diffusional overvoltage was eliminated, and the value of the exchange current
density (i^) could then be found. Exchange current densities were
thus measured at Ni, C and Au electrodes using various melt composi- 2 tions and fuel mixtures. At 650°C, i^ ranged from 16 to 78 mA/cm on 2 2 Ni and from 6.5 to 30 mA/cm on Co. On Au, a value of 16 mA/cm was
found. The activation energy in the 550-700°C range was 6.65 kcal/mole for Ni and 7.31 kcal/mole for Co. The i^ was correlated with the fuel composition by the following equation:
i^ - 1 ° (H_)°'25 (C0L)0'25 (H 0)°'25 (2.2.55) 0 0 2 2 2
Where i^° is the standard exchange current density (i.e., all gases at
1 atm). 69
Lu and Selman (139) applied a similar approach to study the kinetic parameters of a Cu electrode in molten carbonate. They found 2 i^ to range between 16 and 30 mA/cm at 650®C.
Numerous studies have been performed using hydrogen and hydrogen- containing fuel mixtures in molten carbonate fuel cells. However, being of a more applied nature, that literature is beyond the present scope of interest (see Section 2.4.2).
2.2.5.3.5. Hydrocarbon Oxidation. Taking methane as an example, hydrocarbon oxidation can be written as:
CH^ + 4C0g2- ^ scOg + ZWgO + 8e~ (2.2.56)
It is generally accepted that this reaction and analogous reactions of other gaseous hydrocarbons do not occur galvanically, but rather
indirectly via steam reforming and subsequent oxidation of H^ and CO:
CH^ + HgO + CO + 3Hg (2.2.57)
Hg + COgZ" ^ COg + HgO + 2e" (2.2.48)
CO + COgZ" 4. 2C0g + 2e” (2.2.38)
Methane and other gaseous hydrocarbons are electrochemically unreac
tive in molten carbonate as a result of low solubility (68).
In molten carbonate fuel cells, hydrocarbons can be used as the
fuel, but they are steam reformed either outside or inside the cell
(68).
2.4.5.3.6. Additional Anodic Reactions. Dunks et al. (48,140)
have recently reported on electrochemical studies in molten sodium
carbonate at 900°C using gold electrodes. Based on computer modeling
of composite current-voltage curves, three anodic and five cathodic 70 waves were Identified. In the presence of added graphite powder, a large anodic wave developed at -0.56 volts, and was attributed to the following reaction:
COg^" COg + 2e“ (2.2.58)
2- The formation of COg in the presence of CO/COg mixtures was pre viously suggested by Borucka (129,130) , as discussed in Section
2.2.5-3.3. A smaller wave also developed at -0.92 volts, and was attributed to CO oxidation (reaction (2.2.38)).
2.2.5.3. Cathodic Reactions
2.2.5.3.1. Oxygen reduction. The reduction of oxygen dissolved in molten carbonate takes place by the following reaction:
Og + 2C0g + 4e" -» 200^^’ (2.2.59)
Note that this reaction is the reverse of the melt oxidation reaction
(reaction (2.2.33)).
Although reaction (2.2.59) has been widely studied and is of primary importance in fuel cell applications, the current investiga tion was mainly concerned with the anodic half-cell reaction. Hence, oxygen reduction characteristics will not be covered here and the interested reader is referred to Selman and Maru (68).
2.2.5.3.2. Melt Reduction. Depending on the experimental condi tions, cathodic decomposition of a carbonate melt favors one of the following reduction reactions:
M* + e" + M (2.2.60)
COgZ" + I4e" C + 3 0 ^ " (2.2.61)
COgZ" + 2e~ + CO + 2 0 ^ ' (2.2.62) 71
The examination of these cathodic processes and the factors that influence their relative predominance have been carried out by a number of investigators (114,117,119,120,122,141^147).
In general, it has been found that in lithiumrcontaining melts, the deposition of carbon (reaction 2 .2 .61) is the dominant process.
In the absence of lithium, reaction (2.2.60) dominates. Most inves tigators conclude that reaction (2 .2 .62 ) does not occur directly, but
CO evolution may occur in the presence of COg via Boudouard reaction with the electrodeposited carbon, especially at higher temperatures
(above 700°C).
In pure LigCO^ (123) and LigO-LigCO^ mixtures (147), carbon deposition dominates at all levels of current density (as high as 5 2 A/cm ). In the ternary eutectic, the cathodic processes are in fluenced strongly by the specific experimental conditions, as ,i.>. following investigations illustrate.
At 600 and 700°C in the ternary eutectic, Smirnov et al. (146) reported that four different cathodic reactions occurred depending on the current density. As the current density was increased, the suc cessive reactions were; formation of carbides, deposition of carbon, 2 evolution of CO, and deposition of alkali metal (above 400 mA/cm ).
Delimarskii et al. (120) obtained high yields of deposited carbon 2 from a ternary eutectic at 450 mA/cm . At low temperatures (400-
500°C) the yield was quantitative, but at higher temperatures (700-
800“C), the yield decreased to about 70%. The addition of small percentages (less than 10%) of hydroxide to the melt greatly decreasd the carbon yield. 72
Ingram et al.(142) measured the yield of carbon from the ternary eutectic at various current densities, temperatures and gas environments. At low current density and 600®C, no carbon was obtained. As the current density increased, the carbon yield in creased, eventually becoming quantitative. At high current density, the yield was decreased by increasing the temperature to 750*0 or using a COg atmosphere, presumably due to the Boudouard reaction.
Dubois (143-145), found carbon deposition to be the primary process in an acidic ternary eutectic. In a basic ternary melt, the alkali metal deposition rate exceeded that of the carbon deposition reaction.
Although the results of different investigators do not agree totally, it appears that carbon deposition in the ternary eutectic is favored under acidic conditions, at moderate current density (100-400 2 mA/cm ) and low temperature (below 700*0.
To an extent, thermodynamic considerations have been useful in explaining the different cathodic behavior in melts of different composition and acidity/basicity (122,141,142,147). Mechanistic details of the cathodic processes are not well understood, however.
The characteristics of the electrodeposited carbon are of par ticular interst to the present investigation. Ingram et al. (142) investigated the characteristics of electrodeposited carbon in some detail. The rest potential was about 0.85 volts more negative than a commerical, graphitized carbon (i.e., considerably more active). The latter had a rest potential about 0.2 volts more positive than the thermodynamic value in the C/COg couple. It was found that carbon 73 constituted only about 3% of the total deposit, the remainder being solidified salt. Only about half of the deposited carbon could be anodically stripped. The anodic polarization curve featured a very 2 steep current rise, and a current density of several hundred mA/cm could be readily obtained at low overvoltage.
The observed differences in the rest potentials were attributed to alkali metal oxide trapped in the porous structure of the deposited carbon. As reaction (2.2.61) indicates, carbon deposition produces oxide ions, which may reach staturation within the porous carbon deposit. The increase in oxide ion activity results in a negative potential shift, as observed. This interpretation has also been advanced by Smirnov et al. (147).
2.2.6. Corrosion Properties.
Closely related to the electrochemical properties are the corro sion properties of molten carbonates. The study of corrosion in molten carbonates has been the topic of much research
(68,80,91,108,132,133. 148-156). The focus of attention here will be on materials utilized in the present investigation, viz., gold and alumina.
2.2.6.1. Gold. Although gold is the most noble metal, it can undergo oxidation. In aqueous systems, gold oxidation has been widely studied
(157-’160). Oxidation does not take place, however, unless fairly positive potentials are imposed. According to Hoare (157), only one oxide, AUgOg, is important in the Au/Og systems, and this oxide is not formed until a potential above 1.36 volts (vs SHE) is reached. 7%
However, cheralsorption of oxygen on gold can occur at less positive potentials (I6l).
The E-pCOg diagram of Ingram and Janz (108) for gold In molten carbonate has already been presented In Figure 8. Even at 0.0 volts
2- vs. Og, COg/COg , gold Is thermodynamically Immune from attack In acidic melts (pCOg around 0). Corrosion measurements using gold electrodes have been carried out by Janz and co-workers (149,152).
Static corrosion tests In pure COg gave negligible weight loss after
72 hours at 890“C (149). During electrolysis of a ternary eutectic at
700°C under a COg atmosphere, the dissolution of gold contributed less than 0.1 percent of the current (152). A gold electrode used for more than 30 hours of electrolysis at 900*C did show some signs of dissolu tion (pronounced definition of grain boundaries), but gave essentially the same polarization curve as a new specimen (111,152). Thus, even under severe polarization conditions, gold Is quite resistant to corrosion In acidic melts. On the other hand, platinum, which Is normally quite Inert, tends to be corroded by the formation of LlgPtO^
(149,152).
Anderson (91) performed weight loss measurements on gold specimens In basic carbonate melts and NSgO melts. The corrosion rate
Increased significantly as the oxide level was Increased, becoming quite severe In the pure NSgO. The gold dlssoltulon was attributed to the following reaction;
AUgOg + 0^" 4. 2AuOg" (2.2.63)
The initial formation of AUgO^ Is favored In the highly basic melts, as Figure 8 Illustrates. 75
John (80) also found appreciable dissolution in basic sodium carbonate. Using a small number of potential measurements, he was able to construct an approximate Pourbaix diagram for the Au-Na-C-0 system at 900“C.
Thus, it seems safe to say that gold is for all practical pur poses inert in acidic carbonate melts, provided the potential is kept below 0.0 volts, but becomes susceptable to dissolution in basic carbonate melts.
2.2.6.2. Alumina. High-purity, high-density alpha-alumina has been used extensively in molten carbonates, resisting attack at tempera tures up to 1200°C. Other forms of alumina (e.g., lower purity, porous, or powdered) are not nearly as resistant, however. The stability of alumina in molten carbonate is attributed to the forma tion of a film of alkali aluminate (MAIO^), which protects the alumina from further attack. The stability diagram of John (80) shows that aluminate is the stable phase, even in acidic melts. The solubility of aluminate is quite low - John (80) reports a value of 2 x 10 ^ mole
AlOg /mole carbonate at 900“C and Anderson (91) cites Broers and
Ballegoy's (162) value of 1.4 x 10 mole AlOg /mole carbonate at
700*0 in the ternary eutectic. The low solubility of aluminate ac counts for the highly effective passivation. Selman and Maru (68) discuss the properties of alkali aluminates in some detail.
Anderson (91) performed weight loss measurements of alumina crucibles. In acid melts, the weight loss was indeed negligible. In melts with more than 10% alkali metal oxide, the weight loss was substantial. The corrosion in pure oxide was extremely heavy. 76
Thus, it can be concluded that high-purity, high-density alumina is an excellent material for contact with carbonate melts, provided the oxide level is kept low.
2.3. Previous Work Involving Electrochemical Oxidation of
Carbonaceous Materials in Molten Salts
2 .3.1. Carbon Fuel Cells
The direct use of coal or carbon in a fuel cell to produce electricity has been a long standing goal of science and industry, and extensive work has been conducted toward this end. Most fuel cells of the direct carbon type have employed molten electrolytes, mainly because high temperature is necessary to achieve adequate reactivity of the carbon.
Numerous reviews have appeared over the years (163-171). the earliest reviews were published in German by Bechterew (I63) and in
English by Rideal and Evans (164). The review by Haul and Tobler
(165) is considered a classic paper in the fuel cell field, and com prehensively reviews the older work. The reviews by Howard (I66) and
McKee and Adams (167) are also quite detailed, and include later work not covered by Baur and Tobler. Genin (168) reviewed much of the early fuel cell work in a French publication. Hauser (169) reviews the work of a selected few, but in considerable detail. Leibhafsky and Cairns (170) give an eclectic review of the more significant attempts at carbon fuel cells. The latest developments are briefly covered by Selman and Marianowski (171) and Selman and Maru (68). 77
Based on the extensive reviews Just menioned, and on consultation
of many of the original papers, Table 13 has been prepared to sum marize the majority of the work on direct carbon fuel cells in which
molten electrolytes were employed. The work covers a time period of
over one hundred years.
2.3.1.1. Historical Summary. As early as 1855, Becquerel (171)
reported the use of a carbon anode to generate electricity, the
electrolyte being molten potassium nitrate. Numerous other inves
tigators used nitrate melts (174-177,179,182-185). Later, it was
realized that the nitrate chemically oxidized the carbon, setting up
an indirect redox cell with a nitrate/nitrite couple. The
coulometric efficiency of the carbon was greatly reduced due to the
oxidation by the nitrate melt.
The use of hydroxide melts around the turn of the century gave
promising results, and was widely pursued (178,180,181,188-194). The
work by Jacques (188,189) in particular was highly publicized. The
goal was to outperform the steam-driven power plants, which at that
time ran at a very low fuel efficiency. The major limitation with
hydroxide melts is the gradual conversion to carbonate from the
evolved CO^ (See Section 2.2.1). Also, it was demonstrated by Haber
and Brunner (192) that the hydroxide gave rise to a hydrogen cell,
according to;
C + COg + 4NaOH SNa^CO^ + 2Hg (2.3.1)
2Hg + HOH*' 4H2O + i4e" (2.3.2) 78
Table 13. Summary of Fuel Cell Work Using Carbon Anodes in Molten Electrolytes.
Year Investigator Electrolyte Cathode Reference
1855 Becquerel KNOg Pt 172
1864 Gore PbCrOL Ni, Fe 173 NagCOg/CaO/SiOg
1877 Jablochkoff KNOg Fe 174
1882 Brard Nitrate ? 175
1882 Davies Nitrate ? 176
1883 Clark Nitrate ? 177
1883 Archerau NaOH/KOH/NagCOg/KgCOg Cu 178
1886 Langhaus Nitrate ? 179
1888 Bradley NaOH Fe 180
1888 Fabingi and NaOH Pt 181 Farkas
1891 Bull Nitrate Fe 182
1894 Shrewsbury Nitrate Fe 183 et al.
1895 Schmitz Nitrate ? 184
1895 Korda Nitrate CuO/Pt 185 KgCO
1895 Schoop PbO Fe 186
1896 Short PbO Pb 187
1896 Jacques NaOH Fe 188,189
1897 Liebenow and NaOH Fe 190 Strasser
1902 Byrnes NaOH Various, 191 incl. Au 79
Table 13. (continued)
Year Investigator Electrolyte______Cathode Reference
1904 Haber and NaOH/NaMnO, Fe 192 Brunner
1910 Taitelbaum NaOH/NagMnOy Fe 193
1911 Bechterew Various, incl. Various, 163 Carbonates incl. Au
1912 Fischer and NaOH 194 f*3°4 Lepsius
1912 Baur and Na CO /K CO liq. Ag 195 Ehrenberg Borax‘s (Na^Bjo) Cryolite ^ Silicates/KF
1916 Baur, Peterson Borax CuO, PbO 196 and Fuellemann
1918 Reed Borax Au 197
1921 Baur, Treadwell KgCOg/NagCOg FCgOg/FCgOy 198 and Truempler
1923 Rhorer KgCOg/NagCOg Pt,Ag,Cu,Fe 199
1932 Sconzo AgNOg Ag 200
1935 Tamaru and KgCOg/NagCOg/LigCOg Pt,Pd,Au,Ni 201 Kamada Ag,Cu,Fe20g
1935 Baur and Brunner KgCOg/NagCOg/NaCl Pt 202.203
1936 Baur and Barta NagCOg 203.204
1949 McKee and Adams CeOg/WOg 167 F*3°4 1964 Hauser KgCOg/NagCOg/LigCOg Au/Pd 169 80
Table 13. (continued)
Year Investigator______Electrolyte______Cathode Reference
1964 Arkhipov and K C O /NauCO /LI CO. Pt 205 Stepanov ^ ^ ^
1967 Dubois et al. KgCOg/NagCOg/LlgCOg Au/Pd 144,145, 206
1973 Anbar et al.^*) KgCO^/Na^COg/LlgCG^ Pb/PbO,Ag 207-209
1975 Weaver et al. KgCOg/NagCOg/LlgCOg C 210-214
(a) Actually an indirect carbon cell, using a Pb/PbO couple.
With the downfall of hydroxide melts, attention turned toward
other electrolytes, such as borates, silicates and carbonates. The
main conclusion using these higher melting point electrolytes was that
carbon reactivity was too low below about 900“C. However, at this
temperature, CO was the main oxidation product from carbon, and thus
the electrochemical conversion yielded only two Faradays per mole of
carbon.
A further complication that arose with the use of carbonates
(e.g., sodium carbonate) at high temperatures (above 900*C) was the
occurence of the Gay-Lussac reaction:
NagCOg + C + 2Na(g) + CO + COg (2.3.3)
or
NagCOg + 2C -» 2Na(g) -*• 3C0 (2.3.4)
The loss of Na vapor cannot be tolerated If the electrolyte Is to
remain Invariant. Baur and Brunner (203), and Barta (204) studied the
relationship of this reaction to carbon fuel cells. 81
Baur and co-workers eventually abandoned the use of molten electrolytes, and attempted to utilize carbon In cells with solid electrolytes (203.215). Justl et al. (216) later made a similar attempt. The performance was found to be rather poor, and practical difficulties were severe.
Based on work through the 1930*s. It was generally concluded that electricity direct from coal was not economically feasible. Also, with substantial Improvements In the efficiency of steam power plants, the Incentive to utilize coal In a fuel cell correspondingly declined.
Research on fuel cells continued, but with emphasis on gaseous fuels.
In spite of the numerous difficulties uncovered by the early research on carbon fuel cells, a few Investigators have Intermittently challenged the problem. The main line of approach has been to operate
In a low enough temperature range where complete combustion to COg would be favored, yet high enough to achieve rapid kinetics. Although the Individual carbonates typically employed had rather high melting points, the use of carbonate mixtures substantially lowered the melt ing temperature, and paved the way for some promising results. Also, by operating at a lower temperature, the Gay-Lussac reaction would not be a problem. Work along these lines has been reported by Tamaru and
Kamada (201), Hauser (169), Arkhipov and and Stepanov (205), Dubois et al. (206) and most recently by Weaver et al. (210-214).
Tamaru and Kamada (201) determined that COg was the only anode reaction product at 700°C. The measured mass of carbon loss was 93 percent of the theoretical value for complete combustion. Although 82 current densities were not reported, the polarization data suggested only a small contribution by the charcoal anode at 600°CC
Hauser (169) used a graphite anode in the ternary carbonate eutectic, and measured open->circuit potentials and polarization curves over a temperature range of 600^940*0. He also measured anodic gas evolution. The measured open-’circuit potentials were lower than theoretical up to 880°C, but agreed at 940°C. However, the cell purge gas was SOfOg/SOfCOg, which probably did not permit the oxygen ac tivity at the anode to reach equilibrium. At 0.5 volt overpotential, 2 2 the current density was about 20 mA/cm at 700®C and about 40 mA/cm at 800®C. No compensation for IR drop was made. The amount of CO in the product gas was below 1% at 700°C, 1 to 2$ at 800°C, and around
10$ at 870°C. The measured rate of gas evolution agreed fairly well with the theoretical rate based on the applied current level. Hauser also proposed a mechanism consisting of two steps - Boudouard shift, followed by electrochemical oxidation of CO to CO^.
Arkhipov and Stepanov (205) studied the anodic polarization of spectroscopic carbon at 500-900®C in the ternary carbonate eutectic.
Although the work was not specifically directed towards the fuel cell application, it falls into the same category as the investigations under discussion.
The open circuit potentials were -0.6, -0.95, -1.0, -1.2 and -1.3 volts (vs. 33$ Og, 67$ CO^/Pt) at 500, 600, 700, 800 and 900°C respectively. Low values at 500°C were attributed to a lack of equilibrium. High values at 800-900°C were attributed to a mixed potential with alkali metal. In the 600-900“C range, the overvoltage 83 2 at 100 mA/cm was 400-500 mV, the lower values occurring at the high temperatures. The gases evolved from the carbon anode at 600°C and 2 50-500 mA/cm were 96 to 98 percent COg, and 4 to 2 percent CO.
Dubois et al. (144,145,206) studied the open-circuit potentials of several varieties of carbon at 560°C as a function of the oxide
2- activity (pO ). The varieties of carbon included pyrolytic carbon, electrographite, vitreous carbon and charcoal. The carbons were prepared in the form of small rectangular bars. The open-circuit potentials generally fell about 300 to 500 mV below the theoretical 2" value, but exhibited the expected pO dependency. The charcoal gave an exception; the open-circuit potential agreed with the theoretical value under acidic conditions. Electrodeposited carbon was con siderably more electronegative, with an open circuit potential around
-1.8 volts (vs. ), independent of pO . Polarization data on the carbon electrodes was not reported. It was found, however, that prolonged anodic polarization at high current density gave no detectable amounts of CO.
The most recent attempt at a direct carbon fuel cell was carried out at the Stanford Research Institute (SRI) by Weaver and coworkers
(210-214). It is not clear whether the SRI workers were familiar with the prior literature along similar lines (e.g., 169,201,205), since no specific reference to the related studies was cited. Since the SRI work was quite extensive, and has considerable bearing to the present investigation, a detailed review will be given. The SRI work has been briefly reviewed by Salman and Maru (68), and Selman and Marianowski
(171). 64
2.3.1.2, SRI Carbon Fuel Cell. The SRI work originated with the development of a redox-’type fuel cell employing a liquid lead anode in molten carbonate (207-209). The carbon was indirectly oxidized by a redox process involving the Pb/PbO couple. In later studies, the work shifted to the direct use of carbon in a molten carbonate fuel cell
(210-214). The studies involving direct use of carbon are summarized below:
1. Various types of carbon were employed as anodes, including pyrolytic graphite (PG), spectroscopic carbon (SC), fuel cell grade carbon (PC-60), and several varieties of coal - Peabody char,
Pocahontas, Illinois No. 6, Kentucky No. 9 and Decker mine.
2. The coal was shaped into coherent, electrically conducting anodes for experimental measurements. For scale up purposes, two types of electrode configurations were suggested - a fabricated plate of coal- derived carbon, and a basket-type electrode in direct contact with electically conductive coal briquettes or a coal slurry.
3. The presence of added ash in the electrolyte seemed to have little effect on cell performance. Several methods of ash removal were sug gested (213).
4. The open circuit voltages for all types of carbon generally fell close to the theoretical value (-1.0 ± 0.1 volt vs. 33$02/67%C02/Au) at 600-800°C. The coal-derived anodes gave slightly more negative voltages than the pyrolytic graphite and spectroscopic carbon.
5. Detailed polarization studies were reported by Yasuda (212). The electrodes showed a definite aging effect, and most polarization curves were measured using aged (prepolarized) electrodes. At 700°C 85 2 and 100 mA/cm , the overvoltage on a coal-iderived anode (Pocahontas- based) was about 400 mV. This value did not include the IR contribution. At a given current density, the overvoltage was found to increase in the following order:
coal > PC-60 > SC > PG
This trend in reactivity was correlated with a decrease in effective internal surface area due to decreasing porosity.
The effect of scan rate indicated concentration polarization at low current density, but mainly activation polarization at high cur rent density. Scan rates of 1 mV/s were found to give I-V curves equivalent to steady state polarizations. A distinct hysteresis effect was found in the low current density range. The hysteresis was interpreted in terms of CO^ supersaturation at the electrode surface.
6. A separate study of pyrolytic graphite was performed by Ateya in the Appendix of reference (212). The open circuit potential was well below theoretical in the 600-700°C range, and contradicted earlier findings. Exchange current densities were calculated by extrapolating
Tafel lines to the thermodynamic open-circuit potential rather than to the experimental open-circuit potential, to accommodate the assumed presence of a mixed potential. The exchange current densities were
0.004, 0.07 and 1.0 mA/cm^ at 600, 700 and 800°C, respectively. The activation energy was found to be 40 kcal/mole.
7. The IR contribution to the overpotential was quite large (the measured resistance was around 0.5 ohms) and was compensated using the current interruption technique. 86
8. The results of gas analysis depended on the type of carbon anode.
When the anode was pyrolytic graphite or spectroscopic carbon and the temperature was 700®C, the evolved anode gas was 99-100 percent COg, with gaseous current efficiency falling between 95 and 100 percent.
When the anode was coal-’derlved and the temperature was 700 ®C, the evolved anode gas was usually above 90 percent COg at high current density, but dropped off at low current density with a corresponding
Increase In CO percentage. In the earlier study (210), coal-derived anodes gave gaseous current efficiencies between 60 and 80 percent.
In the later study (212), gaseous current efficiencies around 100 percent at high current density (anode voltage positive of ">0.85 volts) were reported.
9. Weight loss measurements of the pyrolytic graphite and spectro-> scoplc carbon gave n values (electrons per carbon atom) generally around 4, but In a few cases It was closer to 2 for the spectroscopic carbon. With the coal-derived anodes, the weight loss was substan tially greater than coulometrlcally expected - ten times too large according to one report (210). The extra weight loss was attributed to mechanical loss or chemical loss by Boudouard reaction.
2.3.1.3. Anode Configurations. Throughout the years, various types of carbon have been used for anodes In carbon fuel cells - amorphous carbons, graphite, wood charcoal, and certain types of coal. Most often, the carbon was In the form of a coherent electrode, such as In the SRI work. This configuration requires the carbon to be electri cally conductive. A few studies have used partlculated carbon In 87 conjunction with some type of nonconsumable contact electrode.
Figures 9-11 illustrate some of these types of carbon fuel cells.
Short's cell (Figure 9) contacted the carbon particles via a conducting feeder tube, the lower portion of which was made of a perforated, non-conducting, corrosion^resistant material, and this lower portion was immersed in the electrolyte (molten PbO). With this configuration, electrically conducting carbon was essential.
In Reed's cell (Figure 10), a graphite grid was covered with crushed anthracite coal, which floated on the electrolyte. The coal layer was thick enough to prevent direct contact of the graphite with the melt.
In the SRI cell (Figure 11), an inert basket electrode was to be partially submerged in the melt to contact particles of carbon directly. Although this design was never tested, it was suggested that the basket could be constructed from stainless steel.
A few additional cells have utilized particulated carbon. Rhores
(199) examined a cell in which ground coal was submerged by confine ment in an iron spoon, which also provided electrical contact.
Taitelbaum (193) used a carbon electrode and added various materials to the anode compartment, including sawdust, petroleum, and charcoal.
Baur, Treadwell and Trurapler (198) appear to have used ground carbon in some of their cells (203).
It is important to note that none of the cells employing particu late carbon in molten electrolytes seem to have had provision for mechanical agitation of the melt. 88
Figure 9. Diagram of Short's Carbon Fuel Cell. Legend*. A-Vessel, B- Lead cathode, C-Wire conductor, D-Electrolyte (molten PbO), E-Carbon, F-Wire conductor, G-Conducting feeder tube, H- Perforatad, non-conducting, corrosion-resistant basket, J- Air pipe, N-Grate, R- Receiving device (motor). Reprinted from Reference (187). 89
3 0 l = ] [
Figure 10. Diagram of Reed's Carbon Fuel Cell. Legend: A-6" Battersea roasting dish. Bisection of 3" Battersea crucible, C-Cover, D-Graphite electrode, E-Aperture for escape of products of combustion, F-Wire conductor to voltmeter, G-Gold foil electrode, H-Wire conductor to voltmeter, K-Broken coal, M-Fused borax with oxides of manganese in solution, N-Communication passages, 0- Oxidizing region, P-Waste gas chamber, R-Reduction region. From Reference (197), p. 91. Reprinted by permission of the publisher, The Electrochemical Society, Inc. 90
ANODE CONTACT MOLTEN CARBONATE
CARBON \ AIR/CO2
POROUS CATHODE
Figure 11. The SRI Conceptual Carbon Fuel Cell. Reprinted from Reference (212). 91
2.3.2. Aluminum Electrowlnnlng
Electrowinning of many metals can be accomplished by electrolysis of molten salts. The most noteworthy example is the electrowinning of aluminum from a cryolite-alumina (Na^AlFg-AlgO^) melt. Other metals produced by electrowinning from molten salts include magnesium, sodium, lithium, cerium, beryllium, calcium, cesium, columbium, lead alloys, zirconium, titanium, and tantalum. Carbon or graphite anodes are almost always employed in these electrowinning operations and therefore the literature on this subject is relevent to the present investigation. An abundant body of literature exists in this context, and numerous reviews are available (10,34,217-220).
Although carbon anodes are used extensively in molten salt electrowinning, most fundamental studies of anode performance have pertained to aluminum manufacture. Literature regarding other molten salt electrowinning operations is more of an applied nature, such as cell design descriptions, and will not be treated here. The more fun damental investigations of carbon anodes in cryolite alumina melts have been reviewed by Randin (34). Salient features will now be discussed.
In aluminum electrowinning, the overall cell reaction is;
EWlgOg + 3C + 4A1 + scOg (2.3.5)
The equilibrium potential of this reaction is 1.163 V at 1010°C.
Commercial cells operate around 1000°C, with a total voltage of around
4.5 volts. About 2.8 volts is taken up by ohmic loses, and the remaining overvoltage is mainly associated with the anode. At a 92 2 typical anode current density of 1 A/cm , the anodic overvoltage is
0.5 volts. The carbon anodes are usually derived from petroleum coke.
The anode reaction can be expressed as
C + 20f" -» COg ♦ He“ (2.3.6)
The exact nature of the ion from which oxygen is discharged has not been verified. The carbon anode preferentially evolves COg, even though CO is thermodynamically favored at 1000“C. The outlet anode gas does contain some 20-50% CO in addition to the COg, and this percentage is usually attributed to reduction of the evolved COg by dissolved or finely dispersed aluminum in the electrolyte. Reduction of COg by the carbon anode can also account for some of the CO production.
The mechanism of reaction (2.3.6) has been studied by numerous
investigators by a variety of electrochemical techniques. Most workers agree that the overpotential is caused by a slow heterogeneous reaction on the anode. It is believed that when oxygen-containing
ions are discharged, intermediate chemisorbed carbon-oxygen complexes
(C^O) are formed, and the rate is limited by formation or desorption of these complexes.
According to Randin (34), two principal reaction mechanisms have been proposed:
Mechanism A
0^" - Oads + 2e“ (2.3.7)
Gads + xC •* C^O (2.3.8)
0 ^ ~ + C^O C^O-Oads + 2e” (2.3.9)
C^O*Oads - COgads + (x-l)C (2.3.10) 93
COgada + COg (2.3.11)
Mechanism B
0^" + Oada + 2e" (2.3.7)
Oads + xC + C^O (2.3.8)
2C^0 ■* COgads (2X-DC (2.3.12)
COgads COg (2.3.11)
Mechanism A involves two charge transfer steps. For both mechanisms,
the rate-determining step(s) is reaction (2.3.8) or reaction (2.3.11)
(i.e., adsorption/desorption steps).
2.3.3. Aditional Investigations
2.3.3.1. Carbonate Melts. Japanese investigators (221) have explored
the use of carbon anodes for production of carbon monoxide by molten
salt electrolysis. This concept has the reverse emphasis compared to
carbon fuel cell work, viz., maximization of CO rather than
minimization. Using an equimolar ternary carbonate electrolyte, a
comparison between graphite and amorphous carbon was made. The cell
current was 100 mA, but the current density was notstated. With a
graphite anode, the percentage CO was low at 650 and 750°C (1 and 20%,
respectively), but CO became predominant at 850°C (70%). With amor
phous carbon (type not specified), the percentage CO was 10% at 650“C,
but rose to 80% at 750°C. Thus, the carbon gave similar results at
temperatures 100°C lower than the graphite. Also, an appreciable
amount of spontaneous thermal decomposition of the melt was noticed at
750“C in the presence of amorphous carbon. 9U
The rate of gas evolution was approximately 0.75 mol/Faraday at temperatures up to 850®C for graphite and 750°C for carbon. The gas data was interpreted in terms of the following anode reactions.
COgZ" + 2C + 3C0 + 2e" (2.3.13)
C0g2" + C + COg + CO + 2e" (2.3.14)
2C0g2" + c -*■ 3CO2 + He" (2.3.15)
Only reaction (2.3.15) agrees with the observed gas rate of 0.75 mol/Faraday; reaction (2.3.13) and (2.3.14) should yield 1.5 and 1.0 mol/Faraday, respectively. However, the high percentage of CO does
not agree with reaction (2.3.15), but favors reaction (2.3.13). This
inconsistency was not fully explained by the authors, but may be due
to the occurrence of the following reactions:
MgO + COg •* MgCOg (2.3.16)
MgO + 2C0 - C + MgCOg (2.3.17) which would lower the measured gas evolution rates. The M^O could
arise from dissociation prior to electrolysis since the cell was
purged with pure helium, or from the cathode reaction since the
cathode compartment was not isolated.
The proposed reaction mechanism involved formation of chemisorbed
oxygen, and subsequent desorption of the oxygenated carbon complex to
form CO or COg. The high percentage of CO was attributed to Boudouard
reaction with the electrode carbon. The reaction steps are as
follows:
COgZ- -► COg + 1/2 Og + 2e" (2.3.18)
C + 1/2 Og -» C-Oads (2.3.19)
C-Oads - CO (2 .3.20) 95 or 2C-0ads + COg + C (2.3.21)
followed by C + COg + 2C0 (2.3.22)
2.3.3.2. Bisulfate Melts. A kinetic study of the electrochemical
oxidation of graphite in bisulphate melts (KHSO^ and NaHSO^ - KHSO^
eutectic) at 100-’320“C was carried out by Arvia et al. (222). The
anodic reaction produced COg, CO and traces of SOg, with a COg/CO
ratio of about 2. The anodic current efficiency was about 90%, assum
ing H Faraday/mol of COg. The activation energy was approximately
42.5 kcal/mol.
A detailed mechanistic analysis was included, which has been
reviewed by Randin (34). The main conclusion is that the kinetics of
the electrode reaction are controlled by a thermal decomposition
reaction, and the surface functional groups (C-0 complexes) on the
graphite were intimately involved in the reaction sequence.
2.3.3.3. Sulfate-»Chloride Melts. Electrochemical oxidation of
graphite has been studied in the following melts: eutectic NagSO^-NaCl
at 700°C (223), NagSO^-NaCl at 850°C (224), S0^^“ in equimolar NaCl-
KCl at 700°C (225), NagSO^-NaCl-KCl at 650°C (226), NagSO^ in eutectic
LiCl-KCl at 600°C (227), CaSO^-NaCl at 740 to 800°C (227) and PbSOy in
eutectic LiCl-KCl at 550-720“C (229). These investigations have been
reviewed by Randin (34). The proposed overall anode reaction for
these sulfate-containing melts is:
SOyZ" + C + SOg + COg + 2e” (2.3.23)
2.3.3.4. Nitrate Melts. Arvia, Triaca and Sustersic (230-232) have
studied the anodic reaction of graphite in molten nitrate from 220 to
470°C. Between 220 and 320°C, the overall anode reaction is: 96
C + 2N0g2" 4. COg + 2N0g + 2e" (2.3.24) 2 Polarization curves in the range 0.5-200 mA/cm fit a Tafel line with a slope of 2RT/F, giving an exchange current density of about 1 2 mA/cm . The activation energy was approximately 6.5 kcal/mol. The rate determining step was thought to be discharge of a nitrate ion on an oxidized surface site.
At temperatures above 350*0, the reaction becomes more complex, with anode products consisting of COg, NO, NgO and NOg. Randin (34) reviewed mechanistic details.
2.3.3.5. Aluroinosilcate Melts. A few studies have been reported dealing with anodic combustion of carbon in molten slag, and these have been reviewed by Randin (34). In oxygen-»containing melts at 1350 to 1450*0, the overall anodic reaction is believed to be:
0 + of" + 00 + 2e” (2.3.25)
Analogous to other melts at lower temperatures, the rate-determining step is believed to be desorption of reaction products from the carbon surface.
2.4. Related Literature
2.4.1. Electrochemical Oxidation of Ooal in Aqueous Systems
A considerable body of literature exists pertaining to electrochemical oxidation of coal and other carbonaceous materials in aqueous systems. A recent review by Park (233) covers many aspects of this subject. Additional information can be found in references
(28,34,166). Investigations have focused on such applications as coal chemistry characterization, production of electricity using fuel 97 cells, and depolarized hydrogen production via coal slurry electrolysis. Regarding energy conversion applications, the low temperatures involved in aqueous systems severely limits carbon reac tivity, precluding practical utilization.
2.4.2. Molten Carbonate Fuel Cells
As discussed in Section 2.3» the use of molten salts as fuel cell electrolytes dates back over 100 years. The most advanced form of the molten salt fuel cell is the gas-fed molten carbonate fuel cell, which has been under systematic development since the 1950's. To date, most of the technical barriers have been overcome, and practical utiliza tion for power generation appears promising.
Much information is available regarding molten carbonate fuel cells, and recent reviews are cited here (68,171,234). State-of-the- art cells typically operate at 650°C, using a molten LigCO^-K^CO^ electrolyte held in a porous body of fine LiAlO^ powder. Anodes are typically porous nickel containing some chromium, and cathodes are 2 porous nickel oxide doped with lithium. Bench scale cells (94 cm ) 2 operating on high Btu gas can generate 760-770 mV at 160 mA/cm . Fuel 2 cell stacks consisting of 8-20 one ft cells have operated for several 2 thousand hours at a current density of 160 mA/cm and conversion efficiency of 42% (171).
2.4.3. Molten Carbonate Coal Gasification
A commercial process for coal gasification in molten sodium carb onate was explored by the M. W. Kellog Co. (235,236) and further developed by Rockwell International (237-239). A good review of the process has been given by Hatt (240). A process development unit 98
(PDU) handling a coal feed of 1 ton/day was eventually built by
Rockwell, and performance appeared promising.
2.4.y. Graphite Oxidation in Molten Carbonate
Recent studies by Stelman et al. (241-243) have focused on
graphite oxidation in sodium carbonate and sodium carbonate/sodium
sulfate melts. The studies were undertaken to gain fundamental infor
mation pertaining to the oxidation of carbon in molten salts, with
application towards catalytic coal gasification. The effects of
temperature, graphite surface area, oxygen concentration, and melt
composition were investigated, and probable mechanisms were discussed.
One mechanism that was considered involved the formation of sodium
carbide by initial reaction of carbon and the Na^CO^ (241). This
mechanism was subsequently ruled out, however.
The addition of sodium sulfate to sodium carbonate melts was
found to increase the oxidation rate significantly. The catalytic
effect of sodium sulfate in molten sodium carbonate has also been
reported by Lefrancois and Barclay (244).
2.4.5. Dispersed Electrodes
A variety of publications were encountered relating to dispersed
electrode systems, in which a feeder electrode contacted conducting
particles dispersed in an electrolyte phase (245-255). The most per
tinent of these studies is the article by Hiddleston and Douglas
(247), who used carbon particles as fluidized bed electrodes. For the
ferrous/ferric system, the current level increased substantially in
the presence of a static bed of carbon, and increased further upon
fluidization of the bed. However, for methanol oxidation, no increase 99 in current was obtained by fluidization of the bed. The results were explained by the fact that fluidization introduces two competing effects: 1) an increase in mass transfer to the surface of particles in the bed, and 2) a decrease in electronic conductivity of the bed. CHAPTER III
EXPERIMENTAL METHODS
3.1. Objectives
The primary objectives of the experimental investigation were as follows:
1. To design and construct an electrochemical cell that would:
a) Permit stirring of a molten carbonate-coal slurry.
b) Provide a gas tight environment under stirred conditions.
c) Withstand temperatures up to 900°C.
d) Generate reliable electrochemical data.
2. To concentrate only on the anode half-cell reaction, and to
relegate overall cell performance to later study.
3. To perform a systematic investigation of the operating system and
identify important parameters. The parameters to be investigated
included:
a) Cell temperature
b) Type of carbonaceous material
c) Concentration of carbonaceous material
d) Particle size of carbonaceous material
e) Stirring rate
f) Anode material
g) Purge gas composition.
100 101
4. To explore a wide range of operating conditions in order to deter
mine optimum performance conditions.
5. To obtain rate data giving Insight into the mechanism of the
anodic reaction(s).
3.2. Apparatus
3.2.1. Electrochemical Cell
A schematic drawing (not to scale) of the electrochemical cell is shown in Figure 12. A detailed description of the various components follows.
3.2.1.1. Working electrodes. Four types of working electrodes (WE's) were constructed, as depicted in the following table.
Table 14. Types of Working Electrodes
Type Material______Diameter____ Exposed Length
I Gold rod 1/8" 1 1/2"
II Gold rod 1/8" 1"
III Gold wire 0.5 mm 1"
IV Graphite rod 1/8" 1 1/2"
The Type I WE was used in the majority of the experiments. The ex posed length dimension refers to the bare portion of the electrode.
The unexposed portion of the electrode was sheathed by 1/4" alumina tubing (McDanel 998). Alumina cement was employed as a sealant to prevent leakage of the melt into the sheathed portion. Two brands of 102
/
Diagram of Electrochemical Cell.
Legend: A-Melt level, B-Alumina stirring shaft, C-Crucible cover, D-Counter electrode assembly, E-Water-cooled lid, F-Thermocouple, G-CE purge inlet, H-CE purge outlet, I-CE terminal, J-Stirrlng motor, K-Water-cooled rotary seal, L-Cell purge outlet, M-Cell purge inlet, N-RE terminal, 0-RE purge outlet, P-RE purge inlet, Q-WE terminal, R-Reference electrode assembly, S-Inconel canister, T-Alumina crucible, U-Gold WE. 103 alumina cement were utilized: Zircar alumina cement (Zircar Products,
Inc.) and Ceramabond 552 (Aremco Products, Inc.). The Zircar alumina cement proved superior in this application.
Inside the alumina tube, a gold lead wire provided contact to the
Type I and Type II gold rod WE’s. Electrical contact was made by spot-welding. For the Type IV graphite rod WE, a platinum lead wire was used, and electrical contact was made with electrically conducting silver epoxy (Epo-Tek H20E, Epoxy Technology, Inc.).
The l/o" gold rod stock was provided courtesy of Battelle
Memorial Institute, and had a nominal purity of 99.9-99.99%. The 0.5 mm gold wire was obtained from Alpha Products, and had a nominal purity of 99.9%. The 1/8” graphite rode was manufactured by United
Carbon Products Co., Inc., and was designated as "ultrapurity spectro scopic graphite."
3.2.1.2. Reference electrodes. For the reference electrode (RE), a
33% Og, 67% COg/COg^ /Au half-cell electrode was utilized. The RE sheaths were made from 1/2" diameter, closed-bottom, alumina tubes
(McDanel 998). Three types of RE sheaths were employed, as distin guished in the following table.
Table 15. Types of Reference Electrode Sheaths
Type Number of Pinholes Size of Pinholes
1 4 " 400 microns
II 1 " 100 microns
III 0 10U
The pinholes were drilled using a focused laser beam, and were located one cm from the bottom of the sheath. In later experiments, it was found advantageous to add alumina powder (80-200 mesh, Fisher
Scientific Co.) to the bottom of the RE sheath, thereby restricting electrolyte diffusion through the pinholes.
The gold electrode, which contacted the carbonate at the bottom of the alumina sheath, consisted of a 1/4" diameter x 2" gold tube spot-welded to a 0.5 mm gold lead wire. The gold tube was provided courtesy of Battelle Memorial Institute, and was high-purity, nominally 99.9-99.99? pure.
During operation, the RE compartment was continuously purged with a 33$ Og, 67$ COg gas mixture. A 1/4" diameter alumina tube placed inside the 1/2" diameter alumina sheath exhausted this purge gas.
Tube connections were made with Swagelock fittings. The gold lead wire exited from the RE assembly through the 1/4" diameter alumina tube, and a Teflon plug provided a gas-tight seal.
There was one preliminary run in which a different type of 2“ reference electrode - a COg/CO^ /Graphite RE - was tested. This RE consisted of a 1/8" graphite rod (ultrapurity. United Carbon Products
Co., Inc.) placed inside the Type I RE sheath. A 100$ COg purge blanketed the RE compartment. The performance of this RE was satis-
factory, but the 33$ Og, 67$ COg/CO^ /Au RE was considered a better choice due to its proven suitability and widespread use.
3.2.1.3. Counter electrodes. In the first few experiments, a counter
electrode (CE) resembling the Type I RE was employed. This CE was
soon replaced by a graphite CE. The graphite CE consisted of a 1/8" 105 spectroscopic graphite rod (ultrapurity, United Carbon Products Co.,
Inc.) placed Inside a 1/2" diameter alumina (McDanel 998) sheath.
Contact to the graphite rod was made with platinum wire using electri cally conducting silver epoxy.
An Inner 1/4" diameter alumina tube with Swagelock fittings provided the plumbing hardware for the CE purge gas. A Teflon plug provided a gas-tight seal where the platinum wire exited the plumbing hardware. Two types of CE sheaths were employed. The first was an opened-bottom alumina tube. The second, which was more commonly used, was a closed-bottom alumina tube containing a 1.5 mm hole In the closed end. The hole was made using a diamond drill bit.
The CE sheaths also contained a 1.5 mm hole in the sidewall, located 5" from the sheath bottom. This hole was positioned above the melt level, and functioned to maintain a balanced pressure with respect to the cell compartment. The hole permitted some exchange of gases between the CE and WE compartments, but the exchange was of little consequence. However, when gas product measurements were made, this additional hole In the CE sheath was not provided, because any mixing of the CE and WE gases was undesirable.
3.2.1.4. Stirring assembly. The stirring assembly consisted of the following; stirring motor, rotary seal, shaft coupling, alumina stir ring shaft, and alumina Impeller. The stirring motor was a 1/40 HP laboratory stirrer (G.K. Heller Corp.), equipped with a GT21 solid state motor controller. It had a dlrect-drlve shaft (0-6000 RPM) and a reduction shaft (0-333 RPM). The rotary seal was a Ferrofluldlcs
MB-250-K-N-089 high speed Instrumentation feedthrough. Cooling of the 106 feedthrough was provided by circulating water through external win dings of copper tubing. A 1/4" to 5/16" stainless steel shaft coupling (Holo 1630%, Bearings, Inc.) connected the rotary feedthrough shaft to the alumina stirring shaft just below the cell lid. The alumina stirring shaft was a 5/16" diameter alumina rod (McDanel 998).
The alumina impeller was connected to the stirring shaft by cutting a slot (using a diamond saw) in the bottom of the stirring shaft and securing the impeller in the slot using high temperature alumina cement (Type 139, L.H. Marshall Co.). The alumina cement was cured at
1300°C. Two sizes of alumina impellers were used. The first was a half “'Circle made by cutting a 32 mm alumina disc (McDanel 998) exactly in half. The second was a flat-sided circle, made from a 32 mm disc by cutting a 25 mm chord across the bottom edge.
3.2.1.5. Alumina crucible. The melt was contained in a McDanel 998 cylindrical alumina crucible. It had a 750 ml capacity, and was 83 mm in diameter and 160 mm in height.
3.2.1.6. Inconel canister. The alumina crucible rested on the bottom of an outer canister, made of Inconel 600 alloy. The canister was
3 1/2" in diameter and 12" in height. It had a 6-hole flange welded on the top for connection to the lid. The canister had several win dings of copper tubing near the flange for cooling.
3.2.1.7. Water-cooled lid. Bolted to the Inconel flange was a water- cooled lid made of brass. Copper cooling coils were soldered on the top and bottom surfaces of the lid. A silicone o-ring provided a gas- tight seal between the lid and the flange. Swagelock fittings 107 equipped with Viton o-rings provided gas tight seals for the feedthroughs.
3.2.1.8. Insulation. Inside the Inconel canister and above the alumina crucible rested 4" of high temperature Insulation. The in sulation consisted of closely packed layers of 1/8" thick refractory felt circles (made from 970 J paper, Allen Refractories Co.), with each layer containing a hole pattern to accommodate the electrodes and stirring shaft. The bottom of the stack was supported by an 89 mm diameter alumina disc (McDanel 998) which contained the same hole pattern as the felt circles. A splash shield of refractory felt was sandwiched between the alumina disc and the top of the alumina crucible.
3.2.1.9. Thermocouple. An Inconel-sheathed chromel-alumel (Type K) thermocouple was employed. The thermocouple was contained inside a
1/4" diameter closed-bottom alumina tube (McDanel 998) to shield it from the melt. The thermocouple was tested for calibration with the melting point of sodium chloride (800°C).
3.2.2. Cell Accessories
3.2.2.1. Electronic components. Figure 13 shows the electronic instrumentation employed for electrochemical measurements. The electronic components included a potentiostat (Aardvark Model V), a voltage scanner (Aardvark Model Scan 2) and an X-Y recorder (Hewlett-
Packard Model 7044A).
3.2.2.2. Gas flow network. The gas flow network is shown in Figure
14. The cell was purged with either 100% CO^ or a 3% 00^/97% gas mixture. The RE and CE assemblies could be purged with the cell purge 108
Potentiostat
CE WE RE Input
Voltage Scanner
CE WE RE
Cell X-Y Recorder
Figure 13. Block Diagram of Electronic Instrumentation. ü D
Bubblers Manometers Flowmeters
Drying A tubes V 0 Electrochemical Cell A A
Gas I Gas Chromatograph Vent Flowrate Supply Measurement o Figure 14. Gas Flow Network. VO 110 gases or with a premixed 33% Og/G?) CO^ reference gas. After leaving the cylinders, the purge gases were passed through drying tubes con-» taining drierite. The cell purge rate could be monitored with either a mid-range flow meter (0-157 cc/min, Cole-Parmer K-3217~06) or a high-range flow meter (0-1694 cc/min, Cole-Parmer K-3217-36). Most of the time only the mid-range flowmeter was used. The purge rates to the RE and CE assemblies were monitored with low-range flow meters (0-
52) cc/min, Cole-Parmer K-3217-04). All flowmeters were precalibrated by the manufacturer. When greater precision was warranted, flowrates were measured directly as described in Section 3.2.3. The manometers were used to assure that balanced presure conditions existed in the
WE, CE and RE compartments. Otherwise, the molten carbonate level in the CE or RE compartments would be too high or too low. Bubblers were used to prevent moisture from back-diffusing into the cell. After exiting the bubblers, the CE and RE purge gases were vented. The cell purge gas was either vented or sent downstream for flowrate measure ment and G.C. analysis.
3.2.2.3. Furnace. The cell was heated by means of a 1700 watt
Lindberg crucible furnace (Type 56622). The furnace temperature was controlled automatically using a Lindberg furnace controller (Type
59344). The furnace jacketed the Inconel canister, except for the upper portion containing the cooling coils and flange, which protruded from the top of the furnace. The furnace and canister were raised into operating position using a heavy duty laboratory Jack. 111
3.2.3 Product gas measurement
Two measurements were made on the product gas stream: flowrate measurement and composition measurement. The flowrate measurement was performed using volumetric displacement of liquid (1 M HCl + 1 M NaCl) from an inverted 50 ml buret. The volume of liquid displaced divided by the time of collection gave the volumetric gas flowrate directly.
The volumetric flowrate could then be converted to a mass or molar flowrate by knowing the atmospheric temperature and pressure. The vapor pressure of the displacement solution was taken into account using vapor pressure data from Perry's Handbook (255).
The composition measurement was performed using gas chromatography. The gas chromatograph was a F & M Model 810, and was equipped with a thermal conductivity detector. Two columns were used in series. The first was 1/8" x 3' stainless steel packed with 80-100 mesh silica gel, operated at 55°C,. The second was 1/8" x 20' stain less steel packed with 80-100 mesh molecular sieve, operated at room temperature. Using a helium carrier gas, the first column separated
COg and the second column separated the remaining gases. The G.C. was equipped with a Beckman 23800 sampling valve. Calibration of the G.C. was performed using the CO and CO^ cylinder gases listed in Section
3.3.4. 112
3.3. Feedstocks
3.3.1. Carbonaceous materials
The carbonaceous materials that were experimentally tested are listed below. Analytical data for activated carbon and coal varieties are given in Appendix A.
Darco activated carbon - manufactured by ICI Americas, Inc., and supplied by Aldrich Chemical Company, Inc.
Mesh sizes; 12-20, 20-Ü0
Kentucky #9 bituminous char - prepared by the Rockwell Hydropyrolysis process and supplied by Westinghouse.
Mesh size: -12
North Dakota lignite - taken from the Beula-Zap seam at the Beulah mine in North Dakota, and supplied by the Energy Research Center,
University of North Dakota.
Mesh size; -9
Primrose anthracite - collected by Pennsylvania State University, and supplied by the Penn State coal data base. Designated as sample number PSOC-867.
Mesh size; -10
Spectroscopic graphite - supplied by Ultra Carbon Corporation.
Specified as UCP-1-100 Briquetting Grade Ultra Spectroscopic Powder.
Mesh size: -100
High purity graphite - supplied by Cerac, Inc. Specified as 99.9$ pure.
Mesh size: -325 113
Carbon graphite powder - supplied by Alfa Products. Specified as
99.5% pure.
Mesh size: 20-60
Synthetic diamond - supplied by Industry.
Mesh size: 100-120
3.3.2. Electrolyte
The ternary carbonate eutectic was prepared by dry mixing 32.1 wt$ lithium carbonate (Baker reagent grade powder), 3%.5 wtX potassium carbonate (Baker reagent grade powder) and 33.% wtX sodium carbonate
(MOB reagent grade powder).
3 .3.3 Additives
Chemicals utilized as additives were: NaOH (Malllnckrodt),
Na^S'QH^O (Malllnckrodt), NatgCgOy (sodium oxalate. Baker), NaAlO^
(Harshaw), AlgO^ (Fisher), Na^O (Alfa), NagOg (Malllnckrodt), TlOg
(Baker), TlgOg (Alfa), SIO^ (sllca gel, Fisher) and FeCO^ (supplier unspecified).
3 .3.4 Gases
Gases were supplied from high pressure cylinders containing the
following compositions:
99.9X CO^ - Matheson bone dry grade
99.5% CO - Matheson CP grade
3.15X COLln - Matheson Certified Standard, mixed from bone-dry
COg and high purity
33% 0^/67% CO^ - Matheson Certified StStandard, mixed from bone-dry
COg and extra dry 0^ 114
3.4. Procedures
3.4.1. Start-up
Prior to each run, the premixed eutectic powder was vacuum dried at 350“C for a minimum of 4 hours. Quantities of activated carbon or coal sufficient to last several runs were vacuum dried at 250°C for a minimum of 4 hours. Graphite and chemical addtives were not dried prior to use. Vacuum was provided by a high-'vacuum rotary pump (Welch
Scientific Company, Model 1405). A dry ice trap collected the evolved moisture. The dried feedstocks were stored in a drierite desiccator until ready for use.
The RE was assembled as follows. The alumina powder (0.5*1.0 g
80-200 mesh), when utilized, was first added to the RE sheath. After insertion of the gold electrode into the RE sheath, 5 g of dried eutectic was added. The reference purge gas hardware was then con nected, and the RE assembly was mounted to the lid. A fresh RE assembly was usually employed each run.
When the closed-bottom CE sheath was utilized, the CE was as sembled as follows. The CE sheath was loaded with 5 g of dried eutectic. The graphite rod was then installed, along with the CE purge gas hardware, and the CE assembly was mounted to the lid. The
CE sheath was positioned such that its end rested on the bottom of the alumina crucible. When the opened-bottom CE sheath was used, it was not prefilled with eutectic.
The gold-to-alumina seal on the WE was recemented as necessary with fresh alumina cement, which was then cured in air at room temperature. Prior to each run, the gold WE was polished with steel 115 wool, etched in 50% HCl for 10-20 seconds, and rinsed with distilled water followed by acetone. When the graphite WE was used, the polish ing and etching steps were not carried out.
The alumina crucible was typically loaded with 350 g of dried eutectic powder, which corresponded to a final melt depth of 1 1/2".
Initial loadings of carbon or additives were mixed in with the eutec tic powder as needed. The crucible was then placed inside the inconel canister.
With the electrodes, stirring shaft and thermocouple mounted to the lid, the lower portion of the cell (crucible, canister and furnace assembly) was slowly raised to bring the cell into the fully assembled position. In this manner, the various alumina feedthroughs pushed easily through the bed of powdered eutectic in the alumina crucible.
When the cell was completely assembled, it was tested for gas leaks. An internal pressure of 1-2 psig was applied to the cell by imposing a flow on the inlet side and sealing all exit lines. The cell was leak-free when the inlet flowmeters all dropped to zero.
The cell was brought to operating temperature using a heating rate of 100“C/hr (50° increments every 1/2 hour). Once steady state conditions were reached, the furnace controller maintained the cell within ±3°C of the desired operating temperature.
During heat-up, a 150 cc/min purge rate of the WE compartment was typically used. The RE assembly was purged with 50 cc/min of either
3% or 100% COg until electrolyte meltdown occurred. After meltdown,
25 cc/min of 33% 0^/67% CO^ was admitted. The CE assembly was purged with 50 cc/min of either 3% or 100% 00^ during heat-up. 116
Once the cell reached the desired operating temperature, the purge rates were decreased. The cell was purged at 50-55 cc/min and the RE was purged at 1-5 cc/min. The CE purge was either turned off or set at 20-30 cc/min with the same gas composition as used in the WE compartment.
When required, carbon or chemical additions were made to the operating cell through the purge gas exit tube (made of 1/4" McDanel
998 alumina).
3.4.2. Collection of Electrochemical Data
With all the operating conditions properly set, the electrochemi cal measurements were begun. The procedure for a typcial run was as
follows. The open circuit potential (OCR) was obtained by switching
the potentiostat to the disabled reference cell potential position
which permitted the high-impedance signal between the RE and WE to be
directly measured. Beginning at the OCR, the WE was polarized at a
linear scan rate (1-100 mV/s) in the positive potential direction to a
predetermined potential limit (typically -0.3 to 0.0 volts vs.
reference). Once the potential limit was reached, the scan direction
was reversed, and the reverse scan proceeded until the OOP was
reached. The first scan was usually taken under unstirred conditions,
with subsequent scans taken under stirred conditions. Scans were
repeated at regular time intervals (usually 1/2 hour) until steady
state conditions prevailed. Additional carbon loadings were then made
to the operating cell as required, and the above procedure was
repeated. 117
The I-V scans were recorded directly on graph paper by means of the X-Y recorder. Calibration of the potentiostat and the X-Y re corder were checked periodically using a high precision digital voltmeter (Fluke 830OA).
3.4.3. Shut-down
After all the data were collected, shut-down commenced. The cell was cooled at 100°C/hr until it reached approximately 500°C. At this temperature, the bottom portion of the cell (crucible, canister and furnace) was disengaged from the lid assembly and lowered away from the electrodes. A separate, water-cooled lid was placed on the canister to minimize heat loss. The furnace was then turned off and tilted at a 45" angle until room temperature conditions prevailed.
The purpose of the 45° tilt was to prevent cracking of the alumina crucible upon solidification of the carbonate.
3 .4.4 . Product Analysis
To monitor product gases evolved from the WE, the flowrate and composition of the cell exit stream were measured on-line, as described in Section 3-2.3. The measurements were performed with the current off to give a baseline, and then with the current on. Current was applied under potentiostatic conditions. The current-time curve was traced on the X-Y recorder using a time-based x axis as the time keeper.
An important aspect in the gas measurement was maintaining a close balance of pressure within the different cell compartments (RE,
CE and WE). Pressure imbalance could cause gas from the CE or RE
(especially the CE, since flow out of the RE was greatly restricted by 118 the alumina powder) to mix with the main cell gas, and give erroneous results. To maintain balanced pressure conditions, the flowrates to
the RE and CE were adjusted until the manometer readings matched that
of the WE compartment.
In one experiment to be described later (Section %.5), a rela->
tlvely thick llght-gray deposit was found on the gold WE. The deposit
was analyzed by EDAX and X^ray diffraction. The EDAX analysis was
provided by the Department of Metallurgical Engineering at OSU, and
was performed on a Model 9100 EDAX system. The XhRay analysis was
provided by the Department of Ceramic Engineering at OSU, and was
performed on a Philips 3100 XRG. CHAPTER IV
EXPERIMENTAL RESULTS
4.1. Overview of Experiments
Table 16 summarizes the runs that were performed during the course of the experimental work. Each run corresponds to a fresh charge of carbonate. Runs T-1 through T-8 served as test runs to verify the functionality of the apparatus and to explore various electrode configurations. Series A was initiated when the apparatus achieved satisfactory operation. Series B was started after it was realized that the stirring effectiveness was inadequate, and a modified stirring arrangement was installed. The data were con^ siderably more consistent in Series B owing to the enhanced stirring effectiveness. Thus, a large part of Series A had to be discounted.
Gas evolution measurement and G.C. analyses were performed in Run C-1, which concluded the experimental program.
After experimenting with various reference electrodes in Series
T, it was decided to utilize the 33t 0^, 67% C O ^ / C O ^ /Au electrode for subsequent runs. Series A and part of Series B utilized the Type
I reference electrode sheath (4 pinholes, approximately 400 microns in diameter). This arrangement usually worked satisfactorily, but in some cases led to large fluctuations in the potential. Starting in
119 120
Table 16. Summary of Completed Experiments
Run Operating No. Temperature, °C Cell Loading Comments T-1 700 Blank (Carbonate only) Begin Series T- test runs T-2 700 2% Darco 12-10 T-3 700 Blank (Carbonate only) T-4 600,700 10% Darco 12-20 T-5 500,600,700,800 10% Darco 12-20 Begin addition of alumina powder in RE sheath T-5 700 10% Darco 12-20 Type III RE sheath T-7 700 10% Darco 12-20 T-8 700 10% Darco 12-20 Graphite RE
A-1 500,600,700,800 10% Darco 12-20 Begin Series A A-2 500,600,700,800 10% Darco 12-20 A-3 700,800 10% Darco 12-20,0.5g FeCO^ Ti02,Na2S,Na2C20y additives A-U 700,800 10% Darco 12-20,2 g AlgO^» Na202.Na0H additive 2 g NaAIOg A-5 700,800,900 10% Darco 12-20 Na2Û additive, WE I, WE III A-6 600,700,800 10% Graphite -325 A-7 700,800 10% Darco 20-40 A-8 700,800,900 10% Darco 20-40 Begin use of shielded CE A-9 600,700,800,900 2,4,5,7,10% Darco 20-40 Gas collection trial A-10 600,700,800,900 1% Darco 20-40 A-11 600,700,800 1% Graphite -325 A-12 600,700,800 1% Darco -325 A-13 600,700,800,900 1% Char -325 WE I, WE III A-in 600,700,800,900 1 % Graphite -325 A-15 600,700,800 1 % Darco 10-40, acid washed A-16 700,800 lOg Darco ash,7g Darco 20-40 WE film analysis A-17 700,800 1% Graphite -325+0.9g ash Gas collection trail A-18 700 1% Graphite 20-60 A-19 700,800 5% Graphite 20-60 WE I, WE II A-20 700 1,2,3,6,10% Darco 20-40 WE I, WE II, WE III 121
Table 16 (continued)
Run Operating No. Temperature, “C Cell Loading Comments B-1 700 1,2$ Darco 20-40+Ti02/Ti20g Series B- Improved stirring B-2 700 1,2% Darco 20-40 + Si02 TigOg additive B-3 700 1% Darco 20-40 + FeCOg ash additive B-4 700 1,2,5,10,15% Darco 20-40 B-5 700 1,2,3,5% Darco 200-325 B-6 800 1,2,5,10,15% Darco 20-40 B-7 600 1,2% Darco 20-40 WE failure B-8 500,600,700 1 % Darco 20-40 Type III RE sheath B-9 600 1 ,2,5,10,15% Darco 20-40 Type II RE (used in all subsequent runs) B-10 500 1,2,5,8,10,15% Darco 20-40 B-11 700 1.2,3,5,10,15% Darco 20-40 B-12 800 1,2,3,5,10,15% Darco 20-40 B-1 3 700 1,2,3,5,10% Darco 50-100 B-in 700 1,2,5,10% Diamonds 100-120 B-15 700 1,2,3,5,10% Char 50-100 B-16 700 1,2,3,5,10% Lignite 30-50 B-1 7 700 1,2,3,5,10% Graphite-100 B-18 700 1,2,3,5,10,15% Anthracite 30-50 B-19 700 1,2,3,5,10% Darco 20-40 Graphite WE
C-1 700 1,5,10% Darco 20-40 Gas analysis
(a) For Series T and A, % loading g carbon/100 g of melt + carbon, For Series B and C, % loading g carbon/100 g of melt. 122
Run B-9, the Type II RE electrode sheath was employed. The smaller pinhole (100 microns) reduced contamination of the RE melt enough to effectively eliminate fluctuations in the reference potential.
In the early •'.ns, a 3% CO^ (balance N^) purge gas blanketed the cell. It was eventually realized that a higher CO^ percentage was needed to prevent melt decomposition, and a switch to 100% COg purge was made. The 100% CO^ purge was utilized part-time in Runs A-3 through A-? and full-time in all runs thereafter.
The high COg percentage precluded meaningful measurement of electrochemical carbon consumption, due to the Boudouard gasification reaction. For instance, in runs that lasted several days, a substan tial portion of the carbon charge was lost by gasification. This gasification loss made it desirable to complete each run in a short time (usually one day).
In some runs, a short duration air purge (15-30 min.) was ad mitted to promote oxidation of the carbon surface and subsequent wetting by the melt. This treatment had a negligible effect on the current-voltage characteristics, and it was discontinued.
Runs defining the effects of temperature and carbon loading gave superior results when the cell was maintained isothermal and the carbon was added incrementally, rather than maintaining a constant carbon loading and incrementing to different operating temperatures.
Changing the temperature to higher levels caused irreversible changes in the carbon charge. The isothermal procedure was used primarily in
Series B. 123
For the remainder of this manuscript, the following distinction in carbon loading will be used;
wt % - g carbon/100 g melt + carbon
% - g carbon/100 g melt
4.2. Open Circuit Potential Data
The results of the open circuit potential (OOP) measurements are presented in Figures 15-17 and Tables 17-18. The OOP data was taken only from runs in which reference electrode fluctuations were not encountered. The OOP was affected significantly by the following parameters: cell temperature, purge gas composition, carbon type,
carbon concentration, run time and cell temperature history. The OOP
was also influenced slightly by the stirring status, e.g., a shift to
slightly more negative values (10-50 mV) occurred when the melt was
not stirred. The OOP values herein reported all refer to stirred
conditions. A positive shift in the OOP frequently occurred after
polarization of the working electrode, but the OOP usually drifted
back to a reproducible value.
4.2.1. Effect of Cell Temperature
In Figure 15, it can be seen that increasing the cell temperature
shifted the OOP to more negative values. A parallel shift in OOP
occurs under different purge gas conditions (compare 100% COg with 3%
COg purge). At 500°C, the OOP falls off considerably under the 100%
COg purge condition. ID
Carbon Loading 1%
3% 5%
A “ 3% C02 Purge B " 100% COS Purge
400 500 800 700 800 900 Temperature (C)
Figure 15- Open Circuit Potential of Darco Activated Carbon versus Temperature. rvj to
3% C02 Purge, 10 Wt% Darco 12-20 Loading 100% C02 Purge, 10 % Darco 20-40 Loading
« t\j
o. o
0 4 8 12 16 20 24 Run Time (hours)
Figure 16. Open Circuit Potential for Darco Activated Carbon Loadings versus Run ro Time. Experimental Conditions; T - 700®C. U 1 o {VI
Temperature (C) e BOO A 7 0 0 a 6 0 0
o o
U CD o ca
0 5 10 1 5 Carbon Loading (g/lOOg melt)
Figure 17. Open Circuit Potential versus Darco Activated Carbon Loading. Experimental Conditions: Purge - 100$ COg. o> 127
Table 17. OCR of Various Carbon Types at 700*C.
Experimental Conditions: Purge - 100% COg.
OOP (Volts vs . Reference)
Carbon Loading (g/IOOg melt) Carbon Type 1 2 3 5 10 15
Darco 20-40 -0.98 -0.99 -0.99 -1.01 -1.02 -1.03
Char 50-100 -0.98 -1.00 -1.00 -1.01 -1.01 -
Lignite 30-50 -0.92 -0.94 -0.95 -0.98 -1.01 -
Anthracite 30-50 -0.96 -0.97 -0.98 -0.98 -1.00 -1 .00
Graphite -100 -0.91 -0.92 -0.93 -0.93 -0.96 -
Diamond 100-120 -0.51 -0.52 - -0.53 -0.55 -
Table 18. Effect of Temperature History on DC? of Darco Activated Carbon.
Experimental Conditions: Carbon Loading - 15% Darco 20-40, Purge - 100% COg.
OOP (Volts vs. Reference) Cell Temperature (°C)
Temperature History 500 600
Isothermal at run temp. -0.58 -0.91
Operated at 700°C -0.96 -0.96
Operated at 800°C -0.95 -0.98 128
4.2.2. Effect of Purge Gas Concentration
Figure 15 shows that a lower concentration of COg in the purge gas shifts the OCR to more negative values. Figure 16 illustrates
that a much longer time is required to reach a steady OOP using a 3%
COg purge compared to the 100$ COg purge.
4.2.3. Effect of Carbon Concentration
Figure 17 shows that an increase in carbon concentration shifts
the OCR to more negative values. The shift in OCR is most pronounced at low carbon loading, and tapers off at high carbon loading.
4.2.4. Effect of Carbon Type
Table 17 presents the OCR of various carbon types at progressive
carbon loadings. At all carbon loadings, a close match in OCR was
found between the Darco activated carbon and the bituminous char.
With the exception of graphite and diamond, the OCR values of the
various carbons matched closely at high carbon loadings. Moderately
lower OCR values were exhibited by lignite at low loadings and by
graphite at all loadings. Diamond gave substantially lower OCR
values.
4.2.5. Effect of Cell Temperature History
The OCR at lower cell temperatures was affected strongly by the
temperature history of the cell, as illustrated in Table 18. By
maintaining the cell isothermal at the indicated run temperature, OCR
values of -0.58 and -0.91 V were obtained at 500 and 600°C,
respectively. However, by running the cell at higher temperatures,
and then returning to the lower temperatures, substantially higher
(more negative) OCR values resulted. 129
4.3. Current-Voltage Data
4. 3 . 1 . Typical Current-Voltage Curves
Figure 18 shows raw current-voltage (I-V) curves as recorded on the X-Y recorder. Each curve represents a forward and return scan, beginning at the OCR. Repeated scans were usually quite reproducible.
Fluctuations in the current were more pronounced under stirred conditions. The degree of current fluctuation exhibited in Figure 18 is comparatively moderate; fluctuations of greater or lesser magnitude were commonly encountered, depending on the operating conditions.
Greater fluctuations were found at lower temperatures. As illustrated in the figure, only a slight hysteresis was present upon return scan, and this was true of most I-V curves (with notable exceptions, as will be discussed).
To facilitate handling of the data, the raw I-V curves were smoothed, digitized, and replotted using a computer graphics program.
All subsequent I-V curves represent replots from the raw data. Unless otherwise noted, I-V curves which showed noticeable hysteresis upon return scan were replotted by averaging the forward and reverse scans.
The current hereafter has been converted to current density using the geometric area of the working electrode.
In the following sections, the effect of run time, purge gas composition, scan rate, stirring rate, carbon type, carbon loading, carbon particle size, cell temperature, cell temperature history, anode size, anode material, carbon impurities, and run replication will be treated individually. The I-V data are presented in Figures
19-40 and Tables 19-21. stirring Conditions A - Stirring at BOO RPM B - No Stirring
in -
—1.2 —1.0 -0.8 —0.8 -0.4 -0.2 0.0 0.2 Voltage (Volts vs Reference)
Figure 18. Typical Raw I-V Curves. Experimental Conditions: T - 700®C, Loading LU 151 Darco 20-40, Scan Rate - 20 mV/s, Purge - 1001 COg. O o Run Time (hrs)
0 .5 1 .5
9 .0 2 3 .0
•rt
O o
-1.5 -1.0 -0.5 0.0 0.5 Voltage (Volts vs Reference)
Figure 19. I-V Curves Showing the Effect of Run Time Under 31 CO^ Purge. Experimental Conditions: I « 700°C, Loading = 10 wtl Darco 12-20, Stirring Rate = 250 RPM, Scan Rate = 20 mV/s. Run Time Chrs)
21
-1.5 - 1.0 — 0 . 5 0.0 0.5 Voltage (Volts vs Reference) Figure 20. I-V Curves Showing the Effect of Run Time Under 100% CO^ Purge. W Experimental Conditions: T » 700°C, Loading - 10 wt% Darco 12-20, fO Stirring Rate =■ 250 RPM, Scan Rate » 20 mV/s. o Purge Conditions 3% CD2, after 50 hrs 100% C02, 16 hrs after A 3% C02, 2.5 hrs after B
-1.6-1.4 —1.2 -1.0 —0.6 —0.6 —0.4 -0.2 0.0 0.2 Voltage (Volts vs Reference) Figure 21. I-V Curves Showing the Effect of Purge Gas Composition. Experimental Conditions: T - 700“C, Loading » 10 wtl Darco 12-20, Stirring Rate - U> 250 RPM, Scan Rate » 20 mV/s (only forward scans shown). w Scan Rate (mV/s) 20
-1.1 -0.9 —0.7 —0.5 —0.3 —0.1 0.1 0.3 0.5 Voltage (Volts vs Reference) Figure 22. I-V Curves Showing the Effect of Scan Rate. Experimental Conditions: T • 700°C, Loading - 10 wtï Darco 12-20, Stirring Rate » 250 RPM, kJJ Purge - 100$ COg, Data from Run A-5. XT o Scan Rate (mV/s) 20
•H
- 1.2 — 1.0 —0.8 -0.8 —0.4 - 0.2 Voltage (Volts vs Reference) Figure 23. i-V Curves Showing the Effect of Scan Rate. Experimental Conditions: T - 700°C, Loading » 15Ï Darco 20-40, Stirring Rate - 500 RPM, Purge - 100$ CO , Data from Run B-11. stirring Rate 1000 RPM BOO RPM E BOO RPM No Stirring < CO
•H O U) (0
C\J -
1.2 1.0 - 0.8 - 0 .8 -0.4 - 0.2 Voltage (Volts vs Reference)
Figure 2U. I-V Curves Showing the Effect of Stirring Rate. Experimental Conditions: T - 700°C, Loading » 5% Darco 20-40, Purge - 100% CO, •ri - 1.2 - 1.0 —0.8 -G.6 -Q.4 - 0.2 Voltage (Volts vs Reference) Figure 25. I-V Curves for Darco Activated Carbon at a Series of Loadings. Experimental Conditions; T - 700°C, Stirring Rate - 600 RPM, W Mesh size - 20-40, Purge - 100$ CO^. Loading (g/lOOg melt) - 1.2 — 1.0 -0.0 —O.B —0.4 - 0.2 Voltage (Volts vs Reference) Figure 26. I-V Curves for Bituminous Char at a Series of Loadings. Experimental Conditions: T ■ 700“C, Stirring Rate - 600 RPM, Mesh size - 50-100, Purge » 100$ COg. 00 o Loading (g/lOOg melt) 10 E O O OJ > •H a o — 1.2 —1.0 —0.8 —0.6 —0.4 —0.2 Voltage (Volts vs Reference) Figure 27. I-V Curves for Lignite at a Series of Loadings. Experimental Conditions: T - 700®C, Stirring Rate - 600 RPM, Mesh size - 30-50, W Purge ■ 100% CO . KO o in Loading (g/lOOg melt) 10 -H O - 1.0 — 0 . 0 —0.6 —0.4 —0.2 - 0.0 Voltage (Volts vs Reference) Figure 28. I-V Curves for Spectroscopic Graphite at a Series of Loadings. Experimental Conditions: T - 700°C, Stirring Rate - 600 RPM, o Mesh size - -100, Purge - 100% CO^. Loading (g/lOOg melt) 10 - 1.2 — 1.0 —0.8 —0.6 —0.4 - 0.2 Voltage (Volts vs Reference) Figure 29. I-V Curves for Anthracite at a Series of Loadings. Experimental Conditions; T - 700°C, Stirring Rate - 600 RPM, Mesh size - 30-50, Purge - 100% COg. A = No Carbon 1% Darco 20-40 •H a o -1.1 -0.9 -0.7 -0.5 -0.3 -0.1 0.1 0.3 0.5 Voltage (Volts vs Reference) Figure 30. I-V Curves Comparing 0% and \% Carbon Loadings. Experimental Conditions: T » 700°C, Stirring Rate - 600 RPM, Purge - 100? CO XT 2* rv) Voltage (volts vs Réf.) » -0.30 = -0.50 “ —0.80 Ti o g 0 5 10 15 Carbon Loading (g/lOOg melt) Figure 31. Current Density versus Darco Carbon Loading at Various Potentials. Experimental Conditions: T - 7Ü0°C, Stirring Rate « 600 RPM, Mesh size - 20-40, Purge » 100$ CO^. Carbon Particle Size A = 200-325 mesh B « 50-100 mesh C “ 20-40 mesh 0.0 2.0 4.0 8.0 8.0 10.0 Carbon loading (g/lOOg melt) Figure 32. Current Density versus Darco Carbon Loading at Various Particle Sizes. Experimental Conditions: T - 700°C, Stirring Rate » 600 RPM (Curves B & C), 500 RPM (Curve A), Voltage - -0.3 Volts, Purge - 100% CO 2 ' o Cell Temperature (C) A = BOO B = 700 C = BOO 0 = 500 •H S2 -1.2 -1.0 -O.B -0.6 -0.4 - 0.2 - 0.0 Voltage (Volts vs Reference) Figure 33. I-V Curves Showing the Effect of Cell Temperature. Experimental Conditions: Loading - 15% Darco 20-40, Stirring Rate - 600 RPM, U1 Purge - 100% CO . Voltage (volts vs Réf.) -0.3 -0.5 — 0 . 8 cr c o o 0.90 0.98 1.06 1.14 1.22 1.30 1/T X 1000 (1/K) Figure 34. Arrhenius Plot for Darco Activated Carbon. Experimental Conditions: Loading » 10% Darco 20-40, Stirring Rate - 600 RPM, Purge = 100$ CO^. Xr en o ni tn - 1.0 - 0 . 0 —G .B —0.4 - 0.2 Voltage (Volts vs Reference) Figure 35. Apparent Activation Energy versus Voltage. Experimental Conditions: Loading - 10% Darco 20-40, Stirring Rate - 600 RPM, Purge - 100% GO. o o ai Temperature History A = BOOC - 10 hrs B = 700C - 27 hrs, BOOC 2 hrs E C = BOOC - 10 hrs. 700C 17 hrs. ü o tris BOOC - 1 hr (0 'V. < E g - 0) O ■P C 0} - 1.2 -1.0 -0.8 —O.B —0.4 - 0.2 Voltage (Volts vs Reference) Figure 36. I-V Curves Showing the Effect of Cell Temperature History. Experimental Conditions: T ■ 600°C, Loading » 151 Darco 20-40, Stirring Rate » XT 600 RPM, Purge - 100$ CO 00 2 " Anode Type. Carbon Loading A = Graphite, 10 % Carbon B = Gold, 10 % Carbon C = Graphite, 0 % Carbon □ = Gold, 0 % Carbon -1.1 -0.9 —0.7 —0.5 —0.3 —0.1 0.1 0.3 Voltage (Volts vs Reference) Figure 37. I-V Curves Comparing Gold and Graphite Anodes. Experimental Conditions: T ■ 700°C, Carbon Loading » Daren 20-40, Stirring Rate = 600 RPM, Purge - 100% COg. VO Scan Rate (mV/s) 100 B » 20 or - 1.2 — 0 . 8 -0.4 - 0.0 0.4 Voltage (Volts vs Reference) Figure 38. I-V Curves Showing the Effect of Added Ash. Experimental Conditions: T - 700°C, Ash Loading - 10.6 g Darco ash, Stirring Rate - 250 RPM, o Purge - 100$ COg. Scan Rate (mV/s) 100 20 - 1.2 —0.8 —0.4 —0.0 0.4 Voltage (Volts vs Reference) Figure 39. I-V Curves Showing the Effect of Added Ash and Carbon. Experimental Conditions: T - 700°C, Ash Loading » 10.6 g Darco ash, Carbon Loading 1 % Darco 20-40, Stirring Rate - 250 RPM, Purge - 100$ CO^. o Loading (g/iOOg melt) 10 Run B-11 Run C-1 >* - 1.2 - 1.0 —0.8 —O.B —0.4 - 0.2 Voltage (Volts vs Reference) Figure 40. I-V Curves Comparing Run B-11 with Run C-1. Experimental Conditions: T « 700“C, Loading » Darco 20-40, Stirring Rate » 600 RPM, VJl Purge - 100$ COg. rv> 153 Table 19. Comparison of the Current Density Achieved Using Different Carbon Types. Experimental Conditions: T - 700°C, Carbon Loading - 10$, Purge - 100$ COg, Stirring Rate - 600 RPM. 2 Carbon Current Density (mA/cm ) Type (mesh) at indicated potential (Volts vs. Reference) -0.Ô0 -0.50 -0.30 Char (50-100) 27.9 136 230 Lignite (30-50) 18.4 108 218 Darco (20-40) 13.2 89.5 136 Graphite (-100) 19.2 52.6 82.4 Anthracite (30-50) 6.05 43.4 54.5 15% Table 20. Current Densities at Three Sizes of Gold Anodes. Experimental Conditions: Run - A-20, T - 700®C, Carbon Loading - 12 % Darco 20-%0, Stirring Rate - 250 RPM, Potential - -0.5 Volts vs. Reference, Purge - 100% COg. Anode Anode Current Current Density 2 Area (cmf) (mA) (mA/cm ) I 3.80 1%0 36.8 II 2.53 90 35.6 III 0.%0 26 65.0 Table 21. Current Densities at Submerged and Dipping Gold Anodes with Graphite Loading. Experimental Conditions: Run - A-19, T - 700°C, Carbon Loading - 5 wt% Graphite 20-60, Stirring Rate - 250 RPM, Potential - 0.0 Volts vs. Reference, Purge - 100% COg. Anode Anode Current Current Density 2 2 Type Area (cm ) (mA) (mA/cm ) Dipping (I) 3.8 300 79 Submerged (II) 2.53 5.5 2.2 155 4.3.2. Effect of Run Time The I-V curves exhibited a definite time dependency, shifting up and sometimes down as time progressed during a given run. As much as possible, it was attempted to obtain time-’Stabilized curves. However, a trade->off had to be made because carbon was consumed by the Boudouard reaction. Figure 19 illustrates the time dependency under 3$ CO^ purge conditions. The curve at time zero was recorded when the cell first arrived at 700°C. Current was flowing only during recording of the curves. The 1-V curves shifted toward higher currents and negative voltages for a long period of time. After 9 hours, a peak developed around -0.35 volts, and became more pronounced as time elapsed. Upon return scan, the peak was closely retraced. Under 100% CO^ purge conditions, the time dependency was reduced substantially, as shown in Figure 20. At long run times, a small peak was still encountered. The time-dependency was further reduced in Series B, presumably due to the improved stirring conditions. The 1-V curves typically shifted slightly at the beginning of the run, but stabilized within 1 to 2 hours. The majority of the 1-V curves that will be presented were taken from Series B, and represent time-stabilized curves. 4.3.3. Effect of Purge Gas Composition The effect of purge gas composition on the time dependency of the 1-V curves has already been presented in Section 4.3.2. In this section, a comparison of time stabilized curves under different purge 156 gas compositions will be given. Figure 21 shows the effect of switch ing from a lower CO^ concentration to a higher CO^ concentration, and then switching back to the low COg concentration. Under 100% COg purge conditions, the I-V curve shifted down considerably, and the OOP shifted from -1.43 to -1.01 volts. After returning to the 3% COg purge, the I-V curve shifted upward toward the initial position. The final I-V curve under 3% COg purge was not fully time-'stabilized, and probably would have continued to shift toward the initial curve if adequate time had been permitted. However, a greater drop in current around -0.25 volts was definitely evident in the final I-V curve. 4.3.4. Effect of Scan Rate Scan rates in the range of 1-100 mV/s were employed during the course of the experimental work, with most data obtained at 1, 10 and 20 mV/s. Scan rates of 100 mV/s were seldom used, but gave basically the same I-V response as the 10 or 20 mV/s scans. Scan rates of 1 mV/s gave variable results. Sometimes the I-V curves coincided with faster scans, but other times they differed substantially. A comparison of I-V curves at two different scan rates is shown in Figure 22. The data were taken from Run A-5. The positive direct tion scan at 1 mV/s closely followed the 20 mV/s scan up to approximately 0.0 volts. At potentials more positive than 0.0 volts, the positive direction 1 raV/s scan was suppressed, and the return 1 mV/s scan showed a substantial hysteresis. After the 1 mV/s return scan, an immediate 20 mV/s scan (not shown) closely retraced the 1 mV/s return scan. However, after a 30 minute wait, a 20 mV/s scan (also not shown) closely retraced the initial 20 mV/s scan. It was 157 suspected that the hysteresis was due to passivation of the electrode. For Series A in general, the hysteresis behavior was most pronounced at 1 mV/s scans and long run times. Faster scan rates of 10 and 20 mV/s usually showed only slight hysteresis. Scan rates of 1 mV/s taken at short run times also showed little hysteresis in most cases. The degree of hysteresis frequently correlated with the mag nitude of the current peak which developed at long run times. When the current peak was absent, the hysteresis usually did not occur. As the current peak increased, the degree of hysteresis also increased. When scans were limited to potentials negative of the peak potential, the hysteresis was substantially reduced or eliminated. The hys teresis at 1 mV/s scan rates was usually not observed when the cell was operated at higher temperatures (800, 900°C). However, when the cell was elevated from 700 to 800“C, and then returned to 700°C, the hysteresis was accentuated. In Series B, the hysteresis was minimized by taking measurements at short run times (one-day runs) and by limiting the return scan potential to values between -0.1 and -0.3 volts. For the most part, 1 mV/s scans coincided closely to the 20 mV/s scans, and little hys teresis was encountered. In several instances, particularly at long run times and high carbon loadings, a different type of hysteresis was encountered, as illustrated in Figure 23. At low overpotential, the 1 mV/s scan gave a considerable dip in current, which then recovered at high overpotential. The return scan at 1 mV/s was not performed. Subsequent 20 mV/s scans differed little from the initial one. 158 The I-V curves presented in following sections were taken from both 1 and 20 mV/s scans. However, In cases where 1 mV/s scans dlf-’ fered from the 20 mV/s scans, only 20 mV/s scans were utilized. 4.3.5. Effect of Stirring Rate In Series A, the stirring blade consisted of a half-clrcle of alumina, 32 mm In diameter, and the stirring rate was most often set at 250 RPM. These stirring conditions were found to give effective mixing In a simulation experiment using water and bouyant plastic particles. However, since visual Inspection of the operating cell was not possible. It was difficult to determine the extent of mixing within the cell. It was eventually realized that the mixing effec tiveness was probably Inadequate. This realization was based on the fact that a large part of the carbon particles did not appear wetted at the end of most runs. Additionally, the 1-V curves showed only minor effects of stirring when compared to non-stlrred conditions. A simulation experiment was performed using conditions very similar to those employed In the cell. In the simulation experiment, a molten salt bath was prepared In an alumina crucible of Identical size as used In the cell, and about 10 wt% of carbon particles were added to the melt. Electrodes were not Installed, It was determined by visual Inspection that the stirring conditions employed In Series A were Indeed Ineffective. The majority of the carbon particles floated on the surface when the melt was stirred, and only a fraction were drawn under the surface. By Increasing the stirring blade size (dimensions described In Section 3.2.1.4.) and Increasing the stirring rate to at least 500 RPM, complete mixing of the carbon particles was observed. 159 The Installation of improved stirring conditions marked the beginning of Series B. The effect of stirring rate on I-V curves is illustrated in Figure 24 for a representative run (Run B-11) in Series B. There is a large difference between the I-V curves under stirred versus non stirred conditions. Under stirred conditions, increasing the stirring rate increases the current level in the high overpotential region. For example, at -0-3 volts, increasing the stirring rate from 600 to 2 1000 RPM increased the current density from 86 to 107 mA/cm . At low overpotential, varying the stirring rate between 600 and 1000 RPM had no effect on the I-V curves, although there still was a difference between stirred and non-stirred conditions. In general, the effect of stirring rate became greater as the carbon loading was increased. At low carbon loadings (e.g., 1,2%) the I-V curves changed little within the 600-1000 RPM range, even at high overpotential. At higher carbon loadings (10,15%), the stirring rate dependency was about the same as in Figure 24. For most I-V curves in Series B, a single stirring rate of 600 RPM was utilized. Higher stirring rates tended to promote splashing of the carbon/carbonate mixture onto the walls of the crucible. 4.3.6. Effect of Carbon Type The type of carbon utilized in the majority of the runs was Darco activated carbon. Additional studies were performed using bituminous char. North Dakota lignite. Primrose anthracite, spectroscopic graphite and industrial diamonds. The diamonds were electrochemically inactive, and therefore no data on diamonds will be presented. 160 Figures 25-29 present the I-V curves of the various carbon types at a series of loading levels. The curves show basically the same features, but the current density at a given potential differs among the carbon types. Table 19 provides a comparison of the various carbon types at three values of potential. The current density achieved with each carbon type decreases in the order listed in the table; the bituminous char gave the highest current density and the anthracite gave the lowest. The reason for the high activity of the bituminous char should not be attributed to the smaller mesh size, as will become clear in Section 4.3.8. One exception to the trend listed in Table 19 is exhibited by the spectroscopic graphite at a potential of -0.8 volts. In this instance, the current density was higher than both the Darco activated carbon and the lignite. 4.3.7. Effect of Carbon Concentration Figure 30 compares the 1-V curves before and after the addition of carbon to the cell. In the absence of carbon, the current level was very low until approximately 0.0 volts, at which point carbonate oxidation (melt decomposition) begins. In the presence of carbon, a wave begins at much more negative voltages, reflecting the electronegativity of the carbon/CO^ redox couple. Figures 25*29 show that an increase in carbon loading results in an upward shift in the 1-V curves. To better depict the effect of carbon loading, the data was cross-plotted as current versus carbon loading at several values of potential. Figure 31 shows such a cross plot for Darco activated carbon. At a given value of potential, the rate of current density increase is high at low carbon loadings, but 161 tapers off at high carbon loadings. The amount of current density increase in going from low to high carbon loadings is highest at the highest overpotentials (most positive voltages). 4.3.8. Effect of Carbon Particle Size The effect of particle size of Darco activated carbon on the current density is illustrated in Figure 32. Three different mesh sizes were used: 20-40, 50-100 and 200-325, with corresponding average particle diameters of 0.63, 0.22 and 0.059 mm, respectively. The 50-100 mesh size was obtained by crushing the 20-40 mesh stock, while the 200-325 mesh size was obtained by crushing a 12-20 mesh stock. Thus, the 200-325 mesh size was derived from a different stock than the 20-40 and 50-100 mesh sizes, but all three were Darco carb ons. The fact that the 200-325 mesh size was derived from a different stock renders a strict comparison difficult. It should also be pointed out that the 200-325 mesh size was quite fluffy and voluminous, which caused insufficient mixing in the cell. The insufficient mixing became apparent at the end of the run, when it was noticed that a large fraction of the carbon particles was not wetted. Furthermore, it was discovered that mixing of the melt caused evolution of carbon fines from the cell, and in this manner a substantial amount of carbon had been ejected from the cell through the exit purge line. Because of these complications with small par ticle sizes, the mesh size was generally limited to +100 mesh. Keeping the above comments in mind, let us now consider Figure 32. At carbon loadings of 1-5%, the current density increased as the particle size decreased. At 10% carbon loading, the current density 162 at the 50-100 mesh size fell below the 20-40 mesh size. This anomalous behavior probably points to some non-unlformlty In mixing, which could not be visually witnessed during cell operation. Data was not obtained at 10$ carbon loading using the 200-325 mesh size because the reference electrode became unstable. The amount of change In current density In going from 20-40 mesh to 50-100 mesh was relatively small. The maximum change occurred at 2 3X carbon loading, where the current Increased from 70 to 87 mA/cm - a 24$ Increase. A larger Increase In current density was obtained by going to the 200-325 mesh size. At 1$ loading, the current density 2 Increased from 37 to 75 mA/cm - a 100$ Increase. 4.3.9. Effect of Cell Temperature The temperature dependency of the I-V curves Is Illustrated In Figure 33. Increasing the temperature Increased the current density by a substantial amount at all voltages. The greatest relative In crease In current density occurred between 600 and 700“C. An Arrhenius plot Is given In Figure 34 at three fixed potentials. Although the data points are fairly scattered, a least squares fit gives activation energies of 17.7,21.5 and 25.2 kcal/mol at voltages of -0.3, -0.5 and -0.8 volts, respectively. The apparent activation energy, G_, can be divided Into chemical and electrical components by the following equation (257): Gg - Gg - onF (E-Egq) (4.3.1) where G^ Is the chemical activation energy and anF(E-E^^) Is the electrical activation energy. The symbols have their usual meaning (a Is the transfer coefficient, n Is the number of electrons, F Is 163 Faraday's constant, E Is the potential, and E^^ Is the equilibrium potential). The chemical activation energy can be obtained froui a plot of versus E, as shown in Figure 35. By extrapolating to the equilibrium potential of -1.0 volts (zero overvoltage), the apparent activation energy becomes equivalent to the chemical activation energy. The chemical activation energy thus determined is 28.*4 kcal/mol. An alternative method of determining the chemical activa^ tion energy will be described in Chapter V (Section 5.2). 4.3.10. Effect of Cell Temperature History The effect of cell temperature history is exemplified in Figure 36. Each I-V curve was recorded at a cell temperature of 600“C. The only difference in operating conditions for each curve is the maximum prior operating temperature of the cell. Curve A represents isother mal conditions, meaning that the cell was maintained at 600°C during the entire run. Curves B and C represent nonisothermal conditions, in which the cell was operated at a higher temperature (700 and 800®C, respectively), and then lowered to 600°C. Comparison of Curves A, B and C from Figure 36 shows that the I-V curves were shifted up sub stantially by prior operation at higher temperatures. The largest shift in the I-V curve was found at the highest prior operating temn perature (800°C). A similar effect of temperature history was found at operating temperatures of 500 and 700“C, although this data is not presented. The substantial effect of temperature history on the I-V curves must be caused by some irreversible change that occurs at higher operating temperatures. 164 4.3.11. Effect of Anode Size As described in Section 3.2.1.1, three sizes of gold anodes were utilized in the cell. Although the Type I gold anode was employed in most experiments, some data were collected using the Type II and Type III gold anodes. Table 20 compares the current densities obtained at each anode size at a potential of -0.5 volts. A similar comparison with similar results could be made at other potentials. The current density agreed closely between Type I and Type II gold anodes. Each of these anodes was made from 1/8 inch gold rod, but they were cut to different lengths. The Type III gold anode gave a current density approximately two-’fold higher than Type I and Type II. The Type III anode was made of 0.5 mm gold wire, which probably had a higher sur face roughness than the 1/8 inch gold rod. A different type of behavior was found with graphite loadings. The Type II and Type III gold anodes were designed to be fully sub merged beneath the melt surface. Table 21 compares the Type II submerged anode with the Type I dipping anode. A drastic reduction in current density was found in the case of the submerged anode. This result gave a good indication that the graphite was not adequately mixed. With the dipping anode, electrical contact was made to a layer of graphite particles on the melt surface, which greatly facilitated electron transfer and anodic discharge by effectively increasing the anodic surface area. 165 4.3.12. Effect of Anode Material Gold anodes were utilized exclusively in all but one experimental run. In Run B-19, a spectroscopic graphite anode was tested. Figure 37 compares the I-V characteristics of a graphite anode and a gold anode; the anodes were of identical geometry (Type x and Type IV, as described in Section 3.2.1.1). In the absence of carbon, the graphite anode gave a substantial corrosion current, whereas the gold anode was essentially inert. In the presence of 10$ Darco carbon, the graphite anode and gold anode gave similar I-V curves. The graphite anode gave somewhat lower current levels at potentials negative of -0.15 volts, and somewhat higher current levels and potentials positive of -0.15 volts. The amount of increase in current caused by the presence of 10$ Darco carbon was substantially less at the graphite anode compared to the gold anode (compare curves B, D and A, C). Passivation of the graphite anode was pronounced at 1 mV/s scan rates. Once passivation occurred, the current density remained low on subsequent scans, and took a long time (overnight) to fully recover. 4.3.13. Effect of Carbon Impurities Since most carbons that were experimentally utilized contained substantial amounts of mineral impurities, it was suspected that these impurities could dissolve in the carbonate melt and have a major influence on the I-V curves. To study the effect of mineral im purities, the following methods were employed; addition of specific impurities to the melt, addition of carbon ash to the melt, utiliza tion of acid-washed carbon, and utilization of purer forms of carbon. 166 y.3.13.1. Results from Series A . Most of the runs which investigated the effect of impurities were performed in Series A. However, since the apparatus was not optimized, especially with regard to the stir ring effectiveness, much of the data is questionable. In spite of this drawback, some useful data was generated, as described below. Initially, it was suspected that most of the current was being producted by oxidation of dissolved minerals or other dissolved species. To test this hypothesis, the following impurity additions were made separately in the presence of 10 wt% Darco carbon: 1.5 g FeCOg, 2.0 g TiOg, 1.0 g NagS'SHgO, U.O g Na^C^O^, 2.0 g NaOH, and 3.0 g NagOg. The current density was not affected significantly by any of these additions. The NaOH addition did seem to reduce the extent of hysteresis. To test the hypothesis that dissolved alumina (aluminate) was responsible for passivation, an initial charge of 2.0 g AlgO^ and 2.0 g NaAlOg was added to the cell along with 10 wt% Darco carbon (Run A-4). The passivation behavior was not altered by the presence of the alumina or aluminate. In Run A-16, the effect of added ash was investigated. The ash was obtained from Darco carbon by air oxidation at 700®C. Oxidizing 35 g of Darco carbon gave 10.6 g of ash. The I-V curves obtained with 10.6 g of ash added to the melt are illustrated in Figures 38 and 39. Figure 38 shows the I-V curves in the absence of carbon. The current level was strongly dependent on the scan rate. Both anodic and cathodic peaks were produced at the highest scan rates (100 mV/s). The cathodic peak indicates the presence of a reducible electroactive 167 species. In the presence of I.Owtf Darco carbon (Figure 39), the current level shifted up considerably. The anodic and cathodic peaks appear even at 20 mV/s, but are shifted to more negative potentials. It is likely that the carbon thermochemically reduced some of the dissolved ash, which increased the concentration of oxidizable reac-> tants and increased the current level. In Run A-15, acid-«washed Darco carbon was tested. The acid washing removed much of the iron impurity and reduced other mineral contents as well. At 1.0 wt% loading, the 1-V curves were not sig nificantly different when compared to non-washed Darco carbon. Ultrapure graphite was tested in Run A-14. At 1.0 wt% loading, no signs of passivation were encountered, even at long run times. 4.3.13.2. Results from Series B . Runs B-1, B-2 and B-3 studied the effects of titanium, silicon and iron impurities, which were suspected to contribute to anode passivation. These particular species were identified from EDAX analysis of an electrode film obtained in Run A-16 (see Section 4.5). All three runs were performed at 700°C. In Run B-1, titanium oxides (TiOg and TigOg) were added to the melt, with and without carbon. The TiOg gave essentially no 1-V response. Addition of 1.0 g TigOg gave two waves: one at -1.0 volts, 2 with a limiting current density of approximately 1 mA/cm , and one at 2 -0.7 volts, with a limiting current density of approximately 2 mA/cm . The addition of 1.0% Darco carbon in the presence of 2.0 g TiOg and 2.0 g TigOg at first gave a very high current density level-around 100 2 mA/cm at -0.3 volts. The high current density was temporary, and after a few scans it dropped off considerably. The stabilized current 168 level was slightly higher than runs without added minerals (about 10% higher at -0.5 volts). Passivation was slightly more pronounced compared to runs without added minerals. In Run B-2, 1.0 g silica gel was added to a carbon^free melt. The silica showed no electrochemical activity. With 1.0% Darco carbon added to the cell, the silica had little effect on the current density level when compared to runs without added silica. However, passiva tion did seem more pronounced. After an overnight stand, the I-V curves shifted down by about two-fold. Cleaning the anode returned the current density to the prior level (i.e., a two-fold increase). In Run B-3, 1.0 g FeCO^ was found to have little effect on the I- V curves at 1.0% Darco carbon loading. Addition of 0.9 g Darco carbon ash increased the current density slightly (about 10% at -0.5 volts). Passivation was not investigated after the ash addition. High purity graphite was employed in Run B-17. Essentially no passivation was found to occur with this type of carbon. 4.3.14. Effect of Run Replication The reproducibility of the I-V data from run to run can be Judged by comparing replicate runs. The reproducibility in Series A was found to be poor, owing largely to the ineffective mixing. The reproducibility improved substantially in Series B and C. Since many different experimental conditions were tested in Series B, and since the apparatus underwent several modifications (such as changing the reference electrode), there were few runs that could considered replications. However, two runs that could be considered replications were Runs B-11 and C-1, and these runs are compared in Figure 40. The 169 reproducibility is quite good at 10% and 5% loadings, but only fair at the 1% loading. At a potential of -0.5 volts, the deviation in cur rent density of Run B-11 versus C-1 is 6, -3, and 10% at 10, 5 and 1% carbon loadings, respectively. 4.4. Gas Evolution Data Two preliminary attempts at gas evolution measurement were made in Runs A-9 and A-17, but the data were mainly qualitative. In Run C-1, a more quantitative measurement of gas evolution was performed. Two types of gas measurements were made; 1) total outlet gas flow rate, with and without current drain, and 2) gas chromatographic analysis of the outlet gas stream composition, with and without cur rent drain. A steady COg purge was required during the mesurements to prevent melt decomposition. Table 22 shows the results of the total gas evolution rate measurements and the calculated gas product yields. The data were obtained at three carbon loadings. The estimated error brackets are included in the table. The net outlet flowrate represents the total gas evolution rate of the current-'producing reaction(s). The gas yield relates the total gas evolution rate to the rate of electron flow. Table 23 gives the results of the gas chromatograph analysis. Carbon oxides (CO and COg) were the only gases found at measurable levels in the outlet gas stream. The outlet COg composition is shown under zero current conditions and with the current set at 425 mA. By knowing the COg composition in the product gas stream and the total 170 Table 22. Total Gas Evolution Rates and Cas Product Yields. Experimental Conditions: Run - C-1, T - 700“C, Carbon Type - Darco 20-40, Potential - -0.3 Volts vs. Reference, Purge - 100$ CO , Stirring Rate - 600 RPM. Î Carbon Current Inlet Outlet Flow rate 'cc/min) Gas Yield^* (g/IOOg melt) (mA) Purge Rate I-off I-on Net (mol/Faraday) (cc/min) 1 95 10.4 17.1 18.2 1.1+0.2 0.71 ± 0.13 5 325 35.7 48.7 51.8 3.U1 .0 0.58 ± 0.20 10 440 35.7 53.2 58.1 4.9+1.0 0.68 ± 0.14 Q__t P (a) Gas Yield - gQ ~ÿ~ j » where ■ net evolution flowrate (cc/min), F - m 96,500 coulombs/Faraday, - gas molar volume (co/mole), I - current (amps). 171 Table 23. Results of Gas Chromatograph Analysis. Experimental Conditions: Run ■ C-1, T - 700°C, Carbon Loading - 10% Darco 20-40, Potential - -0.3 Volts vs Reference, Stirring Rate - 600 RPM, Purge - 100% COg. Parameter Numerical Value Current level 425 mA (a) Outlet COg Composition (I-off) 53.7 mol% f a \ Outlet COg Composition (I-on) 58.5 mol% Inlet COg Flowrate 35.7 cc/min Total Outlet Flowrate (I-off) 53.2 cm/min Total Outlet Flowrate (I-on) 58.1 cc/min Electrochemical COg Evolution Rate^^) 5.4 ± 1.0 cc/min COg Yield^c) 0.78±0.15 mol/Faraday Carbon Efficiency 6.7% (a) Balance CO (b) Calculated from: Q__ LUg,net (^COg^out ^I-on " (^COg^out^I-Off ®COg,net ^ (c) COg yield - — ^ ^— (see Table 22 for nomenclature) m (d) Ratio of electrochemical consumption to Boudouard plus electrochemical consumption. Calculated from: 1/3 QrCOg,net Carbon Eff. ^^CO ^out^I-off * '^COg,net 172 outlet flowrates (from Table 22), the outlet CO^ flowrates could be determined. Baaed on these outlet COg flowrates, the net COg flowrate (Qco net^ and the COg product yield were determined. This COg gas yield was independent of the gas yields reported in Table 22, which were based on total outlet flowrates only. The COg gas yield in Table 23 agrees well with the total gas yields from Table 23, and also with the theoretical value of 0.75 mol/Faraday for the following reaction: C + 2C0g2" 4. 3C0g + 4e" (4.4.1) The carbon efficiency given in Table 23 represents the ratio of the carbon consumption rate by electrochemical reaction to the total carbon consumption rate by the electrochemical and Boudouard reactions. The 6.7? carbon efficiency means that only 6.7 g of carbon were electrochemically utilized for every 100 g of carbon consumed. Thus, the chemical loss of carbon was heavy. 4.5. Surface Behavior of the Gold Anode The same gold anode was used for approximately 45 runs. The accumulated effect of these runs on the anode was minimal. In some cases, the anode did show signs of discoloration or slight surface roughness. However, polishing with steel wool restored the surface to normal. Impurity additions to the melt sometimes resulted in the forma tion of surface films on the gold anode. A surface film was especially noticeable at the conclusion of Run A-l6, in which 10.6 g of Darco ash was added to the melt. The film was light-gray in color. 173 and fairly thick (about 1 rail). It was easily removed from the anode, and was insoluble in 50% HCl. X^Ray diffraction gave no crystalline pattern. EDAX analysis is shown in Table 24. The main component was silicon (probably as SiOg), with lower precentages of titanium and iron. Anode films were also observed when the melt contained added FeCOg, TigOg and SiOg. However, analysis of the film was not per formed in those cases. Table 24. EDAX Analysis of Anode Film Element Composition wt Ï atom % Si 87.1 92.8 Ti 3.49 2.18 Fe 9.45 5.06 CHAPTER V DISCUSSION OF RESULTS 5.1. Thermodynamic Analysis of OOP Data In this section, the experimental OOP data will be compared to thermodynamic values. Equilibrium cell potentials were determined from free energy data as outlined in Section 2.2.5.1. The potential-determining reaction was assumed to be: C 2C0g2" 4. gcOgCW) + He” (5.1.1) The corresponding cell voltage is: "cell ■ "cell • i - i «•'•2' with respect to the CO^/0^ reference electrode. In the presence of carbon, the CO^ pressure above the melt depended on the extent of the Boudouard reaction: COg + C + 2C0 (5.1.3) Assuming equilibrium of the Boudouard reaction, the COg pressure above the melt can be uniquely determined for a given inlet COg pressure and cell temperature. Once the equilibrium COg pressure is known, the equilibrium oxide activity (a .) and alkali metal activity can be determined, along with the cell voltage (OCP). This calculation is outlined in Appendix B. The results of these calculations for two inlet COg pressures are given in Table 25. 17H 175 Table 25. Results of Equilibrium Calculations. Based on equations given In Appendix B. Equilibrium Temperature (°C) Values 500 600 700 800 Boudouard K 0.004007 0.08633 0.9780 6.965 Inlet atm (Bal. PcOg " 0-0315 "2 ) 0.0262 0.0137 0.00310 0.000518 Pq o ^ Pqo (atm) 0.0103 0.0344 0.0551 0.0601 (atm) 0.964 0.952 0.942 0.939 ^2 6.75x10"’° 1.30x10"? 2.25x 10"5 2.66x 10"3 ^oxlde 6.70x10“’^ 4.56x10"* 2.10x 10"5 4.37x10"* inlet Pgo^ - 1.0 atm 0.939 0.746 0.386 0.113 PcOg (at*) Pco (atm) 0.0610 0.254 0.614 0.887 1.88x10~’’ 2.40x10"* 1 .81x10"? 1 .22x 10"5 ^oxlde ""10 4.65x10"T3 2.27x10 5 .65x10"® 7 .71x10"® 176 At high temperature and low inlet CO^ pressure, the Boudouard- based equilibrium CO^ pressure becomes quite low - 0.0031 atm at 700®C and 0.000518 atm at 800“C. The low COg pressure results in a substan tial increase in oxide activity. At 800°C, the equilibrium oxide activity under 3% COg purge was calculated to be 0.00266, so that the carbonate melt could contain as much as “ 0.3% oxide. Under 100% COg purge conditions, the equilibrium COg pressure does not fall ap preciably with respect to the inlet, and the oxide activity remains low. At 800°C, the equilibrium oxide activity is only 1.22 x 10 , or about 0.001$ oxide. The ability to keep a low oxide activity reveals the advantage of using the 100% COg purge in place of the 3% COg purge. This advantage was the primary reason for switching to the 100% COg purge in the experimental program. The long-term time dependency of the OCP (Figure 16) and I-V curves (Figure 19) under 3% COg purge probably reflects a slow dis sociation rate of the melt as it adjusted to the diminished COg pressure. Under the 100% COg purge, the COg pressure was diminished only slightly with respect to the inlet, and steady state was achieved more rapidly. Table 25 reveals that the activity of potassium metal in or above the melt should be minimal below 800°C. The other alkali metals (Na,Li) would have comparably low activités in the melt. A mixed potential involving alkali metal is therefore unlikely. In Table 26 a comparison of the calculated and experimental cell voltages (OCR's) is given. Included in the table is the OCP calcu lated by assuming that the COg pressure above the melt remains 177 Table 26. Comparison of Calculated and Experimental OCP Values. OCP (Volts vs. Reference) Temperature (“C) Basis 500 600 700 800 Inlet Pp. - 0.0315 atm (Bal. "2 ) Inlet pQQ^ ->1.17 -1.18 -1.20 -1.22 Boudouard Equilibrium P.. -1.17 -1.23 -1.35 -1.50 bÜ2 / a \ Experimental Value -0.99 -1.14 -1.29 -1.41 Inlet Pj,Q^ - 1.0 atm Inlet P^o^ -0.993 -0.989 -0.986 -0.982 Boudouard Equilibrium P^^ -0.996 -1.01 -1.05 -1.13 Experimental Value^^^ -0.58 -0.91 -1.03 -1.14 (a) 10 wt$ Darco 12->20 (b) 15% Darco 20^40 178 equal to the inlet COg pressure, i.e., no Boudouard reaction. The Boudouard-based OCP will be denoted as Boudouard-OCP, and the inlet- based OCP will be denoted as nonBoudouard-OCP. At low temperature, where the Boudouard equilibrium constant is low, there is little difference between the two calculated OCP values. As the temperature increases, and the Boudouard equilibrium constant increases, the two calculated OCP values diverge considerably. Under 100% COg purge, the nonBoudouard-OCP values show a tempera ture trend opposite to that of the Boudouard-based OCP values. Since the temperature trend of the experimental OCP values agrees with the Boudouard-OCP values, the assumption of Boudouard equilibrium is a priori better than the assumption of no Boudouard reaction. At 500 and 600“C, the experimental OCP values fall considerably below the calculated OCP values. The low OCP values at these lower temperatures indicate that electrochemical equilibrium between the carbon and carbonate melt (reaction (5.1.1)) was not achieved. At 700 and 800°C, and under 100% COg purge, the experimental OCP values closely agree with the Boudouard-OCP values. Under these conditions, both the Boudouard and carbon/carbonate equilibrium were closely attained. The slight variation between calculated and ex perimental values at 700°C probably indicates that complete Boudouard equilibrium was still not achieved. From Run C-1 gas analysis, P.^ CUg was found to be 0.54 atm at 700“C and 10% Darco loading (Table 23). Using this P_^ value, the OCP calculates to be -1.02 volts, which ^"2 179 agrees exactly with the experimental value of -1.02 volts at 10% Darco loading (Table 17). At 700 and 800°C, and under 3% CO^ purge, the experimental OCP values fall between the Boudouard-OCP and the nonBoudouard-OCP values. This result suggests that the CO^ pressure did not fully reach the Boudouard equilibrium condition. It is also possible that the carb on/carbonate equilibrium was not achieved, but the results under 100% COg purge contradict this possibility. The dependency of the experimental OCP values on carbon loading (Figure 19) is most likely associated with differing degrees of ap-> proach to the Boudouard equilibrium condition. The extent of Boudouard reaction is less at low carbon loadings than at high carbon loadings. However, in contradistinction, the approach to equilibrium does not appear to depend on external surface area of the carbon, since decreasing the particle size from 20-^0 to SO-100 mesh (Run B- 13) did not significantly alter the OCP values. This result suggests that the Boudouard reaction is not limited by the external surface area, in agreement with previous discussion (Section 2.1.6). Thus, the extent of Boudouard reaction would be controlled by the weight of carbon (for a fixed value of the specific internal surface area), and the OCP would reflect the carbon loading and not necessarily the external surface area of the carbon particles. Summarizing the above paragraphs, simultaneous equilibrium of the Boudouard reaction and the carbon/carbonate reaction was favored at temperatures of 700°C or above, 100% (1.0 atm) COg purge, and carbon loadings of 10% or above. 180 The OCP values of the different coal types (Table 17) agree fairly well with the Boudouard-OCP values. Apparently the different carbon networks contained in these coals have essentially the same electrochemical activity. The slight differences in OCP's among the coals may reflect different reactivities toward COg, which influence the degree of approach to Boudouard equilibrium. With graphite, the OCP at 700°C is about 0.1% volts lower than the Boudouard-OCP, but falls within 0.03 volts of the nonBoudouard-OCP. At 700“C, the rate of the graphite-COg reaction is relatively low (see Section 2.1.6), and the Boudouard equilibrium assumption would not be good in this case. The reason for the effect of temperature history on OCP values (Table 18 ) is not well understood. Subjecting the carbon to a higher temperature apparently results in an irreversible increase in the carbon reactivity. The increase in reactivity brings the carb on/carbonate reaction closer to equilibrium after returning to the lower temperatures. The mechanism of this apparent increase in reac tivity is unknown. 5.2. Interpretation of I-V Data There is little doubt that a substantial anodic current was generated from the carbonate melt due to the presence of carbon. The data of Figure 30 together with the literature data reported in Section 2.2.6 verify that the anodic current at potentials negative of 0.0 volts cannot be attributed to oxidation of the gold electrode. Impurities indigenous to the carbon can be discounted as the primary 181 electroactive species based on the experiments discussed In Section 4 .3.13. The dependency of the current density on carbon loading (Figure 31) demonstrates further the participation of carbon In the generation of current. Thus, It Is clear that the carbon Itself Is primarily responsible for the observed I-'V characteristics. However, this conclusion does not Insinuate a necessity of direct participation of carbon In the electrode process, as will be addressed In Section 5.4. The I-V curves for Darco activated carbon at 700°C (Figure 25) can be considered "model" I-V curves for comparison with the other I-V curves. The model I-V curves have two apparent waves of current. The first wave begins at the OCP, and the second wave begins at ap proximately -0.7 volts. A third wave corresponding to melt oxidation occurs around 0.0 volts, but It Is not evident In the model I-V curves due to the -0.3 volt cut-off (Figures 19-22 show the third wave). The first wave shows a linear dependence on carbon loading (Fig. 31, Curve C). The second wave rises to high levels of current density, but the rate of Increase with carbon loading levels off at high loadings (Figure 311 Curves B and A). The stirring rate dependency (Figure 24) Indicates the presence of mass transfer limitations In the anodic process. At low overpoten- tlal and stirring rates above 600 RPM, the I-V curves were Independent of the stirring rate (Figure 24), which Indicates a klnetlcally con trolled process. The fact that gold and graphite anodes gave essentially the same I-V curve at low overpotential (Figure 37) rules out the charge transfer reaction as the rate limiting step. Thus, the 182 low overpotential region (below -0.7 volts) seems to be controlled by a chemical kinetic process at stirring rates of 600 RPM and above. The high overpotential region (positive of -0.7 volts) appears to be controlled by a combination of chemical kinetic (or adsorption/desorption) and mass transport processes. To examine further the I-V characteristics of Darco activated carbon, Tafel plots were constructed, as shown in Figure 41. The non- linearity of the Tafel curves is a further indication of the multifaceted anodic processes. Linear Tafel curves would be rigorously expected only for a single, charge transfer-controlled reaction. At high carbon loadings, the Tafel curves do become somewhat linear in the midrange overpotential region. The negative deviation from linearity at high overpotential is characteristic of systems with mass transport limitations. The potential at which the Tafel curves start to level off is about -0.60 volts, in good agreement with the onset of stirring rate dependence depicted previously in Figure 24. Using the linear portion of the Tafel plots, apparent kinetic parameters were evaluated. The transfer coefficient, an, was evaluated from the following relationship (257); Tafel slope (V/decade) - (5.2.1) The exchange current density was found by extrapolating the linear portion of the Tafel curve to the experimental OCP. The Tafel plot parameters are summarized in Tables 27 and 28. Included in Table 27 are the Tafel parameters for bituminous char and lignite, with all ra Loading {g/lOOg melt) 15 10 o or (0 X < E >. +> •rt o 0) c 0) o c 0) c. O (_ O 3 t H U - 1.2 -1.0 -0.8 -0.6 -0.4 - 0.2 Voltage (Volts vs Reference) Figure *41. Tafel Plots for Darco Activated Carbon. Experimental Conditions: CO T - 700 °C, Stirring Rate - 600 RPM, Purge - 100$ CO w 2" 184 Table 27. Tafel Plot Parameters for Darco Activated Carbon, Bituminous Char and Lignite. Experimental Conditions: Loading - 10%, Stirring Rate ■ 600 RPM, Purge - 100% COg. Tafel slope Transfer Coeff. lo 2 Carbon Type (mV/decade) (an) (mA/cm ) Darco 20-40 360 0.54 3.1 Lignite 30-50 380 0.51 5.0 Char 50-100 430 0.45 9.2 Table 28. Tafel Plot Parameters for Darco Activated Carbon at 600-800°C. Experimental Conditions: Loading - 15%, Stirring Rate - 600 RPM, Purge ■ 600 RPM. Temperature Tafel Slope Transfer Coeff. lo (°C) (mV/decade) (an) (mA/cm^) 600 230 0.76 1.3 700 410 0.47 5.6 800 540 0.39 15 185 carbons being compared at the same loading level (10%). Graphite and anthracite were not included because the Tafel plots for these carbons did not have a linear portion whatsoever. At 700°C, the an values of the three carbon types congregate around 0.5. Making the usual assumption that a - 0.5 gives an n value of 1.0 for the rate-determining charge transfer reaction. However, this analysis breaks down because other results have indicated that the anodic process is not controlled by a charge transfer step. The temperature dependence of the an value (Table 28) indicates that the rate-controlling step is altered by temperature. If it is assumed that the rate controlling step is a chemical reaction step, the temperature dependence of this step would be reflected as a change in the an value. The I-V curves for lignite and bituminous char showed the same general shape as the I-V curves for Darco activated carbon (Figures 25-27), which indicates that the same rate controlling processes were probably in effect. The bituminous char generated the highest current 2 density of the various carbon types - 230 mA/cm at -0.3 volts, and 2 10? loading. A current density of 100 mA/cm was achieved at a volt age of -0.57 volts with the same loading level. Although these current densities are quite respectable, the voltage is too low for commercial purposes. A steeper rise in current density at low over potential (smaller Tafel slope) would be most advantageous. The reason that lignite gave a lower current density than bituminous char may be related to the high volatile matter content of the lignite (see 186 Table 39 in Appendix A). The actual weight of lignite in the cell was substantially less than the initial loading value due to loss of volatile matter upon exposure to the cell temperature. The spectroscopic graphite and anthracite produced I-V curves distinctly different from the Darco activated carbon I-V curves. The I-V curves with graphite (Figure 28) showed nearly linear characteris tics, which may reflect the high electrical conductivity of the graphite. The anthracite I-V curves (Figure 29) leveled off con siderably at high overpotential, which has the appearance of a dominating mass transport limitation. However, little stirring rate dependence was found at high overpotential, which indicates that a different rate controlling process was in effect with anthracite. The lower current densities generated with graphite and anthracite most likely reflect the generally lower chemical reactivity of these forms of carbon. The electrochemical reactivity of the graphite, anthracite, and other carbons does not correlate, though, with the carbon-COg reactivity depicted previously in Table A. The anomalously high current density of graphite at low overpotential (Table 19) may reflect the smaller particle size, or the higher electrical conductivity of the graphite. The particle size dependency on the current density (Figure 32) suggests a certain advantage in using smaller particle sizes. It may be possible to boost the current considerably at low overpotential by operating with very fine particles (e.g., -325 mesh). To further explore the smaller particle sizes, it would be necessary to modify or redesign the apparatus to provide improved mixing characteristics. 187 The chemical activation energy reported In Section 4.3.9 contains contributions from several overlapping rate processes, and should therefore be viewed as a global parameter. For example, the ap pearance of two waves In the I-V curves Implies that different reactions are controlling in the low and high overpotential regions. Thus, the apparent activation energies determined at low and high overpotentials (Figure 34) probably do not apply to the same reaction step, and the chemical activation energy contains contributions from both steps. The chemical activation energy also contains a contribu tion from mass transport processes, so that the magnitude of the chemical activation energy Is probably dependent on the stirring rate. Due to these overlapping rate processes, the chemical activation energy could not be associated with a single rate-controlling step. An alternative method of finding the chemical activation energy Is to construct an Arrhenius plot using logarithmic exchange current densities (258). With this method, extrapolation of the activation energy to the OCP Is unnecessary, since the current densities were already extrapolated to the OCP In the Tafel plots. Using the ex-> change current densities of Table 28, the chemical activation energy was found to be 23 kcal/raol. This value agrees fairly well with the 28.4 kcal/raol found by the first method. It should be pointed out that neither method was very accurate because of the small number of data points Involved. Therefore, an average value of 25 kcal/mol will be reported. The reason for the effect of temperature history on the I-V curves (Figure 36) Is uncertain. The discussion of Section 5.1 188 regarding the effect of temperature history of the OCP is equally applicable here. It should be further noted that a substantially lower activation energy would be obtained if operating temperatures were approached from high to low. Due to the predominantly close correspondence of the I-V curves at different scan rates, the scan rates employed were considered slow enough to achieve steady state polarization conditions (at least in terms of transport boundary layers). The hysteresis behavior that occurred at long run times (Section 4.3.%) is attributed to passiva tion of the electrode, and not to boundary layer depletion. A strong indication of passivation was the sharp drop in current that occurred at long run times under 3$ CO^ purge (Figure 21). The sharp drop probably reflected the onset of passivation. Other results discussed in Section 4.3.4 can best be explained by passivation. For example, the upward shift in the 20 mV/s scan after the 30 minute idle period can be explained by a slow removal of the passive layer. Passivation of the working electrode was probably caused by impurities indigenous to the coal-derived carbons. This conclusion is supported by the following experimental observations: 1. Passivation was not observed with high purity graphite. 2. Darco ash showed definite electrochemical reactivity in the ab sence of carbon. The Darco ash gave a current peak at a potential where passivation was commonly observed (Figure 38). 3. The anode film formed from an ash-contaminated melt was composed of Si, Ti and Fe - primary components of coal ash (Table 3). 189 4. Passivation was most pronounced at long run times, which would correlate with a gradual build-up of dissolved impurities. 5. Passivation usually increased when impurities were intentionally added to the melt. Passivation of gold electrodes in LigCO^-LigO mixtures was pre viously reported by Smirnov et al. (116), as discussed in Section 2.2.5.3.2. The passivation was attributed to the presence of dis solved alumina, which formed a film on the electrode by the following reaction: 2 AlOg" - AlgOg + 0^" (5.2.2) Smirnov et al. (116) also suggested that SiOg and TiOg may have con-« tributed to the passive film. SiOg and TiOg could form by reactions analogous to reaction (5.2.2): SiOgZ" - SiOg + Q^~ (5.2.3) TiOgZ" - TiOg + 0^" (5.2.4) Iron could appear in the anode film by the following reaction: FeOg^" •* FeO + 0^' (5.2.5) Methods of minimizing passivation were not explored in the present investigation. However, one experimental result suggested that the presence of NaOH in the melt reduced the extent of passiva tion (Section 4.3.13). Further experiments are necessary to verify this result, and to investigate other methods of minimizing passivation. In molten electrolytes, the IR contribution to overpotential is usually negligible (Section 2.2.2.7). Operating on this basis, no attempt was made to compensate for IR drop in the I-V curves. 190 However, at high current density, the IR contribution may become sizeable. The IR contribution can be estimated from: IR - (5.2.6) where i is the current density, 1 is the spacing between the WE and RE, and k is the specific conductivity. From Table 9, the specific i — 1 conductivity of the ternary eutectic is 1.838 ohm cm at 700°C. 2 For an approximate 1.0 cm spacing and a current density of 100 mA/cm , the IR drop calculates to be 5*» mV. This IR drop is not negligible, and therefore the I-V curves probably contain a fair degree of ohmic polarization at high overpotential. Future investigations are re quired to experimentally measure the IR contribution at high current density. 5 .3. Comparison of Data with Related Investigations 5.3.1. OCP Data Comparison of OCP data is limited to the relatively recent studies of Hauser (169), Arkhipov and Stepanov (205), Dubois et al. (206) and Weaver et al. (212,214). The OCP data from these workers appear fairly dependable. The large variety of carbon types and variability within each type, together with dissimilar experimemtal conditions, precludes exact comparison of OCP data. However, meaning ful comparisons can be achieved, as discussed below. The following table compares the OCP data of Hauser (169) with the present work. 191 Table 29. Comparison of OCP Data with Data of Hauser (169). OCP (volts) Data of Hauser Data from Figure 15 T(°C) (vs. 50% Og, 50% COg/Pt) (vs. 33%, 67% COg/Au) 600 -0.55 -0.86 - -'0.91 700 -0.69 -’0.98 -■ -1.03 800 -0.82 -1.0»» - -1.14 Hauser's electrode was high grade dense graphite, obtained from National Carbon Company. The cell purge gas and reference electrode gas contained 50% 0^/50% CO^. The difference in potential between the 50% COg, 50% Og/Pt reference electrode and the 33% 0^, 67% CO^/Au reference electrode is within 10 mV, and can be considered negligible. Therefore, the OOP's of Hauser effectively share a common reference potential with the present work. The negative shift in OCP with increasing temperature from Figure 15 agrees with the trend in Table 29. However, at a given tempera ture, the OOP's found in this work were consistently more negative than the OOP's of Hauser. Referring to Table 17, spectroscopic graphite gave an OCP between -0.91 and -0.96 volts at 700°C, depending on the loading level. Hauser's graphite electrode would probably better compare with this spectroscopic graphite. But since the other carbons gave OOP's fairly close to the spectroscopic graphite at 700°C, the difference in carbon variety does not seem to account for the large difference between Hauser's data and the present data. The 192 disagreement in OCP’s can probably be attributed to the use of 50$ 0^ in the purge gas. The high 0^ content in the cell would greatly hinder the establishment of equilibrium conditions at the anode, which would result in a substantial positive shift in the carbon electrode potential. As discussed in Section 2.3.1.1, Arkhipov and Stepanov (205) reported OOP's of a spectroscopic carbon anode, and their OCP data are compared with the present data In the following table. Table 30. Comparison of OCP Data with Data of Arkhipov and Stepanov (205). Open Circuit Potential (volts) Data of Arkhipov and Stepanov Data from Figure 15 T(°C) (vs. 33$ Og, 67$ COg/Pt) (vs. 33$ Og, 67$ COg/Au) 500 -0.60 ' 0.58 - -0.69 600 -0.95 ■>0.86 - -0.91 700 - 1.0 -0.98 - -1.03 800 - 1.2 -1.04 - -1.14 Comparing their values with Figure 15 under 100$ CO^ purge, a close agreement (±0.10 volts) is found at all temperatures. Although the gas environment above the melt was not specified in the work of Arkhipov and Stepanov, it is assumed that 100$ CO^ was employed due to the close agreement with the present data under 100$ CO^ purge. 193 OCP data from Dubois (206) are shown in the following table. Table 31. OCP Data of Dubois. Adapted from Reference (206). OCP (Volts) at 560°C, 1 atm CO, Carbon Type vs. Ag/Ag* (Danner-Rey) vs. 33$ Og, 67$ COg/Au Wood Charcoal -1 .'!9 -0.92 Electrographite -1.15 -0.58 Vitreous -1.12 -0.55 Pyrolytic (PI) -0.98 -0.42 The OOP's were converted to the 33$ 0^ 67$ CO^/Au reference electrode basis by adding 570 mV (see Section 2.2.5.1). The wide range of OOP's among the different varieties of carbon is most likely due to the low melt temperature (560°C), which suppresses the carbon/carbonate half cell equilibrium. Specific reactivities of the various carbons therefore influenced the rest potential significantly. Interpolating from Figure 15, an OCP of approximately -0.82 volts would be expected at 560°C and 1 atm 00^ with the Darco activated carbon. This value falls 0.1 volt below the OCP of wood charcoal, but substantially above the other forms of carbon. It is concluded that the wood charcoal was slightly more reactive toward the carbonate melt than to the Darco activated carbon, whereas the other varieties of carbon were considerably less reactive toward the carbonate melt. 194 Table 32 summarizes the most recent OCP data measured by Weaver and coworkers at SRI (212,214). Although the SRI work was quite extensive, some of the data appears contradictory. For example, the OCP of Pocahontas coal at 700°C fell between -0.98 and -1.00 volts according to reference (212), but reference (214) reports a value of -1.10 volts. Appendix A of reference (212) showed pyrolytic graphite to have a positive shift in OCP with increasing temperature, whereas Appendix B of reference (212) showed a negative shift. There is also a disagreement between Appendix A and Appendix B of reference (212) on the OCP of spectroscopic carbon. Since the CO^ pressure and other experimental conditions (e.g., ash content of the melt, new vs. aged electrode) were sometimes unspecified, it is difficult to account for the discrepancies in the OCP data. For the most part, the OCP data seems to have been measured under 1 atm CO^ pressure. It is likely, however, that the higher OCP values were measured at reduced CO^ pressures. Comparing the data of Table 32 with respective varieties of carbon used in the present work (Figure 15 and Table 17), a close agreement in OCP is generally found. The OCP of spectroscopic graphite used in the present work (-0.91 - -0.96 volts at 700°C) correlates well with the OCP of theSRI spectroscopic carbon. The OCP's of Darco activated carbon and the various coals generally fell within ±0.10 volt of the OCP's of the various SRI coals. The OCP of Darco activated carbon exhibited a greater temperature dependence than the SRI carbons. This conclusion is most evident with Kentucky No. 9 coal, which showed a temperature-independent OCP. This 195 Table 32. OCP data of Weaver and Coworkers. Adapted from References (212,214), OCP (Volts vs. 33% Og, 67% COg/Au) Temperature (®C) Carbon Type 600 700 800 ( 3 Spectroscopic Carbon -0.91 - -0.94 -0.92 - -0.95 -0.95 - -0.98 Spectroscopic Carbon^ - -1.02 - -1.05 -1.03 - -1.10 Pyrolytic Graphite^*) -0.88 - -0.92 -0.87 - -0.91 -0.84 - -0.87 Pyrolytic Graphite^^^ -0.60 - -0.77 -0.85 - -0.92 -0.90 - -0.93 ( 3 Î Pocahontas -0.96 - -0.98 -0.98 - -1.00 -1.05 - -1.08 (c) Pocahontas — -1.10 - Illinois No. 6^°) — -1.10 - (c) Kentucky No. 9 - -1.10 - Kentucky No. 9^^^ -1.04 ± 0.04 -1.04 ± 0.04 -1.04 ± 0.04 (a) Data from Appendix A of Reference (212). (b) Data from Appendix B of Reference (212). (c) Data from Table 3 of Reference (214). (d) Data from Figure 3 of Reference (214). 196 result appears quite anomalous, however, since the other SRI carbons did show a small shift in OCP with temperature. 5.3.2. I-V Data I-V data from the present work are compared with the data of Hauser (169), Arkhipov and Stepanov (205), and Weaver et al. (212,214) in Tables 33, 34 and 35, respectively. The purpose of the comparison is to determine whether the electrode kinetics surpass that of the prior art. Comparisons are made at either fixed overvoltage values or fixed current density values, depending on the applicability of the original data. Referring to Table 33, spectroscopic graphite gave a current density approximately 2.5 times greater than Hauser's dense graphite electrode. The current densities achieved with Darco activated carbon are approximately 3-fold greater than those achieved by Hauser. Hauser found an activation energy of 18 ± 5 kcal/mol based on his I-V data, which is comparatively lower than the 26 kcal/mol found in the present work. Hauser's I-V data gave an values between 0.76 and 1.1, or about two-fold greater than the values shown in Tables 27 and 28. The current density data of Arkhipov and Stepanov (Table 34) are comparatively lower than the present data, but the disparity diminishes with increasing temperature. At 800°C, and 0.4 volt over potential, the current density of Darco cactivated carbon matched their data exactly. The temperature dependence of Arkhipov and Stepanov's I-V data is visibly greater than the present data although an activation energy was not reported. The experimental technique of 197 Table 33- Comparison of I-V Data with Data of Hauser (169). KmA/cm ) at 0.5 Volt Overpotential T(°C) Data of Hauser 15% Darco 20-'*10 10% Graphite -100 600 20 60 - 700 25 95 65 800 *10 120 - Table 3*1. Comparison of I-V Data with Data of Arkhipov and Stepanov (205). I(mA/cm ) at 0.4 Volt Overpotential T(°C) Data of Arkhipov 15% Darco 20-40 10% Graphite -100 and Stepanov 500 1 15 - 600 10 35 - 700 30 60 50 Boo 80 80 - 198 Table 35. Comparison of I-V Data with Data of Weaver and Coworkers (212,214). Carbonaceous Overpotential (volts) Material at 100 mA/cm^, 700°C Pocahontas - SRI 0.40 Spectroscopic Carbon - SRI 0.55 10% Bituminous Char 50-100 0.45 10% Lignite 30-50 0.48 10% Darco 20-40 0.55 10% Spectroscopic Graphite -100 0.76 199 Arkhipov and Stepanov eliminated ohmic overpotential, which may ac count for the higher current densities at the higher temperatures compared to Hauser's data. Comparison of the SRI I-V data is made on the basis of a 100 2 mA/cm current density. Referring to Table 35, the SRI Pocahontas derived anode required the lowest overvoltage to reach this current density. It should be noted, however, that the SRI I-V data did not include ohmic overpotential (see Section 2.3.1.2). From the calcula tion of Section 5.2, an ohmic overpotential of approximately 0.05 2 volts might be expected at 100 mA/cm . If this overpotential con-> tribution is taken into account, the bituminous char overpotential would match the Pocahontas overpotential. Slightly higher overpoten-i tials were exhibited by lignite and Darco activated carbon under the stated carbon loading conditions. The overpotential of the spectro scopic graphite was substantially greater than the SRI spectroscopic carbon. The SRI I-V data for pyrolytic graphite gave an activation energy of MO kcal/mol, which is higher than that found in the present work. Values of an for pyrolytic graphite agreed with the data of Tables 27 and 28 at temperatures of 700°C and below. Above 700°C, their an values remained constant at 0.5, in contrast to the present work. Exchange current densities for pyrolytic graphite were about one order of magnitude lower than the values found in the present work. The lower exchange current densities are attributed to three factors; 1) lower reactivity of the pyrolytic graphite, 2) different anode configurations, and 3) extrapolation of the SRI Tafel curves to the theoretical OCP rather than the experimental OCP. 200 5.3.3. Gas Data The results of gas evolution measurements (Tables 22 and 23) are in agreement with previous findings. The work of Tamaru and Kamada (201), Hauser (169), Arkhipov and Stepanov (205) Weaver et al. (212,211») and Sasaki et al. (221) at temperatures ranging from 500 to 870“C have found gas evolution rates which correspond to the following reaction: C + 2 + 3 COg + 4e" (5.3.1) The theoretical CO^ yield for this reaction is 0.75 mol/Faraday, and the experimental gas yields generally fell within 0.1 mol/Faraday of this theoretical value. In contrast to the present work, the previous investigations have found low CO percentages in the product gas stream (generally below 10) CO) at temperatures below 800®C. However, the anode configuration used in the present work is unlike those in previous investigations, specifically with respect to the carbon-carbonate ratio. The high concentration of carbon in the cell promotes the Boudouard side reac tion, which accounts for the higher CO percentages found in the present work. 5.4. Mechanism of the Anodic Process The electrochemical system in the present investigation is dif ferent from most electrochemical systems in that the primary reactant (carbon) does not dissolve in the electrolyte nor does it comprise the electrode. Rather, the anodic process occurs by means of two separate 201 but interacting heterogeneous solid-liquid interfaces - the carb on/carbonate interface and the electrode/carbonate interface. The present system is somewhat analogous to dispersed catalyst electrodes (245,246) or fluidized bed electrodes (247-255). However, these systems differ in that the primary reactant is usually dissolved, and the particle phase has metallic conductivity. A primary distinction in mechanism should first be made based on whether the interaction between the two heterogeneous surfaces is 1) direct or 2) indirect. Direct interaction necessitates physical contact between the carbon particle and the electrode surface, thereby creating a transient solid-solid interface at the points of contact. Indirect interaction necessitates the participation of an intermediate that is capable of dissolving in the molten electrolyte and reacting at the electrode surface. These two types of interactions are il lustrated in Figure 42. Although these two types of interactions will be considered separately, it is possible that both types occur simultaneously. 5.4.1. Direct Interaction Mechanism The direct interaction mechanism can be expressed in terms of two basic rate processes: 1. Mass transport of carbon particles to the electrode. 2. Electrochemical reaction at one or both of the heterogeneous interfaces. Step number one is promoted by stirring the electrolyte. In an unstirred melt, the carbon floats on the melt surface and direct contact can still be achieved when a bare electrode is used. However, 202 A. Direct Interaction COo Carbon Particle Anode B. Indirect Interaction (R=Reactant, P=Product) COoI P Carbon Particle Anode Figure 42. Illustration of Direct and Indirect Interaction Mechanisms. 203 when a fully submerged electrode was used which was insulated from the carbon surface layer, the unstirred current was not cut off (except in the case of graphite - see Section 4.3.11). This result indicates that at least some of the current is generated by an indirect route. The direct interaction mechanism may still play a dominate role under stirred conditions, however. The electrochemical reaction (Step 2 above) consists of ion discharge and electron transfer steps.' These steps could occur either at the electrode proper or at the carbon particles. Due to the moderate electrical conductivity of the carbon particles, intra particle conduction or collision processes may be important. These types of processes have been discussed in relation to fluidized bed electrodes (255). In the conduction process, electron transfer occurs via short chains of carbon particles extending from the electrode. The intra-particle conduction results in an ohmic potential drop between the electrode and the sites of ion discharge on the carbon particles. In the collision process, the electrical double layer of the dispersed particles becomes charged upon contact with the electrode. Charge transfer occurs by collision with other particles or by the ion discharge reaction. Both of these particle-electrode interaction processes should be affected by the frequency, time and interfacial area of contact with the electrode. These factors should in turn be affected by the stir ring rate and carbon particle size. Since these parameters did have a noticeable effect on the I-V curves, the conduction/collision processes could be partly responsible for rate limitations. 204 Direct conduction or collision processes may be especially rate- controlling in the case of graphite. The nearly linear I-V curves with graphite probably reflect the relatively high electrical conductivity. Held and Gerischer (245) found linear I-V characteris tics in the electrochemical oxidation of by dispersed conducting catalysts. The results discussed in Section 4.3.11 also support the direct contact mechanism in the case of graphite. However, the I-V curves of graphite had very little stirring rate dependency, which seems to contradict the expected contact frequency dependency. 5.4.2. Indirect Interaction Mechanism In the indirect interaction mechanism, the following rate processes should be considered; 1. Mass transport of secondary reactant(s) to carbon particles. 2. Chemical reaction between carbon and secondary reactant(s) to produce intermediate(s). 3. Mass transport of intermediate(s) to electrode surface. 4. Adsorption of intermediate(s) on electrode surface. 5. Electrochemical reaction at electrode surface. 6. Desorption of reaction product(s) from electrode surface. 7. Mass transport of product(s) from electrode surface. 8. Chemical reaction of product(s) with other reactant(s). In general, any of these steps could be rate-controlling. Based on the experimental I-V curves, it appears that more than one rate process is controlling, depending on the potential, stirring rate and specific carbon type. At high overpotential, it is known that mass 205 transport is at least partially rate-controlling. At low overpoten tial, a chemical kinetic or sorption process is rate-controlling. The passivation behavior suggests that more than one intermediate par ticipates in the electrode reaction. The exact identity of the intermediate or intermediates has not been determined. It was originally suspected that iron or other indigenous minerals would act as intermediates, as found in aqueous coal electrochemistry (233). However, the experimental data did not support this hypothesis. Several possible intermediates will be proposed in Section 5.4.3. The interraediate(s) could undergo a cyclic redox process, being oxidized electrochemically at the electrode and reduced chemically by the carbon. Alternatively, the intermediate(s) could be supplied in a form that would not require reductive regeneration. 5.4.3. Plausible Mechanisms of the Overall Anodic Reaction In accordance with present and past experimental results, the overall anodic reaction is assumed to be: C + 2C0g2" 4. scOg + 4e" (5.4.1) Although the anodic reaction itself is probably not the rate-limiting process, it is instructive to consider some plausible mechanisms for this reaction. No attempt will be made to verify any of the proposed mechanisms. The mechanisms are presented in the order of their es timated feasibility as viewed by the author. It is possible that several of the mechanisms occur simultaneously. Mechanism I 2C0g2" ^ Og + 2C0^ + 4e" (5.4.2) 206 C + Og + COg (5.4.3) The electrochemical step occurs at the electrode or at carbon particles in the vicinity of the electrode. The product oxygen ad sorbs on the electrode or carbon particles, or it dissolves in the electrolyte. The local oxygen activity as seen by the electrode is kept low by chemical combination with the carbon particles. The carbon thus plays the role of a chemical depolarizer. Carbon-oxygen surface groups located on the carbon particles may also participate in the reaction. The rate of adsorption or desorp tion of these groups would probably be rate-limiting, based on mechanistic studies of carbon anodes in molten cryolite (Section 2.3.2). This type of mechanism resembles Chamber and Tantram's (124) oxygen concentration cell hypothesis, which was applied to molten carbonate fuel cells operating on gaseous fuels. A similar mechanism was proposed by Sasaki et al. (221) for carbon anodes in molten carb onate. Conduction or collision processes are not necessary in this mechanism if the electrochemical step is confined to the electrode and the product oxygen dissolves in the electrolyte. However, if the electrochemical step occurs on the carbon particles or if the product oxygen adsorbs on the electrode, then conduction or collision processes would be necessary. This mechanism would probably predominate at high overpotential, i.e., at voltages where the oxygen pressure could be maintained close to the equilibrium value. At potentials in the vicinity of -1.0 volts 207 vs. 33% Og, 67% COg/COg^ /Au, the equilibrium 0^ pressure becomes very low (on the order of 10 atm, see Figures 4 and 8), and it is un likely that such low oxygen pressures could be maintained. Mechanism II C + COg + 2C0 (5.4.4) CO + C0g2- - 200^ + 2e" (5.4.5) The Boudouard reaction produces dissolved or adsorbed CO. The CO could adsorb either on the electrode or on the carbon particles. It is known that CO strongly adsorbs on gold, at least at low tempera tures (161,246). Although the solubility of CO in molten carbonate is not great (see Figures 2 and 3), the local concentration of dissolved CO could be maintained by rapid Boudouard reaction. In the electrochemical step, the dissolved or adsorbed CO oxidizes to COg. The standard equilibrium potential (E^*) of reaction (5.4.5) is very close to the standard equilibrium potential of reac tion (5.4.1). Thus, the onset potential of reaction (5.4.5) would be expected to fall in the vicinity of -1.0 volt vs. 33% 0^, 67% COg/COg /Au. The electrochemical step could occur at the electrode or on carbon particles. If the latter, then conduction or collision processes would be necessary. The above sequence was previously advanced by Hauser (169). Mechanism III C0g2" * 0^’ + COg (5.4.6) C + 2of" -► COg + He” (5.4.7) 2“ In this sequence, the 0 ions are supplied by local dissociation 2" 2^ of CO^ . The discharge of 0 occurs at surface sites on the carbon 208 particles. Electrons are transferred from the carbon particle to the electrode by conduction or collision mechanisms. The participation of 0 in carbon electrooxidation has been commonly proposed in other molten salt systems (see Sections 2.3.2 and 2.3.3.). Intermediate steps involving carbon^oxygen surface complexes are likely to occur in the above sequence, as proposed in the case of cryolite melts (Section 2.3.2). Mechanism IV C + COg + 2C0 (5.4.8) CO + COgZ- 4. cOgZ- + cOg (5.4.9) COgZ- cOg + 2e" (5.4.10) The primary distinction in this mechanism is the participation of 2- the intermediate 00^ ion. This ion was previously proposed as an intermediate by Borucka (130,131) as discussed in Section 2.2.5.3.3. The COg formed by reaction (5.4.9) could either be dissolved or adsorbed as in previous mechanisms. Electrochemical oxidation of 2- COg could proceed at the electrode or on carbon particles. Again, conduction and/or collision processes would be necessary in the later case. Borucka (130) found two waves for CO oxidation, one correspond ing to reaction (5.4.10) and a smaller one corresponding to oxidation of dissolved CO. It is possible that both reactions take place in the present system, with a strong overlap of the waves. Mechanism V C + COg + 2C0 (5.4.11 ) CO + COgZ" 4. (5.4,12) 20^0^^" - 4C0g + 4e" (5.4.13) 209 2- This mechanism resembles mechanism IV except that the CgO^ 2"* (oxalate) ion is postulated as an intermediate. The ion was previously proposed by Vogel et al. (131)» as discussed in Section 2.2.5.3.3. The formation of intermediates by reaction of CO and a carbonate melt has strong support by the results of both Vogel et al. (131) and Borucka (129,130). In the presence of carbon, these inter mediates mayattain a greater stability, which would allow higher concentrations to be achieved and therefore higher current densities. Mechanism VI C + 1/2 zCOgZ" ^ (5.4.14) 4. 2C0g2" 4. 3C0g + C^_^0y_2^‘ + 4eT (5.4.15) 4. 2cf" - COg + Oy_2="+ 4e" (5.4.16) In this mechanism, soluble carbon-oxygen compounds of unspecified 2- stoichoimetry are formed by reaction with 00^ . The carbon-oxygen compounds are released from the main coal network as cleaved fragments. An example of such a species would be a highly substituted phenolate group. The alkali metal ions may interact with and stabi lize such compounds in the carbonate melt. Similar carbon-oxygen compounds have been proposed in catalytic coal gasification reactions (see Section 2.1.8.3), although they are usually specified as surface complexes. CHAPTER VI CONCLUSIONS AND RECOMMENDATIONS 6.1 Conclusions Major conclusions from the experimental work are summarized below. 1. A novel approach towards electrochemical oxidation of coal at high temperature has been investigated. The main distinction compared with previous high temperature approaches is that dis persed, particulated carbon was utilized in conjunction with an agitated molten electrolyte. The stirred conditions resulted in rapid mass transport processes in the vicinity of the electrode surface, thereby permitting high current densities. 2. The experimental OCP data agreed well with thermodynamic values for the reaction C + 2C0.2" SCOg + Ue“ (6.1.1) when all of the following conditions were satisfied; a) The temperature was 700®C or above. b) The purge was 100$ CO^. c) The carbon loading was 10$ or above. d) Simultaneous equilibrium of the Boudouard reaction was assumed. 210 211 When the experimental OCP's did not agree with, but fell below (positive of) the thermodynamic values, reaction (6.1.1) or the Boudouard reaction apparently did not achieve equilibrium. 3. The experimental OCP data generally agreed with the data of Arkhipov and Stepanov (205), who used a spectroscopic carbon anode, and the data of Weaver et al. (212,214), who used carbon, graphite and coal-derived anodes. The experimental OCP data did not agree with the data of Hauser (169) and of Dubois et al. (206), but the discrepancies could be rationalized. 2 4. Although current densities above 100 mA/cm were readily achieved in the experimental system, the anodic overvoltage was excessive for commercial fuel cell applications. Future studies will hopefully find methods of reducing the anodic overvoltage. Some possible methods of enhancing cell performance will be presented in Section 6.2. 5. Bituminous char gave the highest current density of the various 2 coals or carbons tested. At 10$ loading and 700°C, 230 mA/cm was achieved at -0.3 volts (vs. 33$ 0^, 67$ /Au). At the same loading, temperature and voltage, lignite gave 218 2 2 mA/cm and Darco activated carbon gave 165 mA/cm . Somewhat 2 lower current densities were found with graphite (80 mA/cm at 2 -0.30 volts) and with anthracite (55 mA/cm at -•0.30 volts) On the basis of current density per unit weight of loading, bituminous coal appears to be the best choice for a coal fuel cell. However, if the lignite were pre-pyrolyzed, it could prove to be just as good or even better than the bituminous char. 212 6. Linear Tafel curves were found under the following conditions; a) low to midrange overpotentlals (between ->0.9. and -0.6 volts vs. 33% Og, 67% COg/COgZ'/Au) b) high carbon loadings (10% and above) c) temperatures of 600°C and above. Values of an ranged between 0.74 and 0.39 at 600-800°C, and compared favorably with the SRI data for pyrolytic graphite. 2 Exchange current densities ranged between 1 and 15 mA/cm and were about one order of magnitude greater than SRI's pyrolytic graphite. The lower exchange current densities of pyrolytic graphite was attributed to three factors: 1) lower reactivity of the pyrolytic grathlte, 2) different anode configurations, and 3) extrapolation of the SRI Tafel curves to the theoretical OCP rather than the experimental OCP. 7. The current density achieved with 10% coal loadings was com parable to the current density achieved by Weaver et al. (212,214) at SRI using coal-derlved anodes. 8. The current level was roughly proportional to carbon loading at low loadings (1-5%), but It leveled off at high loadings (10- 15%). The maximum loading tested was 15%, but higher loadings should be permissible. 9. Decreasing the carbon particle size resulted In higher current densities. At 3% loading of Darco activated carbon, decreasing the particle size from 20-40 mesh to 50-100 mesh Increased the current density by 24%. At 1% loading, decreasing the particle size to 200-325 mesh Increased the current density by 100%. The 213 percent increase in current density diminished at higher carbon loadings. 10. Stirring rates above 600 RPM effectively removed mass transport limitations in the low overpotential region. At high overpoten- tial (typically positive of -0.7 volts), mass transport limitations were still in effect, since the current density was dependent on the stirring rate. 11. Graphite and gold anodes gave similar I-V curves at high carbon loadings, which indicates that electrode kinetics were not rate- controlling. 12. The 100% COg purge gave superior results compared to the 3% COg purge. The 3% COg purge, when brought into contact with high carbon loadings, resulted in substantial melt decomposition. The long-term time dependency of the OCP and I-V data under 3% COg purge was likely due to melt decomposition and gradual build-up of oxide ions. 13. A chemical activation energy of 26 kcal/mol was determined from the I-V data at 500-800°C. This value is viewed as a global rather than a purely kinetic parameter. The chemical activation energy falls between Hauser's (169) value of 18 kcal/mol and Ateya’s (212) value of MO kcal/mol. 1 A. Measurements of the product gas evolution rate and G.C. analyses of the product gas composition were helpful in identifying the overall anode reaction. With Darco activated carbon and a cell temperature of 700°C, the gas data agreed with the stoichiometry of the following overall anode reaction: 211» C + 2 Z 0 ^ ~ * SCOg + '♦e" (6.1.1) The theoretical COg yield for this reaction is 0.75 mol/Faraday, which was confirmed by the experimental gas data. 15. The chemical consumption of carbon by the Boudouard reaction was substantial due to the use of the 100$ COg purge gas. A 6.7$ carbon efficiency (ratio of electrochemical consumption to Boudouard plus electrochemical consumption) was found, based on gas analysis and flow rate data. Due to the heavy chemical loss of carbon, the elecrocheraical carbon stoichiometry of reaction (6.1.1) could not be verified. The heavy chemical loss has negative implications for commercial fuel cell applications. 16. To arrive at the optimum cell temperature in a practical fuel cell, a compromise between current density and carbon utilization efficiency must be made. Ideally, the cell temperature should be as high as possible to maximize the current density, but not so high that the carbon utilization efficiency becomes unacceptably low. The experimental results reveal that it may be very dif ficult at any temperature to simultaneously achieve a high current density (at low overpotential) and a high carbon utiliza tion efficiency. 17. Impurities indigenous to the carbonaceous materials are not the primary electroactive species, and therefore are not responsible for the high current densities achieved. 18. Passivation of the electrode was encountered at long run times. A variety of experiments indicated that dissolved impurities indigenous to the carbonaceous materials were responsible for 215 passivation. The onset of passivation was manifested as a sharp drop in current at potentials between ~0.7 and -0.3 volts. A substantial hysteresis in the I-V curves was also associated with passivation. The passivation was sometimes reversible and some times irreversible, as reflected by repeated I-V scans. It should be noted that passivation may not be a problem even on a long-term basis if the anode potential is kept negative of -0.7 volts. 19. The gold anode held up very well in the carbonate melt. After approximately 45 runs, minimal corrosion was observed. 20. The mechanism of the anodic process appears quite complex. Both direct and indirect interactions between the electrode and carbon particles are probably involved. It is likely that several electroactive intermediates participate in the electrode reaction. A multiplicity of rate-controlling processes appear to be in effect. At low overpotential, a chemical kinetic or sorp tion process is rate-controlling. At high overpotential, a combination of mass transport, chemical kinetic or sorption processes are rate-controlling. Due to the complexity of the chemical/electrochemical/mass transport processes, specific mechanisms of the overall anodic reaction could only be postulated. 216 6.2. Recommendations Recommendations for future work are itemized below. 1. Coal loadings should be pre-pyrolyzed at 700-800“C in an inert atmosphere (e.g., N^) before introduction into the cell. This procedure would minimize caking during loading and prevent ther mal decomposition products from depositing on cooler components of the apparatus. 2. Different anode materials should be tested, such as nickel, copper or silver. Graphite should be further evaluated. The stability of these less expensive anode materials could then be compared to that of gold. Also, the effect of surface roughness of the anode should be investigated. 3. Improved cell performance may be achievable by using hydroxide/carbonate or sulfate/carbonate mixtures. The litera ture indicates that carbon is more reactive in the presence of hydroxide and sulfate. 4. The effect of carbonate cation on cell performance (i.e., current density) should be investigated. A variety of low-melting, two- component eutectics could be used. Pure Li^CO^ could be used at 800°C and above. 5. Carbon loadings could be admixed with a small fraction of graphite particles to increase the effective electrical conduc- tiviy of the dispersed particle phase. This procedure could result in a substantial increase in current density if the anode process is controlled by conduction and collision mechanisms. 217 6. Higher carbon loadings (above 15%) should be tested to determine the upper loading limit that can be tolerated. 7. The mixing conditions within the cell should be modified to effectively accommodate fine particles (->325 mesh). Modifications could entail installation of baffles and a more effective impeller on the stirring shaft. 8. An experimental technique should be employed to determine the IR contribution to the I-V curves. Methods of compensation include voltage feedback (138,139) and current interruption (76,212). 9. Methods to minimize passivation at long run times should be explored. Addition of hydroxide to the melt may be effective. Another method would be to pre-leach the coal in a molten carb onate bath. 10. A more detailed analysis of product gas evolution should be undertaken. Gas measurements should be performed at a variety of potentials and temperatures to determine the anode efficiency over a broad range of operating conditions. Gas measurements should also be performed using different varieties of coal to allow a comparison with the Darco activated carbon. 11. A mechanistic study should be undertaken to distinguish between the direct and indirect interaction mechanisms. It should be possible to eliminate direct electrode-carbon contact by shield ing the electrode. 218 12. For fuel cell applications, the next generation cell design should Include: a) An Og/COg counter electrode of sufficient surface area to handle the anode current. b) A closer anode-cathode spacing (to minimize IR losses). c) A higher anode area to electrolyte volume ratio (to minimize the chemical consumption of carbon). d) A more symmetrical electrode configuration (for uniform current distribution). e) A higher mass transfer coefficient (e.g., higher electrolyte turbulence In conjunction with uniform particle distribution). The feasibility of a continuous or recirculating flow system should also be evaluated. CHAPTER VII SUMMARY A novel approach toward electrochemical conversion of carbonaceous materials (e.g., coal) was investigated. The basic idea was to enhance the electrochemical reactivity of coal slurries by operating at high temperature. To achieve high operating temperatures, a molten carbonate electrolyte (ternary eutectic of lithum, sodium and potassium carbonate) was employed. In the experimental cell, particulated coal or carbon was dispersed by stirring of the melt. Anodic oxidation took place at an inert gold electrode, which also served as the current collector. An alumina-sheathed carbon dioxide/oxygen/gold half-cell served as the reference electrode, and a shielded graphite rod served as the counter electrode. The cell was operated at 500-900“C. Other parameters experimentally investigated were: type of carbonaceous material, concentration of carbonaceous material, stirring rate, anode material, and purge gas composition. The electrochemical measurements entailed generation of steady- state current-voltage curves and open circuit potential data under a wide variety of experimental conditions. Product gas measurements were also performed. 219 220 The open circuit potential data agreed well with theoretical values at temperatures of 700°C and above. The current-voltage data 2 at 700°C showed that current densities of 100 mA/cm could be achieved at overpotentials of approximately 0.5 volts. Measurement of the anode off-gas at 700®C indicated that complete oxidation of carbon to carbon dioxide was achieved. Chemical loss of carbon by the Boudouard reaction was found to be substantial. In order for the system to be commercially attractive as a direct coal-fired fuel cell, the chemical loss of carbon would have to be reduced considerably. Passivation of the anode was sometimes encountered, and was caused by dissolved impurities originating from the coal. Fortunately, passivation was minimal in the low overpotential region (below 0.3 volts). The investigation also included a qualitative mechanistic analysis of the anode process. It was concluded that the anode process occurred by means of both direct and indirect interactions between the anode and the carbon particles. APPENDIX A Analytical Data of Coal and Activated Carbon 221 222 ( a ) Table 36. Analytical Data for Darco Activated Carbon . Test As received Dry Dry, ash free Proximate % Moisture 4.84 - - % Ash 21 .91 23.02 - Ï Volatile matter 4.75 4.99 6.48 % Fixed carbon 68.5 71.98 93.52 Btu/lb 10,283 10,806 14,038 Ultimate % Carbon 70.11 73.68 95.71 % Hydrogen 1 .08 1.13 1.47 % Nitrogen 0.74 0.78 1.01 % Sulfur 0.78 0.82 1.06 % Oxygen 0.54 0.57 0.74 (a) Courtesy of Black Rock Test Labs, Morgantown, West Virginia. 223 Table 37. Physical Data for Darco Activated Carbon^^^ Derivation...... Lignite-based Bulk d e n s i t y ...... 2^4 Ib/ft^ Particle d e n s i t y 0.7 g/cc 2 Total surface area .... 625 m /g Total pore v o l u m e 0.95 cc/g Void v o l u m e ...... 50% Moisture as packed .... 12% max (a) Provided by manufacturer (ICI Americas, Inc., Wilmington, Delaware). 22U (a) Table 38. Analytical Data for Bituminous Char Test 1 As received Dry Dry, ash free Proximate | 1 1 % Moisture | 10.07 - - % Ash 1 15.61 17.35 - 1 Volatile matter) 8.05 8.94 10,82 % Fixed carbon | 66.27 73.71 89.18 1 Ultimate | 1 1 % Carbon | 70.45 78.34 94.79 % Hydrogen | 2.12 1.11 1.34 % Nitrogen | 1.04 1.16 1.40 % Sulfur 1 1.61 0.25 0.30 % Oxygen | 9.17 1.79 2.17 ...... 1 (a) Provided by Westinghouse, Inc., Pittsburgh, Pennsylvania. 225 Table 39. Analytical Data for North Dakota Lignite^®^. Test 1 As received Dry Dry, ash free Proximate | 1 1 i Moisture | 33.60 - - i Ash 1 5.60 41 .42 - $ Volatile matter] 27.50 50.15 45.23 % Fixed carbon | 33.30 8.43 54.77 Btu/lb 1 7219 10,872 11,373 1 Ultimate | 1 ..... - . 1 Carbon | 44.06 66.36 72.47 Hydrogen | 6.15 3.60 3.93 Nitrogen | 0.58 0.87 0.95 Sulfur 1 0.81 1.22 1.33 Oxygen | 42.80 19.52 21.32 - 1 (a) Provided by Energy Research Center, University of North Dakota, Grand Forks, North Dakota. 226 Table *40. Analytical Data for Primrose Anthracite^^^. Test As received Dry Dry, ash free Proximate % Moisture 3.77 - - % Ash 13.39 13.91 - % Volatile matter 3.57 3.71 *4.31 % Fixed carbon 79.27 82.38 95.69 Btu/lb 12,088 12,562 1*4,592 Ultimate Carbon 78.76 81.85 95.07 Hydrogen 1 .02 1.06 1.23 Nitrogen 0.6*4 0.66 0.77 Sulfur 0.50 0.52 0.60 Oxygen 1.90 1.97 2.30 (a) Provided by Penn State Coal Data Base, Pennsylvania State University. APPENDIX B Thermodynamic Calculations 227 228 Equilibrium CO^ Calculations In the general case, the Inlet purge contains a mixture of and COg and the outlet purge contains a mixture of Ng, COg and CO. The amount of CO produced will be governed by the extent of the Boudouard reaction In the cell: C + CO, ^ 2C0 (B.1) CO ,1 "co Let X - (B.2) CO .1 N ,1 N .1 and z - — ^ 0— (B.3) CO .1 ,1 where n Is the number of moles. At a given conversion, moles Ng - z ^ moles COg - (1-x) n^^ ^ moles CO - Zlxn^g ^ total moles - (1+x+z)n^Q ^ 229 Thus. ■ r-H — '■ <=•"> Pco, ■ Too, ’’ • - r H -- ’’ «-5> ’co ■ ’'co ” ' T-T-TTE P «-6> where P is the total pressure and Y is mole fraction. At equlibrium, " ■ ^ ■ (1 . , . - X ) (»-7) For a given temperature and inlet gas composition, the equilibrium conversion was calculated from equation (B.7), and the equilibrium pressures were found from equations (B.4-B.6). Calculation of oxide activity Using the calculated equilibrium P_. and the dissociation con- stants of Table 12, thj values of a „ were found. The carbonate 0^’ activity was assumed to be unity. Calculation of potassium activity The melt was assumed to be pure K^CO^, and ideal solution phase and gas phase behavior was assumed. Potassium vapor can form by the following equation: KgCO (1) + 2C(s) -p 2K(g) + 3C0(g) (B.8) 230 The equilibrium was found from: (B-91 Where K - e"AG°/RT (B.10) The ûG° was calcualted from the JANAF Tables (Table 10). Calculation of Cell Voltage The value of was calculated from the following equation: E .. - E° -, - 1.H86x10~^ T log P_. (W) - il.llHxlo”^ T (B.11) cell cell COg This equation was obtained by substituting numerical values into the equation; ^cell “ ^ cell 4F " 4F '■^COg^” ^^ (B.12) and letting (R) - 0.657 atm and P^ (R) - 0.333 atm. was derived from the free energy data of Table 10. BIBLIOGRAPHY 1. 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