.A· "" ·w:. ·1 SPECIAL.REPORT OF RESEARCH
Conducted in
Departmery:.R.f fuel Technology ). College _of' Mineral Industries §c, · c ·; The .Pennsylvania State University ------:: ~ I CHANGES IN COAL SULFUR DURING CARBONIZATION
by
RICHARD A. ANDERSON
and
T. S. POLANSKY
An Investigatio~ donducted Under the Auspices of the
COAL RESEARCH BOARD
of the
COMMONWEALTH OF PENNSYLVANIA
Special Research Report Number SR-19 August 1, 1960 STATEMENT OF TRANSMITTAL
Special Report SR-19 transmitted herewith has been prepared by
the Coal Research Section of the Mineral In~ustries Experiment Station. Each of the Special Reports presents the results of a phase of one of the research projects supported by the Pennsylvania Coal Research Board or a technical discussion of related research. It is intended to present all of the important resµlts of the Coal Board research in Special Reports, although some of the results may already have been presented in progress reports. The following is a list of Special Research Reports issued previously.
·SR-1. The Crushing of Anthracite May 31, 1958 SR-2 Petrographic Composition and August 1, 1958 Sulfur Content of a Column of Pittsburgh .Seam Coal SR-3 The Thermal Decrepitation of September 15, 1958 Anthracite SR-4 The Crushing of Anthracite with November 1, 1958 ) a Jaw Crusher SR-5 Reactions of a Bituminous Coal February 1, 1959 with Sulfuric Acid SR-6 Laboratory Studies on the Grind April 1, 1959 abili ty of Anthracite and Other Coals SR-7 Coal Characteri.stics and Their April 15, 1959 Relationship to Combustion Techniques SR-8 The Crushing of Anthracite with April 25, 1959 an Impactor-Type Crusher .SR-9 The Igni tibi.1 i ty of Bituminous May 4, 1959 Coal (A Resume' . of a Literature Survey) SR-10 Effect of Gamma Radiation May 6, 1959 and Oxygen at Ambient Tem peratures on the Subsequent Plasticity of Bituminous Coals SR-11 Properties and Reactions May 11, 1959 Exhibited by Anthracite Litho types Un.der Thermal Stress SR-12 Removal of Mineral Matter from June 22, 1959 Anthracite by Chlorination at High Temperatures .Sll-13 Radiation Stability of a Coal .June 25, 1959 Tar Pitch SR-14 The Effect of Nuclear Reactor July 31, 1959 Irradiation During Low Tem perature Carbonization of Bituminous Coals SR-15 Effect of Anthracite and Gamma August 5, 1959 Radiation at Ambient Tempera tures on the Subsequent Plas ticity of Bituminous Coals SR-16 The Isothermal Kinet'ics of August 25,. 1959 Volatile Matter Relea$e from Anthracite SR-17 The Combustion of Dus.t Clouds: November 30, 1959 A Survey of the Literature SR-18 The Ignitibility of Bituminous June 15, 1960 Coal
M. E. Bell, Director M. I. Experiment .Station
------SUMMATION OF RESULTS
The carbonization behavior of sulfur in the coal and its float fraction studied is summarized by the following observatiqns. (1) Pyrite decomposed initially around 350°; decomposition was most extensive between 550° and 650°, and was essentially completed at 700°C, Pyrite decomposition was markedly affected by secondary reactions with volatile matter. (2) Sulfate, present in small quantities in the parent coals, was not found in the cokes prepared above 650°C.
(3) Sulfide formation incr~ased with temperature. The amount of sulfide found below 700°C did not correspond with that predicted for reactions in which one mole of ferrous sulfide is formed for each mole of pyrite decomposed. Above 750° the atomic ratio of sul fide sulfur to non-pyritic iron was greater than unity, indicating the existence of other inorganic sulfides, such as calcium and mag nesium sulfides, and/or a complex iron sulfide. (4) Iron, uncombined with sulfur and resulting from pyrite decomposition, was found in cokes from the whole coal produced at temperatures between 350° and 650°C. The nature of the data does not permit conclusions as to its existence above 650°C in these cokes. The 770°C coke of the float coal contained sulfur free iron,
(5) The organic sulfur content d~creased during carbonization. About 153 of the original organic sulfur was decomposed and evolved at 366°C. At 650° approximately 50 to 603 of the sulfur in the cokes of the whole coal was in organic form, compared to 36.43 in the coal. Above 650° the percent organic sulfur decreased, being about 113 of the total coke sulfur at 885°C. (6) Elemental sulfur was not detected in cokes prepared below 750°C from the whole coal. Above thQS temperature small quantities of free sulfur were observed qualitatively; the amount apparently increased with temperature. It was not detected in the cokes produced from the float fraction of the whole coal.
The investigation of the use of benzoin in conjtlnction with a colorimetric sulfate procedure for the quantitative determination of elemental sulfur was inconclusive.
--::JI ABSTRACT
Changes in the sulfur forms during carbonization of a High Volatile A rank, high sulfur, Pittsburgh Seam coal were investi gated. Possible effects of mineral matter content were investi gated by use of a float fraction of the whole coal. The coals were carbonized in a vertical, static bed unit designed to study the effects of primary and secondary reactions of the sulfur forms. Pyrite was completely decomposed at about 700°C and sul fate sulfur was not observed abo~e 650°C. Formation of inor ganic sulfides, excluding iron sulfide, was noted by the non stoichiometric relationship of sulfide sulfur and non-pyritic iron. Iron, uncombined' with sulfur, was observed in cokes carbonized at temperatures between 350° and 650°C. Organic sulfur began to decompose at about 366°C, and it amounted to approximately 113 of the total coke sulfur at 885°C. El mental sulfur was qualitatively detected in cokes produced above 700°C by benzoin, and a method for determining it quan titatively was investigated. TABLE OF CONTENTS Table of Contents List of Tables List of Figures I. INTRODUCTION Interrelatetl aspects of sulfur in coal and the coke industry 1 Literature review ...... 2 Statement of the problem...... 8 Objectives of the investigation 9 II. PROCEDURE OF THE INVESTIGATION Description of coal and preparatiop of samples .•....•.....• 11 Part I Carbonization Studies
Description o~ carbonization apparatus ...•...... •..... 12 '/: Preparation of adsorbents for :Volatiles from carbonization . 16
Carbonization ~rocedure ..•...... •.....••.....•...... 16 Analytical procedures ...... 17
Part II Quantitative Estimation Of Free Sulfur Preparation of cokes for the addition of elemental sulfur,. 19 Apparatus for conversibn of elemental sulfur to sulfate •••. 20 Apparatus for sulfate determination •.•...•..••....••.••.•.. 20 Experimental procedure ...... 20 I I I • EXPERIMENTAL RESULTS Part I Effect of rapid removal of volatile matter on sulfur fo-rm distribution ...... • ...... 27 Effect of temperature on the forms of sulfur during
carbonization • $ •••••••••••••••••••••••••••••••••••••••••••• 36 Occurrence of elemental sulfur ...... 42 Carbonization of the float coal 43
Analysis of adsorbent materials •••••• 0 ••••••• 0 ••••••••••••• 44
-----::: Part II Qualitative test for free sulfur with benzoin 46 Quantitative determination of pure sulfur . ··· ...... 48 Comparison of colorimetric and gravimetric methods for sulfate determination 49 Determination of total sulfur added to cokes using the Eschka method ...... 50 Application of the sulfur to sulfate conversion and colorimetric determination to cokes with added sulfur ... 50 Extraction of elemental sulfur with carbon disulfide .... 52
IV. DISCUSSION OF RESULTS Part I Decomposition of organic coal sulfur ...... 54 Decomposition of pyrite ...... 55 Formation of organic, sulfide, and elemental sulfur ..... 57 Part I! Quantitative estimation of elemental sulfur ...... 67
V. SUMMARY AND CONCLUSIONS Naure of the investigation ...... 67 Procedure of the investigation ...... 67 Results of the investigation ...... 69 Conclusions ...... 70 Suggestions for future research ...... 72
BIBLIOGRAPHY · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · .. · · · · · · · · · · · · · 75
APPENDIX I Carbonizer operational data'""' ...... 78 Il Modified Mott method of sulfur form analysis ..... 79 III Colorimetric sulfate determination., ...... 81 IV Determination of elemental sulfur in carbonized
samples ...... 0 ...... 0 0 ...... 82
ij
...... List of Tables Page 1. Proximate Analysis and Total Sulfur of Coal Samples 12 2. Total Sulfur, Moisture, and Ash Analyses of Cokes for the 19 Addition of Elemental Sulfur 3. Methods of Sulfur Addition and Amount of Sulfur Added to 25 Cokes · 4. Conditions Qf Benzoin Reaction Tests on Sulfurized Cokes 26 5. Proximate Analysis of Coals and Cokes and Coke Yields 28 6. Analysis of Sulfur and Iron Forms in Coals and Carbonized 30 Samples, Percent on Dry Basis 7. Distribution of Sulfur in Coals and Cokes, Percent of 31 Total Sulfur 8. Analysis of Total Sulfur and Sulfur and Iron Forms of 34 Coals and Cokes, Whole Coal Basis 9. Theoretical Amount of Iron Uncombined with Sulfur in 41 Coals and Cokes 10. Proximate Analysis of Whole and Float Coals and Cokes, 44 and Coke Yields
11. Sulfur and Iron Analysis of Whole and Float Coals and 45 Cokes 12. Total Sulfur and Volatile .Matter Analysis of Adsorbent 46 Materials
13. Results of Qualitative Test for Free Sulfur in Various 47 Coals and Cokes 14. Conditions and Results of Tests on the Quantitative Deter- 48 mination of Pure Elemental Sulfur 15. Colorimetric and Gravimetric Methods of Sulfate Deter- 49 mination ' 16. Comparison of Methods of Conversion and Determination 51 of Elemental Sulfur Added to Cokes List of FiKUres Page 1. Diagram of Frame for Coal Sample Holder 13
2. Diagram of Carbonization Apparatus 15
3. Diagram of Apparatus for Benzoin Reaction and Absorption 21 of Evolved Hydrogen Sulfide
4. Volatile Matter and Sulfur Evolved during Carbonization of 33 Elkhorn Coal
5. Distribution of Sulfur Forms in Coal and Cokes 37
6. Distribution of Pyritic and Sulfide Sulfur in Coal 39 and Cokes
7. Atomic Ratio, Sulfide Sulfur to Non-pyritic Iron 40
8. Distribution of Sulfur Forms in Cokes from a 58 Pittsburgh Seam Coal
9. Calibration Curve for Colorimetric Sulfate Deter- 83 mination 1 I. INTRODUCTION Interrelated Aspects of Sulfur in Coal and the Coke Industry
All coals contain sulfur to some degree, the amount ranging from less than 13 to over 103. Coal sulfur is classified as inorganic and organic in nature. The major inorganic forms are pyrite and marcasite, (generally referred to together as pyrite) which are sulfides of iron represented by the chemical formula Fes . Pyrite is found usually as 2 discrete particles of various sizes but may occur in finely divided microscopic form intermingled with the coal substance. Sulfates are present in coal in minor amounts, usually as calcium sulfate.
In coal analysis, pyrite and sulfate are acid extracted and de termined by standard means. The sulfur remaining after acid extraction is termed organic; its precise chemical nature is not known. Organic sulfur is probably distributed uniformly throughaut~the coal substance and has been described as consisting of heterogeneous riµg structures, (16) thioether(l6,31) and thioalcohol groups.(16) Oxidized or weath ered coal may also contain small amounts of elemental sulf.ur·resulti~g from pyrite oxidation.(1,10,46) Reference is .made to Lowry(31) for a more comprehensive discussion of the occurrence of sulfur in coal.
Upon carbonization of coal, usually 503 or more of the coal sulfur is retained in the coke. Certain restrictions are placed on the sulfur contents of coal and coke for various commercial uses. Metallurgical coke must contain not more than 1.3% sulfur for blast furnace opera tions and not more than 1.03 for foundry use.(2) Greater amounts ad versely affect the quality of the iron and steel products. Other re strictions are placed on the amounts of sulfur contained in the vol atile products of carbonization when the gas is used for domestic con sumption. As the supply of low sulfur coals diminishes, the problem of obtaining low sulfur cokes from high sulfur coals becomes of more immediate importance. It is thus essential to understand the behavior of the various sulfur forms during carbonization and the factors in fluencing the retention of sulfur in coke. It is for this purpose that this investigation was initiated. 2
Literature Review Numerous investigations have been made concerning the distribution of sulfur in the products of coal carbonization. In coke, as well as
in coal, sulfur ~s present in organic and inorganic combination. It is generally agreed that the organic coke sulfur is of a different
nature than the organic coal sulfur. Inor~anically combined sulfur occurs in coke as sulfides, mainly ferrous sulfide, from the de composition of pyrite and of sulf•tes if present in coal.
Several empirical relationships between the coke and coal sulfur contents have been presented. Lowry,(33) from a study of 600 coals
carbonized between 500° and ll00°C~ gave the equation: S(coke) -
0.084 + 0.759 S(coal~. Blayden and Mott(8) obtained a similar equation from the data on 60 samples: S(coke) = 0.82 S(coal). Thiessen's(55) results from 82 samples of Illinois coal were similar to those above. He also related the forms of coal sulfur to total coke sulfur by the equation: S(coke) = 0.62 S + 0.45 S , where S is pyritic coal sul- P o p fur, S is organic coal sulfur, and the S(coke) is given on the whole 0 basis. Kulishenko and Medvedev(27) found this relationship to be: S(coke) = 0.62 S + 0.63 S . M'Callum(38) found that the ratio of p 0 coke sulfur (on the whole coal basis) to coal sulfur varied only from 0.568 to 0.477 for samples with pyritic sulfur contents between 1.28 and 0.133. Powell(47) and Woolhouse(58) stated that the relative proportions of the sulfur forms in coal is unimportant in determining the amount of sulfur retained in coke, only the total sulfur being significant.
While the sulfur forms occurring in coke are readily separable,
the exact nature of the organically held sulfur and the mechani~m by which the various forms are transformed during carbonization are not well known. The presence of a heterogenous matrix of constantly \ changing character, the evolution o.f reactive gases, and the uncertain- ty as to the form of the reacting material make the problem exceed ingly complex. The mechanism of coal carbonization is not a subject for detailed discussion here, although a brie1 description is pertin~nt to this 3 investigation. Kipling(21) preseuted a review of physico-chemical as pects of coal carbonization. The general processes occurring and ,the main products at different temperature levels were given. Two major ) stages of carbonization were described, cbrresponding to temperatures below and above 600°C. Below 600° the main thermal decomposition of the coal substance occurs with release of volatile matter, leaving a carbon skeleton. Above this temperature there is further removal of hydrpgen and oxygen which are fitmly bound to the carbon surface. These processes may be kept in mind when considering the behavior of sulfur during carbonization.
Several reactions of iron pyrite which occur at elevated tem- peratures were given by Jacobson.(20)
(1) 2FeS2 = ~e~ + s2 (2) 2FeS + FeS Fe s (700°C in nitrogen atmosphere) 2 = 3 4 (3) 2Fes + 30 2Feso + 2S (limited oxygen suppply) 2 2 = 3 (4) 2FeS + C 2FeS + CS 2 = 2 React;i.on {I) begins around 500°C.(17) Scortecci and Scortecci(52) discussed the direct reduction of pyrite with carbon and gave the following reactions: (at 760°C) (5) FeS2 = FeS + S(l atm.) (6) s + c cs 2 = 2 They proposed that reaction (5) occurs initially and is followed by reaction (6) in a mixture of 90 parts pyrite to 7 parts of coke. Ferrous sµlfide is extremely stable to heat.(8) Its decomposition to iron and sulfur begins at 1100° in vacuum and at 1600°C equilibrium is entirely with the elements.
The effect of gaseous reactants on the decomposition of pyrite and ferrous sulfide is discussed by several workers. Haarmann(l7) observed that pyrite is rapidly reduced to ferrous sulfide at high temperatures in atmospheres of hydrogen, hydrogen and steam, and nitrogen and steam; hydrogen being most effective. The decom position of ferrous sulfide is slow even at high temperatures, the 4 hydrogen and steam mixture being most effective in this case. Mainz (34) concluded that the decomposition of pyrite in a coke oven n gas atmosphere starts at 280°, increases slowly to 400°, and rapidly to 500°C. Wooolhouse (59) presented the following results: (1) Pyrite is completely decomposed in air at 800° to 900°C. (2) Air and steam mixture is more effective in decomposing pyrite than air alone below 650°, and less effective above 650°C. (3) Above 700°C, ferrous sulfide is attacked by the air-steam mixture. (4) Carbon dioxide is ineffective in decomposing pyrite below 900°c. (5) The reaction, CO+ FeS COS + FeS, occurs at 600°C. 2 = (6) In a streatn of nitrogen, pyrite decomposes to ferrous sul fide and sulfur initially at 500°, completely at 650°C. (7) In a stream of hydrogen, pyrite decomposition is appreciable at 400°, complete at 500°C. (8) Pyrite decomposition starts at 480° in a hydrogen atmosphere and is complete at 530°C. Blayden and Mott (8) stated that equilibrium data for the reaction, FeS + H = Fe + H S, favor formation of ferrous sulfide up to 1200°C. 2 2 In carbonizing coal between 300° and 1000°C, Powell (47) found pyrite decompositon commenced at 300°, was a maximum at 400° to 500°, and was complete at 600°C. Woolhouse (58) agreed with the upper and lower limits and gave the maximum decomposition range as 500° to 550°C. Armstrong and Himus (1) referred to the work of Foerster and Geissler (14) who found about 503 pyrite decomposition at 500°C. Negligible pyrite decomposition has been observed when a low sulfur coal was carbonized below 450°C,(45) Pyrolysis of a high pyrite coal resulted in about 103 decomposition at 500° and complete decomposition between 600° and 700°C.(46) Powell (48) stated that hydrogen facilitates the decomposition of pyrite in coal by effectively maintaining a negligible partial pressure of sulfur over the pyrite, so that the reaction occurs more 5 readily at lower temperatures. Coal containing 0.473 pyrite was car bonized at 500°C and the resultant coke heated in a stream of hydrogen at the same temperature. The untreated coke retained 0.333 pyrite while the hydrogen-treated coke contained only 0.013. The ferrous sulfide content was reduced from 0.123 in a 1000° coke to zero by treatment with hydrogen at 1000°C. Zielke et. al.(60) treated a 600°C coke at 1600°F with hydrogen after devolatilization in nitrogen at the same temperature. Sulfide sulfur was more readily remov~d than organ- ic sulfur. A steam-hydrogen·. mixture was less effective. Kipling.(22) found that steam activation at 890° readily removed sulfur from a 700°C coke in which 853 of the sulfur existed as ferrous sulfide.
The effectiveness of various gases in converting coal sulfur to volatile products was studied by Snow(53) and Mangelsdorf and Broughton
(35) . The experiments were carried out by carboni~ing coal directly in the gas stream rather than carboniz~ng prior to treatment in the gas as Powell did above. Snow used a coal containing 5.343 sulfur, about half pyritic and half organic. Nitrogen, carbon dioxide, carbon monoxide, methane, and ethylene were essentially ineffective at 1000°C. Water gas, anhydrous ammonia, and hydrogen gave successively higher con versions to volatile products. At 800° steam was almost as effective as hydrogen at l000°C. Manglesdorf and Broughton(35) found carbon monoxide and illuminating gas substatially increased the conversion to volatile sulfur compounds at 600°C. Steam, hydrogen, and blue water gas were increasingly more effective. They attributed the difference in their and Snow's results to inherent differences in the coals in vestigated 1 Considerations of the mechanism of desulfurization as regards organic sulfur in coal is somewhat limited. This form is not separ- able as a pure compound, so that its exact., nature is not known. How- ever, by identification of the volatiles from carbonization, some im- portant information has been obtained. Bone and Himus(9) found large amounts of thiophene, thiols, and thioethers'in the volatile products of a high sulfur coal. Leonet(28) gave data for the average distrib ution of sulfur in the carbonizatton products of Belgian coals with an 6
average sulfur content of 1.23. Almost 36% of the total sulfur was found in the tar and ammoniacal liquor. It should be noted that this amount is unusually high. Generally these compounds make up only a small fraction of the volatile sulfurous compounds. Powell(47) found less than 43 of the total sulfur in the tar from a l000°C coke of a high sulfur coal.
Hydrogen sulfide is the preponderant sulfurous gas evolved during carbonization, usually over 953 of the volatile sulfur being in this form.(32) That some organic sulfur must contribute to this produ~t was first pointed out by M'Callum(38) who found large differences be tween the amount of sulfurous volatiles observed and that calculated as resulting only from decomposition of inorganic forms. Evans(ll) found the amount of volatile organic sulfur compounds and ~f hydrogen sulfide increased with total coal sulfur, with no relationship between the two. Powell(47) reported that some organic coal sulfur is de composed giving volatile sulfur compounds, other being converted to hydrogen sulfide. Fuchs(l6) postulated that thiols and thioethers can decompose into olefins and hydrogen sulfide at carbonization temperatures. Hurd(l9) gave reactions for the pyrolitic behavior of many organic sulfur compounds, some of which may be formed during carbonization. Hoffert and Wendtner(l8) presented a survey of reac tions of sulfur, hydrogen sulfide, and mercaptans with unsaturated hydrocarbons. Haarmann(l7) attributed the evolution of hydrogen sul fide during coal carbonization partly to direct removal from the organic coal substance ae.d reactions of organic sulfur compounds with steam and hydrogen.
Further considerations involve the possible interactions ~ all the forms of sulfur present during carbonization. Powell(47) suggest ed that at hi~her temeratures of carbonization, over 500°C, sulfur \ from ferrous sulfide combined with carbon. Terres(54) agreed that p,rt of the inorganic sulfur is transformed to organic sulfur at high tem peratures by reduction of inorganic sulfides to the free metal. When
Mainz(34) carbonized coal with 1.83 added pyrite at 100~ 0 c, the result-
\ 7 ant coke contained more organic sulfur than the-original coal. ' Kulishenko and Medvedev(27), using radioactive tracer techniques, con cluded that pyritic sulfur participates in the formation of all the principle sulfur containing products from the thermal decomposition of coal. Medvedev(40) found that the amount ·of pyritic sulfur con tributing to organic coke sulfur varied with the rank of the coal. Good coking coal of medium rank gave a substantially higher percent transformation to organic sulfur than did very high or low rank coals.
Woolhouse(58) found the atomic ratio of iron to sulfur was greater than unity in 600° coke where pyrite decomposition was complete, but this sulfur deficiency was not observed in the 1000°C coke. He sug gested a transfer of sulfur from the carbon to the iron. Medvedev(40) suggested a mechanism for the transformation of organic sulfur to \ ferrous sulfide which involved the formation of metallic iron.
Evidence for the reaction of sulfur and hydrogen sulfide gas with carbon at temperatures encountered during carbonization i.s not lacking. Carbon disulfide is commercially produced by direct reac tion of carbon and sulfur.(20) Wibaut(57) prepared a stable carbon sulfur complex by heating pure amorphous carbon in sulfur vapor at temperatures between 350° and 900°C. Lewis and Metzner{30) prepared a similar complex at 300° to 600°C
Owen et. al.(44) reacted a number of carbons with hydrogen sul fide under varying pressure and temperature conditions. At 1500°K coke gave a carbon disulfide yield of about 7CY'/o of the calculated equilibrium value. They stated, however, that whether the carbon di sulfide was formed directly by reaction of the hydrogen sulfide with carbon or through intermediate sulfur formation is not clear.
Terres(54) believed that carbon disulfide is a product of s~condary reaction of hydrogen sulfide with coke during carbonization.
Mainz(34) found the concentration of organic sulfur to be greater near the top and walls of the carbonizer oven. This was attributed to the decomposition of sulfurous volatiles, evolved from the center of the coal charge, in the hotter regions of the oven; thus deposit- 8
ing non-volatile organic sulfur. Knapp(26) and Lesoine(29) found large increases in sulfur content when hydrogen sulfide was passed through coke heated to 500° to 1000°C. The majority of the sulfur in crease was organic in nature.
Hydrogen sulfide may also react with the inorg~nic material present in coke. King and Edgcombe(23) found that when 53 by volume of copper, nickel, cast iron, or mild steel was added to coal before carbonization, no hydrogen sulfide was evolved. Aluminum and chro mium, which do not readily form sulfides, nad no effect on the hydro gen sulfide evolution. Kipling(22) carbonized coal samples with and without 2.53 cuprous oxide added, and steam activated the cokes. Both the coke and activated sample retained more sulfur when cuprous oxide had been added. Cuprous sulfide was formed during carbonization to the extent that sulfur was present in the coal, and was not decomposed by steam activation.
The nature of the sulfur retained in coke might now be sum marized. The inorganic form consists of sulfides of varying com position throughout .the range of carbonization temperatures. Powe11(47) noted a definite change in characteristics of the organic sulfur between 400° to 500°C. He suggested that organic sulfur may exist at high temperatures as adsorbed free sulfur or sulfur in solid solution with carbon.(49) Recalling Kipling's(21) suggestion of sur face compounds of hydrogen and oxygen on coke above 600°C, one might expect a similar form for the organic sulfur. Several workers agree on the suggested surface complex.(8,12,54,57,59) In view of the con dit1ons existing during carbonization and the availability of reactants to form such a complex, this suggestion seems reasonable.
Statement of the Problem
The mechanisms involved in the decomposition of sulfur forms present in coal and in the formation of coke sulfur forms during carbonization have not been adequately explained. The effect of such variables as the nature and amount of mineral matter and the 9 release of volatile matter on the kind and amount of sulfur retained in coke require further investigation. In sho~t, a more complete picture as to the interactions of sulfur forms with various coal constituents ~s necessary if the factors contributing to the re tention of sulfur i.n coke are to be clearly defined.
The presence of elemental or free sulfur in high temperature cokes has been suggested, although it has not been detected through chemical analysis. Carbon-sulfur complexes, such as those prepared by Wibaut(57) and Lewis and Metzner(30) by direct reaction of sulfur with carbon, were found to yield free sulfur when heated at temper atures above those at which they were formed. If a carbon-sulfur complex exists in coke, the formation of free sulfur in coke may be analagous to, that in the synthetically prepared complex.
Mechanisms for transference of sulfur to the coke carbon have been suggested by various workers. These might be better understood if the effects due to reactions of volatiles with the sulfur forms could be differentiated from the reactions occurring between the sulfur forms in the solid charge.
Objectives of the Investigation
The objectives of this investigation were:
(1) ~o devise a means of distinguishing the effects of primary and secondary reactions contributing to 'the retention of sulfur during carbonization. (2) To secure data on the behavior. and distribution of sulfur forms during carbonization. (3) To obtain information on variables such as ash and volatile matter content of coals and cokes and their possible effects on the nature and amount of sulfur retained in coke. (4) To determine whether elemental sulfur is in fact formed during carbonization, and if posbible to obtain quantitative data on its occurrene\9.
Consideration of these factors should contribute to a better 10 understanding of the reactions of sulfur forms during carbonization, and may suggest or substantiate mechanisms proposed for the reactions. '~-,/
11
II. PROCEDURE OF THE INVESTIGATION
The investigation consisted of two phases; (1) the design and construction of a static bed, vertical furnace for the study of the behavior of sulfur during carbonization, and (2) the investigation of a qualitative test for possible use in the quantitative deter mination of elemental sulf'ur in coal and coke.
Description of Coal and Preparation of Samples
About 450 pounds of Pittsburgh seam, High Volatile A rank, high sulfur bituminous coal were obtained for use in these studies. The coal, received in seven burlap bags, had been crushed to pass a 3- inch screen. The contents of each bag was spilled onto a clean floor and a portion uf the contents of each bag was transferred into six l.arge steel cans; each can thus contained a fraction of the con tents of each bag. These were flushed witn nitrogen and sealed. The contents of two cans, 141 pounds, were subsequently prepared for laboratory use as descriBed below.
Shale and clay were manually removed from the coal before it was passed through a jaw crusher. The coal was then ground twice in a disc mill in order to produce minus 65-mesh coal. The ground coal was screened to secure a 65 x 150-mesh fraction, which was then washed by mixing with excess water in a large can and decanting off the fines. Finally it was air dried and kept under nitrogen until used. This size, representing about 403 of the coal ground, was chosen on the basis of its compatibilHy with the design of the sample holder used in the static bed carbonization unit. A 150 x
325-mesh fraction was also obtained, washed to ;(remove fines D air dried, and sealed under nitrogen. Portions of both size fractions were separated by flotation with a solution of petroleum ether-carbon tetrachloride of density 1.37. Proximate and total sulfur analyses of these samples are given in Table 1. Their use is discussed in 1subsequent sections. \ 12
Table 1
Proximate Analysis and Total Sulfur of Coal Samples
Sample Moisture Ash(l) V.M. (l) F.C. (1) Total Sulfur (1)
65 x 150 1,62 11.27 37.59 51.14 2.64 65 x 150 1. 79 5.18 39.80 55.02 1. 59 Float 150 x 325 0.98 5.24 39.97 54.79 2.00 Float
(1) Percent on dry basis
Part I Carbonization Studies
Description of Carbonization Apparatus
The design of the carbonization unit was based on considerations of the effects of primary and secondary reactions involving sulfur which would be difficult to differentiate using common carbonization techniques. These effects might be dif£orentiated by reducing the contact time of volatiles with the charge during carbonization. In order to achieve minimum contact time, or largely primary reactions, the diffuston path of volatiles through the charge should be small, and removal of volatiles from the vicinity of the bulk charge should be rapid. The effects of secondary reactions during carbonization could be studied further by allowing the volatiles to contact other materials after being evolved from the coal.
The sample holder design is shown in Figure 1. The framework was
constructed of 3/16-inch, #303 stainless steel strips and covered wtth ''<:
200-mesh, #304 stainless steel screen. The O.D. was io~mm and the length was 20 cm. The holder support tube, which also served as:f.a ther mocouple wellr·'was a stainless steel tube:rof 6 mm O.D. 1!i''The maximum distance of diffusion of volatiles through the bed during carbon ization should correspond to the bed width of 0.7 cm, since the greatest concentration gradient would exist across this lateral di mension. The design also provides for a small lateral thermal grad- 200 Mesh, 304 Stainless Steel Wire Screen, Covers Frame of Sample Holder
Thermocouple Well, 0.6 cm 0.0., 303 Stainless Steel
E u 0 N
Movable Disc, Covers Coal Sample and Centers Thermocouple Well
Four 303 Stainless Steel Strips, 3/16 in. x 1/32 in.
Nut at Base of Thermocouple Well
DIAGRAM OF FRAME FOR COAL SAMPLE HOLDER
FIGURE 1 14
ient through the sample. The coal charge occupied about one half of the sample holder and was covered with the stainless steel mov,ble cover disc during carbonization.
To further increase the lateral passage of volatiles from the charge, a relatively rapid flow of nitrogen, minimum purity 99.93, was maintained to dilute and rapidly remove the volatile carboniza tion products from around the sample holder. A flow of 3.2 liters per minute of nitrogen, metered at room temperature, was used. This corresponded to 33.1 linear cm per second past the sample holder at room temperature.
A second holder, containing either sand or a high temperature coke, was placed directly. above the coal sample holder during car- bonization as shown in Figure 2, so that volatiles could ~iffuse
through it and possibly undergo reactio~. The second holder differed from the coal holder in that its diameter was 23.5 mm rather than 20.0, and 150-mesh screen was used in place of 200-mesh screen. The sand and coke are described fully in the following section. Con tinued diffusion of the sulfurous constituents of the volatile matter through the second holder would occur if appreciable reaction took place to maintain a concentration gradient.
Other considerations in the design of the carbonization unit, shown schematically in Figure 2, were variable heating rates and use at temperatures up to 900°C. The Vycor furnace tube was encased in about 1/8 inch of alundum cement. The lower preheat section and upper reactor section were wound separately with chrome! resistance ribbon, and covered with 1/8 inch of alundum cement. The entire tube was then wrapped with thermo-flex insulating material and mounted vertically on a stand. The preheat section was filled with copper wool to remove oxygen from the nitrogen carrier gas and act as a heat transfer medium. Temperatures were maintained by manual adjustment
\ of two 15-ampere Variacs, and measured from readings o~ a chromel alumel thermocouple in conjunction with a Wheelco #310 potentiometer. The thermocouple was located inside the stainless steel support tube Thermocouple-Potentiometer
14-----#7 Rubber Stopper with 4 Outlet E Grooves for Volatile Matter v q "' r. ~ 29/42 Outer Joint c Stainless Steel Thermocouple Housing, 79 cm Long E v ~ "' E v -----Annular Distance. 0.05 cm '°M Reactor Section (B-CJ 1-+-----Coal Sample Holder, 20 cm Long, 2.0 cm O.D. -----Annular Distance 0.225 cm .,._----Base of Thermocouple Well with Support for Sample Holder E v q Preheater Section (A-B) N "' '1• Thick Wall Vycor Tube 2.45 cm l.D. E v <"! It') ~ i 29/42 Outer Joint Nitrogen Inlet DIAGRAM OF CARBONIZATION APPARATUS FIGURE 2 16 and could be positioned along the length of the sample during the runs Preparation of Adsorbents for Volatiles from Carbonization The materials used in the upper holder for the study of possible secondary reactions of sulfurous volatiles during carbonization were referred to as adsorbents, although the nature of the possible inter actions between them and the volatiles was not known in advance. Dur ing the carbonization run volatiles from the coal sample passed through these materials as was discussed under the apparatus design. Sand was chosen as an inert. A high temperature coke, similar to those produced in the carbonization unit, was also used. Sand adsorbent: Approximately 5 pounds of 30 x 120-mesh sea sand were mixed with 3 liters of 1:2 aqueous hydrochloric acia and heated at about 60°C for 2 hours. The sand was filtered and washed with distilled water until free from chloride. It was then dried at 105° and finally heated at 850°C for about 12 hours to burn off any organic matter. Coke adsorbent: An 850°C coke was produced from the 150 x 325- mesh float derivative coal described in Table 1. A 30-g coal charge, contained in a 5-inch-long, 45-mm-I.D. cylindrical Vycor crucible, was carbonized for 30 minutes in a furnace which had been preheated to the required temperature. The furnace used consisted of an 8-inch-long, 50-mm-I.D., quartz tube, electrically wound and insulated. Three charges were carbonized, then combined, crushed, and screened to ob tain a 60 x 120-mesh fraction. Carbonization Procedure The 65 x 150-mesh coal was carbonized at approximately 100° intervals from about 360° to 860°C. The actual temperatures employ ed, given as the average along the vertical temperature gradient were: 366°, 464°~ 564°, 668°, 670°, 779°, 784°, and 885°C. The 670° and 779°C runs were made using coke adsorbent, sand being used in the re mainder of the ru~s. This coal was also carbonized at 364° and 550°C without the nitrogen sweep. The 65 x 150-mesh float derivative coal 17 also was carbonized at 571° and 770°C. The procedure for carrying out a carbonization run was as follows. The sample and adsorbent holders were asstlmbled as shown in Figure 2, with the weighed coal and adsorbent in place. An 18-g charge of coal was carbonized in each run. The adsorbent weight was 20 g for sand and 10 or 15 g for the coke. The assembly was lowered rapidly into the furnace, which had been preheated to the carbonization temperature. A flow of 3.2 liters of nitrogen per minute, dried by passage through a 4-inch bed of Drierite (anhydrous calcium sulfate), was maintained during the preheat period and carbonization run except in those cases noted above. Temperatures near the bottom and top of the coal sample holder were recorded at intervals of 5 to 15 minutes. When an ad sorbent was used, the temperature in this vicinity was also recorded. After one hour in the carbonization unit the sample was removed and rapidly transferred into a nitrogen flooded container until cool. The cooled coke was removed from the holder, ground to minus 60 mesh if coking had occurred, weighed, and stored in screw cap bottles. Oper ational data are given in Appendix I. Analytical Procedures Proximate analysis of the coals and carbonized samples was done according to the A.S.T.M. method for coke.(2) Total sulfur of all samples was determined by the Eschka method,(2) except that the addi tion of bromine water was eliminated in view of the findings of Marrison and Mott(36) and Battye et. al.(5) who have shown this step to be unnecessary. Sulfur forms in the coals and carbonized samples were determined by a modification of Mott's method.(41,42,43) The modified method differs from Mott's method in that it in cludes a determination of non-pyritic sulfide sulfur. Also, non pyritic iron and pyritic iron were determined by successive acid ex tractions of the same sample rather than their determination in sep arate samples. This latter modification was also employed by van Hees and Early.(56) Tests perfomed in this laboratory have shown the method to be rapid and accurate.(46) The procedure is given in '"- ------18 Appendix II. All samples produced in the invesfiigation,. were tested for free elemental sulfur by the qualitative test described in Part II. Those giving positive tests were analysed by the quantitative method des cribed in Part~IL. The sand and coke adsorbents were tested for elemental sulfur and analysed for total sulfur by the Eschka method. Their volatile matter contents were estimated by means of the standard volatile matter test for coal. Part II Quantitative Estimation of Elemental Sulfur Elemental sulfur has been identified in high tem~erature coke(46) by means of a quali t,ative test which can detect as little as 0. 5 micrograms.(13) Briefly this method consisted of the selective reduction, in situ, of elemental sulfur with benzoin to form hyd rogen sulfide gas which was detected using moist lead acetate paper. The standard methods of analysis for sulfur forms in coal and coke do not include the determi.nation of elemental sulfur, so it seemed desirable to explore the possibl1ties of determining this con stituent. The benzoin reduction offered a conven1.ent method since it was free from interference by both organic and inorganic matter.(13) The reaction of benzoin with elemental sul~ur is: 0 OH 0 0 II I " ft c H C-CH-C H + S c H c-c-c H + H S 6 5 6 5 6 5 6 5 2 A number of methods might then be used to quantitatively deter mine the evolved hydrogen sulfide, either as such or after conversion to a suitable form. A rapid colorimetric method for the determination of sulfate(6,7) was selected for this work since it was applicable over a wide range of concentrations, and the hydrogen sulfjde could be readily converted to sulfate form by oxidation with hydrogen per- oxide in solution. Investigation of the use of benzoin to quantitatively determine elemental sulfur, in conjunction with the colorimetric sulfate deter- 19 mination, was carried out in six consecutive studies. (1) The qual itative test was applied to several coal and coke samples to establish its applicability for these snbstances. (2) Satisfactory condi.tion."s were established for the quantitative conversion of pure elemental sulfur, U.S.P. grade, to hydrogen sulfide using benzoin, and subse quently of hydrogen sulfide to sulfate using hydrogen peroxide. (3) The colorimetric sulfate determination was investigated and com pared with a gravimetric sulfate determination. (4) Four methods were used in adding elemental sulfur to coke prepared for this purpose. (5) The combined benzoin-peroxide conversion and colorimetric sulfate determination were applied to the "sulfurized" coke samples. (6) Car bon disulfide was used to extract elemental sulfur from the coke samples in view of the resul~s of the previous tests. Preparation of Cokes for the Addition of Elemental Sulfur Cokes were prepared at 400° and 600°C. from the 65 x 150-mesh float derivative and at 550°, 650°, and 750°C from the 150 x 325-mesh float derivative. These were chosen because they were free of elemen tal sulfur and represented most of the carbonization temperatures studied. The method of preparation was analogous to that used in the preparation of the 850°C coke described in Part I for use as adsorbent for volatiles, and 60 x 120-mesh fractions were obtained by grinding and screening. The total sulfur, moisture, and ash contents of the cokes are given in Table 2. Table 2 Total Sulfur, Moisture, and Ash Analyses of Cokes for the Addition of Elemental Sulfur Coke Total Sulfurz 3 (1) Moisture z 3 Ashz 3 400() 1.59 0.66 5.56 600° 1. 31 1.89 7.29 550° 1.38 1.24 7.47 650° 1.28 0.78 8.26 750° 1.47 4.19 8.19 (1) Dry basis. 20 Apparatus for Conversion of Elemental Sulfur to Sulfate The reaction vessil for the banzoin reduction of elemental sul fur and the absorption train for evolved hydrogen sulfide gas are shown schematically in Fjgure 3, where the apparatus dimensions are given. The first absorption tube contained 35 ml of 1:6 aqueous ammonium hydroxide and 5 ml of 303 hydrogen peroxide. The second ab sorption tube contained 10 ml of ammoniacal si\ver nitrate solution (ca. O.lN) to test for any sulfide escaping the first tube. Initially the reaction tube was ..i.JrlJllersed about half wa~ in a heated glycerol bath. In subsequent work the tube was heated along its entire length in a standard Fieldner furnace.(2) The nitrogen sweep, minimum purit.y 99.93, was dried by passage through a 4-inch bed of Drierite. Apparatus for Sulfate Determination The instrument used in the colorimetric sulfate determination was a Cenco Photelometer, model 12335, with a 525-millimicron green filter. Experimental Procedures (1) Qualitative test for free sulfur with benzoin. The qualitative test for elemental or free sulfur was performed on 60-mesh coal and coke samples of about 0.01 to 0.05 g mixed with an equal volume of benzoin in a microtest tube (5 x 0.5 cm). The tube was plunged into a giycerol bath at 130° to 140° and raised to 150° to 160°C in 5 minutes as indicated by Feigl and Stark(l3). A strip of moist lead acetate paper was placed in the mouth of the tube to test for hydrogen sulfide. The samples giving positive test were heated similarly but without the addition of benzoin in order to determine if other volatile sulfides were being evolved at the test temperature. (2) Determination of pure sulfur using a gravimetric method. Tests were made on the benzoin reduction of pure elemental sul fur, U.S.P. grade of minimum purity 99.53. Weighed amounts of sulfur were subjected to tests using varying reaction times, temperatures, Pyrex Tubing, 6 mm 1.0. Nitrogen Inlet ----1~ Gas Outlet $ 24/ 40 Outer Joint 11 Tygon Connecting Tubtng Pyrex Reactor Tube 16 cm Long 2.3 cm 0.0. Tube for Ammoniacal 1 Silver Nitrate Solution Sample ----1~ I Absorption Tube, 19 cm Long, 2.5 cm 0.0. for Ammoniacal Hydrogen Peroxide Solution DIAGRAM OF APPARATUS FOR BENZOIN REACTION AND ABSORPTION OF EVOLVED HYDROGEN SULFIDE FIGURE 3 22 and quantities of reactants. The test results are given in Table 14. The sulfur was placed in the reaction tube, the desired amount of benzoin added, and the apparatus assembled as shown in Figure 3, using the glycerol bath preheated to the reaction temperature. The effect iveness of the ammoniacal absorbent was determined by testing for sul fide in the second absorption tube throughout the run. After each run, the ammoniacal absorbent containing the evolved hydrogen sulfide and the hydrogen peroxide was heated slowly to boil ing and boiled until all excess peroxide was decomposed as indicated by the cessation of effervescence. The oxidized solution was then transferred, in whole or a measured aliquot, to a 250-ml beaker, acidified to methyl orange indicator using 1:2 aqueous hydrochloric acid, and 10 ml of 103 barium chloride solution was added. The barium sulfate was determined by standard gravimetric methods. This served to establish satisfactory conditions for the quanti ta-cive re duction of sulfur to hydrogen sulfide and subsequent oxidation to sulfate. (3) Comparison of the colorimetric and gravimetric method of sulfate determination. The colorimetric sulfate determination was investigated and com pared with the gravimetric method. The colorimetric method is based on the reaction between barium chloranilate (barium salt .of ?,5- dichloro-3,6-dihydroxy-p-quinone) and sulfate ion in an acid alcoholic solution, producing the dark purple acid-chloranilate ion. The absorb ance of the resulting purple solution is proportional to the sulfate concentration. Standard potassium sulfate solutions were prepared to obtain the calibration curve of relative intensity versus sulfate concentration, according to the procedure given by Baker Chemical Co.(3) Reagent grade potassium sulfate was used. The procedure and calibration curve for the colorimetric determination are given in Appendix III. Samnles of 0.0675 and 0.0748 g of sulfur were converted to sul fate by the benzoin-peroxide method using the conditions previously 23 described. The resultant sulfate solutions were diluted to a 250-ml standard volume with distilled water, 30 ml taken for colorimetric analysis and 100 ml for gravimetric analysis. A 0.0253-g sample of sulfur was also treated by the benzoin-peroxide method. The result ant sulfate solution was diluted to a 100-ml standard volume and a 30-ml aliquot was taken for colorimetric determination. The results of these determinations are given in Table 15. (4) Methods of addition of elemental sulfur to cokes. The initial method was solid phase mixing in which accurately weighed quantities of sulfur, about 8 to 10 mg, were mixed with about one gram samples of the 750°C coke. This method was used to ascertain whether the presence of the coke would alter the conditions found for the quantitative reduction of pure sulfur by benzoin. The second method was liquid phase adsorption. SQmples of 5 to 9 g of the 400°, 550°, 650°, and 750°C cokes were evacuated at room tem perature for about 60 minutes. Between 25 and 150 ml of saturated sulfur-ethanol solution were admitted to the evacuated sample, depend ing on the concentration of sulfur desired, and the mixture was allow ed to stand one hour· before the ethanol was removed under reduced pressure. This procedure was used in an attempt to deposit sulfur uniformly throughout the sample. In the thi.rd method, an attempt was made to deposit sulfur uni formly in the sample by decomposition of hydrogen sulfide gas on the coke. Five grams of the 600°C coke were placed in a vertical Vycor tube, about 1 cm I.D., through which a stream of ni.trogen was main tained at a flow rate of 200 cc per minute while the sample was being heated to 550°C in an electric furnace. Then hydrogen sulfide was ad mitted to the nitrogen stream at a rate of 85 cc per minute, and the gas mixture was passed through the hot coke for one hour. The hydro gen sulfide was shut off and the sample cooled in the nitrogen stream. The fourth and final method was similar to the third except the gas mixture was heated at 400°C immediately before passing it into the coke sample which was not heated. A 4-g sample of the 650°C coke was used with the same gas flow rates as in the third method. 24 The amount of sulfur added was determined in all cases, except the solid phase mixing where it was known from the weight added, by the difference in the total sulfur content before and after treatment as determined by the Eschka method. C:.D To demonstrate that the Eschka method was applicable to the determination of eleme11tal sulfur, sulfur samples of 0.1060 and 0.0490 g were also determined. Analysis of the quantity of sulfur in the cokes by the various methods of addi tion is given in Table 3. (5) Benzoin method for determination of sulfur added to cokes The benzoin-peroxide conversion of sulfur to sulfate and sub sequent colorimetric determination were applied to the "sulfurized11 cokes. The tests performed are summarized in Table 4. (6) Extraction of elemental sulfur from the "sulfurized" cokes. The use of carbon disulfide as a solvent for sulfur is well known. Its use was indicated by the results of the tests 1 through 13 given in Table 4. The extraction was carried out as follows: An accurately weighed coke sample of about 1 g was placed in a stoppered 250-ml Erlenmeyer flask with 20 ml of carbon disulfide. The mixture was allowed to stand 45 minutes with occasional shaking. The carbon di sulfide was decanted off through Whatman #30 filter paper into the reaction tube used for the benzoin reduction. The coke was again treated with 20 ml of carbon disulfide while the first carbon di- sulfide extract was evaporated off by a stream of air. The coke was finally filtered and washed with 10 ml of carbon disulfide, again catching the filtrate and wash in the reduction tube and evaporating to ''ct ryness " . One half gram of benzoin was added to the reduction tube and the reduction carried out by the procedure given previously. The above procedure was used for samples 15, 16, and 17. The extract from sample 14 was first collected in a 50-ml beaker, then transferred to the reaction tube and evaporated. Because this added step allows for additional error by loss of sulfur during its trans fer, it was eliminated. The results using these procedures are includ ed in Table 16. 25 Table 3 Methods of Sulfur Addition and Amount of Sulfur Added to Cokes Sample Designation --Coke Method of Addition(!) Sulfur Added, mg(2) 1 600° 3 46.9 (3) 2 650° 4 11. 9 (3) 3 650° 2 3.0 (3) 4 400° 2 1. 2 (3) 5 750° 1 7.5 6 II 1 9.6 7 II 1 7.6 8 II 1 8.9 9 " 1 9.4 10 II 1 8.6 11 II 1 7 ..5 12 II 2 1. 9 (3) 13 II 2 4. 8 (3) 14 550° 2 9.4 (3) 15 II 2 1. 9 (3) 16 " 2 8 .1 (3) 17 II 2 8. 1 (3) (1) Methods of sulfur addition: 1. Solid phase mixing, 2. Liquid phase adsorption, 3. Hydrogen sulfide decomposition on hot coke, 4. Hydrogen sulfide decomposition over cool coke. (2) Determined by Eschka method. (3) Mg per g of dry coke. 26 Table 4 Conditions of Benzoin Reaction Tests on 11 Sulfurized11 Cokes Sample Temperature,°C Time, hrs. g Benzoin/g S (1) Heating Unit 1 160 1.0 37/1 Glycerol bath 2 160 1.0 160/1 II II 3 160 1.0 (2) " " 4 160 1.0 II II " ,, II 5 160 1.0 " 6 160 1.0 II Fieldner 7 165 1.0 II II 8 160 1.5 II " 9 360 1.0 " " 10 280 1.0 II II 11 200 1.0 II " 12 200 1.0 " " 13 200 1.0 " II 14 (3) 200 1. 5 " II 15 (3) 200 1.5 " II 16 (3) 200 1. 5 " " 17 (3) 200 1. 5 " II (1) Ca. 1-g coke samples were used in all tests. (2) In tests 3 through 17 about equal quantities of benzoin and coke were used. (3) Samples extracted with carbon disulfide. 27 III. EXPERIMENTAL RESULTS Part I Carbonization Studies The proximate analyses of the coal samples and carbonized samples are given in Table 5. The yields were calculated from the weights of dry coal charged and dry coke remaining after ''carbonization. Some error was introduced by losses in removing the coke from the holder after carbonization because a small amount of coke, estimated to be less than several tenths of a gram, adhered to the thermocouple well. The error from this loss was fairly constant throughout the investiga tion, so that the yields are comparable. Assuming losses as high as one half gram, which were unlikely, the percent error in the yield would not exceed about 43. The average variation of duplicate deter minations for ash and volatile matter was ± 0.083 and ± 0.133 respect ively, except in those cases noted in the table. Effect of Rapid Removal of Volatile Matter on Sulfur Form Distribution Comparison of data for samples carbonized at about the same temperature, but in one case utilizing the nitrogen sweep and in the other without the sweep, indicates the effect of reactions of volatile matter with the sulfur forms during carbonization. Comparative runs were made at temperatures of about 350° (samples 366 and 364A) and 550°C (samples 564 and 550A). In this temperature range the release of volatile matter from primary decomposition of the coal is most extensive. The small variation in temperature is assumed to have negligible effect. The amount of volatile evolution during carbonization in either case was not signtficantly changed by use of the nitrogen sweep, as shown by comparison of the coke yields. However, the amount of vol atiles released from the cokes during the standard volatile matter test at 950° C was greater for the samples which were carbonized with out the nitrogen sweep. About 203 more volatile matter was evolved in the tests from cokes 364A and 550A than from the corresponding cokes prepared using the sweep. This suggests that the organic matter was ~- 28 Table 5 Proximate Analysis of Coals and Cokes and Coke Yields Dry Basis Dry, ash-free Sample (1) Moisture .z.! Ash,% V.M. ,3 F.C. ,3 V.M. ,3 F.C. ,3 Yield,% Coal, 65xl50 1.62 11.27 37.59 51.14 42.36 57.64 Coke, 364A (2,3) 0.19 12.10 34.23 53.67 38.94 61.06 98.7 " 366 0.40 12.26 28.50 59.24 32.48 67.52 96.2 II 464 1.60 14.38 14.79 70.83 17.27 82.73 79.5 II 550A 1.63 15.29 10.85 73.86 12.81 87.19 69.5 II 564 (4) 0.68 15.68 8.97 75.35 10.64 89.36 71.3 ,, 668 2.02 15.89 4.58 79.53 5.45 94.55 67.0 " 670 2.23 16.30 4.65 79.05 5.56 94.44 67.4 II 779 1. 32 18.22 2.17 79.61 2.65 97.35 66.9 II 784 (3) 0.86 17.39 2.32 80.29 2.81 97.19 65.0 II 885 1.49 18.59 2.02 79.39 2.48 97.52 63.5 Float, 65xl50 1. 79 5.18 39.79 55.03 42.00 58.00 Coke, 571F (5) 1.28 7.28 7.28 85.44 7.84 92.16 68.1 II 770F 1.44 8.18 2.66 89.16 2.90 97.30 63.6 (1) Sample designation of cokes corresponds to the average carboniza- tion temperature given in Appendix I. (2) Suffix A denotes coke carbonized without the nitrogen sweep. (3) Duplicate ash determinations varied by more than 0.33. (4) Single ash determination. (5) Suffix F denotes coke produced from the float coal fraction, pre pared by flotation of the whole coal, 65xl50-mesh, in a solution of petroleum ether and carbon tetrachloride, density 1.37. 29 sufficiently changed by the method of carbonization to produce differ ences in the nature of the volatile matter formed during carbonr~~tion and in the resulting cokes. Differences in the sulfur form distribution in these cokes are probably due to the differences in the extent of reaction with the volatile matter. Table 6 gives the analyses of total sulfur and of sulfur and iron forms of the coals and carbonized samples. The iron analyses are included because the pyritic iron values are the basis for calculation of the pyritic sulfur, and the non-pyritic iron con tent is of interest for comparison with the sulfide sulfur content, The average variation of the duplicate sulfur and iron determinations were as follows: Total sulfur, ±0.033; pyritic, ±0.033; sulfide, ±0.053; sulfate, ±0.003; elemental, ±0.02"/o; iron, ±0.033. Those samples for which excessive variation in duplicate determinations was obtained are noted in the table. Table 7 gives the sulfur forms as percent of total sulfur, cal culated from the data in Table 6. The values in Table 7 remain the same whether calculated from the data on the dry basis or on the whole coal basis. Comparison of the samples carbonized at about 350°C shows that more pyrite was decomposed and more sulfide formed in sample 366, with the nitrogen sweep. The differences in the amounts of the inorganic sulfur forms in the cokes is not large, and parallels the small dif ference in evolution of volatile matter under these conditions. It is doubtful that decomposition of pyrite to form ferrous sulfide and elemental sulfur occurred, since it has been reported that this reac tion does not begin until 500°c. (17) The data for samples carbonized around 550°C, where reactions of the inorganic sulfur forms were much more extensive, show that sample 550A, carbonized without the nitrogen sweep, retained only about two thirds as much pyritic sulfur as sample 564. Correspond ingly, nearly twice as much sulfide sulfur was found in sample 550A. \ \ Table 6 Analysis of Sulfur and Iron Forms in Coals and Carbonized Samples, Percent on Dry Basis Sample Total S Pyritic S Sulfide S Sulfate S Elemental S Organic S Pyritic Fe Non-pyritic Fe Coal 2.64 1.61 0.02 0.05 0.00 0.96 1.39 0.11 364A (1) 2.58 1. 74 0.02 0.03 0.00 0.79 1.51 0.10 366 2.54 1.61 0.04 0.03 0.00 0.86 1.40 0.14 464 3.04 1. 76 0.10 0.03 0.00 1.15 1.52 0.35 550A (1) 2.23 0.59 0.53 0.03 0.00 1.08 0.51 1.5~ 564 2.06 0.93 0.28 0.03 0.00 0.82 0.81 1.07 668 2. ...13 0.14 0.92 0.02 0.00 1.05 0.12 2.11 670 2.20 0.07 0.87 0.00 0.00 1.26 0.06 1.90 779 2.39 0.06 1.37 0.00 0.08 0.88 0.05 2.06 784 2.02 0.08 1.23 0.00 0.07 0.64 0.07 1.97 885 2.22 0.06 1. 77 0.00 0.15 0.24 0.06 2.33 Float 1.59 0 .47 0.05 0.02 0.00 1.05 0 .41 0.05 571F 1.32 0.20 0.13 0.01 0.00 0.98 0.17 0.49 770F 1.36 0.06 0.17 0.00 0.00 1.13 0.05 0.60 (1) Duplicate total sulfur determinations varied by more than 0.13 '(,J '! 0 31 Table 7 Distribution of Sulfur Forms in Coals and Cokes, % of Total Sulfur Sample Pyrite Su~fide Sulfate Elemental Organic Coal 60.99 0.76 1.89 0.00 36.36 364A 67.44 0.78 1.16 0.00 30.62 366 63.39 1. 57 1.18 0.00 33.86 464 57.89 3.29 0.99 0.00 37.83 550A 26.46 23.77 1.35 0.00 48.42 564 45.14 13.59 1.46 0.00 39.81 668 6.57 43.19 0.94 0.00 49.30 670 3.18 39.55 0.00 0.00 57.27 779 2.51 57.32 0.00 3.35 36.82 784 3.96 60.89 0.00 3.47 31.68 885 2.70 79.73 0.00 6.76 10.81 Float 29.56 3.14 1.26 0.00 66.04 . 571F 15.l~ 9.85 0 76 0.00 74.24 770F 4.41 12.50 0.00 0.00 83.09 Presumably, the difference in sulfur form distribution is attributable to the reduction of the extent of secondary reaction by removal of volatiles in the nitrogen stream. Since the amount of volatile matter released from both samples was approximately the same(69.5% yield for sample 550A versus 71.3% for sample 564) the relative rate of volatile removal or time of contact of the volatiles with the charge is probably the more significant factor at this temperature. Haarmann(l7) found i'I. straight Xine/relationstiip between tlie• amount of hydrogen sulfide evolved and the percent volatile matter for coals of the same sulfur content. He attributed the formation of hydrogen sulfide during coking to the following: (1) direct splitting off from the organic substance; (2) reactions of organic sulfur compounds evolved with hydrogen or steam; and (3) reaction of pyrite with hydrogen or steam. 32 Volatile matter release and its reactions during carbonization no aoubt also affect organically bound sulfur. Figure_4 presents data obtained from carbonization of a low sulfur Elkhorn coal with about 853 of the sulfur in orgainc form. (45) The amount, of sulfur evolved from the coal during low temperature carbonization is approximately proportional to the amount of volatiles evolved. Since little pyrite decomposition was observed with this coal at the temperatures investi gated, the sulfur evolution is probably largely attributable to the decomposition of organic coal sulfur. Armstrong and Himus(l) gave the conditions favoring elimination of sulfur during coking. They are: (1) immediate removal of the pri mary products of dedomposition with a sulfur free gas; (2) rapid re moval of the mixture from the hot solid to minimize secondary fixation of sulfur; and (3) use of a reactive gas, such as hydrogen or steam, to promote conversion of sulfur forms to hydrogen sulfide. Secondary reactions of the volatiles with the coal may thus con tribute to the nature of sulfur retained in coke. Although a greater amount of pyrite was decomposed in sample 550A where volatiles were more available for secondary reaction with pyrite, Table 6 shows the total sulfur of this sample is somewhat higher than that in sample 564 where volatiles were removed in the nitrogen sweep. Variation in the sulfur determinations for sample 550A was not large enough to account entirely for this difference. Examination of the sulfur forms dis tribution shows that more sulfur was retained in sample 550A in organic form. This sulfur is probably not retained merely because less organic coal sulfur was volatilized during primary decomposition of the coal, since the yield of this coke was about the same as coke 564. It may be the product of secondary reactions of carbon with the sulfurous volatiles formed by reaction with pyrite. Further examination of the data is necessary before these effects may be distinguished. In Table 8 the analyses of total sulfur and sulfur and iron forms of the carbonized samples are given on the whole coal basis. This representation shows more clearly the relative amounts of the sulfur 30 o VOLATILES _, 25 X SULFUR 200 _, < < 0 0 u u Cl) Cl) ~ ~ 0 0 • 11; I I I ·'I 200 250 ~300 350 400 450 TEMPERATURE OF CARBONIZATION, °C. RELATIONSHIP OF VOLATILE MATTER AND SULFUR. EVOLVED DURING CARBONIZATION FIGURE 4 34; retained in the samples. The difference in total sulfur in samples 550A and 564 is 0.103, with 0~183 more organic sulfur in sample 550A. Table 8 Analysis of Total Sulfur and Sulfur and Iron Forms of Coal and Cokes, Whole Coal Basis (1) Sulfur Content 3 Iron Contentz 3 Sample Total Pyritic Sulfide Sulfate Elem'l Organic Pyritic Non_-pyr. Coal 2.64 1.61 0.02 0.05 0.00 0.96 1.39 0.11 364A 2.51 1.69 0.02 0.03 0.00 0.77 1.47 0.10 366 2.41 1. 53 0.04 0.03 0.00 0.81 l.33 0.13 464 2.41 1.40 0.08 0.02 0.00 0.91 1.21 0.28 550A 1.55 0.41 0.37 0.02 0.00 0.75 0.35 1.06 564 1.45 0.66 0.20 0.02 0.00 0.57 0.57 0.75 668 1.43 0.09 0.62 0.01 0.00 0.71 0.08 1.42 670 1.49 0.05 0.59 o.oo 0.00 0.85 0.04 1.29 779 1.59 0.04 0.91 0.00 0.05 0.59 0.03 1.37 784 1.30 0.05 0.79 0.00 0.05 0.41 0.05 1. 27 885 1.41 0.04 1.12 0.00 0.10 0.15 0.04 1.48 Float 1.59 0.47 0.05 0 02 0.00 1.05 0.41 0.05 571F 0.89 0.14 0.09 0.01 0.00 0.65 0.12 0.33 770F 0.86 0.04 0.11 0.00 0.00 0.71 0.03 0.38 (1) Value given in Table 6 times fraction yield of coke. Sample 550A contained 0.083 less total inorganic sulfur than sample 564. It appears that th.e additional organic sulfur found in sample 550A may have resulted from transference of sulfur from the inorganic forms to the carbon, probably by reaction of the decomposition pro ducts of pyrite. The observed differences are not large enough to be 35 conclusive without further evidence. The total sulfur content of the volatile matter from 100 g of coal ca~bonized can be calculated from data for the yields and.total sulfur analyses on the whole coal basis. The amount of volatile matter evolv ed is equal to the difference between 100 and the percent yield of the coke. The amount of sulfur evolved is the difference between the total coal sulfur and the total coke sulfur on the whole coal basis. For sample 564, 28.70 g of volatiles and 1.19 g of sulfur were evolved per 100 g of coal carbonized. The sulfur content of the volatile matter was then 1.19/28.(0 x 100 or 4.143. A similar calculation gives 3.583 for the sulfur content of the volatile matter from sample 550A The theoretical difference in the sulfur content of the volatiles from the two samples can be calculated based on the assumption that pyrite decomposition takes place by reaction with hydrogenous volatiles (probably hydrogen or steam) to form one mole of hydrogen sulfide for each mole of pyrite reacted. The contribution of organic sulfur to hydrogen sulfide formation is taken to be co~stant for the two samples and is not taken into account for the comparison. Sample 564 contained 0.663 pyritic sulfur, sample 550A contained 0.413, with 1.613 present in the coal. By calculations analpgous to those above, the volatiles from the respective cokes during carbonization should contain 3.303 and 3.943 sulfur from pyrite decomposition alone. The volatiles formed during carbonization of sample 550A contained only 3.583 sulfur, including the contribution from organic sulfur. Based on these differ ences it appears that some of the sulfurous volatiles, derived from pyritic sulfur, reacted before leaving the charge, probably forming organic sulfur during coking. It is possible that the volatile re actants were hydrogen sulfide, hydrogen-sulfur free radicals as pro posed by Medvedev,(40) or other sulfur-containing constituents. Pri mary decomposition of pyrite to ferrous sulfide and elemental sulfur is also possible at this temperature, 550°C, and carbon could con ceivably react directly with the released sulfur. ------·--·--·--- 36 Effect of Temperature on the Forms of Sulfur During Carbonization The effect of temperature on the behavior of coal sulfur forms during carbonization is seen in Figure 51 where data from Table 7 are plotted. Significant changes in the relative proportions of the sul fur forms are noted. The inorganic sulfur accounts for about 603 of the total sulfur in the coal, while in the 885°C coke it amounts to about 803 of the total sulfur. Of special interest is the maximum in the organic sulfur curve between 600° and 700°C, and the sharp decrease above 700°C. The maximum percent sulfur in organic form occurs in the temperature region'where greatest pyrite decomposition takes place, and it decreases only after essentially all the pyrite is decomposed. Assuming a mole of sulfide sulfur is formed for each mole of pyrite decomposed, the amount of sulfide observed below 700°C was less than that theoretically possible, again suggesting that a transfer of inorganic to organic sulfur has taken place. The mechanism of the transformation may involve secondary reactions with sulfurous volatiles, or direct transfer of pyritic sulfur to the carbon as pro posed in the previous section. Increasing sulfide formation is noted after pyrite decomposition is cpmplete. The increase is approximately proportional to the organic sulfur decrease in the 700° to 885°C temperature range. This indicates that organically bound sulfur is transferred to the iron or other cations at the higher carbonization temperatures. This is in agree ment with the observations of Woolhouse.(58) The formation of sulfide sulfur must then take place by more than one mechanism. At lower car bonization temperatures, ferrous sulfide is no doubt formed directly as the reaction product of pyrite decomposition. At higher temperatures sulfur is probably transferred from the carbon to the iron. Possibly inorganic sulfides are formed when the carbon becomes less reactive to sulfurous volatile matter. Other inorganic sulfides may also be formed in similar manner. Recent work by Lesoine(51) has shown that addition of 33 by weight of sodium carbonate or calcium carbonate to coal prior to carbonization 100- - XPYRITIC SULFUR o SULFLE)E SULFUR .-ORGANIC SULFUR •ELEMENTAL S-ULFUR 80 I IClli: :::> LL_. :::> V) _. -<( I- 0 1-- LL 40 0 ~ 20 olo , o , """"V , A, X Kl COAL 300 400 500 600 700 800 900 CARBONIZATION TEMPERATURE, °C. DISTRIBUTION OF SULFUR FO-RMS IN COAL AND COKES FIGURE 5 38 increased the amount of sulfur retained in the coke. The increase was accounted for primarily by the formatton of an acid soluble sulfur form, probably sulfide, and of sulfates. Calcium and magnesium com pounds are common constituents of coal mineral matter, usually occurr ing as oxides, carbonates, and sulfates. While d2ta on the composition of the coal mineral matter were not obtained in this study, it is pos sible that sulfides such as calcium sulfide and magnesium sulfide were formed during carbonization. Calcium sulfide may be prepared by re action of hydrogen sulfide 1vi th calcium sulfate and carbonate or with the metal at red heat, and by reaction of carbon disulfide with cal cium oxide at high temperatures.(24) Magnesium sulfide is formed by reaction of carbon disulfide with magnes'itrm oxide at 700° to 900°C. (25) Figure 6 illustrates the effect of carbonization temperature on the distribution of pyritic and sulfide sulfur, using data from Table 8. Pyrite decomposition is continuous and is eAsentially completed at 650°C, while the sulfide curve exhibits a change in slope near this temperature. The slope of the sulfide curve about 650° is about one half the slope at 600°C. Also, the quantity of sulfide sulfur present at 885°C is in excess of the theoretical yield of ferrous sulfide from decomposition of pyrite present in the coal, so that organic sul fur must contribute to sulfide formation. Because the amount of sulfide sulfur is in excess of that re quired for the stoichiometric composition of ferrous sulfide, the existence of other inorganic sulfides or a complex iron sulfide is indicated. Figure 7 gives the atomic ratio of sulfide sulfur to non-pyritic iron as a function of temperature. Only above 750°C is a ratio corresponding to the stoichiometric composition of ferrous sulfide observed. Further increase in temperature increased the ratio until at 885°C it corresponded to the composition Fe s . The 3 4 same ratio was reported by Woolhouse(58) who suggested that a stable complex sulfide, 2FeS:Fes , was formed between 650° and 700°C. Very 2 small amounts of such a sulfide may be formed, as some pyrite was found even at the highest carbonization temperature studied. 1.8------o SULFIDE SULFUR 1.6 X PYRITIC SULFUR 1.4 ~ v) V) 1.2 ~ :::> 0.6 .....u.. :::> V) 0.4 0.2 o.o~-...... ----....---- ...... ----,r------r------r----....., COAL300 400 500 600 700 800 900 CARBONIZATION TEMPERATURE, 0 <1::. DISTRIBUTION OF PYRITIC AND SULFIDE . SULFUR IN COAL AND COKES I FIGURE 6 1.3 1.2 z 1.1 0 !!!: u l.OJ ... s:_o1~HIOMETRIC_COM!.,.°SITIO~ OF !_e:_ E >°' Cl; z 0 0.9 ...... z °':::> .....u.. :::> II) 0.8 w 0 u:..... :::> II) 0.7 0 ~ < 0.6 u°' ~ ....0 < 0.5 . 0 0 I / 0.4 0.3--~--,r--~~-r---~...... ~~~....-~~--~~~--~~..J COAL 300 400 500 600 700 800 900 CARBONIZATION TEMPERATURE, °C. ATOMIC RA TIO, SULFIDE SULFUR/NON-PYRITIC IRON FIGURE 7 41 The formation of sulfides during carbonization can be further investigated if it is assumed that below about 650°C all the sulfide and sulfate sulfur are combined with non-pyritic iron as ferrous sulfide or ferrous sulfate. The amount of sulfur-free iron can then be calculated, since each mole of non-pyritic inorganic pulfur would be combined with a mole of non-pyri tic iron. The differenc .. be tween the observed non-pyritic iron content and the amount required to satisfy the inorganic sulfur is then the sulfur-free iron content of the sample. These data, calculated from data on the whole coal basis, are presented in Table 9. Table 9 Theoretical Amount of Iron Uncombined With Sulfur in Coals and Cokes Sample Inorganic Sulfur, Calculated Observed Sulfur-Free Moles (1) Iron 2 3 I•on2 3 Iron2 3 Coal .0022 0.12 0.11 -0.01 366 .0015 0.08 0.10 0.02 464 .0031 0.17 0.28 0.11 564 .0068 0.38 0.75 0.37 668 .0196 1.09 1.42 0.33 670 .0184 1.03 1.29 0.26 Float .0022 0.12 0.05 -0.07 571F .0031 0.17 0.33 0.16 770F .0034 0.19 0.38 0.19 (1) Excluding pyritic sulfur. For the coal samples insuffic±ent non-pyritic iron is present to be combined with the sulfide and sulfate sulfur. It is to be expected that the sulfate is probably combined with calcium and magnesium. The same may be true of some of the sulfide sulfur, both in the coals and cokes. The sulfur-free iron calculated is therefore the minimum amount possible. The amount increased with the carbonization tempera ture, then decreased slightly at about 670°C. However, the magnitupe 42 of the decrease is within experimental error. A decrease was not observed between 570° and 770°C in the float cokes. Quite possibly there could be substantial sulfur-free iron in the coke even at higher carbonization temperatures, in spite of the fact that sulfur is avail able for reaction with the iron as in the cokes 779, 784, and 885. More data are required to verify these interpretations. The data are conclusive, however, in showing that the decomposition of pyrite to form ferrous sulfide, either by primary or secondary reactions, does not adequately describe the behavior of pyrite during carbonization. The above argument implies that part of the iron in coke is in a form which does not allow for reaction with sulfur, so that non ferrous sulfides, ~g. calcium sulfide and magnesium sulfide, may be formed even where the atomic ratio of sulfide sulfur to non-pyritic iron exceeds unity. On the basis of the available data, this must remain a matter of speculation. Medvedev(40) has postulated the formation of weak coal-iron bonds during carbonization. In any case, it should be kept in mind that iron sulfides may not be present in coke to the extent indicated by the amount of the individual com ponents, iron and sulfide sulfur. Occurrence of Elemental Sulfur Elemental sulfur was observed in small quantity in the whole coal cokes at temperatures above 750°C, the amount increasing with tempera ture. It was not detected in either coke from the float coal, and further presentation will exclude these cokes. The sulfide content of the cokes increased markedly in the same high temperature range, while the organic sulfur content decreased. The formation of free sulfur can be explained by either the decomposition of organic $Ulfur or a decrease in the reactivity of the organic matter with increased temperature. Decomposition could take place to yield free sulfur directly, or it might involve intermediate formation of sulfurous vol atiles which in turn decompose to give free sulfur. Residues of the coke after heating at 950°C (standard volatile. 43 matter test) also gave positive tests for elemental sulfur, except those of the float cokes. Other studies, (46) in which a coal similar to that used in this investigation was carbonized at various tempera tures by a method analagous to the standard test for volatile matter, showed elemental sulfur was present in cokes as low as 500°C. Carbonization of the Float Coal Fraction The carbonizatio~ behavior of the sulfur in the float fraction differed significantly from that in the whole coal. The two coal samples and their cokes are compared in Tables 10 and 11. If the whole coal is compared with the float derivative on the dry, ash free basis, the similarity of the organic coal substance for both samples is apparent. On the dry, ash-free basis the coal samples contain volatile matter in the following amounts: whole coal, 42.363; float coal, 42.003. Similarly the coke yields and volatile matter and fixed carbon of the cokes prepared at about the same temperature differ very little. At about 550°C the float coal appears to have evolved slightly more volatile matter. The volatile matter evolved from the float coal may be of a somewhat different nature than that from the whole coal, since it probably undergoes less secondary reactions due to the presence of less mineral matter and pyrite in the coal. On the same basis, the respective organic sulfur contents were 1.083 in the whole coal and 1.113 in the float coal, the difference being insignificant. The amount of organic sulfur retained in the cokes from both coals at about 550°C were also nearly the same on the dry, ash-free, whole coal basis. At about 770°C the float coal re tained slightly more organic sulfur tnan the whole coal. Because the organic matter in both coals is probably indentical in nature, differences in carbonization behavior of the sulfur may be attributed to other variables. The main differences observed were: (1) The atomic ratio of sulfide sulfur to non-pyritic iron was 0.47 in coke 571F compared to 0.46 in coke 564, but it increased to only 0.50 in coke 770F while increasing to 1.16 in coke 770. (2) Elemen- 44 Table 10 Proximate Analysis of Whole and Float Coals and Cokes, and Coke Yields Dry Basis Dry, Ash-free Basis Sample Ash,% V.M.,3 F.C.,3 Yield,% V.M.,3 F.C.,3 Yield,% Whole 11.27 37.59 51.14 42.36 57.64 Coal Float 5.18 39.79 55.03 42.00 58.00 Coal 564 15.68 8.97 75.35 71.3 10.64 89.36 67.7 571F 7.28 7.28 85.44 68.l 7.84 92.16 66.6 779 18.22 2.17 79.61 66.9 2.65 97.35 61.6 770F 8.18 2.66 89.16 63.6 2.90 97.30 61.6 tal sulfur was found in coke 779 but not in coke 770F. (3) The residues from the volatile matter test (950°C) on cokes of the whole coal gave positive tests for free sulfur wheTeas those from the float cokes did not. Analysis of Adsorbent Materials Analysis of the sand and coke adsorbents used to study the ef fects of secondary reactions revealed that little reaction of the sulfurous volatiles took place on these materials. Visual inspection showed that volatiles did diffuse through the entire bulk of adsorbent and apparently carbonized on the surface at higher temperatures. The sand adsorbents appeared successively darker as higher temperatures were used. No free sulfur was found in any of the adsorbent samples after the carbonization runs. Table 12 lists the adsorbent samples at the temperature of the run with the percent total sulfur determined and that calculated if all sulfurous volatiles would have been retain ed in the material. Data from the coke adsorbent is inconclusive, since apparently sulfur was evolved from the coke even though it had been previously carbonized at 850°C for 30 minutes. 45 Table 11 Sulfur and Iron Analysis of Whole and Float Coals and Cokes Whole Float Cokes 2 F Designates Float Coke Analysis/. 3 (1) Coal Coal 564 571F 779 770F Total S 2.64 1.59 1.45 0.89 1.59 0.86 Pyritic S 1.61 0.47 0.66 0.14 0.04 0.04 Sulfide S 0.02 0.05 0.20 0.09 0.91 0.11 Sulfate S 0.05 0.02 0.02 0.01 0.00 0.00 Elemental S 0.00 0.00 0.00 0.00 0.05 0.00 Organic S 0.96 1.05 0.57 0.65 0.59 0.71 " (2) 1.08 1.11 0.68 0.70 0.72 0.77 Pyritic Fe 1.39 0.41 0.57 0.12 0.03 0.03 Non-pyritic Fe 0.11 0.05 0.75 0.33 1.37 0.38 3 of Total S Pyritic S 60.99 29.56 45.14 15.15 2.51 4.41 Sulfide S 0.76 3.14 13.59 9.85 57.32 12.50 Sulfate S 1.89 1.26 1.46 0.76 0.00 0.00 Elemental S 0.00 0.00 0.00 0.00 3.35 0.00 Organic S 36.36 66.04 39.81 74.24 36.82 83.09 (1) Dry, whole coal basis. (2) Dry, ash-free, whole coal basis. 46 Table 12 Total Sulfur and Volatile Matter Analysis of Adsorbent Materials(!) Sa.mple Adsorbent Sulfur Increase, 3 Cale. Sulfur Increase,% V.M., 3 --.366 Sand 0.01 0.21 0.3 464 San9 0.03 0.21 0.3 564 Sand 0.00 1.06 0.2 668 Sand 0.00 1.07 0.4 670 Coke -0.09 1.36 779 Coke -0.ll 1.86 784 Sand 0.02 1.19 885 Sand 0.00 1.09 0.3 (1) Sulfur determined by the Es.chka method Part II Quantitative Estimation of Elemental Sulfur Qualitative Test for Free Sulfur with Benzoin This test was conducted by heating the sample with benzoin to a final temperature between 150° to 160°C, and testing for evolved hy drogen sulfide produced by reaction of elemental sulfur with benzoin. Lead acetate paper was used to detect,.. the hydrogen sulfide. As little as 0.5 ~icro~rams of sulfur can be detected by the test.(13) Because the test was conducted at an elevated temperature, the results of tests performed on samples not previously heated to the test temperature must be subject to question. For example, a positive test might be ob tained from this type of sample because the temperature used in the test caused release of a volatile sulfide .. ,,,,While. th.is, is_ unlik~~Y ,.,.. the test should be repeated in the usual manner but without adding ben zoin, if a positive test was given initially. In this study a posi tive test was obtained from a raw coal sample. The repeat test with out benzoin was negative. Although the test method is strictly appli cable only to samplrs which have been heat-treated at temperatures higher than 160oC, the range can probably be safely extended to in clude samples with no previous heat-treatment. Table 13 gives the 47 results of tests performed. ori various coals and cokes. Table 13 Results of Qualitative Test for Free Sulfur in Various Coals and Cokes Sample Result 1. Pittsburgh Seam coal used in this study. negative 2. Pittsburgh Seam coal similar to sample #1. (1) positive 3. Elkhorn Kentucky coal. (2) negative 4. Cokes of coal #1 produced below 700°C. (3) negative 5. Cokes of coal #1 produced at about 780°C. (3) positive 6. Coke of coal #1 produced at 885°C. (3) positive 7. Cokes of coal #2 produced below 500°C. (4) negative 8 Cokes of coal #2 from between 500° to 950°C. (4) positive 9. Cokes of coal #3 produced at 700° and 950°C. (4) positive (1) High Volatile A bituminous coal; 2.363 total sulfur with about 543 inorganic and 463 organic. (46) (2) High Volatile A bituminous coal; 0.843 total .sulfur, with about 153 inorganic and 853 organic. (45) (3) Cokes produced in this investigation. (4) Cokes produced in a Fieldner furnace by a method analagous to the standard volatile matter test. (46) 48 Quantitative Determination of Pure Sulfur Elemental sulfur, U.S.P. grade, was determined in four tests by reaction with benzoin, oxidation of the product hydrogen sulfide using hydrogen peroxide, and gravimetric determination of the resultant sul fate. The test conditions and results are summarized in Table 14. Table 14 Conditions and Results of Tests on Quantitative Determination of Pure Elemental Sulfur Test Conditions Test Results Reaction Timef Temperature1 Benzoin Sulfur Sulfur Error( Test Minutes oc . g g g 3 -1 30 130-140 1.0 0.1072 0.0455 57.5 2 30 160 1.0 0.1059 0.0700 33.9 3 60 150 1. 75 0.0521 0.0513 1.53 4 60 160 3.45 0.1055 0 .1049 0.57 The effect of time, temperature, and reagent quantities is ev ident. The reaction is about 703 quantitative in 30 minutes at 160°C using about 1.5 moles of benzoin per mole of sulfur, in contrast to 453 at 130° to 140°C with the other conditons the same. In order to keep the time and temperature of the determination within reasonable limits, a molar ratio of 5 benzoin to 1 sulfur was tried in tests 3 and 4, and the reaction time was increased to one hour. The average percent error of these two determinations was 1.053. The percent error is based on the assumption that the sulfur was 1003 pure. The actual purity was not accurately known, but it was not less than 99.53. If this lower limit of the purity is assumed, the average percent er .ror for .tests 3 and 4 becomes 0. 533. Because the latter two tests in~ dicated that the conditions used were satisfactory for complete reac- • tion, further examination of the variables was not made. 49 Cdmparison of the Colorimetric and Gravimetric Methods for Sulfate Determination Weighed amounts of sulfur, U.S.P., were converted to sulfate by the benzoin-peroxide procedure, and separate aliquots of the sulfate solution were determined by the. gravimetric method and by the color imetric method using barium chloranilate. The procedure for the col orimetric metliod~ is given in Appendix III. The calculated weighti. of sulfur in the aliquot used in the respective determinations is com pared with the weight of sulfur determined in the aliquot by each method,...f in Table 15. Table 15 Colorimetric and Gravimetric Methods of Sulfate Determination Colorimetric Method Gravimetric Method Aliquot S Determined S .Error Aliquot S Determined S Error Test g g 3 L- g 3 1 0.0081 0.0082 1.23 0.0270 0.0267 1.11 2 0.0090 0.0094 4.44 0.0299 0.0298 0.33 3 0.0076 0.0073 3.95 av. 3.21 av. 0.72 The accuracy of the gravimetric method appears to be superior to that of th,e colorimetric method. Using the relationship given by Kulishenko and Medvedev(27) for sulfur in coke, and assuming that 503 of the coke sulfur is inorganic, the maximum free sulfur content can be estimated. The amount of coke sulfur calculated by this relation ship (S k 0.62S + 0.63S ) is 1.603 for the whole coal, so that co e = p o probably about 0.803 would be in inorganic form. The remaining 0.803 is taken to be the maximum possible free sulfur content. Thus, the magnitude of the error by either method for the range of values likely to be found for free sulfur in this investigation is very small. Re sults obtained later in the carbonization work indicated that the er- ror from the colorimetric method was negligible for the amount of free 50 sulfur present in the cokes prepared. The colorimetric method was preferred in this study over the gravimetric methcd for its rapidity and applicability to a wide range of sulfur concentrations. At small concentrations, such as those found in the carbonization work, the accuracy of the gravimetric method suffers from the magnification of filtering and weighing errors. Determination of Total Sulfur Added to Cokes Using the Eschka Method The amounts of sulfur added to the cokes have been presented in Table 3. The application of the Eschka method to samples of pure sulfur weighing 0.1060 and 0.0490 g gave results of 0.1010 and 0.0480 g respectively. The average percent error of the:::e determinations is 3 .3&3. The error f,rom the colorimetric sulfate determination, 3. 213 lies within this experimental error. Application of the Sulfur to Sulfate Conversion and Colorimetric Determination to Cokes with Added Sulfur Table 16 summarizes the data from Tables 3 and 4 and presents the results of the benzoin-peroxide conversion of elemental sulfur in the cokes to sulfate determined by the colorimetric method. It is recalled that in the successful determination of sulfur alone, a molar ratio of 5 benzoin to 1 sulfur was used. It was apparent, however, that this quantity of benzoin, while theoretically in excess of that required for complete reaction, was insufficient to quantitatively react with the sulfur contained in the cokes. For this reason the weight of benzoin used in further tests was about equal to that of the coke. Tests 5 through 8 indicate that this procedure also gave erractic and consistently low results. Therefore, the effect of temperature was sutudied in tests 9 thruugh 11. Test 9, at 360°C, gave high results, possibley because other sulfur forms reacted at this temperature. The tests at 280° and 200°C (10 and 11) gave satisfactory res\l,lts on the samples prepared by solid phase mixing, but when these conditions' were applied to cokes in which sulfur had been added by the liquid adsorp tion method (12 and 13), erratic results were again obtained. These 51 Table 16 Comparison of Methods of Conversion and Determination of Elemental Sulfur Added to Cokes Method of S Total Sulfur Conditions Used for Elemental S Deter'd mg (3) Sample Addition (1) Added 1 m~ ( 2) Benzoin Reaction 1 3 46.9 (4) 160°, 1 hr. 0. 7 (4) 2 4 11.9 (4) 160°, 1 hr. 1.0 (4) 3 2 3 .0 (4) 160°, 1 hr. 1.0 (4) 4 2 1. 2 (4) 160°, 1 hr. 0 .4 (4) 5 1 7.5 160°, 1 hr. 3.5 6 1 9.6 160°, 1 hr. 6.0 7 1 7.6 165°, 1 hr. 6.6 8 1 8.9 160°, 1.5 hr. 3.7 9 1 9.4 360°, 1 hr. 10.9 10 1 8.6 280°, 1 hr. 8.4 11 1 7.5 200°, 1 hr. 7.2 12 2 1. 9 (4) 200°, 1 hr. 1. 9 (4) 13 2 4. 8 (4) 200°, 1 hr. 3.6 (4) 14 2 9. 4 (4) 200 °, 1. 5 hr . 7.0 (4) 15 2 1. 9 (4) 200°, 1. 5 hr .. 1. 9 (4) 16 2 8 .1 (4) 200°, 1.5 hr. 7.9 (4) 17 2 8 .1 (4) 200 °, 1. 5 hr . 8.2 (4) (1) Methods of adding sulfur to cokes: 1. Solid phase mixing. 2. Liquid phase adsorption. 3. Hydrogen sulfide decomposition on hot coke. 4. Hydrogen sulfide decomposition over cool coke. (2) Determined by Eschka method. (3) Benzoin-peroxide conversion, and colorimetric determination. (4) Mg per g of dry coke. 52.., results indicated that free sulfur probably entered into the pore structure of the coke and hence was not entirely contacted by the ben zoin. Extraction of Elemental Sulfur with Carbon Disulfide The difference in results from the determination of pure sulfur and of sulfur in coke suggested extraction of sulfur from the cokes prior to reaction with benzoin. The sulfur was extracted using carbon disulfide in the final four tests (14, 15, 16, and 17), then reacted with 0.5 g of benzoin for 1.5 hours. In test 14 the sulfur extract was first collected in a 50-ml beaker, then transferred to the benzoin reaction tube, evaporated to "dryness", and reacted .with the benzoin. Incomplete transference of the solution is probably the rea son for the low results of the test. Hydrogen sulfide from this test was collected in one adsorption tube during the first hour, in another tube for another 30 minutes. The second tube was found to contain a small amount of hydrogen sulfide, equivalent to .0003 g of sulfur, so that the .reaction time in tests 15, 16, and 17 was extended to 1.5 hours. The average percent error of tests 15, 16, and 17 is 0.853, so this method was subsequently applied to the determination of free sul fur in the cokes prepared in the carbonization work. This degree of accuracy may be off set by the findings in which the highest percent error was 3.383 using the Eschka method to determine pure sulfur, upon which the values from these tests also depend. The procedure for the extraction and conversion of elemental sulfur to sulfate is given in its entirety in Appendix IV. The procedure for the colorimetric meth od is given in Appendix III. It is to be noted that the results of test 1 showed extraordinar ily large differences between the amount of sulfur added and deter. ~ined as elemental sulfur. This sample was prepared by decomposition pf hydrogen sulfide on the coke. Subsequent analysis, using the car ~on disulfide extraction, benzoin-peroxide treatment, and colorimetric petermination, gave a value of 0.083 elemental sulfur compared to 4.693 total sulfur added. The failure of the determination in this 53 case is probably due to the fact that the sulfur was not added in elemental form. 54 IV. DISCUSSION OF RESULTS Part I Carbonization Studies The experimental results have shown that the amount of sulfur retained in coke can be affected by the following factors: (1) the nature of the sulfur forms present in the coal carbonized, (2) the amount of volatile matter released during carbonization, (3) the nature and amount of mineral matter present in the coal, and (4) the method by which the volatile matter is removed during carbonization, The results suggest that the nature of the organic coal substance is a factor in sulfur behavior, in that the characteristics of the vol atile matter released and the coke produced during carbonization prob ably contribute to the amount of sulfur retained or released. These factors are not entirely independent, so that often discussion of one necessitates inclusion of the effects of the others, Possible mechanisms for the decomposition and formation of or ganic and inorganic sulfur forms during carbonization were suggested by investigation of the above variables. While additional work is required to support the mechanisms, given below, the results of the investigation will be discussed on the basis that the behavior of sulfur follows these mechanisms. In this way, definitive experiments for future work will become more apparent, Based on the results of this investigation and in the literature, the behavior of sulfur during carbonization appears to entail the following: Decomposition of Organic Coal Sulfur At low carbonization temperatures, organic sulfur is removed from the coal directly as organic compounds and as hyrdogen sulfide, This has been established by several investigators (9,32,37) and was not investigated in this study. Medvedev and Petrapolskaya(39) found that below 550°C the contribution of organic sulfur to evolved hydrogen sulfide was not appreciable compared to the contribution from pyritic sulfur for a medium to high rank, good coking coal. Carbonization above this temperature resulted in an essentially constant proportion 55 of organic and pyritic sulfur in the hydrogen sulfide·evolved.- The relative contribution of organic and inorganic forms at various carbonization temperatures was found to be a function of the rank of the coal. Powell(47) found that one fourth to one third of the or ganic sulfur is removed as hydrogen sulfide below 500°C. The carbonization behavior of the organic sulfur in the coals studied in this investigation can be estimated by comparison of the organic coal and coke sulfur contents, calculated to the dry, ash free, whole coal basis. Decreases of about 15% and 48% are noted at 366° and 564°C respectively. At and above the higher temperature, contributions from inorganic forms and reaction of the carbon with sulfurous volatiles is probably appreciable, so that further compari son would carry little significance. It is reasonable to expect the decomposition of organic coal sulfur to coincide with that of the organic coal matter in general. On this assumption, the primary de composition of the organic sulfur would occur primarily below 600°C. Decomposition of Pyrite Pyrite is decomposed at relatively low carbonization temperatures. Figure 6 shows the decomposition behavior of pyrite in the coal studied. Decomposition began around 350°, was most marked at 450° to 550°, and essentially none remained above 650°C. Below 500°C, primary .decomp osition of pyrite to form elemental sulfur and ferrous sulfide does not occur.(17) Since pyrite decomposition during coal carbonization is observed at temperatures below 500°C, it must involve secondary reactions. It is known that pyrite reacts with gases such as those present during coking, notably hydrogen and steam, at temperatures as low as 280°C,(34) forming ferrous sulfide and hydrogen sulfide in the case of hydrogenous reactant gases. Since pyrite decomposition during carbonization occurs at corresponding low temperaturess, it has been suggested that the. reaction of pyrite with hydrogen or steam is primarily responsible for its decomposition.(17,47,48,59) Evidence that volatile matter evolution does play an important role in decomposing pyrite was found in this investigation. Data in 56 Table s, for samples carbonized with and without the nitrogen sweep, indicate that around 350°C the extent of pyrite decomposition parallel ed the amount of volatile matter released during carbonization. At about 550°C pyrite decomposition was decreased when volatiles were rapidly removed in a stream of nitrogen, even though the amount of volatiles released changed very little if any. The amount of sulfide sulfur formed below about 750°C did not correspond with that theoretically predicted for reactions, primary or secondary, in which a mole of ferrous sulfide is formed for each mole of pyrite decomposed, Instead, less sulfide sulfur than non pyritic iron was found, indicating that considerable quantities of sulfur-free iron we~e formed. This is shown in Table 9 for cokes prepared between about 350° and 650°C. The 570° and 770° cokes of the float coal also contained iron uncombined with sulfur. Previous carbonization studies(46) on a coal similar to that used. in this in vestigation also showed that a one to one atomic ratio of sulfide sul fur to non-pyritic iron was attained between 600° to 700°C. Below this temperature range a sulfur deficency existed. Woolhouse(58) reported similar results. These results suggest that pyrite may de compose to form elemental iron and sulfur. Apparently the mechanism of pyrite decomposition is more involved than that generally accepted. Medvedev,(40) using radioactive tracer 35 techniques with s in pyrite, reported finding free iron as low as 350° to 400°C. The mechanism for pyrite decomposition which he sug gested can be used to explain the results of this investigation. The pertinent aspects of his mechanism are: (1) formation of the un stable intermediate Fe-S-S-H by combination of pyrite with atomic hydrogen, and (2) decomposition of the intermediate to form ferrous sulfide and the radical 0 S-H, or metallic iron and the radical 0 S-S-H. He also suggested that in good coking coals, pyrite may decompose by combination of the iron with organic free radicals in the carbon structure and formation of elemental sulfur. The sulfur would ..then react forming stable carbon-sulfur bonds in displacing ·the less firmly i 57 bonded iron. These latter reactions occur in the plastic zone from about 450 ° to 500 °c, and are unimportant in non-coking coals. Fo.rmation of Organic. Sulfide.,, and. Elemental Sulfur It is probable that· the formation of these forms during coking is related to the decomposition of pyrite. Already disQ~ssed in part is the formation of ferrous sulfide from pyrite decomposition. Other secondary reactions, as of hydrogen sulfide with carbon or cations probably also affect the formation of the forms of sulfur. It has been shown in this investigation that sulfur from pyrite is probably transferred to the carbon during coking, forming organic coke sulfur. This w~s indicated by a comparison of the sulfur content of the volatile matter calculated from the observed total sulfur decrease with that calculated from pyrite decomposition, and by dif ferences in the observed and theoretical yields of sulfi.de sulfur. These data are given in Table 8. Figure 5 gives the distribution of sulfur forms throughout the ~ange of carbonization temperatures studied. Figure 8 presents the data obtained from a study(46) of the behavior oi sulfur during car bonization of a Pittsburgh Seam, High Volatlle A coal. The analysis of this coal is as follows: moisture, 1.76%; ash, 7.823; volatile matter, 37.963; fixed carbon, 52.463; total sulfur, 2.363; pyritic sulfur, 0,793; sulfate sulfur, 0.443; organic sulfur, 1.083; non pyritic iron, 0.583. The figure shows an increase in the percent sulfur in organic form between 500° and 800° with a maximum at 700°C, corresponding to that in Figure 5. The behavior of pyrite closely parallels that for the coal studied in this investigation, and the similarity in the formation of sulfides has been noted above. Several mechanisms for the transfer of pyritic sulfur to the coke carbon are possible. A solid phase reaction involving direct trans fer of sulfur from the pyritic iron may occur. Intermediate formation of sulfurous volatiles which react with the carbon could also explain the transfer. Because of the apparent effect of volatile matter 100 90t >< ORGANIC SULFUR e PYRITIC SULFUR ~ 80 ::> ~ ..... I o SULFIDE SULFUR ::> 70 V) ..... 60 ....< 0 .... 50 ~ 0 40 .... zw u 30 ~ w a.. 20 10 0 COAL 300 400 500 600 700 800 900 1000 CARBONIZATION TEMPERATURE, °C. DISTRIBUTION OF SULFUR FORMS IN COKES FROM A PITTSBURGHSEAM COAL FIGURE 8 59 evolutioh on'pyrite decomposition, the latter mechanism is probably the more important, although no evidence was found to eliminate the former, Both methods of transfer have been suggested by Medvedev.(40) That not involving sulfurous volatiles is said to be of importance only for good coking coals, presumably because the nature and reactivity of the coke substance formed during carbonization can determine whether or not the re~ction would occur. The free radicals which Medvedev postulates ,F .... 'i could react in a variety of ways with iron (or other cations) or organic matter producing inorganic sulfides, organic coke sulfur, elemental sul fur, and hydrogen sulfide. Specifically, the reactions he gives are: (1) R C-H + 0 SH R C0 + H S (R C-H denotes a coal structure) 3 ' = 3 2 3 (2) 0 S-S-H + H'= H S S + H S 2 2 = 2 (3) 0 S-S-H + R C-H S + H S + R C 0 3 = 2 3 Sulfur atoms formed in the above reactions can react as follows: (4) S + Fe = FeS (5) S + 2R C0 R C-S-CR 3 = 3 3 Knapp(26) found large increases in the organic sulfur content of cokes when they were heated to 800° and 1000°C in a stream of hydrogen sulfide, On selective grinding and screening of 1100°C coke, he characterized two different fractions by their color, apparent hard ness, and volatile matter content, One was black and soft and the other gray and "graphite like". The difference was believed to be due to the temperature gradient within the coke oven, The black por tion was found to be more reactive with hydrogen sulfide than the grey portion, suggesting that hydrogen sulfide may react also at lower temperatures, Lesoine(29) complimented this study and verified that the reaction takes place below 800°C. Using cokes prepared at 500°, 700°, and 900°C he found maximum reactivity at 700° reaction temper ature, He also observed elemental sulfur in the higher temperature cokes after treatment with hyrdogen sulfide. 60 Three types of reactions may account for the transformation of pyri tic sulfur to organic sulfur during carboni.za ti.on. First, hydro gen sulfide, formed dul'"i.ng pyrite decomposition can react with coke to give organic sulfur. Secondly, free radicals such as 0 SH and 0 SSH, from reaction of hydrogen with pyrite would undergo the reactions given above forming hydrogen sulfi.de and elemental sulfur, either of which could subsequently react with the carbon. Thirdly, elemental sulfur formed along with elemental iron from decomposi.ti.on of pyrite can re act with free valences in the carbon giving organic sulfur. Since pyrite decomposition i.s complete at 600° to 700°C these reactions would be most extensive below these temperatures. The formation of sulfi.de and elemental sulfur appear to be close ly related to one another at temperatures above 700°C. The data for the occurrence o.f these sulfur forms are presented in Tables 6, 7, and 8 and Figure 5. Below 700°C the sulfide content is probably due almost entirely to ferrous sulfide from pyrite decompostion. Much smaller amounts of sulfide from the reduction of sulfates may be present. The sulfide content of the cokes continued to increase above 700°, and elemental sulfur was detected only above this temperature. The form ation of other inorganic sulfides such as calcium sulfide and magnesium sulfide is possible under high temperature coking conditions. The presence·of such sulfides was indicated by the atomic ratio of sulfide sulfur to non-pyritic iron above about 750°C, where it exceeds unity. Further indication was the occurrence of sulfide sulfur to a much lesser extent J.n the 770tJ coke of the float coal which contained only about one half the mineral matter in the whole coal The results indicate that above about 700°CJ sulfide and elemental suJfu:r formation in crease in approximate proportion to the decrease of organic sulfur. It is :recalled that the maximum percent sulfur in organic form occutred at about 650°C and thereafter it rapidly decreased The relationship between the behavior of these sulfur forms may involve either the relative stability of organic sulfur at various temperatures of carbonization, or the relative reactivity of the car- 61 bon. If it is accepted that the organic coke sulfur occurs primarily as a carbon-sulfur surface complex, some idea as to its thermal stab ility can be ascertained by compal'ison with the known behavior of this type of complex prepared from pure reactants. Wibaut(57) reacted different types of carbons with sulfur vapor at high temperatures. Pure amorphou,s carbon heated at 700 ° to 900°C with sulfur gave a product from which sulfur was not removed by carbon disulfide or toluene extraction, nor by heating at 1000°C in high vacuum. At lower reaction temperatures, from 350° to 600°, fixed sulfur was again formed, but elemental sulfur was split off by heat- ing at 600° in vacuum, with only small quantities of carbon disulfide being formed. Diamond powder and Ceylon graphite heated at 600° in sulfur vapor for 24 hours did not react. Lewis and Metzner(30) fluidized charcoal (81% carbon) in a stream of nitrogen and sulfur vapor at temperatures of 300° to 600°C. A residue containing 30 to 40% sulfur was formed. When heated in nitro gen almost 75% of the sulfur was removed in elemental form. Removal of all the sulfur with hydrogen did not destroy the structure of the charcoal. They therefore conclude that the sulfur was added as a surface complex, partly chemisorbed and partly physically adsorbed. They agree with Wibaut that higher temperatures of formation cause the chemisorbed sulfur to become more firmly bonded to the carbon. It is apparent that the stability of these complexes is in fluenced by the following variables: (1) the nature of the carbon reactant, (2) the temperature of reaction, and (3) subsequent heat treatment in various atmospheres. Conditions existing during carbon ization would suggest that the organic coke-sulfur complex is quite stable, although conflicting factors do not permit a definite con clusion. The reactive nature of the carbon in the temperature range where most organic coke sulfur is apparently formed, about 500° to 700°C, would suggest a stable complex. The presence of mineral matter • and reactive gases may alter the stability. 62 The effects of complex stability versus carbon reactivity in determining the behavior of organic coke .sulfur can be differentiated by a comparison of 'the behavior of sulfur forms in the float coal with those in the whole coal during carbonization. The coals and cokes referred to are described in Tables 10 and 11,. The main differences in the distribution of sulfur forms in cokes from the respective coals were: (1) The atomic ratio of sulfide sulfur to non-pyritic iron was 0.47 in coke 571F compared to 0.46 in coke 564, but it increased to only 0.50 in coke 770F while increasing to 1.16 in coke 779. (2) Elemental sulfur was found in coke ?:779, . but not in coke 770F. (3) The residues from the volatile matter test at 950°C on cokes of the whole coal gave positive te.sts for free sul fur whereas those from the float coal did not. If the formation of free sulfur and of inorganic sulfides at high carbonization temperatures is attributable to decomposition and/or reaction of the organic coke sulfur because of decreased stability, it must be concluded that the organic sulfur in the 770° float coke did not undergo decomposition as the whole coal did. Be cause little difference is expected in the nature of the organic .substance of the two coal samples, and because its carbonization be havior in both coals was very similar, it is unlikely that the organ ic coke sulfur would be inherently more stable in the float coke than in cokes of the whole coal. Possibly the formation of elemental sulfur may be explained by the differences in the reactivity of the carbon at different carbon ization temperatures rather than by differences in the stability of the organic sulfur with temperature. The release of volatile matter during carbonization no doubt greatly increase.s the reactivity of the organic mass by creating free valence.s. This situation would exist at temperatures through the plastic range to about 600° or 700°C. Abov~_this temperature, the reactivity of the carbon would decrease as the free valences became satisfied by recombination within the 63 carbon, with mineral matter constituents, or by recombination with or ganic free radicals in the volatile matter. Under these conditions the sulfurous products from pyrite de composition, whether sulfur or sulfur-containing volatile matter, would readily react with the cariJoi1 below about 600° or 700°C, and less reaction would be expected at higher temperatures. This would suggest that elemental sulfur may be produced throughout the range of carbonization temperatures, but is detected only at higher temp eratures, where some remains uncombined with the carbon. Free sulfur has been found in cokes carbonized as low as 500°C as is shown for sample 8 in Table 13. This is the Pittsburgh Seam coal described above and pertinent to Figure 8. The sulfur and iron analyses show that a large amount of sulfate and non-pyritic iron are present in the coal, indicating that fairly extensive oxidation of the coal pyrite had occurred f.orming ferrous sulfate. Chatterjee (10) has stated that of the sulfur forms present in coal, pyrite forms free sulfur when oxidized. This coal was found to contain free sulfur. It is possible that a considerable fraction of the reactive carbon surface was also oxidized. In this case, the presence of surface complexes of carbon and oxygen would decrease the number of available reacti.ve sites for combination of the carbon with the sulfur, providing the carbon-oxygen complexes wf"re stable at carbonization temperature,.,, This in effect would reduce the reactivity of the carbon, resulting in the deposition of free sulfur at the lower temperature. Quantita tive data on the occurrence of elemental sulfur in cokes of this coal are not available. Differences in heating rates and soak time may also have affected the behavior of the sulfur. The variation in the reactivity of the coke with temperature may explain the observed differences in the carbonization behavior of the sulfur in the whole coal and float coal. The float coal contained only about one third the amount of pyrite in the whole coal. The percent organic sulfur, on the dry, ash-free, whole coal basis, in both float coal cokes and whole coal cokes was nearly the same at 64 both temperatures studied. This indicates that the same amount of sulfur from pyrite decomposition reacted with the carbon in both cases, but with the whole coal additional sulfur from pyrite decomposition could not be accommodated on the carbon at the higher. carbonization temperature. The large differences in mineral matter content is probably a factor in the much smaller quantity.'. of sulfides found .in coke· 77~F, compared to coke 770. With the whole coal, sulfide formation in creased rapidly only at higher temperatures where the reactivity of the carbonaceous matter was probably decreasing. This implies that the sulfur reacted preferentially with the carbon at lower temperatures and formed inorganic sulfides only when the carbon surface became less reactive. The preferential formation of carbon-sulfur bonds over iron-sulfur bonds is further indicated by the presence of iron, uncombined with sulfur, in the 770°C float coke. In contrast, the 779° coke of the whole coal contained more sulfide sulfur than non pyri tic iron. It has been pointed out that this does not necessarily mean that all of the non-pyritic iron was combined as the sulfide at this temperature, since other inorganic sulfides may account for a significant proportion of the sulfide sulfur. 65 Part II Quantitative Estimation of Elemental .Sulfur The use of benzoin for determination of elemental sulfur involves reaction with the sulfur to form hydrogen sulfide which can be easily detected using lead acetate paper. The reaction is: 000 0 0 fl I ,, ft s + C6H5-C-Cll-C6H5 = H2S + C6H5-c-c-C6H5 Based on the qualitative method given by Feigl and Stark(l3) for the detection of microgram quantities of free sulfur in the presence of organic and inorganic impurities, the benzoin reaction was selected as a possible method for quantitative determinaion of free sulfur in cokes. It was hoped that the method could be applied by reaction of the benzoin with sulfur in situ in the cokes. The results of tests using pure sulfur (U.S.P. grade, minimum pur ity 99.53) given in Table 14 .showed that the reaction between benzoin and sulfur was quantitative to within 1.053 error under the conditions used. These determinations also involved oxidation of the product hydrogen sulfide to sulfate using hydrogen peroxide. The sulfate was determined by precipitation as barium sulfate and using standard gravimetric procedure. Because of inherent filtering and weighing errors in the gravi metric method, which are magnified for small quantities of sulfate, a colorimetric method for sulfate determination at concentrations of 2 to 400 ppm was investigated and compared with the gravimetric deter mination. The procedure for the colorimetric method is given in Appendix III. The test results are given in Table 15. Although the percent error from the gravimetric method was less than that from the colorimetric method in these tests (0.723 versus 3.213) the accuracy of the latter method was adequate for this investigation. It was subsequently employed in the determinations of free sulfur in cokes because of its rapidity and applicability to a wide range of sulfur concentrations. The application of these methods to determining the elemental 66 sulfur content of cokes to which sulfur had been added in known quantity did not give reliable results under the conditions employed. The results in Table 16 indicate that the percent recovery ranged from about 30 to 1003. Further investigations to determine the var iables responsible for the inadequacies of the procedure were not carried out. Cau.ses for the non-quantitative results probably could be ascertained by a detailed study of the effects of particle .size, physical structure, and nature of the analytical sample, and of time and temperature of reaction with benzoin. These studies are suggested by the fact that satisfactory results were obtained when the sulfur was merely mixed with the coke, but unsatisfactory results were obtained for those methods of addition resulting in inclusion of the sulfu,r in the pore structure of the coke. Rather than embarking on such an investigation, solvent extrac tion of the sulfur was used, .since satisfactory results had been ob tained on pure sulfur samples. The solvent used was carbon disulfi~e which has been used in other work(30 1 57) to extract free .sulfur from carbonaceous samples, distinguishing it from bound sulfur. Direct weight determinations of the extracted sulfur was not attempted, because the amounts were small and large error.s could result from in clusion of extraneous matter. Therefore, the extracted sulfur was determined using benzoin-peroxide conversion and colorimetric sulfate determination. This procedure was used in test.s 15, 16, and 17 given in Table 16. Based on the results of these tests it was then used in the analysis of coke samples prepared and discu.ssed in the previous section of this thesis. This phase of the investigation confirmed the formation of free sulfur in the coking process. Although the determination of elemental sulfur in the standard coke samples was apparently successful using carbon disulfide extraction, the uncertainty as to the nature of the free sulfur in cokes does not permit unqualified acceptance of this method as a quantitative procedure. 67 V. SUMMARY AND CONCLUSIONS Low sulfur coke is required primarily for use in metallurgical appplications, notably blast furnace and foundry operations. The problem of producing low sulfur cokes necessitates an understanding of the behavior of sulfur forms during carbonization and of the nature of the sulfur in the coke. Inorganic coke sulfur can be quantitative~ ly determined and is fairly well characterized. Its formation and interactions with organic coke sulfur are not completely understood. Most worlio:ers now agree that organic coke sulfur is a surface complex of carbon and sulfur. Complexes of this type have been pre pared by reaction of elemental sulfur with various carbons. Elemental sulfur has not been previously identified nor determined in coke, although its presence has been suggested. Moreover, the mechanism of formation of the carbon-sulfur complex has not been satisfactorily explained. Nature of the Investigation This investigation was carried out in order to gain information on the behavior of sulfur forms during carbonization. Particular emphasis was placed on attempting to define more clearly the effects of primary and secondary reactions, involving the decomposition and formation of sulfur forms. Possible effects of mineral matter content were investigated. Tests were made to ascertain whether free sulfur was formed during carbonization, as has been suggested, and investiga tion of a method for its quantitative determination was made. On the basis of the data obtained, mechani.sms for the reactions of sulfur forms might be suggested, and the nature of the sulfur retained in coke might be more clearly understood. Procedure of the Investigation A carbonization apparatus was designed and constructed based on considerations which would make possible the differentiation of the effects of volatile matter release on sulfur behavior. The more im- portant features of the unit were: (1) use of a fine mesh stainless 68 steel wire screen basket to contain the coal sample during carboniza tion. This allowed lateral passage of volatiles from the sample with out diffusion through the entire charge. (2) use of a small coal bed width to minimize the length of diffusion of volatile matter through the sample and provide for a small lateral temperature gradient, (3) use of a preheated nitrogen stream at a relatively rapid flow rate to sweep away volatile carbonization products, and (4) use of a second basket-type container placed above the coal holder in the carbonizer tube where secondary reactions of tfie volatiles might take place. Cokes were prepared in the carbonization unit from a high sulfur Pittsburgh Seam, High Volatile A bituminous coal at tempertures be tween 360° and 885°C at about 100° intervals. The total carbonization time was one hour after the coal was placed in the preheated carbon izer. A float fraction of the coal, containing considerably less min eral matter and inorganic sulfur than the whole coal, was also carbon ized at 570° and 770°C. Proximate analyses, total sulfur, and sulfur and iron forms determinations were made on the coals and cokes. Elemental sulfur, which had not been previously determined in coals or cokes, was includ ed in the sulfur form determinations. This determination was based on a qualitative test for free sulfur utilizing benzoin to reduce the sul fur to hydrogen sulfide as reported by Feigl and .fftark.(13) Investigation of the use of the benzoin reaction to determine free sulfur in coal and cokes constituted a second major part of the study. Conditions for quantitative conversion of pure sulfur (U~S.P) to hydrogen sulfide were established. The hydrogen sulfide was sub sequently oxidized with hydrogen peroxide to sulfate, so that a rapid colorimetric method for sulfate determination could be utilized. The colorimetric method was compared with a standard gravimetric procedure \ for sulfate determination. The benzoin-peroxide conversion was then applied to cokes containing known amounts of elemental sulfur. Based on the results of a series of these tests, extraction of free sulfur using carbon disulfide was subsequently used. The extracted sulfur I 69 was determined by the procedure developed fox pure sulfur. Results of th$ Investigation The carbonization behavior of sulfur in the coals studied is summarized by the following observations. (1) Pyrite decomposed initially around 350°, decompQs;ttion was most extensive between 550"' and 650/, and was essentially,cqmpl,~t~d at 700°C. Pyrite decomposition was markedly affected by secondary reactions with volatile matter. Both the amount and rate of removal of volatiles apparently affect its decomposition. (2) Sulfate, present in small quantities in the parent coals was not found in the coke prepared above 650°C. (3) Sulfide formation increased with temperature. The amount of sulfide found below 700°C did not correspond with that predicted for reactions in which one mole of ferrous sulfide is formed for each mole of pyrite decomposed. Above 750°, the atomic ratio of sulfide sulfur to non-pyritic iron was greater than unity, indicating the existence of other inorganic sulfides, such as calcium sulfide and magnesium sulfide, and/or a complex iron sulfide. (4) The organic sulfur content decreased during carbonization. About 153 of the original organic sulfur was decomposed and evolved at 366°C. At 650° approximately 50 to 603 of the sulfur in the cokes of the whole coal was in organic form, compared to 36.43 in the coal. Above 650°the percent organic sulfur decreased, being about 113 of the total coke sulfur at 885°C. (5) Elemental sulfur was not detected in cokes from,, the whole coal prepared below 750°C. Above this temperature small quantities of free sulfur were found; the amount increased with temperature. Free sulfur was not detected in the cokes produced from the float .. fraction. (6) Iron, uncombined with sulfur was found in,cokes from the whole coal produced at temperatures between 350° and 650~C. The nature 70 of the data does not permit definite interpretations as to its exist ence above 650° in these cokes. The 770° coke of the float coal con tai-nea sulfur-free iron. The investigation of the use of benzoin in conjunction with the colorimetric sulfate determination for the determination of elemental _/ sulfur in cokes by the procedures previously given, resulted in the following information. (1) The reaction of benzoin with pure sulfur (U.S.P.) was quantitative to within 1.053 error in one hour at 160°C reaction temperature, using a 5 to 1 molar ratio of benzoin to sulfur. (2) The average percent error from the colorimetric determination in determining weighed amounts of pure sulfur was 3.213. (3) The application of the colorimetric procedure to the direct determination of elemental sulfur added to cokes using benzoin resulted in errors of up to G7%. (4) Carbon disulfide extraction of the sulfurtadded to cokes .. prior to its conversion to sulfate form, gave an average percent error of 0.863 by the colorimetric method. Conclusions (1) Reactions of volatiles with pyrite have a pronounced effect, not only on the amount of pyrite decomposed, but also on the formation of sulfides, organic sulfur, and elemental sulfur through secondary reactions. (2) Several mechanisms for the decomposition reactions of pyrite during carbonization are possible. At temperatures below 50Cf C, the mechanisms probably include reaction with hydrogen and other hydrogen ous gases to form ferrous sulfide and hydrogen sulfide, as suggested by early investigators. Above 5000C the primary decomposition of py rite to ferrous sulfide and elemental sulfur is possible. These two types of reaction can not fully explain the behavior of sulfur and iron forms. This is based on the fact that a stoichiometric relationship between sulfide sulfur!and non-pyriticiiron corresponding to the com- 71 position of ferrous sulfide 7 was .;ot found. The mechanisms proposed l>y Medvedev are supported by the results of this study. These include reaction pf pyrite with free radicals in the coal structure 7 and reaction of pyrite with atomic hydrogen. In the former 7 unstable carbon-iron bonds are formed along with elemental sulfur which can then replace the iron, resulting in a stable carbon-·sulfur combinati")n. In the latter mechanism, the un stable free radicals 0 SH and 0 SSH are formed and subsequently decompose or react with the coal constituents forming organic sulfur, elemental sulfur, inorganic sulfides, and hydrogen sul.fide. (3) Sulfur is transferred from pyrite to the carbon resulting in increased organic sulfur content. The transfer can take place by reaction of the carbon with any of the sulfurous products from pyrite decomposition. The release of primary volatile matter probably facilitates these reactions by creating a more reactive carbon sur face containing free valences. Apparently at temperatures below about 700°C, organic sulfur may be formed in preference to inorganic sulfides. With further increase i.n temperature the amount of organic sulfur decreases suggesting a decrease in the reactivity of the carbon. In the same temperature range, the formation of inorganic sulfides and free sulfur increases. (4) Ferrous sulfide is formed at low temperatures by decomposi tion of pyrite as descr.ibed above. Evidence has been found that at high carbonization temperatures, sulfides are possibly formed only when more sulfur is p:resent than can be accommodated by the carbon. This was suggested primarily by differences .in the carbonization behavior of the sulfur forms in the whole and float coals. (5) Free sulfur i.s formed during carbonization,, It may be the result of either reaction mechanisms proposed by Medvedev for the decomposition of pyrite. Depending on the nature and reactivity of the carbon at various stages of carbonizationy it may react to form a carbon-sulfur complex or remain in elemental form. Whether the free sulfur is amorphous, crystalline, or of some other nature is not known. The presence of mi.neral matter may affect the amount of free 72 sulfur which remains unreacted, since inorganic sulfides may be form ed by reaction of cations with it. (6) Elemental sulfur occurring in coal and coke can be detected by reaction with benzoin at 150 ° to 160°C and testing for the evolved hydrogen sulfide using lead acetate paper. (7) The direct application of the benzoin reaction to quantita tively determine free sulfur in situ in coal and coke is not reliable under the conditions studied. {8) Quantitative estimation of the elemental sulfur content of coal and 'coke can be made using carbon disulfide to extract the sulfur and subsequent reaction with benzoin. Until more is unde~stood of the nature of the elemental sulfur in the coal or coke, this procedure can not be assumed to be completely quantitative. (9) In the Powell method of sulfur form analysis(2) pyritic sul fur is determined by the amount of sulfate obtaj.ned on conversion by ni tri.c acid. This will probably include any free sulfur present and would give a higher pyritic sulfur value than is actually present. Since the modified Mott method of analysis utilizes volumetric deter mination of pyritic iron it would not suffer from this inaccuracy. The difference i.n the values obtained by the two methods might be an indication of the amount of free sulfur present. Suggestions for Future Research More information on the formation and stability of the carbon. sulfur complex under coking conditions is required, Of primary im portance is the effect of the nature of the carbon on complex form- at ion. Characterization of the internal surface and pore structure of a series of carbons or cokes produced throughout the entire car bonization temperature range should be ascertained and correlated to complex formation and stability. Sulfur reactivity data on the cokes might also be obtained. Pretreatment of coals or carbons, eg. partial oxidation, might offer a convenient method for varying the surface properties while retaining other characteristics of the carbon. 73 The mechanism for formation cf the carbon-sulfur complex might be deduced from a study involving adaition ui suilu~ or sulfur com- pounds to coal bef0i'e, during or after C<~rooni.zati.on. Their relative effects on the amount and type of sulfur retained in the coke could suggent the r~actions involved in forming organic sulfu:..· or other sulfur forms in coke. This study suggested that the amount and nature of mineral matter in coal may substantially affect the behavior of sulfur duri.ng carbon ization. Li.ttle work has been done in this field. It is known th.i.t addition of vari.ous metal and inorgani.c compounds can influence the amount of sulfur retained i.n coke. Detailed mineral matter analyses in conjunction with a carbonization study is suggested. A study of systems of carbon-sulfur-cation' would furnish complimentary data. The relative propolt"tions of inorganic and organi.c sulfur in coal can often be varied considerably usi.ng flotation or mechanical methods to remove pyrite in mineral matter. The original mineral matter con tent, less the pyrite, could be feasibly reconstituted by re-addition of appropriate inorganic compounds. In thi:::; way, only the ratio of inorganic to organic sulfur could be varied in the coal. A study of the distribution of sulfur during carbonization of a series of such prepared samples could more clearly define the contri.butions of the various sulfur forms to the final sulfur content of the carbonization pruaucts. Other methods of carbonization using a carbonizer si.milar to that used in this investigation could be utilized to further study the ef- fects of volatile matter on sulfur form behavior Carbonization in vacuum is one suggested techniqueo It is possible that the determinations of sulfur and iron forms in coke do not give a completely accurate picture of the forms present before the coke is cooled. This is, sulfur forms formed at the higher temperatures involved in carbonization may decompose or react during the cooling process. Rapid quenching to very low temperature, eg. liquid nitrogen, could be used to fix the :.r:eaction products prior to 74 analysis. The direct application of the benzoin reaction to quantitatively determine free sulfur in situ in coal and coke may be possible with a detailed study of the factors influencing the reaction. These facto1s probably include the nature and physical structure of the coke and reaction variables such as time and temperature. Parameters such as specific surface areas, pore distribution, and particle size of the analytical sample should be investigated. The effect of grinding should be studied. The nature of the sulfur itself may also affect the quantitativeness of the reaction. More knowledge as to the nature of the carbon-sulfur complex in coke might also be derived from such a study. AHdWOO rrnrn 75 (1) Armstrong, V. and Himus, G.W., Chem. & Ind., No. 17, 543-8 (1939) . (2) American Society for Testing Materials, Standards on Coal and Coke (1954) . (3) Baker Chemical Co. Product Data Sheet for Barium Chloranilate, No. 55801. (4) Bastick, M., Bull. Soc. Chim. France, 814-8 (1956); Chem. Abstr., 50, 11646c {1956) (5) Battye, T., Johnson, R., and Wilkinson, H. C., J. Appl. Chem. (London), i' 119-23 (19~4). (6) Bertolancini, R. J. and Barney, J. E. II, Anal. Chem., 29, 281-3 (1957). ~ (7) Ibid, 30, 202-5 (1958). (8) Blayden, H. E. and Mott, R. A., The British Coke Research Association Seventh Conference (November 7, 1956). (9) Bone, H. A. and Himu:s, G., "Coal, Its Constituti.on and Uses", Longmans, Green and Co., London, pp 140-162 (1936). (10) Chatterjee, N. N., Quart. J. Geol. Mining, Met. Soc. India, 44, 1-8 (1942); Chem. Abstr., 38, 25849 (1944). ~ (11) Evans, C. L., Gas Age, 95, 18-19 (1945). (12) Fanasaka, W., Coal Tar (Japan), 6, 139-43 (1954);Cited in Ref- erence (50). - (13) Feigl, F. and Stark, C., Anal Chem., 27, 1838 (1955). (14) Foerster, F. and Geissler, W., Z. Angrew. Chem.,~' 149 (1929). (15) Foxwell, G. E., Gas J., 217, 39-40 (1937). (16) Fuchs, W., Brenstoff-Chem., 32, 274-6 (1951). (17) Haarmann, A., Brenstoff-Chem., 37, 301-10 (1956). (18) Hoffert, W. H. and Wedtner, K., J. Inst. Petrol., 35, 171-92 (1949) . ~ (19) Hurd, C. D., "The Pyrolysis of Carbon Compounds", The Chemical Catalog Co., New York, pp 696-711 (1929). (20) Jacobson, C. A., "Encyclopedia of Chemical Reactions", Vol. IV, Reinhold Publishing Corp, New York, pp 93-96 (1951) . (21) Kipling, J. J., Science Prog~ess, 37, 657-69 (1949). (22) Kipling, J. J . , Fue 1, 29, 62-3 (1950). (23) King, J. G. and Edgcombe, L. T., Fuel,~' 213-8 (1930). (24) Kirk, R. E. and Othmer, D. F., Editor.s, "Encyclopedia of Chemical Technology", Interscience Encyclopedia Inc., New York, Vol. 2 p. 779 (1948) . 76 (25) Ibid., Vol. 8, p. 615 (1952). (26) Knapp, E. C., M. S. Thesis, Department of Fuel Technology, The Pennsylvania State University (1955). (27) Kulishenko, A. Z. and Medvedev, K. P., Izvest. Akad. Nauk SSSR (O.T.N.), No. 7, 145-9 (1955). (28) Leonet, G., Coke and Gas, 13, 326-7 (1951). (29) Lesoine, L. G., private communication. (30) Lewis, W. K. and Metzner, A. B., Ind Eng. Chem., 46, 849-58 (1954). ~ (31) Lowry, H. H., Editor, "Chemistry of Coal Utilization", John Wiley & Sons Inc., New York, pp 425-30 (1945). (32) Ibid., pp 444-9. (33) Lowry, H. H., Chem. & Ind., No. 38, Gl9 (1950). (34) Mainz, H., Gluckauf, 87, 1045-53 (1051); Chem. Abstr., 46, 4765i (1952). ' - - (35) Maglesdorf, T. A. and Broughton, F. P., Ind. Eng. Chem., 24, 1136-7 (1932). (36) Marrison, F. C. and Mott, R. A., Chem. & Ind., 715-6 (1953). (37) Marshall, E. E. and Draycott, A., "Petrographic, Chemical, and Utilization Studies of the Tangorin. High Organic Sulfur Seam, Greta Coal Measures, New South Wales", University of Sydney (1951). (38) M'Callum, A. L., Chem. Engr., g, 27-8 (1910) .. (39) Medvedev, K. P. and Petrapolskaya, V. M., Izve::;t. Akad. Nauk SSSR (O.T.N.), No. 8, 134-9 (1955). (40) Medvedev, K. P., Izvest Akad. Nauk SSSR (O.T.N.), No. 4, 75-84 (1956) . (41) Mott, R. A., Coke & Gas, 12, 369-70 (1950). (42) Mott, R. A., Gas World, 132, 66-7 (1950). (43) Mott, R. A., Fuel, 29, 53-61 (1950). (44) Owen, A. J. et. al., Trans. Faraday Soc., 49, 1198-1206 (1953) . (45) Polansky, T. S. and Anderson, R. A., "Interim Research Report The Reduction of Sulfur in Coal During Carbonization", Working Report WR-5 to the Coal Researcp Board of the Commonwealth of Pennsylvania from the College of Mineral Industries, The Penn sylvania State University (September 1, 1959). (46) Polansky, T. S. and Anderson, R. A., unreported results. (47) Powell, A. R., Ind. Eng. Chem., _g, 10139-77 (1920). (48) Powell, A. R., Ind. Eng, Chem., _g, 1077-81 (1920). 77 (49) Powell, A. R. ,.:-J. Amer. Chem. Soc., 45, 1-15 (1923). (50) Prien, C. H. , Ind. Eng. Chem., 47, 1911-5 (1955). (51) Report of Bituminous lresearch Activities, Serial No. 57, Depart ment of Fuel Technology, The Mineral Industries Experiment Station, The Pennsylvania Sta:te University (March 12, 1957). (52) Scortecci, A. and Scortecci, M., Metal Progr~ss, 60, 72-5 (1951). (53) Snow, R. D., Ind. Eng~ Chem., 24, 903-9 (1932). (54) Terres, E., Brennstoff-Chem., 35, 225-31 (1954). (55) Thiessen, G., Ind. Eng. Chem., ~· 473-8 (1935). (56) van Hees, W. and Early, E., Fuel,~. 425-8 (1959). (57) Wibaut, J. P., Proc. 3rd International Conference on Bituminous Coal, Vol. 1, 657-73 (1931). (58) Woolhouse, T.,G., Fuel, 14, 259-65 (1935). (59) Ibid., 286-95. (60) Zielke, C. W. et. al., Ind. Eng. Chem., 46, 53-6 (1954) S:?I:::HCTN:!lddV 78 APPENDIX I Carbonizer Operational Data The carbonizer was preheated to the carbonization temperature before admitting the coal sample. The average heating rate during carbonization was about 40° per minute. Temperature gradients were calculated from the temperatures readings at the top and bottom of the coal sample and adsorbent sample. The temperatures of the coke and adsor<_;ent were taken as the average of the temperatures between the bottom and top of the respective sample holders. The table below summarizes this data. Sample Tcz°C (1) Ta 1 °C (2) Gcz°C/inch (3) Ga,°C/inch (4) 3 364 - 4.7 6 366 344 6.0 2.0 8 464' 433 5.8 1.3 12 564 559 7.1 6.0 14 550 15 571 566 2.2 17 668 664 6.2 5.5 19 670 670 1.6 0.0 22 784 765 2.7 4.0 24 779 769 2.4 0.0 26 770 756 4.0 3.8 28 885 371 2.0 (1) Average coke temperature. (2) Average adsorbent temperature. (3) Vertical coke temperature gradient. (4) Vertical adsorbent temperature gradient 79 APPENDIX II Modified Mott Method of Sulfur Form Analysis A two-gram sample of minus 60-mesh coke is weighed into a 250-ml conical flask fitted with a vertical Liebig condenser and inlet for nitrogen gas. Fifty ml of 5N hydrochloric acid are added, the con J denser quickly put in place and the coke-acid mixture boiled for 30 minutes. The evolved hydrogen sulfide is swept into an absorbent I flask containing 75 ml of l :13 aqueous ammonium hydroxide. Non-pyritic iron: t The above mixtl'.re is filtered on Whatman #30 filter paper, washed ~'-. I several times with a total volume of 20 ml of 1:20 aqueous·hydro ~ chlorl.c acid, and 6 times with distilled water. The filtrate is boil ed with 1 ml of saturated bromine water in a 400-ml beaker until excess bromine is expelled. The non-pyritic iron is precipitated as ferric t' hydroxide using 1:1 aqueous ammonium hydroxide. The ferric hydrox i!ic is f.:lltered on Whatman #42 filter paper, washed with hol: dis tilled water, redissolved in 20 ml of 1:4 aqueous sulfuric acid, and determined by titration with potassium dichromate solution (ca. 0.05N) using diphenylamine sulfonate indicator,. The volumetric method can be found in standard texts .on analytical chemistry. Sulfate Sulfur: The filtrate from above is acidified to methyl orange with 1:2 aqueous hydrochloric acid and the sulfate is precipitated by addi'" tion of 10 ml of 103 barium chloride solution. After at least 3 hours digestion at 60°C, the barium sulfate is filtered on Whatman #42 paper, washed, ignited at 850°C, and wei-ghed. Pyritic Iron: The coke residue from the above determinations is replaced into the 250-ml conical flask used for the hydrochloric acid t:xtraction above. Fifty ml of 2N nitric acid are added and the mixture is boil ed for 30 minutes with the condenser in place as before. The mixture is filtered on Whatman #30 filter paper and the filtrate is treated in the same manner as in the non-pyritic iron determination, except 80 that the bromine oxidation is eliminated. Sulfide Sulfur (Non-pyritic): The sulfide in the ammoniacal absorbent used in the initial step It ~- with the hydrochloric acid extraction is determined by the following I procedure. The absorbent solution is transferred quantitatively to ' a 250.-ml beaker and 25 ml of standard (ca. O.lN) silver nitrate are added. The resultant silver sulfide precipitate is digested at room temperature overnight in a dark compartment and filtered on Whatman #42 filter paper. The filtrate is acidified with 35 ml of 5N nitric acid, and the excess silver ion titrated with standard (ca. O.lN) potassium thiocyanate. Calculations: (ml x N) dichromate x 5.585 3 Fe :::: weight sample 3 Pyritic Sulfur 1.15 x 3 Pyritic Iron g Bari.um Sulfate x 13. 735 % Sulfate Sulfur weight sample ~.604 x (ml AgNO x N AgNOa - ml KCNS x N KCNS) 3 Sulfide Sulfur weigh~ sample 81 APPENDIX III Colorimetric Sulfate Determination The details of this method are availbable in references (3), (6), and (7). Briefly, the method depends on the reaction between barium chloranilate (barium salt of 2, 5-dlchloro-3,9-dihydroxy-p-quinone)~ and sulfate ion in acid alcoholic solution, producing a dark purple acid~chloranilate ion. The absorbence of the purple solution, which is propottional to the sulfate concentration, may be measured color imetrically or spectrometrically. The method is applicable to sulfate concentrations in the range of 2 to 400 ppm. Procedure: Standard sulf~te solutions are prepared by dissolving accurately weighed samples of potassium sulfate, reagent grade, i.n distilled water and diluting exactly to the desired concentration in the range of 2 to 400 ppm. An aliquot of less than 40 ml c~ntaining not more than 40 mg of sulfate is placed in a 100-ml beaker. The pH is ad justed to 4 with 1:2 aqueous hydrochloric acid using pH paper. The solution is transferred to a 100-ml volumetric flask. Any remaining solution is transferred into the flask with 10 ml of buffer solution (.05M potassium hydrogen phthalate, pH 4) and 50 ml of 953 ethanol. The total volume is brought to exactly 100 ml wi.th distilled water. About 0.3 g of barium chloranilate is added and the flask is shaken for 20 minutes, after which the solution iS filtered through Whatman #42 filter paper. The filtrate is caught in the photelometer cell and the absorbance read versus a blank stmj.larly prepared. The calibration cu:rve prepared from the standard solution ab sorbences is given in Figure 9. The log relative intensity is plotted versus concentration of sulfate i.n ppm. Unknown solutions are treated as were the standards, and the concentration read from the calibration curve. 82 APPENDIX IV Determination of Elemental Sulfur in Carbonized Samples An accurately weighed coke sample of about one gram is placed in a 250-ml conical flask with 20 ml of carbon disulfide. The mix ture is allowed to stand 45 minutes in the stoppered flask with oc casional shaking. The carbon disulfide is decanted off through What man #30 filter paper into the tube subsequently used for the reaction with benzoin. The coke is again treated with 20 ml of ca.rbon disul fide, while the first filtrate i.s evaporated off in a stream o:t air~ The coke is transferred to the filter paper and washed with IO ml of carbon disulfide, catching the filtrate and wash in the reaction tube. The filtrate i.s evaporated to "dryness" as before and 0. 5 g of benzoin is added to the tube and the apparatus connected as shown in Figure 3, except that the second absorption tube is eliminated. The first absorption tube cont~ins 35 ml of 1:6 ammonium hydrox ide (aqueous) and 5 ml of 303 hydrogen peroxide. The reaction tube is placed in the Fieldner furnace which has been preheated to 20Q°C and is heated for 1.5 hours. After 5 minutes heating, the system is swept for the remainder of the run by a slow nitrogen flow, measured at about 120 bubbles per minute in the absorption tube. The absorption tube is then removed, placed in a water bath, and tne contents heated slowly to boiling. The solution is boiled until excess peroxide is decomposed, about one hour total heating time. Tbe resultant sulfate solution is determined using the colorimetric procedure in Appendix III. j 1.0 0.9 0.8 0.7 0.6 0 ...... 0.5 ~- in zw I- z 0.4 w > i= .... 0.2 0.1 --~~~....-~~---.~~~-r-~~~--~~~--~~~~~~- 400 300 200 100 0 SULFATE CONCENTRATION, ppm CALIBRATION CURVE FOR COLORIMETRIC SULFA TE DETERMINATION FIGURE 9