Fe2o3-Based Oxygen Carriers for Gaseous and Solid-Fueled Chemical Looping Processes
Fe2O3-based Oxygen Carriers for Gaseous and Solid-Fueled Chemical Looping Processes
DISSERTATION
Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University
By
Ankita Majumder
Graduate Program in Chemical Engineering
The Ohio State University
2016
Dissertation Committee:
Professor Liang-Shih Fan, Advisor
Professor David L. Tomasko
Professor Andre F. Palmer
Copyrighted by
Ankita Majumder
2016
Abstract
Chemical looping is an efficient, economic and sustainable means for electricity and/or chemicals production with inherent CO2 sequestration ability. Oxygen carriers play a crucial role in the successful operation of a chemical looping system as their physical and chemical properties dictate the fuel conversion efficiency of the system. They are expected to undergo multiple redox cycles while maintaining their reactivity and mechanical strength in order to improve the overall process economics for commercial viability. This research investigates the behavior of oxygen carriers under different reactive conditions and evaluates their feasibility for biomass chemical looping systems.
The reduction kinetics of OSU’s iron titanium complex metal oxide (ITCMO) oxygen carrier particles are investigated at elevated pressures with H2 and CH4 for application in
OSU’s Shale gas-to-Syngas process. Under CH4, there is almost a 5-fold increase in the reduction rate with an increase in pressure from 1 to 10 atm. Solid characterization revealed increased porosity and surface area at elevated pressures. Faster reaction kinetics at higher pressures can translate into increased processing capacity, reduced reactor sizing, and decreased capital costs. The steam to H2 conversion efficiency of Fe2O3 based oxygen carriers using Al2O3, MgAl2O4 and TiO2 as support materials is investigated in a fixed bed for chemical looping H2 generation. All supported-Fe2O3 based oxygen carriers exhibited >70% steam conversion, close to thermodynamic predictions. Due to its ability to not form complexes with the active material, MgAl2O4-supported Fe2O3 was selected ii for further investigation. Thermogravimetric studies with steam oxidation exhibited excellent recyclability and no significant drop in reactivity. MgAl2O4-supported Fe2O3 also exhibited enhanced steam oxidation kinetics at elevated pressures. Tar derived from biomass pyrolysis is a major concern for biomass thermochemical conversion processes.
For biomass fueled chemical looping processes, it is important to evaluate effects of tars on the oxygen carriers. Fixed bed experiments demonstrated that OSU’s ITCMO oxygen carriers have reasonable reactivity for cracking most biomass-derived tar components. To further enhance the tar cracking ability of Fe2O3-based oxygen carriers, they are combined with traditional tar cracking catalysts. Based on thermogravimetric reactivity and fixed bed tar cracking experiments, NiO is selected as an additive for Fe2O3-based oxygen carriers for biomass chemical looping systems. The outcomes from this research will help in the development of economic and efficient oxygen carriers for the commercialization of the various chemical looping applications.
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Dedicated to my parents for their immeasurable love, support and encouragement
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Acknowledgements
I would like to begin by thanking The Ohio State University and the William G. Lowrie
Department of Chemical and Biomolecular Engineering for giving me the opportunity to work in such state-of-the-art facilities. Having easy access to all the excellent university resources and infrastructure has played a huge role in making my academic experience enjoyable and seamless. I would like to express my deepest gratitude and appreciation for my advisor, Dr. Liang-Shih Fan, for constantly encouraging me to strive for excellence in my work. His boundless enthusiasm and thirst for learning has been a constant source of inspiration for me and his constructive feedback has always helped me grow as a researcher as well as a person. I would like to thank Prof. David Tomasko, Prof. Andre
Palmer, Prof. Jacques Zakin, and Prof. Lisa Hall for serving on my qualifier, candidacy and dissertation committees. Their discussions and feedback provided me with new ideas and directions for my research. I am grateful to the National Science Foundation (NSF) and the United States Department of Energy (US DOE) for their financial support for the projects I have been a part of during this academic career.
The Fan group is full of extremely talented, hardworking and intelligent individuals and I consider myself fortunate to be a part of this group. I would like to thank my seniors, Dr.
Liang Zeng, Dr. Siwei Luo, Dr. Andrew Tong, Dr. Zhenchao Sun and Dr. Dawei Wang for their guidance and support and for always being available for discussions. Their
v invaluable insights have helped shape up my research in many ways. Dr. Niranjani
Deshpande, Dr. Samuel Bayham, Elena Chung, William Wang and Cheng Chung have not only been incredible co-workers, but also great friends, who have provided constant support and made work very enjoyable. It has been a great experience working with all the other Fan group members as well, which include Dr. Lang Qin, Dr. Zhuo Cheng, Dr.
Pengfei He, Dr. Qiang Zhou, Dr. Aining Wang, Omar McGiveron, Alan Wang, Mandar
Kathe, Dikai Xu, Tien-Lin Hsieh, Sourabh Nadgouda, Amoolya Lalsare, Yaswanth
Pottimurthy, Yitao Zhang, and Mengqing Guo. Every member of the group has in many ways helped me grow as researcher and made my experience invaluable. I would also like to mention Camille Ayala and Nicholas Justus, undergraduate researchers who I had the opportunity to mentor. Their keen interest in the work and their insightful questions and suggestions helped me in my research as well.
I want to take this opportunity to thank the technical staff at OSU including Paul Green,
Michael Wilson, Hendrik Colijn, and Cameron Begg for their willingness to help and provide ideas to help solve any technical problems. A special thanks to Angela Bennett,
Susan Tesfai, and Lynn Flanagan for their assistance in completing all my administrative tasks seamlessly in a timely manner. I would like to thank Prof. Bhavik Bakshi and Prof.
Kurt Koelling, who guided me in the completion of my teaching assignments.
I would like to thank all my friends, who have been like a family to me and filled these past few years with a lot of love and laughter. I would like to especially mention my friend Pooja, who has been there for me at all times, even from a distance. Lastly, I want to take this opportunity to thank my parents, Mrinmoy and Krishna Majumder, for their
vi unconditional love and support to grow into the person I am today. They have and continue to remain my pillars of strength and keep me going. Finally, a special thank you my fiancé, Hrishikesh, for always being by my side and encouraging me to strive for more.
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Vita
June 2005 ...... S.S.C., Fr. Agnel Multipurpose High School
June 2007 ...... H.S.C., Fr. Agnel Junior College
June 2011 ...... B.Chem., Institute of Chemical Technology,
Mumbai University
Sept 2011 to present ...... Graduate Research Associate, Department of
Chemical and Biomolecular Engineering, The
Ohio State University
Publications
Luo, S., Majumder, A., Chung, E., Xu, D., Bayham, S., Sun, Z., Zeng L. and Fan, L.-S. (2013). Conversion of Woody Biomass Materials by Chemical Looping Process: Kinetics, Light Tar Cracking, and Moving Bed Reactor Behavior. Ind. Eng. Chem. Res. 52 (39), 14116-14124.
Qin, L., Majumder, A., Fan, J. A., Kopechek, D., and Fan, L.-S. (2014). Evolution of nanoscale morphology in single and binary metal oxide microparticles during reduction and oxidation processes. J. Mat. Chem. A. 2 (41), 17511-17520.
Deshpande, N., Majumder, A., Qin, L., and Fan, L.-S. (2015). High-Pressure Redox Behavior of Iron Oxide-Based Oxygen Carriers for Syngas Generation from Methane. Energy Fuels, 29 (3), 1469-1478.
Majumder, A., Deshpande, N., Zhang, Y., and Fan, L.-S. Investigation of Al2O3, TiO2 and MgAl2O4 as support materials for Fe2O3-based oxygen carriers for chemical looping hydrogen generation. (In preparation).
viii
Majumder, A., Ayala, C., Chung, E., and Fan, L-S. Study of Fe2O3-based composite oxygen carriers for in situ tar cracking in biomass chemical looping. (In preparation).
Luo, S., Zeng, L., Xu, D., Kathe, M., Chung, E., Deshpande, N., Qin, L., Majumder, A., Hsieh, T.-L., Tong, A., Sun, Z., and Fan, L.-S. (2014). Shale gas-to-syngas chemical looping process for stable shale gas conversion to high purity syngas with a H2: CO ratio of 2:1. Energy Environ. Sci. 7(12), 4104-4117.
Luo, S., Bayham, S., Zeng, L., McGiveron, O., Chung, E., Majumder, A., and Fan, L. S. (2014). Conversion of metallurgical coke and coal using a coal direct chemical looping (CDCL) moving bed reactor. Applied Energy, 118, 300-308.
Bayham, S. C., Kim, H. R., Wang, D., Tong, A., Zeng, L., McGiveron, O., Kathe, M. V., Chung, E., Wang, W., Wang, A., Majumder, A., and Fan, L.-S. (2013). Iron-based coal direct chemical looping combustion process: 200-h continuous operation of a 25- kWth subpilot unit. Energy Fuels, 27(3), 1347-1356.
Fields of Study
Major Field: Chemical Engineering
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Table of Contents
Abstract ...... ii
Acknowledgements ...... v
Vita...... viii
List of Figures ...... xiv
List of Tables ...... xxi
1. INTRODUCTION ...... 1
1.1. Chemical Looping Classification ...... 2
1.1.1. Chemical Looping Combustion (CLC) ...... 3
1.1.2. Chemical Looping Partial Oxidation (CLPO)...... 3
1.1.3. Solar Chemical Looping ...... 4
1.2. The Ohio State Chemical Looping Processes ...... 4
1.2.1. OSU Chemical Looping Combustion ...... 5
1.2.2. OSU Chemical Looping Partial Oxidation ...... 7
1.3. Oxygen Carrier Materials Selection ...... 10
1.4. Oxygen Carrier Reactivity ...... 14
1.4.1. Single Metal Oxides ...... 14
1.4.2. Complex and Mixed Metal Oxide Based Materials ...... 23
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1.5. Oxygen Carrier Recyclability and Strength ...... 25
1.6. Thesis Outline ...... 27
2. HIGH PRESSURE REDOX BEHAVIOR OF IRON-OXIDE BASED OXYGEN
CARRIERS ...... 39
2.1. Introduction ...... 39
2.2. Syngas generation at elevated pressures: thermodynamic analysis ...... 43
2.3. Experimental setup, Materials and Procedure ...... 47
2.4. Results and Discussion ...... 49
2.4.1. Reduction in H2 ...... 49
2.4.2. Reduction in CH4 ...... 53
2.4.3. Pressure correction ...... 56
2.4.4. Air oxidation ...... 57
2.4.5. XRD, SEM, EDS, and BET analysis ...... 59
2.5. Concluding Remarks and Future Work ...... 61
3. STEAM OXIDATION OF Fe2O3-BASED OXYGEN CARRIERS ...... 79
3.1. Introduction ...... 79
3.2. Materials ...... 84
3.3. Experimental Methods ...... 85
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3.3.1. Redox with Air ...... 85
3.3.2. Fixed Bed Steam Oxidation ...... 86
3.3.3. Redox with Steam ...... 88
3.3.4. Solid Characterization and Gas Analysis ...... 89
3.3.5. Thermodynamic Analysis ...... 90
3.4. Results and Discussion ...... 90
3.4.1. Redox cycles with air oxidation ...... 90
3.4.2. Fixed bed hydrogen generation ...... 92
3.4.3. Solid Characterization and Phase Evolution ...... 94
3.4.4. Thermodynamic Analysis ...... 97
3.4.5. Redox cycles with steam oxidation ...... 98
3.4.6. Effect of Pressure ...... 99
3.5. Concluding Remarks and Future Work ...... 100
4. BIOMASS CONVERSION AND TAR CRACKING WITH Fe2O3 BASED OXYGEN
CARRIERS ...... 119
4.1. Introduction ...... 119
4.2. Materials ...... 126
4.3. Experimental Procedure ...... 127
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4.3.1. Biomass Decomposition Kinetics ...... 127
4.3.2. Oxygen Carrier Reactivity Screening ...... 128
4.3.3. Fixed Bed Tar Cracking ...... 128
4.3.4. Oxygen Carrier Characterization ...... 130
4.4. Results and Discussion ...... 130
4.4.1. Biomass Decomposition Kinetics ...... 130
4.4.2. Fixed Bed Tar Cracking with ITCMO ...... 135
4.4.3. Composite Oxygen Carrier – Redox Performance ...... 137
4.4.4. Composite Oxygen Carrier – Tar Cracking ...... 139
4.4.5. Composite Oxygen Carriers – NiO Loading ...... 141
4.4.6. Solid Characterization ...... 142
4.5. Concluding Remarks and Future Work ...... 143
REFERENCES ...... 163
APPENDICES ...... 178
A. HIGH PRESSURE AND STEAM EXPERIMENTS ...... 179
B. SUPPLEMENTAL INFORMATION FOR BIOMASS STUDIES ...... 194
C. PERMISSION FROM JOURNAL PUBLICATION ...... 202
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List of Figures
Figure 1.1: Current and projected world energy consumption by source ...... 32
Figure 1.2: Thermodynamic phase diagram of the iron-CO-CO2 system ...... 33
Figure 1.3: Flow schematic for the CDCL process for generation of electricity from coal ...... 34
Figure 1.4: Flow schematic of the SCL process for the cogeneration of H2 and electricity...... 35
Figure 1.5: Flow schematic of the STS process for partial oxidation of CH4 to syngas ...36
Figure 1.6: Ellingham diagram with sections of chemical looping operation ...... 37
o Figure 1.7: Cyclic redox reaction studies for pure Fe2O3 powder at 900 C using H2 and O2-rich gases ...... 38
Figure 2.1: Schematic of Fe-oxide based system for syngas generation from partial oxidation of CH4 ...... 63
Figure 2.2: Simulated equilibrium iron oxide phases and fractional carbon deposition as a function of inlet gas to solid ratios at elevated pressure...... 64
Figure 2.3: Schematic of the experimental setup of the MSB ...... 65
Figure 2.4: The effect of total system pressure on rates of reduction at X = 0.75, and
o constant partial pressure of H2. T = 900 C ...... 66
Figure 2.5: The effect of mole fraction of reducing gas (YH2) on rates of reduction at
X = 0.75, and constant partial pressures. T = 900oC ...... 67 xiv
Figure 2.6: The effect of system pressure on rates of reduction at X = 0.5 and X =
o 0.75, and constant mole fraction of reducing gas YH2 = 50%, T = 900 C ...... 69
Figure 2.7: The effect of partial pressure of reducing gas PPH2 on (a) conversion curves obtained and (b) rates of reduction at constant system pressure P = 5 atm. T =
900oC...... 70
Figure 2.8: Reduction conversion curves obtained using CH4 from the thermogravimetric analysis between 1 and 10 atm at constant mole fraction of
o reducing gas YCH4 = 50%. T = 950 C ...... 71
Figure 2.9: The effect of system pressure on reaction rate for the three-step reduction
o with CH4 as the reducing gas. YCH4 = 50%, T = 950 C, P = 1 to 10 atm ...... 72
Figure 2.10: Pressure data and Reduction conversion obtained based on the original data and the data obtained by applying pressure correction. Reducing gas = CH4 with
o YCH4 = 50%, T = 950 C, and P = 8 atm...... 73
Figure 2.11: Oxidation conversion curves obtained from the thermogravimetric
o analysis between 1 and 10 atm at 푌푂2 = 0.1, T = 900 C ...... 74
Figure 2.12: XRD analysis of (a) reduced and (b) oxidized samples at 1 and 10 atm,
o 900 C. Reducing environment is under H2 with 푌퐻2 = 0.5, and oxidizing environment is air ...... 75
Figure 2.13: SEM and EDS elemental mapping of cross sections of reduced particles.
o Samples reduced under H2, 푌퐻2 = 0.5 and T= 900 C. (a) 1 atm and (b) 10 atm ...... 76 xv
o Figure 2.14: Surface grains in samples reduced under H2, 푌퐻2 = 50% and T= 900 C.
(a) 1 atm and (b) 10 atm ...... 77
Figure 3.1: Schematic of Chemical Looping Hydrogen Generation process ...... 102
Figure 3.2: Schematic of experimental setup for fixed steam oxidation tests ...... 103
Figure 3.3: Schematic of experimental setup of the magnetic suspension balance for the recyclability studies using steam...... 104
Figure 3.4: TGA curves of the 20 redox cycles for (a) MgAl2O4-supported Fe2O3; (b)
TiO2-supported Fe2O3; (c) Al2O3-supported Fe2O3; and (d) Unsupported Fe2O3 ...... 105
Figure 3.5: % Reduction achieved in each redox cycle for the supported Fe2O3 during
20 cycle recyclability test...... 106
Figure 3.6: % Conversion of oxygen carrier during 15th redox cycle (a) Reduction;
(b) Oxidation ...... 107
Figure 3.7: Fixed bed outlet H2 concentration with different oxygen carrier formulation (a) Unsupported Fe2O3; (b) TiO2-supported Fe2O3; (c) Al2O3-supported
Fe2O3; and (d) MgAl2O4-supported Fe2O3 ...... 108
Figure 3.8: Total H2 generated during fixed bed experiments with different oxygen carrier formulations ...... 109
Figure 3.9: Cumulative H2 generated over time during fixed bed steam oxidation with different steam space velocities (a) 0.36 hour-1; (b) 0.72 hour-1 ...... 110
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Figure 3.10: Steam to H2 conversion efficiency of the different oxygen carriers during fixed bed steam oxidation experiments...... 111
Figure 3.11: XRD analysis of the oxygen carriers – fresh and reoxidized after single
o redox cycle with H2 reduction and steam oxidation at 900 C (a) Unsupported Fe2O3;
(b) Al2O3-supported Fe2O3; (c) TiO2-supported Fe2O3; and (d) MgAl2O4-supported
Fe2O3 ...... 112
Figure 3.12: Thermodynamic step-diagram of steam and solid conversions of different oxygen carrier formulations at 900oC, 1 atm ...... 115
Figure 3.13: Weight change of MgAl2O4-supported Fe2O3 sample during 20 redox
o cycle TGA analysis with H2 reduction and steam oxidation at 900 C ...... 116
Figure 3.14: Oxidation conversion values for MgAl2O4-supported Fe2O3 during 20 redox cycle recyclability test ...... 117
Figure 3.15: Effect of pressure on %weight gain vs time during steam oxidation of
MgAl2O4-supported Fe2O3 ...... 118
Figure 4.1: Biomass Chemical Looping Gasification system with high purity hydrogen generation and in-situ CO2 capture ...... 146
Figure 4.2: Dissociative adsorption of phenol over iron oxide139 ...... 148
Figure 4.3: Pyroprobe and fixed bed reactor setup coupled with GC-MS to analyze the product gases...... 150
Figure 4.4: Kissinger plot for biomass devolatilization under N2 and CO2 atmospheres 151 xvii
Figure 4.5: Plots for calculating the kinetic parameters of char gasification. (a) DTG curves for isothermal gasification of char under CO2 atmosphere. (b) Arrhenius plot for isothermal gasification of char under CO2 atmosphere...... 152
Figure 4.6: Arrhenius plots for rate constants of devolatilization and gasification reactions under CO2 atmosphere ...... 153
Figure 4.7: Mass spectrometry data from fixed bed cracking of biomass derived tar
(a) Without oxygen carriers; and (b) With ITCMO particles ...... 154
Figure 4.8: Oxygen transfer capacities of the different oxygen carrier formulations from the TGA experiments ...... 155
Figure 4.9: Redox recyclability of different oxygen carrier formulations from the 20 redox cycle TGA experiments ...... 156
Figure 4.10: GC-MS data from the analysis of the product gas from the fixed bed experiments with different oxygen carriers ...... 157
Figure 4.11: Product chromatogram from the fixed bed experiments comparing reactivity of supported-Fe2O3, with and without NiO ...... 158
Figure 4.12: Product chromatogram from fixed bed cracking of Naphthalene with different composite oxygen carriers ...... 159
Figure 4.13: Product chromatograms from fixed bed experiments comparing reactivity of Fe2O3-based oxygen carriers with NiO content varying from 1-15wt% .....160
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Figure 4.14: XRD spectra of fresh and reacted Fe2O3 based oxygen carrier with 5%
NiO ...... 161
Figure 4.15: SEM images of fresh unreacted Fe2O3 based oxygen carriers with (a)no
NiO; and (b)1wt% NiO ...... 162
Figure A.1: Experimental setup of MSB (Rubotherm GmbH, US-2004-00162) with gas manifold ...... 180
Figure A.2: Working principle of Rubotherm MSB ...... 181
Figure A.3: XRD spectra of steam oxidized samples of MgAl2O4-supported Fe2O3 (a)
1 atm; and (b) 5 atm ...... 183
Figure A.4: XRD spectra of MgAl2O4-supported Fe2O3 (a) Fresh; and (b) After 20
o redox cycles using H2 for reduction and steam for oxidation at 900 C ...... 184
Figure A.5: SEM images of the MgAl2O4-supported Fe2O3 (a) Reduced at 1atm; (b)
Steam oxidized at 1 atm; (c) Reduced at 5atm; (d) Steam oxidized at 5atm; (e) Fresh unreacted; and (f) After 20 redox cycles with H2 and steam ...... 185
Figure A.6: Comparison of rates of reoxidation of ITCMO particles using steam at different pressures ...... 187
Figure A.7: Schematic of cocurrent RGIBBS module...... 192
Figure A.8: Thermodynamic phase diagram of the Fe-H2-H2O system ...... 193
Figure B.1: GC-MS spectrum of biomass derived volatiles without oxygen carriers in fixed bed...... 195 xix
Figure B.2: GC-MS spectrum of napthalene from pyroprobe, without oxygen carriers in fixed bed...... 196
Figure B.3: Experimental setup of fixed bed for biomass derived tar cracking with in- line gas analyzers...... 197
Figure B.4: Arrhenius plot for isothermal gasification of coal derived char under CO2 atmosphere ...... 200
Figure B.5: Comparison of char gasification rates with and without oxygen carriers ....201
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List of Tables
Table 2.1: 푌퐻2 as a function of total system pressure and 푃푃퐻2for Section 2.4.1.1...... 68
Table 3.1: Summary of different iron phases found in steam reoxidized oxygen carrier samples during fixed bed experiments…………………………………………………. 114
Table 4.1: Comparisons of BCL with Biomass IGCC systems with and without carbon capture135……………………………………………………………………….. 147
Table 4.2: Composite oxygen carrier formulations……………………………………. 149
Table A.1: Reactor model setup for simulation of steam oxidation reactions…………. 190
Table A.2: List of chemical species considered for simulation………………………... 191
Table B.1: Ultimate analysis of the biomass sample used for this tar cracking and oxygen carrier development studies…………………………………………………….198
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CHAPTER 1
1. INTRODUCTION
In the next 25 years, the global energy consumption has been projected to increase by
37%.1,2 The current and future energy consumption values are presented in Figure 1.1. As seen in the figure, fossil fuels will still continue to account for about 75% of the total energy demand into 2040. With the increasing concern over emission of greenhouse gases (GHGs) like carbon dioxide (CO2), it has become imperative to develop cost effective and efficient CO2 capture technologies that can be integrated with fossil fuel power plants. Most of the current CO2 capture technologies when integrated into power plants result in high parasitic energy requirements and hence increased fuel consumption, decreased process efficiency, and increased price of electricity with carbon capture. The
U.S. Department of Energy’s (US DOE) road map for CO2 capture technologies compares current and future technologies in terms of their readiness and their cost reduction benefits.3 The current techniques include those using amine solvent separation and cryogenic separation methods. Future techniques for CO2 sequestration and reduction is CO2 emissions include advanced physical and chemical solvents, membrane systems, solid sorbents, biomass co-firing, ionic liquids, metal organic frameworks, chemical looping, oxygen transport membrane, and biological processes.
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Based on this roadmap, chemical looping is considered as a third generation technology, which is the eventual goal of a technology to be developed for carbon capture.3,4
A chemical looping system converts carbonaceous fuels using metal oxide based oxygen carriers circulating between three reactors - a reducer, an oxidizer and a combustor – to produce heat and high purity hydrogen (H2) with simultaneous CO2 sequestration. In the reducer, the fuel is converted to CO2 and H2O by oxygen transfer from the oxygen carriers. The reduced oxygen carriers are then partially oxidized by steam in the oxidizer producing high purity hydrogen and subsequently fully oxidized by air in the combustor.
The energy generated due to the exothermic reactions in the combustor is sufficient to compensate for the heat required to perform the endothermic reactions in the reducer and to produce electricity via steam generation. Since CO2, H2 and heat are generated in separate reactors, this strategy avoids direct contact between the fuel and oxidants. Thus, the process generates a sequestration-ready, concentrated stream of CO2 without the need for any energy intensive separation techniques.
1.1. Chemical Looping Classification
Metal oxide based chemical looping can be classified into three categories based on the reaction conditions in the reducer into: (1) complete/full oxidation; (2) partial/selective oxidation; and (3) solar chemical looping. Category (1) represents the typical chemical looping combustion applications; Category (2) represents the chemical looping gasification or reforming applications; and Category (3) represents applications that use solar energy for the metal oxide reduction-oxidation (redox) reactions, without using any carbonaceous feedstock. 2
1.1.1. Chemical Looping Combustion (CLC)
This chemical looping application using both gaseous and solid carbonaceous fuels, like syngas, natural gas, biomass or coal, has been studied extensively in the past few years.5-8
In the reducer, metal oxide based oxygen carrier particles typically provide lattice oxygen to the fuel to produce complete combustion products CO2 and H2O and the oxygen carriers are reduced in the process. The reduced oxygen carriers are reoxidized completely in the combustor using air, generating high quality heat through the exothermic oxidation reactions. Thus complete oxidation of fuel is used for the generation of heat and electricity, while producing a concentrated stream of CO2 leading to maximum cost reduction benefits.
1.1.2. Chemical Looping Partial Oxidation (CLPO)
This chemical looping application using carbonaceous fuels can produce synthesis gas
9-12 (syngas), which is primarily a mixture of carbon monoxide (CO) and H2. Syngas is used as an intermediate for many gas-to-liquid (GTL) applications such as synthesis of chemicals like methanol and liquid fuels. In CLPO applications, the fuel is partially oxidized to syngas by lattice oxygen transferred from oxygen carriers in the reducer, which are reduced in the process, followed by regeneration in the combustor using air.
The transfer of lattice oxygen to the fuel is controlled in a way so as to maximize the fuel conversion and selectivity to partial oxidation products. In addition to producing syngas from carbonaceous feedstock, the concept of chemical looping can also be potentially used as a one-step solution for the production of chemicals and/or chemical intermediates using multifunctional metal oxide materials. The oxidative coupling of methane (OCM) 3 to ethylene and partial oxidation of butane to maleic anhydride are examples of two such processes that could be carried out via the chemical looping technology. The metal oxide based functional materials in these processes exhibit both catalytic as well as lattice oxygen transfer properties. In such processes, the catalytic metal oxide reacts with a hydrocarbon feedstock to selectively produce chemicals in the reducer, while the reduced catalytic metal oxide is regenerated by oxidation with air in the combustor.13,14
1.1.3. Solar Chemical Looping
Solar chemical looping relies on solar energy as the primary energy source for the reducer. Concentrated heat from solar energy can be directly used to reduce the oxygen carriers to produce oxygen. In the combustor, oxidants such as H2O or CO2 are used to produce H2 or CO, respectively, at high solar energy conversion efficiencies while regenerating the metal oxide.15-18
1.2. The Ohio State Chemical Looping Processes
The uniqueness of all The Ohio State University (OSU) chemical looping systems lies in their simplicity as the solid circulation occurs in a continuous single loop from the top of the reducer through the combustor and riser and back to the reducer. This is primarily a result of using a moving bed reactor as the reducer. The use of the moving bed reducer opens up a wide range of process configuration options for the generation of a variety of products with high efficiency. By simply switching the direction of flow of the fuel relative to the metal oxide in the reducer from the counter-current mode to the cocurrent mode, reaction conditions can be readily changed from full combustion (Category (1)) to gasification or reforming (Category (2)). The solid circulation is achieved with the help 4 of non-mechanical valves, which makes the simplistic design easily scalable. Thus, the
OSU chemical looping processes form a technology platform that can convert a variety of carbonaceous feedstock including coal, biomass, natural gas, and syngas into products such as electricity, syngas, hydrogen, and chemicals.8 Each of these processes are currently at different stages of maturation, starting from lab scale thermogravimetric and fixed bed studies to pilot scale operations.
1.2.1. OSU Chemical Looping Combustion
The CLC process converts hydrocarbon feedstock to CO2 and H2O, using an iron titanium composite metal oxide (ITCMO) oxygen carrier in a countercurrent moving bed reducer. The thermodynamic motivation for using a countercurrent reactor system is illustrated by the iron-CO-CO2 phase diagram, shown in Figure 1.2. The oxygen carrier conversion using a fluidized bed reducer is constrained by the system thermodynamics.
In a well-mixed fluidized-bed reducer, at any point of time the gas inside the fluidized bed and in the fluidized bed outlet has a high partial pressure of CO2, restricting the reduction of Fe2O3. As an example, in a fluidized bed reducer, Fe2O3 cannot be oxidized below the Fe3O4 phase for a 99.9% CO conversion and that corresponds to an 11.1% oxygen carrier conversion. In contrast, in a moving bed reducer with a countercurrent gas-solid contacting pattern, the CO2/CO ratio is high at the solids inlet and low at the solids outlet, which allows for the oxygen carriers to be reduced to an oxidation state lower than Fe3O4. Under the countercurrent moving bed reducer operating condition,
>99.99% conversion of CO can be achieved with an oxygen carrier reduction conversion of about 50%, corresponding to a mixture of FeO/Fe.8 The same principle is applicable
5 for other carbonaceous feedstock including CH4, and coal/biomass volatiles. For a given fuel input, a countercurrent moving bed reducer can result in 30% lower Fe2O3 circulation rate as compared to a fluidized bed reducer.8 Another important advantage of a countercurrent moving bed processing solid fuels, is the fact that the ash separation can be readily achieved due to the difference in particle size between the oxygen carriers and ash derived from solid feedstock like coal or biomass.
1.2.1.1. Coal Direct Chemical Looping (CDCL) Process
A schematic of the coal direct chemical looping (CDCL) process for the generation of electricity is shown in Figure 1.3. In the CDCL process, coal is injected into the middle section of the moving bed reducer. Coal volatiles are carried countercurrently upwards by the enhancing gas, while being oxidized by the ITCMO oxygen carriers entering from the top of the reducer. Coal char flows downwards cocurrently with the oxygen carrier particles and is gasified with CO2 and/or steam as an enhancing gas. The outlet gas stream from the reducer is mainly made up of CO2 and H2O. The CDCL process has been demonstrated at the bench-scale and 25 kWth sub-pilot scale, with over 600 hours of
19,20 cumulative operation. The fully-integrated 25 kWth sub-pilot CDCL unit was successfully run for 200 hours continuously with near-complete conversion of coal to
CO2 and steam, smooth solid circulation, and successful gas-sealing using non- mechanical valves. A 250 KWth pilot plant is currently under construction by the
Babcock & Wilcox Power Generation Group. The US DOE considers carbon capture technologies that can achieve 90% carbon capture without increasing the cost of electricity by more than 35% as transformational technology. The CDCL cost of
6 electricity successfully meets this target and is hence considered a third generation transformational technology.4
1.2.1.2. Syngas Chemical Looping (SCL) Process
The syngas chemical looping (SCL) chemical looping process was developed to convert gaseous fuels like syngas and natural gas in a three reactor iron-based chemical looping scheme to cogenerate H2 and electricity, as shown in Figure 1.4. The SCL reducer oxidizes the fuel using the ITCMO oxygen carriers in a counter-current contact pattern for complete fuel oxidation in the reducer. The counter-current moving bed reducer allows for an Fe2O3 reduction conversion to Fe/FeO phases, which can then be partially oxidized to Fe3O4 using steam in a second reactor called the oxidizer to produce high purity hydrogen, followed by complete reoxidation with air in the combustor.21,22 The use of a single stage fluidized bed reducer restricts the oxygen carrier reduction to FeO/Fe3O4 phases, leading to either no feasible H2 production or unreasonable large solid circulation rates. The SCL process has been successfully tested at the bench-scale and sub-pilot scale, with a pilot scale pressurized system under demonstration at the National Carbon
Capture Center (NCCC) in Wilsonville, Alabama.22-24 A three-day continuous operation of the sub-pilot unit demonstration run was completed showing steady, continuous, and complete syngas conversion in the reducer and high purity H2 production from the oxidizer.22-25
1.2.2. OSU Chemical Looping Partial Oxidation
These chemical looping applications limit the amount of lattice oxygen transferred from the oxygen carriers to the fuel so as to form partial oxidation products instead of 7 complete combustion. In order to restrict the transfer of lattice oxygen, the reducer in these processes is operated as a cocurrent moving bed reactor. The concept has been used to convert different gaseous and solid carbonaceous feedstock like shale gas, coal and biomass into syngas, which is a precursor to various high-valued chemicals and liquid fuels.
1.2.2.1. Shale Gas to Syngas (STS) Process
The shale gas-to-syngas (STS) chemical looping process has been developed for the conversion of gaseous fuels to syngas, with high efficiency and a flexible syngas composition depending on the downstream processing requirements.9 The STS process uses a cocurrent moving bed reducer reactor for producing syngas and a fluidized bed combustor reactor for regenerating the reduced ITCMO oxygen carriers, as presented in
Figure 1.5. The fuel to syngas conversion is obtained in a single step by the transfer of lattice oxygen from the oxygen carriers. The STS process is designed such that the endothermic heat of reaction for syngas production is balanced by the exothermic regeneration of the oxygen carriers in the combustor reactor, making the integrated process autothermal. Optimization of the process requires consideration of multiple parameters like heat transfer capacity of the ITCMO particles, support material type and weight fraction added to the ITCMO mixture, and stoichiometric ratio of active component of ITCMO to the gaseous fuel. The H2:CO ratio in the product syngas can be adjusted based on downstream applications by changing the amount of steam introduced in the reducer, gaseous fuels, ITCMO circulation rate, and gas and solid residence times.
The STS process was demonstrated on a sub-pilot scale moving bed reactor, using
8 methane (CH4) as a model gaseous fuel. The process parameters were adjusted to ensure a product syngas quality directly applicable to the Fischer-Tropsch process for liquid fuel synthesis.
1.2.2.2. Coal-to-Syngas (CTS) Process
The coal-to-syngas (CTS) chemical looping process has been developed for a high efficiency conversion of solid fuels to syngas.26 The CTS process uses a cocurrent moving bed reducer for producing syngas and a fluidized bed combustor reactor for regenerating the reduced oxygen carrier material. The CTS process intensifies conventional gasification systems by eliminating the coal drying system, coal gasifier, air separation unit, and the water-gas shift reactor, thereby resulting in higher overall process efficiencies.27 The oxygen transfer from the ITCMO oxygen carrier to the fuel is controlled so as to maximize the syngas selectivity. Additionally, like the STS process, the syngas quality can be adjusted based on downstream applications, without the need for any additional processing steps. For example, a conventional methanol production system requires syngas with a H2:CO ratio ~2, which is achieved by adding a water-gas shift reactor at the expense of carbon utilization. However, the CTS process can adjust the syngas composition in-situ using a precise ratio of ITCMO to coal and steam to coal, resulting in significantly increased carbon utilization efficiency and process intensification. The CTS reducer has been demonstrated successfully at the bench-scale, using a variety of coal, including bituminous and sub-bituminous.
9
1.2.2.3. Biomass-to-Syngas (BTS) Process
The biomass-to-syngas (BTS) chemical looping process was developed for high efficiency conversion of biomass to syngas. Similar to the CTS process, the BTS process uses a cocurrent moving bed reducer for producing syngas and a fluidized bed combustor for regenerating the reduced oxygen carrier material. Efficient thermochemical conversion of biomass requires effective cracking of tars evolved in the reducer, which can be achieved with the optimum oxygen carrier formulation. The BTS process has been successfully studied at the bench-scale for a variety of biomass, using the ITCMO oxygen carriers.
1.3. Oxygen Carrier Materials Selection
Oxygen carrier materials play a key role in determining the product quality, process efficiency and operating costs of any chemical looping process. During reduction, the oxygen carrier donates the required amount of lattice oxygen for fuel conversion and product synthesis. In the oxidation step, the lattice oxygen vacancies are replenished with oxygen from air while generating heat due to the exothermic oxidation reactions.
Extensive research has been conducted in the design and development of optimum oxygen carrier materials. Oxygen carriers for successful chemical looping operation need to possess certain properties like high oxygen-carrying capacity, high fuel conversion, good redox reactivity, fast kinetics, long term recyclability, high attrition resistance, good heat-carrying capacity, high melting point, resistance to toxicity, easy scalability and low cost. Various materials have been studied for the different chemical looping applications and will be discussed in the following sections.23,28 Initially, only single component 10 materials were investigated for their performance as oxygen carriers. However, the need for oxygen carriers with enhanced reactivity, recyclability, attrition resistance and ionic diffusion led to the investigation of multicomponent-based systems.
As a background, active metal oxides can be selected towards chemical looping applications based on the Ellingham diagram. The Ellingham diagram is a popular tool in metallurgical studies to determine the relative reduction potentials of metal oxides at different temperatures.29 The Ellingham diagram provides information for different metal oxides as oxygen carriers for various chemical looping applications based on their oxidation capabilities.26 In Figure 1.6, the Ellingham diagram has been divided into different sections based on the following four key reactions:
2CO + O2 ↔ 2CO2 Reaction line 1 (1.1)
2H2 + O2 ↔ 2H2O Reaction line 2 (1.2)
2C + O2 ↔ 2CO Reaction line 3 (1.3)
2CH4 + O2 ↔ 2CO + 4H2 Reaction line 4 (1.4)
The different sections have been classified based on the four reaction lines. Based on these sections, the metal oxides have been identified as potential oxygen carriers for different chemical looping processes.
NiO, CoO, CuO, Fe2O3 and Fe3O4 belong in the combustion section (Section A) as they all lie above the reaction lines 1 and 2. These metal oxides have good oxidizing properties and can be used as oxygen carriers for both CLC and CLPO processes. For the small section between the reaction lines 1 and 2, Section E, the metal oxides can be used 11 for CLPO, although with significant amounts of H2O in the syngas product. Metal oxides in this transition region include SnO2. The third section lies between reaction lines 2 and
3 (Section B). Metal oxides lying in this region have average oxidizing properties and can only be used for partial oxidation but not for the complete oxidation processes. CeO2 lies in this section. Metal oxides below reaction line 3 (Sections C and D) lack the potential to be used as oxygen carriers by themselves and are generally considered to be inert. These include Cr2O3 and SiO2. However, they can be used as support materials along with other active metal oxides in order to enhance their oxygen carrier properties. For example, when TiO2, which has very low oxidation ability, is combined with FeO, it forms an
30 FeTiO3 complex which is observed to have higher reactivity as compared to FeO alone.
Thus, addition of the support material in this case improves the oxygen carrier performance. On the other hand, addition of Al2O3 with low oxidation capability to CuO leads to the formation of CuAl2O4, which is highly undesirable as it results in the loss of the chemical looping oxygen uncoupling (CLOU) property of CuO.31 Thus, the addition of the low oxidation materials as supports does not always enhance the oxygen carrier performance. It is, therefore, essential to investigate the behavior of different phases in presence of each other during the development of effective oxygen carriers. CH4 is thermodynamically unstable at temperatures higher than 750oC and spontaneously decomposes to form C and H2 in the absence of an oxygen source. Hence, reaction line 4 is not considered very important for dividing Figure 1.6 into the different sections. It should be noted that the Ellingham diagram represents only a thermodynamic analysis of metal oxides to be used as potential oxygen carrier materials. All the previously
12 mentioned criteria also come into play when selecting oxygen carrier materials for commercial chemical looping applications. With an optimization of all the factors involved, chemical looping systems can then be effectively used to generate a variety of products.
Some of the major challenges in the development of oxygen carriers include synthesizing oxygen carriers with both good reactivity and recyclability. It has been widely reported that most single metal oxide based oxygen carriers suffer steady deterioration in their reactivity and recyclability over multiple redox cycles. Such deactivation is usually a result of decayed morphological properties like decrease in active surface area and pore volume. Because the redox reactions involved in chemical looping processes are mostly conducted at temperatures above 700oC, this deterioration in reactivity and recyclability performance has been attributed to the sintering effect, which leads to the fusing of pore structure and formation of agglomerates.21,32-36 Another major challenge is developing oxygen carriers with high attrition resistance that can sustain their physical strength over multiple redox cycles in the chemical looping systems. It has been shown that the cyclic pore opening-and-closing, which is an integral part of chemical looping processes, is an independent deactivating factor and also results in the loss of physical strength of the oxygen carriers. Most of the research on oxygen carrier development has been focused on the improvement of particle morphological properties, i.e., surface area, pore volume, and pore size distribution, so as to minimize particle sintering at elevated temperatures.21,28,32-
37 However, despite the efforts in morphological enhancement, the deterioration of surface area and pore volume of oxygen carrier particles over time is unavoidable.
13
1.4. Oxygen Carrier Reactivity
During the initial phase of development of the CLC and CLPO applications, single metal oxides were considered as the primary active components in oxygen carriers. However, recent research has been directed towards the use of multiple metal/metal oxide based composite materials for improved performance and stability in these applications. The performance of the oxygen carriers dictates the fuel conversion efficiency, product yields, solid circulation rates and overall operating costs of the chemical looping processes. The behavior and performance of oxygen carriers under redox conditions are dictated largely by the thermochemical properties of their primary metal oxides. The following sections present a brief discussion on metal oxides and their composites, which have been studied for various chemical looping applications.
1.4.1. Single Metal Oxides
1.4.1.1. NiO
NiO is one of the most extensively tested oxygen carriers for both CLC and CLPO applications. NiO has very high selectivity for CO2 and H2O, so it has been used by a number of research groups for CLC processes. In its reduced state, it also has a high propensity for carbon deposition. Hence, for CLPO applications the NiO-to-fuel ratio needs to be controlled within a narrow range to maximize the selectivity for syngas and minimize carbon deposition. Since pure NiO cannot sustain its reactivity for more than a few redox cycles, a considerable amount of research is focused on finding suitable support materials and synthesis techniques.33 The different synthesis techniques investigated include freeze granulation, spray drying, dry impregnation and deposition- 14 precipitation methods. NiO-based oxygen carriers synthesized by the deposition- precipitation methods are found to be more resistant to carbon deposition as compared to those synthesized by dry impregnation.38 Oxygen carriers synthesized using spray drying and freeze granulation have been found to exhibit comparable redox performance.39
MgAl2O4-supported NiO, synthesized by the freeze granulation method exhibited much higher CH4 conversion and lower tendency for carbon deposition as compared to
40 NiAl2O4-supported NiO. Addition of SiO2 as support material for NiO led to the formation stable and unreactive silicates, which resulted in loss of reactivity and
41 recyclability of the oxygen carriers over the first few cycles. Different phases of Al2O3 support have also been investigated as support materials. NiO with α-Al2O3 support exhibited the highest reactivity, while NiO supported with -Al2O3 exhibited the lowest reactivity towards partial oxidation of CH4, which was attributed to the formation of the
42 stable and inert NiAl2O4 phase. Other additives explored for enhanced performance of
NiO-based oxygen carriers include Ca(OH)2 and MgO. Ca(OH)2 doping was found to increase the oxygen carrier attrition resistance.; while using MgO as a promoter helped
39 improve fuel conversion. It was concluded that MgAl2O4-supported NiO performed best for both full oxidation and partial oxidation. However, high materials cost and associated toxicity concerns make NiO unsuitable as the sole active component of oxygen carriers for commercial chemical looping applications.
1.4.1.2. Fe2O3
Similar to NiO, Fe2O3 is another type of material that has been extensively tested for chemical looping applications. However, unlike NiO, the multiple oxidation states of
15
Fe2O3 can be used to control the transfer of lattice oxygen for either partial or complete oxidation of fuels, by combining it with the favorable reactor configuration. Reduction of iron oxide occurs sequentially from Fe2O3 to Fe3O4, to FeO, and to Fe. Fan et al. have extensively reported on utilizing the multiple oxidation states of iron in moving bed reactor configurations designed specifically for the particular chemical looping process to obtain the desired end result.19,24,25,43-46 For example, Luo et al. demonstrated that the combination of ITCMO oxygen carriers and a unique moving bed reactor design can generate syngas at a concentration higher than 90%, with full fuel conversion, minimal carbon deposition and without any steam injection in the reducer.9 The feedstock can be methane, biomass, coal, and other types of carbonaceous fuels. The H2:CO in the product syngas may vary from 1:1 to 3:1 depending on the feedstock and operating conditions and cab be adjusted based on downstream application requirements. Nakayama et al. found that Fe2O3/Y2O3 with Rh2O3 as a promoter could achieve 54% methane conversion
47 with high purity syngas. Steinfeld et al. proposed the production of syngas using Fe3O4 for the partial oxidation of CH4 in a high-temperature reactor heated using solar thermal energy.15 In a chemical looping scheme, the reduced oxygen carriers can be regenerated using steam, thereby producing H2.
In the case of iron oxides, each oxidation state corresponds to a different crystal structure that includes rhombohedral (α-Fe2O3), inverse spinel (Fe3O4), rock salt cubic (FeO), body-centered cubic (Fe) structures. As the oxygen carriers are subjected to consecutive redox cycles, they undergo continuous cyclic volume expansion and contraction, which affects their mechanical strength. Due to the loss in mechanical strength coupled with the
16 sintering effects at high operating temperatures, pure iron oxides tend to lose reactivity and oxygen-carrying capacity over the first few redox cycles. The loss in reactivity of pure Fe2O3 based oxygen carriers over multiple redox cycles is seen in Figure 1.7. The reactivity and recyclability of iron-based oxygen carriers can be improved significantly, with the addition of suitable support materials. For example, Al2O3 and TiO2-supported
Fe2O3 exhibited improved reactivity and mechanical strength over multiple redox cycles,
48 with a 60% Fe2O3-40% Al2O3 mixture by weight showing the best performance.
Phase separation is another cause for the degradation of oxygen carrier materials, as it leads to separation and eventual sintering of the iron phase. Hence, research efforts have also been invested in investigating materials that prevent phase segregation. Liu &
Zachariah doped Al2O3-supported iron oxides with potassium and showed improved
49 reactivity, stability, CO2 product selectivity, and carbon deposition resistance. The K ions improved the binding between Fe and Al, thereby reducing the inclination for phase separation. Li et al. investigated TiO2-supported iron oxide oxygen carriers using an inert marker experiment in combination with density functional theory (DFT) calculations.50
The results indicate that oxidation of pure iron is dominated by the outward diffusion of
Fe cation, whereas in the case of TiO2-supported iron oxide, oxidation is dominated by the inward diffusion of oxygen anion. The difference in the structures of Fe2O3 and TiO2 create a considerable amount of oxygen vacancies on combining, thus enhancing oxygen ion diffusivity. The enhanced ionic diffusivity allows TiO2-supported Fe2O3 to maintain its reactivity over multiple redox cycles even while the pore volume and surface area continue to decrease during this cyclic redox process. Galinsky et al. have reported that
17 the use of multicomponent conductive supports like perovskites, specifically lanthanum strontium ferrite (LSF), increases the reactivity of Fe2O3 up to 5-70 times of the reactivity
51 of TiO2-supported Fe2O3. Galinsky et al. also investigated the effect of
Ca0.8Sr0.2Ti0.8Ni0.2O3 perovskite as a support on the activity and stability of Fe2O3 in redox
52 reactions against CeO2 and MgAl2O4 supports. The perovskite supported Fe2O3 was found to be more active and stable than the other two supports. The CeO2-supported
Fe2O3 was deactivated by 75% within 10 redox cycles and MgAl2O4-supported Fe2O3 lost its structural integrity due to filamentous carbon formation. Both LSF and
2- Ca0.8Sr0.2Ti0.8Ni0.2O3 are believed to facilitate both O and electron transport to and from iron oxide during reduction and oxidation reactions, which explains the superior reactivity of the perovskite-supported Fe2O3. The LSF supported iron oxide carriers were also found to be resistant to sintering and coke deposition over 50 redox cycles. The LSF support is however found to migrate towards the exterior of the oxygen carrier particles over multiple redox cycles. Based on this observation, it was suggested that the LSF support in fact acts as a conductive membrane covering the Fe2O3 oxygen carriers, which is the source of lattice oxygen. The use of perovskite materials as the active oxygen carrier material for chemical looping applications is discussed further in Section 1.4.2.1.
1.4.1.3. CuO
CuO, like Fe2O3, has multiple oxidation states and has varying reduction and oxidation pathways depending on the redox conditions. Kim et al. investigated the mechanism of
CuO reduction by hydrogen in detail using in situ XRD methods. They concluded that at high H2 flow rate, CuO can be directly reduced to metallic Cu. If the H2 flow rate is
18 limited, reduction goes through the formation of Cu2O intermediate. The activation energy for the reduction of Cu2O is about twice that of CuO, making the reaction more difficult. For the oxidation of metallic copper, a two-step pathway has been observed:
2Cu + 0.5O2 ↔ Cu2O (1.5)
Cu2O + 0.5O2 ↔ 2CuO (1.6)
Copper based oxygen carrier particles have several advantages like fast redox kinetics, exothermic reduction and oxidation and low material costs.21 However, a major disadvantage is its low melting temperature, which makes it prone to severe agglomeration leading to problems in solid circulation and recyclability in large scale
23,53 operations. Addition of Al2O3 as a support to CuO-based oxygen carriers was found to improve their stability, however could not prevent its agglomeration and loss of reactivity completely.54,55
CuO has the tendency to decompose at low oxygen partial pressures.56 This is considered undesirable for CLC and CLPO applications because it reduces the oxygen-carrying capacity. However, Mattisson et al. proposed the CLOU process for solid fuel conversion as shown in the Equations (1.7) and (1.8)57:
2CuO ↔ Cu2O + 0.5O2 (1.7)
C + O2 ↔ CO2 (1.8)
CuO based oxygen carriers, while unsuitable for large scale CLC and CLPO applications for their agglomeration issues, can be excellent active materials for the CLOU process.
19
1.4.1.4. MnO2
The redox properties of manganese oxide based oxygen carriers were studied by Kleut
58 using CH4 and H2 for reduction. Manganese oxides are not likely to be reduced to the metallic phase, thus avoiding sintering and carbide formation. Stobbe et al. found that
59 MnO2 reduction to MnO is almost a one-step process. Mn2O3 reduction takes place in two steps through Mn3O4 to MnO.
60 Zafar et al. used Mn2O3/SiO2 as the oxygen carrier particle for CLPO. They found that the active phase and the inert support react irreversibly to form manganese silicate
(Mn2SiO4), which results in a decreased reaction rate just after a few redox cycles. A similar phenomenon was also reported in the case of Al2O3 and TiO2-supported oxygen carriers because of the formation of manganese aluminates and titanates.61 After
28 extensive screening, it was concluded that ZrO2 remained inert in the reactions.
However, it was noted that ZrO2 transformed from monoclinic to tetragonal structures at
1170°C, and this phase transformation could lead to structural deterioration. Similar to copper-based oxygen carrier particles, manganese-based oxygen carrier particles also have the potential for CLOU. Shulman et al. investigated several Mn/Mg-based oxygen carriers in the CLOU process and concluded that they could achieve near-complete
62 conversion of CH4.
1.4.1.5. CeO2
Ceria has been considered as an oxygen carrier material because of its fluorite structure, which allows for easy diffusion of oxygen through the lattice. Otsuka et al. investigated
63 the partial oxidation of CH4 to syngas with cerium oxide per Equation (1.9). 20
CeO2 + xCH4 → CeO2−x + xCO + 2xH2 (1.9)
Thermodynamics suggest that CeO2 can achieve full CH4 conversion with very high selectivity for syngas. From thermodynamic analysis it is known that CeO2 and Ce2O3 can be in equilibrium with high concentration of syngas (>97% CO selectivity).9 The ratio of lattice oxygen to CH4 ([O]:CH4) determines the syngas composition, the reducibility of the oxygen carriers and the possibility of carbon formation. In the case of
CeO2, as long as the [O]:CH4 is maintained >1, CH4 decomposition leading to carbon deposition is inhibited thermodynamically, thus producing high purity syngas.
Various types of dopants have been incorporated into CeO2 to increase its reactivity by enhancing its ionic diffusivity. Gupta et al. found that addition of Sn ions weakens the metal-oxygen bond, resulting in significant enhancement of the oxygen ion diffusivity.64
Doping -Al2O3-supported CeO2 with Pt and Rh promoters could also increase the reaction rate as Pt accelerates the dissociation of the C-H bond.65 It was suggested that introduction of ZrO2 to the Pt-CeO2 system led to the formation of CexZr1xO2 (x> 0.5), with a cubic fluorite structure. The modified structure helps provide the oxygen carrier with a very high oxygen-storage capacity, and hence an enhanced reactivity. The Ce-Zr-
O-based structure has also been investigated as a catalyst for the methane-reforming
66 reaction. Sm and Bi were also investigated as dopants for CeO2 and it was found that while addition of Sm exhibited enhanced reactivity and syngas selectivity, addition of Bi showed reduced selectivity for syngas.67 Although considerable research has been conducted on CeO2 as the primary active oxygen carrier material, the issue of carbon deposition on the catalyst must still be overcome.68-71 Additionally, the thermodynamic
21 limitation of cerium oxide of converting no more than approximately 98% of the CH4 feed to syngas and slow reaction kinetics together with the high oxygen carrier material cost makes a cerium oxide-based partial oxidation process impractical for commercial application.63,72
1.4.1.6. Other Metal Oxides
In addition to these five metal oxides which have been extensively studied for chemical looping applications, there are several other metal oxides that have been investigated for
CLPO applications. Kodama et al. found that WO3 had the potential to convert CH4 to syngas with high selectivity without carbon deposition or the formation of WC phase at
73 1000C. Using ZrO2 as the support material for WO3 improved its reactivity due to reduced grain size and improved dispersion of WO3. They further tested WO3/ZrO2 in a solar furnace at temperatures of 900C-1000C, which demonstrated >80% CH4
74 conversion with >76% CO selectivity. ZnO was also tested for partial oxidation of CH4 in a 5kW prototype solar furnace, where it was able to generate syngas with a 2:1 H2:CO ratio, and ZnO conversion of 90% at 1327C.75 The reaction kinetics were found to be very fast. However, at high temperatures reduced metallic Zn existed in the gas phase, thus requiring a fast quenching step. Similar studies have been carried out on MgO, SiO2, and TiO2; however, none of them so far have been used in any practical applications.73,76,77 Complex formulations made up of different single metal oxides were investigated to improve on the performance of single metal oxides. These studies are discussed in the following sections.
22
1.4.2. Complex and Mixed Metal Oxide Based Materials
As mentioned earlier, the surface area and pore volume of pure metal oxide particles tend to decrease over the cyclic reactions, which leads to drop in reactivity. In the different
CLC and CLPO schemes, along with high fuel conversion and product selectivity, one of the parameters of utmost importance for the success of a commercial scale process is the long term stability of the oxygen carrier particles. Investigation of multicomponent based oxygen carriers has thus gained significant interest. The following sections describe the major complex and/or multi-component metal oxide materials studied for chemical looping applications.
1.4.2.1. Perovskite
Perovskite materials have been widely studied for many different applications and have also been considered as promising oxygen carrier materials for CH4 partial oxidation to syngas. Wei et al. demonstrated that La1xSrxMO3 (MMn, Ni; x0-0.4) and
La1xSrxMnO3α, were able to achieve 16% CH4 conversion with a 75% CO selectivity
78 and H2:CO molar ratio of 2.5:1 at 800C. Partial substitution of La by Sr resulted in an increase in reactivity and a decrease in selectivity. Dai et al. used AFeO3 (A La, Nd,
Eu) synthesized via the sol-gel method and found that CH4 partial oxidation begins with a significant amount of CO2 formation initially, but subsequently, there is a high selectivity
79 for syngas. Among these three perovskite materials, LaFeO3 exhibited the highest
o selectivity, with ~65% CH4 conversion and syngas selectivity as high as 90% at 900 C.
23
The high CH4 conversion and syngas selectivity increased interest in LaFeO3 as the base structure for oxygen carriers for CPLO applications. Of the various synthesis methods investigated for LaFeO3, some include using different complex agents like citric acid and glycine.80 Among different synthesis methods investigated, Mihai et al. found that
LaFeO3 prepared by a DL-tartaric acid-aided method led to a large surface area and good
81 reactivity. Doping LaFeO3 with small amounts of certain metals was found to further improve its reactivity. For example, Mihai et al. found that with the addition of a small amount of Sr, La0.9Sr0.1FeO3 exhibited improved reactivity. They reasoned that this improvement was due to increased oxygen vacancies created by introduction of heteroatoms. Nalbandian et al. found that La0.7Sr0.3Cr0.05Fe0.95O3 mixed with 5% NiO
11 gave excellent performance for CH4 partial oxidation.
1.4.2.2. Ferrite
Fe3O4 is an attractive material for CLC, but has a low selectivity toward CO, as it tends to form complete oxidation products. To enhance its selectivity for partial oxidation, a series of ferrites have been investigated using Fe3O4 as the base material and combining it with other metals to form MxFe3-xO4 (M = Ni, Co, Zn, Cu; 0 82 Ni0.39Fe2.61O4, Co0.39Fe2.61O4, and Zn0.39Fe2.61O4. Under the same experimental conditions, Ni0.39Fe2.61O4 exhibited improved CO yields and selectivity (22% and 72.2%, o respectively at 827 C). However, Co0.39Fe2.61O4 and Zn0.39Fe2.61O4 showed poorer performance as compared to that of Fe3O4. However, when ZrO2 is added as a support for Ni0.39Fe2.61O4, the supported ferrite exhibited low CH4 conversion and high carbon 24 deposition. Because Ni is also widely used as a catalyst for steam methane reforming (SMR), Sturzenegger et al. studied Ni-ferrite as both an oxygen carrier and a SMR catalyst.83 At the early stage of reduction, Ni-ferrite performed as an oxygen carrier, and at the late stage, Ni in the reduced phase acted as a SMR catalyst. Cha et al. studied ZrO2- supported CuxFe3xO4 as oxygen carriers for syngas generation, which exhibited 84 decreased CH4 conversion and selectivity. However, carbon deposition was suppressed when 0.3 CH4 conversion and CO selectivity. Cha et al. also demonstrated that by substituting a portion of the Zr with Ce, the reaction rate for the syngas generation could be enhanced. Of the compositions studied, Cu0.7Fe2.3O4/Ce-ZrO2 (Ce/Zr) was found to have the best performance, with no deactivation observed within 10 cycles.85 The combination of NiO- Cr2O3-MgO also showed promising behavior for CH4 partial oxidation for several redox cycles without significant carbon deposition, at a relatively low temperature of 700C.86 Presence of Cr2O3 helped prevent the formation of NiMgO2 complex. To summarize, oxygen carrier materials to be selected for commercial chemical looping applications are more likely to be either supported and/or composite metal oxide based multicomponent systems, since they exhibit better performance in terms of long-term particle stability, ionic diffusion, attrition resistance, etc. as compared to pure metal oxides. However, the development of robust oxygen carrier particles that can provide high fuel conversion, high selectivity towards the desirable product and superior attrition resistance simultaneously still needs significant research. 25 1.5. Oxygen Carrier Recyclability and Strength Physical integrity of oxygen carriers throughout multiple redox cycles is a critical requirement in particle design criteria. Oxygen carrier particles with a high attrition rate need a high particle make-up rate resulting in a high operating cost of an integrated chemical looping system. The two main causes for particle attrition in the integrated reactor systems include abrasion and fragmentation due to impact and shear stresses. However, ionic diffusion, change of lattice structure, cyclic volume expansion and contraction, heat stress and impurities also induce external and internal physical stresses that affect the physical integrity of the oxygen carriers. Material cost thus is crucial in determining the acceptable particle attrition and makeup rate in the chemical looping systems. During commercial operation of chemical looping systems, the life time of the oxygen carriers in the reactor can vary from days to over a year before they are replaced, corresponding to hundreds or even tens of thousands of redox cycles in the system. Designing carriers that can sustain a long period of active redox operation in a reactor system remain to be the major challenge for the technology. Given the vast and exotic variety of metal oxide formulations and synthesis methods, recyclability and crush strength of the oxygen carriers are the two basic criteria that can be used to screen the chemical looping particles before selecting them for reactor applications. The steady reactivity and recyclability over a thousand cycles and high physical strength throughout the cyclic reactions indicate desired screening results for oxygen carriers before further testing in the flow and reactor systems. It is important to note that the initial strength of the oxygen carriers should not be misinterpreted as an indicator for criteria in oxygen 26 carrier design. Only when the physical strength is sustained over long-term redox cycles should the oxygen carrier be considered for further testing in the reactors. 1.6. Thesis Outline The developments of the chemical looping technology platform for the generation of different kinds of products and the selection of oxygen carrier materials have been discussed so far. The concept of chemical looping has the potential to develop into economic, efficient and environment friendly commercial systems for conversion of various carbonaceous feedstock into desired range of products. However, one of the biggest challenges is directly associated with the metal oxide reaction engineering. The development of viable metal oxide based oxygen carrier particles for applications in fuel combustion, carbon capture, syngas generation, direct chemicals production, or thermochemical water splitting requires extensive knowledge of metal/metal oxide systems including ionic and electron diffusion, morphological transformations, system thermodynamics, redox kinetics, and support, binder and dopant effects. Of equal importance are composite formulations, synthesis methods and associated physical strength that enable the oxygen carrier particles to sustain long-term cyclic redox operation in the integrated systems. The metal oxide redox technology encompasses all facets of particle science and technology from particle synthesis to its flow. System engineering is also an important, integral part of successful process development as it significantly affects the product quality, energy conversion efficiency and its economic impact. However, this thesis primarily focusses on studying and developing Fe2O3-based 27 oxygen carriers for different commercial chemical looping processes and studying their performance under different reactive conditions. A number of important processes like Fischer-Tropsch and methanol synthesis, which use syngas as their feedstock, are operated at elevated pressures between 2-4 MPa. Hence it is economically beneficial for syngas generation processes to be operated at elevated pressures in order to minimize the energy losses associated with compressing the syngas for downstream applications. OSU’s STS process is a one-step solution for generating H2-rich syngas, suitable for gas-to-liquid applications like synthesis of gasoline, diesel and methanol as mentioned in Section 1.2.2.1. Since oxygen carriers are the enabling materials for the STS process, it is important to evaluate their performance at elevated pressures. In Chapter 2, the effect of elevated pressures on the redox kinetics of OSU’s ITCMO particles for application in the STS process has been investigated using a magnetic suspension balance (MSB). The ITCMO particles exhibited superior reduction kinetics at elevated pressures both under H2 and CH4. Under CH4, the reduction rate exhibited almost a 5-fold increase with an increase in pressure from 1 to 10 atm. The increase in the oxidation rates with pressure was less pronounced. Oxygen carrier characterization using X-ray diffraction (XRD), scanning electron microscopy (SEM) and N2 absorption studies with the Brunauer-Emmett-Teller (BET) method revealed that particles reacted at elevated pressures had more uniform grain size, porosity and increased surface area, which explains the faster kinetics. The advantages of faster reaction kinetics at elevated pressures include increased processing capacity or reduced reactor sizes and capital cost for the STS process. 28 Chapter 3 presents an investigation of the hydrogen generation ability of Fe2O3 based oxygen carriers using Al2O3, MgAl2O4 and TiO2 as support materials and compared their reactivity with pure unsupported Fe as the baseline. Fixed bed steam oxidation of all the oxygen carriers exhibited steam to H2 conversion values >70%, close to thermodynamic predictions. XRD analyses revealed that during oxidation of metallic Fe, the FeO phase forms complexes with TiO2 and Al2O3, preventing further oxidation to the Fe3O4 phase. However, in the case of MgAl2O4, metallic Fe can be successfully oxidized to Fe3O4, without any complex formations with the support material. Hence, MgAl2O4-supported Fe2O3 was selected for further recyclability studies in a MSB with H2 reduction and steam oxidation for 20 redox cycles, where it exhibited excellent recyclability and almost no drop in reactivity over time. The steam oxidation kinetics of MgAl2O4-supported Fe2O3 was also investigated, where it exhibited significantly faster kinetics and higher conversion at elevated pressure. All prior oxygen carrier studies have been focused on their reduction ability. This is the first study which evaluates the steam to hydrogen conversion ability of Fe2O3 based oxygen carriers and their recyclability using steam oxidation. As tar derived from biomass pyrolysis is a major concern for many biomass conversion techniques and as chemical looping utilizes a metal oxide to facilitate oxygen transfer, it is important to evaluate effects of tars on the metal oxides. Chapter 4 focuses on examining the decomposition kinetics of soft woody lignocellulosic biomass and investigating oxygen carriers for effective in-situ elimination of tar in the reducer of biomass fueled chemical looping systems. Series of thermogravimetric analyses (TGA) 29 were used to determine the kinetic parameters for devolatilization and gasification of biomass. Although, Fe2O3 is an ideal active material for oxygen carriers, it cannot completely eliminate tar and maintain its reactivity over multiple redox cycles. The performance of several oxygen carrier formulations synthesized using a combination of Fe2O3 and traditional tar cracking catalysts like NiO, K2CO3, dolomite and olivine were examined. Their redox recyclability was evaluated using TGA experiments. Further, their tar cracking ability was investigated in a fixed bed reactor setup coupled with gas chromatography–mass spectrometry (GC-MS), for product gas analysis. The Fe2O3-NiO combination exhibited the best redox recyclability and tar cracking reactivity. Characterization of the reacted solids using XRD established that tar conversion occurs via lattice oxygen transfer. The Fe2O3-NiO combination was selected for further optimization of the NiO content in the oxygen carrier through additional fixed bed studies using biomass derived tar. The outcome of this work is directed towards the development of cost effective and efficient functional materials for thermochemical biomass conversion. The ultimate goal is to screen and develop oxygen carrier formulations for the different chemical looping schemes with various fuel types for different end products. Oxygen carrier formulations tend to perform differently based on the reactive conditions. TiO2- supported Fe2O3 is found to have excellent reactivity in most chemical looping schemes due to enhanced ionic diffusion. It exhibited even more enhanced redox kinetics at elevated pressures. However, for H2 generation using steam, formation of complexes limits its performance. Hence, MgAl2O4 is a more suitable choice of support material for 30 the CLHG scheme, owing to its inherent inertness. For the biomass fueled chemical looping systems, simple Fe2O3-based oxygen carriers are not sufficient to eliminate all the biomass-derived tar in the reducer. Therefore, their reactivity needs to be further enhanced with the addition of NiO to the Fe2O3-based oxygen carriers so as to completely convert all the heavy hydrocarbons in the reducer itself, eliminating the need for any downstream tar removal units. The future work in this area would entail developing each of these formulations into particles with optimum particle size and high attrition resistance, in addition to the excellent redox reactivity, so they can sustain their mechanical structure over hundreds of redox cycles in any large scale integrated chemical looping system. 31 32 Figure 1.1: Current and projected world energy consumption by source Figure 1.2: Thermodynamic phase diagram of the iron-CO-CO2 system 33 Figure 1.3: Flow schematic for the CDCL process for generation of electricity from coal 34 Figure 1.4: Flow schematic of the SCL process for the cogeneration of H2 and electricity 35 Figure 1.5: Flow schematic of the STS process for partial oxidation of CH4 to syngas 36 Figure 1.6: Ellingham diagram with sections of chemical looping operation 37 o Figure 1.7: Cyclic redox reaction studies for pure Fe2O3 powder at 900 C using H2 and O2-rich gases 38 CHAPTER 2 2. HIGH PRESSURE REDOX BEHAVIOR OF IRON-OXIDE BASED OXYGEN CARRIERS Reproduced with permission from Deshpande, N., Majumder, A., Qin, L., and Fan, L.-S. (2015) High-pressure redox behavior of iron-oxide-based oxygen carriers for syngas generation from methane. Energy Fuels. 29(3), 1469-1478. Copyright © 2015 American Chemical Society. I have contributed equally to all the experimental and analytical work included in this chapter and in the preparation of the manuscript for the above-mentioned publication. My contribution involved conducting the high pressure experiments, analyzing the experimental results and characterizing the solids. 2.1. Introduction Transition metal oxides are some of the most versatile functional materials that have found their applications in diverse fields. They are used in lithium ion batteries, solid oxide fuel cells etc. in electrochemistry as well as in making conductor and semiconductor materials.87-89 They have also found applications in the chemical industry where they are used as catalysts and oxygen carriers in a very wide range of processes like selective oxidation, dehydrogenation and chemical looping.23,90,91 Some of these processes are based on the reduction-oxidation (redox) properties of the transition metal 39 oxides. In these processes, it is the oxygen from the metal oxide lattice that participates in the formation of the products, leaving behind vacancies. The vacancies are then replenished by molecular oxygen. The redox behavior of the metal oxides influences their crystal phases and their morphologies and consequently their optical, electrical and chemical properties. In the chemical industry, partial and selective oxidation processes often need to be operated at elevated pressures for upstream and/or downstream applications and from the process economics viewpoint. For example, natural gas or methane (CH4) has garnered a lot of interest in the recent past for the synthesis of valuable chemicals like gasoline, MTBE, alcohols and oxygenates through partial or selective oxidation processes.92 A number of important processes like methanol, ammonia and Fischer-Tropsch synthesis, which are used to synthesize these valuable chemicals, use syngas as their feedstock.93 Syngas for these processes is preferably derived from CH4 because of the lower capital costs and the higher efficiency of the CH4 to syngas conversion systems as compared to coal derived syngas. CH4 is preserved in natural gas fields at high pressures. Also processes like Fischer-Tropsch synthesis and methanol synthesis, which use syngas as their feedstock, operate at elevated pressures between 2-4 MPa.94,95 Thus for conventional syngas generation at ambient pressure, the syngas needs to be compressed prior to being introduced in the system. Hence, it is economically beneficial to carry out syngas generation processes at pressures compatible with downstream applications in order to minimize the energy losses associated with compressing the syngas feedstock for these processes. Gas-to-liquid (GTL) processes, like synthesis of gasoline, diesel and 40 methanol, also require a hydrogen-rich syngas feed with a 2:1 ratio of hydrogen: carbon 96 monoxide (H2:CO). Existing syngas generation processes like steam methane reforming, autothermal reforming, and catalytic partial oxidation of methane are unable to achieve the required syngas quality in a single unit and need additional processing 93 steps. Thus, the single step partial oxidation of CH4 over metal oxides offers an attractive alternative to the existing CH4-to-syngas conversion methods, and the concept is widely utilized in the process of chemical looping reforming. The economic advantages and the limited knowledge of high-pressure partial oxidation of CH4 necessitate a comprehensive study of the impact of elevated pressures on the reactions involved. Chemical looping has been regarded as one of the most promising technologies in the U. 3 S. Department of Energy’s CO2 capture roadmap. The technology is based on high temperature cyclic redox reactions of metal oxide based oxygen carriers between two or more reactors - reducer, combustor and oxidizer - for the conversion of carbonaceous fuel 97 to generate electricity and/or H2. It is designed to inherently produce a sequestration- ready stream of CO2 from the reducer. So far, the chemical looping process has been used 22,98,99 to generate H2 and/or electricity using syngas, coal and biomass as feedstock. Nevertheless, it is a highly versatile process and can be used for syngas generation using natural gas/CH4 as its feedstock. CH4 is converted to syngas in the reducer via oxygen transferred from the metal oxide based oxygen carriers. Such a process was conceptualized at the Ohio State University (OSU) for the utilization of shale gas, termed as the Shale gas to Syngas (STS) Process.9 The schematic of this process is seen in Figure 41 2.1. Syngas with a 2:1 H2: CO ratio can be obtained by controlling the oxygen carrier circulation rate in the system, and thereby the extent of reduction of the oxygen carriers in the reducer. This process has been demonstrated experimentally at various scales at atmospheric pressure. Therefore, syngas generation via chemical looping can be developed into an efficient, economic and environment friendly process that overcomes the issues associated with the existing processes. However, experimental kinetic investigations for the effect of pressure on this system are relatively sparse. Most existing studies on CH4-to-syngas conversion using metal oxides are focused on Nickel (Ni) based complex oxides, due to its catalytic capabilities for the reforming reaction.10,42,94 Some of the other metal oxides studied for CH4-to-syngas application include copper (Cu), iron (Fe), and manganese (Mn).100,101 This study has been conducted from the perspective of such redox systems using Fe-based oxides, which have the potential to be more economical if operated at elevated pressures. The bimetallic system has been investigated at OSU for the STS process in the form of iron-titanium complex metal oxides (ITCMO) particles designed for this process. The operating conditions of STS process have been determined through detailed thermodynamic and process analysis.9 Therefore, it is essential that the ITCMO particles be tested for the effect of pressure on the reactions rates. The present study is intended to accomplish this objective. Developing the solid oxide based redox system for high pressure partial oxidation of CH4 is a multi-optimization problem, which requires careful manipulation of each operating parameter to maximize performance, and to reduce the overall cost. These parameters include (but are not limited to) gas to solid loading ratio, choice of reactor operation such 42 as moving bed vs fluidized bed, co-current and countercurrent gas solid flow, precise control of the gas and solids residence times in each reactor to obtain the desired oxidation state of the solids. Each of these parameters is equally crucial and deserves separate attention and in-depth analysis. Nevertheless, in this study, the reduction and oxidation kinetics of ITCMO oxygen carriers, developed at OSU, have been studied at pressures ranging from 1-10 atm using H2/CH4 for reduction and air for oxidation. Change in reaction kinetics may influence the reducer sizes, the processing capacity and consequently, the process economics. The purpose of this study is to demonstrate the effect of pressure on the reaction rates of the ITCMO particles for CH4-to-syngas conversion. Although the reducing environment of interest is CH4, H2 has been used as the reducing gas for a major part of the study as the H2 reduction reaction is well understood and relatively easier to operate. It provides a clear understanding of the kinetics without the interference of coking, which is observed with CH4 as the reducing gas. Furthermore, the effect of pressure that is discussed in case of H2 can be extrapolated to other reducing environments. The results presented are that of kinetic experiments carried out in a thermogravimetric apparatus. It has been demonstrated in this work that higher pressures are kinetically favorable for H2 and CH4 reduction and to a lesser extent for air re-oxidation of the metal oxides. This work also briefly discusses the advent of coking and its response to elevated pressures. 2.2. Syngas generation at elevated pressures: thermodynamic analysis The thermodynamic analysis of the reducer was conducted using HSC Chemistry (OutoKumptu Research Oy, version 6.0). As stated earlier, the process of partially 43 oxidizing CH4 for production of syngas using the oxygen carrier particles causes coking or C soot formation. Operating the system at elevated pressures exacerbates this condition. To understand the thermodynamic equilibrium limits of operating this system at elevated pressures and its effect on the soot formation, the reducer species were simulated via Gibbs free energy minimization at elevated pressures (up to 10 atm). The Fe-Ti bimetallic system is used for simulating the ITCMO oxygen carrier particles. In the partial oxidation of CH4 to syngas hematite (Fe2O3) is considered as the reactive phase from the ITCMO oxygen carriers. The titanium oxide (TiO2) phase is assumed to be non- reactive. In its simplest form, the theoretical desirable reaction is: 1/3 Fe2O3 + CH4 → CO + 2H2 + 2/3 Fe (2.1) Thus CH4 is partially oxidized to form syngas, a mixture of CO and H2, and the Fe2O3 is completely reduced to metallic Fe. However, thermodynamically, the reduction of Fe2O3 progressively goes through the different reduced phases of iron oxide, namely magnetite (Fe3O4), wüstite (FeO), and finally the completely reduced form of metallic Fe. All of these phases are likely to be present simultaneously. Similarly, along with the formation of H2 and CO, CH4 oxidation also results in complete combustion products (CO2 and H2O) as well as formation of elemental C. Accordingly, the reactive system was simulated with the following species in the gaseous state: CH4, H2, CO, CO2, H2O, and following species in the solid state: C, Fe, FeO, Fe3O4, Fe2O3, Fe2TiO5 and FeTiO3. Isothermal and isobaric systems were simulated at 950˚C, and 1, 5, and 10 atm. The solid loading was assumed to be in the forms of fully oxidized Fe and Ti metallic species. The gaseous input of CH4 was incrementally added, and the outlet species were analyzed for 44 solid and gas equilibrium compositions at minimum Gibbs free energy, at fixed T and P values. As expected, TiO2 remains largely unreacted while oxides of Fe undergo sequential reduction with increasing CH4 loading, from fully oxidized to fully reduced form, both in pure Fe-O as well as Fe-Ti-O complex phases. The equilibrium amount of FeTiO3 and Fe2TiO5 are found to be negligible, and therefore for simplicity, only pure Fe-O phases are considered for the remainder of this discussion. For all pressures, overall CH4 conversion was found to be >99% for the range of gas to solid ratios tested, which was varied between 0.05 to 1.5 of moles of CH4 per mole of Fe2O3. This range was chosen due to the fact that all four oxidation states of Fe are found to exist in this range. The conversion was found to increase with increasing gas to solid ratio. As expected, solid C formation is observed simultaneously with the formation of elemental Fe. This C amount is higher at higher pressures. This is in agreement with our experimental findings, discussed further in Section 2.4.2. For example, at CH4:Fe2O3 ratio of 1.5, comparison of the C formed at 10 atm and 1 atm reveals that the equilibrium C amount at 10 atm is approximately 8 times that of 1 atm. The same comparison between 5 and 1 atm shows that equilibrium C formation at the same ratio is 4.5 times that of 1 atm. The C deposition is shown as a function of gas-solid ratio in Figure 2.2 at 5 atm and 950oC. The figure clearly shows the simultaneous onset of elemental Fe formation and C deposition. 45 In addition, this thermodynamic analysis reveals that increase in system pressure from 1 to 10 atm results in an increase in the formation of CO2 and H2O, along with a slight decrease in the formation of desirable H2 and CO, as well as overall CH4 conversion. This is also supported by Le Chatelier’s principle, according to which Equations (2.2) to (2.5) are favored at higher pressures over Equation (2.1). CO + Fe2O3 CO2 + 2FeO (2.2) CO + FeO Fe + CO2 (2.3) H2 + Fe2O3 H2O + 2FeO (2.4) H2 + FeO H2O + Fe (2.5) However, increase in system pressure is found to have a favorable impact on equilibrium H2:CO ratio, which is desired to be ~2 for downstream processing such as Fischer- Tropsch synthesis. The carbon deposition can be managed by careful manipulation of the gas solid ratios in the moving bed reducer reactor system.17 Nevertheless, the success of this partial oxidation system rests equally on the kinetic factors affecting the reaction. The high pressure operation of Fe-based partial oxidation will require the basic understanding of the manner in which pressure affects the kinetics of each of the reactions involved. Therefore, in the following sections, the effect of pressure on the reduction and oxidation of Fe-Ti oxygen carriers is investigated. 46 2.3. Experimental setup, Materials and Procedure A magnetic suspension balance (MSB, Rubotherm GmbH, US-2004-00162) was used for the high pressure thermogravimetric analysis (TGA) experiments. The schematic of the MSB setup is shown in Figure 2.3. Pure gas bottles of H2, CH4, N2, and air were connected through a battery of mass flow controllers and valves to the TGA assembly. A pressure transducer upstream of the sample cell measures and records the pressure of the sample during the experiment. Downstream of the sample cell, a back pressure regulator (BPR) is installed to regulate the pressure in the sample cell. The gas was preheated prior to entering the TGA assembly by means of heating tapes. The unique working principle of the MSB allows the sample weight and the balance to be connected via magnetic coupling, and therefore the balance is isolated from (and not affected by) the reaction environment. This principle allows the use of high pressure and highly corrosive environments in the sample cell. The section of the TGA housing the magnetic coupling is maintained at 140oC by means of a heat jacket connected to an oil bath. The reaction temperature in the sample cell is maintained independently by means of an electric furnace. The gas mass flow rates were fixed such that the gas space velocity experienced by the sample was constant at all pressures (~1 min-1 for the volume of the sample cell). The sample was heated in the inert flow of N2. When the reaction pressure and temperature were achieved, the gases were switched to introduce the reaction mixture in the sample cell. The sample weight, temperature and pressure were recorded as a function of time. The extent of (%) reduction and oxidation are calculated using the following equations: 47 ∆푤푟 % reduction = 푚푎푥 x 100% (2.6) ∆푤푟푒푑 ∆푤표 % oxidation = 푚푎푥 x 100% (2.7) ∆푤표푥 푚푎푥 푚푎푥 where, ∆푤푟푒푑 is the theoretical maximum weight loss ring reduction and ∆푤표푥 is the theoretical maximum weight gain during oxidation of the oxygen carrier on the basis of the weight of the active component (which is Fe2O3) in the sample. ∆푤푟 and ∆푤표 are the instantaneous sample weight change during reduction and oxidation respectively. Through the remainder of this chapter, a general term X is used to indicate the % reduction and % oxidation alike, as applicable to the discussion. The X values were plotted for each experiment vs time (minutes), to obtain the thermogravimetric conversion curves. These curves were then used to compute the instantaneous rates of reaction, denoted by dX/dt, per minute. Samples of ITCMO particles (0.1 g) were collected after selected experiments and mechanically crushed to a powdered form and sieved to the appropriate particle size. X- Ray Diffraction (XRD) was performed on the powdered specimens with a Rigaku SmartLab X-ray diffractometer. Additionally, specimens were also examined by Scanning Electron Microscopy (SEM) using e-beam imaging of an FEI Helios NanoLab600 DualBeam system. To obtain high quality cross-section imaging, focused ion beam (FIB) was generated using gallium ion source at an accelerating voltage of 30 kV for cross-sectional milling for in-situ SEM observation. The 2-D material mapping was obtained using Oxford Energy Dispersive X-ray Spectrometry (EDS) at an accelerating voltage of 20 kV. Additionally, a NOVA 4200e Quantachrome Brunauer- 48 Emmett-Teller (BET) analyzer was used to measure the pore volume and surface area of samples using N2 sorption. 2.4. Results and Discussion To study the effect of pressure on the rates of reactions involved in partial oxidation system, isothermal experiments of reduction and oxidation were conducted in reducing environment of H2 and CH4, and the oxidizing environment of air. The rates of reactions were then compared by calculating the rates from the solid conversion curves obtained from the TGA. 2.4.1. Reduction in H2 The reduction of the oxygen carrier samples was conducted isothermally at 900°C using H2 as the reducing agent, at various operating conditions of gas concentration and partial pressure. The parameters studied for H2 reduction include partial pressure (푃푃퐻2), total system pressure, and mole fraction of H2 (푌퐻2). The weight loss of the sample corresponds to the total amount of oxygen lost by the sample during reduction. Therefore, the reduction conversion (X) is calculated assuming 100% reduction at complete weight loss of sample, or the most reduced state of Fe. The rates of reaction are calculated graphically at X = 0.5 and 0.75 at the various conditions tested, and exhibit similar trends. 49 2.4.1.1. Constant partial pressure of H2 (푃푃퐻2) The isothermal isobaric experiments were conducted to observe the rates of reaction at different total gas pressures, with constant partial pressure of reducing agent. The TGA conversion curves were compared at different total pressures of the system, at three different partial values of 푃푃퐻2 of 1, 1.5 and 3 atm. At a constant value of 푃푃퐻2, the increase in the overall system pressure resulted in decrease in mole fraction of H2 (푌퐻2). The rate of reaction is found to decrease with increase in total pressure of the system at each value of constant 푃푃퐻2. Similar results have been reported in the past on metal oxide reduction reactions. For example, Garcίa-Labiano et al. have reported decrease in reduction reaction rate with an increase in system pressure at 푃푃퐻2 and 푃푃퐶푂 values of 1 atm for Fe, Cu, and Ni based metal oxides.102 The negative effect of pressure on various reactions has been previously observed by other researchers, and explained by factors such as increase in product gas volume upon reaction, or increased diffusion resistance through the product layer at higher pressures.103-105 Thus, the same set of data is plotted in two different ways. In Figure 2.4 the rates of reaction are plotted against the total system pressure, and in Figure 2.5 the same rate values are plotted against the respective 푌퐻2 values. For example, comparing the experiments conducted at system pressure of 3 atm, the three different experiments at this pressure value would correspond to the three different values of 푃푃퐻2 of 1, 1.5 and 3 atm. The rates of reduction for these three experiments fall on a vertical straight line of Figure 2.4, at x axis value of 3 atm. At this system pressure of 3 atm, the mole fractions 푌퐻2 are, however, widely different. 푃푃퐻2= 1 50 atm at a system pressure of 3 atm results in a 푌퐻2 value of 33%. 푃푃퐻2 = 1.5 atm at the system pressure of 3 atm is at a 푌퐻2 = 50%. And finally, at 푃푃퐻2 = 3 atm, 푌퐻2 is obviously 100%. This is true for all the values of pressure along the x-axis of Figure 2.4, i.e. the curve for 푃푃퐻2 = 3 atm is always at highest mole fractions of the reducing gas (푌퐻2), which seems to be a crucial factor contributing to the superior rates of reaction observed. The entire set of experimental conditions is given in Table 2.1. The curves of rate vs system pressure seem to converge at higher pressure. This can be explained by the fact that the 푃푃퐻2 values chosen here are relatively low, and therefore at high system pressures the 푌퐻2 values are closer. By contrast, at lower system pressure, the rate curves are further apart which correspond to the disparity in 푌퐻2 values at those conditions (see Table 2.1), which increases at lower pressure. In Figure 2.5, the same data is plotted against mole fraction of H2 (푌퐻2). Following the same logic, the rates can be compared at similar values of 푌퐻2. In the vicinity of 푌퐻2 = 30- 33%, even though the mole fraction of H2 is so similar, the system pressure values for the three curves are 3, 5 and 10 atm. Therefore, it is observed that the higher 푃푃퐻2 values expectedly play a part in increasing the rate of reduction. When the rate values are plotted in this manner as a function of 푌퐻2, the plots are linear; indicating a direct proportionality between the rate and mole fraction of reacting gas. However, the plots converge at lower 푌퐻2 values (which correspond to the high pressure experiments, towards the positive x- axis direction in Figure 2.4). Regardless of the manner of analysis, it is evident that at 51 higher values of constant partial pressure of the reducing gas a higher rate of the reduction of solid oxide ITCMO particles is achieved. 2.4.1.2. Constant mole fraction of H2 ( 푌퐻2) Similarly, the reduction experiments were conducted at various pressures between 1 and 10 atm by keeping 푌퐻2 constant at 50% to study the rate of reduction of the ITCMO particles at the varied pressures. In this case, the value of 푃푃퐻2 also inevitably increased with the increase in pressure. It is observed that as the pressure is increased, the slope of the conversion curves increases, indicating higher reaction rates. This is in contrast to the previously reported findings of Garcίa-Labiano et al, who report a slight decrease in reaction rates with increase in pressure at constant mole fraction of CO at 10% value.102 It must be noted in the outset that the rate of reaction is determined by a combination of various factors, such as reactive gas partial pressure, change in diffusivity of reactant and product gas through the porous particle at elevated pressures, relative superficial velocity of gas with respect to solids, etc. As stated before, in the present study, a constant gas linear velocity (space velocity) was used for all experiments. The rates were determined graphically at a fixed conversion value for all the curves, and are plotted in Figure 2.6 at X = 0.5 and at X = 0.75. From Figure 2.6 it can be concluded that there is more than a 100% increase in the reaction rate as the pressure is increased from 1 to 10 atm, when operating at the same mole fraction of the reducing gas, namely, H2. 52 2.4.1.3. Constant pressure of the system Finally, the reduction reactions were studied for the reaction kinetics under constant pressure. For this set of tests, the pressure of the system was maintained at 5 atm and the 푌퐻2was increased from 30% to 100%, thus increasing the partial pressure of the reducing gas (푃푃퐻2 ). If the reaction is conducted under fixed pressure, the rate of reaction is found to increase as expected, with an increase in 푃푃퐻2; this is shown in Figure 2.7. 2.4.2. Reduction in CH4 The same experiments as Section 2.4.1.2 were repeated with CH4 as the reducing gas instead of H2. As indicated in Section 2.2, it was observed that the reduction of oxygen carrier particles with CH4 as the reducing agent results in the formation of elemental C. This is evidenced by the soot formation and weight increase of the sample beyond a certain point. The C deposition is observed when the oxygen carrier reaches a certain degree of reduction conversion, and is always after metallic Fe phase begins to form. Thus, in order to study the reaction kinetics of the reduction of oxygen carrier materials in presence of CH4, the reaction is arrested at or before the initiation of C deposition. Accordingly, experiments were conducted by adjusting the procedure and allowing the maximum possible reduction of particles, till the onset of C deposition. The reduction of Fe2O3 to Fe proceeds through sequential steps of various oxidation states. Here, complete loss of oxygen is considered as 100% conversion. Stage I corresponds to Fe2O3 to Fe3O4 conversion, which translates to 11% reduction conversion (or X = 0.11). Stage II corresponds to Fe3O4 to FeO conversion, which translates to 33% 53 reduction conversion (X = 0.33). At conversions higher than 33%, the stage III is initiated 100 which results in the formation of metallic Fe. Unlike reduction in H2, in case of CH4 reduction these three stages have three distinct reaction rates as seen in Figure 2.8. Further, the different stages react differently to increase in pressure in terms of the rate of reaction. The rate of each reaction stage is studied at various pressures between 1 and 10 atm. At higher pressures, the rate disparity between the three stages is less pronounced, giving a faster overall conversion obtained without three distinct rate stages. It was also observed that the reaction halted at lower conversions, owing to the higher amount of C deposition. These conversion curves were used to compute the reaction rates at different pressures. The evaluation of three separate rate values is warranted for the three distinct stages of reduction. Accordingly, the rate values were calculated from the conversion data obtained and are plotted in Figure 2.9. It can be seen that reaction rates for stages I and III go through a maxima in the range of the pressure tested here. However, the rate of stage II increases exponentially with pressure. Since stage II is the slowest reaction stage, it is the overall rate determining step and therefore any change in the rate of stage II overwhelmingly affects the overall rate of the reduction reaction. For example it can be seen from Figure 2.8 that at 10 atm, 33% reduction is achieved in almost 1/7th of the time taken at 1 atm; and similarly, 60% reduction is achieved in 1/3rd of the time. Increase in pressure, specifically at constant mole fraction of the reacting gas, results in increase in concentration of the active species in the gas phase. This increased concentration of gaseous species inevitably results in increased reaction rate, which is 54 also reported earlier in Section 2.4.1.2 (for H2). Specifically, the role of pressure in the reaction rate is observed to be particularly pronounced in case of reduction of ITCMO particles with CH4. The effect of pressure on reduction kinetics for chemical looping combustion (CLC) scenario has previously been investigated by Garcίa-Labiano et al.102 They studied the reduction kinetics on Fe, Cu and Ni based oxygen carriers at pressures up to 30 atm. At a low constant mole fraction of 10% of H2 and CO, no significant increase in reaction rate was observed with an increase in pressure. However, it was noted that the actual reaction rate was influenced to a certain degree by several factors, of which an important factor was ‘gas dispersion’ that occurs particularly during the initial stage of the reacting gas introduction to the sample cell. The use of a constant molar flowrate across all pressures resulted in a progressively increased gas dispersion effect in the reaction cell at increased pressures. To minimize the progressive gas dispersion effect due to an increase in pressures in the present study, the ITCMO particles were reduced at constant space velocity across all pressures. The use of the constant space velocity leads to an increase in reaction rate of the reduction reaction of the ITCMO particle with increased pressure, as seen in Figure 2.9. For comparison purposes, experiments have also been carried out for the reduction reaction of ITCMO particles with CH4 under the condition of a constant molar flowrate between 1 and 10 atm in the present study. In this case, the space velocity at higher pressure is appreciably lower, giving rise to a larger ‘gas dispersion’ effect. In such experiments, the difference in reaction rates is found to diminish significantly, 55 confirming that the negligible effect of increased pressure on reduction rate observed in reference 102 is attributed to the ‘gas dispersion’ effect with pressures. 2.4.3. Pressure correction The sample weight measurement in the MSB is extremely sensitive to pressure changes in the cell. The system pressure is regulated by the BPR situated downstream of the sample cell. Usually, this BPR enables the system to be maintained at steady pressure value for all the experiments. Thus, all the changes in the sample weight measurement are ‘true’ weight changes, i.e. attributed to the reaction alone. However, the reduction reaction in presence of CH4 is a volume expansion reaction, where one mole of reactant gas is converted to three moles of product according to the Equation (2.1) (other side reactions, such as complete combustion, also result in volume expansion). The rate of this volume expansion is directly proportional to the rate of the reaction. In the fixed volume of the sample cell of MSB, rapid gas volume expansion thus results in a temporary increase in system pressure. The faster reaction results in more volume expansion, and thus has a more pronounced effect on the temporary system pressure change. The three stages in the CH4 reduction reaction occur at different reaction rates as shown in Section 2.4.2. The difference in these rates is also reflected in the trend in the system pressure. During stage I, the pressure increases due to volume expansion (faster reaction = more pronounced effect on pressure). At the onset of stage II, there is sudden appreciable decrease in the rate of reduction reaction, and therefore the pressure buildup starts dissipating by returning to the setpoint value (and further shows a slight increase 56 after stage III initiation). These pressure changes in the system inevitably affect the measurement of sample weight, causing it to change. Therefore, in order to discern the ‘true’ weight change of the sample due to reaction alone, it becomes necessary to correct the measured sample weight for fluctuation due to pressure effect. Thus, a correlation was established between system pressure and sample weight in absence of reaction, by changing the pressure setpoint externally in inert gas and recording the changes in sample weight. This correlation was applied to the measured sample weight during reduction reaction to correct the weight to reflect reaction alone. Figure 2.10 shows the example of this correction applied to compute the conversion. 2.4.4. Air oxidation The effect of pressure was also studied for oxidation reaction for the combustor block discussed in Section 2.1. The oxidation reactions were carried out using air as the oxidizing agent. The sample ITCMO particles were subjected to reduction in H2 followed by air oxidation at constant pressure values (1, 5 and 10 atm). The oxidation conversion curves are compared in Figure 2.11. The increasing trend of reaction rate with pressure is apparent with air oxidation too, although it is not as pronounced as reduction reactions, mainly due to the fact that oxidation reaction with O2 is extremely fast to begin with, with the total oxidation completed in less than 8 minutes even at ambient pressure at the conditions tested here. This slight increase in oxidation rate with pressure is in agreement with a similar rate increase observed by Jin and Ishida in case of Ni-based oxygen carrier particles.106 In case of two different Ni-based particles, effect of pressure was reported to 57 be more pronounced on reduction rates than oxidation, when H2 was used as reducing gas and O2 (air) was used as oxidizing gas, between 1 and 9 atmosphere. It is important to note that while the effect of increasing pressure on the redox kinetics has been investigated in these sets of experiments, a kinetic rate law has not been derived here. The chemical looping systems at OSU have additional parameters like fuel composition and reactor temperatures and pressures already fixed based on extensive simulations, empirical evidence and upstream and downstream applications, which are used to design reactors during scale-up. The redox rate laws are, however, important for general process applications. While, a rate law has not been included in this study, it is noted that determining the intrinsic redox kinetics of the ITCMO oxygen carriers will require the elimination of external and intraparticle gas diffusion resistance and ensuring that chemical reaction is the rate determining step. This can be achieved by conducting a different set of redox experiments using fine powders of oxygen carriers, while varying the gas concentrations and redox temperatures. An ionic diffusion grain model has also been developed at OSU to determine the diffusivity of gases like H2, CH4, and CO in oxygen carrier particles of various sizes. This is pertaining to the intraparticle diffusion resistance and has been developed considering pore diffusion of gas, ionic diffusion within the particle grains as well as chemical reaction on the grain surface. These studies, for determining the intrinsic redox kinetics and the intraparticle diffusion, are however not a part of this thesis. 58 2.4.5. XRD, SEM, EDS, and BET analysis The samples were subjected to XRD analysis after complete reduction (H2) and oxidation, at the lowest and highest pressure tested, viz. 1 atm and 10 atm. Complete reduction of sample is unattainable in CH4, therefore, H2 was used as the reducing medium. The diffraction spectra are shown in Figure 2.12. The samples after reduction exhibit complete reduction in the iron oxide phase at both pressures tested. In addition, the peaks corresponding to TiO2 are diminished at elevated pressure as compared to that of ambient pressure, and the peak corresponding to Fe remains unchanged. The ambient pressure sample exhibits distinct TiO2 peaks along with two prominent peaks characteristic of Fe. In the case of oxidized samples, completely oxidized phases are identified as Fe2O3 and TiO2. In addition, trace amount of complex species are formed at ambient pressures namely Fe2TiO5 and FeTiO3. These complex species are also found to be significantly lesser at sample oxidized at a pressure of 10 atm. These differences in the samples point toward possible mechanistic differences in the reactions conducted at ambient pressure vs high pressure, which might be contributing towards a faster reaction rate at high pressures. However, the exact nature of these differences may only be understood through an in-depth study of the morphological changes on the reaction surface at the microscopic level at elevated pressures. The reacted particles were studied using scanning SEM and elemental mapping according to the technique outlined in Section 2.3. More details of this technique are available elsewhere.107 Figure 2.13(a) and (b) shows the reduced samples produced at 1 and 10 atm, respectively. Figure 2.13(a) shows a typical non-uniform microparticle produced at 59 ambient pressure: a denser part of Ti-rich oxides with a particle size of 40 µm and a porous part containing comparable amount of Fe and Ti oxides with an average grain size of 1-2 µm. It is apparent that subjecting the oxygen carrier particles to high pressure results in more porous particles upon reduction, with an average grain size of 500 nm can be clearly seen in the cross section (Figure 2.13(b)). This strongly suggests that higher pressure results in higher surface porosity compared to ambient pressure, which may explain the difference in reaction rate. Figure 2.14 shows the difference between samples treated under 1 atm and 10 atm. After reduction, the particles processed at 1 atm have a grain size of 1 µm and higher pressures lead to smaller grain size of ~500 nm. Consequently, a reaction pressure at 10 atm can largely promote the increase of overall surface area. These samples were further tested for surface area and pore volume measurements using a BET analyzer. The BJH pore size distribution method was used to find the values of total surface area and pore volume for the four samples tested here. From the analysis, it was discovered that the increase in pressure from 1 to 10 atm resulted in increase in both surface area and pore volume values, for reduced as well as oxidized samples. For the reduced samples, the increase in pressure resulted in surface area change from 7.036 m2/g to 7.227 m2/g whereas the pore volume values increased from 0.014 cm3/g to 0.022 cm3/g. The same trend was observed for oxidized samples, where the change was more pronounced, going from 4.726 m2/g to 15.507 m2/g of surface area values, as well as 0.025 to 0.117 cm3/g of total pore volume. All of these observations serve to explain the increased reduction rates for oxygen carrier particles with increased pressure. Using this preliminary analysis, we conclude that 60 reduction carried out at elevated pressures results in particles with superior surface & intra-particle morphology, as well as uniform small grain structure, which contribute to the high reaction rates observed. A detailed analysis of the relationship between the particle morphology and the reaction rates will result in greater understanding of the mechanism of high pressure redox reactions involved in Fe-based partial oxidation system. Such a study is currently underway and will be published separately. It should be noted that upon re-oxidation no significant morphological difference was found in samples treated at 1 and 10 atm, which is consistent with the small difference in the oxidation reaction rates observed. 2.5. Concluding Remarks and Future Work For chemical synthesis applications, Fe-based partial oxidation process can be applied to produce pressurized syngas from CH4. One such process developed at OSU has been termed as the Shale gas to Syngas (STS) process. The use of the STS process for downstream GTL applications requires the system to be operated at elevated pressures. Thus, it is imperative to investigate the effect of pressure on the operation of such a system, specifically, on the metal oxide particles that supply the oxygen. This study was conducted in order to determine the effect of pressure on the rates of reduction and oxidation reactions for OSU’s ITCMO particles developed for applications such as the STS process. Extensive testing was carried out on the reducer block at the conditions amenable to such a process. It is found that increase in the mole fraction or partial pressure of the reducing gas has a favorable effect on the rate of the reduction reaction of 61 the ITCMO particles over the wide range of conditions tested. Overall, operating the system at higher pressure results in superior reduction reaction rates. Although the primary goal of the present work is to ascertain the effect of pressure on the reaction rates ITCMO particles for CH4 partial oxidation using the STS process, extensive parametric study for reduction was also carried out using H2 as the reducing gas. It is found that the experiments carried out with CH4 exhibit the same trends with the variation of pressure and therefore the pressure effects can be extrapolated. By choosing the experimental methodology to eliminate any gas dispersion effects, the kinetic advantage of higher pressure operation was ascertained in this study. With CH4 as the reducing gas, the ITCMO particles exhibit three distinct reaction stages for reduction. Stage II, which is the slowest and therefore rate determining stage, is the most sensitive to change in pressure; undergoing a 10-fold increase in rate with the increase in pressure from 1 to 10 atm. Thus, any increase in pressure results in a favorable change in rate of reduction reaction by CH4. The improvement in reaction rate is also observed in case of oxidation of reduced ITCMO particles, albeit to a smaller degree. The reacted particles were analyzed using SEM, XRD, and BET techniques to understand the morphological changes on surface and intra-particle level, and the role of these changes in the observed differences in reaction rates. This analysis indicates that conducting the reduction reactions at elevated pressures results in product particle which shows more uniformity with respect to grain sizes, porosity and reactive iron-oxide 62 distribution, with increased surface and intra-particle porosity. The formation in this uniform particle significantly contributes to the superior reaction kinetics. One of the major issues of syngas production using CH4 is the formation of C soot. However, the soot deposition can be managed or eliminated entirely using the control of various other process parameters such as gas to solid loading, reactor residence times, gas injection location and mode of gas-solid contact. Therefore, the advantages of faster reaction kinetics at higher pressures can be realized by circumventing the soot deposition through precise process operation. The advantages of operating the Fe-based partial oxidation system at elevated pressures include increased processing capacity or reduced reactor sizes and capital cost. A high pressure reactor operation is thus desired for the metal oxide partial oxidation process system. Recommendations for future work include determining the optimal operating pressures to be used in this process through rigorous experimental testing on various scales, and thorough process and economic analyses. In any chemical looping process, like the STS process, the oxygen carriers are expected to sustain their reactivity and physical integrity over hundreds of redox cycles. This requires development of oxygen carriers with superior attrition resistance and redox reactivity at elevated pressures. This can be achieved by gaining a fundamental understanding of the reaction mechanisms involved and using that knowledge to engineer oxygen carriers with the desired properties. 63 Figure 2.1: Schematic of Fe-oxide based system for syngas generation from partial oxidation of CH 4 64 Figure 2.2: Simulated equilibrium iron oxide phases and fractional carbon deposition as a function of inlet gas to solid ratios at elevated pressure. T = 950˚C, P = 5 atm. 65 Figure 2.3: Schematic of the experimental setup of the MSB 66 Figure 2.4: The effect of total system pressure on rates of reduction at X = 0.75, and o constant partial pressure of H2. T = 900 C 67 Figure 2.5: The effect of mole fraction of reducing gas (YH2) on rates of reduction at X = 0.75, and constant partial pressures. T = 900oC 68 푷푷푯ퟐ (atm) → 1 1.5 3 Total Pressure (atm) ↓ 1 100% - - 3 33.33% 50% 100% 5 20% 30% 60% 7 - - 42.86% 8 12.50% 18.75% 37.50% 10 10% 15% 30% Table 2.1: YH2 as a function of total system pressure and PPH2for Section 2.4.1.1 69 Figure 2.6: The effect of system pressure on rates of reduction at X = 0.5 and X = o 0.75, and constant mole fraction of reducing gas YH2 = 50%, T = 900 C 70 Figure 2.7: The effect of partial pressure of reducing gas PPH2 on (a) conversion curves obtained and (b) rates of reduction at constant system pressure P = 5 atm. T = 900oC 71 Figure 2.8: Reduction conversion curves obtained using CH4 from the thermogravimetric analysis between 1 and 10 atm at constant mole fraction of o reducing gas YCH4 = 50%. T = 950 C 72 Figure 2.9: The effect of system pressure on reaction rate for the three-step reduction with CH as the reducing gas. Y = 50%, T = 950oC, P = 1 to 10 atm 4 CH4 73 Figure 2.10: Pressure data and Reduction conversion obtained based on the original data and the data obtained by applying pressure correction. Reducing gas = CH4 o with YCH4 = 50%, T = 950 C, and P = 8 atm. 74 Figure 2.11: Oxidation conversion curves obtained from the thermogravimetric analysis between 1 and 10 atm at = 0.1, T = 900oC 푌푂2 75 Figure 2.12: XRD analysis of (a) reduced and (b) oxidized samples at 1 and 10 atm, 900oC. Reducing environment is under H with 푌 = 0.5, and oxidizing environment 2 퐻2 is air 76 Figure 2.13: SEM and EDS elemental mapping of cross sections of reduced o particles. Samples reduced under H2, 푌퐻2 = 0.5 and T= 900 C. (a) 1 atm and (b) 10 atm 77 o Figure 2.14: Surface grains in samples reduced under H2, 푌퐻2 = 50% and T= 900 C. (a) 1 atm and (b) 10 atm 78 CHAPTER 3 3. STEAM OXIDATION OF Fe2O3-BASED OXYGEN CARRIERS 3.1. Introduction Hydrogen is an emission-free or “cleaner” alternate fuel that has found its applicability in a number of end-use technologies and has the potential to satisfy a large percentage of the energy demands. Currently, it has mainly found its use in ammonia synthesis, methanol 108 synthesis and the transportation sector for crude oil refining. Although H2 is the most abundant element in the universe, it does not occur naturally in its elemental form on the earth. Hence, it needs to be derived from other H2-containing sources like fossil fuels, biomass and water. There are very limited options when it comes to H2 production without CO2 emissions, which includes H2 produced by electrolysis of water. These processes however require considerable development, because of their low efficiency and high costs, as far as large scale production is concerned. The next available source of H2 atoms remains in fossil fuels.109-111 Not only do they contain large amounts of hydrogen atoms that can be turned into H2, extracting H2 from fossil fuels has higher energy efficiency over direct water splitting due to the energy stored in the C-C bonds. The predominant ways of H2 production from fossil fuels include steam methane reforming 112-114 (SMR) and coal gasification. Currently, more than 90% of the H2 produced in the US comes from SMR. In this process, CH4 is reacted with steam in a reformer over a 79 Nickel based catalyst to produce syngas. The syngas produced is sent to a water gas shift 115-117 (WGS) reactor to produce more H2. The reactions involved in SMR are shown in Equations (3.1) and (3.2) below. CH4 + H2O CO + 3H2 (3.1) CO + H2O CO2 + H2 (3.2) In general, H2 produced from fossil fuels results in the emission of CO2. The overall reactions in SMR are endothermic in nature. In order to maintain the high heat demand, energy is supplied by combusting additional fuel in the reformer, leading to lowered process efficiencies and higher CO2 emissions. If H2 can be produced from fossil fuels with CO2 sequestration, then it will be possible to produce and use fuels without the emission of any greenhouse gases in the entire cycle. However, the CO2 streams from SMR or gasification processes are often diluted. Hence, CO2 sequestration with high efficiency using either physical or chemical techniques leads to increased capital and operational investments and lowered overall process efficiency. Chemical looping is a cost effective and efficient technology to produce H2 and electricity from fossil fuels, while inherently producing a sequestration ready CO2 stream, without the need for any downstream separation units.21,42,118 Several configurations of the chemical looping technology have been proposed and demonstrated for efficient and economic H2 production from CH4 with simultaneous CO2 separation. Depending on the configuration, steam can either be injected with CH4 or in a separate reactor. The first configuration is based on the concept of SMR, where H2 is produced by reaction between 80 CH4 and steam injected together in the fuel reactor, along with metal oxide based oxygen 119,120 carriers. A WGS downstream is used to increase the H2 yield. The endothermic nature of the reactions in the fuel reactor is countered by the exothermic reoxidation of the oxygen carriers in a separate air reactor. While additional downstream units are required for CO2 separation, the CO2 concentration from the WGS reactor is higher compared to traditional SMR. This is called the chemical looping reforming (CLR) of CH4. In a different configuration, CH4 is partially oxidized to produce H2-rich syngas in the fuel reactor or the reducer by manipulating the gas-solid contact mode to limit the 121-123 oxygen transfer from the oxygen carriers to CH4. This configuration eliminates the need for injection of steam in the reducer. OSU’s STS process is based on this configuration. In a third configuration, there is a third reactor called the oxidizer between the reducer and the combustor.123 This is called the chemical looping hydrogen generation (CLHG). Here, any carbonaceous fuel is oxidized by oxygen transferred from the oxygen carriers to CO2 and steam. The reduced oxygen carriers from the reducer are first sent to the oxidizer, where they are partially reoxidized using steam, producing high purity H2, and then sent to the combustor for complete regeneration. Since CO2, H2 and heat are produced in three separate reactors, this configuration eliminates the need for any downstream processing units like WGS reactor or any other CO2 separation units. Oxygen carriers play a crucial role in the successful operation of a chemical looping system as already discussed. The physical and chemical properties of the oxygen carriers dictate the fuel conversion efficiency, processing capacity, reactor sizing and ultimately the capital and operating costs, which influence the overall success of chemical looping 81 systems. Oxygen carriers are expected to undergo multiple redox cycles without a significant drop in their physical and chemical performance. For the successful operation of a large-scale chemical looping system, the oxygen carrier must possess certain properties, which have been discussed in Chapter 1. Iron oxide (Fe2O3) is an ideal metal oxide that meets most of the criteria for a successful oxygen carrier. Fe2O3-based oxygen carriers have been extensively studied as part of the previous research of this group as well as few other research groups.23,50,124,125 In fact, CLHG using iron oxide to produce 126 H2 is inspired by the steam-iron process developed by Howard Lane. The traditional process was focused more on H2 generation, whereas CLHG is directed towards using fossil fuels for generation of electricity, high valued chemicals and liquid fuels, along with H2 generation coupled with inherent CO2 capture. Figure 3.1 shows a schematic of the CLHG process developed at OSU. In this CLHG process, Fe2O3 in the reducer is reduced to a mix of Fe/FeO on reacting with the fuel. The reduced oxide is then partially oxidized using steam in the oxidizer to Fe3O4 producing H2, according to Equations (3.3) and (3.4) below: 3Fe + 4H2O Fe3O4 + 4H2 (3.3) 3FeO + H2O Fe3O4 + H2 (3.4) Subsequently, Fe3O4 is completely reoxidized to Fe2O3 in the combustor, according to Equation (3.5): 2Fe3O4 + 0.5FeO 3Fe2O3 (3.5) The overall reaction in CLHG is: 82 Fuel + H2O + O2 H2 + CO2 (3.6) Thermodynamics of iron indicate that oxidation with steam to generate H2 is favorable only with the Fe/FeO phases. Thus, the oxygen carriers from the reducer, after converting the fuel, need to be reduced to the Fe/FeO phases to be able to generate H2 in the oxidizer. As discussed in Chapter 1, the chemical looping combustion and gasification processes developed at OSU operate the reducer in a countercurrent moving bed mode, which allows for a greater degree of conversion of the oxygen carriers in the reducer and consequently a lower solid circulation rate.21 Thermodynamic analyses have revealed that countercurrent moving bed operation can achieve about 50% oxygen carrier conversion in the reducer, which in the case of iron oxide, leads to a mixture of FeO and Fe phases. In order for the CLHG process to be efficient and economic, it is important that the oxygen carriers have high steam conversion efficiency as well as good recyclability. Extensive research in this field has shown oxygen carriers perform best when the active metal oxide component is used in conjunction with an inert support material framework. The different support materials tested in conjunction with Fe2O3 for their reactivity and recyclability performance include Al2O3, MgAl2O4, TiO2, SiO2, ZrO2, YSZ (yttria stabilized zirconia).127 However, from the oxygen carrier performance and the process economics viewpoints, Al2O3, TiO2, and MgAl2O4 are the most suitable support material. The aim of this study is to investigate the behavior of Fe2O3-based oxygen carriers supported by Al2O3, MgAl2O4 and TiO2, for the CLHG process using a series of fixed bed and thermogravimetric studies. The different oxygen carrier formulations are tested for their steam conversion efficiency and their reactivity and recyclability using both air 83 and steam as the oxidizing agent. Pure Fe2O3 has also been tested as a baseline reference. Morphological and phase changes have also been considered to study the overall behavior of the different supports and their interaction with the active material, which in this case are different phases of iron oxide, during the course of oxidation with steam. Additionally, a thermodynamic analysis has also been performed to corroborate and explain the experimental observations. The ultimate objective here is to determine the support material best suited for the CLHG process to be used with Fe2O3 as the active material. 3.2. Materials The oxygen carrier samples were prepared by mechanical mixing and sintering. Powdered samples of Fe2O3, MgAl2O4, Al2O3 and TiO2 were obtained from NOAH Technologies Corporation, with >99% purity and <44µ particle size. Fe2O3 is the active material and MgAl2O4, Al2O3 and TiO2 are the support materials in these formulations. Each of the formulations consists of 50% by weight of Fe2O3, the rest balanced by the support material. For each formulation, the powders were first mechanically mixed in a ball mill. The three supported oxygen carrier formulations, along with pure unsupported o Fe2O3 powder were then sintered in a Fisher Scientific Isotemp® muffle furnace at 900 C for 12 hours. The ramp up rate for the furnace is maintained at 10oC/min. 84 3.3. Experimental Methods 3.3.1. Redox with Air The oxygen carriers are first tested for their reactivity kinetics and recyclability in a Setaram SETSYS Evolution Thermogravimetric Analyzer (TGA). During the recyclability tests, each oxygen carrier sample is subjected to 20 consecutive reduction- oxidation (redox) cycles. The oxygen carrier samples are first heated to 900°C under a o constant N2 flow rate with a ramp-up rate of 55 C/min. Once the reaction temperature is reached, during each redox cycle, the oxygen carrier sample is reduced for 10 mins in a 50% H2 stream, balanced by N2 and then oxidized for 10 mins in a 50% air stream, balanced by N2. Reduction and oxidation are alternated by intermittent flushing for 5 mins with N2. The 20 redox cycles are followed by cooling down the TGA under a constant flow of N2. The extent of (%) reduction and oxidation are calculated using the following equations: ∆푤푟 % reduction = 푚푎푥 x 100% (3.7) ∆푤푟푒푑 ∆푤표 % oxidation = 푚푎푥 x 100% (3.8) ∆푤표푥 푚푎푥 푚푎푥 where, ∆푤푟푒푑 is the theoretical maximum weight loss ring reduction and ∆푤표푥 is the theoretical maximum weight gain during oxidation of the oxygen carrier on the basis of the weight of the active component (which is Fe2O3) in the sample. ∆푤푟 and ∆푤표 are the instantaneous sample weight change during reduction and oxidation respectively. 85 3.3.2. Fixed Bed Steam Oxidation The steam to hydrogen conversion ability of the oxygen carrier formulations are tested in a fixed bed reactor setup as shown in Figure 3.2. The reactor used is a 0.5”ID/0.75”OD quartz cylindrical tube held vertically in a PID controlled tubular furnace (MTI Corporation GSL 1100X). Oxygen carrier samples are loaded into the reactor, held at the bottom with a glass frit and packed with quartz wool on top. For each experiment, sample weight used is 10 gm in the case of the supported Fe2O3 formulations. In the case of pure unsupported Fe2O3, in order to make up a sample weight of 10 gm, 5 gm of unsupported Fe2O3 is balanced with 5 gm of quartz chips. All the gases are injected into the reactor through a gad manifold with a battery of mass flow controllers. All gases are preheated by means of heating tapes in a preheater section, prior to entering the reactor. A syringe pump (ISCO Series 100DM) is used to inject water into the preheater section, where it is converted to steam before entering the reactor. A condenser and a desiccant bed downstream of the reactor remove any water from the outlet gas stream, which was then sent to a micro gas chromatograph (µGC) for product analysis. During the fixed bed experiments, the reactor filled with the oxygen carrier sample is o heated to 900 C under a constant flow of N2. Once the reaction temperature is reached, a reducing gas steam of 50% H2, balanced with N2, with a total flow rate of 300 ml/min is introduced into the reactor. H2 is continuously injected until the oxygen carriers are completely reduced to metallic Fe, which is determined from the gas concentrations seen in the µGC. When the outlet gas concentration reaches 50% H2, H2 flow is turned off and the reactor is flushed with N2 until there is no visible trace of H2 in the outlet gas. N2 86 flushing is followed by steam injection for reoxidation of the reduced oxygen carrier samples. Stoichiometric amount of water required to completely convert the reduced Fe to Fe3O4 is injected using the syringe pump. The fixed bed experiments are carried out at two different water flow rates – 0.03 ml/min and 0.06 ml/min – resulting in steam space -1 -1 velocities (SSV) of 0.36 hr and 0.72 hr , respectively. N2 acts as the carrier gas during the steam oxidation step. Gas concentrations are recorded until no trace of H2 is observed in the µGC. The outlet H2 molar flow rate and the steam conversion values are calculated using the following equations: V̊N2−in V out̊ = (3.9) yN2 V H̊ 2 = V out̊ x yH2 (3.11) (3.12) VH2 = ∑ V H̊ 2 V m = H2 (3.13) H2 22.4 mH Steam conversion = 2 x 100 (3.14) mH2O−in where V out̊ and V ̊H2 are the total and H2 volumetric flow rates at the outlet, respectively. V N̊ 2−in is the N2 volumetric flow rate into the reactor. yH2 and yN2 are the concentrations of H2 and N2 at the outlet, respectively. VH2 is the total volume of H2 generated. mH2 and mH2O−in are the total moles of H2 generated and total moles of water injected into the reactor, respectively. 87 3.3.3. Redox with Steam The recyclability of the MgAl2O4-supported Fe2O3 is studied in a magnetic suspension balance (MSB, Rubotherm GmbH, US-2004-00162) setup as shown in Figure 3.3. A gas manifold, syringe pump and preheater section setup similar to the fixed bed experiments are used with the MSB. The unique working principle of the MSB allows the sample weight and the balance to be connected via magnetic coupling, and therefore, keeps the balance isolated from (and not affected by) the reaction environment. This principle allows for the use of high pressure and/or highly corrosive environments in the sample cell and allows for the use of steam as one of the reactive gases. The section of the MSB housing the magnetic coupling is maintained at 140°C by means of a heat jacket connected to an oil bath. The reaction temperature in the sample cell is maintained independently by means of a PID controlled electric furnace. o The sample is heated to 900 C in an inert flow of N2 at 250 ml/min. When the reaction temperature is achieved, a reducing mixture with 50% H2 balanced with N2 and total flow rate of 250 ml/min is introduced into the sample cell. When the sample stops showing any significant change in weight, H2 injection is stopped and the sample cell is flushed with N2 for 10 mins to ensure complete removal of H2. Flushing is followed by water injection at the rate of 0.0935ml/min for 7 mins. The water flow rate is calculated so as to introduce an oxidizing stream of 50% steam, balanced with N2 making a total flow rate of 250 ml/min. A steady flow of N2 is maintained after steam injection is stopped until no significant increase in weight is observed (~53 mins) and then H2 is injected again for the next reduction step. The oxygen carrier sample is subjected to 20 such consecutive redox 88 cycles. The same experimental setup is also used to investigate the effect of pressure on the steam oxidation kinetics of the MgAl2O4-supported Fe2O3 oxygen carrier. A back pressure regulator (BPR) is used downstream of the sample cell to regulate the pressure inside. The gas mass flow rates are maintained such that the gas space velocity experienced by the sample is constant at different pressures (~1 min-1 for the volume of the sample cell). Similar to the recyclability test, the sample is heated to the reaction temperature and pressure in an inert flow of N2. The gases are switched to introduce the reducing gas in the sample cell, followed by N2 flushing and then steam oxidation. The sample weight, temperature, and pressure are recorded as a function of time. The effect of pressure on the steam oxidation kinetics is evaluated using the % weight gain over time during oxidation. 3.3.4. Solid Characterization and Gas Analysis The solid samples from all the experiments are collected, milled and sieved to the appropriate size. The solid samples are then analyzed for the formation of different phases and complexes under X-Ray Diffraction (Rigaku SmartLab Powder XRD). XRD scans are run from 20o to 80o at the rate of 2o/min with an accelerating voltage and filament current of 40kV and 44mA, respectively. A micro Gas Chromatograph (GC, Varian CP4900 by Agilent Technologies) is used to analyze the product species and their concentration in the outlet gas stream during the fixed bed steam oxidation experiments. 89 3.3.5. Thermodynamic Analysis Isothermal modelling of the chemical looping oxidizer performance is completed to analyze the differences in the supported and unsupported Fe2O3 based oxygen carrier formulations for steam oxidation purposes. The theoretical thermodynamic simulation is completed using ASPEN PLUS (v8.4) simulation software, which is a standard and valuable investigation tool used in aiding chemical process design and simulation work. A common set of assumptions are used for the simulations performed. The thermodynamic phase diagram is obtained by using a single stage RGIBBS reactor system in ASPEN. This built-in module determines the equilibrium composition as it minimizes the free energy of the components used. The basic RGIBBS model requires specification of two or three (heat-duty, temperature and pressure) definition parameters. The RGIBBS model has the flexibility of considering all the global components defined as its products or a small subset of the desired products for equilibrium investigations. The thermodynamic properties of the different formulations have been compared in terms of solids and gas conversions. The parameters used are defined in Appendix A, along with details about the thermodynamic conversion calculations. 3.4. Results and Discussion 3.4.1. Redox cycles with air oxidation Figure 3.4 shows the thermogravimetric curves of the 20 redox cycles for each of the oxygen carrier formulations. As discussed in Chapter 1, unsupported Fe2O3 is unable to maintain its recyclability over multiple redox cycles, exhibiting a drop in its reactivity 90 right after the first cycle, as can be seen in Figure 3.4(d). On the other hand, as seen in Figure 3.4(a-c), all the three supported Fe2O3 based oxygen carriers show excellent recyclability, without showing any significant drop in their reactivity over the 20 cycles. MgAl2O4 and Al2O3-supported Fe2O3 require 2-3 redox cycles to get activated and reach their complete reactive potential. This can be more clearly seen in Figure 3.5, where the % reduction of the MgAl2O4 and Al2O3-supported Fe2O3 go through a minimum before increasing and stabilizing. Also, from Figure 3.5, it is seen that TiO2-supported Fe2O3 exhibits the best redox recyclability as it is able to consistently maintain its reactivity close to 100% over the 20 cycles. Recyclability here is defined in terms of the % solid reduction achieved during the reduction step of each redox cycle. After the initial activation, Al2O3-supported Fe2O3 is able to achieve ~96% reduction conversion throughout the 20 cycles, while for MgAl2O4-supported Fe2O3, the reduction conversion is slightly lower at ~86% after the initial activation. Figure 3.6 compares the % th conversion (reduction or oxidation) for all three supported Fe2O3 during the 15 redox cycle in order to examine the effect of support on the reaction kinetics. As seen in Figure 3.6(a), TiO2-supported Fe2O3 exhibited slightly slower reduction kinetics. However, all three supported formulations are able to achieve their respective maximum conversions in ~15mins. On the other hand, when it comes to air oxidation (Figure 3.6(b)), TiO2- supported Fe2O3 showed the fastest kinetics, followed by Al2O3 and then MgAl2O4. The slower oxidation kinetics exhibited by the MgAl2O4-supported Fe2O3 explains the lower % reduction values during the 20 cycle recyclability test shown in Figure 3.5. Since the oxidation time is fixed, the MgAl2O4-supported Fe2O3 particles do not regain all the 91 oxygen lost upon reduction during the oxidation step. As a result, the oxygen available for the following reduction step is low, hence the low reduction conversion. The oxygen carrier performance in terms of reactivity and recyclability during multiple redox cycles with air as the oxidizing agent can be ranked in terms of the support material as TiO2 > Al2O3 > MgAl2O4. This follows the discussion about TiO2-supported Fe2O3 is Chapter 1, 50 where Li et al. have demonstrated that addition of TiO2 to Fe2O3 leads to the creation of oxygen vacancies which allow easier and faster diffusion of oxygen anions through the crystal structure. 3.4.2. Fixed bed hydrogen generation Figure 3.7 compares the outlet H2 concentration values over time for each of the oxygen carriers at the two steam space velocities. As expected, a higher steam space velocity -1 results in a higher value of maximum outlet H2 concentration. At 0.36 hr steam space velocity, the maximum H2 concentration observed at the outlet is between 25-35% for each oxygen carrier formulation, and as the steam space velocity is doubled at 0.72 hr-1, the maximum H2 concentration obtained at the outlet is between 45-55%. In the case of Al2O3-supported Fe2O3 (Figure 3.7(c)) and unsupported Fe2O3 (Figure 3.7(a)), the outlet H2 concentration goes through a second peak before eventually dropping down to zero. This second peak is much more pronounced for the low steam space velocity tests, which suggests some kind of diffusion effect. The probable cause for this second peak in the case of Al2O3-supported Fe2O3 and unsupported Fe2O3 will be discussed further in Section 3.4.3. 92 Figure 3.8 compares the total moles of H2 generated per gram of the oxygen carrier sample during the fixed bed steam oxidation experiments. As seen from the figure, the change in the steam space velocity has no significant effect on the cumulative moles of H2 generated for each oxygen carrier formulation. The steam space velocity does not influence the total H2 generated, as long as the total amount steam sent into the reactor is kept constant. Unsupported Fe2O3 generates on an average 11.6 mmol/g (of active metal oxide) during the steam oxidation experiments. The addition of support material to Fe2O3 slightly improves the total amount of H2 generated. For example, with Al2O3 as the support, the average amount of H2 generated is 12mmol/g, which is a 4% increase from unsupported Fe2O3. With MgAl2O4 as the support, the average H2 generated is 12.4 mmol/g, which is a 7.4% increase, and with TiO2 as the support, on an average 12.7 mmol/g of H2 is generated, which is a 10% increase from unsupported Fe2O3. Thus, there is a small increase in the total amount of H2 produced with the addition of supports, with TiO2-supported Fe2O3 exhibiting the best performance. It is important to note that the main purpose of the addition of the support materials to Fe2O3 is to enhance its recyclability so as to sustain its reactivity over multiple redox cycles. Figure 3.9 shows the cumulative volume of H2 generated during the fixed bed steam oxidation of the reduced oxygen carriers over time. The figure provides a comparison of the effect the addition of different supports has on the rate of H2 generation. The addition of MgAl2O4 and Al2O3 does not show any significant change in the rates of H2 generation from the reduced oxygen carrier (which is metallic Fe in this case). However, the TiO2- supported oxygen carrier exhibited enhanced H2 generation rates. For steam oxidation 93 with both the steam space velocities, TiO2-supported Fe2O3 achieves the maximum rd possible oxidation in about 1/3 of the time taken by Al2O3, MgAl2O4-supported and unsupported Fe2O3. The steam to H2 conversion efficiency of the oxygen carrier formulations is shown in Figure 3.10. The steam conversion is calculated on the basis of stoichiometric amount of steam required to reoxidize metallic Fe to Fe3O4, which is introduced in the reactor during oxidation. Consequently, from Figure 3.8, even Figure 3.10 shows that supported Fe2O3 shows slightly higher steam to H2 conversion efficiency as compared to unsupported Fe2O3. The most important thing to note in Figure 3.10 is that even with stoichiometric amounts of steam, both supported and unsupported oxygen carriers exhibited very high steam to H2 conversion efficiencies. For all the oxygen carriers tested in the fixed bed experiments, including unsupported Fe2O3, the steam conversion efficiency is between 70-80%. 3.4.3. Solid Characterization and Phase Evolution The XRD spectra of fresh and reoxidized samples of all the oxygen carriers from the fixed bed experiments (supported and unsupported) are presented in Figure 3.11 and the different phases observed during the XRD analysis of the various samples have been summarized in Table 3.1. It is important to note here that each oxygen carrier sample is first reduced to their most reduced form (which is metallic Fe) before being reoxidized with steam. From the XRD spectra shown in Figure 3.11(a) and the summary provided in the table, it is observed that unsupported Fe upon reoxidation with steam consists of 94 different phases of iron oxide, indicating incomplete oxidation. All three phases including Fe, FeO and Fe3O4 are present in the reoxidized sample. This again can be explained by the slower oxygen anion diffusion in unsupported Fe, due to the lack of oxygen vacancies, otherwise created by the addition of certain support materials. Unsupported Fe is also known to exhibit sintering effects at elevated temperatures leading to particle agglomeration and loss of active surface area. The sintering effects can affect the inward diffusion of steam into and the outward diffusion of H2 from the sample. This can explain the occurrence of the second peak in Figure 3.7(a) and the fact that it is more pronounced at the lower steam space velocity. In the case of Al2O3-supported Fe2O3, upon reoxidation with steam, the phases seen are Fe, FeO and Fe3O4 as well as a fourth phase FeAl2O4 (known as hercynite), which is a complex formed between the active iron oxide phase FeO and the support material Al2O3 (Figure 3.11(b)). Once the FeAl2O4 phase is formed during steam oxidation, there is no further oxidation to the Fe3O4 phase. Upon further oxidation with air, FeAl2O4 is completely oxidized to Fe2O3 and Al2O3. In case of Al2O3 as a support, the redox reactions proceed by a different pathway, which is as follows: 3Fe2O3 + Al2O3 + H2 2Fe3O4 + H2O (3.15) Fe3O4 + 3Al2O3 + H2 3FeAl2O4 + H2O (3.16) FeAl2O4 + H2 Fe + Al2O3 + H2O (3.17) Thus the Al2O3 does not remain inert and forms the FeAl2O4 spinel structure with the FeO phase. Further oxidation of FeAl2O4 to Fe3O4 using steam is thermodynamically 95 unfavorable and hence majority of the reoxidized sample remains in the FeAl2O4 phase. Studies have also revealed that formation of the hercynite phase tends to affect the gas diffusion into and out from the particles.128 Poor diffusion can be one of the reasons behind the second peak observed in Figure 3.7(b), especially with the lower steam space velocity value. Since Fe2O3 and Al2O3 are mixed in a 1:1 ratio by weight, the molar ratio of FeO and Al2O3 during reoxidation is >1:1. Therefore, there would be excess of FeO left after forming the spinel phase. This excess FeO will be further oxidized to the Fe3O4 phase by steam, which explains the presence of Fe3O4 along with FeAl2O4 in the XRD spectrum of the reoxidized sample. In the case of TiO2-supported Fe2O3, the XRD spectrum (shown in Figure 3.11(c)) of the reoxidized samples exhibits Fe3O4, with a complex phase again, which is FeTiO3 or the ilmenite phase. It is also observed that the fresh unreacted sample of TiO2-supported Fe2O3 contains a pseudobrukite phase, Fe2TiO5, in addition to TiO2 and Fe2O3. Fe2TiO5 is the highest oxidized form of FeTiO3. In fact, sintering the oxygen carrier formulation at high temperatures leads to interaction between Fe2O3 and TiO2 and formation of Fe2TiO5, which means the support is not inert to begin with. Fe2O3 and TiO2 react in a 1:1 ratio to form Fe2TiO5. During synthesis, Fe2O3 and TiO2 are mixed in a 1:2 molar ratio, so ideally the all the Fe2O3 should form the pseudobrukite phase, leaving behind some extra TiO2. But the fact that all three phases, TiO2, Fe2O3 and Fe2TiO5 are seen during XRD indicates that not all the Fe2O3 reacted with the TiO2. The redox reactions in the case of Fe2TiO5 are presented in Equations (3.18) and (3.19): 96 Fe2TiO5 + TiO2 + H2 → 2FeTiO3 + H2O (3.18) FeTiO3 + H2 Fe + TiO2 + H2O (3.19) Once FeTiO3 is formed during oxidation, further oxidation with steam is thermodynamically unfavorable. This is evident in the XRD spectrum of the reoxidized sample which contains FeTiO3, TiO2 and Fe3O4. Majority of the reduced metallic Fe oxidizes to FeO, which then complexes with TiO2 to form ilmenite. Ideally all the FeO should react with TiO2, but the presence of Fe3O4 in the oxidized sample is an indication that some FeO underwent further oxidation without forming a complex with TiO2. The reduced sample of MgAl2O4-supported Fe2O3, upon reoxidation with steam, is oxidized to the FeO and Fe3O4 phases as shown in Figure 3.11(d). No interaction is observed between active iron oxide phases and the support material. There is no unconverted Fe phase observed in this case and MgAl2O4 is the only support material, amongst the three investigated, that remains completely inert during the redox reactions. The experimental observations and results closely match the thermodynamic analysis presented in the following section. 3.4.4. Thermodynamic Analysis The theoretical thermodynamic performance of the different oxygen carrier formulations has been presented in terms of the relationship between their equilibrium gas and solid conversions in Figure 3.12. Both Al2O3 and TiO2-supported oxygen carriers have high steam conversions but only at low solid conversion values. For both these formulations, the formation of complexes with the support material, limits their ability to effectively 97 generate H2 without sacrificing on steam conversion. It is important to note that in the fixed bed steam oxidation experiments, the oxygen carriers were first reduced to metallic Fe before being reoxidized. This explains the high steam conversion values recorded in the case of Al2O3 and TiO2-supported Fe2O3, despite the formation of complexes. However, in a larger scale chemical looping reducer, the solids are reduced to a mixture of Fe/FeO with a larger fraction of FeO phase. The formation of Fe is minimized so as to avoid carbon deposition, which is catalyzed by presence of metallic Fe in the reducer. This means in an integrated process, the steam to H2 conversion efficiencies observed would be significantly lower when complexes like FeAl2O4 and FeTiO3 are formed. In fact, if no Fe is formed in the reducer, then there would be no H2 generation in the oxidizer at all. MgAl2O4 on the other hand does not form any complexes with the active iron oxide phases and is able to achieve higher solid conversion values. In the case of the MgAl2O4-supported Fe2O3, a larger range of Fe oxidation states can be used to generate H2, which means that in an integrated system MgAl2O4-supported iron oxide will still show steam conversion values similar to the results achieved here. It is also important to note that the experimentally calculated steam to H2 conversion efficiencies closely match these thermodynamically predicted values. 3.4.5. Redox cycles with steam oxidation MgAl2O4-supported Fe2O3 was selected for further studies owing to the extremely inert nature of the support material and its ability to oxidize to the highest oxidation state possible without forming any complexes with the active iron oxide phases. The results from the recyclability studies in the MSB are shown in Figure 3.13. As seen in the 98 recyclability data, the oxygen carrier sample was completely reduced during each reduction step, which is indicated by the plateauing of the curve following the drop in weight. The small drop in weight after the plateau represents the N2 flushing. The increase in weight, post the flushing, represents the reoxidation of the reduced sample during steam injection. During the first redox cycle, all the weight lost during reduction is not gained during oxidation. Since complete oxidation of Fe to Fe2O3 is not thermodynamically favorable using steam, the reduced oxygen carrier sample can only be oxidized to Fe3O4. The oxygen carrier conversion achieved during each steam oxidation step remains fairly consistent over the 20 redox cycles as indicated by the conversion values shown in Figure 3.14. Throughout the 20 redox cycles, the oxygen carrier conversion achieved during steam oxidation in each cycle remains consistently between 70-76%. Complete oxidation of Fe to Fe3O4 gives an oxygen carrier conversion value of 89%. The oxygen carrier conversion values achieved during steam oxidation indicate a mix of FeO and Fe3O4 phases, which is also supported by XRD analysis of the reoxidized sample. Thus, MgAl2O4-supported Fe2O3 showed excellent recyclability throughout the experiment, without showing any significant drop in its reactivity over the 20 redox cycles. 3.4.6. Effect of Pressure Operating a chemical looping system at elevated pressures can have economic as well as operational advantages, as discussed in Chapter 2. Hence it is important to investigate the impact of higher pressures on the steam oxidation kinetics of the oxygen carriers. The % weight gain over time during steam oxidation of MgAl2O4-supported Fe2O3 at 1 and 5 99 atm is compared in Figure 3.15. As seen from the figure, increased pressure has a significant impact on the steam oxidation kinetics of MgAl2O4-supported Fe2O3. The reduced oxygen carrier achieves about 16.6% weight in 15 minutes at 5 atm, while in the same time at 1 atm, the weight gain achieved is 10.1%. In the same oxidation time, the weight gain gained at 5atm is 1.64 times that achieved at 1 atm. The advantages of the significantly faster oxidation kinetics at elevated pressures include smaller reactor sizing, increased processing capacity and lower capital costs. Chapter 2 consists of a detailed study of elevated pressures on the reduction and air oxidation kinetics of Fe2O3-based oxygen carriers. 3.5. Concluding Remarks and Future Work Chemical looping has great potential to be developed into a commercial technology for generation of H2 from fossil fuels, while inherently producing a sequestration ready stream of CO2. For a successful CLHG process, in addition to the criteria mentioned in Chapter 1, oxygen carriers also need to exhibit high steam to H2 conversion efficiency as well as good recyclability with steam oxidation. Fe2O3 supported with three different materials, Al2O3, TiO2 and MgAl2O4, have been studied for their oxidation behavior with steam. Steam to H2 conversion efficiencies between 70-80% were obtained during the fixed bed steam oxidation experiments, even with stoichiometric amounts of steam. While TiO2-supported Fe2O3 shows the fastest steam oxidation kinetics, both TiO2 and Al2O3 form complexes with the active FeO, which restricts its further oxidation to Fe3O4. In an integrated chemical looping system, the oxygen carriers are only reduced to FeO in the reducer. Hence, TiO2 or Al2O3-supported Fe2O3 would not form any H2 in the 100 oxidizer due to the formation of the FeTiO3 or FeAl2O4 complex phases respectively. MgAl2O4, on the other hand, continues to remain inert throughout and allows for oxidation of metallic Fe completely to Fe3O4, thus utilizing the entire range of oxidation states available for steam oxidation, which makes MgAl2O4 a suitable choice of support material for CLHG. The experimental results have been corroborated with thermodynamic analyses of the different oxygen carrier formulations. Furthermore, MgAl2O4-supported Fe2O3 exhibited excellent recyclability, consistently maintaining conversions between 70-75% during steam oxidation over 20 redox cycles. Additionally, MgAl2O4-supported Fe2O3 also exhibited significantly enhanced steam oxidation kinetics at an elevated pressure of 5 atm, which can translate into increased processing capacities, reduced reactor sizing as well as capital costs. Future work in this area would involve developing the MgAl2O4-supported Fe2O3 formulation into oxygen carriers with the optimum reactivity as well as attrition resistance for long-term operation in an integrated chemical looping system. The goal is to develop oxygen carriers that not only show excellent reduction and oxidation conversions over hundreds of redox cycles, but also do not show any significant drop in their mechanical strength during the cycles. Both these criteria will ultimately dictate the reactor sizing, efficiency, and capital and operating costs of a chemical looping system. 101 Figure 3.1: Schematic of Chemical Looping Hydrogen Generation process 102 103 Figure 3.2: Schematic of experimental setup for fixed steam oxidation tests Figure 3.3: Schematic of experimental setup of the magnetic suspension balance for the recyclability studies using steam 104 Figure 3.4: TGA curves of the 20 redox cycles for (a) MgAl2O4-supported Fe2O3; (b) TiO2-supported Fe2O3; (c) Al2O3-supported Fe2O3; and (d) Unsupported Fe2O3 105 Figure 3.5: % Reduction achieved in each redox cycle for the supported Fe2O3 during 20 cycle recyclability test 106 Figure 3.6: % Conversion of oxygen carrier during 15th redox cycle (a) Reduction; (b) Oxidation 107 Figure 3.7: Fixed bed outlet H2 concentration with different oxygen carrier formulation (a) Unsupported Fe2O3; (b) TiO2-supported Fe2O3; (c) Al2O3-supported Fe2O3; and (d) MgAl2O4-supported Fe2O3 108 Figure 3.8: Total H2 generated during fixed bed experiments with different oxygen carrier formulations 109 Figure 3.9: Cumulative H2 generated over time during fixed bed steam oxidation with different steam space velocities (a) 0.36 hour-1; (b) 0.72 hour-1 110 Figure 3.10: Steam to H2 conversion efficiency of the different oxygen carriers during fixed bed steam oxidation experiments 111 Continued Figure 3.11: XRD analysis of the oxygen carriers – fresh and reoxidized after single o redox cycle with H2 reduction and steam oxidation at 900 C (a) Unsupported Fe2O3; (b) Al2O3-supported Fe2O3; (c) TiO2-supported Fe2O3; and (d) MgAl2O4-supported Fe2O3 112 Figure 3.11 continued 113 Support → MgAl2O4 TiO2 Al2O3 No Support Phases ↓ Fe FeO Fe3O4 Complex n/a with support Table 3.1: Summary of different iron phases found in steam reoxidized oxygen carrier samples during fixed bed experiments 114 Figure 3.12: Thermodynamic step-diagram of steam and solid conversions of different oxygen carrier formulations at 900oC, 1 atm 115 116 Figure 3.13: Weight change of MgAl2O4-supported Fe2O3 sample during 20 redox cycle TGA o analysis with H2 reduction and steam oxidation at 900 C Figure 3.14: Oxidation conversion values for MgAl2O4-supported Fe2O3 during 20 redox cycle recyclability test 117 Figure 3.15: Effect of pressure on %weight gain vs time during steam oxidation of MgAl2O4-supported Fe2O3 118 CHAPTER 4 4. BIOMASS CONVERSION AND TAR CRACKING WITH Fe2O3 BASED OXYGEN CARRIERS Part of the material in Chapter 4 has been reprinted with permission from Luo, S., Majumder, A., Chung, E., Xu, D., Bayham, S., Sun, Z., Zeng, L., and Fan, L.-S. (2013). Conversion of woody biomass materials by chemical looping process – kinetics, light tar cracking, and moving bed reactor behavior. Ind. Eng. Chem. Res., 52(39), 14116-14124. Copyright © 2013 American Chemical Society. My contribution involved conducting the TGA experiments for biomass decomposition kinetic studies, calculations for the kinetic studies and preparation of the manuscript for the above mentioned publication. 4.1. Introduction Biomass has been garnering increasing attention in the recent years as a renewable source of energy. It can be used to generate electricity, and for the synthesis of hydrogen and valuable liquid fuels. Biomass to energy conversion processes include both biochemical and thermochemical techniques.129 Thermochemical processes offer more flexibility in terms of feedstock requirements, which make them the preferred techniques for traditional biomass to energy conversion processes. However, a major challenge 119 associated with thermochemical biomass conversion processes is the evolution of tar. Tar is a complex mixture of condensable hydrocarbons, which includes single ring to 5-ring aromatic compounds, other oxygen-containing hydrocarbons and complex polycyclic aromatic hydrocarbons.130 Presence of tar in the system is highly undesirable as it hinders the continuous operation of systems, leads to catalyst deactivation, causes the failure of downstream processing equipment and it also is a source of significant loss of usable carbon. The three main thermochemical processes that have been extensively studied are combustion, gasification and pyrolysis. Biomass pyrolysis can yield a variety of bio-oils and valuable liquid fuels by manipulation of the reaction parameters.131 However, it is a very complex process, which needs to be thoroughly researched before it can be commercialized. Biomass combustion and gasification are very similar to coal combustion and gasification processes respectively and have the potential to replace coal successfully.132 Biomass gasification has been successfully demonstrated with the help of various tar cracking and steam reforming catalysts. However, the high moisture content and low energy density of biomass makes these processes energy and cost intensive. This issue can be partly offset by co-firing of biomass and coal to the combustion and gasification processes. The need to employ various carbon capture techniques with these processes further reduces their process efficiency. Hence, it becomes imperative to develop a highly efficient biomass to energy conversion process that can successfully replace the use of fossil fuels. In recent years, chemical looping has developed into a promising technology for the conversion of carbonaceous fuel.99,133,134 It is a cost- 120 effective and environmentally benign technology due to its ability to inherently sequester 3 CO2. The flexibility of chemical looping with respect to fuel types makes it very appealing for the conversion of biomass to energy. BCL has the potential to be developed into a highly efficient biomass to energy conversion process because of its ability to capture CO2 in-situ. Figure 4.1 is a schematic of a BCL system with three reactors – the reducer, the oxidizer and the combustor. As shown in the figure, biomass is injected in the reducer, where it first reacts with the oxygen carriers. The rest of the process is the same, as described in Chapter 1. The heat generated in the combustor is integrated with the reducer by circulating the oxygen carrier particles in order to increase the process efficiency. CO2, H2 and heat are produced in three separate reactors and hence no energy intensive product separation is required. This process has the capacity to produce a H2 stream with >99.9% purity and a CO2 stream with about 99.8% purity. Because of its ability to effectively sequester CO2, a BCL system is carbon negative from the Life Cycle Analysis (LCA) standpoint. Table 4.1 gives a comparison for a 100MWth BCL system and a biomass IGCC (Integrated Gasification Combined Cycle) with and without carbon capture.135 The base biomass IGCC system is only 30% efficient and the efficiency drops down to 21% with about 90% carbon capture. For a BCL system without H2 generation, the theoretical net power generation efficiency is 38% with 100% CO2 capture. With co-generation of H2, the efficiency of the BCL process increases to 69%. Consequently, the cost of power from a BCL system is almost half that from the IGCC system. 121 Oxygen carriers play a crucial role in the successful operation of a chemical looping system as discussed in Chapter 1. The physical and chemical properties of the oxygen carriers dictate the fuel conversion efficiency, processing capacity, reactor sizing and ultimately the capital and operating costs, which influence the overall success of chemical looping systems. Oxygen carriers are expected to undergo multiple redox cycles without a significant drop in their physical and chemical performance. Oxygen carriers perform best when the active metal oxide component is used in conjunction with an inert support material framework. Supported Fe2O3-based oxygen carriers have been extensively studied as part of the previous research of this group.23,30,136 Magnesium aluminate (MgAl2O4) has been found to have certain desirable properties like high physical strength at elevated temperatures, high melting point and high resistance to chemical deactivation, which make it a good choice for support material in chemical 137 looping applications. MgAl2O4-supported Fe2O3 has been previously successfully studied as oxygen carrier material by a number of research groups.32,125 The spinel structure of MgAl2O4 gives it the stability desired in inert support materials. Hence MgAl2O4-supported Fe2O3-based oxygen carriers have been selected for this study. The following redox reactions have been suggested for biomass derived hydrogen production in the BCL process: (2n + m)FexOy + CnH2m → (2n + m)FexOy-1 + nCO2 + mH2O (4.1) FexOy-1 + H2O → FexOy + H2 (4.2) 122 Uddin et al. suggested a possible reaction mechanism where the surface iron (-Fe) connected to the oxide Fe3O4 reacts with biomass derived tar to produce CH4, C2H4, 138 CO, H2O, and H2 in the first stage of the reaction. The produced CO is then converted to CO2 and H2 by the water-gas shift reaction over the iron oxide active site (-Fe-OH), which is different from that for tar decomposition. Over multiple reaction cycles, the activity for water-gas shift reaction decreases, but the activity for tar decomposition is hardly affected. Polychronopoulou et al. proposed another reaction mechanism for the dissociative adsorption of phenol (a secondary tar model molecule) over iron oxide as shown in Figure 4.2.139 Fe2O3 is an ideal candidate for oxygen carrier material for chemical looping. However, its reactivity towards tar is lower compared to the traditional tar cracking catalysts used. Hence, it shows loss in activity over time due to coke deposition caused by the unconverted tar components. On the other hand, the tar cracking catalysts like dolomites, olivines, K2CO3 and NiO have been studied extensively and are effective for tar elimination under various conditions. Some of them in fact have the ability to crack even the most stable tar compounds like naphthalene, benzene etc. For example, Delgado et al. reported that the use of calcined dolomites decreases the tar content from 6.5 wt% to 1.3 wt%.140 Narváez et al. suggested that their addition along with the biomass feed not only improves the product gas quality but also results in a 40% tar reduction.141 The elimination of tar over calcined dolomites occurs mostly due to steam and dry reforming reactions. In spite of these advantages, there are a few critical limitations. Dolomite is very soft and has very low attrition resistance. Dolomite additives generate a large 123 amount of fines during calcination, leading to transport problems between reactor beds. Also the chlorine content in the biomass fuel can react with the calcium to form calcium chloride, which causes the catalytic activity to subside over time. Olivines are attractive as tar cracking catalysts for their low cost and high attrition resistance even at high temperatures.142 While their catalytic activity is comparable to that of dolomites, they perform significantly better in terms of mechanical strength in a fluidized bed environment. Wen et al. and Simell et al. studied the activity of clay minerals as bed additives for tar cracking.143,144 Clay minerals are an attractive catalyst option for tar cracking because of the low cost and relatively fewer disposal problems. However, their catalytic activity is lower than that of dolomites and they do not support the high temperatures (>800oC) required for tar cracking. Char is attractive as a tar cracking catalyst for its easy availability since it is naturally produced in the gasifier.145 Its catalytic activity is comparable to dolomites, but it is heavily dependent on the pore size, surface area and the mineral and ash content of char. It gets rapidly deactivated because of coke formation, which blocks the pores and reduces the surface area. Moreover, it is consumed by gasification reactions caused by steam and CO2 in the reactor and hence needs to be constantly supplied externally. Alkali metals are naturally found in the gasifier as a part of the ash remaining after biomass gasification. This makes them very attractive as tar removal catalysts from the economic standpoint. Additionally, using alkali metals as catalysts solves the problem of handling the ash wastes. Suzuki et al. found that alkali metals are effective in catalyzing gasification reactions by 146 steam and CO2. However, they lose their activity at high temperatures because of 124 particle agglomeration, which leads to loss of contact between the catalyst and char. Another disadvantage is that of catalyst recovery post reaction. Simell et al. found that activated alumina was as effective as dolomites in cracking tar.144 The activity of activated alumina depends on the complex mixture of aluminum, oxygen and hydroxyl ions that combine to produce both acidic and basic sites. However, it can undergo rapid deactivation due to coke formation. Transition metal based catalysts are very effective for steam and dry reforming of methane and other hydrocarbons. Supported nickel catalysts have been extensively studied for their ability to crack tar components. The main advantage of Ni-based catalysts is their ability to eliminate tar completely at temperatures of ~900oC. They are almost 8-10 times more active compared to dolomites under similar operating conditions and also increase the yields of CO and H2 in the product stream.147,148 But sintering effects, sulfur poisoning and coke formation result in rapid deactivation and loss of surface area of the Ni-catalysts. These effects are however reversible and can be controlled by pretreating the feedstock.149,150 Thus it is clear that each one of the traditional tar cracking catalysts has certain drawbacks, which render them unsuitable to be used as oxygen carriers in chemical looping systems by themselves. Biomass conversion occurs in several sequential steps. First devolatilization of the biomass releases the volatiles and tar, leaving behind just char and ash. Oxygen carriers in the BCL process convert volatiles, tar and char to CO2 either directly or indirectly. In the presence of enhancing gases, like steam or CO2, char reacts with the enhancing gas to form CO and/or H2. The resulting gas would be further oxidized to CO2 and H2O by oxygen carriers. The decomposition kinetics of soft woody biomass is studied through 125 TGA experiments. However, the focus of this study is placed in designing oxygen carriers suitable for the complete elimination of tar in the reducer. The effect of using different tar cracking catalysts as additives to Fe2O3 based oxygen carriers on the decomposition of tar components have been studied in fixed bed experiments. The tar cracking catalysts will be used in conjunction with the Fe2O3 based oxygen carriers to enhance the carriers’ ability to crack tar components. TGA experiments have been used to evaluate the oxygen transport capacity, redox reactivity and recyclability of the composite oxygen carriers. They are then tested for their tar cracking ability in a series of fixed bed experiments with a Gas Chromatograph-Mass Spectrometer (GC-MS) connected to the outlet to analyze the reaction products. The fresh and reacted solids have been analyzed using XRD and SEM. Based on the TGA and fixed bed experiments, addition of NiO to Fe2O3 exhibited the best redox reactivity, recyclability and tar cracking ability. The NiO content in the supported-Fe2O3 oxygen carriers is further optimized with more fixed bed tar cracking experiments. The outcome of this study is expected to assist in the synthesis of cost effective and efficient functional materials for thermochemical biomass conversion. 4.2. Materials The biomass used in this study is from BMQ Inc. It is a blend of softwood pines from the Midwest and Southern United States. Dried wood pieces were fed through a pellet mill to form uniform, pressed pellets. In this study, the biomass pellets were further milled for 10 minutes in a ball mill to obtain a powdered form with a particle size of less than 800 microns. The ultimate analysis for the biomass sample is provided in Appendix B. Fe2O3 126 and/or different tar cracking catalysts were mechanically mixed with MgAl2O4 and sintered at 900oC for 12 hours. The composite oxygen carrier formulations prepared are shown in Table 4.2. All formulations contain 50% MgAl2O4 support. The chemicals used were obtained from NOAH Technologies Corporation, with >99% purity and 325 mesh size. 4.3. Experimental Procedure 4.3.1. Biomass Decomposition Kinetics A Setaram SETSYS Evolution TGA was used to perform the experiments to study the decomposition kinetics. It has a maximum temperature limit of 1000oC and a maximum heating rate of 100 K/min. The kinetics of the two stages of biomass decomposition, namely devolatilization and char gasification, were studied using the following three different sets of experiments: 1. Non-isothermal devolatilization of biomass in N2 environment 2. Non-isothermal devolatilization of biomass in CO2 environment 3. Isothermal gasification of biomass derived char in CO2 environment In the non-isothermal experiments, powdered biomass sample was devolatilized by heating it from 20oC to 900oC at a constant heating rate. Each of these tests was carried out at different constant heating rates in the range of 2 and 10 K/min. In the isothermal experiments, each test was carried out by initially devolatilizing the sample in the TGA to obtain biomass derived char. The devolatilization stage involved heating the sample from 20oC to the gasification temperature at a constant heating rate of 55 K/min under a 127 constant flow of nitrogen. Once the desired temperature was reached, the gas was switched from nitrogen to carbon dioxide in order to carry out char gasification, during which the temperature was held constant to maintain isothermal conditions. Gasification was carried out at temperatures between 850oC and 950oC. It should be noted that during all tests, the gas flow rate was kept constant at 100 ml/min and the TGA cooling rate was maintained at a constant of 55 K/min. For every test, the sample was evenly dispersed in the sample holder to minimize any internal heat and gas diffusion influence. 4.3.2. Oxygen Carrier Reactivity Screening The oxygen carrier formulations were then screened in a Setaram SETSYS Evolution TGA for two different criteria – (i) Oxygen Transfer Capacity (OTC), and (ii) Recyclability. In the first set of experiments, to determine the OTC, the TGA was o heated to 900 C under Nitrogen (N2, 100 ml/min), followed by reduction of the oxygen carriers under 66% H2 (200ml/min) balanced with N2 for 30 mins. For the set of experiments on oxygen carrier recyclability, oxygen carrier formulations were subjected to 20 cycles of reduction under H2 (10 mins, 200 ml/min) and oxidation under o air (10 mins, 200 ml/min), alternated with N2 flushing (5 mins, 100 ml/min) at 900 C. 4.3.3. Fixed Bed Tar Cracking The effect of composite oxygen carriers on biomass-derived tar was studied in a fixed bed setup. A cylindrical stainless steel tube (OD = 0.25in) embedded vertically in a tube furnace was used as the reactor for the fixed bed experiments. The reactor was filled with 128 1 gm of oxygen carrier powder, held at the bottom by a thin layer of glass wool. Powdered biomass was pyrolyzed in a CDS 5000 Series Pyroprobe and the pyrolysis products were carried to the fixed bed by the carrier gas. For every experiment, the reactor was first heated to 900oC, and then the pyroprobe was heated to 900oC for 20s to send the volatiles to the fixed bed reactor. Helium (40ml/min) was used as the carrier gas for these tests. Gaseous products exiting the fixed bed reactor at the bottom, along with Helium, were carried to an Agilent 7890B Gas Chromatograph – 5977A Mass Spectrometer (GC-MS). To prevent any of the products from condensing, all transfer lines were maintained at 200oC during the entire length of the experiments. Figure 4.3 shows a schematic of the experimental setup. To further examine the activity of the composite oxygen carriers towards cracking of tar components, naphthalene was chosen as a tar model compound since it is one of the most stable components of biomass derived tar. The oxygen carriers were tested for their ability to crack naphthalene in the same fixed bed experimental setup mentioned above. For these experiments as well, naphthalene was volatilized in the pyroprobe before being carried to the fixed bed by the carrier gas. Based on the TGA tests and the fixed bed cracking of biomass derived tar, it was established that the combination of Fe2O3 with NiO will be selected for further optimization of the oxygen carriers. From the commercialization point of view, it is not only necessary to develop oxygen carriers with high reactivity, but it is also important to optimize the economics. Since NiO is expensive and has certain toxic behavior associated with it, it is important to optimize the NiO content in the oxygen carrier formulations in 129 order to make them more economic and environmentally benign, while still maintaining their reactivity. Hence oxygen carrier formulations with NiO content varying from 1-15% were further tested in a fixed bed setup for their ability to crack biomass derived tar. 4.3.4. Oxygen Carrier Characterization The oxygen carrier formulations were analyzed for the formation of different phases and complexes under X-Ray Diffraction (Rigaku SmartLab Powder XRD). XRD scans are run from 20o to 80o at the rate of 2o/min with an accelerating voltage and filament current of 40kV and 44mA, respectively. An FEI Quanta 200 Scanning Electron Microscope (SEM) is used to examine the effect of addition of supports to the morphology of Fe2O3. 4.4. Results and Discussion 4.4.1. Biomass Decomposition Kinetics 4.4.1.1. Devolatilization Kinetics Biomass devolatilization kinetics was studied under both inert (N2) and reactive (CO2) conditions in the TGA. The general equation for gas-solid reactions with nth order kinetics is: dα = kf(α)npn (4.3) dt g where pg, the partial pressure of the gas used for gasification, was maintained at 1 atm during the experiments in this study. k is the rate constant which is a function of temperature and α is the percentage conversion. They both are defined as: 130 −E k = k exp ( a) (4.4) o RT mo − m α = ( ) x 100 (4.5) mo − m∞ ko is a pre-exponential factor, Ea is the activation energy for the devolatilization reaction and n is the order of the reaction. mo, m∞ and m are the initial, final and instantaneous weights of the sample respectively. Equation (5.16) can be re-written to give: dα −E = k exp( a)(1 − α)n (4.6) dt o RT Kissinger’s method151 is applied by taking time derivative of Equation (4.6) and noting d dα that at the maximum rate of change of conversion ( ) = 0 dt dt d2α d −E −E d n a a n (4.7) 2 = ko [(1 − α) {exp ( )} + exp ( ) (1 − α) ] = 0 dt dt RTm RTm dt Further simplification of Equation (4.7) gives us, Ea 1 dT dα (1 − α) 2 = n (4.8) R Tm dt dt Tm is the temperature at the maximum rate of change of conversion and dT/dt is the rate of change of temperature, which will be denoted as Q henceforth. Substituting Equation (4.6) in Equation (4.8), gives: Ea 1 −Ea n (1 − α) 2 Q = n koexp ( ) (1 − α) (4.9) R Tm RTm Simplifying and rearranging Equation (4.9) will lead to 131 Q −Ea koR n−1 2 = exp ( ) n(1 − α) (4.10) Tm RTm Ea Taking logarithm on both sides of Equation (4.10) gives a linear equation: Q −Ea 1 koR n−1 ln ( 2 ) = + ln [ n(1 − α) ] (4.11) Tm R Tm Ea The mass loss data obtained from the non-isothermal thermogravimetric experiments were used to determine Tm for all the different heating rates. The data from the derivative thermogravimetric (DTG) curves were used to calculate the rate changes of conversions. Devolatilization was carried out at heating rates of 2, 3.5, 5, 6.5, 8 and 10 K/min under both nitrogen and carbon dioxide atmospheres and Tm was found for each experiment. The value of Tm obtained for each test was found to lie between 583K and 613K. The maximum rate of conversion and Tm were both observed to increase with an increase in the heating rate. Figure 4.4 is a plot of Equation (4.11), for devolatilization under both nitrogen and carbon dioxide atmospheres. The slopes of the linear plots give the activation energies (Ea) and the intercepts were used to calculate the values for the pre-exponential factors (ko) for both the inert and reactive conditions. First order reaction kinetics was assumed for simplification as it is generally considered to be a 152 good approximation in existing literature. The Ea values obtained by the Kissinger’s method were 36.81 kcal/mol (154 kJ/mol) in nitrogen atmosphere and 37.16 kcal/mol (155 kJ/mol) in carbon dioxide atmosphere. ko for devolatilization under nitrogen was 11 -1 11 1.27x10 s and for devolatilization under carbon dioxide, the value of ko was 1.68x10 -1 s . As observed, Ea for devolatilization under reactive conditions is slightly higher as 132 compared to the Ea value for devolatilization under inert conditions. The pre-exponential factors (ko) under both conditions were also quite comparable and within the same order of magnitude, with ko for devolatilization under CO2 being slightly, but not significantly, higher. The calculated values of the kinetic parameters suggest that devolatilization in carbon dioxide atmosphere was slightly, but not significantly faster than devolatilization in nitrogen atmosphere. However, it should be noted that decomposition of biomass involves complex reactions, especially in carbon dioxide environments. When carbon dioxide was used for decomposition, devolatilization may be accompanied by simultaneous gasification of the char. Decoupling this effect of possible parallel reaction mechanisms can be difficult. Thus, the applied regression methods may not be the best fit since the multiple reactions are more complex than assumed. However, the kinetic parameters calculated for this lignocellulosic biomass sample were well within the range of values listed in previously reported literature. For different kinds of woody biomass such as beech and pine, the Ea values range from 30 to 50 kcal/mol (125 to 209 kJ/mol).153,154 Comparison with reported literature suggests that the activation energy values for coal devolatilization were in a similar range of 25 to 40 kcal/mol (105 to 167 kJ/mol). However, the frequency factor values of coal devolatilization were many orders of magnitude lower when compared to the values for biomass devolatilization.155 Thus, the devolatilization of biomass was faster when compared to coal devolatilization. 133 4.4.1.2. Char Gasification Kinetics Char gasification kinetics was also studied using Equation (4.3), similar to the 156 devolatilization kinetics. Using that as the starting equation, t0.5, the time required to achieve 50% conversion can be calculated by integrating Equation (4.6): t0.5 1 Ea 0.5 dα ∫ dt = exp ( ) ∫ n (4.12) 0 ko RT 0 (1−α) 0.5 1 E dα a (4.13) t0.5 = exp ( ) ∫ n ko RT (1 − α) 0 The reaction rate averaged over the first 50% conversion is subsequently given by 0.5 −Ea R0.5 = = k′o exp ( ) (4.14) t0.5 RT k’o is the pre-exponential factor which depends on the instantaneous rate of conversion. A linear equation, to model the char gasification kinetics, is obtained by taking logarithm of Equation (4.14): 0.5 Ea 1 ln ( ) = − + ln (k′o) (4.15) t0.5 R T The thermogravimetric data from the isothermal char gasification experiments were used to obtain the values for char conversion as a function of time. Figure 4.5(a) shows the DTG curves for isothermal char gasification at five different temperatures. As seen in the figures, the weight loss occurred in two steps. The initial weight loss step to about 5 mg was due to non-isothermal devolatilization under inert conditions. The second step of weight loss was due to gasification of the char left after 134 devolatilization. The values from the gasification portion of the DTG curves were used to calculate the rate of change of char conversion. Using Equation (4.8), the value for t0.5 for each experiment was determined from the conversion values. As observed, the rate of gasification increased with temperature, and subsequently t0.5 decreased. The value of t0.5 was 9.1 minutes at 850oC, 7 minutes at 875oC, 5.5 minutes at 900oC, 4.5 minutes at 925oC and 3.3 minutes at 950oC. Figure 4.5(b) is a plot of Equation (4.15). For the biomass derived char gasification, the slope of the linear plot was used to determine the activation energy Ea of 27 kcal/mol (113 kJ/mol). The value for k’o calculated from the intercept was 1x104 s-1. Notably, the activation energy for biomass char gasification was lower than that for the biomass devolatilization step. However, the much lower pre-exponential factor value for gasification decreased the overall rate of gasification step as compared to the rate of the devolatilization step. This was also evident from Figure 4.6, which is a plot of the Arrhenius equation for the rate constants of devolatilization and gasification under carbon dioxide atmosphere. The plot essentially shows that at any possible reaction temperature, the reaction rate constant for gasification is always smaller than devolatilization. Thus, at any temperature, gasification is the rate determining step. This was in agreement with results from various previous studies which show that char gasification is the rate limiting step in biomass decomposition. 4.4.2. Fixed Bed Tar Cracking with ITCMO The feasibility of tar cracking by Fe2O3 based oxygen carriers in the fixed bed setup was tested using the ITCMO particles. The ITCMO particles have been extensively studied 135 previously by this group in a number of different chemical looping setups using syngas, methane and coal as the feedstock. Figure 4.7 shows MS data from the fixed bed biomass pyrolysis experiments with and without the ITCMO particles. In Figure 4.7(a) as anticipated, the complex composition of the volatiles makes it difficult to identify all the peaks in the spectra. The peaks around 93+14n (93, 107, 121, etc.) amu indicate the presence of alkyl phenols or phenolic ethers, peaks around 77+14n (77, 91, 105, etc.) amu indicate derivatives of benzene (toluene, xylene, etc.) and peaks >120 amu indicate the presence of more complex heterocyclic ethers and polycyclic aromatic hydrocarbons (PAHs). The change in the peak distribution observed in Figure 4.7(b) is a clear indication of the effect of the presence of oxygen carriers in the bed. Comparison of the two mass spectra in Figure 4.7 (a and b) showed a significant difference in the variety and amounts of chemical species present in the product stream during the two experiments. In Figure 4.7(b), the reduced intensity and number of the peaks indicate that the bed of ITCMO oxygen carriers was successful in cracking many of the aromatics and heavy hydrocarbons, but not all. Carbon analysis of the reacted samples showed no evidence of surface carbon deposition on the oxygen carriers either. The supported Fe2O3 particles however were is unable to convert all the tar components into CO2 and H2O completely. While there is an increased concentration of CO2 in the gaseous effluents from the fixed bed, most the heavy hydrocarbons get converted to lighter ones. Hence there is a need to further enhance the tar cracking reactivity of the Fe2O3-based oxygen carriers with the addition of certain tar cracking catalysts. 136 4.4.3. Composite Oxygen Carrier – Redox Performance The theoretical oxygen transfer capacity of the oxygen carriers is calculated using Equation (4.16): 푤 −푤 % Oxygen transfer capacity = 표푥 푟푒 x 100 (4.16) 0.5푤표푥 where, 푤표푥 and 푤푟푒 are the weights of the oxygen carriers in their most oxidized and most reduced states respectively. The oxygen carrier conversion for each cycle during the TGA recyclability experiments is calculated using Equation (4.17): 푤 −푤 % Solid conversion = 표푥푛 푟푒푛 x 100 (4.17) 0.5푤표푥 where, n is the cycle number and 푤표푥푛 and 푤푟푒푛 are the weights of the oxygen carriers at the beginning and end of reduction step number ‘n’. For these calculations, it is assumed that the support material remains inert and the loss in weight during reduction is due to the loss of oxygen from the active component of the oxygen carrier, i.e. Fe2O3 and the tar cracking catalyst in this case. It is also assumed that the different components in the oxygen carrier formulations are homogenously mixed during the material synthesis stage. Figure 4.8 compares the composite oxygen carriers in terms of their oxygen transfer capacities. From the figure, it is evident that plain supported Fe2O3 has the highest oxygen transfer capacity, which is about 30%, and when part of the Fe2O3 is replaced with a tar cracking catalyst, it leads to a drop in the oxygen transfer capacity. The drop is 137 variable depending on the tar cracking catalyst used. This is an indication that the pure Fe2O3 has higher oxygen transfer capacity than any of the plain tar cracking catalysts or the composite formulations. The figure shows that Fe2O3-K2CO3 and Fe2O3-NiO have oxygen transfer capacities of 29.4% and 26.6%, which are the highest oxygen carrying capacities, after pure Fe2O3. Both Fe2O3-K2CO3 and Fe2O3-NiO have oxygen transfer capacities >25%, which is significantly higher than the rest of the formulations. The tar cracking catalysts by themselves have significantly lower oxygen transfer capacities. Similarly, Fe2O3-dolomite and Fe2O3-olivine have oxygen transfer capacities about 30-35% lower than the other two composite formulations. The redox recyclability of the different oxygen carriers is shown in Figure 4.9. Recyclability of the oxygen carrier is defined in terms of the conversion achieved by the sample during each reduction step during the 20 redox cycle TGA experiments. In terms of recyclability, again Fe2O3, Fe2O3-K2CO3 and Fe2O3-NiO perform better than the rest of the formulations, closely followed by NiO, as seen in the figure. Fe2O3-NiO in fact exhibits the best recyclability, where it is able to maintain its conversion consistently around 26% over the 20 redox cycles. However, both Fe 2O3 and Fe2O3-K2CO3 show a drop in reactivity over the 20 redox cycles. Fe2O3 begins with the highest conversion of 30%, but the conversion drops gradually and gets th stabilized at 27% after the 14 redox cycle. In the case of Fe2O3-K2CO3, again the solid conversion begins with a very high 29% during the first reduction step, close to the Fe2O3 conversion. But the conversion drops significantly right after the first redox cycle. During the second reduction step, there is ~6% drop in the reduction 138 conversion of Fe2O3-K2CO3. After the drop, the solid conversion eventually stabilizes at ~21% after the 7th redox cycle. The large drop in the solid conversion right after the first cycle is an indication of an irreversible loss of oxygen during the first reduction step, which was not replenished during the oxidation step. Hence the oxygen available during the successive reduction steps was significantly lower. It is important to note that tar cracking catalysts, when subjected to the TGA redox cycles, show very low conversion which is reflected in their low oxygen transfer capacities. NiO is the only tar cracking catalyst, which exhibits oxygen transfer capacity and recyclability comparable to the oxygen carriers containing Fe2O3. In the case of K2CO3 and olivine, when added to Fe2O3, the oxygen donated during reduction comes only from the Fe2O3. K2CO3 and olivine do not seem to be participating in the transfer of oxygen. 4.4.4. Composite Oxygen Carrier – Tar Cracking Figure 4.10 compares the spectra of products from biomass pyrolysis without oxygen carriers with those from the fixed bed cracking of tar using the different oxygen carriers. The numbers of product peaks reduce significantly when supported Fe2O3 is added to the bed. The supported Fe2O3 by itself is able to crack many of the heavy hydrocarbons to smaller species. However, not all heavy hydrocarbons are converted. The impact of addition of the tar cracking catalysts to Fe2O3 is different in each case. The addition of olivine to Fe2O3 has a negative impact, where the peaks for products like benzene (~2.53 mins) and naphthalene (~8.07 mins) have higher intensities as compared pure Fe2O3, indicating higher concentration of those species. So in the case of Fe2O3-olivine, most of the heavier hydrocarbons are cracked to the more stable species, but it is less active 139 compared to pure Fe2O3 when it comes to cracking of the stable compounds like benzene, naphthalene and toluene. Fe2O3-dolomite shows slightly improved reactivity towards cracking of biomass derived tar, with reduced intensities of the stable components compared to Fe2O3. However, Fe2O3-NiO and Fe2O3-K2CO3 show the best reactivity towards the biomass derived tar compounds, showing the ability to crack even the most stable compounds like benzene and naphthalene, which are extremely difficult to crack otherwise. The product spectra show almost no residual compounds in the case of Fe2O3- NiO and Fe2O3-K2CO3. Fe2O3-K2CO3 has good reactivity for the tar components, but it shows a drop in its oxygen transfer ability after the first redox cycle from the TGA recyclability tests, indicating an irreversible loss of oxygen during the first reduction step. On the other hand, Fe2O3, which shows the best redox reactivity over multiple cycles and the highest oxygen transfer capacity, is not active enough to crack the more stable tar components like benzene, toluene and naphthalene by itself during tar cracking in the fixed bed tests. Based on the TGA reactivity and recyclability tests and the fixed bed experiments, Fe2O3- NiO is the only oxygen carrier that shows both good long-term oxygen transfer ability as well as good reactivity for tar cracking. The difference in the product chromatograms of the Fe2O3 based oxygen carrier with and without NiO is more clearly exhibited in Figure 4.11, where the same data from Figure 4.10 is plotted but only for Fe2O3 and Fe2O3-NiO. It is clearly seen that addition of NiO can effectively eliminate even the most stable tar components like benzene, toluene and naphthalene. 140 To further study the activity of the composite oxygen carriers, they were tested for their ability to crack one of the most stable components of biomass derived tar - naphthalene. The product chromatograms comparing from the fixed bed cracking of naphthalene are shown in Figure 4.12. Without any oxygen carriers, the effluent gas from the reactor contains naphthalene (~8.1 min) and its derivatives like 1,1’- binaphthalene (~17.5 min) and 2,2’- binaphthalene (~18.5 min). It is evident from the figure that addition of the composite Fe2O3 based oxygen carriers to the fixed bed significantly reduces the concentration of naphthalene and its derivatives in the effluent gases from the reactor. Here again, the Fe2O3-NiO oxygen carriers show better reactivity towards elimination of naphthalene, with very small amounts of residual naphthalene in the effluent stream. With both Fe2O3-K2CO3 and Fe2O3-Olivine, there was still significant amount of residual naphthalene in the products, which is indicated by the high naphthalene peak intensities in the product chromatograms. 4.4.5. Composite Oxygen Carriers – NiO Loading Based on these results, the Fe2O3-NiO combination was selected for further optimization. Since NiO is expensive and relatively more toxic, it is important that its content in the oxygen carrier be minimized from the economic and environmental viewpoints. Hence, oxygen carrier formulations with varying contents of NiO were tested for their tar cracking ability in the fixed bed setup. For these formulations, the support was maintained at 50 wt% and NiO was varied between 1-15wt%, balanced by Fe2O3. Figure 4.13 compares the chromatogram of products from the fixed bed experiments using oxygen carriers with varying NiO content. As seen in the figure, the only significant 141 improvement in tar cracking is observed as the NiO content is increased from 1% to 5%. With the addition of 1% by wt of NiO, the tar cracking ability of the oxygen carrier is significantly improved, but many of the hydrocarbons still remain. As the NiO content is increased to 5wt%, most of these heavy hydrocarbons are eliminated and the only small quantities of benzene, naphthalene and toluene can be seen. As the NiO content is further increased to 10% and 15%, no significant change is observed in the product chromatograms. Hence it is concluded that NiO content of 5wt% in Fe2O3-based oxygen carriers should provide the optimum reactivity towards cracking of biomass derived tar in addition to having all the properties required for effective oxygen carrying materials in chemical looping systems. Anything higher than 5% would increase the oxygen carrier costs and make it more toxic, without significantly improving its performance. 4.4.6. Solid Characterization The tar cracking mechanism largely depends on the oxygen carrier composition. XRD analyses of the reacted samples (in Figure 4.14) showed reduced forms of the active components in the case of Fe2O3-NiO. This is an indication that the lattice oxygen from the oxides participates in the reaction. A possible mechanism in this case is that the tar molecules get adsorbed on the oxygen carrier surface forming radicals and intermediates. The lattice oxygen from the active component of the oxygen carrier reacts with the radicals. Through a series of reactions, the tar compounds get converted into smaller molecules like CO2, CH4, CO, H2O and other lighter hydrocarbons. Complete reoxidation to the initial phases is observed in the case of Fe2O3-NiO with air oxidation. 142 Figure 4.15 shows the SEM images of the oxygen carriers with and without the addition of NiO. 4.5. Concluding Remarks and Future Work With pressing needs for cleaner energy conversion processes, many technologies are being developed which have the ability to sequester greenhouse gases. But most of these processes require some amount of parasitic energy for product separation which reduces the overall process efficiency. The chemical looping process represents one of the most cost-effective and advanced CO2 mitigation strategies. The Ohio State University (OSU) chemical looping system is unique in terms of its flexibility in hydrogen/syngas production, ease in solid fuel handling, and advanced scheme for process intensification. Biomass is a good alternative to fossil fuels, which are a depleting resource. The BCL process is the first biomass chemical looping gasification system and is, based on the exergy analysis, more efficient and versatile than any other competing technologies. It is the solution to the need for an efficient, environment friendly and economic biomass to energy conversion process that can be commercialized. While Fe2O3 is an ideal candidate for oxygen carrier material for chemical looping processes, it is not effective in completely eliminating tar from the system by itself. Hence, different traditional tar cracking catalysts (Olivine, Dolomite, K2CO3 and NiO) have been used in conjunction with Fe2O3 to enhance the carriers’ ability to crack tar. In the TGA tests, the Fe2O3-NiO and Fe2O3-K2CO3 combinations exhibited the highest oxygen transfer capacities, after plain supported-Fe2O3. The Fe2O3-NiO combination also 143 exhibited the best recyclability, where it is able to maintain its oxygen transfer capacity consistently over 20 redox cycles. Fe2O3 by itself is able to eliminate most tar components, but some of the most stable components remain. The best reactivity towards elimination of tar components is exhibited by Fe2O3-NiO and Fe2O3-K2CO3. Addition of NiO eliminates even some of the most stable tar components like benzene, naphthalene and toluene. Due to the excellent recyclability and tar cracking ability exhibited by the Fe2O3-NiO combination, NiO is chosen as the additive for further development of Fe2O3- based oxygen carriers for biomass chemical looping. Since NiO is relatively more expensive and toxic, it is necessary to optimize its content, from the process perspective. Fe2O3-based oxygen carriers with NiO content varied between 1-15% are further tested for their tar cracking ability in the fixed bed. Based on the product chromatograms, it is concluded that addition of ~5% by weight of NiO to Fe2O3-based oxygen carriers will provide the optimum tar cracking ability and redox recyclability, without significantly affecting their looping properties. Future work would involve developing this Fe2O3- based oxygen carrier formulation with NiO as an additive into particles with optimum attrition resistance as well as reactivity for biomass conversion. The ITCMO particles previously developed by this group have been successfully tested for over 100-redox cycles, without showing any significant drop in their reactivity or physical strength. One of the future goals for BCL would include developing composite oxygen carriers with redox and mechanical strength performance similar to that of the ITCMO particles. The results from this study will help further develop oxygen carriers for the successful removal of tar from the reducer and smooth operation of a large scale successful BCL 144 process. The knowledge gained from this study can effectively improve the economic viability and environmental attractiveness of biomass-fueled energy conversion systems. This technology will benefit the biomass conversion value chain with an improved efficiency for biomass energy conversion, a reduced cost for hydrogen production, and an intensified process system for liquid fuels and chemicals production. 145 Figure 4.1: Biomass Chemical Looping Gasification system with high purity hydrogen generation and in-situ CO2 capture 146 Technology BCL Biomass Biomass Capacity (MW) 100 100 100 IGCC IGCC with Biomass Conversion 100 100 C C100apt ure Net Power Generation (%) 38a 30 21b Efficiency (%HHV) CO2 Capture Rate 100 0 90 (% input carbon basis) Cost of Powerc 9.5 18 27 (ca en- bt/asedkWh) on A SPEN simulation of BCL; b - post CO2 scrubber; c - 10USD for CO2 transportation and sequestration cost) Table 4.1: Comparisons of BCL with Biomass IGCC systems with and without carbon capture135 147 139 Figure 4.2: Dissociative adsorption of phenol over iron oxide 148 Oxygen Fe2O3 wt % Tar cracking catalyst, Carrier # wt% 1 50 - 2 30 Olivine, 20 3 30 NiO, 20 4 30 K2CO3, 20 5 30 Dolomite, 20 6 - Olivine, 50 7 - NiO, 50 8 - K2CO3, 50 Table 4.2: Composite oxygen carrier formulations 149 Figure 4.3: Pyroprobe and fixed bed reactor setup coupled with GC-MS to analyze the product gases. 150 Figure 4.4: Kissinger plot for biomass devolatilization under N2 and CO2 atmospheres 151 Figure 4.5: Plots for calculating the kinetic parameters of char gasification. (a) DTG curves for isothermal gasification of char under CO2 atmosphere. (b) Arrhenius plot for isothermal gasification of char under CO2 atmosphere. 152 Figure 4.6: Arrhenius plots for rate constants of devolatilization and gasification reactions under CO2 atmosphere 153 154 Figure 4.7: Mass spectrometry data from fixed bed cracking of biomass derived tar (a) Without oxygen carriers; and (b) With ITCMO particles 155 Figure 4.8: Oxygen transfer capacities of the different oxygen carrier formulations from the TGA experiments 156 Figure 4.9: Redox recyclability of different oxygen carrier formulations from the 20 redox cycle TGA experiments 157 Figure 4.10: GC-MS data from the analysis of the product gas from the fixed bed experiments with different oxygen carriers Figure 4.11: Product chromatogram from the fixed bed experiments comparing reactivity of supported-Fe2O3, with and without NiO 158 Figure 4.12: Product chromatogram from fixed bed cracking of Naphthalene with different composite oxygen carriers 159 Figure 4.13: Product chromatograms from fixed bed experiments comparing reactivity of Fe2O3-based oxygen carriers with NiO content varying from 1-15wt% 160 Figure 4.14: XRD spectra of fresh and reacted Fe2O3 based oxygen carrier with 5% NiO 161 Figure 4.15: SEM images of fresh unreacted Fe2O3 based oxygen carriers with (a)no NiO; and (b)1wt% NiO 162 REFERENCES 1. 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In the MSB, the sample to be measured and the balance are not in direct contact, making it possible to use nearly all kinds of reactive environments. The sample is linked to a suspension magnet, which consists of a permanent magnet, a sensor core and a device for decoupling the sample. An electromagnet, connected to the balance located outside under atmospheric conditions, maintains the suspension magnet in a freely suspended state using an electronic control unit. The magnetic suspension coupling transmits the measuring force from the measuring chamber to the microbalance without any contact between the two. A combination of PID controller and position transducer is used to modulate the voltage on the electromagnet so as to hold the suspension magnet in a vertical position. 179 Figure A.1: Experimental setup of MSB (Rubotherm GmbH, US-2004-00162) with gas manifold 180 Figure A.2: Working principle of Rubotherm MSB 181 Characterization of MgAl2O4-supported Fe2O3 The effect of pressure on the steam oxidation kinetics of MgAl2O4-supported Fe2O3 has been discussed in Section 3.4.6. Figure A.3 shows the XRD spectra of the samples reoxidized with steam at both 1 and 5 atm. While the oxygen carrier formulation exhibited enhanced kinetics at elevated pressures, the sample is seen to oxidize to Fe3O4 under both reactive conditions, as evident from the XRD data. It is also interesting to note that the sample shows traces of Fe2O3 at 1 atm. Although formation of Fe2O3 using steam is thermodynamically unfavorable, the steam oxidation experiments are conducted using excess of steam, which can explain the formation of trace amounts of Fe2O3. The oxygen carrier sample is the limiting reactant in the case of the TGA experiments. However, in the integrated system, the gases will be the limiting agent. Figure A.4 compares the XRD spectrum of fresh and unreacted MgAl2O4-supported Fe2O3 with a reacted sample after 20 redox cycles using H2 for reduction and steam for oxidation. The XRD data corroborates the conversion data from the cyclic redox experiments. Even after 20 redox cycles, the oxygen carrier formulation still maintains its reactivity as the sample is shown to be reoxidized to Fe3O4 completely, based on the XRD spectrum. All the SEM images of the MgAl2O4-supported Fe2O3 samples reacted under various conditions are presented in Figure A.5. 182 Figure A.3: XRD spectra of steam oxidized samples of MgAl2O4-supported Fe2O3 (a) 1 atm; and (b) 5 atm 183 Figure A.4: XRD spectra of MgAl2O4-supported Fe2O3 (a) Fresh; and (b) After 20 o redox cycles using H2 for reduction and steam for oxidation at 900 C 184 Figure A.5: SEM images of the MgAl2O4-supported Fe2O3 (a) Reduced at 1atm; (b) Steam oxidized at 1 atm; (c) Reduced at 5atm; (d) Steam oxidized at 5atm; (e) Fresh unreacted; and (f) After 20 redox cycles with H2 and steam 185 Effect of pressure on steam oxidation kinetics of ITCMO In addition to investigating the effect of pressure on the MgAl2O4-supported Fe2O3, the ITCMO oxygen carriers were also evaluated for their response to elevated pressure during steam reoxidation. Figure A.6 presents the rates of steam oxidation of the reduced ITCMO particles at 1 and10 atm. Like the MgAl2O4-supported Fe2O3 formulation, the ITCMO particles also exhibited enhanced steam oxidation at elevated pressures. The % oxidation achieved during steam oxidation at both pressures was similar. However, at 1 atm, the time required to achieve the highest oxidation state is about 14 mins, while the same oxidation conversion can be achieved in 8.5 mins at 10 atm. The rates are compared by calculating the value of ‘x’ in FeOx, which is calculated as follows: 푤 −푤 푤 x = ( 푡 푟)/( 푖 ) (A.1) 16 160 where, 푤푡 is the instantaneous weight of the oxygen carrier sample during oxidation, 푤푟 is the weight of the sample in its most reduced, state and 푤푖 is the initial weight of the fresh unreacted sample. It is important to note that x=1.5 for Fe2O3, x=1.33 for Fe2O3 and x=1 for FeO. 186 Figure A.6: Comparison of rates of reoxidation of ITCMO particles using steam at different pressures 187 Steam Oxidation Thermodynamic Analysis The common set of assumptions used for the simulations discussed in Section 3.3.5 are presented in Table A.1 and Table A.2. The thermodynamic phase diagram is obtained using a single stage RGIBBS reactor system in ASPEN. The single stage RGIBBS reactor module is shown in Figure A.7. In the oxidizer, the gas-side reaction can be expressed as shown in Equation (A.2): H2O H2 + 0.5 O2 (A.2) Based on the gas side reaction, a gas conversion expression can be expressed depending on the amount of steam converted as shown in Equation (A.3): H O in product Gas conversion = 1 − 2 (A.3) H2O in resource The solids side reaction can be expressed in terms of iron-oxide re-oxidation as follows: FeOx + (y-x)0.5O2 FeOy (A.4) The solids conversion for any specific oxidation state of iron-oxide is shown in Equation (A.5): Instantaneous oxygen present in metal oxide Solids Conversion = (A.5) Oxygen present in the highest oxidation state of metal oxide The thermodynamics of a given metal-oxide system with H2O-H2 based system can be investigated in terms of thermodynamic phase diagrams. The thermodynamic phase diagram of the Fe-H2-H2O system is shown in Figure A.8. This phase diagram can be interpreted in terms of solids and gas conversions at a specific temperature. For example, 188 o if the PH2O/PH2 is 0.1 at the reactor outlet at a temperature of 900 C, the following conclusions can be inferred about the reaction conditions in the oxidizer reactor based on the phase diagram. The gas conversion for a starting steam mole flow of 1 kmol/hr is ~90%. Based on the phase diagram, the solids conversion in equilibrium with a gas conversion of 90% is 100% as Fe phase has no oxygen in it. 189 Parameter Setting Reactor module type RGIBBS Stream Class MIXCISLD Thermodynamic and Physical data Combust, Inorganic, Solids, bank (in order) Aqueous, Pure 22 Base Method PR-BM (Peng-Robinson Base method) Free Water method Steam Tables (Steam-TA) Table A.1: Reactor model setup for simulation of steam oxidation reactions 190 Type: Solid Aluminimum oxide: alpha-corundum Iron-dialuminium tetraoxide (FeAl2O4) (Al2O3) Iron titanium oxide (FeTiO3) Titanium dioxide (TiO2) Di-iron titanium pentoxide (Fe2TiO5) Iron (Fe) Tri-iron carbide (Fe3C) Ferrous oxide (FeO) Magnetite (Fe3O4) Hematite (Fe2O3) Magnesium-Aluminate (MgAl2O4) Type: Conventional + Argon (Ar) H3O (Hydronium ion); - Hydrogen (H2) OH (Hydroxyl ion) Oxygen (O2) Nitrogen (N2) Water (H2O) Nitric oxide (NO) Nitrogen dioxide (NO2) Table A.2: List of chemical species considered for simulation 191 Figure A.7: Schematic of cocurrent RGIBBS module 192 Figure A.8: Thermodynamic phase diagram of the Fe-H2-H2O system 193 APPENDIX B B. SUPPLEMENTAL INFORMATION FOR BIOMASS STUDIES GC-MS product spectra The product chromatogram of biomass derived tars from the fixed bed experiment without oxygen carrier discussed in Section 4.4.4 is presented in Figure B.1. The chromatogram here consists of all component peaks labeled, with all the species present in the gaseous effluent from the fixed bed, without oxygen carriers. It is important to note that in this case, the biomass was pyrolyzed in the pyroprobe at 300oC and the pyrolysis gases were then sent to the fixed bed filled with quartz wool, maintained at 900oC. The product chromatogram from the pyrolysis of naphthalene without oxygen carriers is also presented in Figure B.2. The ultimate analysis of the biomass sample used for the study in Chapter 4 is presented in Table B.1. 194 Figure B.1: GC-MS spectrum of biomass derived volatiles without oxygen carriers in fixed bed. 195 Figure B.2: GC-MS spectrum of napthalene from pyroprobe, without oxygen carriers in fixed bed. 196 Figure B.3: Experimental setup of fixed bed for biomass derived tar cracking with in-line gas analyzers. 197 Wt.% (as Wt.%, Dry Basis received) Residual Ash Carbon Hydrogen Nitrogen Sulfur Chlorine Oxygen Moisture by difference 5.47 1.07 51.33 6.27 <0.1 0.12 0.0127 41.20 Table B.1: Ultimate analysis of the biomass sample used for this tar cracking and oxygen carrier development studies. 198 Char Gasification Additional Information Similar to the biomass char gasification kinetic studies performed in Section 4.4.1.2, the gasification kinetics of char derived from coal was also conducted to support the CDCL process. Using the same procedure for calculating the activation energy for gasification of biomass derived char, the activation energy for char derived from Illinois# 6 coal has also been calculated. The Arrhenius plot for the isothermal gasification of coal derived char is presented in Figure B.4. The slope of the linear plot was used to determine the activation energy Ea of 214 kJ/mol. The value for k’o calculated from the intercept was 9.9x108 s-1. For comparison purposes and to investigate the effect of oxygen carriers on the char gasification kinetics, the rates of coal char gasification were compared with and without the ITCMO oxygen carriers. The plot comparing the gasification rates with and without the oxygen carriers is presented in Figure B.5. As seen from the figure, without the oxygen carrier it takes ~1.5 hours to achieve a 100% char conversion. However, in the presence of the oxygen carriers, the time taken to achieve 100% char conversion is ~45 mins. The char conversion is calculated using 푤 −푤푡 % Char conversion = ( 푖 ) x 100 (B.1) 푤푖−푤푓 where, 푤푖, 푤푓 and 푤푡 are the initial, final and instantaneous sample weights of the char during CO2 gasification. 199 Figure B.4: Arrhenius plot for isothermal gasification of coal derived char under CO2 atmosphere 200 Figure B.5: Comparison of char gasification rates with and without oxygen carriers 201 APPENDIX C C. PERMISSION FROM JOURNAL PUBLICATION The material in Chapter 2 has been reproduced with permission from Deshpande, N., Majumder, A., Qin, L., and Fan, L.-S. (2015). High-pressure redox behavior of iron- oxide-based oxygen carriers for syngas generation from methane. Energy Fuels, 29(3), 1469-1478. Copyright © 2015 American Chemical Society. Part of the material in Chapter 4 has been reprinted with permission from Luo, S., Majumder, A., Chung, E., Xu, D., Bayham, S., Sun, Z., Zeng, L., and Fan, L.-S. (2013). Conversion of woody biomass materials by chemical looping process – kinetics, light tar cracking, and moving bed reactor behavior. Ind. Eng. Chem. Res., 52(39), 14116-14124. Copyright © 2013 American Chemical Society. Permission to use the copyrighted material has been obtained from ACS Publications and is presented in the following pages. 202 203 204