Modeling of Methane Multiple Reforming in Biogas-Fuelled SOFC and Its Application to Operation Analyses by Tran Dang Long Department of Hydrogen Energy Systems Graduate School of Engineering Kyushu University SUBMITTED TO THE GRADUATE SCHOOL OF ENGINEERING IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF ENGINEERING AT THE KYUSHU UNIVERSITY JUNE 2017 Approved by: Assoc. Yusuke Shiratori, advisor/examiner Graduate School of Engineering, Kyushu University Prof. Kazunari Sasaki, co-examiner Graduate School of Engineering, Kyushu University Prof. Kohei Ito, co-examiner Graduate School of Engineering, Kyushu University Prof.
Takuya Kitaoka, co-examiner Graduate School of Bioresource and Bioenvironmental Sciences, Kyushu University Fukuoka, Japan ABSTRACT This research focuses on solid oxide fuel cell (SOFC) operated at high temperature (700±800 oC) with the direct feed of biogas, a gaseous mixture of 55±70 vol% CH4 and 30±45 vol% CO2 obtained from the anaerobic fermentation of organic matters such as garbage, livestock manure and agricultural residues. When the biogas is supplied directly to SOFC, CH4 dry and steam reforming simultaneously occur in a porous Ni- based anode material to produce syngas (Methane multiple-reforming (MMR) process). This type of operation is called direct internal reforming (DIR) operation. Biogas- fuelled DIR-SOFC is a promising technology for sustainable development of a rural area abundant in biomass resources.
For the realization of this technology, prior to system development, operating behavior of it has to be fully understood. However, how to model the complex kinetics of MMR process was a big challenge. In this study, from the reforming data obtained in the series of systematic experiments using Ni-based anode-supported cells (ASCs), a MMR model (model parameters) was inductively generated using the approach of artificial neural network (ANN). The developed MMR model can provide the net consumption and production rates of gaseous species (CH4, CO2, H2O, H2 and CO) involved in the MMR process at arbitrary temperatures and gas compositions.
And, it can be applied for different types of Ni-based catalysts by adjusting a correction factor to compensate the differences in catalytically-active surface area. Computational fluid-dynamics (CFD) calculations, in which mass and heat transports, MMR and electrochemical processes occurring inside the cell were taken into consideration, were conducted for the DIR-SOFC fuelled by biogas. Consistency of the CFD calculation incorporating the MMR model developed in this study (MMR model-incorporated CFD) with the measured performance of SOFC fuelled by CH4-CO2 mixture was confirmed through a three-step model validation process consisting of two model-parameter-tuning steps (model fitting steps with the data experimentally obtained under non-DIR and DIR operations) followed by a validity check whether the established-model can reproduce a performance of DIR-SOFC under an arbitrary i operating condition. The consistency was not achieved by the conventional approach in literature considering MMR as a sum of CH4 dry and steam reforming (ignoring the concurrent effect of CO2 and H2O on the catalytic CH4 conversion).
The MMR model developed in this study was proved to be able to provide more realistic and meaningful estimations for the DIR-SOFCs. In order to enhance thermomechanical stability and output power of DIR-SOFC fuelled by biogas, internal reforming rates have to be properly controlled. For this purpose, two advanced DIR concepts, with the anode gas-barrier mask (Concept-I) and with the in-cell reformer using paper-structured catalyst (PSC) (Concept-II), were investigated by the MMR model-incorporated CFD calculation. Two types of 20 ൈ 50 mm2 ASC, ASC-A and ASC-B, with different thicknesses of anode substrate (Ni- stabilized zirconia) of 950 and 200 ߤ m, respectively, were considered, providing guidelines for selecting a proper cell design depending on the thickness of the anode substrate (in other words the amount of metallic Ni) to obtain a mechanically stable operation with higher power density in the direct feed of simulated biogas mixture (CH4/CO2 = 1) at 800 oC.
For both ASC-A and ASC-B, by adopting Concept-I which can control mass flux of fuel getting into the porous volume of the anode along fuel flow direction, rapid syngas production at the fuel inlet region was suppressed to have homogeneous temperature distribution over the cell. In comparison to the normal ASCs (Normal), about 20% decrease in the maximum thermally-induced stress was estimated with a slight loss (about 8%) of maximum power density for both ASC-A and ASC-B, indicating that the use of anode gas-barrier mask is effective to reduce the risk of electrolyte fracture. Concept-I was confirmed to be a good choice for getting stable operation of DIR- SOFCs. For the feed of 200 mL min±1 simulated biogas, in the cases of Normal and Concept- I, maximum power densities (ܲ௫ ) with thinner anode substrate (ASC-B) were 1.95 W cm±2, respectively, lower than those with thicker one (ASC-A), 1.08 W cm±2, respectively, reflecting that the degree of catalytic CH4 conversion is a predominant factor of the performance.
In fact, by the application of Concept-II, ܲ௫ of ASC-A and ASC-B were boosted up to 1.45 W cm±2, respectively, although ii the risk of electrolyte fracture was increased. The effect of Concept-II was more pronounced for ASC-B with thinner anode substrate, from which H2O (product of the anodic reaction) was easily drained. As a result, buildup of partial pressure of H2O within the anode functional layer under high current densities, leading to the decrease in electromotive force, could be suppressed. This study provided a powerful numerical tool for creating highly efficient and robust DIR-SOFCs operating with biogas.
This dissertation is mainly divided in six parts: overviews of SOFC and conventional modeling approaches for DIR-SOFCs are summarized in General Introduction. Investigation on electrochemical behavior of DIR-SOFC operating with biogas is presented in Chapter 2. In Chapter 3, detailed description of the ANN/FIS- based MMR model is given. CFD model of DIR-SOFC considering MMR and strategy of model validation are described in Chapter 4.
The effectiveness of advanced DIR concepts is discussed in Chapter 5. Finally, important findings and outlook for future work are summarized in Chapter 6. iii ACKNOWLEDGEMENTS The study was conducted under the excellent supervision of Assoc. Yusuke Shiratori whom I gratefully acknowledge for his enthusiasm and many hours of helpful discussion throughout the progress of my thesis.
I wish to express my deep gratitude to Prof. Kazunari Sasaki for giving me the opportunity to realize this thesis in his laboratory. In particular, I greatly appreciate his valuable scientific comments and suggestions in my research. It is an honor for me that he is one of examiners of my thesis.
I am also deeply grateful to Prof. Kohei Ito and Prof. Takuya Kitaoka for being committee members of my thesis. I would also like to thank Assoc.
Hironori Nakajima and Assist. Yuya Tachikawa for their helpful supports in using COMSOL Multiphysics software and valuable discussions on SOFC calculations. I wish to thank to Prof. Akari Hayashi and Assoc.
Masamichi Nishihara for their helpful comments and suggestions in my research. I would like to express my appreciation to Dr. Tran Quang Tuyen for teaching me fundamentals on SOFCs and skills on conducting experiments, as well as accompanying me during my stay in Japan. I especially thank Ms.
Mio Sakamoto, Mr. Atsushi Kubota and Mr. Go Matsumoto, who assisted me to collect experimental results; Ms. Nguyen Thi Giang Huong and Dr.
Pham Hung Cuong who encouraged me all the time; Ms. Tomomi Uchida, who supported me in many things; and all other officemates and students for their support. I also appreciate Saga Ceramic Research Laboratory (Japan) for their supporting the anode-supported half-cells. I gratefully acknowledge to Japan International Cooperation Agency (JICA) and ASEAN University Network/Southeast Asia Engineering Education Development Network (AUN/SEED-Net) for awarding me a scholarship to study in Kyushu iv University; and Japan Science and Technology Agency (JST) and Science and Technology Research Partnership for Sustainable Development (SATREPS) program for financial support on my research.
I greatly appreciate Ms. Akiko Sakono in JICA Kyushu International Center (JICA Kyushu) for helpful supports during my PhD period. Finally, my highest appreciation is addressed to my family: my parents, my sisters and brothers who believe in me and give me any supports without hesitation; my wife, Thuy Ha, who always makes me proud and has never complained for my absence at home; and my beloved children, Vinh Khang and Khanh An, who are my motivation in all circumstances. v TABLE OF CONTENTS Abstract.
iv Table of contents. vi List of figures. ix List of tables. xvii List of symbols.
xviii List of abbreviations. xx Chapter 1: General introduction .2 Solid Oxide Fuel Cells (SOFCs) .4 Direct internal reforming (DIR) operation .3 Overview of modeling approaches for DIR-SOFCs. 21 Chapter 2: Electrochemical behavior of DIR-SOFCs operating with biogas .1 Electrochemical characteristics of Ni-based anodes with H2 and CO .3 Results and discussion .1 Internal reforming behavior under open-circuit condition .2 Electrochemical impedance for simulated biogas mixtures. 39 Chapter 3: Modeling of methane multiple-reforming within the Ni-based anode of an SOFC .2 Determination of model parameters .2 Data post-processing.
62 Chapter 4: Modeling and simulation of a DIR-SOFC operating with biogas .1 A comprehensive CFD model for DIR-SOFCs considering methane multiple-reforming (MMR) .2 Sub-model of mass transport .3 Sub-model of chemical reactions .4 Sub-model of electrochemical reactions.5 Sub-model of heat transport .1 Strategy of model validation.3 Results and discussion .2 Behavior of a DIR-SOFC fuelled by biogas .1 Distribution of gaseous species .3 Distributions of temperature and thermal stress .4 Imperfection of conventional modeling approaches of MMR. 97 Chapter 5: Advanced DIR concepts for SOFCs operating with biogas .2 Results and discussion .1 Case study for the thick anode substrate (ASC-A, ݀ = 950 Pm) .2 Case study for the thin anode substrate (ASC-B, ݀ = 200 Pm) .3 Effect of anode thickness .2 Outlook for future work. 124 Appendix A: Effects of H2O and CO2 on the electrochemical oxidation of Ni- based SOFC anodes with H2 and CO as a fuel. 127 Appendix B: Overview of Artificial Neural Network (ANN).
134 Appendix C: Overview of Fuzzy Inference System (FIS). 140 viii LIST OF FIGURES Fig.1 Biogas-fuelled SOFC as a sustainable power generator.2 Operating mechanism of a SOFC with H2 as a fuel.3 Typical ݅-ܸ characteristics of an SOFC.4 Schematic illustrations of (a) tubular and (b) planar SOFCs [28].5 Schematic illustrations of SOFC single cell configurations [14].6 Carbon formation boundary for humidified biogas mixtures 11 (CH4:CO2:H2O = 0.15)) calculated by HSC Chemistry 9.0 (Outotec, Finland), showing the effect of the degree of humidification on coking prevention within the operating temperature range of SOFCs.7 Calculated electromotive force under open-circuit condition in 12 DIR-SOFC operating with humidified biogas mixtures (CH4:CO2:H2O = 0.15)) without carbon deposition, showing the effect of the degree of humidification on power generation.8 Physical and chemical phenomena in the DIR-SOFC operating with 14 CH4-based fuels.1 Button-type ESC prepared in this study to investigate the 30 electrochemical behaviour of DIR-SOFC operating with the direct feed of simulated biogas mixtures; (a) illustration of cell configuration and (b) photograph of the cell unit. WE ± working electrode (anode); CE ± counter electrode (cathode); and RE ± reference electrode.2 Electrochemical measurement setup for DIR-SOFC fuelled by a 31 simulated biogas mixture; (a) schematic drawing and (b) photograph.3 Internal reforming behavior of ESC with Ni-10ScSZ anode (total 33 anode thickness of about 38 Pm, surface area of 8 ൈ 8 mm2) with 80 mL min±1 of simulated biogas mixtures (CH4:CO2:N2 = 20:݂ைమ :(60 ± ݂ைమ )) measured at 800 oC; (a) total CH4 conversion, (b) net production rates of H2, CO and H2O and (c) H2/CO molar ratio of reformate gas with respect to CO2 inlet flow rate (݂ைమ ).4 Thermodynamically-calculated partial pressure of oxygen in anode 34 side (ைమ ǡ ) with respect to CO2 inlet flow rate (݂ைమ ) at 800 oC for 80 mL min±1 of simulated biogas mixtures (CH4:CO2:N2 = 20:݂ைమ :(60 ± ݂ைమ )).5 Anode-side impedance spectra at 800 oC for the ESC with Ni- 35 10ScSZ measured under open-circuit condition with 80 mL min±1 of different CH4-CO2-N2 mixtures. Spectra for dry and humidified H2 were also plotted for the comparison.