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Title: Numerical modeling of direct carbon solid oxide cells
Authors: Xu, Haoran
Degree: Ph.D.
Issue Date: 2018
Abstract: Solid oxide cells (SOCs) are promising devices for clean energy utilization with high efficiency. A solid oxide cell has a good reversible working mode characteristic. It can work either as a solid oxide fuel cell (SOFC) to generate electrical power from fuels or as a solid oxide electrolysis cell (SOEC) for utilizing excessive renewable power to generate fuels. The SOC is environmentally friendly and has high efficiency in converting energy between fuels and electrical power, which makes it a promising next-generation device in utilizing traditional fossil fuels such as solid carbon. In this work, 2D numerical models are developed for CO and electricity co-generation in direct carbon solid oxide fuel cells (DC-SOFCs). The model is validated by comparing the simulation results with experimental data from the literature. Parametric simulations are conducted to understand the physical/chemical processes in DC-SOFCs. Good performance of DC-SOFC is achievable even at a large distance between the carbon bed and the porous anode, indicating the feasibility of large-scale DC-SOFC applications. The DC-SOFC performance is found to decrease with decreasing temperature due to the decreased Boudouard reaction kinetics. The molar fraction of CO at the anode can be well controlled by adjusting the operating conditions of DC-SOFC, enabling electricity and CO co-generation. Another finding is that the current density in the DC-SOFC increases slightly along the cell length, which is different from that in the H2-fueled SOFC. The anode-supported configuration is found to be beneficial in improving the electrical output of the DC-SOFC but is unfavorable for CO generation. By considering heat transfer effects, parametric simulations are also conducted to investigate the effects of various operating and structural parameters on the thermal behaviors of DC-SOFCs. It is found that the operating parameters greatly influence the DC-SOFC thermal behaviors and the temperature field in DC-SOFC is highly non-uniform. The position of peak temperature in the cell is highly dependent on the operating potential. In addition, it is found that a smaller distance between the carbon bed and the anode electrode is beneficial for improving the temperature uniformity in the DC-SOFC. The breakdown of heat generation/consumption in DC-SOFC shows that the anode processes contribute the most to the temperature variation in the cell. Different from conventional DC-SOFC with CO₂ as gasification agent, a new DC-SOFC using H₂O as gasification agent is proposed and studied systematically by both experiment and simulation. The performance of DC-SOFCs with two agents are compared at different operating potential, temperature and anode inlet gas flow rate. It is found that the H₂O-assisted DC-SOFC significantly outperform the CO₂-assisted DC-SOFC. The higher performance of H₂O-assisted DC-SOFC comes from: (1) higher H₂O-carbon gasification kinetics and (2) lower activation loss of H₂ electrochemical oxidation. It is also found that a higher temperature could greatly improve the performance of both kinds of DC-SOFCs. At a temperature of 1000K and operating voltage of 0.5 V, the current density from the CO₂-assisted DC-SOFC is close to 0 while it is still above 1000 A mˉ² from the H₂O-assisted DC-SOFC, allowing the operation of H₂O assisted DC-SOFC at reduced temperature. It is found that the anode gas flow rate does not significantly affect the performance of DC-SOFC.
To further improve the performance of H₂O assisted DC-SOFCs, Na₂CO₃ is added in solid carbon for enhancing carbon gasification kinetics. The performance of DC-SOFCs with and without catalyst are compared at different operating potential, anode inlet gas flow rate and operating temperature. It is found that adding suitable catalyst can significantly speed up the in-situ steam-carbon gasification reaction and improve the performance of DC-SOFC. The potential of syngas and electricity co-generation from the fuel cell is also evaluated, where the composition of H₂ and CO in syngas can be adjusted by controlling the anode inlet gas flow rate. In addition, the performance DC-SOFCs and the percentage of fuel in the outlet gas are both increased with increasing operating temperature. Lastly, carbon assisted solid oxide electrolysis cell (CA-SOEC) for H₂O electrolysis is proposed for syngas production with easy control of H₂/CO ratio. 2D numerical models are developed to study the effects of operating and design parameters on the CA-SOEC performance. One important finding is that the carbon assisting is effective in lowering the equilibrium potential of SOEC, thus greatly reduces the electrical power consumption for H₂O electrolysis. The CA-SOEC can generate electrical power, CO and H₂ simultaneously at a low current density and sufficiently high temperature. Compared with conventional SOEC for H₂O/CO₂ co-electrolysis, CA-SOEC is advantageous as CO and H₂ are produced in the anode and cathode, respectively. This enables easy control of H₂/CO ratio, which is helpful for subsequent processes to synthesis other chemicals or fuels from syngas. Besides, CA-SOEC can produce electricity rather than consuming it. The model can be used for subsequent design optimization of CA-SOEC for effective energy storage and conversion. The results of this study form a solid foundation for better understanding the chemical/physical processes in DC-SOFCs and CA-SOECs with different kinds of operating conditions. The models can be used for subsequent design optimization of DC-SOFCs and CA-SOECs at a system level for thermal management and effective energy storage and conversion.
Subjects: Hong Kong Polytechnic University -- Dissertations
Solid oxide fuel cells
Fuel cells
Pages: 150 pages : color illustrations
Appears in Collections:Thesis

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