| Author: | Xu, Qidong |
| Title: | Modelling of high temperature methanol-fuelled solid oxide fuel cells |
| Advisors: | Ni, Meng (BRE) |
| Degree: | Ph.D. |
| Year: | 2023 |
| Subject: | Solid oxide fuel cells Methanol as fuel Fuel cells Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Building and Real Estate |
| Pages: | xvii, 98 pages : color illustrations |
| Language: | English |
| Abstract: | Solid oxide fuel cells (SOFCs) have been recognized as one of the most important energy conversion technologies due to high energy efficiency and low emission. In addition, fuel flexibility is the most significant advantage of SOFCs since direct utilization of the carbon-containing fuel (alkanes, alcohols, biomass etc.) is possible at the typical high operating temperature of SOFCs. Among these fuels, Methanol is a promising fuel for SOFCs due to its easy storage and transportation compared with hydrogen. However, in the current literature, computer simulation work on methanol-fuelled SOFCs is limited, and comprehensive understandings of chemical reactions, gas composition and temperature distributions inside the SOFC are unclear. Therefore, a 2D numerical model is developed to study a tubular direct methanol SOFC. A simulation model without considering the heat transfer is developed to study a tubular direct methanol SOFC. The model fully considers the methanol decomposition reaction (MDR) and water gas shift reaction (WGSR) in the anode, electrochemical oxidations of H2 and CO, fluid flow and mass transfer in the cell. The model is validated by the direct methanol SOFC experiment. At a temperature of 1073K, a peak power density of 1.3 W cm-2 is achieved, which is much higher than room temperature direct methanol fuel cells (typically less than 0.1 W cm- 2). Subsequent parametric simulations are conducted to understand the effects of operating and structural parameters on the SOFC performance, such as temperature, potential, anode thickness and cell length. Increasing the temperature enhances chemical/electrochemical reaction rates and ion conduction, leading to improved cell performance. Increasing the anode thickness improves methanol conversion and increases the average current density to some extent. For comparison, a longer cell can also improve methanol conversion but decreases the average cell current density. Thermo-electrochemical modelling study is subsequently conducted to consider heat transfer effects on the electrochemical performance of a methanol-fuelled SOFC. Parametric simulations are performed to investigate the effects of operating potential, steam to carbon ratio, inlet temperature and fuel/air flow rates on the performance of SOFCs. At 1073K, the peak power density of methanol-fuelled SOFC is higher than 10000 W m-2 with a steam to carbon ratio of 1. In addition, the temperature distribution in SOFC could be remarkably affected by the working conditions because of chemical/electrochemical reactions and overpotential losses. Large temperature variation (nearly 180 K) between the inlet and outlet is observed mainly due to greatly improved current density at a low operating potential. Also, temperature reduction could be achieved by increasing the steam to carbon ratio and gas flow rates (higher than 170 SCCM for air and 0.1 ml min-1 for fuel mixture, respectively), which could improve the long-term stability from the perspective of thermal stress but inevitably lower the efficiency of SOFC. Meanwhile, higher inlet temperature not only enhances the power output but improves the uniformity of cell temperature distribution. Further investigation on thermal responses of methanol-fuelled SOFC is also conducted since thermal management is a challenging issue given the non-uniform electrochemical reactions and convective flows within SOFCs. Results show that unlike the low-temperature condition of 873 K where peak temperature gradient occurs in the cell centre, it is likely to appear near the fuel inlet because of rapid temperature rise induced by elevated current density at 1073 K. Despite large heat convection capacity, excessive air could not effectively eliminate the harmful temperature gradient caused by large current density. Fortunately, well control of current density by properly selecting operating potential could result in a local thermal neutral state. Interestingly, the maximum axial temperature gradient could be reduced by about 18% at 973 K and 20% at 1073 K when the air with a 5 K higher temperature is supplied. Also, despite the higher electrochemical performance observed, the cell with a counter-flow arrangement featured by a larger hot area and higher maximum temperature gradients is not preferable for a ceramic SOFC system considering thermal durability. The results of the current study form a basis for subsequent performance enhancement of methanol-fuelled SOFCs by optimization of the cell structure and operating parameters. Besides, this study could also provide insightful thermal information for operating condition selection, structure design and stability assessment of realistic SOFCs running on methanol fuel. |
| Rights: | All rights reserved |
| Access: | open access |
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