Simulation of Variable Operating Condition Performance of Methanol Solid Oxide Fuel Cell Planar Tubular Stack and System

With the advancement of global energy transition, the development of efficient and clean energy conversion technologies becomes crucial to addressing energy shortages and environmental pollution. Solid oxide fuel cells (SOFCs) have attracted much attention due to their high energy conversion efficiency and fuel flexibility, while methanol as a liquid hydrogen carrier offers advantages such as safe storage/transportation and wide availability. However, a systematic research on the off-design performance and optimal design of methanol-SOFC systems remains insufficient, particularly for planar-tubular SOFC stacks through modeling and experimental validation. This study was to establish an accurate mathematical model combined with experimental verification to elucidate the power generation characteristics of methanol -SOFC systems under variable operating conditions, providing theoretical foundations and technical support for efficient and stable system operation.

This study proposed an equilibrium-based SOFC stack model based on the principle of minimizing Gibbs free energy, integrating heat and mass balance calculations, while comprehensively considering the effects of ohmic polarization, activation polarization, and concentration polarization on the voltage. An industrial-sized flat-tube SOFC stack (5 cells) was used to compare the power generation performance of pure hydrogen and methanol reforming gas at 750 ℃. The composition of the reforming gas was analyzed by gas chromatography, and the I–V curve was tested. A system flow model wss constructed using a software named Aspen Plus, and the I–V curves obtained from the SOFC system model simulation aligned closely with the experimental results, confirming the accuracy of the computational model used in this study. The sensitivity analysis was then performed to evaluate the effects of key parameters (i.e., the steam-to-carbon molar ratio (STCR), fuel utilization rate (U_f), and current density) on the system performance.

In the comparation of the performance of the SOFC system under pure hydrogen and methanol reforming gas power generation conditions, the maximum power density difference was only 5.59%, confirming the practicality of methanol fuel. Furthermore, the sensitivity analysis indicates that a) since anode carbon deposition is a key factor leading to stack failure, the impact of the steam-to-carbon molar ratio (STCR) on the system power generation performance is studied. The operating voltage decreases from 3.505 V to 3.226 V, and the system efficiency reduces from 45.95% to 42.35% when the STCR increases from 1 to 4 (a commonly used STCR in industrial processes). A low STCR (=2) can balance both the efficiency and carbon deposition suppression; b) the impact of fuel utilization rate on the system power generation performance is investigated. The system efficiency increases from 35.79% to 57.25% when the fuel utilization rate increases from 55% to 95%. However, the excessive concentration polarization can be avoided, and a fuel utilization rate of 85% is recommended; and c) after determining the system's fuel utilization rate and STCR, the impact of current density on the system power generation performance is studied. The calculated current density range is 0.602–0.733 A·cm^–2, with the operating voltage range of 3.25–3.50 V under the target power range conditions (i.e., P = 147.3–166.9 W). The system efficiency ranges from 38.18% to 43.27% at the optimal current density of 0.704 A·cm^–2.

This study proposed a computational model for the methanol power generation characteristics of flat-tube SOFC stacks, enabling the performance prediction of the methanol-SOFC stack under different operating conditions. The accuracy of this model was validated through modeling and experimental results. The study further clarified the performance behavior of the methanol-SOFC system under varying conditions and proposed optimization ranges for key parameters. The results indicated that methanol fuel could effectively replace pure hydrogen, and the stack output power could be controlled under high voltage conditions. The efficient and stable operation could be achieved via adjusting the steam-to-carbon molar ratio, fuel utilization rate and current density. At an output power of 150 W, the optimal operating parameters were a steam-to-carbon molar ratio of 2, a fuel utilization rate of 85%, and a current density of 0.704 A·cm^–2. Under the optimum conditions, the stack efficiency was 36.71%, and the system efficiency was 42.21%. The research findings could provide an important reference for the engineering application of methanol-SOFC systems and have significant implications for advancing clean energy technologies.