Comparison of off-gas utilization modes for solid oxide fuel cell stacks based on a semi-empirical parametric model

Solid oxide fuel cells (SOFCs) provide a clean and efficient pathway to use a variety of carbon containing fuels for power generation. Reuse or recycle of the high-temperature off-gas can help to improve the fuel or air utilization of SOFC stacks, as well as to improve system efficiency and reduce operating costs. The off-gas utilization mode and its effect on the stack performance need to be further clarified. In this study, a novel semi-empirical parametric stack model was established based on the experimental data of a 3-cell SOFC stack. This parametric model can accurately predict the output performance of real stacks, which was verified by multi-condition experiments. Based on this stack model, one basic process and four off-gas utilization modes were designed and constructed. Comparative analysis of these modes under different operation parameters was carried out. The effects of key operation parameters, including fuel flow rate, operating current, stack temperature, pre-reformer temperature, split ratio and recycle ratio on the stack performance were studied, which were characterized by the electrical efficiency and exergy efficiency of the power module. It was found that the mode with two-stage stacks can get the maximum electrical efficiency (>45%) with the operating current less than 20 A, while the mode with anode off-gas recycled to the pre-reformer instead obtained the maximum electrical efficiency (>35%) when the specified current was larger than 25 A. A comprehensive and in-depth perspective on different stack off-gas utilization modes is provided, which can supply feasible guidance for the next-step overall system design. Besides, the stack modeling and process simulation method developed in this study can provide reference for related research.