Study on the efficiency of SOFC system with anode off-gas recirculation

Abstract

This work is focused on thermodynamic and electrochemical modeling of kW class SOFC system to evaluate its performance and efficiency. The system consists of diesel reformer, SOFC, balance of plants (BOPs) and after-burner. BOPs are composed of pumps and compressors which control flow rates of reactants and products. After-burner combusts unused fuel and supplies the heat energy for cathode air. Recirculation of anode off-gas affects the performance of whole systems (SOFC, reformer, BOP). There are two possible options for recirculation direction: to reformer or to anode of SOFC. Recirculation direction and ratio are major variables. The objective of this research is to define appropriate recirculation ratio and direction. Model for SOFC established in this work is an integrated electrochemical and thermal steady-state model. It is capable of simulating the operating behavior of a fuel cell, i.e. fuel and air utilization, power produced, efficiency, current for different operating conditions. The model was based on the zero-dimensional approach considered as point device, and that is integrated with the process flowsheet model. Anode off-gas recirculation profits the excess fuel and heat energy recycling. System performance and efficiency were varied with recirculation ratio and recirculation direction to reformer or anode of SOFC. Maximum system efficiency archived at recirculation ratio of 0.4 to anode. The change of steam to carbon ratio ($H_2O$/C) and oxygen to carbon ratio ($O_2$/C) was varied to observe system behavior. The relationships between parameters are verified. Oxygen to carbon ratio change was more effective to improve system efficiency. This study can provide design parameters of SOFC system such as recirculation ratio, direction, and inlet conditions to increase the efficiency.