Analysis and optimization of solid oxide fuel cell system with anode and cathode off gas recirculation

Solid oxide fuel cell technology shows great potential in generating electricity. However, insufficient steam in the stack can result in carbon deposition, accelerating cell degradation over prolonged operation. To prevent this, ensuring an adequate supply of external water is necessary, yet resulting in a larger external water purification unit and a greater need for heat for water evaporation. Anode-off gas, which comprises unreacted fuel, steam and CO${}_2$ from the stack, can be strategically recirculated to the external reformer inlet, preventing carbon deposition in the reformer and stack. Further, solid oxide fuel cell system offers the potential to supply high-temperature heat to industrial applications, embodying the concept of combined heat and power plants. To maximize heat availability, cathode-off gas can partially be blended with the fresh air entering the stack. Various anode-off gas and cathode-off gas recirculation configurations are possible, and they have to be systematically analysed and compared. This study models all possible system configurations using different types of anode-off gas (no, cold, warm, hot) and cathode-off gas (no, warm, hot) recirculations. Multi-objective optimization has been conducted, and system performance has been analysed and compared using electrical efficiency, freshwater consumption, thermal efficiency, design complexity, heat availability and heat valorization potential. Further, an in-depth analysis of the impact of decision variables on the objective functions has been performed for different system configurations. These valuable insights serve as a guide to engineers and decision-makers, enabling informed decisions for solid oxide fuel cell system design.