Numerical design and evaluation of ammonia fueled solid oxide fuel cell (SOFC) power systems with different integration architecture: Efficiency, power density and thermal safety

Solid oxide fuel cell (SOFC) power system fueled with the carbon-free fuel ammonia is a promising technique to reduce CO2 in traffic and transportation sector. This study constructed zero-dimensional steady-state models for the SOFC stack and corresponding balance of plant (BOP) components, and conducted efficiency, power density and thermal safety analysis on a kW level ammonia SOFC system. The effects of off-gas recirculation strategy on system performance were firstly studied. The results show that in the basic ammonia SOFC system without off gas recirculation, the fuel and air stream entering the afterburner are strictly determined by the stack operating parameters such as stack fuel utilization and flow rate. This makes its net electrical efficiency be not only low but also impossible to be independently and flexibly adjusted via controlling the fuel thermally consumed in the afterburner. The use of anode off-gas recirculation (AOGR) greatly improves the net electrical efficiency and operating flexibility via decoupling the system fuel utilization from the stack fuel utilization. By contrast, cathode off-gas recirculation (COGR) exhibits adverse effects, which reduces the net electrical efficiency and is not conducive to the thermal stability of the afterburner. In the end, a new system architecture design (SOFC-AOGRSS) is proposed, which adopts both AOGR and steam separation in the anode recirculated off-gas using the vaporization cooling of liquid ammonia. The proposed system could operate at extended stack fuel utilization range of 0.67-0.80 and achieve a net electricity efficiency of 51.94-56.50% without external cooling water. Moreover, with additional external cooling water, the stack fuel utilization range and the net electricity efficiency of system could be extended to 0.58-0.80 and 51.84-57.15% respectively.