A numerical study of fuel recirculation in ammonia-fueled solid oxide fuel cell stacks

Abstract

Fuel recirculation is a system configuration designed to improve the fuel utilization of fuel cells by recirculating the unreacted fuels at the outlet to the fuel inflow stream. In this study, fuel recirculation of a solid oxide fuel cell (SOFC) operated with ammonia is investigated. Recirculation operation results in a notable elevation of nitrogen content within the cell, originating from the decomposition of ammonia within the cell and subsequently reintroduced into the inflow stream. Therefore, the fuel flow rate needs to be increased to maintain the fuel utilization level the same as the base operation, which is an ammonia-fueled operation without recirculation. A multiscale model of an SOFC stack is used to compare the base operation and saturated level of the recirculation operation cases based on their polarization curves and distributions of ammonia, hydrogen, current density, temperature, first principal stress, and nitriding in the fuel electrode. The results show that overall stack performance, as reflected in the polarization curves, remains comparable between the base and recirculation operation cases. The power densities exhibit a difference of less than 2% between the base and recirculation operation cases. Moreover, it is indicated that the recirculation operation cases lead to more uniform hydrogen and current density distributions, lower temperature gradients, and lower thermal stresses. The results demonstrate that the recirculation operation cases lead to a reduction of up to 38% in the maximum tensile stresses within the fuel support layer. Nevertheless, these recirculation operation cases also induce nitriding across a larger portion of the active area. But as shown here this is a challenge that can effectively be addressed by applying nickel coatings to the inlet headers.