Anode Fuel and Steam Recycling for Internal Methane Reforming SOFCs: Analysis of Carbon Deposition

Anode fuel and steam recycling are explored as possible mitigation strategies against carbon deposition in an internal methane reforming solid oxide fuel cell (IR-SOFC) operated at steam-to-carbon ratios (S:Cs) of 0.5–1. Using a detailed computational fluid dynamics model, the cell behavior and spatial extent of carbon deposits within the anode are analyzed based on a thermodynamic analysis accounting for both the cracking and Boudouard reactions for fuel and steam recycling fractions of up to 90% (mass percent). At temperatures close to 1173 K, 50% fuel recycling is found to be an effective mitigation strategy against carbon deposition, with only a minor portion of the cell inlet affected by coking. Steam recycling reduces the extent of carbon deposits by a magnitude comparable to that obtained using fuel recycling, provided that recycling ratios on the order of 25% higher than that for fuel recycling are applied. Steam recycling could therefore be considered advantageous in terms of reduced overall mass flow. The mitigating effect of fuel recycling on the susceptibility to coking at the cell inlet is found to be through the direction of the cracking reaction, while steam recycling has a positive (but slightly less effective) impact on both the Boudouard and cracking reactions. The results suggest that partial anode gas recycling could help extend the operational range of IR-SOFCs to lower fuel humidification levels than typically considered, with reduced thermal stresses and risks of carbon deposits, while reducing system cost and complexity in terms of steam production.