Numerical analysis of an internal methane reforming solid oxide fuel cell with fuel recycling

The development of solid oxide fuel cell (SOFC) systems capable of direct internal reforming (DIR) of methane is being actively pursued. However, a major challenge with current state-of-the-art nickel-based anodes is their propensity to form deteriorous carbon deposits in DIR, unless excess steam is introduced in the fuel. Reduced fuel humidification levels are desirable from the viewpoints of cell performance, reliability and plant economics. This study explores the use of partial recycling of the anode exhaust as a mitigation strategy against carbon deposits at fuel steam-to-carbon ratios less than unity. Using a detailed computational fluid dynamics (CFD) model which couples momentum, heat, mass and charge transport with electrochemical and chemical reactions, the spatial extent of carbon deposition within a SOFC anode is analyzed by accounting for both the cracking and Boudouard reactions, for several fuel humidification and recycling conditions. At temperatures of approximately 1173 K and for inlet fuel molar H2O/CH4 ratios between 0.5 and 1, 50% (mass%) fuel recycling is found to be an effective strategy against carbon deposition. For lower recycling levels at the same fuel compositions, or lower fuel humidification levels (regardless of the recycling level), fuel recycling reduces the risk of coking, but does not eliminate it. The analyses presented suggest that recycling of the anodic fuel stream could help extend the operational range of DIR-SOFCs to lower fuel humidification levels than typically considered, with reduced risks of carbon deposits, while reducing system cost and complexity in terms of steam production. For dry or weakly humidified fuels, additional mitigation strategies would be required.