Anode gas and steam recycling for internal methane reforming SOFCs: Analysis of carbon deposition

The development of solid oxide fuel cell (SOFCs) systems capable of direct internal reforming (DIR) of methane and higher hydrocarbons 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 anodic fuel and steam recycling schemes as possible mitigation strategies against carbon deposits at fuel steam-to-carbon (S:C) 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 operation of a an internal reforming SOFC and spatial extent of carbon deposition within the anode are analyzed based on a thermodynamic analysis accounting for both the cracking and Boudouard reactions, for several fuel humidification and recycling conditions. 50% (mass %) fuel recycling is shown to be an effective mitigation strategy against carbon deposition at inlet xH₂O/xCH₄ ratios of 0.5 to 1, with only a minor portion of the cell inlet region affected by coking. For lower recycling ratios at the same fuel compositions, fuel recycling reduces the risk of coking, but does not eliminate it. For the SOFC configuration studied, at a S:C of 0.5, steam recycling is found to reduce the extent of carbon deposits by a magnitude comparable to that obtained using fuel recycling, providing that steam recycling ratios on order 25% higher than the fuel recycling ratios are applied. Steam recycling may therefore be considered advantageous, in terms of reduced overall mass flow. For a S:C = 0.5, the mitigating effect of steam recycling on the susceptibility to coking is through the directions of the cracking and Boudouard reactions, while fuel recycling has a positive impact on the cracking reaction only. The anodic gas recycling strategies considered could help extend the operational range of DIR-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.