Numerical investigation of the effect of fuel recycling on the susceptibility of a direct internal methane reforming sofc to carbon deposition

There is considerable interest in developing solid oxide fuel cell (SOFCs) systems capable of operating directly on methane via direct internal reforming (DIR). However, a major barrier to DIR is the susceptibility of current state-of-the-art nickel based anodes to carbon deposition, particularly at low fuel humidification levels. Overcoming these difficulties will require improved anode designs and identification of suitable operating conditions. In this study, the potential effectiveness of partial fuel recycling in mitigating the risks of carbon deposits is investigated in a planar DIR-SOFC operated on humidified methane at inlet steam-to-carbon ratios (S:Cs) of 0.1 to 1. This is achieved using a detailed computational fluid dynamics (CFD) model which couples momentum, heat, mass and charge transport with electrochemical and chemical reactions. The model thermodynamically predicts the spatial extent of carbon deposits by accounting for both the cracking and Boudouard reactions, for several fuel humidification and recycling conditions. At temperatures close to 1173 K and for inlet fuel S:Cs of 0.5 to 1, 50% (mass %) fuel recycling is found to be an effective strategy against carbon deposition. For lower recycling ratios at the same fuel compositions, or lower S:C ratios (regardless of the recycling ratio), fuel recycling reduces the risk of coking, but does not eliminate it. The results suggest that partial fuel recycling could contribute to extend the operational range of DIR-SOFCs to lower S:C ratios (0.5 to 1.0) than typically considered, with reduced risks of carbon deposition, while reducing system cost and complexity in terms of steam production. For dry or weakly humidified fuels, additional mitigation strategies would be required.