Numerical SOFC Anode Catalyst Occupation Study: Internal Reforming of Carbonaceous Fuel Mixtures

Internal reforming of light hydrocarbons is one of the major advantages of solid oxide fuel cells (SOFCs). This reforming process includes the risk of carbon formation and/or nickel re-oxidation on the active nickel sites of the porous fuel electrode. Knowledge of the absolute and relative amounts and positions of surface adsorbed species helps to achieve an understanding of the processes that occur and can be used to identify prevalent cell degradation mechanisms induced by internal reforming. This heterogeneous process was numerically investigated at an operating temperature of 800°C by means of a detailed computational fluid dynamics (CFD) model. The surface composition of adsorbed species within the porous structure of a Ni/YSZ SOFC anode caused by different carbon-containing feeds will be discussed. Six fuel mixtures that represent diesel reformates or other carbon containing fuels with varying S/C ratio and methane to carbon monoxide ratio were analyzed in the context of their propensity to coking and oxidation. It was shown that highest carbon surface coverages occur when using feeds with a high methane to carbon monoxide ratio. Fuels that contain only carbon monoxide as carbon precursor lead to considerably lower coverages. Furthermore, surface specific coverage ratios were introduced that were used as effective methods to analyze coking or oxidation of the porous substrate.