Importance of gas-phase kinetics within the anode channel of a solid-oxide fuel cell

Experiments using n-butane as a representative hydrocarbon fuel were conducted under gas-phase conditions similar to those expected in the anode channel of a solid-oxide fuel cell (SOFC). Butane conversion and product formation were monitored in quartz reactor experiments at P similar to 0.8 atm, tau similar to 5 s, and T = 550-800 degreesC. Three different fuel mixtures were used: neat n-butane, 50% n-C4H10/50% H2O, and 50% n-C4H10/50% N-2. These experiments demonstrate that substantial gas-phase chemistry does occur and that this must be accounted for when predicting fuel cell efficiency. These data were compared to predictions using a plug-flow model that incorporated the experimentally measured temperature profile along the reactor. The reaction mechanism used for these simulations consisted of similar to300 species and 2500 elementary reactions and included both pyrolysis and oxidation reactions. Comparisons of the model predictions to the experimental data show that the model, without any modifications, captures the observed strong temperature dependence of n-butane conversion and is also able to capture the changes in product selectivity with temperature for the neat butane and the diluted butane mixtures. The model also properly predicts the observed onset of deposit formation near 700 degreesC. Both conversion and selectivity are shown to be sensitive to only a very small subset of the reactions in the mechanism. Comparison of the rate coefficients of this subset to literature values, where available, are generally reasonable and suggest that this kinetic model is adequate for describing the gas-phase reactions of small hydrocarbons in the anode channels of a SOFC. Additional efforts are required to account for catalytic reactions on the surface of the porous anode.