Heterogeneous and Combined Heterogeneous/Homogeneous Combustion Modeling of SOFC Off-Gases

The heterogeneous (catalytic) and the hetero-/homogeneous (catalytic and gas-phase) combustion processes of solid oxide fuel cell (SOFC) off-gases with compositions typical of a high cell utilization rate are investigated with high-fidelity 2D simulations in a platinum-coated planar channel using detailed hetero-/homogeneous chemistry. The pressures are 1-8 bar; the reactant streams have volumetric H₂ and CO contents 0.7-1.5 and 5.3-9.7%, respectively; H₂O and CO₂ dilutions are ∼40 and ∼50%, respectively; and the global fuel/air equivalence ratio is 0.90. Water inhibits chemically the catalytic oxidation of H₂, as it leads to high H(s) surface coverage that favors the recombinative desorption of H(s) to H₂. On the other hand, H₂O promotes chemically the catalytic oxidation of CO by creating high OH(s) coverage that in turn accelerates the CO consumption. Strong flames are established at the highest H₂ content cases and for pressures p ≥ 3 bar. For all cases with vigorous homogeneous combustion, the catalytic and gas-phase reaction pathways coexist and compete with each other for the consumption of H₂ and CO. The large H₂O content leads to gas-phase production of H₂ via the reaction H₂O + H = H₂ + OH. However, the gas-phase produced H₂ is subsequently consumed by the catalytic pathway, such that nearly complete H₂ conversion is attained at the reactor outlet. Gaseous chemistry does not affect the reactor lengths required for complete H₂ conversion but substantially reduces the corresponding lengths for CO conversion. The H₂ emissions decrease with increasing pressure and are in the range 8-110 ppmv, while the CO emissions increase with rising pressure and span the range 0.3-52 ppmv, thus leading to corrected CO emissions (at 15% O₂) of less than 15 ppmv. Finally, the peak wall temperatures are largely acceptable in terms of catalyst thermal stability.