Heterogeneous and homogeneous combustion of fuel-lean C₃H₈/O₂/N₂ mixtures over rhodium at pressures up to 6 bar

The heterogeneous and homogeneous combustion of C₃H₈/O₂/N₂ mixtures over Rh was studied at pressures 1-6 bar, catalyst surface temperatures 680-1100 K and C₃H₈-to-O₂ equivalence ratios 0.25-0.52. Non-intrusive laser-based measurements were applied in a channel-flow catalytic reactor and involved 1-D Raman spectroscopy of major gas-phase species across the channel boundary layer for assessing the catalytic reactivity and planar laser induced fluorescence (PLIF) of the OH radical for monitoring homogeneous combustion. Simulations were performed using a 2-D CFD code with detailed heterogeneous and homogeneous chemical reaction mechanisms. By comparing the Raman-measured and predicted transverse profiles of the limiting C₃H₈ reactant, the suitability of a detailed surface reaction mechanism was initially evaluated and subsequently a one-step reaction was constructed, which was applicable for the C₃H₈ total oxidation over Rh at 1-6 bar. The catalytic reactivity of C₃H₈ over Rh displayed a ∼p^0.14 pressure dependence, which was substantially lower than a previously reported ∼p^0.70 dependence over Pt. The weak pressure dependence of the C₃H₈ reactivity on Rh suggested caution when selecting catalysts for high-pressure power systems (recuperative microreactors, small-scale turbines) fueled with C₃H₈ or LPG. Comparisons of PLIF-measured and predicted distributions of the OH radical indicated that the employed gas-phase reaction mechanism captured the onset of homogeneous ignition at pressures ≥ 3 bar as well as the ensuing flame shapes. Predicted and measured homogeneous ignition distances agreed within 2.5% at 6 bar. With decreasing pressure, the predictions yielded gradually increasing but still modest underpredictions (up to 11.2% at 3 bar) of the homogeneous ignition distances. The key gas-phase reactions affecting homogeneous combustion at various pressures were finally identified.