Stability of hetero-/homogeneous combustion in propane- and methane-fueled catalytic microreactors: Channel confinement and molecular transport effects

The steady hetero-/homogeneous combustion of lean propane/air and methane/air mixtures in a platinum-coated, plane channel-flow catalytic microreactor has been investigated at pressures of 1 and 5 bar, channel heights of 1.0 and 0.3 mm, and wall thermal conductivities of 2 and 16 W/mK. Stability limits were assessed as a function of fuel type, inlet velocity, and imposed external heat losses. Parametric studies were performed with a full-elliptic, two-dimensional numerical model employing elementary gas-phase (homogeneous) reaction schemes for both fuels, a detailed heterogeneous (catalytic) reaction scheme for methane and a recently developed global reaction step for the oxidation of propane on Pt. Comparisons between the stable combustion regimes of methane and propane revealed a strong impact of the fuel molecular transport properties on the stability and maximum allowable mass throughput. The higher diffusive transport of methane was critical in maintaining wider high inlet velocity stability limits (blowout) compared to those of propane, despite the higher catalytic and gas-phase reactivity of the latter. On the other hand, at the low velocity limits (extinction), propane exhibited a wider stability envelope. Gas-phase chemistry had a strong impact on the blowout limits, even at channel heights as low as 0.3 mm. For the same mass throughput, smaller channel heights tolerated higher heat losses at the extinction branch of the combustion stability envelope thanks to increased transverse fuel transport, while at the same time they exhibited narrower limits at the blowout stability branch due to insufficient residence times at higher inlet velocities. The stable combustion regime of propane increased substantially at 5 bar compared to the same mass throughput at 1 bar, owing to a positive p +0.75 dependence of the propane catalytic reactivity on pressure. Finally, the role of high wall thermal conductivity in widening the blowout limits for both fuels has been demonstrated.