High-pressure catalytic combustion of methane over platinum: In situ experiments and detailed numerical predictions

The catalytic combustion of fuel-lean methane/air premixtures over platinum was investigated experimentally and numerically in the pressure range 4 to 16 bar. Experiments were performed in an optically accessible, laminar channel-flow catalytic reactor. In situ, spatially resolved Raman measurements of major species and temperature over the reactor boundary layer were used to assess the heterogeneous (catalytic) reactivity and planar laser-induced fluorescence (LIF) of the OH radical confirmed the absence of homogeneous (gas-phase) ignition. Numerical predictions were carried out with a two-dimensional elliptic code that included elementary heterogeneous and homogeneous chemical reaction schemes. Comparisons between measurements and numerical predictions have led to the assessment of the high-pressure validity of two different elementary heterogeneous chemical reaction schemes for the complete oxidation of methane over platinum. It was shown that the catalytic reactivity increased with increasing pressure and that crucial in the performance of the heterogeneous reaction schemes was the capture of the decrease in surface free-site availability with increasing pressure. Even in the absence of homogeneous ignition, the contribution of the gaseous reaction pathway to the conversion of methane could not be ignored at high pressures. The delineation of the regimes of significance for both heterogeneous and homogeneous pathways has exemplified the importance of the preignition gaseous chemistry in many practical high-pressure catalytic combustion systems. Sensitivity and reaction flux analyses were carried out on a validated elementary heterogeneous reaction scheme and led to the construction of reduced catalytic reaction schemes capable of reproducing accurately the catalytic methane conversion in the channel-flow configuration as well as in a surface perfectly stirred reactor (SPSR) and, when coupled to a homogeneous reaction scheme, the combined heterogeneous and homogeneous methane conversions. A global catalytic step could not reproduce the measured catalytic reactivity over the entire pressure range 4 to 16 bar.