Homogeneous combustion of fuel-lean syngas mixtures over platinum at elevated pressures and preheats

The gaseous oxidation of H-2/CO/CO2/O-2/N-2 mixtures was investigated experimentally and numerically in a platinum-coated channel at fuel-lean stoichiometries (equivalence ratios phi <= 0.30), H-2:CO molar ratios 0.47-4.54, pressures 2-14 bar, and reactant preheats up to 736 K. Two-dimensional laser induced fluorescence of the OH radical monitored the homogeneous (gaseous) combustion, while 1-D Raman spectroscopy assessed the heterogeneous (catalytic) conversion of H-2 and CO. Numerical simulations, which were carried out with a 2-D elliptic code and detailed hetero-/homogeneous reaction schemes, reproduced the measured onset of homogeneous ignition, the ensuing flame shapes, and the mass-transport-limited catalytic conversion of H-2 and CO. Additional simulations in practical tubular channels with 1 mm diameter have shown that gaseous oxidation was suppressed at atmospheric pressure due to the intrinsic slow gas-phase ignition kinetics in conjunction with the competition from the catalytic pathway for H-2 and CO consumption. At pressures p > 4 bar, homogeneous combustion was largely controlled by flame propagation characteristics due to the near-wall confinement of the established flames. The decrease in laminar mass burning rates at p > 4 bar led to a push of the gaseous combustion zone close to the channel wall, to leakage of H-2 and CO through the flame and, finally, to subsequent catalytic conversion of the leaked fuel components. Radical heterogeneous reactions promoted mildly the onset of homogeneous ignition at p >= 2 bar due to the net desorptive flux of OH over the gaseous induction zone. The catalytically produced H2O had a strong kinetic impact on homogeneous combustion by inhibiting the gaseous oxidation of both H-2 and CO at high H-2:CO ratios and by promoting CO gaseous oxidation at low H-2:CO ratios. The catalytically produced CO2 always inhibited kinetically the gaseous combustion of H-2 and CO, although its effect was much weaker compared to that of H2O. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.