Experimental and numerical investigation of hetero-/homogeneous combustion of CO/H 2/O 2/N 2 mixtures over platinum at pressures up to 5 bar

Abstract

The hetero-/homogeneous combustion of fuel-lean CO/H 2/O 2/N 2 mixtures over platinum is investigated at pressures up to 5 bar, inlet temperatures ( T IN) up to 874 K, and a constant CO:H 2 molar ratio of 2:1. Experiments are performed in an optically accessible channel-flow catalytic reactor and involve planar laser induced fluorescence (LIF) of the OH radical for the assessment of homogeneous (gas-phase) ignition and 1-D Raman measurements of major gas-phase species concentrations over the catalyst boundary layer for the evaluation of the heterogeneous (catalytic) processes. Simulations are carried out with an elliptic 2-D model that includes detailed heterogeneous and homogeneous chemical reaction schemes. The predictions reproduce the Raman-measured catalytic CO and H 2 consumption, and it is further shown that for wall temperatures in the range 975 ⩽ T w ⩽ 1165 K the heterogeneous pathways of CO and H 2 are largely decoupled. However, for wall temperatures below a limiting value of 710–720 K and for the range of pressures and mixture preheats investigated, CO(s) blockage of the surface inhibits the catalytic conversion of both fuel components. The homogeneous ignition distance is well-reproduced by the model for T IN > 426 K, but it is modestly overpredicted at lower T IN. Possible reasons for these modest differences can be the values of third body efficiencies in the gas-phase reaction mechanism. The sensitivity of homogeneous ignition distance on the catalytic reactions is weak, while the H 2/O 2 subset of the CO/H 2/O 2 gaseous reaction mechanism controls the onset of homogeneous ignition. Pure hydrogen hetero-/homogeneous combustion results in flames established very close to the catalytic walls. However, in the presence of CO the gaseous combustion of hydrogen extends well-inside the channel core, thus allowing homogeneous consumption of H 2 at considerably shorter reactor lengths. Finally, implications of the above findings for the design of syngas-based catalytic reactors for power generation systems are discussed.