PRODUCTS IN METHANE COMBUSTION NEAR SURFACES

Emission of carbon monoxide (CO), formaldehyde (CH2O), and unburned methane (CH4) are calculated for premixed methane/air mixtures impinging on a flat surface as functions of surface temperature, equivalence ratio, and strain rate with detailed chemistry involving 46 reversible reactions and 16 species using numerical bifurcation theory. Multiple solutions with different selectivities to stable products are found. On the extinguished branch unburned CH4, molecular hydrogen (H-2), CO, and CH2O dominate, whereas on the ignited branch carbon dioxide (CO2) predominates near the surface. Cold walls can promote the selectivity to CO and CH2O near extinction, and high flow rates can increase considerably the formation of CO, CH2O, and unburned CH4. For example, an ignited stoichiometric methane/air mixture (9.5% CH4 in air) impinging on a surface of 1,000 K is calculated to produce 2 % CO, 150 ppm CH2O, and 3 % unburned CH4 for a strain rate of 500 s-1. Maximum efficiency of CH4 and minimum selectivity to CH2O occur near the stoichiometric ratio, whereas minimum selectivity to CO occurs for fuel lean mixtures. Comparison of combustion near surfaces with freely propagating flames is also shown.