THE OXIDATION OF SOOT AND CARBON-MONOXIDE IN HYDROCARBON DIFFUSION FLAMES

Quantitative OH . concentrations and primary soot particle sizes have been determined in the soot oxidation regions of axisymmetric diffusio n flames burning methane, methane/butane, and methane/1-butene in air at atmospheric pressure. The total carbon flow rate was held constant in these flames while the maximum amount of soot varied by a factor of seven along the centerline. Laser-induced fluorescence measurements o f OH . were placed on an absolute basis by calibration against earlier absorption results. The primary size measurements of the soot particl es were made using thermophoretic sampling and transmission electron m icroscopy. OH . concentrations are greatly reduced in the presence of soot particles. Whereas large super-equilibrium ratios are observed in the high-temperature reaction zones in the absence of soot, the OH . concentrations approach equilibrium values when the soot loading is hi gh. The diminished OH . concentrations are found to arise from reactio ns with the soot particles and only to a minor degree from lower tempe ratures due to soot radiation losses. Analysis of the soot oxidation r ates computed from the primary particle size profiles as a function of time along the flame centerlines shows that OH . is the dominant oxid izer of soot, with O2 making only a small contribution. Higher collisi on efficiencies of OH . reactions with soot particles are found for th e flames containing larger soot concentrations at lower temperatures. A comparison of the soot and CO oxidation rates shows that although CO is inherently more reactive than soot, the soot successfully competes with CO for OH . and hence suppresses CO oxidation for large soot con centrations.