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Oxidation of Soot and Carbon Monoxide in Hydrocarbon Diffusion Flames.


pdf icon Oxidation of Soot and Carbon Monoxide in Hydrocarbon Diffusion Flames. (1114 K)
Puri, R.; Santoro, R. J.; Smyth, K. C.

Combustion and Flame, Vol. 97, 125-144, 1994.

Sponsor:

National Institute of Standards and Technology, Gaithersburg, MD

Keywords:

soot; carbon monoxide; diffusion flames; oxidation; particle size; concentration measurement

Abstract:

Quantitative OH concentrations and primary soot particle sizes have been determined in the soot oxidation regions of axisymmetric diffusion 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 of OH were placed on an absolute basis by calibration against earlier absorption results. The primary size measurements of the soot particles were made using thermophoretic sampling and transmission electron microscopy. 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 high. The diminished OH concentrations are found to arise from reactions with the soot particles and only to a minor degree from lower temperatures due to soot radiation losses. Analysis of the soot oxidation rates computed from the primary particle size profiles as a function of time along the flame centerlines shows that OH is the dominant oxidizer of soot, with O2 making only a small contribution. Higher collision efficiencies of OH reactions with soot particles are found for the 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 concentrations.