Soot Oxidation in Flames

Abstract

The kinetics and mechanisms of soot oxidation were studied at temperatures of 1575 to 1865 K and O₂ mole fractions of 10⁻⁵ to 0.05 in a two-stage atmospheric-pressure combustion system in which soot-laden gas from a primary flame was partially cooled, mixed with oxygen-containing gas, and burned in a downstream premixed flame. The rate of oxidation of soot was measured using light scattering and absorption techniques to determine the particle size and concentration of soot as a function of distance, and hence of time, in the secondary flame. Complementary measurements of soot particle size were performed using probe sampling, electron microscopy, and electrical mobility analysis, and the specific surface area of the soot at different stages of burnout was measured by the BET technique. Temperature and gas composition profiles in the secondary flame were determined using coated thermocouples with electrical compensation for radiation loss, chromatographic analysis of sampled stable gases, and a spectroscopic technique combined with partial equilibrium calculations for radical species. It was found that OH radical is the principal oxidant of soot under these conditions, with O₂ being of secondary importance. The average value of the collision efficiency of OH with soot is found to be 0.28 if the optical (equivalent sphere) diameter of the soot aggregates is used in the calculations, or about 0.13 if the diameter of the individual spherical units within the aggregates is used. The actual value of the collision efficiency is expected to be bounded by these two values. The results indicate that soot burnout rates predicted from the Nagle and Strickland-Constable formula, which assumes O₂ to be the oxidant, are drastically underestimated under fuel-rich flame conditions owing to the neglect of the OH contribution. Breakup of soot aggregates during burnout was observed under fuel lean conditions.