Soot Formation and Oxidation in Laminar Flames (Invited)

The flame and soot structure, including soot primary particle nucleation, surface growth and oxidation properties, of 6 premixed and 20 diffusion flames were studied experimentally for various fuels, at temperatures of 1400-2350 K and at pressures of 13-811 kPa (0.1-8.0 atm). Measurements were made along the axes of flames with the reactants at normal temperature (300 K). The following properties were measured as a function of distance from the burner exit: soot concentrations by deconvoluted laser extinction, soot temperatures by deconvoluted multiline emission, soot structure by thermophoretic sampling and analysis using transmission electron microscopy (TEM), concentrations of major stable gas species by isokinetic sampling and gas chromatography, concentrations of radial species (H, OH, O) by deconvoluted Li/LiOH atomic absorption, and flow velocities by laser velocimetry. The measurements were analyzed to determine local soot surface growth, oxidation and nucleation rates, as well as local flame properties that are thought to affect these rates. The measurements of soot surface growth rates (corrected for soot surface oxidation) were consistent with the measurements in laminar premixed and diffusion flames involving a variety of hydrocarbons at variable pressure. In addition, the growth rates from all the available flames were in good agreement with each other and with existing hydrogenabstraction/carbon-addition (HACA) soot surface growth mechanisms available in the literature yielding values of steric factors on the order of unity, as expected. Measurements of early soot surface oxidation rates (corrected for soot surface growth and prior to consumption of 70 % of the maximum mass of the primary soot particles) in laminar diffusion flames were consistent with all flames regardless of flame types, fuels, temperatures and pressures. The soot surface oxidation rates from all available flames could be explained by reaction with OH, having a collision efficiency of 0.12, and supplemented to only a minor degree by direct soot surface oxidation by O2.