Modeling Formation and Oxidation of Soot in Nonpremixed Flames

A detailed kinetic mechanism of aromatic growth, particulate formation, and oxidation is presented and is tested in nonpremixed laminar flames of methane and ethylene at atmospheric pressure. Model development is refined in strict connection with new experimental data on the formation and oxidation of high molecular mass compounds and incipient particles. Reaction pathways leading to the formation of incipient particles, their transformation to soot, their oxidation, and the oxidation-induced fragmentation of particles and aggregates have been included by using a multisectional approach for the particle process. Predictions within a factor of 2-3 are obtained for major oxidation and pyrolysis products as well as trace aromatic species and particulate concentrations. The newly developed model predicts the concentration of the particles, their sizes, morphology, and chemical properties in nonpremixed flames of methane and ethylene with a wide range of particle formation without any condition-dependent adjustments to the kinetic scheme. A wide range of particle sizes is covered from nanoparticles formed on the fuel side of the flames to larger soot particles and particle aggregates formed in the flame wings. The trend of the H/C ratio of the particles along the flame axis is also predicted well. It decreases to very low values typical of mature soot particles when large aggregates are produced. The new mechanism for particle oxidation, which includes the oxidation-induced fragmentation of particles and aggregates, has shown the importance of accurate modeling of particle oxidation to correctly predict particle burnout and particle size in nonpremixed flames.