Kinetics of Soot Formation in a Flow System Simulating the Lower Part of a Laminar Diffusion Flame

Abstract

The conversion of fuel hydrocarbons into soot in a flow system simulating the lower part of a diffusion flame was investigated. The soot formation was assumed to consist of the decomposition of the fuel into fragment hydrocarbons, and the polymerization of the fragment hydrocarbons into pre-soot hydrocarbons. The pre-soot hydrocarbons are polycyclic hydrocarbons which form soot particles by further polymerization and condensation. These reaction processes were formulated by a set of differential equations on assuming the reactions are first order and reversible. The rate constants of the reactions and equilibrium constants between hydrocarbons were estimated. The rate constant of the decomposition ranged 7 to 50 sec⁻¹, and increased with increases in the reaction temperature and in addition of oxygen. The rate constant of the polymerization was approximately zero at a reaction temperature of 840°C, and increased with an increase in temperature. It increased with addition of oxygen to a maximum value of 2.8 sec⁻¹ at an oxygen ratio of 0.05 and then decreased, at 955°C. The addition of oxygen had little effect on the equilibrium constants which decrease with an increase in temperature i.e. the reverse reactions can be neglected at high temperatures. The equilibrium constant between fuel and fragment hydtocarbons was 0.5 at 840°C and nearly zero at 955°C. The equilibrium constant between fragment and pre-soot hydrocarbons was 4.0 at 905°C and 1.0 at 955°C.