Detailed numerical modeling of PAH formation and growth in non-premixed ethylene and ethane flames

A chemical kinetic mechanism for C₁ and C₂ fuel combustion and PAH growth, previously validated for laminar premixed combustion, has now been modified and applied to opposed flow diffusion flames. Some modifications and extensions have been made to the reaction scheme to take into account recent kinetic investigations, and to reduce the stiffness of the reaction model. Updates have been made to the cyclopentadienyl reactions, indene formation reactions, and aromatic oxidation and decomposition reactions. Reverse reaction rate parameters have been revised to account for numerical stiffness. Opposed flow diffusion flame simulation data for ethylene and ethane flames with the present mechanism are compared to data computed using two other mechanisms from the literature and to experimental data. Whereas the fuel oxidation chemistry in all three mechanisms are essentially the same, the PAH growth pathways vary considerably. The current mechanism considers a detailed set of PAH growth routes, and includes hydrogen atom migration, possible free radical addition schemes, methyl substitution/acetylene addition pathways, cyclopentadienyl moiety in aromatic ring formation, and numerous reactions between aromatic radicals and molecules. It is shown that while bulk flame properties and major species profiles are the same for the three mechanisms, the enhanced PAH growth routes in the present mechanism are necessary to numerically predict the correct order of magnitude of PAHs that were measured in the experimental studies. In particular, predicting concentrations of naphthalene, phenanthrene, and pyrene, to within the correct order of magnitude with the present mechanism show a significant improvement over predictions obtained using mechanisms in the literature. Sensitivity and production rate analyses show that this improvement is attributable to the enhanced PAH growth pathways and updated reaction rates in the present mechanism. The overreaching goal of this research is to generate and fully validate a detailed chemical kinetic mechanism, with as few fitted rates as possible, that can be applied to premixed or diffusion systems, and used with any type of soot model. To that end, in recently published works, the present mechanism has been used to simulate premixed flames, while coupled to a method of moments to determine soot formation, and to simulate diffusion flames, while coupled to a sectional representation for soot formation. The present work extends the validation of the mechanism by applying it to counterflow diffusion flames, for which measurements of large PAH molecules are uniquely available. The validation of PAH growth predictions are of key interest to soot modeling studies as soot inception from PAH combination and PAH condensation are often major constituents of soot production.