The growth of PAHs and soot in the post-flame region

Abstract

The PAHs-C2H2 pathway (PAHs + C2H2 → intermediate → product + H2) has been shown, in theory, to be the important contributor to the growth of polycyclic aromatic hydrocarbons (PAHs) and soot in the post-flame region where H atoms are rare. Calculations of the potential energy surface (PES) using the DFT B3LYP 6-311 + G(d,p) method, and the reaction rate coefficients using the RRKM theory, reveal that armchair and bridge surface sites share similar kinetic characteristics, and are more likely to be the targets of C2H2 molecules in flames compared to zig-zag and 5-membered ring surface sites. Results show that the energy barrier of a 2-H elimination reaction (14–23.8 kcal/mol) is much lower than that of a 1-H elimination (typically 30–40 kcal/mol) for some molecules. The formation of pyrene from phenanthrene via HACA (PAHs + H → PAHs radical (+ C2H2) → intermediate → product + H) and PAHs-C2H2 pathways is investigated using a closed homogeneous zero-dimensional reactor with combustion parameters abstracted from the premixed stagnation C2H4/O2/Ar sooting flame. Results show that the HACA pathway is the dominant pathway for the formation of PAHs and soot surface growth in the main-flame region where H atoms are abundant, but that the PAHs-C2H2 pathway is the preferred pathway in the post-flame region. Our study also suggests that the soot nucleation involving a chemical coalescence of moderate-sized PAHs into a crosslinked three-dimensional structure via the addition reactions of PAHs and PAH radicals in the main-flame region should be considered for inclusion in any soot modeling.