A kinetic model for the formation of aromatic hydrocarbons in premixed laminar flames

Abstract

A detailed chemical kinetic mechanism, including 340 elementary steps and 90 species, has been developed to simulate the formation of aromatic compounds in rich premixed flames of aliphatic hydrocarbons. The mechanism can reproduce the concentration profiles and net rates of benzene and larger aromatic hydrocarbons (two- and three-ring polycyclic aromatic hydrocarbons [PAHs]) in a wide range of temperatures for a slightly sooting, premixed ethylene-oxygen flames. Key sequences of reactions in the formation of aromatics are the combination of resonantly stabilized radicals, whereas the alternative mechanism that involves acetylene addition does not seem to be fast enough to explain the observed formation rates of aromatics in the flames examined. The main routes involved in the formation of the first aromatic ring are the propargyl self-combination and its addition to 1-methylallenyl radicals. Cyclopentadienyl radical combination, propargyl addition to benzyl radicals, and the sequential addition of propargyl radicals to aromatic rings are the controlling steps for the formation of larger aromatic species. The predicted concentration and formation rate of larger aromatics are higher than those of PAHs identified by gas chromatography but similar to those of total aromatic species collected in flames, namely, soot and tar-like material. This result suggests that tar-like material can be considered as the result of a fast reactive coagulation of two- and three-ring PAHs, which form structures with aromatic compounds, eventually connected by aliphatic bonding. Soot inception may be the result of an internal rearrangement of tar without significant contribution of light hydrocarbons, at least in the conditions examined. The model has been used to explore the effect of C/O ratio and temperature on the formation rate and concentration of aromatic hydrocarbons. The net formation rate of aromatics increases monotonically with C/O ratio, and at a fixed C/O ratio, it passes through a maximum as temperature is increased. A comparison with the “bell-shaped” concentration profile of soot collected in flames at different temperatures supports the hypothesis that the formation of two- and three-ring structures may be the rate-determining step in soot formation for slightly sooting regimes.