Formation Process of Soot Precursors in a Laminar Flow Reactor

The Poly-Aromatic Hydrocarbon (PAH) formation process from benzene was studied using a laminar flow reactor and GC-MS. In addition to PAH, acetylene and ethylene were observed. Without oxygen at temperatures over 1070 K, the amount of PAH and C₂ species increased as the benzene concentration decreased. Addition of oxygen caused a linear decrease in the benzene concentration, and almost all of the benzene was consumed under stoichiometric conditions at all temperatures. At 1053 K, the concentrations of PAH and C₂ species were not affected by the addition of oxygen. On the other hand, when the temperature was greater than 1070 K, the amount of PAH formed increased as the equivalence ratio increased, until the equivalence ratio was about 4. Above this equivalence ratio, the amounts decreased. Amounts of phenanthrene and biphenyl were large compared to those of other PAHs, which indicated that the dominant PAH formation path is the formation of phenanthrene via biphenyl. This discussion was based on the experiments which conditions were apart from realistic engine conditions, thus more considerations are needed to apply the mechanism to soot formation process in engines. Simulations with the reaction mechanism proposed by the Massachusetts Institute of Technology combustion research group indicated that PAH formation increased when the temperature was between 1200 and 1350 K. At higher temperatures, decomposition to acetylene was dominant.