Probing PAH formation chemical kinetics from benzene and toluene pyrolysis in a single-pulse shock tube *

Benzene and toluene were pyrolyzed under highly argon-diluted conditions at a nominal pressure of 20 bar in a single-pulse shock tube coupled to gas chromatography/gas chromatography-mass spectrome-try (GC/GC-MS) diagnostics. Concentration evolutions of polycyclic aromatic hydrocarbon (PAH) inter-mediates were measured in a temperature range of 1100-1800 K by analyzing the post-shock gas mixtures. Different PAH speciation behaviors, regarding types, concentrations and formation temperature windows, were observed in the two reaction systems. A kinetic model was proposed to predict and interpret the mea-surements. Through a combination of experimental and modeling efforts, PAH formation patterns from species pools of benzene and toluene pyrolysis were illustrated. In both cases, channels leading to PAHs basically originate from the respective fuel radicals, phenyl and benzyl. Due to the higher thermal stability of benzene, the production of phenyl, and thus most PAH species, occur in higher temperature windows, in comparison to the case of toluene. In benzene pyrolysis, benzyne participates in the formation of crucial PAH species such as naphthalene and acenaphthylene. Phenyl self-recombination takes considerable carbon flux into biphenyl, which serves as an important intermediate leading to acenaphthylene through hydrogen loss and ring closure. The resonantly-stabilized benzyl is abundant in toluene pyrolysis, and its decomposition further produces other resonantly-stabilized radicals such as fulvenallenyl and propargyl. Barrierless addi-tion reactions among these radicals are found to be important sources of PAHs. Fuel-specific pathways have pronounced effects on PAH speciation behaviors, particularly at lower temperatures where fuel depletion is not completed within the reaction time of 4.0 ms. Contributions from the commonly existing Hydrogen-Abstraction-Carbon-Addition (HACA) routes increase with the temperature in both cases. (c) 2020 The Author(s). Published by Elsevier Inc. on behalf of The Combustion Institute. This is an open access article under the CC BY-NC-ND license. ( http://creativecommons.org/licenses/by-nc-nd/4.0/ )

Automated summary

This study investigated the pyrolysis of benzene and toluene under highly diluted argon conditions using GC/GC-MS diagnostics. Different behaviors in polycyclic aromatic hydrocarbon formation were observed in the two reaction systems, with benzene and toluene showing distinct pathways and species. The fuel-specific pathways had pronounced effects on PAH speciation, especially at lower temperatures, where fuel depletion was incomplete.