Effect of C6–C8 aromatic structures on the formation of polycyclic aromatic hydrocarbons in fuel–rich oxidation using an atmospheric flow reactor

• MAHs and PAHs in fuel-rich oxidation of 4 different aromatics were quantified. • A developed kinetic model succeeded in reproducing the experimental data. • Reaction pathway via benzene was commonly important for PAH growth. • Reaction pathways via benzyl, styrene, and o-xylyl were fuel specific. Polycyclic aromatic hydrocarbons (PAHs) abundantly produced from aromatics are recognized as a precursor of soot. Given the presence of various aromatics in real fuels, it is imperative to systematically investigate the PAH formation behavior from aromatic fuels with different structures. Here, the fuel–rich oxidation of four C 6 –C 8 aromatics, benzene, toluene, ethylbenzene, and o–xylene, was examined using an atmospheric–pressure flow reactor at mean gas temperatures of 1000–1350 K, equivalence ratio of 9.0, and residence time of 1.0 s. The concentrations of 7 types of monocyclic aromatic hydrocarbons and 16 types of PAHs in the product gas were quantified using a gas chromatograph mass spectroscopy coupled with electron ionization. A kinetic model that contains not only the oxidation mechanisms of four aromatics but also PAH growth reactions was refined through the incorporation or modification of multiple reactions involving PAHs to reproduce the experimental data. The overall trend for PAH formation was relatively similar among the four fuels, while the formation characteristics of several PAHs were fuel-dependent. The dominant formation pathways of many PAHs were strongly influenced by the fuels, as determined by kinetic analysis. Additionally, it revealed the similarity and difference in the formation pathways of PAHs among the four fuels. For instance, phenyl and cyclopentadienyl, as well as acetylene and propargyl, were identified as critical intermediate species for PAH growth in all fuels examined. Conversely, the importance of several intermediate species, such as benzyl, styrene, and o–xylyl, was fuel–specific, resulting in the different main production pathways toward PAHs.