Exploring pyrolysis and oxidation chemistry of o-xylene at various pressures with special concerns on PAH formation

This work reports an experimental and kinetic modelling investigation on flow reactor pyrolysis and jet-stirred reactor oxidation of o-xylene. The flow reactor pyrolysis experiments were conducted over 1050-1600 K at 0.04 and 1.0 atm. Key products related to fuel decomposition such as o-xylyl radical, o-xylylene, benzocyclobutene and styrene, as well as polycyclic aromatic hydrocarbons (PAHs), were detected by using synchrotron vacuum ultraviolet photoionization mass spectrometry. The jet-stirred reactor oxidation experiments were performed at 10 atm, 800-1200 K, a residence time of 0.5 s and equivalence ratios of 0.5, 1.0 and 2.0. Speciation were conducted by using gas chromatography and Fourier transform infrared spectrometry. A kinetic model of o-xylene was developed from our previous models of aromatic fuels and validated against both present data and experimental data in literature. Styrene is observed as the dominant product in the pyrolysis of o-xylene and it is mainly produced from the isomerization of o-xylylene directly or via benzocyclobutene as an intermediate species. C-9 PAHs (indenyl, indene and indane), phenanthrene and its methyl derivatives were observed as the abundantly produced bicyclic and tricyclic PAHs. The main formation pathways of bicyclic and tricyclic PAHs are found to be different at low and atmospheric pressures, depending on the major precursors produced. Particularly, the self-combination reactions of o-xylyl and the addition reaction of o-xylyl with benzyl and subsequent stepwise H-loss/cyclization reactions are found to be the main sources of methylphenanthrene and dimethylphenanthrene. In the JSR oxidation of o-xylene, toluene and benzene were observed as the abundantly produced aromatic products, while o-methylbenzaldehyde was among the most abundantly produced oxygenated aromatics. modelling analysis reveals that o-xylyl radical is also the dominant fuel consumption product. Its consumption mainly proceeds through the oxidation by HO2 and finally produces o-methylbenzaldehyde. Further decomposition reactions of o-methylbenzaldehyde contribute to the formation of other major oxygenated aromatics such as cresol and benzofuran. Indene and naphthalene were also observed in the oxidation of o-xylene, which are mainly produced from the stepwise H-loss/cyclization reaction sequence of o-methylethylbenzene and decomposition of dibenzofuran, respectively. (C) 2021 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Automated summary

This study investigated the experimental and kinetic modeling of o-xylene pyrolysis and oxidation using flow reactor and jet-stirred reactor. The dominant products observed were styrene in pyrolysis and toluene and benzene in oxidation. The kinetic model of o-xylene was developed and validated against experimental data, showing o-xylyl radical as the dominant fuel consumption product.