Insights into pyrolysis kinetics of xylene isomers behind reflected shock waves

Abstract

This work reports a kinetic study on the pyrolysis of three xylene isomers based on single-pulse shock tube experiments and detailed kinetic modeling. Speciation measurements for the post-shock gas mixtures are obtained via sampling and gas-chromatography-mass spectrometry over a temperature range of 1150−1650 K at a nominal pressure of 20 bar. A sub-mechanism incorporating consumption reactions of xylene isomers as well as subsequent pathways leading to polycyclic aromatic hydrocarbons (PAHs) is developed and integrated into our on-going PAH formation kinetic model. The model can satisfactorily predict the qualitative measurements and accurately characterize both the similarities and fuel-specific features in the pyrolysis of three xylenes. The three isomers decompose at similar rates, though m-xylene exhibits a slightly weaker reactivity. This arises from the fact that m-xylyl, different from its o- and p- counterparts, cannot undergo hydrogen loss to form m-xylylene diradical. Distinct consumption schemes of the three xylyl radicals also result in the different species pools observed at temperatures below 1400 K. A large amount of styrene results from a stepwise isomerization of o-xylylene following dehydrogenation of o-xylyl. An early formation of anthracene is noted as a unique phenomenon in o-xylene pyrolysis, which is attributed to specific reactions of o-xylyl. Due to a relatively high abundance of m-xylyl, the self-recombination product 3,3′-dimethylbibenzyl is among the major species formed at the initial stage of m-xylene pyrolysis. p-Xylyl predominantly converts to p-xylylene which is largely consumed via polymerization processes. Besides, mutual conversions among the three xylyl radicals are found to play an essential role in the pyrolysis of three xylenes. Toluene is measured of significant concentrations in the pyrolysis of all three isomers, which rationalizes that PAH speciation in the pyrolysis of three xylenes and toluene is highly similar at elevated temperatures. Modeling analyses show that apart from the benzyl/toluene chemistry, reactions involving C8 species also have important contributions to PAH formation in xylenes pyrolysis.