A comparative kinetic study on the formation of small hydrocarbons in fuel-rich oxidation of C6–C8 aromatics

Abstract

Aromatic hydrocarbons are one of the main components contained in real fuels and play an important role in their combustion. Although there are many aromatic structures depending on the length and number of the carbon side chains, it is still insufficient to systematically study the reactivity of various aromatics with different structures. In order to unravel the reaction pathways into the small hydrocarbons during combustion of various aromatics, fuel-rich oxidation of four types of C₆–C₈ aromatics, namely, benzene, toluene, ethylbenzene, and o-xylene, was investigated in an atmospheric-pressure flow reactor at mean gas temperatures from 1000 to 1350 K, equivalence ratio of 9.0, and residence time of 1.2 s. The mole fractions of small hydrocarbons from C₁ to C₇ produced from these aromatics were experimentally quantified using gas chromatograph equipped with a flame ionization detector. A kinetic model that includes the reaction mechanism of four types of aromatics was developed based on our previous model. In order to reproduce the experimental data, we revised the model through adding new species/reactions and updating the reaction rate coefficients. The refined model could satisfactorily reproduce not only the present measured data but also various experimental results by other groups. The main consumption pathways of these aromatics were analyzed through the developed model in order to unravel the reaction pathways leading to the small hydrocarbons. Kinetic analysis indicated that the predominant formation pathways of C₁–C₂ species strongly depended on the fuels, while those of C₃–C₅ products were similar with each other. C₃–C₅ products were primarily produced via the reaction pathway involving benzene and phenyl. Because all four fuels used here abundantly produced benzene and phenyl, the main formation pathway of these products was found to be insensitive to the fuels. Conversely, since the chemical structures of the fuels, namely the number and length of carbon chains, strongly affected their consumption pathways, especially the early-stage reactions, the formation pathways of C₁–C₂ species were fuel dependent.