Formation and consumption of single-ring aromatic hydrocarbons and their precursors in premixed acetylene, ethylene and benzene flames

Abstract

Kinetic modeling is becoming a powerful tool for the quantitative description of combustion processes covering different fuels and large ranges of temperature, pressure and equivalence ratio. In the present work, a reaction mechanism which was developed initially for benzene oxidation, and included the formation of polycyclic aromatic hydrocarbons was extended and tested for the combustion of acetylene and ethylene. Thermodynamic and kinetic property data were updated. If available, data were taken from the recent literature. In addition, density functional theory as well as ab initio computations on a CBS-Q and CBS-RAD level, partially published previously, were carried out. Quantum Rice–Ramsperger–Kassel analysis was conducted in order to determine pressure-dependent rate constants of chemically activated reactions. The model was developed and tested using species concentration profiles reported in the literature from molecular beam mass-spectrometry measurements in four unidimensional laminar premixed low-pressure ethylene, acetylene and benzene flames at equivalence ratios (ϕ) of 0.75 and 1.9 (C₂H₄), 2.4 (C₂H₂) and 1.8 (C₆H₆). Predictive capabilities of the model were found to be at least fair and often good to excellent for the consumption of the reactants, the formation of the main combustion products as well as the formation and depletion of major intermediates including radicals. Self-combination of propargyl (C₃H₃) followed by ring closure and rearrangement was the dominant benzene formation pathway in both rich acetylene and ethylene flames. In addition, reaction between vinylacetylene (C₄H₄) and vinyl radical (C₂H₃) contributed to benzene formation in the ϕ = 1.9 ethylene flame. Propargyl formation and consumption pathways which involve reactions between acetylene, allene, propyne and singlet and triplet methylene were assessed. Significant overpredictions of phenoxy radicals indicate the necessity of further investigation of the pressure and temperature dependence and the product distribution of phenyl oxidation. The possible formation of benzoquinones, the ratio of the ortho and para isomers and their degradation pathways are of particular interest.