An experimental and kinetic modeling study of benzene pyrolysis with C2−C3 unsaturated hydrocarbons

Abstract

A combined experimental and kinetic modeling study is carried out to explore the influences of added acetylene, ethylene, propylene, and propyne on the reaction schemes of benzene pyrolysis. Pyrolysis of benzene with and without the presence of the C2- or C3- unsaturated hydrocarbons (HCs) is conducted in a single pulse shock tube coupled to gas chromatography-mass spectrometry technique. A minimum of 30 species is detected in each reaction system, and their mole fraction profiles are obtained over the temperature range 1030–1800 K at a nominal pressure of 20 bar and a nominal reaction time of 4 ms. With updates based on recent theoretical studies, our ongoing detailed kinetic model with 552 species and 4958 reactions can successfully reproduce the decomposition reactivity of the fuels, formation of decomposition products, and the growth of aromatics in the pyrolysis of different fuel mixtures. Various considerations apply to all studied binary mixtures. The addition of C2- and C3- HCs to the reaction system leads to C2H2 formation, and consequently promotes the HACA mechanism starting from phenyl radical (C6H5) at relatively low temperatures. The resulted phenylacetylene, formed through C6H5+C2H2 reaction, promotes the addition-elimination reaction C6H5C2H + C6H5 leading to the enhanced and early formation of all C14H10 PAH isomers including ethynyl biphenyl, methylene-fluorene, diphenylacetylene, phenanthrene, and anthracene. It is also noteworthy that the existence of C2H2 as fuel or its production from C2H4, C3H6, and C3H4-P decomposition results in numerous compounds with ethynyl branches such as ethynyl biphenyl, diethynyl naphthalene, and ethynyl acenaphthylene. Considering the specific features of the different fuel mixtures, the addition of acetylene intensifies the HACA route leading to greater acenaphthylene formation, while naphthalene formation remains similar to the pure benzene case due to the limited H-atoms. Differently, in benzene-C2H4 co-pyrolysis, naphthalene and acenaphthylene are mainly formed through reactions between PAH radicals (phenylacetylene radical and naphthyl radical, respectively) and ethylene. Concerning benzene-C3 co-pyrolysis, indene is the major C9 species resulting from the reaction of benzene/phenyl with C3 fuels. This is an important theoretical pathway to indene which is experimentally probed here for the first time. The high concentration of indene leads to the enhanced formation of naphthalene and acenaphthylene through the reactions of indenyl radical with methyl and propargyl (C3H3) radicals, respectively. Afterwards, the naphthyl radicals participate in the formation of several larger C11–13 PAHs such as methylnaphthalene, ethynylnaphthalene and fluorene through their reactions with CH3, C2H2 and C3H4-P/C3H3, respectively.