Experimental And Kinetic Modeling Investigation On Pyrolysis Of Butylbenzene Isomers

Combustion of fossil fuels accounts for over 80%of total energy supply for modern society.Particularly,gasoline,diesel and jet fuel,provide constantly motive power for combustion engines.Meanwhile,it also causes many environmental and social issues,such as air pollution and energy crisis.Emission of solid particulates from combustion is the main source of particulate matters(eg.PM2.5),which lead to the frequent occurrence of hazy weather in recent years.As one of the major components in practical transportation fuels,aromatic fuels are more likely to generate polycyclic aromatic hydrocarbons and soot than other hydrocarbon fuels,such as alkanes and cycloalkanes.The study of combustion kinetics of aromatic fuels is the building block for the development of surrogate fuel models.At the same time,it also provides a platform to study the mechanism of soot formation,which can be helpful for us to control the generation and emission of soot.Most previous studies on aromatic hydrocarbon combustion focused on benzene,toluene,xylene and ethylbenzene which have simpler molecular structures among aromatic fuels,while the experimental and theoretical calculations studies of complex branched-chain aromatic compounds are limited.Butyl benzene,as a member of aromatic hydrocarbons,was widely used as an important component in kerosene and diesel surrogate fuels.It has four isomeric structures,i.e.n-butylbenzene,iso-butylbenzene,sec-butylbenzene and tert-butylbenzene.Studying the combustion kinetics of butylbenzene isomers can not only compare the influence of alkyl branches on the fuel decomposition,but also be possible to study the synergism effect of benzene ring and different alkyl branches on the generation of polycyclic aromatic hydrocarbons.In this dissertation,the pyrolysis of butylbenzene in a flow tube at various pressures was studied with synchrotron radiation vacuum ultraviolet(SVUV)photoionization mass spectrometry and ultrasonic molecular beam sampling technique.There are two experimental modes used in this work:temperature scan at fixed photon energies and photoelectron energy scan at fixed temperatures.The former mode,which was achieved by changing pyrolysis furnace temperature,can obtain the mole fractions of fuel and pyrolysis products versus temperature at each energy;the latter by changing the photoelectron energy,can get the photoionization efficiency spectra of fuel and pyrolysis products,which are helpful to confirm the species structures and identify the isomers.In the pyrolysis experiments of four butylbenzene isomers,more than 30 pyrolysis products were detected,including free radicals and PAHs,providing a wealth of data for subsequent model development.Due to the different molecular structure of the fuels,the main species generated during the pyrolysis process are different.In the n-butylbenzene pyrolysis experiment,ethylbenzene and ethylene are the main products;in the pyrolysis of sec-butylbenzene,styrene and ethylene are produced in large amount;in the iso-butylbenzene pyrolysis experiment,toluene and propylene are major products;in the tert-butylbenzene pyrolysis experiment,iso-propenylbenzene is identified as the most important product.In the present model,the pyrolysis mechanism of n-butylbenzene was updated based on previous model evaluation and theoretical calculation studies in literature and was incorporated in our previous n-butylbenzene oxidation model.The reaction pathways from the fuel molecules to PAHs were newly updated in the present model.In particular,pressure-dependent rate constants for the unimolecular decomposition reactions of n-butylbenzene were adopted to characterize the experimental data at various pressures.Based on the rate of production(ROP)and the sensitivity analysis,the C-C bond dissociation reaction at the benzyl position is favored,resulting in the formation of benzyl radical and small hydrocarbons.The bimolecular reaction between the small products decomposed by the fuel and the radical containing the benzene ring is an important path to generate di-aromatics.Besides,ring closure reactions followed by subsequent H-loss reactions can also occur to form a bicyclic aromatic hydrocarbon.Finally,butylbenzene isomers pyrolysis experiments were investigated,focusing on the isomeric effects on the fuel decomposition and the formation of polycyclic aromatic hydrocarbons.Based on the product analysis,it can be found that the primary decomposition products during the pyrolysis of four butylbenzene fuels varies greatly due to different alkyl substitution structures.Although the molecular structures of secondary pyrolysis products are basically the same,the mole fractions of some of the products differ greatly.For the formation of PAHs,it was found that the highest PAHs mole fractions can be observed in the pyrolysis experiment of tert-butylbenzene,followed by sec-butylbenzene.The pyrolysis of iso-butylbenzene and n-butylbenzene produces less PAHs.Based on the kinetic analysis,it can be determined that the reaction classes of the four butylbenzene isomers are basically the same,while the formation pathways of the primary products are significantly different due to different fuel molecular structures.Although subsequent decomposition pathways make the molecular structure characteristics of the primary products gradually disappear,the mole fractions of some secondary products are significantly different due to the isomeric effects.Since the generation of PAHs is controlled by the precursor type and its concentration,the fuel isomeric effect on the PAH precursor results in a large difference in the concentration of PAHs in the pyrolysis of the four butylbenzene fuels.