Synchrotron Radiation And Kinetic Modeling Study Of Ozone-assisted Low-Temperature Oxidation Of C1-C4 Alkanes

Compared to gasoline and diesel,natural gas and liquefied petroleum gas are lowcarbon and clean fuels that have been widely used.The rapid development of advanced low-temperature combustion technology has created a demand for low-temperature combustion reaction kinetics mechanisms of natural gas and liquefied petroleum gas.However,due to the low reactivity of the C1-C4 alkane components in natural gas and liquefied petroleum gas,most studies have focused on high-temperature reactions,and validations at low temperatures are still very limited.Ozone is a common and easily available strong oxidant,and it can be used to reduce ignition temperature,enhance combustion reaction activity,and improve combustion stability.Therefore,ozoneassisted combustion provides a method to study the low-temperature oxidation chemistry of unactive fuels.C1-C4 alkanes are the main components of natural gas and liquefied petroleum gas.In this work,we built an ozone-assisted combustion experimental platform based on a jet-stirred reactor(JSR)at the Atomic and Molecular Physics Beamline of the Nation Synchrotron Radiation Laboratory to carry out ozone-assisted low-temperature oxidation of methane,ethane,propane,n-butane,and iso-butane from 350 K to 800 K.By ozone addition,the reaction temperature was reduced to approximately 450 K,which corresponds to the thermal decomposition of ozone to produce highly reactive O atoms.Subsequently,many intermediates and products were measured by the synchrotron radiation vacuum ultraviolet photoionization mass spectrometry(SVUVPIMS),such as aldehydes,ketones,alcohols,olefins,acids,and peroxides.Then,these experimental data were used to optimize kinetic models such as NUIGMech1.1,and the reaction pathways of C1-C4 alkanes in low-temperature oxidation were elucidated.These results provide valuable data for the validation of low-temperature combustion models for natural gas,liquefied petroleum gas,and long-chain hydrocarbon fuels.For the simplest methane,the bimolecular reaction of CH3O2(methyl peroxy radical)dominates the consumption of methane,where the reaction of CH3O2 with O atoms and OH radicals strongly inhibit methane consumption.The bimolecular reaction of CH3O2H(methyl hydroperoxide)is incomplete in NUIGMech1.1,causing its overprediction at low temperatures.For ethane,in addition to the bimolecular reaction,C2H5O2(ethyl peroxy radical)can produce ethylene and HO2 radicals by concerted elimination.The overprediction of ethylene and ethyl hydroperoxide by NUIGMech1.1 is due to the incomplete bimolecular reactions of C2H5O2.Moreover,the decomposition of C2H5O2H(ethyl hydroperoxide)produces a large number of OH radicals,causing the overprediction of ethane.With the addition of 21 new reactions to NUIGMech 1.1,the simulation results were substantially improved for ethane and some intermediates.For propane,n-C3H7O2(n-propyl peroxy radical)could undergo intramolecular hydrogen transfer,2nd oxygen addition and other reactions to produce KHP(ketohydroperoxide).The formation of KHP makes propane more reactive than methane and ethane.In the oxidation of propane,the reaction of n-C3H7O2 showed a strong temperature dependence.The bimolecular reaction,intramolecular hydrogen transfer,and concerted elimination dominated in turn with increasing temperature,which affected the activity of propane and the production of various intermediates.The negative temperature coefficient(NTC)regime in the low-temperature oxidation of propane was also suppressed by the addition of ozone.NUIGMech1.3 underpredicted the formation of acids,and by comparing the relevant reactions in atmospheric chemistry,some bimolecular reactions of acyl peroxide radicals were added to the model,which improved the prediction of formic acid and acetic acid in the model.In addition,since a large amount of propylene was produced during the oxidation of propane,a simplified ozonolysis mechanism was used to simulate the ozone-olefin reactions.Some intermediates that may be produced by the ozonolysis reaction were also identified.For n-butane and iso-butane,the overall reaction pathway is similar to that of propane,where the reaction of peroxy radicals is governed by the competition of intramolecular hydrogen transfer,concerted elimination and bimolecular reactions.For iso-butane,no systematic species identification studies have been performed,so this work presents a detailed identification of the species in the case of iso-butane.Moreover,many intermediates in the reactions of n-butane and iso-butane do not have corresponding formation pathways in the model.Hence,possible reaction pathways to account for their formation were proposed based on the reaction mechanism of peroxyl radicals.Finally,some main products and intermediates in the oxidation of n-butane and iso-butane were compared,and it was found that the reaction trends of fuel and some species were similar when the temperature was below 575 K.It was speculated that under such extremely low-temperature combustion conditions,although the differences in the fuel molecule structures caused the differences in the peroxide concentrations,the decomposition of peroxide was inhibited by the extremely low temperature at this time,and the reaction was actually driven by the oxygen atoms generated by ozone decomposition.Therefore,the differences caused by fuel structure are weakened.