Investigation On C₁-C₂ Reaction Mechanism In Combustion With Insight Into Key Radical Chemistry

Combustion provides more than 85%of global primary energy supply,while it also leads to challenges in fossil energy depletion and air pollutant emissions.To improve combustion efficiency and reduce pollutant emissions from combustion,a better understanding of combustion science is essential.Combustion reaction kinetics focuses on understanding and predicting the chemical behaviors of combustion systems.The core reaction mechanism of C0-C2 fuels is considered as the foundation for the development of combustion kinetic models of transportation fuels.In particular,some reactions in the core reaction mechanism control the key combustion parameters,such as the ignition,heat release rate and flame propagation,etc.In this thesis,the reaction mechanism of C1-C2 fuels（named as C1-C2 reaction mechanism）was developed and validated against comprehensive experimental data.Special attentions are paid to a series of elementary radical reactions that are important at low and intermediate temperatures or contribute to the pollutant formation.Particularly,oxygenated fuels were selected as the target fuels to produce critical radicals at low-and intermediate-temperature regimes.In this way,new validation data for the concerned radical reactions can be provided,either extending previous validation conditions or providing more detailed and stringent validation data.The major contributions of this thesis are listed below.Development of methyl-related growth mechanism:Pyrolysis mechanism is the first step to develop a reaction mechanism and the combination of small radicals can lead to the formation of benzene and PAHs.In order to provide the validation data at intermediate temperatures and various pressures,butane-2,3-dione was selected as the target fuel to serve as an efficient methyl precursor at intermediate temperatures.Its pyrolysis experiment was performed in a flow reactor at 0.04 and 1 atm over 780-1520 K,and also conducted in a jet-stirred reactor at 10 atm between 650-1130 K.Pyrolysis intermediates were measured using several diagnostic methods,including synchrotron vacuum ultra-violet photoionization mass spectrometry,Fourier transform infrared spectroscopy,gas chromatography,etc.These experimental data provide new validation targets for the CH3-related growth mechanism,especially covering the intermediate temperature range（900-1200 K）which has received little attention in the literature.A pyrolysis mechanism of C1-C4 fuels and the formation mechanism of benzene were also developed and validated against previous and newly obtained pyrolysis data.Besides,according to mechanism analyses,the growth pathways from CH3 to benzene are dependent on pressure.At 0.04 atm,addition-elimination reactions play an important role while at 10 atm,the combination reactions are responsible for the carbon chain growth.For the formation of benzene and fulvene at 0.04 atm,C3+C3 pathways are the main source while their contribution drops at 1 and 10 bar.H-abstraction reaction by phenyl radical,C4+C2 pathways and fulvene isomerization reaction also contribute to the formation of benzene or fulvene at atmospheric and high pressures.Development of CO/CH₂O/CH₃OH oxidation mechanism:The reaction mechanisms of CO,CH2O and CH3OH are the foundations for the development of the combustion kinetic models and the present reaction mechanism.In this thesis,methanol was selected as the target fuel to provide new validation data for these reactions.Laminar burning velocity experiments of methanol/air mixture were conducted at 1-10 atm and 423 K in a high-pressure constant-volume cylindrical combustion vessel.The equivalence ratio conditions were extended to?=2.1 compared with literature work.A laminar premixed flat flame of methanol/O2/Ar mixture at the stoichiometric condition and 0.04 atm was also investigated by using synchrotron vacuum ultraviolet photoionization mass spectrometry.Various intermediates including reactive radicals like HCO,CH3 and CH2OH were measured in the experiment.Formic acid was also identified as a major product in the combustion of CH3OH besides CH2O.These obtained data provide new validation targets for the reactions related to HCO,CH2OH and CH3OH.The oxidation mechanism of CO,CH2O and CH3OH was developed and validated against the previous and present experimental data.Besides,the dominant combustion chemistry of methanol under fuel-rich conditions are revealed in this work.Based on the mechanism analyses,HO2 radical dominates the radical pool under very rich conditions,and the reactions involving this radical present large sensitivity.This is because the chain branching pathway HO2?H2O2?2OH becomes competitive while the reaction H+O2=O+OH is inhibited due to the relatively low flame temperature and insufficient H atom production under this condition.Development of intermediate-and high-temperature oxidation mechanism of CH₄and C₂ hydrocarbon fuels:CH4 and C2 hydrocarbons including C2H6,C2H4 and C2H2 are important fuels and major intermediates in combustion.In this thesis,the intermediate-and high-temperature reaction mechanism of CH4,C2H6,C2H4 and C2H2 was constructed based on the evaluation of the rate constants with special concerns on the recent progress in elementary reactions.The reactions that are critical but not well studied were identified in this thesis,which requires further investigations.Besides,in order to provide additional validation data to constrain the uncertainties of the reaction mechanism,the laminar burning velocity experiments of CH4/CO were conducted at an initial temperature of 353 K and pressures of 1,2,5 and 10 atm.Both air and O2/He were used as the oxidizer.These data not only provide validation targets for the high-temperature oxidation mechanism of CH4,but also contribute to a better understanding of the transition chemistry from CH4 to CO under different equivalence ratio and pressure conditions.Based on the present studies,the transition chemistry from methane to dry CO can be monitored by the increasing amount of O atom in the radical pools under all the investigated conditions.Based on the reaction pathway analyses,R167（CH3+CH3（+M）=C2H6（+M））,R28（HCO（+M）=H+CO（+M））,R36（CH4（+M）=CH3+H（+M））and R37（CH4+H=CH3+H2）have major contributions to the transition chemistry.Development of CH₃CHO and C₂H₅CHO mechanism with insight into the low-temperature oxidation mechanisms of CH₃ and C₂H₅:Peroxy and hydroperoxide chemistry play an important role in the low-temperature oxidation mechanism.In this thesis,acetaldehyde and propanal were selected as the target fuels to serve as the precursors of CH3and C2H5,respectively.The low-temperature oxidation experiments of acetaldehyde and propanal were conducted in a jet-stirred reactor at atmospheric pressure and various equivalence ratios by using synchrotron vacuum ultraviolet photoionization mass spectrometry.The sub-mechanisms of acetaldehyde and propanal were developed in this work based on the present theoretical calculations as well as the rate constant evaluations based on literature work,and validated against previously and presently obtained experimental data.Chain branching reactions in different temperature regimes and at various equivalence ratios were identified in this work based on the mechanism analyses.At fuel-lean and low temperatures,the fuel radical relevant pathways play an important role in determining the chain branching,while at rich and intermediate temperatures,the alkyl radical pathways become more and more competitive.Therefore,under fuel-rich and intermediate-temperature conditions,the two fuels can be considered as efficient precursors of CH3 and C2H5,respectively.Various intermediates related to the low-temperature oxidation mechanism of CH3 and C2H5 were measured,including CH2O,CH3OH,CH3OO,CH3OOH,C2H5OH,CH3CHO,C2H5OOH,etc.These species mole fraction data provide stringent constraints for the validation of the low-temperature oxidation mechanism of CH3and C2H5.