The Combustion Kinetics Of Important Elementary Reactions In The Syngas And Biodiesel Systems

Environmental and energy issues are global problems nowadays.In order to solve these problems,lots of countries are actively exploring the advanced clean combustion technologies and looking for the green alternative energy sources.Combustion is one of the main ways of energy supply.Therefore,mastering the detailed combustion mechanism of fuel is very helpful for the development of combustion technology,the promotion of alternative energy and the early realization of carbon peak and carbon neutralization.Combustion reaction kinetic is perceived as a powerful tool to master the mechanism of combustion reaction.In this work,we choose two combustion reaction systems of syngas and biodiesel,and study their combustion reaction kinetics.For the syngas combustion system,the elementary reaction CO+HO₂ is selected as the object.Firstly,we explore the potential energy surface of the water-free and water-assisted reactions,and compare the influence of single molecule H₂O on the reaction barrier and molecular structure.The results show that H₂O can promote the formation of the ring structure and reduce the energy barrier.In addition,the transition state theory（TST）combine with Eckart tunneling correction is used to compare the temperature dependent rate constants of the water-free and water-assisted reactions.The results show that the total reaction rate constants with single molecule H₂O is 1-2orders of magnitude smaller than that in the absence of water,and the difference is greater at low temperature.Therefore,the presence of H₂O does not accelerate the CO+HO₂ reaction.For biodiesel system,it is difficult to study reaction mechanism of long-chain unsaturated fatty acid methyl ester directly.Therefore,the methyl acrylate（MA）with similar functional groups was selected as the research object,and four most common free radicals（H,OH,CH₃,HO₂）in the atmosphere were selected to study the hydrogen abstraction reaction of MA.Firstly,we select six different DFT methods combined with six basis sets to calculate the single point energy,and select the coupled cluster method CCSD（T）-F12/jun-cc-p VTZ to obtain high precision energy as the benchmark to determine a suitable method for the kinetic calculation of this system.The results show that the M062X/maug-cc-p VTZ method has a best chemical performance in this reaction system.In addition,we utilize the multistructural canonical variational transition sate theory with small-curvature tunneling（MS-CVT/SCT）method to calculate the rate constants and branch ratios of all reactions at 500-2000 K,and explore the influence of energy barriers,variational effects,tunneling effects,and MS-T anharmonicity on rate constants and branching fractions.The results show that the H-abstraction reaction at the methyl group of MA is the most favorable.Especially when 500 K<T<1300 K,the H-abstraction reaction by OH radical at the MA methyl position is dominant,which is mainly due to the significant influence of energy barrier and MS-T anharmonicity on the branching fraction;When T>1300 K,the H-abstraction reaction by H at the MA methyl position is dominate.In addition,we compare the rate constants of the reaction MA+OH with or without RC under the pre-equilibrium model and the low-pressure model.The results show that the effect of RC on the rate constant can be ignored at high temperature.