Theoretical Investigations On The Chemical Kinetics For The Hydrogen Abstraction Of Methyl Esters And The Low-temperature Oxidation Of Alkenes

Biodiesel is an important alternative fuel for diesel fuels.For a deep understanding of its combustion characteristics,it is essential to study the chemical kinetics of biodiesel.Biodiesel is composed of fatty acid methyl esters that contain 17¹9 carbon atoms.The two structural characteristics of biodiesel components,having a long carbon chain and C=C double bond?s?,have brought many difficulties to the development of combustion models.To improve the accuracy of biodiesel models for predicting combustion characteristics,this thesis focuses on the reaction kinetics studies of the hydrogen abstraction reaction of fatty acid methyl esters and the the effect of C=C double bond on the low temperature oxidation reaction pathways.As important reactions for fuels consumption,the hydrogen abstraction reactions of fuel molecules by H radicals have a great influence on ignition,flame propagation and products concentrations.Thus,for improving the reliability of long-chain fatty acid methyl ester models,it is very important to obtain precise rate constants,which would require computing very accurate reaction energy barriers and partition functions.Although many high-level quantum methods are available for calculating energies accurately,the use of these methods is limited by the molecular size.Furthermore,in the calculations of partition functions,both the influence of the hindered rotations on the partition functions and the difficulty in hindered rotation treatments are enhanced by the large molecular size of the long-chain fatty acid methyl esters.To solve these problems,a generalized energy-based fragmentation?GEBF?approach is applied to the study of the hydrogen abstraction reactions of C_nH_2n+1COOCH3?n=4,5?for getting accurate reaction energies more efficiently.Then a simplified hindered rotation treatment method is proposed and used for treating hindered internal rotations in the rate constant calculations of the hydrogen abstraction reactions of C₁₅H₃₁COOCH₃ and C₁₈H₃₈.The results show that,for long-chain fatty acid methyl esters,it is more readily to abstract hydrogen atoms from the secondary carbons that are far away from the ester group,which is different from the rate rules of the hydrogen abstractions of short-chain methyl esters.In addition to the hydrogen abstraction reactions,the low-temperature reactivity of the biodiesel is largely influenced by the C=C double bonds in the unsaturated fatty acid methyl esters of biodiesel components,which has not been well studied.To understand how these C=C double bonds affect the low-temperature reactivity of biodiesel fuels,the low-temperature reaction pathways of several representative alkenyl peroxy radicals are investigated.The results show that the reduced low-temperature reactivity of peroxy radicals with one or two double bond?s?can be attributed to several reasons.First of all,there are fewer sites available to add oxygen molecules in alkenyl radicals.Secondly,oxygen additions on the allylic or double-allylic carbons get slower while the reverse dissociation reactions get faster.Thirdly,a few important intramolecular H-shift reactions are inhibited and get slower.Lastly,the intramole cular addition reactions become dominant,suppressing other chain branching and chain propagation pathways.The theoretical results of the hydrogen abstraction reactions of long-chain fatty acid methyl ester would affect the branching ratios of fuel consum ptions and product concentrations directly.In addition,the prediction of the concentrations of some aldehydes can be improved by updating a few low-temperature reaction rate rules in the existing 1-octene model with the results of this study.