Theoretical Study On The Chain Initiation Reactions And Bond Dissociation Enthalpy Of Large Unsaturated Methyl Ester Combustion

Biodiesel mainly consists of various fatty acid methyl esters(FAMEs),and 80%of the constituents are the unsaturated FAMEs.The characteristics of biodiesel molecules include high-molecular-weight,long chain,C=C and C=O.The exploration on the effect of these characteristics on FAMEs’ combustion kinetics can advance better understanding of its combustion characteristics.It is essential to study the bond dissociation enthalpies and barrier heights to obtain accurate rate constants.For determining an appropriate quantum chemical method for studying large biodiesel molecules,the Kohn-Sham density functional theory(DFT)and coupled cluster theory were used to calculate bond dissociation enthalpies(BDEs)of seven fragment molecules of methyl linolenate,and barrier heights of chain initiation reactions of methyl butenoate.The results show that M08-HX and M06-2X have the best performance for BDEs and barrier heights for unsaturated FAMEs,respectively.The long chain of biodiesel molecules leads to multi-structural torsional anharmonic-ity,and C=C,C=O bonds have significant effects on their reaction paths.To examine the effect of these structural characteristics on biodiesel combustion kinetics,this the-sis studied the kinetics of three types of chain initiation reactions for methyl butenoate(methyl 2-butenoate and methyl 3-butenoate)combustion:H abstraction,H addition and decomposition reactions.To explore the applicability of various methods for predicting quantitative rate constants of large FAMEs,three dynamics methods were examined:(1)canonical variational transition state theory(CVT)and conventional transition state the-ory(TST)for rate constants;(2)1-D Eckart(ET),zero-curvature tunneling(ZCT)and small-curvature tunneling(SCT)approximations for transmission coefficients;(3)multi-structural torsional(MS-T)anharmonicity for partition functions and rate constants.The results show that variation reduces the rate constants by a factor of 0.5-1.0;the tunneling has significant effects on rate constants at low temperatures;the ratio of rovibrational par-tition functions based on MS-T to those based on the global minimum-energy structure is up to 11.8 at 2500 K;the contribution of low-energy structures to rovibrational partition function is larger than 90%.Finally,for obtaining accurate rate constants,this thesis utilized the CVT to compute the rate constants,and considered the tunneling effect with the SCT approximation,and employed the multi-structural torsional method(MS-Tor)to correct the MS-T anharmonic effects.Then this thesis explored the effect of C=C on kinetics of methyl butenoate,and studied the branching fraction of hydrogen abstraction and addition on different sites.This thesis utilized highly accurate quantum methods and advanced dynamic methods to calculate the energies and rate constants,respectively.These accurate parameters have significant influence on the development of accurate biodiesel combustion model.The influence of multi-structural torsions and C=C on rate constants of chain initiation reactions of unsaturated FAMEs,explored in this thesis,advances our understanding of the biodiesel combustion.