Chemical Kinetic Mechanism Research On Biodiesel Combustion Process

Biodiesel is a renewable fuel with oxygen content 10% ~ 15% by mass, good power performance and economy performance. The use of biodiesel in diesel engine greatly decreases emissions of pollutants such as carbon monoxide, unburned hydrocarbons, and especially soots emission, thus reducing the growing energy and environmental crisis. Homogeneous charge compression ignition and low-temperature combustion, as a new combustion theory and technology of international combustion engine is developed rapidly in recent years. Essentially, it is a chemical kinetic theory of limited reaction rate by coupling turbulent mixing and chemical kinetics. Therefore, further study of biodiesel combustion chemical kinetics has a significant promotion for developing this new technology.Firstly, it studied the reaction kinetics of biodiesel alternatives in low and high temperature regions using jet stirred reactor model of CHEMKIN PRO software, based on a detailed chemical kinetic mechanism for biodiesel alternatives. At low temperatures, the alternative methyl ester was mainly consumed by the reactions of H-atom abstractions with hydroxyl radicals, forming alkyl-ester radicals. And then, these alkyl-ester radicals reacted through low temperature chain-branching reactions including O2 addition, isomerizations through cyclic transition state, secondly O2 addition and ketohydroperoxide decomposing to radicals and OH.The presence of carbonyl and C=C double bonds made that part of the alkyl-ester radicals decomposed to small unsaturated species and methyl ester species with less carbon atoms at low temperature. It constructed the key reaction paths of main species in the low and high temperature regions, analysed the influence of methyl esters and C=C double bonds on combustion characteristics from aspects of bond energy, rings strain and reaction barrier, and oxygen migration process, providing chemical kinetic foundation for subsequent analysis.By using closed homogeneous model, the ignition delay time of biodiesel was calculated and the chemical kinetic influence of initial temperature, pressure, equivalence ratio and C=C on ignition delay time was analysed. Methyl esters structure made biodiesel have higher cetane number than petroleum diesel. The peak concentration of OH radical occurred earlier, thus induce advanced flame, resulting in a shortened ignition delay time with increasing biodiesel blending ratios. In addition, the presence of C=C double bonds in the aliphatic main chain inhibit low-temperature chain-branching reactions, and the inhibition effect of the C=C double bonds on the low-temperature oxidation reactivity become more pronounced as the double bond moved toward the central position of the aliphatic main chain.Finally, the biodiesel chemical kinetic model including PAHs formation and oxidation process was built. With the shock tube model, the PAHs formation was simulated. The results showed that, with the increasing biodiesel content, the peak concentration of O and OH radical increased, resulting in that more ethyne and propine radicals were oxidized to stable CO2. Additionally, the oxygen atoms from the non-carbonyl part of the methyl ester group caused that more carbon atoms were converted to stable CO2 via OCHO and CH3 OCO at low temperature. Therefore, the concentration of ethyne and propine radicals decreased, thus inhibiting the formation of PAHs. But C=C double bonds lead more unsaturated species formation, thus increased C2H2 and C3H3 formation, promoting the formation of PAHs.