Reaction Mechanism For Novel Clean Fuels And A New Thermodynamic Property Prediction Method For Fuel Molecules

For the purpose of developing chemical kinetic mechanism for clean fuels,this paper is aimed at the development of both new mechanisms for practical application and new methods contributing to the development of reaction mechanism.The study on two types of representative novel clean fuels are presented in this paper.One is Polyoxymethylene Dimethyl Ether?PODE?,which can be synthesized from the oversupplied coal-based C₁ chemicals like methanol and formaldehyde,the other is farnesane,which belongs to the 2nd-generation biofuel that can be produced from non-food and underused feedstock such as lignocellulosic biomass.Both of these two types of fuels have high cetane numbers and good physical property for practical utilization.Hence,they are promising alternatives to diesel or diesel additives.At the same time,they can help decrease the cost of energy or increase the total amount of available energy,and can make a contribution to environment protection.The mechanisms for PODE₃,which is the surrogate molecule for PODE,and farnesane were developed in a systematic way based on the quantum chemistry and chemical kinetic study of the model molecules for PODE₃ and farnesane and a hierarchical structure.PODE₁,the PODE molecule with the degree of polymerization as 1,was selected as the model molecule for PODE₃.The CCSD?t?-F12b/aug-cc-pVTZ//M06-2X/def2-TZVPP or CBS-QB3//B3LYP/CBSB7 level of theory was used to study the reaction classes including hydrogen abstraction,?-scission,and the first oxidation stage in the low-temperature combustion chemistry for PODE₁.Together with the estimation by analogy with similar reactions,a comprehensive mechanism including 225 species and 1082 reactions was developed for PODE₁ and PODE₃.The mechanism for PODE₁ and PODE₃ was further merged with the simplified multi-component mechanism for wide-distillation fuel?WDF?,resulting in a mechanism for oxygenated WDF with PODE,which contains 354 species and 943 reactions.In a similar way,2,6-dimethylheptane?2,6-DMH?was selected as the model molecule for farnesane.The comprehensive mechanisms for 2,6-DMH and farnesane were developed,containing 982 species and 4590 reactions,and 1658 species and 7354 reactions,respectively.The mechanisms proposed in this study were validated with the experiments over a wide range of temperature,pressure,and equivalence ratio conditions in a rapid compression machine,a shock tube,a Homogeneous Charge Compression Ignition engine and a Direct-Injection Compression Ignition engine.This work will be helpful to the popularization and proper utilization of PODE and farnesane.In addition to the mechanism development,we have also developed an automatic and adaptive distance-based group contribution?DBGC?method,which may help develop the mechanism for fuel molecules with better efficiency and accuracy.A database containing the molecular bonding information and the standard enthalpy of formation(H_f,298K)for three classes of species,alkanes,alkenes,and their radicals at the M06-2X/def2-TZVP//B3LYP/6-31G?d?level of theory was constructed.Compared with the conventional group additivity?GA?method,the DBGC method predicts more accurately the H_f,298K for alkanes using the same training sets.Especially for some highly branched large hydrocarbons,the discrepancy from literature data is smaller when the proposed method instead of the conventional GA method is used.When being extended to other molecular classes including alkenes and radicals,the overall accuracy level of this new method is still satisfactory.