Kinetic Studies On Several Typical Molecular Structures Of Diesel And Biodiesel Fuels

Fossil fuels provide the vital support to the development of economy and human society.85% of global energy consumption comes from the combustion of fossil fuels. Effective combustion of fossil fuels and the development of surrogates are two tasks remained to confused humans. Multi-substituted monocyclic aromatics are important components in diesel fuels, and they are also utilized as typical species in diesel surrogates. The chemical effects by differences in number and position of branched chains are the problems to be solved. Biodiesel are used as another kind of diesel surrogates, and esters are the major components. The functions of the typical ester group in the combustion also needs to be fully understood. Thus in our work, the simplest multi-substituted aromatics, xylene isomers, and one of the tri-substituted aromatics,1,2,4-trimethylbenzene, were chosen for the kinetic studies. Besides, methyl propionate, which is the simplest methyl ester with a-C-C bond, was selected to study the effects of ester group in the combustion of methyl esters.The work was carried out in two aspects, one is the experiments, and the other is the kinetic modeling. Synchrotron vacuum ultra violet photoionization mass spectrometer (SVUV-PIMS) was used for the diagnosis in flow reactor pyrolysis and premixed laminar flames. Pyrolysis and combustion species were identified and quantified, including major products, key stable intermediates and radicals. Mole fraction profiles vs. temperature in pyrolysis and mole fraction profiles vs. flame height were achieved. In the aspect of kinetic modeling, new related kinetic models were constructed and validated against the experimental data. Simulation was performed with the software Chemkin-Pro. Rate of product (ROP) analysis and sensitivity analysis were carried out for analyzing important species and reactions, defining the combustion characters. In this thesis, five species are investigated, including o-xylene,p-xylene, m-xylene,1,2,4-trimethylbenzene, and methyl propionate.For xylene isomers, in the pyrolysis processes, unimolecular decomposition and bimolecular reactions by radical attack are the two important kinds of reactions for the fuel initial consumption, among which, the bimolecular reactions are the most crucial. These reactions include H-abstraction reactions and ipso-addition reactions. H-abstraction reactions lead to the formation of related xylyl radicals while ipso-addition reactions lead to the formation of toluene. o-Xylyl and p-xylyl can dehydrogenate to yield related xylylene species. But m-xylyl radical cannot decompose to yield m-xylylene, which is an unstable biradical species. Instead, m-xylyl radical decomposes to p-xylylene via cycloisomerization processes. In the flame, H-abstraction reactions are the dominant processes for the initial fuel decomposition, mainly driven by H and OH. H-abstraction reactions can take place on the methyl groups and benzenoid ring, leading to the formation of xylyl radicals and C6H3(CH3)2 radicals, respectively. In the lean flame, xylyl radicals are dominantly consumed via oxidation processes, leading to the production of toluene and benzene. In the rich flame, xylyl radicals are consumed via unimolecular reactions, just as in the pyrolysis processes. But for m-xylyl radical consumption, oxidation reactions are still significant in the rich flame. The aromatics growth pathways are obviously different between o-xylene and p-/m-xylene systems. o-Xylene processes the molecular structure of adjacent methyls, presenting high tendency to undergo the cyclization processes, which are quite valuable for the formation of indene and naphthalene. In the p-xylene and m-xylene systems, fulvenallenyl radical is one of the most important precursors in aromatic growth. Fulvenallenyl ((?)) radical is formed from fulvenallene ((?)) which is the unimolecular decomposition product of xylyl radicals. Moreover, m-xylyl radical cannot yield xylylene in a comparable flux as those of o-xylyl and p-xylyl, resulting in the gathering of m-xylyl radical and sharing the flux to other reaction pathways. This also causes the high sooting tendency of m-xylene than that of p-xylene.For 1,2,4-trimethylbenzene (124TMB), the various-pressure flow reactor pyrolysis and premixed laminar flames were investigated. Based on the experimental measurement, a detailed kinetic mechanism with 398 species and 2751 reactions were constructed and validated. According to the ROP analysis,124TMB is mainly consumed via reaction by radical attack, especially H-abstraction and ipso-addition reactions. In the low-pressure pyrolysis, unimolecular reactions of fuel are also significant. Initial decomposition yields plenty of dimethylbenzyl radical isomers. Though consumed via different pathways in lean and rich flames, dimethylbenzyl radical is significant for the formation of smaller species under different conditions. Fulvenallenyl radical is the precursor of key aromatic species including naphthalene and phenanthrene. The simulation of the PIE spectrum of m/z=105 in the rich 124TMB flame (line with circles) shows that the signal of m/z= 105 is composed of 80% m-xylyl and 20% p-xylyl, which is very close the predicted ratio of m-xylyl and p-xylyl by the present model (5:1). This demonstrates the abundant production of m-xylyl in the rich 124TMB flame, and verifies the fuel-specific formation pathway of C7H5 ((?)). Moreover, indene may be produced from a process directly related to the decomposition of the fuel.The similarities and differences between xylene isomers and 124TMB includes: (1) Unimolecular dehydrogenation of fuels is significant while methyl-elimination is of little value. (2) Bimolecular reactions are dominant for fuel initial decomposition, including H-abstraction and ipso-addition reactions. With the increase of methyl group, H-abstraction reactions on methyl groups get more valuable for fuel consumption. (3) Dehydrogenation from methyl groups leads to the most important radicals.124TMB yields three different radicals due to the discrepancy of the three methyls. (4) o-Xylyl and p-xylyl can dehydrogenate to o-xylylene and p-xylylene, while dimethyl-benzyl radicals can yield methyl-xylylene isomers. (5) Dimethyl-benzyl radicals present more decomposition pathways than those of xylyl isomers. (6) Oxidation reactions of xylyl isomers and dimethyl-benzyl isomers are major in the lean flames, and unimolecular decomposition are more dominant in rich flames. (7) Fulvenallenyl radical is the precursor for PAH growth.An experimental and kinetic modeling study on methyl propionate (MP) pyrolysis was carried out in the work. The experiment was performed with the flow reactor in the temperature range of 1000-1500 K. at 30 Torr. In the pyrolysis, unimolecular reactions, H-abstraction reactions of the fuel, as well as the sequential decomposition reactions, are vital for MP pyrolysis. Thus the theoretical calculation work was also performed to achieve valuable information on the reaction pathways and the rate coefficients. The calculation results inform of two new formation pathways of CH3CHCO and CH3OH. One is the direct formation via a four-member-ring transition state from the fuel. The other one consists of three steps, including H-immigration, H-rotation and CH3OH elimination. Based on the experimental measurement and theoretical calculation, a new kinetic mechanism of MP was constructed, with 98 species and 493 reactions. The model can satisfactorily simulate most of the mole fractions of pyrolysis species. The decomposition of MP and the formation of products are quite sensitive to the rate constants of initial reactions. ROP analysis indicates that three unimolecular reaction are dominant for the MP consumption, which are MP= CH3+CH2COOCH3, MP= CH3+CH3CH2COO and MP=CH3CHCO+CH3OH.