| Regard to the explosion of world’s population and social economy, the demand of energy supply in this modern society increases rapidly in recent years. It leads to a huge consumption of fossil fuels, such as coal and oil. At the same time, the health of humanity is harmed due to the serious air pollution problems generated by burning fossil fuels. In order to achieve a sustainable development of human society, renewable, highly efficient and clean combustion is urgent and of vital importance. Actually, we are still lack of knowledge on the emission of pollutants in the combustion of fossil fuels, which have been used for quite a long time up to now. It is difficult to perfectly control the pollution during its yielding processes in combustion. Biofuels are the important renewable energy resources,which are considered potential clean substitutes for internal engines. Comparing to ethanol that has been widely doped in the gasoline, butanol (C4H9OH) has higher combustion heat value, better compatibility in engine performance and better hydrophobic property for convenient storage and transportation, which is a very promising alternative biofuel of next generation. However, there is still a lot of work on the characterization of their combustion property and emission of pollutants during the combustion process of themselves and their mixtures with conventional fuels. The development of renewable biofuels remains on primary stage that can not meet the demand of practical applications. It requires foudemental investigations on the combustion kinetics of their mixtures with hydrocarbon fuels. Methane is the main component in nature gas, which is a clean fossil fuel. There are comprehensive investigations on its combustion, and it is a popular model of hydrocarbon fuels in kinetic modeling studies. In current work, methane is chosen as a fundamental fuel of coflow flames for the investigation on PAH formation, focusing on the kinetic mechanism impacted by the diffusion effect and the non-uniformity of chemical species in practical combustion conditions. The doping effect of butanol isomer will also be investigated experimentally and numerically, characterizing its emission of PAH and soot. Coflow flame is a kind of laboratorial flame structure, which combines the impacts of mass diffusion and chemical reactions. Thus it is very similar to real flames. Particularly, it is easy to form PAH and soot in coflow flame, which makes it very suitable for the investigation of the formation mechanism of PAH and soot. In the experiments of current work, synchrotron vacuum ultra-violet photoionization mass spectrometry technique (SVUV-PIMS) is applied in the measurement of coflow flames for the first time in the world. It primarily studied pure methane coflow flames with different N2dilution ratios, and then the doping effect of butanol isomers in methane flames with different doping ratios. Flame species are sampled by quartz probe and then anylized qualitatively and quantitatively by SVUV-PIMS. Photoionization efficiency (PIE) spectra are measured for the identification of combustion species with their mass and ionization energies. In this experiment, about50species are identified with their mass ranging from2to240, including fuel reactants, oxidizer, inert gases, major combustion products, stable small molecule intermediates, mono cyclic hydrocarbons (MAHs), PAHs and some free radicals. Meanwhile, the special distributions of their mole fraction are obtained by taking mass spectra along the central axis of the flame at different photon energies.The major contribution of current work on combustion kinetics is the development of a detailed combustion model for methane and butanol, which could predict PAH formation during their combustion process. This model is consisted of several dependent parts, the methane core mechanism, sub-mechanism of butanols, and sub-mechanism of aromatics. Methane core mechanism is developed based on USC Mech Ⅱ with deep modification,referred to recent theoretical and modeling investigations on the combustion of C0-C4hydrocarbons. The sub-mechanism of butanols is adopted from the former studies in our research group. Comprehensive validations are performed for upgrading and optimizing it. And the sub-mechanism of aromatics is also developed from our former aromatic model. During the modeling study of coflow flames, a well constructed tactics is conducted. The kinetic model is primarily validated and optimized by the data from ideal reactors, and subsequently applied for the numerical simulations of coflow flames. The kinetic modeling of ideal reactor experiments are performed by OpenSMOKE, and the numerical simulation of coflow flames is carried out by laminarSMOKE, which is recently developed for the computational fluid dynamic simulation of reacting flows with detailed kinetic mechanism. Experimental data on the combustion of methane and butanols reported in former literatures are used for the validation and optimization of the kinetic model, including laminar flame speed, pyrolysis in flow reactor, oxidation in jet-stirred reactor, laminar premixed flame and couter flow diffusion flame, etc. Necessary reaction pathways are added, unreasonable reaction pathways are excluded, and inaccurate rate constants are reconsidered, in order to improve the accuracy of model predictions.Based on the modeling results of ideal reactors and coflow flames, rate of production and sensitivity analysis are performed on the combustion of methane and butanol. The discussion on the kinetics of coflow flames mainly focuses on the fuel decomposition, the formation and consumption of aromatic precursors, and the formation of benzene and PAHs. In methane coflow flames, kinetic analysis reveals that methane decomposes to methyl radical, which subsequently combines to form ethane and ethyl radical. The further reactions of these intermediates yield ethylene and vinyl radical, and finally acetylene. Methyl radical could react with acetylene forming propyne, which mainly decomposes to propargyl radical, the most important aromatic precursor. The recombination of propargyl radical is the major pathway for benzene formation in coflow methane flames. Benzene and phenyl are the basis of aromatic growth in coflow flame. Benzyl and other MAHs are formed via the addition of C1-C2intermediates on them. Furthermore, the reactions between benzene, phenyl, benzyl and C2-C3intermediates are the main formation pathways of indene and naphthalene. The addition of small molecule intermediates on large aromatic radicals, like indyl and naphthyl, can produce even larger PAHs. Besides, the dilution of N2will impact the flame temperature, and subsequently change the formation rate of PAH precursors, benzene and PAHs in flame with different dilution ratio.According to the investigation of pure methane coflow flame, the inlet carbon flux is kept constant in flames fueled with different inlet doping ratio of butanol isomers in the research of the doping effect of butanol isomers. This remains identical flame temperature and mole fractions of major combustion products in different flames, whilst highlights the variation of the formation of intermediates influenced by the doping ratio of butanol isomers. With the increment of the butanol doping ratio, there is strong regularity for the formation of benzene and PAHs. They increase as well, since they are formed via the reactions among benzene, phenyl, benzyl and small molecule intermediates, and benzene is obviously the basis of PAHs in these coflow flames. The addition of butanol isomers in methane coflow flame promotes the formation of benzene and PAHs, and the effect increses along with the complexity of the sidechain branching of butanol isomers. Butanol isomers mainly dissociate via unimolecular decomposition reactions, or H abstraction by H atom or methyl radical yielding C4H9O radicals whose following β-scission form smaller hydrocarbons or oxygenates. During this process, their further decomposition and their reactions between methyl radical mainly produced by methane varies the mole fractions of benzene precursors in different butanol isomer doped flames. It subsepuently impacts the efficiencies of the PAH formation pathways and seperates the doping effect of different butanol isomers. For the small molecule intermediates (<C6) that are the primary decomposition products of butanol isomers are dominantly impacted, e.g. n-butanol enhances the concentration of C2species but t-butanol enhances the C3-C4species. Because of the commonality in the benzene formation in butanol doped methane flames, propargyl and its reactions with C3species provide the most contribution. Branched butanol isomers, tert-butanol and iso-butanol, produce C3species more directly and efficiency than straight chain butanol isomers, n-butanol and sec-butanol, therefore more PAHs are formed in their flames. Furthermore, toluene yielded through the recombination of propargyl and vinylacetylene could decompose to benzyl radical, which is another important PAH precursor.1,3-Butadiene and vinylacetylene are easier formed via the decomposition of sec-butanol and tert-butanol than other butanol isomer with the same carbon chain structure. Therefore, toluene and benzyl radical are produced more efficiently in their flames. Meanwhile, the formation of PAHs mainly relies on the addition and recombination of C1-C3intermediates on benzene, phenyl and benzyl radical, which is also a commonality in the PAH formation process in coflow flames. The PAH formation tendency observed in flames doped with different butanol isomers is: tert-butanol>iso-butanol>sec-butanol>n-butanol. |
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