| Combustion of fossil fuels is the most important source of energy supply for modern society, and plays a decisive role in the strategic fields such as energy, industry, transportation and defense. However, the rapid consumption of fossil fuels causes serious energy and environmental problems, which raises the demands in two research topics. On one hand, more efforts should be devoted to the combustion researches of fossil fuels in order to improve the combustion efficiency and reduce the pollutant emissions; on the other hand, the development and combustion researches of biofuels are urgently needed for more plenty supply of renewable energy sources. This paper will focus on the two aspects.In the combustion study of fossil fuels, benzene which has the simplest structure and lowest boil point among all aromatic hydrocarbons was selected as a representative of aromatic fuels in this work. Aromatic hydrocarbons are important components of petroleum-derived oils and their surrogates. They can improve the antiknocking quality and antioxidative stability of gasoline, but reduce the combustion performance of transportation fuels meanwhile. Furthermore, the combustion of aromatics can promote the formation of polycyclic aromatic hydrocarbons (PAHs) and soot due to the existence of benzenoid ring in aromatic molecules, and becomes an ideal system to study their complicated formation mechanism. Thus, investigations on the combustion chemistry of aromatic fuels will benefit the development of kinetic models of surrogate fuels and the understanding of soot formation mechanism. Because of the convenience in experimental work, the simplicity of research system and the role as a start point to study larger aromatics combustion, the combustion chemistry of benzene has been the most important topic in the research of aromatics combustion. Though a lot of experimental studies of benzene combustion have been carried out previously, the validation data for benzene model are still deficient due to the limitations of conventional diagnostic methods and incomprehensive reaction circumstances. In this work, synchrotron radiation vacuum ultraviolet photo ionization mass spectrometry (SVUV-PIMS) was employed to study the low-pressure pyrolysis of benzene and low-pressure laminar premixed flames of benzene at various equivalent ratios (Φ). Based on the validation by experimental results, a detailed kinetic model of benzene was developed.The experimental work includes flow reactor pyrolysis and five laminar premixed flames (Φ=0.75,1.00,1.25,1.50and1.75) at30Torr. Based on the measurements of photoionization mass spectra and photoionization efficiency (PIE) spectra, combustion species including major species, radicals, isomers and PAHs were identified. Compared with the limited number of pyrolysis products, about100flame species with m/z=2～240were detected, including many large monocyclic aromatic hydrocarbons (MAHs) and PAHs in the rich flames. In the pyrolysis experiment, photo ionization mass spectra were measured at several fixed photon energies to obtain the mole fraction profiles of pyrolysis products against temperature. In the flame experiments, mole fraction profiles of major species and intermediates were obtained by scanning burner position at several photon energies. Based on the analysis of experimental temperature and mole fractions profiles, the concentration tendencies of key intermediates with the increase of equivalence ratios were found.The kinetic model of benzene was developed from our previous model of aromatic fuels. CHEMKIN Pro software was used to simulate the pyrolysis and premixed flames. Based on the comparison between the simulated and experimental results and recent literature studies of related reaction pathways, unreasonable reactions were eliminated, inaccurate rate constants were corrected, and benzoquinone sub-mechanism and reactions of some C5oxygenates were added, which improve the performance of this model.Rate of production (ROP) analysis and sensitivity analysis were used to analyze the decomposition pathways of fuels, the formation and consumption pathways of key intermediates and typical large aromatics. In the pyrolysis, benzene is mainly consumed to form phenyl by the H-abstraction reaction via H attack. Sequentially, phenyl can either decompose to smaller products or yield large aromatics via combination reactions with other species. In laminar premixed flames, H-abstraction still controls the decomposition of benzene, and is driven by H attack in the rich flames and by O and OH attack in the lean and stoichiometric flames. In the lean and stoichiometric flames, phenyl is mainly consumed via oxidation reactions to produce phenoxyl radical which is sequentially converted to phenol or oxidized to benzoquinone. In the rich flames, phenyl can also be consumed by unimolecular or radical assisted ring-opening reactions. Smaller hydrocarbon intermediates are mainly produced via consumption processes of phenoxyl and benzoquinone in all flames. For the aromatic growth process in the rich flames, it is mainly initiated by reactions involving phenyl. For example, the formation of MAHs is directly related to the reactions between phenyl and C1-C2intermediates; naphthalene, indene and biphenyl are mainly produced from reactions of phenyl and C3-C6species.In the combustion study of biofuels, ethanol and dimethyl ether (DME), two C2H6O isomers, are selected as the dopants in the benzene flames in order to avoid the limitations in previous hydrocarbon/biofuel combustion studies and help interpret the molecular structure influences of biofuels on hydrocarbon combustion. By maintaining constant C/O ratio, combustion species were identified by using SVUV-PIMS in four benzene/C2H6O flames with inlet C2H6O/(benzene+C2H6O) mole ratios (simplified as C2H6O ratio) of0,15%,30%and50%. The influences of ethanol and DME addition to the concentrations of major products, hydrocarbon and oxygenated intermediates, and large MAHs and PAHs in the rich benzene flame were investigated. A detailed benzene/C2H6O model was developed by adding the sub-mechanisms of ethanol and DME to the benzene model. ROP analysis was performed to analyze the major reaction pathways in the benzene/ethanol and benzene/DME flames.In the benzene/ethanol and benzene/DME flames, with the increase of C2H6O ratio, the concentrations of H2and H2O gradually increase, and those of CO and CO2decrease. For C6and smaller hydrocarbon intermediates, ethanol and DME show great influences on the yield of their decomposition products. For example, ethanol affects C1-C2intermediates, and DME mainly influences CH3and CH4. Besides, the addition of ethanol and DME also promotes the intermediates whose major formation pathways involves the decomposition products of dopants. As the C2H6O ratio increases, the mole fractions of C1-C2oxygenated intermediates including CH2O, CH3CHO, etc. are also affected, which mainly depend on the molecular structures and the primary decomposition processes of the dopants.With the increase of C2H6O ratio, formation of large MAHs and PAHs represents apparent relationships. For example, MAHs like benzyl and toluene have increasing concentrations, while the formation of PAHs such as indene and naphthalene are inhibited. It is concluded that in both benzene and benzene/C2H6O flames, both large MAHs and PAHs are produced from the reactions between phenyl and small species, which indicates that phenyl is the start point of aromatic growth process and the most important precursor of PAHs in these flames. Because the C/O ratio keeps constant in all flames and the H/C ratios in the molecules of dopants are far greater than that in benzene, the oxygen concentrations are limited in both the inlet mixtures and the flames, which reveals the importance of substitution effect of oxygenated fuels to benzene in reducing the formation of PAHs and soot.Based on the present work, pyrolysis and laminar premixed flame experiments at various pressures will be performed in the near future, which will be used for the further development of the present benzene model to broaden its applications at atmospheric and high pressures. Based on this model, experimental and kinetic modeling studies of pyrolysis and laminar premixed flames will be performed for complex aromatic fuels including monosubstituted alkylbenzenes, xylenes and trimethylbenzenes, leading to a further understanding of the combustion chemistry of aromatic fuels. |
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