Experimental And Kinetic Studies On Combustion And Pyrolysis Of Nitroalkanes

Combustion is among the earliest technologies that developed by human. While providing over80%percent of the energy supply for modern world, the combustion of fossil fuels has led to a series of social and environmental problems such as the energy crisis and air pollution. To solve these problems, comprehensive understandings of the chemistry in combustion are required.Nitric oxides (NOx), as a major contributor of photochemical smog and ozone in the urban air, is an important air pollutant that largely produced from combustion processes, especially from internal combustion engines and high temperature furnaces. The formation of NOx has several different routes, among which the fuel-bond nitrogen has the most complex mechanism since NOx is produced through the conversion of N element in the fuel instead of N2in the air.Nitroalkanes are representative nitrogen-contained fuels, which are usually used as propellants and fuel additives. The former investigations on nitroalkane combustion have mainly focused on their pyrolysis, while the traditional diagnostic methods have been challenged by the complexity of the combustion system, which contains many instable intermediates such as radicals. Therefore the development of more precise kinetic models requires more comprehensive investigations.In this dissertation, experimental and kinetic studies are performed on nitromethane and nitroethane premixed flames with three different equivalence ratios at low pressure, as well as the pyrolysis of nitromethane and nitroethane in a flow tube reactor. Relevant kinetic models have been developed and validated against the experimental results.In the experiments, the method of synchrotron vacuum ultraviolet photo ionization mass spectrometry (SVUV-PIMS) combined with molecular beam sampling has been applied. This is a newly developed technology in combustion diagnostics, which enables the identification of unstable intermediates and isomers in combustion. By photoionization efficiency (PIE) scan, many species in the flame and pyrolysis have been identified, including radicals and isomers. Both hydrocarbons and NOx have been detected in the experiments. Besides, the identification of many nitrogenated species has implied the complexity of the N-chemistry. Additionally, species that appear to be produced from the unimolecular water elimination have been identified in the pyrolysis of both nitromethane and nitroethane, indicating the possibility of unknown reactions.Keeping the photon energies as constants, the signal intensities were measured while the sampling position is moved along the axial direction of the flame, from which the mole fraction profiles of the species identified were quantified. Similarly, in the pyrolysis experiments, the mole fraction profiles of the species were quantified as the function of the temperatures. These data can be used to evaluate the importance of relevant reactions, deduce the roles that different species play in the combustion system, and further more validate the kinetic models.In theoretical work, the potential energies of reactants, intermediates and transition states in the unimolecular decomposition process of nitromethane and nitroethane have been calculated. Base on the results, the rate constants for important reactions in these processes are derived through calculations with RRKM theory. Besides, the photoionization thresholds of the unknown species detected in the pyrolysis of nitroethane have also been obtained via quantum calculations. The kinetic models for the combustion of nitromethane and nitroethane are developed and validated against the experimental results. In general, the simulated results are in good agreement with most of the experimental observations. However, the mole fraction profiles of the species newly detected in the pyrolysis experiments, which appear to be from the unimolecular water elimination, are not well predicted. Therefore the models need to be improved by including potential new reaction pathways in the future work.By rate of production (ROP) analysis, the reaction pathway diagrams of nitroalkane combustion have been revealed. The decompositions of nitromethane and nitroethane are mainly through the C-N bond fission, forming hydrocarbons and NOx. Then the hydrocarbons are oxidized while the NOx enhancing this process is reduced. In this process, the fuel-bond nitrogen is converted mostly into NO as the final product. In addition, the reactions of C1and C2hydrocarbon radicals and NOx can lead to a series of nitrogenated species, the further reactions of which can produce N2as another final product and reduce NOx emission.Comparing the reaction pathways in nitromethane and nitroethane flames, both similarities and differences can be observed. For example, their main pyrolysis pathways, the effect of NOx in the oxidation of hydrocarbons and the conversion of nitrogenated intermediates are similar, while there are obvious differences in the contributions of unimolecular decomposition, the interactions of NOx with C1and C2hydrocarbons and the efficiencies of nitrogen conversion into NOx. These observations has reflected their similarities and differences in fuel structures, which can help to further understand the chemistry in the combustion of other nitrogenated fuels.