Combustion Study Of Nitrogen-containing Compounds

This dissertation is to investigate the nitrogen-containing compounds flames, including doped systems (NH₃-doped CH₄ flames), nitro compounds (nitromethane), amines (propylamine and isopropylamine) and heterocycles (pyrrole and pyridine), with vacuum ultraviolet single-photon ionization combined with molecular beam mass spectrometry techniques. This dissertation consists of six chapters. It mainly focuses on the identification and quantification of the combustion intermediates formed in the different nitrogen-containing compounds flames. Moreover, it discusses the formation pathways of NO_X in different nitrogen-containing systems and models the doped systems and nitromethane flames.In Chapter 1, the progress and application of combustion study are briefly described. The purpose and necessity for the combustion study of nitrogen-containing compounds are explained. The formation mechanisms of NO_X are reviewed. The classification and research survey of nitrogen-containing systems are summarized. Some basic concepts are introduced. Moreover, the target, method and importance of the present research are illustrated.In Chapter 2, the theoretical methods, experimental setup and procedure of the combustion studies are demonstrated in detail. A detailed description of the data dealing processes is presented. Especially, the structure of the vacuum ultraviolet (VUV) beamline and combustion endstation, combined with the principles of molecular beam and reflectron mass spectrometry are presented. The powerful combination of molecular beam mass spectrometry (MBMS) with photoionization by tunable VUV synchrotron offers significant improvements over previous combustion diagnostics, including superior signal-to-noise, soft ionization, and tunability in a wide range. These advantages make threshold ionization possible, which can be used to identify isomers and detect active intermediates such as free radicals. In Chapter 3, 11 low-pressure premixed ammonia doped methane/oxygen/argon flames are studied. By measuring the photoionization efficiency (PIE) spectra, combustion intermediates and products are identified. The mole fractions of the flame species are deduced by scanning the burner positions at some selective photon energies. The flame with doped ratio [NH₃]/[CH₄] (R) = 0.5 are modeled with Chemkin 2.0 Premixed codes by using Miller-Bowman (MB), GRI 3.0-Mech (GRI 3.0) and modified mechanisms in this work. The experimental results indicate that the reaction zone is widened with R increasing; the mole fraction profiles of H₂O, NO and N₂ ascend while those profiles of H₂, CO, CO₂ and NO₂ have reverse tendencies. The computational result shows that the MB and GRI 3.0 mechanisms tend to underpredict NO production. The modeling concentration profiles obtained by the present mechanism are in reasonable agreement with the experimental result.In Chapter 4, a low-pressure premixed nitromethane/oxygen/argon flame with equilavence ratio of 1.39 is studied. By scanning photon energies and burner positions, flame species, including Ar, reactants, intermediates and products are measured qulatively and quantitatively. N₂ and NO are found to be the major nitrogenous products, while HCN, HNO, CH₃CN, HNCO/HCNO, H₂C=NH=O, and HONO are nitrogenous intermediates with maximum mole fractions higher than 1.0×10^-3. Moreover, considering the new species identified in this work, a detailed kinetic model comprising 68 species and 307 reactions has been developed. The predictions by the present mechanism are in reasonable agreement with the experimental result.In Chapter 5, propylamine (PA) and isopropylamine (IPA) flames with the same equilavence ratio (φ=1.70) are investigated. About 50 flame species are detected and identified. By scanning burner position at some selective photon energies, mole fractioins of most of the species are deduced. Under flame conditions, N in PA and IPA is mainly converted to HCN and N₂. Since propylamine and isopropylamine have the same chemical composition, a lot of the combustion intermediates are the same, such as methyl radical, ammonia, acetylene, ethylene, ethyl radical, methanimine, nitric oxide, propargyl radical, propyne, allene, 1,3-butadiyne, vinylacetylene, 2-butene, 1,3-cyclopentadiene etc. However, the chemical structures of PA and IPA are different, which leads to some difference of the intermediates pool. Methylene amidogen, ethenol/acetaldehyde, formamide, 2-pyopen-1-imine/cyclopropanimine, 2-propen-1-amine, 1-butanamine and toluene are formed only in the PA flame, while methylamine, ketene, 1-butene, 1-methylethenylamine, 1-pentyl radical/3-pentyl radical, 1,3-cyclohexadiene and cyclohexene are observed only in the IPA flame. Moreover, NO_X formation combined with N conversion pathways in the PA and IPA flames are discussed.In the last Chapter, four pyrrole and pyridine flames are studied under fuel lean and rich conditions. By scanning photoionization mass spectra and photoionization efficiency spectra, combustion intermediates in the flames are identified. By scanning burner positions at some selective photon energies, mole fraction profiles of major species and intermediates are obtained.In the pyrrole flames, N₂, NO and NO₂ are the major nitrogenous products while hydrogen cyanide, isocyanic acid and 2-propenenitrile are the most important nitrogen-containing intermediates. Reaction pathways involving the major species are proposed. The experimental results indicate that 1,3-butadiyne, 1,3-butadiene, 1,3-hexadiene, 2-methylfuran, phenylnitrene, benzonitrile, 4-methylbenzyl radical, 2-vinylpyridine, indolizine and m-tolunitrile exist only in the rich flame, while formamide, formic acid, 1,2-butadiene, isobutyl radical, 2-pentyn-4-one, 1,2,3,6-tetrahydropyridine, 2,4-dimethyloxazole, cyclohexanamine and N-phenyl methanimine are detected only in the lean flame. Moreover, concentrations of C₂H₂ and HCN in the rich flame are higher than that in the lean flame whereas HNCO is facile to be formed in the lean flame. The possible consumption reactions of HCN are acetylene-addition to the triple bond, hydrogen abstraction by OH, reaction with oxygen atom and isomerization to HNC. HNCO participates in radical addition and abstraction reactions and is mainly converted to NO, CO and CO₂. And the main products from C₃H₃N are C₂H₂ and HNCO. In the pyridine flames, about 60 flame species ranged from 15 to 156, including nitrogenous, oxygenated and hydrocarbon intermediates are identified. Under the flame conditions, N in pyridine is mainly converted via the following pathways: (1) N conversion is initiated with H abstraction and O addition, forming the primary products pyriding oxide, 2-pyridinol and o-C₅H₄N radical, and then these species finally convert to NO, NO₂ and N₂ after series of reactions; (2) two ring species, including indolizing, quinoline and 1,8-naphthyridine, are formed through the reactions of the primary nitrogenous intermediates and small unsaturated hydrocarbons. In addition, the results indicate that it is difficult to produce a tricyclic or larger species in pyridine flames. The experimental observations are useful for further insight into the combustion chemistry of nitrogen-containing fuels.