On the oxidation of alkanes and thermodynamic properties of nitrogen-containing compounds: experimental, theoretical, and kinetic modelling studies
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The primary focus of the work presented in this Thesis involved a major re-evaluation of detailed chemical kinetic models describing alkane oxidation under combustion conditions. Combined experimental and theoretical approaches have been undertaken in order to, as comprehensively as practically possible, elucidate the mechanistic pathways of oxidation, as well as to refine numerical values of the fundamental thermochemical kinetic parameters which underpin their description. This was achieved via a broad experimental campaign investigating the oxidation of the pentane isomers (n-, iso-, and neo-pentane), which comprise sufficient structural diversity to be representative of the majority of alkanes. Experimental data were obtained for n-pentane in the high-pressure shock tubes at both NUI Galway and Texas A&M University, one of the twin-opposed piston rapid compression machines at NUI Galway, the atmospheric pressure jet-stirred reactor (JSR) at the Centre National de la Recherche Scientifique (CNRS) Nancy, and the high-pressure JSR at CNRS Orléans. Pressures of approximately 1–20 atm, at temperatures in the range 500–1555 K, at equivalence ratios of 0.3–2.0, and for dilutions in the range 75–99 %, have been investigated. For the branched isomers, ignition delay times have been obtained in the aforementioned shock tube and rapid compression machine facilities under similar conditions to the straight-chained counterpart, with pressure, temperature, equivalence ratio, and dilution ranges of approximately 1–20 atm, 650–1720 K, 0.3–2.0, and 75–99 %, respectively, investigated. These data, alongside literature measurements, served as validation targets for an updated model describing pentane isomer oxidation. Previous models developed in this laboratory describing their oxidation were updated in terms of their level of mechanistic detail, and in their numerical description of the thermochemical kinetic properties of the species and reactions therein. Thermodynamic properties of species pertinent to pentane oxidation were updated based on group additivity values which were recently derived in this laboratory. This was followed by a literature review, conducted as part of this work, of kinetic rate parameters for reactions which are important for describing alkane oxidation under low-temperature conditions (~600–900 K). New recommendations for rate coefficients are made for each important reaction class within the typical low-temperature alkane oxidation scheme. Further refinement of kinetic parameters was undertaken due to experimental and modelling evidence of deficiencies in literature values for an important low-temperature reaction class: cyclisation of hydroperoxyl-alkyl radical to form a cyclic ether and a hydroxyl radical ("Q" ̇OOH ↔ cyclic ether + ȮH). Rate coefficients for 43 examples of this reaction type involving species ranging in size from C2H5O2 to C5H11O2 were determined using density functional theory (DFT) and ab initio approaches. Geometries and ro-vibrational properties were determined using the M06-2X/6-311++G(d,p) level of theory, with single point energies computed using coupled cluster [specifically, CCSD(T)] and second-order Møller-Plesset perturbation theory (MP2) methods, with relatively large basis sets (cc-pVXZ, where X = D,T,Q). Standard statistical thermodynamics and canonical transition state theory were employed to derive the kinetic data of interest. The use of these new rate coefficients in the pentane oxidation model produces favourable agreement with C5 cyclic ether concentration measurements in the JSRs at Nancy and Orléans. These had previously been over-predicted by the model utilising literature rate coefficient values. The combined experimental, modelling, and quantum chemical approaches have led to a comprehensively validated model describing n-pentane oxidation, along with models which can accurately describe the ignition properties of iso- and neo-pentane. Furthermore, by updating recommendations for mechanism development, and for assignment of model parameter values, the findings of this work have influenced model development for alkanes from C3 upwards. Thermodynamic properties [H(T), S(T), and Cp(T)] of nitrogen-containing compounds pertaining to NOx formation mechanisms have been calculated using DFT and ab initio methods. The CBS-APNO, G3, and G4 composite methods were utilised to compute formation enthalpies via the atomisation method. The average values computed by these three methods were previously found to yield results rivalling “chemical accuracy” (arbitrarily, 4.184 kJ mol–1, or 1 kcal mol–1) when tested against a benchmark set of enthalpy of formation values. The effect of vibrational anharmonicities on the thermodynamic properties of the selected compounds was also investigated. The result is a comprehensive database of thermodynamic functions of species relevant to NOx formation, which will underpin subsequent kinetic studies of NOx chemistry. The studies conducted over the course of this work provide valuable information on the combustion chemistry and kinetic modelling of species ubiquitous in gas turbine and reciprocating engine environments.
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