Kinetics and decomposition mechanisms of selected Nitrogen-containing species

Kamal Siddique

This thesis calculates the rate of hydrogen abstraction reactions and the mechanisms of nitrogen oxides (NOx and N2O) reduction, especially those relevant to the oxidation and pyrolysis of nitrogen-rich fuels such as biomass. The dissertation firstly focuses on the interaction of hydrocarbons with the amidogen radical (NH2) and nitrogen dioxide (NO2), before analysing in detail the decomposition of ammonium nitrate (AN) both in gas and liquid media. In addition to this, the moderation of nitrous oxide (N2O) and nitrogen oxides (NOx) via their reaction with a biomass surrogate of catechol was also studied. The underlying aims of the study were to report the mechanisms and kinetic factors controlling the interaction of NH2 and NO2 radicals with a wide array of hydrocarbons, then to map out the prominent reaction pathways prevailing in the decomposition of ammonium nitrate (AN) and conversion of N2O into N2 via dissociative adsorption onto a catechol moiety. Accurate quantum-mechanical calculations probed the hydrogen abstraction reactions from small aliphatic and aromatic hydrocarbons by NH2 and NO2 radicals. Reaction and activation energies for all plausible hydrogen abstraction channels were executed with the accurate chemistry model of CBS-QB3. Reaction rate parameters were obtained based on conventional transition-state theory, accounting for a plausible contribution from tunnelling effects and treating internal rotations as hindered rotors. We established that a linear correlation existed between the strength of the C-H bonds (i.e., primary, secondary, vinylic, and benzylic) and the activation energies for H abstraction channels operated by NH2 and NO2 radicals. Moreover, the meta-hybrid Density Functional Theory (DFT) of M05-2X/6-311+G(d,p) levels elucidated viable systematic conversion routes of N2O into N2 via interaction with a catechol molecule. Two theoretical methodologies were applied to study thermal decomposition of AN in gas and liquid phases. A continuum solvation model density-polarisable continuum model (SMD-PCM) expounds the catalysing effect of water on AN thermal cracking. The solvation model systematically predicts lower activation energies when contrasted with analogous gas phase values. An important part of the thesis investigates the potential of biomass constituents for the so-called selective non-catalytic reduction of NOx into nitrogen molecules. The laboratory-scale rig offers a continuous supply of carrier and reaction gases which run through a tubular reactor coupled with FTIR spectroscopy, micro-GC and a chemiluminescence NOx analyser. The consumption of the biomass surrogate (catechol) is analysed using a triple quadruple mass spectrometer (QQQ-MS) at temperatures starting from 400 °C. Fine-tuning of experimental conditions encompasses residence time and inlet reactant mixing ratios. Above 800 °C, we report more than 80 % NOx reduction efficiency. In summary, our findings throughout the thesis present previously unreported data and new insights pertinent to the combustion chemistry of several selected N-species.

Kinetics and decomposition mechanisms of selected Nitrogen-containing species

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