Kinetic modeling of NOx formation and reduction

Emission of nitrogen oxides from combustion and high temperature industrial processes continues to be a major environmental concern. Nitrogen oxides, collectively termed NOx, are formed either from fixation of N2 in the combustion air at high temperatures or from oxidation of nitrogen chemically bound in the fuel. The concern about NOx emissions and the need to comply with increasingly stringent regulations have motivated a vast amount of research. Despite these efforts, there are still unresolved issues in formation and in situ control of nitrogen oxides. The objective of this thesis work is to contribute to improve the comprehension and knowledge of NOx formation and reduction mechanisms, modifying and perfecting the kinetic model originally developed by the CRECK Modeling group from Politecnico di Milano. The work on NOx formation has been focused on the "prompt" type mechanism, in particular on the proposal, initially advanced by Lin, to replace the initiation step of the mechanism, postulated by Fenimore, with a new route, which proceeds via the formation of the NCN species. Three different sub-mechanisms of NCN formation and consumption proposed in the literature have been tested by simulating the formation of NOx in laminar flames of different typology and under different operating conditions. The sub-mechanism that showed the best agreement with the experimental results was then implemented in the global kinetic model. Numerous experiments on the interactions between NO and various hydrocarbons, both at high and low temperatures, were simulated in order to better characterize the reburning process, which represents a low-cost and effective technique to reduce emissions of nitrogen oxides from fixed combustion systems. Based on kinetic analyses, such as rate of production and sensitivity, further modifications and improvements to the kinetic scheme were adopted, to better characterise this process. Finally, the influence of trace amounts of NO on the oxidation of two model fuels, n-heptane and iso-octane, was studied. The introduction of reactions between alkyl-peroxy radicals (RO2) of these two hydrocarbons with NO and NO2, in analogy to reactions already present in the kinetic scheme for lower molecular weight hydrocarbons, and other modifications of the model led to a better accordance between modeling and experimental results. A better understanding of the interactions between heavy hydrocarbons and NO and, more generally, of the effect of residual burnt-gas composition on combustion characteristics is of vital interest, mainly related to the increasing use of EGR (Exhaust Gas Recirculation) to reduce emissions of NOx from both mobile and stationary sources.