Pemelton, John (2011-08). Shock-Tube Study of Methane Ignition with NO2 and N2O. Master's Thesis.
NOx produced during combustion can persist in the exhaust gases of a gas turbine engine in quantities significant to induce regulatory concerns. There has been much research which has led to important insights into NOx chemistry. One method of NOx reduction is exhaust gas recirculation. In exhaust gas recirculation, a portion of the exhaust gases that exit are redirected to the inlet air stream that enters the combustion chamber, along with fuel. Due to the presence of NOx in the exhaust gases which are subsequently introduced into the burner, knowledge of the effects of NOx on combustion is advantageous. Contrary to general NOx research, little has been conducted to investigate the sensitizing effects of NO2 and N2O addition to methane/oxygen combustion.
Experiments were made with dilute and real fuel air mixtures of CH4/O2/Ar with the addition of NO2 and N2O. The real fuel air concentrations were made with the addition of NO2 only. The equivalence ratios of mixtures made were 0.5, 1 and 2. The experimental pressure range was 1 - 44 atm and the temperature range tested was 1177 - 2095 K. The additives NO2 and N2O were added in concentrations from 831 ppm to 3539 ppm. The results of the mixtures with NO2 have a reduction in ignition delay time across the pressure ranges tested, and the mixtures with N2O show a similar trend. At 1.3 atm, the NO2 831 ppm mixture shows a 65% reduction and shows a 75% reduction at 30 atm. The NO2 mixtures showed a higher decrease in ignition time than the N2O mixtures. The real fuel air mixture also showed a reduction.
Sensitivity Analyses were performed. The two most dominant reactions in the NO2 mixtures are the reaction O+H2 = O+OH and the reaction CH3+NO2 = CH3O+NO. The presence of this second reaction is the means by which NO2 decreases ignition delay time, which is indicated in the experimental results. The reaction produces CH3O which is reactive and can participate in chain propagating reactions, speeding up ignition.
The two dominant reactions for the N2O mixture are the reaction O+H2 = O+OH and, interestingly, the other dominant reaction is the reverse of the initiation reaction in the N2O-mechanism: O+N2+M = N2O+M. The reverse of this reaction is the direct oxidation of nitrous oxide. The O produced in this reaction can then speed up ignition by partaking in propagation reactions, which was experimentally observed.