NO_x Emissions Modeling and Uncertainty for Exhaust-Gas-Diluted Flames

Oxides of nitrogen (NO_x) are pollutants emitted by combustion processes during power generation and transportation that are subject to increasingly-stringent regulations due to their impact on human health and the environment. One NO_x reduction technology being investigated for gas-turbine engines is exhaust gas recirculation (EGR), either through external exhaust-gas recycling or staged combustion. In this study, the effects of different percentages of EGR on NO_x production will be investigated for methane-air and propane-air flames at a selected adiabatic flame temperature of 1800K. The variability and uncertainty of the results obtained by the Gri-Mech 3.0, San-Diego 2005, and the CSE thermochemical mechanisms are assessed. It was found that that key parameters associated with postflame NO emissions can vary up to 192% for peak CH values, 35% for thermal NO production rate, and 81% for flame speed, depending on the mechanism used for the simulation. A linear uncertainty analysis, including both kinetic and thermodynamic parameters, demonstrates that simulated post-flame nitric oxide levels have uncertainties on the order of ±50–60%. The high variability of model predictions, and their relatively-high associated uncertainties, motivates future experiments of NO_x formation in exhaust-gas-diluted flames under engine-relevant conditions to improve and validate combustion and NO_x design tools.