Thermal NOx in stretched laminar opposed-flow diffusion flames with CO/H2/N2 fuel

The effect of flame stretch (variations in velocity and concentration gradients) on thermal NOx formation has been studied in laminar opposed-flow diffusion flames. Detailed chemistry-transport model calculations show agreement (within 150 K for peak flame temperature and within 3 ppm for peak thermal nitric oxid concentrations) with previous experimental measurements in CO/H2/N2 laminar opposed-flow diffusion flames at three different velocity gradients (α = 70, 180, and 330 s−1). Major corrections were required to account for the finite spatial resolution of the probe sampling measurements. Additional model calculations were obtained over a wider range of stretch (α = 0.1–5000 s−1). Calculated NOx concentrations decreased dramatically as flame stretch was increased (with peak NOx values of 2300, 1100, 280, 20, and ≤1 ppm obtained for flames with α = 0.1, 1, 10, 100, and ≥500 s−1, respectively). This decrease was caused by declines in both the reaction time in high temperature flame zones (proportional to α−1) and in the net NOx formation rates. The net NOx formation rates are affected by flame stretch due to changes in peak flame temperature, superequilibrium O atom concentrations, NO destruction reactions, and N2O formation reactions. Most of the NOx in flames at low stretch is formed by the Zeldovich mechanism, while the N2O pathway dominates NOx formation in flames at very high stretch where the peak flame temperatures are lower. Reactions involving the formation and destruction of NO2 occur in lean flame zones, but the amount of NO2 formed is small (≤10 ppm). Both experiments and model calculations show that a very effective way to reduce thermal NOx formation in the forward stagnation regions of laminar opposed-flow diffusion flames (and possibly in turbulent diffusion flames as well) is to increase flame stretch.