Thermal NO x in stretched laminar opposed-flow diffusion flames with CO/H 2/N 2 fuel

Abstract

The effect of flame stretch (variations in velocity and concentration gradients) on thermal NO x 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/H 2/N 2 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 NO x concentrations decreased dramatically as flame stretch was increased (with peak NO x 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 NO x formation rates. The net NO x formation rates are affected by flame stretch due to changes in peak flame temperature, superequilibrium O atom concentrations, NO destruction reactions, and N 2O formation reactions. Most of the NO x in flames at low stretch is formed by the Zeldovich mechanism, while the N 2O pathway dominates NO x formation in flames at very high stretch where the peak flame temperatures are lower. Reactions involving the formation and destruction of NO 2 occur in lean flame zones, but the amount of NO 2 formed is small (≤10 ppm). Both experiments and model calculations show that a very effective way to reduce thermal NO x 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.