Sampling Measurements of Nitric Oxide in Methane + Oxygen + Nitrogen Flames Doped with Ammonia

Probe sampling measurements of the concentrations of nitric oxide in the post-flame zone of methane + oxygen + nitrogen flames doped with ammonia (0.5 % of the fuel) are reported. The goal of this work was to analyze formation of NOx from fuel-N under well-controlled conditions. A Heat Flux method was used for stabilization of non-stretched flames on a perforated plate burner at atmospheric pressure. Dilution ratios of oxygen, O2/(O2+N2), were varied from 0.16 to 0.209. The concentrations of O2, CO, CO2 and NOx were measured by means of a non-cooled quartz probe at different axial distances from the burner. Measured burning velocities for these flames and concentrations of the major species (O2, CO, CO2) agree well with those of the flames of methane + oxygen + nitrogen within an experimental accuracy. The concentrations of NOx in the post-flame zone have a maximum near the stoichiometry. These measurements were compared to model predictions and to similar experiments in flames of methane + oxygen + carbon dioxide doped with ammonia. In (CH4+NH3) + O2 + CO2 mixtures the modeling over-predicts the measured concentrations of NOx however the experimental trends are well reproduced. In (CH4+NH3) + O2 + N2 mixtures the plots of the concentrations of NOx in the post-flame zone as a function of the stoichiometric ratio differ qualitatively from that in (CH4+NH3) + O2 + CO2 mixtures. The modeling is in satisfactory agreement with the experiments in lean flames, while in rich flames it is not