Measurements of NO concentration in NH3-doped CH4 + air flames using saturated laser-induced fluorescence and probe sampling

An experimental study of methane + air flames doped with ammonia (4370 ppm of the fuel) has been performed. The goal of this work was to analyze formation of NOx from fuel-N under well-controlled conditions. The Heat Flux method was used for stabilization of non-stretched adiabatic flames on a perforated plate burner at atmospheric pressure. Laser-saturated fluorescence (LSF) and probe sampling were adopted to measure NO concentrations in the post-flame zone. LSF experiments include two series of measurements: in methane + air flames doped with NO and then in flames doped with NH3. In the lean flames seeded with NO, LSF measurements clearly deviates from the model predictions at higher concentrations of NO seeded, that strongly corroborates existence of the lean NO reburning. The modeling accurately predicts [NO] in the neat flame and shows no consumption of NO up to 170 ppm seeded. In (CH4 + NH3) + air mixtures the NO concentrations measured by LSF are in good agreement with the probe sampling results in the whole range of equivalence ratios 0.65 < Φ < 1.45. No significant impact of the probe sampling on the NOx measurements in the post-flame zone was observed. Present ammonia conversion data are only in agreement with the measurements of Henshaw et al. and disagree with many other earlier experiments. On the other hand they are accurately reproduced by the flame modeling within experimental uncertainties. Therefore the earlier criticism of the models developed by Konnov and by Skreiberg et al. is not substantiated anymore; these mechanisms are capable in predicting ammonia conversion both in lean and in rich flames. Formation of metal complexes in gas cylinders containing ammonia was put forward to explain inconsistencies between present results and earlier experiments.