Influence of Steam Dilution on Nitrogen Oxide Formation in Premixed Methane/Hydrogen Flames

The premixed combustion of natural gas and hydrogen at atmospheric pressure is investigated over a wide range of equivalence ratios and different degrees of steam and nitrogen dilution of up to 30% in the air. Emission formation, local temperatures, OH radical concentrations, and flame shape and position are measured in gas-fired tests. The measurement results confirm that steam effectively inhibits the formation of NOx emissions, even at constant flame temperature. For both natural gas and hydrogen, NOx and CO emissions below 10 ppm were measured up to near-stoichiometric fuel concentrations at wet conditions. Nitrogen dilution does not lead to reduced emissions. A numerical analysis conducted with a reactor network shows that the influence of steam dilution on radical concentrations and NOx formation depends strongly on the degree of humidity and on the equivalence ratio. The thermal, the N2O, and the NNH pathway are significantly constrained at wet conditions mainly due to reduced 0 atom concentrations, whereas prompt NOx can be increased due to higher CH radical concentrations for the natural gas flame. Thus, the inhibiting effect of steam dilution on NOx formation is stronger at high flame temperatures.