Investigation of nitric oxide formation in methane, methane/propane, and methane/hydrogen flames under condensing gas boiler conditions

Abstract

In this study, NOx formation in lean premixed burner-stabilized flames was investigated under condensing gas boiler operating conditions for pure methane, methane/propane, and methane/hydrogen mixtures. Temperature and NOx species profiles were experimentally obtained from thermocouple and wet chemiluminescence detector measurements using in-situ extracted gas samples. Resolved simulations utilizing different NO sub-mechanisms from the GRI 2.11, the GRI 3.0, the CRECK mechanism, and a recent mechanism from Glarborg and co-workers were performed, where the mechanism from Glarborg showed the best agreement with the measured NOx concentrations. While the other mechanisms also agreed fairly well with the experiments, a pathway analysis revealed that this results from error compensation, where the prompt pathway was underestimated, which was balanced by an overestimated contribution of the NNH pathway. Interestingly, the prompt pathway was found to be the major NOx formation route for condensing gas boiler conditions despite the lean mixture. While the temperature level in the post-flame region was too low for the thermal pathway to become dominant, the NNH and N2O pathways reached their chemical equilibrium just downstream of the flame front and therefore did not further contribute to the overall NOx emissions.The partial substitution of methane by hydrogen slightly reduced NOx emissions, which was similarly found in experiment and simulation. The post-flame temperature for methane/hydrogen remained nearly unchanged, because the higher adiabatic flame temperature compared to pure methane was compensated by increased heat losses due to a higher laminar burning velocity. The addition of hydrogen decreased the amount of CH radicals in the flame front, which led to a lower contribution of the most important prompt pathway.Substituting methane by propane increased the temperature level due to a higher adiabatic flame temperature and a slightly smaller laminar burning velocity. While increasing NOx emissions were found in the experiment, the simulation predicted a minimally smaller NOx level compared to pure methane. The higher temperature caused increasing contributions of the thermal, NNH, and N2O pathways. For the prompt pathway, which is again the most important NOx formation route, the temperature trend was superimposed by a decreasing CH concentration in the flame front.