Hydrogen addition effect on NO formation in methane/air lean-premixed flames at elevated pressure

The present study investigates freely propagating methane/hydrogen lean-premixed laminar flames at elevated pressures to understand the hydrogen addition effect of natural gas on the NO formation under the conditions of industrial gas turbine combustors. The detailed chemical kinetic model which was used in the previous study on the NO formation in high pressure methane/air premixed flames was adopted for the present study to analyze NO formation of methane/hydrogen premixed flames. The present mechanism shows good agreement with experimental data for methane/hydrogen mixtures, including ignition delay times, laminar burning velocities, and NO concentration in premixed flames. Hydrogen addition to methane/air mixtures with maintaining methane content leads to the increase of NO concentration in laminar premixed flames due to the higher flame temperature. Methane/hydrogen/argon/air premixed flames are simulated to avoid the flame temperature effect on NO formation over a pressure range of 1–20atm and equivalence ratio of 0.55. Kinetic analyses shows that the N₂O mechanism is important on NO formation for lean flames between the reaction zone and postflame region, and thermal NO is dominant in the postflame zone. The hydrogen addition leads to the increase of NO formation from prompt NO and NNH mechanisms, while NO formation from thermal and N₂O mechanisms are decreased. Additionally, the NO formation in the postflame zone has positive pressure dependencies for thermal NO with an exponent of 0.5. Sensitivity analysis results identify that the initiation reaction step for the thermal NO and the N₂O mechanism related reactions are sensitive to NO formation near the reaction zone.