The Importance of the Nitrous Oxide Pathway to NO_x in Lean-Premixed Combustion

This study addresses the importance of the different chemical pathways responsible for NO_x formation in lean-premixed combustion, and especially the role of the nitrous oxide pathway relative to the traditional Zeldovich pathway. NO_x formation is modeled and computed over a range of operating conditions for the lean-premixed primary zone of gas turbine engine combustors. The primary zone, of uniform fuel-air ratio, is modeled as a micromixed well-stirred reactor, representing the flame zone, followed by a series of plug flow reactors, representing the postflame zone. The fuel is methane. The fuel–air equivalence ratio is varied from 0.5 to 0.7.The chemical reactor model permits study of the three pathways by which NO_x forms, which are the Zeldovich, nitrous oxide, and prompt pathways. Modeling is also performed for the well-stirred reactor alone. Three recently published, complete chemical kinetic mechanisms for the C1–C2 hydrocarbon oxidation and the NO_x formation are applied and compared. Verification of the model is based on the comparison of its NO_x output to experimental results published for atmospheric pressure jet-stirred reactors and for a 10 atm. porous-plate burner. Good agreement between the modeled results and the measurements is obtained for most of the jet-stirred reactor operating range. For the porous-plate burner, the model shows agreement to the NO_x measurements within a factor of two, with close agreement occurring at the leanest and coolest cases examined. For lean-premixed combustion at gas turbine engine conditions, the nitrous oxide pathway is found to be important, though the Zeldovich pathway cannot be neglected. The prompt pathway, however, contributes small-to-negligible NO_x. Whenever the NO_x emission is in the 15 to 30 ppmυ (15 percent O₂, dry) range, the nitrous oxide pathway is predicted to contribute 40 to 45 percent of the NO_x for high-pressure engines (30 atm), and 20 to 35 percent of the NO_x for intermediate pressure engines (10 atm). For conditions producing NO_x of less than 10 ppmυ (15 percent O₂, dry), the nitrous oxide contribution increases steeply and approaches 100 percent. For lean-premixed combustion in the atmospheric pressure jet-stirred reactors, different behavior is found. All three pathways contribute; none can be dismissed. No universal behavior is found for the pressure dependence of the NO_x. It does appear, however, that lean-premixed combustors operated in the vicinity of 10 atm have a relatively weak pressure dependence, whereas combustors operated in the vicinity of 30 atm have an approximately square root pressure dependence of the NO_x.