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

Abstract

This study addresses the importance of the different chemical pathways responsible for NOx formation in lean-premixed combustion, and especially the role of the nitrous oxide pathway relative to the traditional Zeldovich pathway. NOx 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 micro-mixed well-stirred reactor, representing the flame zone, followed by a series of plug flow reactors, representing the post-flame 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 NOx 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 NOx formation are applied and compared. Verification of the model is based on the comparison of its NOx output to experimental results published for atmospheric pressure jet-stirred reactors and for a ten atmosphere 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 NOx 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 NOx. Whenever the NOx emission is in the 15 to 30ppmv (15% O2, dry) range, the nitrous oxide pathway is predicted to contribute 40 to 45% of the NOx for high pressure engines (30atm), and 20 to 35% of the NOx for intermediate pressure engines (10atm). For conditions producing NOx of less than 10ppmv (15% O2, dry), the nitrous oxide contribution increases steeply and approaches 100%. 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 NOx. It does appear, however, that lean-premixed combustors operated in the vicinity of 10atm have a relatively weak pressure dependence, whereas combustors operated in the vicinity of 30atm have an approximately square root pressure dependence of the NOx.

Laser-Induced Fluorescence Measurements of Nitric Oxide Formation in High-Pressure Premixed Methane Flames

“…However, Reisel and Laurendeau (1994j' possibly explaining this result. Nicol et al (1993) found that pressure does affect NO production when they modeled lean, premixed CHJair combustion using three different reaction mechanisms (including the GMK-DB mechanism). They found a weak relationship between NO formation and pressure for lean flames (0.5~r/>~0.7) at pressures up to 10 atm, though the dependence varied with the reaction mechanism, equivalence ratio and pressure.…”

“…Correa and Smooke (1990) also found that NO x is insensitive to pressure in their modeling of lean, premixed laminar CH./air flames. However, in a recent study by Nicol et al (1993),enhanced NO formation was found for lean, premixed flames when using a simple gas-turbine model composed of a wellstirred reactor followed by a series of plug-flow reactors. Despite investigating the importance of various NO formation pathways for several different reaction mechanisms as a function of pressure, no conclusive relationship could be developed between predicted NOx emissions and pressure at 1-30 atm.…”

“…(8) Several additional reactions involving N 20 must be considered when performing detailed kinetic calculations (Nicol et al, 1993). The production of NO through both the thermal-NO and the N 20-intermediate paths is enhanced by super-equilibrium concentrations of OH and 0 in the flame front (Drake et al, 1990).…”

Roles of thermal and radical quenching in emissions of wall‐stabilized hydrogen flames

“…In particular, most studies have calculated the NO mole fraction with the full mechanism and compared this to the NO formed by eliminating one pathway at a time from the full mechanism (for example, Nicol et al, 1995). This procedure does not account for the competitive paths connecting the various pathways.…”

Combustor Stability and Emissions Research Using a Well Stirred Reactor

“…iN Emissions Variables affecting the production of NO. have been extensively studied by Nicol et. al.…”