Nitric-oxide emissions scaling of buoyancy-dominated oxygen-enriched and preheated methane turbulent-jet diffusion flames

Theoretical predictions and measurements of nitric-oxide emissions from buoyancy-controlled weak (≤11%) oxygen-enriched as well as fuel-preheated (<873 K) non-piloted methane turbulent-jet flames are presented. Computational examination of strained opposed-flow diffusion flames underscores flamelet features for air as well as oxygen enrichment. For air- and oxygen-enriched conditions, the NO production is dominated by the sequence of CH + N₂ → HCN + N and N + OH → NO + H. Peak NO-formation shifts from stoichiometric to fuel-rich conditions with oxygen enrichment. Oxygen enrichment increases the Zel'dovich contribution more than the prompt contribution. However, the contribution of thermal NO, as defined by the classical Zel'dovich mechanism, to the total NO remains small even with (<11%) oxygen enrichment. Higher strain rates reduce this effect. A previously developed laminar-flamelet based estimate is extended to incorporate oxygen enrichment as well as fuel preheat. The theory uses reduced chemical-kinetic mechanisms for the flamelet and simplified transport equations for the buoyancy-controlled flame height. Average NO-production rates are obtained from asymptotical examination of a two-reaction-zone oxygen-enriched laminar flamelet with an approximated joint probability-density function for mixture fraction and scalar dissipation. Experimental NO_x emission indices derived from NO and CO₂ measurements of non-preheated, oxygen-enriched (0 ≤ Δ X_O2 ≤ 0.11) methane flames (2.5 × 10³ ≤ Fr ≤ 4.6 × 10⁴) are in excellent agreement (average deviation 13.5%) with the theoretically derived Froude-number correlation. Decreasing downward shifts about the correlation with increasing oxygen enrichment are observed and are consistent with liftoff phenomena. Comparison with a wider range (633 ≤ Fr ≤ 1.6 × 10 ⁶) of non-preheated methane-air flames shows better agreement (average deviation 26%) with the proposed correlation than previously obtainable. The departure of the experimental NO_x emission indices from the flamelet-based predictions for a fuel-preheated, oxygen-enriched (Fr = 4.4 × 10⁴, ΔX_O2 = 0.07) methane flame demonstrates the effects of radiative losses. These losses occur only with substantial (>400 K) preheat and are confirmed by measurements of total radiant fluxes. Measurements of soot-precursor species and soot loading suggest enhanced radiation through enhanced soot loading as potential NO _x-abatement strategy.