Laser-induced fluorescence measurements and modeling of nitric oxide in premixed flames of CO+H2+CH4 and air at high pressures I.: Nitrogen fixation

Laser-induced fluorescence measurements of NO concentrations in flames of synthesized coal-gas having temperatures of 1535 to 1790 K were obtained at pressures of 1.0 to 11.9 atm. Specifically, quantitative measurements of [NO] as a function of equivalence ratio (0.7-1.3), fuel-CH4 (0-14.9%), flow rate and axial distance were obtained in the burnt gas of a series of flat, laminar, premixed low calorific value gas (LCVG) flames containing H-2, CO and CH4 as the combustible species. In these flames there is no fuel-nitrogen. NO concentrations of 1.3 to 6.5 ppm were measured, with [NO] increasing with both pressure and equivalence ratio. The results of the experimental study were then compared to calculations from two different kinetic mechanisms. The first is the GRI mechanism (version 2.11); the second was proposed by Glarborg et al. and modified by Miller and Bowman and by Kilpinen et al. When applied to the combustion of LCV fuels, the GRI mechanism uniformly over-predicted [NO] and failed to predict key experimental trends. Kilpinen's mechanism uniformly under-predicted [NO], but successfully predicted the observed increase in [NO] with pressure. The relative contributions from the different pathways producing NO were determined for both mechanisms. The GRI mechanism greatly over-predicted the contribution from NNH and under-predicted that for prompt NO. By including NNH, predictions using Kilpinen's mechanism should shift closer to the experimental measurements. (C) 2001 by The Combustion Institute.