A Simplified Model for Volatile-N Oxidation

In solid fuel flames, NO is largely formed from the oxidation of volatile nitrogen compounds such as HCN and NH3. To be able to model the nitrogen chemistry in these flames, it is necessary to have an adequate model for volatile-N oxidation. Simple global models for oxidation of HCN and NH3 from the literature should be used cautiously, since their predictive capabilities are limited, particularly under reducing conditions. Models for HCN/NH3/NO conversion based on the systematic reduction of a detailed chemical kinetic model offer high accuracy but rely on input estimates of combustion intermediates, including free radicals. In the present work, simple, semiempirical expressions are presented for estimation of H, O, and OH radicals. Correlations are derived for volatile compositions representative of solid fuels ranging from bituminous coal to biomass, for temperatures of 1200-2000 K, and excess air ratios in the range 0.6 <= lambda <= 2.0. The radical estimation tool is combined with the analytically reduced N-scheme of Pedersen et al. [Combust. Sci. Technol. 1998, 131, 193-223], and the combined model is tested against reference calculations with a comprehensive mechanism. For excess air ratios of lambda >= 0.8 and temperatures of 1400 K and above, the prediction of NO formation from both HCN and NH3 is very good for volatile compositions representing all tested fuels. For lower values of lambda, the predictions are good for biomass and lignite, while they become less accurate for the sub-bituminous and bituminous coals, especially at lower temperatures. The semiempirical correlations for estimating radical concentrations may also be useful in combination with models for other trace species, such as sulfur oxides, organic species, etc.