Simplified Model for Reburning Chemistry

In solid fuel flames, reburn-type reactions are often important for the concentrations of NOx in the near-burner region. To be able to model the nitrogen chemistry in these flames, it is necessary to have an adequate model for volatile/NO interactions. Simple models consisting of global steps or based on partial-equilibrium assumptions have limited predictive capabilities.

Reburning models based on systematic reduction of a detailed chemical kinetic model offer a high accuracy but rely on input estimates of combustion intermediates, including free radicals. In the present work, an analytically reduced nitrogen scheme is combined with simplified correlations for estimation of O/H and hydrocarbon 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 of 0.6 ≤ λ ≤ 2.0. The combined model is tested against reference calculations with a comprehensive mechanism. The results indicate that the approximations in the simplified hydrocarbon radical scheme are satisfactory.

However, when this scheme is combined with the semi-empirical correlations for the O/H radicals, the modeling predictions for the radicals become less accurate. Despite these deviations, the combined model provides a satisfactory prediction of NO under reburning conditions over the range of fuels, temperatures, and stoichiometries tested.