Advanced CFD Post-Processing for Pulverized Fuel Flame Structure and Emissions

The simulation strategy described in this paper provides an alternative to conventional CFD post-processing to estimate exhaust NO_X emissions. The method first analyzes a CFD furnace simulation to specify temperature histories and mixing rates. Then the bulk flow patterns are represented with an equivalent network of idealized reactor elements. Detailed reaction mechanisms are then applied over the reactor network, including the most fully validated reaction mechanisms for coal devolatilization and char oxidation and complete elementary reaction mechanisms for chemistry in the gas phase and in soot. The analysis predicted the NO_X emissions from 1.7 MW_t pilotscale flames of coal and coal/biomass blends within experimental uncertainty over a broad range of O₂ concentrations, with and without staging. The predicted unburned carbon emissions were qualitatively correct, but char reactivity parameters would need to be specified in a one-point calibration for quantitative predictions. The analysis also characterized distinctive chemistry within the flame core, a mixing layer for secondary air entrainment, an overfire air zone, and a char burnout zone. The main practical benefit of the mechanistic complexity is that simulations based on detailed mechanisms require fewer parameter adjustments than CFD simulations whenever different fuels are considered.