Air Staging and Reburning Mechanisms for NOx Abatement in a Laboratory Coal Combustor

Mechanisms that govern the formation and destruction of nitrogenous species in fuel rich zones caused by air staging and reburning were explored in a laboratory coal combustor. The objective was to determine whether the same simplified, but fundamentally based mechanism would predict time resolved profiles of all nitrogenous species, for both NOx abatement procedures. Experimentation was conducted on a 17 kW down-fired pulverized coal combustor, burning a variety of coals, under various air staged and fuel staged configurations. Heterogeneous effects on HCN formation due to the slow release of nitrogen from the coal residue were of minor significance. HCN formation and destruction in fuel rich regimes were governed by homogeneous gas phase kinetics alone. The fixation of N2 by hydrocarbons to form HCN, together with reactions of hydrocarbons radicals with either NO or N atoms, could explain HCN formation, during both air and fuel staging. The inter-conversion of nitrogenous species in the fuel rich zones of air or fuel staging configurations could be adequately described by a simplified mechanism based on known detailed gas phase reactions and partial equilibrium assumptions. Profiles of NO, HCN and NH3 were adequately predicted for both NOx abatement configurations, and the proposed mechanism should prove useful for incorporation into more complicated furnace models to predict NO emissions from practical coal combustion systems.