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Show PAPER NO.15 Air Staging and Reburning Mechanisms for NOx Abatement in a Laboratory Coal Combustor J.O.L.Wendt and J.B.Mereb, University of Arizona, Tucson, AZ 85721 ABSTRACT 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. INTRODUCfION Nitrogen oxides (NOx) emitted from pulverized coal fired,stationary combustion sources are significant contributors to atmospheric pollution. Combustion modifications involving air and fuel staging techniques each require NO and other nitrogenous species to be destroyed under overall reducing environments. Previous work at this laboratory has focused on development of fundamentally based, but global, mechanisms describing time resolved profiles of some nitrogenous species in the reducing zone. This has been completed for the fuel rich, post flame zone in a pulverized coal air staged combustion process (Bose, Dannecker, and Wendt, 1988; Bose and Wendt, 1988) and for the reducing zone produced during the natural gas reburning of pulverized coal flue gases (Mereb and Wendt, 1990, Wendt and Mereb, 1990). Since the mechanisms developed for both cases are fundamentally based one would expect rthem to be identical, but this is not the case. For example, Bose et. al. postulated that the slow release of coal pyrolisis nitrogenous compounds was required ton account for the continuous "source" of HCN observed in the post flame during stage combustion, but Mereb and Wendt showed that such a mechanism was not required to calculate the similar looking profiles of nitrogenous species in the 1 |