PARAMETERS INFLUENCING NITROGENOUS SPECIES FORMATION AND REACTION IN STOKER COAL-FIRED COMBUSTION.

摘要

Stoker coal-fired boilers are significant in terms of coal utilization and environmental impact. Nitrogen oxides (NO(,x)) formation and control in stoker systems have received relatively little research attention. The purpose of this investigation was to characterize the influence of combustion parameters on NO formation mechanisms in the fuel-bed burning regimes of spreader and mass-burning stokers.; The approach utilized an experimental fixed-bed furnace configured to provide specific simulations of either stoker system. In the spreader-stoker configuration large coal particles were fired in a continuous simulation of bed-phase combustion. In the mass-burning stoker configuration the coal-bed was fired in a transient mode to simulate the time/temperature/environmental history of a small segment of a thick fuel-bed.; First-stage stoichiometry was the primary combustion parameter influencing NO formation in both stoker simulations. Substantial reductions in exhaust NO emissions were achieved under staged combustion conditions. Under fuel-rich conditions, fuel nitrogen speciation favored the formation of reduced intermediates (NH(,3) and HCN). Second-stage conversion of fixed nitrogen species was found to be inversely proportional to concentration and dependent on rich-zone residence time and local stoichiometry.; For both stoker simulations, an increase in fuel burning rate resulted in increased NO emissions. In the mass-burning simulation, under excess-air firing, the fuel-bed burned near stoichiometric conditions and adiabatic temperatures. Overfire-air height was shown to affect NO exhaust emissions in the spreader-stoker simulation. Increasing the overfire-air height resulted in decreased NO emissions, under staged conditions. Convective cooling of the first-stage gases resulted in decreased NO emissions. First-stage cooling resulted in increased nitrogenous species concentrations entering the second stage, but decreased species conversion.; Nine coals of varying rank and composition were studied in the stoker simulations. In the spreader-stoker studies, fuel nitrogen speciation favored the formation of HCN for a bituminous coal and NH(,3) for a lignite, fired under fuel-rich conditions. Fixed nitrogen conversion was insensitive to the HCN/NH(,3) ratio. Fuel nitrogen conversion to NO was a function of nitrogen content, volatility and firing conditions.