Prediction of nitrogen oxide formation in turbulent coal flames

Abstract

A model of nitrogen pollutant formation and destruction in pulverized coal reactors, whose foundations were presented recently, is briefly outlined and then evaluated by comparisons of predictions with measurements. The model incorporates effects of turbulence on the NO reactions and subsequent predictions show these effects to be important. Thermal and prompt NO were neglected. The model predicts: 1) An initial decrease followed by a gradual increase in NO emissions with increased swirl number as observed for the particular cases modeled; 2) an increase in NO emissions with increased stoichiometric ratio as observed; at higher values, the model underpredicts NO concentrations, partly because of higher thermal NO contributions which are not modeled, and 3) the observed increase in NO with a decrease in particle size and with increase in moisture percentage. Predicted NO emissions for the four test variables differed by an absolute average of 9% from measured values for 24 comparisons. Comparisons with local NO and HCN concentrations from pulverized-coal flames show some agreement except in regions immediately following ignition where oxygen depletion and devolatilization are overpredicted. Predicted NO effluent emissions for six profile cases differ by an absolute average of 24% from measured values. Results suggest that fuel nitrogen release during devolatilization and gas phase reactions of HCN and oxygen control NO formation, while fuel nitrogen conversion to HCN may be near quantitative and rapid. According to the predictions, NO reduction is dominated by HCN-NO reactions and not by the char-NO reactions, while turbulence has an important impact on the gaseous reactions.