RELATION BETWEEN FUNCTIONAL FORMS OF COAL NITROGEN AND NOX EMISSIONS FROM PULVERIZED COAL COMBUSTION

NOx emissions during pulverized coal combustion, the thermal decomposition behaviour of fuel-bound nitrogen during rapid pyrolysis and the functional forms of coal nitrogen were investigated to develop the general index estimating NOx levels for coals covering a wide range of rank. NOx levels under excess air and two-stage combustion strongly depended on coal type. The effect of nitrogen content in the parent coal on NOx levels was not a continuous relationship, seemingly because of the yields of volatile nitrogen species which evolve during the early stage of combustion. An improved model of NOx formation was proposed to explain the influence of coal type. The dominant factors for NOx reduction were derived on the basis of the improved model. The volatile nitrogen yield and the NH3/HCN ratio strongly affect NOx formation. An NOx index to predict NOx levels was proposed based on the relation between the functional forms of coal nitrogen and the yields of nitrogen-containing species. The index involves the proportion of quaternary, pyrrole- and pyridine-type nitrogen.