IMPORTANCE OF SOLID-FUEL PROPERTIES TO NITROGEN-OXIDE FORMATION THROUGH HCN AND NH3 IN SMALL-PARTICLE COMBUSTION

The formation of nitrogen oxides from fuel-nitrogen through intermedia tes was studied by measuring first fuel-O/fuel-N ratios and nitrogen f unctionality in selected solid fuels. Then the ratios of the yields (f uel-N --> HCN)/(fuel-N --> NH3) in a nearly inert atmosphere at 800-de grees-C in an entrained flow reactor was measured and finally the rati o (fuel-N --> N2O)/(fuel-N --> NO) in an oxidizing atmosphere at 800-d egrees-C. The fuels studied were coal, brown coal, S- and C-type peat, fir bark. birch bark and pine bark, all milled to a particle size < 6 3 mum. The ratios of O/N in the fuel, measured by elemental analysis, ranged from 7 to 150. Nitrogen functionality (mass percent of the tota l nitrogen content) was determined by XPS. The (fuel-N --> HCN)/(fuel- N --> NH3) conversion ratio in the absence of O2, and also the (fuel-N --> N2O)/(fuel-N --> NO) conversion ratio with O2 present, decreased with increasing ratio of fuel-O/fuel-N, but neither ratio decreased re gularly with the increasing ratio of pyrrolic to pyridinic nitrogen in the fuel. Thus, fuel-oxygen plays a more important role than nitrogen functionality in the chemistry of nitrogen oxide formation. The stron g effect of (fuel-O/fuel-N) ratio on the (fuel-N --> HCN)/(fuel-N --> NH3) ratio may be due to the reaction between OH radicals and HCN to f orm NH3 near the fuel particle. The importance of this reaction is con sidered. Charring the fuel sample before combustion led to a sharp dro p in the conversion of fuel-N to N2O compared with the virgin fuels. T hus, heterogeneous combustion reactions produced much less N2O than ho mogeneous combustion reactions.