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 intermediates was studied by measuring first fuel-O/fuel-N ratios and nitrogen functionality in selected solid fuels. Then the ratios of the yields (fuel-N → HCN)/(fuel-N → NH₃) in a nearly inert atmosphere at 800ºC in an entrained flow reactor was measured and finally the ratio (fuel-N → N₂O)/(fuel-N → NO) in an oxidizing atmosphere at 800ºC 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 <63μm. The ratios of O/N in the fuel, measured by elemental analysis, ranged from 7 to 150. Nitrogen functionality (mass percent of the total nitrogen content) was determined by XPS. the (fuel-N → NCN)/(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 regularly 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 strong 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 form NH₃ near the fuel particle. The importance of this reaction is considered. Charring the fuel sample before combustion led to a sharp drop in the conversion of fuel-N to N₂O compared with the virgin fuels. Thus, heterogeneous combustion reactions produced much less N₂O than homogeneous combustion reactions.