Catalytic influence of ash elements on NOx formation in char combustion under fluidized bed conditions

The only source for the formation of nitrogen oxides under fluid bed combustion conditions is coal nitrogen. Most of the reactions proceeding in which nitrogen oxides are formed or reduced are catalytic ones. To investigate the influence of coal ash, comparative experiments were carried out with original, de-ashed and impregnated lignite and hard coal. De-ashing was done with hydrochloric acid. By pyrolytic experiments and elementary analysis, it was shown that the organic coal structure is not changed by de-ashing. Char combustion was carried out in a thermobalance in the temperature range between 500 and 900 degrees C (final pyrolysis temperature=combustion temperature). The NOx formation and reduction reactions are strongly influenced by the catalytic efficiency of the ash. The influence is more marked for lignite than for hard coal. The emissions of NOx are diminished by ash over wide temperature ranges. Individual ash components can cause either a decrease or an increase in NOx emissions. (C) 1997 Elsevier Science Lid. All rights reserved.