RANK DEPENDENCE OF N2O EMISSION IN FLUIDIZED-BED COMBUSTION OF COAL

Emissions of N2O and NO were measured during fluidized-bed combustion of six coals and their chars. The coals ranged in rank from lignite to anthracite. Fractional conversion of coal-bound nitrogen (coal-N) to N2O was found to increase with coal rank. A similar. although much weaker. trend was observed for NO. For a given coal, the sum of coal-N conversions to N2O and NO is practically constant over the whole temperature range (1000-1300 K). X-ray photoelectron spectroscopy determination of nitrogen functionality in coals indicates that the percentage of pyridinic-N in coal increases with coal rank, whereas the percentage of pyrrolic-N exhibits the opposite behaviour. The percentage of coal-N retained in the char was determined from elemental analysis and thermogravimetric data. It was found that coal-N retention in char was substantial for all coals and that it increased with coal rank. The contributions to N2O and NO emissions coming from gas-phase and heterogeneous reactions were evaluated by comparing emissions from coal and char combustion. Results show that although only a relatively small proportion of coal-N is released with the volatiles, gas-phase reactions account for the majority of N2O emitted from a fluidized-bed combustor. This can be explained by the efficient heterogeneous reduction of N2O to N2 which occurs on the char surface immediately after N2O is formed from char-N. The contribution of char-N to N2O emission is substantial only for high-rank materials, in the case of which coal-N retention in char is nearly 100%. In contrast to N2O, char-N was found to be the main source of NO.