The influence of SO2 level and operating conditions on NOx and N2O emissions during fluidised bed combustion of coals

An experimental study on interactions between nitrogen oxides emissions in the presence of SO2 is proposed. Two different coals were burnt in a laboratory-scale fluidised bed (FB) unit in air and in an artificial atmosphere of O2/N2/SO2. The change of operating variables was also considered. Additionally, flow reactor studies on HCN oxidation, which is an important NO/N2O precursor, with and without SO2 present were performed. shown that with increasing temperature or increasing oxygen concentration the emissions of NO increase. N2O emissions increase only slightly with increasing oxygen concentration and show a maximum around a bed temperature of 800°C. A mechanism of homogeneous catalysis operated by SO2 is considered responsible of free radicals (i.e. H, O, OH) recombination under fuel lean conditions. Thus higher SO2 levels increase the emissions of CO, while NO decreases significantly. Due to the reduced destruction by radicals and the lower selectivity in HCN oxidation towards NO, N2O emissions increase at higher temperatures. Apart from the homogeneous interaction between SO2 and NOx and N2O emissions, the addition of limestone has a significant effect due to heterogeneous catalysis at active CaO sites. So the selectivity of HCN and NH3 oxidation towards NO is increased in the presence of limestone. mogeneous tests in the flow reactor confirm the results obtained in the laboratory-scale FB. SO2 inhibits the conversion of HCN and combustible gases (i.e. CH4, CO and H2). It increases the selectivity of HCN oxidation to N2O compared to NO, by changing the formation paths but also decreasing the N2O destruction by the O radical. Modelling results are generally in good agreement with the experimental results.