The influence of SO 2 level and operating conditions on NO x and N 2O emissions during fluidised bed combustion of coals

Abstract

An experimental study on interactions between nitrogen oxides emissions in the presence of SO 2 is proposed. Two different coals were burnt in a laboratory-scale fluidised bed (FB) unit in air and in an artificial atmosphere of O 2/N 2/SO 2. The change of operating variables was also considered. Additionally, flow reactor studies on HCN oxidation, which is an important NO/N 2O precursor, with and without SO 2 present were performed. It was shown that with increasing temperature or increasing oxygen concentration the emissions of NO increase. N 2O 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 SO 2 is considered responsible of free radicals (i.e. H, O, OH) recombination under fuel lean conditions. Thus higher SO 2 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, N 2O emissions increase at higher temperatures. Apart from the homogeneous interaction between SO 2 and NO x and N 2O emissions, the addition of limestone has a significant effect due to heterogeneous catalysis at active CaO sites. So the selectivity of HCN and NH 3 oxidation towards NO is increased in the presence of limestone. The homogeneous tests in the flow reactor confirm the results obtained in the laboratory-scale FB. SO 2 inhibits the conversion of HCN and combustible gases (i.e. CH 4, CO and H 2). It increases the selectivity of HCN oxidation to N 2O compared to NO, by changing the formation paths but also decreasing the N 2O destruction by the O radical. Modelling results are generally in good agreement with the experimental results.