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 labora tory-scale fluidised bed (FB) unit in air and in an artificial atmosphere o f O-2/N-2/SO2. The change of operating variables was also considered. Addit ionally, flow reactor studies on HCN oxidation, which is an important NO/N2 O precursor, with and without SO2 present were performed. It was shown that with increasing temperature or increasing oxygen concentr ation the emissions of NO increase. N2O emissions increase only slightly wi th increasing oxygen concentration and show a maximum around a bed temperat ure of 800 degreesC. A mechanism of homogeneous catalysis operated by SO2 i s considered responsible of free radicals (i.e. H, O, OH) recombination und er fuel lean conditions. Thus higher SO2 levels increase the emissions of C O, while NO decreases significantly. Due to the reduced destruction by radi cals and the lower selectivity in HCN oxidation towards NO, N2O emissions i ncrease at higher temperatures. Apart from the homogeneous interaction betw een SO2 and NOx and N2O emissions, the addition of limestone has a signific ant effect due to heterogeneous catalysis at active CaO sites. So the selec tivity of HCN and NH3 oxidation towards NO is increased in the presence of limestone. The homogeneous tests in the flow reactor confirm the results obtained in t he laboratory-scale FB. SO2 inhibits the conversion of HCN and combustible gases (i.e. CH4, CO and H-2). It increases the selectivity of HCN oxidation to N2O compared to NO, by changing the formation paths but also decreasing the (NO)-O-2 destruction by the O radical. Modelling results are generally in good agreement with the experimental results. (C) 2001 Elsevier Science Ltd. All rights reserved.