Emissions of SO2, NO and N2O in a circulating fluidized bed combustor during co-firing coal and biomass

This paper presents the experimental investigations of the emissions of SO2, NO and N2O in a bench scale circulating fluidized bed combustor for coal combustion and co-firing coal and biomass. The thermal capacity of the combustor is 30 kW. The setup is electrically heated during startup. The influence of the excess air, the degree of the air staging, the biomass share and the feeding position of the fuels on the emissions of SO2, NO and N2O are studied. The results show that an increase in the biomass shares results in an increase of the CO concentration in the flue gas, probably due to the high volatile content of the biomass. In co-firing, the emission of SO2 increased with the increasing biomass share slightly however, non-linear increase relationship between SO2 emission and fuel sulfur content is observed. Air staging decreases the NO emission significantly without raising the SO2 level. Though change the fuel feeding position from riser to downer results in a decrease in the NO emission level, no obvious change is observed for the SO2 level. Taking the coal feeding position R as a reference, the relative NO emission can significantly decrease during co-firing coal and biomass when feeding fuel at position D and keeping the first stage stoichiometry greater than 0.95. The possible mechanisms of the sulfur and nitrogen chemistry at these conditions are discussed and the ways of simultaneous reduction of SO2, NO and N2O are proposed.