Effects of Temperature and Flue Gas Recycle on the SO2 and NOx Emissions in an Oxy-fuel Fluidized Bed Combustor

Abstract

Oxy-fuel combustion consists in burning a fuel with a mixture of nearly pure oxygen and a CO2-rich recycled flue gas resulting in a product flue gas from the boiler containing mainly CO2 and H2O.In this work, the effect of combustion temperature and flue gas recycle composition on pollutant emissions and on the sulfation process, in order to optimize the SO2 retention in circulating fluidized beds (CFBC), were analyzed.The experimental work was carried out with an anthracite coal in a 3 kWth continuous bubbling fluidized bed (BFB) combustor operating in oxy-fuel conditions. It was found that in oxy-combustion conditions in BFB an increase of combustor temperature produced an increase on NO emission and a decrease in the CO and N2O emissions. The optimum temperature to reach the highest sulphur retention was at 900-925°C.Flue gas recirculation was simulated by mixing different gases (CO2, SO2, NO, and steam) in different concentrations. It was observed that SO2 recycle produced an increase on the sulfur retention as a consequence of the higher SO2 concentration existing in the bed. About 65% of the recycled NO was reduced to N2, being the NO converted to N2O lower than 5vol.%. Wet recycle produced an important reduction on NO emission in comparison with dry recycle.