Optimum temperature for sulphur retention in fluidised beds working under oxy-fuel combustion conditions

摘要

Oxy-fuel combustion is one of the leading options for power generation with CO_2 capture. The process consists of burning the fuel with a mixture of nearly pure oxygen and a CO_2-rich recycled flue gas, resulting in a product flue gas from the boiler containing mainly CO_2 and H_2O. Among the possible boiler types, fluidised bed combustors are very appropriate for the oxy-fuel process because they allow the in situ desulphurisation by feeding Ca-based sorbents into the combustor. In this work, the effect of the temperature of the combustor on the retention of the SO_2 generated in the combustion of two coals with very different sulphur content (a lignite and an anthracite) has been studied. The experimental facility used was a bubbling fluidised bed (BFB) combustor of～3 kWth.Tests were conducted under oxy-fuel combustion mode and also under enriched-air combustion mode for comparison reasons. A Spanish limestone "Granicarb" was used as Ca-based sorbent for sulphur retention. The temperatures tested were between 800 and 970 ℃ using Ca/S molar ratios between 0 and 3.rnIt was found that in BFB combustors operating under oxy-fuel combustion conditions the optimum temperature to achieve the highest sulphur retention was 900-925 ℃, whereas operating with enriched air the optimum combustion temperature was 850-870 ℃. Working at the optimum temperature, the SO_2 retentions were lower in oxy-fuel combustion than in enriched air combustion conditions. It was also observed that working with lignite there was 10-15% of sulphur retention by coal ashes, however, working with anthracite the sulphur retention by coal ashes was negligible. This finding was independent of the combustion mode used, oxy-fuel or enriched air.