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Show PAPER NO.2 THE SIGNIFICANCE OF AIR-STAGING ON SULPHUR CAPTURE IN A FLUIDISED BED COMBUSTOR B.M. Gibbs and W.U.Z. Khan* ABSTRACT Staged combustion is an established technique for reducing NO emissions, and limestone addition has been shown to be an effect i ve method of sul phur capture in fluidised bed combustors operating conventionally. This paper has investigated the effect of air-staging on sulphur capture by limestone in a pilot scale 0.3m square fluidised bed combustor. The results showed that wi thout air stagi ng, up to 70% of the sul phur can be retained, and staged operation has little effect on sulphur emissions without limestone addition. However, during staged operation, with limestone addition, sulphur retention decreases by up to 75% as the level of air staging is raised. At high staging levels (at a primary air factor 1 ess than 0.95) sul phur capture is always poor, being less than 10%, irrespective of bed temperature, whilst at lower staging levels, sulphur capture is very temperature sensitive and decreases significantly at bed temperature greater than 850°C. INTRODUCTION The European community is proposing to introduce legislation aimed at limiting NO~, SOx and particulate emissions from coal fired boiler plants. Fluidized oed combustion of coal has the potential for integrating environmental control by simultaneous reduction of NOx and SOx. Studies(I,2) have shown that the operation of a fluidized bed combustor in an air-staged mode can lead to a reduction in NO emissions of 50% or more. Air staging aims at reducing NOx emissions by capitalizing on the enhanced NOx destruct i on that occurs under sub-stoi chi ometri c conditions. In this operational mode the combustion air is separated into a primary air stream which constitutes the fluidizing air supply to the bed and a secondary air stream that is injected hi gher up in the bed or freeboard. All the coal is injected into the primary stage so that the bed is maintained at sub-stoichiometric conditions promoting NO destruction. Combustion is completed following the introduction of secondary air above the bed. The combustor is operated at overall excess air conditions as in conventional operation. Control of S02 emissions can be achieved by the use of sorbents such as limestone or dolomite. The sorbent can be introduced either mixed with the coal or separately. Limestone calcines on entering the fluidized bed combustor to form calcium oxide which then undergoes sulphation with SO~. Concern has been raised however, of the effects of sub-stoichiometrlc operation on the sulphation of CaO. *Department of Fuel & Energy, University of Leeds, LEEDS LS2 9JT, UK . ( 1 ) |