Sulphur capture in fluidized bed boilers: the effect of reductive decomposition of CaSO4

Abstract Sulphur capture by lime was studied in a 16 MW stationary fluidized bed boiler (FBB). A marked fall-off in sulphur capture was noted at temperatures above about 880 °C. The proposed explanation is that the combustion produces reducing conditions in the particle phase, and thus allows for a reductive decomposition of CaSO 4 . This explanation is supported by (i) thermodynamics showing the instability of CaSO 4 under reducing conditions; (ii) in-bed oxygen measurements indicating reducing conditions in the particle phase; (iii) the observed fall-off in sulphur capture with temperature, which is not seen in laboratory tests under oxidizing conditions or in a circulating FBB, where the sorbent particles experience oxidizing conditions to a greater extent; (iv) the observation that the temperature dependence of the sulphur emission is very strong even when the net sulphur capture is zero provided always that CaSO 4 is present and (v) literature data indicating the rate of the proposed reaction.