SO2 capture fluidised-bed boilers: re-emission of SO2 due to reduction of CaSO4

Abstract

Sulphur capture by lime was studied in a 16-MW fluidised-bed boiler. The desulphurization process, i.e. the addition of limestone which is calcined and sulphated, results in an accumulation of CaSO4 in the bed. The CaSO4 is shown to be decomposed to CaO with subsequent release of SO2 at temperatures above 880-890°C, and at an excess air ratio of 1.4. At 930°C the amount of sulphur leaving the boiler as SO2 was more than double the amount of sulphur added to the boiler in the form of fuel sulphur. The decomposition of CaSO4 can be explained by the reaction of CaSO4 with combustion intermediate such as CO and H2 in the dense (particle) phase of the bed. The results explain the decrease in sulphur capture performance with increased temperature observed in fluidised-bed boilers and indicate the important effect or reducing conditions. Increased understanding of these phenomena may provide solutions, with respect to boiler design and operation, that make the use of limestone for desulphurization more efficient. © 1989.