MODEL OF SULFUR CAPTURE IN FLUIDIZED-BED BOILERS UNDER CONDITIONS CHANGING BETWEEN OXIDIZING AND REDUCING

A model treating the sulphur capture and sulphur release reaction is in fluidised-bed boilers under alternating oxidising and reducing conditions has been developed. The model involves two overall rate constants, one for the sulphur capture and the other for reductive decomposition (sulphur release), as well as the temperature dependence of these constants. The constants were determined using experimental data from a fluidised-bed boiler. The values of the constants were used to derive a theoretical maximum sulphation, which is the ratio of end conversion under the conditions in the boiler to the end conversion under oxidising conditions, where ''end'' means at infinite residence time. The theoretical maximum sulphation gives an indication of the reduction in sulphur capture performance imposed by the effect of reducing conditions. The results show that reducing conditions give a lower sorbent utilisation at increased sulphur capture as well as at increased temperature. The detrimental effect of reducing conditions is significant at all temperatures of interest for FBBs, even at temperature below 850-degrees-C. Although the quantitative results refer to the boiler investigated, the qualitative results give an indication of the behaviour of a large number of other FBBs, i.e. those boilers which have a distinct temperature dependence for sulphur capture similar to that of the boiler studied. The general conclusion is that reducing conditions have been greatly underestimated as a limiting factor for sulphur capture in FBBs.