The regeneration of sulphated limestone
Fluidised bed combustion offers potential advantages over conventional power generation systems, particularly with respect to sulphur capture using injected limestone. The stone calcines on entry to the hot bed, forming CaO, and then reacts with SO2 to produce CaSO4. Regenerative schemes aim to reduce the sorbent loading by stripping off the sulphur from the spent limestone which is then reused. This subject of this dissertation is an investigation into the fundamentals of the regeneration of sulphated limestone by reductive decomposition. Following a detailed discussion of the thermodynamic limitations on the reaction system, attention is focussed on the kinetics of the reductive decomposition scheme. The results of a study on the reaction of CaSO4 powder with CO are reported. This made use of two experimental techniques, X-ray powder diffraction and thermogravimetric analysis. These experiments highlighted the major features of the reaction scheme and allowed the study of two special cases, the sulphidation of CaSO4 to produce CaS only and the solid-solid reaction between CaS and CaSO4. The major experimental technique used for this work was the batch addition of limestone to a fluidised bed. After a brief discussion of the results of sulphation experiments, typical regeneration experiments are described. By varying the test conditions as well as performing several special experiments, a mechanism for the overall reaction is deduced. The effect of the operating variables on the product split is then explicable. The evidence suggests that the closed pores resulting from the sulphation reaction lead to strong diffusion resistance on regeneration which controls the rate during the early and middle stages. By utilising high CO2 concentrations the formation of CaS was inhibited; the reaction was then amenable to quantitative analysis which revealed an approximate first order dependence on CO concentration and an activation energy of 110kJ/mol. One method for reducing the quantities of CaS produced is to operate the fluidised bed in a two-zone fashion i.e. with oxidising and reducing regions. An investigation into this reactor configuration is included with particular attention paid to the oxidation of CaS. The results obtained are explicable in terms of the results from the single zone bed and allow the effects of operating variables on the reactor performance to be predicted. Finally, the mathematical modelling of the gas-solid reactions is considered. The changing grain size model is introduced by considering the sulphation of limestone. The final conditions from this model then form the initial conditions for the regeneration model, which considers mildly reducing conditions only. The final model then uses as a basis the mechanism proposed in chapter 5 and is applied to the thermogravimetric analysis results.