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 SO<SUB>2</SUB> to produce CaSO<SUB>4</SUB>. 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 CaSO<SUB>4</SUB> 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 CaSO<SUB>4</SUB> to produce CaS only and the solid-solid reaction between CaS and CaSO<SUB>4</SUB>. 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 CO<SUB>2</SUB> 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.

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Biomass gasification is a technology under development that presents a means of generating hydrogen using renewable energy. While many forms of gasification have been investigated, steam gasification using a dual fluidised bed (DFB) reactor has been shown to efficiently produce high hydrogen content producer gas. The aims of this research were to increase the hydrogen yield from the 100kW DFB gasifier installed at the University of Canterbury, and thereby improve the current state of the art of gasifier operation. Calcium carbonate-based minerals such as calcite and dolomite were shown to be able to improve hydrogen production by absorbing carbon dioxide in the producer gas, promoting the water gas shift reaction. Bed material mixtures of olivine and calcite were the most effective at improving gasifier performance, increasing producer gas yield by 20%, increasing cold gas efficiency by 6% and increasing hydrogen yield by 85%. In addition, the carbon monoxide content was reduced and the ratio of hydrogen to carbon monoxide in the producer gas was ideal for Fisher-Tropsch synthesis of liquid fuels.

The lack of a suitable system for hot gas cleaning is the greatest obstacle to the development of gasification combined-cycle power generation. In this work, a shallow 0.15 m diameter fluidised bed of 1 mm sand particles was used as a filter for 0.5-10 mum gasifier char particles redispersed in nitrogen at 700-900&deg;C and 1 bar a. Earlier work has suggested that, for a correctly-designed low free-area distributor, initial collection efficiencies greater than 99% at 2 could be realised but that poor retention limits the overall filtration efficiency. As an aid to retention, heavy fuel oil was sprayed continuously into the bed using a concentric-tubs atomiser. With this arrangement, theoretically-predicted filtration efficiencies were approached for particles below; 7 mum in size, but the overall performance was seriously marred by secondary aerosol originating from the addition of the oil itself. The results suggested that two modes of filter operation are possible; at lower gas and retention-aid flowrates the challenging aerosol is retained on the collector; at higher gas and retention-aid flov/rates the challenging aerosol is agglomerated and re-emitted from the bed in a distribution with a larger mean size. The operation of various instruments for particle size measurement below 10 mum has been reviewed, and their capabilities have been compared by experiment. The method selected (collection in a liquid impinger, followed by off-line analysis by "Coulter Counter") is described in detail. The addition of a liquid retention-aid to a fluidised bed can cause modification of its fluidisation behaviour, leading eventually to catastrophic defluidisation. Theoretical and experimental aspect of these effects are discussed, leading to the development of an experimental method for direct measurement of interparticle forces, and an expression for the transition condition between Geldart's (1973) groups A and B.

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