Minimization of calcium sulfide in gasification residues by combustion in a circulating fluidized bed

Successful development of the CFBC component of the Air Blown Gasification Cycle (ABGC) requires that residues from the partial gasification of coal are burned efficiently and with environmentally acceptable emissions for both flue gas and solid ashes. The aim of this study was to minimize the amount of residual calcium sulfide (CaS) in the ash; this compound can release H{sub 2}S gas on exposure to moisture, which makes ash disposal more difficult. The CaS can be converted by oxidation to more stable CaSO{sub 4}. Tests have been carried out using a 100 kW CFBC test facility. The excess air level, degree of air staging and bed temperature were varied to establish the best operating conditions to minimize CaS in the ash while giving good combustion performance. Very high conversion of CaS to CaSO{sub 4} was achieved when dolomite sulphur sorbent had been used in the pressurized gasification process. Two dolomites were tested and gave similar results. Less than 1% of the sulphur in the CFBC ash was present in the form of CaS. The degree of conversion was not affected by the combustion conditions. When limestone sulphur sorbent had been used in the gasifier, significant levels of CaS were stillmore » present after combustion. This was true for two different limestones. The reaction was probably limited by diffusion of O{sub 2} into the sorbent particles. In another test SO{sub 2} was added to the CFBC to simulate the exhaust from a regenerable external sulphur removal system, and was retained very effectively with fresh sorbent. Sulfide in the bed ash was minimal. During all the CFBC tests, combustion efficiency and sulphur retention remained very high.« less