Control of Agglomeration during Circulating Fluidized Bed Gasification of a South Australian Low-Rank Coal: Pilot Scale Testing

Abstract

In this paper, results are presented of experiments performed to assess control techniques to counteract the operational problems of agglomeration during gasification of a South Australian low-rank coal in a pilot scale circulating fluidized bed gasification (CFBG) plant. Kaolinite-rich clay additives were used in the experiments in order to reduce the problems associated with sodium present in coals. The effect of refreshing and removing the bed material during continuous gasification was also studied experimentally. Agglomerates were observed to form during tests that occurred without the addition of kaolinite; the joints of the agglomerates were found to be comprised principally of sodium and silicon, presumably as sodium silicates. Sodium silicates were also observed to contribute to deposition of bed material and ash in some parts of gasifier. Deposition occurred mainly just above the air delivery inlets to the riser and at the bottom of the recirculation leg. At both of these points, local higher temperatures than in the other parts of the gasifier would be expected, with temperature having a large influence on the viscosity and stickiness of silicate layers and their tendencies to stick to reactor surfaces. The addition of kaolinite clay reduces the formation of sodium silicates. Instead, sodium reacts with kaolinite to form sodium aluminosilicates, which do not melt at fluidized bed gasification conditions. Agglomeration during normal operating conditions was not observed during clay addition tests. These results further show that the formation of coating and agglomerates in the sand bed can be practically eliminated, and with fine-tuning of the Kingston coal gasification process, the problem of agglomeration and/or defluidization can potentially be overcome during operation of a commercial-scale gasifier.