The major advantage of fluidized bed combustion (FBC) of fossil fuels is the ability to absorb SO_2 by limestone directly in the combustor with high desulfurization efficiency. At the same time, limestone can also capture HCl to form CaCl_2 in the FBC system. This helps reduce corrosion caused by chloride. However, the optimum temperature for capturing hydrogen chloride by limestone in the FBC combustor is <600 deg C, whereas the optimum temperature for capturing SO_2 is 850 deg C. Thus, the decomposition and use of limestone should be looked at in two stages: one is the fluidized bed with its high temperature needed for desulfurization, while the other is the freeboard at relative low temperatures used for the capture of HCl. In order to achieve both high capture efficiencies for SO_x and HCl in an FBC system, a study of the characteristics of limestone decomposition was carried out in a TGA system and a drop tube furnace with high temperature gases flowing through each. The effects of particle size, residence time, temperature, and different types of limestone on the decomposition rate were studied. The results will be useful for choosing the kind of limestone and particle size to get high absorbance efficiencies for SO_x and HCl in an FBC system.
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