Three new granular absorbents were prepared from calcium hydroxide, and applied to an FGD process with internal circulation. The aim of the study was finding the most efficient of the these three applied absorbents for the $SO_2$ removal at high flue gas temperatures. The absorbent is fed to the testing unit at high operation temperature and fluidized inside the FGD system where the sorbent particles react with the $SO_2$ gas. The rate of $SO_2$ decomposition was high in C-type absorbent which had the large surface area. De-SOx characteristics of the current absorbents appeared to be similar to the other conventional agents in this fluidized bed combustor. In particular, the optimum de-SOx condition could be achieved at high mole ratios of Ca to S which can reduce the residual $SO_2$.