A mixing model has been developed to simulate the particle residence time distribution (RTD) in a circulating fluidized bed absorber (CFBA). Also, a gas/solid reaction model for sulfur dioxide (SO2) removal by lime has been developed. For the reaction model that considers RTD distribution inside the core and annulus regions of a CFBA, a macrochemical reaction can be simulated based on microchemical reaction dynamics. The presented model can predict SO2 and lime concentration distributions inside the CFBA, and give the amount of lime needed to remove a given percentage Of SO2. It is found that SO2 concentration decreases with the increase of CFBA distance from the bottom in the core region. However, lime concentration exhibits a very slight variation in the core region. This means that lime is efficiently utilized to remove SO2. The model also predicts that SO2 partial pressure at the exit of the CFBA decreases with the increase in the percentage of fresh lime injected in the CFBA.