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