A reactor model is presented to calculate the sulfur retention and SO2 emission of a fluidized-bed coal combustor with absorption of SO2 by limestone or dolomite. It is based on the two-phase theory of fluidization and a two parameter simplified rate expression for the sulfation reaction. Both parameters can be determined by experimental reaction rate measurements. In the overall reactor model, an average size is assumed for all the stone particles in the bed. Sulfur retention is found to be dependent on only three dimensionless parameters. Comparisons with experimental measurements indicated that the model predicts satisfactorily the dependence of sulfur retention on the Ca/S ratio, sulfur content of the coal, excess air, gas residence time, and sorbent type.