To optimise tile circulating fluidized bed coal combustion (CFBC) with sorbent addition and make predictions about the combustor behaviour in a broad range of operating conditions it is necessary to make a mathematical model of the process. This model was developed integrating hydrodynamic and sulphation kinetic submodels. A simulation, from the point of view of sulphur retention, was made. The variables most affecting the sulphur retention were the pressure drop, the gas velocity, the Ca/S molar ratio, the efficiency of solids recovery by the cyclone and specially the sorbent reactivity and its particle size distribution through the amount of small particle sizes fed in tile combustor.