A mathematical model to describe a circulating fluidized-bed combustor is presented. A modified two-phase model which was used in the bubbling fluidized-bed combustor is considered to simulate the dense zone of the bottom section. For the upper section of the bed the momentum and energy-balance equation are used to predict the temperature and velocity profiles for the gas and the particles. The model performs mass balances for the chemical gas species (O-2, H2O, CO, CO2 and SO2) with consideration being given to the last for retention by limestone particles. The model is applied to typical conditions of a circulating atmospheric fluidized-bed boiler and the simulation results show the expected trends.