The Euler-Euler modeling approach was employed to simulate the coal combustion in a circulating fluidized bed combustor. The unsteady processes of gas-solid two phase flow, heat transfer, mass transfer and chemical reactions were considered. By numerical simulation, distributions of pressure, solid volume fraction, temperature and gas concentrations (O-2, CO2, H2O, CO, NO and SO2) were studied in O-2/CO2 combustion mode (mixture of O-2 and CO2 from gas bottles as oxidant) and O-2/RFG combustion mode (mixture of O-2 and recycled flue gas as oxidant) with O-2 concentration range of 21-40%. Calculations were made for two cases, maintaining the same coal feed rate and maintaining the same oxidant volume flow rate. Simulated temperature distribution, pressure distribution and outlet gas concentrations were validated by the experimental data in 30% O-2/70% CO2 atmosphere. (C) 2013 Elsevier B.V. All rights reserved.