Computational particle-fluid dynamics (CPFD) simulation was carried out to examine the influence of air versus O-2/CO2 as the fluidizing gas on the hydrodynamics of gas-solid flow in a cold-mode circulating fluidized bed. The CPFD simulation results were compared to the experimental data at constant superficial gas velocity, using air or mixed O-2/CO2 in three different concentrations as the fluidizing gas. The simulation results showed that the model successfully captured the experimentally observed trends. A detailed statistical analysis was carried out on the transient pressure data, and results were found to vary depending on whether air or combustion gases (O-2/CO2) were used for fluidization. In all cases, the flow exhibits a typical core-annular flow structure, although for O-2/CO2 gas the solid volume fraction increases near the wall region. The CPFD results provided insights into the gas-solid flow behavior in a fluidized bed combustor riser under an oxy-fuel fluidizing atmosphere. (C) 2017 Elsevier B.V. All rights reserved.