A computational fluid dynamics (CFD) model for simulating the chemical reaction process in a gas-particle circulating fluidized bed (CFB) downer is introduced by combining the two-fluid model (TFM) for the gas-particle turbulent flows and the (c(2)) over bar-epsilon(c) model for the turbulent mass transfer. With the proposed model, the species concentration and solid volume fraction as well as the velocity distributions along the CFB downer are able to be predicted. In mathematical expression of the proposed model, the recently developed formulations of (c(2)) over bar-epsilon(c) is adopted to close the turbulent mass transfer equations so that the turbulent mass diffusivity can be determined without relying on empirical methods. As for the gas-solid two phase turbulent momentum transfer equations, the methodology of k(g) - epsilon(g) - k(p) - epsilon(p) - Theta is used for their closures. To validate the proposed model, simulation is carried out for the catalytic ozone decomposition in a gas-solid CFB downer. The simulation results are compared with the experimental data and satisfactory agreement is found between them in both axial/radial distributions of concentration and solid volume fraction. Furthermore, the simulations reveal that the turbulent mass diffusivity varies along axial and radial directions, and the turbulent Schmidt number is not a constant throughout the CFB downer. (C) 2014 Elsevier B.V. All rights reserved.