Gas-solid flow behavior in a 65 t/h oil shale-fired high-low bed CFB boiler obtained by the revamping of a 75 t/h pulverized coal-fired boiler has been simulated using a Eulerian-Eulerian model (EEM) with kinetic theory of granular flow by the commercial CFD software package, Fluent. Two-dimensional (2D) transient and three-dimensional (3D) steady flows were simulated for the gas and the solid phase, respectively. The comparative study with regard to turbulence and drag model was performed by 20 simulation. The simulated results agreed reasonably with the experimental data and showed that Swirl-modified RNG k-epsilon-Per phase model and Gidaspow drag model could predict preferably the internal circulation process. Gas-solid flow profiles were obtained by 3D steady simulation for solid velocity, pressure, solid volume fraction, and granular temperature and the internal circulation characteristics of the boiler were further understood in detail. The results showed that the pressure difference between the main and side bed and the distributions of solid velocity and volume fraction illustrated the mechanism of internal circulation process. The fluidized velocity in the side beds is lower and wear of immersed tubes is also lower. The granular temperature is higher near the immersed tube bundle. This research established the foundation for the design and large-scale of high-low bed CFB. (C) 2013 Elsevier B.V. All rights reserved.