In this study, the effects of varying the system inlet and outlet boundary conditions were investigated based on three-dimensional computational fluid dynamics (CFD) simulation inside a high solid particle flux circulating fluidized bed riser. Four possible system inlet boundary conditions were considered with varying the gas inlet channel size at 25% (case 1), 50% (case 2), 75% (case 3) and 100% (case 4) of the maximal diameter compared to the system channel (0.050, 0.100, 0.150 and 0.200 m, respectively). The solid volume fraction inside the system decreased as the gas inlet channel size increased. Varying the system outlet boundary conditions by changing the outlet radius from 0.025 m to 0.050, 0.075 or 0.100 m revealed that the solid volume fraction inside the system increased with decreasing outlet radii. These results compare well to those experimental and two-dimensional CFD simulations. In addition, the effects of mesh sizes or computational cells, time-averaged ranges and computational fluid dynamics simulation dimensions were discussed. (C) 2013 Elsevier B.V. All rights reserved.