This study reports a semi-dual fluidized-bed (sDFB) biomass gasifier, which is a novel design of dual fluidized-bed (DFB) with the internal mixing of solid particles between riser and gasifier to enhance the heat and mass transfer. A cold-rig experiment of sDFB (0.8 m width x 0.2 m depth x 3.85 m height) was performed to investigate fluid hydrodynamics and solid circulations. Pressures were sampled at 43 points along the sDFB gasifier. An external circulation rate of sand was measured for 60 s after 2 min of the operating time. In order to estimate the amount of direct back-mixing particles through the gasifier-riser interconnection area, an Eulerian-Eulerian two-dimensional computational fluid dynamics (CFD) model was developed for the cold-rig sDFB. This CFD model included the kinetic theory of granular flow and the k- dispersed turbulence model. The CFD simulation results were validated with the experiment data. About 17% back-mixing of particles through the gasifier-riser interconnection area were obtained from the CFD simulation. This indicates that the sDFB has a possibility of having higher heat and mass transfer than the conventional DFB. (C) 2012 Elsevier B.V. All rights reserved.