This paper presents a three-dimensional model for the simulation of fixed-bed combustion systems. These generally involve two main zones, namely, bed and freeboard. Although commercially available CFD codes have extensively demonstrated their ability to simulate the behavior of the freeboard region, difficulties are found in the simulation of heterogeneous combustion and in the mass-energy interaction between the zones. A unified 3D model for both bed and freeboard simulation is presented here that incorporates the common sub-process models and solves the governing equations for both gases and solids and their interaction within the framework of the CFD code. The model developed is compared with data from an experimental burner, showing a close match. (C) 2011 Elsevier Ltd. All rights reserved.