This paper describes the development and validation of a numerical tool able to simulate biomass combustion in grate-fired systems and support operation and design improvements of these devices. The modeling method is conceived as a compromise between the demand of computing time and the degree of detail in the simulation. As such, it integrates both the bed zone and the freeboard zone on a same 3D grid and assumes the bed as a porous medium, where heterogeneous reactions are simulated by a modified laminar rate model. Liquid water, dry biomass and char are introduced as site species that react on the porous medium surfaces to produce and/or consume gas species (O-2, CO, CO2, H-2, H2O, light hydrocarbons and condensable gases). To validate the numerical tool, predictions have been compared to experimental data gathered in a 250 kW(th) combustion test facility operated with a high quality woody pellet. Once validated, the tool has been applied to characterize the flow patterns as well as the temperature and the main gaseous emissions profiles within the combustion chamber. According to the analysis of the simulation results, significant improvements have been identified concerning not only operation but also design issues.