Numerical modelling is widely used in industry for detailed understanding of the combustion process and for appropriate design and optimization of biomass/waste-fired boilers. This paper presents a numerical study of a 13 MWth waste wood-fired grate boiler, based on the coupled in-bed fuel conversion modelling and freeboard combustion modelling methodology. A 1D model is developed for the conversion of the waste wood in the fuel bed on the grate, providing the appropriate grate inlet condition for the 3D simulation of the freeboard region. Since part of the flue gas is recycled into the boiler as an innovative attempt to improve the boiler performance, a refined weighted-sum-of-grey-gases-model of greater accuracy is developed to better address the impacts of the elevated CO2 and H2O vapour concentrations on radiative heat transfer in the boiler. The impacts of full buoyancy on the turbulent flow are also investigated. The temperature profiles at different ports in the furnace are measured to shed some light on the flow and combustion characteristics in the boiler and also to collect some in-flame data for modelling validation. The overall modelling strategy, the new sub-models and the use of recycled flue gas are all of great benefit or reference for modelling and design of grate-fired boilers. (C) 2016 Elsevier Ltd. All rights reserved.