The eXtended Discrete Element Method (XDEM) is a novel approach to the modelling of the combustion of fuel particles1,2. The method uses a Coupled Continuum and Discrete Model (CCDM) wherein the fuel particles are modelled using the Discrete Element Method (DEM) while the gas-phase is modelled using Computational Fluid Dynamics (CFD). The DEM model is extended to include the necessary theromodynamic state of the particles. This methodology allows the development of a high fidelity model that captures the entire combustion process. In this study the XDEM approach has successfully been used to develop full 3D numerical model of a large feeder-grate type industrial biomass furnace (44 ton/h, 8 MW capacity). A novel co-located partitioning strategy3 is used to solve the large 3D numerical model efficiently on a High Performce Cluster (HPC). The thermal performance of this biomass furnace has been investigated with regards to its fuel and airflow configuration. The numerical model of the furnace includes the introduction of wet fuel particles which are subjected to drying due to conductive and radiative heat transfer before they undergo pyrolysis and gasification reactions. The pyrolysis of the particles yields solid coke which is then also gasified. The gaseous products of the particles’ solid state reactions are transfered to the gas-phase model where the reactions complete; thus yielding the total energy and final products associated with the particles’ combustion. The recirculated flue gas and the secondary air inlets produce a mixing effect within the furnace which increases the gas-phase reactions of the gaseous products. In agreement with emperical observations the burn-out combustion of the biomass particles towards the end of the grate occurs at significantly higher temperatures due to the gasification of the coke created by the initial pyrolysis reactions.