Abstract In this study, the multiphase flow and thermochemical behaviours of char combustion in a bubbling fluidised bed (BFB) are simulated using CFD-DEM approach featuring particle size polydispersity and thermochemical sub-models. The model is first validated in terms of mixing index, particle temperature, and particle diameter. Then, it is applied to examine the contribution of each heat transfer mode and study particle-scale behaviours of char and sand comprehensively. The results show that the polydisperse drag model should be used to accurately reproduce bed hydrodynamics when simulating a BFB system with polydisperse particles. Under the simulation conditions, the particle-averaged heat fluxes to char particles through convection, conduction, radiation, and char reaction take 9.79%, 0.82%, 40.44%, and 48.95%, respectively; the particle-averaged heat fluxes to sand particles through convection, conduction, and radiation take 30.28%, 1.0%, and 68.72%, respectively. For reactive char particles, the radiation and heat of reaction are dominated, while for inert sand particles, the radiation and convection are dominated; and for both particle species, the conduction is negligible. Axial dispersion coefficient is one order of magnitude larger than the horizontal one, demonstrating the dominant role of the introduced gas flow in determining bed hydrodynamics.