Abstract: The CFD-DEM approach coupled with thermochemical submodels is applied to study the heat transfer characteristics during the solid fuel combustion process in a bubbling fluidized bed. The particle flow behavior, gas turbulence, heat transfer between and within two phases, and chemical reactions are integrally considered. The heat transfer submodel involves five modes: conduction between particles, conduction between particle and wall, gas-particle convection, particle-bed radiation, and chemical reaction. The developed model is first verified by comparing the predicted results with experimental data in terms of temperature and gas compositions. Then, the contributions of different heat transfer modes are quantified at a particle scale. For coal particles, heat of reaction is dominant, and radiation and convection also play a significant role. For sand particles, convection and radiation are dominant heat transfer modes. Despite the lower proportion, two conduction modes have an indispensable effect for both particle species. Afterwards, the formation and development of hot spot are elucidated. It is demonstrated that the local high temperature in the reactor is primarily affected by the combustion of the volatile gas rather than char combustion, and the hot spot tends to appear above the surface of bed. Finally, the influences of different particle properties on each heat transfer mode are analyzed. With the increment of volatile content of fuel, the thermal power from convection increases, while the contribution of reaction decreases gradually. For various thermal conductivities, the particle-gas-particle conduction invariably contributes more than particle-particle contact. The particle-wall conduction becomes more important with the increasing thermal conductivity.