Intra-particle temperatures and the emissions of carbon dioxide, carbon monoxide, and total hydrocarbons have been measured during devolatilization and char combustion of large spheres (3-20 mm diameter) of selected fuels. The fuels range from a carbon-rich, bituminous coal to two different, sub-bituminous coals of very different moisture and ash contents, to a highly volatile and ash-rich sewage sludge and to beech wood with the highest volatile content. To obtain results under various fluidized bed combustor conditions and to independently change the governing parameters, the experiments were performed in three different laboratory-scale fluidized bed combustors (FBCs). The beds＇ temperatures varied between 700 and 950 degrees C, the oxygen partial pressure in the fluidizing gas varied from 0 to 21 kPa, the superficial gas velocity from 0.3 to 9 m/s, the voidage of the fluidized particles in the beds was usually 0.5 or 0.997, and the mean diameter of the fluidized particles was 200 or 900 mu m. A dynamic simulation model has been developed to describe the behavior of a single fuel particle during its lifetime in a combustor. The model considers drying, devolatilization, volatile and char combustion, external and internal heat and mass transfer, and changing fuel properties. Calculated temperature profiles and emission data are in good agreement with observations.