Experiments have been performed with coke particles in the freeboard region of a 250 kW Atmospheric Fluidized Bed Combustor (AFBC) with particle sizes ranging from 90 mu m to 360 mu m in the gas temperature range from 1050 K to 1350 K. Partially burned particles were collected at various heights and residence times. The observed morphological development during combustion indicates clear effects of internal burning resulting in an increase of pore volume and internal surface area. A pore model has been developed to describe the evolution of pores and total surface area, and to calculate the intrinsic reactivity and burning rates. The model is based on a combination of a Thiele analysis and the random pore model.