A dynamic simulation model of the particle population in a circulating fluidized bed combustor with external heat exchanger has been developed. It considers the fluid dynamic processes in the various parts of the system, as well as the particle attrition. To handle Multiple solids types simultaneously and to fulfill the mass balances, some of the fluid dynamic sub-models taken from the literature were modified. The model allows to calculate the solids mass flows as well as the corresponding particle size distributions at any point inside the combustion system. The model has been applied to the combustion plant of Stadtwerke Neumunster in Germany, which operates on refuse-derived fuel. The particle balancing model provides new insights into the operating behavior of such a system. In particular, the calculation of the residence time of different particle classes in the system reveals a very broad distribution of size dependent average residence times. ranging from several minutes to a maximum of roughly 40 h. A size fraction exists between 100 and 300 mu m with a maximum average residence time of about 40 h. The Preprint Submitted to Elsevier Science 15 August 2008 simulation provides a means for examining possibilities to control the particle size distribution in the combustion system. It is shown how a recirculation of a fine ash fraction can be used to control the bed particle size distribution in the combustion chamber. (C) 2008 Elsevier B.V. All rights reserved.