An ID model of the circulating fluidized bed (CFB) boiler is developed specifically to predict the material balance in CFB boiler. This model emphasizes on the important factors that influence the ash balance in CFB boilers, such as ash formation, attrition and size reduction, residence time and segregation in dense bed. The corresponding sub-models are discussed in detail. In the simulation of a 135MWe CFB boiler in Zibo power plant, China, the parameters in mass balance model under full load operation, such as segregation parameters and axial decay constant, are optimized. The model can predict the mass balances at different operating loads in the same boiler. (c) 2005 Elsevier Ltd. All rights reserved.