In the present work, a mass balance allows us to predict operating conditions of coal-fired circulating fluidized bed (CFB) combustor under full load operation. A mass balance model has been developed for the particle size distribution and solids mass fluxes of three granular materials supplied and led out in the 966MWth supercritical circulating fluidized bed boiler. This model emphasizes some important factors, such as feed rates, separator characteristics, drainage and recirculation rates of solids and internal processes. In the simulation model the internal processes such as the combustion process, particle shrinkage, fragmentation and attrition were taken into account, respectively. The model can theoretically predict the PSD and mass balance at different operating loads, ash content and points inside the supercritical CFB combustion system. The results of the simulation indicate that they are in accordance with the experimental data from large-scale supercritical CFB boiler. The results of modeling give an opportunity of studying the impact of changes on the solids particle size distribution (PSD) and solids mass fluxes in operating conditions. Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved.