A two-dimensional particle population balance model has been developed for the particle size and density distributions in a circulating fluidized bed (CFB) boiler furnace based on analysis of particle properties and the core-annulus hydrodynamic model. The model, incorporating modules to consider fuel particle fragmentation, char combustion, particle attrition and gas-solid separation, is part of an overall model developed earlier by the authors to simulate the operation of a 12MW CFB boiler. The model predictions for particle population in a CFB furnace agree well with the measurement data.