Knowledge of particle size distributions is crucial for establishing the utilization of solid carbonaceous and sorbents in fluidized bed combustors. Populational models should consider particle shrinking through disintegration and chemical reaction, and particle removal through elutriation and overflow. A modelling review is carried out and a populational model is constructed. Bed surface entrainment and elutriation models are used to deal with particle carryover. Entrainment is assumed to be accomplished through two different parallel mechanisms : the prompt entrainment of very fine particles, and the entrainment through bubble wake ejections. Attrition is identified as the main mechanism of particle disintegration. Concerning carbonaceous particles, attrition and chemical reaction are treated as parallel shrinking effects. Limestone sorbent particles are assumed to shrink only through attrition. The particle population model is solved simultaneously with the entrainment and the elutriation models. The predictions of the interlinked models are in good agreement with available experiment.