Population balance modeling is applied to develop calculation methods for mass inventory of char in fluidized bed combustion (FBC). The focus is on the effect of gas oxygen concentration and the reaction order of char with respect to oxygen, which is important in understanding and simulating combustion in high oxygen concentration. The well-known shrinking particle model describing the single particle combustion is simplified by applying a dimensionless particle size and dimensionless time with a specifically chosen characteristic reference diameter and time constant. Methods and analytical solutions to calculate particle size distribution (PSD), particle combustion time and char mass in FBC are presented. (C) 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.