The Grain-Thermo-Balance (GTB) rig was built for generation of the char, at high heating rates, and for experimental determination of its oxidation rates. A zero-dimensional mathematical model for calculating oxidation rates of millimeter-size char particles was developed to serve as a char particle sub-model in a CFD-based software for predicting performance of fluidized bed boilers. When morphology of the char was determined using mercury porosimetry and the char oxidation rates in the kinetic regime were measured using TGA, the zero-dimensional Shrinking Particle Model was able to reproduce the GTB measurements well - with exception of the last 2% burnout where due to a model singularity the calculated temperatures exceeded the measured values. When the char was generated in TGA, at low heating rates, its intrinsic reactivity was four times lower and the reactivity decrease was attributed to alterations to the char morphology (40% slow-down) and annealing (60%). (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.