To determine the intrinsic gasification rate from experiments performed in a thermogravimetric analyzer, the influences of mass transfer limitations must be evaluated. In this study, a system of nonlinear equations was numerically solved to quantify the mass transfer limitations in three mass transfer steps: external, interparticle, and intra-particle. Unlike previous studies in this area, the reaction was not assumed to be first order and the volume was not assumed to be constant. In this model, the apparent rates of CO2 gasification of petroleum coke and activated carbon, obtained from thermogravimetric analysis, were used to determine the effectiveness factors in each mass transfer step. The model was verified with isothermal experiments at 1223 K and atmospheric pressure. The experimental results and the effectiveness factors calculated with the model showed that the initial mass and particle size were the main physical factors that influenced the gasification rate. The intrinsic reaction rate could only be obtained by thermogravimetric analysis if the effects of these factors on all mass transfer steps were determined. This model also demonstrated the importance of considering the volume change in the reaction of carbon with CO2 to evaluate the mass transfer steps. (C) 2014 Elsevier B.V. All rights reserved.