Reactivity of Gottelborn char over a broad range of particle temperature (1500-3000 K) and partial oxygen pressure (0-100% of 0.7-0.9 MPa) was measured using a shock tube. It was concluded that the contributions of combustion at the internal and external surfaces are of the same order of magnitude. An approximate analytical method based on the Thiele approach was used to calculate the kinetic parameters of the reaction rate with pore growth accounted for by introducing an external surface roughness factor. A computational model was developed to describe the reactions on the internal and external surfaces and the changes in the geometric structure of the char particle. The approximate kinetic results were used as a first approximation in calculations for the same conditions as in the experiments. The calculated values agreed with the measured apparent reaction rates within 27%. The influence of pore growth during burnout was shown to be significant; neglecting this growth would overestimate the total kinetic reaction rate and true activation energy.