Char particles of high ash Indian coal with different sizes are produced using thermogravimetric method at different heating rates. Particle diameter and porosity changes during devolatilization significantly affect char gasification rates. The effect of coal particle size on char production under various heating rates is reviewed. Char gasification is affected by operating conditions such as reaction temperature, char production method, and particle size in addition to chemical composition and physical structure of char. The measurements of the steam and CO2 gasification rate of the produced chars are performed by an isothermal thermo-gravimetric analysis at the temperatures of 900, 950, and 1000 degrees C under ambient pressure conditions. It is found that the reactivity of char gasification is increased along with heating rate of char. Three kinetic models are applied to describe the varying conversion rate: volumetric model, grain model, and random pore model. The activation energy of the gasification is varied based on the char generation method and particle sizes. The activation energy is varying from 122 to 177 kJ mol (-1) for steam gasification and 130 to 214 kJ mol (-1) for CO2 gasification to different sized char particles. The effect of char heating rate on gasification is more pronounced in the activation energies of smaller sized particles. (C) 2015 Elsevier Ltd. All rights reserved.