A comparative study on CO2 gasification kinetics of flax straw char produced by two different processes, namely, torrefaction and pyrolysis was investigated using a TGA (thermogravimetric analyzer). The objective of this research was to investigate the diffusion and surface reactions that occur during the CO2 gasification of the torrefied and pyrolyzed char particles. Gasification experiments were carried out at four different gasification temperatures from 750 to 900 degrees C and for different particle sizes (<90 mu m-925 mu m). The effects of temperature, particle size, and the char type on the rate of CO2 gasification were determined. The 50% conversion rate showed an increase in the reactivity values, along with an increase in the temperature, and decreased with an increase in the particle size for both the char types. The diffusional parameters such as effective diffusivity, effectiveness factor, and the DLI (diffusion limitation index) were calculated from the experimental data for both the char types. BET (Brunauer-Emmett-Teller) analysis and SEM (Scanning Electron Microscopy) analysis were performed to determine the surface area and to study the structural morphology of the char types. Torrefied char had a greater surface area than the pyrolyzed char. (C) 2013 Elsevier Ltd. All rights reserved.