Evaluation of carbon dioxide gasification kinetics on the basis of non-isothermal measurements and CFD modelling of the thermogravimetric analyser

The aim of this study was a thorough analysis of the kinetics of gasification process in an atmosphere of CO2. Firstly, kinetics parameters were determined on the basis of experimental data from non-isothermal thermogravimetric (TGA) measurements at tree heating rates (3 K/min, 10 K/min, 20 K/min) by using Senum&Yang (SY) method. Then, due to the shortcomings of thermogravimetric analysis the three-dimensional CFD-based model were developed to predict heating and decomposition of the coal sample inside the TG analyzer. The model has demonstrated good agreement with the measured data. Moreover, CFD modelling indicates that using a lower heating rate during TGA tests one may estimate more accurate kinetic parameters when compared to the measurements at higher heating rates. This is due to the fact that higher heating rates lead to the formation of larger temperature gradients within the retort and in consequence higher temperature difference between coal sample and heating wall (11 K for 3 K/min, 16 K for 10 K/min and 18 K for 20 K/min). Moreover, discrepancies in heating rate during experiment have significant influence on accuracy of SY method (error up to 20%). Therefore, when the non-isothermal experiments are conducted, the inverse procedure (based on numerical integration of reaction equation and Levenberg-Marquardt algorithm) is preferred for determination of kinetic parameters. Obtained in that way values of activation energy, calculated on the basis of measured (TG) temperature, ranged from 253.7 to 231.2 kJ/mol, for used heating rates, whereas the adjusted values based on CFD modeled temperature were slightly lower i.e. from 247.8 to 223.6 kJ/mol. The second of the kinetic parameters - pre-exponential factor also decrease with the increase in heating rate and where higher than corrected values calculated on the basis of modelled sample temperature. The smallest differences in calculated kinetic parameters were observed during measurement at heating rate of 3 K/min.