Effect of Char Preparation Conditions on Gasification in a Carbon Dioxide Atmosphere

Pyrolysis conditions have a substantial impact on the properties of the resulting char and, consequently, on the kinetics of the gasification process. The aim of this study was to determine the impact of coal pyrolysis on the gasification stage. By applying the thermogravimetric method, kinetic analyses of CO2 gasification of chars derived from Polish "Janina" coal were conducted. Gasification examinations were performed for chars prepared earlier in an argon atmosphere at various heating rates, which, after cooling, were subjected to CO2 gasification (indirect char gasification). Examinations were also carried out in the case of chars formed during heating of coal samples in a CO2 atmosphere (direct char gasification)l-Samples of chars were gasified in. non-isothermal conditions of up to 1100 degrees C under 0.1 MPa pressure at various heating rates, The char gasification reaction order with CO2 was determined with the use of the Coats-Redfern method, and it can be assumed that it is a first-order reaction. The activation energy and pre-exponential factor were calculated using two first-order models: Coats Redfern method and Senum-Yang method. The results were subsequently compared to kinetic parameters calculated on the basis of the model free isoconversional method combined with the model-dependent Coats-Redfern method. Despite the differences in values of kinetic parameters obtained from the use of a given model, all results confirmed that the method of char preparation has an influence on the gasification stage and direct char gasification is more favorable. Activation energy obtained from the use of models based on the first-order reaction ranged between 275 and 296 kJ/mol for direct gasification of chars while the chars gasified indirectly between 307 and 342 kJ/mol, depending upon the heating rate that was used. The model-free isoconversional method confirmed these results. The values for chars gasified directly amounted to E-a = 257-277 kJ/mol, and the values for indirect char gasification obtained by the pyrolysis,process at heating rates of 3) 10, and 20 K/min amounted to E-a = 280-291, 287-309, and 289-305 kJ/mol, respectively.