Modeling the nonisothermal devolatilization kinetics of typical South African coals

Abstract

Multicomponent model fitting was conducted in order to evaluate the devolatilization rate behavior of four typical South African coals, with the aid of nonisothermal thermogravimetry. Rate evaluation was conducted at four different heating rates (5, 10, 25, and 40 K/min) by heating the samples under an inert N2 atmosphere to 950 °C. Evaluation of the kinetic parameters of each coal involved the numerical regression of nonisothermal rate data in MATLAB 7.1.1 according to a pseudocomponent modeling philosophy. The number of pseudocomponents used ranged between three and eight, as larger values induced the risk of over fitting. Quality of fit (QOF) was found to decrease with decreasing heating rate as a result of improved separation of the individual component reactions at the lower heating rates. All four coals showed the occurrence of similar pseudocomponent reactions, although significant differences were observed in the fractional contributions of the different pseudocomponents to the overall reaction rates. Modeling results indicated that the assumption of eight pseudocomponents produced the lowest QOF values and subsequently the best fit to the devolatilization profiles of each coal. For the vitrnite-rich coals (G#5 and TSH), no remarkable decrease in QOF could be observed after 6 pseudocomponent reactions, suggesting that even 6 or 7 pseudocomponent reactions would have provided accurate experimental predictions. Activation energies determined from the selected number of pseudocomponents (between 3 and 8) were found to range between 20 and 250 kJ/mol