Kinetics of the CO2 gasification of woods, torrefied woods, and wood chars. Least squares evaluations by empirical models

The gasification kinetics of chars forming from biomass materials was studied by kinetic equations of type dX/dt = Af(X) exp(- E/(RT)) where X is the conversion of the sample, A is the pre-exponential factor, E is the activation energy and f(X) is a suitable model function. The theoretically deduced f(X) models in the literature are rarely applicable for chars of biomass origin because of chemical and physical inhomogeneities and irregularities. Hence, empirical f(X) functions were determined by a method proposed four years ago (Varhegyi in Energy Fuels 33:2348-2358, 2019). The parameters of the models were obtained by the method of least squares. Thermogravimetric experiments from an earlier work were reevaluated to explore the possibilities of the approaches employed. The experiments belonged to untreated birch and spruce woods; torrefied woods; chars prepared at a higher temperature; and chars formed at high heating rates (ca. 1400 degrees C min(-1)). Common kinetic features were found for the CO2 gasification of the chars studied. The reliability of the results was carefully tested by evaluating smaller and larger groups of the experiments and comparing the results. The method proved to be suitable for the determination of realistic f(X), E, and A from single modulated experiments, too. The models described well the gasification of chars forming from different woods through a wide range of temperature programs and thermal pretreatments.