Empirical Kinetic Models for the Combustion of Charcoals and Biomasses in the Kinetic Regime

Várhegyi, Gábor and Wang, Liang and Skreiberg, Øyvind (2020) Empirical Kinetic Models for the Combustion of Charcoals and Biomasses in the Kinetic Regime. ENERGY & FUELS, 34 (12). pp. 16302-16309. ISSN 0887-0624

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Abstract

An empirical kinetic model was proposed in 2019 and tested extensively on biomass pyrolysis (Várhegyi, G., Energy Fuels 2019, 33, 2348−2358). The model was based on an isoconversional kinetic equation. The functions in the kinetic equation were approximated by mathematical formulas with adjustable parameters, and the parameters were determined by the method of least squares. This procedure ensures that the data calculated from the model would be close to the experimental data. In the present work, this way of modeling was adapted for the combustion of charcoals and lignocellulosic biomasses. The performance of the model was tested by the reevaluation of TGA experiments from earlier publications. In total, 18 experiments belonged to a study of charcoals, while 20 experiments were carried out on wheat straw and willow samples. The corresponding temperature programs included linear, modulated, stepwise, and constant reaction rate (CRR) temperature−time functions. The adjustable parameters of the model were determined by the method of least squares by evaluating groups of experiments together. The procedure aimed at finding best-fitting models for the derivative of the measured reacted fraction. The activation energy, E, was regarded as constant for the whole process. The change of the reactivity during the progress of the reaction was described by the rest of the isoconversional kinetic equation. Model variants with different numbers of adjustable parameters resulted in practically identical E values. It was possible to determine common E values for different samples with only a slight worsening of the fit quality. This procedure allowed an easy comparison of the reactivities of the samples as functions of the reacted fraction.

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