Empirical models with constant and variable activation energy for biomass pyrolysis

Várhegyi, Gábor (2019) Empirical models with constant and variable activation energy for biomass pyrolysis. ENERGY & FUELS, 33 (3). pp. 2348-2358. ISSN 0887-0624 (nyomtatott); 1520-5029 (online)

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Abstract

The so called “model-free” or isoconversional way of kinetic modelling was examined. In this field the available evaluation methods do not aim at an optimal fit for the experimental data. In the present work the functions of the corresponding kinetic equation were approximated by simple versatile formulas; the number of the unknown parameters was kept on reasonably low levels; and the evaluation aimed at the best fit for the experiments by the method of least squares. The considerations and methods were tested on 85 thermogravimetric (TGA) experiments which had been published earlier with different types of kinetic modelling. The experiments belonged to 16 biomass samples including woody biomass, agricultural residues and industrial wastes. The temperature programs comprised constant heating rates, stepwise heating, constant reaction rate heating (CRR), isothermal temperature programs, and a modulated temperature program. The evaluations were based on four to nine experiments for each sample. The best fit was searched for the mass loss rate curves because they reflect better the peculiarities of the pyrolysis than the integral curves. An empirical model with variable activation energy provided good fit for all experimental data. It described the experiments of a biomass sample by 11 parameters. Another model with constant E values provided rougher, but still usable approximations for the data. It allows fast numerical solutions that may be helpful in complex modelling tasks. Both models described the experiments at a variety of temperature programs by a given set of model parameters.

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