Use of autocatalytic kinetics to obtain composition of lignocellulosic materials

Abstract

Non-isothermal thermogravimetric analysis (TGA) data of biomasses and pulps originating from non-wood and alternatives materials (i.e., Tagasaste or rice straw) have been fitted with refined models, which include autocatalytic kinetics. Data sets were obtained for different experimental conditions, such as variations of heating rate and atmosphere, i.e., inert (pyrolysis) versus oxidative atmosphere (combustion). Besides the access to classical kinetic parameters (pre-exponential factor, activation energy, and reaction order), the improved data analysis enabled the determination of the chemical composition of the samples (cellulose, hemicellulose, extractives, lignin). The latter compared very well with those obtained by conventional methods (chemical analysis, HPLC). Given the reduced environmental impact and rapidness of the method, potential applications for research related to new biomasses and industrial processes can be foreseen. The herein implemented method is based on the assumption that samples contain pseudo-components, which independently degrade, and that combustion is the combination of an initial volatilization process (similar to pyrolysis) and a subsequent char oxidation process. Further, it was found that for a reliable modeling of the volatilization stage, extractives should be considered as well, together with the classical pseudo-components: hemicellulose, cellulose and lignin. The char oxidation stage has been simulated as a sum of the oxidation of three char types, one for each main pseudo-component. Importantly, fitting of TGA curves under consideration of autocatalytic kinetics allows the determination of a consistent set of kinetic parameters at different heating rates and leads to significant suppression of the compensation effect. While autocatalysis (characterized by the nucleation order) is not very significant for pyrolysis of biomasses, it can reach high levels for combustion, especially when high heating rates are used. In cellulosic char oxidation a nucleation order larger than one was fitted. The autocatalysis level of the char oxidation can rapidly increase with small modifications of the heating rate (i.e. to pass from 5 to 10 °C/min). In this case, the classically applied nth-order kinetic is particularly insufficient to fit experimental data with the same set of the kinetic parameters.