Pyrolytic degradation of peanut shell: Activation energy dependence on the conversion

This study focuses on the thermo-kinetic analysis of solid peanut shell waste, through dependence of the activation energy with the conversion degree. Three model-free kinetics, Kissinger (K), Friedman (Fr) and Kissinger-Akahira-Sunose (KAS), were applied to thermogravimetric (TGA) data to calculate the effective activation energy E(alpha )during a pyrolysis process. The results obtained by Kissinger's method showed that the pyrolytic breakdown pathway of hemicellulose, cellulose, and lignin in a ligno-cellulosic biomass is independent of the heating rate and can be described through a simple first-order kinetic reaction (f(alpha) = 1 - alpha). The thermo-kinetic behavior obtained by isoconversional methods (Fr and KAS) of the hemi-cellulose degradation shows a progressive and monotonic increase in E-alpha with the conversion, between similar to 140 and similar to 195 kJ mol(-1 )with an average value of 172 kJ mol(-1), which reveals the competitive character of the process (multi-step process). Conversely, in the cellulose degradation, the dependence of E-alpha on alpha, shows the typical behavior of a reaction controlled by a single rate-determining step, with constant average E-alpha values of similar to 209 kJ mol(-1). Meanwhile, the lignin pyrolytic degradation shows an increase in E-alpha from similar to 220 up to similar to 300 kJ mol(-1 ) with the conversion, indicating that this stage is kinetically controlled by an energy barrier that comprises multiple and simultaneous processes. Finally, the kinetic analysis confirmed the absence of autocatalytic reactions (thermally auto-catalyzed process) during the pyrolysis, although the global process is highly exothermic. (C) 2020 Elsevier Ltd. All rights reserved.