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_α 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(α) = 1 - α). The thermo-kinetic behavior obtained by isoconversional methods (Fr and KAS) of the hemicellulose degradation shows a progressive and monotonic increase in E_α with the conversion, between ~140 and ~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_α on α, shows the typical behavior of a reaction controlled by a single rate-determining step, with constant average E_α values of ~209 kJ mol^-1. Meanwhile, the lignin pyrolytic degradation shows an increase in E_α from ~220 up 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.