Stability Investigation of Extracted Lignins from Bagasse, Coconut Husk, RiceStraw, and Corn Stover: Kinetic and Thermodynamic Aspects

Abstract

The influence of different biomasses on lignin extraction impacts the thermal stability of lignin. Four extracted lignins from the soda pulping process were prepared from agricultural wastes, including bagasse, coconut husk, rice straw and corn stover. Kinetic and thermodynamic analyses were utilized to compare and investigate the thermal-oxidative stability behavior of all lignins. Experiments were conducted using the non isothermal method for four heating rates with a thermogravimetric analyzer. The Friedman, FWO, KAS and Starink kinetic methods were used to investigate the oxidative kinetics of lignins. Thermodynamic parameters involving enthalpy (ΔH), Gibbs free energy (ΔG) and entropy (ΔS) were considered for observing thermal characteristics. Thermal degradation of lignin consists of three consecutive regimes: moisture content, lignin degradation and decomposition of residues. All kinetic models showed average activation energies between 132.89 and 144.52 kJ/mol, 109.75 and 121.72 kJ/mol, 156.62 and 167.98 kJ/mol and 160.11 and 171.64 kJ/mol for BG, CH, RS and CS, respectively. The coefficient of determination revealed that all models are promising kinetic methods for calculating kinetic parameters. The fluctuation of kinetic and thermodynamic parameters showed that the thermal oxidative degradation of lignin was a complicated mechanism. The conversion process corresponds to a non-spontaneous endothermic reaction. The results provide valuable information to deeply understand the thermochemical conversion for characterizing the thermal stability of lignins.