Study on pyrolysis of Organosolv lignin impregnated with alkali and alkaline earth metals: Kinetics, thermodynamics, and product characterization

The pyrolysis characteristics and kinetics of Organosolv lignin from pine trees were investigated using isoconventional, generalized master plot, model-based, and Frazer-Suzuki deconvolution methods. With these approaches, the activation energies of Organosolv lignin pyrolysis were determined to be in the range of 70.11–385.58 kJ/mol. In addition, the activation energies of 3 pseudo-reactions were calculated to be 15.71, 204.49, and 32.76 kJ/mol, respectively, by the Frazer-Suzuki deconvolution method. The experimental data of Organosolv lignin pyrolysis were best fitted with the 4th power-law model (P4) with an absolute error of 3.24 %. Entropy (ΔS^o), Gibbs free energy (ΔG^o), and enthalpy (ΔH^o) were also calculated to understand the reaction pathways from a thermodynamic point of view. Based on the pyrolysis mechanisms proposed in this study, the reaction rate constants of different steps were determined. The primary reaction route was identified to be the pyrolysis of Organosolv lignin to liquid products such as bio-oils. Among the alkali and alkaline earth metals (AAEMs) tested, 2.0 wt% Mg showed the most effective on Organosolv lignin pyrolysis, decreasing the mean activation energy (E_a) from 181.67 to 156.55 kJ/mol in the range 0 ≤ X ≤ 0.85. The compositions of gaseous and liquid products formed by pyrolysis were analyzed using a micro-tubing reactor. CO, CO₂, and CH₄ were observed as the main gaseous products, while Organosolv lignin was primarily decomposed into phenol and guaiacol derivatives. The Organosolv lignin was also depolymerized into lower-molecular-weight (LMW) components during the pyrolysis process.