Kinetic Modelling of Hydrothermal Lignin Depolymerisation

Although lignin is one of the most abundant renewable organic materials in the world, it is principally a waste product of the paper industry which is used for the production of heat and power. Hydrothermal lignin depolymerisation aids in facilitating the valorization of lignin in aqueous solutions or suspensions. For the recovery of valuable phenolic products from lignin it is crucial to understand the main reaction pathways of lignin degradation and the reaction kinetics. Batch experiments were carried out for studying the depolymerisation of an enzymatic hydrolysis lignin from spruce wood in near critical water. Phenolic products were extracted from the aqueous phase and quantified via gas chromatography. The main reaction products were grouped (lumped), the main reaction pathways of hydrothermal lignin depolymerisation were discovered and formal kinetic rate coefficients were determined. Optimization of these formal kinetic parameters yielded a satisfying approximation of the experimental yields of phenolic products and describes the most important tendencies over temperature and residence time of solid residue and gas. The model is validated by the comparison with other kinetic studies of the degradation of lignin as well as the decomposition of intermediate phenolics, such as catechols and methoxyphenols.