Lignin Liquefaction: Unraveling the effect of process conditions and sustainable pathways for biofuel production– A comprehensive review

Biomass thermochemical conversion into liquid fuels offers a sustainable alternative to traditional fossil fuels. The progressive shift from fossil fuel-based chemicals to bio-oil, obtained from abundant lignocellulosic biomass, reflects a more eco-conscious approach. Lignin, generated during papermaking and biomass fractionation at a global production volume of approximately 100 million tons per year, holds significant potential as a source for valuable aromatic molecules. These include monocyclic compounds like phenols, vanillin, and guaiacol, as well as bifunctional oligomers. However, depolymerizing lignin poses a significant challenge due to its complex chemical structure. There are various techniques for effectively transforming lignin into valuable liquid fuels. Among these, thermo-chemical liquefaction (TCL) of biomass has gained substantial attention as a promising method. This comprehensive review critically examines the intricate mechanism involved in the transformation of lignin into bio-oil, investigates the factors contributing to the increment of solid residue (reaction intermediates), and explores the wide range applications of bio-oil derived from lignin. Additionally, the review explores an analysis of the kinetics of the reactions involved and proposes sustainable solutions to address various techno-economic challenges encountered in the process. These challenges entail optimizing a multitude of process parameters, devising strategies to prevent reactor plugging, effectively controlling reaction intermediates, and implementing a cyclic approach to catalyst utilization. Drawing insights from the most recent literature in the field, the review meticulously outlines the latest advancements in the optimization of process parameters, with a particular emphasis on the role of catalysts and solvents in the enhancement of bio-oil.