Sustainable Valorization of Forest Waste Hydrolysis Residues to Solid Biofuel: Insights into Conversion Mechanisms and Fuel Properties

The conversion of lignocellulosic biomass into high-value fermentation products generates a lignin-rich hydrolysis residue (LRR), which is predominantly combusted for process heat, offering limited valorization potential. This study investigates the hydrothermal carbonization (HTC) of this residue derived from forest residue biomass (FRB) to produce high-energy-density hydrochar. HTC, a thermochemical conversion process conducted in the presence of water, enables direct processing of wet lignin-rich residues without the need for drying or solvent-based lignin extraction or purification, thereby reducing costs and complexity. Experiments were conducted at 200–280 °C, with a fixed reaction time of 1 h, and the resulting hydrochars were thoroughly characterized for their chemical composition, structural morphology, and thermal behavior. Thermogravimetric analysis confirmed improved pyrolysis properties of the HTC products. Hydrochar yield decreased by 26.26% as the temperature increased from 200 to 280 °C, accompanied by marked improvements in fuel quality. The maximum higher heating value, observed at 280 °C, was 1.75 times greater than that of raw LRR. Elemental analysis and a Van Krevelen evaluation confirmed enhanced carbonization, as evidenced by increasing carbon content and decreasing oxygen content. The specific surface area peaked at 2.66 m2/g at 200 °C before declining with further temperature increases. This study demonstrates a sustainable pathway for valorization of lignin-rich residues from lignocellulosic biorefineries into solid biofuels, advancing circular bioeconomy and offering insights into using HTC for energy and environmental applications.