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.

Increasing Sargassum accumulation across the Gulf of America and the Caribbean has raised environmental, economic, and public health concerns for coastal communities. As Sargassum decomposes, it can impair tourism, aquatic ecosystems, and air quality, highlighting the need for valorization strategies that convert collected Sargassum into useful products. This study evaluates two carbon-product pathways: Case 1, direct pyrolysis of dried Sargassum at 600.00 °C for 30 min; and Case 2, hydrothermal pyrolysis, in which wet Sargassum was hydrothermally carbonized at 220.00 °C for 30 min before pyrolysis. Process configurations, stream balances, capital costs, manufacturing costs, net present values (NPVs), and breakeven selling price sensitivity analyses were compared. It was seen that Case 1 outperformed Case 2, breaking even in year 5 and achieving a year 12 NPV of over 9.60 million USD, whereas Case 2 never broke even and had a year 12 NPV of −66.04 million USD. A breakeven cost sensitivity analysis showed that Case 1 was sensitive to the Sargassum acquisition cost, while Case 2 was sensitive to the hydrothermal reaction temperature. These results indicated that while direct pyrolysis was economically favorable under the modeled assumptions, hydrothermal pyrolysis could become competitive with additional environmental incentives or carbon credits.