Lignin is an abundant byproduct of the pulp and biorefinery industries, remaining underutilized despite its high carbon content and potential for biofuel production. Currently, its applications are limited primarily to low value uses, such as combustion for energy recovery, with only a small fraction being converted into value-added chemicals. Hydrothermal liquefaction (HTL) presents a promising pathway for lignin valorization by converting it into biocrude, a potential renewable fuel. This study investigated the HTL of four distinct waste lignin feedstocks, known as alkaline, dealkaline, organosolv, and lignosulfonate lignin, to assess their biocrude yield and properties. Experiments were conducted at temperatures ranging from 270 to 310°C, with a 30-minute retention time and a 1:8 dry lignin-to-deionized water ratio. Biocrude was separated using dichloromethane and analyzed using elemental analysis, energy content, thermogravimetric analysis, and GCMS. The biocrude yield from alkaline and organosolv lignin varied between 14.9±0.3% to 16.8±0.3% and 14.3±1.3% to 17.1±2.5%, respectively, which is higher than the yield from dealkaline and lignosulfonate lignin. Biocrude yield varied with temperature, increasing for alkaline lignin while decreasing for lignosulfonate at higher temperatures. Alkaline lignin biocrude showed the highest energy content of 31.0±0.2 MJ/kg at 290°C. Additionally, biocrude from lignosulfonate and dealkaline lignin contained significantly higher sulfur content than organosolv and alkaline lignin, which may influence upgrading strategies. Thermogravimetric analysis revealed that biocrude from all lignin types predominantly consists of C8-C20 compounds, with their proportion increasing at higher HTL temperatures. These findings highlight the impact of lignin type on HTL performance and biocrude quality, providing insights for optimizing waste lignin valorization into sustainable biofuels.
