Lignin-driven hydrogen production from biomass via supercritical water gasification: Temperature-dependent efficiency and mechanistic pathways

Hydrogen production via biomass supercritical water gasification (SCWG) presents a promising pathway toward carbon-neutral energy systems, yet the role of lignin—a recalcitrant biomass component—in gasification efficiency and reaction mechanisms remains underexplored. This study systematically investigates SCWG of four lignocellulosic feedstocks with varying lignin abundance (water hyacinth, corn straw, pine sawdust, walnut shell) at 450–600 ℃, elucidating lignin’s temperature-dependent influence on hydrogen yield, selectivity, and reaction pathways. The effects of temperature and lignin abundance on the distribution of SCWG three-phase products were investigated through detailed molecular characterization. The results show that higher reaction temperatures significantly boost total gas yield and hydrogen selectivity, promoting steam reforming and methanation reactions. Critically, lignin’s contribution to gasification efficiency shifts from negligible at 450–500 ℃ to dominant at 550–600 ℃, with walnut shell (highest lignin: 38.2 wt%) achieving peak hydrogen gasification efficiency (98.93 %) and selectivity (61.65 %) at 600 ℃. Liquid-phase analysis identifies phenolic compounds as terminal refractory intermediates, peaking at 79.90 % concentration and underscoring lignin’s resistance to degradation. By decoupling the transformation pathways of cellulose, hemicellulose, lignin, and plant proteins, this work reveals that lignin-derived aromatics act as hydrogen sinks below 500 ℃ but become key hydrogen precursors at higher temperatures. These findings provide a mechanistic blueprint for optimizing SCWG processes tailored to biomass composition, advancing scalable and efficient hydrogen production. The study establishes lignin abundance as a critical lever for tuning gasification outcomes, offering actionable strategies to enhance the sustainability and practicality of SCWG in industrial applications. • Temperature-dependent effect of lignin on H 2 yield was elucidated. • Phenol and furfural serve as the terminal refractory intermediates. • Reaction pathways of cellulose, hemicellulose, lignin, and protein were decoupled. • Dual function of lignin: H 2 sink (<500 ℃) and critical H 2 precursor (>550 ℃).