High-Yield Hydrogen Production from Lignin via Optimized Alkaline Thermal Treatment with CO₂ Sequestration

Current thermochemical methods for hydrogen (H₂) production, such as coal gasification and steam reforming, inevitably produce anthropogenic CO₂. In contrast, biomass-derived H₂ offers a carbon–neutral pathway. Despite its high energy density, lignin—accounting for ~ 30% of lignocellulosic biomass—has been underutilized. This study presents an alkaline thermal treatment (ATT) process that efficiently converts lignin into high-purity H₂ while minimizing CO₂ emissions by sequestering carbon as solid carbonate, potentially rendering the process carbon negative. Compared to conventional gasification, the ATT process operates at 170–450 °C lower temperature. Under optimized conditions, lignin-ATT produced 116.02 mmol H₂/g lignin (2.6 L H₂/g lignin), the highest H₂ yield reported for lignocellulosic biomass, far exceeding cellulose-ATT (39.07 mmol H₂/g cellulose), with a maximum H₂ purity of 94.75%. Importantly, this study confirms that stoichiometric NaOH addition enables H₂ production reaching nearly 97% of the theoretical maximum yield at significantly lower temperatures than steam gasification. The reactivity of different alkaline hydroxides is also compared. This study also demonstrates a sustainable approach using black liquor, real waste lignin, and a NaOH recycling system. This strategy reduces the production costs and generates CaCO₃ as a valuable by-product.