Insights into the temperature dependence of reaction pathways in hydrogen production from model biomass via NaOH thermal treatment

Alkali thermal treatment (ATT) of biomass integrates high-quality H2 production and in-situ carbon storage. Herein, the dominant reactions at different temperatures and the critical roles of NaOH in the formation of H2 and CH4 during ATT were revealed for the first time. NaOH promotes the conversion of carbon from glucose into both solid and gas phases. H2 is primarily derived from NaOH-promoted dehydrogenation of glucose and the alkalinization of active char with NaOH near 300 degrees C. The alkalinization of graphitized char above 400 degrees C contributes to H2 production in the second stage. Meanwhile, CH4 is produced via NaOH-promoted methanation of active char near 330 degrees C. The optimized formula considering actual H2 and CH4 production indicates the NaOH: glucose stoichiometric ratio of 9:1 and H2:CH4 volume ratio of 4:1. A reaction pathway for ATT of glucose is developed to elucidate component distributions in the gas, liquid, and solid phases. The Sankey diagram of energy flow demonstrates an energy recovery of 80.48% for H2 and CH4. Especially, the recovery of NaOH by causticizing reaction can reach 98.28%. This study will contribute to establishing methods aimed at essentially improving the purity and yield of H2 production, thus accelerating the practical application of actual biomass in ATT technology.