Application of molten salt thermoelectric effect in biomass preparation of hydrogen-rich gas, porous biochar and molten salt regeneration

Hydrogen production from biomass pyrolysis is attractive since it allows for green hydrogen production through feedstock and thermal conversion. However, the key limiting factors for hydrogen production are the high oxygen content, uneven heating of biomass pellets during the slow heating process, and insufficient depolymerization due to low reaction temperatures (low gas yields and low hydrogen content). To address these challenges, fast pyrolysis of super Arundo in NaOH-Na₂CO₃ molten salt was carried out in this paper at 450 °C, 550 °C and 650 °C. Considering the sustainable operation and environmental problems caused by the consumption of molten salt, electrochemical electrolysis was proposed to regenerate the molten salt. The experimental results show that at 650 °C, the thermal conversion of biomass in the alkaline molten salt can prepare abundant hydrogen and simultaneously capture CO₂ in situ. Compared to conventional (salt-free) pyrolysis, the percentage of H₂ pyrolysis in molten salt increased from 13.11 % to 81.53 %, while CO₂ decreased from 28.68 % to 2.07 %. It is noted that the mass of H in the gas exceeds the amount of H in the biomass by a factor of 1.67. This indicates that the molten salt was involved in the biomass conversion. Besides, the pyrolyzed carbon (PC) prepared in this molten salt pyrolysis system has a high specific surface area (1960.07m²/g) with many hydroxyl functional groups. Some carbon material is also produced during electrochemical molten salt regeneration, resulting from CO₃²⁻ generated from the C component (C, CO, CO₂) entering the molten salt and re-separated to form OH- and C. Experimental results exhibit that the regeneration ratio of NaOH reaches 24.76 %. The coupling of molten salt thermochemistry and electrochemistry realized carbon-negative utilization of low-carbon resources, this study provides practical guidance for the continuous application of molten salt including regeneration/recycling of molten salt and high-value utilization of waste molten salt.