Efficient Hydrogen Production by Biomass Pyrolysis in Molten Salts: Molten Salts Transformation and Green Regeneration

Abstract

Pyrolysis in alkaline molten salts provides a promising approach for efficient hydrogen production from carbonaceous feedstocks. However, molten salts react with inorganics inherent in feedstocks like biomass and organic solid waste, which in turn affects the organics conversion. The effect of inorganics on the conversion of feedstock and molten salts has not been adequately studied, which is indispensable for the practical operation of molten salt technologies. This study firstly provides insights into the transformation of molten NaOH-Na2CO3 and changes in the physicochemical properties of molten salts under the influence of biomass inorganics by experiments and simulations. Results reveal that ionic bond breaking/reorganization, eutectic salts melting and precipitates dissolution occur sequentially between the inorganics and the molten salts, forming stratification and suppressing hydrogen production. Molten salts form new eutectic salts with NaCl/KCl/SO42−/SiO2 addition, while Mg2+ and Ca2+ are converted into MgCO3/Mg2Cl(OH)3 and CaCO3 precipitates. Inorganics inhibit the hydrogen production in the order: Mg2+ > Ca2+ > K+ > Na+ and SO42− > SiO32− > Cl−. To overcome the stratification and deactivation of molten salts, an innovative green regeneration method is proposed. Under the optimized parameters, the NaOH recovery is 98.14%, the purity is 97.69% and the hydrogen production reaches 69.56 mmol/g-biomass.

Keywords: Molten salt transformation, Regenaration, pyrolysis, Biomass inorganics, Hydrogen