Role of Accumulated Alkali and Alkaline-Earth Chloride Salts in Hydrochar Formation Produced from Lignocellulosic Biomass

Hydrothermal carbonization (HTC) is an effective method for recovering useful carbon-rich solids from biomass and wet organic waste. This study investigated the effects of the type and amount of alkali and alkaline-earth chloride salts (AAECS) in HTC media on the properties of cornstalk hydrochar by analyzing the yield, elemental composition, surface functionality, and morphology, as well as fuel characteristics. The results suggest that the abundance of AAECS in HTC media may increase the aromaticity and energy recovery of hydrochar by facilitating the dehydration and decarboxylation of biomass. Both anions and cations in AAECS are responsible for these variations, especially for Ca2+, which can selectively affect the dehydration reaction of biomass by promoting the formation of anhydride and inhibiting the reaction between carboxyl and hydroxy to form a lactone. In addition, the high concentration of AAECS in HTC media is beneficial for improving the electron-donating capacity of hydrochar. This work could provide important insights into AAECS' effects on hydrochar preparation, which arose from the recirculation of HTC aqueous solution.