Green solvents to enhance hydrochar quality and clarify effects of secondary char

Several limitations hinder the industrial-scale implementation of hydrothermal carbonization (HTC) of biomass, especially the quality of as-carbonized hydrochar. This work investigates solvent extraction of hydrochars to enhance their potential applications. Hydrochars were produced at several HTC temperatures (190, 220, 250 °C) from cellulose and extracted using combinations of green polar solvents (ethyl acetate, acetone, and methanol). Results show that the composition of the extractable fraction resembles that of the HTC liquor, rich in carboxylic acids and furan derivatives, while the non-extractable solid phase shows improved thermal profiles devoid of highly volatile compounds. Carbon microspheres (non-dissolvable secondary char) are unaffected by extraction. The organics adsorbed on the hydrochar surface comprise highly volatile species and solvent washing effectively removes them.