Uncovering the transition between hydrothermal carbonization and liquefaction via secondary char extraction: A case study using food waste

Abstract

Despite the ability to perform both processes in the same reactor, hydrothermal carbonization (HTC) and hydrothermal liquefaction (HTL) are considered two distinct processes differentiated by their reaction temperatures. As temperatures increase from the less severe HTC range into the HTL regime, the product distribution progressively favors an organic bio-oil phase relative to solid hydrochar. Solvents are commonly used to extract bio-oil from the solid residues produced during HTL, and to separate the amorphous secondary char from the coal-like primary char of HTC hydrochars. This suggests secondary char is a HTL biocrude precursor. Lipid-rich food waste was hydrothermally processed between 190 and 340 °C, spanning HTC to HTL conditions. Higher temperatures produce more gas, less liquid, and similar amounts of a progressively less oxygenated hydrochars, suggesting a gradual transition from HTC to HTL. However, analyses of ethanol-separated primary chars and secondary chars tell a different story. While the primary char is progressively more carbonized with temperature, the secondary char composition sharply changes at 250 °C. That is, lipid hydrolysis begins around 220 °C, but proceeds rather completely at 250 °C and above. A lower HTL temperature reduces the energy cost of the hydrothermal process, yet enables full lipid hydrolysis into long chain fatty acids while minimizing recondensation and repolymerization of fatty acids onto the primary char and their subsequent amidation. This maximizes the conversion of lipid-rich feedstocks into liquid fuel precursors with up to 70 % energy recovery.

Keywords: Food waste; Hydrothermal carbonization; Hydrothermal liquefaction; Secondary char; Solvent extraction.