Effect of solvent and feedstock selection on primary and secondary chars produced via hydrothermal carbonization of food wastes

Hydrothermal carbonization is a thermochemical process that converts wet waste biomass into hydrochar, a renewable solid fuel that comprises a coal-like primary phase and an oily secondary phase. The varying oxidation rates of these phases may result in an inefficient energy recovery when combusting the hydrochar, as secondary char is more reactive. Brewer's spent grain, dairy cheese whey and food waste were hydrothermally carbonized at 250 °C. The hydrochars were extracted using six solvents to evaluate the hydrochar partitioning between primary and secondary char phases. Feedstock nature and solvent selection impact the amount and composition of these phases detected. For lipid-rich feedstocks, ethanol extracts up to 50 wt% secondary char enriched in liquid fuel precursors from a solid primary char with enhanced coal-like characteristics. For substrates rich in carbohydrates, proteins, and lignocellulose, less secondary char is produced. Acetone and dichloromethane remove the oily secondary char and maximize primary char yield.