According to EPA, in 2019 37.5 million tons of food waste were disposed of in landfills in the US. Hydrothermal processing is a promising method to convert wet food waste without an energy-intensive drying step necessary for other thermochemical alternatives such as gasification and pyrolysis. Hydrothermal carbonization (HTC) treats wet biomass at a temperature of 180 – 250 °C and pressure up to 8 MPa to generate a hydrochar phase; a coal-like material used as a solid fuel, soil amendment, or activated carbon precursor. Similarly, hydrothermal liquefaction (HTL) treats wet biomass at an elevated temperature between 260 – 400 °C and elevated pressures; conditions that shift the product distribution towards bio-oil formation, a biofuel precursor. Despite the similarities between HTC and HTL processing, they are often considered two distinct processes without comparison amongst themselves. While HTC research focuses on the hydrochar properties and yield, common liquefaction metrics include the composition and yield of the oil phase.<br/>To directly compare these processes, we ran a series of experiments using a representative food waste mixture at temperatures between 190 and 350 °C. Secondary char, an oily phase formed by the polymerization of chemicals in the water phase during hydrothermal processing that sticks to the hydrochar, was extracted through washing with six solvents. Methanol and ethanol solvents showed the greatest extractability of secondary char. GC-MS characterization reveals the presence of short chain acids and aldehydes for samples treated at lower temperatures, while at least 20 wt% of the alcohol extracted into a secondary char phase consists of C14-C16 long chain acids at higher temperatures; likely formed from hydrolysis of the lipid fraction of the food waste and consistent with biooil products found in HTL. This work sheds light on transition between these processes and the fundamental role secondary char plays in hydrothermal processes.

Hydrothermal carbonization and liquefaction of food waste: The role of secondary char

ACS Spring Meeting 2021

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