Coupling Hydrothermal Carbonization of Digestate and Supercritical Water Gasification of Liquid Products

This work investigates the coupling of hydrothermal carbonization (HTC) and supercritical water gasification (SCWG) as a combined process to fully valorise digestate from anaerobic digestion. Firstly, HTC was performed at three different temperatures (180, 220 and 250 °C) to convert digestate to hydrochar (HC) and aqueous HTC liquids (AHLs) and then, SCWG was performed on AHLs to produce a H2-rich gas. Different SCWG temperatures (500, 550 and 600 °C), residence times (15, 30, 60 and 90 s) and AHLs concentrations (6.7, 13.6 and 20.0%) were tested. Increasing SCWG temperature resulted in an increase of the carbon yield and H2 content in the gas phase, for increasing residence time the carbon yield in the gas phase decreased, while increasing the feedstock concentration led to a higher gas generation rate without affecting the carbon yield of the gas. The maximum carbon yield in the gas phase (51%) was reached for tests run at 600 °C, 15 s, and 6.7% of AHLs, the maximum hydrogen content in the gas phase (79%) was reached at 600 °C, 30 s, 6.7% of AHLs, the maximum gas generation rate (9.33 mL min-1) was reached at 600 °C, 30 s, 20.0% of AHLs.