Hydrothermal liquefaction of waste biomass in stirred reactors: One step forward to the integral valorization of municipal sludge

Hydrothermal liquefaction (HTL) of municipal sludge (MS) was performed at 350 °C for 30 min (subcritical water) and at 400 °C for 0 min (supercritical water) at fixed kinetic severity (LogR0 = 8.9) in static and stirred batch reactors to study the effect of the flow regime on the energy recovery (ER) of the process and on the quality of the products. With adopted experimental procedures it was possible to reduce to less than 10% the yield of lost organic compounds, termed volatiles (VT), and to collect and quantify a liquid hydrocarbon fraction (HC) separated from the biocrude (BC). The highest value of the HC yield, 25% w/w, was obtained in supercritical conditions. The C content of the solid residues (SR) and the H/C ratios of the BC increased when the stirred reactor was used. Quite interestingly in this research we have found, to the best of our knowledge for the first time, that the cumulative ER of the product phases was significantly higher than 100% thus indicating that HTL of MS can be energetically driven by renewable thermal energy, such as solar heat, offering a storage option for it.