Effect of Heating Rate on the Performances of HTL Applied to a Sewage Sludge

Abstract

The use of renewable energies and the energy exploitation of residual biomass have become prominent topics in the context of sustainable development goals. From a circular economy perspective, among the different types of biomasses, those defined as “waste” such as sewage sludge are of particular interest since significant volumes of municipal/industrial sludge are discharged into landfills at great cost to the industry and with associated negative environmental impacts. In this context, the hydrothermal liquefaction (HTL) process, working with water in sub-critical conditions, appears to be a promising and still limitedly explored route (as compared with other thermochemical processes) to obtain biofuel from biomass characterised by a high moisture content such as sludge. However, most of the literature studies are based on HTL experiments performed in small-scale batch reactors (generally a few mL), not allowing for proper assessing the effect of thermal transients, which instead occur in larger-scale systems, on product yields and quality. In this work, the set-up of a 500 mL lab-scale HTL apparatus was optimized so as to limit the duration of thermal transients, and preliminary tests were carried out on a municipal sludge to evaluate the yield and quality of the bio-crude obtained at different heating rates.