Kinetic modelling and elemental balances applied to the hydrothermal liquefaction of sewage sludge

Hydrothermal liquefaction (HTL) appears as the most promising solution for converting wet biomass, including sewage sludge, into valuable bio-crude oil while offering advantages such as waste volume reduction, energy and resource recovery, and reduced greenhouse gas emissions. In this study, a kinetic model including four lumped species (i.e. solid residue, aqueous phase, bio-crude and gas) along with a first-order ash dissolution kinetic model was developed based on the experimental data obtained at a heating rate of ∼8 °C min−1 and temperatures of 150–370 °C, utilizing a 500 mL batch apparatus for HTL. While the kinetic parameters were determined from curve-fitting of data obtained at 300 °C and 350 °C, the predictive ability of the model was verified against experimental data obtained at 370 °C. Furthermore, ultimate analysis of lumped species at various operating conditions were used to perform elemental balances and make generalizations about the time evolution of CHNSO elements and ash distribution among various lumped species during the course of HTL. The findings of this study allow determination of both the quantity and quality of all lumped species with minimum reliance on experimental measurements. This modelling approach can be used for optimization of sewage sludge HTL process efficiency in terms of bio-crude selectivity and quality, and wastewater valorisation.