Fast and isothermal hydrothermal liquefaction of sludge at different severities: Reaction products, pathways, and kinetics

Hydrothermal liquefaction has emerged as a preferred means of converting wet biomass to biocrude. In this study, we examined the hydrothermal conversion of sludge over a large range of conditions (300–600 °C, 1–60 min) that included both nominally isothermal processing and non-isothermal processing (rapid heating, short times). An empirical severity index was employed to combine the effects of reaction temperature and holding time into a single parameter. The highest biocrude yields (20.1–30.9 wt%) were achieved at 10⁻¹ < severity index < 10² and about 32% and 63% of the nitrogen in sludge transferred into the biocrude and aqueous phase products, respectively. A Van Krevelen diagram illustrated that dehydration and decarboxylation pathways were important for biocrude and light biocrude formation whereas dehydration paths were more important for heavy biocrude. We developed a quantitative kinetics model that faithfully correlated the effects of time and temperature on the yields of biocrude, aqueous phase products, solid, gas, and volatiles. It also accurately predicted the yields of the product fractions from hydrothermal liquefaction at 350 °C.