Influence of processing conditions and biochemical composition on the hydrothermal liquefaction of digested urban and agricultural wastes

Hydrothermal liquefaction (HTL) of waste biomass is a promising technology to yield an energy dense biocrude oil and a nutrient rich solid phase. An abundant and problematic biomass waste stream potentially suited for HTL is digestate, the side stream of anaerobic digestion. HTL behavior of three digestates from urban and agricultural biogas plants has been studied using response surface methodology to investigate the influence of biochemical composition, temperature, and reaction time on biocrude yield and composition, energy recovery, as well as the mobility of nutrients (N, P, K) among the product phases using KOH as catalyst. A maximum energy recovery of up to 43 % and a carbon recovery of 40 % were achieved for the straw/manure digestate. Mass yields and biocrude composition by GC-MS analysis are shown to be heavily influenced by feedstock composition. High yields of > 80% for the main nutrients Ca, Mg, Fe, and P to the solid residue are observed.