Hydrothermal liquefaction as a treatment technology for anaerobic digestate: A review
Christian Klüpfel, Bomin Yuan, Patrick Biller, Benjamin Herklotz
Abstract
Digestate, the side-product of anaerobic digestion (AD), poses both an environmental and economic problem. Hydrothermal liquefaction (HTL) is discussed as a potential process to treat this waste biomass and yield valuable products, e. g. biocrude. This review outlines the state of the art of digestate treatment and assesses the state of knowledge concerning the HTL of digestate. Options for process integration and synergies in the framework of the proposed biorefinery concept consisting of HTL and AD are discussed. Optimum biocrude yield in the reviewed literature is usually achieved in the range of 300–350 °C and 15–30 min. Feedstock composition varies depending on origin of the digestate. Product characterisation elucidates phenols, fatty acids, and N-heterocycles in the biocrude and significant nutrient load in the hydrochar. Further research should address feedstock dependency, aqueous phase treatment and hydrochar utilisation to develop the process towards large scale application. • Synergy of AD and HTL shows potential for enhanced biomass valorisation. • Optimum processing conditions are found in the range 300–350 °C and 15–30 min. • Digestate origin and HTL conditions significantly affect biocrude composition. • Model-based prediction is impeded by the low availability of data. • Nutrient and physical properties of the hydrochar point towards soil application.