Integrating Anaerobic Digestion and Hydrothermal Liquefaction for Maximum Energy Recovery: A Techno-Economic Assessment and Comparison

Abstract

The increasing global energy demand combined with the growing global population requires technologies to sustainably provide energy. Arguably the most established scheme to convert wet waste biomass to energy is anaerobic digestion (AD) to produce biogas. The side product, digestate, can pose both ecological and economic challenges for the process. Hydrothermal liquefaction (HTL) can be employed to treat and valorize the digestate to produce biocrude oil. In this study, the integration of AD and HTL for sewage sludge, straw/manure, and biogenic household waste is techno-economically assessed and compared to the HTL of the respective undigested biomass. Higher biocrude yield, and thus energy recovery, was found when using the undigested biomass, yet the overall energy recovery is higher by up to 2-fold when combining AD and HTL. The experimental data was used to inform a process model using Aspen Plus software. A preliminary assessment at an input mass stream of 10 kt a–1 reveals minimum biocrude selling prices (MBSP) of biocrude oil between 4.6 and 16.4 € kg–1 depending on feedstock and processing temperature. The economically most viable scenario for both AD + HTL and HTL alone was straw/manure due to higher biocrude yield and higher earnings through biomethane sales. These two cases were scaled up to a simulation of a full-scale plant of 250 kt a–1. The cost analysis reveals that consumption-linked costs contribute >73% of total annual cost. MBSP is 1.6 € kg–1 for AD + HTL and 1.8 € kg–1 for HTL. The sensitivity analysis highlights production quantity and AP treatment as determining price drivers, while energy and labor costs are secondary. Integrating AD and HTL can increase the overall energy yield while enhancing the techno-economic validity of the process.