Hydrothermal liquefaction integrated with wastewater treatment plants – life cycle assessment and technoeconomic analysis of process system options

Abstract

The purpose of this study is the formulation of various scenarios based on two different conceptual design configurations for a sewage sludge-to-fuel pathway via HTL, co-located with a wastewater treatment plant (WWTP), and biocrude upgrading. The first concept refers to decentralized HTL plants assessed for three scenarios of different aqueous phase treatment technologies, coupled with two scenarios of technologies for hydrogen production and a centralized biocrude upgrading plant for diesel and gasoline production. The second concept refers to a decentralized HTL plant followed by a first step of hydrodeoxygenation to stabilize and transfer the treated biocrudes in a central oil refinery for further treatment (e.g., at the FCC cracking units). All cases are assessed with respect to their environmental impacts and their economic profile using the Life Cycle Assessment (LCA) methodology and technoeconomic analysis (TEA). The impact assessment was based on the eighteen mid- and the three endpoint categories of the ReCiPe method. The Global Warming Potential metric ranges between 0.3 and 2.5 kg CO₂-eq. per kg biofuel blend corresponding to GHG emission savings of 35% to 90% compared to fossil diesel. TEA results show production costs of 60–80 € per MW h product. Analysis of results provides background information for design specifications targeting improvement of the environmental and economic performance and, thus, highlighting opportunities for biofuel production and synergies with existing fossil fuel infrastructures.