Optimization of prospective circular economy in sewage sludge to biofuel production pathways via hydrothermal liquefaction using P-graph

Abstract

Hydrothermal liquefaction (HTL) has proven to be an appropriate technology for converting sewage sludge into a valuable resource for renewable energy generation. This study focuses on a prospective analysis of various technological scenarios for sewage sludge-to-fuel pathways via HTL, co-located with a wastewater treatment plant, in support of a circular economy perspective. Four technological foreground scenarios and three prospective background scenarios aligned with the Paris agreement’s climate targets REMIND-SSP2-Base (projecting a 3.5°C temperature rise by the end of the century), PKBudg1150 (aiming to limit the rise to below 2°C), and PKBudg500 (targeting a cap below 1.5°C) are analyzed for sewage sludge-to-fuel conversion in 2030, 2040, and 2050. The superstructure problem of the possible combinations of the developed scenarios is solved using the P-graph studio which is based on the branch and bound approach. The goal of this study is to maximize the objective function (OF) by accounting the credits from avoided GHG emissions, the market value of recovered products, while subtracting operational costs and GHG emission penalties incurred during the biocrude production and upgrading processes. The optimal solution shows a potential OF equal to 858 €/ton of sewage sludge for Pkbudg500 under technological foreground scenario 2 by 2040. The P-graph approach demonstrates that HTL treatment of sewage sludge provides an alternative production pathway within the circular economy concept, capable of identifying optimal and near-optimal solutions for addressing trade-offs between future socio-economic policies and practical implementation for 2030, 2040, and 2050, which are often difficult to monetize.