Techno-economic and life cycle assessment of integrated solid oxide electrolysis cell and Fischer–Tropsch systems for synthetic fuel production from sewage sludge

The European Commission’s Energy Union agenda calls for a major transformation of the energy sector, with a strong emphasis on renewable energy, smart energy systems, and circular economy principles. Among renewable alternatives, liquid biofuels derived from biomass have emerged as promising solutions for decarbonizing the transport sector, which remains heavily reliant on fossil fuels. In this context, synthetic gasoline, jet fuel, and diesel are particularly attractive due to their high energy density and compatibility with existing infrastructure. This study proposes and evaluates an integrated pathway for producing synthetic transportation fuels via Fischer–Tropsch (FT) synthesis from biogas derived through anaerobic digestion of sewage sludge. Two system configurations are comparatively assessed. In the baseline case (AD-FT), the produced biogas is reformed and purified in a pressure swing adsorption (PSA) unit, and the conditioned syngas is fed to the FT reactor followed by downstream fuel upgrading. Process heat is supplied by combusting tail gases in a conventional combustor, with CO₂ released to the atmosphere. In the upgraded case (AD-SOEC-FT), the conventional combustor is replaced by an oxy-fuel unit that produces a CO₂-rich flue gas stream, enabling CO₂ capture and subsequent utilization. The captured CO₂ is directed to a reverse water-gas shift (RWGS) reactor, where it reacts with additional H₂ supplied by an integrated solid oxide electrolysis cell (SOEC) to increase CO production. This CO₂-to-CO conversion improves syngas conditioning prior to FT synthesis and downstream fuel upgrading. The system is assessed using techno-economic analysis (TEA) and life cycle analysis (LCA) to quantify economic feasibility and environmental performance. Key indicators include the levelized cost of liquid fuels (LCOF), total annualized cost, greenhouse gas emissions (GWP100), and climate change impacts for both configurations.