Integration of Reversible Solid Oxide Electrolysis (rSOC) to Wastewater Treatment Plants for Sustainable Green Gas Production and Balancing of Green Power

Abstract

Biogas produced from anaerobic digestion (AD) of sewage sludge and organic wastes at wastewater treatment plants (WWTPs) can present an important energy contribution to a sustainable society. Raw AD biogas contains 50-65 molar% methane (CH4) and 50-35 molar% carbon dioxide (CO2) and can be upgraded to biomethane (with over 98 molar% CH4). During the CO2 separation step of biomethane upgrading, substantial amounts of CO2 are emitted to the atmosphere. A methanation unit can convert this CO2 into sustainable Synthetic Natural Gas (SNG) and replace the conventional CO2 separation unit for biomethane upgrading, but needs addition of hydrogen from water electrolysis. The water electrolysis can be built as a novel high-performance reversible Solid Oxide Electrolysis (rSOC), as demonstrated in the European H2020 project BALANCE. To highlight the zero-emission potential of the rSOC, this paper emphasizes author’s earlier results from the European H2020 project DEMOSOFC, demonstrating zero-local emission SOFC using raw biogas from WWTP sewage sludge and organic wastes. In this paper, profitability estimations for optimal rSOC and methanation investment and operation strategies at a modern city level WWTP in Finland, Europe, are presented. Power used in rSOC electrolysis mode (SOEC) origins from intermittent wind power, while power produced in rSOC fuel cell mode (SOFC) uses raw AD biogas. A dedicated Power-to-X optimization model is used, optimizing power market operations, dimensions of renewable power sources, plant dimensions and operational scheduling. High profitability is shown using European energy crisis market data 2021-2022, resulting in sustainable biomethane net production costs of 30-70 €/MWh.