Optimising power-to-gas integration with wastewater treatment and biogas: A techno-economic assessment of CO2 and by-product utilisation

Production of electrolytic hydrogen and its conversion to methane, also known as power-to-gas (PtG), could play a key role in the transition towards a defossilised energy system. Integration of PtG technology with wastewater treatment and co-digestion presents an opportunity to produce low-carbon methane while simultaneously upgrading biogas, recycling biogenic carbon dioxide and utilising process by-products (heat and oxygen). A model of such an integrated system was developed using real plant data to assess the techno-economic performance through simulation of hourly operation and configuration optimisation. The integrated concept was demonstrated to be a promising option for increasing efficiency and reducing costs and emissions in the PtG system. By-product utilisation increased net energy efficiency from 52.3 to 59.7 %HHV, leading to a reduction in levelised cost of PtG (LCOPtG) of 1.0 % and in net specific emissions of 28.3 % and 2.2 % based on average and marginal grid emission factors respectively. Minimum LCOPtG of 194.6 €/MWhCH4 was achieved, which entailed average and marginal net specific emissions of 37.2 and 635.2 gCO2/kWhCH4, respectively. In the investigated conditions, the optimised PtG configuration produced heat in excess, but could not fulfil oxygen demand at the wastewater treatment plant. Heat integration yielded considerable performance improvements, while oxygen integration provided only minor benefits and slightly increased overall production costs. However, improved economic performance of oxygen integration was shown to be possible depending on local conditions. Although integrating several independent systems introduced the challenge of managing fluctuating heat and oxygen demand, alongside the varying supply of biogas and renewable electricity, the difference in magnitude between by-product generation and demand meant that their utilisation had only a minor impact on system operation.