Complete C-recovery from biogas through upgrading and methanation: a techno-economic and environmental assessment

The share of fossil fuels in the global energy mix remained consistently high for decades, at around 80%, leading to the release of massive amounts of greenhouse gases into the atmosphere, which are responsible for climate change. Awareness of this phenomenon has led to policies and agreements aimed at reducing greenhouse gas emissions. In this context, biomethane can contribute to the achievement of climate neutrality goals through various pathways, firstly by preventing GHG emissions by replacing fossil fuels. Another compelling reason to invest in biomethane is that it is not only a clean but also a secure energy resource, as it can be produced locally, increasing energy security by reducing reliance on imported fossil fuels. A good opportunity to increase the biomethane production comes from sewage sludge produced in wastewater treatment plants (WWT plants) as part of the water cleaning process. This work shows a techno-economic analysis of biomethane production in a WWT plant. The main objective is to investigate the potential for enhancing methane yield by recovering the CO2 removed from biogas during the upgrading process. Indeed, by combining this amount of CO2 with H2 obtained from an electrolyzer, it is possible to produce synthetic natural gas (SNG) using a methanation process. Furthermore, if hydrogen is produced exclusively from renewable energy sources, the resulting SNG can still be classified as biomethane. The study also includes an environmental analysis of the recovery process that examines different production scenarios. Moreover, the possibility of including a photovoltaic system with a battery storage in the plant, to partially satisfy the electric energy demand of the process, is investigated. The technical analysis shows that is possible to increase the production of the plant by +50.6%, from 329.9 kNm3/y when only biomethane is produced through upgrading, to 497.4 kNm3/y by adding 167.5 kNm3/y of SNG obtained from methanation of the recovered CO2. Anyway, electricity consumption increases significantly due to the high demand from the electrolysis unit. From an environmental point of view, when all the electric energy comes from green sources, the SNG is a carbon neutral product. When also electricity from the grid is used, depending on its energy mix, two cases may occur: the CO2 recovered from the upgrading unit can be lower or higher than the one emitted in the atmosphere to produce SNG. The first case is still environmentally convenient respect to the use of fossil fuels, the second one is not. Finally, the economic analysis shows that the SNG produced by the plant is still too expensive compared to natural gas on the market. This is mainly due to the high Capex and huge amount of electricity needed. Considering the Capex reduction projections and using only renewable energy available at its low LCOE, the cost of SNG decreases significantly from a value of 4.84 €/Sm3 in the current scenario down to 1.69 €/Sm3 in 2050.