Biomethane production from biogas obtained in wastewater treatment plants: Process optimization and economic analysis

The methanation of the CO2 present in biogas is getting more and more interest, especially if hydrogen is ob-tained from renewable electricity. The biogas resulting from anaerobic digestion can be separated into CH4-and CO2-rich streams, after which the CO2 goes into the methanation section, though the biogas can also go directly into the reaction section with no separation; both alternatives were studied in this work. The two state-of-the-art methanation reactors, adiabatic and cooled fixed-bed reactors, were considered, with the latter showing a sig-nificant advantage through the formation of different reaction zones inside the reactor, allowing a simpler one -reactor approach. In this work, the use of a CH4-selective membrane was also considered to promote CO2 conversion. The results show exactly that, while the cooled reactor with no previous CO2 separation from the biogas had a CO2 conversion of 91.8 %, the adiabatic reactors showed conversions of 59.6 and 67.2 %, resulting in an overall conversion of 93.0 %. In economic terms, the scenarios studied showed negative net present worth values, considering the present conditions in Portugal and a water treatment plant as case-study, thus requiring the government to provide monetary incentives like feed-in tariffs of 0.050 euro kWh-1 for the scenarios without membrane and 0.052 euro kWh-1 for the scenarios with membrane.