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 obtained 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 significant 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 € kWh−1 for the scenarios without membrane and 0.052 € kWh−1 for the scenarios with membrane.