Membrane electrolysis-assisted CO2 and H2S extraction as innovative pretreatment method for biological biogas upgrading

Turning raw biogas into biomethane as energy carrier requires the selective removal of CO2 in a biogas upgrading process or a total conversion of CO2 to CH4 which is generally energy intensive. During membrane electrolysis, electrical energy can be used to simultaneously remove CO2 (and H2S) and produce H2 as side product. Biogas is thus scrubbed with catholyte and the captured HCO3− and HS− migrate towards the anode. Simultaneously, cathodic H2 mixes with residual biogas in a ratio that can be fine-tuned. We obtained in one step an ideal 4:1 H2:CO2 ratio in the reactor off gas. Subsequently the gas could be further upgraded via chemoautotrophic microbial conversion of CO2 to CH4. Biomethanation delivered biomethane with 98.9 ± 0.9% purity. The electrochemically-assisted scrubbing and stripping of CO2 and H2S resulted in high CO2 removal efficiencies (up to 100%), without addition of chemicals. The system was flexible depending on temporarily available power. Electrochemical biogas upgrading (EBU) can be envisaged as a scalable and decentralized storage of excess or off-peak renewable power, making better use of the power input used to drive a biological CO2 conversion.