Upgrading Biogas to Biomethane Using Membrane Separation

Biogas contains carbon dioxide in the range of 35–45 vol %. Upgrading biogas to biomethane is primarily based on carbon dioxide removal. Biomethane is essentially purified biogas that contains at least 95 vol % methane and it can be either used as fuel for vehicles running on compressed natural gas (CNG) or injected into the natural gas grid. Nowadays, various techniques are used for CO₂ removal from biogas. Among the most commonly used technologies are adsorption, absorption, cryogenic separation, and membrane separation. Currently, in the Czech Republic, no units for biogas upgrading to biomethane are operating. In addition, during the summer months, there is heat overproduction from the co-generation units. In this work, a suitable unit for carbon dioxide separation is proposed. Carbon dioxide separation is possible using membrane separation. Along with carbon dioxide, minor compounds present in biogas such as hydrogen sulfide and water are also separated. The implementation of this unit makes it possible to obtain biomethane form biogas. Membrane separation was tested in a pilot scale using real biogas. All experimental tests were conducted at the Central Waste Water Treatment Plant in Prague. Experimental tests were performed using different types of membranes. For comparison purposes, the following membrane modules materials were chosen: polysulfone and polyimide fiber membranes. Separation of moisture and trace compounds present in biogas was tested for these two types of membrane materials. Other tests were performed using polyimide membranes. Parallel connection of membrane modules was the most effective to remove carbon dioxide from biogas. Purified biomethane contained at least 95 vol % of methane, as is required, even when the highest flow rate was applied of 7 m³ h–¹ of biomethane (measuring conditions: 0.6–0.8 MPa). This small membrane separation unit is recommended for biogas units in wastewater treatment plants.