Biogas upgrading and CO2 removal from biogas

What is Biogas?

Biogas is a renewable gas produced through the anaerobic digestion of organic materials such as agricultural residues, manure, food waste and wastewater sludge. It consists mainly of methane (CH₄) and carbon dioxide (CO₂). The European Biogas Association provides a broader overview of biogas, biomethane and their role in the European energy system.

Anaerobic digestion also produces digestate. This co-product retains valuable nutrients including nitrogen, phosphorus and potassium and can, subject to applicable quality and regulatory requirements, be used in agriculture. The European Biogas Association's 2026 digestate report provides further information on its composition, agricultural use and role in nutrient recycling.

How is CO2 removed from biogas during upgrading?

Biogas upgrading separates CO2 from methane to increase the methane concentration and produce biomethane. Commercial upgrading technologies include membrane separation, water scrubbing, pressure swing adsorption (PSA) and chemical absorption. These processes are described in the European Commission Joint Research Centre report on bioenergy technologies.

Raw biogas contains methane and carbon dioxide together with water and smaller concentrations of other components. During upgrading, CO2 and other unwanted components are separated or reduced so that the methane-rich gas can meet the requirements of its intended application.

The upgrading technology also influences the characteristics of the separated CO2-rich stream. For downstream CO2 recovery, parameters such as CO2 concentration, residual methane, oxygen and nitrogen, moisture, contaminants, pressure, temperature and flow therefore need to be assessed for the individual installation.

When the required gas specification is achieved, biomethane can be injected into the gas grid or used in other energy and transport applications. The European Commission's biomethane overview provides further information on the role of biogas upgrading and biomethane within the European energy system.

The separated CO2-rich stream is a different product stream. Where its composition is suitable, it can be processed in a dedicated CO2 recovery system through purification and liquefaction to produce liquid biogenic CO2. Solveno focuses on this downstream CO2 recovery step and does not supply the upstream biogas upgrading technology.

Recovered biogenic CO2 can have several uses depending on the quality specification achieved and the requirements of the end user. Applications include industrial uses, greenhouses, food and beverage applications and other forms of carbon utilisation. The EBA report on biogenic CO2 from biomethane provides an overview of these markets and applications.

Composition of biogas

The exact composition of raw biogas varies with the feedstock and operating conditions. According to the European Biogas Association, methane and carbon dioxide are its two main components, while water and other gases can also be present.

• Methane (CH₄): Methane is the energy-containing component of biogas. Increasing its concentration is the main objective of conventional biogas upgrading.

• Carbon dioxide (CO₂): CO₂ represents a substantial fraction of raw biogas and is separated from methane during upgrading. The resulting CO2-rich stream can subsequently be evaluated for CO2 recovery.

• Water vapour: Raw biogas normally contains moisture. The required drying depends on the upgrading process and downstream specifications.

• Other components: Depending on the feedstock and plant, biogas can contain components such as hydrogen sulfide (H₂S), nitrogen (N₂), oxygen (O₂) and trace contaminants. Their concentrations must be considered when designing both upgrading and downstream CO2 recovery.

What can be done with biogas?

Biogas can either be used directly for energy production or upgraded to biomethane for additional applications:

1. Electricity and heat: Biogas can be used to generate renewable electricity and heat.

2. Biomethane: Removing CO2 and other unwanted components produces methane-rich biomethane.

3. Gas grid injection and transport: Biomethane that meets the applicable specification can be injected into gas infrastructure or used as a renewable fuel.

4. CO2 recovery: Instead of releasing a suitable CO2-rich stream after upgrading, it can be further purified and liquefied to produce reusable biogenic CO2.

Environmental and economic considerations

Anaerobic digestion converts organic materials into renewable gas while producing digestate that can help recycle nutrients. Upgrading creates biomethane for higher-value applications, while the separated CO2-rich stream may offer an additional opportunity for recovery.

Whether CO2 recovery is technically and economically attractive depends on the actual gas composition, recoverable CO2 flow, operating conditions, required product specification and available market for the recovered CO2. These parameters therefore need to be evaluated for each project individually.

Technical references and further information

• European Biogas Association – Biogases explained
• European Commission Joint Research Centre – Bioenergy technology report
• European Commission – Biomethane
• European Biogas Association – Biogenic CO2 from biomethane