Biogas upgrading technologies and their impact on CO₂ recovery
The biogas upgrading process determines how CO₂ becomes available for recovery: its composition, moisture content, pressure and flow behaviour. These conditions influence the purification required and the energy needed to produce liquid CO₂.
Membranes, pressure swing adsorption, scrubbing and cryogenic processes separate gases in different ways. For operators and engineering companies considering CO₂ recovery, understanding what leaves the upgrading plant helps establish what the downstream installation must do.
Solveno Technologies specialises in CO₂ recovery and liquefaction from biogas upgrading, with low energy consumption as a central design priority. This guide explains the implications of the main upgrading technologies for that part of the project.
Why the upgrading technology matters for CO₂ recovery
Biogas upgrading produces a methane-rich product by separating CO₂ and other components. However, substances removed from the biomethane may remain in the separated CO₂ stream.
A high CO₂ concentration does not, by itself, establish product quality. Residual methane, water, sulfur compounds, oxygen, nitrogen and trace organic compounds can affect the treatment required. Their presence depends on the feedstock, the existing pretreatment and the upgrading configuration.
Two plants using the same upgrading technology can therefore require different CO₂ treatment. The starting point is the actual gas available at the recovery connection.
Membrane separation
Membranes separate gases through differences in permeability. CO₂ passes through the membrane more readily than methane, creating a CO₂-rich stream on the lower-pressure side.
The membrane stages and recycle arrangement influence the composition of this stream, including its residual methane content. Pretreatment protects the membranes, but the resulting CO₂ still needs to be assessed against the requirements for liquefaction and its intended use.
Depending on the gas analysis, downstream treatment can include filtration for residual sulfur and organic compounds, particle removal, drying and separation of methane and other non-condensable gases. The available outlet pressure also influences compression requirements.
Pressure swing adsorption (PSA)
PSA uses solid adsorbents to retain CO₂ under pressure. Lowering the pressure releases the adsorbed gas, while several vessels operate in sequence to maintain biomethane production.
For CO₂ recovery, the relevant conditions are those at the combined off-gas outlet. The vessel cycles can cause variations in gas flow and composition; their extent depends on the plant configuration and any existing buffering.
The recovery system must accommodate that operating profile. Buffering and compressor control may be needed to provide a suitable feed to the downstream process. Residual impurities and moisture determine the additional purification and drying required.
Water scrubbing
Water scrubbing uses the greater solubility of CO₂ in water to separate it from methane. The way the water is regenerated has a direct effect on the gas available for CO₂ recovery.
Where regeneration uses air, the released CO₂ is diluted with nitrogen and oxygen. This is a different starting point from a concentrated CO₂ stream. Adding a dryer and a liquefaction unit alone does not resolve that dilution.
The first assessment is therefore whether a suitable CO₂ stream can be collected. Where recovery is feasible, moisture and any co-separated contaminants must also be addressed. The regeneration arrangement is essential to this assessment.
Amine scrubbing
Amine scrubbing chemically binds CO₂ in a solvent and releases it during regeneration using heat. This can provide a concentrated CO₂ stream for further processing.
Its purification needs depend partly on where contaminants were removed upstream. Sulfur compounds and ammonia may pass into the released CO₂, so a high CO₂ concentration must be considered alongside the trace-component analysis.
The recovery assessment includes cooling, moisture removal and any additional contaminant treatment needed before liquefaction. Heat used to regenerate the amine belongs to the upgrading process and should be distinguished from the electricity required for downstream CO₂ recovery.
Organic physical scrubbing
Physical solvent systems absorb CO₂ and release it during regeneration. Other compounds can also enter the solvent and be released with the CO₂.
The solvent and regeneration method determine the available gas composition. Where air stripping is used, dilution must be assessed before treating the outlet as a feed for CO₂ liquefaction.
The recovery design therefore starts with the actual regeneration gas: its concentration, moisture and contaminants. These establish whether the stream is suitable and which additional treatment would be required.
Cryogenic and hybrid upgrading
Cryogenic processes use low temperatures to separate gas components. Hybrid installations combine separation methods and may already integrate part of the CO₂ purification or liquefaction process.
The relevant question is what CO₂ product or intermediate stream is available at the interface. Its physical state, pressure, temperature and quality determine what remains to be done.
Where treatment is already included upstream, it should be accounted for in the downstream scope. An integrated process must be assessed as a whole to avoid duplicating equipment or energy consumption.
Which purification steps may be needed?
The upgrading technology provides context. The gas analysis and required liquid-CO₂ specification determine the treatment sequence.
| Component or condition | Possible treatment | Relevance to CO₂ recovery |
|---|---|---|
| Water and entrained droplets | Cooling and condensate separation, followed by drying to the required moisture level. | Protects the low-temperature process against ice formation and blockage. |
| Residual sulfur and organic compounds | Suitable adsorption or other contaminant-specific treatment, selected from the gas analysis. | Reduces contaminants that could affect product quality. Filter media must match the substances present. |
| Particles | Particle filtration at appropriate points in the process. | Protects downstream equipment, including against material carried over from adsorption beds. |
| Methane, nitrogen and oxygen | Separation of non-condensable components during CO₂ purification. | Influences achievable product quality, CO₂ recovery yield and the handling of the remaining gas. |
| Flow and pressure variations | Buffering and coordinated compressor control where required. | Matches the available gas supply to the operating requirements of the recovery unit. |
These steps perform different functions. Activated carbon does not replace drying or the separation of non-condensable gases. Equally, liquefaction alone does not establish suitability for food and beverage applications, which also requires source qualification and appropriate quality controls.
How do these differences affect energy consumption?
The energy required per tonne of recovered liquid CO₂ depends on the gas entering the system and the product leaving it. Available pressure affects compression duty. Moisture affects drying requirements. Non-condensable components influence separation and the amount of CO₂ recovered as liquid.
This is why an energy figure needs a defined inlet composition, operating load and product specification. It also needs a clear equipment scope: compression, purification, drying, refrigeration and auxiliary consumption should be identified.
Solveno develops its recovery systems around low energy consumption and evaluates these conditions as part of the integration. The aim is to establish the treatment needed for the available stream and its effect on energy use and operating cost.
For a more detailed explanation of the downstream process, see our guide to CO₂ liquefaction.
CO₂ recovery alongside your upgrading installation
For an existing plant, the assessment starts with the available CO₂-rich stream and its operating conditions. For a new project, the same information can be defined with the upgrading supplier and engineering team.
Solveno’s role is to assess and engineer the CO₂ recovery section, including the necessary treatment and connections to the existing process. Our engineering services support that integration.
To discuss your project, share the upgrading technology, available off-gas analysis, flow, pressure and temperature, together with the intended use of the recovered CO₂. We can then assess suitability and the next steps for an energy-efficient recovery solution.
Technical background
Further background on upgrading technologies and the distribution of impurities is available in Energiforsk’s Biogas upgrading – Technical Review (2016), particularly the sections on impurity handling and purification sequences, and in IEA Bioenergy Task 37’s review of biogas upgrading technologies. These references explain process principles; the treatment required for an individual plant depends on its actual gas conditions.