BiomethaneExplainer
PSA in biogas upgrading: how pressure swing adsorption works
PSA separates CO₂ from methane using cyclic adsorption on a molecular sieve. We discuss the phases of the cycle, methane losses, pretreatment requirements and typical problems.
Other language versions: polski.
PSA (Pressure Swing Adsorption) is a biogas upgrading method in which CO₂ is selectively adsorbed on a solid sorbent (most often a carbon molecular sieve or zeolite) under elevated pressure, and then desorbed after the pressure is reduced. Because a single column becomes saturated, the installation has several (usually four to six) and they operate cyclically, so that the biomethane stream is continuous. PSA is a proven and price-competitive solution, although it requires precise automation.
Phases of the cycle
- Adsorption – dried and compressed biogas (of the order of a few bar) flows through the column. CO₂ is adsorbed on the sorbent, and the methane leaves as the product.
- Pressure equalisation – the saturated column passes gas to a column after regeneration, which recovers pressure energy and part of the methane.
- Depressurisation and desorption – the pressure falls to a level close to atmospheric, often to a vacuum, and the CO₂ is released.
- Purge – part of the clean gas flows through the column to flush out the remaining CO₂.
- Repressurisation – the column is prepared for the next adsorption.
The columns are coupled by valves controlled by a PLC controller. Efficiency depends on the duration of the individual phases, the size of the sorbent grains, and the temperature and humidity of the gas.
How the sorbent "distinguishes" the gases
Carbon sieves work mainly kinetically: CO₂ diffuses into the pores faster than CH₄, so in a short adsorption time the methane does not have time to bind. Zeolites work rather on the principle of adsorption equilibrium and bind CO₂ more strongly thanks to electrostatic interactions. Biogas also contains nitrogen and oxygen, which PSA usually does not remove effectively; their content in the biomethane is therefore close to their content in the feed, which requires quality control at the inlet.
Pretreatment requirements
The sorbent is easily poisoned by hydrogen sulphide, water, volatile organic compounds (VOC), siloxanes and ammonia. Therefore desulphurisation (biological and/or activated carbon filters), drying by cooling and condensation, and coalescing filters are installed before the PSA. A higher H₂S concentration directly shortens the life of the sorbent and increases replacement costs.
Methane losses and energy
Methane losses depend on the number of columns, the equalisation scheme and the afterburning of the off-gas. Modern systems usually achieve around 1% or less, older ones a few percent. The off-gas contains CO₂ and a certain amount of CH₄, so it is often recycled or sent to an oxidiser (RTO). The electricity demand – compression and vacuum – is similar to that of membranes and is of the order of 0.2–0.3 kWh/m³ of biogas; specific values depend on the design and the scope of the balance.
Advantages and disadvantages
| Feature | Assessment |
|---|---|
| CAPEX | usually moderate, competitive at large scale |
| Chemicals and water | not needed (apart from drying) |
| Load flexibility | good within the design range, limited below the minimum |
| Complexity | high: many valves and cycles |
| Sensitivity to contaminants | high (H₂S, VOC, water) |
| CO₂ purity | low, with a methane load |
Valve wear is sometimes an operational problem, because each cycle involves many switchings. Proper maintenance and a stock of spare parts are important for availability.
Questions for the supplier
- What are the guaranteed methane losses across the whole load range?
- What is the expected life of the sorbent and the cost of replacing it?
- Does the off-gas meet emission requirements without additional equipment? If not, what is the cost of the afterburning system?
- How does the system cope with variable inlet composition (e.g. after a change of substrates)?
- What is the electricity consumption for the full system, including drying and compression?
An illustrative example
Assumptions: 1000 m³/h of biogas, 55% CH₄, losses of 1.0%, energy consumption of 0.25 kWh/m³ of biogas, 8000 h/year. Electricity consumption: 1000 × 0.25 × 8000 = 2000 MWh/year. Methane losses: 550 × 0.01 × 8000 = 44 thousand m³ CH₄ ≈ 440 MWh. If the off-gas requires afterburning with fuel, additional costs and emissions arise that must be taken into account in the calculation.
Availability and operation
In a PSA installation failures most often concern not the sorbent but the system of valves and sensors, so it is worth asking about the number of cycles for which the fittings were designed and about the availability of spare parts in Poland. Good practice is remote monitoring of key indicators: pressure differences in the columns, the temperature and dew point of the gas at the inlet, and the CH₄ concentration in the off-gas. A decline in drying quality or a rise in H₂S at the inlet can be caught earlier than permanent damage to the sorbent occurs. Many plants conclude a service contract with a guaranteed response time, and convert the cost of replacing the sorbent into the price of a cubic metre of biomethane, which makes comparison with competing technologies easier.
Summary
PSA is a mature technology with good efficiency, but its profitability depends on the quality of the input gas and the way the off-gas is managed. With a large number of cycles, the reliability of the fittings is key.
