Compressor Lubrication in Biomethane Upgrading and CO₂ Recovery
Biogas places its hardest demands on the compressor at the point where it stops being biogas and starts becoming a product: pipeline biomethane, Bio-CNG, or recovered CO₂.
In most upgrading layouts the compressor sits upstream of full purification, so the lubricant contacts gas that may still carry CO₂, moisture, H₂S, siloxanes and other trace components. Selecting for that duty is not only a matter of matching an ISO viscosity grade: the lubricant also has to manage dissolved gas, oxidation, corrosion, cleanliness and oil separation, and the right balance depends on where the compressor sits in the process and what the gas carries there.
A "biogas compressor" is not one duty
Biogas upgrading to biomethane
Raw biogas is roughly 55–60 % methane, with most of the balance CO₂, plus H₂S, moisture and trace contaminants — useful for on-site heat and power, but not interchangeable with natural gas. Upgrading strips out the CO₂ and contaminants to leave biomethane at 96–99 % methane, a drop-in renewable substitute that can be injected into the gas grid or compressed for transport as Bio-CNG or Bio-LNG. Renewable-gas targets and decarbonisation policy have turned this from a niche into a mainstream route for green gas.
The single most useful idea in lubricant selection for this sector is that the compressor’s position in the process defines its lubricant, not the word “biogas.”
Compression appears at several points, and the gas looks different at each one:
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Membrane and PSA upgrading
Feed-gas compression usually runs before CO₂ is removed, so the lubricant sees raw or partly treated gas: methane, CO₂, moisture, H₂S and siloxanes together. -
Grid injection
Compression of upgraded biomethane, where the gas is clean and dry but duty cycle and oil carryover still drive the choice. -
Bio-CNG
Compression to pressures often around 200–250 bar, which shifts the priority toward cleanliness, oxidation stability and, often, food-grade or other regulatory requirements. -
CO₂ recovery and liquefaction
A separate duty on a CO₂-rich stream, with its own lubricant logic (see below).
Compressor Position and Lubricant Focus
A single upgrading plant rarely runs one lubricant duty. The table below maps each compressor position to the gas it actually sees and the property that should drive lubricant selection there — from wet raw biogas at the front end to the CO₂-rich stream after separation.
How biogas composition stresses the lubricant
For this audience the components themselves need no introduction. What matters is how each one acts on the lubricant.
Carbon Dioxide (CO₂)
It can dissolve into many lubricants more readily than methane and lower the working viscosity under pressure, so the film can run thinner than the data-sheet grade — worth estimating at selection rather than meeting as wear in service.
Hydrogen Sulfide (H₂S)
When wet gas carries it to the compressor, the lubricant faces an acidic, additive-depleting environment — the usual source of acid number rise, wear metals and shortened drain intervals.
Siloxanes
Under heat they form hard, silica-like deposits that foul hot surfaces and the oil circuit, so the lubricant has to carry the deposit-control and oxidation-stability load. Gas treatment removes them; the lubricant cannot.
Ammonia
From nitrogen-rich feedstocks such as manure or poultry waste, it can attack parts of the additive system and some seal materials, so lubricant and elastomer compatibility are worth a check when the feedstock points to it.
Oxygen
The trace amounts left by biological desulphurisation or grid limits accelerate oxidation of the lubricant, depleting its oxidation reserve and pushing viscosity up — so that reserve matters even in otherwise clean service.
What the duty asks of the lubricant
Working viscosity under dilution
-The in-service film after dissolved gas is accounted for, not the fresh-oil ISO grade.
Oxidation stability
Upgrading compressors run continuously; temperature, trace O₂ and contamination all eat oxidation reserve.
Corrosion protection
Bearings, internals and oil-wetted surfaces, especially where H₂S and moisture coexist.
Deposit control
Against oxidation products, thermal stress and siloxane residues.
Oil separation and carryover
Matched to the separator and gas conditions, because carryover becomes a problem in the downstream treatment train.
Materials and process compatibility
Seal elastomers, compressor type, OEM limits, treatment equipment and the end use of the gas.
The upgrading compressor and its lubricant
Most upgrading routes run at elevated pressure, so a feed-gas compressor raises the raw biogas to operating pressure before separation. That places it upstream of full purification, where it sees the gas at its dirtiest. After upgrading, the biomethane is often compressed again for grid injection or Bio-CNG.
That gives the lubricant two jobs at once:
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Surviving raw gas
at the feed compressor the lubricant meets CO₂ dilution, H₂S and moisture, siloxanes and oxidation together, so corrosion protection, oxidation stability and deposit control all matter at the same time. -
Protecting the separation unit
lubricant carried past the separator can foul or damage membranes, adsorbents and scrubbing media, the most expensive parts of the plant. Pretreatment guards against this, but keeping carryover low at the compressor protects it at the source.
For many biomethane and gas-compression duties, PAO is where selection starts: solid oxidation resistance, good thermal and low-temperature behaviour, clean running, and grades across the range needed for continuous service. The real question is rarely whether to use something exotic. It is whether the chosen PAO grade matches the actual H₂S level, moisture, siloxane exposure, pressure, temperature and compressor design.
Two situations justify a closer look beyond a standard PAO. Food-grade (H1) PAO comes into play wherever the gas, the recovered CO₂ or the installation touches food, beverage or otherwise regulated environments — increasingly common as biogenic CO₂ finds a market. Diester and other chemistries are worth evaluating in selected compressor or co-generation duties where dilution behaviour, contamination profile or compressor design pushes against a straight PAO. For many upgrading duties, though, PAO remains the sensible default.
CO₂ recovery and liquefaction
Upgrading leaves more than biomethane: close to half of the raw biogas is CO₂, historically vented. Recovering it returns slip methane for near-complete methane recovery, improves the plant’s carbon intensity, and turns a waste gas into biogenic CO₂ — a renewable product (food ingredient E290) that can replace fossil-sourced CO₂ in food, beverage and industrial use. The economics only tipped recently: tighter CO₂ supply and sustainability pressure since 2022 have made on-site recovery and liquefaction a real revenue stream rather than a nice-to-have.
The route from off-gas to liquid product is well established:
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Collect & scrub
buffer the CO₂-rich stream and wash out foam and water-soluble impurities. -
Compress
raise it from near-atmospheric to liquefaction pressure, roughly 15–20 bar. -
Dry & purify
activated carbon removes trace organics and odour; desiccant driers remove moisture. -
Liquefy
refrigerate to around -20 to -30 °C, stripping out non-condensables (nitrogen, oxygen, residual methane) -
Store & dispatch
hold as liquid, then evaporate to consumers. Food- and beverage-grade CO₂ must meet ISBT/EIGA purity (99.9 % and better, impurities at ppm level), and some waste-derived gas is hard to certify at all.
The compressor sits at the centre of this. It lifts the near-atmospheric stream to liquefaction pressure on continuous duty — often on near-pure CO₂ that still carries moisture before the driers — and what it delivers governs drier loading, condenser performance and final purity.
That puts two specific demands on the lubricant:
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Gas dilution
CO₂ dissolves readily into many lubricants and can lower the working viscosity under pressure. Because the gas here is close to pure at elevated pressure, the effect is more pronounced than in mixed-gas service, so the film can run below the nominal gradestream and wash out foam and water-soluble impurities. -
Oil carryover
anything carried past the separator contaminates the product CO₂ and can foul the carbon beds, driers and condenser. Clean separation matters, and food-grade (H1) lubricant comes into play wherever incidental contact with a food or beverage stream is possible.
For many CO₂ compression duties a PAO is a sound starting point, with PAG considered where pressure, temperature or dilution risk points that way; liquefaction should be judged on its own purity, pressure and temperature rather than carried over from methane-rich biomethane service.
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Frequently Asked Questions
Why is biogas upgrading demanding for compressor lubricants?
The compressor often runs upstream of full purification, so the lubricant contacts raw or partly treated gas containing methane, CO₂, moisture, H₂S, siloxanes and other trace components.
Is biomethane compression the same as natural gas compression?
Not always. Upgraded biomethane can behave like clean gas service, but raw or partly treated biogas carries contaminants that make the duty more severe.
Why is PAO commonly used here?
For its oxidation stability, thermal behaviour and broad grade range. For many biogas-upgrading and biomethane duties it is the practical baseline.
Does CO₂ affect the lubricant?
Yes. CO₂ can dissolve more readily in many compressor lubricants than methane does and can lower the working viscosity under pressure, with the effect strongest in high-CO₂ raw gas and CO₂-rich recovery duty.
Does H₂S affect the lubricant?
Yes — it drives corrosion risk and challenges the additive system, especially with moisture present. The H₂S level should always feed into selection.
Can siloxanes be solved by lubricant choice?
No. Siloxanes are a gas-treatment problem. The lubricant can support cleanliness and deposit control but cannot replace gas cleaning.
Is the same lubricant used for biomethane compression and CO₂ liquefaction?
Not by default. The two duties differ, and liquefaction should be assessed on pressure, temperature, gas purity and compressor design.