Why biogas is a difficult gas to compress
Biogas from an anaerobic digester is mainly methane and carbon dioxide, saturated with water vapour and carrying smaller amounts of hydrogen sulphide (H₂S), ammonia, oxygen and nitrogen from air ingress, siloxanes where the feed includes sewage sludge or some industrial wastes, and fine particles or foam carry-over. Landfill gas is similar but usually more variable and often richer in siloxanes and air ingress.
Each contaminant attacks a different part of the compression train:
- Water condenses as the gas is compressed and cooled, washing out lubricants and combining with H₂S and CO₂ to form corrosive acids.
- H₂S corrodes carbon steel, copper alloys and some bearing materials, degrades lubricating oil and is highly toxic to people.
- CO₂ lowers the heating value and, with water, forms carbonic acid.
- Siloxanes turn into hard silica deposits when burned, damaging engines, burners and catalysts downstream.
- Oxygen from leaks into a low-pressure digester system can create a flammable mixture inside the equipment.
- Particles and foam foul coolers, valves and seals.
So the compressor cannot be chosen from a catalogue flow and pressure alone. It has to be chosen for a gas whose properties are known, with treatment ahead of it that keeps those properties within the maker’s limits.
What pressure does biogas need for each use?
The end use sets the pressure, and very different machines suit each band.
| End use | Typical pressure class | Typical machine type |
|---|---|---|
| Boiler or burner feed, flare | A few tens to a few hundred millibar | Gas-rated side-channel or centrifugal blower |
| Gas engine or CHP feed | Low pressure, set by the engine gas train | Gas blower with pressure control |
| Upgrading plant (membrane, water scrubbing, pressure swing adsorption) | Often in the region of 5–15 bar (72.5–217.6 psig), set by the upgrading process | Oil-injected or oil-free screw, or reciprocating gas compressor |
| Compressed biogas (CBG) for cylinder cascades or vehicles | High pressure, comparable to CNG at about 200–250 bar (2,900.8–3,625.9 psig) | Multi-stage reciprocating CNG-type compressor |
| Grid or pipeline injection | Set by the receiving network | Screw or reciprocating, per network rules |
India’s compressed biogas programme has created many plants that upgrade biogas to biomethane and compress it into cascades for transport. In these plants the high-pressure compressor works on clean, upgraded gas, while the low-pressure feed blower and the upgrading compressor handle raw or partly treated gas. Treat them as separate duties with separate specifications. The biomethane quality itself must meet the applicable Indian standard and the buyer’s requirements; that is the upgrading designer’s responsibility.
The pretreatment train before compression
A typical sequence from digester to compressor looks like this. The exact order and technology depend on the gas analysis and the end use.
- Gas holder or membrane roof to buffer flow and keep a steady suction pressure.
- Condensate trap and sediment pot at low points in the gas line, with safe drainage.
- Desulphurisation: biological (with controlled air or oxygen dosing in the digester or a scrubber), chemical scrubbing, or iron-oxide or activated-carbon media, chosen for the H₂S load.
- Gas cooling to condense water, followed by a separator and reheating so the gas is above its dew point when it enters the machine.
- Particle and droplet filtration.
- Siloxane removal with activated carbon or similar media where the downstream use is sensitive to it.
- Gas analysis for methane, CO₂, O₂ and H₂S at the compressor inlet, with oxygen alarms and shutdown.
- Flame arrestors, isolation valves and pressure-relief arrangements as designed by the gas-system engineer.
Do not let the compressor become the treatment plant. A compressor that receives wet, sour gas will act as a condenser and acid collector, and its oil, valves and coolers will show it within months.
Choosing the compressor or blower
| Technology | Where it fits | Watch points with biogas |
|---|---|---|
| Side-channel blower (gas-tight, ATEX-type construction where required) | Low-pressure boost to boilers, engines and flares | Shaft sealing, material compatibility with H₂S, condensate drain |
| Centrifugal gas blower | Larger low-pressure flows | Changing gas density changes the pressure it makes; check performance at the lowest methane content |
| Oil-injected screw | Upgrading plant feed in the mid-pressure range | Oil chemistry with H₂S and moisture, oil temperature high enough to avoid condensation, separator and oil change intervals from oil analysis |
| Oil-free screw or dry machine | Where oil contact with gas must be avoided | Stage temperatures, coating and seal compatibility |
| Reciprocating gas compressor | High-pressure CBG and some upgrading duties | Valve and ring materials, interstage condensate removal, packing leakage, pulsation |
Whatever the technology, the specification should give the maker the gas analysis range, the worst credible H₂S and moisture case, suction pressure and temperature range, discharge pressure, required flow in normal cubic metres per hour, ambient conditions up to Indian summer highs of 45 °C or more, hazardous-area classification and the duty cycle. Ask the maker to confirm materials, oil type, seals and electrical equipment for that gas.
Interstage cooling and condensate handling
Every stage of compression followed by cooling will drop out more water. With biogas that water is acidic and may carry dissolved H₂S, so it must be drained automatically, collected and disposed of as a process effluent, never just released on the ground. Separators need level-controlled drains suited to the gas, and drain lines must be sealed so gas cannot escape.
Equally important is avoiding condensation where it does harm. In an oil-injected screw, the oil temperature must stay high enough that water does not condense in the oil, but not so high that the oil breaks down faster. Gas-system designers often trace and insulate suction lines in cooler months and in night operation.
Safety boundaries
Biogas is flammable, and H₂S is toxic at low concentrations. A biogas compression installation therefore sits within a wider safety design that the plant owner and its gas-system engineer control.
- Hazardous-area classification of the compressor house and the electrical equipment rated for it.
- Fixed gas detection for methane and H₂S, with ventilation and alarms.
- Oxygen monitoring in the gas stream, because air ingress can create a flammable mixture inside the pipes.
- Pressure relief, flame arrestors and safe venting or flaring of relief discharges.
- Permit-to-work, purging with inert gas before maintenance, and gas-free checks before opening equipment.
- Statutory approvals that apply to high-pressure cylinders and cascades, handled by the plant owner.
We work within those boundaries. We do not certify a gas train or design the pressure protection; we make sure the compressor and its auxiliaries suit the stated duty and are serviced correctly.
Maintenance points for biogas compressors
- Oil analysis at regular intervals, watching acidity (total acid number), water content and wear metals; set oil change intervals from the results.
- Inspection of valves, rings and packing on reciprocating machines for corrosion and deposits.
- Cleaning of coolers and separators, and checking automatic drains on every visit.
- Checking that the pretreatment media, such as desulphurisation and siloxane beds, are changed before breakthrough; a jump in compressor problems often follows spent media.
- Gas detector calibration and safety shutdown tests as per the site schedule.
How Reddy Compressors helps
We sell new and used or refurbished gas compressors, including CNG, booster and process-gas machines, rent them and service them, and supply their spares. We also sell, rent and service blowers of Roots, screw, centrifugal and side-channel types. Our service team, based at Tarnaka, Secunderabad, covers the whole of Telangana and Andhra Pradesh, and we supply equipment and spares across India.
- Review of your gas analysis, pressures and flows against the proposed compressor or blower.
- Supply of gas-rated blowers and compressors from several brands, as an independent multi-brand specialist, not an authorised distributor. We have tie-ups with all major and reputed brands.
- Service, overhaul and oil-analysis-led maintenance of existing biogas, CBG and booster compressors.
- Spares identified from the machine nameplate, model and serial, confirmed before quoting.
- Plant compressed air and instrument air for the rest of the biogas or CBG plant.
Gas-specific design, hazardous-area classification and statutory approvals stay with the plant owner’s specialists, and oxygen, hydrogen or toxic-gas duties follow their own codes and the maker’s design rules.
What to send us
- Gas analysis: methane, CO₂, O₂, H₂S, moisture and siloxanes if measured, with the range seen.
- Required flow, suction pressure and temperature, and discharge pressure.
- End use: burner, engine, upgrading plant, CBG cascade filling or grid injection.
- Existing pretreatment equipment and its condition.
- Hazardous-area classification and site power supply.
- Nameplate photos and fault history for any existing compressor.
Select pretreatment at each equipment interface
Prepare an inlet-condition schedule with normal and worst credible gas composition, pressure, temperature, liquid carry-over and contaminant limits. Ask the compressor supplier to identify accepted limits and the upgrading supplier to identify the product requirement. Design treatment between those agreed boundaries rather than assuming that a generic upstream filter makes raw gas suitable.
Track media condition and any required gas analysis against the applicable limit. A change in feedstock can alter the gas without changing compressor hours. Maintenance planning therefore needs both the machine history and the gas-treatment history. Obtain selected-model performance rather than copying a product-family headline range into the duty sheet.
Frequently asked questions
What kind of compressor is used for biogas?
It depends on the pressure. Low-pressure feed to burners, engines and flares usually uses gas-tight side-channel or centrifugal blowers. Upgrading plants often use screw compressors in the mid-pressure range. Compressed biogas filling at around 200–250 bar uses multi-stage reciprocating compressors similar to CNG machines. All must be built and rated for the gas and its contaminants.
Why must hydrogen sulphide be removed before compressing biogas?
H₂S is corrosive and toxic. With water it attacks steel, copper alloys and bearing materials, and it degrades lubricating oil, shortening valve, seal and cooler life. It also creates a serious health hazard during maintenance. Removing it upstream protects the compressor, the downstream equipment and the people who work on the plant.
Can a normal air compressor be used for biogas?
No. An air compressor is not designed to contain a flammable, toxic gas. Its seals, electrical equipment, materials and lubricant are chosen for air. Biogas needs a gas-tight machine with suitable materials and seals, electrical equipment for the hazardous-area classification, and safety controls designed for the gas.
How do I stop water damaging my biogas compressor?
Where the selected process uses inlet cooling, separate the resulting condensate and provide any reheating specified to avoid unwanted condensation at the compressor inlet. Other treatment arrangements require the same manufacturer-confirmed inlet limits. Fit automatic drains on coolers and separators and keep them working. In oil-injected machines, keep oil temperature high enough that water does not condense in the oil, and use oil analysis to confirm it.
What pressure is compressed biogas stored at?
Compressed biogas for cylinder cascades and vehicle use is typically stored at pressures similar to CNG, in the range of about 200–250 bar (2,900.8–3,625.9 psig), after upgrading to biomethane. The exact pressure, cylinder approvals and fuel quality rules are set by the applicable regulations and the buyer.
How often should a biogas compressor be serviced?
Set the interval from the gas quality, duty and maker’s schedule, then adjust it using oil analysis and inspection findings. Sour or wet gas shortens oil and valve life, so plants with variable gas quality need closer monitoring. Changing desulphurisation and siloxane media on time is often the most effective step in reducing compressor faults.
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Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.