A compressed biomethane plant takes raw biogas through gas cleaning, upgrading, drying, compression and storage before dispensing it at about 250 bar. In a turnkey proposal for a palm oil mill in Indonesia, the plant was sized to deliver 240 Nm³/h of compressed biomethane from existing digesters, with the main design questions being sulphur removal, oxygen control and moisture.
The challenge: fuel-grade gas from mill biogas
Raw biogas from palm oil mill effluent contains hydrogen sulphide, carbon dioxide and water, and often a little oxygen. Vehicle-grade biomethane must be mostly methane (the specification used at least 95%) with very low sulphide, oxygen and moisture. The tender for the plant also required oxygen below 0.5% because the gas would be trucked over a long distance in cylinders.
The approach: the process train
| Stage | Purpose |
|---|---|
| Bio-scrubber (existing, modified, plus a new one) | Bulk hydrogen sulphide removal |
| Activated carbon polishing | Remove remaining sulphide before compression |
| Primary compression and pre-drying | Raise pressure and remove bulk water |
| Temperature swing adsorption dryer | Dry the gas to a dew point of -40°C or lower |
| Two-stage membrane upgrading | Separate carbon dioxide from methane |
| High-pressure compression | Compress to 250 bar(g) |
| Cascade storage and priority panel | Fill from the lowest bank first; discharge from the highest |
| Dispenser | Fill trailers and vehicles |
Removing the air from the bio-scrubbers
Existing bio-scrubbers injected air into the biogas to support the sulphur-oxidising bacteria. That air is the source of the oxygen. The proposal modified them by adding an aeration tank, so the scrubbing liquor is oxidised outside the gas stream. Oxygen then no longer enters the gas.
Polishing with carbon or iron oxide
Enquiries for sulphide adsorbents were based on 450 Nm³/h of raw gas at 0.5 bar(g), with 200 ppm hydrogen sulphide going in and less than 5 ppm (design less than 2 ppm) coming out, in two vessels in lead-lag arrangement. Over six months, that is about 600 kg of hydrogen sulphide. Check: 450 Nm³/h at 200 ppm is about 0.09 Nm³/h of H₂S, around 0.14 kg/h, or roughly 600 kg over 4,380 hours. Both impregnated carbon and iron oxide media were put out to several suppliers.
If oxygen is still too high: catalytic de-oxo
A catalytic option was also put out to enquiry. Oxygen reacts with hydrogen over a precious-metal catalyst at 150 to 220°C to form water: O₂ + 2H₂ → 2H₂O. The target was oxygen below 0.5% from up to 1.0 to 1.2% going in. With hydrogen at 1.1 to 1.2 times the stoichiometric amount, 1% oxygen in 240 Nm³/h needs about 5.3 to 5.8 Nm³/h of hydrogen, which matches the 5 to 6 Nm³/h electrolyser that was enquired about. That is arithmetic from the enquiry data. The final proposal scope did not include this option.
Drying and storage
The temperature swing adsorption dryer uses two beds: one dries the gas while the other is heated and regenerated with dry biomethane. Enquiries asked for a dew point of -40°C or better at pressure, so that no free water or hydrate forms after compression to 250 bar. Storage was specified as three banks (high, medium, low) holding at least 5,000 Nm³, with a discharge of at least 600 Nm³/h to the dispenser.
Why it works
Each stage protects the next. Sulphide would poison the catalyst and corrode compressors. Water would freeze or form hydrates at high pressure. Oxygen in a compressed cylinder is a safety risk. The cascade arrangement keeps the compressor from running every time a vehicle fills.
For UK context on biomethane, see the government’s anaerobic digestion guidance.
What to consider for your own project
- Find the oxygen source first. Air injection into a bio-scrubber can be removed more cheaply than the oxygen can be cleaned up.
- Specify the end use. Trucked cylinders demand a lower oxygen limit.
- Dry to a pressure dew point that suits the final pressure.
- Size storage for the fill pattern, not only the daily volume.
- Define the exclusions. Civil works, power supply and the flare were outside the scope.
For a related flare question on the same type of site, see our post on a biogas flare without a blower.
FAQ
What is compressed biomethane?
Biogas upgraded to mostly methane and compressed, typically to around 200 to 250 bar, to use as a vehicle fuel.
Why remove hydrogen sulphide first?
It is toxic, corrosive and damages compressors and catalysts.
What does a priority panel do?
It directs gas between the storage banks so the highest bank is used first for discharge and the lowest first for filling.
How long did the project take?
The offer allowed 11 months.
Planning a biomethane plant?
Tell us your raw gas flow and composition and we will outline a process train.