Upgrading biogas from palm oil mill effluent to compressed biomethane (CBM) takes five steps after the digester: remove hydrogen sulphide, dry, compress, strip CO₂ and compress again for vehicles. This post describes the chain proposed for a 60 tonnes per hour mill, in which a covered lagoon supplies both the fuel for vehicles and the power for the plant.
The challenge: vehicle-grade gas from a wet, sour source
The mill’s effluent flow was about 840 m³/day at 80,000 mg/l COD, or roughly 54 tonnes of COD a day. A covered lagoon would produce around 43,000 Nm³/day of biogas at 60 to 70% methane. The gas has to meet European standards for biomethane as a transport fuel (EN 16723-1) and gas quality (EN 16726), and be compressed to 250 bar for dispensing. The plant’s own load, under 500 kW, would be met by a gas engine on the same biogas.
The approach: stage by stage
| Stage | Duty | Design figure |
|---|---|---|
| Biological scrubber | Bulk H₂S removal | 5,000 ppm in, 300 ppm out; two vessels |
| Dewatering | Dry the gas | Dew point about 4°C |
| Booster | Move gas through the plant | 300 mbar, duty and standby |
| Compression and cooling | Raise pressure for clean-up | With re-heating of the gas |
| Activated carbon polishing | H₂S polishing | 200 ppm in, 1 ppm out; four vessels |
| Low-temperature cooling | Prepare gas for CO₂ removal | About -25°C |
| Pressure swing adsorption | Remove CO₂ | Controls CO₂ in the product |
| High-pressure compression and dispensing | Fill vehicles | 250 bar |
The flare has more than one job
The PSA exhaust still contains methane and cannot be vented. The flare burns it, together with off-specification gas and surplus, up to 1,500 Nm³/h. It doubles as an emergency flare.
Cooling the feed
Lagoon feed is best at 35 to 40°C, so a POME cooler is included, either an open air-to-effluent type or an enclosed system.
Why the design is a partnership
The proposal combined the lagoon and gas train with specialist upgrading and dispensing equipment from a partner, plus local civil works. Boundaries were the effluent inlet, the effluent spill-point and the dispensing station. A key assumption is that the refuelling station sits near the plant, otherwise a methane pipeline adds cost.
What to consider for your own project
- Treat H₂S in two stages. The biological stage is cheap and the carbon stage guarantees the final figure.
- Plan for the off-gas. Methane in a PSA exhaust must be burned.
- Locate the dispenser sensibly relative to the plant.
- Power the plant from biogas and size the engine for the site load.
- Confirm the gas standard that applies in your market.
For UK background, see the government’s Anaerobic Digestion Strategy and Action Plan.
FAQ
What is compressed biomethane?
Biogas upgraded to natural-gas quality and compressed for use as a vehicle fuel.
Why is the gas cooled to -25°C?
To condition it before CO₂ removal.
Why two H₂S stages?
Carbon alone would be used up quickly on raw gas.
How long does delivery take?
About 10 months from order, subject to the rainy season.
Planning a biomethane project?
Contact us with your effluent flow and COD.