Biogas from a covered lagoon must be desulphurised, dried and pressurised before an engine or upgrading plant can use it. A bioscrubber removes H₂S, a chiller removes moisture and a blower delivers the gas at 300 mbar. This post follows the train in a proposal for a 63 tonnes per hour palm oil mill, sized for 1,250 Nm³/h.
The challenge: gas that is sour, wet and at no pressure
The lagoon produces biogas with about 3,000 ppm of hydrogen sulphide, saturated with water at around 40°C. Engines need low H₂S, low humidity and a regulated supply pressure.
The approach, in order
1. Bioscrubber under suction
Two vertical tanks in parallel, each 13 m high and 3.5 m in diameter, packed with media on which bacteria grow. The bacteria come from the effluent. The gas flows under suction of no more than 25 mbar, because the blower sits after the treatment, and H₂S falls from 3,000 to about 200 ppm in this proposal (a similar proposal quoted 50 ppm). Exit gas is saturated at about 40°C.
2. Chilling
The gas passes through a condenser cooled by a chiller to between 4 and 15°C, where most of the water drops out. The target dew point is 4°C.
3. Blowers
Duty and standby centrifugal blowers, each 1,250 Nm³/h at 300 mbar. Compression heats the gas by roughly 25°C, so the outlet is no more than 40°C and about 40% relative humidity. This reheating is useful: it moves the gas well away from saturation, which protects the engines.
4. Standby flare
An open-flame flare of 1,250 Nm³/h, with a 5:1 turndown. Pipework lets gas bypass the bioscrubber to the flare.
Why the order matters
- The scrubber sits first, in the suction side, because its exit gas is saturated and needs drying after it.
- Drying comes before the blower so that the blower handles dry gas and the heat of compression lowers humidity.
- Safety devices, such as flame and lightning arrestors, are included in the train.
The lagoon and its feed
Effluent first enters a combined structure, a concrete buffer tank that mixes it with recirculated lagoon liquid and controls feed, mixing and overflow. A carbonic acid unit manages pH without chemicals. The lagoon is fed from the bottom along pipes the length of the reactor floor. The POME is first cooled in a cross-flow cooling tower and screened.
What to consider for your own project
- Specify inlet H₂S and the engine’s maximum.
- Keep the scrubber flow above its minimum.
- Allow for a duty and standby blower.
- Locate the powerhouse close to the plant to limit pipework.
See our posts on biological scrubbing with carbon polishing and sizing a covered lagoon.
FAQ
Why is the blower after the scrubber?
So that the scrubber runs under gentle suction and the blower handles treated gas.
Why cool the gas and then heat it?
Cooling removes moisture, and compression reheats the dry gas.
Is a standby blower necessary?
It keeps the engines running during maintenance.
Where does the flare take gas from?
It can bypass the scrubber.
Designing a biogas train?
Contact us with your gas flow and H₂S.