To run a gas engine on lagoon biogas, you have to remove hydrogen sulphide and dry the gas, in that order. A proposal for a palm oil mill in Indonesia did this with three bioscrubber towers followed by a chiller-based dehumidifier, for biogas carrying up to 28,000 ppm of H₂S at about 1,440 Nm³/h.
The challenge: very high sulphide and wet, warm gas
The biogas came from an anaerobic lagoon treating mill wastewater. At 28,000 ppm, H₂S is about 2.8% of the gas by volume, which is far above what an engine can tolerate. The gas also arrives hot (up to 50°C) and saturated. When it cools in the pipework, condensate forms and carries contaminants, including H₂S, that can damage engines, burners and boilers very quickly.
The target was an outlet of 200 ppm H₂S, a reduction of more than 99%, with a dew point of 15°C so that no condensation forms downstream. Particle filtering and gas blowers were outside this scope.
The approach: three towers and a dehumidifier
Bioscrubbers
Three vertical GRP towers, each about 13 m tall and 3.5 m in diameter, hold packing on which sulphur-oxidising bacteria grow. Gas enters at the base and rises through wet packing. Air is introduced to feed the bacteria, and pH is controlled by draining some circulating liquid and replacing it with lagoon effluent, which supplies the carbon and nutrients. Spent liquid is described as safe for a wastewater drain.
Features worth noting at this sulphide level:
- Automatic sulphur cleaning: at high loads, elemental sulphur builds up in the tanks, so an auto-cleaning system is included.
- Multiple access hatches for long-term maintenance.
- Combined pressure relief valve and flame arrester on each tank, plus Zone 2 or intrinsically safe electrics.
- Oxygen cells on each tower, with a pressure switch on each feed line to detect flow loss.
- 10% spare packing supplied.
Dehumidifier
The dehumidifier is a water-cooled shell-and-tube condenser fed by an electric air-cooled chiller. Gas passes through an inlet riser with a coarse screen, then the condenser, then a cyclonic knock-out pot with a polypropylene demister that removes 99% of particles above 6 microns. All gas-wetted parts are 304 stainless steel. The base case lowers the dew point below minimum ambient temperature. The standard configuration can reach about 4°C, and a deeper option at about -30°C would also control siloxanes.
Design basis
| Parameter | Value |
|---|---|
| Gas flow | 1,440 Nm³/h design (sized for up to 1,800 in the summary) |
| Methane | 50 to 60% |
| Inlet H₂S | up to 28,000 ppm |
| Outlet H₂S | 200 ppm |
| Outlet dew point | 15°C |
| Delivery | about 8 months |
Why this order
The bioscrubber adds moisture, because gas leaves it saturated. Drying therefore always follows it. Removing H₂S first also stops it dissolving into condensate and corroding the dehumidifier.
The proposal notes that very low outlet levels may need a polishing step, such as an iron sponge or activated carbon bed, or more bioscrubber area.
What to consider for your own project
- Measure the peak H₂S, not the average. At this level it decides the number of towers.
- Check your engine maker’s limits for H₂S, moisture and dew point.
- Decide where particles are removed; that was excluded here.
- Allow for make-up liquid of suitable quality.
Related reading: horizontal bioscrubber sizing and packed-bed bioscrubber design.
FAQ
Why not dry the gas first?
Because the bioscrubber saturates the gas, drying must come after it.
Why three towers?
The very high sulphide load needs more contact area and bacterial capacity than one vessel gives.
What dew point is needed?
Below the lowest ambient temperature the pipework sees, so nothing condenses.
Can the dehumidifier remove siloxanes?
A deeper chill to about -30°C, followed by carbon, can.
Preparing biogas for an engine?
Share your gas analysis and we will outline a treatment train.