To turn landfill gas into biomethane for a gas network, you must remove hydrogen sulphide, strip out carbon dioxide, dry the gas and filter it, then deliver it at the right pressure. This article follows a 500 Nm³/h feed train proposed to a European gas utility for a landfill near a major city, and shows the purpose of each stage.
The challenge: raw landfill gas is not pipeline gas
Landfill gas arrives saturated with moisture and carries hydrogen sulphide (H₂S), carbon dioxide and fine particles. When the gas cools, condensate forms that carries contaminants including H₂S, which can damage engines, burners and boilers very quickly. For grid injection the demands are tighter still.
The design basis was a maximum of 500 Nm³/h and a minimum of 100 Nm³/h, with 50 to 60% methane, ambient temperatures from 20 to 45°C and inlet H₂S up to 2,000 ppm.
The approach: five stages
1. Bioscrubber for H₂S
Gas passes along a horizontal fibreglass vessel filled with wet packing where bacteria consume H₂S. They need oxygen, supplied by a modulating air valve (under suction) or a variable-speed blower (under pressure) controlled from an oxygen cell at the outlet. The pH is held by controlled drainage and make-up with a nutrient source such as lagoon effluent. The performance guarantee was a drop from up to 2,000 ppm to 200 ppm or less at 500 Nm³/h.
Online H₂S analysers are limited to about 1,000 ppm, so higher readings are checked with colour-change detector tubes.
2. Blower
A stainless steel multi-stage centrifugal blower draws gas through the scrubber and delivers it at about 250 mbar after losses, with PID pressure control.
3. Chiller
Cooling the gas to remove moisture means no condensation in downstream pipework. The design discharge was below 40°C with a controlled dew point.
4. Pressure swing adsorption for CO₂
PSA removes carbon dioxide to the level specified by the network. It is sized for 500 Nm³/h of biogas, with final layout to be confirmed after a site visit. Two oxygen sensors check the residual gas and compare readings; an alarm sounds if they disagree, and shutdown can be linked to high oxygen. An optional cryogenic stage was offered for oxygen removal.
5. Filtration
A glass-fibre filter of EN 779 class F9 (initial resistance 175 Pa, more than 95% average efficiency) protects downstream equipment.
Questions the buyer asked
The client’s clarification questions are instructive:
- Maximum flow is the nominal flow: 500 Nm³/h.
- Permits. The customer obtains construction, activity and environmental permits using technical information from the supplier, who handles equipment certificates and tests.
- Injection pressure. The network injection pressure was 4 bar g, with a regulating valve range of 6 to 16 bar g, so the supplier could choose the best outlet pressure for the technology.
- Condensate. Volumes are small and could be sent to the leachate lagoon, provided it is characterised.
What to consider for your own project
- Know the contaminants. Siloxanes, H₂S and oxygen all matter for upgrading.
- Agree the gas specification with the network operator before choosing technology.
- Get permit responsibilities written down.
- Allow site time. Layout of PSA depends on the visit.
- Plan a flare. Optional flare combustion temperature depends on gas and regulation. See LFTGN05.
UK readers can find background in the Anaerobic Digestion Strategy and Action Plan and LFTGN03.
FAQ
What does PSA do?
It separates carbon dioxide from methane using adsorbent beds and pressure changes.
Why remove H₂S first?
It protects downstream equipment and the adsorbent.
How long does delivery take?
The proposal quoted 5 to 6 months.
Upgrading landfill gas?
Send us your gas analysis and target specification.