Key points
- Raw biogas is not just methane. It also carries CO₂, water vapour, hydrogen sulphide (H₂S) and sometimes siloxanes.
- When H₂S is burned, it forms sulphur oxides, which turn into acid. That acid corrodes engine parts and spoils the lubricating oil.
- Published H₂S limits for gas engines vary widely, and lower is always better for engine life. Your engine maker's own gas specification is the limit to follow.
- Biological desulphurisation, iron-based adsorbents and activated carbon are the most common cleaning methods, often used together.
What is inside raw biogas
| Component | Typical presence | Why it matters |
|---|---|---|
| Methane (CH₄) | 45–75% by volume (IEA) | The fuel. More methane means more energy per m³ |
| Carbon dioxide (CO₂) | Most of the remainder | Does not burn, so it dilutes the gas. Removed only when upgrading to CBG |
| Water vapour | The gas leaves the digester saturated | Condenses in pipes, and mixed with H₂S it forms acids |
| Hydrogen sulphide (H₂S) | Varies widely with the feed. Protein-rich and sulphate-rich wastes give more | Corrosion and oil damage in engines |
| Siloxanes | Mainly in sewage and landfill gas | Burn into hard silica deposits on valves, pistons and spark plugs |
Why H₂S is the main enemy of a biogas engine
H₂S is the gas that smells of rotten eggs. In an engine it burns into sulphur dioxide, and with water that forms acid. The classic GTZ/GATE biogas guidance, reproduced by the FAO, warns that hydrogen sulphide is especially harmful in internal-combustion engines. H₂S and its combustion product quickly lead to severe corrosion and wear.
Researchers at Hochschule Wismar describe the same chain: H₂S becomes sulphuric acid during combustion, which causes corrosion and shortens the life of the engine oil. In practice, that means more frequent oil changes, more wear and more maintenance stops.
How clean is clean enough?
Different sources quote very different limits. An IISc paper notes that the limit for internal-combustion engines depends on the individual engine maker, and that the lower the H₂S, the better for engine life. Many modern gas engines ask for far less than the upper figures in older literature. The safe rule is to use the limit in your engine maker's gas specification and to keep H₂S as low as is practical.
The main cleaning methods
Biological desulphurisation
Sulphur-oxidising bacteria turn H₂S into elemental sulphur using a small, controlled dose of air or oxygen. This happens either in the digester's gas space or in a separate biological filter or scrubber. Running costs are low, but the air dose must be controlled carefully.
Iron-based adsorbents
The gas passes through a bed of iron oxide or iron hydroxide, which binds the sulphur. The GTZ/GATE guidance points to dry desulphurisation with iron-based material as the practical option for small plants. The media must be regenerated or replaced when it is spent.
Activated carbon
Often used as a final “polishing” step for remaining H₂S. It also removes siloxanes, which matters for sewage gas.
Moisture removal
Condensate traps at low points in the pipeline, and gas cooling (a chiller) followed by reheating, keep water out of the engine and help the other cleaning steps work.
In practice, plants combine methods. Biological desulphurisation removes the bulk of the H₂S, moisture removal protects the line, and an adsorbent bed polishes the gas before the engine.
Siloxanes: the sewage-gas problem
Siloxanes are silicon compounds from soaps, shampoos and cosmetics that end up in sewage, and so in sewage sludge and landfills. When burned, they leave hard, glassy silica deposits that scratch cylinders and foul valves and heat exchangers. Gas from manure, food waste or starch effluent usually contains little. Gas from STPs and landfills often needs a dedicated adsorption step. Research on combined adsorbents has reported removing 90% or more of siloxanes at the same time as H₂S.
A simple gas-quality routine
- Measure H₂S and methane regularly with a gas analyser, more often after any change in the feed.
- Drain condensate traps on a fixed schedule.
- Log oil analysis. Rising acidity in the oil is an early warning that gas cleaning needs attention.
- Track the cleaning media. Note when beds were last changed, and watch the H₂S after the bed rise as the media wears out.
- Watch the feed. Protein-rich or sulphate-rich wastes raise H₂S. Plan for that before adding them.
Clean gas is the cheapest insurance for a biogas engine. To see how gas quality affects the electricity you get, read how much power 1 m³ of biogas gives.
Frequently asked questions
What H₂S level is safe for a biogas engine?
Published limits vary widely by engine maker, and lower H₂S always means longer oil and engine life. Follow your own engine maker's gas specification.
What does H₂S do to an engine?
It burns into sulphur oxides that form acids, which corrode the combustion chamber, exhaust and bearings and degrade the lubricating oil.
What is biological desulphurisation?
Sulphur-oxidising bacteria turn H₂S into elemental sulphur using a small, controlled dose of air, either in the digester's gas space or in a separate biological filter.
Do all biogas plants need siloxane removal?
Mainly sewage and landfill gas plants. Siloxanes come from soaps and cosmetics in sewage. Gas from manure, food waste or starch effluent usually contains little.
Sources
- IEA: An introduction to biogas and biomethane
- FAO: Biogas — output and its use (GTZ/GATE guidance)
- Hochschule Wismar: H₂S removal from biogas (research project)
- IISc: Hydrogen sulphide removal from biogas
- Detritus journal: Simultaneous removal of H₂S and siloxanes
Figures are as published in the sources above. Plant results vary with the waste, the design and how the plant is run. Last reviewed 17-10-2026.
