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Gas treatment before the engine: H₂S, moisture and siloxanes in biogas plants

In biogas and landfill projects, the engine is rarely the weak point — the gas train in front of it is. Treated gas is the difference between an overhaul at the planned interval and a cylinder head job nobody budgeted for.

What you are protecting against

Contaminant	Mechanism of damage	Evidence on the engine
Hydrogen sulphide (H₂S)	Forms acidic compounds with combustion water and with the lubricant	Falling oil TBN, rising iron and lead in oil analysis, corroded bearing shells and liner surfaces
Moisture / free water	Condenses in the gas train and intake, disturbs combustion, dilutes oil	Erratic mixture control, rust in the gas train, emulsion in the crankcase
Siloxanes	Burn to a glass-like abrasive ash that deposits on valves, pistons and turbocharger	Deposit build-up, valve recession, turbo imbalance, spark plug and pre-chamber fouling
Particulates and foam	Plug filters, mixers and valves; disturb air/fuel ratio	Rising filter differential pressure, unstable load, frequent filter changes
Ammonia (some digesters)	Combines with sulphur compounds; influences emissions and oil life	Elevated NOx strategy changes, oil degradation

Ballpark targets — always confirm against your engine platform

The numbers below are the ranges commonly used when specifying biogas engines. They are indicative: the acceptable limit depends on the engine platform, the lubricant chosen and the duty cycle. Treat them as a starting point for the discussion with your supplier, not as a substitute for it.

Parameter	Commonly targeted at the engine inlet	Notes
H₂S	Low hundreds of ppm or better; below about 200 ppm is a common design goal	Sour-gas duty is possible with matched oil and a shorter service interval — but it must be a deliberate decision, written into the offer
Moisture	No free liquid; gas comfortably above its dew point at the lowest ambient temperature	Cooling and knock-out, then re-heating of the line in cold climates
Siloxanes	Low single-digit mg/m³ where carbon filters are used	Landfill gas and waste-derived gas are the usual suspects; test for them if the fuel is municipal
Particulates	Filtered to the engine maker's requirement	Coalescing filters plus drip traps; change interval set by measured differential pressure, not by calendar
Pressure stability	Stable at the mixer; today's digester pressure may not be tomorrow's	Buffer volume and a correctly sized booster are part of the treatment scope

Desulphurisation: the two routes

Biological desulphurisation — air or oxygen dosing into the digester headspace, where bacteria convert H₂S into elemental sulphur and sulphates. Low operating cost, no media to buy, but it needs a well-managed digester, careful air dosing, and it reduces methane content slightly. Best suited to digester gas with moderate H₂S and a skilled operator.
Dry media desulphurisation — iron-oxide or impregnated activated carbon in vessels. Predictable outlet quality, easy to monitor, works for high H₂S and for landfill gas. Cost is the media itself and the labour to change it; the spent media needs a disposal route.

Many projects end up with a combination: biological treatment to cut the bulk load in the digester, dry media as the polishing step that guarantees the engine inlet specification. The right split depends on your H₂S profile over a year — which is why a single spot sample is not enough.

Drying and cooling

Biogas leaves a digester saturated with water vapour. As it cools in pipes, in the gas train or in the enclosure on a cold morning, that water becomes liquid — and liquid is what damages engines and instruments. A practical treatment chain usually includes:

Gas cooling with a heat exchanger sized for the worst-case summer condition
Knock-out drums and automatic drain traps at the low points
Coalescing filtration downstream of the knock-out
Line heating or insulation where ambient temperature drops near or below the gas dew point

Monitoring: what to measure, and how often

Gas composition every 3–6 months for CH₄, CO₂, O₂, H₂S and (for municipal waste gas) siloxanes — plus one analysis at commissioning as the baseline
Continuous or frequent H₂S measurement at the engine inlet, so desulphurisation performance is visible before oil analysis shows the damage
Oil analysis every 250–500 running hours — the cheapest early-warning instrument in the plant
Filter differential pressure logged, so filter changes follow reality instead of a fixed calendar

How this changes the commercial side of the project

Two consequences buyers often miss:

Output and consumption guarantees depend on the gas. A guarantee given against assumed gas quality is only valid for that quality. If gas treatment is out of scope, the guarantee margins have to be conservative — and the offer should say so explicitly.
Warranty terms depend on the treatment working. Engine warranties are typically conditional on gas quality being within an agreed window, and on oil analysis being performed. Agree the window, the monitoring routine and who owns each piece before shipment, not after the first failure.

FAQ

Can an engine run on untreated biogas?

It can start and run — and then it will tell you about it in the oil analysis. Raw digester gas with high H₂S and free moisture means a much shorter oil life and an earlier overhaul. If you plan to run untreated, tell the supplier so the oil specification, service interval and warranty position are set honestly from the start.

Who should supply the gas treatment?

Either you or the generator supplier, but someone must own the interface: the gas composition and pressure at the engine inlet. Undefined interfaces between two suppliers are where commissioning delays live.

How do I know if my gas has siloxanes?

Test for them if your feedstock includes municipal waste, sludge, or landfill material. Digester gas from agricultural feedstock is usually low. One lab analysis is far cheaper than a deposit-related turbocharger failure.