The most reliable way to remove siloxanes from landfill gas is to chill the gas to about -30°C so most of the siloxane condenses out, then pass it through activated carbon as a polishing stage. A 3,000 Nm³/h proposal for a UK landfill used exactly that train, aiming to bring siloxane from a design range of 200 to 1,000 mg/m³ down to below 1 mg/Nm³ before the gas reached the engines.
The challenge: silicon that turns into deposits
Siloxanes are organic silicon compounds that end up in landfill gas from everyday products. Cosmetics, detergents, building materials, paper coatings and textiles all contribute. They are not aggressive in terms of emissions, but in a gas engine they burn to form hard white or grey silica deposits on the cylinder head, pistons and valves. A golden lacquer also builds on liner surfaces that the lubricating oil touches.
The consequences are practical:
- The engine has to be stripped and the deposits scraped off by hand.
- Engine oil must be changed more often, and manufacturers recommend monitoring silicon in the oil.
- Engine life is reduced.
The proposal makes a candid point about how this plays out. Two identical engines on two high-silicon sites can behave very differently, so the usual approach has been trial and error. And because siloxanes do not drive exhaust emissions, the decision to treat the gas is made on cost, not compliance.
The approach: a single skid in four steps
The system was a single skid, about 2.2 m wide and 5 m long, designed to sit between the gas extraction system and the engines or boiler. It worked in this order:
- Inlet riser: a galvanised riser with a coarse screen separates free liquids and gives an inspection point for foreign objects.
- First condenser and knock-out: a stainless steel shell-and-tube condenser, fed with chilled water, cools the gas and drops the dew point. Condensate falls into a cyclonic knock-out pot with a polypropylene demister.
- Cryogenic stage: a second condenser takes the gas to about -30°C. Any residual condensate forms as ice and is cleared by thermal swing.
- Filtration and carbon: a 3 micron filter of F9 classification catches fine particles, and a granular activated carbon (GAC) column removes what is left.
Why carbon is the last step, not the first
Carbon alone would be exhausted quickly by a high siloxane load. Chilling does the heavy lifting, and the proposal sizes the carbon bed to deal with the residual 10% or so that passes a cryogenic condenser. That also makes the carbon a buffer against extremes of siloxane concentration, which can vary a lot within a landfill.
Design basis
| Parameter | Value | What it means |
|---|---|---|
| Maximum gas flow | 3,000 Nm³/h | Sized for a large engine installation |
| Methane | 55 to 65% | Typical mature landfill gas |
| Inlet temperature | 25 to 45°C, saturated | Gas arrives wet and warm |
| Dew point after treatment | -30°C | No condensation downstream, even in cold weather |
| Siloxane in | 200 to 1,000 mg/m³ | A wide design range, since the real figure is rarely known |
| Siloxane out | below 1 mg/Nm³ | Target for engine protection |
The scope was a single skid used with a separately supplied flare and booster set. It did not include liquid-handling equipment, so the proposal assumes the inlet gas has already been dewatered upstream.
What it costs to run
The proposal lists the running requirements without hiding them:
- Labour: a technician check once a week, plus a monthly visit from a refrigeration engineer for the chiller.
- Consumables: only activated carbon. An allowance for one replacement a year was suggested, although the carbon takes minimal load and may last longer.
- Electricity: with technician time, one of the two main cost centres.
- Delivery: 18 to 20 weeks ex-works.
Civil work is modest. The recommended base is a reinforced concrete slab about 3.2 m by 6 m and 250 mm deep, with a gravel surround, and no extra planning or permitting was expected because the unit would form part of existing site equipment.
What to consider for your own project
- Sample the gas. Siloxane levels vary widely between sites. A gas analysis tells you whether you need cryogenic treatment or carbon alone.
- Check your engine maker’s limit. Warranty terms often set a siloxane ceiling.
- Dewater first. The skid is not designed for liquids, so bulk condensate must be removed upstream.
- Plan for the carbon. Budget for replacement carbon and make sure a site contractor can change it.
- Prepare the slab and power supply. They are the client’s responsibility and are needed before commissioning.
The Environment Agency publishes landfill gas guidance on the Environment Agency pages of gov.uk, and engine makers set their own fuel gas limits.
FAQ
What do siloxanes do to a gas engine?
They burn to form silica deposits in the combustion chamber and a lacquer on cylinder liners, which shortens engine life and increases oil changes.
Why chill the gas to -30°C?
At that temperature most siloxanes and moisture condense out of the gas, which is far more efficient than relying on carbon alone.
Is activated carbon enough on its own?
For low siloxane levels, possibly. For the high design range in this proposal, chilling takes the bulk and carbon polishes the remainder.
How often must the carbon be replaced?
The proposal suggested budgeting for one change a year, while noting that it might not be needed because the carbon is only a polishing step.
Is siloxane damaging your engines?
Send us your gas analysis and we will advise whether chilling, carbon or both is right for your site.