Siloxane Removal from Biogas: Why Activated Carbon Is Still the First Choice

A Practical Engineering Guide for Reliable Biogas Purification

Quick Answer

Siloxanes may only exist in biogas at a few milligrams per cubic meter, but they can become one of the most expensive contaminants to ignore. Once the gas is burned, siloxanes turn into hard silica (SiO₂), which gradually coats pistons, spark plugs, valves, turbochargers, and heat exchangers. Left untreated, these deposits reduce engine efficiency, increase maintenance costs, and shorten equipment life.
For most commercial biogas projects, activated carbon remains the preferred solution because it combines high removal efficiency with relatively simple operation and reasonable operating costs. The catch is that carbon performance depends on the entire system—not just the carbon itself. Moisture, H₂S, gas temperature, contact time, and vessel design all affect how long the media will last.

Key Takeaways

Activated carbon is currently the most widely adopted commercial technology for siloxane removal in biogas treatment.
Silica deposits formed during combustion are the primary cause of engine wear, not siloxanes themselves.
Adsorption performance depends on pore structure, gas humidity, contact time, and competing contaminants—not simply on iodine value.
No single adsorbent is suitable for every project; technology selection should be based on operating conditions and lifecycle cost.
Well-designed pretreatment and monitoring strategies can significantly extend the service life of activated carbon.

Activated Carbon vs Other Adsorbents at a Glance

Introduction

Whether biogas is used to generate electricity or upgraded into renewable natural gas (RNG), it has one thing in common—it needs to be cleaned before it reaches expensive equipment.

Most operators pay close attention to H₂S and moisture. Siloxanes are easier to overlook, but they often cause the most expensive long-term damage.

This article explains why activated carbon remains the preferred solution, how it compares with other adsorbents, and what engineers should consider when designing a reliable siloxane removal system.

What Are Siloxanes

Siloxanes are volatile silicon‑based compounds. They show up in shampoos, detergents, cosmetics, lubricants, sealants, and silicone rubber. They resist heat and water, which makes them useful in manufacturing, but problematic in biogas.

Most siloxanes in biogas come from human waste. Silicone products go down the drain or into landfills. During digestion or decomposition, volatile siloxanes evaporate into the gas and stay there through collection and transport.

The most common ones in landfill gas and digester gas are cyclic: D4 (octamethylcyclotetrasiloxane) and D5 (decamethylcyclopentasiloxane). Smaller amounts of D3, D6, and linear species may also appear.

Concentrations vary a lot. Municipal WWTPs usually have higher levels due to personal care products. Agricultural digesters have much less. Landfill gas is the most variable, depending on waste age and type.

Always run a gas analysis before picking an adsorbent. Skip that, and you risk early breakthrough or wasted expense.

Why Remove Siloxanes

Siloxanes are stable in the gas phase. The trouble starts when they burn.

Engine, turbine, or boiler temperatures are high enough to break down the silicon compounds. Silicon oxidises to SiO₂ – silica. That mineral is extremely hard.

Unlike gaseous combustion products that exit with exhaust, silica stays inside. It piles up on cylinder heads, pistons, spark plugs, valves, turbochargers, heat exchangers, and catalysts.

Effects:

• Lower efficiency
• Higher fuel consumption
• Raised exhaust temperature
• Frequent spark plug changes
• Poor heat transfer
• More maintenance
• Shorter equipment life
• Higher operating cost

For RNG plants, poor siloxane removal also hurts downstream upgrading gear. Though concentrations are low, the cumulative damage over thousands of hours is big. Preventing silica is far cheaper than removing it later.

How Activated Carbon Works

Activated carbon removes siloxanes mainly by physical adsorption.

Gas flows through the bed. Siloxane molecules diffuse from the gas phase to the carbon surface, then enter the pore network and are trapped by intermolecular forces. No chemical reaction.

The carbon’s pore structure does the job. Three pore sizes matter:

A common myth: higher iodine value means better siloxane removal. Not true. Iodine value mainly measures micropores. Large siloxane molecules need a balanced pore network with good diffusion paths. Two carbons with similar iodine numbers can have very different breakthrough times under the same plant conditions.

Key factors affecting performance:

• Relative humidity
• Gas temperature
• Empty Bed Contact Time (EBCT)
• Gas velocity
• Siloxane concentration
• H₂S and VOC levels
• Particle size and pore distribution

Humidity is often the biggest practical issue. Water vapor competes for sites. If you don’t knock out moisture upstream, capacity drops.

H₂S and other organics also occupy sites or change surface chemistry, shortening bed life. That’s why many plants remove H₂S before the carbon bed, rather than trying to handle everything in a single vessel.

Good engineers don’t just look at BET surface area or iodine number. They demand dynamic adsorption data measured under conditions close to the actual application.

How to Choose the Right Activated Carbon for Siloxane Removal

One of the most common questions from plant operators and EPC contractors is whether coconut shell, coal-based, or pelletized activated carbon performs better for siloxane removal.

The answer is simple: there is no universal “best” activated carbon. The right choice depends on the biogas composition, operating conditions, pressure drop requirements, and maintenance strategy. A carbon that performs well in one project may not deliver the same service life in another.

Coal-Based vs Coconut Shell Activated Carbon

Selecting the carbon base material is often more important than chasing a higher iodine value.

Coal-Based Activated Carbon

Advantages

• Balanced micropore and mesopore structure
• Performs well with mixed siloxanes and VOCs
• Cost-effective for large biogas systems
• Widely used in landfill gas and municipal wastewater projects

Considerations

• Slightly higher dust generation than coconut shell grades
• Product quality varies between manufacturers

Coconut Shell Activated Carbon

Advantages

• High mechanical strength
• Low dust generation
• Excellent hardness
• Suitable where media handling and durability are priorities

Considerations

• Higher cost
• A highly microporous structure is not always ideal for larger siloxane molecules

Granular or Pelletized?

Both forms are widely used, but they are designed for different operating conditions.

Granular activated carbon is often selected where adsorption capacity is the primary concern.

Pelletized activated carbon provides lower pressure drop and better mechanical strength, making it a common choice for larger continuous-flow biogas systems.

Rather than asking which carbon is universally better, engineers usually select the material that best matches the gas composition, vessel design, and operating objectives.

Activated Carbon vs Alternatives

Several commercial options exist. Each has pros and cons, depending on gas quality and economics.

Activated carbon is the most common – high efficiency, simple operation, moderate capital. Bed replacement is easy, vessel design is straightforward, and it handles the typical swings in landfill and digester gas.

Molecular sieves give very high purity and selectivity, but they need bone‑dry feed gas. Capital and operating costs are much higher. Usually only for specialised biomethane upgrading, not CHP.

Silica gel adsorbs siloxanes when dry and can be thermally regenerated. But it’s hydrophilic – not good for wet raw biogas. Best used as a polishing step after drying.

Activated alumina is mainly for dehydration. Not a primary siloxane adsorber in most plants – it’s there to reduce moisture load before the carbon vessel.

Refrigeration/condensation knocks out some siloxanes along with condensed water, helping downstream adsorption. But cooling alone rarely gets siloxanes low enough for engines or RNG specs. It’s a pretreatment, not a standalone fix.

Large biomethane plants increasingly use hybrid trains: cooling + dewatering + H₂S removal + activated carbon + membrane. Higher upfront cost, but often the lowest lifecycle cost for big capacities.

Common Mistakes When Selecting Activated Carbon

Common Mistakes When Selecting Activated Carbon

Many adsorption systems fail to achieve the expected service life not because of poor activated carbon quality, but because of avoidable design and purchasing mistakes.

Mistake 1: Choosing carbon based only on iodine value

A high iodine value does not automatically mean better siloxane removal. Dynamic adsorption performance depends on pore size distribution, gas composition, humidity, and operating conditions.

Mistake 2: Ignoring moisture

Moisture occupies valuable pore volume and can significantly reduce adsorption capacity. Installing proper gas cooling and condensate removal often delivers a greater improvement than simply using more activated carbon.

Mistake 3: Expecting one carbon to remove everything

Trying to remove H₂S, siloxanes, VOCs, and moisture with a single carbon bed usually shortens media life.

For most commercial projects, staged treatment provides better long-term economics.

Mistake 4: Buying the cheapest activated carbon

The purchase price of activated carbon is only part of the operating cost.

Replacement frequency, transportation, disposal, downtime, and maintenance often have a much greater impact on lifecycle cost than the initial media price.

Mistake 5: Skipping outlet gas monitoring

Replacing activated carbon based only on a fixed schedule often wastes adsorption capacity or risks unexpected breakthrough.

Regular outlet gas monitoring provides a more reliable basis for media replacement.

How Engineers Design a System

Selecting carbon isn’t about comparing datasheets. It’s about the whole system – gas composition, operating conditions, vessel geometry, and maintenance plan.

Start with a full gas analysis. Besides CH₄ and CO₂, measure siloxanes, H₂S, moisture, VOCs, and particulates. These interact inside the bed. Miss one, and you get early exhaustion or erratic performance.

Then choose carbon with the right physical properties. Ignore iodine and BET alone – look at pore size distribution, crush strength, abrasion resistance, bulk density, and pressure drop. Dynamic adsorption tests under realistic conditions are far more useful than lab indices.

Vessel design matters. EBCT is critical – the average time gas spends in the bed. Longer EBCT gives more diffusion time, improving removal and extending breakthrough. But longer EBCT means bigger vessels and higher capital. Find the optimum balance; don’t just max it out.

Pressure drop is another constraint. High velocity, poor flow distribution, or dust buildup raise energy use and cut efficiency. Good vessels have inlet distributors to avoid channeling and use the whole bed.

For medium to large plants, lead‑lag configurations work well. The first vessel takes the bulk load, second polishes. When breakthrough starts in the first, the second still protects downstream, so you can change spent carbon without shutting down. This boosts reliability and makes better use of the total capacity.

Monitor outlet gas regularly. Don’t rely on a fixed replacement schedule. Many operators sample outlet quality to catch early breakthrough. That reduces unnecessary changeouts and keeps silica away from engines or upgrading gear.

The goal isn’t maximum theoretical capacity – it’s minimum total cost per cubic metre treated.

Common Design Mistakes

Systems often underperform not because of bad carbon, but due to avoidable errors.

Biggest mistake: picking carbon by iodine value alone. It doesn’t predict siloxane performance. Big molecules need a balanced pore structure with good diffusion.

Second: ignoring moisture. Saturated gas fills pores with water, cutting effective capacity. A simple cooler/knockout upstream often gives more benefit than just adding more carbon.

H₂S matters too. If sulfur is high, upfront desulfurisation extends carbon life and improves siloxane removal.

Flow distribution gets overlooked. Uneven flow creates shortcuts – part of the bed sits unused while breakthrough happens early elsewhere.

And many buyers focus only on carbon price. In reality, media cost is often a small part of total operating expense. Transport, labour, disposal, downtime, and downstream protection often dominate total ownership cost.

Case Study – Municipal WWTP

A municipal wastewater plant with a CHP engine kept having spark plug and cylinder head fouling from silica. They already controlled H₂S, but not siloxanes.

Plant data:

• Source: digester gas
• Flow: 1,200 Nm³/h
• Use: CHP
• Contaminants: H₂S, moisture, siloxanes
• Existing pretreatment: cooling + H₂S removal

After gas analysis, they installed a duplex activated carbon system downstream of desulfurisation. They also improved condensate separation, optimised flow distribution, and started regular outlet monitoring to set replacement timing.

Results:

• Stable siloxane removal
• Much less silica on engine parts
• Longer maintenance intervals
• Higher engine availability
• Lower overall maintenance cost, despite periodic carbon changes

Takeaway: system design and operating strategy often matter more than the carbon grade.

Best Practices

From field experience, these steps work:

• Get a complete gas analysis before choosing adsorbent.
• Remove condensate and excess moisture before the carbon bed.
• Take out H₂S upstream if sulfur levels are significant.
• Design for proper EBCT – don’t undersize vessels.
• Use lead‑lag vessels for continuous operation.
• Monitor outlet gas – don’t rely on calendar‑based changes.
• Select carbon based on dynamic performance and field track record, not just lab numbers.

These guidelines maximise carbon utilisation and cut the risk of breakthrough.

Frequently Asked Questions (FAQ)

1. What is the best adsorbent for siloxane removal from biogas?

There’s no single “best” adsorbent that fits all cases. For most landfill, wastewater, and digester projects, activated carbon offers the most practical balance of efficiency, simplicity, and lifecycle cost. If you need ultra‑high purity or have special process constraints, molecular sieves or hybrid systems may be worth the extra investment.

2. Can activated carbon remove both H₂S and siloxanes?

Yes, but how well depends on the carbon type and operating conditions. Standard activated carbon can take up some of both, while impregnated grades are specifically designed for H₂S. In practice, many plants remove H₂S first to avoid competition and extend the siloxane bed’s service life.

3. How often should activated carbon be replaced?

There isn’t a fixed schedule – it varies with siloxane loading, flow rate, humidity, temperature, pore structure, and EBCT. Rather than guessing, most operators monitor outlet gas quality and change carbon only when breakthrough approaches the plant’s allowable limit.

4. Does a higher iodine value mean better siloxane removal?

Not necessarily. Iodine value mainly indicates micropore volume, but siloxane adsorption also needs adequate mesopores and diffusion pathways. Dynamic performance tests under realistic conditions are far more reliable than iodine number alone.

5. Does humidity affect siloxane adsorption?

Yes, humidity has a major impact. Water vapour competes for the same pore space and can substantially cut effective capacity. Installing cooling and condensate removal upstream of the carbon bed is one of the most cost‑effective improvements you can make.

6. Can activated carbon be regenerated after siloxane adsorption?

Thermal regeneration is possible under controlled conditions, but it rarely restores full original capacity, especially after multiple cycles. For most biogas applications, simply replacing spent media is more practical and economical than setting up regeneration.

7. Why do two plants using the same activated carbon achieve different service lives?

Because performance depends on the whole operating environment – not just the carbon. Differences in siloxane concentration, humidity, H₂S level, temperature, EBCT, vessel design, and flow distribution all affect breakthrough time, so replacement intervals often vary widely even with the same product.

8. How can operators maximize activated carbon service life?

Optimise the entire purification train, not just the adsorbent. Good moisture removal, effective H₂S pretreatment, adequate EBCT, uniform gas distribution, and regular monitoring all contribute to longer bed life and lower operating costs.

9. Need help selecting activated carbon for your biogas project?

XingSen Carbon’s technical team can recommend suitable activated carbon grades based on gas composition, siloxane concentration, operating conditions, and project requirements.

Conclusion

Siloxane removal is now essential for CHP engines, turbines, and RNG plants. Low concentrations cause big cumulative damage via silica deposits.

Activated carbon remains the most widely used commercial solution – good performance, flexible operation, reasonable cost. But reliable results depend on much more than the product.

Successful projects start with gas analysis, then address every design aspect: pretreatment, vessel layout, contact time, monitoring, and maintenance. Projects that take this integrated approach get longer bed life, stable gas quality, and lower lifecycle cost.

As biomethane expands, hybrid systems, better materials, and smarter monitoring are emerging. The best solution is one that delivers required gas quality under real conditions – not the one with the best lab numbers.

References

For technical validation and further reading, the following publications are widely recognized within the biogas and adsorption industries:

1. IEA Bioenergy Task 37. Biogas Upgrading Technologies – Developments and Innovations.
2. U.S. Environmental Protection Agency (EPA). Landfill Gas Energy Project Development Handbook.
3. Renewable Energy. Research articles on siloxane occurrence and removal in biogas upgrading systems.
4. Fuel. Studies on adsorption mechanisms of organosilicon compounds using activated carbon.
5. Environmental Science & Technology (ACS Publications). Research on adsorption behavior of cyclic siloxanes and porous carbon materials.
6. Waste Management. Reviews of biogas contaminants and purification technologies.
7. Journal of Hazardous Materials. Studies on adsorption kinetics and breakthrough behavior of activated carbon.
8. Chemical Engineering Journal. Research on advanced adsorbents for VOC and siloxane removal.

About XingSen Carbon

Selecting activated carbon isn’t about picking the highest iodine number. Different gas compositions need different pore structures, operating strategies, and vessel designs.

XingSen Carbon works with EPC contractors, WWTPs, landfill operators, and RNG producers to optimise adsorption performance and cut lifecycle costs. We supply virgin, impregnated, and custom solutions – our engineering team recommends based on your specific application, not a generic product.