Biogas is one of the most versatile forms of renewable energy available today, produced whenever organic material breaks down in the absence of oxygen. But raw biogas straight from a digester, lagoon, or reactor is far from ready to use. Before it can safely power an engine, boiler, or flare, it needs to pass through a carefully engineered biogas feed train — a sequence of equipment that cleans, dries, filters, and pressurises the gas so it becomes a reliable, long-term fuel.
This article explains what a biogas feed train is, why each stage matters, and how the right combination of technology can turn even difficult, contaminant-heavy biogas into a dependable energy source.
What Is Biogas, and Why Does It Need Treatment?
Biogas can be produced from a wide range of biodegradable substrates — agricultural waste, food processing effluent, palm oil mill effluent (POME), municipal sludge, and more. Regardless of the source, its bulk composition tends to be broadly similar:
| Component | Typical Concentration |
|---|---|
| Methane (CH₄) | 40–65% by volume |
| Carbon dioxide (CO₂) | 30–40% by volume |
| Hydrogen sulphide (H₂S) | Up to several thousand ppm, depending on source |
| Siloxanes | Trace, but damaging if present |
| Water vapour | Fully saturated at point of production |
While methane is the valuable energy carrier, the trace gases can be highly damaging to downstream equipment. Hydrogen sulphide combines with water to form corrosive sulphuric acid, while siloxanes convert to abrasive silicon dioxide particles when burned — both capable of severely damaging engines, turbines, and burners over time.
“It is too easy to accept a low default level only to find out that it is too low and the equipment installed on site needs to be upgraded.” — a costly mistake that proper specification avoids from day one.
The Building Blocks of a Biogas Feed Train
A typical biogas feed train consists of:
- A gas blower (the prime mover), usually with automatic feed or suction pressure control
- Gas cleaning equipment for hydrogen sulphide and/or siloxane reduction
- Dewatering/chilling systems to control moisture content
- Filtration equipment down to micron level for particulates
- Instrumentation for monitoring and control
- Downstream burners, flares, or power generation equipment
These components can be supplied individually or as a complete turnkey package, often mounted on factory-built skids to simplify installation and reduce on-site work.
Biogas Production Options
Biogas feed trains can be paired with a range of digestion technologies, including:
- Dry cell digesters
- Covered lagoons
- Continuously Stirred Tank Reactors (CSTR)
- Upflow Anaerobic Sludge Blanket (UASB) reactors
Biogas Use Options
Once treated, biogas can be directed to multiple end uses:
- Electricity generation
- Compressed Biomethane (CBM) for vehicle fuel or grid injection
- Steam generation
- Direct heat applications
Hydrogen Sulphide Removal
Removing hydrogen sulphide (H₂S) is essential for almost any use of biogas beyond simple flaring. H₂S combines with water to form sulphuric acid, which attacks engines, burners, and steel surfaces alike.
Concentrations vary significantly by feedstock:
| Feedstock | Typical H₂S in Biogas |
|---|---|
| Cassava root processing effluent | 1,000–2,500 ppm (mass) |
| Palm Oil Mill Effluent (POME) | Up to 5,000 ppm |
| Either source (occasional peaks) | Up to 30,000 ppm or more |
A bio-scrubber is generally the recommended solution. It requires no chemical dosing and no specialist equipment — the bacteria involved occur naturally and thrive without external heating in tropical and sub-tropical climates. Make-up water is typically sourced from the digester’s own supernatant effluent, or from fresh water combined with nutrients where needed. For applications requiring very low residual H₂S levels, a polishing system can be added downstream.
Tackling Siloxanes
Siloxanes are silicone-based compounds widely used in consumer products such as detergents, cosmetics, and paper coatings. They frequently end up in wastewater and, because they don’t break down in activated sludge treatment, accumulate in sludge that later undergoes anaerobic digestion.
At digestion temperatures of up to 60°C, siloxanes volatilise into the resulting biogas — a problem sometimes worsened by silicone-based anti-foaming agents added directly to digesters. When burned, siloxanes convert into silicon dioxide particles, essentially fine sand, which grind away at turbines and engines from the inside.
The recommended treatment approach is cryogenic refrigeration (around −30°C) followed by activated carbon polishing where required, achieving long-term siloxane reduction to below 1 mg/Nm³.
Dewatering: Controlling Moisture Before It Causes Damage
Biogas leaves the digester fully saturated with water. Any subsequent cooling — even inside an engine’s charge-air cooler — causes condensation, and that condensate can absorb trace gases like ammonia and hydrogen sulphide to form highly corrosive liquids.
A properly specified chiller drops the biogas dew point to whatever level suits the downstream equipment and environment — sometimes a nominal 20°C is adequate, while other applications require cooling to 4°C combined with trace heating and insulation of downstream pipework.
Critically, a chiller must always be positioned after any wet gas cleaning stage, such as a bio-scrubber — otherwise the dried gas is simply re-saturated, undoing the benefit entirely.
Chiller heat exchangers are typically constructed from stainless steel to ensure long-term reliability in this corrosive service.
Filtration: Protecting Equipment from Particulates
Left uncontrolled, particulates in the gas stream cause abrasive wear and damage downstream. Two main filtration approaches are used:
- Filter pads (stainless steel or polypropylene wire) — effective down to 2 μm, and can double as water-droplet coalescing mesh
- Cyclone separators — effective down to 15 μm, or as fine as 5 μm for high-efficiency designs
For optimum engine protection, best practice is to combine these methods: an inlet mesh filter before the chiller, followed by a cyclone and additional mesh filter after the chiller, and a final micron-filter to guarantee clean, dry gas. All wetted components should ideally be stainless steel, with galvanised carbon steel acceptable for filter and mesh housings where appropriate.
Gas Pumping: Moving Biogas Through the System
Every feed train needs a prime mover to push gas through the treatment stages and deliver it at the correct pressure. The two broad categories are:
- Centrifugal blowers (fans) — generally preferred where practical, due to low noise and the ability to turn down to zero flow without surging (thanks to back-swept blades)
- Positive displacement units — including roots-type blowers, rotary vane compressors, and screw compressors
Blowers should always be protected from water droplets and particulates via an inlet knock-out pot, fitted with flame arresters, and rated for use in appropriately zoned hazardous areas.
Flare Stations: Safety and Compliance
Flaring biogas safely is a highly regulated activity, not a task for amateurs. Regulatory audits — particularly those tied to Clean Development Mechanism (CDM) protocols — scrutinise data recording and record-keeping in detail. Cutting corners on flare station design and monitoring is a fast route to lost revenue and compliance failures.
Safety Integrity Level 2 (SIL2) compliance is increasingly becoming the industry standard for flare station control systems, ensuring safe, auditable operation even where it may initially seem like an excessive requirement.
Why the Right Design Approach Matters
Every biogas source and application is different. Pressure losses through the system, gas temperatures, and relative humidity must all be factored into an optimised design — there is no true “one size fits all” feed train. Getting the specification wrong at the outset (particularly around H₂S capacity) is a common and expensive mistake to correct later.
A well-executed biogas energy recovery project typically aims to:
- Install an anaerobic digester that generates and captures biogas
- Reduce odours and harness the energy value of methane
- Generate renewable electricity or boiler fuel to offset fossil fuel use
- Improve overall wastewater treatment performance
- Where applicable, reduce greenhouse gas emissions and support Emission Reduction Unit creation
Key Takeaways
- Raw biogas typically contains 40–65% methane and 30–40% CO₂, along with damaging trace gases such as H₂S and siloxanes.
- H₂S concentrations can range from 1,000 ppm up to 30,000 ppm or more depending on feedstock, making correct scrubber sizing essential.
- Bio-scrubbers offer a low-maintenance, chemical-free solution for H₂S removal, especially suited to tropical climates.
- Siloxanes are best treated with cryogenic refrigeration (~−30°C) plus activated carbon polishing, reducing levels to below 1 mg/Nm³.
- Dewatering must always occur after wet gas cleaning stages to avoid re-saturating the gas.
- Combined filtration (mesh, cyclone, micron-filter) protects engines and turbines from particulate damage.
- Centrifugal blowers are generally preferred for their low noise and stable turndown performance.
- Flare stations must meet strict safety and regulatory standards, with SIL2 compliance becoming standard practice.
Get in Touch
Designing a biogas feed train that reliably handles your specific feedstock, contaminant levels, and end-use requirements takes real expertise. Whether you need a single component or a complete turnkey system — from bio-scrubbers and chillers to blowers and flare stations — getting the specification right from the start saves significant cost and downtime later. Contact our team to discuss your biogas treatment requirements and find the right solution for your project.