No two biogas streams are identical. The gas leaving a dairy farm digester in the Netherlands has a fundamentally different chemical fingerprint from the gas produced at a municipal wastewater treatment plant in southern Germany or a food waste facility in the UK. Yet all of these gases must be compressed – and the composition of each one directly determines which compressor materials, seal compounds, operating parameters, and maintenance schedules will ensure reliable, long-term performance.
At Fornovo Gas, gas composition analysis is the first step of every compressor engineering project. Before selecting a cylinder bore, specifying a ring compound, or calculating a staging arrangement, our engineering team reviews the complete gas analysis – not just methane and CO₂ percentages, but the full spectrum of trace components that define how the gas will behave inside a reciprocating compressor. This guide explains why each compositional variable matters, what it does to compression equipment, and how custom engineering addresses the challenges. It is written for technical managers responsible for specifying, procuring, or operating biogas compression systems.
Understanding biogas composition
Biogas is a mixture of gases produced by the anaerobic digestion of organic matter. While the major components are methane (CH₄) and carbon dioxide (CO₂), the precise proportions and the trace contaminant profile vary significantly depending on the feedstock, the digestion process, and the operating conditions of the anaerobic digester.
Major components
| Component | Typical range | Role in compression engineering |
| Methane (CH₄) | 50–70% vol. | The valuable component. Methane content determines the calorific value of the gas and influences the compressibility factor and specific heat ratio used in thermodynamic sizing calculations. |
| Carbon dioxide (CO₂) | 25–45% vol. | An inert diluent that increases the molecular weight of the gas mixture. Higher CO₂ content results in higher gas density at suction, but it does not affect the cylinder filling coefficient. From a sizing perspective, its main impact is that it may require the use of special valves. In the presence of water, CO₂ can also form carbonic acid, contributing to potential corrosion. |
| Nitrogen (N₂) | 0–5% vol. | Present in small amounts, especially in landfill gas. Nitrogen is inert but dilutes the methane content, affecting calorific value and compression thermodynamics. |
| Oxygen (O₂) | 0–1% vol. | Should be minimised. Oxygen in biogas indicates air ingress into the digester and creates an explosion risk when combined with methane. |
Trace contaminants: the hidden engineering challenge
While major components determine the thermodynamic behaviour of the gas, trace contaminants mainly create corrosion and material compatibility issues rather than significantly influencing thermodynamics. These include hydrogen sulphide (H₂S), moisture, siloxanes, ammonia (NH₃), VOCs, and halogenated compounds, each potentially affecting compressor materials, seals, valves, and maintenance requirements.
How feedstock determines gas composition
| Feedstock source | Typical CH₄ (%) | Typical H₂S (ppm) | Siloxanes | Moisture | Other notable contaminants |
| Agricultural waste (manure, crop residues) | 55–70% | 100–3,000 | Very low to absent | Saturated | Ammonia (NH₃) from protein-rich substrates |
| Food waste / organic fraction of MSW | 55–70% | 50–1,000 | Low | Saturated | Volatile organic compounds (VOCs), terpenes |
| Sewage sludge (wastewater treatment) | 60–65% | 500–5,000 | Moderate to high | Saturated | Siloxanes from personal care products |
| Landfill gas | 40―55% | 50–2,000 | Moderate to very high | Saturated | Halogenated compounds, higher N₂ and O₂ content |
| Industrial organic waste | 50–70% | Variable | Variable | Saturated | Depends on industrial process; may include solvents |
This variability is the fundamental reason why Fornovo Gas does not offer a one-size-fits-all biogas compressor. Every unit we manufacture is custom-engineered based on the actual gas analysis of the specific project. Our team evaluates the gas composition data alongside the process parameters – pressures, temperatures, flow rates, and end use – to determine the optimal material selections, staging, and operating conditions for each machine.
H₂S corrosion considerations in biogas compression
Hydrogen sulphide (H₂S) is one of the most critical contaminants to consider in biogas compression, mainly because of its corrosive behaviour in the presence of moisture.
In most biogas upgrading plants, H₂S is removed upstream of the compressor through dedicated desulphurisation systems. This is an essential step, as the compressor is designed to operate with treated biogas and not with raw, high-H₂S gas.
When H₂S is present together with water vapour or condensate, it can contribute to acid corrosion and damage gas-wetted components such as cylinders, valves, piston rods, seals and piping. For this reason, both H₂S concentration and gas moisture content must be carefully evaluated during the engineering phase.
For Fornovo Gas compressors, the acceptable H₂S level must be verified project by project. In particular, the DA300 is not suitable for operation with H₂S concentrations above 10 ppm. If higher H₂S values are expected, the gas treatment system must reduce the contaminant level before the gas reaches the compressor.
Correct compressor specification therefore depends not only on the biogas composition, but also on the position and efficiency of the upstream desulphurisation system.
Where does H₂S removal happen in the process chain?
The position of the gas treatment system relative to the compressor is a key factor in defining the operating conditions seen by the machine.
In most biogas upgrading plants, contaminants such as H₂S and siloxanes are removed or significantly reduced before the gas reaches the compressor. As a result, the compressor typically handles conditioned biogas rather than raw digester gas.
A common configuration includes:
- Moisture removal and gas conditioning upstream of the compressor.
- H₂S reduction through biological treatment, activated carbon, iron-based media or other desulphurisation technologies.
- Siloxane removal, when required, through dedicated filtration systems, particularly in landfill gas and wastewater treatment applications.
The exact treatment sequence depends on the upgrading technology and plant layout. For this reason, compressor selection must always be based on the actual gas composition at the compressor inlet rather than on the raw biogas composition measured at the digester outlet.
Fornovo Gas works closely with upgrading technology suppliers and EPC contractors to verify the gas conditions at the compressor suction and ensure that materials, valves, seals and operating parameters are suitable for the expected service conditions.
Siloxanes in biogas applications
- Siloxanes are mainly associated with landfill gas and wastewater treatment plants, where they originate from silicone-based consumer and industrial products. Their presence in agricultural biogas is generally limited, while dedicated filtration systems often remove them before the gas reaches the compressor.
- If not adequately removed, siloxanes can form abrasive silicon-based deposits at elevated temperatures, potentially increasing wear on valves and other components. However, in most upgrading applications the compressor is designed to operate downstream of the gas cleaning system, significantly reducing the risk associated with siloxane contamination.
- As with all contaminants, the relevant parameter for compressor specification is the gas composition at the compressor inlet rather than the raw gas composition upstream of the treatment process.
Moisture management in biogas compression
Biogas leaving an anaerobic digester is invariably saturated with water vapour at the digester’s operating temperature (typically 35–55°C for mesophilic and thermophilic processes respectively). This moisture content, often overlooked in early-stage project specifications, has profound implications for compressor design and operation.
What happens to moisture during compression
When saturated gas is compressed, its pressure increases and the dew point also rises. However, as long as the gas temperature increases during compression, the risk of condensation inside the cylinder is reduced.
Condensation is more likely to occur when the compressed gas is subsequently cooled, especially in intercoolers and aftercoolers, where the gas temperature is brought back closer to ambient conditions. In these sections, water vapour can condense into liquid water if the gas temperature drops below its dew point.
For this reason, condensate management is mainly associated with cooling stages and downstream separation, rather than with the compression stroke itself.
Engineering consequences of moisture in compression
- Corrosion acceleration – moisture is a key factor in promoting corrosion mechanisms associated with acid gases. In the presence of liquid water, contaminants such as H₂S and CO₂ can contribute to the formation of acidic compounds, increasing the risk of corrosion on gas-wetted components. For this reason, moisture management and contaminant control must be considered together when defining compressor operating conditions and material compatibility.
- Lubrication degradation in lubricated systems – in compressors that use oil lubrication in the cylinders, condensed water emulsifies with the lubricating oil, forming a milky mixture that loses its lubricating and sealing properties. This accelerates ring and cylinder wear and can lead to increased oil carryover into the gas stream. Oil-free compressors (such as the Fornovo Gas DA500, DA300, and SA200 Oil Free ranges) eliminate this failure mode entirely because there is no oil to emulsify.
- Hydraulic lock risk – in extreme cases, if liquid water accumulates in the cylinder faster than it can be expelled through the discharge valve, the incompressible liquid can cause a hydraulic lock – a sudden, massive pressure spike that can damage the piston, connecting rod, or crankshaft. Proper drain pots, liquid separation, and cylinder design must account for this risk.
- Intercooler and piping corrosion – condensate collected in intercoolers and drain pots is acidic (due to dissolved CO₂ and H₂S) and must be handled with corrosion-resistant materials and proper disposal procedures.
- Downstream equipment impact – if moisture is not adequately removed during compression, it can carry over to upgrading systems, storage vessels, or grid injection points, where it causes further corrosion, hydrate formation (at high pressures), or failure to meet gas quality specifications.
How Fornovo Gas manages moisture in compressor design for Biogas Compressors
Our engineering approach to moisture management for Biogas compression combines several strategies:
- Material specification for wet gas – all gas-wetted components are specified for wet operating conditions as the default assumption. We do not assume that upstream gas drying will always function correctly – the compressor must tolerate saturated gas without premature degradation.
- Effective interstage separation – intercooler designs include properly sized knockout drums and automatic drain systems to remove condensate before the gas enters the next compression stage.
- DA300 with D13 cylinder technology – this recent Fornovo Gas development specifically expanded our compressor’s capabilities for biogas and wet gas applications, providing enhanced performance in saturated gas conditions while maintaining high efficiency.
- Oil-free technology advantage – by eliminating cylinder lubricating oil, our oil-free compressors remove the water-oil emulsification failure mode entirely. This is one of the less-discussed but highly practical advantages of oil-free compression in biogas applications.
Ammonia, VOCs, and other trace contaminants
Beyond H₂S, siloxanes, and moisture, several other trace components in biogas can influence compressor engineering:
Ammonia (NH₃)
Present primarily in biogas from protein-rich feedstocks (animal manure, food waste, slaughterhouse residues). Ammonia dissolves in condensed water to form ammonium hydroxide, an alkaline solution that attacks copper and brass alloys and can degrade certain elastomeric seal materials. Ammonia concentrations in agricultural biogas can reach 50–100 ppm, occasionally higher. Material selections for compressor seals and instrumentation must account for ammonia exposure in these applications.
Volatile organic compounds (VOCs) and terpenes
Food waste and green waste biogas often contains elevated levels of VOCs and terpenes, which can attack certain polymer materials used in seals, O-rings, and diaphragms. While VOCs are typically present at low concentrations, their chemical diversity means that elastomer compatibility must be verified against the specific VOC profile of the gas, not just generic chemical resistance tables.
Halogenated compounds
Primarily a concern in landfill gas, where chlorinated and fluorinated compounds originate from disposed plastics, solvents, and refrigerants. When combusted or exposed to high temperatures, these compounds release hydrochloric and hydrofluoric acids – among the most aggressive corrosive agents. Compressor material selections for landfill gas must consider halogenated compound exposure, and pre-treatment (activated carbon adsorption) is standard practice for gas with significant halogenated content.
The gas analysis: what to request and how to interpret it
A reliable compressor specification starts with a reliable gas analysis. As a technical manager specifying compression equipment, you should ensure the following parameters are included in the gas analysis report provided to the compressor manufacturer:
| Parameter | Unit | Why it matters for compressor engineering |
| Methane (CH₄) | % vol. | Determines calorific value, compressibility factor, and explosion characteristics (ATEX classification) |
| Carbon dioxide (CO₂) | % vol. | Affects molecular weight, gas density, and corrosion potential (carbonic acid in wet conditions) |
| Nitrogen (N₂) | % vol. | Dilutes methane content; affects thermodynamic properties |
| Oxygen (O₂) | % vol. | Safety-critical for ATEX; must be below 1% in most applications |
| Hydrogen sulphide (H₂S) | ppm or mg/Nm³ | Primary corrosion driver; determines material grade for all gas-wetted components |
| Moisture dew point | °C at specified pressure | Determines condensation behaviour during compression; critical for corrosion assessment |
| Siloxanes (total) | mg/Nm³ | Determines deposit risk; influences staging, intercooling design, and maintenance intervals |
| Ammonia (NH₃) | ppm | Affects elastomer and copper alloy selections |
| Total VOCs | mg/Nm³ or ppm | Influences seal and O-ring material compatibility |
| Halogenated compounds | mg/Nm³ | Critical for landfill gas; determines acid corrosion risk and pre-treatment requirements |
| Suction temperature | °C | Affects gas density at compressor inlet and intercooler design |
| Suction pressure | bar a or bar g | Starting point for staging and power calculations |
Important: a single-point gas analysis captures a snapshot. Biogas composition fluctuates with feedstock changes, seasonal variations, digester operating conditions, and co-digestion ratios. Whenever possible, request multiple analyses over a representative time period, and provide the compressor manufacturer with both the average values and the expected range for each parameter. This allows the engineering team to specify materials and operating conditions that handle the full variability, not just the nominal case.
At Fornovo Gas, we use the gas analysis not only to select materials, but also to validate the thermodynamic sizing calculations. The compressibility factor (Z), specific heat ratio (k), and gas density at each stage are calculated from the actual gas composition – not from generic assumptions for ‘biogas’ – ensuring that the compressor is accurately sized for the real-world gas it will handle.
Customisation requirements: why off-the-shelf compressors fail in biogas
Catalogue compressors designed for natural gas, air, or inert gas service are sometimes offered as lower-cost alternatives for biogas projects. This approach carries significant risks that often materialise within the first 12–24 months of operation:
- Material incompatibility – standard natural gas compressor components are not designed for H₂S, high moisture, or siloxane exposure. Premature corrosion, ring wear, and valve failures result in costly unplanned maintenance and production downtime.
- Incorrect sizing – thermodynamic properties differ between natural gas and biogas. A compressor sized for natural gas at a given flow rate and pressure will be incorrectly sized for biogas of the same stated flow, because the gas density, compressibility, and heat capacity differ.
- Missing certifications – natural gas compressors may not carry the ATEX certification required for biogas applications, where the explosive atmosphere classification differs based on the gas composition.
- Void warranty – operating a compressor outside its design gas specification typically voids the manufacturer’s warranty, leaving the plant operator exposed to the full cost of any failure.
Fornovo Gas compressors are custom-designed and custom-built for each individual project. Our team examines gas composition, process parameters, environmental conditions, and certification requirements before submitting a proposal. This approach costs more upfront than a catalogue selection, but it eliminates the downstream failures, production losses, and safety risks that make off-the-shelf compressors a false economy in biogas service.
