Stationary gas engines — whether running on biogas, natural gas, or in dual-fuel configuration — generate crankcase blow-by gases that are fundamentally different from those produced by diesel engines. If your CHP plant or gas engine installation is experiencing premature filter blockage, oil carry-over into the exhaust aftertreatment system, or corrosion inside the crankcase ventilation (CCV) circuit, the root cause is almost always a mismatch between the blow-by composition and the filter element chemistry. This guide explains why, and how to select the correct filtration solution.
Why Gas Engine Blow-By Is Different
In a diesel engine, blow-by gases are primarily composed of unburnt hydrocarbons, soot particles, and water vapour. Gas engines — particularly those burning biogas — introduce a far more aggressive chemical environment. Biogas derived from anaerobic digestion contains hydrogen sulphide (H₂S), siloxanes, and halogenated compounds that survive combustion and appear in the crankcase atmosphere as acidic aerosols and reactive sulphur species.
Natural gas engines present a cleaner blow-by profile, but still generate significant oil mist loadings at high specific outputs. Dual-fuel engines, which switch between diesel pilot injection and gas combustion, combine characteristics of both: variable blow-by composition depending on the fuel ratio at any given moment, making consistent filtration performance harder to achieve with a single element grade.
Understanding this chemistry is the first step. The second is selecting a filter element that can handle it. For guidance on the broader principles of oil mist separation, see our article on crankcase ventilation filtration.
The Case for K-Type Elements in Acidic CCV Duty
R+F FilterElements offers K-type filter elements specifically engineered for sour gas and acidic condensate environments. Unlike standard RF-C coalescing elements — which use a borosilicate glass microfibre matrix bonded with a standard resin — K-type elements use an acid-resistant binder system and chemically inert drainage layer materials. This makes them suitable for continuous exposure to H₂S-laden blow-by and the sulphuric acid condensate that forms when water vapour and SO₂ combine at lower crankcase temperatures.
For most biogas CHP installations, R+F recommends the RF-C-K series coalescing elements paired with an appropriate housing from the RF-H-420 to RF-H-456 vacuum pump exhaust filter range. These housings are available in 316L stainless steel, which resists the corrosive condensate that would attack aluminium housings within months of installation.
Key Performance Parameters for Gas Engine CCV
Need help selecting the right CCV filter for your gas engine?
Comparing Element Types for Gas Engine CCV Duty
| Parameter | Standard RF-C (Coalescing) | RF-C-K (K-Type, Acid-Resistant) | RF-CS (Vacuum/High-Temp) |
|---|---|---|---|
| Binder system | Standard resin | Acid-resistant resin | Silica-bonded |
| pH tolerance | 5–9 | 2–12 | 4–10 |
| Max temperature | 100 °C | 100 °C | 200 °C |
| H₂S resistance | Limited | Excellent | Moderate |
| Recommended for | Natural gas (clean), compressed air | Biogas CHP, dual-fuel, sour gas | Vacuum pump exhaust, high-temp duty |
| Seal material | NBR standard | FKM/PTFE required | FKM standard |
Use our free Engineering Tool to get a filtration recommendation for your specific application in under 2 minutes.
Sizing the CCV Filter for Your Engine Output
Blow-by flow rate scales with engine displacement and specific output. As a rule of thumb, a 500 kWe biogas CHP unit will generate between 15 and 40 Nm³/h of blow-by gas at full load, depending on engine age and ring condition. Oversizing the filter housing is always preferable in CCV duty: a lower face velocity through the element reduces re-entrainment of coalesced oil droplets and extends service life significantly.
R+F FilterElements offers multi-element housings from the RF-H-440 and RF-H-456 series that accommodate between 4 and 16 elements, allowing flow capacity to be matched precisely to engine output. For dual-fuel engines where blow-by composition varies with fuel ratio, a two-stage arrangement — coarse particulate pre-filter followed by a K-type coalescing stage — provides the most robust protection.
For installations where siloxane contamination is a concern (common in landfill gas and sewage gas applications), an RF-DIA activated carbon adsorber downstream of the coalescing stage will capture siloxane vapours before they reach the engine's turbocharger or exhaust catalyst.
Installation and Maintenance Considerations
Correct installation orientation is critical for coalescing performance. Elements must be mounted vertically with the drain at the bottom to allow coalesced oil to drain by gravity. Horizontal mounting traps oil in the element matrix, accelerating pressure drop rise and reducing effective service life by up to 60%.
Drain lines should be routed to a dedicated oil collection vessel — never back to the engine sump without first confirming that the collected condensate is compatible with the engine oil specification. In biogas applications, the condensate will be acidic and may contain sulphur compounds that will degrade engine oil if returned directly.
Service intervals for K-type elements in biogas duty are typically 2,000–4,000 operating hours, compared with 6,000–8,000 hours for the same element in clean natural gas service. Monitoring differential pressure across the filter is the most reliable indicator of element condition. R+F recommends fitting a differential pressure gauge or transmitter as standard on all CCV filter installations. For more on element selection principles, see our guide on coalescing vs particulate filter elements.
Applicable Standards and Emissions Compliance
In Germany and across the EU, stationary gas engines operating in CHP mode must comply with the TA Luft (Technical Instructions on Air Quality Control) and, for larger installations, the Industrial Emissions Directive (IED). Crankcase ventilation gases must either be returned to the intake (closed CCV) or treated before atmospheric discharge. A correctly specified CCV filter system — using R+F K-type elements in an appropriate stainless steel housing — will achieve the oil mist separation performance required to meet these obligations.
For installations subject to ATEX requirements, R+F housings are available with ATEX-rated drain valves and pressure relief devices. Consult the biogas solutions page for ATEX configuration options.
- In a diesel engine, blow-by gases are primarily composed of unburnt hydrocarbons, soot particles, and water vapour.
- R+F FilterElements offers K-type filter elements specifically engineered for sour gas and acidic condensate environments.
- Blow-by flow rate scales with engine displacement and specific output.
- Correct installation orientation is critical for coalescing performance.
Related Reading
- Crankcase Ventilation Filtration — Principles and Element Selection
- Coalescing vs Particulate Filter Elements — When to Use Which
- ISO 8573-1 Compressed Air Quality Classes Explained
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