by Elohi
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by Elohi
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At Elohi, a Cherokee Measurement & Control Company, we believe knowledge drives success. That’s why we’re launching a monthly educational series focused on measurement and analysis throughout the biogas-to-RNG process. In this first edition, we focus on raw biogas flow measurement and quality analysis, a critical step in ensuring efficient and compliant renewable natural gas (RNG) production.
The Challenges of Raw Biogas Measurement
Unlike processed gas, raw biogas from digesters and landfills comes with unique complexities:
- Saturated with moisture, which causes problems for critical analytical and measurement equipment
- High in contaminants like CO2 and H2S
- Produced at very low pressures, making accurate measurement and analysis complex, necessitating the use of specialized equipment and methodologies
These factors complicate measurement and demand specialized equipment to ensure accurate and reliable results.
Ensuring Regulatory Compliance & Meeting RFS Standards
The EPA’s Renewable Fuel Standard (RFS) mandates strict compliance for gas measurement. While alternative measurement protocols exist, pre-approved equipment can help sites avoid lengthy approval processes.
Key RFS measurement standards include:
- Gas Chromatograph (GC) for BTU Measurement (ASTM D7164)
- Flow Measurement
- Orifice differential pressure (DP) measurement (API 14.3)
- Vortex measurement (API 14.12)
- Sample Handling using API 14.1 to ensure representative samples are analyzed properly
Flow Measurement: The Vortex Advantage
At very low pressures, DP measurement becomes difficult. Instead, Vortex meters, which are widely used in steam applications, are highly effective for measuring biogas straight from the digester.
- Handles wet gas & low pressures effectively
- API 14.12 compliant for biogas applications
Gas Chromatograph
Most GCs comply with ASTM D7164, ensuring compatibility with RFS standards. However, biogas poses additional challenges:
- High moisture content: Moisture introduced to gas analyzers can cause failures.
- Low pressure: GCs require 15-30 psig for proper analysis.
To mitigate these issues, we recommend:
- A heat exchanger (sample chiller) to remove excess moisture before sampling.
- Employing a sample pump to increase pressure from inches of water column to GC-required levels.
- Running the pump above the required pressure and regulating to reduce pulsing
Sample Handling Best Practices (API 14.1)
A proper sample probe should be installed in the middle to upper 1/3 of the pipe per API 14.1.
Additionally:
- Transport samples in insulated or heat-traced tubing to prevent water from freezing.
Beyond Compliance: H2S & O2 Monitoring
Though not required by the EPA, monitoring H2S and O2 is essential for:
- Process control & efficiency
- Digester health monitoring
- Protecting downstream equipment
Options:
- Dedicated H2S & O2 analyzers
- GCs like the Emerson RAI 470XA and ABB NGC 8203 offer landfill/biogas standard applications that include H2S and O2 measurement
Pro Tip: When measuring O2 & H2S via GC, use aluminum calibration gas cylinders—carbon steel LP5 cylinders aren’t suitable.
Why Choose Elohi?
With 20+ years of expertise in natural gas measurement and analysis, Elohi provides:
- Turnkey integration solutions
- Technical support and field services
- Commissioning, training, and scheduled maintenance
Let us help you optimize your biogas measurement and analysis! Contact us today at (918) 446-1611 or click the button below to take the next step!
Stay Tuned for the Next Article in Our Biogas to RNG Series!
In our next article, we’ll dive into RNG/Sales Gas analysis, flow measurement, and sample handling to ensure accuracy, compliance, and efficiency as your gas enters the pipeline.
Subscribe to our email list to stay updated, and be the first to know when our next article is live.
References
Parrott, M. S. 2018. Techniques for Spot Sampling Gas. K2 Controls, Inc. Retrieved from https://asgmt.com/wp-content/uploads/2018/03/036.pdf
