At Elohi, a Cherokee Measurement & Control Company, we empower our clients with renewable natural gas production expertise. In this third edition of our educational series, From Biogas to RNG, we shift focus from upgraded renewable natural gas to tail gas measurement.

Understanding Tail Gas in RNG Production

Tail gas is the off-gas produced when raw biogas is upgraded to renewable natural gas (RNG). Typically rich in carbon dioxide (CO2) and hydrogen sulfide (H2S), this stream is routed to vents, flares, thermal oxidizers, or capture systems for disposal. Accurate measurement of tail gas is critical for operational efficiency and regulatory compliance in RNG facilities.

Why Tail Gas Measurement Matters in Renewable Natural Gas

• Plant Balance: Regulatory agencies mandate a methane (CH4) balance within 5% across raw biogas, upgraded RNG, and tail gas. Imbalances can reduce the value of produced RNG, impacting profitability and slowing RNG projects.
• Emission Reporting: Certain states require detailed air quality reports for vented or flared gases, with H2S often being a key reporting parameter due to its environmental impact.

Challenges in Tail Gas Measurement

Tail gas presents unique measurement challenges that require specialized solutions:

• Inconsistent Flow: In processes like Pressure Swing Adsorption (PSA), tail gas flow varies from low-to-no flow to maximum flow due to cyclic pressure releases. Meters must handle this wide range effectively.
• High H2S Concentrations: Agricultural waste biogas can yield H2S levels exceeding 10,000 ppm in tail gas, concentrated further during methane separation. Many technologies require sample dilution, reducing accuracy and adding complexity.
• Continuous Analysis Needs: Rapid quality changes in tail gas make gas chromatographs (GCs) less effective, as their cycle times (often >5 minutes) can miss critical methane concentration shifts, leading to inaccurate analysis.

Solutions for Effective Tail Gas Measurement

Metering Solutions

• Thermal Mass Meter:
  Ideal for low-pressure, atmospheric conditions, thermal mass meters are RFS-compliant and excel at capturing low-to-no flow scenarios. They are cost-effective, typically installed via a 1” NPT connection, but are less accurate than other technologies and unsuitable for wet gas streams due to moisture-related inaccuracies.

• Vortex Meter:
  RFS-compliant and effective in wet-gas and low-pressure conditions, vortex meters are a robust choice. However, as in-line meters, their costs scale with pipe size, making them less economical for larger lines compared to thermal mass meters.

Quality Analysis Solutions

• Vaisala Multigas Probe MGP262 (CH4 and CO2):
  Utilizes CARBOCAP® infrared technology with a single-beam, dual-wavelength NDIR sensor to measure CH4 and CO2 via their infrared absorption patterns. This in-situ probe offers stable, economical measurements with 1-5% accuracy, requiring no external sample handling.

• NDIR Measurement for CH4:
  Non-Dispersive Infrared (NDIR) technology measures methane by detecting wavelength-specific absorption, providing reliable results for low CH4 levels with 1-2% accuracy, making it a cost-effective middle-ground solution.

 

• Hobre TISOMIC PA-TDL (CH4, H2S, CO2):
  Employs Photoacoustic Tunable Diode Laser (PA-TDL) spectroscopy, where laser-excited gas molecules produce sound waves for precise concentration measurements. This premium solution measures percent-level H2S, CO2, and CH4 with top-tier accuracy of 1% or better.

Conclusion

Effective tail gas measurement is essential for maintaining plant balance, meeting regulatory requirements, and optimizing RNG production. By leveraging advanced metering and quality analysis technologies, operators can address the challenges of inconsistent flow, high H2S concentrations, and rapid quality changes, ensuring compliance and maximizing operational efficiency.

Why Choose Elohi?

With over 20 years of expertise in natural gas and RNG measurement, Elohi is your trusted partner for:

• Turnkey Integration: Seamless solutions from equipment selection to commissioning.
• Expert Field Service: Field services, training, and scheduled maintenance tailored to RNG.
• Pipeline Compliance: Proven systems to meet custody transfer and RFS standards.

Let us optimize your RNG measurement and analysis. Call us today at (918) 446-1611 or click here to get started!

For more insights on biogas and RNG measurement solutions, stay tuned for our next newsletter!