by Elohi
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by Elohi
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At Elohi, a Cherokee Measurement & Control Company, we empower our clients with renewable natural gas production expertise. In this second edition of our educational series, we shift focus from raw biogas to upgraded renewable natural gas measurement and quality analysis—a critical step in delivering pipeline quality gas that meets custody transfer tariff specifications and complies with natural gas pipeline standards.
The Challenges of Upgraded RNG Measurement
Upgraded RNG is primarily methane, a significant step up from raw biogas. Still, natural gas pipeline specifications demand precision in measuring trace contaminants at low levels of parts-per-million (ppm):
- Hydrogen Sulfide (H2S), moisture (H2O), and oxygen (O2): natural gas pipelines require high-accuracy measurement of these contaminants to ensure gas quality and protect infrastructure.
- Total Sulfur and Siloxanes: Some pipelines mandate continuous or periodic monitoring to prevent corrosion and equipment damage.
- Post-Compression Dynamics: Because RNG is typically measured after it becomes compressed natural gas (CNG) careful pressure regulation is required to maintain analyzer performance without compromising gas quality.
These challenges necessitate specialized equipment and meticulous sample handling to deliver pipeline-ready RNG.
Ensuring Regulatory Compliance & Meeting RFS Standards
The EPA’s Renewable Fuel Standard (RFS) remains consistent from raw biogas to renewable natural gas, requiring:
- Gas Chromatograph (GC) for lower heating value BTU measurement per ASTM D7164.
- Flow Measurement adhering to standards like orifice differential pressure (API 14.3) or Vortex measurement (API 14.12).
- Sample Handling per API 14.1 to ensure representative samples.
Beyond RFS, custody transfer tariff specs demand precise measurement of contaminants to meet pipeline requirements, making equipment selection critical.
Recommended Equipment for RNG Analysis
For RNG, accuracy, repeatability, and reliability are paramount. Here’s what we recommend:
Gas Chromatograph: C6+ Analysis
- Emerson Rosemount Analytical 470XA GC
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- 3-minute cycle time for C6+, carbon dioxide (CO2), and nitrogen (N2) analysis, meeting pipeline specs.
- Balances application needs, price, and lead time for efficient deployment.
- Compliant with ASTM D7164 for RFS BTU measurement.
Contaminant Analyzers for H2S, Moisture, and O2
Pipelines require low-ppm measurement of H2S, H2O, and O2. There are typically two tiers of analytical measurement for these compounds:
- Economic Option: Electrochemical for H2S & O2
- Cost-effective for continuous H2S and O2 measurement.
- Best suited as a check or monitoring tool rather than custody transfer, especially for narrow pipeline specs.
- Requires consistent ambient conditions and frequent maintenance (e.g., puck replacement, regular validation).
- Performance Options: Tunable Diode Laser and Optical Quenched Fluorescence for H2S, H2O, and O2
- TDLAS (Tunable Diode Laser Absorption Spectroscopy) and PA-TDL (Photo-Acoustic Tunable Diode Laser) are industry standards for custody transfer with H2S and H2O contaminants.
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- QF (Quenched Fluorescence) is the ideal performance version for O2.
- High accuracy and repeatability, largely unaffected by other gases.
- Minimal consumables, longer uptime, and smart remote monitoring/troubleshooting software.
- Siloxanes and Total Sulfur: Most pipelines accept periodic lab analysis, but continuous monitoring options are available for compliance.
Flow Measurement: The Orifice Advantage
RNG flow measurement follows the same RFS standards as raw biogas.
- Orifice differential pressure (DP) measurement(API 14.3)
- Vortex measurement (API 14.12)
- Alternative Measurement Protocol (AMP)
Unlike biogas, RNG is dry gas with post-compression pressure. Pipeline specs will typically require a “custody transfer” meter, giving orifice metering an advantage per API 14.3. Differential Pressure (DP) measurement is a tried-and-true method that allows a wide range of turndown and flexibility with pipe size by changing orifice plate size. A high-quality multivariable transmitter (differential pressure, static pressure, and temperature) is required for accurate analysis.
The Daniel Senior Orifice Fitting is a dual-chamber device that is the most widely used means of measuring natural gas, with over 1 million worldwide. The dual chamber allows plate inspection and changes under pressure without flow interruption.
Higher performance meters include Multi-Path Ultrasonic and Coriolis. Using these meters will require an additional step when permitting a facility, as they must be on the AMP list. However, many technologies and manufacturers are approved on this list, so it’s not the burden it once was.
Sample Handling Best Practices (API 14.1)
Proper sample handling ensures accurate RNG analysis, especially for low-ppm contaminants. Key practices include:
- Sample Probe Placement: Install probes in the middle of the upper 1/3 of the pipe per API 14.1.
- Pressure Regulation: Post-compression RNG requires regulation to 15-25 psig for analyzers. Run phase curves to stay 30°F above the hydrocarbon dew point (HCDP) to prevent liquid dropout.
- Specialized Tubing: SilcoNert-coated tubing is used for low-ppm H2S and H2O measurement. Stainless steel can absorb sulfur and moisture, leading to inaccurate readings.
- Example: Summer heat can release absorbed H2S, falsely indicating high levels and risking pipeline shut-ins.
- Insulated/Heat-Traced Tubing: Prevents freezing and ensures sample integrity during transport.
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 Support: 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 below to get started!
Stay Tuned for the Next Article in Our Biogas to RNG Series!
Next month, we’ll explore Tail Gas and Flare Measurement—key to emissions compliance and process optimization. Subscribe to our email list to stay updated and be the first to read it!
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
- API 14.1: Gas Sampling Standards.
- ASTM D7164: Standard Practice for On-line/At-line Heating Value Determination.
FAQs
RNG can be produced from various sources, including landfills, wastewater treatment plants, and livestock operations. Landfills are a significant source of RNG, as the decomposition of waste in landfills produces a gas rich in methane. Wastewater treatment plants also produce RNG, as the treatment process involves decomposing organic matter to produce methane-rich gas. Livestock operations, such as dairy and pig farms, can also produce RNG, as the manure from these operations can produce biogas. The production of RNG from these sources involves using anaerobic digesters, which are systems that break down organic matter without oxygen to produce biogas. The biogas is then upgraded to produce RNG, which can be used as a replacement for traditional natural gas.
Renewable natural gas (RNG) is a term used to describe biogas that has been upgraded for use in place of fossil natural gas. RNG is essentially biogas that has been processed to purity standards, making it fully interchangeable with conventional natural gas. The production of RNG involves the decomposition of organic matter, such as food waste, agricultural waste, and sewage sludge, to produce a gas that is composed primarily of methane and carbon dioxide. RNG can be used as a replacement for traditional natural gas in various applications, including heating, electricity generation, and vehicle fuel.
Co-digestion processes, which involve combining multiple types of organic materials, play a crucial role in the efficient generation of biogas. The description of RNG has been developed by the EPA’s voluntary programs, emphasizing the need for specific standards and guidelines in its processing. Various components, such as flow meters and gas chromatographs, are critical in measuring and analyzing RNG, ensuring compliance and optimizing performance. Anaerobic digesters play a vital role in stabilizing and optimizing methane production from various feedstocks, capturing methane emissions, and converting them into RNG for energy generation or as a transportation fuel.
