RNG has quickly emerged as a major source of non-fossil methane.
In response, the EPA and others have tightened measurement and analysis requirements, which continue to evolve as the industry matures.
A wide variety of technologies are used in the traditional natural gas market for flow measurement and compositional analysis. Flow measurement is achieved using a variety of differential pressure devices including orifice, Venturi and insertion probes. Positive displacement meters and turbine meters are also commonly used, along with both ultrasonic and Coriolis-based metering. Gas composition is most commonly determined in the natural gas market by gas chromatography, a proven technique used extensively in analytical chemistry across a broad range of applications. Optical devices are used widely for the detection of natural gas contaminants, for example, H2S, moisture, CO2 and oxygen.
Measurement and analysis at RNG facilities is required at more than one sample location. Initial measurements are made on the "raw" biogas gathered from the feedstock source, such as from an anaerobic digester or landfill, and additional measurements are required after upgrading but prior to injection into the natural gas grid. The requirements at each location are different since composition, pressure and temperature vary significantly.
The development of the RNG market has relied heavily on financial incentives associated with California’s Low Carbon Fuel Standard and the EPA’s Renewable Fuel Standard (RFS) program. As such, the EPA is responsible for the development of equitable rules to ensure the integrity of the RNG injected into both transmission pipelines and natural gas distribution systems.
In July 2023, the EPA issued its final Set Rule, formally titled RFS Program: Standards for 2023–2025 and Other Changes, codified in 40 Code of Federal Regulations (CFR) Parts 80 and 1090. These rules restricted the selection of flow measurement methods and compositional analysis based on the recognition of certain other industry standards. The allowable technologies for flow measurement were differential orifice flow meter (API MPMS 14.1), thermal mass flow meter (EN 17526) and vortex flow meter (API MPMS 14.12). The only allowable technology for compositional analysis was gas chromatography (ASTM D7164). While these technologies were adequate, designers and users alike requested a broader selection for a variety of performance reasons. Soon after its release, the EPA began to issue exceptions to the original Set Rule, known as 40 CFR 80.135(c) or (d), allowing additional methods.
The first exception issued allowed the use of certain gas Coriolis meters for the measurement of flow rate. Coriolis meters gained wide popularity in the mid-90s and early 2000s, offering the advantages of wide measurement range and fluid independence. Coriolis meters are also well suited for smaller diameter applications, such as two-to-three-inch lines, common in RNG production locations. Shortly after the use of Coriolis meters (AGA Report No.11) was issued, exceptions for certain ultrasonic flow meters followed.
Ultrasonic gas flow meters (AGA Report No.9) gained considerable popularity in the mid-90s because of their wide measurement range, especially compared with differential devices in larger pipe diameters.
To date, no exceptions to gas chromatography have been issued for the compositional measurement of RNG. The ability of gas chromatography to produce extremely accurate and repeatable results has set a high bar for alternative technologies, and additional gas standards have been slow to develop. The one weakness of gas chromatography is analysis speed, usually requiring several minutes to produce a full compositional analysis whereas optical devices respond to changes in seconds.
Clues to future changes to the EPA Set Rules can be found at epa.gov or by visiting federalregister.gov/d/2023-16541 since a petition must be made for consideration. While it is difficult to predict the exact changes, additional allowances are expected as industry experience deepens and the challenges are better understood.
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