Wobbe Index and Calorific Value Measurement: Natural Gas Quality
Wobbe index, calorific value, CARI and specific gravity measurement in natural gas: online and portable Wobbe analyzers and gas quality verification.
What is the Wobbe index and why is it measured?
The Wobbe index is a measure of how much energy a fuel gas delivers at a burner. It is obtained by dividing the calorific value by the square root of the gas's specific gravity, and it shows whether gases of different composition can be interchanged on the same burner. Because natural gas composition varies by source, deviation in the Wobbe index affects combustion stability, emissions and turbine or burner performance. The Wobbe index and calorific value are therefore tracked in gas distribution and transmission lines, at metering stations and at gas turbine plants. CARI (calorific air requirement index) and specific gravity are also parameters assessed together for combustion tuning.
Where is the Wobbe index measured?
- Natural gas transmission and distribution lines: At metering and quality stations.
- Gas turbine and boiler plants: In fuel gas quality and commissioning work.
- Custody transfer points: In quality verification.
- Site verification: In checking fixed online analyzers with a portable reference instrument.
How is the Wobbe index measured?
Wobbe index analyzers use the combustion properties of the gas, or calculate the index from a specific gravity cell and calorific value measurement. Online instruments measure continuously with sample pressure held within a set range, and transfer the value to the control system through analog outputs. Portable instruments allow one instrument to be used at several measurement points; they give the Wobbe index, CARI, calorific value and specific gravity together.
Choosing a Wobbe analyzer
- Fixed online installation or a portable instrument for site verification
- Sample pressure and gas preparation
- Measurement range and output signal
- Periodic calibration plan
Wobbe index measurement with Tetra
Tetra supplies the Hobre HiGAS portable natural gas quality analyzer and the Hobre WIM Compas Flex online Wobbe index analyzer in Türkiye. It also provides installation, commissioning, calibration and service.
Frequently asked questions
What unit are the Wobbe index and calorific value expressed in?
Both are generally given in MJ/m³ or kWh/m³, at specific reference combustion and metering temperatures. Using the higher calorific value gives the upper Wobbe index, and using the lower calorific value gives the lower Wobbe index. When comparing values from different sources, check that the reference conditions are the same.
What is the difference between a Wobbe analyzer and a gas chromatograph?
A gas chromatograph measures components individually and calculates calorific value and Wobbe index from the composition, but its analysis cycle takes minutes. Direct Wobbe analyzers give the index much faster by measuring combustion properties or specific gravity. Direct analyzers are preferred in applications needing a fast response, such as gas turbine combustion control, and chromatographs at points requiring a commercial composition certificate.
How is a Wobbe index analyzer calibrated?
Calibration is performed with certified reference gases of known Wobbe index; gases representing the lower and upper parts of the measurement range are generally used. Calibration frequency is determined by instrument type, process criticality and manufacturer recommendation. Comparing fixed analyzers with a portable instrument on site provides additional verification.
How does blending hydrogen into natural gas affect the Wobbe index?
Hydrogen's volumetric calorific value and density differ from those of methane, so the Wobbe index and calorific value change as the blend ratio increases. This can affect burner and turbine combustion settings. For hydrogen-containing gases, the analyzer's suitability for this composition and its measurement range should be confirmed at the selection stage.
Products measuring this parameter
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