Composition in mole percent (dry basis). If the analysis does not total 100, the result is normalised and flagged.
The Calculation, Explained
HHV and LHV — one gas, two heating values
Burning a hydrocarbon produces water. If that water is condensed, its latent heat is recovered and the fuel is credited with the higher (gross) heating value, HHV. In a gas turbine or boiler the water leaves as vapour in the exhaust, so the usable energy is the lower (net) heating value, LHV. The difference is exactly the latent heat of the water formed:
For a mixture, each component contributes in proportion to its mole fraction. Component molar heating values are the standard tabulated data of ISO 6976 (combustion reference 25 °C). Volumetric values use the ideal-gas molar volume at normal conditions, 22.414 L/mol at 0 °C and 1.01325 bar:
The Wobbe index — why fuel systems care
Through a given fuel nozzle at a given pressure drop, the energy flow depends on the volumetric flow and the density together — and the combination collapses into the Wobbe index. Two gases with the same Wobbe index deliver the same heat release through the same nozzle without any hardware change; that is why gas interchangeability (pipeline gas vs RLNG blends, for instance) is judged on Wobbe, not on heating value alone. A shift beyond roughly ±5% from the fuel a combustor was tuned for deserves engineering attention: flame stability, emissions and dynamics all feel it.
Assumptions and applicability
- Dry gas, mole (volume) percent basis, as reported by a gas chromatograph (ISO 6974 analysis). Heavier ends are represented as n-pentane; H2S and water vapour are not modelled.
- Ideal-gas behaviour at the reference state. Real-gas compressibility shifts volumetric heating values by well under 1% at reference conditions, but matters at pipeline pressure — custody-transfer calculations apply ISO 6976 with compressibility corrections.
- Basis declared: combustion reference 25 °C, metering reference 0 °C. Contracts using 15 °C or 60 °F bases give slightly different volumetric numbers for the same gas — check the basis before disputing a bill.
Worked example (the defaults above)
A typical pipeline-quality gas — 92% methane with modest heavier ends, 2.3% nitrogen, 1% CO2 — gives: molecular weight 17.50 kg/kmol, specific gravity 0.604, HHV 40.33 MJ/Nm³, LHV 36.41 MJ/Nm³ (46.6 MJ/kg), and Wobbe index 51.9 MJ/Nm³. Feed the LHV and SG from here straight into the Fuel Flow Calculator for your plant.
References
ISO 6976 — Natural gas: calculation of calorific values, density, relative density and Wobbe indices from composition; ISO 6974 — determination of composition by gas chromatography.
Frequently Asked Questions
What is the Wobbe index used for?
The Wobbe index judges whether two fuel gases are interchangeable in the same burner or gas turbine fuel system. Gases with the same Wobbe index deliver the same heat release through the same nozzle at the same pressure, so a fuel change within roughly plus or minus 5% Wobbe usually needs no hardware change.
What is the difference between HHV and LHV?
HHV (gross heating value) credits the fuel with the latent heat of the water formed during combustion, assuming it condenses. LHV (net) does not, because in real engines and boilers that water leaves as vapour in the exhaust. For natural gas LHV is about 90% of HHV. Gas turbines are rated on LHV.
What is a typical Wobbe index for natural gas?
Pipeline natural gas typically has a gross Wobbe index around 48 to 54 MJ per normal cubic metre. Lean gases with high nitrogen or CO2 content sit lower; rich gases with more ethane and propane sit higher.
Related calculators: Fuel Flow Calculator · Unit Converter