ISO 6976

Natural gas - Calculation of calorific values, density, relative density and Wobbe index from composition

This standard calculates calorific values (molar, mass and volume based) along with other properties from a gas composition.


Molar Mass


Molar Mass

Symbol Description Units
M Molar mass of the mixture kg/kmol
Xj Mole fraction of component j -
Mj Molar mass of component j kg/kmol

Standard Compression Factor


ISO_6976_StdCompressibility

Symbol Description Units
zs Standard compression factor -
Xj Mole fraction of component j -
bj Summation factor for component j -

The summation factor for each component (bj) is taken from ISO 6976, Table 2 – Summation factors for components of natural gases at metering reference conditions of 15°C and 101.325 kPa.

Relative Density


Relative Density

Symbol Description Units
ρr Relative density -
Xj Mole fraction of component j -
Mj Molar mass of component j kg/kmol
Zs Standard compression factor -

Constant Description Value Units
zair Compression factor of dry air (1995) 0.99958 -
Mair Molar mass of dry air (1995) 28.9626 kg/kmol
zair Compression factor of dry air (2016) 0.999595 -
Mair Molar mass of dry air (2016) 28.96546 kg/kmol

The molar mass of dry air (Mair) and the compression factor of dry air (zair) are taken from ISO 6976 - 1995 - Annex B or ISO 6976 - 2016 - Annex A Properties of dry air, as appropriate

Calorific value on a molar basis - 1995


ISO_6976_CVMolar

Symbol Description Units
CVc Calorific value on molar basis KJ/mol
xj Mole fraction of component j -
CVj Calorific value of component j KJ/mol

The component calorific value (CVj) is taken from ISO 6976 - 1995 - Table 3, Calorific values for components of natural gases at various combustion reference conditions for the ideal gas on a molar basis.

Calculation of calorific value on a molar basis - 2016



The Superior calorific value is calculated as per 1995 using ISO 6976 - 2016 - Table 3, Gross calorific values on a molar basis for components of natural gas in the ideal gas state at various combustion reference temperatures

The Inferior calorific value is calculated as follows
ISO_6976_SupCVMolar

Symbol Description Units
CVc Calorific value on molar basis KJ/mol
xj Mole fraction of component j -
bj Number of hydrogen atoms present in each molecule of component j -
L Enthalpy of vaporization of water -

Calorific Value by Mass - 1995 Definitive and 2016


ISO_6976_CVMassD

Symbol Description Units
CVm Calorific value based on mass MJ/kg
CVc Calorific value on molar basis KJ/mol
M Molar mass of the mixture kg/kmol

Calorific Value by Mass - 1995 - Alternative


ISO_6976_CVMass

Symbol Description Units
CVm Calorific value based on mass MJ/kg
xj Mole fraction of component j -
Mj Molar mass of component j kg/kmol
M Molar mass of the mixture kg/kmol
CVj Calorific value of component j MJ/kg

The component calorific value (CVj) is taken from ISO 6976 - 1995 - Table 4, Calorific values for components of natural gases at various combustion reference conditions for the ideal gas on a mass basis.

Calorific Value by Volume - 1995 - Definitive and 2016


ISO_6976_CVVolumeD

Symbol Description Units
CVv Calorific value based on volume MJ/Sm3
CVc Calorific value on molar basis KJ/mol
pb Base pressure bar a
tb Base temperature K

Constant Description Value Units
R Universal Gas Constant 8.314459848484848 J/mol/K

Calorific Value by Volume - 1995 - Alternative


ISO_6976_CVVolume

Symbol Description Units
CVv Calorific value based on volume MJ/Sm3
xj Mole fraction of component j -
CVj Calorific value of component j MJ/Sm3
Zs Standard compression factor -

The component calorific value (CVj) is taken from ISO 6976 Table 5, Calorific values for components of natural gases at various combustion reference conditions for the ideal gas on a volumetric basis.

References