Wobbe index

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The Wobbe index (WI) or Wobbe number^[1] is an indicator of the interchangeability of fuel gases such as natural gas, liquefied petroleum gas (LPG), and town gas and is frequently defined in the specifications of gas supply and transport utilities.

If ${\displaystyle V_{C}}$ is the higher heating value, or higher calorific value, and ${\displaystyle G_{S}}$ is the specific gravity, the Wobbe index, ${\displaystyle I_{W}}$, is defined as:

${\displaystyle I_{W}={\frac {V_{C}}{\sqrt {G_{S}}}}.}$

${\displaystyle G_{S}={\frac {\rho _{STP}}{\rho _{air,STP}}}={\frac {M}{M_{air}}}}$

${\displaystyle \rho _{STP}}$ is the density of the gas at standard conditions, the definition of which changed in 1982. Published Wobbe data may be using 0 °C, 15 °C, 15.56 °C, 20 °C or 25 °C. EU directives on gas quality use 15 °C in accordance with ISO 13443 and ISO 6976.

${\displaystyle \rho _{air,STP}}$ is the density of air at standard conditions, ${\displaystyle M}$ is the molar mass of the gas and ${\displaystyle M_{air}}$ is the molar mass of air which is about 28.96 kg/kmol. . The Wobbe index is used to compare the combustion energy output of different composition fuel gases in an appliance (fire, cooker etc.). If two fuels have identical Wobbe indices then for given pressure and valve settings the energy output will also be identical. Typically variations of up to 5% are allowed as these would not be noticeable to the consumer.

The Wobbe index is a critical factor to minimise the impact of the changeover when analyzing the use of substitute natural gas (SNG) fuels such as propane-air mixtures. The Wobbe index also requires the addition of propane to some upgraded biomethane products,^[2] particularly in regions where natural gas has a high calorific value such as Sweden.^[3]

The Wobbe index has its origins in the 1920's with Italian physicist and engineer Goffredo Wobbe.^[4]