The Wobbe index is used to compare the combustion energy output of different composition fuel gases in an appliance (fire, cooker etc.).
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.
[4] Note that these Wobbe numbers below are not calculated at 15 °C and so are not correct according to ISO 13443, see Standard temperature and pressure.
There are three ranges or "families" of fuel gases that have been internationally agreed based on Wobbe index.
In spite of its usefulness, Wobbe index alone is not a good indicator of the interchangeability of two or more gases, or mixtures of them.
It is necessary to bear in mind other criteria while determining the plenty substitution of a fuel by other, different of the one used to adjust the burning system.