The Wobbe Index or Wobbe number is an indicator of the interchangeability of fuel gases such as natural gas, liquefied petroleum gas, and town gas and is frequently defined in the specifications of gas supply and transport utilities. If is the higher heating value, or higher calorific value, and is the specific gravity, the Wobbe Index,, is defined as: is the density of the gas at standard conditions, is the density of air at standard conditions, is the molar mass of the gas and 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. 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 fuels such as propane-air mixtures. The Wobbe Index also requires the addition of propane to some upgraded biomethane products, particularly in regions where natural gas has a high calorific value such as Sweden
The Wobbe index is expressed in MJ/Nm³. In the case of natural gas, the typical heating value is around 39 MJ/Nm³ and the specific gravity is approximately 0.59, giving a typical Wobbe index of 51 MJ/Nm³. There are three ranges or "families" of fuel gases that have been internationally agreed based on Wobbe index. Family 1 covers manufactured gases, family 2 covers natural gases and family 3 covers liquefied petroleum gas. Combustion equipment is typically designed to burn a fuel gas within a particular family: hydrogen-rich town gas, natural gas or LPG.
Other flame characteristics and composition limits may determine the acceptability of the replacement gas, e.g. flame speed, "yellow tipping" due to incomplete combustion, sulfur content, oxygen content, etc.
Limitations
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.
