Wobbe index

The Wobbe Index (WI) is the main indicator of the interchangeability of fuel gases such as natural gas LPG and Town Gas and is frequently defined in the specifications of gas supply and transport utilities. If V_C is the higher heating value, or calorific value, and G_S is the specific gravity, the Wobbe Index, I_W, is defined as

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In details, the above equation can be translated into Wobbe Index = higher heating value/(square root of gas specific gravity).

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 SNG fuels such as propane-air mixtures.

Wobbe Index (kcal per standard cubic meter) of common fuel gases

Usage

The Wobbe index is commonly expressed in Btu per standard cubic foot or megajoules per standard cubic meter (1000 BTU/scf = 37.3 MJ/m³). In the case of natural gas (molar mass 17 g/mol), the typical heating value is around 1,050 Btu per cubic foot and the specific gravity is approximately 0.59, giving a typical Wobbe index of 1,367.

There are three ranges or "families" of fuel gasses that have been internationally agreed based on Wobbe Index. Family 1 covers manufactured gases, family 2 covers natural gases (with high and low ranges) and family 3 covers liquefied petroleum gas (LPG). 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, sulphur content, oxygen content etc..