Wobbe index

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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. It is defined as:

      Wobbe Index = higher heating value/(square root of gas specific gravity)

It 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 a given valve setting 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.

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

 Fuel Gas      Upper Index  Lower Index
 Hydrogen         11,528      9,715 
 Methane          12,735     11,452
 Ethane           16,298     14,931
 Ethylene         15,253     14,344
 Natural gas      12,837     11,597
 Propane          19,376     17,817
 Propylene        18,413     17,180
 n-butane         22,066     20,336
 Iso-butane       21,980     20,247
 Butylene-1       21,142     19,728
 LPG              20,755     19,106
 Acetylene        14,655     14,141
 Carbon monoxide   3,060      3,060

[edit] 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/Sm3). In the case of natural gas (Mol. Wt. 17), 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 a particular family - hydrogen-rich Town Gas, Natural Gas or LPG.

 Family  Wobbe Index range (MJ/Sm3)    Type of gas
 1            22.5 – 30             Town gas / Syn Gas
 2 L            39 – 45                  Natural
   H          45.5 – 55
 3            73.5 – 87.5                 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..