The Wobbe Index (WI) or Wobbe number[1] is an indicator of the interchangeability of fuel gases such as natural gas, liquified petroleum gas (LPG), and town gas and is frequently defined in the specifications of gas supply and transport utilities. If is the higher heating value, or calorific value, and is the specific gravity, the Wobbe Index, , is defined as
That is, 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 substitute natural gas (SNG) fuels such as propane-air mixtures..
Fuel gas | Upper index Kcal/Sm3 |
Lower index Kcal/Sm3 |
Upper index MJ/Sm3 |
Lower index MJ/Sm3 |
Hydrogen | 11,528 | 9,715 | 48.23 | 40.65 |
Methane | 12,735 | 11,452 | 53.28 | 47.91 |
Ethane | 16,298 | 14,931 | 68.19 | 62.47 |
Ethylene | 15,253 | 14,344 | 63.82 | 60.01 |
Natural gas | 12,837 | 11,597 | 53.71 | 48.52 |
Propane | 19,376 | 17,817 | 81.07 | 74.54 |
Propylene | 18,413 | 17,180 | 77.04 | 71.88 |
n-butane | 22,066 | 20,336 | 92.32 | 85.08 |
Iso-butane | 21,980 | 20,247 | 91.96 | 84.71 |
Butylene-1 | 21,142 | 19,728 | 88.46 | 82.54 |
LPG | 20,755 | 19,106 | 86.84 | 79.94 |
Acetylene | 14,655 | 14,141 | 61.32 | 59.16 |
Carbon monoxide | 3,060 | 3,060 | 12.80 | 12.80 |
The Wobbe index is commonly expressed in British Thermal Units (BTU) per standard cubic foot or megajoules per standard cubic meter (1000 BTU/scf = 37.3 MJ/Sm3). 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 BTU/scf (51 MJ/m3).
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 (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.
Family | Type of gas | Wobbe Index range (MJ/Sm3) | Wobbe number range from [1] |
1 | Town gas / Syngas | 22.5 – 30 | 24 – 29 |
2 L | Natural | 39 – 45 | |
2 H | 45.5 – 55 | 48 – 53 | |
3 | LPG | 73.5 – 87.5 | 72 – 87 |
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.
[[es:Índice de Wobbe]