Wobbe index

The Wobbe Index (WI) or Wobbe number[1] is an indicator of the interchangeability of fuel gases such as natural gas, liquefied 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 higher calorific value, and is the specific gravity, the Wobbe Index, , is defined as:

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. 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 [3]

Wobbe Index of common fuel gases [4]

Fuel gas Upper index
Kcal/Nm3 		Lower index
Kcal/Nm3 		Upper index
MJ/Nm3 		Lower index
MJ/Nm3
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
Note: 1 Joule = 2.3901×10−4 kcal.

Usage

The Wobbe index is commonly expressed in either British Thermal Units (BTU) per standard cubic foot, or else megajoules per standard cubic meter (1,000 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/Nm3)     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.

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.[5]

References

  1. 1 2 Treloar, R.D. (2005). Gas Installation Technology. Blackwell. p. 24. ISBN 978-1-4051-1880-4.
  2. ↑ http://www.biogasmax.eu/media/d3_3v2biogasmax_iwes_v2f_nov2010__052078600_1109_10022011.pdf
  3. ↑ B .
  4. ↑ Wobbe Index and Fuel Gases
  5. ↑ Flórez-Orrego,Daniel, Escola Politécnica da USP. "Fuel Gas Interchangeability". Retrieved 8 Sep 2012.
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