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
VC
GS
IW
IW=
| VC | |
| \sqrt{GS |
GS=
| \rhoSTP | |
| \rhoair,STP |
=
| M | |
| Mair |
\rhoSTP
\rhoair,STP
M
Mair
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]
The Wobbe index has its origins in the 1920's with Italian physicist and engineer Goffredo Wobbe.[4]
Note that these Wobbe numbers below are not calculated at 15 °C and so are not correct according to ISO 13443, see Standard temperature and pressure.
| Fuel gas | Upper index kcal/Nm³ | Upper index MJ/m³ | Lower index MJ/m³ | ||
|---|---|---|---|---|---|
| 11,528 | 9,715 | 48.23 | 40.65 | ||
| 12,735 | 11,452 | 53.28 | 47.91 | ||
| 16,298 | 14,931 | 68.19 | 62.47 | ||
| 15,253 | 14,344 | 63.82 | 60.01 | ||
| 12,837 | 11,597 | 53.71 | 48.52 | ||
| 19,376 | 17,817 | 81.07 | 74.54 | ||
| 18,413 | 17,180 | 77.04 | 71.88 | ||
| 22,066 | 20,336 | 92.32 | 85.08 | ||
| 21,980 | 20,247 | 91.96 | 84.71 | ||
| 21,142 | 19,728 | 88.46 | 82.54 | ||
| 20,755 | 19,106 | 86.84 | 79.94 | ||
| 14,655 | 14,141 | 61.32 | 59.16 | ||
| 3,060 | 3,060 | 12.80 | 12.80 |
Note: 1 Joule = .
The Wobbe index is expressed in MJ/Nm³ (where 'Nm³' indicates'm³ in Normal conditions), or sometimes in BTU/scf. In the case of natural gas (molar mass 17 g/mol), the typical heating value is around 39 MJ/Nm³ (1,050 BTU/scf) and the specific gravity is approximately 0.59, giving a typical Wobbe index of 51 MJ/Nm³ (1,367 BTU/scf).
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 | |
|---|---|---|---|---|
| 1 | 22.5 – 30.0 | 24.0 – 29.0 | ||
| 2 L | 39.0 – 45.0 | |||
| 2 H | 45.5 – 55.0 | 48.0 – 53.0 | ||
| 3 | 73.5 – 87.5 | 72.0 – 87.0 |
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.
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.[6]