Wobbe index

The Wobbe Index (WI) or Wobbe number 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 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..

Wobbe Index of common fuel gases

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.