Wobbe index Information & Wobbe index Links at HealthHaven.com

The Wobbe Index (WI) 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 $V C$ is the higher heating value, or calorific value, and $G S$ is the specific gravity, the Wobbe Index, $I W$ , is defined as

$I_W = \frac {V_C} {\sqrt{G_S}}.$

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.

[edit] Wobbe Index of common fuel gases

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

Note: 1 Joule = 2.3901×10⁻⁴ kcal.

[edit] Usage

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	Wobbe Index range (MJ/Sm³)	Type of gas
1	22.5 – 30	Town gas / Syngas
2 L	39 – 45	Natural
2 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, sulfur content, oxygen content, etc.

[edit] External links

Wobbe Index and Fuel Gases