The Wobbe Index or Wobbe number is an indicator of the interchangeability of fuel

Fuel

Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...

 gases such as natural gas

Natural gas

Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...

, liquified petroleum gas

Liquified petroleum gas

Liquefied petroleum gas is a flammable mixture of hydrocarbon gases used as a fuel in heating appliances and vehicles. It is increasingly used as an aerosol propellant and a refrigerant, replacing chlorofluorocarbons in an effort to reduce damage to the ozone layer...

 (LPG), and town gas

Coal gas

Coal gas is a flammable gaseous fuel made by the destructive distillation of coal containing a variety of calorific gases including hydrogen, carbon monoxide, methane and volatile hydrocarbons together with small quantities of non-calorific gases such as carbon dioxide and nitrogen...

 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

Specific gravity

Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...

, 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

Substitute natural gas

Substitute natural gas, or SNG, can be produced from fossil fuels such as lignite coal or from biofuels, when it is named Bio-SNG.In the Great Plains, USA, approximately 15 TWh/year of SNG from lignite coal is injected into the national gas grid...

(SNG) fuels such as propane

Propane

Propane is a three-carbon alkane with the molecular formula , normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central...

-air mixtures..

Wobbe Index of common fuel gases

Fuel gas	Upper index Kcal/Sm3 Standard conditions for temperature and pressure Standard condition for temperature and pressure are standard sets of conditions for experimental measurements established to allow comparisons to be made between different sets of data...	Lower index Kcal/Sm3 Standard conditions for temperature and pressure Standard condition for temperature and pressure are standard sets of conditions for experimental measurements established to allow comparisons to be made between different sets of data...	Upper index MJ/Sm3 Standard conditions for temperature and pressure Standard condition for temperature and pressure are standard sets of conditions for experimental measurements established to allow comparisons to be made between different sets of data...	Lower index MJ/Sm3 Standard conditions for temperature and pressure Standard condition for temperature and pressure are standard sets of conditions for experimental measurements established to allow comparisons to be made between different sets of data...
Hydrogen Hydrogen Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...	11,528	9,715	48.23	40.65
Methane Methane Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...	12,735	11,452	53.28	47.91
Ethane Ethane Ethane is a chemical compound with chemical formula C2H6. It is the only two-carbon alkane that is an aliphatic hydrocarbon. At standard temperature and pressure, ethane is a colorless, odorless gas....	16,298	14,931	68.19	62.47
Ethylene Ethylene Ethylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...	15,253	14,344	63.82	60.01
Natural gas Natural gas Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...	12,837	11,597	53.71	48.52
Propane Propane Propane is a three-carbon alkane with the molecular formula , normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central...	19,376	17,817	81.07	74.54
Propylene Propylene Propene, also known as propylene or methylethylene, is an unsaturated organic compound having the chemical formula C3H6. It has one double bond, and is the second simplest member of the alkene class of hydrocarbons, and it is also second in natural abundance.-Properties:At room temperature and...	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 Acetylene Acetylene is the chemical compound with the formula C2H2. It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in pure form and thus is usually handled as a solution.As an alkyne, acetylene is unsaturated because...	14,655	14,141	61.32	59.16
Carbon monoxide Carbon monoxide Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...	3,060	3,060	12.80	12.80

Note: 1 Joule

Joule

The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...

 = .

Usage

The Wobbe index is commonly expressed in British Thermal Unit

British thermal unit

The British thermal unit is a traditional unit of energy equal to about 1055 joules. It is approximately the amount of energy needed to heat of water, which is exactly one tenth of a UK gallon or about 0.1198 US gallons, from 39°F to 40°F...

s (BTU) per standard cubic foot or megajoules per standard cubic meter

Standard conditions for temperature and pressure

Standard condition for temperature and pressure are standard sets of conditions for experimental measurements established to allow comparisons to be made between different sets of data...

 (1000 BTU/scf = 37.3 MJ/Sm³). In the case of natural gas (molar mass

Molar mass

Molar mass, symbol M, is a physical property of a given substance , namely its mass per amount of substance. The base SI unit for mass is the kilogram and that for amount of substance is the mole. Thus, the derived unit for molar mass is kg/mol...

 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 Standard conditions for temperature and pressure Standard condition for temperature and pressure are standard sets of conditions for experimental measurements established to allow comparisons to be made between different sets of data... )	Wobbe number range from
1	Town gas / Syngas Syngas Syngas is the name given to a gas mixture that contains varying amounts of carbon monoxide and hydrogen. Examples of production methods include steam reforming of natural gas or liquid hydrocarbons to produce hydrogen, the gasification of coal, biomass, and in some types of waste-to-energy...	22.5 – 30	24 – 29
2 L	Natural Natural gas Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most 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

Flame speed

The flame speed is the measured rate of expansion of the flame front in a combustion reaction. Whereas flame speed is generally used for a fuel, a related term is explosive velocity, which is the same relationship measured for an explosive. Combustion engineers differentiate between the laminar...

, "yellow tipping" due to incomplete combustion, sulfur content, oxygen content, etc.

http://www.documentation.emersonprocess.com/groups/public/documents/application_notes-tech._briefs/d301334x012.pdf?&xBusinessUnit=Remote%20Automation%20Solutions

References