Summary

The Wobbe Index is a measure of the interchangeability of fuel gases and their relative ability to deliver energy. It gives an indication of whether a turbine or burner will be able to run on an alternative fuel source without tuning or physical modifications.

Definitions

\( \text{HHV} \):Higher Heating Value (energy/volume)
\(I_{w}\):Wobbe Index
\(RD\):Relative Density \(\rho_{gas}/\rho_{air}\), \(\rho_{air} = 1.205\) kg/m3


Wobbe Index Definition

The Wobbe Index for a fuel gas can be calculated using its higher heating value and relative density as follows:

\[ \displaystyle I_{w} = \cfrac{\text{HHV}}{\sqrt{RD}} \]

The Wobbe index is the ratio of the specific energy of a gas to its relative flow rate as shown by its constituent components:

  1. - The Higher Heating Value, which is a measure of the energy per volume for the gas being considered.
  2. - The inverse of the root of the relative density, which is a good approximation of the relative mass flow rate of a gas through a fixed orifice at a fixed pressure.

By determining the ratio of the energy content of the fuel gas along with the relative mass flow we arrive at a number proportional to the energy delivered to a burner or turbine. Where two fuel gases have the same Wobbe Index, they will deliver the same amount of energy when supplied at the same pressure.

Units

The Wobbe Index may be expressed in either British Thermal Units per standard cubic foot, \(\text{BTU}/\text{SCF}\), or megajoules per standard meter cubed, \(\text{MJ}/\text{Sm}^{3}\). Where the units are not given it is likely that the units are the more traditional \(\text{BTU}/\text{SCF}\). The units are often deliberately omitted to prevent confusion with the actual energy per volume of the fuel gas.

It is important to note that Wobbe index values must not be used in place of energy of combustion even though it may have the same units.

Limitations

The Wobbe index should not be used as the sole criteria by which to judge the feasibility of a fuel swap out. There are several limitations of the Wobbe index that need to be considered separately:

  • Liquid Drop Out - It is possible to make a mixture that will have a suitable Wobbe Index but will be prone to liquid drop out, which in turn will effect the operation of combustion equipment. In the mixture of carbon dioxide and pentane which simulates natural gas, significant amounts of the pentane will liquefy under typical operating conditions.
  • Reactive Components - The presences of reactive components like nitrogen, carbon monoxide and alkenes may cause problems for fuel system components.


Wobbe Index Values for Common Fuels

The below table gives some approximate Wobbe index for common fuel gases.

Gas \(I_{w}\) () \(I_{w}\) ()
Hydrogen45.51220
Methane49.91339
Ethane63.61708
Propane74.82008
Butane84.42266
Pentane92.52482
Acetylene57.01531
Natural Gas46 - 521235 - 1396
Liquefied Natural Gas (LNG)52.81417

Consequences of Incorrect Wobbe Index Fuels

High Wobbe Index

When the Wobbe Index of the replacement fuel is too high the fuel it will deliver too much energy and will likely to require more oxygen for complete combustion. This can result in the following negative effects:

  • Over Heating - Higher than expected flame temperature leading to damage to the burner, turbine or equipment being fired.
  • Excessive carbon monoxide formation - Caused by incomplete combustion, if allowed to build up can be harmful or fatal to people.
  • Soot build up - Increased potential for soot to build up due to incomplete combustion.
  • Explosive gases - In cases where the air supply is insufficient, quantities of carbon monoxide and hydrogen may build up to a point that the atmosphere becomes explosive when air is re-admitted.
  • Inefficient combustion - Failure to extract the maximum energy from the fuel.

Low Wobbe Index

Where the Wobble index of the replacement fuel is too low the fuel will not deliver enough energy. The can result in the following negative effects:

  • Ignition problems - The equipment may not ignite if the energy content of the gas is too low.
  • Flame instability and flashback - An unstable flame, and flashback through the fuel/air supply tube can occur. This may snuff the flame and damage equipment. When the flame is snuffed a hazard is caused by the accumulation of un-burnt fuel gas being released.


Further Reading

  1. Plant Engineer's Reference Book, Second Edition
  2. Mechanical Engineers Reference Book