What is fuel Wobbe Index and why is it used?

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AJ Stack Solutions Trainer · Accepted Answer

Gaseous fuels are made up of different hydrocarbon constituents in varying quantities, with each constituent of a different calorific value and density.  The fuel may also contain non-combustible components such as nitrogen and carbon dioxide.  A gas turbine requires a specific amount of heat energy to produce a unit of output power so it is important to determine how much of a specific fuel is required to deliver the necessary fuel energy.    Wobbe Index is a measure of the energy density of a gaseous fuel.  Mathematically, it is expressed as:  [attach]2[/attach] LHV is the lower heating value of the fuel.  SG is the fuel specific gravity with air having an SG=1.  Wobbe Index provides a measurement for designing the fuel system volume to ensure sufficient fuel flow to the engine.  Fuel system volume encompasses the fuel valve, injectors and manifold.  Standard utility natural gas that is >95% C1 (methane) will have a Wobbe Index of approximately 1200 BTU/SCF.  As the LHV drops or the density increases, a larger volume of fuel, thus a larger fuel system, will be needed to deliver the required fuel energy.  As the LHV increases or the density decreases, a smaller volume of fuel, thus a smaller fuel system, is required.     A particular fuel system size will normally be able to accommodate a range of Wobbe Index, typically +10% from a nominal value.  Below is a chart showing the Wobbe Index of typical fuels encountered in the Oil and Gas and Power Generation industries. Low Wobbe fuels (<800 W.I.) typically have large quantities of carbon dioxide.  Coke Oven Gas contains hydrogen, which is very volatile, but relatively low in energy.  [attach]3[/attach]

Control Systems [Solved] What is fuel Wobbe Index and why is it used?

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