Chemical & Process Technology: Use Wobbe Index to Manage Fuel Quality to Gas Burner

Thursday, October 2, 2008

Use Wobbe Index to Manage Fuel Quality to Gas Burner

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Gas turbine generator or gas turbine driven compressor is common used in refinery, LNG and gas plant. These turbine basically will burn gas fuel from the plant itself and hot flue gas is passed through the gas turbine. Gas turbine is then rotate and drive motor or compressor to generate power and head.

Flow and Heating Value Changes in Gas burner
Gas fuel burnt in gas burner which typically has fixed orifice nozzles. Heat output from the fixed orifice burner is proportional to flow (Q) and heating value (HV) of the gas fuel. As fuel composition change, flow and heating value will have to be changed to maintain a correct heat output. However, the magnitud of flow and heating value changes may not change linearly or there is not fix relation between flow and heating value. How to relate these two parameters ?

Continuous Changes in Fuel Heating Value (HV)
Gas fuel from refinery, LNG or gas plant is normally a mixtures of gas from several sources i.e. waste gas with low, medium and high heating value. A typical example is fuel gas system in LNG plant. Fuel source can be

End Flash Gas which contains very high inerts (as high as 50%-55% Nitrogen level) and low heating value
Flash gas from Amine regeneration unit which contains high level of CO2 and H2S, Hydrocarbon component varies from ethane to Decane including BTEX
Demethaniser overhead which contain high Methane level
Boil-off gas (BOD) which contains very high level of methane and low level of nitrogen
Flash gas from Ethane, Propone, and LPG storage
Make-up which composition varies from Methane to Decane

These gases will have large differences in composition, high heating value(HHV), low heating value(LHV) , specific gravity (SG), etc. As the flow for each sources may change due to dynamic of the plant and above value will change dynamically from time to time. How to manage the dynamic changes ?

How to manage ?
How to manage a the flow and heating value which may vary in different magnitude and continuous variation in source heating value whilst maintaining a constant heat input into the gas turbine ? What are the parameter to be maintained or limited ?

Wobbe Index is the parameter. Mr. Wobbe found that

Flow is proportion to gas specific gravity (SG) and;
Heating Value is also proportion to gas specific gravity (SG)

Wobbe Index (WI) is define as

WI = HHV / Sqrt (SG)

where
Sqrt = Square root of
HHV = High Heating Value (Btu/Scf)*
SG = Specific Gravity (MWgas / 28.96)

* Some may use Lower Heating Value to define WI

Wobbe Index is used to compare the combustion energy output of different composition fuel gases. Two fuels with identical Wobbe Index at given pressure and valve setting (orifice size) the energy output will be identical. The variation in WI is typically upto 5% (but maximum could be 10% for some manufacturer).

Thus, plant fuel gas designer shall design the fuel gas system such that the fuel gas mixture feeding into the gas turbine meeting the WI limitation. In the event of any upset or interference of any fuel supply source, the control system shall be able to maintain the WI within the limitation.

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Labels: Combustion, Design