Use Wobbe Index to Manage Fuel Quality to Gas Burner

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.

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 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

This is the parameter that gives a relative measure of the mix/heating value. 
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.

Calculations of Specific Gravity, Calorific Value & Wobbe Index Table Excerpted From The Institute of Gas Technology, bulletin No. 32

• Wobbe Index is a measure of the amount of energy delivered to a burner via an injector (orifice). The energy input is a linear function of Wobbe index.
• Two gases differing in composition but having the same Wobbe Index will deliver the same amount of energy for any given injector/orifice under the same injector pressure. 
• In natural gas appliances, the gas flow is restricted by passing it through an orifice (hole). The Wobbe Index is useful because for any fixed orifice size and gas pressure, any gas compositions that have the same Wobbe number will deliver the same amount of heat energy or expressed as interchangeability of varying gas compositions.

Interchangeability; Wobbe Index

According to ISO-13686, gas interchangeability indicates the degree of substitutability between the combustion characteristics of different gas types.

Gas with one particular composition is interchangeable with another having a different composition, only if the quality of combustion remains within the specified range.

The Mexican NOM uses the Wobbe Index to deal with gas interchangeability. This index shows the relationship between heat value and the specific gravity of gas, as a flow of energy at constant pressure.

Gas with different compositions but the same Wobbe Index provides the same amount of energy at the same pressure (Fig. 3).

This relationship is represented by a flow equation at the flow point, as follows: W=HHV/ (squareroot of)sg ; where W= Wobbe Index; HHV=high heat value; and sg=specific gravity.

Sources: 

http://www.ogj.com/articles/print/volume-102/issue-25/transportation/mexicos-cre-issues-new-gas-quality-specs.html

http://www.analyticexpert.com/category/gas-analysis/