1. The document discusses a gas turbine generator site with power demands of 23MW. The gas turbines on site are Siemens SGT-400 models rated at 13.9MW each. 2. It describes challenges with using higher hydrocarbon and hydrogen-rich fuels in gas turbines, such as increased risk of flashback and combustion instability. Heavier fuels require heating to maintain a suitable modified Wobbe index. 3. Solutions proposed include heating the fuel gas to adjust its Wobbe index, and rig testing of combustors with higher calorific value, high hydrogen fuels to evaluate performance.

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GAS TURBINE GENERATOR
 Site power demand: 23MW
 Make – Siemens
 Model – SGT- 400 (3+1
Units)
 Rating: 13.9 MW (ISO),
10.2MW site rating @45°C.
 DLE combustors
 Speed – 9500 RPM
2 – SALIENT FEATURES OF MAIN EQUIPMENT,
PROBLEMS, DECISIONS

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Typical Gas Turbine
Typical Gas Turbine

3. Gas Turbine Combustor
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Blue Lines represents Compressed Air
from Axial compressor
Red Lines Represents Fuel Gas and
Hot Gases
DLN / DLE
Combustor

4. Comparison of Flame Speeds
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Flame Speed

5. Ar-Ratawi Fuel Gas composition
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Heavier Hydrocarbon, Flash back
H2, Flash back
Inert Gases,
Blown Out
Fuel Gas Mapping
• High hydrocarbon fuels are significantly more reactive and energetic (Similar to H2) compared to
natural gas.
• Wobbe Index: The index defines the heating value of a quantity of gas that will flow through a
hole of a given size in a given amount of time. The higher the index the higher the heating value.

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 Blown Out. Blown out occurs when the flame becomes detached from the location where it is anchored and
is physically “blown out” of the combustor. Due to low WI, Mass flow rate of fuel through injector will increase
(As it is required to provide the same Heat release) resulting in Pressure drop with possible flame
detachment Result : Dynamic Pressure Amplitudes Solution : Changing the Fuel Injector Nozzle design
(High inert gas components)
 Flashback. Flashback occurs when the turbulent flame speed exceeds the flow velocity (Gas Velocity of gas
before combustion), allowing the flame to propagate upstream into the premixing section (Which is not
designed for higher temperature). (High Hydrogen and Higher Hydrocarbons components)
 When Flashback occurs acoustic oscillations in the premixer section causes fluctuations in the fuel and/or air
supply rates, thus producing a reactive mixture whose equivalence ratio changes periodically in time. Result :
Mixture fluctuation is transferred to the flame where it produces heat release oscillations that drive the
Combustion instability.
 Combustion Instability. Combustion instabilities are characterized by large amplitude pressure oscillations
that are driven by unsteady heat release
 Due to Heavier Hydrocarbons / Hydrogen in fuel
composition will Increase the flame speed and change
flame Centre of Mass (Increasing the Burner Metal
Temp.).
 Chances of Hot spot due to shorter convection
time in the combustor.

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 Modified Wobbe Index (MWI): By increasing the fuel supply temp wobbe index is altered from
ISO conditions. The fuel supply temperature impacts the actual volumetric fuel flow.
“All gas mixtures that have the same Wobbe number will give the same amount of heat.”
MWI=LHV/(SG * Tgas) 0.5
LHV = Lower heating value of the gas in Btu/scf
SG = Specific gravity of the gas fuel related to air
Tgas= Absolute temperature of the gas fuel in oR
Ethane content vol % 10 15 20 25 30 35 40 45 50
Min Supply Temp C 2.5 3.0 11.0 20.0 29.0 37.0 46.0 54.0 63.0
Dew Point C < -20 < -20 < -20 < -20 < -20 < -20 < -20 < -20 < -20
Temp Corr Wobbe Index kj/m3 48214 48939 48991 48957 48926 48975 48946 48993 48965
Wobbe Index @ iso kj/m3 47156 47909 48650 49380 50101 50811 51513 52205 52888
For standard SGT-400 operation,
Siemens generally accept fuels within
a MWI range 37 – 49MJ/m3.
SOLUTION 1 : Heating the Fuel Gas (Heavier
Hydrocarbon)

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SOLUTION 2: RIG TESTING OF COMBUSTORS (High Calorific value High H2)
COMBUSTORS

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TEST ARRANGMENT

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TYPICAL RESULTS