Optimal Design of Gas Turbine Power Station

P M V SubbaraoProfessor

Mechanical Engineering Department

More Ideas for better fuel Economy…….

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1872, Dr Franz Stikze’s Paradox

Condition for Compact Gas Turbine Power Plant

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max0min0max, T

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TTcw pnet

12 min0max0max0max, TTTcw pnet

At maximum power:

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max00 TT

TT exhaust

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TTT

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netcompact

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min0max0max, 11

TTT

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Important Comments:

What if I am not interested in Compactness.

Should I prefer high Pressure Ratio for Efficient Plant?

Why the plant is compact at this condition?

What else can be inferred form this condition?

The state-of-the-art

• The newer large industrial gas turbines size have increased and capable of generating as much as 200 MW at 50 Hz.

• The turbine entry temperature has increased to 12600C, and the pressure ratio is 16:1.

• Typical simple cycle efficiencies on natural gas are 35%. • The ABB GT 13 E2 is rated at 164 MW gross output on natural

gas, with an efficiency of 35.7%. • The pressure ratio is 15:1. • The combustion system is designed for low Nox production.

• The dry Nox is less than 25 ppm on natural gas. • The turbine entry temperature is 11000C and the exhaust

temperature is 5250C. • The turbine has five stages, and the first two rotor stages and

the first three stator stages are cooled; • the roots of the last two stages are also cooled.

• Siemens power corporation described their model V84.3.

• This is rated at 152 MW at an efficiency of 36.1%. The pressure ratio is 16:1.

• Six burners designed for low Nox emissions are

installed in each chamber.

• The turbine entry temperature is 12900C and the

exhaust temperature is 550 C. • The turbine has four stages and the first three

rotating stages are air cooled. • The effectiveness of the cooling is improved by inter-

cooling the cooling air after it is with drawn from the compressor.

• General Electric and European Gas Turbines have jointly developed the MS9001F 50Hz engine.

• This unit generates 215 MW at an efficiency of 35%. • The engine uses an 18 stage compressor with an overall

compression ratio of order of 20:1. • The gas turbine has three stages, with the first two stages

cooled. • Turbine entry temperature is 1288 C. • These large high efficiency units can be used for peak lopping

purposes. • The research for more efficient gas turbine-based power

generation cycles has been underway for some time. The aims are:

• - Higher turbine entry gas temperature, - Higher compressor efficiency and capability

• The different manufactures participated and initiated the collaborative advanced gas turbine.

• The outcome of their effort include a variety of advanced cycle options, including intercooling, humid air turbine, steam injection, reheat combustor and chemical recuperation.

• The U.S. Department of Energy (DOE) has initiated a development program called the advanced turbine system (ATS).

• The aim of ATS is to achieve over 60% efficiency, with low Nox

and suitable operating costs at the end of a 10-year program. • They pictured the program with increasing in firing temperature

up to over 1427 C and changes in cycle, as intercooling, reheat combustors, massive moisture injection and chemical recuperation.

Fuel  Natural gas

 Frequency  60 Hz

 Gross Electrical output  187.7 MW*

 Gross Electrical efficiency  36.9 %

 Gross Heat rate  9251 Btu/kWh 

 Turbine speed  3600 rpm

 Compressor pressure ratio  32:1

 Exhaust gas flow  445 kg/s

 Exhaust gas temperature  612 °C

 NOx emissions (corr. to 15% O2,dry)  < 25 vppm

GT24 (ISO 2314 : 1989)

Fuel  Natural gas

 Frequency  60 Hz

 Gross Electrical output  187.7 MW*

 Gross Electrical efficiency  36.9 %

 Gross Heat rate  9251 Btu/kWh 

 Turbine speed  3600 rpm

 Compressor pressure ratio  32:1

 Exhaust gas flow  445 kg/s

 Exhaust gas temperature  612 °C

 NOx emissions (corr. to 15% O2,dry)  < 25 vppm

9756 kJ/kWh

The Ideal Machine

• 1824: Sadi Carnot, who founded the science of thermodynamics, identified several fundamental ideas that would be incorporated in later internal combustion engines: – He noted that air compressed by a ratio of 15 to 1 would

be hot enough (200°C) to ignite dry wood.– He recommended compressing the air before combustion.

Fuel could then be added by "an easily invented injector".– Carnot realized that the cylinder walls would require

cooling to permit continuous operation. – Later, Diesel thought he could avoid this, but found out

the hard way. – He noted that usable heat would be available in the

exhaust, and recommended passing it under a water boiler.

Developments in Gas Turbine Cycles

1. The wet compression (WC) cycle

2. The steam injected gas turbine (STIG) cycle

3. The integrated WC & STIG (SWC) cycle

4. Themo-chemical Recuperation cycles

Wet compression

• One of the most effective ways to increase the gas turbine power output is to reduce the amount of work required for its compressor.

• A gas turbine compressor consumes about 30 to 50% of work produced by the turbine.

The wet compression (WC) cycle

Intake AirInlet Duct

Water InjectionCompressor

Combustor

Fuel

Turbine

G

Representing wet compression process on P-V diagramW isothermal = f-1-2T-g-f (isothermal)

Wwet compression = f-1-2K-g-f (wet compression)

W isentropic = f-1-2S-g-f (isentropic)

W polytropic = f-1-2n-g-f (polytropic)

1

2s2k 2n2T

P 1

P 2

f

g

0

P

V

The wet compression (WC) cycle

• The wet compression cycle has the following benefits over the simple cycle.

1. Lower compressor work

2. Higher turbine work

3. Higher cycle efficiency

ISENTROPIC INDEX OF WET COMPRESSION PROCESS

• Isentropic index of wet compression can be obtained from the equation

1 1

L dw k

R dT k

Where k=Isentropic index of wet compression, dw/dT = Evaporative rate kg/k, L= Latent heat kJ/kg, R=Gas constant of humid air kJ/kg k.

ACTUAL WET COMPRESSION INDEX

• Actual wet compression index can be obtained from the equation

1

1 1 1 1

m L dw n

m R dT n

Wherem=polytropic index of actual wet compression process,n=polytropic index of actual dry air compression

Compressor work with wet compression

• Compressor work with wet compression is a function of

1. Pressure ratio ,

2. Evaporative rate dw/dT and

3. Geometry of the compressor.

• Wet compression work is much lower than that of dry air compression work.

• The higher is the pressure ratio, more the saving in compressor work.

Variation of wet compression work with pressure ratio

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pressure ratio

wo

rk w

(kj/k

g)

W_diW_daW_wiW_wa

(Evaporative rate dw/dT=7.5e-4 kg/k)

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0 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006 0.0007 0.0008

Evaporation rate dw/dT(kg/k)

work

(kj/k

g)

W_diW_daW_wa

Pressure ratio = 7

VARIATION OF WET COMPRESSION WORK WITH THE EVAPORATIVE RATE FOR A GIVEN PRESSURE

RATIO

REAL WET COMPRESSION WORK CONSIDERING OFF DESIGN BEHAVIOUR

• For calculation purposes, if the design (dry) value of the polytropic efficiency is assumed to be maintained throughout the compression process, it is tantamount to the operation of the compressor at increased operating pressure ratio.

Comparison of Work Input For Wet and Dry Compression Considering Off-Design Behaviour

Sl

no

Evaporative rate, kg/k

Operating

Pr. ratio

Real wet work

kJ/kg

Dry work

KJ/kg

1 0 10.2 343.269 343.269

2 0.00015 11.5597 316.649 370.415

3 0.00035 11.5737 284.812 370.683

4 0.00075 11.6017 255.000 371.218

ACTUAL WET COMPRESSION WORK CONSIDERING OFF DESIGN BEHAVIOUR

200225250275300325350375400

0 0.0002 0.0004 0.0006 0.0008

Evaporative rate, dw/dT,kg/k

Wor

k in

put k

j/kg

DesignconditionOff-designcondition

Fuel  Natural gas

 Frequency  60 Hz

 Gross Electrical output  187.7 MW*

 Gross Electrical efficiency  36.9 %

 Gross Heat rate  9251 Btu/kWh 

 Turbine speed  3600 rpm

 Compressor pressure ratio  32:1

 Exhaust gas flow  445 kg/s

 Exhaust gas temperature  612 °C

 NOx emissions (corr. to 15% O2,dry)  < 25 vppm

9756 kJ/kWh

WaterSuper Heated Steam

The steam injected gas turbine (STIG) cycleThe steam injected gas turbine (STIG) cycle

G

Compressor Turbine

Combustor

Fuel

Injection Steam

Exhaust

HRSGwaterpump

Intake Air

The steam injected gas turbine (STIG) cycle

• Steam injection into the combustion chamber of a gas turbine is one of the ways to achieve power augmentation and efficiency gain.

• In a steam injected gas turbine (STIG), the heat of exhaust gasses of the gas turbine is used to produce steam in a heat recovery steam generator.

• The steam is injected into the combustion chamber or before entering the combustion chamber (i.e. in the compressor discharge).

• STIG cycle has higher cycle efficiency than the WC cycle.• STIG cycle gives higher net work out put than the WC

cycle up to a pressure ratio of 7.

The integrated WC & STIG (SWC) cycle

G

Intake Air

Water injection

Inlet Duct

Compressor Turbine

Combustor

Fuel

Injection Steam

Exhaust

HRSGwaterpump

The integrated WC & STIG (SWC) cycle

• It has the combined benefit of the advantage of higher efficiency of STIG cycle and higher net work output of WC cycle.

• But its cycle efficiency is less than that of the STIG cycle owing to the need for higher heat input.

COMPARISION BETWEEN SIMPLE, WC, STIG AND INTEGRATED WC & STIG CYCLES

2022

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283032

3436

384042

4446

4850

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Net work output, MW

Cycle

eff

icie

ncy,

%

simple cycle w et compression STIG WC & STIG

Cycle efficiency versus pressure ratio

2025303540455055

5 7 9 11 13 15 17 19

Pressure ratio

Cyc

le e

ffici

ency

%

simple cycle w et compression cycle STIG cycle WC & STIG cycle

Net work output versus pressure ratio

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Pressure ratio

Net

wo

rk o

utp

ut M

W

simple cycle w et compression cycle STIG cycle WC & STIG cycle

Comparison of typical parameters of simple, WC,STIG and SWC cycles.

cycle Pressure ratio

PR

Evaporative rate, kg/k

Net work output, MW

Cycle efficiency

%

Fuel mass flow rate, kg/sec

Steam mass flow rate, kg/sec

simple 11 0 151.13 31.28 11.04 0

WC 11 7.5e-4 232.75 35.35 15.04 0

STIG 11 0 215.65 39.63 12.43 54.49

SWC 11 7.5e-4 303.11 38.16 18.15 54.49

Future work

There are many areas and challenges which can be explored further to this work. They are:

• Economic feasibility of these cycles need to be studied.

• Compressor life reduction due to water injection. (because of the off design running conditions that prevail in reality).

• The difficulties involved in designing a turbine to handle large mass flow rates of combustion gasses and steam.

• The effect of steam injection in reducing NOX emissions.

A tree converts disorder to order with a little help from the Sun

Clues from Nature to get Better Fuel

• One of such clue is Thermo Chemical Recuperation

• The major reactions involved in Steam-TCR are well known, and the overall reaction for a general hydrocarbon fuel, CnHm, is:

The formation of carbon must be minimized in the operation of the reformer to minimize fouling of heat transfer surfaces, blinding of catalyst particles, plugging of flow paths and carbon losses.

The theoretical merits of the Steam-TCR concept are based on the overall endothermic nature of the reforming chemical reactions, and the formation of a low-thermal-value fuel gas replacing the high-thermal-value turbine fuel, with both factors contributing to improved efficiency

Steam-TCR Power Plant Cycle Diagram

Flue Gas-TCR Power Plant Cycle Diagram

Model TCR Cycle

The chemical Reactions in Flue Gas TCR Cycle

• Combustion of Methane with 100% theoretical air.

222224 52.7252.72 NOHCONOCH

222222 52.7352.72 NOHCOHNOHCO

22222222 04.15452.7252.73 NOHCONONOHCOH

Thermochemical recuperator: Reforming of Flue Gas Only

• Combustion of reformed flue gas :

• Combustion of reformed flue gas and methane mixture:

22222224 52.732304.1542 NOHCOCOHNOHCOCH

222222222 56.226352.7204.15323 NOHCONONOHCOCOH

• Thermochemical recuperator: Reforming of Flue gas with methane

The chemical Reactions in Flue Gas & Methane TCR Cycle

First Law Analysis of Thermochemical Recuperator

No work transfer, no heat transfer, change in kinetic and potential energies are negligible

CVout

outin

inCV WgzVhmgzVhmQ

22

outoutinin hmhm

Turbine ExhaustCooled exhust

Fuel & Flue gasReformed fuel

Energy lost by turbine exhaust = Increase in energy of reformed gas.

fluegasfluegasfuelfuelrfuelrfueloutgingg hmhmhmhhm ,,

Generalized Recuperation Reaction

OHyxNCOyxCOHx

NOHCOyxCH

2222

2224

331252.73

52.72

fluegasfluegasfuelfuelrfuelrfueloutgingg hmhmhmhhm ,,

Analysis of Reformation Process

Study of Optimal TCR Cycle

Parameter Simple Brayton TCR Brayton

Flue Gas Recirculation 0% 70%

Mass flow rate of air 462 kg/s 135 kg/s

Power input to compressor 155.2MW 44.5MW

Fuel 8.4kg/s 7.35kg/s

Flue gas compressor -- 114MW

Net Power output 134.7MW 141.8MW

Efficiency 32.1% 38.6%

Steam generation 252kg/s 41kg/s

Reduction of CO2 Emissions

• Increasing CO2 content in atmosphere is one of the factor for Global Warming.

• Power Generated CO2 is responsible.

• Kaya’s Equation:

SE

CO

GDP

E

POP

GDPPOPCO 2

2 eatomospher oemission t

• Where

• POP : Population that demands and consume energy

• GDP/POP: Per capita gross domestic product, reflecting standard of living.

• E/GDP: Energy generated per gross domestic product, the energy intensity.

• CO2/E : Emission per unit energy generation, the carbon intensity

• S: Natural and induced removal emission product from atmosphere into a sink.

Carbon dioxide Sinks • Biosphere sinks : Natural Resources

• Geosphere Sinks: Natural Resources with anthropogenic intervention.

• Material Sinks: Anthropogenic Resoruces

Carbon Sequesterizaton

Partial Oxidation Cycles

Partial Oxidation Cycle