Gas Turbine WHRU - Introduction

GAS TURBINE WASTE HEAT

RECOVERY UNITS (WHRU)

Aug 2012

AcknowledgmentGPSA Section 15

Pictures from many sources, suppliers, internet

Gas Turbine

Prime mover to drive compressors and generators

2 Types: Aero-derivatives and industrial Aero-derivatives are light weight; preferred offshore and are

used to 25 MW

Heavier industrial units are used for higher duties and on onshore

Ambient air (gas) is compressed in an axial air compressor to 10 to 20 bar similar to BFW pump in a steam system

Fuel is fired in a combustion chamber into hot compressed air. Hot and pressurized flue gas is let down in pressure first thru a "gas producer" turbine -similar to steam turbine, to provide energy for inlet compression

Balance energy in the hot gas is used for jet-action in an aircraft. In industrial unit, it is recovered thru a "power turbine" to run a compressor or generator

Inlet compression takes 2/3 energy developed in turbines. Recycled energy!

GT fuel efficiency: 30-40% based on Xs air and Exhaust Temp

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ExhaustAir


GT Single Vs Split Shaft

Industrial GTs are made of single shaft - that is air compressor, power turbine and driven equipment (compressor or generator) all share the same common shaft Single shaft is more stable, lesser bearings but requires

higher starting power. Narrow speed range to avoid surging in inlet air compressor

Aero-derivatives have dual shafts, the air compressor components in one shaft and power turbine and driven unit in another shaft Air compressor is run at its best speed and power

turbine runs at speeds to suit the driven equipment

Single shaft: Power generation with a gearbox; Variable speed 2 shaft for compressor/ pumps

Driven unit can be near the air-end ("cold end drive) or exhaust end ("hot end" drive)

Driven Compressor/

Generator

ExhaustAir 100 u

Fuel35 u

65 u

GTs efficiency: 35%

GT exhaust heat can be used to generate steam or heat oil in a WHRUGT + exhaust steam turbine provide a combined cycle power plant with 45% efficiency

Gas Turbine Performance

Rated by Power Delivered, kW and Heat Rate, kJ/kWh energy supplied to get a kWh at ISO conditions: 15C; 0 m elevation, atmospheric pressure of 101.325 kPa a and 0%

Relative Humidity - dry air, without any inlet loss (due to air filter) and outlet loss (due to silencers/WHRU)

Ambient Temp & Altitude Correction: Inlet air compressor is a constant volume machine. Altitude and air temperature impact air density and mass flow and power delivered

Inlet air filter and outlet WHRU/ silencer impact air flow and head developed hence power delivered The linked image cannot be displayed. The file may have been moved, renamed, or deleted. Verify that the link points to the correct file and location.

Suitable corrections are then applied for site

conditions of these variables to derive site

delivered power, heat rate

Gas Turbine Typical Data

Higher the compression ratio, hotter the gas,

less fuel required to reach a given combustion

zone T1 temperature

Make Power, kW Effcy, % Comp

Ratio

Xs Air, % Exhaust

T, C

Solar Taurus 70 8,140 35.3 15 240 510

Solar Mars 100 11,860 34.4 17.5 260 485

GE LM 2500 25,060 37.1 18 200 566

Nuovo Pignone MS5002 28,340 29.2 8.4 280 516

Rolls Royce RB 211 30,910 38.6 20.8 250 490

Gas Turbine WHRU

WHRU

GTs employ 250 - 350% excess air to limit combustion

chamber metal temperature

Exhaust at 500C with 15% O2: Source of waste heat - either

as hot combustion air or in a waste heat recovery exchanger

Combustion Air: Extensive ducting and load co-ordination

GT Vs heater load. Post firing 2-3 times of GT based on Xs air

Waste Heat: Because of high Xs air, heat recovery is limited.

Usually WHRU duty GT load. As air flow remains same, GT

part load exhaust temperature is low

Xs air reduction: Large machines use Inlet Guide Vanes.

IGV reduce air to 80% during part load. Below 80%

inlet Axial Flow Compressor will surge

Driven Compressor/

Generator

ExhaustAir 100 u

Fuel

35 u

30 u

GTs efficiency: 35%

35 u

Exhaust

65 u

With WHRU

70 %

As combustion air

Low NOX burners operate with lean fuel: air mix. They

employ recycle loop across air compressor to limit Xs air

in combustion chamber during part load


GT WHRU

Heat hot oil, hot water, crude, amine, regen gas

generate and superheat steam

Finned tubes, similar to heater convection section

CIBAS: (concentric integral bypass and silencer). Finned helical coils in annulus between outer casing and internal bypass

WHRU

Bypass





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WHRU

Bypass Bypass

Common WHRU

for 2 Turbines

Parts

soot-blowers, by-pass stack, auxiliary firing (oil or

gas firing)

Diverter dampers control outlet temperature



GT WHRU Supplementary Firing

Lower the ambient temperature, lower is exhaust temperature; Exhaust temperature is also low at GT part load

Additional firing in WHRU inlet duct boosts GT exhaust temp or waste heat recovery, specially during GT part load

As exhaust has 15-18% O2, fuel (oil or gas or both ) can be directly fired in the duct

Duct Burners



GT WHRU Supplementary Firing

Duct burners: Many small burners across duct

cross section


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GT WHRU Ducting

A fired heater convection is usually castable

cement or ceramic fiber lined internally on

carbon steel casing. In GT WHRU gases travel at

high velocity (Heater P = 0.5 WC (o.125 kPa);

WHRU P = 6 WC (1.5 kPa)) that will erode

insulation

GT /WHRU ducts are usually made of SS sheets

with external mineral wool type of insulation

THANK YOU

Gas Turbine WHRU - Introduction

Documents

shaft air compressor

power turbine

air compressor components

axial air compressor

xs air

inlet air filter

hot compressed air

onshore ambient air