GAS-FIRED COMBINED-CYCLE POWER PLANTS … gas-fired combined-cycle power plant combines two thermal processes so well that the resulting electricity ge-neration is as efficient as

HOW DO THEY WORK?GAS-FIRED COMBINED-CYCLE POWER PLANTS

A company of

Cover picture: Gas turbine compressor with combustion chamber

CONTENTS

At a glance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 05

1. How does a gas-fired combined-cycle power plant work? . . . . . . . . . . . . . . . . . 06

1.1 The gas-fired combined-cycle power plant – in model form . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 07

1.2 The gas turbine: the heart of the power plant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 08

1.3 The water/steam cycle: more than just hot air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 09

1.4 The high-voltage system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1.5 The control station: control and security . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2. What are the positive aspects of a gas-fired combined-cycle power plant? . . . . 12

2.1 The issue of efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.2 Cost matters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.3 Modern combustion technology: a plus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3. EGL projects in Italy (inserts) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

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1 – Model of an EGL gas-fired combined-cycle power plant

AT A GLANCE

A gas-fired combined-cycle power plant, also known as a Combined Cycle Gas Tur-bine Power Plant, combines the strengths of two thermal processes in ideal fashion:electricity production using a gas turbine together with a steam turbine. The acronymnormally used to describe this system is CCGT.

Around two thirds of the electrical power generated is produced by the gas turbine.In a similar way as an aeroplane engine, a mixture of compressed air and fuel is com-busted. The hot gases that this process creates drive the turbine and, with it, the gene-rator that is coupled to it.

The rest of the electrical power generated, roughly a third, is produced by the steamturbine using the hot exhaust gases leaving the gas turbine. In the heat recovery steamgenerator (HRSG) the exhaust gases transfer their heat to the circulating water: thepressurised water vaporises, causing the temperature in the system to rise. The steamdrives the steam turbine and, with it, the generator that is coupled to it.

Gas-fired combined-cycle power plants are technologically advanced and usedthroughout the world. Compared with other types of power plant, they are highly effi-cient: the state-of-the-art EGL plants in Italy achieve a percentage in the region of 56,meaning that the energy supplied in the form of fuel is converted into electricity as efficiently as technically possible.

Investment costs are comparatively low because the main components are largelystandardised. The extremely powerful gas turbine allows for a compact power plant,which minimises construction time to around two and a half years.

Gas-fired combined-cycle power plants are built modular, consisting of severalblocks. Each block is practically a self-contained power plant that can be operated inde-pendently. The standardised EGL power plants all have two blocks with a combined yieldof around 760 megawatts. One of these power plants could cover about ten percent ofSwitzerland’s annual energy needs.

With modern combustion engineering, the emission of harmful exhaust gases canbe controlled and minimised. Compared with all other conventional thermal plants, suchas coal-fired power plants, carbon dioxide and nitric oxide emissions are lowest whenplants are fired with natural gas.

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The gas-fired combined-cycle power plant combines twothermal processes so well that the resulting electricity ge-neration is as efficient as possible. Around two thirds ofthe electricity is produced by the gas turbine, and the re-maining third by the steam turbine coupled to it. Poweredby the expanding fuel gases (gas turbine) or the steam(steam turbine), a mechanical rotation takes place in thetwo turbines, which is then converted by the generatorsinto electrical energy.

The EGL power plants are designed as multi-shaft in-stallations. This means that the two turbines are coupledto separate generators. – This is in contrast to single-shaftinstallations, where both turbines power the same genera-tor. – The EGL power plants in Italy consist of two blockseach, so they each have four drive machines (two gas andtwo steam turbines) and four driven machines (genera-tors).

1. HOW DOES A GAS-FIRED COMBINED-CYCLE POWER PLANT WORK?

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1 – Gas turbine2 – End winding of generator

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1. Ambient air is drawn in via filter stages and compressed in the compressor. 2. Gas turbine: air is compressed, na-

tural gas is mixed in. Combustion takes place, generating hot gases under high-pressure. The turbine powers the ge-

nerator and the compressor. 3. Heat recovery steam generator: water is vaporised using the hot exhaust gases from

the gas turbine. 4. Steam turbine: the steam powers the turbine. The resulting mechanical energy is transferred to the

generator. 5. Condenser: here the exhaust steam from the steam turbine is converted back into water by means of air

cooling. 6. Generators: here the mechanical energy from the turbines is converted into electricity.

1.1 The gas-fired combined-cycle power plant – in model form


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exhaust gas stack

compressor gas supply combustionchamber

gas turbine

generator

generator

high-voltage lines

high-voltage lines steam lines

high pressuresteam turbine

medium andlow pressure steam turbine

condenser

heat recovery steam generator

air intake

air intake

water

feed-waterpump

steam

turbine

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1

6

2

3

5

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1.2 The gas turbine: the heart of the power plant

The gas turbine is the first stage in the process of produ-cing electricity. The gas turbine compressor draws in airfrom the environment via a filter (1. on the model). Thisair is compressed in the compressor (2. on the model),which means that it is elevated to a higher pressure, andthen directed into the combustion chamber. Fuel is fedinto this chamber in the form of natural gas, and combu-stion takes place. This process produces hot gases thatare allowed to “relax” in the turbine, which means thatthey are brought to virtually ambient pressure. The gasspreads out and expands. The energy that this releasesis converted into a mechanical rotation just like a toy balloon when air is escaping from it. Here too the pres-sure compensation (expansion) brings about the motion,the action of the balloon “darting off”. The mechanicalrotation powers the compressor and the generator. Thegenerator converts this energy into electricity.

When the hot gas exits the turbine as exhaust gas,it has a temperature of around 600°C. This heat energyis then transfered to the water in the heat recoverysteam generator. The second stage of electricity genera-tion then takes place: the water/steam cycle where thepressurised water is heated and vaporised.

We regard the gas turbine as the heart of the powerplant. Not only does it produce two thirds of the electri-city, it also gives the power plant its name. Diesel oil

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1 – Heat recovery steam generator2 – Gas turbine compressor

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could also possibly be used as fuel, but the EGL plantsin Italy have not been designed for this. Natural gas haslower exhaust gas emissions compared to diesel oil. Inaddition, the cost and effort needed to maintain and ser-vice turbines powered with natural gas is usually lower. Ageneral overhaul, where heavily used parts have to be re-placed, tends to be necessary only once every three ye-ars. During such maintenance inspection times, it paysfor a power plant to have more than one block. One partof the plant can carry on producing electricity normallyeven if the other is undergoing inspection and is thereforeout of action for some time.

All EGL power plants use type V94.3A2 gas turbines.They are manufactured by the Italian company Ansaldo,under a licence provided by Siemens. The turbine weighsroughly 300 tons and yields a nominal electrical output of260 megawatts. By way of comparison, around 3,500 VWGolfs together would be needed to produce this kind ofoutput.

1.3 The water/steam cycle: more than just hot air

A third of the total electrical output comes from the steamturbine (4. on the model). The water/steam cycle usesthe heat energy of the exhaust gases that would other-wise go to waste in the gas turbine process. This heat isused to generate water vapour, which produces electricity

with the help of a steam turbine. The water/steam cycleis closed, which means that it is always the same waterthat is heated, vaporised and then converted back intowater in the condenser.

The heat recovery steam generator (3. on the model)is a large and complex configuration consisting of bund-les of pipes and drums. It has three areas, each with adifferent pressure level: one high, one medium and onelow. By dividing into these three levels, it is possible toharness an impressive amount of the energy contained inthe exhaust gas.

In all EGL power plants the boiler is roughly 45 metreshigh and 17 metres wide.

The steam turbine (4. on the model), like the boiler, isdivided into three pressure levels: one high, one mediumand one low. The respective area of the steam boiler sup-plies the steam turbine with the correct steam. This isthen allowed to “relax” in the turbine; in other words, it releases pressure. The turbine converts the steamenergy into a mechanical rotation that is then transferredto the generator. There, it is transformed into electricity.

The steam turbines for all EGL power plants in Italy aremanufactured by Ansaldo under a licence provided byBBC (ABB).

The generator’s nominal output is 132 megawatts. Ifyou again compare this output with a VW Golf, you wouldneed another 1,800 vehicles to produce this level of output.

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1 – Rotor of high pressure steam turbine2 – Ventilators of condenser

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The condenser (5. on the model), on account of its size,is the most striking component of the power plant. Thesteam exits the turbine under vacuum, i.e. negative pres-sure. It flows through pipes a few metres in diameter intothe condenser, which is cooled by air. Large ventilatorsadd ambient air, which cools the steam right down until itbecomes water again: it is condensed. The feed-waterpump then returns this “condensate” to the boiler. Andthe cycle starts afresh.

1.4 The high-voltage system

The alternating current generated in the plant cannot bestored. Power plants therefore need an electrical systemthat reliably conveys the electricity produced to consu-mers. The power plant’s own transformers convert theelectricity produced so that it can be fed directly into thehigh-voltage network. In the event of a disruption to thehigh-voltage network, the plant automatically begins torun down in a safe mode. As soon as the electricity gridis available again, and the corresponding demand exists,the power plant automatically connects to the grid againand supplies the required output.

1.5 The control system: control and security

The control system in the plant’s central control roomsteers, controls and monitors all the processes and ope-rations in the power plant. It records all the major se-quences and process variables, and assists human inter-vention. It is also able to draw comparisons betweentarget and actual conditions by itself and to respond tothem. In addition, the continuous feedbacks sent by themany sensors in the power plant trigger automatic re-sponses.

Normally speaking, all standard operation sequenceswithin the power plant are fully automated from start tofinish. If necessary, operating staff can purposefully in-tervene and make improvements.

The system collects and stores a large volume of ope-rational data that is important for specific analyses. The historical data record also makes it possible to de-termine maintenance and inspection times precisely.


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2.1 The issue of efficiency

A plant’s availability is the key to its cost efficiency: towhat extent is the power plant ready for operation (afterscheduled and unscheduled downtimes)? Gas-fired com-bined-cycle power plants can achieve very high values up-wards of 95 percent if maintained well. This means thatthey are connected to the grid roughly 345 days of theyear on average.

Compared with other types of power plant, gas-firedcombined-cycle power plants boast a high level of effi-ciency. They convert the energy introduced in the form ofnatural gas into electricity with the least amount of lossthat is technically feasible. It is not possible to achievezero loss given that friction and heat transfer alwayscause some energy to be lost. The gas turbine managesa level of efficiency of around 35 percent. The water/steam cycle coupled to it significantly raises the effi-ciency for the entire plant by using the heat energy emit-ted by the exhaust gases from the gas turbine to producesteam and, therefore, to generate electricity.

This way, the EGL plants in Italy achieve excellent va-lues of roughly 56 percent. Coal-fired power plants tendto be 10 to 15 percentage points below this level. Whileit is true that plant efficiency is slightly lower if cooling isdone with air, as opposed to cooling with water, this doesoffer greater independence for plant operation and pro-tects natural water resources.

But it is not just the features of the plant itself that arekey to its cost efficiency. Other decisive factors includethe location and the professionalism of plant operation.An important consideration in the choice of location is

2. WHAT ARE THE POSITIVE ASPECTS OF A GAS-FIRED COMBINED-CYCLE POWER PLANT?

1 – Rotor of low pressure steam turbine2 – Condenser

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the proximity to a high-voltage power line (380 kilovolts).Short distances mean that transmission losses can be li-mited. In addition, it is important that line capacities toconsumer centres are adequate. Like connection to thehigh-voltage network, connection to the natural gas supplyis also very important. Therefore, an ideal location for apower plant is one that is close to the high-voltage networkand to the natural gas pipeline.

Well trained, experienced personnel are a basic requi-rement for reliable operation of a power plant, and one onwhich EGL places special emphasis. It recruits its staffearly and offers continuous training for their specialist du-ties. The EGL employees responsible for operation on siteensure the availability of the plant. The EGL specialists athead office make decisions regarding the use of thepower plant and production volumes.

2.2 Cost matters

The investment costs for a gas-fired combined-cyclepower plant are lower than those for coal-fired powerplants and other conventional thermal plants, standing ataround 0.6 million euros per megawatt of installed capa-city, whereas for coal-fired power plants, for example,twice that amount can be expected. The reasons for thisare the largely standardised main components of this typeof power plant and the short construction time of just twoand a half years or so. Depending on the design of theplant, it is even possible to go into operation with just thegas turbine first and to complete the water/steam cycle in parallel. That way, the investor can start generating in-

2. WHAT ARE THE POSITIVE ASPECTS OF A GAS-FIRED COMBINED-CYCLE POWER PLANT?

come from the sale of electricity earlier, thereby achievinga better return. A gas-fired combined-cycle power planthas a useful life of 25 to 30 years, which is comparablewith other types of power plant.

Production costs for electricity, measured in euros permegawatt hour, depend very heavily on fuel costs. Withtoday’s natural gas prices (as at autumn 2006), theirshare can account for up to 70 percent. This heavy depen-dency suggests that it is better to use gas-fired combined-cycle power plants as medium load electricity generatingplants: this means taking advantage of their flexibility andonly operating them when electricity market prices are at-tractive.

The block size that has been chosen for all EGL powerplants (380 megawatts each), guarantees attractive eco-nomies of scale, measured on production costs. Smallerunits require more specific investment expenditure andtherefore generate fewer profits.

2.3 Modern combustion technology: a plus

Gas-fired combined-cycle power plants emit greenhousegases. Compared with all other fossil-thermal plants, thatare all plants with combustion processes, the lowestemissions of carbon dioxide and nitrogen oxide (NOx) arefrom plants operated with natural gas. The values forcoal-fired power plants, for example, are roughly twice ashigh. Natural gas is regarded as a relatively clean fuel.Continuous monitoring of the combustion also helps tocontrol and minimise the emissions.

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Editor/contact

EGL Elektrizitäts-Gesellschaft Laufenburg AGCorporate CommunicationsLerzenstrasse 10CH-8953 DietikonTel. +41 44 749 41 [email protected]

Photos: EGLMay only be reproduced with written permission of the editor

www.egl.ch

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GAS-FIRED COMBINED-CYCLE POWER PLANTS … gas-fired combined-cycle power plant combines two thermal processes so well that the resulting electricity ge-neration is as efficient as