Combined Cylce Power Plants

7/23/2019 Combined Cylce Power Plants

http://slidepdf.com/reader/full/combined-cylce-power-plants 1/23

Power EngineeringPlant Operations

Combined Cycle Power Plants - Calpine

Revision Date: September 2005 Filename: C:\BCIT\Steam Lab - Tours\Power and Process\Combined CylcePower Plants.doc

Page 1 of 23

Gas Turbines

Many modern power plants use gas turbine driven electric generators. Gas turbines can vary

from small portable power generators producing a few kilowatts up to large foundation mountedturbine generators capable of producing 300 megawatts or more. These units may be aeroderivative type gas turbines or stationary design units. Aero-derivative gas turbines are basicallyairplane jet engines which have been modified to operate as stationary turbines. These werepopular in early gas turbine plants since the turbines and experienced maintenance personnelwere readily available. Modern plants, however, usually incorporate gas turbines designed forstationary use. These turbines operate on the same principles as aero-derivative gas turbinesbut are much more sturdily constructed to meet generating demand and, therefore, they areheavier.

As discussed in Plant Theory, a gas turbine consists of a compressor which draws in air and

compresses it. The compressed air is then discharged to a combustion chamber where fuel isadded and burned to heat the air. The hot air is expanded through a turbine providing torque todrive the turbine. In most cases the turbine is mounted on the same shaft as the compressorand drives the compressor directly. However, twin shaft turbines are also fairly common wherethe turbine may be separated into two sections with one section driving the compressor and thesecond section on a separate shaft which is connected to the load. In any case, about 60% ofthe power produced by the turbine is required to drive the compressor.

The gas turbine shown in this figure is asimple open cycle single shaft machine.Simple cycle refers to the fact that the hot

exhaust gases are not used for otherprocesses. In many modern plants, theexhaust gases are used to boil water andproduce steam. The steam is then usuallyused for heating or to drive a separate steamturbine. When used for a steam turbine it isreferred to as a combined cycle plant. Opencycle refers to the fact that compressordraws in air from the atmosphere and theturbine discharges gases to the atmosphere.In some cases called a closed cycle gas

turbine, air or another working fluid is recirculated through the turbine and compressorrepeatedly. The compressed air is heated in a heat exchanger before entering the turbine. Theexhaust gases are then cooled in another heat exchanger and returned to the compressor inlet.These units are not commonly used in Power Plants in North America.






Page 2 of 23

Actual gas turbine generators are very compact. The air and gas flow through the unit isnormally in one direction. The compressors are normally axial units using several stages tocompress the air. The air travels axially along the shaft direction and is discharged into several

combustors arranged around the turbine (in a magazine type combustor). The hot gases arethen discharged from the combustors directly into the turbine and travel the same direction asthe air through the compressor. A modern gas turbine is shown in the figure below.

Air enters the compressor through an inlet duct. For a large stationary turbine the air enteringthe turbine is conditioned. The conditioning includes passing the air through screens to removedebris (birds) that might be drawn into the inlet of the compressor. The air is then filtered toremove dust that would otherwise build up on the compressor blades causing reduced efficiencyand increased vibration. The air may then be heated in cold weather to reduce the chance of iceformation on the inlet guide valves and compressor blades. In summer, the air is usually cooledto increase the gas turbine efficiency by increasing the mass flow through the unit.






Page 3 of 23

The air then passes through inlet guide valves and into the axial compressor. The inlet guidevanes control the air flow to the compressor and turbine and in conjunction with fuel flow. Airand fuel flow are regulated to control speed and surging on startup and to control the load when

the unit is running although gas turbine generators normally run at full load. In the example inthe figure, the air is compressed by passing through 15 compressor stages. Typically the air iscompressed to about 10 bar or 1000 kPa (145 psi). During compression the air is heated toapproximately 300 oC (570oF).

From the compressor, the air is discharged to the combustion chambers. In the example in thefigure, magazine type combustors are used. These consist of a number of combustionchambers fitted around the periphery of the compressor outlet. Typically, 10 or more combustionchambers may be used. Only a small portion of the compressed air is required for combustion.The remaining air provides mass flow to drive the turbine after heating during the combustionprocess. In addition, the excess air is used to cool the combustion chamber and the first stages

of the turbine blading. In the combustion process, the air is heated to about 1300oC (2375oF).The hot gases are discharged from the combustion chambers trough nozzles that direct thegases into the first stage of the turbine at a velocity of approximately 80 meters/sec (260 ft/sec).Typically the turbine consists of 3 or 4 stages of reaction blades. After expanding through theturbine stages, the gases are discharged at about 550oC (1000oF).

Note that the turbine shown in the figure is a single shaft unit with the turbine and compressormounted on one shaft. The shaft is then supported in two bearings. The front end orcompressor bearing supports the shaft in a radial position and also includes an axial thrustbearing to ensure that the shaft is positioned correctly in the horizontal plane. The rear bearingis a radial bearing only. This allows the turbine to expand slight toward the exhaust end as the

unit is warmed up. The rear bearing is mounted inside a housing that is located in the turbineexhaust duct. The housing is rigidly supported by struts that are hollow to allow cooling air andlubricating oil to be circulated around the bearing housing when the turbine is in operation.






Page 4 of 23

Sequential Combustor Gas Turbines

Some modern high efficiency gas turbines utilize a sequential combustor configuration. In this

system, the compressed air is supplied to a low NOx combustion chamber where fuel is injected.the hot gasses from this combustor is discharged through nozzles to a high pressure turbine.The exhaust gases from this single stage turbine are still at a fairly high pressure. These gasesare discharged to a second or sequential combustor. Additional fuel is burned in the secondcombustor and the hot gases from this combustor are discharged through nozzles into the lowpressure stages of the turbine. This is shown in the figure below representing an Alstomsequential combustor gas turbine.

Air is drawn into the compressor through variable inlet guide vanes (VIGVs). The air iscompressed and discharged into a ring of EV combustors. These low NOx combustors aredesigned to provide combustion at low temperature by addition of steam or water into the

combustor in addition to reducing fuel supplied to this first combustion chamber. This type ofcombustor is referred to as environmental (EV) combustor due to the low NOx formation. The airrich gases from the EV combustors are passed through the high pressure turbine stage to asecond ring of combustors called sequential environmental (SEV) combustors. Four lowpressure turbine stages convert the heat energy in the gases from the SEV combustors tomechanical energy.






Page 5 of 23

This 3 dimensional drawing of the same turbine shows the ring headers that supply fuel to theEV and SEV combustors. The number of ring headers depends on the fuel burned and the NOxcontrol used. For dual fuel gas turbines there will be a ring header for natural gas, a second ringheader for fuel oil and a third ring header for atomizing air or steam. In addition, there is often aring header for water or steam injection into the combustors.

Ring Headers






Page 6 of 23

Auxiliary Systems

In addition to the gas turbine itself, a large gas turbine power plant requires several auxiliary

systems depending on the configuration of the plant and the type of fuel being burned. In asimple cycle gas turbine power plant the auxiliary systems include fuel handling and conditioning systems, inlet air systems, exhaust discharge systems, water or steam injection systems, blade washing systems, governor and lube oil systems, fire protection systems, electrical distribution systems, and controls and interlock systems.

Air Inlet Systems

Although most of these systems are similar to conventional steam power plants, some areunique to gas turbine power plants such as the air inlet and exhaust systems, the water or steaminjection systems and the blade washing systems.

Air inlet systems for a large gas turbine include screens and filters to remove debris, air heatingsystems to avoid freezing conditions in winter, and air cooling systems to improve turbine

efficiency. To reduce noiseproduced by the turbine inlet

blades, a silencer is normallyinstalled in the inlet plenum of thecompressor.

The filters consist of a largenumber of paper cartridges whichcan be replaced as necessary. Air

jets are commonly installed at theoutlet side of each filter cartridge.Compressed air is blasted at thecartridge reversing the air flow

momentarily. This reversed airflow is used to clean dirt from thefilter and increase the filters usefullife.






Page 7 of 23

Since the efficiency of a gas turbine depends on the mass of air passing through the turbineblades every second, the greater the air density, the higher the efficiency. On hot days the airdensity may be lower than ideal for turbine operation. To counteract this, the air is often cooled

using evaporative coolers. These consist of a fabric media which is soaked with water. As theair passes through the fabric, water is evaporated and heat of evaporation is removed from theair causing it to be cooled. The fabric is kept wet using water sprays or by using a continuousfabric belt like a vertical conveyor which is submerged at the bottom of its travel in a sump filledwith water. This evaporative cooler also assists in removing dust from the inlet air. The watermust be circulated through filters and blowdown used to control the amount of dirt build-up.

In cold weather, moisture in the air may result in ice build-up on the first stage compressorblades and inlet guide vanes. To avoid this, a small amount of air can be bled off thecompressor exhaust and returned to the inlet duct. This air is hot due the compression. Mixingthe hot compressed air with incoming air keeps the air temperature high enough to avoid

freezing problems.

Exhaust Systems

The exhaust gases are discharged from the turbine through exhaust ducting. This ducting isdesigned to minimize noise using perforated silencers installed inside the duct. The gases are

then discharged toatmosphere through a stackwhich is tall enough todisperse the gases.Continuous monitoring of the

exhaust gases from a turbineis mandated by governmentagencies in most countries.This consists of continuouslydrawing a sample of gasfrom the stack and passing itthrough electronic analyzerswhich test the gases foremissions such as carbonmonoxide, nitrous oxides(NOx) and sulphur gases.






Page 8 of 23

NOx Control

Of particular concern with gas turbines is the nitrous oxide content of the exhaust gases. This is

due to the inherent NOx production in gas turbine combustors. This has been greatly reduced inrecent years although upset operation can result in high NOx production in modern units as well.Many gas turbines are equipped with selective catalytic reduction systems to reduce NOxcontent of the exhaust gases. This consists of injecting ammonia into the flue gases at theexhaust of the turbine. The gases are then passed over a catalyst grid where the ammoniareacts with nitrous oxides to form nitrogen and water. SCR is normally used in conjunction withwater or steam injection for NOx control.

Thermal NOx is produced in a gas turbine combustor or other combustion process whennitrogen is oxidized. Nitrogen is considered to be an inert gas and does not react easily withother elements such as oxygen. However, when enough energy is added to the reaction,

nitrogen can be made to oxidize forming nitrous oxides (NO and NO2). Nitrous oxides (NOx)cause pollution by increasing acid rain and ground level ozone. This results in the formation ofan amber haze apparent over many cities. Although SCR can be used to remove NOx from theexhaust gases of a turbine, the best way to control this pollutant is to avoid its production.

Since energy is required to form NOx, the hotter the flame temperature, the more NOx isproduced. In a boiler this flame temperature can be controlled by staging the addition ofcombustion air and by recirculating flue gases to the furnace to increase mass flow through thefurnace and cool the flame. In a gas turbine the combustion process is limited by the size of thecombustors. The combustors are not very large and therefore combustion must happen rapidlyas the air and fuel move through the combustor. To reduce the combustion temperature in these

units, water or steam is often injected into the combustion chamber as well as the fuel. Water ismost effective because it evaporates in the combustion chamber providing evaporative cooling.

Also, water lines required for the cooling are much smaller than those required for steam.However, since the water evaporates in the combustion chamber, much of the combustionenergy is used for the evaporation process reducing turbine efficiency. This is made up by theincreased mass flow through the turbine due to the added water.

When using steam for NOx control the steam cools the combustion process by dilution andevaporative cooling is not a factor. As a result, more steam is required but the energy requiredto evaporate water is not removed from the combustion zone. As a result steam injection resultsin a considerable increase in turbine efficiency.






Page 9 of 23

Blade Washing

The inlet air filtration system can never completely eliminate all impurities. For this reason,

“washing” of the compressor blades must be carried out whenever blade fouling becomes toosevere. Due to the large volume of air that is passed through the gas turbine, the compressorblades must be washed on a regular basis (often once a week). Regular blade washing can becarried out on line (with the gas turbine operating) or when the unit is stopped. Demineralizedwater and detergent are commonly used for washing. The solution is sprayed into the inlet of theturbine compressor through permanently mounted blade washing nozzles located in front of theinlet guide vanes. Each turbine or group of turbines has a blade washing skid consisting ofmixing tanks, pumps and flow controls. Blade washing is controlled automatically by the processcomputers. When fouling is suspected to be more than normal, the turbine can be shut downand put on turning gear for blade washing. This allows and longer wash period with greaterquantities of water used.

Gas Turbine Cooling System

The limiting factor to the power output of a gas turbine is the blading at the inlet stages of theturbine. If materials can be made to withstand higher temperatures, much higher efficiency gasturbines could be produced. In the past, gas turbines were very low efficiency but advances inmetallurgy and blade cooling have increased turbine efficiency dramatically in the past fewyears. Modern gas turbine blades are carefully machined with cooling air passages and air fromthe compressor is forced through these cooling passages to keep the blades within safeoperating temperatures. Air is also injected into the labyrinth seals between stages of theturbines.

In addition to blade cooling, parts of the turbine casing, the turbine supporting legs and theexhaust end bearing housing are normally air cooled. In some cases these might even be watercooled. Normally a fan or blower is used to force air between the turbine casing and thesurrounding housing and through the support legs and bearing housing.






Page 10 of 23

The figure shows air cooling passages for a Siemens silo combustor type gas turbine. Most of the airfrom the compressor is drawn off to the combustors, located on each side of the turbine, to beheated. Some of the compressed air, however, bypasses the combustors and is injected into thecooling channels on the stationary turbine casing. From here the air passes through openings intothe stationary turbine blades and seals. For cooling the moving blades, air from the compressordischarge passes through holes in the turbine shaft into the hollow shaft center. The air then flowsout through openings in the turbine blade wheels and into the first stage turbine blades where itpasses through cooling channels machined into these blades. Some air passing through the shaft isalso used to seal the ends of the stationary blades.

For cooling air required at a lower pressure, such as the exhaust end of the turbine, it can bedrawn off the compressor at the desired stage. This “bleed” air is then piped to the requiredlocation on the turbine and through cooling channels into the turbine.






Page 11 of 23

Advantages and Disadvantages of Gas Turbine Power Plants

Gas turbine power plants, when compared to conventional steam turbine power plants, have

some important advantages which make them desirable for power generation. One of theadvantages is that they can be started up very quickly. A conventional boiler-turbine power plantmay require 12 hours or more to start up from cold since the boiler must be filled with water andthe boiler, turbine and piping warmed gradually to minimize stress. A gas turbine can be startedfrom cold and generating at maximum capacity in a few minutes. Another advantage is capitolcost. Due to the need to build a boiler, complex piping systems, water treatment, and coolingwater facilities, a steam turbine power plant is much more expensive to build than a gas turbineplant of the same capacity. In a gas turbine power plant only a turbine and generator is requiredalthough several auxiliary systems must also be installed as discussed earlier in this document.Due to the reduced size of the units, gas turbine power plants are commonly built using moduleswhich are built at the manufactures facility as package units and delivered to the site for

assembly and construction of interconnecting equipment. As a result, costs are further reducedand construction times are much shorter than steam turbine power plants.

One major disadvantage of gas turbine power plants when compared to steam turbines hasbeen efficiency. In the past, due to the high exhaust temperatures of gas turbines, theirefficiency was lower than steam turbines. However, modern gas turbines often have higherefficiency than condensing steam turbines. In addition, gas turbines are often used in combinedcycle operation when the hot exhaust gases are used to generate steam in a heat recoverysteam generator. This greatly increases the plant efficiency and combined cycle gas turbinepower plants with cogeneration capabilities can have efficiencies exceeding 60%. Comparedwith 35% for steam turbine power plants using condensing turbines.

The main disadvantage to gas turbine power plants compared to steam turbine plants is that thefuel that can be burned is limited. Without very complex and expensive processing systems, gasturbines cannot be fuelled using inexpensive fuels and, for the most part, are limited to naturalgas and light oils. Boilers can easily be made to burn waste fuels such as hog fuel or garbage.Boilers can also burn chemical fuels such as black liquor produced in the paper making process.In many chemical processing plants, heat is a buy product of the process. For example whenproducing urea for fertilizer, a large amount of heat is generated and this heat must be removedfrom the process using cooling water. The cooling water is boiled and can be used to drivesteam turbines. Also, coal is commonly used as boiler fuel because it is much cheaper. Coalcannot be burned in a simple open cycle gas turbine.






Page 12 of 23

Simple Cycle vs Combined Cycle

A simple cycle gas turbine power plant consists of one or more gas turbines which exhaust to

atmosphere directly from the turbine. Since the exhaust gases are discharged at a temperatureof about 550oC, a large amount of heat is wasted. In order to increase the efficiency, combinedcycle gas turbine power plants are often used. Combined cycle refers to the use a gas turbinecycle combined with a steam turbine cycle. In a combined cycle power plant, the hot exhaustgases from the gas turbine are used in a heat recovery steam generator (HRSG) to producesteam. The steam produced is then used to drive a steam turbine. The gas turbine and steamturbine may be used to drive separate electric generators or they may be connected to acommon shaft driving one generator.

In many plants two gas turbines are combined with one steam turbine. This is because theexhaust gases from two gas turbines contain enough heat to produce steam to drive a steam

turbine of the same power output as the gas turbines. With this configuration, three generatorscan be used that are all the same. This reduces maintenance training and spare partsrequirements. In some combined cycle power plants, by-pass dampers are used which vent theturbine exhaust to atmosphere before the HRSG. This allows the gas turbine to be run withoutusing the HRSG and also allows faster startup as the HRSG must be warmed up slowly to avoidthermal stress. With other units the HRSG cannot be by-passed and the exhaust gases alwayspass through the HRSG. This reduces one of the main advantages of gas turbine power plantscompared with steam turbine plants, which is their rapid startup time. Another disadvantage ofcombined cycle power plants is that the initial cost and maintenance costs are increased makingthem less desirable when compared to conventional steam turbine and boiler plants. Howeverthe much improved efficiency of combined cycle make it a common choice for new continuous

operating plants.

Additional systems required for a combined cycle power plant that are not normally required forsimple cycle include: heat recovery steam generator (discussed below), steam turbine generator and all of the auxiliary systems, feedwater treatment system (normally demineralizers), water treatement chemical injection system, treated water storage tanks and pumps, deaerator, feedwater pumps,

turbine condenser, cooling tower and cooling tower water treatment system, cooling water pumps, and steam and water piping systems.






Page 13 of 23

Heat Recovery Steam Generators

The main difference between a simple cycle gas turbine power plant and a combined cycle gas

turbine power plant is the HRSGs. The turbine is mostly unchanged except that because steamis produced in a combined cycle power plant, steam is usually used for NOx control at thecombustors whereas water is use in simple cycle plants where steam is not available. After theturbine, exhaust gases are passed through a steam generator to recover heat from the gasesthat is used to produce steam and drive a steam turbine. There are two main categories ofHRSG: vertical and horizontal.

This designation is dependant on the direction of the gas flow through the HRSG. A horizontaltype HRSG is shown in the figure below meaning that the hot gases from the gas turbine flowhorizontally through the HRSG.






Page 14 of 23

In this example there are three pressure stages in the HRSG. Normally two or three pressuresof steam are produced. This is done to maximize efficiency since higher pressure steamimproves the efficiency of the steam turbine but as the gases from the gas turbine are cooled

they cannot produce steam at high pressures because the saturation temperature is increased.Therefore, low pressure steam is produced at the back end of the HRSG. This low pressuresteam can be used for heating in the deaerator or for a heating process such as a paper mill or itmay be used to drive a low pressure turbine as in this example. The low pressure turbine isactually the last stages of the main steam turbine in this case. High pressure steam is the mainsource of propulsion for the steam turbine but low pressure steam is admitted and passedthrough the last few turbine stages to improve plant efficiency and use the excess low pressuresteam. The low pressure steam is also used in the deaerator.

As shown in the figure, there are many elements in the HRSG to extract as much heat aspossible from the exhaust gases. Most of these elements consist of an upper and lower header

connected by a series of vertical tubes. If the turbine burns mostly natural gas as a fuel, thetubes are usually finned to increase the surface area and heat transfer. For oil fired units, thefirst elements may not be finned to reduce soot build-up and sootblowers would be installed oneach element.

Notice that feedwater for the high pressure stage enters the HRSG at the back end and passesthrough an economizer. From this (No. 5) economizer the feedwater passes through fouradditional economizers before entering the high pressure steam drum. These economizers areplaced in sequentially higher temperature zones to gradually increase feedwater temperature.The water is then boiled in the tubes of the high temperature element, or evaporator, and thesteam generated rises back up to the steam drum where it is separated from the water. The

water is recirculated through the high pressure element and the steam leaves the steam drumthrough separators similar to those used in conventional high pressure boilers.

The high pressure steam goes through a high pressure low temperature superheater. After theprimary superheater, the steam passes through a desuperheater for final steam temperaturecontrol and then the steam passes through a secondary or high pressure high temperaturesuperheater. All of the high pressure steam produced is sent to the steam turbine. This type ofcombined cycle plant is normally operated at full load when in operation and other plants in thesystem are set up to vary as load changes.

Gas turbine plants should run at maximum power as much as possible (simple or combined

cycle). Gas turbines do not operate very efficiently at reduced loads. This is because theturbine must rotate at 3600 rpm at all loads due to the electrical frequency requirements. Thismeans that the compressor always runs at the same speed and therefore compresses the sameamount of air. At reduced load the same amount of power is required to drive the compressorand since this is 2/3 of the turbine power produced, as output power to the generator is reducedthe efficiency falls quickly. In extreme conditions, the compressor may require more power thanthe turbine can produce as the fuel to the combustor is cut back. This can result in efficiencybelow 0% resulting in power draw from the grid.






Page 15 of 23

Gas Turbines which are required to drive varying loads are commonly two shaft machines.These have one shaft with the compressor and a primary turbine. The exhaust from the primary

turbine is then discharged to a secondary turbine which is on a separate, output shaft. Thegenerator or other load is attached to the secondary shaft. In this configuration, the primaryshaft speed can be allowed to change as the turbine load changes while the speed of thesecondary shaft remains constant.

Since all the steam produced in the high pressure HRSG goes to the turbine and the turbinespeed is controlled the distribution grid, the governor valves on a combined cycle steam turbineare normally required only on startup for turbine control. During normal operation these valvescontrol the pressure in the HP section of the HRSG.

Feedwater for the intermediate stage

element of the HRSG passes through asingle economizer before entering thesteam drum. This heated water is alsoused in the fuel gas preheater. Since thenatural gas is supplied to the site at about500 psi and the pressure is reduced to 60psi for the gas turbine, the natural gas iscooled dramatically at the reducing valves.Hot water is used to reheat the natural gasin gas heaters.

Steam generated in the elements of theintermediate pressure section of the HRSGis separated in the IP steam drum andflows through a single superheaterelement. This steam is then used for NOxcontrol by injection into the gas turbine(GT) combustors.

Feedwater is supplied to the low pressuresection of the HRSG through a smalleconomizer. Steam generated in the low

pressure elements is draw off the steamdrum and used to heat the deaerator.Excess low pressure steem is directed tothe turbine where it passes through the laststages and into the turbine condenser.

H.P SteamDrum

I.P SteamDrum

L.P Steam

Drum

Outlet toStack

TurbineExhaust






Page 16 of 23

This HRSG is equipped with duct burners. These consist of natural gas or oil fired burnerslocated in the duct at the inlet of the gas turbine. Duct burners allow the temperature of theexhaust gases from the turbine to be increased. The increased gas temperature results in

increased steam generation and raises the temperature of the gases passing through the lastelements in the HRSG. Since duct burners are not as efficient as the combined cycle, they arenormally only used in cold weather or when the low pressure section of the turbine is notgenerating enough steam for the deaerator.

The figure also shows the location of the ammonia injection nozzles and the SCR catalyst gridused to reduce NOx content of the exhaust gases.

A three dimensional representation of a similar horizontal HRSG is shown in the second figure.This unit is similar to the previous diagram with a high pressure, intermediate pressure and lowpressure steam generating section. As shown here, the vertical elements are closely spaced in

the HRSG duct to allow room for the steam generating elements and the many economizersections and superheaters for the high pressure and intermediate pressure stages.






Page 17 of 23

Cogeneration Power Plants

A condensing steam turbine is inherently a low efficiency device. This is because the steam

leaving the turbine still contains the latent heat that was added in the boiler or HRSG. This latentheat comprises about 70% of the heat in the steam entering the turbine and is lost to the coolingwater in the condenser. Therefore, steam turbine plants cannot achieve more than about 30%thermal efficiency. However, if the steam exhausted from the turbine can be used in a heatingprocess where the steam is condensed and the latent heat absorbed by a useful process, theoverall plant efficiency is greatly increased because this latent heat is not lost. In a plant thatrequires large amount of steam such as a paper mill, steam is usually generated at highpressure and then passed through a turbine to reduce the pressure. Once lowered to thedesired pressure it is used for heating the process. This use of the same steam for theproduction of electricity and then for heating is called Cogeneration.






Page 18 of 23

The figure shows a simplified flow diagram for a typical combined cycle power plant that alsoprovides steam to a process. This is a combined cycle, cogeneration plant. As discussedpreviously, two gas turbines with HRSGs are used in many combined cycle plants. The HRSGs

produce steam at three pressures. The high pressure steam is used to drive a steam turbine,the intermediate pressure steam is used for NOx control at the gas turbine combustors and thelow pressure steam is used for heating the deaerator. Some low pressure steam is also used inthe turbine. Typically the high pressure section operates at a pressure of about 6200 kPa (900psi), the intermediate stage at about 2400 kPa (350 psi) and the low pressure stage at 35 kPa (5psi). The lower the pressure of the low pressure stage the more efficient the HRSG because thesaturation temperature is reduced and therefore a reduced stack temperature can be achievedwith less heat loss.

The turbine exhausts to a condenser. Cooling water is circulated through the condenser tocondense the steam. The condensate is then pumped from the condenser back to the deaerator

to be reused as feedwater.

The cooling water used by the condenser is returned to the cooling tower where the heatabsorbed in the condenser is removed by spraying this water down through the cooling towerwhile air rises upward. The air evaporates some of the water causing the water to be cooled.This water is then pumped back through the condenser.

As shown, the steam turbine is equipped with an extraction connection. This connection allowssteam to be drawn off the turbine at the desired pressure and used for heating a process orsupplementing a steam system in a paper mill, refinery or some other process requiring a lot ofsteam. Since the steam extracted from the turbine is used for heating, the efficiency of the plant

is increased.






Page 19 of 23

The figure also shows asimplified flow for thefeedwater system. Feedwater

is pumped from the deaeratorstorage tank to the HRSGsteam drums througheconomizers. The feedwaterpump is a multi stage pumpproviding high enoughpressure supply water to thehigh pressure section of theHRSG. However, the lowpressure sections of theHRSG do not require this high

pressure. Often feedwaterpumps with interstage bleedconnections are used for thistype of operation. The

feedwater pump may have five or six stages in order to produce adequate pressure for the highand intermediate pressure sections of the HRSG. At the discharge of the second or third stage,feedwater is drawn off to the low pressure stage of the HRSG. A cross section of an interstagebleed type feedwater pump is shown in this figure.






Page 20 of 23

Island Cogeneration Plant

A combined cycle cogeneration plant has been installed in Campbell River. Run by Calpine, this

plant sells electrical power to BC Hydro to improve voltage in the mid island region and reducethe dependence on power transmitted through the undersea cables from the lower mainland toVancouver Island. In addition, steam is extracted from the turbine and sold to the Elk Falls papermill adjacent to the power plant. The plant has a maximum electrical capacity of 250 megawattsbut is soon to undergo some enhancements that will increase the capacity to 300 megawatts.This is approximately the power requirement of the paper mill and the entire Campbell Riverarea.

The large building in this photo is the turbine building housing both the steam turbine and the gasturbine. The HRSG is shown in front of this building and the offices and maintenance buildingsare on the right side. The building in front of the HRSG stack is the water treatment buildingwhere demineralizers are used to produce pure water for the boilers and water injection for NOxcontrol. When using water for NOx control, the water must be very pure to avoid the build-up ofdeposits on the gas turbine blading. The large tank with the concrete dyke around it is the fueloil tank and the other large tank by the turbine building is for treated water storage.






Page 21 of 23

On the left side of the turbine building is the cooling tower using three cells. The cooling towersupplies cooling water for the steam turbine condenser. When entering the site you will drivethrough a gate and then up the road at the bottom right of the photo. Continue along this road

and park in front of the office building where you will meet the tour guides.

Unlike the plantsdiscussed previously,Island Cogen has asingle gas turbineand HRSG. Inaddition, the gasturbine and steamturbine drive a singleelectric generator.

The gas turbine isconnected directly tothe generator shaftand provides powerto drive the generatoras long as the gasturbine is running.The steam turbinedrives the generatorthrough a clutchwhich allows the

steam turbine to bedisconnected fromthe generator. This isnecessary to allowthe gas turbine tostart up and produceenough hot exhaustto generate steam inthe sections of theHRSG before thesteam turbine is

started.

During this startup period, any steam produced is vented to atmosphere. Once the steam is upto operating pressure in the HRSG, the steam turbine is started. The combined output from thegas turbine and steam turbine produce the total 300 megawatt output of the generator.






Page 22 of 23

The HRSG has three sections similar to the system discussed earlier. The high pressure steamis used to drive the steam turbine. Intermediate pressure steam is also used to drive the steamturbine injected at a lower pressure stage. The low pressure steam is used for the deaerator and

any excess low pressure steam is passed through the last stages of the turbine to thecondenser. NOx control is achieved in this system by injecting demineralized water. Sincesteam is not available at startup, this allows NOx control even during startup or when the plant isoperated in simple cycle mode.

This is a cogeneration plant with extraction steam drawn off the turbine and used in the adjacentpaper mill. The exhaust steam from the turbine is condensed in a water cooled condenser.Cooling water is supplied from a cooling tower sump and the water is recirculated to the coolingtower.

Because of the need for

high and low pressurefeedwater supply for theHRSG sections,separate feedwaterpumps are used. A lowpressure feedwaterpump supplies water tothe low pressure sectionand high pressurefeedwater pumps supplythe intermediate and

high pressure sections.

The exhaust gases fromthe Gas Turbine passupward through theHRSG and the tubesections are positionedhorizontally. As shownin the figure, there areseveral elementslocated in the gas pass

including the highpressure, intermediatepressure and lowpressure evaporatorsand the economizersand superheaters.





Revision Date: September 2005 Filename: C:\BCIT\Steam Lab - Tours\Power and Process\Combined CylcePower Plants doc

Page 23 of 23

The high pressure superheater is located at the bottom of the HRSG where the hot gases enterfrom the turbine. Above that are the intermediate pressure superheater and the high pressureevaporator. A primary superheater for the intermediate pressure system is located after the HP

evaporator as well as a HP economizer. After the HP economizer, the gases pass across the IPevaporator, the HP and IP economizer and the low pressure evaporator. Before exiting to thestack, the gases pass across another economizer for each section of the HRSG.

The figure is a crude representation of the HRSG and the actual positioning of the elements ismore complex with the tube sections being intermingled and the gas flow directed through thetubes using baffles to direct and restrict the flow. As shown, low pressure steam used for thedeaerator and back end of the turbine is not superheated. This steam comes directly off the lowpressure steam drum. Intermediate pressure and high pressure steam is superheated usingelements located at the hot gas inlet to the HRSG.

Documents

Combined Cylce Power Plants