Gas Turbine Power Plants - · PDF file14 Gas Turbine Power Plants 14·1. Introduction The gas turbine in its simplest form is a heat engine operating by means of a series of processes

14Gas Turbine Power Plants

14·1. Introduction

The gas turbine in its simplest form is a heat engine operatingby means of a series of processes consisting of compression ofair drawn from the atmosphere, increase of air temperature bythe combustion of fuel in the air, expansion of hot gases toatmosphere, the who"le being a continuous flow process. It isthus similar to gasoline and diesel engines in its working mediumand internal combustion, but is akin to the steam turbine in itsaspect of the steady flow of the working medium. The compressionand expansion processes are carried out in turbomachines, thatis by means of rotating elements in which the energy transferbetween fluid and rotor is effected by means of kinetic action,rather than by positive displacement as in reciprocatingmachinery. Thus in its simplest form a gas turbine consists of acompressor, a combustion chamber, and a turbine unit. Airwhich acts as a working fluid is compressed in the compressor andenergy is added to it in the combustion chamber. The high energyfluid is then expands in the turbine and thus mechanical energyis produced. Part of this energy is used up in driving the compressor,which is usually mounted on the same shaft as that of turbine,and rest of the energy may be utilised for various purposes.Since the compressor is coupled with the turbine shaft, the comepressor absorbs some of power produced, by the turbine andhence lowers the efficiency. The network is therefore the differencebetween the turbine work and work required by the compressor todrive it.

The gas turbine prime mover was first used in 1939 for largecentral station service. Since then several stations have beenbuilt with gas turbines to drive electric generators. Gas turbinegenerators have been built and electrical outputs upto 100 MW.In some situations gas turbines are the cheapest type of plantsavailable. These situations are when they are used as intermittentor peak load plants in combination with the base load plants.These are particularly useful and economical when the amountof energy required is a small part of the total energy to be suppliedby the whole system and the lcpad factor of the plant is less

GAS TURBINE POWER PLANTS 589

than 15%. In a large system the size of the gas turbine plantsnormally employed varies from 10 to 30 MW.

These plants are cheaper in capital cost compared withsteam stations of the same size. Also the fixed charges of theseplants are comparatively lower than those for steam plants.The thermal efficiency of gas turbine plants is however lowercompared to that of condensing steam 'plants (20 to 25% comparedto 25 to 30%). No doubt a lower thermal efficiency results inincreased fuel costs at low load factors, but this is compensated bylower fixed charges as well as lower operating and maintenancecharges.

A gas turbine plant has the advantage of high reliability,flexibility, low start up time and less space requirements. They areideally suitable as peak load plants. At some places they are alsoused as base load plants.

In India the 70 MW gas turbine plant at Namrup in Assamworks as base load plant with natural gas as fuel. Uran-Gas turbinepower plant in Maharastra is the second power plant established inthe country.

14·2.Application of gas Turbine Plants

Gas turbine plants have the following applications:

1. To drive generators and supply peak loads to steam, dieselor hydro plants.

2. To work as combination plants.

3. To supply mechanical drive for auxiliaries.

These plants are suited for peak load purposes as alreadymentioned because their fuel costs are some what higher while theirinitial costs are low when these plants are used with conventionalboilers they may be used for

(a) supercharging or

(b) heat recovery from exhaust gases.

In supercharging system air is supplied to the boilers underpressure by a compressor mounted on the common shaft with turbineand gases formed as a result of combustion after coming out of theboiler; work in the gas turbine before passing through the economiserand exhausting through the chimney. The turbine drives thecompressor and also generators, producing some additional powerfor the station.

590 POWER PLANT TECHNOLOGY

In the exhaust heat recovery cycle the gas turbine plant isfitted with usual combustor and gets the gas supply from thecombustor. The gases after expanding in the turbine enter the boilerand transfer part of the heat, to the boiler tubes.

In the supercharging system heat transfer in the boiler increasesby about 7 to 8%, while in the exhaust heat recovery cycle the heatrates are improved by about 4 to 5%. Also in the later case nomechanical draught is required because due to pressure of exhaustgases the furnace is under positive pressure.

The gas turbine is widely used in air craft. There are manyinstallations in ships as propulsion unit. Attempts are also beingmade to develop it as an engine for automobile use. There is a widerange of industrial applications ranging from petro-chemical, thermalprocess industries to generate utility industries.

14·3. Types of Gas Turbine Plants

On the basis of combustion process the gas turbine may beclassified as follows:

1. Continuous combustion of constant pressure type, the cycleworking on this principle is called Joule or Brayton cycle.\

2. The explosion or constant volume cycle; the cycle workingon this principle is called Atkinson cycle.

Another classification based on the path of the working ~1Jbstance,it is classified as :

(i) Open cycle gas turbine in which working fluid entersfrom atmosphere and exhausts to atmosphere. The workingsubstance air first is compressed in the compressor, and aftercompression, its temperature is raised by burning fuel in it.The products of combustion along with the excess air arepassed through the turbine, developing power and then exhaustedinto the atmosphere. For next cycle, fresh air is taken in thecompressor.

(ii) Closed cycle gas turbine, in which working fluid is confinedwithin the plant. The air is heated in an air heater (refer Fig. 14·3·1(b) by burning fuel externally. The working air does not come incontact with the products of combustion.'The hot air expands in theturbine and then cooled in a precooler and supplied back to thecompressor. The same working fluid circulates over and again inthe system.

GAS TURBINE POWER PLANTS

Combustion chamber

Compressor

Air inta~e

(a) Open cycle.-'-

FUE L

c.c.

Gases.out

591

Compressor Gas turbine

Heat exchanger

Fig. 14·3·1. Schematic diagram of open cycle andclosed cycle gas turbines.

Gas turbine power plants can be anyone of the followingtype.

(a) Simple cycle Gas turbine power plant.

(b) Combined cycle Gas turbine power plant.

(c) Co-generation Gas turbine power plant.

(a) Simple cycle Gas turbine power plant. It is based onBrayton cycle as stated above in which air is compressed to a higherpressure with the help of compressor and temperature of air firingfuel in the combustion chamber before expanding in the turbine.The difference between work output in expansion process and tl:1ework input in compression process is the net oqtput of Gas turbine.which will be converted into electricity.


(b) Combined cycle Gas turbine Power plant. This typeof power plant is combination of simple Brayton cycle gas turbineand Rankine steam cycle as bottoming cycle. Exhaust gases fromGas turbine whose temperature is of the order of 550°C are led theheat recovery steam generator to generate steam which in turndrives steam turbine producing additional power. This cycle derivesthe advantage of higher temperature achieved in Brayton cycle andlower heat rejection (sink) temperature of Rankine cycle. Grossefficiency of the order of 47% can be achieved in such combinedcycle power plant which is higher that super critical pressureconventional power plant.

(c) Co-generation Gas turbine power plant: These powerplants are similar to combinedcyclepower plants; the basic differencebeing that the steam generated in the heat recovery steam generatorby the gas turbine exhaust gases is used for process applicationeither fully or partially instead of generating electricity only.

14.4. Open and Closed Cycle Gas Turbine

(1) Open Cycle Gas Turbine. The arrangement that hasproved most successful in the continuous combustion or constantpressure gas turbine which is described as follows:

A simple open cycle gas turbine plant consists of the turbineitself, a compressor mounted on the shaft or coupled to the turbine,the combustor, and auxiliaries, such as starting device, auxiliarylubrication system, fuel system, the dust system etc. A modifiedplant may have in addition to the above, an intercooler, a regeneratorand a reheater. The arrangement of a simple gas turbine plant isshown in Fig. 14·4·1.

Fuel

c

Combustion/chamber"

3

Poweroutput

Air fromatmosphere Exhaust gas to

atmosphere(a) Schematic diagram of gas turbine cycle.

(;AS TURBINE POWER PLANTS 593

p

i2

4Constant pressureheat rejection

--- "\r

(b) P.V. diagram.3

\~';;;;,o~

\r-\

T

\Turbine

2/ 2

\worki \

~~-r/ f,t/4I II -----'il,-=- 'ill.,

cp --- ••

(e) T-4J diagram.

}<'ig.14·4·1. Open cycle gas turbine.

It will be noted that the essential components are three innumber, namely, air compressor, a combustion chamber (combustor)and a turbine. The method of operation is as follows:

Air enters the air compressor in which it is compressed, througha pressure compression ratio of some 4 or 6 : 1. There are someinstallations in which the pressure compression ratio is as high as10 : 1 or even 18 : 1, although these llre not common. The quantityof the working fluid and speed required are more, so generally, acentrifugal ,or an axial compressor is employed.

Centrifugal compressors are often used in small gas turbines.An axial-flow compressor consists of sets of moving and fixed blades,resembling a turbine in reverse. In traversing the passages betweenthe blades, the kinetic (motion) energy of the gas imparted by therotation is changed into pressure (internal) energy (i.e. the pressureof the gas is increased). In the centrifugal compressor, air taken in


near the shaft of a rotating impeller blade is accelerated outward bycentrifugal force. At the periphery, the high-speed air enters a diffuser,that is, a nozzle designed to convert kinetic energy into pressureenergy.

The compressed air is passed from the air compressor into thecombustion chamber through a duct. If there are several combustionchambers then the take off volute from the air compressor will haveducts feeding the combustion chamber equispaced around it. In thecombustion chamber fuel, which is usually a fuel oil, such as gas oilor kerosene is sprayed inform a burner and is burnt continuously.Thus the air passing through the combustion chamber has itstemperature and volume increased while its pressure remainsconstant. Due to combustion, heat is added to the working fluidfrom T2 to T3. The product of combustion from the combustion chamberare expanded in the turbine from P3 to atmosphere.

It will be noted upon inspecting Fig. 14·4·1 (a) that the turbineis coupled back to the air compressor by a coupling shaft. On theother side of tl1e turbine there is a coupling by means of which theturbine can be coupled to drive some external equipment. From thisit will be observed that, in this case part of the turbine output isused to drive the air compressor and it is the net output whichappears for driving external equipment.

Due to continuous combustion which occurs in the combustionchamber, steps are taken to ensure that temperatures do not becometoo high. This is usually dealt with by supplying considerable excessair above the required for complete combustion. A special shroud isusually built round the burner in order to meter the air to thecombustion space. This ensures that there is good burning of thefuel and that further air is fixed with the very hot combustionproducts further down to the combustion chamber. This brings thefinal combustion product temperature down to something workablebefore entry to the turbine. The mass (or weight) of air supplied tothe compressor is three to four times the amount required theoreticallyfor complete combustion (about 50 to 60 parts by weight of air toone part of fuel). The excess air mixes with the very hot combustionproducts and moderates the temperature of the gas somewhat, thusprotecting both the combustor and the turbine blades from damage.

The turbine illustrated in Fig. 14·4·1 (a) is arranged to developshaft power. This being the case, the turbine would be designed toextract as much energy from the combustion products as possiblebefore they are passed to exhaust.

On the other hand, the gas turbine has a very wide use as ajet


propulsion unit for air craft. The basic element of the jet propulsionunit are the same. There is however, no power out shaft and theturbine itself its built just large enough to drive the air compressorand auxiliaries, such as fuel-pump and oil pump are necessary. Inthese circumstances the combustion products will leave the turbinestill with a high energy content. They are then passed rearward ofinto a nozzle from which they issue with a high velocity and thusthey provide the necessary thrust for propulsion of the air craft.

Gas turbines are not self starting machine as the reciprocatinginternal combustion engine it is necessary only to turn the engineover one compression, the engine will fire and then it will pick upspeed on its own. The gas turbine will not start simply turning theburner on. It must first be motored up to some minimum speed,called the 'coming in speed' before the fuel is turned on.When thisspeed has been reached, the fuel is turned on ignited and the turbinewill then pick up speed of its own. The turbine rotor is usuallymotored upto 'coming in speed' by a starter motor. This can eitherbe electrical or some times it is a small turbine.

C,C.II Combustionchamber

Gas turbine

Exhaustgases

Fig. 14·4·2. Simple cycle gliB turbine plant.

AC

Genera"tor

The speed of the gas turbines varies considerably. It can be aslow as 3000 rev/min. and as high as 35,000 rev/mm. Reduction gearboxes are fitted to high-speed turbines for coupling to externalequipment in order to reduce this speed.

Turbine of output as high as 20,000 kW have now been built,and air consumption as much as 130 kg/s is recorded.

2. Closed Cycle Gas Turbine. In an open cycle gas turbineplant, the fuel is mixed with air in the combustor and combustiongases are expanded in the gas turbine; the hot gases cause erosionand corrosion of turbine blades. To minimize these superior quality


Accumulators

Air heater

-r~Jp",:':,Cooling medium

Jo'ig. 14·4·3. Simple closed cycle glIS turbine.

Fuel'

of filel has to be used in the combustion chamber. The trouble ordrawback of an open cycle plant is overcome in a closed cycle plant,where the fuel does not mix with the working medium air or gas.

Since the close cycle continuously circulates the same workingfluid, air or gases of a higlier density than air, the heat added mustbe supplied through a heat exchanger from an external source andthe heat rejected from the system must be through a heat exchangerand a cooling medium. A schematic sketch of a simple closed cyclegas turbine is shown in Fig. 14·4·3. Combustion of the fuel takesplace in the air heater and is external to the working medium of thesystem. The working fluid leaving the turbine is cooled down by thecooling water in the precooler and is recirculated to the compressor.

Gases ofcombustion

The advantages of this system over that of the open cycle are:

L R.educed size. The density of the working fluid is increasedin the closed cycle by placing the system under an initial overallhigh pressure. Also since the working- medium is not required tosupport combustion, it is not mandatory that it should be air. It ispossible to use a gas of heavier density and higher specific heatthan air, such as the monoatomic gases; krypton, argon, Xenon andmercury vapour. This increase in the density, red4ces the physicalsize of all components and ducts of the system for the same poweroutput and permits the use of higher temperatures for a given stresslimit. Other working mediums may be helium, hydrogen or neon.The heat conductivity of hydrogen is about 6-8 times that of air andtherefore requires smaller heat exchangers.

<;AS TURBINE POWER PLANTS 597

2. Fuel. The closedcycle utilizing external heating can use aninexpensive solid fuel, such as coal.

3. No contamination. Since the working medium doesnot contain the gases of combustion the turbine and the generatorare not subjected to carbon deposits and should remain relativelyclean. There is absence of risk of corrosion and abrasion· of theinterior of the turbines. The compressor should remain free ofdust and other foreign deposits since the working medium canbe cleaned before being put into the systems. This means that\the periodic cleaning of the component is not necessary and thecomponent efficienciesshould not change appreeiably with continuedoperation. Thus continued operation should not reduce the thermalefficiently.

4. Improved part load efficiency. The control of a closedcycle system is different from the open cycle.The power output of aclosed cycle gas turbine can be controlled by changing the massflow. The system pressure is proportional to the gas mass flow. Bychanging the pressure and mass flow, output changes but thetemperature drop remains the same. Constant temperatures lead toconstant heat drop and constant velocities in the turbine bladingand hence the velocity triangles and consequently the turbine andcompressor efficiencies remain constant for every power output. Incase of an open cycle gas turbine the power control is affected bycontrolling temperature which affects the efficiency of the turbineat part load.

5. Fluid friction loss is reduced due to the higher Reyhold'snumber.

6. Improvement in the rate of heat transmission.

7. The regulation of the closed cycle gas turbine is simpler.The power output at constant speed can be varied by addingor subtracting the working fluid and thus altering the chargeweight.

Disadvantages of the closed cycle as compared to open cyclegas turbine engine are:

1. The use of high pressure requires a strong heat exchanger.

2. The complexity and cost of the system particularly in theload control, is increased. Since the system is under an initial highpressure with a working medium other than air, it is necessary thatthe system be gas tight. This add~to the cost and increases theengineering problems.

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_______ ~~ ._. __ .••. _. ••••••• JL •• J..IJ.l.l i, " .

POWER PLANT TECHNOLOGY

3. It is a dependent system Cooling water must be providedfor the precooler. This eliminates the use of the system as an'aeronautical' engine. The provision of cooling water is not a problemin marine propulsion and many land based applications but this isa disadvantages as compared to an open cycle plant.

4. A heavy, large air heater is required. The air heater isrelatively inefficient compared to the internal combustion chamberof the open cycle gas turbine engine. Poor combustion efficiencyresults due to losses as radiation and other, since heat transfer isindirect.

Inspite of the disadvantages and complexities of theclosed cycle, it is higher in efficiency, smaller in weight andspace, and is easier to adopt to marine propulsion than theopen cycle gas turbine. It has a comparable or better efficiencythan steam plants of the same power output with a great saving inweight and space.

14·5. Work Output and Thermal Efficiency of ConstantPressure Gas Turbine Plant

The ideal gas turbine cycle using isentropic compression andisentropic expansion is called constant pressure cycle or Brayton orJoule cycle. Such a cycle is never possible in practice due toirreversibilities introduced in the operation on account of leakage,turbulence and internal friction. The actual processes of compressionand expansion are not isentropic, and temperature of air (or gas) atthe end of compression and at the end of expansion are higher thanthose in the case of an ideal cycle. The representation on pressurevolume and temperature entropy planes is shown in Fig. 14·5·1.The actual cycle is represented by points 1, 2', 3, 4' and the ideal bypoints 1, 2,3, 4.

Since the compressor is coupled to the turbine then.

Net work output-Turbine output-compressor work

But compressor work = rha CPa (Ti - T1)

where rha = mass of air flow/second

CPa = specific heat of air at constant pressure

T~= final compression absolute temperature~

Tl = intake absolute temperature.

...(14·5·1)

...(14·5·2)

GAS TURBINE POWER PLANTS 59f

6

2 . 2/--~-_.

\

\

\

l.,/'.\IsenlroplC\,

3 IT

Volume - cp,Entropy _

where

Fig. 14·5·1. Ideal and actual gas turbine cycles onP-V and T-f diagrams.

But the final compression temperature is above the normaladiabatic compression temperature du,e to turbulence as stated above.The frictionless adiabatic temperature is calculated using gas lawsand is obtained from the equation,

r. -1

T2 = Tl (~)----r;;- ...(14.5.3)

Ya = adiabatic index for air.

The connection between the frictionless adiabatic compressiontemperature T2' and the final compression temperature Ti, is bymeans of the adiabatic or isentropic efficiency equation,

. T2-T1 ()lsentrop1c1]comp= 'r.' T ... 14·5·42 - 1

Knowing the isentropic efficiency of the compressor, T2 can becalculated.

Now to consider the turbine, by a similar analogy to that used forthe air compressor, the turbine output is obtained from the equation.

Turbine output = mt Cpt (T3 - T';) ..(14·5·5)

where mt = mass of the combustion products through turbineper second /

CPt = specific heat of combustion products ~t constantpressure

T3= inlet absolute t~perature of the turbine

T'; = exhaust absolute temperature of the turbine;


where

Now the final exhaust temperature from the turbine will beabove the frictionless adiabatic exhaust temperature as a result ofturbulence, etc., which occurs in the turbine.

In a similar way to that adopted in the case of the aircompressor, so these two temperatures are connected by the equation

. T3 - T4IsentropIc TJturb - m T.3 - 4

Isentropic TJturb = Isentropic efficiency at the turbine

T4 = Frictionless adiabatic absolute exhausttemperature.

T4 may be calculated from the gas law equation

...(14·5· 7)

where

r, - 1

T4 = T3 (~;)r;-It = adiabatic index for the combustion products

through the turbine.

Net Workoutput = mt CPt (T3 - T';) - ma Cpa (Ti - T1) ... (14·5·8)

Now the mass of fuel used is usually small compared with themass of air, and hence the mass of the fuel is often neglected. If thisis the case, then

m; = ma = m', say

If the fuel is neglected the it can be considered that

CPt = CPa = Cp say.

Then from Equation (14·5·8).

Net turbine output

= mCp (T3 - T4) - mCp (Ti - T1)

= mCp {(T3 - T4) - (Ti - TJ)} ...(14·5·9)

If m: = mass of air in kg/s, then from equation (14·5·9)

Net power outPUt of the turbine

= mCp {(T3 - T4) - (Ti - T1)}

Substituting for (T3 - T4) and (Ti - T1) in terms of (T3 - T4)

and (T2 - TI)


j 1 C [r- 1 ]

. 1 p--

=mCpTJt1'a 1- r r~lI-Tk1'1 r r -1 ...(14·5·10)

where r is the compression ratio, and r is the ratio of the twospecific heats at constant pressure and at constant volume. Thisexpression can be differentiated w.r.t. compression ratio r, keeping1'a and 1'1 as constants, and equated to zero to find the value of r formaximum net work.

The energy received in the gas turbine is in the combustionchamber at constant pressure. In the combustion chamber thetemperature is raised from 1'';' to 1\ If the fuel mass is neglected asbefore, then energy received at constant pressure in combustionchamber

Now thermal efficiency

= ritCp (T:l - 1'2)

= change in enthalpy

_ work output- energy input

Which from equations (14·5·9) and (14·5·10) becomes

...(14·5·11)

...(14·5·12)=

11=Thermal mCp {(I:1 - 1't) - (1'2 - I'd}

--' mCp(T:1 - 1'2)

(T:l - 1';) - (1'2 - 1'1)

(T:l - 1'2)

With TIc = 11t = 100%, the following relationship is obtained forideal conditions.

rr

r = (~~)2(r-· I)

)

602

---~=~------------------__-IIIIIIIIlnRII


i.e., the maximum network from the plant, theoretically, thetemperature of t)1e gas at the end of compression is the same as thetemperature at the end of expansion.

Heat supplied to plant = Cp (Ta - Ti)

= mC p (Ta - T2) approximately

[ r- 1]= mC p Ta - Tlr) 7

[ r- 1]= mCpT1 ~: - (r)-r-

Thermal efficiency T/th = net work

[ r- 1 ]';C T1 r-r-- 1

... (14·5·13)

... (14·5·14)

From equation (14·5·14) it is evident that thermal efficiency ofgas turbine plant depends upon the ratio of compression, theefficiencies of compression and expansion, the turbine inlettemperature and compressor inlet temperature.

The expression (14·5·14) can be differentiated with respectto r and equated to zero to find the compression ratio formaximum thermal efficiency. This results in the following expressionfor r.


If we assume

nc = 100%, i.e. under ideal conditions

603

...(14·5·15)

y

Then ry-1 = ~: ... (14·5.16)

The equation gives the optimum compression ratio for maximumthermal efficiency.

Example 14·5·1. A continuous combustion constant pressuregas turbine takes in air at 0·95 kg Icm2 (93 kN I m2) with a temperatureof 20°C. A rotary air compressor compresses the air to a pressure of5·70 kglcm2 (558 kN 1m2), with an isentropic efficiency of 83%. Thecompressed air is passed to a combustion chamber in which itstemperature is raised to 867·C. From the combustion chamber thehigh temperature air passes into a gas turbine in which it is expandedto 0·95 kg I cm2 (93 kN I m2) with an isentropic efficiency of 80%. Foran air flow of 10 kg Is and neglecting the fuel mass as small, determine :

(a) the net power output of the plant if the turbine is coupled tothe compressor;

(b) the thermal efficiency of the plant.

Take r= 1·4, Cp = 0·24 keall kg°K)

(= 1·00 kJ / kg oK)

Solution. For the compressor (refer Fig. 14·5·1.)r-1

T2 = T1 (~~)-r-1·4 - 1

= 293·50:4= 293 x 1·5837 = 464°K

Isentropic efficiency for the compressor

T2 - T11/c = mJ T.12 - 1

T2 - T1 _ 464 - 293 = 206 KTi. - T1 = 1/c - 0.83

604

.;.


Ti = 206 + 294 = 500 K.

t2 = 500 - 273 = 227°C

For the turbine

r- 1

T4 = T3 (~:)-r-= 510~2~~:: 7200K

Isentropic efficiency of turbine

~1 - T,£7J - -'--t- ~1-T4

73 - T'; :: (T3 - T4)rlt

= (1140 - 720) 0·80 = 336 K

T,£= T3 - 336 = 1140 - 336 = 804 K

t" = 531°C

Net power out put (MK8)

= mCp {(T3 - T';) - (Ti - T1)}

= 10 x 0·24 (336·- 206) = 312 kcal/s

= 312 x 427 :: 1776HP. Ans.75

Net power output (81 system)

= 10 x 1 x (336 - 206) :: 1300 kW Ans.

Thermal Work output7J:: Energy input

rhCp {(T3 - T4) - (Ti - T1)}

= rhCp (~1- T2)

(~1- T,£)- (Ti - T1) _ 336 - 206 = 0.2031:: I "'~ - 1140 - 5003 - 1.21

= 20·31% Aus.

14·6. Methods to Improve Thermal Efficiency of Gas TurbinePlant.

The efficiency and the specific work output of the simplegas turbine cycle is quite low inspite of increased componentefficiencies.

GAS TURBINE POWER PLANl'S 605

Some modifications improve the thermal efficiency of a simpleopen cycle gas burbine, they are:

(1) Regeneration (4) Gas Temperature(2) Intercooling and (5) Pressure ratio(3) Reheating (6) Combined cycle and Co-generation

(1) Regeneration. One of the main reasons for the lowefficiency of a simple gas turbine plant is the large amount of heatwhich is rejected in the turbine exhaust. Due to limitations ofmaximum turbine inlet temperature ann the pressure ratio whichmay be used with it, the turbine exhaust temperature is alwaysgreater than the temperature at the outlet of the compressor. So, ifthis temperature difference is used to increase the temperature ofthe compressed air before entering the combustion chamber and,thereby, reducing the heat which must be supplied in the combustionchamber for a given turbine inlet temperature, an improvement inefficiency can be attained. This utilization of heat in turbine exhaustcan be affected in a heat exchanger called re-generator. In the regenerator the heat energy from the exhaust gases is transferred tothe compressed air, before it enters the combustion chamber.Therefore, by this process there will be a saving in fuel used in thecombustion chamber, if the same final temperature of the combustiongases is to be attained and also there will be a reduction of wasteheat, thus there will be improvement in the cycle thermal efficiency.Fig. 14·6·1 shows a schematic diagram of such an arrangement. Theexhaust gases from the turbine pass through the regenerator andgive their heat to the compressed air, before it enters the combustionchamber, thereby reducing the amount of heat which must be suppliedin combustion chamber to get a given turbine inlet temperature T:l'Thus regeneration improves fuel economy. The power output will beslightly reduced because of the pressure losses in regenerator andits associated pipework.

6- ...•..-Heatexchanger

2

Air intake

Gasturbine

----- Generator

)

Fig. 14·6·1. Temperature entropy diagram for regenerative cycle.


The energy recovered from the exhaust in actual gas turbinesvaries from 50 to 90%. They operate most commonly between 70%and 80%, recovery. They percentage recovery of the heat exchangeris called its effectiveness. The thermal efficiency of gas turbineswithout heat exchanger is usually in the range of 15% to 20%. Witha heat exchanger fitted, the thermal efficiency is pushed upto therange 20% to 30%.

The temperature entropy diagram for the turbine arrangementwith heat exchanger is illustrated in Fig. 14·6·2. It will be noted

tT

tCompressed airtemp. ·lncreasein combustion

JChamber4' t TExhaust temp.

drop in HEl Max. temp.drop in H.E.J

ct>-Fig. 14·6·2. Temperpture entropy diagram for regenerative cycle.

that the maximum exhaust temperature drop available in the

exchanger = (T4 - Ti), since Ti is the lowest temperature in theheat exchanger. The effectiveness of the regeneration is definedas:

e = Effectiveness = Rise in air temperatureMax. possible rise

_ T., - Ti- T4 - Ti

Or it can be written as

EmCp (T4 - Ti) = mCp (T4 - n) = mCp (T., - Ti)

assuming Tit and Cp constant throughout then

E (T4 - Ti) = (T4 - T6) = (T5 - Ti)

...(14·6·1) .

...(14·6·2)


Equation (14·6·2) will -enable the exhaust temperature fromthe exchanger, T6 to be determined. The nett turbine output will beas before .

...(14·6·3)

However the energy required from the fuel is that required toincrease the temperature from Ts to T3.

...(14·6·4)

This is evidently less than that which would be required ifno heat exchanger was fitted in which case the temperatureincrease required from the fuel would be from Ti. to Ts. Thuswith a heat exchanger the thermal efficiency of the plant isincreased.

Thermal 11 = mCp {(7:1 - TI)- (Ti. - TI)}. r'<p (7:1 -7:,;

and again assuming m and Cp constant throughout,

...(14·6·5)

...(14·6·6)(T3 - T';) - (Ti. - TJ)

Thermal 1] = (T3_ T5)

Example 14-6·1.In a gas turbine cycle, the compressor compressesair from 100 kPa and 22°C to 600 kPa. The turbine inlet temperatureis 800°C. It is lmown that a regenerator with 80% effu:iency is available,the isentropic efficiencies of the compressor and the turbine are 0·90and 0·85 respectively. Determine the improvement in the efficiencyresulting from the installation of the regenerator. Assume y == ]·4and Cp = ]·03 kJ / kg K.

Solution. Data given

PI = 100 kPa,

P2 = 600 kPa,

TI = 273 + 22 = 295 Ie

T3 = 273 + 800 = 1073 K

e = regenerator effectiveness = 0·8

1]is comp = 0·90, 1]is turbine = 0·85

First we will determine the thermal efficiency without regeneratorand then with regenerator and calculate the improvement in theefficiency due to the regenerator.

Referring to figure 14·6·2.


t~W0:::J~<t0:W(L~W1-'

30 Ok

i.e.

ENTROPY ~

Fig. 14·6·2 K T.S. diagram for cxmplc (14·6·2).

For the isentropic process 1 - 2 and 3 -- 4

_ (~)(7- I)/rT2- TI PI

= 295 (0)(14- 1)/1.4 = 492 K

( )(7- I)/rSimilarly • T4 = T:I P2

PI.

-. 1073 X 6.4/14 = 643 K

Equation for the isentropic compressor efficiency is

T2 -TI11i", =- ~--1' on rearrangement12 -- I

'T" _ T T2 - TI12 - 1 + ----7Jis C

= 295 + 492...:..-295 = 514 K0·90

The expression for the isentropic turbine efficiency is

1;1 - T4T/is turbine =,." T1;1 - 4

T4 = T3 - T/isl (T3 - 1',.,)

= 1073 - 0·85 (1073 - 643) = 707·5 K.


Also E = .0.80 = T.., - Ti."" rn'4 - 12

T5 = 0·80 (707·5 - 514) + 514 = 669 K

For thermal efficiency without regeneration

Cp (T3 - T4) - Cp (Ti. - T1)

71th = Cp (T3 - T2)

_ (1073 - 707·5) - (514 - 295)- 1073 - 514

= 0·26 or 26%

Thermal efficiency with regeneration

Cp (T.1- T4) - Cp (Ti. - T1)71th =

Cp (T.1 - T5)

_ (1073 - 707·5) - (514 - 295)- 1073 - 669

= 0·36 or 36%

The improvement in the thermal efficiency of the plant, due toregenerator instaJJation is

= 0·36 - 0·26 = 38m0.26 70

(2) Intercooling. A regenerator, as discussed above does notchange the work output of a gas turbine cycle. Two possible methodsfor increasing the work output are:

(i) by reducing the work of compression, and

(ii) by increasing the work done by the turbine.

IntercooJing is used for decreasing the workdone on thecompressor. One of the ways to achieve this is to cool the air after ithas been partiaJJy compressed, and this is accomplished by employingmulti-stage compression and intercooJing between stages. Usually 2to 3 stages of compression are used. IntercooJing improves the thermalefficiency, air rate and work ratio. Thus, if intercooling is used thesize of the turbine and compressor can be reduced for the sameoutput or alternatively greater work can be obtained from the plantof the same size.

Fig. 14·6·3 shows the schematic diagram of a two stage intercooledgas turbine and Fig. 14·6·4 shows the indicator diagram for a two


Combustionchamber

Compressor

2

Intercoole r

3

Exhaust

Fig. 14·6·3. Schematic diagram of a two-stage intercooled gas turbine.S

T

i

Entropy,¢ _Fig. 14·6·4. T-¢ diagram for t\\O stage intcrcoolcd gas turbine cycle.

stage compressor with intercooler. By employing multistagecompression with intercooling between stages, the compression processin the compressor can be made to approach isothermal compressionwhich requires less powr~r than the adiabatic compression. In theideal state of intercooling the fluid should be cooled to its ambienttemperature, i.e. the temperature of the fluid before compression, ineach'stage ; and there should be no loss of pressure in the system.Also the maximum advantage of intercooling is obtained when thepressure ratio for each stage is the same. Maximum advantage ofintercooling occurs when pressure ratio is high, compression efficiencyis low and regeneration is employed.


(3) Reheating. It is arfother method of increasing the specificwork output of the cycle. An intercooling improves compressorperformance, reheating improves the output from the turbine dueto multiple heating. The gain in work output is obtained be cause ofdivergence of constant pressure lines on T • ¢> diagram, with anincrease in temperature. Thus for the same expansion ratio if theexhaust from one stage is reheated in a separate combustion chamberand expanded, more output will be obtained than that obtained byexpansion in a single stage. Fig. 14·6·5 shows the schematic diagramof a reheat gas turbine plant and Fig. 14·6·6 the corresponding T-¢>

diagram.Reheater

3

Compressor HPTurbine

lPTurbine

Generator

6"

Fig. 14·6·5. Schematic diagram of a reheat gas turbine plant.

T

t

-4>Fig. 14·6·6. T-~ diagram of reheat cycle.

Reheating involves extra equipment of combustors and hightemperature resistance material for construction which adds to cost.Also, the complication of spliting the turbine and of producing suitlablecontrols may offsets much of the gain by use of reheater in m~nycases.


In order to increase the thermal efficiency of a Brayton cycle,we can inaease the pressure ratio. However, as the pressure ratiois increased, the t<!mperature at the outlet of the compressor increases,causing problems with seals and metal fatigue. Also, the physicalsize of a compressor incr~ases with the increase in the pressureratio. To minimize such problems, compression is accomplished intwo or more stages. Ideally, in intercooling the compressed air atthe exit of one stage is cooled to the inlet temperature of that stageand then compressed in the next stage. The compressor work requiredmay be reduced by dividing the compression into two or more stages,and to cool the air or working fluid between them. Most of the heatof compression may then be removed by inter-cooling. The effectof i.nter cooling, when carried to the theoretical limits, is to havethe work of compression approach an isothermal process (i.e.,compression at a constant temperature). Ideally, intercoolingthe compressed air at the exit of one stage is cooled to the inlettemperature of that stage and then compressed in the next stage.'i'he total work of compression in the cycle is the sum of the workfor each compression stage. From a practical stand point, however,the effect of inter-cooling is to reduce the work of compression requiredto achieve a given pressure. The reduction of compressor work achievedin this manner results in an increase in the overall gas turbineoutput and usually improves the overall plant efficiency.

In a reverse manner, the output of the turbine may be increasedby dividing the expansion of the working media into a numberof steps and the gas reheated between them. The reheating of thegas or working media back to the limiting turbine-inlet temperatureallows a greater portion of the expansion to take place at highertemperatures. The theoretical limit of reheating would of course, bean isothermal expansion at the turbine inlet temperature. Again,from a practical stand point, the result of reheating is an increasein the output of the turbine through the same expansion pressurerange although it has a negligible effect upon the overall efficiency.However, when reheating is properly utilised in conjunction withregeneration, the increase in overall efficiency is appreciable.

The mechanical components and T-S diagram for the Braytoncycle with intercooling and reheat is shown in l<'ig.(14·6·7). As canbe seen from the T-S d'iagram in the figure, both intercooling andreheat increase the network available from the cycle. For the cycleshown in Fig. (b).

The thermal efficiency of the cycle with intercooling and reheatcan then be calculated in the usual manner. If the compressor andthe turbines have isentropic efficiencies of less than 100 percentand if there is also regeneration, the analysis becomes a bit morecomplex but presents no extraordinary difficulty.


(a)

t--

11:::!wt--

ENTROPY S ~

(b)

Fig. 14·6·7. Brayton cycle with inter cooling and reheater.

(a) Mechanical components, (b) T-S. diagram.

We = (h4 - h3) + (hz - hi)

WT = (hn - N;) + (h7 - hS)

qin = (hn - h4) + (h7 - N;)

613


The open cycle gas turbine with regenerator, intercooler, andreheater is shown in Fig. (14·6·8) with T. S. diagram.

REGENERATOR

8 TURBINE

GENERATOR

9

T

REHEATER

INTERCOOLER

CaOUNG' MEDIAM

(a)

ENTROPY ..•S

(b)

Fig. 14·6·S. Brayton cycle with regenerator, intercooler, and rcheater.(a) Mechanical components (b) T-B diagram.


We = (112- hi) + (h4 - h3)

Wr = (~ - h7) + (hs - 119)

qin = (~ - ~) + (hs - h7)

The thermal efficiency of the cycle is expressed as

_ [(~ - h7)+ (hs - h9)J-=.[(h2 - hi) + (h4 - h3))

17th - (~ _ ~) + (hs - h7)

615

... (14·6·7)

Gas Temperature. The thermal efficiency of a gas turbine, asdefined earlier, depends in the first place on the intake gastemperature, which should be as high as possible. In practice, thistemperature is limited by the potential for blade damage. Gastemperatures are commonly in the range from 800 to 900°C. By theuse of special alloys and protective refractory coatings for the blades,the temperature can be increased to about 1250°C or so. For stillhigher temperature, it would probably be necessary to use specialmeans of cooling the blades. However, the increase in thermalefficiency resulting from an increase in gas temperature must bebalanced against the greater cost of the turbine.

(5) Pressure ratio. The thermal efficiency of a gas turbineis related to the pressure ratio (i.e. the pressure in the combustorrelative to the ex.haust gas pressure). Upto a point, an increase inthe pressure ratio, to about 10 at moderate gas intake temperaturesor to 20 at high temperatures, is accompanied by an increase inefficiency once again, however, the increased cost of the equipmentmust be taken into account.

Combined cycle and Cogeneration. Another approach toincreasing the efficiency of fuel utilization would be in a combined cycle or cogeneration system. The still hot exhaust gas from theturbine provides the heat for generating steam in a waste heatboiler. The steam is then used to operate a steam turbine i.e. combinedcycle generation. Alternatively, the hot gas might be used to produceprocess heat i.e. cogeneration.

Example 14·6.2. In a two stage gas turbine cycle with idealinter cooling and reheat, the pressure ratio in each stage is 3·5. Theinlet conditions are 300 K and 100 k pa and the temperature at theinlet to the turbines is 1300 K. A regenerator with an efficiency of70% is used to improve the efficiency. Determine the compressorwork, the turbine work, and the thermal efficiency of the cycle; Taker= 1·4, and Cp = 1·03 kJ / kg K.


Solution. T.8. diagram for Two-stage regenerative gasturbine cycle with ideal intercooling and reheat is shown inFig. (14·6·7).

For isentropic process 1·2

( / )(r- I)/rWe haveT2 = TI '7PI

= 300 X (3·5)°·4/1.4 = 429·10 K

= T4 also

Ideal intercooling and reheating is to be considered.(r- I)

Likewise T7 = Tg = T6 (3'5f -r-= 1300 (3·5)-04/14

= 908·80 K.

The compressor input is

We = Cp [(T4 - T3) + (T2 - T1)]

= 2 Cp (T2- T1)

= 2 x 1·03 (429·10 - 300)

= 265·95 kJ/kg Ans.

The turbine output is

WI = Cp [(T6- T7) + (T8 - Tg)]

= 2 Cp (T6 - T7)

= 2 x 1·03 (1300 - 908·80)

= 805·70 kJ/kg Ans.

For the regenerator

0.7 = (1:<; - T4)(Tg - T4)

or T5 = 0·7 (908·80 - 429·10) + 429·10

= 764·90 K.

The heat supplied is given by

qif/t'=(h6- h5) + (h8- h7)

= Cp [(T6 - T5) + (T8 - T7)]

= 1·03 [(1300 - 764·90) + (1300 - 908·80)]

= 954kJ/kg


Now thermal efficiency can be calculated

Turbine work - compressor work11th = Heat input

= WT - We

805·70 - 265·95954

= 0·565 or 56·5% Ans.

617

14·7. Main Components of A Gas Turbine Plant

The basic gas turbine components are:

(1) Compressor,

(2) Combustion chamber,(3) Turbine and

(4) Heat exchangers.

1. Compressor. A gas turbine compressor should be able tohandle a relatively large volume of air or working media and deliveringit at 4 to 6 atmospheric pressure with the highest possible efficiencies,moreover, the compressor should be such as can be coupled to theturbine shaft which runs at very high speed ranging from about 600rpm to 40,000 rpm. On the above basic requirements, only a centrifugalor axial compressors can be employed. Reciprocating compressorscan not be used, because it sutTers from a number of disadvantages,such as, inertia of moving parts, sliding friction of the piston insidethe cylinder, limitations in speed, etc. and are not considered suitablefor use in gas turbine plants. However, a version of this compressorin the 'free piston' design, which eliminates use of crank shaft andconnecting rods and is at present being developed for use in theseplants.

The centrifugal compressor consists of a rotor called impellerprovided with vanes and moving in a casing or scroll. The inletsection at the hub of the impeller on one side, called the inducer, iscurved to minimize entry losses and is provided with vanes to directthe air to the eye. A schematic diagram of a radial bladed centrifugalcompressor is shown in Fig. 14·7·1. Air is given a whirling motionat high velocity by the impeller and is thrown out of it by centrifugalforce. The static pressure of air increases to the tip. A stationary Ipassage surrounding the impeller diffuser helps to convert most o~the velocity head into pressure head as the air has a high velocitywhen it leaves the impeller. The impeller converts the mechanical


Diffuserthroat

Depth of

vaned

diffuser

(b)

(a)

Collector

Vaneless

diffuser

(e)

Fig. 14·7·1. Schematic of a centrifugal compressor.

energy imparted to air by the rotation of the impeller into pressureand kinetic energy. The pressure rise in the impeller is due todiffusion action (i.e. the relative velocity decreases from inlet tooutlet due to diverging channel area) and the centrifugal action (i.e.the air enters at lower diameter and comes out at higher diameter).The rest of the kinetic energy available at the tip of the impeller isconverted into pressure energy in the vaneless and vaned diffuser.The vane less diffuser converts some part of the kinetic energy into


pressure energy and stabilizes the flow so that it enters the blededdiffuser without shock. From the vaned diffuser the air is collectedin the volute casing and comes out from the outlet pipe. For the gasturbine (instead of putting the volute casing), a 90° bend is providedto take air to the combustion chambers. The present day practice isto design the centrifugal compressor such that about half the pressurerise occurs in the impeller and half in the diffuser.

The impeller blades are made in two types, the radial bladesand the backward curved blades. Radial bladed impeller is suitablewhere low weight and dimension are required, whereas the backwardturned blade is suitable where higher efficiency is preferred. In thegas turbine radial bladed impeller is used due to lighter constructionand less stressed impeller. A pressure ratio of 4·5 : 1 may be obtainedin a single stage centrifugal compressor. In a multistage centrifugaltwo or more impellers operating in series on a single shaft areprovided in a single casing. The effect of multi-staging is to increasethe delivery pressure of air, as air compressed in one stage of machineis fed into the next stage for further compression and pressure ismultiplied in each stage. The overall efficiency of a multistagecompressor is lower than the efficiency of individual stages. Labyrinthpackings provide sealing effect on the air and prevent leakage betweenthe impellers of various stages and from inside the compressor tooutside through shaft end connections. The compressor dischargecan be controlled by varying the speed.

The centrifugal compressor is superior to the axial flowcompressor in that a high pressure ratio can be obtained in a shortrugged single stage machine. It is relatively insensitive to surfacedeposits, has a wider stability range and less expensive. Howeverthe efficiency is lower, the diameter greater and it is not as readilyadoptable to multistaging.

For higher pressure ratios multistage centrifugal compressordoes not prove to be as useful as an equivalent axial flow compressor.Therefore, when high pressure ratios are needed, axial compressoris advantageous and it always used for industrial gas turbineinstallations. Although, the axial compressor is heavier than thecentrifugal compressor but it has higher efficiency than the centrifugalcompressor.

The axil fZow compressor consist of a rotor and a stator asshown in Fig. 14·7·2. The rotor (i.e. moving element) consists ofrows of moving blades and the stator (i.e. stationary component)

consists of a rows of stationary blades. Some part of the kinetic (energy imparted to air by the rotor is converted into pressure energyin the rotor due to diffusion action and the rest is converted in the


Rotating blades

Guideblades

ROTOR

Casing

Stat ionaryblades

Fig. 14·7·2. Arrangement of rotor and stator in axiall10w compressor.

stator. The stat,or blades also redirects the air into an angle suitablefor entry to the succeeding rows of moving blades. The rotor as wellas the stator blade channels are of diverging type. A row of movingblade with a succeeding row of stationery blades is called a stage ofaxial compressor. Blades are usually made of air foil section.

The important characteristics of the axial flow compressor areits high peak efficiencies, adoptability to multi staging to obtain higheroverall pressure ratio, high flow rate capabilities, and relativelysmall diameter. However, the axial flow compressor is sensitive tochanges in air flow and rpm, which results in a rapid drop off inefficiency, i.e. the stability range of speeds for good efficiencies issmal1. These latter characteristics limit the part load capabilities;of this type of compressor and are considered undesirable in someinstallations.

(3) Combustion Chamber (Combustor) : Generally the airfuel ratio in open gas turbine varies from 50 : 1 to 250 : 1, to keepthe turbine inlet temperature down to permissible limits. Thecombustion process taking place inside the combustion chamber isquite important because it is in this process that energy, which islater converted into work by the turbine is supplied. Therefore, thecombustion chamber should provide thorough mixing of fuel and airas well as combustion products and air so that complete combustionand uniform temperature distribution in the combustion gases maybe achieved. Combustion slwuld take place at high efficiency becauselosses incurred in the combustion process have a direct effect on thethermal efficiency of the gas turbine cycle. Further the pressurelosses in the combustion chamber should be low and the combustionchamber should provide sufficient volume and length for complete


combustion of the fuel. Hence requirements of a comhustion chamberare:

(a) lower pressure loss;

(b) high combustion efficiency;(c) good flame stability;(d) low carbon deposit in the combustion chamber, turbine

and regenerators;(e) low weight and frontal area;(n reliability and serviceability with reasonal1ife, and

(g) through mixing of cold air with the hot products ofcombustion.

The types of construction of the combustion chambers in useare in general:

(1) tubular or 'can' counter flow.

(2) tubular or can straight-through flow, and(3) annular parallel flow.

Although theoretically the annular chamber possesses advantagesover the 'can type', this has not been realized in practice. Also, fortest and for replacement of burned out chambers, the 'can' type ischeaper and more practical. For these two reasons, the 'can' typepredominates in current practice.

A typical combustion chamber design employs an outer cylindricalshell with a conical inner sleeve which is provided with ports orslots along its length. At the cone apex is fitted a nozzle throughwhich fuel is sprayed in a conical pattern into the sleeve, with anigniting device or sparking nearly. (Refer Fig. 14·7·3.) A few airports provided close to the location of the nozzle, supply the combustionair directly to the fuel and are fitted with vanes to produce a whirlingmotion of oil and thereby to create turbulence. The rest of thE. airadmitted ahead of the combustion zone serves to cool the combustionchamber and the outlet gases.

I\\"

Outer shell/T- ".'--'--,,---- Nozzle

--I --~UL~-~~~'~~ ~conic~~::eve ;~~:~:gt ~~:I

Gases

Fig. 14·7·3. Arrangement of a combustor.


The combustion chamber in the open cycle gas turbine engineis the most efficient component of the gas turbine. Efficiencies ofbetween 95 and 98 percent are obtained over a fairly large operatingrange. The combustion chamber of a closed cycle gas turbine engineis actually a heat exchanger. The heat added to the working fluidair or gases of higher density, must be supplied through a heatexchanger from an external source. The working media is thus notcontaminated with the products of combustion. The problem hasbeen to design an efficient heat exchanger of a practical size, capableof supplying the heat addition required. However cheaper fuels suchas soft coal may be used in the heat exchanger of a closed cycle gasturbine.

(3) The gas turbine. The construction and shape of the gasturbine blades are very similar to that of steam turbines. It differsonly in the blading material, the means for cooling the bearings andhighly stressed parts, the thermal distortion due to highertemperatures, and high ratio of blade length to wheel diameter toaccommodate large gas flows. The main requirements for the gasturbines are light weight, high efficiency, ability to operate at hightemperatures for long periods, reliability and serviceability. Specialcooling arrangements for the blades may some times be used in gasturbines. These include supply of cooling air near the rim or use ofdifferent materials for rim and hub sections. The blade speed isselected on the strength consideration of the wheel.

The arrangement of the rotor and stator blades in the gasturbine is similar to that of steam turbine. As in the case of steamturbines, gas turbines may be irripulse or reaction. If the entirepressure drop of the turbine occurs across the fixed blades, thedesign is impulse type, while if this drop takes place in the movingblades, the fixed blades serving only as deflectors, the design iscalled reaction type.

Generally the blades are made of Nimic 80 alloy (heat resisting).

(4) Heat Exchangers. Regenerator and the intercooler arethe heat exchangers used in gas turbine plants. In the heat exchangers,heat transfer takes place between exhaust gases and cool incomingair, while in the int.ercooler the heat transfer occurs between thehot air under compression and cooling water. Since water has amuch better heat transfer coefficient than do air and gases, thesurface required for the same amount of heat transfer is much lessin the case of the intercooler than for the regenerator.

The regenerator is generally shell and tube construction, withgas flowing inside the tubes and air flowing outside, in oppositedirection.


14·8. Auxiliaries and Controls

Gas turbine engines need additional equipment to serve the

main comP9nents ; these include; starting motor or engine, auxiliarylubricating oil pump, fuel control system, oil coolers and filters,inlet and exhaust mumers (silencers), air and gas ducts and plantcontrol panel. In addition there are also automatic devices for alarmand shut down.

The starting motor or engine drives the gas turbine anacompressor through a clutch and step up gear. The starting gear ismounted on the shaft at one end. The clutch is often made to workunder air pressure. The rotation of the turbine-compressor shaft,for about 5 minutes at speed of 500 to 1000 rpm results ineliminationof un burnt fuel from the air-gas flow system. Speed is then increasedto 4000 to 5000 rpm and fuel is allowed to enter the compressorwhere it is made to ignite and gases produced are passed on to theturbine. The turbine then slowly starts under influence of the gases,and at about 6000 rpm, the starting motor is shutdown and clutchdisconnected automatically. Feeding in additional fuel brings theturbine upto rated speed.

Often the drive for the lubricating oil pump is taken from thestarting-up gear. High pressure oil from the pump is supplied to thehydraulic control system and low-pressure lubricating oil for thegas turbine, gears and driven apparatus. A separate motor-driverpump usually acts as stand-by if main pump fails. A failure of thelubricating pump system results in stopping of the unit automatically.

The fuel feed is made responsive to the speed governor, forgenerator drive. A reduction in speed (when load increases) opensthe fuel valves to restore normal speed. A rise in speed (when loaddecreases) closes the fuel valves to lower the rate of fuel feed andrestore speed.

The duct system includes the main connection between thecompressor and combustor, and between the combustor and theturbine in the simple cycle plant, and an addition of other suchconnection when additional heat exchangers are employed. Compressorinlet air usually enters t.he gas turbine unit from outdoors througha filter and duct. The filter proves necessary because a slight buildup of solids (fouling) on compressor blading can seriously reduce itsefficiency. Both filter and ducts must be sized to minimize air-pressuredrop. Any undue loss in pressure directly reduces the capacity ofthe unit. The exhaust duct and stack must also be size to minimizepressure drop because this loss raises the turbine-exhaust pressureand reduces turbine capacity and efficiency. Exhaust gas from the


stack must not be allowed to recirculate to the compressor intake;this can be minimized by increasing stack height, which also raisespressure loss. Another important point regarding ducts in to supportthem suitably so that vibrations are reduced to minimum. Theyshould be stift enough to resists vibration caused by the air andgas-Cows. Furthermore, the ducts should be capable of taking upthe expansion at joints due to changes in temperature, and so adequateexpansion joints should be incorporated wherever necessary. Theexhaust duct and connections between combustor and turbine, inparticular, should be capable of standing high temperatures (about500°C for simple open cycle turbines and 350°C for regenerativecycle).

Filters of various types are used on air compressor inlets. Theymay be oil bath type or dry type. The viscous types are use a filteringmaterial dipped in oil that catches air-brone particles as they passthrough. They dry type filters use glass fibres as the trapping agent.The air capacity through the filter should not increase about 2 mIsee, which gives a pressure drop of about 13 to 19 mm of watergauge.

Silencers may be used at the inlet and exhaust of air and gasrespectively. The air velocity through the inlet muller may be about60 mlsec.

14·9.Fuels for Gas Turbine Plants

Gas turbines can use a wide variety of fuels, solid, liquid andgases. The ideal fuel is ofcourse natural gas but this is not alwaysavailable. Natural gas which is mainly methane has a very highcalorific value and is generally used for auxiliary power generationwithin the oil fields. Blast furnace and producer gas can also beused for these plants.

Liquid fuels of petroleum origin such as distillate oils or residualfuels (including fuel oils, furnace oils, boiler fuel oils) are mostcommonly used for such plants. These fuels are generally costly.When using such fuels one has to be very careful that the fuel usedpossess proper volatility, viscosity and calorific value. Moisture andsuspended impurities should not be there, as they may clog thesmall passage ofthe.nozzles and damage valves and plungers of thefuel pumps. Residual oil usually contain sodium, vanadium, andcalcium as part of the ash constituent. They corrode hot metals andbuild up hard deposits that choke gas passage in the blading. Residualoil may be treated by heating it, then mixing it, then mixing it with5% of water. Two centrifuges in series receive the mixture to removethe water that takes with it most of the sodi urn originally in oil.


The increased use of heavy oils has been limited by the effectof vanadium corrosion and deposits build up on blades. Distillatefuels burns more easy than doresiduals fuels.Thereforewhen startingthe unit for cold initially distillate fuels feed into the combustorafter which residual fuels may be fed. In cold climate it may benecessary to preheat the residual fuels.

Solid fuels (for example pulverised coal) may be used but theycreate coal handling and ash handling problems. The efficiency ofcoal fired gas turbine plant is lower than that of oil fired plant.Present day gas turbine plants use mainly natural gas liquid petroleumfuels.

14·10. Plant Layout

In the case of a gas turbine plant the main building is theturbine house in which major portion of the plant as well as auxiliariesare installed. In many respects it is similar to the steam plantturbine house.

The fuel oil storage tanks are arranged outside but adjoiningthe turbine house. In some installations even heat exchangers areplaced out doors.

The rotating parts of the plant form a very small part of thetotal volume of the plant since it is the intercoolers, combustionchambers, heat exchangers, waste heat boilers and interconnectingducts work which have to be arranged and accommodated. It isthese components which occupythe major portion of the total space.

Airfilter

Alternator

g~~motor

Intercooler.

///"

--' /"

--[,../ I I H.P--//1 I Turbine

--- -'''0-

LPTurbine

Heat exchanger

~-,:\

,. I, \" \

____ 1

CombusflOchamber

Fig. 14·10.1. Layout of a gas turbine power plant..

A typical layout of gas turbine plant is shown in Fig. 14·10·1.

626 POWER PLANT TECHNOLOG'Y

The purpose of the air filter is to clean air. From this air filter airflows to the L.P. compressor. From there the compressed air entersH.P. compressor via intercooler. The air leaving the H.P. compressorenters heat exchanger, the-hot air from there flows to the combustionchamber. Products of combustion are first expanded in H.P. turbineand than in L.P. turbine.

The layout of a gas turbine plant has a very importanteffect on the overall performance of the plant. Since there may be aloss of as much as 20% of power developed in the interconnectingducts with a large number of sharp bends. Great care has thereforeto be exercised in the design and layout of the air as well as gascircuits.

14·11.Comparison of Gas Turbine Plants with Other Plants

(A) Comparison with Steam Power Plants

1. Space requirement for a gas turbine plant is smallercompared to a condensing steam plant of equal size.

2. A gas turbine plant can be started quickly and has ashort starting time in comparison to steam plant.

3. The capital cost of a gas turbine plant is lower than thatof a comparable steam power plant.

4. The circulating water consumption is less in comparsionto that of a steam turbine plant of the same size. Thismakes site selection easier. In water scarcity areas theyhave great application.

5. These plants can be readily located in cities and industrialcentres very near to the areas of heavy power demand.

6. The gas turbine plant uses fewer auxiliaries comparedwith steam plant. Therefore smaller size of the gas turbinecomponents enables complete work tested units to betransported to the site.

7. Storage of fuel is much smaller and its handling is easy.

8. The fuel consumption during starting and shutting-downperiods is low.

9. Foundations and buildings are less costly.

10. Time for installation required is less

11. Number of personnel required for operation is hardlyone-third compared with that for a steam plant of samesize.


12. Problems of coal and ash handling as encounter in case, ofsteam plants are eliminated in open cycle gas turbineplants using gas or liquid fuel.

13. The components and circuits of a gas turbine plant maybe arranged to give the most economic results in any givensituation. This is not possible in case of steam power plant.

14. A gas turbine plant becomes more economical for operatingbelow a given load factor as saving on the capital chargesoutweighs the additional cost of fuel.

15. The heat rate of gas turbine is gerierally higher than theheat rate of steam turbine.

16. Specific weight of steam turbine is generally more thantwice of the specific weight of gas turbine.

17. The operation of turbine is simpler and its capital andmaintenance costs are lower than those of steam turbineplant.

(B) Comparison with Diesel Power Plants

1. As compared to diesel power plants, gas turbine have highermechanical efficiency due to fewer sliding parts inconstruction. While the adiabatic expansion of gases inthe cylinder of diesel engine is incomplete, the gas turbineallows for a more or less complete expansion of gaseswhich increases power output.

2. Gas turbine plants have easier maintenance and reducedattendance charges.

3. Gas turbine plants have lower cost of buildings and smallersite area.

4. Gas turbine being rotating machine is well balanced at allspeeds, so less vibrations.

5. There is greater flexibility in design of a gas turbine plantas the processes of compression, combustion and expansionoccur independent units unlike diesel plant in whichoperations occurs in the cylinder of the engine.

6. The gas turbine is a compact powerful machine and specificweights are low, as 15 kglh.p. (20 kgIkW), as compared to85 kglh.p. (112 kglkW) for diesel engine.

7. The gas turbine, is able to operate with lower graaeR offuel oils than is possible with diesel engines. Also lowgrade waste gases may be utiliz~ as fuel.


8. Heat rate of a gas turbine is generally better than theheat rate of a diesel engine.

9. Water requirements are much less in gas turbine plant incontrast to a diesel plant.

However, the gas turbine plant has lower thermal efficiency ascompared with a diesel plant, as a great deal of its power output isused to run the compressor. The diesel plant is somewhat easier tostart and needs less elaborate cooling arrangements.

14·12. Combination Gas Turbine Cycle

Gas turbines after several advantages for Jifferent type of servicepeak load, emergency standby, base load, hydrostation stand-by etc.In some of th.ese services the quick starting ability makes the gasturbine plant desirable.

The combination gas turbine-steam turbme cycles aims atutilizing the heat of exhaust gases from the gas turbine and thus, toimprove the ov~rall plant efficiency. The heat content of gas turbineexhaust is quite substantial. Gas turbine exhaust has a temperatureof around 500°C. The oxygen content in this exhaust is around 16%compared with 21% in atmospheric air. A simple cycle gas turbineplant wastes this energy to atmosphere, while a regenerative gasturbine plant recovers much of this heat to raise overall thermalefficiency. But instead we can use the gas turbine exhaust as a heatsource for a steam plant cycle.

The combined steam and gas turbine cycle provides the highestefficiency turbine system available at the present time. The efficiencyof the combined cycle is higher than efficiency of a standardregenerative cycle gas turbine.

There are three popular designs of the combination cycles:

1. Gas turbine exhaust gases used for feed water heating,

2. Employing the exhaust gases as combustion air. in thesteam boiler, and

3. Employing the gases from a suppercharged boiler to expandin the gas turbine.

Fig. 14·12·1. shows a combined cycle in which the gas turbineexhaust passes through a heat exchanger to feed water for the boilerof the system plant. When this arrangement is used, bleeding ofsteam from the steam turbine (for the purpose of fed water heating)is not necessary. The full steam supply to the steam turbine isavailable for expansion and producing mechanical power.

GAS TURBINE POWER PLANTS 62fl

Air in

Fuel tocombustor Feed water

heater

Turbine exhaust

Tostacks

Fig. 14·12·1. Use of exhaust gases to heat feed water of steam cycle.

If bleeding is also used, the requirement of bled steam is muchless than what would be required when no feedwater heating withexhaust gases is employed. Arrangement is shown in figure, usingboth exhaust gases and bled steam for the feedwater heating. Furtherthe gas inlet temperature to turbine can be increased and this resultsin an overall increase in efficiency of the plant.

Fig. 14·12·2 shows a combined cycle in which the gas turbineexhaust is used as preheated air for the boiler of the steam plant.

Air in

G.T.

To stock

rGGenerator

Gas turbine 12xhoust

Baiter

Fig. 14·12·2. Combined gas and steam plant (Heat reccvery boiler).

The gas turbine exhaust has around 16% oxygen which is enough tosupport combustion in the boiler. Supplementary fuel and air canbe fed to the boiler, which would be larger than the conventionalboiler. About 5% improvement in plant heat rate can be obtained bythe use of combined cycle.

Fig. 14·12·3 shows a flow diagram for the supercharged boiler.Here the combustors of the gas turbine unit are replaced by a steamgenerator having a supercharged fumace, and the gas-turbine exhaust,heats the feedwater before it enters the boiler. The heat transferrate in the boiler are increased due to the high density of air. So,the boiler weight get reduced by as much as about 50%. Heat ratealso gets improved by about 7 to 8 per cent. The station capacity isalso increased and there is only a slight increase in the coolingwater arrangement.


To stack

Feed water

Flue gases

Exhaustgases

Feed water

Alternator Star(lngmotor

tU~brne~~F=()

Fig. 14·12·3. Flow diagram of supercharged boiler cycle.

14·13. Advantages and Disadvantages of the Gas TurbinePower plants.

Advantages

(1) Low Installation cost. Presently, the installation cost /MW capacity for a conventional fossil fuel power plant of unit size200/500 MW is nearly Rs 10 millions, whereas, it is only Rs. 6millions in case of combined cycle power plant of capacity 300 MWwhich comprises of two nos. Gas turbine of 100 MW each and asteam turbine of 100 MW. In case of simple cycle, Gasturbine powerplant installation costJMW capacity is only Rs 35 millions for a unitof size 100 MW.

(2) Higher Efficiency. Combined cycle plant efficiency is ofthe order of 42-47% which are nearly 10-20% more efficient thanfossil fuel conventional power plants.

(3) High Reliability/Availability. Combined cycle power plantis highly reliable to the extent of 85% to 90%. Some combined cyclepower plants achieved even 95% reliability for years long. As perthe North American Electric Reliability Council (NERC) which collectsand analyses data of such electricity generating plants, simple cycleGas turbine power plants achieved a reliability of 95·7%. Thesefigures are very high when compared to reliability figures of theorder of 65% generally achieved for conventional power plants.Meantime between failure CMTBF) which is mean operating durationbetween two forced outages, for Gas turbine power plant is above1000 hours whereas it is nearly 500 hrs for conventional powerplants.

(:AS TURBINE POWER PLANTS 631

(4) Low Gestation Time. The installation time for a simplecycle gas turbine power plant capacity can be installed in 16-18months and the rest of the capacity which is steam cycle plant canbe added in 12-14 months more. Some manufacturers keep gas turbineunits as off-shelf items, since gas turbines are of standard equipment.These durations are very much on favourable side when comparedto installation time of 48 and 60 months for conventional 200 MWand 500 MW plants respectively. This advantage contributes to lessinterest charges during construction and escalation of power plantcost due to inflation, faster returns on the investment and indirectlyhelps in improving the national economy in promoting the unrestrictedgrowth.

(5) Fast Starting Characteristics. Gas turbine power plantscan achieve full load within 20-30 minutes from cold start condition.Even in combined power plants, two-third (2/3) of full load can berealised within 20-30 minutes by operating gas turbines in simplecycle with the help of by-pass stack. Entire combined cycle plantcan be brought to full load within, 1/2 - 2 hrs (one and half-twohrs). This fast starting characteristics make them favourable to runas peaking power plants or two shifts in a day mode unlike inconventional thermal power plants.

(6) Less Water Requirements. Simple cycle gas turbine powerplants need neglibrible amount of cooling water for its auxiliariesonly. However, this water consumption may also'be eliminated byresorting to air cooling methods. Water consumption is nearly 40%of the requirement of conventional thermal power plants. In combinedcycle power plants since steam cycle generates only one third (1/3)of the total power output. If availability of water for condensercooling purpose in difficult even for this quantity, air cooled typecondenser can be envisaged. Efficiency deterioration due to highercondenser back pressure is not as much as in conventional powerplants due to only one-third (1/3) of power output contributed bysteam cycle in combined cycle power plants.

(7) Less Pollution Problems. Gas turbine power plants donot have dust pollution problem unlike in coal fired power plants.Thermal pollution is also comparatively less due to higher efficiencyin case of combined cycle plants. There is no ground water/waterpollution around ash disposal area due to ash dumping in coal firedpower plant. Pollution due to blow down from cooling water systemis also less when compared to conventional thermal power plants.

Though gas turbine power plants emit more oxides of nitrogen(Nox) when compared to conventional thermal power plants. Theseemissions can be controlled easily to/the acceptable levels by stearil!


water injection into gas turbine combustion chamber unlike inconventional thermal power plants.

(8) Flexibility in locating Power plants. Gas turbine powerplants can be located in areas where minimal infrastructural facilitiesare available. There is no absolute requirement of site to be connectedby rail road unlike in coal fired power plant to transport vast quantitiesof coal to the power plant.

One of the main factors to decide the location of a thermalpower station is availability of sufficient quantity of water for itsconsumption in the near vicinity. Since combined cycle power plantsneed only 40% of water requirement of conventional thermal powerplant, its flexibility with respect to water availability is more.Combined cycle power plants can be located even in deserts byenvisaging air cooled condenser with marginal loss in its thermalefficiency unlike conventional thermal power plants.

The requirement ofland for combined cycle power plant is only10 to 12% of coal fired power plant of equal capacity. There is norequirement of land for disposing ash and storing coal. Landrequirement for water storage is also less unless on perennial watersupply facility is existing. Less land requirement and minima] pollutionproblems favour these p]ants· to be located near load centres likecities.

(9) Less man/MW Ratio. Gas turbine power plants need lessman power to erect, operate, and maintain than conventional thermalpower plants. So human management problems are less comparatively.

(10) Clean operating Conditions. These power plants canbe kept in absolutely dean and tidy condition unlike coal fired powerplants which adds to the morale of work force.

Disadvantages

(1) Need of Good Quality Fuels. Gas turbine power plantscan be operated only with gaseous fuels mainly natura] gas or liquidfuels such as HSD, naptha, natura] Gasoline Liquid (NGL), heavyresidual oils etc., which are scarce in our country and their inevitable(or unavoidable) requirement in other industries such as petrochemical industry, fertilizer industry and transport industry.

Leser Plant Life. Gas turbine power plants have operatinglife of around 15-20 years when compared to 25 years for conventionalpower plants. Gas turbine power plants life depends mainly on thetype of fuel used and the actual combustion temperatures subjectedover design combustion temperature. With heavy residua] fuels such


as LSHS, HPS etc., gas turbine life deteriorates faster than withclear fuels such as natural gas, HSD etc.

Uneconomical Partial Load Operation. Gas turbine powerplants efficiency is considerably low when they are operated at partialloads. However, with inlet guide vane system, efficiency deteriorationcan be checked upto 80% ofrated load.

14·14. Prospects of Gas Turbine Power Plants in India

The application of gas turbine power plants can be foreseen inthe following fields in India:

(a) base load gas turbine power plants

(b) peak load gas turbine power plants

(c) Captive power combined cycle plants

(d) Retrofitting of old combined cycle and uneconomical powerplants.

(e) Co-generation gas turbine plants.

Base Load Gas Turbine Power Plants. Power supply situationin our country is becoming worse year by year due to under utilizationof existing capacity and faster pace of demand growth. Power shortagecan be mitigated on crash programmes by installing combined cycleutility power plants due to their less gestation periods and lowinstallation cost in this age of diminishing capital availability. Though,India has vast coal deposits but these deposits are mainly concentratedin central and eastern parts. Our coals have less calorific value andhigh ash content which cost excessively in transporting to the powerplants located in extreme south, south western regions western regionsand north western region. At present 40% of the rail freight ismainly due to coal transportation in India. Fortunately some of theabove regions have fairly good other hydro-carbon deposits such asoil and natural gas.

India's gas reserves have increased five folds during the lastten years and these are expected to increase further. Present gasreserves is of the order of 906·31 billion cubic metres by the end ofthe seventh plan. To exploit the natural gas reserves, pipe linenetworks interconnecting the production and consumption points tomeet the requirements of various industries are being contemplated.

Phase - 1 of this pipe network is based on the proven reserveswhile phase II & III are base on the additional proven reservesthrough conversion of prognosticated resources. Phase 1 is HBJpipe line from Hazira (Gujarat) to Babrala (D.P.) which will supply


N. G. (Natural Gas) to three combined cycle power plants of totalcapacity 1600 MW being under execution. There is further scope oftwo more combined cycle power plants of total capacity nearly 1000MW, since plants to install some gas based fertiliser plants alongthis pipe line are not materialising. Phase 1A is an extension ofHBJ pipe line from Auraiya to Kapurthala, while phase 1 B is apipe line connecting Bombay south terminal to Banglore which willcater natural gas to combined cycle power plants of capacity 2875MW. Phase II envisages a pipe line network extension in southernsector upto 'T'rivandrum and its connection to Northern pipe linenetwork, which will cater to 13 combined cycle power plants of totalcapacity 3875 MW. In phase III, the North East and Eastern part ofthe gas fields are proposed to be connected to the gas grid, creatingscope for some more combined cycle power plants.

So, it can be visualised that there is a scope of atleast 10,000MW capacity gas based combined cycle power plants in India by2000 AD.

There is scope for simple cycle gas turbine base load powerplants to utilize the associated gas from crude oil wells which willbe flared otherwise in Bombay High region and north eastern region,mainly due to their low gestation periods and least installation cost.One example of this type of power plant is Uran Gas Turbine PowerStation (MSEB), with installed capacity of 672 MW. In 1986-87nearly 2718 million cubic metres of Natural gas was flared which iscolossal wastage of natural resource.

Combined cycle power plants need not depend on the availabilityof liquid or gaseous fuels in entire future. Coal can be gasified toproduce lower caloric value gas which can be utilised in pit headcombined cycle power plants most economically for power generation.Coal gasification technology is on the threshold of commercialutilization. Uneconomical coal deposits by present technology whichare deep in earth and of less seam thickness, can be exploitedeconomically only by underground coal gasification technology,which will enhance scope for pit head combined cycle power plantsfurther.

Peak Load Gas Turbine Power Plants. These power plantsare mainly simple cycle gas turbine power plants because of theirshorter gestation period, low cost of installation and fast startingcharacteristics though their thermal efficiency is relativelyunfavourable. All large load centres in India, need this type of powerplants to stabilize the grid when frequency is falling either due tooverdrawing of power or less feeding to grid due to failure of fewoperating power plants. These power plants can also rectify the


complete grid failures very quickly since they can achieve their fullload within 20 minutes restoring partial. Supply to the grid cateringto essential load requirements and to restart the tripped powerplants. Gas turbine units operating on simple cyclewill be the idealsolution to act as spinning reserve to cater to peak demand anddemand-fluctuations of the grid.

The primary fuel for these plants will be gas or liquid fuel. Thegenerator of these gas turbine power plants can also be utili sed assynchrounous condenser to improve power factor of the grid, whengas turbine power plants are not generating power.

Captive Power Combined Cycle Power Plants. When unitcapacities are below 100 MW, combined cycle power plants are bestsuitable to generate electricity at lesser cost and than coal firedconventional thermal power plants even at existing liquid/gaseousfuel prices in India due to their higher efficiency of the order of 4045%, low cost of installationlkW and high reliability. At present,unit capacities of the order of 100 MW and below are being mainlyinstalled as captive power plants since most of the regional grids inIndia can accommodate larger size single unit of 200 MW and more.

Retrofitting of old and uneconomical power plants. Someof the power plants which can not generate electricity at economicalcost due to their less design thermal efficiencycan be converted intocombined cycle power plants. This modification can be done byreplacing the existing steam generation by HRSG (Heat recoverysteam generator) and Gas Turbine, such that existing steam cyclefacilities can be utilised as bottoming cycle to the gas turbine.Sometimes, it is also possible to use Gas turbine exhaust gases as apractical source of heat energy in already existing coal fired steamgenerator by doing moderate alternations in the steam generator.

Co-Generation Gas Turbine Power Plants. Efficiencies ofthe order of 80-85% can be achieved in these power plants. Thesepower plants find its application in process industries likepetrochemical, fertilizers, paper industries etc, where large quantitiesof steam and auxiliary power are required. In some applicationsinstead of producing steam by the gas turbine exhaust gases, exhaustgases can be used directly for heating\ requirement such as incentralised adsorption, refl;geration/air-~onditioning plants, food!processing, plastic industries etc. The cost of power generated bythese plants is less than the larger size utility coal fired powerprojects.

Co-generation systems are not only decentralised but alsointegrated systems of energy based on the total energy concept.


These systems are ideally suited for process industries such as sugar,paper, petrochemicals, fertilizers and several other industries whichrequire both process heat and electricity.

Several variations in gas turbine co-generation systems suchas supplementary firing and the waste heat recovery boilers to satisfythe varying or peak energy (either heat or electricity) loads, backpressure steam turbine in case of the combined cycle, duel fuelsystems etc. are possible making their application broad based. In arecent study conducted by Haigler Baily & Co. (USA) and NationalProductivity council for the Department of Non-conventional Energysources, it has been estimated that the two states of Gujarat andMaharashtra alone have a co-generation potential of more than 2000MW. The above study has further established that Gas Turbineswith Co-generation are generally more economical than the otherco--generation methods adopted by many of our industries.

Questions

14·1. Describe with the help of a suitable sketch, the operation of a continuouscombustion, constant pressure gas turbine.

14·2. A gas turbine plant delivers 1712 kW (200 hop) and operates such thatinlet pressure and temperature at the compressor is 9·807 N/sq em(1 kg/sq cm) and 15°C, and that of turbine is 39·23 N/sq.cm. (4 kg/sq.cm) and 700°C. Calculate the isentropic efficiency of the turbine andthe requisite mass flow of air in kg/sec if the compressor efficiency is85% and overall thermal efficiency is 21%. (Ans. 81%, 9·57 kg/sec.)

14·3. In a continuous combustion constant pressure, gas turbine, air is takeinto a rotary compressor at a pressure of 100 kN/m2 and temperature18°C. It is compressed through a pressure ratio of 5 : 1 with an isentropicefficiency of 85%. From the compressor, the compressed air is passed toa combustion chamber, where its temperature is raised to 810°C. Fromthe combustion chamber, the high temperature air is passed to a gasturbine in which it is expanded down to 100 kN/m' with an isentropicefficiency of 88%. From the turbine, the air is passed to exhaust. Ifthe air used is 4·5 kg/s and neglecting the mass of fuel as small,determine,

(a) the net power output of the turbine plant if the turbine is coupledto the compressor;

(b) the thermal efficiency of the plant. (Ans. 688 kW, 25·7%)

14·4 In a gas turbine plant, working on the Brayton cycle with a regeneratorof 75% effectiveness, the air at the inlet to the compressor is at 0·1MPa, 30°C, the pressure ratio is 6, and the maximum cycle temperatureis 900°C. If the turbine and compressor have each an efficiency of 80%,find the percentage incrpase in the cycle efficiency due to regeneration.

(Ans. 42·56%)

14·5. In a gas turbine plant working on the Brayton cycle the air at the inletis at 27°C, 0·1 Pa. The pressure ratio is 6·25 and the maximum


temperature is 800°C. The turbine and compressor efficiencies are each80%. Calculate:

(i) the compressor work per kg of air,

(ii) the turbine work per kg of air,

(iii) the heat supplied per kg of air

(iv) the cycle efficiency, and

(v) the turbine exhaust temperature.

(Ana. (i) 259·4 k.Jlkg (ii) 351·68 kJlkg (iii) 569·43 kJlkg(iv) 16·2% (v) 723 K)

14·6. In a Brayton cycle gas turbine plant, the air from the compressorpasses through a heat exchanger heated by the exhaust gases from thelow pressure turbine, and then into the high pressure combustionchamber. The high pressure turbine drives the compressor only. Theexhaust gases from the high pressure turbine pass through the lowpressure combustion chamber to the low pressure turbine which iscoupled to an external load or generator. The following data refer tothe plant.

Pressure compression ratio in the compressor, 4 : 1

Isentropic efficiency of compressor;

Isentropic efficiency of H.P. turbine,

Isentropic efficiency of LP turbine,

Mechanical efficiency of drive to Compressor

0·86

0·84

0·80

0·92

In the heat exchanger 75% of the available heat is transferred to theair.

Temperature ofthe gases entering HP turbine,

Temperature of gases entering LP turbine,

Atmospheric temperature and pressure are 15°C and 1·03 kg flcm'(0·1 MPa) respectively.

Assuming that the specific heat of air and gas is 0·24(1·03 kJlkg K).Determine

14·7.

14·8.

14·9.

14·10.

14·11.

(i) the pressure of the gases entering the low pressure turbine;

(ii) the overall efficiency. (Ans. 1·6555 kgllcm' 25·3%)

Describe briefly a closed cycle gas turbine plant. What are the advantages

of closed cycle? .

What are the main fuels which arc used for gas turbine'~lant?

Describe the recent developments introduced in the sim9;e gas turbinecyclE)and the result of each on plant heat rate. /

What are the important considerations to be taken account while decidingabout layout of a gas turbine power plant.

Describe the controls and auxiliaries necessary in a gas turbine plant.How is the plant started and what are the safety devices employed?


14·12. What are the fuels used in gas turbine plants and what fuel characteristicssuit such plants best? Discuss the recent trends to use solid fuels insuch plants.

14·13. What are combination cycles and why have these been developed?

Describe the principal combination cycles using gas turbine-cum-steamplants for power production with advantages of each.

14·14. Compare the gas turbine plants with steam turbine power plants anddiesel power plants.

Objectives Type Questions

14·1. Thermodynamic cycle on which a gas turbine works:

(a) Brayton or Atkinson cycle(b) Rankine cycle (c) Joule cycle(d) Erricson cycle.

14·2. Open cycle gas turbine works on cycle.

(a) Brayton or Atkinson(c) Joule cycle

(b) Rankine cycle(d) Erricson cycle.

(b) lower

14·3. The thermal efficiency of gas turbine plants is as comparedto condensing steam plants.

(a) higher(c) same.

14·4. Gas turbines for power generation are normally used .

(a) to supply peak load requirements(b) to supply base load requirements(c) both a and b.

14·5. The air fuel ratio in a gas turbine is of the, order of

(a) 7: 1(c) 50: 1

(b) 15: 1(d) 120: 1

14·6. The pressure ratio in open cycle gas turbine is of the order of

(a) 12: 1(c) 18: 1

(b) 9: 1(d) 6: 1

14·7. For starting gas turbine, the turbine rotor is usually motoredupto 'coming in' speed which is equal to

(a) rated speed of the gas turbine

(b) t of the rated speed of the gas turbine(c) no relation with speed of the turbine.


(b) high

(b) decreases

(b) thermal efficiency

(b) reheating(d) both a and b

14·8. The pressure ratio for an closed cycle gas turbine comparedto open cycle gas turbine of some power is .

(a) low(c) same.

14·9. The thermal efficiency of a simple gas turbine for a giventurbine inlet temperature with increase in pressure ratio:

(a) increases(c) remains same.

14·10. In gas turbines, high thermal efficiency is obtained in .....

(a) open cycle (b) closed cycle(c) in both the cycles.

14·11. Efficiency of the gas turbine cycles increased by .

(a) regeneration (b) intercooling(c) reheating (d) all of the above.

14·12. The blades of the gas turbine rotor are made of

(a) carbon steel (b) high alloy steel(c) stainless steel(d) high nickel alloy (Nimic 80).

14·13. In a gas turbine plant, a regenerator increases .

(a) work output(c) pressure ratio.

14·14. Work output of the gas turbine cycle is increased byemploying ....

(a) inter cooling(c) regeneration(e) a, band c.

14·15. Maximum temperature in a gas turbi~ is of the orderof '

(a) 700°C(c) 1500°C

(b) 1000"C(d) 2000°C.

14·16. The fuel for gas turbine can be .

(a) coal gas(c) producer gas(e) anyone of the above.

(b) blast furnace gas(d) pulverized coal

640

~..', .


(b) lobe(d) centrifugal

14·17. In a gas turbine the type air compressor can be employed.

(a) reciprocating(c) axial flow(e) cor d any.

14·18. The combustion efficiency of a gas turbine using a goodcombustor is of the order of .

(a) 80% (b) 90% (c) 98%.

14·19. Maximum combuxstion pressure in a gas turbine is ascompared to diesel engine.

(a)less (b)more (c)same.

14·20.Capital cost of a gas turbine plant is ............ than that of a

steam power plant of same capacity(a)

higher (b)lower (c)same

Answers1.

(a) 2.(a) 3. (b)4.(a) 5.(c)6.

(d) 7.(b) 8. (b)9.(c) 10.(b)

11. (d)12. (d)13. (b)14.(d) 15.(a)

16. (e)

17. (e)18. (c)19.(a) 20.(b).

Documents

Gas Turbine Power Plants - · PDF file14 Gas Turbine Power Plants 14·1. Introduction The gas turbine in its simplest form is a heat engine operating by means of a series of processes