Combined-Cycle Gas and Steam Turbine Power Plants

Copyright O 1997 by PennWell Publishing Company 142 1 South Sheridafl.0. Box 1260 Tulsa, Oklahoma 74101

ISBN 0 - 8 7 8 1 4 - 7 3 b - 5

All rights reserved. No part of this book may be reproduced, stored in a retrieval system, or transcribed in any form or by any means, electronic or mechanical, including photocopying and recording, without the prior written permission of the publisher.

Printed in the United States of America.

CONTENTS

...................................................... ogeneration

pter 5 ....................................................... Components

Combined-Cycle Plants for

......................... trol and Automation

......... rating and Part-Load Behavior..

arison of The Combined-Cycle Plant Other Thermal Power Stations .......

onmental Considerations.. ..............

pmental Trends ............................

ical Combined-Cycle ready Built.. .............................

Chapter 12 ....................................................... Conclusions 353

............................................................ Conversions.. 355

Symbols Used.. .......................................................... 357

Indices Used ............................................................. 359

Appendix 1 ................................................................ 3f3

................................. Definition of Terms and Symbols 371

Bibliography ............................................................. 37'7

Chapter I

INTRODUCTION

rature has often suggested combining two or more ther- ithin a single power plant. In all cases, the inten- crease efficiency over that of single cycles. Thermal n be combined in this way whether they operate e or with differing working media. However, a com-

cycles with different working media is more inter- se their advantages can complement one another.

the cycles can be classed as a "topping" and a "bot- . The first cycle, to which most of the heat is sup- the "topping cycle." The waste heat it produces in a second process which operates at a lower

vel and is therefore referred to as a "bottoming

on of the working media makes it possible to process that makes optimum thermodynamic the upper range of temperatures and returns

nvironment at as low a temperature level the "topping" and "bottoming" cycles are

h e , only one combined cycle has found e combination gas turbinelsteam turbine lants of this type have burned generally y-liquid fuels or gases.)

2 COMBINED CYCLE GAS & STEAM TURBINE POWER PLANTS INTRODUCTION 3

Fig. 1 is a simplified flow diagram for an installation of this It therefore is quite reasonable to use the steam process for type, in which an open-cycle gas turbine is followed by a steam process. The heat given off by the gas turbine is used to gen- ine power plants were not more widely used even earlier erate steam. clearly been due to the historical development of the gas

ine. Only in recent years have gas turbines attained inlet Other combinations are also possible, e.g., a mercury vapor

process or replacing the water with organic fluids or ammonia.

The mercury vapor process is no longer of interest today since even conventional steam power plants achieve higher efficienc-

orld totals more than 30,000 MW.

ies. Organic fluids or ammonia have certain advantages over water in the low temperature range, such as reduced volume flows, no wetness. However, the disadvantages, i.e., development costs, environmental impact, etc., appear great enough to prevent their ever replacing the steam process in a combined-cycle power plant. The discussion that follows deals mainly with the combination of an open-cycle gas turbine with a waterhteam cycle. Cer- tain special applications using closed-cycle gas turbines will also be dealt with briefly.

Why has the combination gas turbinehteam turbine power plant, unlike other combined-cycle power plants, managed to find wide acceptance? Two main reasons can be given:

@ It is made up of components that have already proven themselves in power plants with a single cycle. Devel- opment costs are therefore low.

The steam process uses water, which is likewise inexpensive flow diagram of a combination gas turbinelsteam turbine power

and widely available, but better suited for the medium and low temperature ranges. The waste heat from a modern gas turbine

4. Steam turbine has a temperature level advantageous for a good steam process. 5. Condenser

6. Fuel supply

Chapter 2

MODYNAMIC PRINCIPLES OF PLANT

Considerations

t efficiency is the maximum efficiency of an ideal

Carnot efficiency

Temperature of the energy supplied

Temperature of the environment

the efficiencies of real processes are lower since s involved. A distinction is drawn between energetic losses. Energetic losses are mainly heat n and convection), and are thus energy that is ess. Exergetic losses, on the other hand, are in- sed by irreverisible processes in accordance with of thermodynamics [I].

o major reasons why the efficiencies of real pro- er than the Carnot efficiency:

erature differential in the heat being supplied ry great. In a conventional steam power plant,

e maximum steam temperature is only about

6 COMBINED CYCLE GAS & STEAM TURBlNE POWER PLANTS

810K (980°F), while the combustion temperature in the boiler is approx. 2000 K. Then, too, the temperature of the waste heat from the process is higher than the ambient temperature. Both heat exchange processes cause losses.

The best way to improve the process efficiency is to reduce these losses, which can be accomplished by raising the maximum temperature in the cycle, or by releasing the waste heat at as low a temperature as possible.

The interest in combined-cycles arises particularly from these two considerations. By its nature, no single cycle can make both improvements to an equal extent. It thus seems reasonable to combine two cycles: one with high process temperatures, and the other with a good cold end.

In an open-cycle gas turbine, the process temperatures attainable are very high because its energy is supplied directly to the cycle without heat exchangers. The exhaust heat temperature, however, is also quite high. In the steam cycle, the maximum process temperature is not very high, but the exhaust heat is returned to the environment on the cold end at a very low temperature.

Combining a gas turbine and a steam turbine thus offers the best possible basis for a high-efficiency thermal process (Table 2-1).

The last line in the table shows the "Carnot efficiencies" of the various processes, i.e., the efficiencies that would be attainable if the processes took place without internal exergetic losses. Although that naturally is not the case, this figure can be used as an indicator of the quality of a thermal process. The value shown makes clear just how interesting the combined-cycle power plant is when compared to the single-cycle processes. Even a sophisticated installation such as a reheat steam turbine power plant has a theroretical Carnot efficiency 10 to 15 points lower

MODYNAMIC PRINCIPLES OF THE COMBINED-CYCLE PLANT 7

hat of a combined-cycle plant. On the other hand, the ex- c losses in the combined cycle are higher because the tem-

e differential for exchanging heat between the exhausts e gas turbine and the waterlsteam cycle is relatively great. s clear why the differences between the actual effici- ttained by a combined-cycle power plant and the other s are not quite that large.

n by Fig. 2-1, which compares the temperaturelentropy f the four processes, the combined cycle best utilizes ature differential in the heat supplied, even though additional exergetic loss between the gas and the

-1: Thermodynamic Comparison of Gas Turbine, Steam Turbine, and Combined-Cycle Power Plants

Reheat Reheat Power Plant

ed, in K 950 - 1000 640 - 700 550 - 630 950 - 1000 (1250 - 1340) (690 - 800) (530 - 675) (1250 - 1340)

500 - 550 320 - 350 320 - 350 320 - 350 (440 - 530) (115 - 170) (115- 170) (115- 170)

10 COMBINED CYCLE GAS & STEAM TURBINE POWER PLANTS

Combining these two equations yields:

2.2.1 The Effect of Additional Firing in the Waste Heat Boiler on Overall Efficiency

Substituting Equations (4) and (7) into Equation (2), one obtains:

VGT GT + VST (QSF + Q G T [ I - VGTI) TIK =

(8) QGT + QSF

Additional firing in the waste heat boiler improves the overall efficiency of the combined-cycle installation whenever:

Differentiation of Equation (8) produces the inequality:

a V S T @GT + QSF) - VST QSF + [= QGT (1 - VGT)] - (10)

G ~ + b F ) - V ~ Q G T (1- vGT)J > O

This yields:

RMOD YNAMlC PRlNClPLES OF THE COMBINED-CYCLE PLANT I I

nee the second term of the inequality is equal to I(, the in- ality reduces to:

(12)

)I is none other than the heat to the steam cycle. The formula thus becomes:

(13)

tion (13) means that increasing the additional firing im- the efficiency of the combined-cycle plant only if it im- the efficiency of the steam process. The greater the

nee is between the efficiencies of the combined-cycle and er the temperature is of the heat

the steam process, the more effective that improve- Ll be. For that reason, additional firing is becoming less

cy of the combined-cycle instal- eases far more rapidly than that of the steam process, y increasing the difference (TK - ST). In view of the

is generally better to burn the modern gas turbine, because the heat is supplied to

ess at a temperature level higher than that in the steam

bined-cycle installations with firing are discussed in more detail in Section 3.2 below.

12 COMBINED CYCLE GAS & STEAM TURBINE POWER PLANTS MODYNAMIC PRINCIPLES OF THE COMBINED-CYCLE PLANT 13

2.2.2 Efficiency of Combined-Cycle PlanLs without 2-2: Allowable Reduction in Steam Process Efficiency Additional Firing in the Waste Heat Boiler as a Function of Gas Turbine Efficiency (Steam

Without additional firing, Equation (8) can be written as fol- process efficiency = 0.25)

lows: (14)

~ G T 0.2 0.3 0.4 VGT - QGT + VST . QGT (1 - TIGT) = sGT + (1 - sG VK =

QGT - - *ST 0.94 1.07 1.25

Differentiation makes it possible to estimate the effect that a change in efficiency of the gas turbine has on overall efficiency:

a 7 7 ~ = 1 + --- a VST (1 - VGT) - VST (15) in the steam cycle. But a gas turbine with a maxi-

a VGT a VGT ciency still does not provide an optimum combined- nt. For example- with a constant turbine inlet

Increasing the gas turbine efficiency improves the overall ef- ure- a gas turbine with a very high pressure ratio at-

ficiency only if: her efficiency that a machine with a moderate pres- wever, the efficiency of the combined-cycle plant

a r]K (16) nd machine is sigmficantly better because the steam > 0 a VGT ollows operates far more efficiently with the higher temperature and produces a greater output.

From Equation (15) one obtains: I ows the efficiency of the gas turbine alone as a

afl < 1 - ssr (17) e turbine inlet and exhaust temperatures. The max-

-- 1 - ~ ] S T cy is reached when the exhaust gas temperatures

~ G T (A low exhaust temperature means a high pres- Improving the gas turbine efficiency is helpful only if it does

not cause too great a drop in the efficiency of the steam Process.

ST ws the overall efficiency of the combined-cycle

Table 2-2 shows the maximum allowable reduction- - y. Compared to Fig. 2-2a, the optimum point has as a function of the gas turbine efficiency. ~ G T higher exhaust temperatures from the gas tur-

omical considerations, present-day gas turbines This table indicates that the higher the efficiency of the gas imized with respect not to efficiency but to max-

turbine, the greater may be the reduction in efficiency of the nsity. Fortunately, this optimum coincides fairly steam process. The proportion of the overall output being pro- h the optimum efficiency of the combined-cycle vided by the gas turbine increases, reducing the effect of lower It- most of today's gas turbines are optimally

ined-cycle installations.


Gas turbines of a more complicated design, i.e., with intermediate cooling in the compressor or recuperator, are less suitable for combined cycles. They normally have low exhaust gas temperatures, so that the efficiency of the steam turbine can only be low. We shall not discuss a reheat gas turbine here since this type of machine has disappeared from the market due to its complexity. a

In summary, it may be said that:

The gas turbine with the highest efficiency does not necessarily produce the best overall efficiency of the combined-cycle plant. The turbine inlet temperature is a far more important factor.

Similar considerations also apply with regard to the efficiency of the steam cycle. These, however,are less important because the gas turbine is generally the "standard machine." The exhaust heat available for the steam process is thus a given, and the problem lies only in its maximum conversion into mechanical energy (refer on this point to Section 2.3.)

b

Efficiency of Gas Turbines in cornbined-Cycle the Turbine Inlet and Exhaust Gas T~~~~~~~~~~~

18 COMBINED CYCLE G A S & STEAM TURBINE N I W E R H A N T S SYSTEM LAYOUTS 19

In addition to this physical limitation, there is also a chemical- Combined-cycle plants without additional firing often are made limitation on energetic use of the exhaust gases imposed by low P of several gas turbines and waste heat boilers that supply temperature corrosion. This corrosion, caused by sulphur, oc- to a single steam turbine. In the following, we generally curs whenever the exhaust gases are cooled below a certain tern- only of one gas turbine and one waste heat boiler, but perature, the sulphuric acid dewpoint. youts can also be adapted for several gas turbines. Because

implest system is typical of all, it has been discussed more In a waste heat boiler, the heat transfer On the flue gas side tail, and the other possibilities have then been derived from

is not as good as on the steam or water side. For that reason1 the surface temperature of the pipes on the flue gas side is aP- proximately the same as the water or steam temperature. If these ingle-Pressure System pipes are to be protected against an attack of low temperature corrosion, the feedwater temperature must remiin approximately plest arrangement for a combined-cycle plant is a single- as high as the acid dewpoint. Thus, a high stack temperature for the flue gases does no good if the temperature of the feed- f one or more gas turbines with a single-pressure waste water is too low (refer also to Section 5.2). Low temperature corrosion can occur even when burning fuels containing no sul- and a single-stage feedwater preheater in the de- phur if the temperature drops below the water dewpoint. e Steam for the deaerator is tapped from the steam

3.1 Combined-Cycle Plants without Additional Firing ste heat boiler consists of three paas: In combined-cycle plants without additional firing, all the fuel

is burned in the gas turbine. The steam turbine then utilizes the dwater preheater (economizer), which is by the flue gases;

exhaust heat from the gas turbine, with no additional source of thermal energy. This type of combined-Cycle plant is already in widespread use because it is simple and inexpensive and high efficiencies can be attained with modern gas turbines.

ural circulation.

combined-cycle plants is quite large because attempts have be made to improve the quality of the heat exchange between t Ble-Pressure System

flue gas and the water or steam by using complex systems. s the heat balance in a typical single-pressUre has led to systems that utilize the exhaust heat well both e getically and energetically. enerator aPProx. 35 kgls (277,200 lb/hr) steam

with an output of 35 MW. Because of the good


Figure 3-1 SYSTEM LQYOUTS 21

- HEAT TRANSFER

Fig. 3-1: TemperatureEXeat Diagram: Ideal Heat Exchange

22 COMBINED CYCLE G A S & STEAM TURBINE POWER PLANTS

Figure 3-3

SYSTEM LAYOUTS 23

Fig. 3-3: Flow diagram of the single-pressure system

1 Compressor 6 Economizer ll Feedwater tank/ 2 Gas turbine 7 Boiler drum deaerator 3 Bypass stack 8 Steam turbine 12 Feedwater pump 4 Superheater 9 Condenser 13 Condensate pump 5 Evaporator 10 Steam bypass

24 C O M B I N E D CYCLE G A S & STEAM TURBINE POWER P U N T S SYSTEM U Y O U T S 25

hewn in Fig. 3-7, the smallest temperature differ- river-water cooling system, pressure in the condenser is 0-04 between the water and the exhaust gases in the bar (0.58 psia), resulting in a gross efficiency of the irlstauation omizer is on the warmer end of the heat ex-

of 45% (Table 3-1, page 30). ger. 'I'hat means: the amount of steam production lble does not depend on the feedwater tempera- . In a conventional steam generator, on the other

Noteworthy is the poor energetic utilization of the exhaust the sr.nallest temperature difference is on the heat from the gas turbine. Together with the relatively low live end of the economizer because the water flow steam data, this produces a fairly modest efficiency in the stearn larger in proportion to the flue gas flow. AS a process. Fig. 3-5 shows the energy flow. t, the WnoUnt of steam production possible de-

s on the feedwater temperature.

45% of the thermal energy supplied is converted into electri- wS two examples of conventional steam generators cal energy. The rest is removed in the condenser (28.3%) or g feed-water temperatures. ~t is obvious that with

through the stack (25.2%) or is lost elsewhere (1.5%). rmce in temperature at the end of the econo-

Fig. 3-6 shows the exergy flow of the Same plant. The heat t available for evaporation and superheating is

that has to be removed in the condenser is only about half that greater where the feedwater temperature is high-

0s a conventional steam power plant of the same size. m ~ o u n t of live steam produced by a conventional increased by raising the feedwater temperature.

One significant difference between a Conventional steam plant and the steam process in a combined-cycle plant lies in the boil rnbient Conditions

f eedwater preheating. A conventional Steam plant attains a ter efficiency if the temperature of the feed-water is brou s here only the effect that different ambient to a high level by means of multi-stage preheating. In a c on the design point for the installation. HOW

bined-cycle power plant, however, the boiler feedwater m sioned combined-cycle plant behaves will be be as cold as possible, with the limit determined by low te on 7, Operating and Part-Load Behavior. Those perature corrosion: the temperature of the water must not e valid, however, only for the steam turbine significantly below the dewpoint for sulphuric acid. There a two reasons for this difference:

mbient conditions. This can be justified 0 Normally, a conventional steam generator is equippe

with a regenerative air preheater that can further ut gas turbine that has been optimized for

ilize the energy remaining in the flue gases after the f 15°C (59°F) does not look significantly economizer. There is nothing like that in a waste he t has been designed for, say, 40°C (104°F). boiler, so that the energy remaining in the ~xhaus t ng a new machine would thus not be gases after the economizer is lost.

26 COMBINED CYCLE GAS & STEAM TURBINE POWER PLANTS SYSTEM LAYOUTS 27

Figure 3-5

30,l %

25,2 1 4 , 9 7 4

Fig. 3-5: Energy Flow Diagram for the Single-Pressure Combined-Cycle Plant

Q Energy input Diagram of the Single-Pressure Combined-Cycle Plant V1 Loss in condenser V2 Loss in stack V3 Loss due to radiation in waste heat boiler V4 Loss in flue gas bypass V5 Loss in generator and radiation, gas turbine V6 Loss in generator and radiation, steam turbine GT Electricity produced in the gas turbine ST Electricity produced in the steam turbine

e steam turbine ste heat boiler

SYSTEM LQYOUTS 29


Table 3-1: Main Technical Data of the Single-Pressure Increasing the air temperature reduces the density of Combined-Cycle Plant the air, and thereby reduces the air mass flow drawn

y the compressor increases in Gas turbine output take temperature (in K), without Steam turbine output being a corresponding increase in the output

Station service power required

Net power output of plant 101 500 kW capacity of the turbine re- 228 000 kW pressure before the turbine is re-

Thermal energy supplied (Diesel fuel) decreases as the air Efficiency of gas turbine in reduces the pressure ra-

157 000 kW same principle applies in- Heat contained in exhaust gases compressor, but because its Utilization rate for waste heat energy* an of the turbine, the total bal- Efficiency of the steam process

Gross efficiency of the plant ows this change in a temperaturelentropy diagram. s that the exhaust gas temperature becomes higher

ses. This is because the turbine pres- * 100% utilization if the exhaust gases are cooled down to 15 OC (59 OF duced while the inlet temperature remains con-

avior of the exhaust gas temperature explains The situation is different on the steam end of the steam tu that the air temperature has on the efficiency

cycle plant differs from that which it has on the sure of, say, 0.2 bar (2.9 psia) can no longer function Proper he gas turbine alone. if the pressure is only 0.04 bar (0.58 psia).

e efficiencies of the gas turbine and plant as a function of the air temperature,

the air temperature, air pressure, and cooling water temper aining otherwise unchanged. As it ture. The relative humidity is important only if the water the air temperature even has a slightly cooling the condenser is recooled in a wet Cooling tower. ficiency of the combined-cycle plant,

erature in the gas turbine exhaust rai- Air Temperature team process (Fig. 3-1 1) enough to more

There are three reasons why the air temperature has a 1 uced efficiency of the gas turbine influence on the power output and efficiency of an open-c

urprising when one remembers the Car- n (1)). The rise in the final temperature

SYSTEM U Y O U T S 33

34 COMBINED CYCLE GAS & STEAM TURBINE POWER PUNTS

Figure 3-11

-20 -10 0 + I 0 + 2 0 3 0 40 5 0

A I R TEMPERATURE

Fig. 3-11: Relative Efficiency of the Steam Process in Combined-Cycle Plants Function of the Air Temperature Cooling water temperature 20°C (68°F)

SYSTEhd LAYOUTS 35

ression causes a slight increase in the average temper-

es only if the temperature of the water cooling the

air-cooled condenser, the efficiency of the steam nges because the condenser pressure is now different.

nges with the air temperature when the cooling

e case with direct air-cooled condensation.

m its efficiency. Here the reduced flows of air es play a more important role than the exhaust

how the power outputs of the gas turbine and le plant change depending on the air temper-

ite Elevation

e air pressure on the efficiency of a gas tur- ro if the temperatures remain unchanged. On

36 COMBINED CYCLE GAS & STEAM TURBINE POWER PLANTS SYSTEM LA YOUTS

taken in, which varies in proportion to the intake pressure a thereby also affects the flow of exhaust gas. The exhaust heat

well to the real situation, this then causes a similar variati in the power output from the steam turbine.

Because the power outputs of the gas turbine and the st turbine vary in proportion to the air pressure, the total po output of the combined-cycle plant varies correspondingly. efficiency of the plant remains constant, however, since the thermal energy supplied and the air flow are varying in p portion to the air pressure.

Cooling media for the Condenser

To condense the steam, a cooling medium must be used to car A I R TEMPERATURE

which has a high specific thermal capacity and good heat tra perature on the Efficiency of Combined-Cycle Plants

fer properties. Where water is in short supply, cooling can done in air in a wet cooling tower; where no water is availab an air-cooled condenser or a dry cooling tower are necessa

The temperature of the cooling medium affects the efficien

producing a greater useful enthalpy drop in the steam turbi

pressure as a function of the design temperature for the cooli medium. There are three different cases:

1 O C

with


Figure 3-13

-20 -10 0 + I 0 + 2 0 3 0 40 5

A I R T E M P E R A T U R E

Fig. 3-13: Effect of the Air Temperature on the Efficiency of Combined-Cycle with Direct Air-Cooled Condensation

SYSTEM LAYOUTS 39

SYSTEM LAYOUTS 41

42 COMBINED CYCLE GAS & STEAM TURBINE POWER PUNTS SYSTEM UYOUTS 43

@ direct water cooling e Steam Pressure water cooling, with water recooled in a wet cooling steam pressure does not tower . 3-17 shows how the ef-

@ direct air cooling on the live steam pres- is striking that the best efficiency is attained even while

The greatest vacuums are attained with direct water cooling, steam pressure is quite low. the least with direct condensation with air. In the comparison, it must also be borne in mind that the water temperature is gen an increased efficiency erally lower than that of the air. steam cycle due to the greater enthalpy gradient

. The rate of waste heat energy utilization in the For the wet cooling tower, a relative air humidity of 60% h , however, drops off sharply. The overall effici-

been assumed. am process is the product of the rate of energy

The Effect of the Most Important Design Parameters the efficiency of the waterlsteam cycle. There

on Power Output and Efficiency at approx. 30 bar (435 psia).

When dimensioning a combined-cycle plant, the gas t of energy utilization in design is generally a given, since the gas turbine is a sta oiler: the temperaturelheat diagrams are for dized machine. of 15 and 60 bar (203

e steam pressure, there The free parameters for the design involve the steam pr energy available for evaporation and super-

and it is mainly these that are discussed below. One mu ure is correspondingly forget, however, that the output of the steam turbine is int of the evaporator is the same in both approx. 30 to 40% of the total power output. Optimizati ce area of the heat exchanger is therefore the steam process can therefore only influence that porti result, the stack temperature at 15 bar is

Another important point: The efficiency of the steam r than at 60 bar, which means that the eing better utilized.

is always proportional to the output of the steam turbine, si in a plant without additional firing, the thermal energy supp lso greatly affects the to the steam process is a given. ved in the condenser (Fig. 3-19). The

en pressures are lower since a greater Live Steam Data removed from the exhaust gases and

The selection of the live steam data for a combined-cycle p at a lower efficiency. with a single-pressure system is a compromise between o mum energetic and optimum exergetic utilization of the ke it advisable to raise heat from the gas turbine. The main determining factor is e the thermodynamic optimum. This live steam pressure selected.

44 COMBINED CYCLE G A S & STEAM TURBINE POWER PLAN SYSTEM LAYOUTS 45

@ a reduction in the exhaust steam flow, or, if the si of the steam turbine remains unchanged, smaller e i~nproved efficiency of the waterlsteam cycle haust losses. ompensates for the slight drop in the rate of

@ a smaller condenser y utilization. Moreover, for the steam turbine, @ a reduction of the cooling water requirement e steam temperature means less erosion in the

use of the reduced water content in the steam). Especially in the case of power plants with expensive air

condensers, this can mean considerably lower costs. re of the gas turbine exhaust gas provides the

Live steam flows greater than that in the example sho in temperature is necessary between the ex- the optimum toward higher live steam pressures, since live steam in order to limit the size of the

over, too high a live steam temperature can rtionate increase in plant costs since a great

combined-cycle plants with several gas turbines to sele team turbine. In most cases, however, the steam pressure that is above the optimum. The reduced rature sets the limit for the live steam tem- flow that results makes it possible to employ piping and

of live steam is reduced. The optimum live steam pres depends on the total amount of live steam: increasing t ood rate of waste heat energy utilization, improves efficiency in the high pressure section of e feedwater should be kept as low as pos- turbine. With a larger volume flow, longer blades are re in the first row, which reduces the edging losses.

Live Steam Temperature in Section 3.1.1, preheating has been In contrast to the live steam pressure, raising the live

temperature always brings with it a slight increase in effic Id improve the efficiency but it has not (Fig. 3-20). There are two reasons for this improvement because the solutions shown in Sections increased superheating: arly better. Dividing preheating into sev-

mprove the rate of energy utilization in this improved thermodynamics of the cycle, e system, which is the greatest disadvantage increased steam turbine efficiency due to reduc m. Even with minimum feedwater temper- wetness in the low pressure section. mperature remains at approx. 200 O C (392


Figure 3-21

SYSTEM UYOUTS 51

nser Pressure

nce on the effici- py drop in the steam

anges sharply (Fig. 3-22). An increase in the pressure decrease in power output. However, plant costs are

nt of the Waste Heat Boiler

inch point") of the er, which affects the amount of steam generated -7). By reducing the pinch point, the rate of en-

However, the surface of the heat exchanger in- entially, which quickly sets a limit for the utiliza-

he waste heat boiler should be such that the s as low as possible. This nd efficiency of the gas the turbine. In present-

oss is approximately 0.8% for each 1% recovered in the steam

rate of recovery is 35%.

of the gas turbine exhaust gas is im- cle. If the turbine inlet ine with a higher ex-

Fig. 3-21: ~ f f ~ ~ t of the Feedwater Temperature trw on the r overall efficiency produces process and Rate of Waste Heat Energy Utilization tical cornpressor and

tfw = Feedwater Temperature [Other terms as in Fig. 3-17]

bar

SYSTEM M Y O U T S 57

a Low

I-)


10% of that required for the version using w , however, is low compared to the improvement in ator itself can be of the natural circulation or the for

mplicated and accordingly low in cost. Even if the very high levels of sulphur, the feedwater can be

In this second design, it is sometimes possi sufficiently high a temperature without any re- arate low pressure drum. The feedwater tank the ficiency worth mentioning. as a low-pressure drum, resulting in a simp feed pumps or drum level controls are required. ows the temperature/heat diagram for the waste cause of the two-phase flow, special care he exhaust gases are cooled by approximately an designing the piping and the introduction of the O C (90 OF') in the preheating loop in order to warm. mixture into the feed-water tank. te to 130 O C (266 O F ) .

Example of a Single-Pressure Combined-Cycle Technical Data of the Single-Pressure Plant with a Preheating Loop ined-Cycle Plant with a Preheating Loop

This is shown in Fig. 3-27, using the same gas turbin the example for the simple single-pressure system (Fig.

the steam turbine 36 800 kW Table 3-3 lists the main technical data of this system 1 200 kW

equipped with a low pressure evaporator. 104 000 kW

Compared to the simple single-pressure system, it att 228 000 kW

nifieantly higher steam turbine output, improvi 30.0 % ciency by 2.5%. This is because in this case no steam 157 200 kW

from the turbine. As a result, the entire live ste 72.5 %

pand to the condenser pressure. But the larger volu 23.4 %

exhaust steam produced is a certain disadvantage si 46.1 %

mensions of the steam turbine exhaust and the cond

The increase in the amount of heat to be remove xhaust gases are cooled down to 15 O C (59 OF)

condenser is more than proportional to the increas output. The energy utilization rate of the waste heat tal Conditions

by about 15% while the power output from t increases only by 8%, since the additional exhaust heat ct the combined-cycle plant with a pre- is at a low temperature level. The rate for convertin ately the same way as the simple single- chanical energy (exergy) is therefore modest. The in &on 3.1.2). We will therefore no treat

60 COMBINED CYCLE GAS & STEAM TURBINE POWER PUNTS SYSTEM LAYOUTS

separately the various parameters that depend on the e ment.

Fig. 3-29 shows the effects that the temperatures of and the cooling water have on the power output and ef of the plant as a whole. It is obvious that a rise in air ature causes a reduction in power output and a s ment in overall efficiency. On the other hand, a high tem for the cooling water affects both parameters negativ

Effect of the Most Important Design Parameters on Power Output and Efficiency

The effect of most parameters is similar to that for the single-pressure system (See Section 3.1.1).

Live Steam Data

The effects of live steam pressure and Live steam tem on the efficiency of the steam cycle are practically the for a simple single-pressure system. The optimum live ste sure is at approximately the same level. Slight shifts t higher pressure can result due to a larger exhaust steam flow.

However, installing a preheating loop in the waste imposes a limit on the minimum live steam pressu seen from Fig. 3-30, the flue gas temperature af omizer drops when the live steam pressure falls. minimum temperature of the water in the boiler by the sulphuric acid dewpoint, the amount of use the preheating loop is reduced correspondingly.

If a high feedwater temperature is required, t pressure selected must not be too low. Otherwise a the preheating would have to be done in a low pressure

66 COMBINED CYCLE GAS & STEAM TURBINE POWER PLAN SYSTEM LAYOUTS 67

A low pressure preheater has a negative effect on the ptimum. In many cases, the low pressure evaporator process efficiency because less heat is recovered from t no great expense, produce more steam than required haust gases. However, the reduced wetness and exit 10s t the feedwater and that excess steam could be con- the turbine to a large extent compensate for that negative 0 mechanical energy if it were admitted into the tur-

suitable point. To do this, the steam turbine must Condenser Pressure m admissions: one for high pressure, and another

The effect of the condenser pressure on the efficiency ure steam (two-pressure turbine). steam process is similar to that in the simple single-pressu tern, but the change in efficiency is somewhat more prono ows a system of this type, further equipped with

because the exhaust steam flow is about 10 to 15% gre ure pre-heaters. This not only provides better util- waste heat as mentioned above, but also makes

Pinch Point of the Waste Heat Boiler dynamic use of the low pressure steam. A larger the low pressure steam can flow into the turbine

The effect that the pinch point of the waste heat pressure preheater, while the feedwater is be- on the efficiency of the steam process is similar to tha n the first section using low quality steam. ple single-pressure boiler (cf. Section 3.1.1). However tion of the pinch point affects not only the surface pressure steam reaches the turbine, it can be evaporator and the economizer but also that of the p ated. The thermodynamic advantage of doing loop. There are two reasons for this: s minimal because the pressure drop between

e and the drum is increased. This reduces the @ The flue gas temperature after the economizer

reducing the amount of heat available for the generated because the saturation temperature heating loop. re evaporator is raised. If the water separation

@ The heat required for feedwater heating increa ive enough, the saturated steam can be sent since a greater flow of feedwater is needed for creased steam production. The preheating loop take up more energy. lphur or sulphur-free fuels, further improve-

becomes possible. When the dewpoint is low Other Parameters t gases can preheat a more or less significant

We will not investigate the effects of the other desi ter in a low temperature economizer. Fig. eters here because they differ only insignificantly fro le burning sulphur-free natural gas. The in a simple single-pressure system. eated far enough in a deaerator so that

the water dewpoint of the exhaust ga-

3.1.3 Two-Pressure System OF). Because this temperature is so low, ce in this case under a vacuum. Following

A single-pressure system with a preheating loop pro arator, all the feedwater is heated in a ter waste heat utilization than a simple single-press Nevertheless, that utilization is neither energeticall

SYSTEM MYOUTS 69

bar

:ondenser

70 COMBINED CYCLE GAS Br STEAM TURBINE POWER PLANTS SYSTEM LAYOUTS

Figure 3-33

Fig. 3-33: Simplified Flow Diagram for a Two-Pressure System for Fuel Flow Diagram for a Two-Pressure System for Sulphur Contain Sulphur

1 Compressor 10 High pressure steam bypass ll Feedwater tanwdeaerator

LO High pressure steam bypass 2 Gas turbine

I2 High pressure feed pump 11 Feedwater tanwdeaerator

3 Flue gas bypass (optional) I2 High pressure feed pump 4 High pressure superheater 13 Condensate pump 13 Condensate pump 5 High pressure evaporator 14 Low pressure feed pump

I5 Low pressure evaporator 14 Low pressure feed pump 6 High pressure economizer 16 Low pressure boiler drum

I5 Low pressure evaporator 7 High pressure boiler drum

17 Low pressure preheater 16 Low pressure boiler drum

8 Steam turbine 17 Low pressure economizer 9 Condenser IH Low pressure steam bypass

72 COMBINED CYCLE GAS & STEAM TURBINE POWER PLANTS SYSTEM UYOUTS 73

low pressure economizer to approximately the saturation te ost of the feedwater preheating is still being accomplished perature of the low pressure steam. It is then admitted to t exhaust gas heat. The boiler feed-water temperature must low pressure drum. Next, a high pressure feedwater pump below the water dewpoint (if the fuel is sulphur-free) culates the feedwater for the high pressure evaporator from id dewpoint (if it contains sulphur). low pressure drum into the high pressure steam generator. this case, too, it is possible to supply the low pressure steam advantage here is the reduced efficiency resulting from

the turbine either as saturated steam or as slightly superheat ing higher quality steam from the turbine. Moreover, densation pressure is low, it may become necessary e another low pressure pre-heater heated with ex-

In addition to this system, there are further variants p team in order to reduce wetness at the end of the tur- Most of these are not as good thermodynamically, but o ould reduce the power output slightly further. tain operational advantages.

f Two-Pressure Combined-Cycle Plants One example is shown in Fig. 3-35, where the high pr ss here examples of two typical two-pressure

and low pressure feed-water are separated directly after th plants, both based on the same gas turbine as water tank. The low pressure economizer shown in Fig. -pressure systems. The first is designed for burn- therefore divided into a low pressure economizer for t nd for burning sulphur-free natural gas. pressure feedwater and a high pressure economizer for t step in preheating the high pressure feedwater. This syst wo-pressure system for fuels containing sulphur the following advantages:

s the main technical data for this unit. The ma-

@ better availability, since the high pressure portio rom the single-pressure system with a preheat- remain in operation even if either the low press 3-stage feedwater preheating. Two pump or the circulating pump fails aters heated with extraction steam reduce

@ fewer problems with steaming out in the low m required for the deaearator, which sup- economizer during part-load operation. mount of excess steam to the low pressure

e it produces additional mechanical energy. On the other hand, a slight reduction of about 5% i ssure has been raised to 60 bar (870 psia) in

sure steam generation must be accepted in most cas e efficiency of the steam process. Unlike the

Another possibility that operates without vacuum ems, this system is not significantly affected

is shown in Fig. 3-36. The deaerator here operates heat utilization in the high pressure portion

overpressure. To do this, it requires extraction steam o oiler because the heat that is not utilized is

quality than that in a system with vacuum deaeration. ow pressure portion. Table 3-3 (page 83)

flows within reasonable limits, the condensate is preh hnical data of this plant.

the feed-water in a water-to-water heat exchanger. Th

SYSTEM LAYOUTS 75

SYSTEM LAYOUTS 79

use its temperature is low, the additional heat absorbed ciently be converted into work. A large portion of off again in the condenser. The exhaust steam flow

urbine and the cooling water system are approximately

the improvement in efficiency is great enough so that

ows the heat flow diagram for Example 2. Com- ple single-pressure system (See Fig. 3-5), the sharp

temperatureheat diagram of the steam gen- s, approximately 70% of the heat exchange high pressure portion and approx. 30% in the

re and low pressure steam generation respec-

g water temperatures, for the plant zer. As in the case of single-pressure air temperature affects overall ef- ly. In this case, the gradient is even

80 COMBINED CYCLE GAS & STEAM TURBINE POWER PLANTS SYSTEM MYOUTS 81

Figure 3-39

EXHAUST GAS

HEAT TRANSFER

Fig. 3-39: Energy Flow Diagram for the Two-Pressure Combined-C Low Pressure Economizer

Q Energy input V1 Condenser Loss V2 Stack Loss V3 Loss due to Radiation in the Waste Heat

1 bar

Steam


s of the two curves explains the contrary functions

to generate high quality steam, that of the the remaining waste heat as fully as possible, mplished only if the pressure in the evapor-

ow. However, there are two reasons why the

flow of steam becomes very large, result- pondingly large duct cross-sections.

the rate of waste heat energy utilization in ction of the low pressure live steam pressure.

here, too, be as high as possible, without

q~~ Eficiency of steam process

P ~ , ~ . ~ ~ h w pressure live steam pressure before turbine vaporator, to be sure, a higher superheat- High temperature live steam temperature 475% High pressure live steam pressure super-heater provides the advant- Low pressure live steam temperature

1, the lack of superheating is com-

e high pressure steam after eam at the mixing point in

SYSTEM LAYOUTS 89

2 4 0 260

the Efficiency c

O C

)f the

92 COMBINED CYCLE G A S & STEAM TURBINE POWER PLANTS

Figure 3-47

Fig. 3-47:

SYSTEM LAYOUTS 93

en the high pressure and the low pressure steam portions. reason, it is about half as great as with single-pressure

where the useful pressure drop in the low pressure steam imately half that of the high pressure steam. For that e pinch point selected for the low pressure portion

so not be too low.

ary: With a two-pressure system, the pinch points of gh pressure and the low pressure evaporators have fect on the efficiency of the steam proces than with

sure systems. If equal economic value is attached to cy, then, the pinch points selected for two-pressure uld be larger than those for single-pressure systems. eration is purely academic, however, since two- ems are selected only where efficiency is valued hat in turn means low pinch points.

ows the relative efficiency of the steam process of the pinch points of the high and low pressure

xhaust Gas Temerature

in the exhaust gas temperature lowers the effi- am process. This reduction, however, is less pro- n with the single-pressure system (Cf., Fig. 3-24) y utilization rate does not drop off as quickly. turbine exhaust gas temperature is, the more re system makes. Fig. 3-49 shows the ratio encies of the two-pressure and the simple

ses as a function of the gas turbine exhaust a theoretical exhaust gas temperature of

is ratio is pracically equal to 1. This fact ems that have supplementary firing (refer

mgle- Gas

96 COMBINED CYCLE GAS & STEAM TURBINE POWER PLAN SYSTEM MYOUTS 97

3.1.4 Special Systems em with steam injection is finding steam at the suit- In addition to the four systems discussed in Section 3. re level. Live steam is generally either at too high

3.1.3, there are also others that can at times prove usef re live steam) or too low (low pressure live steam) will cite two examples: epending on the gas turbine and the load involved,

level required for steam injection is, at least for @ A system with steam or water injection int a1 gas turbines, between 15 and 25 bar (203 and

turbine to reduce nitrogen oxide emissions ( ng reduced high pressure steam is the simplest and @ A system using a single waste heat boiler for t ensive solution but is exergetically undesirable.

turbines ows an improved solution which employs a three-

The system with steam or water injection is taking on er with a standard high pressure portion, a importance as environmental protection regulations be sure evaporator for generating the injection steam, more stringent; the system using a single waste heat sure portion for preheating the feedwater. The two gas turbines is of interest mainly for smaller machi relatively complicated. It can be simplified by unit power ratings of between 5 and 30 MW. m from the turbine (Fig. 3-51), which makes it

the standard systems without additional equip- System with Steam or Water Injection into the Gas Turbine

Environmental protection laws such as those curre to which of these two systems is the better must fect in the USA, Japan, and in most European countri one case to the next. It is certain that the three- that the NOx levels in the exhaust be very low. With p ment attains a slightly higher efficiency at full gas turbine combustors, special measures must be t der to maintain these levels.

e disadvantage of a solution employing steam One way to reduce the formation of NOx during urbine might be part-load operation in instal-

is by lowering the temperature of the flame, sine ral gas turbines. Unless all the gas turbines are of the reaction producing NOx is noticeably rapid ressure at the extraction point decreases so high temperatures. Injecting water or steam into th st cases inadequate. It thus becomes neces- can produce the temperature reduction desired (ref to live steam, which again negatively affects 9.1). e-pressure system is better in this regard.

f the amount of injection steam generated For gas turbines alone, with no waste heat boile enough live steam need be used to cover

to inject water but efficiency is lower than with ste

mparison between the single-pressure sys- loop without steam injection into the gas system with injection of extracted steam

100 COMBINED CYCLE GAS & STEAM TURBINE POWER PUIN SYSTEM UIYOUTS 101

or with water injection. This comparison, of course, do sharp drop in efficiency, approx. 2% with steam and al- in itself possess any absolute validity because the amou % with water injection explains why all gas turbine man- steam injected varies with specifications and type of gas tu rers are working on the development of dry low NOx It does, however, show trends. es as presented in Section 10.3 in order to attain low

sion levels without requiring water or steam injection. The comparison shows that the system with steam inj

has a slightly higher overall power output and less w possible to conceive of a solution that no longer has to be dissipated in the condenser than the dry system. rbine at all: All of the steam is directed into the fact indicates that there is less waste heat from the turbin 91. Fig. 3-52 shows one such system which could the condenser, which also makes the installation less expe as a peaking unit in countries where water is plen- Dis-advantages, on the other hand, are its lower effic e and attains an efficiency higher than that of the greater amount of additional water required. In so alone. However, if the steam flow injected is then, the solution with steam injection can be more e e than approx. 2 - 4% of the air mass flow, major that the standard solution. The prerequisite for this, s must be made to the gas turbine, principally mo- is having available a low cost source of additional w he compressor. This system is therefore only of system with water injection has the lowest efficiency b interest. Because its efficiency is lower and its put is approx. 7% greater. Water consumption is less ion far greater than with the normal combined because the water has a better cooling effect than st for its economical application is quite limited.

Table 3-5: Comparison of the single-pressure system highly sophisticated systems are being marketed with preheater loop with and without st r such names as STIG (Steam-Injected Gas Tur- or water injection, Fuel Oil #2 etc. [45], [46]. They all suffer from the disad-

er consumption is high, and that their efficiency

Injection in the gas turbine Water Steam Dry ot as high as in normal combined-cycle plants. ire specially designed gas turbines, which g their acceptance. STIG systems are

Gas turbine output 73 800 76 000 68 400 k r smaller cogeneration plants with aero- Steam turbine output 38 600 31 200 36 800 k . Fig. 3-53 shows one such system, with

255 700 239 500 228 000 k for two gas turbines. Station service power 1 300 1 200 1 200 k Net power output 111 100 106 500 104 000

d to simplify and reduce the costs of the Waste heat in condenser 78 900 61 800 76 100 k r the combined-cycle plant includes sev-

interest mainly with smaller gas tur- s possible with this system whenever

r is provided to serve two gas turbines.

102 C O M B I N E D CYCLE GAS & STEAM TURBINE POWER PLANTS SYSTEM LAYOUTS

The cost reduction is less with larger machines. The advantage of this solution (Fig. 3-54) are:

@ savings for the evaporator

@ simpler steam circuit

required, a cost increase results which cancels out to a

The reduction in availability can be considered mod the unfired waste heat boiler is a reliable component

In summary, it can be stated that this arrangement esting only for combined-cycle plants employing small bines that one intends to equip with a flue gas bypass

exhaust gas channel, since these can be built without pensive flue gas ducts. The boiler can be placed bet

ment without a steam turbine, with 100% steam injection in th

Uf Steam injection ll Feedwater tanwdeaerator

ipment) 12 High pressure feedwater pump In an open-cycle gas turbine, only 25-35% of the 13 Condensate pump

14 Low pressure evaporator 15 Low pressure feedwater pump

used for an supplementary firing in the steam gen 16 L ~ W pressure drum

renders the combined-cycle process even more v regard to design, operation, and choice of fuel.

Earlier combined-cycle installations generally h tary firing. The fact that that is frequently no 1 today can be attributed to progress in the devel

LC gas


gas turbine. Thermodynamic interest in supplementary mbined-cycle Plants with Limited decreases as the gas turbine inlet temperature rises (Secti pplementary Firing Fig. 3-55 (p. 109) shows the efliciency of the combined-c lementary firing heats the exhausts gas to at most cess using the gas turbine inlet temperature as a par OC (1472 to 1672 OF). The arrangement of the steam The curves are valid for single-pressure steam circuits ed is similar to that of installations without supple- supplementary firing. Older gas turbines had low tur temperatures. With these machines, an increase in ring. Up to temperatures of 750 OC (1382 OF), simple ture to 750°C (1382°F) improves overall efficiency. B boilers can be used, without cooling of the combus- point, supplementary firing brings increases only in that point, a cooling similar to that used

tional steam generator is necessary.

In gas turbines with inlet temperatures in excess ed are oil or gas. With a simple waste heat boiler (1832 OF), the gain is negligible even in the lower rang combustion chambers, gas is the best fuel because pressure processes, only a slight gain in efficiency can iation and ease of ignition. with a supplementary firing to 750°C (1382 OF). Co pressure processes, however, attain their maximum ows that the efficiency attains a maximum at a when utilizing the waste heat alone. er the supplementary firing) of 750 O C (1382 OF).

use the heat exchange in the economizer is op- As gas turbine inlet temperatures keep increasing, since the curves for flue gas and water tem-

tance of supplementary firing will diminish even allel. The exchange of heat can therefore take ertheless, the increased operating and fuel flexi mum loss of exergy. Fig. 3-56 (p. 110) shows combined-cycle with supplementary firing may b eat diagrams for temperatures of 500" (932" in special cases. Particularly in installations use and 1000°C (1832°F) after supplementary fir- ation of heat and power, this arrangement make e temperature curves in the economizer are control the electrical and thermal outputs separ he minimum difference in temperature on Section 4). This pattern is the same as that for a waste

supplementary firing (refer to Sect. 1). At Combined-cycle installations with supplementa r hand, the minimum difference in temper-

one of two categories: the water end-is at the inlet to the rn corresponds to hat of a conventional

o units with limited supplementary firing, w similar to units without supplementary firi

o units with maximum supplementary firin most of the oxygen contained in the gas the supplementary firing is the limit case hausts is utilized. This type of power pla nee in temperature along the entire econ- the conventional steam process. t the exhausts can practically be cooled

r temperature, thereby eliminating the evaporators (Sections 3.1.2 and 3.1.3).

108 COMBINED CYCLE GAS & STEAM TURB SYSTEM OllYOUTS 1

Unlike conventional power plants, the feedwater tempe here depends solely on the sulphuric acid dewpoint (Section Thermodynamic improvement by multi-stage preheating higher temperatures serves no purpose. This pattern corr to that of combined-cycle plants without a supplementa waste heat boiler.

Example of a Combined-Cycle Unit with Limited Supplementary Firing

Fig. 3-57 shows the heat balance of a typi plant with supplementary firing to 750 O C (1 the gas turbine is the same 70 MW machine as that us examples without supplementary firing. T sulphur-free natural gas, which produces results opti regard to efficiency. Natural gas has the fu it can be burned easily in a waste heat bo combustion chamber. With oil, even that is (Section 5.2). The basic arrangement for this insta same as that for the purely one-pressure system in 3. the fuel contains no sulphur, the feedwater be reduced to 60 OC (140°F). Deaeration therefor

the refired combined-cycle installation as a function of th under a vacuum. perature after supplementary firing and gas turbine inlet

Fig. 3-58 shows the corresponding Temperature of the combined-cycle plant

One can see the optimum temperature pattern in th resulting in a low stack temperature. This is low pressure evaporator would bring not further i ization of the waste heat energy at full load. At pa ever, or when the supplementary firing is switched temperature rises. For installations that are fr at part loads, it can thus make economic sense rangement with a preheater loop. The sa applies to plants with a temperature after su lower than 750°C (1382 OF) at their design poin

110 COMBlNED CYCLE GAS & STEAM TURBINE POWER

Figure 3-66

a l

Fig. 3-56: Temperaturaeat diagram for 500' (93Z°F), 750" (1382°F) ( (1832°F) (c) after supplementary firing

t Temperature in "C Q/Qk Heat exchanged A Flue gas B WaterISteam

SYSTEM LAYOUTS 711


The Influence of Ambient Conditions natural gas. When burning oil, the paths of the curves for on Power Output and Efficiency single-pressure system would not be significantly changed,

there would be less difference between the single and If the temperature after supplementary firing is constant, t essure systems (Section 3.6).

effects of air pressure and air temperature are similar to th in installations without supplementary firing. These two des hould note the decreasing difference between the effi- parameters have a very pronounced influence on the pow s of single and two-pressure systems as the flue gas tem- put, but the efficiency remains to a large extent unaffe re after supplementary firing increases. The curve in Fig.

ms that the two-pressure process provides no advan- Because the steam turbine is providing a greater po the single-pressure process at temperatures above

the total power output, the temperature of the condens 2 OF). Here, too, the improvement at low flue gas tem- ing medium has a stronger effect on the overall power o is greater with the two-pressure process. It makes no and the overall efficiency. Its effect on the steam process at all for the steam turbine whether this flue gas tem- is similar to that in plants without supplementary firin s attained directly from the gas turbine or by means 3-22). However, because of the higher live steam data, t entary firing. The results indicated in Section 3.1.3 of enthalpy drop is slightly reduced. re also valid for installations with supplementary fir-

The Influence of the Most Important Design Parameters on Power Output and Efficiency ows another reason why the machine behaves in Flue gas Temperature after Supplementary Firi e, the rate of energy utilization in the single-

The temperature after the supplementary firing i continues to rise as the temperature after the

important design parameter because it strongly infl ring increases, up to 750 OC (1382 OF). This is

power output and the design of the plant. sing stack temperature and the increasing tem- tial between the steam generator inlet and out-

Fig. 3-59 shows how relative power output and ate of thermal energy utilization increases as

pend on the temperature after supplementary fi s, the improvement possible with a two- tom limit, 525 OC (977 OF), represents utilization of t omes continually smaller.

waste heat alone. Two different systems have b the diagrams:

for a combined-cycle plant with supple- @ a single-pressure system (Fig. 3-57) 50 OC (1382 OF) are comparable to those @ a two-pressure system (Fig. 3-34) rbine plant with the same power out-

e. The live steam temperatures have The basis for comparison is the single-pr o the pressure, according to values

out supplementary firing. Calculations assu wer plants. As is the case in

l6 COMBINED CYCLE GAS & STEAM TURBINE SYSTEM UYOUTS 117

Figure 3-59

firing is practically unaffected by the live reduction in exergy losses at higher live

re directly affects overall efficiency of the

on in the steam generator, the tempera-

kes it necessary to preheat to a higher eating should take place in several stages.

be employed to reach the required 110

the higher feedwater temperature.

an 750 OC (1382 O F ) , a system with a pre- asonable way to make optimum use of . The more often the unit is run at part

ombined-cycle plants, the increased

Fig. 3-59: Effect of the Temperature after Supplement output being provided by the steam Efficiency of the Combined-Cycle Plant and Utilization

ser pressure is of more importance nts which merely utilize the waste

P ~ / P R ~ ~ Relative Power output of the combined-cycle plant

VK/VREF Relative efficiency of the combined-cycle plant WB Rate of utilization of waste heat energy kF Flue gas temperature after supplementary firing ative effect of a change in condenser * Reference = Plant without supplementary firing (52

118 COMBINED CYCLE GAS & STEAM TURBINE POWER SYSTEM LAYOUTS 119

effect it has on the power output of the steam

cycle plants have a greater exhaust steam flo sing only part of the oxygen remaining in

put, since there is less preheating of the feed- ine and a steam turbine with reheat. In a reason, the condenser pressure exerts a greater i pe, the regenerative air pre-heater usu- power output from the steam turbine.

flue gas is being used as the combustion 3.2.2 Combined-Cycle Plants with Max

Supplementary Firing w to flue gas flow is much greater than ue to considerations of exergy only a

The basic idea for combined-cycle plants wit should be directed through the econo- ited supplementary firing in the waste heat bo d with flue gas. The rest flows as nor- the best possible use of the gas turbine's wast

rt-flow economizer is therefore ideal not the gas turbine but the conventional steam water and steam temperatures run was to provide a prior gas turbine in order to i

-free natural gas, the energy is the proved air blower with an air heater built in. to provide electricity. pressure portion of the feed water

nt is advantageous, however, only if This approach is reflected in the ratio of t usts is very low (Fig. 3-62). steam and the gas turbines. Depending on the

air, this is between 4 and 10, as compared to cycle plants without supplementary firing.

eases the availability of the a conventional steam power

cy in operation of the fan is a unit with steam reheating a water heating.

eat importance here.

The number of possible systems av tically all known steam processes ca

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SYSTEM LAYOUTS 129

steam power plant because of thermody- Thus, in most cases, no extraction points

the steam turbine, which increases the turbine. However, the generator and

e able to handle this additional power cases that can lead to restrictions.

ignificant gain in efficiency that can be ng. The power output of the plant as n tripled. A supplementary firing could output because the gas turbine exhausts

vering a portion of the heat demand of

128 COMBINED CYCLE GAS & STEAM TURBINE Po

Figure 3-64

Fig. 3-64: Combined-Cycle Plant with Existing Steam Turbine

1 Gas Turbine 9 Condenser 2 Compressor 10 Steam bypass 3 Flue gas bypass ll Feedwater tanwdeaerator 4 Superheater 12 HP Feedwater pump 5 Evaporator 13 6 Economizer 14 LP evaporator 7 Drum 25 LP Feedwater pump 8 Steam Turbine 16 LP drum

1 700 1 200 kW

107 000 228 000 kW

plementary firing in repowering is the tation to the live steam data of the orig-

ign of the gas turbine has been stan- its on the amount of steam that can

o not necessarily lie close to the design turbine.

130 COMBINED CYCLE GAS & STEAM TURBINE PO SYSTEM LAYOUTS 737

Another approach to repowering can be used cle ~nstallations with dern steam Power plants equipped with reheat Gas Turbines [63 to 681 To improve the efficiency of such a unit, the fie be replaced in supplying the oxygen needed for c losed-cycle process is, however, much less installing a new gas turbine before the existing tor. Here, the existing steam boiler continues in sisting of compressor, heater, turbine, be adapted to its new operating mode. The modific ly, the medium can be any gas1 but al- are due mainly to the much higher temperat at have been built employ air. Helium bine exhausts (approx. 500 OC/ 932 OF as camp ossible media, with helium in particular 572 - 662 OF for the fresh air after an air preh nuclear power stations.

Parts that must be modified are the: gas turbine can be raised by employing

@ burners ems, e.g., a recuperator, compressor @ fresh air ducts ng, or the like. However, as was the

@ perhaps the reheater as turbines, the simpler arrangements

A waste heat recovery system must be installe generator to handle most of the condensate and f the closed-cycle gas turbine is the great

ing. The complete plant appears similar to tho lection of the fuel. In addition to oil or 3-60 and 3-62. 1 can also be used. Its main disadvantage

lied to the process via a heat exchanger, The major problems that arise with this typ e inlet temperature to levels bwer than

are in connection with: turbines. According to [64], efficiencies Id be attainable. Fig. 3-66 shows the way

a) space availability for installation of the g of the combined-cycle process depends and the waste heat recovery system ssure ratio and the gas turbine inlet tern-

b) adaptation of the boiler and the overall cept to the new mode of operation

tial application involves nuclear Power For steam Power plants that burn gas or oil ed reactors (high temperature reactors

one very interesting possibility for raising s). In these plants, the gas would be in than 10% and power output by 20 to 30% a reactor. The maximum process temper- vestment Costs. With coal-burning units, ther the highest process temperatures attain- economic gain because the conversion itself is there is less improvement in efficiency.

today at 950 O C (1742 OF).

134 COMBINED CYCLE GAS & STEAM TURBINE POWER SYSTEM L4YOUTS 135

The use of combined-cycle plants with closed- nt advantage is the small, compact steam gen- bines can be considered for the following applic e and the greater speed

r is better than in a con- @ burning coal in a fluidized bed rator. Consequently, the surface required

high temperature reactors. hat theoretically should

Fig. 3-67 shows one possible arrangement with combustion, a closed-cycle air or helium the second type. Here, un- sequent reheating steam process. A com oying a simple charging group, advant- a high temperature reactor, helium turbine, a gas inlet temperature. The steam can be seen in Fig. 3-68. In neither case can a aces the gas turbine combustor. Net effi- be expected in the near future because are within the range of possibility. That economic hurdles are too great. have no thermodynamic

mbined-cycle plants with 3.5 Pressurized Steam Generators even these values are

by simple combined-cycle plants with- As one final combination we should mention a g which today exceed 50%. The ques- a pressurized steam generator. This type of' pow

to whether there will be any economic fall into one of two categories: gas-fired plant of this type in the future.

0 installations with a simple charging group, r efficiency, it has the following further or may not provide (a small amount of) ele power mbined-cycle plant with

0 installations with gas turbines and subse mizers

uiring a special steam generator Fig. 3-69 shows the diagram of the principl

first of these arrangements. Power plants lik n of the gas turbine and steam quite early: the more than 100 Velox boil veri operate in this way. However, such ins real future (at least for oil or gas-burning pla cannot provide any genuine thermodynamic the following advantages: conventional steam plants. The gas turbi a very low temperature level because it is or- though this advantage is from the steam generator as a working m ut by the subsequent economizer is being made of the high temperature pot machine, and its power output is for that re process possible because Typical efficiencies of such units are in th of the steam generator.

SYSTEM LQYOUTS 137

140 COMBINED CYCLE GAS & STEAM TURBINE POWER P u

The balance is more likely to be on the negative sid oil or gas-fired plant.

Plants with pressurized steam generators could ne still become very interesting in the future because t a possibility of burning coal cleanly in a pressurized bed combustor (PFBC). Refer to Section 10 below for information about such plants.

3.6 Summary and Evaluation of the Vario Arrangements Possible

In our evaluation of the various arrangements p will omit from consideration systems with closed-cy bines or pressurized steam generators because they of academic interest only. The following six exarnp compared below:

0 the single-pressure sytem (Fig. 3-4)

the single-pressure system with a preheater 3-27)

@ the two-pressure system for fuels containin (Fig. 3-37)

@ the two-pressure system for fuels containin phur (Fig. 3-38)

@ the combined-cycle plant with limited suppl firing (Fig. 3-57)

@ the combined-cycle plant with maximum s tary firing (Fig. 3-60)

All arrangements are based on the same gas t approx. 70 MW and are as a result directly compa

Gas Coal

ST Steam turbine Suppl. Supplementary

-fired steam generator with a low pressure part-flow econo

point is the high net efficiency of the gas- ystem. The unit with limited supplementary

r, far behind: its efficiency is less only by Its power output, however, is about 40% am turbine produces about twice as much ent can be of interest whenever a higher 'nable with utilization of the waste heat reover, the specific investment costs re- to be lower than those for the two-pres-

142 COMBINED CYCLE GAS & STEAM TURBINE POWER P

sure system. But this type of plant is more complex become less and less attractive in the future as gas tur temperatures continue to increase.

Efficiency is only one of the important criteria wh on the selection of a power plant. A second is the is difficult in a Handbook of this type to provide ex Only comparative prices are possible. The basis fo is in all cases the simple single-pressure system a ative prices are valid as specific prices for install comparable power rating (Table 3-10).

Table 3-10: Comparison of Specific Prices for the

SYSTEM LAYOUTS 143

nt consideration is the amount of cooling wa-

arison of the Amount of Cooling Water The net efficiency of the combined-cycle with m

plementary firing is poor, but it can cover 70% o quirements with coal, which is frequently an adva

133.4 670

3180 15940

109.0 145.3 348.8

21.9 46.5

supplementary firing need be- er installed MW because in these

ional low exergy heat is being supplied m the exhaust gas. Because this energy into mechanical energy, it must in large again in the condenser.

unt of cooling water required, the maximum supplementary firing turns

Various Systems, in %

Single- Single- 2-Pressure 2-Pressure Limited Pressure Pressure, Fuel with Fuel with Suppl.

Preheater Sulphur no Sulphur Firing Loop

Relative price 100" 101-103 105-108 106-110 103-110

* Basis for comparison

The higher specific price for the unit with a c rating when employing maximum supplemen to the fact that a steam plant is more expensi bine, which means that the relative price for a proportionally large steam component will This is especially true for a plant with a coal-bu today usually requires installation of a scrubb sulphur from the exhaust gases.

146 COMBINED CYCLE GAS & STEAM TURBINE P O

portance today. Their advantages include:

high efficiency

a simple steam process ED-CYCLE PLANTS

@ low investment costs COGEIVERATION quick installation

simple operation and maintenance. dvantage of a combined-cycle applies wer plant that produces power alone,

their low live steam data (Table 3-15). The provide heat or process steam as well. high availability ratings and provide easy ope diagram of such an installation with a nance.

he combined-cycle plant er plant is even more pronounced in

used only to generate e heat input to the pro-

n a combined-cycle plant

plants have to supply heat at the same

Live steam ss in temperature drop is the same in pressure,bar e loss in combined-cycles is smaller be- Live steam ilable is larger (Fig. 4-2). temperature, OC

Feedwater d-cycle installations can be considered temperature, O C

No. of feedwater pre-heaters 1 lant with a backpressure turbine

lant with an extraction/condensing

a waste heat boiler (Fig. 4-4)

152 COMBINED CYCLE GAS & STEAM TURBINE PO BINED-CYCLE PLANTS FOR COGENERATION 153

Example of a Combined-Cycle Industrial PO el of the process steam and the power coef-

Just as we did for power generation alone importance for design, because the pressure

further considerations here on one given exa a gas-burning plant with supplementary firing,

ws the effect on the overall output of the is very high, the use a steam turbine be-

ince its pressure differential is then too e steam process reduces to a waste heat

The Effect of the Most Important Desig

As with combined-cycle plants used for g nt of the plant is affected mainly by three alone, the air temperature is of particular im for power output. In industrial processes, the generally does not depend on the ambient uel supplied directly to the boiler result, one is often compelled to select th temperature for design purposes.

el of the process steam Table 4-1: Main Technical Data of the Com

Industrial Power Plant akes it possible to lower the power co-

Gas turbine power output

Backpressure steam turbine power output cient is high. That feature must not,

Station service power a disadvantage since for industries oefficients it is often a better idea

Net power output of the plant from the connected grid and to gen-

Heat input to the gas turbine (LHV) nal steam generators. Fig. 4-7 shows

Heat input to the supplementary firing (LHV) depends on the temperature after Process steam flow

Process steam pressure

Thermal energy of process steam g turbine offers greater design and Rate of fuel utilization irection of higher power coefficients. Power coefficient the turbine makes it possible to in-

Electrical yield produced at the cost of process steam procedure works out unfavorably on

BINED-CYCLE PUNTS FOR COGENERAUON 157

ency. As shown in Fig. 4-8, when the power

eneration. As soon as the power coefficient

is the case with mixed process steam pro- of the process steam is being produced in

fficiency of power production drops off ra-

Important Parameters

esign data, it is necessary to distinguish unfired waste heat boiler. With supple-

power plants. Just as for plants used to e feedwater temperature must be as low in a good utilization of the heat in the

pressure should, however, be higher enthalpy drop between the live steam s is especially true if a relatively high

for the process steam. Poorer heat util-

Fig. 4-7: Effect of the Temperature aker the Coefficient (*Case without Supple

low pressure systems are used only

Power Coefficient Flue gas temperature after supplemen essure steam into the turbine because

ential available between the low pres-

MBINED-CYCLE PLANTS FOR COGENERAUON 163

picted is based on a single-pressure steam pro- ncrease the rate of utilization of the waste heat

ed by a heating circuit heated by the flue gas. wn, this circuit is built as a closed system. g the district heating water directly would

it would mean greater problems in operation.

the example shown, the steam turbine could ondensing turbine, which would provide a

xibility. Whether or not such an added in- orthwhile depends mainly on the value at- nal electricity which could be produced. m using an extractionlcondensing turbine.

en a complete two-pressure system could erequisites for this however would be that ttached to the electrical power produced

ating water supply temperature be low d be a reasonable enthalpy drop between and the heating condenser(s).

mportant Ambient Conditions

nts for district heating, the strong effect has on the power output is more likely ce maximum output is demanded when

ial power plants, the temperature level affects the power output of the steam the temperatures selected for the dis-

d be as low as possible. The design tem- mpromise between maximum electrical r transportation of the heat.

164 COMBINED CYCLE GAS & STEAM TURBINE POWER MBINED-CYCLE PLQNTS FOR COGENERATION 165

As in combined-cycle industrial power plank, the pressure should also be higher than in power planb generation alone. Levels of between 40 and 70 bar (5 cle plants used with seawater desalinization

a1 power output and the flow of process steam psig) are typical for optimum design of installations plementary firing. ed independently of one another. A supplemen-

refore recommended. Fig. 4-13 shows the prin- eat balance of such a power station. 4.3 Power Plants Coupled with Seawater

Desalinization Units tion process is based on a "Multiflash" system

Combination of a power station with a seawate water is treated with polysulphate. The heating

unit is one especially interesting application fo 1.2 bar (approx. 17 psia) and the specific heat

Combined-cycle plants are outstandingly well su kJ/kg (108 Btu Ab) distillate. This corresponds

pose because such power plants are generally 20-stage desalinization plant. The main tech-

countries and ideal fuels for combined-cycle n in Table 4-3. therefore easily available at a reasonable cos

this type becomes very interesting if the value

Larger seawater desalinization plants are alw ctrical yield is high. Even if the process heat

use the multi-stage "multiflash" process. One si loss, the electrical efficiency reaches approx-

nieal requirement imposed by the process of s gh a value is attained in a conventional steam

is that the maximum temperature of the water when only electricity is being produced. The limited. The reason for this is the way in whic nt is less suitable if the ratio between fresh

treated to prevent CaC03 deposits. Generally t 1 power must be high. The power coefficient volves treatment with polysulphate or sulphuric In such a case, either the additional steam

salinization unit must be supplied from an With polysulphate, the maximum temperatur different type of power plant must be cho-

water can be heated is 90 OC (194 OF); with sulp oefficients can be attained using O C (248 OF). The resulting heating steam pressur g steam turbines. However, the significant 1 and 2.5 bar (14 and 36 psia), which provide i ng only a negligible amount of cooling wa- for a combined cycle plant because the usabl s is lost and the unit becomes more com- in the back-pressure steam turbine is high, ensu trical output.

a, a, 5. Eli;

COMPONENTS

in the most important component in the com- am turbine power plant. The combined-cycle le to become a competitive thermal process of the rapid development in the direction of

e inlet temperatures.

evelopment in the turbine, there has also been in the compressor. Today the compressor can er mass flows and higher pressure ratios, mak-

in considerably higher power outputs and costs and improve efficiency.

storical development of maximum air flows emperatures. Inlet temperatures are higher

industrial gas turbines. In a jet turbine, t role: procurement and maintenance an with stationary gas turbines, where

overhauls are demanded. For that rea- higher inlet temperatures and greater

gressed more rapidly in jet turbines than

es can be classified into one of three cat-

bines derived from steam turbine

172 COMBINED CYCLE GAS & STEAM TURBINE POWER Pi

* industrial gas turbines derived original technology

* the aero-derivative turbine, consisting followed by a power turbine

The last of these is normally a two-shaft turbine iable speed for the compressor and the driving tur an advantage with regard to part-load efficiency sinc of air taken in is reduced due to the lower spe advantage when operating with a generator, how there is no compressor braking the power turb shedding. Two-shaft turbines are usually used or pump drives, where the operating speed of t bine is also variable. On the other hand, turbi two types are practially always built as single- when used to drive a generator with an output to 20 MW. Fig. 5-2 to 5-4 show typical modern each of the three main types.

One important fact is that the machines h dardized. As a result, they can be built as sto possible shorter lead-times and lower prices. Bec dardization, there are only a few types of m on the market and it is never possible to buy a power capacity. However, the advantages b ardization outweigh this consideration.

Table 5-1 shows the characteristic data of mod used for combined-cycle installations.

COMPONENTS 173

n Technical Data of the Most Common Turbines Available on the Market

10 - 18

950 - 1150 O C

480 - 570 OC

30 - 500 kg/s

ith gas turbines lies in how fast their de- rogressing. They are developed primarily

ine alone. But because fuel costs are also mpt is being made to make a correspond- turbine efficiency and to reduce the speci- quired for them. As a result, turbine inlet en very quickly, which has not in the past effect on availability.

e, this situation has changed and gas tur- very high reliability. They can therefore ase-load or medium-load combined-cycle

same high availability as conventional

ications, fouling in the compressor and oncern. Compressor fouling occurs be- erates in an open cycle, drawing in air ed completely. Turbine fouling becomes "dirty" fuels as crude or residual oils

tZ: 2 GAS TURBINE INLET TEMPERATURE u $: m

Fig. 5-2: A typical Industrial Gas Turbine

178 COMBINED CYCLE GAS & STEAM TURBINE POWER PL4 COMPONENTS 179

e turbine is due mainly to the ash contained in in the additives used to inhibit high temperature

filters most frequently employed are self-cleaning selecting the correct type of additives. It is less peaking or medium-load operation because of

in order to limit the rate of fouling. ng effect produced by start-up and shut-down.

It is, however, impossible to keep the compressor dation in combined-cycle plants after 1000 - 2000

clean. The fouling that results causes losses in outp ciency that are greater in single-cycle gas turbin

output from the combined-cycle plant

cycle plants, some of the losses can be recovered efficiency of the combined-cycle plant

on a clean fuel:

ing of the cold turbine and compressor makes 0 Reduction in output from the combined-cy

3to6% 0 Reduction in efficiency of the combined-c on problems were one of the major causes

2 t o 3 % . Because of the use of better blading ma-

Two types of cleaning can be used to help recov problems of this nature have today prac-

a) a "dry" cleaning, using nutshells or

b) washing

age to protective coatings on the compre it requires shutting down and cooling the the links connecting the gas and the to wash at low speed- typically ignition e three main types: chine cold. The machine must therefore b for approx. 24 hours. The compressor wa ithout supplementary firing with the gas turbine at full load, but thi ith supplementary firing low-speed washing.

180 COMBINED CYCLE GAS & STEAM TURBINE POWER COMPONENTS 787

As can be seen from Section 3, the first of these t m waste heat boiler must fulfill the following-

most interesting and the remarks that follow theref tradictory-conditions:

trate mainly on it. aste heat utilization must be high (high

5.2.1 Waste Heat Boilers without on the flue gas side must be low in or- Supplementary Firing losses in power output and efficiency

A waste heat boiler without supplementary firi a heat exchanger. However, the requirements i ture corrosion must be prevented

eration in a combined-cycle installation pose sp gradient permissible during start-up must that are often underestimated. In particular, pr made to accommodate the short start-up time of t Even so, the waste heat boiler is a simple compo

fficult to meet the first two of these con-

reliability and availability. ime. Because of the low temperature, the

ce- transfer by means of radiation is

Waste heat boilers without supplementary firi pletely by convection. Since the differ- according to two principles: between the exhaust gas and the water

11 in order to attain a good rate of waste steam generators with forced circulation ( urfaces required for the heat exchange are ty pel , an large pressure losses unless the speed steam generators with natural circulatio low, which would again increase the size

surface. However, this problem can be Either type of waste heat boiler can be us using small-diameter finned tubes. An-

cycle plant. A forced circulation boiler has adv tube diameter is the small amount of that render it especially suitable for combined-c . This means that the thermal capacity

quick changes in load. minimum space requirements arising fr design eing built today have very low pinch fast, easy start-up drops on the flue gas side. Values of suitability for designs with a low pinch pressure losses of 25 to 30 mbar (10

less sensitivity to steaming out in the

The main advantage of a natural circulat modern forced-circulation waste heat

circulation pumps are needed.

the heat exchangers sus-

186 COMBINED CYCLE GAS & STEAM TURBINE POWER PL4 COMPONENTS 187

Figure 5-7 m design of a Waste Heat Boiler

ler, one should strive for an etween cost and gain. The cost depends mainly on change surface installed. The indicator generally used

r (the minimum difference re between the water and the flue gas). As can be 3-22, the area of the evaporator increases exponen-

e temperature differential decreases, while the in- inear. For that reason, the ctor determining the heat-

installations where a high value is attached to 5 K (18 to 27 OF); where

alued lower, it can increase to 15 to 20 K (27 to

stitutes 40 to 50% of the total o 60% remain practically

In an extreme case, too se pressure losses on the

resulting reduction in power ncy of the gas turbine is greater than the power by the steam turbine. A pressure drop of 10

r output and efficiency by ap- a portion of this loss can be recovered in the

sign of the waste heat boiler e gas turbine. The rapid expansions

Fig. 5-7: Acid Dewpoint as a Function of the Sul e accommodated taking suit- Conversion from S o 2 to ~ 0 3 , and the ch as suspension of the tube bundles,

T~ Acid dew points S Sulphur content of fuel X Rate of conversion from so2 to so3 ate of loading arises from the drum. h Excess air ratio ssible, the walls of the drum should

188 COMBINED CYCLE GAS & STEAM TURBINE POWER COMPONENTS 189

be as thin as possible, which can be done provided the economizer. In order to keep this within live steam pressure is low. onomizer is generally so dimensioned that the

outlet is slightly under-cooled a t full load. Optimum steam pressures for installations without tween the feedwater temperature and its sa-

tary firing are low, 30 to 70 bar (420 to 1000 psig), ture is known as the "approach tempera- advantage for quick start-up. causes a reduction in the amount of steam

ld be kept as small as possible, typically 5 to Another operating problem is the volumetric ch

the evaporator during start-up. The large differe volume between water and steam at low and me cause large amounts of water to be expelled fr eat Boiler with Limited ator at the start of the evaporation process. Th able to take up most of this water since other eration of a waste heat boiler with limited amount of water would be lost through the e g is the same as that for the unfired boiler. of the drum during each start-up. The gross volu esigns available for the firing itself. Units should therefore- depending on starting time gas temperature of approx. 750 O C (1382 water loss- be 1.5 to 2.5 times as great as the entary firing can be built with simple duct aporator steam in normal operation. This pro iring cooled combustion chamber walls (I?&. change can be held within bounds by empl so that at least no steaming out occurs in t improve part-load efficiency and behavior of t icularly well suited to burning natural gas, plant, it is possible to operate the boiler at no problem in attaining a uniform ternper-

er the burners and the radiation to the walls The system is generally operated at a low er is low. For that reason, most of the

the steam turbine load is lower than a t full burn natural gas. There are systems accomplished by employing sliding pressu t because they involve major problems, Chap. 7). For example, for half-load for th a to look for a different solution when- whole, the waste heat boiler in a system e of oil must be burned. A cooled corn- and 1 steam turbine can be operated with o burners such as that used in turbines running a t full load. The live s ators is one good method of providing sliding pressure operation is only half as . 5-9 shows how it is constructed. The load, which causes the volume flows in t

atural circulation portion, used to cool superheater, and in the live steam duct to , and a forced circulation portion. In in operation is still a t full load while

bine is a t full load.) One consequence ry throttling of the flue gas flow, it may steam pressure at part load is that some o e burners directly with cooling air from

COMPONENTS 191

192 COMBINED CYCLE GAS & STEAM TURBINE POWER P U COMPONENTS 193

If the burners are supplied directly with flue gas, th er heating superfluous. Therefore, in order to make flow of the flue gas must be throttled down by appro to cool the exhaust gas after the steam generator to 30 mbar (8 to 12 in. WG), corresponding to the pressu emperature, an additional economizer is provided through the burner. This reduces the power output of es over a portion of the feedwater preheating from turbine by approx. 1.5 % . rative preheating. The best arrangement divides the

tween the economizer and the high pressure feed The great advantage of supplementary firing with c n the fuel is gas, an additional low pressure part-

bustion chambers lies in iLs operating flexibility. Th zer improves efficiency. The fuel burned in the the supplementary firing can be varied within a broad oil, gas, or pulverized coal. Fig. 5-11 shows an ex-

and the maximum temperature is no longer restricte ore detailed information, see Ref. [1021. (1382 OF). This system thus is particularly ideal for co whenever a broad control range is required for the pro flow at various gas turbine loads.

ine used for a modern combined-cycle instal- Fig. 5-10 shows another system with 2-stage firi machine with relatively low live steam data.

the output of the supplementary firing of waste he he following main characteristics: uncooled combustion chambers can be increased. gas is heated after the gas turbine in a first stag ature not to exceed 750 OC (1382 OF). This is fo ing in a first heat exchanger (e.g., evaporator The exhaust gases can then be reheated in a mes are of particular importance because the tary firing before they flow through the final ts are often used as medium-load units with heat boiler. ut-downs. These features are required above

ithout supplementary firing. With a fired 5.2.3 Steam Generator with Maximu that arise are similar to those in conventional

Supplementary Firing

With this type of steam generator, the exha eam turbine used for a combined-cycle in- gas turbine are used primarily as oxygen carri mentary firing. This is a single-cylinder tent of the exhausts is small in comparison ble exhaust section. Because the tur- firing in the boiler. It is therefore no longe g pressure operation, no control stage a waste heat boiler. e are likewise no extraction points be-

he feedwater takes place in the waste The design of a steam generator of this ty ing such a steam turbine, one must re- tical to that of a conventional boiler, ex am temperature is lower during part- generative air preheater. The gas turbi

intain an approximately constant at a temperature of 480 to 550 OC (896 t

COMPONENTS 195

200 COMBIiVED CYCLE GAS & STEAM TURBINE POWER PLQN COMPONENTS 201

Figure 5-14 isition. It must provide assurance of safe and reliable

machine, the steam process should likewise be corres-

operation of the plant as a whole, thereby reducing erator error. For this reason, the control and auto-

n though the process itself is fairly simple. Modern

nd raises the availability of the power station. op controls as an example, Fig. 5-15 shows the

erarchic system of this type. A hlghly automated 1 system encompasses three hierarchic levels:

vel, all individual drives are controlled and mon- y devices in the switchgear act directly on the relays. These signals are sent both to the logical f the drive level and to the higher hierarchic

rocess are gathered together into functional ontrol circuit on this level encompass inter-

ctional groups are:

Fig. 5-14: Typical Single-Line Diagram for a C

1 Gas turbine generator 2 Steam turbine generator 3 or 3' Gas turbine block transformer(s) 10 4 Steam turbine block transformer 5,6,7 Station service transformers s the logical control circuits that link

nother. These include, for exam-

202 COMBINED CYCLE GAS & STEAM TURBINE POWER PLAN COMPONENTS 203

bines or overriding logical controls that coordinate the op g river or seawater

irect cooling with a wet cooling tower installations. ect air cooling in an air condenser

As in conventional plants, process computers are t hree, no cooling water is required. In- ing an important role in combined-cycle installations, cooling requires additional water to replace evapora-

in losses. The amount needed is approximately equal optimum operation and maintenance. Some of the t e first variant with direct water cool-

water in an amount approximately 40 to 50 times n that of the condensate.

of the intervals between cleanings and overhauls, et lants are equipped with flue gas bypasses

5.7 Other Components n of the gas turbine(s), the design and

In addition to the major components mentioned gas damper(s) play a special role. This

combined-cycle power plant also includes much m et the following requirements:

and many other systems similar to those in conve ovide a tight seal both to the boiler and to power stations. For example:

e condenser rt-up or shut-down, it must take over the f the firing in a conventional boiler.

* cooling system ty must be high. If it malfunctions, the gas * feedwater tank / deaerator the boiler go out of service.

* feed pumps pers are provided for the bypass

condensate pumps they must be interlocked with an never both be closed si-

piping and fittings

* condenser ejector system flue gas damper with only one flap valve.

* water treatment plant hut off either the boiler or the bypass stack. * compressed air supply at both paths can never be closed at the same

* flue gas bypass

* steam turbine bypass

204 COM

Figure

Fig. 5-15: Hi

Chapter 6

CONTROL AND AUTOMATION

y power plant must cover the demand of an electrical grid . The equipment discussed below is

modern combined-cycle installations not only regulate lso to assure the safe start-up and

their proper dynamic behavior.

Concept of Closed-Loop Control

d-loop controls for a power station can be grouped into

which adjusts the output of

ndary control circuits which maintain the im- ers within permissible limits. eratures, or pressure are like- "

ically, this leads to the hierarchic structure of the 1 system already mentioned in Section 5.6.

y/Load Contr 01

ing of several gas turbines and there are several possible way to adjust the

output of the gas turbines is adjusted

he steam and the gas turbines are ad-

90 7


The first method is generally applied for plants without plementary firing because varying the steam turbine load duces only a temporary effect. Over th from the steam turbine automatically adjusts to the amou heat being supplied by the gas turbine. suggested that a load or frequency c be provided in plants without suppl sudden increases or decreases in load. Additional compl and the poorer efficiencies at full and part loads, and ex the fact that the gas turbine(s) generate(s) approx. tw of the total power output, argue generally for a solutio out a control for the steam turbine power output.

Fig. 6-1 shows the design of the closed-loop load con the output of the steam turbine is not controlled. The e cycle is operating with purely sliding pressure; th bine inlet valves are fully open. This is the mode best suited for high part-load effici the steam turbine exhaust during low level control is not absolutely necessary because put of the plant can be adjusted by changing t the individual gas turbine controls.

The output of the gas turbine is amount of fuel supplied. In the upper lo times also be adjusted by varying the am iable guide vanes in the first stage of the co used to accomplish this. With a system of t inlet temperature remains constant between 80% load. Below that level, the temperatu in order to protect the last turbine stag atures.

If supplementary firing has been p idea to equip the steam turbine with a lo process then operates in a manner simila tional steam plant.

210 COMBINED CYCLE GAS & STEAM TURBINE POWER P U N CONTROL AND AUTOMATION 21 1

Fig. 6-2 shows one possible design for the closed-loop Con maintain safe operating conditions. These will be described

e The turbine inlet valve regulates the power output the steam turbine. rum level control:

e The amount of steam generated is varied to fit demand by adjusting the supplementary firing in orde is normally a three-element control system which forms

to maintain a constant live steam pressure. rial from the feedwater and live steam flows and the level the drum. This signal is used to position the feedwater

A control system of this type can be of advantage w valve. Low Pressure drums are often equipped only with fairly large jumps and changes in power plant load are r ement control that uses only the level within the drum The steam turbine is then capable of taking over a P the load surges. This affects the service life of the ga positively because it reduces the changes in load and steam temperature control:

changes in temperature in that turbine. of the low flue gas temperatures, no control of the temperature is absolutely necessary with unfired waste

up to this point, no distinction has been made b rs. If such a control is provided, it serves more as a and frequency control systems. In principle, the re n as an actual control. Its purpose is generally to re- valid for both. However, one must bear in mind t erature peaks under extreme operating conditions per load range, the gas turbine can be switched & operation. For that reason, the cooling often ature control. In that case, neither fluctuations i er the superheater and not between two portions load have any influence on the plant: it is con eater as in a conventional steam generator. Nor- the turbine inlet temperature. ssure feedwater is injected into the live steam

In conclusion, it can be said that combined- m down to the required temperature. However,

very well suited to rapid load changes. The $3 deposits in the turbine, it is necessary to operate

extremely quickly because their time cons fully demineralized water. This is impossible in

soon as the fuel valve opens, more added P a1 plants, which means that the temperature of

able on the shaft. Gas turbine load jumps 0 must be controlled either by mixing it with sat-

sible, but they cannot be recommended sin xtracted after the drum or by means of a heat

detrimental for the life expectancy of the tu change in load produces thermal stresses re control extending over a broad load range cause of the change in turbine inlet tern waste-heat boiler operation because the tur-

mPeratUres drop off rapidly during part-load 6.1.2 Secondary Closed Control L s turbine. In plants with supplementary fir-

Fig. 6-3 shows the essential closed con s are more like those of a conventional steam quired in a combined-cycle process with elevated gas temperatures are possible in

CONTROL A N D AUTOMATION 2 13

274 COMBINED CYCLE GAS & STEAM TURBINE POWER PLANTS CONTROL AND AUTOMATION 275

these cases, it is important that the temperatures of the ste and the superheater tubes be maintained within safe limits. 9 steam turbine control valves

do this, the superheater is divided into two or three sectio steam turbine bypass

with water injected between them. 9 starting valve

c. Feedwater temperature:

turbine bypass7 which can provide several advantages: load range, drop significantly below the acid or water de

fore that the feedwater temperature er start-up times

a more or less constant level corresponding approxi the acid dewpoint.

Whenever steam is being extracted from a turbine b

the or into a starting condenser. waste heat boiler, the opposite problem occurs du operation of the gas turbines: more low Pressur is frequently omitted due to economical

crated than required. This excess energy must be d specifically for the following reasons:

generated, or by directing the excess low Press

d. Live steam pressure:

ous. However, such a control is necessary f

pressure operation, on the other hand7 t s bypass is fairly expensive. gaged at all times.

ere are also many advantages in having Bow this control is accomplished igned flue gas bypass:

of the power plant. In principle one

216 COMBINED CYCLE GAS & STEAM TURBINE POWER PLANTS CONTROL A N D AUTOMATION 217

greater flexibility in operation r, increased availability, since the gas turbine can con-

tinue in operation even when the boiler is unavailabl Other control loops

@ reduced risk of explosion within the boiler because the gas turbine can be lit off in bypass operation for lube oil and control oil

sure, etc. will not be discussed here.

cycle plant with an unfired heat recovery boiler. Combined-Cycle Plant Table (6-1: Various Control Systems Used to Regulate ined-c~cle Power plants are usually started up and shut

Live Steam P ~ ? s S U ~ ~

tI-01~ for units to be activated during start- down from the central control room. Whether the

to be issued to the individual drives or drive goups ng staff or from a higher-level automatic starting

ust be decided on a case-by-case basis. In base-load s that operate with only a few starts, full automation Normal operation (sliding pressure) -

ity. The starting Normal operation (fixed pressure) - riding on whether or not the plant has Operation at low load

Steam turbine switch-off or trip

Waste heat boiler switch-off

YPass is provided, the gas turbine can be started, and loaded practically independently of the steam

as the gas turbine is on line, the steam process 0 possible to switch on the is in load operation. The

operation, pressure is held constant by used to adjust the heat supply to the require- heat boiler. It may be necessary to place a

sure only in such special cases as start-up, e gas temperature during a cold start of the tary firing shut-down, etc. the superheater cannot handle the full flue

I h u t being cooled. The steam turbine can e. ~ e v e l in feedwater tank and hot ta are high enough, which

These are controlled by regulating the the steam must have reached about 40 to

cycle make-up water. What level is a*i t least 50 to 80

216 COMBINED CYCLE GAS & STEAM TURBINE P(

@ greater flexibility in operation

@ increased availability, since the gas turbi,,, bU1. -- tinue in operation even when the boiler is unavail

@ reduced risk of explosion within the boiler becaus the gas turbine can be lit off in bypass operation

Table 6-1 shows which live steam pressure control syste its are used for the various modes of operation of a con cycle plant with an unfired heat recovery boiler.

Table (6-1: Various Control Systems Used to Regulate Live Steam Pressure

Start-up Shut-down Normal operation (sliding pressure) -

Normal operation (fixed pressure) -

Operation at low load Steam turbine switch-off or trip Waste heat boiler switch-off

the gas turbine is in load operation. The operation, pressure is held constant by var e used to adjust the heat supply to the require- to the boiler. The steam turbine takes over heat boiler. It may be necessary to place a

e gas temperature during a cold start of the tary firing shut-down, etc. the superheater cannot handle the full flue

I h u t being cooled. The steam turbine can e. ~ e v e l in feedwater tank and hotwe as the steam data are high enough, which

t the steam must have reached about 40 to

cycle make-up water. What level is a*i re and be-superheated by at least 50 to 80

218 COMBINED CYCLE GAS & STEAM TURBINE POWER PLANTS CONTROL AND AUTOMATION 219

The by~ass stack is used for light-off of the gas turbine. If the .2 Dynamic Behavior "

flue gas damper closes 100% tight, no purging of the waste hea e dynamic behavior of modern combined-cycle plants is char- boiler is required prior to start-up since no fuel can get into t boiler even if fuel system malfunctions should occur. If the fl gas damper is not 100% tight, it is best to purge the boiler be starting the gas turbine, particularly after a malfunction or ove the gas turbine can be started and loaded quickly. overhaul. Use its reaction time is also short, it is capable of following

changes and surges in load. Generally, load changes are Until the steam turbine takes over the full steam flow, uated only by adjusting the turbine inlet temperature. AS

excess steam flows across the steam turbine bypass or the s It7 every major change in load reduces the life expectancy ing valve. ~f supplementary firing has been provided, it turbines more than would be the case for a steam tur- not be lit until the gas turbine is at full load and the ste ere are variable guide vanes at the compressor inlet, bine has taken over the entire steam flow. With the SuPP uce the thWmal Stresses in the upper load range because tary firing on, the steam process can then be further 10 The plant is shut down by shedding load from the gas

One factor negatively affecting expected service life (or first, from the supplementary firing). Once the flue

m~lo~in€! only throttling of the fuel supply. bine is shut down. The boiler and the gas turbine are the unloaded and shut off. The bypass damper must be Process with an unfired waste heat boiler has low the boiler damper closed before shutting down the g ata. It is therefore quite capable of following the

P times of the gas turbine. However, the steam tur- b. Plants without a flue gas bypass

Here attention must be paid to the Steam Process ing the gas turbine since the entire flow of flue gas P the boiler. Particularly during a cold start of the was bined-c~cle plants with ratings between 100 and the gas turbine must not be loaded at full speed 0 be started within the following times:

temperature change in the boiler drum would exc mum level permissible. A second minor problem (after 8 to 14 hr. at standstill): 20 to 50

light-off of the gas turbine, during which a high : 60 to 120 minutes

the flue gas temperature.

In order to prevent explosions, it Can be advi as turbines are already at full load after 10 to

with oil-fired gas turbines, to purge the b i l e of the Power output is already available after

the gas turbine. This is done by operating the in a cold start.

nition speed for a few minutes, without ign the start-up and shut-down of combined-cycl without flue gas bypasses are very similar.

OPERATING AND PART-LOIPD BEHAVIOR

ay in which a power plant responds to changes in its conditions (ambient conditions, part load) is of great im-

ce both for its economy and its safe operation. It is there- rtant to have precise knowledge of both the steady-state dynamic operating behavior of the plant.

cal calculation of the dynamic behavior is costly and or that reason, one frequently limits himself to op- rience in other similar plants or to estimates. A more tion of the behavior would certainly be advantage- usually omitted due to considerations of time and es, however, the calculations of steady-state oper- -load behavior should be completed.

sed for Calculations

of the steady-state part-load and operating be- am portion in a combined-cycle plant differs that for a conventional steam plant. The dif-

mainly the boiler and the operating mode of aste heat boiler, the heat is transfered mainly ection, while in a conventional boiler it takes

e of a combined-cycle plant functions most e sliding pressure - sliding temperature pro-

'uncontrolled." The steam data are deter-

223

224 COMBINED CYCLE GAS & STEAM TURBINE POWER PLANTS OPERATING AND PART-LOAD BEHAVIOR 225

mined only by the exhaust flow and exhaust temperature of the striking, but the efficiency of the combined cycle is ap- gas turbine and by the swallowing capacity of the steam tur- proximately 50% higher at every point than that of the gas turbine. In contrast, a conventional plant is generally operated at bine. In other words: the ratio between the outputs of the gas a fixed pressure, i.e., the live steam pressure and temperature e remains approximately constant remain constant. That simplifies calculations because the s l ~ a m ~ o s s the entire range of loads. Fig. 7-2 shows the curves for pressure and the steam temperature are known in advance. The wer outputs and live steam data. steam turbine and the boiler can therefore be considered inde- pendent of one another. Calculations for the gas turbine are no n based on purely sliding pressure problem since one is dealing with standard machi ation above 50% load. Below that point, the five steam pres- correction curves are available to account for changes in a is held constant by means of the steam turbine inlet valves.

bient conditions and for part-load operation. g. 7-1 is the quick deterioration ciency at Part loads. This is due to load control of the gas

Statement of the Problem e (by changing the turbine inlet temperature). At pad-load

Calculating the operating behavior of an installation dire ingle-shaft machines, the flow of intake air remains prac-

from the geometry of that unit would be very ti s reduced, the turbine inlet tern-

The process can, however, be simplified by referring all the average temperature of the

to the thermodynamic data at the design point- If that ng supplied and thus the efficiency. Improvements could

point is known, general equations (the Law of cones, he fer law, etc.) can be used to reduce the calcula r taken in during part-load a reasonable number of equations, without the nece considering the dimensions of the unit itself. lling several gas turbines the details of the method for calculation described in R can be found in the Appendix to this Chapter.

7.2 Part-Load Behavior vanes in the cornpressor

A careful economic evaluation must also gi ting the intake air

to the part-load behavior of a power plant. Power also have as high an efficiency as possible at ~art-loa dern combined-cycle plants without supplementar achines are available only for low ratings: they

efficiency of the plant as a whole depends main1 in Section 5.1. Compressors

ficiency of the gas turbine. et guide vanes are offered for use in combined- several suppliers of gas turbines. Using this

Fig. 7-1 shows the efficiency curves for a gas wer output can be rduced to approx. 80 to 85% combined-cycle plant, based on the full-load ge in the turbine inlet temperature. Below that gas turbine. The similarity of the paths of these t reduced to avoid overheat-

226 COMBINED CYCLE GAS & STUM TURBINE POWER PLANTS OPERATING AND PART-LOAD BEHAVIOR 227

Fig. 7-1: Part-load Efficiency of the Gas Turbine and the Comb

Combined-cycle plant Outputs and Live Steam Data of a Combined-Cycle Plant a t Part

Relative power output r output of the combined-cycle plant Relative efficiency e output to gas turbine output

Reference 100% load of gas turbine

228 COMBINED CYCLE GAS & STEAM TURBINE POWER PL4NTS OPERA TING AND PART-L OA D BEIfA VIOR 229

Fig. 7-3 shows the part-load efficiency of a combined-ey

ing the intake air is that there are no particular difficu doing it. This increases the efficiency of a combined-cycl (refer to Section 7.3), but, of course, a source of heat is re

low pressure steam from a waste heat boiler. Fig. 7-5 s

per load range, the efficiency here is even better th

its effectiveness drops off when the ambient temer The air can only be heated to approx. 50 - 55 O C ( without exceeding the limit imposed by compressor

To make an even greater improvement in part-load a design employing several gas turbines should be

dividual gas turbines at part loads. The other gas tu run at a higher load and a higher inlet temperature, erts a positive effect on the overall efficiency. 0

one steam turbine. The load of the plant as a wh

C C PLANT W I T H VARIABLE IGV

@ down to 75%, there is a parallel reduction i -- CC PLANT WITHOUT VARIABLE IGV all four gas turbines

@ at 75% one gas turbine is shut down cy of a Combined-Cycle Plant with Variable Inlet

down to 50%, there is a parallel reductio the three remaining gas turbines

e combined-cycle plant @ at 50%, a second gas turbine is shut dow

guide vane control


Figure 7-4

OPERATING AND PART-LOAD BEHAVIOR 231

OPERATlNG A N D PART-LOAD BEHAVIOR 233

ith this mode of operation, the efficiency at 75%, 50%, and load is practically as high as that at full load.

The Effect of Ambient Temperatures

e power output of the gas turbine and the combined- . The behavior of the plant designed responds to

ir temperature rises while the vacuum within the con-

a constant temperature, air is used either directly

r comparable to that indicated by the curves in Sec- ce the steam turbine designed for this case has been for a given condenser pressure.

g Water Temperature

the cooling water temperature affects the volume st steam. Quite quickly, the exhaust steam vol-

This either increases exit losses when the

ction 2, in which the size of the turbine is to the temperature of the cooling water.

effect of the cooling water temperature on the power output of a steam process.

234 COMBINED CYCLE GAS & STEAM TURBINE PLANTS OPERA TlNG AND PART-LOAD BEHA VlOR 235

.4 Aceptance Tests and Commissioning

Acceptance Tests

for a combined-cycle plant are always a spe- blem because the gas turbine, waste heat boiler, and steam all interact upon one another. It is therefore best to award

cts for combined-cycle plants to a single general contrac- o will assume responsibility for supplying the entire plant

semi-turnkey basis and who gilarantees the fficiency of the plant as whole. For combined-

nts, it is easier in any event to measure the values gu- for overall plant performance than those for each ma-

vidually. The amount of waste heat supplied te heat boiler by the gas turbine, in particular, cannot

red accurately. When overall values are guaranteed, rical output, and ambient conditions of the e measured. These are quantities which can

ined with relative exactness.

es guaranteed for a power plant or one of ponents are valid only if all ambient or design con-

Suitable correction curves must be used any deviations. Thus, for example, the effect that

as on the power output and efficiency of must be measured and - if need be- correction

wer stations and for gas turbines, the me- Fig 7-7: Effect d Cooling Water ~ e m p e r a t u ~ ~ on the rrections are described in the standards (e.g.,

cycle Plant (Operating Performance) tc.). No standards have yet been established tCW Cooling water temperature plants. We will therefore indicate below one

is problem that has proven useful.

that a limit must be defined around the plant onents and systems supplied by a given

236 COMBINED CYCLE GAS & STEAM TURBINE POWER PMNTS OPERATING AND PART-LOAD BEI-IAVIOR 237

contractor are included, and only those components and SYs is way, one takes into account the fact that the model ~ 1 1 others for which he is not responsible are excluded- or calculations is, strictly speaking, valid only for the guaranteed values must clearly define the ambient or des tical (guaranteed) installation, not for the installation as conditions for which they are valid, i.e., the conditions as ally has been built. This method is especially suitable if sured at the limit defining his responsibility. In the c ter programs are used for making the corrections, since combined-cycle plant used for power generation only an best calculate the behavior of the theoretical plant. plied on a turnkey basis, such values and the margins tions to be defined might be as follows: standards, however, recommend a second, reverse pro-

he values measured are corrected to the design con-

a. Guaranteed values he guarantee, since in many cases, guarantees must r several load points. In a contract, it is best to make

e Overall power output of the combined-cycle pla for this fact by using a guaranteed weighted average e efficiency

b. Design or Ambient Conditions for the Guarante e marginal conditions are generally used for all load e Air temperature e can scarcely assume that the ambient conditions e Air pressure ill actually remain unchanged while all load points

e Relative humidity asured. The theoretically correct procedure thus

e Cooling water temperature, and flow (if cool thematically and contractually to unsolvable prob- pumps are supplied by another party) lating the average value measured. On the other

procedure indicated in the standards is used, e Frequency

nts are corrected to warranty conditions. There e Power factor of the generators blem in comparing guarantees with measurements.

tical arguments favoring the first of these proce- c. Comparison between Measured and Guar countered by recognizing that the ambient condi-

TWO different methods can be used for doing the theoretical and the actual plants in similar the values measured are compared with those g ure in the standards is therefore not incorrect.

mbient conditions must remain within bounds

0 Correction of the guaranteed values to t rements. This is another reason why the cor- conditions at the time of measurement e valid both for the theoretical and for the ac-

0 Comparison of the data measured with t ill not cause significant error. rected guarantees

238 COMBINED CYCLE GAS & STEAM TURBINE P O W PLANTS

Correction of the Measured Power Output of a Combined-Cycle Plant

It has proven to be a good idea to correct the power outp of the gas turbine and the steam turbine separately. For the turbine, the usual correction curves are used to take into acco the effects produced by air temperature, air pressure, rotatio speed, etc.

The power output measured for the steam turbine is con using curves that show the indirect effects of air tempe air pressure, and gas turbine speed on the steam proce the direct effect of the cooling water temperature. To ca these curves, it is best to use a computer model which si the steam process as a whole (refer to Section 7.1). Cha the data of the ambient air produce changes in the gas t exhaust data and these latter affect the power outpu steam turbine.

The advantage of this procedure is that it can, wi adaptations, be used even if the gas turbine is put i tion at a somewhat earlier date than the steam turbin ing to the standards, both the gas turbine and the st must be measured as new machines. That necessarily a certain time interval will separate the gas turbin turbine measurements.

To demonstrate that the guarantees have be aranteed power output is compared with the pow sured and corrected.

The power output, measured and corrected, is lows:

Pic-~orr = PST-~orr + PGT-~orr

OPERATING AND PART-LOAD BEHAVIOR 239

the Measured Efficiency %Cycle Plant

:ed efficiency of a combined-cycle plant without firing may be written in the form:

compare the measurements, the heat flow supplied must rrected. The measured and corrected efficiency is then ob- d from the equation:

(20)

od idea to present the correction curves for gas tur- input can be corrected directly

ouring via the efficiency of the gas turbine. This pro- ined-cycle plants with supple-

portant whether or not the waste heat boiler is a flue gas bypass. If so, the gas turbine can be

process. Moreover, an earlier uite conceivable, since stan- r lead time than steam tur-

re designed and built on a case-by-case basis.

ing of the steam tubine can be similiar to that steam turbine plant, with the flue gas damp- the boiler firing. If there is no flue gas by-

heat boiler must be put into 1 to one another. The gas turbine cannot start

dy for it and, inversely, the ue gas available for it.


Chapter 8 attention must be paid to coordinating the operating mode the waste heat boiler, the gas, and the steam turbines. Thi

POWER STAT1

s 2 and 3 show that combined-cycle plants are thermo-

ental impact, fuel availability, etc. must also be taken

lowing, we will compare the combined-cycle plant

competition comes at present from steam and gas

lant can also be a genuine alternative. The high odern diesel engines is comparable to that of a

W, it is thoroughly conceivable that diesel e the optimum choice. On the other hand,

estment and maintenance costs are higher bined-cycle plant, without compensating

icult to attain low emission levels with

242 COMBINED CYCLE GAS & STEAM TURBINE COMPARISON IF THE COMBINED-CYCLE PLANT 243

investmants (annuity factor) and on the load factor of the plant,

power stations: which? in turn, depends on the load, the operating time desired, to 8.1.4). Capital costs are also in-

e steam turbine plants

e gas turbine plants he specific fuel costs are inversely proportional to the aver-

combined-cycle plants of the installation. This average efficiency must

~h~ main range of ratings under consideration is between t, be confused with the thermal efficiency at rated d . It is defined as follows:

and 500 MW.

(21)

ration alone because their relative costs increase as the Cy, which takes into account the rating decreases. They are best used mainly for industri tfict heating power stations, but even for these applicatio

economical size is apProx. 10 MW. hut-down losses

8.1 Economy other heat and energy losses, e.g., due Every industrial plant strives to keep production ng, operator error, etc.

as possible, and power plants are no exception in t mistrative costs include: Political factors and environmental protection legisla

on and administration (staff

le costs of operation and repair (maintenance, ement parts, etc.)

dude three types of ~0sts: he various costs for a power station

e capital costs different times. For that reason, for financial cal- e fuel costs

operation and administrative ~ 0 s t s g into commercial operation. These 0Unt.s are referred to as "present valuew.

The first of these depends on the price and rate for the plant, on interest, Or on the desired

244 COMBINED CYCLE GAS & STEAM TURBINE POWER PLANTS COMPARISON IF THE COMBINED-CYCLE PLANT 245

The simplified formula used to figure the present value 1.1 Cornparison of efficiency all expenses is: t today's fuel prices, the thermal efficiency is the crucial fac-

for installations operated at medium and base load. For that isite for having an econom-

8-1 shows how the thermal efficiency at rated load for s of power plant under consideration depends on the

have been further broken

commissioning, such as the price of the plant, e o plants with and without reheating. tion interest, etc.); in monetary units reheating steam plants have not been considered be-

TNj Equivalent utilization time at rated Power outp mong the combined-cycle per annum; T N ~ = energy generated during the ly limited supplementary vided by the rated output

yF price of fuel, in monetary units Per kW hr. akes clear the thermodynamic superiority of the le. Surpassed by far are the gas turbines which, igh turbine inlet temperature of approx. 1100 O C

$b Annuity factor in 11% 9 = ly attain an efficiency of 30% to 35%.

interest rate arison of Price

Amortization period in years is the most important criterion for se- w the specific investment costs for the

p Rated power output in kW power plant depend on their power output. u Operating and adminstrative costs, inch turnkey installation including machine

insurance, in monetary units per annum ot as workshop, offices, staff facilities, and n based on 1988 price levels and

The power production costs can be derived fr ts, and do not include interest during value using the following formula: data shown merely indicate trends: appro-

be taken in applying them, since very er plant: erection site,

ation, impediments to

Below we will discuss and compare the tors affecting the economy of a power plant f

248 COMBINED CYCLE GAS & STEAM TURBINE POWER PLANTS COMPARISON IF THE COMBINED-CYCLE PLANT 249

for the gas turbine, which have contributed significantly t wide-spread acceptance.

Steam power stations are significantly more expensi a combined-cycle power plant. A coal-burning plant, fo values for availability can be stated that will be valid for ple, costs two to three times as much as a combined-cyc ses since such factors as preventive maintenance and op-

with the same power output. Modern combined-cycle plants are therefore simpler and less expensive than stea

Nuclear power plants have not been included in this rison because the investment costs required for them dependent upon political and other local considerat ed plants are as follows:

8.1.3 Comparison of Operating and 88 - 95 %

Administrative Costs turbine plants (oil or gas-fired) 85 - 90 %

80 - 85 %

affect the economy of a power plant only slightly. T ed-cycle plants (gas-fired) 85 - 90 %

only to 5 to 10% as much as the fuel costs. res are valid for plants operated at base load; they

ps and shut-downs greatly reduce Me expectancy

than a steam turbine alone. Little staff and main nd therefore increase the scheduled maintenance

quired. A steam power plant requires more staff nance costs are higher. Combined-cycle plants fall ors determining plant availability are:

like a gas turbine plant, and those with maxim tary firing more like a steam turbine power pl

8.1.4 Comparison of Availability ion (base, medium, or peak load)

The availability of a power plant greatly a Whenever a unit is down, the electricity mu nd skill of the operating and mainte- ated in another power station or- if there is


8.1.5 Comparison of Construction Time

The time required for construction affects the economy unit. The longer it takes, the larger the capital rqeuireme be written off, since construction interest, price increas materials, insurance, and taxes during the construction add to the price of the plant.

Fig. 8-3 shows the amount of time required to build t ious types of power plants. The gas turbine, because of i dardized design, can be built with the shortest lead time has encouraged its widespread acceptance. More time is up to the completion of a combined-cycle plant. One c ever, commission the gas turbines prior to the stea so that from 60 to 70% of the power output is avail the same time as would be required for a gas turbine po This is a great advantage over a conventional which can deliver power only after two to

8.1.6 Comparison of Economy

The diagrams below show the effects of the mos parameters on the economy of a power plant:

1. Fig. 8-4 to 8-6: Dependence of Cost of Pow on Fuel Prices, for 50, 200, and 500 MW pla

2. Fig. 8-7 to 8-9: Dependence of Cost of Pow on the Equivalent Utilization Time, for 50, MW plants

3. Fig. 8-10: Dependence of Cost of Power G Annuity Factor, for a 200 MW plant.

A combined-cycle power plant has less fu steam power plant. The question can therefor an oil or gas burning combined-cycle plant i

COMPARISON IF THE COMBINED-CYCLE PUNT

-3

260 COMBINED CYCLE GAS & STEAM TURBINE POWER PUNTS COMPARISON IF THE COMBINED-CYCLE PUNT 261

-- - - " - -

@ risk of a supply shortage due to political interfer such as war, boycott, etc.

@ political opposition to nuclear power plants

@ environmental protection

The result of all these factors may well be that the fuel is may be one other than that which appears best at of plant construction.

The long-term source of fuel can be taken into account o to a limited extent, by including in the calculations of econo costs an estimated price increase for the fuel during the expect service life of the unit. However such estimates should be h dled with care. In any case, the greater the fuel flexibility

tions about long-term developments in the prices for the vari- A second requirement for burning heavy oil or crude in a gas ous possible fuels. In this regard, the following aspects can be- turbine is the correct treatment of the fuel, generally by means come important in selecting the type of power station to be built:

le to remove or inhibit elements that cause high temperature long-term availability of the fuel at a reasonable cost rrosion, such as vanadium, sodium, etc. The specific problems

~ence, volved with the burning of coal in a combined-cycle plant are t with in more detail in Section 9.2.

le 8-1: Various Types of Power plants and the Fuels They can Burn

selected the time

the plant chosen, the less the risk from possible increases in f prices.

Table 8-1 lists the fuels that can be burned in the various p plants today.

The fuel flexibility of combined-cycle plants is less th of steam power plants. Some gas turbines can burn he or crude, provided the machine is designed to do so. In gas turbines are more suitable than those derived from nology. It is easier to burn special fuels in gas turbines w combustors than in those with several smaller combust annular combustor, since the latter are more sensitive to in flame length, radiation, etc.

lies to the gas turbine. In the supplementary firing, however, n be burned just as well as in a conventional steam generator.

nly in the supplementary firing.

used as gas turbine fuels. However, adaptations .ed if the heat value i s low.

Chapter 9

ENVIRONMENTAL CONSIDERATIONS

264 COMBINED CYCLE GAS & STEAM TURBINE POWER PLANTS ENVIRONMENTAL CONSIDERATIONS 265

hydrocarbons. The large air flows have the further advanta of strongly diluting the pollutants. that level, the flame temperature is higher but there is less

. Above that level, NOx decreases because an over- for use in heavily populated areas. Particularly when the burned is natural gas, the only toxic emissions contained in exhausts are NO and NO2. The NOx (NO + NO21 level is low excess air ratios are beneficial from the point of view

f~xmation but are very detrimental to efficiency and cause NO, generates nitric acid (H2NO3) in the atmosphere, and t together with sulphuric acid (H2S04) is One of the factors re ible for acid rain.

Y, gas turbine combustors operate with an excess air

9.1 Reduction of Nox Emissions prox. 1 at full load, ensuring a good, stable combus- the entire load range. Obviously, NOx emissions will

NOx is produced in large quantities only at very high t h unless special precautions are taken. Typically, NO, he exhaust gases after mixing with the cooling and air are in the range from 120 to 300 vppm.

after a very long time. The situation in a gas turbine plest way to reduce the NOx concentration is to cool

ich can easily be accomplished by injecting water it. Fig. 9-4 shows the reduction factors for NO,

combustor are thus: iNected, indicated by the coefficient D , the ra-

the excess air ratio of the combustion ( h ) e flows of water or steam and fuel. At a ratio ical reduction factor is approx. 5 with water, and steam. Steam is less efficient than water because

e the duration of the combustion

temperatures are high, such as those found in t m a gas-fired gas turbine or a combined-cycle vppm. In some cases, even 25 vppm is attain-

Fig. 9-3 also shows how concentrations of e following disadvantages:

--) 7 N2 + 12 Hz0

ly these are well-proven systems, but they entail the

270 COMBINED CYCLE GAS & STEAM TURBINE PO - - * - '--

@ Large amounts of demineralized water are required

@ The efficiency of the combined-cycle plant is lower particularly if water injection is used.

The fact that the output capability of the same plant is h especially with water injection, only partly compensates for disadvantages.

Fig. 9-5 shows how steam and water injection affect th put and efficiency of a combined-cycle plant, as a funct the ratio of water to fuel, 0 . With a ratio 0 = 1, the foll changes in output and efficiency from those in a normal c cle without injection may be considered as typical:

WER PLAN13 ENVIRONMENTAL CONSIDERATIONS 277

those with injection systems. Section 10.3 provides further, e detailed information about these combustors, referred to ry low NOx combustors.

e local regulations, e.g., those in California or Tokyo, re- NOx emission levels much lower than 40 vppm. In these

erally necessary to install a reduction system in haust system. These systems, known as "Selective Cat- eduction" (SCR) systems, inject ammonia (NH3) into the gases before a catalyst and can thereby remove approx. the NOx from them. The chemical reactions involved

+ 4NH3 + 0 2 --b 4N2 + 6H20 Table 9-1: output and Efficiency of a Combined-Cycle

with Water or Steam Injection, as Compared the Same Plant without Injection

(equal to about 20% of the

ent costs are high (the life expectancy of the s between 4 and 8 years).

talled in the center of the evaporator of the waste heat boiler,

With steam injection, the cycle is similar to the exa &ion functions properly only at tempera-

00 OC (572 and 752 OF) .

in fig. 3-50, but there is a dual steam admission into ammonia is necessary.

A 3-pressure cycle as shown in fig. 3-49 would re slightly lower because of the increased in efficiency caused by the steam iaection but th in the waste heat boiler.

SCR system for installation in a heat plant (refer also to Section 3.1.4).

s in conjunction with steam or water idec- hnicall~ possible to attain an NO, level in

from a corr)-bined-cycle plant of less than 10


Figure 9-5

ENVIRONMENTAL CONSIDERA TIONS 273

274 COMBINED CYCLE GAS & STEAM TURBINE POWER PLANTS EN v i R o N M ~ ~ ~ ~ ~ CONSIDERA TIONS 275

9-2 shows the mounts of Waste heat that must be dis- atedl as percentages of the primary energy input. cooling tow-

at least in a coal-burning plant- much higher. have been taken into consideration, i.e. , the condenser considered as being cooled with river or sea water.

9.2 SO, Emissions as turbine requires practically no cooling water, which has The concentrations of SO2 and SO3 produced depend ma greatly to its widespread acceptance in water-poor

on the quality of the fuel. Because gas turbines generally clean fuels, this is less of a problem in combined-cycle than in coal-burning power stations. The latter plants ca 9-2: of the Heat to be Dissipated ever, be equipped with scrubbers which reduce sulph as Percentages of the Energy input sions by approximately 90% by converting the So2 into of Paris. A similar system could in theory be installed

phur content is therefore a less costly solution.

9.3 Waste Heat Rejection

Another environmental problem is the ~ a s t e he power station supplies to the environment. Here1 t

of waste heat remaining.

in which that heat is given off the the enviro portant. The effect is less if the Power plant stead of giving off its heat to a river or the se

advantage over a steam plant in that it r much cooling water.

g lower injectio

NOx emission levels without ~n (dry low NOx combustors)

Chapter 10

DEVELOPMENTAL TRENDS

trends in development are in three directions:

ward increased efficiency and power output of the

rd the use of coal in combined-cycle plants

rd attaining lower NOx emission levels without r or steam injection (dry low NOx combustors)

of these trends is a continuation of the development d to the break-through of the combined-cycle plant

t to use fuels other than oil or natural gas in le plants or gas turbines is not new. Development ntrating mainly on the utilization of coal in plants egrated Coal Gasification Combined-Cycle (IGCC)

Fluidized Bed Combustor (PFBC) plants.

as Turbines

ct of a high gas turbine inlet temperature on as turbine or a combined-cycle plant has ed in Section 2 (refer to Fig. 2-2). It seems

further improvement mainly in the direc- et temperatures, which has become possi-

opment of new materials and improved arch projects here are concerned mainly ling of the hot gas path of the gas turbine. g technologies, for example, employing

277

278 COMBINED CYCLE GAS & STmM TURBINE POWER PLANTS DEVELOPMENTAL TRENDS 279

water or steam cooling systems, do not yet appear close to a stage of development where they could be used in a real gas turbine

at making it possible to use coal

For these reasons, work at present is concentrating On the tly Or in combined-cycle installations. T~~ pat.s

velopment of new materials, e.g., ' superallo been suggested and seem close to commercial realization:

sion strength (ODs) alloys, directionally solidified (DS) bla and of more efficient air-cooling systems employing

o improved film cooling

o impingement cooling A Combined Cycle Plant with coal Gasification

The use of ceramic materials in gas turbines, mainly fo sification of coal is a Very old technology. Before nat-

ing, still appears far from ready for commercial applica as introduced on the market, coal gasification was used

cause of the very low reliability that must be expected on in urban areas. ~t is also used

blades. Ceramics are presently being used on1 ently in the chemical and petrochemical industries

in the hot gas path of a few gas turbines. raw materials for chemical processes.

Parallel to this, improvements are also being made to S based on partial combustion

the heat required for the gasi- cannot by fully exploited only if the pressu ess in which the coal is converted, generally chine is increased to an appropriate level. High uni also being attained by increasing the air flow IAro 1 C02, CH4, HgS, and H20. The

s involved are: pressor. With modern blading, Compressors are ab volume flows that seemed utopian Just a few Years - CO + H2 the use of transonic stages, it is already Pas valent outputs and pressure ratios with many fewe stages. - CHq + Hz0

With these improvements, combined-cycle is endothermic, the coal must be

power capacities of more than 200 MW for 50 tine: either air or oxygen into and 150 MW for 60 HZ applications mean low herefore be classified either as: combined-cycle power plants will b€?Come ev ing for large power stations. It can be expect produce a gas with a low ~ ~ 1 - combined-cycle plants will, within the next 1 ~ ~ i c a l l ~ 5000 to 6000 k ~ / k ~ (100 to 120 ficiencies of 52 to 55% or more (LHV).

280 COMBINED CYCLE G A S & STEAM TURBINE POWER PLANTS DEVELOPMENTAL TRENDS 28 1

g. 10-1 shows the working principle of these various types. gasifiers which are closest to large-scale commercial appli-

~ t ~ / ~ ~ f ) , approx. one-third the calorific value of

11, Dow, Prenflovv), both oxygen-blown.

Either type of gasifier can be employed in combined-cy plications; the advantages and disadvantages of each iers can also be classified according to their operating pres-

e., whether they operate at atmospheric pressure or at cussed further below.

Table 10-1 shows the composition of typical obtai bined-cycle plant applications because the gas pressure

the two different gasification processes. t reason, gasifiers for combined-cycle plants typically op- t 20 to 30 bar (275 to 420 psig). Table 10-2 shows the clas- on of the major gasification processes.

ning fuel gas than by cleaning the combustion gas -fired boiler. The main reasons for this are:

ume flow of the coal gas to be treated is less that of the exhaust gas from the boiler.

akes it economically feasible to remove more 5% of the sulphur, as compared to only 90% in bber of a coal-fired boiler.

uutants, i.e., heavy metals, chlorides, etc., oved in the gasification process.

In addition to this general chSsification ba ium used for oxidation, a distinction must on the flow pattern in the gasifier itself. Th

esdphurization processes, 5 to 10 times

1) fixed bed gasifiers with counter- roduct is produced, since the sulphur

2) fluidized bed gasifiers with bubbli 0 molecules of products like gypsum.

L MBtu m t u

mtrained Entrained Fluidized Fluidized . > 5l Co

\ Yes Yes rri I Yes I Yes Yes Yes (Yes) 2 ) (Yes) 2 ) 8 .- - syngas coo&'=& Yes Yes yes

I - quench 1

STEAM& ca.150Oo ASH or SLAG SLAG

COUNTER - FLOW

E N T R A I N E D FLOW

PARALLEL - FLOW

Fig. 10-1: Design Principles of the Main Types of Gasifiers

284 COMBINED CYCLE GAS & STEAM TURBINE POWER PLANTS DEVELOPMENTAL TRENDS 285

5) ~ o s t oxygen-blown gasifiers produce unleachable slag- enerated in the gasification plant, which demonstrates clearly waste disposal is far easier than with the ash produced e importance of integrating the gasification process into the by coal-fired boilers. mbined-cycle plant.

For all these reasons, it is clear that coal gasification may w be very interesting for power generation in the nlost promising method to produce a truly clean coal-burni e gas and the coal. Heat recovery after the gasifier is there-

plant. far less important, and such plants therefore usually gen-

Coal gasification can be either a system integrated into combined-cycle plant, or a non-integrated system, with the ish Gas-Lur@ slagging gasifiers. It can be seen quite clearly sification and combined-cycle plants quite dist e degree of integration is far less important here than other. Only the first of these two possibilitie interesting at present, since integration, with its good utili of the waste heat generated in the gasification process, greatly improves the overall efficiency of the entire sYs difference is that the gas produced in a fixed-bed gas-

One major consideration when designing an integrat lication combined-cycle plant (IGCC) is the temperature the gas is to be cleaned. All successful indus heavy hydrocarbons. used today work at near ambient temperatu that the coal gas must be cooled down after it leaves provides the main technical data of the IGCC plant ifiers. If the gas was very hot, it is important that t be recovered efficiently. One of the main di of 34% under IS0 conditions. fixed-bed gasifiers and entrained-bed gasifiers is temperature of the former is typically approx. 5 (900 - 1100 OF) and that of the latter approx. 1500 O

This is why entrained-bed gasifiers are equipped Illinois No. 6 ers to generate either saturated or superheated lue of the coal gas 12 500 kJkg (290 Btu/scf) steam. This steam is then used in the combined- 99 % generate more power. 4 135 mg/GJ (75 vppm)

577 MW The example shown in Fig. 10-2 is a typical I 173 MW

on a Shell entrained bed gasifier unit. The ste 110 MW the gasifiers and that from the heat-recovery ption for auxiliaries 33 MW of the combined-cycle plant both flow through 250 MW sure section of the steam turbine. About 40% 43.3 %


These figures show the efficiency that can be expected fro gas turbines with higher inlet temperatures an IGCC plant employing commercially proven componentsin 19 dry low NOx combustors TO reduce the NOx emissions to the low level indicated for th hot gas clean-up two plants, the fuel gas is mixed with water vapor. The costs such a plant are in approximately the same range as those introduction of cleaning systems operating at 400 to 500 modern Coal-fired units with scrubbers and de-NOx system 2 to 932 OF) would improve the outlook for air-blown gas-

At Present, this type of gasifier is at a disadvantage because er exergetic and energetic losses that occur during cool- gas prior to the gas treatment. The gas flow is approx-

TWO fairly large IGCC plants already built demonstrate th three times as neat as in the case of oxygen-blown gasifiers.

is a proven technology. The first of these is the 100 MW n as the fuel gas only needs to be cooled down to 400 to water plant in California, a demonstration plant that has be , that disadvantage will be greatly reduced. Air-blown

crating since 1984 with a Texaco gasifier. Although this PI n systems obviously have the advantage that no air sep-

an efficiency of only 31.3%, the main purpose in build* not to achieve the highest efficiency possible, but to sh the integration of coal gasif'ication and a combined-cycle PI er field for possible application of coal gasification could actually work. The second of these is at the Dew ehemic neration of power and chemical raw mat&& in the in Plaquemine, LA, with a total output of aPPrOx. 160 (e.g., power and methanol).

One very interesting feature of IGCC plants is their for phased or staged construction, i. e., an IGCC plant ssurized Fluidized Bed Cornbustion.

pleted in two or three steps: nd pressurized fluidized bed combustion systems

Step 1: the gas turbine portion different from that on which the IGCC is based. Step 2: conversion to a combined-cycle plant ion, one will want to burn coal cleanly, utilizing Step 3: coal gasification of the gas-fired combined-cycle plant. Essentially,

gas turbines with a hlgh turbine inlet temper- The advantage of this method for building an IGC

to this, PFBC takes its point of departure from course, the relatively small capital requirement for team power plants. steps. Larger investments do not have to be made when economical and operation considerations requ news, subsequent flue gas desulphurization and of the gas-fired plant into a coal-fired unit. very involved and expensive. For that reason,

both atmospheric and pressurized, desulphur- It can be expected that the efficiencies of I

hed by reaction of the sulphur with h e s t o n e year 2000 will be between 46 and 48% (LHV) du

on. Since the temperature must not exceed improvements: OF), relatively little NOx is produced as well.

DEVELOPMENTAL TRENDS 289

L TRENDS 291


peratures and the fact that the gas turbine is of a special d advantage of these arrangements is that the turbine is op- which cannot fully utilize the high temperature potential a on a clean medium. Their disadvantage, however, is the able with modern gas turbines because of restrictions on the rface area required for the atmospheric fluidized bed. A perature within the fluidized bed. Then too- unlike in plants problem lies in selection of the material for the fluidized a low temperature gas turbine, the economizer in a plant ling tubes, which are not as efficiently cooled. The heat a high temperature gas turbine operates at atmospheric pre r coefficient of air is lower than that of water or steam. because it is situated after the gas turbine. This affects the co ness of the plant. Dry LOW NOx Combustors

Suggestions for using the full temperature potential of as shown in Section 9.1, the methods commonly used to-

turbine do, of course, exist, but whether or not they c uce NOx emissions are indeed effective, but they entail

alized commercially is an open question, since they make t more complicated and problematic. These are based on gasifying the coal or directing it through a pyrolysis bef water consumption

plying it to the fluidized bed. The gas produced thereby uction in the efficiency of the combined-cycle

following the fluidized bed to bring the gas turbine inlet ature to the levels usual for normal turbines. Overall e arly in the case of base-load installations, this impacts of 48 to 50% should be attainable with such systems. on plant economy. Moreover, the high rate of water

How the PFBC process might make a breakthrough is not unproblematic from an environmental point

ket in competition to IGCC and conventional steam hese reasons, various development projects are cur-

is a question that remains to be answered. If succe ess, directed toward reducing NOx emissions by in the first four large-scale plants currently under combustion technology. Theoretically, thew are ..

the prospects for the process could well be very go t such development can take:

In addition to these two types of installations with -lean combustion (A < 1)

there are also suggestions for using a hot air turbine. ch combustion (A > 1) has been shown in Section 3.4 with a closed-cycl Similar systems employing an open air-process have e dependence of the NOx concentrations on

. Only a small amount of NOx can form in an stion, despite the high flame temperature, be-

In both cases, the air is used as the medium to co cely any oxygen available for that to happen. bed. It thereafter expands in the turbine before be to attain a complete combustion, there must bustion air in the open process. -up combustion stage in which there is almost

e to the lower temperature. This approach is steam generators and is referred to as "staged

s"

298 COMBINED CYCLE

Figure 10-7

FLA

GAS & STEAM TURBINE POWER

& - F U E L

Fig. 10-7: Premix Burner

I O N

ANTS DEVELOPMENTAL TRENDS 299

employed due to that factor. This type of burner has scarcely er worked well with liquid fuels.

ere are a few gas turbines from various manufacturers in com- cia1 operation that have dry low NOx combustors of this type, ating on gas only.

10-8 shows a combustor of this type equipped with 36 burn- ar to that in Fig. 10-7; Fig. 10-8 shows a photograph of

rners viewed from below, with the swirl baskets of the 36 hat are used to contain the flames. This combustor is on a 70 MW gas turbine. 36 burnem were selected in lve the problem of part-load operation. To prevent the

ratio in the burners from becorning too high at part loads, xtinguishing the flame, the burners can be switched on groups. Fig. 10-9 shows the loads at which the various

e switched on or off, and the correspondmg changes in trations in the exhausts. In all, there are five groups

this combustor: Fig. 10-10 shows how they are con- e fuel end and controlled with a single control valve.

other solutions to the problems at part load, such as, a more or less progressive switch-over from premix normal diffusion combustion, or the installation of

on the burner which allows more or less air to escape load involved. This also makes it possible to as- excess air ratio and a stable flame.

bustion technolo@, there are large gas turbines eration with NOx emissions ranging, in gas-fired ll load, between 25 and 75 vppm at 15% 0 2 , de-

sign. Further development of this type of burner the direction of even lower emission levels and, greater fuel flexibility. Of particular importance,

sible in the future to burn liquid fuels and coal in H2 or CO.

4 D Y BUILT

SOME TYPICAL COMBINED-CYCLE PUNTS ALREAD Y BUILT 309

Table 11-1: Main Technical Data of the 120 MW Figure 11-3 Combined-Cycle Plant at Korneuburg

lend special interest to the burning of residual. The oil is w and dosed with additives for combustion So as to preven temperature corrosion. Table 11-2 contains the main techn

The gas turbines and the waste heat boilers are built outdo the gas turbines in a light enclosure (Fig. 11-51. On the ot hand, the steam turbine is accommodated in a conventional chine building.

Table 11-2: Main Technical Data for the Tunghsiao Combined-Cycle Plant, Unit 3

Air temperature

Cooling water temperature Gas turbine power output at base load

Steam turbine power output

Station service power 11 Suprheater l2 High pressure drum

Net power output of the plant High pressure circulating pump

Net efficiency of the plant (LHV) 14 Bypass flap valves

Condenser pressure

Stack temperature Condenser

Feedwater temperature 19 Condensate pumps

Gas turbine exhaust gas flow evaporator 20 Steam bypasses

Gas turbine exhaust gas temperature

Gas turbine inlet temperature

Live steam flow Live steam data (at base load)

SOME TYPICAL COMBINED-CYCLE PLANTS ALREADY BUILT 313

11.1.3 The 1200 MW Trakya Combined-Cycle Plant TEK, Turkey [Ill]

m turbine. The gas turbines are rated at 95 MW under site ditions. The steam turbine, with a single casing and two low

he plant, which is a typical dual pressure cycle.

with a dry cooling tower built on the Hungarian Heller

: the steam is condensed by mixing it with cooling wa-

11-5: Layout of the Gas Turbines and Waste Heat r& the le 11-3 shows the main technical data.

Combined-Cyc\e Power plant (Unit 3) 1-3: Main Technical Data for one 300 MW unit of

1 Gas turbine 3 Bypass 4 Waste heat boiler

2 Control block

15 OC natural gas

: 2 x 9 7 . 5 2 x 104.5 MW

118 MW

3.7 MW

323.3 MW

50 %

93 kg/sec temperature : 477 498 OC pressure : 49.4 53.9 bar

20 kg/sec

197 OC pressure : 4.9 5.2 bar

99 OC

51 OC

SOME TYPIaL COMBINED-CYCLE PLANTS A LREADY BUILT 3 75

pressure live steam flow : 150 kg/sec pressure live steam data : 64.7 bar

500 O C at Nigashi-Nigata, Japan w pressure live stearn flow : 44.1 kg/sec

The 1090 MW Higashi-Nigata combined-cycle plant owne w pressure live steam data : 5.9 bar ~ ~ h u k u Electric comprises two blocks, each with three 133 saturated

0.043 bar

109 O C

The plant has a dual pressure steam cycle 11-7 shows the general layout of the power plant and ~ i g .

condenser is cooled directly with sea water.

Combined-Cycle Plants with Supplementary Firing

low a level, the gas turbines have been equipped wi Power Station of the Stadtwerke

MW combined-cycle block consists of a 60 MW gas tur- cry steam generator. Table 11-4 shows the main te MW gas turbine, and a 300 MW reheat steam turbine.

to operation between 1974 and 1976. The gas turbines

Table 11-4: Main Technical Data of One Unit of

11-10 show the structural design of the plant, which and shut-down fully automatically. Each gas

Air temperature capacity flue gas bypass, malung single-cycle

Cooling water temperature : e- At any given time, one of the two gas turbines Gas turbine power output : bed-cycle operation along with the steam gen- Steam turbine power output : other is available for solo operation in the upper Station service power Power output of the plant : 1090 MW Efficiency of the plant ( M V ) :

ped with a fresh air fan. Table 11-5 shows the

BUILT 317

SOME TYPICA L COMBINED-CYCLE PLANTS ALREADY BUILT 321

Table 11-5: Main Technical Data of the Combined-Cycle Plant at Lausward

turbine power output (bypass operation) 74183 MW

turbine power output (combined-cycle operation) 62170 MW

300 MW

236 kgls

188 bar

540 OC

40.6 OC

540 OC 0.028 bar

378 kgls

890/935 OC ut when burning oil (LHV) 34.9 MW

43.8 %

e 750 MW Power Plant Gersteinwerk,

combined-cycle plant consists of a gas-burning tur- by a coal-burning forced-circulation steam genera-

ns on load, to operate the steam process by itself, turbine. One of the fresh air fans also supplies

23 Steam air preheater 24 Fresh air ventilation fan level in the gas turbine exhausts is not sufficient 25 Fresh air dud to steam

X Silencer 27 Burners ue gas is sent to a desulphurization unit after it 28 Flue gas d u d m generator. The uncleaned portion is mixed with 3 Gas turbines 1 and 2 2 Gas turbine exhaust duct

to steam generator Gas turbine exhaust stack for single-cycle operation

322 COMBINED CYCLE GAS & STmM TURBINE POWER PLANTS T Y P l a ~ COMBINED-CYCLE PLANTS ALREADY T 323

the cleaned to attain a stack temperature of approx- 70 OC- The Figure 11-lla power plant is cooled using a natural circulation wet cooling tower.

~ i ~ . 11-12 shows the principle of design and Fig. 11-13 the layout. Table 11-6 contains the most i m p o ~ n t technical data.

~ ~ b l ~ 11-6: M& Technical Data (Combined-Cycle Opention of the Power Plant Gersteinwerk, Block K

Fuel for steam generatof

Air temperature

Relative humidity Gas turbine power output

Steam turbine power output

Net power plant output Net efficiency of the installation (LHV) Gas turbine exhaust gas flow Gas turbine exhaust gas temperature Feedwater tank with 32 Gas turbine exhaust 41 steam turbogroup

spray denerator duct to steam 15 Feedwater pumps transformer

generator Gas turbine inlet temperature 16 nigh pressure 42 Gas turbogroup 33 Gas turbine exhaust

feedwater heater transformers stack for single-cycle 43 Station =nice Live steam flow 17 Part-now economizers operation transformers

34 Gas turbine exhaust 44 control room Live steam data before turbine F"ll-flow economizer duct to exhaust gas 45 M~~~~~~ hall crane 22 Boiler circulating heat exchanger (to be 46 Gas turbine machine

Live steam data after reheater, before turbine

steam generator district heating(to be

27 Air enclosure hall installed later) 50 Reducing station for

39 H~draulically operated natural gas dampers

30 Gas turbines 1 and 2 40 sealing air supply 31 Air intake shaft for

gas turbine

hi^ was designed to allow later ~~xivers i coal gassication system, with the gas produced be gas turbine and the char in the boiler.

-

326 COMBINED CYCLE GAS & STEAM TURBINE POWER PLANTS SOME TYP164~ COMBINED-CYCL E PLANTS ALREADY BUILT 327

11.2.3 Nemweg Unit 7, EB Amsterdam, ble l l -7: Main Technical Data of the Hemweg 7 the Netherlands (Repowering) [I 121 Power Plant

This plant is a very interesting example of the repowering an existing steam turbine plant. Unlike the example discussed

weg 7 formerly was a modem 500 MW steam turbine plant

efficiency of this plant, which uses relatively expensive has been converted into a combined-cycle plant. The resu rbine power output is similar to that with maximum supplementary firing in turbine power output

Neither the boiler, nor the steam turbine, nor the feedwat ing system was originally designed for operation with a rbine inlet temperature bine instead of an air blower and regenerative air pr rbine exhaust temperature

rbine exhaust flow

tern were therefore necessary. The following changes made on the boiler to allow it to operate on hot gas t hausts instead of preheated air:

@ new ducts to the burners

@ new wind box with gas burners

@ additional new low and high pressure econo

Fig. 11-14 shows the thermal diagram of the Hem bined cycle after repowering, Fig. 11-15 the layout

in Table 11-7.


11.3 Combined-Cycle Plants for Cogeneration

11.3.1 Pegus Unit 12, Utrecht, the Netherlands

The Pegus Unit 12 combined-cycle pla one of the most modern combined-cycle ceived as a cogeneration plant suplying heat to three district heating systems and power to the @id. The m esting feature of this plant is that despite its being a cogen tion plant, it is capable of supplying power summer, at the extremely high efficiency of ne shows the principle of design of the plant, 150 MW gas-burning industrial gas turbine equipped wit low NOx combustor that produces less than 50 vppm o without the injection of water or steam. Fig. 11-17 sh model of the 150 MW gas turbine.

The energy in the exhaust gas is reco 3-pressure boiler, with the steam from the being reheated. The intermediate pressure steam is steam turbine. The low pressure steam is produced 13 Gas turbogroup trans-

high pressure water and is used for feedwater he former

9 Intake duct 14 Steam turbogroup deaerator, with the excess being piped to the steam P lo Fresh air ventilation E 10 k v switchgear

hot water from the flash system is either used for Gypsurn storage b w voltage switchgear

heating systems or, in summer, to heat condensate

The steam turbine has three casings. T the high pressure turbine is reheated in the b mediate pressure turbine has a second admiss mediate pressure steam and three extractio heating steam to four district heating con is not extracted is fed to the low pressure turb signed to operate at optimum efficiency even w being extracted.

SOME TYPICAL COMBINED-C~CLE PLANTS ALREADY BUILT 331

Figure 11-15

334 COMBINED CYCLE GAS & STEAM TURBINE POWER PUNTS SOME TYPICAL COMBINED-CYCLE PUNTS ALREADY BUILT 335

11.3.2 The Combhed-Cycle Gas and S Table 11-9: Main Technical Data of the Combined-Cycle Cogeneration Plant, Unit 10, at PEWS, in Cogeneration Plant, Unit 10, at PEGUS, Utrecht U treeht, the Netherlands (Repowerhg) [N]

This combined-cycle plant started operation in late 1978. It equipped with two 30 MW gas turbines extraction/condeming turbine. One important feature is the ste turbine, which was built as long ago as 1959 condensing turbine. During the repowering, an extraction installed between the high pressure and the lo turbine power output

This makes it possible to operate either as a heat turbine power output

or as a straight condensing plant (Fig. 11-20). The exhaus both the two gas turbines are supplied to two single-pressure heat boilers. At the end of the boilers, there is which recovers the heat that remains downstream generator. This is used partly for district heatin feedwater preheating. Because the fuel burne gas, the exhausts can be cooled to 100°C (212 OF). For technical data, refer to Table 11-9.

336 COMBINED CYCLE GAS & STEAM TURBINE POWER PLANTS SOME TYPICAL COMBINED-CYCLE PLQNTS ALREAD y BUI

figure 11-17 Figure 11-18

18 COMBINED CYCLE GAS & STlEAM TURBINE POWER PLANTS SOME TYPICAL COMBINED-CYCLE PL4 NTS ALREADY BUILT 339

A comparison of the data for maximum heating output with those for maximum electrical output shows the great flexibility of this installation. In winter, it provides heat and electricity at n energetic utilization rate of 82% and in summer it produces lectricity at an afficiency of approx. 41%.

Fig. 11-21 shows the layout of the power plant. The gas tur- es and the waste heat boilers are installed in the converted

iler house of the former steam plant. F'g. 11-22 shows one of turbines, Fig. 11-23 the repowered steam turbine with traction and cross-over pipes.

.3 The Combined-Cycle Cogeneration Plant H 415 of Elektromark AG at Hagen, Germany

plant is another interesting example of the cogeneration electricity, The 220 MW combined-cycleplant started

ion in 1981. It consists of two gas-burning 75 MW gas s, two two-pressure waste heat boilers, and one extrac-

ing turbine (Fig. 11-24). It supplies process steam at .5 bar (170 and 50 psig) to a paper mill and district

water at a max. temperature of 110 OC (230 O F ) to a large plant nearby. Provision has been made for expansion

ide additional district heating in the future.

aste heat boilers each have a high pressure and a low steam generator with a economizer and a superheater. pressure steam is fed either into the process steam sys-

e steam turbine. Table 11-10 shows the data at the oint of the installation. The overall layout of the entire

ain that shown in Fig. 11-25. Fig. 11-26 shows a gen- of the power station.

342 COMBINED CYCLE GAS & STEaM TURBINE POWER PLANTS SOME TYPICAL COMBINED-CYCLE PLANTS ALREADY BUILT 343

Figure 11-22 Figure 11-23

Firr. 11-22: Gas turbine for the 100 MW Combined-Cyclc Cogeneration Plant, Unit 1%

344 COMBINED CYCLE GAS & STOIM TURBINE POWER PLANTS SOME TYPICAL COMBINED-CYCLE PLANTS ALREADY BUILT 345

Figure 11-24 Figure 11-25

Fig. 1

346 COMBINED CYCLE GAS & STmM TURBINE PLANTS SOME TYPICAL COMBINED-CYCLE PLANTS ALREADY BUILT 347

Table 11-10: Main Technical Data of the Combined-C~cle e l~ t r i ca l power to the grid and generates 31.5 kg/s (250,000 l b h ) Cogeneration Plant at Hagen of wet steam (85% quality) at 70 bar (1000 psig). This process

steam is used for enhanced oil recovery, i.e., it is injected into the @;round to increase the production from the oil wells located around the power plant.

Air temperature Gas turbine power output . 11-27 shows the thermal diagram of the combined cycle. Steam turbine power output gas-fired gas turbines rated at 43 MU' under Total electrical power output der normal operating conditions, steam is in- Electrical yield (LHV) bustor to reduce NOx emissions. In case of Process steam flow (12.8 bar) ergency, water injection is also possible. The exhaust from one Process steam flow (4.5 bar) the gas turbines is directed to a heat recovery boiler which Process heat uality process steam (steam iqjection boiler), Rate of energy utilization (LHV) the flue gas from the other is used to generate superheated

in a normal dual pressure boiler (steam electric boiler). Both Gas turbine exhaust temperature

pressure and low pressure steam are expanded in the Gas turbine exhaust flow ondensing steam turbine. An interconnecting duct Gas turbine inlet temperature s it possible to control the process steam flow. If less pro- High pressure live steam flow required, some of the flue gas from the first gas High pressure live steam pressure e is ducted over to the steam electric boiler. The cooling High pressure live steam temperature is cooled back down in a forced-draft wet cooling tower. LOW pressure live steam flow

LOW pressure live steam pressure extremely low emission levels from this power plant are

LOW pressure live steam temperature ost interesting feature: The concentration of NOx ust had to be held below 7 vppm at 15% 02. The

or water into the combustor is not by itself o attain this low a level. Selective catalytic reduction ms had to be installed in both boilers. These systems

11.3.4 The 100 M W Combined-Cycle P1 onia to convert the NOx into nitrogen and water. The

~~s-placerita , ~alifornia; USA [ eactions have already been indicated in Section 9.1. atalytic reduction works properly and with good con- ciency only within a temperature window of about

the temperature drops below 250 to 300 OC O F ) , the reaction is too slow. Above 400 to 450 OC OF) the ammonia is converted into NOx. The SCR un-

348 COMBINED CYCLE GAS & STmM TURBINE POWER PLANTS SOME TYPICAL COMBINED-CYCLE PLANTS ALREADY BUILT 349

its therefore must be installed in the evaporators of the heat re- Table 11-12: Main Technical Data of the AES placefita covery boilers, thereby dividing them into two sections- Combined-Cycle Plant

A catalytic converter for CO has also been installed in each boiler to reduce by oxidation the emission of carbon monoxide from

natural gas

less than 10 vppm to approx. 1 vppm after the turbine. The basic Air temperature O C 24 Relative humidity

reaction here is 5% 26.7 Gas turbine power output MW 2 x 43.8

2 60 + 02 --b 2 (202 Steam turbine power output MW 15.0 Station service power

Expensive acoustical precautions were taken to reduce n MW 2.6

Net power output levels at 240 m (800 ft) to less than 39 dbA, including the

MW 100 rocess steam flow

of a cooling tower with low speed fans, special building wa kg/s 31.5

large silencers on the gas turbines, etc. bar 70 bar I 85% quality

Table 11-11 shows the levels of the most important pollu Live steam flow kg/s 24.4 after the gas turbines and after the boilers. ve steam pressure bar 41.4

Table 11-11: Emission levels from the AES Placerita Pla

bar 5.3 bar / saturated

115 1 123 bine inlet temperature OC 1085

Fig. 11-28 shows its layout.

350 COMBINED CYCLE GAS & STEAM TURBINE POWER -PUNTS SOME TYPICA L COMBINED-CYCLE PLQ NTS ALREADY BUILT 351

F'igure 11-26 Figure 11-27

' the AF,S Placerita Combined-Cycle Plant

SCR catalytic converter 31 SE-boiler feedwater Ammonia injection tank SI-boiler feedwater pre- 38 Deaeratorlfeedheate~ heater 39 Replacement water SE-boiler feedwater pumps 40 Steam turbine (ST) Condensate return pump 41 ST turbogenerator Stack silencer 42 HP stop and control SE-boiler HP economizer valve (Part 2) 47 LP stop and control SE-boiler economizer valve (Part 1) 44 Bleed steam line SE-boiler HP evaporator 45 Back-up steam SE-boiler HI? superheater bypass to deaerator SE-boiler LP economizer 46 Back-up steam bypas SE-boiler LP evaporator to steam injection SE-boiler HP drum 47 HP steam bypass sect SE-boiler LP drum 48 LP steam bypass SE-boiler KP feed- section water pumps 49 Steam condenser SE-boipr LP feed- FX) Condensate pumps water pumps


possible. This makes it possible to adjust to the growth in demand for energy in a grid. In a third step, coal gasification can be installed if there is too sharp an increase in the price of gas or oil.

simplicity of operation Conversion of the main units used A combined-cycle plant without supplementary firing is significantly simpler to run than a conventional steam plant. Moreover, because combined-cycle plants to obtain are generally operated fully automatically, they are also especially suitable for use where operating staff 14.504 psi is less experienced. 1.055 kJ

@ low environmental impact 1.8 Gas-burning combined-cycle plants in particular are

'Rankine

ideally suitable for use in heavily populated regions 0.30480 m because of their high efficiency and their low emis- 3.7854 sion levels for pollutants. In particular, the very low

1

nitrogen oxide levels of clean combined-cycle plants 2.54 cm

will be one of their most attractive features. Further- 0.94781 Btu more, gas-fired combined-cycle plants produce per kwh only 40% of the C02 produced by a coal-fired

2.20046 lb plant. 0.26417 US gal

@ advantages for cogeneration of heat and electricity 3.2808 ft The good thermodynamic properties of the combine 1.0936 cycle are highly desifable here. Electrical yields yd more than 40% are quite common in heating or 0.068948 bar strial power plants with a backpressure turbine.

The limited fuel flexibility of the combined-cycle pla version formulas greatest disadvantage for use in countries where oi in short supply. However, combined-cycle plants wi sification or PFBC plants could in the future becom alternative to conventional coal-fired steam powe flue gas scrubbing. Net efficiencies of more than 4 tainly be attained, which would permit the en sound and economical use of coal in a combined-c K + 273

eating Value (LHV).

..rr

SYMBOLS USED

Rate of heat utilization in waste-heat boiler

Carnot efficiency

Velocity

Outside diameter of a tube or pipe

Inside diameter of a tube or pipe

Frequency

Enthalpy

Difference in enthalpy

Isentropic efficiency

Heat flow coefficient

Mass flow

Rotational speed

Nusselt number

Power output

Pressure

Power coefficient

Difference in pressure

Pressure loss on flue gas end in waste-heat boiler

Polytropic efficiency

Polytropic efficiency for dry steam

Prandtl number

Heat flow

Heat flow, amount of heat

Reynolds number

Surface area

Temperature in K

INDICES USED

Air Outlet from a heat exchanger or feedwater heater Cooling water Inlet to a heat exchanger or feedwater heater Economizer Exhaust gases Flue gas Gas turbine Heater High pressure Combined-cycle plant / Waste-heat boiler At the generator terminals Low pressure Live steam Medium pressure Process steam Steam Supplementary firing Steam turbine, steam process Supplied Waste-heat boiler Waterhteam circuit Design point Gas turbine inlet Gas turbine outlet Flue gas temperature after supplementary firing Stage group of a turbine: inlet Stage group of a turbine: outlet

J

APPENDIX I

CALCULATION OF THE OPERATING PERFORMANCE OF COMBINED-CYCLE INSTALLATIONS (Refer to Section 7.1)

1. Equations for the heat exchangers

The equations of energy, impulse, and continuity are used to calculate the steady-state behavior of economizers. The conti-

uity equation comes down in the steady state to:

(24)

e impulse equation can be simplified into:

Ap = (geometry) (25)

owever, because the pressure losses both in the economizer n the evaporator has a negligible influence on the energy ions, the assumption

(26)

. In this case, the pressures along the heat exchanger re- nstant, on both the gas and water sides. The energy equa-

r a small section dx of a heat exchanger, which can be approximately as a tube, can be written as follows:

= k . A t e n . d - d x . (27)

362 COMBINED CYCLE GAS & STEAM TURBINE POWER PLANTS APPENDIX I 363

If it is assumed that the heat transfer coefficient k remains At the design point, Equations (30) and (31) become:

constant over the entire length of the heat exchanger (econo- j& = ko . S - Atmo. mizer or evaporator), Equation 27 becomes: (32)

L QJ = k . S J A ~ (x). -

0 L Dividing Equation (30) by Equation (32) and Equation (31) by

cannot be In the general case, the expression b (33) yields the formulas: grated. The heat exchanger must therefore be dealt with in th small element. (34)

In the special cases of a heat exchanger with counter or allel flow, however, integration is possible assuming that specific heat capacities of both media along the heat exchang remain constant.

Subtracting Equation (35) from (34) produces: The result of the integration is the logarithmic average

for the difference in temperature, which can be written in form: (36)

r: ALE - A ~ A = At,. his is the non-dimensional, global equation of heat transfer .I A1 (x) =

0 A [E the heat exchanger. If, in addition, Equation (31) is taken In (G) t flow coefficient k is known, a

tem of equations is obtained that defines the heat exchanger. This average value can also be used for a recuperator

evaporator. The heat exchangers do not, in fact, operate i cordance with an ideal counterflow principle, but the error main negligible. owing equation:

Substituting Equation (29) into Equation (28) yields: (37)

Q = k S . At,. wever, the relative values k/ko appear in the heat transfer

From Equation (24), the amount of heat exchanged can tion. From this: pressed as follows:

Q = lj? . ah, = m G . AhG. s


The heat transfer coefficients on the gas end of the econo- BY substituting mG/S for c~ QG, one obtains: mizer and the evaporator ( a G) are from 0.1 to 0.01 times as large as those on the steam end ( a s). Moreover, both values always lizG . dl Re = shift in the same direction (+ +, - -) V G . s ' (45)

For these reasons, the following relationship may be used: Then, substituting this expression into Equation (43), the geometric parameters disappear:

k a G K = . - = - ko aGo

The a-value on the gas end can be calculated as follows using the Nusselt number: If the mass flow is constant, all that remains is:

N u G = C . Re" - Pr" = aG - dl AG - (47)

Here, C, m, and n are constants that depend mainly upon t For m, one can use 0.57 for pipes that are offset from and 0.62 geometry involved. From this, the following expression is o or pipes that are lined up with one another. tained:

a~ = C ' - A G S Rem. Pr". oes not vary greatly and depends practically only on the prop-

If this is substituted into Equation (39), the geometric replaced with the following approx-

C' disappears:

AG + Rem Pr" K = (48) A G O . Rer Prt;'

For gases, the Prandtl number is almost exactly a co Theref ore :

are 1 the average gas temperatures along the heat ex- erating point. This produces for the

AG Re " e value of K : K =. ---- .

AGO -

For the Reynolds number, the following expression (49)

CG . PG ' dl ds to a slight extent on the geo- Re = of the boiler. TIG


It is more complicated to calculate an exact value for I( in the In steam turbines, the pressure ratio is always very small. This case of a superheater because the heat transfer on the steam makes it possible to replace the quadratic expression with I. The end is poorer than that in the evaporatgr. ratio of the absorption capacities is likewise close to I.

When all of these equations have been obtained for all parts What remains is then:

of the boiler, the waste-heat boiler has been defined mathemat- ically. Similar equations can also be formulated for calculating ms = ~ z i v a n the condenser. ti',, V a

When calculating the economizer and the evaporator of a drum At a constant rotational speed, the efficiency of a stage de- boiler, the problem arises that the state of the feedwater at the pends only upon the enthalpy drop involved. In part-load op- inlet to the drum is not clearly defined. For that reason, two eration, however, no relatively great change occurs in that different cases must be considered: gradient except in the last stages. Because this means that the

reatest portion of the machine is operating at a constant effi-

@ The feedwater is supplied with steam at its entry into iency, it can be assumed that the polytropic efficiency remains the drum (partial steam out in the economizer) onstant. The turbine efficiency is calculated in the same way

@ The feedwater is undercooled at its exit from the s for the design point. economizer and must be heated to saturation temperature in the drum. The following formulas are used to calculate efficiency:

3. The Steam Turbine For parts of the turbine operating in the superheated zone:

Most steam turbines in combined-cycle plants operate in s Vpol. tr. = const ant ing pressure operation and generally have no control stage

(52)

nozzle groups. This simplifies calculations, because simula or parts in the saturated steam: of the control stage and the inlet valv

Vpol = Vpol. tr. - (1 - ~ a ) + (1 - xa)

A portion of a steam turbine with no 2 (53) one equation for its absorption capacity and one for its ency. The absorption capacity is approximately using th polytropic efficiency selected should be such that the de- of Cones. In general, according to Ref. [I]: (50) ower output is once again actually attained in the design

3-T - Pa "S = "a0 . ollowing equation is used to determine the adiabatic ef- . J";; SO NO . Pao v a

'lpoi

- (2)+ x - 1 :

368 COMBINEL) CYCLE GAS & STEAM TURBINE POWER PUNTS APPENDIX I 369

These equations make it possible to establish the expansion for further iteration. The procedure is repeated until all three line of the steam turbine. The power output of the steam equations have been fulfilled. turbine can be determined from this by allowing for dummy piston, exhaust, generator, and mechanical losses. The The energy and heat transfer equations for the economizer dummy piston losses in single-flow reaction turbines are and the evaporator can be used to determine a second approx- approx. proportional to the live steam pressure, and are imation for live steam pressure. typically between 400 to 600 kW. Mechanical losses range from 150 to 250 kW and exhaust losses at full load are generally in If the feedwater tank is in sliding pressure operation, a first the range of 20 to 35 kJkg steam. estimate for feedwater temperature is also necessary.

The new value obtained for live steam pressure is then used 4. Solving the System of Equations to continue calculation of the superheater and the turbine until

Taken together, all the equations in the waste-heat boiler, th all equations for the boiler and the Law of Cones agree. The next

steam turbine, etc. produce a system which can only be solve step is to calculate the preheating of the feedwater. This is used-

by iteration. if the pressure in the feedwater tank varies- to find a new approximation for feedwater temperature. The boiler is then re-

The following values are known: calculated, using this new value. Finally, the condenser pressure and extraction flow are determined in another iteration. Then,

thermodynamic data in the design point from this information, one can determine the power output of the marginal conditions for the particular operation the steam turbine. to be calculated (exhaust data for the gas turbine, cooling water data, etc.) operating mode of the feedwater tank (sliding or fixe pressure)

Gas and Steam Tables

The following information must be found:

behavior of the steam circuit

Fig. Appendix-1 shows the method used for solution. One s with the superheater, inputting into the computer a fi mate for live steam temperature and pressure. Using th of Cones and the energy equation, one can then calcul live steam flow and the gas temperature following the heater. Next, from the heat transfer equation, a new va live steam temperature can be determined. This is the


Figure Appendix-1

1. APPROXIMATION

L

SUPERHEATER

+ C

PREHATING S Y S T E M

7. + + F W <\"> I

CONDENSER r - 7 - - I OUTPUT OF STEAM 1

Fig. Appendix 1: Calculation of Operating and Part-Load Behavior: Met Solving the System of Equations

DEFINITION O F TERMS AND SYMBOLS

The selection of terms and symbols below has been based on national and international definitions and expanded- where it appeared necessary- with additional terms and symbols. The selection was adapted to the special technical requirements of this book.

Term Symbol Unit D e f i n i t i o n

Annual service hours Tan,. hrlyr Number of hours per year (oper- (Annual operating time) ating time per year) during which

a unit (or a group of units) was or is to be operated, continuously or with interruptions

Annual utilization time TNj hr/yr The annual utilization time for nominal output nominal output is obtained by

dividing the operating output during the operating period by the nominal power output.

oach temperature ST K Undercooling of the feedwater at the inlet to the boiler drum (difference between actual temperature of feedwater and saturation)

lability factor in kAN - The quotient obtained by divid- s of nominal work ing the work available PV.Tv by

the nominal work

pv 'Tv kAN = -

371

372 COMBINED CYCLE GAS & STEAM TURBINE POWER PLQNTS DEFINIUON OF TERMS AND SYMBOLS 373

Term Symbol Unit D e f i n i t i o n Symbol Unit D e f i n i t i o n

Availability (time) T The availability (time) of a power Power coefficient PC kWslkJ The flow coefficient of a plant station or power plant unit is ob- cogenerating heat and electricity tained by dividing the availabil- or is obtained by dividing the net ity (the sum of operating time electrical power generated in a plus time a t readiness) by the (J kWs/kJ given time span by the usable nominal time: heat generated in that same time

span, both limited to the limit of

"Bottoming cycle" A thermal process that operates the plant

E(e) kJ/kg The maximum technological perature process work obtainable from a system

in accordance with the Second

Efficiency of power 'T~EL Law of Thermodynamics if the In cogeneration plants, the e

generation system is brought reversibly into equilibrium with its environ-

a1 power output by the additional fuel supplie

team turbine of the turbine

process electrical power output their components is obtained by

dividing their outage time due to plied to the steam proc malfunction by the sum of the process with waste hea operating time plus outage time tion alone, it can be fou the formula:

nerator output P~~~ kW The generator output of a power Efficiency of the The efficiency of th (kVA) station or a power plant block is steam1 water cycle water cycle is obtained the power available at the gener-

ing the electrical output ator terminals. The generator output is the gross output.

water or steam in the

374 COMBINED CYCLE GAS & STEAM TURBINE POWER PLANTS DEFINITION OF TERMS AND SYMBOLS 375

Term Symbol Unit D e f i n i t i o n Term Syntbol Unit D e f i n i t i o n

Heat output Q 1-1 kW The heat output to cover non- with the full production capacity block-connected heat demand, continually in operation.

e.g., heat supplied to a district The two work measurements heating system must be of the same type, gross

or net.

IS0 conditions Standard environmental conditions per ISO: Reliability (time) 'T The reliability (time) is obtained Total air temperature 15 O C by dividing the operating time by Total air pressure 1.013 bar the sum of operating time plus Relative humidity GO % outage time due to malfunctions:

Pinch point of a waste r Station service power PEIqEL) kw The station service power of a heat boiler power station or power plant

block is the amount of power required to drive all motor-driven block auxiliaries and ancillaries (power consumpt,ion of the mo- tors), plus the electrical losses in

electricity is equal to the q station service transformers and tient obtained by dividing electrical transmission losses

within the power station. usable heat generated in a

Thermal efficiency The thermal efficiency of a power station or a power plant

same time span. block generating electricity alone is obtained by dividing the elec-

Rate of waste heat util- 0 WR - The quotient obtained b ization

available to the waste A thermal process operating in the upper range of temperatures, followed by a low temperature process.

production in that ti ess or moisture The energy losses due to wetness vided by the work in the wet steam section of the

{" turbine

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380 COMBINED CYCLE GAS & STEAM TURBINE POWER PMNTS

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384 COMBINED CYCLE GAS & S T M M TURBINE POWER PUNTS BIBLIOGRAPHY 385

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Documents

Combined-Cycle Gas and Steam Turbine Power Plants