Summary of the Development of Open-Cycle Gas Turbine-Steam ...

-- UNION CARBIDE CORPORATION FOR THE UNITED STATES ;s%q~ G

DEPARTMENT OF E N E R G Y ~ ~ ~ -, - C;

Summary of the Development of Open-Cycle Gas

Turbine-Steam Cycles

M. E. Lackey A. S. Thompson

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency Thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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This report was prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither theUnitedStatesGovernment nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference hereln to any specific commercial product, process, or service by trade name, trademark, manufact~rrer. Or Otherwioo, doea not r~wutruuarlly ccinstttute or imply its . endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state nr reflect thosa of the United 9 tales Government Or any agency theroof.

* -

oRN~/T'M-6252 D i s t . Category UC-90f

#

C o n t r a c t No. W-7405-eng-26

Engineer ing Technology.Divis$on

.SUMMARY OF THE DEVELOPMENT OF OPEN-CYCLE GAS TURBINE-STEAM CYCLES

M. E . Lackey A. S. Thompson

Date P u b l i s h e d - September 1980

Prepared by t h e OAK RIDGE NATIONAL LABORATORY

Oak Ridge, Tennessee 37830 o p e r a t e d by

UNION CARBIDE CORPORATION f o r t h e

DEPARTMENT OF ENERGY

~ I S T R I B Y T I O ~ ~ OF THIS [10CI1l3'1EHT IS IlIILIb3iT tl

This is one of a set of companion reports evaluating and comparing

advanced energy conversion systems on the basis of their research and

development experience. The reports are listed below.

ORNLITM-6236 - Summary of the Research and Development Effort on the Supercritical C02 Cycle

ORNLITM-6237 - Summary of the Research and Development Effort on Closed-Cycle Gas Turbines

ORNLITM-6250 - Summary of the Research and Development Effort on Steam Plants for Electric Utility Service

ORNLITM-6251 - Research and Development Experience and Current Status of Development of Open-Cycle Gas Turbines

- ..

ORNLITM-6252 - Summary of the Development of Open-Cycle Gas Turbine- Steam Cycles

, ORNLITM-6253 Summary of the Research and Development Effort on Open-Cycle Coal-Fired Gas Turbines

ORNL/TM-6254 - Summary of Research and Development Effort on Air and Water Cooling of Gas Turbine Blades

ORNLITM-6255 - Summary of the Research and Development Effort on Ceramic Gas Turbines

ORNL/TM-6256 - Summary of the Research and Development Effort on Alkali Metal Vapur Cycles

ORNLITM-6257 - Summary of the Research and Development Effort on Open-Cycle MHD Systems

O R N L / T M - ~ ~ ~ ~ - Summary -of -the Research and Development ~f f ort on Fuel Cells

-rs3nI 19bf-s~~ y3k3orlfuA ysI i sV .-s~esnn9T yd Ld:lor.noqe rf31.sn~.~R

ORNLITM-6303 - Comparison of Advanced Energy Conversion Systems on . . - . the Basis of Research and Development Experience.

Executive Summary Report

iii

CONTENTS

Page

.................................................... ACKNOWLEDGMENT v

ABSTRACT .......................................................... 1

...................................................... INTRODUCTION 2

BACKGROUND ........................................................ 2

System Description ........................................ 6

Gas turbine with a waste heat recovery boiler ............... 6 Integrated coal gasification and combined-cycle plant complex ........................................ 7 Basic performance characteristics ........................... 9 Combined cycle with the turbine air heated by tubes in a fluidized bed ......................................... 10

Summary of Program Costs and Running Time ...................... 10

RECOMMENDED DEVELOPMENT PROGRAM ................................... 10

PRINCIPAL PROBLEMS AND PARAMETERS ................................. 11

Matching the Steam Cycle to the Gas Turbine Cycle .............. 12

Practicable Gas Turbine Inlet Temperatures ..................... 16

Efficient Utilization of Energy in Coal ........................ 17

Effects of coal conversion process efficiency and heat recovery temperature .............................. 17 Characteristics of typical coal gasification process ........ 20 Plant integration problems .................................. 23

Efficiency ..................................................... 25

Fuel Flexibility ........................................ 32

Reliability and Availability ................................... 33

CURRENT STATlTS OF DEVELORMENT ...................................... 36 REFERENCES .................................................. 40

APPENDIX A . COMBINED-CYCLE POWER PLANT INSTALLATIONS ............. 45

..................... APPENDIX B . POTENTIAL FOR FUTURE DEVELOPMENT 49

THIS PAGE

W A S INTENTIONALLY

LEFT BLANK

v

ACKNOWLEDGMENT

A s p e c i a l d e b t of a p p r e c i a t i o n i s owed S. A l p e r t , R . Duncan, and

R. J a f f e e of EPRI, R . W. Foster-Pegg of Westinghouse, and H. Hartman,

M. Myers, J . P. N i c h o l s , and G . Samuels of ORNL f o r t h e i n v a l u a b l e i n f o r -

ma t ion , p a p e r s , and r e p o r t s t h a t t h e y k i n d l y s u p p l i e d . I n a d d i t i o n ,

s p e c i a l t h a n k s a r e due R. W. Fos ter -Pegg, M. L. Myers, and G. Samuels

f o r t h e i r d e t a i l e d c r i t i c i s m s and comments f o l l o w i n g t h e i r r e v i e w s o f

t h e d r a f t r e p o r t .

SUMMARY OF THE DEVELOPMENT OF OPEN-CYCLE GAS TURBINE-STEAM CYCLES

M. E. Lackey A. S. Thompson

ABSTRACT

Combined-cycle plants employing gas turbine cycles superimposed on ccnventional steam plants are well developed. Nearly 200 units are operating in the U.S. on clean fuels (natural gas or distillate fuel oils) and giving overall thermal efficiencies 'as high as 42%. Future plants will have to use coal or coal- derived fuels, and this presents problems because gas turbines are very sensitive to particulates and contaminants in the fuel such as sulfur, potassium, lead, etc. If clean liquid or high- Btu gaseous fuels are made from coal, it appears that the conversion efficiency will be no more than 67%. Thus, the overall efficiency of utilization of coal would be less than if it were burned in a conventional steam plant unless the permissible gas turbine inlet temperature can be increased to %1500°C (2732OF). Coupling a combined-cycle power plant directly to a low-Btu coal gasifier increases the fuel conversion efficiency and per- mits salvaging waste heat from the gasifier for feedwater heating in the steam cycle. By using a gas turbine inlet temperature of 1315OC (2400°~), well above the current maximum of ~~1040°C (1904"F), an overall efficiency of ~ ~ 4 0 % has been estimated for the integrated plant. However. as discussed in com- p a ~ i i o n reports, it is doubtful that operatinn with gas turbine inlet temperatures above llOO°C (2012°F) will prove practicable in base-load plants. Recent difficulties with Fe203 deposits on air-cooled turbine blades operating at temperatures above 1100°C (2012°~j with ofily 0.06 ppu~ ul: particulates in the air stream make it questionable whether it will be practicable to operate a gas turbine coupled to a coal gasifier with turbine inlet temperatures above %llOO°C (2012°F). The inherent brit- tleness of ceramics makes ceramic turbines subject to failure from thermal stresses while the high heat flux to water-cooled blades coupled with the intricacy of the water cooling pas- sages required make that approach a doubtful prospect for 1315°C (2400°F) gas turbines.

INTRODUCTION

This is one of a series of topical reports summarizing the research

and development (R&D) effort on various phases of advanced power conver-

sion systems. '-I1 This report is especially concerned with the applica-

tion of the gas turbine in combined-cycle electric utility plants.

The first portion of this report presents a background history of

combined-cycle utility plants up to 1977, and the next section discusses

recommended development programs and priorities. This is followed by

a presentation of the principal problem areas, performance parameters,

and figures of merit characteristic of the system to provide perspective

on the various problems. The principal parameters and figures of merit

include such quantities as the operating life, turbine inlet temperature,

cycle efficiency, etc. Subsequent sections summarize the current status

and experience in combined-cycle installations.

This work was carried out at the request of the Office of Program

Planning and Analysis, Fos,sil Energy Program, DOE, with funds provided

for a general appraisal of advanced fossil energy systems.

BACKGROUND

A combined cycle is defined as a combination of several cycles which

could each operate independently. The several cycles operate with dif-

ferent working fluids or else they would be classified as compound cycles.

Commercial applications of combined cycles have included diesel-steam,

mercury-steam, and gas turbine-steam. Other combined cycles, such as

steam-organic fluid, helium-ammonia,' alkali metalsteam, MHD-steam, etc.,

have been proposed for commercial operation.

The peak temperature in the mercury-steam cycle of the 1930's was

.limited to 482OC (900°F) by materials problems and hence lost its effi-

ciency advantage when it became possible to build 540°C (1000°F) steam

plants. The relatively small generating capacity per unit and the low

temperature of the steam produced by the diesel-steam cycle limits its

u s e t o s p e c i a l a p p l i c a t i o n s . The combined g a s t u r b i n m t e a m c y c l e h a s

been adap ted t o s e r v e t h e l a r g e u t i l i t y producer a s w e l l as i n d u s t r i a l

p l a n t s t h a t r e q u i r e b o t h e l e c t r i c i t y and p r o c e s s steam p r o d u c t i o n . A s

i n s i m p l e open-cycle g a s t u r b i n e s , t h e the rmal e f f i c i e n c y depends h e a v i l y

o n t h e g a s t u r b i n e component e f f i c i e n c y . Many e a r l y s t u d i e s y i e l d e d h igh

e s t i m a t e d the rmal e f f i c i e n c i e s f o r g a s t u r b i n e c y c l e s , y e t i n c a s e s i n

which a p l a n t was a c t u a l l y b u i l t t h e measured the rmal e f f i c i e n c y o f t e n

f e l l f a r below t h e o r i g i n a l e s t i m a t e . I n f a c t , i n t h e f i r s t g a s t u r b i n e

e v e r b u i l t (des igned by S t o l z e i n 1 8 7 2 ) , t h e compressor power i n p u t ex-

ceeded t h e t u r b i n e power o u t p u t . '' It was n o t u n t i l t h r e e decades l a t e r

t h a t a n e t u s e f u l power o u t p u t was o b t a i n e d from a con t inuous combustion

g a s t u r b i n e b u i l t and t e s t e d by C h a r l e s Lemale and Rene Armengaud. 1 3

However, t h e the rmal e f f i c i e n c y was o n l y 3%.

The f i r s t commercial a p p l i c a t i o n of t h e g a s t u r b i n e (which i n c i -

d e n t a l l y used a n a x i a l compressor) o c c u r r e d i n 1932 w i t h t h e adven t of

t h e Velox p r e s s u r i z e d b o i l e r developed by Brown, Bover i and Company of

S w i t z e r l a n d . 1 4 The Velox b o i l e r i s p r e s s u r i z e d by a compressor d r i v e n by

a g a s t u r b i n e w i t h t h e h o t f l u e g a s from t h e b d i l e r . T h i s was t h e f i r s t

commercial u s e of t h e combined g a s t u r b i n e s t e a m c y c l e . Over 100 u n i t s

have been b u i l t f o r commercial s e r v i c e and 34 f o r U.S. Navy d e s t r o y e r s ,

a l l of which were f o r o i l o r g a s f i r i n g . ' ' Owing t o t h e low component e f -

f i c i e n c i e s (compressor 73%, t u r b i n e 83%) and t h e r e l a t i v e l y low t u r b i n e

i n l e t t e m p e r a t u r e o f 450 t o 510°C i n t h e s e e a r l y u n i t s , t h e t u r b i n e o u t p u t

was o n l y s u f f i c i e n t t o power t h e combustion a i r compressor. A somewhat

similar a p p l i c a t i o n of t h e g a s t u r b i n e compressor d r i v e sys tem was used

i n t h e Houdry c a t a l y t i c - c r a c k i n g p r o c e s s i n 1936 by t h e Sun O i l Company

a t i t s Marcus Hook, Pennsy lvan ia , p l a n t . 1 6 T h i s i n s t a l l a t i o n n o t o n l y

used t h e t u r b i n e t o supp ly t h e compressor d r i v e power, b u t a l s o used t h e

h e a t i n t h e t u r b i n e exhaus t t o r a i s e p r o c e s s s team; t h e r e was s u f f i c i e n . ~

e x c e s s power from t h e t u r b i n e o v e r t h e compressor power requ i rement t o

d r i v e a n e l e c t r i c g e n e r a t o r .

For e l e c t r i c u t i l i t y s e r v i c e t h e f i r s t power s t a t i o n employing o n l y

g a s t u r b i n e s was developed i n 1939 by t h e Escher Wyss Engineer ing Works

i n Z u r i c h , S w i . t z ~ . r l ; l n d . ' ~ This p l a n t was a c losed-cyc le sys tem employing

regeneration and intercooling that produced 2000 kW(e) at a thermal ef-

ficiency of 31.6%. In 1949, the Oklahoma Gas and Electric Company in-

stalled at their Belle Isle Station a 3500-kW(e) locomotive drive gas

turbine in combination with an existing medium-pressure steam system. 18'19

The overall plant cycle efficiency was increased from approximately 22%

to 26% by the addition of the gas turbine.

The price advantage of coal and residual fuel oils used in conven-

tional fossil fuel steam generators over the distillate fuel oil or

natural gas required for high-temperature gas turbines did not favor the

installation of gas turbinesteam combined-cycle power plants by utilities

during the 1950's and 1960's. During the late 1960's and early 1970fs, the

demand for repowering existing plants, coupled with increased construction

costs, led to a substantial demand for combined-cycle installations. The

rate of installation of new units to give the current electrical utility

power generation capacity of approximately 20,000 MW(e) is shown in Fig. 1. 19-28 A tabulation of 103 combined cycle power plants with a total of

179 units that were in operation or on order as of June 1976 is given in

Appendix A (Ref. 29).

Inspection of Fig. 1 shows the effects of the present shortage and

high cost of high-grade fuels on the installation of gas turbinesteam

combined cycles. There were orders for oniy two installations in 1976 as

compared to an average of 13 per year for the previous five years. The

present fuel outlook indicates that, in the foreseeable future, coal will

be the primary U.S. fossil fuel source for new central stations. The di-

rect coal-fired gas tur'bine, as indicated in another report in this series, 4

is not likely to prove practicable because of turbine b1ad.e erosion and de-

posits caused by flyash, but clean gaseous or liquid fuels'derived from

coal offer promising possibilities for fueling gas turbinesteam cycles.

Liquid and high-Btu gaseous fuels have the major advantage that they can

be stored and transported, but if this course is taken the heat losses in

the conversion process cannot be utilized for feedwater heating in the

steam-electric plant to improve the overall efficiency of the conversion of

the chemical energy in the coal into electrical energy. A higher overall

energy conversion efficiency can be obtained by integrating a low-Btu coal

gasification plant directly with a combined-cycle electric power plant.

1940 1950 1960 1970 1980

ORDER YEAR

Fig . 1. Gas t u r b i n e s t e a m combined-cycle power p l a n t i n s t a l l a t i u n s f o r e l e c t r i c u t i l i t y power gene ra t i on (1949-1976) .

Thus t h e most heav i ly funded approach c u r r e n t l y e n t a i l s coupl ing a

coal-fueled gas t u r b i n e s t e a m cyc l e p lan t ' t o a low-Btu coa l g a s i f i c a t i o n

process . The l a t t e r should y i e l d a f u e l ga s .w i th a s u l f u r con ten t below

~ ~ ~ r o x i m a t e l ~ O.2%, s o t h a t no s t a c k gas c l e a n u p n w i l l be r equ i r ed and a

p a r t i c u l a t e conten t below approximately 1 ppm so t h a t t u r b i n e b l ade ero-

s i o n and d e p o s i t s w i l l probably no t be a problem wi th t u r b i n e i n l e t tem-

p e r a t u r e s up t o %llOO°C (2012OF). The problems of a t t a i n i n g t h i s objec-

t i v e i n t h e c o a l g a s i f i c a t i o n process a r e t r e a t e d i n Refs. 30 t o 32; a

b r i e f summary of t h e formidable problems involved is presen ted l a t e r i n

t h i s r e p o r t .

System Desc r ip t i on

The v a s t m a j o r i t y of t h e combined gas turbine-s team c y c l e p l a n t s

b u i l t t o d a t e have been b a s i c a l l y gas t u r b i n e i n s t a l l a t i o n s w i th a waste

h e a t recovery b o i l e r i n t h e t u r b i n e exhaust t o d r i v e a steam bottoming

c y c l e . The peak steam tempera ture i n such a system is l i m i t e d t o some-

t h i n g less than t h e p re sen t upper va lue of 540°C u n l e s s t h e gas t u r b i n e

i n l e t t empera ture i s i n t h e 1050 t o 1100°C range. Base-load gas t ~ i r h i n ~ . ~

o p e r a t i n g i n t h i s t empera ture range reprecent t h r reaches of Chc

p r e s e n t s t a t e uf t h e a r t of gas t i l rbfne development. For gas turbi l ies

o p e r a t i n g a t lower i n l e t t empera tures , modern steam t.ernperatures can be

a t t a i n e d by a f t e r b u r n i n g t o r a i s e t h e temperature of t h e t u r b i n e exhaust .

A thermodynamic a n a l y s i s of t h i s approach, presented i n Appendix B, shows

t h a t when a n a f t e r b u r n e r i s employed and t h e gas turhine-s team system i s

des igned t o g ive t h e maximum work output per pound of a i r handled (which

a c t s t o r educe t h e c a p i t a l c o s t of t h e i n s t a l l a t i o n ) , t h e system a l s o

y i e l d s t h e maximum thermal e f f i c i e n c y .

Gas t u r b i n e wi th a waste hea t recovery b o i l e r

A f lowsheet f o r a s imple o i l - o r gas - f i red gas t u r b i n e wi th a hea t

recovery s team b o i l e r and steam t u r b i n e bottoming cyc l e is shown i n F ig .

2 . T h i s system has t h e advantage of s i m p l i c i t y , but t h e thermal e f f i c i e n c y

i s l i m i t e d because t h e peak tempera ture i n t h e steam c y c l e is r e l a t i v e l y

low - perhaps 4 3 0 ° C (806°F). Inasmuch as t h e g a s t i i rbtne burner ope ra t ec

w i t h %300% excess a i r , an a f t e r b u r n e r can be added between t h e t u r b i n e

and t h e s team gene ra to r t o i n c r e a s e t h e steam temperature . This i nc reases

t h e c y c l e e f f i c i e n c y t o approximately 35% and a l s o i n c r e a s e s t h e o11tp11t

o b t a i n a b l e from t h e p l a n t a s a whole w i th a given s i z e gas t u r b i n e . 3 3

Thus t h i s approach i s commonly employed. The system performance could

a l s o be improved f u r t h e r by increasi-ng t h e gas t u r b i n e i n l e t temperature ,

which g i v e s a h igher exhaust t empera ture and more f avo rab l e cond i t i ons i n

t h e s team c y c l e . Assuming advanced gas t u r b i n e des igns w i t h t u r b i n e i n l e t

t empera tu re s around 1370°C (2500°F), i n t h e ECAS s tudy i t was es t imated

t h a t a p l a n t thermal e f f i c i e n c y of %47% might be a t t a i n e d . 3 4 - 3 6

ORNL DWG 7 7 14273 A

t EXHAUST STACK

I STEAM NERATOR *

GENERATOR

SIMPLE- CYCLE GAS-TURBINE

Fig . 2. Flowsheet f o r a simple-cycle gas t u r b i n m t e a m combined cyc l e .

F igure 3 is a f lowsheet f o r a p l a n t i n ~ u n e n , Germany, t h e only com-

bined-cycle system coupled t o a c o a l g a s i f i c a t i o n p l a n t t h a t has been

b u i l t ar lywllere i n t h e world t o date. .37 Note t h a t t h e c l e a n f u e l gas is

burned a t a p r e s s u r e of 1 0 atm t o g i v e a flame temperature of 1400°C, and

much of t h e hea t of combustion i s removed by t h e steam gene ra to r tubes so

t h a t t h c ho t gas i s fed t o t h e gas t u r b i n e a t 820°C. The ope ra t i on of t h i s

steam gene ra to r i n t h e high-temperature and -pressure reg ion a l lows t h e gas

s i d e temperature d i f f e r e n c e and t h e gas s i d e hea t t r a n s f e r c o e f f i c i e n t t o

be increased by f a c t o r s of approximately 3 and 6, r e s p e c t i v e l y , over cun-

v e n t i o n a l steam gene ra to r s f o r combined c y c l e s such a s t h a t of Fig. 2.

This r e s u l t s i n a major r educ t ion i n t h e s u r f a c e a r e a and t h e c a p i t a l c o s t

of t h e rteam gene ra to r . The ho t exhaust gas l eav ing t h e gas t u r b i n e a t

400°C i s used f o r feedwater hea t ing . The Lurg i g a s i f i e r s a r e supp l i ed

wi th about 12% of t h e a i r compressor flow, which is boosted from 10 t o 20

atm w i t h a smal l compressor t h a t i s powered by an expansion t u r b i n e t h a t

r ecove r s much of t h e p re s su re energy i n t h e gas s t ream flowing from t h e

g a s i f i e r s t o t h e steam gene ra to r combustion chamber.

ORNL-DWG 77-14269

COAL

111 HEATER

Fig. 3. Flowsheet for the integrated gasification combined-cycle ~ u n e n plant.

The p l a n t as designed was expected t o have a n o v e r a l l e f f i c i e n c y

o f 36.9%, w i t h approx imate ly 40% of t h e e l e c t r i c power be ing g e n e r a t e d

by t h e g a s t u r b i n e . 3 7 Both t h e o u t p u t of t h e g a s t u r b i n e and t h e over-

a l l the rmal e f f i c i e n c y cou ld b e i n c r e a s e d i f i t were p o s s i b l e t o employ

a h i g h e r t u r b i n e i n l e t t empera tu re ; hence r e c e n t d e s i g n s t u d i e s i n t h e

U . S. ( e .g . , E C A S ~ 4, 5, have assumed t u r b i n e i n l e t t empera tu res a s h i g h a s

1370°C (2500°F), which i s f a r h i g h e r t h a n t h e 820°C t u r b i n e i n l e t tem-

p e r a t u r e o f t h e ~ u n e n p l a n t . T h i s s t e p w a s t aken because , u n l e s s t h e

g a s t u r b i n e i n l e t t empera tu re can be i n c r e a s e d t o over 1100°C (2000°F),

t h e p l a n t i s n o t c o m p e t i t i v e w i t h a c o n v e n t i o n a l c o a l - f i r e d s u p e r c r i t i c a l

p r e s s u r e steam p l a n t w i t h wet l i m e s t o n e s t a c k g a s s c r u b b e r s . Thus, a

c r u c i a l q u e s t i o n i s t h e f e a s i b i l i t y of a c h i e v i n g t h e s e markedly h i g h e r

g a s t u r b i n e i n l e t t e m p e r a t u r e s i n a ,base-load p l a n t . w i t h a f u e l t h a t

p robab ly w i l l n o t be as c l e a n as t h e n a t u r a l g a s o r d i s t i l l a t e f u e l o i l s

c u r r e n t l y used i n g a s t u r b i n e s .

B a s i c performance c h a r a c t e r i s t i c s

Perhaps t h e most impor tan t advan tage of c o n v e n t i o n a l open-cycle gas

t u r b i n e s is t h a t t h e y can be s t a r t e d up q u i c k l y ( i n minu tes ) and respond

r a p i d l y t o a b r u p t changes i n t h e e l e c t r i c a l l o a d . I f a steam bottoming

c y c l e i s added t o one of t h e s e open-cycle g a s t u r b i n e s , one can s t i l l

g e t good response t o l o a d changes bu t t h e s t a r t u p t ime is g r e a t e r . When

a c o a l g a s i f i c a t i o n p l a n t i s t i g h t l y coupled t o a combined g a s turbine--

s team p l a n t , t h e s i t u a t i o n becomes comple te ly d i f f e r e n t . The v o l u m e t r i c

f low r a t e of t h e low-Btu g a s i s s o g r e a t t h a t i t i s i m p r a c t i c a l t o s t o r e

i t ; hence i t must be used a s r a p i d l y a s i t i s g e n e r a t e d . The c o a l g a s i -

f i e r s have a h i g h h e a t c a p a c i t y and t a k e a l o n g t i m e t o h e a t up; t h e

e f f e c t i v e n e s s of t h e c o a l g a s i f i c a t i o n p r o c e s s i s s e n s i t i v e t o p r e s s u r e ,

t e m p e r a t u r e , and f low r a t e c o n d i t i o n s , s o t h a t t h e g a s i f i e r s o p e r a t e w e l l

o v e r o n l y a narrow range o f c o n d i t i o n s . Thus, p l a n t s of t h i s t y p e r e q u i r e

1 2 t o 20 h r t o g e t on l i n e from a c o l d s ta r t , a r e r e l a t i v e l y i n f l e x i b l e ,

and hence can be employed o n l y i n base-load s e r v i c e .

Combined cycle with the turbine air heated bv tubes in a fluidized bed

The complexity and high capital cost of a coal gasifier and gas tur-

bine erosion and deposits caused by ash in the coal led R. W. Foster-Pegg

to propose a combined cycle. in which the air from the compressor would

be heated in a tube bank in a fluidized bed so that the gas turbine would

operate on clean air. About 33% of the hot air leaving the turbine

would then go to the fluidized bed as combustion air and the balance could

be used to generate steam. This system looks especially attractive for

industrial cogeneration applications.

Summarv of Program Costs and Runnine Time

As in the development of the steam system, it is difficult to estimate

the cost of the development of combined-cycle systems because the basic

R&D costs have fallen on either conventional steam or conventional gas

turbine systems. Thus, the incremental costs of the combined system have

been written off against each commercial installation and are buried in

capital and operating costs. However, the R&D costs for the U.S. program

on coal gasification have totaled about $10' since 1965. The total opera-

ting time on commercial combined cycle systems in the U S , is over

6,000,000 hr.

RECOMMENDED DEVELOPMENT PROGRAM

To acllieve a thermal efficiency as high as that of coal-fired steam

plants, combined-cydle plants must be tightly integrated with coal gasifi-

cation plants to utilize the relatively large amounts of heat that would

otherwise be lost in the coal gasifi.c.sti.nn process. T h i ~ tight integratio~i

implies far more complex and difficult operation and control problems than

have been encountered heretofore in central power stations. These problems

and their implications relative to the manning and staffing requirements

for the operation and maintenance of integrated plants will require a de-

velopment program both for the manpower supply and the redirection of

the management of utilities to include the disciplines required for the

operation of a chemical plant.

The ope ra t i ng h i s t o r y of e x i s t i n g coa l g a s i f i c a t i o n p l a n t s , s u l f u r

removal systems, and combined-cycle p l a n t s should be examined t o e s t ab -

l i s h t h e inc idence of both forced and scheduled outages of t he se u n i t s

and t h e a s s o c i a t e d amounts of downtime. The r e s u l t s should then be ap-

p l i e d t o y i e l d an e s t ima te of t h e r e l i a b i l i t y and a v a i l a b i l i t y of t h e

i n t e g r a t e d p l a n t . I f t he e s t i m a t e s prove t o be d i s appo in t ing ly low, t he

p o s s i b i l i t i e s f o r improving the r e l i a b i l i t y and a v a i l a b i l i t y should be

examined because they could i n f luence t h e d i r e c t i o n of t h e program.

I f t h e c a p i t a l , f u e l , and ope ra t i ng c o s t of a combined-cycle-coal

g a s i f i c a t i o n p l a n t complex i s t o be apprec iab ly lower than t h a t f o r a

convent iona l coa l - f i r ed steam p l a n t , it w i l l be necessary t o i n c r e a s e

t h e gas t u r b i n e i n l e t temperature s u b s t a n t i a l l y over t h e h ighes t va lues

t h a t have proved p r a c t i c a b l e t o d a t e f o r c e n t r a l s t a t i o n base-load s e r v i c e .

The advantages t h a t h igher gas t u r b i n e i n l e t temperatures would confer

should n o t be taken f o r g ran ted because t h e r e i s reason t o doubt t h a t

any of t h e t h r e e approaches under a c t i v e development (a i r -cooled , water-

cooled , o r ceramic t u r b i n e b lades) w i l l prove successf u l . s ? I n pa r t i cu -

l a r , t o keep c o s t s reasonable , t h e f u e l w i l l probably have a h igher s u l f u r

and p a r t i c u l a t e conten t than n a t u r a l gas o r d i s t i l l a t e f u e l s . Thus a

.heavy f i n a n c i a l commitment t o t h i s type of system should be cont ingent

on a demonstrat ion t h a t a . g a s t u r b i n e i n l e t temperature i n t h e 1200 t o

1370°C range i s f e a s i b l e w i th f u e l from a c o a l g a s i f i c a t i o n p l a n t . Such

a test has been proposed us ing a Curtiss-Wright t u r b i n e w i t h a i r -cooled

b l s d e ~ fue l ed by t h e s l agg ing g a s i f i . ~ . ~ i n the Westfield p l a n t i n Scot land .

It appears t h a t such a t e s t could be run exped i t i ous ly and should prove

eminent ly worthwhile because i t should demonstrate whether t h e complex and

r e l a t i v e l y d e l i c a t e c o n s t r u c t i o n r equ i r ed t o make i t p o s s i b l e t o ope ra t e

a i r -cooled t u r b i n e b l ades wi th a %1370°C (2500 '~ ) gas temperature w i l l be

p r a c t i c a b l e wi th low-Btu gas from a c o a l g a s i f i c a t i o n p l a n t .

PRINCIPAL PROBLEMS AND PARAMETERS

The gas t u r b i n ~ t e a m combined cyc l e u t i l i z i n g p re sen t technology

and operaring exper ience can demonstrate e f f i c i e n c i e s of 40 t o 42% com-

pared t o 38% f o r t h e most modern coa l - f i r ed steam p l a n t . 3 9 9 4 0 I n o rde r

t o a t t a i n t h e s e e f f i c i e n c i e s , t h e gas t u r b i n e must ope ra t e wi th an i n l e t

t empera ture of 1000 t o llOO°C and a p a r t i c u l a t e content i n t he combustion

g a s of l e s s than 1 ppm. 9 4 0 4 1 This i n t u r n r e q u i r e s t h a t t h e t u r b i n e

b e supp l i ed wi th c l e a n f u e l s such a s n a t u r a l gas o r d i s t i l l a t e o i l . In-

c r e a s i n g t h e t u r b i n e i n l e t temperature w i l l r e s u l t i n an inc rease i n t he

combined-cycle e f f i c i e n c y because i t i n c r e a s e s both t h e gas t u rb ine e f f i -

c i ency and t h e tempera ture of t h e gas t u r b i n e exhaust from which hea t i s

s u p p l i e d t o t h e steam system ( s e e Fig. 4 ) .

Matching t h e Steam Cycle t o t h e Gas Turbine Cycle

When r a i s i n g steam by cool ing a hot gas , one is confronted by a

fundamental hea t t r a n s f e r problem. That is , t h e bulk of t h e hea t is

absorbed i n t h e water b o i l e r a t t h e s a t u r a t i o n temperature, which must

b e almost a s low a s t h e e x i t temperature of t h e gas i n order t o provide

a p o s i t i v e temperature d i f f e r e n t i a l throughout t h e l eng th of t h e b o i l e r .

The s i t u a t i o n i s i l l u s t r a t e d i n Fig. 5 f o r t y p i c a l steam condi t ions .

Note t h e "pinch poin t" a t t h e l e f t of t h e diagram, which determines t h e

upper l i m i t of t h e steam p res su re t h a t can be used wi th a given tu rb ine

exhaus t tempera ture i f no a f t e r b u r n e r is employed. Note a l s o t h a t super-

h e a t i n g g i v e s l i t t l e i n c r e a s e i n steam c y c l e e f f i c i e n c y r e l a t i v e t o an

i n c r e a s e i n s a t u r a t i o n temperature (Fig. 6 ) . This i s because a s t h e

supe rhea t ing i s inc reased , t h e temperature-entropy diagram depa r t s pro-

g r e s s i v e l y more from t h a t f o r t h e i d e a l r ec t angu la r Carnot cyc l e diagram

( s e e Appendix B ) . The p r i n c i p a l va lue of superhea t ing i s t h a t it eases .

problems w i t h t u r b i n e e r o s i o n and mois ture churning l o s s e s .

One way of g e t t i n g h e a t ' i n t o t h e steam cyc le a t a h igher temperature

i s t o u s e a dua l p r e s s u r e c y c l e wi th t h e higher p re s su re p o r t i o n absorbing

h a l f t h e h e a t from t h e hot gas and t h e lower p re s su re po r t ion absorbing

t h e o t h e r h a l f . Th i s e n t a i l s pass ing t h e higher temperature steam through

a n i n t e r m e d i a t e p re s su re t u r b i n e from which i t l eaves a t t h e same p r e s s ~ ~ r ~ .

as t h a t of t h e steam from t h e lower p re s su re b o i l e r so t h a t t h e two steam

s t reams can feed t h e low-pressure t u rb ine . The increased c o s t of t h e

more complex steam system i s smal l r e l a t i v e t o t h e inc rease i n output

ob ta ined . F igure 7 i n d i c a t e s t h e temperature d i s t r i b u t i o n i n a dual

~ 2 1 ) 800 1 1000 I 1200 I 1400 1 1

TURBINE INLET TEMPERATURE

Fig. 4 . Effects of gas turbine in l e t temperature on the turbine outlet temperature and the overall thermal efficiency of a combined cycle with o i l f i r ing and component e f f ic ienc ies of 90%.

14

ORNL-DWG 78-1 1394

SUPERHEATER

PINCH POINT

=90 psia

FEED HEATER

CYCLE EFFICIENCY = 27%

I I I I

FRACTION OF HEAT TRANSFERRED ( % )

Fig. 5. Temperature d i s t r i b u t i o n i n t h e b o i l e r a s estimated f o r a t y p i c a l gas turbine ,

p ressure b o i l e r f o r t h e same hot gas condit ions a s used i n Fig. 5. The

est imated e f f i c i e n c i e s f o r the steam cycles and f o r t h e combined dltal pres-

s u r e cyc le are indicated i n Figs. 5 and 7. I f the turbine i n l e t and out-

l e t temperatures were increased, t h e pressures and temperatures i n t h e

steam cyc les could be increased t o give an increase i n the o v e r a l l steam

c y c l e e f f i c iency .

F i g steam cy

ORNL-DWG 69-14096

16

OHNL- DWG 78-1 1395

-

- 620 psia

CYCLE EFFICIENCY = 37.5%

CYCLE EFFICIENCY = 27%

I I

FRACTION OF HEAT TRANSFERRED (96)

Fig. 7. Temperature d i s t r i b u t i o n s i n t h e steam generator estimated f o r a dual pressure cycle.

P rac t i cab le Gas Turbine I n l e t Temperatures

Present base-load f i r i n g temperatures ( i .e . , turbines operat ing a t

least 6600 h r lyea r f o r t h r e e years without major maintenance) a r e i n the

range of 1000 t o 1 0 5 0 ~ ~ . ~ ~ Increasing t h e allowable base-load f i r i n g tem-

pe ra tu res w i l l r e q u i r e t h e development of improved methods of tu rb ine

b lade cooling, possibly water cooling, o r improved blade materials such

a s ceramics . The p rospec t s f o r t h e succes s of t h e s e approaches a r e ex-

amined i n o t h e r r e p o r t s i n t h i s ser ies .5y6 Ceramic b l ades do no t appear

p r a c t i c a b l e , t h e po in t of d imin ish ing r e t u r n s seems t o be nea r f o r a i r

coo l ing , and water cool ing seems t o y i e l d des igns t h a t a r e no t on ly ex-

t remely complex i n o rde r t o keep thermal stresses t o t o l e r a b l e l e v e l s ,

bu t a l s o have hea t f l u x e s t h a t a r e so h igh t h a t t h e r e i s l i t t l e margin

i n t h e coo i ing system t o accommodate d e v i a t i o n s from i d e a l i t y t h a t might

y i e l d burnout ' hea t f l uxes . A s a consequence, commercial a p p l i c a t i o n is

f a r from c e r t a i n .

E f f i c i e n t U t i l i z a t i o n of Energy i n Coal

There a r e two b a s i c approaches t o t h e use of coal-der ived f u e l s i n

combined cyc l e s . The f i r s t i n t e g r a t e s t h e c o a l g a s i f i c a t i o n p l a n t and

t h e combined-cycle u t i l i t y p l a n t , s o t h a t hea t l o s s e s from t h e c o a l gas i -

f i c a t i o n process can be used t o advantage i n t h e steam cyc l e ; t h e second

l o c a t e s t h e c o a l g a s i f i c a t i o n p l a n t and t h e combined-cycle u t i l i t y p l an t

a t d i f f e r e n t sites. The "cold-gas e f f i c i e n c i e s " claimed f o r v a r i o u s coa l

conversion systems range from 65 t o 902."343-45 However, t h e w r i t e r s

have been unable t o f i n d any t e s t d a t a ob ta ined from an a c t u a l p i l o t o r

demonstrat ion p l an t t h a t y i e l d e f f i c i e n c i e s of conversion of t h e chemical

energy i n t h e coa l i n t o chemical energy i n t h e product i n excess of 61%

a f t e r a l lowances f o r energy i n p u t s represen ted by steam, compressed a i r ,

oxygen ( i f used) , and c i r c u l a t i n g pumps toge the r w i th l o s s e s i n t h e gas

c leanup t r a i n f o r removing s u l f u r and p a r t i c u l a t e s . lr a h igher va lue of

75% i s talcen as p o s s i b l e , t h i s wou1.d r ep re sen t a 25% e f f i c i e n c y pena l ty

t h a t would be app l i ed t o combined-cycle p l a n t s no t i n t e g r a t e d w i t h t h e

c o a l conversion p l a n t ; t h i s would g i v e t h e combined c y c l e p l an t a h igher

f u c l concumption than a conventi.nna.1. steain p l a n t , Thus, f o r an a t t r a c t i v e

o v e r a l l thermal e f f i c i e n c y , t h e more promising approach appears t o be t o

i n t e g r a t e t h e c o a l g a s i f i c a t i o n p l a n t wi th t h e combined-cycle p l a n t .

E f f e c t s of c o a l conversion process e f f i c i e n c y and h e a t recovery temperature

Close i n t e g r a t i o n of a c o a l g a s i f i c a t i o n p l a n t w i th a combined cyc l e

t h a t u t i l i z e s t h e waste hea t from t h e g a s i f i e r g ives a system whose o v e r a l l

t he rma l e f f i c i e n c y i s d i f f i c u l t t o app ra i s e . To ge t some pe r spec t ive ,

one can draw an envelope around t h e c o a l g a s i f i c a t i o n p l a n t . To a f i r s t

approximat ion t h e chemical energy i n t h e c o a l flowing i n t o t h e p l a n t w i l l

b e equa l t o t h e chemical energy i n t h e f u e l gas l eav ing t h e p l a n t , p lu s

t h e was t e h e a t recovered i n t h e form of steam a t a u s e f u l temperature , p lu s

t h e unrecoverab le hea t l o s s e s t o t h e sur roundings o r t o low-temperature

c o o l i n g s t reams. I d e a l l y , t h e f u e l gas from t h e g a s i f i e r would flow t o

t h e bu rne r of t h e combined c y c l e a t a h igh temperature , so t h a t t h e s ens i -

b l e h e a t i n t h i s f u e l g a s would a l s o be p a r t of t h e energy ou tpu t ; however,

i n most g a s i f i c a t i o n p roces se s t h e gas must be cooled t o remove s u l f u r and

p a r t i c u l a t e s . Hence, t h e s e n s i b l e hea t i n t h e f u e l gas is u s u a l l y sma l l ,

s o t h a t t h e parameter "cold g a s conversion e f f i c i e n c y " i s commonly used.

An examinat ion of d a t a f o r t y p i c a l p l a n t s i n d i c a t e s t h a t t h e h e a t

l o s s e s t o t h e sur roundings p l u s a l lowances f o r pumping power ( i nc lud ing

t h e thermal e f f i c i e n c y of t h e system f o r gene ra t i ng t h a t power) commonly

run roughly 7%. The s e n s i b l e hea t recovered from t h e c o a l g a s i f i c a t i o n

p r o c e s s can be u t i l i z e d e i t h e r f o r feedwater hea t ing o r f o r a low-pressure

s team c y c l e ; t h e o v e r a l l p l a n t thermal e f f i c i e n c y w i l l be t h e same i n

e i t h e r ca se . For purposes of t h i s a n a l y s i s , i t i s s impler t o t r e a t t h i s

in te rmedia te - tempera ture h e a t a s i f i t were u t i l i z e d i n a s imple steam

c y c l e which (with al lowances f o r convent iona l component e f f i c i e n c i e s )

would g i v e c y c l e e f f i c i e n c i e s of 12, 25.5, 32, and 33.5% f o r steam tem-

p e r a t u r e s of 93, 204, 316, and 427OC (200, 400, 600, and 800°F), respec-

t i v e l y . Thus, t h e r e a r e t h r e e major independent v a r i a b l e s a f f e c t i n g t h e

o v e r a l l power p l a n t thermal e f f i c i e n c y : t h e c o a l t o f u e l gas conversion

e t t i c i e n c y , t h e tempera ture of waste hea t recovery from t h e c o a l ga s i -

f i c a t i o n p l a n t , and t h e gas t u r b i n e c y c l e i n l e t temperature . This g ives

t o o many v a r i a b l e s f 0 r . a s imple c h a r t . One way of t r e a t i n g t h e problem

i s t o c a l c u l a t e f i r s t what can be c a l l e d an " e f f e c t i v e co ld gas thermal

e f f i c i e n c y , " which i s t h e co ld gas conversion e f f i c i e n c y p l u s t h e product

of t h e f r a c t i o n of t h e chemical energy i n t h e c o a l t h a t is recovered a s

s team a t a n i n t e rmed ia t e temperature and t h e r a t i o of t h e thermal e f f i -

c i ency of t h e steam c y c l e f o r t h a t temperature t o t h e thermal e f f i c i e n c y

of t h e combined c y c l e a t t h e s p e c i f i e d gas t u r b i n e i n l e t temperature ( s ee

F ig . 4 ) . The r e s u l t i n g v a l u e s a r e shown i n t h e lower p a r t of Fig. 8 f o r

ORNL OWG 78-10383

EFFECTIVE COLD GAS THERMAL EFFICIENCY (9"

-

, - 0 9 5 0 4050 ( 1 5 0 1250 1350

W a WF TURBINE INLET TEMPERATURE ( 'C) >u 05 UU - g L + z a a W W =b COLD GAS THERMAL EFFICIENCY (To)

5 0 6 0 7 0 8 0

93(2001 L - / - r I I , . I

950 1050 ( (50 1250 1350

TURBINE INLET TEMPERATURE (OC)

Fig. 8. E f f e c t s of co ld gas e f f i c i e n c y , s e n s i b l e heat r e j e c t i o n tem- p e r a t u r e , and t u r b i n e f i r i n g temperature .on in tegr ,a ted c o a l g a s i f i c a t i o n combined-cycle power p l a n t s .

a s e r i e s of cold gas thermal e f f i c i e n c i e s . Note t h e marked inc rease i n

t h e " e f f e c t i v e cold gas thermal e f f i c i ency" wi th an inc rease i n t h e tem-

p e r a t u r e a t which hea t i s recovered from t h e g a s i f i e r . The o v e r a l l thermal

e f f i c i e n c y of t h e p l a n t complex can be found from t h e c h a r t a t t h e top of

Fig. 8 by ucing t h e e f f e c t i v e c n l d gas thermal e f f i c i e n c y and the gas t u r -

b ine i n l e t temperature. F igure 8 shows t h a t , f o r a gas t u r b i n e i n l e t tem-

p e r a t u r e of 1100°C (2010°F), an o p t i m i s t i c a l l y high co ld gas conversion

e f f i c i e n c y of 80%, and recovery of waste hea t from t h e g a s i f i e r a t 204°C

(40n°F), t h e o v e r a l l thermal e f f i c i e n c y of t he p l an t complex is only %36%,

about t h e same a s f o r a convent ional coa l - f i r ed steam p l a n t wi th s t a c k gas

s c r u b b e r s . Thus e i t h e r a h ighe r waste hea t recovery temperature o r a gas

t u r b i n e i n l e t t empera ture of over 1100°C (2010°F) i s needed t o make t h e

sys tem a t t r a c t i v e .

C h a r a c t e r i s t i c s o f t y p i c a l c o a l g a s i f i c a t i o n processes

The g e n e r a l s u b j e c t of c o a l g a s i f i e r s s u i t a b l e f o r coupl ing t o a

combined c y c l e i s beyond t h e scope of t h i s r e p o r t , but an o u t l i n e of t h e

c h a r a c t e r i s t i c s and problems of t y p i c a l g a s i f i e r s is needed t o provide some

i n s i g h t i n t o t h e l i m i t a t i o n s t h a t t h e v a r i o u s types of g a s i f i e r impose on

a combined-cycle p l a n t . A s poin ted out i n an e x c e l l e n t a r t i c l e on t h i s

se t of problems, t h e v a r i o u s g a s i f i e r types under development can be

c l a s s i f i e d as moving-bed, f lu id ized-bed , o r entrained-f low u n i t s ' ( s ee

Tab le 1 ) .

I n t h e moving-bed g a s i f i e r s , raw c o a l is fed i n t o t h e top whi le steam

and a i r ( o r oxygen) e n t e r nea r t h e bottom and pass up through t h e bed i n

coun te r f low wi th t h e c o a l . Low-Btu f u e l gas l eaves a t t h e top a t '~500°C

(932°F) and ash i s d ischarged from t h e bottom. The bed i s s t i r r e d

mechanica l ly by a s lowly r o t a t i n g paddle , and t h i s r e q u i r e s t h a t a

noncaking c o a l be used w i t h a minimum of f i n e s . Temperatures vary widely

from t h e bottom t o t h e t o p of t h e bed, ranging from '~1200°C ( 2 2 0 0 " ~ ) i n

t h e lower p a r t t o ' ~ 5 0 0 ~ ~ (932°F) a t t h e top . Many u n i t s of t h i s type

oh g a s i f i e r ( i . e . , t h e Lurg i ) have been used commercially and give a

c o l d gas convers ion e f f i c i e n c y of %60%. The process i n h e r e n t l y y i e l d s

a g a s w i t h a high t a r con ten t which may be removed wi th a water sc rubber .

However, thorough c l ean ing is d i f f i c u l t , and t h e t a r s f o u l hea t exchangers

and o t h e r equipment ( a major problem i n t h e ~ u n e n p l a n t of F ig . 3 ) . A

v a r i a t i o n of t h i s type i s t h e s l agg ing bottom g a s i f i e r developed by t h e

B r i t i s h Gas Company. Oxygen r a t h e r than a i r -is used t o ~r,%ve temperature

o f Q~1500"C (2730°F) a t t h e bottom of t h e g a s i f i e r s o t h a t t h e a sh is above

t h e f u s i o n po in t and i s removed a s molten s l a g . This g r e a t l y reduces car -

bon l o s s e s i n t h e a s h and makes i t p o s s i b l e t o handle f i n e s i n t h e c o a l .

The f lu id ized-bed g a s i f i e r s o p e r a t e w i th a f l u i d i z e d bed of a sh o r

sand he ld a t a cons t an t temperature of 870 t o 1040°C (1600 t o 1900°F).

Coal o r c h a r i s f ed i n t o t h e bed, which i s f l u i d i z e d wi th steam and a i r o r

oxygen. . The high t e m p e r a t u r e . i n t h e gas l eav ing t h e bed is expected t o

Table 1,. General comparison of c o a l g a s i f i c a t i o n r e a c t o r t y p e s 4 4

- -

Function

Moving bed F lu id ized Entrained

Slagging Dry ash bottom bed flow

Zapac i ty Low High In te rmedia te High

A b i l i t y t o handle caking c o a l s without pretreatment Moderate Moderate Poor Exce l l en t

Temperature of ope ra t i on , OF 2000400 2800-800 1600-1900 3000-1700' N P

Temperature c o n t r o l Poor Poor Good , Moderate

Ref rac tory prob1em.s Moderate Poor Moderate Poor

By-product t a r formation Y e s Yes Poss ib ly probably no t

S j i l i t y t o e x t r a c t a sh low i n carbon , Moderate Good Moderate Good

4 3 i l i t y t o consume f i n e carbon p a r t i c l e s Poor Good Probably poor Good

make i t ' p o s s i b l e t o avoid t r o u b l e w i th t a r s i n t h e f u e l gas . Unfor tuna te ly ,

t h e r e i s n o t enough o p e r a t i n g exper ience w i th t h i s t ype of g a s i f i e r t o de-

f i n e t h e performance c h a r a c t e r i s t i c s t h a t can be a t t a i n e d .

The en t ra ined- f low g a s i f i e r systems supply pu lver ized c o a l , steam,

and a i r o r oxygen t o t h e bottom of t h e g a s i f i e r . These s w i r l through

t h e g a s i f i e r a t s u f f i c i e n t l y h igh v e l o c i t i e s t h a t t h e c o a l and a sh p a r t i -

c l e s a r e e n t r a i n e d i n t h e gas throughout i ts t r a n s i t through t h e r e a c t o r

v e s s e l . About 30% of t h e c o a l , t oge the r wi th t h e steam and preheated a i r

o r oxygen, i s i n j e c t e d t a n g e n t i a l l y i n t o a combustion chamber r eg ion a t

t h e bottom of t h e g a s i f i e r where t h e c o a l is burned e s s e n t i a l l y s t o i c h i -

o m e t r i c a l l y t o g i v e a tempera ture of %1870°c ( 3 4 0 0 ' ~ ) . The ho t gases pass

upward i n t o t h e g a s i f y i n g zone, where t h e balance of t h e c o a l feed i s i n -

j e c t e d and much of t h e s e n s i b l e hea t i n t h e ho t gas absorbed i n endothermic

r e a c t i o n s t o y i e l d t h e f u e l gas . The h igh v e l o c i t i e s reduce t h e t r a n s i t

t ime s o t h a t t h e carbon i s no t a l l consumed i n t he f i r s t pa s s and hence

t h e e n t r a i n e d ash i n t h e e x i t ga s i s removed and r e c i r c u l a t e d . Ash co l -

l e c t e d a s molten s l a g on t h e w a l l s of t h e combustion chamber is dra ined

o f f , quenched, and removed a s a water s l u r r y .

One must look c l o s e l y a t t h e energy inpu t and output of a g a s i f i e r

t o g e t a p roper va lue f o r i ts conversion e f f i c i e n c y . Take, f o r example,

t h e Texaco e n t r a i n e d bed g a s i f i e r of Ref. 45. A t y p i c a l set of d a t a is

t h a t from run W 1 on page 26 f o r a test us ing a s feed a t a r r y r e s i d u e from

a c o a l l i q u e f a c t i o n p l a n t . The cold gas conversion e f f i c i e n c y was a h igh

83.5%. However, a l lowance should be made f o r t he power r equ i r ed t o make

t h e oxygen supp l i ed t o t h e g a s i f i e r ( i . e . , 420 k ~ / t o n of 02) o r 717 Btu / lb

02 9 and when t h e thermal e f f i c i e n c y of t h e thermodynamic c y c l e f o r gen-

e r a t i n g t h i s power i s taken i n t o account , one f i n d s t h a t t h e co r r ec t ed

convers ion e f f i c i e n c y i s reduced t o %72%. A f u r t h e r allowance f o r t h e

energy i n t h e steam fed t o t h e g a s i f i e r reduces t h e co r r ec t ed co ld gas

convers ion e f f i c i e n c y t o 70%. A s u l f u r removal system would degrade i t

f u r t h e r by about two p o i n t s , and pumping l o s s e s would amount t o another

p o i n t o r two. Thus, t h e co ld gas e f f i c i e n c y a f t e r c o r r e c t i o n f o r a l l

t h e s e f a c t o r s would be %66% - n o t 83.5%. Fu r the r , t h e energy l o s s e s repre- . . .

s e n t i n g t h e d i f f e r e n c e between 83.5% and 66% would y i e l d h e a t a t too low

a tempera ture t o be of much v a l u e i n t h e steam cyc l e .

P l a n t i n t e g r a t i o n problems

An e x c e l l e n t t rea tment of t h e economics of i n t e g r a t i n g c o a l g a s i f i e r s

w i th combined cyc l e p l a n t s is presen ted i n a r e c e n t EPRI r e p o r t 3 2 which

g i v e s many d e t a i l s on t h e equipment r equ i r ed f o r t h e c o a l g a s i f i c a t i o n

and f u e l gas c leanup systems. There is enough d e t a i l e d d a t a i n t a b l e s

and f lowshee ts t o provide good q u a n t i t a t i v e e s t i m a t e s of t h e p r i n c i p a l

l o s s e s f o r each of t h e seven systems s tud i ed . Table 2 summarizes key

d a t a f o r t h e s e seven systems f o r which t h e des ign cond i t i ons were chosen

s o t h a t t h e r e s u l t s should be comparable. For example, t h e gas t u r b i n e

i n l e t temperature and p re s su re r a t i o , t he steam cyc l e cond i t i ons , and the

s t a c k gas temperatures were t h e same i n a l l cases . The des ign power output

v a r i e d somewhat, bu t only 210%. It i s i n t e r e s t i n g t o no t e t h a t , when t h e

p r i n c i p a l l o s s e s a r e taken i n t o account , t he h i g h e s t o v e r a l l thermal e f f i -

c iency f o r any of t h e systems was 40.6% (based on t h e f u e l h ighe r hea t ing

va lue , HHV), which i s much less than t h e 46.8% of t h e Westinghouse ECAS

s tudy . 3 4

To b r ing about t h e wide use of coal-der ived f u e l s i n combined gas

t u r b i n e t e a m p l a n t s , two deve10'~ments a r e necessary. The f i r s t i s the

development of an e f f i c i e n t c o a l g a s i f i c a t i o n process t h a t w i l l handle

a wide range of c o a l s wi th vary ing ana lyses and c h a r a c t e r i s t i c s ; t h e

second i s t h e development of a gas t u r b i n e t h a t can u t i l i z e t h e coa l -

der ived f u e l s wi th a t u r b i n e i n l e t temperature above llOO°C.

I n a t tempt ing t o a p p r a i s e t h e p rospec t s f o r improving t h e conversion

e f f i c i e n c y of c o a l g a s i f i e r s , i t should be remembered t h a t coal . g a s i f i c a -

t i o n has been used commercially f o r 175 yea r s and' t h a t around $10' has

been spen t on c o a l conversion i n t h e U.S. i n t h e p a s t 12 yea r s . S t i l l

n o p i l o t p l a n t has y e t given a co r r ec t ed conversion e f f i c i e n c y ( i nc lud ing

al lowances f o r s u l f u r removal) of more than %67% and none has opera ted

f o r over a year without s e r i o u s t r o u b l e .

The p r i n c i p a l problems i n t h e development of t h e gas t u r b i n e s t e a m

c y c l e i t s e l f a r e l a r g e l y t hose a s s o c i a t e d wi th i nc reas ing t h e pe rmis s ib l e

i n l e t temperature f o r t h e gas t u r b i n e . The combined gas t u r b i n e s t e a m

c y c l e f u e l r a t e improves approximately 2% f o r a r i s e of 55°C i n t h e gas

t u r b i n e i n l e t temperature and 7.8% f o r an i nc rease of 1% i n t h e average

component e f f i c i e n c y ( s e e Appendix B ) .

Table 2 . S u m ~ y of key performance Cata for zeal gasifier gas turbine combined .cycle power plant

Foster Combustion Texaco Texaco Lurgi BGC slagger E'L;ir MACW MXSC

Wheeler Ihgineering EXTC MTC EAHC WHC EALC (slurry feed) (dry feed)

Gasification and gas cleaning system

Coal feed rate. lb/hr m.f. Oxygen or airfcoal ratioBa lb/lb m. E. Oxidant temperature, O F

Steam/coal ratio. lb/lb m.E. Slurry water!coal ratio, l':/lb m.f. Gasifier exir pressure, pstg . Crude gas temperature, OF . Crude gas H W (dry basis) .' Btu/SCF Temperature of fuel gas to gas turbine,

Power system

Gas turbine inlet temperatxe, OF Pressure ratio Gas turbine exhaust temperature, O F

Steam conditions, psig/OF/"F Condensing .p:essure, in. Hs abs. Stack temperature, "F Gas turbine power.e Steam turbine power ,'MU Power consumed, MW Net system power, MU

Overall System

Process and deaerater make-p water, 2,207 334 497 1.031 157 362 1,072 gpm/1000 MW Cooling tower makeup water, gpm/100C MW 5,698 5,882 6,125 6,003 : ,439 7.588 7,255 Cooling water circulation rate, gpmlEIW 366 307 341 321 243 347 352 Cooling tower heat rejectian, % of ma1 HHV 33.9 33.8 36.8 33.2 27.6 38.7 35.6 Air cooler heat rejection, % of coal! HHV 6.5 4.7 3.2 7.2 r -9 5.2 4.6 Net heat rate, Btu/kWh 9,762 3,41@ 8,428 8,876 8.959 8,813 8,928 Overall system efficiency '(coal + pcuer), 34.96 40.6 40.5 38.5 X.1 38.7 38.2

% of coal HHV

.a Dry basis, 100% 02 for >xygen b l m case.

b~asifier exit pressure is -0.5 ic.. H20.

'~vera~e gasifier pressure.

d~xcluding the HHV of HzS, COzS, a d ??HI.

e ~ t generazor terminals.

Efficiency

The p o s s i b i l i t i e s of improving t h e thermal e f f i c i ency of t h e gas

tu rb ine cycle i t s e l f deserve some discuss ion t o ind ica te t h e p o s s i b i l i t y

of taking t h i s course t o improve t h e e f f i c iency of the combined cycle.

Considering t h e a c t u a l i n s t a l l a t i o n s of gas turbines by e l e c t r i c u t i l i t i e s ,

a few were made i n t h e l a t e 19401s, approximately 50 i n s t a l l a t i o n s were

made during t h e 1950's, and wide acceptance of t h e u n i t s began i n the

1960's with 24 u n i t s being i n s t a l l e d i n t h e 1960 t o 1962 period. This

t rend continued i n t o the 1970's with an estimated 20% of the new i n s t a l l e d

capaci ty of u t i l i t i e s i n 1971 being gas turbine driven.23 The majori ty of

t h e u n i t s i n s t a l l e d in the 1950 t o 1976 period were simple open-cycle

systems i n s t a l l e d a s generator d r ives f o r peaking and standby service .

To ob ta in a higher gas tu rb ine cycle ef f ic iency, a small number of regen-

e r a t i v e cycle u n i t s were i n s t a l l e d by u t i l i t i e s , Figure 9 shows the

e f f e c t s of tu rb ine i n l e t temperature upon t h e thermal e f f i c i ency f o r

some t y p i c a l u t i l i t y i n s t a l l a t i o n s of both simple open-cycle gas turbines

and open cycles with regenerat ion b u i l t during t h e 1950 t o 1976 period. 1 9-2 8

Inspection of Fig. 9 ind ica tes t h a t , a f t e r design, c a p i t a l , and cos t com-

promises w e r e made, l i t t l e w a s gained i n e f f i c iency by regenerat ion a t

tu rb ine i n l e t temperatures below approximately 900°C.

Figure 1 0 shows t h e re la t ionsh ip between t h e thermal e f f i c i ency , t h e

compressor pressure r a t i o , and t h e turbine i n l e t temperature f o r a series

of t y p i c a l modern gas turbines. The da ta f o r t h i s graph were adapted from

d a t a given i n Ref. 46 and a r e represen ta t ive of t h e p tesent s t a t e of t h e

a r t . The ca lcu la t ions were based upon compressor, turbine , and combustion

effic5encie.s of 89, 88, and LOO%, respect ively . It w a s a l so assumed t h a t

a t an i n l e t temperature of 870°c, 2% of t h e a i r is bled from t h e compres-

s o r f o r cooling t h e seal s t ruc tu re ; none is required f o r turbine blade

cooling. A t 1370°C (2500°F), 2% of t h e compressor a i r flow is f o r cooling

t h e seal s t r u c t u r e and 10% f o r cooling t h e turbine. It was f u r t h e r as-

sumed t h a t t h e 10% a i r f low f o r tu rb ine cooling var ied l i n e a r l y between

t h e tu rb ine i n l e t temperatures of 870 and 1370°C. It can be seen from

inspect ion of Fig. 1 0 t h a t increasing t h e tu rb ine i n l e t temperature up

ORNL-DWG 77-10205 A

500 600 700 800 900 1000 1100 1 ZOO ' - 7 - , r .7-w,r. :7

TURBINE INLET TMPERATURE ("c ) 1. .L? 1- .+,,,,<; , - + . , ; & ~ !$ i.; :-s+ep 1 . '".',,-,,:'\-- . F,.-

Fig. 9. Reported turbine inlet temperature and thermal efficiency for unirs installed during the 195U-1976 period compared to the ideal eff ic iency of a s imple open and a regenerative cycle with a compressor eff ic iency of 88% and a turbine efficiency of 90%.

Fig. 10. Effects of compressor pressure ratio and turbine in le t temperature on the thermal efficiency of open-cycle gas turbines.

t o %137OoC inc reases t h e thermal ef f ic iency, but above 1370°C t h e in-

c reas ing l o s s e s from a i r cooling begin t o more than o f f s e t t h e e f f e c t s

of inc reas ing temperature. Note a l s o t h a t t o take f u l l advantage of in-

c reas ing t u r b i n e i n l e t temperature, t h e compressor pressure r a t i o must be

increased.

Figure 11, which is superimposed upon t h e ca lcula ted values from

Fig. 10, shows the chronological development of u t i l i t y turbines a s indi-

ca ted by t h e range of gas t u r b i n e i n l e t temperatures and pressure r a t i o s

f o r . new models introduced. 9 - 2 8 9 4 7 AS can be seen, t h e manufacturers of

gas t u r b i n e s have been increas ing both t h e r a t i n g s f o r the compressor

p ressure r a t i o and t h e t u r b i n e i n l e t temperature i n order t o obta in higher

c y c l e e f f i c i e n c i e s ( i n l i n e with t h e a n a l y t i c a l l y derived r e l a t i o n s of Ap-

pendix B) . The cyc le e f f i c i e n c i e s genera l ly a r e about two points below

t h e values read from Fig. 10. The maximum values reported f o r the turbine

i n l e t temperature and t h e cycle e f f i c iency of peaking p l a n t s offered by

manufacturers and i n s t a l l e d by u t i l i t i e s i n t h e 1950 t o 1976 period a r e

shown i n Fig. 12. 9-28 9 The apparent d i s p a r i t y between t h e i n s t a l l e d

and o f fe red temperature ranges is caused by t h e delay between the placing

of o rders and the a c t u a l i n s t a l l a t i o n of t h e equipment. Note t h a t , in

spite of l a r g e increases i n t h e tu rb ine f n l e t temperature, t h e improvement

i n t h e thermal e f f i c i e n c y of open-cycle gas turbines seems t o have reached

t h e po in t of diminishing re tu rns . This stems from increases in l o s s e s

assoc ia ted with more d r a s t i c cooling of t h e turbine blades. 5

The c y c l e e f f i c i e n c y of gas turbine-steam combined-cycle p lan t s in-

s t a l l e d i n t h e 1950 t o 1975 period i s shown i n Fig. 13. 1 8 , 1 9 , 3 3 , 4 7 - 5 5 pOr

comparison, t h e e f f i c i e n c y of open-cycle gas turbine systems with regen-

e r a t i o n from Fig. 9 and t h e e f f i c iency of t h e gas turbine-steam combined

system from Fig. 1 3 a r e shown i n Fig. 14 as a funct ion of t h e tu rb ine

i n l e t temperature. Note t h a t e f f i c i e n c i e s i n Fig. 13 are based upon t h e

h igher heat ing value and those i n Fig. 9 a r e based upon t h e lower heating

value. A s can be seen from Fig. 14, t h e e f f i c iency of t h e combined cycle

is genera l ly super ior t o t h a t of an open cycle with regenerat ion. Another

important f a c t o r favoring t h e combined cycle is t h a t t h e much b e t t e r heat

t r a n s f e r c o e f f i c i e n t on t h e water s i d e of t h e heat exchangers used f o r .the

ORNL-DWG 77-10204 A

-_ _. _ _ . _ . . - MANUFACTURER LINE OF MAXIMUM

TEMPERATURE (OC)

Fig. 11. Reported performance of open-cycle gas turbines introduced during the 1949-1980 period.

ORNL-DWG 77-10200 A

1940 1950 1960 1973 1980 1990 YEAR

F i g . 12. Gas turbine in let temperatureqand cycle eff ic iencies for peaking units during the 1950-1976 period.

q: I: T' p ,I*-

? ,,'; .-

- I.?; . . - -

ORNL-DWG 77-10199 A

1950 1960 1970 1980 YEAR

Fig. 13. Plant thermal efficiency (HHV) and turbine in l e t temperature for gas turbinesteam combined systems installed during the 19504975 period. IS0 conditions, No. 2 fuel o i l , base-load rating.

ORNL-DWG 77-10198 A

TURBINE INLET TEMPERATURE ("c)

Fig. 14. Influence of t h e gas turbine i n l e t temperature on the thermal e f f i c i ency of open gas tu rb ine cycles with regenera t ive and combined gas turbine--steam cycles .

combined cyc le l eads t o s u b s t a n t i a l l y lower c a p i t a l cos t s a s compared t o

t h e open-gas tu rb ine cycle wi th regeneration.

Fuel F l e x i b i l i t y

Since the cos t of t h e f u e l is a s i g n i f i c a n t f r a c t i o n of the power

c o s t , t h e c o s t and a v a i l a b i l i t y of s u i t a b l e f u e l s g r e a t l y influence the

u s e of gas turbines by u t i l i t i e s . The need f o r higher thermal e f f i c i ency

r e q u i r e s t h a t the gas tu rb ine opera te with t h e maximum i n l e t temperature

that Pechnology will allow. The choice of turbine inlet temperature,

however, must be tempered by the effects of corrosion, erosion, and foul-

ing that may result from contaminants in the fuel.

There are two basic types of fuel that a gas turbine may accommodate.

These are the "clean" fuels, which include natural gas, distillate oils,

and other derived fuels that are relatively free from contaminants; and

the "heavy" fuels, which include crudes, residuals, heavy distillates,

and coal-derived fuels contaminated with sulfur, chlorine, and trace

metals. . .

The "clean" fuel, when protected from contamination before firing,

provides a satisfactory fuel for the present state-of-the-art gas tur-

b i n e ~ . ~ ~ The "heavy" fuels are unsuitable in the "as-received" condition

and require cleaning before firing. The major contaminants that must be

removed are those containing sodium, potassium, lead, and vanadium. The

combined concentration of these four elements is usually limited to a

maximum value of 3 ppm.

As the present sources of gas turbine fuels increase in cost and

decrease in availability,, the gas turbine fuels will be derived from

coal, shale, and tar sands. These fuels will probably have different

contaminants and firing characteristics than the petroleum-derived fuels

and will in turn require turbine system modifications.

The properties of the derived fuels produced will represent a com-

promise between high fuel quality, practical economics, and the maximum

utilization of natural resources.

Reliability and Availability

The reliability and availability of industrial and central station

gas turbines have steadily improved since their introduction after World

War 11. Various individual utilities have presently accumulated operatingb

experience with gas turbines of the order 10,000,000 hr . The application and general maintenance of the gas turbine are prob-

ably the two most important factors affecting the reliability and avail-

ability of the units. Edison Electric Institute made a study of equipment

availability of 35 systems employing jet engines (1076 unit years) and 66

systems employing gas t u r b i n e s designed f o r u t i l i t y ope ra t i on (2183 u n i t

y e a r s ) i n peaking s e r v i c e . The a v a i l a b i l i t y and s t a r t i n g r e l i a b i l i t y

f o r t h e 3259 u n i t y e a r s of s e r v i c e were 86.5 and 81.8%, r e s p e c t i v e l y . The

mean t i m e between fo rced outages was 666 h r . Reducing t h e ope ra t i ng tem-

p e r a t u r e of t he g a s t u r b i n e i s r epo r t ed t o be t h e most r e l i a b l e mode of

p r e v e n t i v e maintenance. I n 1975, Commonwealth Edison r epo r t ed a $250,000

annual r educ t ion i n t h e maintenance c o s t s f o r t h e o p e r a t i o n of 65 gas t u r -

b i n e s by reducing t h e t u r b i n e i n l e t temperature an average of 20°C below

t h e r a t e d va lue . " Typica l o p e r a t i n g condition^^^ f o r a peaking p l a n t

w i t h a i r -cooled nozz l e s and b l ades a r e given i n Table 3 .

Table 3 . Opera t ing c o n s t r a i n t s f o r a gas t u r b i n e peaking p l a n t fue led w i th n a t u r a l gas

Turbine i n l e t Operat ing hours1 Operat i on temperature number of s t a r t s

phase ( " 0 between i n s p e c t i o n s

a

Base 1000 Peaking 1065 Rcscrve 1100

a Thc inopcc t i on involves the i u l e t ~ i u z z l e and t h o f i r c t row of bladco. Expceted l i f e at Ldse- load c o n d i t i o n s i s es t imated a t 30,000 h r be fo re a major overhaul .

The t i m e between major overhauls is heav i ly dependent on t h e flie.1,

employed. For a g iven t u r b i n e i n l e t temperature t h e - b e s t r e s u l t s a r e

ob t a ined w i t h n a t u r a i gas ; w i th No. 2 f u e l o i l t h e time between major over-

h a u l s i s reduced by 20% and w i t h r e s i d u a l f u e l . o i 1 t h e t i m e between over-

h a u l s must be reduced by around 60% r e l a t i v e t o ope ra t i on wi th n a t u r a l

gas . 6 1 These r educ t ions i n l i f e have stemmed from t h e c o r r o s i v e e f f e c t s

of s u l f u r , vanadium, a l k a l i me ta l s , and c h l o r i d e s i n t h e f u e l . The

e f f e c t s a r e complex and a r e heav i ly dependent on t h e p a r t i c u l a r combina-

t i o n of t h e s e contaminants . I n p r a c t i c e , t h e usua l procedure is t o re -

duce t h e peak t u r b i n e i n l e t temperature f o r ope ra t i on wi th t h e d i r t i e r

fuels. The operating life also depends on the incidence of starts and

stops, which determine the amount of thermal strain cycling and hence

influence the development of cracks, particularly in the combustors.

Thus the operating life is much greater for gas turbines that operate

continuously at 'their design loads in industrial service or are employed

by utilities in base-load combined-cycle plants. A comparison of in-

dustrial and utility experience indicates that operation of the turbine

at continuous base-load conditions with no more than one start per 1000

hr.of operation would extend the inspection period from 'L3000 hr to be-

tween 8000 and 30,000 hr, depending upon the cost to the utility of down-

time . Adequate preventive maintenance and crew training can increase the

reliability and availability of both peaking and combined-cycle plants.

A case history of the Vogtle plant of the Geogia Power Company indicates

the value of preventive maintenance in a peaking plant. 6 2 Over a period

of two years the percentages for the reliability and availability improved

from the mid-eighties to the mid-nineties as a result of a change in pre-

ventive maintenance from none to adequate. Corresponding improvements

were experienced with combined-cycle plants as a result of crew training

and experience. A combined-cycle power plant operated by the City of

Clarksdale, Mississippi, showed an improvement in availability from ap-

proximately 70% at plant startup to 91.8% two years later. A combined-

cycle plant operated in the Netherlands by the Dow Chemical Company has

given results even better than the Clarksdale experience; c 3 the avail-

ability of the Dow plant improved from 88% at startup to 98.5% in a period

of 3 to 5 years.

Integration of a gas turbinesteam combined-cycle plant with a coal

gasification plant represents a new requirement for utility operators;

that is, operation of ;I rel.atively complex chemical plant in addition to

the electrical generation plant. This effective quantum jump in required

operator ability will most probably result in a lower reliability and

availability for the integrated plants during their formative years and

a lufiger period for shakedown than was experienced with the combined-cycle

plants. The fuel quality from an integrated coal gasification plant .will

probably be such that the gas turbine life, reliability, and/or the allow-

able turbine inlet temperature will be reduced. The overall reliability

and availability of the integrated plant must include the coal gasification

plant, which will likely result in low values considering the complexity

and severe corrosion conditions in the gasification system.

CURRENT STATUS OF DEVELOPMENT

As indicated in previous sections, the development of the gas t1.1rhine.

and the gas turbinesteam combined cycle are interrela.ted.. The develop-

ment status of the gas turbine is explored in cbmpanion reports in this

seyieg.7,=,6

The gas turbinesteam plants have been adopted by industrial and

utility plants during the past 25 years to satisfy special local condi-

tions. These conditions have incl.i.ided a need for high-temperature process

heat, repowering existing sites, and the availability of low-cost high-

grade fuel. 18,19,49,50 Approximately 6,000,000 hr of operating experience

with the gas turbinesteam combined-cycle plants in the U.S. has shown

that with adequate preventive maintenance and sufficient crew training,

the reliability and availability can reach the 95% range. 3 3 . 6 3

With the present state of development of gas turbines, a combined-

cycle plant operating with clean fuel. can be constructed to yield a cycle

efficiency of approximately 42%. However, in view of the present

clean fuel shortage, coal must be considered as the prin.r.i,pal fuel source

for a mmbined cycle.

The combination of a coal gasification plant and a gas turbinesteam

combined cycle might have (in the near-term development stages) a r.ycle

efficiency of approximately 40% , The development nf the con l l i i~ed

coal gasification and utility power plant to yield higher efficiencies

will have to provide solutions to the problems of erosion, corrosion, and

deposits at the relatively high temperature required by the gas turbine.

Construction of an experimental integrated hlgh-pressure gasification

plant and a combined gas turbinesteam electrical generation plant was

started in 1969 by Steinkohlen-~lektrizitat AG (STEAG) at the Kellermann

Power Station at ~unen near ~usseldorf , Germany. A simplified flowsheet

for the integrated gasification combined-cycle ~unen plant is shown in '

Fig. 3.' The plant as designed was expected to have an efficiency of 36.9%.

The solids content of the fuel gas after the scrubber is specified to be

1 to 2 mg/m3.

The plant began operation in January 1973. Early operations en-

countered difficulties with the Lurgi gasifiers and with inadequate solids

removal from the fuel gas by the two-stage water scrubber (i.e., particu-

late concentrations in the 50-mg/m3 range). 64 These and other problems

continued to handicap operation, so that at the end of 1977 the plant was

still. not in regular service. This must be considered significant because

the hen plant is the only one in the world in which a coal gasification plant has been integrated with a combined cycle. Further, the Lurgi gasi-

fiers are widely regarded as the most reliable of the various coal gasifi-

cation systems.

The Lurgi technology as represented in the ~unen plant has several

inherent disadvantages: relatively low capacity, high steam consumption,

inability to handle caking coals or large quantities of fine material,

and inability to recycle tars. 4 4 Various other coal gasification reactor

types are under present investigation. Some of these are in the paper

stage; others, such as the British Gas slagging gasifier located at West-

field, Scotland, and a two-stage gasi.fier operated by Westinghouse at

Waltz Mill, Pennsylvania, are in the pilot stage.

A major problem limiting the life of coal gasifiers is H2S corrosion

in the reducing atmnspheres that prevail. A recent paper65 on this sub-

ject indicates that one practical way to cope with the problem is to re-

' duce the operating pressure in the gasifier. This reduces the partial

pressure of the H2S, and this in turn reduces the corrosion rate. Figure

15 shows the corrosion rate for 0.5% Cr steel as a function of both tem-

perature and H2S partial pressure. The corrosion rate for 300 series

stainless steels i s about 10% of that for the ferritic low-chromium alloy

of Fig. 15.

Both Curtiss-Wright and General Electric, under contract with DOE,

are investigating combined cycles in which coal is burned directly in a

pre~surized f 1uidi.zrl.d bed combustor. In the Curtiss-Wright system,

(Fig. 16) the bed is cooled by air which is blended with the combustion

TEMPERATURE ( OF

Fig . 15. Corrosion r a t e ( i n . / y e a r ) of f e r r i t i c steel (1.52 Cr) i n hydrogen s u l f i d e . 6 5

gases ." The hot g a s mix ture i s used t o d r i v e a t u r h i n e and a waste hea t

b o i l e r . Approximately 70% of t h e power ou tput is generated by t h e gas

t u r b i n e and about 30X by t h e steam t u r b i n e .

I n t h e General E l e c t r i c approach, t h e f luidized-bed combustor i s

cooled by steam. The combustion gases from the p re s su r i zed fi1rn.a.c.e a r e

used t o d r i v e a g a s t u r b i n e , and t h e steam i s used t o d r i v e a steam tu r -

b ine . Approximately 70% of t h e power ou tput is genera ted by t h e steam

t u r b i n e and about 30% by t h e g a s t u rb ine .

These approaches u t i l i z i n g t h e p re s su r i zed f l u i d i z e d bed combustor

have i n common t h e problem of combustion gas c leanup (namely, s u l f u r

and p a r t i c u l a t e removal) . The d e t a i l de s igns of t h e s e systems w i - l l most

p robably r e f l e c t t r a d e o f f s between t h e gas c leanup system and t h e a b i l i t y

ORNL-DWG 78--11397

Fig . 16. S i m p l i f i e d f l o w s h e e t f o r t h e Curt iss-Wright p r e s s u r i z e d f l u i d i z e d bed-gas . t u r b i n e s t e a m . t u r b i n e combined-cycle power sys tem. 6 7

of t u r b i n e d e s i g n s and c o n s t r u c t i o n m a t e r i a l s t o t o l e r a t e p a r t i c u l a t e s i n

t h e g a s s t ream.

Commercial g a s t u r b i n e s have been t r o u b l e d by d e p o s i t s stemming from

d u s t i n t h e i n l e t a i r s t r eam. A s a consequence, t h e eng ine m a n u f a c t u r e r s

recommend t h a t a i r f i l t e r s be i n c o r p o r a t e d i n t h e i n s t a l l a t i o n t o keep t h e

d u s t c o n t e n t t o l e s s t h a n %2 ppm, and t h i s is commonly found n e c e s s a r y

i n many urban environments. However, r e c e n t e x p e r i e n c e i n d i c a t e s t h a t

even t i g h t e r r e s t r i c t i o n s are r e q u i r e d as t h e t u r b i n e i n l e t t e m p e r a t u r e

i s i n c r e a s e d . One o f t h e f i r s t s t r o n g i n d i c a t i o n s of t h i s was encounte red

i n a 1974 g a s t u r b i n e a c c e p t a n c e test a t t h e P h i l a d e l p h i a Navy Yard.

S u b s t a n t i a l d e p o s i t s were n o t e d a f t e r o n l y %200 h r of o p e r a t i o n under

c y c l i n g c o n d i t i o n s i n which t h e t u r b i n e i n l e t t empera tu re w a s i n t h e

2100°F (1150°C) range abou t h a l f t h e t i m e . A f t e r %3000 h r t h e s e d e p o s i t s

b locked c o o l i n g a i r d i s c h a r g e p o r t s i n t h e b l a d e s s o t h a t t h e y were damaged

by o v e r h e a t i n g . Subsequent i n v e s t i g a t i o n d i s c l o s e d t h a t t h e b u l k o f t h e

d e p o s i t c o n s i s t e d o f submicron-s ize FezOs p a r t i c l e s t h a t e n t e r e d t h e

eng ine w i t h t h e i n l e t a i r and had a p a r t i c u l a t e c o n t e n t of o n l y 0.06 ppm.

F u r t h e r i n v e s t i g a t i o n d i s c l o s e d t h a t P h i l a d e l p h i a a i r was n o t d i r t i e r i n

t h i s r e s p e c t t h a n t h e a i r i n most l o c a l i t i e s and t h a t t h e d e p o s i t s d i d n o t

form when t h e nominal t u r b i n e i n l e t t empera tu re was l i m i t e d t o ' ~ 1 8 0 0 ° F

(982°C). Note t h a t t h e m e l t i n g p o i n t o f Fe203 is 2860°F (1570°C), which

i m p l i e s a s i n t e r i n g t e m p e r a t u r e of Q2030°F (1110°C). T h i s a p p e a r s t o be

t h e r e a s o n t h a t t h i s t y p e o f d e p o s i t d i d n o t prove a problem u n t i l opera-

t i o n s a t a t u r b i n e i n l e t t e m p e r a t u r e of 2100°F (1150°C) were i n i t i a t e d .

Note a l s o t h a t c o a l a s h commonly c o n t a i n s %5% Fez03 . Thus, t h e impl ica-

t i o n s of t h i s e x p e r i e n c e a r e s e r i o u s indeed . It may n o t be p r a c t i c a b l e

t o o p e r a t e g a s t u r b i n e s i n base- load s e r v i c e w i t h t u r b i n e i n l e t temgera-

t u r e s above ?.llOO°C (2012°F) because o f d.i f f i..ci.il.t.ies w i t h Fe203 d e p o s i t s

vn t u r b i n e h l a d e s . '

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S. Moskowitz, " P r e s s l ~ r i xe.d Fl-uidized Bed P i l o t E l e c t r i c P l a n t , " Pro- ceedings of the Fluidized Bed Combustion TechnoZogy Exchange Workshop, C~~k ' - ' i IU44l -P- l , Vol. 1 ( A p r i l 1977) .

Appendix A

COMBINED-CYCLE POWER PLANT INSTALLATIONS

Table A-1 gives representative gas turbinesteam combined-cycle

power plants designed for electric utility power generation that were

in operation or on order,as of June 1976. The listing does not include

process applications (where the steam is used in the process and not

for electric power generation).

he data in Table A-1, ,taken from Ref. 29, were derived from data published during the 1966 to 1976 period. The information was cross-

checked for accuracy against manufacturer's installation lists.

Teble A-1. Cc .mhined-cyc le power p l a n t i n s t a l l a t i o n s a s of J u n e 1 , 1 9 - 6 ( R e f . 29)

Gas t u r b i n e Zombined-cycle O p r r a t i o n

Customer and s i t e Order t ime Fuel C ~ t p u t No. CE 3 u t p u t N o . of t ime Mfr.

I:KW) u n i c s :MW) p l a n t s

Comments

A u s t r i a Hohe Wand Timelkam Lir.z N.E. Ag T h e f s s Simmering Vienna Tiwag

Belgium I n t e r b r a b a n t S o c o l i e

Czechos lovak ia B r a t i s l a v a CSSR S t a t e E l e c . B r a t i s l a v e

F i n l a n d Nesteoy LKS L a h t i

F rance V i t r y Sur S e i n e I , I1 Emile Hachet

Germany S t a d t w e r k e Munich S t a d t w e r k e Brunswig A l t b a c h I1 Essen-Steag-Lunen G e r s t e i n w e r k Vew Emden IV NWK Rober t F rank PREAG Marbach, EVS CWH PLT I, 11 VEW Ensland A S t a d t w e r k e Munich Franken I G?AG GK Weser, Ve l the im VEW Emsland B,,C S t a d t w e r k e 3 u s s e l d o r f S t a d t w e r k e 3 u i s b u r g I11 Wiesbaden Ksl Mainz STW Saarbru:ken Bewag-Berli2

I t a l y Cornigl iano-Genova

K WU 1 2 1 80 1 1962 1965 Gz s 41.9% e f f i c i e n c y LCV BBC 7 0 1 1 1972 1975 Gzs-011 S t a l 7 4 1 1 1973 1976 Gz s KWU 73 1 i46 1 1974 1977 GES 46.6% e f f i c i e n c y LCV K WU 6 6 1 1 1975 1970 Gzs- o 9

ACEC - 92 1 135 1 1974 1976 0-1, g a s ACEC

P r v n i P r v n i

F i a t S t a l

BBC

KWU KWU KWU KWU KWU K WU BBC BBC KWU KWU KWU KWU BBC KW BBC KW

A l s t h A l s t h

BBC

Gas

Or1 C i t y h e a t i n g Gas

D i s t r i c t h e a t i n g D i s t r i c t h e a t i n g

Gas 42 .0% e f f i c i e n c y LCV Coke % a s

D i s t r i c t h e a t i n g D i s t r i c t h e a t i n g 38.2% e f f i c i e n c y LCV

Coa- g a s / o i l Coal ~ a s i f i c a t i o n Gas G a s 43.0% e f f i c i e n c y LCV Gas Gas 'o i l Gas Gas GasJo51 a s G%s f o i l &sf o i l GIS f o i l Gas D i s t r i c t h e a t i n g oil (11s/oil G a s / o i l

D i s t r i c t h e a t i n g D i s t r i c t h e a t i n g

Table A-1 ( con t inued)

Gas t u r b ~ n e Combined c y c l e Opera t i o n

Customer and s i t e o u t p u t NO. of o u t p u t NO. of Order time t ime

Fue l Comments Mfr.

(MW) u n i t s (MW) p l a n t s

J a p a n Maruzumi Paper b MH I 1 6 1 3 3 1 1965 1968 P.esid. o i l S a k a i d e Shikoku E l e c Fwr MH I 3 0 1 225 1 1966 1970 Coke g a s 42.1% e f f i c i e n c y LCV

Mexico Comision Fed E l g t r i c i d a d W 1 4 0 2 260 3 1973 1975176 O i l

N e t h e r l a n d s Amercentralie-PNEM BBC 1 6 2 1 1968 1971 Gas / o i l Royal Dutch S a l t GE 1 7 1 1 1969 1972 Gas Geertruiden-PNEM BBC 7 6 1 1 1973 1976 Gas De L ' A z o t e - S l u i a i l F i a t 3 0 1 1 1974 1976 O i l f g a s

Penama Cana l Cana l Company Balboa W 24 2 1 1962 1963 O i l

P u e r t o Rico Water Resources A u t h o r i t y GE 1 8 0 4 310 2 1974 1976177 O i l

S. Korea Korea E l e c t r i c GE 2 60 4 396 2 1976 . 1978 O i l

U.S.S.R. Nevinomyssk Len 3 7 1 200 1 1965 1969 Ga s

Venezuela Cadafe E l Tabazo WCan 1 5 1 1 1970 1972 O i l Repowering s team t u r b i n e

Repowering s team t u r b i n e



GT o p e r a t i o n 1975

Yugos lav ia Toplana

Ur.ited S t a t e s Apache Ar izona C i t y F a i r b a n k s Alaska C i t y F a i r b a n k s Alaska Oklahoma Gas 6 E l e c Chugach E l e c t r i c Alaska Community P.S. &w Mexico Empire R i v e r t o n Kansas West Texas U t i l i t i e s Dow Chemical Texas Sou th land Paper Texas U n i v e r s i t y Texas Wolver ine REA Michigan Ottawa Wtr 6 L i g h t Kansas C i t y Wyandotte Michigan Dow Chemical Texas

F i a t 90 N a p h t h a f r e s i d . D i s t r i c t h e a t i n g

NG 38.5% e f f i c i e n c y LHV O i l Repowering s team t u r b i n e O i l NG 39.7% e f f i c i e n c y LCV O i l Repowering s team t u r b i n e NG O i l NG 40.7% e f f i c i e n c y LCV NG NG NG O i l NG G a s f o i l NG


Table '3-1 1:cont inued:

Ga:; t u r b i n e Combined-cycle

Customer and s i t e Order time O p e r a t i o n Fue l Comments Output No. of Output :Jo. of t ime

Mfr. (W) u n i t s (MW) . > l a n t s

Uni ted S t a t e s i c o n t i n u e d ) C i t y C l a r k s d a l e M i s s i s s i ? p i Gulf Coas t Aluminum Duke Power Riverbend N.C. Duke Power Buck, N.C. C i t y Hutch inson Minnesota Dow Chemical Texas GE Lynn, Mass Kansas Power & L i g h t C i t y D e t r o i t Michigan Houston L i g h t & Power P u b l i c S e r v i c e Oklahoma S t . J o e Power & L i g h t C e n t r a l Iowa Co-Op C e n t r a l Vermont Pub S e r v Duquesne L i g h t Penn E l Paso Texas GPU J e r s e y C e n t r a l J a c k s o n v i l l e E l e c Auth J a c k s o n v i l l e E l e c Auth J e r s e y C e n t r a l L o u i s i a n a Power & L i g h t Ohio Edison Pub S e r v i c e E&G New J e r s e y So C a l Ed ison So C a l Ed ison So C a l Ed ison Southwest Fub S e r v T e x a s Ar izona P u b l i c S e r v i c e B r a i n t r e e M a s s a c h u s e t t s C-W Wood-Ridge New J e r s e y F l o r i d a Power & L i g h t P a c i f i c Gas & E l e c P o r t l a n d Gen E l e c Oregon S a l t R iver Ar izona San Diego Gas &, E l e c C e n t r a l Iowa Co-Op Dow Chemiczl Texas Burbank C a l i f o r n i a Chugach A l i s k a So Ca l ECison Wes te rn Farmers Oklahoma P a c i f i c Gas & E l e c

GE Turbd TPM TPM

G E W GE GE W GE W W W W GE W W W GE GE GE GE TPN Turbd W GE W GE Turbd cw W TPM GE GE Turbd W GE TPM Turbd GE GE ?

PG K c G a s l o i l G a s l o i l O i l EX: G a s l o i l

G a s l o i l E;G N: C i l C i l C i l C i l

C i l C i l C il C il CG C il b G EGIoi l E G O i l FG FG C l i l

FG O i l SG O i l KG NG G a s l o i l BG C a s l o i l O i l O i l O i l O i l

Repowering s team t u r b i n e Repowering s team t u r b i n e

Repowering s team t u r b i n e GT o p e r a t i o n i n 1973

Repowering s team t u r b i n e Repowering s team t u r b i n e Canceled

Canceled GT o p e r a t i o n i n 1973 GT o p e r a t i o n i n 1974 GT o p e r a t i o n i n 1973 GT o p e r a t i o n i n 1973 GT o p e r a t i o n i n 1973

Repowering s team t u r b i n e I n s t o r a g e Canceled Repowering s team t u r b i n e

Converted t o g a s i f i e d c o a l

GT o p e r a t i o n i n 1974

Canceled

Repowering s team t u r b i n e Repowering s team t u r b i n e GT o p e r a t i o n i n 1980181

C o n t r a c t n o t awarded y e t

Appendix B

POTENTIAL FOR FUTURE DEVELOPMENT

Theore t i ca l Cons idera t ions

One way t o cons ider t h e t h e o r e t i c a l a s p e c t s of t h e performance

of s e p a r a t e gas and steam t u r b i n e c y c l e s and of combinations of t h e s e

i s t o p l o t t h e i r c y c l e s on a temperature-entropy diagram and compare t he

r e s u l t s w i th those of t h e t h e o r e t i c a l Carnot cyc l e . One s ta tement of

t h e second law of thermodynamics i s t h a t no r e a l power p l a n t c y c l e opera-

t i n g between given upper and lower temperature l i m i t s can have an e f f i -

c i ency g r e a t e r than t h a t of a Carnot c y c l e ope ra t i ng between t h e same

tempera ture l i m i t s . The performance of t h e t h e o r e t i c a l Carnot c y c l e

t h e r e f o r e r e p r e s e n t s t h e upper l i m i t t o t h e performance of r e a l cyc l e s .

F igu re B-1 shows schemat ica l ly a Carnot cyc l e , i n which h e a t i s added a t

t h e h i g h e s t temperature of t h e gas t u r b i n e c y c l e and r e j e c t e d a t ambient

temperature . The h ighes t temperature is des igna ted T2 and t h e ambient

t empera ture TI. On t h e Carnot c y c l e diagram, a l l t h e a r e a under t h e l i n e

T 2 and above t h e temperature of a b s o l u t e zero T = 0 r e p r e s e n t s t h e hea t

added t o t h e c y c l e from t h e high-temperature source , and a l l t h e a r e a

below t h e l i n e T I and above a b s o l u t e ze ro r e p r e s e n t s t h e hea t r e j e c t e d

t o t h e atmosphere. The d i f f e r e n c e between t h e s e a r e a s , t h e a rea* be-

tween t h e l i n e s T2 and T I , r e p r e s e n t s t h e work done by t h e Carnot cyc l e .

The r a t i o of t h e work done t o t h e hea t added i s t h e Carnot e f f i c i e n c y E

and i s

F igu re B-2 shows t h e d e f i c i e n c i e s i n performance of a gas t u r b i n e com-

pared wi th t h e i d e a l i z e d Carnot c y c l e ope ra t i ng between. t h e temperature

A Only f o r r e v e r s i b l e processes , f o r which t h e r e a r e no l o s s e s caused

by f r i c t i o n , e t c . , do a r e a s on t h e temperature-entropy diagram rep re sen t h e a t and work. Real power p l a n t p rocesses can only be approximately so re ,presen ted .

ORNL-DWG 77-10430

I ORNL-DWG 77-lC43:

52 ENTROPY. S

5 1 52 ENTROPY. S

F i g . B-1. Carnot cyc l e . Fig. B-2. Gas t u r b i n e cyc l e .

l i m i t s T2 and TI i n t h e form of t h e a r e a s l e f t o u t s i d e t h e cyc l e i n t h e

r eg ion r e p r e s e n t i n g Carnot work.

F igu re s B-3 and,B-4 show somewhat d i f f e r e n t arrangements of combined

c y c l e s i n which some.of t h e hea t exhausted from t h e gas t u r b i n e is used

f o r making steam i n t h e steam p l a n t . It i s apparent t h a t t h e diagrams of

t h e combined cyc l e s have somewhat less a r e a l e f t b lank i n t h e r eg ions

r ep re sen t ing Carnot work and t h e r e f o r e may have h ighe r thermal e f f i c i e n -

c i e s than e i t h e r of t h e s e p a r a t e cyc l e s . I n t h e cyc l e represen ted by

Fig. B - 3 , t h e exhaust hea t from t h e gas t u r b i n e i s used t o provide a l l

t h e feedwater hea t and p a r t of t h e hea t of evapora t ion . For t h e steam

s e c t i o n of t h e p l a n t , t h e remaining h e a t of evapora t ion and t h e superhea t

and r ehea t a r e provided by a hea t source o t h e r than t h e s e n s i b l e hea t i n

t h e t u r b i n e exhaust - probably a f t e r b u r n e r . I n t h e cyc l e represen ted by

F ig . B-4, t h e gas t u r b i n e exhaust provides a l l t he h e a t r equ i r ed f o r the

ENTROPY. 5

Fig . B-3. Combined c y c l e w i th t h e ' s t e a m cyc l e h e a t requirement supp l i ed p a r t l y by t h e gas t u r b i n e c y c l e and p a r t l y by a burner i n t h e gao t u r b i n c cxhauot otrcam.

ENTHROPY. S

P i g . B-4. Combined c y c l e wi th t h e steam cyc l e hea t requirement en- t i r e l y supp l i ed by t h e gas t u r b i n e cyc l e .

s team s e c t i o n of t h e plant: Many o t h e r combinations a r e p o s s i b l e , in -

c l u d i n g t h e u s e of s team b l e d from t h e steam t u r b i n e a t an i n t e rmed ia t e

p r e s s u r e f o r feedwater h e a t i n g o r p rocess a p p l i c a t i o n . The performance

of each combination must be considered w i t h i n t h e con f ines of t h e system

a p p l i c a e l o n .

~nxim~im' E f f i c i ency of a Combined Cycbe wi th a F i r ed B o i l e r

Consider a s a n example t h e c a l c u l a t e d performance of a combined gas

t u r b i n e s t e a m power cyc l e t h a t c o n s i s t s of a r e f i r e d 1 0 MPa-538°C-5380C

(1500 psia-lOOO°F-lOOOOF) steam cyc l e topped wi th a gas t u rb ine . The

s team p a r t of the c y c l e h a s a thermal e f f i c i e n c y of 40.7% and t h e char-

a c t e r i s t i c s given i n Table B-1.

5 3

Table B-1. Steam c y c l e performance'

- -- -

Low-pressure High-pressure T o t a l system system

Evaporat ion p r e s s u r e , MPa ( p s i a ) 3.59 (540) Evaporat ion temperature , O C (OF) 244 (471) Net work, W / k g 1226 Heat added, kJ /kg , 1295

Fecdwater 94 2 Evaporat ion Superheat 353

Thermal e f f i c i e n c y , %

The g a s t u r b i n e i s o p e r a t e d a t an o v e r a l l t empera tu re r a t i o (T) of

t u r b i n e i n l e t t empera tu re t o ambient t empera tu re of 4 , which r e p r e s e n t s

approx imate ly t h e p r e s e n t gas t u r b i n e o p e r a t i n g c o n d i t i o n s . The average

component e f f i c i e n c y (p) i n t h e gas t u r b i n e c y c l e is taken a s 98%, and

t h e c y c l e p r e s s u r e r a t i o is v a r i e d t o de te rmine t h e e f f e c t s upon t h e over-

a l l c y c l e e f f i c i e n c y .

It can be s e e n i n Tab le B-2 t h a t t h e h i g h e s t the rmal e f f i c i e n c y f o r

t h e combined c y c l e o c c u r s when t h e gas t u r b i n e c y c l e i s des igned f o r maxi-

mum work. A t h e o r e t i c a l a n a l y s i s o f t h i s sys tem shows t h a t t h i s r e s u l t

i s t o be expec ted .

The temperature-entropy IT-S) diagram of t h e combined c y c l e is shown'

s c h e m a t i c a l l y i n F i g . B-5. .The symbols used i n the, f o l l o w i n g development

have t h e s i g n i f i c a n c e i n d i c a t e d i n t h e f i g u r e , where Q1 is t h e h e a t added

t o t h e g a s t u r b i n e from a n e x t e r n a l s o u r c e , W is t h e work done by t h e g a s t

t u r b i n e c y c l e , and t h e h e a t r e j e c t e d from t h e g a s t u r b i n e c y c l e i s t h e sum

of Q3, t h e h e a t r e j e c t e d t o t h e a tmosphere , and Q 2 , t h e h e a t t r a n s f e r r e d

from t h e g a s t u r b i n e exhaust t o t h e s team b o i l e r . The t o t a l h e a t added

t o th'e steam c y c l e i s t h e sum Q of two q u a n t i t i e s : Q 2 , t h e h e a t from Che

g a s t u r b i n e e x h a u s t , and Q 4 , t h e h e a t s u p p l i e d from a n e x t e r n a l s o u r c e .

The work done by t h e steam c y c l e i s Ws, and t h e h e a t r e j e c t e d t o t h e a t -

mosphere from t h e . s t e a m c y c l e is Q s . The e f f i c i e n c y of the combined

a Table B-2. Gas t u r b i n e and combined-cycle perfcrmarce

Compressor Compressor Gas t u r b i n e Gas tu rb ine exhams t G a s t u r b i n e Exhaust. hea t Ex te rna l heat Combined

Combined c y c l e t empera tu re p r e s s u r e e f f i c i e n c y ,

't tempera turn , work, t o steam. 92 e f f i c i e n c y , E

r a t i o , T r a t i o , (X) 'exh t (kJ,kg) t.0 steam, Qb c y c l e work, W (=c: (X)

a Rat io o f f low of a i r co flow of stEam, m = 4 . 4 9 6 ; tempera ture r a t i o a c r o s s c y c l e , T = 4 .

ORNL-DWG 77-10424

ENTROPY. S

F i g . B-5. T-S diagram f o r a g a s t u r b i n m t e a m combined c y c l e .

c y c l e is g i v e n by t h e e x p r e s s i o n

From t h e diagram,

4 4 = Q - 4 2

~ r o m the f i r s t l aw of thermodynamics,

Combining t h e s e exp res s ions g i v e s , f o r t h e e f f i c i e n c y ,

The thermal e f f i c i e n c y of t h e combined c y c l e and t h e work output of t h e

gas t u r b i n e cyc l e w i l l depend.on the choice of t h e compressor temperature

r a t i o T whereas, t h e q u a n t i t i e s , Ws, Q , and Q 3 a r e independent of t h e c

c h o i c e of compressor r a t i o . I t is d e s i r e d t o f i n d whether t h e r e is a

c h o i c e o i t h e compressor temperature r a t i o which maximizes t h e thermal

e f £ i c i e n c y . This c o n d i t i o n would be expected t o occur when d ~ / d ~ = 0, C

o r from Eq. ( 4 ) , when

S i n c e t h e q u a n t i t y i n parentheses can never be zero ( t h e e f f i c i e n c y of

t h e s team c y c l e cannot be g r e a t e r than u n i t y ) , i t is requi red t h a t

T h i s i s t h e requirement f o r a gas t u r b i n e maximum work cyc le and i n d i c a t e s

t h a t t h e maximum e f f i c i e n c y of t h e combined c y c l e w i l l occur when t h e gas

t u r b i n e c y c l e i s designed f o r maximum work. The r e s u l t s i n Table B-2 in-

d i c a t e t h a t t h i s expec ta t ion is indeed s a t i s f i e d f o r t he c a l c u l a t i o n s made

her@.

It would be expected from cons ide ra t ion of t h e Carnot cyc l e t h a t f o r

maximum e f f i c i e n c y t h e - t e m p e r a t u r e d i f f e r e n c e s , AT1 and AT2 (see Fig. B-5) ,

should be made a s smal l a s p o s s i b l e w i t h i n t h e r e s t r a i n t s of reasonably

s i z e d equipment a s determined by cons ide ra t ions of hea t t r a n s f e r . An in-

v e s t i g a t i o n shows t h a t t h i s i s t h e case . Equation (I), f o r t h e thermal

e f f i c i e n c y of t he combined c y c l e , can be w r i t t e n

When this expression is differentiated with respect to the two tempera-

ture differences, the result is

where K is a positive number, h represents the energy required to heat

feedwater, and Es is the thermal efficiency of the steam cycle. Therefore,

the efficiency of the combined cycle is highest when the two temperature

differences are as low as possible and preferably equal. This result

is also seen in Fig. B-6, where the maximum efficiency occurs when the

mass flow ratio is adjusted to give equal values of the temperature dif-

f erences . Figures B-7 to B-9 show the effects on the thermal efficiency of

changing one of the design conditions used for the calculation of the

results of Table B-2, namely, T = 4 for the overall cycle temperature

ratio (turbine inlet temperature = 927"C), p = 0.9 for the average com-

ponent efficiency of the gas turbine cycle, and AT = 28°C for the tem-

perature difference across the feedwater heater. The results of these

figures can be briefly summarized as follows:

1. The fuel. rate of the combined gas turbinesteam cycle improves

about 2% for a rise of 56°C in the turbine inlet temperature (the fuel

rate for the gas turbine cycle itself improves about 7.5% for a rise of

56°C).

2. The fuel rate for the comblned cycle improves about 2.7% for an

increase of 1% in the average component efficiency in the gas turbine

cycle (the fuel ra te . for 'the gas turbine cycle itself improves about 17.7%

for an increase of 1% in component efficiency).

3. The fuel rate for the combined cycle worsens by somewhat more

than 2.5% for each additional 28°C rise in the temperature difference

(AT) between the air and water.inqthe feedwater heater.

ORNL-DWG 77-10427 A

W S S FLOW RATE. m ( l b , A!R/lb, STEAM) GAS WRBINE CYCLE TENPERATURE RATIO

Fig. B-6. Thermal ePficisncy of combined cycle vs mass flow rate.

Fig. B-7. Zombined cycle thermal efficiency as a function of the gas turbine cycle temperature rati~,.

59

O R N L DWG 77 10431 A

TEMPERATURE D l FFERENCE ( O C I

Fig. B-8. Combined cycle thermal efficiency as a function of the heat exchanger temperature difference.

O R N L - OWG 71 10426 A

AVERAGE COMPONENT E F F I C I E N C Y 1 % ) . .

F i g . R-9. Combined cycle thermal efficiency as a function.of the average gas ~urbine comporient efficiency.

Because t h e e f f i c i e n c y and power o u t p u t from t h e b a s i c s team c y c l e

are b o t h h i g h e r t h a n t h o s e of t h e g a s t u r b i n e , t h e e f f i c i e n c y of t h e com-

b i n e d c y c l e i s less dependent on changes i n t h e gas t u r b i n e c y c l e t h a n

i s t h e ' g a s t u r b i n e c y c l e i t s e l f . T h i s means a l s o t h a t r a t h e r l a r g e i m -

provements i n gas t u r b i n e c y c l e performance a r e needed t o make s i g n i f i -

c a n t improvements i n t h e performance of t h e combined c y c l e .

OEUJLITM-6252 D i s t . Category UC-90f

I n t e r n a l D i s t r i b u t i o n

E. C. Fox 32. A. P. Fraas ( c o n s u l t a n t ) 33. T. G. Godfrey 34. R. L. Graves 35. R. S. Holcomb 36. J. M. Holmes '37. J. E. Jones , J r . 38. M. E. Lackey 3 9. R. E. MacPherson 40. L. E. McNeese 41-42. T. W . P i c k e l 43.

J . L. Rich M. W . Rosenthal I. Spiewak H. E. Trammel1 D. B. Trauger G. P. Zimmerman ORNL Pa t e n t Off i c e Cen t r a l Research Library Document Reference Sec t ion Laboratory Records Department Laboratory Records, RC

Ex te rna l D i s t r i b u t i o n

S . Alpe r t , EPRI, 3412 H,illview Ave., Palo A l to , C a l i f . 94304 D. H. Archer, Westinghouse Research Labora tory , 1310 Beulah Road, P i t t s b u r g h , Pa. 15235 J. T. B a r t i s , Planning and System Engineering, DOE F o s s i l Energy, MS-C-164, Germantown, Washington, D.C. 20545 D. H. Broadbent, Nat iona l Coal Board (IEA Se rv i ce s L td . ) , 14/15 Lower Grosvenor P l ace , London, England R. D. Brooks, General E l e c t r i c Co., Energy Systems and Tech- nology Div i s ion , Bldg. 2 , Schenectady, N.Y. 12345 Robert Brookshire , Tennessee Val ley Author i ty , 440 Commerce Union Bank Bldg., Chattanooga, Tenn. 37401 John Byam, Morgantown Energy Technology Center , P.O. Box 880, Morgantown, W.Va. 26505 N. H. Coates, The Mitre Cnrp., Westgate Research Park , McLean, Va. 22101 A. Cohn, EPRX, 3412 Millview Ave., Pa lo A l t o , C a l i f . 94304 H. G. Co rne i l , Exxon E n t e r p r i s e s , Inc. , 1251 Avenue of t h e Americas, N e w York, N.Y. 10020 Russe l l Covel l , Combustion Engineer ing, Inc. , 1000 Prospec t H i l l Road, Windsor, Conn. 06095 E. L. Daman, Fos t e r Wheeler Corp., 110 S. Orange Ave., Living- s t o n , N . J . 07039 Shel ton E h r l i c h , EPRI, 3412 Hi l l v i ew Ave., P.O. Box 10412, Palo Al to , C a l i f . 94304 John E u s t i s , DOE, Washington, D.C. 20545 E. C. Feher , TRW, Mail S t a t i o n 0112270, 1 Space Pa rk , Redondo Beach, C a l i f . 90278 T. J. F i t z g e r a l d , Oregon S t a t e Un ive r s i t y , Chemcial Engineering n ~ p a r t m e n t , C o r v a l l i s , Ore. 97331 K. W . Fosrer-Pegg, Westinghouse E l e c t r i c Corp., G36 Turbine Engine Div is ion , P.O. Box 9175, Ph i l ade lph ia , Pa. 19113

Steven Freedman, Coal Conversion and U t i l i z a t i o n , D iv i s ion o f

F o s s i l Energy Research, DOE, Washington, D.C. 20545 J. A. Fullam, Ingersoll-Rand Co., Process Compressor and Expanders, Turbo Products D iv i s ion , Ph i l l i p sbu rgh , N . J . 08865 F. D. Gmeindl, Morgantown Energy Technology Center , P.O. Box 880, Morgantown, W.Va. 26505 G. H. Goff , Research and Engineer ing, Bechtel Corp., 50 Beale S t . , San F ranc i sco , C a l i f . 94119 J e r r y Golden, Tennessee Val ley Author i ty , Knoxvil le Of f i ce Com- p l e x , 400 Commerce Avenue, WlOA19, Knoxvil le , Tenn. 37902 W. B. Ha r r i son , Southern Company Se rv i ce s , Inc . , P.O. Box 2625, Birmingham, Ala. 35202 H. R. Hazard, B a t t e l l e Columbus Labora to r i e s , 505 King Avenue, Columbus. Ohio 43201 T. A. HeLrLck, E ~ ~ L u L L Company, J e E i n n e ~ ~ e , Pa. 15644 8 . C . Hoke, Exxon Research and Engineering Co., P.O. Box 8 , Linden, N . . J . 070.36 J . J . Horgan, United Technologies Corporat ion, P.O. Box 109, South Windsor, Conn. 06109 H. R. Hoy, D i r e c t o r , Leatherhead Laboratory, Nat iona l Coal Board (BCURA), Ltd . , Randal l s Road, Leatherhead, Sur rey , England

D. L. Kea i rns , Westinghouse Research Laboratory, 1310 Beulah Road, P i t t s b u r g h , Pa. 15235 C. W . Knudson, General E l e c t r i c Co., Energy Systems and Tech- nology D i v i s i o n , Bldg. 2 , Schenectady, N.Y. 12345 L. R. Lawrence, Jr . , Gas Research I n s t i t u t e , 3424 S. S t a t e S t . , Chicago, I l l . 60616 J. F. Louis , Massachuset ts I n s t i t u t e of Technology, 77 Massachu- s e t t s Avenue, Energy Labora tory , Room 31-254, Cambridge, Mass. 02139 T. E. Lund, EPRI, 3412 Hi l lv iew Avenue, P.O. Box 10412, Palo A l to , C a l i f . 94304 J. J. Markowsky, American E l e c t r i c Company, 2 Broadway, N e w York, N.Y. 10004 C. H. Marston, General E l e c t r i c Company, 1 River Road, Schenec- t ndy , N.Y. 12345 M. J . Mayfield, Tennessee Val ley Au tho r i t y , 1020 CliesLnut Street Tower-11, Chattanooga, Tenn. 37401 J, L. Morgan, General Elcctric Co., I River Road, S r h e n ~ r t a d y , N.Y. 12345 Seymour Moskowitz, Curtiss-Wright Corp., One Pas sa i c S t r e e t , Wood-Ridge, N . J . 07075 Abolhassan Nazemi, The Mitre C O ~ ~ . / M E T R E K D iv i s ion , Westgate Research Park, McLean, Va. 22101 W . T. Newberry, Tennessee Val ley Au tho r i t y , 1020 Chestnut S t r e e t Tower-11, Chattanooga, Tenn. 37401 Kent P h i l i p s , Combustion Power Company, Inc. , Mamlo Park, C a l i f . 94025 J i m Powell , Brookhaven Nat iona l Labora tory , Department of Ap- p l i e d Sc ience , Upton, N.Y. 11973 Lynn Rubow, G i l b e r t Assoc i a t e s , P.O. Box 1498, Reading, Pa. 19603

L. I. Shure , NASA-Lewis Research C e n t e r , 21000 Brookpark Road, Cleve land , Ohio 44135 Lyle S i x , AiResearch Mfg. Co., 402 South 3 6 t h S t r e e t , Phoenix , Ar iz . 85034 J. W . Smith , Babcock and Wilcox, 20 South Van Buren Avenue, Barber ton , Ohio 44203 E. V . Somers, Westinghouse E l e c t r i c Corp., Research and Develop- ment C e n t e r , C h u r c h i l l Boro, P i t t s b u r g h , Pa. 15235 A. M. S q u i r e s , Department of Chemical E n g i n e e r i n g , V i r g i n i a P o l y t e c h n i c I n s t i t u t e , Blackburg, Va. Beno S t e r n l i c h t , Mechanical Technology, Inc . , 968 Albany Shalcer Road, b t h a m , N.Y. 12110 A. S. Thompson, P.O. Box 118, S p e r r y v i l l e , Va. 22740 R. H. T o u r i n , Program D i r e c t o r , New York S t a t e ERDA, 230 Park Avenue, New York, N.Y. 10017 J.. G. V l a h a k i s , DOE, Washington, D.C. 20545 G. E. Voe lker , F o s s i l F u e l U t i l i z a t i o n , DOE F o s s i l Energy, M.S.-E-178, Germantown, Washington, D.C. 20545 J. F. Weinhold, Tennessee Val ley A u t h o r i t y , 1345 Commerce Union Bank Bldg., Chat tanooga, Tenn. 37401 G. C. Weth, F o s s i l F u e l U t i l i z a t i o n , DOE F o s s i l Energy, MS-E- 178, Germantown, Washington, D.C. 20545 D. M. Willyoung, Genera l E l e c t r i c Co., 1 River Road, Schenec- t a d y , N.Y. 12345 H. W. Wi thers , Tennessee V a l l e y A u t h o r i t y , 1020 Ches tnu t S t r e e t , Tower-11, Chat tanooga, Tenn. 37401 F. A. Zenz, F. A. Zenz, Inc . , -P .O. Box 205, G a r r i s o n , N.Y. 10524 W . Zimmerman, Genera l E l e c t r i c Co., Advanced Energy Program, Evendale , Ohio 45215 O f f i c e of A s s i s t a n t Manager f o r Energy Research and Development DOE, ORO, Oak Ridge, Tenn. 37830 Given d i g t r i b u t i o n a s shown i n DOEITIC-4500 under c a t e g o r y UC-90f

Summary of the Development of Open-Cycle Gas Turbine-Steam ...

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