study the performance of the combined gas turbine-steam cycle for power generation

8/13/2019 Study the Performance of the Combined Gas Turbine-Steam Cycle for Power Generation

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Mathematical Theory and Modeling www.iiste.org

ISSN 2224-5804 (Paer! ISSN 2225-0522 ("nline!

#ol.$% No.&2% 20&$

20

Study the Performance of the Combined Gas Turbine-Steam

Cycle for Power Generation

'r. Saa )asso* +hmed,niersity o /aylon1ollege o ngineering'eartment o MechanicalIra3-/aylon

e-mailsaaya6yahoo.com

Abstract

In this research% a theoretical and analytical st*dy o gas t*rine-steam t*rine comined cycle or ower

generation was carried o*t to imroe the erormance o the thermal ower lant.

+ thermodynamic analysis o the comined cycle thro*gh a tyical attern was done. +roriate ass*mtions

were made or two 7inds o comined cycles% simle gas t*rine-steam t*rine and reheat gas t*rine-steam

t*rine comined cycles% s*ch that the res*lts otained wo*ld closely aroimate act*al data. Then the

e3*ations o the comined cycle calc*lations were deried. 9o*r cases were st*died to choose the otim*m gas

t*rine and steam t*rine conditions or the comined cycle.

The reheat gas t*rine-steam t*rine comined cycle is etter as comared to the simle gas t*rine-steam

t*rine comined cycle% eca*se the o*t*t er *nit mass o air low is signiicantly imroed y $5-4&:% andthe eiciency is higher y 4 : or the comined cycle *tili;ing the reheat gas t*rine% which oints to otential

cost saing or s*ch a cycle.

It was o*nd that the otim*m conditions o the gas t*rine cycle in the comined cycle mode occ*r at the

maim*m net wor7 conditions. The eiciency o the waste heat oiler or high eha*st temerat*re ranges

(maim*m :! is comarale to the eiciency o the conentional oilers. The waste heat oiler has the

added adantage o simlicity% thereore this tye o comined cycle *sing waste heat oiler has great romise

or ower generation.

Keywords Combined gas-steam cycle, gas turbine cycle

List of Symbols:

Latin Symbols

Symbol Definition

1? 1om*stion ress*re ?oss1 Seciic heat at 1onstant ress*re ([email protected]!

9*el to +ir Batio (7g *el7gair!

h nthaly (7@7g!

? 1om*stion and Badiation ?oss

m Mass low rate (7gsec!

P Press*re (Pa!C Deat S*lied (A@Ag!

r Press*re Batio

T Temerat*re(A!

E Eor7 o*t*t (E!

Greek SymbolsFD25 1aloriic #al*e at 25

o1

< iciency

Subscript

&%2%$%4%5%G The state as shown in 9ig*res(&%2%$%4!

2s%4s%Gs Isentroic case or each state

a +ir+dd +dded

/ /oiler

1 1omressor

1om 1omined

eg ha*st gas

g Haso "erall

s Steam

sg Stac7 gases

s* S*erheated


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2&

T T*rine

1. Introduction

lectrical ower generation has *ndergone a reol*tion since 1990. The gas-steam comined ower

lant has come o age and there is the romise o more adanced gas t*rine cycles ased on aero-deriatiemachines. '*ring the 1990s, a s*ite o com*ter codes were deeloed at 1amridge to analy;es adanced

ower cycles at a more detailed leel than *s*al. 9or s*ch calc*lations% it is most imortant to handle thethermodynamics as rigoro*sly as ossile(Horlock, J.H., et.al., 1998, 2000).

In recent ast the gassteam comined cycle ased ower lants hae ecome o*lar as these oer

more eectie *tili;ation o the ossil *el energy. These oer higher thermal eiciency as comared to the gas

t*rine ased lant or the steam t*rine ased lants in isolation the erormance o the gassteam comined

cycle ower lant deends *on the erormance o toing and ottoming cycle. Has t*rine is seen to oer

high seciic wor7 o*t*t i the t*rine inlet temerat*re (!)co*ld e increased. Th*s% with increased !theerormance o the heat recoery steam generator (H"#$)and consec*tiely the steam t*rine imroes therey%

oering imroement in comined cycle erormance (%ada&, J.'., #ing,r *.,200+).

The literat*re has oten s*ggested comining two or more thermal cycles within a single ower lant. In

all cases the intention was to increase eiciency oer that o single cycles. Thermal rocesses can e comined

in this way whether they oerate with the same or with diering wor7ing media. Doweer% a comination ocycles with dierent wor7ing media is more interesting eca*se their adantage can comlement one another.

Normally the cycles can e classed as a toingand a bottoming cycle. The irst cycle to which

most o the heat is s*lied is called the toing cycle. The waste heat rod*ces is then *tili;ed in a second

rocess which oerates at a lower temerat*re leel and is thereore reerred to as a bottoming cycle. , to

the resent time % only one comined cycle has o*nd wide accetance the comination gas t*rinesteam

t*rine ower lant (elo/er, "., 199).

+n alternatie regenerator conig*ration has een imroed y (allenback,'..,2002) thro*gh

imroing the eiciency o gas t*rine cycle.("a&i umer, et.al.,200)has energy losses in dierent gas t*rine

cycle comonents. It is osered that the irreersiility in eha*st gases is low which indicates eectie

*tili;ation o heat energy% *t the seciic wor7 o*t*t o the t*rine decreases.

Henerally with increase the t*rine inlet temerat*re the seciic wor7 o gas t*rine increases. /*t

increase in the t*rine inlet temerat*re has strict metall*rgical limitations in terms o maim*m temerat*re

that the t*rine stage co*ld with stand. ("a&i umar, 3., #ita "ama, .4., 200) analy;ed the eect o inletcooling on heat recoery steam generator (H"#$) erormance. It is o*nd that the inlet cooling red*ces the

wor7 in*t o the comressor and increases the mass low rate o air.

Similarly the eiciency o steam cycle can e imroed y increasing the temerat*re o steam entered

into the steam t*rine. The maim*m temerat*re o steam that can e *sed in steam t*rine is considered rom

metall*rgical oint o iew o t*rine lades. The mass low rate o steam and steam temerat*re deends on the

amo*nt o heat aailale in the gas t*rine eha*st. ("a&i umar, et.al., 200+)has done erormance sim*lation

o (H"#$) in comined cycle ower lant. They disc*ssed the eect o ario*s arameters li7e inch oint%

aroach oint% steam ress*re% steam temerat*re% gas low rate on the erormance o the (H"#$).

("a&i umar, et.al., 200)hae een analy;ed dierent heat recoery steam generator conig*rations o

single ress*re and d*al ress*re. The comined cycle eiciency with dierent heat recoery steam generator

conig*rations hae een analy;ed arametrically y *sing irst law second law o thermodynamics% it was

osered that in the d*al cycle high ress*re steam t*rine ress*re m*st e high low ress*re steam t*rine

ress*re m*st e low or etter heat recoery rom heat recoery steam generator. (#rini&as, ., et.al., 200)hae een analy;ed the eect o JnK eed water heaters (low! on erormance o a steam ower cycle with a

generali;ed mathematical orm*lation. The erormance calc*lations were orm*lated searately to single /.5.

and etended to 6n7 /.5.sor arametric st*dy. The otim*m led steam temerat*re ratio is o*nd at 0.with

single/.5.at gien wor7ing conditions. Similarly the otim*m ress*re in a steam reheated is otained at 20

2: o the oiler ress*re irresectie o the n*mer o heaters. The res*lts show that the maim*m gain in the

eiciency o cycle is otaining with the irst /5and the increment diminishes with the addition o the n*mer

o heaters. +lso% has een eamined the imroements in eiciency with increases in oiler ress*re% t*rine

inlet temerat*re and *rnace temerat*re.

2-alculations of Gas !urbine ycle

2-1:alculations of Simple Gas !urbine ycle

+ schematic diagram% "i#ure $1%% reresents the simle gas t*rine cycle and "i#ure $2%reresent the cycle on

!-Sdiagram. +ir at atmosheric ress*re is comressed rom state 1to state 2y the comressor. 9*el is added


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2$

2-1-2: (nalysis of Simple Gas !urbine ycle:

12 5rp ==== ;(1)

(((( ))))

a

a 1

1s2 %rp$!!

==== ;(2)

The act*al temerat*re o air at the end o comression is gien y

1s212

%!!$!!

++++==== ;(


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24

are then eanded to state '% where again *el is added in the second com*stion chamer to heat the gases tostate (*ll reheat!. Then% the hot gases are eanded thro*gh the second gas t*rine to state 6 (atmosheric

ress*re!. + schematic diagram o the reheat gas t*rine reresented in "i#ure $'%% where the n*mers on the

schematic corresond with the n*mers o the !-Sdiagram. The *el added to the irst com*stion chamer isf1

and or the second com*stion chamerf2.!

S

1

2s2

&

6s

6's

'

"i#ure $&%: !-S Dia#ram of 8e)eat Gas !urbine ycle.

G.!. G.!.

..

..

Gas Generatoro*er !urbine1

2 &

'

6

f

f

1

2

"i#ure $'%: Sc)ematic Dia#ram of 8e)eat Gas !urbine ycle.

2-2-1: (ssumption of 8e)eat Gas !urbine ycle:In reheat gas t*rine cycle% the same ass*mtions were made or the simle gas t*rine cycle% in addition to

&- The second com*stion ress*re loss=&/.2- The isentroic eiciency o the second gas t*rine also was e3*al to =/.$- The ecess air or the irst gas t*rine was set to '00 / and &00 / in the second gas t*rine or

eansions.

2-2-2: (nalysis of 8e)eat Gas !urbine ycle:The roerties o oints 142% and &are the same as in the simle cycle.

otes:the oints corresond with the oints o "i#ure $&%.

The act*al temerat*re o oint 'is calc*lation rom euation $1'%elow

1! ++ ====

%!!$p%f1%$m1$%!!$p 'a1aL12air ++++==== & ;(1)

The isentroic temerat*re at oint 4

(((( ))))

!

'&s'

!!!

==== ;(1)

The ress*re at oint '


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25

==== #as

1#as

&s' %rp31.$!!

1#as#as

'

&

'

&

!

!

====

;(1+)

Then the ress*re at oint 5 can e calc*lated as shown elow

%L1$ 2' ==== ;(1)Ehere L2-Second com*stion ress*re loss ercentage

Since 6 is 7nown rom the ass*mtion o the simle gas t*rine cycle or ' then we m*st ind the

ratio o $36%in order to ind the temerat*re at oint G as ollow

==== #as

1#as

6

s6

%3$

!

!

;(18)

Then% the act*al temerat*re at oint 6

%!!.$!! s6!6 ==== ;(19)Dence the net wor7 o*t*t

%!!$p%ff1%$m1$++ 6#as21aL1!net ++++++++======== & ;(20)+nd the heat s*lied is calc*lated as ollow according to mass and energy low diagram "i#ure $b% or the

second com*stion chamer

'#a1aL#as21aL

2airaLasaL1add

!p%f1%$m1$!p%ff1%$m1$$

!p%m1$!p%m1%$f1$,

++++++++++++

++++++++====

&&

&&

;(21)

ma h2

m

ma m h$

1.#.

( !1om*stion

1hamer

ma

m& h4

m2

ma m& m 2 h5

1.#.

(! ( !( !1om*stion

1hamer

(a! (!

"i#ure $%: ;ass and


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2G

Gen

S.!.

".+.7.

ondenser

oiler

!o Stack


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2>

f*)sup

s#e#

))

))

airofmass

steamofmass

==== ;(22)

To determine the mass o the leed steam or eamle% consider the adiaatic miing rocess at the irst

eed heater% in which Im& 7g o steam o enthaly hI% mi with (((( ))))Is mm && 7g o water o enthaly h8% to gie

Sm& 7g o water o enthaly h&0

%))$m%))$m 10sII ==== && ;(2


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28

22owas chosen as a inch oint o the comined cycles calc*lations% *t or the last two cases o

1'Aand 1Ain reheat gas t*rine-steam t*rine comined cycle% i the inch oint at the eaorator is 7et

the same as or the other cases% the dierence etween the stac7 temerat*re and the eed water temerat*re (the

inch oint at the economi;er! ecomes ery small in the range o 0 to o. This wo*ld oio*sly lead to a ery

large economi;er% which wo*ld e *neconomical and thereore not desirale. In order to aoid this% the inchoint at the economi;er was made as 22 oand or this case the inch oint at eaorator comes to e &'oand

2oetween the water steam circ*it and the gas circ*it in the waste heat oiler.

!emp.

Steam


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2L

The eha*st temerat*re decreases when the ress*re ratio increases at a constant theoretical airercentage% eca*se o greater eansion% th*s the eha*st temerat*re ehaes oosite to the maim*m

temerat*re with the ress*re ratio. 9or a gien ress*re ratio% the eha*st temerat*re decreases as the

theoretical air ercentage increases. This ehaior is the same as the maim*m temerat*re.

"i#ure$9%: ;aBimum !emperature and


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$0

"i#ure $11%:


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$&

"i#ure $1&% shows the relation etween the cycle eiciency and the net wor7 as a *nction o theress*re ratio or ario*s maim*m temerat*res. It is clear rom the lot that or a gien maim*m cycle

eiciency is higher than the otim*m ress*re ratio or maim*m net wor7% eca*se at high ress*re ratio the

heat added ecomes less which ma7es the eiciency increase *ntil a certain ress*re ratio% then it ecomes

decreases% eca*se the net wor7 ecomes ery small. +t a constant ress*re ratio the cycle eiciency and the networ7 increases as the maim*m temerat*re increases% eca*se greater eansion is ossile.

"i#ure$1&%:ycle


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$2

"i#ure $1%:+ork 8atio ?ersus 8.

&-2: 8esults of 8e)eat Gas !urbine ycle:

Two sets o calc*lations hae een made or the reheat gas t*rine cycle.

&-2-1: onstant (3" 8atio:

In this set o calc*lations only the '00/o theoretical air was chosen.

"i#ure$16%shows a lot o the net wor7 ers*s the ress*re ratio. It can e shown that the net wor7

increases when the ress*re ratio increases% eca*se the maim*m temerat*re is increased d*e to increase o the

o*tlet comressor temerat*re% the increases ecome less and less as the ress*re ratio increases% eca*se more

wor7 is needed or comressor.

+lso it can e shown on the same "i#ure$16% that the cycle eiciency increases as the ress*re ratio

increase% eca*se more net wor7 is rod*ced% *t these increases ecome less and less% eca*se the net wor7

ecomes less and less as more wor7 is needed or the comressor.

The wor7 ratio decreases as the ress*re ratio increases% eca*se more wor7 is needed or the

comressor as shown in "i#ure $16%. The maim*m temerat*re increases as the ress*re ratio increases d*e toincrease o the o*tlet comressor temerat*re.

"i#ure $16%:ycle


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$$

"i#ure $1%:!'4!64 and 36?ersus 8.

&-2-2: onstant ;aBimum !emperature:

In the same o the simle gas t*rine cycle% the set o calc*lations rom 11'' to 1 Awere *sed asthe maim*m temerat*re.

!able $&% gies the otim*m ress*re ratios at which the net wor7 and the cycle eiciency are

maim*m.

!able $&%: Cptimum ressure 8atios for ;aBimum ycle


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$4

"i#ure $19% shows the relation etween the cycle eiciency and the ress*re ratios or ario*smaim*m temerat*res. +t a constant maim*m temerat*re% the cycle eiciency increases when the ress*re

ratio increases *ntil a certain al*e o ress*re ratio and then it egins to decreases with the increase o the

ress*re ratio% eca*se the net wor7 ecomes less. +t a constant ress*re ratio% the cycle eiciency increases

with the increase o the maim*m temerat*re% eca*se net wor7 ecomes more d*e to greater eansion.

"i#ure $19%: ycle


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#ol.$% No.&2% 20&$

$5

"i#ure $21%: !6?ersus 8 for 8e)eat Gas !urbine ycle.

"i#ure $22% shows the relation etween the ower t*rine eansion ratio and the ress*re ratio or

dierent maim*m temerat*res.

+t a constant ress*re ratio% the ower t*rine eansion ratio increases as the maim*m temerat*re

increases% eca*se the eit enthaly rom the irst gas t*rine increases% so that the eit relatie ress*re

increases which ca*ses the increase in the ress*re o state . +t a constant maim*m temerat*re% the ower

t*rine eansion ratio increases when the ress*re ratio increases *ntil a certain al*e o ress*re ratio% the

ower t*rine eansion ratio egins to decreases% eca*se the greater eansion occ*rs in the irst gas t*rine

d*e to more comressor wor7 ma7es the eit enthaly ecomes less% so that the eit relatie ress*re ecomes

less which leads to the decrease in the ress*re o state .

"i#ure$22%: 36?ersus 8 for 8e)eat Gas !urbine ycle.

"i#ure $2&% shows the relation etween the wor7 ratio and the ress*re ratio or ario*s maim*m

temerat*res. It is the same as in the simle gas t*rine cycle.

0 16 2' &2 '0

ressure 8atio

600

00

1000

1200

1'00

!6$A%

!@11''A

[email protected]

!@1&66A

[email protected]

!@1'00A

0 16 2' &2 '0

ressure 8atio

0.0

1.0

2.0

&.0

'.0

.0

6.0

$1.3

16%

!@11''A

[email protected]

!@1&66A

!@1' A

[email protected] A


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#ol.$% No.&2% 20&$

$G

"i#ure$2&%: +ork 8atio ?ersus 8 for 8e)eat Gas !urbine ycle.

&-&: 8esults of t)e ombined ycle:

The res*lts o the simle gas t*rine-steam t*rine and the reheat-steam t*rine comined cycles aregien in the same lots to ma7e a comarison etween them.

The res*lts are ill*strated in "i#ures $2'% to $29%. "i#ure $2'%shows the relation etween the eha*st

temerat*re and the maim*m temerat*re or oth simle and reheat gas t*rine cycle. The eha*st

temerat*re increases as the maim*m temerat*re increases and it is clear that the eha*st temerat*re or the

reheat gas t*rine is greater than the eha*st temerat*re or the simle gas t*rine% which means that the heat

energy aailale in the eha*st gases is more in the reheat gas t*rine cycle.

"i#ure $2'%:


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$>

"i#ure $2%: Stack !emperature ?ersus ;aB. !emperature.

"i#ure $26%shows a lot o the oiler eiciency ers*s the maim*m temerat*re. Ehen the maim*m

temerat*re increases the oiler eiciency increases in the oth comined cycles. The oiler eiciency o the

comined cycle *sing the reheat gas t*rine is greater than the oiler eiciency o the comined cycle *sing the

simle gas t*rine y $10-1% /% eca*se o the lower stac7 temerat*re in the comined cycle *sing reheat gas

t*rine.

"i#ure $26%: >oiler


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$8

"i#ure $2%: C?erall Steam ycle


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$L

"i#ure $29%: ombined ycle ?ergy nalysis o/ @odern Aossil-Auel 'o5er 'lants,

+SM @. ng. 9or Has T*rine and Power &28% -&->.

-Dorloc7% @. D.% )o*ng% @./.% Manirda% H.% &LL8% e "ational >//iciency o/ Aossil-Auel 'o5er 'lants,+SM

Symosi*m on thermodynamics Int. Mech. ng.% No.% 1aliornia.

-Aehlhoer% B.% &LL>% Combined-Cycle $as and #team 'o5er 'lant,PennEell P*lishing 1omany% T*lsa-

"7lahoma.-Bai A*mar% N.% et.al.% 2005% >?ergy analysis o/ $as urbine 'o5er 'lant 5it lternati&e Con/iguration o/

"egenerator,Proceeding on 1'% 2nd

International ergy nergy nironmental Symosi*m (IS2!% Aos%

Hreece% #I-&$.

-Bai A*mar% N.% et.al.% 200G% 'er/ormance #imulation o/ Heat "eco&ery #team $enerator in Combined Cycle'o5er 'lant, Proceeding &8

th National and &>

th ISDMT-+SM Deat and Mass Transer 1onerence% India%

.&>8&-&>8>.-Bai A*mar% N.% et.al.% 200>% ermodynamic nalysis o/ Heat "eco&ery #team $enerator in Combined Cycle

'o5er 'lant,Thermal Science% #ol.&&% No.4% .&4$-&5G.

-Bai A*mar% N.% Sita Bama BaQ*% +.#.% 2005% e #tudy o/ te >//ects o/ $as urbine !nlet Cooling on 'lant

and H"#$ 'er/ormance, Proceedings% National 1onerence on +dances in Mechanical ngineering% India%

.55-G0.

-Bice% I.H.% &L80% e Combined "eeat $as urbine #team urbine Cycle,+SM% #ol.&02% @an*ary% . $5-4L.

-Srinias% T.% H*ta% +.#.S.S.A.S.% Beddy% /.#.%200>% $eneraliBed ermodynamic nalysis o/ #team 'o5er

Cycles 5it n 3umber o/ Aeed5ater Heaters,Int. @. o Thermodynamics% 'ecemer%#ol.&0 (No.4!% . &>>-

&85%

-)ada% @.P.% Singh% ".% 200G% ermodynamic nalysis o/ ir Cooled #imle $as and #team Combined Cycle'lant,@o*rnal o Instit*tion o ngineers (India!-Mechanical ngineering% #ol.8G% .222.

600 00 1000 1200 1'00 1600 100;aBimum !emperature $A%

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study the performance of the combined gas turbine-steam cycle for power generation

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