Lecture11-CombinedCycleCombinedCyclewithHeatRecovery

8/9/2019 Lecture11-CombinedCycleCombinedCyclewithHeatRecovery

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Page 1 of 5Lecture 11

Lecture 11: Combined Cycle,

with Heat RecoveryCombined Heat and Power

When combustion engines are used only for generating electricity, most of the usable energy from fuel combustion is lost in the form of heat – resultingin systems that are only 20 - 30% ecient !enerating systems can be madegreater than "0% ecient by reco#ering and using $aste heat from thecombustion rocess – this strategy is more commonly &no$n as'(ogeneration' or '(ombined )eat and Po$er'

(ogeneration, as discussed in re#ious lectures, $here #aor o$er systemused for district heating *or cooling+ he resent alications build onrecogniing that the e.haust gas temerature of a simle gas turbine istyically $ell abo#e ambient temerature and thus hot gas e.iting theturbine has signi/cant thermodynamic utility that might be harnessedeconomically

Combined Cycle, General

(ombined cycle o$er generation combines t$o cycles for oeration, namelythe gas turbine cycle *rayton (ycle+ and the #aor o$er *or an&ine+ cycle

n a gas turbine o$er lant, natural gas and comressed air undergocombustion he resultant high ressure gas dri#es the gas turbine $hich inturn roduces electricity lthough it is clean and fast in starting u, the gasturbine o$er lant su4ers from lo$ thermo eciency of about 25 to 30%6uch of the energy is $asted in the form of gas turbine e.haust

Figure 11.1: P-v, T – s and Schematic of Air Standard Brayton Cyce




he hot e.haust gas from the gas turbine, instead of being released as $aste,is catured and channeled to the steam turbine $here steam is heated by thee.haust to dri#e the turbine he combined cycle o$er generation ma&esuse of the merits of the high temerature *1100 to 17508(+ gas turbine cycleand the lo$er temerature *590 to 7508(+ steam turbine cycle

Combined Cycle with Heat – Recovery Boiler

n the gas turbine lant, atmoshericair enters through the comressorand into the combustor *orcombustion chamber+ $here fuel*usually natural gas+ is added(ombustion ta&es lace and the hotgas dri#es the turbine, $hich in turn

dri#es the generator and roduceselectricity

he gas turbine is usually oerated$ith a high air-fuel ratio,aro.imately 900 ercenttheoretical air, to ma&e sucient aira#ailable in the gas-turbine e.haustfor further combustion

he hot :ue gas from the gas turbineenters a heat e.changer, sometimes

&no$n as heat reco#ery boiler or heat reco#ery steam generator, $here it isused to heat u the steam he suerheated steam is then used to dri#e thesteam turbine $hich in turn dri#es the generator to roduce electricity hee.it steam from the steam turbinegoes through a condenser and thenbac& to the heat e.changer $herethe cycle reeats itself

n large combined-cycle lants used foe base-load oeration, $here eciencyis of rime imortance, searate sulementary /ring e;uiment isinterosed bet$een the gas turbine and the heat reco#ery boiler he steam-turbine outut is usually greater than the gas-turbine outut by <=1 he

steam cycle is therefore designed for high eciency $ith reheat and a fullcomlement of feed$ater heaters forced draft fan may be installed aheadof the sulementary /ring to oerate the steam cycle on its o$n $hen thegas turbine is o4

>ariations of the cycle sho$n in ?igure 112 are used to e.tract the ma.imumamount of energy from the gas lea#ing the heat-reco#ery boiler beforee.hausting it to the stac& @eending uon the temerature of that gas, it

Figure 11.!: Com"ined gas tur"ine–va#or #o$er #ant.



Page of 5Lecture 11

may be used for *1+ artial heating *regeneration+ of the comressed airlea#ing the comressor, *2+ feed$ater heating of the steam cycle in a closed-tye feed$ater heating, or *3+ generating steam in a dual – or multi-ressuresteam cycle

here are #arious tyes of combined cycle o$er generation, the most

imortant of $hich comriseA

1 (ombined cycle $ith sulementary /ring,2 (ombined cycle $ith regeneration andBor feed$ater heating,3 (ombined cycle $ith multi-ressure steam cycle,9 (ombined cycle for nuclear o$er lants

!"ciency

he combined cycle has the gas turbineCs high a#erage temerature of heataddition and the #aor cycleCs lo$ a#erage temerature of heat reDection, andthus a thermal eciency greater than either cycle $ould ha#e indi#idually?or many alications combined cycles are a good choice, and they areincreasingly being used $orld$ide for electric o$er generation

With reference to ?ig 112, the thermal eciency of the combined cycle is

η= gas vap

´

$here is the net o$er de#eloed by the gas turbine and is

the net o$er de#eloed by the #aor cycle denotes the total rate of

heat transfer to the combined cycle, including additional heat transfer, if any,to suerheat the #aor entering the #aor turbine he e#aluation of the;uantities aearing in abo#e e;uation follo$s the rocedures described inthe sections on #aor cycles and gas turbines

he relation for the energy transferred from the gas cycle to the #aor cyclefor the system of ?ig 112 is obtained by alying the mass and energy ratebalances to a control #olume enclosing the heat-reco#ery steam generator?or steady-state oeration, negligible heat transfer $ith the surroundings,and no signi/cant changes in &inetic and otential energy, the result is

− −



Page # of 5Lecture 11

$here and are the mass :o$ rates of the gas and #aor,

resecti#ely

s $itnessed by relations such as abo#e e;uations, combined cycleerformance can be analyed using mass and energy balances

ecause gas turbines are not yet built in sies as large as steam turbines,combined cycles are often built in combinations of more than one gas turbinelus one steam turbine Euch combinations sho$ certain ad#antages, notonly in higher total lant outut but also in higher a#ailability, :e.ibility inser#ice, and art-load eciency

$dvanta%e&

combined cycle o$er generation system o4ers many ad#antages

s most combined cycle generation systems use natural gas as the fuel, theen#ironmental emissions are lo$ here is less ollution roduced comaredto con#entional steam or gas turbine o$er lants s such, comle. ande.ensi#e en#ironmental control systems are not needed ransortation of fuel #ia ielines is also easier than that of coal and oil

he gas turbine ortion of the combined cycle system is easy to install hismeans a short schedule of about 1 year from order to oeration $hile thesteam turbine ortion can oerate $ithin another year his can ro#ide thegrid $ith o$er earlier than $ith other systems

he combined cycle o$er generation system also o4ers ;uic& art-load

starting ?or e.amle, the !F 6odel-"000 gas turbine is able to roducema.imum outut of 1G<6W $ithin 30 minutes $hile the steam turbineortion ta&es about an hour to oerate from a cold start t can oerate o#er a$ide range of loads and is suitable for meeting ea& o$er re;uirement andalso base load Eulementary /ring can be used to increase steam turbineoutut in times of increased outut demand Hn the other hand, the gasturbine can be stoed $hen there is a decrease in demand

t is also cheaer to build a combined cycle o$er lant than coal, nuclear orrene$able energy o$er lant ts caital cost is lo$er than that of steamturbine lant

'i&advanta%e&

he combined cycle o$er generation system is not $ithout itsdisad#antages

s natural gas is the fuel used, its higher cost comared to coal and oil $illresult in higher oerating cost he system is also less :e.ible $ith regards to




the tyes of fuel to be used, thus this limit in resources means the suly of fuel is critical to the lantIs oeration

lso, the combined cycle o$er generation system is a combination of t$otechnologies, the comle.ity $ill result in higher maintenance cost and also it$ill re;uire highly s&illed and better trained oerating sta4s

he caital cost of a combined cycle o$er generation lant is higher thanthat of gas turbine lant

(uture

here are many o$er generation systems a#ailable in the $orld lthoughnuclear o$er lants are clean and ecient, they are e.ensi#e andcomle. hey are also less easily acceted by the oulation due to thee.erience of the hree 6ile sland and (hernobyl incidents Hn the otherhand, combined cycle o$er lants o4er :e.ibility in terms of sie and

oututs, and are faster to build than nuclear o$er lants he ability toincrease and decrease outut on demand gi#es combined cycle o$er lantsan uer hand in the cometiti#e o$er mar&ets

he FarthIs suly of natural gas has been estimated to last "0-100 years,and as long as natural gas rice remains lo$, combined cycle o$ergeneration $ill ha#e the cometiti#e edge against other forms of generationlso, $ith ad#ancement in technology, the caital cost of combined cyclegeneration has droed from JEK700B&W in 1GG0 to less than JEK350B&Wtoday

Etudies in the Jnited ingdom ha#e also redicted that by 2050, 90 - 50% of

JIs o$er suly $ill be dominated by combined cycle o$er lants

?rom these, $ith continual imro#ement to the system, $e can see thatcombined cycle o$er generation $ill become the mainstay in the o$erindustry for at least the ne.t fe$ decades

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