Lecture13-CombinedCycle with MP Steam

8/9/2019 Lecture13-CombinedCycle with MP Steam

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Page 1of 5Lecture 13

Lecture 13: Combined Cycle

with Multi-pressure SteamCombined Cycle with Multi-pressure Steam

A combined cycle with multi-pressure steam reduces the temperature of thegas leaving the heat-recovery boiler and hence results in increased eciencyof the plant as a whole. With steam cycles operating around 13 psia !"bar#$ saturation temperatures around %&%o' !3o(#$ and feedwater into the)*+ at about ,%o' !13o(#$ the gas temperature leaving the )*+ to thestac is still at about 3 to /o' !1% to ,o(#. 0ome of the energy leavingwith that gas can be utilied in a multi-pressure steam cycle. 2he simplest

such cycle is a dual-pressure one$ although triple-pressure cycles have beenconsidered.

A dual-pressure cycle !'ig. 13.1#shows a heat-recovery boiler with twosteam circuits in it. ne$ a high-pressure circuit$ feeds steam to thesteam turbine at its inlet4 the other$ alow-pressure circuit$ feeds the steamto the same turbine at a lower-pressure stage. A correspondingtemperature-enthalpy diagram of

both gas and steam circuits in the)*+ is shown in 'ig 13.,.

56haust gas leaving the gas turbineenters supplementary 7ring !0'# and/ and the heat-recovery boiler !)*+#at %$ leaving it to the stac !02# at .(ondensate leaves the steamcondenser !(# at 8 and enters thecondensate pump !(P# and twoclosed-type feedwater heater !'W)#and one open-type deaerating heater

!9A#. :t then enters the boiler feedpump !+'P# at "$ where it is pumpedto 1 to a lower pressure than that ofsteam ma6imum. Process 1-11 isfeedwater heating in a low-pressureeconomier$ followed by evaporationto 1, and superheat to 13.


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0uperheated low-pressure steam at 13 enters the steam turbine at a low-pressure stage.

Water from low pressure steam drum at 11 is pumped by a booster pump!+P# to 1/ and goes to the high-pressure economier. 5vaporation occursfrom 1% to 1 and superheat to 1&.

)igh pressure superheated steam at1& enters the steam turbine 7rststage.

:t can be seen from 2-) diagram !'ig 13.,# that low-pressure steam boils at a temperature !1,# below thatof high-pressure steam !1#$ and hence there are twopinch points between the gas line and the saturatedsteam lines. :t can also be seen that a single high-pressure steam circuit could be represented by 1;-1%-1-1& with gas leaving to the stac at ;. Adding thelow-pressure circuit allows the gas to leave at a lowertemperature !#$ thus e6tracting more energy from itand increasing the overall cycle eciency.

An e6ample of a dual-pressure combined cycle is the 9onge- in )olland. :t has a gas-turbine output of &.& >W and asteam-turbine output of /&.3 >W and attains a remarable /.1 percenteciency at a standard air condition of 1%o( !%"o'# and 1 atm$ with only one9A feedwater heater.

A proposed triple-pressure combined cycle generates steam at anintermediate pressure between the two steam-turbine inlets. 2his steam is

in?ected into the gas-turbine combustion chamber to reduce nitrogen o6ideemissions to meet =6rigid standards$ if applicable. 2he water$ thus lost$must of course be continually made up.

A Combined Cycle for uclear !owerplants

A combined cycle for nuclear plant presupposes that a high-temperature gas-cooled nuclear reactor !)2


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)elium at 1 !'ig 13.3 !a# and !b## is compressed in a helium compressor !)e(#to ,. :t is then preheated in a regenerator !*# to 3$ enters the )2


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Page "of 5Lecture 13

#nte$rated %asi&cation Combined-Cycle !ower !lants

'or decades vapor power plants fueled by coal have been the worhorses of.0. electricity generation. )owever$ human-health and environmental-impact issues lined to coal combustion have placed this type of powergeneration under a cloud. :n light of large coal reserves and the criticalimportance of electricity to our society$ ma?or governmental and private-sector eBorts are aimed at developing alternative power generationtechnologies using coal$ but with fewer adverse eBects. :n this section$ weconsider one such technologyC integrated gasi7cation combined-cycle !:


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2aing a closer loo at 'ig. 13./$ better :

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Lecture13-CombinedCycle with MP Steam