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The Rankine Cycle:Workhorse of the Coal-fired Utility Industry

4 Burns & M cDonnell

By Steve Voss, P.E., and

Greg Gould, P.E.The steam cycle in a power plant is

haracterized by the maximum oper-

ating p ressure of the cycle. The typ ical

team turbine power p lant operates

on the Rankine cycle, the workhorse

of the coal-fired u tility ind ustry.

Subcritical Rankine CycleThe main feature of the Rankine cycle

s the compression or p um ping that

occurs w hen th e w orking fluid, w ater,s in the liquid p hase. The amou nt of

nergy ava ilable for extraction by the

working fluid is depend ent on the

operating temperature and p ressure of

he fluid. Raising the steam pressure

or steam temperature improves effi-

iency.

Why is it desirable to raise pressures

and temperatures? Figure 1 rep resents

he Rankine cycle. The upper line rep-

esents the steam temp erature and

pressure generated by the boiler. Theower line represents the condensing

portion of the steam cycle. The d iffer-

nce between the upp er and lower

ines determines how mu ch energy

an be extracted by the steam turbine,

and thu s the efficiency of the cycle.

The condenser op erates at a tempera-

ure and pressure dictated by external

ond itions such as the tem perature of

he atmosph ere or the cooling w ater

emp eratu re. It is not feasible to sub-

tantially lower th is line. The only

way to im prove the cycle efficiency is

by pu shing the up per line higher.

In a typical coal-fired steam cycle

pow er plant the operating pressure is

2400 psi. Steam temp eratu re is typ ical-ly 1000 or 1050 deg rees F. Steam tem-

perature is limited by available mate-

rials that can survive at elevated tem-

peratures. Most larger units have a

reheat cycle (shown as line from p oint

2 to 3 in figure 1), where the steam is

prod uced in the boiler, passes through

a portion of the turbine, is "reheated"

in the boiler and then goes through

the remainder of the turbine. This

increases the efficiency of the cycle

without increasing the m aximu m

steam temperatures.

The operating pressure of convention-

al coal-fired power plan ts can be clas-

sified as subcritical or supercritical.

The critical point is where the temper-

ature and pressure are such that the

fluid is no longer classified exclusive-

ly as liqu id or ga s. It is thought of as a

fluid above th e critical poin t. The criti-

cal point for water is slightly above

3200 psi. Figure 1 shows a typical 2400

psi su bcritical Rankine cycle with sin-gle reheat. The critical point is show n

slightly above the cycle show n. Figure

2 shows a similar single reheat cycle,

but opera ting at 3500 psi, or in the

sup ercritical range. Increased efficien-cy is represented by the increase in

area und er the curve, approximately

as shown in Figure 3.

Increasing the steam pressure

improves cycle efficiency. It also pro-

vides the opportu nity to go to a "dou -

ble reheat" cycle, which allows even

more imp rovemen t in overall efficien-

cy. The overall net efficiency for a typ-

ical su bcritical coal-fired un it is abou t

10,000 Btu/ kWh. Increasing the initial

steam pressu re to 3500 psi from 2400

psi improves the heat rate by about

1.5%. The efficiency of a unit with

3500 psi initial steam p ressure and

double reheat is about 4% better than

a typ ical subcritical un it. For a 600

MW unit bu rning $1.20 per m illion

Btu fuel with an 80% annu al capacity

factor, this represents an ann ual cost

savings of abou t $2 million.

Existing subcritical units in the Un ited

States typically have a steam drumwhere the working fluid circulates

through the water w alls either by heat

transfer and gravity in the case of nat-

Figure 1Temperature Entropy Diagram

Subcrit ical Rankine Cycle

Rankine Cycle Legend

1-2 HP Turbine Expansion2-3 Reheat3-4 IP/ LP Turbine Expansion4-5 Condenser5-6 FeedwaterHeating/ Pumping6-1 Boiler

Critical Point


2/35TechBriefs

ural circulation, or with the addition

of pump s in the case of forced circula-

tion.

Supercritical Benson CycleSupercritical un its use a once-through

design, also referred to as the Benson

cycle. In a once-through boiler the

fluid p asses through th e unit one

time, and there is no recirculation as

takes place in the water walls of a typ-

ical d rum -type boiler. Since there is no

thick-walled steam d rum , the startup

time and ram p rates for a once-

through unit can be significantlyreduced from that required for a

drum -type u nit.

Why Arent There More

Supercritical Units?So if supercritical units are more effi-

cient and have better startup and

ram p r ate characteristics, why isnt

supercritical the right answer for any

new coal-fired unit?

First, there is history. Most coal-firedplants in the United States are subcrit-

ical. The first commercial pow er p lant

using a sup ercritical steam cycle was

placed into service in 1957. By the

mid-1960s, about half of all U.S. units

being ordered were supercritical. The

purchase of supercritical units in the

United States dropped off dramatical-

ly in the 1970s, primarily because of

the onset of base-loaded nuclear

pow er stations. Plants designed to

burn fossil fuels during this time peri-od were built to follow load, and the

subcritical cycle w as selected because

experience with cycling sup ercritical

un its (wh ich w ere all originally

designed for base load operation) was

minim al. Also, supercritical un its that

had been built in the United States up

to that point suffered from a variety of

problems.

Second, typical U.S. supercritical units

suffered more from the rapid increase

in un it size than from technology.Most of the sup ercritical un its in the

United States were designed for coal

firing. More than half had pressurized

furnaces and one-quarter of the super-

critical units were equipped with d ou-

ble reheat sections. During develop-

ment of the supercritical unit in the

1960s, the average fossil unit grew in

size from 247 MW to 500 MW.

While the U.S. generally quit bu ild ing

large coal-fired units in the 1980s, they

continued to be constructed inEurope, Japan, an d elsewhere aroun d

the w orld. There have been consider-

able advances in d esign an d operation

of supercritical units. Units now have

improved bypass systems, which sim-

plify startup. N ew u nits are also

designed to op erate with sliding p res-

sure, which improves load change

characteristics. Many of th e "supercrit-

ical-related" problems w ith the ear ly

sup ercritical un its have been resolved

with new designs.


Supercritical Rankine Cycle

Third, since the once-through desig

does not have a p lace for blowdow

from the system, the w ater enterinthe boiler must be of a much better

quality than in dru m-type un its. A

condensate polishing system and c

er attention to system water qu alit

are both necessary to su ccessfully

operate a supercritical unit.

Supercritical un its are also more su

ceptible to water indu ction than

drum-type units.

Fourth , controllability of a once-

through unit is tougher than a druunit. Once-through design require

faster responding controls and ada

tive tuning over the entire load ran

This is much easier to accomp lish

with todays Distributed Control

Systems (DCSs) than with the old

crete comp onent electronic control

systems. The need for better contro

systems was known back in the 19

but the advancement of Direct Dig

Control (DDC) proved unsuccessfu

Rankine Cycle Legend

1-2 HP Turbine Expansion2-3 Reheat3-4 IP/ LP Turbine Expansion4-5 Condenser

5-6 FeedwaterHeating/ Pumping6-1 Boiler

Critical Point


3/36 Burns & M cDonnell

Steve Voss is manager of theplant services group in Burns &

McDonnells Energy Group. He hasa bachelors degree in mechanical

engineering from the University ofMissouri-Rolla. He has worked for

Burns & McDonnell since 1990.

Greg Gould is an associate vicepresident and manager of engi-

neering in Burns & McDonnellsEnergy Group. He has a bache-

lors degree in mechanical engi-neering from the University of

Nebraska. He has worked forBurns & McDonnell since 1975.

because it required the use of redun -

dant m ainframe compu ters, wh ich d id

not provide the reliable control

equired for pow er plants.

Lastly, there is th e high er capital cost.

The added capital cost of a supercriti-

al unit over a d rum -type unit ranges

rom no change to 3-5% depending on

he source of the information.

Where Is All This Headed?The last big hurdle is overcoming a

echnology that is decades old. The

atest developments are aimed at even

higher thermal efficiencies: 4500 psi

with tem peratu res of 1500 F results inas mu ch as 20% better therm al effi-

iency than conventional dru m-type

un its. The limiting factors are the

materials of constru ction th at can

withstand extreme conditions and

wh at advances in metallurgy and

eramics can solve the p roblems.

The construction of coal-fired base-

oad pow er has been all but non -exis-

ent in the United States for the last 20

years. A few projects have been com-

pleted here and there, but the m ajorityof the technological advances have


Super vs. Subcritical Rankine Cycle

taken p lace overseas, wh ere the mar-

ket for coal-fired boilers ha s been bet-

ter. Since 1997, over 22,000 MW of

coal-fired generation has been built in

Europe. In that same time, less than10% of that number has been built in

the Un ited States. An interesting trend

to note is that over 80% of the over-

seas un its are sup ercritical. There are

approximately 360 sup ercritical un its

worldwide.

What About Unit Availability?The ava ilability of sup ercritical units

bu ilt since 1990 is every bit as high as

the subcritical units. The early super-

critical unit pop ulation in th e U.S. hasa d ark cloud that followed it around

due to availability problems. Some of

the problems can be attributed to the

sup ercritical cycle, but just a s man y

can be attributed to the fact that the

supercritical units are on the average

newer units that w ere built to tighter

emission control standard s and have

had more control equipment. As any

statistician or maintenance person can

confirm, a system w ith more m oving

parts has a higher potential for failure

than a simple system.

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