T he drivers may be different, but thedestination—higher efficiency—isthe same worldwide. As a primary

component of current efforts to reduce theenvironmental impact of burning low-costcoal, new and more-efficient steam plantdesigns are once again being considered bythe U.S. generation industry.

Even though current market conditionsin the U.S. tend to favor diversification oftechnologies and operating capabilities, thelowest-cost generating units will still befirst in line for dispatching. The present andexpected makeup of regional generatingfleets in the U.S. generally indicate that anymodern supercritical, coal-fired unit willhave a significant fuel cost advantage andcould be dispatched at costs approachingthose of current nuclear plants.

Although seasonal and daily load reduc-tions could be plausible in the long term,much of any new supercritical coal-firedcapacity will not be frequently shut downor continually load-cycled. This is onemajor difference between the market condi-tions and practices of the U.S. and Europe,and a main reason why it should not beassumed that the pressure-control mode andtechnology prevalent in Europe should beembodied in the bulk of new unit construc-tion in the U.S.

To advance plant efficiencies to 40%(HHV) and beyond, supercritical steamconditions (higher than 3,208 psia) areemployed. Operation at these pressures,where there is no phase distinction betweenliquid and vapor, requires unique steamgenerator design features, most notably infurnace circuitry and components. Withinthis category of steam generators, thedesign is also very much influenced by theintended operating mode: constant pressureor sliding pressure (see box).

STEAM GENERATOR DESIGN

Constant and sliding-pressureoptions for new supercritical plantsSliding-pressure, supercritical plants are all the rage. They generally include

certain design features developed for markets and operating environ-ments outside the U.S., where new coal-fired plants have been built inrecent decades. U.S. market conditions are different, and considerablecapital cost savings—with negligible operating cost differences—arepossible if technology options are considered for the next wave of super-critical and ultra-supercritical steam plants.

By Brian P. Vitalis, Riley Power Inc., a subsidiary of Babcock Power Inc.

2 www.powermag.platts.com POWER | January/February 2006

Constant pressure implies stable pressure of the steam generator and main steam line overthe unit’s load range. Meanwhile, the basic nature of a simple, rotating turbine is to requireless pressure as load and flow rate are reduced, and if the main steam pressure is limitedto only that required for each load, this mode is referred to as pure sliding pressure.

However, when we speak generally of “sliding pressure,” we often mean modified slidingpressure, as shown in Figure 1. This mode has a limited amount of pressure throttling toprovide a modest amount of fast-response load reserve. A unit under constant pressure willhave significant load reserve at any reduced load, due to its significant pressure throttlingor the availability of admission valve(s). By opening the throttle valve or an admissionvalve, the pressure in the turbine and steam generator move toward equalization. The sud-den reduction of pressure in the steam generator prompts an instantaneous expulsion ofsteam mass due to the increase in a specific volume of steam within the confines of thesystem, and it provides a temporary load increase even before the fuel-handling and -firingsystem can be loaded to support any sustained higher load.

Pure sliding-pressure operation does not offer this kind of load or frequency responseand is therefore generally not practiced. Note that for a typical 3,800-psia steam pressurerating, a (modified) sliding-pressure steam generator operates at subcritical pressures atall loads below about 73% maximum continuous rating (MCR).

Steam pressure vs. load

100

80

60

40

20

0

Subcritical

Constant pressure

Modified slid

ing pressure

Pure sliding pressure

Load (% of MCR)

Pres

sure

(% o

f ful

l rat

ing)

0 10 20 30 40 50 60 70 80 90 100

Note: MCR = maximum continuous rating

1. Steam generator operating modes

Source: Riley Power Inc.

Beyond the apparent differences incomponent and construction design fea-tures, the choice of mode has broaderimplications, for example, on overall fur-nace sizing differences and materialsoptions. These less-discussed differencescan have a noticeable impact on cost andcan become even more significant as steamconditions are gradually advanced towardultra-supercritical conditions in pursuit ofgreater efficiency and reduced emissions.Plant designers should factor these steamgenerator design implications into theirstrategic planning and their developmentof specifications for new plants to arrive atthe most cost-effective generation portfoliofor particular U.S. and regional marketenvironments.

Design for sliding pressureMarket conditions in Europe and Japan—including shutdowns and rapid and continualload ramping of supercritical coal-firedplants—foster priorities and operatingpractices different from those in the U.S. Inpart, these conditions have justified thedevelopment and expense of sliding-pressuredesigns overseas. For instance, to handlerapid and continual load ramping (which is ofparticular value due to high local fuel costs),turbine temperature transients are minimizedby operating in sliding-pressure mode. Thisrequires certain drastic adaptations of thesteam generator design, which—for currentsteam conditions—are apparently worth theinvestment given European and Japanesemarket realities (except that the implied lowcapacity factor means a longer paybackperiod for the higher capital investment).

In sliding-pressure operation, becausethe steam generator operates under bothsupercritical and subcritical conditions asload is var ied , the furnace must bedesigned to accommodate both single- andtwo-phase fluid flow. Because the twopressure regimes and the wide variation influid specific volume make continualforced recirculation rather impractical, it isappropriate to use a once-through design,in which flow rate through the furnace isdirectly proportional to load. Steam flowrate and velocity through the furnace tubesare critical for cooling the tubes, and withflow proportional to load, low-load opera-tion presents a challenge to proper furnacetube cooling.

Further, in sliding-pressure mode at lowload, the fluid is subcritical, posing specificchallenges to heat transfer and tube cool-ing. Both departure from nucleate boilingand steam dry-out carry the potential forelevated tube metal temperatures. Theseconditions are mitigated or avoided, in part,

by providing sufficient steam mass flowdensity at subcritical, once-through, lowloads. Designing for proper steam coolingeffect at low loads produces very highsteam mass flow density and pressure dropat full load in a once-through design.Therefore, specifying minimum once-through load should be done with carefulconsideration of its consequences at fullload. Below the minimum design once-through flow rate, recirculation pumps areusually used to protect the furnace.

Sufficiently high steam mass flow densi-ty at once-through loads is provided by useof a small flow area. Because the furnaceperimeter has certain minimum limitationsdue to conventional firing configurationsand slag control, the challenge of providinga small flow area to envelop a relativelylarge furnace enclosure requires specialplumbing arrangements. But because slid-ing pressure operation involves two-phasefluid over most of the load range, multiplefurnace passes with up-down-up flow direc-tion become difficult to manage, making asingle upward flow progression preferable.

The upward flow progression in a singlepass is achieved with fewer tubes by layingthe wall tubes down at a low inclination

angle rather than hanging the tubes verti-cally. A given transverse dimension of afurnace wall normally covered by nine ver-tical tubes and membrane fins can bespanned by only three inclined tubes of thesame tube and membrane size (Figure 2).Although the furnace cross-section remainsrectangular, this inclined tube arrangementis often called a “spiral” design due to theoverall progression of each tube upwardand around the furnace. The tube inclina-tion angle is typically 10 to 20 degreesfrom horizontal, so the tube length is threeto five times greater than the vertical dis-tance gained.

Special internally rifled tubing couldallow a lower steam mass flow density andthe use of vertical tubes, but the range ofoperating conditions under sliding-pressureoperation would make such a systemdesign quite challenging.

Figure 3 is an example of a sliding-pressure unit designed for Powder RiverBasin (PRB) coal, with a spiral arrange-ment in the high heat-flux zone of thelower furnace. Although much experiencehas been gained and many lessons learnedfrom such a furnace wal l des ign, i tremains a complicated structure to design,

STEAM GENERATOR DESIGN

January/February 2006 | POWER 3

Furnace wall dimension

2. Spiral arrangement. The furnace circuit flow area and the tube count can bereduced by inclining the wall tubing at a low angle. Source: Riley Power Inc.

Forging capMembrane wall

HeaderLoad transfer

weld pads Supportstraps

Supportstraps 50

725 MWg3,800 psia/

1,055/1,051FPRB coal

3. Sliding-pressure, once-through furnace construction. The lower wallswith inclined tubing are supported by external support straps. Source: Riley Power Inc.

fabricate, erect, and maintain. Once thetubes rise into a sufficiently low heat-fluxzone, the expensive arrangement is termi-nated and a transition is made to verticaltubes in the upper furnace. The transitionis commonly accomplished by a ring offorgings around the perimeter of the fur-nace and an external ring mixing header.The walls composed of inclined tubes arenot self-supporting, so an “exoskeleton”support system is used, consisting of verti-cal support straps and load transfer bymany welded lugs over the wall surfaces.

Constant pressureTwo-phase heat transfer crises are notencountered in furnaces maintained atsupercritical pressure, so constant-pressureoperation allows greater flexibility and theuse of a conventional design. By employingfurnace recirculation smoothly over theentire operating range, low load does notdictate furnace design. As a result, a fur-nace can be designed with:

■ Vertical, self-supporting, smooth-bore tubes.■ A single upward pass with the same simple

construction as a conventional drum unit.■ No intermediate mixing or external piping.

Figure 4 shows a 400-MW Riley Powerrecirculating supercritical unit with thesefeatures. It has powered South CarolinaElectric & Gas Co.’s Wateree Station Units1 and 2 since 1970.

Beyond plumbingIn addition to incorporating these con-structional differences, a sliding-pressurefurnace (evaporator system) must be sizedto yield a greater outlet enthalpy (energycontent of steam), so it requires a greaterheat duty and furnace size.

To illustrate this, Figure 5 compares thesteam generator operating conditions andtrends on an enthalpy-pressure steam dia-gram. This steam property diagram is usedto trace the rising heat content (enthalpy) ofthe steam as it flows and loses pressurethrough the boiler (the series of circled datamarkers and dashed lines at right).

Sliding-pressure operation during loadreductions moves the furnace operation intothe subcritical, two-phase region at loadsbelow 70% to 75% MCR. The nearly hori-zontal dashed lines in Figure 5 indicate thetrend of furnace inlet and outlet conditionsover the load range. To accommodate thetwo-phase boiling condition of steam, thereare specific steamside conditions that mustbe fulfilled at the minimum once-throughload, and so it is sometimes low load—rather than full load—that determines the

heat duty and size of the furnace or evapo-rator system. Those conditions are:

■ The economizer size is limited to pre-vent steaming within it.

■ The furnace size must be sufficient toproduce dry steam in once-through modeto prevent introduction of liquid waterinto superheaters.

These requirements are indicated inFigure 5 at the 35% of MCR load condi-

tion. A furnace sized for a certain mini-mum once-through load produces theindicated conditions at full load, includ-ing the total heating duty (the arrow onthe far r ight) and the furnace outletenthalpy and temperature. Accordingly,the selection of minimum once-throughload has consequences not only on thesteam flow area and the full-load pressuredrop; it also drives the overall furnacesize and operating steam and metal tem-peratures. It is interesting to note that the

STEAM GENERATOR DESIGN

4 POWER | January/February 2006

Detail A

A

4. Constant-pressure, recirculating unit. This design features vertical, self-supporting,smooth-bore furnace tubing in a single upward pass. Source: Riley Power Inc.

Notes: MCR = maximum continuous rating; SH = superheater

35%MCR

100%MCR

Slid

ing

Furnace

EconomizerEnth

alpy

(Btu

/lb)

Pressure (psia)

Two-phase(boiling)region

Vapor

Liquid

SH1SH2

SH3

0 1,000 2,000 3,000 4,000 5,000

1,500

1,000

500

0

5. Enthalpy-pressure steam diagram. In sliding-pressure operation, the furnacemust absorb proportionately as much energy as a typical, 1,500-psia industrial boiler. Source:Riley Power Inc.

sliding-pressure furnace is essentiallysized as one would size the evaporatorsystem for a 1,500-psia industrial unit.Often, these medium-pressure industrialunits employ a boiler bank or convectiveevaporator section to supplement theboiling heat duty while limiting the fur-nace size.

In contrast, constant-pressure units stay inthe supercritical, single-phase region andtherefore have no such waterside sizingcriterion. Figure 6 shows in blue theoperating conditions of the constant-pressure, Riley Power recirculating unit overthe same load range. The usual gas-sidefurnace sizing criteria that apply to anyoperating pressure unit—such as firingarrangement requirements, residence time

and burnout, emissions considerations, andexit gas temperature limits for slagging andfouling control—will dictate. Depending onthe particular fuel and fireside conditions,the constant-pressure furnace could be sizedas indicated (the large blue arrow). Notethat, although the sliding-pressure furnacemust be sized like an industrial boiler, theconstant-pressure furnace can be sized asone would a high-pressure subcritical,natural-circulation unit (Figure 7).

But unlike natural-circulation units, thesupercritical unit remains flexible in itsperformance, because it does not have afixed evaporator (furnace) end point. Evap-orative and superheat duty can be shiftedbetween furnace and convective surfacesin response to changes in fuel, slagging, or

other conditions. This feature is not limitedto Benson, Sulzer, or other once-throughdesigns, and the constant-pressure designretains this flexibility at all loads. By com-parison, a sliding-pressure unit has lessflexibility as pressure is reduced and themargin above saturation (two-phase boil-ing) decreases.

Nearly as important as this size differ-ence, the furnace outlet temperature of theconstant-pressure unit can be significantlyless than that from the sliding-pressure unit(due to this enthalpy difference). Further-more, the thermodynamics of steam aresuch that, at the greater outlet enthalpylevel required for the sliding-pressure unit,temperature is much more sensitive to dif-ferences in enthalpy between furnacetubes. This increased sensitivity is partlymitigated by the heat absorption equalizingeffect of the spiral tube arrangementaround the sliding-pressure furnace.

These are especially important points forextension to ultra-supercritical conditions,where it is found that sliding-pressuredesigns will have very high furnace outlettemperatures (approaching 1,000F to1,100F) and may require advanced alloysfor the furnace walls. The various materialsresearch efforts being conducted worldwidefor ultra-supercritical plants are strugglingwith this issue, partly due to the exclusiveassumption of sliding pressure. Though thefurnace outlet temperature with constantpressure also continues to rise, the potentialreduction compared to sliding pressurebecomes greater—and furnace materialsooptions are comparatively broader—as thefinal steam conditions are advanced.

A visible differenceA constant-pressure furnace designedaccording to the universal gas-side criteriaresults in a furnace outlet steam enthalpy ofabout 1,050 Btu/lb (at 760F). The equiva-lent sliding-pressure furnace is about 20%larger in order to yield the required outletenthalpy of 1,150 Btu/lb (at 790 to 800F).Because the larger furnace is effectivelyaccomplishing some of the superheat dutyat higher loads, the radiant superheater canbe reduced accordingly, but the net costincrease is positive. Additionally, a particu-lar advantage of the Riley Power recirculat-ing supercritical design is that it does notrequire intermediate furnace mixing. Thatnot only reduces associated piping costs butalso permits the use of a close-coupledbackpass and eliminates the tunnel sectionthat would otherwise be required.

The primary differences in furnace con-struction and size are estimated to result in4% to 5% greater overall boiler cost for

STEAM GENERATOR DESIGN

January/February 2006 | POWER 5

Slid

ing

Furnace

EconomizerEnth

alpy

(Btu

/lb)

Pressure (psia)

Two-phase(boiling)region

Vapor

Liquid

SH1SH2

SH3

0 1,000 2,000 3,000 4,000 5,000

1,500

1,000

500

0

Cons

tant

Notes: MCR = maximum continuous rating; SH = superheater

6. Constant- and sliding-pressure operating trends. The constant-pressurefurnace size is not driven by the significant heat of vaporization at lower pressures. Source:Riley Power Inc.

Slid

ing

Furnace

EconomizerEnth

alpy

(Btu

/lb)

Pressure (psia)

Two-phase(boiling)region

Vapor

Liquid

SH1SH2

SH3

0 1,000 2,000 3,000 4,000 5,000

1,500

1,000

500

0

Cons

tantTypical utility

subcritical2,600 psia

Notes: MCR = maximum continuous rating; SH = superheater

7. Relative furnace heating duty. Although the sliding-pressure furnace must besized like an industrial boiler, the constant-pressure furnace can be sized as one would expectfor a high-pressure subcritical, natural-circulation unit. Source: Riley Power Inc.

sliding-pressure designs. For a 650-MWunit, this differential amounts to about $6million to $7 million, including materialsand erection. This cost differential is due toonly the tube circuitry, intimate support,erection, and overall furnace size differ-ences. It does not include further potentialdifferences in tube materials; tunnel passelimination; cycling design requirements;and steel, building, or foundation differ-ences—all of which lead to even greatercosts for a typical sliding-pressure design.

Is it worth it?Can the additional capital investment in asliding-pressure plant be recovered byoperating cost advantages in the U.S. mar-ket? With uncertainty about long-rangeload dispatching, the efficiency of newplants at low loads becomes important forconsidering a plant’s payback of capitaland, indeed, for dispatch competition.Many people have been under the impres-sion that sliding-pressure units offer betterefficiency (lower heat rate) than constant-pressure units at reduced loads. The extentto which this is true depends greatly on theturbine control mode, and so a closerreview of heat rate differentials is in order.

Though old, throttle-control turbines atconstant pressure indeed suffer in efficien-cy at part loads, comparative data from tur-bine manufacturers indicate that modern,nozzle-control turbines at constant pressurehave nearly the same efficiency as at slid-ing pressure across the load range. This ismainly due to the sequential use of the tur-bine admission valves, and at several loads(the “valve best points”) the remainingvalves are fully open and there is negligiblethrottling loss before the first turbine stage.

Using differential heat rate data from tur-bine manufacturers, heat rates were evaluat-ed for both constant- and sliding-pressuresystems, with both throttle and nozzle con-trol. Plant operating costs were evaluated atall loads for each turbine control modeusing a detailed economic model includingfuel, reagent, and emissions costs accordingto typical U.S. conditions.

Even assuming a nightly load reductionto 35% to 80% every night over an entire20-year evaluation period, the presentvalue of the difference in operating costs iscalculated to be only $0.5 million for PRBcoal firing and less than $1 million forhigh-sulfur bituminous coal firing of amodern 650-MW unit with nozzle control.

As Figure 8 makes clear, the present valueof 20 years of operating cost savings is notnearly enough to justify the additional $6million to $7 million capital investmentrequired for the sliding-pressure steamgenerator. Meanwhile, the sliding-pressureturbine cost savings are reportedly estimat-ed to be on the order of $0.5 million andwould be partly offset by any additionalfeedwater heater and steam generator coststo handle sliding pressure and any associ-ated load and pressure cycling.

For cycling service?For completeness, it should be recognizedthat continual load cycling and fast start-upabilities may be of particular value for a lim-ited number of units in each region of theU.S., though the value is relatively difficultto quantify. Sliding pressure may be justifiedand viable where such features are especiallyvalued, but development of these abilitieswith constant-pressure systems should not beoverlooked. Nevertheless, it is widelybelieved that any continual load cycling ofnew coal units, beyond controlled nightlyreductions, will be for a relatively small pro-portion, to be strategically determined foreach grid region. The significant operating

STEAM GENERATOR DESIGN

6 POWER | January/February 2006

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cost advantage of new supercritical units willgive these units preference for load dispatch.

In addition, America’s installed naturalgas–fired capacity—now almost 200 GW—represents a sizeable sunk investment in gen-

eration that is well suited for peaking duty.Though it is expensive to operate, this capac-ity is available to meet peak loads and is rela-tively easy to start up and shut down. Thiscreates a different environment from that of

the 1970s, when such peaking capacity wasnot available and utilities were caught notbeing able to easily cycle their baseloadedunits when a recession hit. Independentpower producers considering new coal-firedunits should recognize that—armed witheconomically efficient generation fired bycoal rather than by natural gas—their role incontributing to the regional grid load andtheir priority on the dispatch curve will beentirely different, moving from the peakingrole into the baseload and average-load roles.

Regarding start-up, it should be noted thatnot all of the start-up systems and featuresemployed on modern generating units aroundthe world are inherently or exclusively appli-cable to sliding-pressure operation, and theexpense of once-through sliding-pressuresteam generators need not be assumed to gainsuch features. The Riley Power recirculatingunits in operation since 1970 already provethe successful application of recirculation tofacilitate start-up of a constant-pressuresupercritical unit. For the future generation ofcoal-fired plants in the U.S., other modernstart-up features can be developed and inte-grated with appropriate plant designs for therange of expected domestic needs, for bothconstant- and sliding-pressure applications. ■

STEAM GENERATOR DESIGN

January/February 2006 | POWER 7

Notes: Two-shift operation: day 100% load; night load as indicated

Turbine control method and night load

Mill

ions

($)

Nozzle

35% night load Random night load

Powder River Basin $1.00 Powder River Basin $1.40 Bituminous $1.8010

9

8

7

6

5

4

3

2

1

0

Throttle

35% night load Random night load

0.2 0.2 0.3 0.4 0.6 0.9

4.7

6.3

9.3

3.6

4.8

7.2

8. Investment payback. The chart shows simple 20-year present value of operatingcost savings with sliding pressure on a 650-MW unit. Additional cost for a sliding-pressuresteam generator is estimated as $6 million to $7 million. Source: Riley Power Inc.

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