Chapter-2 History of Supercritical Boilers

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1.2 HISTORY OF SUPERCRITICAL BOILERS

Once through boilers has long been the vision of boiler design engineers. In

the United States, patents for once through boiler concepts date from as early

as 1824. hese early inventors !ere undoubtedly motivated by the desire to

improve the product safety because of the notoriety of pressure vessel

failures associated !ith the early fire tube and !ater tube boilers. "hile

advances in the boiler industry in the late 18##s such as the developments by

he $abcoc% & "ilco' (ompany )founded in 18*+ significantly improved

product safety, interest continued in the development of once through boilers

both as a !ay to eliminate the need for the steam drum and !ith the hope

that the design !ould better cope !ith impurities contained in the !ater.$&"-s research in once through boilers dates from 11* !hen boiler

research !as begun at the company-s $ayonne, /e! 0ersey aboratory. In

%eeping !ith the technology of the time, this early research unit !as operated

at a pressure of 4 3a. he first significant commercial application of once

through boilers !as made by ar% $enson, a (echoslova%ian inventor,

!hen he in 125 provided 46ton7hr unit for nglish lectric (o., td. at 9ugby,

ngland. his unit !as designed to operate at critical pressure !ith the belief

that operating at this pressure,

"here there is not density difference bet!een steam and !ater, !ould avoid

boiler tube overheating and solids deposition.

ar% $enson continued his development !or%, !hich included the installation

in 15# of a 1156ton7hr unit in $elgium. i%e the unit for nglish lectric, this

unit !as intended to operate at critical pressure. Operating at critical

pressure, ho!ever, did not fulfill the hoped elimination of problems by

operating at critical pressure, fulfilled and it !as necessary to reduce the

boiler operating pressure to overcome problems !ith tube failures. In this

case, the boiler inventor-s vision outreached the technology available at this

time for both tube materials and !ater chemistry control. /onetheless, these

early units !ere successful in operation and served as the foundation for the

boiler development !or% that set the direction for uropean boiler

development. Siemens ultimately ac:uired ar% $enson-s concepts and it is

from these concepts that the $enson $oiler echnology no! licensed

!orld!ide by Siemens !as developed. $&" also continued !ith their

e'perimental !or% on once through boilers in the 12#s as boiler and po!er

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supercritical application. he vision of the supercritical po!er plant !as also

held by >merican lectric 3o!er and @enerallectric )for the steam turbine.

>merican lectric 3o!er entered into contract !ith both $&" and @eneral

lectric to build the !orld-s first ultra supercritical po!er plant. his 12; "

installation at the 3hilo 3lant operated at main steam condition of 51 3a and

*21( !ith t!o stages of reheating first to ;*;( and then to ;58(. he

decision to proceed !ith this plant !as made in 1;5 and operation !as

begun in 1;+. "hile the intent of the plant !as to demonstrate the feasibility

of the supercritical pressure cycle, this unit !as commercially operated until

1+. he boiler used $&"-s cyclone firing technology and !as e:uipped

!ith three cyclone furnaces. he boiler arrangement, sho!n in =igure 2.1, is

based on horiontal gas flo! over the maAority of the convection heat transfer

surfaces.

Figure#2.1 12$ %& Boiler

his arrangement is :uite different than the typical boiler arrangement of

today, but !as very similar to the natural circulation boiler arrangements of

the time, such as the # ", 1# 3a boiler sho!n in =igure 2.2. he furnace

tube arrangement !as :uite differentfrom that used in boilers of the $enson

technology design. he $enson technology boilers, because they !ere

designed for operationin the sub critical regime, made use of the meandering

tube arrangement !here the tubes passed completely around the furnace

enclosure as a means of obtaining more uniform heatabsorption from tube to

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tube. his construction !as necessaryfor the sub critical design as a means

to minimie the temperaturedifference bet!een tubes. he meandering tube

design usedthe refractory and s%in casing construction.Since the 3hilo unit

operated in the supercritical regime, the concern for differential heat

absorption and the resulting impacton tube temperature difference !as not

nearly as great.herefore, this unit !as designed !ith vertical tubes follo!ing

the construction techni:ues already employed in natural circulation boilers.

=ollo!ing the practice of United States boilerconstruction, the design made

use of partial membraning to minimie the amount of refractory and s%in

casing. In order toobtain the necessary mass flu' !ithin the tubes to provide

ade:uate heat transfer and tube cooling, a multiple pass arrangement !as

adopted. $ecause the unit !as intended only for operation in the supercritical

regime, this design strategy !as successful. he rapid development of the

$&" once through boiler paralleled the rapid e'pansion of the United States

Utility Industry in the 1*#s and 1+#s. he second and third boiler contracts

!ere also !ith >merican lectric 3o!er, but these 4;# " systems !ere

ordered before the 3hilo unit !ent into service. $oth boilers are to!er style

designs

Figure 2.2 '( %& Dru! Boiler) 1'$* +e,ign

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since it !as recognied that the horiontal style of the 3hilo design could not

lend itself to larger sies )See =igure 2.5.

Figure 2.- $( %& AEP /ree+ /oiler 0UP#2

he second contract, for the $reed Station !as cyclone fired !hile the third

contract, for the Sporn Station !as pulveried coal fired reflecting the

difference in coal characteristics. he steam conditions !ere reduced

some!hat from the 3hilo unit to 24 3a !ith main steam temperature of

;*;( and t!o stages of reheat to ;*;(. $y the fourth contract, a 2;# "

subcritical oil and gas fired units for Southern (alifornia dison, the design

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had evolved to the (arolina configuration used today for both once through

and drum type pulveried coal fired boilers )=igure2.4.

Figure2. 22$

%& Boiler

0ust 12 years after the 12; " unit began operation, the 11;# " unit for

ennessee Balley >uthority began operation and a fe! years later the first of

the 15## " series, also for ennessee Balley >uthority, began operation.

>s noted earlier, the ninth of the 15## " series began operation at the

Cimmer 3o!er 3lant in 1#. )=igure 2.;

$&" has supplied 8 supercritical boilers !ith an installed capacity of *#,###

" including 42,### " firing coal. his chart in many !ays reflects the

trend in the United States utility industry. ?uring the 1*#s there !as rapid

gro!th in po!er plant sie !ith most of the large units being the supercritical

cycle. >s economic conditions changed in the early and mid 1+#s the

industry reverted to favoring the subcritical steam cycle using the natural

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circulation drum boiler due to the lo!er installed cost of the subcritical plant.

"hile the supercritical cycle !as more efficient, this efficiency improvement

did not Austify the increased capital cost under the economic constraints

imposed upon the utilities. "ith the decline in the United States economy into

the

Figure 2.$ 1-(( %& i!!er Boiler

18#s the gro!th rate of electric po!er demand declined significantly and this

caused a further change in the utility buying pattern as they could no longer

Austify the larger po!er plants )*## " sie and greater. he trend to!ard

smaller sie installations further impaired the economics of the supercritical

steam cycle.



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Suercri"ic3l /oiler 4urn3ce +e,ign

=igure 2.* presents the boiler side elevation generated for the preliminary

boiler design based on the conventional spiral !ound furnace and the vertical

internally ribbed tube furnace. $oth furnace designs have an opposed !all

firing burner arrangement so as to avoid e'cessive furnace height and large

performance variations !ith varying mill combinations. he final superheat

and reheat heating surfaces are of proven pendent design, !hich resist slag

build6up. he second pass comprises typical convective surfaceD primary

reheat, primary superheat and economiser ban%s. he second pass has the

flue gas in do!n!ard flo! in a series gas path arrangement and re:uires flue

gas recirculation for reheat steam temperature control at part load conditions.

Figure 2.* *(( %& ,uercri"ic3l /oiler, "5o 3,, +e,ign

he layout of each furnace has been derived on the basis for lo! /O'

emissions, generous residence time for fuel burnout and to minimise the

accumulation of slagging7ash deposits. o ensure ade:uate residence time

for burnout and lo! volumetric heat release rate for lo! /O', a large volume

furnace design !ith e'panses of !ater6cooled !alls !as necessary.



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Sir3l 5oun+ 4urn3ce +e,ign

he spiral !ound furnace, based on $enson technology and itsui $abcoc%

supercritical boiler design, has been demonstrated in over 4# units,

amounting to over 14,###"e, on plants such as eri 3ori );;#"e in

=inland and Eem!eg )*;#"e in Eolland. >s sho!n in =igure 2.+ belo!,

the furnace !alls from the hopper inlet to the furnace arch are of spiral !ound

construction to minimise the number of tubes re:uired in its construction and

provide a high mass flu' to ensure sufficient cooling of the furnace tubes.

ransfer headers at the outlet of the spiral provides flo! mi'ing and pressure

e:ualisation for the !ater7steam mi'ture before it enters the membraned

vertical tube section in the top half of the furnace and the furnace rear !all

sling tubes.

Figure 2.6 T5o#P3,, Boiler De,ign,

Sir3l v,. 7er"ic3l Tu/e Furn3ce Arr3nge!en",

In order to minimise capital costs the t!o6pass boiler designs have been

designed !ithout a vestibule bet!een the furnace and the second pass. his

novel feature introduces the need for innovative support of the common rear

furnace !all and second pass front !all. If the furnace design dictates that the

spiral outlet header needs to be located at a level e:uivalent to the furnace

arch then the straps in the rear !all !ill be e'posed to the hot combustion

gases in the rear pass before the economiser ban%. > hot strap design for the

rear !all is therefore necessary and a design has been developed to carry the

load of the lo!er furnace !hen subAected to the hot gases of the second

pass.



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7er"ic3l "u/e 4urn3ce +e,ign

(ompared to the conventional spiral !ound furnace, the lo! fluid mass flu'

vertical internally ribbed tube furnace benefits from lo!er capital and

operating costs. he main advantages identified areF 6

Lo5er C3i"3l Co,",8

G Self6supporting tubes hence simplifying part of the boiler support system.

G limination of transition headers at spiral7vertical interface.

G Simpler ash hopper tubing geometry.Lo5er Oer3"ing Co,",8

G o!er overall boiler pressure drop, hence lo!er au'iliary po!er load

resulting

in higher plant output and higher efficiency.

G H3ositive flo! characteristic- automatically compensates for variations in

furnace absorptions compared to the negative flo! characteristics of the

spiral furnace re:uiring pressure balancing and positive mi'ing methods.

G Simple and economic tube repair. > brief comparison of the spiral !ound furnace versus the vertical tube

furnace is

presented in =igure 2.8.

Sir3l 5oun+ 4urn3ce 7er"ic3l "u/e 4urn3ce



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Figure 2.9 Co!3ri,on, o4 Sir3l &oun+ 3n+ 7er"ic3lTu/e Furn3ce,

Furn3ce Tu/e %3"eri3l,

he !all temperature of the furnace tubes is a critical parameter in boiler

designs for advanced steam conditions due to the materials of manufacture.

=urnace !alls use materials for !hich pre6 and post6!eld heat treatment is

not re:uired. he feed !ater temperature and the amount of heating provided

by the economiser dictate the !aterside inlet conditions to the furnace. =or

the turbine steam conditions of 2# bar7*##(7*2#( the furnace front !all,

side!alls and lo!er rear !alls for the t!o designs under consideration may

be constructed of the 1J (r material ?I/ 15(ro44. he revie! of the

thermo6hydraulic stability of the furnace circuits also recommended that the

arch tubes be manufactured from the material 2174J (r content

?I/ +(roBi$1#61# )e:uivalent to >S 24 material.

Boiler +e,ign

he boiler design has been based on the $S / 12;26design code. In

addition to this some analysis !as underta%en to arrive at the design margins

re:uired for the pressure part scantlings based on upsets to gas and steam6

side imbalance, heat flu' profiles and heat imbalance.

Suerhe3"er : Rehe3"er

he superheater and reheater !ith IS$62### advanced steam conditions are

similar in arrangement to those for a conventional subcritical plant. he

superheater tubes are designed to operate at some 5;< above the final main

steam temperature. he higher steam temperatures impose additional

stresses on the superheater material and increase the rates of both fireside

and steam6side corrosion7o'idation and to offer good service lifetimes,

austenitic materials of high chromium content )2#J62;J !ould need to be

used. !o grades of steel identified in the materials revie! !hich appear

suitable for the application and !hich are commercially available are /=+#

and E95(. Other grades may be considered as long6term corrosion and

steam o'idation data become available.

He3+er,) %3ni4ol+, : Pie,

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Eeaders, manifolds and steam pipes as thic% section components can limit

the permissible rate of load change of the plant. itsui $abcoc% design

features and theuse of the high6grade materials identified in the proAect have

been used to ensure that these components have a reduced !all thic%ness

and hence minimised operational constraints.

/=*1* )32 and E(12> )3122 steels appear prime candidates for use

!here ferritic materials are re:uired. Eo!ever, !elds in thic% section headers

and pipe!or% are li%ely to be susceptible to ype IB crac%ing and allo!able

stresses !ill ultimately be limited by the need to account for this

phenomenon. Similar consideration !ill also need to be paid to the

performance of transition Aoints. itsui $abcoc% has over 5# years of

e'perience of dissimilar !elds, for e'ample ferritic7austentic !elds for the final

superheaters at the **#"e boilers of ?ra' 3o!er Station. he ferritic

materials are the preferred choice for the boiler designs considered in this

proAect.

"ith regard to the choice of austenitic materials, the some!hat lo!er metal

temperatures e'pected in headers and pipe!or% mean that less e'pensive

grades !ith lo!er chromium contents can be considered than for tubing. >

grade suitable for the present application is seen to be K5(r/io/1+15

steel. > characteristic of the austenitic materials is their relatively poor

thermo6elastic behaviour and this must be ta%en into consideration at the

design stage by conducting creep fatigue assessments.

SUPERCRITICAL BOILER 1*

Chapter-2 History of Supercritical Boilers

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