supercritical benson boiler

LATEST EXPERIENCE OF COAL FIRED SUPERCRITICAL SLIDING PRESSURE

OPERATION BOILER AND APPLICATION FOR OVERSEAS UTILITY

Hajime Kimura Junichiro Matsuda

Kazuhito Sakai Babcock-Hitachi K.K.

ABSTRACT

In the fossil power utility field, a supercritical sliding pressure operation type unit has been developed

as the most advanced generating system combining higher efficiency, with low emissions technology.

Globally, this type of system has been the major trend for recent thermal plants. The use of low grade

coals i.e. low volatile, high ash content, for economic reasons provides additional challenges to meet

the international demands for improved efficiency and low emissions.

Babcock-Hitachi K.K. (BHK) has supplied a number of coal-fired boilers for thermal power plants in

Japan. The contribution to the improvement of power plant efficiency and the success of reduction in

NOx emission has been favourably received by all utilities.

Our most recent boiler experience relates to 1050MW power plant (Electric Power Development Co.

Japan, Tachibana-Wan Thermal Power Station No.2 boiler; 3000t/h steam output at Maximum

continuous rating) combining advanced steam conditions; pressure 25.0MPa, temperature

600oC/610oC and achieved the guaranteed boiler efficiency and strict requirements for NOx

emissions and unburned carbon in ash throughout the performance test. Subsequently, a 1000MW

power plant (The Tokyo Electric Power Co. Inc. Japan, Hitachi-Naka Thermal Power Station No.1

boiler, 2870t/h steam output at Maximum continuous rating) is now under commissioning, to provide

similar advanced steam conditions, pressure 25.4MPa and temperature 604oC/602oC. The boiler

was designed to meet challenging technical improvements and will be a benchmark for the boilers

with High Capacity Coal Fired Supercritical Sliding Pressure.

BHK have established a standardised design concept for coal fired supercritical sliding pressure

operation boilers for worldwide utilities based on our experience of designing and constructing

similar units over the previous decade.

This paper describes the principal characteristics of a coal fired supercritical sliding pressure

operation boiler and suitable design parameters to utilise various grades of coal, considering

economics and reliable operation in accordance with our experiences in Japan and in the export

maker for this technology.

1. Introduction

In the late 1950s, in Japan, the number of large capacity supercritical pressure oil fossil fired power

plants increased as an alternative to former smaller capacities sub critical pressure fossil fired plants

using domestic coal for fuel. In the early 1970s, energy dependence on imported oil reached

approximately 80%. Significant oil price increases and subsequently occurred twice, in 1973 and

1978, initiating a total review of domestic electricity economics triggered a need for fuel

diversification and energy saving. At that time, the demand for LNG increased as the most immediate

effective substitute fuel. After the 1980s, imported coal became the main energy resource to coping

with a stable supply and the mixing of electric power resources. With the increase of nuclear power

plants to accommodate base load operations, the associated variations of electric load demands mean

that the most new power units tended to be designed for cyclic duties.

Fig. 1 shows the general trends of utility boiler supplied by BHK in Japan. Various capacities of BHK

Benson Boilers have been supplied to cope with aforementioned utility power plants demands.

Subsequently, improvement in performance and technology has been the subject of BHK resource,

which will be incorporated into future project as they become available.

Fig. 1 General Trends of Utility Boilers Supplied by BHK in Japan

2. Improvement of Steam Conditions

Higher steam conditions were initiated through global environmental issues, i.e. to reduce greenhouse

gases, especially CO2 emissions and by improving plant efficiency. Fig. 2 shows a record of steam

parameter improvements established by BHK in Japan. The first ‘USC (Ultra Super Critical)’ plant

in Japan was built in 1989 employing gas fired boilers with steam conditions of

31MPa/566oC/566oC/566oC. At that time, the newly installed coal fired plants had a typical live

steam pressure of 24.1MPa, though steam temperatures increased gradually. The most advanced

steam condition currently in commercial operation in Japan is 25.0MPa/600oC/610oC, at the

Tachibana-Wan No. 2 boiler of Electric Power Development Co. Ltd. supplied by BHK in 2000.

This trend is being maintained with plants currently under commissioning, and with Hitachi-Naka No.

2 Unit of The Tokyo Electric Power Co. Inc., the steam parameters of which are 25.4MPa/

604oC/602oC.

Fig. 2 and 3 show typical improvements of steam condition and plant efficiency for the latest plant.

C/O Year

Goi 2u

Anegasaki 1u

Kashima 6uSodegaura 3u

Hirono 2u

FuelSteam Temp.

Boiler Type Super./ O.T(V.P.)Sub./ Super. / O.T.(C.P.)Sub. / N.C.538/566(538)538/538,(℃) 566/566 593/593,600/600566/566

Import CoalOilDomestic Coal Gas

Chiba4u

Takehara 3uChita 2-1u

Shinchi 1u

Hekinan 2u

Coal

Gas

Oil

Matsuura 1u

Nanao-Ohta 1u(USC)

Matsuura 2u(USC)

Higashi-Ohgishima 1u (USC)

LegendN.C. : Nutural CirculationO.T. : Once-Through TypeSub. : Sub. Critical PressureSuper.: Super-Critical PressureV.P. : Variable PressureC.P. : Constant PressureUSC : Ultra Super-Critical Plant

(USC)

(USC)

Ow ase- Mita 3u

Gobo 2u Nanko 2uNoshiro 1u

566/593

(USC)

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 20050

200

400

600

800

1,000

Unit

capa

city

(MW

)

Haramachi 2uHitachi-Naka 1u

Tachibanawan 2u

Tachibanaw an 1u

C/O Year

Goi 2u

Anegasaki 1u


Hirono 2u

FuelSteam Temp.



Chiba4u


Shinchi 1u

Hekinan 2u

Coal

Gas

Oil

Matsuura 1u

Nanao-Ohta 1u(USC)

Matsuura 2u(USC)



(USC)

(USC)

Ow ase- Mita 3u


566/593

(USC)

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 20050

200

400

600

800

1,000

Unit

capa

city

(MW

)


Tachibanawan 2u

Tachibanaw an 1u

C/O Year

Goi 2u

Anegasaki 1u


Hirono 2u

FuelSteam Temp.



Chiba4u


Shinchi 1u

Hekinan 2u

Coal

Gas

Oil

Matsuura 1u

Nanao-Ohta 1u(USC)

Matsuura 2u(USC)



(USC)

(USC)

Ow ase- Mita 3u


566/593

(USC)

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 20050

200

400

600

800

1,000

Unit

capa

city

(MW

)

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 20050

200

400

600

800

1,000

Unit

capa

city

(MW

)


Tachibanawan 2u

Tachibanaw an 1u

Fig. 2 Improvement of Steam Condition in Japan Fig. 3 Improvement of Plant Efficiency

3. Boiler Design Feature

3.1 Once-through Boiler (BENSON boiler for sliding pressure operation)

Benson type boilers have been developed and designed for variable sliding pressure operation plants

of high efficiency at all loads, which is suitable for both base and middle load operations. The start-up

system consists of a steam/water separator, a boiler circulation pump and associated piping, which

ensure a quick, smooth start-up and shutdown of the plant and easy operability. Table 1 summarises

the design future of the BENSON boiler in comparison with other boiler types.

A spirally wound water wall construction with ribbed tubing is applied to the furnace to enable have

sufficient mass flow velocity in the water wall tubes under variable loads to prevent departure from

nucleate boiling (DNB) and to achieve uniform water temperature distribution at the furnace outlet

when operating below critical pressure and without pseudo DNB when operating above critical

pressure (Fig. 4). All heated water walls are arranged with upward fluid flows.

Fig.4 Comparison of Mass Velocity in Furnace Wall Tube in Sliding Operation Mode

Tachibanawan No.2 (1050MW)

Haramachi No.2 (1000MW)

Matsuura No.2 (1000MW)

Nanao-Ohta No.1 (500MW)

Shinchi No.1 (1000MW)

Noshiro No.1 (600MW)

24.1MPa/538/566℃

24.1MPa/566/593℃

24.5MPa/600/600℃

24.1MPa/593/593℃

Hekinan No.2 (700MW)

1990 20001995 2005

Year

25.0MPa/600/610℃

1985 2010

Matsuura No.1 (1000MW)

40

41

42

43

44

16.6MPa 16.6MPa 24.1MPa 24.1MPa 24.1MPa 24.1MPa 24.5MPa

Plan

t effi

cien

cy (%

)

00 1,000 2,000 3,000 4,000 6,000

30 %MCR 50 %MCR 75%MCR MCR BMCR

Multi Ribbed TubeSmooth Tube

5,000

3,000

00 1,000 2,000 3,000 4,000 6,000

Multi Ribbed TubeSmooth

Mean Design Mass Velocity

5,000

3,0002.0

2.5

1.5

1.0

0.5

MPag

Mean Critical Mass velocity

Main Steam Flow (Mlb )

0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

Pressu

reat

Furnac

e Inlet

Pressureat SH Outle

t

X 103 kg/m2s

00 1,000 2,000 3,000 4,000 6,000

30 %MCR 50 %MCR 75%MCR MCR BMCR

Multi Ribbed TubeSmooth Tube

5,000

3,000

00 1,000 2,000 3,000 4,000 6,000

Multi Ribbed TubeSmooth

Mean Design Mass Velocity

5,000

3,0002.0

2.5

1.5

1.0

0.5

MPag

Mean Critical Mass velocity

Main Steam Flow (Mlb )

0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

Pressu

reat

Furnac

e Inlet

Pressureat SH Outle

t

X 103 kg/m2s

Ribbed Tube

Table 1 Boiler Type and Furnace Construction

FurnaceConstruction

Mixing Bottles

OperatingPressure

Sub-Critical(Constant or Sliding)

UP Boiler Benson BoilerNC Boiler

Dow

n Co

mer

DRUM

Steam FeedWater

Vertical TypeVertical Type(UP-UP Type)

Feed Water

Mix

ing

Bott

les

Spiral Type

Sub-Critical or Supercritical

(Constant Pressure)

Sub-Critical to Supercritical Region(Sliding Pressure)

Mixing Bottles arenot Necessary

Mixing Bottles are Necessary to Reduce Effect of Heat Flux

Unbalance

Mixing Bottles arenot Necessary By Spiral Type Water

Wall

Feed Water

ApplicableSteam PressureThrough FurnaceEnclosure TubesFluid StabilityTemperature UniformityMass Flow RateVariable Pressure ?

Max. Unit Capacityin Operation

Furnace EnclosureConstructionTube O/D (mm)

Start-up Time (min.)(Hot start)

Allowable Min. Load (%)

Load Change Rate

Subcritical Supercritical &Subcritical

Supercritical &Subcritical

Self BalanceBetter

Approx. 13%YES

BaseBase100%NO

Much BetterMuch Better

100%Wide Range

15 35 - 45 25 - 35 (OT Mode)15 (Circ. Mode)

Base Slightly Higher Higher

120 - 150with TB By-pass 250 120 - 150

with TB By-pass

57.0 - 63.5 22.5 - 31.8 31.8 - 38.1

800 MW 1,300 MW 1,000 MW

Start-upBypass System

Heat Lossduring Start Up

•Not installed•Operation of drain 　　valves and　vent valves 　is necessary

•Main valve is installed inthe main steam line

•Shift operation of start-up valves is necessary

•Operation of drain valves and vent valves is necessary

•Simplified start-up bypasssystem

•Shift operation of start-upvalves is not necessary

•Operation of drain valves 　　and　vent valves is 　　　　　necessary

•Continuous blowing(in case of bad water quality)

•Warming of start-up bypass 　system

•Warming of start-up bypass 　system•Heat recovery of circulated 　water by BCP

1SH 2SHCAGE

DRUM

W/WECO

HP-HTR BFP

DEAER

COND

TURBINE

1SH 2SH

CAGE

FLASHTANK

W/W

ECO

HP-HTR BFP

DEAER

COND

TURBINE

1SH 2SH

CAGE

W/W

ECO

HP-HTR BFP

DEAER

COND

3SH

TURBINE

BCP

DRAINTANK

SEPARATOR

Furnace Construction

Vertical Vertical Spiral

Notes NC:Natural Circulation OT:Once-Through Cric.:Circulation O/D:Outside Diameter

FurnaceConstruction

Mixing Bottles

OperatingPressure

Sub-Critical(Constant or Sliding)

UP Boiler Benson BoilerNC Boiler

Dow

n Co

mer

DRUM

Steam FeedWater

Dow

n Co

mer

DRUM

Steam FeedWater

Vertical TypeVertical Type(UP-UP Type)

Feed Water

Mix

ing

Bott

les

Feed Water

Mix

ing

Bott

les

Spiral Type

Sub-Critical or Supercritical

(Constant Pressure)

Sub-Critical to Supercritical Region(Sliding Pressure)

Mixing Bottles arenot Necessary

Mixing Bottles are Necessary to Reduce Effect of Heat Flux

Unbalance

Mixing Bottles arenot Necessary By Spiral Type Water

Wall

Feed WaterFeed Water

ApplicableSteam PressureThrough FurnaceEnclosure TubesFluid StabilityTemperature UniformityMass Flow RateVariable Pressure ?

Max. Unit Capacityin Operation

Furnace EnclosureConstructionTube O/D (mm)

Start-up Time (min.)(Hot start)

Allowable Min. Load (%)

Load Change Rate

Subcritical Supercritical &Subcritical

Supercritical &Subcritical

Self BalanceBetter

Approx. 13%YES

BaseBase100%NO

Much BetterMuch Better

100%Wide Range

15 35 - 45 25 - 35 (OT Mode)15 (Circ. Mode)

Base Slightly Higher Higher

120 - 150with TB By-pass 250 120 - 150

with TB By-pass

57.0 - 63.5 22.5 - 31.8 31.8 - 38.1

800 MW 1,300 MW 1,000 MW

Start-upBypass System

Heat Lossduring Start Up

•Not installed•Operation of drain 　　valves and　vent valves 　is necessary

•Main valve is installed inthe main steam line

•Shift operation of start-up valves is necessary

•Operation of drain valves and vent valves is necessary

•Simplified start-up bypasssystem

•Shift operation of start-upvalves is not necessary

•Operation of drain valves 　　and　vent valves is 　　　　　necessary

•Continuous blowing(in case of bad water quality)

•Warming of start-up bypass 　system

•Warming of start-up bypass 　system•Heat recovery of circulated 　water by BCP

1SH 2SHCAGE

DRUM

W/WECO

HP-HTR BFP

DEAER

COND

TURBINE

1SH 2SH

CAGE

FLASHTANK

W/W

ECO

HP-HTR BFP

DEAER

COND

TURBINE

1SH 2SH

CAGE

W/W

ECO

HP-HTR BFP

DEAER

COND

3SH

TURBINE

BCP

DRAINTANK

SEPARATOR

Furnace Construction

Vertical Vertical Spiral

Notes NC:Natural Circulation OT:Once-Through Cric.:Circulation O/D:Outside Diameter

800MW 1,300MW 1,050MW

3.2 Sliding Pressure Operation

The sliding pressure operation is a control system in which the main steam is controlled by sliding

pressure in proportion to the generation output. Steam quality at the turbine inlet can be changed at

constant volume flows while keeping the turbine governing valve open. Utilising sliding pressure, the

thermal efficiency of the steam turbine is improved at partial operating loads through decreasing

thermodynamic efficiency as follows, by comparison with constant pressure type operation:

(1) A smaller governing value loss enables improvement of high pressure turbine internal efficiency

(2) Decrease of feed water pump throughput

(3) Boiler reheat steam temperature can be maintained at higher levels because of higher

temperatures in high-pressure turbine exhaust steam.

Fig. 5 Feature of Coal Firing Supercritical Sliding Pressure Operation Boil

4. Pressure Parts Material

4.1 Materials for conventional super critical boilers

Table 2 lists the typical materials used for conventional super critical boilers with steam conditions of

24.1MPa/538oC/566oC, and Fig. 6 shows allowable stresses of the boiler materials. The selection of

appropriate and economical material for boiler pressure parts depends on a number of factors such as

SLIDING PRESSURE OPERATION

0 20 40 60 80 100-3

-2-1012345

0 20 40 60 80 10005

1015202530

Turbine Load (%)

Turbine Load (%)

Mai

n St

eam

Pre

ss. (

Mpa

)de

grad

atio

n

A

B

C

Improvement of Turbine Heat Rate dueto Sliding Pressure Operation

A slight governing valve loss enables improvementin the HP turbine internal efficiency : A

Decrease of feed water pump input : B

In comparison to constant pressure operations, a slidingtype enables much improvement in plant efficiencyunder partial load operations.

Rel

ativ

e co

mpa

rison

of

Hea

t Rat

e (%

)im

prov

emen

t

Unit output control method by sliding pressureis as follows.By the sliding pressure in proportion to the generatoroutput, steam quantity at turbine inlet can bechanged at a constant volume flow while keepinggoverning valve open.

Boiler reheat steam temperature can be maintainedhigh because of high temperature of HP turbineexhaust steam : C

Thermodynamic loss byfalling in pressure


0 20 40 60 80 100-3

-2-1012345

0 20 40 60 80 10005

1015202530

Turbine Load (%)

Turbine Load (%)

Mai

n St

eam

Pre

ss. (

Mpa

)de

grad

atio

n

A

B

C





Rel

ativ

e co

mpa

rison

of

Hea

t Rat

e (%

)im

prov

emen

t



0 20 40 60 80 100-3

-2-1012345

0 20 40 60 80 10005

1015202530

Turbine Load (%)

Turbine Load (%)

Mai

n St

eam

Pre

ss. (

Mpa

)de

grad

atio

n

A

B

C





Rel

ativ

e co

mpa

rison

of

Hea

t Rat

e (%

)im

prov

emen

t


Boiler reheat steam temperature can be maintainedhigh because of high temperature of HP turbineexhaust steam : C

Thermodynamic loss byfalling in pressure

strength properties, corrosion resistance, metallurgical stability and so on. It is therefore necessary to

select the optimum steel, considering all relevant factors at anticipated metal temperatures.

Steam conditions:24.1MPa/538C/566CPressure part Metal

Temperature Materials

Economizer 300～350C Carbon steel(STB510)

Furnace wall 350～500C0.5Mo(STBA13)0.5Cr0.5Mo(STBA20)1Cr0.5Mo(STBA22)

Superheater 450～590C

0.5Mo(STBA13)0.5Cr0.5Mo(STBA20)1Cr0.5Mo(STBA22)2.25Cr1Mo(STBA24)18Cr10NiTi(SUS321HTB)

Tubing

Reheater 350～610C

Carbon steel(STB340) 0.5Mo(STBA13)1Cr0.5Mo(STBA22)2.25Cr1Mo(STBA24)18Cr10NiTi(SUS321HTB)

Superheater headerMain steam pipe

550C2.25Cr1Mo(STPA24)

HeaderPiping

Reheater header Hot reheat pipe 570C 2.25Cr1Mo

(STPA24,SCMV4)

To establish adequate safety margins and service life, the characteristics of the steel must be given

due consideration during the design phase. Economy dictates that the lowest alloy with properties

suitable to the conditions should be used, stepping up from carbon steel to molybdenum steel and to

chromium-molybdenum steel as temperatures increase. For metal temperatures approaching about

550oC, lower-alloy ferritic steels up to and including 2.25% chromium are usually considered

adequate. Stainless steels are used at higher temperatures, where conditions require an increase in

resistance to corrosion and oxidation. However, stainless steel tubes usually have a higher carbon

content in order to increase creep rupture strength. In spite of the sensitisation due to the higher

carbon content during the usage in elevated temperature service, no stress corrosion cracking has

been experienced in stainless tubes. This may be related to the fact that the inside surface of tubes

contacts with dry steam.

The steam headers and pipes connecting the boiler and turbine are highly important components of

the power plant. Such piping must be carefully designed and erected to absorb thermal expansion and

vibratory stresses. Some stainless steel piping previously used in the power plant industry exhibited

300 400 500 600 7000

50

100

150

Temperature (C)

Allo

wab

le s

tres

s (M

Pa)

2.25Cr1Mo steel(STBA24)

Carbon steel(STB510)

Stainless steel(SUS321HTB)

0.5Mo steel(STBA13)

Table 2 Typical Materials for Conventional Super Critical Boiler

Fig. 6 Allowable Stresses of Boiler Materials

[A213TP321H]

[A209T1a]

[A213T22]

serious cracking problems caused by high thermal stresses due to higher thermal expansion

coefficients, as experienced under service conditions1). Therefore, such thick walled components

should be constructed using ferritic type steel pipes whose thermal expansion coefficient is relatively

low.

4.2 Materials for advanced super critical boiler

Fig. 7 shows a comparison of allowable stresses between conventional and advanced chromium

molybdenum steel pipe material. For high temperature headers and pipes associated with superheaters

and reheaters, STPA28 (Mod.9Cr1Mo) developed by Oak Ridge National Laboratories is suitable due

to its high temperature strength and excellent resistance to oxidation. Since the late 1980’s the steel

[STPA29] has been widely used for advanced power plants with the steam conditions of

approximately 25MPa/600oC/600oC in Japan and Europe. STPA29 (NF616) subsequently developed

by Nippon Steel and SUS410J3TP (HCM12A) developed by Sumitomo Metal have higher creep

strengths than that of STPA28 (T91), and these steels have been used for an advanced power plant

with steam conditions of 25MPa/600oC/610oC.

Fig. 7 Comparison of Allowable Stresses between Conventional and

Advanced CrMo Steel Pipes

Fig. 8 shows a comparison of allowable stresses between conventional and advanced stainless steel

tubes. Newly developed austenitic stainless steels such as SUS304J1HTB (SUPER304H) developed

by Sumitomo Metal and SUS310J2TB (NF709) developed by Nippon Steel have extremely high

creep rupture strength and the allowable stresses are twice as high value compared to SUS321HTB at

500 550 600 650 7000

50

100

150

Temperature (C)

Allo

wab

le s

tres

s (M

Pa)

STPA24(2.25%Cr)

STPA28(9%Cr)

STPA29(9%Cr)

SUS410J3TP(11%Cr)

STPA24J1(2.25%Cr)

[A335P91]

[A336P92]

[A335P122] [A335P23]

[A335P22]

650oC. These steels have been applied to high temperature superheater tubes. For severe corrosion

loads SUS310J3TB (HR3C) developed by Sumitomo Metal can be used because of its higher

chromium content.

Fig. 8 Comparison of Allowable Stresses between Conventional and Advanced Stainless Steel Tubes

Another issue that must be considered when selecting material for high temperature tubing is the

resistance to coal ash corrosion caused by coal sulphur content. Fig. 9 shows the effect of SO2

content on corrosion loss. At SO2 content of 0.1% or less (corresponding to about 1% sulphur in

coal) or less, corrosion loss is negligible for austenitic stainless steels containing 18% chromium2).

When the sulphur content of coal is around 5% (corresponding to about 0.5% SO2 in fuel gas), it will

be necessary to use a high-chromium austenitic stainless steels such as SUS310J1TB (HR3C).

Fig. 10 shows the effect of steam temperatures on steam oxide scale thickness. With increasing steam

temperatures, materials with an improved steam oxidation resistance require to be used for

superheater and reheater tubes. Spalled steam oxide scales have the potential to plug steam flows and

erode turbine components. The use of high chromium content or fine-grained stainless steel tubes is

effective in minimizing steam oxidation troubles. Fig. 10 also shows that internally shotblasted

stainless steel tube containing 18% chromium has similar resistance to steam oxidation as high

chromium stainless steel at temperatures up to 700oC2).

0

Allo

wab

le S

tress

SUS321HTB[A213TP321H](18Cr-10Ni-Ti)

SUS304J1HTB[A213UNS S30432](18Cr-9Ni-3Cu-Cb-N)

SUS310J1TB[A213TP310HCbN] (25Cr-20Ni-Cb-V)

SUS310J2TB[NF709] (20Cr-25Ni-1.5Mo)

18Cr-10Ni-Ti-Cb(Tempaloy A1)

550 600 650 700 750

50

100

150

1000 1100 1200 1300 1400

(MPa) (1000psi)20

15

10

5

(oF)

(oC)0

Allo

wab

le S

tress

SUS321HTB[A213TP321H](18Cr-10Ni-Ti)

SUS304J1HTB[A213UNS S30432](18Cr-9Ni-3Cu-Cb-N)

SUS310J1TB[A213TP310HCbN] (25Cr-20Ni-Cb-V)

SUS310J2TB[NF709] (20Cr-25Ni-1.5Mo)

18Cr-10Ni-Ti-Cb(Tempaloy A1)

550 600 650 700 750550 600 650 700 750

50

100

150

1000 1100 1200 1300 1400

(MPa) (1000psi)20

15

10

5

(oF)

(oC)

5. Combustion System

For environmental protection, the Hitachi NR2 burner has been applied to achieve low levels of

emission of NOx (oxides of nitrogen) and high levels of combustion efficiency. Following on from

the development of Hitachi’s NR burner in which an innovative concept of in-flame NOx reduction is

applied, BHK further developed the NR2 and NR3 burners progressively. These burners features a

stable and strengthened flame in terms of high-temperature reduction and achieved extremely low

levels of NOx emission and high levels of combustion efficiency 3). The Hitachi NR burner series is

based on an improved in flame NOx-reduction technology, which incorporates two innovative

devices: a pulverised-coal (PC) concentrator and a space creator. The configuration of the burners is

shown in Fig. 11.

Fig, 11 Configuration of NR Series Burners together with Improvement Trend

0.01 0.02 0.05 0.1 0.2 0.5 10

50

100

150

200

SO2 content in fuel gas (%)

Temperature:650C Time:20h Ash:1.5MNa2SO4-1.5MK2SO4-1MFe2O3

Corr

osio

n lo

ss (m

g/cm

)2

SUS310J2TB(20%Cr)

SUS310J1TB(25%Cr)

SUS304J1HTB(18%Cr)

Temperature (C)

Inne

r sc

ale

thic

knes

s (μ

m)

0

10

20

30

40

50

550 600 650 700 750 800

Time:1,000h

SUS304J1HTB(18%Cr)

SUS310J1TB(25%Cr)

SUS310J2TB(20%Cr)

ShotblastedSUS304J1HTB

(18%Cr)

Fig. 9 Effect of SO2 Content on Coal AshCorrosion Loss of Stainless Steel Tubes

Fig. 10 Effect of Temperature on Steam Oxide Scale of Stainless Steel Tubes

1980 1985 1990 1995 20000

100

200

300

400

NO

x (6

%O

2,pp

m)

300 ppm(100%)Coal propertyFuel Ratio：2.2Nitrogen:1.8%Two Stage Combustion

Dual Burner(Conventional)

NR2 BurnerNR Burner NR3 Burner

175 ppm(60%)150 ppm(50%) 125 ppm(40%)

NR2 BurnerDual Burner NR3 BurnerNR BurnerDelayed Combustion Rapid ignition（In Flame NOx Reduction）

Guide sleeveFlame StabilizingRing

Swirler

Space CreatorFlame StabilizingRing

P.C. Concentrator

Flame StabilizingRing

＋Baffle Plate

Guide sleeve

P.C. ConcentratorSpin Vane

Air register

（100 ppm(33%)）Coal propertyFuel Ratio：1.8Nitrogen:1.5%Two Stage Combustion

1980 1985 1990 1995 20000

100

200

300

400

NO

x (6

%O

2,pp

m)

300 ppm(100%)Coal propertyFuel Ratio：2.2Nitrogen:1.8%Two Stage Combustion

Dual Burner(Conventional)

NR2 BurnerNR Burner NR3 Burner

175 ppm(60%)150 ppm(50%) 125 ppm(40%)

NR2 BurnerDual Burner NR3 BurnerNR BurnerDelayed Combustion Rapid ignition（In Flame NOx Reduction）

Guide sleeveGuide sleeveFlame StabilizingRingFlame StabilizingRing

SwirlerSwirler

Space CreatorSpace CreatorFlame StabilizingRingFlame StabilizingRing

P.C. ConcentratorP.C. Concentrator

Flame StabilizingRing

＋Baffle Plate

Guide sleeveGuide sleeve

P.C. ConcentratorP.C. ConcentratorSpin Vane

Air register

（100 ppm(33%)）Coal propertyFuel Ratio：1.8Nitrogen:1.5%Two Stage Combustion

Coal propertyFuel Ratio：1.8Nitrogen:1.5%Two Stage Combustion

These burners enable a low level of excess air at only 15% at economiser’s outlet. This is important

when firing any various kinds of imported coals.

6. Latest Boiler Experience

The Tachibana-Wan Thermal Power Station No. 2 Unit (1,050MW) was completed for the Electric

Power Development Co. Ltd. (EPDC) in December of 2000. It applies advanced steam temperatures

of 25.0MPa and 600oC/ 610oC. The 3,000-t/h coal fired USC boiler for the generating plant was

supplied by BHK, and during the commissioning and test period, it was confirmed the boiler attained

all of its design performance specifications. To achieve these high-level steam parameters, newly

developed high-strength materials were utilised in the high-temperature areas, and the latest

combustion technology was used for environmental protection and also ensure a high level of

efficiency, operating reliability and plant availability.

Subsequently, a 1000MW power plant (The Tokyo Electric Power Co. Inc. Japan, Hitachi-Naka

Thermal Power Station No.1 boiler, 2870t/h steam output at Maximum continuous rating) is now

under commissioning, and will provide the advanced steam condition, pressure 25.4MPa and

temperatures 604oC/602oC.

6.1 Main Design Features of the Boilers

Fig. 12 is a cross-sectional schematic views of the Tachibana-Wan No.2 boiler and Hitachi-Naka

No.1 boiler. As both boilers are medium-load plants burning coals with a wide range of grades,

various factors were considered in designing the sliding-pressure boiler operation with advanced

steam parameters. All these advanced plants burn a wide variety of world traded coals from Australia,

Indonesia, South Africa etc. The main specification of the boilers is summarised in Table 3.

During the design engineering process of the Hitachi-Naka No.1 boiler, many special considerations

were implemented to ensure improvement of operability, maintainability and reliability together with

plant performance development. These efforts were synonymous with the activity for standardisation

of the BHK BENSON Boiler such that targets are considered with the demands of power plant users.

As more types of coals are imported from overseas, achievement of this standardization can be easily

transferred to meet world-wide operating requirements.

Hitachi-Naka No.1 Boiler Tachibana-Wan No.2 Boiler

Fig. 12 Cross-Sectional Schematic Views

Table 3 Main Specification of Boilers

Item Specification Hitachi-Naka No.1 Tachibana-Wan No.2 Boiler Type Babcock-Hitachi Supercritical Sliding Pressure

Operation Benson Boiler Main 2,870 t/h 3,000 t/h Steam Flow Reheat 2,407 t/h 2,490 t/h

Steam Pressure Main 25.4 MPa(g) 25.0 MPa(g) Main 604 oC 600 oCSteam Temperature Reheat 602 oC 610 oC

MCR

Economiser Inlet: Feedwater Temperature 286.9 oC 288 oC

Combustion System Pulverised Coal Fired Draught System Balanced Draught System

Main Water-Fuel Ratio Control and Stage AttemperationSteam Temperature Control System Reheat Parallel Gas Dampering and Spray Attemperation Main 30% ECR to MCR 35% ECR to MCR Steam Temperature Control Range Reheat 50% ECR to MCR 35% ECR to MCR

MCR: Maximum Continuous Rating, ECR: Economical Continuous Rating

The principal design features are as follows:

a) One of the best features of this Benson type boiler is the spirally wound water wall

No Installation of GRF

気水分離器

二次過熱器三次過熱器

再熱器

一次過熱器

節炭器

脱硝装置

空気予熱器

ボイラ循環ポンプ微粉炭器

バンカ

ＡＡポート

バーナ

一次通風機押込通風機

Steam Separator

Mills FDFBRP PAF

Secondary Superheater

Tertiary Superheater

Primary Supergeater

ﾊﾞーﾅ

Reheater

AAﾎﾟーﾄ

EconomiserBunkers

SCR

AH

Mills FDFBRP

PAF

SecondarySuperheater


Evaporator

Secondary Reheater

Economiser

Multi-Cyclone GRF

AH

SCR

Final Superheater

Primary Reheater

Primary Superheater

Steam Separator

Horizontal Reheater Coils

Steam Cooled Roof

Bunkers

No Installation of GRF

気水分離器


再熱器

一次過熱器

節炭器

脱硝装置

空気予熱器


バンカ

ＡＡポート

バーナ


Steam Separator

Mills FDFBRP PAF



Primary Supergeater

ﾊﾞーﾅ

Reheater

AAﾎﾟーﾄ

EconomiserBunkers

SCR

AH

Mills FDFBRP

PAF



Evaporator

Secondary Reheater

Economiser

Multi-Cyclone GRF

AH

SCR

Final Superheater

Primary Reheater

Primary Superheater

Steam Separator


Steam Cooled Roof

Bunkers

気水分離器


再熱器

一次過熱器

節炭器

脱硝装置

空気予熱器


バンカ

ＡＡポート

バーナ


Steam Separator

Mills FDFBRP PAF



Primary Supergeater

ﾊﾞーﾅ

Reheater

AAﾎﾟーﾄ

EconomiserBunkers

SCR

AH

Mills FDFBRP

PAF



Evaporator

Secondary Reheater

Economiser

Multi-Cyclone GRF

AH

SCR

Final Superheater

Primary Reheater

Primary Superheater

Steam Separator


Steam Cooled Roof

Bunkers

arrangement at the lower furnace wall. In particular, Hitachi-Naka No.1 boiler equipped the

mixing manifolds at the outlet of spiral water wall to ensure a more uniform metal

temperature particularly at lower load conditions.

b) The boiler and furnace walls are suspended from overhead steel work such that the

progressive expansion of pressure parts is in a downward direction and there are no relative

expansion effects experienced between the furnace wall sections. The furnace walls compose

an all-welded membrane construction, which ensures complete gas tightness, and less in

erection time at site.

c) The combustion gas flows upward from the furnace area then turns into the pendant

convection pass section where pendant surfaces are located to efficiently absorb heat from

the hot gas. The gas then continues to flow down through the rear horizontal convection pass

area.

d) The primary superheaters and reheaters are located in a parallel and horizontal convection

pass arrangement together with the economizers. This also enables a sufficient amount of

reheater heating surface to be installed in this zone to permit allow quick responses for steam

temperature control by a gas biasing system. To improve this effect, all reheater-heating

surfaces are positioned in the horizontal convection.

e) A steam/water separator is used during the start-up and shutdown, and at loads lower than

minimum once-through load for smooth and reliable operation. In the case of Hitachi-Naka

No.1 boiler, the steam separator is positioned at the boiler front side. With this arrangement,

the all roof and cage walls are of steam-cooled surface and designed to mitigate the potential

of excessive temperature deviations within panels.

6.2 Achievements in the Commissioning

Hitachi-Naka No.1 boiler is currently at the commissioning stage [early 2003], however,

Tachibana-Wan No.2 boiler successfully entered commercial operation in December 2000. Stable and

reliable operation with the advanced steam conditions has been confirmed in both static and dynamic

modes. The main features of boiler performance have also been confirmed and are described below.

Boiler Performance

Fig. 13 shows the steam-water characteristics at each turbine load. The main steam and reheat steam

temperatures achieved anticipated values without causing any concerns. The backend gas temperature

and amounts of excess air and unburned carbon in ash (UBC) were significantly below the design

values. The boiler efficiency was found satisfactory across the load range.

Fig. 13 Steam and Water Temperature and Main Steam Pressure Characteristics

Combustion Performance

The results of combustion testing are summarised in Fig. 14. The A-type coal has a high fuel ratio

500 1,000 1,500 2,000 2,500 3,0000

10

20

30

200

300

400

500

600

Steam & Water Temperature (℃)

788M W525M W315M W 1050M W M C R

M ain Steam Flow (t/h)

Superheater O utlet

Superheater Intlet

Econom izer Intlet

Econom izer Outntlet

● : M easured (SH) , ■ : M easured (RH), : Anticipated

Reheater O utlet

Reheater Inlet

Main Steam Pressure

(MPa(g))

●

●

●

●

●

●●

●■

●

■ ■●

●

●

●

●●

●●

■

● ● ● ●■

■ ■ ■

(fixed carbon to volatile matter = 2:1) and nitrogen content, which means that the simultaneous

reduction of NOx and UBC is extremely difficult. In burning the A-type coal, the reduction in UBC

was confirmed, as were lower NOx emission levels. The same trend held when burning the B-type

coal, with its higher fuel ratio (=2.4) and nitrogen. Fig. 15 shows the flame during burning of B-type

coal at a load of 35% ECR (368MW). A very bright and stable flame was maintained at the tip of the

fuel nozzle at the low coal-firing load of 35% of ECR, along with stable boiler performance.

Fig. 14 Combustion Test Results at Rated Load Fig. 15 Flame of Hitachi NR2 Burner

7. Application to Overseas Projects

There are several coal fired generating plant projects proceeding overseas with more projects entering

the final contact stage. The current precedent project is the Genesee Power Generating Station Phase

3 boiler being provided for EPCOR in Canada (for 495MW Power Plant) and this project is now

entering the peak of construction stage [spring 2003].

For the Genesee Power Generating Station Phase 3 boiler, the same design of BHK reference boiler,

Nanao-Ohta No.1 boiler of Hokuriku Electric Power Co. (for 500MW Power Plant), was adopted.

This was due to not only the similar design steam condition required but also that Nanao-Ohta No.1

boiler has achieved excellent availability records. To be able to apply the same proven design

characteristics from the Nanao-Ohta No.1 boiler with modification was greatly appreciated by the

client.

0

5

120 160 200

NOx Emission (p p m, at 6%O2)

Unb

urne

d C

arbo

n in

Ash

(%

)

A coa l

B coa l

The reference plant, Nanao-Ohta No.1 boiler, is summarised in Fig. 16 and its specification is

described in Table 4. Fig, together with the specification of Genesee Power Generation Station Phase

3 boiler. Fig. 17 indicating which design considerations were applied from the design of Nanao-Ohta

No.1 boiler to Genesee Power Generating Station Phase 3 boiler.

Fig. 16 Main Future of Nanao-Ohta No.1 boiler

Table 4 Main Specifications of Nanao-Ohta No.1 and Genesee No. 3 Boiler

Item Specification Nanao-Ota No.1 Genesee No. 3 Boiler Type Babcock-Hitachi Supercritical Sliding Pressure

Operation Benson Boiler Main 1,510 t/h 1,450 t/h Steam Flow Reheat 1,158 t/h 1,192 t/h

Steam Pressure Main 25.0 MPa(g) 25.0 MPa(g) Main 570 oC 570 oCSteam Temperature Reheat 595 oC 568 oC

MCR

Economiser Inlet: Feedwater Temperature 289 oC 284 oC

Combustion System Pulverised Coal Fired Draught System Balanced Draught System

Main Water-Fuel Ratio Control and Stage AttemperationSteam Temperature Control System Reheat Parallel Gas Dampering and Spray Attemperation Main 28% MCR to MCR 35% ECR to MCR Steam Temperature Control Range Reheat 50% ECR to MCR 50% ECR to MCR

MCR: Maximum Continuous Rating, ECR: Economical Continuous Rating

Features

2. Imported variety coal combustion(Australia, Indonesia, South Africa, etc.)

···115 kinds of CoalGCV (AD kJ/kg) 20,660 - 33,240Inherent Moist. (%) 1.5 – 25.2Ash (%) 1.1 - 19.0Volatile Matter (%) 24.2 – 43.6Fixed Carbon (%) 35.6 – 61.8Fuel Ratio (-) 0.9 - 2.4Sulphur (%) 0.2 - 1.5Nitrogen (%) 0.7 – 1.9HGI (-) 40 - 89Ash Softening Temp (ºC) 1,100 - 1,500Ash Fusion Temp. (ºC) 1,180 - 1,600

1. Advanced steam condition 25MPa, 570ºC/595ºC

Features




Features




Fig. 17 Design Considerations Applied to Genesee Power Generating Station Phase 3 Boiler

Another overseas project, Council Bluffs Energy Centre No.3 Boiler for MidAmerican Energy

Company in USA (for 790MW Power Plant), is presently at the detailed engineering stage. Special

consideration will take into account the coal properties applying to Powder River Basin (PRB), which

can provide severe slagging in the boiler furnace. To cope with this undesirable coal property, the

furnace design is required to accommodate sufficiently large sectional area. The Standard design

established for Hitachi –Naka No.1 Boiler will therefore be fully applied to Council Bluffs Energy

Centre No.3 Boiler. Fig. 18 shows cross-sectional schematic views of the Council Bluffs Energy

Centre No.3 Boiler.

Fig. 18 Council Bluffs Energy Centre No.3 Boiler

Nanao-Ota500MW

Genesee #3

Width

Depth

Height

16.6 m

13.3 m

53.6 m

16.6 m

15.3 m

55.1 m

Volume 10240 m3 12070 m3

Plant Area 220 m2 254 m2

High Reliability 500mw Class Standard Design Philosophy Based On 7 Years Successful Operation Of Nanao Ota #2

Expanded Furnace Plane Area and Volume by +2m of depth to meet Genesee coal characteristics Reliable Design Philosophy confirmed from 7 years successful operation of Nanao-Ota boiler.


Standard 500MW Class Boiler

CANADA EPCOR Genesee 3

Boiler circulation pump

Hitachi NR3burners

Tertiary superheater

Secondarysuperheater

Over air ports

Final superheater

Secondary reheater

Water separator

Primary reheater

Primary superheater

Economizer

Air heater

Forced draught fan

Bunkers

Nanao-Ota500MW

Genesee #3

Width

Depth

Height

16.6 m

13.3 m

53.6 m

16.6 m

15.3 m

55.1 m

Volume 10240 m3 12070 m3

Plant Area 220 m2 254 m2

High Reliability 500mw Class Standard Design Philosophy Based On 7 Years Successful Operation Of Nanao Ota #2



Standard 500MW Class Boiler

CANADA EPCOR Genesee 3


Hitachi NR3burners



Over air ports

Final superheater

Secondary reheater

Water separator

Primary reheater

Primary superheater

Economizer

Air heater

Forced draught fan

Bunkers


Hitachi NR3burners



Over air ports

Final superheater

Secondary reheater

Water separator

Primary reheater

Primary superheater

Economizer

Air heater

Forced draught fan

Bunkers

Hitachi NR3burners



Over air ports

Final superheater

Secondary reheater

Water separator

Primary reheater

Primary superheater

Economizer

Air heater

Forced draught fan

Bunkers

B abcock-H itachi Supercritical Sliding PressureO peration B enson Boiler

M C R Steam Flow 2,531 t/hPlant Capacity 790 M W (Net)Steam C ondition 25.3 M Pa(g)/566℃/593℃

B oiler Type

8. Summary

Utility capacity and steam conditions have rapidly increased in Japan, to meet the demand for high

efficiency and low emission pollution in generating plant. To cope with these demands, the BHK

Benson technology has been and continues to be improved.

As evidence of this improvement, Tachibana-Wan No. 2 boiler with the advanced steam condition of

25.0MPa/ 600oC/ 610oC entered service in December of 2000. It was confirmed that this latest model

coal fired boiler, which was designed to operate satisfactory with a wide range of coal fuels and to

have improved countermeasures against environmental pollution, achieved all performance design

specifications during its commissioning and testing period.

Consequently, Hitachi-Naka No. 1 Boiler with advanced steam condition of 25.4MPa/ 604oC/ 602oC

is under construction. In this boiler unit, many special considerations have been implemented to

improve operability, maintainability and reliability together with performance development and these

innovative design features will be applied to future overseas projects.

BHK will continue to research and add improvements in performance and technology as their

contribution to global welfare in the form of better technologies for energy and environmental

systems.

References

1) T.C.McGough, J.V.Pigford, P.A.Lafferty, S.Tomasevich et al.: Selection and Fabrication of

Replacement Main Steam Piping for the Eddystone No. 1 Supercritical Pressure Unit, Welding

Journal, Vol.64, No.1 (1985) pp.29-36.

2) K.Sakai, S.Morita : The design of a 1000MW coal-fired boiler with the advanced steam

conditions of 593oC/593oC, International Conference on Advanced Steam Plant, IMechE

Conference Transaction 1997-2, (21-22 May 1997) pp.155-167.

3) S. Morita et al., “ Development of Extremely Low NOx Pulverized Coal Burners by Using the

Concept of ‘in-Flame’ NOx Reduction,” ICOPE-93, Vol. 2, pp. 325-330 (1993)

supercritical benson boiler

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