Latest Technology of Highly
Efficient Coal-Fired Thermal
P o w e r P l a n t s a n d F u t u reProspects
TAKAO HASHIMOTO*1

YOSHINORI TANAKA*1

MASAHIKO HOKANO*2

DAIJIRO HIRASAKI*1

Against the background of soaring crude oil prices, coal, which has a comparatively low unit price per caloriﬁc power and asteady price with less ﬂuctuations, is expected to be used more widely in the future. On the other hand, the annual electric power
generated in Japan by thermal power plants using fossil fuels such as coal, LNG and petroleum in 2005 was approximately 60% of
the total power, so that control of CO2 emissions from thermal power plants is the most important factor for limiting greenhouse effectgas emissions. Since coal amounts to about 25% of the annual power generation, it is necessary for countermeasures against global
warming to take measures to limit CO2 emissions through highly efﬁcient power generation.

This most sophisticated coal-fired thermal power plant
has the efﬁciency of 43% at generator terminal (HHV base),and reduces CO 2 emissions intensity by 3% of conventional
plants.
1.1 Turbine
Figure 2 shows the world’s largest 600 MW class turbine
which is a two-casing type and consists of a combined casing
for a high-pressure turbine and an intermediate-pressure
turbine, and a single low-pressure turbine casing. The highpressure and intermediate pressure turbines adopt hightemperature materials and cooling structures of provendesign, to withstand the high-temperature steam conditions,and the blade rows and overall frame size are optimizedin order to secure the performance and reliability of theshaft dynamics, realizing a compact design. Further, highperformance and highly reliable technology is used in the1. S t a t u s o f h i g h l y e f f i c i e n t c o a l - f i r e d p o w e r
generat ion technolog y
For conventional thermal power plants, each unit capacity
has been increased and high-temperature and high-pressure
steam conditions have been promoted to improve the thermal
efﬁciency as shown in Fig. 1.
The Hirono No. 5 Thermal Power Station of Tokyo ElectricPower Company is a coal-ﬁred thermal power plant adopting
the ultra super critical ( USC) conditions of 24.5 MPa ×
600/600°C, the highest level in the world, and has continued
highly rel iable operat ion since it st a r ted commercial
operation in July, 2004.

: Temperature: Pressure

610°C
600°C

500
400

538°C
24.5MPa
24.1MPa

593°C
40
31.0MPa

16.6MPa300
2001910 1920 1930 1940 1950 1960 1970 1980 1990 2000
Year302010Steam pressure (MPa)Steam temperature (°C)

566°C

600Fig. 1 Trends of steam conditions in thermal power plants
The current steam conditions are improved to a pressure of 31 MPaand a temperature of 610°C.

Fig. 2 External view of Hirono No. 5 steam turbine

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 1 (Mar. 2008)*1 Power Systems Headquarters*2 Nagasaki Shipyard & Machinery Works11

low-pressure turbine, such as a 48-inch steel ISB (integralshroud blade), a high-performance exhaust hood, and a bearing
base structure directly supported by the foundations.
As for the high-temperature materials, the new 12Cr
forged steel is used for the high-and-intermediate-pressure
turbine rotor, 12Cr forged steel is used for the inner casing,and 9Cr steel for the turbine inlet valves and inter-connecting
piping between the valves and the casing. The longitudinal
center section of the rotor as well as the blade grooves at theintermediate-pressure inlet section are cooled by passing
steam from the control stage outlet to the intermediatepressure turbine along the rotor. In addition, a thermal
shield is installed so that the inner surface of the outer casingat the intermediate-pressure turbine is not directly exposedto the 600°C reheat steam, and cooling steam from the highpressure turbine exhaust is led into a space between thethermal shield and the outer casing.
1.2 Boiler
T he boiler shown in Fig. 3 adopts low NOx and low
unburned carbon combustion technology applying an A-PM
(advanced-pollution minimum) burner and MRS (Mitsubishi
rotary separator) pulverizer for Mitsubishi circular ﬁring in
addition to the vertical-tube furnace using high-temperaturematerial and riﬂed tubes.

However, there are many issues to overcome such as thedevelopment and production of new materials suitable forhigh-temperature and high-pressure use, the development of
manufacturing technology and a design which is reliable andeconomically feasible. The development schedule is foreseen
as shown in Fig. 4. A period of nine years is set for theproject starting from the elementary development including
material development, to veriﬁcation tests such as ﬁeld tests.
A period of ﬁve years is allotted for demonstration tests, withthe details not yet determined.
2.1 Material development
In order to withstand the 700°C steam conditions, it isnecessary to develop materials unlike conventional materials,materials with ultra high-temperature strength. Since theferrite steel and austenite stainless steel used in conventional
USC are not suitable for 700°C temperatures, the applicationof Ni or Fe-Ni-base alloy with sufficiently high allowablestress as the main materials of A-USC boilers and turbines isindispensable.
Turbine manufacturers have independently developed
various materials as candidate materials for turbine rotors,and MHI has developed LTES700R, a Ni-base alloy with lowthermal expansion.
As for boiler materials, Ni-base alloys such as Alloy617,CCA617, Haynes230, Alloy740 and Nimonic263 in addition toHR6W, an Fe-Ni-base alloy currently under development inJapan, are candidates.
The development of these materials requires the storageand evaluation of long-term creep data and thermal fatigue
data in addition to knowledge about their characteristics
over long-term use under high temperatures for enhanced
reliability. In addition, since expensive Ni-base alloys areused, it is important to minimize scope by optimizationand to establish a design method that ensures economic
efﬁciency.

2. Development of the next generation, advanced
ultra super critical plant (A-USC)
With the limitation of CO2 emissions becoming an urgent
problem in recent years, increasing efficiency by further
increasing steam conditions (higher in temperature andhigher in pressure) of thermal power plants are an effectivemeans of reducing CO 2 emissions. As in Europe andAmerica, there is also a movement in Japan to promote, asa national project, the development of 700°C class A-USCs,
with the net power plant efﬁciency aimed at 46% (HHV base).

Fig. 3 Side view of Hirono No. 5 boiler

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Technical Review Vol. 45 No. 1 (Mar. 2008)12

2.2 Development of manufacturing technology
In order to apply these materials, the development of
manufacturing technology including processing, welding,and inspection is required.
(1) In order to use Ni-base alloy for turbine rotors, themanufacturability of large steel ingots of Ni-base alloy
has to be improved. From the economic point of view, it isnecessary to establish welding and inspection technology
for the welded sections to realize a welded rotor using Nibase alloy in the high-temperature section, and high Cr
steel at both ends, which are exposed to comparatively
low temperatures.
(2) An Ni-base alloy or an Fe-Ni-base alloy should be usedinstead of ferrite or austenite steel for the main steam
pipes and reheat steam pipes, and bending technology
for materials used under high temperature and highpressure is required. In addition, overall verification isneeded, such as the development of weld materials forjoints made of these materials and dissimilar metals, theestablishment of welding methods, the optimization ofwelding conditions, knowledge of the material structure,

1styearSystemdesignSystem design anddevelopmentof design implementationMaterial
development
BoilerElementarydevelopment
Turbine2ndyear

3rdyearstrength and toughness of the joints, heat treatment
processes and weld inspection methods.
2.3 Consideration of structure(1) Since Ni-base alloy and Fe-Ni-base alloy will be used forthe main steam pipes and reheat steam pipes, the thermal
expansion will be larger than for conventional ferrite
steel, calling for attention to the piping routes and system
conﬁguration.
(2) Since the valves will be exposed to high temperatureand high pressure steam, it is necessar y to secure
structural reliability such as the operating and shut-off
performances of the sliding sections and the durability ofthe sealing section under high thermal stress conditions.
(3) Since the desuperheaters (attemperators) within theboiler are exposed to low-temperature spray water underhigh-temperature steam under a cycling thermal stressenvironment, attention must be paid to the structure andthe nozzle material.
(4) The turbine also needs its structure developing which
includes cooling and reducing the thermal stress inthe high-temperature section in addition to material

4thyear5thyearr

6thyear7thyear8thyear9thyear

Basic design, layout optimizationand economic evaluation

Development of new materials forlarge-diameter pipes and heat-exchange
tubes, and material modification
High-temperature, long-term, material tests (30,000 - 70,000 hours)

Verification ofmaterial
manufacturability

Establishment of welding
technology, tests and bending
tests

Structure andelementary
technology

Development of elementary
technology including cooling andsealing

Materialdevelopment

Selectspecifications ofmaterial

100,000 hours

Prototype of full-scale
components

Large-scale welding technology andprototypes of rotors, casings, etc.
High-temperature, long-term, material tests (30,000 - 70,000 hours)
Hightemperature
valves

Structural,elementaryand material
development

Field tests and rotation tests(including high-temperature valves)

Prototype design

100,000 hours

Manufacturing of prototype

Basicdesign

DetaildesignManufacturingand installation

Test andevaluation

Demonstration
plantFig. 4 National project schedule for A-USC technical development (plan)

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Technical Review Vol. 45 No. 1 (Mar. 2008)13

development. Study of a cooling system suitable for thestrength of the Ni-base material is also necessary.

3. Conclusion
The continuous research and development of new materials
and manufacturing technology, which are indispensable to
Japan’s energy security for coal-ﬁred thermal power and torealize the high steam conditions of A-USCs, is an urgent
issue. Thus, innovative technical developments are required
to improve the thermal efficiency of thermal power plants
and to ensure CO2 reductions, contributing to the preventionof global warming.

Takao Hashimoto

Yoshinori Tanaka

Masahiko Hokano

Daijiro Hirasaki

References
1. Monma, H. et al., Commencement of the Commercial Operation of600 MW Unit, "Hirono No.5 Thermal Power Station of The Tokyo
Electric Power Co., Inc.", Mitsubishi Heavy Industries TechnicalReview Vol.41 No.5 (2004)
2. Accomplishment Report by The Japan Society of MechanicalEngineers Special Interest Group for 700°C Class Advanced Ultra
Supercritical (A-USC) Thermal Power Generation Technology,
P-SCD353 (2007)

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 1 (Mar. 2008)14