Project Subjects
Utilization of Low Grade Fuels
Background and Objective
Coal supply and demand is going to be tight due to the increase of coal consumption in developing countries, and securing a coal supply is getting worse. This laboratory has been developing combustion technologies
to improve the availability of low grade fuels in a wide load range. To increase sub-bituminous coal (up to 30%moisture) utilization in a conventional power plant designed for bituminous coal, this project develops co-combustion technologies of bituminous and sub-bituminous coals which enables a high mixing rate of sub-bituminous
coal and stable combustion at low load. It also develops a countermeasure for sulﬁdation, evaluates fouling andslugging properties of ash, and clariﬁes the effect on the air pollution control facilities and trace element emissions, caused by the use of low grade fuels.

Main results
1. Co-ﬁring technology of low grade fuels with bituminous coal
The grinding characteristic of bituminous and sub-bituminous coals in a roller mill was investigatedto raise the blending ratio of sub-bituminous coal. The high moisture in sub-bituminous coal caused roller slip
during the milling process and resulted in low grindability comparing with bituminous coal. It was also clariﬁedthat the blending ratio of sub-bituminous could be improved up to 50% from 30% by maintaining the mill exit
temperature at 60°
C.
2. Countermeasures for sulﬁdation corrosion
A combustion control and coating technologies are under development as countermeasures which prevent
sulﬁdation corrosion of coal ﬁred boiler walls. Combustion tests using a coal combustion test furnace showed thatH2S formed between the burner and the two staged combustion air port (Fig. 1), and H2S concentration became
higher when low fuel ratio coals were used (Fig. 2) [M 09004, M 09010].
The coating was applied to some thermal power plants to test the effect on the prevention of sulﬁdation.
Some data was also obtained from the coating tests to verify the equation of corrosion rate. In addition, to support
the maintenance planning of boiler walls, a corrosion estimation program based on ﬂue gas compositions wasadded to the software which evaluated the sulﬁdation corrosion condition.
3. Characterization of ash adhesion properties
To evaluate ash adhesion characteristics on boiler tubes, the initial ash adhesion and the growth of attachedash layer were investigated using a cooling probe that simulated a boiler tube and an ash adhesion test piece without cooling, respectively (Fig. 3). Using these methods, the ash adhesion characteristics of various coals wereobserved in the coal combustion test furnace (Fig. 4).
4. Trace element behavior in coal combustion power plant
Estimation of trace element transfer from coal into coal ﬂy ash and ﬂue gas is useful for the selection
and the use of coal, which improve the environmental protection of the plant and the control of ﬂy ash quality.
Trace element behaviors, such as vaporization during combustion, transfer from gas to ash particles in ﬂue gas,chemical reactions in a wet FGD, were investigated to estimates the partition of trace elements in a power plant.

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Stable Power Supply Technology
400

Burner
バーナ

H2 S

H2S

1.0

Burner
バーナ

H2

2.0

[ppm]

3.0

H2S [ppm]

300

0.0

SK (Fuel ratio:1.01)
TH (Fuel ratio:1.00)
NL (Fuel ratio:1.98)
WB (Fuel ratio:2.17)

200
100

バーナからの距離 [m]000.01.0Burner
バーナ

CO1.0Burner
バーナ
0.0

1.5

22.5

33.0[%]

Fig. 2 H2S concentrations on the central axis of
the coal combustion test furnace obtained
from four different coals
Lower fuel ratio coals (SK, TH) showed higher H2S concentrations than higher fuel ratio coals (NL, WB).

3.0[%]

3.0

2.0

1Position from a burner [m]バーナからの距離 [m]

0.0

O2

2.0

0.5[%]

バーナからの距離 [m]

1.0

2.0
Position
fromaburner
バーナからの距離 [m]Coal ATest piece

Fig. 1 H2S, H2, CO and O2 concentration proﬁlesin the coal combustion test furnace for SK
coal
H2S was formed between the burner and to two stage combustion air port. H2S concentration proﬁle was similar to
those of H2 and CO.

Pulverized coal+Primary air

Video camera

Melted andsintered

Coal B
Test piece
Flow down

Furnace
exit

Combustion furnace

Cooling probe

Coal CVideocamera

Coolant

Test piece

Fragile・・・

Non-cooling test piece

Cross-section at the ash adhesion test position

Fig. 3 Ash adhesion test device installed in thecoal combustion test furnace.
The cooling probe simulates initial ash adhesion on boiler
tube surfaces. The non-cooled test piece was used toobserve the adhered ash layer on the test piece surfaces at
the furnace gas temperature.

Fig. 4 Ashes layers formed on non-cooling testpieces.
The differences in ash adhesion caused by ash melting
point, additives, etc. were observed. The ash of coal A
was sintered and adhered strongly to the test piece. Thecoal B ash was melted and ﬂowed down. The coal C ash
was not melted and was fragile.

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