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Heat recovery from melted blast
furnace slag
– Control of clinker size by particle
spraying into falling melted slag –
Kouta Suzuki, Tadaaki Shimizu
Niigata University, Japan
72nd IEA-FBC, Budapest, Hungary, Apr.28-29, 2016
Blast furnace slag production in Japan
Production of blast furnace slag
290 kg-slag/1000 kg iron
Annual production of slag is 24 Mt.
For one large scale blast furnace, production rate of
melted slag (at about 1500℃) is 100 – 150t/hour.
Solidified by natural cooling in atmosphere or rapid
cooling by water. Heat recovery has not yet been
done.
Potential of melted slag as heat source
Melted slag at 1500℃ has sensible and latent heat
of about 400 – 500 cal/g.
If half of the heat of melted slag is to be recovered
in Japan, the total heat recovery will be equivalent
to about 1 million tonnes of coal.
=0.5% of annual coal import of Japan
=1.1% of coking coal consumption of Japan
Problems with heat recovery from melted slag
Phase change from liquid to solid by cooling.
Accumulation of solid slag on to heat transfer
surface if solidified.
Insufficient heat recovery if heat removal in from
only liquid phase (sensible heat of liquid) is to
be done.
Heat recovery after solidifying (cooling) reduces
temperature difference
Need to new technology to heat recovery
Fluidized bed heat recovery system • Melted slag is
dropped into the bed
↓
• Solidification in bed
and heat transfer to
immersed boiler
tubes
↓
• Removal of solidified
slag from the bottom
↓
• Crushing a part of the
solidified slag and
use as bed material
Importance of size control of slag droplets
• If the size of slag
droplets is too big,
they sedimentation
velocity will be too
fast.
• Insufficient heat
recovery, too hot
materials in the
bottom, difficulty in
withdrawal.
• Control of size is
necessary.
Control of size (1) Agitation of FB
Screw transporter
Fluidized bed
Motor
Fluidizing air
Fluidizing
air
Agitator
M
M
Agitation of bed
•Mechanical force to
shear droplets during
solidification
•Control is easy (by
rotation rate).
•Heat-resistant metal is
necessary.
•Erosion problem.
Control of size (2) Solid injection
Injection of solid-gas
flow to melted wax
• Rapid cooling and
covering slag by
particles (“dry”
surface)
• Application of CFB
technology.
• High rate of solid
transportation is
necessary.
WAX
AIR
Fluidized
Bed
Down-
comer Riser
B
A
AIR
AIR AIR
Melted slag
This work
Objective of this work
Injection of solid-gas two-phase flow using cold model.
●Measurement of solid transportation rate to
find out suitable design of transportation
system
●Control of solid size using simulated melted
slag (wax)
Cold model
FB main body
Bed material
Silica balloon
Dp.: 0.37 mm
Dens.:870kg/m3
Slag (wax)
1-Hexadecanol
M.P. 49 ℃
Dens.: 800 kg/m3
Wax
Down-
comer
Air
Riser 0.51g/s
80℃
Height :60 cm
Cross sec.:16 cm sq
Gas vel: 0.076m/s
Air
247mm
447mm
Small FB
Different
height of
downcomer
Cold model (transportation system)
Small-FB
Height:70mm
Length:90mm
Width:A:20mm, B:30mm,
C:40mm, D:60mm,
E:90mm
Downcomer
ID:20mm
height:247mm,447mm
Riser
ID :9mm
Length:600mm,800mm
Air
Down-
comer
Riser
Small Fluidized Bed
FB
Measurement of transportation rate
Down-
comer
Air
Air
Riser
●Solid sampling at
the exit of riser
●Effect of design
parameters and gas
feed rate on solid
transportation rate Air
Injection of solid-gas to falling wax
Down-
comer
Air
Air
Riser
Wax 0.51g/s, 80℃ ●Melted wax (simulated
slag) was continuously
fed to fluidized bed
●Solid-gas flow was
injected to wax above
the bed
●Clinkers were sampled
after the experiment.
Size distribution was
measured by sieving.
Solid transportation rate
Width of Small-FB = 20 mm
There existed optimal gas feed rate condition to
maximize solid transportation rate.
70mm
20mm
90mm
20mm
FB
Riser
Downcommer
>Ut
Max flow >Umf
2Ut
Solid transportation rate
70mm
40mm
90mm
40mm
FB
Riser
Downcommer
Optimal gas feed: Riser gas vel. = 2Ut;
Fluidizing velocity =1.5 Umf
1.5Umf
Width of Small-FB = 40 mm
2Ut
Max flow
Solid transportation rate
70mm
90mm
90mm
90mm
FB
Riser
Downcommer
When the bottom fluidized bed was too big, solid
transportation did not occur even when gas velocity
in riser was higher than Ut.
(E)
Width of Small-FB = 90 mm
Max flow
2Umf
Effect of design on transportation rate
There was an
optimal width of
bottom small-FB
to maximize solid
transportation.
Width: A:20mm
B:30mm
C:40mm
D:60mm
E:90mm
Resistance of solid horizontal flow
12 L/min
For designs A-C (narrower fluidized beds at the bottom),
solid flow rate differed among designs. Pressure drop in
the horizontal direction was measured.
Solid flow
Downcomer Riser
⊿P
Horizontal pressure drop in small FB
Reverse
direction
For narrow small FB in the bottom, pressure gradient
in reverse direction was observed. Resistance
Pressure difference at
a gas flow of 12 L/min
12 L/min
装置幅20mm
装置幅30mm
装置幅40mm
Effect of gas feed rate on horizontal
pressure difference
Narrow bedWide bed
Design of solid transportation
Higher downcomer is favorable to provide
head for solid transportation.
Riser gas velocity : about 2Ut
Fluidization in bottom small FB at about 1.5Umf
Appropriate width of bottom small-FB
Solid injection into falling melted wax
Reduction of coarse clinkers Size control possible.
Type C system, gas feed rate =12 L/min (Optimum
condition to maximize solid transportation rate
Conclusion
As a measure to control solidified clinker size for
a fluidized bed heat recovery system, injection of
solid-gas two-phase flow was proposed
There was an oprimum design and operation
condition to maximize solid transportation for
solid-gas injection.
Size of clinkers can be controlled by injecting
solid-gas two phase flow to falling melted wax in
the freeboard.
吹き付け量測定
Fig.5 Concept of the present process
ある点を過ぎると吹き付け量が低下する。
→下降管に空気の逆流が発生。
層内への空気流量の増加により、上昇管内の圧力損失が大きくなったからだと考えられる。
0
2
4
6
8
10
12
0.00 0.02 0.04 0.06 0.08
流動化ガス速度 [m/s]
粒子
質量
速度
[g/
s]
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
上昇
管ガ
ス速
度 [
m/s
]
粒子質量速度(下降管247mm)粒子質量流速(下降管447mm)最小流動化速度終端速度上昇管ガス速度
気固2相流質量流量
最大流量
最小流動化速度以上で、上昇管ガス速度が終端速度を超えるように設計できた。
0
2
4
6
8
10
12
0.00 0.02 0.04 0.06 0.08
流動化ガス速度 [m/s]
粒子
質量
速度
[g/
s]
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
上昇
管ガ
ス速
度 [
m/s
]
粒子質量速度(下降管247mm)粒子質量流速(下降管447mm)最小流動化速度終端速度上昇管ガス速度
気固2相流質量流量
流動化ガス速度、上昇管ガス速度の関係を変えて検討。
粒子がうまく上昇しなかった
0
2
4
6
8
10
12
0.00 0.02 0.04 0.06 0.08
流動化ガス速度 [m/s]
粒子
質量
速度
[g/
s]
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
上昇
管ガ
ス速
度 [
m/s
]
粒子質量速度(下降管247mm)粒子質量流速(下降管447mm)最小流動化速度終端速度上昇管ガス速度
気固2相流質量流量
下降管が長いほうが吹きつけ量が多くなった。 吹きつけ駆動力となる粒子ヘッドが増加したため。
下降管447mm
下降管247mm
0
2
4
6
8
10
12
0.00 0.02 0.04 0.06 0.08
流動化ガス速度 [m/s]
粒子
質量
速度
[g/
s]
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
上昇
管ガ
ス速
度 [
m/s
]
粒子質量速度(下降管247mm)粒子質量流速(下降管447mm)最小流動化速度終端速度上昇管ガス速度
気固2相流質量流量
流動化ガス速度の増加に伴い吹きつけ量が減少。 流量の増加に伴い、圧損が大きくなるため。
吹きつけによるせん断効果
0
10
20
30
40
50
60
70
80
90
100
<3.36mm 3.36~6mm 6mm~10mm 10mm<Clinker size
Mas
s fr
action
[%]
吹きつけなし吹きつけあり(下降管247mm)吹きつけあり(下降管447mm)
吹きつけにより粗大クリンカーの発生を抑制