72 nd IEA-FBC, Budapest, Hungary, Apr.28-29, 2016 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
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 Agitation of bed Mechanical force to shear droplets during solidification Agitator M Control is easy (by rotation rate). Heat-resistant metal is necessary. Erosion problem. Fluidizing air Fluidizing air Fluidized bed Screw transporter M Motor
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. This Fluidized work Bed AIR Melted WAX slag AIR Downcomer AIR AIR Riser B A
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)
Experimental
Cold model FB main body Height :60 cm Cross sec.:16 cm sq 0.51g/s 80 Wax Riser Gas vel: 0.076m/s Bed material Silica balloon Downcomer Different height of downcomer D p.: 0.37 mm 247mm Dens.:870kg/m 3 Air Slag (wax) 1-Hexadecanol M.P. 49 Dens.: 800 kg/m 3 Air Small FB 447mm
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 FB Air Downcomer Riser ID :9mm Length:600mm,800mm Small Fluidized Bed
Measurement of transportation rate Solid sampling at the exit of riser Downcomer Riser Effect of design parameters and gas feed rate on solid transportation rate Air Air Air
Injection of solid-gas to falling wax 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. Wax 0.51g/s, 80 Air Air Downcomer Riser
Solid transportation rate Width of Small-FB = 20 mm FB Downcommer Max flow >U mf 90mm >U t Riser 70mm 20mm 2U t 20mm There existed optimal gas feed rate condition to maximize solid transportation rate.
Solid transportation rate Width of Small-FB = 40 mm FB Downcommer Max flow 90mm 1.5U mf 2U t Riser 70mm 40mm 40mm Optimal gas feed: Riser gas vel. = 2U t ; Fluidizing velocity =1.5 U mf
Solid transportation rate Width of Small-FB = 90 mm FB Downcommer (E) Max flow 90mm Riser 90mm 2U mf 90mm 70mm When the bottom fluidized bed was too big, solid transportation did not occur even when gas velocity in riser was higher than U t.
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 For designs A-C (narrower fluidized beds at the bottom), solid flow rate differed among designs. Pressure drop in the horizontal direction was measured. Downcomer Riser Solid flow 12 L/min P
Horizontal pressure drop in small FB Effect of gas feed rate on horizontal pressure difference 12 L/min 装置幅 20mm 装置幅 30mm 装置幅 40mm Pressure difference at a gas flow of 12 L/min Narrow bed Wide bed Reverse direction For narrow small FB in the bottom, pressure gradient in reverse direction was observed. Resistance
Design of solid transportation Higher downcomer is favorable to provide head for solid transportation. Riser gas velocity : about 2U t Fluidization in bottom small FB at about 1.5U mf Appropriate width of bottom small-fb
Solid injection into falling melted wax Type C system, gas feed rate =12 L/min (Optimum condition to maximize solid transportation rate Reduction of coarse clinkers Size control possible.
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
吹き付け量測定 下降管が長い方が吹き付け量が多くなった 下降管の延長により 下降管と上昇管との圧力差が大きくなったため
流動化速度に対する吹き付け量 装置幅 20mm 装置幅 30mm 装置幅 40mm 装置幅 60mm 装置幅 90mm 最小流動化速度
流動化速度に対する圧損 装置幅 20mm 装置幅 30mm 装置幅 40mm
上昇管圧力 ( 装置 C)
上昇管と下降管の圧力差 ( 装置 C)
結果と考察
粒子質量速度 [g/s] 上昇管ガス速度 [m/s] 気固 2 相流質量流量 12 10 8 6 4 2 0 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 最大流量 0.00 0.02 0.04 0.06 0.08 流動化ガス速度 [m/s] 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 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
粒子質量速度 [g/s] 上昇管ガス速度 [m/s] 気固 2 相流質量流量 12 10 8 6 4 2 0 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 粒子がうまく上昇しなかった 0.00 0.02 0.04 0.06 0.08 流動化ガス速度 [m/s] 流動化ガス速度 上昇管ガス速度の関係を変えて検討 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
粒子質量速度 [g/s] 上昇管ガス速度 [m/s] 気固 2 相流質量流量 12 10 8 6 4 2 0 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 下降管 447mm 下降管 247mm 0.00 0.02 0.04 0.06 0.08 流動化ガス速度 [m/s] 吹きつけ駆動力となる粒子ヘッドが増加したため 下降管が長いほうが吹きつけ量が多くなった 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
粒子質量速度 [g/s] 上昇管ガス速度 [m/s] 気固 2 相流質量流量 12 10 8 6 4 2 0 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 0.00 0.02 0.04 0.06 0.08 流動化ガス速度 [m/s] 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 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
Mass fraction [%] 吹きつけによるせん断効果 100 90 80 70 60 50 40 30 20 10 0 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) <3.36mm 3.36~6mm 6mm~10mm 10mm< Clinker size 吹きつけにより粗大クリンカーの発生を抑制
結言 流動媒体の吹きつけ量を測定し 吹きつけ量と流動化速度 上昇管速度との関係を示した 連続的に滴下される溶融ワックスに気固 2 相流を吹きつけることによりワックスを細かくせん断することができた 吹きつけに必要な空気量は総空気量の約 10% でプロセス運転にかかるコストとして問題ないと思われる