UDC 669.162.263.43:669.162.263.23

Burden and Gas Distribution Considering Blast Furnace

Aerodynamics*

By Kiichi NARITA,** Shin-ichi INABA,** Masakata SHIMIZU,** Arata YAMAGUCHI,** Isao KOBAYASHI** and Ken-ichi OKIMOTO**

Synopsis

The transitional behavior of burden and gas distribution in the blast

furnace was studied aerodynamically by a blast furnace model. When ore is charged under a gas flow, a coke column (hereinafter referred to as the central coke column) forms in the central region. On the basis o f the experimental observation, the formation process of the column consists of the following three steps.

(1) With the progressive covering of the stack surface by the flow of ore from the periphery to the center, the furnace gas concentrates to the central region.

(2) The coke layer expands upward in the region where the gas velocity exceeds the minimum fluidization velocity of the coke layer.

(3) Some of the coke lump in the expanded layer are pushed into the central part by the flow of charged ore.

The central coke column is formed more easily by pellet charging rather than by sinter charging. The gas velocity in the central part has been related to the thickness and area of ore layer and the permeability ratio of ore to coke, and it has been expressed by the following equations.

Uc = 1IAc Aw Uc UT AT + AT Uw

U H ] ' 0.778 A -0.483 n Co ~ ~ c U

w R K2 AT

where, A is the sectional area, U, the gas velocity, H, the ore thickness, R, the throat radius, K1 and K2i the permeability indices of coke and ore, respectively, C0 and n, constants, suffix C and W, the central region without ore and the peripheral region covered with ore, respectively, and suffix T, the throat. The radius of central coke column has been estimated from three relations to be about 0.58 m.

The pressure drop in the stack of blast furnace is decreased by the

formation of a central coke column, while the distribution of gas through the coke layers in the softening-melting zone hardly changes.

I. Introduction

In previous studies on the burden distribution at the blast furnace top carried out under the static con-dition, it is reported that the central gas flow is re-strained by the increase of ore/coke at the central

part when a large quantity of pellets is charged. Recently, in dessectioned blast furnaces with many

pellets charged,"2~ no pellets were observed at the central part, and the central gas flow was indicated to be strong. Especially, in the dessection of Amaga-saki No. 1 BF,2~ the central part of small-coke layer expanded over the upper two layers. This suggests that the coke was fluidized at the central part. The mechanism of coke fluidization in the central part has been phenomenally explained by Nishio and Ariyama,3~ but the fundamental factors have not suf-ficiently been analyzed.

In this work, the charging behavior of burden, especially formation of a central coke column, has

been investigated aerodynamically by a model furnace under the practical conditions, and change of top gas

velocity distribution during ore charging has been

analyzed. Furthermore, the influence of central coke column on the gas flow through the cohesive zone in

the furnace has been studied.

II. Distributions of Burden and Gas Velocity

1. Experimental Method

The experimental apparatus is shown in Fig. 1. This is a 1/20 model of Kakogawa No. 1 BF and 180° segment (throat diameter : 430 mm). The charging level can be kept constant by a table feeder ® set at the bottom. The stack surface profile and the layer thickness are measured every charging. Air is blown from tuyeres. The distribution of gas velocity in the throat is measured by four anemometers QQ , which are ra-dially placed 80 mm above the stack surface. The

pressure distribution in the stack is measured by pressure taps {o . Experimental conditions are as follows : 1) Charging materials : 2 ' 4 mm pellet, 3 5 mm

sinter and 5 10 mm coke 2) Coke base : 1.52 kg/charge (Average layer thick- ness is about 40 mm at the throat.) 3) Weight ratio of ore to coke : 4.0 4) Charging method : bell charging 5) Average top gas velocity: 0, 0.4, 0.8, 1.0 and

1.2 m/s

Fig. 1. Experimental apparatus.

*

**

Originally published in Tetsu-to-Hagane, 66 (1980), 459, in Japanese. English version received May 9, Central Research Laboratory, Kobe Steel, Ltd., Wakinohama-cho, Chuo-ku, Kobe 651.

1980.

Research Article (405)

(406) Transactions ISIJ, Vol. 21, 1981

6) Charging level: 130 mm (At this level, the ore/ coke distribution is similar to that of full size

model. )4)

2. Charging Behavior

Photograph 1(a) shows the effect of top gas velocity on charging behaviors of coke and ore. Under the no gas flow condition, the layer surface angle of coke by CS~ is nearly equal to that of sinter, and each layer thickness is uniform in radial direction. In the case of the ore layer thickness in the central part is remarkably increased because of a small in-clination of pellet layer. Layer surface angles de-crease with the increase of the top gas velocity. At 0.8 m/s of top gas velocity, the mixed layer of coke and ore begins to be observed in the central part . In the range of 0.8 1.2 m/s, the coke layer is ex-

panded in the central part at the time of ore charging in both cases of CS~ and CJ,P~. The expanded coke is pushed into the central part by charged ore, and a central coke column is formed. Once a central coke column is formed, the supply of ore into the cen-ter is restrained and ore layer thickness increases in the intermediate and peripheral regions. At 1.2 m/s, the coke column is very large and some of the coke is violently spouted and piled on the ore layer to form a flat surface.

Figure 2 shows the effect of top gas velocity on the radial ore/coke distribution. In both cases of C~SJ and with increase of top gas velocity, the ore/ coke ratio decreases in the central part and increases in the periphery. For the ore/coke ratio at the central part is very small above 1.0 m/s of top gas velocity. Photograph 1(b) shows appearances of the central coke column when ore is charged twice (C~O1~O2~) at 1.0 m/s of top gas velocity. Comparing with Photo. 1(a), it is found that twice charging of ore is easier to form the central coke column. The volume of central coke column is the largest for and decreases in order of C~S~S~, CAP J P~, C~ (S • P)-

(S • P)~ and Here, (S • P)~ means mixed charging of equal weights of sinter and pellets. Figure 3 shows the distributions of top gas velocity and ore/coke ratio. The change of charging sequence from CS~ to C~S~S~ makes the ore/coke ratio in-crease at the peripheral part. This is caused not only by the spread of central coke column due to the twice charging of ore but also by the change of layer surface angle depending on the charging volume of burden.4 In other words, the twice charging raises the inclina-tion and increases the peripheral layer thickness to such a degree enough to lower the permeability, and hence, the gas flow is concentrated in the central

Photo. 1.

Appearance of

tion.

burden distribu-

Transactions ISIJ, Vol. 21, 1981 (407)

region and the fluidization of coke is widely developed. Exchanging the charging sequence from C~S~S J to C P~ S J , the central coke column spreads more widely as shown in Fig. 3. Pellet layer has such a small inclination that ore/coke is decreased at the periphery and increased remarkably at the intermediate.

The radial distribution of top gas velocity is closely related to that of ore/coke. The difference of top

gas velocity between and is recognized in the periphery and the intermediate. In both cases of CPS~ and C~SJ S~, a very large central column is formed, and the gas velocity is large in a broad area of the central part. For C~ P J P J , however, the gas velocity at the center is small in spite of the formation of a large central coke column. This reason is that

pellets sink into the void of the central coke column during the descent of stack and act as a resistance against gas flow.

III. Change of Central Gas Velocity at Ore Charg- ing

1. Experiment

The progressive covering of the stack surface with ore is modeled as shown in Fig. 4. The radial dis-tribution of gas velocity above the stack is measured under the steady state. The particles of permeability index K1 are piled with height H and area AW on the stack of permeability index K2 (K1 >K2) and surface angle. The permeability index K is defined in the following equation.5j

dP/L = U2-~ ..................(1)

where,

K--Gi -( 1 1+p (1 ~ ~~ 1+~ i

. ---- --

gc ~5s,DP ~3

Fig. 2. Effect of top gas velocity on ore/coke distribution.Fig. 4. Schematic model

central part Uc.

for estimation of gas velocity in

Fig .3. Radial distribution of ore/coke and gas in various charging sequences. (Ur= 1.0 m/s)

( 408 ) Transactions ISIJ, Vol. 21, 1981

In order to achieve the experimental condition in Fig. 4 and to keep the ore layer from corumbling, the progressive side of ore layer is supported by cylindrical parting strips. The radial distribution of

gas velocity above the stack is measured by anemo-meters at four radial positions. The permeability index K1 and K2 are estimated by substituting the

pressure drop in the packed bed in Eq. (1), because it is difficult to measure the parameters of Eq. (1)'.

jS is obtained as 0.23 from the experiment of the packed bed. K1 and K2 are varied by the particle diameter (Dr). Uc and UW are the averages of gas velocity in the area covered and not covered with ore, respectively.

2. Experimental Results

Figure 5 shows variations of top gas velocity dis-tribution, which are observed during the progressive covering of the stack surface with ore of a constant layer thickness. Immediately after charging ore, the radial distributions of the velocity have a peak at the intermediate region, and the gas velocity at the cen-tral part slightly changed. The enhancement of gas flow in the intermediate region is caused by the decrease of the permeability in the peripheral region where ore was piled. After the stack surface was covered with ore, the peak shifts toward the center. When the peak arrives at the center, the central gas velocity has a maximum. In the case of average top

gas velocity above 0.8 m/s, the coke layer is spouted in the central region. The change of central gas velocity at the time of ore charging is analyzed as follows. It is assumed that the gas velocity changed in the radial direction depending on the permeability and that the gas pressure is uniform at a distance L below the stack surface. From Fig. 4, Uc and UW are expressed as Eqs. (2) and (3) in the case of 0=0°.

Uc-p = 4P/(K2•L•~R• pl-~) ...............(2)

UW P = 4P/{(Ki•H+K2•L)•p~•pl-~} .........(3)

From the material balance

Uc 1 tAG + (Aw/(U0 UT AT AT UW

Applying L=R6~ to Eqs. (2) and (3), Uc/UW in Eq. (4) is expressed as a function of K1/K2 and H/R.

Ucl UW = f (K1/K2- H/R) ..................(5)

The relation of Eq. (5) has been experimentally studied, and is shown in Fig. 6. The data points are represented by straight lines in a logarithmic scale, and the lines are parallel in the vertical direction. Considering the area of ore, AW(=AT-Ac), Uc/UW can be expressed as Eq. (6), whose relation is shown in Fig. 7 (marked o).

Uc H K1 0.778 Ac -0.483 UW R K2 AT

The right side of Eq. (6) is called the biased gas coef-ficient. Figure 7 also shows that Eq. (6) can be applied to experimental results for 0=15° and 25°. Uc/UW is finally arranged as Eq. (7).

Uc H K, 0.778 Ac -0.483 n

W 2

where Ca and n depend on 0 as shown in Fig. 8. Equation (7) indicates that the ratio of the average gas velocity in the central region to that in the peripheral region is dependent only on the piling con-dition of ore and that the permeability ratio and is independent of the gas flow rate. Relative gas ve-

Fig. 5. Changes of gas velocity

ing.

distribution at ore charg-

Fig . 6. Effect of permeability ratio (H/R)(K1/K2)°•778 on gas velocity ratio of central and peripheral part Ucf

UW.

Fig. 7. Effect of biased gas coefficient (H/R)(K1/K2)o.778

(Ac/AT)-o.aas on gas velocity ratio of central and peripheral part Uc/ UW.

Transactions ISIJ, Vol. 21, 1981 (409)

locity at the central section (Uc/UT) is obtained by substituting Eq. (7) to Eq. (4). As shown in Fig. 7, Eq. (7) is accurately applied for biased gas coefficients larger than 0.1 without fluidization of coke.

Iv. Discussion

1. Comparison between Observed and Calculated Gas Ve- locities in the Central Region

On applying Eq. (7) to the practical aerodynamic behavior, it is necessary to estimate the average layer thickness (H) of charged ore. In this work, H and Ac are measured from the photographs taken at a 0.2 s interval during ore charging. Figure 9 shows the comparison between the observed and the cal-culated gas velocities in the central region. It is found that (U)cal is slightly higher than (Uc)°bS since the void fraction of ore layer at the time of charging is larger than that at stationary state.

2. Estimation of Coke Fluidization Area in the Central Region of Blast Furnace

The fluidization area of coke layer at the time of

pellet charging is estimated from Uc derived from Eqs. (4) and (7) and the incipient fluidization velocity of coke layer. The permeability indices of the coke and pellet layers are calculated from particle size distributions in Table 1 by a method of Fukutake and Okabe.'~ The calculated results are given in the right end column of Table 1. On using Eq. (7), Kl was considered as the permeability index of pellet, and K2 as the average value of pellet and coke. It is assumed that the inclination of coke layer (8) is 30° and ore slides along the coke surface. H/R was varied from 0.02 to 0.16. Figure 10 shows the relation between the relative gas velocities at the central part (Uc/UT) and the area covered with pellet (Aw/Ar). The incipient fluidiza-tion velocity (Umf) of the coke in Table 1 is estimated to be 2.6 m/s using the equation presented by C. Y. Wen and Y. H. Yu8~ for the top gas temperature= 120°C and top gas pressure=2.0 kg/cm2. From Fig. 10, when Uc = 2.6 m/s and UT= 1.0 m/s, the fluidi-zation area of coke layer is r/R=0.48 at H/R=0.10.

3. Estimation of Central Coke Column Volume in the Blast Furnace

To estimate the volume of central coke column in the blast furnace, the formation process of the column is modeled as Fig. 11 based on the experimental ob-

Fig. 8. Effect of 8 on Co and n.

Fig, 9. Comparison between observed

velocity in central part.

and calculated gas

Fig. 10, Variation of Uc/ UT at pellets charging.

Table 1. Particle size and permeability index of coke and pellets.

(410) Transactions ISIJ, Vol. 21, 1981

servation. In Fig. 11, wde is the surface of coke layer just

before the charging of ore. The surface of coke col-umn is assumed to be flat. At the time of ore charg-ing, the furnace gas is concentrated in the central region owing to the progressive covering of the stack surface from the periphery to the center by charged ore (a - + b -f c). When ore reaches point c, the coke layer is expanded within the area of radius r where the gas velocity exceeds the incipient fluidization ve-locity. The expanded coke layer (cdg f) is pushed into the central region by the charged ore and is reformed as a central coke column of radius R~. The volume of coke layer with radius r before expan-sion, V~ (cdeijk), is expressed as Eq. (8).

Vc = 7rr2• {law-(R-r).40} - --(r3-R~)•4B ...(8) 3

lcw = Vc~+ (R3-R3)•48 /7rR2 ............(9) 3

40 = tan B~ - tan Bo

The expanded volume, 4 V~, is :

4V~ = a• V. ........................(10)

where a is the expansion ratio of coke layer. The void fraction of the expanded layer (e1) is related to the friction before expansion (so) as Eq. (11).

El = (E°+a)/(1+a) .....................(11)

The volume of central coke column, V~, is obtained as Eq. (12).

V~ = 7rR~{low+(R-R~)•48} ...............(12)

low = VaT-'r (R3-3•R•R~+2R~)•4a l~(R2-R~) 3

........................(13)

Assuming that the coke in the expanded portion of a coke layer gathered to the central region with a maxi-mum void fraction (emax), Eq. (14) is derived.

(1-Emax)V' = (1-~L)•4Vo = a.(1-er)• V,......(14)

Defining Y as Eq. (15), V is written as Eq. (16).

l-1 a 1-s0 r= a.---- = -.- - 1-S max (1 +a) 1 _ amax

V' - c -' r V ~ c (16)

The radius of a central coke column (Re) can be estimated from Eqs. (8), (9), (12), (13), (15) and

(16) by the knowledge of s, emax and a. It is well known that the void fraction of a stationary coke layer (E°) is about 0.5. The maximum void fraction is obtained by substituting the maximum expansion ratio amax to Eq. (11). amax was measured from the height of the packed bed which was once fluidized and shut down. Figure 12 shows variations of amax and amax with the particle diameter (Dr) of coke. The maximum void fraction of coke bed with par-ticles above 10 mm diameter is about 0.57 regardless of the bed length. Meanwhile, by evaluating R~, 40, low, law and r in Eqs. (8), (12), (15) and (16), the expansion ratio a is estimated as follows.

a = 0.113-0.134

Where R~, 48, low and lcw were measured from Photo. 1 and r is determined from Eq. (7). In the case of

a=0.123, is evaluated as 0.126 from Eq. (15). Applying these results to the blast furnace conditions in Table 2, the radius of central coke column is finally estimated as R~ = 0.58 m.

4. Influence of Central Coke Column on the Gas Flow in Blast Furnace

When a central coke column is formed in the blast furnace, the layer structure in granular zone is varied and the gas flow distribution is changed by the forma-tion of a softening-melting zone without a top cohe-sive layer.9~ In this investigation, by the blast furnace model in which simulated softening-melting layers were placed, the influence of the central coke column on the gas flow distribution has been studied.

Fig. 11. Schematic

column.

drawing on formation of central coke

Fig. 12. Effect of particle diameter on maximum expansion

rate and void fraction of coke bed.

Table 2. Conditions for

coke column.

estimation of region of central

Transactions ISIT, Vol. 21, 1981 (411)

1, Experiment

As shown in Fig. 13, pellet (1 '..'5 mm dia. ), coke

(5-.10 mm dia.) and plastic plates to simulate soft-ening-melting layers were used around a central coke column of radius R~ in the model of Fig. 1. The softening-melting zones of types A and B are of inverted V shape with different height. Air was blown at 3.6 Nm3/min from tuyeres, and the static

gas pressure distributions were measured with probes of 3 mm diameter. The gas flow distribution in the furnace can be estimated from the pressure distribu-tion. 2. Effect of Central Coke Column on Pressure Distribu- tion

Figure 14 shows the effect of the central coke column on the radial distribution of static gas pres-sure. The gas pressure in each coke slit is high at the inner side and low at the outer side in both types A and B. With increase in the radius of central coke column (R~/R), all the pressure lines shifted in

parallel to lower pressures, and the total pressure drop in the furnace decreased.

Figure 15 shows the vertical pressure distributions. In this figure, Pinner and Pouter are the axial pressure

distributions at the center and at the periphery, re-

spectively. Three pressure curves at the same posi-tion are drawn so that they overlap each other below the middle of the shaft, because they are parallel in

this part. From Fig. 15, it is found in both types A and B

that the formation of a central coke column decreased

the pressure drop only in the upper granular zone and had little influence on the drop in the lower zone

where the cohesive layers exist. The decrease in

pressure drop in the granular zone is resulted from the concentration of gas flow to the central coke col-

umn. Considering that the pressure drop in vertical and horizontal directions did not significantly change

in the lower part, it is supposed that a cross flow took

place in the upper granular zone.

Fig. 13. Schematic drawing of softening-melting zones with

different height in model.

Fig. 15. Effect of the central coke column

on pressure distributions in vertical

direction.

Fig. 14. Radial distribution of static pressure in coke slits.

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3. Effect of Configuration of Softening-Melting Zone on Pressure Distribution Figure 16 shows the effect of the configuration of

the softening-melting zone on the pressure distribu-tion in the vertical direction. It is found that the

transformation of the softening-melting zone profile influenced only on the pressure distribution of the

lower part and not on that in the upper granular zone. This means that gas leaving from the soften-

ing-melting zone was rapidly redistributed in granular

zone, while the gas volume through the coke slit de-

pended on the configuration of the softening-melting zone. The pressure drop in the lower part of the

shaft is higher in type B than in type A. From these results, it is realized that the softening-melting zone

plays a role of the buffle plate against the gas flow and that the rise of the top cohesive layer does not always decrease the pressure drop in the furance.l0~

4. Effects of the Central Coke Column and the Softening- Melting Zone on Gas Flow Distribution

The gas flow distribution through the softening-melting zone is shown in Figs. 17 and 18. ` Slit gas' and ` peripheral gas ' stand for the gas volume through the coke slit in the softening-melting zone and the uprising gas in the peripheral granular zone, respec-tively. The volume of each slit gas was estimated by substituting the value of the pressure drop in a coke slit in Eq. (1) for jl = 0.23, and the volume of the

peripheral gas is the sum of all the slit gas volumes up to a given height. From Fig. 17, it is found that the central coke column had little effect on the gas dis-tribution in both types A and B.

Meanwhile, as shown in Fig. 18, gas distribution from coke slits is markedly influenced by the con-figuration of the softening-melting zone, and the gas volume passing through the lower slits in type A is larger than that of type B.

Fig. 16. Effect of the softening-melting

zone profile on pressure distribu-

tions in vertical direction.

Fig. 17. Effect of central coke column on gas volume

through coke slits in softening-melting zone and

on uprising gas volume in peripheral zone.

Fig. 18. Effect of softening-melting zone profile

volume through coke slits in its zone

uprising gas volume in peripheral zone.

on

and

gas

on

Transactions ISI1, Vol. 21, 1981 (413)

From these results, it is suggested that the gas flow in the lower part of the furnace is mainly determined by the configuration of the softening-melting zone and is distributed so as to keep the horizontal pres-sure distribution uniform at the top level of the cohesive zone. It is concluded that the central coke column decreases the total pressure drop in the blast furnace and has little influence on the gas distribu-tion in the lower part of the blast furnace.

V. Conclusion

The behavior of charged burden has been studied aerodynamically by a model furnace, particularly concerning the formation of a central coke column. The column volume varied depending on the perme-ability of charged ore and the charging sequence. The change of gas velocity in the central part at the time of ore charging has been analyzed considering the permeability and piling profile of ore. The pres-ence of a central coke column decreased the pressure drop in the furnace. It seems that a central coke column is formed in the practical blast furnace.

Nomenclature

Ac, AW : sectional area of central part without ore and peripheral part with ore covered, re-

spectively (m2) AT : sectional area of throat (m2) Go, C1: constant, respectively (-) g~ : gravitational conversion factor (kg • m/Kg . s2)

H: thickness of ore layer (m) K1, K2 : permeability index defined in Eq. (1)

L : distance from stack surface (m) law, low : layer thickness of coke and ore, respectively,

at the periphery (m) AP : pressure drop (Kg/m2)

R : throat radius (m) R~ : radius of central coke column (m) r: radial distance (m)

Uo: superficial gas velocity based on empty column (m/s)

Umf : incipient fluidization gas velocity of coke layer (m/s)

UT: average gas velocity at throat (m/s) Vc : volume of coke layer with radius r (m3) Vc, Vo : charging volume of coke and ore, respec-

tively (m3) a : expansion ratio of coke layer (-)

Amax : maximum expansion ratio of coke layer

(-) i : coefficient depending on Reynolds number

Eo, E1: void fraction of coke layer before and after expansion, respectively (-)

Emax : maximum void fraction of coke layer (-)

11 : viscosity of gas (kg/m• s) p : density of gas (kg/m3)

oS : shape factor of particle (-) 8: inclination of stack surface (°)

oc, Bo : surface inclination of coke and ore layers, respectively (°)

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