Relationship between Shape of Raceway and Productivity of

Blast Furnace Taking Account of Properties of Coke

Sampled at Tuyere Level*

By Tetsu NISHI,** Hiroshi HARAGUCHI,** Yoshiaki MIURA,** Satoshi SAKURAI,** Katsuya ONO** and Hideo KANOSHIMA***

Synopsis

When the productivity of No. 2 blast furnace at Sakai Works, JV'ippon Steel Corporation, was cut down from 2.08 tad • m3 because of the economic deterioration in Japan in the latter half of 1970's, coke was sampled at the tuyere level seven times in the course of the cutdown. On the basis of the test result of the properties of coke sampled, the fol-lowings were concluded:

(1) As the production rate decreases, the point of the maximum tem-perature in the raceway moves toward the tuyere and the temperature of the dead-man decreases.

(2) Coke becomes fragile because of the long time exposure to high temperatures at the lower part of the furnace. Therefore, the amount of coke fines in the raceway increases with increasing kinetic enemy of blast, and in consequence, gas flows upward rather along the wall side than in the center. On the other hand, the decrease in the amount of coke fine, which is brought about by the decrease in the kinetic energy of blast, results in a smooth operation which is attributed to the uniform gas distribution. How-ever, in the case of lower kinetic energy of blast, the temperature of the dead-man decreases and, consequently, softened and half-melted materials form in the region.

I. Introduction

To cope with the prolonged substantial cutbacks in crude steel production since the oil crisis, Sakai Works of Nippon Steel Corporation was compelled to reduce the output from its blast furnaces. These blast fur-naces were operated at low productivity levels as had scarcely been experienced before; that is, the mini-mum productivity (monthly average) was 1.39 t/d • m3

(December, 1978) at No. 1 BF (inner volume: 2 501 m3) and 1.23 tjd • m3 (August, 1978) at No. 2 BF (inner volume : 2 797 m3). The Japanese steel industry had experienced such low productivity levels also in the recession of 19623, when the minimum productivity was 1.0 tad • m3 at a 1 000 m3 class BF, 1.2 tfd • m3 at a 1 500 m3 class BF, and 1.4 t f d • m3 at a 2 000 m3 class BF.'} At that time, as a measure to adapt operations to the low productivity levels, the tuyere diameter was reduced to maintain the blast velocity at the tuyere on a normal level. As a result, operations of the above-mentioned low productivity were possible, but increases in the fuel rate could not be avoided. In the blast-furnace operations of low productivity

at Sakai Works, an attempt was made this time to maintain the fuel rate on a normal level. The mea-sures taken included the improvement of the distribu-tion of gas flow and burden, nitrogen enrichment, reduction of oil volume, improvement of permeability

mainly by the improvement of the quality of charge materials and improvement of the reduction efficien-cy. As a result, the fuel rate was 448 kg/t-pig (aver-age of blast furnaces) in May, 1978 and 446 kgf t-pig in February, 1979. Thus, the low-productivity oper-ations suitable for the production cutbacks were established.

In the analysis of blast-furnace operations, it is im-

portant to understand the behavior of the raceway. There are reports by Gotlib,2~ the Iron and Steel In-stitute of Japan,3~ Nakamura et a1.,4 Hatano et a1.,5 and Tate et a1.6''> The behavior of coke near the raceway is important also when the quality of blast-furnace coke is evaluated. Therefore, this matter has been minutely examined by the dissection of blast furnaces.8'9~ However, the dissection of blast fur-naces is, so to speak, the dissection of dead bodies and knowledge that can be obtained by this means is limited. Accordingly, the blast-furnace operations have recently been analyzed at Nippon Steel Corp. by sampling coke from blast furnace tuyeres during shutdowns and investigating the properties of coke in comparison with coke before charging, with emphasis laid on the behavior of the raceway.10 14) To analyze the behavior of the raceway during the above-men-tioned operations of low productivity and low fuel rates at Sakai Works, samples of coke were taken seven times (from December, 1975 to September, 1978) from the No. 5 tuyere at No. 2 blast furnace. This report describes the results of the analyses of the

properties of these samples.

II. State of Blast-furnace Operations When Sampling Was Made from Tuyere

The blast-furnace operations during this period characterized by the following measures were taken to mitigate the decrease in the raceway size associated with the decrease in the kinetic energy of blast : (1) reduction of the tuyere diameter, (2) installation of tuyeres with a 10 deg declination, (3) nitrogen blow-ing, and (4) change of the charging method to con-trol extreme gas flows at the wall. Table 1 shows the main indices of blast-furnace operations when sampling was carried out seven times. At Sakai Works, the reduction_ of blast volume is adopted as the principal means of reducing balst furnace output. The blast volume for No. 2 blast furnace was reduced from 4 116 Nm3/min (productivity: 2.08 t/d • m3) in

**

Originally published in Tetsu-to-Hagane, 66 (1980), 1820, in Japanese; Formerly presented to the 99 th ISIJ Meeting, April 1980, at The University of Tokyo in Tokyo. English version received April 17, 1981. Process Technology R & D Laboratories, Nippon Steel Corporation, Edamitsu, Yawatahigashi-ku, Kitakyushu 805. Sakai Works, Nippon Steel Corporation, Chikko-yawata-cho, Sakai 590.

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December, 1975 to 2 501 Nm3/min (productivity: 1.34 tad. m3) in September, 1978. In this case, the tuyere blast velocity was increased

from 257 to 288 m/s by reducing the tuyere diameter from 140 to 130 mm to prevent the raceway from be-coming shallow because of reduced blast velocity at the tuyere caused by the reduction of blast volume. However, when the blast volume is decreased for fur-ther productivity reduction, the tuyere blast velocity is maintained by increasing the nitrogen volume, not by reducing the tuyere diameter. As a result, the tuyere blast velocity is almost the same as the high

productivity level of 2.08 tad • m3. The fuel rate de-creases with decreasing productivity. It decreased from 481 kgf t-pig at a productivity level of 2.08 t/ d • m3 to 478 kg/t-pig at a productivity level of 1.34 t/ d. m3. The charged coke has crushing strengths

(DI') of 83.8 to 84.6 and this property is almost con-stant. However, the mean size of coke is somewhat large in the case of low-productivity operations.

III. Experiment Method

1. Sampling of Coke from Tuyere

The blow pipe and tuyere proper at No. 5 tuyere were removed and a sampler was set there. After sampling, the sampling pipe was extracted and both ends of the pipe were closed with taphole clay. The

pipe was then cooled from the outside by water spray. After cooling, the upper half of the pipe was cut open and the inside was observed and photographed. After that, the entire cored sample was divided into equal lengths of 200 mm and used as the samples for various tests. The sketch and specification of the sampler are shown in Fig. 1 and Table 2, respectively.15~ The

sampling rate is as shown in Fig. 2 and

proximately 75 % as shown below:

Length of cored sample in pipe (m)

averages ap-

Penetration of sampling pipe into x 100 = 75 the blast furnace (m)

Open spaces are found near the tuyeres. Coke keeps moving round in the raceway when the blast furnace is in operation, and it cannot be said that all the sam-

ples taken are the coke stayed within the raceway dur-ing the operation. It is probable, instead, that coke

Table 1. Operation results of Sakai No. 2 BF.

Table 2. Specification of sampling apparatus.

Fig. 1 Apparatus for sampling from tuyere. (unit: mm)

Transactions ISIJ, Vol. 22, 1982 (289)

which has descended during the shutdown is included. However, since it is impossible to make a distinction

between both, the samples taken were treated as the

coke within the raceway in analyzing the data this time. As described below, the samples taken by the

sampler contain also metal and slag and, therefore, coke was separated from them and taken out to in-

vestigate its properties. The coke samples thus sep-arated are hereinafter referred to as " coke from tuyere."

2. Procedure for the Preparation of Cored Investigation of Coke Properties

1. Preparation of Cored Samples

As mentioned above, slag, metal and

Samples

coke

and

were

separated and the entire sample divided into equal lengths of 200 mm, in accordance with the procedure

illustrated in Fig. 3. Coke was further classified by

grain size. The position of each sample is such that it corresponds to the sample length starting from the

sample end that is nearest to the tuyere opening. The

position of samples is expressed as " the distance from the tip of tuyere." 2. Investigation of Coke Properties The items of the coke quality measurement are

proximate analysis,16) ultimate analysis,l7) composi-tion of ash,18) reactivity, porosity,19> microstrength,2o) and drum strength by I-type drum.21)

3. Estimation of Furnace Temperature and Raceway 1. Furnace Temperature

The temperature in the furnace was estimated from the graphitization of the coke from tuyere. The charged coke was first reheated to various tempera-tures. Then, the relationship between the reheating temperature and the half value breadth* of (002) line spectrum of graphite obtained by X-ray diffraction (powder method) of coke treated was found. Based on this result, the calibration curves shown in Fig. 4 were prepared. As shown in this figure, graphitiza-tion varies greatly depending on the reheating condi-tions. The furnace temperature was estimated from the calibration curve III shown in Fig. 4, since it was

judged at the dissection of Kukioka No. 4 blast fur-nace that a mixed charge of coke and ore shows the best correspondence to the furnace temperature. Be-fore the X-ray diffraction, the samples were subjected to HC1+HF treatment to remove Si09 from ash and the effect of Si02 on the diffraction profile of (002)

Fig . 2. Amount of sample in the sampl ing pipe.

Fig. 3.

Procedure for the preparation of

cored sample and the size analysis.

* Measurement of graphitization

Diffraction was carried out using an X-ray diffraction device (made by Shimazu Seisakusho, Ltd., XD-5) and the half-value breadth of (002) line spectrum was measured.

Measuring conditions : Target: Go, Filter: Fe, Voltage: 35 kV, Current: 20 mA, Time constant: 1 s, Divergence slit: 0.5 deg, Receiving slit: 0.3 mm

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( 290 ) Transactions ISIJ, Vol. 22, 1982

line spectrum of graphite was eliminated in this man-ner. 2. Raceway

Two types of raceway are conceivable; a physical raceway (to be estimated from the piling structure) and a chemical raceway (to be estimated from tem-

perature and gas composition).2~ This time, the chemical raceway was estimated. The central point of the curve showing an abrupt change in the raceway temperature was defined as " the wall of raceway" as shown in Fig. 5 and the raceway depth was ex-

pressed by the distance from the tip of tuyere to this

wall. Further, the area from the tuyere to the wall of raceway was considered to be within the raceway and the area from the wall of raceway toward the furnace center was defined as the dead-man.

Iv. Experiment Results

1. Appearance and State of Sampling

Figure 6 shows the appearances of examples and the sampling rates of coke, metal and slag, the size distribution of coke, and the temperature in the fur-nace estimated. As is apparent from an examination of the coke from tuyere, dark brown coke is loosely

piled on the tuyere side, followed by medium-size coke to which metal and slag adhere, roundish me-dium-size and fine coke, then black coke fines, and large-and medium size coke in this order. This tend-ency of size distribution is almost constant although

Fig. 4. Relationship between the half-value breadth of

(002) line spectrum and the reheating temperature of coke sample.

Fig. 6 (a). Example of the size distribution and amount of

coke in the sampling pipe at a high productivity.

Fig. 5. Definition of th e depth of the raceway.

Fig. 6 (b). Example of the size distribution and amount of

coke in the sampling pipe at a low productivity.

Transactions ISIJ, Vol. 22, 1982 (291)

there are some differences depending on the operat-ing conditions.

The samples of coke from tuyere taken this time are characterized by that softened and half melted mate-rials as shown in Photo. 1 are present in the samples

(E, F and G) taken from March 31, 1978 onwards (productivity: 1.49 t/d.m3). This may be caused by the fact that the temperature of the dead-man has decreased with decreasing productivity. The amount of small-size coke of -25 mm accounts for approxi-mately 60 % when the productivity of the furnace ranges from 1.8 to 2.2 t/d.m3, but it decreases with decreasing productivity to approximately 50 % and the mean size of coke in the furnace increases. The coke, metal and slag ratios of sample are 60 to 80 %, 20 to 30 % and less than 10 %, respectively, when the productivity of the furnace is 1.8 to 2.2 t/d. m3. The coke ratio of sample decreases to a level of 20 % with the productivity of less than 1.50 t/d.m3 because of the presence of softened and half-melted materials in the area 1.4 m from the tip of tuyere toward the dead-man.

2. State of Breakage of Coke

Figure 7 shows the state of generation of --3 mm coke fines of all the samples. The ratio of --3 mm coke fines increases in the sequence of A, B and C as the productivity of the furnace decreases from 2.08 to 1.70 t/d. m3 and the maximum ratio has a tendency to move toward the tuyere. However, when the produc-tivity of the furnace decreases further (D -~ E -~ F -+ G 1.58 t/d.m3 -~ 1.34 t/d.m3), this tendency reverses itself; that is, the ratio of -3 mm coke fines decreases

and the maximum rate moves toward the innermost of the raceway.

3. Properties of Coke from Tuyere The results of the investigation of the properties of coke sampled from the tuyere were arranged in terms of the relation to the productivity of the furnace. 1. Size Figure 8 shows the relationship between the pro-ductivity of the furnace and the mean size of coke. This result reveals that the mean size of coke in the furnace increases with decreasing productivity. What is especially conspicuous is the fact that the rate of +25 mm lump coke which seems to be present in the area of the raceway 0.6 to 0.8 m from the tip of the tuyere increases when the productivity of the furnace is below a specific level (1.6 t/d . m3). This seems to have a close relation to the operating conditions of the blast furnace, which will be described later. 2. Strength Figure 9 shows the relationship between the pro-ductivity of the furnace and the strength of coke (by I-type drum). At the productivity of below 1.5 t/ d.m3, the coke strength decreases about 5 % on the average. 3. Other Coke Properties Figure 10 shows the relationship between the pro-ductivity of the furnace and other properties of coke

Photo. 1. Softened and melted

(productivity in Sept.

materials

11, 1978:

in cored sample. 1.34 t/d-m3 )

Fig. 7. Change of

from the tip

- 3 mm coke fines with

of tuyere.

the distance

Fig. 8. Relationship

nace and the

level.

between the productivity of

mean size of coke sampled

the fur-

at tuyere

Fig. 9. Relationship

nace and the

level.

between the

strength of

productivity of the fur-

coke sampled at tuyere

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from tuyere. When the productivity of the furnace is below approximately 1.6 t/d • m3, the rate of in-crease in the reactivity is high and the rate of increase in the MSI (micro-strength index) is low. This sug-

gests the degradation of coke. The amount of alkalis in coke increases, since the alkalis deposited on coke above the tuyere level volatilize in smaller amounts with decreasing tuyere flame temperature associated with the low-productivity operations. As mentioned above, an investigation was made into the properties of coke sampled from the tuyere. As a result, it was found that the size of coke in the furnace increases with decreasing productivity of the furnace. However, no clear relationship was found between the state of generation of --r3 mm coke fines and the productivity. It is presumed from the prop-erties of coke such as the strength that the deteriora-tion of coke aggravates when the residence time* of coke in the furnace increased as the productivity of the furnace decreases. Figure 11 shows the relation between the productivity of the furnace and the re-sidence time of coke in the furnace, which was found on the basis of the operational data given in Table 1.

4. Behavior of Raceway

Figure 12 shows the temperatures in the furnace estimated from all the samples. The furnace temper-atures estimated, both the maximum and the mini-mum, tend to decrease as the productivity of the fur-nace decreases in order of A, B, C, etc. Further, the

position where the temperature drop begins, moves toward the tuyere as the productivity of the furnace decreases. As a result, the depth of the raceway de-creases from 1.7 m to approximately 0.6 m, as shown in Fig. 13. Moreover, the temperature of the dead-man decreases because of the decrease in the produc-tivity. This seems to lead to the formation of the above-mentioned softened and half-melted materials.

5. Softened and Half-melted Zone at Dead-man

When the productivity of the furnace decreases and the temperature of the dead-man drops (E, F and G), a zone of softened and half melted materials, as men-tioned above and shown in Photo. 1, is observed in the dead-man. The temperature of the dead-man at this time is estimated to be below 1 200 °C, not high enough for the burden of the blast furnace to melt completely and drop. Unreduced burden materials may have descended to the dead-man.

Fig. 10. Relationship between the productivity of the fur-

nace and the properties of coke sampled at the

tuyere level.

Fig. 11. Relationship between the productivity of the fur-

nace and the residence time of coke in the

furnace.

Fig. 12. Distribution of the

at tuyere level.

temperature along the radius

Transactions ISIT, Vol. 22, 1982 (293)

V. Discussion

On the basis of the above-mentioned results, some considerations were tried in connection with the blast-furnace operating methods.

1. Relationships between BF Operating Methods and the Properties of Coke from Tuyere and the Behavior of Raceway during Shift to Low Productivity Levels

When the productivity of the furnace was reduced from 2.08 to 1.72 t/d • m3, the tuyere blast velocity was increased from 257 to 286 m/s by reducing the tuyere diameter from 140 to 130 mm, and the depth of the raceway was maintained in this manner. As shown in Figs. 6 and 7, however, the ratio of -3 mm coke fines increased when the tuyere blast velocity was in-creased. In addition, the raceway became unexpect-edly shallow.

Therefore, in the blast-furnace operations at low

productivity levels carried out thereafter, the tuyere diameter was kept constant at 130 mm in spite of a decrease in the blast volume, and the blast velocity at tuyere was decreased at the expense of some raceway depth. From these results, the changes in the coke

properties at tuyere and the condition of the raceway were expected to be influenced by the blast velocity at tuyere, or the kinetic energy of blast. The coke

properties and the conditions of raceway were studied with the blast velocity at tuyere as a variable. The results of this consideration are shown in Fig. 14. The formula for calculating the kinetic energy of blast used here is that proposed by Okabe.22~ As is apparent from these results, coke at tuyere level is

pulverized and the amount of coke fines increases when the furnace is operated with a large kinetic energy of blast (a hard blast)* to secure the raceway depth as the productivity of the furnace decreases. So, when the kinetic energy of blast is decreased (a soft blast)* to avoid the increase in the amount of coke fines, the degree of pulverization of coke at tuyere level becomes small and the mean size be-comes large. However, the strength of coke decreases in this case.

These phenomena may be explained by the fol-lowing interpretation : although coke deteriorates

greatly and becomes fragile because of the long resi-dence in the furnace caused by the reduction of pro-ductivity, it is not crushed in the raceway because of the small kinetic energy of blast and large grains re-main within the furnace. To investigate this point further, values of raceway depth calculated are ex-

pressed as relative ratios on the basis of a high produc-tivity level (2.08 t/d•m3). Table 3 shows the relation between the ratio of raceway depth and the blast velocity at tuyere. As is apparent from Table 3, the depth of the raceway must increase with increasing blast velocity at tuyere. However, the depth of the raceway estimated from the temperature distribution in the raceway decreases with decreasing productivity on both sides of a specific level of kinetic energy of blast shown in Fig. 15, that is, both with a soft blast and with a hard blast. On the other hand, the con-dition of coke sampled from tuyere shows a consider-able difference between both cases, as can be seen from Fig. 14. The reason why the raceway decreases in spite of a large kinetic energy of blast may be that coke that has become fragile while descending in the furnace is crushed and pulverized by the physical force of the kinetic energy of blast and, therefore, the blast cannot easily reach the deep part of the raceway.

Fig. 13. Relationship between the productivity of the fur-

nace and the depth of raceway.

Fig. 14. Relationship between the kinetic energy of blast

and the properties of coke sampled at tuyere level.

* For convenience of explanation,

called a soft blast.

a blast with large kinetic energy is called a hard blast, whereas a blast with small kinetic energy is

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(294) Transactions ISI7, Vol. 22, 1982

The reason why the raceway decreases with decreas-

ing kinetic energy of blast may be caused by a physi-

cal phenomenon that the kinetic energy of blast is

insufficient. Thus the same decrease in the depth of the raceway was found to be brought about by very

different causes.

2. Gas Flow in Raceway Considered from Mechanism of Generation of Coke Fines

As described above, at low productivity levels the

properties of coke sampled, in particular the amount of -3 mm fines generated, and the condition of the raceway were different from those experienced in the conventional operations. So, some considerations were given to the mechanism of generation of coke fines (-3 mm coke fines) and the behavior of the raceway. It is reported2~ that the gas composition in the

raceway generally levels off with high CO2 until the furnace wall at raceway is reached, when CO2 sud-denly drops to almost nil shown in Fig. 16. From this, it is considered3,4> that the CO2 generated by the combustion of coke reacts with coke near the wall of raceway to form CO, leading to the deterioration of coke, with the result that coke fines are generated by the impact exerted when coke moves around in the raceway. Therefore, it might be thought that when the solution losses are large the temperature drop in the deep part of the raceway becomes large because of the endothermic character of this reaction and that the amount of coke fines generated at the wall of raceway becomes large.

In the process in which the operation was shifted to low productivity levels this time, the above-men-tioned relationship was not observed between the gen-eration of coke fines and the temperature in the deep area of the raceway (the dead-man), as is apparent

from Fig. 17. The temperature in the deep area of the raceway (the dead-man) was found to increase when the amount of coke fines generated is large. To explain this condition, schemes are shown in Fig. 18, in which the interpretation described in Section V. 1 is incorporated. In the process in which the operation is shifted to

a low productivity level, the residence time of coke in

Table 3. Relationship between the tuyere blast ve-

locity and the ratio of raceway depth.

Fig. 15. Estimated depth of the raceway.

Fig. 16. Change of gas composition in the raceway.2

Fig. 17. Relationship between - 3 mm coke fines and the

temperature of dead-man at tuyere zone.

Fig. 18. Schematic drawings of the pilin

the gas flow at the tuyere part.g state of coke and

Transactions Is", Vol. 22, 1982 (295)

the furnace is long and the amount of alkalis trapped is large because of the low furnace temperature at tuyere level. For these reasons, coke is more sub-

jected to the solution loss reaction near the dropping zone and the softened and half-melted zone, and de-scends to the tuyere part in a fragile state. In this case, the raceway decreases in size by the generation of coke fines resulting from the crushing of coke if the kinetic energy of blast is increased by reducing the tuyere diameter according to the reduced blast vol-ume in order to secure the depth of the raceway. As a result, the gas flow becomes nonuniform and the

gases show a stronger tendency to flow along the wall side. On the other hand, if the kinetic energy of blast is reduced without changing the tuyere diameter in spite of the reduced blast volume to prevent such a phenomenon, the kinetic energy of blast in the race-way becomes small and this brings about the reduc-tion of raceway size. However, unlike the case of the reduction of raceway size caused by the generation of coke fines, the size of coke at the wall of raceway and in the dead-man is large and, therefore, the gases flow uniformly also to the furnace center. Thus it was found from the condition of the raceway that the same

phenomenon of the reduction of raceway size has a large different effect on the gas flow in the furnace depending on its mechanism.

3. Relationships between Behavior of Raceway and BF Operating Indices

Figure 19 shows the results of an investigation of the relationships between the kinetic energy of blast and the blast-furnace operating indices (fuel rate

[F.R.] and utilization rate of top gas ['ico] ). When the kinetic energy of blast is increased, the gases flow nonuniformly along the wall side, as mentioned above. This phenomenon can also be presumed from the fact that as shown in Fig. 20, the temperature of stave coolers increases when the kinetic energy of blast is increased. Since the fuel rate increases and the utili-zation rate of top gas decreases, the operational re-sults are not very satisfactory. On the other hand, if the kinetic energy of blast is reduced, the gas flow be-comes uniform, resulting in a decrease in the fuel rate and an increase in the utilization rate of top gas. Therefore, the results of this blast-furnace operation are satisfactory.

However, as mentioned above, softened and half-melted materials are apt to be formed since the tem-

peratures in the raceway and the dead-man drop as the productivity of the furnace decreases. It is dif-ficult for such softened and half-melted materials to be completely melted at lower positions. These ma-terials to be completely melted at lower positions. These materials may not be completely melted unless they come into contact with melted iron or slag. It was found at the dissection of Oita No. 1 blast furnace that in a large blast furnace projections are formed in the furnace bottom when low-productivity operations are carried out. One of the causes of such projec-tions in the furnace bottom may be the softened and

half melted materials in the dead-man. Once pro-

jections are formed in the furnace bottom, it takes a long time to melt them. Therefore, if additional heat is given to the furnace in order to increase the produc-tivity abruptly during a stable furnace operation at a low productivity level, the temperature of the hearth wall will rise before the softened and half melted ma-terials in the dead-man are completely melted, thus

posing a problem in the furnace control. It is dif-ficult to shift abruptly the furnace operation from a low to a high productivity level. Thus it seems to be undesirable from the standpoint of blast-furnace operations that softened and half melted materials are formed in the furnace center when the furnace is oper-ated at a low productivity level.

VI. Conclusion

When the operation of No. 2 blast furnace at Sakai

Works was shifted to the low-productivity operation, coke at tuyere level was sampled and its properties

were investigated. Based on the results of this in-

vestigation, an examination was made as to the rela-

Fig. 19. Relationship

and the fuel

gas, ~Ico•

between

rate and

the

the

kinetic energy of blast

utilization rate of top

Fig. 20. Relationship between kinetic energy of blast and

the temperature of stave-cooler at the lower part

of the shaft.

( 296 ) Transactions ISIJ, Vol. 22, 1982

tionships between the kinetic energy of blast and the condition of the raceway and coke properties. The following results were obtained:

(1) As the productivity of a blast furnace de-creases, the residence time of coke within the furnace becomes long and coke becomes fragile compared with the case of high-productivity operations. If large kinetic energy is given to coke by increasing the kinet-ic energy of blast in order to secure the depth of the raceway, the amount of coke fines generated increases and these coke fines accumulate in the deep area of the raceway. This brings about gas flows along the wall side and, therefore, has an adverse effect on the blast-furnace operations.

(2) If the kinetic energy of blast is decreased, the kinetic energy of blast in the raceway becomes insuf-ficient and the raceway size becomes small. In this case, however, the generation of coke fines is prevent-ed and the size of coke in the deep area of the raceway is large. Therefore, the gas flow is uniform, which produces a good effect of blast-furnace operations.

(3) Reducing the kinetic energy of blast during a low-productivity operation is a desirable measure from the standpoints of the condition of the raceway and gas flow. However, when this measure is taken, the temperature of the dead-man drops, resulting in the formation of softened and half-melted materials. The softened and half-melted materials may be one of the causes of projections in the furnace bottom.

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Research Article