BLAST FURNACE--BURDEN AND GAS DISTRIBUTION

(PROCESS TECHNOLOGY DIVISION) MONDAY AFTERNOON, APRIL 18, 1977

The session on Blast Furnace-Burden and Gas Distribution convened at 2:00 pm. The Chairmen were:

R. W. Bouman J. Szekely Bethlehem Steel Corp. MIT Bethlehem, Pa. Cambridge, Mass.

iron ores and coals. Presently, Japanese blast furnaces are running almost exclusively on im- ported ores and coals.

BURDEN AND GAS DISTRIBUTION '

IN THE BLAST FURNACE.

Shin Hashimoto, Akira Suzuki

and Hiromitsu Yoshimoto

Blast Furnace Department, Yawata Works

Nippon Steel Corporation.

53 blast furnaces are presently in operation in Japan, among them are included ten of large- scale blast furnaces with inner volumes of about 4,000 m3 or more. High productivity (over 2.0 t/m3/d), low fuel consumption (about 400 kg of coke and 60 kg of heavy oil per ton of iron) and hlgh cumulative iron production per single cam- paign are characteristics of Japanese blast furnaces.

Theoretical study of the behavior of solids, gases and liquids have provided reliable methods for the estimation of reaction in the blast fur- nace. Dissection of a few quenched blast furnaces have given us a fairly clear picture of what is going on in the operating blast furnace. The data have proved the importance of the pattern of the cohesive zone in the furnace.

The key to the high-efficiency operations of large blast furnaces, with proper protection of furnace walls, is to control the pattern of the cohesive zone, which directly influences upon the gas distribution on the lower part of the furnace, so that this pattern is maintained in a conical form with solid-geometrical uniformity in the circumferential direction of the furnace.

This paper reports on the large blast furnace operation, which incorporates these latest find- ings and makes full use of gas distribution measurements and movable armors, with primary emphasis on the followings:

(1) Recent large blast furnace operations,

( 2 ) Burden and gas distribution controls in large blast furnaces.

A. Introduction.

Since the early stages of its development, the Japanese iron- and steelindustry has made efforts to reduce coal consumption. The major motive of these efforts has been that there has never been an abundance of natural resources of

Since the post-war reconstruction period, the effort has been focussed on the coke saving in blast furnaces. For this purpose all the scientific knowledges have been utilized for analysis of the reactions taking place in the furnace. Large sums of investment were laid on the modernization of blast furnaces and burden preparation facilities that hadbbeen heavily damaged during the war. In the late 19501s, new blast furnaces with inner volumes over 1600 m3 were blown-in at the Tobata area of NSC's Yawata Works. Subsequently, a number of new blast fur- naces were built. The general tendency in this "modernization" has always been towards a smaller and smaller number of larger and larger produc- tion units. In the case of Yawata Works, 10 blast furnaces were in operation in 1968. pro- ducing a total of about 20,000 t/d at a fuel consumption of about 530 kg./t. In 1976, 3 blast furnaces are producing about the same amount of iron at a fuel consumption of about 470 kg./t. (Fig. 1)

This tendency towards a smaller number of larger production units has strengthened the requirements for the higher plant availability and the more stable production from individual blast furnace. Accordingly, expensive new ma- terials have come into use and many devices have come into use for primary diagnosis of the de- terioration of individual equipment in the earlier stages. These materials and devices caused the expansion of the investments on individual blast furnace. And this expansion, in turn, has spurred the tendency towards smaller number of larger production units. The large amount of investment for the individual blast furnace can be economically justified by the low operating cost and long campaign life with ex- tremely high cumulative production. The high production, (e. g. over 2 t/m3/d) low fuel con- sumption (about 460 kg./t) and high cumulative production (over 20 million tons or over 5000 t/m3/campaign) of the large blast furnace have thus enhanced the economic advantages of the process.

In the early 1 9 7 0 ' ~ ~ the first 10,000-t/d- blast furnaces, with inner volumes of around 4000 m3, were put into operation. At the end of 1976, the largest blast furnace in operation has an inner volume of over 5000 m3, and the standard size of "large blast furnaces" in Japan is about 4000 m3. (Fig. 2) Construction and operation of these large blast furnaces have been made possi- ble through extensive application of many recent developments in the various fields of science. However, the most important basis has been the recent development in the study of the reactions in the furnace.

From the viewpoint of reactions in the furnace, the general tendency has been towards higher and higher pressure and temperature. Top pressure and blast temperature rapidly rose in this period. At the same time, a number of devices came into practical use for monitoring

21 .ooo Product ion

. (t/d) 20.000

18.000

- 550 Fuel Consumption (kg/t)

14

Number 12

10

8

6

4

2

0

Hearth diameter

12

Number n=53 10

8

6

4

- 500

- 450

2

0 r

- 400

5 10 15 20 25 30 35 40 45 50 , , $ \ , , ' , I \ \ , 5 10 I5 20 25 30 35 40 45 50 55

(.. IO"m3) Inner volume

F i g . 2 Japanese blast furnaces in operation

Top pressure ( I / w )

- 1.500

- 1.000

- 500

- 0

t 15'00' Accidental shut-downs 10'00' (H /month /B~)

5'00'

Hangs

( t2h" ;BF I 1

1968 1969 1970 1971 1972 1973 1974 1975 1976 J a n - S e p t

Fig. l Blast furnace performance a t Yawata Works

and regulating the reactions in the furnace. These devices also required a considerable amount of investment on every blast furnace and have had something to do with the tendency towards the smaller number of larger units mentioned before. They have considerably changed the nature of the blast furnace operation. Large blast furnaces are presently operated with many data from inst- ruments monitoring the reactions in the furnace, and the burden distribution control has proved to be a reliable method for the elaborate control of reactions.

The performance of the blast furnaces has been greatly improved, productivity of individual furnaces increased, fuel rate decreased and large blast furnaces are smoothly running for months without hangs. In case of slack demand for iron, the blast furnaces are operated with decreased blast volumes for lower production rates without seriously imparing the fuel economy.

B. Reactions in the blast furnace.

1) General description

The blast furnace process is a kind of coun- tercurrent reaction process in the charge column. Charging, blowing, molten materials treatment and high pressure gas cleaning are four major phases of blast furnace operation.

The reactions in the blast furnace are not so much complicated, however, their control has remained for a long time in the realm of opera- tor's skill. The blast furnace could be operated only by skilled operators who changed charging or blowing condition on the basis of his own limited personal experiences. Until .recently, the theoretical calculations could find only limited practical applications. This was because of the lack of information on the behavior or mass of burden and coke in the furnace. Although -there remain so many things to be clarified in the future, we have just arrived at the gate of scientific operation of the blast furnace.

The dissection of a few quenched blast fur- naces between 1968 and 1971 had provided a fairly clear picture of the reactions in the charge - column; its temperature profile, procedure of reduction reaction, concentration of certain elements, etc. (1) s(5) On the basis of the dis- section, we can now.divide the charge column into five zones, i. e. granular zone, cohesive zone, active coke zone, raceway and hearth. (Fig. 3.) The granular zone.is the highest part of the column where both ores and coke retain the layer they had when they were charged. The cohesive zone consists of the coke layers and the "cohesive layer", a half molten mass of iron ore particles sticking to each other. The active coke zone consists exclusively of coke, among which the ~olten iron and slag drop down. The raceway is a partly vacant space where the coke burns with a violent agitation due to kinetic energy of the blast from the tuyere. ,The hearth is packed with coke soaked with molten metal and slag.

Fundamental study on the penetration of the gas through the bed of irregularly shaped solid materials has provided a practical method for the estimation of pressure drops in the v.irious parts of the charge column. Theoretical study on the heat and mass exchange between solids and gases has been of great use for the estimation of pro- cedure of heating reduction and melting of ores. The dissection of the blown-out blast furnaces have been of great help for the application of the recent theoretical findings to the practical operation of the blast furnace.

These theoretical formulae refer only to the reactions taking place uniformly in the entire cross section of the furnace. However, in the operating blast furnaces, the reactions do not and cannot take place uniformly. The dissection of quenched blast furnaces has amply clarified how the procedure of the reaction at the center is different from that at the periphery. Here lies the necessity to study the burden and gas distribution in the blast furnace.

2) Reactions in the blast furnace

Hot metal and slag are the products of a series of physical, thermal and chemical reac- tions. These reactions proceed differently in the five zone of the charge column; i. e. granular zone, cohesive zone, active coke zone, raceway and hearth. Even in the same zone, the reactions ~roceed differently from center to periphery. The heat transfer, chemical reduction and melting are the three major reactions that take place at individual zones under counter- current condition.

Countercurrent phenomenon.

Stable countercurrent condition is the most important basis of a smooth operation of the furnace. In the granular zone, the counter- current phenomenon refers to the downward move- ment of solid particles against the up-streaming gas. Here, Ergun's equation gives the pressure drop through the charge column. (6)

This equation gives also some suggestions for the improvement of blast furnace operation. Void fraction of the charge column at high tem- perature zones has the strongest effect on the stability of countercurrent condition. This has led us to the improvement of size analysis and room and high temperature mechanical strength of the burden and coke. Sintered ores are crushed and screened to sizes between 5 and 50 mm and coke to sizes between 25 and 100 mm before being charged into the furnace. Next important factor is the velocity of the gas. This has led us to higher and higher top pressure. As a result of these improvements, the gas velocity in the granular zone of the modern blast furnaces is being kept below 3 m/sec (superficial gas velo- city being below 1.5 m/sec) and the pressure drop

Fig. 3 F i v e zones in the blast furnace.

is being kept below 30 g/cm2/m.

The cohesive layer is a dense mass of semi- reduced iron ores that blocks the gas flow, therefore, the countercurrent in the cohesive zone refers to the re-distribution of the gas through coke-packed opening between cohesive layers, that we call "slits". A sufficiently large "slit area" is necessary for the stable countercurrent condi- tion and ultimately for the efficient operation of the blast furnace.

A sufficiently high void fraction is neces- sary both for the smooth countercurrent at the active coke zone and for the smooth flow of metal and slag in the hearth, too.

Heat transfer.

The temperature profile in the furnace can be estimated primarily by two factors; i. e. the theoretical combustion temperature (Tf) and the ratio of water equivalents of gas and solids (Ws/Wg).

The higher Tf means the greater temperature differences between the gas and the molten materials. Therefore, a higher Tf is always pre- ferable from the viewpoint of heat transfer. In the case of large blast furnaces, with large hearth diameters, the high Tf and the high wind velocity at tuyere are indispensable to keep the central part of the hearth at high temperatures. As a matter of fact, Tf slightly over 2300°C is thought to be necessary for the operation of the blast furnace at low fuel rates. At the same time, in the case of large blast furnaces, the wind velocity at the tuyere is usually kept - between 250 and 280 m/sec to ensure the kinetic energy at the raceway between 12,000 and 15,000 kg-rnlsec.

Ws/Wg determines the temperature profile in the furnace, especially that in the granular zone. As the fuel rate lowers, the Ws/Wg generally tends to increase. And the oxygen enrichment of the blast, too, strengthens this tendency. If the Ws/Wg goes over a certain limit, tlie burden in the upper part of the furnace is not heated up to temperatures high enough for the reduction by gas. In such cases, the burden comes down deep into the furnace without being.reduced by the gas, resulting in a low utilization of the gas in the furnace and a rough burden descent. The pattern of the cohesive zone is also affected by Ws/Wg. Too much oxygen enrichment of the blast sometimes gives rough operation of the furnace with wide variations in metal composition.

Reduction.

Data from the dissection of blast furnaces have shown that the reduction of the iron ore begins at the upper part of the furnace. When the ore reaches the cohesive zone, its degree of reduction is about 70 %. The semi-reduced iron ore particles.stick together to form a plate-like

cohesive layer under the pressure of the charge column above it.

The properties of burden materials (e. -g. size and chemical composition) have a direct influence on the procedure of the reduction by gas. From the viewpoint of reduction reaction, smaller sized iron ores are preferable. In actual practice, however, the sintered ores are crushed and screened to sizes between 5 and 5 0 m in order to keep good gas permeability. The FeO content of the sinter is closely controlled be- cause -it has a strong effect both on the reduci- bility and on the degradation of sinter in the furnace.

Uniform countercurrent of the charge and the gas is the basis of the efficient reduction by gas. If a strong local gas flow is formed in a limited part of the cross section, considerable amount of the gas goes to the top of the furnace without reducing the iron ores, and the overall utilization. of the gas becomes lower. In this case, the ores in other parts of the- cross sec- tion have less chance to be heated and reduced by the gas. Consequently, the unreduced ores go deeper into the'furnace without being sufficient- ly reduced to form,the cohesive layers, and the pattern of cohesive zone changes as a result of par,tial channeling of the gas in the granular zone.

Melting.

Iron ores are melted exclusively on the inner and lower surface of the cohesive layer. The melting characteristics of the slag forming materials in the burden have a great deal to do with the shape of inner surface and the width of the cohesive layer. If the burden consists of two kinds of ores with high and low melting points of the slag forming materials, these two kinds of ores melt at different temperatures. In such cases, the width of the cohesive layer is larger. Just the reverse is the case of the . blast furnace running exclusively on basic sinter.

If the primary slag, thus formed at the lower surface of the cohesive zone, has a high viscosity, the void fraction in the active coke . zone becomes lower. In this regard, the good melting characteristics of the burden components is very important for the efficiency of the blast furnace.

3) Burden and gas distribution. .

~ f f iciency of the blast furnace operation is always measured in terms of production, fuel consumption and the life of lining. All of these have dlrect effects on the economy of the process.

The daily production of the furnace is normally limited by the volume of wind that the furnace can "take". If the wind volume is in- creased beyond a certain limit, the descent of burden becomes deteriorated. It is a kind of deterioration in the countercurrent condition.

The fuel rate is primarily dependent upon the utilization of the gas in the furnace. Finally, the life of lining is directly dependent upon the thermal load to which the lining is exposed. The high utilization of gas and the low thermal load upon the lining can be expected only with a stable countercurrent condition in the furnace. In the case of stave cooled blast furnaces, the thermal load on the lining can be continuously measured. And the average thermal load can be kept to a level of about 10,000 kcal/m2/h by the appropriate control of the gas distribution in the furnace. This gas distribution control has favorable effects both on the life of lining and on the fuel economy.

The non-uniformity of the gas flow inevitably involves a certain decrease in the utilization of thermal and chemical potential of gas. However, in actual practice, a certain degree of centra- lized gas flow is taken as a necessary condition for a stable countercurrent in the blast furnace. This poses the question of minimum centralized gas flow necessary for smooth operation of the blast furnace.

Cohesive zone.

So far as the bell charging system is used, a considerable size segregation takes place when the burden and the coke are charged. As a re- sult of this segregation, the larger particles gather nearer to the center and the smaller particles accumulate at the periphery. (Fig. 4) Consequently, the permeability at the center is normally higher than that on the periphery. This tendency always results in a centralized gas flow. The surface of the charge column is nor- mally V-shaped. The V-shaped surface always gives a shorter path of gas at the center than at the periphery. This is another factor that strengthens the tendency towards the centralized gas flow. On the other hand, the angle of repose of the coke is always steeper than that of the ore burden. This difference in the angles of repose results in thicker ore layers at the center than at the periphery. However, this tendency is thought to be lessened by the fact that the ore charge sweeps the coke into the center but the coke charge does not. As the ore burden layer always has lower permeability than the coke layer, the difference in the angle of repose should have adverse effects upon the centralized gas flow. The three factors,that are the size segregation, the V-shaped surface and the difference in the angles of-repose, normally result in a more ore less centralized gas flow, so far as the gas in the lower part of the furnace can penetrate up to the center of the furnace

As a result of the centralized gas flow, the ores charged at the center encounter largeramounts of gas and they are heated and reduced earlier than those charged at the periphery. It is quite clear that the centralized gas flow results in the earlier formation of the cohesive layer at the center than at the periphery. Thus, a certain degree of centralized gas flow calls for a conical shape of cohesive zone, which, in our

experience, has proved to be one of the most im- portant bases of the smooth operation of blast furnace. As a matter of fact, the centralized gas flow is normally observed in smoothly operat- ing blast furnaces.

The.gas going out of the slits of the cohe- sive zone is at temperatures between 1200 and 1400°C. It consists exclusively of N2, CO and Hz. On its way upwards to the top of the furnace, the gas first reduces the iron oxides in the col- umn and then heats the burden and coke. Accord- ingly, C02 and Hz0 appear and increase in the gas and the temperature of the gas gradually decreases. The temperature and chemical compo- sition of the gas at any point in or above the granular zone give a clue to the amount of burden and coke that the gas has already encountered. As a matter of fact, conical shaped cohesive zone gives generally higher temperatures and lower C02 contents of the gas at the center and lower temperatures and higher.CO2 contents of the gas at the periphery.

The importance of the burden distribution was pointed out by Germans as early as in the late 1950's. (7) At that time, the V-shaped surface of charge column with a slight concent- ration of the fines at the periphery was recom- mended as a reliable method for smooth operation of blast furnace. This is in good agreement with our recent experiences and technical data.

The conical shaped cohesive zone is, thus, one of the indispensable requirements for smooth operation of the blast furnace. Its slit area should be large enough to allow the gas to pass without excessive pressure drop. At the same time, the position of the cohesive layer at the periphery should be sufficiently high so that the molten metal and slag dropping down can be sufficiently superheated before coming into the tuyere raceway. The slit area and the position of peripheral cohesive zone are the two most important factors that we have to take into consideration for the efficient operation of blast furnace.

Mathematical calculation of the pattern of the cohesive zone is still an interesting subject of discussion. Many formulae have been proposed for the calculation based on chemical composition and temperature of the gas in or above the granu- lar zone. However, no one has yet given a sufficiently accurate answer, because the hori- zontal movement of gas and the effects of size distribution on the chemical reaction and heat transfer have not yet been correctly taken into calculation.

Pressure measurement at the shaft lining and some of the acoustic measurements can be utilized as good indicators for the estimation of position of the cohesive zone.

Practical applications.

For many years, the distribution of the gas at the top of the blast furnace has attracted

Fig.4 Gas flow in the blast furnace.

the attention of blast furnace men. The charging sequence has sometimes been changed for the re- gulation of the gas flow distribution. However. the alteration in the charging conditions were made mostly in an empirical way. But, in the light of the data obtained by the dissection and in the light of the recent findings in the aero- dynamic calculations, the pattern of the cohesive zone was found to have a substantial effect upon the performance of the furnace.

The movable armor' plates and Paul Wurth charging chute have provided a very reliable and effective method of burden distribution control. These are very important especially for the smooth and efficient operation of large blast furnaces with large throat diameters.

C. Operation of large' blast furnaces.

1) General description.

Scientific operation of the modern blast furnace is the realization of three stationary reactions (i. e. heat transfer, reduction and melting) in five separate zones (i. e. granular zone, cohesive zone, active coke zone, raceway and hearth) in an industrial scale.

As the blast furnace process is a literally continuous process, a primary requirement for its high efficiency is the trouble free operation of all machinery for charging and blowing the furnace and for gas cleaning and molten materials treatment. (Fig. 5.) Not only the average daily production but also the fuel consumption are strongly dependent upon its plant availability. Therefore, due attention must be paid to keep high plant availability.

The requirement for the high plant availa- bility is especially strong for large blast furnaces. In the case of large blast furnaces operating at high pressures, it takes consider- able time to bring the furnace out of normal operation and back again into normal operation. At the same time, the shut-down sometimes causes changes in the gas distribution and heat effici- ency of the furnace. In such cases, a consider- able time is required to restore the initial performance of the furnace.

As the blast furnace process is a counter- current process, stable supply of good quality raw maerials and fuel is another requirement for its high efficiency. Fluctuations in the pro- perties of burden or coke directly influence the descent of the charge column in the furnace. This is the reason why the Japanese iron- and steelindustry has laid large amounts of invest- ment for the modernization of sintering strand and coke ovens. Large blast furnaces always have large throat diameters that arouses a strong size segregation and makes the furnace strongly sen- sitive to the fluctuations in the size distribu- tion of the burden and coke. Close control of size distribution is especially important for the large blast furnaces.

In one occasion, the size distribution of sinter changed, coaser and finer fraction in- creased, resulting in a slight increase in mean size. (Fig. 6) The gas flow became too strongly centralized and the utilization of the gas lowered, at the same time, the thermal load on the cooling stave increased.

High plant availability and good quality charge materials are the two essential bases of the blast furnace operation. Further, this is true especially for the heavy equipped large blast furnaces. The heavy equipped large blast furnace is a sophisticated system of furnace, machinery and instrumentation, that can be operated only with good quality burden and coke.

On the basis of the above, large blast furnaces in Japan are presently operated always at highest possible pressure and temperature of reaction. These blast furnaces are normally operated at blast temperatures over 1200°C and at top pressures around 2.5 kg./cm2. The high top pressure has remarkably improved the counter- current condition and provided a substantial basis for the improvement in fuel consumptions. The high blast temperatures have made it possible to utilize considerable amount of additional fuel and have had a great deal to do with the over- all fuel saving. In our normal practice, the top pressure and the blast temperature are kept at the highest that the furnace is designed for. The blowing condition is no longer changed for the regulation of the reactions in the furnace, because the highest possible temperature is the best for fuel saving. The humidification of blast is not practised on most of the large blast furnaces, because it always lowers the combustion temperature in the raceway. Accordingly, the heat balance of the furnace is adjusted at first by some changes in the fuel injection rate and then by some changes in the burden-to-coke ratio.

Another important factor for the smooth operation of the large blast furnace is the "dry hearth". In order to keep the level of molten materials in the hearth sufficiently low, the large blast furnaces are normally cast continu- ously. In case of large blast furnaces producing about 9,000 t/d, iron and slag are normally cast from 1 tap-hole and at the end of each cast another tap-hole is opened for the next cast. (Fig. 7) Casting rate from the tap-hole is normally 6 to 7 tons of iron and about 2 tons of slag per minute in the average.

For example, Tobata No.1 Blast Furnace, quoted in the figure, is designed and operated in such a way that in no case the surface of the molten slag comes over a level of 2.0 m below tuyere level. This, in our experience, is the highest permissible level, above which the raceway is thought to be deformed by the molten slag. The tap-hole is located 5 m below tuyeres and the operating standard stipulates "if, for any reason, the slag does not come within 60 minutes of the commencement of a cast or one torpedo (250 t) is not filled up within 60 minutes, another tap-hole should be opened for simultaneous casting".

C H A R G I N G GAS CLEANING

+ r----------- I L --,------- LOWER GAS VELOCITY; 1

HlGH TEMP. BLOWING TREATMENT .c - - - - - - - - r ~IG~COMBUSTION 7

TDRY HEARTH 7 + I TEMPERATURE I L- - - - - --,,A L ,,-,,-,-,, I

J

HlGH VOID FRACTION I a t HIGH TEMP. ZONE

I B U R D E N and C O K E Q U A L I T Y C O N T R O L

; STATIONARY REACTION i L,-,--,,,,---d

Fig5. Operation of the blast furnace.

'

. B A S E S TROUBLE FREE OPERATION

of MACHINERY I

100

80 Sinter size distribution 60

1 ,, 20

0

Mean size 22

of sinter 20

(m.1 18

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c o z

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6

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25 30 5 July

Fig.6 Effect of sinter size distribution change (Tobata No. 1 BF)

I Fig. 7 Casting large blast furnace

No.3T.H.

No.4T.H.

O c t . 2 8 . 1976 Tobata No.1 B.F.

Metal

Slag B

Stand by

I I Under repair

€a B I

The charging condition, however, is some- times changed for the improvement of the effici- ency of reactions in the furnace. (Fig; 8) The circumferential uniformity is the first thing that the blast furnace operators must pay atten- tion to. In order to get a good circumferential uniformity, the charging system has to be elabo- rately adjusted and the worn out liners on the big bell or hopper have to be replaced all in one shut-down. The circumferential uniformity becomes more and more important as the throat diameter becomes larger.

In one occasion, partial wear of the big bell liner resulted in a strong deterioration in the circumferential uniformity of the charge column. This in turn resulted in a wide fluctu- ation in the chemical composition of iron from tap-hole to tap-hole. (Fig. 9)

The final item of adjustment is the radial distribution of burden and coke. The movable armor plates are normally used for this purpose and the adjustment is made always with the co- hesive zone in mind. In our normal practice, the adjustment is made for the purpose of long term optimization, and, consequently, it is made only after a certain discussion among the metallur- gists concerned. Once the position of the armor plates is changed, the effect of the change is thoroughly studied with the help of many sensors, which give informations on the reactions in the new situation. (Fig. 10) Thermocouples installed on the cooling staves or in the lining, which primarily indicate the thermal load on the lining, are also used for the estimation of the gas flow in the furnace. The movable armor plates are normally.used in the same way for months until a better pattern of use is found as a result of intensive study.

The operational practice slightly changes from furnace to furnace, but the common feature is that the furnace is operated under the same condition for a long time and the performance is intensively studied for further improvement..

2) Examples of.efficient blast furnaces.

Kukioka No.4 Blast Furnace.

This medium sized blast furnace, with a hearth diameter of 8.8 m and an inner volume of 1540 m3, was built in 1971 as a replacement for the old No.4 Blast Furnace in Yawata Area of NSC's Yawata Works. The ultimate target of this blast furnace was to practically confirm the lowest limit of fuel~consumption. Small as it is, it is equipped with many devices for moni- toring and regulating the reactions in the furna- ce. The gas sampling probe above the charge column has been operated every shift since a few days after the blowing-in and provided data on the distribution of chemical composition and the temperature of the top gas. In October, 1973, the furnace marked the lowest fuel rate of 434 kg./t. An extremely low thermal load, at that time, suggested a strongly centralized gas flow in the middle part of the furnace but the

peripheral part of upper shaft was strongly heated.

In the first five years of campaign, it produced a total of about 6.4 million tons or about 4170 tons/m3 at an average fuel consumption of slightly less than 460 kg./t. (Fig. 11, Fig. 12) Under a slack demand for hot metal, it is presently running at a production rate of about 3100 t/d at a fuel rate of about 460 kg./t. (Fig. 13)

The gas distribution in the upper part of this furnace is now being measured continuously with a fixed temperature probe. It is also monitored intermittently every shift with the help of two probes, one above the charge column and the other in the charge column. (Fig. 14) Until March 1976, the furnace had operated on a burden containing about 70 % of sinter produced at a sinter strand nearby and directly sent by belt conveyor. In April, the sinter supply source was changed to another source about 10 km. from the blast furnace. This change resulted in a considerable decrease in the gas temperature, with resultant fluctuations in metal composition. As a first trial the movable armor plate position was changed to strengthen the centralized gas flow. However, the result was poor, the burden descent was slightly deteriorated and the fluc- tuations in the metal composition was not im- proved. After a few months of unsuccessful trial in this way, the movable armor plate position was again changed to the former pattern. At the same time the oxygen enrichment of the blast .was stop- ped in order to decrease Ws/Wg and to fully utilize the peripheral part of the higher part of shaft. Consequently, the temperature at the higher part of the granular zone rose, indicating a moderate change in the pattern of cohesive zone. The gas utilization steadily increased and fuel rate gradually decreased. These changes took place very slowly and the change is still going on. (Table I)

During the past five years of operation, this furnace has lost five pipes of the stave cooler. But the furnace shell is lasting quite well. The furnace is expected to be in service for another 2.5 years for a total cumulative production of about 9 million tons or nearly 6000 t/m3/ campaign.

Kimitsu No.3 Blast Furnace.

This is NSC's first 10,000-t/d- blast furnace. The furnace itself has a hearth diameter of 13.4 m and an inner volume of 4063 m3. It was blown-in in September, 1971, and has already produced a cumulative total of more than 16 million tons. (Fig. 15) The ultimate target of this furnace, at the design stage, was the produc- tion of, 10,000 t/d.

Since the start-up period, this furnace has always been in successful operation under strict control in many aspects. The furnace and its auxiliary equipment were designed and constructed for an expected high production rate of over 10,000 t/d. A new sinter strand was put into

+o. 1 Si% of metal from N o . 2 T . H . o -average Si%

(%) -0.1

-0 2

S% of metal + O 01 from N o . 2 T . H . -average S% 0

( % I -0.01

+ 10 Temp. of metal from N o . 2 T . H . O -average metal temp.

(C) -20

Fig.9 Effect of partial wear of big bell liner (Tobata No.4 6. F . )

PERFORMANCE

PRODUCTION FUEL RATE

-

FEED

FOR-

WARD BURDEN and LINING LIFE ADJUST- COKE QUALITY

CHARGING CONDITION

COMPARISON WITH STANDARD ANALY SlS FOR OPTlMlZATlON n

Fig. 8 Metallurgical adjustment of the blast furnace.

FURNACE and AUX. MACHINERY

Gas sampl~ng probe(T A )

Fig.1 0 Sensors to rnon~tor reactions.

25

20

Cumulat~ve 15 P r o d u c t ~ o n ( m ~ l l ~ o n tons)

I (1

5

0 0 1000 2000 3000 4000 5000

Inner Volume ( m 3 1

Fig. I I lnner volume vs cumulative production

K u K u k ~ o k a N S C T Tobata N S C M r Muroran N S C Km K ~ m ~ t s u N S C N Nagoya N S C 0 O ~ t a N S C F Fukuyarna N K K M z M ~ z u s h ~ m a Kawasak~

o K s Kash~ma Sum~torno T I Kk Kakogawa Kobe

K r Kure N ~ s s h ~ n

I Y I 1 I I

- 3500 Production

- 3000

- 2500

Fuel rate ( k 6 1 t )

b

5 000 K m l

- K r l M z I

~rn; 0 . 0 .

K s l M r 2 Ti!

Cumulat~ve Product~on ' OoO

per Inner Volume

3 000 - K u Kuk~oka NSC T Tobata NSC Mr Muroran NSC Km K ~ m ~ t s u NSC N Nagoya NSC 0 O ~ t a NSC F Fukuyama NKK Mz M~zush~ma Kawasakl K s Kash~ma Sum~tomo Kk Kakogawa Kobe Kr Kure N ~ s s h ~ n

0 1 2 3 4 5 6 7 8

Years

Fig. 12 Cumulat~ve production on un~t I n n e r volume

- 7

I Fig .I3 'operating data, Kukioka No .4 B.F

Cumulative Production

( y ; ; ~ )

360 . 6 - 5 - 4

- ; - 1 .O

-./a

-.-*-* I*---

/*-*-*- /*

-./* *-*-*-

*-*-*- 1971 1972 1973 1974 1975 1976 0 Ja A JC 0 Ja A JC 0 Ja A JC 0 Ja A J1 0 Ja A Jt

/ \ \ \ \ \ \ \ ' , \ \ \ \ \ I / \ D M J n S D M J n S D M J n S D M J n S D M J n S

Fig. 14 Gas distribution Kukioka N o . 4 B.F.

August 1976 I

I total fuel (kglt) 1 434 1 453 1 468 1 431 1 469 1 459 457 (volume (NrnVmin)( 2.6851 2 .4581 2 .5451 6 .4231 6 .9291 6 ,2601 6.271

6, - 600 E

al 0 'u

E I= - - z -0

Shaf t probe a m 5 E C 'Z .- ln s a .- CI - e C

S

May 1976

0

March 1976. ,-.. Gas sarnpl~ng ;: - X-X probe

b-• Fixed probe 0-0 Shaft probe

Table I Operating data

p r e s s u r e ( g l d ) 2.530 2.107 2.200 3.837 3.986 4.115 4,130 1 B l a s t 1 temperature ( c ) 1.241 1.228 1.226 1.313 1.301 1.279 1.280 moisture g 1 2 . 3 7 1 1 2 . 3 7 . 4 . 5 . 9 1 15.5 1 ll.9 1

40

- x

( C O ~ / ( C O ~ C O Z ) (%) 1 49.8 50.1 50.2 51 .9 48.3 1 49.2 50 .6 B u r d e n 1 s i n t e r ( % ) I 6 8 . 6 1 8 1 . 3 7 9 . 5 1 9 2 . 5 81 .01 8 1 . 2 78.9

Blast Furnace Period

T o p G a s

Kimitsu No.3 Kukioka N o . 4

Production

Fuel Rates

P i g I r o n 1 S k4) 1 0.027 1 ,0 .038 1 0 .028 1 0.034 1 0.033 1 0.027 1 0 .023 1

Mary75 9.323 2.295

365 66

Oct:73 4.128 2.680

372 62

Tobata No.1

dai ly ( t l d ) u n i t (tlm'ld) c o k e (kglt) o i l (kgft)

temperature ( t ) C O z (?6) C O ( W ) H z (%)

I

(temperature ( t ) 1 1 . 5 0 5 1 . 5 0 2 1.5201 1.5071 1,5271 1 .5301 1,530 I volume (kglt) 1 281 / 314 325 1 312 316 1 325 312

Oct:76 9.277 2.283

418 51

Sept:76 8.447 2.040

398 61

pe l le t (76) 1 15.7 1 14.7 12.2 1 7 . 5 10.0 1 11.6 11.2

Mar76 3.114 2.022

396 52

Oct:76 8.607 2.079

395 62

131 21.8 22.0

4.3

I si ( % ) I 0 . 4 4 0 . 5 8 0 .611 0 . 3 1 1 0 . 4 4 1 0 . 4 3 0.40

Oct:76 3.115 2.022

419 47

S l a g

H a n g s (hangsfmonth) 0 1 0 1 0 1 0 1 0 1 0 1 0 F l u e d u s t (knit) 1 13.3 1 8 . 0 1 5.2 1 3 . 6 5 . 6 1 5 . 2 5 . 8

177 21.4 21.3

3 . 3

1 C a O / S i O z 1 1 . 3 3 1 . 2 7 1 . 2 7 1 1 .231 1 . 2 2 1 1 . 2 3 1.22

S i O z ( 4 ) A1203 (%) C a O (%) M g O (%)

Shut down time(hr-min)

128 21 .I 20 .9

2 . 8

0 1 12'47' 18'48' 1 14'03' 24'46' 1 22'07' 1 10'03'

34.5 15.1 45.9

2 . 1

108 22.7 21.0

4 . 0

33.2 14.1 42.3

6 . 5

123 21 .O 22.5

3 . 4

34.7 11.9 44.1

7.3

153 21 .6 22 .3

4 . 0

152 21 .8 21 .3

4 . 1

34.3 14.7 41.9

1 . 6

34.2 13.9 41.6

6 . 2

33.8 14.5 41.5

5 . 4

33.9 13.9 41.4

6 . 7

(t/d) (t/rn3/d)

Productlon

-10000

- 9000

-8000

- 7000

- 6000

- 2 5 0 \o-~\,

w-

/ - 2 0

- 1 5

Fuel rate

(kg/t)

-16

-12

- 460

- 440

-420

-400

-380 Cumulat~ve

Productton

(::!:I - 8 /- /.-• - 4 -0-• -./.- ./--. - 0

1971 1972 1973 1974 1975 1976 0 Ja A J C 0 Ja A J C 0 Ja A J C 0 Ja A J C 0 J a A J C

I I I I \ \ \ I I \ \ \ I \ I \ \ \ \ I D M J n S D M J n S D M J n S D M J n S D M J n S

Fig.15 Operating data, Kimitsu N o . 3 B . F

operation just before the blowing-in of this blast furnace. Accordingly, this.furnace has always been running almost exclusively on high quality self-fluxing sinter burden. Since the basic design period, a special task force has been continuously working for optimizing the reactions in the furnace. The furnace profile was decided with due attention to the pattern of the cohesive zone. Many devices were introduced for monitor- ing the reactions in the furnace, with many others presently being studied or in the develop- ment stage.

The gas distribution of this furnace is .monitored-with a horizontal probe in the upper part of shaft. The burden distribution is re- gulated by the NSC type movable armor plates.

A series of theoretical studies and practi- cal test operations has been carried out on this furnace. In one occasion, the monthly average fuel rate was lowered to 431 kg./t, including 66 kg. of oil per ton of iron. These fundamental studies and practical experience have provided a reliable foundation for the smooth and efficient operation of this blast furnace. The furnace is presetnly operated with gas distribution shown in Fig. 16. The high temperature at the center shows the centralized gas flow in this part of the furnace. On the other hand, a slight rise in temperature at the periphery shows the suf- ficient heating of the charge column at the per?phery. Experiences in the past have shown that too low a temperature at the periphery some- times leads to rough operation of the furnace. because, under such conditions, the cohesive layer at the perfphery goes too low and the re- sultant insufficient super heating of the molten materials at the periphery sometimes induces troubles around the tuyeres. (Table I)

The target of this furnace, now, after 5 years of operation, is the highest cumulative production. The furnace is expected to produce a cumulative total of about 23 million tons in one campaign of about 7 years

Tobata No.1 Blast Furnace.

This furnace was blown-in on March 22, 1975 in the Tobata Area of Yawata Works. It has a hearth diameter of 13.6 m and an inner volume of 4,140 m3. As a second version of the 4,000 m3 blast furnace, this furnace was designed to meet the optimum combination of the reduction in the initial investment and reduction in the operating costs. For this purpose, the width of charging belt conveyor was reduced from former standard of 2.0 m to 1.8 m, the space of the cast floor was reduced from 6,500 m2 to 5,600 m2 and the pressure equalizing lines for the charging system was simplified. On the other hand, many improve- ments were made in the individual equipment to cope with difficulties in the operation of large blast furnaces. For example, the entire space of cast floor was made flatter and connected with a inclined ramp way to the ground level. This type of cast floor allows the heavy machinery for civil engineering (e. g. back-hoes and dump trucks) go to any tap-holes and to any part of

. ..

troughs and runners by itself without help of cranes. The total length of metal and slag run- ners was reduced to about 2 1 3 of the former stan- dard. Extensive use of computer and CRT system for the quality control of burden and coke is another feature of this blast furnace.

Under unfavorable economic conditions; the furnace has not yet reached the full capacity production of 10,000 t/d. However, the fuel consumption has steadily decreased since the beginning of campaign. (Fig. 17, Table I)

The underlying idea of the operation of this blast furnace has always been good fuel economy and long campaign life. Every possible care was taken for these purposes. As a result of these efforts, the fuel consumption is low and the re- fractory lining is still remaining even at the lower part of the shaft and upper part of the belly after 20 months of operation.

The gas distribution of this furnace is mo- nitored continuously by two fixed temperature probes and intermittently by a gas sampling probe above the charge column. (Fig. 18) An additional horizontal probe in the upper part of the shaft is now under construction for future fuel saving. The experiences in the past have shown that sharply centralized gas flow and moderately high temperature at the peripheral part of the upper part of the shaft give smooth countercurrent condition and high fuel efficiency also on this furnace.

At the beginning of April, 1976, a new sinter strand was put into service in the Wakamatsu Area of Yawata Works. Since the new sinter came into use, the performance of Tobata No.1 Blast Furnace has heen gradually improved.

Tobata No. 1 Blast ~u=nace has just started campaign. It is expected to play a leading role in the future development of ironmaking techno- logy.

3) Other developments.

T'JO still larger blast furnaces came into operation in 1975 and 1976 in NSC's Kimitsu and Oita Works. They are Kimitsu No.4 Blast Furnace and Oita No.2 Blast Furnace, with inner volumes of 4,930 m3 and 5,070 m3, respectively. The performance of these blast furnaces will be dis- cussed in the future.

All of the above mentioned blast furnaces and some others in and out of NSC are expected to contribute a great deal to the development of the ironmaking technology in the future.

D. Summary.

53 blast furnaces are presently in operation in Japan, among them are included 10 large units with inner volumes of about 4,000 m3 or more. These blast furnaces are being operated with strict attention to the reactions in the furnace.

- Mar. 1975

c* Nou. 1976

Fig. 16 Gas distributim, Kimitsu No.3 B.F (Shaft probo)

./.-. - - = = ./.-. c'

- I - 4 ? 0

Cvmulattve

1976 M A M J J A S O N D J F M I M J J A S O A P A U U U E C O E A E A P A U U U E C R R Y N L G P T V C N B R R V N L G P T

Fig .17 Operating data. Tobata No. l B . F

50 . c o z

CO+ C 0 2 40 .

(%) 30 .

2 0

600 ,

500 . Temperature

(c) 400 .

300 .

200 .

100 .

0 periphery center periphery

Fig. I 8 Gas distribution, Tobata No.1 B.F

x---x---*---x,

'7 \ \ \ \ \ \ \ \ \ \

x---x Gas sampling probe

' 0-0 Fixed probe S.E.-N.W.

@-+ F ixed probe E.N-W.S

Oct . 1976

Various devices are being used for monitoring and regulating the reactions. As a result, the pro- duction of these furnaces is high and the fuel consumption low.

Heat transfer, chemical reduction and melt- ing are the three most important reactions that directly determine the efficiency of the blast furnace. These reactions can be understood as taking place,under countercurrent condition, in five separate zones in the furnace: i. e. granular zone, cohesive zone, active coke zone, raceway and hearth. The performance of the blast furnace can be easily explained with the pattern of cohesive zone, especially with its "slit area" and its position at the periphery.

In the daily practice, the stable operation of all auxiliary equipment and the consistent supply of good quality burden and coke form the bases of the good performance of the blast fur- nace. On the basis of the above, modern blast furnaces are being operated with more or less centralized gas flow in the upper part of the furnace. This type of gas distribution is under- stood as reflecting the good pattern of the co- hesive zone. The operations of Kukioka No.4, Kimitsu No.3 and Tobata No.1 Blast Furnaces were quoted as typical examples.

Nomenclature

AP ; pressure drop (~~/m*);

AL ; charge column height (m)

E ; void fraction of charge column (-).

; shape factor (-) . d ; particle diameter (m).

p ; viscosity of gas (kgfm-s).

u ; superficial gas velocity (mfs).

p ; density of gas (kg/m3)

2 gc ; conversion factor (kg.m/Kg.s ) .

Cs ; specific heat of solid (kca1fkg-deg.C).

Cg ; specific heat of gas (kca1fkg.deg.C).

Gs ; mass flow rate of solid (kgfhr).

Gg ; mass flow rate of gas (kgfhr).

Acknowledgements

Deputy General Superintendent, Yawata Works, who first suggested to us the dissection of the blown- out blast furnace in 1968 and opened the way to the scientific operation of the blast furnace.

The authors wish to express heartfelt thanks also to Mr. M. Yoshinaga, General Manager, last Furnace Department, Yawata Works, for his valuable advices in the past 8 years, ,during that time, the performance of Yawata Blast Furnaces has been re- markably improved.

References

1. Kodama, K. and Hashimoto, S.', "A Considera-. tion on Blast Furnace Operation, Featuring the Use of Water Quenching." Proceedings of Inter- national Conference on the Science and Technology of Iron and Steel. Part I. Suppl. Trans. Iron, and Steel Institute of Japan. Vol. 11, 1971, pp. 112-117.

2. Kanbara, K., Sasaki, M., Okuno, Y. and Katayama, T., "The Formation of the Melting Zone in Blast Furnace". Proceedings, AIME. Iron- making Conference, 33 (1974) pp. 416-422.

3. Kanbara, K., Hagiwara, T., Shigemi, A,, Kondo, S., Kanayama, Y., Wakabayashi, K. and Hiramoto, N., "Dissection of Blast Furnaces and Their In- side State." ~etsu-to-~a~an6. Vol. 62, April, 1976, pp. 535-546.

4. Shimomura, Y., Nishikawa, K., Arino, S., Katayama, T., Hida, Y. and Isoyama, T., "On the Inside State of the Lumpy Zone of Blast Furnace." Tetsu-to-Hagang. Vol. 6 2 , April, 1976, pp. 547-558.

5. Sasaki, M., Ono, K., Suzuki, A,, Okuno, Y., Yoshizawa, K. and Nakamura, T., "Formation and Melt-down of Softening-Melting Zone in Blast Furnace." Tetsu-to-Hagang. Vol. 62, April, 1976, pp. 559-569.

6. Ergun, S., "Fluid Flow through Packed Columns." Chemical Engineering Progress. Vol. 48, 1952, pp. 89-94.

7. Winters, C., "Ein neuer Hochofen mit 9 m Gestell-durchmesser." Stahl und Eisen. Heft 80, 14, April 1960, pp. 465-473.

The fundamental idea on the operation of the blast furnace, described above, has been arrived at as a result of repeated weekly discussions with Mr. N. Inagaki, Manager in charge of pro- jecting new blast furnaces in Yawata Works. The authors are much thankful for his cooperations.

The authors are grateful to Dr. T. Kato,