US3690632A

US3690632A - Blast furnace control based on measurement of pressures at spaced points along the height of the furnace

Info

Publication number: US3690632A
Application number: US102615A
Authority: US
Inventors: William A Munson
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1970-12-30
Filing date: 1970-12-30
Publication date: 1972-09-12
Anticipated expiration: 1989-09-12
Also published as: CA960036A; FR2120065B1; FR2120065A1; JPS5514121B1; IT944588B

Abstract

A system for automatically controlling the addition of moisture, the addition of enrichment fuels and/or the temperature of a hot blast for a blast furnace as a function of the ratio of (1) the pressure differential between the pressure in the furnace at the tuyeres and at a point slightly above the mantle to (2) the pressure differential at said point slightly above the mantle and that at the top of the furnace.

Description

D United States Patent 1 3,690,632 Munson [451 Sept. 12, 1972 BLAST FURNACE CONTROL BASED 2,625,386 1/1953 Leone ..75/41 UX ON MEASUREMENT OF PRESSURES 3,581,070 5/1971 Tsujihata ..7S/4l X AT SPACED POINTS ALONG THE 2,822,257 2/ 1958 Hanna et a1. ..75/4l HEIGHT OF THE FURNACE 3,560,197 2/1971 Shellenlberger et al ..75/41 [72] Inventor: 311mm A. Munson, Williamsville, Primary Examiner Spencer Overholser I I J Assistant Examiner-John E. Roethel Assignw Westinghouse Electric po n, Attorney-J 1-1. Henson, R. G. Brodahl and J. J.

Pittsburgh, Pa. I Wood 22 F! d: Dec. 30, 1970 l l 57 ABSTRACT [21] Appl. No.: 102,615

A system for automatically controlling the addition of moisture, the addition of enrichment fuels and/or the (g1 temperature of a hot blast for a blast furnace as a f ti n f the ti f( l) the pressure differential [58] Flew of Search 30; 75/4! 42 between the pressure in the furnace at the tuyeres and at a point slightly above the mantle to (2) the pressure [56] kgferences I differ'entialat said point slightly above the mantle and UNITED STATES S} that at thetop of the furnace. 1,997,603 4/1935 Spalding ..75/42 UX 9 Claims, 4 Drawing Figures I l I 34 I -I 36 FRoM BLOWER I 1 88 AL TV k$ I I '4 =5- as I I TO STACK r ENR T T ENT 2 I I I 54 I 40 I I STEAM I 86 ENRICHMENT HOT BLAST HOT FUEL VALVE TEMPERATURE g i gggf BLAST CON"ROL CONTROL CONTROL ,re E I a2 '84 58 5s a 9O I I DETERMINE ACTUAL 98 HO'I'B AST I I Pl-P2/P2-P3AND RAI:E l OUTPUT COMPARE WITH "1 PROGRAM DESIRED I I 92 I 94 i 96 I ENRICHMENT HOT BLAST MOISTURE TEMPERATURE I PROGRAM PROGRAM PROGRAM MANUAL INPUT DESIRED PRESSURE RATIO, HOT BLAST TEMPERATURE 8 RATE, FUEL ENRICHMENT B MOISTURE CONTENT P'ATE'NTED SEP 12 1912 INPUT SHEET 1 [IF 3 68 P3 [2 l I Q 1 1 TL Fl F2 T ,M I/ I I8 I I 38 36 FROM BLOWER 66 I4 I I as Jr W l 54 1/ LJE 46 I 5 48 I TO STACKr l r T I T I 7 l 7 l {STEAM 1( ENRICHMENT HOT BLAST HOT FUEL VALVE TEMPERATURE gg z gggf BLAST CONTROL CONTROL CONTROL ,78 L 1:82 '84 r90 9s DETERMINE ACTUAL HOT BLAST Pl-P2/P2-P3AND RATE OUTPUT COMPARE WITH 1 PROGRAM k DESIRED i ENRICHMENT HOT BLAST MOISTURE TEMPERATURE i PROGRAM PROGRAM PROGRAM MANUAL INPUT DESIRED PRESSURE RATIO, HOT BLAST TEMPERATURE 8 RATE FUEL ENRICHMENTB MOISTURE CONTENT FIG. I

PMENTEDSEP I 2 I972 SHEET 2 0F 3 N In FIG. 2 D. O. I E 8'.

OIRECT REDUCTION CARBON DESIRED RATIO P|P2/P2P3 loo MANUAL INPUT Pl f IO DETERMINE ACTUAL COMPARE F I63 2\ p2 P|P2,P2-P3 ACTUAL a RATlO RATIO TO '03 I p -p2/ p2-p3 DESIRED RATIO P3 ,COMPENsATINO FACTOR H4\ H8\ |30 l28 DESIRED COMPENsATE COMPENsATE DESIRED FOR RATIO COMPARATOR FOR RATIO MOIsTURE TEMPERATURE VARIATION VARIATION MANUAL I32 I ACTUAL T A TEMPERATURE I COMPARATOR U "2 CONTROL MOISTURE I BUTTERFLY ggs IRgg vALvE 36 56 H L RATE I24 I l26 "ggtfgggf I CONTROL COMPARATOR COLD BLAST vALvE as I ACTUAL VARY HOT BLAST POSI ON RATE vALvE 4s MEJMPENSATE (I40 CONTROL 4 DESIRED VALVE 52 COMPARATOR 523,2?33 ENRICI-IMENT ACTUAL ENRICHMENT PATENTEDsEP 12 m2 SHEET 3 BF 3 FIG. 4

36 FROM BLOWER F 88 1 7 A (V A 32 46 l T 48 I o STACK I r ENRICHMENT A 7 FUEL l 7 I STEAM M 5 ENRICHMENT HOT BLAST T HOT FUEL vALvE TEMPERATURE 28: 1:35 BLAST CONTROL CONTROL CONTROL L 9 F'SLLBTRACTO Pl-PZ I52 Pl-PZ /|62 'SUBTRACTOR r DIVIDER COMPARATOR T I86 1 I74 V I88 MULTIPLIER COMPARATOR 4 /DESIREDLT I92 I68 78 ik I94 (53 MULTIPLIER COMPARATOR r DESIRED Fl DPERATORS g L '76 CONSOLE I I96 I82 "k 200 F MULTIPLIER COMPARATOR 1 ,DES|REDM 204 205 F2\ COMPARATOR DESlREDF2 ;2o2

BLAST FURNACE CONTROL BASED ON MEASUREMENT OF PRESSURES AT SPACED POINTS ALONG THE HEIGHT OF THE FURNACE BACKGROUND OF THE INVENTION As is known, the usual blast furnace consists of two major parts. In the upper part, called the stack, the burden materials comprising iron ore, coke and limestone (i.e., stones) are preheated and partially reduced by carbon monoxide from the lower part. The coke is also preheated in the stack so that by the time it reaches the combustion zone at the lower portion of the furnace it will produce flame temperatures high enough for the final smelting of the iron-bearing materials. In the lower part of the furnace (i.e., the bosh and hearth), the combustion of the preheated coke takes place; the final reduction of the burden is completed; and the iron and slag are melted. A hot blast of air, forced into the lower portion of the furnace through thetuyeres, supplies a hot air draft which travels upwardly through the furnace and supplies oxygen for the combustion of the coke. i

In a blast furnace of this type, the main source of heat is the combustion of the carbon of the coke by the air blast in the lower part of the furnace. However, because of the high temperature in this region of the furnace (above 2,000 F) and the excess carbon in the hearth, the carbon can be burned only to carbon monoxide, not to carbon dioxide. The carbon monoxide is used for reducing the iron oxide of the burden. This reduction takes place in two distinctly different ways, depending on whether the temperature of the reaction is above or below 2,000 F. When the iron oxide is reduced below 2,000 F in the stack portion of the furnace, the chemical reaction is called indirect reduction. In this reaction, the carbon monoxide passing upwardly through the stack reacts with the iron oxide to produce iron and carbon dioxide. This carbon dioxide does not react further and leaves the furnace through the top. Since the amount of carbon monoxide available in the stack is limited, only about half the burden can be reduced indirectly.

The second type of reduction, called direct reduction, takes place in the power portion of the furnace by a reaction which cannot proceed until the temperature of the material exceeds 1,800 F. In contrast to indirect reduction, direct reduction absorbs large quantities of heat. While carbon monoxide accounts for most of the reduction of the iron oxide in the direct reduction process, the carbon dioxide that is formed is not stable because of the high temperature of the reaction and the presence of excess carbon, so it is converted back to carbon monoxide. Consequently, the overall effect of the reaction is the same as though hot carbon had been the reducing agent.

In the operation of a blast furnace, it is essential that sufficient carbon monoxide be available in the upper stack portion of the furnace to reduce the iron oxide to molten, metallic iron. A lack of sufficient carbon monoxide slows down the rate of indirect reduction with the result that large amounts of iron oxide become melted before being reduced to metallic iron. When this occurs, the melted iron oxide comes in direct contact with coke and is reduced by it to iron. Since the melting temperature of iron is higher than that of iron oxide the iron so formed solidifies, freezes the coke particles together and causes the furnace to hang. It is theorized that this same effect can occur when the direct reduction zone moves upwardly because of lack of sufficient heat in the upper portions. But, regardless of the theory adopted as to'why hanging occurs, its effect is to cause the burden to wedge within the stack, interrupting the uniform descent of the stock. The material underneath the wedged material continues to move and a void is created which tends to increase in size until the bridge collapses, causing a sudden downward movement of the stock above. In sever cases, this causes a sudden increase in gas pressure and an explosion effect. Thus, it is essential at all times to generate enough carbon monoxide and/or heat in the lower portion of the furnace to complete the indirect reduction process in the stack portion above.

' The amount of carbon monoxide and heat generated in the lower portion of the furnace can be varied by varying the moisture content of the hot blast, by adding an enrichmentmaterial such as a hydrocarbon gas to the hot blast, or by varyingthe hot blast temperature. Normally, these variables are controlled by the furnaceman based upon his judgment of the conditions existing in the furnace, andspecifically his judgment as to the proportion of direct-to-indirect reduction going on internally within the vessel.

It has been shown that it is possible to determine the amount of direct reduction to indirect reduction going on in a furnace by the use of pressure taps, one of which is located at the furnace tuyeres, another of which is located slightly above the mantle of the furnace, and the third of which is located at the top of the furnace. This is described, for example, in a paper by R. L. Stephenson, United States Steel Corporation entitled Analysis and Control of the Blast Furnace Operation, presented and prepared in the Troisiemes Journies Intemationales de Siderurgie, Luxembourg October 1962. Specifically, it has been found that when the ratio of the difference between the pressure at the tuyeres and the pressure above the mantle, to the difference between the pressure at the mantle and the pressure at the top of the furnace increases, the direct reduction portionof the furnace is rising, meaning that there is a lack of sufficient carbon monoxide in the upper portion of the furnace. When this happens, it is known that a hanging condition is occurring; but while this phenomenon has been known for some time, no suitable means has heretofore been devised for utilizing this information to automatically vary the composition and/or temperature of the hot blast to bring the furnace back into the desired equilibrium condition.

SUMMARY OF THE INVENTION In accordance with the invention, a system is provided for controlling the ratio of direct-to-indirect reduction in a blast furnace including means for producing electrical signals proportional to the pressure within the blast furnace at the bottom portion thereof, at the top portion thereof, and at a point intermediate the top and bottom. Further means are provided responsive to the aforesaid electrical signals for controlling a characteristic of the hot blast to maintain a desired ratio of direct-to-indirect reduction within the furnace.

richment fuel added to the hot blast, the hot blast temperature, or any combination of two or more of these parameters. The pressures are preferably measured at the tuyeres, at a point just above the furnace mantle, and at the top of the furnace. The difference between the pressure at the tuyeres and that above the mantle is compared with the difference between that above the mantle and at the top of the furnace to provide a ratio for correcting electrical signals proportional to the desired hot blast temperature, desired moisture content and desired enrichment as determined by the furnace operator. These corrected values are then compared with the actual values of temperature, moisture and enrichment as derived from measuring instruments to provide feedback control signals for the hot blast system. In this manner, and since the desired values derived from theoretical considerations are constantly monitored by the ratio of the aforesaid pressure dif- DESCRIPTION OF THE PREFERRED EMBODIMENTS The above and other objects and features of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings which form a part of this specification, and in which:

FIG. 1 is a schematic illustration of one embodiment of the invention wherein a general purpose computer is employed to control the characteristicsof the hot blast as a function of the pressures at spaced points within the furnace;

FIG. 2 is a plot of direct reduction versus the ratio of pressure difi'erentials between the mantle and the top and bottom of the blast furnace;

FIG. 3 is a flow diagram from which a suitable computer program can be devised for the system of FIG. 1;

FIG. 4 is a schematic circuit diagram of an alternative embodiment of the invention which performs the same function as that of FIG. 1 but wherein hard-wired logic elements are employed rather than a general purpose computer.

With reference now to the drawings, and particularly to FIG. 1, there is shown schematically a blast furnace having an upper stack portion 12, an intermediate bosh portion 14 and a lower hearth portion 16. The stack portion 12 and bosh portion 14 intersect at a mantle l8 underneath which supporting columns are circumferentially spaced around the furnace, the purpose of these columns being to support the stack portion 12.

Surrounding the bosh portion 14 is the usual bustle pipe 22 connected through tuyeres 24 to the interior of the hearth portion 16. The bustle pipe 22, in turn, is

connected to a hot blast main 26 having a branch line 28 connected to the output of a blast furnace stove 30.

The inlet to the blast furnace stove 30, in turn, is connected through a second branch line 32 back to the hot blast main 26. With this arrangement, it will be appreciated that a portion of the blast will flow through the branch line 32, along the dotted lines 34 within the stove 30, and through branch line 28 back to the hot blast main 26.

Some of the air in the cold blast main from a blower. not shown, passes through a bypass 36 around the stove 30. Within the bypass 36 is a butterfly valve 38 which can be opened or closed by a hot blast temperature control mechanism 40 connected to the valve 38 through a mechanical linkage 42. As will be appreciated, manipulation of the butterfly valve 38 will vary the temperature of the hot blast passing into the furnace through the tuyeres 24. That is, as the butterfly valve 38 is opened further, more air will bypass the stove 30 to be moved with that in branch line 28 which has passed through'thestove 30. Conversely, as the butterfly valve 38 is closed, more air will be forced to flow through the stove 30, ultimately raising the temperature of the blast passing through the tuyeres 24..

In the usual blast furnace installation, there are three stoves, each of which comprises a brick-lined regenerator enclosed in a circular steel shell with a fiat bottom and a dome-shaped top. However, only a single stove is shown herein for purposes of simplicity. Essentially, the stove consists of two parts. The first is a combustion chamber in which natural or blast furnace gases burn. The second part comprises the checkerwork, which contains a multiplicity of small passageways through which the products of combustion from a burned gas pass. In this manner, heat is stored within the refractory brick checkerwork, which heat is subsequently used to heat a blast of air forced into the furnace through the tuyeres. During heating of the stove 30, for example, a combustible fuel is introduced into the bottom of the stove through conduit 44 where it burns in a combustion chamber, with the products of combustion passing upwardly through the combustion chamber in a direction opposite to that indicated by the arrows, and then flows downwardly through the checkerwork and to conduit 46 where it passes to a stack, not shown.

When three stoves are used, any one of these is said to be on-wind, inasmuch as it is heating a relatively cool gas to provide the hot blast. At the same time, the other two stoves are said to be on-gas inasmuch as they are being heated. When the heat has been extracted from one stove, the hottest stove of the remaining two is then switched into the system; while the stove just switched out of the system again starts the heating process. In this discussion and the drawing of FIG. 1, it will be assumed that the stove 30 is heated and that the air is passing upwardly through the brick checkerwork and then downwardly to the branch line 28 where it is mixed with cooler air from bypass 36 before it passes to the tuyeres 24.

As was mentioned above, it is desirable to have a specified proportion of direct-to-indirect reduction within the furnace. 10. An excess amount of direct reduction can result, for example, from too hot a blast. The temperature of the blast can be cooled by the addition of moisture in the .form of steam through valve 48 which is, in turn, controlled by a moisture control device 50. Similarly, in cases where insuflicient indirect reduction occurs because of a lack of carbon monoxide, enrichment fuels can be added via valve 52, this valve being controlled by an enrichment fuel valve control 54. Finally, the ratio of direct-to-indirect reduction can be varied by varying the temperature of the hot blast via valve 38 and control 40.

The present invention provides a hot blast control system wherein the amount of moisture in the hot blast is varied, the amount of enrichment fuel added is varied, or the temperature of the hot blast is varied to control the proportion of direct-to-indirect reduction and thus prevent hanging up within the furnace. In FIG. 1, the ratio of direct-to-indirect reduction is controlled by means of a computer, generally indicated by the reference numeral 56; however the computer can be replaced by hard-wired logic as will hereinafter be described. The computer 56 is provided with the usual input and

output panels

58 and 60 together .with a manual input console 62 by which the furnace operator enters into the computer electrical signals proportional to certain desired operating parameters. The feedback signals applied to the input panel 58 include a pressure signal P1, derived from pressure sensor 64, proportional to the pressure in the blast furnace at the tuyeres; a pressure signal P2, derived from pressure sensor 66, proportional to the pressure within the furnace directly above the mantle 18; and a pressure signal P3, derived from pressure sensor 68, proportional to the top furnace pressure. Also fed back to the input panel 58 is a signal T proportional to the hot blast temperature derived from temperature sensor 70; a signal F1 proportional to the enrichment fuel rate derived from flow meter 72; a signal M proportional to the moisture con tent of the hot blast as derived from dew point detector 74; and a signal F2 derived from flow meter 76 proportional to the hot blast flow rate.

The output control signals derived from the output panel 60 include a signal on lead 78 for the enrichment fuel valve control 54; a signal on lead 80 for the hot blast temperature control 40; a signal on lead 82 for the moisture control device 50; and a signal on lead 84 for a hot blast control device 86 which, in turn, controls a main cold blast valve 88 and, hence, the flow rate of air into the furnace. The manual inputs to the computer 56 from the console 62 include electrical signals proportional to the desired ratio of Pl-P2 to P2-P3; the

desired hot blast temperature, the desired hot blast rate, the desired fuel enrichment and the desired moisture content. These are derived by the operator from theoretical considerations and experience and fed into the computer 56.

The relationship of the ratio of P1-P2 to P2-P3 to the amount of direct reduction is shown in FIG. 2. An increase in the difference between P1 and P2 increases the ratio and indicates that P2 and P3 are equalizing. This, in turn, means that the hot blast above the mantle is encountering restrictions which occur when the furnace begins to hang. As the ratio increases, the amount of direct reduction increases also due to a lack of sufficient carbon monoxide in the top of the furnace. The ratio of P1-P2 to P2-P3 can be reduced, as well as the amount of direct reduction, in various ways. This includes increasing the amount of enrichment fuels added to the furnace, decreasing the hot blast temperature and increasing the hot blast moisture content.

Variation in any one of these will vary the coke rate and, hence, the efficiency of the furnace. Normally, the furnace operator will select the optimum conditions and these varied by the control system of the invention primarily to avoid a hanging condition.

The entire control system is based upon the comparison of the desired values entered by the operator with the actual, measured values to derive error signals for varying fuel enrichment, hot blast temperature, moisture and the hot blast flow rate control. However. in addition, the desired ratio of Pl-P2 to P2-P3 is compared with the actual ratio of these pressure differentials to derive a correction factor for varying the feedback error signals. Thus, the computer 56 is provided with a program including a Subprogram 90 for determining the actual ratio of P1-P2 to P2-P3 and comparing it with the desired ratio as determined by the operator from console 62 to derive a correction factor. This correction factor, in turn, is applied to an enrichment program 92, a hot blast temperature program 94 and a moisture program 96. In each of these programs, the desired enrichment, hot blast temperature and moisture are compared with the actual values and are corrected in accordance with the correction factor derived from program 90 to produce output signals on leads 78, and 82. The computer 56 also includes a separate hot blast rate program 98 which compares desired hot blast rate with actual hot blast rate to derive an output signal on lead 84 without correction to maintain the desired ratio of direct-to-indirect reduction.

The flow diagram for the program for computer 56 is shown in FIG. 3. The pressure P1 is shown as block 100, pressure P2 is shown as block 102 and pressure P3 is shown as block 103. These three pressures are sent to a determiner, shown as block 104, which is used to determine the actual pressure differences P1-P2 and P2-P3 and to determine the ratio of Pl-P2 to P2-P3. This ratio of P1-P2 to P2-P3 is the ratio of direct reduction to indirect reduction being carried on internally within the vessel.

As was explained above, direct reduction in the lower part of the furnace takes place with carbon monoxide reducing the iron oxide; while the carbon dioxide is converted back to carbon monoxide. On the other hand, indirect reduction which takes place in the upper part of the furnace occurs with carbon monoxide being converted into carbon dioxide which passes off through the top of the furnace. When insufficient carbon monoxide is available in the indirect reduction zone, the iron in the upper part of the furnace solidifies and freezes the coke particles together and causes the furnace to hang. Under these conditions, there will be very little difference between the pressures P1 and P2; while the pressure difierence between P2 and P3 will increase. It can be theorized that this is due to the fact that an arch has formed above the mantle 18 which acts as a baffle for the hot blast passing upwardly through the furnace. In any event, as the ratio of P1-P2 to P2-P3 increases, it is an indication that insufficient indirect reduction is occurring and that if it continues long enough, the furnace will hang.

Block 106 of FIG. 3 represents the desired ratio of P1-P2 to P2-P3 as entered by the furnace operator via console 62. This manual input is worked out ahead of time based upon the burden characteristics together with the furnace characteristics such as vessel size, hot blast quantity available, maximum hot blast temperature, moisture limits and enrichment materials available. These enrichment materials can be either oil, tars, coil or perhaps natural gas. In block 108 of the flow diagram of FIG. 3, the desired ratio of P1-P2 to P2-P3 is compared with the actual ratio from block 104 to provide a compensation factor.

The hot blast itself has certain properties which have been predetermined. These include a desired temperature, which is represented by block 110 in FIG. 3 and the desired hot blast amount or rate identified as block 1 12. These two particular values, desired hot blast temperature and desired hot blast rate, are values which are dependent upon the stove characteristics, the temperature being a result of the size of the stoves, the type of brick checkerwork in them and the size of the burners associated with the stoves together with the number of stoves for the furnace. The hot blast amount or rate is a function of the size of the blowing machine. These are all physical limitations imposed on the overall system and have maximum limiting values which cannot be exceeded. The desired hot blast temperature and rate are entered into the computer via the operators console 62.

The desired hot blast rate from block 110 is compensated or varied in block 114 as a function of the output of block 108, this being the aforesaid correction factor due to a variation in desired and actual pressure ratios. This then gives an output comprising desired, but corrected, compensated hot blast temperature which is compared against the actual temperature from block 116 as derived from temperature sensor 70 (FIG. 1). Assuming that the actual temperature varies from the desired, corrected hot blast temperature, an error is generated by block 118 which is used in block 120 to vary the position of the butterfly valve 38 via hot blast temperature control circuit 40 (FIG. 1).

The desired hot blast amount, block 112, is compared against the actual hot blast flow from block 122, as derived from flow meter 76, in block 124. Assuming that the two are not the same, block 124 commands block 126 to varythe hot blast flow rate via valve 88 and hot blast control 86 of FIG. 1. Note that there is no compensation in hot blast rate for variations in the ratio of direct-to-indirect reduction. Furthermore, the hot blast rate will be the same regardless of the position of butterfly valve 38 since, if the air does not pass through the stove 30 it will pass through the bypass 36.

Block 128 of the flow diagram of FIG. 3 represents the desired moisture value as determined by the operator via control console 62. This manual input is based on the maximum moisture that can be used. This can run as high as 16 or 20 grains of moisture in the hot blast or it can be as low as 7 grains or even below that value. The next block 130 in the flow diagram of FIG. 3 is used to compensate the desired moisture output of block 128 as a function of the difference between the actual and desired pressure ratios as derived from block 108. The output of block 130 is then compared in block 132 with the output of block 134 which is the actual moisture content of the hot blast as determined by dew point determining device 74. The difference, if any, from block 132 is then applied to block 136 to command the moisture control 50 to vary the position of valve 48, this being represented by the block 138 in FIG. 3.

Block 140 in FIG. 3 represents the desired fuel enrichment to the hot blast and is again a manual input to the system based on materials available for enrichment and the characteristics of the total overall blast furnace configuration. In block 142 the desired enrichment is again compensated for the difference between the actual and desired pressure ratios as derived from block 108. This compensated value of desired enrichment is then sent to a comparator, block 144, where it is compared with the actual amount of enrichment as derived from flow meter 72, block 147 of FIG. 3. Assuming that the two are not the same, block 144 then commands the enrichment fuel valve control 54 to vary the position of valve 52 as represented by block 146 in FIG. 3.

The operation of the system will now be described with respect to the use of the foregoing ratios and particularly in the direct reduction zone which occurs between

pressure sensors

64 and 66 at the tuyeres and slightly above the mantle 18. With this zone assumed as being at the point of desired operation, and working with a ratio of perhaps 40 to 50 percent of the reduction being carried out by direct reduction with the remainder as indirect reduction above the mantle 18, the necessary correction should be taken if this ratio goes up which would imply that the direct reduction zone is moving above the pressure sensor 66. As the ratio of P1-P2 to P2-P3 increases, the direct reduction portion of the chemical equilibrium of the vessel moves above the mantle 18 into an area whereby the molten material solidifies and there is not enough heat available to maintain the iron in a molten condition. Hence, the coke becomes coated by the solidification of iron and presents a baffle to the hot blast, thereby raising the pressure P2 at sensor 66 whereby P2 approaches P1. When this occurs, the ratio of P1-P2 to P2-P3 decreases. Furthermore, when this happens, the furnace begins to hang and the entire system can hold up with little or nothing happening. That is, the material will no longer continue to move downwardly through the fumace.

To prevent this condition from occurring, the control system of the invention takes immediate remedial action when it finds the pressure ratio of Pl-P2 to P2-P3 increasing. That is, the system is dynamic and takes corrective action before hanging occurs. When the interface between the direct and indirect zones moves up and the ratio of the pressure differences increases, immediate corrective action is taken to move the interface downwardly. Similarly, when the interface moves down and the ratio decreases, immediate corrective action is taken to bring the system back into the desired operating ratio. Ordinarily, the first step in the remedial action is to increase the amount of moisture in the hot blast. By increasing the moisture, it can be theorized that the amount of heat available for heating up material in the vessel is reduced, meaning that the amount of heat available for liquefying the iron oxide is reduced. Thus, by reducing the heat available above the mantle by the addition of moisture, the direct reduction zone is brought back down below the mantle level. This, then, reduces the ratio of P1-P2 to P2-P3 and brings the ratio back in line with the desired value.

Another method of controlling the reaction is to cause the pressure ratio to move back to the desired value by the increased addition of enrichment materials. This creates more carbon monoxide within the furnace, such that additional carbon monoxide is available in the indirect reduction zone to eliminate the hanging condition. Furthermore, by increasing either the moisture of the hot blast or the enrichment additions to the hot blast, heat is taken to convert these from their solid or liquid state to gas. As a result, the amount of heat available above the mantle level is reduced.

A final method which can be used to control and reduce the direct reduction zone is by lowering the hot blast temperature itself. This-again accomplishes the basic end result of reducing the amount of heat that is being introduced into the furnace at the tuyere level to bring the direct reduction zone back down below the mantle level. i

If it is found instead that the ratio of P1-P2 to P2-P3 is decreasing, it is an indication that the direct-to-indirect reduction zone interface is falling below the mantle level and, as such, the furnace is not accomplishing as much direct reduction as desired. This is compensated for by the opposite of which is done when the ratio increases. That is, the amount of moisture is reduced, the amount of enrichment fuel added is reduced, or the hot blast temperature is increased. Any one of these can be essentially an instantaneous type of compensation to give immediate correction to the direct-to-indirect ratio of reduction and bring it back in line with that desired by forcing the direct reduction interfaceto rise upor falldown within the furnace and approach the mantle level. I

The determination of the best value of 'direct-to-indirect reduction is somewhat of a trial and error approach in that as the ratio goes up, the amount of coke that is being introduced into the burden comes down. The end result being sought is to achieve the minimum coking rate for the furnace with a maximum direct-toindirect ratio of reduction without having the furnace going into a hanging condition. Accordingly, this desired ratio is initially calculated for the furnace taking into account the materials that will be fed into the furnace and the furnace characteristics together with the maximum temperature and blast rate which can be introduced. Once this has been determined, a systematic approach is used to nibble away at improving the ratio while decreasing the coking rate until the ratio of direct-to-indirect reduction begins to increase at a high rate. When this occurs, it is necessary to reduce the direct-to-indirect reduction ratio, increase the coke rate slightly, and obtain a stable operation of the furnace at that point. The system described enables the furnace operator to maintain this ratio without exceeding and then going into a hanging condition. At the same time, it prevents working too far below the desired level and results in an efficient furnace operation by compensatingimmediately for any changes that are known and recognized by the system.

With reference now to FIG. 4, a hard-wired logic system is shown for accomplishing the same function as the computer system of FIGS. 1 and 3. Elements in FIG. 4 which correspond to those of FIGS. 1 and 3 are identified by like reference numerals. The system may be either analog or digital in nature; although a digital system is preferred. In this respect, the term "electrical signal as hereinafter used means either an analog signal or a digital signal of ON and OFF bits. Electrical signals from

pressure sensors

64 and 66 proportional to the pressures P1 and P2, respectively, are applied to a subtractor circuit to derive, on lead 152, an electrical signal proportional to P1-P2. Similarly, signals derived from the

pressure sensors

66 and 68 proportional to P2 and P3 are applied to subtractor circuit 154 to derive, on lead 156, an electrical signal proportional to P2-P3. These electrical signals on

leads

152 and 156 are applied to the two inputs of a divider circuit 158 to derive an electrical signal proportional to the ratio of P1-P2 to P2-P3. This electrical signal is then compared in comparator 160 with an electrical signal on lead v162 from an operators console 163 proportional to the desired ratio of P1-P2 to P2-P3. The output of the comparator 160 on lead 164 is applied to three

multipliers

166, 168 and 170.

If the system is digital in nature, then the signal on lead 164, when the desired and actual ratios are the same, would represent unity. When the actual ratio is above the desired ratio, it would be less than unity; and

when the desired ratio exceeds the actual ratio it would.

be above unity. On the other hand, if the system is analog in nature, then the output of the comparator on lead 164 will be a signal whose polarity indicates whether the actual ratio exceeds or falls below the desired ratio and whose magnitude is a function of the deviation between the two. I

Also applied to the multiplier 166, tobe multiplied by the signal on lead 164, is a signal on lead 172 from the operator's console 163 proportional to desired hot. blast temperature. The output of the multiplier 166 on lead 174, therefore, is the corrected, compensated desired temperature. Similarly, a signal on lead 176 proportional to the desired fuel enrichment rate is applied to the multiplier 168 along with the signal on lead 164 such that the output of the multiplier on lead 178 is proportional to compensated, corrected enrichment fuel rate. Finally, a signal on lead 180 from operators console 163 proportional to desired moisture content is multiplied in multiplier 170 with the correction signal on lead 164 to derive a signal on lead 182 which is proportional to compensated, corrected moisture content.

The signal on lead 174 is compared in comparator 184 with a signal on lead 186 proportional to actual hot blast temperature as derived from temperature sensor 70. The difierence between these two (i.e., actual temperature and corrected, compensated temperature) generates a signal on lead 188 which is applied to the hot blast temperature control 40. Similarly, the dif ference between the signal on lead 178 and a signal on lead 190 proportional to actual enrichment fuel rate are compared in comparator 192 to produce a signal on lead 194 for the enrichment fuel valve control 54. Finally, the signal on lead 182 proportional to corrected, compensated desired moisture content is compared with a signal on lead 196 proportional to actual moisture content as derived from dew point determining device 74 in comparator 198. The output of comparator 198, therefore, is a signal on lead 200 which is applied to the moisture control device 50.

As was mentioned above, there is no correction for variation in direct-to-indirect reduction for hot blast rate. Accordingly, a signal on lead 202 from operators console 163 proportional to the desired hot blast rate is compared in comparator 204 with a signal on lead 206 proportional to actual hot blast rate as derived from flow meter 76. The output of the comparator on lead 208 is an error signal which is applied to the hot blast control device 86 for controlling the cold blast valve 88.

From the foregoing, it will be appreciated that the function of the hard-wired logic system shown in FIG. 4 is essentially the same as that of the computer-control system shown in FIGS. 1 and 3.

Although the invention has been shown in connection with certain specific embodiments, it will be readily apparent to those skilled in the art that various changes in form and arrangement of parts may be made to suit requirements without departing from the spirit and scope of the invention.

I claim as my invention:

1. In a system for controlling a characteristic of a hot blast for a blast furnace, the combination of'rneans for producing electrical signals proportional to the pressure within the blast furnace at the bottom portion thereof, at the top portion thereof and at a point intermediate the top and bottom, computer means responsive to said electrical signals for determining the ratio of (l) the difference between the pressure at the bottom of the furnace and at said intermediate point to (2) the difference between the pressure at said intermediate point and that at the top of said furnace, and apparatus coupled to said computer means for controlling a characteristic of said hot blast as a function of said ratio of the pressure differentials to maintain a desired ratio of direct-to-indirect reduction within the furnace.

2. The system of claim 1 wherein said point intermediate the top and bottom is directly above the furnace mantle.

3. The system of claim 1 wherein said characteristic which is controlled comprises the temperature of the hot blast.

4. The system of claim 3 including means in said computer means for producing a signal proportional to the ratio of said pressure differentials, means under the control of a furnace operator for producing a first signal proportional to desired hot blast temperature, means for producing a second electrical signal proportional to the actual hot blast temperature, means for modifying said first signal by said signal proportional to the ratio of said pressure differentials, and means for comparing said second signal with said modified first signal to derive an error signal for controlling the temperature of said hot blast.

5. The system of claim 1 wherein the characteristic which is controlled comprises the moisture content of the hot blast.

6. The system of claim 5 including means in said computer means for producing a signal proportional to the ratio of said pressure differentials, means under the control of a furnace operator for producing a first signal proportional to desired moisture content of the hot blast, means for producing a second signal proport' al t 'stu co tent of e hot blast, ri ans gr first s rgnal by s a id signal proportional to the ratio of said pressure difl erentials, and means for comparing said second signal with said modified first signal to derive an error signal for controlling the moisture content of said hot blast.

7. The system of claim 1 wherein the characteristic which is controlled comprises the amount of enrichment fuels added to said hot blast.

8. The system of claim 5 including means in said computer means for producing a signal proportional to the ratio of said pressure differentials, means under the control of a furnace operator for producing a first signal proportional to desired enrichment fuel addition of the hot blast, means for producing a second signal proportional to actual enrichment fuel addition of the hot blast, means for modifying said first signal by said signal proportional to the ratio of said pressure differentials, and means for comparing said second signal with said modified first signal to derive an error signal for controlling the enrichment fuel addition of said hot blast.

9. The system of claim 1 including means for controlling the addition of enrichment fuels to said hot blast, means for controlling said hot blast temperature, means for controlling the moisture content of said hot blast, and electrical circuit apparatus responsive to said electrical signals proportional to pressure for regulating said control means.

Claims

1. In a system for controlling a characteristic of a hot blast for a blast furnace, the combination of means for producing electrical signals proportional to the pressure within the blast furnace at the bottom portion thereof, at the top portion thereof and at a point intermediate the top and bottom, computer means responsive to said electrical signals for determining the ratio of (1) the difference between the pressure at the bottom of the furnace and at said intermediate point to (2) the difference between the pressure at said intermediate point and that at the top of said furnace, and apparatus coupled to said computer means for controlling a characteristic of said hot blast as a function of said ratio of the pressure differentials to maintain a desired ratio of direct-to-indirect reduction within the furnace.

6. The system of claim 5 including means in said computer means for producing a signal proportional to the ratio of said pressure differentials, means under the control of a furnace operator for producing a first signal proportional to desired moisture content of the hot blast, means for producing a second signal proportional to actual moisture content of the hot blast, means for modifying said first signal by said signal proportional to the ratio of said pressure differentials, and means for comparing said second signal with said modified first signal to derive an error signal for controlling the moisture content of said hot blast.