Pulverized Fuel Delivery System For A Blast Furnace

Abstract

A pulverized fuel delivery system for a blast furnace in which pulverized coal is delivered in dense phase fluidized form into the blast furnace from gas pressurized tanks that are placed in communication, one at a time, in cyclical sequence with a pneumatic transport means. The tank gas pressure is regulated in accordance with the blast furnace wind rate to control the weight flow rate of pulverized coal into the furnace and the transport gas flow rate is regulated in accordance with the fuel weight flow rate to maintain a prescribed transport gas flow rate per pound of coal delivered to the furnace.

Parent Case Text

This application is a continuation of Ser. No. 799,773, filed Feb. 17, 1969, and now abandoned.

Description

This invention relates in general to pulverized fuel handling equipment and more particularly to a pulverized fuel delivery method and a system which is capable of injecting pulverized coal into a blast furnace to replace a portion of the coke normally consumed thereby.

In the smelting of iron ore in a blast furnace, coke has been traditionally the material used to provide the carbon and heat necessary for the smelting process. Coke, which normally constitutes about one-third of the furnace charge, is about the most expensive commodity in the production of iron. Consequently, replacement of a portion of the coke used with cheaper coal is of economic importance.

Various prior art systems are available for injecting pulverized coal into a blast furnace to replace part of the coke otherwise consumed, as for example the pulverized coal firing system described by U.S. Pat. No. 3,150,962 issued to L. Pearson, and that described by U.S. Pat. No. 3,301,544 issued to N.W. Eft et al.

The present invention provides a somewhat more sophisticated pulverized coal delivery system for a blast furnace which is capable of automatic operation to meet the varying coal requirements of the blast furnace. In the system of the invention, pulverized coal is delivered in dense phase fluidized form into the blast furnace from gas pressurized tanks that are placed in communication one at a time, in cyclical sequence with a pneumatic transport means. The tank gas pressure is regulated in accordance with the blast furnace wind rate to control the weight flow rate of pulverized coal into the furnace. The transport gas flow rate is regulated in accordance with the pulverized coal weight flow rate so as to maintain a prescribed transport gas flow rate per pound of coal delivered into the furnace.

Within the system, means are provided for sensing the rate at which pulverized coal is withdrawn from the tanks and for regulating, in accordance with the coal withdrawal rate, the output of pulverized coal supply means that replenishes the tanks so as to maintain a predetermined total quantity of pulverized coal stored in the system. This assures that there will be an adequate reserve of pulverized coal always available for delivery to the furnace even though the coal consumption rate may fluctuate over a wide range.

The pulverized coal delivery system includes a coal pulverizer which operates to convert the coal as delivered into a dried, pulverized product, a reservoir which receives and stores the pulverized coal output of the pulverizer system and distributor means connected to the reservoir and to feed tanks associated therewith whereby the furnace is supplied from a feed tank through pneumatic transport means. This distributor means also includes multiple valved coal lines for controlling coal flow from the reservoir to the individual feed tanks to replenish them one at a time in a cyclical sequence that is in staggered relation with respect to the coal delivery sequence from the feed tanks to the furnace. Three or more feed tanks are provided so that while one tank is feeding coal to the furnace, the second tank in the sequence is in reserve, filled with coal and pressurized, and thus is instantly available for feeding the furnace as soon as the first tank approaches the empty condition, while the third tank in the sequence is being refilled from the reservoir. With this arrangement uninterrupted coal feed to the furnace is assured, since regardless of which particular tank is feeding the furnace, there will normally be a pressurized full tank of pulverized fuel available in reserve.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects obtained by its use, reference should be had to the accompanying drawings and descriptive matter in which there is illustrated and described a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing:

FIG. 1 is a schamatic diagram of a pulverized coal delivery system according to a preferred embodiment of the invention.

FIG. 2 is a schematic diagram illustrating in, greater detail the controls associated with the system of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

In the pulverized coal delivery system 10 illustrated by way of example in FIG. 1, raw coal is withdrawn from a storage bunker 11 and flows by gravity through a shut-off valve 13, which is open when system 10 is in operation, to feeder 12. Feeder 12 supplies coal to a pulverizer 14 at a rate which can be adjusted by regulating the setting of a variable speed drive means 12A associated with feeder 12 to correspondingly regulate the pulverized coal output rate of pulverizer 14.

The pulverizer 14 operates to convert the raw coal into dried pulverized coal having a fineness suitable for injection into a blast furnace 15.

As shown, a separately fired air heater 16 connected to pulverizer 14 by a duct 22 supplies hot primary air to pulverizer 14 to dry the coal and to subsequently convey coal product output through a pipe 18 to the inlet of a cyclone separator 17. The hot primary air is produced by burning natural gas, admitted to heater 16 through a pipe 19, with air being supplied by a primary air fan 20 connected to heater 16 by a duct 21. To allow proportioning of the primary air flow rate according to the coal rate through pulverizer 14, the fan 20 is provided with an adjustable damper 20A.

The air-coal mixture that enters the cyclone separator 17 is centrifugally separated, with the coal passing to a storage reservoir 23 by gravity flow by way of a coal line 24 which is provided with a normally open shut-off valve 25. The extremely fine coal particles entrained in the primary air as it leaves separator 17 are carried along with the air through a pipe 26 to a bag filter-house 27 or other functionally similar medium and collected therein. The cleaned (or purified) primary air stream is then vented to the atmosphere and the collected, coal fines are fed to storage reservoir 23 through a coal line 28 provided with a normally open shut-off valve 29. Moisture evaporated from the coal during the pulverizing process is vented with the primary air.

If desired a plurality of pulverized coal preparation units can be operated in parallel to supply coal to storage reservoir 23, since with multiple units, intermittent operation, maintenance, or emergency servicing of any single unit can be accommodated without necessitating a shut-down of the delivery system 10. In lieu of spare pulverizing capacity afforded by multiple coal preparation units, an auxiliary storage reservoir (not shown) can be provided. The auxiliary tank could be suitably connected to the coal lines 24 and 28 to receive some or all of the pulverizer output in excess of the then current needs of the furnace 15.

The storage reservoir tank 23 is suitably vented so as to operate at atmospheric pressure and serves to provide sufficient storage of pulverized coal to supply a plurality of batch tanks 31A, 31B and 31C which feed the furnace 15. Tanks 31A-C are located at a lower elevation than reservoir tank 23 and are connected thereto by a plurality of corresponding coal distributor lines 30A, 30B, 30C respectively.

Coal distributor lines 30A-C are provided with remotely operable shut-off valves 32A, 32B, 32C respectively that serve to control the flow of pulverized coal from reservoir tank 23 to the individual batch tanks 31A-C. The tanks 31A-C are placed in communication with a pneumatic transport line 33 by means of corresponding coal outlet lines 34A-C provided with respective coal outflow control valves 35A-C that can be selectively opened to permit coal flow from selected tanks 31A-C, one at a time, to furnace 15 through line 33, and closed to isolate from line 33 those tanks 31A-C other than the one currently selected to supply coal to furnace 15.

Transport line 33 is supplied with the compressed air needed for pneumatic conveyance of the coal by a compressed air source 36, the outlet of which is connected to line 33 through a control valve 37 and a check valve 38.

At the blast furnace 15, line 33 communicates with one or more distributors 39 from which a multiplicity of coal pipes 40 lead to the individual tuyeres 41 of furnace 15, in a manner similar to that described in U.S. Pat. No. 3,150,962 to L. Pearson, and in U.S. Pat. No. 3,204,942 to W. J. Matthys et al. The number of distributors 39 as well as the number of tuyeres 41 served by each distributor 39 can be varied according to the requirements of the blast furnace 15. Each of the pipes 40 is provided with a nozzle 42 which extends into the tuyere 41 to quickly mix the coal with the blast air and thereby promote rapid combustion.

Inert gas or air is used for pressurizing the tanks 31A-C and also to aerate the coal contents of the tanks and storage reservoir 23. For such purposes, a compressed gas source 50 is provided with a delivery pressure sufficient to maintain dense phase coal flow from any given tank 31A-C into transport line 33 at the maximum anticipated furnace 15 demand rate and against the maximum expected back pressure of the furnace tuyeres 41. The choice of an inert gas for pressurizing and aerating is favored because it prevents ignition of the coal within the reservoir 23 and in tanks 31A-C.

In addition to the coal inlet valves 32A-C and the coal outlet valves 35A-C, the tanks 31A-C are provided with valves 52A-C, 53A-C, and 54A-C and 55A-C respectively to accomplish the pressurization, aerating, and venting functions required in the operation of the system 10. The pressurizing valves 52A-C are connected by suitably arranged piping to the compressed inert gas source 50 through a check valve 56 and control valve 57, and to the upper portions of their respective tanks 31A-C and when open serve to pressurize the coal contents of the tank. Aeration valves 53A-C are connected to their respective tanks 31A-C and to source 50 by suitably arranged piping in parallel with corresponding valves 52A-C and serve, when open, to introduce inert gas into the lower portion of tanks 31A-C for aerating the coal therein. Valves 54A-C serve, when open, to vent their respective tanks 31A-C to the atmosphere or to a bag filterhouse, as dictated by dust loading.

Valves 55A-C are connected via suitable piping to the storage reservoir 23 and to their respective tanks 31A-C and serve, when open, to equalize the pressures between tanks 31A-C and the reservoir 23. Reservoir 23 is aerated with inert gas passed through a conduit connecting the reservoir and the source 50, and having suitably positioned control valve 60 and check valve 61.

In the operation of system 10, each of the tanks 31A-C is alternately filled, pressurized, and emptied to feed the furnace 15 in a predetermined cyclical sequence. For example, while tank 31A is feeding the furnace 15, tank 3lB is in standby status, filled with coal and pressurized with inert gas, while tank 31C is being filled with coal from reservoir 23.

Accordingly, it can be stated that each tank 31A, 31B, 3lC must necessarily be in one of three operative states, namely: 1. The active mode, characterized by the tank being isolated from the reservoir 23, in communication with the transport line 33, and pressurized to deliver coal to the furnace 15; 2. The standby mode, characterized by the tank being isolated from both the reservoir 23 and the transport line 33, filled with coal and pressurized; 3. The refill mode, characterized by the tank being isolated from the transport line 33, in gas pressure equalizing communication with the reservoir 23, and in coal-flow communication with the reservoir 23, to receive coal therefrom.

After a tank (31A, 31B, 31C) which has been in the active mode becomes empty, that tank is switched to the refill mode, the tank which was in the standby mode is concurrently switched to the active mode so that the furnace 15 may be continuously supplied with coal without interruption or overlap, and the most recently filled tank is switched to the standby mode. The tanks 31A-C and the coal lines 30A-C from reservoir 23 are proportioned so that the tank in the refill mode will be filled with coal to its intended capacity before the then currently active tank becomes empty. Thus when switching tanks, that tank which was in the refill mode is switched to the standby mode so that there is always one tank filled and pressurized and therefore ready to immediately replace the active tank.

One of the advantages of providing three tanks 31A-C is that should it be necessary to remove one tank and its associated control system from service, for maintenance and for repair, it is still possible to continuously supply the furnace 15 using only the two remaining tanks. In such case, the two tanks would alternate between the active and refill modes.

If desired to provide additional batch tank storage capacity, this can be done simply be adding to the system 10 extra tanks equipped with valves and connected in a manner similar to the tanks 31A-C. With more than three tanks 31A-C, there will be available two or more standby tanks.

Preferably, the valves 53A-C are left open during all operating modes to assure satisfactory fluidization of the coal contents of the respective tanks 31A-C.

To place the individual tanks 31A-C in the active, standby and refill modes of operation, their respectively associated coal inlet valves 32A-C, coal outlet valves 35A-C and pressure equalization valves 55A-C are set in the states indicated by the following Table I:

TABLE I

Required Valve States Active Standby Refill Valve Mode Mode Mode _________________________________________________________________________ _ Coal Inlet(32A- C) Closed Closed Open Coal Outlet(35A- C) Open Closed Closed Equalization(55A-C) Closed Closed Open _________________________________________________________________________ _

The states of the pressurization and vent valves 52A-C and 54A-C associated with whichever tank is in the active mode are set by control signals derived on the basis of the difference between the actual coal flow rate to furnace 15 and the required or demand coal flow rate thereto. Should the actual coal flow rate be less than the demand rate, a control signal is applied to open the pressurization valve 52A-C thus increasing the tank gas pressure to raise the actual coal flow rate. Conversely, should the actual coal flow rate be greater than the demand rate, a control signal is applied to open the vent valve 54A-C thereby reducing the tank gas pressure to correspondingly reduce the actual coal flow rate.

To prevent abrupt changes in the coal flow rate to furnace 15 when a standby tank is about to be switched to the active mode, the states of the pressurization and vent valves 52A-C and 54A-C associated with the tank in the standby mode is set by control signals so as to maintain the gas pressure in the standby tank the same as that within the active tank.

At the end of the Active Mode, the tank contains only a small amount of coal, but it is pressurized with gas. Neither valves 55A-C or valves 32A-C may be opened until this gas pressure is relieved, for to do so would result in high pressure gas rushing into the reservoir 23 and forcing large amounts of pulverized coal out of its vent. Therefore, the first event of the Refill Mode is to vent the tank to the atmosphere. This could be done through valves 54A-C or through separate valves in parallel with valves 54A-C. When the pressure in the tank falls to atmospheric pressure (actually almost to atmospheric pressure), valves 54A-C (or its equivalent) is closed and valves 55A-C and 32A-C are opened to allow coal to flow from the reservoir to the tank and to allow the displaced (probably dirty) gas to flow from the tank to the reservoir. When the tank is filled with coal, valves 55A-C and 32-C are closed, valve 54A-C (or its equivalent) remains closed, and valve 52A-C is opened to pressurize the tank. When proper tank pressure is attained, the Refill Mode ends and the Standby Mode begins.

It should be noted that in order to minimize the loading of the gas source 50, the pressurization and vent valves 52A-C and 54A-C for any given tank are never opened simultaneously.

FIG. 2 illustrates by way of example a typical control system 100 which can be used to regulate the operation of the valves 32A-C, 35A-C, 52A-C, 54A-C and 55A-C as to place the tanks 31A-C in their active, standby, and refill modes according to a predetermined sequence, and to regulate the coal output of the pulverizing unit such that a predetermined total weight of coal is maintained within storage reservoir 23 and tanks 31A-C regardless of changes in the coal delivery rate to furnace 15.

Associated with control system 100 are a set of weight measuring transducers 101A-D which are arranged to sense the weight of tanks 31A-C and reservoir 23 respectively, and to establish signals W1, W2, W3, and W23 corresponding to the weight of the coal in the tanks 31A, 31B, 31C and reservoir 23 respectively. These weight signals are applied to a signal summing means 102 which provides an output signal W.sub.T representing the total weight of all the coal currently stored in the system 10 and available for delivery to furnace 15. The weight signals W1, W2, W3, are also applied to respective differentiators 103A, 103B and 103C which derive therefrom corresponding output signals Q.sub.1, Q.sub.2, and Q.sub.3 representing the rates at which the coal content weights of tanks 31A, 31B and 31C are changing. From the signals Q.sub.1, Q.sub.2, and Q.sub.3, it is possible to determine at any time which of the tanks 31A-C is in the active mode, which tank or tanks are on standby, and which tank is being refilled. For such purpose, the signals Q.sub.1, Q.sub.2, Q.sub.3, are applied to a polarity discriminator 104 which produces an output signals, Q.sub.C equivalent in magnitude to the one of the signals Q.sub.1, Q.sub.2, Q.sub.3 that represents a negative rate of weight change.

For example, when tank 31A is in the active mode, tank 31B on standby and tank 31C is being refilled, signal Q.sub.1 will be negative and its magnitude will represent the coal flow rate to furnace 15, signal Q.sub.2 will be zero, since there is no coal flow, and signal Q.sub.3 will be positive with its magnitude representing the rate at which coal is being transferred from storage reservoir 23 to the tank 31C.

The total stored coal weight signal W.sub.T is applied to an error detector 105 along with a reference signal W.sub.R established by an adjustable selector 205 and representing the total stored coal weight that is to be maintained. Error detector 105 provides an output signal E.sub.W representing the difference, or error between the signals W.sub.T and W.sub.R. Error signal E.sub.W is applied to a feeder speed controller 106 that regulates the output speed of the drive means 12A for feeder 12 (see FIG. 1) to correspondingly regulate the coal output of pulverizer 14 in accordance with the value of signal E.sub.W so as to null the error between W.sub.T and W.sub.R. Thus, the operation of pulverizer 14 is controlled to continuously introduce pulverized coal into the system 10 at the same rate at which pulverized coal leaves the system 10, and therefore, on a steady state basis the coal inflow rate to storage reservoir 23 will be equal to the actual delivery rate Q.sub.C to furnace 15.

The feeder speed controller 106 is connected to another controller, 107 which regulates the operation of the primary air fan damper 20A in accordance with the feeder 12 speed to maintain a given weight proportion between the primary air flow through pulverizer 14 and the coal flow therethrough.

To determine the pulverized coal requirements of furnace 15, flow transducer 207 is connected to the inlet of bustle pipe 51 (see FIG. 1) to sense the combustion air flow rate, or wind rate, into furnace 15, and to provide an output signal Q.sub.W representative thereof. The wind rate signal Q.sub.W is applied to an adjustable signal translator 108, which establishes an output signal Q.sub.CR representing the coal demand rate of furnace 15. The signal ratio represents the number of pounds of pulverized coal to be injected into furnace 15 per 1,000 CFM of combustion air and can be adjusted to vary the percentage of coke replacement by pulverized coal in furnace 15 operation.

The coal demand rate signal Q.sub.CR is applied to a signal convertor 109, along with the coal changes in weight rate signal Q.sub.C obtained from discriminator 104. On the basis of the two input signals Q.sub.CR and Q.sub.C, signal convertor 109 establishes an output signal P.sub.R representing the value of the gas pressure required in the tank 31A-C which then is feeding furnace 15 in order to null the difference between the actual coal delivery rate and the demand rate as indicated by signals Q.sub.C and Q.sub.CR, and thereby maintain a steady state delivery rate equal to the demand rate.

Each of the tanks 31A-C is provided with individual transducers 110A, 110B, 110C respectively, which sense the gas pressure in associated tanks 31A-C and provide output signals P.sub.1, P.sub.2, P.sub.3, indicative of the values of the pressurizing gas pressure than prevailing in tanks 31A, 31B, and 31C respectively.

For the purpose of controlling the gas pressure in each of the tanks 31A-C, the signals P.sub.1, P.sub.2, and P.sub.3 are applied to corresponding error detectors 111A, 111B, and 111C respectively, the output signal P.sub.R is also applied to each of the error detectors 111A-C. Error detector 111A establishes an error signal E.sub.P1 corresponding to the difference between the gas pressure in tank 31A and the required gas pressure indicated by signal P.sub.R. Similarly, error detectors 111B and 111C establish error signals E.sub.P2 and E.sub.P3 corresponding to the differences between the required gas pressure and that existing in tanks 31B and 31C respectively.

The tank pressure error signals E.sub.P1, E.sub.P2, and E.sub.P3 are individually applied to corresponding valve controllers 112A, 112B, 112C that regulate the operation of the associated pressuring valves 52A-C and vent valves 54A-C in accordance with the information presented by signals E.sub.P1, E.sub.P2, E.sub.P3 and with mode indicator signals that are also applied to controllers 112A-C via respective input lines 113A, 113B, 113C as will be more fully described later.

The operation of the controllers 112A-C is best explained by considering a typical example in which tank 31A is active, tank 31B is on standby, and tank 31C is being refilled. Under such conditions, controller 112A, which has signal output lines connected to the operating solenoids of the valves 52A and 54A associated with tank 31A will regulate the opening and closing of the valves 52A, 54A in accordance with the signal E.sub.P1, so as to null the pressure error represented thereby. If signal E.sub.P1 indicates that the tank 31A pressure is greater, by a predetermined threshold value, than the required value, controller 112A will cause the pressurizing valve 52A to be held closed and the vent valve 54A to be opened until the tank 31A pressure decreases to the required value, upon which occurrence, vent valve 54A will be closed by controller 112A. Conversely, should signal E.sub.P1 indicate that the tank 31A pressure is lower, by a predetermined threshold value, than the required pressure, controller 112A will cause the vent valve 54A to be held closed and the pressurizing valve 52A to be opened until the tank 31A pressure increases to the required value, upon which occurrence, valve 52A will be closed by controller 112A.

Controller 112B has signal output lines connected to the operating solenoids of the valves 52B and 54B associated with tank 31B, and when tank 31B is either active or on standby, controller 112B regulates the opening and closing of valves 52B and 54B to maintain the tank 31B gas pressure equal to the required value set by signal P.sub.R, in the same manner as previously described in connection with controller 112A.

With regard to controller 112C and the valves 52C and 54C associated with the tank 31C that is in the refill mode, it should be noted that such mode requires that the tank be vented to a low pressure receiver (not shown). Accordingly, where tank 31C is being refilled, the mode indicator signal applied to controller 112C via line 113C and identifying tank 31C as being in the refill mode causes said controller 112C to hold the pressurizing valve 52C closed, and the vent valve 54C open regardless of the value of the signal E.sub.P3.

Although in the example given, individual tanks 31A-C were identified as being in specific operating modes, it will be understood that during the normal operating cycle of the system 10, the tanks 31A-C will be placed in their other modes in accordance with a set sequential pattern, and the controllers 112A-C will function (1) to maintain the active tank at the pressure required to satisfy the coal demand rate, (2) to maintain the standby tank at the same pressure as the active tank, and (3) to maintain venting of the refill tank during the filling operation regardless of which of the tanks 31A-C are in a particular mode.

A typical mode sequencing program for the tanks 31A-C is given by the following Table II:

TABLE II Sequence Active Standby Refill Period Tank Tank Tank 1 31A 31B 31C 2 31B 31C 31A 3 31C 31A 31B 4 31A 31B 31C

from Table II it can be noted that the tank mode pattern is repeated after every third period in the sequence because there are three tanks 31A-C and each is capable of assuming the three different modes.

The setting of the tanks 31A-C in the sequence of modes prescribed by Table II is accomplished by means of a sequential controller 114 having input lines 115A-C which receive the tank weight signals W1, W2, and W3 provided by transducers 101A-C, and which has three sets of output lines 116A-C, one set for each corresponding tank 31A-C. Each output line set 116A-C includes three output lines A, S, R that carry mode command signals for establishing the corresponding tank 31A-C in the active, standby, and refill modes respectively.

The sequential controller 114 output line groups 116A-C are connected to corresponding valve controllers 117A-C to regulate the operation thereof.

The valve controllers 117A-C have output lines connected to the operating solenoids of the coal inlet, coal outlet and pressure equalizing vents valves 32A-C, 35A-C and 55A-C. The refill (R) lines of the line groups 116A-C are connected to the mode indicator input lines 113A-C of respective valve controllers 112A-C.

The valve controllers 117A-C themselves are so constructed and arranged that a mode command signal applied to the active (A) line of any controller 117A-C causes that controller to set the three valve groups, (32A, 35A, 55A), (32B, 35B, 55B), (32C, 35C, 55C) associated with the controller in the states required to place the corresponding tank 31A-C in the active mode. Similarly, a mode command signal applied to the standby (S) line of a particular controller 117A-C causes it to set its three valve group in the states required to place the corresponding tank 31A-C in the standby mode. Likewise, a mode command signal applied to the refill (R) line of a controller 117A-C causes it to set its three valve group in the states required to place the corresponding tank 31A-C in the refill mode. In addition, a mode control signal on the R line of a controller 117A-C is carried via the connected lines 113A-C to the corresponding controller 112A-C to set the valves (52A and 54A), (52B and 54B) or (52C and 54C) associated therewith in the states required when the corresponding tank 31A-C is in the refill mode.

Under normal operating conditions, there will be some coal left in whatever tank 31A-C is switched from the active mode to the refill mode, and also, at the instant the switch occurs, such tank will be pressurized with gas. To prevent any blowback of coal from a tank 31A-C into the reservoir 23, as might occur upon initiation of the refill mode, the valves 32A-C and 55A-C associated with the refill mode tank 31A-C are held closed until the gas pressure therein is reduced to approximately atmospheric pressure through the opened vent valve 54A-C. This can be accomplished by using conventional pressure sensitive interlock switch means (not shown) in conjunction with valves 32A-C and 55A-C to sense gas pressure in the corresponding tanks 31A-C and permit opening of valves 32A-C and 55A-C only when the refill tank pressure is substantially atmospheric.

At any given time, the controller 114 applies only one mode command signal per output line set. For example, in the first period of the sequence defined by Table II, controller 114 applies a mode command signal to the A line of the set 116A to put tank 31A in the active mode, applies a second mode command signal to the S line of the set 116B to put tank 31B in standby mode, and applies a third mode command signal to the R line of set 116C to put tank 31C in the refill mode. During the course of such first period, the coal weight in tank 31A will decrease and the coal weight in tank 31C will increase. Should the coal weight in tank 31C reach a predetermined maximum value as indicated by the W3 signal before the coal weight in tank 31A reaches a predetermined minimum value as indicated by the W1 signal, controller 114 will switch the third mode command signal from the R line of set 116C to the S line to put tank 31C into the standby mode, thereby eliminating any possibility of overfilling tank 31C.

For sustained pulverized coal delivery to furnace 15 without interruptions, it is necessary that the coal transfer rates from storage reservoir 23 to the individual tanks 31A-C during their refill modes be at least equal to, and preferably somewhat greater than the maximum anticipated coal outflow rate of any tank 31A-C during its active mode. Consequently, it is to be expected during any typical period of operation that the tank which at the beginning of the period was in the refill mode will become filled with coal up to its maximum coal weight before the active tank is emptied to its minimum coal weight, which of course, need not be a completely empty condition. W.J.

In such case, during the latter part of the first period of operation, there will be two tanks, 31B and 31C on standby, and the one tank 31A active.

When the coal weight in the active tank 31A reaches the predetermined minimum value, controller 114 terminates the first period of operation and initiates the second by applying mode command signals to the R line of set 116A, the A line of set ll6B, and the S line of set ll6C. Here again, should the coal weight in the refill tank 31A reach the maximum value before the end of the second period, controller 114 will switch the command signal from the R line of set 116A to the S line to place tank 31A on standby. The second period of operation ends and the third period commences when the coal weight in tank 31B reaches the minimum value.

At the end of the third period, which occurs when the coal weight in tank 31C reaches the minimum value, controller 114 sets the tanks 31A-C in the same modes as in the first period, and the tank mode program cycle is repeated.

In the operation of the blast furnace 15, the combustion air is supplied at a temperature normally 1,000.degree. F or greater above the temperature of the carrier gas, which is preferably air, introduced into line 33 (see FIG. 1) for transporting coal into furnace 15. For efficient furnace 15 operation, it is important to minimize the amount of cold air injected into the furnace 15 along with the coal, since whatever cold air goes into the furnace 15 necessitates using a portion of the fuel to supply the heat necessary to raise the temperature of the coal-air mixture to furnace 15 operating temperature. On the other hand, if the flow of carrier air in the transport line 33 is too low for the coal flow rate, there is the danger of compacting and plugging the line. Accordingly, there are optimal carrier air flow to coal flow ratios which are conducive to efficient operation of blast furnace 15.

The invention provides means for automatically regulating the flow of carrier air into transport line 33 in accordance with the coal flow rate, or more precisely in accordance with the coal demand rate of furnace 15.

For such purpose, the coal demand rate signal Q.sub.CR from signal translator 108 is applied to another signal translator 120 that provides an output signal Q.sub.AR representing the required flow rate of carrier air to be supplied to transport line 33 for the coal flow rate represented by signal Q.sub.CR.

The signal Q.sub.AR is applied to an error detector 121 along with a signal Q.sub.A derived from a flow transducer 122 connected to the carrier gas source 36 to sense the flow rate of carrier air into line 33. Signal Q.sub.A represents the actual flow rate of carrier air. Error detector 121 supplies to the input of a valve controller 123 an error signal E.sub.A representing the difference between the actual and the required carrier air flow rates. Controller 123 has an output connected to the operating solenoid of valve 37 and serves to vary the effective opening of valve 37 in accordance with signal E.sub.A so as to null the carrier air flow rate error represented thereby and thus maintain on a steady state basis a carrier air flow rate equal to that prescribed for the coal demand rate.

It should be noted that while the carrier air flow rate could be regulated on the basis of the actually existing coal flow rate, regulation on the basis of the coal demand rate is preferable since it tends to minimize the effects of system time constant lags.

As can be appreciated by the artisan, the pulverized coal delivery system 10 can be operated manually instead of automatically as provided by the control system 100, in which case the operation of the various valves associated with the tanks 23 and 31A-C, and the pulverivation unit would be operated by personnel monitoring the various weight, pressure and flow transducers provided in the system.

While in accordance with the provisions of the statutes there is illustrated and described herein a specific embodiment of the invention, those skilled in the art will understand that changes may be made in the form of the invention covered by the claims, and that certain features of the invention may sometimes be used to advantage without a corresponding use of the other features.

Claims

What is claimed is:

1. A pulverized fuel delivery system for a blast furnace which comprises a pulverizer, a feeder for controlled delivery of raw material to the pulverizer and means for regulating the flow of hot carrier medium to and through the pulverizer, control means coordinating raw material and carrier medium flow to the pulverizer, means for passing the mixture of pulverized fuel and carrier medium from the pulverizer to a separating zone wherein the fuel and carrier medium are separated with the pulverized fuel being discharged into a storage reservoir, a plurality of pressurisable tanks positioned beneath the storage reservoir for gravitational flow of pulverized fuel selectively to the pressureable tanks, means for continuously weighing the pulverized fuel in each of the tanks and the total weight of fuel in the reservoir and the tanks, pneumatic transport means for conveying pulverized fuel from the tanks into the blast furnace, fuel outflow control means connected to each of the tanks and the transport means, said outflow control means being operable to connect the tanks one at a time in predetermined sequence with said transport means to supply same with pulverized fuel, means for pressurizing the tank connected with the transport means in accordance with a condition of said blast furnace to control the rate at which pulverized fuel is delivered into the blast furnace, and control means responsive to the total weight of fuel in the reservoir and tanks to regulate the flow of carrier medium and raw material to the pulverizer.

2. A pulverized fuel delivery system according to claim 1 including means for pressurizing the next tank in said sequence to the same pressure as the tank communicated with said transport means to minimize discontinuities in the rate at which pulverized fuel is delivered to the blast furnace when the communication of the original tank with said transport means is terminated and said next tank is communicated with the transport means.

3. A pulverized fuel delivery system according to claim 1 including weighing means for sensing the quantity of pulverized fuel remaining in the tank currently communicated with said transport means and establishing a low fuel weight condition indicator signal whenever such quantity of fuel is below a predetermined amount, and wherein said outflow control means is responsive to said indicator signal to terminate the communication with said transport means of the tank currently supplying pulverized fuel thereto and communicate the next tank in said cyclical sequence with said transport means.

4. A pulverized fuel delivery system according to claim 1 wherein distributor means connects said reservoir to each of said tanks to gravitationally deliver pulverized fuel from said reservoir to each of said tanks in cyclical sequence, the fuel delivery sequence of said distributor means being in staggered order with respect to the tank-to-transport means communication sequence of said outflow control means so as to supply pulverized fuel to tanks not communicated with said transport means.

5. A pulverized fuel delivery system according to claim 1 wherein said tank pressurization means includes means for supplying pressurized inert gas to the tank communicated with said transport means, and means for regulating the inert gas pressure inside said tank in accordance with the blast furnace wind rate to maintain a pulverized fuel delivery rate into the blast furnace which is in a predetermined functional relationship to the wind rate.