CA1061550A - Cupola furnace waste gas recuperative system and method for operating same - Google Patents

Abstract

CUPOLA FURNACE WASTE GAS RECUPERATIVE
SYSTEM AND METHOD FOR OPERATING SAME

Abstract of the Disclosure An improved cupola furnace waste gas recuperative system and method for collecting waste furnace gases, cleaning them, and burning same in a recuperator-heat exchanger for preheating incoming furnace blast air. The improvement includes a method and means for the recirculation of waste cupola furnace gases through a portion of the scrubber or cleaning system for the purposes of; controlling the pressure at the furnace gas take-off chamber, preventing gases from escaping through the charging hopper, preventing excess in-draft of air, and maintaining cleaning efficiency at any gas flow rate. The improvement also includes a control means whereby the actual top gas pressure is regulated as a function of the incoming blast air.

Description

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Thls invention relates to a cupola furnace waste gas recuperative system and method for operating same, and more partlcularly to means and method for controlling gas pressure at the top of the furnace by recirculat~on of a portion of the waste furnace gases through a portion of a gas cleaning system.
In order to protect the environment ~rom harmful industrial a~r pollution, methods and apparatus have been proposed for conditioning waste gases which are a by-product of iron-making furnace operations. Capturing such effluent waste gases is especially difficult in a cupola furnace because, as opposed to present blast furnace operations, the 5 top of a top-charging cupola furnace is opened to the at-mosphere.
Presently reported cupola waste gas recuperat~ve ~ -apparatus generally are divided into two categories. One category o~ apparatus mixes atmospheric alr with the hot waste gases emitted from the furnace causing their combustion.
The hot products of combustlon together with particulate pollutants are next passed over heat exchange surfaces for heating cold incoming rurnace blast air on the opposite side of those surfaces which ~s introduced into the furnace through .
., a blast air main, bustle pipe, and tuyeres. The waste gases r are then cleaned by any method o~ scrubbing or filtering to remove the partlculates and pollutants before releasing same to the atm~sphere. Waste combustion gases are moved throug~
the recuperative system by an exhaust blower or fan means~ ~ -preferably located in a portion of the system through which the cleaned gas flows.
A second category ~f apparatus captures cupola fur-nace waste gas in a "below the charge door gas take-off",

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then conditions, cools, and scrubs the gases to remove pollutants and particulate matter. Cleaned gases are sub- i sequently introduced into a combustion chamber by means of a - blower or compressor~ where the~y are mixed with air and burned. The resulting hot products of combustion are passed through a heat exchanger for heating cold incom~ng cupola -;furnace blast air and then released to tne atmosphere.
Cleaned cupola furnace gases, not required for heating of blast air, may be used for other purposes such as for firing ln a waste heat boiler.
One of the advantages of this apparatus is the heat exchange surfaces require little or no cleaning as particulate 5 contaminants are removed ~rom the waste gases prior to burning them. This invention relates to the second category of appara-tus hereina~ter called a clean gas recuperator. Pr~sent clean gas recuperators ha~e several shortcomings.
A problem exists with known clean gas recuperative apparatus which utilizes a closing valve between the cupola furnace and the gas ~leaning apparatus and rec-uperator for the purpose of controlling the gas take-off chamber pressure because additional systems are required to maintain cleaning '.
efficiency at any flow rate. In this ~onnection, problems exist wlth known apparatus used for cIeaning the ~aste gases, commonly a wet orific~ scrubber. A wet orifice scrubber separates particles from gases by wetting the particles, accelerati~g the mixture through a venturi orifice~ and the~
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diverting the gas from the path of the particles in the dis-charge section of the scrubber. The efficiency of a wet orifice scrubber depends upon the pressure di~ferential F
through the oririce. In hereto~ore known wast~ ga~ recupera-ti.ve apparatus, it is customary to make the orifice o~ the :~

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.... . . . . . . ' 5a3 scrubber variable in order to maintain a minimum required pressure di~ferential across that orifice for maintaining cleaning efficiency at reduced flow rates. ~his requ1res a separate pressure differential control with its associated additional maintenance and wear problems.
Also, a variable speled exhaust blower and its . assoc~ated control devices are necessary if a closing valve is utilized in the recuperator.
Another problem exists in preventing the exhauster or blower from surging when the flow rate of the scrubber is less than 50~ o~ the design flow.
An addltional problem in the existing apparatus is that no workable system other than a manual control is provided to govern the amount of in-draft air brought in l;
through the open top of the cupola during its operation ; relative to the~amount of waste gases generatedO For safe operation of a clean gas recuperative system, the amount of indra~t air at the top of the cupola furnace should be close-ly controlled at all times~ If excessi~e ~ir is drawn in and mixed ~ith the was~e gas, its oxygen content can cause accidental explosive combustion of the waste gases resulting ~.:
in danger to l~fe and prope~ty. ~
Applicant1s invent~on solves the above problem~s :
associated with prior recuperative apparatus by remo~ing the .
direct valve means connection between the furnace and re-cuperatiYe system and adding means ~or recircula~ing waste gases in a controlied manner through the portion`of the`sys ~c tem which ~emoves the pollutant particles. Controllably recirculating the clean waste gases aids in determining the ~--:, ,, ;. .. .
~aste gas pressure at the top of the furnace, and mainta~ n- -lng the efficiency of particulate matter removal from the j_ ~ -4- . ~

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waste furnace gases by maintainlng full flow through a constant orifice venturi scrubber. Full flow through the venturi gas cleaning portion of the system eliminates any surging in the fan or blower.
Applicant~s invention also includes a control system not heretofore known or utiliæed which safely inte-, grates the operation of the furnace with the operations ofthe gas cleaner and the recuperator ~or any furnace gas flow rate~ ' ,10 It is therefore an ob~ect o~ the invention to provide'a new and improved method and system for cupola furnace waste gas recuperation.
An important object of the invention is t,o provide an apparatus for controllably recirculating waste furnace gases through at least a portion o~ the waste gas recupera-tive system.
Another object o~ the invention is the provision o~ a waste gas recuperative system which inte~rally ~unctions with the cupola ~urnace because barrier means therebetween is eliminated and which is capable Or controlling the amount of indraPt air in proportion to the amount of gases generated to provide safe and explosion free operations at any flow rate ~p to full design ~low. , ~, A stlll further object of the inventlon is to provide a control apparatus for the entire system includ~ng control means in the recirculation means for determining ~as pressure at the'top of the ~urnace, while maintaining the ,...
e~ficiency of the gas cleaning apparatus without the need for ,, a variable orifice scrubber. , ~ .
Other objects, featuresg and advantages o~ the , .

i~vention will be apparent from the following detailed , : , , .
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disclosure, taken in conjunction with the accompanying sheets of drawings, wherein like reference numerals refer to li~e parts, in which:
FIG. 1 iS a diagram of a cupola furnace and a waste gas recuperative system forming one embodiment of the invention operatively connected thereto;
FIG. 2 is a perspective view of a cupola and of the portion of the recuperative system through which recirculation takes place;
FIG. 3 is a vertical elevational view of the portion of the waste gas recuperative system through which recirculation of the waste gases takes place;
FIG. 4 is a horizontal plan view of the cupola and the entire waste gas recuperative system of FIG. 1 including the ; incoming blast air apparatus;
FIG. 5 is an enlarged fragmentary vertical elevational view taken on line 9-9 of FIG. 4 of the recirculation means of the invention wherein the primary duct valve means is open and the emergency duct valve means is closed as in normal operation;
FIG. 6 is a view corresponding to FIG. 5 wherein-the emergency duct valve means is open as in operation at cupola shutdown; and FIG. 7 is a schematic diagram of the control system which integrates the operation of the cleaning system and recuperator with the cupola furnace.
Referring to FIGS. 1 and 2, a conventional cupola furnace is indicated generally at 10. It includes a stack 11 w-~thin which the charge (not shown) is located. A bustle pipe 12 surrounds the bottom portion of the stack 11, and a pluraltiy of tuyeres 13 connect the bustle pipe 12 with that bottom ':

portion and provldes a passageway for blast air which is blown into the cupola l0. At the top of ~he cupola is a cylindrical charge hopper 14 and a top cover 15 which is movable to open or close the top of the furnace. Between the charge hopper 14 and the stack 11 is an annular gas take-off chamber 16 which surrounds the lower part of the charge hopper below the charge level maintained therein~ and forms the coupling between the cupola 10 and the waste gas clean-ing system, shown generally at 17. Take-off chamber 16 is refractory-lined and has ducts 20 extending diametrically .
from opposite sides thereof.
Hot waste furnace gases exit the stack 11 and trave at low velocity through the take-of~ chamber 16~ ducts 20, and i into quenchers 21 of known type. Quenchers 21 are vertically - oriented chambers each having water spray nozzles (not shown) i-facing inwardly o~ the quencher which emit water sprays into the dirty gases passing therethrough. Within the quenchers hot gases are c~oled to approximately saturation temperature ; and water vapor is added to the gases to very nearly satura-tion. Heavy dust particles and excess water collect on the - conical bottom of the quenchers and are washed away through the draln connection to a disposal tank. The downward travel-lng gases are then deflected upwardly through~gas ducts 22 Ea h gas duct 22 joins at its upper end to a duct 22a which leads into a venturi gas scrubber, shown generally L
at 23. Prior art waste gas recuperators have a positively closing valve means located ln the ducting means between the quenchers 21 and the scrubber entrance 24 in the common duct ~2a which controls the waste gas flow through the recuperator. f:: .
Applicant~s lmprovements allow the cupola and recuperator system to be interconnected without such valve means since .

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gas flow ls controlled by recirculation means d~scussed below, The venturi entrance duct 24 contains a serles of spray nozzles (not shown) facing inwardly of the duct which emit scrubbing water c~vering the entire cross section of the venturi. At the middle of the scrubber is a reduced constant diameter converging portion 25 through which the gas and particles therein are accelerat.ed. Due to a lower pressure at ~he discharge side o~ the scrubber, caused by the suction 3 o~ an exhauster or ~lower 33, the mixture is accelerated through the narrow ori~ice 25 Scrubbing water is introduced into the stream prior to passing through the orifice. The accelerating gas and particle stream shears the water stream into very small droplets or mist. Due to di~ferential f velocities between water droplets and particles and intensive turbulence, the particles are wetted by the water, agglomerate, and are consequently separated from the gases ~Yhen the stream is subjected to changes in direction in the discharge section of the scrubber.
In the cyclonic separator or mist eleimlnator 30, any particulate matter remaining in the gas is removed by ,~
means o~ centri~ugal action and also deposited in slurry tank 310 The cleaned and cooled gas is drawn ~rom the top ~ ~ .
- o~ the separator 30 through a gas line 32 into the inlet of L
an exhaust ~an~ indicated generally at 33. Rotation o~ the ~ ~ ~
impeller o.~ ~an 33 creates a Yacuum at its inlet. This -vacuum pulls.the gasës through the quencher 21, venturi - scrubber 23, and cyclonic separator 30, assures that gases in the ~urnace stack 11 do not escape to the atmosphere, and ~ .:
normally pulls small controlled amounts of environmental air th.rough the charge materials in hopper 14 lnto the stack 11 :, 8- ... .

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-of the cupo].a 10. The fan 33, also supplies a posltive pressure at its discharge ~nd. Thls positive pressure is then utilized to.force ~ases through the combustion chamber and heat exchanger of the recuperative system~ The ~an 33 is driven by an electric motor 33a From the exhaust of the fan 33 the cleaned and cooled waste gas travels up riser duct 35 in~o the clean gas main 36. A bleed stack 40 and a bleed valve 41 are connected to the clean gas main and serve to bleed off excess gas not required for burning in the 10 . recuperator 43. The bleed stack 40 may vent directly to atmosphere where permitted ~otleverg it will usually combine with other gas lines for heating purposes elsewher.e in the . plant. r From the main 36, the cleaned and cooled gas passes through downcomer 42, across control valves 42a, 4?b, and ` into the recuperator~heat exchanger, sho~n generally 43 in - FIGSo 1 and 4. Valves 42a~ 42b, control the amount of gases passing into the combustion chamber. Valve 42a controls the temperature of the blast air exiting the recuperator 43.
Val~e 42b closes the ~low of waste gases to the recuperator in the event an unsafe condition ~xists. In FIG. 4 the .!
. complete apparatus is shown including two recuperators j.
- 43~43 in parallel whereas in.the diagram of FIG. 1 only one recupPrator ls shown to simplify the explanation of `. operation. -~edundant recuperators allow furnace operation .. ~
while on~ iecuperator is being repaired. The first portion r of each-recuperator-heat exchanger 43 is the:combustion chamber.
shown at 44. Each combustion chamber 44 has an inlet 45 to . feed oxygen carrying air into the chamber and a pilot burner sectlon 46 which may be fueled by a commercial gas or oil.
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~ Air inlet 45 is connected by duct 47 to a plurality of ~ .
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combustion ~ir fans 48 which control the amount of alr fed into the combustion chamber. Typically, one o~ the three combustion air fans 48 shown in ~IG. 4 is for stand-by use only. The cleaned and cooled waste gases then enter the-combustion chamber 443 are burned, and raised to a high tem-perature. The combustion or f:Lue gases then pass into heat exchanger 50 and over heat exchanger tubes 51, which contain counterflow moving fresh blast a~r brought in through the intake duct 52 by air compressors 53. On~ of the three air compressors 53 is generally for emergency use only.
The hPat ~rom combusted waste gases is transfcrred to the blast air in heat exchanger 50 preheating it to a ~ desirable temperaturer From tubes 51 inside each heat ex- !
- changer 50, the preheated blast air ~lo~s through ducts 49 into the blast air main 54 and thence to the bustle pipe 12, through tuyeres 13, and intv cupola furnace 10. A blast air bleed vent 55 together with bleed valve ~5a and blast shut-off valve 56 provide for temporary or emergency shut~off o~
blast air to the cupola~
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The waste gases having been partially burned in the furnace 10~ cleaned, cooled, and completely burned a second -time in combustion chamber 44 ha~e chemically become safe for exhausting into the atmosphere through s~ack 60, i.e.~ they contain a dust loading of less than .05 grains/cu.ft.
- The apparatus of applicant's lnvention includes a rec~rculation duct system, shown generally at 61 intercon .. . . ..
nected or extending between the positive pressure side of gas moving and 33, at the clean gas main 36~ back to a - port~on o~ the gas cleaning system, the inlet 24 of the venturi scrubber 23. More specificallyg the recirculation ducting means 61 includes a primary recirculation duct o2, -shown most clearly in FI~S. 5 and 6, having a primary valve control means 63 positioned therein for determining the flo~
through the duct, and an emergency secondary recirculation duct 64 including a secondary recirculation control ~alve 65 for controlling the waste gas flow through the duct.
The recirculation duct means 61 connects two por~
. tions of the waste gas recuperative apparatus on either side of fan 33, thereby creating a semi-closed ci.rculatory path o~ waste gas ducting which is capable of operating in-.10 dependently of the cupola furnace 10, i.e~, the blower 33,may remain running without harming the system after the cupola 10 has shut down. The independent ducting circulatory path created by recirculation ducting means 61 is capable of temporarily storlng cleaned waste cupola gases when the cupola 10 is out of operation.
Also,~an increased flow of clean waste gases through recirculation duct means 61 decreases the negative pressure di~ferential between the cupola 10 and the recuperative waste ~-gas system 17 thereby performing the same function as the prior art valve means which physically closed off the cupola 10 from the re~uperative system 17. The uninterrupted joinder of ~he cupola 10 to the recuperative system 17 allows the ~.
- totality of the ~urnace and accouterment to function to~ether ~n a much more ef~icient manner.
The rec~rculation duct means 61 is also capable o~
maintainin~ the pressure drop across the venturi scrubber 23 at a desirable level whethër the cupola furnace 10 is ih or . .
out of operation. The efficiency of a venturi scrubber is direc~ly related to the pressure drop across the scrubber which ~:
determines the maximum speed the gases and particles therein '.
a~.tain accelerating across the venturi. In heretofore known ..

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waste gas recuperative apparatus, when the cupola furnace has been deactlvated, the venturi scrubber pressure differen-tial has dropped to zero because the air moving means, i.e., the ~ans, were also deactivated. In starting up a cupola and recuperative apparatus, waste gases were passed across the - venturi scrubber until an adequate pressure different~al was built up therein for efficient particle serar~tion. Therefore, ¦
substantial amounts of waste gases were not sufficiently cleaned un~il an adequate pressure differential was reached.
The control apparatus which integrates the safe operation of the cupola ~urnace and the gas cleaner and re- L
cuperator is shown schematically in FIG. 7. In order to monitor the physical conditions in the furnace-cleaner~
recuperator system, sensor transmitters are positioned at various locations therein to provide input into the contro apparatus. Among these are a pressure transmitter 70 and a ~low transmitter 71 positioned at the intake duct 52 to each blast air compressor 53, Signals from the transmitters are sent into an air weight controller-recorder, generally i . . ..
at 72, which includes ~eans for llnearizing tha transmitter signals at 73O The linearized signal for each compresor is then documented on recorder 74 and passed into flow con-troller 75. Controller 75 determines the air f`low through compressor 53 by means of operating a plurality o~ guide ~anes or a butterfly valve3 symbolized at 76, at the compressor in-let throu~h a current to pressure converter ~t 80. The linearized signals from each air weight controller-recorder ...
are also added together and recorded by a total flow indicator, generally at 81. m e total air flow signal is then fed into --the master pressure controller 82 whosa function is discussed below. ---. . . ~ .

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~ 55~ ( Another sensor, a differen~ial pressure trans-mitter 83, is located at the cupola furnace gas take-off chamber 16. Transmitter 83 sends a si~rlal representing the difference between atmospheric pressure and the gas take-off chamber pressure to the differential pressure controller 84.
The pressure differential from transmit~er 83 monitors the ~ressure in the gas take-off chamber 16. The differential pressure controller sends a signal which operates the primary and secondary recirculation val~es 63, 65 respectively The master pressure controller 82 adjusts the set point of the differential pressure controller ~4 allowing it to correctly control the recirculation valves 63, 65 for any rate of blast air flow through the cupola furnace 10. Alsol if one of the local override switches at A, B, C, D, etc. close, the set . point of the differential pressule cvntroller 84 is nulled .
there~y openlng recirculation val~es 63 and 65 to dec.rease the Yacuum in the charge hopper 14 to zero. The local override switches are connected to various detect~rs located .
throughout the furnace and recuperative system which are -20 .... discussed belo~.
In operation, the secondary recirculation valve 65, .`.
which may be a butterfly val~e or other known type, ln second-ary recirculation duct 64 is normally closed as in FIG. 5. .
The primar~ recirculation valve-63, similarly a butterfly or ..
other known type Yalve, is normally partially open aliowing .
an approxim~tely 10% recirculation of cupola waste gases. The .-operation or recirculation valves 63., 65 may be influenc~d . ...
- . by several means. Primarily, the amount that valves 63, 65 are ..-opened is inversely related to the negative pressure near ... ~
the top o~ cupola furnace 10. In other words, as the cupola .: . .

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1~ phased out of operation, the amount of blast air is substantially reduced and negative pressure increases at the top of stack 11. ~en this occurs, primary recirculation valve 63 opens allowing the vacuum at: the top of throat 11 to decrease. If the change is drastic, secondary recircula-tion valve 65 in larger duct 64 is opened as shown in FIG. 6 to substantially decrease the vacuum at the top of throat 11.
Primary valve 63 may operate between closed and open posi-tions in a low range of vacuum. Secondary valve 65 opera es at a high ~acuum range, the lower end of which overlaps ; the top of the operating range for valve 63. Therefore, valve 65 begins openin~ shortly before valve 63 is fully open thereby avoiding flat spots duxing changes in the recirculation flow. This action prevents the possibility of an explosion at the top of the furnace 10 or in the gas take-~ off chamber 16 which would be caused by drawing in too much ; oxygen laden air through the interface of the top cov'er 15 and charge hopper 14 with a high vacuum in the top of the furnace.
- The air would combustively combine with the waste gases which are at approximately 500F. and normally contain 18-20% carbon monoxide. A gas analyser ~not shown) is positioned in the system to read the CO, H, and 2 levels in the gas, High hydrogen content ma, mean a tuyere,water jacket has ruptured, a potentially ex~losive situation~ ' Also, the extent recirculation valves 63 and 65 are open is conjointly dependent upon the amount of blast air ~ ' ; 1Owing into the furance. The recuperation system proportionalizes the vacuum in hopper 14 with the blast air flowing into ~he stack 11 for the entire range of blast air flow rates.

, -14-Control of the recirculation valves is further influenced by the level of charye in hopper 14. Con-ventionally, radio-active sensors 70a-71a are located at two different levels across thè furnace charye hopper 1~.
I~en the charge therein reaches the lower level 71a, an in-dication is given to close top cover 15 and thereby prevent excess oxygen from being drawn into the take-off chamber.
As the furnace is temporarily deactivated, the recircula-tion valves are opened as mentioned previously. Then the cover may be reopened and the furnace is recharged to upper level 70a adding iron making matter by char~ing means 18 which may be a conveyor belt, hopper, skip hoistr or the like.
It will be understood that modiications and variations may be effected without departing from the scope : of the novel concepts of the present invention, but lt is understood that this application is limited only by the scope f th appended cla:ms, _ .

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Claims (2)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a cupola furnace, a waste gas recuperative system including in combination; ducting means for passage of waste gases therethrough leading from said furnace, water spray means in said ducting for cooling said waste gases and adding mass to pollutant particles therein by wetting them to facilitate their separation, means in said ducting for separating said wetted particles from said waste gases, means for moving said waste gases through said ducting, a recuperator having a combustion chamber for combusting said waste gases, and a heat exchanger for transferring the heat of combustion of said waste gases to blast air which is thereafter fed into said cupola furnace, the improvement comprising; recirculation ducting means for diverting waste gases before they reach the combustion chamber and for returning same to the ducting means at a location upflow of said separating means, said recirculation ducting means includes a primary duct having a valve means therein which is normally at least partially open during operation, and a secondary duct for intermittent use having a diameter greater than that of said primary duct including a valve means therein which is normally at least partially open during operation, and a secondary duct for intermittent use having a diameter greater than that of said primary duct including a valve means therein which is normally closed when said furnace is in operation.

2. The waste gas recuperative system of claim 1 further including control means for opening and closing at least said primary and secondary ducting valve means in order to proportion-alize a vacuum at the top of the furnace with a flow of blast air to the furnace and thereby control the amount of air indrafted into the top of the furnace.