Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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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 .
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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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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 ~--
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~aste gas pressure at the top of the furnace, and mainta~ n- -
lng the efficiency of particulate matter removal from the
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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
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e~ficiency of the gas cleaning apparatus without the need for ,,
a variable orifice scrubber. ,
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Other objects, featuresg and advantages o~ the
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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
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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
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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
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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
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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
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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,
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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
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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
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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
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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
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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
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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.
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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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