Note: Descriptions are shown in the official language in which they were submitted.
~2-198 ~8 2
Docket No. 95B161 PATENT
- 1 -
FURNACE WASTE GAS COMBUSTION CONTROL
The present invention relates to an apparatus and method of operating
a coke consuming furnace and relates particularly, but not exclusively, to a
waste gas combustion control system for a cupola or similar furnace.
Background of the Invention
Cupolas are widely used in foundries to melt pig iron, scrap iron, steel
scrap or mixtures thereof. In order to operate a conventional cupola, a red
hot bed of coke is established at its bottom. The coke bed is maintained at
the desired temperature by supplying an air blast through tuyeres which
direct the air at relatively low velocity into the bed. A charge comprising
alternate layers of metal to be melted and coke is fed into the shaft of the
cupola. Hot gases created by the exothermic reaction of the air blast with
the coke bed flow upwards through the shaft of the cupola and heat the
metal by convection sufficiently for a region of molten metal to be created
immediately above the coke bed. The molten metal percolates through the
coke bed and is superheated by radiation from the coke. From time to time
molten metal is tapped off from the bottom of the cupola into a ladle for use
in the foundry. Alternatively, the molten metal may be continuously tapped
and collected in a suitable receiver. Although the coke in the bed is
progressively consumed by the reaction with the oxygen component of the
air blast, the coke layers in the charge will replenish the bed and the coke
bed is maintained at an adequate depth throughout the operation of the
cupola. It is also conventional to include within the charge limestone or
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Docket No. 95B161 - 2 -
other slag-forming agent, ferrosilicon or other suitable ferroalloys so as to
improve the metallurgical properties of the metal during the melting
operation.
While the above mentioned cupola provides a relatively efficient
method of melting scrap iron, it does suffer from one major disadvantage,
namely that there is emitted from the top of the cupola a visible smoke or
fume which is heavily laden with particles. Although it is possible to treat
such smoke or fume to reduce its content of particles so as to render it less
suitable for discharge to the atmosphere, the cost of so doing is high.
A cupola which aims to at least reduce the above mentioned
disadvantages is described in EP-A-0554022 which provides a method of
operating a vertical shaft furnace comprising: establishing a hot coke bed in
a bottom region of the furnace; charging the furnace with metal to be melted
and with coke; burning at least one stream of fuel with a stoichiometric
excess of oxygen over that required for complete combustion of the fuel
thereby forming a hot gas mixture including oxygen; introducing the hot gas
mixture into the shaft furnace and allowing it to pass upwardly through the
charge in the furnace, oxygen for the hot gas mixture thereby reacting with
the coke charge such that a part of the coke charge is consumed, the heat
being provided to the metal via the hot gas mixture and by reaction between
the oxygen and the coke being sufficient to melt the metal without the
necessity of being an air blast supplied to the furnace, and the molten metal
so formed flowing downwardly under gravity through the hot coke bed;
introducing at least one jet of oxygen or oxygen-enriched air into the hot
coke bed so as to maintain the bed at a temperature sufficient to superheat
the molten metal as it passes through the hot coke bed; and discharging
superheated molten metal from the furnace.
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Docket No. 95B161 - 3 -
The apparatus of the above mentioned patent states that the
significant reduction in visible fume emitted therefrom, in comparison with
conventional hot blast and cold blast cupolas, was attributable to an ability
(through the combustion of said at least one stream of fuel) to generate a
high temperature oxygen-containing gas mixture. This gas mixture is
typically produced at a temperature of from 900~C to 1 1 00~C. Such
temperatures are well in excess of those at which the air enters the shaft of
a conventional hot blast or cold blast cupola. This high temperature is
conducive to the creation of conditions in which gas-borne particles of coke
and the like are more readily oxidized to gaseous products than in
conventional arrangements, with the result that the amount of visible fume
emitted from the cupola shaft is significantly reduced.
It is an object of the present invention to improve still further the
cleaning up of exhaust gases emitted from such cupolas.
Summary of the Invention
Accordingly, the present invention provides a method of operating a
coke-consuming furnace comprising the steps of: establishing a hot coke bed
towards the bottom of the furnace; charging the furnace with metal to be
melted, thus establishing a layer of metal to be melted immediately above
the hot coke bed; introducing an oxygen-containing gas stream into the hot
coke bed, thereby reacting with the coke such that part of the coke charge
is consumed, an exhaust gas produced and heat provided to the metal by
said reaction such as to melt the metal; allowing molten metal so formed to
flow downwardly under gravity through the hot coke bed and extracting said
molten metal from the furnace; characterized by the further steps of
determining the temperature of the exhaust gas, measuring at least one of
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Docket No. 95B161 - 4 -
the CO, CO2 or ~2 component levels therein and altering the oxygen
concentration within the exhaust gas in accordance therewith by introducing
in a controlled manner a further quantity of oxygen-containing gas at one or
more points below, in or above the charge thereby causing any exhaust gas
CO to react with the introduced ~2 to form CO2, and combustion destruction
of carbon particles or other combustibles within said exhaust gas.
Brief Description of the Drawings
The following drawings are illustrative of embodiments of the
invention and are not intended to limit the invention as encompassed by the
claims forming part of the application.
FIG. 1 is a cross sectional view of a cupola in accordance with the
present invention;
FIG. 2 is a schematic representation of the control system of the
present apparatus;
FIG. 3 is a diagrammatic representation of a cupola according to the
present invention in combination with a filtration system; and
FIG. 4 is a diagrammatic representation of a further embodiment of a
cupola in accordance with the present invention, in combination with
filtration system.
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Docket No. 95B161 - 5 -
Detailed Description of the Preferred Embodiment
In the method of the present invention comprises an improvement in
the operation of a coke-consuming furnace wherein a hot coke bed is
established towards the bottom of the furnace, a layer of metal to be melted
is formed thereover and heat is provided into the coke bed by the
introduction of an oxygen-containing gas stream thereby melting the metal
and forming an exhaust gas. The temperature of the exhaust gas is
measured as well as at least one of the components thereof, and a further
quantity of oxygen-containing gas is introduced into the furnace to cause
substantially all of the carbon monoxide in the exhaust gas to form carbon
dioxide and cause combustion destruction of carbon particles.
Preferably, the component of the exhaust gas to be measured is
carbon monoxide which is preferably measured at the furnace gas outlet.
The addition of oxygen-containing gas to the furnace is in relation to the
amount of C0 detected and is generally adjusted so that substantially all of
the carbon monoxide in the exhaust gas is reacted to form carbon dioxide.
By "substantially all" is meant that the concentration of C0 in the exhaust
gas is reduced to less than about 1%, and preferably to about 200ppm.
The method may also comprise measuring the ~2 component level
within the exhaust gas, and adjusting the quantity of oxygen-continuing gas
introduced so as to maintain the ~2 level within the range of 5% to 15%,
more preferably between 8% and 10%. Again, the ~2 component level may
be measured at the furnace exhaust gas outlet.
While the oxygen-containing gas may be introduced below or into the
charge, it is preferably introduced into the furnace above the upper surface
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Docket No. 95B161 - 6 -
of the charge and directly into the exhaust gas. Suitably, this
oxygen-containing gas is air or air-enriched with an additional 1% to
10% ~21 preferably 2% to 4% ~2-
Advantageously, the method includes the step of monitoring theexhaust gas temperature and initiating control over a heating means for
raising the temperature thereof should it fall towards a predetermined value,
preferably at least 500~C. Preferably, the heating means is in the form of an
air or oxy/fuel burner also employed for creating a hot oxygen rich exhaust
gas which is directed for passage upwardly through the coke charge thereby
allowing some excess oxygen to react with the coke charge such that part
of the coke charge is consumed and heat is generated in excess of that
necessary for metal melting which produces a rise in the exhaust gas
temperature. Preferably, the heating means is activated before the exhaust
gas temperature drops to the predetermined minimum temperature and the
rate of supply of any fuel/air and/or oxygen thereto is controlled in
accordance with a predetermined operating characteristic.
Advantageously, the method includes the step of introducing a further
charge of coke and metal into the furnace while excluding or substantially
excluding any ingress of air therewith, preferably through a lock hopper
having inner and outer doors so that the charge is isolated from the external
atmosphere before the inner doors are opened and the charge introduced to
the furnace.
The temperature of the exhaust gas is preferably measured at a point
substantially level with the lowermost portion of the charge door through
which charges of metal and coke are introduced into the furnace. These
charges are preferably added so as to maintain the mean upper surface of
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Docket No. 95B161 - 7 -
the charge in the furnace at least 1 m, and preferably 2m, below the
lowermost portion of the charge door, oxygen-containing gas being
introduced into the exhaust gas above the upper surface of the charge in the
furnace. This allows the carbon monoxide in the upwardly-moving exhaust
gas sufficient time to react with the introduced oxygen to form carbon
dioxide. The addition of oxygen-containing gas is controlled by monitoring
the exhaust gas temperature at the lowermost edge of the charge door, so
that between 25% and 100% of the carbon monoxide is converted to
carbon dioxide. Preferably, between 50% to 90% of the carbon monoxide is
converted to carbon dioxide, leaving at least a small amount of carbon
monoxide in the exhaust gas at the level of the lowermost edge of the
charge door. This is advantageous since it is difficult to ensure that the
charge door is perfectly sealed, and therefore a small amount of air can be
allowed to enter the furnace through the charge to react with the small
amount of carbon monoxide remaining in the exhaust gas thereby converting
substantially all of it to carbon dioxide.
Additionally, or alternatively, the exhaust gas temperature is measured
at the exhaust gas outlet from the furnace and the further quantity of
oxygen-containing air being introduced into the exhaust gas adjusted in
relation thereto. The temperature of the exhaust gas at the outlet is
preferable maintained at between 600~C and 900~C, preferably between
650~C and 850~C.
In systems in which the exhaust gas is conveyed through ductwork to
a dust collection device, the temperature of the exhaust gas passing through
the ductwork may be measured and air admitted into the ductwork at a point
upstream of the furnace exhaust gas outlet and downstream of the location
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Docket No.95B161 - 8 -
at which the said temperature is measured in order to control the
temperature at which the exhaust gas enters the dust collection device.
Turning to the drawings and referring to FIG. 1, a cupola 10
comprises a vertical shaft 12 extending from a floor 14 and towards an
exhaust gas outlet 16 (best seen in FIG. 3). The shaft 12 is defined by a
cylindrical wall 20 formed of a refractory brick with an inner refractory lining22 typically of a silica-based refractory. The top of the cupola 10 forms an
outlet 24 for hot gases. The cupola 10 has a charge door arrangement
shown generally at 26 which comprises a lock hopper 28 and chute
arrangement 31 to be described in detail later herein. The cupola 10 is
further provided with a plurality of oxy/fuel burners one of which is shown at
30. Each burner comprises a supply of oxygen 32 and fuel 34 and suitable
control valves 36, 38 for controlling the flow thereof to burner 30. Each
burner is positioned for creating a hot combustion exhaust gas stream in a
plenum chamber 40 adjacent the coke bed 42 thereby allowing for the
production of a fully developed exhaust gas stream prior to its introduction
into the coke bed for reasons which will be described in detail later herein.
Towards the bottom of coke bed 42 there is provided a lance 44 for
the introduction of oxygen, oxygen-enriched air or natural air to the base of
the coke bed 42. A suitable control valve 46 and actuator 48 are provided
when initiating and maintaining control over the flow to lance 44. A further
oxygen supply arrangement 50 is provided immediately above the level of
any charge 52 within the cupola itself. Arrangement 50 comprises a
plurality of apertures 54 circumfrentially spaced around the circumference of
the cupola for allowing the introduction of air or oxygen-enriched air (up to
50% but preferably between 20 and 30% oxygen) into the exhaust gas G
virtually as soon as it passes from charge 52. As shown in FIG. 1, the
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Docket No.95B161 - 9 -
arrangement 50 might conveniently comprise supply pipe 56 connected to a
source of liquid oxygen (best seen in FIG. 2) and a control valve and actuator
arrangement 60 and 62 respectively which controls the flow of oxygen to an
annular supply duct 64 from which each of inlets 54 are fed. The lock
hopper arrangement 26 comprises a first and second pair of doors 70, 72
and suitable actuators 74, 76. In use, the lower doors 72 are moved to their
closed position and a charge 78 of scrap metal and coke is passed through
open doors 70 which are then closed therebehind. Once the charge is
loaded into the lock hopper and the doors closed therebehind, little if any air
will be drawn into the cupola when the charge is introduced. Consequently,
little if any cooling of the furnace exhaust gas G will take place during the
introduction of a fresh charge. The advantages of this arrangement are
explained in more detail later herein. An exhaust gas analyzer, shown
schematically at 80 is provided for analyzing the exhaust gas and
determining one or more of the temperature, carbon monoxide, carbon
dioxide and oxygen component levels thereof. The analyzer itself is operably
linked to a central control apparatus shown at 82 in FIG. 2 which initiates
control over the flow of fuel and/or oxygen into the apparatus in a manner to
be described in detail later herein. Means, shown schematically in the form
of opturator door 84 and actuator 86 are provided for allowing or inhibiting
the introduction of ambient air into the exhaust gas at a point downstream
of the oxygen injection thereby facilitating dilution thereof and the reduction
in its temperature.
In order to operate the cupola 10 of FlGs. 1 and 2, a bed of silica sand
is established on the floor 12a of the furnace up to the level of the bottom
of a tap hole 102 from which slag 104 can be removed from the furnace.
The coke bed 42 is then established to a level 42a above the burner 30 by
introducing coke into the cupola 10 through lock hopper 26. The bed 42 is
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Docket No. 95B 1 6 1 -1 0 -
then ignited by burner 30 or by means of a gas poker (not shown) which can
be introduced into the bed 42 through a bottom door (not shown) in the side
of the cupola 10. Next, normal operation of burner or burners 30 is
commenced thereby producing a hot exhaust gas 106 within the plenum
chamber 40 and then introducing this into the hot coke bed 42. Burners 30
are capable of being operated with excess air or oxygen, that is to say with
air or oxygen at a rate in excess of the stoichiometric requirement for
complete combustion of the fuel. The walls of the cupola are pre-heated by
hot combustion products from burners 30 for a period of up to 30 minutes.
During this period no excess air is supplied to the burners 30. Five minutes
before the end of this period, injection of pure oxygen into the coke bed 42
via lances 44 is commenced. The injection of oxygen into the coke bed 42
accelerates the rate of combustion of the coke and causes its temperature to
rise rapidly. During the final 5 minutes of pre-heating, the coke bed is made
up again to the level at which combustion was commenced. At the end of
pre-heating, the cupola 10 is loaded through lock hopper 26 with a charge
78 comprising scrap iron and coke and possibly steel, ferrosilicon and
limestone or other slagging agent. This charging is performed such that
layers of metal alternate with layers of coke. The amount of the charge is
regulated so that the top layer thereof is below the level of inlets 54.
In operation of the cupola 10 to melt the ferrous metal, the
combustion air to burner 30 is preferably enriched in oxygen. In addition,
the burners 30 are operated with up to 100% excess air or enriched
air/oxygen. The flame 106 of each burner 30 quickly extends into plenum
chamber 40 such that a hot gas mixture including oxygen leaves each flame
and ascends the cupola 10, thereby heating the ferrous metal by convection.
In addition, the oxygen in the hot gas mixture reacts with coke to generate
additional heat. The resulting hot gas mixture emanating from the top of the
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Docket No. 95B161 - 1 1 -
charge will typically have a temperature well in excess of 500~C and
normally above 750~C comprises a number of components to be described in
detail later herein.
The molten metal in the lowest of the layers begins to melt by virtue
of being heated by the hot gas mixture leaving the burners 30. A region of
molten metal is thus created and the limestone reacts with ash in the coke
to form a slag. The molten metal falls under gravity into the coke bed 42
and trickles therethrough. Typically, the molten metal is in a super heated
state as it encounters coke bed 42. During its residence in bed 42 the
molten ferrous metal is further superheated by radiant heat emanating from
the coke which is maintained at a suitably high temperature by the
continuous injection of oxygen at high velocity through lances 44. A small
amount of the coke is dissolved in the molten ferrous metal, thereby
increasing its carbon content to a predetermined level. In addition, the
silicon also dissolves in the ferrous metal. If desired, the carbon level of theferrous metal can be further enhanced by direct introduction of graphite into
the molten metal through a port (not shown) specially adapted for this
purpose. If the temperature of the molten metal is sufficiently high, there
will also be a reduction of silica at the interface between the coke and
molten slag 104 with the result that additional silicon is incorporated into themolten ferrous metal. The molten metal 100 and the slag 104 may be run
off through respective holes 110 and 102. It can therefore be appreciated
that the charge will gradually sink downwards through the cupola 30. In
addition, the reaction between the oxygen and the coke in bed 42 will cause
gradual erosion of the bed. However, the height of the bed is restored each
time melting of a layer of metal is completed since the next coke layer then
merges with bed 42. In order to enable molten metal to be produced
throughout a chosen period of time, fresh charges are periodically loaded
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Docket No.95B161 - 12 -
into the cupola through lock hopper 26. It has been observed that tap
temperatures in the order of 1500~C have been maintained over a period of
time, while being able to operate the cupola with a maximum rate of
production of molten metal some four times in excess of a minimum rate.
Turning now more specifically to aspects of the present invention, it
will be appreciated that the off-gas G from the top of bed 52 will contain a
number of different components. Typically, this exhaust gas will include
carbon monoxide, carbon dioxide, nitrous oxide, sulfur dioxide, carbon
particles and other combustibles. These component levels, together with the
temperature of the exhaust gas, are monitored by exhaust gas analyzer 80
positioned above the top of the charge bed itself. As best seen in FIG. 2,
exhaust gas analyzer 80 is operably linked to a central control apparatus
shown schematically at 82. The control apparatus 82 is operably linked for
initiating control over valves 46, 60, 36 and 38 via actuators 48, 62,120
and 122. The rate of oxygen and/or fuel supplied to various portions of the
cupola 10 may therefore be controlled in accordance with predetermined
operating requirements. Also shown in FIG. 2 are sources of oxygen or
oxygen-enriched air 124 for supply to valves 46, 60 and 36 together with a
source of fuel 126 for supply to valve 38. If the source of oxygen is a
liquefied source, vaporizers 128, 130 and 132 are provided downstream of
their respective valves 60, 46 and 36 to allow the vaporization of the
oxygen prior to its delivery. The source of fuel 126 may be liquid or gaseous
and hence, where appropriate, a pump 140 may be provided for pumping the
fluid to valve 38. Each of actuators 74, 76 and 86 are operably connected
to central controller 82 which initiates operation thereof in a manner
described in detail later herein.
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Docket No. 95B161 -13 -
Operation of the control system shown generally in FIG. 2 comprises
the steps of monitoring the various components of the exhaust gas and
adjusting the flow of oxygen, fuel and/or air as and when necessary. In
more detail, the control system 82 must achieve two main functions. First, it
must operate such as to ensure the exhaust gas G entering region R
(adjacent inlets 54) is above the auto ignition temperature of any of the
exhaust gas components thereby facilitating the complete combustion
thereof. Second, it must ensure that sufficient free oxygen is available within
this region to facilitate combustion or reaction of the undesirable
components.
The temperature of the exhaust gas within region R can be maintained
in any one of a number of different ways. For example, the apparatus can
be operated so as to increase the quantity of fuel and/or oxygen or
oxygen-enriched air to burner 30, thereby creating an increase in the
temperature of the gas stream emanating therefrom or providing excess
oxygen to the coke bed for reaction therein and the production of extra heat.
This gas mixture is typically produced at a temperature of from 900~C to
1 100~C and causes a consequential increase in the temperature of coke bed
42 and hence exhaust gas G. To some extent, the exhaust gas temperature
can be increased by supplying additional oxygen through inlet 54 such as it
reacts exothermically with any carbon monoxide contained in the exhaust
gas. This heating effect is however not as significant as the prior mentioned
method.
Preferably, the control apparatus 82 is operable to monitor the
temperature of the exhaust gas and initiate control so as to raise the
temperature thereof in advance of the temperature falling below a
predetermined value. Modern control systems well known to those skilled in
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Docket No. 95B161 -14 -
the art and therefore not described in detail herein, may be employed to
ensure efficient and accurate temperature control is achieved without
wasting fuel and/or oxygen. Side by side with the temperature control
steps, control system 82 is employed to monitor one or more of the
remaining component levels and initiate further control over the system to
ensure destruction of any undesirable components. For example, the level of
carbon monoxide in the exhaust gas can be monitored and the oxygen
supply adjusted accordingly. When excess carbon monoxide exists the
exhaust gas effectively produces a reducing atmosphere and, when the CO
is reduced to low levels, all the combustibles are essentially removed by
burning (provided the temperature in region R is above the auto ignition
temperature thereof). The reaction of carbon monoxide with oxygen is
exothermic and is easily mixed with the further carbon dioxide being emitted
in the exhaust gas.
Once the carbon monoxide has been reacted to form CO2, any further
oxygen introduced into region R is available for the combustion destruction
of any carbon particles or other combustibles within the exhaust gas.
Control of any additional oxygen over and above that required to react
carbon monoxide to carbon dioxide is particularly important as oxygen is
expensive and any wasting thereof has a significant impact on the economic
operation of the cupola itself. In practice, the control system 82 is operated
so as to ensure substantially complete reaction of carbon monoxide to
carbon dioxide and substantially complete combustion of any carbon
particles or other combustibles in the exhaust gas by the supply of additional
oxygen while curtailing the supply of oxygen over and above this
requirement. If the oxygen level in region R is too high, the control system
automatically adjusts the supply rate thereby eliminating wastage thereof.
This process, in combination with the maintenance of a temperature in
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Docket No. 95B161 -15 -
region R equal to or greater than that of the auto ignition temperature of the
undesirable components in the waste gas G is effective in producing an
exhaust gas having virtually no visible smoke.
While it will be appreciated that control of the oxygen supply can be
initiated upon detection of any particular carbon monoxide level, it has been
found that levels of 100 to 1000 ppm, preferably 500 ppm, are particularly
convenient levels to employ in the control system. Also, while it will be
appreciated that the auto ignition temperature is different for each
component, components such as simple oils and the like can be destroyed at
temperatures as low as 500~C. Consequently, if it is desirable to ensure
destruction of just simple components, the temperature in the region R may
be maintained at or above a temperature as low as 500~C. Combustion of
more complex components may require a much higher temperature and
hence operation up to and including 1 1 00~C in region R is also encompassed
by the present invention. Simple oils and the like can be combusted at a
temperature of only 200~ C to 300~C.
Referring now to FIG. 4, an apparatus comprising a cupola 10 in
combination with a dust collection device, or filtration system 150 which is
in accordance with the present invention is shown. Three exhaust gas
analyzers 80a, 80b and 80c are provided. The first exhaust gas analyzer
80a measures the temperature T1 of the exhaust gas in the furnace, as will
be further described below. The second exhaust gas analyzer 80b measures
the temperature T2 of the exhaust gas, its carbon monoxide content and its
oxygen content, all at a point adjacent the furnace exhaust gas outlet 24.
The third analyzer 80c measures the exhaust gas temperature T3 in the
ductwork leading from the furnace outlet 24 to the inlet to the dust
collection device 150.
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Docket No. 95B161 -16 -
In normal operation of the apparatus of FIG. 4, the composition of the
exhaust gas immediately above the surface of the charge 52 is principally
carbon monoxide and nitrogen; the amount of carbon monoxide is dependent
on the ratio of the coke to charge, the carbon monoxide level usually being
between 15% and 30%. The temperature of the exhaust immediately above
the charge 52 is normally greater than 750~C.
Oxygen-containing gas is admitted via arrangement 50 as described
above. This gas is air, preferably air enriched with between 1% and 10%
oxygen, more preferably 2% to 4% oxygen, the precise amount of oxygen
enrichment being calculated to ensure that between 25% and 100% (and
preferably between 50% and 90%) of the carbon monoxide is converted to
carbon dioxide. It is advantageous to provide a slightly sub-stoichiometric
amount of oxygen in the gas admitted through arrangement 50, so that most
but not all of the carbon monoxide is converted to carbon dioxide. This
allows for the remaining carbon monoxide to be converted to carbon dioxide
by reacting with any air which leaks into the furnace 10 via the charge door
26a; it is more convenient and/or less difficult and expensive to allow for a
small amount of air leakage through the charge door 26a, than to provide an
air-tight sealing arrangement thereat.
The CO~CO2 reaction occurring above the surface of the charge 52
raises the temperature of the exhaust gas and the measured temperature T1
is indicative of the extent to which that reaction has been completed. As is
explained above, the control apparatus 82 is operative to adjust the furnace
conditions to achieve a predetermined temperate measurement T1 which is
indicative of the extent of completion of the CO~CO2 reaction. In the event
T1 falls unduly, allowing the charge to combust so that the charge level falls
will encourage T1 to rise. The furnace is charged so as to maintain a set
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Docket No. 95B1 61 -1 7 -
distance D between the lower edge of the charge door 26a and the upper
surface of the charge 52. This distance is preferably 2m, and in any event
not less than 1 m, in order to allow for combustion of nearly all of the carbon
monoxide in the gas injected through arrangement 50 to form carbon dioxide
before the upwardly-rising exhaust gas reaches the lower edge of the charge
door 26a.
The ingress of air through the charge door 26a is reduced and/or
controlled by the design of the charge door arrangement 26 or other suitable
known device and by balancing the draft on the stack 160 so as to reduce
the negative pressure at the furnace outlet 24, as is known in the art.
Preferably the charge door 26a is made as small as possible, in order to be
able to more accurately predict/control the ingress of air therethrough.
The addition of more air into the exhaust region allows the substantial
completion of the CO~C02 reaction. This exothermic reaction also consists
in the combustion of other combustibles within the exhaust gas. The
second exhaust gas analyzer 80b measures the temperature and carbon
monoxide and oxygen levels in the exhaust gas at or adjacent the furnace
outlet 24. The temperature level should be between 600~C and 900~C,
preferably between 650~C and 850~C. The carbon monoxide level should be
less than 1%, preferably less than 200ppm, although in practice it has been
found that a carbon monoxide level of about 100ppm can be consistently
achieved using the method of the present invention. The oxygen level
should be between 5% and 15%, preferably between 8% and 12%. In
order to achieve/maintain these levels, the oxygen-containing gas admitted
through arrangement 50 is varied, for example, by varying the volume of air
admitted and/or by varying the amount of oxygen-enrichment thereof.
Allowing a certain amount of air to enter the ductwork via arrangement 90
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Docket No. 95B161 -18 -
will also have a minimal effect on the temperature and composition of the
exhaust gas. In practise, however, the temperature and composition of the
exhaust gas are primarily adjusted by varying the oxygen-containing gas
admitted through arrangement 50.
Maintaining the exhaust gas temperature and composition levels
within the ranges stated above (as measured at the furnace outlet 24)
ensures that substantially all of the carbon monoxide is converted to carbon
dioxide and nearly all other combustibles, such as fine coke particles,
droplets of oil, greases and other hydrocarbons and their vapors, emitted
from the cupola 10 are burned before the exhaust gas enters the dust
collection device 150. The C0 level in the exhaust gas at the furnace outlet
24 is indicative of the extent to which the other combustible components
have been burned. A low C0 level is also desirable in itself, to avoid the risk
of fire or an explosion resulting from the CO~C02 reaction in the device
150. The volatile constituents, which are largely the cause of undesirable
smoke and odor, are, when solid or liquid in form, generally smaller than
20~m, and the above method of operation substantially eliminates all such
small particles leaving only 'grits' (ie particles greater than 20~m in size) tobe dealt with by the dust collection device 150. Since the dust collection
device 150 need only deal with such 'grits' (eg oxides of iron, silicon or
aluminium and large particles of carbon), and not with smaller particles or
other volatile and potentially dangerous and/or undesirable constituents, the
device 150 may be simpler in design and hence less expensive. Moreover,
the combustion of the volatile constituents of the exhaust gas reduces
condensation and the risk of fire or explosion in the dust collection device
1 50.
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Docket No. 95B1 61 -1 9 -
The actuation of the air-admitting arrangement 90 is primarily in
response to the temperature T3 measured by analyzer 80c, the amount of air
admitted being varied so as to control the temperature of the exhaust gas
entering the dust collecting device 150, in combination with the cooling of
the exhaust gas by conduction with the ductwork as it flows towards the
device 150, so as to ensure the exhaust gas entering the device 150 does
so at a predetermined temperature, or within a predetermined temperature
range, thus allowing the device 150 to operate efficiently. The air admitted
via arrangement 90 is able to cool the exhaust gas efficiently since, as
described above, the processes of combustion are substantially complete by
the time the exhaust gases reaches the furnace outlet 24; the addition of air
via arrangement 90 is therefore only beneficial, and there is no risk that this
might cause fire or an explosion.
It has been found that the method in accordance with this invention is
applicable to all types of furnace, including both conventional cupola
furnaces and cupola furnaces of the type described in EP-A-0554022.
