Note: Descriptions are shown in the official language in which they were submitted.
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A METHOD AND APPARATUS FOR REDUCING A FEED
MATERIAL IN A ROTARY HEARTH FURNACE
Field of the Invention
This invention relates to a rotary hearth furnace and a
process of reducing a feed material in a rotary hearth furnace.
More particularly, this invention relates to a rotary hearth
furnace having an improved flue system and a process of reducing
a feed material in a rotary hearth furnace.
Background of the Invention
A typical rotary hearth furnace includes an annular inner
refractory wall, an annular outer refractory wall and an annular
hearth disposed between the inner and outer walls. The hearth is
movably supported on an array of rollers about its circumference.
Disposed between the inner and outer walls and above the hearth
is a stationary roof. A plurality of burners are positioned along
the inner and/or outer walls and fire into the annular space
above the rotating hearth within the stationary roof to heat a
feed material that is typically conveyed on the rotating hearth
through various zones, e.g., loading zone, process zone and
discharge zone.
In operation, the feed material to be heated is placed
directly on the hearth in the loading zone and then conveyed
through the process zone wherein the feed material is subjected
to radiant heating and process gases conducive to chemical
reaction as the feed material is conveyed around the hearth path.
The processed feed material is then removed from the rotating
hearth in the discharge zone.
In a rotary hearth furnace, such as shown in U.S. Patent Nos.
4,597,564 and 4,622,905, gases that are produced in the rotary
hearth furnace are exhausted from a flue positioned adjacent
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the loading zone and away from the discharge zone of the furnace. The flue is
positioned adjacent the loading zone and away from the discharge zone of the
furnace to create a counter flow system drawing gases from the discharge zone
to
the loading zone, i.e., the effluent flows counter to the direction of
rotation of the
hearth for maximum exposure time to the feed material to be heated. Although
the
flue systems have performed satisfactorily, it has been found that the gases
produced in a typical counter flow type rotary hearth furnace near the
discharge
zone tend to short circuit the intended gas flow through the water seal tunnel
and
flow directly to the flue, thereby avoiding the process zone and the loading
zone.
Furthermore, it will also be appreciated that when the flue is positioned
adjacent or
proximate either the loading zone or the discharge zone a pressure
differential is
obtained at the loading zone and the discharge zone conducive to the escape of
the
furnace gases from the rotary hearth furnace through the loading zone and the
discharge zone.
Furthermore, in the direct reduction of iron process, a high CO/CO2 ratio-in
the last zone of the furnace is required to prevent back oxidation of the
direct
reduced iron (DRI). It will be appreciated that to maintain high CO/CO'
ratios, the
burners must be operated ut relatively low air to fuel ratios (less than 6.2
to 1).
These low air to fuel ratios translate to unacceptably low available heat
values, i. ., .
84.5 BTU/ft' of natural gas at an air/fuel ratio of 6.12, which translates
into high
fuel usage. Three countenneasures are available to improve the available heat
problem. The first is to recover energy from the furnace exhaust gases to
preheat
combustion air, the second is to replace some or all of the combustion air
with
oxygen, and the third is to combine preheated combustion air with oxygen
enrichment. Table 1 summarizes the effect of preheating combustion air and
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oxygen enrichment on the available heat, and on the pounds of natural gas
consumed per pound of direct reduced iron (DRI) produced.
Table 1 Available Heat and Oxygen Enrichment at CO/M ratio of about 2
Preheat
Percent Temperature Air/Fuel Ratio Available Natural Gas
Heat
Oxygen Degrees F Volume/Volume BTU/ft3 fuel lbs/lb Direct
Reduced Iron
21 70 6.12 84
21 1000 6.12 193 0.118
21 ' 200 6.12 218 0.106
21 1400 6.12 243 0.096
25 1000 5.12 221 0.107
25 1200 5.12 242 0.098
25 1400 5.12 263 0.090
30 1000 4.27 248 0.098
30 1200 4.27 265 0.092
30 1400 4.27 283 0.086
In view of the foregoing, it will be appreciated that there is a significant
need for an improved rotary hearth furnace that is directed to problems of the
prior art. It is an object of the present invention to provide a rotary
:iearth turnace
having iriiproved process gas flow. It is another object of the prcseat
invention to
provide a iotary hearth furnace that prevents the process gas from short
circuiting
the process zone through either the loading zone and/or the dischai ge zone of
the
furnace. /'-,nother object of the present invention is to provide a rotary
hearth
furnace that efficiently utilizes the available energy to reduce the feed
material in a
rotary hearth furnace. Yet another object of the present invention is to
provide a
rotary hearth furnace that efficiently reduces the amount of stack gases
exiting the
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flue of a rotary hearth furnace such that the size of the exhaust equipment
may be
reduced. Another object of the present invention is to provide a draft away
from
the loading zone to eliminate release of potentially toxic vapors arising from
the
organic or carbonaceous binders used in preparation of the feed material
through
the slots of the feeder. It will be appreciated that this allows the process
gas to be
combined with oxygen from air and to burn thereby releasing heat within the
preheat zone of the furnace. Yet another object of the present invention is to
provide a rotary hearth furnace that is simple and economical to manufacture.
Summary of the Invention
Briefly, according to this invention there is provided a rotary hearth furnace
for reducing a feed material. The rotary hearth furnace includes a rotating
hearth
disposed in a refractory lined enclosure and mounted for rotary movement. The
enclosure includes an annular inner wall, an annular outer wall and a roof.
The
enclosure is operatively sealed to the hearth and divided into a plurality of
zones
including at least a loading zone, a preheat zone, a process zone and a
discharge
zone. The fumace further includes a plurality of burners positioned in at
least the
outer wall of the enclosure to provide a controlled temperature within the
rotary
hearth furnace and a flue positioned between the l;rch-eat zone and the
process zone
of the furnace to exhaust combustion gases from the burners and process gases
resulting from the processing of the feed material.
Description of the Drawings
Further features and other objects and advantages of this invention will
become clear from the following detailed description made with reference to
the
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drawings in which:
FIG. 1 is a top view of rotary hearth furnace; and
FIG. 2 is a sectional view taken along line 2-2 of FIG. 1.
Detailed Description of the Preferred Embodiments
Referring to the figures, wherein like reference characters represent like
elements, there is shown a rotary hearth furnace 10. It should be noted that
for
purposes of clarity certain details of construction of the rotary hearth
furnace 10
are not provided in view of such details being conventional and well within
the
skill of the art once the invention is disclosed and explained. For example,
burners,
blowers, piping and duct work and the like as required for the handling of
gaseous
and particulate solid materials may be any such known commercially available
components with the exception that such components may be modified as
necessary by one skilled in the art to be employed in the overall system of
the
present invention as discussed herein. Reference is made to the Chemical
Engineer's Handbook, 7th Edition, McGraw Hill, New York 1984; Kelly, E., G.,
Introduction To Mineral Processing, John Wiley & Sons, Inc., 1982, and to the
chemical engineering industry literatur:; generally for detailed descriptions
of the
various apparatus and processing stru(.ture and conditions.
Referring to the figures, there shown a rotary hearth furnace 10 including
a hearth 12 mounted for rotary movement about its center in the
counterclockwise
direction. The hearth 12 is disposed in a doughnut shaped enclosure and
supported
on an array of rollers around its circumference as well known in the art. The
enclosure includes an annular inner sidewall 14 and an annular outer sidewall
16.
The annular sidewalls 14 and 16 are preferably disposed vertically and are
made of
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a refractory material of a type well known in the art. Positioned between and
connecting the uppermost end of the inner and outer sidewalls 14 and 16 is a
stationary roof 18. The enclosure is operatively sealed to the hearth 12 by
water
seals (not shown) as well known in the art.
Suitable burners 20 of a conventional design are positioned in the vertical
outer sidewall 14 and/or inner sidewall 16 of the enclosure. The burners 20
may be
supplied with a suitable fuel such as oil, pulverized coal and/or gas and
combusted
with air. The burners 20 are operably fired to provide a controlled
temperature
within the rotary hearth furnace 10 for reducing the feed material. When
processing a feed material containing and releasing coal volatiles, selected
burners
are utilized as air inlets for the purpose of burning the combustible gases
otherwise
present within the furnace enclosure. When operating with highly reducing
gases
in the final quadrant of the processing zone as further described herein, air
only is
introduced in selected burners to partially burn the combustible gases.
The rotary hearth furnace 10 is typically divided into a plurality of zones
including at least a loading zone 28, a preheat zone 26, a process zone 24 ana
a
discharge zone 22. Each zone may be separated from an adjacent zone by a
barrier
curtain (not shown) which is constructed of an alloy or ceramic fiber suitable
to
withstand high temperatures and corrosive atmospheres within the zones as is
v: ell
known in the art. As used herein the term "zone" refers to a separate
artificial
section of the rotary heaith furnace wherein the principal activity that is
occurriikg
is different from a prinripal activity occurring in another section of the
furnace,
e.g., loading, preheating, processing and discharging, etc. Each zone may be
further subdivided into sequential quadrants. As used herein the term
"quadrant"
refers to a separate section of each zone of the furnace. As shown in the
figures,
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the quadrants are of equal size.
In the loading zone 28, a feed material that is to be reduced is distributed
onto the rotating hearth 12 of the rotary hearth furnace 10. The feed material
may
be any suitable material that is to be reduced by heating or that is to be
exposed to
a process gas in a controlled atmosphere. The feed material may include
carbonaceous material, such as coal material, a coal material containing
mixture, a
petroleum coke re and a petroleum coke containing mixture. The feed material
may also include virgin, i.e., untreated or unprocessed, metal oxide
concentrates
and natural ore fines such as hematite, limonite, magnetite, taconite,
siderite,
pyrites and chrornire and/or metal processing mill waste, electric arc furnace
dust,
rolling mill scale, or the like, collected as a result of normal metal making
operations or a mixture thereof. The feed material may contain volatiles such
as a
coal material or a coal material containing mixture and the like, or the feed
material may be free of volatiles such as a coke material. The feed material
may be
in particulate, compact or pellet form as well known in the art.
The feed material is preferably uniformly distributed onto the hearth 12 of
the rotary hearth furnace 10 by a conventional feeder such as an electric
vibratory
feeder o: a profiled star wheel which extends through an outer s-dewall 16 of
the
furnace a suitable distance above the surface of the hearth. Ir; a preferred
embod:ment, the feed material is placed generally one layer dePp directly on
the
rotating ~'~earth 12 to facilitate uniform treatment of the feed ma:erial and
prevent
variations in the degree of reaction between highly exposed and less highly
exposed feed material.
After the feed material is charged into the loading zone 28, the feed material
is transported within the enclosure along the hearth path to the preheat zone
26,
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then into the process zone 24. The preheat zone 26 operates at a lower
temperature
than the process zone 24, i.e., 1800 degrees Fahrenheit and 2200 degrees
Fahrenheit versus 2300 degrees Fahrenheit and 2600 degrees Fahrenheit, to
minimize objectionable thermal transient conditions which might otherwise lead
to
spalling of the feed material. As shown in FIG. 1, the preheat zone 26 extends
from the downstream end of the loading zone 28 to the entry area of the
process
zone 24.
The process zone 24 is further subdivided into three sequential continguous
quadrants identified as 1, 2, and 3. Each quadrant includes an entry area and
an
exit area. Quadrant 1 extends from the downstream end of the preheat zone 26
to
the upstream entry area of quadrant 2, quadrant 2 extends from the exit area
of
quadrant 1 to the entry area of quadrant 3, quadrant 3 extends from the exit
area of
quadrant 2 to the upstream end of the discharge zone 22.
In the preheat zone 26, the burners 20 are fired to obtain a desired zone
temperature between 1800 degrees Fahrenheit and 2200 degrees Fahrenheit at an
air to fuel ratio sufficient to burn the volatile organic matter released from
the feed
material as the major source of fuel. In the process zone 24, the burners 20
are
fired to obtain a desired furnace temperature of be'ween 2300 degrees
Fahrenheit
and 2600 degrees Fahrenheit and an atmosphere c. )nducive fbr the reduction of
the
feed material. The feed material is reduced by sur-iecting the feed material
to
radiant heating and the action of combustion gases from the burners 20 and,
depending upon the feed material, to process gases evolved from the processing
of
the feed material during travel around the hearth path. Air may also be
introduced
as needed to the process zone 24 of the furnace to combust with any excess
carbon
monoxide and hydrogen from the combustion process to form carbon dioxide and
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water vapor and release heat to maintain a desired hearth temperature for the
treatment of the feed material in the process zone.
After the feed material is reduced in the process zone 24, the reduced feed
material is removed from the rotating hearth in the discharge zone 22. For
example, the reduced feed material may be discharged from the discharge zone
22
by a helical screw disposed across and spaced above the hearth. The reduced
feed
material may then be discharged to a soaking pit and the like for further
processing
as well known in the art.
In an alternative embodiment, the rotary hearth furnace 10 may also
include a warming zone (not shown). The warming zone of the rotary hearth
furnace 10 is located immediately before the loading zone for the introduction
of
the feed material. The warming zone, void of feed material, may be heated to a
desired temperature prior to loading of the feed material. It will be
appreciated that
warming of the hearth void of feed material immediately prior to charging of
the
feed material allows for the warming of the entire upper surface of the hearth
furnace and for radiant heating of the subsequently introduced feed material
from
the top and for conductive and radiant heating of the feed mmerial from the
bottom. It will be appreciated that dedicated warming of a zone of the rotary
hearth
furnace void of feed material allows the furnace to warm the rotating hearth
and
achieve a constant loading zone temperature as opposed to a rotary hearth
furnace
that experiences a cooling effect caused by the continuous charging of cold
feed
material to the rotating hearth of the furnace.
The roof 18 of the rotary hearth furnace 10 includes a flue 30 positioned
within the area of the process zone 24 of the furnace between the preheat zone
26
and the discharge zone 22. By placing the flue 30 within the process zone 24,
the
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feed material, process gas and combustion gases flow within the preheat zone
26
and are combined with oxygen from combustion air and burn releasing heat
within
the preheat zone 26 and allow process gas and combustion gases to flow from
the
discharge zone 22 and combine with oxygen from air and/or oxygen enriched air
and burn thereby also releasing heat within the furnace 10.
The flue 30 may be positioned anywhere between the exit area of the
preheat zone 26 and the entry area of quadrant 3 of the process zone. In a
preferred embodiment of the invention, the rotary hearth furnace 10 includes a
flue
30 positioned at approximately the exit area of the preheat zone 26 or the
exit area
of quadrant 2 of the process zone. It will be appreciated that by positioning
th-- flue
30 at approximately the exit area of the preheat zone 26, the loading zone and
the
discharge zone of the furnace may be maintained at a pressure equal to
atmospheric pressure to preclude furnace gases from escaping through the
loading
or discharge mechanism in a positive pressure situation, and to preclude
unwanted
air from entering the furnace in a negative pressure situation.
As shown in FIG. 2, the flue 30 includes a series of interconnected
horizontally extending afterburner chambers 32 and one or more vertically
exter. ding stacks 34. 'L he bottom of the chambers 32 are conical shaped and
a:[s
as drop out chambers to and as a collection area as further described below.
Operatively connectcJ to the bottom of each chamber 32 is a dust removal v~lve
36 of a type well known in the art. The dust removal valve 36 facilitates
accL:ss
and removal of dust and particulate matter that falls out and is collected in
the
afterburner chambers 32.
It will be appreciated that the hot exhaust stack gases within the flue 30
leave the rotary hearth furnace 10 containing waste sensible energy and
chemical
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energy. The waste sensible energy is in the form of heat and the chemical
energy
in the form of organic volatiles, carbon monoxide and hydrogen. The stack
gases
may also carry particulates consisting of fine metallic oxides and/or
carbonaceous
material. In order to achieve acceptable emission standards for the stack
gases
from the rotary hearth furnace 10, combustion air and/or oxygen is introduced
to
the afterburner chambers 32 via an air pipe 38 to combust with the organic
volatiles, other combustible gases and carbonaceous particulates from the
rotary
hearth furnace. The unburned particles within the stack gases then settle out
within the collection area of the downstream after burner chambers 32 for
removal
through dust removal valves 36.
It will be appreciated that the gases leave the rotary hearth furnace 10 at
temperatures ranging from 1800 degrees Fahrenheit to 2350 degrees Fahrenheit,
and rise to temperatures in excess of 2500 degrees Fahrenheit when combined
with
combustion air and/or oxygen. Temperatures in excess of 2500 degrees
Fahrenheit
are favorable to the formation of nitrogen oxides. Accordingly, it is
necessary to
control the combustion temperature within the afterburner chambers 32 to
facilitate
low particulate carrvover, acceptable hydrocarbon emissions, acceptable carbon
monoxi&; emissions, and acceptable low NOx emissions. The a*1erbunier
chaml,e: 32 temperature is controlled for the after burning procoss to less
than
about 1 'L-")00 degrees Fahrenheit to prevent oxidation of nitrogen to form
nitrogen
oxides. The temperature in the afterburner chamber 32 is controlled by a water
spray c uench. The water spray quench injects water droplets into the stream
of
combustion air and/or oxygen by atomizing the water droplets and cooling the
temperature within the afterburner chamber 32 and the flue 30. In a preferred
embodiment, water droplets are injected via a plurality of fluid nozzles 40
within
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the flue 30. The fluid nozzles 40 may be most any suitable nozzle such as Flo
Max
air atomizing nozzles from Spraying Systems Co.
It will be appreciated that in the case of the operation of a typical rotary
hearth furnace, combustion gases from the burners 20 and gases resulting from
processing of the feed material are exhausted near the loading zone 26 of the
furnace to provide maximum exposure of the feed material to the exhaust gases
and the process gases. However, it has been found that a significant portion
of
exhaust gases and combustible rich process gases produced in a typical rotary
hearth furnace tend to flow directly to the flue and short-circuit the
reduction zone.
Various aspects of the present invention will be further clarified by a
consideration of the following examples, which are intended to be purely
exemplary of the invention.
Examples
The rotary hearth furnace is subdivided into two zones, a preheat zone 26
and process zone 24. The process zone 24 of the rotary hearth furnace is
subdivided into three equal quadrants (1, 2, and 3) as shown in FIG. 1 to
evaluate
the effect of the position of the flue on stack gas Ilow and energy
consumption.
Stack gas flow is the measured flow rate of p:ot:ucts of combustion and
process
gas evolved from the furnace. Energy consump*ion is the measured energy
consumed in reducing a feed material.
In the examples it is assumed that about 105,032 lbs/hr. of a feed material
comprising about 79 weight percent agglomerated low silica hematite ore and
about 21 weight percent low volatile bituminous coal is reduced in a rotary
hearth
furnace. The rotary hearth furnace is maintained at a constant operating
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temperature of about 2350 degrees Fahrenheit and an operating CO/CO2 ratio of
betNveen about 2-3 to maintain a reducing atmosphere within quadrant 3 of the
furnace. The combustion air for delivery to all quadrants is preheated to
about
1200 degrees Fahrenheit prior to introduction into the furnace.
The quantity of natural gas fuel supplied to the rotary hearth furnace to
maintain a constant furnace temperature of 2350 degrees Fahrenheit is
determined
as a function of the position of the flue when located at the exit area of
quadrants
1, 2, and 3 the exit area of the preheat zone 26 and the exit area of the
Charging
Zone. The results are provided in Table 2.
Table 2
Flue Position Natural Gas Flow Rate (lbs/hr)
Quadrant 3 12,535
Quadrant 2 11,614
Quadrant 1 9,006
Preheat Zone 26 7,035
Downstream of Charaine Zone 11.300
As shown in Table 2, thE Iargest requirement of natural gas will occur when
the flue is positioned at the exit area of quadrant 3. Furthermore, the
smallest
requirement of natural gas will or; ur when the flue is positioned at the exit
area of
the preheat zone 26.
The volume of stack gases from the rotary hearth furnace is determined as a
function of the position of the flue at the exit area of quadrants 3, 2, 1 and
preheat
zone 26, and immediately downstream of the Charging Zone. The results are
provided below in Tables 3- 5.
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Table 3
Furnace Exhaust (lbs/hr)
Flue Position H20 H, N2 CO CO2 Coal Total
Volatile
Quadrant 3 33,968 1,085 149,261 36,504 30,379 0 251,197
Quadrant 2 33,176 919 145,375 34,150 31,550 0 245,170
Quadrant 1 32,493 327 135,964 29,139 32,269 0 230,192
Preheat zone 29,079 167 124,275 23,352 35,957 0 212,860
26
Downstream 24,548 369 137,022 26,213 41,823 3,903 233.878
Discharge
Zone
Table 4
Afterburner Air (lbs/hr)
Flue Position Combustion Cooling Total AFB Air
Quadrant 3 127,730 347,405 475,135
Qu~:drant 2 116,824 318,798 435,622
Quadrant 1 83,905 253,870 337,775
Preheat zone 26 85,033 163,593 248,526
Downstream 122,358 338,749 461,107
Discharge Zone
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Table 5
Flue Position Gas to PAS Cooling Water Total Stack Gas
(lbs/hr) (lbs/hr) (lbs/hr)
Quadrant 3 726,332 219,422 945,754
Quadrant 2 680,792 206,368 887,160
Quadrant 1 567,967 173,140 741,107
Preheat zone 26 461,386 141,481 602,867
Downstream 694,985 203,993 898,978
Discharge Zone
As shown in Table 5, the largest flow rate of stack gas will occur when the
flue is positioned at the exit area of quadrant 3. Furthermore, the smallest
flow
rate of stack gas will occur when the flue is positioned at the exit area of
the
preheat zone 26.
Coal volatiles are released in the preheat zone 26 of the furnace with little
appreciable metalization and may be burned with a low CO/CO2 ratio. In
quadrants
1 and 2, the CO/CO2 ratio becomes more important to limit re-oxidation of the
reduced iron in the feed material. In quadrant 3, the CO/CO,:atio must be
between
about 1.5-3.5 to suppress re-oxidation of the reduced iron. The CO and other
reducing gases produced in quadrant 3 are burned in quadrants 1 and 2 to
provide
energy to the reduction process. Air is added to the furnace to combust with
CO
anci H2 and to reduce the volume of stack gas.
In yet another embodiment of the process of the present invention, the
preheated combustion air for quadrant 3 may be enriched with 95% purity
oxygen.
The oxygen content is typically increased from about 21% to as much as 30%.
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This increase improves the available heat from a nominal 218 to 265 BTU/ft3
fuel.
The patents and documents referenced herein are hereby incorporated by
reference.
Having described presently preferred embodiments of the invention, it is to
be understood that it may be otherwise embodied within the scope of the
appended
claims.
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