U.S. patent application number 12/668440 was filed with the patent office on 2010-07-29 for exhaust gas processing system and method for rotary hearth type reducing furnace.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel Ltd.). Invention is credited to Hideaki Fujimoto, Takao Harada, Hiroshi Sugitatsu.
Application Number | 20100186644 12/668440 |
Document ID | / |
Family ID | 40228435 |
Filed Date | 2010-07-29 |
United States Patent
Application |
20100186644 |
Kind Code |
A1 |
Sugitatsu; Hiroshi ; et
al. |
July 29, 2010 |
EXHAUST GAS PROCESSING SYSTEM AND METHOD FOR ROTARY HEARTH TYPE
REDUCING FURNACE
Abstract
Provided are an exhaust gas processing system and method capable
of effectively utilizing the sensible heat of an exhaust gas for
preheating air for burner combustion in a rotary hearth type
reducing furnace while preventing troubles caused by adhesion of
dust such as blockage in an exhaust gas processing facility for a
rotary hearth type reducing furnace and corrosive deterioration in
the facility without increasing the facility costs excessively. The
system comprises a radiant heat exchanger 2 for heat exchange
between an exhaust gas exhausted from the rotary hearth type
reducing furnace and air. The radiant heat exchanger 2 includes an
inner cylinder 22 made of metal surrounding a space through which
the exhaust gas flows; an outer cylinder 21 disposed on an outer
side in a radial direction of the inner cylinder to define a flow
channel between the inner cylinder and the outer cylinder 21 for
allowing the air to flow so as to exchange heat with the exhaust
gas; and a highly thermal conductive refractory 23 applied to an
inner side of the inner cylinder 22 so as to cover an inner surface
thereof.
Inventors: |
Sugitatsu; Hiroshi; (Hyogo,
JP) ; Harada; Takao; (Hyogo, JP) ; Fujimoto;
Hideaki; (Hyogo, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND MAIER & NEUSTADT, L.L.P.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
Kabushiki Kaisha Kobe Seiko Sho
(Kobe Steel Ltd.)
Hyogo
JP
|
Family ID: |
40228435 |
Appl. No.: |
12/668440 |
Filed: |
June 20, 2008 |
PCT Filed: |
June 20, 2008 |
PCT NO: |
PCT/JP08/61324 |
371 Date: |
January 11, 2010 |
Current U.S.
Class: |
110/203 ;
165/164 |
Current CPC
Class: |
C21C 2100/06 20130101;
Y02P 10/20 20151101; Y02P 10/212 20151101; F27D 17/004 20130101;
F28F 19/00 20130101; C21B 13/105 20130101; F27D 17/001 20130101;
F28F 19/02 20130101; Y02P 10/136 20151101; C22B 7/02 20130101; F27D
17/008 20130101; F27B 9/16 20130101; F28F 13/18 20130101; C22B
1/216 20130101; F28D 7/106 20130101; Y02P 10/134 20151101; F27B
9/3044 20130101; F28D 21/001 20130101 |
Class at
Publication: |
110/203 ;
165/164 |
International
Class: |
F23J 15/06 20060101
F23J015/06; F28D 7/00 20060101 F28D007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 10, 2007 |
JP |
2007-181217 |
Claims
1. An exhaust gas processing system for processing an exhaust gas
exhausted from a rotary hearth type reducing furnace, comprising: a
radiant heat exchanger configured to perform a heat exchange
between the exhaust gas exhausted from the rotary hearth type
reducing furnace and air, wherein the radiant heat exchanger
includes: an inner cylinder made of metal surrounding a space
through which the exhaust gas flows; an outer cylinder disposed on
an outer side in a radial direction of the inner cylinder to define
a flow channel between the inner cylinder and the outer cylinder
for allowing the air therethrough so as to exchange heat with the
exhaust gas; and a highly thermal conductive refractory applied to
an inner side of the inner cylinder so as to cover an inner surface
of the inner cylinder.
2. The exhaust gas processing system for the rotary hearth type
reducing furnace according to claim 1, wherein: the highly thermal
conductive refractory contains any one of SiC, ZrB.sub.2, and BN as
a main constituent.
3. The exhaust gas processing system for the rotary hearth type
reducing furnace according to claim 2, wherein: the highly thermal
conductive refractory is a silicon carbide refractory containing
SiC as the main constituent.
4. The exhaust gas processing system for the rotary hearth type
reducing furnace according to claim 3, wherein: an SiC content in
the silicon carbide refractory is 60% or greater by mass.
5. The exhaust gas processing system for the rotary hearth type
reducing furnace according to claim 1, further comprising: heat
exchanger inlet gas temperature control means for controlling a
temperature of the exhaust gas at an inlet of the heat exchanger to
be a target temperature of 1100.degree. C. or above.
6. The exhaust gas processing system for the rotary hearth type
reducing furnace according to claim 5, wherein: the heat exchanger
inlet gas temperature control means controls the temperature of the
exhaust gas at the inlet of the heat exchanger to be a target
temperature of 1200.degree. C. or above.
7. The exhaust gas processing system for the rotary hearth type
reducing furnace according to claim 5, wherein: the heat exchanger
inlet gas temperature control means controls the temperature of the
exhaust gas at the inlet of the heat exchanger by introducing air
into the exhaust gas from the rotary hearth type reducing
furnace.
8. The exhaust gas processing system for the rotary hearth type
reducing furnace according to claim 5, wherein: the heat exchanger
inlet gas temperature control means controls the temperature of the
exhaust gas at the inlet of the heat exchanger by introducing a
cooled exhaust combustion gas into the exhaust gas from the rotary
hearth type reducing furnace.
9. The exhaust gas processing system for the rotary hearth type
reducing furnace according to claim 1, further comprising: an
exhaust gas cooling tower provided at a latter stage of the heat
exchanger to forcibly cool the exhaust gas exhausted from an outlet
of the heat exchanger.
10. The exhaust gas processing system for the rotary hearth type
reducing furnace according to claim 9, further comprising: a
refractory duct from the outlet of the heat exchanger to the
exhaust gas cooling tower, wherein the refractory duct includes a
duct main body made of metal and a refractory applied so as to
cover at least a part of an inner surface of the duct main
body.
11. A method of peeling dust adhering onto a heat transfer surface
in the exhaust gas processing system for the rotary hearth type
reducing furnace set forth in claim 1, comprising: temporarily
lowering a temperature of the surface below a temperature of the
surface during a reducing operation to generate cracking which
promotes peeling of a dust layer adhering onto a surface of the
highly thermal conductive refractory, in the dust layer.
12. An exhaust gas processing method for processing an exhaust gas
exhausted from a rotary hearth type reducing furnace, comprising:
introducing the exhaust gas exhausted from the rotary hearth type
reducing furnace into a heat exchanger for performing a heat
exchange between the exhaust gas and air to preheat the air,
wherein the heat exchanger is a radiant heat exchanger that
includes: an inner cylinder surrounding a space through which the
exhaust gas flows; an outer cylinder disposed on an outer side in a
radial direction of the inner cylinder to define a flow channel
between the inner cylinder and the outer cylinder for allowing the
air to flow therethrough; and a highly thermal conductive
refractory disposed so as to cover an inner surface of the inner
cylinder.
13. The exhaust gas processing method for the rotary hearth type
reducing furnace according to claim 12, further comprising:
controlling a temperature of the exhaust gas at an inlet of the
heat exchanger to be a target temperature of 1100.degree. C. or
above.
14. The exhaust gas processing method for the rotary hearth type
reducing furnace according to claim 13, wherein: the target
temperature of the temperature of the exhaust gas at the inlet of
the heat exchanger is 1200.degree. C. or above.
15. The exhaust gas processing method for the rotary hearth type
reducing furnace according to claim 12, wherein: the heat exchanger
performs heat exchange between the exhaust gas and the air so as to
lower an exhaust gas temperature at an outlet of the heat exchanger
to a temperature that is 600.degree. C. or above but lower than an
exhaust gas temperature at an inlet of the heat exchanger; and the
method further comprises forcibly cooling the exhaust gas to lower
the temperature thereof from 600.degree. C. to 200.degree. C.
within 5 seconds by introducing the exhaust gas exhausted from the
heat exchanger into an exhaust gas cooling tower.
Description
TECHNICAL FIELD
[0001] The present invention relates to a system and a method for
processing an exhaust gas generated in a rotary hearth type
reducing furnace (hereinafter, referred to also as the rotary
hearth furnace) by reducing carbon composite metal oxide
agglomerates, and more particularly, relates to a structure of a
heat exchanger for effective recovery of the sensible heat of the
exhaust gas and an exhaust gas processing technique using the
structure.
BACKGROUND ART
[0002] There is limited a conventional reduced iron manufacturing
process by a requirement of an expensive natural gas as a reductant
and a location of the plant which is normally restricted to a
producing region of the natural gas. Under these circumstances,
attention has been recently focused on a method of manufacturing
reduced iron using coal as the reductant; coal is relatively
inexpensive and eases the geographical restriction on the plant
location.
[0003] For example, Patent Document 1 discloses such a
manufacturing method of reduced iron using coal as the reductant,
the method comprising charging carbon composite metal oxide
agglomerates (hereinafter, simply referred to also as agglomerates)
that is a mass of a powdery mixture of iron ore and coal into a
rotary hearth furnace and heating the agglomerates under
high-temperature atmosphere to reduce it.
[0004] In this process, for effective use of the sensible heat of
the exhaust gas, the first stage in an exhaust gas channel is
provided with a heat exchanger to cool the exhaust gas indirectly
by air and to preheat the air, which is utilized for burner
combustion to heat the rotary hearth furnace. The heat exchange
between the high-temperature exhaust gas and the air for burner
combustion as above can raise the preheating temperature of the air
for burner combustion sufficiently to achieve a reduced iron
manufacturing process with an excellent energy efficiency.
[0005] In addition, by using steel mill waste instead of the iron
ore, this process is now being applied to a waste processing
process to remove zinc, lead, and so forth in the raw material by
means of reducing evaporation.
[0006] In a case where the steel mill waste is used as a raw
material, zinc and lead in the agglomerates are reduced and
evaporate in the form of metal vapor or evaporate in the form of
chloride in part under high-temperature atmosphere inside the
rotary hearth furnace to be isolated and removed from the
agglomerates, thereby producing a large volume of dust containing
not only the zinc and the lead but also other impurity elements,
such as alkaline metal and halogen. When the dust is introduced
into an exhaust gas processing facility outside the furnace
together with the exhaust gas, the dust firmly adheres onto a metal
heat transfer surface in the heat exchanger, a duct surface or the
like, and grows up there. The growth of the dust deteriorates the
heat transfer efficiency of the heat exchanger, blocks the exhaust
gas flow channel, or shortens the life of the facility caused by
metal corrosion.
[0007] There have been made the following various proposals as
countermeasures against such problems as the adherence of the dust
causing blockage in the exhaust gas processing facility for the
rotary hearth furnace or early corrosive deterioration of the
facility.
[0008] 1) Patent Documents 2 and 3 disclose a method comprising
installing a waste heat boiler using a water coolant for the first
stage of the exhaust gas processing for the rotary hearth furnace,
installing a heat exchanger using an air coolant for the next
stage, controlling the inner surface temperature of the waste heat
boiler to be 400.degree. C. or below and removing dust adhering
onto the inner surface of the waste heat boiler by means of soot
blow or by the hammering mechanism.
[0009] This method, however, requires an addition of the waste heat
boiler to the exhaust gas processing facility, which involves an
increase in the facility costs. Further, the inlet gas temperature
of the heat exchanger at the latter stage controlled to be, for
example, as low as about 600.degree. C. prevents sufficiently
raising the temperature of the preheating air for burner combustion
in comparison with the process described in Patent Document 1 using
the heat exchanger provided at the first stage of the exhaust gas
processing. This deteriorates the energy efficiency of the rotary
hearth furnace. In addition, this method requires mechanical dust
removing means such as soot blow or the hammering mechanism, which
is likely to cause facility troubles.
[0010] 2) Patent Document 4 discloses a method comprising
installing a radiant heat exchanger using an air coolant for the
first stage in the exhaust gas processing facility for the rotary
hearth furnace to lower the temperature of the exhaust gas flowing
out from the rotary hearth furnace, which is about 1200.degree. C.,
to about 1000.degree. C., thereafter precooling the gas to about
750.degree. C. in a water-cooled duct and further rapidly cooling
the gas to 250.degree. C. or below by introducing cold air into the
exhaust gas.
[0011] This method, as with the method described in Patent Document
1 above, also cannot avoid the problems, such as adhesion of dust
onto the inner surface of the heat exchanger or corrosive
deterioration of the inner surface.
[0012] 3) Patent Document 5 discloses an heat transfer tube for
heat exchanger which tube has an outer cylinder formed of porous
ceramic chiefly made of SiC. This heat transfer tube is a single
tube formed of ceramic alone, the use of which is extremely
restricted. Specifically, this type of cylinder is capable of
ensuring a required strength in a case where its diameter is small,
while hard to use due to its insufficient strength in a case where
a considerably large inner diameter is required, for example, when
there is a need to let a large volume of exhaust gas pass through
the interior of the tube.
[0013] Patent Document 1: JP-A-11-279611
[0014] Patent Document 2: JP-A-2000-169906
[0015] Patent Document 3: JP-A-2003-90686
[0016] Patent Document 4: JP-A-11-342314
[0017] Patent Document 5: JP-A-2001-48650
DISCLOSURE OF THE INVENTION
[0018] In view of the foregoing, the present invention has an
object to provide exhaust gas processing system and method capable
of effectively utilizing the sensible heat of an exhaust gas for
preheating air for burner combustion in the rotary hearth type
reducing furnace while preventing troubles due to adhesion of dust,
such as blockage in the exhaust gas processing facility for the
rotary hearth type reducing furnace and corrosive deterioration of
the facility, without excessive increase in equipment cost.
[0019] In order to accomplish the object, an exhaust gas processing
system for a rotary hearth type reducing furnace of the invention
includes a radiant heat exchanger configured to perform a heat
exchange between an exhaust gas exhausted from the rotary hearth
type reducing furnace and a coolant, the radiant heat exchanger
including: an inner cylinder made of metal surrounding a space
through which the exhaust gas flows; an outer cylinder disposed on
an outer side in a radial direction of the inner cylinder to define
a flow channel between the inner cylinder and the outer cylinder
for allowing air to flow therethrough so as to exchange heat with
the exhaust gas; and a highly thermal conductive refractory applied
on an inner side of the inner cylinder so as to cover an inner
surface of the inner cylinder.
[0020] Also, an exhaust gas processing method for a rotary hearth
type reducing furnace of the invention includes introducing an
exhaust gas exhausted from the rotary hearth type reducing furnace
into a heat exchanger for heat exchange between the exhaust gas and
air to preheat the air, wherein the heat exchanger is a radiant
heat exchanger that includes: an inner cylinder surrounding a space
through which the exhaust gas flows; an outer cylinder disposed on
an outer side in a radial direction of the inner cylinder to define
a flow channel between the inner cylinder and the outer cylinder
for allowing the air to flow therethrough; and a highly thermal
conductive refractory disposed so as to cover an inner surface of
the inner cylinder.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a flow diagram schematically showing the
configuration of an exhaust gas processing system for a rotary
hearth type reducing furnace according to an embodiment of the
invention.
[0022] FIG. 2 is a perspective view showing in detail a radiant
heat exchanger included in the exhaust gas processing system.
[0023] FIG. 3 is a graph showing the relation between the elapsed
number of days for operation and the overall heat transfer
coefficient on the heat transfer surface of the radiant heat
exchanger in a case where an SiC content in a silicon carbide
refractory is 60% by mass.
[0024] FIG. 4 is a graph showing the relation between the elapsed
number of days for operation and the preheating air temperature in
a case where an SiC content in a silicon carbide refractory is 60%
by mass.
[0025] FIG. 5 is a graph showing the relation between the elapsed
number of days for operation and the overall heat transfer
coefficient on the heat transfer surface of the radiant heat
exchanger in a case where an SiC content in a silicon carbide
refractory is 80% by mass.
[0026] FIG. 6 is a graph showing the relation between the elapsed
number of days for operation and the preheating air temperature in
a case where an SiC content in a silicon carbide refractory is 80%
by mass.
[0027] FIG. 7 is a cross section showing the temperature profile on
the heat transfer surface of the radiant heat exchanger.
[0028] FIG. 8 is a flow diagram schematically showing the
configuration of an exhaust gas processing system for a rotary
hearth type reducing furnace according to a comparative
example.
BEST MODE FOR CARRYING OUT THE INVENTION
[0029] Hereinafter, preferred embodiments of the invention will be
described in detail with reference to the drawings.
[0030] FIG. 1 schematically shows the configuration of an exhaust
gas processing system for a rotary hearth type reducing furnace
according to one embodiment of the invention. The exhaust gas
processing system includes a rotary hearth furnace 1 that exhausts
an exhaust gas G, a radiant heat exchanger 2 that performs a heat
exchange between the exhaust gas G exhausted from the rotary hearth
furnace 1 and air A for burner combustion used in the rotary hearth
furnace 1, an exhaust gas cooling tower 4 connected to an outlet of
the heat exchanger 2 via a refractory duct 3 to rapidly cool the
exhaust gas introduced therein via the refractory duct 3, a dust
collector 5 that applies dust removal processing to the rapidly
cooled exhaust gas, and an induced draft fan 6 that exhausts the
exhaust gas to which the dust removal has been applied, through a
stack 7.
[0031] Hereinafter, a further detailed description will be given
along the flow of the exhaust gas G.
[0032] Heat Exchanger 2 [0033] From the rotary hearth furnace 1 is
exhausted the exhaust gas G having a temperature of 1100 to
1450.degree. C. . The high-temperature exhaust gas, the temperature
of which is adjusted (described in detail below) as needed, is
introduced into the heat exchanger 2 to exchange heat with the air
A for burner combustion used in the rotary hearth furnace 1.
[0034] FIG. 2 shows the heat exchanger 2 in detail. The heat
exchanger 2 includes an outer cylinder 21 made of metal, an inner
cylinder 22 made of metal and disposed on the inner side of the
outer cylinder 21 so as to keep a distance from the inner surface
thereof, and a highly thermal conductive refractory 23 laid on the
inner cylinder 22 so as to cover the ginner surface of the inner
cylinder 22. The inner cylinder 22 and the refractory 23 surround
an exhaust gas flow channel through which the exhaust gas G as the
heat source is flown. Meanwhile, the inner cylinder 22 and the
outer cylinder 21 define an air flow channel therebetween for
allowing the air A as a coolant to flow through the air flow
channel. Examples of suitable metals forming the inner cylinder 22
and the outer cylinder 21 include but not limited to steel plates
made of carbon steel and stainless steel.
[0035] Dust contained in the exhaust gas G at high concentration
comes into contact with the surface of the highly thermal
conductive refractory 23 and is cooled to turn into a solid , thus
eventually forming an adhesion layer on the surface. However, the
adhesion layer of the dust formed on the surface of the highly
thermal conductive refractory 23 hardly grows because it is brittle
and readily peels off, thus having a small thickness. The adhesion
layer therefore hardly gives influences to the heat transfer
efficiency of the heat exchanger 2 and will never block the exhaust
gas flow channel.
[0036] The refractory 23 covers the inner surface of the inner
cylinder 22 to inhibit corrosive dust from direct contact with the
inner cylinder 22, thereby prevents corrosive deterioration. The
thin adhesion layer of the dust formed on the surface of the
refractory 23 can rather contribute to prevention of abrasion or
wear of the refractory 23 due to contact with the dust flowing with
the exhaust gas. This eliminates the need for the mechanical
removing means for removing dust during operation as are disclosed
in Patent Documents 2 and 3. Although increase in the thickness of
the adhesion layer of the dust is capable of deteriorating the heat
transfer efficiency or causing a blockage of a flow channel of the
exhaust gas, the adhesion layer of the dust is so brittle and so
likely to peel off that the removal of the adhesion layer is easy.
For example, the removal can be easily performed by preliminarily
forming maintenance holes at the inlet portion and the outlet
portion of the heat exchanger and inserting a metal rod into the
heat exchanger through the maintenance holes to poke the adhesion
layer at the time of maintenance inserted.
[0037] Further, temporarily lowering the temperature of the surface
of the highly thermal conductive refractory 23 below the
temperature of the surface during the reduction operation can
provide the dust layer adhering onto the surface with cracking
which promotes peeling of the dust layer in. This makes it possible
to easily remove the dust layer without a work of inserting a rod
or the like through the maintenance holes provided in advance to
poke the dust layer.
[0038] It is sufficient to perform this operation once a month for
example. The time for each operation (a time over which the
temperature is lowered) preferably continues for 8 to 12 hours in
accordance with the daily maintenance or the like However, even an
operation over a time as short as 30 minutes to 1 hour also can
achieve the effect described above.
[0039] This operation can be performed while the raw material is
charged into the rotary hearth furnace; however, it is preferably
performed as idle driving when no raw material is charged,
specifically during the daily maintenance or the like. For example,
setting the heat exchanger inlet temperature to 1,350.degree. C.
during the reduction operation and setting the heat exchanger inlet
temperature to 1,050.degree. C. (that is, lowering the gas
temperature by about 300.degree. C.) during the idle driving, can
cause thermal contraction in the dust layer adhering onto the
surface of the highly thermal conductive refractory 23 to form the
cracking therein, which can promote the peeling of the dust
layer.
[0040] An amount of the exhaust gas and an amount of combustion air
during the idle driving are smaller than those during the reduction
operation. For example, an amount of the exhaust gas, which is
11,700 Nm.sup.3/h during the reduction operation, decreases to
about 6,000 Nm.sup.3/h, and an amount of preheating air, which is
13,100 Nm.sup.3/h during the reduction operation, decreases to
about 6,000 Nm.sup.3/h. However, intentional increase in an amount
of preheating air during the idle driving also can lower the
surface temperature of the highly thermal conductive refractory 23,
and extra preheating air generated by the increase may be released
to atmosphere.
[0041] Examples of the highly thermal conductive refractory 23
include refractory containing SiC, ZrB.sub.2, and BN having high
thermal conductivity as a main constituent. The refractory excel
not only in thermal conductivity but also in wear resistance and
corrosion resistance. Accordingly, they are preferable in terms of
resistance against an exhaust gas from the rotary hearth furnace
that contains corrosive dust at high concentration.
[0042] Among all, the most strongly recommended is a refractory
containing SiC as a main constituent (silicon carbide refractory),
which is relatively inexpensive. A silicon carbide refractory which
contains more SiC is more preferable in terms of ensuring the
thermal conductivity. The SiC content is preferably 60% or greater
by mass and more preferably 70% or greater by mass. The thermal
conductivity of the silicon carbide refractory is, for example,
4.65 W/ (mK) when the SiC content is 30% by mass, 8.72 W/(mK) when
the SiC content is 60% by mass, and 11.1 W/(mK) when SiC content is
78% (the rest are Al.sub.2O.sub.3 and SiO.sub.2 and the value of
the thermal conductivity is indicated at 1,000.degree. C. in each
case). In addition, refractory containing SiC as a main constituent
has an advantage that its surface forms an SiO.sub.2 membrane with
excellent corrosive resistance.
[0043] The thinner the highly thermal conductive refractory 23, the
more preferable to ensure an overall heat transfer coefficient of
the heat exchanger 2; however, in consideration with the upper
temperature limit of the inner cylinder 22 made of metal, wear of
the refractory 23, applicability of the refractory 23 to the inner
surface of the inner cylinder 22, and so forth into account, it is
preferable to adjust the thickness in a range of 30 mm to 100 mm
both inclusive as needed. The refractory 23 can be applied, for
example, by welding plural anchors made of metal to the inner
surface of the inner cylinder 22, installing a framework, and
injecting a castable refractory into the framework. the castable
refractory is preferably injected by means of vibration filling,
which increases the denseness of the refractory applied to prevent
deterioration of thermal conductivity and suppress adhesion of
dust. Further, the refractory is preferably finished with a smooth
surface. It is important to control moisture during
application.
[0044] The temperature of the exhaust gas G to be introduced into
the heat exchanger 2 (that is, the gas at the inlet of the heat
exchanger 2) is preferably adjusted, by specific heat exchanger
inlet gas temperature control means 8, to the target temperature of
1,100.degree. C. or above and more preferably 1,200.degree. C. or
above but lower than the temperature of the exhaust gas G exhausted
from the rotary hearth furnace 1. The conventional heat exchanger
without the refractory for covering the inner surface of the inner
cylinder made of metal limits the temperature of the exhaust gas to
be introduced therein to about 1,000.degree. C. or lower because of
the restriction by the heat resistance of the inner cylinder, while
the exhaust gas processing system of the invention comprises the
refractory covering the inner surface of the inner cylinder to
allow a high-temperature exhaust gas to be introduced therein.
[0045] As the heat exchanger inlet gas temperature control means 8,
preferably used is a means of introducing air into the exhaust gas
G from the rotary hearth furnace 1 or a means of introducing a
cooled exhaust combustion gas into the exhaust gas G ,for example.
In addition, a means for directly spraying cooling water into the
exhaust gas G or a means indirectly cooling the exhaust gas G by
letting the exhaust gas G circulate through a water-cooled tube is
also available.
[0046] While the control means using cooling water as described
above decreases a total amount of the sensible heat of the exhaust
gas G, the means for introducing air or a cooled exhaust combustion
gas into the exhaust gas G allows the exhaust gas G to be
introduced into the heat exchanger 2 without decrease in a total
amount of the sensible heat, which can enhance the heat recovery
efficiency. In particular, the means for introducing a cooled
exhaust combustion gas into the exhaust gas G is more preferable
because it can increase the radiation ratio of the exhaust gas G to
increase an amount of radiant heat transfer in comparison with the
means for introducing air since the exhaust combustion gas contains
CO.sub.2 components and H.sub.2O components having a high radiation
ratio.
[0047] As the cooled exhaust combustion gas may be used part of the
exhaust gas G after the cooling and the dust removal drawn from the
exit side of the dust collector 5 or the exit side of the induced
draft fan 6 of the exhaust gas processing system, for example.
Alternatively, an exhaust gas from a facility other than the
exhaust gas processing system, for example, another combustion
furnace or a drying furnace, may be used as the exhaust combustion
gas.
[0048] In a case where the heat resistance of the refractory 23 is
sufficient to eliminate particular limitation of the inlet
temperature of the heat exchanger 2 , the control on the inlet
temperature by the heat exchanger inlet gas temperature control
means 8 (that is, the cooling of the exhaust gas G from the rotary
hearth furnace 1) may be omitted, or the heat exchanger inlet gas
temperature control means 8 may be adapted for raising the inlet
temperature conversely, in order to increase the heat recovery rate
in the heat exchanger 2. The omission of cooling of the exhaust gas
G may increase the inlet temperature contrarily due to inflammable
components, such as a CO gas, remaining in the exhaust gas G.
Further, in a case where the exhaust gas G contains the inflammable
components or a vapor of heavy metal such as zinc, an introduction
of a gas containing oxygen (for example, air or an exhaust
combustion gas) into the exhaust gas G can achieve promoting
combustion (oxidation) of the inflammable components and the vapor
to perform the control for actively raising the temperature of the
exhaust gas G (that is, raising the heat exchanger inlet
temperature) .
[0049] The outlet temperature of the heat exchanger 2 is preferably
maintained at a high temperature no lower than the pre-set
temperature (for example, 700.degree. C.). This reduces an amount
of dust adhering onto the surface of the highly thermal conductive
refractory 23 inside the heat exchanger 2 to enable the operation
over a longer period. The outlet temperature is possible to be
adjusted by changing the setting of the inlet gas temperature of
the heat exchanger 2 and/or changing a flow rate of air inside the
heat exchanger 2. Since the components that evaporate by reactions
inside the rotary hearth furnace 1 mostly contain low melting point
compounds in a gaseous form and slightly contain compounds having a
melting point exceeding 600.degree. C., keeping the temperature of
the surface of the highly thermal conductive refractory 23 no lower
than 600.degree. C. effectively decreases an amount of dust
adhering onto the surface.
[0050] Although in this embedment is used only the single radiant
heat exchanger 2, in the invention may be used a plurality of
radiant heat exchangers interconnected in series and/or in
parallel. The specific number and form of connection may be
determined in comprehensive consideration with a manufacturing
scale of the reduced metal (that is, a scale of amount of the
exhaust gas to be generated), a location of installment, facility
costs, and so forth.
[0051] Although illustrated in this embodiment is the radiant heat
exchanger 2 of a parallel current flow type in which the exhaust
gas G and air flow in the same direction, it may be of a
countercurrent flow type in which the exhaust gas G and air flow in
opposite directions.
[0052] Refractory Duct 3, Exhaust Gas Cooling Tower 4, and
Others
[0053] The exhaust gas G exhausted from the heat exchanger 2 is
introduced into the exhaust gas cooling tower 4 through the
refractory duct 3. The refractory duct 3 includes a duct main body
made of metal and a refractory applied so as to cover at least a
part of the inner surface of the duct main body (generally a
high-temperature portion; preferably the entire portion).
[0054] The refractory duct 3 can be changed into a water-cooled
duct. However, the refractory duct 3, the refractory of which
protects the inner surface of the duct main body from corrosion,
can have a longer life. In the water-cooled duct, decrease in the
temperature of the duct surface prevents high-temperature corrosion
(dissolved salt corrosion) on the inner surface of the duct to some
extent; however, when there is generated a region having a
temperature lower than the dew point of acid due to a drift of the
exhaust gas or the like inside the duct, acid corrosion of the
metal forming the duct main body may possibly occur in this region.
On the contrary, the refractory of the refractory duct 3 can
inhibit the exhaust gas from coming into contact with the inner
surface (metal surface) of the duct main body to prevent both the
high-temperature corrosion and the acid corrosion of the inner
surface.
[0055] The refractory used for the refractory duct 3 is not limited
to the highly thermal conductive refractory applied to the heat
exchanger 2, but various kinds of refractory can be used with
reference to the temperature during used. For example, an
inexpensive alumina-silica refractory can be also used. In a case
where the duct main body has an extensible tube portion for
absorbing thermal expansion, to which the refractory is hard to be
applied, a heat insulator such as glass wool may be applied to the
extensible tube portion.
[0056] It is more preferable to apply the refractory as above also
to the interior of the exhaust gas cooling tower 4 because of the
same reason as described above. In this case, the exhaust gas
temperature drops considerably in the downstream side portion of
the exhaust gas cooling tower 4, which allows the refractory to be
applied only to the upstream side portion. However, for the exhaust
gas cooling tower in which the exhaust gas flows from top to bottom
inside the tower, the refractory may be applied across the entire
surface because application of the refractory only to the top
portion that is the upstream side portion complicates the
structure.
[0057] In a case where the exhaust gas contains chlorine
components, the exhaust gas temperature at the inlet of the exhaust
gas cooling tower 4 is set to 600.degree. C. or above and more
preferably 800.degree. C. or above in order to suppress synthesis
(resynthesis) of dioxins, and then the exhaust gas is rapidly
cooled inside the exhaust gas cooling tower 4 to 200.degree. C. or
below and more preferably 180.degree. C. or below. Even when the
chlorine components or dioxins is contained in the raw material of
the exhaust gas, rapid heat up of the raw material inside the
rotary hearth furnace 1 under high-temperature atmosphere at
1,100.degree. C. or above almost prevents synthesis of dioxins from
the chlorine components. Moreover, even the dioxins which can be
present in the raw material will be almost completely decomposed.
Hence, the hot exhaust gas G fresh out of the rotary hearth furnace
1 contains little dioxins.
[0058] The exhaust gas temperature at the inlet of the exhaust gas
cooling tower 4 can be adjusted by changing the setting of the
inlet gas temperature of the heat exchanger 2 or by changing a flow
rate of air inside the heat exchanger 2, as with the adjustment of
the outlet temperature of the heat exchanger 2.
[0059] Concerning the cooling rate of the exhaust gas in the
exhaust gas cooling tower 4, preferable is such a rapid cooling
(forced cooling) as lower the temperature of the exhaust gas from
600.degree. C. to 200.degree. C. within 5 seconds. The rapid
cooling is achieved, for example, by spraying water into the
exhaust gas, introducing diluted air, making heat exchange with
another fluid, or a combination of the foregoing.
[0060] Although the exhaust gas G in the embodiment is introduced
into the exhaust gas cooling tower 4 from the heat exchanger 2 by
way of the refractory duct 3, it is permitted to perform heat
recovery using a waste heat boiler or a heat exchanger of another
type before the exhaust gas G is introduced into the exhaust gas
cooling tower 4.
[0061] Spraying of water and introduction of diluted air into the
exhaust gas G inside the exhaust gas cooling tower 4 is performed
on the precondition that the dust collector 5 for removing dust
from the exhaust gas G exhausted from the exhaust gas cooling tower
4 is of a dry type using a bag filter or the like. In a case where
a dust collector of a wet type, such as a Venturi scrubber that
brings a large volume of cooling water into direct contact with an
exhaust gas, is used as the dust collector 5, harmful impurity
elements dissolve into the cooling water, which may require
wastewater disposal separately.
[0062] The exhaust gas G cooled to 200.degree. C. or below and
preferably 180.degree. C. or below in the exhaust gas cooling tower
4 is subjected to dust removal processing in the dust collector 5
formed of the bag filter or the like, and then exhausted to
atmosphere through the stack 7 by the induced draft fan 6.
EXAMPLES
Example 1
(Confirmation of the Effects of the Basic Configuration of the
Invention)
[0063] In order to confirm the effects of the invention,
experiments using the exhaust gas processing system shown in FIG. 1
were conducted. In Example 1 was used, as the radiant heat
exchanger 2 , a exchanger including an inner cylinder 22 formed of
a steel plate made of stainless steel with an inner diameter of
1.55 m, a length of 8.2 m, and a heat transfer area of 40 m.sup.2
and a refractory 23 formed of a silicon carbide refractory (the SiC
content is 60% by mass and the rest are chiefly Al.sub.2O.sub.3 and
SiO.sub.2) with the thickness of 50 mm and laid to the inner
cylinder 22 by casting so as to cover the inner surface thereof. As
the exhaust gas cooling tower 4 is used one in which spraying water
and introduction of air at normal temperature into the exhaust gas
is performed.
[0064] An exhaust gas at about 1,450.degree. C. was exhausted from
the rotary hearth furnace 1 at a flow rate of about 11,600
Nm.sup.3/h, and air at normal temperature was introduced into the
exhaust gas at a flow rate of about 1,130 Nm.sup.3/h to adjust the
temperature of the exhaust gas to about 1,350.degree. C. The
exhaust gas subjected to the temperature adjustment was then
introduced into the heat exchanger 2. Inside the heat exchanger 2,
air at normal temperature as the coolant was flown at about 13,130
Nm.sup.3/h. The exhaust gas temperature at the inlet of the exhaust
gas cooling tower 4 was controlled to be 800.degree. C. or above,
and the exhaust gas temperature at the outlet of the exhaust gas
cooling tower 4 was controlled to be 180.degree. C. or below.
Inside the exhaust gas cooling tower 4, the exhaust gas was rapidly
cooled so that the temperature of the exhaust gas dropped from
1,070.degree. C. to 170.degree. C. in about 2 seconds.
[0065] The operation was continued for about 190 days under the
conditions specified above, while changes of the overall heat
transfer coefficient and the preheating air temperature in the heat
exchanger 2 were examined during this period. The results of the
examination are set forth in FIG. 3 and FIG. 4. In these drawings,
the indication of "60%" means that the SiC content in the silicon
carbide refractory was 60% by mass. The marks with an indication of
"(I)" represent data during idle driving when the rotary hearth
furnace was heated merely to maintain the atmosphere temperature
and no agglomerates were charged therein. The marks with an
indication of "(O)" represent data during the reduction operation
when agglomerates were charged into the rotary hearth furnace to
manufacture reduced iron (the same applies to FIG. 5 and FIG. 6
described below) .
[0066] As can be obvious from these drawings, noticeable changes
were not observed in both the overall heat transfer coefficient and
the preheating air temperature of the heat exchanger, even after
the operation over a long period. Furthermore, observation of the
interior of the inner cylinder of the heat exchanger 2 after the
operation over about 190 days showed that no falling off of the
silicon carbide refractory was occurred and a thin adhesion layer,
which was formed on the refractory surface, could be easily removed
by an operator poking it manually with a metal rod.
[0067] These results show that applying the highly thermal
conductive refractory to the inner cylinder of the heat exchanger
so as to cover the inner surface (heat transfer surface) thereof
makes it possible to effectively recover the sensible heat of the
exhaust gas without the blockage of the exhaust gas flow channel
and corrosion of the heat transfer surface caused by adhesion onto
the heat transfer surface.
[0068] In addition, analysis of the components of the exhaust gas
at the outlet of the dust collector 5 formed of a bag filter
revealed no synthesis (resynthesis) of dioxins. The synthesis seems
to be prevented by contribution of rapid cooling inside the exhaust
gas cooling tower 4
Example 2
(Confirmation of Influence of SiC Content in Silicon Carbide
Refractory)
[0069] In Example 2, in the exhaust gas processing system according
to Example 1 above, a refractory having a SiC content of 80% by
mass was used as the silicon carbide refractory applied on the
inner side of the inner cylinder 22 of the heat exchanger 2 instead
of the silicon carbide refractory having a SiC content of 60% by
mass. The operation was continued for about 190 days under the
operation conditions same as those in Example 1.
[0070] FIG. 5 and FIG. 6 show changes of the overall heat transfer
coefficient and the preheating air temperature of the heat
exchanger 2 during the operation period. In FIG. 5, data during the
reduction operation on and after about 150 days contain a rather
low overall heat transfer coefficient, which does not represent
deterioration of the heat transfer performance but is attributed to
a reduction in amount of heat exchange because of the heat exchange
performed under conditions where an amount of the preheating air
was smaller than the reference amount (about 13,130 Nm.sup.3/h)
specified above.
[0071] These drawings obviously show no significant changes in the
overall heat transfer coefficient and the preheating air
temperature of the heat exchanger during the operation over a long
period as with Example 1 above. Moreover, the visual observation of
the interior of the inner cylinder of the heat exchanger 2 after
the operation over about 190 days showed that no falling off of the
silicon carbide refractory were occurred and the refractory surface
were covered with a thin adhesion layer, which was easily removable
by a workman poking it manually with a metal rod, as with Example 1
above.
[0072] The following Table 1 indicates average values of the
overall heat transfer coefficient and the preheating temperature
during the respective periods shown in FIG. 3 through FIG. 6 for
comparison.
[0073] Table 1
[0074] Table 1 shows that increase in the SiC content in the
silicon carbide refractory from 60% by mass to 80% by mass elevates
the overall heat transfer coefficient of the heat exchanger by
about two times and the preheating air temperature by about
150.degree. C. during the reduction operation, respectively, which
means that the heat transfer efficiency can be improved with an
increase of the SiC content.
[0075] FIG. 7 shows the result of the calculation of the
temperature profile in the vicinity of the heat transfer surface of
the heat exchanger 2 during the reduction operation in a case where
the silicon carbide refractory having the SiC content of 80% by
mass is applied. FIG. 7 allows it to be estimated that the
refractory surface temperature was always no higher than
850.degree. C. and the inner surface temperature of the inner
cylinder steel plate is always no higher than 650.degree. C., and
these temperatures were well below the upper temperature limits of
the respective materials.
EXAMPLE 3
(Influence of Thickness of Silicon Carbide Refractory)
[0076] The thickness of the silicon carbide refractory was 50 mm in
each of Examples 1 and 2 above. Refractory thickness of 100 mm
reduces an amount of heat exchange by about 15% compared with the
thickness of 50 mm, and thickness of 200 mm reduces an amount of
heat exchange drops by about 27% compared with the thickness of 50
mm. It is therefore preferable that the highly thermal conductive
refractory is made as thin as in its application.
Example 4
(Confirmation of Influence of Adjusting Means for Exhaust Gas
Temperature at Heat Exchanger Inlet)
[0077] In Example 2 above, adjustment of the exhaust gas
temperature at the heat exchanger inlet by introduction of air at
normal temperature made the exhaust gas composition at the heat
exchanger inlet 3.6% by volume of O.sub.2, 7.6% by volume of
CO.sub.2, 14.6% by volume of H.sub.2O, and 74.2% by volume of
N.sub.2.
[0078] In example 4, the exhaust gas temperature at the heat
exchanger inlet was adjusted by introduction of a cooled exhaust
combustion gas. Specifically, differently from Example 2 above, the
temperature of the exhaust gas was adjusted to about 1,350.degree.
C. by introducing, at a flow rate of about 1,150 Nm.sup.3/h, the
exhaust gas after the cooling and the dust removal (composition:
9.3% by volume of O.sub.2, 5.0% by volume of CO.sub.2, 28.6% by
volume of HAD, and 57.1% by volume of N.sub.2, and the temperature:
170.degree. C.) taken out from the exit side of the induced draft
fan 6 into the exhaust gas at about 1,450.degree. C. exhausted at a
flow rate of 11,600 Nm.sup.3/h from the rotary hearth furnace 1 as
in Example 2 above. The exhaust gas after the temperature
adjustment was then introduced into the heat exchanger 2.
[0079] This experiment confirmed that an increase in the contents
of CO.sup.2 and H.sub.2O contents with a high radiation ratio in
the exhaust gas after the temperature adjustment achieved the
average temperature at about 400.degree. C. higher than the average
temperature (375.degree. C.) of the preheating air during reduction
operation obtained in Example 2 above, further increasing an amount
of the preheating air by about 25% in comparison with Example 2
above.
[0080] Moreover, it was confirmed that periodical idle operation
(intentionally decreasing the inlet temperature of the heat
exchanger 2) apart from the reduction operation under the
conditions specified above could remove the dust effectively.
Specifically, an operation for setting both the exhaust gas flow
rate and the preheating air amount at the heat exchanger inlet to
6,000 Nm.sup.3/h to lower the exhaust gas temperature at the heat
exchanger inlet to about 1,050.degree. C. for 12 hours, caused
cracking in the dust layer adhering to the surface of the highly
thermal conductive refractory 23 inside the heat exchanger 2 to
peel off the dust layer, thus eliminating the need for poking work
to remove the dust layer.
Comparative Example 1
[0081] In a comparative example was used the exhaust gas processing
system shown in FIG. 8, that is, an exhaust gas processing system
including a rotary hearth furnace 1, a cooled-air duct 13, an
exhaust gas cooling tower 4, a heat exchanger 12, a dust collector
5, an induced draft fan 6, and a stack 7. As the heat exchanger 12
was used a plate-type heat exchanger (heat transfer area 350
m.sup.2). The plate-type heat exchanger 12, in which no refractory
was applied to the heat transfer surface thereof, requires cooling
the exhaust gas in the exhaust gas cooling tower 4 to let the
exhaust gas temperature at the inlet of the heat exchanger 12 400
to 600.degree. C. for suppressing the formation of the adhesion
layer on the heat transfer surface and corrosion of the heat
transfer surface.
[0082] Accordingly, there were performed rapidly spraying water and
introducing diluted air inside the exhaust gas cooling tower 4 for
the exhaust gas at about 1,450.degree. C. exhausted from the rotary
hearth furnace 1 at a flow rate of about 11,600 Nm.sup.3/h as in
Examples 1 and 2 above to lower the temperature of the exhaust gas
to about 500.degree. C. The exhaust gas was then introduced into
the plate-type heat exchanger 12 so as to exchange heat with air at
normal temperature flowing in about 13,130 Nm.sup.3/h. As a
consequence, it was confirmed that preheating air at about
350.degree. C. was obtained but its temperature was hard to be
increased even by an enlargement of the heat transfer surface
because of the low exhaust gas temperature at the inlet of the heat
exchanger 12.
[0083] On the contrary, in Example 1 and Example 2, it was possible
to obtain the average temperatures of the preheating air during the
reduction operation at 227.degree. C. and 375.degree. C.,
respectively, with the heat transfer area of 40 m.sup.2. Moreover,
the sufficiently high exhaust gas temperature at the inlet of the
heat exchanger 2 allows the preheating air temperature to be
further raised by enlargement of the heat transfer surface.
Specifically, Both enlargement of the heat transfer area to about
75 m.sup.2 in Example 1 and enlargement of the heat transfer area
to about 45 m.sup.2 in Example 2 allows the preheating air
temperature at 400.degree. C. to be easily obtained.
[0084] In addition, in Example 1 and Example 2, the refractory
applied so as to cover the metal surface (specifically, the inner
surface of the duct main body of the refractory duct 3 and the
inner surface of the exhaust gas cooling tower 4) facing the
exhaust gas flow channel from the outlet of the heat exchanger 2 to
the outlet of the exhaust gas cooling tower 4 prevented a corrosion
of the meal surface regardless of the continuous operation over a
period as long as 380 days. In contrast to this, in the comparative
example where the refractory to cover the metal surface is not
applied to the water-cooled duct 13 and the exhaust gas cooling
tower 4, corrosion occurred on the metal surface (the metal surface
that comes in direct contact with the exhaust gas). This corrosion
is acid corrosion and is assumed to have occurred in a region where
the exhaust gas temperature locally dropped below the dew point of
acid due to a drift or the like of the exhaust gas.
[0085] The comparison between Examples 1 through 4 and Comparative
Example obviously shows the following. In the conventional exhaust
gas cooling system with no refractory applied to the heat transfer
surface of the heat exchanger and the heat transfer surface
exposed, the temperature of the exhaust gas to be introduced into
the heat exchanger is required to be lowered in order to suppress
adhesion of the dust or the like onto the heat transfer surface and
corrosion of the heat transfer surface, which involves a problem of
extreme decrease in the heat transfer efficiency in the heat
exchanger. On the contrary, in the exhaust gas cooling system of
the invention, the highly thermal conductive refractory applied so
as to cover the heat transfer surface makes it possible to easily
achieve both the suppression of adhesion to the heat transfer
surface and corrosion of the heat transfer surface end maintenance
or enhancement of the heat transfer efficiency.
[0086] As described above, an exhaust gas processing system of the
invention comprises a radiant heat exchanger into which an exhaust
gas exhausted from the rotary hearth type reducing furnace is
introduced, the radiant heat exchanger including: an inner cylinder
surrounding a space through which the exhaust gas flows; an outer
cylinder disposed on an outer side in a radial direction of the
inner cylinder to define a flow channel between the inner cylinder
and the outer cylinder for allowing a coolant to flow through so as
to exchange heat with the exhaust gas; and a highly thermal
conductive refractory disposed so as to cover an inner surface of
the inner cylinder. This system does not allow dust to adhere onto
the highly thermal conductive refractory but weakly to be easily
peeled off, in comparison with dust which adheres onto the inner
surface in the conventional system in which the inner surface of
the inner cylinder made of metal is exposed, thus preventing
blockage due to the dust adhesion. In addition, the highly thermal
conductive refractory can inhibit corrosive dust from coming into
direct contact with the inner cylinder made of metal to prevent
corrosive deterioration of the inner cylinder.
[0087] Furthermore, temporarily lowering the temperature of the
surface of the highly thermal conductive refractory below the
temperature of the surface during the reduction operation allows
cracking to be generated in the dust layer adhering onto the
surface to promote peeling thereof, thus facilitating the removal
of the dust layer.
[0088] In addition, the refractory with high thermal conductivity
does not deteriorate the overall heat transfer coefficient of the
heat exchanger significantly. Moreover, forming a portion to come
into direct contact with the exhaust gas with the refractory having
higher upper temperature limit than metals allows the temperature
of the exhaust gas introduced into the heat exchanger to be higher,
thus enabling a heat transfer rate between the exhaust gas and air
to be as high as or rather higher than the conventionally
achievable rate. This also enables an effective utilization of the
sensible heat of the exhaust gas for preheating air for burner
combustion in the rotary hearth type reducing furnace.
[0089] Furthermore, the refractory, which suppresses corrosion of
the inner cylinder and troubles caused by adhesion of dust onto the
inner cylinder, can provide effects such as extension of life of
the facility, enhancement of productivity resulting from an
increase of the operating rate, a reduction in driving costs owing
to a reduction in a heat loss, and a reduction in the maintenance
costs.
[0090] Moreover, these effects, which can be achieved by merely
application of the highly thermal conductive refractory to the
inner side of the inner cylinder of the existing heat exchanger,
require no additional facility such as a waste heat boiler inside
the furnace which increases the facility costs.
[0091] As the highly thermal conductive refractory, a highly
thermal conductive refractory mainly containing anyone of SiC,
ZrB.sub.2, and BN is suitable. A silicon carbide refractory mainly
containing SiC is particularly preferable. Further, a silicon
carbide refractory with an SiC content of 60% or more by mass can
ensure high heat transfer efficiency.
[0092] In the exhaust gas processing system, the temperature of the
exhaust gas to be introduced into the heat exchanger is allowed to
be high to increase the heat recovery ratio. Specifically, the
system preferably comprises heat exchanger inlet gas temperature
control means for controlling a temperature of the exhaust gas at
an inlet of the heat exchanger to be a target temperature of
1100.degree. C. or above. Further, it is more preferable that the
heat exchanger inlet gas temperature control means controls the
temperature of the exhaust gas at the inlet of the heat exchanger
to be the target temperature of 1200.degree. C. or above.
[0093] For example, the heat exchanger inlet gas temperature
control means controls the temperature of the exhaust gas at the
inlet of the heat exchanger by introducing air or a cooled exhaust
combustion gas into the exhaust gas from the rotary hearth type
reducing furnace.
[0094] Further, in a system comprising an exhaust gas cooling tower
provided at a latter stage of the heat exchanger to forcibly cool
the exhaust gas exhausted from an outlet of the heat exchanger, the
forced cooling can effectively suppress synthesis (resynthesis) of
dioxins.
[0095] Furthermore, in a system comprising a refractory duct from
the outlet of the heat exchanger to the exhaust gas cooling tower,
the refractory duct including a duct main body made of metal and a
refractory applied so as to cover at least a part of an inner
surface of the duct main body, the refractory can prevent both
high-temperature corrosion and acid corrosion of the duct main
body.
[0096] The invention also provides an exhaust gas processing method
for processing an exhaust gas exhausted from a rotary hearth type
reducing furnace, including introducing the exhaust gas exhausted
from the rotary hearth type reducing furnace into a heat exchanger
for heat exchange between the exhaust gas and air to preheating the
air, wherein the heat exchanger is a radiant heat exchanger that
includes: an inner cylinder surrounding a space through which the
exhaust gas flows; an outer cylinder disposed on an outer side in a
radial direction of the inner cylinder to define a flow channel
between the inner cylinder and the outer cylinder for allowing the
air to flow therethrough; and a highly thermal conductive
refractory disposed so as to cover an inner surface of the inner
cylinder.
[0097] In this method, a temperature of the exhaust gas at an inlet
of the heat exchanger is preferably controlled to be a target
temperature of at 1100.degree. C. or above, and more preferably to
1200.degree. C. or above.
[0098] Also, it is preferable that the heat exchanger performs a
heat exchange between the exhaust gas and the air to lower an
exhaust gas temperature at an outlet of the heat exchanger to a
temperature of 600.degree. C. or above and the method further
comprises forcibly cooling the exhaust gas to lower the temperature
thereof from 600.degree. C. to 200.degree. C. within 5 seconds by
introducing the exhaust gas exhausted from the heat exchanger into
an exhaust gas cooling tower.
* * * * *