US20070075468A1

US20070075468A1 - Cooling exhaust gases from smelting furnace

Info

Publication number: US20070075468A1
Application number: US11/543,307
Authority: US
Inventors: Herbert Huning
Original assignee: Oschatz GmbH
Current assignee: Oschatz GmbH
Priority date: 2005-10-05
Filing date: 2006-10-04
Publication date: 2007-04-05
Also published as: CN101025338A; CA2561827A1; AU2006225254A1; BRPI0604230A; EP1772692A1

Abstract

In combination with a furnace having a top from which very hot gases are exhausted, a cooling system has an outwardly open collar fixed to the top of the furnace and open inward into the furnace, a tubular exhaust stack adjacent the furnace, a connecting duct extending from the stack and fitted concentrically to the collar, a network of heat-exchange tubes lining the top of the furnace. The collar, and the connecting duct, and means for circulating through all of the tubes a coolant at generally the same temperature.

Description

FIELD OF THE INVENTION

The present invention relates to a system for cooling exhaust gas. More particularly this invention concerns the exhaust gases from a pig-iron reduction smelting furnace.

BACKGROUND OF THE INVENTION

In the production of pig iron, the furnace produces a great deal of hot exhaust gases that, on the one hand, contain valuable recoverable heat, and on the other hand should not be discharged directly into the atmosphere. Typically a tubular exhaust conduit or stack is attached to the top of the furnace. A connecting conduit or duct extends from the stack to the furnace to conduct the hot gases from the furnace to the stack. As a rule both the top of the furnace and the stack are cooled by passing a cooling fluid through them to a temperature Ti, as is the connecting duct. The top of the furnace normally has a connection collar to which the connecting conduit is fitted.
The furnace of the device according to the invention generates exhaust gases at a superatmospheric pressure, typically about 0.8 bar above atmospheric pressure. The exhaust gas enters into the exhaust duct with a relatively high temperature of about 1450° C. In the gas-tight and cooled exhaust duct, the exhaust gases are cooled to a temperature that is suitable for, e.g. preheating ore. The cooling of the exhaust or flue gases is done with a coolant that is conducted through cooling pipes in the walls of the exhaust duct. The coolant or fluid is normally boiling water at a temperature of for example 260° C.
Devices are known in which the inside wall of the furnace is cooled by a coolant pumped through cooling pipes lining the top of the furnace. As coolant, liquid water with a temperature of for example 60° C. is conducted through the cooling pipes of the inside wall of the furnace. Following cooling of the inside wall of the furnace, the heated liquid water, which at this point has a temperature of 80° C. is disposed of without utilizing the absorbed heat. These known devices have the disadvantage that different thermal expansions, especially vertical thermal expansions of the various system components, result due to the temperature differential between the cooling of the furnace inside wall on the one hand, and the exhaust duct on the other. This means that elaborately equipped compensators need to be installed in order to compensate for these thermal expansions. Furthermore, with many known devices, the cooling of the inside wall of the furnace, as well as certain areas thereof is unsatisfactory.

OBJECTS OF THE INVENTION

It is therefore an object of the present invention to provide an improved exhaust-gas cooling system.
Another object is the provision of such an improved exhaust-gas cooling system that overcomes the above-given disadvantages, in particular that efficiently cools the gases with relatively simple but effective equipment.

SUMMARY OF THE INVENTION

In combination with a furnace having a top from which very hot gases are exhausted, a cooling system has according to the invention an outwardly open collar fixed to the top of the furnace and open inward into the furnace, a tubular exhaust stack adjacent the furnace, a connecting duct extending from the stack and fitted concentrically to the collar, a network of heat-exchange tubes lining the top of the furnace. The collar, and the connecting duct, and means for circulating through all of the tubes a coolant at generally the same temperature.
Thus the invention is characterized in that the coolant for cooling the top part of the furnace may be used for cooling both the inside wall of the furnace and the branch collars or sockets, and cooling pipes for cooling the upper part of the furnace extend to the collar of which there is at least one.
The temperature Ti of the coolant refers to the temperature at which the coolant is fed to the exhaust stack or the connection duct and to the furnace to be cooled. The coolant for cooling the top of the furnace likewise has the same temperature Ti as the coolant for cooling the exhaust stack, which means that within the scope of the invention, the coolant for cooling the furnace likewise essentially has the same temperature Ti. The temperature of the coolant for cooling the top of the furnace may therefore vary up or down by as much as 15° C., preferably 10° C., and even more preferably 5° C. from the temperature Ti of the coolant for the exhaust stack. According to an especially preferred embodiment, the temperature of the coolant for cooling the top of the furnace varies only up or down by 0° C. to 2° C. from the temperature Ti of the coolant for the exhaust stack.
It is within the scope of the invention that the device according to the invention preferably is operated with gas overpressure, i.e. that the exhaust gas enters the exhaust stack from the furnace at superatmospheric pressure so that the exhaust gas may have a pressure of about 0.8 bar above atmospheric pressure. Such gas overpressure is associated with certain constraints or considerable mechanical stresses of the system components. Nevertheless, the device according to the invention operates flawlessly, even at such a gas overpressure, when implementing the features according to the invention.
According to the invention, at least one collar, which is connected between the top of the furnace and the exhaust stack , is likewise cooled with the coolant of the furnace. The connecting collar concerns an attachment branch for the end of the exhaust stack facing the furnace. The coolant for cooling the furnace may therefore initially be used for cooling the inside wall of the furnace, and subsequently fed into the connecting collar in order to cool the latter. The coolant with the temperature Ti, however, may also be fed in parallel into the connecting collar for cooling the latter and the top of the furnace in order to cool the top.
For this purpose, the temperature Ti should exceed 150° C. , preferably 200° C. , more preferably 230° C., and most preferably 240° C.
It is within the scope of the invention that the temperature Ti is between 240° C. and 280° C., preferably between 250° C. and 270° C. The temperature according to an especially preferred embodiment of the invention is 260° C., or more or less 260° C. Thus both the coolant for cooling the inside wall of the furnace and the coolant for cooling the exhaust gas duct has this temperature Ti. The temperature Ti is the temperature of the coolant fed to the inside wall of the furnace or the exhaust stack. Advantageously, the coolant for cooling the walls of the collars has the temperature Ti.
It is within the scope of the invention that the cooling medium for the furnace and the inside wall of the furnace and/or the connecting collar of the furnace is boiling water. Advantageously, both the top of the furnace and the connecting collar are cooled with boiling water. Cooling with boiling water ensures that a water/water vapor mixture develops from the boiling water, when cooled. Thus there is evaporative cooling.
It is within the scope of the invention that the exhaust stack and the connection duct are also cooled with boiling water. The evaporative cooling for the furnace or the top of the furnace has very special advantages. The generated steam may therefore be used very efficiently for heat or energy recovery, as opposed to the devices known from the related art. According to the invention the coolant for the furnace is pumped through cooling tubes or pipes that line the inside of the top part of the furnace. Preferably, a cooling jacket is formed or coiled from these cooling pipes at the inside wall of the furnace or at the inside wall of the top part of the furnace.
According to the invention, the cooling pipes for cooling the furnace and/or the top part of the furnace also extend into the collar, of which there is at least one. The cooling pipes are advantageously form coils in the collar, and thus line the wall of the collar as a cooling jacket.
According to a preferred embodiment of the invention, the coolant initially flows through the cooling pipes lining the inside wall of the furnace, and subsequently into cooling pipes that line the wall of the collar. This shared cooling for furnace and connecting collar has proven to be successful. By using evaporative cooling for the device according to the invention it is possible to use relatively small cooling pipe diameters, allowing the cooling pipes, as well, to be coiled with minimal bending radii relative to the cooling jacket. The diameters of the cooling pipes used for the furnace and/or connecting collar are advantageously below 60 mm, and preferably in the 30-50 mm range. Due to the very small bending radii of the cooling pipes, cooling jackets may be realized allowing for very efficient cooling of the inside wall of the furnace and of the wall of the collars.
In an embodiment with two collars provided at the furnace, a narrow inside wall area of the furnace between these two collars may be cooled in a simple and operationally reliable way. This embodiment will be explained in more detail below.
Evaporative cooling also has the advantage relative to the cooling of the inside wall of a furnace with removal of liquid water known from the related art that corrosion and deposition problems in the cooling pipes may be largely avoided.
Basically, for cooling the inside wall of the furnace and/or the collar, of which there is at least one, a continuous cooling jacket consisting of a coiled cooling pipe is used. According to an especially preferred embodiment of the invention, the cooling pipes, however, form a cooling jacket consisting of a plurality of cooling jacket sections through which flows may pass separately and preferably parallel, and that may be blocked individually, if required. The individual turns or pipes of the jacket directly abut one another so as to completely cover or line the interior of the space they are in. Hence, the cooling pipes according to this embodiment form separate cooling coils. An especial advantage of this embodiment is that defective cooling jacket parts may be turned off without compromising the cooling effect of the other cooling jacket sections. In case of a leak in a cooling pipe, the whole device needs therefore not be shut down; instead a defective cooling jacket section may be disconnected and exchanged, provided the other cooling jacket sections continue to operate.
According to an embodiment of the invention already mentioned above, two adjacent collars are provided at the top of the furnace, and the angle between them does not exceed 100°, advantageously 95°, preferably 90°, and very preferably 85°. According to an especially preferred embodiment of the invention, the angle between the adjacent collars is about 80°. The angle is thereby measured between the center lines or axes of the normally cylindrical collars. For an embodiment of the device according to the invention as space-saving as possible and of little volume, a minimum angle between the collars is chosen. This will result in a very narrow area of the inside wall of the furnace between both collars, where cooling is problematic with devices known from the related art. It is highly recommendable that this narrow space also be cooled so as to avoid any problems.
According to a very preferred embodiment of the device according to the invention, cooling pipes are guided out of the first connecting collar over the narrow inside wall section of the furnace between both attachment or intake holes of the collars, the cooling pipes then pass into the second connecting collar to cool it. This will allow simple and efficient cooling of the narrow space. This is especially the case, since within the scope of the invention cooling involves evaporative cooling, which makes possible small pipe diameters and especially small bending radii of the cooling pipes. Hence, trouble-free cooling, including of the narrow space, may be achieved with the device according to the invention.
According to a very preferred embodiment of the invention, cooling pipes extending along the inside wall in order to cool the furnace pass into the first connecting collar to cool the latter, and then again pass out of the first collar. Subsequently, these cooling pipes are pass over the inside wall section of the furnace between both attachment holes of the collars, and then pass into the second connecting collar to cool the latter, and then again pass out of the second collar. Then, these cooling pipes may extend further along the inside wall of the furnace. As already emphasized above, evaporative cooling with boiling water enables small bending radii, allowing trouble-free coiling, as well fitting of the cooling pipes to the system sections.
The invention is based on the recognition that unwanted relative thermal expansion, especially vertical thermal expansion between the furnace and exhaust stack may be avoided in an efficient and operationally reliable way by cooling the furnace with a cooling medium at the same temperature as the exhaust stack. In contrast to the devices known from the related art, it is possible to omit elaborate compensators in the transition area between the furnace and exhaust stack. The invention is furthermore based on the knowledge that especially efficient cooling of the furnace and thus also effective avoidance of disturbing thermal expansion may be achieved if evaporative cooling is used during cooling of the furnace. Moreover, cooling with boiling water advantageously enables efficient heat recovery, which is not easily done with liquid cooling water used in accordance with the prior art. When using evaporative cooling for the furnace, signs of corrosion and deposits in the cooling pipes on the walls of the furnace may furthermore largely be avoided.
The invention is also based on the recognition that by using the cooling measures according the invention, i.e. evaporative cooling, cooling may be done relatively smoothly including at the connecting collar attached to the furnace, and in areas that are not easily accessible. This is primarily due to the fact that evaporative cooling makes possible the use of cooling pipes with a small diameter, and thus also allows for small bending radii of the coiled cooling pipes. A further advantage of the invention is the possible avoidance of adverse condensation of sulfur dioxide on the cooled inside walls of the furnace, when cooling same at a relatively high temperature Ti (e.g., 260° C.). Finally, it should be emphasized that the device according to the invention is designed in a relatively simple and uncomplicated way, making it relatively inexpensive to manufacture.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features, and advantages will become more readily apparent from the following description, reference being made to the accompanying drawing in which:
FIG. 1 is a partly schematic view of the furnace and cooled exhaust-gas system according to the invention;
FIG. 2 is a large-scale horizontal section through a detail of the system;
FIG. 3 is a large-scale view of a detail if FIG. 2;
FIG. 4 is a top view. of a double-collar furnace top in accordance with the invention; and
FIG. 5 is a developed view from inside of the structure of FIG. 4.

SPECIFIC DESCRIPTION

As seen in FIG. 1 a melt reduction furnace 1 for the production of pig iron is associated with a tubular exhaust-gas stack 3 for removal and cooling of the exhaust gases is attached at the top part 2 of the furnace. The exhaust gas enters the exhaust stack 3 with a temperature of about 1450° C. and under a gas overpressure of about 0.8 bar above atmospheric pressure.
A connecting duct 4 attached to the furnace 1 is cooled by means of a cooling medium having a temperature Ti and coming from a supply 16. In the example, this cooling medium is boiling water and is fed to the exhaust stack 3 at a temperature Ti of 260° C. The cooling medium moves through cooling pipes 9 of the exhaust stack 3. The pipes 9 in the example form the primary wall of the exhaust stack 3.
According to the invention, a top part 2 of the furnace 1 where the connecting duct 4 is attached is also cooled with a cooling medium of the same temperature Ti as the cooling medium for cooling the exhaust stack 3. In the example, the cooling medium for cooling the top part 2 of the furnace 1 thus also has a temperature of 260° C. This temperature Ti refers to the cooling medium fed to the furnace 1. The cooling medium in the top part 2 of the furnace 1 moves through cooling pipes 10 of the furnace 1, and these cooling pipes 10 are provided at the inside wall of the furnace 1 of the top part 2 in the example. The cooling pipes 10 are here coiled into a cooling jacket 11. According to a preferred embodiment of the invention, the cooling jacket 11 consists of a plurality of cooling jacket sections in a way that is not shown in further detail, and these may be separately connected and disconnected. It is within the scope of the invention that boiling water is likewise used for cooling the top part 2 of the furnace 1. Evaporative cooling for the furnace 1 produces significant advantages, as already explained more extensively above. According to an embodiment of the invention, parts of the lower furnace section may also be cooled in the same way as the top part of the furnace.
In the example according to FIGS. 1 to 3, the furnace 1 has at its top part 2 a connecting collar 12 that is provided for attaching the exhaust stack 3 or connecting duct 4. According to the invention this connecting collar 12.is cooled with the cooling medium also used for cooling the top part 2 of the furnace 1. In other words, the cooling medium for the furnace 1 is conducted through cooling pipes 10 provided both at the inside wall of the furnace 1 and at the inside wall of the connecting collar 12. Hence, the cooling medium for the connecting collar 12 likewise involves preferably boiling water at the temperature Ti.
FIGS. 2 and 3 show the attachment of the exhaust stack 3 and the connecting duct 4 to the connecting collar 12 of the furnace 1. The connecting duct 4 is fitted coaxially into the connecting collar 12 and fastened to the connecting collar 12. The cooling pipes 9 form the primary wall of the connecting duct 4. The cooling pipes 10 form the primary wall of the connecting collar 12. The cooling pipes 9 and 10 both advantageously carry boiling water with a temperature Ti of 260° C. as the cooling medium. As already described above, relative thermal expansion between furnace 1 or connecting collar 12 and exhaust stack 3 may be effectively avoided in this way. Both the connecting duct 4 and the connecting collar 12 have an outside insulation layer 13.
Preferably as shown in the example, the cooling pipes provided at the inside wall of the top part 2 of the furnace 1 extend from this inside wall into the connecting collar 12 and overlay the pipes 10 lining the furnace top.1 and collar 12. The cooling pipes 10 are therefore coiled not just for the cooling jacket 11 in furnace 1, but also, as it were, into the connecting collar 12. In other words, the cooling pipes 10 for cooling the furnace 1 extend into connecting collar 12, as well. This is explained in more detail below based on a special embodiment.
It can also be seen in FIG. 1 that the exhaust stack 3 is supported on a floor 17 by support elements or vertical supports 8. The vertical supports 6 are preferably heated with a medium, as is the case in the example, and this medium for the vertical supports 8 comes from the supply 16 and has the same temperature Ti as the cooling medium for the exhaust stack 3 and as the cooling medium for the furnace 1 or connecting collar 12. Hence, in the example, the medium for heating the vertical supports 8 also has the temperature Ti of 260° C. The medium for the vertical supports 8 likewise involves preferably boiling water. For this purpose, the vertical supports 8 are hollow, for example pipes, through which the medium flows. By heating the vertical supports 8, relative thermal expansion of the connecting ducts 4 and the posts 8 may effectively be avoided, so that no related stresses are produced. This entails the considerable advantage of eliminating the need for compensators between the individual sections of the exhaust stack 3 to take up thermal expansion. The vertical supports 8 are supported at the bottom on one single load uptake surface 17. Since both the top part 2 of the furnace 1 and the connecting collar 12, as well as the exhaust stack 3 are cooled with a cooling medium of the same temperature Ti, the above-mentioned load uptake surface for the supporting elements may be defined in a very simple and accurate way. The supporting elements or the vertical supports 8 according to the preferred embodiment and in the example according to FIG. 1 are formed are pivoted at their upper and lower ends, that is each vertical support 8 is attached via a hinge joint to the exhaust stack 3, and preferably via a hinge joint to the floor 17. This has the advantage that horizontal thermal expansions may likewise be absorbed or compensated without any problems.
FIG. 1 furthermore shows that the exhaust stack 3 has the above-mentioned horizontal or slightly inclined connecting duct 4 that continues into a first vertical connecting duct or stack section 5 in which the exhaust gas is conducted upward. This first vertical exhaust stack section 5 is connected to a downwardly U-shaped baffle section 6 that in turn is connected to a second vertical stack section 7, in which the exhaust gas moves downward. In FIG. 1, the further treatment of exhaust gas after the exhaust stack 3 is not shown. The cooled exhaust gas in the exhaust stack 3 may, for instance, serve to preheat ore for the production of steel.
In the example according to FIGS. 4 and 5, two adjacent collars 12 and 14 are provided at the top part 2 of furnace 1. In the example, the angle between these two collars 12, 14 as measured between the center lines or axes of the collars 12 and 14 is 80°. This angle is chosen as small as possible in order to make the system as compact as possible. In similar prior-art systems, this makes the very narrow space between collars 12 and 14 impossible to cool. The invention is based on the knowledge that efficient cooling of this narrow space or inside wall section 15 of the furnace is essential for long-term and functionally reliable operation of the device. According to the invention efficient cooling of the inside wall section 15 of the furnace between both collars 12 and 14 is made possible as shown in FIG. 5 in that the cooling pipes 10 that are coiled into a cooling jacket 11 of the furnace 1 initially extend to the right side along the inside wall of furnace 1. These cooling pipes then pass as coils into the first connecting collar 12, whereupon on the left side of the first connecting collar 12, the cooling pipes again pass out of this connecting collar 12, and then over the narrow inside wall section 15 of the furnace. This ensures very efficient cooling of the inside furnace wall section 15. The cooling pipes 10 on the left side of the inside wall section 15 of the furnace then pass out as coils into the second connecting collar 14, and on the left side of this second connecting collar 14, they again pass out of the connecting collar 14 and further along the inside wall of furnace 1. Guiding the cooling pipes 10 in this way is possible, since the invention uses evaporative cooling, and because evaporative cooling allows cooling pipes 10 of a very small diameter, so that the cooling pipes 10 may also have very small bend radii, enabling them to be fitted to this complex shape between the two collars 12 and 14.

Claims

1. In combination with a furnace having a top from which very hot gases are exhausted, a cooling system comprising:

an outwardly open collar fixed to the top of the furnace and open inward into the furnace;

a tubular exhaust stack adjacent the furnace;

a connecting duct extending from the stack and fitted concentrically to the collar;

a network of heat-exchange tubes lining the top of the furnace, the collar, and the connecting duct; and

means for circulating through all of the tubes a coolant at generally the same temperature.

2. The exhaust-gas cooling system defined in claim 1 wherein coolant is at a temperature above 150° C.

3. The exhaust-gas cooling system defined in claim 2 wherein the coolant is above 200° C.

4. The exhaust-gas cooling system defined in claim 1 wherein the coolant is boiling water.

5. The exhaust-gas cooling system defined in claim 1 wherein the network of tubes comprises a multiplicity of tube sections that are individually controllable.

6. The exhaust-gas cooling system defined in claim 1 wherein the furnace top has two of the collars extending at an angle of at most 100° to each other.

7. The exhaust-gas cooling system defined in claim 6 wherein the two collars extend at an angle of at most 85° to each other.

8. The exhaust-gas cooling system defined in claim 6 wherein the tubes lining one of the collars extend continuously into the furnace top and line the furnace top between the collars and then extend into the other of the collars.

9. The exhaust-gas cooling system defined in claim 1 wherein the tubes lining the connecting duct project into the collar and overlap the tubes lining the collar.