WO2001069162A1

WO2001069162A1 - Duct for conditioning dusty gases by evaporative cooling

Info

Publication number: WO2001069162A1
Application number: PCT/EP2001/002713
Authority: WO
Inventors: Gianfranco Velcich; Stefano Boscolo; Roberto Buttazzoni
Original assignee: Alstom (Switzerland) Ltd
Priority date: 2000-03-14
Filing date: 2001-03-09
Publication date: 2001-09-20
Also published as: ITMI20000517A1; IT1317608B1; AU5827001A

Abstract

A duct (20) for the conditioning of dusty gases by evaporative cooling comprising: an outer supporting shell (24); a metallic structure (25) comprising one or more adjacent longitudinal portions (21, 22, 23) which are arranged inside the outer shell (24) and comprise one or more metallic panels being rigidly coupled to the outer shell (24) so as to have at least one first surface which is adapted to form a region for the flow of the dusty gases (5). At least one of the metallic panels is rigidly coupled to the outer shell so that it can be subjected to a transverse load which is adapted to move the first surface substantially at right angles to the main direction of motion of the dusty gases (5).

Description

DUCT FOR CONDITIONING DUSTY GASES BY EVAPORATIVE COOLING

Technical Field

The present invention relates to a duct for the conditioning of dusty gases by evaporative cooling.

The present invention is particularly suitable for use in exhaust gas treatment units for steelworks and/or smelters, for cement plants or for waste disposal plants, and is described with particular reference to this type of industrial plant without limiting in any way the scope of its application. A typical exhaust gas treatment system for steelworks 1 which uses a duct for the conditioning of dusty gases by evaporative cooling is described with reference to Figure 1. In this plant, the gases 5 that exit from the furnace, after being mixed with air, access the exhaust gas treatment unit through a duct 2 which is connected to a decantation chamber 3. From the decantation chamber, the gases 5 access a conditioning duct 4, from which they are conveyed, after the conditioning process, to subsequent filtration stages (not shown in the figure).

The duct shown in Figure 1 is commonly known as "up-flow" type; in it, the gases 5 move substantially vertically upward with respect to the ground. Other types of duct that are used are so-called "down-flow" ducts, in which the gases 5 move substantially vertically but in the opposite direction, and so-called "horizontal" ones, in which the gases 5 move substantially horizontally with respect to the ground.

The gases 5 comprise gaseous mixtures characterized by a high concentration of dust (dust concentration values of approximately 15-20 g/Nm³ are common) and by very high temperatures (600-650 °C or more at the intake of the duct 4).

Inside the duct 4, the process for conditioning the dusty gases 5 by evaporative cooling is based on the injection of an amount of finely atomized water within the hot gas stream. Injection occurs, in the duct designated by the reference numeral 1, at a region at the base of the duct 4 by using injection lances which are schematically designated by the reference numeral 9. Obviously, in "down- flow" or "horizontal" ducts injection can occur in regions other than the one indicated in Figure 1.

A detail related to the structure of the wall 6 of the duct 4 is described with reference to Figure 2 and is designated by the reference numeral 40 in Figures 1 and 2.

The wall 6 is generally provided by means of a steel skin 10 which is internally protected by a layer of refractory material 12 and by a layer of ceramic fiber 11.

The layer of refractory material 12 is exposed to the motion of the dusty gases 5. The refractory material 12 substantially acts as a protection against wear caused by excessive heating, while the layer of ceramic fiber 11 allows to achieve thermal insulation of the duct. Background Art

It is known that conventional ducts for the conditioning of dusty gases by evaporative cooling have several drawbacks.

One of the main problems is constituted by the formation of dust deposits on the inner side of the wall 6 of the duct 4.

Substantially, dust deposits form due to reasons related to fluid dynamics. The distribution of the stream of gas inside the duct 4 has turbulence regions which generate transverse motions. The consequent reduced evaporation effectiveness causes the deposition of water droplets on the wall 6, consequently forming "clusters" of deposited dust. Accordingly, initiation regions form which entail the rapid growth of a permanent layer of deposited dust which can rapidly become several centimeters thick.

The formation of dust deposits is also facilitated by the operating mode of this type of industrial plant, which is very often discontinuous, with cycles of 15-20 minutes. The presence of permanent dust deposits entails a partial obstruction of the duct 4, with considerable load losses.

This decreases the efficiency of the conditioning process and accordingly entails the need to build longer ducts in order to achieve a satisfactory level of purification of the dusty gases 5.

Accordingly, it is necessary to perform extraordinary maintenance and cleaning, which frequently entail plant shutdown periods, consequently increasing the operating costs.

Extraordinary maintenance and cleaning (which in some cases are fortnightly) consist in removing the permanently deposited layer of dust. These procedures are generally performed manually with the aid of mechanical means.

This requires the adoption of particular precautions in order to ensure the safety of the operators, who have to work inside the duct 4. Moreover, very often it has been found in practice that the adoption of dedicated mechanical means (for example pneumatic hammers) can cause damage to the layers of ceramic and fired-clay materials.

In an attempt to solve the problem of dust deposition inside the duct 4, which has very often limited the use, in industrial plants, of ducts for conditioning dusty gases by evaporative cooling, a solution has been adopted in the art which entails internally lining the wall 6 with a layer of stainless steel 13 in order to reduce the adhesion of the dust.

However, in practice it has been observed that this type of solution does not satisfactorily solve the problem of dust deposition. Dust accumulation certainly occurs more slowly than in the previously described situation, but the need to perform extraordinary maintenance remains, with all the drawbacks that have already been described. Disclosure of the Invention

The aim of the present invention is to provide a duct for the conditioning of dusty gases by evaporative cooling which allows to reduce to negligible levels the formation of permanent deposits at the surface of the duct that is exposed to the motion of the dusty gases.

Within this aim, an object of the present invention is to provide a duct for the conditioning of dusty gases which allows to drastically reduce the need to perform extraordinary maintenance and cleaning.

Another object of the present invention is to provide a duct for the conditioning of dusty gases which allows to perform any maintenance and cleaning by working from the outside of the duct, without requiring plant shutdown periods. Another object of the present invention is to provide a duct for the conditioning of dusty gases which can be easily used in various exhaust gas treatment units, particularly exhaust gas treatment units related to steelworks and/or smelters, cement plants and waste disposal plants.

Another object of the present invention is to provide a duct for the conditioning of dusty gases which is relatively compact, easy to install and at low costs.

This aim and these and other objects which will become better apparent hereinafter are fully achieved by a duct for the conditioning of dusty gases by evaporative cooling, comprising: an outer supporting shell; a metallic structure comprising one or more adjacent longitudinal portions which are arranged inside said outer shell and comprise one or more metallic panels being rigidly coupled to said outer shell so as to have at least one first surface which is adapted to form a region for the flow of said dusty gases; characterized in that at least one of said metallic panels is rigidly coupled to said outer shell so that it can be subjected to a transverse load which is adapted to move said first surface substantially at right angles to the main direction of motion of said dusty gases.

Brief Description of the Drawings Further characteristics and advantages of the present invention will become better apparent from the following detailed description of preferred but not exclusive embodiments thereof, illustrated only by way of non- limitative example in the accompanying drawings, wherein:

Figure 1 is a view of an exhaust gas treatment unit which comprises a conventional duct for the conditioning of dusty gases by evaporative cooling;

Figure 2 is a view of a detail of the conventional duct for the conditioning of dusty gases by evaporative cooling shown in Figure 1;

Figure 3 is a view of an embodiment of a duct for the conditioning of dusty gases by evaporative cooling according to the present invention;

Figure 4 is a view of a detail of an embodiment of the duct for the conditioning of dusty gases by evaporative cooling according to the present invention;

Figure 5 is a view of another detail of another embodiment of the duct for the conditioning of dusty gases by evaporative cooling according to the present invention;

Figure 6 is a view of another detail of a further embodiment of the duct for the conditioning of dusty gases by evaporative cooling according to the present invention. Ways of carrying out the Invention

The structure of a duct 20 for the conditioning of dusty gases by evaporative cooling according to the present invention is illustrated schematically with reference to Figure 3.

The duct 20 has an outer supporting shell 24 and a metallic structure 25 which comprises one or more longitudinal adjacent portions which are arranged inside the outer shell. In the embodiment of the present invention shown in Figure 3, which illustrates a conditioning duct of the "up-flow" type, the metallic structure 25 comprises adjacent longitudinal metallic portions 21, 22 and 23. The metallic portions 21, 22 and 23 are arranged so as to be longitudinally adjacent within the shell 24. Accordingly, they form a metallic structure 25 which is arranged along the main direction of motion of the dusty exhaust gases 5.

Preferably, the duct 20 comprises at least one longitudinal portion 26 made of fired-clay material which is arranged inside the metallic shell 24 and is connected to a decantation chamber 27.

The portion 26 and the decantation chamber 27 can be conveniently provided by adopting structures and methods which are known in the art.

A detail 401 of the duct 20 is illustrated with reference to Figure 4, allowing to illustrate the present invention more clearly. The portions 21, 22 and 23 comprise one or more metallic panels 30 being rigidly coupled to the outer shell 24 so as to have at least one first surface 31 which is adapted to form a flow region 32 for the dusty gases 5. Advantageously, the metallic panels 30 can be made of metallic materials which are resistant to the high temperatures that are typical of dusty gases 5, preferably stainless steel.

In the duct 20, at least one of the metallic panels 30 is rigidly coupled to the outer shell 24 so that it can be subjected to a transverse load (arrow 33) which is adapted to cause the movement 34 of the first surface 31 substantially at right angles to the main direction of motion of the dusty gases 5.

Preferably, the metallic panels 31 are rigidly coupled to the outer shell 24 so as to form an interspace 35 between the outer shell 24 and the metallic structure 25. For this purpose, it is possible to use supporting elements 37 so as to rigidly couple the panels 30, easily maintaining the seal of the duct 20 with respect to the outside environment.

Advantageously, the interspace 35 is connected to the outside environment by adjustable closure/opening elements (not shown), which are constituted for example by a gate, so as to allow, if necessary, the inflow of air from the outside environment into the interspace 35. The transverse load 33 can be generated at least partially by the variation in the difference between the pressure of the region 32 where the dusty gases flow (reference PI) and the pressure of the interspace 35 (reference P2). Such pressure difference variation generates vibratory movements in the panel which determine a movement 34 (advantageously in the elastic mode) of the surface 31. In particular, the variation in pressure difference is particularly high when a plant cycle closes. When a plant cycle closes, the inside of the duct, in which there is usually a pressure PI which is negative with respect to the ambient pressure, in fact suddenly reaches the pressure of the environment outside the duct. To summarize, when a plant cycle closes, one has PI = atmospheric pressure: this causes a transverse load on the panel 30 which is due to the imbalance of a preceding equilibrium (characterized by the difference in pressure between PI and P2).

It should be noted that in this case the variation in the pressure difference is generated intrinsically by the variation of the pressure PI, but it might also be easily generated by appropriately varying the interspace pressure P2 as well.

The presence of vibratory motions of the surface 31 effectively reduces the formation of dust deposits, drastically reducing the accretion rate of the accumulated permanent layer. The intrinsic instability of the surface 31, achieved by means of the particular arrangement of the metallic panels 30, prevents the formation of so-called "clusters" or regions of dust deposit activation.

The presence of the interspace 35 is particularly important for another function which is synergistic with the above-described one. The interspace can in fact receive from the outside environment an inflow of air at relatively low temperature by means of the opening of the above-described adjustment elements.

This causes a sudden cooling of the panels 30. Accordingly, any deposited dust layer is subjected to a temperature differential between the surface in contact with the panel 30 (which is cooled) and the surface exposed to the motion of the exhaust gases 5 (at high temperature).

Due to the nature of the deposited dust, this forms cracks which can facilitate the removal and fall of the deposited layer of dust.

In a preferred embodiment, the transverse load 33 can be generated at least partially by actuation means 36 which are operatively connected to one or more of the panels 30.

In particular, the actuation means 36 can allow actuation by an operator who is located outside the duct 20. The actuation means 36 can be constituted for example by one or more jacks which are adapted to generate the transverse load 33 in predefined points of the panel 30, so as to cause the movement 34 (preferably in the elastic mode) of the surface 31. This embodiment is particularly advantageous. It in fact allows to clean the duct 20 without requiring plant shutdown periods.

It should be noted that also this type of intervention can be performed very frequently (for example daily), so as to decisively prevent the formation of a permanent deposit inside the duct.

The panels 30 can have a structure which can be optimized so as to increase the effectiveness of dust deposit prevention. The structure can be optimized so as to obtain the intended vibratory motions for each panel 30. In particular, one or more of the adjacent metallic portions 21, 22 and 23 can comprise a single metallic panel 30 whose structure is such that the first surface 31 substantially corresponds to the lateral surface of a cylinder (as shown in Figure 5) or of a polyhedron (not shown).

Likewise, one or more of the adjacent metallic portions can comprise a plurality of metallic panels 31 whose structure is such that the first surface 31 substantially corresponds to the lateral surface of a sector of a cylinder (as shown in Figure 6) or of a polyhedron (not shown). The number of portions 21-23 to be comprised within the metallic structure 25, the number of metallic panels 30 for each portion and the definition of the size ratios between the base "b", the height "h" and the radius of curvature "r" of each panel can be easily calculated according to requirements.

Considerable design flexibility is thus possible, making the duct according to the present invention suitable for "up-flow", "down-flow" or "horizontal" applications. In practice it has been observed that the duct for the conditioning of dusty gases by evaporative cooling according to the present invention fully achieves the intended aim and objects.

The duct according to the present invention is used particularly and effectively in exhaust gas treatment units for steelworks and/or smelters, for cement plants or for waste disposal plants. Thanks to the considerable reduction in duct soiling levels it is in fact possible to provide exhaust gas treatment units which are more compact without incurring excessive load losses due to excessive dust deposition.

The duct according to the present invention has further proved to be easy to install and has a relatively low cost with respect to the solutions commonly provided in the art.

The duct for the conditioning of dusty gases by evaporative cooling according to the present invention is susceptible of modifications and variations, all of which are within the scope of the inventive concept; all the details may further be replaced with technically equivalent elements. In practice, the materials employed, so long as they are compatible with the specific use, as well as the individual components, may be any according to requirements and to the state of the art.

The disclosures in Italian Patent Application No. MI2000A000517 from which this application claims priority are incorporated herein by reference.

Claims

1. A duct for the conditioning of dusty gases by evaporative cooling, comprising: an outer supporting shell; a metallic structure comprising one or more adjacent longitudinal portions which are arranged inside said outer shell and comprise one or more metallic panels being rigidly coupled to said outer shell so as to have at least one first surface which is adapted to form a region for the flow of said dusty gases; characterized in that at least one of said metallic panels is rigidly coupled to said outer shell so that it can be subjected to a transverse load which is adapted to move said first surface substantially at right angles to the main direction of motion of said dusty gases.

2. The duct for the conditioning of dusty gases by evaporative cooling according to claim 1, characterized in that said one or more metallic panels are rigidly coupled to said outer shell so as to form an interspace between said outer shell and said metallic structure.

3. The duct for the conditioning of dusty gases by evaporative cooling according to claim 2, characterized in that said interspace is connected to the outside environment by adjustable closure/opening elements, so as to allow the inflow of air from the outside environment into said interspace.

4. The duct for the conditioning of dusty gases by evaporative cooling according to claim 3, characterized in that said air inflow is adapted to cool said metallic panels.

5. The duct for the conditioning of dusty gases by evaporative cooling according to the preceding claims, characterized in that said transverse load is generated at least partially by the variation in the difference between the pressure related to said dusty gas flow region and the pressure related to said interspace.

6. The duct for the conditioning of dusty gases by evaporative cooling according to claim 1, characterized in that said transverse load is generated at least partially by actuation means which are operatively connected to one or more of said panels.

7. The duct for the conditioning of dusty gases by evaporative cooling according to claim 6, characterized in that said actuation means can be actuated by an operator located outside said duct.

8. The duct for the conditioning of dusty gases by evaporative cooling according to one or more of the preceding claims, characterized in that at least one of said adjacent portions comprises a single metallic panel whose structure is such that said first surface substantially corresponds to the lateral surface of a cylinder.

9. The duct for the conditioning of dusty gases by evaporative cooling according to one or more of claims 1 to 7, characterized in that at least one of said adjacent portions comprises a single metallic panel whose structure is such that said first surface substantially corresponds to the lateral surface of a polyhedron.

10. The duct for the conditioning of dusty gases by evaporative cooling according to one or more of claims 1 to 7, characterized in that at least one of said adjacent portions comprises a plurality of panels whose structure is such that said first surface substantially corresponds to the lateral surface of a cylindrical sector.

11. The duct for the conditioning of dusty gases by evaporative cooling according to one or more of claims 1 to 7, characterized in that at least one of said adjacent portions comprises a plurality of panels whose structure is such that said first surface substantially corresponds to the lateral surface of a sector of a polyhedron.

12. The duct for the conditioning of dusty gases by evaporative cooling according to one or more of the preceding claims, characterized in that it is of the down-flow or up-flow or horizontal-flow type.

13. An exhaust gas treatment unit for steelworks and/or smelters, characterized in that it comprises a duct for the conditioning of dusty gases by evaporative cooling according to one or more of the preceding claims.

14. The exhaust gas treatment unit for steelworks and/or smelters according to claim 13, characterized in that said duct comprises at least one longitudinal portion made of fired-clay material which is arranged inside said metallic shell and is connected to a decantation chamber.

15. An exhaust gas treatment unit for cement plants, characterized in that it comprises a duct for the conditioning of dusty gases by evaporative cooling according to one or more of claims 1 to 12.

16. An exhaust gas treatment unit for industrial and/or domestic waste disposal systems, characterized in that it comprises a duct for the conditioning of dusty gases by evaporative cooling according to one or more of claims 1 to 12.