WO1996021500A1

WO1996021500A1 - Dust cleaner

Info

Publication number: WO1996021500A1
Application number: PCT/SE1996/000013
Authority: WO
Inventors: Roine Brännström; Ingvar Hultmark
Original assignee: Abb Carbon Ab
Priority date: 1995-01-12
Filing date: 1996-01-11
Publication date: 1996-07-18
Also published as: FI972942A0; EP0802819A1; JPH10512185A; SE9500091L; FI972942A; SE9500091D0

Abstract

The present invention comprises a cyclone adapted for flue-gas cleaning and designed such that particles, flowing towards the wall of the cyclone, from a gas flow inside the cyclone with angles of incidence to the wall greater than zero are avoided. This is achieved by making the necessary area reduction of the cyclone, at the lower part thereof, even and continuous. The transition (1d) in the cyclone from a first cylindrical shell surface (1a, 1b) with a larger cross section to a cylindrical cyclone leg (1c) at the lower-most part of the cyclone is made evenly tapering without any discontinuities in the decrease of the circular cross section area, which is achieved in that the shell surface of the cyclone at the transition (1d) exhibits a double-curved surface or consists of a number of consecutively arranged conical shells with a decreasing cross section area.

Description

Dust cleaner

TECHNICAL FIELD

The present invention relates to cleaning plants for separa¬ tion of particles from gases. Preferably, a device according to the invention is utilized for separation of hot particles from hot gases in combustion plants in so-called cyclones.

BACKGROUND ART

For separation of particles from gases, cyclones are usually used. Such a cyclone is typically shaped with an upper cylin¬ drical shell which changes into a conical shell connected with the first shell, below this, and with a downwardly directed tip. A gas from which particles are to be removed is led in at the upper part of the cylindrical shell, tangentially to the periphery thereof. The gas and the particles will thereby by forced into a spiral movement, a vortex, from a higher to a lower level inside the wall of the cyclone, that is, the above-mentioned shells with a downwardly tapering, circular cross section. During the downward flow, the velocity of the gas flow increases, whereby heavier particles occurring in the gas vortex are thrown out towards the cyclone wall and then fall down into cyclone legs, which form dust outlets from the cyclone at the lower, narrowest part of the conical shell. The gas thus cleaned from particles is discharged at the top at an outlet located centrally in the uppermost cylindrical part of the cyclone.

In a combustion plant for unclean fuels, such as coal, gases which are to drive gas turbines must be cleaned from erosive particles in gas cleaners, which may consist of the above- mentioned cyclones. In a gas turbine, the highest possible gas temperature at the inlet of the turbine is aimed at. This means that gas cleaners which separate dust from, for example, combustion gases which are passed to a gas turbine operate at the temperature which the combustion gases have when they leave a co bustor in the combustion plant.

A combustion plant of, for example PFBC type, operates with a gas temperature which may amount to 950°C. The high tempera¬ ture entails considerable stresses in cyclones for cleaning the combustion gases before these are supplied to a turbine. Especially great are the problems in the lowermost part of the cyclone and at the outlet thereof, that is, at the cyclone leg. The high velocity of the greatly corrosive, erosive particles in the gas mass and the high temperature reduce the strength of the cyclone material and deteriorate its resis¬ tance to abrasion. A combination of corrosion and erosion is often obtained in this environment.

In spite of different forms of cooling of cyclones and different variants of the design of the cyclones and the cyclone legs, problems with the hard wear on the cyclone material from dust in the gas still remain. It has therefore become necessary to provide cyclones with an erosion-resistant material, usually in the form of a lining. This lining may be formed from a chemical material, which is a technique which has long been well-known. In already existing PFBC energy plants, the cyclones have been internally lined with a high- resistant ceramic material. Such a device is described, for example, in Swedish patent application 90029240.

Another problem during dust separation in, for example, PFBC plants is that fires may occur inside the cyclone. This is caused, among other things, by the fact that the hot gas which is to be cleaned from particles, primarily ash, also contains unburnt coal particles, that the temperature is high, and that the gas contains oxygen residues.

In case of a fire in the cyclone, the constituents thereof are heated to such an extent that the resistance of the cyclone to abrasion is further deteriorated. To reduce the heating of the constituents of the cyclone, the cyclone may be cooled. One example of an embodiment of cyclone cooling is shown in Swedish patent 459986.

Another way of reducing the risk of too high temperatures of the cyclone material is to eliminate the risks of fire by ensuring that the oxygen percentage in the gas inside the cyclone where a fire may arise is kept low.

As mentioned, a cyclone is usually designed with a downwardly tapering cone with a transition into a cylindrical part, a cyclone leg, which is connected to the tip of the mentioned cone. Generally, the cyclone leg consists of a cylindrical tube.

Because of the design of the cyclone, turbulence arises at certain places, especially at transitions between, for example, conical and cylindrical cyclone portions, whereby a difference in speed arises between the gas in the cyclone vortex and the particles flowing with the gas. This contri- butes to a mixing, which increases the probability that un- burnt particles will have access to oxygen, and hence increa¬ ses the risk of fire. In the turbulent portions mentioned, the particles in the gas do not always slide along the tapering shell surface of the cyclone cone, but sometimes hit this surface at a certain impact angle. Since the speed of the particles is high, this considerably increases the erosion of the cyclone material.

All the necessary conditions for a fire, such as oxygen, a high temperature, and a burnable substance, may exist in the cyclone. It is probably for this reason that a cyclone, which is used for cleaning of hot flue gases which are passed to a gas turbine, is usually more or less burning. By changing the design of the cyclone, however, it is possible to affect the extent to which the cyclone is burning.

If the design of the cyclone is changed, the relative speed between gas and particles in the gas flow may be influenced. This is shown in experimental investigations, where, for example, a cross introduced in the centre of the cyclone leg has increased the fire activity. In the same way, the fires increase when the cyclone is internally provided with rough wall surfaces. In both these cases, it is the relative speed between gas and particles mutually that has increased because of the particles having been slowed down. This increase of the relative speed between gas and particles gives an increased combustion in the interior of the cyclone.

As regards erosion in the cyclone, the erosion is influenced by a variety of different parameters. The size, speed, den¬ sity, hardness, and shape of the particle are some of these parameters. Other parameters are the composition of the gas, which, for example, may be corrosive or reducing. The proper¬ ties of the cyclone material are also of importance. Oxide layers or ceramics, for example, function well from an erosion-inhibiting point of view. Another very important factor is the angle of incidence of the particles with respect to the inner surface of the cyclone upon impact, when partic¬ les collide with the cyclone surface.

If a fire arises inside a cyclone, this results, inter alia, in a lower oxygen content and hence a risk of a reducing environment. In this reducing environment, metal oxides on the surface layer of surrounding metal surfaces are degraded. These oxides are very hard, brittle and hence resistant to erosive influence. Probably, the impact angles between particles and the metal surfaces mentioned are predominantly small, whereby the good properties of the metal oxides act as erosion-inhibiting means. It is thus desirable and important that fire zones be avoided in the cyclone also from the point of view of erosion.

At the high speeds at which the particles are travelling when they strike against the cyclone wall, it has been found that the angle of incidence of the particles upon impact has a con¬ siderable influence on the erosion. For this reason, it is desirable with a cyclone design which, as far as possible, eliminates particles falling against the cyclone wall at a angle of incidence larger than zero. That is to say, if particles touch the cyclone wall at all, this should be done in a movement parallel to the wall. This is achieved by pre¬ venting speed differences arising between gas and particles present in the gas. In the present invention, a design with these desired properties in a cyclone is described, which solves the disadvantages of the above-mentioned kind with which conventional cyclones are associated. The principle of this solution is to disturb the flow as little as possible inside the cyclone.

SUMMARY OF THE INVENTION

According to the present invention, a cyclone for flue gas cleaning is designed such that particles, flowing towards the wall of the cyclone, from a gas flow inside the cyclone at angles of incidence to the wall larger than zero are avoided. This is achieved by making the necessary area reduction of the cyclone in the lower part thereof even and continuous. The transition from a cylindrical shell surface in the uppermost part of the cyclone with a larger cross-section area than a cylindrical cyclone leg at the lowermost part of the cyclone is made by means of a transition part evenly tapering without any discontinuities in the decrease of the circular cross- section area. According to the invention, the shell surface of the transition part in the cyclone exhibits a double-curved surface.

In the region between the upper cylindrical shell surface and the transition part of the cyclone, as well as between the lower cylindrical shell surface and the transition part, a shell surface with the shape of a frustum of a cone may be applied. In this connection, one or both of these mentioned conical shell surfaces may be applied adjoining the transition part. These conical shell surfaces may be used to simplify the design and render the cyclone according to the invention less expensive. A cyclone according to the invention is more com¬ plicated and expensive to manufacture than a conventional cyclone. Still, the extended service life with respect to erosion and the advantages described provide obvious benefits, for example when utilizing the invention for cleaning of hot gases with particle contents.

By providing a successive area reduction of the cyclone accor¬ ding to the above, so chosen that the speed of the gas flow becomes constant and as low as possible, the lowest erosion and the lowest propensity to fire are attained, and, in connection therewith, an additionally reduced erosion because of the influence exerted by fires on the resistance of the materials. The low erosion is obtained because of a lower speed of the particles and by avoiding discontinuities, for example by transitions from cylindrical to conical shell sur¬ faces, or inversely.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a vertical cross section through a cyclone according to the invention.

Figure 2 shows a diagram of the distribution of the erosion and the particle speed at different levels of a cyclone according to the invention.

Figure 3a illustrates a curve of the dependence of the erosion on the angles of incidence of particles to a surface made of two different materials.

Figure 3b shows the angle of incidence for a particle which hits a surface on a cyclone wall.

Figure 4 shows a cross section of a cyclone according to the prior art, with a diagram of the distribution of the erosion and the particle speed at different levels of such a cyclone inserted at the side. Figure 5 shows an alternative embodiment of a cyclone accor¬ ding to the invention, where the cone part and the cyclone leg have been lined with an erosion-resistant material.

Figure 6 shows the location of erosive particles near the wall of the cyclone at the transition between the cone part and the cyclone leg in a conventional cyclone.

Erosion which occurs at a surface where particles hit the surface at various angles is illustrated in Figure 3 for two different materials of surfaces which are subjected to partic¬ les flowing towards and through them. The curve shown in con¬ tinuous line applies to a surface of a soft metallic material, whereas the dotted curve relates to a hard and brittle mate- rial, for example a ceramic. In the figure, the erosion as a function of the angle of incidence of the particle to the sur¬ face has been drawn. As is clear from the curves, the erosion increases greatly at an increasing angle of incidence of the particle. An increase of the angle of incidence of a particle from, for example 0.1° to 1.0° may increase its erosive influ¬ ence ten times. This dependence is particularly remarkable at a surface consisting of a soft metallic material. The most interesting thing about the curves according to Figure 3 is that, at the angle of incidence zero, the erosion is practi- cally non-existent. The intention of the present invention is to avoid angles of incidence greater than zero of the partic¬ les when these make contact with the shell surface of the cyclone. This means that particles near the shell surface slide along this surface.

The erosion at a surface which is subjected to a particle flow of the type described in the invention is also greatly depen¬ dent on the speed of the particles. This situation may be described in the relationship

Erosion = k ^• vP where p = 2.5 - 3.5 depending on the combination of parameters as mentioned above, v is the particle speed and k a constant. This parameter p is very annoying because of the great increase in speed of gas and particles which takes place when the diameter of the cyclone in the cone part of a cyclone decreases. As an example it may be mentioned that the diameter may decrease from 800 mm to 150 mm. Thus, it is extremely important that the particle speed be maintained at a low level without any peaks.

Figure 4 shows a cyclone according to well-tried technique, where the lower part of the cyclone is designed with a cone part which suddenly changes into a cylindrical tube, which constitutes the cyclone leg. The adjoining diagram illustrates a curve which reproduces the particle speed of particles in a gas flow through the cyclone as a function of the position of the particles in the cyclone. As will be clear from the dia¬ gram, the speed curve exhibits a smaller peak at the area A and a larger maximum at the area B. In the area A, particles hit the shell surface of the cyclone at an angle considerably larger than zero, which is due to the fact that the inlet into the cyclone is located here. Consequently, erosion arises in this area in spite of the particle speed being relatively low. This erosion is not, however, of an annoying kind.

The area B exhibits a strong maximum for the particle speed, which in turn leads to a very extensive erosion in this area. As is clear from the diagram, however, the erosion occurs somewhat below that point where the speed of the particles has its maximum. This great speed increase which the particles exhibit at the curve peak C is due to the speed having increa^¬ sed with a decreasing circumference of the cyclone when par¬ ticles in a gas flow in the cone part are forced to run in spirals along a periphery which becomes increasingly narrower. When subsequently the particles penetrate into the cyclone leg with its constant cross-section area, the particle speed decreases, inter alia because of the friction. At the relatively abrupt transition between the cone part and the tube (the cyclone leg) in the cyclone, it is probably so that particles which are sliding in spirals along the shell surface of the cone part on their way downwards at the sudden change of the cyclone symmetry, due to inertial forces, briefly leave the proximity to the shell surface and then again, because of centrifugal forces, are thrown outwards towards the cylindrical shell surface of the cyclone leg. This causes particles, which during the movement towards the cylinder wall at the area B collide with the wall, to fall towards the wall at an angle of incidence which has suddenly increased and become considerably greater than zero. This increased angle of incidence and the high speed of the particles result in the heavy erosion which occurs in the area B. This situation is shown in Figure 6, where typical particle positions are marked in a conventional cyclone at the border between a conical shell surface lb and a cylindrical tube, the cyclone leg lc. Particle positions at three different cross sections of the cyclone are reproduced, from which it is clear that particles which at section E are located near the wall of the cyclone, immediately below the sharply marked transition between the cone part lb and the cylinder part lc at section F, leave the proximity to the wall and then later, through the influence of the centrifugal force in the gas vortex in the tube lc, again assume a position near the cylinder wall at section G. Approximately at section G, particles consequently flow towards the cyclone wall at high speeds and at relatively high angles of incidence, which brings about maximum erosion on the cyclone wall in this area (at G) .

Another worry which arises at a sharp transition between different cyclone parts (cyclone cone and cyclone leg) , as is the case at the area around section E, is that a coating 15 of ash particles tend to form between the sections E and G, this coating 15 being largest at the section F, where the coating may amount to thicknesses of about 3 mm. See Figure 4. The coating may cause problems in the discharge system, provided downstream of the cyclone, for the removal of separated cyclone ash. It has proved that the coating is easily detached in connection with stoppage of a plant served by the cyclone. In case of such a stoppage, the cyclone is cooled down to approximately room temperature, whereby the cyclone material (usually some type of stainless steel) and the coating 15 decrease in volume to varying degrees because these materials have different coefficients of liner expansion, which results in the coating cracking and loosening from the cyclone wall. Pieces falling down from the coating may thus easily cause a stop and difficulties in the ash discharge system.

The solution to the problems described is, according to the invention, to design the transition from the conical part to the cyclone leg in the cyclone such that the flow is not dis- turbed as described. With the described solution according to the invention, problems with both erosion and coating are prevented.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the invention will be described with reference to the accompanying drawings.

Figure 1 shows a vertical cross section of a cyclone 1 accor- ding to the invention. In its upper part, this cyclone is designed in accordance with conventional technique with an inlet 2 for the gases which are to be cleaned. Gas flowing into the cyclone is set in rotation by a casing, which in the upper part of the cyclone is designed as a cylinder part la. The rotation is brought about by arranging the inlet 2 tangen- tially to the cylinder part la at the upper portion thereof. The casing then changes into a conical part lb, where the gas vortex which occurred when the gas was forces into a rotary movement, is accelerated to a higher speed, since the circu - ference of the cross-section area of the cone in the cone part lb of the cyclone, over which the gas flow travels, becomes increasingly narrower when the gas moves downwards in the conical part lb. The cylinder part la together with the coni- cal part lb, when there is such a conical part lb, is compri¬ sed in what is here referred to as a first shell part. Down¬ stream of the first shell part of the cyclone 1, there is a tubular cyclone leg lc. At the lowermost part of the cyclone leg, there is an outlet 3, where ash or other impurities removed from the gas flow may be emptied. Between the first shell part and the cyclone leg lc, the cyclone has a transi¬ tion part Id with an even and continuous area reduction of the cross-section area from the smallest cross-section area of the conical part lb down to the cross-section area of the cyclone leg lc. Expressed in different terms, the transition part Id has a double-curved shell surface, since the transition part Id consists of a shell which is shaped as a surface of revolu¬ tion which is formed by an arc rotating one turn around the symmetry axis of the cyclone. With this definition, the tran¬ sition part Id is geometrically connected to the first shell part la, lb of the cyclone and the cyclone leg lc, respec¬ tively, by the tangent to the ends of the rotating arc coin¬ ciding with the generatrices of the first shell part la, lb and the cyclone leg lc, respectively, in each position of the arc during the rotation. According to the prior art, a discon¬ tinuity with respect to the above-mentioned area reduction occurred at the joint between the conical part lb and the cyclone leg lc. The intention of the invention is to eliminate this discontinuity.

In an alternative variant of the embodiment, the cone part lb is omitted, in which case the connection between the transi¬ tion part Id and the cylinder part la is made without any intermediate body, in this case without the cone part lb. In an additional alternative, the cyclone is designed with a lower conical part between the transition part Id and the cyclone leg lc with or without the cone part lb, to render the manufacture of the cyclone simpler and less expensive. However, this is not a preferred embodiment.

Cleaned gas is discharged from the cyclone, at the top thereof, at a second outlet 11. The diagram in Figure 2 illustrates how the speed 4 of the gas flow is changed when the gas flows through different sections of the cyclone. From this it is clear, among other things, that the speed is maintained constant and relatively low while the gas flows through the transition part Id of the cyclone 1. Another curve 5 in the same diagram illustrates the magnitude of the erosion in different parts of the cyclone. From this erosion curve it may be read that the erosion has no maxima in the tapering part of the cyclone, which means that the erosion is maintained at a low level and distributed over a larger surface. Nor do the previously mentioned relative speeds between the gas flow and particles in the gas occur with this described embodiment, which results in a considerable reduc¬ tion of the fire propensity, a fact which also reduces the erosive effect according to the above.

The material in the shell surface in the transition part Id is preferably a hard material, for example white cast iron. A further improvement of the erosion resistance in the cyclone 1 is- achieved by a lining of primarily the transition area Id by means of a ceramic. All the parts and surfaces are machined so as to obtain an absolutely even surface structure. Different parts of the cyclone 1 are jointly machined in joints so as to minimize joints and gaps.

According to Figure 5, the cyclone 1 in the transition region Id may be lined with a wear-resistant, heat-resistant and erosion-resistant material. A lining 6 is arranged inside a gas-tight casing 7. The casing 7 can then be made with a simpler and cheaper construction. The casing 7 is connected to the lower conical part lb of the cyclone 1 by means of a flange 8. The flange 8 may consist of a welded flange or be joined in some other way. The intention is that the casing 7 with the internal lining 6 is to be easily detachable for maintenance. The lining 6 is made of a ceramic or of some other hard material, for example white cast iron and shall have an internal shape as described for the transition part Id above. The lining 6 is composed in an optional number of sections 6a - 6c or as one whole. When composing the lining by means of sections, these may be provided with lining flanges 9, so as to make possible joining with different types of joints. The lining sections 6, 6a - 6c may alternatively be stacked loosely on top of each other and be supported by annularly arranged support points as disclosed in Swedish patent application 9002924. Between the lining 6 and the casing 7 there is a gap 10, which may be closed or arranged such that cooling air may be caused to circulate in the gap 10 for cooling the lining 6 and/or the casing 7.

A disadvantage with a design of a cyclone according to the invention is that it leads to a higher investment cost, which, however, is rapidly paid back in the form of longer intervals between service requirement and component replacements, and a greatly reduced risk of operational disturbances.

Claims

1. A device (1) for separation of particles from hot gases, wherein the device has a shell which comprises a first cylin- drical shell part (la, lb) and a second cylindrical shell part (lc) , wherein the first shell part has a circular cross sec¬ tion and a larger area than the second shell part which also has a circular cross section, wherein the first shell part (la) has a tangentially arranged inlet (2) for hot uncleaned gases and an outlet for cleaned gases (11), whereas the second shell part (lc) at one of its ends has an outlet (3) for sepa¬ rated material and wherein a third shell part with circular cross section constitutes a transition part (Id) between the first and second shell parts, characterized in that the transition part (Id) exhibits an even and continuous area reduction without discontinuities in the cross-section area from the cross-section area of the first shell part (la, lb) to the cross-section area of the second shell part (lc) , and that the shell surface of the transition part is double- curved.

2. A device according to claim 1, characterized in that the first shell part comprises a cylinder part (la), which changes into a conical part (lb) , whereby the conical part of the first shell part connects with the transition part (Id) .

3. A device according to any of the preceding claims, characterized in that the transition part (Id) consists of a shell which is formed as a surface of revolution which is formed by an arc rotating one turn around the symmetry axis of the device, wherein the transition part (Id) is connected to the first shell part (la, lb) of the cyclone and the second shell part (lc) , respectively, by the tangent to the ends of the rotating arc coinciding with adjoining generatrices of the first shell part (la, lb) and the second shell part (lc), respectively, in each position of the arc during rotation.

4. A device according to claim 3, characterized in that the inside of the transition part (Id) is freed from irregulari¬ ties by machining by means of turning, milling, or laser machining.

5. A device according to any of the preceding claims, characterized in that^' the transition part (Id) is an inter¬ nal lining (6) for the casing (7) of the device.

6. A device according to claim 5,' characterized in that the lining (6) is divided in the axial direction into tubular sections (6a - 6c) .

7. A device according to claim 5 or 6, characterized in that the lining is made of a heat-resistant and erosion-resistant material, for example a ceramic or white cast iron.