CYCLONE FOR SEPARATION OF A PARTICULATE MATERIAL FROM A GAS

Abstract: A cyclone (3) for separation of a particulate material from a gas, for example ashes and/or consumed bed material, which in a PFBC power plant leave/leaves a bed vessel together with combustion gases. According to the invention the lower part of the cyclone (3) is formed with a bottom (3d) through which an outlet tube (40) for separated material projects into the cyclone (3), the outlet tube together with said bottom (3d) and the surrounding cyclone wall (41) forming an annular space (42), open on its top.

Description: The invention relates to a cyclone for separation of a particulate material from a gas according to the precharacterising part of Claim 1. The invention relates primarily to the cleaning of combustion gases in a PFBC power plant. The term "PFBC" is formed of the initial letters in the expression Pressurized Fluidized Bed Combustion. During separation of material in a cyclone, a circulating vortex is formed and solid particles are accumulated on the wall of the cyclone. In the lower conical part of the cyclone, the particle concentration and the particle velocity relative to the cyclone wall increase with decreasing diameter of said conical part and the erosive impact on the cyclone wall increases. Thus, from the view point of wear, the cyclone outlet constitutes a critical portion. One way of solving this wear problem has been the insertion of devices (vortex eliminator) which reduce the speed of rotation of the vortex. Another way has been to construct the outlet part of or to line it with hardened wear resistant materials, for example Ni-hard, ceramics, basalt, or the like. It is the object of the invention to develop a cyclone of the above-mentioned kind with a greatly reduced liability to erosive wear in its outlet part without being forced to take refuge to decreased vortex speed or expensive materials. To achieve this object the invention suggests a cyclon according to the introductory part of Claim 1, which is characterized by the features of the characterizing part of Claim 1. Further developments of the invention are characterized by the features of the additional claims. The invention brings about a considerable reduction of erosion in the outlet part of the cyclone by means of a relatively inexpensive and robust design. Thus the invention increases the life time of the cyclone, and the operational availability of the power plant, and reduces the maintainance and operating costs of the plant. The dimension of the bottom diameter of the cyclone should be at least three times that of the outer diameter of the outlet tube. The outlet tube should project into the cyclone by a distance which is at least twice the outer diameter of the tube. The annular space is suitably given such a depth that a layer or cushion of dust can form at the bottom. The lowermost part of the cyclone is suitably cylindrical. It may be provided with vortex-damping means. Although the invention is primarily designed for PFBC power plants, it is also applicable to other plants in which erosive particulate material is to be separated from gases, for example in carbon gasification plants. The invention will now be described in greater detail with reference to the accompanying drawings showing - by way of example - in Figure 1 a conventional PFBC power plant to which the invention may be applied, Figure 2 a first embodiment of a cyclone according to the invention, Figures 3 and 4 alternative embodiments of a cyclone according to the invention. In Figure 1 a pressure vessel 1 surrounds a bed vessel 2 and a gas cleaning plant, symbolized by only one cyclone 3. In reality such a cleaning part usually consists of a plurality of cyclones connected in series and/or in parallel. The bed vessel 2 is provided with a bottom 4 with air nozzles 5 and fuel nozzles 6. The bed vessel 2 contains a bed 7 of a particulate bed material containing or completely consisting of a sulphur absorbent such as lime or dolomite. In the lower part of the bed vessel or combustion chamber 2 there are water-supplied tubes 8 for generating steam and thereby cooling the bed 7. Fuel is supplied to the bed 7 via the conduit 10 and nozzles 6. Air for combustion of this fuel and for fluidization of the bed material is supplied to the combustion chamber 2 from the space 12 between the pressure vessel 1 and the combustion chamber 2 via the nozzles 5 in the bottom 4. Bed material is supplied to the bed 7 through a conduit 13 and is removed through a conduit 14. The combustion gases are accumulated in the freeboard 15 above the bed 7 and are passed via the conduit 16 to the cyclone 3, where dust is separated and discharged via the conduit 17. Through the conduit 18 the gases are passed to a turbine 20 driving a compressor 21 and a generator 22. The compressor supplies compressed air to the space 12. the gases leaving the turbine 20 are passed through a feed water preheater and from there to a chimney. Steam generated in the tubes 8 is led to the steam turbine 27 which drives a generator 28. The expanded steam is led to a condenser and from there to a feed water tank. By means of a feed water pump, the condensate is returned to the tubes 8 of the bed vessel 2 via a conduit 30. The ash conveying pipe 17a, 17b comprises a pressure reducing device 31, which is dimensioned such as to obtain a gas flow necessary for removal of separated material at the pressure difference prevailing between the gas cleaner and a receiving container 32, in which the pressure is approximately equal to atmospheric pressure. The pressure reducing device 31 for removal of separated material from the cyclone 3 is designed as a cooler for dust and the gas conveying the dust. It may be of the kind disclosed in EP-B-0108505 or in EP-A-86100806. As shown in Figure 2, the cyclone 3 may be formed with an upper cylindrical part 3a, an intermediate part 3b, and a lower cylindrical part 3c with a plane bottom 3d. An outlet tube 40 is connected to the bottom 3d and projects somewhat into the lower cylindrical part 3c. The tube 40, the wall 41 in the lower cylindrical part of the cyclone 3, and the bottom 3d form an annular space 42 open at its top. This space 42 is suitably given such a depth that, in operation, a cushion 43 of separated material is formed at the bottom 3d, which cushion prevents erosion of the bottom plate. The tube 40 is connected to or integral with the tube 17 which conducts separated particulate material to the pressure reducing device 31. The cyclone part 3c is given such a diameter that the vortex of gas and particles has such a low velocity as to cause only insignificant erosion on the wall surface 41 in the part 3c. The twice enforced diversion of the gas stream in the space 42 before entering into the tube 40, as shown by the arrows 44, also produces a retarding effect which reduces the vortex velocity. The diversion and the retardation reduce the wear of the bottom 3d, or eliminate it completely by the formation of the dust cushion 43. The cushion 43 also contributes to a more efficient retardation. Hardly any erosion at all occurs in the outlet tube 40. By diverting the gas-particle stream by 180° before entering into the outlet tube, a concentration of the particles to the centre of the outlet tube is obtained, which results in a virtually complete elimination of tube wear. This effect is known per se from the above-mentioned EP-A-86100806. The tube 40 and the outflow therethrough are dimensioned so as to obtain also a sufficient exhaustion of particles from the central part of the cyclone. With the embodiment according to Figure 3, a great pressure reduction is obtained at the actual outlet from the cyclone 3. The pipe 17 connected to the cyclone 3 has a considerably larger diameter than the outlet tube 40. The lower end of the tube 40 may be connected to the bottom 3d, or, as shown in Figure 3, may be extended by a part 40b projecting down into the pipe 17. In the embodiment according to Figure 4, the lower part of the cyclone 3 is formed with several cylindrical or conical parts, arranged in series, with successively decreasing diameter towards the outlet, namely, part 3c with a bottom 3d, part 3e with a bott om 3f, and part 3g with a bottom 3h. The outlet tube 40 projects into the cyclone part 3c, an outlet tube 45 projects into the cyclone part 3e, and an outlet tube 46 projects into the cyclone part 3g. The latter outlet tube 46 is connected to the conveying pipe 17. The cyclone 3 can be cooled by the surrounding compressed air in the space 12 between the bed vessel 2 and the pressure vessel 1, or by water. To improve the cooling effect, as shown in Figure 4, the lower parts 3c, 3e, 3g of the cyclone 3 may be surrounded by cooling jackets 50 in the case of air cooling, or cooling jackets 60 in the case of water cooling. In the case of a PFBC plant with cyclones 3 placed in a pressure vessel 1 filled with compressed air, the surrounding air in the space 12 has a high density and hence also a high cooling capacity. By a cooling jacket 50, which in Figure 4 surrounds the part 3e of the cyclone 3, the cooling effect is improved by air from the space 12 being forced to pass along the part 3e through gap 51, formed by the jacket 50, on its way to the bottom nozzles 5 of the bed vessel 2. The cooling effect can be improved by providing the part 3e with longitudinal or helical flanges 52. In a power plant with a fluidized bed 7, which operates at a pressure insignificantly exceeding the atmospheric pressure and in which the cyclone 3 is not surrounded by compressed air with high density and good cooling capacity, cooling by water may be an alternative solution. By a cooling jacket 60, which in Figure 4 surrounds the part 3g, the lower part of the cyclone 3 can be cooled. Cooling water is supplied to the space 61 between the cooling jacket 60 and the part 3g via the conduit 62 and is transported away via the conduit 63. Also cyclones in PFBC power plants may, of course, be water-cooled. The invention has been shown as applied to a PFBC power plant. However, the cyclone may be used in all types of plants involving cleaning of dust-laden gas. In plants with a low or insignificant pressure difference between the cyclone and a receiving container for separated dust, it may be necessary to place a suction fan in the outlet conduit from the cyclone. The abrasion resistance and the life time of a cyclone will, of course, be extra high if abrasion-resistant materials are used also in connection with the embodiments according to the invention.

Claims: 1. A cyclone (3) for separation of a particulate material from a gas, for example ashes and/or consumed bed material, which for example in a PFBC power plant leave/leaves a bed vessel (2) together with combustion gases, characterized in that the lower part of the cyclone (3) is formed with a bottom (3d) through which an outlet tube (40) for separated material projects into the cyclone (3), the outlet tube together with said bottom (3d) and the surrounding cyclone wall (41) forming an annular space (42), open on its top. 2. Cyclone according to Claim 1, characterized in that the diameter of the bottom (3d) of the cyclone (3) is at least three times the outer diameter of the outlet tube (40). 3. Cyclone according to Claim 1 or 2, characterized in that the depth of the annular space (42) is at least twice the outer diameter of the outlet tube (40). 4. Cyclone according to any of the preceding Claims, characterized in that the lower part (3c) of the cyclone (3), surrounding the outlet tube (40), is cylindrical. 5. Cyclone according to any of the preceding Claims, characterized in that the outlet tube (40) opens out into a space (12) with a larger diameter than that of the outlet tube (40). 6. Cyclone according to any of the preceding Claims, characterized in that the cyclone (3) is formed with a gradually decreasing cross section by arranging series-connected parts (3c, 3e, 3g) with consecutively smaller diam eters each of these parts having a bottom (3d, 3f, 3h) through which an outlet tube (40, 45, 46) for separated material projects towards the main part of the cyclone (3), and that each of said parts, together with the associated bottom (3d, 3f, 3h) and part (3c, 3e, 3g), forms an annular space (42) open on its top. 7. Cyclone according to any of the preceding Claims, characterized in that the lower part (3c, 3e, 3g) of the cyclone (3) is surrounded by a cooling jacket (50) which guides a gas stream along said lower part (3c, 3e, 3g). 8. Cyclone according to any of the preceding claims 1-6, characterized in that the lower part (3c, 3e, 3g) of the cyclone (3) is surrounded by a cooling jacket (60) for cooling by water.

Inventors: BRAENNSTROEM, ROINE, VEENHUIZEN, DIRK, Brännström, Roine
Applicants: ASEA STAL AKTIEBOLAG
Application year: 1987
Main classification: B04C5
IPC codes: B04C5, F23J15, B01D45