POWER PLANT WITH COMBUSTION OF A FUEL IN A FLUIDIZED BED

Abstract: Power plant, particularly a the PFBC type, with combustion of a fuel in a fluidized bed containing a particulate sulphur-absorbing material. A discharge device for bed material and ashes comprises a quantity controlling crusher (56) which breaks down slag lumps so that the material can be transported away via a pneumatic pressure reducing transport device (64), or which grinds down slag lumps and other bed material so that unconsumed absorbent in the bed material is exposed, whereafter this material is returned to the bed (26). The plant includes a vertical crusher (56), to which an outlet (54) from the bed vessel (12) is eccentrically connected. The crusher (56) has a housing (110) with a stationary grinding portion (112) and a rotat­ ing grinding portion (116) with a plate (132) receiving bed material. Above the plate (132) there are a number of scrap­ ers (184a-184g) each scraper forming a gap of different height with the plate 132. Thus each scraper scrapes off a layer of material during the rotation of the plate (132), resulting in an even distribution of the material along the periphery (182) of the plate (132). This ensures uniform wear around the stationary grinding portion (112).

Description: The invention relates to a power plant with combustion of a fuel in a fluidized bed according to the precharacterising part of Claim 1. Such a power plant is known from the EP-A-0 167 992. The invention primarily refers to a so called PFBC power plant, the abbreviation "PFBC" being derived from the term "Pressurized Fluidized Bed Combustion". In the power plant a sulphur-containing fuel is burnt in a fluidized bed containing a particulate sulphur-absorbing material in a bed vessel. A discharge device for bed material and ashes includes a crusher having a controlled capacity. The crusher breaks down slag lumps so that the material can be transported away via a pressure reducing pneumatic transport device, or grinds down bed material so that unconsumed absorbent in the bed material is exposed and utilized by feeding it back into the bed. Absorbent must be permanently supplied to the bed in dependence on the sulphur content in the fuel. Bed material and ashes formed from the fuel or slag particles which do not accompany the combustion gases must be continuously or intermittently removed to prevent the bed level from rising. Fine-grained material, as finer fractions of ashes and worn bed material, accompanying the combustion gases, are separated in a cleaning plant and transported away therefrom. Bed material, ashes and slag have to be removed by being tapped off from the bed vessel through a controllable discharge device. The EP-A-0 167 992 describes a power plant with a fluidizable bed in a combustion chamber. The bed material also contains sulphur absorbing material. This power plant is designed to burn moist coal. In order to give such fuel better conveying properties to avoid clogging in the tubes, the fuel is dried in a rotating channel that is supplied with hot combustion gases. To increase this drying process part of the bed material extracted from the bed vessel is crushed in a crusher and mixed with the moist fuel in said rotating channel. The structural details of the crusher are not disclosed. Means for automatic control of the bed level are not provided. The invention aims at developing a power plant of the above-mentioned kind with a discharge device which allows effective control of the bed level, permits efficient utilization of the sulphur absorbent and functions in a away that ensures a long life of the components of the discharge device. To achieve this aim the invention suggests a power plant 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. According to the invention, the discharge device of the plant includes a crusher with a vertical shaft located below the bed vessel. This crusher is supplied with bed material from the bed vessel via a tube extending from the bed vessel. The crusher comprises a housing with a ring forming the stationary crushing or grinding portion of the crusher. A vertical shaft journalled in the housing supports a conical, circular body, forming the rotating grinding portion of the crusher, and a substantially horizontal disk. The plate and the grinding portion may be constructed as one unit and formed as a truncated cone. The upper surface of the truncated cone forms the disk. A supply tube for bed material in the upper part of the housing is connected to the outlet tube of the ash chamber and opens out above the plate, eccentrically in relation to the axis of rotation of the rotating grinding portion of the crusher. In the housing, above the plate supported by the shaft, there are a number of stationary scrapers which distribute the bed material coming down through the outlet tube such that bed material falling down between the grinding portions of the crusher is evenly distributed along the periphery of the plate. This results in uniform wear around the stationary grinding portion. The scrapers can be designed and arranged in different ways. They are suitably located at different levels above the plate and have an orientation which deviates from the radial direction, suitably arcuate or helical as the blades of the impeller of a centrifugal pump. Since the gap between the plate and scrapers are successively smaller in the direction of rotation, each one of the scrapers removes a material layer when the disk is rotating. This results in a substantially even distribution of material along the periphery and in uniform wear in the crusher. It would also be possible to use scrapers at the same level but having different lengths in the radial direction. The invention will now be described in greater detail with reference to the accompanying drawings showing - by way of example - in Figure 1 schematically the invention applied to a PFBC power plant, Figure 2 the crusher included in the power plant according to the invention, Figure 3 a section taken along line A-A in Figure 2, displaying an embodiment of scrapers, included in the crusher. In Figure 1 numeral 10 designates a pressure vessel enclosing a bed vessel 12 and a gas cleaning plant symbolized by one cyclone 14 only. In reality the cleaning plant usually consist of parallel-connected groups of series-connected cyclones. The lower part of the bed vessel 12 includes an air distributor 16, which divides the bed vessel 12 into a combustion chamber 18 and an ash chamber 20. The air distributor 16 consists of a number of elongated air distribution chambers 22 with air nozzles 24. The chambers 22 communicate with the space 25 inside the pressure vessel 10 and outside the bed vessel 12. This space 25 contains combustion air under pressure. Between the space 25 and the chambers 22 there are arranged members (not shown) for control of the air supply. Through the nozzles 24 the combustion chamber 18 is supplied with air for fluidizing the bed 26 of particulate material containing a sulphur absorbent and for combustion of a fuel which is supplied to the bed 26 from a fuel storage (not shown) through the fuel pipe 28. Fresh bed material is supplied to the bed 26 from a bed material storage (not shown) via the pipe 30. Above the bed surface 32 there is a freeboard 34 for collecting the combustion gases. From the freeboard 34 the combustion gases are passed via a pipe 36 to the cleaning plant 14 from where the cleaned gases are led via a pipe 38 to a turbine 40. The turbine 40 drives a generator 41 and a compressor 42. Combustion air compressed in the compressor 42 is supplied to the space 25 via a pipe 44. Dust separated in the cleaning plant 14 is transported away to a collecting container (not shown) through a pipe 37. The bed 26 contains tubes 46 for generating steam for driving a steam turbine (not shown) thereby simultaneously cooling the bed 26. Between the elongated air distribution chambers 22 there are openings 48, through which bed material from the bed 26 may pass to the ash chamber 20. The material 50 in the ash chamber 20 consists of ashes, slag and consumed and unconsumed sulphur absorbent. The ash chamber 20 comprises tubes 52 for cooling the material 50. The coolant in these tubes 52 may, for example, consist of combustion air, steam or water. The material 50 is removed from the ash chamber 20 via the outlet tube 54 and a crusher 56. The tube 54 may comprise a valve 55. In one embodiment of the invention, the crusher 56 may be constructed in such a way that it only crushes slag lumps to a grain size of about 7-10 mm, whereas all other material having a grain size of about 3-6 mm pass freely. In this case slag is broken down so that the material fed out through the crusher 56 is given a maximum grain size appropriate for pneumatic transportation. The pressure in the crusher 56 should in a PFBC power plant be approximately equal to the pressure in the bed vessel 12. The outlet 58 of the crusher 56 is connected, either directly or via an ejector 60, to a pressure reducing discharge device 64 of the kind described in EP-A-0 108 505. The ejector 60 may be provided with propellant gas directly from the space 25 or via a booster compressor, which on its suction side is connected to the space 25 in the pressure vessel 10. In another embodiment of the invention, the crusher 56 may be designed in such a way that it crushes both slag lumps and other fed-out bed material. Unconsumed absorbent in the interior of bed material particles is then exposed, and the crushed material is returned to the bed 26. The outlet 58 of the crusher 56 is then connected to the combustion chamber 18 via the ejector 60 and the dash-lined return pipe 62. For measuring the depth of the bed 26, differential pressure sensors 75,77 are provided, which are connected with one side via pipes 74,76 to points 70,72 on different levels in the bed vessel 12, and which are connected with their other side, via a common pipe 78, to a point 80 in the bed vessel 12 approximately on a level with the air distributor 16. In reality more than two differential pressure sensors are provided. These pressure sensors 70,72 are connected to a signal processing device 82 via pipes 84,86. A temperature sensor 88 in the bed 26 is also connected, via the pipe 90, to the signal processing device 82. A desired value, determined by the power level, is supplied to the signal processing device 82 from a power control equipment (not shown) via the pipe 92. A control equipment 96 for a supply device (not shown) for fresh bed material is connected to the signal processing device 82 by a pipe 94 and to the supply device by a pipe 98. A control device 100 is connected to the signal processing device 82 by the pipe 102 and to the driving motor 106 of the crusher 56 by a pipe 104. At constant power output, the feeding out of material 50 from the ash chamber 20 is controlled by controlling the speed of the motor 106 and hence of the crusher 56, so that the bed level 32 is maintained constant. The amount of removed material is equal to the amount of supplied bed material and coal ash formed which does not accompany the combustion gases on their way out from the freeboard 34. When the power is reduced, the bed level is lowered by increasing the speed of the crusher 56, and when the power is increased, the bed level is increased by stopping the crusher or reducing its speed and/or increasing the supply of fresh bed material. Figure 2 shows a section of the vertical crusher 56 included in the invention. The crusher 56 comprises a housing 110 with a stationary grinding portion 112 and a rotor 114 with a rotating grinding portion 116. The housing 110 consists of a lower part 110a and an upper part 110b, which parts are formed with flanges 110c and 110d, respectively, and joined together by means of a bolted joint 118. Despite cooling in the ash chamber 20, the material 50 has a relatively high temperature, up to 400°C, when leaving the ash chamber 20. The parts of the crusher 56 are thus subjected to a high temperature, which must be taken into consideration when designing the crusher. The stationary grinding portion 112 is constructed as a replaceable ring resting on a flange 120 in the lower part 110a of the housing 110 in such a way that it may expand radially. The grinding portion 112 is retained by a resilient centering ring 122 with a flange 122a which is clamped between the flanges 110c and 110d. The space between the stationary grinding portion 112 and the housing 110 is filled with heat insulating material 124. The rotor 114 of the crusher 56 is journalled in an upper bearing 126 in a sleeve 128 which slides into the lower part 110a of the crusher housing 110, and in a lower bearing in the drive motor 106. The upper bearing 126 is suitably positioned on a level with the grinding portions 112 and 116. The rotor 114 comprises a shaft 130 which is connected at the bottom to the output shaft of the motor 106, to the suitably annular rotating grinding portion 116 supported by the shaft 130, and to the suitably plane plate 132 connected to the grinding portion 116. The grinding portion 116, the shaft 130 and the plate 132 jointly form a space which is filled with a heat-insulating material 134. The shaft 130 and the grinding portion 116 are joined together by a bolted joint 133. The shaft 130 is internally water-cooled. Cooling water is introduced and discharged through a swivel 136. The shaft 130 is provided with a bore 138. The cooling water is introduced into a central tube 140 in the bore 138 so as to obtain satisfactory cooling of the upper part of the shaft 130 and of the bearing 126. The cooling water is returned through the gap 142 between the wall of the bore 138 and the tube 140. Between the shaft 130 and the sleeve 128 there is formed an annular gap 144. At the lower part of the sleeve 128 there is a sealing device 147 between the sleeve 128 and the shaft 130. Through the connection 146 the gap 144 is supplied with air or other gas, partly for cooling the sleeve 128 and partly for cleaning the space 148 between the rotating grinding portion 116 and the disk 152 which is joined to the sleeve 128 by a bolted joint 150. As shown by the arrow, the gas flows through the gap 144, the openings 154, the gap 156, the space 148, and the gap 158 to the collecting space 160 for crushed material. The sleeve 128 is externally provided with heat insulating material 162. The lower part 110a of the housing 110 is internally provided with a layer of abrasion-resistant, heat insulating material 164. The upper part 110b of the crusher housing 110 is internally provided with a plate 168 which is joined by means of a bolted joint 170 to a flange 172. Insulating material 174 is provided in the space above the disk 168. Through the upper part 110b of the crusher housing 110 there passes an inlet tube 176. The inlet tube 176 is joined to a flange on the outlet tube 54 by means of a flange 178 and a bolted joint. The inlet tube 176 is located eccentrically in relation to the vertical axis C-C of the crusher 56. The inlet tube 176 opens out at such a height above the plate 132 of the rotor 114 that the largest slag lumps, 40-60 mm, that may pass the gap 48 or rectangular openings between the chambers 22 of the air distributors, are able to pass between the orifice 180 and the plate 132. The distance from the edge 182 of the plate 132 is so large that bed material falling down through the tube 176 does not of its own run over the edge 182 of the plate 132. The disk 168 is provided on its lower side with a number of scrapers 184a-184g which have an orientation deviating from the radial direction. The scrapers 184a-184g are suitably arcuate, as shown in Figure 3. The scrapers 184a-184g have different vertical extensions so that between them and the plate 132 there are formed gaps 186a-186d of a successively diminishing height. When the rotor 114 rotates, as indicated by the arrow 188 in Figure 3, material on the plate 132 will be scraped off by the scrapers 184a-184g. The first scraper 184a scrapes off a first material layer, the second scraper 184b scrapes off a second material, and so on, because of the fact that the gaps 186a-d between the scrapers 184a-184g successively decrease in vertical size. In this way the material is distributed relatively evenly along the periphery 182 of the plate 132 to the conical gap 190 between the two grinding portions 112 and 116 of the crusher 56. In the conical gap 190 the slag lumps are crushed. In those cases where the crusher 56 only is to ensure that larger pieces of material are crushed to such a size, suitably 7-10 mm, that they may pass freely through a pressure reducing pneumatic discharge device, the gap 192 between the grinding portions 112 and 116 has such a size that the main part of the bed material having a size of about 3-6 mm is able to fall freely through the gap 192 into the space 160 without giving rise to wear. The outlet 194 is connected to the outlet pipe 58. In those cases where the crusher 56 is also used for crushing bed material particles to expose unconsumed absorbent contained in the inner part of the particles, the gap 192 has such a size as to obtain a fine-grained material with a large effective absorption surface of the unutilized absorbent.

Claims: Power plant with combustion of a fuel in a fluidized bed housed in a bed vessel (12) comprising supply means for supplying sulphur absorbent, crushed material and fuel to the bed (26), a compressor (42) for supplying the bed vessel (12) with air for fluidization of the bed material (26) and combustion of the fuel, and a discharge means comprising a crusher (56) with controllable discharge capacity for removal of bed material from the bed vessel (12) via an outlet tube (54) extending from the bed vessel (12), characterized in that the crusher (56) comprises a housing (110) with a ring (112), arranged in the housing (110), forming the stationary grinding portion of the crusher (56), a vertical shaft (130) journalled in the housing (110), a substantially horizontal plate (132) supported by said shaft (130), a rotating grinding portion (116) supported by said shaft (130), a supply tube (176), fitted in the upper part (110b) of the housing (110), which is connected to an outlet tube (54) from the bed vessel (12), which supply tube (176) opens out above said plate (132) eccentrically in relation to the axis of rotation (C-C) of the crusher (56), and a plurality of stationary scrapers (184a-184g), arranged in the housing (110) above said plate (132), these scrapers being shaped and dimensioned such as to distribute the bed material falling down between the grinding portions (112,116) along the periphery (182) of the plate (132). Power plant according to Claim 1, characterized in that the scrapers (184a-184g) are arranged at different heights above said plate (132). Power plant according to Claim 1 or 2, characterized in that the scrapers (184a-184g) have different radial extensions. Power plant according to Claim 2, characterized in that the scrapers (184a-184g) are arcuate. Power plant according to any of the preceding Claims, characterized in that the outlet of the crusher (56) is connected to the bed vessel (12) by a pipe (62) for returning crushed material to the bed vessel (12). Power plant according to any of Claims 1 to 4, characterized in that the combustion in the bed vessel (12) takes place at a pressure considerably exceeding the atmospheric pressure (PFBC plant) and that the crusher (56) is connected, on its outlet side, to a pneumatic, pressure reducing transport device (64), in which pressure reduction is brought about by changing repeatedly the gas/material flow in overflow chambers between series-connected tube parts. Power plant according to any of Claims 1 to 4, characterized in that the bed vessel (12) is enclosed in a pressure vessel (10) with a space (25) between the pressure vessel (10) and the bed vessel (12) containing compressed combustion air, from which space (25) the bed vessel (12) is supplied with air for fluidization of the bed (26) and combustion of the fuel, and that the crusher (56) is connected on its outlet side to a pneumatic, pressure reducing transport device (64), in which pressure reduction is brought about through repeated diversion of gas/material flow in connection chambers between series-connected tube parts. Power plant according to any of Claims 1 to 4, characterized in that the bed vessel (12) is enclosed within a pressure vessel (10) with a space (25) between the pressure vessel (10) and the bed vessel (12) containing compressed combustion air, from which space (25) the bed vessel (12) is supplied with air for fluidization of the bed (26) and combustion of the fuel, and that the crusher (56) is connected on its outlet side to the bed vessel (12) by means of a pneumatic conveying pipe (62) for returning crushed bed material to the bed (26).

Inventors: BRAENNSTROEM, ROINE, LINDBLAD, EVERT, MALMKVIST, LEOPOLD, MANSSON, MARTIN, THOREN,MATS, Brännström, Roine, Thorén,Mats
Applicants: ASEA STAL AKTIEBOLAG
Application year: 1988
Main classification: F23C11
IPC codes: B02C2, F23C10, F22B31