GRATE FOR FLUIDIZED BED BOILER

Abstract: The invention relates to a grate construction for use in the bottom section of the combustion chamber of a fluidized-bed boiler, the construction comprising a grate (1) equipped with a plurality of parallel cooling pipes (3; 18), an air-plenum chamber (2) adapted below the grate (1), means (14) adapted to the grate (1) for injecting fluidizing gas from the air-plenum chamber (2) to a fluidizing zone in the bottom section of the combustion chamber, and means adapted to the grate (1) for removal of solids from above the grate (1). In the construction according to the invention, the means for removal of solids comprise at least one solids removal trough (7), which is adapted to the grate (1) so as to flare wide open at the grate level with a therefrom downward tapering and at its bottom end open structure that is comprised of at least two opposed slanted surfaces (8, 9) between which the solids to be discharged is arranged to flow downward.

Description: GRATE FOR FLUIDIZED BED BOILER The invention relates to a grate according to the preamble of claim 1 for a fluidized-bed boiler. In a fluidized-bed boiler, fuel is combusted and partially gasified in a fluidized bed which is maintained above the bottom of the boiler combustion chamber and is formed by the bed material with the fuel mixed therewith. Conventionally, the bed material is sand. The fluidization of the bed is maintained by blowing a fluidizing gas such as air at a high velocity upward into the bed via the boiler bottom. Fluidized-bed boilers are intended for combustion of solid fuels and they are particularly suitable for firing with easily gasifiable fuels such as wood and peat . The fuel is fed into the fluidized-bed boiler via fuel infeed ports adapted to the walls of the combustion chamber of the boiler. The bottom of the combustion chamber in a fluidized-bed boiler is formed by a grate, conventionally having thereunder located an air-plenum chamber wherefrom fluidizing air is injected into the combustion chamber via air infeed openings such as nozzles adapted to the grate. Additionally, the grate has adapted thereto discharge means via which coarse aggregates of solids accumulated on the grate are removed. The solids to be removed can be, e.g., uncombusted fuel, rocks or other tramp material that has traveled into the combustion chamber with the fuel infeed. While the bed material may also form coarse aggregates through sintering, the amount of such solids in a properly controlled combustion process is small as compared with the coarse solids entering with the fuel. If the solids accumulated on the grate cannot be removed sufficiently effectively, it begins to interfere with the fluidization of the bed and intake of combustion air. For the removal of solids accumulated on the grate, a plurality of different types of arrangements have been developed, one of which is disclosed in international application publication WO 99/14530. The grate described in the publication for a fluidized-bed boiler comprises a plurality of substantially upright -oriented pockets opening into the combustion chamber, thus serving to remove coarse solids from the bed material falling onto the grate. The pockets reach into an air-plenum chamber located below the grate. The horizontal cross section of the ash removal pockets at the level of the grate has the shape of an elongated slot, while the cross section of the slots taken in the vertical plane is downward tapering. Hence, the ash removal pockets divide the grate surface on one hand into substantially horizontal areas serving as the air infeed areas proper into which a major portion of the fluidizing-air injection nozzles are adapted and, on the other hand, into ash removal areas along which the coarse solids to be removed can flow into one or greater number of ash removal pockets. In the art, the term open grate area refers to that summed area of the ash removal means over which said means opening into the combustion chamber form a free surface in the overall area of the grate. In a fluidized- bed boiler, the required amount of open grate area is determined on the basis of such factors as the fuel to be fired in the boiler. In the above-described grate boiler construction, longitudinally running walls of the ash removal pockets opening into the combustion chamber are made upright planar, whereby a large open grate area can be attained only by locating a great number of ash removal pockets close to each other or, alternatively, by using wide pockets. The width of the pocket is limited to the diameter of the ash discharge conduit connected to the bottom of the pocket. Ash discharge conduits are conventionally made using a 300 mm standard pipe, which means that the use of ash discharge conduits larger than this causes a severe constraint to the dimensioning of other parts of ash discharge conduits and devices connected thereto. Additionally, as a sufficiently wide space must be left between the pockets to fit infeed nozzles for combustion air injected into the combustion chamber, it is difficult to locate the pockets close to each other. Due to the above-described reasons, the open area in the above-described grate construction cannot be made larger than about 50 % maximum of the overall area of the grate without causing essential disturbance in the operation of the boiler. It is an object of the invention to provide for a fluidized-bed boiler an entirely novel grate construction, wherein the open area can be made large without disturbance to the boiler operation. The goal of the invention is achieved by virtue of providing the grate of the combustion chamber of a fluidized-bed boiler with elongated troughs that flare wide open at the grate level with a therefrom downward tapering and at its bottom end open structure via which the solids accumulated on the grate is removed. The troughs are comprised of at least two opposed slanted surfaces disposed at a distance from each other so that their mutual angle of inclination in regard to the vertical direction is advantageously 20° to 55°. The solids removal troughs are advantageously constructed from parallel cooling pipes having therebetween fins on which the infeed nozzles of the fluidizing gas are mounted. To the bottom section of the removal troughs are adapted conduits through which the solids fallen into the troughs is discharged into ash removal conduits. More specifically, the grate construction according to the invention is characterized by what is stated in the characterizing part of claim 1. The invention offers significant benefits By way of making the solids removal troughs downward tapering, it is possible to provide a large open area on the grate, whereby the solids removal can be made over a wide portion of the grate surface. The proportion of the open area can be varied in the design stage of the grate through modifying the number and depth of the solids removal troughs. In a grate according to the invention, the open area of the grate can be varied within a range of about 10-100 % of the overall area of the grate, thus making it possible to optimize the boiler design for different types of fuels. By designing the solids removal troughs wide and locating them at the fuel infeed openings, a major portion of the coarse aggregates such as rocks and tramp iron entering the combustion chamber along with the fuel feed can be made to fall directly into the solids removal troughs rather than onto the grate. The erosion, which attacks the cooling pipes constituting the troughs through abrasion by the sand and other solids falling into the solids removal troughs, remains at a minor level inasmuch a protective layer of sand is stagnant on the cooling pipes and therebetween due to the sufficiently wide tapering angle of the trough. Moreover, a grate according to the invention can be designed for a sufficiently large open area with a small number of solids removal troughs, thereby making the grate simple in construction and cost-efficient to manufacture. In the following, the invention is examined in detail by making reference to the attached drawings wherein FIG. 1 shows schematically in a perspective view a grate according to the invention; FIG. 2 shows an end view of another embodiment of a grate according to the invention; FIG. 3 shows the grate of FIG. 2 in an end view taken along line A-A; FIG. 4 shows schematically in a perspective view a third embodiment of a grate according to the invention; FIG. 5a shows schematically in a longitudinally sectional view a solids removal trough having a pocket-shaped solids removal nozzle adapted to the bottom of the trough; and FIG. 5b shows schematically in a longitudinally sectional view a solids removal trough having a plurality of solids removal conduits adapted to the bottom of the trough. A grate assembly according to the invention and designed to be located in the bottom section of the combustion chamber of a fluidized-bed boiler comprises a grate 1 adapted to operate between side walls 4 and between the front wall 5 and the rear wall 6 of the boiler combustion chamber so that an air-plenum chamber 2 remains below the grate. To the grate 1 are adapted elongated troughs 7 for solids removal that flare wide open at the grate level and are formed by at least two opposed slanted surfaces 8, 9 disposed at a distance from each other. The solids removal troughs 7 have a downward tapering shape. To the bottom section of the solids removal troughs 7 are adapted solids removal conduits 15 that are routed via the interior of the air-plenum chamber 2 in a downward direction so as to pass the solids removed from the grate 1 further forward into ash discharge pipes 19. The grate 1 is water-cooled for which purpose it is made into, e.g., a panel of tubes comprising a plurality of parallel cooling pipes 3 connected to each other by their fins 10. The cooling water is passed into the cooling pipes 3 from at least one feed pipe 11 which is adapted to pass crosswise over the width of the grate 1 and whereto the cooling pipes 3 are connected. The grate construction illustrated in FIG. 1 comprises one cooling water feed pipe 11, while the constructions shown in FIGS. 2 and 3 have two cooling water feed pipes 11. The cooling water feed pipe 11 is placed between the front wall 5 and the rear wall 6 in the combustion chamber, whereby the pipe is aligned at least substantially orthogonal to the cooling pipes 3 of the grate 1. The cooling water feed pipe 11 divides the grate 1 in two separate sections. The cooling water flows in the cooling pipes 3 from the center area of the grate 1 toward the sides walls 4. At the edge of the grate level, the cooling pipes 3 are bent upward so that they form a portion of the surface of the combustion chamber side walls 4. In a close proximity to the lower section of the side walls 4 are located tubular chambers 12 having connected thereto a plurality of parallel cooling pipes 13 that in combination with the upward-bent cooling pipes 3 form the side walls 4 of the combustion chamber. Advantageously, the side walls 4 are formed into pipe panels wherein the cooling pipes 3 of the grate 1 and the cooling pipes 13 connected to the tubular chambers 12 run in parallel with each other in an alternating manner. In this arrangement, the pitch of the pipes forming the side walls 4 is twice as dense as that of pipes in the grate 1. Nozzles 14 wherefrom the fluidizing air is blown into the combustion chamber are mounted on the fins 10 spanned between the parallel running cooling pipes 3. Both the level of the grate 1 and the slanted surfaces 8, 9 of the solids removal trough 7 are provided with the nozzles 14. The fluidizing gas is passed to the nozzles 14 from the air-plenum chamber 2 located below the grate 1. The surfaces 8, 9 forming the solids removal troughs 7 of the grate 1, as well as the level of the grate 1 itself, are comprised of the parallel running cooling pipes 3 and the fins 10 spanned therebetween. As is evident from FIG. 3, the cooling pipes 3 of the solids removal troughs 7 first rise to a suitable height as they leave the cooling water feed pipes 11 that are located in the center portion of the grate 1. Next, the pipes are bent toward the side walls 4 of the combustion chamber so as to make them run substantially straight to the side walls 4 in a slightly upward inclined direction, whereby any steam possibly formed in the cooling pipes 3 can easier escape along the pipe bore. The surfaces 8, 9 forming the solids removal troughs 7 are advantageously slanted so that the cooling pipes 3 of the surfaces 8, 9 are protected against erosive wear from bed sand and other solids falling into the troughs. For this reason, the inclination angle α of the surfaces 8, 9 of solids removal trough 7 in regard to the vertical plane must be sufficiently large in order to leave on and between the cooling pipes 3 of the surfaces 8, 9 under the influence of friction a layer of sand and other solids that protects the pipes against abrasive wear. Instructions for the dimensioning of the angle α can be found, e.g., on pages 1-2 and 1-3 in handbook Desktop Design Manual published by Particulate Solid Research Inc. (PSRI) . According to this publication, the inclination angle of a wall from the vertical plane at which all nonadhering solids having no sharp edges, such as conventional fluidized-bed materials, can flow down from a silo structure similar to the solids removal troughs 7 employed in the invention is normally smaller than 25°. Hence, as a guideline, the angle α should be made larger than 25° in order to prevent not all of the sand and solids resting on and between the cooling pipes 3 of the surfaces 8, 9 of the solids removal troughs 7 from flowing downward. In the embodiment according to the invention, the angle α is advantageously in the range of 20-55°, most advantageously 35-45°. When the angle α is selected, such factors as the shape of particulates in the bed material must be taken into account . For instance, the angle α must be made wider for particles having blunt edges than for sharp-edged particles. Also the temperature of the bed material affects its flow properties. Hot sand particles, for instance, flow easier than cold sand particles. The number and depth of the solids removal troughs 7 determine the size of the open grate area. By making the solids removal troughs 7 deep, its is possible to provide a large open area of the grate 1, because in such a construction the solids removal troughs 7 are wide and occupy a larger portion of the overall area of the grate 1. The depth of the solids removal troughs 7 is limited by the vertical height of the nozzles 14 that are mounted in the trough 7 for blowing the fluidizing gas. In practice the maximum height of the nozzles 14 is about 700 mm, because of the fact that the nozzles 14 must retain their rigidity even when embedded in the hot bed material glowing at a temperature of about 800 °C . Hence, the depth of the solids removal troughs 7 can hardly exceed 600 mm, which means that the maximum practical width of the solids removal troughs 7 is about 2000 mm. The open area of a grate 1 designed according to the invention may be made as large as 90 % of the overall area of the grate 1 without causing problems in boiler operation. Advantageously, the grate 1 is constructed such that the solids removal troughs 7 are located below the fuel infeed openings of the combustion chamber walls as seen in the longitudinal direction along the axes of the fuel infeed openings, whereby rocks, tramp metal and other coarse aggregates entering the combustion chamber with the fuel will fall directly into the solids removal troughs 7. Solids fallen into the solids removal troughs 7 is transported forward via solids removal conduits 15 communicating with the bottom sections of the troughs 7 to ash discharge pipes 19, wherefrom the solids is removed by means of auger conveyors, for instance. The parallel running conduits 15 are inclined to converge with each other so as to finally join with each other in the air- plenum chamber 2. In this manner, the solids collected via two conduits 15 can be passed via a single connection to the ash discharge pipe 19. The entry openings of the conduits 15 are adapted at the level of the bottom sections of the troughs 7. At the level of the entry ends of the adjacent conduits 15, there is mounted a planar plate 20 that prevents solids from falling into the air- plenum chamber 2. Herein, the walls 17 of the converging conduits 15 with the plate 20 form into the air-plenum chamber 2 a triangle-shaped downward-tapering opening through which air can flow. The width of the solids removal conduits 15 in the lateral direction of the solids removal troughs 7 is advantageously at least substantially equal to the standard diameter of about 300 mm generally selected for the ash discharge pipes 19 to be connected to the bottom ends of the conduits. Hence, if the entry end of the conduit 15 is about 600 mm wide, the width of the plate 20 in the longitudinal direction of the trough 7 is typically about 300 mm. The angle β between the exterior surface 16 of the conduit 15 and the horizontal plane is advantageously made wider than the free flow angle of the solids on an inclined surface, whereby the flow of the solids takes place at a substantially constant speed over the entire cross section of the conduit 15. Hence, the angle β must be greater than 30° (cf. pages 1-2 and 1-3, Desktop Design Manual, PSRI) . By virtue of the conduit arrangement according to the invention, solids can be removed via a single connection to the ash discharge pipe 19 from an area which generally is wider than what can be achieved through equipping the bottom sections of the troughs 7 with pocket-shaped solids removal nozzles connected thereto. To understand this difference, it must be borne in mind that during the operation of a fluidized-bed boiler, the troughs 7 are filled with a plentiful amount of solids that flows via solids removal structures connected to their bottom section into the ash discharge pipes 19. However, in the construction according to the invention, the internal friction of the solids does not essentially affect the solids flow in the conduit 15 if the inclination angle β of the exterior surface 16 of the conduit in regard to the horizontal plane is larger than the angle of repose of solids on an inclined surf ce. The internal friction of the solids flow does not have any effect on the flow until at the entry end of the conduit 15 which is located at the bottom section of the solids removal trough 7, where the solids can flow into the conduit 15 from an area having a width flaring upward at an angle of about 30° from the vertical plane laterally from the entry opening edges of the conduit 15 (cf. pages 1-2 and 1-3, Desktop Design Manual, PSRI) . FIG. 5b schematically elucidates the flow of solids in a trough 7 having a plurality of convergently inclined conduits 15 connected to its bottom section. FIG. 5a, respectively, elucidates the flow of solids in a trough 7 having downward tapering pockets connected to its bottom section. The solids flow takes place toward a discharge conduit 22 adapted to the bottom of the pocket. Herein, the internal friction of the solids in a similar manner constrains the width of the area wherefrom solids can be removed to an area having a width flaring upward at an angle of about 30° laterally from the edges of the exit opening. Hence, it is easy to see that the width of the area wherefrom solids can be removed from a trough 7 is determined by the distance of the exit opening from the top level 21 of the solids layer accumulated in the trough 7. As can be seen from the exemplifying embodiment illustrated in FIG. 5b, the conduits 15 are advantageously disposed at a distance from each other on the level of the bottom section of the trough 7 such that the thus formed overall solids removal area extends over the entire top level 21 of the solids layer remaining between the conduits 15, whereby the conduit construction according to the embodiment of the invention allows solids to be removed via the solids discharge conduit 22 into the ash discharge pipe 19 over an area which is about 20 % wider than that achievable by conventional solids removal means of the pocket type. In fact, the wider the solids removal area can be made the farther apart from each other can be disposed the adjacent ash discharge pipes 19. In the embodiments shown in FIG. 5a and 5b, an assumption is made that the distance from the top level of the solids discharge conduit 22 to the bottom section of the trough 7 is substantially equal to the distance from the bottom section of the trough 7 to the top level 21 of the solids layer accumulated in the trough 7. Pocket-shaped solids removal means are loaded by a substantially greater amount of solids than what loads the conduits 15 used in the construction according to the invention, which means that more massive structures must be used for conventional pockets than for the conduits 15. Moreover, the conduits 15 cause less obstruction to solids flow in the interior structures of the air-plenum chamber 2 than pockets of a larger structure. In the embodiments shown in FIGS. 2 and 3, the cooling pipes 3 of the solids removal trough 7 and the grate 1 run substantially parallel to the longitudinal direction of the solids removal troughs 7. In the embodiment of FIG. 4, wherein the grate 1 has only one solids removal trough 7, the cooling pipes 18 of the grate 1 and the trough 7 may also be disposed running in the lateral direction of the solids removal trough 7. Then, the parallel cooling pipes 18 run from the bottom section of the trough 7 along its slanted surfaces 8, 9 upward toward the front wall 5 and rear wall 6 of the combustion chamber. At the walls 5, 6, the cooling pipes 18 are bend upward, whereby they form a portion of the wall surfaces 5, 6. Cooling water is passed into the cooling pipes 18, e.g., from cooling water feed pipes 11 that are disposed in a close proximity to the wall surfaces 5, 6 and aligned at least substantially orthogonal to the cooling pipes 18. To the cooling water feed pipes 11 are connect- ed a plurality of parallel cooling pipes 13 that in combination with the upward-bent cooling pipes 18 of the solids removal trough 7 form the front wall 5 and the rear wall 6 of the combustion chamber. As to its other details, the embodiment shown in FIG. 4 is similar to those shown in FIGS . 2 and 3. In addition to those described above, the invention may have different embodiments. The surfaces 8, 9 forming the solids removal trough 7 may have, e.g., a slightly concave or convex shape. Furthermore, only one cooling water feed pipe can be used in the manner shown in FIG. 1 instead of having two feed pipes 11 running across the grate 1, whereby the cooling water flow is divided from the common feed pipe 11 to both sides of the grate 1. Cooling water may additionally be fed into the cooling pipes 3 of the grate 1 and the solids removal troughs 7 and to the cooling pipes 13 of the combustion chamber side walls 4 from the same cooling water feed pipe 11 as is shown in the embodiment of FIG. 4. When necessary, the cooling pipes forming the solids removal troughs 7 may also be bent in the lateral direction of the solids removal troughs 7 in order to divide the solids removal troughs 7 into three-dimensionally taper- ing funnels. Between the adjacent solids removal conduits 15 may be disposed guides which project upward from the level of the planar plates 20 so as to direct the solids toward the solids removal conduits 15, whereby the cross section of the guides may have a shape of an upward tapering equilateral triangle, for instance.

Claims: What is claimed is: 1. Grate construction for use in the bottom section of the combustion chamber of a fluidized-bed boiler, the construction comprising - a grate (1) equipped with a plurality of parallel cooling pipes (3; 18), - an air-plenum chamber (2) adapted below the grate (1) , - means (14) adapted to the grate (1) for injecting fluidizing gas from the air-plenum chamber (2) to a fluidizing zone in the bottom section of the combustion chamber, and - means adapted to the grate (1) for removal of solids from above the grate (1) , c h a r a c t e r i z e d in that - said means for removal of solids comprise at least one solids removal trough (7) , which is adapted to the grate (1) so as to flare wide open at the grate level with a therefrom downward tapering and at its bottom end open structure that is comprised of at least two opposed slanted surfaces (8, 9) between which the solids to be discharged is arranged to flow downward. 2. Grate structure according to claim 1, c h a r a c t e r i z e d in that, to the bottom end of said solids removal trough (7) and disposed at a distance from each other, there are adapted solids removal conduits (15) for the purpose of removing solids from the solids removal trough (7) to an ash discharge pipe (19) . 3. Grate structure according to claim 2, c h a r a c - t e r i z e d in that a planar plate (20) is adapted at the level of the entry ends of the adjacent solids removal conduits (15) . 4. Grate structure according to claim 2, c h a r a c - t e r i z e d in that the adjacent solids removal conduits (15) are inclined to converge with each other so as to join with each other in the interior of said air- plenum chamber (2) . 5. Grate structure according to claim 4, c h a r a c t e r i z e d in that the angle (β) between the exterior surface (16) of the solids removal conduit (15) and the horizontal plane is not smaller than 30°. 6. Grate structure according to claim 1, c h a r a c t e r i z e d in that the surfaces (8, 9) forming the solids removal trough (7) are comprised of cooling pipes (3) disposed parallel to each other and running longitudinally along the surface of the solids removal trough (7) . 7. Grate structure according to claim 1, c h a r a c - t e r i z e d in that the surfaces (8, 9) forming the solids removal trough (7) are comprised of cooling pipes (18) disposed parallel to each other and running transversely along the surface of the solids removal trough (7) . 8. Grate structure according to claim 1, 6 or 7 , c h a r a c t e r i z e d in that said means (14) for injecting fluidizing gas from the air-plenum chamber (2) to a fluidizing zone in the bottom section of the combustion chamber are adapted on fins (10) spanned between the grate (1) and the cooling pipes (3; 18) of the solids removal troughs (7) . 9. Grate structure according to claim 1, c h a r a c t e r i z e d in that the inclination angle (α) of the surfaces (8, 9) forming the solids removal trough (7) from the vertical plane is in the range of 20-55°, most advantageously 35-45°. 10. Grate structure according to claim 1, 6 or 7 , c h a r a c t e r i z e d by at least one cooling water feed pipe (11) which is adapted to pass at least substantially orthogonal to the cooling pipes (3; 18) of the grate (1) and the solids removal trough (7) and has the cooling pipes (3; 18) of the grate (1) and the solids removal trough (7) connected thereto. 11. Grate structure according to claim 1, 6 or 7 , c h a r a c t e r i z e d in that said cooling pipes (3; 18) are bent upward at the edge of the grate (1) so as to make them form the walls (4; 5, 6) of the combustion chamber . 12. Grate structure according to claim 1, 6 or 11, c h a r a c t e r i z e d in that the walls (4; 5, 6) of the combustion chamber are comprised of the cooling pipes (3; 18) of the grate (1) and the cooling pipes (13) connected to either a tubular chamber (12) or a feed pipe (11) of cooling water, the cooling pipes being adapted to run in parallel with each other in an alternating manner. 13. Grate structure according to claim 1, c h a r a c t e r i z e d in that the solids removal trough (7) is located below a fuel infeed opening of the combustion chamber wall as seen in the longitudinal direction along the axes of the fuel infeed openings . 14. Grate structure according to any one of foregoing claims 1, 6, 7 or 8, c h a r a c t e r i z e d in that said means (14) for injecting fluidizing gas into the fluidization zone are mounted on the slanted surfaces (8, 9) of the solids removal trough (7) .

Inventors: BJOERKLOEF SVEN, HIRVENOJA JUKKA, LATVA-SOMPPI JOUKO, LEHTONEN OLOF, BJOERKLOEF, SVEN, HIRVENOJA, JUKKA, LATVA-SOMPPI, JOUKO, LEHTONEN, OLOF
Applicants: BJOERKLOEF SVEN, FORTUM ENGINEERING OY, HIRVENOJA JUKKA, LATVA SOMPPI JOUKO, LEHTONEN OLOF, BJOERKLOEF, SVEN, HIRVENOJA, JUKKA, LATVA-SOMPPI, JOUKO, LEHTONEN, OLOF
Application year: 2000
Main classification: F23C10
IPC codes: F23H3, F23H15