BRIEF DESCRIPTION OF THE DRAWINGS
In the following, the invention will be described in more detail with reference to the appended drawings, in which
FIG. 1 shows the lower part of the furnace in a cross-sectional view,
FIG. 2 shows a cross-section of the grate at one element in plane A-A of FIG. 1,
FIGS. 3 to 5 show different types of elements in cross-sectional views, and
FIG. 6 shows the grate in cross-section along plane A-A of FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is a cross-sectional view showing the lower part of the furnace 1 of a fluidized bed boiler, limited from underneath by a horizontal grate 2. The grate consists of parallel longitudinal hollow elements 3 with means 4 for supplying fluidizing air upwards into the furnace. FIG. 1 shows, in a side view, a single grate element 3 provided at certain intervals in the longitudinal direction with air nozzles used as means 4 for supplying fluidizing air. The elements with the air nozzles are arranged at certain intervals in the transverse direction so that they form a grate with openings left between the elements 3 as shown in FIG. 6. Coarse material can be discharged from the bed through the openings into a discharge unit underneath the grate.
From the sides, the furnace is limited by vertical walls 5 with heat transfer tubes for transferring energy, released during the combustion, into a heat transfer medium flowing in the tubes. The heat transfer medium is water which evaporates in the tubes. The water circulations of the evaporator circuit of the fluidized bed boiler and the other heat transfer surfaces for recovering energy may be known as such, and they will not be discussed in more detail, as they are not involved in the invention. The supply of fuel and secondary air into the furnace may be implemented by conventional arrangements and they will not be described in more detail.
FIG. 1 also shows an additional heat transfer surface 6 in the lower part of the furnace, extending between opposite walls 5 through the lower part of the furnace 1 in the horizontal direction. The function of the heat transfer surface 6 is to cool the bed in case the fuel is of such a quality that the recommended maximum combustion temperature is exceeded. This additional heat transfer surface consists of an array of heat transfer tubes 6a placed on top of each other and mounted directly on top of the element 3, in parallel with the same. Thus, the element 3 supports the tubes 6a along their whole length from underneath. The lower edge of the bundle constituted of tubes is thus integrated as a part of the element 3, and it is not exposed inside the furnace, subject to the erosive effect of the fluidizing air and the fluidized bed material nor to various vibrations. The tubes 6a are made of steel, and they are covered with a mass or a coating to protect them. The structures protecting the tubes from the conditions of the fluidized bed will be described in more detail hereinbelow.
FIG. 1 shows, in a side view, only one heat transfer surface 6 placed on top of a corresponding element 3. However, there may be several similar heat transfer surfaces 6 placed on adjacent elements 3 of the grate. It is possible to provide each element 3 of the grate with a heat transfer surface composed of tubes 6a, or to place heat transfer surfaces 6 more sparsely so that they are fewer in number than the elongated elements 3. In particular, it is advantageous to leave at least the outermost elongated elements 3 without a heat transfer surface, because these elements are close to a parallel side wall whose heat transfer surface cools the bed in the marginal area sufficiently. At the same time, the development of narrow points close to the side of the furnace is avoided. There may also be heat transfer surfaces 6 in the central area of the grate 2, distributed so that only a part of the elements 3, for example every second element 3, is equipped with a heat transfer surface.
FIG. 1 also shows the connection of the heat transfer surface to the circulation of medium in the boiler. A heat transfer medium, to which the heat of the furnace 1 is transferred, flows through the tubes 6a of the heat transfer surface. The tubes 6a are connected to the rest of the tube system of the boiler, wherein the same heat transfer medium flows therein. Thus, the flow of the medium inside the tubes 6a of the heat transfer surface 6 occurs spontaneously as part of the medium circulation in the boiler, and separate circulating pumps will not be needed. FIG. 1 shows a downcomer pipe 7 from a drum in the upper part of the boiler, inlet tubes 8 being branched off the downcomer pipe 7 for supplying water into the tubes 6a of the heat transfer surfaces 6 (only one inlet tube 8 and one heat transfer surface 6 are shown in the figure). The opposite ends of the tubes 6a of the heat transfer surface 6 are connected to the tubes of the wall 5 of the furnace by means of a connecting tube 9. Thus, the cooling of the heat transfer surface 6 is implemented as a part of the evaporator circuit operating by the principle of natural circulation in the boiler, and evaporation takes place in the tubes 6a of the heat transfer surface. The ends of the heat transfer surface 6 are led through the walls 5 of the furnace 1 in a gas-tight manner, and its connections to the medium circulation (evaporator circuit) of the boiler are outside the furnace 1. Further, in the area outside the furnace, there is no need to support and shield the heat transfer surface 6 from underneath.
By a suitable tubing, the flow of the heat transfer medium can also be provided so that the flows are in opposite directions in different heat transfer surfaces 6.
The figure also shows cooling channels 3a for cooling the elongated grate element 3 arranged, for example, by the principle disclosed in U.S. Pat. No. 5,743,197. The entire disclosure of the U.S. Pat. No. 5,743,197 is incorporated herein by reference. Also these cooling channels 3a are a part of the evaporator circuit operating by the principle of natural circulation in the boiler, and their supply water can also be taken from the downcomer pipe 7. FIG. 1 shows an inlet tube 10 for the cooling tubes 3a of the element, connected to the downcomer pipe 7. At the opposite end, the cooling tubes 3a are connected to the heat transfer tubes of the wall 5.
FIG. 2 shows, in a cross-sectional view, a grate element 3 integrated to a single structural element, and a heat transfer surface 6. The elongated grate element 3 is a so-called box beam, inside which fluidizing air flows. The element 3 is used, in a way, as a supporting beam for the heat transfer surface 6. As shown in the figure, the heat transfer surface 6 has, in a cross-sectional view perpendicular to the longitudinal direction of the element 3, the general shape of an upright rectangle, whose long flanks are substantially parallel and vertical. The element 3 and the heat transfer surface 4 jointly form a profile which has substantially the same shape over its whole length, the lower part consisting of the element 3 and the upper part consisting of the narrower heat transfer surface 6. The heat transfer surface is mounted on the upper wall of the element 3, which in FIG. 2 is a structure having the shape of a saddle roof with the shape of an inverted V. The lowermost pipe 6a of the heat transfer surface is mounted to the ridge of the upper wall by means of a vertical web plate.
FIG. 2 also shows nozzles used as means 4 for supplying fluidizing air, which are connected to the hollow inside of the element 3, into which the fluidizing air is fed. In the cross direction, the nozzles 4 are placed at a sufficient distance from the heat transfer surface 6. The nozzle pipes of the nozzles are arranged to be oriented to the sides so that the nozzle openings 4a at their top end are distributed as evenly as possible in the area of the grate 2, to secure even distribution of the fluidizing air. This principle is disclosed in U.S. Pat. No. 5,966,839. The entire disclosure of the U.S. Pat. No. 5,966,839 is incorporated herein by reference Furthermore, it is advantageous to place the nozzle openings for the fluidizing air at a suitable distance from the heat transfer surface 6 in the lateral direction.
Furthermore, the figure also shows a protective layer 6b forming the outer surface of the heat transfer surface and placed around the heat transfer tubes 6a to shield them. The protective layer may be made of, for example, a known protective mass used in boilers. The protective mass used may be, for example, a silicon carbide mass with a high coefficient of thermal conductivity. The heat transfer tubes 6a are pinned (pins 6c) to improve the heat transfer and to increase the adhesion between the mass and the tubes. As shown in the figure, the protective layer 6b may also extend over the upper wall of the element 3 wider than the width of the heat transfer surface 6, which feature reinforces the structure and simultaneously protects the upper part of the box beam.
In view of the heat transfer, it is also advantageous that the lowermost tube 6a of the heat transfer surface is above the nozzle plane determined by the nozzle openings 4a of the nozzles 4, above which plane also the fluidized bed material is moving.
FIGS. 3 to 5 show other structural arrangements which differ from the profile of FIG. 3 primarily with respect to the structure of the element 3 (box beam). In FIG. 3, the element 3 is similar to that in FIG. 2 in its general cross-sectional shape, but there are no cooling channels 3c in its corners and walls. In this uncooled beam, the protective layer 6b extends around the whole beam. The profile of FIG. 4 is characterized in the downwards tapering of the rectangular lower part of the element 3, and the cooling channels 3c are included. The protective layer 6b also covers the upper wall of the element 3 in the same way as in FIG. 2. The element 3 of FIG. 5, in turn, has a circular cross-sectional shape and is an uncooled beam (without cooling channels 3a), and it is protected with a mass consisting of a different material than the protective layer of the heat transfer surface 6. Also in this case, the lowermost tube 6a is connected to the element 3 by means of a plate.
In practice, the heat transfer surface can be manufactured and installed in such a way that the pinned tubes 6a are welded together to form a “tube bundle”, in which the tubes are horizontal and on top of each other, and this bundle is attached to the element 3, for example, by welding. In FIGS. 2 to 5, the tubes 6a of the tube bundle are connected to each other with plates. After the tubes have been connected to each other and installed on top of the element 3, a protective layer can be formed around the tube bundle, for example, with the above-described mass. The heat transfer surfaces 6 can be formed in both existing fluidized bed boilers, in connection with their maintenance operations, in which case they are mounted on top of existing elements of the grate, for example on top of box beams, or it can be made ready in new boilers. Thus, for example the box beam and the heat transfer surface as well as the nozzles connected to the box beam can be made as a prefabricated element for assembling the grate of the fluidized bed boiler from a plurality of such elements.
The number of heat transfer tubes in the heat transfer surface 6 may vary. It is advantageously at least three, preferably 4 to 10.
The invention is well suited to be also used in an adjustable beam grate, in which the width of the fluidized area is adjusted by beam-specific control means, which control the supply of fluidizing air into the single box beams or parts thereof. Such a beam grate is disclosed in U.S. Pat. No. 6,782,848. The entire disclosure of the U.S. Pat. No. 6,782,848 is incorporated herein by reference
The invention is not restricted to the structures and profile shapes described above, but it can be modified within the scope of the inventive idea presented in the claims. The material for manufacturing the elements 3 and the tubes 6a is a suitable heat-resistant metal, such as steel. The heat transfer tubes 6a may also be attached on top of each other and to the underlying element 3 without protection, if only a strong support is to be achieved over the whole length of the tube bundle. Similarly, the protective layer 6b may only be provided over the length where protection for the tubes is needed because of the conditions. The cross-sectional shape of the heat transfer surface 6 may also be slightly conical, that is, it is wider in the lower part than in the upper part, and its side walls are not exactly parallel. Furthermore, in the furnace 1, the heat transfer tubes 6a do not need to be supported to the element 3 over their whole length but only over the length where this is allowed by the structure of the element 3.
The need for circulating gas used for cooling decreases mathematically by 30 to 100%, when the fluidized bed boiler is equipped with the heat transfer surfaces according to the invention, which increases the efficiency of the electricity production of the boiler.
Moreover, the invention is not limited to any specific type of a fluidized bed boiler. The invention is well suited for bubbling fluidized bed boilers, thanks to their temperature profile, but it can be used in both circulating and bubbling fluidized bed boilers.
Patent Application
20070245935
