Fluidized-bed boiler

Abstract

 O A fluidized-bed boiler having at its bottom part a plurality of fluidizing cells (3a, 3b, 3c and 3d) for fluidizing a fluidizable layer (7) thereabove is fired for starting by an oil burner (15) so mounted as to inject its combustion flame directly into one (3d) of the fluidizing cells.

Description

BACKGROUND OF THE INVENTION

[0001] This invention relates generally to fluidized-bed apparatuses and more particularly to a fluidized-bed boiler in which its starting or firing device is simplified.

[0002] As is known, a fluidized-bed boiler, in general, has a boiler housing structure, a group of fluidizing cells covering substantially the entire bottom of the boiler structure, a fluidizable medium of particulate material laid over the fluidizing cells, and evaporation tubes imbeddedly disposed within the fluidizable medium. The fluidizing cells are each supplied with pressurized air injected upward thereinto through their bottoms except one cell which is not thus supplied with air but functions as a starting or firing cell. A fluid fuel is injected upwardly .into the fluidizable medium at a point immediately above each cell through a respective fuel feed pipe.

[0003] A fluidized-bed boiler, in general, has a number of advantageous features, among which is high fuel efficiency owing to the high rate of heat transfer from the combustion gases and the fluidized medium to the water being evaporated. However, a problematic feature of the conventional fluidized-bed boiler is the general use of a firing device of elaborate design for supplying hot air at a temperature of the order of 800°C into the above mentioned starting cell for the purpose of igniting or firing the fuel being injected thereabove. This feature and its drawbacks as well as the general construction of the known fluidized-bed boiler will be described in greater detail hereinafter.

SUMMARY OF THE INVENTION

[0004] It is an object of this invention to provide a fluidized-bed boiler in which the firing means is simplified.

[0005] According to this invention, briefly summarized, there is provided a fluidized-bed boiler comprising:

a boiler housing structure; a plurality of fluidizing cells installed in the lower part of the housing structure and respectively supplied with pressurized air through an air supply duct; a fluidizable layer disposed above and covering the fluidizing cells; fuel feed pipes for injecting fuel upwardly into the fluidizable layer at positions above respective fluidizing cells; evaporation tubes installed at positions to be imbedded in the fluidizable layer; and firing means for igniting the fuel thus injected, characterized in that the firing means comprises burner means mounted to inject the combustion flame thereof directly into one of the fluidizing cells.

[0006] The nature, utility, and further features of this invention will be more clearly apparent from the following detailed description when read in conjunction with the accompanying drawing briefly described below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] In the drawing:

FIGURE 1 is a schematic elevation showing an example of a fluidized-bed boiler according to this invention;

FIGURE 2 is a fragmentary elevation, on an enlarged scale, showing a part of FIGURE 1; and

FIGURE 3 is a schematic elevation similar to FIGURE 1 but showing a conventional fluidized-bed boiler.

DETAILED DESCRIPTION OF THE INVENTION

[0008] As conducive to a full understanding of this invention, the general nature and limitations of the conventional fluidized-bed boiler will first be briefly described with reference to FIGURE 3.

[0009] The boiler structure 1 of this known fluidized-bed boiler has at its bottom part a plurality of fluidizing cells 3a, 3b, and 3c supplied with pressurized air from below from an air supply duct 2 and a starting cell 3d which is not connected to the air supply duct 2. Immediately above these cells 3a through 3d, a fluidizable layer 7 filled with a desulfurizing material 6 is horizontally disposed in a wall-to-wall manner to completely cover the cells. A combustion chamber 4 is formed above the fluidizable layer 7. Evaporation tubes 5 are imbedded in the fluidizable layer 7, into which fuel is fed through fuel feed pipes 8, 8, .... passing through the cells 3a through 3d.

[0010] The above described fluidized-bed boiler is started by a firing device 9 comprising a hot-air furnace 12 connected at its outlet end by an expansion joint 13 to a hot-air duct 14 for conducting hot air into the above mentioned starting cell 3d, an oil burner 10 for heating the interior of the hot-air furnace 12, and an air pipe 11 for supplying air into the furnace 12.

[0011] This fluidized-bed boiler is started by supplying and spreading the desulfurizing material 6 to a prescribed level thereby to form the fluidizable layer 7 and supplying fluidizing air to the fluidizing cells 3a, 3b, and 3c thereby to fluidize the desulfurizing material 6. The oil burner 10 of "the firing device 9 is then ignited, and air is supplied through the air pipe 11 thereby to generate hot air in the hot-air furnace 12 and to supply this hot air into the starting cell 3d. Fuel is then fed through the fuel feed pipes 8 into the fluidized layer 7. The fuel in the fluidized layer portion above the starting cell 3d is then first ignited by the above mentioned hot air injected by way of the starting cell 3d into the fluidized layer 7. Thereafter, the state of firing and combustion is propagated successively to the fluidized layer portions above the other fluidizing cells 3c, 3b, and 3a until the fluidized layer 7 above all fluidizing cells are undergoing fluidized-bed combustion.

[0012] In a conventional fluidized-bed boiler of the above described character, however, the firing device 9 requires, in addition to an oil burner 10, at least a hot-air generating chamber or furnace 12, an air pipe 11, an expansion joint 13, and a hot-air duct 14. As a consequence, the device becomes structurally large as a firing device, requiring a large installation space, and is also uneconomical.

[0013] This invention overcomes the above described problem by providing a fluidized-bed boiler in which the firing device is greatly simplified as described hereinbelow with respect to one embodiment of the invention and with reference to FIGURE 1. In FIGURE 1 those parts which are the same as or equivalent to corresponding parts in FIGURE 3. are designated by the same reference numerals.

[0014] The fluidized-bed boiler shown in FIGURE 1 has a boiler structure 1 provided at its bottom part with a plurality of fluidizing cells 3a through 3d, which are supplied with pressurized air from below from an air supply duct 16 in the form of a manifold and through respective air ducts provided with flow-control dampers 18, 18, .... The fluidizing cells 3a through 3d are in the form of compartments defined by so- called membrane walls 21 each comprising a row of parallel water tubes lying in substantially the same plane and spacer fins joining adjacent water tubes. A fluidizable layer 17 of desulfurizing material 6 is provided in the boiler structure 1 above the fluidizing cells 3a through 3d, and evaporation tubes 5 are imbeddedly disposed within the fluidizable layer 17 similarly as in the aforedescribed known fluidized-bed boiler. Each of the fluidizing cells 3a through 3d is provided with a respective fuel feed pipe 8, which at its injection end is directed upward and terminates in a Tee-shape fitting 19, as shown in FIG. 2, functioning as a fuel injector.

[0015] As in the aforedescribed known boiler, a desulfurizing material is used in the fluidizable layer 17 because, by its use in this manner, desulfurization can be accomplished simultaneously with combustion. In the case of desulfurization within the boiler furnace with the use of limestone as the desulfurization material, the limestone first undergoes calcining within the fluidized bed to become quick lime, which thereafter reacts with So₂ formed by the combustion of the fuel and thereby accomplishes desulfurization within the fluidized layer as indicated by the following formulas.

[0016] Also as in the known fluidized-bed boiler, the principle of the fluidized-bed combustion is utilized by installing evaporation tubes 5 in imbedded state in the fluidizable layer 17 of the desulfurizing material 6, whereby the thermal conductivity of the heat transfer from the fluidized medium to the water in the evaporation tubes 5 is greatly increased by the counter flow effect. Thus, the thermal conductivity is from several times to even as high as ten times that in an ordinary boiler. Furthermore, the combustion temperature can be held at a low value by the heat transmitting surface within the fluidized layer. For this reason the generation of thermal NOx can be suppressed, whereby the evaporation tubes 5 can be installed within the fluidized layer.

[0017] An important feature of this invention is that, as in the embodiment shown in FIGURE 1, the pressurized air supply duct 16 differs from the air duct 2 shown in FIGURE 3 in that it supplies pressurized air also to the fluidizing cell 3d. Furthermore, an oil burner 15 is mounted directly on a side wall 20 of the fluidizing cell 3d, which thereby becomes a starting cell. The fluidizing cell 3d is located at the end of the row of the fluidizing cells 3a, 3_b, 3c and 3d, and the side wall 20 of the cell 3d forms a part of the outer side wall of the boiler housing structure 1. The oil burner 15 is so oriented that the combustion flame thereof will be directed toward that wall 21 of the fluidizing cell 3d, which forms a partition between the cell 3d and the adjoining cell 3c.

[0018] The fluidized-bed boiler of the above described construction according to this invention is started in the following manner. First, the oil burner 15 is ignited, and pressurized air is supplied through the air supply duct 16 into the fluidizing cells 3a, 3b, and 3c and the starting cell 3d. The air within the starting cell 3d is heated by the oil burner 15 to a temperature of the order of 800°C and is injected into the fluidizable layer l7 through nozzles 22 thereby to fluidize the desulfurizing material 6.

[0019] Fuel is then fed through the fuel feed pipes 8, 8, .... into the fluidizable layer 17. As a consequence, first, the fluidizable layer portion above the starting cell 3d is fired by the hot air injected thereinto from the starting cell 3d. Thereafter, the state of firing and combustion is propagated successively to the fluidized layer portions above the other fluidizing cells 3c, 3b, and 3a until the fluidized layer 17 above all fluidizing cells are undergoing fluidized-bed combustion.

[0020] As described above, in the fluidized-bed boiler of this invention, an oil burner is mounted to inject its flame directly into one of the fluidizing cells, which thereby is used as a starting cell. For this reason, an elaborate, large-scale hot-air furnace, as those used heretofore, becomes unnecessary, and the air for combustion of the oil burner can be supplied by using a part of the pressurized air supply duct of the cells.

[0021] As one result, an expansion joint for overcoming problems arising from combustion vibration of the hot-air duct and the boiler and elongations due to heat becomes unnecessary. Another result is that the firing device is simplified and its air pipe system is dispensed with so that the firing device does not require a large installation space. Therefore the overall installation cost of the fluidized-bed boiler is appreciably reduced.

Claims

1. A fluidized-bed boiler having a boiler housing structure (I), a plurality of fluidizing cells (3a, 3b, 3c, 3d) installed in the lower part of the housing structure (1) and respectively supplied with pressurized air through an air supply duct (16), a fluidizable layer (17) disposed above and covering the fluidizing cells, fuel feed pipes (8) for injecting fuel upwardly into the fluidizable layer (17) at positions above respective fluidizing cells, evaporation tubes (5) installed at positions to be imbedded in the fluidizable layer (17), and firing means for igniting the fuel thus injected, characterized in that the firing means comprises burner means (15) mounted to inject the combustion flame thereof directly into one (3d) of the fluidizing cells (3a, 3b, 3c, 3d).

2. A fluidized-bed boiler as claimed in claim 1 wherein said fluidizing cells (3a, 3b, 3c, 3d) are disposed in a row and said one fluidizing cell (3d) is so located at the end of the row that a side wall thereof remote from the other fluidizing cells-(3a, 3b, 3c) forms a part of the outer side wall of the boiler housing structure (1) and wherein said burner means (15) is mounted through said side wall of the one fluidizing cell (3d).

3. A fluidizing-bed boiler as claimed in claim 2 wherein said burner means is an oil burner (15).

4. A fluidizing-bed boiler as claimed in claim 2 wherein said burner means (15) is so oriented that the combustion flame thereof will be directed toward the wall of said one fluidizing cell (3d), forming a partition between itself (3d) and the adjoining fluidizing cell (3c).

Drawing