EP0236647B1 - Fluidized-bed heat generator with a device for ash removal and heat recovery - Google Patents

Abstract

A fluidized bed heat generator with improved collector for ash removal and heat recovery comprises a central hearth having a fluidization grid at its lower end. The side wall of the hearth is enclosed by a hollow peripheral enclosure to define an intermediate volume therebetween which is connected both to the upper part of the hearth and to fume tubes. These fume tubes extend through the peripheral enclosure and open into a fume exit conduit such that fumes and entrained ash from the hearth can be directed via the intermediate volume and the fume tubes to the fume exit conduit. Water is arranged to circulate within the peripheral enclosure around the hearth and fume tubes.

Description

The subject of the invention is a moderate power thermal generator with a fluidized bed comprising improved means for evacuating the centers and recovering heat, usable in particular for solid fuels in particles (coals, granules of household waste, etc.). .) or pasty (heavy petroleum fractions, residual sludge, etc.).

A generator of this type operating according to the fluidized bed combustion technique has the advantage of using a wide range of fuels. However, the production of such a generator poses construction and industrialization problems linked to the evacuation of the centers and the treatment of the fumes.

So far these problems have been solved partially and imperfectly; existing known generators are ill-suited to the combustion of ashy products and they require the installation of bulky and expensive smoke boxes.

For this reason, known generators of monobloc construction have a low power; more powerful generators generally use in combination one or more cells with water tubes (fireplaces), connected to a separate cell with smoke tubes, the overall size of which is consequently large. The prefabrication in the factory and the assembly of such systems are also relatively long.

For example, GB-A 2 109 096 discloses a steam locomotive boiler in which there is a vertical hearth with a fluidization grid in which water tubes pass; from the top of this hearth leave smoke tubes which, after a 90 ° bend, extend horizontally to feed from above a vertical ash separation chamber which is installed next to the hearth: from the upper part of this chamber vertical horizontal tubes which lead the smoke to a vertical smoke pipe; the whole of the hearth, the smoke tubes, the vertical chamber, the horizontal tubes leading to the smoke duct, is included in a general exterior envelope containing the water and the vapor. It is undeniable that this steam generator is of the type extended horizontally in one direction, from the loading opening of the hearth to the final flue, as is appropriate on a locomotive.

Another document known from GB-A 2 077 616 is another steam generator having a fluidized bed hearth; this generator is called in this document "horizontal boiler" to mean that it comprises several horizontal passages of the combustion gases, from the hearth to a smoke box. The hearth is elongated in a horizontal direction and the fumes are directed first towards the rear below this hearth then forwards above the same hearth.

In fact, the boilers described in these two documents do not escape the drawbacks mentioned above; they are designed to be of the horizontally elongated type and are composed of bulky members which are connected to each other in a rather complex and costly manner.

The main object of the invention is to achieve a generator of the type indicated above with which the difficulties set out above are overcome substantially if not totally, in a simple and effective manner.

• - un foyer central vertical à paroi latérale et à grille de fluidisation pour supporter un lit fluidisé, avec des ouvertures supérieures de sortie des fumées,
• - une enveloppe périphérique creuse à paroi supérieure et à moyens d'arrivée et de sortie d'eau de circulation, enveloppant le foyer central,
• - un conduit de sortie de fumées relié au foyer central, est caractérisé, selon l'invention, en ce que l'enveloppe périphérique est substantiellement concentrique au foyer central et elle limite avec ce dernier au moins un volume intermédiaire qui est mis en communication avec le foyer central dans sa partie supérieure par lesdites ouvertures pour fumées et qui est raccordé à sa partie inférieure au niveau de la grille de fluidisation à des tubes de fumées verticaux par une première extrémité inférieure de ceux-ci, ces tubes de fumées s'étendant vers le haut parallèlement audit foyer central à grille de fluidisation, à l'intérieur de ladite enveloppe périphérique, et étant raccordés à leur seconde extrémité supérieure avec le conduit de sortie de fumée, ce dernier étant supporté au-dessus du foyer central par la paroi supérieure de l'enveloppe périphérique.

A thermal generator comprising:

• - a vertical central fireplace with side wall and fluidization grid to support a fluidized bed, with upper smoke outlet openings,
• - a hollow peripheral envelope with an upper wall and with means for the inlet and outlet of circulating water, enveloping the central hearth,
• - A smoke outlet duct connected to the central hearth, is characterized, according to the invention, in that the peripheral envelope is substantially concentric with the central hearth and it limits with the latter at least one intermediate volume which is placed in communication with the central hearth in its upper part by said openings for smoke and which is connected at its lower part at the level of the fluidization grid to vertical smoke tubes by a first lower end thereof, these smoke tubes extending upwardly parallel to said central focus with fluidizing grid, inside said peripheral envelope, and being connected at their second upper end with the smoke outlet duct, the latter being supported above the central hearth by the wall upper of the peripheral envelope.

The intermediate volume thus created is advantageously provided at its lower part with a collector for collecting and extracting the ash entrained by the fumes.

The intermediate volume constitutes a primary dust collector which is integrated into the generator upstream of the smoke tubes and which protects these tubes against the risks of fouling and premature erosion. This arrangement avoids the use of a smoke box which is often produced by mechanical welding and which is attached using flanges on thermal generator.

The compactness of the generator according to the invention is improved and the prefabrication is facilitated.

According to a particular characteristic, the fluidized bed hearth has at least one side wall with water tubes surrounded by the peripheral envelope with a symmetrical arrangement which avoids the problems of differential expansion and resistance over time of the generator.

According to a more particular characteristic, the vertical wall is made up of water tubes joined in a sealed manner by membranes; this construction method facilitates industrial production of the generator and improves heat transfer.

In an exemplary embodiment of the invention, the fluidization grid is part of the enclosure filled with water which is placed in communication with the peripheral envelope. This design allows a lowering of the temperature of the grid, which allows its manufacture in economical material. In addition, all the walls delimiting the fluidized bed, including the grid itself, thus participate in the heat exchange. This makes it possible to reduce the exchange surfaces to be immersed in the bed; it follows that the tubes constituting these exchange surfaces can be arranged more freely, avoiding the areas where a strong erosion would occur.

The fluidization grid used in the thermal generator according to the invention is in itself of a known type; this may in particular be a grid with openings flared upwards, such as those which are the subject of documents FR-A- Nos 2 171 945, 2 519 877, 85-08320, 85-15580.

Preferably, at least one of the walls of the hearth has one (or more sieuurss) opening (s) provided (s) with a selective closing means; this opening makes it possible to regulate the level of the fluidized bed.

According to a variant, the ash collector contains a water circulation loop connected to the hollow peripheral envelope. This loop completes the thermal exhaustion of the ashes and further improves the efficiency of the unit.

According to another particular characteristic, the generator has a device for re-injecting the ash collected by a final dust collector (for example a multi-cyclone) connected to the smoke circuit at the outlet of the boiler.

This device thus makes it possible to improve the desulfurization in situ, to reduce unburnt residues and to limit the extraction points of the ashes.

This device can be produced, for example, pneumatically, bypassing the arrival of fluidizing air or secondary air or mechanically by conveying using a conveyor screw.

In addition to this reinjection, it is possible to recycle the ash collected from the internal dust collector, in whole or in part, for example as a means of self-feeding the bed with inert materials.

• - la figure 1 est une représentation schématique en coupe par un plan vertical selon I-I de la figure 2 d'un générateur thermique conforme à l'invention ;
• - la figure 2 est une vue en coupe par un plan horizontal selon II-II de la figure 1,
• - la figure 3 est une vue partielle en coupe montrant une variante de réalisation de la partie inférieure du générateur de la figure 1.

We will now give, without limiting intention and without excluding any variant, a description of an exemplary embodiment of the invention. Reference is made to the accompanying drawings in which:

• - Figure 1 is a schematic representation in section through a vertical plane along II of Figure 2 of a thermal generator according to the invention;
• FIG. 2 is a sectional view through a horizontal plane along II-II of FIG. 1,
• - Figure 3 is a partial sectional view showing an alternative embodiment of the lower part of the generator of Figure 1.

A generator according to the invention comprises a hearth 1 with a fluidized bed, with side wall, of a type known per se and a peripheral casing 2 which envelops the hearth 1 by limiting with the side wall 3 of the latter an intermediate volume 4. The hearth 1 could be cylindrical as well as the peripheral envelope 2, so that the intermediate volume 4 would be an annular volume. At the upper part of the hearth 1, openings 5 put it in communication with the intermediate volume 4.

In the present example, as shown in FIG. 2, the hearth 1 has a generally rectangular configuration; the peripheral envelope 2 is cylindrical and it limits, with the hearth 1, two separate and opposite intermediate volumes 4A, 4B which are each located on a long side of the hearth 1. The two long sides 3A, 3B of the hearth 1 are formed by tubes of parallel, spaced 3C water, connected to each other in a sealed manner by 3D intermediate membranes, in a manner known per se. At the upper extreme part of the hearth 1 certain tubes, preferably one out of two tubes, are offset towards the outside of the hearth 1 and the intermediate 3D membranes are removed in the extreme offset part of each tube 3C. The upper openings 5 are thus obtained through which the fumes from the hearth 1 can circulate.

In the example described here, the hearth 1 is closed at its upper part, at a level higher than that of the openings 5, by an upper wall 1A which projects beyond the hearth 1 and which also closes the intermediate volumes 4A, 4B. The fumes can enter the latter only through the upper openings 5. In addition, due to the existence of this upper wall 1A, the peripheral envelope 2 also covers the hearth 1 above this upper wall 1A. In this way, the peripheral envelope extends around the side wall and the upper wall of the hearth 1; it wraps the latter completely on all sides. At its top, the peripheral envelope 2 is closed by an upper wall 6. The latter supports a smoke outlet duct 7.

In the present example, the peripheral envelope 2 is divided by interior, radial partitions 6A, 6B, 6C, 6D forming four compartments opposite two by two. Two opposite compartments, of larger dimension in circumferential direction 8A and 8C contain vertical tubes 9, spaced apart, which extend between the lower wall 10 of the peripheral envelope 2 and its upper wall 6. These tubes 9 pass through this upper wall 6 and open into the smoke outlet duct 7. At its opposite lower end each tube 9 passes through the lower wall 10 and opens into an ash collector 11. An ash collector 11 is provided under each of the two compartments 8A, 8C of the peripheral envelope 2. Each manifold 11 extends below the lower wall 10 and it communicates by a lower passage 11 A with a corresponding intermediate volume 4A, 4B. The combustion fumes coming from the upper passages 5 descend into the intermediate volumes 4A, 4B, pass through the collectors 11 and rise in the tubes 9 to reach the smoke outlet duct 7.

Each collector 11 has a depth sufficient for ashes to collect therein without the circulation of smoke being interrupted. In addition, as shown only on the left-hand side of FIG. 1, each collector 11 is equipped at 12 with pneumatic means known per se, for extracting the ashes and for reintroducing these ashes inside the fluidized bed, to ensure their recycling.

The two other opposite compartments 8B, 8D are provided, respectively, with a water inlet 13 and a water outlet 14. In addition, these two compartments are connected directly, through the hearth 1, by tubes of water 15 (visible only in FIG. 1) which are drowned during operation in the bed 16 of fluidized combustible material. At the desired upper level that the bed 16 can reach, there is provided in the side wall 3 of the hearth 1 at least one overflow opening 17, known per se, which opens into a compartment 4B for lowering the fumes and ash entrained with these. A pipe 18 crosses from top to bottom the smoke outlet duct 7, the upper wall 6 of the peripheral envelope 2, the upper wall 1A of the hearth 1 to extend inside the latter and stop at the above the upper level of the fluidized bed 16. This pipe 18 serves to supply the hearth 1 with fuel. It will be noted that the invention is compatible with other modes of supplying the hearth 1 with fuel. Preferably, the pipe 18 is cooled by a forced water circulation loop connected in bypass to the general supply input / output circuit.

In FIG. 1, the grid 19 is represented as a hollow grid having an internal volume 19A into which the water tubes 3 open, by their lower end. These tubes 3 are therefore joined to the peripheral envelope 2 at the level of the grid 19. As a variant, as shown in FIG. 3, the fluidization grid 19 ′ is of conventional construction and it rests on a hollow enclosure 20 which supports it and whose internal volume is placed in communication with the internal volume of the peripheral envelope 2. Of course, this hollow enclosure 20 is traversed in a sealed manner by tubes 21 which supply fluidization gas to the usual orifices of the grid 19 ' . According to another variant, the grid is of conventional construction; it is neither cooled nor supported by a cooled hollow enclosure.

In the two cases of FIG. 1 where the grid 19 is hollow and of FIG. 3 where the grid 19 'rests on a hollow enclosure 20, a reduction in the average temperature of the grid is obtained during operation, which makes it possible to 'use a more economical material for its realization.

In all circumstances, even if it is not intended to use a hollow grid 19 or a grid 19 'supported by a hollow enclosure 20, there is a water inlet 22 at a lower point of the peripheral envelope 2 and an upper point a water outlet 23. During operation, the water circulates in the tubes 3 which constitute the side wall of the hearth 1; it also flows along the upper wall 1A of the same focus; it still flows along the outer wall which limits the intermediate volume 4 and around the tubes 9 where the fumes circulate; it further circulates, preferably through the hollow grid 19 or through the hollow enclosure 20 which supports the grid 19 '. In this way, the heat contained in the fumes and ashes is recovered. It is also possible to place in the lower internal part of each ash collector 11 a mouth 24 for circulation of water connected to the interior volume of the peripheral envelope 2 as can be seen in FIGS. 1 and 3.

Almost all of the calories in the ashes can be recovered. At the same time, the ashes are collected without bulky material means and can then be disposed of easily, either to dispose of them definitively or to recycle them in the fluidized bed.

Under certain circumstances, it may be necessary to eliminate the tubes 15 which pass through the fluidized bed 16. In this case, the compartments 8B, 8D can also be eliminated; the peripheral envelope 2 then becomes a single envelope annularly surrounding the side wall of the hearth 1. The latter can be cylindrical and the intermediate volume 4 can also annularly surround the side wall of the hearth 1.

Claims (14)

1. A heat generator comprising:

- a central vertical firebox (1) having a side wall and a fluidisation grate (19, 19') for supporting a fluidised bed, with upper flue gas outlet openings (5),

- a hollow peripheral casing (2) having an upper wall (6) and inlet and outlet means for circulating water, which encloses the central firebox (1), and

- a flue gas outlet duct (7) connected to the central firebox (1),

characterised in that the peripheral casing (2) is substantially concentric with the central firebox (1) and it defines therewith at least one intermediate volume (4) which is communicated with the central firebox (1) in its upper portion by way of said flue gas openings (5) and which is connected at its lower portion at the level of the fluidisation grate (19, 19') to vertical flue gas tubes (9) by way of a first lower end of the tubes (9), the tubes (9) extending upwardly parallel to said central firebox (1) with fluidisation grate (19, 19') within said peripheral casing (2) and being connected at their second upper end to the flue gas outlet duct (7), the latter being supported above the central firebox (1) by the upper wall (6) of the peripheral casing (2).

2. A generator according to claim 1 characterised in that or each intermediate volume (4) is provided in its lower portion with at least one collector (11) for gathering and extracting ash entrained by the flue gas.

3. A generator according to claim 1 wherein the central firebox (1) has a fluidisation grate (19, 19'), a side wall and an upper wall characterised in that the peripheral casing (2) encloses the central firebox (1) round the side wall (3) and the upper wall (1A) of the firebox.

4. A generator according to claim 1 characterised in that the central firebox (1) has a side wall formed by water tubes (3C) which are close to each other and which are sealingly joined by intermediate diaphragms (3B) and which are communicated by way of their opposite ends to the peripheral casing (2).

5. A generator according to claim 1 characterised in that at least one communicating passage (17) is provided through the side wall (3) of the central firebox (1) for selectively communicating the internal volume of said central firebox (1) with the intermediate volume (4) substantially at the maximum level of the fluidised bed during operation.

6. A generator according to claim 1 characterised in that the side wall (3) of the central firebox (1) is formed by water tubes (3C) which are close to each other and which are joined sealingly by intermediate diaphragms (3D) and communication of the intermediate volume (4) with the central firebox (1) is effected by a diversion of a part of the length of some of said water tubes (3C) with respect to the array of the other water tubes (3C), said diversion locally eliminating the sealing effect between said water tubes (3C).

7. A generator according to claim 2 characterised in that the or each collector (11) contains a water circulation circuit (24) connected to the peripheral casing (2).

8. A generator according to claims 3 and 4 in combination characterised in that the water tubes (3C) are connected to the peripheral casing (2) by way of their upper end which is remote from the grate (19) and which opens into the zone of said peripheral casing (2) which covers the upper wall of the central firebox (1).

9. A generator according to claim 8 characterised in that some of the water tubes (3C) rave an upper end part which is diverted outwardly with respect to the central firebox (1), the diversion eliminating the sealing effect between said tubes with their diverted end part and the other water tubes (3C) to communicate the central firebox (1) with the intermediate volume (4).

10. A generator according to claim 8 characterised in that the flue gas outlet duct (7) is disposed immediately above the zone of the intermediate volume which covers the upper wall (1A) of the central firebox (1) and the flue gas tubes (9) extend over their entire length in the peripheral casing (2), being connected by their opposite ends on the one hand to the intermediate volume (4) and on the other hand to the flue gas outlet duct (7).

11. A generator according to claim 4 characterised in that the fluidisation grate (19) is hollow with an internal volume (19A) connected to the peripheral casing (2) while the water tubes (3C) extend from said grate (19) and are communicated with the internal volume thereof, their connection to the peripheral casing (2) being provided by way of the internal volume of said grate (19).

12. A generator according to claim 4 characterised in that the fluidisation grate (19') is supported by a hollow enclosure (20) which is communicated with the peripheral casing (2).

13. A generator according to claim 1 characterised in that it comprises a final dust removing device mounted on the flue gas outlet circuit (7) for collecting at least a part of the ash entrained by the flue gas and a device for re-injection of the collected ash into the firebox (1).

14. A generator according to claim 2 characterised in that it comprises a means for introducing the ash collected by said collector (11) into the interior of the fluidised bed.

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