EP0216677B1 - Boiler with a circulating fluidized bed - Google Patents

Abstract

Boiler with a circulating fluidized bed, comprising a fluidization column completely lined with refractory material, a recirculation cyclone (2) and a duct (3) for recycling the solid materials. At least one heat-exchanger component (25, 26, 27) is placed in the recirculation cyclone (2). Pipe lines (30) enable air to be injected in tangential directions into the upper part of the cyclone (2) and thereby to increase the vortex effect. The combustion takes place substantially in the upper part of the cyclone (2). The fluidization column (1) comprises a widened section (1a) in its lower part.

Description

The invention relates to a circulating fluidized bed boiler using the heat produced by the combustion of circulating materials.

Such boilers generally comprise a main body constituting an elongated fluidization chamber arranged with its vertical axis, a recirculation cyclone in communication with the upper part of the fluidization chamber, a recirculation duct or leg bringing the lower part of the cyclone into communication with the lower part of the main body, as well as one or more heat exchanger elements in which the heating and vaporization of water are ensured by thermal contact with the gases and solid materials at high temperature circulating in the boiler. Combustible materials and possibly non-combustible materials in the form of solid particles are introduced at the base of the fluidization chamber and are suspended in an oxidizing gas which is generally air flowing from bottom to top in this chamber, with a speed sufficient to drive a substantial part of the solid particles upwards from the fluidization chamber and from there into the recirculation cyclone.

Three main types of fluidized bed boilers are known, which differ mainly in the structure and the functions of the main body constituting in particular the fluidization chamber.

In a first type of boiler, the main body has membrane walls constituting the heat exchanger and combustion develops in this main body, within a fluidized medium circulating at a speed of the order of 5 to 6 meters / second. The inner walls of the main body which constitute the exchange surfaces directly collect the heat developed by the combustion which is regulated so as to operate in the optimal zone for the fuel desulphurization temperature, while operating with a reasonable excess of air. This type of boiler has relatively satisfactory operating characteristics; these performances are however limited by the fact that the density of suspension of the solids is still low (10 to 30 kg / Nm ³ of smoke), by the fact that the exchange coefficient whose value is generally between 50 and 80 kcal / m ² h ^O C is limited by the presence of a layer of solid materials pressed against the exchange walls and by the fact that the pre-gasification zone for solid materials, at the base of the main body, is very small volume, so that the nitrogen oxide emissions in the gases are relatively large due to the fact that a significant excess of air is used (of the order of 20%). A second type of circulating fluidized bed boiler comprises a main body, the internal wall of which is partially coated with refractory material and partially provided with membrane walls, in its upper part. An external heat exchanger is associated with the boiler and receives solid particles taken from the recirculation leg, at the outlet of the cyclone and reinjected into the main body. The removal of solids and their cooling at the level of the external exchanger ensure thermal equilibrium so as to operate in the optimum temperature zone for desulfurization of the fuel with a reasonable excess of air which is always around 20%. The fluidization speeds in the main body are slightly higher than for the first type of boiler (6, at 8 m / s).

However, the density of suspended solids and the heat exchange coefficient in the upper part of the main body remain roughly identical to what they were in the case of boilers of the first type. On the other hand, the exchange coefficient reaches a value of around 300 to 350 kcal / m ² h ° C in the external exchanger which operates in a fluidized bed at low speed to limit the erosion of the exchanger tubes immersed in the fluidized bed.

However, such a boiler has the drawback of requiring an external exchanger and circuits of fluidizing agents, gas evacuation and sampling and reinjection of solids associated with this external exchanger; the solids sampling circuit must include a valve for controlling the quantities sampled. As a result, the construction of such boilers requires relatively large additional investments.

A third type of boiler as described in the document “Power, Vol. 129, No. 2 February 1985, pages S1-S16, NY USA ”has a main body of small dimensions entirely coated on its internal surface with refractory material and in communication, as in other types of boilers, with a recirculating cyclone. This type of boiler also includes an external exchanger, the exchange walls of which are brought into contact with particles taken from the recirculation leg, particles which are then reinjected into the main body. In this type of boiler, the fluidization speed is very high (8 to 10 m / s) and the heat transfer takes place entirely outside the main circulation loop of the fluidized bed. The disadvantage of this type of boiler is that it requires, as before, an additional solid circulation loop on which the external exchanger is located, which results in an increased construction cost. It is also necessary to introduce particles of very large particle size at the base of the main body, these particles not being circulated and making it possible to artificially create a dense phase capable of trapping and converting the fine particles of fuel which are reintroduced at the base of main body.

The object of the invention is therefore to provide a circulating fluidized bed boiler using the heat produced by the combustion of circulating materials and comprising an elongated fluidization chamber with vertical axis entirely coated, on its internal surface, with a layer of refractory material, at the base of which a combustible material and possibly a non-combustible solid material are introduced in the form of particles, as well as an oxidizing gas for suspending said solid materials circulating from bottom to top in the chamber, at least a recirculation cyclone with a vertical axis, in communication with the upper part of the fluidization chamber by a conduit disposed substantially tangentially with respect to the recirculation cyclone, a solids recycling conduit bringing the lower part of the cyclone into communication recirculation with the lower part of the fluidization chamber and at least one heat exchanger element eur in which circulates water to be heated and vaporized, the external exchange surface of which comes into contact with hot gases and solid matter in circulation, a boiler which makes it possible to obtain very good operating performance and the costs of construction and maintenance are limited.

For this purpose, at least one heat exchanger element is placed inside the recirculation cyclone, at least in its upper part which comprises on the one hand means ensuring the combustion of the gases and combustible solids circulating in this part of the cyclone and on the other hand a zone where its internal wall is coated with a refractory material, the fluidization chamber or column comprising, in its lower part, an enlarged zone into which the combustible material to be fluidized is introduced and the fluidizing air.

In a preferred embodiment, the recirculation cyclone comprises, at least in its upper part, means for injecting air into the circulating fluidized bed arranged in directions substantially tangential to the cyclone.

In order to clearly understand the invention, a description will now be given, by way of nonlimiting example, with reference to the appended figures, an embodiment of a circulating fluidized bed boiler according to the invention.

• La Fig. 2 est une vue en coupe suivant B-B de la Fig. 1 ;
• La Fig. 3 est une vue suivant F de la Fig. 1 ;
• La Fig. 4 est une vue en coupe suivant C-C de la Fig. 1.

Fig. 1 is a view in vertical section, along AA in FIG. 2, of a circulating fluidized bed boiler according to the invention;

• Fig. 2 is a sectional view along BB of FIG. 1;
• Fig. 3 is a view along F of FIG. 1;
• Fig. 4 is a sectional view along CC of FIG. 1.

In Fig. 1, the entire boiler is seen comprising a main body 1 of elongated shape with a vertical axis, a recirculation cyclone of cylindro-frustoconical shape 2 and a recirculation leg 3.

The upper part of the main body 1 is placed in communication with the upper part of the cyclone 2, by means of a duct 4 directed tangentially with respect to the cyclone 2, as can be seen in FIG. 2. The recirculation leg 3, in the form of tc j ", as can be seen in FIG. 3, puts the lower part of the recirculation cyclone 2 into communication with the lower part of the main body 1. The components 1, 2, 3 and 4 of the boiler constitute the circulation loop of the fluidized bed.

The lower part of the main body 1 receives the solid materials which are introduced into the terminal chute 3a of the recirculation leg 3 through a pipe 7; these solid materials are constituted, for example, by particles of coal which constitute the fuel and particles of limestone which constitute a desulfurizing and heat-transferable material. These solid materials are received by a fluidization grid 5 placed at the lower end of the main body 1 under which opens a primary air injection pipe 8 carrying out the fluidization of the solid materials in the lower part of the main body 1 which constitutes the fluidization chamber of the boiler.

As can be seen in Figs. 3 and 4, this lower part of the main body 1 has a square section enlarged with respect to the current rectangular section of the main body visible in FIG. 2.

As can be seen in FIG. 1, this lower part 1a receives, in addition to the fluidization means 5, 8, the starting burner 9 of the boiler, a preheating burner 10, an ash discharge duct 12 and thermocouples 13 allowing the determination of the temperature in this area of the boiler, for process control.

The inner surface of the main body 1 constituting the fluidization chamber is entirely coated with a refractory material 14 ensuring the protection of its external metallic envelope on which is inserted an expansion joint 15 in the lower part. Thermocouples 16 pass through the wall of the main body 1 in its upper part. The main body 1 also includes a safety valve 18, at its upper end connected to the duct 4 for the tangential arrival of the bed circulating in the recirculation cyclone 2.

The cyclone 2 has an upper part 20 entirely coated with refractory material and in communication at its upper part with a chamber 21 for evacuating the fumes which are taken via a pipe 22 internally coated with refractory, to an installation for thermal recovery and dust removal of the type usually used, as for example in FR-A-2,527,478, in association with circulating bed boilers.

The recirculation cyclone 2 comprises, below its part 20 coated with a refractory, a part 24 of cylindrical shape, the inner wall of which carries a heat exchanger 25 in which circulates water, the entire inner wall of the part 24 of the cyclone constituting an exchange surface. The smoke evacuation chamber 21 is placed in communication with the part 20 of the cyclone by means of a separator tube 27, the wall of which constitutes a heat exchanger wall in which water circulates.

The lower part 26 of cyclone 2, of frustoconical shape, comprises over the greater part of its height, a membrane wall in which water circulates constituting a heat exchanger 28 and in its lower part, over a small height, a wall 29 covered internally with refractory material, this lower part being in direct communication with the recirculation leg 3 also covered with refractory material.

Lines 30 for introducing secondary air into the cyclone, in a direction substantially tangential with respect to the latter, are placed, both at the level of the upper part 20 and of the cylindrical upper part 24 of this cyclone. Thermocouples 32 and 33 pass through the wall of the cyclone, at its part 20 and its part 24 respectively. Support tabs 34 are welded to the outer surface of the frustoconical part 26 of the cyclone.

The lower part of the recirculation cyclone 2 is in communication with the recirculation leg 3, the shape of which is visible by referring to FIGS. 1 and 3. This recirculation leg has a "J" shape which allows the formation of an airlock for the gas circulating in the lower part 1a of the fluidization chamber and thus avoiding the passage of this gas from the main body 1 to the cyclone 2, via the recirculation conduit 3. The solid matter separated from the gases reaching a the lower part of the cyclone and consisting mainly of combustion ash and heat-transferable limestone particles pour into the vertical inlet part of the recirculation duct 3 and are then received on the horizontal bottom of this duct; the recirculation of these solid particles into particles is ensured by recirculation air injection nozzles 35 passing through the horizontal bottom of the recirculation leg 3. The solid materials recirculated by the nozzles 35 inclined towards the outlet of the leg 3 go up in its vertical outlet part from which they flow into the chute 3a communicating with the part 1a of the fluidization chamber. A part of the solids arriving in the recirculation leg 3 can be evacuated by a pipe 36 which is connected to a circuit on which there is an external heat exchanger, a valve for controlling the solids extraction flow rate allowing adjustment of this extraction. The circuit of the external exchanger closes on the recirculation leg 3 through a pipe 37 for reinjection of the cooled solid materials.

The operation of the boiler according to the invention which has been described is as follows: coal and limestone in particulate form are introduced into the lower part 1a of the fluidization chamber and primary air is introduced under the grid 5 by through the conduits 8. The primary air flowing from bottom to top in the fluidization chamber 1 carries the solid particles upwards from this chamber. The enlarged part 1a at the base of this chamber 1 makes it possible to trap particles with a large particle size, the narrowing of the chamber prevents entrainment towards the upper part.

When the boiler is started, the start-up burner 9 makes it possible to raise the temperature in the enclosure, which then allows the introduction of the fuel which therefore undergoes prepyrolysis and gasification in zone 1a. The process is carried out in a reducing regime in this zone 1a, the air introduced at the base of the chamber 1 being limited to the quantity of air necessary for the fluidization.

This mode of operation with a secondary reaction zone at the base of the chamber 1 makes it possible in particular to reduce the discharges of nitrogen oxide in the gases and to avoid the entrainment of large fuel particles towards the upper part of the cyclone where these particles could damage the refractory lining material. These particles with a large particle size can be fractured and converted inside the zone 1a. This zone 1a also has the advantage of favoring the mixing between the fuel introduced and the solid matter recirculated by the leg 3.

In chamber 1, in addition to the fluidization, a pyrolysis and a more or less complete gasification of the fuel are carried out, so that the fluidized bed arriving in the upper part of the cyclone is constituted by a mixture of gas and solid materials containing a high proportion of combustible materials. This mixture undergoes a vortex movement upon entry of the cyclone and constitutes a vortex, which, with the combined action of the impact of the particles on the impact surface in refractory 20, considerably reduces the speed of vertical circulation of materials in the cyclone and increases the density of matter in the form of solid particles in the gases. This vortex effect is increased by the staged and tangential injections of secondary air injected through the conduits 30. This contributes to maintaining a high density of solid particles in the cylindrical zone of the cyclone. Secondary air is injected in sufficient quantity to achieve a high combustion of the circulating materials and to maintain a clearly oxidizing atmosphere in the upper part of the cyclone. Under the effect of the temperature and the oxidizing atmosphere, the desulfurization of combustible materials is carried out by formation of stable CaSo 4 sulfate from the calcium sulfide previously formed by reaction of the sulfur in the fuel with the limestone particles.

So in the upper part 20, 24 of cyclone 2 a reaction zone in which the vortex effect increases the residence time of solid particles. The circulation of gases and particles also promotes contact and heat exchanges with the heat exchanger 25 in such a way that the exchange surface swept by a gas at high speed is not covered by a layer. solids and therefore the heat exchange coefficient remains high. The walls of the separator tube 27 promoting the separation of gases and solid particles also constitute exchange surfaces recovering part of the heat of combustion. The burnt gases carrying fine solid particles in suspension are entrained, inside the vertical tube 27, towards the chamber 21 to constitute the fumes extracted by the conduit 22. The solid particles recovered by the cyclone fall back into the frustoconical part 26 of this cyclone and are cooled in contact with the walls of the heat exchanger 28 constituting a part of the internal wall of the truncated cone 26.

The cooled solid materials then fall back into the recirculation duct 3 to be recycled inside the lower part 1a of the chamber 1. The ash formed is periodically removed by the extraction duct 12.

When a circuit with an external heat exchanger is used, there is an additional means for removing heat from the recycled solid materials.

We therefore see that the main characteristics of the device, namely * the presence of a fluidization chamber entirely coated with refractory and the arrangement of heat exchanger bodies in the recirculation cyclone and more particularly in its upper part, allow '' obtain operation with higher solids densities and gas residence times in the reaction zone much longer, for the same height of the installations. In the fluidization chamber preceding the recirculation cyclone, only the pyrolysis and partial gasification of the fuel are carried out, the main reaction zone being located in the upper part of the cyclone. The heat exchanges in this area are greatly increased by the vortex effect, so that the heat exchange surfaces can be considerably reduced, of the order of 30%. This will have considerably reduced the size and the cost of construction of the boiler and its ancillary installations. Tangential injections in the upper part of the cyclone can reinforce these effects.

The upper zone 20 of the cyclone coated with refractory material makes it possible to slow down the particles in circulation and therefore to reduce the erosion of the exchange walls located below the zone 20.

The existence of a secondary reaction zone of enlarged section at the base of the fluidization column makes it possible to obtain other advantages as regards the conduct of the process.

The constructive provisions adopted allow the pressurization of the equipment which, therefore, can be used in a direct cycle die, that is to say without external exchanger.

The invention is not limited to the embodiment which has been described. This is how the heat exchanger elements can be arranged in a different way from that which has been described; for example, these heat exchanger elements can be strictly limited to the cylindrical upper part of the recirculation cyclone. These heat exchangers can be produced in the form of membrane walls of any type comprising means for distributing water and recovering steam.

The installation can be carried out in modular form, a single main body constituting the fluidization column being connected for their supply to several recirculation cyclones containing heat exchangers.

Recirculation cyclones can be of any known type, from the moment they allow the provision of heat exchangers on their internal wall. In particular, they can be of the counter-current type as in the embodiment which has just been described or with streams of recovered gases and particles circulating in the same direction. The size of these cyclones can be arbitrary since the effects of tangential gas circulation can be controlled by means of gas injections at different stepped locations in the cyclone.

Finally, the boiler according to the invention can be supplied with any fuel or other material in the form of particles, its pipe being particularly flexible, thanks to the internal means or to the annexed means available for adjusting. combustion and circulation of the fluidized bed.

Claims (9)

1. A boiler having a circulating fluidized bed using the heat generated by the combustion of circulating materials and comprising : an elongate fluidization chamber (1) of vertical axis whose inner surface is completely lined with a layer of refractory material (14), a combustible material and if necessary an incombustible solid material being introduced at the base of the boiler in the form of particles together with an oxidizing gas suspending the solid materials circulating upwardly in the chamber (1) ; at least one recirculation cyclone (2) of vertical axis which communicates with the upper portion of the fluidization chamber (1) via a duct (4) disposed substantially tangentially of the recirculation cyclone (2) ; a duct (3) for recycling solid materials via which the lower portion of the recirculation cyclone (2) is brought into communication with the lower portion of the fluidization chamber (1) ; and at least one heat exchanger component (25, 26, 27) in which the water which is to be heated and vaporized circulates and whose outer transfer surface comes into contact with the hot circulating gases and solid materials, characterized in that at least one heat exchanger component (25, 26, 27) is placed inside the recirculation cyclone (2) at least in its upper portion, which comprises on the one hand means (30) ensuring the combustion of the gases and combustible solids circulating in such portion of the cyclone (2), and on the other hand a zone in which its inner wall is lined with a refractory material, the fluidization chamber or column (1) comprising in its lower portion a widened zone (1a) into which the combustible material to be fluidized and the fluidization air are introduced.

2. A fluidized bed boiler according to claim 1, characterized in that the recirculation cyclone (2) comprises at least in its upper portion (20, 24), means for injecting air (30) into the circulating fluidized bed, which are arranged in substantially tangential directions in relation to the cyclone (2).

3. A fluidized bed boiler according either of claims 1 and 2, characterized in that the zone of the cyclone lined with the refractory material is disposed above the heat exchanger components (25, 26, 27) placed in the cyclone and undergoing the impact of the circulating solid particles and ensuring their deceleration.

4. A fluidized bed boiler according to any of claims 1, 2 and 3, characterized in that the widened zone (1a) is connected to the general part of the fluidization chamber (1), of smaller cross-section, via a constriction enabling the particles of solid materials of large particle size to be trapped, such materials being prepyrolyzed and gasified in the widened zone (1a).

5. A fluidized bed boiler according to claim 4, characterized in that the duct (3) for recirculating the solid materials discharges at its outlet end into the widened zone (1a) in which the recirculated solid materials and the new solid materials introduced at the base of the fluidization chamber (1) are mixed.

6. A fluidized bed boiler according to any of claims 1, 2, 3, 4 and 5, characterized in that it comprises a separator tube (27) which is disposed in the upper portion of the recirculation cyclone (2) along its axis and discharges at one of its ends into the cyclone and at its other end into a flue gas discharge chamber (21) and is formed by an exchanger wall (27) in which water circulates.

7. A fluidized bed boiler according to any of claims 1, 2, 3, 4, 5 and 6, characterized in that the inner wall of the lower portion (26) of the cyclone (2), which is frustoconical in shape, is formed by an exchanger wall (28).

8. A fluidized bed boiler according to any of claims 1 to 7, characterized in that an external heat exchanger is included in a circuit branching off from the duct (3) for recirculating the solid materials by means of pipelines (36, 37) for extracting and reinjecting solid materials.

9. A fluidized bed boiler according to claim 2, characterized in that the means (30) for injecting air are disposed in a stepped manner in the vertical direction in the upper portion of the recirculation cyclone (2).

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