EP0099281B1 - Process for the combustion in a fluidized bed of low grade fuels, especially coal or oil shales - Google Patents

Abstract

1. Process for the combustion in a fluidized bed of low grade fuels, especially of coal or oil shales, on a travelling grate (5) inclined upwards from the front to the rear, with blowing compartments (9a, 9b, 9c, 9d, 9e) of air any other oxidant gas arranged below the grate (5) which is fed at fore with fuel, the front area of the said grate constituting a fluidisation zone (F) and the rear part of the said grate forming at a higher level than that of the fluidized bed (12) a transport section (T) of the ashes in form of a permanent layer, characterized in that : - starting from a fixed point (15), behind the fluidization zone (F), one maintains an insufficient blowing to generate the fluidization, so that between the fluidization zone (F) and the transport section (T) an intermediate settling zone (S) takes form, where ashes deposit along a bank (14), the said bank extending through the whole length of the settling zone (S), being renewed by successive parallel sections in proportion to the displacement of the grate (S), and - the flow rate of the blowing air is distributed in the blowing compartments (9a, 9b, 9c, 9d, 9e) in such a way as to create a methodical outflow of the fuel from to the rear of the fluidized bed (12), allowing the ashes to be brought in the settling zones (S) with a view to extract them.

Description

The present invention relates to the combustion in a fluidized bed of lean fuels, in particular coal or bituminous shales.

The technique of combustion in a fluidized bed has been known for a long time. An installation is advantageously used for this purpose, consisting of a substantially vertical front wall, provided with a fuel supply device in the form of particles, two side walls, a movable grid inclined upwards from front to rear. , said grid being arranged between the two side walls, at a certain distance from the latter, and a plurality of boxes for blowing air or any other oxidizing gas disposed under the grid. An installation of this kind is described in patent FR-A-1,092,540. Patent FR-A-1,440,039 describes a similar installation further comprising a plurality of valves, each of which being assigned to a box of particular blowing.

In current applications of such installations, relatively rich fuels are used which burn at a temperature of at least 1200 ° C. At such a temperature, the ash agglomerates in the form of bottom ash which is deposited at the bottom of the bed over the entire length thereof. These agglomerates are extracted by the grid itself through the free surface of the bed, behind the fluidization zone. A moderate blowing, insufficient to cause fluidization, can advantageously be maintained towards the rear of the grid, at a level higher than that of the bed, to prevent the agglomerates transported by the grid from sticking to it.

It has already been thought of extending the field of application of this technique to the combustion of poor fuels with a high ash content.

The difficulty is that the combustion temperature is often less than 1000 ° C and in any case less than the temperature necessary for the formation of agglomerates. It then becomes practically impossible to evacuate the ashes, or at least their evacuation results in the simultaneous evacuation of a relatively large quantity of unburnt materials. The thermal efficiency of combustion is very poor, which is all the more annoying the higher the ash content.

The object of the invention is essentially to overcome this difficulty.

It consists above all in forming between the fluidization zone and the transport zone, an intermediate sedimentation zone, and in establishing a methodical circulation of the fuel so as to bring into said sedimentation zone almost all of the ash which is deposited there after the fuel has remained in the fluidized bed for the minimum time necessary for its complete combustion.

• - l'on maintient à partir d'un point fixe, à l'arrière de la zone de fluidisation, un soufflage insuffisant pour provoquer une fluidisation, de sorte qu'il se forme entre la zone de fluidisation et la zone de transport, une zone intermédiaire de sédimentation dans laquelle les cendres se déposent suivant un talus, ledit talus qui s'étend sur toute la longueur de la zone de sédimentation, se renouvelant par tranches parallèles successives au fur et à mesure du déplacement de la grille, et
• - l'on répartit le débit d'air de soufflage dans les caissons de manière à créer un écoulement méthodique du combustible de l'avant vers l'arrière du lit fluidisé, permettant ainsi l'amenée des cendres dans la zone de sédimentation en vue de leur extraction.

A more specific subject of the invention is a method of combustion in a fluidized bed of poor fuels, in particular coal shale or bituminous, on a grid inclined upwards from front to rear, air-blowing boxes or any other oxidizing gas being arranged below the grid which is supplied with fuel from the front, the front part of said grid constituting a fluidization zone, and the rear part of said grid constituting, at a level higher than that fluidized bed, an ash transport zone in the form of a fixed layer, characterized in that:

• - Maintaining from a fixed point, behind the fluidization zone, an insufficient blowing to cause fluidization, so that it forms between the fluidization zone and the transport zone, a intermediate sedimentation zone in which the ashes are deposited along an embankment, said embankment which extends over the entire length of the sedimentation zone, being renewed by successive parallel sections as the grate moves, and
• - the blowing air flow is distributed in the boxes so as to create a methodical flow of fuel from the front to the rear of the fluidized bed, thus allowing the ashes to be brought into the sedimentation zone in view of their extraction.

The level and the extent of the fluidized bed are adjusted by modifying the position of said fixed point.

The speed of the grate is adjusted, for a given fuel flow rate and a given ash content, in order to obtain a minimum residence time of the fuel in the fluidized bed for complete combustion, and a predetermined thickness of the fixed layer of ash.

The level and the extent of the fluidized bed are adjusted by modifying the number of boxes put into service with a sufficient flow rate to cause fluidization, on the one hand, and the distribution of the air flow rate between said boxes, on the other hand go.

The invention will be better understood by referring to the description which follows, given with reference to the appended drawings, concerning an embodiment of an installation for implementing the method according to the invention, given by way of example not limiting.

Figure 1 shows schematically in longitudinal section such an installation.

Figure 2 is a cross-sectional view of this installation.

In the figures, the reference 1 designates the front wall 1 of the installation and the references 2 and 3 its side walls. The wall 1 is provided with a pipe 4 for supplying fuel in the form of particles from a not shown crushing device.

A grid 5 inclined upwards from front to rear is arranged between the walls 2 and 3 at a certain distance from them. This grid advantageously constituted by the upper strand of a endless chain, receives the fuel leaving the pipe 4.

The reference 6 designates the fuel slope formed against the wall 1, while the references 7 and 8 designate the slope formed against the side walls 2 and 3.

Below the grid 5 are deposited boxes for blowing air or any other oxidizing gas. These boxes, five in number in the example shown, are respectively marked 9a, 9b, 9c, 9d and 9e from the front to the rear of the grid between the drums 10 and 11 thereof.

Only the boxes 9a, 9b and 9c deliver an air flow sufficient to cause fluidization of the fuel in the bed 12 which forms above said boxes. The boxes 9d and 9e deliver an amount of air insufficient to cause fluidization. The area above the casing 9d constitutes an ash sedimentation area which will be discussed more explicitly below, while the area above the casing 9e constitutes, in a manner known per se, an area for transporting ash and unclog grid 5.

Each box is associated with a valve for adjusting the air flow rate coming from a fan, not shown, these valves are respectively marked 13a, 13b, 13c, 13d, 13e. These valves make it possible to distribute the total air flow so that this flow decreases progressively from front to rear, which causes inside the bed 12, a methodical flow of fuel. The term “methodical flow” is understood to mean a flow from the front to the rear at a speed such that the residence time of the fuel in the bed 12 is equal to the minimum necessary to ensure its complete combustion, while the only ashes are brought to the rear end of the bed 12 where they pass from the fluidization zone F to the sedimentation zone S. In this latter zone the ashes are deposited along a slope 14 whose height is equal to that of the fixed layer extracted from the bed by the displacement of the grid 5 towards the transport zone T. The embankment 14 extends over the entire length of the sedimentation zone S and it is renewed in successive sections as the grid advances .

It can be seen that the sedimentation zone S extends over the entire length of the box 9d, from a fixed point 15 materialized by the trace on the plane of the figure of the upper end of the front face of the box 9d. We can change the position of this point forward by adjusting the valve 13c so that the box 9c no longer delivers sufficient flow to ensure fluidization. The sedimentation zone S moves and no longer covers only the caisson 9c, the transport zone T covering the caissons 9d and 9e. One can, of course, move this point backwards by admitting on the contrary in the box 9d a flow sufficient to cause fluidization. The sedimentation zone S moves and covers the only box 9e, while the very short transport zone T is devoid of boxes. The level N and the extent of the fluidization zone F are therefore adjusted by modifying the position of point 15 by means of the valves 13a, 13b, 13c, 3d and 13e.

On the other hand, the speed of the grate can be adjusted for a given fuel flow rate and an ash content thereof, in order to act correlatively on the minimum stay time of the fuel in the bed 12 for combustion. complete, as well as to obtain a fixed layer of optimum thickness in the transport zone T.

The following will give, by way of example, the characteristics of a pilot installation carried out by the applicant and the test results obtained:

The number of boxes may differ from five. It is even advantageous, to ensure a finer adjustment of the position of point 15, to provide, for a given grid length, a greater number of boxes of shorter length. One can also consider, instead of acting on the number of boxes, to provide inside each of them an adjustable partitioning device.

It is also possible to carry out a given program, by providing a remote control of the valves 13a, 13b, 13c, 13d, 13e adjustable according to the various parameters (ash content, speed of the grate, height of the layer, distribution of the air flow and fuel). These different parameters can be recorded on the panel of a control panel.

Claims (4)

1. Process for the combustion in a fluidized bed of low grade fuels, especially of coal or oil shales, on a travelling grate (5) inclined upwards from the front to the rear, with blowing compartments (9a, 9b, 9c, 9d, 9e) of air or any other oxidant gas arranged below the grate (5) which is fed at fore with fuel, the front area of the said grate constituting a fluidisation zone (F) and the rear part of the said grate forming at a higher level than that of the fluidized bed (12) a transport section (T) of the ashes in form of a permanet layer, characterized in that:

- starting from a fixed point (15), behind the fluidization zone F), one maintains an insufficient blowing to generate the fluidization, so that between the fluidization zone (F) and the transport section (T) an intermediate settling zone (S) takes form, where ashes deposit along a bank (14), the said bank extending through the whole length of the settling zone (S), being renewed by successive parallel sections in proportion to the displacement of the grate (S), and

- the flow rate of the blowing air is distributed in the blowing compartments (9a, 9b, 9c, 9d, 9e) in such a way as to create a methodical outflow of the fuel from the front to the rear of the fluidized bed (12), allowing the ashes to be brought in the settling zone (S) with a view to extract them.

2. Process according to claim 1, characterized in that the level (N) and the extent of the fluidized bed (12) are adjusted by changing the position of the said fixed point.

3. Process according to claims 1 or 2, characterized in that the speed of the grate (5) is adjusted for a given output of the fuel and its given ash content, in order to obtain the minimal residing time of the fuel in the fluidized bed (12) for complete combustion and a predetermined thickness of the permanent layer of ashes.

4. Process according to claim 2, characterized in that the level (N) and the extent of the fluidized bed (12) are adjusted by changing the number of compartments (9a, 9b, 9c, 9d, 9e) set into operation with a sufficient flow rate as to generate the fluidization, on one hand, and the distribution of the air flor rate between the said compartments on the other hand.

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