EP0308300B1 - Combustion process in a fluidised bed and installation for carrying out the process - Google Patents

Description

The present invention relates to solid fuel fireplaces in which combustion is carried out in two stages: a first stage where the fuel is burned in a fluidized bed with a substoichiometric amount of primary air and a second stage where the gases emerging from the fluidized bed and the fine particles of fuel which they entrain burn with secondary air admitted into the combustion chamber at a certain height above the fluidized bed.

More particularly, the invention relates to fluidized bed fireplaces of the aforementioned type known in particular from publication FR-A-2 583 305. This known installation or boiler comprises a vertical combustion chamber, the bottom of which is constituted by a movable grate such as the upper strand of an endless chain, inclined upwards from front to rear in the direction of advancement of the fuel, this grid forming a fluidization support traversed by the primary blowing air. The temperature within the fluidized flit is sufficient to allow agglomeration of ash in bottom ash which is discharged at the upper end of the grid thus forming a mechanical means of extraction by conveyor. Since the fuel used generally contains sulfur, this known installation relates more particularly to the desulfurization of gases leaving the fluidized bed hearth. All of the fly ash is separated from the flue gases in a primary dust collector while the desulphurizing agent and the products of the desulphurization reaction are separated from the fumes therefore in particular of the fly ash in a secondary dust collector while a portion of the desulphurizing agent and the products of the desulphurization reaction thus separated are discharged through a purge line. All of the solid particles, separated in the primary dust collector, are recycled at the first stage of combustion below the secondary air inlet, the fly ash being reinjected either just above the fluidized bed or within it itself. fluidized bed while the other part of the desulphurizing agent and the products of the desulphurization reaction, separated in the secondary dust collector, is recycled to the second stage of combustion, that is to say above the inlet secondary air and possibly at two different levels depending on the combustion regime, namely a lower level for operation at low load and an upper level for operation at high load, the fresh desulfurizing agent also being introduced in the second stage of combustion above the secondary air supply. There are therefore two separate circulations according to respectively two separate closed circuits, namely: the circulation of desulphurizing agent and products of the desulphurization reaction, on the one hand, and the circulation of fly ash, on the other hand.

The disadvantage of known fireplaces of this type, in which the ashes and soot, which are entrained by the gases emerging from the fluidized flit, are separated in one or more dust collectors and reinjected into the fluidized bed or just above that -this lies in the fact that the soot hardly burns in the fluidized bed or just above it because they are in a reducing medium and can be trapped by the ashes and evacuated with them, which reduces the yield combustion.

The European patent specification EP-A-206 066 reveals a combustion device forming a circulating fluidized bed boiler (or turbulent layer).

This known system has the drawback of a relatively organically complex structure therefore of a relatively high cost as well as of a technical result or effect which is only partially satisfactory. In fact, the operating mode of this known system results in obtaining a temperature maintained substantially constant at a desired value by regulation by means of forced cooling of all or part of the recycled ash, so that the flow rate of recirculation is determined therefore imposed by the desired constant temperature thereby resulting in a very high recirculation rate (ratio of recycled flow to purged flow) thereby increasing the risk of erosion and energy expenditure, the temperature conditioning l 'emission of nitrogen oxides and the effectiveness of the desulfurizing agent (injected with carbon into the fluidized bed). This known system thus endeavors to simultaneously deal with all the problems simultaneously concerning respectively the combustion, the emission of nitrogen oxides, sulfur dioxide etc. Furthermore, the possibility, in this known system, of recycling all of the separated solid materials has the drawback of causing, over time, an increase in the solids content of the recirculation circuit.

The present invention essentially aims to eliminate or at least overcome the drawbacks of the aforementioned known systems by creating a method of combustion of a solid fuel containing sulfur, comprising a first stage where the fuel is burned in a fluidized bed with a quantity substoichiometric primary combustion and fluidizing air and a second stage in the area of which a desulfurizing agent is injected and where the gases emerging from the fluidized flit and the fine particles of fuel or soot entrained by these gases burn with secondary air, the major part of the ash produced in the first stage of the combustion being evacuated at this stage while the solid particles separated from the fumes resulting from the combustion in the second stage, with the exception of a purge flow eliminating part of said solid particles entrained by the gases, are recycled to combustion for the circulate in a closed circuit. In this process, the problem posed above is solved in accordance with the invention by the fact that all of said separate solid particles consist of a mixture of fly ash and products of the desulphurization reaction and their recycled part is recycled in the second stage. combustion so as to obtain, only at this stage, a single circulation in a closed circuit with several passages of a mixture of fine particles of fuel or soot, fly ash, desulfurizing agent and products of the desulfurization reaction with a circulating charge rate equal to several (for example about ten) times the purge rate.

Advantageously, the particles, recycled in the second stage of combustion, are injected at two different levels depending on the combustion regime: a first level close to the intake level of secondary air, for walking at low load and a higher level for high load walking. The recycled particles are preferably reinjected pneumatically by means of secondary air.

The invention also relates to a boiler forming installation with a fluidized bed hearth for the implementation of the aforementioned process, of the type comprising a combustion chamber, the lower part of which constitutes a fluidized bed reactor, with a bottom forming an inclined grid means for evacuating the ashes at the upper end of the hearth and under which are arranged means for blowing air for fluidization and primary combustion while above the hearth are placed means for introducing fuel in the fluidized bed and the upper part of the chamber includes secondary air intake nozzles and means for injecting a desulphurizing agent, located at a certain height above the fluidized bed and said nozzles secondary air intake; dust collector means connected to the outlet of the upper part of said chamber as well as means for reinjecting, into the combustion chamber, part of the solid particles separated from the combustion gases in said dust collector means and purge means for withdrawing the other part of said separate solid particles, characterized in that said fuel introduction means are arranged so as to introduce it into the most deep of the fluidized bed and said reinjection means opening into said combustion chamber at or above said secondary air intake nozzles while said purge means are arranged to remove a mixture of fly ash and products of the desulfurization reaction with a flow such that the flow of the circulating charge is equal to several times the purge flow.

• La figure 1 est un schéma illustrant le procédé de l'invention ; et
• La figure 2 est une représentation schématique d'une chaudière équipée d'un foyer conforme à l'invention.

Other characteristics of the invention will appear on reading the description which follows and refers to the accompanying drawing which shows, by way of nonlimiting example, an embodiment of the invention and in which:

• Figure 1 is a diagram illustrating the method of the invention; and
• Figure 2 is a schematic representation of a boiler equipped with a fireplace according to the invention.

In the diagram in FIG. 1, the reference numbers 1 and 2 represent reactors, 3 and 4 are convection heat exchangers, 5 is a primary dust collector and 6 is a final dust collector.

In reactor 1, the fuel introduced in 7 is fluidized by the air admitted in 8, under an inclined and gas-permeable floor. The ash produced by combustion in a fluidized bed is removed at 9 by overflow or defluidization above the upper end of the hearth, or extracted from the bed, in the form of bottom ash, by a conveyor if the temperature of the bed is sufficient for cause agglomeration of ash. The air flow blown at 8 is less than the stoichiometric flow, so that the gases produced which emanate from the bed are reducing. These gases, which are charged with fly ash and fine fuel particles, are introduced into reactor 2 where secondary air is also introduced, at 11, and if necessary a desulfurizing agent, such as limestone or lime, in finely divided form, at 13. The gases and the fuel particles which they transport burn with the secondary air to produce hot gases which successively pass through the exchanger 3, the dust collector 5, the exchanger 4 and the dust collector 6. The dust separated from the gases in the dust collectors 5 and 6 is returned to the reactor 2, at 15, except for a small fraction which is withdrawn at 17, to remove the ash and the reaction products of desulfurization. By controlling the flow rate of the dust reinjected at 15, the circulating charge is maintained equal to several times the purge flow rate, so that each particle of desulphurizing agent and of fuel passes several times through the reactor 2 which makes it possible to obtain very good yields combustion and desulfurization.

In the boiler of FIG. 2, the two combustion zones are superimposed, without separation, in the same enclosure. This boiler comprises a fluidized bed hearth 10, the bottom of which is formed by the upper strand, inclined with an endless chain 12. This bottom constitutes the bottom of a vertical chamber 18 whose walls and ceiling are formed, so known, of sealed tube panels, which are connected, at their upper end, to a vapor collecting balloon 20. The chamber 18 is connected to a horizontal chamber 22 in which are placed bundles of tubes 24 connected to the balloon 20 and tubes 26 where the vapor collected in the balloon 20 is superheated. The fumes produced in chamber 18 and leaving chamber 22 pass successively through a primary dust collector 28, constituted for example by a multicyclone, an exchanger 30, which could be an economizer, and a final dust collector 32, for example a bag filter.

The air necessary for the combustion of the fuel is partly blown through the chain 12 to ensure the fluidization of the solid fuel and its partial combustion, and partly at a certain distance above the bed, by means of nozzles 34.

The solid fuel is introduced through a chute 14 into the deepest part of the bed and a desulfurizing agent is injected at 36 into the upper part of the combustion chamber 18.

The dust collected in the chamber 22, in the dust collectors 28 and 32 and in the exchanger 30 is reinjected, for example pneumatically, at 37 into the combustion chamber 18, at the level of the nozzles 34 or above it; we can, in particular, use secondary air to reinject the dust. Part of the dust is drawn off, continuously or discontinuously, by a purge circuit 38 to remove the products of the desulfurization reaction and the ashes. The circulation rate - ratio of the flow rate of the circulating charge to the purge flow rate - is of the order of 10. Provision may be made for re-injecting the dust at two different levels according to the operating mode of the boiler: a lower level for the walking at low load and a higher level for walking at high load.

If the combustion in the fluidized bed 10 takes place at a temperature sufficient to allow the ash to agglomerate in the form of bottom ash, these settle on the chain 12 and are discharged through it at its upper end. If the temperature of the fluidized bed is not sufficient to cause agglomeration of the ashes, they are evacuated by overflow or defluidization at the upper end of the hearth. In all cases, most of the ash is thus extracted from the bed, the rest being entrained by the gases emerging from the bed. Combustible gases emerging from the bed and fines fuel particles entrained by them, with the fly ash, burn above the bed 10 with the secondary air introduced by the nozzles 34.

It can be considered that the fluidized bed 10 constitutes the first stage of a two-stage reactor, the second of which is similar to a circulating fluidized bed and where the desulphurization and combustion of the soot are carried out entirely. However, it is distinguished from conventional circulating fluidized beds by the fact that it does not have a fluidization grid and that the circulating charge is much more reduced, for a given rate of circulation of the desulfurizing agent, since the major part of the ashes is extracted from the bed 10 in the form of clinkers, so that the particles are kept in suspension in the gases in this second stage. Consequently, the content of desulphurization products in the fraction extracted by the purge circuit 38 is significantly higher, which facilitates the subsequent treatment thereof; the dust concentration of the chambers 18 and 22 is lower and the walls of these chambers as well as the tubes of the bundles 24 and 26 are subjected to less severe erosion.

Claims (4)

1. Method of combustion of a solid fuel containing sulphur, comprising a first stage wherein the fuel is burnt in a fluidized bed with a substoechimetric amount of primary combustion and fluidizing air and a second stage in the zone of which is injected a desulphurizing agent and wherein the gases which are evolving from the fluidized bed and the fine fuel particles or soots carried along by these gases are burning with secondary air, the major part of the ashes produced in the first stage of the combustion being discharged at this stage whereas the solid particles separated from the smokes resulting from the combustion at the second stage, except for a draining flow rate eliminating one part of the said solid particles carried along by the gases, are recycled to the combustion to make them circulate in a closed circuit, characterized in that all the said separated solid particles consist of a mixture of fly-ashes and of products from the desulphurization reaction and their recycled part is recycled to this second stage of the combustion so as to obtain at this stage only, a single circulation in a closed circuit with several passages of a mixture of fine fuel particles or soots, of fly-ashes, of desulphurizing agent and of products of the desulphurization reaction with a circulating batch flow rate equal to several times (for example about ten times) the draining flow rate.
2. Method according to claim 1, characterized in that the said particles recycled to the second stage are injected at two different levels as a function of the combustion operating conditions, namely: a first level near to the secondary air intake level for a small batch load running and a higher level for the great batch load running.
3. Method according to claim 1 or 2, characterized in that the said recycled particles are pnuematically reinjected by means of secondary air.
4. Eqipment forming a boiler with a furnace with a fluidized bed for carrying out the method according to one of the foregoing claims, comprising: a combustion chamber (18) the lower part of which constitutes a fluidized bed reactor (10) with a hearth forming an inclined grate (12) provided with means for the discharge of the ashes at the upper end of the hearth and below which are disposed means for blowing primary fluidization and comubstion air whereas above the hearth (12) are placed means (14) for the introduction of fuel into the fluidized bed (10); and the upper chamber part of which comprises secondary air intake nozzles (34) and means (36) for the injection of a desulphurizing agent (36), located at some height above the fluidized bed (10) and of the said secondary air intake nozzles (34); de-dusting means (28, 32) connected to the outlet of the upper part of the said chamber (18) as well as means (37) for reinjecting into the combustion chamber (18) one part of the solid particles separated from the combustion gases within the said de-dusting means (28, 32) and draining means (38) for drawing off the other part of the said separated solid particles, characterized in that the said means (14) for the introduction of fuel are disposed so as to introduce the latter into the deepest part of the fluidized bed (10) and the said reinjection means (37) are opening into the said combustion chamber (18) at the level of or above the said secondary air intake nozzles (34) whereas the said draining means (38) are arranged to remove a mixture of fly-ashes and of products from the desulphurization reaction with such a flow rate that the circulating batch load flow rate be equal to several times the draining flow rate.

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