EP3475609B1 - Method for burning fuel in a cylindrical combustion chamber - Google Patents

Description

• une projection, à partir d'un brûleur, d'un jet de combustible solide pulvérulent déplacé par un air de transport, et éventuellement d'un flux d'air primaire, et
• une alimentation en un gaz comburant suivant un sens de propagation de manière à former un courant de gaz comburant autour du jet de combustible projeté par le brûleur, à une température occasionnant une combustion du combustible.

The present invention relates to a fuel combustion process in a cylindrical combustion chamber, comprising, in this combustion chamber,

• a projection, from a burner, of a jet of pulverulent solid fuel displaced by transport air, and optionally by a flow of primary air, and
• a supply of an oxidizing gas in a direction of propagation so as to form a current of oxidizing gas around the jet of fuel projected by the burner, at a temperature causing combustion of the fuel.

In the field of the calcination of mineral rocks, in particular limestone and dolomitic rocks, various types of furnaces are used, in particular rotary furnaces, shaft furnaces, and in particular annular straight furnaces.

These straight annular furnaces use upper and lower combustion chambers to heat the material. The lower combustion chambers are originally designed to operate with natural gas as fuel and this burns almost instantaneously.

However, it is becoming increasingly desirable to be able to replace, in these furnaces currently in service, the fuel gas with a less expensive fuel, in particular a pulverulent solid fuel of the powdered coal, coke or lignite type, grapeseed , olive pits, sawdust, etc.

An example of a burner not in accordance with the invention is represented schematically according to an axial section on the picture 3a and in perspective on the figure 4a . The burner comprises a fuel conduit 120 surrounded by a cylindrical sleeve 121 comprising a flared portion 127 towards the end of the nose of the burner and comprising a plurality of holes 128. The cylindrical sleeve 121 forms with the conduit 120 an annular space through which passes a combustible gas 126. The sleeve 121 and the conduit 120 are included in an outer casing 122 (shown on the picture 3a , not shown on the figure 4a ) so that the nose of the conduit 120 protrudes from the flared portion 127 of the sleeve 121 and from the nose of the outer casing 122, that the flared portion 127 of the sleeve is recessed with respect to the nose of the outer casing 122. Axial primary air 105 can circulate between a space formed by the outer casing 122 and the sleeve 121, as well as by the plurality of holes 128 on the flared part of the sleeve.

The supply of the burners in the lower combustion chambers of annular calcining furnaces with such a solid pulverulent fuel has however proved, during experimental tests carried out by the applicant, to be difficult to appropriate. In fact, the combustion is incomplete, which leads to combustion of the unburned matter no longer in the combustion chamber, but in the bed of material and even in the inner cylinder of the annular furnace, through which the hot flue gases are recovered. This results in a deterioration in the quality of the baked product (loss of reactivity) and in productivity (frequent stops of the oven to clean it). And we come to have to continue to implement combustible gas in combination with pulverulent solid fuel to avoid these problems. The expected price reduction is thus greatly reduced.

It should be noted that the lower combustion chambers of annular straight furnaces are small and short. They are sized for natural gas which burns instantaneously according to the “as soon as mixed, as soon as burned” law of homogeneous combustion (gas-gas combustion). In these chambers, too, the air required for combustion arrives premixed with recirculated flue gases, which have a reduced oxygen concentration.

When solid fuel is projected into the combustion chamber, the situation is different, we are faced with heterogeneous combustion (solid-gas) where the law of “as soon as mixed, as soon as burned” no longer applies. The burning time is much higher and depends on a lot factors, such as particle size, reactivity of the solid surface, availability of oxygen near the solid surface.

Simply replacing gaseous fuel with solid fuel in existing combustion chambers has therefore proven to be quite problematic.

In order to improve the combustion of solid fuel, mechanical devices have already been provided which force the solid fuel to mix more intimately with the oxidant (see for example BE 1015604 , which describes a method and a combustion chamber according to the preamble of claims 1 and 10 respectively, and EP2143998 ).

However, such systems remain complicated and expensive to manufacture and especially to maintain. They present significant risks of malfunction, such as clogging, rapid wear of mechanical parts, etc.

The object of the present invention is to remedy these drawbacks and therefore to propose a combustion method applicable in the combustion chambers of furnaces, in particular of existing furnaces, which is effective with a consumption of only pulverulent solid fuel.

To solve this problem, a combustion process has been provided as indicated at the beginning, in which the jet of solid fuel has an axial component of projection in the same direction as said direction of propagation of the combustion gas in the cylindrical combustion chamber and wherein the ratio of the specific momentum rate of the burner to the specific momentum rate of the combustion gas is equal to or less than 1.0 and greater than zero.

The specific momentum rate is the measure of the force of a jet (eg burner jet or oxidant current) divided by the burner power.

où

• Qmcs = débit massique du combustible solide (kg/sec),
• Qmat = débit massique de l'air de transport (kg/sec),
• Qmap = débit massique de l'air primaire (kg/sec),
• Vinj = vitesse d'injection axiale du combustible (m/sec)
• Vap = vitesse d'injection axiale de l'air primaire, et
• P = puissance du brûleur (MW).

The specific momentum flow rate of the burner used is calculated according to the following equation (1): $$\mathrm{gas}\_\mathrm{br}\widehat{\mathrm{a}}\mathrm{their}=\left(\mathrm{Qmcs}+\mathrm{Qmat}\right)\times \mathrm{vinj}/\mathrm{P}+\mathrm{Qmap}\times \mathrm{steam}/\mathrm{P},$$

or

• Qmcs = solid fuel mass flow (kg/sec),
• Qmat = transport air mass flow (kg/sec),
• Qmap = primary air mass flow (kg/sec),
• Vinj = axial fuel injection velocity (m/sec)
• Vap = axial injection speed of the primary air, and
• P = burner power (MW).

• Pour le combustible
  • Vinj = Qvat/Sb, où
  • Qvat = débit volumique réel de l'air de transport (m³/sec), et
  • Sb = section droite du conduit d'injection du combustible dans le brûleur (m²).
• Pour l'air primaire
  • Vap= Qvap/Sap
  • Qvap = débit volumique réel de l'air primaire (m3/sec)
  • Sap = section droite du conduit d'injection de l'air primaire dans le brûleur (m2)

The axial injection speed is calculated according to the following equation (2):

• For the fuel
  • Vinj = Qvat/Sb, where
  • Qvat = actual transport air volume flow (m ³ /sec), and
  • Sb = straight section of the fuel injection duct in the burner (m ² ).
• For primary air
  • Vap= Qvap/Sap
  • Qvap = actual primary air volume flow (m3/sec)
  • Sap = straight section of the primary air injection duct in the burner (m2)

• P = Qmcs x PCI, où
• PCI = pouvoir calorifique inférieur du combustible (MJ/kg).

The burner power is calculated according to the following equation (3):

• P = Qmcs x PCI, where
• LCV = lower calorific value of the fuel (MJ/kg).

• Qmgc = débit massique du gaz comburant (kg/sec), et
• Vgc = vitesse axiale du gaz comburant autour du jet de combustible solide (m/sec).

The specific momentum flow rate of the oxidizing gas is calculated according to the following equation (4): $$\mathrm{gas}\_\mathrm{oxidizer}=\mathrm{Qmgc}\times \mathrm{vgc}/\mathrm{P},\mathrm{or}$$

• Qmgc = mass flow rate of oxidizing gas (kg/sec), and
• Vgc = axial velocity of the combustion gas around the solid fuel jet (m/sec).

• Vgc = Qvgc/Sch, où
• Qvgc = débit volumique du gaz comburant (m³/sec), et
• Sch = section droite de la chambre de combustion (m²).

The axial velocity of the combustion gas is calculated according to the following equation (5):

• Vgc = Qvgc/Sch, where
• Qvgc = volume flow rate of oxidizing gas (m ³ /sec), and
• Sch = cross section of the combustion chamber (m ² ).

The basic principle in the design of burners is that a burner must have a significant and sufficient momentum flow rate (injection speed x mass flow rate) for the central fuel jet to be able to draw in the oxidizer arriving at its periphery, thereby forcing the fuel/oxidant mixture, which accelerates combustion. The aerodynamics of a flame of traditional design is therefore determined by the burner itself (see figure 1 ).

On the contrary, the process according to the present invention is based on an aerodynamics which is determined by the oxidant arriving in the combustion chamber. The oxidizer here forces the fuel to enter its current by adapting the momentum rate of the burner to that of the oxidizer (see figure 2 ). It is therefore no longer the jet of fuel which is the driving force, it is the fuel which is driven by the oxidizer. This results in an increased residence time of the fuel, with the effect of making it possible to use a fuel only in a solid pulverulent form and to obtain total combustion of this fuel in the combustion chamber.

To adapt this momentum flow rate of the burner, it is possible, for example, to increase the fuel injection section in the nose of the burner, which has the immediate effect of reducing the fuel injection speed while maintaining unchanged the flow rates of fuel and oxidizer and the speed of the oxidizer and this has no influence on the operation of the furnace itself. This is a minor and easy modification to the burner nose, with immediate effect on the claimed ratio between specific momentum rates being adapted to become equal to or less than 1.0. Preferably, this ratio will be between 0.5 and 0.9.

According to one embodiment of the method according to the invention, the cylindrical combustion chamber has first and second axial ends and the jet of pulverulent solid fuel is projected by the burner from the first axial end of the combustion chamber towards the second axial end. Advantageously, the burner is arranged in a peephole provided in the front wall of the first end of the combustion chamber. The jet of solid fuel can thus come into contact with the oxidizer over the entire length of the combustion chamber.

According to the invention, the combustion gas is mainly a flue gas recirculated, for example from the calcining furnace. This flue gas can be enriched with oxygen, for example by supplying air.

Advantageously, the combustion gas is fed tangentially into the combustion chamber at said first end thereof, so as to form a helical current of combustion gas around the jet of fuel projected by the burner. This favors the fuel-oxidizer mixture. Naturally, provision can also be made for the combustion gas to be fed into the combustion chamber at said first end of the latter, parallel to its axis and around the jet of fuel projected by the burner. The propagation of the combustion gas must in any case follow a direction of propagation towards the downstream end of the combustion chamber.

In order to further promote the fuel-oxidizer mixture, provision may be made, according to the invention, for partial or total rotation of the jet of fuel transported by transport air. This can for example be obtained by giving a rotational movement to the conveying air, with the aid of guiding fins.

The method according to the invention is intended to be preferably implemented in a lower combustion chamber of a straight annular kiln for calcining limestone or dolomitic rock.

The present invention also relates to such a combustion chamber comprising, at a first axial end, a burner arranged to project a jet of pulverulent solid fuel into this chamber, and optionally an axial primary air flow, and a supply inlet for an oxidizing gas arranged so as to form a current of oxidizing gas in a direction of propagation around the jet of fuel projected by the burner, the burner being arranged to project the solid fuel in an axial component of projection having the same direction as the direction propagation of the current of oxidizing gas in the cylindrical combustion chamber, so as to allow the implementation of the method according to the invention. It also relates to a straight annular kiln for calcining limestone or dolomitic rock, comprising at least one such combustion chamber as well as a straight annular kiln for calcining limestone or dolomitic rock, implementing a process according to the invention.

• La figure 1 représente de manière schématique une projection non conforme à l'invention d'un jet de combustible solide pulvérulent dans un four rotatif conventionnel.
• La figure 2 représente de manière schématique une projection suivant l'invention d'un combustible solide pulvérulent dans une chambre de combustion par exemple de four de calcination droit annulaire.
• La figure 3a représente une vue schématique selon une coupe longitudinale d'un brûleur non conforme à l'invention.
• La figure 3b représente une vue en coupe axiale d'un brûleur utilisable pour la mise en oeuvre du procédé suivant l'invention.
• La figure 4a représente une vue schématique en perspective d'un brûleur non conforme à l'invention.
• La figure 4b représente une vue schématique en perspective d'une forme de réalisation d'un brûleur utilisable pour la mise en œuvre du procédé suivant l'invention.
• La figure 4c représente une vue schématique en perspective d'une autre forme de réalisation d'un brûleur utilisable pour la mise en œuvre du procédé selon l'invention.
• La figure 5 représente une vue en coupe axiale d'un four de calcination droit annulaire pourvu de chambres de combustion inférieures mettant en œuvre le procédé suivant l'invention.

The invention will now be described in more detail with reference to the accompanying drawings given without limitation.

• The figure 1 schematically represents a projection not in accordance with the invention of a jet of pulverulent solid fuel in a conventional rotary kiln.
• The picture 2 schematically represents a projection according to the invention of a pulverulent solid fuel in a combustion chamber, for example of an annular straight calcining furnace.
• The picture 3a represents a schematic view according to a longitudinal section of a burner not in accordance with the invention.
• The figure 3b shows a view in axial section of a burner that can be used to implement the method according to the invention.
• The figure 4a represents a schematic perspective view of a burner not in accordance with the invention.
• The figure 4b represents a schematic perspective view of an embodiment of a burner usable for the implementation of the method according to the invention.
• The figure 4c shows a schematic perspective view of another embodiment of a burner usable for implementing the method according to the invention.
• The figure 5 shows a view in axial section of a right annular calcining furnace provided with lower combustion chambers implementing the process according to the invention.

In the various drawings, identical elements bear the same references.

It is usual to implement, in the combustion chambers of industrial rotary kilns, burners which are supplied solely with pulverulent solid fuel. The conditions provided for the operation of burners of this type, whose power is 66 MW, in a rotary kiln whose throughput is 1100t/day are summarized in table 1 below. <b>Table 1</b> Debit T (°C) Speed (m/sec) Gas (N/MW) Oxidizer* 165806 m ³ /h 550 2.5 0.74 transportation air 22127 m ³ /h 50 60 6.10 Coal 9000 kg/h 50 60 2.28 total burner 8.38 Gas_burner/Gax_comburant 11.29 *The oxidizer in this case is air.

As can be seen, the specific momentum flow rate of the burner (transport air + coal) is much higher than that of the oxidizer.

On the figure 1 this combustion chamber 1 is illustrated schematically. The fuel is projected by the burner 2 at a very high injection speed 3 and the injection cone 4 formed by the fuel projected out of the nose of the burner has a very tapered shape. Thanks to this high injection speed, the oxidizer 5, fed around the jet of fuel, is sucked into it.

As appears from the figure 5 , a conventional annular straight furnace for calcining limestone or dolomitic rock comprises an outer cylinder 6 and an inner cylinder 7 forming an annular space 8 into which the material to be fired descends. The raw material is introduced from the top of the furnace at 9 and the cooked product is discharged from the bottom at 10. The fuel is injected at two levels, through several upper 11 and lower 12 combustion chambers (from 4 to 6 chambers depending on oven capacity). In general, 1/3 of the fuel is injected into chambers 11 and 2/3 into chambers 12. All of the fumes from the upper chambers 11 and part of the fumes from the lower chambers 12 are drawn upwards by a ventilation fan. draw 13, therefore against the current of the movement of the material load. In this zone, a counter-current calcination takes place. The other part of the flue gases from the lower combustion chambers 12 is drawn downwards by a depression created at the level of the intake vents 14 provided in the inner cylinder 7, lower than the combustion chambers 12. This is the co-current calcination zone. At the level of the vents, the fumes from the co-current calcination zone mix with the cooling air introduced at the bottom of the furnace at 15. This mixture forms the recirculation fumes which, at 16, are recovered from the inner cylinder 7 and returned to the lower combustion chambers 12 to become the combustion gas there. Through a conduit 17, this oxidizing gas 31 arrives at each of the chambers 12 tangentially to the axis of the chamber and therefore to the jet of the burner 18 injected axially. As a result, the combustion gas 31 acquires a rotational movement which induces a centrifugal force pushing the combustion gas 31 towards the walls of the cylindrical combustion chamber.

Experimental tests were then carried out to apply to each of the combustion chambers of such a conventional annular calcining furnace a supply of the burner solely with pulverulent solid fuel.

The figure 3b shows an axial section of a burner embodiment according to the invention. The burner comprises a sleeve 21 comprising a central duct 20 through which the pulverulent solid fuel is fed. The sleeve 21 further comprises at least one additional conduit 23 through which combustible gas 26 can be supplied when the oven is switched on, and only then. An outer casing 22 envelops sleeve 21 and forms with it a space through which axial primary air 5 can be supplied to aid combustion. The outer casing 22 comprises a portion 19 whose internal diameter progressively reduces towards the nose of the burner, and the sleeve comprises a portion 27 whose external diameter gradually increases towards the nose of the burner so as to reduce the space between the nose of the outer casing 22 and the nose of the sleeve 21. This reduction in space between the outer casing 22 and the sleeve makes it possible to increase the injection speed of the axial primary air 5 into the combustion chamber without having to provide a high flow of axial primary air. The nose of the outer casing 22, the nose of the sleeve 21, the nose of the central conduit 20 and the nose of said at least one additional conduit 23 pass through a plane orthogonal to the axis 30 of the burner.

The figure 4b shows a perspective view of a first embodiment of the burner according to the invention. The sleeve 21 comprises a central duct 20 through which the pulverulent solid fuel is fed. The sleeve 21 further comprises an additional duct 23 forming a thin annular space, through which combustible gas 26 can be supplied at the moment of ignition of the furnace, and only at that moment. An outer casing 22 (not shown in the figure 4b ) envelops sleeve 21 and forms a space through which axial primary air 5 can be supplied to aid combustion.

The figure 4c shows a perspective view of another embodiment of the burner according to the invention. The sleeve 21 comprises a central duct 20 through which the pulverulent solid fuel is fed. The sleeve 21 further comprises a plurality of additional ducts 23 distributed around the central duct 20, these additional ducts 23 through which combustible gas 26 can be supplied when the oven is switched on, and only at this time. An outer casing 22 (not shown in the figure 4c ) envelops sleeve 21 and forms a space through which axial primary air 5 can be supplied to aid combustion.

According to other possible embodiments of the burner, a reduction in the space between the sleeve 21 and the outer casing 22 can be achieved only by reducing the internal diameter of the casing 22 at the level of the nose of the burner and keeping the external diameter of the sleeve 21 constant, or alternatively by increasing the external diameter of the sleeve 21 at the level of the nose of the burner while keeping the internal diameter of the casing 22 constant. This reduction in space between the sleeve 21 and the casing makes it possible to provide a higher primary air injection speed at the outlet of the burner.

According to another possible embodiment of the burner, the internal diameter of the envelope 22, the external diameter of the sleeve 21 and the space between the sleeve 21 and the external envelope remain constant. In this case, the axial primary air flow or the volume of the sleeve 21 or the volume of the interior of the casing 22 are adapted to allow the axial primary air to exit at a predefined speed at the nose of the burner.

The figure 5 shows a diagram of an annular furnace and includes a representation of one embodiment of a cylindrical combustion chamber 12 according to the invention. In this embodiment, the combustion chamber comprises an inlet forming the casing 22 of the burner 18 and the axis 30 of the burner is preferably located in the axis 30' of the cylindrical combustion chamber 12. The combustion chamber 12 further comprises an oxidizing gas inlet 31 located tangentially with respect to the axis 30, 30' of the burner and of the cylindrical combustion chamber, as described above. The combustion gases are then evacuated from the combustion chamber through a conduit.

The conditions provided for the operation of a burner as described using the example of the figure 3b , whose power is 1.13 MW, in an annular kiln equipped with 4 lower combustion chambers and whose lime flow rate is 150t/day, are summarized in Table 2 below. <b>Table 2</b> Debit T(°C) Speed (m/sec) Gas (N/MW) Oxidizer* 10995 m ³ /h 700 6.1 5.99 Axial primary air 700 m ³ /h 200 34 1.89 transportation air 199 m ³ /h 50 60 3.19 Coal 185 kg/h 50 60 2.73 total burner 7.81 Gas_burner/Gax_comburant 1.30 *The oxidizer in this case is formed from the recirculation gases.

The injection speed of the fuel transported by air is obtained by passing through the pipe 20 which has a section of 0.001 m ² . The "force" of the burner, i.e. its specific flow rate of momentum (axial primary air + transport air + coal) is still slightly greater than that of the oxidizer, but it is insufficient to suck the oxidizer into the fuel. It is not to be compared with that of the rotary kiln described above. And we therefore observe an unsatisfactory combustion with a furnace having the disadvantages described above.

Provision has now been made, for an annular calcining kiln having the same lime flow rate of 150 t/day and equipped with identical lower combustion chambers with burners of the same power, to reduce the specific momentum flow rate of the burner, contrary to what the person skilled in the art would have imagined on the basis of his knowledge. The new conditions applied are those indicated in Table 3. <b>Table 3</b> Debit T(°C) Speed (m/sec) Gas (N/MW) Oxidizer* 10995 m ³ /h 700 6.1 5.99 Axial primary air 700 m ³ /h 200 34 1.89 transportation air 199 m ³ /h 50 35 1.85 Coal 185 kg/h 50 35 1.58 total burner 5.32 Gas_burner/Gax_comburant 0.88 *The oxidizer in this case is formed from the recirculation gases.

As can be seen, only the injection speed of the pulverulent solid fuel displaced by the transport air has been modified, to almost half of its value. Such a modification could be obtained by adapting the section of the duct 20, to a value of 0.002 m ² . This minor modification led to the obtaining of a ratio between the specific momentum flow rate of the burner and the specific momentum flow rate of the oxidant clearly lower than 1.

Surprisingly, it was then noted that this simple modification gave rise to a flame, initiated very quickly, as quickly as with natural gas, and above all that, now, it was the fuel which was sucked into the helical flow of oxidizing gas.

This phenomenon is represented schematically on the figure 2 . Given its low injection speed 3, the fuel projected by the burner 2 forms a more open projection cone 4 and it is also sucked into the current of combustion gas which becomes the engine.

An identical experiment was carried out on a burner, whose power is 1.81 MW, in a straight annular furnace equipped with 5 combustion chambers and whose flow rate is 300t/day. The operating conditions with a burner with a cross-section of the fuel supply duct of 0.001 m ² are given in table 4. <b>Table 4</b> Debit T(°C) Speed (m/sec) Gas (N/MW) Oxidizer* 17592 m ³ /h 700 6.2 6.13 Axial primary air 1400 m ³ /h 200 34 1.89 transportation air 318 m ³ /h 50 60 3.19 Coal 296 kg/h 50 60 2.73 total burner 7.81 Gas_burner/Gax_comburant 1.27 *The oxidizer in this case is formed from the recirculation gases.

This result proved to be unsatisfactory for obtaining a satisfactory fuel-oxidizer mixture in the combustion chamber and therefore complete combustion of the pulverulent solid fuel therein.

By modifying the fuel injection speed, by enlarging the section of the injection duct to 0.002 m ² , the conditions given in table 5 below are obtained: <b>Table 5</b> Debit T(°C) Speed (m/sec) Gas (N/MW) Oxidizer* 17592 m ³ /h 700 6.2 6.13 Axial primary air 1400 m ³ /h 200 34 1.89 transportation air 318 m ³ /h 50 35 1.86 Coal 296 kg/h 50 35 1.59 total burner 5.34 Gas_burner/Gax_comburant 0.87 *The oxidizer in this case is formed from the recirculation gases.

This arrangement allows a residence time of the particles increased drastically in the combustion chamber and therefore the oxygen is better available and the combustion is complete inside the combustion chamber.

It should be understood that the present invention is in no way limited to the embodiments indicated above and that many modifications can be made thereto without departing from the scope of the appended claims.

It is for example possible to add a movement specific to the burner, either by adding rotation fins in the pulverulent solid fuel circuit, or by adding rotation fins to the transport air circuit or to the axial primary air circuit, or yet another combination of these measures. It is also possible to add to the periphery of the burner an additional air circuit brought into rotation to help the opening of the fuel projection cone in the chamber.

It is also possible to inject the fuel directly into the stream of oxidizing gas, for example at the point of arrival of the latter in the combustion chamber, but before its rotation.

It is also quite conceivable not to supply primary air to the burner, which can modify the values of the ratio claimed with respect to those obtained with a burner in which primary air is supplied.

In a burner without primary air, when implementing a jet of fuel at an injection speed Vinj equal to 15 m/sec, the claimed ratio can even become equal to 0.25. At an injection speed Vinj of 45 m/sec, it will then be 0.74.

Claims (12)

1. A method for burning fuel in a cylindrical combustion chamber (1), comprising, in this combustion chamber,

   - a projection, from a burner (2), of a jet of pulverulent solid fuel (3), carried by a conveying air, and possibly of a flow of primary air, and

   - a supply of an oxidising gas (5) in a direction of propagation such as to form a stream of oxidising gas around the jet of fuel projected by the burner, at a temperature that causes combustion of the fuel,

   characterised in that the jet of solid fuel has an axial component of projection in a same direction as said direction of propagation of the oxidising gas in the cylindrical combustion chamber, and in that the ratio between the specific momentum rate of the burner and the specific momentum rate of the oxidising gas is equal to or less than 1.0 and greater than zero.
2. The method according to claim 1, characterised in that the ratio of the specific momentum rate of the burner to the specific momentum rate of the oxidising gas is between 0.25 and 0.9.
3. The method according to either of claims 1 or 2, characterised in that the cylindrical combustion chamber has a first and a second axial end and in that the jet of pulverulent solid fuel is projected by the burner from the first axial end of the combustion chamber towards the second axial end.
4. The method according to claim 3, characterised in that the oxidising gas is supplied tangentially into the combustion chamber at said first end thereof, such as to form a helical stream of oxidising gas around the jet of fuel projected by the burner.
5. The method according to claim 3, characterised in that the oxidising gas is supplied into the combustion chamber at said first end thereof, parallel to its axis and around the jet of fuel projected by the burner.
6. The method according to any one of claims 1 to 5, characterised in that it comprises a partial or total rotation of the jet of fuel carried by conveying air.
7. The method according to any one of claims 1 to 6, characterised in that it comprises a partial or total rotation of the flow of primary air.
8. The method according to any one of claims 1 to 7, characterized in that the oxidising gas is a recirculated flue gas.
9. The method according to any one of claims 1 to 8, characterized in that said combustion chamber is a lower combustion chamber of an annular shaft kiln for mineral rock calcination.
10. A cylindrical combustion chamber (1) comprising, at a first axial end, a burner (2) arranged to project a pulverulent solid fuel (3) in this chamber and a supply inlet for an oxidising gas (5) arranged to form a stream of oxidising gas in a direction of propagation around the jet of fuel projected by the burner, characterised in that the burner is arranged to project the solid fuel along an axial component of projection having a same direction as said direction of propagation of the gases in the cylindrical combustion chamber, this chamber being arranged and dimensioned for the implementation of the method according to any of claims 1 to 9.
11. An annular shaft kiln for mineral rock calcination, comprising at least one combustion chamber according to claim 10.
12. An implementation of a method according to any one of claims 1 to 9 in an annular shaft kiln for mineral rock calcination.

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