WO2018002151A1

WO2018002151A1 - Method for burning fuel in a cylindrical combustion chamber

Info

Publication number: WO2018002151A1
Application number: PCT/EP2017/066018
Authority: WO
Inventors: Ziad Habib
Original assignee: S.A. Lhoist Recherche Et Developpement
Priority date: 2016-06-28
Filing date: 2017-06-28
Publication date: 2018-01-04
Also published as: ES2915900T3; BE1023896B1; PL3475609T3; BE1024784B1; EP3475609B1; FR3053102B1; EP3475609A1; SI3475609T1; BE1024784A9; HRP20220707T1; BE1024784B9; FR3053102A1; BE1024784A1

Abstract

Method for burning fuel in a cylindrical combustion chamber, comprising, in this combustion chamber, a projection, from a burner, of a jet of pulverulent solid fuel which is carried in conveying air, and possibly of a flow of primary air, and a supply of oxidising gas 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, the jet of solid fuel having an axial component of projection in the same direction as the said direction of propagation of the oxidising gas in the cylindrical combustion chamber and the ratio between the specific rate of change of momentum of the burner and the specific rate of change of momentum of the oxidising gas being equal to or less than 1.0 and greater than zero.

Description

Fuel combustion process in a chamber

cylindrical combustion

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

a projection, from a burner, of a powdery solid fuel jet displaced by a transport air, and possibly of a primary air flow, and

- A supply of an oxidizing gas along a direction of propagation so as to form a stream of oxidant 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 of limestone and dolomitic rocks, different types of furnaces are used, in particular rotary kilns, vat furnaces, and in particular straight annular furnaces.

These annular straight furnaces implement, for heating the material, upper and lower combustion chambers. The lower combustion chambers are originally designed to work with natural gas as fuel and it burns almost instantaneously.

Now, it becomes more and more desirable to be able to replace, in these furnaces currently in service, the fuel gas with a less expensive fuel, in particular a powdery solid fuel such as coal powder, coke or lignite, grape seeds. , olive stones, sawdust, etc.

An example not according to the invention of burner is shown schematically in axial section in Figure 3a and in perspective in Figure 4a. The burner includes a fuel conduit 120 surrounded by a cylindrical sleeve 121 including a flared portion 127 toward the end of the nose of the burner and including a plurality of holes 128. The cylindrical sleeve 121 forms with the duct 120 an annular space through which a combustible gas 126 passes. The sleeve 121 and the duct 120 are included in an outer casing 122 (shown in FIG. 3a, not shown in FIG. 4a) so that the nose of the conduit 120 protrudes from the flared portion 127 of the sleeve 121 and the nose of the outer casing 122, that the flared portion 127 of the sleeve is recessed relative to the nose of the outer casing 122. Primary air axial 105 can flow 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 portion of the sleeve.

The feeding of the burners in the lower combustion chambers of annular calcination furnaces by such a solid powdery fuel has, however, proved, in experimental tests carried out by the Applicant, that it is hardly appropriate. In fact, the combustion is incomplete, which leads to a combustion of unburnt not in the combustion chamber, but in the bed of material and even in the inner cylinder of the annular furnace, by which the hot flue gas is recovered. This results in a deterioration of the quality of the cooked product (loss of reactivity) and productivity (frequent stops of the oven to clean it). And we have to continue to implement fuel gas in combination with powdery solid fuel to avoid these problems. The expected price reduction is thus greatly reduced.

It should be noted that the lower combustion chambers of the annular straight furnaces are small and short. They are sized for natural gas that instantly burns according to the law of "immediately mixed, immediately burned" with a homogeneous combustion (gas-gas combustion). In these chambers also the air required for combustion comes premixed with flue gas recirculated, which have a reduced concentration of oxygen.

When solid fuel is projected into the combustion chamber, the situation is different, one is before a heterogeneous combustion (soiide-gas) where the law of "immediately mixed, immediately burned" is no longer applicable. The burning time is much higher and depends on a lot factors, such as particle size, reactivity of the solid surface, oxygen availability near the solid surface.

Simply replacing gaseous fuel with solid fuel in existing combustion chambers has proved to be a real problem.

In order to improve the combustion of the 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 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 present invention aims to remedy these drawbacks and therefore to provide a combustion process applicable in the furnace combustion chambers, in particular existing furnaces, which is effective with a consumption of only solid powdery fuel.

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

The specific flow rate of momentum is the measurement of the force of a jet (for example burner jet or oxidant flow) divided by the power of the burner.

The specific flow rate of the burner used is calculated according to the following equation (1):

Gax_brugger = (Qmcs + Qmat) x Vinj / P + Qmap x Vap / P, where

Qmcs = mass flow rate of the solid fuel (kg / sec),

Qmat = mass flow rate of the transport air (kg / sec),

Qmap = mass flow rate of the primary air (kg / sec), Vinj = axial fuel injection speed (m / sec)

Vap = axial injection rate of the primary air, and

P = burner power (MW).

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

Vinj = Qvat / Sb, where

Qvat = actual flow rate of the transport air (m ³ / sec), and

Sb = cross-section of the fuel injection duct in the burner (m ² ).

For primary air

Vap = Qvap / Sap

Qvap = actual volume flow of the primary air (m3 / sec)

Sap = cross section of the primary air injection duct in the burner

(M2)

The power of the burner is calculated according to the following equation (3): P = Qmcs x PCI, where

PCI = lower calorific value of the fuel (MJ / kg).

The specific flow rate of the oxidizing gas is calculated according to the following equation (4):

Gax_comburant = Qmgc x Vgc / P, where

Qmgc = mass flow rate of the oxidizing gas (kg / sec), and

Vgc = axial velocity of the oxidant gas around the solid fuel jet (m / sec).

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

Vgc = Qvgc / Sch, where

Qvgc = volume flow rate of the oxidizing gas (m ³ / sec), and

Sch = cross section of the combustion chamber (m ² ).

The basic principle in the design of the burners is that a burner must have a flow of momentum (injection speed x mass flow) large and sufficient for the central jet of fuel to suck the oxidant arriving at its periphery, thus forcing the fuel / oxidant mixture, which accelerates combustion. The aerodynamics of a Traditional flame design is therefore determined by the burner itself (see Figure 1).

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

To adapt this burner momentum flow rate, 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 injection speed of the fuel while maintaining unchanged the fuel and oxidant flows and the speed of the oxidizer and which has no influence on the operation of the furnace itself. This is a minor and easy modification of the nose of the burner, with immediate effect on the claimed ratio between the specific flow rates of momentum which is 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 process according to the invention, the cylindrical combustion chamber has a first and a second axial end and the powdery solid fuel jet is projected by the burner from the first axial end of the combustion chamber to the second axial end. Advantageously, the burner is arranged in a hole provided in the front wall of the first end of the combustion chamber. The solid fuel jet can thus come into contact with the oxidant along the entire length of the combustion chamber.

According to the invention, the oxidizing gas is mainly a recirculated flue gas, for example from the calcination furnace. This flue gas can be enriched with oxygen, for example by a supply of air. Advantageously, the oxidizing gas is fed tangentially into the combustion chamber at said first end thereof, so as to form a helical stream of oxidizing gas around the jet of fuel projected by the burner. This favors the fuel-oxidant mixture. Of course, it is also possible for the oxidizing gas to be fed into the combustion chamber at said first end thereof, parallel to its axis and around the jet of fuel projected by the burner. The propagation of the oxidizing gas must in any case follow a direction of propagation towards the downstream end of the combustion chamber.

To further promote further the fuel-oxidant mixture can be provided, according to the invention, a partial or total rotation of the fuel jet transported by the transport air. This can for example be obtained by giving a rotational movement to the transport air, with the aid of guide vanes.

The process according to the invention is intended to be preferably carried out in a lower annular furnace of calcareous or dolomitic rock calcination furnace.

The present invention also relates to such a combustion chamber comprising, at a first axial end, a burner arranged to project a powdery solid fuel jet into this chamber, and optionally an axial primary air flow, and a feed inlet for an oxidizing gas arranged so as to form a stream of oxidizing gas along a direction of propagation around the jet of fuel projected by the burner, the burner being arranged to project the solid fuel according to an axial projection component having the same direction as the direction propagation of the combustion gas stream in the cylindrical combustion chamber, so as to allow the implementation of the method according to the invention. It also relates to an annular calcic or dolomitic rock calcination furnace, comprising at least one such combustion chamber and an annular calcic or dolomitic rock calcination furnace, implementing a method according to the invention. The invention will now be described in more detail with reference to the accompanying drawings given without limitation.

Figure 1 schematically shows a projection not according to the invention of a pulverulent solid fuel jet in a conventional rotary kiln.

FIG. 2 schematically represents a projection according to the invention of a pulverulent solid fuel in a combustion chamber, for example of an annular right annealing furnace.

Figure 3a shows a schematic view in longitudinal section of a burner not according to the invention.

Figure 3b shows an axial sectional view of a burner used for carrying out the method according to the invention.

Figure 4a shows a schematic perspective view of a burner not according to the invention.

FIG. 4b represents a schematic perspective view of an embodiment of a burner that can be used for carrying out the method according to the invention.

FIG. 4c represents a schematic perspective view of another embodiment of a burner that can be used for carrying out the method according to the invention.

Figure 5 shows an axial sectional view of an annular right annealing furnace provided with lower combustion chambers implementing the method according to the invention.

On the different drawings the identical elements bear the same references.

It is customary to use, in the combustion chambers of industrial rotary kilns, burners that are fed solely with pulverulent solid fuel. The conditions for the operation of burners of this kind, whose power is 66 MW, in a rotary kiln with a flow rate of 1100 t / day are summarized in Table 1 below. Table 1

* The oxidizer is in this case air.

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

In Figure 1 this combustion chamber 1 is schematically illustrated. The fuel is thrown 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 oxidant 5, fed around the fuel jet, is sucked into it.

As is apparent from FIG. 5, a conventional annular right furnace for the calcination of calcareous 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. The raw material is introduced from the top of the oven at 9 and the cooked product is discharged from below at 10. The fuel is injected at two levels, through several upper and lower combustion chambers 11 (from 4 to 6 chambers according to the capacity of the oven). In general, 1/3 of the fuel is injected into the chambers 11 and 2/3 in the chambers 12. All the fumes of the upper chambers 11 and a portion of the fumes of the lower chambers 12 are pulled upwards by a fan of draw 13, so against the flow of the material charge. In this zone a countercurrent calcination occurs. The other part of the flue gas The lower combustion chambers 12 are drawn downwards by a depression created at the intake openings 14 provided in the inner cylinder 7, which is lower than the combustion chambers 12. This is the calcination zone at the same time. gills the fumes of the co-current calcination zone mix with the cooling air introduced at the bottom of the oven 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 oxidizing gas. 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 oxidizing gas 31 acquires a rotational movement which induces a centrifugal force pushing the oxidizing 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 calcination furnace a supply of the burner solely powdery solid fuel.

Figure 3b shows an axial section of a burner embodiment according to the invention. The burner comprises a sleeve 21 comprising a central conduit 20 through which the powdery solid fuel is fed. The sleeve 21 further comprises at least one additional conduit 23 through which fuel gas 26 can be supplied at the time of ignition of the furnace, and only at that time. An outer shell 22 surrounds the 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 is progressively reduced towards the nose of the burner, and the sleeve comprises a portion 27 whose external diameter increases progressively towards the nose of the burner so as to reduce the space between 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 in the combustion chamber without have 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 duct 20 and the nose dudtt at least one additional duct 23 pass through a plane orthogonal to the axis 30 of the burner. FIG. 4b represents a perspective view of a first embodiment of a burner according to the invention. The sleeve 21 comprises a central conduit 20 through which the solid powdery fuel is fed. The sleeve 21 further comprises an additional conduit 23 forming a thin annular space, through which fuel gas 26 can be supplied at the time of ignition of the oven, and only at that time. An outer shell 22 (not shown in FIG. 4b) surrounds the sleeve 21 and forms a space through which axial primary air can be supplied to aid combustion.

FIG. 4c represents a perspective view of another embodiment of the burner according to the invention. The sleeve 21 comprises a central conduit 20 through which the solid powdery fuel is fed. The sleeve 21 further comprises a plurality of additional conduits 23 distributed around the central duct 20, these additional ducts 23 through which fuel gas 26 can be supplied at the time of ignition of the oven, and only at that time. An outer shell 22 (not shown in FIG. 4c) surrounds the 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 space reduction between the sleeve 21 and the outer casing 22 can be achieved only by decreasing the internal diameter of the casing 22 at the nose of the burner and keeping the external diameter of the sleeve 21 constant, or alternatively by increasing the outer diameter of the sleeve 21 at the nose of the burner while keeping the internal diameter of the envelope 22 constant. This reduction in space between the sleeve 21 and the envelope makes it possible to provide a higher primary air injection rate at the exit of the burner.

According to another possible embodiment of the burner, the internal diameter of the casing 22, the outer diameter of the sleeve 21 and the space between the sleeve 21 and the outer casing remain constant. In this case, the axial primary air flow rate or the volume of the sleeve 21 or the volume of the inside of the casing 22 are adapted to allow the axial primary air to exit at a predetermined speed to the nose of the burner. Figure 5 shows a schematic of an annular furnace and includes a representation of an 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 a combustion gas inlet 31 located tangentially with respect to the axis 30 _" 30 'of the burner and the cylindrical combustion chamber, as described above. The combustion gases are then removed from the combustion chamber by a conduit.

The conditions provided for the operation of a burner as described with the aid of the example of FIG. 3b, whose power is 1.13 MW, in an annular furnace with four lower combustion chambers and whose lime flow is 150t / day, are summarized in Table 2 below.

Table 2

^* The oxidant is in this case formed of recirculation gases.

The injection speed of the fuel transported by air is obtained by passing through the conduit 20 which has a section of 0.001 m ² . The "force" of the burner, ie its specific flow rate (axial primary air + transport air + coal) is still slightly higher than that of the oxidant, but it is insufficient to suck the oxidant into the fuel. It is not to be compared with that of the rotary kiln described above. And so we observe an unsatisfactory combustion with an oven with the disadvantages described above. It has now been provided, for an annular calcination furnace having the same lime flow rate of 150 t / day and with identical lower combustion chambers with burners of the same power, to reduce the specific flow rate of the burner momentum, contrary to what the skilled person would have imagined on the basis of his knowledge. The new conditions applied are those shown in Table 3.

Table 3

^* The oxidant is in this case formed of recirculation gases.

As can be seen, only the injection speed of the pulverulent solid fuel displaced by the transport air has been changed 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 induced a ratio between the specific flow rate of the burner's momentum and the specific flow rate of the oxidant significantly less than 1.

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

This phenomenon is shown diagrammatically in FIG. 2. Given its low injection speed 3, the fuel projected by the burner 2 forms a more open projection cone 4 and is further drawn into the stream of oxidizing gas which becomes the engine. An identical experiment was carried out on a burner, whose power is 1.81 MW, in an annular right furnace with 5 combustion chambers and whose flow rate is 300t / day. The operating conditions with a burner whose section of the fuel supply duct is 0.001 m ² are given in Table 4.

Table 4

* The oxidant is in this case formed of recirculation gases.

This result proved unsatisfactory to obtain a satisfactory fuel-oxidant mixture in the combustion chamber and therefore a total combustion of the powdery solid fuel therein.

By modifying the injection speed of the fuel, by enlarging the section of the injection duct to 0.002 m ² , the conditions given in Table 5 below are obtained:

Table 5

Flow rate T (° C) Speed Gax (N / MW)

(M / sec)

^~ . Dmburant * 700 6.2 6.13

Axial primary air 1400 n / h 200 34 1, 89

Transport air 318 m ³ / h 50 35 1, 86

Coal 296 kg / h 50 35 1, 59

Total burner 5.34

Gax_brûleur / Gax_comburant 0.87 ^* The oxidant is in this case formed of recirculation gases.

This arrangement allows a residence time of the particles increased drastically in the combustion chamber and thus 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 set out above and that many modifications can be made without departing from the scope of the appended claims.

For example, a clean movement of the burner can be added, either by adding rotating vanes in the powdery solid fuel circuit, or by adding rotating vanes to the transport air circuit or the axial primary air circuit, or still a combination of these measures. It is also possible to add, at the periphery of the burner, an additional air circuit rotated to assist in opening the cone for throwing the fuel into the chamber.

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

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

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

Claims

A method of fuel combustion in a cylindrical combustion chamber comprising, in this combustion chamber,

- A supply of an oxidizing gas along a direction of propagation so as to form a stream of oxidant gas around the jet of fuel projected by the burner, at a temperature causing combustion of the fuel,

characterized in that the solid fuel jet has an axial projection component in the same direction as said direction of propagation of the combustion gas in the cylindrical combustion chamber, and in that the ratio between the specific flow rate of the burner and specific flow rate of the oxidizing gas is equal to or less than 1, 0 and greater than zero.

2. Method according to claim 1, characterized in that the ratio between the specific flow rate of the burner movement and specific flow rate of the oxidizer gas is between 0.25 and 0.9.

3. A method according to either of claims 1 and 2, characterized in that the cylindrical combustion chamber has a first and a second axial ends and that the jet of solid powdery fuel is projected by the burner since the first axial end of the combustion chamber to the second axial end.

4. A method according to claim 3, characterized in that the oxidizing gas is fed tangentially into the combustion chamber at said first end thereof, so as to form a helical stream of oxidant gas around the jet of fuel projected by the burner.

5. Method according to claim 3, characterized in that the oxidizing gas is fed into the combustion chamber at said first end of it, parallel to its axis and around the jet of fuel projected by the burner.

6. Process according to any one of claims 1 to 5, characterized in that it comprises a partial or total rotation of the fuel jet transported by the transport air.

7. Process according to any one of claims 1 to 6, characterized in that it comprises a partial or total rotation of the primary air flow.

8. Process according to any one of claims 1 to 7, characterized in that the oxidizing gas is a flue gas recirculated.

9. Process according to any one of claims 1 to 8, characterized in that said combustion chamber is a lower combustion chamber of annular right furnace mineral rock calcination.

10. A cylindrical combustion chamber comprising, at a first axial end, a burner arranged to project solid powdery fuel into this chamber and a feed inlet for an oxidizing gas arranged so as to form a stream of oxidizing gas in a direction of propagation around the jet of fuel projected by the burner, characterized in that the burner is arranged to project the solid fuel according to an axial projection component having the same direction as said direction of propagation of the gases in the cylindrical combustion chamber, this chamber being arranged for carrying out the method according to any one of claims 1 to 9.

11. annular right mineral rock calcining furnace, comprising at least one combustion chamber according to claim 10.

12. Annular right mineral rock calcination furnace, implementing a method according to any one of claims 1 to 11.