BE1024784A9 - Fuel combustion process in a cylindrical combustion chamber - Google Patents

Abstract

A method of fuel combustion in a cylindrical combustion chamber comprising, in said combustion chamber, a projection, from a burner, of a pulverulent solid fuel jet displaced by a transport air, and optionally a primary air flow, and a supply of an oxidizing gas in 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 a combustion of the fuel, the fuel jet solid having an axial projection component in the same direction as said direction of propagation of the combustion gas in the cylindrical combustion chamber and the ratio between the specific flow rate of the burner momentum and the specific flow rate of the combustion gas being equal to or less than 1.0 and greater than zero.

Description

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The present invention relates to a fuel combustion method in a cylindrical combustion chamber, comprising, in this combustion chamber, - a projection, from a burner, of a jet of pulverulent solid fuel displaced by air transport, and possibly a primary air flow, 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 a fuel combustion.

In the field of caïcination of mineral rocks, in particular of limestone and doiomitic rocks, different types of ovens are used, in particular rotary ovens, vat ovens, and in particular straight annular ovens.

These straight annular ovens use, for heating the material, upper and lower combustion chambers. The lower combustion chambers were originally designed to work with natural gas as fuel and it burns almost instantly.

However, it is becoming more and more desirable to be able to replace, in these ovens currently in service, the combustible gas by a less expensive fuel, in particular a solid pulverulent fuel of the coal, coke or lignite powder type, grape seeds. , olive pits, sawdust, etc.

An example not in accordance with the burner invention is shown diagrammatically in an axial section in FIG. 3a and in perspective in FIG. 4a. The burner comprises a fuel duct 120 surrounded by a cylindrical sleeve 121 comprising a flared portion 127 towards the end of the burner nose and comprising a plurality of holes 128. The cylindrical sleeve 121 forms with the duct 120 an annular space through which passes a combustible gas 126. The sleeve 121 and the conduit 120 are included in an external envelope 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 of the nose of the external envelope 122, that the flared portion 127 of the sleeve is set back with respect to the nose of the external envelope 122. Primary axial air 105 can circulate between a space formed by the external envelope 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 calcination ovens with such a solid pulverulent fuel has, however, proved, during experimental tests carried out by the applicant, hardly suitable. In fact, the combustion is incomplete, which leads to unburnt back combustion no longer in the combustion chamber, but in the bed of material and even in the interior cylinder of the annular furnace, by which the hot smoke gases are recovered. This results in a deterioration in the quality of the cooked product (loss of reactivity) and in productivity (frequent stopping of the oven for cleaning). And we end up having to continue to use 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 straight annular ovens are small and short. Elies are sized for natural gas which burns instantly according to the law of "immediately mixed, immediately burned" of a homogeneous combustion (gas-gas combustion). Also in these chambers the air necessary for combustion arrives premixed with recirculated smoke gases, which have a reduced oxygen concentration.

When solid fuel is sprayed into the combustion chamber, the situation is different, we are faced with heterogeneous combustion (solid-gas) where the law of "immediately mixed, immediately burned" is no longer applicable. The combustion time is much longer and depends on many factors, such as its particle size, its reactivity on the solid surface, its availability of oxygen near the solid surface.

Simply replacing gaseous fuel with solid fuel in existing combustion chambers has therefore proven to be really 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 1015004 and EP2143998). However, such systems remain complicated and costly to manufacture and especially to maintain. They present significant risks of malfunction, such as blockages, 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 ovens, in particular of existing ovens, which is effective with consumption only of pulverulent solid fuel.

To solve this problem, a combustion process has been provided as indicated at the beginning, in which 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 between specific flow rate of momentum of the burner and specific flow rate of momentum of the oxidizing gas is equal to or less than 1.0 and greater than zero.

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

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

Gax__burner ~ (Gmcs + Qmat) x Vin] / P + Gmap x Vap / P, where Gmcs = mass flow rate of solid fuel (kg / sec),

Gmat = mass flow of transport air (kg / sec),

Gmap = mass flow of primary air (kg / sec),

Vinj = axial fuel injection speed (m / sec)

Vap ~ axial injection speed of the primary air, and P ~ burner power (MW).

The axial injection speed is calculated according to the following equation (2): For Vinj fuel - Qvat / Sb, where

Qvat = real volume flow of transport air (m3 / sec). and Sb "cross section of the fuel injection pipe in the burner (m2). For primary air Vap- Qvap / Sap

Qvap - real volume flow of primary air (m3 / sec)

Sap - cross section of the primary air injection duct into the burner (rr? 2)

The burner power is calculated according to the following equation (P): P - Gmcs x PCI, where PCI ~ lower calorific value of the fuel (MJ / kg).

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

Gax „oxidizer = Qmgc x Vgc / P, where Qmgc - mass flow of oxidant gas (kg / sec), and Vgc" axial speed of the oxidant gas around the solid fuel jet (m / sec).

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

Vgc ™ Gvgc / Sch, where

Gvgc = volume flow rate of the oxidizing gas (m3 / sec), and Sch - cross section of the combustion chamber (m2).

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 fuel jet to be able to suck the oxidant arriving at its periphery, thus forcing the fuel / oxidizer mixture, which accelerates combustion. The aerodynamics of a traditional design flame 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 oxidizer here forces the fuel to enter its current by adapting the flow rate of momentum of the burner to that of the oxidant (see Figure 2). It is therefore no longer the fuel jet which is driving, it is the fuel which is entrained by the oxidant. This results in an increased residence time of the fuel, with the effect of using a fuel only in a pulverulent solid form and of obtaining a total combustion of this fuel in the combustion chamber.

To adapt this flow rate of movement of the burner, it is possible, for example, to increase the fuel injection section in the burner nose, which has the immediate effect of reducing the fuel injection speed while keeping unchanged. the fuel and oxidant flow rates and the speed of the oxidant and which has no influence on the operation of the furnace itself, this is a minor and easy modification of the burner nose, with immediate effect on the claimed ratio between the specific flows of momentum which is adapted so as to become equal to or less than 1.0. Preferably the 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 an opening 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 over the entire length of the combustion chamber.

According to the invention, the oxidizing gas is mainly a flue gas recirculated, for example from the calcination oven. This smoke gas can be enriched with oxygen, for example by supplying air.

Advantageously, the oxidizing gas is fed tangentially into the combustion chamber at said first end of the latter, so as to form a helical stream of oxidizing gas around the jet of fuel projected by the burner, This promotes the fuel-oxidant mixture, It is of course also possible to provide that the oxidizing gas is supplied to the combustion chamber at said first end thereof, parallel to its axis and around the fuel jet 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 the fuel-oxidizer mixture, provision may be made, according to the invention, for a partial or total rotation of the fuel jet transported by transport air. This can for example be obtained by giving a rotation movement to the transport air, using guide vanes,

The process according to the invention is intended to be preferably carried out in a lower combustion chamber of an annular straight furnace for calcining limestone or doiomitic 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 according to an axial component of projection having the same direction as the direction of 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 oven for calcining limestone or dolomitic rock, comprising at least one such combustion chamber as well as a straight annular oven for calcining limestone or dolomitic rock, implementing a process 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 in accordance with the invention of a jet of pulverulent solid fuel 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 a straight annular calcination furnace.

FIG. 3a represents a schematic view according to a longitudinal section of a burner not in accordance with the invention.

FIG. 3b represents a view in axial section of a burner which can be used for implementing 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 which can be used for implementing the method according to the invention.

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

FIG. 5 represents a view in axial section of a straight annular calcination furnace provided with lower combustion chambers implementing the method according to the invention.

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

It is customary to use, in the combustion chambers of industrial rotary ovens, burners which are supplied solely with pulverulent solid fuel. The conditions provided for the operation of burners of this kind, whose power is 86 MW, in a rotary oven with a flow rate of 11 ÛOt / day are summarized in Table 1 below.

Table 1

* The oxidizer in this case is air.

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

In Figure 1 this combustion chamber 1 is illustrated schematically. The fuel is sprayed by the burner 2 at a very high injection speed 3 and the injection cone 4 formed by the fuel sprayed out of the burner nose has a very tapered shape. Thanks to this high injection speed ie the oxidizer 5, supplied around the fuel jet, is drawn into it,

As shown in FIG. 5, a straight annular oven used for calcining calcareous or doomite rock comprises an outer cylinder 6 and an inner cylinder 7 forming an annular space 8 in which the material descends to be cooked. The raw material is introduced through the top of the oven at 9 and the cooked product is discharged through 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). Generally, 1/3 of the fuel is injected into the chambers 11 and 2/3 into the 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 blower. print 13, therefore against the flow of the movement of its charge of matter. In this zone H calcination occurs against the current. The other part of the flue gases from the lower combustion chambers 12 is drawn down by a vacuum created at the intake openings 14 provided in the inner cylinder 7, lower than the combustion chambers 12, that is ia caicination zone by cocurrent, At the smoke gills of ia caicination zone in co-current mix with the cooling air introduced at the bottom of the oven in 15. This mixture forms the recirculation smoke which. at 16, are recovered from the lower cylinder 7 and brought back to the lower combustion chambers 12 to become the oxidizing gas there. By a conduit 17, this oxidizing gas 31 arrives at each of the chambers 12 tangentially to the tax 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 its 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 only with solid pulverulent combustion.

Figure 3b shows an axial section of an embodiment of the burner according to the invention. The burner comprises a sleeve 21 comprising a central duct 20 through which the pulverulent solid fuel is brought, The sleeve 21 further comprises at least one additional duct 23 through which combustible gas 26 can be supplied at the time of the ignition of the furnace, and only then. An external envelope 22 envelops the sleeve 21 and forms with it a space through which axial primary air 5 can be supplied to aid combustion. The external envelope 22 comprises a portion 19 whose internal diameter is gradually reduced towards the burner nose, and the sleeve comprises a portion 27 whose external diameter gradually increases towards the burner nose so as to reduce the space between the nose of the external 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 speed of injection of the axial primary air 5 into its combustion chamber without having to provide a high flow of primary air axiai. The nose of the outer casing 22, the nose of the sleeve 21, the nose of the central duct 20 and the nose of said at least one additional duct 23 pass through a plane orthogonal to the axis 30 of the burner.

Figure 4b shows a perspective view of a first embodiment of a burner according to the invention. The sleeve 21 comprises a central duct 20 through which the pulverulent solid fuel is brought. The sleeve 21 further comprises an additional conduit 23 forming a thin annular space, through which combustible gas 26 can be supplied at the time of ignition of the furnace, and only at that time. An outer shell 22 (not shown in Figure 4b) envelops the sleeve 21 and forms a space through which axial primary air 5 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 duct 20 through which the pulverulent solid fuel is brought. The sleeve 21 further comprises a plurality of additional conduits 23 distributed around the central conduit 20, these additional conduits 23 through which combustible gas 26 can be supplied at the time of the ignition of the furnace, and only at that time. An outer jacket 22 (not shown in Figure 4c) wraps 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 reduction in 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 burner nose and keeping the external diameter of the sleeve 21 constant, or alternatively by increasing the external diameter of the sleeve 21 at the nose of the noisemaker while keeping the internal diameter of the envelope 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 casing 22, the external diameter of the sleeve 21 and the space between the sleeve 21 and the external casing 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 burner nose.

FIG. 5 represents a diagram 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 burner 30 of the burner is preferably located in the axis 30 'of the cylindrical combustion chamber 12. The combustion chamber 12 comprises at in addition to a combustion gas inlet 31 located tangentially 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 pipe

The conditions provided for the operation of a burner as described with the example of FIG. 3b, the power of which is 1.13 MW, in an annular furnace provided with 4 lower combustion chambers and the lime flow rate is 150t / day, are summarized in Table 2 below.

Table 2

* The oxidizer is in this case formed by recirculation gases.

The injection speed of the fuel transported by air is obtained by passage through the duct 20 which has a section of 0.001 m2, The "force" of the burner, that is to say its specific flow rate of momentum (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 there is therefore an unsatisfactory combustion with an oven having the drawbacks described above.

Provision has now been made, for an annular calcination furnace having the same lime flow rate of 150 t / day and provided with identical lower combustion chambers with burners of the same power, to reduce the specific flow rate of momentum of the burner, on the contrary of what the skilled person would have imagined on the basis of his knowledge. The new conditions applied are those indicated in Table 3.

Table 3

* The oxidizer 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 was changed, to almost half its value. Such a modification could be obtained by adapting the section of the duct 20, to a value of 0.002 m2. This minor modification induced the obtaining of a ratio between specific flow rate of momentum of the burner and specific flow rate of momentum of oxidant significantly less than 1. Surprisingly, it was then found that this simple modification gave place in a flame, initiated very quickly, as quickly as with natural gas, and especially that, now, it was the fuel which was sucked in the helical current 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 spray cone 4 and it is also sucked 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 a straight annular oven with 5 combustion chambers and whose flow rate is 30 liters / day. The operating conditions with a burner whose cross section of the fuel supply duct is 0.001 m2 are given in table 4.

Table 4

* The oxidizer is in this case formed of redrculating gases.

This result was found to be unsatisfactory for obtaining a satisfactory fuel-oxidant mixture in the combustion chamber and therefore complete combustion of the puerulent solid fuel therein.

By modifying the fuel injection speed, by enlarging the cross section of the injection pipe to 0.002 m2, the conditions given in table 5 below are obtained:

Table 5

* The oxidizer is in this case formed of recirculating 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.

One can for example add a clean movement to the burner, either by adding rotation fins in the powdery solid fuel circuit, or by adding rotation blades to the transport air circuit or to the axial primary air circuit, or still a combination of these measures. You can also add an additional air circuit at the periphery of the burner, which is rotated to help open the fuel spray 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 thereof in the combustion chamber, but before it is rotated.

It is also quite possible not to supply primary air to the burner, which can modify the values of the ratio claimed 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 become equal to 0.25. At a Vinj injection speed of 45 m / sec, it will then be 0.74.

Claims (12)

1. A method of burning fuel in a cylindrical combustion chamber, comprising, in this combustion chamber, - a projection, from a burner, of a jet of pulverulent solid fuel displaced by a transport air, and optionally of a primary air flow, 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, characterized in that 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 in that the ratio between specific flow rate of momentum of the burner and specific flow rate of momentum 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 momentum of the burner and the specific rate of momentum of the oxidizing gas is between 0.25 and 0.9. 3. Method according to either of claims 1 and 2, characterized in that the cylindrical combustion chamber has first and second axial ends 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. Method according to claim 3, characterized in that the oxidant gas is fed tangentially into the combustion chamber at said first end of the latter, so as to form a heiocidal 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 supplied to the combustion chamber at said first end of this, parallel to its axis and around the jet of fuel projected by the burner. 6. Method 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 transport air. 7. Method 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. Method according to any one of claims 1 to 7, characterized in that the oxidizing gas is a recirculated smoke gas. 9. Method according to any one of claims 1 to 8, characterized in that said combustion chamber is a lower combustion chamber of a straight annular furnace for mining rock mining. 10. Cylindrical combustion chamber comprising, at a first axial end, a burner arranged to project solid powdery fuel into this chamber 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, characterized in that the burner is arranged to project the solid fuel along 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 the implementation of the method according to any one of claims 1 to 9. 11. Straight annular furnace for calcining mineral rock, comprising at least one combustion chamber according to claim 10. 12. Oven right annuiaire of caicination of mineral rock, implementing a method according to any one of claims 1 to 11.