ES2915900T3 - Fuel combustion procedure in a cylindrical combustion chamber - Google Patents

Abstract

Fuel combustion process in a cylindrical combustion chamber (1), comprising, in this combustion chamber, - a projection, from a burner (2), of a jet of powdery solid fuel (3), displaced by a conveying air, and possibly a primary air flow, and - a supply of an oxidizing gas (5) in a direction of propagation so that a stream of oxidizing gas is formed around the jet of fuel projected by the burner, at a temperature that causes combustion of the fuel, characterized in that the jet of solid fuel has an axial component of projection in the same direction as said direction of propagation of the oxidizing 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 oxidizing gas is equal to or less than 1.0 and greater than zero.

Description

DESCRIPTION

Fuel combustion procedure in a cylindrical combustion chamber

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 pulverulent solid fuel jet displaced by conveying 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 that causes the combustion of the fuel.

In the field of calcination of mineral rocks, in particular calcareous and dolomitic rocks, different types of kilns are used, namely rotary kilns, shaft kilns and in particular vertical annular kilns.

These vertical annular furnaces use upper and lower combustion chambers to heat the material. The lower combustion chambers are originally designed to run on natural gas as fuel and it burns almost instantly.

However, it is increasingly desirable to be able to replace fuel gas with a less expensive fuel in these furnaces currently in service, in particular a pulverulent solid fuel such as powdered coal, coke or lignite, grape seeds, olive pits , sawdust, etc

An example of a burner not according to the invention is shown schematically in axial section in figure 3a and in perspective in figure 4a. The burner comprises a fuel conduit 120 surrounded by a cylindrical sleeve 121 comprising a portion 127 widened towards the tip end of the burner and comprising a plurality of holes 128. The cylindrical sleeve 121 forms with the conduit 120 an annular space at through which a combustible gas 126 passes. The sleeve 121 and conduit 120 are included in an outer casing 122 (shown in Figure 3a, not shown in Figure 4a) such that the tip of the conduit 120 protrudes from the enlarged portion 127 of the sleeve 121 and the tip of the outer casing 122, that the enlarged portion 127 of the sleeve is recessed with respect to the tip 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 through the plurality of holes 128 in the enlarged portion of the sleeve.

However, feeding the burners of the lower combustion chambers of the annular calcination furnaces with a powdery solid fuel of this type has proven, during experimental tests carried out by the applicant, hardly appropriate. In fact, the combustion is incomplete, which leads to a 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 waste products are recovered. hot combustion gases. This translates into a deterioration in the quality of the baked product (loss of reactivity) and in productivity (frequent stops of the oven to clean it). And it becomes necessary to continue using fuel gas in combination with powdery solid fuel to avoid these problems. Therefore, the expected price reduction is greatly reduced.

It should be noted that the lower combustion chambers of vertical annular furnaces are small and short. They are sized for natural gas that burns instantly according to the law of homogeneous combustion (gas-gas combustion) “as soon as it is mixed, it burns”. The air necessary for combustion also arrives in these chambers, previously mixed with recirculated combustion gases, which have a reduced oxygen concentration.

When solid fuel is projected into the combustion chamber, the situation is different, a heterogeneous combustion (solid-gas) occurs where the law of “as soon as it is mixed, it burns” no longer applies. The burning time is much longer and depends on many factors, such as the size of the particles, the reactivity of the solid surface, the availability of oxygen near the solid surface.

Therefore, simply replacing gaseous fuel with solid fuel in existing combustion chambers has turned out to be really problematic.

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

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

The purpose 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 efficient with the consumption of only powdery solid fuel.

To solve this problem, a combustion procedure has been provided as indicated at the beginning, in which the solid fuel jet has an axial component of projection in the same direction as said direction of propagation of the oxidizing gas in the cylindrical combustion chamber. and wherein the ratio of the burner specific momentum rate to the oxidizer gas specific momentum rate is equal to or less than 1.0 and greater than zero. Specific momentum ratio is the measure of the force of a jet (for example, burner jet or oxidizer stream) divided by burner power.

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

Gax_burner = (Qmcs Qmat) x Viny / P Qmap x Vap / P, where

Qmcs = solid fuel mass flow rate (kg/s),

Qmat = mass flow rate of conveying air (kg/s),

Qmap = primary air mass flow rate (kg/s),

Viny = axial fuel injection velocity (m/s)

Vap = primary air axial injection velocity, and

P = burner power (MW).

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

for the fuel

Viny = Qvat / Sb, where

Qvat = actual volumetric flow rate of conveying air (m3/s), and

Sb = straight section of the fuel injection pipe in the burner (m2).

for primary air

Vap= Qvap / Sap

Qvap = actual volumetric flow rate of primary air (m3/s)

Sap = straight section of the primary air injection duct in the burner (m2).

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

P = Qmcs x PCI, where

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

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

Gax_comburente = Qmgc x Vgc / P, where

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

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

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

Vgc = Qvgc/Sch, where

Qvgc = volumetric flow rate of the oxidizing gas (m3/s), and

Sch = straight section of the combustion chamber (m2).

The basic principle in burner design is that a burner must have a significant and sufficient momentum rate (injection velocity x mass flow rate) so that the central jet of fuel can attract the comburent arriving at its periphery, thus forcing mixing. of fuel / oxidant, which accelerates combustion. Therefore, the aerodynamics of a traditionally designed flame are determined by the burner itself (see Figure 1).

On the contrary, the method according to the present invention is based on aerodynamics that are determined by the oxidizer that reaches the combustion chamber. Here, the oxidant forces the fuel into its current by adapting the torque of the burner to that of the oxidizer (see figure 2). Therefore, it is no longer the fuel jet that is the driving force, it is the fuel that is dragged through the oxidizer. This results in a longer residence time of the fuel, having as an effect the possibility of using a fuel only in powdery solid form and obtaining a total combustion of this fuel in the combustion chamber.

To adapt this burner torque rate, it is possible, for example, to envisage increasing the fuel injection section at the burner tip, which has the immediate effect of reducing the fuel injection speed while keeping the fuel flow rates unchanged. 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 of the burner tip, with an immediate effect on the claimed ratio of specific moment rates which is adapted to be equal to or less than 1.0. Preferably, this ratio will be between 0.5 and 0.9.

According to an embodiment of the method according to the invention, the cylindrical combustion chamber has first and second axial ends and the pulverulent solid fuel jet 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. Therefore, the solid fuel jet can come into contact with the oxidant throughout the 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 means of an air supply.

Advantageously, the combustion gas is fed tangentially into the combustion chamber through said first end thereof, so that a helical current of combustion gas is formed around the jet of fuel projected by the burner. This favors the mixing of fuel-oxidant. Naturally, provision can also be made for the combustion gas to be fed into the combustion chamber through 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.

In order to further promote fuel-oxidant mixing, provision can be made, according to the invention, for a partial or total rotation of the jet of fuel transported by conveying air. This can be obtained, for example, by providing a rotational movement to the conveying air, with the help of guide fins.

The process according to the invention is preferably intended to be carried out in a lower combustion chamber of a vertical annular furnace for calcining calcareous 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 powdery solid fuel into this chamber, and optionally an axial primary air flow, and an inlet supply for an oxidizing gas arranged in such a way that a stream of oxidizing gas is formed 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 projection component having the same direction as the propagation direction of the oxidizing gas current in the cylindrical combustion chamber, so that the method according to the invention can be put into practice. It also relates to a vertical annular kiln for calcining calcareous or dolomitic rock, comprising at least one combustion chamber of this type, as well as a vertical annular kiln for calcining calcareous or dolomitic rock, implementing a method according to the invention.

The invention will now be described in more detail with reference to the accompanying drawings provided in a non-limiting manner.

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

Figure 2 schematically represents a projection according to the invention of a pulverulent solid fuel in a combustion chamber, for example of a vertical annular calcination furnace.

Figure 3a represents a schematic view according to a longitudinal section of a burner that is not according to the invention.

Figure 3b represents an axial sectional view of a burner that can be used to implement the process according to the invention.

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

Figure 4b represents a schematic perspective view of an embodiment of a burner that can be used to implement the method according to the invention.

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

Figure 5 represents an axial sectional view of a vertical annular calcination furnace provided with lower combustion chambers that implements 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 that are fed only with powdery solid fuel. The conditions foreseen for the operation of burners of this type, whose power is 66 MW, in a rotary kiln whose flow rate is 1100 t/day, are summarized in table 1 below.

Table 1

*The oxidant in this case is air.

As can be seen, the specific momentum rate of the burner (carbon transport air) is much higher than that of the comburent.

In figure 1 this combustion chamber 1 is schematically illustrated. 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 burner tip has a very conical shape. Thanks to this high injection speed, the oxidant 5, fed around the fuel jet, is sucked into the latter.

As can be seen in Figure 5, a conventional annular vertical kiln for calcining calcareous rock 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 at the top of the furnace at 9 and the fired product is discharged at the bottom at 10. The fuel is injected at two levels, through several upper and 12 lower combustion chambers 11 (from 4 to 6 chambers depending on oven capacity). In general, 1/3 of the fuel is injected into the chambers 11 and 2/3 into the chambers 12. All the fumes from the upper chambers 11 and part of the fumes from the lower chambers 12 are sucked upwards by a fan 13 of suction, therefore countercurrent with respect to the movement of the material load. Countercurrent calcination takes place in this zone. The other part of the combustion gases from the lower combustion chambers 12 is sucked downwards by a depression created at the level of the return 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 into the lower part of the furnace at 15. This mixture forms the recirculation fumes which, at 16, are recovered from cylinder 7 interior and are carried to the lower combustion chambers 12 to be converted into the combustion gas. Through a conduit 17, this oxidizing gas 31 reaches each of the chambers 12 tangentially to the axis of the chamber and therefore to the burner jet 18 injected axially. As a result, the oxidizing gas 31 acquires a rotational movement that induces a centrifugal force that pushes the oxidizing gas 31 towards the walls of the cylindrical combustion chamber.

Subsequently, experimental tests were carried out to apply to each of the combustion chambers of such a conventional annular calcining kiln a burner feed with only pulverulent solid fuel.

Figure 3b represents 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 fed at the time of ignition of the furnace, and only at that time. An outer casing 22 surrounds the sleeve 21 and forms with it a space through which axial primary air 5 can be fed to aid combustion. The outer casing 22 comprises a portion 19 whose internal diameter progressively reduces towards the burner tip, and the sleeve comprises a portion 27 whose external diameter progressively increases towards the burner tip such that the space between the casing tip 22 outer and the tip of the sleeve 21. This reduction of space between the outer casing 22 and the sleeve allows to increase the speed of injection of the axial primary air 5 in the combustion chamber without having to provide a high flow rate of axial primary air. The tip of the outer casing 22, the tip of the sleeve 21, the tip of the central duct 20 and the tip of said at least one additional duct 23 pass through a plane orthogonal to the axis 30 of the burner.

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

Figure 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 pulverulent solid fuel is fed. The sleeve 21 further comprises a plurality of additional ducts 23 distributed around the central duct 20, additional ducts 23 through which fuel gas 26 can be fed at the time of ignition of the furnace, and only at this time. An outer cover 22 (not shown in Fig. 4c) surrounds the sleeve 21 and forms a space through which axial primary air 5 can be fed to aid combustion.

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

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 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 interior of the casing 22 are adapted to allow the axial primary air to exit at a predefined speed through the tip of the burner.

Figure 5 represents a schematic of an annular furnace and comprises 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 housing 22 of burner 18 and burner axis 30 is preferably located on axis 30' of 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 duct.

The conditions foreseen for the operation of a burner such as the one described with the help of the example of figure 3b, whose power is 1.13 MW, in an annular furnace equipped with 4 lower combustion chambers and whose lime flow rate is 150 t/day, are summarized in table 2 below.

Table 2

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

The injection speed of the airborne fuel is obtained by passing through the conduit 20 having a section of 0.001 m2. The “force” of the burner, that is, its specific moment rate (primary air axial coal transport air) is still slightly greater than that of the oxidizer, but it is insufficient to suck the oxidizer into the fuel. It should not be compared to that of the rotary kiln described above. And therefore unsatisfactory combustion is observed with a furnace having the above-described disadvantages.

It has now been envisaged, for a ring calcining kiln with 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 moment rate of the burner, unlike what the person skilled in the art would have imagined based on his knowledge. The new conditions applied are those indicated in Table 3.

Table 3

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

As can be seen, only the injection speed of the pulverulent solid fuel displaced by the transport air has been modified, up to almost half of its value. Such a modification can be obtained by adapting the section of the duct 20 to a value of 0.002 m2. This small modification has led to obtaining a ratio between the specific momentum rate of the burner and the specific momentum rate of the oxidizer clearly less than 1. Surprisingly, it was then observed that this simple modification gave rise to a flame, started very quickly , as quickly as with natural gas, and above all that, now, it was the fuel that was sucked into the helical current of combustion gas.

This phenomenon is schematically represented in figure 2. Given its low injection speed 3, the fuel projected by the burner 2 forms a more open projection cone 4 and is also sucked in by a current of comburent gas that becomes a driving force.

An identical experiment was carried out with a burner, whose power is 1.81 MW, in a vertical annular furnace equipped with 5 combustion chambers and whose flow rate is 300 t/day. The operating conditions with a burner whose section of the fuel supply pipe is 0.001 m2 are given in table 4.

Table 4

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

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

Modifying the fuel injection speed, enlarging the section of the injection duct to 0.002 m2, the conditions given in table 5 below are obtained:

Table 5

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

This arrangement makes it possible to drastically increase the residence time of the particles in the combustion chamber and therefore oxygen is more available and combustion is complete inside the combustion chamber. It is to be understood that the present invention is in no way limited to the embodiments set forth above and that many modifications may be made thereto without departing from the scope of the appended claims.

For example, it is possible to add a specific movement to the burner, either by adding rotation fins in the pulverulent solid fuel circuit, or by adding rotation fins to the conveying air circuit or to the axial primary air circuit, or even another combination of these measures. It is also possible to add an additional rotating air circuit to the burner periphery to help open the fuel projection cone in the chamber.

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

It is also entirely conceivable not to supply primary air to the burner, which may modify the values of the claimed ratio with respect 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 a Viny injection speed equal to 15 m/s, the claimed ratio can even be equal to 0.25. At a Viny injection speed of 45 m/s, it will then be 0.74.

Claims (12)

1. Fuel combustion process in a cylindrical combustion chamber (1), comprising, in this combustion chamber, - a projection, from a burner (2), of a powdery solid fuel jet (3), displaced by conveying air, and possibly by a primary air flow, and - a supply of an oxidizing gas (5) according to a direction of propagation so that a current of oxidizing gas is formed around the jet of fuel projected by the burner, at a temperature that causes the combustion of the fuel, characterized in that the jet of solid fuel has an axial component of projection in the same direction as said direction of propagation of the comburent gas in the cylindrical combustion chamber, and in that the ratio between the specific momentum rate of the burner and the rate of specific moment of the oxidizing gas is equal to or less than 1.0 and greater than zero. Method according to claim 1, characterized in that the ratio between the specific momentum rate of the burner and the specific momentum rate of the comburent gas is between 0.25 and 0.9. 3. Method according to one or the other of claims 1 and 2, characterized in that the cylindrical combustion chamber has a first and a second axial end and in that the powdery solid fuel jet 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 combustion gas is fed tangentially into the combustion chamber through said first end thereof, so that a helical current of combustion gas is formed around the jet of fuel projected by the burner. . Method according to claim 3, characterized in that the combustion gas is fed into the combustion chamber through said first end thereof, parallel to its axis and around the jet of fuel projected by the burner. Method according to any one of Claims 1 to 5, characterized in that it comprises a partial or total rotation of the jet of fuel transported by the conveying air. 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. Process according to any one of claims 1 to 7, characterized in that the comburent gas is a recirculated combustion gas. Method according to any one of claims 1 to 8, characterized in that said combustion chamber is a lower combustion chamber of a vertical annular furnace for calcining mineral rock. 10. Cylindrical combustion chamber (1) comprising, at a first axial end, a burner (2) arranged to project a powdery solid fuel (3) into this chamber and a supply inlet for an oxidizing gas (5) arranged so that a stream of oxidizing gas is formed according to 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 component of projection that has the same direction that 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 one of claims 1 to 9. 11. Vertical annular furnace for the calcination of mineral rock, comprising at least one combustion chamber according to claim 10. 12. Implementation of a process according to any of claims 1 to 9 in a vertical annular furnace for the calcination of mineral rock.