BE1023896B1 - METHOD FOR FUEL COMBUSTION IN A TUBULAR COMBUSTION CHAMBER - Google Patents

Abstract

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

Description

Fuel combustion method in a tubular combustion chamber

The present invention relates to a fuel combustion method in a tubular combustion chamber, comprising, in this combustion chamber, an axial projection, from a burner, of a pulverulent solid fuel jet displaced by a transport air, and possibly a primary air flow, and - a supply of an oxidizing gas so as to form a stream of oxidizing gas around the jet of fuel projected by the burner, at a temperature causing a 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 use, 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. The supply of existing burners in the lower combustion chambers of annular calcination furnaces known by such solid powdery fuel has, however, proved, in experimental tests carried out by the applicant, difficult to 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 in front of a heterogeneous combustion (solid-gas) where the law of "immediately mixed, immediately burned" is no longer applicable. The burning time is much higher and depends on many 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 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 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, in which the ratio between the specific flow rate of the burner momentum and the specific flow rate of the oxidizing 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_burner = (Qmcs + Qmat) x Vinj / P + Qmap x Vp / 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 velocity of the primary air, and P = burner power (MW).

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

Qvat = actual flow rate of the transport air (m3 / sec), and Sb = straight section of the fuel injection duct in the burner (m2). 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 of the oxidant 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 (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 jet of fuel to suck the oxidant arriving at its periphery, thus 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 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 burner nose, with immediate effect on the ratio claimed between specific flow rates that are 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 tubular combustion chamber has a first and a second axial ends 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.

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 for axially projecting a powdery solid fuel jet into this chamber and optionally an axial primary air flow and a feed inlet for a oxidizing gas arranged to form a stream of oxidant gas around the jet of fuel projected by the burner, so as to allow the implementation of the method according to the invention. It also relates to an annular calcic or dolomitic calcination furnace, comprising at least one such combustion chamber and an annular calcined calcareous or dolomitic 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 an annular right annealing furnace.

FIG. 3 represents an axial sectional view of a burner that can be used for carrying out the method according to the invention.

FIG. 4 represents an axial sectional view of an annular straight calcination furnace provided with lower combustion chambers embodying the process 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 provided for the operation of burners of this type, whose power is 66 MW, in a rotary kiln whose flow rate is 110Ot / 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. 4, 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 gases of the lower combustion chambers 12 is pulled downwards by a depression created at the inlet openings 14 provided in the inner cylinder 7, which is lower than the combustion chambers 12. calcination zone by cocourant. At the level of the gills the fumes of the co-current calcination zone are mixed with the cooling air introduced at the bottom of the oven at 15. This mixture forms the recirculating fumes which, at 16, are recovered from the inner cylinder 7 and brought back to the lower combustion chambers 12 to become the oxidizing gas. Through a conduit 17, this oxidizing gas 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 acquires a rotational movement which induces a centrifugal force pushing the oxidizing gas towards the walls of the tubular 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.

In Figure 3 there is shown the nose 19 of one of the burners used. This has a central duct through which the pulverulent solid fuel is fed. A cylinder 21 surrounds the conduit 20 so as to release a thin annular space, through which fuel gas can be supplied at the time of ignition of the furnace, and only at that time. Finally, between the outer shell 22 of the nose of the burner and the cylinder 21, axial primary air can be fed to aid combustion.

The conditions provided for the operation of such a burner, whose power is 1.13 MW, in an annular furnace with 4 lower combustion chambers and whose lime flow rate is 150 t / day, are summarized in the table. 2 below.

Table 2

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

The injection rate 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 m2. This minor modification induced a ratio between the specific flow rate of the burner momentum and the specific flow rate of the oxidant significantly less than 1. Surprisingly, it was found that this simple modification gave It is a flame, initiated very quickly, as quickly as with natural gas, and especially now that it was the fuel that was sucked into the helical stream 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

* 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 (12)

A method of fuel combustion in a tubular combustion chamber, comprising, in this combustion chamber, an axial projection, from a burner, of a powdery solid fuel jet displaced by a transport air, and optionally a primary air flow, and - a supply of an oxidizing gas 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, characterized in that the ratio between the specific flow rate of the burner momentum and the 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. Method according to either of claims 1 and 2, characterized in that the tubular combustion chamber has a first and a second axial ends and in that the powdery solid fuel jet 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 thereof, 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 tubular combustion chamber comprising, at a first axial end, a burner arranged to axially project a powdery solid fuel in this chamber and a feed inlet for a combustion gas arranged to form a stream of oxidant gas around the jet. fuel jet projected by the burner, this chamber being arranged for the implementation of 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.