EP0323299B1 - Apparatus for ensuring a staged combustion of a fuel-oxidant mixture reducing emission of nox - Google Patents

Abstract

The invention relates to a process and an apparatus intended to effect the combustion of a mixture of air and liquid or gaseous fuel, wherein the injection of the fuel is staged without that of the air for combustion being staged. According to the invention, in addition to the primary means (20, 34) for injecting fuel into the passage (10, 14) supplying air to the burner, secondary injection means (24) mounted inside said passage are provided and comprise a main part (24a) which is extended by a terminal part (24b) bent in such a manner that the end of said terminal part is situated downstream of the end of the primary means (20, 34) and at a distance from the axis (2) of said passage which is greater than the minimum section of the latter. The invention is applicable both to so-called parallel air admission burners and to burners having swirl vanes.

Description

The present invention relates to burners for liquid or gaseous fuels intended to be mounted opposite an orifice provided for this purpose on the wall of an enclosure, such as the combustion chamber or the hearth of an oven or a boiler.

In general, the burner of the invention comprises an air box with a central axis, primary and secondary means for injecting fuel (s) passing through said air box, the secondary injection means comprising a part main extended at the location of an elbow by a terminal part which ends beyond the end of the primary injection means.

Such a burner is described in publication EP-A-76 036.

The object of the present invention is to provide improvements to this type of existing burners which make it possible to improve their efficiency, in particular in this context of reducing the nitrogen oxides produced, without requiring a complete change of the burner.

• la partie principale des moyens secondaires d'injection s'étend, en partie au moins, suivant un axe sensiblement parallèle à l'axe de la boîte à air, ou bien présentant avec lui un angle compris entre 5 et 30° environ,
• et la partie terminale de ces mêmes moyens secondaires d'injection se termine à une distance de l'axe central de la boîte à air supérieure à la distance minimale qui sépare ce même axe de celui de ladite partie principale des moyens secondaires d'injection.

To this end, the burner of the invention is characterized in that:

• the main part of the secondary injection means extends, at least in part, along an axis substantially parallel to the axis of the air box, or else having with it an angle of between approximately 5 and 30 °,
• and the terminal part of these same secondary injection means ends at a distance from the central axis of the air box greater than the minimum distance which separates this same axis from that of said main part of the secondary injection means.

Thus, without having to pierce the wall of the wall or that of the burner, it will be possible to obtain a staged combustion with a fuel injection which will take place towards the periphery and downstream of the first central zone of combustion, effectively reducing the amount of nitrogen oxides produced.

On many burners, with combustion instability and pollution problems, there is often that of delicate assembly or disassembly of the parts.

Another object of the present invention is therefore to provide a constitution of burners allowing easy mounting or dismounting, thereby increasing their flexibility of use.

It is for this purpose that, in accordance with another characteristic of the invention, can the secondary fuel injection means be pivotally mounted, their terminal part being able to have a shape and dimensions such that, by pivoting these means around the axis of the main part, the end of this terminal part will be brought inside a fictitious, cylindrical envelope substantially coaxial with the central axis of the air box and having a lower or equal section to the corresponding minimum section of the latter.

In addition to the burner as just presented, the invention also relates to such a burner mounted on the wall of its combustion chamber substantially opposite an axial orifice formed through the wall, this orifice in the wall connecting to the air box of the burner to form with it a single channel for the combustion air and the primary and secondary injection means extending in this channel.

In practice, to operate the burner of the invention, injecting by said secondary injection means, preferably at least 20% of the total fuel supplied to the burner.

If these lances are not supplied with fuel, while the burner is in service, they may advantageously be cooled by circulation of an incombustible gas.

According to a further complementary characteristic, these secondary means will advantageously extend at least in part, inside the orifice of the wall, near the wall which peripherally limits the latter, substantially following the shape of this wall. In this way we will reduce the turbulence problems, we will limit the deposits of unburnt on the lances and we will promote the efficiency of the downstream combustion zone.

• la figure 1 est une vue schématique en coupe longitudinale, par un plan sensiblement médian, d'un premier mode de réalisation du brûleur de l'invention,
• la figure 2 est une vue de face, suivant la flèche II de la figure 1, d'une partie du brûleur,
• les figures 3 à 6 sont des vues schématiques, en coupe longitudinale identique à celle de la figure 1, de diverses variantes de réalisation du brûleur de l'invention,
• la figure 7 est une vue suivant la flèche VII du brûleur de la figure 6,
• la figure 8 est une vue schématique en coupe partielle selon la ligne VIII-VIII du brûleur illustré sur cette même figure 6,
• et la figure 9 est une vue en coupe longitudinale identique à celle des figures 3 à 6, illustrant un autre mode de réalisation du brûleur de l'invention.

Other characteristics and advantages of the invention will appear from the following description made with reference to the accompanying drawings in which:

• FIG. 1 is a schematic view in longitudinal section, through a substantially median plane, of a first embodiment of the burner of the invention,
• FIG. 2 is a front view, along arrow II of FIG. 1, of part of the burner,
• FIGS. 3 to 6 are schematic views, in longitudinal section identical to that of FIG. 1, of various alternative embodiments of the burner of the invention,
• FIG. 7 is a view along arrow VII of the burner of FIG. 6,
• FIG. 8 is a schematic view in partial section along the line VIII-VIII of the burner illustrated in this same FIG. 6,
• and FIG. 9 is a view in longitudinal section identical to that of FIGS. 3 to 6, illustrating another embodiment of the burner of the invention.

It will now be noted that in the different figures, the same parts or elements have been identified in the same way.

This being so, let us first refer to FIGS. 1 and 2 where a first mode of illustration is illustrated. realization of a burner, of the turbulence type. To receive this burner, an orifice 14 has been formed in the wall 12 of a surrounding wall, for example oven or boiler. The orifice 14 here has a substantially frustoconical shape flaring inwardly marked inside 1 of the enclosure to be heated.

If we refer more particularly to Figure 1, we see that the burner in question is fixed (for example by screwing, not shown) on the outer face 12a of the wall 12 of the enclosure, so that its duct 10 air intake, which here has a shape converging in the general direction of flow towards the enclosure, is placed substantially coaxially with the axis 2 of the orifice 14 into which it opens and with whom it forms a circulation channel for the combustion air.

As illustrated, the duct 10 is placed in a box or a register 16 for supplying pressurized air with which it communicates.

In addition, it is closed at its upstream end by a wall 23 and the air, or more generally the oxidizing fluid chosen, passes from the box 16 into the duct by peripheral slots equipped with fins or vents (shown diagrammatically at 18 ) which communicate to the air a rotational movement creating a depression in the axis 2 of the burner and favoring the mixture of fuel and combustion air.

Figure 1, we identified AM and AV respectively upstream and downstream of the burner with reference to the direction of circulation of the fluids which flow from the rear to the front of this same burner. An arrow 3 indicates the direction in question.

The fuel is here, for the first part, supplied via a rod 20 which is disposed substantially in the axis 2 of the channel and forms the primary means of injection, thus defining a first combustion zone.

The injection end of the cane which is located approximately at the neck 4 of the air intake channel is equipped with a flame cone 22 whose role is to create turbulence in the air stream and promote combustion. The rod 20 is fixed to the rear wall 23 of the air box 16, for example by means of a flange or the like (not shown).

In order to stagger the fuel injection and thus define a second combustion zone located downstream of the first, the burner is also equipped with secondary injection means comprising so-called auxiliary tubular lances 24 which extend up to downstream of the aforementioned primary means (in this case the rod 20).

According to the invention, the auxiliary lances 24 are mounted inside the channel formed by the conduit 10 and the orifice 14. In addition, each of these lances 24 is shaped so as to comprise a main part 24a which s here extends substantially parallel to the axis 2 of the channel, upstream from its neck 4, and an end portion 24b which extends beyond the neck, opposite the orifice 14, this main portion relative to which it is bent , so as to move away from the axis 2. Preferably, the end portion 24b will extend, close along the walls of the orifice 14, substantially parallel to one of its generatrices.

Figure 1 it is clearly seen that in section the distance D between the injection end of the auxiliary lances and the axis 2 of the air supply channel is greater than the distance R at the neck 4 of this same channel.

To ensure a directed injection of fuel at this second combustion zone, one or more orifices 26 will be drilled towards the end of the lances 24 and the axes of these orifices will be oriented (angle α in FIG. 1) so that the part of the fuel injected at this level either according to the frustoconical envelope surface or in the form of a hyperboloid, convergent or divergent depending on the type of combustion favored.

It will be noted that the section of the orifices 26 will determine the speed of exit of the fuel which will generally be between about 10 and 200 meters per second.

In practice, the number of auxiliary lances 24 will generally be between 3 and 16 and these lances will most often be regularly distributed around the primary injection rod 20 (FIG. 2). That said, in order to facilitate reading, FIG. 1 shows only two of the auxiliary lances.

In general, the flow of fuel, most often gaseous, injected by means of these auxiliary lances 24 will be between approximately 10 and 60% of the total flow of fuel supplied to the burner.

Preferably, the lances 24 will be fixed individually in their "working" position on the rear wall 23 of the air box, for example by means of flanges (not shown). In this way, after removing the attachment, it will be possible to pivot the lances 24 so as to place them in a "folded" position illustrated in dashed lines in FIG. 2. The end part of these lances will then be, as in seen, located inside a fictitious substantially cylindrical envelope coaxial with the air supply channel and having a lower section, or at the equal limit, to the section at the neck 4 of the latter. According to the invention, in order to allow easy removal of the lances 24 from the burner, after having placed them in their said folded position, the length l₁ separating, in a plane 5 substantially perpendicular to the axis 2, the main parts of two adjacent auxiliary lances will be at least slightly greater than the length l₂ of their terminal part, projected in this same plane 5 (see FIG. 2).

Thus, in the "folded" position, the end portion 24b of the lances can easily pass between the neck 4 of the channel and the widest edge of the flame cone 22 and it is understood that by pulling the lance considered towards the rear of the burner, substantially in axis 2, it will be possible to remove it easily without stopping the burner. Of course, there will then be provided in the wall 23 passages of suitable size and shape.

Let us now refer to FIG. 3 to see illustrated a second embodiment of the invention which differs from the previous one essentially by the shape of the cane 20 for primary injection of fuel (for example gas) which ends, at the level from the neck 4 of the air supply channel, by a series of injection fingers 50 arranged substantially radially, in a star.

It will be noted that the burner illustrated is also of the type with peripheral air intake comprising a register with turbulence fans.

The arrangement and shape of the auxiliary lances 24 for secondary fuel injection which are distributed in the air channel, around the rod 20, are entirely comparable to those of the previous embodiment.

FIG. 4 shows a third alternative embodiment of the invention which has the particularity, compared to the previous ones, of its primary fuel injection system, the shape of its auxiliary lances and that of the orifice which has been formed in the wall to receive the burner.

More specifically, the primary injection rod 20 is here disposed inside a tube 60 from which it opens only at its injection end 20a and into which a fuel, liquid or gaseous, other than the one who feeds the cane. The latter being supplied with combustible gas, the tubular conduit 60 could, for example, receive fuel.

Regarding the orifice 14 formed in the wall, this has, in the considered direction of flow, first of all a substantially cylindrical shape which then connects to a sort of truncated hyperbola going "to flare" to enclosure 1.

As for the auxiliary lances 24 of which only two have been shown, their main part 24a extends up to the level of the entrance forming a neck 4 of the orifice 14 along concurrent axes 61 moving in closer towards the orifice, their end portion 24b then conforming substantially to the shape of the walls of this same orifice, inside which it opens.

In practice, the angular difference γ between the axes 61 of the main part of the lances and that 2 of the rod 20 and the tube 60 will be relatively small and, in general, between approximately 5 and 30 °.

It will be noted that the burner illustrated in this FIG. 4 is also of the type with peripheral air admission comprising an air box 16 provided with turbulence vents.

Referring to Figure 5, there is shown a fourth embodiment of the invention.

This variant can be compared to that of FIG. 1 by the frustoconical shape of the walls of the orifice 14, by the provision in the axis 2 of the air intake channel of a cane 20 for primary fuel injection. which opens onto the canal at its neck 4 and by the arrangement of the auxiliary lances 24 (of which again only two have been shown). In addition, it is still a burner of the peripheral air intake type, but here comprising two air boxes 16, 16 ', the duct 10' of one, 16 ', opens into the duct 10 on the other 16.

This being so, the present embodiment is essentially distinguished by the provision of a tubular crown 70 in the form of a torus which extends at the periphery of the air intake channel at the connection between the duct 10 and the orifice 14.

In practice, the crown 70 which is connected to a fuel supply tube 71 will be used to inject this same fuel through orifices (not shown) provided for this purpose, substantially at the level where the rod 20 injects its own, defining thus the primary combustion zone.

With such a burner, it could for example provide for injecting combustible gas through the crown 70 and fuel through the cane 20, the auxiliary lances 24 being able to inject gas into the second combustion zone.

Let us now refer to Figures 6 to 8 to see illustrated a fifth embodiment of the invention.

The burner shown here is of the multilance type with axial or parallel air flow.

More specifically, it is a burner comprising a box or box 30 supplied with pressurized air and opening onto the duct 10, without the air having been subjected to rotation, as was the case in the modes previous work.

In addition, the present burner is characterized in that it comprises, to ensure the primary injection of the fuel and in addition to its central rod 20 around which they are distributed, so-called "primary" lances 34.

In the example illustrated, the primary lances in question are each inserted between two adjacent auxiliary lances 24 (FIG. 7) and open, slightly downstream from the injection end 20a of the rod 20, inside the orifice 14, which here has a substantially circular cylindrical shape with its cylinder axis disposed in axis 2.

Figure 6 shows only one of the primary spears. It will be noted that the latter extends substantially parallel to the axis 2 of the channel and close to its walls. It will also be noted that the rod 20 carries, which surrounds it, a sort of disc 33 provided with fins intended to ensure a rotation of the air around the axis 2.

Given the shape of the orifice 14, the auxiliary lances 24 are here mounted so as to extend by their main part 24a, substantially parallel and close to the walls of the orifice, up to the level of the face 12b from the wall 12 oriented towards the inside of the enclosure, and to be extended by their end portion 24b inside this same enclosure, moving away of axis 2.

In practice, this terminal part which may be rectilinear or curvilinear (see FIG. 7) will extend substantially parallel to the internal wall 12b of the wall of the enclosure.

In the case under consideration, the terminal part of the lances 24 is oriented, in the same plane, at approximately 90 ° from their main part.

As can be clearly seen in FIG. 6, each of the terminal parts in question may be extended by a short end part 24c directed obliquely and comprising at least one injector 31 with its orifice 32 for ejecting fuel. The oblique orientation of this end part may in particular be located in the plane containing the main part 24a of the lances and the junction point between their end and end parts (angle α, FIG. 6).

Also, this same end part 24c could extend along an axis directed obliquely to the plane containing the main part of the auxiliary lances and the junction point between the end and end parts (angle β, FIG. 8). In this case, the fuel injection angle at the auxiliary lances will therefore be given by the orientation of the end part in question and conditioned by the angles α and β chosen.

Nevertheless, one could have provided end portions 24b without end portion, the fuel injection angle then being given at this level by the orientation (angles α and / or β) of the orifice 32 of the injectors 31.

In practice, taking as an axis parallel to that of the air intake channel, an angle α between -40 ° and + 70 ° and an angle β between -60 ° and + 60 ° will ensure , depending on the type of combustion chosen, a convergent or divergent injection of the fuel jets at the end of the auxiliary lances.

Regarding the dimensions of the parts terminal (and possibly extreme), we will preferably adapt, as for the previous embodiments, the length l₁ which separates the main parts of two adjacent auxiliary lances, so that it is at least slightly greater than the length l₂ of their terminal part (possibly extended by the extreme part).

In the present case, it will also be preferable to provide, still in section, a length l₃ separating two adjacent primary lances at least slightly greater than the length l₂ (FIG. 7).

Thus, if care is taken to fix the lances on the burner by providing in the rear wall 23 'of the conduit 10 grooves shaped to allow the auxiliary lances 24 to rotate around the general axis 2 of the inlet channel air, independently of the primary lances 34, it will suffice to remove them from the burner, to rotate them on the one hand around the axis of their main part 24a, so as to place them in the "folded" position ( illustrated in dashed lines in FIG. 7) and, on the other hand around the axis 2, so that they can be removed towards the rear of the burner without them coming up against the primary lances.

In this regard, it will be noted from the study of FIG. 7, that in the folded position, the terminal part (possibly extended by the end part) of the auxiliary lances will then extend between the wall 15 of the orifice 14 and the outer edge 35 of the turbulence disc 33.

Let us finally look at Figure 9 which illustrates a sixth embodiment of the burner of the invention.

Like the previous one, this burner is of the type with primary multi-lances and with parallel air intake.

It therefore comprises a series of primary lances 34 distributed around a central rod 20 (although the provision thereof is not essential) and the pressurized air which passes from the box 30 into the duct 10 and then into the orifice 14 does not undergo, at this level, any rotation movement.

As illustrated, the orifice 14 which has been formed in the wall 12 of the enclosure has here a first frustoconical shape, convergent in the direction of flow then, substantially cylindrical circular of axis 2 then finally frustoconical divergent. As a result, both the primary lances 34 and the main facing part 24a of the auxiliary lances which extend up to the level of the neck of the orifice 14 are directed in concurrent directions going towards this same orifice. As already indicated with reference to Figure 4, the angular deviation will in this case generally be between 5 and 30 ° approximately.

Since FIG. 9 a central rod 20 has been provided, it will be noted that it opens into the orifice 14 slightly downstream of the injection end of the primary lances 34.

From the study of this figure 9 it will also be noted, once again, that, still for the sake of clarity, only a primary lance and an auxiliary lance have been represented, although their arrangement is in practice and in general that in Figure 7.

With regard to the auxiliary lances 24, it will be noted that, over their length situated opposite the walls of the orifice 14, they extend in accordance with the invention, towards the periphery of the orifice, of which they substantially follow the wall contour and that, given its first shape "neck", they extend into the enclosure 1 by an end portion 24b and an end portion 24c of the type described in relation to Figures 6 to 8, the part terminal 24b extending parallel to the wall 12b of the wall.

The various mainly envisaged embodiments of the burner of the invention having been described, we will now present the general principle of operation.

Before doing so, it seems important to note, at this stage of the description, that all the envisaged embodiments are capable of being combined or associated with each other.

That said, any of these embodiments operates according to the invention, so that at a given time of combustion, the burner is supplied with a total quantity of air, under suitable pressure, at least sufficient for burning, under substantially stoichiometric conditions, a total amount of fuel (s).

The supplied combustion air will therefore circulate in its entirety inside the channel formed by the duct 10 and the orifice 14, before leading to the hearth of the enclosure 1.

If the invention does not provide for a staging of the intake of combustion air, it does however provide for a staging of the fuel injection. To this end, we will therefore use the primary injection means (20 and / or 34) to inject, at a first level located upstream of the hearth, part of the total amount of fuel necessary to obtain combustion under substantially stoichiometric conditions. with the air in circulation, while the remaining quantity of fuel is injected by the auxiliary lances 24 at a second level located downstream of the first, so as to then substantially reach the stoichiometric combustion conditions. At the level of this second zone, combustion will therefore take place between the excess air which has not been consumed at the level of the first combustion zone and the part of the fuel which has been injected by the auxiliary lances. Given the relative arrangement of the primary and secondary injection means, it is clear that the flame resulting from the secondary combustion will envelop and extend, inside the enclosure, the flame resulting from the primary combustion.

It will have been noted that in the figures, the ignition means of the burner have not been shown. It has been omitted for the sake of clarity. However, it must be clear that such means will indeed be provided so that the air-fuel mixture can be ignited at the level of the two aforementioned combustion zones. As known per se, these means could for example consist of electrodes suitably insulated and supplied with suitable high voltage.

Finally, it will be noted that if, in general, the air supply pressure of the burner will be higher than atmospheric pressure, it could also be envisaged to supply this same burner with subatmospheric pressure.

Claims (11)

1. Burner for liquid or gaseous fuels comprising an air box (16, 16', 30) having a central axis (2), primary means (20, 34) and secondary means (24) for injecting fuel(s) passing through the said air box, the secondary injection means (24) comprising a main part (24a) extended at the location of a bend by an end part (24b) which terminates beyond the end (20a, 22, 50) of the primary injection means (20, 34),
   characterised in that:
   the main part (24a) of the secondary injection means (24) extends at least partially in a direction (61) which is substantially parallel to the axis (2) of the air box, or which is at an angle of between approximately 5° and 30° thereto;
   and the end part (24b) of these secondary injection means terminates at a distance (D) from the central axis (2) of the air box which is greater than the minimum distance (D - 1₂) separating this axis (2) from that of the main part (24a) of the secondary injection means.
2. Burner according to Claim 1, characterised in that the secondary means for injecting fuel(s) (24) is pivotably mounted on the burner, and the end part (24b) thereof has a form and dimensions such that when these means are pivoted about the axis of the main part (24a), the end of the end part (24b) is drawn into an imaginary cylindrical envelope, which is substantially coaxial to the central axis (2) of the air box and has a cross-section smaller than or equal to the minimum corresponding cross-section (2R) of the latter.
3. Burner according to Claim 1 or Claim 2, characterised in that the end part (24b) of the secondary injection means (24) is extended by an end part (24c) disposed obliquely relative to the said end part.
4. Burner according to any one of the preceding Claims, characterised in that the primary injection means (20, 34) comprise a plurality of nozzles (34) disposed about a central injection pipe (20) extending substantially along the axis (2) of the air box, these nozzles each being interposed between two consecutive secondary injection means (24).
5. Burner according to any one of the preceding Claims, characterized in that the end part (24b) of the secondary injection means (24) is curved.
6. Burner according to any one of the preceding Claims, characterized in that the end part (24b) of the secondary injection means extends substantially perpendicularly to the main part thereof (24a).
7. Combustion unit comprising a burner according to any one of the preceding Claims, mounted on the wall (12) of a combustion chamber (1), substantially opposite an axial aperture (14) which passes through the wall, and / or the aperture (14) in the wall is connected to the air box (16, 16', 30) of the burner in order to form therewith a single duct for the combustion air, the primary means (20, 34) and secondary means (24) of injection extending in this duct.
8. Unit according to Claim 7, characterised in that the secondary injection means (24) extends at least partially into the aperture (14) in the wall, in the vicinity of the wall which delimits the periphery of the wall, substantially following the outline of this wall.
9. Unit according to either of Claims 7 or 8, characterised in that the secondary injection means (24) passes entirely through the aperture (14) in the wall (12) of the chamber and extends inside this chamber.
10. Unit according to either of Claims 7 or 8, characterised in that the main part (24a) of the secondary injection means (24) extends through the aperture (14) in the wall substantially as far as the surface (12b) of this wall which faces the inside of the chamber (1) and is extended inside this chamber by the end part (24b).
11. Unit according to any one of Claims 7 to 10, characterised in that the end part (24b) of the secondary injection means extends about the aperture (14) of the wall substantially parallel to the surface (12b) of this wall which faces the inside of the chamber (1).

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