EP0384277B1 - Method and combustion installation for the reduction of nitrogen oxide formation during the combustion of fossil fuels - Google Patents

Abstract

In the method for reducing the nitrogen oxide formation in the combustion of fossil fuels in a combustion chamber, flue gases are, after preceding removal of heat, recycled to the burner side of the combustion chamber and, due to the injector action of the flame, introduced around the burner into the combustion space in a part quantity reducing the combustion temperature to a temperature level which is at most equal to the limiting temperature for the formation of nitrogen oxides. As a result, the flame is surrounded by - cool - flue gases and cooled. In a firing installation, a combustion chamber arranged within a housing carrying a flow of a heat carrier medium has a "hot" combustion space which is surrounded by a shell accommodated at a distance from the combustion chamber walls. Flue gas flow paths extend between the shell and the combustion chamber, and at least one inflow path branches off from these for recycling a part stream of the - cooled - flue gas into the combustion space, this path leading into the latter in the vicinity of a burner or flame tube and cooling the combustion flame. <IMAGE>

Description

The invention relates to a method for reducing nitrogen oxide formation when burning fossil fuels of gaseous, liquid or fine-grained consistency in a combustion chamber equipped with at least one burner, in which at least a partial flow of the flue gases is returned to the burner side and recirculated after at least part of its thermal energy has been released the burner or a flame tube associated with it is re-inserted into the combustion chamber.

Furthermore, the invention relates to a firing system used to carry out the method, in particular designed as a boiler for building heating systems.

From DE-OS 36 01 000 a water boiler with a hot combustion chamber is already known, in which, in order to reduce the nitrogen oxide content in the combustion gases, part of the previously cooled flue gases extends over a flame tube of the burner Recirculating annular gap is returned to the combustion chamber.

DE-PS-37 38 623 also discloses a boiler with flue gas recirculation which, at the burner end of the combustion chamber, has a baffle plate that closes the combustion chamber at a distance from a boiler door that communicates the burner holder with an injector channel, through a subset of the then partially cooled flue gases into the combustion chamber is traceable. This flue gas recirculation is intended to reduce the nitrogen oxide content in the exhaust gas by lowering the oxygen partial pressure.

Similarly, DE-OS 36 28 293 discloses a boiler with flue gas recirculation in this case, however, only indoors in the combustion chamber. An injector channel surrounding the flame is arranged within the combustion chamber. This injector channel is used to suck in a partial flow of the flue gases, which have already been cooled by previous heat extraction, from the burner side into the combustion chamber.

From DE-OS 29 27 193 a hot water boiler, used as a water heater, is known in which a part of the flue gases flows back into the combustion chamber to equalize the pressure. The combustion chamber is surrounded by a combustion chamber wall which is formed by a pipe screw or hollow body carrying water.

Unsatisfactory with the known boilers seems their lack of adaptability to the respective conditions of use. Accordingly, the object of the present invention is to provide an improved method for reducing the formation of nitrogen oxides when burning fossil fuels and to create a combustion system which is used to carry out the method, in particular designed as a boiler for building heating, and which optimizes the combustion process, that is to say while maintaining good combustion efficiency enable effective nitrogen oxide reduction.

From a firing point of view, this problem is solved, starting from the method specified at the outset, in that the recirculating flue gases, after previous heat removal, are at a combustion temperature at a temperature level at most equal to the limit temperature for the formation of nitrogen oxides corresponding subset around the burner in the combustion chamber.

The recirculating reintroduction of the flue gases previously cooled by heat extraction in such a subset enables the combustion process to be optimized in a desirable manner, and indeed for fuels of different composition and composition.

It has proven to be advantageous if, according to a further development of the method according to the invention, the flue gases are let into the combustion chamber in a partial quantity which keeps the combustion temperature in the combustion chamber at about 1200 ° C. after prior heat removal.

In-depth tests have shown that in the combustion of fuels of the type specified in the preamble of claim 1, which may be natural gas, light or heavy heating oil, but also, for example, coal dust, only at combustion temperatures in excess of 1200 ° C. significant amounts of nitrogen oxides.

In terms of device technology, the object on which the invention is based is achieved by creating a firing system, in particular a boiler for building heating systems, in which a heat transfer medium, such as water, leading housing, a combustion chamber with at least one burner is accommodated and the combustion chamber has a combustion chamber which is surrounded by a jacket which extends essentially over its entire length and is surrounded with flow paths for returning the flue gases after their directional deflection on the side of the combustion chamber facing away from the burner , which extend between the jacket and a wall delimiting the combustion chamber, at least some of these flow paths opening into a burner-side exhaust gas chamber, which in turn is connected to the combustion chamber via an inflow path that extends around the burner or a flame tube associated with the latter, that a flame developed by the burner or the flame tube is essentially completely enclosed by the flue gas partial flow returned to the combustion chamber, and the cross section of the inflow path opening into the combustion chamber it is changeable for the recirculating flue gases.

The furnace according to the invention thus has a "hot" combustion chamber. Hot combustion chambers have proven themselves to achieve good combustion efficiency. With sufficient presence of oxygen, a practically complete burnout takes place in such combustion chambers, but undesirably high nitrogen oxide emissions occur because excess quantities of oxygen and nitrogen present in the combustion air oxidize to nitrogen oxides at the combustion temperatures that occur.

Although the combustion system according to the invention also has a "hot" combustion chamber, combustion of the flame at a temperature level below or at most equal to the limit temperature for the formation of nitrogen oxides takes place as a result of cooling the flame by recirculating - cool - flue gases in the above-mentioned partial quantity. This creates a very simple firing system, which can be easily adapted to, for example, different fuel properties by changing the cross-section of the inflow path leading into the combustion chamber for the recirculating flue gases, and in which the formation of nitrogen oxides while maintaining a high level of combustion efficiency is largely achieved is prevented.

The arrangement of an exhaust gas space that extends at least partially around the burner or a flame tube enables a part of the flue gas to be returned to the combustion chamber in a structurally simple manner. In a further development of the invention, the inflow path connecting the exhaust gas chamber to the combustion chamber consists of a gap extending all around the burner or a flame tube, the width of which for sucking in a part of the - cooled - flue gases into the combustion chamber compared to one of the exhaust gas chamber on that of the combustion chamber wall facing away from the side can be changed by means of a sleeve which can be adjusted in the longitudinal direction of the combustion chamber and which adjoins the burner on the jacket surrounding the combustion chamber.

Another development of the invention provides that the jacket enclosing the combustion chamber essentially over its entire length is pot-shaped and provided with a bottom on the burner side, the burner or a flame tube protruding into the combustion chamber through an opening in the bottom and this opening being excessively large so opposite the burner or flame tube that an annular channel for sucking in flue gas from the exhaust gas space extends into the combustion chamber around the burner or flame tube.

Even with such a design, a sleeve which can be displaced in the longitudinal direction of the combustion chamber can be arranged in the opening in the bottom of the jacket surrounding the combustion chamber in order to adjust the amount of exhaust gas which can be sucked into the combustion chamber, and by means of its axial adjustment the width of an inflow gap between this sleeve and one of the exhaust chamber on the Combustion chamber side facing the wall to be adjustable.

With such a configuration, a good enclosure of the flame extending from the burner or flame tube in the combustion chamber with - cooled - flue gases and thus in the interest of reducing nitrogen oxide formation depending on the quantity of cooled flue gases supplied to the combustion chamber, ensures an effective reduction in the combustion temperature.

If according to a further development, the sleeve slidably received in the opening in the bottom of the jacket surrounding the combustion chamber on the to the combustion chamber pointing side has a funnel-shaped widening section, it is possible in a simple manner to enclose the flame developed from the burner or the flame tube over a large axial length with flue gas and thereby effectively cool it.

Another important embodiment provides that when the combustion chamber extends essentially horizontally in the housing, the flue gases recirculated in the upper part between the jacket surrounding the combustion chamber and the combustion chamber are fed directly to a flue gas outlet, whereas the flue gases recirculated in the lower part are introduced into a flue gas-side flue space and from there, in part due to the injector effect of the flame around the burner or a flame tube around the combustion chamber.

The flue gases recirculated in the upper part between the casing surrounding the combustion chamber and the combustion chamber have higher temperatures than the flue gases recirculated in the lower half due to thermal buoyancy. In view of the discharge of the flue gases recirculated in the upper half into the flue gas outlet and the use of part of the flue gases recirculated in the lower half of the combustion chamber to lower the combustion temperature, the flame temperature and can be reduced extremely effectively with the same amount of flue gases recirculated to the combustion chamber a correspondingly high reduction in nitrogen oxide emissions.

From a structural point of view, it has proven to be expedient in the aforementioned embodiment if, in the upper half of the casing surrounding the combustion chamber, an approximately semicircular collar extends from the burner-side end thereof and extends to the burner-side insulating plate, which collar extends in the upper part of the combustion chamber - Separates flow paths from the exhaust gas chamber so that a partial quantity can only be sucked into the combustion chamber from the flue gases which are returned in the area of the lower combustion chamber half.

It has also proven to be advantageous if defined inflow paths for sucking in cool flue gases are arranged in a bottom which closes the jacket on the burner side around an opening arranged coaxially to the burner or flame tube and into which the burner or flame tube projects, and further if means for at least partial closure of these inflow paths are provided, which allow a quantity limitation of the flue gases that can be sucked into the combustion chamber as a result of the injector effect of the flame.

Another important embodiment of the invention provides that on the side of the combustion chamber opposite the burner there is a pot-shaped additional heating surface which, in view of the heating surface enlargement caused thereby and its reaching into the combustion chamber, leads to a noticeable improvement in the - firing-related - efficiency contributes.

The additional heating surface arranged at the end of the combustion chamber opposite the burner can expediently be conical, cylindrical or in the form of a pocket and, in the interest of improved heat transfer, also have ribs on the side in contact with the flue gas.

Another important development of the last-mentioned embodiment provides that when the combustion chamber is arranged horizontally, the additional heating surface protruding into the latter opposite the burner is arranged offset upwards from the combustion chamber axis and that the flue gas discharge paths in the lower part of the combustion chamber are located at the end of the combustion chamber facing away from the burner have larger cross sections than in the upper part. Such a design increases the flow resistance in the upper area. This prevents the majority of the flue gas flow from flowing out of the combustion chamber in the upper region as a result of thermal buoyancy. By sensible choice of the eccentricity of the additional heating surface, the flue gas outflow from the combustion chamber can be largely evened out.

Likewise in the interest of the best possible - technical firing - efficiency of the furnace, another development provides that the jacket surrounding the combustion chamber extends axially beyond the actual combustion chamber formed by a finned tube.

Fig. 1

einen Heizkessel mit sich horizontal erstrekkender Brennkammer in einer Längsschnittansicht,

Fig. 2

einen Heizkessel mit sich senkrecht erstreckender Brennkammer in einer Ansicht wie in Fig. 1,

Fig. 3

ebenfalls in einer Ansicht wie in Fig. 1 einen Heizkessel mit liegender Brennkammer und brennerseitig mittels eines Bodens ähnlich der Ausführungsform nach Fig. 2 geschlossenen Verbrennungsraum,

Fig. 4

in einer ausschnittsweisen Ansicht eine weitere Abwandlungsform einer Abgasrückführung,

Fig. 5

eine Querschnittansicht gemäß der Schnittlinie V-V in Fig. 4,

Fig. 6

in einer Ansicht wie in Fig. 1 einen Heizkessel mit einer unsymmetrisch zur Längsachse der Brennkammer angeordneten Heizfläche auf der vom Brenner abgewandten Seite der Brennkammer.

Some exemplary embodiments of the invention will be explained below with reference to the accompanying drawings. Schematic views show:

Fig. 1

a boiler with a horizontally extending combustion chamber in a longitudinal sectional view,

Fig. 2

a boiler with a vertically extending combustion chamber in a view as in Fig. 1,

Fig. 3

likewise in a view as in FIG. 1 a boiler with a lying combustion chamber and combustion chamber closed on the burner side by means of a base similar to the embodiment according to FIG. 2,

Fig. 4

a partial view of a further modification of exhaust gas recirculation,

Fig. 5

3 shows a cross-sectional view along the section line VV in FIG. 4,

Fig. 6

in a view as in FIG. 1 a boiler with a heating surface arranged asymmetrically to the longitudinal axis of the combustion chamber on the side of the combustion chamber facing away from the burner.

In the boiler 10 illustrated in Fig. 1 is within a water-carrying housing 11 made of sheet steel, which in turn is from one here in detail jacket 12 of interest is surrounded by insulating material, a horizontally extending and essentially cylindrical combustion chamber 14 is accommodated. At one end of the combustion chamber 14 there is a burner 15 with a flame tube 16, while the end facing away from the burner is closed off by a heating surface 17 molded into the combustion chamber like a pot.

The cylindrical part of the combustion chamber 14 consists of a finned tube 18 with fins 19 extending radially inwards, between which flow paths extending parallel to one another extend. A cylindrical jacket 20 is received within the combustion chamber 14 at a radial distance from the combustion chamber walls formed by the finned tube, which ends at a distance from the heating surface 17 of the combustion chamber facing away from the burner side and encloses a combustion chamber 22.

An exhaust gas chamber 25 is arranged between a burner plate 23 carrying the burner 15 with an insulating plate 24 assigned to the combustion chamber, through which the flame tube 16 of the burner extends, and the cylindrical jacket 20 surrounding the combustion chamber 22 the flue gas flow paths 26, which extend at a radial distance therefrom, extend into the flue gas flow paths 26. Furthermore, a flue gas outlet 27 extends from the exhaust gas chamber 25. On the burner side, a sleeve 28 adjoins the cylindrical jacket 20 accommodated in the combustion chamber 14, and for the purpose of setting a sleeve 28 between it and the flame tube 16 surrounding insulating plate 24 annularly around the flame tube extending gap 30 axially movable according to double arrow 29 and can be determined in the respective setting position.

When the boiler is in operation, the fuel used is burned in a flame that extends from the flame tube 16 into the combustion chamber 22. After the combustion in the combustion chamber, the flue gases flow around the end of the cylindrical jacket 20 arranged in the combustion chamber 14 away from the flame tube and between the latter and the combustion chamber in the flow paths 26 formed by the circumferentially spaced ribs 19 of the finned tube 18 to the burner side, in order there to enter the exhaust gas chamber 25 and then to be discharged via the flue gas outlet 27. During the return flow to the exhaust gas chamber 25, the flue gases give off their thermal energy largely via the finned tube 18 of the combustion chamber 14 to the water 32 received as heat transfer medium in the housing 11 and thus enter the exhaust gas chamber 25 in the cooled state.

As a result of the injector effect emanating from the flame in the combustion chamber 22, from the exhaust gas chamber 25 which extends in a ring around the flame tube, cooled flue gases are sucked into the combustion chamber through the annular gap 30 formed between the axially adjustable sleeve 28 and the insulating plate 24 assigned to the burner plate 23, which smoke gases are sucked into the combustion chamber Essentially completely surround the flame and thereby reduce the combustion temperature in the combustion chamber.

In the interest of an effective reduction or prevention of nitrogen oxide formation during combustion, the combustion temperature must be set to a temperature level which is below the limit temperature which is decisive for the formation of nitrogen oxides. This is achieved in a simple manner by regulating the quantity of the cool smoke gases flowing into the combustion chamber 22 through the said annular gap 30, by appropriately adjusting the width of the inflow gap between the burner-side end face of the adjustable sleeve 28 and the insulating plate 24 surrounding the flame tube 16 of the burner.

The embodiment of FIG. 2 differs in particular from the embodiment of FIG. 1 that within a water-carrying housing 11 ', which is surrounded by a jacket 12' made of insulating material, a cylindrical combustion chamber 14 'extends vertically. This combustion chamber, which is also completed on one side by a burner plate 23 with an inner insulating plate 24 and on the other end face has a bottom in the manner of a spherical cap 17 ', in its cylindrical part in turn consists of a finned tube 18 with a radially inward direction Ribs 19 which are parallel to each other in the axial direction. Within the section formed by the finned tube, a cylindrical jacket 20 is also inserted into this combustion chamber, which, as in the embodiment according to FIG. 1, is on the side of the burner facing side extends to a certain extent beyond the finned tube 18 and is closed on the burner side by a conical bottom 34.

Coaxial to the flame tube 16 extending through the insulating plate 24 of the burner 15 arranged on the burner plate 23 is within a provided with a collar 35 recess of the conical bottom 34 of the cylindrical shell 20 which surrounds the combustion chamber, in turn a cylindrical sleeve 28 'axially movably received, which has oversize compared to the flame tube 16 of the burner. In view of this excess extends between the flame tube 16 and the sleeve 28 'an annular inflow gap for cooled flue gases which are sucked into the combustion chamber during operation of the boiler by the injector effect emanating from the flame and which largely completely enclose the flame extending flame tube.

The sleeve 28 'is axially displaceable for the purpose of adjusting the width of the gap 30' between the sleeve 28 'and the exhaust gas chamber 25 on the end facing away from the combustion chamber 22 insulating plate 24. This is a precise adjustment of the amount of flue gas returned to the combustion chamber for cooling the flame possible depending on the requirements of the respective application.

The boiler illustrated in Fig. 3 has Again, like the embodiment according to FIG. 1, a horizontally arranged combustion chamber 14, but in accordance with the embodiment according to FIG. 2, the jacket 20 surrounding the combustion chamber is provided on the burner side with a conical bottom 34, in which coaxial to the flame tube 16 of the burner 15 a sleeve is axially movable and can be locked in any axial position. This sleeve has a section 36 which widens slightly conically towards the combustion chamber 22 and which favors the inclusion of the flame extending from the flame tube with cool exhaust gases which are sucked out of the exhaust gas chamber into the combustion chamber as a result of the injector effect.

In the embodiment illustrated in FIG. 4, the cylindrical casing 20 surrounding the combustion chamber 22 is closed on the burner side by a straight bottom 34 '. Compared to the embodiment according to FIG. 1, the burner-side closure of the jacket by means of a straight - or also conical - bottom has proven to be advantageous in that the exhaust gas space becomes larger and the resistance on the flue gas side becomes smaller. This favors a rapid flow through the combustion chamber with exhaust gases sucked into it and thus contributes to reduced nitrogen oxide formation.

4, like the exemplary embodiments according to FIGS. 1 and 3, is a construction with a horizontally arranged Combustion chamber 14. When the combustion chamber is arranged horizontally, the channels extending in the upper region between the casing 20 surrounding the combustion chamber 22 and the combustion chamber 14 are subjected to a higher thermal load due to the thermal buoyancy of the heating gases than the channels leading to the exhaust gas chamber 25 in the lower region. 4 and 5, by extending between the straight bottom 34 'and the insulating plate 24 penetrated by the flame tube 16 of the burner 15, a semicircular collar 38 which flows back through the flow paths in the upper half of the combustion chamber - Thermally more heavily loaded - introduces flue gas directly into the flue gas outlet 27. The flue gas flowing back in the lower half, however, enters the burner-side exhaust gas chamber 25 and through a coaxial to the flame tube 16 in the bottom 34 'received sleeve 28 into the combustion chamber. Since the flue gases returned to the combustion chamber 22 have cooled more than the flue gases flowing back to the flue gas outlet 27 in the upper part of the combustion chamber, this leads to a further reduction in nitrogen oxide formation.

Fig. 5 illustrates in a section corresponding to the section VV in Fig. 4, another variant in which around a straight bottom 34 'of the jacket surrounding the combustion chamber 20 coaxially penetrating the flame tube of the burner recess in the upper half of several holes 40, however larger flow cross-sections in the lower half having elongated holes 41 are arranged around the flame tube. In a manner not shown, means for partially or completely covering the bores or elongated holes can also be provided, which in turn leads to a simple adaptability of a boiler designed in this way to the requirements of the respective application.

The boiler 10 illustrated in FIG. 6 'differs from the embodiment according to FIG. 1 only in that the additional heating surface 17' extending on the side facing away from the burner into the combustion chamber, which is essentially frustoconical, not symmetrical to the longitudinal axis of the combustion chamber arranged, but is offset upwards. Accordingly, have in the upper part of the combustion chamber between the jacket 20 and the additional heating surface 17 'extending discharge paths for the combustion gases smaller cross-sections than the discharge paths in the lower part. In view of these smaller cross-sections, there are increased flow resistances in the upper region of the combustion chamber, which, without prejudice to a naturally occurring thermal buoyancy in the combustion chamber, results in a certain homogenization of the mass flow of the flue gases and consequently a largely uniform flow through all areas of the between the combustion chamber and the jacket assigned to it extending flue gas flow paths is ensured.

Claims (11)

1. A method for reducing NO_x-formation when combusting fossil fuels of gaseous, liquid or fine granular consistency in a burner chamber (14,14') equipped with at least one burner (15) and having a combustion chamber (22) characterized in that

   a) the flue gases are led outside of the combustion chamber (22) and thereby cooled whilst giving off a part of their heat content,

   b) a part stream of the thus cooled flue gases are recycled to the burner side and are therefrom recycled recirculating into the combustion chamber (22),

   c) the part stream of the recirculating flue gases being recycled from the burner side to the combustion chamber (22) are controlled by adjusting the cross-sectional area of the gas delivery channel (30,30',40,41) that discharges into the combustion chamber so that the combustion temperature within the combustion chamber (22) is restricted to about 1200°C.
2. A combustion appliance for carrying out the method according to claim 1, and in particular a domestic heating boiler having
      a casing (11, 11') which functions to conduct heatcarrying medium, such as water, and a burner chamber (14, 14') which is equipped with at least one burner (15) and arranged within the casing;
      a combustion chamber (22) being included in the burner chamber (14, 14') and being surrounded by a non-cooled shell (20) which extends essentially over the whole length of the combustion chamber whereby a hot combustion is provided, characterized by that for the purpose of returning the flue gases subsequent to changing the flow direction thereof at the side of the burner chamber (14, 14') opposite said burner (15), the burner chamber (14, 14') has disposed around the periphery thereof a plurality of gas delivery channels (26) which extend axially between the shell (20) and a wall which delimits the burner chamber (14, 14') at one end thereof;
      that at least part of said gas delivery channels discharge into an exhaust gas chamber (25) which is arranged on the burner side and which, in turn, is connected with the combustion chamber (22) over a gas delivery channel (30, 30', 40, 41) which extends around the burner, and a flame tube which embraces said burner and
      that means are provided for adjusting the cross-sectional area of the recycled flue gases delivery channel (30, 30', 40, 41) that discharges into the combustion chamber (22).
3. A combustion appliance according to claim 2, characterized in that the gas delivery channel (30, 30', 40, 41) connecting the exhaust gas chamber (25) with the combustion chamber (22) is comprised of a gap (30, 30') which extends around the burner and a flame tube (16) respectively.
4. A combustion appliance according to claim 3, characterized in that the width of the gap can be adjusted by means of a sleeve (28) which is axially moveable in the longitudinal direction of the burner chamber (14), such that a given volume of cooled flue gas will be drawn by suction into the combustion chamber (22) transversely over a wall (24) which terminates the exhaust gas chamber (25) on the side opposite the combustion chamber (22), said sleeve (28) connecting with the shell (20) surrounding the combustion chamber on the burner side.
5. A combustion appliance according to claim 3, characterized in that the shell (20) surrounds the combustion chamber (22) over essentially the whole of length of said chamber; in that the shell (20) has a pot-like shape and, on the burner side, is provided with a bottom member (34); in that the burner and a flame tube (16) extend into the combustion chamber through an opening in said bottom member; and in that the dimension of the opening in relation to the burner and the flame tube is such as to form around the burner and the flame tube an annular channel which functions to draw flue gas out of the exhaust gas chamber (25) and into te combustion chamber by suction.
6. A combustion appliance according to claim 5, characterized by a sleeve (28') which can be moved in the longitudinal direction of the burner chamber for the purpose of adjusting the size of the opening in the bottom (34) of the shell (20) surrounding the combustion chamber in order to draw a determined volume of gas into the combustion chamber (22) by suction, whereby axial movement of said sleeve (28') adjusts the width of a flow gap (30') between the sleeve and an exhaust gas chamber terminating wall (24) on the side opposite to the combustion chamber.
7. A combustion appliance according to claim 6, characterized in that the side of the sleeve (28') displaceably arranged in the bottom (34) of the shell (20) surrounding the combustion chamber 22 facing towards the combustion chamber has a funnel-shaped flared section (36).
8. A combustion appliance according to any one of claims 2-7, characterized in that when the burner chamber (14) is arranged essentially horizontally in the water conducting casing (11), the flue gases returned to the upper part between the shell (20) surrounding the combustion chamber (22) and the combustion chamber (14) are returned immediately to a flue gas outtake (27), whereas the flue gases returned in the bottom part, between the shell and the burner chamber, are introduced into an exhaust gas chamber (25) on the burner side and extracted therefrom by suction partly by the injector effect generated by the flame around the burner and a flame tube (16) and into the combustion chamber.
9. A combustion appliance according to claim 8, characterized by approximately semi-circular collars (38) which are sufficiently large to extend in the upper part of the shell (20) surrounding the combustion chamber (22) from its burner side end to an insulating plate (24) on the burner side and which define the flue gas flow channels which extend in the upper part of the burner chamber.
10. A combustion appliance according to claim 8 or 9, characterized in that gas delivery channels (40, 41) through which cool flue gases are sucked are defined in a bottom (34') which terminates the shell (20) on the burner side and which extend coaxially with the burners and the flame tube (16) around an opening through which the burner and the flame tube (16) extend.
11. A combustion appliance according to Claim 10, characterized by means for at least partially closing the gas delivery channels (40, 41) which function to suck cool flue gases into the combustion chamber (22).

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