The present invention relates to a method of operating a burner, and a burner, in particular a radiant heating tube burner. A burner of the type of interest can be operated with liquid and/or gaseous fuels. It is also possible, for example, to achieve a cooler flame and/or better mixing of fuel and combustion air or also with flue gas by recirculating combustion flue gases to a combustion zone and/or a mixing zone.

By recirculation it is possible to reduce the average and the maximum temperature of the flame such that the thermally generated nitrogen oxide emission is reduced.

If liquid fuels are used, the recirculation of combustion flue gases can be utilized to evaporate the liquid fuels when they are combusted.

Background of the invention In many technical applications in the field of manufacturing industrial furnaces, it is necessary to transfer heat to matter in a protective or reactive gas. Electrical heaters or radiant heating tubes, which are usually fired with gas burners, provide indirect heat to the furnaces used. The radiant heating tube, which is of metal or a ceramic material, is constructed in such a way that in operation it transfers the heat to the matter by means of radiation.

While the realization of requirements with respect to the combustion in the field of gas-fired radiant heating tubes have greatly advanced during the last few years, the combustion of oil as a fuel in a radiant heating tube is an industrial problem. If conventional burner technology is used there are exceedingly high NO, ; emissions and problems of deposit formation resulting from the incombustible components of the fuel oil. The use of combustion air preheating can also lead to problems with fuel

atomization, since the maximally allowable temperatures in the area of the oil nozzle are far below the desired combustion air preheating temperature.

To solve these problems, a burner with a recuperator for radiant heating tubes was developed in the Lehrgebiet fur Energie-und Stofftransport/EST (Department of Heat and Mass Transfer), RWTH Aachen, Germany. This burner, in its oil burner version, comprises various components such as a recuperator and flame tube, as well as a mixing assembly and an evaporation tube. The components are arranged in such a way that flue gas can be recirculated in the evaporation or mixing tube and into the flame tube. The combustion air is supplied to the burner in a step-wise manner. Thus the primary air enters into the evaporation or mixing tube and the secondary air enters into the flame tube. It is possible to differentiate between the states of. stationary operation and cold start-up operation. In the latter case, the recirculation of flue gas into the evaporation tube must be prevented. For further details, please refer to the publication"Low-NOx Strahlheizrohr-Brenner fur flussige und gasförmige Brennstoffe" ("Low-NOx radiant heating tube burners for liquid and gaseous fuels"), by Dipl. -Ing. A. Munko, Dr. Ing. F. Kleine-Jager, Prof. Dr. Ing. H. Kohne, Lehrgebiet für Energie-und Stofftransport (Dept. of Heat and Mass Transfer), RWTH Aachen, Germany, Dr. Ing. W. Diemar, LOI Thermprocess, Essen, Germany, GWI, volume 5,2002. In particular, in this burner the liquid fuel is fed into the evaporating tube through an axially arranged jet head by means of a commercially available swirl-pressure atomization nozzle. The primary air is fed into the evaporating tube via tubes which are concentrically arranged with respect to the oil nozzle. The high exit speed of the primary air results in a recirculation of flue gas into the evaporation tube due to the injector effect. The recirculated flue gas flows out of the area of the outer tube through an annular slot between the recuperator and the evaporating tube.

In the evaporating tube, heat is transferred to the fuel by hot flue gas, the heat transfer from the hot tube wall and upstream heat radiation from the glowing flame

tube. This heat supply results in complete evaporation of the liquid fuel so that at the end of the evaporating tube, a completely gaseous, homogeneous mixture of fuel, primary air and flue gas is provided. If the burner is operated with gaseous fuel, the described task of fuel evaporation is eliminated. In that case, the evaporating tube serves to homogeneously mix combustion gas, primary air and flue gas. The diameter and length of the evaporating tube are chosen such that there is no ignition of the fuel-air mixture within the evaporating tube. While the conditions for auto- ignition are given here, the time the mixture remains in the tube is shorter than the ignition delay. The mixture enters the flame tube in a non-combusted state. At the transition from the evaporating tube to the flame tube, the secondary air preheated in the recuperator is fed in. Due to the high exit impulse of the secondary air from the feeding nozzles, flue gas recirculation is achieved in the flame tube. In the flame tube, the mixing of the gas streams and the stabilization of the flame is achieved. In order to ensure high efficiency of the radiant heating tube, the ratio of the preheated secondary air flow and the non-preheated primary air flow must be adjusted to be as high as possible. Then there is a strongly sub-stoichiometric fuel-air mixture in the evaporating tube. The stationary operation of the burner is characterized by the auto- ignition of the fuel-air mixture and the stabilization of the flame in the flame tube.

Stationary operation is therefore only possible, after the radiant heating tube components have reached a high temperature level.

As indicated before, the secondary air preheated in the recuperator is fed to the entry opening of the flame tube via a plurality of feeding nozzles. A portion of the hot recirculated flue gases is passed to these feeding nozzles in countercurrent in an annular channel around the evaporating tube. This embodiment with feeding nozzles is relatively complex and expensive to manufacture.

DE 39 30 569 A1 discloses a mixing assembly for a burner with intake openings at the upstream end of the flame tube through which, due to the injector effect of the flow, flue gases from the environment are sucked into the flame tube. However, the

combustion flue gases are not fed into a mixing zone separate from the combustion zone in which the fuel is mixed with the primary air. It is therefore obvious that a homogeneous mixture of the different material flows cannot be achieved before the combustion zone. This means that the formation of so-called hot spots, which critically contribute to the thermal formation of nitrogen oxides, cannot be avoided.

From EP 0 463 218 B 1, a method and an apparatus for burning a fuel in a combustion chamber is known, by means of which the oxidation of fuels with air, while effectively utilizing the heat generated by the oxidation, is carried out and only a small amount of nitrogen oxides is generated. The method suggested here is carried out with normal combustion air and extremely high combustion exhaust air recycling (r > 2), so that even when the air is completely preheated (e = 1) the maximum temperatures arising with oxidation of less than 1500°C are below that which results when the fuel is combusted in flames. Recycling the combustion gases partially cooled down by useful heat and mixing them with the preheated combustion air is done in the following way. The combustion air in the form of air jets exiting from nozzles is fed through an essentially fuel-free area, for example, in which the air jets can be enveloped by and mixed with the combustion gases sucked in by the injector effect. It is deemed advantageous that the air jets are arranged in a ring and fuel is fed in the area surrounded by the air jets at a predetermined distance behind the nozzle opening.

DE 199 25 715 Al explains a method and an apparatus for the homogeneous mixing of combustion air and combustion flue gases. The flue gases are mixed with air by internal recirculation, i. e. due to the injector effect of air, for example, in a place where there is no conversion of the fuel, so that the mixture becomes homogeneous.

Basically, the hot combustion flue gases are recycled, however, in a mixing zone after they have transferred a certain amount of heat to air in a heat exchanger. The recirculated hot flue gases are introduced into a flow chamber in the same direction as the air and mixed there.

Finally, DE 23 03 280 A should also be mentioned, from which a burner with flue gas recycling is known. Again, the hot flue gases are recycled from the combustion chamber to the exit of the fuel by the injector effect of the combustion air. The fuel- air mixture formed after the fuel exit is mixed with the combustion air in a mixing tube, before it enters the combustion chamber behind the mixing tube and burns.

The combustion apparatus described in EP 0 386 732 is a two-fuel burner in which the air supply channel is closed off by a nozzle plate having first and second nozzles.

A tube referred to as a flame tube in this document is arranged at a small distance from the nozzle plate, so that a flue gas circulating opening results. Another tube is coaxially disposed to the flame tube and spaced from the nozzle plate. The tube surrounds the nozzles and extends beyond their openings and into the flame tube.

The recycled flue gas flows, split into two flue gas flows, together with the combustion air which is also split into two air flows by the nozzles, into the flame tube in such a manner that there are two separate flows of mixtures of flue gas and combustion air. This is supposed to achieve flame stabilization, which would eliminate the provision of internal structures for flame support.

US 4,708, 638 A discloses a burner provided with an annular channel through which secondary air is supplied. The annular channel for the supply of the secondary air is intersected by channels for supplying the flue gas.

US 4, 575, 332 A, just like US 4,708, 638 A, shows a burner implementation to reduce the formation of NOx, in which burnt-out flue gas is sucked from the firing chamber due to the injector effect of the primary air and supplied to the flame base between the primary air intake and the secondary air intake. As in the US 4, 708, 638 A, there are connection tubes to feed the secondary air through the hot, recirculated combustion flue gases.

Summary of the invention According to a first aspect of the present invention, for the first time, a method of operating a burner is disclosed in which a portion of the flue gas of the hot flue gases exiting a combustion zone is recycled and secondary air is mixed with a fuel in the combustion zone. The secondary air and the portion of the flue gas freely intersect each other in a transition area in a countercurrent. In particular, with such a method it is possible to avoid using a complex nozzle or tube configuration to achieve the intersection of the two flows of secondary air and hot flue gases.

In another embodiment of the present invention, the portion of the flue gases is recycled from the hot flue gases exiting from a combustion zone to a mixing zone.

The fuel is mixed with primary air in the mixing zone. The secondary air is then supplied to the fuel-primary-air mixture in a combustion zone. In other words: a method according to the present invention can also operate solely with a secondary air supply. In that case, the term"fuel-primary-air mixture"is to be replaced by "fuel". A supply of primary air is therefore optimal.

According to the present invention, it may thus be advantageous to mix the fuel- primary-air mixture exiting from the mixing zone with secondary air and to recycle a portion of the hot flue gases exiting from the combustion zone to the mixing zone.

Herein the secondary air is passed through the first portion of the flue gas in a freely intersecting way in countercurrent in a transition area or space. The transition area or space can be between the mixing zone and the combustion zone and can be configured, for example, as an annular slot or a cylinder. The portion of the flue gas intersecting the secondary air can be fed, for example, in countercurrent to the fuel- primary-air mixture flowing in the mixing zone to the beginning of the mixing zone.

For this purpose a mixing tube is present, for example, which is surrounded by a tube concentrically arranged thereto. The portion of the flue gas is then fed through the thus defined annular space.

The secondary air is mixed, for example, with the fuel-primary-air mixture at the exit from the mixing zone and/or in the area of the entry of the fuel-primary-air mixture into the combustion zone. Again, the flows are guided in such a way that the secondary air freely intersects, in countercurrent, the hot flue gases exiting from the combustion zone, which are recycled to the mixing zone. Unlike the prior art approaches for burners, for indirect or direct firing, a complicated and expensively manufactured nozzle and tube feeding of secondary air is eliminated. In particular, the plurality of feeding channels for the introduction of the secondary air into the combustion zone, such as shown in the initially mentioned publication"Low-NOx- Strahlheizrohr-Brenner fur flussige und gasförmige Brennstoffe" ("Low-NOx radiant heating tube burner for liquid and gaseous fuels"), can be done without. It should also be noted that an exemplary embodiment of the present invention can be formed like a burner as shown in the mentioned publication. Moreover, such a burner can also be provided not only for direct firing, but also for indirect firing. In the latter case, the burner described here is equipped with a radiant heating tube, but it does not have, as mentioned above, secondary-air-supply tubes which extend into the combustion zone in order to have the flows intersect.

According to another aspect of the present invention, the guiding of the flows can be optimized by recycling the portion of the flue gas to the mixing zone in such a way that at least part of its heat is transferred to the secondary air. This serves to preheat the secondary air which improves the combustion process in the combustion zone by having the secondary air introduce a greater amount of heat.

Another exemplary embodiment of the present invention provides that the first portion of the flue gas is recycled to the mixing zone in such a way that, in addition to transferring heat to the secondary air, a portion of its heat is also transferred to the fuel-primary-air mixture in the mixing zone. This exemplary flue gas recycling also improves the mixing and/or combustion processes in the mixing or combustion zone, respectively.

In the above explained exhaust recirculation it may be advantageous to feed the portion of the flue gas both in countercurrent to the fuel-air mixture and in countercurrent to the secondary air between these two flows. This results in a heat transfer from the portion of the flue gas both to the secondary air flowing in countercurrent and to the fuel-primary-air mixture in the mixing zone, in a simple construction.

A simple construction for feeding the different flows provides that the portion of the flue gas is guided both to the fuel-primary-air mixture and to the secondary air by a partition. The partition can be formed, for example, by one or more tubes.

In another exemplary embodiment of the present invention, the mixing zone is formed by a mixing tube, within which the fuel is mixed with primary air. On the outside of the mixing tube, the flue gas is recirculated to the entry of the mixing zone, and the secondary air flows outside of the flue gas recirculation.

According to another aspect of the present invention it may be advantageous to recirculate a portion of the hot flue gases to the entry of the combustion zone. In this exemplary embodiment, the flue gases are thus not only recirculated to the mixing zone, but also to the combustion zone.

Another exemplary embodiment of the method according to the present invention provides that a portion of the flue gases is passed through a heat exchanger and then exhausted from the burner. In the heat exchanger, this portion of flue gas is fed in countercurrent to the secondary air. This is to achieve additional heating of the secondary air, which flows in countercurrent in the heat exchanger, and therefore to increase the energy potential at the entry into the combustion zone. For example, the heat exchanger may also be formed as a recuperator integrated in the burner.

Basically, however, it is also possible to configure the heat exchanger so that it is separate from the burner.

Simple feeding of the flows can be achieved according to another exemplary embodiment of the method according to the present invention by not splitting the hot flue gases into portions until the hot flue gases have flowed to the beginning of the combustion zone in countercurrent to the fuel-primary-air mixture in the combustion zone. For example, such a feeding of the flows can be achieved by defining the combustion zone by means of a combustion tube. The fuel-primary-air mixture, which is mixed with the secondary air, then flows within the combustion tube. The flue gas flows on the outside of the combustion tube and is split into the above- explained portions of flue gas in the area of the entry of the combustion tube. A portion of flue gas can, in turn, be split into a first portion and a second portion, wherein the first portion is recirculated to the mixing zone, while the second portion is recirculated to the combustion zone. Another portion of flue gas can be extracted and then fed through a heat exchanger or recuperator.

The feeding and introduction of the flows can be improved, for example, by having a portion of flue gas flow into the above-explained transition area (free space) between the mixing zone and the combustion zone in such a way that the portion of flue gas is introduced into the transition area in countercurrent to the secondary air. The portion of flue gas also flows radially further outside into the transition area than the secondary air introduced from the opposite side. In other words, the introduction openings for the secondary air are at a position radially further to the inside in the transition area than the opposite introduction openings for the portion of flue gas. For initiating the flue gas recirculation in the desired way, it may be advantageous for the fuel to be injected into the mixing zone with an impulse. It is also possible, however, to achieve this without an impulse. It should be noted that only one introduction opening for the secondary air may be sufficient, which would then be formed as an annular slot. The same also applies to the introduction opening for the portion of flue gas. With direct firing, the portion of flue gas freely flows into the transition area.

According to another exemplary embodiment of the method according to the present invention, a portion of the flue gases is passed through a heat exchanger or recuperator and then exhausted from the burner. The primary air and the secondary air are then fed together in the recuperator in countercurrent to this portion of flue gas. This also achieves heating of the later primary air in a simple way.

According to still another exemplary embodiment of the method according to the present invention, the primary air and secondary air commonly flow over part of the distance in the recuperator. Then the primary air exits the recuperator through at least one exit opening and is then fed to the area of the fuel supply. The secondary air freely exits the end of the recuperator in such a way that it freely intersects the recirculated portion of flue gas in the transition area in countercurrent.

According to yet another exemplary embodiment of the method of the present invention, the primary air and the secondary air commonly flow to the end of the recuperator, after which the primary air is fed into the area of the fuel intake by means of a flow redirection means. The secondary air freely exits the end of the recuperator in such a way that it freely intersects the recirculated portion of flue gas in the transition area in countercurrent.

According to another aspect of the present invention, a burner according to the present invention for liquid or gaseous fuels comprises a fuel supply means for supplying the fuel. A secondary air supply means is also provided, with the aid of which secondary air is fed into a combustion zone. A fuel combustion zone is also present, and a flue gas recycling means is provided, with the aid of which at least a portion of the hot flue gases exiting the combustion zone is recycled such that the secondary air and the recirculated portion of the first portion of flue gases freely intersect each other in a transition area in countercurrent.

An exemplary embodiment of the present invention comprises a mixing zone in which fuel and primary air are mixed with each other. The fuel is introduced into the mixing zone with the aid of the fuel supply means. The secondary air supply means feeds the secondary air into the combustion zone downstream from the mixing zone, where the fuel-primary-air mixture is mixed and ignited together with the secondary air.

Another exemplary embodiment of the present invention provides that a primary air supply means is present for supplying primary air into the mixing zone. The combustion zone in which the fuel-primary-air mixture reacts enriched by the secondary air is positioned downstream from the mixing zone. The flue gas recycling means can be configured such that a portion of the flue gases is recirculated into the mixing zone and a portion of the flue gases is recirculated into the combustion zone.

It can be advantageous for the combustion zone to consist of a combustion tube, and for a flue gas recycling zone to be formed between the outer surface of the combustion tube and a radiant heating tube arranged coaxially thereto. This flue gas recycling zone opens into the transition area in the area of the end face of the combustion tube at a distance to the end face of the burner. One or more circumferentially evenly spaced exit openings for the secondary air open out into the transition area, which is configured, for example, as an annular slot or a cylindrical cavity between the mixing zone and the combustion zone. For example, the openings for the secondary air are arranged radially further to the inside than the exit zone of the flue gases, which opens out in countercurrent into the transition area, i. e. the gap between the combustion tube and the exit opening of the secondary air.

Another exemplary embodiment provides that the mixing zone be formed by a mixing tube which, in turn, is surrounded by a coaxial tube spaced from the mixing tube. The annular opening of the introduction orifice, which is formed by this arrangement, opens out into the transition area so that a portion of the hot flue gases

is recirculated into this annular gap and sucked to the introduction opening in the mixing zone.

Advantageously, the primary air may flow through a plurality of openings into an area in front of the mixing tube, and the ring of introduction flow openings for the primary air can concentrically surround the openings for the injection of fuel.

Another exemplary embodiment of the burner according to the present invention provides that on the outside of the tube surrounding the mixing tube, the recuperator is provided for supplying secondary air, through which at least a portion of the flue gases exiting the combustion chamber is fed in countercurrent.

Generally, it may be provided for all above described burners according to the present invention, for direct or indirect firing, that the fuel supply means is spatially separated from the hot furnace areas or combustion zones, by being spatially separate. In particular, there may be a separation from the mixing zone and the combustion zone.

This spatial separation of the fuel supply means and the hot furnace or combustion areas is advantageous not only from the point of view of making a homogeneous fuel-air-combustion-exhaust-air mixture possible, but it also prevents overheating of the fuel. Another advantage of this burner design may be that the combustion flue gases which have already lost part of their heat, are recirculated into the mixing zone and the combustion zone, which are spatially separate from each other, so that the temperature of the recirculated combustion gases is lower than in burner designs, in which the recirculated flue gas is taken from the combustion chamber and has nearly flame temperature. Advantageously, in a burner design according to the present invention, both gaseous and liquid fuels can be used. By the dosed introduction of combustion flue gases, the fuel is preheated so that when the fuel is liquid it is at least partially evaporated. This approach also allows for at least a portion of the

combustion air to be preheated by means of pre-cooling the combustion gases. The so-called overall recirculation ratio, i. e. the ratio of the overall recirculated combustion mass flow and the overall mass flow of combustion air, may well be increased at will with burners of the present invention.

Short description of the drawings For further explanation and better understanding, several exemplary embodiments will be described below in more detail with reference to the accompanying drawings, in which: Fig. 1 is a longitudinal sectional view of a first embodiment of a burner of the present invention, which is formed as a radiant heating tube; Fig. 2 is a partially cutaway elevational view of a radiant heating tube burner in which the radiant heating tube is formed as U-shaped tube; Fig. 3 is a longitudinal sectional view of another embodiment of a burner according to the present invention similar to the burner of Fig. 2, without a mixing tube; Fig. 4 is a table of numerous further variants of the embodiments shown in Figs. 1 to 3 of a burner of the present invention; Fig. 5 is a longitudinal sectional view of another embodiment of a burner of the present invention which is only equipped with one combustion tube in the furnace to be heated, but in a directly radiating configuration; Fig. 6 is a longitudinal sectional view of another embodiment of a burner according to the present invention which is similar to the one shown in Fig. 5, without a recuperator, without a combustion tube and without a mixing tube;

Fig. 7 is a list in the form of a table of the variants of a burner according to the present invention for direct firing; Fig. 8 is a longitudinal sectional view of another embodiment of a burner according to the present invention in which the primary air is preheated together with the secondary air in a recuperator, wherein the top half of the figure shows the burner integrated into the wall of a furnace for direct firing and the bottom half of the figure shows the burner with a radiant heating tube for indirect firing; and Fig. 9 shows a longitudinal sectional view of another embodiment of a burner according to the present invention, in which unlike the burner shown in Fig.

8, primary air is passed right up to the end of the recuperator and, after the primary air is redirected, it then flows into the mixing zone, wherein, in Fig.

9, again, the top half of the figure shows the burner integrated into the wall of a furnace for direct firing while the bottom half of the figure shows the burner with a radiant heating tube for indirect firing.

It should also be noted that in all figures the same components are identified by the same reference numerals and that in the above-mentioned longitudinal sectional views, the top half of the figure shows in schematic form a stationary operation while the bottom half of the figure shows a cold start-up operation.

Detailed description of exemplary embodiments of the invention Fig. 1 shows a longitudinal sectional view of an embodiment of a burner according to the present invention for indirect firing. The burner comprises a radiant heating tube 1 which is configured as a barrel type radiant heating tube 20. In the conventional manner, the barrel type radiant heating tube 20 is arranged in furnace 21. The interior of furnace 21 is indirectly heated by radiant heat. Unlike a direct firing without a

barrel or radiant heating tube 1, the two atmospheres in barrel type radiant heating tube 20 and in the interior of furnace 21, remain separate from each other.

In the radiant heating tube, as shown in Fig. 1, a recuperator 2 is arranged in an annular fashion around a center axis of radiant heating tube 1. Recuperator 2 has two flow channels separate from each other in the present example, an outer flow channel for outflowing flue gas T2AB and an inner flow channel for inflowing secondary air SL. On the inside, recuperator 2 is defined by a tube 3. Tube 3, in turn, is divided into a front area 5 and a back area 4 by a partition 8. The back area 4, in turn, is divided into outer and inner areas by another tube 4b. In this way, in a cold start-up operation, for example, a countercurrent can be generated in the outer area in order to prevent recirculation of hot flue gases during cold start-up. It must be noted, however, that tube 4b is not a requirement in order to carry out a smooth cold start- up and stationary operation.

Partition 8 comprises a plurality of openings 9,12 and 17. Primary air flows through openings 9 into mixing zone 6. In a cold start-up operation, blocking air flows through openings 17 into the front portion 5 of mixing zone 6 in order to prevent, in cold start-up operation, flue gases from being recirculated into mixing zone 6. In the present embodiment, the primary air PL and the blocking air are separated from each other in the back area 4 by a tube 46. A fuel supply means 10 is arranged centrally and coaxially with the center axis of the burner, in which, through its center channel 11, fuel is injected into space 5 with or without an impulse. The fuel passes into space 5 via openings 12. In space 5, a mixing tube 7 is arranged coaxially with the center axis. Mixing tube 7 and tube 3 thus form an annular channel 30 extending from a front end 31 to the beginning of mixing zone 6 of mixing tube 7.

A flame tube 13 is arranged downstream from mixing zone 6. Combustion tube 13 is coaxially aligned with the center axis of the burner. It must be noted that flame tube 13 has a length which is shorter than radiant tube 1 so that hot flue gases can exit at

the downstream end of flame tube 13 and can flow back into annular space 18 formed by flame tube 13 and radiant tube 1. Flame tube 13 is arranged at its end face 13a at a distance from exit 15 of the flow channel for the secondary air SL. This forms a transition space 16 between flame tube 13 and the exit opening of the secondary air SL.

In particular, it must be noted that the secondary air SL is not fed to the combustion zone 14 within combustion tube 13 with the aid of small feeding tubes, but that the flue gas flows TAB and SL freely intersect each other in countercurrent. The term in countercurrent means that the flue gas flow TAB and the secondary air flow SL continue to flow in opposite directions. For short periods of time, the flow directions can also enclose angles of more or less 90°, but after a certain distance they flow in opposite directions. In the present case, hot flue gas TAB flows through annular channel 18 in countercurrent to the flow direction of fuel B. In transition space 16, a portion of the hot flue gas flow TAB is fed into recuperator 2, a first portion TIAB is sucked into transition area 16 by the injector effect and is split into a partial flue gas flow T, IAB which flows into the combustion zone 14 and a portion Tl2AB which continues to flow in the annular gap 30 between tube 3 and mixing tube 7 in countercurrent to the secondary air SL and the fuel-primary-air mixture in mixing zone 6 in mixing tube 7 and is redirected at the end of recuperator 2 into mixing zone 6 by the injector effect of the primary air PL. The injector effect of the secondary air flow SL and the primary air flow PL must be adapted to each other to optimize the exhaust air flows.

As already mentioned, Fig. 2 shows an elevational view of a burner in a U-shaped radiant tube 20. Here, hot flue gas is redirected at the end of comubustion tube 13, which has a certain length, into annular space 18 and recycled in the direction of the burner. Radiant tube 20 and combustion tube 13 can have strongly differing lengths.

Fig. 3 shows a burner similar to the one shown in Fig. 2. However, there is no mixing tube 7 in mixing zone 6. A portion of flue gas is not extracted either. The annular gap between components 1 and 23 is closed in the present embodiment, it could be open, however, to exhaust flue gases. The basic guidance of the flue gas in an intersecting flow with the secondary air SL is identical to the one of Figs. 1 and 2.

As can be seen from the table according to Fig. 4, a burner according to the present invention as shown in Figs. 1 to 3 can be used for indirect firing with a recuperator, without a recuperator, with a combustion or mixing tube, without a mixing tube, with a combustion tube, without a combustion tube. Further variants are also possible, in which the fuel is injected into mixing zone 6 with or without an impulse.

The longitudinal sectional view of Fig. 5 shows another exemplary embodiment of a burner according to the present invention for indirect firing. Unlike the above- explained inventive embodiments of a burner, there is a direct contact between the furnace atmosphere and the burner flame and the hot flue gases. Due to the arrangement and configuration of the feeding of the secondary air SL, there is still, however, a recirculation of hot flue gases T I AB despite the elimination of the radiant tube. The whole of the burner is not within a portion of radiant tube 1, but in a sheath 22, which, however, in turn, as with the above-explained embodiment of a burner of the present invention according to Fig. 1, has a recuperator 2. The recirculation of the flue gases TU RAB and TAB is identical to the recirculation in the previously explained embodiments of a burner according to the present invention. In a transition area 16 between exit opening 15 for secondary air SL and the end face 13a of combustion tube 13, there is, again, a free flow of the two gas flows of the secondary air SL and the flue gas TIAB in couiitercurrent in the spirit of the present invention. Part of the hot flue gas TIAB is sucked into combustion tube 13 together with the secondary air SL, a different part is sucked into annular gap 30 between tube 3 and mixing tube 7 and mixed with fuel B in mixing zone 6 formed by mixing tube 7.

As mentioned before, by the introduction of a partial flow T2AB of the hot flue gases TAB through recuperator 2, the secondary air SL is preheated, before it enters combustion zone 14. In the same way, as with the previously explained burners with indirect firing, in the exemplary embodiment shown here of a burner of the present invention with direct firing, a portion of the hot flue gases is fed in annular zone 30 in countercurrent to the secondary air SL and the mixture in mixing zone 6 so that a portion of the heat of the hot flue gases T12AB is transferred both to the secondary air SL passed through the recuperator and to the fuel-air mixture in mixing zone 6.

In another embodiment of a burner according to the present invention shown in Fig.

6 for direct firing, unlike the embodiment of an inventive burner shown in Fig. 5, flame tube 13 has also been eliminated. There is no recuperator integrated in sheath 22 on the outside of the burner. As in Fig. 3, the outer annular gap is closed off, so that there is not flue gas extraction. If desired, however, a portion of hot flue gases could be extracted through it. To do this, the annular gap would have to be open.

Mixing zone 6 is not defined by mixing tube 7 but by outer tube 3 which forms the inner circumferential surface of the secondary air supply. Again, the flue gases TAB are recirculated due to the configuration of the supply of the secondary air SL and the primary air PL so that hot flue gases also enter into mixing zone 6.

Finally, Fig. 7 shows a list in the form of a table of the possible variants of burners of the present invention with direct firing. Again, it is possible to configure the burner of the present invention with or without a recuperator, with a mixing tube, without a mixing tube, with a combustion tube or without a combustion tube. As with the previously explained embodiments of inventive burners with indirect firing, it is also possible to supply fuel with or without an impulse.

It can be seen from the explained embodiments, in particular, that with all burners according to the present invention, both with direct and indirect firing, a mixing zone

6 is spatially separate from a combustion zone 14. To achieve this, the fuel supply means is further back than a secondary air supply means. In particular, the primary air supply means is arranged in the area of the fuel supply means while the secondary air is only introduced downstream after mixing zone 6.

Incidentally, in should be noted that in all embodiments, as shown in the bottom halves of the various figures, in cold start-up operation, the recirculation of hot flue gases T12AB is reduced or prevented by a special air introduction through exit opening 17, i. e. an air countercurrent is built up which reduces recirculation in the mixing zone 6, for example, of hot flue gases from T12AB- Finally, it should be noted that the burner can also be configured in such a way that switching from a cold start-up operation to stationary operation or using a special air introduction through exit openings 17 for a cold start-up operation is not necessary.

In Fig. 8, a longitudinal sectional view of another exemplary embodiment of a burner according to the present invention is shown, which can be used for direct firing (top half of Fig. 8) or for indirect firing in a radiant heating tube 1 (bottom half of Fig. 8).

Unlike the burner of the present invention shown in Fig. 5, the primary air PL is preheated together with the secondary air SL in a recuperator 2. In this embodiment no mixing tube is used. As can be seen from Fig. 8, the primary air PL and the secondary air SL commonly flow from left to right within recuperator 2. In front of partition 8, one or more openings 40 are present in recuperator 2, through which air can flow into the back portion 4 of partition 8 and where it can flow, now as primary air PL, into mixing zone 6 through openings 12. However, only part of the air flow flowing in recuperator 2 escapes through openings 40, so that the other part escapes as secondary air SL at the exit end or the front end 31 of the mixing zone and flows in free countercurrent to the recirculated hot flue gas TAB or its partial air flow T, IAB-

Unlike the top half of Fig. 8, a radiant tube 1 is present in the bottom half of the figure, in which a flame tube 13 extends coaxially. Flame tube 13 ends at a distance from the front end 31 so that flue gases TAB recirculated there are recirculated in countercurrent to the secondary air SL and flow into combustion zone 14.

Fig. 9 shows another exemplary embodiment of a burner according to the present invention. Again, the top half of the figure shows a burner of the present invention for direct firing while the bottom half of the figure shows a burner of the present invention for indirect firing in a radiant tube 1. It can be seen that as with the burner approach shown in Fig. 8 primary air PL and secondary air SL is commonly passed from left to right in a recuperator 2. Prior to the exit of the secondary air SL from the front end 31 of the burner, a redirection means or redirecting channel is present which feeds the primary air PL to the back space of partition 8. The preheated primary air PL then escapes and is mixed with the fuel B. The secondary air SL, as with the other burners of the present invention, flows in free countercurrent to the recirculated hot flue gases T, I AB or TAB. The burner shown in Fig. 9 is thus different from the burner shown in Fig. 8, in that primary air PL is passed right up to the end area of recuperator 2. After the primary air PL is redirected, it flows into mixing zone 6.