This invention concerns a method of gas combustion, for example of natural gas or propane, in industrial furnaces, especially glass furnaces or furnaces for metal melting, by the help of oxygen or air and a burner for realization of this method.

In the industrial furnaces, there are due to reason of reaching of very high temperatures necessary for thermal treatment of the material, i.e. melted glass or heated metal, used different systems of gas burning by the help of oxygen or air. Combustion systems consist of burners which are placed in the furnace with simultaneous input of the fuel gas and additional gas in the form of oxygen or air. Both these gases react at the temperature over the ignition temperature of the fuel gas, burn and herewith add energy to the process. For accomplishment of optimal heat transfer and at the same time for the protection of heat proof material of the furnace against high temperature of the flame it is necessary to design these burners in the way for the flame to reach good heat transfer in the space of the furnace and simultaneously not to melt heat proof walling of the furnace area. This optimal flame is obtained using specific construction of the burner with accurate setting of the flow and outflow velocity of the gas and oxygen.

The solution of this problem was described for example in CZ 279526 or WO 2007/048429 where with aim to reach improved control of the flame was used setting with one central flow of the gas and one concentric flow of the oxygen. Yet this solution does not enable setting of outflow velocity in the wide range of values and does not enable to sufficiently control the shape of the flame. Alternative solution is type of the burner with more oxygen and gas inlets as it is described in EP 0 687 853 and EP 0 762 050. According to these solutions it is difficult to properly set the burner and reach optimal conditions for burner performance and at the same time these solutions require use of complicated burner shaped piece. In the device according to US 5,299,929 are used two flows of oxygen which are led through flat slot above and under central flow of the gas with target to create two zones of burning. This solution simplifies control of the burner but does not enable gentle and accurate setting of the gas and reacting oxygen. In the design according to US 6, 190, 158 was used burner with central flow of the oxygen, which is concentrically surrounded by the gas and the gas is then also surrounded by the oxygen. This solution does enable good gas and oxygen regulation but the presence of oxygen in the central area of the flame decreases radiation efficiency of the burner and negatively influences emission of the nitrogen oxides (NOx). For solution of sufficient regula- tion ability of the flame was in US 6,685,463 used configuration for burning of the gas by atmospheric air, where the gas is input into through central nozzle and subsequently surrounded by the flow of primary air into which is angle wise added another flow of secondary combustion air. Disadvantage of this solution is use of the air as an oxidation reagent which causes heat efficiency decrease of the burner. Further disadvantage is angle wise input of the secondary air into the burner which causes increase of the turbulence in the flame and increased NOx emissions.

The aim of this invention is to solve at least some of the above mentioned complications and to provide a new method suitable for combustion of fuel gas, for example natural gas or propane, by the help of additional gas, preferably oxygen. Preferably accurate setting of combustion process with high radiation efficiency and low NOx emission is enabled. Likewise is the task of the invention to introduce design of the burner for realization of this method.

This problem is solved by the method according to claim 1. The method of gas combustion in industrial furnaces, especially glass furnaces or furnaces for metal melting, by the help of multi nozzle burner with controllable regulation of the flow of fuel gas as well as of additional gas, comprises that the fuel gas is input into cavity of the burner through at least one central gas nozzle with simultaneous input into of two independent flows of the additional gas in the way that the fuel gas is surrounded by concentric flow of primary additional gas which is then surrounded by concentric flow of secondary additional gas. In a preferred design the fuel gas is input into the burner in velocity (Vp) ranging between values 1 to 30 m/s, whereas at first the primary additional gas is input into the burner in controllable capacity amount, which ranges between 10 to 90 weight % necessary for stoichiometric combustion of the fuel gas, in velocity (V ₀ i), whose value is given by relation and also the secondary additional gas is input into the burner in controllable capacity amount, which ranges from 90 to 10 weight % necessary for stoichiometric combustion of the fuel gas, in velocity (V ₀₂ ), whose value is given by relation

In an optimal case the amount and the velocity of the input fuel gas, primary additional gas and secondary additional gas are continuously controllable and as the fuel gas is used methane or propane in mixture with other hydrocarbons and gases and as the additional gas is used oxygen, air or another oxidant with oxygen content over 80 weight %.

Next the invention provides a burner for gas combustion in industrial furnaces, especially glass furnaces or furnaces for metal melting, comprising an injector and a shaped piece of the burner, which is fixed on outlet of the injector, where the injector comprises either from an inner gas nozzle equipped on an inlet with a main regulation valve of input of the fuel gas or from the inner gas nozzle and an insertion gas nozzle equipped with an auxiliary valve, whereas the inner gas nozzle is surrounded by a first concentric nozzle equipped on inlet with a regulation valve of input of the primary additional gas and the first concentric nozzle is around the perimeter surrounded by a second concentric nozzle, which is equipped with a regulation valve of input of secondary additional gas and which forms the body of the injector.

Likewise is beneficial when between a flow cross section (0) of the fuel gas, given either by a flow cross section (0-i) of the inner gas nozzle or by sum of flow the cross section (0-i ₎ of the inner gas nozzle and a flow cross section (0 ₂ ) of the insertion gas nozzle and a flow cross section (0) of the additional gas given by sum of a flow cross section (0- ₁ ) of the first concentric nozzle and a flow cross section (0 ₂ ) of the second concentric nozzle is valid relation

0 : 0 = 1 : (0,9 to 1 ,8) whereas under condition ø = 0-1 + 0 ₂ is valid relation 1 : 02 = 1 : (0,3 to 2,2).

Further then is advantageous when the regulation valve of inflow of the primary additional gas and the regulation valve of inflow of the secondary additional gas are via a control unit of the additional gas connected into a block of control of combustion process where, also via a control unit of the fuel gas, is connected the main regulation valve of inflow of the fuel gas with the auxiliary regulation valve, whereas the block of the control of combustion process is connected to an evaluating and control block equipped with control elements.

With this presented solution is reached new and higher effect herein that through quite simple construction lay out is possible to achieve accurate setting of combustion process with high radiation efficiency and low NOx emission. Continually controllable inlets of the fuel gas and the additional gas the enable accurate and gentle setting of shape and kinetic of the flame and enable sufficient regulation ability of the luminescence of the flame and influence over NOx production. With the present invention a method of gas combustion in industrial furnaces, especially in glass furnaces or furnaces for metal melting by the help of a multi nozzle burner with controllable flow of a fuel gas and additional gas according to claim 1 is provided. Therein fuel gas is input into a cavity of the burner by at least one central gas nozzle with simultaneous input of two independent flows of the additional gas in the way that the fuel gas is surrounded by a concentric flow of a primary additional gas which is then surrounded by a concentric flow of a secondary additional gas, the primary and secondary additional gas being oxygen, air with an oxygen content over 80 weight % or another oxidant with an oxygen content over 80 weight %. According to the invention the fuel gas is blown into the burner at a fuel velocity (VP) ranging between 1 to 30 m/s, whereas the primary additional gas is input into the burner in an amount, especially in a controllable capacity amount, which ranges between 10 to 90 weight % necessary for stoichiometric combustion of the fuel gas, in at a first velocity (V01 ), whose value is given by relation and the secondary additional gas is input into the burner in an amount, especially in a con- trollable capacity amount, which ranges from 90 to 10 weight % necessary for stoichiometric combustion of the fuel gas, in a second velocity (V02), whose value is given by relation

The inventions emanates from the finding that the low velocities of fuel gas and additional as well as the velocity relations of fuel gas and first and second additional gas lead to a gas flow in the cavity, which has reduced turbulences, in particular is nearly free of turbulences. Thus a flame building is supported, in which in a first reaction state fuel gas reacts with the primary additional gas due to the temperature conditions in an industrial furnace by forming a zone rich in exhaust gas, which surrounds the central fuel gas flow depleted of oxidant. This zone rich in exhaust gas is then surrounded by the secondary additional gas. The secondary additional gas reacts with the fuel gas only in a second reaction state. Preferably this second combustion takes place predominantly in the furnace chamber and not in the cavity of the burner. This is also supported by the low velocity of the gases. The relocation of the second combustion from the cavity of the burner to the furnace chamber prevents high temperatures within the burner, which could cause damages to the burner and the kiln lining near the burner. A flame building as described also leads to a luminous and visible flame in the fur- nace chamber, which is helpful for control measures.

The preferred value of the relation between the fuel gas velocity and the velocity of the primary or the secondary additional gas depends on the respective amount of the respective additional gas. The higher the amount of the respective the higher is favourably the velocity of the respective additional gas. Preferably the velocity of the fuel gas is within a range of 15 to 25 m/s, even more preferably between 17 and 23 m/s. The amount of the primary and secondary additional gas is preferably in the range of 20 to 80 weight % necessary for stoichiometric combustion of the fuel gas, whereby preferably the two amounts in sum provide 100 weight % necessary for stoichiometric combustion of the fuel gas. Preferably 80 weight % necessary for stoichiometric combustion of the fuel gas are provided by the secon- dary additional gas. This supports that the relocation of the second combustion and the hot zone into the furnace chamber.

In an development of the method according the invention the primary additional gas consists of the same gas as the secondary additional gas. With this development a feeding of the primary and secondary additional can be realized by a common line to the burner. In a favourable development the amount and the velocity of the input fuel gas, primary additional gas and secondary additional gas are continually controllable. This supports the control of the adjustment of the flame building. Preferably the fuel gas is methane or propane in mixture with other hydrocarbons and gases.

According to a second aspect of the invention a burner of claim 6 is provided for a gas combustion in industrial furnaces, especially glass furnaces or furnaces for metal melting. The burner comprises an injector and a shaped piece, which is fixed on an outlet of the injector , the shaped piece comprising a cavity and the injector comprising an inner gas nozzle for input of a fuel gas, wherein the inner gas nozzle is surrounded by a first concentric nozzle for input of a primary additional gas and the first concentric nozzle is around the perimeter surrounded by a second concentric nozzle for input of a secondary additional gas and which forms the outer body of the injector, wherein a tip of the second concentric nozzle is arranged in the cavity at the same distance or at a smaller distance to an outlet of the cavity than a tip of the first concentric nozzle.

The burner according the invention supports also the building of the flame, in which in a first reaction state fuel gas reacts with the primary additional gas due to the temperature conditions in an industrial furnace by forming a zone rich in exhaust gas, which surrounds the central fuel gas flow depleted of oxidant, this zone rich in exhaust gas is then surrounded by the secondary additional gas. The secondary additional gas reacts with the fuel gas only in a second reaction state. Preferably this second combustion takes place predominantly in the furnace chamber and not in the cavity of the burner. This is due to the tip of the second concentric nozzle being arranged in the cavity at the same distance or at a smaller distance to an outlet of the cavity than a tip of the first concentric nozzle. This means the first reaction between the fuel gas and the primary additional gas takes is predominantly not disturbed by the secondary additional gas which flows nearly free of turbulences around the flow of fuel gas with primary additional gas. Especially, when the tip of the second concentric nozzle is arranged in the cavity at a smaller distance to the outlet of the cavity than a tip of the first concentric nozzle, the first reaction of fuel gas and primary additional gas takes before the flow of secondary additional gas reaches the flow of fuel gas and primary additional gas. This leads to flows in the cavity which are nearly free of turbulences and the zone of the second combustion being shifted towards the furnace chamber. The latter effect is supported by lowering the distance between the tip of the second concentric nozzle and the outlet of the cavity.

In an development of the burner the injector additionally comprises an insertion gas nozzle arranged in the center of the inner gas nozzle for input of the fuel gas. Preferably the relation between a fuel flow cross section (0) of the fuel gas, given either by a flow cross section (0-i ) of the inner gas nozzle or by a sum of the flow cross section (0-i ) of the inner gas nozzle and a flow cross section (0 ₂ ) of the insertion gas nozzle, and an additional gas flow cross section (ø) of the additional gas, given by sum of a first flow cross section (0-1 ) of the first concentric nozzle and a second flow cross section (0 ₂ ) of the second concentric nozzle is given as 0 : 0 = 1 : (0,9 to 1 ,8) and the relation between the first flow cross section (0-1 ) and the second flow cross section (0 ₂ ) is given as 0-1 : 0 ₂ = 1 : (0,3 to 2,2).

In a preferable development a thickness of the inner wall of the second concentric nozzle, which separates the primary additional gas and the secondary additional gas, is at the tip of the second concentric nozzle in the range of 0.25 to 3.0 times of the inner diameter of the inner gas nozzle. Such a wall thickness helps to shift the zone of the second combustion towards the furnace chamber as the interference of the secondary additional gas with the fuel gas and the primary additional gas is further delayed, herein in axial direction. The concrete relation between inner diameter of the inner gas nozzle and thickness of the the inner wall of the second concentric nozzle at the tip of the second concentric nozzle depends on the inner diameter of the inner gas nozzle, the greater the diameter the smaller the relation. Preferable thicknesses are in the range of 10 to 30 mm.

In further developments of the burner the inner wall of the second concentric nozzle at the tip of the second concentric nozzle is built by a pipe building an outer wall of the first concentric nozzle or by an additional bush arranged on the outer wall of the first concentric nozzle. If the inner wall is built by the pipe building the outer wall of the first concentric nozzle this pipe only has to be prolongated with respect to an inner wall of the first concentric nozzle. As the pipes are normally made of metals, for developments with short distances between tip of the second concentric nozzle and the outlet of the cavity and thus high thermal loads an additional bush is preferred. The additional bush can be made of metal or, preferably of a ceramic, especially a high temperature ceramic.

Preferably the tip of the second concentric nozzle is arranged nearer to an outlet of the cavity with respect to a tip of the first concentric nozzle in a way that the distance between an entry point of the primary additional gas and an entry point of the secondary additional gas is in the range of 1.0 to 8.0 times of the inner diameter of the inner gas nozzle. This distance helps to shift the zone of the second combustion towards the furnace chamber as the interference of the secondary additional gas with the fuel gas and the primary additional gas is further delayed, herein in longitudinal direction. In a further development of the burner the second concentric nozzle is movably arranged in the cavity to vary the distance between the tip of the first concentric nozzle and the tip of the second concentric nozzle. In this development the distance between first and second concentric nozzle can be varied in order to optimise the flame. In a further development an inlet of the first concentric nozzle, an inlet of the inner gas nozzle and an inlet of the second concentric nozzle are arranged in one plane, wherein the inlet of the inner gas nozzle is arranged in the same direction as the cavity of the burner, the inlet of the first concentric nozzle forms an angle in a range of 15 to 45 degrees, preferably 15 to 35 degrees, with the inlet of the inner gas nozzle and the inlet of the second concentric nozzle forms an angle in a range of 15 to 45 degrees, preferably 15 to 35 degrees, with the inlet of the inner gas nozzle. With this development the turbulences in the cavity are further minimized as the flows of additional gases reaches the cavity already nearly without turbulences. The larger a distance between the inlet of the nozzle and the tip of nozzle, the steeper angles are allowed. Steeper angles a favourable in view of the manufacturing. In a further development a main regulation valve is assigned to, in particular is arranged at, the inlet of the inner gas nozzle, a first regulation valve is assigned to, in particular is arranged at, the inlet of the first concentric nozzle and a second regulation valve is assigned to, in particular is arranged at, the inlet of the second concentric nozzle and additionally an auxiliary regulation valve is assigned to, in particular is arranged at, the inlet of the insertion gas nozzle if present.

Preferably the first regulation valve for input of the first flow of additional gas and the second regulation valve for input of the second flow of additional gas are connected through a control unit for the additional gas to a block of combustion process control, whereas the main regulation valve for input of the fuel gas is also connected through a control unit for the fuel gas to the block of combustion process control and whereas the block of combustion process control is connected to an evaluation and control block equipped with control elements. In a further development the main regulation valve of input of the fuel gas with the auxiliary regulation valve is connected through a control unit for the fuel gas to the block of combustion process control. According to a third aspect the invention relates to an industrial furnace of claim 17, especially a glass furnace or a furnace for metal melting, with at least one burner of the invention wherein the at least one burner is arranged on a sidewall of the furnace. Particular embodiments of invention design are schematically illustrated in enclosed drawings, where:

Fig. 1 is a lengthwise schematic cut of basic design of the burner,

Fig.2 is a lengthwise schematic cut of an alternative design of the burner with two gas nozzles,

Fig.3 is a scheme of a flame structure in basic design of the burner according to the Fig.1 , and

Fig.4 is a lengthwise schematic cut of the alternative design of the burner from the Fig.2 with illustrated connection of the regulation valves of particular nozzles to the block of control and operating of combustion process,

Fig.5 is a cross-sectional drawing of a preferred embodiment of a burner according to the invention;

Fig.6 is a three-dimensional drawing of the preferred embodiment of Fig.5;

Fig.7 is a cross-sectional drawing of a burner detail of a modified preferred embodiment of a burner according to the invention, with a bush;

Fig.8 is a schematic drawing of a burner detail to illustrate the principle of delaying interferences between a secondary and a primary additional gas with fuel gas.

The drawings which illustrate introduced invention and subsequently described examples of particular designs do not in any case limit scope of the protection mentioned in definition, but only clarify essence of the invention.

In a basic design illustrated in Fig.1 the burner consists of an injector 1 and a shaped piece of the burner 2, which is fixed on outlet of the injector and is formed in a way that the diameter of its inner cylindrical cavity 21 basically corresponds with outer diameter of the body of the injector 1. The injector 1 consists of three coaxially placed pipe nozzles 1 1 , 12 and 13. The inner gas nozzle 1 1 is equipped on inlet with a main regulation valve 1 1 1 of inlet of the fuel gas and is surrounded by a first concentric nozzle 12 equipped on inlet with a regulation valve 121 of inlet of the primary additional gas. This first concentric nozzle 12 is around its perimeter surrounded by a second concentric nozzle 13 which is equipped with a regulation valve 131 of inlet of the secondary additional gas and which forms the body of the injector 1 . As it is visible from the Fig.4 both regulation valves 121 and 131 are via a control unit 4 of the additional gas connected to a block 5 of control of combustion process, where is also via a control unit 6 of the fuel gas connected the main regulation valve 1 1 1 of inlet of the fuel gas. The block 5 of the control of the combustion process is then connected to an evaluation and control block 7, which is equipped with non-illustrated control units and preferably contains computer.

In an alternative design of the burner illustrated in the Fig.2 the injector 1 is also equipped with an additional insertion gas nozzle 14 equipped with an auxiliary regulation valve 141 , which is then connected via the control unit 6 of the fuel gas into the block 5 of the control of combustion process.

The injector 1 is then made in the way to have between a flow cross section (0) of the fuel gas, given in case of basic design of the burner by a flow cross section (0-i ) of the inner gas nozzle 1 1 and in case of alternative design of the burner or by sum of the flow cross section (0 _{1 )} of inner gas nozzle 1 1 and a flow cross section (0 ₂ ) of the insertion gas nozzle 14 and a flow cross section (ø) of the additional gas given by sum of a flow cross section (0-1 ) of the first concentric nozzle 12 and a flow cross section (0 ₂ ) of the second concentric nozzle 13 valid relation 0 : 0 = 1 : (0,9 to 1 ,8) whereas under condition 0 = 0-1 + 0 ₂ is valid relation 1 : 02 = 1 : (0,3 to 2,2).

During performance of the burner of basic construction design according to the Fig .1 the fuel gas is blown at velocity V _P ranging between 1 to 30 m/s from the inner gas nozzle 1 1 into the central part of the cavity 21 of the shaped piece 2 of the burner where it is mixed with the primary additional gas inlet from the first concentric nozzle 12 at outflow velocity V ₀ i whose value is given by relation

Vp : V01 = 1 : (0,5 to 3,0), whereas the primary additional gas is into the cavity 21 inlet in controllable capacity amount, which ranges between 10 to 90 weight % necessary for stoichiometric combustion of the fuel gas. During the flow through the cavity 21 of the shaped piece 2 comes to, thanks to temperature conditions in an industrial furnace, into which is the burner installed, to ignition of the fuel gas and reaction CH ₄ with oxygen of the additional gas to formation of C0 ₂ and H ₂ 0. With regard to relatively low velocity of the fuel gas and the additional gas there is no significant turbulent flow which results in decreased temperature of the flame and in forming of the zone rich in exhaust gas surrounding the flow of the fuel gas depleted of oxidant, thus oxygen. This phenomenon causes slow down of the burning and due to radiation from the space of the furnace comes to formation of carbon radicals in the area of the reacting flame. The structure of the flame for basic design of the burner from Fig.1 is then illustrated in Fig.3.

Simultaneously there is from the second concentric nozzle 13 blown into the cavity 21 of the shaped piece 2 secondary additional gas, which surrounds the reacting fuel gas and the primary additional gas and enables perfect combustion of all reactive products. The secon- dary additional gas is again inlet in controllable capacity amount, which ranges between 90 to 10 weight % necessary for stoichiometric combustion of the fuel gas, whereas controllable outflow velocity V ₀ 2 of the secondary additional gas is with regard to outflow velocity V _P of the fuel gas given in relation. Continually controllable inlets of the fuel gas into the nozzles 1 1 , 12, 13, 14 equipped with the regulation valves 1 1 1 , 121 , 131 , 141 enable accurate and gentle setting of the kinetic of the flame and enable its sufficient controllability with possibility of influence of NOx emission. As the fuel gas is according to the area of use of the burner preferably used methane or propane in mixture with other hydrocarbons and gases. As the additional gas is preferably used oxygen, air or another oxidant with oxygen content over 80 weight %.

The method of gas combustion for example of natural gas or propane, namely by the help of oxygen or air, and the burner for performance of this method is aimed for use in industrial furnaces, above all glass furnaces or furnaces for metal melting.

Fig.5 shows a burner 500 for gas combustion in industrial furnaces consisting of an injector 501 and a shaped piece 502, which is fixed on an outlet of the injector 501 . The shaped piece 502 comprises a cavity 505 and the injector 501 comprises an inner gas nozzle 51 1 for input of a fuel gas 521 wherein the inner gas nozzle 51 1 is surrounded by a first concentric nozzle 512 for input of a primary additional gas 522 and the first concentric nozzle 512 is around the perimeter surrounded by a second concentric nozzle 513 for input of a secondary additional gas 523 and which forms the outer body of the injector 501 , wherein a tip 533 of the second concentric nozzle 513 is arranged in the cavity 505 at the same distance to an outlet 506 of the cavity 505 as a tip 532 of the first concentric nozzle. The injector 501 is fixed to the shaped piece 502 in a detachable manner with a fixing device 551. In the shown embodiment an inlet 542 of the first concentric nozzle 512, an inlet 541 of the inner gas nozzle 51 1 and an inlet 543 of the second concentric nozzle 513 are arranged in one plane, wherein the inlet 541 of the inner gas nozzle 51 1 is arranged in the same direction as the cavity 505 of the burner, the inlet 542 of the first concentric nozzle 512 forms an angle in a range of 15 to 45 degrees with the inlet 541 of the inner gas nozzle and the inlet 543 of the second concentric nozzle 513 forms an angle in a range of 15 to 45 degrees with the inlet 541 of the inner gas nozzle. This embodiment leads to reduced turbulences, in particular nearly turbulence-free flow of the gases 521 , 522, 523 in the cavity 505. Fig.6 shows the burner 500 of Fig.5, wherein the injector 501 with the inlet 542 of the first concentric nozzle 512, the inlet 541 of the inner gas nozzle 51 1 and the inlet 543 of the second concentric nozzle 513 is fixed with the fixing device 551 to the shaped piece 502.

Fig.7 shows a modified burner 700 based on the burner 500 of Fig.5. In the modified burner 700 the tip 733 of the second concentric nozzle 713 is arranged in the cavity 705 at a smaller distance to an outlet 706 of the cavity 705 than a tip 732 of the first concentric nozzle 712. An inner wall 763 of the second concentric nozzle 713 which separates the primary additional gas and the secondary additional gas at the tip 733 of the second concentric nozzle is built by by an additional bush 770 arranged on the outer wall 762 of the first concentric nozzle 712. In the shown embodiment the thickness of the inner wall 763 of the second concentric nozzle, is at the tip of the second concentric nozzle 0.3 times of the inner diameter of the inner gas nozzle. The additional bush 770 is made of a high-temperature ceramic in order to withstand the high temperatures in the vicinity of the outlet 706 of the cavity 705 of the burner. In order to prevent additional turbulences due to the bush, the bush 770 has a skewed edge facing the channel of the second concentric nozzle in this embodiment. Fig.8 illustrates the principle of further delaying the interferences between secondary additional gas on the one hand and on the other hand fuel gas together with primary additional gas and the shift of the second combustion zone, in which the secondary additional gas reacts with the fuel gas to the furnace chamber due construction means on an embodiment of a burner 800. In one aspect the interference between secondary additional gas 823 and fuel gas 821 together with primary additional gas 822 is delayed in longitudinal direction by the distance B between the tip 832 of the first concentric nozzle and the tip 833 of the second concentric nozzle. With this arrangement the fuel gas 821 can react with the primary additional gas 822 without interference with the secondary additional gas 823 in a first stage. At the tip 833 the flow of fuel gas and primary additional gas is then surrounded by the secondary additional gas. The distance B is preferably 1 to 8 times the inner diameter of the of the inner gas nozzle. Due to the low velocities of the gases no further turbulences occur and the second combustion zone is shifted towards the furnace chamber. The effect can be further intensified by minimizing the distance C between the tip 833 of the second concentric nozzle and the outlet 806 of the cavity 805, for instance by with an arrangement comprising the tip of the second concentric nozzle at a position indicated with 833'. For a delay of the interference in axial direction the thickness A of the inner wall 863 of the second concentric nozzle, is increased to increase the axial distance between the primary additional gas and the secondary additional gas. The thickness A is in this purpose preferably 0,25 to 3 times the inner diameter of the of the inner gas nozzle 81 1.