EP4008955A1 - Device and method for supplying combustion air and exhaust gas recirculation for a burner - Google Patents

Abstract

The invention relates to a device (2) and a method for supplying combustion air and recirculating exhaust gas for a burner (1) with a combustion chamber (10) and a burner (1) with a device (2) for supplying combustion air and recirculating exhaust gas, with combustion air being recirculated by means of several distributed along the central axis (A) of propulsion nozzles (21) while sucking in exhaust gases from the combustion chamber (10) to a mixing chamber (22) downstream of the propulsion nozzles (21), the combustion air exiting from the propulsion nozzles (21) is in the mixing chamber (22). is mixed with exhaust gases flowing out of the combustion chamber (10) and sucked back by means of the driving nozzles (21) to form a combustion air/exhaust gas mixture and the combustion air/exhaust gas mixture is fed to a reaction zone downstream of the mixing chamber (22).

Description

FIELD OF APPLICATION AND PRIOR ART

The invention relates to a device and a method for supplying combustion air and recirculating exhaust gas for a burner and a burner with a device for supplying combustion air and recirculating exhaust gas.

Hydrogen, in particular so-called green hydrogen, which is obtained by water splitting from renewable energies such as wind energy, solar energy or hydropower, or from biomass, is becoming increasingly important as an energy source, initially as an admixture to natural gas and later as a pure gas. Although hydrogen burns with almost no emissions, oxygen and nitrogen are components of the combustion air, meaning that nitrogen oxides can also form when hydrogen is used. Thermal nitrogen oxide formation starts at high temperatures and then increases exponentially with temperature. Due to the high reaction speed of the hydrogen, the thermal nitrogen oxide formation increases significantly when using hydrogen compared to using pure natural gas. For example, with burners without special measures, around 50 ppm nitrogen oxide is produced in the exhaust gas with natural gas (CH4) and with hydrogen more than 100 ppm.

Exhaust gas recirculation or exhaust gas recirculation is known as an effective measure against the thermal formation of nitrogen oxides in the exhaust gas from furnaces, with the oxygen content being reduced by recirculating the exhaust gas and the flame temperature thus being lowered. In the context of the application, an exhaust gas recirculation ratio (EGR) is defined as the ratio of the mass flows of the recirculated or recirculated exhaust gas and the combustion air supplied (m _A /m _L ). The exhaust gases are also referred to as combustion exhaust gases or combustion gases.

A distinction is usually made between external and internal exhaust gas recirculation. With external exhaust gas recirculation, exhaust gas is routed out of the combustion chamber and a partial exhaust gas flow is removed from the exhaust tract, for example in a chimney, and mixed with the combustion air or the fuel before or when it flows into a combustion chamber. EGR can be regulated to a desired ratio by means of suitable controls. A significant disadvantage of the external exhaust gas recirculation is an increase in the amount of exhaust gas, which is why areas for extracting the heat have to be increased accordingly.

With internal flue gas recirculation, flue gas or combustion gas present in a combustion chamber is returned to the reaction zone with the impulse of the combustion air. If a temperature in the combustion chamber is above an ignition temperature of the fuel, the exhaust gas recirculation ratio can be increased at will since flame stability is irrelevant.

For example, is off EP 0 463 218 A1 discloses a method and a device for burning fuel in a combustion chamber, with combustion air emerging from a nozzle device having a number of nozzles arranged in a ring with sucked-back exhaust gases that have been partially cooled in the combustion chamber, forming a combustion air/exhaust gas mixture that has at least the ignition temperature in an exhaust gas recirculation ratio EGR >= 2 is miscible.

On the other hand, if the temperature in the combustion chamber is below the ignition temperature, the exhaust gas recirculation ratio must be limited in order to avoid the flame going out.

TASK AND SOLUTION

It is an object of the invention to create a device and a method for supplying combustion air and internal exhaust gas recirculation for a burner, in particular for low-temperature processes, with a defined exhaust gas recirculation ratio.

According to a first aspect, a device for combustion air supply and exhaust gas recirculation for a burner with a combustion chamber is created, the device comprising a plurality of propulsion nozzles distributed around a central axis and fluidly connected to a combustion air supply and a mixing chamber downstream of the propulsion nozzles, the propulsion nozzles and the mixing chamber form a jet pump, and wherein in the mixing chamber, combustion air emerging from the propulsion nozzles can be mixed with exhaust gases flowing out of the combustion chamber and sucked back by means of the propulsion nozzles to form a combustion air/exhaust gas mixture and the combustion air/exhaust gas mixture can be fed to a reaction zone downstream of the mixing chamber.

A space which is delimited from the environment and has a defined cross-section, which is provided between the propulsion nozzles and the reaction zone of the combustion chamber, is referred to as the mixing chamber. The cross section of the mixing chamber can be suitably selected by a person skilled in the art depending on the application. In advantageous configurations, the cross section is Flow direction constant, in one embodiment in an inlet and / or an outlet a converging or diverging cross-section for improved inflow or outflow is provided.

The distributed motive nozzles and the mixing chamber form a jet pump, with the exhaust gas recirculation ratio of the combustion air/exhaust gas mixture conveyed by the jet pump depending on a cross-sectional ratio of the mixing chamber and the motive nozzles and on operating parameters such as a temperature of the recirculated exhaust gas. The exhaust gas recirculation ratio can thus be specified in advance by a person skilled in the art by suitably designing the mixing chamber and the propulsion nozzles, for example up to a limit of flame stability, for specific operating parameters. In other words, the cross section of the mixing chamber is matched to an outlet cross section of the propulsion nozzles and a number of propulsion nozzles.

In one configuration, an end of the mixing chamber facing the propulsion nozzles is at least partially spaced in the direction of flow from a wall on which the propulsion nozzles are arranged, so that a circumferential or interrupted gap is created, which acts as a suction opening of the jet pump, through which an exhaust gas is sucked back and in the mixing chamber can be conveyed. In another embodiment, an intake chamber with an opening for intake of exhaust gas is provided upstream of the mixing chamber. In use, an end of the mixing chamber facing the combustion chamber is arranged upstream of an outlet opening of a fuel supply, with a distance being suitably selectable by a person skilled in the art depending on the application.

Similar to an external exhaust gas recirculation, the combustion air and the exhaust gas are mixed with one another before being mixed with the fuel in a defined exhaust gas recirculation ratio that may be dependent on operating parameters, without the amount of exhaust gas in an exhaust tract having to be increased for this purpose, as is the case with external exhaust gas recirculation.

The flame temperature is lowered by the flue gas recirculation. The nitrogen oxide formation rate is around 10 ⁴ ppm/s at flame temperatures of around 2000 °C for conventional fuels and drops to around 10 ppm/s at 1500 °C. At low flame temperatures and residence times in the range of tenths of a second, single-digit nitrogen oxide values in the exhaust gas can be achieved.

The arrangement of the propulsion nozzles distributed around a central axis is also referred to as a ring-shaped arrangement in connection with the application. In one embodiment, the propulsion nozzles are arranged in parallel. In other configurations, axes of the propulsion nozzles are inclined with respect to the central axis. The design of the jet pump with a plurality of propulsion nozzles distributed around a central axis and a mixing chamber downstream of these propulsion nozzles creates a small-sized jet pump which can be integrated into existing burners of the usual dimensions. The device is therefore also suitable for retrofitting existing systems.

The device with a jet pump formed by the mixing chamber and the driving nozzles is suitable both for burners in a power range with a few kW and for burners with a MW power range.

In one embodiment, a mixing chamber with an annular cross section is provided. An inner diameter of the mixing chamber is selected in such a way that the mixing chamber can be arranged around a fuel lance provided coaxially to the central axis during use.

Depending on the application and size of the burner, the number of driving nozzles can be suitably determined by a person skilled in the art. In one embodiment, eight or more propulsion nozzles are provided, distributed uniformly around the central axis. This creates a good suction effect, in particular for a mixing chamber with a feed opening in the form of an annular gap.

A cross-sectional ratio of the mixing chamber and the driving nozzles of the jet pump is designed in order to achieve a specific exhaust gas recirculation ratio AGR, a resulting cross-section of all driving nozzles being referred to as the cross-section of the driving nozzles. In one embodiment, the aspect ratio of the mixing chamber and the propulsion nozzles is less than or equal to 20.

As mentioned above, an exhaust gas recirculation ratio EGR that is optimal for avoiding pollutants also depends on operating parameters. For example, depending on the temperature of the recirculated exhaust gas, an exhaust gas recirculation ratio EGR of 1 to 1.5 with an oxygen content of the combustion air/exhaust gas mixture of between approx. 10% and approx. 12% is required in order to lower the flame temperature to 1500°C.

In one embodiment, therefore, a bypass channel is provided, by means of which combustion air can be fed to the reaction zone, bypassing the driving nozzles. This makes it possible, for example, to reduce the EGR for flame stability by directing part of the combustion air past the propellant nozzles via the bypass channel. In one embodiment, the bypass channel is designed as an annular gap channel, which is arranged around a fuel lance during use and runs in sections between the mixing chamber and the fuel lance. In one embodiment, nozzle openings are provided at an outlet end of the bypass duct in order to achieve rapid and complete mixing of the combustion air supplied via the bypass duct with the combustion air/exhaust gas mixture of the jet pump.

In one embodiment, an adjustable bypass valve is provided in the bypass channel. In one embodiment, the bypass valve can only be adjusted between an open position and a closed position. In other configurations, a continuously or steplessly adjustable bypass valve is provided. In configurations, the bypass valve is adjusted by means of a controllable or regulatable actuating device, with the bypass valve being opened or closed or a passage being varied, depending on the configuration, by means of regulating or control interventions. By means of the bypass valve, an oxygen content of the combustion air/exhaust gas mixture for the combustion can be changed and, in particular, kept within a defined range for flame stability by a variable supply of additional combustion air.

As an alternative or in addition to a bypass channel, an adjustable valve is provided in one embodiment in an intake opening for the back-sucked exhaust gas, with the valve preferably being adjustable continuously or steplessly. By means of the valve in the intake opening for the sucked-back exhaust gas, an oxygen content of the combustion air/exhaust gas mixture for the combustion can be changed and, in particular, kept within a defined range for flame stability through a variable supply of exhaust gas.

In one embodiment, a probe is provided for an oxygen measurement. The probe is preferably provided upstream of outlet openings of a fuel supply and thus upstream of a flame. The oxygen content of the mixture, determined with the probe, can be determined and can be varied by regulating or controlling interventions on the bypass valve and/or on the valve in the intake opening for the recirculated exhaust gas.

Alternatively or additionally, a sensor for measuring the temperature of the recirculated exhaust gas is provided in one embodiment. An exhaust gas recirculation ratio optimized for a temperature of the exhaust gas can be determined and set by regulating or controlling interventions on the bypass valve and/or on the valve in the intake opening, preferably by measuring the oxygen content.

According to a second aspect, a burner is created, comprising a device for combustion air supply and exhaust gas recirculation with a jet pump, wherein the jet pump has a preferably annular gap-shaped mixing chamber and several propulsion nozzles arranged in a ring around a central axis, and a fuel lance arranged coaxially to the central axis with outlet openings . The outlet openings are arranged downstream of an outlet opening of the mixing chamber, with a distance being suitably selectable by the person skilled in the art. To improve flame stability, in one embodiment, a baffle plate is provided upstream of the outlet openings of the fuel lance. The burner thus created can be installed in a conventional chamber.

In one embodiment, a flame tube is provided which delimits a reaction zone transversely to the direction of flow. The exhaust gas can flow in an annular gap between a wall of the chamber and the flame tube to the jet pump and/or to an exhaust gas outlet. In one embodiment, the flame tube is arranged directly adjacent to the mixing chamber. A length of the flame tube can be suitably selected by those skilled in the art depending on the fuel. In one embodiment, for operation with fuels with a low reaction rate, such as natural gas, an extended flame tube is chosen for an extended residence time to ensure burnout. However, since a dwell time also influences the formation of nitrogen oxides, a short flame tube is provided in other configurations.

In one embodiment, the fuel lance includes an ignition device or a pilot burner. An outlet opening of the ignition device or the pilot burner is preferably offset with respect to the outlet openings of the fuel lance for normal operation.

According to a third aspect, a method for supplying combustion air and recirculating exhaust gas for a burner with a combustion chamber is created, with combustion air by means of a plurality of propulsion nozzles arranged distributed around a central axis, with exhaust gases being drawn in from the combustion chamber, to a mixing chamber downstream of the propulsion nozzles, in the mixing chamber the combustion air emerging from the propulsion nozzles with the exhaust gases flowing out of the combustion chamber and sucked back by means of the propulsion nozzles to form a combustion air/exhaust gas mixture is mixed and the combustion air/exhaust gas mixture is fed to a reaction zone downstream of the mixing chamber.

The propulsion nozzles and the mixing chamber form a jet pump, by means of which a combustion air/exhaust gas mixture with a defined EGR can be fed to the reaction zone depending on certain operating parameters.

In one embodiment, it is provided that combustion air is selectively supplied to the reaction zone via a bypass channel, bypassing the driving nozzles. A proportion of the combustion air supplied via the bypass duct is preferably variable in order to carry out an adjustment as a function of specific operating parameters.

For this purpose, in one embodiment, an oxygen content of a mixture of the combustion air supplied via the bypass duct and the combustion air/exhaust gas mixture is monitored and a quantity of the combustion air supplied via the bypass duct is adjusted to maintain a defined oxygen content.

Alternatively or additionally, another embodiment provides for a temperature of the recirculated exhaust gas to be recorded and for a quantity of the combustion air supplied via the bypass channel to be adjusted as a function of the recorded temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1:

eine geschnittene Seitenansicht eines Brenners mit einer Vorrichtung zur Verbrennungsluftzufuhr und Abgasrezirkulation;

Fig. 2:

den Brenner gemäß Fig. 1 in einer geschnittenen Draufsicht gemäß einer Markierung II-II in Fig. 1;

Fig. 3:

eine geschnittene Seitenansicht eines Brenners ähnlich Fig. 1 mit einer Vorrichtung zur Verbrennungsluftzufuhr und Abgasrezirkulation;

Fig. 4:

den Brenner gemäß Fig. 3 in einer geschnittenen Draufsicht gemäß einer Markierung IV-IV in Fig. 3;

Fig. 5:

einen Brenner ähnlich Fig. 1 in einer geschnittenen Seitenansicht mit einer Kammer.

Further advantages and aspects of the invention result from the claims and from the description of exemplary embodiments of the invention, which are explained below with reference to the figures. show:

Figure 1:

a sectional side view of a burner with a device for combustion air supply and exhaust gas recirculation;

Figure 2:

according to the burner 1 in a sectional plan view according to a marking II-II in 1 ;

Figure 3:

a sectional side view of a burner similar 1 with a device for combustion air supply and exhaust gas recirculation;

Figure 4:

according to the burner 3 in a sectional plan view according to a marking IV-IV in 3 ;

Figure 5:

similar to a burner 1 in a sectional side view with a chamber.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Figures 1 and 2 show a burner 1 with a combustion chamber 10 and with a device 2 for combustion air supply and exhaust gas recirculation in a sectional side view and in a sectional top view according to marking II-II in 1 .

The burner 1 shown has a fuel feed 3 with a feed connector 30 , a fuel lance 31 running coaxially to a central axis A and outlet nozzles 32 . In the exemplary embodiment shown, a flame holder 4 is provided upstream of the outlet nozzles 31 for stabilizing a flame front. The fuel supply 3 shown also includes an internal pilot burner or ignition device 34. The ignition device 34 is arranged in a pipe 35, which delimits a channel for the fuel supply in the fuel lance 31 of the fuel supply. The combustion chamber 10 is delimited by a flame tube 12 transversely to the direction of flow.

The device 2 comprises a combustion air supply with a supply nozzle 20, several, in the illustrated embodiment, sixteen fluidically connected to the combustion air supply, arranged around the central axis A and distributed around the fuel lance 31 propulsion nozzles 21 and a mixing chamber 22 downstream of the propulsion nozzles 21. The propulsion nozzles 21 and the mixing chamber 22 form a jet pump. The combustion air supplied by means of the driving nozzles 21 serves as a driving medium, by which a pumping effect is generated, so that an exhaust gas flowing out of the combustion chamber 10 is sucked in via an intake opening 25 provided between the driving nozzles 21 and the mixing chamber 22 . In the mixing chamber 22, the combustion air emerging from the driving nozzles 21 is mixed with the exhaust gases flowing out of the combustion chamber 10 and sucked back by means of the driving nozzles 21 to form a combustion air/exhaust gas mixture, and the combustion air/exhaust gas mixture is conveyed downstream of the mixing chamber 22 to a reaction zone in the combustion chamber 10 fed. The mixing chamber 22 of the device 2 shown has an annular cross-section and surrounds the fuel lance 31. The flame tube 12 connects to the mixing chamber 22. FIG. In the illustrated embodiment, the flame tube 12 and the mixing chamber 22 are realized by a common component. In other configurations, separate components are provided.

The in the Figures 1 and 2 The device 2 shown also has a bypass channel 23, by means of which combustion air can be fed to the reaction zone, bypassing the driving nozzles 21. In the exemplary embodiment shown, the bypass channel 23 is designed as an annular channel running coaxially to the central axis A between the fuel lance 31 and the mixing chamber 22 . The bypass duct 23 ends downstream of the mixing chamber 22 and upstream of the flame holder 4. For rapid and complete mixing of the combustion air supplied via the bypass duct 23 with the combustion air/exhaust gas mixture coming from the mixing chamber 22, in the exemplary embodiment shown there are nozzle openings at an outlet end of the bypass duct 23 230 provided. In the exemplary embodiment shown, a continuously or steplessly adjustable bypass valve 232 is provided in the bypass channel 23 .

Downstream of the mixing chamber 22 and in the illustrated embodiment downstream of the outlet end of the bypass channel 23 and upstream of the flame holder 4 and the outlet nozzles 32 of the fuel feed 3, a probe 5 for an oxygen measurement is provided.

A sensor 6 is also provided for measuring the temperature of the recirculated exhaust gas. In the exemplary embodiment shown, the measuring sensor 6 is arranged in the region of the suction opening 25 of the jet pump formed by the mixing chamber 22 and the driving nozzles 21 .

An exhaust gas recirculation ratio of the combustion air/exhaust gas mixture conveyed by the jet pump depends on a cross-sectional ratio of the mixing chamber 22 and the driving nozzles 21 and on operating parameters such as a temperature of the recirculated exhaust gas.

In order to reduce a flame temperature to 1500°C, an exhaust gas recirculation ratio of 1 to 1.5 is required, depending on the temperature of the recirculated exhaust gas. A cross-sectional ratio of the mixing chamber 22 and the propulsion nozzles 21 is corresponding to the Those skilled in the art designed to be suitable for a temperature range of the recirculated exhaust gas. In the illustrated embodiment, the cross-sectional ratio is selected to be less than 20. The mixing chamber 22 shown has funnel-shaped inlet and outlet areas. A cross section of the mixing chamber 22 is determined in an intermediate section with a constant cross section.

If it is necessary to reduce an exhaust gas recirculation ratio during operation to maintain flame stability, for example due to deviations in the temperature of the recirculated exhaust gas, part of the combustion air can be supplied via the bypass channel 23 in the illustrated exemplary embodiment. An oxygen content can be detected using the probe 5 and can be regulated to a specific value using the bypass valve 232 .

Figures 3 and 4 show a burner 1 with a combustion chamber 10 and with a device 2 for combustion air supply and exhaust gas recirculation in a sectional side view and in a sectional top view according to marking II-II in 1 . The burner 1 according to Figures 3 and 4 is similar to the burner 1 according to the Figures 1 and 2 and uniform reference numbers are used for identical components. A detailed description of components already described is omitted.

In contrast to the embodiment according to the Figures 1 and 2 has the device 2 according to the Figures 3 and 4 no bypass channel 23 on. Instead, a continuously or steplessly adjustable valve 27 is provided in the intake opening 25 for the sucked-back exhaust gas. If it is necessary to reduce an exhaust gas recirculation ratio during operation to obtain flame stability, in the exemplary embodiment according to FIG Figures 3 and 4 a recirculation of exhaust gas by means of the valve 27 can be reduced. It is analogous to the embodiment according to the Figures 1 and 2 an oxygen content of the combustion air/exhaust gas mixture upstream of the outlet nozzles 32 of the fuel supply can be detected by means of the probe 5 and—in contrast to the exemplary embodiment according to FIGS Figures 1 and 2 - Adjustable to a specific value using the valve 27. In the exemplary embodiment shown, an annular cavity remains between the fuel lance 31 of the fuel feed 3 and the mixing chamber 22, which cavity can be used, for example, for routing the cable of the probe 5. In other configurations, an inner diameter of the annular mixing chamber 22 is equal to an outer diameter of the channel 31, so that no cavity remains.

figure 5 shows burner 1 according to FIG 1 and a heating space 7 delimited by a housing 70. In the illustrated embodiment, a double-walled housing 70 is provided. In the double-walled housing 70 there is a coiled tubing 71 through which a medium to be heated is passed. The exhaust gas or combustion gas is routed through the double-walled housing 70 to an outlet 72 and in the process heats the medium routed in the coiled tubing. In addition, to avoid the formation of thermal nitrogen oxide, part of the exhaust gas is sucked in by the jet pump formed by the driving nozzles 21 and the mixing chamber 22 and is mixed with the combustion air.

In contrast to the configuration according to the Figures 1 and 2 is in accordance with the design figure 5 for operation with low reaction rate fuels such as natural gas, an extended flame tube 112 is provided for increased residence time to ensure burnout.

Claims (15)

Device for supplying combustion air and recirculating exhaust gas for a burner (1) with a combustion chamber (10), the device (2) comprising a plurality of propulsion nozzles (21) distributed around a central axis (A) and fluidically connected to a supply of combustion air, characterized in that a mixing chamber (22) downstream of the driving nozzles (21) is provided, with the driving nozzles (21) and the mixing chamber (22) forming a jet pump, and with combustion air emerging from the driving nozzles (21) in the mixing chamber (22) also coming out of the combustion chamber (10) exhaust gases flowing out and sucked back by means of the propulsion nozzles (21) can be mixed to form a combustion air/exhaust gas mixture and the combustion air/exhaust gas mixture can be fed to a reaction zone downstream of the mixing chamber (22). Device according to Claim 1, characterized in that the mixing chamber (22) has an annular cross-section. Device according to Claim 1 or 2, characterized in that eight or more propulsion nozzles (21) are provided, distributed uniformly around the central axis (A). Device according to Claim 1, 2 or 3, characterized in that a cross-sectional ratio of the mixing chamber (22) and the driving nozzles (21) is less than or equal to 20. Device according to one of Claims 1 to 4, characterized in that a bypass channel (23) is provided, by means of which combustion air can be fed to the reaction zone, bypassing the driving nozzles (21), nozzle openings (230) preferably being provided at an outlet end of the bypass channel (23) are provided. Device according to Claim 5, characterized in that an adjustable bypass valve (231) is provided in the bypass channel (23), the bypass valve (231) preferably being adjustable continuously or steplessly. Device according to one of Claims 1 to 6, characterized in that an adjustable valve (27) is installed in an intake opening (25) for the sucked-back exhaust gas. is provided, wherein preferably the valve (27) is continuously or steplessly adjustable. Device according to one of Claims 1 to 7, characterized in that a probe (5) for measuring oxygen, preferably upstream of outlet openings of a fuel supply (3), and/or a sensor (6) for measuring the temperature of the recirculated exhaust gas is provided. Burner comprising a device according to one of Claims 1 to 9 and a fuel lance (31) which is arranged coaxially to the central axis (A) and has outlet openings (32). Burner according to Claim 9, characterized in that a flame tube (12, 112) is provided which delimits the combustion chamber (10) transversely to the direction of flow. Burner according to Claim 9 or 10, characterized in that the fuel supply (3) comprises an ignition device (34) or a pilot burner. Method for supplying combustion air and recirculating exhaust gas for a burner (1) with a combustion chamber (10), in which combustion air is compressed by means of a plurality of propulsion nozzles (21) distributed around a central axis (A), with exhaust gases being sucked in from the combustion chamber (10) of one of the propulsion nozzles (21 ) is fed to the downstream mixing chamber (22), in the mixing chamber (22) the combustion air emerging from the propulsion nozzles (21) is mixed with exhaust gases flowing out of the combustion chamber (10) and sucked back by means of the propulsion nozzles (21) to form a combustion air/exhaust gas mixture and the combustion air/exhaust gas mixture is fed to a reaction zone downstream of the mixing chamber (22). Process according to Claim 12, characterized in that combustion air is optionally supplied to the reaction zone via a bypass duct (23), bypassing the propellant nozzles (21). The method according to claim 13, characterized in that monitors an oxygen content of a mixture of the selectively via the bypass channel (23) supplied combustion air and the combustion air / exhaust gas mixture and a quantity of the Bypass channel (23) supplied combustion air is adjusted to maintain a defined oxygen content. Method according to Claim 12, 13 or 14, characterized in that a temperature of the recirculated exhaust gas is recorded and a quantity of the combustion air supplied via the bypass duct (23) is adjusted as a function of the recorded temperature.

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