Process and apparatus for low-NO _x combustion

The invention relates to a process and an apparatus for low-NO _x combustion using fuel and oxidizing agent and/or furnace off-gases and/or carbon dioxide and/or steam.

In the known low-NO _x combustion, the furnace off-gases, which are sucked in by a blower, sheath the burner flame, thereby reducing the flame temperature and consequently the thermal emission of NO _x .

However, this conventional combustion has the significant drawback that the furnace off-gases which are recirculated in the furnace installation are not completely mixed with the oxidizing agent, and consequently the stipulated emission of NO _x in the off-gas can only be realized at additional cost.

High investment costs are inevitable with the known low-NO _x combustion, and costs are additionally incurred for maintenance of the installation, in particular the highly loaded blower and the pipelines. Moreover, external energy is required to operate the blower.

Therefore, it is an object of the present invention to provide a process and an apparatus which allow economical and low-pollutant (low-NO _x ) combustion in conven- tional furnace installations.

This object is achieved by a process having the features of Claim 1 and by an apparatus having the features of Claim 13.

Advantageous refinements of the invention are given in the subclaims.

According to the invention, a mixture of oxidizing agent and/or furnace off-gas and/or carbon dioxide and/or steam is burnt with the fuel, which is fed to the

burner separately, by means of the burner, which is arranged in a burner block in a refractory lining of a furnace installation.

For this purpose, the oxidizing agent is fed to an injector at a pressure of from 0.2 to 40 bar and advantageously having been heated from 20 to 900 ⁰ C in a heat exchanger by means of furnace off-gas. The oxidizing agent may also be fed to the injector directly without being heated.

The oxidizing agent, which expands as it flows out of the nozzle (which is axially displaceable in the injector at the flow end side), generates a gas jet at a velocity of from 20 to 660 m/s, and thereby generates a reduced pressure in the injector, the sucking action of which sucks either furnace off-gas and/or carbon dioxide (CO ₂ ) and/or superheated steam generated from water through heat exchange with furnace off-gas into the jet of oxidizing agent, and this mixture is then fed to the burner, with temperature balancing, in a line connecting the injector to the burner.

A conventional blowing nozzle or some other equivalent technical means can also be used instead of the injector, which is advantageously arranged in a stack provided for discharging the furnace off-gases from the combustion chamber of the furnace installation.

As an alternative to the oxidizing agent, it is possible for fuel gas at a pressure of from 0.2 to 40 bar to be fed to the injector, hi this case, the oxidizing agent is added to the burner.

The mixture of oxidizing agent and/or furnace off-gases and/or carbon dioxide (CO ₂ ) and/or steam, which is fed to the burner at a temperature of from 20°C to 1600 ⁰ C, preferably 900 ⁰ C, and at a velocity of from 5 to 70 m/s, has an oxygen content of at least 5% by volume.

The burner, which is, for example, arranged set back in the burner block, is advantageously a parallel-flow burner with two tubes (inner tube and outer tube) arranged substantially coaxially with respect to one another for feeding fuel and oxi-

dizing agent and/or furnace off-gases and/or carbon dioxide and/or steam to the burner mouth. The fuel or the oxidizing-agent mixture may be passed to the burner mouth through the inner tube or through the outer tube.

The oxidizing agent used is an oxygen-containing medium with an oxygen content of at least 10% by volume.

The fuel used may be any conventional gaseous or liquid fuel, particularly advantageously natural gas.

The injector, which is advantageously operated with the oxidizing agent, is equipped with an axially displaceable nozzle for controlling the intake quantity and concentration and temperature of the mixture fed to the burner. This eliminates the need to supply the injector with external energy, which entails additional costs.

The heat exchanger which is used to heat the oxygen, carbon dioxide and the water and is advantageously arranged in the stack that discharges the furnace off-gases from the combustion chamber of the furnace installation is advantageously a conventional recuperator or regenerator.

The burner used is preferably a conventional parallel-flow burner with at least one feed for the oxidizing agent and at least one feed for the fuel, preferably comprising two cylindrical, concentrically arranged tubes.

The burner design according to the invention allows the mixture of oxidizing agent and/or furnace off-gases and/or carbon dioxide (CO ₂ ) and/or steam to flow out of the burner mouth of the burner at a velocity which is 0.3 to 4 times higher than the fuel, with the result that a total momentum flux, based on the burner power, of from 1.5 to 8 NMW and a ratio of the momentum flux densities of the mixture of oxidizing agent and furnace off-gases to fuel of from 0.8 to 31 are ensured, and as a result a power density of from 0.2 to 0.5 KW/mm ² is reached at the outlet of the burner block.

The outlet velocity of the mixture of oxidizing agent and/or furnace off-gases and/or carbon dioxide (CO ₂ ) and/or steam is between 20 and 80 m/s at the burner mouth.

The burner may also be arranged on the off-gas side of the furnace installation, preferably in the stack which discharges the furnace off-gases from the combustion chamber of the furnace installation, or at any other location which is suitable for its intended use in the furnace wall surrounding the combustion chamber of the furnace installation.

It is also possible for the injector and the heat exchanger to be arranged in the burner. An injector/heat exchanger arrangement of this type is advantageous if the furnace off-gas is extracted through an annular gap around the burner mouth, as for example in the case of rotary drum furnaces, in particular when the burner is in- stalled on the off-gas side of the furnace. In this case, the mixture of oxfdizing agent and/or furnace off-gas and/or carbon dioxide and/or steam is recuperatively heated by the furnace off-gases.

The lines which carry the oxidizing agent, the furnace off-gas, the carbon dioxide and the steam consist of heat-resistant and corrosion-resistant NiCr or ODS alloys and are provided with an insulation which ensures the required thermal protection from the inside and/or the outside and preferably ceramic fibres.

The burner block which includes the burner preferably has a cylindrical opening.

The burner is equipped with a UV light receiver for flame monitoring.

The mixture of oxidizing agent and/or furnace off-gas and/or carbon dioxide and/or steam which is fed to the burner in accordance with the invention reduces the reaction rate of the combustion, since the reactions of the oxygen with the fuel are impeded by the CO ₂ and/or H ₂ O molecules.

The mixing of the oxidizing agent with furnace gas and/or carbon dioxide and/or

steam results in the formation of a voluminous combustion flame with a high concentration of carbon dioxide and steam. The greater volume of the flame compared to that achieved with known combustion, and the higher concentration of carbon dioxide and/or steam in the burner flame significantly increase the gas radiation of carbon dioxide and/or steam, which takes place in the spectral region in radiation bands, with the result that the material to be treated can be heated by a flame temperature which lowers the levels of NO _x in the off-gas. The radiation bands which are relevant to carbon dioxide are in the range from 2.4 to 3 μm, 4 to 4.8 μm, 12.5 to 16.4 μm, and those which are relevant to steam are in the range from 1.7 to 2 μm, 2.2 to 3 μm and 12 to 30 μm.

As a result of the high-viscosity mixture of oxidizing agent and/or furnace off- gases and/or carbon dioxide and/or steam being fed to the burner at a temperature of from 20°C to 1600 ⁰ C, preferably 900°C, this mixture is mixed in such a manner with the fuel at the burner mouth that the combustion takes place at a flame temperature of from 800°C to 2700°C, which significantly reduces the thermal NO _x off-gas potential of the furnace installation.

The mixture of oxidizing agent and/or furnace off-gases and/or carbon dioxide and/or steam which is fed to the burner, as well as the burner which is used in accordance with the invention, causes the fuel to be at least partially self-carburized in the fuel tube of the burner and, owing to the design of the burner, in the fuel-rich core of the burner flame. The self-carburization or decomposition takes place in oxygen-free zones and at temperatures of greater than 1000°C in the case of hy- drocarbons, so as to form soot. The heating of the soot particles in the burner flame leads to continuous radiation in the range from 0.2 to 20 micrometers and therefore to cooling of the flame, so that the NO _x off-gas levels from the furnace installation are additionally lowered.

A further advantage is the improved heating of lower layers, e.g. in a glass melt bath, since liquid glass is semi-transparent to wavelengths in the range from 0.3 to 4 micrometers.

The NO _x off-gas levels are additionally reduced by the use of preferably low-TM ₂ oxidizing agent mixtures and fuels.

The circulating furnace gases cause nitrogen oxides which are present in the com- bustion chamber of the furnace installation to be fed to the burner flame, and these nitrogen oxides are then reduced to form nitrogen (N ₂ ) in the fuel-rich zones of the burner flame.

The very long, soft and visible flames generated in the combustion chamber of the furnace installation allow particularly advantageous low-NO _x combustion in aluminium holding furnaces and rotary drum furnaces.

Moreover, the combustion according to the invention is stable and low-noise. The noise level is 50-80 Decibels.

With the low-NO _x combustion according to the invention - unlike with the known flame-free combustion - the flame radiation in the visible region advantageously increases the heat transfer to the material to be treated.

The high concentration and volume of CO ₂ /H ₂ O vapour in the burner flame additionally increases the gas radiation of CO ₂ and/or H ₂ O vapour, which takes place in the spectral region in radiation bands, in such a manner as to ensure improved heat transfer to the material to be treated, e.g. when melting glass.

Furthermore, the turbulence and swirling during combustion, which have a disruptive influence when dust-containing products are introduced, are reduced.

The injector insert significantly reduces the wear and maintenance costs for the furnace installation incurred, for example, with a blower consisting of expensive heat-resistant materials which has hitherto been used. Moreover, the supply of external energy which has hitherto been required to operate the blower is no longer necessary.

Furthermore, the thermal loading and therefore wear to the pipe tubes is reduced, since the mixing of the oxidizing agent with furnace off-gases and/or carbon dioxide and/or steam lowers the temperature of the media that are to be transported.

In addition, primary energy can be saved through preheating of the oxygen used as oxidizing agent and/or carbon dioxide and/or steam by furnace off-gases in the heat exchanger, and as a result the operating costs of the furnace installation can be reduced further.

The Io W-NO _x combustion according to the invention, with a uniform temperature distribution at a low temperature level (burner flame) in the combustion chamber and therefore with a significantly reduced NO _x off-gas potential can be used in any conventional furnace installation, particularly advantageously in aluminium holding furnaces or glass-melting furnaces.

The invention is explained in more detail below on the basis of an exemplary embodiment illustrated in the drawing, in which:

Fig. 1 diagrammatically depicts a furnace installation with combustion apparatus;

Fig. 2 diagrammatically depicts a further furnace installation with combustion apparatus;

Fig. 3 diagrammatically depicts a third furnace installation with combustion appa- ratus.

The furnace installation illustrated in Fig. 1 comprises a refractory lining 1 which surrounds a combustion chamber and has an off-gas opening 19 and a stack 2, which discharges the furnace off-gases, and pipeline 3 as well as a burner block 4 with a burner 5, the burner 5 being connected by a pipeline 7 to an injector 6 and to a heat exchanger 8 arranged in the stack 2.

The furnace off-gases which flow out of the combustion chamber through the off-

gas opening 19 are cooled as they flow around the heat exchanger 8 and then flow out of the furnace installation through the stack 2.

The gaseous oxygen, which is used as oxidizing agent at a temperature of from -20 to 4O ⁰ C and at a pressure of from 0.2 to 40 bar, flows into the heat exchanger 8 through an inlet 9.

The oxygen flowing through the heat exchanger 8, which is designed as a recuperator or regenerator, is heated by the furnace off-gases flowing around the heat exchanger 8 and flows through an outlet 10 of the heat exchanger 8 into the injector 6 through an inlet 11 at a temperature of from 20 to 900 ⁰ C.

The oxygen which flows out of the outflow nozzle 12 of the injector 6 at a velocity of from 20 to 660 m/s expands, thereby generating an oxygen jet flowing at a ve- locity of from 20 to 660 m/s.

The high flow velocity of the oxygen jet generates a reduced pressure at position 13 in the injector 6, the sucking action of which reduced pressure sucks the furnace off-gases out of the combustion chamber through the pipeline 3 into the oxygen jet, and in the pipeline 7, which is designed as a mixing section of length x, they are mixed with the oxygen jet, with temperature balancing, after which the mixture of oxygen and furnace off-gases is fed, at a temperature of from 20 to 1600 ⁰ C, through a connection 14 to the burner 5, which via a further connection 15 is supplied with natural gas as gaseous fuel.

The pipelines carrying the oxygen and the furnace off-gases consist of a heat- resistant NiCr or ODS alloy and are provided on the inner side with a thermal protection and/or on the outer side with a thermal insulation, e.g. comprising ceramic fibres or ceramic blocks.

The burner 5, which is used as a parallel-flow burner, advantageously has an inner tube and an outer tube, with the natural gas used as gaseous fuel flowing to the burner mouth 16 through the fuel tube 18, which is arranged as the inner tube, and

the mixture of oxygen and furnace off-gas flowing to the burner mouth 16 through the outer tube, which accommodates fuel tube 18 and is designed as an annular gap 21, generating a long, soft and visible burner flame 17 in the combustion chamber of the furnace installation for heating material that is to be treated.

Partial self-carburization of the fuel takes place in the fuel tube 18 of the burner 5 through recuperative heat exchange with the mixture of oxidizing agent and furnace off-gases.

The burner structure according to the invention allows the mixture of oxidizing agent and furnace off-gases to flow out bf the burner mouth 16 of the burner at a velocity which is 0.3 to 4 times higher than the fuel, with the result that a total momentum flux, based on the burner power, of from 1.5 to 8 N/MW and a ratio of the momentum flux densities of the mixture of oxidizing agent and furnace off- gases to fuel of from 0.8 to 31 are ensured, and as a result a power density of from 0.2 to 0.5 KW/mm ² is reached at the outlet of the burner block 4.

The mixture of oxidizing agent and furnace off-gases flows out of the burner mouth 16 at a velocity of from 20 to 80 m/s.

The burner flame which burns the material that is to be treated in the combustion chamber has a flame temperature of from 800 °C to 2700 ⁰ C.

The burner block 4 which accommodates the burner 5 has a preferably cylindrical opening.

The burner is advantageously equipped with a UV light receiver 20 for flame monitoring.

The furnace installation which is diagrammatically depicted in Fig. 2 is advantageously used if the furnace off-gases are ladened with dust or other substances which are aggressive or promote oxidation. This furnace installation comprises the refractory lining 1, which surrounds a combustion chamber of a furnace installa-

tion and has an off-gas opening 19, and a stack 2, which discharges the furnace off-gas and accommodates the heat exchanger 8, as well as the burner block 4, which contains the burner 5 and is connected by a pipeline 7 to the injector 6 and the heat exchanger 8.

The furnace off-gases which flow out of the combustion chamber through the off- gas opening 19 are cooled as they flow around the heat exchanger 8, which is supplied with water, and then flow out of the furnace installation via the stack 2.

As it flows through the heat exchanger 8, the water which is fed to the heat exchanger 8 through the inlet 9 is evaporated through heat exchange with the furnace off-gas flowing around the heat exchanger 8 and then flows into the injector 6 at position 13 as superheated steam at a temperature of from 20 to 900°C.

The gaseous oxygen, which is used as oxidizing agent at a temperature of from -20 to 40°C and a pressure of from 0.2 to 40 bar, flows into the injector 6 through the inlet 11. The oxygen jet expanding as it flows out of the outflow nozzle 12 of the injector 6 increases its flow velocity to 20 to 340 m/s, with the result that a reduced pressure is generated at position 13 in the injector 6, the sucking action of which reduced pressure sucks the superheated steam into the oxygen jet flowing through the injector 6 at position 13 and mixes it with the oxygen jet, with temperature balancing, in the pipeline 7, which is designed as mixing section of length x, and the oxygen/steam mixture flows, at a temperature of from 20 to 1600°C, through connection 14 into the burner 5, which is supplied through connection 15 with natural gas as gaseous fuel.

The pipelines carrying the oxygen and the steam consist of a heat-resistant and corrosion-resistant NiCr or ODS alloy and are designed from the inside with a thermal protection or from the outside with a thermal insulation, e.g. comprising a ceramic fibre or ceramic block.

The burner 5, which is used as a parallel-flow burner, advantageously has an inner tube and an outer tube, natural gas which is used as gaseous fuel flowing to the

burner mouth 16 through the fuel tube 18, which is arranged as an inner tube, and the mixture of oxygen and steam flowing to the burner mouth 16 through the outer tube, which accommodates the fuel tube 18 and is designed as an annular gap 21, thereby generating the long, soft and visible burner flame 17 with a flame tempera- tare of from 800°C to 2700°C in the combustion chamber of the furnace installation for heating material that is to be treated.

Partial self-carburization of the fuel takes place in the fuel tube 18 of the burner 5 through recuperative heat exchange with the mixture of oxidizing agent and steam.

The burner design according to the invention allows the mixture of oxidizing agent and steam to flow out of the burner mouth 16 of the burner at a velocity which is 0.3 to 4 times higher than the fuel, with the result that a total momentum flux, based on the burner power, of from 1.5 to 8 N/MW and a ratio of the momentum flux densities of the mixture of oxidizing agent and steam to fuel of from 0.8 to 31 are ensured, and as a result a power density of from 0.2 to 0.5 KW/mm is reached at the outlet of the burner block 4.

The mixture of oxidizing agent and steam flows out of the burner mouth 16 at a velocity of from 20 to 80 m/s.

The burner block 4 has a preferably cylindrical opening.

The burner is equipped with a UV light receiver 20 for flame monitoring.

The furnace installation which is diagrammatically depicted in Fig. 3 is used if the furnace off-gases are ladened with dust or other aggressive or oxidation-promoting substances. This furnace installation comprises the refractory lining 1, which surrounds a combustion chamber and has an off-gas opening 19, and the stack 2, which is designed to discharge the furnace off-gas and contains the heat exchanger 8, as well as the burner block 4 with burner 5, burner 5 being connected to the injector 6 and to the heat exchanger 8 by a pipeline 7.

The exhaust gases which flow out of the combustion chamber through the off-gas opening 19 are cooled as they flow around the heat exchanger 8, which is supplied with carbon dioxide, and then flow out of the furnace installation through the stack 2.

Liquid or preferably gaseous carbon dioxide which is supplied through the inlet 9 of the heat exchanger 8 is heated to 20°C to 900°C through heat exchange with the furnace off-gas flowing around the heat exchanger 8 and flows through the outlet 10 into the injector 6 at position 13.

The gaseous oxygen, which is used as oxidizing agent at a temperature of from -20 to 4O ⁰ C and a pressure of from 0.2 to 40 bar, is fed to the injector 6 through the inlet 11. The oxygen flowing through the injector 6 expands as it flows out of the outflow nozzle 12 of the injector, so that its flow velocity is increased to from 20 to 340 m/s, with the result that a reduced pressure is generated in the injector 6 at position 13, the sucking action of which reduced pressure sucks the carbon dioxide into the oxygen jet, with the carbon dioxide being mixed with the oxygen jet, with temperature balancing, in the pipeline 7, which is designed as a mixing section with a length x, and then the mixture of oxygen and carbon dioxide flows, at a temperature of from 20 to 1600 ⁰ C, through connection 14 into the burner 5, which is supplied via a further connection 15 with natural gas as gaseous fuel.

The pipelines carrying the oxygen and the carbon dioxide consist of a heat- resistant and corrosion-resistant NiCr or ODS alloy and are provided on the inner side with a thermal protection and/or on the outer side with a thermal insulation, e.g. comprising ceramic fibres.

The burner 5, which is used as a parallel-flow burner, advantageously has an inner tube and an outer tube, with natural gas used as gaseous fuel being fed to the burner mouth 16 through the fuel tube 18, which is arranged as the inner tube, and the mixture of oxygen and carbon dioxide being fed to the burner mouth 16 through the outer tube, which accommodates the fuel tube 18 and is designed as an annular gap 21, producing a long, soft and visible burner flame 17 with a flame

temperature of from 800-2700°C in the combustion chamber of the furnace installation for heating material that is to be treated.

Partial self-carburization of the fuel takes place in the fuel tube 18 of the burner 5 through recuperative heat exchange with the mixture of oxidizing agent and carbon dioxide.

The burner design according to the invention allows the mixture of oxidizing agent and carbon dioxide to flow out of the burner mouth 16 of the burner at a velocity which is 0.3 to 4 times higher than the fuel, with the result that a total momentum flux, based on the burner power, of from 1.5 to 8 N/MW and a ratio of the momentum flux densities of the mixture of oxidizing agent and carbon dioxide to fuel of from 0.8 to 31 are ensured, and as a result a power density of from 0.2 to 0.5 KW/mm ² is reached at the outlet of the burner block 4.

The mixture of oxidizing agent and carbon dioxide flows out of the burner mouth 16 at a velocity of from 20 to 80 m/s.

The burner block 4 has a preferably cylindrical opening.

The burner is equipped with a UV light receiver 20 for flame monitoring.

List of designations

Refractory lining

Stack (furnace off-gas)

Pipeline (furnace off-gas)

Burner block

Burner

Injector

Pipeline

Heat exchanger

Inlet (8)

Outlet (8)

Inlet (6)

Outflow nozzle (6)

Position (6)

Connection (5)

Connection (5)

Burner mouth

Burner flame

Fuel tube

Off-gas opening

UV light receiver

Annular gap