EP0021035B1 - Operating process for premix burners and burner for carrying out the process - Google Patents

Description

The invention relates to a method for operating premix burners under normal or increased pressure with gaseous fuels, or with fuels which are liquid at normal temperature and completely vaporized before combustion, at low combustion temperatures with formation of low-emission gases, and a burner for carrying out the method.

During the combustion of gaseous and liquid fuels, the nitrogen oxides NO and NO _z ', collectively referred to as NO _x , are formed as pollutants in the exhaust gas. These pollutants contaminate the air and can have a negative effect on the material in the furnace or in contact with the burner exhaust gases in some furnaces. Therefore, efforts are made to keep the NO _x content in the exhaust gas as low as possible. The causes of the NO _x formation are known, and several measures for reducing the NO _x content in the exhaust gas are also known, for example

• zwei oder mehrstufige Verbrennung,
• Abgasrezirkulation durch Vorbeiführen der Abgase an der Flamme mittels Rückführungskanälen oder speziellen Brennerkonstruktionen,
• überstöchiometrische Verbrennung.

Lowering the combustion temperature through direct flame cooling, e.g. water injection or cooled burning surfaces,

• two or multi-stage combustion,
• Exhaust gas recirculation by passing the exhaust gases past the flame using recirculation channels or special burner designs,
• superstoichiometric combustion.

In spite of this, no burner for industry or trade is known to date, which has an extremely low NO _x content in the exhaust gas at high output (specific output based on the passage area or the passage volume of the burner). This also applies to a known premix burner (DE-B-1 526 031), in which the gas and air are passed into a mixing chamber through separate sectors of a perforated plate that can be changed in the passage cross-section to adjust the output of the burner, and in this in a certain chamber the entire output range of the burner is mixed in a constant quantitative ratio, after which the mixture passes through a burner plate which has an ordinary perforation of flame holes which allow the same mixture quantities to pass through.

The object of the invention is to provide a method for operating premix burners with which gaseous and / or vaporous fuels can be burned at normal or elevated pressure so that on the one hand complete combustion at low combustion temperatures with the formation of exhaust gases with extremely low NO _{x -} Held takes place, but on the other hand a high burner output is achieved and reliable combustion burning over a large output range is achieved, as well as creating a burner specially designed and suitable for carrying out this method.

According to the invention, this object is achieved by the measures mentioned in claims 1 to 9. According to the method according to the invention, a large amount of cooling gas is therefore used to reduce the NO _x formation, in particular at high specific burner outputs, at which the NO _x formation and tendency towards NO _x emission usually increases with the exhaust gases . The reduction in flame speed caused by the use of the cooling gas nevertheless permits stable combustion, because of the simultaneous application of the special flame design during the combustion of the mixture and because of the shielding of the flame until it is completely burned out.

A corresponding operation would not be possible with the known premix burner (DE-B-1 526 031) mentioned above. With this burner, if the amount of air compared to the amount of gas, ie the air ratio, were increased, the pressure losses in the burner would increase disproportionately with the increase in the air ratio, so that the burner output would have to decrease at the specified fan pressures. Furthermore, the mixture in front of the burner plate is accelerated by the narrowing mixing chamber, which is also too short for homogeneous mixing, and if the air ratio were also increased and the air ratios previously allowed for conventional premixing burners were increased, the mixture speeds would become too large to allow the flame to be held when the flame speed decreases at the same time as the air ratio increases, in particular since the known burner does not have a correspondingly adapted flame design for the possibility of preventing the flame from tearing off the burner plate when the air ratio is increased and has no means of effectively shielding the flame. The lack of effective flame shielding in the known burner also helps to make the combustion taking place incomplete, which leads to the combustion products in the furnace chamber following the burner plate reacting and producing uncontrolled temperature peaks which favor the formation of NO _x .

The method according to the invention and the structure and mode of operation of the burner according to the invention are explained below with reference to FIGS. 1 and 2.

NO _x is formed on the one hand from the nitrogen bound in the fuel and on the other hand thermally from free nitrogen, which is present in particular in the air and possibly also in the fuel, for example in natural gas. The thermal NO _x formation is preferably carried out at high combustion temperatures, for example natural gas from approximately 1600 ° C. A low combustion temperature and thus a low NO _x content in the exhaust gas can be achieved according to the inventive method for fuels with a low proportion of bound nitrogen by homogeneously mixing the combustion air / fuel mixture before the combustion with a cooling gas. This cooling gas can contain air, exhaust gas, Steam or a mixture of two or all of these components. In order to theoretically achieve 1 ppm NO _x in the exhaust gas (parts per million based on air-free and dry exhaust gas), a setting of the theoretical combustion temperature of 1330 ° is required at a pressure of 1 bar and when using air at 20 ° C as the cooling gas C required.

In FIG. 1, the dependence of the theoretical combustion temperature on the mass flow ratio e at different air temperatures T _i and exhaust gas temperatures T _{2 is} shown using the example of the combustion of natural gas.

The mass flow ratio e is defined as the ratio of a first mass flow, which is composed of a fuel quantity, a combustion air quantity and a cooling gas quantity, to a second mass flow, which is composed of the same fuel quantity and the combustion air quantity required for the stoichiometric combustion. The theoretical combustion temperature results from the heat calorific value and the enthalpies of the materials fed to the burner without heat exchange with the environment, with complete combustion of the fuel to C0 ₂ and H ₂ 0. The enthalpies are determined by quantities, temperatures and specific heat capacities of the substances.

In Figure 1, the solid curves of a first family of curves show which combustion temperatures are reached as a function of the mass flow ratio e if the fuel natural gas is mixed homogeneously with air at the temperature T, indicated on the solid curves before combustion, if so in that The mass flow ratio defined above, the first mass flow does not contain any recirculated exhaust gas as cooling gas and air quantities of different sizes are used as cooling gas.

The dashed curves of a second family of curves show in FIG. 1 the combustion temperatures that occur as a function of the mass flow ratio e, if the first mass flow of the mass flow ratio defined above contains an air quantity that is equal to the air quantity required for stoichiometric combustion in the second mass flow, and if the first mass flow contains recirculated exhaust gas as cooling gas. It applies to the dashed curves that the supplied combustion air has a temperature of 20 ° C and that the exhaust gas serving as cooling gas has the temperature T ₂ indicated in each case on the dashed curves.

The dashed curves represent only an example for the determination of the theoretical combustion temperature or the mass flow ratio. For the sake of clarity, the corresponding curves have not been shown for cases in which differently tempered water vapor is used as cooling gas or that a differently tempered cooling gas with combustion air is mixed at a temperature other than 20 ° C. Such curves can be calculated using the specific data published in relevant manuals and the like.

It can be seen from Figure 1 that in order to reach a theoretical combustion temperature of e.g. 1300 ° C when using only combustion air of 20 ° C at the same time as cooling gas (bottom solid curve) the mass flow ratio is e = 1.74, while when using combustion air of 20 ° C mixed with exhaust gas of 100 ° C as cooling gas (bottom dashed line Curve) the mass flow ratio e = 1.70.

Experiments carried out both in the laboratory and in operational use have shown NO _x values of 1.5 ppm (air-free, dry) when using natural gas as fuel and air as combustion and cooling gas, with a theoretical combustion temperature of 1300 ° C was set. This shows that the theoretical values mentioned above are largely achieved in practice. With the burners that have been customary to date, the NO _x content in the exhaust gas averages 50-500 ppm (air-free, dry).

At low combustion temperatures (e.g. natural gas below approx. 1600 ° C) the combustion rate becomes so low that the combustion can be unstable and that further cooling of the flame can easily stabilize combustion intermediates such as CO and formaldehyde. These difficulties are avoided if the burner is designed according to the invention.

The burner according to the invention is suitable for all fuels which are in gaseous or vapor form before combustion and which can be mixed homogeneously with the combustion air and the cooling gas. The burner can be operated under normal pressure as well as under increased pressure.

An embodiment of the burner according to the invention is shown in Figure 2. The method according to the invention and the burner are described below.

The mixing tube 1 must be supplied with fuel 2, combustion air 3 and cooling gas 4. The combustion air is fed to the mixing tube, for example, by a blower, not shown in FIG. 2.

If air is used as the cooling gas, this air is supplied in the same way. If exhaust gas or water vapor serve as cooling gas, these can be conveyed together with the combustion air by a fan if their temperature or the temperature of the air-cooling mixture is permissible for the fan. Otherwise, the cooling gas as well as the fuel can reach the mixing tube directly, e.g. by injector action. To shorten the mixing tube, the fuel can also be fed upstream of the blower.

The burner head 5 is connected to the mixing tube 1, and its cross section 6 at the connection to the mixing tube 1 is, for example, twice the cross section of the mixing tube. This abrupt transition to a larger flow cross-section creates a tear-off edge for the flow. The burner head 5 then expands ko nisch to, for example, 4.5 times the cross section of the mixing tube. Instead of the conical shape of the burner head jacket shown, curved jacket shapes are also possible. At the end of the burner head there is a burner plate 7 which has a large main flame bore 8 and a plurality of small bores 9 which are arranged in a plurality of concentric rings around the main flame bore 8 and serve to form the holding flames. Depending on the size of the burner head, there may be several main flame holes in the burner plate. In addition, the small bores 9 can be replaced by corresponding slot-shaped openings. The burner plate can consist of both metal and ceramic material. The distances between the holding flame bores 9, which together have a slightly smaller free cross-section than the main flame bore 8, are selected so that they ensure a perfect ignition from the outermost holding flames to the main flame and a mutual stabilization of the holding flames. While the main flame bore 8 runs parallel to the burner axis, at least the holding flame bores 9, which are located in the outermost ring, are inclined at an angle of, for example, approximately 40 ° to the burner axis. The outermost holding flame ring is stabilized in this way by backflows on the cylindrical wall of the burner mouth 10, which adjoins the burner plate 7.

The burner mouth 10 is only a short piece cylindrical and then tapers conically, for example to 2.9 times the cross section of the mixing tube. Analogously to the burner head, the lateral surface of the burner mouth can be either conical, as shown in FIG. 2, or curved. The burner plate 7 can also be made conical or curved instead of the flat shape shown.

In order to protect the resulting flame from cooling from the outside and to prevent undesired penetration of foreign gases into the flame or combustion area, which would have the negative effects described above, the burner mouth 10 is connected to a flame protection cover 11. In Figure 2 it is shown as a cylindrical tube, the inside diameter of which corresponds to the largest outside diameter of the free-burning flame. Another advantageous embodiment, not shown, of the flame protection cover consists of a conically expanded and subsequently cylindrical tube, which is therefore adapted to the shape of the flame. The flame protection cover is designed in such a way that it does not hinder or restrict the flame. The flame protection cover 11 prevents the flame from being cooled further by contact with air and / or exhaust gas from the environment and would thereby be prevented from completely burning out. It has proven to be advantageous to provide the inside of the burner mouth 10 and the flame protection cover 11 with a catalytically ineffective material or at low ambient temperatures with thermal insulation, e.g. Ceramics to line. The task of a flame protection cover can also be fulfilled by a combustion chamber that does not dissipate useful heat and in which the flame can burn out completely.

With the method according to the invention, it is possible for the first time to burn homogeneous mixtures of the type mentioned with very high mass flow ratios in a manner that is reliable and low in pollutants. By setting the mass flow ratio, a desired combustion temperature can be set in the manner described above. Because the mixing of the burner gases with foreign gases, such as air or exhaust gas, which are present in the vicinity of the burner, is largely avoided, the flame temperature remains so homogeneous that the thermal NO _x formation largely corresponds to the NO _x formation in the theoretical Combustion temperature corresponds.

Despite the simplest construction, the burner according to the invention is characterized, among other things, by a quiet, stable, low-pollutant combustion over a large output range.

The possible uses for the subject matter of the invention are extremely versatile. These include, for example, the generation of exhaust gas-air mixtures for heating and drying food, the heating of boilers and industrial ovens of all kinds and the generation of drive gas for gas turbines. In all of these cases, because of the unusually low NO _x content in the exhaust gas, the subject of the invention can make a valuable contribution to air pollution control.

Claims (9)

1. Process for the operation of pre-mixture burners under normal or elevated pressure with gaseous fuels, or with fuels which are liquid at normal temperature and completely vaporised before combustion, at low combustion temperatures, forming waste gases having a low content of harmful substances, characterised in that a homogeneous mixture which is composed of the fuel in gas or vapour form, a combustion air quantity required for the complete combustion of the fuel and a cooling gas quantity serving for the setting of the combustion temperature of 1100to 1700°C, preferably 1200to 1300°C, is fed to the burner, in that the combustion of the homogeneous mixture takes place in a manner known per se in at least one central main flame which is surrounded by several support flame rings, and in that the occurring flame is protected over its length against access of ambient air and/or waste gas and against cooling to the exterior or heating from the exterior until the completion of burning.

2. Process according to Claim 1, characterised in that air and/or waste gas and/or water vapour is used as cooling gas in the unburned mixture fed to the burner.

3. Process according to Claim 1 or 2, characterised in that the quantity of the cooling gas amounts to 20 to 600% of the quantity of air required for the complete combustion of the fuel.

4. Apparatus for carrying out the process according to one of Claims 1 to 3, characterised by a mixer pipe (1) with supply conduits for the fuel (2), for the combustion air (3) and for the cooling gas (4), a burner head (5) adjoining the mixer pipe (1), which has at its entry end connected to the mixer pipe a cross-section of 1.1 to 3.8 times, preferably 1.8 to 2.7 times, the mixer pipe cross-section and widens to its exit end to a cross-section of 2.0 to 6.8 times, preferably 3.2 to 4.8 times, the mixer pipe cross-section, a burner plate (7) arranged at the widened exit end of the burner head (5), which in a manner known per se contains at least one large main flame bore (8) parallel to the burner axis and several small support flame openings (9) surrounding the main flame bore in several concentric rings, where at least the support flame openings in the outermost ring extend at an angle of 10 to 70°, preferably 25 to 45°, to the burner axis, a burner mouth (10) adjoining the burner plate (7) which with a cross-section corresponding to the widened exit end of the burner head is made initially cylindrical and then narrows to 1.4 to 4.9 times, preferably 2.3 to 3.5 times, the mixer pipe cross-section, and by a flame guard (11) adjoining the narrowed end of the burner mouth (10), which guard surrounds the flame over its length as far as the position of completion of flame combustion and the internal diameter of which corresponds to the maximum external diameter of the free-burning flame.

5. Apparatus according to Claim 4, characterised in that the flame guard (11) consists of a cylindrical tube extending from the narrowed end of the burner mouth (10) to the completely burned-out flame end.

6. Apparatus according to Claim 4, characterised in that the flame guard (11) consists of a tube which, beginning at the narrowed end of the burner mouth (10), firstly widens conically and then extends cylindrically as far as the completely burned-out flame end.

7. Apparatus according to one of Claims 4 to 6, characterised in that the burner mouth (10) and the flame guard (11) are formed by a combustion chamber connected to the burner head (5) with burner plate (7), which chamber conducts away practically no useful heat.

8. Apparatus according to one of Claims 4 to 7, characterised in that the burner mouth (10) and the flame guard (11) are internally lined with a catalytically ineffective material.

9. Apparatus according to Claim 8, characterised in that the burner plate (7), the burner mouth (10) and the flame guard (11) are produced from and/or lined with a thermally insulating ceramic material.

Images