EP0510783B1 - Device for the combustion of fluid or gaseous fuels - Google Patents

Abstract

Device for the combustion of fluid and gaseous fuels with a burner nozzle (12) arranged in a burner pipe (10), a device (13) for supplying the fuel to the burner nozzle (12), a device (15) for supplying the combustion air into the burner pipe (10), which device is essentially arranged around the device (13) for supplying the fuel, an insert (14) for returning combustion products into the combustion zone, which insert is inserted into the burner pipe (10) at the burner-side end, the return of the combustion products taking place essentially in an intermediate space (16) between the insert (14) and the burner pipe (10), and a device (18) for tangential introduction of air into the combustion air supply (15) in order to create at least inside the insert (14) a supercritical swirling flow. <IMAGE>

Description

The invention relates to a device for the combustion of flowable or gaseous fuels, with a burner nozzle arranged in a burner tube, a device for supplying the fuel to the burner nozzle, a device for supplying combustion air into the burner pipe, essentially around the device for supplying the fuel is arranged around, and an insert for returning combustion products into the combustion zone, which is inserted into the burner tube at the end of the burner, the return of the combustion products taking place essentially in a space between the insert and the burner tube.

Firing systems with facilities for the recirculation of combustion products and / or exhaust gases serve the purpose of a to achieve as complete a combustion as possible. The aim is to implement the energy content contained in the fuel as completely as possible. Another goal is to reduce the _NOx emissions by lowering the temperature of the flame is reduced. NO _x is generated either from bound nitrogen, for example from heavy heating oil, or from the free nitrogen in the ambient air used for combustion, in which it contains about 78%. During combustion, nitrogen is oxidized to NO if it is exposed to temperatures above approx. 1400 ° C. Such temperatures usually occur in gas and oil burner flames if no measures are taken to reduce the temperature.

Another problem is that flames are unevenly hot, ie areas of higher temperature (temperature peaks) are embedded in "normal" areas of the flame. The longer the nitrogen remains in such temperature peaks, the more it is naturally converted into NO. The corresponding relationships have been known for a long time.

Attempts have also been made for a long time to reduce the thermal NO _x formation from the atmospheric nitrogen by exhaust gas recirculation with a corresponding reduction in temperature. With such a recirculation, the combustion gases act as inert gases, which, because there is no oxygen in them, alone bring about a reduction in the temperature. If these gases are also at a reduced temperature, this is a desirable side effect.

EP-A-0 386 732 already describes a combustion device for a two-fuel burner with an internal recirculation. However, the device there has a nozzle plate on which a plurality of nozzles is arranged, which are channeled differently in their air flow through a tube before they are mixed in a flame tube. The device described there has a very complicated construction, the apparently necessary to achieve a sufficiently stable recirculation in the construction with two pipes.

In any case, a disadvantage of such a device is that, given the large number of nozzles, maintenance is very complicated and expensive, and even minor disturbances can have major disadvantages in operation.

Another furnace for reducing nitrogen oxide formation when burning fossil materials is described in EP-A-0 384 277, this method expressly aimed at enclosing and cooling the flame of "cool" flue gases. A swirling is not provided and cannot take place to a sufficient extent in the structural design of a combustion system proposed there. The effect of adding inert gases during combustion to reduce NO _x formation is therefore not used and the system must remain suboptimal.

Another boiler for the combustion of liquid and gaseous fuels is proposed in DE-OS 36 28 293, in which an injector channel is already described, which surrounds the flame and allows circulation outside of it. The construction there, however, provides for a forced return of the flue gas between the injector channel and a frame with a welded-on floor, which is placed in the opposite direction around the injector channel. Such a construction seems to be necessary in order to achieve a sufficient recirculation of combustion gases at all, but this construction in no way mixes the flue gases with the gases to be burned in front of the burner; at most, edge cooling will take place. Due to the various deflections of the flue gas, there is a considerable backlog effect of the exhaust gases, which is not conducive to combustion and the maintenance of a stable flame.

A device for oil burners in which a recirculation takes place with good mixing of the gases to be burned with the flue gases is described in DE-OS 40 08 692, in which, however, a baffle plate together with a fan causes the mixing.

Such an embodiment is technically very complex, on the other hand there is an increased need for maintenance due to the existing fan and in this document, too, only the aim is to cool the edge region of the flame with cooler combustion gases. Such a device can therefore only reduce the NO _x reduction and suffice with great effort.

A further device for burning liquid or gaseous fuels, in which swirling takes place during burning, is described in EP-A-0 404 731, in which a toroidal combustion zone is achieved by intensive swirling and corresponding guide means. However, no internal recirculation of the combustion gases is proposed, so that this means of reducing NO _x remains unused and the device therefore leaves something to be desired in terms of its efficiency.

Finally, DE-OS 39 23 238 is to be mentioned, in which a device for recycling combustion products is described, in which the air-fuel-flue gas mixture is already mixed before the combustion zone, in which there is also a device for supplying the combustion air which is arranged around the burner nozzle, is provided and which is equipped with a device for returning the combustion products into the combustion zone. Furthermore, on the inside of the burner tube there are nozzles for introducing a part of the combustion air, which serve to generate a low pressure behind these nozzles. However, the device there does not yet have the possibilities of internal recirculation with simple ones constructive features developed, so that the device described there still drives a considerable design effort, and on the other hand suffers unnecessary losses due to the relatively extensive exhaust gas recirculation realized there.

From CH-A-327648 a method for achieving a combustion with high heat development is known. The fact should be exploited that the combustion can be accelerated if active ions, radicals or particles, which originate from the fuel at the start of combustion, are moved or diffused through a combustible mixture. For this purpose, a burner was constructed with which a return of the largely unburned gases from the area of the incipient combustion is branched off and returned to the mixing point of the as yet unlit fuel with air, where these products enter the fuel-air mixture and after the ignition point be carried along.

In another context, EP-A-0 001 437 generally describes the generation of a supercritical swirl flow.

GB-A-609 135 discloses a graduated air injection via air nozzles which are partly tangential and partly non-tangential and which lead to a very special flow behavior. This is done to effect a staged combustion, in particular to be able to effectively break down organic nitrogen bound to the fuel.

The object of the present invention is therefore to achieve the combustion of a flowable and / or gaseous fuel as completely as possible with the formation of as little NO _x as possible by means of optimal mixing, in each case optimal mixing ratios and an optimal residence time of the combustion gases in the flame with the simplest possible constructive means.

According to the invention this object is achieved in a generic firing system by the features of claim 1.

This makes it possible to effectively mix the combustion products, in particular the exhaust gases with a low oxygen content, into the medium to be burned, for example the oil mist which is already distributed in the combustion air. Since there are very high differences in viscosity between the hot and the cold gases, this mixing is by no means possible without problems, but encounters great difficulties. The very strong swirl of the combustion air leads to the fact that the combustion gases supplied laterally over a large extent are mixed in very well. This happens in front of the flame that burns inside the insert. The very short distances in connection with the good mixing, which does not allow temperature peaks to occur, makes it possible that the temperature of the mixed-in combustion products does not have to be lowered significantly.
This means that all heat can be removed at one point, and no devices for cooling the flue gases need to be provided.

By "supercritical swirl" is to be understood that the air is set in such a strong rotational movement that a central backflow is formed. If an air flow which is set in rotation in this way encounters an expansion of the space leading it, strong turbulences take place. These turbulences are used in the present invention to bring about particularly good mixing between the cold fresh air, the supplied fuel and the returned combustion products.

The combustion products are thereby recycled in such a way that they have a significantly lower temperature than the flame. Nevertheless, the combustion gases are much hotter than the freshly supplied air and the fuel and have a significantly different viscosity, but this does not have a disadvantageous effect on the mixing, since this is ensured by the measures according to the invention.

In the device according to the invention, additional secondary air nozzles, which are arranged around the burner nozzle, are provided. Secondary air nozzles of this type, which in a further preferred embodiment are designed with an inclination that is essentially variable towards the axis of the burner tube, have the advantage that the air introduction conditions can be optimized to the conditions present in the respective individual combustion chamber.

Because a correspondingly high proportion of the air is blown in through the secondary air nozzles, these have a significant influence on the flow behavior at the start of use. They lead the swirled and mixed air out of the Swirling or mixing area by creating a vacuum behind them. This negative pressure further leads to an increased supply of recirculated combustion products through the space remaining between the insert and the burner tube.

The proportion of the combustion air which is blown in through the secondary air nozzles is preferably up to 90% of the total amount of air.

It has proven to be advantageous that the secondary air nozzles are set at an angle of up to 30 °, preferably 10-25 °, inwards to the axis of the burner tube. Such a setting causes a particularly favorable flow behavior for a stable flame with a large return quantity of combustion products.

Fig. 1

eine schematische Darstellung des Heizkessels nach einem Ausführungsbeispiel der Erfindung in Längsschnittdarstellung;

Fig. 2

die Einrichtung zur tangentialen Lufteinleitung in Querschnittsdarstellung entlang der Linie A-B in Fig. 1;

Fig. 3

die Strömungsverhältnisse beim Einsatz eines Einsatzes der erfindungsgemäßen Vorrichtung in das Brennerrohr ohne gleichzeitige Verdrallung der Luft; und

Fig. 4

die Strömungsverhältnisse in einem Brennerrohr der erfindungsgemäßen Vorrichtung mit Verdrallung der Verbrennungsluft.

The invention is explained in detail below with reference to the accompanying drawings. It shows:

Fig. 1

is a schematic representation of the boiler according to an embodiment of the invention in longitudinal section;

Fig. 2

the device for tangential air introduction in cross-sectional view along the line AB in Fig. 1;

Fig. 3

the flow conditions when using an insert of the device according to the invention in the burner tube without simultaneous swirling of the air; and

Fig. 4

the flow conditions in a burner tube of the device according to the invention with swirl of the combustion air.

The device according to the invention is shown in FIG. 1. The burner tube 10 is shown with the exhaust outlet 22 on the left side and the burner nozzle 12 on the right side. The fuel is supplied via the fuel supply 13. The insert 14 is inserted into the burner tube 10, a space 16 being left between the insert 14 and the burner tube 10 for the backflow of the combustion products. A distance between the insert 14 and the end of the burner tube facing the burner can be clearly seen. Combustion air is introduced tangentially into the air supply 15 around the burner 12 and then swirled into the burner tube 10 through the conical constriction 26 of the air supply. This swirl becomes supercritical due to the enlargement 24 of the space available here in the burner tube 10 or in the insert 14, so that a central backflow can form. So that this backflow does not extend into the air supply 15, the outer wall 28 is provided around the burner nozzle 12 with an end surface 30 which serves to limit these flow conditions.

Secondary air nozzles 20 are also provided which terminate approximately with the burner. They can be arranged - if necessary variably - inclined inwards up to an angle of approx. 30 ° and serve on the one hand to support the swirling and mixing, and on the other hand to help the exhaust gas recirculating flow due to the negative pressure that is behind it generate, reinforce.

The insert 14 according to the invention will have a diameter of approximately 80% of the total burner tube diameter. Its length will make up 20 to 70% of the burner tube length. The temperature of the combustion will be reduced from approx. 1600 ° C to 1300 ° C with these measures. The dwell time in the area of very high temperatures is also reduced. Overall, a reduction of up to 50% of the NO _x concentration in oil and up to 75% in natural gas can be achieved. In the burner tube 10 itself, the velocity of the gases will be low, so the air pressure resistance is not increased unnecessarily.

In the device 18 for tangential air introduction shown in FIG. 2, a slide 32 is shown with a round end edge 34, which serves to be able to vary the tangential velocity component of the air with the same volume flows (swirl adjustment).

3 and 4 schematically illustrate how the swirl of the supplied combustion air leads to improved recirculation of combustion products and better mixing.

Thanks to the optimal mixing, more inert gas can be added and the combustion temperature can be further reduced without incomplete combustion occurring.

Reference list

10th

Burner tube

12th

Burner nozzle

13

Fuel supply

14

commitment

15

Combustion air supply

16

Space

18th

Device for tangential air introduction

20th

Secondary air nozzles

22

Exhaust outlet

24th

extension

26

Narrowing

28

Outer wall

30th

Final area

32

Slider

34

Trailing edge

Claims (4)

1. A device for burning flowable or gaseous fuels, comprising a burner nozzle (12) disposed in a burner tube (10), a means (13) for supplying fuel to the nozzle (12) of a means (15) for supplying air for combustion to the burner tube (10), the means (15) being disposed substantially around the means (13) for supplying fuel, and an insert (14) for returning combustion products to the combustion zone and disposed in the burner tube (10) at the burner end, the combustion products being recycled substantially in a space (16) between the insert (14) and the burner tube (10), characterised in that a means (18) is provided for tangential introduction of air into the combustion-air supply means (15) in order to generate a supercritical swirl flow at least inside the insert (14); in that additional secondary-air nozzles (20) are provided around the burner nozzle (12) and inject combustion air in a proportion high enough to generate a negative pressure behind it; and in that the insert (14) has a diameter of about 80% of the total burner-tube diameter and a length of 20 to 70% of the total burner-tube length.
2. A device according to claim 1, characterised in that the secondary-air nozzles (20) are disposed at a variable angle, substantially relative to the axis of the burner tube (10).
3. A device according to claim 1 or 2, characterised in that the proportion of air for combustion injected through the secondary air nozzles (20) is up to 90% of total amount of air.
4. A device according to any of claims 1 to 3,
   characterised in that the secondary-air nozzles (20) are disposed at an angle of up to 30°, preferably 10 - 25°, inwards towards the axis of the burner tube (10).

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