EP0433790A1 - Burner - Google Patents

Abstract

A burner (1), with a conical shape opening in the direction of flow, is composed of two part cone members (2, 3) which are positioned on top of one another and the central axes (2a, 3a) of which run in a displaced manner in relation to one another in the longitudinal direction. From this displacement, a tangential inlet slot to the interior (17) of the burner (1) is in each case formed over the length of the burner (1). The fuel supply takes place centrally via a nozzle (9) and tangentially in the region of the inlet slots via a fuel pipe (10, 11) in each case, which is provided with fuel openings (21) which there take on the injection of the fuel (6). Above each inlet slot, a duct is formed, which is equipped with an injector (12, 13). Additional fuel (4) is introduced by this injector. The air/fuel mixture with fuel from the injector (12, 13) and/or fuel from the fuel pipe (10, 11) flows generally in the form of an air/fuel mixture (8) through the tangential inlet slots into the interior (17) of the burner (1). There, if necessary, further mixing with the fuel (5) from the nozzle (9) takes place. <IMAGE>

Description

The present invention relates to a burner according to the preamble of claim 1. It also relates to a method for operating such a burner.

STATE OF THE ART

From EP-A1-0 321 809 a burner has become known which consists of two half-hollow partial cone bodies which are offset from one another. The conical shape of the partial conical bodies shown in the figure there extends in the direction of flow over a certain fixed angle. The aforementioned displacement of the partial cone bodies relative to one another creates on both sides of the burner body a tangential entry slot over the entire length of the burner, the width of which corresponds to the respective offset of the central axes of the partial cone bodies with respect to one another, and through which the combustion air flows into the interior of the burner.

A fuel nozzle is placed in the interior at the beginning of the burner, the fuel injection of which preferably starts in the center of the mutually offset central axes of the partial cone bodies. Further fuel nozzles are provided in the area of the tangential inlet slots. Liquid fuel is preferably introduced through the central fuel nozzle, while the fuel nozzles in the region of the tangential inlet slots are preferably operated with a gaseous fuel. If such a burner is now operated with a medium-calorific gas, which generally contains highly flammable hydrogen, there is a concrete risk that the combustion air that is brought in is present and mix this gas so strongly already in the entry area, at the point of their meeting, that the mixture may ignite prematurely. This in turn would lead to a diffusion-like combustion with a greatly increased NO _x emission. In addition, it can be ascertained that with such a mixture of air / gas, shear layers can easily arise, which results in instability of the mixing process as a result of strong turbulence. If pressure pulsations occur due to the instability mentioned above, this also leads to strong vibrations in the system.

OBJECT OF THE INVENTION

The invention seeks to remedy this. The invention, as characterized in the claims, is based on the object of providing measures in a burner of the type mentioned at the outset which prevent the mixture from igniting prematurely when a medium-calorific gas is used as fuel. The measures should also enable the mixing process to be stabilized.

The main advantage of the invention is that the NO _x emissions remain low because there is no premature ignition.

Another important advantage of the invention is that the injector, which forms the solution according to the task, makes it possible not to change the flow field of the burner used, despite the high mass flow share of the medium-calorific gas in the air / gas mixture. This is achieved with the aid of a suitable distribution of a number of injector bores of the same size or with the aid of an arrangement of bores whose diameter varies in a suitable manner. Here is the Density of the gas inlet holes (P _GB ) proportional to the radially averaged combustion air inlet velocity through the tangential air inlet slots of the burner.

Furthermore, the injector according to the invention does not allow shear layers to arise during the mixing process: these shear layers, which always arise when the velocity of the gaseous fuel at the mixing location is greater than the air velocity, cause strong turbulence, which triggers instability in the system. By designing the injector so that the two media meet at the mixing location at almost the same speed, there is no turbulence; there are also no pressure pulsations which would have a negative effect on the mixing and burning process, so that vibrations on the system are excluded. The mixing process is designed for full load with regard to the flow rate of the gaseous fuel: the gaseous fuel is "breathed" into the air flow almost without pressure. Further advantages of the invention relate to the avoidance of acoustic hardness when the fuel is injected: by designing the gap width and the length of the injector accordingly, the flow can recover before leaving the injector to such an extent that the aforementioned acoustic hardness cannot arise.

Another advantage of the invention can be seen in the fact that even combustion of gases with a low calorific value is conceivable in suitable temperature and pressure ranges.

Advantageous and expedient forms of watering of the task solution according to the invention are characterized in the further claims.

An exemplary embodiment of the invention is explained in more detail below with reference to the drawing. All elements not necessary for the immediate understanding of the invention have been omitted. The direction of flow of the different media is indicated by arrows. In the different figures, the same elements are each provided with the same reference symbols.

BRIEF DESCRIPTION OF THE FIGURES

Fig.1

eine perspektivische Darstellung des Brenners, entsprechend aufgeschnitten, mit angedeuteter tangentialer Luftzuführung und

Fig.2

einen Schnitt durch die Ebene II-II von Fig. 1, in einer schematischen, vereinfachten Darstellung.

It shows:

Fig. 1

a perspective view of the burner, cut open accordingly, with indicated tangential air supply and

Fig. 2

a section through the plane II-II of Fig. 1, in a schematic, simplified representation.

DESCRIPTION OF THE EMBODIMENT

To better understand the structure of the burner 1, FIGS. 1 and 2 should be used simultaneously. Furthermore, so that FIG. 1 remains clear, the injectors shown in FIG. 2 have not been included.

Fig. 1 shows a burner 1, which consists of two half, hollow partial cone bodies 2, 3, which are offset from one another. The conical shape of the partial conical bodies 2, 3 shown has a certain fixed angle in the flow direction. Of course, the partial cone bodies 2, 3 can have an increasing taper (convex shape) or a decreasing taper (concave shape) in the direction of flow. The latter two forms are not recorded in the drawing, since they can be easily traced. Which form is ultimately used depends on the various parameters of the combustion process. The form shown in the drawing is preferably used. The offset of the respective central axis 2a, 3a (see FIG. 2) of the partial cone bodies 2, 3 relative to one another creates a tangential entry slot 2b, 3b with a certain entry slot width S on both sides of the burner 1 in the flow direction (see FIG. 2), through which the combustion air 8 (air / fuel mixture) flows in the interior 17 of the burner 1. The entry slot width S is a measure that results from the displacement of the two central axes 2a, 3a of the partial cone bodies 2, 3. The two partial cone bodies 2, 3 each have a cylindrical initial part 2c, 3c, which, like the partial cone bodies 2, 3, also run offset from one another, so that the tangential entry slots 2b, 3b are present from the start. The burner 1 can of course describe a purely conical shape, that is to say without a cylindrical starting body. A nozzle is accommodated in this cylindrical starting body, which is preferably operated with a liquid fuel 5 and whose fuel injection 15 is preferably placed in the center of the two central axes 2a, 3a. As a further fuel supply, the two partial cone bodies 2, 3 each have a fuel line 10, 11, which are provided in the flow direction with openings 21 which are distributed over the entire length of the fuel lines. A gaseous fuel 6 is preferably introduced through the fuel lines 10, 11, this fuel being injected in the region of the tangential inlet slots 2b, 3b, as can be seen particularly well from FIG. 2. The burner 1 also has a fuel supply, preferably a gaseous fuel 4, which takes place via injectors 12, 13, which also act in the region of the tangential inlet slots 2b, 3b via a number of gas bores 14, as can be seen comprehensively from FIG. 2. For the description in this regard, reference is made to FIG. 2. It is basically the case that the burner 1 can be operated via individual fuel feeds or through a mixed operation with the available fuel options. On the combustion chamber side 22, the burner 1 has a collar-shaped wall 20, through which, if necessary, bores (not shown) are provided, through which dilution air or cooling air is fed to the front part of the combustion chamber 22. The liquid fuel 5, which is preferably introduced into the burner 1 through the nozzle 9, is injected into the interior 17 at an acute angle, in such a way that the most conical spray pattern is obtained in the burner outlet plane. This fuel injection 15 can be an air-assisted atomization or a pressure atomization. The conical liquid fuel profile 16 is enclosed by a trangentially flowing combustion air stream 8 and an axially brought in further air stream 7a. About the composition of the tangential inflowing air / fuel mixture 8 is discussed in more detail in the description of FIG. 2. In the axial direction of the burner 1, the concentration of the injected liquid fuel 5 is continuously reduced by an air flow or by the air / fuel mixture 8. If gaseous fuel 6 is used via the two fuel lines 10, 11, the mixture formation begins with the invisible air supply (see FIG. 2, item 7) directly in the area of the tangential inlet slots 2b, 3b, corresponding to the fuel openings 21 provided there The injection of liquid fuel 5 via the nozzle 9 in the area of the vortex run, ie in the area of a backflow zone 18 that is formed, the optimal, homogeneous fuel concentration is achieved over the cross section. The combustion process of each air / fuel mixture then begins at the top of this backflow zone 18. Only at this point can a stable flame front 19 arise. A flashback of the flame inside the burner 1 like this With known premixing sections, there is always no need to worry, whereas complicated flame holders are there to remedy the situation. If, in general, the air used (see FIG. 2, item 7) is preheated at most, accelerated, holistic evaporation of the liquid fuel 5 occurs before the point at the outlet of the burner 1 is reached at which the combustion process of the mixture begins. The degree of evaporation is dependent on the size of the burner 1, the drop size and the temperature of the air streams 7a, 7 respectively. of the air / fuel mixture 8 dependent. Regardless of whether, in addition to the homogeneous droplet mixing by means of a low-temperature combustion air flow or in addition partial or complete droplet evaporation is achieved by preheated combustion air, the nitrogen oxide and carbon monoxide emissions are low if the air excess is at least 60%, which means an additional one precaution to minimize _NOx emissions is available. In the case of complete vaporization of the fuel used before entering the combustion zone, the pollutant emission values are lowest. The same applies to near-stoichiometric operation when the excess air is replaced by recirculating flue gas. When designing the partial cone bodies 2, 3 with regard to the cone angle and width of the tangential inlet slots 2b, 3b, narrow limits must be observed so that the desired flow field of the air with its return flow zone 18 is established in the area of the burner mouth for flame stabilization. In general, it can be said that a reduction in the tangential inlet slots 2b, 3b, that is to say a reduction in the inlet width S (see FIG. 2), shifts the backflow zone 18 further upstream, which would cause the mixture to ignite earlier, however. In the meantime, it should be noted that the backflow zone 18, which is once geometrically fixed, is inherently position-stable, because the swirl number increases in the direction of flow in the region of the cone shape of the burner 1. The axial speed leaves further influence each other by axially feeding the air flow 7a already mentioned. The design of the burner 1 is ideally suited, given a given overall length of a burner 1, to adapt the size of the tangential inlet slots 2b, 3b to the requirements by pushing the partial cone bodies 2, 3 towards or away from one another, which means that the distance between the two central axes 2a, 3a reduced or increases, and accordingly the entry slot width S also changes, as can be seen particularly well from FIG. 2. Of course, the partial cone bodies 2, 3 can also be moved relative to one another in another plane. Seen in this way, the burner 1 can be individually adapted without changing its focal length.

wobei α der Öffnungswinkel des Brenners 1 (Siehe Fig. 1) ist, S die Eintrittsschlitzbreite bezeichnet und R der mittlere Radius der jeweils betrachteten Stelle des Eintrittsschlitzes 2b, 3b ist (Siehe Fig. 1). Die Richtungen der Gasbohrungen 14 sollten vorzugsweise mit der vorherrschenden Strömungrichtung im Eintrittsschlitz 2b, 3b zusammenfallen. Dabei ist es wichtig, dass das gasförmige Brennstoff 4 die eigentliche Drosselung beim Eintritt aus dem Gaszufuhrkanal 12a, 13a in die Gasbohrungen 14 erfährt. Da mittelkalorische Gase in der Regel leicht entzündlichen Wasserstoff enthalten, sind die Gasbohrungen 14 so auszulegen, das sie nicht frei in den Innenraum 17 des Brenners 1 ausblasen können. Diese Gasbohrungen 14 münden in einen Gasinjektorkanal 12b, 13b, der sich bis zum Eintrittsschlitz 2b, 3b erstreckt. Vorteilhaft ist es, wenn dieser Kanal in Längsrichtung mehrfach durch nicht ersichtliche Strömungsbleche unterteilt ist, damit das gasförmige Brennstoff 4 unter Auslegebedingungen, beispielsweise Vollast, in Richtung der Verbrennungsluftströmung kanalisiert wird. Des weiteren wird damit Beihilfe geleistet, dass das gasförmige Brennstoff 4 mit der jeweiligen Geschwindigkeit der herangeführten Luft 7 im Bereich der Eintrittsschlitze 2b, 3b bläst. Damit wird verhindert, dass sich die Luft 7 und das zum Einsatz gelangende mittelkalorische Gas 4 bereits im Eintrittsbereich in den Innenraum 17 des Brenners 1 stark durchmischen kann, denn dies würde zwangsläufig zu einer verfrühten Zündung führen, welche eine diffusionsartige Verbrennung mit stark erhöhten NO_x-Emissionen nach sich zieht. Um diese angestrebten Ziele zu erreichen, wird der Übergang von den Gasbohrungen 14 zum nachfolgenden Gasinjektorkanal 12b, 13b vorzugsweise als Borda-Carnot-Erweiterung ausgebildet. Was die Mindestlänge des Gasinjektorkanals betrifft, so wird hier mit Vorteil auf die übliche Regel der 3 - 5 hydraulischen Durchmesser resp. 6 - 10 Spaltbreite zurückgegriffen. Bei einer solchen Auslegung ist Gewähr vorhanden, dass sich die beruhigte Gasströmung 4 "hauchartig" mit der Luftströmung 7 vermischen kann, wodurch auch die akustische Härte beim Mischvorgang vermieden wird.2 is a section approximately in the middle of the burner 1, according to section plane II-II from FIG. 1. The inlets 23, 24, which are arranged axially symmetrically and which open into the interior 17 of the burner 1, each contain an injector 12, 13, which extends over the entire tangential length of the burner 1. The injector 12, 13 is designed in such a way that the gaseous fuel 4, which is preferably used, flows from a gas supply pipe 12a, 13a through which a gas can flow, via a number of gas bores 14, into a gas injector channel (injection channel) 12b, 13b. This extends into the area of the tangential entry slot 2b, 3b. The width of the injector 12, 13 is designed such that the air 7 that is brought in flows along the flanks of the injector 12, 13, and begins to mix with the gaseous fuel 4 in the region of the trangential inlet slot 2b, 3b, after which the air / Fuel mixture 8 is formed. Of fundamental importance is the property of the injector 12, 13 of not significantly changing the flow field of the burner 1 despite the high mass flow fraction of the medium-calorific gas used in the air / gas mixture. This is achieved with the help of a suitable distribution of the gas holes 14 of the same size or with the help an arrangement of holes, the diameter of which varies in a suitable manner. The density of the gas bores, called P _GB , is proportional to the radially averaged speed of the air 7 in the inlet slots 2b, 3b of the burner 1, and follows the following formula:

where α is the opening angle of the burner 1 (see FIG. 1), S denotes the entry slot width and R is the mean radius of the position of the entry slot 2b, 3b in question (see FIG. 1). The directions of the gas bores 14 should preferably coincide with the prevailing flow direction in the inlet slot 2b, 3b. It is important that the gaseous fuel 4 undergoes the actual throttling when it enters the gas bores 14 from the gas supply channel 12a, 13a. Since medium-calorific gases generally contain highly flammable hydrogen, the gas holes 14 are to be designed in such a way that they cannot be freely blown into the interior 17 of the burner 1. These gas bores 14 open into a gas injector channel 12b, 13b, which extends to the inlet slot 2b, 3b. It is advantageous if this channel is divided several times in the longitudinal direction by flow plates that cannot be seen, so that the gaseous fuel 4 is channeled in the direction of the combustion air flow under design conditions, for example full load. Furthermore, aid is provided in that the gaseous fuel 4 blows at the respective speed of the air 7 brought in in the region of the inlet slots 2b, 3b. This prevents the air 7 and the medium-calorific gas 4 that is used can already be mixed thoroughly in the inlet area into the interior 17 of the burner 1, because this would inevitably lead to premature ignition, which entails a diffusion-like combustion with greatly increased NO _x emissions. In order to achieve these desired goals, the transition from the gas holes 14 to the subsequent gas injector channel 12b, 13b is preferably designed as a Borda-Carnot extension. As far as the minimum length of the gas injector channel is concerned, the usual rule of 3-5 hydraulic diameters resp. 6 - 10 gap width used. With such a design, there is a guarantee that the calmed gas flow 4 can "breath-like" mix with the air flow 7, which also avoids the acoustic hardness during the mixing process.

Claims (6)

Burner, consisting essentially of at least two partial cone bodies positioned one on top of the other with a conical shape opening in the direction of flow, the central axes of these partial cone bodies being offset with respect to one another in the longitudinal direction, in such a way that tangential entry slots to the interior of the burner are formed over the length of the burner, characterized in that that a channel (23, 24) extends above each inlet slot (2b, 3b), outside the burner (1) formed by the partial cone bodies (2, 3), in which an injector (12, 13) for a fuel (4th ) that the fuel (4) flows out of the injector (12, 13) in the area of the inlet slot (2b, 3b) and can be mixed there with an air stream (7) flowing through the channel (23, 24). Burner according to claim 1, characterized in that the injector (12, 13) consists of a feed channel (12a, 13a) for the fuel (4) extending in the flow direction of the burner (1), that the feed channel (12a, 13a) in The direction of flow of the fuel (4) has a number of bores (14) such that the bores (14) open into an injector channel (12b, 13b) which extends in the region of the inlet slot (2b, 3b). Burner according to claim 2, characterized in that the transition from the bores (14) to the subsequent injector channel (12b, 13b) is formed by a Borda-Carnot extension. Burner according to Claim 2, characterized in that the density (P _GB ) of the bores (14) is proportional to the radially averaged inlet speed of the air (7) in the region of the inlet slot (2b, 3b) of the burner (1), according to the following formula:

where a is the opening angle of the tapered burner (1), S is the entry slot width, R is the mean radius of the point of the entry slot (2b, 3b) in question. Burner according to claim 2, characterized in that flow aids for the fuel (4) are provided in the injector channel (12b, 13b) for an adjustment to the flow direction of the air flow 7 and the combustion air (8). A method of operating a burner according to claim 1, characterized in that the fuel (4) by the injector (12, 13) is a gaseous gas, the rate of inflow into the interior (17) of the burner (1) is equal to or less than the speed of the Air flow (7), which mixes at least with the fuel (4) in the area of the inlet slots (2b, 3b).

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