Patent Grant
7972133
1. Field of Endeavor
The present invention relates to a burner for operating a heat generator, wherein such a burner has a swirler for a combustion air flow, and also means for injecting at least one fuel into the combustion air flow. Downstream of the swirler, a mixing path is arranged, and in the region radially outside the discharge opening of the mixing path of the burner there is at least one nozzle for feeding liquid pilot fuel. Furthermore, the present invention relates to a method for operating such a burner.
2. Brief Description of the Related Art
Premix burners, as proposed for example in EP 0 321 809 B1, are burners in which a fuel, gaseous or liquid, is first mixed with the combustion air and after this mixing process is combusted in the flame. In the case of the type of such a premix burner which is proposed in EP 0 321 809 B1, a plurality of conical wall elements are provided, wherein these wall elements are arranged in an offset manner to each other in such a way that inlet slots for the combustion air into the interior of the burner are formed between them. In this region, therefore, a swirl is generated, and the swirled flow which is formed therein is then transferred into a mixing path. In such a burner both liquid as well as gaseous fuels can be combusted, wherein the former are preferably fed on the axis of the burner via a fuel lance, and the latter are fed in the region of the inlet slots, typically via a multiplicity of exit orifices which are arranged in series. Such burners are characterized by an outstanding stability of the flame and also by excellent pollutant values (low NOx values) and efficient heat generation.
A further improvement of such a construction is described, for example, in documents EP 0 704 657 B1 and EP 0 780 629 B1. In this case, a mixing path is also arranged downstream of the swirler formed by the conical wall elements, and specific transfer passages, which ensure an ideal transfer of the flow which is formed in the swirler into the mixing path, are provided at the inlet of this mixing path.
In the case of such burners, the fact that they have the tendency to become unstable, if for example they are controlled under low-load conditions or under transient conditions with a low fuel supply, is problematical. This is because, inter alia, such burners ideally have to be operated close to the lean quenching limit in order to have the aforementioned advantages. If the fuel supply is lowered below a critical value, then quench pulsations can occur, that is to say a quenching of the flame can be caused as a result of oscillations in the combustion chamber (so-called thermoacoustic instabilities).
In order to avoid such problems, a so-called pilot mode had been proposed on a number of occasions, that is to say an operating mode in which special additional fuel nozzles, which can be controlled under such low-load conditions or in the case of transient conditions, are arranged at suitable places of the burner or in the combustion chamber.
So, for example, EP 0 994 300 B1 describes the possibility of injecting gaseous pilot fuel in the case of a burner of the type as is described in EP 0 704 657 B1 or in EP 0 780 629 B1, this virtually being at the front edge of the mixing path, wherein swirl generators are additionally arranged in the region of the outlet of this pilot fuel. As a result of the vortex plaits which are created on the swirl generators, an increased mixing of the combustion air with the pilot fuel is brought about, and higher stability of the combustion process and lower pollutant values accordingly. As a result, the effect can be achieved of the operating range of such a burner being able to be extended to the bottom end with constant pollutant values.
Another possibility for feeding gaseous pilot fuel is described in EP 0 931 980 B1, wherein the gas in a discharge ring of the burner, after mixing with combustion air, is ignited by an ignition unit and injected into the combustion chamber.
While the aforementioned systems relate exclusively to the feed of gaseous pilot fuel, EP-A-1 389 713 in addition also describes the feed of liquid pilot fuel, after mixing with combustion air, into the combustion chamber very close to the discharge opening of the burner, this feed also being on the front outlet edge which faces the combustion chamber and specifically from a conical flank of the discharge ring which is bevelled outwards and towards the burner rear wall. Since liquid fuels on the one hand as a rule are more easily combustible, the pilot mode can also be maintained beyond the partial load, and since when feeding liquid fuel after shutting down it is not mandatory to be purged with air, this is of great advantage.
In order to get a grip on the problem of the excessive heat which occurs in the region of the outlet edge, the feed via fuel pipes with discharge openings arranged at their ends is described in EP-A-1 389 713, wherein the discharge openings do not lead directly into the combustion chamber, but, rather, lead into an encompassing cavity in the discharge ring which is arranged in the region of the outlet edge directly next to the burner opening and which is purged with combustion air and has holes which are arranged above the discharge openings or nozzles respectively and through which the liquid fuel can discharge into the combustion chamber from the said flanks. In order to be able to ensure the stability of the pilot flame, the fuel is introduced into the combustion chamber in a jet which is arranged in a plane which includes the axis of the burner. It is specified that the jet with the axis of the burner forms an angle within the range of 15 to 600. The discharge openings are indeed exposed to circumflow on their surface which faces the combustion chamber by the combustion air which is fed in the ring, but the cooling still has optimization requirements because an uneven distribution of the air through the air ring occurs, and consequently an uneven cooling. There is also the fact that the cold fuel in this case gives rise to a high temperature gradient which leads to high stresses.
For better mixing of the liquid fuel with the combustion air, it is necessary, moreover, to arrange swirl generators for the liquid fuel in the feed line upstream of the nozzle which is arranged at the discharge opening. It is specifically disclosed that, for example, a perforated plate, with at least two holes for the generation of such turbulence and which is installed in the pipe cross sections of the feed pipe, can be used.
Since the pilot nozzle for the liquid fuel is integrated in the discharge ring in a fixed manner, and the same purging air is used as for the gas pilot, there is a further disadvantage of the solution which is known from EP-A-1 389 713, in that in case of damage, the entire burner head has to be exchanged which gives rise to high costs.
One of numerous aspects of the present invention includes an improved burner which can be operated with liquid fuel in pilot mode. In particular, stable operation with low pollutant values can be achieved, as well as avoidance of overheating of components. Furthermore, a construction which is modularized as far as possible can be provided, which for example allows replacement of the elements of the pilot burner. Specifically, it concerns the improvement of a burner for operating a heat generator in this case, wherein the burner comprises a swirler for a combustion air flow, and also means for injecting at least one fuel into the combustion air flow, wherein a mixing path is arranged downstream of the swirler, and wherein at least one nozzle for feeding liquid pilot fuel is arranged in the region radially outside the discharge opening of the mixing path of the burner. In principle, therefore, it concerns a burner of the type as is described in EP 0 321 809 B1, wherein in addition, as this is described for example in EP 0 704 657 B1 or in EP 0 780 629 B1, transfer passages can be arranged between the swirler and the mixing path.
Another aspect of the present invention includes the at least one nozzle being arranged in a burner front plate, wherein in a front face of the burner front plate, which is arranged essentially parallel to a combustion chamber rear wall, at least one discharge opening is provided, through which the liquid pilot fuel discharges into the combustion chamber. This burner front plate with its front face which is arranged parallel to the combustion chamber rear wall, which is arranged outside the discharge opening of the burner, allows the feed of pilot fuel to be integrated into the burner, but to be arranged nevertheless at sufficient distance from the discharge opening of the burner. In this way, overheating of constructional components of the burner occurring during pilot mode can be avoided. As a result of a direct feed of screening air (purging air), the atomization of the liquid pilot fuel is assisted and coking is avoided, and also a local backflow is prevented. Moreover, as a result of the arrangement in the front face, a better atomization of the fuel can be ensured. The injection angle in this case can be kept smaller in comparison to the prior art, since injection is carried out far enough from the burner outlet edge.
Furthermore, a modular type of construction is advantageously possible, that is to say on account of the fact that the elements of the pilot burner are not arranged in the discharge ring of the burner, as in the case of EP-A-1 389 713, these elements are better accessible and can be easily exchanged, which saves costs.
Specifically, a burner of the aforementioned type typically has a central section which adjoins the burner opening and which, with regard to a burner axis, is formed in a manner in which it slopes radially outwards and conically rearwards, and forms a bevelled flank. The burner front plate can now be formed in one piece with such a section, that is to say, can have a central section which adjoins the burner opening and which, with regard to a burner axis, is formed in a manner in which it slopes radially outwards and conically rearwards, and forms a bevelled flank. In this case, the at least one discharge opening, with regard to the burner axis, is arranged radially outside this flank according to a preferred embodiment of the invention.
Alternatively, it is possible that a discharge ring is arranged between the burner front plate and the burner opening, and which, with regard to a burner axis, is formed in a manner in which it slopes radially outwards and conically rearwards, and forms a bevelled flank. Also in this case, the discharge opening, with regard to the burner axis, is arranged radially outside this flank.
A further preferred embodiment of the invention is characterized in that the burner front plate has a plurality of discharge openings which are arranged in an encompassing manner, wherein the burner front plate has at least one inlet opening, in most cases provided behind a rear wall of the combustion chamber, and through which combustion air from outside can enter the burner front plate and, as a result of the pressure drop towards the combustion chamber, can flow through the discharge openings. In this way, an optimum cooling of the edge region and also of the burner front plate can be ensured.
According to one embodiment of the invention, one nozzle only per burner is arranged behind a discharge opening.
Preferably, it is possible to form the nozzle as a plain jet or as a pressure swirl nozzle. A pressure swirl nozzle is preferred in this case, at least with regard to the pollutant values.
A pressure swirl nozzle is a nozzle in which the fuel under high pressure is first guided via, for example, tangentially extending slots into a swirl chamber and then leaves this swirl chamber via a nozzle orifice. Consequently, a spray cone results, in which the fuel is broken up into extremely fine particles (in addition to this, compare, for example, Lueger, Lexikon der Technik, Stuttgart, 1965, Band 7, Seite 600 (Lueger, Dictionary of Technology, Stuttgart, 1965, volume 7, page 600)).
One aspect of the invention, therefore, is that a conventional plain jet injection, as this is described in EP-A-1 389 713, is not to be used, but rather a completely specific nozzle formation is to be used, that is to say a pressure swirl nozzle. That the use of a pressure swirl nozzle in connection with the pilot injection is on the whole possible, is quite unexpected. The fact that overheating in the region of the nozzle has to be avoided is problematical when injecting liquid fuel in the edge region of the burner, that is to say, in direct proximity of the combustion chamber. This can already be largely achieved by the arrangement of the pilot burner in the region of the front face of a burner front plate. When using a nozzle according to EP-A-1 389 713, this is partially ensured since the jet of fuel can be carried a long way into the combustion chamber and accordingly the flame is mostly, but not always, sufficiently far away from the rear wall of the combustion chamber. In the case of the fine droplet structure of a pressure swirl nozzle, it would have been generally to be expected for the flame to be located much too close to the rear wall and that, as a result, an excessive heating in the region of the nozzle would have to occur. Surprisingly, it now turns out, this is not the case.
A further embodiment exemplifying principles of the invention concerns a pressure swirl nozzle which produces a hollow spray cone and not a full fuel cone. For example, nozzles, as are described in EP 0 924 461 B1 or in EP 0 794 383 B1, can be used, but other constructions are also possible.
It is advantageous if the nozzle is arranged in a cavity in the burner front plate, which has a discharge opening to the combustion chamber through which the spray cone which is produced by the nozzle enters the combustion chamber, wherein the nozzle orifice is set back from the discharge opening with regard to the combustion chamber. This cavity is preferably a cavity which is essentially cylindrical, at least in the region of the nozzle and downstream of the nozzle, and in particular the inside diameter of this cavity is preferably equal to or smaller than the inside diameter of the discharge opening. The nozzle orifice is preferably offset rearwards by up to 50 mm from the front edge of the discharge opening which faces the combustion chamber.
An ideal combustion characteristic of the pilot flame can be realized if such a cavity has at least one inlet opening through which combustion air from outside enters the cavity and, as a result of the pressure drop towards the combustion chamber, can flow through the discharge openings. Consequently, a combustion air flow results, which virtually encompasses the spray cone and can ensure an optimum transporting into the combustion chamber and an enveloping of this spray cone. This is especially the case when the nozzle is arranged at the end of an essentially cylindrically formed fuel pipe which projects into the essentially cylindrical cavity and concentrically to this, so that the combustion air flows around the spray cone in an essentially encircling manner. This screening air (purging air) assists the atomization, and coking of the injector and local backflowing are advantageously avoided. The injection of the liquid pilot fuel is therefore carried out separately and is positioned with separate purging air in the case of each nozzle.
The discharge opening is preferably at least the same size as the cylindrical cavity in order to avoid flow losses. In order to be able to adjust the conditions, it proves to be advantageous to provide means upstream of the nozzle by which the throughflow cross section for combustion air in the cavity can be adjusted.
The nozzle is advantageously oriented in such a way that the principal axis of the spray cone which is produced by the nozzle is arranged in a plane which is formed by the principal axis and the central axis of the burner, wherein an angle γ in the range of +/−45°, preferably in the region of 0°, is included between the principal axis of the spray cone which is produced by the nozzle (with a spray cone angle β in the range of 0 to 90°) and the axis of the burner.
In this case, it is also possible to deviate from the plane by a tilt angle δ and in this way to ensure an injection virtually parallel, or at least obliquely at a small angle, to the rotational direction of the combustion air flow which discharges from the main opening of the burner.
Furthermore, the present invention relates to a method for operating a burner as described above. The method is especially characterized in that liquid fuel through the nozzle is used for producing pilot flames at least at low load or under transient conditions. As a result of the specific design of the nozzle, it is possible to control the pilot flame for stabilization both at nominal load and high load respectively.
The invention is to be subsequently explained in more detail based on exemplary embodiments in connection with the drawings. In the drawings:
The fuel which discharges from this fuel nozzle 7 forms a fuel cone 11 and is picked up by the tangentially inflowing combustion air 9 and enveloped by this, and a conical column consisting of a mixture of fuel and combustion air is formed. Gaseous fuel can be fed in the region of the tangential inlet slots 8 via additional fuel nozzles 12.
A mixing path 3 is connected downstream to this swirler 2. Transfer passages 6 are arranged in the transition from the swirler 2 to the mixing path 3, which assist the flow in this region and ensure an optimum entry into the mixing path 3. The mixing path 3 essentially includes a cylindrical tube. A burner front plate 32, which delimits the burner towards the combustion chamber 16 and also, if necessary, a discharge ring 4 completely on the inside, are now arranged at the end of this tube which faces the combustion chamber 16.
In the region of this burner front plate 32 or of the discharge ring 4, devices are provided in order to feed gaseous fuel for the pilot mode, as this is described for example in EP 0 931 980 B1 or in EP 0 994 300 B1. Furthermore, a feed for liquid pilot fuel is now also provided in the region of the burner front plate 32 or is integrated into this. For this purpose, a fuel pipe 17 is provided, which on its end which faces the combustion chamber has a pressure swirl nozzle 20 or a conventional pressure jet.
The at least one nozzle 20 is arranged in the burner front plate 32. At least one discharge opening 15, through which the pilot fuel discharges into the combustion chamber 16, is provided in a front face 34 of the burner front plate 32, which is arranged essentially parallel to a combustion chamber rear wall 28.
The orientation of this pressure swirl nozzle or plain jet 20 can be arranged parallel to the axis 29 of the burner (see lower spray cone 21 with a spray cone angle β in
In
As is indicated by the broken line, it is possible to provide a separate discharge ring 4 with a bevelled edge 33, but it is also possible to form the projection of such a discharge ring 4 integrally with the burner front plate 32 as one element.
While the invention has been described in detail with reference to exemplary embodiments thereof, it will be apparent to one skilled in the art that various changes can be made, and equivalents employed, without departing from the scope of the invention. The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents. The entirety of each of the aforementioned documents is incorporated by reference herein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 477/06 | Mar 2006 | CH | national |
This application is a Continuation of, and claims priority under 35 U.S.C. §120 to, International application no. PCT/EP2007/052031, filed 5 Mar. 2007, and claims priority therethrough under 35 U.S.C. §§119, 365 to Swiss application no. 0477/06, filed 27 Mar. 2006, the entireties of which are incorporated by reference herein.
| Number | Name | Date | Kind |
|---|---|---|---|
| 1886866 | Blakesley | Nov 1932 | A |
| 1920186 | Carter | Aug 1933 | A |
| 5073104 | Kemlo | Dec 1991 | A |
| 5081843 | Ishibashi et al. | Jan 1992 | A |
| 5660044 | Bonciani et al. | Aug 1997 | A |
| 5713327 | Tilton et al. | Feb 1998 | A |
| 5951882 | Simmons et al. | Sep 1999 | A |
| 6186775 | Jansohn et al. | Feb 2001 | B1 |
| 6210152 | Haffner et al. | Apr 2001 | B1 |
| 6331109 | Paikert et al. | Dec 2001 | B1 |
| 7140183 | Ruck et al. | Nov 2006 | B2 |
| 7520745 | Oomens et al. | Apr 2009 | B2 |
| 20050164138 | Ruck et al. | Jul 2005 | A1 |
| 20070172783 | Stephens et al. | Jul 2007 | A1 |
| 20070172784 | Stephens et al. | Jul 2007 | A1 |
| 20070172785 | Stephens et al. | Jul 2007 | A1 |
| Number | Date | Country |
|---|---|---|
| 0321809 | Jun 1989 | EP |
| 0670456 | Sep 1995 | EP |
| 0704657 | Apr 1996 | EP |
| 0780629 | Jun 1997 | EP |
| 0794383 | Sep 1997 | EP |
| 0924461 | Jun 1999 | EP |
| 0931980 | Jul 1999 | EP |
| 0994300 | Apr 2000 | EP |
| 1389713 | Aug 2002 | EP |
| 1321715 | Jun 2003 | EP |
| W02007110298 | Oct 2007 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 20090081599 A1 | Mar 2009 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2007/052031 | Mar 2007 | US |
| Child | 12238792 | US |
