Abstract:
A method and device for operating a combined burner for liquid and gaseous fuels for the purpose of generating hot gases functions to raise the lean stability limit of the gas flame without impairing the atomization of the liquid fuel and improve the regulating range of the burner. According to the invention, this is achieved when the inflow rate and/or swirl of the blast air (5) into the inner burner space (16) is controlled. To this end, the blast air (5), during operation with gaseous fuel (6), is throttled back by injection of pilot fuel into the blast air, and additionally swirled by swirl generators in the burner. In addition, active regulation of the blast air inflow rate is effected at the burner inlet during the use of both gaseous fuel and liquid fuel.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method and a device for operating a combined burner for liquid and gaseous fuels for the purpose of generating hot gases. 
     2. Discussion of Background 
     To achieve the lowest possible NOx emissions, burners are operated close to their lean extinguishing limit. This results in the disadvantage that the regulating range of the burners is greatly restricted. In order to remove this disadvantage, individual burners are switched off during partial load of the gas turbine so that the remaining burners can be operated in their stability range. But this is accompanied by an impairment in the temperature distribution over the periphery. 
     A further possibility of improving the regulating range of the burner is to enrich the fuel gases with additional fuel near the axis of the burner, which is also called internal piloting. As a result, the stability range of the burners is extended by the injection of a pilot gas to such an extent that reliable operation is guaranteed. To alternatively operate a burner with gas or fuel oil, a method is known in which the fuel oil used as an alternative to the pilot gas is atomized by means of an airblast nozzle. In this method, air is injected to atomize the fuel oil near the axis, i.e. in the center of the burner. But this is done not only during the fuel-oil atomization but also during pilot operation with gas, in which, however, no blast air is required for the atomization. This additional air destabilizes the pilot-gas flame on the one hand by making the mixture leaner and on the other hand by the oncoming air flow itself. The destabilizing leads to a clear reduction in the lean extinguishing limit of the gas flame. 
     SUMMARY OF THE INVENTION 
     Accordingly, one object of the invention, in attempting to avoid these disadvantages, is to provide a method and a device for operating a combined burner for liquid and gaseous fuels for the purpose of generating hot gases, which method and device raise the lean stability limit of the gas flame without impairing the atomization of the liquid fuel and improve the regulating range of the burner. 
     According to the invention, this is achieved when, in a method in which, the inflow of the blast air into the inner burner space is controlled. To this end, the blast air, during operation with gaseous fuel, is throttled back or throttled back and additionally swirled, or active regulation of its inflow is effected during the use of both gaseous fuel and liquid fuel. 
     The throttling-back is advantageously achieved by displacing the blast air by means of the pilot gas. For this purpose, the pilot-gas passage leads out in the air-feed line or in the outer and/or inner air passage so that the pilot gas is directed into the blast air inside the airblast nozzle or upstream in the area directly in front of it. The injection point lies sufficiently far from the air-inlet opening of the burner that the gaseous fuel cannot flow back into the plenum in front of the burner. 
     In this method, the pilot gas is injected to, the blast air at a higher pressure than the blast air. Therefore on the one hand it throttles the inflow of the blast air and on the other hand is at least partly mixed with this air before entering the inner burner space. The throttling of the air feed leads to the desired enrichment of the fuel gases and the early mixing of the pilot gas with the blast air for reducing the oncoming flow of the gas flame. Stabilization of the flame and an improvement in the lean extinguishing limit are thereby achieved during pilot-gas operation without having to dispense with the possibility of the advantageous fuel-oil atomization by means of an airblast nozzle. 
     It is especially convenient when a jump in cross section of the burner wall is formed at the transition of the pilot-gas/air mixture from the airblast nozzle to the inner burner space. By the separation of the flow behind the jump in cross section, the pilot-gas/air mixture is kept at the burner axis and thus the lean extinguishing limit is further improved. The contour of the airblast nozzle at the atomization cross section remains unchanged and its function is not impaired. 
     In the method, in which the control of the inflow of the blast air into the inner burner space is effected by throttling-back and increased swirl, the pilot gas is injected into the outer air passage against the direction of flow of the blast air. By this type of injection, it is possible to largely throttle back the inflow of blast air, in particular its axial impulse, which is troublesome during operation of the burner with gaseous fuel. The injection of the pilot gas into the outer air passage takes place tangentially and either against or in the direction of rotation of the main burner air. As a result of the tangential injection of the pilot gas, a swirl is additionally imparted to the blast air. If this swirl is orientated in the opposite direction to the direction of rotation of the main burner air of the burner, increased friction and thus mixing of the two air flows occurs in the inner burner space. Thus the axial impulse of the blast air is weakened and the vortex breakdown, i.e. the breakdown of the fuel mixture, is upstream into the burner. On the other hand, if a swirl equidirectional to the direction of rotation of the main burner air is imparted to the blast air, this strengthens the vortex core of the fuel mixture in the burner axis so that the vortex breakdown is intensified and likewise displaced in the direction of the nozzle. In this way, the tangential injection of the pilot gas into the blast air, irrespective of the swirl direction, leads to an improvement in the flame maintenance and thus to stabilization of the combustion. The same effects can be achieved by introducing pilot gas already swirled beforehand into the blast air. To this end, at least one spacer is arranged between the burner wall and the intermediate wall of pilot-gas passage and outer air passage and is preferably of wound design. It serves to center the fuel-feed sleeve in the burner and in its preferred design produces the swirl of the fed pilot gas. As an alternative, the swirl can also be brought about by means of separately arranged swirl generators. 
     When the pilot-gas passage leads into the air-feed line upstream in the area in front of the airblast nozzle, the pilot gas is directed at this point into all the blast air and is mixed with it so that the pilot-gas/air mixture formed flows through both air passages of the airblast nozzle. This results in the additional advantage of increased throttling-back of the air and thus the further enrichment of the pilot-gas/air mixture provided for internal piloting. In addition, improved premixing of the pilot gas with the blast air occurs. Similar advantages can be achieved when the pilot gas is directed inside the airblast nozzle into both air passages. In this variant, however, the blast air can be throttled back to an even greater extent. 
     In another embodiment of the invention, the entry of the blast air into the inner burner space is actively regulated. This is done by regulating the inflow of the blast air from the plenum into the burner during the use of both gaseous and liquid fuel. To this end, a drivable adjusting mechanism is arranged on the fuel lance or the burner connection piece, which adjusting mechanism at least partly closes the burner air-inlet opening for the blast air during operation of the burner with gaseous fuel. If the blast air is required only for the atomization of liquid fuel, the fuel pressure of the liquid fuel can advantageously be utilized to actuate the adjusting mechanism and thus to open the air-inlet openings of the burner. The pressure drop in the combustion chamber upon completion of the fuel feed then serves as counterpressure to the closing of the air-inlet openings. In addition to the advantages of the achievement according to the invention which have been described hitherto, it is possible in this embodiment to adapt the inflow of the blast air to the actual load state of the burner. For this purpose, the inflow of the blast air is regulated separately. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings of a plurality of exemplary embodiments of the invention illustrating various burners provided in each case with an airblast nozzle, wherein: 
     FIG. 1 shows a schematic representation of the arrangement of a burner equipped with an airblast nozzle; 
     FIG. 2 shows a partial longitudinal section of the burner FIG. 1; 
     FIG. 3 shows a partial longitudinal section of the burner in another embodiment; 
     FIG. 4 shows a partial longitudinal section of the burner in a further embodiment; 
     FIG. 5 shows a partial longitudinal section of the burner in a next embodiment; 
     FIG. 6 shows an enlarged detail from FIG. 5; 
     FIG. 7 shows a section VII--VII through the airblast nozzle according to FIG. 6; 
     FIG. 8 shows a cross section VIII--VIII through the burner according to FIG. 1, in the configuration according to FIGS. 5 to 7, in simplified representation; 
     FIG. 9 shows a representation in accordance with FIG. 7 but with bores directed in the opposite direction; 
     FIG. 10 shows a representation in accordance with FIG. 8 but in the configuration according to FIG. 9; 
     FIG. 11 shows a partial longitudinal section of the burner in a further embodiment; 
     FIG. 12 shows an enlarged detail from FIG.11; 
     FIG. 13 shows a section XIII--XIII through the airblast nozzle in accordance with FIG. 12; 
     FIG. 14 shows a longitudinal section of the burner in a next embodiment; 
     FIG. 15 shows a longitudinal section of the burner in a further embodiment; 
     FIG. 16 shows an enlarged detail in accordance with FIG. 15 in a further embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, only the elements essential for understanding the invention are shown and the direction of flow of the working media is designated by arrows, an airblast nozzle 2 is arranged in the upstream end of a burner 1 designed as a double-cone burner 1. It is supplied with liquid fuel 4 and blast air 5 via a fuel lance 3 connected to the double-cone burner 1. In addition, the fuel lance 3 delivers the gaseous fuel 6 for the double-cone burner 1, which receives its main burner air 7 from the space inside the burner hood 8. The blast air 5 can also be fed directly from a plenum 34 located outside the burner hood 8. In addition, to enrich the fuel gases near the axis of the double-cone burner 1 via the fuel lance 3, gaseous fuel, so-called pilot gas 9, is additionally injected into the burner 1. This pilot gas 9 flows into the burner chamber 10 downstream (FIG. 1). 
     The airblast nozzle 2 has an inner air passage 11 and an outer air passage 12. A pilot-gas passage 13 is arranged concentrically outward of the inner air passage 11 and outer air passage 12. The two air passages 11, 12 are connected upstream to an air-feed line 14 and lead into the inner burner space 16 at the atomization cross section 15 of the airblast nozzle 2. The air-feed line 14 and the outer air passage 12 are separated from the pilot-gas passage 13 by an intermediate wall 17 (FIGS. 2 to 4). 
     The intermediate wall 17 ends in the direction of flow upstream of the atomization cross section 15 of the airblast nozzle 2. The pilot-gas passage 13 thereby merges directly into the outer air passage 12. The orifice 18 is arranged inside the airblast nozzle 2 and thus substantially closer to the atomization cross section 15 than to the air-inlet opening 19, shown in FIG. 14, of the double-cone burner 1. A jump 20 in cross section of the burner wall 21 is formed at the atomization cross section 15. A spacer 22 is arranged between the burner wall 21 and the intermediate wall 17 of pilot-gas passage 13 and outer air passage 12 and is of wound design (FIG. 2). 
     During operation with gaseous fuel 6, the pilot gas 9 is already directed through the orifice 18 into the blast air 5. The pilot gas 9 is mixed with blast air 5 thus simultaneously throttles the blast air 5 inflow. The resulting pilot-gas/air mixture 23, directly after entering the inner burner space 16, is mixed with the blast air 5 which has flowed through the inner air passage 11. In the process, the wound design of the spacer 22 results in a swirl of the pilot gas 9 penetrating into the blast air 5. This swirl imparts the desired rotary impulse to the pilot-gas/air relative to the rotating main burner air 7. A plurality of separate swirl generators 24 designed as annular grooves can also be arranged in the pilot-gas passage 13. In this way, a swirl of the pilot gas 9 or of the pilot-gas/air mixture 23 is likewise brought about (FIG. 3). 
     During operation with liquid fuel 4, this liquid fuel 4 is directed into the airblast nozzle 2 via a fuel-oil line 25 arranged centrally in the fuel lance 3, is finely atomized there by means of the blast air 5 and then passes into the inner burner space 16 for premixing with the main burner air 7 (FIG. 3). 
     In another exemplary embodiment, the pilot-gas passage 13 ends further upstream in the area in front of the airblast nozzle 2, and the orifice 18 is likewise formed in this area. Thus the blast air 5 is mixed with the pilot gas 9 already before the airblast nozzle 2 (FIG. 4). 
     In a further exemplary embodiment, a plurality of uniformly distributed bores 26 are arranged in the intermediate wall 17 of pilot-gas passage 13 and outer air passage 12. They lead tangentially into the outer air passage 12 and are orientated in the opposite direction to both the direction of flow of the blast air 5 and to the direction of rotation of the main burner air 7 of the burner 1 (FIGS. 5 to 7). The blast air 5 is thereby throttled back to an increased extent. In addition, a counter-swirl of the pilot-gas/air mixture 23 and of the main burner air 7 occurs in the inner burner space 16 (FIG. 8). Thus better premixing of the fuel mixture 27 inside the burner 1 is achieved, the axial impulse of the blast air 5 is weakened and the vortex breakdown is displaced into the burner 1 (FIG. 1). 
     In a next exemplary embodiment, the bores 26 are likewise orientated against the direction of flow of the blast air 5 but in the direction of rotation of the main burner air 7 (FIG. 9). In this way, a commonly directed swirl of the pilot-gas/air mixture 23 and the main burner air 7 is obtained in the inner burner space 16 (FIG. 10). This commonly directed swirl intensifies the vortex formation in the area of the burner axis 28 and likewise displaces the vortex breakdown into the burner 1. Thus this solution also helps to improve the flame maintenance and thus stabilize the combustion. 
     In a further exemplary embodiment, the pilot-gas passage 13 leads into both air passages 11, 12 inside the airblast nozzle 2. To this end, a plurality of fastening elements 30 provided with one radial blind bore 29 each are arranged on the intermediate wall 17 in the area of the airblast nozzle 2. The blind bores 29 connect the pilot-gas passage 13 to the outer air passage 12 and the inner air passage 11 via a first opening 31 and a second opening 32, respectively. The blast air 5 is thereby throttled back in both air passages 11, 12 (FIGS. 11 to 13). 
     In another exemplary embodiment, the double-cone burner 1 is fastened in the burner hood 8 by means of a burner connection piece 33. The air-inlet opening 19 for the blast air 5 flowing in from the plenum 34 is integrated in the burner connection piece 33. To feed the liquid fuel 4, the fuel lance 3 adjoins the burner connection piece 33 upstream. Arranged on the fuel lance 3 is an adjusting mechanism 35 designed as an axially displaceable sleeve 37 provided with a projection 36 (FIG. 14). The adjusting mechanism 35 can also be arranged on the burner connection piece 33. It is controlled by a drive (not shown). By in each case two adjusting mechanisms 35 being connected to one another via a linkage (likewise not shown), the inflow of the blast air 5 into two double-cone burners 1 can advantageously be regulated by means of a common drive. A single drive can of course also be provided for the adjusting mechanisms 35 of all double-cone burners 1 of a gas turbine. 
     During operation of the double-cone burner 1 with gaseous fuel 6, the sleeve 37 closes the air-inlet opening 19 for the blast air 5 and thus prevents it from flowing into the double-cone burner 1 from the plenum 34. By partial closing of the air-inlet opening 19, it is likewise possible to regulate actively the inflow of the blast air 5 into the inner burner space 16 in accordance with the load state. 
     However, if only liquid fuel 4 is to be atomized with the blast air 5, the adjusting mechanism 35 is actuated when a fuel pressure of the liquid fuel 4 is applied and thus opens the air-inlet openings 19 of the double-cone burner. The pressure drop in the combustion chamber is utilized as counterpressure to the closing of the air-inlet openings 19. 
     In another exemplary embodiment, the adjusting mechanism 35 is arranged on a tube 39 acting on the air-inlet opening 19 of the burner connection piece 33, concentrically enclosing the fuel lance 3 and provided with two radial feed openings 3 for the blast air 5, and is likewise designed as an axially displaceable sleeve 40 (FIG. 15). Here, it is possible by appropriate displacement of the sleeve 40 to throttle back the feed of the blast air 5 completely or partly. 
     In a further exemplary embodiment, the adjusting mechanism 35 is designed as rotatably mounted sleeve 41 arranged on the tube 39 concentrically enclosing the fuel lance 3 (FIG. 16). The metering or the complete interruption of the feed of the blast air 5 is realized in this variant of the invention by turning the sleeve 41. To this end, a recess 42 is provided in it, which recess 42 corresponds with the feed opening 38 during operation with liquid fuel 3 but can be closed during operation with gaseous fuel 6. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.