Abstract:
A premix burner is provided for a gas turbine, in the form of a double-cone burner, which has two partial cone shells which are arranged nested one inside the other, forming air inlet ducts between them, through which combustion air from the outside flows into a conical inner space of the premix burner. Linear rows of holes of injection openings, which extend transversely to the flow direction of the combustion air, are arranged on the outer walls of the air inlet ducts and through which a gaseous fuel is injected into the combustion air which flows past transversely to them. A method for reworking such premix burners is also provided.

Description:
FIELD OF INVENTION 
     The present invention relates to the field of gas turbine technology. It refers to a premix burner for a gas turbine and also refers to a method for reworking such premix burners. 
     BACKGROUND 
     The present invention starts from a premix burner for a gas turbine in the form of a so-called “double-cone burner”, as is known, for example, from U.S. Pat. No. 5,921,770, which is incorporated by reference. The first figure of this application is generally reproduced here as  FIG. 1 . 
     The premix burner  10  according to  FIG. 1  is comprised of two hollow partial cone shells  11 ,  12  which extend along an axis ( 29  in  FIG. 2 ) and are nested one inside the other in an offset manner in relation to each other. The offset of the respective center axis or longitudinal symmetry axis of the partial cone shells  11 ,  12  in relation to each other creates, on both sides, in a mirror-image arrangement, a tangential air inlet duct  18 ,  19  in each case through which combustion air  20  flows into the conical inner space  30  of the burner. The two partial cone shells  11 ,  12  have in each case an entry section in the form of a cylinder  14 ,  15 . Accommodated in the region of the cylinders  14 ,  15  is a nozzle  24  for atomizing a preferably liquid fuel  23  which, after combustion together with the injected combustion air  20 , forms a flame front  28 . 
     Naturally, the premix burner  10  can be of purely conical design, that is to say without the cylinders  14 ,  15 . The partial cone shells  11 ,  12  furthermore have in each case a fuel line  16 ,  17  which are arranged along the tangential air inlet ducts  18 ,  19  and provided with injection openings  21  in the form of linear rows of holes through which a gaseous fuel  22  is injected into the combustion air  20  which flows past there, as is represented by means of arrows. These fuel lines  16 ,  17  are preferably placed at the latest at the end of the tangential inflow before entry into the inner space  30  in order to ensure optimum air/fuel mixing. 
     Towards the combustion chamber  25 , the premix burner  10  has a front plate  13 , serving as an anchor for the partial cone shells  11 ,  12 , with a number of holes  26  through which cooling air  27  can be fed to the front section of the combustion chamber  25  as required. 
     The design and arrangement of the injection openings  21  for the gaseous fuel  22  has considerable influence upon the mixing of the fuel with the combustion air  20 . The fuel  22  is injected into the air inlet passage  18 ,  19  of the premix burner  10  perpendicularly to the air flow. Mixing of the fuel  22  with the air is influenced both by the location of the injection openings  21  and by the flow velocity of the gaseous fuel. 
     In premix burners of the described type in use up to now, use is made of injection openings  21  which are represented as a row of holes R 1  in  FIG. 2 , wherein such a row of holes is associated in each case with each of the two air inlet ducts  18 ,  19 . If natural gas is used as the gaseous fuel, 32 injection openings  21  with a small outside diameter are arranged in the row of holes R 1 . 
     It has now transpired that during operation of such premix burners the mixing-through of the combustion air and the gaseous fuel can be improved more in order to lower the peak values of the flame temperature in the burner and therefore to reduce pollutant emissions (for example NOx). 
     SUMMARY 
     The present disclosure is directed to a premix burner for a gas turbine, in the form of a double-cone burner, having two partial cone shells arranged nested one inside the other, forming air inlet ducts between them, through which outside combustion air flows into a conical inner space of the premix burner. Linear rows of holes of injection openings, which extend transversely to a flow direction of the combustion air, are arranged on the outer walls of the air inlet ducts and through which a gaseous fuel is injected into the combustion air which flows past transversely to them. The injection openings have a diameter ratio of a diameter of the injection opening to an effective outlet diameter of the premix burner between 0.011 and 0.015. 
     In another aspect, the present disclosure is directed to a premix burner for a gas turbine, in the form of a double-cone burner, having two partial cone shells arranged nested one inside the other, forming air inlet ducts between them, through which outside combustion air flows into a conical inner space of the premix burner. Linear rows of holes of injection openings, which extend transversely to a flow direction of the combustion air, are arranged on the outer walls of the air inlet ducts and through which a gaseous fuel is injected into the combustion air which flows past transversely to them, the injection openings have a diameter ratio of a diameter of the injection opening to an effective outlet diameter of the premix burner which is greater than 0.015 and less than 0.017. 
     In a further aspect, the present disclosure is directed to a method for reworking premix burners for a gas turbine, in the form of a double-cone burner, having two partial cone shells arranged nested one inside the other, forming air inlet ducts between them, through which outside combustion air flows into a conical inner space of the premix burner. Linear rows of holes of injection openings, which extend transversely to the flow direction of the combustion air, are arranged on the outer walls of the air inlet ducts and through which a gaseous fuel is injected into the combustion air which flows past transversely to them. The method includes closing every other hole of a row of holes of injection openings and enlarging the diameter of remaining injection openings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention shall subsequently be explained in more detail based on exemplary embodiments in conjunction with the drawings. In the drawings: 
         FIG. 1  shows in a perspective, partially sectioned side view a known premix burner of the double-cone type, as is suitable for realization of the invention; and 
         FIG. 2  shows different rows of holes of injection openings of known and new configurations in relation to the premix burner. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Introduction to the Embodiments 
     It is therefore the object of the invention to create a premix burner of the type referred to in the introduction, which is significantly improved with regard to the intermixing of combustion air and gaseous fuel. 
     The object is achieved by means of the entirety of the features of claim  1 . It is preferable for the solution according to the invention that the injection openings are enlarged in their diameter. This enlargement, however, must be limited to a specific range. Furthermore, it was discovered that the absolute size of the diameter is not critical for achieving good results but a diameter ratio of diameter of the injection opening  21  to effective outlet diameter of the premix burner  10  in each case is. In this case, the diameter of a circle which has the same area as the outlet opening of the premix burner is to be understood as an effective outlet diameter of the premix burner. 
     A typical conventional hole diameter of a burner for natural gas with high methane content led to a diameter ratio of 0.0086 for example when using the newly introduced ratio of diameter of the injection opening  21  to effective outlet diameter of the premix burner  10 . For a gaseous fuel with a lower calorific value, a diameter ratio of diameter of the injection opening  21  to effective outlet diameter of the premix burner  10  of 0.0097 was used, for example. 
     For the best intermixing and combustion, a range of diameter ratios of diameter of the injection opening to effective outlet diameter of the premix burner, which lies between 0.011 and 0.015, has newly been determined. For operation with a gaseous fuel with a calorific value which lies at least 20% below the calorific value of methane, a widened range of diameter ratios of diameter of the injection opening to effective outlet diameter of the premix burner is proposed which is greater than 0.015 and less than 0.017. Overall, this results in an advantageous range of diameter ratios of 0.011 to 0.017. Correspondingly, the distance between the injection openings is also increased or the overall number of injection openings is reduced. 
     The injection openings were conventionally kept as small as possible in order to enable a good intermixing. A minimum size, however, was necessary in order to minimize the pressure losses which arise during injection of the fuel. 
     As a result of the new design of the rows of holes with larger diameter, a higher impulse of gas jets coming from the injection openings ensues, leading to an increased penetration of the transversely-flowing combustion air and therefore to improved mixing. With the improved mixing, the flame temperatures even out, which is accompanied by a reduction of temperature peaks and of pollutant emissions which are caused by them. 
     In a further aspect of the disclosure, it is sought to specify a height of the air inlet ducts, into which the combustion gas  2  is introduced into the premix burner, in a range which is adapted to the injection opening and which leads to good mixing-through with low pressure loss and stable combustion. In combination with the stated ratios of the diameter of the injection opening to effective outlet diameter of the premix burner, in each case a ratio of diameter of the injection opening to height of the air inlet duct which lies between 0.097 and 0.153 is advantageous. 
     In a further development of the invention, in each case a ratio of the sum of the areas of the injection openings to effective outlet diameter of the premix burner should be selected in an advantageous range. For the proposed hole diameter ranges, said range lies between 0.0051 and 0.0097. 
     According to one development of the invention, all the injection openings of a row of holes have the same diameter and are equidistant. 
     In another development of the invention, the distance between adjacent injection openings of a row of holes is approximately 16 mm. 
     For operation with natural gas, it is possible furthermore to specify an advantageous range of the ratio of diameter of the injection opening to height of the air inlet ducts which lies between 0.109 and 0.124. In combination with the specified hole diameter ranges, in particular, two particularly advantageous partial ranges of the ratio of diameter of the injection opening to height of the air inlet ducts have been determined. These are the ranges of 0.109 to 0.112 and 0.119 to 0.124. 
     In another development of the invention, the premix burner is intended for operation with natural gas as the gaseous fuel, and the ratio of hole diameter of the injection openings to the effective outlet diameter of the premix burner is 0.012 in each case. 
     In a further development of the invention, the premix burner is intended for operation with a gaseous fuel which has a calorific value which lies at least 20% below the calorific value of methane, and the injection openings have in each case a diameter ratio of diameter of the injection opening to effective outlet diameter of the premix burner of 0.0137. 
     For operation with a gaseous fuel with a calorific value which lies at least 20% below the calorific value of methane, it is possible furthermore to specify an advantageous range of the ratio of diameter of the injection opening to height of the air inlet ducts which lies between 0.123 and 0.140. In combination with the specified hole diameter range, in particular, two particularly advantageous partial ranges of the ratio of diameter of the injection opening to height of the air inlet ducts have been discovered. These are the ranges of 0.123 to 0.128 and 0.134 to 0.140. 
     The combustion gas speed into the injection openings must, on the one hand, be high enough to attain good mixing-through, but on the other hand should be low in order to keep pressure losses in the combustion gas system low and thereby eliminate, or minimize, a compression of the combustion gas, which may be required depending on the pressure level of the gas supply system, before the introduction. Here, the combustion gas speed into the injection openings is proportional to the gas quantity and inversely proportional to the sum of the areas of the injection openings of a burner. Typically, the combustion gas quantity introduced into a burner is also proportional to the burner size. The ratio of the sum of the areas of the injection openings of a burner to the effective outlet area of the premix burner is projected as a characteristic variable for an optimum burner selection, wherein the effective outlet diameter corresponding to the effective outlet area is typically used as a measure for the burner size. For operation with natural gas, a ratio which lies between 0.005 and 0.008 was found to be an advantageous ratio of the sum of the areas of the injection openings to effective outlet area of the premix burner. For operation with a gaseous fuel with a calorific value at least 20% below the calorific value of methane, a ratio which lies between 0.007 and 0.010 was discovered to be an advantageous ratio of the sum of the areas of the injection openings to the effective outlet area of the premix burner. 
     According to one development of the invention, two parallel rows of holes with doubled hole distance between the injection openings, the holes of which are arranged in an offset manner in relation to each other, are provided per air inlet duct in each case. As a result of the different injection positions, combustion stability can be positively influenced. 
     According to a further development of the invention, one row of holes with injection openings is provided per air inlet duct in each case. 
     In addition to the new-type premix burner, a method for reworking such premix burners is a subject of the invention. It is the object of the method to rework a conventional premix burner with small injection openings with minimum cost so that a new-type premix burner with larger injection openings is obtained. For this purpose, it is proposed to close every other hole of a row of holes of injection openings and to enlarge the diameter of the remaining injection opening. For closing, the holes are welded up or soldered up, for example. A small stopper can also be used, for example. 
     In one development of the invention, the injection opening which lies nearest the outlet of the premix burner to the combustion chamber is closed. Starting from there, one hole is bored out and one hole closed alternately in each case. 
     In one development of the invention, the injection opening, which lies nearest the outlet of the premix burner to the combustion chamber, is bored out. Starting from there, one hole is closed and one hole bored out alternately in each case. 
     According to one development of the invention, the diameter of the remaining injection openings is enlarged so that its outlet area is doubled. 
     DETAILED DESCRIPTION 
     In  FIG. 2 , one half of a premix burner  10  of the double-cone type is shown, as is used in large gas turbines. Shown is the conical character of the premix burner  10 , which is delimited towards the combustion chamber (to the right in  FIG. 2 ) by means of a front plate  13 . Also shown is an air inlet duct  18 , on the outer side of which a fuel line  16  for the gaseous fuel is transversely arranged. 
     In conventional premix burners, the gaseous fuel is injected into the air inlet duct  18  through injection openings  21  which in shape and arrangement form the depicted row of holes R 1 . In this case, it involves 32 injection openings  21  with a diameter ratio of 0.0086 (for natural gas; 0.0097 for a gas with lower calorific value), which have a distance from each other of 8 mm and are therefore distributed over a length L of 8×31 mm. From the outer side of the front plate  13 , the row of holes R 1  has a distance of 15 mm. 
     In order to now achieve here more intense fuel jets, the row of holes R 1  is replaced by the row of holes R 2  or R 3 , in which provision is made for only 16 injection openings  21  with an increased diameter ratio of 0.011 and a distance d of 16 mm in each case. So that the sum of all the flow cross sections of the injection openings compared with the hole row R 1  remains the same, the fewer individual jets, however, are more intense and therefore reach deeper into the flow of combustion air and lead to a significant improvement of intermixing. The distance of the row of holes to the front plate  13  in this case can remain unaltered compared with the row of holes R 1  (row of holes R 2 ; distance a 1 ). It is also conceivable, however, to increase this distance from 15 mm to 23 mm (row of holes R 3 ; distance a 2 ), as a result of which the region of a stable combustion is shifted to lower temperatures. 
     The diameter ratio of 0.012 for the injection openings  21  of the rows of holes R 2  and R 3  is provided for the use of natural gas. If, instead of natural gas, a gaseous fuel with a calorific value of less than 80% of the calorific value of methane is injected, the injection openings  21  preferably all have a diameter ratio of 0.014. 
     In the embodiment R 5 , provision is made for two parallel rows of holes with injection openings which are offset in relation to each other so that the two rows of holes are positioned “by a stagger” in relation to each other. The distance between the holes of a row of holes in this case is doubled to 2×d. 
     The distribution of the mass flow of gaseous fuel to considerably fewer injection openings with larger diameter is essential for improved intermixing, combustion and pollutant emission. Contrary to the expectation according to which for a better mixing-through a large number of small injection holes with correspondingly high pressure loss during injection would lead to improved mixing-through, emissions can be reduced on account of the greater penetration depth with larger holes. It is understood that the diameters and distances apart of the injection openings  21  in a row of holes can have certain variations within the scope of the invention in order to be able to compensate for unevenness in the combustion air flow. 
     LIST OF DESIGNATIONS 
     
         
           10  Premix burner 
           11 ,  12  Partial cone shell 
           13  Front plate 
           14 ,  15  Cylinder 
           16 ,  17  Fuel line 
           18 ,  19  Air inlet duct 
           20  Combustion air 
           21  Injection opening 
           22  Fuel (gaseous) 
           23  Fuel (liquid) 
           24  Nozzle 
           25  Combustion chamber 
           26  Hole 
           27  Cooling air 
           28  Flame front 
           29  Axis 
           30  Inner space (conical) 
         a 1 , a 2  Distance 
         d Distance 
         H Height of the air inlet ducts 
         L Length 
         R 1 , . . . ,R 5  Row of holes