Abstract:
A burner for a gas combustor and a method of operating the burner are disclosed. The burner includes a front surface area divided into a plurality of subareas and inlets arranged on the front surface area such that each subarea is encircled by at least four inlets and such that during operation of the burner, a gas recirculation in the combustor is facilitated corresponding to each subarea.

Description:
FIELD OF INVENTION 
       [0001]    The present invention relates to a gas combustor burner particularly arrangement of inlets in the burner which supplies combustible gas to the combustor and its operation for fuel staging. 
       BACKGROUND OF INVENTION 
       [0002]    Gas turbines are used to convert heat energy to mechanical energy, for example in power plants. Gas turbines have combustion chambers in which a fuel is burned with air. The combustion chambers of gas turbine plants are supplied with liquid and/or gaseous fuel using burner consisting of one or more nozzle or inlets. The burner can also be used to carry air required for the combustion. For the optimal operation of the gas compressors, stable flames should be formed in the combustion chamber during combustion. Common techniques for flame stabilization include the formation of small eddies or recirculation zones in the combustion chamber. The temperature in the recirculation zone needs to be above a threshold especially at lower load conditions to sustain combustion which will result in stable operation. 
         [0003]    A flame will be inherently unstable if the energy release from the combustion is insufficient to raise the temperature to a level at which combustion is self sustaining under conditions of heat loss including radiation to and from the flame and under hot gas recirculation. The ability to keep a sustainable temperature to maintain combustion at different loads especially at lower loads and simultaneously achieve emission targets is a great challenge in gas turbine operations. 
       SUMMARY OF INVENTION 
       [0004]    In view of the foregoing, an embodiment herein includes a burner for a gas combustor. The burner comprises a front surface area divided into a plurality of subareas. Inlets are arranged on the front surface area such that each subarea is encircled by at least four inlets such that during operation of the burner, a gas recirculation in the combustor is facilitated corresponding to each subarea. 
         [0005]    In view of the foregoing, another embodiment herein includes a method of operating a burner with a plurality of inlets on the front surface area of the burner to provide combustible gas to a combustor. The method comprises dividing the front surface area into a plurality of subareas, and for at least one subarea selecting at least four inlets which are encircling the respective subarea. The method further comprise providing combustible gas only through the selected inlets such that a gas recirculation in the combustor is facilitated corresponding to each subarea. 
         [0006]    The underlying idea here is to provide gas recirculation inside a combustor by providing and operating inlets in at least one subarea which is smaller than that of front surface area of the burner. By operating the inlets encircling the subarea a gas recirculation or gas recirculations, if more than one subarea are operated, is formed which can maintain the sustainable temperature for the combustion. The number of subareas operated is based on the load of the gas turbine which can also be directly mapped to the combustor load. Operating the inlets in a subarea or subareas also enables staging the supply of combustible gas to the combustor for combustion. The combustion resulting in the creating of gas recirculation which is hot thereby resulting in maintaining the required temperature throughout the entire load range. By operating a single smaller subarea at lower loads, the combustion associated with that subarea can be sustained. This combustion creates hot recirculation which also provides stability to flames in the combustor provided for the combustion. As the load increases other subareas can be made operational to supply the combustible gas to the combustor. Practically, at least four inlets are required to realize a gas recirculation in a subarea. The staging of the combustible gas referred in the invention should also be interpreted as the staging of the fuel since the inlets are generally supplied continuous by air in industrial operations. 
         [0007]    According to a preferred embodiment, the burner further comprises a pilot inlet in at least one of the subareas. This pilot inlet helps in supplying flames to provide adequate temperature to start the combustion process. 
         [0008]    In an alternative embodiment, the inlets encircling the subarea are spaced equally. This enables to create a stable recirculation using the combustible gases injected into the combustor by the inlets in the subarea. 
         [0009]    In another alternative embodiment, the front surface area comprises of two or three or more subareas. Having plurality of subareas enables more flexibility or control of the supply of the fuels to the combustor for combustion. That means the supply of the fuels to the combustor can be staged. The more the number of subareas, more the number of staging that can be realized. During staging, the amount of combustible gas supplied through the inlets in one or more subareas are regulated or controlled based on the load of gas turbine. Also simultaneous operation of multiple subareas will result in plurality of gas recirculation, which further provides more tuning flexibility regarding thermo acoustic oscillations in the gas combustor. Practically two or three gas recirculations are optimal even though more gas recirculations can be realized for the operation. 
         [0010]    In another alternative embodiment, the shape of said burner is circular or elliptical. The shape of the burner enables to arrange the inlets in multitude of possibilities to get a gas recirculation. 
         [0011]    In another alternative embodiment, the subareas formed by the arrangement of the inlets on the front surface area have symmetrical configuration. Symmetrical configuration of the subareas and corresponding inlets arranged will enable to provide more stability to the combustor, if operated simultaneously. 
         [0012]    In another alternative embodiment, the subareas formed by the arrangement of the inlets on the front surface area has asymmetrical configuration. This enables the burner to have subareas having different area configurations and also possibly with different number of inlets encircling them which further helps in staging the combustible gas by selecting the required subarea based on the combustor load. For example if the load is very low, the smallest subarea can be selected for the operation. 
         [0013]    In another alternative embodiment adjacent subareas on the front surface area of the burner are adapted to use at least one inlet in common. This helps in effective utilization of the front surface area of the burner to generate effective gas recirculation. 
         [0014]    In another alternative embodiment, the inlets arranged on the front surface area, to supply the combustible gas to the combustor, are of at least two different diameters. This will help in the operation of the required subarea which further regulates the flow of combustible gas based on the load requirement. For example, at lower loads the inlets with smaller diameter can be operated and at higher loads, when more combustible gas is required the inlets with larger diameter could be used. 
         [0015]    In another alternative embodiment, said burner operates on pre-mixed jet flames. Combustion systems based on pre-mixed jet flames offer special advantages over for example, swirl stabilized systems from the thermo-acoustic point of view, owing to the distributed heat release zones and the absence of swirl induced vortices. By appropriately selecting the jet impulse, small scale eddy structures can be created which dissipate the acoustically induced fluctuations of heat release, thereby suppressing the pressure pulsations which are typical for swirl stabilized flames. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The present invention is further described hereinafter with reference to illustrated embodiments shown in the accompanying drawings, in which: 
           [0017]      FIG. 1  illustrates an elliptical burner where inlets are arranged in the front surface area of the burner so as to form two sub areas according to an embodiment of the invention, 
           [0018]      FIG. 2  illustrates a circular burner where inlets are arranged in the front surface area of the burner so as to form multiple sub areas according to an embodiment of the invention with a central pilot inlet, 
           [0019]      FIG. 3  illustrates a circular burner where inlets are arranged in the front surface area of the burner so as to form multiple sub areas according to an embodiment of the invention with a pilot inlet in one of the subareas, 
           [0020]      FIG. 4  illustrates a combustion chamber, showing the gas recirculation, when operating multiple subareas in a burner as shown in  FIG. 1 , and 
           [0021]      FIG. 5  illustrates a combustion chamber, showing the gas recirculation, when operating two subareas using a burner as shown in  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0022]    It is known that undesired thermoacoustic oscillations frequently occur in combustors of gas turbines. The term “thermoacoustic oscillations” designates mutually self reinforcing thermal and acoustic disruptions. In the process, high oscillation amplitudes can occur, which can lead to undesired effects, such as to high mechanical loading of the combustor and increased NOx emissions as a result of inhomogeneous combustion. In order to ensure a high output in relation to pulsations and emissions over a wide operating range, further tuning of the fuel distribution and active or passive control of the combustion oscillations may be necessary. 
         [0023]      FIG. 1  illustrates an elliptical burner  100 , where inlets are arranged in the front surface area  101  of the burner so as to form two subareas  102  and  104  according to an embodiment of the invention. The inlets  105  along with the common inlets  106  encircle the subarea  102 . Likewise, inlets  107  along with the common inlets  106  encircle the subarea  104 . During operation of the burner, i.e. during combustion, a gas recirculation in the combustor is facilitated or produced corresponding to each subarea  102  and  104  if all the inlets of the burner are operated. It is to be noted that in this embodiment no inlet is used as a pilot inlet to start the combustion and the inlets encircling the subarea are substantially spaced at equal distance between them. Piloting could even be realized by a separate burner if required or even any other heating means can be used to provide the required temperature for the start of the combustion. 
         [0024]    In  FIG. 1 , the inlets are placed in the front surface area to form two almost identical or symmetrical subareas. The upper subarea  102  and the lower subarea  104  have symmetrical configurations. The number of subareas in the  FIG. 1  is limited to two, but practically the burner could be realized by three or more subareas. For example at low load operation of the gas turbine, the required combustion temperature could be maintaining by supplying the combustible gas, i.e. the fuel and air mixture through the inlets of any one of the subarea  102  or  104 . The inlets in the other subarea will be supplied only with air, resulting in a flame in only one of the two subareas for which the combustible gas was supplied. The flame will generate a hot gas recirculation. The air flowing through the other subarea also results in a recirculation, which will not be hot, but is referred as a cold recirculation here after for the explanation and understanding. 
         [0025]    Also it is possible to merge the subareas during operation. At high load operation all the inlets of the burner are used to supply combustible gas to the combustor. The operation of all inlets in the burner results in two hot recirculations; one formed by the inlets of the subarea  102  and another formed by the inlets of the subarea  104 . 
         [0026]      FIG. 2  illustrates a circular burner  200 , where inlets are arranged in the front surface area  201  of the burner so as to form multiple sub areas  202 ,  204  and  206  according to an embodiment of the invention with a central pilot inlet  209 . The centralised pilot could be used for any one or all of the subareas  202 ,  204  and  206 . The placement of the inlets in the front surface of the burner as shown in  FIG. 2  provides an asymmetric configuration of the subarea, since for example the number of the inlets which encircles each subarea varies. Also it should be noted that the size, more specifically the diameter of few of the inlets in the subarea  206  differs from that of the inlets in subarea  202  or  204 . Also between adjacent subareas there are inlets which are common. As shown in  FIG. 2 , inlet  203  is common to subarea  202  and  204 . Likewise, inlet  205  is common to subarea  202  and  206  and also inlet  207  is common to subarea  204  and  206 . As previously mentioned the pilot inlet  209  is common to all the subareas. When the load on the gas turbine is low, the inlets in the subarea  202  or  204  is operated and when the load increases the inlets in other subareas including the subarea  206  having larger diameter could be used to supply the combustible gas. Based on the load, different combinations of operation of the subareas are possible enabling the staging of the combustible gas supply to the combustor thereby resulting in complete combustion throughout the load range and less emissions. Flame stability is also achieved using the hot gas recirculation formed in the combustion in the combustor. The hot recirculation formed after the combustion gives enough temperature to sustain the combustion in the region of recirculation resulting in a stable flame. 
         [0027]      FIG. 3  illustrates a circular burner  300  where inlets are arranged in the front surface area  301  of the burner so as to form multiple subareas  302 ,  304  and  306  according to an embodiment of the invention with a pilot inlet  308  in one of the subarea  306 . In the specific arrangement, the pilot inlet can initiate the combustion first and at low loads the subarea  306  could be operated. As the load increases other subareas can be made operational by controlling the respective inlets to form multiple hot recirculations. 
         [0028]      FIG. 4  illustrates a combustion chamber, showing the gas recirculation, when using a burner as shown in  FIG. 1 .  FIG. 4  shows a combustion chamber  400 , of a can-type combustor. The combustion chamber has an internal space  402  enclosed by a wall  401 , which is generally cylindrical. On the inlet side  403 , a burner  404  having plurality of inlets placed on the front surface of the burner as shown in  FIG. 1  is placed. The burner is considered to be an elliptical burner as shown in  FIG. 1 . By operating all the inlets in both the subareas the gas recirculation inside the combustion chamber  400  will be as shown in the  FIG. 4 , shown by the arrows. The gas recirculation arrows  406  shows the recirculation in the upper region of combustion chamber  400  formed by the inlets in the subarea  102  of  FIG. 1 . The gas recirculation arrows  407  shows the recirculation in the lower region of combustion chamber  400  formed by the inlets in the subarea  104  of  FIG. 1 . These recirculations provide the flame stabilization mechanism. By staging of the combustible gas through the inlets, the number of hot recirculation can be controlled. The number of hot recirculation required can also be based on the operational load conditions, the required stability and emission requirements. 
         [0029]      FIG. 5  illustrates a combustion chamber  500 , showing the gas recirculation when using a burner as shown in  FIG. 1 . The combustion chamber has an internal space  502  enclosed by a wall  501 , which is generally cylindrical. On the inlet side  503 , a burner  504  having plurality of inlets are placed. The inlets placed in the front surface of the burner are similar to that discussed and shown in  FIG. 1 . By staging the fuel only through one sub area for example, through all the inlets in the subarea  102  of  FIG. 1 , the gas recirculation inside the combustion chamber  500  will look like what is shown in  FIG. 5 . The gas recirculation arrows  508  show the hot recirculation formed by the operation of inlets in the subarea  102  and recirculation arrows  506  shown in dotted lines indicate the cold recirculation formed by the flow of air through the inlets in the subarea  104 . By operating the inlets in a single or inlets in multiple subareas based on the load, the fuel staging can be achieved and thereby the number of hot recirculation inside the combustor can be controlled for getting flame stability. 
         [0030]    Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternate embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that such modifications can be made without departing from the embodiments of the present invention as defined.