Abstract:
A gas turbine combustion system includes a cylindrical combustor, a plurality of combustion sections in an arrangement spaced apart in an axial direction of the combustor, a plurality of fuel supply lines independently connected to the combustion sections, respectively, premixed fuel supply sections respectively provided for the fuel supply lines for supplying a premixed fuel, a diffusion combustion fuel supply section for supplying a diffusion combustion fuel to the combustion sections, and a control switching over the fuel supply sections to selectively supply either one of the premixed fuel and the diffusion combustion fuel. The premixed fuel at a first combustion stage is burned while the premixed fuel of subsequent stage is ignited by a high-temperature gas generated from combustion of the premixed fuel of a preceding combustion stage.

Description:
This application is a Division of application Ser. No. 08/854,749, filed on May 12, 1997, (U.S. Pat. No. 5,802,854) wich is a continuation of application Ser. No. 08/394,275 filed on Feb. 24, 1995, now abandoned. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Various other objects, features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood from the following detailed description when considered in connection with the accompanying drawings in which like reference characters designate like or corresponding parts throughout the several views and wherein: 
     FIG. 1 illustrates an embodiment of a gas turbine combustion system according to the present invention 
     FIG. 2 is a cross-sectional view of part of the gas turbine combustion system of FIG. 1; 
     FIG. 3 is a view explaining the function of the embodiment shown in FIG. 1; 
     FIG. 4 is an enlarged view of the pilot burner in the embodiment shown in FIG. 1; 
     FIG. 5 illustrates a fuel system of the embodiment shown in FIG. 1; 
     FIG. 6 illustrates a combustion portion of another embodiment of the present invention; 
     FIG. 7 illustrates a combustion portion of still another embodiment of the present invention; 
     FIG. 8 illustrates a modification of a micro burner employed in the embodiment shown in FIG. 1; 
     FIG. 9 illustrates an igniter which may be replaced with the micro burner employed in the embodiment shown in FIG. 1; 
     FIG. 10 is a graphic representation showing control characteristics of a computing element of the embodiment shown in FIG. 1; 
     FIG. 11 is a flowchart illustrating the function of the embodiment shown in FIG. 1; 
     FIG. 12 illustrates NOx characteristics of the prior art; 
     FIG. 13 illustrates NOx characteristics of the prior art; 
     FIG. 14 illustrates the relation between NOx or Co and the proportion of a diffusion fuel flow rate; 
     FIG. 15 illustrates the relation between NOx and the combustion range premixed equivalent ratio  15 ; and 
     FIGS. 16A &amp; 16B illustrate the relation between the wall surface cooling ratio and the fuel outlet equivalent ratio. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of a gas turbine combustion system according to the present invention will be described below with reference to the accompanying drawings. 
     FIG. 1 illustrates the structure of the gas turbine combustion system according to the prevent embodiment. As shown in the figure, the combustion system is provided with a combustor  1  having a cylindrical, for example, structure closed at one end by a header H and including a first combustion chamber  2   a  having a three-stage combustion portion, and a second combustion chamber  2   b  having a two-stage combustion portion. The first combustion chamber  2   a  has a structure in which a pair of inner tubes  1   a  and  1   b  having small diameters are coupled to each other in the direction of a gas stream. 
     The small-diameter inner tube la located on an upstream side in the first combustion chamber  2   a  is provided with a pilot burner  3 , premixing units  4   a  and at least one micro burner  5   a  (which may be a heater rod heated by an electric heater or other ignition device designed to discharge ignition energy by utilizing electric or magnetic energy). The pilot burner  3  is on the other end mounted to the header H. The small-diameter inner tube  1   b  located on a downstream side in the first combustion chamber  2   a  is provided with premixing units  4   b  and at least one micro burner  5   b.  The premixing units  4   a  or  4   b , each having a configuration of a premixing duct, are arrayed in a number ranging from 4 to 8 in a peripheral direction of the inner tube  1   a  or  1   b.  Fuel nozzles  6   a  and  6   b  are disposed at air inlets of the premixing units  4   a  and  4   b , respectively. 
     The second combustion chamber  2   b  includes an inner tube  7  having a diameter larger than those of the inner tubes  1   a  and  1   b,  premixing units  4   c  and  4   d  and at least one micro burner  5   c.  The premixing units  4   c  or  4   d , each having a configuration of a premixing duct, are arrayed in a number ranging from 4 to 8 in a peripheral direction of the large-diameter inner tube  7 . 
     Fuel nozzles  6   c  and  6   d  are disposed at upstream sides of the premixing units  4   c  and  4   d , respectively. The premixing units  4   a ,  4   b ,  4   c  and  4   d  are fixed to a dummy inner tube  9  by means of supports  8   a  and  8   b  (only part of which is illustrated). The axial position of the dummy inner tube  9  is set by supports  11  fixed to a casing  10  so that the dummy inner tube  9  can receive thrusts acting on the small-diameter inner tubes  1   a  and  1   b  and the large-diameter inner tube  7 . 
     An inner wall  12  of a tail pipe and an outer wall  13  of a tail pipe  13  are provided downstream of the large-diameter inner tube  7 . The tail pipe outer wall  13  is formed with a large number of cooling holes  14 . Similarly, a flow sleeve  15 , having a large number of cooling holes  16 , is provided on an outer peripheral side of the large-diameter inner tube  7 . A tie-in portion between the large-diameter inner tube  7  and the tail pipe inner wall  12  and a tie-in portion between the flow sleeve  15  and the tail pipe outer wall  13  are sealed by means of spring seals  17 , respectively. 
     A premixed fuel injection port  18  of the first stage is provided at the upstream end of the small-diameter inner tube  1   a.  Outlets of the premixing units  4   a ,  4   b ,  4   c  and  4   d  provided in the inner tubes  1   a ,  1   b  and  7  serve as premixed fuel injection ports of the second, third, fourth and fifth stages  19   a ,  19   b ,  19   c  and  19   d , respectively. The premixed fuel injection ports of the second, third, fourth and fifth stages  19   a ,  19   b ,  19   c  and  19   d  are disposed at predetermined intervals which ensure that the series combustion can be conducted adequately in the axial direction of the combustor. The premixed fuel may be injected from the injection ports  19   a ,  19   b ,  19   c  and  19   d  toward the center of the combustor. The injection ports may also be disposed in a spiral fashion so that the gas stream can have a swirling component, as shown in FIG.  2 . 
     The pilot burner  3  includes a diffusion fuel nozzle  20  located along a central axis of the small-diameter inner tube  1   a,  a premixed fuel nozzle  21  and a swirler  22 . A peripheral wall constituting the portion of the pilot burner  3  located upstream of the swirler  22  has a large number of air holes  23 . The burning state of the pilot burner  3  is illustrated in FIG.  3 . Operation of the pilot burner  3  is described herebelow. 
     FIG. 4 illustrates the structure of the pilot burner  3  in greater detail. A distal end of a pilot diffusion fuel supply pipe  24  has injection holes  25 . The injection holes  25  are located close to and in opposed relation with a nozzle distal end  26 . The nozzle distal end  26  has injection holes  27  and  28  through which a diffusion fuel is injected. 
     The micro burners  5   a,  serving as ignition sources, are provided near the central portion of the nozzle distal end  26  and an inverted flow area  29 . A flow passage  30  is formed on an outer peripheral side of the pipe  24 . A distal end of the flow passage  30  has an injection port  31  through which a premixed fuel, which is a mixture of a combustion air and a fuel, is injected into the combustion chamber. 
     As shown in FIG. 1, a fuel supply system  32  has a fuel pressure adjusting valve  33  and a fuel flow rate adjusting valve  34  and is designed to supply a fuel to the fuel nozzles  6   a  to  6   d  through cutoff valves  35  and  36 , a fuel flow rate adjusting valve  37 , a distributing valve  38  and fuel flow rate adjusting valves,  39   a ,  39   b ,  39   c  and  39   d.    
     FIG. 5 illustrates a configuration of the fuel supply system. A fuel N, which has passed through the pressure adjusting valve  33  and the flow rate adjusting valve  34 , is distributed into two systems. 
     One of the two systems extends through the cutoff valve  36  and is then divided into two system lines. One of these two system lines is in turn divided into a line  41   a  which extends through a flow meter  40   a  and the flow rate adjusting valve  39   a  and a line  41   b  which extends through a flow meter  40   b  and the flow rate adjusting valve  39   b  while the other one of the system lines extends through a flow meter  40   e  and the flow rate adjusting valve  39   e  and is divided into a line  41   e  which extends through the flow rate adjusting valve  38  and another line  41   f.    
     The system line which extends through the flow rate adjusting valve  34  extends through the cutoff valve  35  and is then divided into a line  41   c  which extends through a flow meter  40   c  and the flow rate adjusting valve  39   c , and a line  41   d  which extends through a flow meter  40   d  and the flow rate adjusting valve  39   d.    
     Signals S 101 , S 102 , S 103 , S 104  and S 105  output from all the above-described adjusting valves, the cutoff valves, the flow meters and so on, an output signal S 106  of a generator  51   a  and a load signal S 107  are supplied to a computing element  42 . The computing element  42  controls the input signals according to the load signal  107  on the basis of a schedule input in the computing element  42 . Reference numeral  51   b  denotes a denitration device and reference numeral  51   c  denotes a chimney. 
     Operation of the combustor  1  is described hereinbelow. 
     First, the flow of air will be explained with reference to FIGS. 3 and 5. As shown in FIG. 5, part of high-temperature/high-pressure air A 0  ejected from an air compressor  50  is used to cool a turbine  51 . Part of air A 0  is supplied to the combustor  1  as a combustor air A 1 . The combustor air A 1  passes through the tail pipe cooling holes  14  and  16  and flows into a gap  52  as an impinging jet A 2  to cool the tail pipe inner wall  12  and the large-diameter inner tube  7  due to a convection flow. 
     The impinging jet A 2  does not flow into the combustor  1  at the region of the tail pipe inner wall  12  and the large-diameter inner tube  7  so that it can flow into the premixing duct units  4   a ,  4   b ,  4   c  and  4   d  as combustion airs A 3 , A 4 , A 5  and A 6 , respectively. The impinging air A 2  also flows into the pilot burner  3  through the combustion air holes  23  as a combustion air A 7 . The impinging air A 2  also flows downstream in the gap  52  so that it can be used as a film cooling air A 8  of the small-diameter inner tubes  1   a  and  1   b.    
     The flow of air and fuel in the pilot burner  3  will be described below. 
     The combustion air A 7  which has flowed from the air holes  23  shown in FIG. 4 is swirled by the swirler  22  so that it has angular momentum. The resulting swhirling air flows into the small-diameter inner tube  1   a  through the injection, port  31 . The injection port  31  shown in FIG. 4 corresponds to the premixed fuel injection port  18  of the first stage shown in FIG. 2. A pilot diffusion fuel N 1  ejects, as a jet, through the holes  25  formed at the downstream side of the pipe  24  to cool the nozzle distal end  26  by the convection flow, and then flows into the small-diameter inner tube  1   a  through the injection port  27  as a diffusion fuel N 2 . The diffusion fuel N 2 , is ignited by, for example, an igniter  53  provided on the peripheral wall of the small-diameter inner tube  1   a  to form a pilot flame F 1 . After ignition, the diffusion fuel N 1  is gradually replaced with a premixed fuel N 3  in response to the signal S 103  from the computing element  42 . 
     The premixed fuel N 3  is showered through the premixed fuel nozzle  21  as a fuel N 4 . The fuel N 4  is uniformly premixed with the combustion air A 7 . A resultant premixed fuel N 5  increases its speed to a velocity twice the turbulent combustion speed or more as it swirls downstream and then flows into the small-diameter inner tube  1   a  from the premixed fuel injection port  18  of the first stage, i.e. the injection port  31 . At that time, no backfire occurs from the pilot flame F 1  because the velocity of the fuel is twice the turbulent combustion speed or more. By the time the fuel replacement is completed, all the pilot flame F 1  becomes a premixed mixture flame obtained from the premixed mixture fuel N 3 , and hence generation of NOx is almost reduced to zero. 
     Next, the flow of fuel in the combustor inner tube and the combustion method will be described hereunder. 
     First, the pilot flame F 1  is formed in the small-diameter inner tube  1   a  by the above-described method. The flame F 1  is stabilized because of a desired combination of the pilot diffusion fuel N 1  with the pilot premixed fuel N 3 . After the pilot flame F 1  has been formed, the fuel having a flow rate controlled on the basis of the output signal S 103  of the computing element  42  is uniformly mixed with air in the premixing unit  4   a.  A resultant premixed fuel N 4  flows into the small-diameter inner tube  1   a  through the premixed fuel injection ports  19   a  of the second stage. 
     The premixed fuel N 4  is ignited and burned by the pilot flame F 1  located upstream of the premixed fuel N 4  to form a premixed flame F 2 . Next, a premixed fuel N 5  of the third stage similarly flows into the small-diameter inner tube  1   b  from the premixed fuel injection ports  19   b  of the third stage. The premixed fuel N 5  is ignited and burned by the total amount of combustion gas obtained by adding the pilot flame F 1  to the premixed flame F 2  located upstream of the premixed fuel N 5  thereby to form a premixed flame F 3 . Premixed fuels N 6  and N 7  of the fourth and fifth stages respectively form premixed flames F 4  and F 5  by the same process as that of the second and third stages. 
     The computing element  42  controls the respective fuel flow rates such that the premixed fuels N 1 , N 2 , N 3 , N 4  and N 5  have a combustion temperature, less than 1600° C., which ensures generation of no NOx. Consequently, NOx characteristics (i) (see FIG. 12) can be made low over the entire gas turbine load region, unlike NOx characteristics (b) (see FIG. 12) of a conventional low NOx combustor, and the NOx objective value (h) (see FIG. 12) can thus be achieved. 
     Flames are stabilized by the adoption of so-called “series combustion” in which the premixed fuels of the first, second, third, fourth and fifth stages are ignited and burned in series by the high-temperature gas located upstream thereof to expand a flame. 
     Cooling of the combustor inner tube will be discussed. 
     A large part of the air supplied from the air compressor  50  to the combustor  1  passes through the impinging cooling holes  14  and  16  respectively formed in the tail outer tube  13  and the flow sleeve  15 , and then collides against the tail inner tube  12  and the large-diameter inner tube  7  as the impinging jet A 2  to cool the wall surfaces thereof by the convection flow. 
     The impinging jet A 2  does not enter the combustor at the tail inner tube  13  but flows into the combustor as the combustion airs A 3 , A 4 , A 5  and A 6  of the premixing units  4   a ,  4   b ,  4   c  and  4   d  and as the combustion air A 7  of the pilot burner  3 . 
     At the small-diameter inner tubes  1   a  and  1   b  corresponding to the first combustion chamber  2   a,  less than 20% of the combustion air A 1  flows into the combustor as a film cooling air to cool the inner surface thereof. That is, only cooling of the outer surface is conducted at the tail inner tube  12 , so that the air to be used as a film cooling air can be used as combustion airs A 3 , A 4 , A 5 , A 6  and A 7 , thus increasing the amount of combustion air. Consequently, a desired premixed fuel air ratio assuring a combustion temperature, less than 1600° C., which ensures generation of no NOx can be set, and a reduction in the NOx can thus be achieved. 
     The computing element  42  which performs the above-described combustion method will be discussed. 
     As shown in FIG. 10, premixed fuel flow rates W 1  through W 5  of the five stages are stored beforehand as functions relative to a gas turbine load in the computing element  42  for the five stages of fuel lines. A total of the premixed fuel flow rates W 1  to W 5  is equal to a total fuel flow rate W 0 . The premixed fuel flow rates W 1  to W 5  of the five stages are obtained by the signal S 103  using the flow rate adjusting valves  37 ,  39   a ,  39   b ,  39   c  and  39   d  relative to the load signal S 107 . 
     Referring to FIG. 11, where a load increases, the fuel of the first stage is replaced (step  1101 ), and then the premixed fuels of the respective stages are increased in sequence (steps  1102  to  1105 ). 
     Where a load decreases, the fuel flow rates of the respective stages are reduced in sequence starting with the fifth stage in the manner reversed to that shown in FIG.  11 . Since an air flow rate Wa relative to the gas turbine load is substantially fixed, the combustor outlet temperature is determined by controlling the total fuel flow rate W 0 . 
     As shown in FIG. 4, the micro burners  5   a  for causing a small flame to issue are provided near the inverted flow regions of the inner tubes  1   a ,  1   b  and  7  to effectively stabilize the flames. 
     The above-described embodiment of the present invention is not restrictive and susceptible to various changes, modifications, variations and adaptations as will occur to those skilled in the art. FIGS. 6 through 9 illustrate such modifications of the present invention. 
     In the modification shown in FIG. 6, the fuel injection ports  18 ,  19   a ,  19   b ,  19   c  and  19   d  shown in FIG. 1 are modified such that they have an annular arrangement surrounded by double cylinders. That is, a combustion air A 10  is swirled by a swirler  60  so that it has an annular momentum, and then flows into the cylinder from a fuel injection port  61   a ,  61   b ,  61   c ,  61   d  or  61   e  of the first, second, third, fourth or fifth stage. A fuel N 10  is supplied to the respective injection ports through separate fuel supply systems, as in the case shown in FIG.  1 . The premixed flames F 1  through F 5  are formed continuously in the axial direction of an inner tube  62  correspondingly with the fuel injection ports  61   a  through  61   e  of the first, second, third, fourth and fifth stages to achieve series combustion. 
     In the modification shown in FIG. 7, although a pilot burner  63  is substantially the same as that of the embodiment shown in FIGS. 1,  5  to  8 , multi-burner type cylindrical premixing units  66  fixed to a second combustion chamber  64   b  (located downstream of a first combustion chamber  64   a ) are arrayed in the peripheral direction of the combustion chamber. Such an array is provided at two positions in the axial direction of the combustor. Swirlers  67  are provided in each of premixing units  66  to provide uniform premixing even in a short flow passage. 
     In this modification, flames are formed in series starting from the upstream side in the same manner as those of the above-described embodiment to form premixed flames F 11 , and generation of NOx can thus be effectively restricted. 
     FIGS. 8 and 9 illustrate modifications of the micro burner shown in FIG.  1 . 
     The modification shown in FIG. 8 contemplates a micro burner  5   a  having a configuration which assures premixed combustion by a self-holding flame. That is, the distal end portion of the premixed fuel injection port  18  ( 19   a , - - -) is widened so that eddy currents can be generated in the distal end portion to form self-holding flames  70 . This configuration achieves further stabilization of flames. A heat-resistant coating layer  71  is formed at the distal end portion of the injection port. 
     In the modification shown in FIG. 9, an igniter is structured by a heating rod  81  having a high-temperature portion  80  whose temperature is increased to a value ensuring ignition by means of electrical energy. In this modification, the premixed fuel injection port  18  is formed wide, as in the case of the modification shown in FIG. 8, to form a staying region  82  of a fuel A. 
     The gas turbine combustor according to the present invention has been described above in its various embodiments and modifications. It is, however, to be emphasized that the present invention can be applied to various types of gas turbines which employ a gaseous or liquid fuel. 
     As will be understood from the foregoing description, in the gas turbine combustion system according to the present invention, simultaneous achievement of the super lean combustion condition, stable flame combustion and combustor wall surface cooling, which would conventionally be difficult, is made possible. As a result, NOx can be reduced to a desired aimed value or less (&lt;10 ppm) over the entire operation range. A great reduction in NOx enables scale-down or elimination of a denitration device, reduces the operation cost including a reduction in an amount of ammonia consumed, and contributes to global environment purification.