TECHNICAL FIELD 
       [0001]    The present invention relates to a burner, a combustor, and a gas turbine. 
       BACKGROUND 
       [0002]    In recent years, from the viewpoints of prevention of global warming and effective use of resources, gas turbines are requested to use a byproduct hydrogen gas secondarily generated from a manufacturing process of a petrochemical plant etc. in addition to a natural gas that is a main fuel of the gas turbines. 
         [0003]    Patent Document 1 discloses a gas turbine combustor having a combustion cylinder forming a combustion chamber thereinside, a casing covering the outside of the combustion cylinder and forming a flow path of a compressed air (hereinafter referred to as “combustion air”) supplied from a compressor around the combustion cylinder, a first fuel nozzle corresponding to a main burner disposed upstream of the combustion cylinder and injecting a first fuel (coal gasification gas) into the combustion chamber, and a plurality of second fuel nozzles corresponding to reheating burners disposed downstream of the first fuel nozzle and penetrating a circumferential wall of the combustion cylinder from the casing, and the gas turbine combustor injects a second fuel (hydrogen-containing gas) radially inward from the circumferential wall of the combustion cylinder into the combustion chamber so as to diffuse and combust the second fuel in a combustion product gas. 
         [0004]    On the other hand, a lean premixed combustion method (Dry Low Emission combustion method) is attracting attention as a method of suppressing a NOx emission amount without using water or steam and, in recent years, gas turbines having combustors (DLE combustors) employing this combustion method are operating at plant facilities etc. 
         [0005]    Therefore, the present applicant has proposed a combustor of a gas turbine including a reheating burner injecting a lean premixed gas acquired by preliminarily mixing a combustion air, a hydrocarbon-based first fuel (e.g., a natural gas), and a second fuel (e.g., a hydrogen gas) in a downstream region in a combustion chamber of a DLE combustor in PCT/JP2014/065657 (unpublished). 
         [0006]    This reheating burner is a burner mixing the combustion air introduced into a premixing flow path from the upstream side and the first and second fuels in the premixing flow path to generate the premixed gas and injecting the premixed gas from the downstream side into the combustion chamber for combustion, and has first and second fuel injection holes injecting the first and second fuels into a premixing chamber. 
       PRIOR ART DOCUMENT 
     Patent Document 
       [0007]    Patent Document 1: Japanese Patent Application Publication No. 2011-75174 
       SUMMARY OF THE INVENTION 
       [0008]    Considering the generation of the premixed gas composed of the combustion air, the first fuel, and the second fuel in the structure of the reheating burner proposed by the present applicant, the hydrogen gas has an extremely small specific gravity as compared to the natural gas (density of hydrogen gas: 0.09 kg/m3N, density of natural gas: 0.62 kg/m3N). Therefore, the hydrogen gas itself injected from the second fuel injection hole has a small penetrating force (i.e., a kinetic energy when the hydrogen gas is injected), so that stirring and mixing with other fluids become insufficient in the premixing flow path, which makes it difficult to generate a lean premixed gas having a uniform concentration distribution. Thus, a local high-temperature region generated during combustion results in an increase in NOx emission amount and, therefore, room for further improvement exists. 
         [0009]    It is therefore an object of the present invention to provide a burner capable of premixing a hydrocarbon-based first fuel (e.g., natural gas), a second fuel (e.g., hydrogen gas), and a combustion air and injecting a lean premixed gas having a uniform concentration distribution into a combustor combustion chamber and capable of suppressing a NOx emission amount, a combustor equipped with the burner, and a gas turbine. 
         [0010]    A burner of the present invention is a burner mixing a combustion air introduced into a premixing flow path from an upstream side and a fuel in the premixing flow path to generate a premixed gas and injecting the premixed gas from a downstream side into a combustion chamber for combustion, the burner comprising an outer cylinder having the premixing flow path formed inside; a first air introduction part supplying the combustion air from the outer edge toward the center of the outer cylinder on the upstream side of the premixing flow path; a first fuel introduction part introducing a first fuel into the premixing flow path; and a second fuel introduction part introducing a second fuel having a specific gravity smaller than the first fuel into the premixing flow path, wherein the second fuel introduction part is formed projecting from an upstream-side end portion of the premixing flow path toward the downstream side into the premixing flow path, wherein the second fuel introduction part has a plurality of second fuel injection nozzles injecting the second fuel to a compressed air introduced from the first air introduction part, wherein the second fuel is injected from the second fuel injection nozzles to the combustion air to generate a primary air-fuel mixture, and wherein the first fuel is introduced from the first fuel introduction part to the primary air-fuel mixture to generate a secondary air-fuel mixture. 
         [0011]    According to this construction, the second fuel is injected from the second fuel injection nozzles to the combustion air flowing from the outer edge toward the center of the outer cylinder on the upstream side of the premixing flow path to generate the first air-fuel mixture. In this case, the second fuel is injected into the flow of the combustion air from the second fuel injection nozzles projecting from the upstream-side end portion of the premixing flow path into the premixing flow path, avoiding a low speed area (viscous boundary layer) of the compressed air generated in the vicinity of the upstream-side end portion of the premixing flow path. Therefore, for example, even in the case of the second fuel having a small specific gravity and a low penetrating force like a hydrogen gas, the lean primary air-fuel mixture having a uniform concentration distribution is generated without a risk of retention in the low flow area described above. Subsequently, the first fuel is introduced from the first fuel introduction part to the primary air-fuel mixture to generate the secondary air-fuel mixture (premixed gas). In this case, since the first fuel has a greater specific gravity than the second fuel, the first fuel and the primary air-fuel mixture are sufficiently stirred and mixed so that the lean secondary air-fuel mixture is generated with a more uniform concentration distribution than the primary air-fuel mixture. As a result, the lean premixed gas having a uniform concentration distribution is supplied to the combustion chamber, and the NOx amount can be suppressed in a combustion exhaust gas. 
         [0012]    The first fuel introduction part included in the burner may have a first fuel injection nozzle projecting concentrically with the outer cylinder from the upstream-side end portion of the premixing flow path into the premixing flow path and injecting the first fuel toward the outer edge of the outer cylinder. 
         [0013]    According to this construction, the primary air-fuel mixture flowing from the outer edge toward the center of the outer cylinder on the upstream side of the premixing flow path flows along the outer circumference of the first fuel injection nozzle toward the downstream side of the premixing flow path and, subsequently, the first fuel is injected from the first fuel injection nozzle to the primary air-fuel mixture to generate the secondary air-fuel mixture. In this case, since the first fuel is injected in a direction intersecting with the flow direction of the primary air-fuel mixture, the mixing of the first fuel and the primary air-fuel mixture is promoted, so that the secondary air-fuel mixture is generated with a uniform concentration distribution. As a result, the lean premixed gas having a uniform concentration distribution is supplied to the combustion chamber, and NOx can be suppressed in the combustion exhaust gas. 
         [0014]    The burner may comprise a straightening protrusion part projecting concentrically with the outer cylinder from the upstream-side end portion of the premixing flow path into the premixing flow path; the first fuel introduction part included in the burner may be formed in the upstream-side end portion of the premixing flow path on the outer edge side relative to the straightening protrusion part; and the first fuel introduction part may have a plurality of first fuel injection holes inclined toward the outer edge of the outer cylinder. 
         [0015]    According to this construction, the secondary air-fuel mixture is generated by injecting the primary air-fuel mixture from the first fuel injection holes inclined toward the outer edge of the outer cylinder to the primary air-fuel mixture flowing from the outer edge toward the center of the outer cylinder on the upstream side of the premixing flow path. In this case, since the first fuel is injected in a direction intersecting with the flow of the primary air-fuel mixture, the mixing of the primary fuel-air mixture and the first fuel is promoted in the premixing flow path, so that the secondary air-fuel mixture is generated with a uniform concentration distribution. As a result, the lean premixed gas having a uniform concentration distribution is supplied to the combustion chamber, and the generation of NOx can be suppressed. Additionally, since the secondary air-fuel mixture changes the direction and flows along the straightening protrusion part toward the downstream side and is injected into the combustion chamber without lowering the speed, a backfire can be suppressed. 
         [0016]    The burner may comprise a straightening protrusion part projecting concentrically with the outer cylinder from the upstream-side end portion of the premixing flow path into the premixing flow path, and the first fuel introduction part may include a plurality of first fuel injection nozzles injecting the first fuel from the outer edge toward the center of the outer cylinder on the downstream side relative to the first air introduction part. 
         [0017]    According to this construction, the primary air-fuel mixture flowing from the outer edge toward the center of the outer cylinder on the upstream side of the premixing flow path changes the direction and flows along the straightening protrusion part toward the downstream side. Subsequently, the first fuel is injected from the first fuel injection holes toward the center of the outer cylinder to the primary air-fuel mixture to generate the secondary air-fuel mixture. In this case, since the first fuel is injected in a direction intersecting with the flow of the primary air-fuel mixture, the mixing of the primary fuel-air mixture and the first fuel is promoted in the premixing flow path, so that the secondary air-fuel mixture is generated with a uniform concentration distribution. As a result, the lean premixed gas having a uniform concentration distribution is supplied to the combustion chamber, and the generation of NOx can be suppressed. Additionally, since the primary air-fuel mixture flows along the straightening protrusion part toward the downstream side without lowering the speed, a reduction in flow speed is suppressed when the direction is changed. Therefore, the premixed gas is injected into the combustion chamber while maintaining a sufficient flow speed, so that the backfire can be suppressed. 
         [0018]    The burner may comprise a second air introduction part introducing the combustion air from the outer edge of the outer cylinder into the premixing flow path, on the downstream side relative to the first air introduction part. The outer cylinder may be made up of a first cylindrical body on the upstream side and a second cylindrical body on the downstream side arranged coaxially with each other; the first cylindrical body and the second cylindrical body may be arranged to partially overlap in the direction of the axis; and the second air introduction part may be defined by the first cylindrical body and the second cylindrical body and may be an annular gap gradually decreasing in diameter from the upstream side to the downstream side. In this case, the inner diameter of the second cylindrical body may be substantially the same as the inner diameter of the first cylindrical body on the downstream side thereof. A portion of the combustion air is blown onto the outer circumference of the first cylindrical body and is then introduced as a secondary air into the second air introduction part. By introducing the secondary air from the second air introduction part, the retention of the premixed gas can be suppressed in a boundary layer. The secondary air is uniformly straightened while flowing through the second air introduction part from the upstream side to the downstream side. Since the secondary air is fed into the premixing flow path, the retention of the air-fuel mixture can more effectively be suppressed in the boundary layer. On the other hand, the secondary air flows through the annular gap gradually decreasing in diameter from the upstream side to the downstream side and thereby can form a flow guiding the premixed gas retained in the boundary layer toward the center of the flow path. If the inner diameter of the second cylindrical body is made substantially the same as the inner diameter of the first cylindrical body on the downstream side thereof, the flow rate of the premixed gas flowing through the premixing flow path can be balanced. 
         [0019]    According to this construction, the occurrence of the low speed area is restrained in the vicinity of the inner surface of the outer cylinder and the backfire can be suppressed. 
         [0020]    The first fuel may be a natural gas or a liquefied natural gas, and the second fuel may be a hydrogen gas or a hydrogen-containing gas. 
         [0021]    A combustor of the present invention is a gas turbine combustor of a comprising a combustion cylinder forming a combustion chamber combusting a fuel; a premixing type main burner disposed upstream of the combustion cylinder; and a reheating burner disposed through a downstream-side circumferential wall portion of the combustion cylinder, wherein the reheating burner is the burner according to any of the above descriptions. 
         [0022]    This construction enables provision of a combustor having a reheating burner capable of injecting a premixed gas having a uniform concentration distribution into the combustion chamber of the combustor and capable of suppressing a NOx emission amount. 
         [0023]    Furthermore, a gas turbine of the present invention comprises the combustor described above. 
         [0024]    This construction enables provision of a gas turbine equipped with a combustor capable of suppressing a NOx emission amount. 
         [0025]    The present invention can provide the burner capable of suppressing a NOx emission amount by premixing the first fuel (e.g., natural gas), the second fuel (e.g., hydrogen gas), and the combustion air and injecting the premixed gas having a uniform concentration distribution into the combustor combustion chamber, the combustor equipped with the burner, and the gas turbine. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0026]      FIG. 1  is a diagram of a general construction of a gas turbine according to an embodiment of the present invention. 
           [0027]      FIG. 2  is a longitudinal cross section of a combustor according to one embodiment of the present invention. 
           [0028]      FIG. 3  is a longitudinal cross section of a reheating burner according to a first embodiment of the present invention. 
           [0029]      FIG. 4  is a transverse section of a premixing flow path when viewed in a direction A-A of  FIG. 3 . 
           [0030]      FIG. 5  is a diagram of a modified example of the first embodiment. 
           [0031]      FIG. 6  is a longitudinal cross section of a reheating burner according to a second embodiment of the present invention. 
           [0032]      FIG. 7  is a longitudinal cross section of a first example of a reheating burner according to a third embodiment of the present invention. 
           [0033]      FIG. 8  is a longitudinal cross section of a second example of the reheating burner according to the third embodiment of the present invention. 
           [0034]      FIG. 9  is a vertical cross section of a third example of the reheating burner according to the third embodiment of the present invention. 
           [0035]      FIG. 10  is a longitudinal cross section of a fourth example of the reheating burner according to the third embodiment of the present invention. 
           [0036]      FIG. 11  is a longitudinal cross section of a reheating burner according to a fourth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0037]    A burner, a combustor, and a gas turbine according to embodiments of the present invention will now be described with reference to the accompanying drawings. The following description is merely an exemplification of a form of the present invention and is not intended to limit the technical scope of the present invention, the application of the present invention, or the use thereof. 
       First Embodiment 
       [0038]    A general construction and function of a gas turbine is shown in  FIG. 1 . In the gas turbine  1 , a compressor  2  sucks an atmospheric air to generate a compressed air  200 . The compressed air  200  is combusted together with a fuel in a combustor  3  to generate a high-temperature high-pressure combustion product gas (hereinafter referred to as “combustion exhaust gas  300 ”). The combustion exhaust gas  300  is supplied to a turbine  4  and used for rotating a rotor  5 . The rotation of the rotor  5  is transmitted to the compressor  2  and used for generating the compressed air  200  (hereinafter referred to as “combustion air  200 ”), while the rotation of the rotor  5  is transmitted to a generator  6  and used for electric generation, for example. 
         [0039]      FIG. 2  shows the combustor  3 . In this embodiment, the combustor  3  is a reverse-flow can-type combustor in which a flow direction of the compressed air  200  supplied from the compressor (see  FIG. 1 ) (a direction from the top to the bottom of FIG.  1 ) and a flow direction of the combustion exhaust gas  300  (a direction from the bottom to the top of  FIG. 1 ) are opposed to each other. The combustor may be an annular type having a plurality of fuel injection valves on a circumference thereof. 
         [0040]    The combustor  3  includes a combustion cylinder  34  and a casing  35  concentrically arranged on a central axis  302 . A burner unit  30  is attached to the top of the combustion cylinder  34 , and a combustion chamber  33  for combusting a fuel etc. injected from the burner unit  30  is formed inside the combustion cylinder  34 . The combustion cylinder  34  is surrounded by a cylindrical casing  35  so that an annular combustion air flow path  37  is formed between the combustion cylinder  34  and the casing  35 , in which the combustion air  200  supplied from the compressor flows. The casing  35  and the combustion cylinder  34  support a plurality of reheating burners  36  on the downstream side relative to the burner unit  30 . 
         [0041]    In this embodiment, the burner unit  30  is disposed along the central axis  302  and includes a premixing type main burner  31  for injecting a premixed gas generated by mixing the fuel and the combustion air  200  into the combustion chamber  33  and a diffusion combustion type pilot burner  32  for injecting the fuel directly into the combustion chamber  33 . The main burner  31  is concentrically disposed around the pilot burner  32 . The main burner  31  and the pilot burner  32  are in communication with a first fuel supply source  305  (natural gas supply source) through a piping  304 . 
         [0042]    In this embodiment, the main burner  31  has an outer cylinder  310  and an inner cylinder  312  arranged concentrically along the central axis  302 . In this embodiment, as shown in the figure, the inner cylinder  312  also serves as a combustion air injection cylinder  322   b  of the pilot burner  32  described later. An annular space between the outer cylinder  310  and the inner cylinder  312  is used as a premixing flow path  314  for mixing the fuel and the combustion air. The premixing flow path  314  has one end opened to the combustion chamber  33  and the other end opened radially outward through a plurality of air intake ports  315  to the combustion air flow path  37 . A plurality of main fuel nozzles  316  for injecting a first fuel is arranged radially outside the air intake ports  315 . Although not shown, preferably, the plurality of the air intake ports  315  and the plurality of the main fuel nozzles  316  corresponding thereto are arranged at regular intervals in the circumferential direction around the central axis  302 . Although not shown, the main fuel nozzles  316  each have a plurality of fuel injection holes (not shown) formed at a position facing the air intake port  315  to inject the first fuel toward the air intake port  315  and are connected to the first fuel supply source  305  (natural gas supply source) through a piping  304   a  including a flow regulating valve so that, when the flow regulating valve is opened at the time of normal operation, the fuel supplied from the first fuel supply source  305  is supplied from the air intake ports  315  to the premixing flow path  314  along with the combustion air supplied from the combustion air flow path  37  and is mixed in the premixing flow path  314 , and the premixed gas is injected into the combustion chamber  33 . In this embodiment, a plurality of swirl vanes (swirlers)  317  is provided in the air intake ports  315  to impart a swirling force to the combustion air flowing into the premixing flow path  314  so as to promote premixing with the first fuel. 
         [0043]    The pilot burner  32  includes a fuel injection cylinder  322   a  extending along the central axis  302  and a combustion air injection cylinder  322   b  concentrically covering the fuel injection cylinder  322   a , and a fuel injection path (not shown) formed in the fuel injection cylinder  322   a  is connected to the first fuel supply source  305  (natural gas supply source) through a piping  304   b  including a flow regulating valve so that, when the flow regulating valve is opened at the time of startup, the natural gas supplied from the first fuel supply source is injected into the combustion chamber. An annular air flow path  324  is formed between the fuel injection cylinder  322   a  and the combustion air injection cylinder  322   b  and has one end connected to the combustion air flow path  37  and the other end connected to the combustion chamber so that the compressed air supplied from the compressor is injected into the combustion chamber. 
         [0044]    The reheating burners  36  are each attached to the casing  35  and the combustion cylinder  34  along four axes  360  included on a plane orthogonal to the central axis  302  and circumferentially arranged at equal intervals. As described in detail later, the reheating burners  36  are connected to a first fuel supply source  305  (natural gas supply source) and a second fuel supply source  307  (hydrogen gas supply source) through a piping including a flow regulating valve and are configured such that, when the flow regulating valve is opened at the time of high-load operation, the first fuel and the second fuel can be mixed with the combustion air taken in from the combustion air flow path  37  to generate a premixed gas so as to inject the premixed gas into the combustion chamber. The first fuel refers to a gas containing 60 vol % or more hydrocarbons and 10 vol % or less hydrogen gas, or a liquid containing 60 vol % or more hydrocarbons. The second fuel refers to a gas containing 50 vol % or more hydrogen. In this embodiment, a natural gas is illustrated as an example of the first fuel and a hydrogen gas is illustrated as an example of the second fuel. 
         [0045]    The operation of the combustor  3  so constructed will hereinafter be described with reference to  FIG. 2 . As shown in  FIG. 2 , when the gas turbine (not shown) is started, the flow regulating valve is opened, and the natural gas supplied from a main fuel supply source to the pilot burner  32  is injected into the combustion chamber  33 . The gas is diffusively mixed in the combustion chamber  33  with the combustion air injected from the annular air flow path  324  into the combustion chamber  33  and is ignited by an ignition source not shown to form a pilot flame from diffusion combustion. 
         [0046]    When the gas turbine shifts to a normal operation, the premixed gas injected from the premixing flow path  314  of the main burner  31  is ignited by the pilot flame in the combustion chamber  33  and is combusted in a primary combustion region S 1  on the upstream side of the combustion chamber  33 . By combusting a lean premixed gas, the combustion flame temperature in the combustion chamber  33  decreases and an amount of NOx in the combustion exhaust gas of the main burner is suppressed. 
         [0047]    When high-load combustion is requested so as to raise the output of the gas turbine, a premixed gas of the natural gas, the hydrogen gas, and the combustion air  200  generated in the reheating burners  36  is introduced into the combustion chamber  33  and is mixed with the combustion exhaust gas of the main burner  31  and combusted in a secondary combustion region S 2  on the downstream side relative to the primary combustion region S 1 . By combusting a lean premixed gas, an amount of NOx in the combustion exhaust gas is suppressed. 
         [0048]    A reheating burner according to an embodiment of the present invention will be described with reference to the accompanying drawings. 
         [0049]    The reheating burner  36  according to a first embodiment of the present invention is shown in  FIG. 3 .  FIG. 3  shows a cross section corresponding to that in  FIG. 2 , and  FIG. 4  shows a cross section taken along A-A indicated by arrows of  FIG. 3 . In the following description related to the structure and the operation of the reheating burner  36 , the terms “upstream side” and “downstream side” are used with respect to a flow direction of a fluid in the reheating burner  36 . 
         [0050]    As shown in  FIG. 3 , the reheating burner  36  includes an outer cylinder  364  having a plurality of construction elements, for example, a head block  361 , a first cylindrical part  362 , and a second cylindrical part  363  arranged in order from the outside toward the inside on the axis  360  in a radial direction with respect to the central axis  302  of the combustor  3 . The head block  361  is fitted and fixed to an attaching hole  352  formed in the casing  35 , and a flange part  365  of the first cylindrical part  362  is fixed to the head block  361  via a plurality of coupling pieces  366 , while the second cylindrical part  363  is fitted and fixed to a through-hole  340  formed in the combustion cylinder  34 . A premixing flow path  367  for mixing the fuel and the combustion air  200  is formed as an internal space surrounded by the head block  361 , the first cylinder  362 , and the second cylindrical part  363 . 
         [0051]    The reheating burner  36  also includes a first fuel introduction part  368  for introducing the natural gas supplied from the first fuel supply source into the premixing flow path  367 , a second fuel introduction part  369  for introducing the hydrogen gas supplied from the second fuel supply source into the premixing flow path  367 , and a first air introduction part  370  for introducing the combustion air  200  from the combustion air flow path  37  into the premixing flow path  367 . 
         [0052]    The first air introduction part  370  is formed as a plurality of gap spaces (air intake ports) surrounded by the flange part  365  of the first cylindrical part  362 , the head block  361 , and the plurality of the coupling pieces  366  coupling the flange part and the head block, so that a portion of the compressed air  200  (the combustion air  200 ) flowing through the combustion air flow path  37  can be introduced from the first air introduction part  370  into the premixing flow path  367 . The combustion air  200  introduced into the premixing flow path  367  flows from the outer edge (radially outer side) toward the center (radially inner side) of the outer cylinder  364 . The coupling pieces  366  are arranged at equal intervals of 45 degrees on a circumference concentric with the outer cylinder  364  and are arranged at circumferential positions separated from second fuel injection nozzles  384  described later, and air intake holes are arranged at circumferential positions corresponding to the second fuel injection nozzles  384 . 
         [0053]    The first fuel introduction part  368  includes a first fuel supply path  380  extending in the head block  361  along the axis  360  from the upstream side to the downstream side and a first fuel injection nozzle  381  having a bottomed cylindrical shape projecting from a downstream-side wall surface of the head block  361  along the axis  360  into the premixing flow path  367 . 
         [0054]    The upstream side of the first fuel supply path  380  is in communication with the first fuel supply source through a piping  306  including a flow regulating valve, and the downstream side of the first fuel supply path  380  is in communication with the premixing flow path  367  through a plurality of first fuel injection holes  382  formed by radially penetrating a circumferential wall of the first fuel injection nozzle  381 . The first fuel injection holes  382  are arranged at equal intervals in the circumferential direction and the axial direction. The holes are arranged at intervals of 90 degrees in the circumferential direction. With such a construction, the natural gas supplied from the first fuel supply source is injected via the first fuel supply path  380  and the first fuel injection nozzle  381  into the premixing flow path  367 . 
         [0055]    The second fuel introduction part  369  has a second fuel supply path  383  extending in the head block  361  from the upstream side to the downstream side and a plurality of cylindrical second fuel injection nozzles  384  projecting from the downstream-side wall surface of the head block  361  into the premixing flow path  367 . The upstream side of the second fuel supply path  383  is connected to the second fuel supply source through a piping  308  including a flow regulating valve. The downstream side of the second fuel supply path  383  has an annular flow path  385  formed surrounding the first fuel supply path  380  and spreading concentrically with the outer cylinder  364 . The downstream side of the annular flow path  385  is in communications with the premixing flow path  367  through the internal spaces of the second fuel injection nozzles  384 . The second fuel injection nozzles  384  are arranged at equal intervals of 45 degrees on a circumference concentric with the outer cylinder  364  and extend in parallel with the outer cylinder. With such a construction, the hydrogen gas supplied from the second fuel supply source is injected via the second fuel supply path  383  and the second fuel injection nozzles  384  into the premixing flow path  367 . 
         [0056]    The operation of the reheating burner  36  having the construction described above will hereinafter be described with reference to  FIGS. 2, 3, and 4 . The combustion air  200  introduced from the first air introduction part  370  into the premixing flow path  367  flows from the outer edge toward the center of the outer cylinder  364  on the upstream side of the premixing flow path  367 , and the hydrogen gas is then injected from the second fuel injection nozzles  384  to the combustion air  200  to generate a primary air-fuel mixture. In this case, the hydrogen gas is injected into the flow of the combustion air  200  from the second fuel injection nozzles  384  projecting from the upstream-side end portion of the premixing flow path  367  (the downstream-side wall surface of the head block  361 ) into the premixing flow path  367 , avoiding a low speed area (viscous boundary layer) generated in the vicinity of the upstream-side end portion of the premixing flow path  367  (in the vicinity of the downstream-side wall surface of the head block  361 ). Therefore, even in the case of the hydrogen gas having a small specific gravity and a low penetrating force, the lean primary air-fuel mixture having a uniform concentration distribution is generated without a risk of retention in the low flow area described above. Subsequently, the primary air-fuel mixture changes the direction and flows along the outer circumference of the first fuel injection nozzle  381  toward the downstream side of the premixing flow path  367  before being mixed with the natural gas injected from the first fuel injection nozzle  381  to generate a secondary air-fuel mixture. 
         [0057]    In this case, since the natural gas has a greater specific gravity than the hydrogen gas, the natural gas and the primary air-fuel mixture are sufficiently stirred and mixed so that the lean secondary air-fuel mixture is generated with a more uniform concentration distribution than the primary air-fuel mixture. Additionally, since the first fuel (natural gas) is injected from the first fuel injection nozzle  381  in a direction intersecting with the flow direction of the primary air-fuel mixture, the mixing of the first fuel and the primary air-fuel mixture is promoted so that the concentration distribution of the secondary air-fuel mixture becomes uniform. As a result, a lean premixed gas  700  (secondary air-fuel mixture) having a uniform concentration distribution is supplied to the secondary combustion region S 2  downstream of the primary combustion region S 1  of the combustion chamber  33 , and the NOx amount can be suppressed in the combustion exhaust gas. 
         [0058]    The reheating burner according to the first embodiment described above can variously be modified. For example, as shown in  FIG. 5 , the reheating burner may be configured to inject the hydrogen gas from second fuel injection holes  386  formed in the circumferential walls of the second fuel injection nozzles  384  to a flow of the combustion air  200  in a direction opposite to the flow. According to the construction, since the hydrogen gas injected from the second fuel injection holes  386  collides with the combustion air  200 , the dispersion effect of the hydrogen gas is improved. As a result, the mixing of the hydrogen gas and the combustion air  200  is promoted, so that the more uniform primary air-fuel mixture can be generated. In the case of this modification example, the number of the second fuel injection holes  386  may be one; however, by making a plurality of the second fuel injection holes  386  as shown in  FIG. 5 , the dispersion effect of the hydrogen gas is further improved and the facilitation of mixing of the hydrogen gas and the combustion air can be expected. 
       Second Embodiment 
       [0059]    A reheating burner according to a second embodiment of the present invention will be described.  FIG. 6  shows the reheating burner  36  according to the second embodiment of the present invention. The basic structure of the reheating burner  36  according to this embodiment is the same as the reheating burner  36  according to the first embodiment described with reference to  FIG. 3  and, therefore, the same constituent portions are denoted by the same reference numerals and will not be described. 
         [0060]    The reheating burner  36  according to this embodiment has two points different from the reheating burner  36  according to the first embodiment described with reference to  FIG. 3  in that an inverted conical straightening protrusion part  390  extending in the premixing flow path  367  coaxially with the outer cylinder  364  is formed on the downstream-side wall surface of the head block  361  and that the first fuel introduction part  368  is configured to inject the natural gas from a plurality of first fuel injection holes  391  surrounding the straightening protrusion part  390 . The upstream side of the first fuel injection holes  391  is in communication with the first fuel supply path  380  and the downstream side of the first fuel injection holes  391  is in communication with the premixing flow path  367 . The first fuel injection holes  391  are arranged on the circumference concentric with the outer cylinder  364  at equal intervals at circumferential positions corresponding to the second fuel injection nozzles  384  and the first air introduction part  370 . The first fuel injection holes  391  are located closer than the second fuel injection nozzles  384  to the center of the outer cylinder  364  and are inclined toward the outer edge (radially outward) of the outer cylinder  364  from the upstream side to the downstream side. 
         [0061]    The operation of the reheating burner  36  having the construction described above will be described. To the primary air-fuel mixture flowing from the outer edge toward the center of the outer cylinder  364  on the upstream side of the premixing flow path  367 , the first fuel (natural gas) is injected from a plurality of the first fuel injection holes  391  formed in the downstream-side wall surface of the head block  361  (the upstream-side end portion of the premixing flow path  367 ) to generate the secondary air-fuel mixture. In this case, since the first fuel is injected in a direction intersecting with the flow of the primary air-fuel mixture, the mixing of the primary air-fuel mixture and the first fuel in the premixing flow path  367  is promoted, so that the secondary air-fuel mixture (premixed gas) having a uniform concentration is generated. As a result, the lean premixed gas  700  (secondary air-fuel mixture) having a uniform concentration distribution is supplied to the secondary combustion region S 2  downstream of the primary combustion region S 1  of the combustion chamber  33 , and NOx can be suppressed in the combustion exhaust gas. Additionally, since the secondary air-fuel mixture flows to the downstream side without lowering the flow speed along the straightening protrusion part  390  and is injected into the combustion chamber  33 , a backfire can be restrained from occurring due to a reduction in flow speed of the secondary air-fuel mixture. 
         [0062]    Although an inverted conical straightening protrusion part  390  is employed in this embodiment, the shape of the straightening protrusion part  390  is not limited to an inverted conical shape. The part may have any outer circumferential shape capable of guiding the primary air-fuel mixture from the base end side to the distal end side. In particular, the part may have any shape as long as the cross-sectional area decreases from the base end side toward the distal end side, and may have a partial spherical shape, for example. 
       Third Embodiment 
       [0063]    A reheating burner according to a third embodiment of the present invention will be described.  FIGS. 7 to 10  show variations of the reheating burner  36  according to the third embodiment of the present invention. The structure of the reheating burner  36  of this embodiment is the same as the reheating burner  36  according to the first embodiment described with reference to  FIG. 3  except that the burner has a second air introduction part  393  for introducing the combustion air  200  into the premixing flow path  367  on the downstream side relative to the first fuel introduction part  368  and, therefore, the same constituent portions are denoted by the same reference numerals and will not be described. 
         [0064]      FIG. 7  shows a first example of the reheating burner according to the third embodiment of the present invention. The second air introduction part  393  of the first example is a gap formed between the first cylindrical part  362  (the first cylindrical body) and the second cylindrical part  363  (the second cylindrical body). As shown in  FIG. 7 , the combustion air  200  flowing through the combustion air flow path  37  is distributed into a primary air  201  flowing in from the first air introduction part  370  and a secondary air  202  flowing in from the second air introduction part  393  before being introduced into the premixing flow path  367 . 
         [0065]    The secondary air  202  flowing into the premixing flow path  367  from the second air introduction part  393  suppresses occurrence of a low speed area in the vicinity of the inner wall surface of the second cylindrical part  363 . As a result, a backfire can be prevented from being caused by movement of a combustion flame formed in the combustion chamber  33  to the vicinity of the inner wall surface of the second cylindrical part  363 . 
         [0066]      FIG. 8  shows a second example of the reheating burner according to the third embodiment of the present invention. The reheating burner  36  of the second example includes a second cylindrical part  363 A having a diameter larger than the first cylindrical part  362  and has a construction in which an upstream-side end portion of the second cylindrical part  363 A and a downstream-side end portion of the first cylindrical part  362  are overlapped in the axial direction of the outer cylinder. The second air introduction part  393  of the second example is an annular gap formed between the outer circumferential surface of the first cylindrical part  362  and the inner circumferential surface of the second cylindrical part  363 A. The secondary air  202  introduced into the premixing flow path  367  from the second air introduction part  393  is straightened while flowing through the annular gap from the upstream side to the downstream side, so as to flow intensively in the vicinity of the inner wall surface of the second cylindrical part  363 A having a high concentration of the secondary air-fuel mixture  700 , and is therefore more effective than the first example. 
         [0067]      FIG. 9  shows a third example of the reheating burner according to the third embodiment of the present invention. The reheating burner  36  of the third example has a construction for increasing the flow speed of the premixed gas  700  injected through the premixing flow path  367  into the combustion chamber  33 . In the second air introduction part  393  in this construction, the annular gap defined by the first cylindrical part  362  and the second cylindrical part  363  gradually decreases in diameter toward the downstream side of the reheating burner  36 . Specifically, in the reheating burner  36  of the third example, an inner circumferential surface  363 B of the second air introduction part  393  in the second cylindrical part  363 A gradually decreases in diameter from the upstream side to the downstream side. A tapered part  394  gradually decreasing in diameter from the upstream side to the downstream side is formed on the outer circumferential surface of the downstream-side end portion of the first cylindrical part  362  at a position facing the inner circumferential surface  363 B. In the third example, the inner diameter of the second cylindrical part  363  may be substantially the same as the inner diameter of the first cylindrical part  362  on the downstream side thereof. The reheating burner  36  of the third example is the same as the reheating burner  36  of the second example shown in  FIG. 8  except the construction described above and, therefore, the same constituent portions are denoted by the same reference numerals and will not be described. The reheating burner  36  of the third example having the construction described above produces the following effects. In particular, a portion of the compressed air  200  is blown onto the outer circumference of the first cylindrical part  362  and is then introduced as the secondary air  202  into the second air introduction part  393 . By introducing the secondary air  202  from the second air introduction part  393 , the retention of the premixed gas  700  can be suppressed in a boundary layer. The secondary air  202  is uniformly straightened while flowing through the second air introduction part  393  from the upstream side to the downstream side. Since the secondary air  202  is fed into the premixing flow path  367 , the retention of the air-fuel mixture can more effectively be suppressed in the boundary layer. On the other hand, the secondary air  202  flows through the annular gap (the tapered part  394 ) gradually decreasing in diameter from the upstream side to the downstream side and thereby can forma flow guiding the premixed gas  700  retained in the boundary layer toward the center of the flow path (the radial inner side of the second cylindrical part  363 A). If the inner diameter of the second cylindrical part  363  is made substantially the same as the inner diameter of the first cylindrical part  362  on the downstream side thereof, the flow rate of the premixed gas  700  flowing through the premixing flow path  367  can be balanced. As a result, the occurrence of the low speed area is further suppressed in the vicinity of the inner wall surface of the second cylindrical part  363 A, so that the backfire can effectively be prevented from being caused by movement of a combustion flame formed in the combustion chamber  33  to the vicinity of the inner wall surface of the second cylindrical part  363 A. 
         [0068]      FIG. 10  shows a fourth example of the reheating burner according to the third embodiment of the present invention. The reheating burner  36  of the fourth example includes the second cylindrical part  363 A having a diameter larger than the first cylindrical part  362  and has a construction in which the upstream-side end portion of the second cylindrical part  363 A is fixed to the flange part  365  of the first cylindrical part  362 . The second air introduction part  393  of the third example is a plurality of inflow ports formed in a circumferential wall portion of the second cylindrical part  363 A. This reheating burner  36  of the fourth example can produce the same effects as the second example. 
         [0069]    In the reheating burner  36  according to the third embodiment of the present invention described above, the ratio between the primary air  201  flowing in from the first air introduction part  370  and the secondary air  202  flowing in from the second air introduction part  393  may normally be 1:1; however, it is confirmed in the experiments by the inventors that the ratio of the primary air  201  may be increased if consideration is given to the reduction of NOx and that the ratio of the secondary air  202  may be increased if consideration is given to the backfire prevention. 
       Fourth Embodiment 
       [0070]    A reheating burner according to a fourth embodiment of the present invention will be described.  FIG. 11  shows the reheating burner  36  according to the fourth embodiment of the present invention. In the reheating burner  36  of this embodiment, the same constituent portions as those of the reheating burner  36  according to the first and second embodiments described with reference to  FIGS. 3 and 6 , respectively, are denoted by the same reference numeral and will not be described. 
         [0071]    As shown in  FIG. 11 , the reheating burner  36  according to this embodiment has three points different from the reheating burner  36  according to the first embodiment described with reference to  FIG. 3  in that the first fuel introduction part  368  is configured to inject the natural gas from a plurality of first fuel injection holes  395  circumferentially arranged at equal intervals in the first cylindrical part  362 , that the same straightening protrusion part  390  as the second embodiment is formed on the downstream-side wall surface of the head block  361 , and that the second fuel supply path  383  is formed along the axis  360 . 
         [0072]    As shown in  FIG. 11 , the first fuel introduction part  368  is made up of a columnar passage part  396  formed on the upstream side of the head block  361 , a first annular passage part  397  formed on the downstream side of the head block  361 , a branch passage part  398  formed on the downstream side relative to the first annular passage  397  and extending from the downstream side of the head block  361  through the coupling pieces  366  to the first cylindrical part  362 , and a second annular passage part  399  formed in the flange part  365  of the first cylindrical part  362  and allowing branch passages  398  to join together. The first annular passage part  397  is disposed concentrically with the outer cylinder  364  to surround the second fuel supply passage  383 . The branch passage part  398  has two branch passages and is configured to penetrate two circumferentially opposed coupling pieces  366 . The second annular passage part  399  is disposed concentrically with the outer cylinder  364 . As shown in the figure, the plurality of the first fuel injection holes  395  is circumferentially formed at equal intervals in the inner surface of the first cylindrical part  362 . The first fuel injection holes  395  extend radially outward to communicate with the second annular passage part  399 . 
         [0073]    As shown in the figure, the second fuel supply path  383  extends from the upstream side to the downstream side along the axis  360  and has the upstream side connected to the second fuel supply source through the piping  308  having a flow regulating valve and the downstream side to which the second fuel injection nozzles  384  are connected through a header portion  385 A. 
         [0074]    It is noted that the reheating burner  36  of this embodiment can employ a construction in which the secondary air  202  is introduced into the premixing flow path  367  as described in the first to fourth examples of the reheating burner  36  of the third embodiment. 
         [0075]    The operation of the reheating burner  36  so constructed will hereinafter be described with reference to  FIG. 11 . 
         [0076]    Since the primary air-fuel mixture flowing from the outer edge toward the center of the outer cylinder  364  on the upstream side of the premixing flow path  367  flows along the straightening protrusion part  390  to the downstream side without lowering the flow speed and is injected into the combustion chamber  33  as the premixed gas  700  (secondary air-fuel mixture) mixed with the first fuel, the backfire can be restrained from occurring due to a reduction in flow speed of the premixed gas. In this case, since the first fuel (natural gas) has a greater specific gravity than the second fuel (hydrogen gas), the first fuel and the primary air-fuel mixture are sufficiently stirred and mixed by the first fuel, so that the lean premixed gas  700  is generated with a more uniform concentration distribution than the primary air-fuel mixture. Additionally, since the first fuel is injected in a direction intersecting with the flow direction of the primary air-fuel mixture, the mixing of the first fuel and the primary air-fuel mixture is promoted so that the concentration distribution becomes uniform. As a result, the lean premixed gas  700  having a uniform concentration distribution is injected to the secondary combustion region S 2  downstream of the primary combustion region S 1  of the combustion chamber  33 , and NOx can be suppressed in the combustion exhaust gas. 
       EXPLANATIONS OF LETTERS OR NUMERALS 
       [0000]    
       
           1  gas turbine 
           2  compressor 
           3  combustor 
           4  turbine 
           5  rotor 
           6  generator 
           31  main burner 
           32  pilot burner 
           33  combustion chamber 
           34  combustion cylinder 
           36  reheating burner (fuel injection device) 
           37  combustion air flow path (air flow path) 
           200  compressed air (combustion air) 
           300  combustion exhaust gas 
           360  axis 
           361  head block 
           362  first cylindrical part 
           363  second cylindrical part 
           364  outer cylinder 
           366  coupling piece 
           367  premixing flow path 
           368  first fuel introduction part 
           369  second fuel introduction part 
           370  first air introduction part 
           380  first fuel supply path 
           381  first fuel injection nozzle 
           382  first fuel injection hole 
           383  second fuel supply path 
           384  second fuel injection nozzle 
           390  straightening protrusion part 
           393  second air introduction part 
           700  premixed gas
The present invention provides a burner, a combustor equipped with the burner, and a gas turbine, with which it is possible to premix a first hydrocarbon-based fuel (for example, natural gas), a second fuel (for example, hydrogen gas), and combustion air, and to spray into the combustion chamber of the combustor a thin and uniform concentration distribution of the premixed air, and with which it is possible to suppress the amount of NOx discharged. On the upstream side of the premix flow path, hydrogen gas is sprayed from second fuel spray nozzles, which project into the premix flow path, into the flow of the combustion air flowing toward the center from the outer edge of an outer cylinder, whereby a primary air-fuel mixture having a uniform concentration distribution is generated without affecting a low-speed region of the combustion air. Natural gas is then sprayed from first fuel spray nozzles into the primary air-fuel mixture, whereby the natural gas, which has a high specific gravity, and the primary air-fuel mixture are adequately mixed in a stirring fashion, and a secondary air-fuel mixture (premixed air) is generated that is lean and has a more uniform concentration distribution than the first air-fuel mixture. By combusting this type of premixed air in the combustion chamber, NOx in the combustion exhaust gas can be suppressed.