DESCRIPTION COMBUSTION BURNER AND BOILER INCLUDING THE SAME TECHNICAL FIELD : [0001] The present invention relates to a combustion burner and a boiler including the combustion burner, and more particularly, to a combustion burner capable of reducing the emission amount of nitrogen oxides (NOx) and a boiler including the combustion burner. BACKGROUND ART [0002] Conventional combustion burners typically employ a configuration to stabilize the outer flame of combustion flame. In this configuration, a high-temperature and high-oxygen area is formed in an outer peripheral part of the combustion flame, resulting in an increase in the emission amount of NOx. As an example of such conventional combustion burners employing this configuration, a technology described in Patent Document 1 is known. [0003] [Patent Document 1] Japanese Patent No. 2781740 DISCLOSURE OF INVENTION PROBLEM TO BE SOLVED BY THE INVENTION [0004] The present invention has an object to provide a combustion burner capable of reducing the emission amount of NOx and a boiler including the combustion burner. MEANS FOR SOLVING PROBLEM [0005] According to an aspect of the present invention, a combustion burner includes: a fuel nozzle that injects fuel gas prepared by mixing solid fuel and primary air; a secondary air nozzle that injects secondary air from outer periphery of the fuel nozzle; and a flame holder that is arranged in an opening of the fuel nozzle. The flame holder has a splitting shape that widens in a flow direction of the fuel gas, and when seen in cross section along a direction in which the flame holder widens, the cross section passing through a central axis of the fuel nozzle, a maximum distance h from the central axis of the fuel nozzle to a widened end of the flame holder and an inside diameter r of the opening of the fuel nozzle satisfy h/(r/2)<0.6, EFFECT OF THE INVENTION [0006] Because the combustion burner according to the present invention achieves inner flame stabilization of combustion flame (flame stabilization in a central area of the opening of the fuel nozzle), an outer peripheral part of the combustion flame is kept at low temperature compared with configurations for outer flame stabilization of combustion flame (flame stabilization in the outer periphery of the fuel nozzle or flame stabilization in an area near the inner wall surface of the opening of the fuel nozzle). Therefore, with the secondary air, the temperature of the outer peripheral part of the combustion flame in a high oxygen atmosphere can be lowered. This is advantageous in that the emission amount of NOx in the outer peripheral part of the combustion flame is reduced. BRIEF DESCRIPTION OF DRAWINGS [0007] [Fig. 1] Fig. 1 is a configuration diagram of a combustion burner according to an embodiment of the present invention. [Fig. 2] Fig. 2 is a front view of an opening of the combustion burner illustrated in Fig. 1. [Fig. 3] Fig. 3 is a schematic for explaining a flame holder in the combustion burner illustrated in Fig. 1. [Fig. 4] Fig. 4 is a schematic for explaining effects of the combustion burner illustrated in Fig. 1. [Fig. 5] Fig. 5 is a graph of performance test results of the combustion burner illustrated in Fig. 1. [Fig. 6] Fig. 6 is a schematic for explaining effects of the flame holder illustrated in Fig. 3. [Fig. 7] Fig. 7 is a graph of performance test results of the combustion burner. [Fig. 8] Fig. 8 is a schematic for explaining a flow straightening structure in the combustion burner illustrated in Fig. 1. [Fig. 9] Fig. 9 is a schematic for explaining a flow straightening ring of the flow straightening structure illustrated in Fig. 8. [Fig. 10] Fig. 10 is a schematic for explaining a modification of the combustion burner illustrated in Fig. 1. [Fig. 11] Fig. 11 is a schematic for explaining a modification of the combustion burner illustrated in Fig. 1. [Fig. 12] Fig. 12 is a schematic for explaining a modification of the combustion burner illustrated in Fig. 1. [Fig. 13] Fig. 13 is a graph of performance test results of the combustion burner. [Fig. 14] Fig. 14 is a schematic for explaining a modification of the combustion burner illustrated in Fig. 1. [Fig. 15] Fig. 15 is a schematic for explaining a modification of the combustion burner illustrated in Fig. 1. [Fig. 16] Fig. 16 is a schematic for explaining a modification of the combustion burner illustrated in Fig. 1. [Fig. 17] Fig. 17 is a schematic for explaining a modification of the combustion burner illustrated in Fig. 1. [Fig. 18] Fig. 18 is a schematic for explaining a modification of the combustion burner illustrated in Fig. 1. [Fig. 19] Fig. 19 is a schematic for explaining a modification of the combustion burner illustrated in Fig. 1. [Fig. 20] Fig. 20 is a schematic for explaining the emission amount of NOx when the combustion burner illustrated in Fig. 1 is applied to a boiler employing an additional-air system. [Fig. 21] Fig. 21 is a schematic for explaining the emission amount of NOx when the combustion burner illustrated in Fig. 1 is applied to the boiler employing the additional-air system. [Fig. 22] Fig. 22 is a configuration diagram of a typical pulverized coal combustion boiler. BEST MODE (S) FOR CARRYING OUT THE INVENTION [0008] The present invention will now be described in detail with reference to the accompanying drawings. This embodiment is not intended to limit the present invention. Components in the embodiment include components that are replaceable and obviously replaceable while maintaining unity of the invention. A plurality of modifications described in the embodiment can be combined in any manner within the scope obvious to those skilled in the art. [0009] [Pulverized Coal Combustion Boiler] Fig. 22 is a configuration diagram of a typical pulverized coal combustion boiler. This pulverized coal combustion boiler 100 is a boiler that burns pulverized coal to produce thermal energy and is used for power generation or industrial applications, for example. [0010] The pulverized coal combustion boiler 100 includes a furnace 110, a combustion apparatus 120, and a steam generating apparatus 130 (see Fig. 22). The furnace 110 is a furnace for burning pulverized coal, and includes a combustion chamber 111 and a flue gas duct 112 connected above the combustion chamber 111. The combustion apparatus 120 is an apparatus that burns pulverized coal, and includes combustion burners 121, pulverized coal supply systems 122 supplying pulverized coal to the respective combustion burners 121, and an air supply system 123 supplying secondary air to the combustion burners 121. The combustion apparatus 120 is so arranged that the combustion burners 121 are connected to the combustion chamber 111 of the furnace 110. In the combustion apparatus 120, the air supply system 123 supplies additional air for completing oxidation and combustion of pulverized coal to the combustion chamber 111. The steam generating apparatus 130 is an apparatus that heats water fed to the boiler through heat exchange with fuel gas to generate steam, and includes an economizer 131, a reheater 132, a superheater 133, and a steam drum (not illustrated). The steam generating apparatus 130 is so configured that the economizer 131, the reheater 132, and the superheater 133 are arranged stepwise on the flue gas duct 112 of the furnace 110. [0011] In the pulverized coal combustion boiler 100, first, in the combustion apparatus 120, the pulverized coal supply system 122 supplies pulverized coal and primary air to the combustion burner 121, and the air supply system 123 supplies secondary air for combustion to the combustion burner 121 (see Fig. 22). Subsequently, the combustion burner 121 ignites fuel gas containing pulverized coal, primary air, and secondary air and injects the fuel gas into the combustion chamber 111. Consequently, the fuel gas burns in the combustion chamber 111, whereby fuel gas is produced. The fuel gas is then discharged from the combustion chamber 111 through the flue gas duct 112. In this process, the steam generating apparatus 130 causes heat exchange between the fuel gas and water fed to the boiler to generate steam. The steam is to be supplied to an external plant (a steam turbine, for example). [0012] In the pulverized coal combustion boiler 100, the sum of the supply amount of primary air and the supply amount of secondary air is set to be less than a theoretical air volume with respect to the supply amount of pulverized coal, whereby the combustion chamber 111 is maintained at a reduction atmosphere. NOx emitted as a result of combustion of the pulverized coal is reduced in the combustion chamber 111, and additional air (AA) is additionally supplied thereafter, whereby oxidation and combustion of the pulverized coal are completed (additional-air system). Thus, the emission amount of NOx due to combustion of the pulverized coal is decreased. [0013] [Combustion Burner] Fig. 1 is a configuration diagram of a combustion burner according to an embodiment of the present invention, and is a sectional view of the combustion burner in its height direction along its central axis. Fig. 2 is a front view of an opening of the combustion burner illustrated in Fig. 1. [0014] This combustion burner 1 is a solid fuel combustion burner for burning solid fuel, and is used as the combustion burner 121 in the pulverized coal combustion boiler 100 illustrated in Fig. 22, for example. An example will now be given in which pulverized coal is used as solid fuel, and the combustion burner 1 is applied to the pulverized coal combustion boiler 100. [0015] The combustion burner 1 includes a fuel nozzle 2, a main secondary air nozzle 3, a secondary air nozzle 4, and a flame holder 5 (see Figs. 1 and 2). The fuel nozzle 2 is a nozzle that injects fuel gas (primary air containing solid fuel) prepared by mixing pulverized coal (solid fuel) and primary air. The main secondary air nozzle 3 is a nozzle that injects main secondary air (coal secondary air) into the outer periphery of the fuel gas injected by the fuel nozzle 2. The secondary air nozzle 4 is a nozzle that injects secondary air into the outer periphery of the main secondary air injected by the main secondary air nozzle 3. The flame holder 5 is a device used for igniting the fuel gas and stabilizing the flame, and is arranged in an opening 21 of the fuel nozzle 2. [0016] For example, in the present embodiment, the fuel nozzle 2 and the main secondary air nozzle 3 each have an elongated tubular structure, and have rectangular openings 21 and 31, respectively (see Figs. 1 and 2). With the fuel nozzle 2 at the center, the main secondary air nozzle 3 is arranged on the outer side, whereby a double tube is formed. The secondary air nozzle 4 has a double-tube structure, and has a ring-shaped opening 41. In the inner ring of the secondary air nozzle 4, the fuel nozzle 2 and the main secondary air nozzle 3 are inserted and arranged. Accordingly, with the opening 21 of the fuel nozzle 2 at the center, the opening 31 of the main secondary air nozzle 3 is arranged on the outer side of the opening 21, and the opening 41 of the secondary air nozzle 4 is arranged on the outer side of the opening 31. The openings 21 to 41 of these nozzles 2 to 4 are aligned and arranged coplanarly. The flame holder 5 is supported by a plate member (not illustrated) on the upstream side of the fuel gas, and is arranged in the opening 21 of the fuel nozzle 2. The downstream end (widened end) of the flame holder 5 and the openings 21 to 41 of these nozzles 2 to 4 are aligned coplanarly. [0017] In the combustion burner 1, the fuel gas prepared by mixing pulverized coal and primary air is injected through the opening 21 of the fuel nozzle 2 (see Fig. 1). In this process, the fuel gas is branched at the flame holder 5 in the opening 21 of the fuel nozzle 2, and then ignited and burnt to be fuel gas. To the outer periphery of the fuel gas, the main secondary air is injected through the opening 31 of the main secondary air nozzle 3, whereby the combustion of the fuel gas is facilitated. To the outer periphery of combustion flame, the secondary air is supplied through the opening 41 of the secondary air nozzle 4, whereby the outer peripheral part of the combustion flame is cooled down. [0018] [Arrangement of Flame Holder] In the combustion burner 1, to reduce the emission amount of NOx as a result of the combustion of pulverized coal, the arrangement of the flame holder 5 relative to the opening 21 of the fuel nozzle 2 is optimized, which will be described below. [0019] First, when seen in cross section along a direction in which the flame holder 5 widens, the cross section passing through the central axis of the fuel nozzle 2, the flame holder 5 has a splitting shape that widens in the flow direction of fuel gas (mixed gas of pulverized coal and primary air) (see Figs. 1 and 3). In addition, a maximum distance h from the central axis of the fuel nozzle 2 to the widened end (the downstream end of the splitting shape) of the flame holder 5 and an inside diameter r of the opening 21 of the fuel nozzle 2 satisfy h/(r/2)<0.6. [0020] For example, in the present embodiment, the fuel nozzle 2 has the rectangular opening 21, and is so arranged that its height direction is aligned with the vertical direction and its width direction is aligned with the horizontal direction (see Figs. 1 and 2). In the opening 21 of the fuel nozzle 2, the flame holder 5 is arranged. The flame holder 5 has a splitting shape that widens in the flow direction of the fuel gas, and has an elongated shape in the direction perpendicular to the widening direction. The flame holder 5 has its longitudinal direction aligned with the width direction of the fuel nozzle 2, and substantially transects the opening 21 of the fuel nozzle 2 in the width direction of the opening 21. Furthermore, the flame holder 5 is arranged on the central line of the opening 21 of the fuel nozzle 2, thereby bisecting the opening 21 of the fuel nozzle 2 in the height direction of the opening 21. [0021] The flame holder 5 has a substantially isosceles triangular cross section and an elongated, substantially prismatic shape (see Figs. 1 and 3). When seen in cross section along the axial direction of the fuel nozzle 2, the flame holder 5 is arranged on the central axis of the fuel nozzle 2. Specifically, the flame holder 5 has its vertex directed to the upstream side of the fuel gas and its bottom arranged in alignment with the opening 21 of the fuel nozzle 2. Accordingly, the flame holder 5 has a splitting shape that widens in the flow direction of the fuel gas. In addition, the flame holder 5 has a splitting angle (the vertex angle of the isosceles triangle) 9 and a splitting width (the base length of the isosceles triangle) L set at respective predetermined sizes. [0022] The flame holder 5 having such a splitting shape is arranged in a central area of the opening 21 of the fuel nozzle 2 (see Figs. 1 and 2). The "central area" of the opening 21 herein means an area where, with the flame holder 5 having a splitting shape that widens in the flow direction of the fuel gas, when seen in cross section along the direction in which the flame holder 5 widens, the cross section passing through the central axis of the fuel nozzle 2, the maximum distance h from the central axis of the fuel nozzle 2 to the widened end (the downstream end of the splitting shape) of the flame holder 5 and the inside diameter r of the opening 21 of the fuel nozzle 2 satisfy h/(r/2)<0.6. In the present embodiment, because the flame holder 5 is arranged on the central axis of the fuel nozzle 2, the maximum distance h from the central axis of the fuel nozzle 2 to the widened end of the flame holder 5 is a half L/2 of the splitting width of the flame holder 5. [0023] In the combustion burner 1, because the flame holder 5 has the splitting shape, the fuel gas is branched at the flame holder 5 in the opening 21 of the fuel nozzle 2 (see Fig. 1). In this configuration, the flame holder 5 is arranged in the central area of the opening 21 of the fuel nozzle 2, and the fuel gas is ignited and flame is stabilized in this central area. Thus, inner flame stabilization of the combustion flame (flame stabilization in the central area of the opening 21 of the fuel nozzle 2) is achieved. [0024] In this configuration, compared with configurations (not illustrated) for outer flame stabilization of combustion flame (flame stabilization in the outer periphery of the fuel nozzle or flame stabilization in an area near the inner wall surface of the opening of the fuel nozzle), an outer peripheral part Y of the combustion flame is kept at low temperature (see Fig. 4). Therefore, with the secondary air, the temperature of the outer peripheral part Y of the combustion flame in a high oxygen atmosphere can be lowered. Thus, the emission amount of NOx in the outer peripheral part Y of the combustion flame is reduced. [0025] Fig. 5 is a graph of performance test results of the combustion burner illustrated in Fig. 1, depicting test results of the relationship between a position h/(r/2) of the flame holder 5 in the opening 21 of the fuel nozzle 2 and the emission amount of NOx. [0026] This performance test measured, in the combustion burner 1 illustrated in Fig. 1, the emission amount of NOx, with the distance h of the flame holder 5 varied. The inside diameter r of the fuel nozzle 2, the splitting angle 0 and the splitting width L of the flame holder 5, for example, were set constant. The emission amount of NOx is represented in relative values to a configuration that stabilizes the outer flame of combustion flame (a configuration in which a flame holder is arranged on the outer periphery of a fuel nozzle, see Patent Document 1) (i.e., h/(r/2)=l). [0027] As the test results represent, it can be observed that the emission amount of NOx decreases as the position of the flame holder 5 comes closer to the center of the opening 21 of the fuel nozzle 2 (see Fig. 5). Specifically, with the position of the flame holder 5 satisfying h/(r/2)<0.6, the emission amount of NOx decreases by equal to or more than 10%, exhibiting advantageous properties. [0028] In the combustion burner 1, it is preferable that the ends of the flame holder 5 in the longitudinal direction and the inner wall surface of the opening 21 of the fuel nozzle 2 come into contact with each other. In the typical design, however, a minute gap d of some millimeters each is defined between the ends of the flame holder 5 and the inner wall surface of the fuel nozzle 2 in consideration of thermal expansion of members (see Fig. 2). Accordingly, in the configuration in which the ends of the flame holder 5 and the inner wall surface of the fuel nozzle 2 are arranged close to each other, the ends of the flame holder 5 are exposed to radiation from the combustion flame. As a result, flame propagation proceeds from the ends of the flame holder 5 to the inside, which is preferable. [0029] [Splitting Angle and Splitting Width of Flame Holder] In the combustion burner 1, to suppress the emission amount of NOx as a result of the combustion of the solid fuel, it is preferable that the splitting shape of the flame holder 5 be optimized, which will be described below. [0030] As mentioned earlier, in the combustion burner 1, the flame holder 5 has the splitting shape to branch the fuel gas (see Fig. 3). In this configuration, it is preferable that the flame holder 5 have a splitting shape with a triangular cross section with its vertex directed to the upstream side of the flow direction of the fuel gas (see Fig. 6 (a)). With the flame holder 5 having such a triangular cross section, branched fuel gas flows along the side surfaces of the flame holder 5 and is drawn into the base side due to differential pressure. This makes it hard for the fuel gas to diffuse outward in the radial direction of the flame holder 5, and therefore, inner flame stabilization of combustion flame is secured properly (or enhanced). Consequently, the outer peripheral part Y of the combustion flame (see Fig. 4) is kept at low temperature, whereby the emission amount of NOx due to mixing with secondary air is reduced. [0031] In a configuration in which a flame holder has a plate-like splitting shape (see Fig. 6 (b)), branched fuel gas flows toward the inner wall surface of a fuel nozzle from the flame holder. This is a typical configuration in conventional combustion burners in which fuel gas is branched at the flame holder and guided along the inner wall surface of the fuel nozzle. In this configuration, an area near the inner wall surface of the fuel nozzle becomes fuel gas rich compared with a central area of the fuel nozzle, and the outer peripheral part Y of the combustion flame has higher temperature than an inner part X (see Fig. 4). As a result, in the outer peripheral part Y of the combustion flame, the emission amount of NOx due to mixing with secondary air can increase. [0032] In the configuration described above, it is preferable that the splitting angle G of the flame holder 5 having a triangular cross section be 9<90 (degrees) (see Fig. 3). It is further preferable that the splitting angle 9 of the flame holder 5 be 9<60 (degrees). Under such conditions, branched fuel gas is prevented from diffusing toward wall surface sides without the fuel nozzle, whereby inner flame stabilization of combustion flame is ensured more properly. [0033] For example, in the present embodiment, the flame holder 5 has a splitting shape with an isosceles triangular cross section, and the splitting angle 9 is set to be 9<90 (degrees) (see Fig. 3). In addition, because the flame holder 5 is arranged symmetrically with respect to the flow direction of the fuel gas, each side inclined angle (9/2) is set below 30 (degrees). [0034] Furthermore, in the configuration described above, it is preferable that the splitting width L of the flame holder 5 with a triangular cross section and the inside diameter r of the opening 21 of the fuel nozzle 2 satisfy 0.06