POWDER FUEL BURNER

Abstract

A powder fuel burner in a combustion chamber of a boiler includes: a cylindrical air-fuel mixture conveying tube, through which an air-fuel mixture is conveyed and which injects, from a distal end, the air-fuel mixture of powder fuel and conveying air into the combustion chamber; a swirl vane mounted at the distal end of the tube; a rod-shaped center flow blocker mounted on a center axis of the tube upstream of the swirl vane in a conveying direction of the air-fuel mixture, the center flow blocker including a cylindrical surface concentric with the tube and extending in a direction of the center axis; and a cylindrical peripheral flow blocker mounted along an inner wall surface of the tube between the center flow blocker and the swirl vane, the peripheral flow blocker including a cylindrical inner peripheral surface concentric with the tube and extending in the direction of the center axis.

Claims

1. A powder fuel burner mounted in a combustion chamber of a boiler, the powder fuel burner comprising: a cylindrical air-fuel mixture conveying tube, through which an air-fuel mixture is conveyed and which injects, from a distal end thereof, the air-fuel mixture into the combustion chamber, the air-fuel mixture being a mixture of a powder fuel and conveying air; a swirl vane mounted at the distal end of the air-fuel mixture conveying tube; a rod-shaped center flow blocker mounted on a center axis of the air-fuel mixture conveying tube at a position upstream of the swirl vane in a conveying direction of the air-fuel mixture, the center flow blocker including a cylindrical surface that is concentric with the air-fuel mixture conveying tube and that extends in a direction of the center axis; and a cylindrical peripheral flow blocker mounted along an inner wall surface of the air-fuel mixture conveying tube between the center flow blocker and the swirl vane, the peripheral flow blocker including a cylindrical inner peripheral surface that is concentric with the air-fuel mixture conveying tube and that extends in the direction of the center axis.

2. The powder fuel burner according to claim 1, wherein a diameter of the cylindrical surface of the center flow blocker is greater than or equal to a diameter of the cylindrical inner peripheral surface of the peripheral flow blocker.

3. The powder fuel burner according to claim 1, wherein the center flow blocker includes curved surfaces at both ends of the cylindrical surface, respectively, the curved surfaces protruding from the respective ends of the cylindrical surface in the direction of the center axis to form conical side surfaces, respectively, such that angles formed by the center axis and the respective curved surfaces are each less than or equal to 20 degrees, and the peripheral flow blocker includes curved surfaces at both ends of the cylindrical inner peripheral surface, respectively, the curved surfaces protruding from the respective ends of the inner peripheral surface in the direction of the center axis and connecting to the inner wall surface of the air-fuel mixture conveying tube to form truncated conical side surfaces, respectively, such that angles formed by the inner wall surface of the air-fuel mixture conveying tube and the respective curved surfaces are each less than or equal to 20 degrees.

4. The powder fuel burner according to claim 1, wherein the center flow blocker is configured such that a position of the center flow blocker in the direction of the center axis is adjustable.

5. The powder fuel burner according to claim 1, wherein the swirl vane includes: a cylindrical structure that is concentric with the air-fuel mixture conveying tube and that is, at the distal end of the air-fuel mixture conveying tube, mounted inside the air-fuel mixture conveying tube, the cylindrical structure having an internal diameter that is greater than or equal to a diameter of the cylindrical inner peripheral surface of the peripheral flow blocker; and vanes mounted to an outer peripheral surface of the cylindrical structure.

6. The powder fuel burner according to claim 1, wherein the powder fuel is a petroleum residue fuel, a biomass fuel, or a mixed fuel of biomass and pulverized coal.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0010] FIG. 1 is a schematic diagram showing one example of a combustion system including a boiler including powder fuel burners according to an embodiment of the present disclosure.

[0011] FIG. 2 is a schematic sectional view showing one example of a powder fuel burner according to the present embodiment.

[0012] FIG. 3 is a sectional view taken along line A-A of FIG. 2.

[0013] FIG. 4 shows one example of a swirl vane as seen from the proximal end side of an air-fuel mixture conveying tube.

[0014] FIG. 5 is a schematic sectional view showing an essential part of the powder fuel burner, in which a different swirl vane is used.

[0015] FIG. 6 is a schematic sectional view showing an essential part of the powder fuel burner, in which a different swirl vane is used.

DESCRIPTION OF EMBODIMENTS

[0016] Hereinafter, a preferred embodiment of the present disclosure is described with reference to the drawings. In the drawings, the same or corresponding elements are denoted by the same reference signs, and repeating the same descriptions is avoided below. The drawings show each component schematically in order to facilitate the understanding thereof. Therefore, in some cases, the drawings may not display accurate shapes, accurate dimensional ratios, etc.

Embodiment

[0017] FIG. 1 is a schematic diagram showing one example of a combustion system including a boiler including powder fuel burners according to the present embodiment.

[0018] The combustion system shown in FIG. 1 includes a fuel feeder 20 and a boiler 100. The fuel feeder 20 includes a fuel storage tank 21, a weighing feeder 22, and a pulverizer 23. The fuel storage tank 21 stores therein a flame-retardant fuel, for example, a petroleum residue such as petroleum coke. The fuel stored in the fuel storage tank 21 is fed to the weighing feeder 22, and the fuel in a predetermined weight is fed from the weighing feeder 22 to the pulverizer 23. The pulverizer 23 grinds the fuel fed from the weighing feeder 22 into powder, and feeds an air-fuel mixture that is a mixture of the resulting powder fuel and conveying air to powder fuel burners 5 of the boiler 100. An A/F, i.e., an air-fuel ratio, of the air-fuel mixture fed from the pulverizer 23 to the powder fuel burners 5 is about 1.8.

[0019] The boiler 100 is configured as an inverted vertical furnace. The boiler 100 includes; a combustion chamber 1, which is, for example, a cylindrical chamber with a rectangular cross section; and a path 12 connected to the combustion chamber 1 via a gas outlet 11.

[0020] The combustion chamber 1 includes: a high-temperature reducing combustion chamber 2, which is located at the upper end of the combustion chamber 1; a low-temperature oxidizing combustion chamber 3, which is located below the high-temperature reducing combustion chamber 2; and a restrictor 4, which connects the high-temperature reducing combustion chamber 2 and the low-temperature oxidizing combustion chamber 3 to each other. The restrictor 4 is a passage at which the horizontal cross sectional area of the combustion chamber 1 is reduced by 20 to 50% compared to the horizontal cross sectional area of the other part of the combustion chamber 1.

[0021] The walls of the high-temperature reducing combustion chamber 2 are covered by a fire-resistant material 6, which can withstand a predetermined internal temperature of a high-temperature furnace. The powder fuel burners 5 are located on each of two side walls of the high-temperature reducing combustion chamber 2, the two side walls facing each other. In this example, a total of eight powder fuel burners 5 are located on these two side walls. The eight burners 5 are located in such a manner that the flame axes of the respective burners do not face each other, but are parallel to each other. In the high-temperature reducing combustion chamber 2, the fuel fed from the burners 5 is subjected to initial-stage combustion in a high-temperature reducing atmosphere. As a result, combustion gas is generated. The amount of generated combustion gas increases due to the fuel being newly fed. Consequently, the generated combustion gas is pushed out from the high-temperature reducing combustion chamber 2, and flows downward through the restrictor 4 to the low-temperature oxidizing combustion chamber 3.

[0022] The side walls of the low-temperature oxidizing combustion chamber 3 have a water-cooled wall structure, by which the high-temperature combustion gas that has flowed downward through the restrictor 4 from the high-temperature reducing combustion chamber 2 is cooled down. In the low-temperature oxidizing combustion chamber 3, a single line of air nozzles 7, or multiple lines of air nozzles 7, are located on a furnace wall that is spaced apart downward from the restrictor 4. The air nozzles 7 are nozzles to feed air for second-stage combustion. In a second-stage combustion region 10 below the air nozzles 7, with the air fed from the air nozzles 7, uncombusted gas in the combustion gas is subjected to second-stage combustion in a low-temperature oxidizing atmosphere. Combustion ash accumulated at the bottom of the furnace is discharged to the outside of the furnace through an ash outlet 8.

[0023] The gas outlet 11, which leads to the path 12, is located on a lower side surface of the second-stage combustion region 10. The flow of combustion gas generated in the second-stage combustion region 10 is inverted in a U shape, and then the combustion gas flows into the path 12. The path 12 includes steam superheater tubes 13 and an economizer 14. The combustion gas that has flowed into the path 12 exchanges heat with boiler water at the steam superheater tubes 13 and the economizer 14, and is then fed to the following treatment step. Combustion ash that flows with the combustion gas is discharged through an ash outlet 15.

[0024] In the boiler 100, the fuel fed from the powder fuel burners 5 is subjected to initial-stage combustion in a high-temperature reducing atmosphere in the high-temperature reducing combustion chamber 2, and then further subjected to second-stage combustion in a low-temperature oxidizing atmosphere in the low-temperature oxidizing combustion chamber 3. Consequently, the amount of NOx generation is reduced.

[0025] FIG. 2 is a schematic sectional view showing one example of a powder fuel burner 5 according to the present embodiment. FIG. 3 is a sectional view take along line A-A of FIG. 2.

[0026] The powder fuel burner 5 shown in FIG. 2 includes: a cylindrical air-fuel mixture conveying tube 51, through which the air-fuel mixture fed from the pulverizer 23 is conveyed; and a wind box 52. The wind box 52 includes: a secondary air feeding passage 53, through which secondary air is fed to the high-temperature reducing combustion chamber 2; and a tertiary air feeding passage 54, through which tertiary air swirled by a swirl vane 55 is fed to the high-temperature reducing combustion chamber 2. That is, the air-fuel mixture is injected from the distal end of the air-fuel mixture conveying tube 51 into the high-temperature reducing combustion chamber 2, and the secondary air and the tertiary air, each of which is combustion air, are fed from around the distal end of the air-fuel mixture conveying tube 51 to the high-temperature reducing combustion chamber 2.

[0027] On the distal end of the air-fuel mixture conveying tube 51, there is a flame stabilizing plate 51a, which is integrated with the air-fuel mixture conveying tube 51. A swirl vane 64 and a short internal cylinder 65 are mounted at the distal end of the air-fuel mixture conveying tube 51. The proximal end of the air-fuel mixture conveying tube 51 is connected via an elbow tube 58 to an air-fuel mixture feeding tube 57, through which the air-fuel mixture from the pulverizer 23 is fed. An ignition burner 61, which is a gun-type heavy oil burner or gas burner, is located on a center axis 5a of the air-fuel mixture conveying tube 51.

[0028] The swirl vane 64 is fixed near the distal end of the ignition burner 61. FIG. 4 shows one example of the swirl vane 64 as seen from the proximal end side of the air-fuel mixture conveying tube 51. The swirl vane 64 is a well-known swirl vane that includes: a cylindrical structure 64a mounted to the ignition burner 61; and flat-plate vanes 64b. The swirl vane 64 is located inside the internal cylinder 65. The internal cylinder 65 is concentric with the air-fuel mixture conveying tube 51 and mounted inside the air-fuel mixture conveying tube 51. The vanes 64b are mounted to the outer peripheral surface of the cylindrical structure 64a at regular intervals, such that each vane 64b has a predetermined angle relative to the center line of the cylindrical structure 64a. The center line of the cylindrical structure 64a coincides with the center axis 5a, and the cylindrical structure 64a is mounted concentrically with the air-fuel mixture conveying tube 51.

[0029] In the powder fuel burner 5, at the start of the operation of the boiler 100, first, heavy oil or gas injected from the ignition burner 61 is combusted. Then, after the high-temperature reducing combustion chamber 2 is warmed up, the air-fuel mixture is injected from the powder fuel burner 5, and combustion of the powder fuel in the air-fuel mixture is started. Thereafter, the injection of the heavy oil or gas from the ignition burner 61 is stopped at a suitable time, and the combustion of the powder fuel in the air-fuel mixture injected from the powder fuel burner 5 is continued.

[0030] In the powder fuel burner 5 of the present embodiment, at a position upstream of the swirl vane 64 in the conveying direction of the air-fuel mixture, a rod-shaped center flow blocker 62 is mounted on the center axis 5a of the air-fuel mixture conveying tube 51. The center flow blocker 62 includes a cylindrical surface 62a, which is concentric with the air-fuel mixture conveying tube 51 and which extends in the direction of the center axis 5a. Further, a cylindrical peripheral flow blocker 63 is mounted along an inner wall surface 51s of the air-fuel mixture conveying tube 51 between the center flow blocker 62 and the swirl vane 64. The peripheral flow blocker 63 includes a cylindrical inner peripheral surface 63a, which is concentric with the air-fuel mixture conveying tube 51 and which extends in the direction of the center axis 5a.

[0031] According to the powder fuel burner 5, the center flow blocker 62 causes the air-fuel mixture flowing inside the air-fuel mixture conveying tube 51 to flow in a peripheral region inside the tube, and thereafter, the direction of the flow of the air-fuel mixture is oriented toward a central region inside the tube by the peripheral flow blocker 63, so that the air-fuel mixture flows in the central region inside the tube in the same direction as the direction of the center axis 5a. Thereafter, when the air-fuel mixture has passed the region where the peripheral flow blocker 63 is mounted, the air-fuel mixture flows in a manner to spread over the entire internal space of the tube. Then, the air-fuel mixture is swirled by the swirl vane 64, and injected into the combustion chamber 1. Here, also after the air-fuel mixture traveling straight in the same direction as the direction of the center axis 5a has passed the region where the peripheral flow blocker 63 is mounted, the particles of the powder fuel contained in the air-fuel mixture are to travel straight due to the inertial force. Accordingly, as indicated by arrows S1, the air-fuel mixture having a high powder fuel concentration, i.e., the air-fuel mixture having a low A/F, flows in the central region of the air-fuel mixture conveying tube 51, and passes through the swirl vane 64 to be injected into an ignition region R1 of the combustion chamber 1. Also, as indicated by arrows S2, the air-fuel mixture having a low powder fuel concentration, i.e., the air-fuel mixture having a high A/F, flows near the tubular wall of the air-fuel mixture conveying tube 51. Therefore, even when the air-fuel mixture having a high A/F is fed to the burner 5, the A/F of the air-fuel mixture injected into the combustion chamber 1 can be locally reduced. That is, even when a flame-retardant powder fuel, such as a petroleum residue fuel, having a higher A/F than a suitable A/F for its ignition and combustion is directly fed from the pulverizer 23, the A/F of the air-fuel mixture injected into the combustion chamber 1 can be locally reduced to a suitable A/F for the ignition and combustion of the powder fuel. Consequently, the ignitibility can be improved, and stable combustion can be achieved. In this example, the A/F of the air-fuel mixture fed from the pulverizer 23 is about 1.8. A suitable A/F of the air-fuel mixture for the ignition and combustion of petroleum coke, which is one example of a flame-retardant powder fuel, is preferably less than or equal to 1.5, and more preferably about 1.0.

[0032] Hereinafter, the center flow blocker 62 and the peripheral flow blocker 63 are described in detail with reference to FIG. 5. FIG. 5 is a schematic sectional view showing an essential part of the powder fuel burner 5, in which a swirl vane 66 different from the above-described swirl vane 64 is used. In the case of the example shown in FIG. 5, the powder fuel burner 5 includes the swirl vane 66 instead of the swirl vane 64 and the internal cylinder 65 shown in FIG. 2, and except the swirl vane 66, the configuration of the powder fuel burner 5 shown in FIG. 5 is the same as the configuration of the powder fuel burner 5 shown in FIG. 2.

[0033] As shown in FIG. 5, a length L1 of the cylindrical surface 62a of the center flow blocker 62 in the direction of the center axis 5a is a predetermined length greater than or equal to a particular value. Consequently, the flow direction of powder particles contained in the air-fuel mixture that has passed the face of the cylindrical surface 62a is parallel to the direction of the center axis 5a. Also, a length L2 of the cylindrical inner peripheral surface 63a of the peripheral flow blocker 63 in the direction of the center axis 5a is a predetermined length greater than or equal to a particular value. Consequently, the flow direction of powder particles contained in the air-fuel mixture that has passed the face of the inner peripheral surface 63a is parallel to the direction of the center axis 5a.

[0034] The diameter of the cylindrical surface 62a of the center flow blocker 62 is greater than or equal to the diameter of the cylindrical inner peripheral surface 63a of the peripheral flow blocker 63. In other words, the sum of the diameter of the cylindrical surface 62a of the center flow blocker 62 and a value that is twice the distance from the inner wall surface 51s of the air-fuel mixture conveying tube 51 to the inner peripheral surface 63a of the peripheral flow blocker 63 is greater than or equal to the internal diameter of the air-fuel mixture conveying tube 51. Accordingly, in a case where the inside of the air-fuel mixture conveying tube 51 is seen in the direction of the center axis 5a, for example, as shown in FIG. 3, it appears that there is no gap between the center flow blocker 62 and the peripheral flow blocker 63. According to this configuration, the flow direction of the entire air-fuel mixture flowing in the air-fuel mixture conveying tube 51 is restricted by the center flow blocker 62 and the peripheral flow blocker 63. This makes it possible to make the flow of the air-fuel mixture a desired flow, and contributes to locally reducing the A/F of the air-fuel mixture injected into the combustion chamber 1.

[0035] The center flow blocker 62 includes curved surfaces 62b and 62c at both ends of the cylindrical surface 62a, respectively. The curved surfaces 62b and 62c protrude from the respective ends of the cylindrical surface 62a in the direction of the center axis 5a to form conical side surfaces, respectively. Angles e and f formed by the center axis 5a and the respective curved surfaces 62b and 62c are each preferably less than or equal to 20 degrees. Consequently, the generation of a separation vortex is suppressed before and after the air-fuel mixture flowing in the air-fuel mixture conveying tube 51 passes the center flow blocker 62, and the flow of the air-fuel mixture can be made a desired flow. The curved surface 62b protrudes from the upstream end of the cylindrical surface 62a toward the upstream side in the conveying direction of the air-fuel mixture, and the curved surface 62c protrudes from the downstream end of the cylindrical surface 62a toward the downstream side in the conveying direction of the air-fuel mixture.

[0036] The peripheral flow blocker 63 includes curved surfaces 63b and 63c at both ends of the cylindrical inner peripheral surface 63a, respectively. The curved surfaces 63b and 63c protrude from the respective ends of the inner peripheral surface 63a in the direction of the center axis 5a, and connect to the inner wall surface 51s of the air-fuel mixture conveying tube 51 to form truncated conical side surfaces, respectively. Angles g and h formed by the inner wall surface 51s of the air-fuel mixture conveying tube 51 and the respective curved surfaces 63b and 63c are each preferably less than or equal to 20 degrees. Consequently, the generation of a separation vortex is suppressed before and after the air-fuel mixture flowing in the air-fuel mixture conveying tube 51 passes the peripheral flow blocker 63, and the flow of the air-fuel mixture can be made a desired flow. Thus, this configuration makes it possible to make the flow of the air-fuel mixture in the air-fuel mixture conveying tube 51 a desired flow, and contributes to locally reducing the A/F of the air-fuel mixture injected into the combustion chamber 1. The curved surface 63b protrudes from the upstream end of the inner peripheral surface 63a toward the upstream side in the conveying direction of the air-fuel mixture, and the curved surface 63c protrudes from the downstream end of the inner peripheral surface 63a toward the downstream side in the conveying direction of the air-fuel mixture.

[0037] The center flow blocker 62 may be configured such that the position of the center flow blocker 62 in the direction of the center axis 5a is adjustable. In this case, for example, the center flow blocker 62 may be slidably mounted to the ignition burner 61 such that the center flow blocker 62 is slidable in the direction of the center axis 5a, and an operating lever may be connected to the center flow blocker 62 and extended to the outside of the air-fuel mixture conveying tube 51 so that the operating lever can be moved in the direction of the center axis 5a. This configuration makes it possible to move the center flow blocker 62 in the direction of the center axis 5a, and thereby the distance between the downstream-end curved surface 62c of the center flow blocker 62 and the upstream-end curved surface 63b of the peripheral flow blocker 63 can be adjusted. The less the distance, the higher the concentration of the powder fuel in the air-fuel mixture that has passed the peripheral flow blocker 63 and that is in a region closer to the center of the air-fuel mixture conveying tube 51. Thus, by adjusting the distance, proper concentration distribution can be achieved.

[0038] The swirl vane 66 shown in FIG. 5 includes: a cylindrical structure 66a, which is, at the distal end of the air-fuel mixture conveying tube 51, mounted inside the air-fuel mixture conveying tube 51; and flat-plate vanes 66b. The vanes 66b are mounted to the outer peripheral surface of the cylindrical structure 66a at regular intervals, such that each vane 66b has a predetermined angle relative to the center line of the cylindrical structure 66a. The center line of the cylindrical structure 66a coincides with the center axis 5a, and the cylindrical structure 66a is mounted concentrically with the air-fuel mixture conveying tube 51. That is, roughly speaking, the swirl vane 66 is configured such that, in the above-described swirl vane 64, the diameter of the cylindrical structure 64a is increased. The internal diameter of the cylindrical structure 66a of the swirl vane 66 is greater than or equal to the diameter of the cylindrical inner peripheral surface 63a of the peripheral flow blocker 63.

[0039] For example, in a central region near the center of the swirl vane 64 shown in FIG. 4, the vanes 64b are densely located. There is a case where the swirl vane 64 in the central region flicks off a lot of powder fuel particles to the outside of the ignition region R1. In such a case, by using the swirl vane 66 shown in FIG. 5, a large amount of powder fuel particles can be fed to the ignition region R1 from the vicinity of the center of the air-fuel mixture conveying tube 51. This contributes to locally reducing the A/F of the air-fuel mixture fed to the ignition region R1 of the combustion chamber 1.

[0040] FIG. 6 is a schematic sectional view showing an essential part of the powder fuel burner 5, in which a swirl vane 67 different from the above-described swirl vanes 64 and 66 is used. Roughly speaking, the swirl vane 67 is configured such that, in the above-described swirl vane 64 shown in FIG. 2, the diameter of the swirl vane 64 is increased by increasing the length of the vanes 64b, and also, the internal cylinder 65 is absent. In some cases, the swirl vane 67 thus configured may be used.

[0041] In the present embodiment, in a case where the ignition burner 61 is not mounted inside the air-fuel mixture conveying tube 51, but mounted separately, a center axis rod instead of the ignition burner 61 may be mounted on the center axis 5a, and the center flow blocker 62 may be mounted to the center axis rod. In this case, the swirl vane 64 in FIG. 2 or the swirl vane 67 in FIG. 6 may also be mounted to the center axis rod.

[0042] The flame-retardant powder fuel in the present embodiment may be a petroleum residue fuel, a biomass fuel, or a mixed fuel of biomass and pulverized coal. Each of these fuels is less combustible than pulverized coal.

[0043] From the foregoing description, numerous modifications and other embodiments of the present disclosure are obvious to those skilled in the art. Accordingly, the foregoing description is to be construed as illustrative only, and is provided for the purpose of teaching those skilled in the art the best mode for carrying out the present disclosure. The structural and/or functional details may be substantially modified without departing from the scope of the present disclosure.

Summary

[0044] A powder fuel burner according to a first aspect of the present disclosure is a powder fuel burner mounted in a combustion chamber of a boiler, the powder fuel burner including: a cylindrical air-fuel mixture conveying tube, through which an air-fuel mixture is conveyed and which injects, from a distal end thereof, the air-fuel mixture into the combustion chamber, the air-fuel mixture being a mixture of a powder fuel and conveying air; a swirl vane mounted at the distal end of the air-fuel mixture conveying tube; a rod-shaped center flow blocker mounted on a center axis of the air-fuel mixture conveying tube at a position upstream of the swirl vane in a conveying direction of the air-fuel mixture, the center flow blocker including a cylindrical surface that is concentric with the air-fuel mixture conveying tube and that extends in a direction of the center axis; and a cylindrical peripheral flow blocker mounted along an inner wall surface of the air-fuel mixture conveying tube between the center flow blocker and the swirl vane, the peripheral flow blocker including a cylindrical inner peripheral surface that is concentric with the air-fuel mixture conveying tube and that extends in the direction of the center axis.

[0045] According to the above configuration, the center flow blocker causes the air-fuel mixture flowing inside the air-fuel mixture conveying tube to flow in a peripheral region inside the tube, and thereafter, the direction of the flow of the air-fuel mixture is oriented toward a central region inside the tube by the peripheral flow blocker, so that the air-fuel mixture flows in the central region inside the tube in the same direction as the direction of the center axis. Thereafter, when the air-fuel mixture has passed the region where the peripheral flow blocker is mounted, the air-fuel mixture flows in a manner to spread over the entire internal space of the tube. Then, the air-fuel mixture is swirled by the swirl vane, and injected into the combustion chamber. Here, also after the air-fuel mixture traveling straight in the same direction as the direction of the center axis has passed the region where the peripheral flow blocker is mounted, the particles of the powder fuel contained in the air-fuel mixture are to travel straight due to inertial force. Accordingly, the air-fuel mixture in the state of having a high powder fuel concentration in the central region of the air-fuel mixture conveying tube and having a low powder fuel concentration in the other region of the air-fuel mixture conveying tube passes through the swirl vane to be injected into the combustion chamber. Therefore, even when the air-fuel mixture having a high A/F is fed to the burner, the A/F of the air-fuel mixture injected into the combustion chamber can be locally reduced. That is, even when a flame-retardant powder fuel, such as a petroleum residue fuel, having a higher A/F than a suitable A/F for its ignition and combustion is directly fed from the pulverizer, the A/F of the air-fuel mixture injected into the combustion chamber can be locally reduced to a suitable A/F for the ignition and combustion of the powder fuel. Consequently, the ignitibility can be improved, and stable combustion can be achieved.

[0046] A powder fuel burner according to a second aspect of the present disclosure is configured such that, in the powder fuel burner according to the first aspect, a diameter of the cylindrical surface of the center flow blocker is greater than or equal to a diameter of the cylindrical inner peripheral surface of the peripheral flow blocker. According to this configuration, the flow direction of the entire air-fuel mixture flowing in the air-fuel mixture conveying tube is restricted by the center flow blocker and the peripheral flow blocker. This makes it possible to make the flow of the air-fuel mixture a desired flow, and contributes to locally reducing the A/F of the air-fuel mixture injected into the combustion chamber.

[0047] A powder fuel burner according to a third aspect of the present disclosure is configured such that, in the powder fuel burner according to the first or second aspect, the center flow blocker includes curved surfaces at both ends of the cylindrical surface, respectively, the curved surfaces protruding from the respective ends of the cylindrical surface in the direction of the center axis to form conical side surfaces, respectively, such that angles formed by the center axis and the respective curved surfaces are each less than or equal to 20 degrees, and the peripheral flow blocker includes curved surfaces at both ends of the cylindrical inner peripheral surface, respectively, the curved surfaces protruding from the respective ends of the inner peripheral surface in the direction of the center axis and connecting to the inner wall surface of the air-fuel mixture conveying tube to form truncated conical side surfaces, respectively, such that angles formed by the inner wall surface of the air-fuel mixture conveying tube and the respective curved surfaces are each less than or equal to 20 degrees. Consequently, the generation of a separation vortex is suppressed before and after the air-fuel mixture flowing in the air-fuel mixture conveying tube passes the center flow blocker, and also, the generation of a separation vortex is suppressed before and after the air-fuel mixture flowing in the air-fuel mixture conveying tube passes the peripheral flow blocker. Thus, this configuration makes it possible to suppress the generation of a separation vortex in the air-fuel mixture conveying tube and to make the flow of the air-fuel mixture in the air-fuel mixture conveying tube a desired flow, and contributes to locally reducing the A/F of the air-fuel mixture injected into the combustion chamber.

[0048] A powder fuel burner according to a fourth aspect of the present disclosure is configured such that, in the powder fuel burner according to any one of the first to third aspects, the center flow blocker is configured such that a position of the center flow blocker in the direction of the center axis is adjustable. This configuration makes it possible to move the center flow blocker in the direction of the center axis, and thereby the distance between the center flow blocker and the peripheral flow blocker can be adjusted. The less the distance, the higher the concentration of the powder fuel in the air-fuel mixture that has passed the peripheral flow blocker and that is in a region closer to the center of the air-fuel mixture conveying tube. Thus, by adjusting the distance, proper concentration distribution can be achieved.

[0049] A powder fuel burner according to a fifth aspect of the present disclosure is configured such that, in the powder fuel burner according to any one of the first to fourth aspects, the swirl vane includes: a cylindrical structure that is concentric with the air-fuel mixture conveying tube and that is, at the distal end of the air-fuel mixture conveying tube, mounted inside the air-fuel mixture conveying tube, the cylindrical structure having an internal diameter that is greater than or equal to a diameter of the cylindrical inner peripheral surface of the peripheral flow blocker; and vanes mounted to an outer peripheral surface of the cylindrical structure. According to this configuration, a large amount of powder fuel particles can be fed to the ignition chamber from the vicinity of the center of the air-fuel mixture conveying tube. This contributes to locally reducing the A/F of the air-fuel mixture injected into the combustion chamber.

[0050] A powder fuel burner according to a sixth aspect of the present disclosure is configured such that, in the powder fuel burner according to any one of the first to fifth aspects, the powder fuel is a petroleum residue fuel, a biomass fuel, or a mixed fuel of biomass and pulverized coal.

REFERENCE SIGNS LIST

[0051] 1 combustion chamber of a boiler [0052] 5 powder fuel burner [0053] 51 air-fuel mixture conveying tube [0054] 62 center flow blocker [0055] 62a cylindrical surface of the center flow blocker [0056] 63 peripheral flow blocker [0057] 63a inner peripheral surface of the peripheral flow blocker [0058] 64, 66, 67 swirl vane [0059] 66a cylindrical structure of the swirl vane [0060] 66b vane of the swirl vane