Pulverized coal burner

Abstract: The pulverized coal burner suitable for lowering the concentration of nitrogen comprises a pulverized coal nozzle 10 for jetting a mixture of pulverized coal and primary air, a secondary air nozzle 11 and a tertiary air nozzle 12, concentrically arranged around the outer periphery of the pulverized coal nozzle 10, and a tube expanded portion 20 at the end of a partition wall separating two adjacent air nozzles. A flow shift means for shifting the secondary air in the secondary air nozzle so as to flow along the tube expanded portion, for example a guide plate 30, is provided. The secondary air comes to be jetted outward by the guide plate, and mixing of the secondary air and the tertiary air with pulverized coals is delayed, whereby an amount of occurrence of NOx can be decreased.

Description: BACKGROUND OF THE INVENTION The present invention relates to a pulverized coal burner as described in the preamble of claim 1 and disclosed in EP 0 314 928 A1, EP 0 445 938 A1, US-A-4 545 307 or WO 95/13502. In general, for burners, suppression of NOx (nitrogen oxides) formation during combustion is a subject matter to be solved. Particularly, coal includes a larger amount of nitrogen, compared with gaseous fuel and liquid fuel. Therefore, it is more important to decrease NOx produced by combustion of pulverized coals than in a case of combustion of gaseous fuel or liquid fuel. NOx produced by combustion of pulverized coals is almost all NOx that is produced by oxidizing nitrogen contained in coal, that is, so-called fuel NOx. In order to decrease the fuel NOx, various burner structures and burning methods have been studied. As one of the burning methods, there is a method forming a low oxygen concentration region within flame and reducing (deoxidizing) NOx. For example, JP A 1-305206 (US patent 4,930,430), JP A 3-211304, JP A 3-110308, US patent 5,231,937, US patent 5,680,823, etc. disclose a method of producing flame of low oxygen concentration atmosphere and completely burning coal, and a structure having a fuel nozzle for pneumatically transferring coal at the center thereof and an air injecting nozzle arranged outside the fuel nozzle. According to those prior arts, a reducing flame region of a low oxygen concentration is formed within the flame, reducing reactions of NOx are progressed in the reducing flame region, and an amount of NOx occurred within flame is suppressed to be small. Further, the JP A 1-305206 discloses a method of stabilization of flame by providing, at an outlet end portion of a nozzle, an obstacle against the flow direction of gas. Further, JP A 3-311304, JP A 3-110308 and US patent 5, 231, 937 disclose stabilization of flame by providing a flame stabilizing ring at the tip of a pulverized coal nozzle. According to those prior arts, recirculating zones are formed downstream of the tip of the pulverized coal nozzle by providing the flame stabilizing ring or obstacle at the tip of the pulverized coal nozzle. Since a high temperature gas stays in the recirculating zones, ignition of pulverized coals progresses and the stability of flame can be raised. However, in the above-mentioned prior arts, NOx formation has not been sufficiently suppressed as yet. EP 0 314 928 A1, EP 0 445 938 A1, US-A-4,545,307 or WO 95/13502 disclose a pulverized coal burner comprising a pulverized coal nozzle for jetting or spouting a mixture of pulverized coals and primary air, a secondary air nozzle concentrically arranged around the outer peripheral wall of the pulverized coal nozzle, a tertiary air nozzle concentrically arranged around an outer peripheral wall of the secondary air nozzle, an expanded portion at the downstream end of the outer peripheral wall of the secondary air nozzle, a flow shift means for shifting secondary air jetted from the secondary air nozzle in a radially outward direction, and a flame stabilizing ring provided at the downstream end of the outer peripheral wall of the pulverized coal nozzle and upstream of the flow shift means directly adjacent thereto. The flow shift means according to EP 0 314 928 A1 (Fig. 11) comprises an annulus of the downstream end of the outer peripheral wall of the coal nozzle extending radially over a predetermined distance into the downstream end of the coal nozzle to define a flame stabilizing ring and into the downstream end of the secondary air nozzle to define an obstacle which is bent radially outwardly in the direction of the expanded portion which a deflection angle of less than 30° with respect to the central axis of the burner and ending in the same radial plane with the tip of the expanded portion. A very similar flow shift means is known from EP 0 445 938 A1 (Fig. 8) wherein the radially outwardly bent portion of the obstacle ends upstream of the tip of the expanded portion. This is also the case with the bent portion of US-A-4,545,307 which is radially outwardly bent along a part of a circle and has a deflection angle at its downstream end of 0° to a maximum of 60°. The burner of WO 95/13502 has a rectangular structure in cross section and comprises peripherally spaced flow shift means having a radially outwardly bent portion at the end of the coal nozzle defining a deflection angle with the central axis of the burner of 15 to 25° and having its end extending somewhat downstream of the expanded portion. It is the object of the invention to provide a coal burner of the generic kind which can further decrease NOx formation. This object is achieved with the pulverized coal burner of claim 1 preferred embodiments of which are described in the subclaims 2 to 10. By separating the inner flame stabilizing ring formed at the side of the pulverized coal nozzle and the outer ring or guide plate formed in the side of the secondary air nozzle so that they are distant from each other in the flow direction, the recirculating flows downstream of the Inner and outer rings are axially deviated, thereby expanding the recirculating flow formation zone. As a result, a large recirculating flow is formed. The place in which the recirculating flow or flows are formed is lower in oxygen concentration than the place in which such recirculating flows are not formed, so that a large recirculating flow forming region makes a higher effect of reducing NOx and unbumt substances. The pulverized coal burner in which the secondary. air nozzle and tertiary air nozzle are concentrically arranged around the outer periphery of the pulverized coal nozzle and configurated according to the invention aims to suppress NOx formation by forming a NOx reducing zone of a low oxygen concentration by primary air and carry out complete combustion by forming an oxidizing flame region by mixing the secondary air and tertiary air with the flow at a downstream side of the NOx reducing region. The later the mixing of the secondary air and tertiary air with pulverized coals becomes, the larger NOx reducing zone is formed, so that an effect of suppressing the NOx formation can be raised. On the other hand, pulverized coal itself is not good in ignitability, and under the condition that oxygen is short, the pulverized coal is uneasy to be ignited but flame is easily extinguished. In order to stably form flame under the condition of air shortage, it is desirable to pull a high temperature combustion gas present in the after flow of the flame to a position close to the outlet of the pulverized coal nozzle. By forming a low pressure portion at a downstream side of the tip of a partition wall separating or partitioning the pulverized coal nozzle and the secondary air nozzle, a recirculating zone is formed there, and the high temperature combustion gas comes to be pulled back. When the recirculating zone is formed, air flowing outside the recirculating zone has a tendency to be pulled to the inside by the recirculating zone. However, if the recirculating zone is formed to spread in a perpendicular direction to the axis of the pulverized coal nozzle and be large in the axial direction, the air flowing outside the recirculating zone becomes slow in pullback and does not flow back close to the outlet of the pulverized coal nozzle. According to the present invention, since secondary air comes to flow outwardly along the expanded portion of the tip of outer peripheral wall of the secondary air nozzle, the size of recirculating zone formed at a downstream side of the partition wall separating the pulverized coal nozzle and the secondary air nozzle becomes large, whereby pullback of the secondary air becomes slow. Further, by a large-sized recirculating zone, the ignitability of pulverized coals becomes good and flame becomes uneasy to be extinguished. The angle of the above-mentioned outer ring or guide plate is preferably in a range of 80 to 90 against the central axis of the pulverized coal nozzle. The tip of the guide plate is positioned downstream of the tip of the expanded portion provided on the outer peripheral wall of the secondary air nozzle. By such an arrangement, after the secondary air flowing in the secondary air nozzle flows out of the nozzle, the flow direction is changed radially outwardly, and the secondary air flows toward the tertiary air flow so as to impinge thereon. Thereby, the flow of tertiary air comes to be shifted further outwardly, and mixing of the tertiary air comes to be delayed. The tip of the guide plate and the tip of the expanded portion are desirable to be separated by a distance in a range of from 5 mm or more to 50 mm or less. When the distance is too small, the effect is small, and when too large, the secondary air expands after leaving the nozzle and the velocity of the flow becomes slow, whereby an effect of shifting the tertiary air toward the outside becomes small. The tip of the guide plate also is desirable to be positioned at an upstream side of the tip of the outer peripheral wall of the tertiary air nozzle. The outer peripheral wall, usually, is jointly served as a furnace wall of a boiler in many cases. Combustion and slug are adhered to the furnace wall, and the substances and slug, in a case of large amount, may reaches to from several kg to several hundred kg. In order to prevent the burner from being broken by falling of them, the tip of the guide plate is preferable not to project into the inside of the furnace from the furnace wall jointly served as the outer peripheral wall of the tertiary air nozzle. For the tertiary air nozzle, it is preferable that outward force has been already applied when the tertiary air is jetted from the tertiary air nozzle, therefore, it is preferable to provide a swirler inside the tertiary air nozzle. Further, it is preferable to have outwardly expanded the end portion of the outer peripheral wall of the tertiary air nozzle. Still further, it is preferable to have outwardly expanded the end portion of the inner peripheral wall of the tertiary air nozzle. By making the burner so that secondary air flows along the expanded portion provided on the outer peripheral wall of the secondary air nozzle, a recirculating zone is unlikely to be formed between the secondary air nozzle and the tertiary air nozzle, whereby pullback of the tertiary air also becomes slow. The angle of the guide plate with the central axis of the coal nozzle should be greater than that of the expanded portion provided on the outer peripheral wall of the secondary air nozzle. The flow shift means may comprise a gas jet nozzle for jetting a gas toward the secondary air flowing in the vicinity of the outlet of the secondary air nozzle and shifting the secondary air in the radially outward direction. By shifting a secondary air flow in the radially outward direction as in the present invention, it becomes possible to delay mixing of secondary air and tertiary air with pulverized coals and form a large NOx reducing zone. Further, by a large recirculating zone between the pulverized coal nozzle and the secondary air nozzle, the ignitability of pulverized coals is improved to be easily ignited, additionally, such an effect can be attained that an air-short NOx reducing zone comes to be stably formed. It is desirable to further provide, within the secondary nozzle, a flow path narrowing member or obstacle for narrowing the flow path of the secondary air nozzle to make the flow velocity faster. It is possible to direct the flow of tertiary air in a further radially outward direction by changing, by the guide plate, the flow direction of the secondary air made faster in flow velocity by the flow path narrowing member, and then spouting it from the secondary air nozzle. The flow path narrowing member can be provided at the inner peripheral wall or outer peripheral wall of the secondary air nozzle, however, it is preferable for it to be provided at the inner peripheral wall side, because it is possible to more rapidly change the direction of a secondary air flow in the redially outward direction. Further, it is possible to form slits in this flame stabilizing ring or in the guide plate provided at the tip of inner peripheral wall of the secondary air nozzle. The slits have an effect of suppressing thermal deformation of the flame stabilizing ring or the guide plate. Further they have an effect of making it easy to form a recirculating zone at a downstream side of the flame stabilizing ring or the guide plate. According to the present invention, since the flow shift means for deflecting the secondary air jetted from the secondary air nozzle in the radially outward direction of the secondary air nozzle is provided, the secondary air flows in the radially outward direction the recirculating zone formed downstream of the partition wall between the pulverized coal nozzle and the secondary air nozzle moves in the radially outward direction, and the scale thereof also can be enlarged. As a result, mixing of pulverized coal and secondary air, tertiary air in the vicinity of the burner is suppressed, the pulverized coal burns under the condition of low oxygen concentration atmosphere in the vicinity of the burner, and NOx formation can be effectively decreased. Embodiments of the invention are described by way of examples referring to the attached drawings, in which Fig. 1(a) is a sectional view of a pulverized coal burner of a first embodiment of the present invention; Fig. 1(b) is an enlarged view of a part of Fig. 1(a); Fig. 1(c) is an enlarged view as Fig. 1(a) showing a modification of Fig. 1(b) that is not within the scope of the invention; Fig. 2 is a sectional view of an end portion of a nozzle of a conventional pulverized coal burner, which is shown for caparison with the first embodiment of the present invention; Fig. 3 is a sectional view of a pulverized coal burner of a second embodiment of the present invention; Fig. 4 is a sectional view of a nozzle end portion of a pulverized coal burner of a third embodiment of the present invention; Fig. 5 is a sectional view of a pulverized coal burner of a forth embodiment of the present invention; Fig. 6 is a sectional view of a pulverized coal burner of a fifth embodiment of the present invention; and In Figs. 1(a), 1(b) and 1(c), 10 denotes a pulverized coal nozzle which is connected to a transfer tube (not shown) at an upstream side and transfers and supplies pulverized coals together with primary air. 11 denotes a secondary air nozzle for jetting secondary air. The secondary air nozzle 11 has a flow path formed around the outer periphery of the pulverized coal nozzle 10 and shaped in a circular cross-section which is concentric with the pulverized coal nozzle 10. 12 denotes a tertiary air nozzle for jetting tertiary air, which has a flow path formed around the outer periphery of the secondary air nozzle 11 and shaped in a circular cross-section which is concentric with the secondary air nozzle 11. A flow rate distribution among primary air, secondary air and tertiary air is 1-2: 1: 3-7, for example, and the distribution is made so that the pulverized coals are completely burnt by the tertiary air. 13 denotes inflowing pulverized coals and primary air. 14 and 15 denote inflowing secondary air and tertiary air, respectively. 16 denotes an oil gun provided in the pulverized coal nozzle 10 so as to axially extend to a position in the vicinity of the outlet of the nozzle 10. The oil gun 16 is used for assisting combustion at the time of burner starting or low load combustion. 17 denotes a venturi tube making small the inner diameter of the pulverized coal nozzle 10 to prevent the pulverized coals from backfiring. 18 denotes a flame stabilizing ring provided at the end of a partition wall 28 partitioning the pulverized coal nozzle 10 and the secondary air nozzle 11 and separating the primary air and secondary air to expand a recirculating zone 31. 19 denotes a burner throat forming a furnace wall and served also as an outer peripheral wall of the tertiary nozzle 12. 20 denotes a guide sleeve provided at the end of a partition wall 21 separating the secondary air nozzle 11 and the tertiary air nozzle 12, which sleeve also is referred to as a tube expanded portion in the present invention. 22 denotes a swirler for swirling tertiary air along the periphery of the secondary air nozzle 11. The swirler 22 employs air swirling vanes usually called as resistor vanes in this embodiment. 23 denotes a side plate for inflowing secondary air. 24 denotes water pipes provided on the furnace wall 19. 25 denotes a wind box in which secondary air is introduced. 26 denotes a damper for adjusting secondary air. 27 denotes a swirler for swirling secondary air along the periphery of the pulverized coal nozzle, and the swirler 27 employs air swirling vanes usually called as vanes in this embodiment. 28 denotes the partition wall between the pulverized coal nozzle 10 and the secondary air nozzle 11. 30 denotes a guide plate provided at the end of the inner peripheral wall of the secondary air nozzle 11 for jetting the secondary air toward the radially outer side. 31 denotes the recirculating zones formed between jetting regions of the pulverized coal nozzle 10 and the secondary air nozzle 11. 52 denotes a secondary air flow. 53 denotes a tertiary air flow. 65a denotes an obstacle for flow path narrowing which is a part of the flame stabilizing ring 18 and provided in the inner peripheral portion of the secondary air nozzle 11. Next, a burning operation of the present embodiment will be described, referring to Figs. 1(a) and 1(b). As the pulverized coal burner starts up combustion, since the air downstream of the partition wall 28 is taken in the the air jetted from each nozzle, the pressure downstream of the partition wall 28 decreases, and a recirculating zone 31 is formed. Since the flame stabilizing ring 18 is provided at the end portion of the partition wall 28, primary air and secondary air are separated from each other, and the recirculating zone 31 expands. Since a high temperature gas stays within the recirculating zone 31, ignition of pulverized coals progresses, the stability of flame is improved. Thereby, the flame is stably formed by pulverized coals and primary air in the vicinity of the outlet of the pulverized coal nozzle 10. Further, consumption of oxygen progresses within the flame, a NOx reducing zone expands and it is possible to decrease an amount of NOx formation. Further, since the combustion of coal progresses, unburnt carbon in combustion ashes left after combustion decreases. Further, since the swirlers 22, 27 are provided, secondary air and tertiary air are jetted as swirling flows, the negative pressure downstream of the flame stabilizing ring 18 is raised by the centrifugal force of the air, the recirculating zone expands further. Thereby, mixing of the secondary air and tertiary air with the pulverized coals in the vicinity of the burner is delayed, and the concentration of oxygen within the flame decreases, so that the NOx reducing zone expands. In the present embodiment, further, since the guide plate 30 is provided at the end portion of the inner peripheral wall of the secondary air nozzle 11 as a means for deflecting a secondary air flow 52 jetted from the secondary air nozzle 11 in the radially outward direction, the secondary air is jetted in the radially outward direction, the mixing of the secondary air and tertiary air with the pulverized coals is delayed further, and the recirculating zone downstream of the flame stabilizing ring 18 expands. Therefore, the combustion of the pulverized coals in this recirculating zone region is promoted, NOx formtion and unburnt carbon can be decreased further. The combustion conditions at this time will be explained, comparing with the conventional structure in Fig. 2 in which the guide plate is not provided. In Fig. 2, the flow path of tertiary air 53 is bent by the guide sleeve 20 formed in a tapered cylindrical shape, and the tertiary air is jetted outward. On the other hand, the flow path of the secondary air nozzle 11 is expanded outward at the nozzle outlet by the guide sleeve 20. Since air flows straightly by its inertia, secondary air is apt to flow along the burner axis (a dashed line in Fig. 2), and there occurs a pressure drop in a reverse direction (hereunder, referred to as adverse pressure gradient) to a jetting direction of air flow along the guide sleeve 20, whereby a recirculating zone 54 is formed downstream of the guide sleeve 20. By this recirculating zone 54, a flow directed to the center (the dashed line in Fig. 2) is induced in the tertiary air 53, and the tertiary air is mixed early with the pulverized coals, so that the NOx reducing zone is narrowed. On the contrary, in the present embodiment, as shown in Fig. 1(b), secondary air 52 is jetted in an outer peripheral direction by the guide plate 30. Therefore, formation of a recirculating zone at a downstream side of the guide sleeve 20 separating the secondary air nozzle 11 and the tertiary air nozzle 12 is prevented or suppressed. Further, in particular, since the burner is constructed so that the secondary air 52 is jetted radially more outwardly than tertiary air 53, the flow of the tertiary air 53 is further directed to the outer peripheral direction by the momentum of secondary air 52 jetted in the outer peripheral direction. Therefore, mixing of the secondary air arid tertiary air with the pulverized coals in the vicinity of burner is delayed, the concentration of oxygen within the flame is lowered, and the NOx reducing zone expands, whereby NOx occurred within the flame can be decreased. Further, since the tip of the guide plate 30 is disposed closer to the burner axis (a dashed line in Fig. 1(b)) side than the tip of the guide sleeve 20, the secondary air is apt to flow radially more outwardly and a recirculating zone is unlikely to occur downstream of the guide sleeve 20. In this embodiment, the flow path of the secondary air nozzle 11 is narrowed near its outlet by the flame stabilizing ring 18, whereby the secondary air made larger in flow velocity by the flow path narrowing is jetted, so that tertiary air can be further delayed in mixting with coal. In this manner, according to this embodiment, secondary air is jetted in the radially outward direction from the secondary air nozzle 11 by the guide plate 30 provided on the secondary air nozzle 11. Further, the adverse pressure gradient at the downstream side of the partition wall 21 between the secondary air nozzle 11 and the tertiary air nozzle 12 becomes small, so that tertiary air also is jetted in the radially outward direction from the tertiary air nozzle 12 disposed at the outer peripheral wall of the secondary air nozzle 11. Therefore, mixing of pulverized coal and combustion air with pulverized coals in the vicinity of the burner is suppressed, the pulverized coals are burnt in the vicinity of the burner under the condition of low oxygen concentration, whereby an amount of NOx formation can be reduced. As an example, a combustion test was conducted in a combustion furnace (500 kg/h), using the pulverized coal burner (a distance between the guide sleeve 20 and the guide plate 30 is 10 mm) as shown in Figs. 1(a) and 1(b) and the burner shown in Fig. 2. The result is shown in a table 1. The concentration of NOx after combustion by the burner of Figs. 1(a) and 1(b) was 103 ppm (6 vol% O 2 ), while the NOx concentration by the burner of Fig. 2 was 111 ppm (6 vol% O 2 ). An effect of decreasing a NOx formation amount by the present invention was acknowledged. Burner Structures NOx (ppm; 6%vol. O 2 -concentration basis) Unburnt Carbon in Ashes (wt%) Without Guide Plate (Fig. 2) 111 ppm 6.0 With Guide plate (Fig. 1(b)) 103 ppm 6.0 With Guide Plate (Fig. 1(c)) 107 ppm 6.0 In the burner of Fig. 1(c), the guide plate 30 is shifted axially to a more upstream side than the tip of the sleeve 20. The secondary air 52 is changed outwardly in its flow direction by the guide plate 30, however, the flow in the radially outward direction is prevented by the sleeve 20. Therefore, the secondary air jetted from the burner flows directed more to a direction of the central axis than in the case where the guide plate 30 is arranged at a more downstream side in the burner axis direction than the tip of the guide sleeve 20 as shown in Fig. 1(b). Therefore, as shown in Fig. 1(c), a recirculating zone 54 is apt to be formed in a downstream side of the guide sleeve 20. Flows are induced in the tertiary air 53 by the recirculating zone 54. Since the flows toward the central axis are apt to be induced in the tertiary air 53, mixing between the tertiary air and the pulverized coals is advanced in time and a NOx reducing zone is narrowed. As an example, using the burner as shown in Fig. 1(c) (the tip of the guide plate 30 is positioned at a place upstream of the tip of the guide sleeve 20 by 10 mm in the burner axis direction), a combustion test was conducted at a coal supply rate of 500 kg/h. The result is shown in the table 1. At this time, the NOx concentration at the combustion furnace outlet of the burner shown in Fig. 1(b) was 103 ppm (6% oxygen concentration basis), while the NOx concentration by the burner shown in Fig. 1(c) was 107 ppm (6% oxygen concentration basis) on the basis of the same unburnt carbon amount, and NOx formation was raised more than in the case where the guide plate 30 is positioned more downstream of the tip of the sleeve in the burner axis direction. Fig. 3 is a sectional view of a pulverized coal burner of the second embodiment. This embodiment is different from the first embodiment of Figs. 1(a) and 1(b) in that an angle 55 of the guide plate 30 and an angle 56 of the guide sleeve 20 each are made adjustable, and the other structure is the same as that of the first embodiment. According to this embodiment, by adjusting operation of the angle 55 of the guide plate 30 and the angle 56 of the guide sleeve 20, the angles of the guide plate 30 and guide sleeve 20 are adjusted depending on supply amounts of pulverized coal, primary air and combustion air, whereby it is possible to form a further suitable recirculating zone region and effectively decrease NOx and unburnt carbon, as compared with the first embodiment. By setting the angle 55 of the guide plate 30 to 60° to 90°, preferably 80° to 90°, it is possible to prevent formation of recirculating zone between secondary air and tertiary air, and to form a large recirculating zone at a downstream side of the guide plate 30. Fig. 4 is a sectional view of a nozzle end portion of a pulverized coal burner of a third embodiment. The third embodiment is characterized in that a gas jet nozzle 63 for jetting a gas toward the radially outward direction is provided within the secondary air nozzle 11 or in a region of the nozzle outlet as a means for deflecting a secondary air flow jetted from the secondary air nozzle 11 in the radially outward direction of the secondary air nozzle 11, as shown in Fig. 4. The other structure is approximately the same as that of the first embodiment. As the gas, air, combustion exhaust gas, inert gas such as nitrogen, steam, etc. can be used. According to the third embodiment, secondary air jetted from the secondary air nozzle 11 flows along the outer periphery by the momentum of the gas jetted from the gas jet nozzle 63. In order to make the momentum large, it is desirable that the flow velocity of gas jetted from the gas jet nozzle 63 is faster than the flow velocity of air jetted from the secondary air nozzle 11. With the burner of this structure, the recirculating zone formed downstream of the partition wall 28 expands, ignition of pulverized coals is promoted by the recirculating zone, and consumption of oxygen progresses, whereby it is possible to expand a region of a low oxygen concentration atmosphere within the flame and to effectively decrease NOx and unburnt carbon. In each of the pulverized coal burners of the above-mentioned embodiments, since the means for deflecting the secondary air jetted from the secondary air nozzle in the radially outward direction of the secondary air nozzle is provided, the secondary air flows in the radially outward direction, and a recirculating zone becomes unlikely to be formed downstream of the partition wall partitioning the secondary air nozzle and the tertiary air nozzle positioned at the outer periphery side of the secondary air nozzle. In the region of recirculating zone, pressure drop in a reverse direction to a jetting direction of air flow (adverse pressure gradient) is caused. Therefore, air flowing along the recirculating zone changes in flow direction by the adverse pressure gradient and air flowing outside the recirculating zone is apt to flow toward the primary air side. However, in the present invention, since the secondary air is jetted toward the radially outward direction, the primary air and secondary air are separated from each other and flow as they are separated. Therefore, the adverse pressure gradient becomes strong at the downstream side of the partition wall of the pulverized coal nozzle and the secondary air nozzle, and the recirculating zone formed in the region of the adverse pressure gradient expands. In the recirculating zone formed between the primary air and the secondary air, a high temperature gas stays, stabilizes the ignition of pulverized coal and flame. Expansion of the recirculating zone promotes ignition of pulverized coal by the high temperature gas. Since consumption of oxygen progresses by the ignition; a region of low oxygen concentration atmosphere within the flame expands, whereby it is possible to decrease an amount of NOx formation and an anount of unburnt carbon in the combustion ashes. Further, since the stability of ignition of pulverized coal and flame is improved, an effect that a distance necessary for combustion is shortened and the apparatus itself can be small-sized comes to be attained. Further, since flame becomes stable even in a case where the concentration of pulverized coal becomes small as at the time of low load operation, a possible range of combustion of only pulverized-coals by the pulverized coal burner without assistance of any other kinds of fuel is expanded. A forth embodiment of the invention shown in Fig. 5 is characterized in that a ring 30 having a plane perpendicular to directions of a primary air flow and secondary air flow is provided at the end portion of the partition wall 28 as a means for deflecting a secondary air flow jetted from the secondary air nozzle 11 in the radially outward direction of the secondary air nozzle 11 and forming a recirculating zone at a downstream side of the partition wall 28, as shown in Fig. 7. The other structure is approximately the same as that of the first embodiment. In Fig. 5, the ring 30 is formed of an inner ring 301 formed at the side of the pulverized coal nozzle 10 and an outer ring 302 formed in the side of the secondary air nozzle 11. The ring 30 causes turbulence in the primary air and secondary air by the ring 30, whereby the recirculating zone formed downstream of the ring 30 develops. In the forth embodiment, further, the positions of the inner ring 301 and outer ring 302 are separated from each other in the flow direction. As a result, in the recirculating zone formed downstream of the ring 30, slippage (or difference)in flow direction occurs between the pulverized coal flow side and the air flow side, and the recirculating zone 31 is formed so as to extend in the flow direction and so that gas is rolled back from the downstream side. In this manner, the recirculating zone region can be expanded, and the region of low oxygen concentration atmosphere within the flame also can be expanded, so that an amount of NOx formation and an amount of unburnt carbon in the combustion ashes can be effectively decreased. Further, it is possible to improve the ignition of pulverized coals and the stability of flame, and to shorten the distance necessary for combustion. Further, since the flame is stabilized even in a case where the concentration of pulverized coal decreases as at the time of combustion under a low load, a range in which it is possible to burn only pulverized coals by the pulverized coal burner is expanded. A fifth embodiment of the invention shown in Fig. 6 is characterized in that the ring 30 provided at the end portion of the partition wall 28 is provided with a large thickness portion 303 (10 mm thick, for example) at the secondary air nozzle inner wall side of the ring 30, as a means for deflecting a secondary air flow jetted from the secondary air nozzle 11 in the radially outward direction of the secondary air nozzle 11 and forming a recirculating zone at a downstream side of the partition wall 28, as shown in Fig. 6. The other structure is approximately the same as that of the forth embodiment. According to the fifth embodiment, the flow path of the secondary air nozzle 11 is narrowed by the large thickness portion 303, the secondary air is made faster in velocity when the air passes at the large thickness portion 303, the air impinges on the outer ring 302, and then it is jetted in the radially outward direction. As a result, it is possible to form expanded a recirculating zone 31, and expand the region of low oxygen concentration atmosphere within flame, so that an amount of NOx formation and unburnt carbon in the combustion ashes can be effectively decreased, and it is possible to improve the ignition of pulverized coal and the stability of flame. Further, in each of the forth and fifth embodiments, the outer ring 302 of the ring 30 is made in a uniform ring, however, the outer ring 302 can be made in notched shape or concave-convex shape at the peripheral portion of the end portion thereof, when necessary. By forming it in such a shape, thermal deformation of the ring can be damped, further, the turbulence downstream of the outer ring 302 increases, and the recirculating zone develops further. Further, the concave-convex notch can be formed in the inner ring 301 side in addition to the outer ring 302.

Claims: A pulverized coal burner comprising a pulverized coal nozzle (10) for jetting or spouting a mixture (13) of pulverized coal and primary air, a secondary air nozzle (11) concentrically arranged around an outer peripheral wall (28) of the pulverized coal nozzle (10), a tertiary air nozzle (12) concentrically arranged around an outer peripheral wall (21) of the secondary air nozzle (11), an expanded portion (20) at the downstream end of the outer peripheral wall (21) of the secondary air nozzle (11), flow shift means (30, 63, 302) for shifting secondary air (52) jetted from the secondary air nozzle (11) in a radially outward direction, and a flame stabilizing ring (18, 301) provided at the downstream end of the outer peripheral wall (28) of the pulverized coal nozzle (10) and upstream of the flow shift means (30, 63, 302), characterized in that the flow shift means comprises an outer ring (30, 302) positioned at the downstream end of the flow shift means, separated from the flame stabilizing ring (18, 301) in the flow direction, and having a deflection angle (55) in the range of 60° to 90° with respect to the central axis of the pulverized coal nozzle (10) for deflecting the secondary air (52) in the radially outward direction. Burner according to claim 1, characterized In that the deflection angle is in a range of 80° to 90°. Burner according to claim 1 or 2, characterized in that the outer ring (30, 302) is provided downstream of a flow path narrowing member (65a, 303) arranged in the secondary air nozzle (11). Burner according to claim 3, characterized in that the flow path narrowing member (65a, 303) is provided on the inner peripheral wall (28) of the secondary air nozzle (11). Burner according to one of the preceding claims, characterized in that the outer ring (302) is arranged with a deflection angle (55) which is greater than that (56) of the expanded portion (20) of the secondary air nozzle (11). Burner according to one of the preceding claims, characterized in that the distance between the downstream end of the expanded portion (20) formed on the outer peripheral wall (21) of the secondary air nozzle (11) and the further downstream end of the outer ring (30, 302) is in a range of 5 to 50 mm. Burner according to one of the preceding claims, characterized in that the outer ring (30, 302) is provided with slits or is notched or has a concave-convex shape on its peripheral end portion. Burner according to one of the preceding claims, characterized in that the flow shift means comprises a gas jet nozzle (63) for jetting a gas toward the secondary air (52) so that the secondary air flowing in the secondary air nozzle (11) is shifted in a radially outward direction. Burner according to one of the preceding claims, characterized in that the end portion (19) of the tertiary air nozzle (12) is outwardly expanded. Burner according to one of the preceding claims, characterized in that the tertiary air nozzle (12) is provided with a swirler (15) for swirling and jetting the tertiary air (53).

Inventors: JIMBO TADASHI, KIYAMA KENJI, KOBAYASHI HIRONOBU, KURAMASHI KOUJI, MORITA SHIGEKI, NOMURA SHIN-ICHIRO, OKAZAKI HIROFUMI, SHIMOGORI MIKI, TSUMURA TOSHIKAZU, JIMBO, TADASHI, KIYAMA, KENJI, KOBAYASHI, HIRONOBU, KURAMASHI, KOUJI, MORITA, SHIGEKI, NOMURA, SHIN-ICHIRO, OKAZAKI, HIROFUMI, SHIMOGORI, MIKI, TSUMURA, TOSHIKAZU
Applicants: BABCOCK HITACHI KK, HITACHI LTD, BABCOCK HITACHI K.K., HITACHI, LTD.
Application year: 1998
Main classification: F23D1
IPC codes: F23D1, F23C99