Method for combusting pulverized coal

Abstract: An object is to provide a pulverized coal combustion burner and a combustion method by the burner in which occurrence of NOx is small and unburnt carbons in combustion ashes are small. In a combustion method by a pulverized coal combustion burner which is provided with a pulverized coal nozzle (10) for injecting a mixture (13) of pulverized coal and air and an air nozzle (11) for injecting air (14), a combustion flame formed by the above-mentioned pulverized coal combustion burner forms a zone (17) of gas phase air ratio of 1 or less at a radially central portion of the flame and a zone (16) of gas phase air ratio of more than 1 outside the zone, in the vicinity of the burner jet port, and a zone of gas phase air ratio of 1 or less in the downstream side of the flame.

Description: The invention relates to a method for combusting pulverized coal and air in a combustion flame by using a combustion furnace with two combustion stages, wherein the first combustion stage comprises a burner for injecting a fluid mixture of pulverized coal and air into the furnace for effecting an incomplete combustion of the coal, and the second combustion stage comprises an air supply means arranged at a downstream side of the burner of the first stage for supplying the remainder of air necessary to complete combustion thereby forming a zone of complete combustion. In such pulverized coal combustion burners, occurrence of NOx during combustion is a large problem. Particularly, coal has a larger content of nitrogen, compared with gaseous fuel and liquid fuel. Therefore, it is more difficult to decrease NOx produced by combustion of pulverized coals than in the case of combustion of gaseous fuel or liquid fuel. NOx produced by combustion of pulverized coal 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 combustion methods have been studied. As one of the burning methods, there is a method of forming a low oxygen concentration zone within flame and utilizing reducing reaction of NOx which becomes active when the oxygen concentration is low. For example, JP A 1-305206, JP A 3-211304, JP A 9-170714, JP A 3-110308, etc. disclose a method of producing flame (reducing 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. That is, in those methods, a low oxygen concentration zone is formed inside the flame, reducing reactions of NOx are progressed in the reducing flame zone, and an amount of NOx occurred within flame is suppressed to be small. Further, JP A 3-211304, JP A 9-170714 and JP A 3-110308 disclose formation of recirculating flows at a downstream side of the tip of a pulverized coal nozzle by providing a flame stabilizing ring or obstacle at the tip of the pulverized coal nozzle. That is, since a high temperature gas stays inside the recirculating flows, ignition of pulverized coals progresses and the stability of flame can be raised. In general, since the ignitability of coal is not better than the other fuel, it is difficult to raise the ignitability of the coal even if the above-mentioned various methods are adopted. Therefore, in combustion of coal, consumption of oxygen does not progress and a reducing zone is hard to be formed. In order to form a reducing zone, it is necessary to suppress mixing of fuel and air jetted from an air nozzle in the vicinity of the pulverized coal nozzle. Therefore, hitherto, in general, the mixing with fuel is suppressed by supplying the air to be supplied from the air nozzle in swirling flow. However, when strong swirling is imparted to air, mixing of the air and fuel does not progress even at a downstream portion (more than three times as large as the diameter of a burner throat) separated from the burner due to centrifugal force, and it is hard to effect complete combustion. Therefore, in this kind of pulverized coal combustion, there is the problem that NOx is easy to occur and unburnt carbons is left in combustion ashes of pulverized coal. With a two-stage combustion according to the generic kind as described in the first section of the specification and disclosed in US-A-4 545 307 and WO 95/13502 in relation to prior art, an air-deficient zone is formed in the burner zone of the combustion furnace, and an amount of air corresponding to this deficient amount of air is supplied downstream of the burner zone to effect complete combustion, whereby combustion over the whole of the combustion furnace is improved thereby reducing the amount of NO x discharged. However, with such a two-stage combustion half-burned coal particles, called char, are formed in the air-deficient zone of the burner such that a large free space is required in the furnace for complete combustion of the char with the additional air fed downstream of the burners. Although the two-stage combustion is rather efficient in lowering NO x emissions of the combustion, it has still certain limitations such as unburned carbon and unstable flame conditions. In order to form the air-deficient zone very close to the tip, according to US-A-4 545 307 and WO 95/13502 an improved low-NO x -burner is provided allowing to eliminate the two-stage combustion and use only a one-stage combustion burner. Such an one-stage combustion burner as disclosed in US-A-4 545 307 comprises a central pipe inserted into a burner throat on the lateral wall of a combustion furnace and having an injection port provided on its end facing to the furnace space with a bluff body in the form of an annular dish. The inner circumference of the dish extends partly into the injection port, while the dish itself has a cross section of a quarter circle extending into an air nozzle for secondary air surrounding the central pipe. The end of the air nozzle facing to the furnace space is an outwardly fiaring truncated cone with a cone angle of 30 to 50°. This air nozzle of secondary air is surrounded by an air nozzle for ternary air surrounded by the furnace wall. The mixture of pulverized coal and air is jetted from the pipe through the restriction of the bluff body into the furnace with an air ratio of 1 or less forming a reducing flame of high temperature in which the nitrogen compounds of the coal are decomposed into volatile nitrogen compounds and nitrogen compounds contained in the char. The secondary air exiting out of the secondary air nozzle between the bluff body and the truncated cone with a whirling force forms an oxidizing flame in form of circulating eddy surrounding and enclosing the reducing flame as a sandwich and oxidizing volatile nitrogen from the high temperature reducing flame and nitrogen from the air to general NO. The reducing flame is followed by a reducing denitration zone, which does not expand and in which the NO formed in the oxidizing flame is reacted with reducing intermediate products of the high temperature reducing flame to form N 2 . The ternary air ejected with a powerful whirling force from the ternary air nozzle between the outside of the truncated cone and the furnace wall is fed downstream of the denitration zone, where N-containing char and unburned matters are completely burnt. With the burner of the described structure the reducing flame is completely surrounded by oxidizing air, until the coal is completely burned with a tow NO x . WO 95/13502 describes a very similar burner used for the same purpose, i.e. to obtain complete combustion in one stage. With this burner the deflection angle of the guide sleeves for the nozzles of the secondary and ternary air comprise 15 to 25° in relation to the central axis of the coal pipe. The swirl number of the secondary air stream is 0,5 to 1,0 and the speed of the stream is 2 to 3 times greater than the speed with which the fluid mixture of pulverized coal and air is jetted. With the burner of this structure a central reducing flame zone is produced surrounded by a secondary recirculation zone which in turn is surrounded by a vigorous turbulent combustion zone around which the ternary air flows to an end zone of complete combustion near the main vortex generated by the tangential injection of the burner with regard to the furnace. EP 0 445 938 describes a two-stage burner for pulverized coal having a coal duct for pulverized coal and primary combustion air and a secondary combustion air duct such that the coal and primary air and secondary air mix outside the outlet nozzles of the duct in a mixing zone at which combustion occurs. An annular tertiary air passage is formed around the secondary air passage and is ending in an outlet nozzle. The combustion air comprises the igniting secondary air and tertiary air for the complete combustion. This is because the excess fuel combustion region is formed at the central portion of the flame to promote the reduction of the NO x by the secondary air and the mixture. In order to facilitate the formation of the excess fuel region, the mixing between the secondary and tertiary air flow at the exit of the burner is suppressed by means of a gap interposed between the secondary and the tertiary air passages. For establishing in the flame a large circulating flow at a high temperature secondary air and tertiary air are injected by swirling. Combustion of the unburned content of the burner combustion zone is completed by the after air. Flame configurations for two-stage combustion of the prior art are discussed in relation to Fig. 2 as well as 5 and 6 of the description of the figures. It is the object of the invention to provide a method of the generic kind referring to two combustion stages allowing a combustion with high efficiency, very low NO x and nearly no unburned carbons in the combustion ashes. This object is obtained with a method of the generic kind in that in the first combustion stage the fluid mixture of the pulverized coal and air is jetted in a straight stream from a pulverized coal nozzle of the burner such as to form an ignition zone followed by a first zone with a gas phase ratio of 1 or less at a relatively central portion of the flame in a flame front stage portion, an air stream is jetted from an air nozzle enclosing the pulverized coal nozzle on two opposite sides or concentrically, and having a guide plate or a guide plate and a flame stabilizing ring on its downstream end, which air stream is jetted without being swirled or in a week swirling stream of swirl number of 0,8 or less in a direction separating from the pulverized coal nozzle at an angel of 30° to 50° to the central axis of the pulverized coal nozzle by the correspondingly inclined guide plate with a speed that is two times to three times larger then the speed with which the fluid mixture of pulverized coal and air is jetted from the pulverized coal nozzle, thereby forming a second zone with a gas phase air ration of larger then 1 outside said first zone in the flame front stage portion, and mixing the pulverized coal flowing at a central portion of the flame from the flame front stage portion to a downstream flame rear stage portion with the air flowing from the second zone towards the center of the flame at the flame rear stage portion, thereby forming a third zone with a gas phase air ration of 1 or less in the flame rear stage portion. With the method according to the invention the flame rearstage portion is separated from the burner nozzle outlet by a distance of three times as long as the burner throat diameter or more. The mentioned gas phase air ratio is the ratio between a real air quantity and an air quantity necessary for effecting complete combustion of gaseous components emitted from the pulverized coal. As stated, the first stage of the two combustion stages comprises the front stage portion and a flame rearstage portion downstream of the flame frontstage portion. In the forntstage portion, the flame is created in such a way that it has a reducing core with low oxygen, low NO x , but unburned coal partides. This core is surrounded by a oxidizing flame with high oxygen, low NO x and low coal concentration. In the flame rearstage portion the core flame part and the outside flame part are mixed by the flow of the surrounding flame part towards the center and by radially spreading the reducing flame of the core within which the majority of the pulverized coal has passed. These measures lead to a flame in the rearstage portion having a low, but not very low oxygen and low NO x . In the second stage air is supplied downstream from the first stage for coming to a complete combustion, the combustion gases of which have surprisingly low NO x and a very high combustion efficiency, which means that there is nearly no unburned coal within the coal ashes. As stated above, the combustion flame formed by the above-mentioned pulverized coal combustion burner has, in the vicinity of the jet port of the burner, a zone of a gas phase air ratio of 1 or less formed at a radially central portion of the flame and a zone of a gas phase air ratio of more than 1 formed outside the zone, so that oxygen is consumed by combustion reaction in the central portion of the pulverized coal flame and reducing flame of low oxygen concentration is formed. Since the concentration of fuel is low at the radial outside of the reducing flame, consumption of oxygen does not progress and oxidization flame of high oxygen concentration is formed. Further, since combustion is effected so that a uniform air ratio zone of a gas phase air ratio of 1 or less and a variation range of the gas phase air ratio of 0.2 or less is formed inside the flame at a downstream side, air jetted from the air nozzle and pulverized coal flowing at a central portion of the flame are mixed with each other at a flame rear stage portion. Since oxygen consumption has progressed in the flame front stage portion of reducing flame and oxidizing flame, the reducing flame of a low oxygen concentration spreads radially in the flame rear stage portion, therefore, the majority of the pulverized coal passes in the reducing zone, so that NOx occurred by the oxidizing flame in the flame front stage portion also is reduced, further, an air distribution becomes uniform, a zone of an extremely low gas phase air ratio is not formed, therefore, combustion reaction progresses, and it is possible to improve the combustion efficiency and reduce unburnt carbons in combustion ashes. Embodiments of the invention and of the prior art are now described referring to the drawings, in which Fig. 1 is a vertical sectional side view of an embodiment of a pulverized coal combustion burner of the present invention; Fig. 2 is a vertical sectional side view of a conventional pulverized coal combustion burner; Fig. 3 is a diagram showing examination results by the pulverized coal combustion burner of the present invention and the conventional pulverized coal combustion burner; Fig. 4 is a vertical sectional side view of another embodiment of a pulverized coal combustion burner of the present invention; Fig. 5 is a vertical sectional side view of a conventional pulverized coal combustion burner; Fig. 6 is a vertical sectional side view of a conventional pulverized coal combustion burner; Fig. 7 is an enlarged side view of a main part of another embodiment of a pulverized coal combustion burner of the present invention; Fig. 8 is an enlarged side view of a main part of another embodiment of a pulverized coal combustion burner of the present invention; Fig. 9 is a vertical sectional side view of another embodiment of a pulverized coal combustion burner of the present invention; Fig. 10 is a front view of the pulverized coal combustion burner of Fig. 9; Fig. 11 is a front view of another embodiment of a pulverized coal combustion burner of the present invention; Figs. 12A and 12B each are a diagram of a gas phase air ratio distribution; and Figs 13A and 13B each are a vertical sectional side view of a conventional pulverized coal combustion burner. EMBODIMENT 1 A reference number 10 denotes a pulverized coal nozzle for pneumatically transferring pulverized coal, the upstream side of which is not shown but connected to a transfer conduit. A reference number 11 is an air nozzle provided outside the pulverized coal nozzle 10, a reference number 12 denotes a furnace space for combustion of pulverized coal and air jetted from the pulverized coal combustion burner. An arrow 13 shows a stream of pulverized coal jetted from the pulverized coal nozzle 10 and an arrow 14 shows a stream of air jetted from the air nozzle 11. A reference number 99 denotes an oil gun provided for assisting combustion. In this first embodiment, a method (two stage combustion method) is taken wherein a quantity of air supplied from the burner is made a little smaller than a quantity of air necessary for effecting complete combustion of pulverized coal and the remainder of the necessary air is supplied at a downstream side. A reference number 19 denotes an air supply means therefor, that is, an air nozzle for second stage combustion, and a reference number 20 denotes an air stream supplied therefrom. A reference number 18 denotes a combustion zone of second stage combustion air and pulverized coal supplied from the burner. In this embodiment, air jetted from the air nozzle is jetted out from the burner, and then flows separately from the center of flame at a front stage portion of the flame and then flows toward the center of the flame at a rear stage portion of the flame (at a separate position from the burner nozzle outlet by more than a distance of three times as long as a burner throat diameter). Therefore, mixing of air jetted from the air nozzle and pulverized coal flowing at the center of the flame is suppressed in the flame front stage portion, and at a downstream side of an ignition zone 15, oxygen is consumed at the central portion of pulverized coal flame by combustion reaction and reducing flame 17 of low oxygen concentration is formed. Further, consumption of oxygen does not progress because of low fuel concentration at a radially outside of the reducing flame 17, so that oxidizing flame 16 of high oxygen concentration is formed. Further, mixing of air jetted from the air nozzle and pulverized coal flowing at the central portion of the flame in the rear stage portion of the flame spreads radially the reducing flame of low oxygen concentration in the flame rear stage portion because oxygen consumption has progressed in the flame front stage portion composed of the reducing flame and the oxidizing flame. In the present invention, a radial direction of flame means a direction crossing an arrow 13 at right angles, which arrow shows a direction of a pulverized coal flow. It is a flame expansion direction in a radial direction of the burner. In this manner, in order to cause a flow of the air jetted from the air nozzle to separate from the central axis in the flame front stage portion and then mix with the pulverized coal flow flowing at the center at the flame rear stage portion, the air is jetted in a direction separate from the pulverized coal nozzle at an angle of 30° or larger and 50° or smaller to the central axis of the pulverized coal nozzle so as to be in a straight flow or in a weak swirling flow of a swirl number of 0.8 or less. Here, the swirl number can be obtained from the following equation: Swirl number = (momentum in a swirling direction) ÷ (axial momentum x throat outer diameter). In comparison with the first embodiment shown in Fig. 1, in a conventional pulverized coal burner shown in Fig. 2, air is jetted from an air nozzle 11 in a swirling flow swirled by strong swirling force of swirl number of 0.8 or more, so that the air after being jetted flows separately from the center and it is not mixed with a central portion even in the flame rear stage portion. Therefore, it has been separated into reducing flame 17 at the flame central portion and oxidizing flame 16 at the outside thereof, even in the flame rear stage portion. In Fig. 3, there is shown an examination result of a relation between a ratio (abscissa) of an air quantity and a pulverized coal quantity and the concentration (ordinate) of NOx at the furnace outlet. A curve P shows the performance of the conventional pulverized coal burner and a curve Q the performance of the pulverized coal combustion burner of the present embodiment shown in Fig. 1. As is apparent from the diagram, it will be noted that the pulverized coal combustion burner of the present invention has a relatively low occurrence ratio of NOx compared with the conventional burner irrespective of largeness of the air ratio. In the conventional burner by which the oxidizing flame 16 and reducing flame 17 flow separately form each other, reduction reaction of NOx progresses in the reducing flame at the flame central portion and NOx emission is small. However, since NOx occurs in the oxidizing flame spreading radially outward of the reducing flame, a quantity of NOx emission from the whole flame becomes large. Further, in the reducing flame, in a case where a gas phase air ratio (a ratio between a real air quantity and an air quantity necessary for effecting complete combustion of gaseous components emitted from pulverized coal) is too low, for example, 0.6, combustion reaction is delayed, so that unburnt substances increases, and there is a fear that it causes a decrease in combustion efficiency and becomes a bar to effective use of combustion ashes due to an increase of unburnt carbons in combustion ashes. As in the first embodiment, in the case of a method (two stage combustion method) in which an air quantity supplied from the burner is made smaller than that necessary for complete combustion of pulverized coal and the remainder of the necessary air is supplied downstream, since combustion of pulverized coal does not progress, NOx occurring at the portion mixing with air for second stage combustion increases. On the contrary, in the previous embodiment of the present invention, reducing flame spreads in a radial direction in the flame rear stage portion, therefore, the majority of pulverized coal passes in the reducing zone, so that NOx produced in the oxidizing flame of the flame front stage portion is also reduced. Further, as compared with the conventional burner, since an air distribution becomes uniform, a zone of a extremely low gas phase air ratio is not formed. Therefore, combustion reaction progresses more than the conventional burner example shown in Fig. 2, and the combustion efficiency is improved and unburnt carbons in combustion ashes are reduced. Further, since the combustion reaction of pulverized coal has progressed before mixing with air for second stage combustion, NOx occurring by mixing with the air for second stage combustion becomes small. EMBODIMENT 2 In Fig. 4, an air nozzle is separated into two, a secondary air nozzle 32 and a tertiary air nozzle 33. Here, the secondary air nozzle 32 serves to provide a spacing between the pulverized coal nozzle 10 and the tertiary air nozzle 33. In the case where the pulverized coal nozzle and the tertiary air nozzle are spaced from each other, the burner is damaged by burning and can not be used when secondary air is not flowed from the secondary air nozzle 32. Therefore, secondary air is flowed from the secondary air nozzle 32 as a cooling gas. A quantity of the secondary air is sufficient to be 1/3 the quantity of tertiary air. In order to flow secondary air along a guide plate 21 described later and distance it from the pulverized coal nozzle 10, some device is taken on the shape of a flame stabilizing ring 31. That is, a tip portion of the flame stabilizing ring 31 extends outward in the radial direction. Further, a venturi 24 and a spindle-shaped obstacle 25 are provided at a central portion of the pulverized coal nozzle 10. Since pulverized coal flows toward the outer periphery along the obstacle 25, the concentration of pulverized coal is raised in the vicinity of the flame stabilizing ring 31, whereby the pulverized coal is ignited earlier in the vicinity of the flame stabilizing ring 31 and a zone of reducing flame 17 expands. Further, the present embodiment shown in Fig. 4 differs from the conventional burner and is provided with the guide plate 21 on the wall, at the pulverized nozzle side, of the outlet of the tertiary air nozzle 33. By this guide plate 21, the direction of tertiary air flowing in parallel with the central axis of the pulverized coal nozzle at the throat portion 22 is bent in a radially outer direction. The inclination angle 34 of the guide plate 21 to the central axis of the nozzle is set 30° to 50°. Therefore, the tertiary air is jetted from the burner at an angle of 30° to 50° to the central axis of the pulverized coal nozzle. After the tertiary air is jetted from the tertiary air nozzle, the air flows separately from the center of flame in the flame front portion and then flows toward the flame center in the flame rear stage portion (in the portion separate from the burner nozzle outlet by a distance of three times as long as the burner throat diameter), as shown by an arrow 14. In this manner, in the flame front stage portion, without progress of mixing of the tertiary air jetted from the tertiary air nozzle and the pulverized coal flowing at the center of the flame, oxygen is consumed by combustion reaction at the central portion of the pulverized coal flame and reducing flame 17 of the low oxygen concentration is formed, at a downstream side of an ignition zone 15. Further, since oxygen consumption does not progress because of low fuel concentration at a radially outer side of the reducing flame 17, oxidizing flame 16 of high oxygen concentration is formed. Further, tertiary air jetted from the tertiary air nozzle 33 and pulverized coal flowing at the central portion of flame are mixed in the flame rear stage portion. At this time, since oxygen consumption has progressed in the flame front stage portion composed of the reducing flame 17 and oxidizing flame 16, reducing flame of low oxygen concentration spreads in the radial direction in the flame rear stage portion. Since reducing flame spread radially in the flame in the flame rear stage portion, the majority of pulverized coal passes in the reducing zone, whereby NOx occurred by oxidizing flame of the flame front stage is also reduced. Further, as compared with the conventional burner, a distribution of air becomes uniform, so that a zone of extremely low gas phase air ratio is not formed. Therefore, combustion reaction progresses and improvement of combustion efficiency and reduction of unburnt carbons in combustion ashes are carried out, more than in the conventional burner shown in Fig. 5. Further, since combustion reaction of pulverized coal has progressed before mixing with second stage combustion air, NOx occurring by mixing with the second stage combustion air becomes small. In this manner, in order to flow tertiary air from the tertiary air nozzle to separate from the central axis in the flame front stage portion and mix it with pulverized coal flowing at the center in the flame rear stage portion, it is desirable to jet the above-mentioned tertiary air at an angle of 30° to 50° to the central axis of the pulverized coal nozzle and supply the tertiary air in a straight stream or in a weak swirling stream. Thereby, since centrifugal force of the tertiary air is small, mixing with pulverized coal is promoted in the flame rear stage portion. Further, it is desirable to jet the tertiary air at a higher speed than the pulverized coal flow jetted from the pulverized coal nozzle. At this time, the momentum of the tertiary air flow becomes larger than that of the pulverized coal flow, so that it becomes difficult for the jetting direction of tertiary air to be influenced by the pulverized coal flow. Therefore, it is suppressed to mix tertiary air and pulverized coal in the vicinity of the burner. Further, as in the second embodiment shown in Fig. 4, the guide plate 21 is desirable to extend radially outward more than an extension line of the outer peripheral wall of the throat portion 22 which has a flow path parallel with the central axis of the pulverized coal nozzle. Tertiary air flows in parallel with a pulverized coal flow and a jetting direction thereof is changed by the guide plate 21 in the throat portion. However, in the case where the guide plate is short as shown in Fig. 6, a flow the direction of which is not changed by the guide plate as shown by an arrow 34 is formed, whereby the flow becomes easy to mix with the pulverized coal flow at a position close to the burner. With this construction, since the tertiary air and pulverized coal are mixed at an ignition time, a flame temperature is lowered and the ignition is delayed, whereby a reducing zone becomes difficult to be formed, so that the concentration of NOx at the furnace outlet increases. Further, in the case where air nozzle is separated radially into a plurality of air nozzles as in the present embodiment, since it is possible that an injection ratio of air is changed by the respective air nozzles, it is possible that an emission quantity of NOx and unburnt carbons in combustion ashes can be made suitable by adjusting a mixing position and mixing ratio of air and pulverized coal. EMBODIMENT 3 In the nozzle portion of the pulverized coal burner of Fig. 7, the guide plate 21 is provided on the wall of an outlet of a tertiary air nozzle 33 on the pulverized nozzle side. A flow path at the tertiary air nozzle side of the guide plate is formed to have a curved surface for the tertiary air flow so that the flow path changes smoothly. In Fig. 8, when a flow course of the tertiary air flowing in the tertiary air nozzle is bent by the guide plate 21, a stay zone 35 in which the flow is delayed is formed at a connecting portion between the throat portion and the guide plate. The guide plate 21 is raised in temperature by radiation from the flame inside the furnace. The guide plate 21 is cooled by convection heat transfer of the air flowing there and heat conduction in the material constructing the guide plate. When the stay zone 35 is formed, the convection heat transfer in the stay zone decreases, so that the temperature of the guide plate rises and the possibility of burning damage increases. The stay zone is not formed by smoothing the flow course as shown in Fig. 7. At this time, the guide plate 21 can be cooled by convection heat transfer of the air flow. Further, since the structural member of the connecting portion between the guide plate and the throat portion becomes thick, heat conduction in the structural member becomes more, whereby the temperature of the guide plate is suppressed to rise and the durability thereof can be raised. EMBODIMENT 4 In Fig. 9, a reference number 10 denotes a pulverized coal burner for pneumatically transferring pulverized coal, the upstream side of which is not shown but connected to a transfer conduit. A reference number 11 denotes an air nozzle provided so as to surround the pulverized coal burner. The pulverized coal nozzle 10 is divided into a plurality of nozzles and the air nozzle can be also divided into a plurality of air nozzles. Further, a reference number 12 denotes a furnace space for combustion of pulverized coal and air jetted from the burner. An arrow 13 denotes a stream of pulverized coal jetted from the pulverized coal nozzle and an arrow 14 denotes a stream of air jetted from the air nozzle. Further, in this embodiment, a method (two stage combustion method) is used in which a quantity of air jetted from the burner is made slightly smaller than the quantity of air necessary for complete combustion of pulverized coal, and the remainder of the necessary air is supplied downstream. A reference number 19 denotes an air nozzle for second combustion air, and an arrow 20 denotes a flow of the second stage combustion air. A reference number 18 denotes a combustion zone of second combustion air and pulverized coal supplied from the burner. In the forth embodiment, the air jetted from the air nozzle flows separately from the center in the flame front stage portion and then flows toward the center of the flame in the flame rear stage portion (at a position separated from the burner outlet by distance of three times as long as the burner throat diameter), after being jetted from the burner. Therefore, mixing of air jetted from the air nozzle and the pulverized coal flowing at the center of flame is suppressed in the flame front stage portion, and in a downstream side of an ignition zone 15, oxygen is consumed by combustion reaction at the central portion of pulverized coal flame and reducing flame 17 of low oxygen concentration is formed. Further, since oxygen consumption does not progress in a radially outer side of the reducing flame 17 because of low oxygen concentration, oxidizing flame 16 of high oxygen concentration is formed. Further, in the flame rear stage portion, when air jetted from the air nozzle and pulverized coal flowing at the central portion of flame are mixed, since oxygen consumption has been progressed in the flame front stage portion composed of the reducing flame and the oxidizing flame, the reducing flame of low oxygen concentration spreads in the radial direction in the flame rear stage portion. In this manner, in order to flow air jetted from the air nozzle separately from the central axis in the flame front stage portion and mix it with pulverized coal flowing at the center in the flame rear stage portion, the above-mentioned air is jetted at an angle of more than 30° and less than 50° to the central axis of the pulverized coal nozzle. In the embodiment shown in Fig. 9, the reducing flame spreading radially in the flame rear stage portion spreads inside the flame. Therefore, since the majority of pulverized coal passes in the reducing zone, NOx occurred by the oxidizing flame of the flame front stage is also reduced. Further, a distribution of air becomes uniform as compared with the conventional burner, so that a zone of an extremely low gas phase air ratio is not formed. Therefore, combustion reaction progresses, improvement of combustion efficiency and reduction of unburnt carbons in combustion ashes are brought about. Further, since combustion reaction of pulverized coal has progressed before mixing with second stage combustion air, NOx occurring by mixing with the second stage combustion air becomes small. EMBODIMENT 5 As shown in Fig. 11 the air nozzle of the fifth embodiment is composed of a plurality of the air nozzles 11 and provided around the pulverized coal nozzle 10 so as to surround the nozzle 10. The outlet to the furnace of each air nozzle 11 is inclined at an angle of more than 30° and less than 50° to the central axis of the pulverized coal nozzle, and air is jetted from the air nozzles 11 at an angle of more than 30° and less than 50° to the central axis of the pulverized coal nozzle. In the fifth embodiment, the air jetted from the air nozzles 11 flows separately from the center in the flame front stage portion and then flows toward the center of the flame in the flame rear stage portion (at a position separated from the burner outlet by distance of three times as long as the burner throat diameter), as shown by an arrow 14, after being jetted from the burner. Therefore, mixing of air jetted from the air nozzles 11 and the pulverized coal flowing at the center of flame is suppressed in the flame front stage portion, and in a downstream side of an ignition zone 15, oxygen is consumed by combustion reaction at the central portion of pulverized coal flame and reducing flame 17 of low oxygen concentration is formed. Further, since oxygen consumption does not progress in a radially outer side of the reducing flame 17 because of low oxygen concentration, oxidizing flame 16 of high oxygen concentration is formed. Further, in the flame rear stage portion, when the air jetted from the air nozzles and the pulverized coal flowing at the central portion of flame are mixed, since oxygen consumption has been progressed in the flame front stage portion composed of the reducing flame and the oxidizing flame, the reducing flame of low oxygen concentration spreads in the radial direction in the flame rear stage portion. In this manner, in order to flow air jetted from the air nozzles separately from the central axis in the flame front stage portion and mix it with pulverized coal flowing at the center in the flame rear stage portion, the above-mentioned air is jetted at an angle of more than 30° and less than 50° to the central axis of the pulverized coal nozzle. Therefore, since the majority of pulverized coal passes in the reducing zone, NOx occurred by the oxidizing flame of the flame front stage is also reduced. Further, a distribution of air becomes uniform as compared with the case where air is jetted from an air nozzle 11 at an angle of less than 30° to the central axis of the pulverized coal nozzle, so that a zone of an extremely low gas phase air ratio is not formed. Therefore, combustion reaction progresses, improvement of combustion efficiency and reduction of unburnt carbons in combustion ashes are brought about. Further, since combustion reaction of pulverized coal has progressed before mixing with second stage combustion air, NOx occurring by mixing with the second stage combustion air becomes small. EMBODIMENT 6 Figs. 12A and 12B show comparison of gas distribution inside the pulverized coal furnace by conventional burner and the embodiment of the present invention. Here, gas phase air ratios are shown as gas concentration distribution. As mentioned above, the gas phase air ratio is a ratio of a real air quantity and a quantity of air necessary for complete combustion of components discharged as gas from pulverized coal. A zone of gas phase air ratio of 1 or less represents reducing flame of low oxygen concentration, and a zone of 1 or more represents oxidizing flame. The gas phase air ratio is calculated by obtaining each element amount from the concentration of gas components and from oxygen atomic numbers necessary for complete combustion of the each element and oxygen atomic numbers really contained in the gas components. The lower side of each of Figs. 12A and 12B, the upper side thereof and the right end thereof represent the central axis, the furnace wall and the furnace outlet, respectively. The pulverized coal burner is mounted on the left end of the furnace in Figs. 12A, 12B, and an air injection inlet for second combustion air is provided on a furnace side wall downstream by about 6 m from the pulverized coal burner. Fig. 12A is a distribution of gas phase air ratios in the case where the conventional pulverized coal burner shown in Fig. 13A is used, and Fig. 12B is the distribution of gas phase air ratios in the case where the pulverized coal burner of the present invention shown in Fig. 13B is used. In the conventional pulverized coal burner shown in Fig. 12A and Fig. 13A, strong swirling is imparted to the air jetted from the air nozzle of the burner, and the air flows closely to the side wall separate from the central axis, as shown by an arrow of Fig. 12A. Therefore, gas phase air ratios in the zone from the burner to a position 6 m separate from the burner are separated into oxidizing flame of more than 1 in the vicinity of the side wall and reducing flame of less than 1 near to the central axis. On the contrary, in the pulverized coal burner of the present embodiment shown in Fig. 12B and Fig. 13B, the air jetted from the air nozzle of the burner has weak swirl imparted as compared with the conventional burner, and it is jetted in a direction separating from the pulverized coal nozzle at an angle of more than 30° and less than 50° to the central axis of the pulverized coal nozzle. Therefore, as shown by an arrow in Fig. Fig. 12B, air jetted from the air nozzle flows separately from the central axis near the burner (in the zone from the burner to a position distanced by 3 m from the burner) and flows toward the central axis at a downstream side of the zone. Therefore, a reducing flame zone of a gas phase air ratio of 1 or less spreads radially inside the furnace at a flame downstream side, that is, in the zone before the injection inlet for second stage combustion air. Therefore, since the majority of pulverized coal passes in the reducing zone, NOx occurred by the oxidizing flame of the flame front stage is also reduced. Further, a distribution of air becomes uniform as compared with the conventional burner as shown in Fig. 12A, so that a zone of an extremely low gas phase air ratio is not formed. Therefore, combustion reaction progresses, improvement of combustion efficiency and reduction of unburnt carbons in combustion ashes are brought about. Further, since combustion reaction of pulverized coal has progressed before mixing with second stage combustion air, NOx occurring by mixing with the second stage combustion air becomes small.

Claims: Method for combusting pulverized coal and air in a combustion flame by using a combustion furnace (12) with two combustion stages, wherein in the first combustion stage a fluid mixture of the pulverized coal and air is jetted for effecting an incomplete combustion of the coal, and in the second combustion stage the remainder of air (20) necessary to complete the combustion is supplied downstream the first stage, thereby forming a zone (18) of complete combustion, wherein in the first combustion stage the fluid mixture of pulverized coal and air is jetted in a straight stream (13) to form an ignition zone (15) followed by a first zone with a gas phase air ratio of 1 or less at a radially central portion of the flame in a flame front stage portion, and wherein an air stream (14) is jetted on two opposite sides or concentrically with respect to the flow of fluid mixture of pulverized coal and air with a speed that is two to three times larger than the speed with which the fluid mixture of pulverized coal and air is jetted from the pulverized coal nozzle (10), thereby forming a second zone (16) with a gas phase air ratio of larger than 1 outside said first zone in the flame front stage portion, characterized in that the air stream (14) is jetted without being swirled or in a weak swirling stream of swirl number of 0,8 or less, and in a direction having an angle of 30° to 50° to the central axis of the flow of fluid mixture of pulverized coal and air, and in that the pulverized coal flowing at a central portion of the flame from the flame front stage portion to a downstream flame rear stage portion is mixed with the air flowing from the second zone (16) towards the center of the flame at the flame rear stage portion, thereby forming a third zone (17) with a gas phase air ratio of 1 or less in the flame rear stage portion, wherein the third zone (17) is spread in the radial direction in the flame rear stage portion and the air in the third zone (17) is uniformly distributed. Method according to claim 1, characterized in that a distance of more than three times the length of the burner throat diameter is used to separate the flame rear stage portion from the burner outlet. Method according to claim 1 or 2, characterized in that the range of the gas phase air ratio in the third zone (17) is varied to be 0.2 or less.

Inventors: JIMBO TADASHI, KIYAMA KENJI, KOBAYASHI HIRONOBU, KURAMASHI KOUJI, MORITA SHIGEKI, NOMURA SHINICHIRO, OKAZAKI HIROFUMI, SHIMOGORI MIKI, TSUMURA TOSHIKAZU, JIMBO, TADASHI, KIYAMA, KENJI, KOBAYASHI, HIRONOBU, KURAMASHI, KOUJI, MORITA, SHIGEKI, NOMURA, SHINICHIRO, OKAZAKI, HIROFUMI, SHIMOGORI, MIKI, TSUMURA, TOSHIKAZU
Applicants: BABCOCK HITACHI KK, HITACHI LTD, BABCOCK HITACHI K.K., HITACHI, LTD.
Application year: 1999
Main classification: F23D1
IPC codes: F23C6, F23D1, F23C7, F23N1, F23K3