Burner apparatus for pulverized coal

Abstract: A burner comprises a pulverized coal supply pipe (73), a starter oil burner (71) extending within the pulverized coal supply pipe (73) to define therebetween a tubular passage through which a mixture of combustion air and pulverized coal passes into a furnace (11), a flame holder provided at an outer periphery of one end of the pulverized coal supply pipe (73) facing to the furnace (11), a cylindrical member (74) disposed in the tubular passage for dividing a part thereof into two coaxial passage parts (731,732), and a valve (75) adapted to close an axial end opening of the cylindrical member for varying a concentration of pulverized coal in a radial outer passage part (731) of the coaxial passage parts.

Description: The present invention relates to a burner apparatus for pulverized coal, and more particularly to a burner apparatus for pulverized coal used in combustion system which includes a coal mill directly connected to such burner apparatus. Due to the change in the recent fuel situation, coal is taking the place of heavy oil. In particular, in commercial thermal power stations, larger-scale boilers have been increasing, where coal is exclusively used as their fuel. On the other hand, in order to meet the recent power demand, the thermal power station boiler is increasing the difference between a maximum load thereof and a minimum one, and is being used with adjusting the load thereof instead of a base load operation. If such thermal power station boiler is operated with changing a boiler pressure according to the load thereof, namely a full load operation is conducted in a super critical pressure condition and a partial load operation is conducted in a sub-super critical pressure condition, a power generation efficiency in the partial load operation is increased by some percentage. Therefore, in the thermal power stations where coal is exclusively used as their fuel, the boilers are scarce, which are operated in full load condition at all times. It is becoming normal that the boilers are operated with changing their loads among 75% load, 50% load and 25% load in the daytime, and they are stopped in the night time. Namely it is becoming common that such boilers are frequently started and stopped, or they work under daily start stop operation (hereinunder referred as "DSS operation"). Further, in the boilers for DSS operation, where coal is exclusively used as their fuel, it is scarce that they are operated with using pulverized coal only as their fuel during an entirely load range, or from a start (no load) to a full load. Among boilers where coal is exclusively used as their fuel, light oil, heavy oil, gas, or the like is used as their auxiliary fuel on the start or low load operation thereof. The reason is that no heating air is fed to the coal mill so as to warm it up from the boilers where coal is exclusively used as their fuel on the start thereof. Therefore, it is impossible to operate the coal mill, thereby unable to grind the coal to the pulverized coal. Further, it is impossible for the coal mill to obtain a sufficient turndown ratio on the low load operation, and the pulverized coal is poor in ignition. These are the reasons why light oil, heavy oil, gas, or the like is used in the burners of the boilers where coal is exclusively used as their fuel. For example, in case that light oil and heavy oil are used as auxiliary fuel, at first, light oil is supplied to a burner from the start thereof to the 15% load thereof. Subsequently, heavy oil is changed over to light oil, during from the 15% load to the 40% load. Beyond 40% load, heavy oil and pulverised coal is mixed together and supplied to the burner. The amount of heavy oil is gradually reduced, on the contrary the amount of pulverised coal is gradually increased to rise a mixture ratio of pulverized coal to the heavy oil. Finally, only pulverised coal is supplied to the burner. In such burner which uses not only pulverized coal but also auxiliary fuel, auxiliary fuel is supplied to the burner every time of start and stop of operation thereof, which occurs frequently. Therefore, the amount of auxiliary fuel to be consumed is extremely increased. Further, in case that the load of the coal mill is low, or on the start of the coal mill, the concentration of the pulverized coal in the mixture of pulverized coal and combustion air is low. Then the ignition of the pulverized coal in the burner is unstable, thereby increasing the amount of unburnt component (carbons etc.) in the fly ash. Therefore, this increases the risk of reduction of combustion efficiency in the boiler. EP-A-0 260 382 discloses a low NOx burner comprising a pulverized coal nozzle for injecting a mixture of pulverized coal and primary air. A liquid fuel nozzle is disposed within the pulverized coal nozzle for providing a jet of liquid fuel such as heavy oil and the like when a combustion furnace is preheated. On an outer end of the pulverised coal nozzle an outwardly flared flame holder for generating a swirl flow is disposed. The object of the present invention is to provide a burner which requires a reduced amount of auxiliary fuel and which allows a stable ignition of the pulverized coal on the low load operation and an effective partial load operation with safety. This object will be solved by a burner comprising the features of claim 1. Advantages of the present invention become more clearly from the following description of the preferred embodiments described with referring to the accompanying drawings. Fig. 1 is a fragmentary enlarged sectional view showing a burner according to one embodiment of the present invention; Fig. 2 is a fragmentary sectional view showing the burner shown in Fig. 1, attached to the pulverized coal combustion boiler; Fig. 3 is a schematic view showing a pulverized coal combustion boiler system incooperating therein the burner shown in Figs. 1 and 2; Figs. 4 and 5 are graphs showing characteristics of the burner; Fig. 6 is a perspective view showing a cylindrical member used in another embodiment of the invention; Figs. 7 and 8 fragmentary sectional views showing a burner according to a still another embodiment of the invention; Figs. 9 to 11 showing the plug positions according to the load of the burner; Figs. 12 and 13 are fragmentary sectional views showing a burner according to the other embodiment of the invention; Figs. 14 to 16 showing the flow of mixture according to the load of the burner; Figs. 17 and 18 are fragmentary sectional views showing the burners according to another embodiments of the invention, respectively. A burner apparatus according to an embodiment of the present invention shown in Fig. 1 is incooperated within a pulverized coal combustion boiler system shown in Fig. 3. The boiler system includes a pulverized coal combustion boiler 1 with a boiler furnace 11, a coal mill 2, a coal bunker 3, a heat exchanger 4, a heavy oil tank 5, a light oil tank 6, a plurality of pulverized coal burners 7, and a wind box 18. The pulverized coal burner 7 comprises, as shown in Figs. 1 and 2, a heavy oil starter burner 71 sheathed with a guide sleeve 72 and connected to the heavy oil tank 5, a light oil ignition burner disposed adjacent to an injection end of the heavy oil starter burner 71 and connected to the light oil tank 6, and a pulverized coal supply pipe 73 disposed to surround the guide sleeve 72. The wind box 18 comprises a secondary air register 182 and a third air register 183. On the start of the boiler 1, at first the heavy oil starter burner 71 is ignited by the light oil ignition burner. Heavy oil is exclusively fed to the starter burner 71 to reach a load level of the boiler substantially equal to 25% to 35% of a full load thereof. After an interior temperature of the furnace 11 is raised enough, pulverized coal is fed from the coal mill 2 to furnace 11 through the pulverised coal supply pipe 73 and then is burnt in the furnace 11. Thereafter, the amount of heavy oil to be supplied to the heavy oil starter burner 71 is gradually decreased so that the pulverized coal is exclusively fed to the furnace 11. Hot air from the heat exchanger 4 in which the hot air is heat-exchanged with exhaust gas from the boiler 1 is fed not only to the coal mill 2 as a primary combustion air but also to the wind box 18 as supplementary combustion air. The primary combustion air serves not only to remove the mist of water sticked on the coal supplied from the coal bunker 3 but also to classify the ground coal in a classifier (not shown) disposed in the coal mill 2. Further, the primary combustion air carries the pulverized coal from the coal mill 2 to the pulverized coal supply pipe 73. As shown in Figs. 1 and 2, a tubular passage defined between the supply pipe 73 and the guide sleeve 72 is divided at an end portion thereof into two coaxial tubular sub-passages 731 and 732 by means of a cylindrical member 74 and a valve 75. The cylindrical member 74 is provided at a periphery thereof with a plurality of slits 741 and has a truncated conical end portion 743 with a valve seat opening 742 formed therein. The valve 75 includes a valve element 751 and a stem 752 to which the valve element 751 is attached, and is adapted to be axially moved by an actuator 753 to abut the seat opening 742 to close it. The cylindrical member 74 is so disposed that an sectional area of the radially outer sub-passage 731 is extremely small compared with that of the radially inner sub-passage 732. The supplementary combustion air from the heat-exchanger 4 is divided in the wind box 18 into a secondary combustion air B and a third combustion air C by means of a dividing sleeve 181. They are swirled through the respective registers 182 and 183, and then supplied into the furnace 11. The operation of the cylindrical member 74 and the valve 75 will be described hereinunder with referring to Fig. 1. They divide a mixture A into three flows, namely a high concentration flow Ac passing through the radial outer sub-passage 731, a low concentration flow A R passing through the radially inner sub-passage 732 via the slits 741, and a bypass flow A B passing through the radially inner sub-passage 732 via the seat opening 742. The bypass flow A B is controlled by moving the valve 75 axially. The truncated conical end portion 743 of the cylindrical member 74 separates the pulverised coal from the mixture A due to inertia thereof, and feeds it radial outwards. In order to keep a flame stable in the pulverized coal burner, it is necessary to obtain a higher concentration of the pulverized coal in the mixture and to reduce a velocity of the pulverized coal. In general, in the pulverized coal burner to which the coal mill is connected, if the load of the burner is reduced, the coal mill degrades the grinding performance thereof, thereby lowering the concentration of the pulverized coal in the mixture. Therefore, it is necessary to raise the concentration so as to keep a flame stability. To this end, in this embodiment, in case of a low load of the burner, the valve element 751 is moved to close the seat opening 742 as shown by a chain line. In this state, a more primary combustion air in the mixture is introduced into the radially inner sub-passage 732, then the concentration of the pulverized coal in the mixture flowing through the radially outer sub-passage 731 is increased, thereby keeping the flame stability. To the contrary, in case of a high load of the burner, the coal mill operates fully to raise the concentration of the pulverized coal in the mixture. In this occasion, the valve element 751 is moved traversely to open the seat opening 742 as shown by a solid line so as to permit the mixture of a high concentration of the pulverized coal to flow both through the radially outer and the radially inner sub-passages 731 and 732. This prevents the pressure difference between the sub-passages 731 and 732 from raising and reduce the velocity of the pulverized coal in the mixture and pressure drop of primary air which carries the pulverized coal to burner, thereby preventing the burner 7 from being damaged due to wear which is caused by collision between the pulverized coal and the burner element. Accordingly, since regardless of the burner load, the mixture of a high concentration of the pulverized coal is always fed radially outwards into the furnace 11, the stable combustion can be always obtained. Figs. 4 and 5 show characteristics of change of concentration of the pulverized coal in the mixture flowing through the radially outer sub-passage. The axis of abscissa of Figs. 4 and 5 represent a distribution ratio of the primary combustion air, namely, a ratio A out/A pul of the air flow rate in the mixture flowing through the radially outer sub-passage 731 to the air flow rate in the mixture flowing through the pulverized coal supply pipe 73. The axis of ordinate of Fig. 4 represents a concentration ratio of the pulverized coal, namely a ratio C out/C pul of the pulverized coal flow rate in the mixture flowing through the radially outer sub-passage 731 to the pulverized coal flow rate in the mixture flowing through the pulverized coal supply pipe 73. The axis of ordinate of Fig. 5 represents a ratio of the cross-sectional area So of the radially outer sub-passage 731 to the Si of the radially inner sub-passage 732. In the above-mentioned embodiment, in case that the mixture ratio C/A of the pulverized coal flow rate to the combustion air flow rate in the mixture flowing through the pulverized coal supply pipe 73 is 0.2, if the cylindrical member 74 is so arranged that the distribution ratio A out/A pul becomes equal to or less than 40%, it becomes possible to keep the mixture ratio of the pulverized coal flow rate to the combustion air flow rate of the mixture flowing through the radially outer sub-passage 731 in a high level, e.g. 30% to 45%. Namely, a high concentration ratio of the pulverized coal can be obtained in the mixture flowing through the radially outer sub-passage 731. Therefore, as apparent from Fig. 5, in order to obtain 40% or less distribution ratio A out/A pul, it is necessary to make the ratio So/Si of the cross-sectional areas less than 60%. With taking the stability of ignition into the consideration, it is practical that the mixture ratio of the pulverized coal flow rate to the combustion air flow rate in the mixture flowing through the radially outer sub-passage 731 is 30% or more. Therefore, it is preferable that the relations of the distribution ratio A out/A pul, of the concentration ratio C out/C pul, and of the cross-sectional area ratio So/Si are in the hatched areas in Figs. 4 and 5. Further, in this embodiment, a flame holder 76 is provided at one end of the pulverized coal supply pipe 73. Mixture of a higher concentration of pulverized coal flows along the flame holder 76 and then the flame holder 76 prevents the swirl of the supplementary combustion air from affecting the mixture from the pulverized coal supply pipe 73, thereby obtaining a steady flame. On the contrary, in the prior art with no flame holder, the supplementary combustion air affects the mixture from the pulverized coal supply pipe to cause an inverse flow. The flame is retained only in a zone in the boiler where the velocity of the inverse flow is smaller than the flame propagation velocity. Therefore, though pulverized coal is diffused fully, the flame becomes unstable. A cylindrical member 174 used in another embodiment of the present invention is provided with a plurality of ribs 175 as shown in Fig. 6. The cylindrical member 74 of the above-mentioned embodiment doesn't have such ribs. The rib 175 is located on the truncated conical end portion adjacent to the slit 1741. The ribs 175 restrain the pulverized coal in the mixture from being introduced into the radially inner sub-passage. A burner 17 according to still another embodiment, as shown in Fig. 7, comprises a plug 77 instead of the valve 75. The plug 77 includes a plug element 771 in the form of a tubular, opposite ends of which are cut aslant, and a long hollow stem 772 to which the plug element 771 is attached. The plug element 771 is moved axially by an actuator 773 according to the change of burner load (Fig. 8). A tubular member 78 having an outer peripheral wall 781 and an inner peripheral wall 782 is attached to an end portion of a guide sleeve 79. The tubular member 78 is provided at the outer peripheral wall 781 thereof with a plurality of openings 783 equiangularly spaced from each other, and has a truncated conical end portion 784 with an axial end opening 785. The inner periphery of the end opening 785 extends radially inward to a passage of the plug element 771. A guide tab 786 provided at a downstream side periphery of the respective openings 783 extends radially inward beyond the passage of the plug element 771. The tubular member 78 is axially movable. A tubular pulverized coal supply passage is divided at an end portion of the supply pipe 73 into two coaxial tubular sub-passages 731 and 732 by means of the tubular member 78. On a high load operation of the burner, the plug element 771 is located in a position designated by a solid line in Fig. 7. According this, pulverized coal flows through both of sub-passages 731 and 732. To the contrary, on a low load operation of the burner, the plug element 771 is moved to a position designated by a chain line so as to close the end opening 785. In this occasion, due to the separation effect of the tubular member 78, the pulverized coal in the mixture is moved radially outwards. As a result, a rich mixture flows through the radially outer sub-passage 731 and a lean mixture flows through the radially inner sub-passage 732. On an extremely low load operation of the burner, the plug element 771 is further moved to a position designated by a broken line in Fig. 7. Namely, the tubular member 78 is moved to a position designated by a broken line in Fig. 7 with the end opening 785 being closed by the plug element 771. As a result, a rich mixture flows through the radially outer sub-passage 731 and a lean mixture flows through the radially inner sub-passage 732. Since the tubular member 78 is extended into the furnace 11, even though an ejection velocity of the mixture from the radially outer sub-passage 731 is reduced, the ignition can be taken place certainly in a space between the flame holder 76 and the tubular member 78. Further, this delays mixing of mixture flowing through the radially outer sub-passage 731 with mixture flowing through the radially inner sub-passage 732, thereby improving the stability of flame. The details of the above-mentioned operations will be described hereinunder with referring to Figs. 9 to 11. On the high load operation of the burner, e.g. 40% load or more, as shown in Fig. 9, the plug element 771 is apart from the tubular member 78. In this occasion, the mixture supplied from the coal mill is rich and of a flow rate enough to make the flame steady. Therefore, in order to restrain the pressure loss, as much mixture as possible is to flow through the radially inner sub-passage 732. On the intermediate load operation of the burner, e.g. 25% to 40% burner load, as shown in Fig. 10, the plug element 771 is moved to close the end opening 785 of the tubular member 78. Accordingly, the mixture flows towards the radially outer sub-passage 731. Air in the mixture is introduced into the radially inner sub-passage 732 through the openings 783, thereby rich mixture flows through the radially outer sub-passage 731 and lean mixture flows through the radially inner sub-passage 732. The rich mixture is retained by the flame holder 76, thereby improving the stability of flame. In this occasion, 70% to 90% of the pulverized coal in the mixture flowing through the supply pipe 73 is fed to the radially outer sub-passage 731 and only 5% to 39% of primary air is fed to the radially outer sub-passage 731. Therefore, the concentration of the pulverized coal in the mixture flowing through the radially outer sub-passage 731 is 2 to 4.5 times as rich as that flowing through the supply pipe 73, and then the mixture can be obtained, the concentration of pulverized coal in which is enough to make the flame stable. To the contrary, on the extremely low load operation of the burner, e.g. 15% to 25% burner load, as shown in Fig. 11, the tubular member 78 is moved into the furnace 11 with the end opening 785 being closed by the plug element 771. Air in the mixture is introduced into the radially inner sub-passage 732 through the openings 783, thereby rich mixture flows through the radially outer sub-passage 731 and lean mixture flows through the radially inner sub-passage 732. The tubular member 78 which extends into the furnace can delay the dilution of rich mixture ejecting from the radially outer sub-passage 731 with lean mixture ejecting from the radially inner sub-passage 732. Therefore, even on the extremely low load operation of the burner, a stable combustion can be obtained. Further, since a low velocity zone of the rich mixture is formed by the flame holder 76, the flame is retained stable. A burner 27 according to another embodiment, as shown in Figs. 12 and 13, comprises a guide sleeve 79 provided with a pilot member 791 and a throat nozzle 80 axially movable within the coal supply pipe 73 so as to cooperate with the pilot member 791. Further, the burner 27 comprises an upstream side duct 81 and a downstream side duct 82 spaced from the duct 81. Both ducts 81 and 82 are disposed within the coal supply pipe 73 and are axially aligned with each other. They cooperate with each other to divide a tubular pulverized coal passage defined between the guide sleeve 79 and the coal supply pipe 73 to provide a radially outer sub-passage 731 and a radially inner sub-passage 732. Opposite ends of each of the pilot member 791 and the throat nozzle 80 are cut aslant. The relative positional relationship between the slant surfaces of the pilot member 791 and the throat nozzle 80 is varied so as to change the direction of the mixture to be fed into the furnace 11. Further, in this embodiment, in addition to the flame holder 75 provided at the pulverized coal pipe 73, another flame holder 83 is provided at one end of the downstream side duct 82. On a high load operation of the burner, as shown in Fig. 14, the throat nozzle 80 is located on the upstream side of the pilot member 791. Accordingly, as the mixture passes through the space defined between the throat nozzle 80 and the pilot member 791, the pulverized coal is separated from the mixture due to inertia thereof and is directed to the radially outer sub-passage 731. A large part of air in the mixture flowing through the radial inner sub-passage 732 is separated therefrom and sucked into the radially outer sub-passage 731 and mixed with the mixture flowing through the radially outer sub-passage 731. Therefore, rich mixture flows through the radially outer sub-passage 731 and lean mixture flows through the radially inner sub-passage 732 through the ducts 81 and 82 are so inherently disposed that a larger amount of pulverized coal is fed to the radial inner sub-passage 732. The rich mixture is burnt stably by means of the flame holder 76. In this occasion, the sub-passages 731 and 732 aren't throttled, thereby reducing a passage resistance and reducing a pressure difference in the burner and simultaneously the velocity of pulverized coal is kept a low level, thereby preventing the pulverized coal from wearing away the parts of the burner. On a low load operation of the burner, as shown in Fig. 15, the throat nozzle 80 is located on the downstream side of the pilot member 791. Accordingly, as the mixture passes through the space defined between the throat nozzle 80 and the pilot member 791, almost of the pulverized coal is directed to the radially inner sub-passage 732. A large part of air in the mixture flowing through the radial inner sub-passage 732 is separated therefrom, and sucked into the radial outer sub-passage 731 and mixed with the mixture flowing the radial outer sub-passage 731. Therefore, rich mixture flows through the radially inner sub-passage 732 and lean mixture flows through the radially outer sub-passage 731. The rich mixture is burnt stably by means of the flame holder 83. On an extremely low load operation of the burner as shown in Fig. 16, the throat nozzle 80 is located on the downsteam side of the pilot member 791 and abutted against the upstream side duct 81. Accordingly, all of pulverized coal is directed to the radially inner sub-passage 732. Therefore, rich mixture flows through the radially inner sub-passage 732 and is burnt stably by means of the flame holder 83. In order to obtain a good combustion in the burner, it is preferable that the ducts are so arranged as to meet the following relationships, namely the ratio (So + Si)/S is between 0.5 and 0.9, the ratio Si/(Si + So) is less than 0.4, and Sr is greater than So, where So represents a minimum cross-sectional area of the radially inner sub-passage 732; Si represents a minimum cross-sectional area of the radially outer sub-passage 731; and Sr represents a minimum area of the slit between the ducts 81 and 82. Burners according to still another embodiments of the present invention are shown in Figs. 17 and 18, respectively. In the burner shown in Fig. 17, a pivotable deflector 84 is used instead of the throat nozzle 80. The deflector 84 is pivoted according to the load of the burner so as to change the direction of the mixture from the coal mill. In the burner shown in Fig. 18, the pivotable deflector 84 and a bent duct 85 are substituted for the throat nozzle 80 and the pilot member 791, respectively. It should be understood that these burners can enjoy the meritorious advantages of the above-mentioned embodiments as well.

Claims: A burner apparatus for pulverized coal comprising: a pulverized coal supply pipe (73), starter means (71) extending within said pulverized coal supply pipe (73) to define therebetween a tubular passage through which a mixture of combustion air and pulverized coal passes into a furnace (11), and a flame holder (75) provided at an outer periphery of one end of said pulverized coal supply pipe (73) facing said furnace (11), characterized in that a dividing means is disposed within the pulverized coal supply pipe (73) for dividing a part of the tubular passage into a radially outer sub-passage (731) and a radially inner sub-passage (732) having different sectional areas, and a varying means is arranged within the pulverized coal supply pipe (73), said varying means including a bypass passage (741) for connecting the radially inner and outer sub-passage (732, 731) and being movable for controlling an amount of pulverized coal flowing though the radially inner sub-passage (732). A burner apparatus according to claim 1, characterized in that the varying means is axially movable for varying the opening degree of a seat opening (742; 785) of the dividing means. A burner apparatus according to claim 1 or 2, characterized in that said dividing means include a cylindrical member (74; 174) disposed within said tubular passage so as to make a radial dimension in a cross-section of said radial outer passage part (731) smaller than that of said radial inner passage part (732) and that said bypass passage part (741, 742) is provided in said cylindrical member (74; 174). A burner apparatus according to claim 1, characterized in that the dividing means is axially movable. A burner apparatus according to claim 1, characterized in that the dividing means comprises an upstream duct (81) and a downstream duct (82) axially aligned with each other and that the varying means comprises an axially movable throat nozzle (80) and a pilot member (791) each having sloped surfaces cooperating within one another to change the direction of the mixture. A burner according to claim 5, characterized in that a deflector (84) pivotally mounted at the pulverized coal supply pipe (73) is used instead of the throat nozzle (80). A burner according to claim 5, characterized in that a pivotable deflector (84) and a bent duct (85) are used instead of the throat nozzle (80) and the pilot member (791).

Inventors: BABA AKIRA, KAKU HIROYUKI, KOBAYASHI HIRONOBU, MORITA SHIGEKI, OKIURA KUNIO, OYATSU NORIYUKI, BABA, AKIRA, KAKU, HIROYUKI, KOBAYASHI, HIRONOBU, MORITA, SHIGEKI, OKIURA, KUNIO, OYATSU, NORIYUKI
Applicants: BABCOCK HITACHI KK, BABCOCK-HITACHI KABUSHIKI KAISHA
Application year: 1990
Main classification: F23D1
IPC codes: F23D1