DUST COAL IGNITING BURNER DEVICE

Abstract: Several burners each have a primary sleeve into which a mixture of pulverised coal and primary air is supplied and outside which is a secondary air supply. The exit end of the primary sleeve has a swirl vane provided in it and leads to a large diamter portion and a flame protector. A ceramic igniter and a coal concn. detector are provided in the large diameter portion. The portion serves to slow the flow of the air and pulverised coal mixture. The swirl vane creates a region of high coal concn. suitable for ignition and its degree of opening is controlled by output from the coal cocn. detector.

Description: TECHNICAL FIELD The present invention relates to an ignition burner apparatus for pulverized coal, and more particularly to an ignition burner apparatus for directly igniting pulverized coal. BACKGROUND ART Recently, in this country, there is a tendency such that, in order to reduce dependency on petroleum in view of the insufficiency of the supply of crude oil, industrial plants used especially for burning the crude oil are being changed to those used especially for coal. In particular, with industrial thermal power generation boilers, large capacity thermal power plants especially for coal are now under construction. On the other hand, it is a feature of recent electric power demand that a difference in load between a maximum load and a minimum load is increased with the increase of the atomic power generation. There is a tendency that boilers for thermal power generation are modified from those for base loads to load adjustment type boilers. It is possible to enchance the power generation efficiency by several percents under a partial load operation, by a variable pressure operation boiler which operates in a super critical pressure region in a so-called full load operation mode in which the thermal power generation boilers are operated by changing pressures in response to the load and which operates in a subcritical pressure region in a partial load operation mode. For that reason, there are a decreased number of boilers especially for burning coal, which are always operated under the full load. The boilers especially for burning coal are converted to boilers for intermediate loads, which perform a so-called daily start/stop (hereinafter simply referred to as DSS) operational mode in which in the daytime the load is changed among 75% load, 50% load and 25% load and in the night-time the operation is stopped. Also, there are a small number of the coal burning boilers for performing the DSS operational mode, which operate over the full load region only with pulverized coal from the start to the full load operation. Even in the coal burning boilers, readily ignitable auxially fuel such as light oil, heavy oil, gas and etc. other than pulverized coal is used for the start or low-load operation. The reason for this is that, in the starting mode, it is impossible to obtain exhaust gas or heated air from the boiler for warming up a mill, and hence it is impossible to operate the mill for pulverizing coal. Also, the light oil, heavy oil and gas are used for the reasons that it is impossible to keep a "turn down" ratio of the mill under the low load and the ignition property of the pulverized coal itself is deteriorated. For example, in the case where the light oil and heavy oil are used in the starting mode, the light oil is burned as fuel from the start to the 15% load, the fuel is switched over from the light oil to the heavy oil in the range of 15% load to 40% load, and over 40% load the mixture of the heavy oil and the pulverized coal is burned and over 40% load, the mixture is burned while gradually decreasing the amount of the heavy oil and gradually increasing the amount of the pulverized coal, thereby increasing the mixture burning rate of the pulverized coal for transferring to the substantial coal burning. On the other hand, in the case where the boiler load is reduced from the full load to the low load, the pulverized coal fuel is burned to 35% load to operate the boiler as the coal burning boiler, and below the 35% load, the boiler is operated by auxiliary fuel such as heavy oil, light oil and gas. As described above, in the coal burning thermal power plant which performs the DSS operational mode, it is general to use the pulverized coal and the readily ignitable auxiliary fuel such as light oil, heavy oil and gas. Fig. 4 is a schematic diagram showing a conventional pulverized coal burning boiler. In Fig. 4, pulverized coal burners 4, 5, 6, 7, 8 and 9 are arranged in this order from a bottom to a top of a boiler furnace 1 in a front wall 2 and a rear wall 3 of the boiler furnace 1. After-air-ports 10 and 11 are provided above the pulverized coal burners 8 and 9 for reducing generation of NOx. Air is supplied from front wall wind boxes 12 and rear wall wind boxes 13 to the respective pulverized coal burners 4, 5, 6, 7, 8 and 9 and from a front wall wind box 14 and a rear wall wind box 15 to the after-air-ports 10 and 11, respectively. On the other hand, the supply of coal to the pulverized coal burners 4, 5, 6, 7, 8 and 9 is performed as follows. Namely, coal within a coal banker 16 is fed from a coal feeder 17 to a mill 18 and is pulverized within the mill 18. Rough particles contained in the pulverized coal in the mill 18 are classified by a classifier (not shown) and is returned back to the pulverizing section within the mill 18. The pulverized coal is fed from the pulverized coal supplying means, i.e., the mill 18 to the respective burners 4, 5, 6, 7, 8 and 9 by the delivery means including pulverized coal pipes 23, a blower 50 and the like. Namely, the delivery of the pulverized coal is performed by generating air flow from an air duct 22 through the mill 18 and the pulverized coal pipes 23 to the burners 4, 5, 6, 7, 8 and 9 by means of the blower 50. The burning air for the front wall wind boxes 12, the rear wall wind boxes 13, the front wall wind box 14 and the rear wall wind box 15 is pressurized by a forced draft fan 19 and thereafter is preheated in an air heater 20. The air is supplied through an air passage 21, an air flow adjustment damper or restrictor 24 and air passage 25 to the wind boxes 12, 13, 14 and 15. Also, gas is supplied to a hopper 26 by a gas recirculation fan 27 and a gas recirculation passage 28 for controlling a steam temperature during a partial load of the boiler. A gas duct 29 for mixing the gas with the burning air of an air passage 25 from an outlet of the gas recirculation fan 27 is provided for reducing the generation of the NOx. The foregoing description relates to the general flows of the burning air, the gas and the pulverized coal in the pulverized coal boiler. It should be noted that the respective burners 4, 5, 6, 7, 8 and 9 are provided with igniters. Fig. 5 is a enlarged view showing a detail of the fine coal burner shown in Fig. 4. In Fig. 5, the reference numerals are used to show the same components as in Fig. 4. Namely, reference numeral 1 denotes a boiler furnace, numeral 2 denotes a front wall, numeral 3 denotes a rear wall, numerals 4, 5, 6, 7, 8 and 9 denote pulverized coal burners, numerals 12 and 13 denote front and rear wall wind boxes, respectively, and reference numeral 23 denotes a pulverized coal pipe. Reference numeral 30 denotes an air resistor and reference numeral 31 denotes a plasma igniter. The development and research of the pilot burner so constructed that the auxiliary fuel having good ignitability such as light oil, heavy oil, gas and the like for the pulverized coal boiler have been advanced. As a typical example, the development of the device provided with a plasma igniter 31 for directly igniting the pulverized coal using a plasma arc is realized as shown in Fig. 5. The igniter device provided with such a plasma igniter 31 is of the type such that a high temperature heat source at 1,500 to 2,000°C is provided for directly igniting the pulverized coal without any auxiliary fuel such as light oil, heavy oil and gas. However, this system involves a serious problem and is not actually used since the ignition of the plasma arc needs a heat source near to 2,000°C with large energy of 60 to 80 kW at the ignition stage, thereby discharging a great amount of thermal NOx at the ignition with the pulverized coal burners 4, 5, 6, 7, 8 and 9. In the conventional pulverized coal burning boiler, the light oil and heavy oil that have a good ignitability are used as auxiliary fuel, and in the load change mode due to the DSS operation, the heavy oil is used for the burner starting fuel and the light oil is used for the igniters in view of the ignitanility and operativity. Three different kinds of fuel including pulverized coal as the main fuel are needed for the conventional boiler. Thus, there is a disadvantage that the instrument cost and running cost in connection with the transportation, storage and maintenance of the respective fuels are required. Also, in the direct ignition system using the plasma arc as described above, the ignition energy and the thermal source temperature are too high. Therefore, this system causes such a problem that a great amount of NOx is generated in the ignition operation. US-A-2 096 945 discloses a pulverized igniting burner apparatus including a pulverized coal supplying source, a first sleeve having a first diameter, which supplies a pulverized coal/air-mixture in a downstream direction of the sleeve, a rotary vane disposed within the first sleeve close to the downstream and there off, a second sleeve being connected to the downstream and of the first sleeve and having a second diameter larger than said first diameter and means for maintaining a flame. These means of maintaining a flame make use of continuous electrical sparks and effect a central region of the second sleeve. In view of the foregoing defects inherent to the conventional system, an object of the invention is to provide a burner device for directly igniting the pulverized coal with high reliability in ignition without discharging NOx larger than necessary and without auxiliary fuel. Incidentally, Japanese Patent Unexamined Publication No. 61-184309 and USP 4,545,307 show the other prior art relating to the present invention. DISCLOSURE OF THE INVENTION The object of the present invention is solved in accordance with the features of the main claim, dependent claims are directed on preferred embodiments. The present invention applies three steps so as to ignite the coal/air mixture. First, an enlargement is provided so as to reduce the flow speed of the mixture. Second, at the downstream end of the first sleeve a rotary vane is disposed so as to increase the concentration of ignitable material. These two steps are state of the art. At the downstream end of the enlarged part, what is called the second sleeve, an abrupt constriction is disposed which causes eddies at the upstream and downstream side thereof. By means of these eddies the constriction causes an improved passing-by of the mixture at a heated ceramic igniter. Therefore, volatile components contained in the coal are ignited and cause on their part ignition of the coal itself. As ignition of the mixture takes place at a lower energy level, N0x-generation and emission is reduced. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross-sectional view showing a primary part of a pulverized coal burner according to an embodiment of the invention; Fig. 2 is a view showing a structure of the burner shown in Fig. 1; Fig. 3 is a graph showing an ignition characteristic curve of the ignition burner shown in Fig. 1; Fig. 4 is a schematic view showing a system of a pulverized coal burning boiler; and Fig. 5 is a cross-sectional view showing a pulverized coal ignition burner provided with a plasma igniter. BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the invention will now be described with reference to the accompanying drawings. Fig. 1 shows a primary part of a pulverized ignition burner apparatus in accordance with the embodiment of the invention, Fig. 2 shows a structural feature of the pulverized coal ignition burner shown in Fig. 1, and Fig. 3 is a graph showing an ignition characteristic curve of the pulverized coal ignition burner apparatus shown in Fig. 1, wherein a ratio (C/A) of pulverized coal to air is shown in ordinate and an air flow rate (m/s) at a nozzle outlet of the pulverized coal burner is shown in abscissa. In Figs. 1 and 2, reference numerals 4, 5, 6, 7, 8 and 9 denote pulverized coal burners. A mixture flow of pulverized coal 33 and primary air 34 is supplied to a primary sleeve 32 from the mill 18 and the coal pipe 23 shown in Fig. 4. Secondary air 35 is supplied around the primary sleeve 32. These components are constructed in the same manner as in the conventional system. Reference numeral 36 denotes a rotary vane for imparting a swirl motion to the mixture flow of the pulverized coal 33 and the primary air 34, numeral 37 denotes a large diameter portion formed at a distal end of the primary sleeve 32, numeral 38 denotes a flame maintaining means, numeral 39 denotes an ignition region, formed in the large diameter portion 37, in which the pulverized coal flow rate is slower than that within the primary sleeve 32, reference numeral 40 denotes an eddy circulation flow of the pulverized coal 33, and reference numeral 41 denotes a ceramic igniter. Reference numeral 42 denotes a C/A detector, reference numeral 43 denotes an opening adjuster for the rotary vane 36, numeral 44 denotes a heater electric source device for the ceramic igniter 41, numeral 45 denotes a controller and numeral 46 denotes flame. The thus constructed pulverized coal ignition burner apparatus according to the embodiment includes, as shown in Fig. 2, the primary sleeve 32 for supplying the pulverized coal 33 and the primary air 34, the rotary vane 36 for imparting swirl motions to the mixture of the pulverized coal 33 and the primary air 34 to cause rich/lean distribution in the mixture flow, the large diameter portion 37, the flame maintenance means 38, the opening adjuster 43 for adjusting the opening of the rotary vane 36, the C/A detector 42 for detecting the pulverized coal density (C/A), the ceramic igniter 41 for performing the ignition to the pulverized coal, the heater electric source device 44, and the controller 45 for controlling the opening of the rotary vane 36 in response to the signal outputted from the C/A detector 42 and for applying current/voltage to the heater 41 to impart the ignition command signal. Fig. 3 shows experimental results of the ignition characteristics in the case where the ceramic igniter 41 is inserted into the mixture flow. As is apparent from Fig. 3, it will be understood that, in order to stably ignite the mixture flow of the pulverized coal 33 and the primary air 34, the pulverized coal density (C/A) should meet the condition, C/A ≧ approximately 0.5, and the air flow v should meet the condition, v ≦ approximately 10 m/s. Also, Fig. 3 shows a requirement that the ignitability is liable to be affected by the flow rate as the amount of the pulverized coal is decreased from A to B and from B to C, and hence in order to perform the stable ignition, it is necessary to reduce the flow rate. On the other hand, it is generally understood in view of the relationship of specific weight of the pulverized coal 33 or the like that the upper limit is defined by the value of the relationship, C/A ≦ 0.5. Also, the burner shape is designed so that the flow rate in the primary sleeve 32 is defined by the relationship of v > 15 m/s in view of the aspect of reducing the backfire. Therefore, in order to directly ignite the pulverized coal 33 by using the heater such as ceramic igniter 41, it is necessary to modify the burner structure as proposed in accordance with the embodiment of the invention. The effect of the ignition according to the embodiment will now be described with reference to Figs. 1 and 2. The mixture flow of the pulverized coal 33 and the primary air 34 supplied at a flow rate of 15 to 20 m/s within the primary sleeve 32 is subjected to the swirl motion by the rotary vane 36 provided within the primary sleeve 32 and made of ceramics that are superior in heat resistance and wear resistance. As shown in Fig, 1, the ignition region 39 in which the pulverized coal density is high is formed within an inner surface of the large diameter portion of the primary sleeve 32. It is necessary to set a suitable C/A according to the amount of the coal as shown in Fig. 3 in order to stably ignite the coal. According to the foregoing embodiment, the density of the pulverized coal within the large diameter portion 37 is detected by using the C/A detector 42 using the laser beam, and the opening of the rotary vane 36 is controlled in response to the detected signal by means of the opening adjuster 43 and the controller 45. If swirl motion would be excessively applied to the mixture by the rotary vane 36, the pressure loss would be remarkable. Therefore, it is possible to avoid the practical problem by controlling the opening of the rotary vane 36 so that the value of C/A is retained within a range of 0.5 ≦ C/A ≦ 2. There are air flow rate conditions as shown in Fig. 3 for other affecting parameters for stably igniting the coal. However, according to the embodiment, the flow rate is decreased from the range 15 to 20 m/s to 10 m/s or less by increasing diameters of the outlet ports of the pulverized coal burners 4, 5, 6, 7, 8 and 9, that is, by providing the large diameter portions 37 at the distal ends of the primary sleeve 32. Furthermore, as shown in Fig. 1, the mixture flow is collided with the flame maintenance means 38 as shown in Fig. 1, so that the eddy recirculation flows 40 are formed in the vicinity of the flame maintenance means 38. The flow rate of the eddy recirculation flows 40 is in the low flow region of 0 to 5 m/s in terms of absolute values and is within the region where the ignition and flame maintenance are well performed. Namely, the ignition region 39 suitable for the direct ignition of the pulverized coal having a high pulverized coal density and at the low flow rate is formed at the inner surface of the outlet of each of the pulverized coal burners 4, 5, 6, 7, 8 and 9. Subsequently, the particles of the pulverized coal 33 in the large diameter portion 37 are collided with the ceramic igniter 41 heated at 1,000 to 1,200°C set within the ignition region 39. As a result, a volatile component contained in the pulverized coal 33 is continuously ignited to form the flame 46 within the eddy recirculation flows 40. As described above, according to the embodiment of the invention, it is possible to directly ignite the pulverized coal in a positive and stable manner without generation of the thermal NOx unlike the igniter such as the conventional plasma igniter. Although the pulverized coal density is increased by the rotary vane 36 in accordance with the embodiment of the invention, the invention is not limited to the embodiment shown. It is possible to enhance the pulverized coal density by supplying the pulverized coal from another bottle laid on another place to the interior of the primary sleeve 32. Also, it is possible to increase the pulverized coal density within the primary sleeve 32 by extracting the primary air 34 from the primary sleeve 32. INDUSTRIAL APPLICABILITY According to the present invention, since the pulverized coal is directly ignited, it is unnecessary to use the auxiliary fuel such as light oil, heavy oil and gas, and in addition, to reduce the thermal NOx generation at the ignition operation. Also, if the pulverized coal directly igniting burner according to the invention is applied to the pulverized coal burning boiler, it is possibly to sum up three systems for the light oil, heavy oil and pulverized coal into a single system for the pulverized coal, so that maintenance for additional instruments and additional fuel supply may be dispensed with.

Claims: 1. A pulverized coal igniting burner apparatus including a pulverized coal supplying source (18) a first sleeve (32) having a first diameter, supplying a pulverized coal/air-mixture in a downstream direction of the sleeve, a rotary vane (36) disposed within the first sleeve close to the downstream end thereof, a second sleeve (37) being connected to the downstream end of the first sleeve (32) and having a second diameter larger than said first diameter, igniting means (41) for igniting the pulverized coal in the second sleeve (37) and means (38) for maintaining the flame, characterized in that the means (38, 41) for maintaining the flame is disposed at the inner peripheral wall of said second sleeve (37) and comprises an abrupt constriction disposed at the downstream end of the second sleeve (37) such as to cause eddies in the vicinity of the means for maintaining the flame. 2. An apparatus according to claim 1, characterized in further comprising means (42) for detecting density of the pulverized coal within the second sleeve and means for controlling opening of said rotary vane (36) in response to a signal outputted from said detecting means (42). 3. An apparatus according to claim 1 or 2, characterized in that the transition between the first and the second sleeve is substantially conus-like.

Inventors: BABA, AKIRA, HAMADA, IKUHISA, SAKAGUCHI, YASUHIDE, SUGITA, RYUICHI, TSUMURA, TOSHIKAZU
Applicants: BABCOCK-HITACHI KABUSHIKI KAISHA
Application year: 1987
Main classification: F23D1
IPC codes: F23D1