Patent Application
20090000532
This application claims the priority to DE Patent Application Serial No. 10 2007 030 269.1-13, entitled “Pulverized Coal Burner for Firing Fuel Which is Fed by Dense Phase Conveyance,” filed Jun. 28, 2007. The specification of the above-identified application is incorporated herewith by reference.
The present invention is directed towards a burner, especially a pulverized coal burner, with a fuel conveying tube and a primary air tube which is concentrically arranged within it, wherein the primary air tube on the mouth discharge side terminates at a distance to the mouth opening of the fuel conveying tube, and the burner is connected, or can be connected, to a feed line which conveys pulverous fuel in dense phase. Furthermore, the invention is directed towards a method for combustion of particulate fuel, especially pulverized coal, preferably dry brown coal, in a burner with primary air tube and fuel conveying tube, wherein the fuel is fed to the burner by dense phase conveyance and is conveyed by dense phase conveyance inside the burner along the longitudinal axis of the burner and mixed with combustion air.
For the combustion of particulate fuel, especially pulverized coal and pulverized dry brown coal, it is known to combust this in burners which in addition to a core air feed have a primary air feed, a secondary air feed and a tertiary air feed. Such burners are especially used in conjunction with the firing of furnaces of large steam generators. The primary air feed, secondary air feed and tertiary air feed in this case are formed and arranged in the form of annular conveying cross sections, which are arranged concentrically to each other, or in the form of concentric arrangements coaxially around a central core air tube, in which an oil burner lance or suchlike can be arranged. The pulverous fuel in this case as a rule is guided together with the primary combustion air (primary air) in a primary air tube inside the burner to the burner mouth. Swirlers, which are arranged on the outer side of the core jacket tube, are then provided in this primary air tube and impart a desired swirl to the pulverized coal/primary air mixture, so that as a result the combustion is optimized, but at least improved, especially with regard to a low-NOX combustion of the fuel. These burners as a rule are used in plants in which the pulverous fuel is conveyed by means of a pneumatic conveyor from the mill directly to the burner. Attached to this is the disadvantage of being operated with low transporting gas loads and high conveying speeds in the process. This leads to an extensive wear of the transporting tubes. Furthermore, the conveying tubes have relatively large dimensions (conveying cross sections).
For avoiding these disadvantages, a generic-type burner and a generic-type method, in which the pulverous fuel is pneumatically conveyed to the burner, and in the burner, by means of dense phase in a dense phase conveying system, have already been proposed in DE 197 15 373 A1. With this, pulverized dry brown coal is conveyed in a single stage by means of a dense phase conveying system, and with a transporting gas load of at least 60 kg of pulverized coal/kg of transporting gas, or with a conveyed stream density of between 100 and 350 kg of pulverized coal/m3, is fed to a steam generator burner. For controlling the mass stream of pulverized coal and the loading of the conveying lines, water vapour or a hot inert gas, such as flue gas or nitrogen, is used as pneumatic conveying medium. In this case, burners are described in DE 197 15 973 A1, in which the fuel is fed to the burner in a central feed line, and then, during discharge from this central line, is mixed with primary and secondary air streams which issue from annular passage-form openings arranged concentrically around it, is dispersed, and distributed into the furnace in a suspended state. Another burner has a central primary air tube which is concentrically encompassed by secondary and tertiary air lines. With this burner, the fuel is introduced into the burner in a pulverized coal pipe which is guided in a coiled manner around the primary air tube, and inside the burner is discharged into the secondary air passage at a distance from the mouth opening of the burner.
This known burner indeed already has the advantage that the combustion air streams, which are fed to the burner, and the fuel stream are spatially separated from each other so that a dense phase conveyance of the fuel is possible, wherein transporting gas loads of over 20 kg of pulverized coal/kg of transporting gas and conveying gas speeds of 3 to about 20 m/s at pressures of <10 bar, are understood by dense phase conveyance in the present application, but the combustion is not yet optimized with regard to a low-NOX combustion. In particular, with the so-called DS (swirl flow) burners, which are characterized by an especially low-NOx combustion, it is not possible to feed the pulverous fuel to the burner either in the central core air tube or in a central air tube, or via an additional fuel conveying line which is guided in a coiled manner in the secondary air feed passage. In order to achieve a low-NOX combustion, it is a technical aim to enrich the fuel concentration in the radially viewed outermost zone of the primary air stream which is guided in the burner, before its discharge into the furnace, wherein this enrichment is to be carried out uniformly over the circumference of the primary air jet. In the case of the burners which are known from DE 197 15 973 A1, this aim is not achievable. In the case of the burner with feed of fuel by dense phase conveyance through a central conveying tube, the feed of the fuel is directly centred in the region of the extension of the longitudinal axis of the burner, so that the primary air stream which is arranged around it just has no enrichment by fuel particles in its radial outer region. In the case of the other burner which is known from this publication, the fuel is indeed conveyed in a secondary air stream which in the radial direction is circumferentially arranged around the primary air stream. However, this is carried out with a coiled conveying tube of small cross section so that the fuel discharges into the secondary air only in a small area of the annular passage which conveys the secondary air, and consequently there is no uniform distribution of the pulverous fuel over the cross section of the entire annular conveying passage of the secondary air. Also, in this case, the fuel is conveyed into the secondary air stream so that an enrichment of the outer region of the primary air stream with fuel is not carried out.
A pulverized coal burner for the steam-oxygen gasification of pulverized coal which is introduced in dense phase, is indeed already known from DD 251 476 A3, in which the fuel, which is uniformly distributed around a central feed of a combustion gas and of an oxidizing agent, via an annular passage cross section, is fed by dense phase conveyance. In this case, however, the admixing of a steam-oxidizing agent mixture is only then carried out in the further, subsequent stream path outside a combustion chamber. This burner serves for the gasification of the fuel and for producing a gasifying gas, which is why in addition to the oxidizing agent steam is also blown into the pulverized coal stream. This technology, however, cannot be used with a burner for the firing of a furnace of a steam generator.
The present invention relates to a low-NOX burner, which is suitable for firing the furnace of a steam generator, with a dense phase conveyance of fuel, without disadvantageously affecting the low-NOX combustion characteristic of the burner.
According to an exemplary embodiment of the invention, the inside space of the primary air tube is connected, or being connected, to a primary air feed line, and the fuel conveying space which is formed between the primary air tube and the fuel conveying tube being able to be connected, or being connected, to the feed line which feeds pulverous fuel in dense phase to the burner.
According to an exemplary embodiment of the present invention, the fuel is conveyed inside the burner in an annular passage-form fuel conveying space which is formed radially outside a primary air line, and also by the primary air stream which issues from the primary air line being admixed in a swirled state with the fuel stream after discharge from the fuel conveying space inside the fuel conveying tube.
One of advantages of the present invention is that a solution is now created by which it is possible, without disadvantageously affecting the low-NOX combustion characteristic of the burner, to feed pulverous fuel to the burner by dense phase conveyance and to first mix the fuel with combustion air in the burner. This is achieved by an annular passage being formed and provided, in which just before the mouth region on the discharge side of the burner to the furnace, the pulverous fuel, especially pulverized dry brown coal, is conveyed in the burner along the longitudinal axis of the burner. This annular passage in this case is concentrically circumferentially arranged around the primary air stream. The primary air stream is now furthermore swirled, for which swirlers are arranged and formed in the primary air stream at a suitable point, so that at the end of the annular passage-form conveying channel it mixes with the fuel which is conveyed by dense phase conveyance, so that consequently at the mouth end on the discharge side of the burner to the furnace the radially outer region of the primary air stream is circumferentially uniformly enriched with the fuel, or there is fuel exclusively in this region. By means of this measure, fuel is therefore conveyed by dense phase conveyance almost only in the outer circumferential region of the primary air stream. This does not disadvantageously affect the low-NOX combustion characteristics of the respective burner, on the contrary these are assisted, boosted and optimized as a result. By means of the invention, it is possible to maintain the swirling of the primary air stream and to distribute the pulverized coal stream or coal mass stream uniformly over the circumference of the primary air stream on its radial outer edge region. Since, furthermore, the swirlers which are provided for swirling the primary air stream are not arranged in the pulverized coal stream or coal mass stream, these are also not exposed to the abrasive wear of the fuel particles, so that the burner according to the invention is characterized as well by a reduced wear compared with the conveyance of fuel in the primary air mixture.
In order to swirl the primary air stream and to be able to also use the dense phase conveyance which is provided according to the invention with a swirl flow burner, it is expedient if at least one swirler is arranged in the inside space of the primary air tube, which the invention provides in a development.
In order to also be able to operate the burner with an oil burner lance or suchlike, it is advantageous according to a development of the invention if a core air tube is arranged concentrically to the primary air tube within it. An oil burner lance or suchlike, as this is known from customary burners, can then be arranged in the core air tube.
Since the conveyance of the particulate fuel, especially pulverous fuel, is carried out in dense phase, large conveying cross sections, such as annular passage cross sections or pipeline cross sections, are not necessary for this conveyance, so that the invention is furthermore characterized in that the radial distance between core air tube and primary air tube is greater than the radial distance between primary air tube and fuel conveying tube.
In this case, it is furthermore especially advantageous and especially expedient if the core air tube extends in the longitudinal axial direction beyond the mouth end on the discharge side of the primary air tube into the region of the mouth opening on the discharge side of the fuel conveying tube. As a result of this, a mixing chamber, in which the swirled primary air stream and the dense stream conveyed stream of fuel are mixed with each other, is formed inside the burner at the mouth end on the discharge side of the burner to the furnace. As a result of this, it is especially possible to re-use the geometry of previously existing burners and to retrofit these simply by introducing a new primary air tube for the combustion of fuel which is conveyed in dense phase. The originally available primary air tube then becomes the fuel conveying tube. Greater or more costly constructional steps are not to be otherwise taken on the furnace which is provided with burners.
Since for conveying the primary air stream a furthermore relatively larger opening cross section is made available, it is possible to arrange there a plurality of swirlers, to arrange these on the outer generated surface of the core air tube, especially when a central coaxial core air tube is available. The invention, therefore, furthermore provides that a plurality of swirlers, preferably in the form of a vane ring, circumferentially distributed on the outer surface of the core air tube, are arranged radially on the outer side in the annular passage to the primary air tube.
Furthermore, it is advantageous if the burner according to the invention is also equipped with an oil burner ignition lance, for which reason, according to a development of the invention, an ignition lance, especially an oil burner ignition lance, is arranged coaxially in the core air tube.
ince with the configuration according to the invention especially low-NOX burners are to be provided, it is expedient if these additionally also have a secondary air feed and a tertiary air feed. The invention, therefore, is further characterized by a secondary air feed which encompasses the fuel conveying tube, and also a tertiary air feed which encompasses the secondary air feed.
In a further expedient development, it is then also expedient, especially for low-NOX burners, if a radially inwards extending stabilizing ring is arranged at the end on the combustion chamber mouth side of the fuel conveying tube.
The method according to the invention in an expedient development first of all provides that the primary air is admixed with the fuel stream in the mouth region of the burner in a mixing chamber before discharging into the furnace. As a result of this, the possibility arises of using the geometry of existing burners and of creating an intensive mixture of primary air and pulverous fuel for low-NOX combustion. Finally, in this case then it is still especially advantageous if air, especially heated air, recirculated flue gas or a mixture of air and recirculated flue gas, is fed to the burner as primary air.
The present invention is subsequently exemplarily explained in more detail with reference to a drawing.
FIGURE shows, in schematic view in detail and according to an exemplary embodiment of the present invention, a cross section along a longitudinal axis of an end on a combustion chamber side of a low-NOX burner which is arranged in the wall of a combustion chamber or of a furnace of a steam generator, especially of a large power plant.
The burner 1, on the discharge side, leads into the combustion chamber or into the furnace 2 of a steam generator and is arranged in its circumferential wall. The burner 1 has an oil burner ignition lance 4 which is arranged centrally along its longitudinal axis 3. This is arranged in the centre of a core air tube 5 which coaxially encompasses the oil burner ignition lance 4. The air guiding device 6, which is arranged around the oil burner ignition lance 4 at its end which faces the furnace 2, ends flush with the mouth end on the burner discharge side of a fuel conveying tube 7, in which primary air, which is fed to the burner through a primary air line 18 and mixed with particulate fuel which is fed through the fuel conveying tube 7, discharges into the furnace 2. The fuel conveying tube 7 is concentrically arranged around the oil burner ignition lance 4 and also around the core air tube 5, and with the outer generated surface of a primary air tube 9, and, downstream of the end of the primary air tube towards the furnace 2, with the outer generated surface of the core air tube 5, forms in each case an annular passage-form conveying cross section. At the end on the combustion chamber discharge side of the fuel conveying tube 7, an air deflecting throat 8, which extends over the whole circumference of the fuel conveying tube 7, is formed on the outer side. On the inner side of the fuel conveying tube 7, a stabilizing ring 14, which is provided with teeth, extends in this region radially inwards into the conveying cross section of the fuel conveying tube 7 and forms the termination of the fuel conveying tube 7 at this point. The core air tube 5 terminates on the burner inner side at a distance to the mouth end on the combustion chamber discharge side of the fuel conveying tube 7 in the region of the air guiding device 6.
In the annular passage-form conveying cross section, which is formed between core air tube 5 and fuel conveying tube 7, the primary air tube 9 is arranged coaxially to the core air tube 5 and to the fuel conveying tube 7 so that a fuel conveying space 13 is formed with an annular passage-form conveying cross section. The mouth end of the primary air tube 9 which faces the furnace 2 terminates at a distance from the mouth ends on the burner discharge side both of the core air tube 5 and of the fuel conveying tube 7, so that on the burner inner side a mixing cross section or mixing chamber 10 is formed. The end on the mouth side of the primary air tube 9 is located approximately in the region of the arrangement of swirlers 15 which in the case of low-NOX burners are customarily provided in the primary air stream. In the exemplary embodiment, the primary air tube 9 extends into the burner 1 to such an extent that its end on the mouth side is arranged in the positioning region of adjustable swirlers 11, 12 which are located in a secondary air feed 19 and in a tertiary air feed 20. The primary air tube 9, furthermore, is arranged relative to the core air tube 5 and to the fuel conveying tube 7 in such a way that its radial distance to the core air tube 5 is greater than to the fuel conveying tube 7. Therefore, the annular passage-form fuel conveying space 13 is formed between the outer side of the primary air tube 9 and the inner side of the fuel conveying tube 7. This annular fuel conveying space 13 is connected, or can be connected, to a feed line, which is not shown, by which fuel is pneumatically fed by dense phase conveyance to the burner 1, and then is also pneumatically conveyed in the fuel conveying space 13 in dense phase. The fuel is particulate fuel, especially pulverized coal, preferably pulverous dry brown coal. The dense phase conveyance is carried out with a load of 20-90 kg of pulverized coal/kg of conveying gas, and a conveying speed of 3 to 20 m/s at pressures of <10 bar. The radial distance between the outer side of the primary air tube 9 and the inner side of the fuel conveying tube 7, and consequently the passage height of the fuel conveying cross section or fuel conveying space 13, can be approximately as great as the length by which the teeth of the stabilizing ring 14 extend radially inwards in the direction of the burner axis 3.
Primary air is fed to the burner 1 through the annular passage-form conveying cross section which is formed between the inner side of the primary air tube 9 and the outer side of the core air tube 5. The primary air can be air, especially preheated air, but can also be recirculated flue gas or a mixture of recirculated flue gas and air. The primary air, which is conveyed through the primary air line 18 with circular passage-form conveying cross section, when discharging is transferred into a swirled stream by means of adjustable swirlers 15 which are arranged on the inner side on the primary air tube 9 between core air tube 5 and primary air tube 9 at the end on the discharge side. The swirlers 15 are formed in the form of a vane ring consisting of guide vanes which are arranged in a uniformly distributed manner on the outer circumference of the core air tube 5.
Furthermore, the burner 1, as known from customary burners, has a secondary air tube 16 which is arranged on the outer side coaxially around the fuel conveying tube 7, and a tertiary air tube 17 which in turn is arranged on the outer side coaxially at a distance around the secondary air tube. Secondary air is fed through the secondary air tube 16 to the furnace 2, and tertiary air is fed through the tertiary air tube 17 to the furnace 2, as this is known from known burners for creating a multistage low-NOX combustion.
For controlling a possibly desired swirling both of the primary air and of the secondary air in the secondary air feed and of the tertiary air in the tertiary air feed, it is provided that the respective swirlers 15, 11 and 12 which are arranged therein are formed with adjustment capability. However, it is also possible to form the swirlers 15, 11 and/or 12 in each case without adjustment capability.
During operation of the burner 1 according to the invention, pulverized coal is pneumatically conveyed through the fuel conveying space 13 by dense phase conveyance inside the burner 1 along the burner axis 3. This fuel conveying space 13 is located radially outside the primary air line 18 through which primary air is conveyed and which is formed by the inside space of the primary air tube 9 and the outer generated surface of the core air tube 5. The supplied primary air is swirled in the discharge region of the primary air line 18, or of the primary air conveying cross section, by means of swirlers 15, and mixes with the dense stream of fuel which leaves the fuel conveying space 13. In the mixing cross section or mixing space 10, which is formed in this region of the burner 1, a mixing-through of the two components, which are primary air and fuel, is carried out, forming an enrichment of the radial outer region of the developing air stream with fuel, so that the coal material is conveyed into the region of the teeth of the stabilizing ring 14 and then discharges into the furnace 2. The primary air stream which is loaded with coal particles in this way leaves the burner 1 with a conveying speed of about 15-25 m/s, preferably 18-20 m/s.
With the burner construction according to the invention, it is therefore possible to convey fuel by pneumatic dense phase conveyance into the direct discharge region of a burner 1 and to first mix the fuel with primary air there. Only the smaller cross sections which are required for the dense phase conveyance are necessarily made available. Moreover, the swirlers 15 which cause swirling of the primary air are not exposed to the pulverized coal stream, so that these are not subjected to wear loading by pulverized coal particles.
Even if the embodiment above represents a construction with a centrally arranged ignition burner, then, however, it is also possible to provide burners which are constructed without core air feed and ignition burners. The necessary geometric conditions are then created depending upon burner construction. According to the invention, it is only important in this case that primary air is conveyed centrally on the inner side in a primary air tube with regard to the longitudinal axis of the burner, and then, in a swirled state, is admixed with a fuel stream which is conveyed radially outside the primary air stream in dense phase.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102007030269.1-13 | Jun 2007 | DE | national |
