Abstract:
Disclosed is a burner for particulate fuel, in particular made of biomass, with a primary tube and a core tube arranged in the primary tube. The primary tube and the core tube form a primary tube gap and the primary tube gap is configured to guide a flow of particulate fuel and gaseous combustion means from an inlet-side end to an outlet-side opening of the primary tube. In order to prevent the drawbacks occurring when using coarse-grain particles, preferably biomass, as a fuel for dust firing, or at least to reduce them without having to accept an increased outlay for equipment and/or additional energy losses, at least one device is provided for centring the flow within the primary tube in the region of the outlet-side end of the primary tube.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1) Field of the Invention 
         [0002]    The invention relates to a burner for particulate fuel, in particular made of biomass, with a primary tube and a core tube arranged in the primary tube, the primary tube and the core tube forming a primary tube gap and the primary tube gap being configured to guide a flow of particulate fuel and gaseous combustion means from an inlet-side end to an outlet-side opening of the primary tube. 
         [0003]    2) Description of Prior Art 
         [0004]    Burners for the combustion of particulate fuels, such as, in particular, coal, in a combustion chamber have been known for some time. Dust firing is also referred to in this connection. 
         [0005]    A burner of this type is described, for example, in EP 0 571 704 A2. The burner has a core tube, which has air flowing through it, and has a burner gun for igniting the particulate fuel. Arranged concentrically with respect to the core tube is a primary tube, which, with the core tube, forms an annular gap, which is connected at its rear end to a dust line. A mixture of coal particles and primary combustion means (primary air) is supplied via the dust line to the burner. The mixture of coal particles and combustion means is made to rotate by means of a swirling body arranged in the annular gap, so the coal particles are concentrated in the outer region of the annular gap. 
         [0006]    Additionally provided concentrically with respect to the primary tube are a secondary tube and a tertiary tube, which, with the respective inner tube, define a secondary and a tertiary annular gap, which have secondary and tertiary combustion means (secondary air and tertiary air) flowing through them. Swirling bodies are also provided in the secondary and the tertiary annular gaps in order to impress a swirl on the combustion means. Conical widenings in the wall of the combustion chamber are provided at the outlet-side end of the secondary tube and the tertiary tube. 
         [0007]    Provided at the outlet-side end of the primary tube is a so-called flame-holder, which has a radially inwardly directed edge leading to stalling and to turbulence of the coal particles. Thus, a flow is produced, which is directed into the combustion chamber, with a high degree of turbulence and coal particle concentration. This flow is “surrounded” by the flows leaving the core tube, the secondary annular gap and the tertiary annular gap. Owing to the high degree of turbulence of the particle-rich flow, the volatile components are very rapidly expelled from the coal particles. Because of the high particle concentration, the air ratio is strongly sub-stoichiometric, so less nitrogen oxides (NOx) are formed. 
         [0008]    The burners of the type mentioned can basically also be used to burn particulate fuels other than coal, for example biomass. For this purpose, the biomass has to be very finely ground, however, which, because of the usually fibrous and tough structure of conventional biomasses, is linked with an increased outlay for equipment and energy. In particular, the fine grinding of biomass often entails a high degree of wear of the equipment used for this. Biomasses are therefore not generally ground so finely as coal. In the case of hard coal, the particle size is typically 90% smaller than 90 μm and, in the case of brown coal, 90% smaller than 200 μm. On the other hand, in biomass, a mean particle size of about 1 mm is desired. 
         [0009]    The volatile components of the biomass particles are already expelled more slowly because of their size, which can impair stable combustion of the biomass. In addition, a correspondingly larger quantity of air, the so-called carrying air, has to be used in order to transport the larger biomass particles, free of deposits, from the crusher through the burner into the combustion chamber. The larger carrying air quantity flowing through the annular gap between the primary tube and the core tube can, together with a delayed release of volatile components, lead to a local air excess during the combustion. As a result of this, more nitrogen oxides are formed. 
         [0010]    The present invention is therefore based on the object of preventing the drawbacks occurring during the use of coarse-grain particles, preferably biomass, as a fuel for dust firing, or at least to reduce them, without an increased outlay for equipment and/or additional energy losses having to be accepted. 
       SUMMARY OF THE INVENTION 
       [0011]    This object is achieved in a burner of the type mentioned at the outset and described in more detail above in that at least one device is provided for centring the flow within the primary tube in the region of the outlet-side end of the primary tube. 
         [0012]    The invention recognised that the drawback of an increased carrying air quantity and larger particle diameters in the combustion of coarse-grain fuels, such as biomass, can be in any case partially compensated by a fluidic deflection of a part of the primary air in the direction of the core zone of the burner mouth. The deflection makes it possible to guide a part of the primary air around the flame-holder or to guide it centrally through the latter, without this part of the primary air arriving in the turbulent particle flow zone adjoining the flame-holder. This only takes place at a later point in time, at which the turbulent particle flow zone has widened and the volatile components of the fuel particles have escaped to a greater extent. The particle concentration is consequently high in the particle flow downstream of the flame-holder. Consequently, the flame of the burner can be stabilised despite a delayed escape of volatile components. In addition, the oxygen concentration in the particle flow downstream of the flame-holder is clearly sub-stoichiometric, which counteracts the formation of nitrogen oxides. 
         [0013]    The deflection of a part of the primary air in a central region of the burner is made possible by the swirling of the primary air in the primary annular gap, which concentrates the fuel particles in the outer region of the primary annular gap and feeds them to the flame-holder. The concentration of the fuel particles in the outer region of the primary annular gap is accompanied by a depletion of particles in the inner region of the primary air flow. The invention makes use of the invention to divert a part of the primary air into the central region of the burner without this having significant effects on the transport of the fuel particles. However, it is necessary for the partial diversion of the primary air flow to adapt the burner geometry in such a way as to provide space for the primary air flow to be deflected. This space is not present in conventional burner geometries. 
         [0014]    According to the invention, it is unnecessary for the fuel particles to be transported by an air flow through the primary tube gap even if this is appropriate for cost reasons. Instead of air, another combustion means known per se could also be used. It would even be conceivable to use an oxygen-free gas if the oxygen required for the combustion is otherwise provided. For the sake of simplicity, the term primary air is used, however, below. 
         [0015]    There are also basically no limits with regard to the core tube. Gas having oxygen or an oxygen-free gas can flow through the core tube, which may, in particular, be expedient to cool the core tube. As an alternative or in addition, a burner gun can be received in the core tube to provide a support or ignition flame. Quite in general, core tubes may be provided which are constructed in a manner known per se. As a result of the deflection of a part of the primary air flow, if necessary, a flow through the core tube can be dispensed with. This may also favour the deflection of the primary air into the central region of the burner behind the core tube. 
         [0016]    The person skilled in the art will recognise that the core tube and the primary tube preferably have circular cross-sections and are arranged concentrically with respect to one another as this is favoured in terms of flow technology. The core tube and the primary tube then form a primary tube gap in the form of a symmetric annular gap. Basically, there could be a deviation both from circular cross-sections of the core tube and primary tube and/or from a concentric arrangement of these tubes, even if, as a rule, this is less preferred. However, for the sake of simplicity, in the present case, only the terms core tube and primary tube are used instead of core channel and primary channel without this inevitably having to be interpreted in a restrictive manner. 
         [0017]    In a first configuration of the invention, the core tube ends before the primary tube, viewed in the longitudinal direction of the burner. The inner part of the primary air can therefore arrive on time in front of the flame-holder in a central region of the burner and, uninfluenced by the flame-holder, in a flow that is laminar as far as possible, bypass the latter there. So an inner part of the primary air can reach the central region or core region of the burner and can form a uniform flow as far as possible, an adequate spacing has to be provided between the outlet-side end of the core tube and the outlet-side end of the primary tube or the flame-holder—if present. In the longitudinal direction of the primary tube, this spacing should be at least 50% of the mean radial width of the primary tube gap. From a flow point of view, it is more favourable, however, if this spacing is at least 75%, in particular at least 100%. A very short spacing in comparison to this may be counter-productive, in particular if the core tube ends abruptly. The flow close to the core tube in the primary gap can be stalled there and extend the turbulent region after the flame-holder. Thus, specifically no part flow of the primary air flow is guided around the flame-holder and the turbulent region of particle-rich flow adjoining the latter. 
         [0018]    Alternatively or in addition, the core tube may taper toward its outlet-side end. The tapering of the end of the tube, compared to an abrupt end of the core tube, has the advantage that the flow can be guided more uniformly. Stalling and turbulences can thus be avoided in the region of the inner part of the primary air flow. It is particularly preferred if a tapering of the core tube is accompanied by a longitudinal-side spacing of the flame-holder or opening of the primary tube, on the one hand, and the outlet-side end of the core tube, on the other hand. 
         [0019]    For flow reasons, the core tube may taper constantly toward its outlet-side end. The tapering may be uniform or non-uniform here. It is favourable in terms of flow technology if the tapering decreases to the outlet-side end in order to avoid stalling before the end of the core tube. 
         [0020]    To even out the flow, it is preferred, in particular in the case of a round core tube, if the latter tapers conically toward its outlet-side end. In this case, the angle of inclination of the cone should not be too great to avoid a stall. Angles of inclination of less than 20° are preferred here. In order to avoid a stall even at higher flow speeds, angles of inclination of less than 10° are appropriate. During testing, particularly good results were achieved with angles of inclination of about 7°, if necessary with a deviation of ±1°. 
         [0021]    To support the deflection of a part of the primary air, a deflection device may be provided at the outlet-side end and outside the core tube in the primary tube to deflect the flow guided close to the core tube in the primary tube gap inwardly. As a result, it can, for example, be ensured that the desired proportion of primary air is also diverted in the direction of the centre. In addition, the deflected part flow can be guided in a more laminar manner owing to the additional surfaces of the deflection device. The deflection device preferably projects into the primary tube gap, in particular into the primary air flow. 
         [0022]    In order to avoid a deflection of the fuel particles into the core region of the burner, the deflection device may be configured to deflect about 30% by volume to 70% by volume of the air flow in the primary tube gap. In this case, it is appropriate if the deflection device approximately extends into the primary tube gap, preferably radially, to over 30% to 70% of the width of the primary tube gap. Particularly good results are achieved if the deflection device is configured to deflect about 40% by volume to 60% by volume of the air flow in the primary tube gap and/or extends therein to over 40% to 60% of the gap width of the primary tube gap. 
         [0023]    Provided between the core tube and the deflection device is preferably a flow channel, through which the deflected primary air flow is guided. In this case, it is particularly preferred from the technical flow point of view if the free flow cross section in the flow channel of the deflection device remains constant. An unfavourable variation with respect to energy of the flow speed can thus be avoided. 
         [0024]    As an alternative or in addition to further devices, at least one flow director may be provided in the primary tube gap to influence the swirl of a part close to the core tube of the flow guided in the primary tube gap. A widening of the primary air flow after leaving the primary tube gap can be counteracted, for example, by influencing the swirl of at least the flow close to the core tube, which favours the centring of the part flow of the primary air close to the core tube. A plurality of flow directors, preferably distributed over the periphery of the primary tube gap, may also be provided. The number of flow directors should preferably increase here with the diameter of the primary tube. 
         [0025]    The at least one flow director may be oriented in the longitudinal direction of the primary tube. The swirl of at least one part of the primary air flow close to the core tube is at least weakened thereby, which can have a favourable effect on the flow conditions. In order to direct the flow but not to lastingly disrupt it, the at least one flow director should be much wider in the longitudinal direction than in the peripheral direction of the primary tube gap. 
         [0026]    The at least one flow director may, as an alternative thereto, also be oriented transverse to the longitudinal direction, i.e. partially in the peripheral direction, of the primary tube. In this case, the orientation of the at least one flow director may differ from an orientation in the longitudinal direction of the primary tube in such a way that the swirl of the primary air flow is intensified by the at least one flow director, at least for a part of the primary air flow close to the core tube. The at least one flow director may, however, also reduce the swirl of the primary air flow in an also possible orientation pointing more in the longitudinal direction of the primary tube. The at least one flow director may, however, also be oriented in the opposite direction to the swirl direction of the primary air flow. This may, for example, lead to the fact that the swirl direction of the primary air flow is reversed at least for a part of the primary air flow close to the core tube. In order to intensify the swirl of the primary air flow in regions, it may be expedient to incline the at least one flow director by 35° to 45° relative to the longitudinal direction of the primary tube. In order to weaken the swirl of the primary air flow in regions, it may be favourable to incline the at least one flow director by less than 25°, in particular less than 15°, relative to the longitudinal direction of the primary tube. 
         [0027]    Depending on the boundary conditions in terms of flow technology, each of these orientations of the at least one flow director may entail positive effects. Basically, by means of a swirl, which has a different strength or is differently directed, of parts of the primary air flow, a separation in terms of flow technology of these parts can be achieved, as the latter have different properties in terms of flow technology. To enable a control of the burner, the at least one flow director may be variable with regard to its orientation, i.e. incline compared to the core tube. 
         [0028]    It is also conceivable for the at least one flow director to have a varying incline in the longitudinal direction of the primary tube in order to achieve a gradual change in the swirl direction of the part of the primary air flow close to the core tube. Alternatively or in addition, however, a plurality of flow directors or groups of flow directors distributed over the periphery of the core tube may also be provided one after the other. In this case, it is particularly preferred if the incline changes relative to the longitudinal direction of the primary tube from flow director to flow director or from one group of flow directors to the next group of flow directors in the longitudinal direction of the primary tube. 
         [0029]    Moreover, it can easily be achieved that the swirl of the outer flow in the primary tube gap remains uninfluenced by the at least one flow director in order to not impair the flame stability. For this purpose, the flow director is not provided in the last outer 20% of the primary tube gap. If the outer 30% or even 40% of the primary tube gap can be kept free of flow directors, this is favoured in terms of flow technology. The outer flow director-free region may, for example, be increased in that the number of flow directors arranged on the peripheral side is increased. 
         [0030]    To adjust defined flow conditions in the primary tube gap, it may be preferred if the at least one flow director is connected downstream in the flow direction of the primary air flow from a swirling device to impress a swirl on the primary air flow. The swirling device is, in this case, in particular provided in the primary tube gap even if the flow can basically already be made to rotate by being supplied to the primary air gap. The impressing of the swirl on the primary air flow may take place by means of swirling bodies, for example in the form of guide vanes or guide plates. These may preferably be inclined by 20° to 30°, in particular about 25°, compared to the longitudinal direction of the primary tube. 
         [0031]    The at least one flow director is preferably provided in front of a deflection device in the flow direction, so the primary air flow of the deflection device can be supplied in a suitable manner. In this case, the flow director may, if necessary, be provided directly in front of the deflection device. It may even be provided that the flow director and the deflection device are connected to one another in order to rule out a possible impairment of the flow in the intermediate space. In order to avoid increased abrasion by fuel particles, wear-resistant materials, such as hard welding applications or ceramics, may be used for the flow directors. 
         [0032]    A flame-holder projecting inwardly into the flow of the primary tube may be provided at the outlet-side end of the primary tube to stabilise the flame. The edge of the flame-holder preferably pointing radially inwardly may be continuous or interrupted. A toothed edge, which can produce a high degree of turbulence, is also conceivable. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    The invention will be described in more detail below with the aid of drawings showing only embodiments. In the drawings 
           [0034]      FIG. 1  shows a first embodiment of the burner according to the invention in a longitudinal section; 
           [0035]      FIG. 2  shows a detail of the burner according to  FIG. 1  in a longitudinal section; 
           [0036]      FIG. 3  shows a detail of a second embodiment of the burner according to the invention in a longitudinal section; 
           [0037]      FIG. 4  shows a detail of a third embodiment of the burner according to the invention in a longitudinal section; and 
           [0038]      FIG. 5  shows a detail of a fourth embodiment of the burner according to the invention in a longitudinal section. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0039]      FIG. 1  shows a longitudinal section through a burner  1 , which is arranged in a wall W of a combustion chamber F. The inner part of the burner  1  from  FIG. 1  is shown to an enlarged scale in  FIG. 2  for improved clarity. 
         [0040]    A core tube  2 , in which a burner gun, not shown, can be provided, is provided in the centre of the burner  1 . Other devices are also possible, which are shown here purely schematically. The core tube  2  is arranged concentrically with respect to a primary tube  3 , so a peripheral concentric primary tube gap  4  is provided between the core tube  2  and the primary tube  3 . A mixture of particulate biomass and combustion means, the primary air, is supplied to said primary tube gap by devices, not shown. Provided in the primary tube gap  4  is a swirling device  5  in the form of guide vanes which are set at about 25° relative to the longitudinal extent of the primary tube and which make the primary air flow rotate. The biomass particles then migrate in the flow direction, because of centrifugal forces, to the outside, where the biomass particle concentration increases, while it accordingly decreases in a region close to the core tube. A flame-holder  7 , which defines the outlet opening  8  of the primary tube  3 , is provided at the outlet-side end  6  of the primary tube  3 . A toothed edge  9 , which points radially inwardly, is provided on the inside of the flame-holder  7  and comes into contact with the primary air flow and the biomass particles and, following this, ensures a swirling of the flow, which is indicated in  FIG. 1  by the sharply curved arrows A. 
         [0041]    A secondary tube  10  which, with the primary tube  3 , forms a secondary tube gap  11 , is provided concentrically with respect to the primary tube  3 . The secondary tube gap  11  has secondary air flowing through it, said secondary air having a swirl impressed on it by means of swirling devices  12  in the form of guide vanes set relative to the longitudinal extent of the primary tube in the secondary tube gap  11 . The secondary air does not have to be air in the actual sense. Provided at the outlet-side end  13  of the secondary tube  10  is a secondary groove  14 , which is a conical widening of the secondary tube  10  and deflects the secondary air flow radially outwardly. 
         [0042]    Provided on the outlet-side end  6  of the primary tube  3  is an outwardly pointing primary groove  15  in the form of a conical widening, which contributes to the outward deflection of the secondary air flow and leads to a stalling at the flame-holder  7 . This stalling assists the configuration of the turbulent swirling of the biomass particles after the flame-holder  7 , as is shown by the arrows B in  FIG. 1 . 
         [0043]    Arranged concentrically with respect to the secondary tube  10  is a tertiary tube  16 , which, with the secondary tube  10 , forms a tertiary tube gap  17 . The tertiary air is guided to the combustion chamber F in the tertiary tube gap  17 , this not having to be air in the traditional sense, which is made to rotate by means of swirling devices  18  in the tertiary tube gap  17 . The tertiary tube  16 , at its outlet-side end  19 , has a conical widening, which is also called a muffle  20  and preferably has a larger angle of inclination than the secondary groove  14 . The muffle  20  is used to deflect the tertiary tube flow outwardly. For the purpose of cooling, cooling lines L associated with the muffle  20  are provided in the wall W of the combustion chamber F. In the shown and to this extent preferred burner  1 , the secondary groove  14  is set back inwardly relative to the muffle  20 . The secondary groove  14  could, however, also be configured aligned with the muffle  20 , in particular flush with the wall W of the combustion chamber F. 
         [0044]    The outlet-side end  21  of the core tube  2  does not only end significantly in front of the flame-holder  7 . The core tube  2 , at the outlet-side end  21 , also has a conical taper  22 . The axial spacing D between the core tube  2  and the flame-holder  7 , in the shown and to this extent preferred burner  1 , is at least equal to, if not greater than, the radial spacing R between the core tube  2  and the primary tube  3 , in other words the width of the primary tube gap  4 . 
         [0045]    Accordingly, the outer diameter of the core tube  2  in the region of the outlet-side end  21  decreases with an increasing closeness to the outlet-side end  21  in the longitudinal direction. In the shown and to this extent preferred embodiment, the conical taper  22  at the outlet-side end  21  has a constant angle of inclination a of substantially 7°. As a result of this configuration of the core tube  2  and the axial spacing D between the core tube  2  and the flame-holder  7 , a part flow of the primary air close to the core tube is deflected at the outlet-side end  21  of the core tube  2  and thereafter in the direction of the axial core region of the burner  1 . A centring of the primary air flow at the outlet-side end of the core tube  2 , in particular, however, at the outlet-side end of the primary tube  3 , thus takes place. This centring, as illustrated by the arrows C in  FIG. 1 , leads to a part of the primary air being deflected centrally around the flame-holder  7 , in particular around the edge  9 , which is directed inwardly, of the flame-holder  7 , without this part flow arriving directly in the highly turbulent particle-rich flow region produced by the flame-holder  7 . At a later time, at which the centrally deflected part flow of the primary air is located further in the interior of the combustion chamber F, the deflected part flow may, however, very well come into close contact with the fuel particles, if necessary, in order to oxidise them. 
         [0046]      FIG. 3  shows a detail of a burner  30  in a longitudinal section in accordance with  FIGS. 1 and 2 . The same components have been given the same reference numerals here. The important difference between the burners  1 ,  30  shown in  FIG. 1  and  FIG. 3  is that the core tube  2 , peripherally on its outer lateral surface  31 , has a plurality of flow directors  32 , which are thin in the peripheral direction. The flow directors  32  extend parallel to the longitudinal extent of the burner  30  or the core tube  2  and therefore deflect a part of the primary air in the axial direction. 
         [0047]    The flow directors could, however, alternatively also be inclined to the left or right, i.e. extend both in the longitudinal direction and transverse to the longitudinal direction of the primary tube  2 , similarly to that which is the case with the swirling devices. Depending on in which direction and with which incline the flow directors are inclined in the peripheral direction of the core tube, the swirl of the part of the primary air flow close to the core tube is intensified or weakened. An incline of greater than 45° to 90° is basically less preferred here as the primary air flow is thus clearly decelerated. 
         [0048]    The flow directors  32  of the shown and to this extent preferred burner  30  allow the rotation of the primary air to be eliminated at least for a part of the primary air flow close to the core tube. In the burner  30  shown, the outer primary air part flow adjoining the primary tube  3  is not influenced by the flow directors  32 . This primary air flow thus continues to rotate. For this purpose, the radial extent of the flow directors  32  in the shown and to this extent preferred burner  30  merely corresponds to about 40% of the radial spacing R between the core tube  2  and the primary tube  3 . 
         [0049]    The substantially axial core flow in the primary tube gap  4  is particularly well deflected into a central region of the burner  1  by the conical region  22  of the core tube  2  and the axial spacing D from the flame-holder  7 , as indicated by the arrow C in  FIG. 3 . 
         [0050]      FIG. 4  shows a detail of a burner  40  in the longitudinal section, which in addition to the burner  30  according to  FIG. 3 , has a deflection device  41 . The deflection device  41  is associated with the outlet-side end  21  of the core tube  2  and forms a concentric annular gap adjoining the core tube  2 . In the shown and to this extent preferred burner  40 , the deflection device  41  covers the conically tapering portion  22  of the core tube  2 , which is formed in the embodiment by a reduction in the material thickness of the core tube  2 . Before the conically tapering portion  22 , the deflection device  41  forms an inlet region  42 , in which the flow is oriented substantially axially, but not radially. The inlet region  42  may be formed by a concentric tube sleeve. In the region of the conically tapering portion  22  of the core tube  2 , the deflection device  41  in the shown and to this extent preferred burner has a portion tapering at the same angle of inclination a as the core tube  2 . So that the flow cross section in the deflection device  41  does not decrease too sharply, the conical portion of the deflection device  41  may also be slightly less inclined, if necessary, than the conical portion  22  of the core tube  2 , so a constant flow cross section is provided, for example, in the deflection device  41 . The deflection device  41  is preferably configured as an axially peripheral component, which ends in the same plane as the core tube  2 . The deflection device  41  is spaced apart from the flow directors  43  and, in the shown and to this extent preferred burner  40 , has a substantially similar radial overall height as the flow directors  43 . 
         [0051]      FIG. 5  shows the detail of a burner  50 , in which the flow directors  51  arranged distributed over the periphery of the core tube  2  are directly connected to the deflection device  52 . Put more simply, the flow directors  51  guide the part flow close to the core tube in the primary tube gap  4  into the deflection device  52 , which is configured as an axially peripheral component. In the burner  50  shown in  FIG. 4 , the deflection device  52  extends further in the direction of the outlet-side end  6  of the primary tube  3  or the flame-holder  7 , than the core tube  2 . The deflection device  52  thus ultimately projects relative to the core tube  2  in the flow direction for sealing off relative to the turbulences produced by the flame-holder  7 .