Abstract:
A burner nozzle for delivering fuel to a burner flame in a furnace includes an inner cylinder and an outer cylinder that are both hollow. The inner cylinder is at least partly disposed within the outer cylinder and axially aligned with it, and is movable in an axial direction relative to the outer cylinder. One end of the inner cylinder has at least one outward projection extending in a radial direction from the outer surface of the cylinder, this projection serving to decrease the free cross-sectional area between the inner cylinder and the outer cylinder at that end of the inner cylinder.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a nozzle for a power station burner, in particular to a nozzle that is adjustable for different fuel types, and to a method for the use thereof. 
     BACKGROUND TO THE INVENTION 
     Biomass or waste fuels (e.g. wood pellets, wood chips, miscanthus, straw, olive cake, palm kernels, sugarcane, corncobs, groundnut shells, refuse derived fuel and solid recovered fuel) have become increasingly popular for use in firing power stations. However, they have not completely replaced coal, and so it is desirable to provide burners for power station furnaces that are able to be operated with both types of fuels. 
     Due to the different combustion characteristics of coal and biomass, the provision of such burners is technically challenging. In particular, it is desirable to provide burners that may be quickly and easily re-configured for use with a different fuel. 
     SUMMARY OF THE INVENTION 
     Biomass and coal fuels are typically delivered into a furnace in pulverised, particulate, or shredded form. The present inventors have found that a significant difference in the combustion characteristics of biomass and coal lies in the different particle velocities that are required to form a stable flame at the mouth of the burner. 
     Therefore, at its most general, the present invention may provide a fuel nozzle for a burner, in which the free cross-sectional area of the nozzle at its exit is adjustable. The free cross-sectional area denotes the portion of the nozzle exit that is available for particle flow therethrough, that is, the portion of the nozzle exit that is unobstructed. As is well-known in this technical field, a high free cross-sectional area will result in low fuel particle velocity. Conversely, a low free cross-sectional area will result in high fuel particle velocity. 
     The free cross-sectional area of the exit is adjustable by providing one or more obstructions that may be moved between a position at the nozzle exit and a position upstream of the nozzle exit. It is thought that when the one or more obstructions are located upstream of the nozzle exit, the fuel particles by-passing the obstruction have sufficient time to re-distribute around the nozzle area and slow down to the desired velocity once they reach the nozzle exit. Conversely, when the obstruction is positioned at the nozzle exit, the particles exit the nozzle with high velocity. 
     It is desirable that the mechanism for adjusting the free cross-sectional area of the nozzle exit is compact and interferes as little as possible with the operation of the burner. 
     Therefore, in a first aspect, the present invention may provide a burner nozzle for delivering fuel to a burner flame in a furnace, the nozzle comprising an inner cylinder and an outer cylinder, the inner and outer cylinders being hollow and the inner cylinder being at least partly disposed within the outer cylinder and axially aligned therewith, the inner cylinder being movable in an axial direction relative to the outer cylinder,
         wherein one end of the inner cylinder has at least one outward projection extending in a radial direction from the outer surface thereof, the at least one outward projection serving to decrease the free cross-sectional area between the inner cylinder and the outer cylinder at that end of the inner cylinder.       

     In general, the at least one outward projection is located at the downstream end of the inner cylinder, that is, at the end facing the nozzle exit. 
     This arrangement allows the free cross-sectional area at the nozzle exit to be adjusted relatively easily, simply by moving the inner cylinder along a longitudinal axis of the burner. There is no need to dismantle or substitute any of the existing parts of the burner with alternative or new parts. This helps to provide the burner with a high level of flexibility, such that it can easily be adapted to burn a different fuel. In certain cases this arrangement may allow adjacent burners to be operated under different modes of operation, e.g. such that each burner burns a different fuel. 
     Typically, the upstream end of the inner cylinder protrudes from the burner, and so the axial position of the cylinder may be manipulated by means of this protruding end. Effectively, therefore, the burner configuration may be adjusted externally to the burner. 
     Typically, the nozzle comprises a plurality of outward projections disposed at one end of the inner cylinder and projecting in a radial direction from the outer surface thereof. In general, these projections are disposed in a radially symmetrical distribution about the inner cylinder. This helps to ensure that the fuel particles leave the nozzle exit in a uniformly distributed manner. 
     Preferably, the at least one outward projection is configured such that, when viewed along an axial direction of the nozzle, the outward projection tapers in a radially inward direction of the nozzle. This helps to ensure that the radially inner portion of the nozzle exit is not obstructed excessively and that there is an acceptable fuel particle density around the longitudinal axis of the burner. 
     Preferably, the at least one outward projection subtends an angle of between 30° and 50′, more preferably between 35° and 45°, at the longitudinal axis of the nozzle. 
     Preferably, there is sufficient clearance between the inner surface of the outer cylinder and the at least one outward projection to allow for dynamic adjustment of the axial position of the inner cylinder. Typically, the gap between the inner surface of the outer cylinder and the at least one outward projection is less than 5 mm, preferably less than 4 mm. 
     Typically, the at least one outward projection is provided with a ridge at its radially outermost extent, the ridge extending in an axial direction of the nozzle and contacting the inner surface of the outer cylinder. This helps to ensure that the inner cylinder remains centred within the nozzle. 
     Typically, the free cross-sectional area between the inner cylinder and the outer cylinder at the location of the at least one outward projection is less than 80%, preferably less than 60%, more preferably less than 50%, of the total cross-sectional area between the inner cylinder and the outer cylinder. Thus, the arrangement according to the first aspect of the invention is capable of providing large differences in free cross-sectional area at the nozzle exit, so as to adapt the burner for use with different fuels. 
     Typically, the outer cylinder is provided at one end thereof with at least one inward projection extending in a radially inward direction thereof. In general, when viewed along the longitudinal axis of the nozzle, the inward projection tapers in a radially inward direction of the nozzle. Preferably, the inward projection subtends an angle in the range of 10° to 20°, more preferably 12° to 18°, at the longitudinal axis of the nozzle. 
     In certain embodiments, the inward projection extends less than half the distance between the outer cylinder and the inner cylinder. This helps to ensure that there is an acceptable fuel density around the longitudinal axis of the burner. 
     In general, the outer cylinder is provided at one end thereof with a plurality of inward projections extending in a radially inward direction thereof. Typically, the plurality of inward projections are arranged in a radially symmetrical distribution around the outer cylinder. 
     The inward projections may help to provide radially distributed fuel-rich and fuel-lean zones immediately downstream of the nozzle exit. The fuel-rich zones tend to provide oxygen-lean environments within the resultant flame, such that NO x  emissions are reduced. Nitrogen oxides are pollutants that are regulated globally and so it is desirable to inhibit their formation. 
     In general, the nozzle has equal numbers of inward projections and outward projections, the inward and outward projections being arranged such that, when viewed along a longitudinal axis of the nozzle, the inward projections are each disposed between a pair of adjacent outward projections. Typically, the inward projections are each disposed midway between a pair of adjacent outward projections. 
     It has been found that different fuels require different airflow patterns around the burner flame in order to achieve a good balance of efficient combustion with low levels of harmful emissions (such as NO x  emissions). Typically, a burner is provided with two air sources for mixing with the fuel as it exits the nozzle. A first, radially inward air source helps to create an internal recirculation zone (IRZ) immediately downstream of the nozzle exit, while a second radially outward air source provides oxygen to allow combustion of the fuel as it escapes the IRZ. 
     Therefore, in a second aspect, the present invention may provide a burner comprising a nozzle according to the first aspect of the invention, and first and second air sources, the air sources each being disposed around the nozzle in a ring shape that is centred on the longitudinal axis of the nozzle,
         wherein the flow rate from the first air source is adjustable relative to the flow rate from the second air source.       

     The first and second air sources are typically provided with swirlers to give angular momentum to the air flow passing through them. 
     In a third aspect, the present invention may provide a method of adjusting the operating conditions of a burner for use with different fuels, comprising the steps of
         providing a burner comprising a nozzle according to the first aspect of the invention;   moving the inner cylinder of the nozzle along its longitudinal axis between a first position in which the outward projections are axially aligned with an end of the outer cylinder, and a second position in which the outward projections are axially displaced from an end of the outer cylinder.       

     Typically, the burner is a burner according to the second method of the invention, and the method comprises the further step of adjusting the flow rate from the first air source relative to the second air source. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described by way of example with reference to the following Figures in which: 
         FIG. 1  shows a schematic cross-sectional view of a burner comprising a nozzle according to an embodiment of the first aspect of the invention, the nozzle being arranged according to a first configuration; 
         FIG. 2  shows a schematic cross-sectional view of a nozzle according to a second embodiment of the first aspect of the invention, arranged according to a second configuration; 
         FIG. 3  shows a schematic plan view of the nozzle of the burner of  FIG. 1  arranged in the first configuration; 
         FIG. 4  shows a schematic plan view of the nozzle of the burner of  FIG. 1  arranged in the second configuration; 
         FIG. 5  shows a schematic view of  FIG. 4 , including the dimensions of the nozzle; 
         FIG. 6  shows a schematic perspective view of a magnified portion of the nozzle of the burner of  FIG. 1 ; 
         FIGS. 7 and 8  show schematic perspective views of the nozzle of the burner of  FIG. 1  arranged according to the first configuration; 
         FIGS. 9 and 10  show schematic perspective views of the nozzle of the burner of  FIG. 1 , arranged according to the second configuration; 
         FIGS. 11 and 12  show schematic perspective view of the upstream ends of the nozzles of  FIGS. 8 and 10  respectively. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , a burner  10  is mounted in the wall of a furnace (not shown) and has a flame side  11  that faces into the interior of the furnace. The burner comprises a plurality of concentric tubes. A core air tube  12  houses a gas igniter and an oil burner  14 . A ring-shaped nozzle  16  is disposed around the core air tube  12  and is concentric with the core air tube. The nozzle  16  comprises an inner cylinder  18  and an outer cylinder  20  that is concentric with the inner cylinder  18 . 
     The end of the inner cylinder  18  that is adjacent the core air tube is provided with outward projections  22  that extend in a radially outward direction of the cylinder  18 . The outward projections  22  also extend axially along a limited portion of the length of the inner cylinder  18 . The surfaces of the outward projections that face towards the interior of the furnace (that is, in a downstream direction of the nozzle) are oriented at an oblique angle of 58° relative to the longitudinal axis of the burner. Effectively, these surfaces together provide an interrupted generally concave surface about the longitudinal axis of the burner. The surfaces of the outward projections that face away from the interior of the furnace (that is, in an upstream direction of the nozzle) extend in a lateral direction from the burner axis. 
     The end of the outer cylinder  20  at the nozzle exit (that is, the end adjacent to the core air tube  12 ) is provided with inward projections  24  that extend in a radially inward direction of the outer cylinder  20 . The inward projections  24  also extend axially along a limited portion of the length of the outer cylinder  20 . The surfaces of the inward projections that face towards the interior of the furnace (that is, in a downstream direction of the nozzle) are oriented at an oblique angle of 58° relative to the longitudinal axis of the burner. Effectively, these surfaces together provide an interrupted generally concave surface about the longitudinal axis of the burner. The surfaces of the outward projections that face away from the interior of the furnace (that is, in an upstream direction of the nozzle) extend in a lateral direction from the burner axis. 
       FIG. 1  shows the nozzle arranged in a first configuration, that is, the position of the inner cylinder  18  along the longitudinal axis of the burner is such that the outward projections lie within the burner and are displaced from the nozzle exit. 
     A first air source  26  is provided in the shape of a ring that is disposed outwardly of the outer cylinder  20  and is concentric with it. The first air source has a swirler  28  to provide angular momentum to the air travelling through it. 
     A second air source  30  is provided in the shape of a ring that is disposed outwardly of the first air source  26  and is concentric with it. The second air source has a swirler  32  to provide angular momentum to the air travelling through it. 
     A fuel connection  33  provides a path for delivering fuel to the nozzle. 
       FIG. 2  shows a nozzle in a second configuration. The nozzle has slightly different dimensions to the one shown in  FIG. 1 , but this is does not affect the basic principle of its operation. Features  11 ′,  12 ′,  14 ′  18 ′,  20 ′, and  33 ′ correspond to features  11 ,  12 ,  14 ,  18 ,  20 , and  33  of  FIG. 1  respectively. The inner tube  18  is axially displaced relative to its position in  FIG. 1 , such that the outward projections are located at the nozzle exit. That is, the axial position of the outward projections corresponds to the axial position of the inward projections. 
       FIGS. 3 and 4  show the nozzle of the burner of  FIG. 1  in its first and second configurations respectively. The nozzle is viewed from the nozzle exit. Like numerals indicate like features. 
     The outward projections  22  are arranged radially symmetrically about the longitudinal axis of the burner. Similarly, the inward projections  24  are arranged radially symmetrically about the longitudinal axis of the burner. Each outward projection is positioned midway between adjacent inward projections, and each inward projection is positioned midway between adjacent outward projections. 
     The outward projections  22  taper in a radially inward direction of the burner and each subtend an angle of 42° at the longitudinal axis of the burner. The inner projections  24  taper in a radially inward direction of the burner and each subtend an angle of 14° at the longitudinal axis of the burner. These dimensions are shown in  FIG. 5 . 
     When the outward and inward projections are axially aligned (as in  FIG. 4 ), the free cross-sectional area at the nozzle exit is reduced by 42% relative to the configuration in which the outward projections are axially displaced upstream of the nozzle exit (as in  FIG. 3 ). 
     There is a clearance of 3 mm between the outward projections and the inner surface of the outer cylinder, except where the outward projections are provided with ridges  22   a  that extend in a longitudinal direction of the burner and contact the inner surface of the outer cylinder (see  FIG. 6 ). 
       FIGS. 7 and 8  show the nozzle of the burner of  FIG. 1  in its first configuration. Like numerals indicate like features. 
       FIGS. 9 and 10  show the nozzle of the burner of  FIG. 1  in its second configuration. Like numerals indicate like features. 
     The upstream end of the inner cylinder  18  is provided with a flange  40  that is mounted on rods  42  that are secured to the fuel connection  33 , the flange being slidable along those rods. In the second configuration of the nozzle, the downstream ends of the inner and outer cylinders coincide and the flange lies flush against the fuel connection  33  such that it may be bolted thereto. In the first configuration of the nozzle, the inner cylinder  18  is displaced relative to the outer cylinder in an axial direction of the nozzle. Thus the upstream end of the inner cylinder protrudes from the fuel connection  33 . 
       FIGS. 11 and 12  show detail views of the upstream portions of  FIGS. 8 and 10  respectively. Like numerals indicate like features. 
     In use, the gas igniter lights the oil burner  14  which is used to pre-heat the boiler before the fuel can be fired. Core air is fed through the burner by a small fan (not shown) to aid combustion of the oil and gas. 
     Pulverised fuel (e.g. coal or biomass) is driven down the nozzle  16  into the furnace, conveyed by a carrier airstream. In the case that a low fuel exit velocity is desired (for example, in the case that biomass fuel is being used), the nozzle is arranged in its first configuration, i.e. the outward projections are located upstream of the nozzle exit. In this configuration, the free cross-sectional area at the nozzle exit is high, resulting in low fuel velocity. In the case that a high fuel exit velocity is desired (for example, in the case that coal fuel is being used), then the nozzle is arranged in its second configuration. In this configuration, the axial positions of the outward and inward projections  22 , 24  coincide, such that the free cross-sectional area at the nozzle exit is low, resulting in high fuel velocity. 
     Pre-heated air is driven through the first and second air sources. The relative air flow rates through the two sources are adjusted depending on the fuel type. For example, in the case that the fuel is biomass the flow rates of the first and second sources are in the ratio 2:1, whereas in the case that the fuel is coal, the ratio is reversed. The swirlers  28 , 32  provide the exiting air with angular momentum, so as to promote the formation of an internal recirculation zone at the burner exit.