INGVARSSON, Robert (Gunrabol 112, Åmål, S-662 91, SE)
CLAIMS
1. A burner for the combustion of granular, solid fuel, for instance wood-flour pellets, chips or the like, comprising a preferably horizontal combustion chamber (16) with a grid in the form of a surrounding rotatable combustion chamber drum (32) of sheet metal provided with a plurality of openings (33), through which primary air is intended to be inserted for the combustion in the combustion chamber (16), c h a r a cterised in that stud-like nozzles (34, 34') are mounted at least in a majority of the opening (33), and that the nozzles (34, 34') are mounted to extend into the combustion chamber drum (32) at a distance of at least the wall thickness of the drum (32).
2. A burner according to claim 1, characterised in that the nozzles (34, 34') have an outer diameter or any similar cross-section size in the region of 15 to 40 mm, preferably about 20 to 30 mm.
3. A burner according to claim 1 or 2, characterised in that the nozzles (34, 34') are mounted to extend into the combustion chamber drum (32) at a distance in the order of at least two times the wall thickness of the drum (32), preferably at a distance in the order of at least three times the wall thickness of the drum (32).
4. A burner according to any of claims 1 to 3, characterised in that the nozzles (34, 34') are provided with a central inlet opening (38) ending into at least one deflected outlet opening (32), wherein the diameter (d{) of said central inlet opening (38) is at least 20 % smaller, preferably at least 40 % smaller than the outer diameter (D) of the nozzle (34).
5. A burner according to claim 4, characterised in that the nozzles (34) are provided with three deflected outlet openings (39, 39', 39") being distributed around the circumference of the nozzle (32), wherein the diameter (dθ of said central inlet opening (38) is at least 20 % larger, preferably at least 40 % larger than the diameter
(d 3 ) of said outlet opening (39).
6. A burner according to any of claims 1 to 5, characterised in that a plurality of nozzles (34) are welded to the wall of the drum (32).
7. A burner according to any of claims 1 to 6, characterised in that a plurality of nozzles (34') has a threaded base portion (36') intended to engage a corresponding threaded portion of the primary air openings (33) where the nozzles (34') are to be mounted, and that the threaded nozzles (34') are screwed on the drum (32).
8. A burner according to claim 7, characterised in mat the threaded nozzles (34') are provided with at least two engagement surfaces (40) evenly distributed around the circumference of the nozzle (34') to give possibility to engagement by a tool for screwing on the nozzles (34').
9. A burner according to claim 8, characterised in that the threaded nozzles (34') have hexagonal outer sides (40).
10. A burner according to any of claims 1 to 9, characterised in that the nozzles (34, 34') are mounted in at least a majority of the openings (33).
11. A boiler, characterised in that it comprises a burner according to any of claims 1 to 10. |
BURNER FOR COMBUSTION OF GRANULAR, SOLID FUEL
TECHNICAL FIELD The present invention relates to a burner for the combustion of granular, solid fuel, for instance wood-flour pellets, chips and the like, comprising a preferably horizontal combustion chamber with a grid in the form of a surrounding rotatable combustion chamber drum provided with a plurality of openings, through which primary air is intended to be inserted for the combustion in the combustion chamber.
PRIOR ART
Such a burner is previously known through for instance SE 519 605 C2 and it has proved to be an essential improvement over previously known burner constructions, where extensive and time-consuming work is required to replace or repair a combustion chamber, which has been burnt through, or a secondary combustion chamber. The main reason for the inner walls of the combustion and secondary combustion chambers, respectively, being deformed and holes arising therein is considered to depend on the fact that the flame jet being formed by the burning flue gases and by the air supplied by a blower land at too short a distance from said inner walls. With burners described in SE 519 605 C2, it is possible to displace and to limit, respectively, in a determined way the combustion centre of the flame jet and hence the "volume" of the flame jet, i.e. the axial and radial temperature distribution of the flame jet.
However, it has proved to be desirable to achieve an additional extension of the lifetime of the combustion chamber. The area around the primary air opening in the drum wall is subject to wear, as the heat radiation from a small flame created when air enters at a high pressure returns towards the drum wall and erodes material around the opening with the formation of a crater with a diameter of about 20 mm.
DISCLOSURE OF THE INVENTION
Thus, the object of the present invention is to provide a combustion chamber drum with improved lifetime.
With a burner of the kind initially mentioned this object is according to the invention achieved by mounting stud-like nozzles at least in the majority of the openings. In this way, the material thickness at each opening may be increased in the area which is subject to crater formation, so that the lifetime of the combustion chamber drum is
increased. Further, the somewhat surprising advantage is gained that the drum and parts belonging to the drum get a reduced heat supply, which further positively influences the lifetime of the drum and hence also of thereto belonging parts.
Preferably, the nozzles have an outer diameter or a corresponding size of the cross section, which is chosen in such a manner that the material of the nozzles will be subject to the erosion phenomena while the drum wall will be spared.
Further, the nozzles are preferably mounted to extend into the combustion chamber drum at a distance corresponding to at least the wall thickness of the drum. As the combustion chamber drum rotates, the risk for sintering is reduced essentially, and as the nozzles extend into the combustion chamber drum, a continuous mixing and recirculation of the granular, solid fuel is more safely obtained, so that an almost complete combustion of the fuel is secured.
Still more preferred, the nozzles are mounted to extend into the combustion chamber drum at a distance in the order of at least two, preferably three times the wall thickness of the drum.
Suitably, the nozzles are provided with a central inlet opening ending in at least one deflected outlet opening. In this way, it is possible easily to create surfaces around the outlet which are not influenced in the same degree by a "rebounding flame", when then favourable non-eroding angle conditions may easily be obtained. Further, the fact is that this arrangement may reduce the probability for outlet openings being blocked when the burner is not in a low load position, i.e. when the drum at times partly stands still.
Preferably, each nozzle is provided with at least two, more preferred at least three deflected outlet openings distributed around the circumference of the nozzle. In such a way, a more favourably distributed supply of primary air may be obtained.
Further, it is suitable that a plurality of the nozzles is welded to the drum wall. The welding of the nozzles from the outside of the drum may easily be automated.
Alternatively, a plurality of the nozzles have a threaded portion, which is intended to engage the corresponding threaded portion of the openings where the nozzles are to be mounted, and the threaded nozzles are firmly screwed into the drum.
Then, the threaded nozzles are suitably provided with at least two engagement surfaces, which are evenly distributed around the circumference of the nozzle to create the possibility of engagement by a tool for screwing on the nozzles. Preferably, the threaded nozzles have hexagonal outer sides, so that the nozzles may be screwed on with conventional nut setters.
If some of the openings are less exposed to crater formation than others, one may consider not mounting nozzles in all openings. Suitably, nozzles are, however, mounted in at least the majority of the openings.
BRIEF DESCRIPTION OF THE ENCLOSED DRAWINGS
Below, the invention will be described more in detail with reference to preferred embodiments and to the enclosed drawings.
Fig. 1 is a schematic cross-sectional view of a burner with a combustion chamber drum for the combustion of granular, solid fuel, for instance wood-flour pellets, chips or the like, according to a preferred embodiment of the present invention mounted on a conventional boiler.
Fig. 2 is a side view of a preferred embodiment of a stud-shaped nozzle intended to be welded to the combustion chamber drum.
Fig. 3 is an end view of the stud-shaped nozzle of Fig. 2.
Fig. 4 is a perspective view of a cross section of a combustion chamber drum with welded nozzles according to Figs. 2 and 3 with a portion of the drum wall removed in order to show the inside of the drum.
Fig. 5 is a perspective view of a cross section of a combustion chamber drum with screwed nozzles which outside have hexagonal sides, with a portion of the drum wall removed in order to show the inside of the drum.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS With reference to Fig. 1, a cross section is schematically shown through parts of the burner 1 for the combustion of granular, solid fuel according to the invention, said burner 1 being mounted at a conventional boiler 2 for heating of for instance a building (not shown). Said granular, solid fuel may for instance comprise compacted wood-flour
pellets or briquettes, chips or the like with a suitable diameter of about 6 to 12 mm. The burner 1 comprises also a dispensing unit 3, and a smaller fuel store 4 incorporated in the actual dispensing unit 3, said smaller fuel store 4 may either be topped up manually, and then normally a couple of times a week, or automatically (not shown) via at least one fuel conveyor from a fuel store detached from the dispensing unit 3, said store suitably being arranged at a distance from said dispensing unit 3. In order to obtain an even supply of the fuel in question and to prevent that the fuel forms an accumulation of fuel preventing further conveying of the fuel, said smaller store 4 has suitably somewhat inclined boundary surfaces 5, which form a hopper mouth 6 open at the bottom, at which hopper mouth 6 a screw conveyor 7 is also arranged. The dispensing unit comprises a motor 8 with a gearbox for driving the screw conveyor 7, which is rotatably arranged in an essentially horizontal, suitably rigid feed pipe 9 in order to automatically discharge the fuel via the hopper mouth 6 of the fuel store 4 and on down via a suitably vertical or essentially vertically inclined, rigid down-pipe or flexible down-hose 10 to an essentially horizontally arranged fuel feed device 11 in the burner 1. The boiler 2 also comprises a water-based heat supply system (not shown in more detail), for example a water-based circulation system provided with radiators, having water-cooled surfaces arranged inside the boiler 2. In the embodiments shown in the figures, the burner 1 is arranged essentially horizontally, but in other embodiments, not shown, the burner 1 may instead be arranged with a certain vertical inclination in relation to the boiler space 12 of the boiler.
The fuel feed device 11 comprises an additional screw conveyor 13 with a driving motor 14, said screw conveyor 13 being rotatably arranged inside a fuel feed pipe 15 for automatic dispensing of the fuel from the down-pipe or down-hose 10 of the dispensing unit 3 and further into a combustion chamber 16, which in the embodiment shown is essentially horizontally arranged, and which has a grid in the form of a combustion chamber drum 32 surrounding the chamber. The fuel feed pipe 15 ending in the centre of rotation of the combustion chamber drum 32 has a circular cross-section and functions also as an axis of rotation for rotating parts of the burner 1. A driving motor 17 for the rotation of said parts is schematically shown in Fig. 1. The fact that the fuel feed pipe 15 ends in the centre of rotation of the combustion chamber drum 32 implies that fuel is centrally supplied. Air may then be radially supplied outside the fuel feed pipe 15. The central fuel feed implies that fuel may be supplied at a distance from the combustion chamber 16. Hence, fuel may be supplied to a comparatively cold site. Consequently, the risk for, for example, return leakage is reduced because of the fact
that seals are unable to remain tight at high temperatures. This is an advantage of a central feed compared with peripheral feeding of fuel to the combustion chamber 16.
The burner 1 further comprises at least one blower 18 having at least one air outlet 19, 20 for the supply of air to the combustion part 21 of the burner 1, which is arranged inside the boiler space 12 of the boiler 2, via one or more air inlet pipes 22, 23 and from the air inlet pipes 22, 23 on via a plurality of essentially elongated air ducts, essentially separated from and parallel to one another, or via one or more air chambers 24, 25, essentially surrounding the fuel feed pipe 15 and the combustion chamber 16, for the supply of primary air to the combustion chamber 16 and secondary air to a secondary combustion chamber 26 arranged downstream of the combustion chamber 16, that is to say furthest away from the combustion part 21 of the burner 1, via a secondary air distributor 26 A which separates the combustion chamber 16 from the secondary combustion chamber 26. The primary air is intended for a primary combustion of the fuel to flue gases and for conveying said gases and any fly ash thereby formed from the combustion chamber 16 to the secondary combustion chamber 26 via an outlet 27 arranged through the secondary air distributor 26 A between the combustion chambers 16 and the secondary combustion chamber 26. The secondary air is intended for a secondary combustion of the flue gases and for conveying the flue gases on into the boiler space 12 of the boiler 2 in order to transmit the combustion heat to the heat supply system of the boiler 2, but also to discharge the fly ash and any other remaining residual products out of the combustion part 21. The secondary air distributor 26 A is designed to blow the secondary air radially inwards so that in the secondary combustion chamber 26 the flame will be concentrated and located at a distance from the wall of the secondary combustion chamber 26.
The connection 28, see figure 1, between the down-pipe or the down-hose 10 and the fuel feed pipe 15, and the connection 29 between the air outlet pipes 19, 20 of the blower 18 and the air inlet pipes 22, 23 of the burner 1 are arranged in a way suitable for the burner 1 , not closer shown in the figures. For example, the fuel feed and air inlet pipes 15, 22, 23 in question may, at said connections 28, 29, comprise a plurality of openings (not shown) arranged around the circumference of the pipes 15, 22, 23 for the passage of the fuel or the air, at the same time as the very connections 28, 29 each consist of a device (not shown in more detail) surrounding each pipe 15, 22, 23 with a connection opening to the connecting ends of the down-pipe or down hose 10, or of the air outlet pipes 19, 20 respectively.
The blower 18, which in the embodiment shown in figure 1 is mounted to the burner 1 adjacent to the boiler 2, may, of course, also be arranged in the rear part 30 of the burner 1, i.e. at a larger distance from the boiler 2. The blower 18 has a suitably silent running and speed-controlled motor 31 with a built-in thermal contact which breaks in the event of overload.
The combustion chamber drum 32 is provided with a plurality of openings 33 intended for the insertion of primary air for the combustion in the combustion chamber 16. The drum 32 normally has an inner annular cross-section but may also, if desired, have a polygonal cross-section for tumbling the fuel at rotation of the drum 32. It is also known, at the inside of the drum 32 to arrange longitudinal or screw-line shaped laminae to improve the tumbling of the fuel.
The lower portion of the combustion chamber drum 32 is thus a ratable grid for the primary combustion, i.e. the gasification of the fuel, on which grid a fuel bed rests during intermittent or continuous air percolation. The outlet 27 of the combustion chamber 16 for the flue gases through the secondary air distributor 26A into the secondary combustion chamber 26 constitutes also an outlet for any fly ash formed, and the secondary combustion chamber in its turn has an outlet opening 47 in the boiler space 12. At a rotating combustion chamber drum 32 with discharge flanges (not shown) the conceivable, but heavily undesired, accumulation of ash and sinter slag will also be fed towards the outlet 27 from the combustion chamber 16 and further into the secondary combustion chamber 26 via the secondary air distributor 26A. Such an accumulation contains solid combustion products which have not yet been entirely burnt, possibly also a minor amount of unburnt fuel. The energy content of said products is utilized by the subsequent combustion in the secondary combustion chamber 26.
The secondary air distributor 26A comprises a fan 49 in order to simultaneously expel all solid and gaseous combustion products during the said secondary combustion, so that no residual products can block the air inlet openings 48 from the air ducts or the air chambers 24 to the secondary air distributor 26 A and in order to move the centre of the secondary combustion, and hence the hottest part of the flame, away from the combustion chamber 16 and further into the secondary combustion chamber 26, so that a substantial part of the secondary combustion will also take place inside the boiler space 12 of the boiler 2 and outside and at a distance from the combustion part 21.
As mentioned above, the area around a primary air opening 33 in the wall of the drum 32 is subject to wear as heat radiation from a small flame, which is created when air enters at a high pressure, returns to the wall of the drum 32 and erodes the material around the opening 33 which results in the formation of a crater with a diameter being somewhat larger than the diameter of the opening 33 itself; in a usually occurring case a crater with a diameter of about 20 mm is created. This is avoided according to the invention by mounting stud-like nozzles 34 at least in a majority of the openings 33, preferably all, in the combustion chamber drum 32. hi this way, one may at every opening provided with a nozzle 34 increase the material thickness in the area exposed to crater formation, so that the lifetime of the combustion chamber drum is increased.
A preferred embodiment of the nozzles 34 is shown in Figs. 2 and 3. The nozzle 34 comprises a main portion 35, which is intended to be arranged at the inside of the drum 32 and which in this embodiment is cylindrical, a base portion 36 which is also cylindrical and which extends axially from one end of the main portion 35, said base portion being intended to fit into a primary air opening 33 in the drum 32, and a outlet portion 37 at the other end of the main portion 35, which outlet portion has the shape of a truncated cone. The top rake angle of the cone may be in the order of 90°.
For the percolation of the primary air, each nozzle 34 is provided with a central inlet opening 38, which suitably ends into at least one deflected outlet opening 39 located in the truncated cone-shaped surface of the outlet portion 37. hi this way, many advantages are gained, e.g. that it will be easy to make the openings 39 and that the surrounding surfaces get normal forming angles in relation to the centre line of the opening, whereby "straight rebound" of the flame is eliminated. Further, the probability is reduced that the outlet opening could be blocked when the burner is in it low load position. As may be seen from Figs. 2 and 3, each nozzle 34 is preferably provided with a plurality, e.g. three, deflected outlet openings 39, 39', 39", which may be evenly distributed around the circumference of the nozzle 34. hi this way, an evenly distributed supply of primary air is achieved. It should, however, be emphasized that tests have shown that the nozzles 34 are advantageously placed with the openings at random directions, so that a kind of "chaotic flow order" is created within the drum 32, as it has proved that surprisingly good efficiency is then obtained.
The main portion 35, within which a central inlet opening 38 runs, has preferably an outer diameter D in the region of 15 to 40 mm, depending on application, however, often about 20 mm is preferred and may in many cases be utilized as a kind of a
"standard nozzle" for drums 32 of different sizes. By utilizing a thickness D exceeding the diameter (d{) for said central inlet opening 38 by at least 20 %, the material of the nozzles 34 will be subject to erosion, while the wall of the drum 32 is spared. As already mention, the nozzles 34 are preferably provided with a plurality, e.g. three, deflected outlet openings 39, 39', 39", wherein the diameter (di) of said central inlet opening 38 is at least 20 % larger, preferably at least 40 % larger, than the diameter (d 3 ) of said outlet opening 39. Preferably, the total cross-section area of the outlet openings 39, 39', 39" corresponds to the cross-section area of the central inlet opening 38. It should be understood that in certain applications more than three outlet openings 39 may advantageously be utilized, as well as that it in certain applications may be an advantage to reduce the number of openings 33 in the drum 32 and instead use nozzles with more openings in order in this way to gain the advantage, partly to have to make fewer openings in the drum (cheaper), partly also to obtain more outlets (better turbulence) with almost the same total outlet area.
As mentioned above, the nozzles 34 are intended to be mounted so that they extend into the combustion chamber drum 32. Preferably, they have such a length that they extend at a distance which at least corresponds to the wall thickness of the drum 32, whereby the lifetime of the drum 32 is increased. As the combustion chamber drum rotates, the risk for sintering is reduced essentially, and as the nozzles 34 extend into the drum 32, a continuous mixing and recirculation of the granular, solid fuel is more safely obtained, so that an almost complete combustion of the fuel is secured.
A still more increased lifetime of the drum and improved mixing is achieved if the nozzles 34 are mounted to extend into the combustion chamber drum 32 at a distance in the order of at least two, preferably at least three, times the wall thickness of the drum 32. If the drum consists of a plate of a thickness of e.g. 6 mm, the stud-like nozzles 34 suitably extend at a distance in the order of 20 mm. With such long stud-like nozzles 34, also the above mentioned laminae for tumbling of the fuel may be neglected.
In the perspective view in Fig. 4 a cross section of a combustion chamber drum 32 is shown with welded nozzles 34 according to Figs. 2 and 3 with a portion of the drum wall removed in order to show the inside of the drum. The tubular base portion 36 of the nozzles 34 has a free end which is bevelled on the inside and the outside, and it has a length suitably being some millimetres larger than the wall thickness of the drum 32.
Through the bevelling of the outside an annular recess is formed between the free end of the base portion 36 and the surrounding primary air opening 33, which recess is
intended for the weld joint formed at the welding, which may easily be automated. The bevelling of the inside improves the flow conditions of the primary air at the entrance to the inlet opening 38.
Alternatively, as shown in Fig. 5, a plurality of the nozzles34' have a threaded base portion 36', which is intended to engage the corresponding threaded portion of the primary air openings 33 where the nozzles 34' are to be mounted, and the threaded nozzles 34' are firmly screwed into the drum 32.
Then, the threaded nozzles 34' are suitably provided with at least two engagement surfaces 40', which are evenly distributed around the circumference of the main portion 35' of the nozzle 34' to create possibility for engagement by a tool, not shown, for screwing on the nozzles 34'. Preferably, the threaded nozzles 34' have hexagonal outer sides 40', so that the nozzles 34' may be screwed on by means of conventional nut setters.
If some of the primary air openings 33 are less exposed to crater formation than others, one may consider not to mount nozzles in all openings. However, nozzles are suitably mounted in at least the majority of the openings. Further, Fig. 1 shows a pattern for the primary air openings 33, wherein the openings form a substantially square pattern of openings. Preferably, one pattern of openings is, however, used where the single rows of openings are displaced in relation to each others, so that the pattern of openings has been turned 45° as is shown in Figs. 4 and 5.