WO2007139475A1

WO2007139475A1 - A pellet burner

Info

Publication number: WO2007139475A1
Application number: PCT/SE2007/000521
Authority: WO
Inventors: Brage Norin
Original assignee: Aktiebolaget Swebo Flis O Energi
Priority date: 2006-05-30
Filing date: 2007-05-30
Publication date: 2007-12-06
Also published as: SE529932C2; EP2032896A1; US20090205545A1; SE0601191L

Abstract

Burner for particulate fuel, above all pellets such as bio-pellets, comprising a chute (10) having a longitudinal axis (11), the chute having a generally horizontally oriented longitudinal axis and having an inner wall of an essentially cup-shapedly upwardly open cross-section shape, a feeding device (30-34) for feeding a bed (20) of the particulate matter in and along the chute (10), and a device (40) for supply of primary air to the bed (20) in the chute (10), characterized in that the primary air supply device (40) is arranged to, without contact, direct primary air from above toward the free top- side of the bed (20) in the chute.

Description

Pellet burner

The invention relates to burner of the kind that is defined in the preamble to the appended claim 1.

Thus, the invention relates to a burner of the general kind that comprises a chute having a generally horizontally oriented longitudinal axis and having an inner wall that essentially has an upwardly open cup-shaped cross-section shape, a feeding device for feeding a bed of particulate fuel, particularly pellets in and along the chute, the bed being burnt in the chute during transportation along the same, and a device for supply of primary air to the bed in the chute.

A drawback of burners previously known from practice and of the kind in question relates to the fact that a substantial part of the primary air is led into the bed via open- ings via the cup surface of the chute carrying the bed. In this way, burning of said pellets occurs close by the openings, and then the temperature becomes particularly high close by said openings, so that formed combustion deposits/ashes vitrify and get stuck against the chute in the area near the openings. Accordingly, said vitrified deposits of ashes become situated on the sliding surface of the pellet bed in the chute, and block or interfere with the advancement of the pellet bed along the chute. In that connection, the pellet bed will encounter a greater and greater friction against the chute during the advancement thereof. The feed-in of new pellets always finds the easiest way and finally avoids the obstacle formed by the pellet bed because of the increased friction mentioned above. The feed-in of new pellets can no longer manage to press the pellet bed forward in the chute in a controllable way. The pellets begin to climb the obstacle formed and the entire burning is disturbed.

Therefore, an object of the invention is to provide a burner device by means of which said drawbacks are entirely or partly obviated.

The object is achieved by the invention.

The invention is defined in the appended independent device claim.

Embodiments of the invention are defined in the appended dependent claims. In addition to primary air, also secondary air is supplied in order to guarantee controlled burning of combustible gases escaping from the bed. If the pellet ashes melt, a stabilizing crust of vitrified combustion deposits/ashes will be formed on the free top- surface layer of the bed, which is not in contact with the chute. The friction against the chute is not increased thereby, and fed-in new pellets can easily proceed to press the fuel bed forward in a controlled way. Primary air should of course also be directed toward the central parts of the free top surface of the bed in order for the top side of the entire pellet bed should be reached by primary air.

In the following, the invention will be described by way of examples, reference being made to the appended drawing.

Figure 1 shows a schematic vertical longitudinal section through a pellet burner.

Figure 2 shows a schematic cross section taken along the drawing II— II in Figure 1.

Figure 3 shows in a section corresponding to Figure 2, another embodiment of the chute of the burner and devices for supply of primary and secondary air.

Figure 4 shows an oblique perspective view over a tested embodiment of the burner.

Figures 1 and 2 show a burner for particulate fuel, particularly pellets, above all so- called bio-pellets, which usually consist of cellulose material, foremost waste wood that has been transferred into the form of longitudinal sections of compressed and extruded particle material.

The burner is shown to contain a chute 10 having a longitudinal axis 11, the chute being oriented with the bottom thereof essentially horizontally, in the direction of the longitudinal axis 11. The chute has an upwardly open cup-shaped cross section and receives a bed 20 on the upwardly facing side thereof. The inside of the upper edge portions 11 of the chute 10 are shown generally vertical, while the lower part of the chute is rounded and slopes inward toward the longitudinal centre plane of the chute. A feed pipe 30 for pellets mouths in a screw conveyor 31 having a horizontal driving shaft 32 that is rotationally driven by a motor 33. Pellets propelled by the screw 31 are driven into the bottom region of the chute 10 at one end thereof. When the screw 31 feeds pellets through a tubular pipe part 34 extending in through an adjacent end wall 36 of the chute 10. In this way, a bed 20 of pellets progressing along the chute is formed in the chute 10. Primary air is led in from above toward the free top surface of the bed 20 in the chute 10 by means of a nozzle device 40. The nozzle device 40 is located not to touch the particle bed 20, and to blow primary air from above downward toward the free top surface of the particle bed 20 in the chute 10. The outlet opening of the pipe part 34 is well covered by pellets also on the topside.

The bed 20 is set on fire near the wall 36 and moves under the impact of the screw conveyor 31 towards the right in Figure 1, that is away from the wall 36, while burning, the cross-section area of the bed decreasing. The length of the chute 10 is suitably selected so that in normal operating conditions, the particle bed is essentially entirely consumed when it reaches the end of the chute 10, so that combustion deposits then can fall down from the chute in order to be taken care of in a way conventional per se.

Figures 1 and 2 should be interpreted so that the surface of the chute carrying the bed 20 is smooth and continuous and allows the bed 20 to slide along the chute in a well- controlled way.

Preferably, in that connection, the chute 10 has an essentially constant cross-section along the length thereof. Since the cross-section area of the bed decreases in the dis- placement direction of the bed, the side walls of the chute can be given a decreasing height in the direction away from the inlet end 36 of the chute, such as is outlined in Figure 1.

The device 40 for supply of primary air suitably has a nozzle device 41 that leads in the primary air from above downward toward the bed 20 along a substantial longitudinal section of the bed 20. Therein, the device 40 may hold a nozzle device 42 for introduction of secondary air toward the area above the bed 40, for burning of combustible gases escaping from the bed. The nozzle device 42 may also be arranged to introduce air along a substantial part of the length of the bed and chute. As is seen in Figure 2, the device 41 may comprise one or more nozzles 410 that direct air downward toward the free top surface of the bed 20 near the upper edge portion 11 of the chute. Furthermore, the device 41 may contain one or more additional nozzles 411 that blow in primary air over the central parts of the free top surface of the bed such as viewed in the cross section according to Figure 2, the nozzles 411 suitably also being arranged to supply primary air along a substantial longitudinal section of the chute 10. The openings 410 (or the corresponding slot) lead in a part of the primary air close by the inside of the respective wall of the chute above the bed. In this way, it is also attained that coke and soot deposited on the exposed insides of the chute are burnt away.

At each side of the chute, it may be enough with one nozzle 410 if this is in the form of an elongate slot along the chute. Furthermore, it may be enough with a single nozzle 411 and 42, respectively, if the same are in the form of a slot. In other embodiments, said slots may be replaced by a respective row of spaced-apart nozzles. The primary air may naturally be supplied via a single slot or row of nozzles, if the same are formed to produce a flow pattern of a desired character. Such as is indicated in Figure 2, it is desirable that the primary air is introduced toward the top surface of the bed 20 in such a way that a crust 21 of combustion deposits is imparted to the free top surface of the bed 20. The crust 21 lets through combustion gas from the bed 20, but blocks small particles released by the pellets of the bed 20 from escaping upward from the bed 20. The crust 21 principally consists of vitrified ashes.

Figure 3 illustrates a variant in which the chute 10 is made double- walled, and primary and secondary air is introduced into the space 12 in the wall of the chute 10 and is led out to the upper end portion 14 of the wall provided with nozzle openings that afford primary and secondary air flows in the patterns accounted for in connection with Fig- ure 2.

The double- wall structure, and accordingly also the hollow space 12, also extends along each one of the upper edges of the chute such as is shown in Figure 3, a row of nozzles 41 being directed downward vertically close by the inside of the side wall 11 of the chute 10, and a row of nozzles 411 being directed toward the centre area of the top surface of the bed 20. Furthermore, there is a row of nozzles 42 directed generally horizontally in over the area above the bed 20 to supply secondary air for final burning of combustible gases escaping from the bed. The row of nozzles 42 may be terminated at a distance from the downstream end of the chute 10, such as also is seen in Figure 4. In Figure 4, a duct 140 is shown along the upper edge of the respective side wall 11 of the chute 10 along the length of the chute, the duct sections 140 being fed with air in one end, preferably from the end closest to the wall 36 and being closed in the other end located at the downstream end of the chute 10. Said ducts are shown to have the centre thereof in a vertical plane tangent to the inside of the side- wall part 11 of the chute. In Figure 4, it can be seen that one of the duct sections 140 has a row of nozzles 42 as well as rows of nozzles 410 and 411, respectively. In Figure 4, it can also be seen that the walls of the chute 10 have a decreasing height in the direction away from the wall 36.

Claims

1 Burner for particulate fuel, above all pellets such as bio-pellets, comprising a chute (10) having a longitudinal axis (11), the chute having a generally horizon- tally oriented longitudinal axis and having an inner wall of an essentially cup-shapedly upwardly open cross-section shape, a feeding device (30-34) for feeding a bed (20) of the particulate matter in and along the chute (10), and a device (40) for supply of primary air to the bed (20) in the chute (10), the primary air supply device (40) being arranged to, without contact, direct primary air from above toward the free topside of the bed (20) in the chute, characterized in that the supply device (40) has one or more primary air outlet openings (410) that along a substantial portion of the length of the chute (10) direct primary air downward close by the inside of each one of the upper edge parts (11) of the chute (10).

2. Burner according to claim 1 , characterized in that primary air supply device comprises one or more outlet openings (411) that direct primary air toward the free topside of the bed in the transversally central area of the bed.

3. Burner according to claim 1 or 2, characterized in that the primary air supply device has one or more air outlet openings directed to lead primary air over substantially the entire width of the free surface of the bed along at least one longitudinal section of the chute (10), whereby a crust (21) of combustion deposits being established on the top surface of the bed (20) when burning the particle bed, if the ashes vitrify.

4. Burner according to any one of claims 1-3, characterized in that one or more of said at least one primary air outlet opening of the supply device are in the form of a slot extending along a substantial part of the length of the chute.

5. Burner according to any one of claims 1—4, characterized in that the air outlet openings for primary air are in the form of a row of openings extending along a substantial part of the length of the chute.

6. Burner according to any one of claims 1-5, characterized in that the sup- ply device has one or more openings for blowing in secondary air toward an area above the bed (20) in the chute (10).

7. Burner according to any one of claims 1-6, characterized in that the chute is double- walled, and that primary air and possibly also secondary air is led through a hollow space (12) in the double-wall chute to a supply device (40) extending along the upper edge of the respective wall of the chute (10).

8. Burner according to any one of claims 1-7, characterized in that the air supply device (14, 40) extends along the upper edge of the respective side wall of the chute (10).