This invention relates to a burner for the burning of solid fuels, such as pellets, comprising a cartridge that may be inserted into a central heater, said cartridge delimiting a combustion chamber that is open relative to the interior of the heater and having a bottom, below which there is at least one cavity that is open outwardly, from the central heater, but closed inwardly, to which cavity air may be supplied from outside, the bottom along opposing longitudinal sides transposing into protruding side walls, in which there are orifices for the supply of air from the cavity into the combustion chamber, so as to create of a combustible gas mixture together with gasified constituents from a glow hearth located on said bottom, to which hearth fuel is supplied from above via a fuel feed canal.

During the last decades, several different burners for the burning of pellets or other solid fuels have been developed and commercialized, usually for biological material as the main raw material. The common feature of previously known burners is that they have a moderate efficiency due to incomplete combustion. In practice, the reason for the incomplete combustion is to be found in that the supply air taken from outside is mixed in a mediocre way with the constituents that are gasified from the glow hearth created by the fuel, and in that the fuel in the glow hearth partly remains unburnt due to insufficient supply of air oxygen. A contributory cause is also that the gas mixture formed in the combustion chamber has a limited dwell time in the chamber. Besides resulting in a bad economy in general, the incomplete combustion leads to a number of secondary inconveniences, such as an unnecessarily large ash forming, sooting exhaust gasses, etc.

As an example of previously known burners, reference is made to US-A-4 593 629, which describes a burner of the type as generally defined in the introduction. According to this previously known burner, the necessary air is supplied to the combustion chamber via a number of small orifices, which are provided solely in the side walls of the cartridge and distributed substantially evenly along their entire length. This involves that the supply of oxygen-rich air to the interior of the glow hearth becomes mediocre.

The present invention aims at removing the above mentioned inconveniences of previously known burners for burning solid fuels and creating an improved burner. A primary object of the invention is to provide a burner with the capability of on one hand supplying optimal amounts of oxygen-rich air to the interior of an established glow hearth, and on the other hand homogenizing the gas mixture composed of gasified fuel constituents and incoming secondary air and guaranteeing a comparatively long dwell time for it, in the combustion chamber. Yet another object is to provide a burner that is constructively simple and cheap to manufacture, at the same time as it should be compact, i.e., occupying a minimum of space in connection with a central heater. It is also an object to provide a burner that may work during long periods of time without any service, primarily in the form of ash removal, and in addition is neat to handle at the rarely occurring, but necessary service occasions. Furthermore, it should be possible to demount and mount, respectively, the burner in a simple way.

According to the invention, at least the primary object is attained by the characteristics that are defined in the characterizing clause of claim 1. Further, advantageous embodiments of the burner according to the invention are defined in the dependent claims.

In the drawings:

Fig 1

is a schematic section through a central heater and a burner according to the invention connected thereto,

Fig 2

is a schematic section showing only the burner, in enlarged scale,

Fig 3

is a partly transparent perspective view of the burner in a still more enlarged scale, and

Fig 4

is a perspective exploded view showing different components comprised in the burner.

In Fig 1, reference numeral 1 designates a central heater with an internal combustion chamber 2 and an outlet 3 for flue gases. In connection with an opening 4 in a front wall 5 of the heater is mounted a burner according to the invention, designated 6 in its entirety. As may be seen in Fig 1 and 3, this burner comprises a cartridge 7 that may be inserted into the central heater, which cartridge delimits combustion chamber 8, that is open relative to the interior of the heater and is composed of an upper part 9 and a lower part 10. The cartridge lower part 10 consists of a box construction that is open outwardly from the heater, but closed inwardly, with a lying central bottom 11 and two side walls 12 protruding from said bottom, in which side walls are orifices for the supply of air into the combustion chamber. Besides the bottom 11 and the side walls 12, the box construction 10 also comprises a secondary bottom 13 located at a level below the bottom 11. Along opposing longitudinal sides, the secondary bottom 13 transposes into two vertical side pieces 14, which at their tops transpose into planar, horizontal portions 15, which in turn at inner edges transpose into the side walls 12. As may be seen in Fig 3, the side walls 12 are advantageously inclined (in a direction upwards/outwards) in order to form a trough- or groove-shaped configuration together with the planar horizontal bottom 11. The parts 11, 12, 13, 14 and 15 together delimit a cross-sectionally substantially U-shaped cavity 33 (see Fig 2) which at an inner end is closed by means of a gable wall 16. However, at its opposed end, the cavity is open.

As described so far, the burner is substantially previously disclosed in the above-mentioned PCT/DK83/00114.

According to the present invention, the cartridge 7 is detachably connected with an outer housing 17 which, when mounted, is located on the outside of the central heater. More specifically, the lower part 10 of the cartridge is introduced into a first opening 18 (see Fig 4) in a front wall 19 of the housing 17. Outwardly from the vertical front wall 19 extends a cross-sectionally quadrangular cover with two vertical side walls 20 and two horizontal upper and lower walls 21, 22. These walls are jointly connected with an outer, vertical wall 23, on which is mounted a fan 24 for blowing in air into the interior of the housing. Through the interior of the housing extends a tube 25 which forms a fuel feed canal, whose one, upper end transposes into a vertical tube socket 26, to which a suitable fuel supply conduit, e.g. a hose, may be connected. The opposing end of the fuel feed canal ends in an opening 27 in the front wall 19 of the housing 17. More particularly, the inlet opening 27 is placed centrally, immediately above the opening 18, into which the cartridge lower part 10 is introduced. The tube 25, which is inclined at a suitable angle, e.g. 45°, between its opposed openings, may advantageously have a rectangular cross-sectional shape, in order to form a shaft, through which solid fuel bodies, such as pellets, may without any resistance fall or slide onto the cartridge bottom 11. It should be observed that the tube 25 at its opposed ends is air-proof-connected to the walls 19 and 21, respectively. In other words, air from the interior of the housing 17 cannot penetrate into the tube 25.

According to a characteristic feature of the invention, the air supply orifices in the cartridge lower part 10 have the shape of elongated slots 28 placed in the region between the central bottom plate 11 and each individual side wall 12. More specifically, the individual slot 28 has a first portion penetrating the bottom plate 11 per se and a second portion penetrating the adjacent side wall 12. As may be clearly seen in Fig 3, each one of the slots 28 is oblique relative to the axial extension of the cartridge. In practice, the bottom 11 and the side walls 12, respectively, may be made of 2 mm thick sheet metal and the individual slot 28 may have a width of 2 to 2,5 mm and a total length of about 20 to 40 mm, suitably about 30 mm, it being most suitable that about 2/3 of the total length is comprised in the bottom plate 11, while 1/3 is comprised in the adjacent side wall.

At a certain distance inside the outer end of the cartridge lower part 10, is provided a vertical wall 29 whose upper edge is situated on the same level as the upper planar portions 15 of the cartridge lower part. When the cartridge lower part is mounted in the opening 18, then the latter is closed so that air cannot pass out axially into the groove that is open upwards, which groove is delimited between the side walls 12 and the bottom plate 11. Therefore, air that shall pass through the slots 28 is forced to pass through the cavity 33 within the cartridge lower part 10. As may be clearly seen in Fig 3 as well as in Fig 4, the slots 28 are provided only along a restricted region of the entire length of the bottom plate 11. More specifically, the slots are arranged in a region adjacent to the air-intercepting wall 29. In practice, the length of this region may lie within the range of 25 to 50%, suitably about 33% of the distance between the wall 29 and the opposed gable wall 16. The section of the bottom plate 11 that extends between the innermost slot 28 and the gable wall 16, is devoid of any sort of holes. On the other hand, in each side wall 12 is recessed at least one orifice 30 that is located relatively near the inner free end of the cartridge lower part. Tertiary air may be supplied to the gas mixture in the combustion chamber through this orifice 30.

In practice, the length of the bottom plate 11 - when calculated between the two walls 16, 29 - may lie within the range of 150 to 300 mm, e.g. 200 mm, and the width of the bottom plate may amount to 40 to 90 mm, e.g. 50 to 60 mm.

According to the preferred embodiment as shown in the drawings, two holes in the shape of tube pieces 31 are provided on either side of the fuel feed inlet opening 27, for the supply of air into the combustion chamber 8. By means of the fan 24, an overpressure is created in the interior of housing 17, whereby air will pass through the tube pieces 31 while creating substantially axial air jets in the combustion chamber.

As may be seen in Fig 3 and 4, the upper part 9 of the cartridge 7 is cross-sectionally arched. Suitably, a central part is semi-circularly arched, the semi-circle shape transposing into planar, vertical side pieces of limited height. Most suitably, the upper part 9 is detachably connected with the lower part 10 by means of suitable means that are not shown in more detail.

It should also be mentioned that an upwardly facing, central edge 32 of the gable wall 16 is located somewhat higher than the bottom plate 11. The protruding part of the gable wall 16 forms a stop that counteracts the falling down of fuel from the bottom plate 11.

When the burner, after having been placed into the central heater in the way as illustrated in Fig 1, is to be brought into operation, then a glow hearth is effected in a suitable way on the bottom plate 11 of the cartridge lower part 10, and the fan 24 is started and fuel is supplied from a suitable fuel supply (not shown), more precisely via the feed inlet tube 25. By means of the fan, supply air is blown into the cavity 33 of the cartridge lower part via the outwardly open end, and the air is pressed through the slots 28. Then the portions of the slots 28 being comprised in the bottom plate 11 will substantially supply primary air that penetrates through the glow hearth in a direction from below, while the air that is pressed in through the portions of the slots being comprised in the side walls, mainly forms secondary air which partly penetrates the glow hearth and partly sweeps frontwards along the top side of the glow hearth. Performed tests have shown that the air that penetrates through the slots 28 forms mutually independent, rotating vortices which, when having passed the glow hearth (i.e., the solid fuel during pyrolysis), consist of combustible gas. By setting the vortices into rotation, the gases are well mixed with each other, at the same time as the rotary motion cares for a comparatively long dwell time in the combustion chamber. Further secondary air is supplied via the two tube pieces 31 in the form of two separate jets which, when they meet the rotating gas vortices, give rise to a turbulence that attends to an intensive mixing of the gasses rising from below and the secondary air, while guaranteeing a homogenous gas mixture. This gas mixture is burnt during its continued transportation towards the outlet opening of the cartridge, to finally burn as a flame into the interior of the central heater. More specifically, the final combustion of the gas mixture takes place around the region of the orifices 30, through which tertiary air is led into the immediate proximity of the opening of the combustion chamber.

Besides supplying secondary air into the outer end of combustion chamber 8, the air jets led in via the tube pieces 31 bring about an ejector effect that counteracts an erroneous direction of flow of the gas mixture. In other words, these air jets counteract a back burning in the burner.

By its cross-sectionally arched-like or vaulted shape, the cap-like upper part 9 of the cartridge guarantees that the gas mixture obtained at the outer end of the combustion chamber may rotate at a smallest possible resistance.

Like the lower part 10 of the cartridge, the upper part 9 may advantageously be made of simple sheet metal.

Numerous tests have shown that the most apparent advantage of the burner according to the invention consists in the fact that the combustion of the solid fuel becomes substantially complete. This involves that the efficiency of the burner becomes high, at the same time as the formation of ashes and soot is reduced to a minimum. Another advantage is that the burner by its constructive simplicity may be produced at low cost. At the same time, required service may be performed in a smooth way; primarily due to the fact that the cartridge that delimits the combustion chamber is detachably connected to the external housing through which fuel and air supply is made.

The term "elongated slot" should be interpreted in its widest sense. Thus, instead of shaping the individual slot as a continuous opening, it is feasible to form a plurality of smaller orifices in a row, behind each other.

The invention is not restricted solely to the embodiment as described above and shown in the drawings. Thus, the cartridge that delimits the combustion chamber may be modified in many ways with reference to its shape and dimensions within the scope of the appended claims. Thus, instead of assembling the cartridge of two mutually detachably connected pieces, it is possible to make the cartridge as one single integrated, tube-like sleeve, on whose bottom side the necessary free space is provided for the air supply via the oblique slots. The bottom in the cartridge, on which the glow hearth rests, also does not need to be planar, and the adjoining side walls may be shaped in another way than as planar plates or walls. Thus, it is possible to provide a bottom that is cross-sectionally arched and transposes into the respective side walls without any pronounced break-lines or bends. Therefore, it is possible to form the cartridge as a cylindrical or cross-sectionally oval tube, in which two sets of slots are provided on either side of a central, vaulted bottom. However, independently of the geometrical shape of the cartridge bottom, the slots should have their outer ends located higher than the inner ends in order to care for the in-blowing of primary air from below as well as secondary air from the side. It should also be pointed out that the cartridge in its entirety may be permanently joined to the external housing.