US 4,989,521 describes a burner where pellets or the like solid fuel is fed through a hopper to a heat zone. In the heat zone the fuel burns and gradually drops through holes in a shelf as the fuel turns into ashes. When the ashes drop through the holes in the shelf, more solid fuel from the hopper is fed to the heat zone. The feeding of the solid fuel to the heat zone takes place by gravity forces, and thus there is no need for a means for feeding the pellets that require electrical power.

EP 0 854 324 describes a stove also for burning solid fuel such as pellets. The stove comprises a hopper for containing the solid fuel. The fuel is discharged onto a grate at the bottom of the hopper. The grate is sloped so that the fuel pellets roll away from the slot and as the pellets are burned more pellets are discharged from the hopper through the slot to the grate. The pellets are discharged solely by means of gravity.

US 4,261,269 describes a furnace comprising a central hopper for feeding waste material to be burned and a number tubes provided at an inner wall of the hopper. The number of tubes constitutes ducts for fresh air to be supplied to a combustion space in a conically shaped furnace muffle. Subsequent to the waste material having been burned in the combustion space, ashes from the combustion space drop, by means of gravity, through a central opening in a deepest portion of the furnace muffle. However, if not all of the waste material is fully burned, the non-burned waste material may clog up the central opening.

BRIEF DESCRIPTION OF THE INVENTION It is the object of the present invention to provide a method and an apparatus for burning of solid fuel, which method and apparatus is more compact in relation to the amount of heat being produced, where the apparatus is produced so that the production, the assembly and the subsequent use and maintenance is considerably eased, and where the

burning of the solid fuel is sufficiently efficient and reliable, thereby ensuring safe and endurable use of the apparatus without the risk of loss of operation.

This object is obtained by a method comprising introducing the solid fuel through a hopper having a disc-shaped, alternatively an annular, cross-section and said hopper being positioned in a central part of the burner, and introducing primary combustion air into the primary combustion zone through a first air inlet placed circumferentially in relation to the hopper and said introduction of primary combustion air being introduced along an annular circumference to the hopper, and discharging primary combustion gasses and ashes from the primary combustion zone centrally through a number of central passages, the ashes being discharged primarily by means of a draught established through the numbers of central passages.

By providing the solid fuel in a central part of the burner and providing at least the primary combustion air circumferentially around the solid fuel hopper and lastly discharging ashes primarily by means of draught through the apparatus, and only optionally also by means of gravity, a very homogeneous and safe burning of the solid fuel is obtained.

Because of the primary air being introduced circumferentially to the hopper and thus circumferentially to the discharge of the solid fuel where the solid fuel is burned, not only does the burning of the solid fuel benefit from the symmetry of an a annular cross section, but the risk of any build up of fuel or the risk of an un-homogenous burning is purely incidental and is not caused by the way the solid fuel and the primary combustion air is introduced into the primary combustion zone. Because of the ashes being discharged primarily by means of the draught, the necessity of discharging the ashes by means of gravity can be avoided. This results in that no risk evolves of clogging up of the number of central passages, because ashes or any possible non-burned material cannot be discharged solely by means of gravity.

In a preferred embodiment the method furthermore comprises introducing secondary combustion air into the secondary combustion zone through a second air inlet placed circumferentially in relation to first inlet and said secondary combustion air being introduced along an annular circumference to the first inlet.

By also having secondary combustion air introduced circumferentially to the hopper and circumferentially to the introduction of primary combustion air, the aforementioned advantages are also obtained in relation to the introduction of the secondary combustion air. The secondary combustion air is preferably introduced in a part of the combustion zone lying at a central distance being further away from the introduction of the primary air and

even further away from the introduction of the solid fuel. Thereby, the primary combustion and the secondary combustion are established along a longitudinal distance of the burner and extending along a longitudinal line. This further enhances the homogenous and safe burning of the solid fuel due to the combustion gasses not having to perfrom several changes of direction during conduct through the burner, which decreases the efficiency obtained by the burning of the fuel and of the primary combustion gasses.

In a preferred embodiment the longitudinal direction along which the solid fuel, the primary combustion gasses and the secondary combustion gasses are discharged and conducted through the burner is a vertical downwards direction. Thereby, the discharge of the fuel and the discharge of ashes from burning the fuel may be directed through the burner by means of gravitation without the need for electrically or manually conducting the fuel and the ashes through the burner. In alternative embodiments the fuel, the primary combustion gasses and the secondary combustion gasses may be discharged and conducted through the burner in other directions such as horizontally, vertically upwards or obliquely downwards or upwards. However, this may raise a need for discharge means for conducting the fuel and the ashes through the burner.

An apparatus according to the invention comprises a hopper for introducing solid fuel into a primary combustion zone and comprising a passage between the primary combustion zone and a secondary combustion zone for introducing primary combustion gasses from the primary combustion zone to the second combustion zone, and said hopper constituting a duct having a disc-shaped, alternatively an annular, cross-section and said hopper being positioned in a central part of the burner, and said apparatus comprising a first air inlet also constituting a duct having an annular cross section and being placed circumferentially in relation to the hopper, and said number of passages being positioned in relation to an outlet of the hopper in such a manner that ashes from the fuel having been burned in the primary combustion zone cannot be discharged through the number of passages solely by means of gravity. Preferably, the apparatus furthermore also comprises a second air inlet also constituting a duct having an annular cross section and being placed circumferentially in relation to the first inlet.

Providing an apparatus with these features fulfils the needs of the method in a sufficient but nevertheless efficient manner. The parts needed for the apparatus, and the individual mutual assembling of the parts may take place like Chinese boxes. There is no need for welding or in any other manner joining the individual parts. This eases the initial assembly of the burner but it also eases any subsequent separation of the individual part during maintenance or dismantling before transportation.

In order to divide the primary combustion zone from the secondary combustion zone, in a preferred embodiment the orifice of the first air inlet is divided from the orifice of the second air inlet by a partition, said partition being provided with holes constituting the passage between the primary combustion zone and the secondary combustion zone, and in an even more preferred embodiment, the partition is a plate being placed opposite the orifices of the hopper and of the first air intake, said plate being made of a fire retardant material, preferably a fire resistant material, more preferably made of the material Skamol@.

A simple partition made of a fire retardant material and preferably a ceramic plate made by the company Skamols makes it possible to easily separate the two combustion zones without the need for specially designed grates having a special technical effect. The plate is as simple a partition as possible and by providing the plate with holes it is ensured that both primary combustion gasses and ashes from the primary combustion may be discharged to the secondary combustion zone.

The apparatus according to the invention is preferably intended for different kinds of solid fuels depending on the kind of solid fuel being available, being the cheapest, being the best or other criteria for selecting a certain kind of solid fuel. Also, for a certain kind of solid fuel, the burning of the fuel may necessitate that different amounts of fuel are discharged to the primary combustion zone, either during start of the burning where a larger amount of fuel may be needed or due to different levels of moisture in the solid fuel such as pellets or wooden chips where fuel with a higher level of moisture may necessitate, a larger amount of fuel.

Accordingly, the apparatus according to the invention is preferably capable of providing a mutual distance between the orifice of the hopper and the orifice of the first air inlet, said mutual distance being adjustable in order to adjust the amount of solid fuel introduced into the primary combustion zone.

In a preferred embodiment the adjustment of the mutual distance is effected by adjusting the central longitudinal position of the orifice of the hopper in relation to a longitudinal position of the partition between the primary combustion zone and the secondary combustion zone. Such adjustment of the mutual distance may be effected simply by raising or lowering the hopper in relation to the partition between the primary combustion zone and the secondary combustion zone, said raising or lowering preferably taking place by means of a number of actuators.

The partition between the primary combustion zone and the secondary combustion zone is intended for dividing the two zones. However, the partition itself may not prevent the fuel from discharging from the hopper, through primary combustion zone, through the passages in the partition and directly to the secondary combustion zone without being fully or even partly burned in the primary combustion zone. This may especially be the case where the orifice of the hopper, the primary combustion zone and the secondary combustion zone lies along a longitudinal line and the line extends vertically downwards.

Therefore, in embodiments of the apparatus where the above risk is present, the apparatus is preferably provided with an obstruction in order to avoid the solid fuel from discharging directly from the orifice of the hopper to the passage without passing and resting in the primary combustion zone for a sufficient amount of time for being burned.

A preferred obstruction is constituted by a cone, said cone having a peak directed towards the orifice of the hopper and said cone having a surface oppositely directed in relation to the peak and being directed towards the passage between the primary combustion zone and the secondary combustion zone. By using a cone as the obstruction, the geometry of the obstruction helps preventing any build up of so-called concave of solid fuel in the hopper. This may be the case if the fuel is pellets or if the fuel is moist. Also, the cone ensures a smooth and even distribution of the fuel into the primary combustion zone.

Although the risk of any build up of non-burned fuel and/or ashes is limited due to the symmetric and homogeneous burning of the fuel, a preferred embodiment of the apparatus according to the invention comprises means capable of subjecting any build up of non-burned fuel and/or ashes to a pressurised fluid, preferably pressurised air, in order to mechanically destroy the build up, which fuel and/or ashes because of the build up otherwise would be prevented from being discharged from the primary combustion zone.

DETAILED DESCRIPTION OF THE INVENTION The invention will hereafter be described more in detail with reference to the accompanying drawings, where fig. 1 and fig. 2 is a longitudinal cross section and a transverse cross section, respectively, of a possible embodiment of an apparatus according to the invention, and fig. 3 and fig. 4 is a longitudinal cross-section and a transverse cross-section, respectively, of an alternative embodiment of an apparatus according to the invention.

Fig. 1 is a schematic view of a longitudinal cross section of a burner for burning solid fuels.

In the embodiment shown, the burner consists of a number of mutually circumferentially ducts arranged around a central duct. The central duct 1 constitutes a hopper for introducing the solid fuel into the burner. Preferably, the ducts are cylindrical, and more preferably the ducts are circular cylindrical (see fig. 2). However, some or even all of the ducts may be cylindrical with another transverse cross section than cylindrical, such as oval, triangular, rectangular, polygonal or even other cross sections. Alternatively, also some or all of the ducts may be conical along their longitudinal extension.

As mentioned, a central duct 1 constitutes the hopper for introducing the solid fuel. The hopper has an inlet 2, on top of which a lid 3 is provided. The solid fuel such as pellets are fed into the hopper by lifting the lid 3 and pouring the solid fuel down the hopper.

Afterwards, the lid 3 may be closed again, and the lid is preferably provided with a packing (not shown), which is resistant towards heat so that air is not led down the hopper 1 via the inlet 2, when the lid 3 is closed. Under the influence of gravitation, the fuel then falls down to an outlet provided at an orifice 4 of the hopper 1. The orifice 4 of the hopper 1 opens towards a primary combustion zone P.

The primary combustion zone P is delimited at the bottom by a top surface of a partition constituted by a plate 5 made of a fire retardant material and a cone 6 also made of a fire retardant material and placed on a top surface of the plate. Alternatively to delimiting the bottom of the top surface by a plate and a cone, the partition may be a grate with no cone provided. The firstly mentioned embodiment will preferably be used in small-scale burners such as a 12 kW burner. The secondly mentioned embodiment will preferably be used in large-scale burners such as a 200 kW burner. This is because, in large scale burners the solid fuel, by itself, will form a cone-like core inside the fuel at the centre of the grate. This is because the primary combustion air exhibits a less pressure loss at the boundaries of the grate nearest to the passage for the primary combustion air. Thereby, the necessary a so-called incandescent ring for creating the primary combustion gasses is formed by itself.

However, each of the embodiments may be used in small-scale burners as well as in large- scale burners, depending on the actual scale of the brurner and depending on the kind of solid fuel, i. e. the material, which the solid fuel is made of, and the size of the pellets or other lumpy constituents, which constitute the solid fuel. In the following, the firstly mentioned embodiment will be referred to for describing the invention. However, this do not deprive the person skilled in the art of adopting one or more features of this firstly mentioned embodiment, when carrying out the secondly mentioned embodiment.

Preferably, the plate 5 and the cone 6 are made of a ceramic, more preferably of a ceramic made by the company Skamolg. Thus, when the solid fuel falls down the hopper, the fuel is stopped from falling further by the cone 6 and the plate 5. The plate 5 is supported by supporting bars 7 or a supporting disc extending sideways from a first air inlet, which is further described below. The cone 6 is supported by the plate 5 on small stilts 8 in such a way that a small distance d is provided between the bottom of the cone 6 and the top surface of the plate 5.

The plate 5 is provided with a passage in the shape of a hole 9 positioned below the cone 6. The outlet of the fuel into the primary combustion zone P is restricted by a distance D between the orifice 4 of the hopper 1 and the plate 5. The distance D between the orifice 4 of the hopper 1 and the top surface of the plate 5 may be adjusted. In the embodiment shown, adjustment may take place by means of actuators 10 provided between a top section of first inlet duct and the hopper. Thereby, the hopper 1 may be raised or lowered in order to either increase or decrease the distance D between the orifice 4 of the hopper and the top surface of the plate 5. Increase or decrease of distance D may be an advantage in order to consider the possibility of burning different types of fuels. As example, burning of fuels such as more or less humid wood chips, saw dust or the like fuels with a relatively low thermal energy per unit of weight require a larger distance than burning of fuels such as sludge-based fuels or wood pellets with a relatively high thermal energy per unit of weight, because the amount of e. g. wood chips, which have to be led to the primary combustion zone per unit of time is greater than the amount of e. g. wooden pellets, which have to be led to the combustion zone in order to maintain a certain combustion.

Because of the cone 6, the fuel will be directed sideways when leaving the orifice 4 of the hopper. Thus, the cone 6 constitutes an obstruction preventing the fuel from just passing directly from the orifice 4 of the hopper to the hole 9 in the plate 5. The cone 6 is preferably used along with fuels such as wood pellets having a high thermal energy per unit of weight and/or which require relatively long time of residence in the primary combustion zone. For other fuels such as wood chips and/or fuels which require a shorter time of residence in the combustion zone, the obstruction will preferably be a grate or the like in stead of a cone. A grate permits residence of a larger amount of fuel in the primary combustion zone. At the same time, the hole in the plate 5 will preferably be larger, or the plate will be provided with more holes in order to allow a greater flow of ashes from the primary combustion zone because of the shorter time of residence for the fuel in the combustion zone. The grate may be formed like a number of rings with a mutual distance, and the grate is placed in the hole 9 and is secured to the plate 5.

A first inlet duct 11 for introducing primary combustion air is placed circumferentially to the hopper 1. The first inlet has a vertical section 12 and a top section 13. The top section 13 is provided with an intake 14 for primary combustion air. The primary combustion air is directed from the top section 13 to the vertical section 12 of the first inlet duct 11 and down the first inlet duct 11 to the primary combustion zone P. A primary combustion of the solid fuel takes place above the plate 5 and beside the cone 6 within the first inlet duct 11.

During combustion, a so-called incandescent ring forms constituted by burning fuel turning from non-burned state to ashes. Primary combustion gasses together with ashes from the primary combustion is directed from the incandescent ring towards the small distance d between the bottom of the cone 6 and the top surface of the plate 5 and further down through the central hole 9 in the plate 5.

The ashes are directed to the central hole 9 by means of draught being established, when burning the solid fuel in the primary combustion zone. In the embodiment shown, the plate 5 has a substantially horizontal top surface. Thus, ashes cannot in any way be directed to the central hole in the plate by means of gravity. This must be effected by the said draught, preferably a natural draught, being established, when the primary combustion gasses are directed through the passage constituted by the central hole.

In an alternative embodiment, the plate 5 may have a more or less conically shaped top surface with the point of the cone being directed downwards towards the passage constituted by the central hole. All of the primary gasses will be the primary means for directing the ashes to the central hole. By shaping the plate slightly conically, also gravity may aid in directing the ashes to the central hole. However, the conicity must never be so great that gravity alone can direct the ashes to the central hole. If so, any possible non- burned solid fuel may drop down to the centre of the conically shaped plate and clog up the central hole, thereby harming burning of the solid fuel in the primary combustion zone.

In still an alternative embodiment, the plate may have a more or less conically shaped surface, however with the point of the cone being directed upwards towards the centre of the plate, but still with the passage constituted by the central hole being provided in the plate. Additional passages than the central hole may be provided along outer boundaries of the plate. Thereby, a first part of the primary combustion gasses may be directed towards the central hole and another part, preferably a minor part, of the primary combustion gasses may be directed towards the additional passages along the boundaries of the plate.

In this embodiment, by means of the other part of the primary combustion gasses and by the aid of gravity, the ashes may be directed towards the additional passages along the boundaries of the plate. However, also in this embodiment, the conicity must never be so great that gravity alone can direct the ashes to the additional passages. If so, any possible

non-burned solid fuel may drop down to the boundaries of the conically shaped plate and clog up the additional passages along the boundaries of the plate, thereby harming burning of the solid fuel in the primary combustion zone.

A secondary combustion zone S is established beneath a bottom surface of the plate 5.

Thus, the secondary combustion zone S and the primary combustion zone P are divided by the plate. The secondary combustion zone P is delimited at the bottom by a shield 15 made from a fire retardant material. Thus, when the primary combustion air is directed through the central hole 9 in the plate 5, the primary combustion air is introduced into the secondary combustion zone S. As mentioned, the ashes formed during the primary combustion are discharged together with the primary combustion air trough the central hole 9 in the plate 5. However, the shield 15 is provided with a central hole 16 and therefore the ashes discharged through the central hole 9 of the plate 5 will, due to gravitation, be further discharged trough the central hole 16 in the shield 15. The shield 15 is supported by wires or chains (not shown) suspended from a top part of a boiler being part of the burner. The shield 15 may be supported by other means, but the advantage of supporting the shield in wires or chains is that installation and possible removing of the shield during maintenance of the burner is made very easy.

A second inlet duct 17 for introducing secondary combustion air is established circumferentially to the first inlet duct 11 and thus also circumferentially to the hopper 1.

The second inlet duct 17 also has a vertical section 18 and a top section 19, and the top section 19 is provided with an intake 20 for secondary combustion air. The second inlet 17 is formed between an outer surface 21 of the first inlet duct 11 and an inner surface 22 of a boiler. The secondary combustion air is directed from the top section 19 to the vertical section 18 of the second inlet duct 17 and down the second inlet duct 17 to the secondary combustion zone S. A secondary combustion of the combustion gasses from the primary combustion takes place between the bottom surface of the plate 5 and the shield 15.

Secondary combustion gasses is directed from the secondary combustion zone S towards the central hole 16 in the shield 15 and further down through the central hole in the shield.

The secondary combustion gasses are discharged as flue gasses vertically downwards through the central hole 16 in the shield and into a furnace chamber 23 and further upwards through a number of flue gas ducts 24. The number of flue gas ducts 24 extends through the boiler 25 from the furnace chamber 23 to a top section 26 of the burner, said top section 26 being provided with a flue gas outlet 27. During passage in the flue gas ducts 24 from the furnace chamber 23 to the top section 26 to the flue gas outlet 27 the flue gasses heat water in the boiler 25. Preferably, walls 28 and a bottom 29 of the furnace chamber 23 constitutes part of the boiler, i. e. the walls 28 and the bottom 29 also contain

water (not shown), so that the flue gasses not only heat the water in the boiler 25 during passage along the flue gas ducts 24, but also heat the water in the boiler 25 during conduct away from the hole 16 in the shield 15 and to the flue gas ducts 24.

The bottom 29 of the furnace chamber functions as ash pit for collecting ashes from the primary combustion being discharged through the central hole in the plate, through the secondary combustion zone and further through the central hole in the shield into the furnace chamber and to the bottom of the furnace chamber. A means is provided for extracting the ashes (not shown) such as an ash pan, a door (see fig. 2) for gaining access to the ash pit or other suitable means.

Fig. 2 is also a schematic view of a transverse cross section of the burner according to the invention shown in fig. 1. The hopper 1 constitutes the central duct for feeding the solid fuel into the burner. The vertical section 12 of the first inlet duct 11 is shown circumferentially to the hopper and the vertical section 18 of the second inlet duct 17 is shown circumferentially to the first inlet duct 11. The boiler 25 is shown circumferentially to the vertical part 18 of the second inlet duct 17. As mentioned, the second inlet duct 17 is formed by the outer surface 21 of the first inlet duct 11 and the inner surface 22 of the boiler 25.

A number of flue gas ducts 24 extend inside the boiler 25 and, as mentioned, the flue gasses passing the number of flue gas ducts 24 heat the water in the boiler. In the embodiment shown, the number of flue gas ducts 24 is constituted by eight pipes passing through the boiler. The ducts 24 may be fewer or more and may have another cross- section than circular. Perhaps the number of flue gas ducts may be only one duct being an annular duct extending along an entire 360° circumference. Also, the number of flue gas ducts may be provided with fins extending either or both on the inner surface or/and on the outer surface of the ducts in order to increase the heat transfer rate between the flue gasses and the water in the boiler.

The flue gas outlet 27 is shown directed parallel to the transverse cross section. Other directions of the flue gas outlet will of course be possible and it will be possible to provide more flue gas outlet than just one. A door 30 with a hinge 31 and a handle 32 is provided for enabling access to the furnace chamber (see fig. 1) in order to extract ashes from the bottom of the furnace chamber. Other means for extracting ashes from the furnace chamber such as an ash pit may be provided in stead of or additionally to the door.

Fig. 3 schematically shows an alternative embodiment of an apparatus according to the invention. The alternative embodiment is very alike the embodiment shown in fig. 1, the

only difference being the introduction of an inner pipe 33 provided in the centre of the hopper 1 and booster pipes 34 provided in the first air inlet duct 11.

The inner pipe 33 is intended for passing down the solid fuel fully or partly down through the hopper 1 inside the hopper, before being discharged to and being burned in the primary combustion zone. The pipe 33 has a circumference being significantly smaller than a circumference of the hopper 1 so that a space is established, said space creating a distance between the inner pipe and side walls of the hopper. This results in the solid fuel not being in contact with the side walls of the hopper along the extension of the inner pipe.

Because of the hopper extending all way down to the primary combustion zone, heat from the primary combustion is passed upwards in the material, which the side walls of the hopper are made of. Subsequently, there is the risk of the heat in the material, which the side walls of the hopper are made of, being passed further into the fuel, before the fuel is passed to the primary combustion zone. This may result in the solid fuel fully or partly glowing down, before it reaches the primary combustion zone P. By providing the inner pipe creating a distance between the fuel and the side walls of the hopper, this risk is eliminated. A distant end 36 of the pipe is positioned between the inlet 2 of the hopper 1 and the orifice 4 of the hopper 1 so that the distant end 36 of the inner pipe 33 does not extend all the way to the orifice 4 near the combustion zone.

As mentioned. distance between the solid fuel and the side walls of the hopper ensures that the solid fuel will not be heated from heat emerging from the combustion zone and up through the material, which the side walls of the hopper are made of-a heating, which may cause the solid fuel glowing down. If so, this would cause the burner in malfunctioning, because the fuel would be more or less combusted, when it reaches the primary combustion zone. The risk of the solid fuel fully or partly glowing down in the hopper is especially the case with fuel having a high thermal energy per weight unit, i. e. is easy to ignite such as wooden pellets, which resides in the hopper for a relative long time, because combustion of such fuel in the primary combustion zone takes place more slowly.

The booster pipes 34 may be provided in order to boost the combustion in the primary combustion zone. The booster pipes 34 may be used both as booster pipes for boosting the combustion, but may also be used as means for introducing pressurised fluid such as pressurised air, as previously mentioned, in order to destroy any possible build up of non- burned solid fuel and/or ashes in the combustion zone. In the embodiment shown, four booster pipes 34 are provided, but more or fewer pipes may be provided.

However, it is preferred that the booster pipes 34 are evenly distributed along the circumference of first air inlet duct 11 in order to boost the combustion homogeneously

and/or in order to being able to destroy any build up along the entire circumference of the combustion zone. Boosting the combustion may be needed, as mentioned, in order to start the combustion or in order to aid the combustion if a lot of water from the boiler suddenly is drawn from the boiler necessitating added heat to the boiler.

The inner pipe 33 and the booster pipes 34 may be provided both in the same burner or may be provided separately, i. e. either the inner pipe or the booster pipes provided. The technical effect of the inner pipe 33 and the technical effect of the booster pipes 34 are independent of each other. Also, the booster pipes 34 may be used either as means for destroying any build up or may be used for boosting the combustion, or the booster pipes may be used both as for destroying and for boosting.

In the embodiments of the apparatus according to the invention shown in fig. 1, fig. 2 and in fig. 3, fig. 4 all parts of the burner are preferably made of sheet iron, apart from the partition constituting a partition and the cone constituting an obstruction. Sheet steel is easy to manufacture, to machine and to weld, if needed. Due to the fact that only very few parts of the burner according to the invention need to be mutually joined, e. g. by welding, the tendency of sheet steel towards buckling because of differences in temperature is of no problem. However, some or all parts of the burner may of cause be made of other materials such as cast iron and stainless steel having a less tendency towards buckling and/or having a less tendency towards being corroded. Perhaps the partition and the cone may be made of cast iron or be made of a combination of ceramic materials, cast iron and stainless steel. Also, at least the boiler and preferably the whole burner is provided with an insulation in order to minimise any loss of heat from the water in the boiler and any loss of heat from the primary and the secondary combustion, unless the boiler and/or the burner is intended for emitting heat to the surroundings by convection heat or radiation heat.