A DEVICE AND A METHOD FOR COMBUSTION

Abstract: A device (1) and a method for combustion of fuel, in particular solid fuels such as pellets of wood. The device (1) comprises a combustion chamber (2) with at least one gas inlet (8) and at least one gas outlet (9), said gas inlet being arranged to conduct oxygenous gas, for example air, substantially tangentially into the combustion chamber for the purpose of achieving a whirling motion in said combustion chamber and said gas outlet being arranged substantially centrally, in relation to a centre axis (10) of the combustion chamber, for letting combustion gas out in one end of the combustion chamber in a direction substantially parallel to said centre axis. Furthermore, the device includes a fuel holder (3) arranged in the combustion chamber for exposing the fuel (11) to gas present in the combustion chamber and promote mixing of the oxygen contained in the gas with the pyrolysis gas originating from the fuel.

Description: A device and a method for combustion FIELD OF THE INVENTION The present invention relates to a combustion device according to the preamble of claim 1. Although the current fuel may be any fuel that one want to burn up, including solid fuels, liquid fuels as well as gaseous fuels, below it is exemplified a device for combusting pellets of wood. In addition, the invention relates to a method for combustion according to the preamble of claim 26. The invention has a series of different applications within the field of combusting of different fuels for the purpose of generating heat. PRIOR ART For combusting of different types of fuels there are several different methods disposal. However, fundamental when using combustion methods is in every case that such a complete combustion as possible is attained. To reach a good combustion it is required among other things that the oxygen quantity supplied is adapted to the fuel quantity to be burned and that the oxygen and the fuel are well mixed. That is so because combustion is an oxidation process where a combustible substance, for example carbon, combines with oxygen to a combustion product, which combustion product is inflamed at a temperature specific for the substance during that the energy chemically bound in the fuel is converted to heat. It has previously been found that a theoretically optimum air quantity may be determined. To receive a power of 1 kilowatt (1 kW) about 1 ,2 m 3 is needed. However, this air quantity is based on an almost complete mixing of oxygen and combustible substance, so that all oxygen is consumed in the oxidation process, which in practice hardly is accomplished. Besides that an incomplete combustion process leads to that the energy content of the fuel is poorly used, i.e. that a low efficiency is received, it also leads to a series of other inconveniences such as discharge of undesired chemical compounds that could be harmful for the environment. By way of example may be mentioned that in combustion with deficiency of air (under stochiometrical combustion) carbon monoxide (CO) may be formed, whereas in excess of air (over stochiometrical combustion) nitrogen oxide (NO x ) tend to be formed. A method used in combustion is to conduct the supplied oxy- genuous gas, usually air, so that a whirling motion arises in a combustion chamber. The principle in formation of the whirling motion is that when a gas is conducted substantially tangentially into a receptacle a rotary motion is formed. If the current receptacle is lack of a closure in its upper end the whirling motion will decrease quickly, which occurs independently of where the tangentiell inflow of gas takes place. However, if the receptacle is closed by a cover including a restricted central opening, a strong and permanent whirling motion will be achieved in the receptacle. If the gas is conducted into the upper end of the receptacle, where the cover is placed, a whirling motion around the periphery of the receptacle and a flow down along the periphery of the receptacle occur and when the gas reach the bot- tom of the receptacle the gas change the direction of flow during maintaining of the whirling motion, so that the gas moves whirling upwards in a central part of the receptacle and then out of the hole in the cover. The Swedish patent No. 109 608 describes a combustion device designed for solid fuels in which a whirling motion of the air conducted is used for the combustion. The device according to the Swedish patent 109 608 has a vertical reaction chamber with an outlet directed upwards and a tangentially inlet for supplying air. The fuel is conducted in the lower part of the chamber by means of a screw device so that the entire bottom of the chamber is covered of the fuel. The device is intended to work in a way so that when air is conducted tangentially a whirl arises in the chamber which whirl should be able to lift the fuel, divide the fuel into pieces and maintain the combustion and lead the com- bustion gases upwards towards the central outlet of the chamber. This device has several drawbacks and undesired effects, among other things a very large supply of combustion air into the chamber is required for maintaining the rotation. This depends on that the air supplied to a great extent is used for lifting and dividing the fuel. Furthermore, the rising combustion gases from the fuel will counter act the formation of whirls in the chamber. This causes that the device must be supplied an air quantity that is unnecessary large for combusting the fuel and that it has to be an overpressure in the chamber, which in turn requires special arrangements regarding the filling of fuel for avoiding undesired outflow of gas. Solutions based on introduction of large quantities of air obstruct an optimum combustion and, as described above, undesired combustion products may be formed therethrough. In addition, in combusting solid fu- els, the device requires that the fuel is very finely divided already when it is introduced into the chamber, i.e. probably it is required that the fuel is in form of powder and that it has a very low percentage of moisture. Even if the fuel is finely divided, the method of operation of the device will lead to that smaller or greater amounts of incompletely burned particles stick on the periphery surface of the chamber and/or are discharged out of the chamber. A further drawback of the device is that even in the event of that the fuel is burned substantially completely there is no solution described for managing arised ash particles but instead it is mentioned that these should be thrown out through the outlet together with the gases. Since the basic problems of combustion devices today, including the devices that work with whirling formation, partly are the problems to mix the oxygen contained in the air with the com- bustible substance in a proper way and partly to supply optimum oxygen quantity, these devices do not work satisfactory as regards maximally utilisation of the energy content of the fuel or in the best case satisfactory only at a power specific for the device. THE OBJECT OF THE INVENTION The object of the present invention is to describe how one in a simply cheaply and very efficient way is able to burn different types of fuels, for example a solid fuel in the form of pellets of wood, so that the drawbacks of the combustion devices according to prior art substantially have been reduced or eliminated. SUMMARY OF THE INVENTION The object mentioned above is obtained according to the invention in that the device includes a fuel holder arranged in the combustion chamber for exposing the fuel to gas present in the combustion chamber and promote mixing of the oxygen con- tained in the gas with the pyrolysis gas originating from the fuel. It has been found in practical experiments, when firing pellets of wood in the device according to the invention, that the application of the fuel holder leads to a very efficient combustion in consequence of a nearly complete mixing of oxygen and the pyrolysis gas, which in turn depends on that the fuel accommo- dated in the holder will have a large effective surface exposed to the gas containing the oxygen. According to a preferred embodiment of the invention, in which the combustion chamber has a limiting surface and the fuel holder is arranged so that the fuel at least partly is at a distance from the limiting surface, a very efficient combustion is received by that the oxygenuous gas introduced in the combustion chamber is able to whirl between a large exposure surface of the fuel and the limiting surface of the combustion chamber. Especially, it is advantageously to combine that with another preferred embodiment, in which the fuel holder at least in one part of its surface has holes for exposing the fuel for gas present in the combustion chamber. According to a further preferred embodiment of the invention, in which the fuel holder has at least two disc elements for containing the fuel between the elements in a substantially unifor- mously thick layer, the oxygenous gas is brought in an even higher degree to come in contact with the fuel. If the layer is made relatively thin the fuel obtains a large exposure surface in relation to the volume of the fuel, which promote the mixing of oxygen with pyrolysis gas as well as an uniform combustion of the fuel throughout its volume. Another preferred embodiment of the invention, in which the device comprises members for maintaining and/or intensifying the whirling motion present in the combustion chamber, enables a very efficient mixing and in particular in combination with a fur- ther embodiment, in which said members comprise a whirling creator including a tube placed substantially parallel to the centre axis of the combustion chamber, which tube has an inlet end and an outlet end coinciding with the outlet of the combustion chamber, said inlet end being provided with one or more cuts in the envelop surface of the tube for letting gas in, and particularly together with yet another embodiment in which at least one guide blade is arranged at one or more of the cuts at the inlet end of the whirling creator for guiding the gas into the tube, a very efficient whirling motion of the gas flowing through the combustion chamber is received and in addition it is by that possible, if so is desired, to use a high gas flow in the chamber without causing any overpressure therein. In the whirling creator the whirling formation goes on and the undesired laminar flow is pressed, whereby an efficient mixing and burning are maintained until nearly complete combustion has occurred. Another embodiment according to the invention, in which the device has means for bringing a part of the combustion gases that have left the combustion chamber back to the combustion chamber, enables that the gas flow may be maintained despite that the air supply is reduced for the purpose of reducing the admitting of oxygen into the chamber, which is desirable for example when the combustion device works during lower load. Another preferred embodiment of the invention in which the de- vice comprises means for guiding the whirling motion of the gas in the combustion chamber, and particularly together with an embodiment, in which the guide means includes at least one guide rail, results in a possibility to guide the gas flow around the fuel holder and by that around the fuel in a controlled way for receiving a long mixing distance. This leads to other advantages, in addition to the quite combustion technical ones, such as for example the possibility to make the combustion device considerably smaller than a combustion device with a comparing mixing distance and having a linear gas flow. Another embodiment according to the invention, in which the device comprises members for conducting at least a part of the gas in direction towards the fuel, and especially one embodiment, in which the conducting member includes at least one conducting rail, said device enabling that a certain part of the gas being conducted towards the fuel for exposing the fuel to oxygen present in the gas to an even greater extent and promoting the necess'ary mixing of oxygen and pyrolysis gas. According to a another embodiment of the invention, in which the device comprises means for collecting ashes, and a separator and/or a collecting receptacle are arranged to separate and/or collect the ash before it is admitted to leave the combustion chamber together with the combustion gases, important advantages are obtained since the ash collecting may be per- formed inside the combustion chamber so that the ash will not leave the combustion chamber together with the gas, which spares for example a furnace connected to the chamber. By the design of the combustion chamber according to the invention and current gas flow therein, the most of the fly ash arising after combustion will be thrown out to the periphery and by that the most of the fly ash may be collected by the separator and/or the collecting receptacle in an easy and efficient way. According to an embodiment of the invention in which the device comprises means for removing the ashes from the collecting receptacle by means of water flushing, it is possible to continuously remove the ash arised. The characteristics of the method according to the invention are defined in greater detail in the description and in the accompanying claims. Further features and advantages of the invention will be described more in detail in the following. BRIEF DESCRIPTION OF THE DRAWINGS With reference to the appended drawings, below follows a description of preferred embodiments of the invention cited as ex- amples. ings: is a partly cut view from above of an embodiment of the device according to the invention, which has a vertical combustion chamber, is a section along the line II-II in fig 1 that shows the combustion chamber, including a fuel holder, as seen from the side, is a cut view of an embodiment of the device according to the invention, which has a horisontal combustion chamber, designed for so-called two way-gas flow, said device including a guide rail, is a cross-section of the device according to the invention in fig 3, is a cut view of an embodiment of the device accord- ing to the invention, which has one horisontal combustion chamber, designed for so-called three way- gas flow, said device including several conducting rails, is a cross section of the device according to the invention in fig 5, is a perspective view of a whirling creator which is included in the devices according to the invention in fig 3-6, is a perspective view of a fuel holder which is included in the device according to the invention in fig 3 and 4, is a perspective view of a fuel holder which is included in the device according to the invention in fig 5 and 6, Fig 10 is a flow chart of a plant including the device according to the invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS In fig 1 and 2 is illustrated an embodiment of the combustion device 1 according to the invention comprising a vertical combustion chamber 2 and a fuel holder 3. The combustion chamber 2 has a substantially cylindrical limiting surface 4, which surface is defined by a jacket 5, a cover 6 and a bottom 7. Even if the combustion chamber 2 in principle may be of any shape, the shape that is symmetrical with respect to rotation is preferred, and in practice the chamber will probably be designed just as a cylinder. Other embodiments of the device 1 include a horisontal combustion chamber 2, which will be described closer in the following. Further, the combustion chamber is provided with an inlet 8 in the jacket 5 for introducing oxygenuous gas, usually air, in a tangentiell direction. In the cover 6 of the combustion chamber an outlet 9 is arranged to discharge combustion gases produced in the combustion chamber 2 substantially parallel to the centre axis 10 of the combustion chamber 2. Inside the combustion chamber 2 the fuel holder 3 is arranged to hold and/or accommodate fuel 1 1 for the purpose of exposing the fuel 1 1 to the oxygenuous gas present in the combustion chamber 2 and therethrough promote mixing of oxygen with py- rolysis gas originating from the fuel 1 1 . Although the fuel holder 3 in this embodiment is shaped as a cup with one open upper end and with several holes in its surface for promoting gas exposure of the fuel 1 1 , and placed centrally at a distance above the bottom 7 of the combustion chamber 2, it will be pointed out that the fuel holder 3 may be shaped according to several different embodiments and be placed in several different ways inside the combustion chamber 2 to provide for different types of combustion chambers 2, different types of fuels 1 1 , different methods of refilling fuel 1 1 etc. The expressions "fuel holder" 3 and "holder" 3 here include dif- ferent kinds of gripping devices, plain support surfaces and containers for holding and/or accommodating fuel 1 1 in place. By experiments it has been found that a holder 3 that gives the fuel 1 1 a large exposure surface in relation to the volume of fuel in many cases is desirable, since one limiting factor for efficient combustion, at least for solid fuels 1 1 , is the size of the surface at which the combustion may take place. For that reason a fuel holder 3 of the container type having a large surface is often advantageous, even though a number of geometrical shapes of the fuel holder 3 are possible. Among them, for example holders having box-shape and holders 3 having spherical, cylindrical, or conical/double conical shape and tubular holders 3 with arbitrary cross section. Most of the types of holders 3 are suitably gas permeable. That may be provided for example by that the surface of the holder 3 is perforated, i.e. has several holes or by that the holder is manufactured of a thread material or a net material so that it receives a structure similar to a net. In practical experiments it has been found that a fuel holder 3 of two disc elements, between which elements the fuel is accom- modated, is a design very favourable for efficient combustion, which design will be described more in detail in connection with another embodiment of the device 1 . The introducing of fuel to the chamber 2, i.e. refilling of fuel 1 1 in the holder 3, may be performed in several different ways known per se. Among others, it may be mentioned different types of conventional mechanical feed mechanisms, for example different feed gears. An alternative introduce of fuel is that fuel 11 is supplied via a fuel pipe, which opens in connection to the fuel holder 3, by letting the fuel 1 1 to simply fall down into the holder 3 by way of the gravitation. In fig 3 and 4 one embodiment of the device 1 according to the invention having a horisontal combustion chamber 2, designed for so-called two way-gas flow, is illustrated. The definition two way-gas flow is explained below. The combustion chamber 2 has a substantially cylindrical limiting surface 4. The chamber 2 comprises a fuel inlet 12 for filling of fuel, a gas inlet 8 for introducing oxygenuous gas, usually air, and a gas outlet 9 for letting combusting gas out of said chamber 2. In this embodiment the combustion device 1 of the invention comprises members 13 for maintaining and/or intensifying a whirling motion present in the combustion chamber. Said members 13 include a so-called whirling creator 14 arranged along the centre axis of the combustion chamber 2. Furthermore, the device includes a fuel holder 3a, means 15 for guiding the whirling motion of the gas and means 16 for collecting ashes. The whirling creator 14 includes a tube 17 placed substantially parallel to the centre axis of the combustion chamber 2, which tube 17 has an inlet end 18 and an outlet end 19 coinciding with the outlet 9 of the combustion chamber 2, said gable 20 of the inlet end 18 being closed and said gable 21 of the outlet end being open and said inlet end 18 being provided with one or more cuts 22 in the envelop surface of the tube for inletting of gas. In the embodiments described the tube 17 of the whirling creator 14 has circularly cross section, which probably is the most appropriate, although other shapes of the cross section also are realisable. At respective cut 22 one guide blade 23 is arranged to guide the gas into the whirling creator tube 17. The whirling creator 14 is also illustrated in fig 7, in which the gable 20 of the inlet end 18 has no closing for the purpose of illustrating the cuts 22 and guide blades 23 of the whirling creator 14 more clearly. In practice, as already mentioned, the whirling creator 14 is closed at the inlet end 18 so that gas is able to flow in through said cuts 22 only. Previously mentioned "two way-gas flow" means that oxygenuous gas is brought to flow tangentially into the combustion chamber 2, in the proximity of a first end 24 of the combustion chamber 2, for creating a whirling motion. The gas flow, according to the aforesaid discussion, will receive one component of motion (directed to the left in fig 3) parallel to the centre axis of the combustion chamber. Thus, the gas is brought to whirl mainly between the limiting surface 4 of the combustion cham- ber 2 and the fuel holder 3a at the same time as the gas flows towards the other end 25 of the chamber 2. When the gas arrives to the other end 25 the gas is brought by means of the guide blades 23 of the whirling creator 14 to flow into the inlet end 18 through the cuts 22 at the same time as the whirling mo- tion of the gas is concentrated and intensified. Thus, inside the whirling creator a strong, turbulent flow arises which besides has a component of motion parallel to the centre axis of the combustion chamber and directed in the opposite direction in relation to the incoming gas (i.e. directed to the right in fig 3). The whirling formation is important among other things for obtaining good mixing of oxygen supplied and the pyrolysis gases originating from the fuel 1 1. By the rotation of the air around the fuel 1 1 an efficient mixing distance is received which is consid- erably longer than in the case of a substantially completely linear flow. The rotation increasing effect of the whirling creator 14 enables to use a high gas flow in the chamber 2 without causing an overpressure therein. In the described embodiment it is instead a small underpressure in the combustion chamber 2, which has great advantages, for example the introducing of fuel is considerably facilitated. The turbulent flow of the outgoing gasflow in the whirling creator 14 also contributes to the maintaining of a very efficient mixing and final combusting of any incompletely burned particles until the combustion gases leave the combustion chamber 2. The fuel holder 3a according to the embodiment in fig 3 and 4 is illustrated also in fig 8. The holder 3a has two disc elements 26 for accommodating fuel 1 1 between the elements 26 in a substantially uniformously thick and comparatively thin layer. The disc elements 26 are curved and in the present embodiment in a way so that two substantially circular limiting surfaces 27, 28, defined by a radius r1 and a radius r2, respectively, is created for the fuel 1 1 accommodated in the fuel holder 3a. The internal limiting surface 27 defined by r1 coincides suitably with the ex- ternal diameter of the whirling creator tube 17. Thus, the internal limiting surface 27 of the fuel holder 3a is in this example of an embodiment formed by a tube with circular cross section, i.e. the fuel holder 3a has one through channel 50 arranged centrally in the holder 3a. The external limiting surface 28 of the fuel holder 3a has a tubular shape with a longitudinal slot 29, i.e. in the surface there is one longitudinal opening 29, which leads to that the external limiting surface 28 of the fuel holder 3a has a shape of a tube with a circular cross section where a part of the envelope surface of the tube has been removed. This opening is intended to be directed against the fuel inlet 12 so that the fuel 1 1 supplied via the fuel inlet 12 may be introduced into the space between the two limiting surfaces 27, 28 of the fuel holder 3a. Furthermore, the external limiting surface 28 of the fuel holder 3a is gas permeable. To be more precise, that surface 28 is perforated, i.e. it has several holes for exposing the fuel 1 1 accommodated in the holder 3a and for enabling the required gas flow therethrough. Since the fuel 1 1 , in the way it is arranged in the fuel holder 3a, receives a large exposing surface in relation to its volume and because of the whirling formation in the combustion chamber 2 contributes to a long mixing distance and that the fuel 1 1 is accommodated as a relatively thin layer and uniformously thick layer, a very efficient combustion with almost optimum oxygen utilisation may be achieved. In fig 3 is illustrated how the means 15 for guiding the whirling motion of the gas is arranged in the space between the fuel holder 3a and the limiting surface 4 of the combustion chamber 2. The guide means 15 is realised by means of a guide rail 30 which extends helically along the fuel holder 3a. The guide rail 30 enables to guide the gas flow around the fuel holder 3a and by that around the fuel 1 1 in a way that is controlled for obtaining for example a long mixing distance. The guide rail 30 may be used for promoting the whirling formation and reducing the com- ponent of motion of the gas along the centre axis of the combustion chamber 2 and by that obtain a long mixing distance in a manner that is controlled. By that a good mixing between the oxygenuous gas and the pyrolysis gas is obtained and also the advantage that the combustion chamber 2 and by that the com- bustion device 1 may be designed substantially smaller than a combustion device with a comparing mixing distance, in which the gas flow is linear. In the example of an embodiment the helical guide rail 30 has an increasing pitch in the direction towards the inlet end 18 of the whirling creator 14. This embodiment im- plies that one take into consideration that the entering gas expands and gets a larger volume during its way through the combustion chamber 2. It should be pointed out that said guide rail 30 constitutes an example only how the whirling motion may be controlled in the combustion chamber 2. As an example, the guide rail 30 must not constitute a continuous segment, but instead the guide means 15 may for example include a number of smaller guide rails spaced apart along the periphery of the fuel holder 3a and/or the limiting surface 4 of the combustion chamber 2. For obtaining the best possible control of the gas it would be most suitable that the height of the guide rail/rails 30 is substantially equal to the distance between the external limiting surface 28 of the fuel holder 3a and the limiting surface 4 of the combustion chamber, although it is of course possible to adapt the height so that it is formed a slot between respective rail 30 and the limiting surface 4 of the combustion chamber and as an alternative between respective rail 30 and the external limiting surface 28 of the fuel holder 3a. In the device 1 according to the invention the means 16 com- prised for ash collecting includes a collecting receptacle 31 in the periphery of the combustion chamber 2. This receptacle 31 is suitably formed in the bottom 7 of the combustion chamber 2. By the design of the combustion chamber 2 according to the invention and the combustion method, which influence the gas flow in the combustion chamber 2, the most of the fly ash arising after combustion will be thrown out to the periphery and by that the most of the fly ash may be collected in the collecting receptacle 31 in an easy and efficient way. By that, a great advantage is that a large part of the ash may be taken care of before the ash is admitted to leave the chamber 2 together with the combustion gases, which means that a considerably smaller part of the ash is supplied to a furnace connected to the combustion chamber 2 in comparison with many of the combustion devices within the current field already known. The collecting receptacle 31 extends suitably along substantially the whole length of the combustion chamber 2 and forms a channel in which the fly ash is collected. The collecting recepta- cle 31 may also be completed with any form of separator 32, for example a plate, for guiding the ash to the receptacle 31 . In this example of embodiment the bottom surface of the collecting receptacle 31 slopes towards an ash discharge 33 for enabling removal of the ash out of the collecting receptacle 31 by means of water flushing. In fig 5 and 6 is illustrated an embodiment of the device 1 according to the invention having a horisontal combustion chamber 2, designed for so-called three way-gas flow. This embodiment includes a number of construction characteristics in common to the embodiment of so-called two way-gas flow described above. For that reason, only the characteristics specific to the embodiment of the three way-gas flow will be described in the following and for the rest reference is made to the embodiment already described. Said "three way-gas flow" means that oxygenuous gas is brought, via the gas inlet 8, to flow tangentially into the combustion chamber 2, in proximity of an end 34 of the fuel holder 3b, for the creating of a whirling motion. The combustion cham- ber 2 comprises suitably any kind of dividing wall between the gas inlet 8 and the inlet end 18 of the whirling creator 14 for preventing the gas to flow directly to the inlet end 18. The gas flow, in accordance with previous discussion, will receive one component of motion (directed to the right in fig 5) parallel to the centre axis of the combustion chamber. Thus, the gas is brought to whirl mainly between the limiting surface 4 of the combustion chamber 2 and the fuel holder 3b simultaneously as the gas flows in a direction towards the other end 35 of the fuel holder 3b and at the gable 36 of the combustion chamber 2 the gas is brought to receive a component of motion in the opposite direction (i.e. directed to the left in fig 5) and according to the theory previously described, the whirling motion now continuous between the outer surface of the whirling creator tube 17 and the internal limiting surface 27b of the fuel holder 3b to arrive thereafter to the inlet end 18 of the whirling creator 14, whereupon the gas flows by analogy with the embodiment previously described of two way-gas flow and finally out of the combustion chamber 2 via the outlet end 19 of the whirling creator. The purpose of this embodiment is to further increase the exposing surface of the fuel 1 1 and further extend the mixing distance for mixing of the oxygenuous gas with pyrolysis gas originating from the fuel 1 1 . By this method and the design of the device, thus is enabled that the fuel 1 1 is exposed to the oxygenuous gas on its internal limiting surface 27b as well as on its external limiting surface 28b. In fig 9 the fuel holder 3b is separately shown. The holder 3b has nearly the same geometry as the holder 3a in the previous described embodiment according to fig 8. However, the fuel holder 3b has been adapted for three way-gas flow by making the radius R1 , which defines the internal limiting surface 27b of the holder 3b, larger than the radius r1 which defines the external diameter of the whirling creator tube 17. This is done for enabling the gas to pass between the internal limiting surface 27b of the fuel holder 3b and the whirling creator tube 17. Fur- thermore, in this example of embodiment also the internal limiting surface 27b of the fuel holder 3b is perforated, i.e. similar to the external limiting surface 28b it has a number of holes for exposing the fuel 1 1 to gas present in the combustion chamber 2. The device 1 according to the invention comprises, in this embodiment, members 37 for conducting at least a part of the gas in a direction towards the fuel 1 1 . The conducting member 37 may include one or more conducting rails 38. In the example of embodiment the rails 38 are arranged along the fuel holder 3b and respective rail 38 forms an angel B with the external limiting surface 28b of the fuel holder 3b, which is best illustrated in fig 6. As an alternative the conducting rail/conducting rails 38 may extend along the limiting surface 4 or the combustion chamber 2 and instead form an angel with the limiting surface 4 of the combustion chamber 2. Furthermore, the conducting rails 38 could have a curved cross section so that the rails 38 in greater extent follow the fuel holder 3b and/or the limiting surface 4 of the combustion chamber 2. Irrespectively of the arrangement of the conducting rails 38 along the fuel holder 3b or along the lim- iting surface 4 of the combustion chamber 2, or a combination of these embodiments, the principle is that the conducting rails 38 conduct a part of the whirling gas towards the fuel holder 3b for exposing the fuel 1 1 to a larger extent to the oxygenuous gas and promoting an efficient combustion. In the example of an embodiments have been described for the case of two way-gas flow that guide rails 30 may be used to guide and control the whirling information around the fuel holder 3a and in the case of three way-gas flow has been described above of how conducting rails 38 may be used to conduct a part of the gas towards the fuel holder 3b and thereby towards the fuel 1 1 . However, it should be pointed out that guide rails 30 may be applied also in the embodiment with three way-gas flow and that conducting rails 38 as described for the embodiment of three way-gas flow may be applied in the embodiment of two way-gas flow as well. Thus, guide rails 30 and conducting rails 38 and combinations thereof may be applied in both embodiments of two way-gas flow and three way-gas flow. The design of the fuel holders 3a, 3b in fig 8 and fig 9, respectively, enables that different coefficients of fullness of fuel 1 1 , in holder 3a and 3b, may be achieved in the both cases. This is an important characteristic of the fuel holder 3 for the reason that the effective output of the combustion device 1 may be con- trolled therethrough. With a relatively small quantity of fuel and a quantity of oxygen adapted thereto, a lower output is received in relation to the application of a large quantity of fuel and a larger quantity of oxygen associated therewith, that gives a higher output of the device 1. Another embodiment of the fuel holder 3 is to create a shorter distance between the elements 26 at the bottom of the holder 3 than otherwise between the disc elements 26 by mutual displacing of the disc elements 26. That embodiment will contribute to a large exposing surface even when only a small quantity of fuel is supplied to the holder 3, since, when supplying fuel 1 1 , the holder 3 first is filled with fuel 1 1 in the bottom. In fig 10 is schematically illustrated an installation including the device 1 according to the invention. The installation includes briefly a silo 39 from which the fuel 1 1 is sucked into a feed device 40 and then the fuel 1 1 is dosed and supplied to the device 1 of the invention for combustion. The device 1 is connected to a conventional heating boiler 41 for extracting heat. From the heating boiler 41 the combustion gases are conducted to an apparatus 42 for purifying of the combustion gases where the gases of combustion are purified and further heat are extracted from the gases of combustion by means of a heat exchanger 43. After the purifying of combustion gases the gas is discharged through a chimney 44. In all embodiment examples described the device suitably comprises means 45 for bringing a part of the combustion gases that have left the combustion chamber back to the combustion chamber 2. This is schematically illustrated in fig 10. A pipe 46 including a valve 47 and control equipments 48 required con- nects the pipe outgoing from the combustion gas purifying apparatus 42 with the combustion device 1 . The return of the combustion gases may be performed in a separate inlet 49 in the combustion chamber 2 for return or by mixing the combustion gases with the oxygenuous gas required for the combustion, usually air, before the air and the combustion gases are supplied into the combustion chamber via the gas inlet 8 of the combustion chamber. Other alternatives, such as bringing back the combustion gases before these have passed through the apparatus 42 for purifying combustion gas, are also possible. Returning of combustion gases in the device 1 according to the invention has the advantage that thereby is enabled that the gas flow in the combustion chamber 2 may be maintained in spite of that the supply of air is reduced, in most cases with the purpose to reduce the inletting of air into the chamber 2, which is desired for example when the combustion device 1 works at a lower load. Although the device 1 in the schematically installation is con- nected to a heating boiler 41 in such a way that a pipe connects the outlet 9 of the combustion chamber 2 of the device 1 and the heating boiler 41 , it is an alternative and realisable solution that the combustion chamber 2 is arranged inside a conventional heating boiler 41 . The device 1 comprises in the example means 51 for removing collected ash by water flushing. Said means 51 may suitably include pipes, connection components, valves and control equipment. In the description exemplifying embodiments have been described, which should be considered only as examples and not constitute any restrictions for the basic idea of the invention. Several modifications are possible within the scope of the invention. For example the fuel may be constituted of almost any kind of solid fuel including wood, coal, coke, peat, energy forest, sewage sludge, refuse and waste etc. In this connection should be mentioned that in practical experiments it has been found that in the combustion chamber of the combustion device according to the invention a high temperature may be achieved if so is desired, which enables that the device with advantage may also be used in combusting of so-called "difficult to split-fuels". The method according to the invention and the device may in a somewhat modified embodiment advantageously also be used for liquid fuels. One way is to design such a fuel holder so that it is able to accommodate a liquid fuel, but practically the best way is probably to use a liquid fuel by supplying the fuel in connection with the introducing of the oxygenuous gas to the combustion chamber. Then, the combustion may be performed in an ef- ficient way since by that one may use the good mixing characteristics of the device as described previously regarding long mixing distance, strong whirling formation etc. Other modifications within the scope of the idea of the invention are for example that the combustion chamber is provided with several inlet for oxygenuous gas and/or combustion gases brought back, introducing of secondary air to the whirling creator, to arrange guide rails and/or conducting rails between the whirling creator and the internal limiting surface of the fuel holder in the case of three way-gas flow, to arrange an agitator for stirring the fuel in the fuel holder for the purpose of exposing the fuel better and others.

Claims: Claims 1. A combustion device (1 ) comprising a combustion chamber (2) with at least one gas inlet (8) and at least one gas outlet (9), said gas inlet being arranged to conduct oxygenous gas, for example air, substantially tangentially into the combustion chamber for the purpose of achieving a whirling motion in said combustion chamber and said gas outlet being arranged substantially centrally, in relation to a centre axis (10) of the combustion chamber, for letting combustion gas out in one end of the combustion chamber in a direction substantially parallel to said centre axis, characterized in that the device includes a fuel holder (3) arranged in the combustion chamber for exposing the fuel (1 1 ) to gas present in the combus- tion chamber and promote mixing of the oxygen contained in the gas with the pyrolysis gas originating from the fuel. 2. A device according to claim 1 , characterized in that the fuel holder (3) comprises at least one container for accommo- dating the fuel (1 1 ) therein. 3. A device according to claim 1 or 2, characterized in that the combustion chamber (2) has a limiting surface (4) and that the fuel holder (3) is arranged so that the fuel (1 1 ) at least partly is at a distance from the limiting surface. 4. A device according to claim 3, characterized in that the limiting surface (4) of the combustion chamber (2) at least in one part is substantially symmetrical with respect to rotation. 5. A device according to claim 4, characterized in that the limiting surface (4) of the combustion chamber (2) forms substantially a cylinder. 6. A device according to any previous claim, characterized in that the fuel holder (3) at least partly is gas permeable. 7. A device according to claim 6, characterized in that the fuel holder (3) at least in one part of its surface has holes for exposing the fuel for gas present in the combustion chamber (2). 8. A device according to any previous claim, characterized in that the fuel holder (3a) has at least one through channel (50) arranged substantially centrally in the holder. 9. A device according to any previous claim, characterized in that the fuel holder (3a, 3b) has at least two disc elements (26) for containing the fuel between the elements in a substantially uniformously thick layer. 10. A device according to claim 9, characterized in that the disc elements (26) are curved. 1 1 . A device according to claim 10, characterized in that the disc elements (26) extend substantially parallel with the central axis of the combustion chamber, said disc elements being defined by a radius r1 and a radius r2, respectively, and said radii r1 , r2 having the proportions r2>r1 >0 in one cross-section of the fuel holder (3a, 3b). 12. A device according to any previous claim, characterized in that the device (1 ) comprises members (13) for maintaining and/or intensifying the whirling motion present in the combustion chamber. 13. A device according to claim 12, characterized in that said members (13) comprise a whirling creator (14) including a tube (17) placed substantially parallel to the centre axis of the combustion chamber, which tube has an inlet end (18) and an outlet end (19) coinciding with the outlet of the combustion chamber, said inlet end being provided with one or more cuts (22) in the envelope surface of the tube to let gas in. 14. A device according to claim 13, characterized in that at least one guide blade (23) is arranged at one or more of the cuts (22) at the inlet end (18) of the whirling creator (14) for guiding the gas into the tube (17) of the whirling creator. 15. A device according to any previous claim, characterized in that the device (1 ) has means (45) for bringing a part of the combustion gases that have left the combustion chamber back to the combustion chamber (2). 16. A device according to any previous claim, characterized in that the device (1 ) comprises means (15) for guiding the whirling motion of the gas in the combustion chamber (2). 17. A device according to claim 16, characterized in that the guide means (15) includes at least one guide rail (30). 18. A device according to claim 17, characterized in that the guide rail (30) extends substantially helically. 19. A device according to claim 18, characterized in that the helical guide rail (30) has a varying pitch. 20. A device according to any previous claim, characterized in that the device (1 ) comprises members (37) for conducting at least a part of the gas in direction towards the fuel. 21 . A device according to claim 20, characterized in that the conducting member (37) includes at least one conducting rail (38). 22. A device according to any previous claim, characterized in that the device (1 ) comprises means (16) for collecting ashes. 23. A device according to claim 22, characterized in that the ash collecting means (16) includes a separator (32) and/or a collecting receptacle (31 ). 24. A device according to claim 23, characterized in that the separator (32) and/or the collecting receptacle (31 ) are arranged to separate and/or collect the ash before it is admitted to leave the combustion chamber together with the combus- tion gases. 25. A device according to claim 23 or 24, characterized in that the device (1 ) comprises means (51 ) for removing the ashes from the collecting receptacle (31 ) by means of water flush- ing. 26. A method for combustion of fuel, in particular including solid fuels such as pellets of wood, in which the fuel is introduced into a combustion chamber (2) and an oxygenous gas, for example air, is introduced in a tangential direction through at least one inlet (8) into the combustion chamber for the purpose of achieving a whirling motion in the combustion chamber and the combustion gases are discharged from the combustion chamber substantially centrally, in relation to a centre axis (10) of the combustion chamber, at an outlet (9) in one end of the combustion chamber in a direction substantially parallel to said centre axis, characterized in that the fuel is kept in the combustion chamber by means of a holder (3) so that the fuel is exposed to gas present in the combus- tion chamber and so that the mixing of the oxygen contained in the gas with the pyrolysis gas originating from the fuel is promoted. 27. A method according to claim 26, characterized in that the gas present in the combustion chamber (2) downstream of the inlet (8) is influenced to maintain and/or intensify its whirling motion by means of one or more members (13).

Inventors: LARSSON BERTIL, LARSSON, BERTIL
Applicants: LARSSON BERTIL, LARSSON, BERTIL
Application year: 1999
Main classification: F23B1
IPC codes: F23C3, F23M9