Background to the invention The burning of gas intensive biofuels, such as pellets, briquets, wood chips and the like, is associated with a plurality of problems. One specific problem in all biofuel combustions is to find combustion devices that allow for a high reduction speed for the carbon particles in the combustion zone. If a retort or a combustion chamber is used, the burning of the highly energetic fuels having a low degree of moisture content causes the fuels to gasify at a low effect output before they have reached the retort or the combusition chamber. High resistance in the throughput causes difficulties to direct a uniform blow-through of the fuel mass, which results in local burnings, where the main part of the airstream will flow through, while other parts of the fuel will be unaffected.

A particular problem is that the melting point for ash often is easily reached.

In connection to this, if a retort or an equivalent device is used, a deposition of slag and uncombusted fuel is easily created, which in turn disturbs the desired distribution of air and fuel.

Another problem in combustions is that salts condensates in the temperature range of 600-1000 °C, as does certain gases within the fuel. This causes depositions to form on the outside surfaces of the combustion device. These depositions are hard to remove and may clog the air holes disposed in the combustion device.

The present invention is intended to overcome these problems and simultaneously ensure an efficient combustion during all burning conditions. This and other advantages are obtained with the combustion device according to the present invention.

Brief description of the drawing Fig. 1 shows a cross-section of one embodiment of the present invention.

Description of the invention As is clear from Fig. 1, the combustion device according to the invention comprises a combustion chamber 1, which essentially consists of an excentric input part 2, a cylindrical middle part 3, and a conical output part 4. A number of apertures 12 for air-blasting, in general marked with X in the figure, are disposed in the input part 2, the middle part 3, and the output part 4 to supply preheated air on the one hand through the face of the fuel mass and on the other hand through the created gas volume, wherein the blowing is performed in marked zones of primary air, secondary ari and tertiary air, from the input part 2 to the output part 4. Primary air defines the fuel evaporation air, secondary air defines the gas combustion air, and tertiary air is defined as the air in the final combustion of the rests. In addition to the abovementioned parts the combustion device is equipped with a fuel grating in the lower part of the conical shaped output part 4. A grating is here defined as a device that provides small evacuation paths out of the combustion chamber 1. The grating is provided to allow the combusted fuel particles to fall down through the grating and out of the combustion chamber, while the burned gases leave the combustion chamber through the apertures in the conically shaped output part 4.

During operation the fuel is fed horisontally into the combustion chamber 1 through a feeding pipe 7 by means of a feeding screw 6 disposed in the feeding pipe 7. With help from organs, such as a blower, the combustion air is supplied around the feeding pipe 7 of the feeding screws 6, therebye cooling the fuel. The fuel is preferably ignited by means of an electrical ignition device 10 disposed in the end of the feeding pipe 7. The end of the feeding pipe is adjacent to the excentric input part 2. The combustion air continues towards the excentric input part 2 and cools this part. The combustion air then continues in a pipe 9, that surrounds the combustion chamber and a cylindrically shaped heat shield 8, which in turn surrounds a large part of the combustion chamber 1, preferably it surrounds the whole of the middle part and at least 2/3 of the conically shaped output part 4. After having travelled over the heat shield 8, the combustion air first encounters the conically shaped output part 4, and then shifts direction approximately 180°C and passes in under the heat shield 8. In this position the air is heated by the combustion chamber, but in the contact with the output part 4 heat is absorbed from the output part 4, which means that the air will be pre- heated at the same time as a cooling of the output part 4 is obtained. Finally, the

combustion air enters the cylindrical middle part 3 as pre-heated primary air.

Since the side channels have a divergent orientation with regard to the fuel input direction a loosening of the fuel mass as well as a smaller volume height is obtained, which gives an increased contact surface for the supplied primary air.

The abovementioned heat shield have three functions. The first function is to shield the cylindrical pipe 9 from the heat that is radiated from the combustion chamber 1. Without this shielding the cylindrical pipe would implicitly influence the temperature in the combustion zones. The second purpose is to force the combustion air to cool the conical output part 4 in the combustion device, and therebye implicitly supply a lower temperature with the secondary air, which in turn reduces the creation of nitrogen oxides in the final stage of the combustion.

The third purpose is to preheat the primary air that is to gasify the fuel under a sufficiently short time so that the creation of carbon monoxide radically decreases and so that the chemical process instead goes directly to carbon dioxide.

The main part of the combustion takes place in the primary zone when the preheated air quickly gasifies the fuel. Here, the combustion air is turbulent and directed against the feeding direction of the fuel. The rising burn gases might contain partially uncombusted parts that encounter the air supplied through the apertures in the upper part of the cylindrically shaped middle part 3, wherebye gases emanating from the fuel are trapped and mixed with fire flames from the front of the fuel. These gases are now forced, through the increase in volume, towards the conical output part 4 where they are mixed with air supplied from apartures in the conical part 4. This is where the final combustion of the remaining uncombusted gases takes place.

During the gasification, the heat shield 8 that supports the preheating of the primary air also provides for a preheating of the moisture embedded in the fuel, which means that overheated vapour will be present in the process. The overheated vapour is heavier than the other gases and is transported out of the combustion chamber through the grating 5, where the overheated vapour encounters carbon particles which leads to a reduction reaction since carbon in combination with water creates carbon dioxide.

As mentioned above, the output aparture 4 is provided with a grating 5 in the lower part. The grating 5 prevents carbon particles larger than a certain size from leaving the combustion zone before they have been"prepared"with overheated vapour. The remains of the carbon that are still present after the

combustion cannot leave the combustion zone since they are built by carbon chrystals that are drawn to materials in the form of fluid or gas. They can be removed by treatment with overheated water vapour, which is a by-product in the combustion of biofuel.

The grating in the lower part of the conically shaped output aperture is a way of overcoming the problem that the time required to burn the carbon particles completely cannot be achieved under normal conditions. The reason for this is partly the amount of fuel and the supply of air, but it also depends on the compositions that carbon particles bind on their surface without forming chemical bonds. If the uncombusted carbon particles did not fall down through the surface of the grating, the combustion zone would decrease in the same degree, whereby the increased volume of carbon particles would affect the combustion negatively, partly through increased volume-pressure relations, partly through a lower combustion temperature, something that in practice would mean shorter retention times and an increased output of uncombusted gases and indirectly a lower heating effect.

Characteristic for all combustions of biofuels is that the gas generation and the reductions always reach an equilibrium that is directly linked to the reaction temperature. In a combustion device according to the present invention, the combustion air is preheated in all combustion zones except the ash zone. It is always well calibrated in relation to the supplied amount of fuel in the combustion zone.