METHOD AND DEVICE FOR BURNING DISINTEGRATED PLANT-BASED FUEL

Abstract: Method and arrangement for burning crush-like plant-based fuel. The arrangement comprises a boiler (1) designed for burning dry fuel (18). When moist fuel (18) is fed to the arrangement, the heat energy obtained from combustion gases of the boiler is led to dry the fuel (18). The heat energy which is not needed for drying the fuel (18) is conducted to heat the combustion air of the boiler (1). Then the burning procedure is easy to implement technically and the boiler can be provided with an advantageous structure. Further, fuel (18) of variable moisture can be fed to the arrangement.

Description: METHOD AND DEVICE FOR BURNING DISINTEGRATED PLANT-BASED FUEL FIELD OF THE INVENTION The invention relates to a method of burning crush-like plant-based fuel, in which method fuel is burned in a boiler designed for burning dry fuel, fuel is dried with the heat energy obtained from combustion gases of the boiler and fed after that into a combustion chamber of the boiler for burning. The invention relates further to an arrangement for burning crush¬ like plant-based fuel, the arrangement comprising a boiler designed for burn- ing dry fuel, a feeding apparatus for feeding fuel into a combustion chamber of the boiler, a drying apparatus for drying the fuel to be fed to the arrangement and means for feeding the heat energy of combustion gases of the boiler into the drying apparatus. BACKGROUND OF THE INVENTION In fairly small chip burning plants of a size class typically below 2 MW, a use of boilers planned for burning dry chips is most preferable, as far as purchase price is concerned. Such plants are poorly adapted to variations in the moisture of the fuel, i.e. they are not capable of burning for instance un- seasoned chips, bark or sawdust. When boilers are planned in such a way that also unseasoned fuel can be burned in them, the price of the plant easily rises high. Primary price increasing factors are increased masonry of combustion chamber, enlarged convection part and preheaters of combustion air. A plant planned for burning moist fuel is also poorly suited for dry fuel, because dry fuel raises the temperature of the combustion chamber high and the plant will be subjected to resistance problems, for instance. If necessary, dry fuel must even be wetted. A plant planned for burning moist fuel is also poorly suited for driving small powers because of difficult power control, particularly when dry fuel is used. Finnish publication 780 822 discloses a fuel drying apparatus of a boiler. The drying apparatus comprises a fuel tank, from which the fuel is transported to a combustion chamber of the boiler. Combustion gases from the boiler are led into a closed space below the fuel tank, whereby the com¬ bustion gases heat the fuel. By this arrangement, moist fuel can be dried be- fore it is led into the boiler, but when already dry fuel is fed into the fuel tank, the combustion gases dry it further completely unnecessarily, which causes a fire risk in the fuel tank, for instance. Finnish publication 60 435 also discloses a fuel drying apparatus. The drying apparatus comprises a vertically longitudinal flat chamber heated by combustion gases, in which chamber there are horizontal screw conveyors on each other and under the screw conveyors chutes, in which the fuel passes from one screw conveyor to another back and forth and downwards. The combustion gases are directed to flow upwards in the chamber, whereby they dry fuel. The arrangement is inconvenient and complicated and the drying pro- cedure cannot be controlled in any way, which means that when, for instance, dry fuel is fed it is dried unnecessarily and there is an obvious fire risk in the drying apparatus. German Offenlegungsschrift 39 13 885 discloses a boiler arrange¬ ment for burning moist fuel. This arrangement comprises a slanting grate, to the upper end of which the fuel is led. The fuel is dried on the upper part of the grate by conducting combustion gases for the purpose of drying. The fuel dries on the drying part of the grate and it is burnt on the remaining part of the grate. This arrangement has a big and expensive structure and it is not suitable for burning dry fuel. SUMMARY OF THE INVENTION The object of this invention is to provide a method and an arrange¬ ment not showing the above-mentioned drawbacks and making it possible to burn crush-like plant-based fuel of variable moisture. The method according to the invention is characterized in that fuel the moisture of which can vary between dry and wet is used and that the com¬ bustion gases are conducted to heat air and that heat and flow amounts of this air are dimensioned in such a way that the air in question is sufficient for dry¬ ing the fuel in all moisture conditions of the fuel and that the heated air is di- rected in such a way that the air not needed for drying fuel is conducted into the boiler to serve as combustion air. Further, the arrangement according to the invention is characterized in that the arrangement comprises a heat exchanger and an air duct, the heat exchanger being arranged to heat the air flowing in the air duct with the heat energy of the combustion gases of the boiler, that the heat exchanger and the air flow amount are dimensioned in such a way that the air flowing in the air duct is sufficient for drying the fuel in all moisture conditions of the fuel and that the arrangement comprises means for conducting the air flowing in the air duct and not needed for drying the fuel into the boiler to serve as combustion air. An essential idea of the invention is that the boiler of the arrange¬ ment is designed to burn dry fuel and that when moist or wet fuel is fed to the arrangement, the heat energy obtained from the combustion gases of the boiler is utilized for drying the fuel and the heat energy not needed for drying the fuel is directed to heat the combustion air of the boiler. Further, the idea of an embodiment consists in that the combustion gases are conducted to a heat exchanger heating the air which is led either to dry fuel or to serve as combus¬ tion air of the boiler, as per need. The idea of another preferred embodiment is that the fuel is dried by a wire dryer, the air heated with the heat energy of the combustion gases being conducted through the wire. The idea of a third pre- ferred embodiment is that a recovery of the heat of drying air and the air flow to be led through the chips are dimensioned in such a way that the air flowing out through the chips is substantially saturated with water vapour. The idea of a further fourth preferred embodiment is that an air distribution either for a purpose of drying fuel or for serving as combustion air of the boiler is con- trolled on the basis of the temperature in the boiler. An advantage of the invention is that fuel of variable moisture can be fed to the arrangement for burning. Another advantage is that the boiler is designed to use dry fuel, due to which the burning procedure is easy to im¬ plement technically and the boiler can be provided with an advantageous structure. It is then possible to use for instance several different alternative grate solutions, high temperature can be achieved for the combustion cham¬ ber, due to which a great part of the heat energy can be recovered as radiation heat, little masonry is needed in the boiler, the fuel can be burnt accurately causing minor outlets, and further, condensation and corrosion problems can be avoided. In addition, the temperature of the combustion gases can be rather high, due to which the convection part of the boiler can be dimensioned to be small and the aging of the boiler is not significant. Moreover, when dry fuel is used, the heat energy of the combustion gases can be recovered through the combustion air back to the system to be utilized. The efficiency of the arrangement is also rather high irrespective of the moisture values of the fuel. Further, the arrangement can be built in such a way that its total price will be low. Likewise, the rather efficient arrangements can be implemented in such a manner that they can be positioned in movable containers, for in¬ stance. BRIEF DESCRIPTION OF THE DRAWING The invention will be described in more detail in the attached figure showing an arrangement according to the invention schematically. DESCRIPTION OF THE INVENTION The figure shows a boiler 1 comprising a combustion chamber 1a and a convection part 1b in a manner fully known per se. Further, the figure shows a feeding apparatus 2 of fuel 18 schematically. The fuel 18 can be fed into the boiler 1 e.g. by a screw conveyor or by using so-called stoker burning, for instance, or some other solution fully known per se. Since the feeding ap- paratus 2 of fuel 18 is fully known per se, it is not discussed further in this con¬ nection. Combustion gases are led out of the boiler 1 through a combustion gas duct 3. The combustion gases are conducted according to arrows A through a heat exchanger 4. The heat exchanger 4 heats the air flowing in an air duct 5. The air flows in the air duct 5 according to arrows B. The air heated by the heat exchanger 4 is conducted through a drying air duct 6 to a drying apparatus 7 to dry moist fuel 18. The heat exchanger 4 can either be a sepa¬ rate heat exchanger or an integral part of the boiler 1. The drying apparatus 7 comprises an air permeable wire 8 forming an endless loop and arranged to go round rolls 9a and 9b. A chamber 10 is ar- ranged in the middle of the loop formed by the wire 8. The heated air is con¬ ducted through the drying air duct 6 into the chamber 10. The heated air is blown out of the chamber 10 through the upper part of the wire 8, whereby the air also flows through the fuel 18 above the wire 8 according to arrows C and D. The fuel 18 is fed on the wire 8 through a feed channel 11 , for instance. The fuel 18 may consist e.g. of dry or unseasoned chips, bark, sawdust, grain screenings, waste from carpentry industry, sod peat or some other suitable plant-based crush-like fuel. The first end of the chamber 10 is preferably sealed against the first roll 9a and the second end against the wire 8. By flow¬ ing through the fuel 18, the heated air extracts moisture from it. The tempera- ture of the drying air and the speed of the wire 8 are preferably dimensioned in such a way that the air flowing out through the fuel 18 is substantially satu¬ rated with water vapour. The rolls 9a and 9b going round, i.e. the movement of the wire 8, may either be continuous or intermittent controlled by a step motor, for instance. However, the most essential thing is that the temperature and the outlet velocity of the drying air as well as the movement and surface area of the wire 8 are dimensioned in such a way that the fuel 18 can be dried suffi¬ ciently dry. In the case according to the attached figure, moist fuel 18 is fed to the drying apparatus of the arrangement according to the invention, whereby all the air heated by the heat exchanger 4 is directed by means of a damper 12 of the drying air duct 6 and a damper 14 of a combustion air duct 13 to dry the fuel 18. Combustion air is then led from a secondary air duct 15 through the combustion air duct 13 into the boiler. The attached figure shows the pro¬ cedure of leading combustion air in a simplified manner, but naturally, it is possible to lead both primary and secondary combustion air into the boiler in a manner fully known per se. When the fuel 18 is not so wet that all the air heated by the heat exchanger 4 has to be led to dry the fuel 18, the damper 12 of the drying air duct 6 is closed partly and the damper 14 of the combus¬ tion air duct 13 is opened partly, whereby part of the heated air is conducted to serve as combustion air of the boiler. Further, when the fuel 18 fed to the ar¬ rangement is so dry that no drying at all is needed, the damper 12 of the dry¬ ing air duct 6 is closed entirely and the damper 14 of the combustion air duct 13 is opened in such a way that all the air heated by the heat exchanger 4 is conducted to serve as combustion air. For the sake of clarity, the attached fig- ure does not show fans required for moving air in the ducts. The dampers 12 and 14 can be controlled by a control device 17. An information on the temperature of the combustion chamber 1a detected by a sensor 16 is given to the control device 17. When rather moist fuel 18 changes into dryer, the fuel going into the combustion chamber 1a after the drying apparatus 7 is dryer than before and the temperature of the combustion chamber 1a rises. The sensor 16 detects this temperature rise, due to which the control device 17 controls the dampers 12 and 14 in such a way that a greater part than before of the air heated by the heat exchanger 4 will serve as combustion air. When the temperature rises further, the control device 17 closes the damper 12 of the drying air duct 6 entirely and all the combustion air can be preheated. When the fuel changes into moister than before, the temperature of the combustion chamber 1a begins to fall and the control de¬ vice 17 controls the air heated by the air exchanger 4 by means of the damp¬ ers 12 and 14 to dry the fuel 18 more than before. The dampers 12 and 14 can also be controlled on the basis of the outlet temperature or outlet moisture of the drying air, for instance, or on the basis of some other suitable basic data. The boiler 1 of the arrangement according to the invention is de¬ signed for dry fuel, which in this connection signifies that special solutions necessary for burning moist fuel need not be taken into consideration in the structure of the boiler 1. For instance, prices of solutions to feed and grate problems of the fuel may be low, without the reliability of operation suffering, however. The starting point of the designing may, for instance, be that such fuel is dry fuel which always has a moisture of 35 % or less. In the structure of the boiler, attention can then easily be paid to the fact that, for instance, fuel of a moisture of 10 % or even quite tinder-dry fuel is fed to the arrangement. Cal¬ culations have proved that the moisture of the fuel to be fed to the arrange¬ ment may be for instance 60 %, and nevertheless, the efficiency of the ar¬ rangement can be kept rather good. When fuels dryer than this are used, the efficiency is better still, of course. When dryer fuel is used, the temperature of the combustion chamber 1a rises, which intensifies the heat transfer to the boiler. The combustion gases can be removed from the boiler 1 rather hot, at about 300 to 400 degrees, preferably 350 to 400 degrees, for instance. Then the convection part 1b of the boiler can be made small, and simultane- ously, the problems with the arrangement getting dirty are decreased. The temperature of the combustion gases can be dropped by the heat exchanger 4 to about 100 degrees, for instance. Then the air in the air duct 5 can be heated by the heat exchanger 4 rather well. The figure and the related description are only intended to illustrate the idea of the invention. As to the details, the invention may vary in the scope of the claims. So, prior art technique can be used for storing fuel and feeding it to the drying apparatus, for instance.

Claims: CLAIMS 1. Method of burning crush-like plant-based fuel, in which method fuel (18) is burned in a boiler (1) designed for burning dry fuel, fuel (18) is dried with the heat energy obtained from combustion gases of the boiler (1) and fed after that into a combustion chamber (1a) of the boiler (1) for burning, characterized in that fuel the moisture of which can vary between dry and wet is used and that the combustion gases are conducted to heat air and that heat and flow amounts of this air are dimensioned in such a way that the air in question is sufficient for drying the fuel (18) in all moisture conditions of the fuel (18) and that the heated air is directed in such a way that the air not needed for drying fuel (18) is conducted into the boiler (1) to serve as com¬ bustion air. 2. Method according to claim 1 , cha racterized in that the fuel (18) is dried by feeding it on an air permeable wire (8) and by blowing air heated with the heat energy of the combustion gases through the wire (8) and the fuel (18). 3. Method according to claim 2, cha racterized in that the air heating with combustion gases and the air flow to be led through the fuel (18) are dimensioned in such a way that the air flowing out through the fuel (18) is substantially saturated with water vapour. 4. Method according to any of the foregoing claims, c h a ra c ¬ terized in that the combustion gases of the boiler (1) have a temperature of 300 to 400 degrees when they are removed from the boiler (1). 5. Method according to any of the foregoing claims, c h a r a c - t e r i z e d in that the temperature of the combustion chamber (1a) of the boiler (1) is measured and, on the basis of the measurement, the air heated with the heat energy of the combustion gases is directed to dry the fuel (18) and to serve as combustion air of the boiler (1), as per need. 6. Arrangement for burning crush-like plant-based fuel, the ar- rangement comprising a boiler (1) designed for burning dry fuel, a feeding ap¬ paratus (2) for feeding fuel (18) into a combustion chamber (1a) of the boiler (1), a drying apparatus (7) for drying the fuel (18) to be fed to the arrangement and means for feeding the heat energy of the combustion gases of the boiler (1) into the drying apparatus (7), characterized in that the arrange- ment comprises a heat exchanger (4) and an air duct (5), the heat exchanger (4) being arranged to heat the air flowing in the air duct (5) with the heat en- ergy of the combustion gases of the boiler (1), that the heat exchanger (4) and the air flow amount are dimensioned in such a way that the air flowing in the air duct (5) is sufficient for drying the fuel (18) in all moisture conditions of the fuel (18) and that the arrangement comprises means for conducting the air flowing in the air duct (5) and not needed for drying the fuel (18) into the boiler (1) to serve as combustion air. 7. Arrangement according to claim 6, characterized in that the drying apparatus (7) comprises an air permeable wire (8), means for feed¬ ing fuel (18) above the wire (8) and means for conducting the heated air through the wire (8) for drying the fuel (18). 8. Arrangement according to claim 6 or 7, characterized in that the arrangement comprises dampers (12, 14) for controlling the flow of the air heated by the heat exchanger (4) to dry fuel (18) and to serve as combus¬ tion air of the boiler (1), a sensor (16) for measuring the temperature of the combustion chamber (1a) of the boiler (1) and a control device (17) for control¬ ling the dampers (12, 14) on the basis of the measurement result of the sensor (16). AMENDED CLAIMS [received by the International Bureau on 6 October 1997 (06.10.97); original claims 1 and 6 amended; remaining claims unchanged (2 pages)] 1. Method of burning crush-like plant-based fuel, in which method fuel (18) is burned in a boiler (1) designed for burning dry fuel, fuel the mois¬ ture of which can vary between dry and wet is used, fuel (18) is dried with air heated with the heat energy obtained from combustion gases of the boiler (1 ) and fed after that into a combustion chamber (1a) of the boiler (1) for burning, ch a ra cte ri zed in that heat and flow amounts of the air are dimen¬ sioned tn such a way that the air in question is sufficient for drying the fuel (18) in all moisture conditions of the fuel (18) and that the flow amount of the heated air is controlled in such a way that the air not needed for drying fuel (18) is conducted into the boiier (1 ) to serve as combustion air 2. Method according to claim 1, characterized in that the fuel (18) is dried by feeding it on an air permeable wire (8) and by blowing air heated with the heat energy of the combustion gases through the wire (8) and the fuel (18) 3. Method according to claim 2, characterized in that the air heating with combustion gases and the air flow to be led through the fuel (18) are dimensioned in such a way that the air flowing out through the fuel (18) is substantially saturated with water vapour. 4. Method according to any of the foregoing claims, c ha ra c ¬ terized in that the combustion gases of the boiler (1) have a temperature of 300 to 400 degrees when they are removed from the boiler (1 ) 5. Method according to any of the foregoing ctaims c ha ra c ¬ te ri ze d in that the temperature of the combustion chamber (1a) of the boiler (1) is measured and, on the basis of the measurement, the air heated with the heat energy of the combustion gases is directed to dry the fuel (18) and to serve as combustion air of the boiler (1), as per need. 6. Arrangement for burning crush-like plant-based fuel, the ar¬ rangement comprising a boiler (1) designed for burning dry fuel, a feeding ap- paratus (2) for feeding fuel (18) into a combustion chamber (1a) of the boiler (1), a drying apparatus (7) for drying the fuel (18) to be fed to the arrangement, a heat exchanger (4) and an air duct (5), the heat exchanger (4) being ar¬ ranged to heat the air flowing in the air duct (5) with the heat energy of the combustion gases of the boiler (1) and means for feeding the heated air into the drying apparatus (7), cha racterized in that the heat exchanger (4) and the air flow amount are dimensioned in such a way that the air flowing in the air duct (5) is sufficient for drying the fuel (18) in all moisture conditions of the fuel (18) and that the arrangement comprises means for controlling the air flow amount by means conducting the air flowing in the air duct (5) and not needed for drying the fuel (18) into the boiler (1) to serve as combustion air. 7. Arrangement according to claim 6, characterized in that the drying apparatus (7) comprises an air permeable wire (8), means for feed¬ ing fuel (18) above the wire (8) and means for conducting the heated air through the wire (8) for drying the fuel (18). 8. Arrangement according to claim 6 or 7, characterized in that the arrangement comprises dampers (12, 14) for controlling the flow of the air heated by the heat exchanger (4) to dry fuel (18) and to serve as combus¬ tion air of the boiler (1), a sensor (16) for measuring the temperature of the combustion chamber (1a) of the boiler (1) and a control device (17) for control¬ ling the dampers (12, 14) on the basis of the measurement result of the sensor (16).

Inventors: HONKASALO MARTTI, HONKASALO, MARTTI
Applicants: HONKASALO MARTTI, HONKASALO, MARTTI
Application year: 1997
Main classification: F23G5
IPC codes: F23G5, F23G7, F23K1, F23N1, F23N5