Method and device for burning disintegrated plant-based fuel

A method of burning plant-based fuel is involved in which the plant-based fuel is burned in a boiler designed for burning dry fuel and with the plant-based fuel being used having a moisture value which can vary in range from dry to wet. The method involves heating air with heat energy from combustion gases of the boiler to provide a source of heated air and providing heat and flow amounts of the heated air, with derived heat energy from the combustion gases, to the fuel in an amount sufficient for drying the fuel in any condition presented amongst the range between dry and wet. The flow of the heated air directed to the fuel is controlled to provide sufficient drying of the fuel before use of the fuel in the boiler and such that the heated air not needed for drying the fuel is conducted into the boiler to serve as heated combustion air.

FIELD OF THE INVENTION
 The invention relates to a method of burning disintegrated 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 the 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 disintegrated
 plant-based fuel, the arrangement comprising a boiler designed for burning
 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,
 the 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, unseasoned 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 higher. Primary price increasing factors
 are additional masonry for the combustion chamber, an 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
 small power level requirements 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
 combustion gases heat the fuel. By this arrangement, moist fuel can be
 dried before 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 procedure 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 arrangement 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 arrangement not
 characterized by the above-mentioned drawbacks and making it possible to
 burn disintegrated 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
 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 in all moisture conditions of the fuel and
 that the heated air is directed 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 arrangement 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 combustion air of the boiler, as needed. 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 preferred 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 vapor. 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 controlled 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
 implement technically and the boiler can be provided with an advantageous
 structure. It is then possible to use, for instance, several different
 alternative boiler grate solutions, high temperature can be achieved for
 the combustion chamber, 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 instance.

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 apparatus 2 of fuel 18 is fully known per se, it is not
 discussed further in this connection. 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 separate 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 around rolls 9a and 9b. A chamber 10 is arranged
 in the middle of the loop formed by the wire 8. The heated air is
 conducted through the drying air duct 6 into the chamber 10. The heated
 air is blown out of the chamber 10 through the upper, permeable support
 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
 disintegrated 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
 flowing through the fuel 18, the heated air extracts moisture from it. The
 temperature 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 saturated with water vapor. The rolls 9a and 9b going
 around, i.e. the movement of the wire 8, may either be continuous or
 intermittent, and 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 sufficiently 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. Thus, drying air duct 6 and combustion air
 duct 13 received the heated air flowing in air duct 5. 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 procedure 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
 multipositional damper 12 of the drying air duct 6 is closed partly thus,
 drying air duct 6 and combustion air duct 13 received the heated air
 flowing in air duct 5, and the multipositional damper 14 of the combustion
 air duct 13 is opened partly, thus, drying air duct 6 and combustion air
 duct 13 received the heated air flowing in air duct 5, whereby part of the
 heated air is conducted to serve as combustion air of the boiler. Further,
 when the fuel 18 fed to the arrangement is so dry that no drying at all is
 needed, the damper 12 of the drying 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 figure does not show fans
 required for moving air in the ducts.
 The dampers 12 and 14 can be controlled by a control device 17. 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
 drier fuel, 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 a
 moister fuel than before, the temperature of the combustion chamber la
 begins to fall and the control device 17 controls the air heated by the
 air exchanger 4 by means of the dampers 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 designed 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. Calculations have proved that
 the moisture of the fuel to be fed to the arrangement may be for instance
 60%, and nevertheless, the efficiency of the arrangement can be kept
 rather good. When drier fuels than this are used, the efficiency is better
 still, of course. When drier 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 in a rather hot
 state, (e.g., at about 300 to 400.degree. C. degrees, and more preferably
 350 to 400.degree. C. degrees, for instance). Then the convection part 1b
 of the boiler can be made small, and simultaneously, 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.degree. C.
 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 techniques can be used for storing fuel
 and feeding it to the drying apparatus, for instance.