Furnaces – Process – Treating fuel constituent or combustion product
Reexamination Certificate
1998-11-18
2001-05-29
Lazarus, Ira S. (Department: 3743)
Furnaces
Process
Treating fuel constituent or combustion product
C110S188000, C110S224000, C110S254000, C110S302000, C110S234000, C110S346000
Reexamination Certificate
active
06237511
ABSTRACT:
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.
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Ciric Ljiljana V.
Lazarus Ira S.
Smith Gambrell & Russell
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