A fluidized-bed boiler is a boiler where the fuel is burnt and partly gasified in a fluidized bed above the bottom of the boiler, the bed being formed by a mixture of incombustible bed material and fuel. The fluidized state of the bed is maintained by blowing in fluidizing gas, usually air, at a great velocity through nozzles in the bottom. Fluidized-bed boilers are intended for solid fuels and are particularly well suited for fuels which are readily gasifiable, such as wood or peat, whereby the temperature of the bed can be controlled by controlling the degree of gasification by means of the air distribution. A fluidized-bed boiler usually consists of two parts, the lower part comprising a fluidized bed of a height of about one metre, the fuel being burnt and partly gasified in this bed. The top part is the freeboard. Fuel which has been gasified in the fluidized bed rises into the freeboard where it is completely burnt, and the combustion is controlled by feeding secondary air into the freeboard in order to ensure complete combustion. The walls of the freeboard are usually water-cooled and heat transmission to the walls usually takes place as gas radiation.

The temperature of the fluidized bed is about 750 to 900 °C. The temperature should not decline too much because this reduces the efficiency of the combustion. Exceeding the upper limit may quickly lead to sintering of the bed as the melting cinder forms clots of the fluidized bed material. With readily gasified fuels the temperature may be controlled by varying the amount of air blown into the bed, whereby a larger part of the fuel can be gasified by reducing the amount of air, thus reducing combustion and lowering the temperature of the bed. Hereby a larger part of the fuel is burnt in the freeboard. In the case of fuels which are not easily gasified, such as coal, the temperature often rises uncontrollably, wherefore these cannot be used as the main fuel. Controlling the temperature of a fluidized bed is also problematic when the quality of the fuel and the performance level of the boiler vary.

When burning coal, circulating fluidized bed techniques are usually used where part of the bed material and the fuel are circulated back into the bed via the top part of the boiler. In this solution a cyclone or another efficient separator is needed to separate the circulating material from flue gases, wherefore the construction is more expensive than that of a fluidized-bed boiler.

Circulating fluidized-bed boilers have been described, for example, in US documents Nos. 5,054,436 and 4,766,851.

Fluidized-bed boilers are usually designed for a certain fuel and variations in the type, humidity and thermal value of the fuel will make it more difficult to control the boiler. A factor which strongly influences the operation of the boiler is the amount of alkaline mixtures such as alkali salts contained in the fuel. These mixtures melt at low temperatures and are vaporized in the furnace of the boiler. The molten material accumulates on the surfaces of the furnace and on the fluidized bed particles, glueing the particles to one another. The bed material is gradually sintered to the degree of unexploitability and must be replaced. Due to sintering, materials containing alkaline components cannot be burnt in fluidized-bed boilers without special arrangements.

US Patent Specification No. 3,907,674 describes a method for burning slurries of low thermal value and containing alkali chlorides in an ordinary fluidized-bed boiler. In this method, reactive silicon oxide and alkali metal oxides are introduced into the boiler as admixtures to prevent the accumulation of sticky compounds in the fluidized bed. Such admixtures must be used in large amounts because fine particles which have the best reactivity cannot be used as they are rapidly removed from the fluidized bed and the furnace with exhaust gases almost without having reacted. Admixture particles of sufficient size which remain in the fluidized bed for a sufficient length of time do not react very efficiently wherefore they are needed in large amounts. In an ordinary fluidized bed the reactor load must be kept relatively constant during combustion because low temperatures have a negative effect on combustion and high temperatures give rise to agglomeration of the particles. Admixtures must be used in very large amounts in order to prevent agglomeration particularly when combusting fuels of high thermal value containing alkaline components in conventional fluidized-bed reactors.

PCT Application No. FI88/00098 describes a method for combusting fuels containing alkaline components in a circulating fluidized-bed boiler. The method is particularly intended for the combustion of lignite and brown coal. In this method the fuel is fed into the furnace of a circulating fluidized-bed boiler and prior to being fed into the furnace it is admixed with a reaction agent which reacts with the alkali mixtures contained in the fuel such that alkali metal compounds are produced during combustion having a high melting point. The temperature of the furnace is kept lower than the melting point of the alkali metal compounds formed. The reaction agent contains silicon oxide, metal oxide or hydroxide from the group comprising aluminium, calcium, magnesium, iron, titanium and mixtures thereof. Kaolin is an advantageous reaction agent because it contains several oxides of the above-mentioned agents. Kaolin is also relatively inexpensive.

The problem hampering the above-mentioned method in the case of dry fuels is the even mixing of the admixture with the fuel and the need for very accurate temperature control. The method is only suited for circulating fluidized-bed boilers in which the temperature can be effectively controlled by controlling the flow of circulating material and where admixture particles possibly exhausted from the furnace are recovered from the flue gases in a cyclone. In this manner unreacted admixture can be brought back to react in the fluidized bed and the need for admixture is reduced. In a fluidized- bed boiler any admixture added in particle form would be removed from the boiler, which results in the problems described for the US Patent above.

The aim of the present invention is to achieve a method for more effectively preventing sintering of the bed occurring when fuels containing alkali mixtures are burnt than has been possible according to prior-art solutions.

The invention is based on adding the admixture which reacts with alkali mixtures in the form of an aqueous solution, advantageously by admixing it with dry fuel.

More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1.

The invention offers considerable benefits.

One significant advantage provided by the invention is that deinking slurry from the paper industry can be used as the admixture. As a major part of the material used in coating paper comprises kaolin, deinking slurry separated from recycled paper can as such be used as an admixture to bind the alkali salts in the fuel. When wet, the slurry adheres well even to dry fuel and is evenly mixed.

Naturally, even other agents can be used as admixtures, such as ball clay, deinking slurry ash, and other generally used agents mentioned above which react with alkalis. The admixture is introduced admixed with water either as a clearly aqueous solution or as a damp slurry which is well admixed with the fuel. In the furnace the admixture is close to the alkali compounds in the fuel and will therefore react efficiently and does not have the time to be exhausted from the fluidized bed with the flue gases. Thus the method can advantageously be used to prevent the sintering of even the beds of fluidized-bed boilers without using any great excess of admixture. By means of the damp admixture it is possible to reduce the thermal value of a dry fuel, should it be necessary to burn such fuel as an alternative fuel in a boiler which is primarily intended for damp fuel. Thus the invention makes it possible to effectively exploit biomass waste to which alkali mixtures have been added while processing the material. Such material may be, e. g. plywood and gluelam waste where considerable amounts of sodium hydroxide are used to regulate the acidity of the glue. Some types of biomass, such as straw, are highly alkaline in themselves, wherefore auxiliary agents are needed in fluidized-bed combustion when these materials are burnt.

In the following, the invention is described in closer detail with reference to the annexed drawings.

Fig. 1 is a schematic representation of a fluidized-bed boiler with its fuel-feeding system. Only the furnace 1 of the boiler is depicted which inside the boiler is divided into a fluidized bed 2 and a freeboard 3 above the bed, the freeboard mainly containing gaseous substances. The boiler also comprises air inlet means, heat transmission surfaces and exhaust channels for flue gases as needed, which are not shown here. The fuel is fed into the furnace 1 from a storage hopper 5 via a charging hopper 4. The fuel is transferred from the storage hopper 5 to the charging hopper by means of a conveyor 9 which is selected according to the fuel used, a suitable conveyor for dry fuel being, e. g. a belt or a screw conveyor. As the pressure in the furnace 1 is greater than the ambient atmospheric pressure, a blocking stoker 7 preventing any reflux must be provided between the charging hopper 4 and the furnace 1. The fuel is taken from the charging hopper 4 to the blocking stoker 7 by means of an unloader 6 and from the blocking stoker 7 it is transferred to a skew feeding tower 8 via which the fuel will fall into the fluidized bed 2 of the furnace.

In this embodiment the admixture is added to the fuel from the admixture hopper 10 on the conveyor 9 right after the fuel has been discharged from the storage hopper 5. The mixing may take place in many ways and the most advantageous method depends on the construction of the conveyor. Generally speaking, however, the admixture is most advantageously added into the fuel quite simply by spraying by means of spray nozzles. The admixture metering may occur on a continuous or a periodic basis. The manner of feeding and metering is also affected by the stage'at which the admixture is added. If the admixture is added on the conveyor 9, the feed of the admixture must be periodical because, as a rule, the conveyor is operated periodically. If the admixture is added in connection with the unloader 6, the feed must be continuous because the unloader operates on a continuous basis. The manner of metering can also be selected according to the need for admixture and if only a small amount is needed it is often more advantageous to meter it periodically, and when large amounts are fed it is easy to implement continuous metering. Independent of which feeding method is used, however, it is necessary to see to it that there is a sufficient amount of admixture in the furnace, i. e. a sufficient amount of admixture must always be admixed with the fuel.

The amount of admixture added may be made dependent on the efficiency of the boiler or the amount of gaseous alkalis in the furnace or on changes occurring in the area of the superheater heat delivery surfaces. The regulation of the introduced amount may also be based on a combined measurement of these variables or, if the alkaline strength of the fuel is known, the amount of admixture may possibly be set solely on the basis thereof.

The admixture may be added to the fuel at any point prior to feeding the fuel into the furnace or a dryer. If the fuel used is produced at a site different from the site of combustion, the admixture may be admixed with the fuel where the fuel comes from. This may be of particular advantage when using biofuels, but when fuel already containing admixture is used, facilities for complementary charging of admixture must be provided, if the quality of the fuel varies extensively and if its alkaline strength cannot be accurately estimated. At the place of use of the fuel the admixture may be added to the fuel at any point before the blocking stoker.

According to the invention the admixture is always admixed in the form of a aqueous solution. This provides good adhesion of the admixture to dry fuel and even with wet fuel, an aqueous solution or slurry is more easily mixed and the admixture is more evenly distributed into the fuel. If the admixture is admixed with wet fuel which is dried prior to combustion, the admixture is added before the drying whereby it remains on the surface of dry fuel or, in the case of finely divided fuels, is admixed with the fuel and adheres to the particles.

The admixture may be any agent which reacts with alkalis, such as silicon oxide, a metal oxide or hydroxide from the group of metals comprising aluminium, calcium, magnesium, iron, titanium, and alloys thereof. The above-mentioned agents are richly contained in kaolin where the active agent is kaolinite. In addition, ball clay may be used which contains more impurities than kaolin but is still usable as an admixture in fuel. Deinking waste produced in the deinking of recycled paper is rich in kaolin and is as such in the form of a slurry, wherefore such waste or the cinder of suspended deinking waste can advantageously be used in accordance with the invention. Deinking waste has previously been unusable waste and the invention makes its reuse possible in an advantageous manner. The water content of the admixture may vary greatly and the water content may, if needed, even be exploited to control the thermal value of the fuel.