FI103349B - Procedure for preventing sintering in a fluidized bed - Google Patents

Description

103349

The present invention relates to a method according to the preamble of claim 1 for preventing sintering of bed material by low melting point alkali metal alloys contained in a circulating fluidized bed boiler or a fluidized bed fluidized bed fuel.

10 A fluidized bed boiler is a boiler in which the fuel burns and partially gasses in the fluidized bed above the bottom of the boiler, consisting of a mixture of non-combustible bed material and fuel. The fluidization of the bed is maintained by blowing through the nozzles on the bottom with high speed fluidizing gas, usually air.

Fluidized bed boilers are designed for solid fuels and are particularly well suited to easily gasifiable fuels such as wood and peat, whereby the temperature of the bed can be controlled by adjusting the degree of gasification with the help of the il-20 manifold. A fluidized bed boiler usually consists of two parts, the lower one being a fluidized bed approximately one meter high, in which the fuel burns and partially gasifies. The upper part is the after fire mode. In the fluidized bed, the gasified fuel rises to the afterburner, where it is finally burned and the combustion is controlled by supplying the afterburner: ·. secondary air to ensure complete combustion. Aftermath • · «'• t. the walls of the combustion chamber are usually water cooled and the heat transfer to the walls is mainly by gas radiation.

The temperature of the fluidized bed is about 750 to 900 ° C. The temperature should not fall too low as this will reduce the efficiency of the combustion. Exceeding the upper limit can quickly lead to: sintering of the bed with melting ash forming lumps in the fluidized bed material. With easily gasizable fuels 35, the temperature can be controlled by varying the amount of air blown into the bed, reducing the amount of air to gasify the gas and reducing combustion and lowering the bed temperature. In this case, most of the fuel will burn in the afterburner. With poorly gaseous fuels, such as coal, the temperature in the bed often rises uncontrollably and cannot be used as the main fuel. Fluid bed temperature control is also problematic when fuel quality and boiler power levels vary.

Coal combustion typically uses a circulating bed technology whereby some of the bed material and fuel is recycled through the top of the boiler back to the bed. This solution requires a cyclone or other efficient separating device for separating the circulating material from the flue gases, making the structure more expensive than a bed bed boiler. Circulating bed-15 boilers are described, for example, in US 5,054,436 and 4,766,851.

Fluidized-bed boilers are usually designed for a particular fuel and changes in fuel quality, humidity, and calorific value make it difficult to control the boiler. One factor that strongly influences the operation of the boiler is the amount of alkaline mixtures in the fuel, such as alkali salts. These alloys melt at low temperatures and evaporate in the boiler furnace. The molten material accumulates on the surfaces of the furnace • '· 25 and the fluidized bed particles, gluing the particles together. The bed material is gradually being sintered and needs to be replaced. Due to sintering, materials containing temporal substances cannot be burned in fluidized bed boilers without special arrangements.

·. US Patent 3,907,674 describes a process for burning low calorific and alkali chloride-containing slurries in a conventional fluidized bed boiler. In this process, reactive silica and alkali metal oxides are added to the boiler; . which prevent the accumulation of sticky compounds in the fluidized bed 103349. Large amounts of additives have to be used because fine particles with the highest reactivity cannot be used because they are rapidly removed from the fluidized bed and the furnace with virtually no reactivity with the exhaust gases.

5 Sufficiently large additive particles that remain long enough in the fluidized bed do not react very effectively and should be added in large quantities. In a conventional fluidized bed, the reactor load has to be kept relatively stable during combustion because low temperatures 10 have a negative effect on combustion and high temperatures cause particle agglomeration. Particularly when using high-calorific and high-temporal grade fuels in conventional Lei bed reactors, a high amount of additives must be used to prevent agglomeration.

International Application PCT / FI88 / 00098 describes a method for burning fuels containing alkaline substances in a circulating bed boiler. The method is intended in particular for burning lignite and lignite. In this process, the fuel is fed to the fire chamber of a circulating bed boiler and mixed with a reactant, which reacts with the alkali alloys contained in the fuel prior to feeding to the furnace, to produce high melting point alkali metal compounds. The temperature of the furnace is kept below the melting point of the alkali metal compounds formed. The reactant contains silica, metal oxide or hydroxide from the group consisting of aluminum, calcium, magnesium, iron, titanium and mixtures of these substances. Kaolin is a preferred reaction agent because it contains several oxides of the above mentioned substances. Kaolin is also relatively inexpensive.

. The problem of the above method with dry fuels is the uniform mixing of the additive with the fuel and the need for very precise temperature control. The method is suitable only for circulating fluidized bed boilers where the temperature can be effectively controlled by adjusting the flow rate and in which any additive particles which may leave the furnace are recovered from the flue gases in the cyclone. In this way, the unreacted additive is reacted again in the fluidized bed and the amount of additive required is reduced. The additive in particulate form in a fluidized bed boiler would be removed from the boiler, with the same problems as the solution described in the aforementioned United States Patent.

The object of the present invention is to provide a method by which the sintering of the bed during the combustion of fuels containing alkali mixtures can be prevented more advantageously than in the prior art solutions.

The invention is based on the addition of an additive which reacts with alkali mixtures as an aqueous solution, preferably by mixing it with dry fuel.

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

| * · - 25 The invention provides considerable advantages.

One significant advantage of the invention is that the deinking sludge of the paper industry can be used as an additive. Since most of the material used in paper coating .. 30 is kaolin, the deinking sludge refined from recycled paper is • «> a useful additive in its fuel time •« * · *. binding of additional salts. When wet, the sludge adheres well to the fuel and mixes evenly. Naturally, other substances may be used as an additive, such as 35 ball clay, ash from the de-inking sludge and others; . commonly used alkali-reacting substances. The additive is added in water mixed either as a clear aqueous solution or as a moist slurry that mixes well with the fuel. In the furnace, the additive is close to the alkali compounds of the fuel and thus reacts effectively and does not have time to exit the fluidized bed with the flue gases. Thus, the method can advantageously prevent the sintering of the bed of even fluidized bed boilers without the use of a large excess of additive. The damp additive can be used to calculate the calorific value of dry fuel if such fuel 10 has to be burned as an alternative fuel in a boiler for the damp fuel itself. Thus, the invention makes it possible to efficiently utilize biomass waste to which alkali mixtures have been added when processing the material. Such materials include, for example, plywood and glue waste, where considerable amounts of sodium hydroxide are used to regulate the acidity of the glue. Some biomasses, such as straw, are very timely in themselves, so incineration with fluidized bed combustion is also required.

20

The invention will now be explained in more detail with reference to the accompanying drawings.

Figure 1 schematically shows a fluidized bed boiler with combustion feed systems. Only the boiler • · *. · The furnace 1 is shown, which is divided into a fluidized bed 2 and a freeboard 3 above it, which contains mainly gaseous substances. The boiler also includes the necessary air inlets, heat transfer surfaces and flue gas:. 30 drain channels not shown here. The fuel · '... is fed to the furnace 1 from the storage silo 5 via the supply silo 4. The fuel is transferred from the storage silo 5 to the feed silo by a conveyor 9 which is selected according to the fuel to be used and for dry fuels the conveyor 35 is suitable, for example a belt or screw conveyor. Since the furnace 1 has a higher pressure than the ambient air pressure, there must be a backflow preventive barrier feeder 7 between the feed silo 4 6 103349 and the furnace 1. The fuel is transferred to the shutter feeder 7 from the

In this embodiment, the additive is mixed with the fuel from the additive silo 10 by conveyor 9 immediately after removal from the storage silo 5. Mixing can take place in many ways and the most preferred mixing method depends on the design of the conveyor. Usually, however, it is most advantageous to inject the additive into the fuel simply by injecting nozzles. The administration of the additive may be continuous or intermittent. The manner in which the additive is fed is also influenced by the method of feeding and dispensing. If the additive is fed by the conveyor 9, the supply of the additive must be intermittent, since the conveyor is usually operated intermittently. If the additive is supplied in conjunction with the discharger 6, the feed must be continuous since the discharger operates continuously. The mode of administration can also be selected according to the need of the additive, and if the need for the additive is low, it is often preferable to administer it periodically, and when using high feed rates, continuous dosing is easy. However, all supply methods must ensure that there is sufficient additive: '· 25 in the furnace, that is, when mixed with fuel • * · *.

The amount of additive feed can be tied to boiler power or gaseous alkali content in the furnace or to changes in the temperature range of the superheater 30. Adjustment of the feed rate may also be based on a combination measurement of these quantities or, if the alkali content of the fuel is known, the additive amount may be adjusted, if any, on its own.

'' 35 i The additive may be blended with the fuel at any time prior to being fed to the furnace or dryer.

If the fuel used is generated at a location other than the one being burned, the additive may be mixed with the fuel at its source. This method may be particularly advantageous when using bio-5 fuels, but when using pre-doped fuel, one must be prepared for supplemental feed if the quality of the fuel varies widely and its alkali content cannot be accurately estimated. At the fuel application site, the additive may be added to the fuel 10 at any time prior to the shutter feeder.

According to the invention, the additive is always mixed as an aqueous solution. In this case, the additive adheres to very dry fuel and even when mixed with wet fuel, it is easier to mix the aqueous solution or slurry and the additive is more evenly distributed in the fuel. If the additive is mixed with a wet fuel that is dried prior to combustion, the additive is mixed prior to drying to remain on the surface of the dry fuel or to mix with the fuel in the fine fuel and adhere to the particles.

The additive may be any alkali-reactive substance such as silica, metal oxide or hydroxide from the group of metals including aluminum, calcium, magnesium, iron, titanium and mixtures of these substances. The abovementioned substances are abundant in kaolin, the active ingredient of which is kaolinite. In addition, it is possible to use spherical clay, which contains more impurities than kaolin, but which is still useful as a fuel additive.

·. 30 The deinking waste from cleaning paper for recyclable paper contains high levels of • kaolin and is already present as a slurry. thus, it or the slurry ash slurry ash can be used advantageously in accordance with the invention. Decontamination: The waste has previously been unusable and the invention makes it possible to reuse it in an advantageous manner.

! The water content of the additive can vary widely and the water content of the 8 103349 can even regulate the calorific value of the fuel if necessary.

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Claims (16)

103349 A method for preventing sintering of bed material of low melting point 5 alloys containing low melting point fuel contained in a fuel of a circulating fluidized bed or a fluidized bed boiler, characterized in that the fuel is admixed with the low-melting liquor containing fuel before being fed to the furnace. 2. A method according to claim 1, characterized in that the additive is a deinking waste. 15 3. A method according to claim 1, characterized in that the additive is ash from the deinking residue. A process according to claim 1, characterized in that the additive is ball clay. A process according to claim 1, characterized in that the additive is kaolin. I '· 25 Method according to one of the preceding claims, characterized in that the amount of additive is determined on the basis of the boiler power. A method according to any one of the preceding claims, characterized in that the amount of additive is determined by the amount of gaseous alkali in the furnace. r «· '8. Method according to one of the preceding claims, characterized in that the amount of additive is determined on the basis of changes * in the area of the superheater lips. 103349 Method according to one of the preceding claims, characterized in that the amount of additive is determined on the basis of the amount of alkali in the fuel. 5 Method according to one of Claims 6 to 9, characterized in that the additive is continuously mixed with the fuel. Method according to one of Claims 6 to 9, characterized in that the additive is mixed with the fuel periodically. Method according to one of Claims 6 to 9, characterized in that at least a part of the additive is mixed with the fuel at its place of origin or collection 11a. Process according to one of the preceding claims, characterized in that the fuel used is at least partially alkali-containing glue or plywood waste. A process according to any one of claims 1 to 12, characterized in that the fuel used is at least partially alkaline biomass, such as straw. • · «·« · 15. Use of low-melting point de-inking in recirculating fluidized bed or fluidized bed boiler fuel for cleaning of recycled paper; 30 to prevent sintering of the bed material caused by alkali alloys. • · 16. Use of ash from the purification of recycled paper to prevent sintering of the low-melting alkali metal alloys 103349 contained in the circulating fluidized bed or bed fluidized bed fuel. <* • »4 · i f • f 9 • · 4 · t 0 • · 4 4 4« 12 103349