JPH06126252A - Method for improving quality of fly ash - Google Patents

Abstract

PURPOSE:To efficiently reduce the unburned carbon at the time of air-classifying fly ash in the coal-fired power plant, etc., by specifying the temp. of the fly ash grain in the classification part of a separator. CONSTITUTION:The fly ash in a raw powder tank 3 is passed through a raw powder measuring device 4 and introduced into a classifying air current 9. The classifying air current is formed by mixing cooling air 11 into the high- temp. combustion gas 10 from a hot stove 2, and the fly ash is dried, heated to a specified temp. and introduced into a separator 1. At this time, the temp. of the current 9 is controlled so that the temp. of the fly ash grain is adjusted to 70-350 deg.C at the classification part of the separator 1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention provides a method for efficiently reducing unburned carbon in fly ash of fly ash recovered from exhaust gas by an electrostatic precipitator or the like in a coal-fired thermal power plant. To do.

[0002]

2. Description of the Related Art Fly ash has been used as a cement admixture or a concrete aggregate after being recovered from exhaust gas by an electrostatic precipitator or the like in a coal-fired power plant. However, in recent years, the use of low-priced or low-grade coal to reduce power generation costs at thermal power plants, and low-temperature combustion of coal to reduce pollution of nitrogen oxides (NOx) and to deal with pollution regulations, A large amount of unburned matter, especially carbon, remains in the fly ash, which causes the quality of the fly ash to deteriorate. That is, for example, when the fly ash containing a large amount of such unburned carbon is used for fly ash cement, the cement becomes black or causes a decrease in strength, so that it cannot be used as it is. It was happening.

Therefore, as means for removing residual unburned carbon in the dry state of fly ash, a method of burning unburned carbon, a method of separating by utilizing the density difference between unburned carbon and pure fly ash, and unburned Methods such as classification based on particle size utilizing the large amount of carbon contained in relatively large fly ash particles have been studied, and various methods using these methods alone or in combination have been devised. ing.

However, the conventional method, for example, the method of burning unburned matter, requires a large amount of fuel, and has the disadvantage of high treatment cost. Further, as a conventional method of utilizing the difference in density, a fluidized bed formed by blowing a compressed gas from below the fly ash layer is used to form an unburned carbon layer having a smaller density than pure fly ash. A method is known in which the material is collected by floating it on the upper part, but in order to achieve the purpose of this method, it is important that the entire bed be in a uniform fluidized state. Since the separation due to the difference is not performed and the result is mixing on the contrary, it is necessary to adjust the particle size composition of the fly ash to be supplied, or to supplementally add vibration to the entire layer, which complicates the processing step and the apparatus. However, there was a problem that the operating cost of was high. Furthermore, in the method by classification, it is possible to obtain the target product in the sieve type classification, but the amount of treatment per hour is small, and in the airflow type classification, large fly ash particles containing a large amount of unburned carbon, The drag and centrifugal force due to the swirling of the air flow strongly act to collapse the unburned carbon, and the carbon particles are collected on the side that collects the fine particles, so that the target product cannot be obtained. It was

[0005]

DISCLOSURE OF THE INVENTION The present invention provides efficient comparison of unburned carbon from fly ash containing a large amount of unburned carbon recovered from coal combustion gas by an electrostatic precipitator or the like, that is, with a small energy consumption. The present invention provides a method of separating a large amount of treatment in a short time with a relatively simple facility.

[0006]

According to the present invention, when air classifying fly ash by an air separator, the temperature of the fly ash particles in the classifying portion of the separator is set to 7
The temperature is adjusted by heating the fly ash before supplying it to the air separator so that the temperature can be 0 to 350 ° C. and the moisture of the fly ash can be reduced to improve the dispersibility of the fly ash before classification. It was configured. Further, as another method, fly ash is supplied to the separator at room temperature, and when the fly ash reaches the classification part of the separator, a classification air flow having a temperature of 70 to 350 ° C. is supplied to the separator. It is also possible to classify. At this time, the heat source for heating the fly ash or the classified air flow may be heat obtained by burning fossil fuel or electric heater. It is also possible to use hot waste water and exhaust gas from power plants, chemical plants, cement plants and the like.

On the other hand, it is possible to circulate the classified air flow in order to reduce the loss when operating the equipment incorporating the present invention. Furthermore, in a coal-fired power plant, it is also possible to directly or cool the exhaust gas containing fly ash at about 150 to 350 ° C. and introduce it into an air separator for classification. Since it is difficult to directly measure the temperature in the classification part of fly ash, it is possible to substitute it by measuring the temperature of the accompanying classification airflow. Further, it is not always necessary to measure the temperature of the classifying unit, and the temperature of the separator outlet may be measured to estimate the temperature of the classifying unit.

[0008]

The basic principle of the present invention is to classify particles according to particle size by utilizing the fact that a large amount of unburned carbon is contained in relatively large fly ash particles as described above. Fly ash is a powder containing fine particles that are well-burned and spherical, and the average particle size is about 10 to 20 μm. The particles with poor burning included in this are larger than the spherical fine particles and have a sponge-like shape. The particles are particles, and a large amount of unburned carbon is attached to the surface, or unburned carbon is connected between pure fly ash particles.

It can be seen that the use of such properties of fly ash makes it possible to reduce unburned carbon by sieving. As described above, unburned carbon also has a carbon species of fuel coal and its combustion. Some of them are very brittle and fragile depending on the condition, which makes air classification difficult because it has the advantage of being capable of mass processing. In addition, fly ash has high hygroscopicity because it contains fine particles and sponge-like particles, and since a water film crosslinks between particles to aggregate, it is difficult to separate into single particles during air classification. have. And
Since the water film of normally used fly ash is crosslinked at room temperature, if fly ash is dispersed in the air flow and a large force is applied to classify it with centrifugal force or inertial force, the water film The part of the carbon that is more fragile than the crosslinks collapses, and the broken carbon particles enter the separator's fine powder collecting side, making it impossible to reduce unburned carbon. At this time, air flow drag acting on fly ash particles and agglomerated particles, centrifugal force, and the like are stronger as the particles are larger.

Therefore, when heat is applied to the fly ash to raise the temperature to 70 to 350 ° C. as in the method of the present invention, the fly ash particles are crosslinked with each other to evaporate water and separate agglomerated particles. The apparent particle size is reduced, and the air flow drag force and centrifugal force acting on the particle are reduced, which makes it difficult for the unburned carbon to collapse. In this way, by removing large fly ash particles containing a large amount of unburned carbon by classification with an air separator, it becomes possible to easily obtain high-quality fly ash with a low carbon content.

However, according to the present invention, the cross-linking water of fly ash in the classification section is reduced, that is, moisture is measured in the analysis of general chemical components. Since it is difficult to measure the moisture content of fly ash in the section continuously and precisely, the temperature was used as a substitute. In addition, there is a method of reducing pressure as a method of reducing moisture, but this method is not adopted because it is difficult to apply to existing equipment and the efficiency of moisture reduction is not good. Here, the heating temperature is 7
The reason for not lowering the temperature below 0 ° C. is to efficiently evaporate the crosslinking water, and the reason for not higher than 350 ° C. is to reduce the loss of heating energy.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described with reference to the drawings. Figure 1
It is an example of an apparatus for implementing the present invention. This device is one in which a heavy oil combustion gas can be introduced as a classification air flow into a general air classification system.

The fly ash to be classified is put into the classifying air stream 9 from the raw powder tank 3 through the raw powder measuring device 4. The classification airflow 9 is a mixture of the high temperature combustion gas 10 from the hot stove 2 and the cooling air 11. The fly ash reaches the separator 1 while being dried and heated to a predetermined temperature. Here, the amount of the classification airflow 9 attracted by the exhaust fan 6 and the rotation speed of the rotary rotor 1a are adjusted according to the target fineness of the fine powder, and the raw powder is classified into fine powder and coarse powder, respectively, from the bag filter 5. Fine powder outlet 7,
And, it is collected through the coarse powder discharge air lock feeder 8.

In Table 1, in the method of the present invention, using the fly ash obtained at a certain coal-fired power plant, with air at room temperature and heavy oil combustion gas such that the gas temperature at the inlet of the classification section becomes 75 ° C, The results of classification with the same classification air flow rate, separator rotor speed, and raw powder supply amount are shown.

[0015]

[Table 1]

As can be seen from the table, the one classified at room temperature shows that the unburned carbon of the classified product (fine powder) increased more than that of the raw powder, and the carbon collapsed. It can be seen that the use of No. 1 reduced the amount of unburned carbon and improved the quality of fly ash. Further, it can be seen that since the dispersibility of the raw powder is improved, the fine powder recovery rate is improved, and more fine powder can be taken from the same amount of raw powder.

[0017]

As described above, the present invention has the following remarkable effects in improving the quality of dry fly ash discharged from a coal-fired boiler or the like. (1) Compared with the conventional unburned carbon reduction method, even if fly ash, which is prone to the collapse of unburned carbon, is used, classification can be performed while suppressing the collapse of unburned carbon. You can get fly ash. (2) Since the dispersibility of the raw powder is improved, more products (fine powder) can be collected even if the same amount of raw powder as the conventional one is used, and the characteristics of the air classifier capable of large-scale processing are improved. It can be used more effectively.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention.

[Explanation of symbols] 1 air separator 1a rotating rotor 2 hot blast stove 3 raw powder tank 4 raw powder measuring device 5 bag filter 6 exhaust fan 7 product (fine powder) discharge port 8 coarse powder discharge air lock feeder 9 classification air flow 10 heavy oil combustion gas 11 cooling air

Claims (1)

[Claims] 1. A method for reducing unburned carbon in fly ash, wherein the temperature of the fly ash particles in the classifying portion of the separator is 70 ° C. to 350 ° C. when the fly ash is air classified by an air separator.