Patent Application
20080216658
(a) Field of the Invention The present invention relates to a wet-type electrostatic precipitator, and more particularly to a wet-type electrostatic precipitator having a discharge electrode arranged along a flow path of a gas to be treated.
(b) Description of the related arts
Exhaust gas exhausted from a furnace that employs heavy oil or coal as a fuel contains sulfur oxides generated from sulfur contents in the fuel. Therefore, from such exhaust gas exhausted from a boiler for a thermal power plant, dust is removed by a dry-type electrostatic precipitator, then, sulfur oxides are removed by a wet-type desulfurizer, and finally, the exhaust gas is guided to a wet-type electrostatic precipitator where a mist or the like is removed and then the resultant is emitted to the atmosphere, as disclosed in the Japanese Unexamined Patent Application No. 2002-45643.
Although the sulfur oxide contained in this type of the exhaust gas is mainly sulfur dioxide, sulfur trioxide of several +ppm level is contained. The sulfur trioxide react readily with water to become sulfuric acid, which is condensed to form a sulfuric acid mist when a gas temperature becomes the dew point of sulfuric acid or lower. Since the sulfuric acid mist has strong corrosivity, the temperature of the exhaust gas is kept to the temperature higher than the dew point of the sulfuric acid (e.g., about 170° C.), at a stage before the wet-type desulfurizer. However, when the exhaust gas is guided to the wet-type desulfurizer to rapidly cool the same to about 55° C. that is the dew point of water, a micromist of sulfuric acid is formed. The micromist is difficult to be removed by the wet-type desulfurizer, so that it is removed at the wet-type electrostatic precipitator at a later stage.
In the wet-type electrostatic precipitator, a mist such as the micromist of sulfuric acid in the exhaust gas transmitted from the wet-type desulfurizer and remaining dusts are collected to a dust-collecting electrode based on a principle of electrostatic precipitation. The collected mist itself forms a wet film on the surface of the dust-collecting electrode and naturally flows down. When the amount of the mist is small and the natural flow-down is difficult to occur, washwater is flown all times or intermittently from above the dust-collecting electrode so as to flow down the mist and dusts collected on the dust-collecting electrode.
However, when the gas to be treated containing a corrosive mist such as sulfuric acid mist is treated in the wet-type electrostatic precipitator, a mist is collected to the dust-collecting electrode, whereby the discharge electrode is likely to be dried. Therefore, when the corrosive mist in the gas to be treated is adhered onto the discharge electrode, the corrosive mist is enriched due to the dryness. Accordingly, the discharge electrode is corroded, thereby entailing a problem of decreasing the usable life of the discharge electrode. In order to improve the problem described above, it is considered that washwater is sprayed from above the discharge electrode to flow down the corrosive mist adhered onto the discharge electrode. However, in such way, the sprayed water droplets are flown by a gas flow, and thus the water droplets cannot reach the lower part of the discharge electrode. Therefore, it is difficult to flow down the corrosive mist adhered onto the discharge electrode entirely. Further, if the particle diameter of the sprayed water droplet is increased so as to prevent the water droplet from being flown by the gas flow, most of the water droplets are collected to the dust-collecting electrode. Therefore, sufficient washing effect cannot be attained, and conversely, there arises a problem that the water droplets induce sparks.
The object of the present invention is to remedy the aforesaid problems of the conventional technique, and to provide a wet-type electrostatic precipitator that can supply water all over a discharge electrode, and can prevent corrosion of the discharge electrode, even if a gas to be treated containing a corrosive mist is treated.
In order to achieve the foregoing object, a wet-type electrostatic precipitator according to the present invention is a wet-type electrostatic precipitator having a discharge electrode arranged along a flow path of a gas to be treated, including spraying means that is arranged at the upstream side of the discharge electrode and is capable of spraying a water mist or water vapor, wherein the water mist or water vapor sprayed from the spraying means forms a wet film on the surface of the discharge electrode.
In the wet-type electrostatic precipitator according to the present invention, the spraying means is a spray nozzle that sprays a water vapor or mist having a particle diameter of less than 10 μm. It is desirable that control means capable of intermittently controlling the flow rate of the sprayed water mist or water vapor is mounted to the spraying means. For the wet-type electrostatic precipitator according to the present invention, a roughening process for roughening the surface of the discharge electrode is performed.
The water mist or water vapor sprayed from the spraying means flows along the flow path of the gas to be treated from the upstream side to the downstream side of the discharge electrode, and successively adhere onto the discharge electrode during this process and thus a wet film is formed on the surface of the discharge electrode. The wet film serves as a protective film against a corrosive mist. Specifically, even if the corrosive mist is adhered onto the discharge electrode, the corrosive mist is sufficiently diluted by the wet film. Therefore, the corrosive force is reduced to thereby remarkably prevent the corrosion of the discharge electrode. When the thickness of the wet film increases, it naturally flows down by its own weight. Accordingly, the wet film is not grown to a certain thickness or more. The wet film keeps on being renewed by a condensed water or mist that is newly adhered, therefore the function as the protective film is not deteriorated.
If the spraying means sprays a water vapor or mist having a particle diameter of less than 10 μm, the wet film can be easily formed all over the discharge electrode in the widthwise direction and depth direction. Providing the control means, which can control the flow rate of the water mist or water vapor sprayed from the spraying means, is mounted, makes it possible to automatically execute the intermittent washing operation on the discharge electrode. Therefore, the corrosion of the discharge electrode can further be prevented. Since the roughening process for roughening the surface of the discharge electrode is performed, the wet film is satisfactorily be formed and maintained on the discharge electrode, whereby the corrosion preventing effect is enhanced.
As shown in
As shown in
As a result, the water vapors sprayed from the spray nozzles 28 flow along the flow path of the gas to be treated 10 from the upstream side toward the downstream side of the discharge electrodes 14. When the water vapor is saturated vapor of about 100° C., and the temperature of the gas to be treated 10 is about 50 to 60° C., the sprayed water vapors are cooled by the gas to be treated 10, and some of them are condensed to form fine water droplets. Therefore, the condensed fine water droplets successively adhere onto the discharge frame 18 or discharge wires 20 composing the discharge electrode 14 from the upstream side to the downstream side of the discharge electrode 14, thereby forming a wet film on the entire surface of the discharge electrode 14. This wet film serves as a protective film against the corrosive mist. Specifically, even if the corrosive mist adheres onto the discharge electrode 14, the corrosive mist is sufficiently diluted by the wet film, with the result that the corrosive force is reduced. Accordingly, the corrosion of the discharge electrode 14 can remarkably be prevented. When the thickness of the wet film increases, it naturally flows down by its own weight. Accordingly, the wet film is not grown to a certain thickness or more. The wet film keeps on being renewed by a condensed water or mist that is newly adhered, therefore the function as the protective film is not deteriorated.
It is desirable that the spray nozzle 28 sprays the water vapor with the flow rate substantially equal to the flow rate of the gas in order to cause the sprayed water vapor to be easily carried on the gas flow of the gas to be treated. The reason of this is as follows. In the electrostatic precipitator, the exhaust gas is rectified and passes in order to disperse the airflow is uniformly. Therefore, preventing the disturbance in the air flow caused by the sprayed water vapor as much as possible is effective for maintaining the dust-collection function. As shown in
A roughening process for roughening the surface of the discharge electrode 14 is desirably performed. Examples of usable roughening process include filing process, blast process, dimple process, channeling process, etc. When such a roughening process is performed, the formation and maintenance of the wet film on the discharge electrode become satisfactory, whereby the corrosion preventing operation is enhanced. Since some of the discharge wires at the downstream side of the gas, among the great number of discharge wires 20 that are the main components of the discharge electrode 14, are readily dried, the roughening process is desirably performed on the surface of discharge wires 20, particularly in such part.
As described above, the wet-type electrostatic precipitator of the present embodiment can form a wet film all over the discharge electrode without flowing wash water from above the discharge electrode, even if a gas to be treated containing a corrosive mist is treated, whereby the corrosion of the discharge electrode can be prevented.
In the above-mentioned each embodiment, the case in which saturated steam is sprayed from the spray nozzle 28 is described. However, the water sprayed from the spraying means according to the present invention is not limited to the saturated steam. Unsaturated steam, superheated steam, or moist air sufficiently containing water vapor may be employed, and in this case, the similar effect can be attained. When the temperature of the gas to be treated 10 is so high exceeding 100° C., the formation of water droplets due to the condensation of water vapor cannot be expected. Therefore, in this case, water droplets are desirably sprayed from the spraying means.
It is to be noted that a water droplet having a particle diameter of 10 μm or more readily falls down by its own weight, or is readily charged and attracted by the dust-collecting electrodes. Thus, it is not effective for forming a wet film on the discharge electrodes. Accordingly, it is desirable that the water mist having a particle diameter of less than 10 μm, more preferably a water mist having a particle diameter adjusted to approximately 1 μm is sprayed, in the case of a water mist. When the particle diameter is less than 10 μm, the moving speed by electrostatic force is overwhelmingly predominant to the free fall speed by gravity, and this is the more outstanding with the particle of the more decreased diameter.
The spraying means according to the present invention is not limited to the spray nozzle 28 described in the aforesaid each embodiment. For example, the structure in which water is sprayed from a continuous thin slit may be employed. The spraying means is not limited to a fixed type. A moving type or a movable type that changes the spraying direction may be employed.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2007-054705 | Mar 2007 | JP | national |
