Electrostatic dust collector

Abstract: There are provided a dust collector and method for collecting dust in which the rarefaction of a dielectric at the rear part of electric field forming means is prevented, whereby the collecting efficiency can be increased. The dust collector includes charging means 1 for charging a substance to be collected 9, such as dust and mist, contained in a gas; spray means 2 for spraying a dielectric 10 on the substance to be collected 9 charged by the charging means 1; electric field forming means 3, having first and second electrodes 11 and 12 for forming a direct current electric field, for dielectrically polarizing the dielectric 10 sprayed by the spray means 2 by means of the direct current electric field; and dielectric collecting means 16 for collecting the dielectric 10 which has arrested the substance to be collected 9, the spray means 2 being provided with grounding means 17 and 18 for electrically grounding the dielectric 10 before being sprayed to let a charge of the dielectric 10 escape.

Description: FIELD OF THE INVENTION AND RELATED ART STATEMENT The present invention relates to a dust collector, which is used to remove dust, mist, and the like contained in a gas. In order to efficiently collect fine dust (submicron particles), mist, and the like, the applicant has before proposed a dust collector in Japanese Patent Provisional Publication No. 10-174899 (No. 174899/1998). This dust collector includes charging means for charging a substance to be collected such as dust and mist contained in a gas, spray means for spraying a dielectric on the substance to be collected charged by the charging means, electric field forming means for forming an electric field for dielectrically polarizing the dielectric sprayed from the spray means, and dielectric collecting means for collecting the dielectric which has arrested the substance to be collected. The above-described dust collector has a high voltage applied electrode 100 and a ground electrode 200, shown in FIG. 7, as the electric field forming means, and allows an exhaust gas containing the substance to be collected such as dust and mist (in this example, SO 3 mist indicated by the black dots in the figure) 300 and a dielectric (in this example, water mist) 400 sprayed from the spray means to flow between the electrodes 100 and 200. The substance to be collected 300 has been charged, for example, negatively in advance by the charging means. On the other hand, the dielectric 400 is dielectrically polarized by a direct current electric field formed between the electrodes 100 and 200. Therefore, the substance to be collected 300 is collected by the dielectric 400 by means of the Coulomb's force acting between the particles of dielectric 400. When an alternating voltage is applied between the electrodes 100 and 200 as shown in FIG. 8, the polarization polarity of the dielectric 400 changes with time, and the charged substance to be collected moves in a zigzag form. Thus, the substance to be collected 300 is collected by the dielectric 400 by means of the Coulomb's force acting between the particles of dielectric 400. According to this dust collector of the earlier application, submicron particles can be collected efficiently despite the compact configuration. OBJECT AND SUMMARY OF THE INVENTION In order to further increase the efficiency in collecting the substance to be collected 300, it is necessary for the dielectric 400 to exist enough up to the upper part (rear part) of the electrodes 100 and 200. In the conventional collector, however, the dielectric shows a tendency to rarefy at the upper part (rear part) of the electrodes 100 and 200. The inventors found that the aforementioned tendency is ascribed to the charging of the dielectric sprayed from the spray means. Specifically, the particles of dielectric sprayed from the spray means are charged positively or negatively because the particles of dielectric exchange charges at the boundary of a pipe through which the dielectric itself flows. Therefore, the dielectric 400 having been charged positively or negatively is sprayed from the spray means, which is a cause of bringing about the aforementioned tendency as described below. In FIG. 9 corresponding to FIG. 7, the circle mark applied to the side of the particle of dielectric 400 indicates the charging state of the particle of dielectric 400. If the charged dielectric 400 is supplied between the electrodes 100 and 200, the positively charged dielectric 400 is attracted to the electrode 100, and the negatively charged dielectric 400 is attracted to the electrode 200 by means of the Coulomb's force. Therefore, most of the dielectric 400 is collected by the electrodes 100 and 200 before it arrives at the upper part (rear part) of the electrodes 100 and 200. FIG. 10 shows a case where an alternating electric field is applied to between the electrodes 100 and 200. In this case, the charged dielectric 400 goes while being swayed to right and left with the change cycles of alternating electric field. At this time, the particles of dielectric 400 having a positive and negative charge are attracted to one another and aggregate, so that the distribution concentration of the dielectric 400 decreases toward the upper part of the electrodes 100 and 200. That is, even if an alternating electric field is applied to between the electrodes 100 and 200, the dielectric 400 rarefies at the upper part of the electrodes 100 and 200. EP-A-0 808 660 discloses a dust collector with the feature of the preamble portion of claim 1. The present invention has been made in view of the above situation, and accordingly an object thereof is to provide a dust collector and method for collecting dust in which the rarefaction of dielectric at the rear part of electric field forming means is prevented, whereby the collecting efficiency can be increased. To achieve the above object, the present invention provides a dust collector, comprising charging means for charging a substance to be collected, such as dust and mist, contained in a gas; spray means for spraying a dielectric on the substance to be collected charged by the charging means; electric field forming means, having first and second electrodes for forming a direct current electric field, for dielectrically polarizing the dielectric sprayed by the spray means by means of the direct current electric field; dielectric collecting means for collecting the dielectric which has arrested the substance to be collected; and grounding means, provided in the spray means, for electrically grounding the dielectric before being sprayed, wherein a charge of the dielectric is caused to escape by the grounding means so that the dielectric is made electrically neutral. According to the present invention, since the electrically neutral dielectric is sprayed from the spray means, the arrest of the sprayed dielectric by the electrode of the electric field forming means is restrained. Therefore, a shortage of dielectric in the rear zone of an electric field forming section is prevented, so that the efficiency in collecting the substance to be collected is increased. A metallic net is used as the grounding means, and the net is disposed in a flow path of the dielectric in the spray means so as to traverse the flow path. With the use of the metallic net as de-electrifying means, a satisfactory de-electrifying effect can be achieved without obstructing the flow of the dielectric. In the dust collector described above, a plurality of stages of the pair of the spray means and the electric field forming means can be disposed. According to this configuration, the substance to be collected is collected in a dust collecting section of each stage, so that a very high dust collecting efficiency can be obtained. In this configuration, fresh water is sprayed from spray means of at least the most downstream stage of the plurality of spray means, and circulating water is sprayed from spray means excluding the spray means which sprays fresh water. According to this configuration, since fresh water is sprayed from spray means of at least the most downstream stage, the collecting efficiency is further increased. Therefore, this configuration is especially advantageous in preventing the outflow of harmful substances. The spray means of the most downstream stage can be provided with a nozzle for atomizing the fresh water to an average diameter not larger than 50 µm. If such a nozzle is provided, the nozzle is not clogged, thereby maintaining a high dust collecting efficiency, and the quantity of fresh water used can be decreased. The dust collectors described above can be configured so as to further comprise a dielectric circulating system for supplying the dielectric from a dielectric storage tank to the spray means and for returning the sprayed dielectric from the spray means to the storage tank; dielectric supply means for supplying a fresh dielectric to the dielectric storage tank; dielectric discharge means for discharging the dielectric in the dielectric storage tank; absorbent charging means for charging an absorbent in the dielectric storage tank, the absorbent being used to absorb a reaction product produced by a substance in the gas; and control means for controlling the quantity of dielectric supplied by the dielectric supply means and the quantity of dielectric discharged by the dielectric discharge means so that the concentration of the reaction product exhibits a value within a given range and for controlling the quantity of absorbent charged by the absorbent charging means so that the pH value of the dielectric exhibits a value within a given range. According to this configuration, the deterioration in dielectric can be prevented, and also harmful gas can be absorbed and removed positively. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic longitudinal sectional view showing a general construction of a dust collector in accordance with the present invention; FIG. 2 is a schematic perspective view showing a construction of a preliminary charging section; FIG. 3 is a schematic perspective view showing a construction of a dust collecting section; FIG. 4 is a sectional view showing a construction of a spray section; FIG. 5 is a sectional view showing another construction of the spray section; FIG. 6 is a schematic sectional view showing another embodiment of the dust collector in accordance with the present invention; FIG. 7 is an explanatory view showing a general principle of dust collection in a direct current electric field; FIG. 8 is an explanatory view showing a general principle of dust collection in an alternating electric field; FIG. 9 is an explanatory view typically showing behavior of the particles of dielectric in the direct current electric field in a conventional dust collector; and FIG. 10 is an explanatory view typically showing behavior of the particles of dielectric in the alternating electric field in a conventional dust collector. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS FIG. 1 is a schematic longitudinal sectional view showing a general construction of a dust collector to which the present invention is applied. This dust collector has a preliminary charging section 1, a spray section 2, and a dust collecting section 3. The preliminary charging section 1 includes, as shown in FIG. 2, a plurality of ground electrodes (positive electrodes) 4 arranged in parallel and discharge electrodes (negative electrodes) 5 disposed between the ground electrodes 4. The discharge electrode 5 is configured so that a plurality of (three, in this example) conductive rods 5a are disposed vertically in a plane parallel with the ground electrode 4, and a large number of spine-like portions 5b are arranged in the vertical direction of the rod 5a at appropriate intervals. The spray section 2 is, as shown in FIG. 3, provided with a large number of nozzles 6 for spraying a dielectric, which are arranged under the dust collecting section 3. The nozzles 6 are formed on a plurality of pipes 7 arranged horizontally at appropriate intervals. As shown in FIG. 1, the pipe 7 is connected to a dielectric storage tank 8 via a pipe 13. Therefore, if a dielectric (water in this example) 10 in the storage tank 8 is drawn up by a pump P interposed in the pipe 13, the mist-like dielectric 10 is sprayed from the nozzles 6. The dust collecting section 3 includes, as shown in FIG. 3, a plurality of ground electrodes 11 arranged in parallel and high voltage applied electrodes 12 interposed between the ground electrodes 11. In the dust collector constructed as described above, as indicated by the arrow mark in FIG. 1, an exhaust gas from which dust is to be removed (for example, an exhaust gas generated when coal, heavy oil, or the like is burned) is introduced into the preliminary charging section 1. The exhaust gas passes between the ground electrode 4 and the discharge electrode 5 shown in FIG. 2. At this time, a substance to be collected such as dust, mist, and the like contained in the exhaust gas is provided with a charge by corona discharge occurring between the electrodes 4 and 5. In this example, by the provision of the charge, the substance to be collected is charged negatively. The exhaust gas having passed through the preliminary charging section 1 flows into a gas absorbing zone 15 shown in FIG. 1, and then, after flowing upward, it is introduced into the dust collecting section 3 together with the dielectric 10 sprayed from the spray section 2. The sprayed dielectric 10 is dielectrically polarized by a direct current electric field or an alternating electric field acting between the electrodes 11 and 12 (see FIG. 3) of the dust collecting section 3. Therefore, the negatively charged substance to be collected sticks to the dielectric 10 by means of the Coulomb's force acting between the particles of dielectric 10. The dielectric to which the substance to be collected has stuck is recovered in a dielectric collecting section 16 consisting of a demister or the like. Therefore, a clean gas from which the substance to be collected has been removed is discharged from the dielectric collecting section 16. Since this dust collector is applied to the treatment of a harmful gas, the sprayed dielectric 10 absorbs some of the harmful gas. Specifically, for example, in the case where the dust-containing gas contains a harmful gas such as SOx, the dielectric 10 absorbs the SOx during the time when the dielectric 10 is used by being circulated. If the dielectric 10 absorbs a harmful gas in this manner, the pH value of the dielectric 10 decreases, so that a problem of corrosion etc. arises. In this dust collector, therefore, in order to solve the above problem, there are provided a fresh water supply pipe 51 in which a valve 50 is interposed, a discharge pipe 53 in which a valve 52 is interposed, an absorbent supply pipe 55 in which a valve 54 is interposed, and a controller 56 or the like for controlling the valves 50, 52 and 54. Specifically, the dielectric 10 in the storage tank 8 contains a reaction product according to the absorption amount (treatment amount) of SOx or the like contained in the dust-containing gas. Therefore, the controller 56 controls, based on the output of a concentration sensor 57 for detecting the in-liquid concentration of the reaction product, the valves 50 and 52 so that the in-liquid concentration exhibits a value within a given range. That is to say, the controller 56 regulates the quantity of fresh water poured into the tank 8 and the quantity of dielectric 10 discharged from the tank 8. Also, the controller 56 controls, based on the output of a pH sensor 58 for detecting the pH concentration of the dielectric 10 in the tank 8, the valve 54 so that the pH concentration exhibits a value within a given range. That is to say, the controller 56 regulates the quantity of absorbent (for example, NaOH and Mg) charged into the tank 8 to absorb the reaction product. If the in-liquid concentration of the reaction product and the pH value of the dielectric 10 are controlled as described above, not only the corrosion or the like can be prevented, but also the harmful gas can be removed positively by utilizing the harmful gas absorbing function of the dielectric 10. Although the in-liquid concentration of the reaction product is controlled based on the output of the concentration sensor 57 in the above description, the concentration control can be carried out without the use of the concentration sensor 57. Specifically, since the average degree of increase in the in-liquid concentration is known in advance by an experiment etc., the quantity of fresh dielectric (fresh water) poured into the tank 8 and the quantity of dielectric discharged from the tank 8, which correspond to the degree of increase, are determined in advance, and the valves 50 and 52 are controlled so that the poured quantity and discharged quantity are attained. Thereby, the in-liquid concentration of the reaction product can be made within a given range. First, embodiments in which the direct current electric field is formed between the electrodes 11 and 12 shown in FIG. 3 will be explained. (Embodiment 1) As described above, the dielectric 10 sprayed from the spray section 2 has been charged positively or negatively. When the direct current electric field is formed between the electrodes 11 and 12 of the dust collecting section 3, the charging of the dielectric 10 decreases the efficiency in collecting the substance to be collected for the aforementioned reason (sticking of the dielectric to the electrode) explained with reference to FIG. 9. Thereupon, in the dust collector of embodiment 1, the spray section is formed as shown in FIG. 4. This spray section is configured so that an earth net 17 is disposed in the nozzle 6, and an earth net 18 is disposed at a slightly downstream position from the position where the nozzle 6 is disposed in the pipe 7. The earth nets 17 and 18, which are made of a metal, are provided so as to traverse the flow path of the dielectric 10. The pipe 7 and the nozzle 6 are grounded, so that the earth nets 17 and 18 fitted to these elements are also grounded. The charged dielectric 10 flowing through the pipe 7 is de-electrified during the time when it passes through the earth nets 17 and 18. As a result, the dielectric 10 that has been de-electrified, that is, that is electrically neutral, is sprayed from the nozzle 6. The de-electrified dielectric 10 having been sprayed from the nozzle 6 is not subjected to the Coulomb's force created by the direct current electric field between the electrodes 11 and 12 when it is introduced to between the electrodes 11 and 12 shown in FIG. 3. Therefore, most of the dielectric 10 moves toward the upper part (rear part) of the electrodes 11 and 12 without being arrested by the electrodes 11 and 12. As a result, even at the upper part of the electrodes 11 and 12, the substance to be collected is efficiently collected by the dielectric 10. With the use of the earth nets 17 and 18 as de-electrifying means, a satisfactory de-electrifying effect can be achieved without obstructing the flow of the dielectric 10. In the spray section 2, a two fluid nozzle as shown in FIG. 5 can be used. For this two fluid nozzle 60, the dielectric 10 is introduced from the side of the nozzle 60 via an introduction pipe 61, and at the same time, a pressurized air is introduced via an air supply pipe 62 continuous with the lower part of the nozzle 60, so that the dielectric 10 can be sprayed from the tip end of the nozzle 60. When this two fluid nozzle 60 is used, an earth net 20 is disposed at the outlet of the introduction pipe 61, and an earth net 21 is disposed at a slightly downstream position from the position where the nozzle 60 is disposed in the pipe 7. Thereby, the de-electrified dielectric 10 is sprayed from the nozzle 60 as in the case of the nozzle 6 shown in FIG. 4. (Embodiment 2) FIG. 6 shows an embodiment in which a plurality of stages (two stages in this example) of the pair of the spray section 2 and the dust collecting section 3 are disposed in the direction of the gas flow. This embodiment can be applied to both the case where the direct current electric field is formed between the electrodes 11 and 12 of the dust collecting section 3 and the case where the alternating electric field is formed. According to this configuration, the substance to be collected that has not been collected in the first-stage dust collecting section 3 is collected in the second-stage dust collecting section 3, so that a very high dust collecting efficiency can be attained. In this embodiment, circulating water is used as the dielectric 10 supplied to the first-stage spray section 2, and fresh water is used as the dielectric 10 supplied to the second-stage spray section 2. Thus, the outflow of harmful substances contained in the dielectric 10 from the demister 16 can be restrained to the utmost. In this embodiment as well, as in the case of the dust collector shown in FIG. 1, there are provided dielectric supply/discharge means and absorbent charging means, having the valves 50, 52 and 54, the controller 56, the sensors 57 and 58, and the like. Therefore, the concentration of the reaction product in the dielectric 10 can be controlled so as to be a concentration within a given range, and also the pH value of the dielectric 10 can be controlled so as to be a value within a given range. In this embodiment, however, the fresh water supply valve 50 is provided in the supply pipe 7 of the second-stage spray section 2. Although the number of stages of the pair of the spray section 2 and the dust collecting section 3 is two in this embodiment, the number of stages can be set at three or more. In this case, fresh water may be supplied to at least the final-stage spray section 2. Also, when the outflow of harmful substances poses no problem, it is a matter of course that circulating water can be sprayed even in the final-stage spray section 2. It is preferable that the nozzle 6 of the spray section 2 for spraying the fresh water as the dielectric 10 have a function of being capable of atomizing the fresh water to an average diameter not larger than 50 µm to decrease the quantity of fresh water used and to increase the dust collecting efficiency. The reason for this will be described below. In the case where fine dust or mist such as SO 3 is the substance to be collected, in order to efficiently collect the substance to be collected, it is necessary only that water mist be caused to float as close as possible to the substance to be collected. In order to cause the water mist to float close to the substance to be collected, the water mist must be atomized as small as possible. The reason for this is that even when the same quantity of dielectric is sprayed, the smaller the particles of the water mist are, the larger the number of scattered particles is, and resultantly, the water mist can be brought close to the substance to be collected. Because fresh water contains no foreign matter, the nozzle 6 having a function of being capable of atomizing the fresh water to, for example, an average diameter not larger than 50 µm can be used. As a nozzle having such a function, there are well known a one fluid nozzle in which the spray pressure is high (for example, 5 kg/cm 2 G) and the foreign matter passing diameter is not larger than 1 mm, a two fluid nozzle additionally using assist air, and the like. Since a solid matter etc. of the substance collected in the circulating water exist as impurities in the circulating water, when the circulating water is used as the dielectric, the foreign matter passing diameter of nozzle cannot be decreased. Therefore, it is necessary to use a general-purpose one fluid nozzle or two fluid nozzle to spray the circulating water. In this case, the average diameter of the obtained water mist is at the level of about 100 to 200 µm at least. Comparing the case where a general nozzle for spraying water mist having an average diameter of 170 µm is used with the case where a special nozzle for spraying water mist having an average diameter of 20 µm is used, the necessary quantity of water for obtaining the same dust collecting efficiency differs greatly. In an experiment, it has been verified that the necessary quantity of water in the latter case is decreased to 1/8 or less of the former case. The circulating water can be used in a large quantity. However, the quantity of the fresh water used must be decreased for the reason of the necessity of decreasing a utility and for other reasons. In the embodiment shown in FIG. 6, a general-purpose nozzle is used as the nozzle 6 of the first-stage spray section 2, which sprays circulating water as the dielectric 10, and a special nozzle capable of atomizing fresh water to an average diameter not larger than 50 µm is used as the nozzle 6 of the second-stage spray section 2, which sprays the fresh water as the dielectric 10. Thereby, the nozzle is not clogged, thereby maintaining a high dust collecting efficiency, and the quantity of fresh water used is decreased. Although water is used as the sprayed dielectric 10 in the embodiments described above, the dielectric 10 is selected appropriately according to the composition of the substance to be collected 9. For example, when the gas containing the substance to be collected 9 is an acidic gas such as hydrogen chloride or sulfur dioxide, an alkaline absorbing solution etc. represented by an aqueous solution of sodium hydroxide are used as the dielectric 10, so that gas absorption can also be effected. Also, the sprayed dielectric 10 is not limited to a liquid. For example, powder of activated carbon etc. having a charging function can be used as the dielectric 10. The dielectric consisting of liquid such as water and the dielectric consisting of the powder can be sprayed at the same time, or a mixture of the liquid and powder can be sprayed. Further, although the dielectric 10 is sprayed upward in the embodiments described above, the dielectric 10 may be sprayed downward or horizontally. Still further, although the exhaust gas having passed through the preliminary charging section 1 is moved along the f low path directed from the downside to the upside, the exhaust gas can be moved along a flow path directed horizontally. However, the movement of the exhaust gas along the flow path directed from the downside to the upside is more advantageous in increasing the efficiency in collecting the substance to be collected. The reason for this is that a nonuniform distribution of the substance to be collected in the exhaust gas caused by the action of the gravity is not formed, so that the substance to be collected is distributed uniformly.

Claims: A dust collector, comprising: charging means (1) for charging a substance which is contained in a gas and which is to be collected; spray means (2) for spraying a dielectric (10) on said substance to be collected charged by said charging means (1); electric field forming means (3), having first and second electrodes (11,12) for forming a direct current electric field, for dielectrically polarizing said dielectric (10) sprayed by said spray means (2) by means of said direct current electric field; and dielectric collecting means (16) for collecting said dielectric (10) which has arrested said substance to be collected; characterized by comprising grounding means (17,18;20,21) provided in said spray means (2), for electrically grounding said dielectric (10) before being sprayed for causing a charge of said dielectric (10) to escape by said grounding means (17,18;20,21) so that said dielectric (10) is made electrically neutral; wherein a metallic net (17,18;20,21) is used as said grounding means, and said net (17,18;20,21) is disposed in a flow path of said dielectric (10) in said spray means (2) so as to traverse said flow path. The dust collector according to claim 1, wherein a plurality of stages of pairs of said spray means (2) and said electric field forming means are (3) disposed. The dust collector according to claim 2, wherein fresh water is adapted to be sprayed from spray means (2) of at least the most downstream stage of said plurality of stages, and circulating water is adapted to be sprayed from spray means excluding said spray means which is adapted to spray fresh water. The dust collector according to claim 3, wherein said spray means (2) of the most downstream stage has a nozzle (6) for atomizing said fresh water to an average diameter not larger than 50 µm. The dust collector according to any one of claims 1 to 4, further comprising: a dielectric circulating system for supplying said dielectric (10) from a dielectric storage tank (8) to said spray means (2) and for returning the sprayed dielectric (10) to said storage tank (8); dielectric supply means (50,51) for supplying a fresh dielectric (10) to said dielectric storage tank (8); dielectric removal means (52,53) for removing said dielectric (10) in said dielectric storage tank (8); absorbent supply means (54,55) for supplying an absorbent into said dielectric storage tank (8), said absorbent being used to absorb a reaction product produced by a substance in said gas; and control means (56) for controlling the quantity of dielectric (10) supplied by said dielectric supply means (50,51) and the quantity of dielectric removed by said removal means (52,53) so that the concentration of said reaction product exhibits a value within a given range and for controlling the quantity of absorbent supplied by said absorbent supply means (54,55) so that the pH value of said dielectric (10) exhibits a value within a given range.

Inventors: TOMIMATSU KAZUTAKA, UEDA YASUTOSHI, TOMIMATSU, KAZUTAKA, UEDA, YASUTOSHI, Tomimatsu, Kazutaka, c/o Mitsubishi Heavy Indust., Ueda, Yasutoshi, c/o Mitsubishi Heavy Industries
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI HEAVY INDUSTRIES, LTD.
Application year: 2000
Main classification: B03C3
IPC codes: B03C3