Abstract:
A dust collector for collecting dust, in which the rarefaction of a dielectric at the rear part of electric field forming apparatus is prevented, whereby the collecting efficiency can be increased. The dust collector includes a charging device for charging a substance to be collected, such as dust and mist, contained in a gas; a sprayer device for spraying a dielectric on the substance to be collected which is charged by the charging device; an electric field forming device, having first and second electrodes and which form a direct current electric field and dielectrically polarize the dielectric sprayed by the spray device; and a dielectric collecting device for collecting the dielectric which has arrested the substance to be collected. The spray device includes grounding device and for electrically grounding the dielectric before being sprayed to let a charge of the dielectric escape.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Divisional of U.S. patent application Ser. No. 09/635,430 filed Aug. 10, 2000, now U.S. Pat. No. 6,500,240, the entire contents of which are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION AND RELATED ART STATEMENT 
     The present invention relates to a dust collector and method for collecting dust, 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. 24, 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&#39;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. 25, 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&#39;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. 26 corresponding to FIG. 24, 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&#39;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. 27 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 . 
     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 can be 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. 
     Also, 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; and dielectric collecting means for collecting the dielectric which has arrested the substance to be collected, wherein a plurality of corona discharge sections arranged in the flow direction of the gas at given intervals are formed on the opposed surfaces of the first and second electrodes to generate band-shaped uniform corona discharge perpendicular to the gas flow, and the dielectric is provided with a charge of reverse polarity alternately by the corona discharge. 
     According to the present invention, the dielectric goes in a zigzag form to the rear zone of the electric field forming means under the action of the charge developed by discharge of the corona discharge section, so that the substance to be collected can be collected very efficiently. 
     The arrangement interval between the corona discharge sections on the first electrode and the arrangement interval between the corona discharge sections on the second electrode are preferably set so as to be equal to each other. Also, both of the corona discharge sections are preferably provided so as to have an arrangement phase difference of ½ of the arrangement interval in the flow direction of the gas. According to this configuration, corona discharge on the electrodes of the electric field forming section does not oppose, so that the occurrence of spark discharge can be restrained. 
     The rear parts of the first and second electrodes can be extended, and a plurality of the corona discharge sections can be formed in the flow direction of the gas on one of these extensions only. According to this configuration, the dielectric can be collected at the extension of the electrode of the electric field forming section, so that a demister can be omitted. 
     Further, 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; and dielectric collecting means for collecting the dielectric which has arrested the substance to be collected, wherein the distribution of the dielectric sprayed by the spray means is set so that the distribution of the dielectric at the rear part of the first and second electrodes is uniformed. 
     According to the present invention, the dielectric can be caused to exist uniformly in the rear zone of the electric field forming section, so that the collecting efficiency is increased. 
     Sill further, 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; and dielectric collecting means for collecting the dielectric which has arrested the substance to be collected, wherein the spray means is provided with charge providing means for providing the dielectric before being sprayed with a charge having a reverse polarity of the charging polarity of the substance to be collected. 
     According to the present invention, a repelling force acts between the particles of sprayed dielectric, so that the aggregation of the particles of dielectric in the electric field forming section is prevented, thereby increasing the collecting efficiency. 
     The charge providing means can be configured so as to supply ionized air to the dielectric before being sprayed. According to this configuration, the dielectric is charged via the ionized air. 
     Also, the charge providing means can be configured so that magnetism in the direction perpendicular to the flow direction of the dielectric is applied to the dielectric before being sprayed. According to this configuration, the dielectric is charged by the action of the magnetism. 
     In the dust collectors 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 freshwater. 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. 
     A method for collecting dust in accordance with the present invention comprises a first step of charging a substance to be collected, such as dust and mist, contained in a gas; a second step of causing the gas having undergone the first step to flow from the downside to the upside; a third step of spraying a dielectric on the substance to be collected contained in the gas flowing from the downside to the upside; a fourth step of dielectrically polarizing the sprayed dielectric and of causing the dielectric to arrest the substance to be collected by means of the Coulomb&#39;s force created by the polarization; and a fifth step of collecting the dielectric which has arrested the substance to be arrested. 
     According to the present invention, the gas in which the substance to be collected has been charged is moved from the downside to the upside, so that a nonuniform distribution of the substance to be collected caused by the action of the gravity is not formed. Therefore, the substance to be collected is distributed uniformly, and is collected efficiently. 
    
    
     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 perspective view showing another construction of the dust collecting section; 
     FIG. 7 is a schematic perspective view showing a construction of a corona discharge section; 
     FIG. 8 is a partial perspective view showing a mode of discharge of the corona discharge section; 
     FIG. 9 is a plan view showing a construction of small protrusions constituting the corona discharge section; 
     FIG. 10 is a sectional view taken along the line A—A of FIG. 9; 
     FIG. 11 is a sectional view taken along the line B—B of FIG. 9; 
     FIG. 12 is a plan view showing another construction of the small protrusions constituting the corona discharge section; 
     FIG. 13 is a sectional view taken along the line C—C of FIG. 12; 
     FIG. 14 is a sectional view taken along the line D—D of FIG. 12; 
     FIG. 15 is a plan view showing another construction of the corona discharge section; 
     FIG. 16 is a sectional view taken along the line E—E of FIG. 15; 
     FIG. 17 is a sectional view taken along the line F—F of FIG. 15; 
     FIG. 18 is a schematic sectional view showing a general distribution mode of dielectric in the dust collecting section; 
     FIG. 19 is a schematic sectional view typically showing a spray mode of dielectric in the dust collector in accordance with the present invention; 
     FIG. 20 is a sectional view showing a construction of the spray section used in the dust collector in accordance with the present invention; 
     FIG. 21 is a sectional view showing another construction of the spray section used in the dust collector in accordance with the present invention; 
     FIG. 22 is a perspective view for explaining the operation of the spray section shown in FIG. 21; 
     FIG. 23 is a schematic sectional view showing another embodiment of the dust collector in accordance with the present invention; 
     FIG. 24 is an explanatory view showing a general principle of dust collection in a direct current electric field; 
     FIG. 25 is an explanatory view showing a general principle of dust collection in an alternating electric field; 
     FIG. 26 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. 27 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  5   a  are disposed vertically in a plane parallel with the ground electrode  4 , and a large number of spine-like portions  5   b  are arranged in the vertical direction of the rod  5   a  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&#39;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 (freshwater) 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.  26 . 
     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 upstream 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&#39;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 upstream 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 . 
     FIG. 6 shows an embodiment in which a plurality of corona discharge sections  110  and  120  arranged in the flow direction of the gas are formed on the opposed surfaces of the electrodes  11  and  12  of the dust collecting section  3 , respectively. In this embodiment as well, the direct current electric field is formed between the electrodes  11  and  12 . 
     As shown in FIG. 7, the corona discharge sections  110  and  120  are located at intervals of L, and have an arrangement phase difference of L/2 with respect to each other in the flow direction of the exhaust gas. 
     The corona discharge sections  110  and  120  each have a configuration in which small protrusions  110   a  and  120   a  are disposed closely with a pitch P in the direction perpendicular to the gas flow. Therefore, as shown in FIG. 8, a band-shaped corona current can be supplied from the corona discharge section  110  ( 120 ) to the opposed electrode  12  ( 11 ). 
     In FIG. 6, when its initial charging polarity is negative, the dielectric  10  going between the electrodes  11  and  12  is transferred to the electrode  11  by the Coulomb&#39;s force created by the direct current electric field between the electrodes  11  and  12 . 
     The corona discharge sections  110  and  120  release the positive and negative charges, respectively, by corona discharge between the electrodes. Therefore, the dielectric  10  transferred to the electrode  11  is charged positively by the charge released from the corona discharge section  110 , with the result that the dielectric  10  is transferred to the electrode  12 . The dielectric  10  transferred to the electrode  12  is charged negatively by the charge released from the corona discharge section  120 , so that the dielectric  10  is transferred again to the electrode  12 . That is to say, the dielectric  10  transfers while being provided with a charge of reverse polarity alternately. 
     Thus, the dielectric  10  (water mist in this example) goes upward between the electrodes  11  and  12  while transferring in a zigzag form, and is dielectrically polarized by the electric field acting between the electrodes  11  and  12 . On the other hand, the particles of substance to be collected (SO 3  mist in this example)  9  indicated by the black dots scarcely move in the direction such as to traverse the gas flow (right and left direction in FIG.  6 ). As a result, the dielectric  10  goes in a zigzag form while collecting the substance to be collected  9  by means of the Coulomb&#39;s force acting between the particles of dielectric  10 . 
     The particle size of the dielectric  10  is appreciably larger than that of the substance to be collected  9 , so that the quantity of charge given to a unit weight of the dielectric  10  per unit time is considerably larger than that of the substance to be collected  9 . The above-described operation such that the dielectric  10  collects the substance to be collected  9  while going in a zigzag form is attained by a difference in the quantity of charge given to a unit weight per unit time. 
     According to this embodiment 2 in which the charges developed by the discharge of the corona discharge sections  110  and  120  are utilized, the dielectric  10  can be caused to exist up to the upper part of the electrodes  11  and  12 , so that the efficiency in collecting the substance to be collected  9  is increased. 
     If the arrangement interval L between the corona discharge sections  110  and  120  is set so as to be smaller than the given interval, the discharge sections  110  and  120  are opposed to each other and a locally high electric field is formed in a spot form, so that there is a fear of the occurrence of spark discharge. Therefore, the arrangement interval L is preferably set so as to be L≧d (d denotes a distance between the electrodes  11  and  12 ). 
     In this embodiment 2, the upper ends (rear end) of the electrodes  11  and  12  are extended by an appropriate length D, and the corona discharge sections  120  are formed at the extension of the electrode  12  only. In this configuration, the dielectric  10  that has arrested the substance to be collected  9  and has arrived at the extensions of the electrodes  11  and  12  is finally attracted and collected by the electrode  11 , that is, the extension of the electrode  11  has a function of collecting the dielectric  10 . Therefore, the demister  16  shown in FIG. 1 can be omitted. 
     The corona discharge sections  110  may be formed at the extension of the electrode  11  only. In this case, the dielectric  10  that has arrested the substance to be collected  9  is finally attracted and collected by the electrode  12 . 
     FIG. 9 is a plan view showing an example of the small protrusions  110   a ,  120   a  constituting the corona discharge section  110 ,  120 . FIGS. 10 and 11 are sectional views taken along the lines A—A and B—B of FIG. 9, respectively. The small protrusion  110   a ,  120   a  shown in these figures is formed into a triangular shape by cutting and raising a metal plate forming the electrode  11 ,  12 . These protrusions  110   a ,  120   a , having a sharp tip end, are advantageous in concentrating the electric field. 
     FIG. 12 is a plan view showing another example of the small protrusions  110   a ,  120   a . FIGS. 13 and 14 are sectional views taken along the lines C—C and D—D of FIG. 12, respectively. This small protrusion  110   a ,  120   a  is formed by welding a spine-like stud to the electrode  11 ,  12 . 
     FIG. 15 is a plan view showing another construction of the corona discharge section  110 ,  120 . FIGS. 16 and 17 are sectional views taken along the lines E—E and F—F of FIG. 15, respectively. The corona discharge section  110 ,  120  is made up of conductive electrode reinforcing pipes  19   a  fixed to both sides of the electrode  11 ,  12  and small-diameter conductive wires  19   c  stretched between the electrode reinforcing pipes  19   a  via conductive wire mounting pieces  19   b.    
     According to this corona discharge section  110 ,  120 , a band-shaped corona current can be supplied from the wire  19   c  of the discharge section  110 ,  120  to the opposed electrode  12 ,  11 . 
     FIG. 18 shows a distribution mode of dielectric  10  in the dust collecting section  3  in the case where the direct current electric field is formed between the electrodes  11  and  12  and the dielectric  10  sprayed from the spray section  2  is charged negatively. As shown in FIG. 18, the distribution of the dielectric  10  is uniform in the lower zone of the electrodes  11  and  12 , but much of the dielectric  10  is distributed on the side of the electrode  11  in the upper zone thereof. The reason for this is that the negatively charged dielectric  10  is attracted to the positive electrode  11  as it transfers to the upper part of the electrodes  11  and  12 . 
     If a nonuniform distribution of the dielectric  10  is formed in the upper zone of the electrodes  11  and  12  as described above, the efficiency in collecting the substance to be collected decreases in the upper zone. 
     (Embodiment 3) 
     FIG. 19 shows another embodiment of the present invention in which the above problem is solved. In this embodiment, the distance between the electrodes  11  and  12  is increased, and the right and left nozzles  6  of the spray section are substantially shifted from the middle position between the electrodes  11  and  12  to a position close to the electrode  12 . 
     According to this configuration, since the dielectric  10  sprayed from both of the right and left nozzles  6  is supplied to the periphery of the electrode  12 , much of dielectric  10  is distributed on the side of the electrode  12 . 
     The dielectric  10 , which has been charged negatively, transfers upward in the dust collecting section  3  while being subjected to an attracting force from the positive electrode  11 . Therefore, the dielectric  10 , which has initially been distributed more on the side of the electrode  12 , is uniformly distributed at the upper part of the dust collecting section  3 . 
     According to this embodiment 3, the dielectric  10  can be caused to exist uniformly at the upper part (rear part) of the dust collecting section  3 , so that the substance to be collected  9  can be collected enough even at the upper part, resulting in an increase in the collecting efficiency. 
     Even in the case where the dielectric  10  is charged positively, the distribution of the dielectric sprayed from the spray section is set so that the distribution of the dielectric  10  is made uniform at the rear part of the electrodes  11  and  12 . 
     Next, an embodiment in which the alternating electric field is formed between the electrodes  11  and  12  shown in FIG. 3 will be explained. 
     (Embodiment 4) 
     When the alternating electric field is formed between the electrodes  11  and  12 , as described with reference to FIG. 27, there occurs a phenomenon that the particles of dielectric  10  aggregate each other. In order to prevent the aggregation of the particles of dielectric  10 , it is necessary only that the mist  10  be charged in advance so as to have the same polarity. This is because the particles of dielectric  10  repel each other due to the charging. 
     Thereupon, in the dust collector of this embodiment 4, the spray section  2  is configured as shown in FIG.  20 . This spray section  2  has a charging section  25  provided at a slightly upstream position from the nozzle  6  in the pipe  7  to obtain the charged dielectric  10 . The charging section  25  includes an air supply pipe  26  whose tip end is open in the pipe  7 , an electrode  27  projecting in the air supply pipe  26 , and a direct current source  28  for applying a high voltage to the electrode  27 . 
     When pressurized air is introduced into the air supply pipe  26 , the air is provided with a positive charge from the electrode  27 , so that the air is ionized positively. The positively ionized air is injected into the dielectric  10  in the pipe  7  as bubbles from the tip end of the air supply pipe  26 , so that the dielectric  10  is positively charged by the positive ion of the air. As the result, the positively charged dielectric  10  is sprayed from the nozzle  6 . 
     The positively charged particles of dielectric  10  are subjected to a repelling force therebetween, so that they do not aggregate between the electrodes  11  and  12  in the dust collecting section  3 . Therefore, the dielectric  10  exists enough even at the upper part of the dust collecting section  3 , thereby increasing the efficiency in collecting the substance to be collected. 
     The spray section  2  shown in FIG. 21 uses a magnet  31 ,  32  as a means for obtaining the charged dielectric  10 . The magnet  31 ,  32  is disposed at a slightly upstream position from the nozzle  6  in the pipe  7  so that the tip end portions thereof are opposed to each other in the pipe  7 . The magnet  31 ,  32  is housed in a case  33  having electrical insulating quality and non-magnetism. 
     Between the tip end portions of the magnet  31 ,  32 , a magnetic flux B is produced as shown in FIG.  22 . The dielectric (water in this example)  10  flows in the X direction perpendicular to the Z direction of the magnetic flux B, so that an electromotive force e in the direction (Y direction) perpendicular to the X and Y directions is created. The electromotive force e is created based on Lorentz&#39;s law. 
     Ions and electrons in the dielectric  10  move in the direction of the electromotive force e or the direction opposite to this according to the polarity thereof. Electrodes  33 A and  33 B are disposed on one side and the other side of the flow path of the dielectric  10  so as to be perpendicular to the direction of the electromotive force e. The electrode  33 A, which is located in the direction opposite to the direction of the electromotive force e, is grounded. 
     The dielectric  10  passes through an electric field formed between the electrodes  33 A and  33 B by the electromotive force e. Therefore, the negative ions and electrons in the dielectric  10  flow out via the grounded electrode  33 A. As a result, positive ions remain in the dielectric  10  having passed through between the electrodes  33 A and  33 B. That is to say, the dielectric  10  is charged positively by passing through between the electrodes  33 A and  33 B. 
     The positively charged dielectric  10  is supplied to the nozzle  6  shown in FIG. 21, so that the positively charged dielectric  10  is sprayed from the nozzle  6 . Thereafter, the positively charged dielectric  10  transfers up to the upper part of the dust collecting section  3  without being aggregated, as described above. Therefore, a shortage of the dielectric  10  at the upper part can be avoided. 
     In the embodiment shown in FIGS. 20 and 21, the dielectric  10  is charged positively based on the fact that the charging polarity of the substance to be collected  9  in the preliminary charging section  1  is negative. In the case where the charging polarity of the substance to be collected  9  is positive, the dielectric  10  is charged negatively. In this case, the dielectric  10  can be charged negatively by using charging means corresponding to the charging means shown in FIGS. 20 and 21. 
     (Embodiment 5) 
     FIG. 23 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 ⅛ 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. 25, 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 flow 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.