Combustion exhaust gas treatment system Number:6,773,680 from the United States Patent and Trademark Office (PTO) owispatent Five combustion exhaust gas treatment systems capable of removing dust and selenium (Se) in combustion exhaust gas and making harmless are proposed: (1) combustion exhaust gas is cooled to 350.degree. C. or less, dust is separated, Se is transformed into insoluble compound, and Se is separated; (2) combustion exhaust gas is cooled to 350.degree. C. or less, dust is separated, Se elution preventive agent is added, and dust is formed into scale; (3) dust in combustion exhaust gas is collected by dust collector, dust is formed into slurry by making use of part of circulation liquid in desulfurization apparatus, and tetravalent Se in dust slurry is transformed into insoluble compound, which is separated into solid and liquid; (4) dust is separated from combustion exhaust gas by dust collector, and it is heated to gasify Se, and is led into desulfurization apparatus, etc., and formed into slurry, and Se is made into insoluble compound; and (5) a treating agent for making tetravalent Se insoluble is added to part of circulation liquid in desulfurization apparatus, and it is sprayed into combustion exhaust gas to make Se insoluble.<H Combustion, treatment, Combustion, exha, 6773680.html, Patent, pto, 6773680

Title: Combustion exhaust gas treatment system

Abstract: Five combustion exhaust gas treatment systems capable of removing dust and selenium (Se) in combustion exhaust gas and making harmless are proposed: (1) combustion exhaust gas is cooled to 350.degree. C. or less, dust is separated, Se is transformed into insoluble compound, and Se is separated; (2) combustion exhaust gas is cooled to 350.degree. C. or less, dust is separated, Se elution preventive agent is added, and dust is formed into scale; (3) dust in combustion exhaust gas is collected by dust collector, dust is formed into slurry by making use of part of circulation liquid in desulfurization apparatus, and tetravalent Se in dust slurry is transformed into insoluble compound, which is separated into solid and liquid; (4) dust is separated from combustion exhaust gas by dust collector, and it is heated to gasify Se, and is led into desulfurization apparatus, etc., and formed into slurry, and Se is made into insoluble compound; and (5) a treating agent for making tetravalent Se insoluble is added to part of circulation liquid in desulfurization apparatus, and it is sprayed into combustion exhaust gas to make Se insoluble.

Patent Number: 6,773,680 Issued on 08/10/2004 to Shimizu, et al.

Inventors: Shimizu; Taku (Tokyo, JP), Kimura; Kazuaki (Tokyo, JP), Okazoe; Kiyoshi (Tokyo, JP), Tatani; Atsushi (Tokyo, JP), Ukawa; Naohiko (Hiroshima, JP), Hino; Masao (Hiroshima, JP), Okino; Susumu (Hiroshima, JP), Haruki; Takashi (Hiroshima, JP), Takashina; Toru (Hiroshima, JP), Ochi; Eiji (Tokyo, JP)
Assignee: Mitsubishi Jukogyo Kabushiki Kaisha (

Appl. No.: 10/294,269

Filed: November 14, 2002

Related U.S. Patent Documents


Application Number	Filing Date	Patent Number	Issue Date
407042	Sep., 1999	6511636	Jan., 2003
059675	Apr., 1998
592116	Jan., 1996	6113863	Sep., 2000

Foreign Application Priority Data


Mar 30, 1995 [JP]			7-72778
Mar 30, 1995 [JP]			7-72779
Apr 07, 1995 [JP]			7-82460
May 30, 1995 [JP]			7-131991
Aug 29, 1995 [JP]			7-219970

Current U.S. Class: 422/173 ; 422/139; 422/140; 422/146; 422/168; 422/169; 422/172; 422/177

Current International Class: B01D 53/50 (20060101); B01D 53/46 (20060101); F23J 15/00 (20060101); F23J 15/02 (20060101)

Field of Search: 422/139,140,141,145,146,147,168,169,170,171,172,177,211,217,173 423/510,210,215.5,508

References Cited [Referenced By]

U.S. Patent Documents


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3966889	June 1976	Kakuta et al.
4029752	June 1977	Cahn
4617180	October 1986	Vogg
4620492	November 1986	Vogg et al.
4696804	September 1987	Shinoda et al.
4873065	October 1989	Braun et al.
4917732	April 1990	Baker
4994246	February 1991	Moser et al.
5192514	March 1993	Dismukes
5266210	November 1993	McLaughlin
5512072	April 1996	Laslo
5686053	November 1997	Kikkawa et al.
5756052	May 1998	Suzumura et al.
5965095	October 1999	Owens et al.

Foreign Patent Documents


0 304 412	Feb., 1989	EP
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WO 89/08493	Sep., 1989	WO

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Primary Examiner: Johnson; Jerry D.
Assistant Examiner: Doroshenk; Alexa A.
Attorney, Agent or Firm: Trop, Pruner & Hu, P.C.

Parent Case Text

This is a divisional of prior U.S. patent application Ser. No. 09/407,042, filed Sep. 27, 1999, which issued on Jan. 28, 2003 as U.S. Pat. No. 6,511,636, which is a divisional of prior U.S. patent application Ser. No. 09/059,675, filed Apr. 13, 1998, now abandoned, which is a divisional of prior U.S. patent application Ser. No. 08/592,116, filed Jan. 26, 1996, which issued on Sep. 5, 2000 as U.S. Pat. No. 6,113 863.

Claims

What is claimed is:

1. A combustion exhaust gas treatment system for treating combustion exhaust gas containing sulfurous acid, dust and Se components, comprising a dust collector for removing dust from the combustion exhaust gas, a desulfurization apparatus having an absorption column in which an absorbent slurry for absorbing and removing sulfurous acid circulates, and means for heating the dust removed by the dust collector to a temperature for gasification of Se in the dust, wherein the gas generated by heating the dust by the heating means is fed into the desulfurization apparatus together with combustion exhaust gas released from the dust collector, and Se is dissolved and captured in the slurry in the desulfurization apparatus, and a means for mixing a treating agent for making the tetravalent Se insoluble in the slurry.

2. A combustion exhaust gas treatment system of claim 1, further comprising oxidation-reduction reaction control means for controlling the oxidation-reduction reaction in the desulfurization apparatus, so that the hexavalent Se in the slurry in the desulfurization apparatus may be reduced to tetravalent Se, by sulfurous acid in the slurry.

3. A combustion exhaust gas treatment system for treating combustion exhaust gas containing sulfurous acid, dust and Se components, comprising a dust collector for removing dust from the combustion exhaust gas, a desulfurization apparatus having an absorption column in which an absorbent slurry for absorbing and removing sulfurous acid circulates and a cooling and dust collecting column disposed upstream of the absorption column, and means for heating the dust removed by the dust collector to a temperature for gasification of Se in the dust, wherein the gas generated by heating the dust by the heating means is fed into the desulfurization apparatus together with combustion exhaust gas released from the dust collector, and Se is dissolved and captured in a circulation liquid in the cooling and dust collecting column, and a means for mixing a treating agent for making the tetravalent Se insoluble in the the circulation liquid.

4. A combustion exhaust gas treatment system of any one of claims 1 to 3, wherein the dust collecting means comprises a plurality of recovery units for separating and collecting dust from the inlet side to the outlet side of the combustion exhaust gas, and only the dust separated and collected from the recovery unit at the outlet side is fed into the heating means.

5. A combustion exhaust gas treatment system of any one of claims 1 to 3, further comprising sorting means for sorting the dust separated by the dust collecting means into a large particle size group and a small particle size group, wherein only the small particle size dust group is introduced into the heating means.

Description

FIELD OF THE INVENTION AND RELATED ARTS

The present invention relates to a combustion exhaust gas treatment system, more particularly to a combustion exhaust gas treatment system capable of easily removing dust and selenium (Se) In combustion exhaust gas containing dust and Se components, and making harmless.

Hitherto, as the combustion exhaust gas treatment system installed in a thermal power plant or the like, a combustion exhaust gas treatment system comprising a dust collector (usually an electrostatic precipitator) for removing fly ash and other dust from the combustion exhaust gas, and a combustion exhaust gas desulfurization apparatus for absorbing sulfurous acid in the combustion exhaust gas is generally employed.

Moreover, conventionally, as the combustion exhaust gas treatment system installed in a thermal power plant or the like, a combustion exhaust gas treatment system comprising a dry dust collector (usually an electrostatic precipitator) for removing fly ash and other dust from the combustion exhaust gas, and a wet combustion exhaust gas desulfurization apparatus for absorbing sulfurous gas in the combustion exhaust gas by contacting with an absorbent slurry (for example, slurry containing calcium compound) in the absorption column, and separating and recovering gypsum as byproduct from the slurry in the absorption column is generally employed.

Recently, handling of harmful impurities contained in the combustion exhaust gas aside from sulfur oxides is posing a problem. In particular, in the combustion exhaust gas treatment system for coal fired boiler, the harmfulness of selenium (Se) contained at a maximum level of about 10 mg/kg in coal is a problem lately, and its harmless treatment is demanded.

Meanwhile, Se exists as tetravalent Se (main form: selenious acid SeO.sub.3.sup.2-) which is easy to treat by making insoluble by treating agent, and hexavalent Se (main form: selenic acid SeO.sub.4.sup.2-) which is hard to treat by making insoluble, end in particular the hexavalent Se is high in solubility (solubility at 20.degree. C. is 95%) and is easy to elute. Besides, this Se has a toxicity similar to that of arsenic compound, and disaster cases and emission regulations are known overseas, and it is newly added to the list of regulated items also in Japan, and is controlled by the environmental standard (0.01 mg/liter), discharge standard (0.1 mg/liter), and elution standard in landfill (0.3 mg/liter).

FIG. 24 shows an example of prior art of combustion exhaust gas treatment system of this type (an example of combustion exhaust gas treatment system for coal fired boiler). In FIG. 24 and FIG. 25, the combustion exhaust gas A emitted from a coal fired boiler 1 is sent into a denitration apparatus 2 installed downstream of the boiler 1 to be rid of nitrogen oxides (NOx), and passes through an air heater 3 and a heat recovery unit 4 of gas-gas heater (GGH), and is introduced into an electrostatic precipitator (EP) 5, in which fly ash and dust are removed. In succession, the combustion exhaust gas is guided into a wet combustion exhaust gas desulfurization apparatus 7 by a fan 6, and sulfurous gas Is removed in this desulfurization apparatus 7, and after passing through a reheater 8 of the gas-gas heater (GGH), it is led into a stack 10 by a fan 9, and is released into the atmosphere through the stack 10 (FIG. 25).

On the other hand, fly ash and dust removed in the electrostatic precipitator 5 are discharged from plural hoppers 5a (dust recovery units) formed in the electrostatic precipitator 5, and are conveyed and collected in batch by a conveyor 11. Thus collected dust B is either recycled as cement material or the like, or discarded in an ash disposal yard (FIG. 24).

Herein, the desulfurization apparatus 7 comprises an absorption column, for example, in which combustion exhaust gas is introduced, and by contact of combustion exhaust gas with absorbent slurry (usually slurry containing calcium compound) in this absorption column, the sulfurous acid in the combustion exhaust gas is absorbed in wet process, and usually from the slurry in the absorption column, gypsum is separated and collected as byproduct.

Incidentally, the heat recovery unit 4 of the gas-gas heater (GGH) may be also disposed immediately before the desulfurization apparatus 7 as shown in FIG. 26.

In these combustion exhaust gas treatment systems, most of Se in coal (Se in combustion exhaust gas) is condensed at the downstream side of the air heater 3 (that is, the position before introduction Into the electrostatic precipitator 5), and is removed by the electrostatic precipitator 5 in a state being contained in the dust in combustion exhaust gas, and is directly mixed in the refuse in the ash disposal yard or in the cement material. To render Se harmless by conforming to the elution standard, it requires a complicated and costly aftertreatment of, for example, diluting the ash removed by the electrostatic precipitator 5 in a huge volume of water.

OBJECTS AND SUMMARY OF THE INVENTIONS

In the light of the prior arts, it is an object of the invention to present a combustion exhaust gas treatment system capable of easily removing and making harmless the Se components contained in combustion exhaust gas.

To achieve the object, five inventions are proposed.

A. First Invention:

(1) A combustion exhaust gas treatment system for treating combustion exhaust gas containing dust and Se components, comprising means for cooling combustion exhaust gas to 350.degree. C. or less, dust collecting means for separating dust in the combustion exhaust gas, and Se treating means for transforming the existent form of Se in the dust into an insoluble compound by adding water and treating agent to the dust separated by the dust collecting means.

(2) A combustion exhaust gas treatment system for treating combustion exhaust gas containing dust and Se components, comprising means for cooling combustion exhaust gas to 350.degree. C. or less, dust collecting means for separating dust in the combustion exhaust gas, Se treating means for transforming the existent form of Se in the dust into an insoluble compound by adding water and treating agent to the dust separated by the dust collecting means, and solid-liquid separating means for separating the slurry containing insoluble Se compound discharged from the Se treating means into solid and liquid.

Preferred embodiments of the first invention includes the following combustion exhaust gas treatment systems (3) to (6).

(3) A combustion exhaust gas treatment system of (1) or (2), wherein the dust collecting means is a dust collecting apparatus constituted by forming a plurality of recovery units for separating and collecting dust from the inlet side to the outlet side of the combustion exhaust gas, separating and collecting the dust collected from the recovery unit at the inlet side of the combustion exhaust gas,and the dust collected at the outlet side separately, and introducing only the dust collected at the outlet side into the Se treating means.

(4) A combustion exhaust gas treatment system of (1) or (2), further comprising sorting means for sorting the dust separated by the dust collecting means into large particle size group and small particle size group, wherein only the small particle size dust sorted by the sorting means is introduced into the Se treating means to make Se insoluble.

(5) A combustion exhaust gas treatment system of (3) or (4), further comprising means for mixing the dust making Se insoluble by the Se treating means, and the remaining dust not making Se insoluble, so that the moisture content may be 20% or less.

(6) A combustion exhaust gas treatment system of (5), further comprising means for forming the dust mixed by the mixing means into scale.

In the combustion exhaust gas treatment system of the first invention, the method of making Se insoluble may be realized in various forms, and representative forms include a method of forming slurry by adding water to the dust containing Se components separated and recovered by the dust collecting means, adding treating agent to the slurry to make Se insoluble, and separating into solid and liquid, and a method of spraying a solution of treating agent to the dust to immerse uniformly, and making Se insoluble.

In the combustion exhaust gas treatment system of the first invention, most of the Se in the combustion exhaust gas is condensed as being cooled by the cooling means, and is removed by the dust collecting means in a state being contained in the dust. In consequence, the treating agent is added by the Se treating means to the dust separated by the dust collecting means, and the existent form of the Se in dust is transformed into an insoluble compound. Accordingly, if the dust is discarded same as in the prior art, the Se elution standard is satisfied, and the Se is made harmless easily without requiring complicated aftertreatment.

By making Se insoluble only in the dust separated and collected from the specific recovery unit at the outlet side of the combustion exhaust gas in the dust collecting means, the required amount of treating agent and capacity of Se treating means can be reduced, and the Se is made harmless easily and at low cost.

That is, according to the study by the present inventors, it is known that more Se is contained (deposited) in the smaller particle size dust (ash) separated and collected from the specific recovery unit at the outlet side, and the Se is made harmless on the whole only by applying insoluble treatment on the dust of smaller particle size.

Moreover, by the same reason, by making Se insoluble only in the dust of smaller particle size sorted by the sorting means, the required amount of treating agent and capacity of Se treating means can be reduced, and the Se is made harmless more easily and economically.

Incidentally, in the case of the apparatus for making Se insoluble only in part of the dust, by further comprising the mixing means, the dust making Se insoluble, and the remaining dust without Se insoluble treatment are mixed so that the moisture content may be 20% or less, and therefore the moisture content in the dust can be easily lowered and handling is made easier in the disposal process of dust, without installing any huge equipment such as solid-liquid separator that requires wastewater (filtrate) treatment.

Moreover, by further comprising means for making the dust mixed by the mixing means into scale form, handling in dust disposal process is much easier.

The combustion exhaust gas treatment system of the first invention is intended to separate Se components contained in the combustion exhaust gas from the combustion exhaust gas together with dust, and making harmless by further making it insoluble. The dust being rid of dust including Se components is further led into the wet combustion exhaust gas desulfurization apparatus, and SO.sub.2 is removed, and is made completely harmless by proper treatment by ordinary method, and is released into the atmosphere.

B. Second Invention:

According to the system by the first invention, although the Se in the flue can be easily made harmless, but in order to form slurry by adding water to the separated and collected dust, and to make Se insoluble by adding and mixing insoluble treating agent to separated into solid and liquid, it requires wastewater treatment apparatus, solid-liquid separating apparatus, and other devices.

A second invention presents a combustion exhaust gas treatment system capable of easily making harmless the Se contained in the combustion exhaust gas.

(1) A combustion exhaust gas treatment system for removing harmful matter in combustion exhaust gas, comprising means for cooling combustion exhaust gas to 350.degree. C. or less, dust collecting means for separating dust in the combustion exhaust gas, and mixing means for adding and mixing Se elution preventive agent and humidifying liquid or solution of Se elution preventive agent to the dust separated by the dust collecting means.

(2) A combustion exhaust gas treatment system for removing harmful matter in combustion exhaust gas, comprising means for cooling combustion exhaust gas to 350.degree. C. or less, dust collecting means for separating dust in the combustion exhaust gas, mixing means for adding and mixing Se elution preventive agent and humidifying liquid or solution of Se elution preventive agent to the dust separated by the dust collecting means, and means for forming the dust mixed with the Se elution preventive agent and humidifying agent or solution of Se elution preventive agent by the mixing means into scale form.

Preferred embodiments of the second invention involve the combustion exhaust gas treatment systems (3) and (4).

(3) A combustion exhaust gas treatment system of (1) or (2), wherein the dust collecting means is a dust collecting apparatus constituted by forming a plurality of recovery units for separating and collecting dust from the inlet side to the outlet side of the combustion exhaust gas, separating and collecting the dust collected from the recovery unit at the inlet side of the combustion exhaust gas and the dust collected at the outlet side separately, and introducing only the dust collected at the outlet side into the mixing means.

(4) A combustion exhaust gas treatment system of (1) or (2), further comprising sorting means for sorting the dust separated by the dust collecting means into large particle size group and small particle size group, wherein only the small particle size dust sorted by the sorting means is introduced into the mixing means.

In the combustion exhaust gas treatment system of the first invention, most of the Se in the combustion exhaust gas is condensed as being cooled by the cooling means, and is removed by the dust collecting means in a state being contained in the dust. In consequence, the Se elution preventive agent and humidifying liquid or solution of Se elution preventive agent are added by the mixing means to the dust separated by the dust collecting means, and the existent form of the Se in dust is transformed into an insoluble compound. Accordingly, if the dust is discarded same as in the prior art, the Se elution standard is satisfied, and the Se is made harmless easily without requiring complicated aftertreatment. Still more, because of the constitution of adding humidifying liquid, Se elution preventive agent or solution of Se elution preventive agent, and mixing the dust and Se elution preventive agent, or further forming the mixture into scale, as compared with the constitution of forming the dust into slurry and mixing Se insoluble treating agent and the separating into solid and liquid and discarding, it does not require large equipment or apparatus for wastewater (filtrate) treating facility or solid-liquid separator, and handling in disposal of dust is much easier.

By making insoluble only in the dust separated and collected from the specific recovery unit at the outlet side of the combustion exhaust gas in the dust collecting means by introducing into the mixing means, the required amount of Se elution preventive agent and capacity of mixing means and scale forming means can be reduced, and the Se is made harmless easily and at low cost.

That is, according to the study by the present inventors, it is known that more Se is contained (deposited) in the smaller particle size dust (ash) separated and collected from the specific recovery unit at the outlet side, and the Se is made harmless on the whole only by applying insoluble treatment on the dust of smaller particle size.

Moreover, by the same reason, by making Se insoluble only in the dust of smaller particle size sorted by the sorting means, the required amount of treating agent and capacity of Se treating means can be reduced, and the Se is made harmless more easily and economically.

C. Third Invention:

(1) A combustion exhaust gas treatment system for treating combustion exhaust gas containing sulfurous acid, dust and Se components, comprising a dry dust collector for removing dust in the combustion exhaust gas, a desulfurization apparatus having a desulfurization column for circulating an absorbent slurry for absorbing and removing sulfurous acid, repulsing means for dissolving the dust removed by the dry dust collector in water to form into slurry, treating agent feeding means for feeding a treating agent for making tetravalent Se insoluble into the dust slurry obtained in the repulping means, means for separating the dust slurry containing Se made insoluble into solid and liquid, and a piping system for introducing the separation liquid discharged from the separating means into the absorbent slurry.

(2) A combustion exhaust gas treatment system of (1), wherein part of the circulating liquid forming the slurry of the desulfurization system is introduced into the repulping means, and is used as the solvent in the repulping means.

(3) A combustion exhaust gas treatment system of (1) or (2), further comprising means for feeding filter additive to the dust slurry.

(4) A combustion exhaust gas treatment system for treating combustion exhaust gas containing sulfurous acid, dust and Se components, comprising a dry dust collector for removing dust in the combustion exhaust gas, a desulfurization apparatus having a desulfurization column for circulating an absorbent slurry for absorbing and removing sulfurous acid, means for introducing the dust removed by the dry dust collector into the absorbent slurry, and treating agent feeding means for feeding a treating agent for making tetravalent Se insoluble into the absorbent slurry.

(5) A combustion exhaust gas treatment system for treating combustion exhaust gas containing sulfurous acid, dust and Se components, comprising a desulfurization apparatus having a desulfurization column for circulating an absorbent slurry for absorbing and removing sulfurous acid in the combustion exhaust gas, and treating agent feeding means for feeding a treating agent for making tetravalent Se insoluble into the absorbent slurry, wherein the combustion exhaust gas is introduced directly Into the desulfurization column.

(6) A combustion exhaust gas treatment system of any one of (1) to (5), further comprising oxidation-reduction reaction control means for controlling the oxidation-reduction reaction in the desulfurization apparatus, so that the hexavalent Se mixed in the slurry in the desulfurization apparatus may be reduced by the sulfurous acid in the slurry to be tetravalent.

(7) A combustion exhaust gas treatment system for treating combustion exhaust gas containing sulfurous acid, dust and Se components, comprising a dry dust collector for removing dust in the combustion exhaust gas, a desulfurization apparatus having a cooling and dust collecting column disposed upstream of an absorption column, and possessing a desulfurization column for circulating an absorbent slurry for absorbing and removing sulfurous acid, means for feeding the dust removed by the dry dust collector into the circulating slurry in the cooling and dust collecting column, and means for feeding a treating agent for making tetravalent Se insoluble into the circulating slurry in the cooling and dust collecting column.

(8) A combustion exhaust gas treatment system for treating combustion exhaust gas containing sulfurous acid, dust and Se components, comprising a desulfurization apparatus having a cooling and dust collecting column disposed upstream of an absorption column, and possessing a desulfurization column for circulating an absorbent slurry for absorbing and removing sulfurous acid, and means for feeding a treating agent for making tetravalent Se insoluble into the circulating slurry in the cooling and dust collecting column, wherein the combustion exhaust gas is introduced directly into the cooling and dust collecting column.

(9) A combustion exhaust gas treatment system of any one of (1) to (8), further comprising a wastewater treatment apparatus for treating wastewater from the desulfurization apparatus, n treating agent feeding means for feeding a treating agent for making tetravalent Se insoluble to the impurity slurry separated by this wastewater treatment apparatus.

Any combustion exhaust gas treatment system of the second invention comprises means for feeding a treating agent for making insoluble the tetravalent Se dissolved in the slurry. The treating agent for making tetravalent Se insoluble includes, for example, FeCl.sub.3 , Fe.sub.2 (SO.sub.4).sub.3, chelating agent (e.g. Epolus MX-7 of Miyoshi Resin), and high molecular heavy metal capturing agent (e.g. Epofloc L-1 of Miyoshi Resin). The reaction of these treating agent for making insoluble tetravalent Se is described later.

In the combustion exhaust gas treatment system of (1), most of Se in the combustion exhaust gas is removed by the dry dust collector in a state being included in the dust, and the dust contacts with water or liquid (solvent) in the repulping means, and is dissolved into the liquid in the process of forming into slurry. As the slurry forming liquid (solvent), aside from the water supplied from outside the system, the treated water or slurry coming from each process in the system may be used. Of the Se components dissolved in the dust slurry, at least tetravalent Se is made insoluble by treating agent, and is discharged to the solid phase side by the separating means (into the dust cake). On the other hand, the filtrate separated by the separating means is introduced into the slurry in the desulfurization apparatus. Accordingly, hexavalent Se is also included in the combustion exhaust gas, and if it is not made insoluble by the treating agent and is dissolved in the filtrate, most of the hexavalent Se is introduced into the slurry in the desulfurization apparatus, and reacts with the sulfurous acid absorbed from the combustion exhaust gas into the slurry, and is reduced to return to tetravalent Se.

Therefore, if hexavalent Se is contained in the combustion exhaust gas, in the stationary state, Se components dissolved in the slurry in the desulfurization apparatus or the circulation liquid composing this slurry are mainly tetravalent Se components, and in the wastewater treatment apparatus for treating discharge of circulating liquid of the desulfurization apparatus, only by making this tetravalent Se insoluble, the Se elution standard may be easily satisfied, and the desulfurization apparatus can be used as reduction reaction apparatus for hexavalent Se, so that the constitution of the entire system may be simplified.

In the combustion exhaust gas treatment system of (2), since part of the circulating liquid for composing the slurry of the desulfurization apparatus in the combustion exhaust gas treatment system of (1) is used as solvent in the repulping means, as compared with the constitution of feeding water separately, the quantity (circulation) and consumption of water can be reduced.

In the combustion exhaust gas treatment system of (3), since filter additive is charged into the mixing means or separating means, the dehydrating performance in the separating means is enhanced, and a solid matter (dust cake) low in moisture content and easy to handle is obtained. As the filter additive, gypsum used in the desulfurization process or the like may be used.

In the combustion exhaust gas treatment system of (4), most of Se in combustion exhaust gas is removed by the dry dust collector in a state being contained in the dust, and is directly led into the slurry in the desulfurization apparatus, repulped in the desulfurization apparatus, and mixed with a treating agent for making insoluble. Accordingly, at least tetravalent Se components of the Se components contained in the combustion exhaust gas are mostly made insoluble directly by the treating agent in the desulfurization apparatus, and mixed in the solid matter (gypsum, etc.) separated and formed from the slurry in the desulfurization apparatus and discharged, and the remaining Se components are also easily made insoluble and solidified by the insoluble treating agent In the wastewater treatment apparatus for treating discharge of circulating liquid in the desulfurization apparatus. Besides, if hexavalent Se components are contained in the combustion exhaust gas, most of hexavalent Se reacts with the sulfurous acid absorbed from the combustion exhaust gas in the slurry in the desulfurization apparatus to be reduced to change to tetravalent Se, which is also made insoluble by the treating agent in the desulfurization apparatus and is mixed into the solid matter (gypsum, etc.) separated and formed from the slurry in the desulfurization apparatus and discharged, or easily made insoluble and solidified in the wastewater treatment apparatus.

Therefore, in this treatment system, too, the Se elution standard can be easily satisfied, and moreover since the desulfurization apparatus functions both as reducing reaction apparatus of hexavalent Se and as repulping means of dust, the constitution of the entire system may be further simplified.

In the combustion exhaust gas treatment system of (5), most of Se in combustion exhaust gas is directly introduced into the desulfurization apparatus together with the combustion exhaust gas in a state being contained in dust, and is mixed into the absorbent slurry in, for example, the absorption column of the desulfurization apparatus, and is repulped and mixed with the treating agent. Accordingly, at least tetravalent Se components of the Se components contained in the combustion exhaust gas are mostly made insoluble directly by the treating agent in the desulfurization apparatus, and mixed in the solid matter (gypsum, etc.) separated and formed from the slurry in the desulfurization apparatus and discharged, or easily made insoluble and solidified by adding an insoluble treating agent in the wastewater treatment apparatus for treating discharge of circulating liquid in the desulfurization apparatus. Besides, if hexavalent Se components are contained in the combustion exhaust gas, most of hexavalent Se reacts with the sulfurous acid absorbed from the combustion exhaust gas in the slurry in the desulfurization apparatus to be reduced to change to tetravalent Se, which is also made insoluble by the treating agent in the desulfurization apparatus and is mixed into the solid matter (gypsum, etc.) separated and formed from the slurry in the desulfurization apparatus and discharged, or easily made insoluble and solidified in the wastewater treatment apparatus.

Therefore, in this treatment system, too, the Se elution standard can be easily satisfied, and moreover since the desulfurization apparatus functions as the dust collector, reducing reaction apparatus of hexavalent Se, and repulping means of dust, the constitution of the entire system may be further simplified, as compared with the constitution for installing the dust collector, repulping means and others separately.

In the combustion exhaust gas treatment system of (6), the oxidation-reduction reaction control means controls the oxidation-reduction reaction in the desulfurization apparatus so that the hexavalent Se mixing in the slurry in the desulfurization apparatus may be reduced almost completely by sulfurous acid in the slurry to be tetravalent. Accordingly, if hexavalent SE is contained in the combustion exhaust gas, this hexavalent Se can be almost completely changed into tetravalent form in the desulfurization apparatus, so that the Se in the combustion exhaust gas may be made insoluble more easily and completely.

In the combustion exhaust gas treatment system of (7), most of Se in combustion exhaust gas is removed by the dry dust collector in a state being contained in the dust, and is directly introduced into the liquid in the cooling and dust collecting column of the desulfurization apparatus, and is repulped in the cooling and dust collecting column. In the dust slurry formed by feeding the dust containing Se into the liquid in the cooling and dust collecting column and repulping, a treating agent for making tetravalent Se insoluble is mixed. Accordingly, at least tetravalent Se components of Se components contained in the combustion exhaust gas are directly made insoluble by the treating agent in the desulfurization apparatus, and discharged into the solid-phase side by separating means (into the dust cake), or are easily made insoluble by addition of insoluble treating agent in the subsequent treatment of the separated water, so as to be discharged. Ig hexavalent Se components are contained in the Se components in the combustion exhaust gas, most of hexavalent Se components react with sulfurous acid absorbed from the combustion exhaust gas into the liquid in the cooling and dust collecting column to changed to reduced tetravalent Se components, which are also made insoluble by the treating agent, and discharged into the solid-phase side by separating means (into the dust cake), so as to be treated to be harmless.

Therefore, in this treatment system, too, the Se elution standard may be easily satisfied. Moreover, since the desulfurization apparatus functions also as reducing reaction apparatus of hexavalent Se or as repulping means of dust, the constitution of the entire system may be further simplified. In this treatment system, since dust is not mixed into the slurry in the absorption column of the desulfurization apparatus, the performance of the desulfurization apparatus such as desulfurization rate may be maintained high.

In the combustion exhaust gas treatment system of (6), most of Se in combustion exhaust gas is directly fed in the cooling and dust collecting column of the desulfurization apparatus together with the combustion exhaust gas In a state being contained in the dusts in the combustion exhaust gas, and is repulped in this cooling and dust collecting column. In the dust slurry repulped as the dust containing Se is fed into the liquid in the cooling and dust collecting column, a treating agent for making tetravalent Se insoluble is mixed. Accordingly, at least tetravalent Se components of Se components contained in the combustion exhaust gas are directly made insoluble by the treating agent in the desulfurization apparatus, and discharged to the solid-phase side (in the dust cake) by the separating means for separating the dust slurry into solid and liquid, or is easily made Insoluble by the addition of insoluble treating agent in the subsequent treatment of separated water so as to be discarded. If hexavalent Se components are contained in the combustion exhaust gas, most of these hexavalent Se components react with the sulfurous acid absorbed from the combustion exhaust gas in the liquid in the cooling and dust collecting column to transform to reduced tetravalent Se, which is also made insoluble by the treating agent, and is discharged to the solid-phase side (in the dust cake) by the separating means, and is made harmless.

Therefore, in this treatment system, too, the Se elution standard can be easily satisfied, and moreover since the desulfurization apparatus functions as the dust collector, reducing reaction apparatus of hexavalent Se, and repulsing means of dust, the constitution of the entire system may be further simplified, as compared with the constitution for installing the dust collector, repulping means and others separately. In addition, in this treatment system, dust is not mixed into the slurry In the absorption column of the desulfurization apparatus, and the performance of the desulfurization apparatus such as desulfurization rate may be maintained high.

In the combustion exhaust gas treatment system of (9), in a prior stage of treatment by solidifying the impurities in the wastewater treating device in the desulfurization apparatus, a treating agent for making tetravalent Se insoluble is added, and the Se eluting in the wastewater can be made insoluble to change into disposable form. In the combustion exhaust gas treatment system of (1) to (8), Se in the combustion exhaust gas can be almost completely made insoluble, but this system is effective in the case where Se is not to be made insoluble, for example, it is necessary to treat Se concentrated in the circulation, being dissolved in the absorption liquid slurry due to reduction of hexavalent Se, mixing of Se into the gypsum to be collected in the desulfurization process must be avoided. In the apparatus of (4) to (8), depending on the conditions of treatment, the insoluble treating agent may be supplied only in the wastewater treating apparatus.

D. Fourth Invention:

(1) A combustion exhaust gas treatment system for treating combustion exhaust gas containing dust and Se components, comprising a dust collector for removing dust from the combustion exhaust gas, and means for heating the dust removed by the dust collector to a temperature for gasification of Se in the dust.

(2) A combustion exhaust gas treatment system for treating combustion exhaust gas containing sulfurous acid, dust and Se components, comprising a dust collector for removing dust from the combustion exhaust gas, a desulfurization apparatus having an absorption column for circulating an absorbent slurry for absorbing and removing sulfurous acid, and means for heating the dust removed by the dust collector to a temperature for gasification of Se in the dust, wherein the gas generated by heating the dust by the heating means is fed into the desulfurization apparatus together with the combustion exhaust gas, and Se is dissolved and captured in the slurry in the desulfurization apparatus, and a treating agent for making the tetravalent Se insoluble is mixed in the treating process of slurry.

(3) A combustion exhaust gas treatment system of (2), further comprising oxidation-reduction reaction control means for controlling the oxidation-reduction reaction in the desulfurization apparatus, so that the hexavalent Se mixed in the slurry in the desulfurization apparatus may be reduced by sulfurous acid in the slurry to be tetravalent Se.

(4) A combustion exhaust gas treatment system for treating combustion exhaust gas containing sulfurous acid, dust and Se components, comprising a dust collector for removing dust from the combustion exhaust gas, a desulfurization apparatus having an absorption column for circulating an absorbent slurry for Absorbing and removing sulfurous acid and a cooling and dust collecting column disposed upstream of the absorption column, and means for heating the dust removed by the dust collector to a temperature for gasification of Se in the dust, wherein the gas generated by heating the dust by the heating means is fed into the desulfurization apparatus together with the combustion exhaust gas, and Se is dissolved and captured in the circulation liquid in the cooling and dust collecting column, and a treating agent for making the tetravalent Se insoluble is mixed in the treating process of the circulation liquid.

(5) A combustion exhaust gas treatment system of any one of (1) to (4), wherein the dust collecting means comprises a plurality of recovery units for separating and collecting dust, from the inlet side to the outlet side of the combustion exhaust gas, the dust collected from the recovery unit at the inlet side of the combustion exhaust gas and the dust collected from the outlet side are individually separated and collected, and only the dust separated and collected from the recovery unit at the outlet side is fed into the heating means.

(6) A combustion exhaust gas treatment system of any one of (1) to (4), further comprising sorting means for sorting the dust separated by the dust collecting means into large particle size group and small particle size group, wherein only the small particle size dust sorted by the sorting means is introduced into the heating means.

(7) A combustion exhaust gas treatment system of any one of (1) to (6), wherein the heating means is capable of heating the dust to any temperature in a range of 100 to 1200.degree. C.

In the combustion exhaust gas treatment system of (1), most of Se in combustion exhaust gas is removed by the dust collector in a state of being contained in fly ash or dust, and is heated and gasified by the heating means. Accordingly, almost no Se is left over in the dust after treatment, and the Se elution standard is satisfied, and hence it can be directly recycled as cement material or discarded.

In the combustion exhaust gas treatment system of (2), most of Se in combustion exhaust gas is removed by the dust collector in a state of being contained in fly ash or dust, and is heated and gasified by the heating means. The gasified Se is fed into the desulfurization apparatus together with the combustion exhaust gas being rid of dust, and is dissolved and captured in the absorbent slurry. In the treating process of the absorbent slurry, it is mixed with a treating agent for making tetravalent Se insoluble, and is made insoluble. That is, at least tetravalent Se is directly made insoluble by the treating agent in the desulfurization apparatus, and is discharged as being mixed in the solid matter (gypsum, etc.) separated and formed from the slurry in the desulfurization apparatus, or is made insoluble by the treating agent in the wastewater treating apparatus for treating the discharge of circulation liquid in the desulfurization apparatus, and is easily solidified.

Therefore, when the content of hexavalent Se is small in the absorption liquid in the desulfurization apparatus, only by making tetravalent Se insoluble, the Se elution standard can be satisfied without releasing Se into the atmosphere. Moreover, in the constitution where Se is separated from the dust by the heating means and is introduced into the desulfurization apparatus, without feeding the entire dust into the desulfurization apparatus, the dust can be recycled easily, and lowering of desulfurization performance in the desulfurization apparatus can be avoided.

In the combustion exhaust gas treatment system of (3), the oxidation-reduction reaction control means controls the oxidation-reduction reaction of the slurry in the desulfurization apparatus so that the hexavalent Se mixed in the slurry in the desulfurization apparatus may be almost completely reduced by the sulfurous acid in the slurry to become tetravalent Accordingly, the hexavalent Se can be changed to tetravalent almost completely in the desulfurization apparatus, and the Se in combustion exhaust gas can be easily and completely made insoluble.

In the combustion exhaust gas treatment system of (4), most of Se in combustion exhaust gas is removed by the dust collector in a state of being contained in fly ash or dust, and is heated and gasified by the heating means. The gasified Se is fed into the cooling and dust collecting column of the desulfurization apparatus together with the combustion exhaust gas being rid of dust, and is dissolved and captured in the circulation liquid. In the treating process of the circulation liquid, it is mixed with a treating agent for making tetravalent Se insoluble, and is made insoluble. That is, at least tetravalent Se is directly discharged to the solid-phase side by solid-liquid separating means or the like connected to the cooling and dust collecting column of the desulfurization apparatus, or is made insoluble by the treating agent in the wastewater treating apparatus for treating the discharge of circulation liquid in the desulfurization apparatus, and is easily solidified. Besides, most hexavalent Se reacts with sulfurous acid absorbed from the combustion exhaust gas in the liquid in the cooling and dust collecting column and is reduced to be tetravalent Se, and is made insoluble by the treating agent, and is discharged to the solid-phase side by the separating means so as to be made harmless.

Therefore, in this system, too, the Se elution standard can be satisfied easily without releasing Se into the atmosphere, and the cooling and dust collecting column of the desulfurization apparatus functions also as hexavalent Se reduction reaction facility, so that the constitution of the entire system may be simplified. Also in this system, Se or other dust rarely mixes into the slurry in the absorption column of the desulfurization apparatus, and the desulfurization performance in the desulfurization apparatus can be maintained high, and moreover gypsum of high quality can be collected as byproduct.

In the combustion exhaust gas treatment system of (5), only the dust separated and collected from a specific recovery unit at the outlet side of the combustion exhaust gas in the dust collecting means is fed into the heating means, and Se is gasified and removed, and therefore the required capacity of the heating means may be reduced. Moreover, in the subsequent desulfurization apparatus and others, the required amount of the treating agent for making Se insoluble is also reduced, so that Se is made harmless more easily and inexpensively.

That is, according to the study by the present inventors, it is known that more Se is contained (deposited) in the smaller particle size dust (ash) separated and collected from the specific recovery unit at the outlet side, and the Se is made harmless on the whole only by heating dust of smaller particle size, and applying insoluble treatment on gasified Se.

In the combustion exhaust gas treatment system of (6), Se is gasified and separated by feeding only the dust of small particle size sorted by the sorting means into the heating means, and therefore the required capacity of the heating means may be reduced. Moreover, in the subsequent desulfurization apparatus and others, the required amount of the treating agent for making Se insoluble is also reduced, so that Se is made harmless more easily and inexpensively.

That is, according to the study by the present inventors, it is known that more Se is contained (deposited) in the smaller particle size dust (ash), and the Se is made harmless on the whole only by heating dust of smaller particle size, and applying insoluble treatment on gasified Se.

In the combustion exhaust gas treatment system of (7), the heating temperature of dust by the heating means is 100 to 1200.degree. C., and therefore recondensation of gasified Se into dust is prevented, and Se can be easily removed from dust, and the Se elution standard of dust is satisfied.

E. Fifth Invention:

(1) A combustion exhaust gas treatment system for treating combustion exhaust gas containing sulfurous acid, dust and Se components, comprising a dust collector for removing dust in the combustion exhaust gas, a desulfurization apparatus having an absorption column for circulating an absorbent slurry for absorbing and removing sulfurous acid, means for mixing a treating agent for making at least tetravalent Se insoluble, to a circulation liquid composing the absorbent slurry extracted from the desulfurization apparatus, and means for spraying the circulation liquid mixed with the treating agent by the mixing agent, into a combustion exhaust gas lead-in passage upstream of the dust collector.

(2) A combustion exhaust gas treatment system for treating combustion exhaust gas containing sulfurous acid, dust and Se components, comprising a desulfurization apparatus having an absorption column for circulating an absorbent slurry for absorbing and removing sulfurous acid in the combustion exhaust gas, means for mixing a treating agent for making at least tetravalent Se insoluble, to a circulation liquid composing the absorbent slurry extracted from the desulfurization apparatus, and means for spraying the circulation liquid mixed with the treating agent by the mixing agent, into a combustion exhaust gas lead-in passage upstream of the desulfurization apparatus, wherein the combustion exhaust gas is directly fed into the desulfurization apparatus.

(3) A combustion exhaust gas treatment system for treating combustion exhaust gas containing sulfurous acid, dust and Se components, comprising a desulfurization apparatus having an absorption column for circulating an absorbent slurry for absorbing and removing sulfurous acid in the combustion exhaust gas, with a cooling and dust collecting column disposed upstream of the absorption column, means for separating the circulation slurry extracted from the cooling and dust collecting column into solid and liquid, means for mixing a treating agent for making at least tetravalent Se insoluble, to the separated liquid discharged from the separating means, and means for spraying the separated liquid mixed with the treating agent by the mixing agent, into a combustion exhaust gas lead-in passage upstream of the desulfurization apparatus, wherein the combustion exhaust gas is directly fed into the cooling and dust collecting column of the desulfurization apparatus.

(4) A combustion exhaust gas treatment system of any one of (1) to (3), further comprising oxidation-reduction reaction control means for controlling the oxidation-reduction reaction in the desulfurization apparatus so that the hexavalent Se mixed in the slurry in the desulfurization apparatus may be reduced by the sulfurous acid in the slurry to become tetravalent.

In the combustion exhaust gas treatment system of (1), most of Se in combustion exhaust gas is removed by the dust collector in a state being contained in the dust, but at least tetravalent Se thereof is, before being removed by the dust collector, made insoluble by reacting with the treating agent for making at least tetravalent Se insoluble (hereinafter merely called treating agent) mixed in the circulation liquid of the desulfurization apparatus sprayed into the combustion exhaust gas lead-in passage by the spraying means. Accordingly, when the other Se content than tetravalent Se is less, the Se elution standard may be satisfied if the dust after treatment may be directly recycled or discarded.

If hexavalent and other Se or other impurities mix into the slurry in the desulfurization apparatus, most of hexavalent Se reacts with sulfurous acid absorbed from the combustion exhaust gas in the slurry in the desulfurization apparatus, and is reduced to change to tetravalent Se, and mainly tetravalent Se exists in the circulation liquid in the desulfurization apparatus. Consequently, this Se and other impurities are led into the mixing means as the circulation liquid is extracted, and mixed with the treating agent, and sprayed into the combustion exhaust gas lead-in passage, and most Se is removed by the dust collector, together with dust, in an insoluble state. If Se and other impurities are slightly mixed into the desulfurization apparatus without being removed by the dust collector, the Se and other impurities are prevented from being accumulated excessively in the circulation liquid of the desulfurization apparatus by the functions of the mixing means and spraying means, so that the wastewater treating apparatus for treating the wastewater of the desulfurization apparatus is not needed.

Moreover, by using the desulfurization apparatus having a cooling and dust collecting column upstream of the absorption column, fine dust particles not captured by the electrostatic precipitator can be captured in the cooling and dust collecting column, and hardly mix into the slurry of the absorption column, and therefore higher desulfurization performance is achieved, and the collected gypsum is higher in quality.

In the combustion exhaust gas treatment system of (2), most Se In combustion exhaust gas is directly fed into the desulfurization apparatus, together with the combustion exhaust gas, in a state being contained in the dust, but at least tetravalent Se thereof is made insoluble by reacting with the treating agent mixed in the circulation liquid of the desulfurization apparatus sprayed into the combustion exhaust gas lead-in passage by the spraying means. Accordingly, at least tetravalent Se contained In the combustion exhaust gas is directly discharged as being mixed into the solid matter (gypsum, etc.) separated and formed from the slurry in the desulfurization apparatus.

If hexavalent Se mixed into the desulfurization apparatus, most of the hexavalent Se reacts with sulfurous acid absorbed in the slurry in the desulfurization apparatus, and is reduced to be tetravalent Se, and also finally reacts with the treating agent added by the mixing means to be insoluble, and is discharged as being mixed in the solid matter (dust cake, etc.) separated and formed in the desulfurization apparatus.

Therefore, in this system, too, the Se elution standard ban be easily satisfied, and without requiring wastewater treating apparatus, the Se and others are prevented from being accumulated excessively in the absorption liquid in the desulfurization apparatus.

In the combustion exhaust gas treatment system of (3), most Se in combustion exhaust gas is directly fed into the cooling and dust collecting column of the desulfurization apparatus, together with the combustion exhaust gas, in a state being contained in the dust, but at leas