WO2012176634A1 - Exhaust gas treatment apparatus and orp control method therefor - Google Patents

Abstract

This exhaust gas treatment apparatus has: a redox agent supplying means for supplying a redox agent, which produces hydrogen chloride and ammonia when vaporized, to a discharge flue for the exhaust gas from a boiler; a reduction/denitration device that has a denitration catalyst for reducing the nitrogen oxide in the exhaust gas using ammonia and oxidizing the mercury in the presence of hydrogen chloride; a wet desulfurization device for absorbing and removing the sulfur oxide in the exhaust gas and the oxidized mercury in the reduction/denitration device using an absorbent; an oxidation reduction potential (ORP) meter for measuring the ORP of the absorbent; a solid-liquid separating means for separating the solids and the mercury from the liquids in the desulfurization waste water discharged from the wet desulfurization device; an electrolyser for electrolysing the separated liquid separated by the solid-liquid separating means to produce a hypochlorous acid; and a controlling means for supplying the hypochlorous acid to the absorbent to control the ORP of the absorbent.

Description

Exhaust gas treatment device and ORP control method of exhaust gas treatment device

The present invention relates to an exhaust gas treatment device for removing mercury contained in exhaust gas from a boiler and an ORP control method for the exhaust gas treatment device.

Because exhaust gas discharged from boilers, which are combustion devices such as thermal power plants, may contain metal mercury (Hg ⁰ ) in addition to soot, sulfur oxides (SOx) and nitrogen oxides (NOx) Conventionally, various devices and methods for removing mercury in exhaust gas have been devised.

Usually, a boiler is provided with a wet desulfurization device for removing sulfur content in exhaust gas. In a flue gas treatment facility in which a desulfurization device is attached to such a boiler as an exhaust gas treatment device, since divalent mercury oxide is soluble in water, mercury can be easily collected by the desulfurization device. Widely known.

Thus, in recent years, various devices and methods for treating this metallic mercury have been devised in combination with a denitration device that reduces NOx and a wet desulfurization device that uses an alkaline absorbent as an SOx absorbent.

As a method for treating metallic mercury in a large volume of exhaust gas, as a conventional desulfurization method, lime--using a gas-liquid contact type desulfurization apparatus mainly by a reaction shown in the following formulas (1) and (2): The plaster method is used.
SO ₂ + CaCO ₃ + 1 / 2H ₂ O → CaSO ₃ · 1 / 2H ₂ O + CO ₂ (absorption) (1)
CaSO ₃ .1 / 2H ₂ O + 3 / 2H ₂ O + 1 / 2O ₂ → CaSO ₄ .2H ₂ O (oxidation) (2)

In the gas-liquid contact type desulfurization apparatus, mercury oxide (Hg ²⁺ ) is adsorbed and absorbed in a lime gypsum slurry absorbing liquid (hereinafter also referred to as “slurry”, “slurry absorbing liquid” or “alkali absorbing liquid”). Fixed and removed mercury. At this time, by adding an oxidizing agent to the slurry absorbing liquid, the slurry absorbing liquid is brought into an oxidized state to prevent reduction of mercury oxide (Hg ²⁺ ) (Hg ²⁺ → Hg ⁰ ), and to the gas phase. A method for suppressing re-scattering of zero-valent mercury (Hg ⁰ ) has been proposed (see, for example, Patent Document 1).

As a method of supplying the oxidizing agent to the slurry absorbing solution, a part of the slurry absorbing solution is extracted in order to suppress oxidation inhibition by the nitrogen-sulfur compound (hereinafter referred to as NS compound) contained in the slurry absorbing solution. A flue gas desulfurization method has been proposed in which the filtrate after solid-liquid separation is electrolyzed in the presence of chloride ions and supplied to a desulfurization apparatus as make-up water (see, for example, Patent Document 2).

US Pat. No. 7,758,829 Japanese Patent No. 2695680

However, since the flue gas desulfurization apparatus and method described in Patent Document 1 add an oxidizing agent to the slurry absorbent, the oxidizing agent added by the reducing substance (oxidation inhibitor) contained in the slurry absorbent is not included. There is a problem in that the oxidant must be supplied excessively in order to be consumed.

In addition, if a reducing substance (oxidation inhibitor) is contained in the slurry absorbent, an appropriate amount of oxidizing agent is added to control the oxidation-reduction potential (ORP) within a suitable range. There is a problem that it is difficult.

In addition, the flue gas desulfurization method described in Patent Document 2 is for electrolyzing the filtrate of the slurry absorbent and supplying it to the desulfurization apparatus. However, in order to decompose the NS compound contained in the slurry absorbent. In addition, the pH of the slurry absorbing solution or the slurry absorbing solution is adjusted to 3 to 4, and the redox potential (ORP) is not controlled for the purpose of removing mercury contained in the exhaust gas.

Furthermore, in a system that does not drain desulfurization drainage as in the flue gas desulfurization method described in Patent Document 2 (no drainage), there is a problem that chloride ions for generating a chloric acid compound by electrolysis are insufficient. is there.

Therefore, when there is a reducing substance (oxidation inhibitor) in the exhaust gas and the slurry absorption liquid, a predetermined oxidation state (oxidation reduction potential (ORP) value is 100 mV or more and 200 mV or less) with an appropriate amount of oxidant. Therefore, it is desired to efficiently remove mercury in exhaust gas by suppressing re-scattering of zero-valent mercury (Hg ⁰ ) into the gas phase.

The present invention has been made in view of the above, and it is possible to efficiently remove mercury contained in exhaust gas from a boiler, and to suppress oxidation inhibition in desulfurization, and an ORP of the exhaust gas treatment device. An object is to provide a control method.

In order to solve the above-described problems and achieve the object, an exhaust gas treatment apparatus according to claim 1 of the present invention generates hydrogen chloride and ammonia when vaporized in a flue for exhausting exhaust gas from a boiler. A reduction oxidation auxiliary agent supplying means for supplying a reduction oxidation auxiliary, a reduction denitration apparatus having a denitration catalyst for reducing nitrogen oxides in the exhaust gas with ammonia and oxidizing mercury in the presence of hydrogen chloride; and in the exhaust gas The sulfur oxides and mercury oxidized in the reductive denitration device are absorbed and removed by the absorption liquid, the redox potential meter for measuring the redox potential of the absorption liquid, and the wet desulfurization apparatus. Solid-liquid separation means for separating the solid and mercury in the desulfurization effluent and the liquid, and electrolysis for generating hypochlorous acid by electrolyzing the separated liquid separated by the solid-liquid separation means And having a location, and control means for controlling the oxidation-reduction potential of the absorbing solution by supplying the hypochlorous acid to the absorption liquid, the.

The exhaust gas treatment apparatus according to claim 2 of the present invention is the exhaust gas treatment apparatus according to claim 1, wherein the separation liquid is a dehydrated filtrate separated by the solid-liquid separation means, or a heavy metal is removed from the dehydrated filtrate. It is a liquid.

An exhaust gas treatment apparatus according to a third aspect of the present invention is the exhaust gas treatment apparatus according to the first or second aspect, wherein the electrical conductivity measuring means for measuring the electrical conductivity of the separation liquid and the oxidant raw material are supplied to the electrolysis apparatus. An oxidant raw material supply means for supplying the oxidant raw material when the measured electrical conductivity is equal to or lower than a predetermined value.

The exhaust gas treatment apparatus according to claim 4 of the present invention is the exhaust gas treatment apparatus according to any one of claims 1 to 3, wherein the control means sets the oxidation-reduction potential of the absorption liquid to a range of 100 mV to 200 mV, or the absorption. The sulfite ion in the liquid is controlled to be 0.1 mmol / L or more and 2.0 mmol / L or less.

An exhaust gas treatment apparatus according to claim 5 of the present invention is the exhaust gas treatment apparatus according to any one of claims 1 to 4, wherein the hypochlorous acid, the dehydrated filtrate, or one or both of the treatment liquids, A mixing tank for mixing the mixed liquid and a mixed liquid return line for returning the mixed liquid mixed in the mixing tank to the wet desulfurization apparatus, and the control means measures an oxidation-reduction potential of the mixed liquid. The hypochlorous acid is supplied to the mixed solution so that the redox potential of the mixed solution is in the range of 100 mV to 200 mV, or the sulfite ion in the mixed solution is 0.1 mmol / L to 2.0 mmol / L. Control is as follows.

An exhaust gas treatment apparatus according to claim 6 of the present invention is the exhaust gas treatment apparatus according to any one of claims 1 to 5, wherein the electrolyzer generates hypochlorous acid from the separated liquid with a pulsed current. Features.

Moreover, the ORP control method of the exhaust gas treatment apparatus according to claim 7 of the present invention is a reduction in which a reducing oxidation assistant that generates hydrogen chloride and ammonia when vaporized is supplied into a flue that exhausts exhaust gas from a boiler. An oxidizing aid supplying step, a reducing denitration step having a denitration catalyst that reduces nitrogen oxides in the exhaust gas with ammonia and oxidizes mercury in the presence of hydrogen chloride, sulfur oxides in the exhaust gas and the reductive denitration Wet desulfurization step of absorbing and removing mercury oxidized in the process by the absorption liquid, oxidation-reduction potential measurement step of measuring the oxidation-reduction potential of the absorption liquid, solid content in the desulfurization effluent discharged from the wet desulfurization process, and A solid-liquid separation step for separating mercury from a liquid component, an electrolysis step for electrolyzing the separated liquid separated in the solid-liquid separation step to produce hypochlorous acid, and the absorption The supplied hypochlorous acid and having a control step of controlling the redox potential of the absorbing solution to.

An ORP control method for an exhaust gas treatment apparatus according to claim 8 of the present invention is the above-described ORP control method according to claim 7, wherein the separation liquid is a dehydrated filtrate separated in the solid-liquid separation step, or heavy metal from the dehydrated filtrate. The treatment liquid is removed.

Further, the ORP control method for an exhaust gas treatment apparatus according to claim 9 of the present invention is the method according to claim 7 or 8, wherein the electrical conductivity measurement step for measuring the electrical conductivity of the separated liquid and the oxidation step for oxidation are performed. An oxidant raw material supply step for supplying an oxidant raw material, wherein the oxidant raw material supply step supplies the oxidant raw material when the measured electrical conductivity is equal to or lower than a predetermined value.

An ORP control method for an exhaust gas treatment apparatus according to a tenth aspect of the present invention is the method according to any one of the seventh to ninth aspects, wherein the control step sets the oxidation-reduction potential of the absorbent to a range of 100 mV to 200 mV. Alternatively, the sulfite ion in the absorbing solution is controlled to be 0.1 mmol / L or more and 2.0 mmol / L or less.

An exhaust gas treatment apparatus ORP control method according to an eleventh aspect of the present invention is the method according to any one of the seventh to tenth aspects, wherein the hypochlorous acid, the dehydrated filtrate, or the treatment liquid is selected. Or a mixing step for mixing the two, and a mixed solution returning step for returning the mixed solution mixed in the mixing step to the wet desulfurization step, and the control step is an oxidation-reduction of the mixed solution The potential is measured, and the hypochlorous acid is supplied to the mixed solution so that the oxidation-reduction potential of the mixed solution is in the range of 100 mV to 200 mV, or the sulfite ion in the mixed solution is 0.1 mmol / L to 2 It is characterized by being controlled to 0.0 mmol / L or less.

An ORP control method for an exhaust gas treatment apparatus according to claim 12 of the present invention provides the ORP control method according to any one of claims 7 to 11, wherein the electrolysis step generates hypochlorous acid from the separated liquid with a pulsed current. It is characterized by generating.

According to the exhaust gas treatment device and the ORP control method of the exhaust gas treatment device of the present invention, it is possible to efficiently remove mercury contained in the exhaust gas from the boiler and to suppress the inhibition of oxidation during desulfurization.

FIG. 1 is a configuration diagram of an exhaust gas treatment apparatus according to the first embodiment. FIG. 2 is a configuration diagram of the exhaust gas treatment apparatus according to the second embodiment. FIG. 3 is a configuration diagram of the exhaust gas treatment apparatus according to the third embodiment. FIG. 4 is a configuration diagram of the exhaust gas treatment apparatus according to the fourth embodiment. FIG. 5 is a configuration diagram of the exhaust gas treatment apparatus according to the fourth embodiment. FIG. 6 is a configuration diagram of the exhaust gas treatment apparatus according to the fourth embodiment.

Hereinafter, embodiments of the exhaust gas treatment device and the ORP control method of the exhaust gas treatment device according to the present invention will be described in detail with reference to the drawings. In addition, this invention is not limited by the content described in the following Examples. In addition, constituent elements in the embodiments described below include those that can be easily assumed by those skilled in the art, those that are substantially the same, and those in the so-called equivalent range. Furthermore, the constituent elements disclosed in the embodiments described below can be appropriately combined.

<Exhaust gas treatment equipment>
The configuration of the exhaust gas treatment apparatus according to this embodiment will be described. FIG. 1 is a schematic configuration diagram of an exhaust gas treatment apparatus 10A according to the present embodiment. As shown in FIG. 1, the exhaust gas treatment device 10 </ b> A according to the present embodiment is an exhaust gas treatment device 10 </ b> A that removes NOx, SOx, and Hg contained in the exhaust gas 22 generated from the boiler 11. The exhaust gas 22 discharged from the boiler 11 that combusts the fuel 21 is purified through the processes in the reduction denitration device 12, the air preheater 13, the dust collector 14, and the wet desulfurization device 15, and then the chimney 16. Discharged to the outdoors.

The exhaust gas treatment apparatus 10 </ b> A includes a reduction oxidation aid supply means 31 for spraying an NH ₄ Cl solution containing ammonium chloride (NH ₄ Cl) as the reduction oxidation aid 32 in the flue 23 downstream of the boiler 11, and the exhaust gas 22. NOx contained therein is reduced with NH ₃ gas, and a reduction denitration device 12 having a denitration catalyst that oxidizes metallic mercury (Hg ⁰ ) in the presence of HCl gas, and an air preheater (AH) that exchanges heat between the denitrated exhaust gas 22 ： Air heater) 13, dust collector (ESP: Electrostatic Precipitator) 14 for removing dust in the denitrated exhaust gas 22, SOx in the exhaust gas 22, and divalent Hg ² oxidized in the reductive denitration device 12 ⁺ a a wet desulfurization system 15 be removed using lime gypsum slurry (alkaline absorbing solution) 34, the lime gypsum slurry 34 used for desulfurization reaction is extracted from the bottom of the column 15a, is solids A solid-liquid separation means 41 for solid-liquid separation and silver dehydrated cake containing (HgSO _4, etc.) (gypsum) 42 and dried filtrate (separated liquid) 46, separated dewatered filtrate (separated liquid) 46 Hypochlorous acid (HClO) is generated by electrolyzing chlorine ions (Cl ⁻ ) contained in the supply line L ₄ supplied to the electrolyzer 54 for reuse and the separation liquid (dehydrated filtrate) 46. the electrolyzer 54, a supply line L ₆ supplied to the wet desulfurization system 15 as a solution 55 of hypochlorite oxidizing agent generated by electrolysis device 54 (hypochlorous acid), not reused dehydrated filtrate (Separation liquid) 36, wastewater treatment equipment 43 for performing wastewater treatment such as removal of suspended matters and heavy metals, pH adjustment of the dehydrated filtrate (separation liquid) 36 to be discharged, and oxidizing agent (hypochlorous acid) The electric conductivity of the solution 55 is measured by an electric conductivity (EC) meter 61. The oxidant raw material (chlorine compound) 53 is supplied from the oxidant raw material supply means 52 into the electrolyzer 54, and the redox potential of the lime gypsum slurry (alkali absorbing liquid) 34 is measured by the redox potentiometer 58. And control means 51 for controlling. In FIG. 1, V ₁ , V ₂ , and V ₄ indicate open / close valves.

(Reducing oxidation aid supply means)
The reductive oxidation aid supplying means 31 uses a reductive denitration device 12 as a reductive oxidation aid 32 on the downstream side of the boiler 11 with an NH ₄ Cl solution containing ammonium chloride (NH ₄ Cl) adjusted to a predetermined concentration in the solution tank. Is supplied into the flue 23 on the upstream side.

In the present embodiment, NH ₄ Cl is used as an example of the reduction oxidation aid 32, but this embodiment is not limited to this, and the reduction oxidation aid 32 is oxidized with an oxidizing gas when vaporized. Any substance capable of producing reducing gas can be used. In the present embodiment, the reduction oxidation assistant 32 is an oxidation (Hg ⁰ → Hg ²⁺ ) of metallic mercury (Hg ⁰ ) contained in the exhaust gas 22 in the presence of an oxidizing gas (HCl gas) on the denitration catalyst. ) And an oxidizing aid used to reduce NOx contained in the exhaust gas 22 with a denitration catalyst using a reducing gas. In this embodiment, HCl gas is used as the oxidizing gas, and NH ₃ gas is used as the reducing gas.

The NH ₄ Cl solution is supplied from the reduction oxidation aid supply means 31 to the exhaust gas 22 discharged from the boiler 11. The reduction oxidation auxiliary agent supply means 31 sprays the NH ₄ Cl solution in a liquid state on the exhaust gas 22 and is contained in the exhaust gas 22 on the denitration catalyst filled in the denitration catalyst layer filled in the reduction denitration device 12. NOx is reduced and Hg ⁰ is oxidized.

The droplets of the NH ₄ Cl solution sprayed into the flue 23 from the nozzle head of the spraying device are evaporated and vaporized by the high-temperature atmosphere temperature of the exhaust gas 22 to generate fine NH ₄ Cl solid particles. As in (3), it decomposes into HCl and NH ₃ and sublimes. Accordingly, the NH ₄ Cl solution sprayed from the spray device is decomposed to generate HCl and NH ₃ , and NH ₃ gas and HCl gas can be supplied into the flue 23.
NH ₄ Cl → NH ₃ + HCl (3)

NH ₃ gas acts as a reducing agent, and HCl gas acts as a mercury chlorinating agent (oxidation aid). That is, on the denitration catalyst filled in the reductive denitration apparatus 12, NH ₃ gas undergoes a reduction reaction with NOx in the exhaust gas 22 as shown in the following equation (4), and HCl gas as shown in the following equation (5). In addition, the oxidation reaction proceeds with Hg ⁰ in the exhaust gas 22. HCl gas reductively denitrates NH ₃ on a denitration catalyst, oxidizes metallic mercury (Hg ⁰ ) (Hg ⁰ → Hg ²⁺ ) to form water-soluble mercury chloride (HgCl ₂ ), and is installed on the downstream side The wet desulfurization apparatus 15 dissolves HgCl ₂ in water to remove mercury contained in the exhaust gas 22.
4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O (4)
Hg ⁰ + 1 / 2O ₂ + 2HCl → HgCl ₂ + H ₂ O (5)

In this example, ammonium halides such as ammonium bromide (NH ₄ Br) and ammonium iodide (NH ₄ I) other than NH ₄ Cl were used as the reduction oxidation aid 32, and a solution dissolved in water was used. It may be used. Further, a mixed solution of NH ₄ Cl and aqueous ammonia or hydrochloric acid may be used. Moreover, although the reduction oxidation aid 32 has been described as a liquid, the present invention is not limited to this, and a gas (for example, hydrogen chloride gas and ammonia gas) may be supplied.

In this embodiment, instead of the reduction oxidation aid 32, an oxidation aid and a reduction agent may be supplied. An HCl solution can be exemplified as the oxidation aid, but the present embodiment is not limited to this, and the oxidation aid can be used as long as it generates an oxidizing gas when vaporized. . Examples thereof include hydrogen halides such as hydrogen bromide (HBr) and hydrogen iodide (HI). Moreover, although the oxidation aid has been described as a liquid, the present invention is not limited to this, and a gas (for example, hydrogen chloride gas) may be supplied. An NH ₃ solution can be exemplified as the reducing agent, but the present embodiment is not limited to this, and any reducing agent that generates a reducing gas when vaporized can be used. For example, urea ((NH ₂ ) ₂ CO) can be used. Although the reducing agent has been described as a liquid, the present invention is not limited to this, and a gas (for example, ammonia gas) may be supplied.

As a means for supplying the NH ₄ Cl solution, the HCl solution, and the NH ₃ solution into the flue 23 of the boiler 11, for example, a two-fluid nozzle or the like may be used. In addition, a present Example is not limited to this, You may use the injection nozzle for normal liquid spraying. The supply amounts of the NH ₄ Cl solution, the HCl solution, and the NH ₃ solution can be arbitrarily adjusted.

Although the temperature of the exhaust gas 22 in the flue 23 depends on the combustion conditions of the boiler 11, it is preferably 320 ° C. or higher and 420 ° C. or lower, more preferably 320 ° C. or higher and 380 ° C. or lower, and further preferably 350 ° C. or higher and 380 ° C. or lower. . This is because NOx denitration reaction and Hg oxidation reaction can be efficiently generated on the denitration catalyst in these temperature zones.

In the present embodiment, any one or more of the reducing oxidation assistant (NH ₄ Cl solution) 32, the oxidation assistant (HCl solution), and the reducing agent (NH ₃ solution) are provided on the downstream side of the boiler 11. It can be supplied into the flue 23 on the upstream side of the reductive denitration device 12. For example, when the HCl component is insufficient in the NH ₄ Cl solution due to the content of the exhaust gas 22 component, two of the NH ₄ Cl solution and the HCl solution may be supplied, or when the HN ₃ component is insufficient May supply two of NH ₄ Cl solution and NH ₃ solution. Further, two of an HCl solution and an NH ₃ solution may be supplied. Further solution _of NH ₄ Cl, HCl solutions, may be supplied with three NH ₃ solution. Further, NOx only supply of the reducing oxidizing aid (NH ₄ Cl solution) 32, SOx, if possible removal of Hg is, ₍₄ Cl solution NH) reducing oxidizing aid 32 one may be supplied.

Therefore, in the exhaust gas treatment apparatus 10A according to the present embodiment, the supply amounts of the NH ₄ Cl solution, the HCl solution, and the NH ₃ solution are obtained by determining the contents of NOx, SOx, and Hg contained in the exhaust gas 22, respectively. , Can be adjusted by its value. Further, FIG. 1 shows a configuration in which the reduction oxidation assistant (NH ₄ Cl solution) 32 is supplied. However, the present invention is not limited to this, and the above three solutions are mixed and supplied. May be. In addition to the HCl solution, NH ₃ solution, and NH ₄ Cl solution, a solution in which the reduction oxidation aid 32 is dissolved, a solution in which the reduction agent is dissolved, and a solution in which the oxidation aid (mercury chlorinating agent) is dissolved. Further, a plurality of these may be mixed and supplied.

(Reduction denitration equipment)
The reductive denitration device 12 has a denitration catalyst (not shown) that reduces NOx in the exhaust gas 22 with NH ₃ gas and oxidizes metallic mercury (Hg ⁰ ) in the presence of HCl gas. As shown in FIG. 1, the exhaust gas 22 contains, for example, HCl gas and NH ₃ gas generated from droplets of the NH ₄ Cl solution sprayed into the flue 23 from the reducing oxidation assistant supply means 31, The reduced denitration device 12 is fed. In the reductive denitration device 12, NH ₃ gas generated by decomposition of NH ₄ Cl is used for NOx reductive denitration, and HCl gas is used for Hg oxidation, and NOx and Hg are removed from the exhaust gas 22.

That is, NH ₃ gas reduces and denitrates NOx as in the following formula (6) on the denitration catalyst filled in the denitration catalyst layer filled in the reductive denitration device 12, and HCl gas represents the following formula (7 Hg is oxidized (chlorinated) as shown in FIG.
4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O (6)
Hg + 1 / 2O ₂ + 2HCl → HgCl ₂ + H ₂ O (7)

The reduction denitration device 12 includes one denitration catalyst layer, but the present embodiment is not limited to this, and the reduction denitration device 12 appropriately changes the number of denitration catalyst layers according to the denitration performance. be able to.

After the NOx reduction in the exhaust gas 22 and the oxidation of Hg are performed in the reductive denitration device 12, the exhaust gas 22 removes dust in the exhaust gas 22 after recovering the air preheater 13 that recovers the heat in the exhaust gas 22. After passing through a dust collector (ESP) 14 and dedusting, it is sent to a wet desulfurization apparatus 15 for desulfurization treatment. Examples of the dust collector 14 include, but are not limited to, an inertial dust collector, an electric dust collector, a centrifugal dust collector, a filtration dust collector, and a cleaning dust collector.

(Wet desulfurization equipment)
The wet desulfurization apparatus 15 removes SOx and oxidized divalent Hg ²⁺ in the exhaust gas 22 after removing the dust in a wet manner. In the wet desulfurization apparatus 15, the exhaust gas 22 is fed from the wall surface side of the tower bottom in the apparatus main body, and the lime gypsum slurry 34 used as the alkali absorption liquid is supplied from the lime gypsum slurry supply means 33 into the apparatus main body from the slurry absorption liquid supply line. And jetted from the nozzle 15c toward the tower top side 15b. The exhaust gas 22 rising from the bottom side in the apparatus main body and the lime gypsum slurry 34 jetted from the nozzle 15 c are brought into gas-liquid contact with each other, and HgCl ₂ and SOx in the exhaust gas 22 are in a lime gypsum slurry 34. It is absorbed in, separated and removed from the exhaust gas 22, and the exhaust gas 22 is purified. The exhaust gas 22 purified by the lime gypsum slurry 34 is discharged from the tower top side 15b, and then discharged from the chimney 16 to the outside (hereinafter referred to as “outside of the system”) as the treated exhaust gas 24.

Lime gypsum slurry 34 used for the desulfurization reaction of the exhaust gas 22 is supplied to the tower bottom 15a of the wet desulfurization apparatus 15 from the slurry absorbent supply line. Limestone gypsum slurry 34, a lime slurry CaCO ₃ dissolved limestone powder in water, SOx lime and the flue gas 22 and then the gypsum slurry CaSO ₄ obtained by further oxidation reaction, is produced by mixing with water. As the lime gypsum slurry 34, for example, a pumped liquid (L ₁ ) of the liquid stored in the tower bottom 15 a of the apparatus main body of the wet desulfurization apparatus 15 is used. In the apparatus main body, SOx in the exhaust gas 22 reacts with lime (CaCO ₃ ) in the lime-gypsum slurry 34 as shown in the following formula (8).
CaCO ₃ + SO ₂ + 0.5H ₂ O → CaSO ₃ .0.5H ₂ O + CO ₂ (8)

On the other hand, the lime-gypsum slurry 34 that has absorbed SOx in the exhaust gas 22 is mixed with water 35 supplied into the apparatus main body, and an oxidant (hypochlorous acid) containing oxygen supplied to the tower bottom 15a of the apparatus main body. The solution 55 is oxidized. At this time, the lime gypsum slurry 34 that has flowed down in the apparatus main body undergoes a reaction such as the following formula (9) with water 35 and oxygen (air).
CaSO ₃ · 0.5H ₂ O + 0.5O ₂ + 1.5H ₂ O → CaSO ₄ · 2H ₂ O (9)

In this way, SO _x in the exhaust gas 22 is captured in the form of gypsum CaSO ₄ .2H ₂ O in the wet desulfurization apparatus 15. At this time, since mercury chloride (HgCl ₂ ) in the exhaust gas 22 is water-soluble, it is transferred to the lime gypsum slurry 34 side.

In this embodiment, lime gypsum slurry 34 is used as the alkali absorbing liquid, but other solutions can be used as the alkali absorbing liquid as long as it can absorb HgCl ₂ in the exhaust gas 22. Examples of the alkali absorbing solution other than the lime gypsum method include sodium hydroxide solution, sodium sulfite solution, ammonia water, magnesium hydroxide solution and the like. In this case, as a method for promoting the removal of mercury, for example, a method of mixing a chelating agent or a polymer flocculant as a heavy metal scavenger can be used in combination.

The method of supplying the lime gypsum slurry 34 is not limited to the method of jetting from the nozzle 15c toward the tower top side 15b, and may be caused to flow downward from the nozzle 15c so as to face the exhaust gas 22, for example.

(Solid-liquid separation means)
In this embodiment, the solid-liquid separation means 41 is provided on the downstream side of the wet desulfurization apparatus 15. The solid-liquid separation means 41 extracts the lime gypsum slurry 34 used for the desulfurization reaction stored in the tower bottom 15a of the wet desulfurization apparatus 15 from the tower bottom 15a (L ₂ ), and a mercury salt that is a solid content by solid-liquid separation processing. It is separated into a dehydrated cake (gypsum) 42 containing (HgSO ₄ or the like) and a separation liquid (dehydrated filtrate) 46 which is a liquid component.

As the solid-liquid separation means 41, for example, a belt filter, a gravity precipitation concentration tank, a liquid cyclone, a centrifuge, a decanter type centrifugal sedimentator, or the like is used. The separated dehydrated cake (gypsum) 42 is discharged out of the system of the exhaust gas treatment apparatus 10A. The separation liquid (dehydrated filtrate) 46 is supplied to the electrolysis apparatus 54 through the supply line L _4, and a solution 55 of hypochlorous acid (oxidant) generated by electrolysis in the electrolysis apparatus 54 is obtained. It is supplied to the wet desulfurization apparatus 15 (L ₆ ) and reused. The remaining separation liquid (dehydrated filtrate) 36 that has not been reused is fed (L ₃ ) to the wastewater treatment device 43 and subjected to wastewater treatment.

(Wastewater treatment equipment)
In this embodiment, a waste water treatment device 43 is provided on the downstream side of the solid-liquid separation means 41. The waste water treatment device 43 removes suspended matter, heavy metals, mercury, etc. 44 in the remaining separation liquid (dehydrated filtrate) 36 that has not been supplied from the solid-liquid separation means 41 to the electrolysis apparatus 54. The suspended matter, heavy metal, mercury, etc. 44 in the removed separated liquid (dehydrated filtrate) 36 are discharged out of the system. The treatment liquid from which the suspended matter, heavy metal, mercury, and the like 44 have been removed is subjected to wastewater treatment such as pH adjustment. This waste water-treated treatment liquid is treated as waste water 45.

Examples of the treatment method of the suspended matter, heavy metal, mercury and the like 44 in the waste water treatment apparatus 43 include a precipitation method, an ion exchange method, an adsorption method, and the like. Examples of the precipitation method include, for example, a method using a mixed liquid of ferrous sulfate and sodium sulfide (precipitation method), a chelating agent or an aggregating agent as a heavy metal scavenger, or a method using “mercury chelate” which selectively reacts with mercury. And so on. Examples of the ion exchange method include a method using an ion exchanger. Examples of the adsorption method include activated carbon adsorption and a method using a chelate resin. The present embodiment is not limited to the above processing method.

Further, final treatment such as waste water treatment and detoxification treatment as described above may be performed by a final treatment apparatus (not shown). As the waste water treatment in this final treatment device, a known final treatment device such as a sulfide coagulation sedimentation treatment device, a chelating agent treatment device, a chelate resin treatment device, an ion exchange resin treatment device, or an activated carbon treatment device is applied. do it. Moreover, as a detoxification processing apparatus which is not illustrated, a cement solidification processing apparatus can be mentioned, for example.

(Electrolysis device)
In the present embodiment, an electrolyzer 54 that supplies the wet desulfurizer 15 after electrolyzing the separated liquid (dehydrated filtrate) 46 processed by the solid-liquid separator 41 is provided. The electrolyzer 54 electrolyzes chlorine ions (Cl ⁻ ) contained in the separated liquid (dehydrated filtrate) 46 to generate hypochlorous acid (HClO). Hypochlorous acid generated in the electrolysis apparatus 54 is supplied to the wet desulfurization apparatus 15 as a solution 55 of an oxidizing agent (hypochlorous acid) (L ₆ ). The supply amount of the oxidant (hypochlorous acid) solution 55 supplied to the wet desulfurization apparatus 15 is adjusted by an open / close valve V ₄ controlled by the control means 51.

In the present embodiment, a separated liquid (dehydrated filtrate) 46 containing abundant chlorine ions (for example, about 20000 mg / L) is used as the electrolytic solution supplied to the electrolyzer 54. Chlorine ions in the separation liquid (dehydrated filtrate) 46 undergo a reaction represented by the following formula (10) by electrolysis.
2Cl ⁻ → Cl ₂ + 2e ⁻ (10)

The generated Cl ₂ dissolves in water (H ₂ O) to form hypochlorous acid (HClO) and dissolves as ClO ⁻ ions. In this embodiment, the generated hypochlorite electrolysis device 54 ^- to be supplied to the slurry absorption liquid 34 of the wet desulfurization system 15 (ClO ion-containing) as a solution 55 of the oxidizing agent (hypochlorite).

Note that a commercially available electrolyzer or the like can be generally used as the electrolyzer 54 used in this embodiment.

In the present embodiment, when all the separated liquid (dehydrated filtrate) 46 is reused and wastewater is not discharged (hereinafter referred to as “no drainage system”), hypochlorous acid is generated by the electrolyzer 54. There is a risk of lack of chlorine ions. Therefore, the electric conductivity (EC) of the separated liquid (dehydrated filtrate) 46 to be reused, the inside of the electrolyzer 54, and the solution 55 of the oxidizing agent (hypochlorous acid) supplied to the wet desulfurizer 15 is obtained. When the electric conductivity becomes a predetermined value or less as measured by the meter 61, it is determined that the chlorine ion is insufficient, and the oxidant raw material (with the oxidant raw material supply means 52 in the electrolyzer 54). (Chlorine compound) 53 is supplied. Thereby, the shortage of chlorine ions can be solved.

In addition, when the electric conductivity is equal to or lower than a predetermined value, the pH may be controlled in the range of 5 or more and 7 or less in order to improve the performance (electrolysis efficiency) of the electrolytic solution, or the electrolysis device 54. You may make it raise the output of.

Note that the electric conductivity is equal to or less than a predetermined value is the electric conductivity in the electrolyzer 54 <the electric conductivity of the solution 55 of the oxidizing agent (hypochlorous acid) supplied to the wet desulfurizer 15. Is the case. The range of electrical conductivity is preferably 1 mS / cm or more and 100 μS / cm or less, more preferably 2 mS / cm or more and 60 mS / cm or less.

Examples of the oxidant raw material (chlorine compound) 53 added to the electrolyzer 54 include a calcium chloride (CaCl ₂ ) solution, a sodium chloride (NaCl) solution, and a hydrochloric acid (HCL) solution. Among these, since Ca ⁺ and Cl ⁻ are components originally contained in the slurry absorbing liquid 34, a calcium chloride (CaCl ₂ ) solution is preferable.

In addition, it can also be judged by the oxidation-reduction potential (ORP) mentioned later other than judging the shortage of a chlorine ion by electrical conductivity. In this case, although not shown, the ORP meter 58 measures and compares the ORP of the solution 55 of the oxidizing agent (hypochlorous acid) supplied to the electrolyzer 54 and the wet desulfurizer 15. In this case, the oxidation-reduction potential (ORP) is equal to or less than a predetermined value because, like the electrical conductivity, the oxidation-reduction potential in the electrolyzer 54 <the oxidant (hypochlorous acid) supplied to the wet desulfurizer 15 In this case, the redox potential of the solution 55 is obtained.

Since the electrolyzer 54 of the present embodiment electrolyzes the above desulfurized wastewater separation liquid (dehydrated filtrate) 46, scales such as Ca (OH) ₂ and Mg (OH) ₂ are generated on the electrode surface. There is a risk. In the electrolysis apparatus 54 of the present embodiment, the generation of scale on the electrode surface can be suppressed by making the current applied to the electrode into a pulsed current. According to this pulse electrolysis method, for example, the time during which Ca ²⁺ adheres to the electrode can be shortened, so that the generation of scale on the electrode surface can be suppressed.

Further, as the electrode of the electrolyzer 54, for example, titanium (Ti) can be used for the cathode, platinum (Pt), iridium oxide (IrO ₂ ), or the like can be used for the anode. Thereby, hypochlorous acid (ClO ⁻ ) can be stably generated and supplied to the slurry absorbent 34 of the wet desulfurization apparatus 15.

In this example, when scale is generated on the electrode surface and the efficiency of generating hypochlorous acid is reduced, the scale is removed by acid cleaning with an acidic chemical (for example, hydrochloric acid HCl, citric acid, etc.). Can be removed.

<Control of redox potential>
The control of the oxidation-reduction potential (hereinafter also referred to as ORP) in the present embodiment is carried out by using the separation liquid (dehydrated filtrate) 46 after the solid-liquid separation treatment of the lime gypsum slurry 34 used for the desulfurization reaction in the wet desulfurization apparatus 15. A solution 55 of hypochlorous acid (oxidant) generated by electrolysis in the electrolysis device 54 is supplied to the wet desulfurization device 15 to perform ORP control of the slurry absorbing liquid (lime gypsum slurry) 34. As a result, an appropriate amount of the oxidizing agent (hypochlorous acid) solution 55 can be generated and supplied. In addition, a wet solution for supplying an oxidant (hypochlorous acid) solution 55 generated by electrolyzing a separated liquid (dehydrated filtrate) 46 obtained by solid-liquid separation of desulfurized wastewater to the wet desulfurization apparatus 15. The desulfurization waste water can be recycled in the desulfurization device 15.

The exhaust gas treatment apparatus 10 </ b> A of the present embodiment electrolyzes a control means 51 that controls the ORP of the slurry absorbing liquid (lime gypsum slurry) 34 in the tower bottom 15 a of the wet desulfurization apparatus 15, and an oxidant raw material (chlorine compound) 53. A solution of an oxidant raw material supply means 52 to be supplied to 54, an oxidation-reduction potentiometer (hereinafter also referred to as an ORP meter) 58 for measuring ORP, and an oxidant (hypochlorous acid) generated by the electrolyzer 54. The electric conductivity of the supply line L ₆ for supplying 55 to the wet denitration device 15, and the dehydrated filtrate 46, the electrolyzer 54, and the solution 55 of the oxidizing agent (hypochlorous acid) supplied to the wet desulfurization device 15. An electrical conductivity meter (hereinafter also referred to as an EC meter) 61 for measurement is provided.

The control means 51 measures the ORP value of the slurry absorbing liquid (lime gypsum slurry) 34 at the tower bottom 15 a of the wet desulfurization device 15 by the ORP meter 58. Based on the measured ORP value, the supply amount of the solution 55 of the oxidizing agent (hypochlorous acid) supplied to the tower bottom 15a of the wet desulfurization apparatus 15 is controlled. Oxidizing agent supplied to the bottom portion 15a of the wet desulfurization system 15 by controlling the ORP of the slurry absorbing solution 34 by adjusting and controlling the opening and closing valve V ₄ the supply of the solution 55 (hypochlorous acid), It prevents the oxidized Hg collected in the slurry absorbing liquid 34 stored in the tower bottom 15a of the wet desulfurization apparatus 15 from being reduced (Hg ²⁺ → Hg ⁰ ) and released from the chimney 16. Can be prevented.

As the ORP control, when the ORP measured by the ORP meter 58 is lower than a predetermined value, the supply amount of the oxidizing agent (hypochlorous acid) solution 55 supplied to the tower bottom 15a of the wet desulfurization apparatus 15 is increased. To. Specifically, the following four can be mentioned, for example. 1) The oxidant raw material 53 is supplied to the electrolyzer 54 so as to increase the electrolysis efficiency (ratio in which chlorine is generated from the current), thereby increasing the ClO ⁻ ion concentration in the electrolytic solution. 2) By making the pH of the electrolytic solution of the electrolyzer 54 acidic, HCLO (hypochlorous acid) having strong oxidizing power is increased. Since the relationship between pH and HCLO can be explained by HCLO⇔CLO ⁻ + H ⁺ , when pH is set to the acidic side, the equilibrium shifts to the left side and HCLO increases. For this pH adjustment, for example, a solution of sulfuric acid, hydrochloric acid, carbonic acid or the like can be used. 3) Increase the amount of electricity supplied to the electrolyzer 54. For example, the CLO ⁻ ion concentration in the electrolyte can be increased by increasing the current value. 4) The supply flow rate of the electrolytic solution generated by the electrolyzer 54 is increased. This hypochlorous acid generated by the electrolyzer 54 ^- increasing the feed flow rate to be supplied to the slurry absorption liquid 34 of the wet desulfurization system 15 as a solution 55 (ClO ion-containing) an oxidizing agent (hypochlorite) It is.

The ORP of the slurry absorbing liquid (lime gypsum slurry) 34 in the wet desulfurization apparatus 15 is preferably in the range of, for example, 0 mV or more and 600 mV or less in order to prevent re-scattering of Hg from the slurry absorbing liquid 34. More preferably, it is in the range of 50 mV to 300 mV, and most preferably in the range of 100 mV to 200 mV. More optimally, the range is 150 mV or more and less than (less than) 200 mV. This is because if the ORP is within the above range, Hg collected as HgCl ₂ in the slurry absorbing liquid 34 is a stable region, and re-scattering into the atmosphere can be prevented.

Further, since the oxidizing agent solution 55 is hypochlorous acid (ClO ⁻ ) -containing electrolytic solution, the oxidizing power is stronger than oxygen (air) used for general ORP control, so the ORP meter in the slurry absorbing liquid 34 is used. The potential of 58 can be in the range of at least 100 mV and 200 mV. By maintaining the potential of the ORP meter 58 in the slurry absorbing liquid 34 in a range of at least 100 mV to 200 mV, reduction of mercury oxide (Hg ²⁺ ) (Hg ²⁺ → Hg ⁰ ) is prevented, and Scattering can be prevented.

Reduction of mercury oxide (Hg ²⁺ ) (Hg ²⁺ → Hg ⁰ ) causes mercury re-scattering because the potential of the ORP meter 58 is in the range of 100 mV or less. It is sufficient that the potential of 58 is 100 mV or more. When the potential of the ORP meter 58 is set to 200 mV or more, both the amount and concentration of the oxidant to be supplied are excessive, and it is not very economical.

Further, the exhaust gas 22 of a general coal-fired boiler may contain selenium (Se) as well as mercury. Selenium in the exhaust gas 22 is absorbed by the slurry absorbent 34 of the wet desulfurization device 15 and removed from the exhaust gas 22. Examples of a method for treating selenium in the slurry absorbing liquid 34 include a coprecipitation method using iron oxide and an adsorption method using activated carbon or activated alumina. This is an effective treatment method for tetravalent Se (IV). Yes, it is not an effective processing method for hexavalent Se (VI).

On the other hand, when the potential of the ORP meter 58 is 200 mV or more, oxidation of tetravalent Se (IV) is promoted to hexavalent Se (VI), and it may be difficult to treat selenium (Se). There is a problem. Therefore, for the reasons described above, by setting the oxidation-reduction potential within the range of 100 mV or more and less than 200 mV, it is possible to efficiently and economically prevent the reduction of mercury, reduce re-scattering, and treat selenium (Se). It can prevent side effects such as deterioration.

In this embodiment, the ORP of the slurry absorbing liquid 34 is used as an index for determination. However, in addition to the ORP determination, the concentration of sulfite ion (SO ₃ ²⁻ ) may be used as an index for determination. When controlling the concentration of sulfite ions contained in the slurry absorbent 34, the concentration of sulfite ions is, for example, 0.1 mmol / L or more and 2.0 mmol / L or less, preferably 0.5 mmol / L or more and 1 0.0 mmol / L or less, more preferably 1.0 mmol / L or less.

Since it is known that the sulfite ion concentration and the oxidation-reduction potential (ORP) show a good correlation (for example, JP-A-11-169657), the concentration of sulfite ions can be controlled by controlling the ORP. It is. In this case, for example, the value of sulfite ion (SO ₃ ²⁻ ) corresponding to the ORP value “150 mV to 200 mV” is approximately “1.0 mmol / L”. Therefore, when judging at one point, the value of sulfite ion (SO ₃ ²⁻ ) may be judged as “1.0 mmol / L”. Therefore, the concentration of sulfite ion (SO ₃ ²⁻ ) can be used as an index for judgment instead of control by ORP.

In the present embodiment, the ORP control of the exhaust gas treatment apparatus 10A is not the slurry absorbing liquid (lime gypsum slurry) 34, but the electrolyzing apparatus 54 for the separated liquid (dehydrated filtrate) 46 that is returned to the wet desulfurization apparatus 15 and reused. Since the solution 55 of the oxidizing agent (hypochlorous acid) generated by electrolysis in step 1 is supplied to the wet desulfurization apparatus 15, it is not necessary to supply the solution 55 of the oxidizing agent (hypochlorous acid) excessively. The ORP of the slurry absorbing liquid 34 can be controlled by the amount 55 of the oxidizing agent (hypochlorous acid) solution 55. In addition, a wet solution for supplying an oxidant (hypochlorous acid) solution 55 generated by electrolyzing a separated liquid (dehydrated filtrate) 46 obtained by solid-liquid separation of desulfurized wastewater to the wet desulfurization apparatus 15. The desulfurization waste water can be recycled in the desulfurization device 15.

As described above, the exhaust gas treatment apparatus 10A according to the first embodiment includes the solid-liquid separation means 41 for performing the solid-liquid separation process on the slurry absorbing liquid (lime gypsum slurry) 34 used for the desulfurization reaction, and the separated separation liquid. An electrolysis device 54 that electrolyzes (dehydrated filtrate) 46 to produce a solution 55 of an oxidant (hypochlorous acid), and an oxidant material that supplies an oxidant material (chlorine compound) 53 to the electrolysis device 54 solution supply means 52, a supply line L ₆ for supplying the generated oxidizing agent solution 55 (hypochlorous acid) to the wet desulfurization system 15, oxidant wet desulfurization system 15 (hypochlorite) 55 And a control means 51 for performing ORP control of the slurry absorbing liquid 34.

According to the exhaust gas treatment apparatus 10A according to the first embodiment, the separation liquid (dehydration filter) is removed when the sulfur oxide (SOx) and the divalent Hg ²⁺ oxidized in the reduction denitration apparatus 12 are removed by the wet desulfurization apparatus 15. Liquid) 46 is electrolyzed by an electrolyzer 54, and a solution 55 of oxidant (hypochlorous acid) is supplied to the wet desulfurizer 15 to perform ORP control. A solution 55 of (hypochlorous acid) can be supplied, NOx, SOx, particularly Hg contained in the exhaust gas 22 can be efficiently removed, and oxidation inhibition in desulfurization can be suppressed. In addition, a wet solution for supplying an oxidant (hypochlorous acid) solution 55 generated by electrolyzing a separated liquid (dehydrated filtrate) 46 obtained by solid-liquid separation of desulfurized wastewater to the wet desulfurization apparatus 15. The desulfurization waste water can be recycled in the desulfurization device 15.

FIG. 2 is a schematic configuration diagram of the exhaust gas treatment apparatus 10B according to the present embodiment. In addition, the same code | symbol is attached | subjected to the structure same as Example 1 mentioned above, and the duplicate description is abbreviate | omitted. As shown in FIG. 2, the exhaust gas treatment apparatus 10 </ b> B according to the present embodiment recycles the separation liquid (dehydrated filtrate) 36 separated by the solid-liquid separation means 41 and the treatment liquid 47 treated by the waste water treatment apparatus 43. A solution 55 of the oxidizing agent (hypochlorous acid) generated by electrolysis using the electrolyzer 54 is supplied to the wet desulfurizer 15. In addition, the ORP control of this embodiment is performed by supplying a solution 55 of an oxidizing agent (hypochlorous acid) generated by electrolyzing the treatment liquid 47 by the electrolysis apparatus 54 to the wet desulfurization apparatus 15 and supplying the slurry absorbing liquid 34. ORP control is performed.

In this embodiment, the electrical conductivity of the solution 55 of the oxidizing agent (hypochlorous acid) supplied to the separation liquid (treatment liquid) 47, the electrolysis apparatus 54, and the wet desulfurization apparatus 15 is measured by the EC meter 61. When the electrical conductivity falls below a predetermined value, it is determined that the chlorine ions are insufficient, and the oxidant material (chlorine compound) 53 is supplied into the electrolyzer 54 by the oxidant material supply means 52. It is supposed to be. Thereby, the shortage of chlorine ions can be solved.

Note that the electric conductivity is equal to or less than a predetermined value is the electric conductivity in the electrolyzer 54 <the electric conductivity of the solution 55 of the oxidizing agent (hypochlorous acid) supplied to the wet desulfurizer 15. Is the case.

The treatment liquid 47 treated by the waste water treatment apparatus 43 is further removed of suspended matter, heavy metals, mercury, and the like 44 as compared with the dehydrated filtrate 36, and the treatment liquid 47 is electrolyzed by the electrolysis apparatus 54. When the generated oxidant (hypochlorous acid) solution 55 is supplied to the wet desulfurization apparatus 15, the suspension, heavy metal, mercury, etc. contained in the slurry absorbing liquid (lime gypsum slurry) 34 in the wet desulfurization apparatus 15 The density of 44 can be reduced. That is, the total amount of mercury contained in the slurry absorbing liquid (lime gypsum slurry) 34 can be reduced.

According to the ORP control of the exhaust gas treatment apparatus 10B according to the second embodiment, the concentration of metallic mercury re-scattered to the exhaust gas 22 side can be reduced as the metallic mercury concentration in the slurry absorbing liquid 34 decreases. As a result, the mercury concentration in the treated exhaust gas 24 discharged from the wet desulfurization apparatus 15 can be reduced. Further, a solution 55 of an oxidizing agent (hypochlorous acid) generated by electrolyzing a treatment liquid 47 obtained by further treating a separated liquid (dehydrated filtrate) 36 obtained by solid-liquid separation of desulfurized wastewater with an electrolyzer 54 is subjected to wet desulfurization. The desulfurization effluent can be recycled in the wet desulfurization device 15 for supply to the device 15.

FIG. 3 is a schematic configuration diagram of an exhaust gas treatment apparatus 10C according to the present embodiment. In addition, the same code | symbol is attached | subjected to the structure same as Example 1 and Example 2 mentioned above, and the duplicate description is abbreviate | omitted. As shown in FIG. 3, the exhaust gas treatment apparatus 10 </ b> C according to the present embodiment is a wastewater treatment apparatus for separating a part of the separation liquid (dehydrated filtrate) 46 processed by the solid-liquid separation means 41 and the remaining dehydrated filtrate 36. One or both of the separation liquid (processing liquid) 47 processed in 43 is supplied to the electrolysis apparatus 54 through the supply lines L ₄ and L ₅ and is generated by electrolysis in the electrolysis apparatus 54 ( A solution 55 of hypochlorous acid) is supplied to the wet desulfurization apparatus 15.

Further, the ORP control of this embodiment is performed by using a solution 55 of an oxidizing agent (hypochlorous acid) generated by electrolyzing either one or both of the dehydrated filtrate 46 and the treatment liquid 47 with an electrolyzer 54. This is supplied to the wet desulfurization apparatus 15 to perform ORP control of the slurry absorbent 34.

In this embodiment, the electric conductivity of the solution 55 of the oxidant (hypochlorous acid) supplied to the separation liquid (dehydrated filtrate 46, treatment liquid 47), the electrolysis apparatus 54, and the wet desulfurization apparatus 15 is measured with an EC meter. When the electric conductivity becomes a predetermined value or less as measured by 61, it is determined that the chlorine ion is insufficient, and the oxidant raw material (chlorine) is supplied to the electrolyzer 54 by the oxidant raw material supply means 52. Compound) 53 is supplied. Thereby, the shortage of chlorine ions can be solved.

Note that the electric conductivity is equal to or less than a predetermined value is the electric conductivity in the electrolyzer 54 <the electric conductivity of the solution 55 of the oxidizing agent (hypochlorous acid) supplied to the wet desulfurizer 15. Is the case.

The supply amount of the dehydrated filtrate 46 and the treatment liquid 47 to the electrolyzer 54 is determined and controlled based on the amount of components contained in the slurry absorbing liquid 34. Examples of components contained in the slurry absorbing liquid 34 include gypsum, sulfite gypsum, calcium carbonate, mercury, and heavy metals.

Also, it is preferable to consider water balance such as reuse of desulfurization drainage discharged from the wet desulfurization apparatus 15 so that a minimum amount of water (industrial water) 35 can be replenished.

According to the ORP control of the exhaust gas treatment apparatus 10C in the third embodiment, the concentration of metallic mercury re-scattered to the exhaust gas 22 side can be reduced as the metallic mercury concentration in the slurry absorbing liquid 34 decreases. As a result, the mercury concentration in the treated exhaust gas 24 discharged from the wet desulfurization device 15 can be reduced. In addition, a solution 55 of an oxidizing agent (hypochlorous acid) generated by electrolyzing with an electrolyzer 54 by reusing separated liquids (dehydrated filtrate 46 and processing liquid 47) obtained by solid-liquid separation of desulfurized wastewater. In order to supply the wet desulfurization apparatus 15, the desulfurization waste water can be recycled in the wet desulfurization apparatus 15.

Next, the exhaust gas treatment apparatuses 10D to 10F according to the present embodiment will be described with reference to FIGS. 4 to 6 are schematic configuration diagrams of the exhaust gas treatment apparatuses 10D to 10F according to the present embodiment. Note that the same components as those in the first to third embodiments are denoted by the same reference numerals, and redundant description is omitted. As shown in FIGS. 4 to 6, the exhaust gas treatment apparatuses 10D to 10F according to the present embodiment are separated from the separation liquid (dehydrated filtrate 46, treatment liquid 47) by the electrolysis apparatus 54 in addition to the configurations of the first to third embodiments. ) And the mixing tank 56 for mixing one or both of the dehydrated filtrate 48 and the treatment liquid 49, and the solid-liquid separation means 41. Supply lines L ₄ and L ₅ for supplying either or both of a part of the treated dehydrated filtrate 46 and a part of the treated liquid 47 treated by the waste water treatment device 43 to the electrolyzer 54, and dehydration Supply lines L ₆ and L ₇ for supplying one or both of the remaining filtrate 48 and the remaining treatment liquid 49 to the mixing tank 56, and the oxidizing agent (hypochlorous acid) solution 55 are mixed into the mixing tank 56. to the supply line L ₈ and supplies the mixture 57 of the mixing tank 56 the wet desulfurization system The mixed liquid return line L ₉ for returning to 15 and a line for supplying water (industrial water) 37 to the mixing tank 56 are provided. In FIG. 4 to FIG. 6, V ₁ to V ₄ indicate open / close valves.

In this embodiment, the ORP is controlled by supplying the mixed liquid 57 in which the redox potential of the mixed liquid 57 mixed in the mixing tank 56 is controlled to the slurry absorbing liquid 34 of the wet desulfurization apparatus 15 (L ₉ ). 34 ORP control is performed.

The oxidation-reduction potential of the mixed solution 57 mixed in the mixing tank 56 is, for example, in the range of 0 mV to 600 mV in order to prevent re-scattering of Hg from the slurry absorbent 34 in the wet desulfurization apparatus 15. Is preferred. More preferably, it is in the range of 50 mV to 300 mV, and most preferably in the range of 100 mV to 200 mV. More optimally, the range is 150 mV or more and 200 mV or less. In this embodiment, since the oxidation-reduction potential of the mixed liquid 57 supplied to the slurry absorbing liquid 34 of the wet desulfurization apparatus 15 is within the above range, for example, the oxidation-reduction potential of the mixed liquid 57 is in the range of 100 mV to 200 mV. When the ORP control is performed, the oxidation-reduction potential of the slurry absorbing liquid 34 of the wet desulfurization apparatus 15 is less than 200 mV, and does not exceed 200 mV.

Therefore, if the oxidation-reduction potential of the mixed liquid 57 is within the above range, Hg collected as HgCl ₂ in the slurry absorbing liquid 34 of the wet desulfurization apparatus 15 is a stable region and prevents re-scattering into the atmosphere. be able to. More preferably, by setting the oxidation-reduction potential in the range of 150 mV or more and 200 mV or less, reduction of mercury can be efficiently and economically prevented, re-scattering can be reduced, and as described in Example 1, selenium (Se ) Side effects such as deterioration of processability can be prevented.

In this example, the EC meter 61 measures the electrical conductivity of the separation liquid (dehydrated filtrates 46 and 48 and treatment liquids 47 and 49), the electrolysis apparatus 54, and the mixed liquid 57 supplied to the wet desulfurization apparatus 15. When the electrical conductivity becomes a predetermined value or less, it is determined that the chlorine ions are insufficient, and the oxidant material (chlorine compound) 53 is contained in the electrolyzer 54 by the oxidant material supply means 52. To supply. Thereby, the shortage of chlorine ions can be solved.

Note that the electric conductivity is equal to or lower than a predetermined value is a case where the electric conductivity in the electrolysis apparatus 54 <the electric conductivity of the mixed liquid 57 supplied to the wet desulfurization apparatus 15.

According to the ORP control of the exhaust gas treatment apparatuses 10D to 10F in the fourth embodiment, it is possible to reduce the concentration of metallic mercury re-scattered to the exhaust gas 22 side as the metallic mercury concentration in the slurry absorbing liquid 34 decreases. As a result, the mercury concentration in the treated exhaust gas 24 discharged from the wet desulfurization device 15 can be reduced.

Further, a solution of an oxidizing agent (hypochlorous acid) generated by electrolyzing a separation liquid (a treatment liquid 47 obtained by treating the dehydrated filtrate 46 and the dehydrated filtrate 36) obtained by solid-liquid separation of desulfurized wastewater with an electrolyzer 54. Since 55 is supplied to the wet desulfurization apparatus 15, the desulfurization waste water can be recycled in the wet desulfurization apparatus 15.

Further, since the redox potential of the mixed liquid 57 mixed in the mixing tank 56 before being supplied to the slurry absorbing liquid 34 is controlled within the range of 100 mV to 200 mV, the redox of the slurry absorbing liquid 34 of the wet desulfurization apparatus 15 is controlled. The potential will be below 200 mV and will not be above 200 mV.

As described above, the exhaust gas treatment apparatus 10 according to the present embodiment includes the solid-liquid separation means 41 for performing the solid-liquid separation treatment on the lime gypsum slurry 34 used for the desulfurization reaction, and the separated liquid (dehydrated filtrate 46 and An electrolyzer 54 for electrolyzing the treatment liquid 47) to produce an oxidant (hypochlorous acid) solution 55; and an oxidant material supply means for supplying an oxidant material (chlorine compound) 53 to the electrolyzer 54. 52, supply lines L ₆ and L ₈ and a mixed liquid return line L ₉ for supplying the generated oxidant (hypochlorous acid) solution 55 to the wet desulfurization apparatus 15, and an oxidant ( And a control means 51 for supplying the solution 55 of hypochlorous acid and controlling the ORP of the slurry absorbent 34.

As described above, according to the exhaust gas treatment apparatus 10 according to the present embodiment, when the wet oxide desulfurization apparatus 15 removes the sulfur oxide (SOx) and the divalent Hg ²⁺ oxidized in the reductive denitration apparatus 12, the separated liquid. An oxidant (hypochlorous acid) solution 55 generated by electrolyzing (dehydrated filtrate 46 and treatment liquid 47) with an electrolyzer 54 is supplied to the wet desulfurizer 15 to perform ORP control of the slurry absorbent 34. By doing so, an appropriate amount of an oxidant (hypochlorous acid) solution 55 can be supplied, and NOx, SOx, especially Hg contained in the exhaust gas 22 can be efficiently removed, and desulfurization is performed. Oxidation inhibition in can be suppressed.

Further, according to the ORP control of the exhaust gas treatment apparatus 10 in the present embodiment, the concentration of metallic mercury re-scattered to the exhaust gas 22 side can be reduced as the metallic mercury concentration in the slurry absorbing liquid 34 decreases. As a result, the mercury concentration in the treated exhaust gas 24 discharged from the wet desulfurization device 15 can be reduced.

Furthermore, according to the ORP control of this embodiment, it is possible to efficiently and economically prevent the reduction of mercury, reduce re-scattering, and prevent side effects such as deterioration of the processability of selenium (Se). it can.

Further, a solution 55 of an oxidizing agent (hypochlorous acid) generated by electrolyzing the separated liquid (dehydrated filtrate 46 and treatment liquid 47) obtained by solid-liquid separation of the desulfurized wastewater with the electrolyzer 54 is supplied to the wet desulfurizer 15. In order to supply, desulfurization waste water can be recycled within the wet desulfurization apparatus 15.

10 Exhaust gas treatment device 11 Boiler 12 Reduction denitration device 13 Air preheater (Air heater: AH)
14 Dust collector (ESP)
15 wet desulfurization system 15a column bottom 15b top portion 15c nozzle 16 chimney 21 fuel 22 gas 23 flue 24 treated flue gas 31 reduction-oxidation auxiliary agent supply means 32 reduction-oxidation auxiliary agent (ammonium chloride (NH ₄ Cl) solution)
33 Lime-gypsum slurry supply means 34 Lime-gypsum slurry (slurry, slurry absorbent, alkali absorbent)
35, 37 water (industrial water)
36, 46, 48 Separation liquid (dehydrated filtrate)
41 Solid-liquid separation means 42 Dehydrated cake (gypsum)
43 Wastewater treatment equipment 44 Suspensions, heavy metals, mercury, etc. 45 Wastewater 47, 49 Separation liquid (treatment liquid)
51 Control means 52 Oxidant raw material supply means 53 Oxidant raw material (chlorine compound)
54 Electrolyzer 55 Solution of oxidizing agent (hypochlorous acid) 56 Mixing tank 57 Mixed solution 58 Redox potential meter (ORP meter)

Claims (12)

1. Reducing oxidation aid supply means for supplying a reduction oxidation aid that generates hydrogen chloride and ammonia when vaporized in a flue that discharges exhaust gas from the boiler;
   A reduction denitration apparatus having a denitration catalyst that reduces nitrogen oxides in the exhaust gas with ammonia and oxidizes mercury in the presence of hydrogen chloride;
   A wet desulfurization device that absorbs and removes sulfur oxides in the exhaust gas and mercury oxidized in the reductive denitration device with an absorbing solution;
   An oxidation-reduction potentiometer for measuring the oxidation-reduction potential of the absorbing solution;
   Solid-liquid separation means for separating the solid and mercury in the desulfurization effluent discharged from the wet desulfurization apparatus, and the liquid, and
   An electrolysis apparatus for electrolyzing the separated liquid separated by the solid-liquid separation means to produce hypochlorous acid;
   Control means for controlling the redox potential of the absorbing liquid by supplying the hypochlorous acid to the absorbing liquid;
   An exhaust gas treatment apparatus comprising:
2. The exhaust gas treatment apparatus according to claim 1, wherein the separation liquid is a dehydrated filtrate separated by the solid-liquid separation means, or a treatment liquid obtained by removing heavy metals from the dehydrated filtrate.
3. Electrical conductivity measuring means for measuring the electrical conductivity of the separated liquid;
   An oxidant raw material supply means for supplying an oxidant raw material to the electrolyzer;
   Have
   3. The exhaust gas treatment apparatus according to claim 1, wherein the oxidant material supply unit supplies the oxidant material when the measured electrical conductivity is equal to or lower than a predetermined value.
4. The control means controls the oxidation-reduction potential of the absorption liquid to a range of 100 mV to 200 mV, or controls the sulfite ions in the absorption liquid to 0.1 mmol / L or more and 2.0 mmol / L or less. The exhaust gas treatment apparatus according to any one of claims 1 to 3.
5. A mixing tank for mixing the hypochlorous acid, the dehydrated filtrate, or one or both of the treatment liquids;
   A mixed liquid return line for returning the mixed liquid mixed in the mixing tank to the wet desulfurization apparatus;
   Have
   The control means measures the oxidation-reduction potential of the mixed solution, supplies the hypochlorous acid to the mixed solution, and sets the oxidation-reduction potential of the mixed solution in the range of 100 mV to 200 mV, or in the mixed solution The exhaust gas treatment apparatus according to any one of claims 1 to 4, wherein the sulfite ion is controlled to be 0.1 mmol / L or more and 2.0 mmol / L or less.
6. The exhaust gas treatment apparatus according to any one of claims 1 to 5, wherein the electrolyzer generates hypochlorous acid from the separated liquid with a pulsed current.
7. A reduction oxidation auxiliary agent supplying step for supplying a reduction oxidation auxiliary for generating hydrogen chloride and ammonia when vaporized in a flue for discharging exhaust gas from the boiler;
   A reduction denitration step having a denitration catalyst that reduces nitrogen oxides in the exhaust gas with ammonia and oxidizes mercury in the presence of hydrogen chloride; and
   A wet desulfurization step of absorbing and removing sulfur oxides in the exhaust gas and mercury oxidized in the reductive denitration step with an absorbent;
   A redox potential measuring step for measuring a redox potential of the absorbing solution;
   A solid-liquid separation step for separating the solid and mercury in the desulfurization effluent discharged from the wet desulfurization step, and the liquid, and
   An electrolysis step of electrolyzing the separated liquid separated in the solid-liquid separation step to produce hypochlorous acid;
   A control step of supplying the hypochlorous acid to the absorption liquid to control a redox potential of the absorption liquid;
   An ORP control method for an exhaust gas treatment apparatus, comprising:
8. 8. The ORP control method for an exhaust gas treatment apparatus according to claim 7, wherein the separation liquid is a dehydrated filtrate separated in the solid-liquid separation step or a treatment liquid obtained by removing heavy metals from the dehydrated filtrate. .
9. An electrical conductivity measurement step for measuring electrical conductivity of the separated liquid;
   An oxidant raw material supply step for supplying an oxidant raw material to the electrolysis step;
   Have
   9. The ORP control method for an exhaust gas treatment apparatus according to claim 7, wherein the oxidant raw material supply step supplies an oxidant raw material when the measured electrical conductivity is equal to or lower than a predetermined value.
10. In the control step, the oxidation-reduction potential of the absorbing solution is controlled in a range of 100 mV to 200 mV, or the sulfite ion in the absorbing solution is controlled to 0.1 mmol / L to 2.0 mmol / L. The ORP control method for an exhaust gas treatment apparatus according to any one of claims 7 to 9.
11. A mixing step for mixing the hypochlorous acid with either one or both of the dehydrated filtrate and the treatment liquid;
    A liquid mixture returning step for returning the liquid mixture mixed in the mixing step to the wet desulfurization step;
    Have
    In the control step, the oxidation-reduction potential of the mixed solution is measured, and the hypochlorous acid is supplied to the mixed solution so that the oxidation-reduction potential of the mixed solution is in the range of 100 mV to 200 mV, or in the mixed solution. The method of controlling an ORP of an exhaust gas treatment apparatus according to any one of claims 7 to 10, wherein the sulfite ion is controlled to be 0.1 mmol / L or more and 2.0 mmol / L or less.
12. The ORP control method for an exhaust gas treatment apparatus according to any one of claims 7 to 11, wherein the electrolysis step generates hypochlorous acid from the separated liquid with a pulsed current.

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