JP2016120438A - Wet-type desulfurization apparatus and wet-type desulfurization method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wet-type desulfurization apparatus and a wet-type desulfurization method which removes metal mercury in exhaust gas with a small amount of oxidizer and prevents scattering of a harmful substance originated from the oxidizer to the exhaust gas downstream side.SOLUTION: A wet-type desulfurization apparatus provided with an absorption column 1 which has a spray nozzle 8 for removing sulfur oxide in exhaust gas according to gas-liquid contact between the exhaust gas of a boiler or the like and absorption liquid and a plurality of stages of mist eliminators 2 disposed in the exhaust gas flow direction on the downstream side of the spray nozzle is provided with a washing device 50 in which water containing no mercury oxidizer is used for washing of at least one stage of mist eliminator 2 from the exhaust gas flow downstream side toward the upstream side is used and water containing the mercury oxidizer is used for washing of the mist eliminator 2 besides at least one stage of mist eliminator. Therein, the used water containing the oxidizer is circulated and used, and a part of the circulated water and the used water containing no oxidizer are supplied to the absorption column. Mercury is efficiently removed by the mercury oxidizer and, at the same time, even when a harmful substance originated from the mercury oxidizer is generated, the harmful substance is removed by the water containing no mercury oxidizer.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a flue gas treatment apparatus and a flue gas treatment method for removing harmful components in exhaust gas generated from a combustion apparatus such as a boiler installed in a thermal power plant or factory, and in particular, removes mercury in the exhaust gas. The present invention relates to a wet desulfurization apparatus and a wet desulfurization method.

  In fossil fuel combustion furnaces using coal, heavy oil, residual oil, etc., such as boilers installed in thermal power plants and factories, in order to remove sulfur contained in exhaust gases, etc., from the viewpoint of environmental conservation Desulfurization equipment is installed. There are various types of desulfurization equipment, but wet desulfurization using the limestone-gypsum method is the main method in large power plants in developed countries, where strict regulations are required. Many high spray systems are used.

FIG. 6 shows a system of a conventional wet desulfurization apparatus.
Coal and combustion air are supplied to a boiler (not shown) to burn the coal. And the exhaust gas discharged | emitted by the combustion reaction of coal is introduce | transduced into the absorption tower 1 of the wet desulfurization apparatus 20 through the denitration apparatus, the dry-type dust collector, etc. which are not shown in figure.

  In the wet desulfurization apparatus 20, a desulfurization absorption liquid containing an absorbent such as limestone (calcium carbonate) or slurry containing lime is sprayed as fine droplets from the desulfurization spray nozzle 8 in the absorption tower 1, and the desulfurization absorption liquid droplets. By contacting the exhaust gas with the exhaust gas, sulfur oxides (SOx) in the exhaust gas together with acid gases such as dust, hydrogen chloride (HCl), and hydrogen fluoride (HF) in the exhaust gas are absorbed on the surface of the absorbing droplets of the desulfurization spray nozzle 8. Is absorbed and removed chemically.

The absorbing solution that has absorbed SOx (mainly SO ₂ ) once accumulates in the circulation tank 9 at the bottom of the absorption tower 1, is oxidized by oxygen in the air supplied from the air supply line 11, is stirred by the stirrer 10, and then is sulfuric acid. Produces calcium (gypsum). The circulation tank 9 is supplied with an absorbent from the limestone slurry supply device 3 in accordance with the amount of SOx contained in the exhaust gas from the boiler or the like. The slurry-like absorption liquid in the circulation tank 9 is pressurized by the absorption liquid circulation pump 7 and supplied from the absorption liquid circulation pipe 18 to the desulfurization spray nozzle 8 in the upper part of the absorption tower 1.

  Further, the absorbent extracted from the circulation tank 9 or the absorbent circulation pipe 18 is separated from the gypsum by the gypsum dehydrator 4 and the gypsum 12 is recovered. The absorption liquid separated from the gypsum 12 is used for adjusting an absorbent such as limestone slurry, returned to the absorption tower 1 from the absorption liquid return line 48, or partly sent to the waste water treatment device 5 and discharged. Or

The exhaust gas treated in the absorption tower 1 is discharged from the chimney 6 through the mist eliminator 2. The exhaust gas from which SO ₂ has been removed by gas-liquid contact with the desulfurization absorbent 8 from the desulfurization spray nozzle 8 contains fine spray droplets (mist), and is supplied by the mist eliminator 2 installed at the outlet of the absorption tower 1. Mist is removed. In the mist eliminator 2, the mist eliminator 2 is washed with industrial water or the like in order to prevent clogging due to solid content such as gypsum contained in the mist, and the used washing water is allowed to flow down to the absorption tower 1.

The exhaust gas discharged from the boiler contains not only nitrogen oxides and sulfur oxides, but also mercury in the order of μg / m ³ N. The form of mercury in exhaust gas is classified into zero-valent metallic mercury (Hg ⁰ form) and divalent mercury oxide (Hg ²⁺ form). Divalent mercury oxide (hereinafter referred to as “mercury oxide”) has a lower vapor pressure than zero-valent metallic mercury (hereinafter referred to as “metal mercury”), and is easily adsorbed to solid particles such as soot dust and liquids such as desulfurization absorption liquid. However, since it is absorbed, it is easy to remove from the exhaust gas.

  As a method for removing mercury in exhaust gas, a method of adding mercury adsorbent such as activated carbon to the exhaust gas and removing it with a dust removal device, or a method of oxidizing mercury with a denitration catalyst or additive and absorbing and removing it with a desulfurization device Etc. In the latter method, only the mercury oxide can be absorbed and removed by the desulfurization apparatus, and the metallic mercury that has not been oxidized by the oxidizing means upstream of the desulfurization apparatus is discharged to the atmosphere without being removed. It will be. It has also been confirmed that a part of mercury (mercury oxide) absorbed in the desulfurization absorption liquid is reduced in the liquid and re-released into the exhaust gas as metallic mercury.

  The coal that is mainly used in Japan at present has a small amount of Hg content, so the amount of this re-release is very small. However, there are cases where coal containing Hg at a high concentration is used overseas. Given the increasing severity, it is necessary to suppress the phenomenon of mercury re-emission.

  Patent Document 1 below discloses a mercury removing device that oxidizes and removes zero-valent mercury in exhaust gas. This mercury removal apparatus converts a zero-valent mercury gas into a divalent mercury gas by bringing a solution containing a trivalent iron component into a mercury oxidant and having an aqueous solution with a pH of 1 or less in contact with the gas to be treated. Then, it is dissolved in an aqueous solution and separated and removed from the gas to be treated. Specifically, in a two-column type desulfurization apparatus, exhaust gas is cooled and removed in a cooling tower installed upstream of the exhaust gas flow path of the absorption tower, and mercury oxidant is added to achieve zero valence in the exhaust gas. The mercury gas is oxidized to divalent mercury gas. Therefore, by adding a mercury oxidizer addition device, pH meter, and pH adjuster addition device to the cooling tower arranged upstream of the exhaust gas flow path of a general two-column desulfurization device, existing thermal power generation It can be easily introduced at a place.

  Patent Document 2 below discloses an exhaust gas treatment system that efficiently removes trace components in exhaust gas such as fluorine and mercury in exhaust gas. In this system, a venturi scrubber that has improved the removal performance of trace components in exhaust gas by increasing the gas flow rate by the throat part is installed downstream of the exhaust gas flow path of the desulfurization unit, and the alkali liquid is used as the absorption liquid to remove the exhaust gas. The fluorine removal performance is enhanced. Further, by providing an oxidizer between the desulfurizer and the venturi scrubber, the zero-valent mercury gas in the exhaust gas is oxidized to a divalent mercury gas and absorbed and removed by the venturi scrubber.

In Patent Document 3 below, mercury in exhaust gas from a waste incinerator is oxidized to mercuric chloride and mercuric chloride by hydrochloric acid and chlorine contained in the exhaust gas, and acidic gas components such as sulfur dioxide and hydrochloric acid are added. the absorption liquid harmful gas removing apparatus for removing, but is removed readily dissolved, it is described that the mercury dissolved in the absorption liquid is being reduced metal mercury by sO ₂ and sO _3. Therefore, the pH is adjusted to 6-9 from the mercury removal rate, the amount of chlorine gas generated from the absorbing solution, the consumption of sodium hydroxide, etc. when sodium hypochlorite is added to the absorbing solution to change the pH of the absorbing solution. Thus, the mercury dissolved in the absorbing solution is prevented from being reduced by SO ₂ or SO ₃ .

  Furthermore, in Patent Document 4 below, in an exhaust gas treatment apparatus equipped with a two-column desulfurization apparatus or a single-column desulfurization apparatus and a wet electrostatic precipitator, mercury is added to the circulating liquid of the cooling tower, the desulfurization absorption liquid, or the supply water of the wet electrostatic precipitator. An apparatus and method for removing mercury in exhaust gas by adding a remover is disclosed.

  Further, in Patent Document 5 below, in a system for removing mercury oxidized by a denitration catalyst with a desulfurization apparatus, an oxidizing agent such as sodium chlorite is added to the desulfurization absorption liquid, or a cleaning liquid is sprayed at the outlet of the desulfurization apparatus. Thus, a method of removing mercury by separating and circulating the liquid with a mist eliminator on the downstream side of the exhaust gas flow path is disclosed.

JP 2006-136856 A JP 2008-302345 A JP 62-30532 A Japanese Patent Laid-Open No. 10-216476 JP 2004-313833 A

In the invention described in Patent Document 1, a zero-valent mercury gas is oxidized and removed using an aqueous solution having a pH of 1 or less and containing a mercury oxidizing agent containing trivalent iron as a component. Since it is a cooling tower on the upstream side of the desulfurization apparatus, the metal mercury re-released from the desulfurization apparatus cannot be removed. Further, considering that the mercury oxidant is consumed by SO _{2 in} the exhaust gas before being introduced into the desulfurization apparatus, it is necessary to supply a large amount of the mercury oxidant. Furthermore, when an oxidant composed of trivalent iron consumed by oxidation of zero-valent mercury gas in exhaust gas or reaction with SO _{2 in} exhaust gas is oxidized by oxygen in the exhaust gas, 2 in the aqueous solution. Valent mercury may be reduced and released again into the exhaust gas.

  In addition, in the invention described in Patent Document 2, a venturi scrubber is installed on the downstream side of the exhaust gas flow path of the desulfurization device to improve the removal performance of trace components in the exhaust gas such as fluorine. It cannot be removed. Moreover, when an oxidation apparatus is provided between the desulfurization apparatus and the venturi scrubber to oxidize zero-valent mercury gas, the cost is large and operation control is difficult. Further, when an alkaline liquid is used to improve the performance of removing fluorine in the exhaust gas, mercury absorbed and removed in the venturi scrubber may be reduced in the liquid and re-released into the exhaust gas.

In the invention described in Patent Document 3, sodium hypochlorite is added to the absorbing solution to adjust the pH to 6 to 9, and the mercury dissolved in the absorbing solution is prevented from being reduced by SO ₂ or SO ₃ . However, sodium hypochlorite added to the absorbent is consumed by sulfur oxides in the exhaust gas. Therefore, the oxidation effect of mercury by sodium hypochlorite cannot be expected so much.

The invention described in Patent Document 4 discloses a method for removing metallic mercury in exhaust gas using a mercury removing agent in an exhaust gas treatment apparatus equipped with a two-column desulfurization apparatus, a single-column desulfurization apparatus, and a wet electric dust collector. However, various oxidizing agents and adsorbents are listed as mercury removing agents. When an oxidizing agent is added to a circulating fluid that comes into contact with exhaust gas containing high concentration of SO ₂ such as a cooling tower or a desulfurization absorption tower, these mercury removing agents will be consumed by SO ₂ and consumed. Becomes enormous. Also, when iron or manganese salts are used as mercury removal agents, mercury may be reduced and released again into the exhaust gas along with the oxidation-reduction reaction that occurs in the absorption liquid circulated in the cooling tower and desulfurization absorption tower. There is also.

  Furthermore, in the invention described in Patent Document 5, it is described that mercury is recovered in a mist eliminator installed at the outlet of the desulfurization absorption tower. However, when sodium hypochlorite is used as an oxidizing agent, oxidation is performed. Since chlorine is released by the reaction, it is necessary to consider the scattering of chlorine to the downstream side of the exhaust gas passage. The reason is that if no measures are taken, the amount of chlorine emitted from the chimney will increase.

  An object of the present invention is to oxidize and remove metallic mercury in exhaust gas with a small amount of oxidant to reduce the amount of mercury discharged to the outside air, and harmful substances derived from the substance used as the oxidant It is provision of the wet desulfurization apparatus and wet desulfurization method which prevent scattering to a downstream side.

The object of the present invention can be achieved by adopting the following constitution.
The invention described in claim 1 includes a gas-liquid contact portion (8) for absorbing and removing sulfur oxides in the exhaust gas by gas-liquid contact between the exhaust gas discharged from the combustion apparatus including the boiler and the absorbing liquid, and the gas-liquid A wet type equipped with an absorption tower (1) downstream of the exhaust gas flow path of the contact portion (8) and provided with a plurality of stages in the exhaust gas flow direction and having a mist eliminator (2) for removing mist in the exhaust gas. In the desulfurization apparatus, a cleaning device (26) for cleaning the mist eliminator (2) is provided, and the cleaning device (26) is arranged from the downstream side to the upstream side of the exhaust gas flow path in the plurality of mist eliminators (2). On the other hand, at least one mist eliminator (2) is cleaned with a non-mercury oxidizer cleaning section (26b) that uses water that does not contain a mercury oxidizer, and other mist eliminators (2) with a mercury oxidizer. Use water containing A mercury oxidant cleaning part (26a), a circulation supply part (28) for extracting the used cleaning water from the mercury oxidant cleaning part (26a) and circulatingly supplying it to the mercury oxidant cleaning part (26a); In the mercury oxidant cleaning section (26a), a part of the used cleaning water is extracted and supplied to the absorption tower (1) together with the used cleaning water in the non-mercury oxidant cleaning section (26b). (32) is a wet desulfurization apparatus.

  The invention according to claim 2 is a pH adjuster supply section (15) for supplying a pH adjuster to the mercury oxidizer cleaning section (26a), and a mercury oxidizer cleaning section (15) from the pH adjuster supply section (15). 26a) a pH adjusting agent amount adjusting unit (36) for adjusting the supply amount of the pH adjusting agent, and a pH measuring unit (19) for measuring the pH of the wash water after use in the mercury oxidizing agent cleaning unit (26a). An oxidant supply unit (14) for supplying a mercury oxidant to the mercury oxidant cleaning unit (26a), and a mercury oxidant from the oxidant supply unit (14) to the mercury oxidant cleaning unit (26a). An oxidant amount adjusting unit (30) for adjusting the supply amount, an ORP measuring unit (16) for measuring the ORP of cleaning water after use in the mercury oxidant cleaning unit (26a), and the pH measuring unit (19) Based on pH measured by ORP and ORP measured by ORP measurement unit (16) Which is a wet desulfurization apparatus according to claim 1, wherein the provided pH adjusting agent amount adjuster (36) and a control unit for controlling oxidant amount adjusting portion (30) and (17).

  The invention described in claim 3 is an exhaust gas return supply section (42) for supplying exhaust gas treated in the absorption tower (1) to the mercury oxidant cleaning section (26a), and mercury oxidation from the exhaust gas return supply section (42). An exhaust gas amount adjusting unit (40) for adjusting the amount of exhaust gas supplied to the agent cleaning unit (26a), a pH measuring unit (19) for measuring the pH of the cleaning water after use in the mercury oxidant cleaning unit (26a), An oxidant supply unit (14) for supplying a mercury oxidant to the mercury oxidant cleaning unit (26a), and a supply of the mercury oxidant from the oxidant supply unit (14) to the mercury oxidant cleaning unit (26a). An oxidant amount adjusting unit (30) for adjusting the amount, an ORP measuring unit (16) for measuring the ORP of cleaning water after use in the mercury oxidant cleaning unit (26a), and the pH measuring unit (19) Measured pH and ORP measured by ORP measurement unit (16) Which is a wet desulfurization apparatus according to claim 1, wherein the provided gas amount adjusting unit (40), and a control unit for controlling oxidant amount adjusting portion (30) and (17) Zui.

  The invention according to claim 4 comprises a gas-liquid contact portion (8) for absorbing and removing sulfur oxides in the exhaust gas by gas-liquid contact between the exhaust gas discharged from the combustion apparatus including the boiler and the absorbing liquid, and the gas-liquid A plurality of stages in the exhaust gas flow direction at the downstream side of the exhaust gas flow path of the contact portion (8), and an absorption tower (1) having a mist eliminator (2) for removing mist in the exhaust gas, In the wet desulfurization method for removing sulfur oxides, at least one mist eliminator (2) of the plurality of mist eliminators (2) from the downstream side to the upstream side of the exhaust gas passage contains a mercury oxidant. The other mist eliminator (2) is washed with water containing mercury oxidizer, and the water containing mercury oxidizer after use is circulated as wash water for mist eliminator (2). Use a wet desulfurization method using water containing no mercury oxidation agent after use in a part and washing water containing mercury oxidation agent after use in cleaning as makeup water for the absorption tower (1).

  The invention according to claim 5 measures the pH and ORP of the water containing the mercury oxidizer after use for cleaning, and supplies the pH adjuster and the mercury oxidizer to the water containing the mercury oxidizer based on the measured values. The wet desulfurization method according to claim 4 used for cleaning.

  The invention according to claim 6 measures the pH and ORP of water containing mercury oxidant after use for cleaning, and the exhaust gas treated in the absorption tower (1) with water containing mercury oxidant based on the measured values. The wet desulfurization method according to claim 4, wherein a mercury oxidant is supplied and used for cleaning.

(Function)
According to the present invention, the mercury oxidant is mixed with the cleaning water of the mist eliminator installed on the downstream side of the gas-liquid contact part of the wet desulfurization apparatus, so that the exhaust gas and the mercury oxidant desulfurized in the mist eliminator are mixed. In contact, mercury is oxidized and removed. Since sulfur oxides in the exhaust gas are removed at the gas-liquid contact area, the mist eliminator on the downstream side of the exhaust gas suppresses the consumption of the mercury oxidant by the sulfur oxides. Metallic mercury is oxidized and removed.

  As described above, since the metallic mercury that has not been removed in the gas-liquid contact portion of the wet desulfurization apparatus is efficiently removed on the downstream side of the gas-liquid contact portion, the concentration of mercury discharged to the outside air can be reduced. In addition, conventionally, it has been confirmed that mercury oxide removed by a wet desulfurization apparatus is reduced in the desulfurization absorption liquid and is released again into the exhaust gas as metallic mercury. Since this is oxidized and removed by the mist eliminator, such a problem does not occur.

  In particular, when lignite is used as the fuel, the moisture content is high, so there is a concern that the amount of moisture in the combustion exhaust gas will increase, resulting in an increase in the desulfurization absorption liquid temperature and an increase in the amount of mercury re-released. . However, according to the present invention, the metal mercury concentration in the exhaust gas at the outlet of the desulfurization apparatus can be kept low.

  As the mercury oxidizing agent, a chlorine-based oxidizing agent such as hypochlorite or an oxidizing agent containing a metal such as copper, iron or manganese can be used as an aqueous solution. In particular, when a chlorine-based oxidant such as hypochlorite is used, chlorine and hydrogen chloride are released downstream of the contact part of the exhaust gas and the mercury oxidant. Among the eliminators, at least one mist eliminator from the downstream side of the exhaust gas flow path is formed by using water containing no mercury oxidant, for example, industrial water or alkaline aqueous solution such as sodium hydroxide aqueous solution as cleaning water. Hydrogen chloride can be recovered from the exhaust gas.

  In addition, in this specification, industrial water is water used for industrial production, and usually has a pH in a neutral range (about 6 to 8). Specifically, cooling, washing, Water used for product processing, etc., and purified water of sewage is used in addition to water, groundwater, river water.

  Therefore, according to the invention described in claim 1 or claim 4, the mist eliminator installed on the downstream side of the gas-liquid contact portion of the wet desulfurization apparatus is washed with water containing a mercury oxidizing agent, thereby Mercury is oxidized and removed. In addition, by using cleaning water that does not contain a mercury oxidant in the at least one mist eliminator from the downstream side of the exhaust gas flow path, even if harmful substances derived from the mercury oxidant are generated, they are simultaneously removed.

  And the water containing a mercury oxidizing agent can be used effectively by recycling the mercury oxidizing agent. Further, a part of the water containing the mercury oxidant after use and the water not containing the mercury oxidant after use can be used as makeup water for the absorption tower.

  Mercury oxidizer is consumed by reaction with metallic mercury. Mercury oxidation performance is also affected by the pH of the wash water. That is, in order to reduce the mercury concentration in the exhaust gas at the outlet of the desulfurization apparatus, it is necessary to efficiently oxidize metallic mercury in the exhaust gas. Therefore, according to the invention of claim 2 or claim 5, in addition to the action of the invention of claim 1 or claim 4, while measuring the pH and ORP (redox potential) of the wash water, By replenishing the mercury oxidant, waste and shortage of the mercury oxidant can be prevented. Moreover, the removal performance of mercury can be improved by adjusting the pH of the washing water to a range in which metallic mercury is oxidized with high efficiency by the pH adjuster.

  And according to invention of Claim 3 or Claim 6, in addition to the effect | action of the said invention of Claim 1 or Claim 4, it replenishes mercury oxidizing agent, measuring pH and ORP of washing water. By doing so, waste and shortage of the mercury oxidant can be prevented. Moreover, the removal performance of mercury can be improved by adjusting the pH of the cleaning water with the treated exhaust gas to a range in which metallic mercury is oxidized with high efficiency.

According to the present invention, mercury released from a wet desulfurization apparatus can be efficiently removed from exhaust gas with a small amount of oxidizing agent. Specifically, the following effects are exhibited.
According to the first or fourth aspect of the invention, mercury is efficiently used by using cleaning water containing a mercury oxidizer for cleaning a mist eliminator provided downstream of the gas-liquid contact portion of the wet desulfurization apparatus. Therefore, the mercury concentration discharged to the outside air can be reduced. Further, by using cleaning water that does not contain a mercury oxidant as the cleaning water downstream of the exhaust gas flow path of the mist eliminator, harmful substances derived from the mercury oxidant are simultaneously removed.

  Further, the mercury oxidant can be effectively used by circulating water containing the mercury oxidant. Further, a part of the water containing the mercury oxidant after use and the water not containing the mercury oxidant after use can be used as make-up water for the absorption tower, so that make-up water can be saved.

  According to the invention described in claim 2 or 5, in addition to the effect of the invention described in claim 1 or 4, the mercury oxidant is replenished while measuring the pH and ORP of the washing water. Thus, waste and shortage of mercury oxidizer can be prevented. Moreover, the pH of the cleaning water can be adjusted to an appropriate range in which metallic mercury is oxidized with high efficiency by the pH adjuster, and the mercury removal performance can be improved.

  According to the invention described in claim 3 or claim 6, in addition to the effect of the invention described in claim 1 or 4, the mercury oxidant is replenished while measuring the pH and ORP of the washing water. Thus, waste and shortage of mercury oxidizer can be prevented. Further, by effectively using the treated exhaust gas, the pH of the cleaning water can be adjusted to an appropriate range in which metallic mercury is oxidized with high efficiency, and the mercury removal performance can be improved.

It is a figure which shows the system | strain of the wet desulfurization apparatus which is one Example (Example 1) of this invention. It is the figure which showed the relationship between the addition amount of the mercury oxidizing agent and ORP on conditions with constant pH. It is the figure which showed the relationship between the pH of sodium hypochlorite aqueous solution at the time of using sodium hypochlorite as a mercury oxidizing agent, and the removal performance of metallic mercury. It is the figure which showed the relationship between pH of sodium hypochlorite aqueous solution at the time of using sodium hypochlorite as a mercury oxidizing agent, and the discharge | release amount of chlorine. It is a figure which shows the system | strain of the wet desulfurization apparatus of the other Example (Example 2) of this invention. It is a figure which shows the system | strain of the wet desulfurization apparatus of a prior art.

  Hereinafter, embodiments of the present invention will be described.

  In FIG. 1, the system | strain of the wet desulfurization apparatus which is one Example of this invention is shown. In the wet desulfurization apparatus shown in FIG. 1, a part of the members having the same reference numerals as those of the wet desulfurization apparatus shown in FIG. The wet desulfurization apparatus shown in FIG. 1 assumes a wet desulfurization apparatus for a coal-fired boiler, but can be applied to other combustion furnaces as appropriate.

Coal and combustion air are supplied to a boiler (not shown) to burn the coal, and the combustion exhaust gas is introduced into the absorption tower 1 of the wet desulfurization device 20 via a denitration device, a dry dust collector and the like (not shown).
In the wet desulfurization apparatus 20, in the absorption tower 1, desulfurization absorption liquid containing an absorbent such as slurry containing limestone or lime is sprayed as fine droplets from the desulfurization spray nozzle 8 (gas-liquid contact portion), and the desulfurization absorption liquid By bringing the droplet and the exhaust gas into gas-liquid contact, SOx in the exhaust gas is chemically absorbed and removed by the absorbing droplet surface of the desulfurization spray nozzle 8 together with soot dust and acidic gas such as HCl and HF in the exhaust gas.

The absorbing solution that has absorbed SOx (mainly SO ₂ ) once accumulates in the circulation tank 9 at the bottom of the absorption tower 1, is oxidized by oxygen in the air supplied from the air supply line 11, and is stirred by the stirrer 10 to form gypsum. Is generated. The circulation tank 9 is supplied with an absorbent from the limestone slurry supply device 3 in accordance with the amount of SOx contained in the exhaust gas from the boiler or the like. The slurry-like absorption liquid in the circulation tank 9 is pressurized by the absorption liquid circulation pump 7 and supplied from the absorption liquid circulation pipe 18 to the desulfurization spray nozzle 8 in the upper part of the absorption tower 1.

  The absorbent extracted from the circulation tank 9 or the absorbent circulation pipe 18 is separated from the gypsum by the gypsum dehydrator 4 and the gypsum 12 is recovered. The collected gypsum 12 is reused as building materials. Further, the absorption liquid separated from the gypsum 12 is used for adjusting an absorbent such as limestone slurry, returned to the absorption tower 1 from the absorption liquid return line 48, or partly sent to the waste water treatment device 5. It is discharged.

The exhaust gas treated in the absorption tower 1 is discharged from the chimney 6 through the mist eliminator 2. The exhaust gas from which SO ₂ has been removed by gas-liquid contact with the desulfurization absorbing liquid from the desulfurization spray nozzle 8 contains fine mist, and is removed by the mist eliminator 2 installed at the outlet of the absorption tower 1. In the mist eliminator 2, a mist eliminator cleaning device 50 for cleaning the mist eliminator 2 is installed in order to prevent clogging due to solid content such as gypsum contained in the mist.

  In this embodiment, a mercury oxidant (for example, sodium hypochlorite) is supplied to the cleaning water tank 22 (originally containing industrial water) of the mist eliminator cleaning device 50 as an oxidant supply line (oxidant supply unit) 14. The exhaust gas after desulfurization treatment and the cleaning water containing the mercury oxidizing agent are brought into contact with each other to remove metallic mercury in the exhaust gas. As the mercury oxidizing agent, in addition to a chlorine-based oxidizing agent such as hypochlorite, an oxidizing agent containing a metal such as copper, iron, or manganese can be used as an aqueous solution.

The exhaust gas containing SO ₂ introduced into the absorption tower 1 is removed by the desulfurization absorbent from the desulfurization spray nozzle 8 and introduced into the mist eliminator 2. The mist eliminator 2 is cleaned by spraying cleaning water containing a mercury oxidizing agent from the cleaning spray nozzle 26 through the cleaning water line 24.

  The mist eliminator 2 is installed in a plurality of stages in the flow direction of the exhaust gas, and at least one stage of the mist eliminator 2 from the downstream side to the upstream side of the exhaust gas passage uses water containing no mercury oxidizing agent. In the illustrated example, a two-stage mist eliminator 2 is installed, and cleaning water containing mercury oxidant is used for the cleaning spray nozzle (mercury oxidant cleaning unit) 26a on the upstream side of the exhaust gas flow. The non-mercury oxidant cleaning part) 26b shows a case where industrial water not containing a mercury oxidant is used.

  A three-stage mist eliminator 2 is installed, cleaning water containing mercury oxidant is used in the second stage upstream of the exhaust gas flow, and industrial water not containing mercury oxidant is used in the first stage downstream. May be used. From the viewpoint of mercury removal, it is preferable that the number of stages using washing water containing a mercury oxidizing agent is larger than the number of stages using washing water containing no mercury oxidizing agent. The industrial water is supplied from the industrial water line 13 to the downstream cleaning spray nozzle 26b.

  When a chlorine-based oxidant such as hypochlorite is used, chlorine, hydrogen chloride, and the like are released by the oxidation reaction, but industrial water is sprayed from the downstream cleaning spray nozzle 26b. Can be absorbed and removed. In addition to industrial water, chlorine or hydrogen chloride can be recovered from the exhaust gas by using an alkaline aqueous solution such as an aqueous sodium hydroxide solution as washing water.

  According to the present embodiment, the mercury is efficiently removed by bringing the exhaust gas after the desulfurization treatment into contact with the cleaning water containing the mercury oxidizing agent, so that the concentration of mercury discharged to the outside air can be reduced. Further, by using cleaning water containing no mercury oxidizing agent for the downstream cleaning spray nozzle 26b, even if released chlorine derived from hypochlorite is generated, it is simultaneously removed.

  Further, a part of the upstream cleaning liquid containing the mercury oxidant is returned to the cleaning water tank 22 from the cleaning water circulation line 28 and recycled. The washing water circulation line 28, the washing water tank 22, the washing water line 24, and the like constitute a washing water circulation supply unit. Then, a part of the cleaning liquid is extracted from the cleaning water circulation line 28 and returned to the circulation tank 9 and the desulfurization spray nozzle 8 in the absorption tower 1, so that the absorption liquid can be effectively used, and the makeup water of the wet desulfurization apparatus 20 can be saved. Is also connected.

  Further, the cleaning liquid not containing the mercury oxidizer on the downstream side is also allowed to flow from the cleaning water flow line (makeup water supply unit) 32 to the circulation tank 9 of the absorption tower 1 after the mist eliminator 2 is cleaned. Use as makeup water.

The reason for this is that the upstream cleaning solution contains an oxidizing agent and is circulated for its effective use, while the downstream cleaning solution may contain impurities such as chlorine and SO _2. This is because it is a very small amount and can be sufficiently used as industrial water as makeup water for the absorption tower. Further, although the upstream cleaning solution contains Cl, Cl is also contained in the absorbing solution, and since Hg is also a very small amount, there is a particular problem even if it is used as makeup water for the wet desulfurization apparatus 20. Absent.

  Then, the ORP measuring device (which may be an ORP meter or the like) 16 measures the ORP of the cleaning water in the cleaning water tank 22 or the cleaning water line 24, and the mercury oxidant so as to obtain a predetermined ORP under a constant pH condition. The supply amount is controlled. Specifically, the control device 17 controls the adjustment valve (oxidant amount adjustment unit) 30 of the oxidant supply line 14. Further, the pH of the cleaning water in the cleaning water tank 22 or the cleaning water line 24 is measured by a pH measuring device 19 (which may be a pH meter or the like), and the adjusting agent supply line is controlled by the control device 17 so that the pH is maintained at a predetermined value. Fifteen adjustment valves (pH adjuster amount adjustment unit) 36 are controlled. In addition, although a separate control apparatus may be used for adjustment of ORP and pH, by using the same control apparatus 17, it becomes a simple structure.

FIG. 2 shows the relationship between the addition amount of mercury oxidant (expressed as the concentration of oxidant) and ORP under a constant pH condition.
Sodium hypochlorite was added to industrial water to measure pH and ORP. The pH was adjusted with hydrochloric acid. As shown in this figure, there is a correlation between the oxidant concentration and the ORP. When the pH is a predetermined value (constant) (for example, pH = 6) is indicated by a solid line X, when the pH is lower than the predetermined value (for example, pH = 5), the ORP becomes high (dotted line X1), and the pH is When it is high (for example, pH = 7), the ORP becomes low (dotted line X2). Thus, although the ORP to be adjusted varies depending on the pH, the relationship between the oxidant concentration (unit: mg / L) and the ORP (unit: mV) at each pH is stored in the control device 17, and from this relationship The supply of mercury oxidant is controlled.

  That is, the ORP is not less than a predetermined value A while adjusting the pH, which is changed by adding a mercury oxidizing agent or absorbing a trace amount of harmful substances remaining in the exhaust gas, to be kept constant with a pH adjusting agent (for example, hydrochloric acid). Mercury oxidizer is added so that The predetermined value A is preferably 600 mV or more. The pH range will be described later, but ORP capable of maintaining an effective mercury oxidation rate even at pH 7 is 600 mV or more.

  Since the relationship between the ORP and the amount of mercury oxidant added under a constant pH condition varies depending on the industrial water used, for example, the amount of oxidizer added at an ORP of 600 mV at pH 7 varies depending on the industrial water.

  First, a sufficient amount of mercury oxidant is supplied (the initial supply amount is determined so as to obtain a predetermined ORP in advance), and the mercury oxidant is consumed and ORP falls below a predetermined value A (600 mV). When it comes, it supplies mercury oxidant. The pH that is kept constant will be described in detail below.

  FIG. 3 shows the relationship between the pH of the sodium hypochlorite aqueous solution and the metal mercury removal performance when sodium hypochlorite is used as the mercury oxidizing agent, and FIG. 4 shows the sodium hypochlorite aqueous solution. Shows the relationship between the pH and the amount of chlorine released.

As a measuring method, using a small spray device, a gas in which mercury vapor was mixed with air and a spray solution were brought into contact with each other. A solution obtained by adding sodium hypochlorite to industrial water was used as a spray solution, and the pH was adjusted with a pH adjuster (hydrochloric acid). That is, the concentration (amount) of sodium hypochlorite was constant, and only the pH was changed. The Hg concentration was measured at the inlet and outlet of the spray device, and the Cl concentration in the gas was measured at the outlet of the spray device. As measurement conditions, the inlet metal mercury concentration was 1 μg / m ³ N, the gas amount was 10 NL / min, and the spray liquid amount was 500 ml / min.

  As shown in FIG. 3 and FIG. 4, it can be seen that when the pH of the sodium hypochlorite aqueous solution is lowered, the metal mercury removal performance is improved, but the amount of chlorine released from sodium hypochlorite is increased. Therefore, when sodium hypochlorite aqueous solution is used as the mercury oxidant, the pH of the cleaning water is in the range of 5 to 7, that is, in a proper range in which metal mercury is oxidized with high efficiency, thereby releasing chlorine. A high metal mercury removal rate can be obtained while keeping the amount within the regulation range.

When the metal mercury removal rate is improved by adjusting the pH, if the pH exceeds 7, the improvement of the removal rate cannot be expected, and if the pH falls below 5, the amount of chlorine discharged from the chimney 6 is regulated (Cl ₂ as 30 mg / Nm ³ ) may be exceeded. Therefore, the pH is preferably set from the range of 5-7. By maintaining ORP at 600 mV or higher, an effective mercury oxidation rate can be maintained even at pH 7.

According to FIG. 4, although the Cl concentration is slightly higher at pH 5, it is removed to some extent by the downstream washing water not containing the mercury oxidizer, so the Cl concentration at the smoke outlet is slightly lowered.
Further, the pH range varies depending on the type of mercury oxidant and the type of harmful substance derived from the mercury oxidant, and is appropriately determined from the relationship between the removal performance of metallic mercury and the amount of harmful substance.

Thus, the adjustment of the oxidant supply amount is aimed at ORP control under a constant pH condition, and the pH adjustment is aimed at improving the mercury oxidation performance.
In this embodiment, sodium hypochlorite is used as the mercury oxidizing agent, and the pH of the cleaning liquid in the cleaning water tank 22 is measured by the pH measuring device 19 so that the measured value becomes 5 to 7. By controlling the adjustment valve 36 of the supply line 15 and supplying hydrochloric acid as a pH adjuster, a high metal mercury removal rate can be obtained while suppressing the amount of released chlorine.

  In this embodiment, the pH and ORP of the cleaning liquid in the cleaning water tank 22 for supplying the mercury oxidizing agent and the pH adjusting agent are measured, but the merging line to the cleaning water circulation line 28 and the cleaning water flow down line 32 is measured. pH and ORP are measured and supplied into the wash water tank 22 from the amount of change from the initial value (preset or measured) of the wash water in the wash water tank 22 or the wash water line 24 at the start of control. The structure which calculates | requires the quantity of a mercury oxidizing agent and a pH adjuster, and controls these supply quantities may be sufficient.

  That is, the measurement location of pH and ORP may be different from the supply location of the pH adjuster or mercury oxidant. In short, it is only necessary that the cleaning water of the upstream mist eliminator 2 can maintain a predetermined pH and ORP. For example, a pH adjusting agent or a mercury oxidizing agent may be directly supplied to the upstream cleaning spray nozzle 26a.

FIG. 5 shows another embodiment. In addition, description of the member of the same code | symbol as the wet desulfurization apparatus of FIG. 1 is abbreviate | omitted.
In this embodiment, not only the mercury oxidant but also the treated exhaust gas containing carbon dioxide is introduced into the cleaning water tank 22 of the mist eliminator cleaning device 50 from the exhaust gas return line (exhaust gas return supply unit) 42. That is, it differs from the desulfurization apparatus of FIG. 1 in that an exhaust gas return line 42 and an exhaust gas adjustment valve (exhaust gas amount adjusting unit) 40 are provided instead of the adjusting agent supply line 15 and its adjustment valve 36. And by controlling the exhaust gas regulating valve 40 of the exhaust gas return line 42 so that the pH of the washing water becomes 5 to 7, a high metal mercury removal rate can be obtained while suppressing the amount of released chlorine as in the first embodiment. It is done.

Since the exhaust gas contains a large amount of carbon dioxide generated by the combustion of coal, by returning the pH of the cleaning liquid inclined to alkalinity to the acidic side due to the influence of sodium hypochlorite used as a mercury oxidant, The pH can be adjusted to an appropriate range. In addition, since SO ₂ and metallic mercury in the exhaust gas are removed, there is no problem even if it is returned to the absorption tower 1.

Compared with Example 1, it is not necessary to use a pH adjusting agent by effectively using the treated exhaust gas, so that the cost is reduced.
Also in this embodiment, at least one stage of the mist eliminator 2 from the downstream side to the upstream side of the exhaust gas passage uses water that does not contain a mercury oxidizing agent (for example, industrial water or aqueous sodium hydroxide solution). . This cleaning liquid is allowed to flow down from the cleaning water flow line 32 to the circulation tank 9 of the absorption tower 1 after being cleaned by the mist eliminator 2 and is used as makeup water for the absorption tower 1. The upstream cleaning liquid after cleaning the mist eliminator 2 may be returned from the cleaning water circulation line 28 to the circulation tank 9 and the desulfurization spray nozzle 8 in the absorption tower 1.

Also in this embodiment, the measurement location of pH and ORP may be different from the supply location of the pH adjuster or mercury oxidant. Further, the supply amount control of the exhaust gas and the mercury oxidant may be the same as the supply amount control of the pH adjuster and the mercury oxidant in Example 1.
Therefore, the present embodiment can achieve the same effects as those of the first embodiment.

  In other combustion furnaces than boilers, it may be used as a technique for removing Hg in exhaust gas.

DESCRIPTION OF SYMBOLS 1 Absorption tower 2 Mist eliminator 3 Limestone slurry supply device 4 Gypsum dehydrator 5 Waste water treatment device 6 Chimney 7 Absorption liquid circulation pump 8 Desulfurization spray nozzle 9 Circulation tank 10 Stirrer 11 Air supply line 12 Gypsum 13 Industrial water line 14 Oxidant supply line 15 Adjusting agent supply line 16 ORP measuring device 17 Control device 18 Absorbing liquid circulation pipe 19 pH measuring device 20 Wet desulfurization device 22 Washing water tank 24 Washing water line 26 Washing spray nozzle 28 Washing water circulation line 30, 36, 40 Regulating valve 32 Washing water flow down Line 42 Exhaust gas return line 48 Absorbent return line 50 Mist eliminator cleaning device

Claims (6)

A gas-liquid contact part that absorbs and removes sulfur oxides in the exhaust gas by gas-liquid contact between the exhaust gas discharged from the combustion apparatus including the boiler and the absorbent, and a downstream side of the exhaust gas flow path of the gas-liquid contact part. In a wet desulfurization apparatus provided with a plurality of stages in the exhaust gas flow direction and having an absorption tower having a mist eliminator that removes mist in the exhaust gas,
A cleaning device for cleaning the mist eliminator is provided,
The cleaning apparatus is a non-mercury oxidant cleaning that uses water that does not contain a mercury oxidant for cleaning at least one mist eliminator from the downstream side to the upstream side of the exhaust gas flow path among the plurality of mist eliminators. And the other mist eliminator is equipped with a mercury oxidizer cleaning section using water containing mercury oxidizer,
A circulation supply unit for extracting cleaning water after use in the mercury oxidant cleaning unit and circulatingly supplying the mercury oxidant cleaning unit,
A wet type characterized in that a part of the used cleaning water is extracted in the mercury oxidant cleaning unit, and a makeup water supply unit is provided in the non-mercury oxidant cleaning unit to supply the absorption tower together with the used cleaning water. Desulfurization equipment. A pH adjuster supply section for supplying a pH adjuster to the mercury oxidizing agent cleaning section;
A pH adjuster amount adjusting unit for adjusting a supply amount of the pH adjuster from the pH adjuster supplying unit to the mercury oxidizing agent cleaning unit;
A pH measurement unit for measuring the pH of the cleaning water after use in the mercury oxidant cleaning unit;
An oxidizing agent supply unit for supplying a mercury oxidizing agent to the mercury oxidizing agent cleaning unit;
An oxidant amount adjusting unit for adjusting a supply amount of the mercury oxidant from the oxidant supply unit to the mercury oxidant cleaning unit;
An ORP measuring unit for measuring the ORP of the cleaning water after use in the mercury oxidizing agent cleaning unit;
The control unit for controlling the pH adjusting agent amount adjusting unit and the oxidizing agent amount adjusting unit based on the pH measured by the pH measuring unit and the ORP measured by the ORP measuring unit. The wet desulfurization apparatus according to 1. An exhaust gas return supply unit for supplying the exhaust gas treated in the absorption tower to the mercury oxidant cleaning unit;
An exhaust gas amount adjusting unit for adjusting an exhaust gas supply amount from the exhaust gas return supply unit to the mercury oxidant cleaning unit;
A pH measurement unit for measuring the pH of the cleaning water after use in the mercury oxidant cleaning unit;
An oxidizing agent supply unit for supplying a mercury oxidizing agent to the mercury oxidizing agent cleaning unit;
An oxidant amount adjusting unit for adjusting a supply amount of the mercury oxidant from the oxidant supply unit to the mercury oxidant cleaning unit;
An ORP measuring unit for measuring the ORP of the cleaning water after use in the mercury oxidizing agent cleaning unit;
The control unit for controlling the exhaust gas amount adjusting unit and the oxidant amount adjusting unit based on the pH measured by the pH measuring unit and the ORP measured by the ORP measuring unit is provided. Wet desulfurization equipment. A gas-liquid contact part that absorbs and removes sulfur oxides in the exhaust gas by gas-liquid contact between the exhaust gas discharged from the combustion apparatus including the boiler and the absorbent, and a downstream side of the exhaust gas flow path of the gas-liquid contact part. In the wet desulfurization method for removing sulfur oxides in the exhaust gas by an absorption tower having a plurality of stages in the flow direction of the exhaust gas and having a mist eliminator that removes mist in the exhaust gas,
Among the plurality of mist eliminators, at least one mist eliminator is washed with water containing no mercury oxidant from the downstream side to the upstream side of the exhaust gas flow path, and the other mist eliminators contain a mercury oxidant. Water containing mercury oxidant after use for cleaning is recycled as mist eliminator wash water, and part of water containing mercury oxidant used for cleaning and mercury oxidation after use for cleaning A wet desulfurization method characterized in that water not containing an agent is used as makeup water for an absorption tower.   Measuring pH and ORP of water containing mercury oxidant after use for cleaning, supplying pH adjusting agent and mercury oxidant to water containing mercury oxidant based on the measured value, and using for cleaning The wet desulfurization method according to claim 4, wherein the wet desulfurization method is performed.   Measure the pH and ORP of the water containing mercury oxidant after use for cleaning, and supply the exhaust gas and mercury oxidant treated in the absorption tower to the water containing mercury oxidant based on the measured value. The wet desulfurization method according to claim 4, which is used.

Images