JP2009208078A - Method for treating mercury in tail gas, and treatment system for tail gas - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for treating a tail gas that can remove mercury and enables efficient operation and continuous performance without having a bad effect on an apparatus in a system. <P>SOLUTION: The method for treating mercury in a tail gas includes subjecting the tail gas containing a nitrogen oxide, a sulfur oxide and mercury to reduction denitration in the presence of a solid catalyst after the addition of a chlorination agent and then performing wet desulfurization with an alkaline lean liquid. A predicted value of an inlet mercury concentration before the reduction denitration is calculated based on the mercury concentration in the tail gas measured after the wet desulfurization. The feed rate of the chlorination agent to be added before the reduction denitration is controlled based on the amounts of change of the above predicted value and a standard inlet mercury concentration by the method for treating mercury in a tail gas and a treatment system for the tail gas is provided. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a flue gas treatment method, that is, a mercury removal method in a flue gas treatment system, and more particularly to a method for effectively removing metallic mercury from exhaust gas in a system for desulfurizing a large amount of exhaust gas.

There are harmful trace substances such as mercury in coal and heavy oil-fired exhaust gas, and it is generally difficult to remove them in current smoke treatment systems. Mercury is considered to exist mainly in the exhaust gas as metallic mercury (Hg) or mercury chloride (HgCl ₂ ). HgCl ₂ is easily absorbed by water and can be removed by a desulfurization absorption tower or the like. However, since mercury mercury (Hg) has extremely low solubility in water, it is not absorbed by the desulfurization absorption tower. Mercury vapor may be discharged from the chimney. Therefore, conventionally, an activated carbon adsorption method, a sodium hypochlorite absorption method, or the like is used as the Hg removal technique.

  As the activated carbon adsorption method, a method of blowing activated carbon powder into exhaust gas and collecting it with a bag filter has already been put into practical use. However, there is no example of applying mainly to waste incineration exhaust gas to large-capacity gas such as power plant exhaust gas. Further, as a sodium hypochlorite absorption method, for example, an additive such as sodium hypochlorite in cooling water of a cooling tower or absorption liquid of a desulfurization absorption tower, or supply water or circulating water of a wet electric dust collector A method of directly adding is known. However, all add additives to the main equipment of the exhaust gas treatment plant, and there is a concern that the essential functions are hindered by the additives. For example, since the cooling tower has a low pH, a large amount of oxidizing agent is required, a peroxidation material is generated in the absorption tower, and sulfur dioxide is oxidized and the acidity becomes strong in the wet electrostatic precipitator. Moreover, it mainly targets waste incineration exhaust gas and is not suitable for large-capacity gas such as power plant exhaust gas.

  On the other hand, as described above, metallic mercury is difficult to dissolve in water and thus passes through the desulfurization apparatus. However, if it can be made water-soluble, it can be removed by the desulfurization apparatus. In view of this, it is conceivable that in a denitration apparatus filled with a catalyst, metal mercury is converted to mercury chloride which is easily soluble in water on the catalyst, so that it can be removed by a downstream desulfurization apparatus. That is, a flue gas treatment method in which a chlorinating agent (hydrogen chloride or the like) that converts metallic mercury into mercury chloride is injected before the denitration apparatus is effective.

  However, adding more than necessary chlorinating agent has become a causative substance for the flue in the system and the downstream device, which ultimately shortens the life of the plant equipment. In addition, when a certain amount of chlorinating agent is simply injected, the utility cost increases.

  That is, after the denitration apparatus, it is usually arranged in the order of an air heater, a dust collector, a gas gas heater (heat exchanger), and a desulfurization absorption tower. In particular, a device using a chlorinating agent in a heat exchanger where cooling is performed. The effect on corrosion and breakage is great. Also, in the desulfurization absorption tower, since the chlorinating agent is mixed, the chlorine concentration of the absorption liquid rises, and the corrosion and breakage of the metal part in the tower becomes a problem.

  Furthermore, when the chlorine concentration in the desulfurization absorption tower rises, there arises a problem that the oxidation performance at the time of desulfurization or the desulfurization performance itself is lowered, which may cause the performance of the entire system to be lowered. Further, as the salt concentration increases, the foaming property of the absorbing liquid increases, and there is a possibility that the operating power increases due to an increase in the pressure loss in the absorption tower.

Japanese Patent Laid-Open No. 10-230137 Japanese Patent Laid-Open No. 2-191526 JP-A-9-308817

  In view of the above problems, the present inventors have provided a mercury removal method in a flue gas treatment system capable of removing mercury, particularly metal mercury vapor, contained in a large volume gas such as power plant exhaust gas. We have intensively studied to develop a method that enables efficient operation and maintenance of the system without adversely affecting the downstream equipment by appropriately controlling and adjusting the amount of chlorinating agent added for removal. . As a result, the present inventors do not simply introduce the chlorinating agent at the preceding stage of the desulfurization apparatus, but, for example, at the outlet of the desulfurization absorption tower, the dust collector outlet, or the reheater outlet after desulfurization. It has been found that such problems can be solved by continuously monitoring the amount of chlorinating agent and appropriately adding the necessary and sufficient amount of chlorinating agent. The present invention has been completed from such a viewpoint.

That is, the present invention is a mercury treatment method in exhaust gas in which exhaust gas containing nitrogen oxides, sulfur oxides and mercury is subjected to reductive denitration treatment in the presence of a solid catalyst after addition of a chlorinating agent, and then subjected to wet desulfurization with an alkali absorbent. And measuring the mercury concentration of the exhaust gas after the wet desulfurization, calculating a predicted value of the inlet mercury concentration before the reductive denitration treatment based on the mercury concentration, and an amount of change between the predicted value and the reference inlet mercury concentration Therefore, the present invention provides a method for treating mercury in exhaust gas, characterized in that the supply amount of a chlorinating agent added in the previous stage of reductive denitration treatment is adjusted. In the present invention, the boiler load signal is attributed to the determination of the reference inlet mercury concentration. The mercury concentration in the exhaust gas after the wet desulfurization can be measured at the front stage of any of the wet dust collector, the reheater or the chimney provided downstream of the desulfurization absorption tower. In addition to the desulfurization absorption tower, mercury chloride is also removed by the cooling tower when the cooling tower is installed at the front stage of the desulfurization absorption tower, so it can be measured at the front stage of the desulfurization absorption tower provided at the subsequent stage. . Here, the reference inlet mercury concentration is calculated from the mercury concentration measured at the boiler outlet in advance for each coal type or the mercury content in the coal because the inlet mercury concentration changes depending on the boiler load and coal type. Expressed as the total concentration of mercury and mercury chloride.

Further, in a form to be a reference of the present invention , exhaust gas containing nitrogen oxides, sulfur oxides and mercury is subjected to reductive denitration treatment under a solid catalyst after addition of a chlorinating agent, and then wet desulfurized with an alkali absorbent. A method for treating mercury in exhaust gas, comprising measuring a mercury concentration of the exhaust gas before wet desulfurization, calculating a predicted value of outlet mercury concentration after wet desulfurization based on the mercury concentration, and calculating the predicted value and a reference outlet An object of the present invention is to provide a method for treating mercury in exhaust gas, characterized in that the supply amount of a chlorinating agent added at the previous stage of reductive denitration treatment is adjusted from the amount of change from mercury concentration. The mercury concentration in the exhaust gas before wet desulfurization can be measured at any stage before any of a denitration device, an air heater (A / H), a heat exchanger, an electrostatic precipitator, or a desulfurization device. Here, the reference outlet mercury concentration is the outlet target mercury concentration, but since there is a response delay in normal control, it is a value obtained by subtracting the fluctuation width of the concentration from the upper limit value of the discharged mercury concentration.

Furthermore, in another form of reference of the present invention , exhaust gas containing nitrogen oxides, sulfur oxides and mercury is subjected to reductive denitration treatment under a solid catalyst after addition of a chlorinating agent, and then wet desulfurized with an alkali absorbent. A method for treating mercury in exhaust gas, measuring mercury concentration of the exhaust gas before and after wet desulfurization, and adding the mercury concentration before the reductive denitration treatment from the amount of change between the mercury concentration and the reference mercury concentration The present invention provides a method for treating mercury in exhaust gas, characterized by adjusting the supply amount of a chlorinating agent. According to such a method, the accuracy of the mercury concentration prediction value can be improved by detecting the operation load signal of the boiler load, the electrostatic precipitator, and the desulfurization apparatus.

Further, the present specification describes wet desulfurization by adding a chlorinating agent from a chlorinating agent supply device to exhaust gas containing nitrogen oxides, sulfur oxides and mercury, and performing a denitration treatment under a solid catalyst of a reducing denitration device. An exhaust gas treatment system for performing desulfurization with an alkali absorbing liquid in the apparatus, wherein a mercury concentration meter is provided at a subsequent stage of the wet desulfurization device, and from a calculator and a controller connected to the mercury concentration meter. The present invention also provides an exhaust gas treatment system characterized by sending a flow rate adjustment signal to the chlorinating agent supply device. Here, the above computing unit mainly calculates the estimated value of the inlet mercury concentration before the reductive denitration treatment based on the mercury concentration in the exhaust gas measured after wet desulfurization and before the wet dust collector, reheater or chimney. calculate. The regulator adjusts the chlorinating agent supply flow rate based on a deviation signal between the inlet mercury concentration calculated by the calculator and a preset reference inlet mercury concentration.

According to the present invention, in the removal of mercury in a flue gas treatment system capable of removing mercury, particularly metal mercury vapor, contained in a large volume gas such as power plant exhaust gas, the amount of hydrogen chloride added for mercury removal By properly controlling and adjusting the system, it is possible to efficiently operate and maintain the performance of the system without adversely affecting the downstream device. More specifically, for the devices such as air heaters, dust collectors, gas gas heaters (heat exchangers), and desulfurization absorption towers that are installed downstream of the denitration equipment, the problem of corrosion and damage due to the addition of an excessive amount of chlorinating agent is effective. Can be prevented. Also, the increase in chlorine concentration in the desulfurization absorption tower prevents the oxidation performance and desulfurization performance during desulfurization from decreasing, or prevents the foaming property of the absorbent from increasing, maintaining the overall system performance including desulfurization performance or improving the performance. Can be achieved. Furthermore, according to the present invention, the utility cost during operation can be kept low by optimizing the supply amount of a chlorinating agent such as hydrogen chloride and reducing the amount of excessive addition.

  According to the present invention, in the removal of mercury in a flue gas treatment system capable of removing mercury, particularly metal mercury vapor, contained in a large volume gas such as power plant exhaust gas, the amount of hydrogen chloride added for mercury removal By properly controlling and adjusting the system, it is possible to efficiently operate and maintain the performance of the system without adversely affecting the downstream device. According to the present invention, there is a problem of corrosion and damage due to the addition of an excessive amount of chlorinating agent in devices such as an air heater, a dust collector, a gas gas heater (heat exchanger), a desulfurization absorption tower, etc. provided downstream of the denitration device. It can be effectively prevented. In particular, it avoids the problem of corrosion of heat exchangers that are cooled, and the problem of corrosion and breakage of metal parts due to the increase of chlorine concentration in the absorption liquid due to the mixing of hydrogen chloride in the desulfurization absorption tower. it can. In addition, the increase in chlorine concentration in the desulfurization absorption tower reduces the oxidation performance and desulfurization performance at the time of desulfurization, or prevents the foaming property of the absorbent from increasing, thereby maintaining and improving the performance of the entire system, including desulfurization performance. You can plan. Furthermore, according to the present invention, the utility cost during operation can be kept low by optimizing the supply amount of a chlorinating agent such as hydrogen chloride and reducing the amount of excessive addition.

It is a general | schematic process flowchart which shows an example of the mercury processing method of this invention. It is a process flow figure showing an outline of a testing device used in Example 1 of the present invention. It is a figure showing the conditions and measurement result in Example 1 at the time of using the system of FIG. It is a process flowchart which shows one example of the control system in the case of implementing the mercury processing method of this invention. It is a process flowchart which shows another example used as the reference of the control system in the case of implementing the mercury processing method of this invention. It is a process flowchart which shows another example used as the reference of the control system in the case of implementing the mercury processing method of this invention. It is a figure showing the conditions and measurement result in Example 1 at the time of using the system of FIG. It is a figure showing the conditions and measurement result in Example 1 at the time of using the system of FIG.

  In the exhaust gas treatment of the present invention, exhaust gas containing nitrogen oxides (NOx), sulfur oxides (SOx) and mercury (Hg) is subjected to reductive denitration treatment under a solid catalyst after addition of a chlorinating agent, and then wet with an alkali absorbent. Desulfurization is performed. Then, the mercury concentration is measured for the exhaust gas before and / or after the wet desulfurization, and based on the mercury concentration, the inlet mercury concentration before the reducing denitration treatment or the predicted value of the outlet mercury concentration after the desulfurization treatment And the supply amount of the chlorinating agent to be added before the reductive denitration treatment is adjusted from the amount of change between the predicted value and the reference mercury concentration. As a result, the chlorinating agent to be added can effectively act on metallic mercury, and an adverse effect on the system due to an excessive amount of the chlorinating agent can be avoided.

As the method for controlling the addition amount of the chlorinating agent in the present invention, the following three methods are mainly considered.
That is, a method of detecting the outlet mercury concentration and controlling it with a deviation signal between the predicted value of the inlet mercury concentration and the inlet reference concentration (part 1), detecting the mercury concentration before desulfurization, and predicting the outlet mercury concentration and the outlet reference concentration Control method with deviation signal ( reference form 1 ), mercury concentration before desulfurization and outlet mercury concentration are detected, deviation signal between mercury concentration before desulfurization and reference inlet concentration, and outlet mercury concentration and desulfurization This is a method of controlling by both the deviation signal from the predicted value of the outlet mercury concentration calculated from the pre-mercury concentration ( form 2 for reference ). An example of a system using these methods is shown in FIG. 4 (part 1), FIG. 5 ( reference form 1 ), and FIG. 6 ( reference form 2 ).
Hereinafter, specific embodiments of the processing method of the present invention will be described in detail with reference to the accompanying drawings.

Embodiment (Part 1)
In the present invention, exhaust gas containing NOx, SOx and mercury is subjected to reductive denitration treatment in the presence of a solid catalyst after addition of a chlorinating agent, and then wet desulfurized with an alkali absorbent. When carrying out such a treatment method, in the desulfurization absorption tower which is an apparatus in the system, the exhaust gas comes into contact with an absorption liquid such as a lime slurry circulating liquid, and SOx is absorbed and removed. In addition, mercury chloride (HgCl ₂ ) out of mercury contained in the exhaust gas is dissolved and removed in the absorbing solution. However, metallic mercury (Hg) out of mercury is extremely low in solubility in water as usual, so it is not removed by the absorbing solution, but is contained in the desulfurization exhaust gas as metallic mercury vapor and passes through the desulfurization absorption tower 6. End up. Therefore, in the present invention, a chlorinating agent is added immediately before the desulfurization apparatus to convert metallic mercury into water-soluble mercury chloride, and then led to the desulfurization absorption tower.

  In the present invention, the chlorinating agent 3 is usually added immediately before the denitration treatment. Although various shapes can be considered for the catalyst in the reductive denitration apparatus, it is generally a titania-based oxidation catalyst having a honeycomb shape or the like. Since metal mercury can be oxidized on this catalyst (about 90% or more), a chlorinating agent such as hydrogen chloride is introduced in the preceding stage. In other words, the solid catalyst in the denitration apparatus acts as an original denitration catalyst and at the same time acts as a catalyst for converting metallic mercury into mercury chloride.

  FIG. 1 shows an outline of a system when the processing method according to this embodiment is used. In the system of FIG. 1, the exhaust gas that has passed through the denitration device 2 provided downstream of the boiler 1 passes through an air heater (A / H) 3, a heat exchanger 5 that recovers thermal energy, and a dust collector 4. It is introduced into the desulfurization absorption tower 6. Here, the dust collector 4 is not particularly limited as long as it can roughly collect dust before the exhaust gas is introduced into the desulfurization absorption tower 6. The desulfurization absorption tower 6 may be a two-column desulfurization apparatus generally used in flue gas treatment or a desulfurization apparatus in which a cooling tower is installed in front of the absorption tower, and is not particularly limited. In the desulfurization system by the wet method as described above, a wet dust collector 7 and a reheater 8 are provided in the downstream of the desulfurization absorption tower 6, and the exhaust gas passes through these devices from the chimney 9 into the atmosphere. To be released. Here, in the reheater 8, the combustion exhaust gas whose temperature has been lowered is heated by the heat energy recovered by the heat exchanger 5 in the previous stage of the desulfurization absorption tower 6. This is because, if the exhaust gas whose temperature has been lowered is discharged as it is from the chimney, white smoke due to water vapor is generated. Therefore, when the combustion exhaust gas is discharged, the purified gas is heated to be a high temperature gas and then discharged, and heat is supplied in the latter stage of the desulfurization device 6 of the wet process equipment. A heat exchanger 8 is provided.

And in this Embodiment, as shown in FIG. 1, the chlorinating agent supply apparatus 3 is installed in the flow path from the boiler 1 to the reductive denitration apparatus 2. In addition, an ammonia injection device (not shown) for injecting NH ₃ supplied from the ammonia tank 3 into the exhaust gas is also provided. The exhaust gas from the boiler 1 is introduced into the reduction denitration device 2. The exhaust gas into which NH ₃ and HCl are injected undergoes a reaction between NH ₃ and NOx in a reductive denitration apparatus, and at the same time, metal Hg is oxidized to HgCl ₂ in the presence of HCl. Air heaters 3, via the heat exchanger 5, etc., after removing the soot in an electric precipitator 8, wet desulfurizer 9 removal of SO ₂ in the exhaust gas at the same time HgC1 ₂ removal is performed. Excess HCl is contained in the exhaust gas exiting the reductive denitration device, but is not discharged from the chimney because it is absorbed by the alkaline aqueous solution such as lime milk in the desulfurization device. The present invention relates to an exhaust gas treatment method in which NOx in exhaust gas is removed by a reduction denitration device, and SO ₂ is removed by a wet desulfurization device that uses an antkari absorbent as an absorbent, and a chlorinating agent is added to the upstream side of the denitration device. to is a exhaust gas treating method, NH ₃ is so necessary for denitrification, without the addition of NH ₃ in the upstream side of the reduction denitrator, mercury by chlorinating agent in the presence of a catalyst of the reduction denitrator It is possible to convert to chloride and remove mercury with wet desulfurization equipment. The exhaust gas from which Hg has been removed in the desulfurization absorption tower 6 is introduced into the reheater 8, heated by the heat energy recovered by the heat exchanger 5, and discharged from the chimney 9.

  Thus, in this invention, after adding a chlorinating agent to exhaust gas, it processes in a solid catalyst and removes the mercury in the exhaust gas converted into water-soluble in a wet desulfurization process. However, depending on the amount of chlorinating agent supplied, the added amount is too small and a large amount of metal mercury remains and is discharged, or the added amount is too great and causes serious deterioration such as corrosion and desulfurization performance of piping and equipment. Problems can arise. Therefore, in the present embodiment, the supply amount of the chlorinating agent is calculated by measuring the mercury concentration of the exhaust gas after wet desulfurization, and calculating the predicted value of the inlet mercury concentration at the inlet of the reductive denitration process based on the mercury concentration. The amount to be added before the reductive denitration treatment is adjusted from the amount of change between the value and the already set reference inlet mercury concentration.

Here, a method for controlling the amount of mercury added when hydrogen chloride is used as a chlorinating agent will be described. The oxidation reaction of mercury by hydrogen chloride can be expressed by the following formula.
Hg + 2HCl + 1 / 2O ₂ → HgCl ₂ + H ₂ O
If the oxidation rate of mercury to mercury chloride can be expressed as r _ox = kp _Hgin ^a · p _{O 2} ^b · p _HCl ^c , the O ₂ and Hg concentrations cannot be adjusted because they depend on boiler load, coal type, etc. The rate at which mercury is oxidized to mercury chloride can be adjusted by increasing or decreasing the supply. For example, assuming that mercury in coal-fired exhaust gas exists in two forms of Hg ⁰ and HgCl ₂ , the outlet mercury concentration can be expressed by the following equation (1).

p _ToHgout = p _ToHgin − [p _Hgin (1−η _ox ) · η <sub>Hg
+ (P Hgin · η ox +</sub> p HgCl2in) · ηHgCl 2]
... (1)
p _ToHgin : inlet total mercury partial pressure
p _ToHgout : outlet total mercury partial pressure
p _Hgin : Inlet Hg ⁰ partial pressure
p _HgCl2in : Inlet HgCl ₂ partial pressure
η _ox : Metal mercury oxidation rate
η _Hg : Metal mercury removal rate in flue gas treatment system
η _HgCl2 : Mercury chloride removal rate in the flue gas treatment system Here, the mercury partial pressure at the inlet varies depending on the boiler load and the coal type. Further, the mercury removal rate in the flue gas treatment system is the removal rate in a dust collector, a desulfurization absorption tower, etc., and depends on these operating conditions.

Generally removal rate eta _Hg of metallic mercury Hg ⁰ is lower than the removal rate eta _HgCl2 of mercury chloride HgCl _2, mercury removal rate as flue gas treatment system is improved if the oxidation to more mercuric chloride metallic mercury. Moreover, since the oxidation rate η _ox of metallic mercury depends on the HCl supply amount, the catalyst filling amount, the catalyst temperature, etc., the oxidation rate of metallic mercury can be improved by increasing the HCl supply amount. Therefore, in the present embodiment, the supply amount of the chlorinating agent and the oxidation rate of metallic mercury are controlled using the system shown in FIG.

  The outlet mercury concentration A is detected at any of the desulfurization absorption tower outlet 11a (or the cooling tower outlet when a cooling tower is provided), the wet dust collector outlet 11b, or the reheater outlet 11c. From the supply flow rate of the chlorinating agent (HCl) and the mercury removal rate in the flue gas treatment system (removal rate in each device), the calculator 15 calculates the predicted value of the inlet mercury concentration. The predicted value Y is compared with a preset reference inlet mercury concentration based on the coal type and boiler load signal X, and the flow rate Z of the chlorinating agent by the regulator 16 is supplied to the chlorinating agent supply valve 10 from the amount of change. Sent as a signal. By adjusting the chlorinating agent supply valve 10, the supply amount of the chlorinating agent 3 is controlled to an appropriate flow rate. In this way, by inputting the boiler load signal X to the outlet Hg concentration prediction calculator 15, it is possible to cope with boiler load fluctuations and to prevent a load follow-up delay due to the detection delay of the outlet mercury concentration. Also, since the removal rate of each mercury compound in the flue gas treatment system depends on the operating conditions of the electrostatic precipitator and desulfurizer, the operating conditions of the electrostatic precipitator and desulfurizer, such as the electric field strength in the electrostatic precipitator, the circulation flow rate in the desulfurizer It is also possible to control the chlorinating agent supply flow rate by calculating a more accurate predicted mercury concentration by inputting such a signal to the computing unit 15. As a result, it is possible to cope with fluctuations in operating conditions in the electrostatic precipitator and the desulfurization apparatus, and it is possible to control the flow rate more appropriately.

  Exhaust gas targeted by the present invention is, for example, boiler exhaust gas from a thermal power plant or factory that burns fuel containing sulfur or mercury such as coal or heavy oil, or metal factory, oil refinery, petrochemical factory, etc. It is a heating furnace exhaust gas, and generally has a low NOx concentration and a large amount of discharge containing carbon dioxide, oxygen, SOx, dust, or moisture. The reductive denitration method used in the present invention is usually a method in which ammonia is used as a reducing agent and NOx in exhaust gas is reduced to nitrogen in the presence of a solid catalyst. Ammonia is injected in a conventional manner. The solid catalyst used for reductive denitration in the present invention is, for example, a metal oxide or sulfate such as V, W, Mo, Ni, Co, Fe, Cr, Mn, Cu, Pt, Ru, Rh, Pd, Ir, or the like. These noble metals, or a mixture thereof, can be used which are supported on titania, silica, zirconia or a composite oxide thereof, or zeolite or the like as a support.

The chlorinating agent used in the present invention is one in which mercury in exhaust gas reacts with the mercury chlorinating agent in the presence of the catalyst to generate HgCl ₂ or HgCl, such as hydrogen chloride (HCl), ammonium chloride. , Chlorine, hypochlorous acid, ammonium hypochlorite, chlorous acid, ammonium chlorite, chloric acid, ammonium chlorate, perchloric acid, ammonium perchlorate, other amine salts of the above acids, other salts, etc. Is mentioned. For the addition of hydrogen chloride, hydrogen chloride may be used as a chemical, or hydrochloric acid which is an aqueous solution may be used. The concentration of hydrochloric acid is not particularly limited, and examples thereof include concentrated hydrochloric acid to dilute hydrochloric acid of about 5%. As a device for adding hydrogen chloride to exhaust gas, a conventional metering pump for chemicals can be used. The addition of salts such as ammonium chloride is preferably carried out using an aqueous salt solution. The chlorinating agent may be added before or after adding ammonia to the exhaust gas. As the wet desulfurization apparatus, a conventional apparatus can be used. Examples of the absorbent used for wet desulfurization include aqueous solutions (alkali absorbent) of absorbents such as calcium carbonate, calcium oxide, calcium hydroxide, sodium carbonate, and caustic soda.

In general, the amount of metallic mercury in the exhaust gas is about 10 μg / m ³ · N at the outlet of the boiler, and about several μg / m ³ · N at the chimney. And depending on the target exhaust gas, such as coal-fired exhaust gas, hydrogen chloride is already contained to some extent, for example, at a concentration of about 10 to 30 ppm. Therefore, when a certain amount of addition is performed, the residual metal mercury concentration after the denitration apparatus due to the difference in these target exhaust gases also varies, so it is difficult to remove the harmful effects caused by the excessive amount addition. According to the present invention, the mercury concentration after removal of mercury chloride by the desulfurization absorption tower is measured, and an appropriate amount of chlorinating agent to be added is supplied. The quantity is not supplied and adverse effects on the system can be avoided. In addition, the utility cost during operation can be kept as low as possible compared to the case where a constant amount is always added. Specifically, for example, when a constant amount of about 10 to 100 ppm is added to the exhaust gas at any time as the concentration of the chlorinating agent added to the exhaust gas using coal or heavy oil as the fuel, the operation control of the present invention is performed. By performing, the supply amount is changed between about 30 to 50%, and the mercury removal can be surely performed. As a result, the amount of chlorinating agent added can be reduced by 50 to 70%, leading to a significant cost reduction.

Reference form (part 1)
FIG. 5 shows an example of a system used by the processing method according to this embodiment . As an exhaust gas treatment system, wet desulphurization equipment is used by adding a chlorinating agent from a chlorinating agent supply device to exhaust gas containing nitrogen oxides, sulfur oxides and mercury, and performing a denitration treatment under the solid catalyst of the reduction denitration device. The desulfurization is performed with the alkali absorbing liquid in the inside, which is the same as the processing system of the above-described embodiment (part 1), but the control method for adding the chlorinating agent is different. In the present embodiment, mercury concentration B before desulfurization is detected at any location (desulfurization device upstream) such as a denitration device inlet, an A / H inlet, a heat exchanger inlet, a dust collector inlet, etc. As in 1), calculate the outlet mercury concentration predicted value from the chlorinating agent flow rate and the removal rate of the flue gas treatment system (removal rate at each device), and set the preset reference outlet mercury concentration (outlet mercury concentration target value) and The chlorinating agent supply flow rate is controlled from the deviation signal. The predicted mercury concentration at the outlet is calculated based on the above equation (1). Here, when controlling based on the measurement result of the dust collector outlet concentration, the mercury removal rate in the dust collector 4 is subtracted from the mercury removal rate (η _Hg , η _{HgCl 2} ) in the preset flue gas treatment system. Calculate the predicted mercury concentration at the outlet.

In the system of the present embodiment , a mercury concentration meter is provided at one of the above detection points in front of the wet desulfurization apparatus, and flow rate adjustment signals from the calculator 15 and the controller 16 connected to the mercury concentration meter. Z is sent to the chlorinating agent supply device. Thereby, an appropriate amount of chlorinating agent can be added to the exhaust gas upstream of the denitration apparatus. In addition, by inputting the boiler load signal and the operation signal from the dust collector and desulfurizer to the outlet mercury concentration prediction calculator 15, it is possible to cope with the boiler load fluctuation and the fluctuation of the operating conditions of the dust collector and desulfurizer, and the outlet mercury concentration. Load follow-up delay due to detection delay can be prevented.

Reference form (part 2)
FIG. 6 shows an example of a system used by the processing method according to this embodiment. As an exhaust gas treatment system, a chlorinating agent is added to exhaust gas from a chlorinating agent supply device, denitration treatment is performed under the solid catalyst of the reduction denitration device, and desulfurization is performed with an alkaline absorbent in the wet desulfurization device. Yes, it is the same as the processing system of the above embodiment (part 1). In the present embodiment, as in the second embodiment (part 2), the mercury concentration B before desulfurization at any position (pre-desulfurization apparatus) such as the denitration apparatus inlet, A / H inlet, heat exchanger inlet, dust collector inlet, etc. Is detected. On the other hand, the outlet mercury concentration A is detected at any position such as a desulfurization apparatus outlet (or a cooling tower outlet if there is a cooling tower), a wet dust collector outlet, or a reheater outlet.

  Next, in the computing unit 15, the deviation signal between the reference inlet mercury concentration and the mercury concentration before desulfurization set according to the boiler load and the coal type, the chlorinating agent flow rate and the removal rate of the flue gas treatment system (removal rate at each device) From the deviation signal from the preset reference outlet mercury concentration (outlet mercury concentration target value), the chlorinating agent supply flow rate is adjusted by the chlorinating agent supply device via the regulator 16. Control. Moreover, by inputting the boiler load signal and the operation signal of the dust collector and desulfurizer to the mercury concentration prediction calculator 15, it is possible to cope with the fluctuation of the boiler load and the operating condition of the dust collector and desulfurizer, and due to the detection delay of the mercury concentration. Load follow-up delay can be prevented. In order to confirm the adjustment effect of the added chlorinating agent in the present invention, the following experiment was performed, but the present invention is not limited to the description of these examples.

Example 1
An experiment for adjusting the amount of hydrogen chloride to be supplied was performed based on the monitoring result of the continuous mercury meter 14 using the test apparatus of FIG. In the test apparatus of the present embodiment, the mercury supply apparatus 12 is provided in the preceding stage of the denitration catalyst 13 because it is necessary to vary the concentration of mercury contained in the exhaust gas in order to investigate the appropriate amount of hydrogen chloride to be added. As test conditions, the gas amount is 280 m ³ N / h (w), the denitration catalyst SV is 8000 h ⁻¹ , the catalyst temperature is 300 to 360 ° C., the mercury concentration is 10 to 100 μg / m ³ N, and the desulfurization absorption tower temperature is 50. ° C.

In this embodiment, by mercury feeder 12, the inlet mercury concentration in the exhaust gas and 100 [mu] g / ^{m 3} N, is changed to 30 [mu] g / ^{m 3} N after a predetermined time has elapsed, then returned again to 100μg / ^{m 3} N. The supply amount of hydrogen chloride 3 as a chlorinating agent was operated so as to be a constant concentration of 100 ppm without being changed. The target value for the outlet mercury concentration is a constant 15 μg / m ³ N. During this time, the outlet mercury concentration was measured by the continuous mercury meter 14. The result of using the system shown in FIG. 4 in the embodiment (part 1) is shown in the graph of FIG. From this result, it was confirmed that the measured value of the outlet mercury concentration changed in the same manner as the inlet mercury concentration increased or decreased. Therefore, the outlet mercury concentration may be lower than the target value, and there is a case where the supply amount of hydrogen chloride is excessive. If the change with time of the addition concentration that avoids such an excessive supply of the chlorinating agent is shown, it becomes as shown by a dotted line in FIG. From this example, it became clear that the target outlet mercury concentration can be adjusted by grasping the inlet mercury concentration and adjusting the amount of chlorinating agent to be added. In addition, under the same conditions, the results of using the system shown in FIG. 5 in the reference form (part 1) are shown in FIG. 7, and the system shown in FIG. 6 in the form of reference (part 2) is used. The results are shown in FIG.

1 boiler 2 denitration equipment 3 air heater (A / H)
DESCRIPTION OF SYMBOLS 4 Dust collector 5 Heat exchanger 6 Desulfurization absorption tower 7 Wet dust collector 8 Reheater 9 Chimney 10 Chlorination agent supply valve 11 Mercury concentration measuring part 12 Mercury supply device 13 Chlorination agent supply device 14 Continuous mercury meter 15 Calculation 16 adjuster 17 supply adjuster

Claims (4)

  An exhaust gas containing nitrogen oxides, sulfur oxides and mercury is subjected to reductive denitration treatment in the presence of a solid catalyst after addition of a chlorinating agent, and then subjected to wet desulfurization with an alkali absorbing liquid. Measure the mercury concentration of the exhaust gas after desulfurization, calculate the predicted value of the inlet mercury concentration before the reductive denitration treatment based on the mercury concentration, and reduce the denitration treatment from the amount of change between the predicted value and the reference inlet mercury concentration A method for treating mercury in exhaust gas, characterized in that the supply amount of a chlorinating agent added in the preceding stage is adjusted.   An exhaust gas containing nitrogen oxides, sulfur oxides and mercury is subjected to reductive denitration treatment in the presence of a solid catalyst after addition of a chlorinating agent, and then subjected to wet desulfurization with an alkali absorbing liquid. The mercury concentration is measured for the exhaust gas before desulfurization, and the predicted value of the outlet mercury concentration after wet desulfurization is calculated based on the mercury concentration. From the amount of change between the predicted value and the reference outlet mercury concentration, the reduction denitration treatment A method for treating mercury in exhaust gas, characterized in that the supply amount of a chlorinating agent added in the preceding stage is adjusted.   An exhaust gas containing nitrogen oxides, sulfur oxides and mercury is subjected to reductive denitration treatment in the presence of a solid catalyst after addition of a chlorinating agent, and then subjected to wet desulfurization with an alkali absorbing liquid. The mercury concentration is measured for the exhaust gas before and after desulfurization, and the supply amount of the chlorinating agent to be added in the first stage of the reductive denitration treatment is adjusted from the amount of change between the mercury concentration and the reference mercury concentration. Mercury treatment method in exhaust gas.   Add the chlorinating agent from the chlorinating agent supply device to the exhaust gas containing nitrogen oxides, sulfur oxides and mercury, perform denitration treatment under the solid catalyst of the reducing denitration device, and convert it into the alkaline absorbent in the wet desulfurization device An exhaust gas treatment system for performing desulfurization, wherein a mercury concentration meter is provided at a subsequent stage of the wet desulfurization device, and a flow rate adjustment signal from an arithmetic unit and a controller connected to the mercury concentration meter is chlorinated. An exhaust gas treatment system characterized by being sent to an agent supply device.

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