JP2008238057A - Absorbent material of metal mercury in exhaust gas and method for removing metal mercury using it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an absorbent material suitable for absorbing and removing metal mercury in an exhaust gas. <P>SOLUTION: The absorbent material for absorbing the metal mercury in the exhaust gas is characterized by that molybdic anhydride or tungsten trioxide is deposited for 5atom% or more to an inactive carrier such as silica, titania, alumina or zirconia. Even under a condition that a Cl concentration is low, by bringing the exhaust gas containing the metal mercury into contact with a metal mercury absorbent material whose main component is molybdic anhydride or tungsten trioxide, the metal mercury concentration in the exhaust gas is reduced. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an adsorbent for metallic mercury in exhaust gas and a method for removing metallic mercury using the same, and more particularly to a technology for removing metallic mercury in exhaust gas discharged from a coal-fired power plant.

Mercury, which is a harmful substance, is contained in exhaust gas generated by burning fossil fuels such as coal with a combustion apparatus such as a boiler used in a thermal power plant. Mercury is considered to exist in the exhaust gas as mercury in the metal form (Hg) or as oxidized form mercury, and in the presence of oxidized form mercury such as mercury chloride (HgCl ₂ ) Since it is easily absorbed, it is relatively easy to capture and remove. However, when it exists as mercury in a metal form, it is hardly absorbed by water and is difficult to remove.

  Therefore, as a conventional mercury removal technique, an activated carbon adsorption method is known in which an adsorbent such as activated carbon is blown into exhaust gas, and after adsorbing mercury, the adsorbent is recovered by a bag filter. However, the activated carbon adsorption method can obtain a high mercury removal rate, but it requires a large amount of activated carbon, and therefore has a problem that the operating cost is high and the life of the dust collector is shortened.

  Also, using a solid catalyst that has an oxidation function of mercury, metal form mercury (hereinafter referred to as metal mercury) is oxidized to oxidation form mercury with a low vapor pressure and removed with a downstream dust removal device or desulfurization device. A catalytic oxidation method is known (for example, Patent Document 1). However, in the catalytic oxidation method described in Patent Document 1, when the Cl concentration in the exhaust gas is low, the oxidation efficiency at the catalyst is lowered, and thus it is difficult to obtain a high mercury removal rate.

  Therefore, a method is known in which halogen compounds such as HCl are injected into the flue on the upstream side of the catalyst tower to improve the oxidation efficiency of mercury in the exhaust gas in the catalyst tower and increase the mercury removal efficiency in the downstream equipment. (For example, Patent Document 2). However, if a halogen compound is injected, the oxidation efficiency of the catalyst is increased, but it is necessary to continuously add the halogen compound, so that the operating cost is high and the piping is corroded by the chlorinating agent. Become.

  On the other hand, it is not removal of mercury in exhaust gas, but as a technique for removing mercury in natural gas condensate, it has been proposed to adsorb and remove with an adsorbent in which a sulfide such as molybdenum is supported on a carrier such as alumina (for example, Patent Document 3).

  Further, although the use of the adsorbent is not specifically described, an adsorbent in which molybdenum trioxide is supported as an active metal component on an alumina-containing porous inorganic oxide support has been proposed (for example, Patent Documents). 4).

JP-A-2005-125211 JP 2001-198434 A JP-A-2-2873 JP2005-13930

  However, Patent Document 3 describes that molybdenum sulfide has excellent activity for adsorption of mercury, but describes that molybdenum oxide does not adsorb mercury.

By the way, SO ₂ is generally contained in exhaust gas. However, if a mercury adsorbent with strong oxidation activity is used, corrosivity becomes a problem when SO ₂ is oxidized to SO ₃ . No consideration is given to whether or not the adsorbent described in No. 4 has the ability to adsorb mercury in exhaust gas and the oxidation activity for SO ₂ .

  An object of the present invention is to provide an adsorbent suitable for adsorbing and removing metallic mercury in exhaust gas.

In order to solve the above problems, the adsorbent for adsorbing metallic mercury in the exhaust gas of the present invention has molybdenum trioxide or tungsten trioxide supported by 5 atom% or more on an inert carrier such as silica, titania, alumina, zirconia. It is characterized by.
That is, the present inventors have studied a method that can efficiently remove metallic mercury in exhaust gas even under low Cl concentration conditions. As a result, molybdenum trioxide and tungsten trioxide have a high adsorption capacity for metallic mercury. And came to adopt a method of reducing the concentration of metal mercury in the exhaust gas by contacting the exhaust gas containing metal mercury with a metal mercury adsorbent mainly composed of molybdenum trioxide or tungsten trioxide. .

Here, although the adsorption mechanism of metallic mercury on molybdenum trioxide or tungsten trioxide is not clear, both chemical adsorption and physical adsorption as described below are conceivable.
(A) Metallic mercury in the exhaust gas reacts with double-bonded oxygen present in molybdenum trioxide or tungsten trioxide to form Hg—O bonds as shown in formula (1).
Hg + O = Mo → Hg-O-Mo (1)
(B) Oxygen in molybdenum trioxide or tungsten trioxide or mercury oxidized by oxygen in exhaust gas is adsorbed in multiple layers.

  Therefore, if the surface area of the adsorbent is increased by using an inert carrier such as silica, titania, alumina, or zirconia as the porous body, the number of adsorption points of molybdenum trioxide and tungsten trioxide and the adsorption capacity of the adsorbent increase. Mercury adsorption removal rate and adsorbent usage period can be improved.

  Further, in the method for removing metallic mercury using the mercury adsorbing material of the present invention, since no additive such as activated carbon or halogen compound is required, the operating cost is low, and even in the exhaust gas having a low metallic mercury Cl concentration, It is possible to efficiently adsorb and remove metallic mercury.

Further, when SO ₂ is contained in the exhaust gas, if an adsorbent having oxidation activity is used, as shown in the formula (2), SO ₃ generation reaction that causes corrosion of purple smoke and piping occurs. In this regard, in the metal mercury adsorbent of the present invention, the contained components are only inert components such as silica, titania, alumina, zirconia, and molybdenum trioxide and tungsten trioxide having very low oxidation activity against SO ₂ , According to the metal mercury adsorbent of the present invention, generation of SO ₃ is not a problem.
SO ₂ + O ₃ → SO ₃ + O ₂ (2)
Further, the content of molybdenum trioxide or tungsten trioxide is preferably supported by 5 atom% or more, and more preferably 5 to 20 atom%. If it is 5 atom% or less, the effect of dispersion is small, and if it is 20 atom% or more, the surface area decreases due to sintering of molybdenum trioxide or tungsten trioxide at the time of firing, so the effect does not increase so much.

  In addition, the adsorbent is preferably formed in a plate shape or a honeycomb shape. For exhaust gas with a large amount of dust, wear and clogging can be prevented by making the shape of the adsorbent plate. When the amount of dust is small, the surface area can be made larger than that of the plate shape by forming the honeycomb shape.

  Further, after introducing exhaust gas containing nitrogen monoxide and metal mercury into the denitration device to reduce the concentration of nitrogen monoxide, the exhaust gas is brought into contact with the adsorbent, thereby preventing the metal mercury from adsorbing to the adsorbent. Since the concentrations of nitric oxide and ammonia are low, a high metal mercury removal rate can be obtained.

  Moreover, it is preferable that the temperature of the exhaust gas which contacts an adsorbent shall be 300-400 degreeC. In this case, the surface area of the adsorbent can be maintained high, and a high metal mercury removal rate can be obtained.

  According to the present invention, an adsorbent suitable for adsorbing and removing metallic mercury in exhaust gas can be provided.

  Hereinafter, the present invention will be described based on embodiments.

(Embodiment 1)
FIG. 1 is a diagram showing a system configuration of an embodiment of an exhaust gas treatment system of the present invention. As shown in the figure, the exhaust gas treatment system includes a boiler 1, a denitration device 2, a heat exchanger 3, a dry electric dust collector 4, a wet desulfurization device 5, a wet electric dust collector 6, a chimney 7, an ammonia injection device 8, and an adsorbent packed bed 9. Each device is connected by an exhaust gas duct or the like.

The combustion exhaust gas of coal discharged from the boiler 1 is injected with NH ₃ by an ammonia injection device 8. The exhaust gas into which NH ₃ has been injected is introduced into the denitration apparatus 2 and the reaction between NH ₃ and NO _X is performed. The exhaust gas from which NO _X has been removed is introduced into the adsorbent packed bed 9, where mercury in the exhaust gas is adsorbed and removed. The exhaust gas from which the mercury has been removed passes through the heat exchanger 3, the soot is removed by the dry electrostatic precipitator 4, and then the SO ₂ in the exhaust gas is removed by the wet desulfurization device 5. After removing soot and dust with the wet electrostatic precipitator 6, it is discharged from the chimney 7 to the atmosphere.

  In the present embodiment, the adsorbent packed layer 9 has a plurality of mercury adsorbents arranged side by side in a packed box, and the exhaust gas contacts the mercury adsorbent in the process of passing through the adsorbent packed layer 9 so that mercury is adsorbed and removed. Is done. The mercury adsorbent is formed by supporting molybdenum trioxide or tungsten trioxide on an inert carrier (for example, silica, titania, alumina, zirconia) having a high surface area.

  Although the adsorption mechanism of metallic mercury on molybdenum trioxide and tungsten trioxide is not clear, both chemical adsorption and physical adsorption are conceivable as shown in the above formulas (A), (B), and (1). .

  The raw material of molybdenum trioxide used as the adsorbing component of the adsorbent of the present embodiment is generally well-known alkali molybdate such as ammonium molybdate and sodium molybdate, as well as molybdic acid and molybdenum trioxide itself. Can be used. Here, as the alkali molybdate, it is preferable to use any of ammonium molybdate, molybdic acid, and molybdenum trioxide because the alkali component may adversely affect the mercury adsorption ability. The same applies to the raw material of tungsten trioxide, and it is preferable to use ammonium tungstate, tungstic acid, or tungsten trioxide.

The carrier supporting molybdenum trioxide or tungsten trioxide is not particularly limited as long as it is an inert carrier. However, when a porous inert carrier having a surface area of 100 m ² / g or more is used, adsorption is performed. Since the surface area of the material can be increased, the metal mercury removal rate can be improved.

  As a method for preparing the mercury adsorbent, a method is used in which the powders of the molybdenum raw material or tungsten raw material and the inert carrier are kneaded, and the paste is applied to a net-like material such as a wire net or a metal lath, and then dried and fired. be able to. Alternatively, a method may be used in which a mixed slurry of molybdenum or tungsten and an inert carrier is coated on a plate-like or honeycomb-like inert carrier, and then dried and fired. Here, as the inert carrier, for example, a honeycomb-like carrier obtained by corrugating an inorganic fiber sheet, a nonwoven fabric sheet made of inorganic fiber, a ceramic honeycomb carrier such as cordierite or alumina, and an inorganic fiber yarn such as E glass fiber are woven in a net shape. A net-like material, a net-like material such as a wire net or a metal lath carrying inorganic fibers can be used.

  Further, a method of impregnating the above plate-like or honeycomb-like inert carrier with a solution in which molybdenum raw material or tungsten raw material is dissolved, carrying molybdenum or tungsten, and drying and firing the same may be used.

  The firing temperature in these preparation methods may be set to 300 to 600 ° C. in any case, but if the firing is performed on the low temperature side of about 300 to 400 ° C., the surface area of the adsorbent can be kept high.

  Further, it is only necessary that an adsorption component such as molybdenum trioxide or tungsten trioxide is supported by 5 atom% or more on an inert carrier such as silica, titania, alumina, zirconia. If the amount is less than 5 atom%, the effect of dispersion is small, and the upper limit is not particularly limited. The amount is preferably 5 to 20 atom%.

  For exhaust gas with a lot of dust, such as soot, wear and clogging can be prevented by making the mercury adsorbent into a plate shape. On the other hand, when the amount of dust is small, the surface area can be made larger than the plate shape by forming the honeycomb shape.

  Although the installation location of the adsorbent packed bed 9 is not particularly limited, it may be installed on either the upstream side or the downstream side of the denitration device 2, and in particular, the downstream side of the denitration device 2 as shown in FIG. If it is installed, the concentration of nitric oxide and ammonia, which prevent the metal mercury from adsorbing to the adsorbent, is low, so that a high metal mercury removal rate can be obtained.

  In addition, when the adsorbent packed bed 9 is installed on the upstream side of the denitration device 2, it is adsorbed on the upstream side of the ammonia injection device 8 in order to prevent a reduction in the removal rate of metallic mercury due to inhibition of adsorption of metallic mercury by ammonia. It is preferable to install the material filling layer 9. Furthermore, in any of these cases, the exhaust gas temperature is preferably in the vicinity of 300 to 400 ° C.

  In the method for removing metallic mercury using the adsorbent according to the present embodiment, an additive such as activated carbon or a halogen compound is not required, so that the operating cost is low. Further, in the conventional catalytic oxidation method, when the Cl concentration in the exhaust gas is low, the oxidation efficiency of the metal mercury in the catalyst is reduced and the removal rate of the metal mercury is reduced, but in this embodiment, the Cl concentration is low. It is possible to efficiently adsorb and remove metallic mercury even in low exhaust gas.

Further, when SO ₂ is contained in the exhaust gas, as shown in the above formula (2), the generation reaction of SO _{3 that} causes the corrosion of purple smoke and piping occurs, but the mercury adsorbent of the present embodiment so-containing component, silica, titania, alumina, and inactive ingredients, such as zirconia, oxidation activity is very low molybdenum trioxide for sO _2, because it is composed of tungsten trioxide, to worry about occurrence of sO ₃ There is no need.

  Next, an example in which the present embodiment is applied and analysis was performed by experiment and a comparative example thereof will be described with reference to FIG. 1, Table 1, and Table 2. This embodiment is referred to as Test Example 1.

  As shown in FIG. 1, in Test Example 1, an experiment was performed assuming that an adsorbent packed bed 9 was installed on the downstream side of the denitration apparatus 2. At this time, since the exhaust gas introduced into the adsorbent packed bed 9 is a gas after the denitration treatment, in this test, the test was performed under conditions in which nitrogen monoxide and ammonia do not exist (Table 1 Test Example 1). The adsorbents prepared in Examples 1 to 7 and Comparative Examples 1 and 2 shown below were applied to the adsorbent packed bed 9, and the removal rate of metallic mercury in the gas after 5 hours from passing gas was measured. The results are summarized in Table 2.

  Silica powder, ammonium molybdate, water and silica-based inorganic fibers were added and kneaded using a kneader to prepare a catalyst paste having a composition of Si / Mo = 95/5 (atomic ratio). Separately, a SUS430 band steel is processed with a metal lath to create a reticulated base material having an opening of about 3 mm, and the paste is placed on this base material and passed through a pressure roller, thereby allowing the inter-mesh of the base material and The paste was pressure-bonded to the surface and molded into a plate-like product having a thickness of 0.7 mm. The plate-like material was dried at 150 ° C. for 2 hours and then fired in the atmosphere at 400 ° C. for 2 hours to obtain a plate-like adsorbent.

  A plate-like adsorbent was obtained in the same manner as in Example 1 except that ammonium molybdate was changed to ammonium paratungstate.

  A plate-like adsorbent was obtained in the same manner as in Example 1 except that the silica powder was changed to titania powder.

  A plate-like adsorbent was obtained in the same manner as in Example 3 except that ammonium molybdate was changed to ammonium paratungstate.

  A plate-like adsorbent was obtained in the same manner as in Example 3 except that Ti / Mo was changed to 90/10.

  Titania powder, ammonium paratungstate, water and silica-based inorganic fibers were added and kneaded using a kneader to prepare a catalyst paste having a composition of Ti / W = 95 / 2.5 (atomic ratio). Separately, a SUS430 band steel is processed with a metal lath to create a reticulated base material having an opening of about 2 mm, and the paste is placed on this base material and passed through a pressure roller, thereby allowing the inter-mesh of the base material and The paste was pressure-bonded to the surface and molded into a plate-like product having a thickness of 0.7 mm. The plate was dried at 150 ° C. for 2 hours and then impregnated with an ammonium molybdate solution. This was dried again at 150 ° C. for 2 hours and then calcined at 400 ° C. in the atmosphere for 2 hours to obtain a plate-like adsorbent having a composition of Ti / Mo / W = 95 / 2.5 / 2.5 (atomic ratio). Obtained.

A porous cordierite honeycomb carrier mainly composed of alumina and silica was impregnated with an ammonium molybdate solution. This was dried at 150 ° C. for 2 hours and then fired in the atmosphere at 400 ° C. for 2 hours to obtain a honeycomb-shaped adsorbent having a composition of Ti / Mo = 95/5 (atomic ratio).
(Comparative Example 1)

A plate-like adsorbent was obtained in the same manner as in Example 1 without adding ammonium molybdate.
(Comparative Example 2)

  A plate-like adsorbent was obtained in the same manner as in Example 3 below without adding ammonium molybdate.

表２の結果より、三酸化モリブデン及び三酸化タングステンを吸着材として用いると、排ガス中の金属水銀を除去することが可能である。また、担体が同じである場合、三酸化モリブデンを吸着材としたときの金属水銀の除去率は三酸化タングステンより高い値を示した。また、吸着成分が同じ場合、チタニアを担体とした場合の金属水銀の除去率はシリカよりも高い値を示した。

From the results in Table 2, it is possible to remove metallic mercury in the exhaust gas when molybdenum trioxide and tungsten trioxide are used as adsorbents. When the same carrier was used, the removal rate of metallic mercury when molybdenum trioxide was used as the adsorbent was higher than that of tungsten trioxide. Further, when the adsorbing components were the same, the removal rate of metallic mercury when titania was used as a carrier was higher than that of silica.

  As shown in Example 6, a mixture of molybdenum trioxide and tungsten trioxide adsorbed components in a ratio of 1: 1 is composed of molybdenum trioxide (Example 3) and tungsten trioxide having the same component amount (Example 3). Example 4) An intermediate value of the removal rate of metallic mercury was shown. Furthermore, the removal rate of metallic mercury was greatly improved by increasing the amount of adsorbent components.

(Embodiment 2)
FIG. 2 is a diagram showing a system configuration of an exhaust gas treatment system according to another embodiment of the present invention. Hereinafter, it is called Test Example 2.

  As shown in the figure, this embodiment differs from the first embodiment in that an adsorbent packed bed 9 is installed on the upstream side of the ammonia injection device 8 and the denitration device 2 as shown in FIG. At this time, since the exhaust gas introduced into the adsorbent packed bed 9 is an untreated gas and is a gas before ammonia is injected, this test is performed under conditions where only nitrogen monoxide exists (Table 1-Test Example). 2). Moreover, the removal rate of metallic mercury in the gas after 5 hours from the gas flow was measured using the same adsorbents of Examples 1 to 4 of Embodiment 1. The results are summarized in Table 3.

表３の結果より、排ガス中の金属水銀の除去性能は、一酸化窒素が吸着材への金属水銀の吸着を妨げているため、試験例１の結果と比較すると少し低くなったが、吸着材による金属水銀の除去効果は認められた。

From the results of Table 3, the removal performance of metallic mercury in the exhaust gas was slightly lower than that of Test Example 1 because nitrogen monoxide hindered the adsorption of metallic mercury on the adsorbent, but the adsorbent The removal effect of metal mercury was recognized.

(Embodiment 3)
FIG. 3 is a diagram showing a system configuration of an exhaust gas treatment system according to another embodiment of the present invention. Hereinafter, this is referred to as Test Example 3.

  As shown in the figure, the present embodiment is different from the first embodiment in that an adsorbent packed layer 9 is installed between the ammonia injection device 8 and the denitration device 2. At this time, since the exhaust gas introduced into the adsorbent packed bed 9 is an untreated gas after the ammonia injection, this test was performed under conditions where both nitrogen monoxide and ammonia exist (Table 1-Test Example 3). It was. Using the same adsorbent as in Examples 1 to 4 of Embodiment 1, the removal rate of metallic mercury in the gas after 5 hours from the gas flow was measured. The results are summarized in Table 4.

金属水銀の除去性能は、一酸化窒素、アンモニア両方の影響を受けるため、試験例１、２の結果と比較すると最も低くなったが、吸着材による金属水銀の除去効果は認められた。この場所に吸着材充填層を設置する場合は、吸着材成分量を増加させて、金属水銀の除去性能を高める必要がある。

The metal mercury removal performance was affected by both nitrogen monoxide and ammonia, and thus was the lowest compared with the results of Test Examples 1 and 2, but the metal mercury removal effect by the adsorbent was recognized. When an adsorbent packed bed is installed at this location, it is necessary to increase the amount of adsorbent components and improve the removal performance of metallic mercury.

  From the results of Test Example 1 to Test Example 3, it can be seen that the highest metal mercury removal rate can be obtained when both the concentrations of nitric oxide and ammonia are low. Therefore, it is most preferable to install the mercury adsorbent at the outlet of the denitration apparatus that hardly contains nitric oxide or ammonia.

It is a figure which shows the system | strain structure of embodiment of the exhaust gas processing system of this invention. It is a figure which shows the system configuration | structure of the exhaust gas processing system which is other one Embodiment of this invention. It is a figure which shows the system configuration | structure of the exhaust gas processing system which is other one Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Boiler 2 Denitration device 3 Heat exchanger 4 Dry type electrostatic precipitator 5 Wet desulfurization device 6 Wet electrostatic precipitator 7 Chimney 8 Ammonia injection device 9 Adsorbent packed bed

Claims (5)

  An adsorbent for metallic mercury in exhaust gas, in which molybdenum atom trioxide or tungsten trioxide is supported at 5 atom% or more on an inert carrier such as silica, titania, alumina, zirconia.   The adsorbent according to claim 1, wherein the adsorbent is formed in a plate shape or a honeycomb shape.   A method for removing metallic mercury in exhaust gas, wherein the adsorbent according to claim 1 or 2 is brought into contact with exhaust gas containing metallic mercury to reduce the concentration of metallic mercury in the exhaust gas.   An exhaust gas containing nitrogen monoxide and metal mercury is introduced into the denitration device to reduce the concentration of nitric oxide, and then brought into contact with the adsorbent according to claim 1 or 2 to reduce the concentration of metal mercury in the exhaust gas. A method for removing metallic mercury in exhaust gas, characterized in that:   5. The method for removing metallic mercury in exhaust gas according to claim 3 or 4, wherein the temperature of the exhaust gas in contact with the adsorbent is 300 to 400 ° C.

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