JP5186699B2 - Oxidation catalyst for metallic mercury in exhaust gas and method for oxidizing metallic mercury using the catalyst - Google Patents

Description

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

  In recent years, there has been a concern about the depletion of oil, and thermal power generation using coal as a fuel with abundant reserves and less localized unevenness will play an important role as a stable source of long-term energy in the future. It is expected to carry.

  On the other hand, about trace elements (mercury (Hg), selenium (Se), arsenic (As), cadmium (Cd), lead (Pd), etc.) contained in coal generated by coal combustion at a concentration of 0.1% or less Is known to be toxic to living organisms when the permissible concentration in the environment is exceeded. In particular, elemental mercury with a high vapor pressure is easily released into the atmosphere, and even if it is a trace amount, exposure to the human body can cause neuropathy, kidney damage, and cardiovascular disease. Also, when inorganic mercury released into the atmosphere is absorbed by lakes and seas, inorganic mercury changes into organic mercury (methyl mercury), which causes teratogenic problems in the human body via seafood. For this reason, strict regulations on mercury emissions are being imposed around the world, particularly in the United States.

  Mercury emitted from thermal power plants exists in metallic and oxidized forms. Of these, mercury in metallic form does not adhere to soot and other particles, and is close to atmospheric temperature. However, since it remains in the gas phase, it is difficult to remove it with a dust remover such as an electric dust collector or a bag filter. On the other hand, mercury in the oxidized form, especially mercury chloride produced by reaction with hydrogen chloride or chlorine in the exhaust gas, has a low vapor pressure and is soluble, and is easily immobilized in ash and in the absorbent of desulfurization equipment. Therefore, it can be recovered and removed with a dust removing device such as an electrostatic precipitator or a desulfurization device relatively easily.

For this reason, there is a practical study of using a solid catalyst having a mercury oxidation function to oxidize metallic mercury to mercury oxide with a low vapor pressure by contacting it with a solid catalyst and removing it with a downstream dust removal device or desulfurization device. It is being advanced. As such a solid catalyst, for example, a general denitration catalyst (Ti--) having titanium (Ti) as a main component, and an oxide such as vanadium (V), molybdenum (Mo), tungsten (W) added thereto is added. Mo-V catalysts or Ti-WV catalysts) are well known. These catalysts can efficiently oxidize metallic mercury to mercury oxide in the presence of a halogen compound (Patent Documents 1 and 2). In addition to Mo, W, and V, copper (Cu), manganese (Mn), iron (Fe), and the like are known to act as oxidation catalyst components for metallic mercury (Patent Documents 3 to 5).
JP 2005-125211 A JP 2003-53142 A Japanese Patent Laid-Open No. 10-230137 JP 2006-29673 A JP 2004-237244 A

In the oxidation process of Hg using a Ti-Mo-V catalyst or Ti-WV catalyst as described above, when the temperature of the exhaust gas is high, when the low halogen compound concentration in the exhaust gas, or reducing such as NH ₃ and SO ₂ In the case where a reactive gas is present, the oxidation efficiency of metallic mercury often decreases. Furthermore, in recent years, the SO ₂ concentration in the exhaust gas has exceeded 3000 ppm due to the deterioration of the carbon quality. Therefore, in the conventional denitration catalyst, in the denitration equipment in which the NH ₃ concentration is increased in addition to SO ₂ , Hg It was difficult to efficiently oxidize the material. As a general means for solving this problem, there is a method of increasing the total amount of catalyst active sites contributing to the oxidation reaction of metallic mercury by increasing the amount of catalyst loaded in the denitration apparatus and the amount of vanadium as an active component supported. However, this method can improve the oxidation activity of metallic mercury in the denitration apparatus, but increases the oxidation activity of SO _{2 as} the catalyst active point increases. Further, even when Cu, Mn, Fe, or the like is used as an oxidation active component of metallic mercury, the reduction in the oxidation efficiency of metallic mercury due to the coexistence of NH _{3 in} the exhaust gas cannot be greatly improved.

An object of the present invention is to provide a catalyst having high Hg oxidation activity while maintaining high denitration activity and low SO ₂ oxidation property in high-temperature exhaust gas in which SO ₂ and NH ₃ exist, and oxidation treatment of metallic mercury using the catalyst It is to provide a method.

As a result of intensive studies to solve the above problems, the present inventors have used a catalyst containing iridium (Ir) in a catalyst mainly composed of titanium (Ti) and vanadium (V). The present inventors have found that a high Hg oxidation rate can be achieved without increasing the SO ₂ oxidation rate even in exhaust gas with a high SO ₂ concentration, and have reached the present invention. That is, the invention claimed in the present application is as follows.

(1) A catalyst that oxidizes metallic mercury in exhaust gas in which ammonia (NH ₃ ) and sulfur dioxide (SO ₂ ) exist, and includes titanium (Ti), molybdenum (Mo) or tungsten (W), and vanadium (V ) Containing iridium (Ir) in the main component of the catalyst , and the concentration of the iridium is in the range of 10 to 100 ppm with respect to the total weight of the catalyst. .
(2) A method for oxidizing metal mercury in exhaust gas, characterized in that the metal mercury in exhaust gas is oxidized using the catalyst according to (1) .
(3) A soluble iridium compound is dissolved in water, an aqueous solution of the iridium compound and titanium oxide (TiO ₂ ) are kneaded in advance, and then a molybdenum compound or a tungsten compound and a vanadium compound are added ( A method for producing a catalyst according to 1) .

[Principle / Action]
The following equation (1) is a general equilibrium equation of mercury oxidation reaction in the presence of HCl.
Hg + 2HCl + 1 / 2O ₂ ⇔ HgCl ₂ + H ₂ O (1)
The equilibrium reaction of equation (1) can be shifted to the right by using a catalyst, and metal mercury can be oxidized. However, when NH ₃ coexists in the gas, NH ₃ is adsorbed at the active site in the catalyst, and the active site where Hg and HCl can react decreases, so the mercury oxidation reaction of the formula (1) that occurs on the catalyst It is believed that the rate is reduced and the reaction rate is reduced.
Further, when SO ₂ is present in the exhaust gas, it acts as a reducing substance SO ₂ is HgCl _2, since the HgCl ₂ produced oxidized in the catalyst is reduced to Hg, as SO ₂ concentration increases, the catalytic Hg oxidation performance Will be reduced.

As a general countermeasure in such a case, the catalytic activity with respect to the amount of gas to be treated is increased by increasing the content of the active component (for example, vanadium) in the catalyst or by increasing the amount of catalyst used per amount of exhaust gas. A method of increasing the oxidation reaction rate of Hg by increasing the total number of points is employed. However, in such a method, the oxidation reaction (side reaction) of SO ₂ as shown in the following formula (2) is also promoted, and SO ₃ that generates purple smoke due to the generation of SO ₃ and corrosion of piping is caused. Has the disadvantage of generating a large amount.
SO ₂ + 1 / 2O ₂ → SO ₃ (2)

Even when Cu, Mn, Fe or the like is used as the Hg oxidation active component instead of the vanadium compound, inhibition of the Hg oxidation reaction due to adsorption of NH _{3 to} the active site or sulfation of the active component due to the reaction with SO ₂ Therefore, it is difficult to improve the decrease in the Hg oxidation rate in the presence of NH ₃ or SO ₂ .

Accordingly, the present inventors have lower oxidation activity for SO _2, the result of searching a catalyst capable of oxidizing Hg in high efficiency in the presence of SO ₂ and NH _3, titanium (Ti), molybdenum (Mo) or tungsten (W), a catalyst comprising a combination of vanadium (V) as a main component and iridium (Ir) shows a high Hg oxidation activity in a system in which NH ₃ and SO ₂ coexist. Reached.

According to the catalyst of the present invention, it is possible to achieve high Hg oxidation activity while maintaining high denitration activity and low SO ₂ oxidation property compared to conventional denitration catalysts. In particular, even in exhaust gas with a high SO ₂ concentration, high Hg oxidation activity can be maintained, so even in exhaust gas burned with low-quality coal with a high S content, high Hg oxidation with a low SO ₂ oxidation rate. Rate can be obtained.

  Furthermore, since the catalyst of the present invention can be used even in high-temperature exhaust gas, it becomes possible to simultaneously perform denitration and Hg oxidation in a denitration apparatus used near 350 ° C to 400 ° C. . For this reason, it is not necessary to newly install a catalyst for Hg oxidation in the middle of the flue where the exhaust gas temperature is low, and an increase in utility cost can be suppressed.

ADVANTAGE OF THE INVENTION According to this invention, the fall of the Hg oxidation performance at the time of using in the waste gas which uses the low quality coal containing high concentration S content as a fuel can be improved. Moreover, since the SO ₂ oxidation rate in the exhaust gas can be kept low, troubles such as purple smoke and sulfuric acid corrosion of the steel frame material caused by SO ₃ generated by the oxidation reaction of SO ₂ can be reduced.

The catalyst used in the present invention can be obtained by combining Ir with a metal oxide composed of TiO ₂ , WO ₃ or MoO ₃ , and V ₂ O ₅ . If the Ir concentration to be added is too small, the effect of Ir addition is not sufficient, and if it is too large, the degree of dispersion of Ir deteriorates, and the contribution to not only Hg oxidation activity but also SO ₂ oxidation activity increases. High initial activity cannot be obtained, and the SO ₂ oxidation rate increases. Therefore, good results are easily obtained when the amount of Ir added is in the range of 10 to 100 ppm with respect to the total catalyst weight.

The raw material used for the catalyst preparation can be any of oxides and salts of each component, but if a soluble compound such as iridium chloride or iridium chloride is selected as the Ir raw material, it will be within the pores of TiO _2. It can be supported with high dispersion and can not only increase the initial activity, but also suppress adsorption and poisoning of SO _{2 in} the exhaust gas, so that good results are easily obtained.

For this reason, as a method for preparing the catalyst, iridium chloride or iridium chloride is dissolved in water, the Ir aqueous solution and titanium oxide (TiO ₂ ) are kneaded in advance, and then a molybdenum compound or a tungsten compound and a vanadium compound are mixed. It is best to prepare the catalyst by adding it.
In addition, as a method for processing the shape of the catalyst, a method in which the catalyst in paste form is applied to a net-like material such as a wire mesh or a metal lath and then dried and fired. The method of drying and baking after coating to an inert support | carrier is mentioned. What is necessary is just to set the calcination temperature at this time to 400-600 degreeC in any case.

  Among the above preparation methods, the method of kneading each catalyst raw material, applying the paste into a net-like material such as a wire mesh or a metal lath, and drying and calcining it is just a matter of changing the water content of the paste, and finely adjusting the catalyst. Since the pore volume can be freely selected, it is relatively easy to ensure the catalyst performance. In addition, the method of drying and firing after coating the catalyst slurry on a plate-like or honeycomb-like inert carrier can localize the active component at a high concentration on the catalyst surface in contact with the exhaust gas. Therefore, the Hg oxidation reaction can be further promoted, and good results can be easily obtained.

Hereinafter, the present invention will be described in detail using specific examples.
Example 1
Mix titanium oxide (specific surface area approx. 130m ² / g) and iridium tetrachloride solution, knead well in advance, add ammonium metatungstate and ammonium metavanadate, water, silica sol, silica ceramic fiber and knead thoroughly with kneader Thus, a catalyst paste was obtained. Place the resulting paste on a 0.6 mm thick substrate made of metal lath processed SUS430 steel plate with a thickness of 0.2 mm and pass it through a pressure roller to crimp the catalyst paste between the mesh and the surface of the substrate Applied. This was dried at 150 ° C. for 2 hours and then calcined in the atmosphere at 500 ° C. for 2 hours to obtain a plate-like catalyst. The composition of this catalyst is Ti / W / V = 94.6 / 5 / 0.4 atomic ratio, the catalyst coating amount is 700 g / m ² , and the Ir addition amount is 10 ppm with respect to the catalyst coating weight.

Example 2
The amount of Ir added to the catalyst of Example 1 was increased, and the catalyst was obtained in the same manner as the others. The composition of this catalyst is Ti / W / V = 94.6 / 5 / 0.4 atomic ratio, the catalyst coating amount is 700 g / m ² , and the Ir addition amount is 100 ppm with respect to the catalyst coating weight.

Example 3
In the preparation method of Example 1, an inert plate-like carrier mainly composed of TiO ₂ was obtained without adding Ir, W and V raw materials (700 g / m ² ). Separately, the catalyst paste of Example 1 was calcined and pulverized at 500 ° C. for 2 hours in the atmosphere, and then the catalyst powder was mixed with water to obtain a 28% by weight catalyst slurry. The catalyst slurry was impregnated with the plate carrier and dried to obtain a catalyst having a catalyst surface coated with about 120 g / m ² of the catalyst component.

Comparative Example 1
In the catalyst preparation of Example 1, the catalyst was prepared without adding iridium tetrachloride. The composition of this catalyst is Ti / W / V = 94.6 / 5 / 0.4 atomic ratio, the Ir addition amount is 0, and the catalyst coating amount is 700 g / m ² .
Comparative Example 2
In the catalyst preparation of Example 1, a catalyst having a catalyst composition of Ti / W / V = 94.6 / 5 / 0.4 atomic ratio, a catalyst coating amount of 700 g / m ² , and an Ir addition amount of 5 ppm with respect to the catalyst coating weight was prepared. .
Comparative Example 3
In the catalyst preparation of Example 1, a catalyst having a catalyst composition of Ti / W / V = 94.6 / 5 / 0.4 atomic ratio, a catalyst coating amount of 700 g / m ² , and an Ir addition amount of 200 ppm with respect to the catalyst coating weight was prepared. .

Test example 1
In order to compare the catalyst performance of Examples 1 to 3 and Comparative Examples 1 to 3, the NOx removal rate and the Hg oxidation rate were measured under the conditions shown in Table 1, and the SO ₂ oxidation rate was measured under the conditions shown in Table 2.
These results are summarized in Table 3. From this table, it can be seen that the performance of the catalyst of the present invention is higher in Hg oxidation performance than the comparative catalyst. Further, as in Comparative Example 2, when the amount of Ir added is too small, the Hg oxidation rate is not improved, while when the amount of Ir added is too large, the SO ₂ oxidation rate increases rapidly. For this reason, in carrying out the present invention, it is effective to set the range of the Ir addition amount described in claim 2. Further, the Hg oxidation rate of the catalyst of Example 3 in which the Ir-containing catalyst was surface-coated on an inert support of TiO ₂ was further increased. This is considered to be because Hg in the exhaust gas is easily oxidized because the Ir component is present at a high concentration on the catalyst surface that is easily in contact with the exhaust gas.
Test example 2
In order to confirm the effect of the catalyst of the present invention on high concentration of SO _2, the Hg oxidation rate was measured under the conditions of Table 1 with the SO ₂ concentration increased to 3000 ppm.

The results are summarized in Table 4. As is clear from this table, the catalyst of the present invention has a smaller decrease in the Hg oxidation rate even when the SO ₂ concentration is increased, and the effect of Ir addition increases as the SO ₂ concentration increases. You can see that For this reason, it can be said that the catalyst of the present invention is a catalyst that can exert a great effect in exhaust gas using low quality coal having a high S content as fuel.

Claims (3)

A catalyst that oxidizes metallic mercury in exhaust gas in the presence of ammonia (NH ₃ ) and sulfur dioxide (SO ₂ ), mainly titanium (Ti), molybdenum (Mo) or tungsten (W), and vanadium (V). An oxidation catalyst for metallic mercury in exhaust gas, characterized in that the catalyst as a component contains iridium (Ir) and the concentration of the iridium is in the range of 10 to 100 ppm with respect to the total weight of the catalyst. An oxidation treatment method for metallic mercury in exhaust gas, characterized in that the metallic mercury in exhaust gas is oxidized using the catalyst according to claim 1 . Soluble iridium compound is dissolved in water, after previous mixing with an aqueous solution and the titanium oxide of the iridium compound (TiO _2), in claim 1, characterized in that the addition of a molybdenum compound or tungsten compound, a vanadium compound The manufacturing method of the catalyst of description.