JPS6244975B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a catalyst for treating nitrogen oxide (NOx)-containing gas, and in particular to a catalyst for treating gas containing nitrogen oxides (NOx), particularly for treating large amounts of dust and sulfur oxides (SOx) emitted from heavy oil-fired boilers, coal-fired boilers, and combustion furnaces attached to various chemical equipment. The present invention relates to a method for producing a catalyst suitable for use in detoxifying and removing nitrogen oxides contained in exhaust gas. Methods for removing NOx from exhaust gas include adsorption methods, oxidation absorption methods, solidification collection methods, and catalytic reduction methods, but catalytic reduction methods that do not require post-treatment are economical and
Since it is technically advantageous, development efforts are being made in various fields. There are two possible methods for the catalytic reduction reaction depending on the selection of the reducing agent, but the selective catalytic reduction method is economically advantageous as it is not affected by the presence or absence of oxygen in the exhaust gas. The present invention relates to a catalyst that can be applied to this reaction, and since it is particularly advantageous to use NH ₃ as the reducing agent, the present invention will be further detailed below using this method. Conventionally, porous refractory materials such as alumina, titania, zirconia, silica, diatomaceous earth, and zeolite have been used alone or in combination as carriers for catalysts applied to selective catalytic reduction processes, but none of these materials have been granulated. This method has disadvantages in that it is expensive to use, and the production cost of the catalyst increases because it supports an activation-imparting component. By the way, distilled oil and gas fuel (e.g. LNG,
Since there is almost no dust in the exhaust gas from LPG), it is possible to granulate the aforementioned carrier into any shape such as spherical or cylindrical, and bring the exhaust gas into contact with the catalyst layer perpendicularly. be. On the other hand, in order to develop exhaust gas treatment technology that contains a large amount of dust, such as heavy oil naphtha, coal-fired boiler exhaust gas, garbage incinerator exhaust gas, and coke oven exhaust gas, it is necessary to take measures to prevent dust from accumulating in the catalyst layer. . For this purpose, methods are being considered, such as making the catalyst shape into a cylindrical honeycomb shape to make it easier for the dust to pass through, and moving the granular catalyst to scatter the adhering dust. A method of preventing dust adhesion by flowing is also promising, and the present inventors are also working hard to develop a catalyst that can be applied to this method. The optimal catalyst shape for this method is a plate-like structure, and the above-mentioned alumina, titania, zirconia, silica,
Since it is currently difficult to form porous refractory materials such as diatomaceous earth and zeolite into large plates, a support material that can be produced at low cost, is lightweight, and has considerable strength is desired. Furthermore, since the exhaust gas from boilers using fuel such as heavy oil and coal contains SOx in addition to dust, it is also desirable that the material be stable against SOx. Taking the above points into consideration, the present inventors conducted research on carriers and catalysts that meet these requirements, and as a result, they developed the method described in Japanese Patent Application No. 16874/1983, which uses silicic acid and lime as the main raw materials. Invention of a catalyst in which an active ingredient is supported on a calcium silicate carrier obtained by mixing an inorganic additive such as asbestos glass fiber as necessary with a slurry material and reacting the mixture under heating and pressure, and a method for producing the same. Further development of this led to the invention of a method for producing a lightweight and extremely stable catalyst against SOx. That is, the above-mentioned catalyst proposed earlier mixes inexpensive silicic acid raw materials and lime raw materials as a carrier with inorganic additives such as asbestos and glass fiber as necessary, and suspends the slurry in water. ~20Kg/
The active slurry of calcium silicate crystals obtained by proceeding with the hydrothermal synthesis reaction and crystallization is formed by heating at 150 to 300℃ under a pressure of about cm ² and stirring, or by temporarily discontinuing stirring, and dried. This product uses a calcium silicate molded body made from carbon dioxide and supports an active ingredient as a catalyst, and its major features are that it can be manufactured at low cost, is lightweight, and has excellent strength. However, when applied to exhaust gas containing a large amount of SOx, such as from a coal-fired boiler, calcium silicate, which is a carrier base material, cannot coexist with moisture.
SOx reacts rapidly with _SO3 , especially SO3, which separates into gypsum and silica, resulting in decreased strength and thermal deformation, resulting in cracks, so it cannot be said to be perfect as a catalyst carrier. The reaction between calcium silicate and SO ₃ in the exhaust gas proceeds according to the following equation and separates into gypsum and silica.
When a strong acidic gas such as SO ₃ comes into sudden contact with the silica crystals, the strength of the separated silica crystals becomes weaker, which reduces the strength of the catalyst structure itself, and also causes cracks due to thermal distortion, reducing the supporting strength of the catalyst. This will result in a decline. 6CaO・6SiO ₂・H ₂ O＋6SO ₃ +nH ₂ O →6CaSO ₄ +6SiO ₂・nH ₂ O Therefore, the present inventors investigated the calcium silicate composition.
In order to avoid a rapid reaction between CaO and SO ₃ , we have repeatedly conducted experimental studies on a pretreatment method for the surface layer of the carrier that supports at least _the catalyst component.
This problem can be solved by neutralizing CaO, which easily reacts with oxidants, in advance with a weakly acidic substance and separating and converting it into silica crystals whose strength does not change. The present invention was completed based on these favorable findings. That is, when the present inventors brought the above-mentioned molded body into contact with an aqueous solution of oxalic acid and its compounds in the process of investigating the neutralization treatment of a calcium silicate-based molded body, the calcium silicate constituting the molded body Although it is converted into calcium oxalate and silica gel, the shape of the converted molded product does not substantially change and maintains its strength, and the silica-calcium oxalate composite thus obtained is The present invention was completed based on the surprising fact that the molded product retains its initial state and strength even when brought into contact with strong acids such as sulfuric acid and hydrochloric acid. The present invention has been completed based on this knowledge, and involves contacting a calcium silicate-based molded body with an aqueous solution of oxalic acid or its compound, using the resulting silica-calcium oxalate composite as a carrier, and using it as a catalyst. Next, a method for producing a catalyst for exhaust gas treatment is proposed, in which a slurry containing a catalyst component is applied, added, and then dried and fired. The calcium silicate molded body used as the starting material in the present invention includes a manufacturing method thereof,
Any of various molded bodies containing calcium silicate as a main component can be used regardless of the crystallinity, crystal system, etc. of the calcium silicate constituting the molded body. For example, regarding calcium silicate, depending on the manufacturing method, products with different crystallinity such as xonotlite, tobermolite, phosiyaite, diairolite, and quasi-crystalline calcium silicate (CSHn) can be obtained, and any of them can be used. Particularly in terms of suitability as a catalyst carrier, those made of xonotrite crystals (6CaO.6SiO ₂ .H ₂ O) are preferred because they have excellent strength and thermal properties.
In the present invention, separation of a calcium silicate-based molded body into calcium oxalate and silica can be easily carried out by bringing the molded body into contact with an aqueous solution of oxalic acid or its compound. in this case,
In order to complete the reaction with oxalic acid in a short time, it is better to carry out the reaction by heating rather than at room temperature, and as a method for contacting it, it is preferable to immerse it in the above liquid. The concentration of oxalic acid is not particularly limited as long as it is sufficient to complete the reaction; however, increasing the concentration more than necessary in relation to solubility will not be effective. In addition to oxalic acid, compounds thereof such as ammonium oxalate, iron oxalate, and vanadyl oxalate can also be used in the present invention. Thus, the calcium silicate constituting the molded body is converted into silica gel and fine crystals of calcium oxalate by contact with an aqueous solution of oxalic acid and its compounds. In other words, pseudocrystalline silica gel retains the chain structure of SiO ₄ tetrahedrons that make up the skeleton structure of calcium silicate crystals, and maintains its crystal form due to the chain structure, and extremely fine particles that exist attached to the pseudocrystalline silica gel. Calcium oxalate is produced. An example of the reaction with oxalic acid in the present invention using calcium silicate consisting of xonotlite crystals is as follows. 6CaO・6SiO ₂・H ₂ O＋6H ₂ C ₂ O ₄・2H ₂ O →6CaC ₂ O ₄・H ₂ O＋6SiO ₂・nH ₂ O Pseudo-crystalline silica gel and ultrafine calcium oxalate are obtained by the reaction formula shown above. This fact is confirmed by electron microscopy, X-ray diffraction, and chemical analysis results. When the silica-calcium oxalate composite obtained as described above is brought into contact with exhaust gas containing a large amount of SOx, only the liberated calcium oxalate reacts with SOx and the pseudocrystalline silica gel becomes SiO ₄ While maintaining the chain structure of tetrahedrons, this remains as a strength component, and temporarily, SOx
Even if a sudden reaction occurs, there is no risk of an extreme decrease in strength or cracks due to thermal distortion. Next is a method for supporting the active ingredient in order to use the silica-calcium oxalate complex as a denitrification catalyst, which is similar to the conventional method for supporting on alumina, titania, silica, diatomaceous earth, and zeolite. Operation is possible. Generally, kneading methods, impregnation methods, coating methods, etc. are known as supporting methods, but in order to support the silica-calcium oxalate composite, the molded body is considered to be a mere base material and the surface is coated with catalytic activity. A method of applying the components is preferred. Application methods include dipping, spraying,
Although any method such as brush coating is applicable, the dipping method is preferable in terms of practicality. A preferred method of application is to first immerse the calcium silicate molded body in a solution of oxalic acid or its compound;
A method of converting calcium silicate, which is a component of the above-mentioned molded body, into a composite of silica and calcium oxalate, and immersing it in a slurry containing a catalyst component as it is or once dried, or the above-mentioned oxalic acid treatment. In order to improve the adhesion of the composite material converted into silica and calcium oxalate with the catalytic active component, it is immersed in a colloidal solution (hereinafter abbreviated as silica sol) in which ultrafine particles of silicic anhydride are dispersed in water. Then, any method such as applying the slurry by dipping it in a slurry containing a catalyst component can be applied. Next, the catalyst components that can be applied to the present invention are conventionally known vanadium,
Metal oxides such as dungsten, chromium, iron, copper, nickel, molybdenum, and titanium, or salt compounds thereof can be used, and the above components can be used alone or in combination of two or more. At this time, a method may be used in which the catalyst itself is made into a slurry solution, the carrier is immersed in the slurry solution, and the carrier is dried, but in the method of the present invention, the catalyst is immersed in a slurry solution containing as many components as possible by drying or baking. By subsequently drying and firing, the active substance is characterized in that its stability is increased and its bondability with the base carrier is increased. Hereinafter, the present invention will be explained in detail with reference to Examples. [Example 1] A raw material slurry with a molar ratio of lime and silicic acid of 0.98 and a water to solid content ratio of 12/1 was prepared at 12 kg/cm ²
Zonotlite crystals (6CaO・
Potassium glass fiber and asbestos were added to the slurry of 6SiO ₂ H ₂ O) at 3wt% and 6wt% based on the weight of the xonotrite crystals, and after thorough mixing, the slurry was placed in a mold and dehydrated by pressing. 120-150
It was dried at ℃ for 10 hours to obtain a calcium silicate molded body. This molded body was immersed in an aqueous solution containing 5 wt % oxalic acid at a temperature of 50° C. for 3 minutes to obtain a silica-calcium oxalate composite. [Example 2] The calcium silicate molded body obtained in Example 1 was immersed in an aqueous solution containing 1% ammonium oxalate at a temperature of 60° C. for 3 minutes to obtain a silica-calcium oxalate composite. [Example 3] The molar ratio of lime and silicic acid was set to 0.83, the weight ratio of water to solids was set to 12/1, and a hydrothermal reaction was carried out with stirring at 8 kg/cm ² (175°C) to form tobermolite crystals ( Obtain a slurry of 5CaO・6SiO ₂・5H ₂ O), and add glass fiber and asbestos to this by 3wt relative to the weight of the above crystals.
% and 6 wt %, and this was molded and dried in the same manner as in Example 1 to obtain a calcium silicate molded body. This molded body was immersed in an aqueous solution containing 10 wt % oxalic acid at a temperature of 40° C. for 1 minute to obtain a silica-calcium oxalate composite molded body. [Example 4] When the silica-calcium oxalate composites obtained in Examples 1, 2, and 3 were observed by X-ray diffraction, xonotrite crystals (6CaO・6SiO ₂・H ₂ O), which constitute the starting material, were observed. and tobermolite crystals (5CaO・6SiO ₂・5H ₂ O) disappeared, and calcium oxalate crystals were confirmed in their place. [Example 5] The silica-calcium oxalate composite molded bodies obtained in Examples 1, 2, and 3 were each cut into a size of 200 x 400 x 10 mm, and 20 g of vanadium pentoxide was mixed with 50 g of oxalic acid and water. Each catalyst was immersed in a slurry solution of 1200 g of dissolved titanium oxide and 80 g of tungstic acid for about 30 seconds, dried at 150°C for 5 hours, and then calcined at 550°C for 3 hours to obtain each catalyst. [Example 6] The silica-calcium oxalate composite molded body obtained in Example 1 was cut into a size of 200 x 400 x 10 mm, and mixed with 25 g of ammonium metavanadate and 100 g of ammonium molybdate, 1000 g of water, and monoethanol. 1000 g of titanium oxide in a solution of 300 g of amine
150 g for about 30 seconds in a slurry solution mixed with
After drying at 550°C for 5 hours, the catalyst was then calcined at 550°C for 3 hours. [Example 7] The silica-calcium oxalate composite molded body obtained in Example 1 was cut into a size of 200 x 400 x 10 mm, and mixed with 20 to 21% silicic anhydride and sodium oxide.
Colloidal solution containing 0.04% or less (PH3-4)
Example 5
The catalyst was immersed for about 30 seconds in a slurry liquid containing a catalytically active component, dried and calcined in the same manner as in Example 5 to obtain a catalyst. [Test Example 1] To check the SOx resistance of the catalysts of the present invention obtained in Examples 5, 6, and 7, SOx was 1000 to 1500 ppm, SO ₃ was 20 to
A catalyst was placed in the exhaust gas of a heavy oil-fired boiler containing 30ppm, and a durability test was conducted at 350-380℃.
Even after 5,000 hours, there was no abnormality, and no extreme decrease in strength, occurrence of cracks, or decrease in the strength of supporting the active substance was observed. On the other hand, a comparison catalyst prepared using a calcium silicate molded body not treated with oxalic acid as a carrier according to Example 5 showed a decrease in the supporting strength of the crack-generating active substance, and the strength also decreased to about 1/3. . Table 1 shows changes in strength and appearance after the durability test.

[Test example 2]

The plate-shaped catalysts obtained in Examples 5 and 6 were fixedly installed in a reactor of a denitrification device connected to the exhaust gas path of a coal-fired boiler at intervals of 15 mm so as to flow in parallel with the exhaust gas. An example of an embodiment of a parallel flow plate-shaped catalytic reactor is shown in FIG. In FIG. 1, 1 is a reactor of a denitrification device, 2 is a flow direction of exhaust gas, 3 is an ammonia injection device, and 4 is a denitrification catalyst of the present invention. Boiler exhaust gas containing NOx was passed through the reactor, and a catalyst performance test was conducted under the test conditions shown in Table 2, and the denitrification performance shown in FIG. 2 was obtained.

[Test example 3]

The catalyst obtained in Example 7 was installed in a denitrification device under the method conditions of Test Example 2, and the performance test of the catalyst was conducted for about 5,000 hours to observe changes over time in denitration performance and catalyst strength, and changes in appearance. The results are shown in FIG. Figure 3 shows the change in denitrification performance and strength over time after testing for about 5,000 hours. A in the figure shows NH ₃
[ppm] / NOx [ppm] = 1.0 shows the change in denitrification rate. B ₁ shows the change in bending strength of the same catalyst. B ₂ shows the change in the conventional catalyst, that is, the calcium silicate molded body not treated with oxalic acid, in Example 7. This figure shows the change in bending strength of the catalyst prepared according to the method. Further, as a result of looking at the occurrence of cracks and the decrease in the strength of supporting the active material, no abnormalities were observed in the catalyst of the present invention, but abnormalities occurred in the conventional comparative catalyst after about 3000 hours. [Test Example 4] In order to check the thermal expansion and contraction of the catalyst tested for 5000 hours in Test Example 3, the thermal expansion and contraction was measured according to the hot linear expansion and contraction rate test method of JRS2617, and the results shown in FIGS. 4 and 5 were obtained. As a result, it was confirmed that the catalyst of the present invention exhibits small thermal changes and extremely excellent durability. FIG. 4 shows the hot expansion/contraction curve of the catalyst of the present invention, in which the curve a shows the value before the test, and the curve a' shows the value after the 5000 hour test. Further, FIG. 5 shows the hot expansion/contraction curve of a conventional catalyst as a comparison material, where b is the curve before the test and b' is the curve after the 5000 hour test.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of one embodiment of a reactor equipped with a parallel flow plate catalyst, showing an example of the use of the catalyst according to the present invention, and FIGS. 2 to 5 are graphs showing the performance of the catalyst according to the present invention. . 1...Reactor, 3...Injection device, 4...Catalyst.

Claims (1)

[Claims] 1. The silica obtained by contacting a calcium silicate molded body with an aqueous solution of oxalic acid or its compound.
A method for producing a catalyst for exhaust gas treatment, which comprises adding a slurry containing a catalytically active component to a calcium oxalate composite, followed by drying and firing.