JP5156173B2 - Method for producing catalyst for removing nitrogen oxides - Google Patents

Description

  The present invention relates to a method for producing a catalyst for removing nitrogen oxides, and in particular, exhaust gas containing sulfur oxide (SOx) such as nitrogen oxide (NOx) and coal fired boilers contained in high-temperature exhaust gas at 450 to 600 ° C. The present invention relates to a method for producing a catalyst for removing NOx contained therein with high efficiency.

  In recent years, in the United States, so-called simple cycle gas turbines, in which gas turbines are constructed and operated independently, are increasing in order to make up for power shortages and to cope with peak power consumption. Since the equipment used for these is constructed in the suburbs of the city, it is necessary to efficiently decompose and purify NOx in the exhaust gas. However, in the simple cycle gas turbine power generation, it is necessary to treat the denitration device at a high temperature of 450 to 600 ° C. immediately after the gas turbine outlet, so a denitration catalyst having high performance and life at a high temperature is required.

In the United States, boilers using low-grade coal types such as subbituminous coal (PRB coal) and bituminous coal are on the rise. Bituminous coal produced especially in the eastern United States contains a large amount of S (1 to 3%), and the SO ₂ concentration in the exhaust gas is as high as 2000 to 3000 ppm, but part of this SO ₂ removes NOx. Oxidized on the denitration catalyst provided for the production to become sulfur trioxide (SO ₃ ), (1) Corrosion of downstream equipment, (2) Reaction with leaked ammonia (NH ₃ ) to produce ammonium sulfate, and downstream This causes problems such as blocking the air heater. Therefore, a bituminous coal-fired boiler requires a denitration catalyst that has high denitration activity that suppresses the amount of NH ₃ and SO ₃ generated at the outlet as much as possible and that reduces the oxidation rate of SO ₂ to SO ₃ . On the other hand, in general, when trying to control the activity by increasing or decreasing the active ingredient, increasing the active ingredient improves the denitration activity, but also increases the oxidative activity, while reducing the active ingredient to lower the oxidative activity. Then, since the denitration activity is also reduced, there is a problem that it is difficult to obtain a catalyst having high denitration activity and low SO ₂ oxidation activity.

On the other hand, the catalyst of titanium dioxide as (support) (TiO ₂₎ is a well which has a 300 meters ² / g close specific surface area alone, up to 50 to 60 m ² / g by baking for example 600 ° C. The specific surface area decreases, heat sintering tends to occur, and the activity and durability deteriorate. In contrast, when modifying the SiO ₂ to TiO ₂ by adding silicon dioxide (SiO ₂₎ to TiO _2, the heat sintering hardly occurs due compared to TiO ₂ alone, even after firing at 600 ° C. 80 m ² / it g near the specific surface area is obtained, also catalyst carrying active ingredients to, are known to have an oxidation rate to SO ₃ in a high denitration activity and a low SO ₂ (SO ₂ oxidation rate). Therefore, a number of methods for producing a catalyst in which an active component is supported on a composite oxide of TiO ₂ and SiO ₂ have been proposed for use in NOx removal where durability at high temperatures and a low SO ₂ oxidation rate are desired. (Patent Documents 1 to 5).

On the other hand, when the present inventors mixed titanium oxide having a hydroxyl group on the surface with tungstic acid or a salt thereof in the presence of water, the hydroxyl group of TiO ₂ and tungstic acid were condensed to form a bridge between TiO ₂ crystals. When this is calcined, fine pores of 50 mm or less comparable to the zeolite pores are formed, and when impregnating V as a catalytic active component in this pore, not only shows extremely high activity, but also V compound We have found a phenomenon that can prevent the sintering of TiO ₂ due to the action of (Patent Document 6).

Japanese Patent Publication No.57-30532 Japanese Patent Publication No.62-14339 JP 08-257399 A JP 2000-254493 A JP 2001-113170 A Japanese Patent Laid-Open No. 08-229407

The above-described conventional method for producing TiO ₂ —SiO ₂ composite oxide mainly includes inorganic and organic compounds that are baked to produce titanium oxide as a titanium source, such as titanium tetrachloride, titanium sulfate, etc. Inorganic titanium compounds or organic titanium compounds such as titanium oxalate and tetraisopropyl titanate are used as silicon sources, together with inorganic silicon compounds such as colloidal silica, water glass, fine-particle silicon, silicon tetrachloride and silica gel, and organic silicon compounds such as tetraethyl silicate. In general, a method of precipitating by a coprecipitation method and then drying and baking is generally used. However, this method has the following effects: (1) Increasing silica content to increase heat resistance has an adverse effect on denitration activity. (2) Since SiO ₂ is added during the production of TiO ₂ , the Ti / Si ratio When producing catalysts with different compositions, it is not easy to change the catalyst specifications or control the catalyst performance. (3) Since the titanium oxide-silica composite compound obtained by the conventional method has poor binding properties, it is difficult to obtain a high-strength catalyst. There was a problem that should be improved.

In view of the above-mentioned problems of the prior art, the object of the present invention is to prevent sintering of TiO ₂ to suppress high heat resistance, to suppress oxidation activity of SO ₂ to SO ₃ and to obtain high denitration activity. Nitrogen oxidation that can control a Ti-Si ratio that is easier than before and that can produce a high-strength catalyst with high bonding properties and in the simplest way possible without going through complicated manufacturing processes. An object of the present invention is to provide a method for producing an object removal catalyst.

In order to achieve the above-mentioned problems, the present inventors have conducted extensive research to develop a catalyst that has titanium oxide as a main component and has dramatically improved both heat resistance and activity. As a result, the present invention is as follows. It reached.
TiO ₂ undergoes sintering, that is, crystal growth by heating, the specific surface area is reduced, and heat resistance is deteriorated. In order to prevent this, it was considered effective to cover the periphery of the TiO ₂ particles with silica particles and prevent the TiO ₂ from coming into contact with each other to bond and grow crystals. To achieve this, a titanium oxide, and a silica having a smaller particle diameter than the titanium oxide particles were mixed in the presence of water, by covering the surfaces of titanium oxide particles with silica particles, TiO ₂ crystals between It was found that the growth of TiO ₂ crystals due to heat can be prevented by preventing contact, and the decrease in specific surface area can be prevented even at high temperatures. When the silica particle diameter is larger than titanium oxide, silica cannot enter between the secondary aggregated titanium oxide particles, and even if it enters, the titanium oxide particles cannot be covered with the silica particles.

In the method of the present invention, a silica sol having a fine particle diameter and a silica sol are used in order to more effectively express the above-described catalyst micropore formation effect by tungsten and the above-described titanium oxide crystal growth prevention effect by fine silica. A sol obtained by mixing an optionally mixed tungstic acid or a salt thereof is mixed with titanium oxide. As a result, it was found that a synergistic effect obtained by using these in combination with W and silica can be obtained rather than using W and silica alone, and an improvement in catalytic activity and an improvement in heat resistance can be obtained.
That is, the invention claimed in the present application is as follows.

(1) Titanium oxide having an average particle size of 15 nm or less as the first component, colloidal silica having an average particle size smaller than the first component as the second component, and soluble salts of tungsten or molybdenum as the third component And a catalyst slurry or paste obtained by mixing water with a carrier, or extruding into a honeycomb shape and then firing, followed by firing a method for producing a catalyst for removing nitrogen oxides.

(2) When mixing the compound of the first component, the second component and the third component with water, a compound of one or more elements selected from cerium, vanadium or niobium is further added and mixed as the fourth component. The method according to (1), characterized in that:

(3) A catalyst slurry or paste obtained by mixing the fourth component compound and water into the mixture of the first component, the second component, and the third component compound is supported on a carrier or a honeycomb. (2) The method according to (2), characterized by firing after extrusion into a shape.

(4) The carrier is a corrugated honeycomb carrier obtained by corrugating a metal or inorganic fiber network, an inorganic fiber woven fabric, or a silica alumina inorganic fiber sheet (1) to (3) ) Any one of the methods.

  In the present invention, the titanium oxide raw material as the first component may have an average particle diameter of 15 nm or less, and a hydrous titanium oxide or a dried product of a titanium oxide sol can be used. If the average particle size is 15 nm or less, the specific surface area is large, and the silica is easily covered with the titanium oxide surface layer, and the effect is easily obtained. In the case of titanium oxide having a particle diameter larger than 15 nm, the specific surface area is small, and it is difficult to obtain the effect because silica hardly enters the gaps between the titanium oxide particles.

  On the other hand, the silica raw material as the second component is silica having an average particle size smaller than the crystallite size of the titanium oxide, specifically, the average particle size is in the range of 2 to 10 nm, preferably 2 to 5 nm. It is important to be a colloidal silica. When the average particle diameter is larger than the crystallite diameter of titanium oxide, it is difficult to obtain an effect of preventing titanium oxide crystal growth by entering into the titanium oxide secondary particles formed by aggregation of titanium oxide particles. On the other hand, colloidal silica sols having an average particle size of 2 nm or less are unstable in liquid properties and easily gelled, thus causing problems in handling. The addition amount is 1 to 15 wt%, preferably 5 to 10 wt% of the titanium oxide weight. If the amount added is small, heat resistance and strength are deteriorated, and if it is too large, pores are clogged and the activity tends to be lowered.

The tungsten (W) raw material, which is the third component, is a MO ₄ type ion (M: W) or a soluble salt of the corresponding metal, and a salt that can be arbitrarily mixed with colloidal silica, such as ammonium metatungstate. Preferably used. By using a raw material in which silica and a W raw material are intimately mixed, W and silica can form a bridge between titanium oxide particles, thereby preventing crystal growth of titanium oxide. The amount of addition is 1 to 20 atomic%. For the molybdenum (Mo) raw material which is the same third component, an MO ₄ type ion (M: Mo) or a soluble salt of the corresponding metal, and an ammonium salt such as ammonium molybdate is preferable. Since ammonium molybdate is easily gelled with silica, it is easier to obtain the effect by separately mixing with titanium oxide than adding it in a mixed state with silica. The amount of addition is 1 to 20 atomic%.

Also, when W is used, the heat resistance at high temperature is excellent. Therefore, when high heat resistance is required, especially when removing NOx in high-temperature exhaust gas of 550 ° C or higher such as simple cycle gas turbine power generation, W is compared to 500 ° C or lower. Use of Mo gives good results for low temperature exhaust gas. When W is added in a small amount, the heat resistance is deteriorated, and when it is too large, the ratio of titanium oxide holding the active ingredient is decreased to cause a decrease in activity. Desirably, 5 to 15 atomic% is suitable. Since Mo causes a decrease in performance when the amount is too much or too little, 5 to 15 atomic% is also appropriate.
For the cerium (Ce) raw material, which is the fourth component, in addition to salts such as cerium nitrate, cerium oxide sol in which CeO ₂ is dispersed in water containing organic alkali or acid as a stabilizer is used. it can. The addition amount is 0 to 10 atomic%. When the addition amount is small, it is difficult to obtain high activity. Desirably, the range of 1 to 5 atomic% tends to give good results.

Furthermore, the vanadium (V) raw material, which is the same fourth component, can be a salt such as ammonium salt such as ammonium metavanadate, but a raw material that is strongly adsorbed to titanium oxide such as vanadyl sulfate is SO _2. Since it is easy to raise oxidation activity, it is not preferable. The addition amount of V is 0 to 10 atomic%, and when it is small, high activity is difficult to obtain, and when it is too large, the SO ₂ oxidation rate is likely to increase. Desirably, the range of 1 to 5 atomic% tends to give good results.
As the niobium (Nb) raw material, niobic acid, a sol-like material in which niobium oxide is dispersed in water containing an organic alkali or acid as a stabilizer, and the like are used, a good result is obtained. The addition amount is 0 to 10 atomic%. When the addition amount is small, it is difficult to obtain high activity. Desirably, the range of 1 to 5 atomic% tends to give good results.

  In the method of the present invention, as the addition order of the catalytically active component, after adding and mixing the second component silica and the third component W, Mo raw material to the first component titanium oxide, the fourth component Ce, V and / or Nb compounds are preferably added. By doing so, a bridge is formed between the titanium oxide particles by silica and W or Mo, and then added Ce, V and Nb are supported between the titanium oxide particles, and these are strongly adsorbed on the titanium oxide and oxidized. Prevents sintering of titanium. The mixture of the first, second and third components when adding the fourth component compound may be a mixture with water (slurry or paste), or a mixture after drying and baking. .

  The carrier supporting the catalyst slurry or paste containing the above compound includes a honeycomb carrier obtained by corrugating an inorganic fiber sheet as described above, an inorganic fiber nonwoven fabric, a net such as a wire mesh or a metal lath, and an inorganic material such as E-glass fiber. A net or the like obtained by weaving fiber yarns is used. These may be reinforced with a known reinforcing agent, or provided with a coating layer for the purpose of preventing an increase in adhesion of the catalyst component and oxidation of the metal substrate.

Any supporting method may be used. However, when using a metal or ceramic mesh, if the mesh is small, a catalyst paste with a moisture content of 30-35% and inorganic fibers added. A method of applying so as to fill the mesh with a roller can be employed. On the other hand, for inorganic corrugated honeycombs, ceramic non-woven fabrics, ceramic honeycomb carriers, etc., a method of immersing in a slurry having a catalyst component of 30 to 50 wt% and coating the catalyst slurry on the fiber gap or the surface is suitable. In addition, a method of forming a honeycomb paste by extruding a catalyst paste having a moisture content of 30 to 35% and adding inorganic fibers with a mold is also possible.
The catalyst slurry or catalyst paste supported on various substrates by the above method is subjected to treatments such as cutting, molding and deformation as necessary, and then dried by a known means such as air drying or hot air drying. It is calcined at 500 to 700 ° C. and used as a catalyst.

According to the present invention, it is possible to prevent sintering of TiO ₂ , to have high heat resistance, to suppress oxidation activity of SO ₂ to SO ₃ , to have high denitration activity, and to be more Ti-Si than conventional. A high-strength catalyst that is easy to control the ratio and has high binding properties can be manufactured by a simple method without going through a complicated manufacturing process.

Furthermore, in the conventional method, a TiO ₂ —SiO ₂ compound is prepared in advance, and then an active ingredient, inorganic fiber, binder, etc. are mixed to obtain a catalyst slurry or catalyst paste, which is supported on a carrier or extruded into a honeycomb shape. A molding method is used, but the TiO ₂ —SiO ₂ compound obtained by the conventional method has poor bonding properties, so that it is difficult to obtain strength. On the other hand, in the method of the present invention, the formation of the TiO ₂ —SiO ₂ compound and the loading of the active component can be performed in one step, and the silica sol added at this time has the function as an inorganic binder as it is, so that the machine can be easily used. A catalyst with high mechanical strength can be obtained. A coal-fired combustion boiler containing S content contains a large amount of ash in exhaust gas, and there is a concern that the catalyst may be worn by the combustion ash. However, with the method of the present invention, a high-strength catalyst can be obtained. it can.

[Example 1]
Low-temperature dry titanium oxide (G5 manufactured by Millennium, surface area 275 m ² / g, average particle size of 6 nm or more) 120 g, ammonium metatungstate aqueous solution ((NH ₄ ) ₆ · H ₂ W ₁₂ O ₄₀ · XH ₂ O, WO ₃ 79g, fine silica sol (manufactured by Nissan Chemical Co., OXS sol, concentration 10wt%, average particle size 5nm) is mixed with 120g, evaporated to dryness with stirring on a sand bath, and 150 ℃ in the atmosphere And dried for 5 hours to obtain a Ti—W—Si compound. The content at this time is 9/1 in terms of Ti / W atomic ratio, and silica is 10 wt% of the titanium oxide weight.

[Example 2]
A Ti-Mo-Si compound was obtained in the same manner as in Example 1 except that the ammonium metatungstate of Example 1 was changed to 29.4 g of ammonium molybdate ((NH ₄ ) ₆ · Mo ₇ O ₂₄ · 4H ₂ O). It was. The content at this time is 9/1 in terms of Ti / Mo atomic ratio, and silica is 10 wt% of the titanium oxide weight.

[Example 3]
A Ti—W—Si compound was obtained in the same manner as in Example 1, except that the fine silica sol of Example 1 was changed to 1.6 kg of silica sol having an average particle diameter of 2.1 nm (Nissan Chemical Co., Ltd., concentration: 3 wt%). The content at this time is 9/1 in terms of Ti / W atomic ratio, and silica is 10 wt% of the titanium oxide weight.

[Examples 4 and 5]
A Ti—W—Si compound was obtained in the same manner as in Example 1 except that the fine silica sol of Example 1 was changed to 60 and 180 g. The content at this time is 9/1 in terms of Ti / W atomic ratio, and silica is 5, 15 wt% of the titanium oxide weight.

[Examples 6 to 8]
109 g of the dried powder of Ti—W—Si compound obtained in Example 1 was pulverized in a mortar, and each was mixed with 1.28 g of ammonium metavanadate (NH ₄ VO ₃ ), niobium oxide sol (manufactured by Taki Chemical Co., Niobium Sol, 5 Niobium oxide concentration 10wt%) 14.5g, Cerium oxide sol (manufactured by Taki Chemical Co., Nidral U-15, cerium oxide concentration 15wt%) 12.4g and water 150g were added, evaporated to dryness in a sand bath, and 150 in air Dry at 5 ° C. for 5 hours. Ti-W-Si-V compound, Ti-W-Si-Nb compound, and Ti-W-Si-Ce compound were obtained respectively. The content at this time is 9/1 in Ti / W atomic ratio, Ti / V, Ti / Nb, Ti / Ce are all in 99/1 atomic ratio, and silica is 10wt% of titanium oxide weight. is there.

[Comparative Example 1]
A Ti—W compound was obtained in the same manner as in Example 1 except that the fine silica sol of Example 1 was changed to silica sol (manufactured by Nissan Chemical Co., Ltd., OL sol, average particle size 45 nm). The content at this time is 9/1 in terms of Ti / W atomic ratio.
[Comparative Example 2]
Only the titanium oxide used in Example 1 was mixed with water, evaporated to dryness while stirring on a sand bath, and dried in air at 150 ° C. for 5 hours.
[Comparative Example 3 ]
In the same manner as in Example 6, ammonium metavanadate was added to the titanium oxide used in Example 1, and thereafter, Ti-V compound was obtained in the same manner as in Example 6. The Ti / V ratio at this time is 99/1 in atomic ratio.

150 ° C. dried products of the compounds obtained in Examples 1 to 8 and Comparative Examples 1 to 3 were calcined at 600 ° C. for 2 hours and 20 hours, respectively. The specific surface area of the dried product and the fired product obtained was measured by the BET specific surface area measurement method, and the crystallite diameter of titanium oxide was measured by an X-ray diffractometer.
Table 1 shows the compositions of the compounds of Examples and Comparative Examples, and Table 2 shows the measurement results of specific surface area and titanium oxide crystallite diameter. The specific surface area of only titanium oxide shown in Comparative Example 2 is as high as 270 m ² / g or more by drying alone, but decreases to 1/5 or less by firing at 600 ° C. In Comparative Example 3 in which V is added thereto, the specific surface area is further greatly reduced and the crystal growth is increased. On the other hand, the catalysts of the examples all show a small reduction in specific surface area after firing at 600 ° C. and a small growth of the crystallite diameter of titanium oxide, indicating that the method of the present invention has a great effect of preventing sintering of titanium oxide. In Comparative Example 1 using a silica sol having a large silica particle diameter, the effect of preventing sintering is small, and it is clear how superior the method of the present invention is compared with the results of Example 3 using a smaller fine particle silica sol. .

[Example 9]
As an example for realizing the catalyst of the present invention, a production example of a plate catalyst to which the present invention is applied will be shown.
Titanium oxide (Ishihara Sangyo Co., Ltd., specific surface area 250m ² / g ) 12kg, ammonium metatungstate 4.25kg (powder, 93% as WO ₃ ), silica sol (Nissan Chemical Co., OXS sol) 8.1kg and water kneader After mixing for 20 minutes, add 399 g of ammonium metavanadate and knead for 20 minutes, and knead for 30 minutes while gradually adding 3.2 kg of silica-alumina ceramic fiber (manufactured by TOSHIBA FINEFLEX). % Catalyst paste was obtained. The obtained paste is placed on a metal lath processed base material made of SUS430 steel plate with a thickness of 0.2 mm, sandwiched between two polyethylene sheets, and passed through a pair of pressure rollers between the mesh and the surface of the metal lath base material. Applied. This was air-dried and then calcined at 500 ° C. for 2 hours to obtain a plate catalyst. The thickness of the obtained catalyst was 0.9 mm.

[Comparative Example 4 ]
To 40 kg of metatitanate slurry containing 30% TiO ₂ and 2.7% sulfate radical, 5.78 kg of ethyl silicate was added and mixed with a kneader for 3 hours. Further, 4.04 kg of ammonium metatungstate (powder, 93% as WO ₃ ) was added and kneaded while heating to obtain a paste with a moisture content of 32%. This was extruded and granulated into a 3 mmφ cylindrical shape, dried at 150 ° C. for 12 hours, and then pulverized with a hammer mill to obtain a Ti / W / Si powder.
12 kg of the resulting catalyst powder, 270 g of ammonium metavanadate and water were added and kneaded for 20 minutes. Silica alumina ceramic fiber (manufactured by Toshiba Fineflex) was kneaded for 30 minutes while gradually adding 3.3 kg. Latency properties were developed, and a catalyst paste with a moisture content higher than that of Example 9 was obtained with a paste moisture of 34%. When the obtained paste was applied to the substrate in the same manner as in Example 9, a phenomenon that the paste adhered to the roller was observed. The obtained catalyst was air-dried and then calcined at 500 ° C. for 2 hours to obtain a plate catalyst. The thickness of the obtained catalyst was 1.3 mm.

The catalyst obtained in Example 9 and Comparative Example 4 was measured for the denitration rate at 350 ° C. under the conditions shown in Table 3, and the SO ₂ oxidation rate at 380 ° C. under the conditions shown in Table 4. In addition, to measure the wear strength of the catalyst, a catalyst cut to 100 mm x 100 mm is installed at an angle of 45 °, and 8 kg of a grid with a diameter of about 0.7 mm is naturally dropped from the upper 50 cm onto the catalyst surface. The test was conducted. The results are shown in Table 5. The catalyst of Example 9 has high strength and high denitration performance and a low SO ₂ oxidation rate. However, the catalyst of Comparative Example 3 has low strength and a good molded article cannot be obtained, and the denitration performance is low and SO ₂ is low. High oxidation rate. From this, it is clear that the method of the present invention is a production method superior to the conventional method.

According to the present invention, a catalyst for removing nitrogen oxides that can obtain high activity even at a low concentration can be obtained, and NOx in high-temperature exhaust gas such as gas turbine exhaust gas that does not have HRSG can be efficiently purified. An apparatus can be realized.

Claims (4)

Titanium oxide having an average particle size of 15 nm or less as the first component, colloidal silica having an average particle size smaller than the first component as the second component, soluble salts of tungsten or molybdenum as the third component, water And a catalyst slurry or paste obtained by mixing with a carrier, or extruded into a honeycomb and then fired, and then a method for producing a catalyst for removing nitrogen oxides. When mixing the compound of the first component, the second component and the third component and water, adding a compound of one or more elements selected from cerium, vanadium or niobium as the fourth component and mixing The method of claim 1, characterized in that: A catalyst slurry or paste obtained by mixing the fourth component compound and water into the mixture of the first component, the second component and the third component compound is supported on a carrier or extruded into a honeycomb shape. The method according to claim 2, wherein firing is performed after molding. 4. The corrugated honeycomb carrier obtained by corrugating a metal or inorganic fiber network, an inorganic fiber woven fabric, or a silica-alumina inorganic fiber sheet. The method described.