JPS6243736B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a catalyst for removing nitrogen oxides from exhaust gas emitted from natural engines, various combustion furnaces, industries using nitric acid and nitrates, and the like. More specifically, the present invention provides a catalyst for reducing nitrogen oxides by heating exhaust gas containing nitrogen oxides to a temperature in the range of 150 to 650°C in the presence of ammonia, the catalyst comprising a catalyst component. (A) titanium oxide and (B) vanadium oxide and/or sulfate (however, rutile type and/or
(excluding those in which vanadium oxide is supported on a titanium oxide support with an anatase type crystal structure), and the content of titanium and vanadium is more than 96% and less than 99.9% in terms of their atomic percentages, and the content of titanium and vanadium is more than 96% and less than 99.9%, and The present invention relates to a nitrogen oxide reduction catalyst characterized in that the nitrogen oxide reduction catalyst is less than 4% and 0.1% or more. Conventionally, several examples are known as catalysts that reduce nitrogen oxides, such as NO and NO ₂ , to nitrogen. For example, many catalysts are known, including copper-based catalysts such as copper oxide-alumina, copper oxide-silica, and copper-chromite, platinum group-based catalysts such as platinum-alumina, and rare earth catalysts such as cerium and lanthanum. According to the results of additional tests, these known catalyst systems each have drawbacks in terms of durability, activity, catalyst cost, etc. Regarding durability, most of them are poisoned by trace amounts of sulfur-containing substances such as sulfur dioxide gas contained in the flue. Therefore, at present, it is not possible to obtain a good catalyst for reducing and removing nitrogen oxides that is free from practical problems. Based on the above recognition, the present inventors have developed highly active reduction catalysts for nitrogen oxides, especially NO and NO ₂ .
We conducted extensive research to find inexpensive and durable catalysts, and as a result, we discovered that a catalyst containing titanium and vanadium exhibits extremely good activity and is an inexpensive catalyst with excellent durability. It was completed. In the reduction and removal of nitrogen oxides using the catalyst of the present invention, the characteristics of the catalyst of the present invention are most clearly exhibited when ammonia is used as a reducing agent. In general, combustion exhaust gas from which nitrogen oxides are removed contains oxygen in an amount several tens of times the mole of nitrogen oxides, and it is well known that hydrogen is used as a reducing agent to reduce nitrogen oxides. When carbon monoxide, hydrocarbons, etc. are used, the reducing agent is first consumed by oxygen, so the amount of reducing agent required reaches several tens of moles or more than the amount required to reduce nitrogen oxides. On the other hand, it is known that when ammonia or hydrogen sulfide is used as a reducing agent, the loss of the reducing agent is extremely small. Furthermore, conventionally known catalysts containing platinum and copper have strong oxidizing activity against ammonia, and increasing the reaction temperature leads to the formation of nitrogen oxides from ammonia, resulting in a sharp decrease in the removal rate of nitrogen oxides. . For this reason, when nitrogen oxides are reduced and removed using ammonia using conventional catalysts, nitrogen oxides can only be effectively removed within a very narrow temperature range, so it is difficult to remove nitrogen oxides on an industrial scale. In order to implement this method, strict control of the reaction temperature is required, which is expected to make operation difficult. When nitrogen oxides are reduced and removed by ammonia using the catalyst of the present invention, the reaction temperature is 150 to 650.
The catalyst of the present invention has an epoch-making performance capable of removing nitrogen oxides with an almost complete removal rate in the temperature range of °C, and has extremely good durability. With the advent of the present invention, the removal of nitrogen oxides from flue gas using ammonia as a reducing agent has been provided as an industrially advantageous method. As stated above, the present invention is characterized by its catalyst, and the catalyst of the present invention contains (A) titanium oxide and (B) vanadium oxide and/or sulfate as catalyst components. (However, rutile type and/or
(excluding those in which vanadium oxide is supported on a titanium oxide support with an anatase type crystal structure), and the content of titanium and vanadium is more than 96% and less than 99.9% in terms of their atomic percentages, and the content of titanium and vanadium is more than 96% and less than 99.9%, and It is characterized by being less than 4% and 0.1% or more. The catalyst of the present invention has extremely high performance as described above, but it can be easily manufactured by conventional methods such as precipitation, coprecipitation, and kneading methods that are normally used in the manufacture of catalysts. . Further, as the final catalyst molding method, any molding method can be adopted depending on the purpose, such as a usual extrusion molding method, tablet molding method, or rolling granulation method.
Alternatively, a small amount of the catalyst may be supported on a porous carrier such as silica or alumina, or a carrier component such as silica, alumina, zirconia, or magnesia may be simultaneously mixed with a compound that can form the catalyst component of the present invention by calcining its water-soluble salt. Of course, it can also be mixed with the catalyst and used by co-precipitation, or by sufficiently kneading the hydroxide or oxide of the carrier component and the catalyst component-forming compound of the present invention. As the titanium raw material for preparing the catalyst of the present invention, various compounds that produce titanium oxide when heated (calcined), such as titanium hydroxide, titanic acid, and titanium sulfate, can be used. In addition, various titanium salts commonly used in catalyst preparation, such as titanium halides and titanium sulfate, are precipitated with aqueous ammonia, caustic alkali, alkali carbonate, etc. to form hydroxides, and then thermally decomposed to generate oxides. The method is also a preferred method. Preferable vanadium raw materials include vanadium oxide and ammonium metavanadate. Of course, vanadyl sulfate, vanadyl chloride, vanadium chloride, etc. can also be used. Here, an example of the preparation method will be given and its contents will be explained in more detail. Hydroxide precipitated by adding aqueous ammonia to a predetermined amount of titanium tetrachloride aqueous solution is thoroughly washed with distilled water. Next, a predetermined amount of an aqueous solution of ammonium metavanadate is added thereto, thoroughly mixed, and then extrusion molded, or directly dried and molded into tablets. Further, during molding, part or all of the catalyst mixture may be thermally decomposed in advance to form an oxide mixture, which is then extruded or tableted. The molded product obtained as described above is finally fired at a temperature of 300 to 700°C for about 1 to 10 hours and subjected to reaction. The above catalyst preparation method is just one example, and it goes without saying that catalysts with good performance can also be obtained using various other commonly used preparation methods. Nitrogen oxides treated with the catalyst of the present invention include:
NO, N ₂ O ₃ , NO ₂ , N ₂ O ₄ and N ₂ O ₅ are included, but it is difficult to separate and analyze all of these compounds when they coexist, but for example, ammonia can be used as a reducing agent. In this case, the following reactions are considered to be typical. 3NO ₂ +4NH ₃ →7/2N ₂ +6H ₂ O 4NO+4NH ₃ +O ₂ →4N ₂ +6H ₂ O To carry out the nitrogen oxide removal reaction, the chemistry required to completely react ammonia with nitrogen oxides in the exhaust gas is required. The concentration range is about 1 to 10 times the stoichiometric ratio, preferably about 1 to 2 times the stoichiometric ratio. In addition, the obtained mixed gas passes over the catalyst at a space velocity (based on the sky column) of 2000 ~
100,000/hour, preferably in the range of 5,000 to 60,000/hour. The reaction temperature is in the range of 150-650°C, preferably 200-550°C. The pressure is not particularly limited, but can range from atmospheric pressure to about 10 kg/cm ² G or more. As for the type of reactor in which the present invention is carried out, since the flow rate is large, various measures are required, but in principle, ordinary fixed bed, moving bed, fluidized bed, etc. reactors can be used. Next, the present invention will be specifically explained with reference to Examples. <Reference Example 1> Take 189.7 g of titanium tetrachloride [TiCl ₄ ] and pour it into 1000 ml of ice water. Add 3N ammonia water to this to neutralize it. Separate the formed precipitate and wash thoroughly with distilled water. Take 189 g of the cake thus obtained (26.6 g as TiO ₂ , equivalent to 1/3 mole), add 2.05 g of ammonium metavanadate dissolved in 200 ml of distilled water, and thoroughly knead the mixture to evaporate water. After drying the resulting cake, add about 25 ml of distilled water, wet-mill it for 30 minutes using a Laikai machine, and extrude it into a size of 6 mm in diameter. The obtained molded product is dried and then fired in a Matsufuru furnace at 500°C for 5 hours. The thus obtained catalyst has an atomic ratio of Ti:V=9.5:0.5. This was crushed to a size of 10 to 20 meshes and used in the following reaction. The reaction tube is a quartz reaction tube with an inner diameter of 16.5 mm, and has a quartz thermocouple insertion tube with an outer diameter of 5 mm inside, and the outside is heated with an electric furnace. The feed gas has the following composition: NH ₃ 700-800ppm NO 600-700ppm SO ₂ 482ppm O ₂ 4% H ₂ O 7.14% N ₂ balance Gas with this composition is passed at a space velocity (NTP equivalent sky column standard, hereinafter referred to as SV) of 50000 Hr ^-1 . The conversion rate of NO obtained by sequentially changing the reaction temperature (=
Reacted NO/supplied NO x 100) is shown in Table 1. Note that NO was measured using a NO/NO _x analyzer (Model 951 manufactured by Toshiba Beckman). <Examples 1 to 3, Reference Examples 2 to 5> A reaction was carried out under the same conditions as in Reference Example 1 using a catalyst prepared in the same manner as in Reference Example 1 except that the composition ratio of titanium and vanadium was changed. The results obtained are listed in Table-1. <Comparative Example 1> This example is an example of a catalyst made only of titanium. Table 1 shows the results of a reaction using a titanium oxide catalyst prepared in the same manner as in Reference Example 1 except that vanadium was not used. <Comparative Example 2> This example is an example of a catalyst consisting only of vanadium. The powder obtained by heating and decomposing 58.5g of ammonium metavanadate at 330℃ for 1 hour in a Matsufuru furnace,
Add 1% by weight of polyethylene oxide (trade name: Alcotx E-30, manufactured by Meisei Chemical Industry Co., Ltd.), add a small amount of distilled water, wet mill, and extrude to a diameter of 6 mm. The obtained molded product was fired in a Matsufuru furnace at a firing temperature of 500°C for 5 hours. The obtained catalyst was pulverized into 10 to 20 meshes and reacted in the same manner as in Reference Example 1. The results are shown in Table 1.

[Table] <Examples 4 to 8> The samples were prepared in the same manner as in Example 1, only the firing temperature was changed, and the results shown in Table 2 were obtained. The reaction conditions at this time are as follows. NH ₃ 600ppm SV=50000Tr ^-1 NO 520ppm SO ₂ 500ppm SO ₃ 550ppm O ₂ 4% H ₂ O 10% Note that SO ₃ was introduced by air oxidation of SO ₂ . SO ₂ oxidation conditions, JGC Chemical N801 (V ₂ O ₅ - care soil) catalyst 10-20 meshes, 16.6 ml, SV=
2300Hr ^-1 , reaction temperature 300℃

[Table] <Example 9> In this example, the results of a long-term life test using exhaust gas from a heavy oil boiler as the processing gas will be described. In this case, the diameter of the reactor is
The 150mm catalyst loading is approx. The catalyst was prepared in the same manner as shown in Example-1 and had a diameter of 6 mm.
A piece molded to a length of 6 mm was used as is. The reaction was carried out by adding ammonia to the exhaust gas from a heavy oil boiler, and its composition is as follows. NH ₃ 160-220ppm NO _x 150-210ppm SO ₂ 1000-1300ppm SO ₃ 30-70ppm H ₂ O 8-11% O ₂ 3-6% CO ₂ 11-14% Soot and dust 60-90mg/NM ³ N ₂ balance This gas was mixed at SV=10000h ^-1 and reaction temperature 300℃.
Table 3 shows the results obtained after continuous circulation for 2000 hours.
Shown below.

[Table] <Example 10> In this example, a case will be described in which metatitanic acid is used as a raw material for titanium oxide. Take 100 g of metatitanic acid slurry with a water content of about 60% and 1.76 g of ammonium metavanadate, and add 100 g of water to this.
Add cc and mix well. After drying this slurry at 150° C., about 1 g of graphite is added thereto, and then it is compressed into a cylindrical tablet with a diameter of 6 mm and a height of 6 mm. After baking this molded product at 500℃ for 2 hours,
The mixture was ground into 10 to 20 mesh pieces and subjected to an activity test. This catalyst has an atomic ratio of titanium and vanadium of 9.7:
Contains at a ratio of 0.3. Table 4 shows the results of a test using 4 c.c. of this catalyst and flowing a reaction gas having the following composition at SV=59000 h ^-1 . NH ₃ 300ppm NO 250ppm SO ₂ 500ppm CO ₂ 12% O ₂ 3% H ₂ O 15% N ₂ balance

[Table] <Example 11> In this example, a case will be described in which vanadyl sulfate [VOSO ₄ .6H ₂ O] is used as a raw material for vanadium. Dissolve 189.7 g of titanium tetrachloride in 1000 ml of ice water and neutralize by adding 3N ammonia water. Separate the formed precipitate and wash thoroughly with distilled water. 183g of cake thus obtained (26.6g1/3 as _TiO2 )
Add 1.68 g of vanadyl sulfate dissolved in 100 ml of water and thoroughly knead to evaporate water. After drying the resulting cake, add about 25 ml of distilled water, wet-mill it for 30 minutes in a mill, and then extrude it into a diameter of 6 mm. After drying, the obtained molded product is fired in a Matsufuru furnace at 400°C for 5 hours. The catalyst thus obtained has a metal atomic ratio of Ti:V=9.7:0.3, and since the thermal decomposition temperature of vanadyl sulfate in air is 400° C. or higher, the catalyst has sulfate groups. This catalyst was pulverized to a size of 10 to 20 meshes and reacted in the same manner as in Example 1 to obtain the results shown in Table 5 below.

[Table] <Example 12> A catalyst having the composition shown in Table 6 below was prepared in the same manner as in Example 1, and 10 ml of this was ground into 10 to 20 meshes and poured into the interior. A quartz reaction tube with an inner diameter of 20 mm and a quartz thermocouple insertion tube with a diameter of 5 mm was filled with the mixture and heated in an electric furnace. A gas having the following composition was passed through this at SV = 10000 Hr ^-1 . SO ₂ : 540-570 ppm O ₂ : 4% N ₂ : Residual The analysis of the reaction products is carried out using the method of "separate determination of sulfur dioxide and sulfuric acid mist (SO ₃ ) using heated common salt" [Japan Analyst, 23 , 356 (1974)] ] The oxidation reaction rate of SO ₂ is defined by the following formula: Oxidation reaction rate of SO ₂ (%) = (1-SO ₃ /SO ₂ +SO ₃ )×100 The results are shown in Table 6 below.

[Table] <Example 13, Comparative Example 3> Catalyst (A) shown in Table 7 below (implemented) was prepared in the same manner as Example 1 except that the composition ratio of titanium and vanadium was changed as shown in Table 7 below. Example 13) was prepared. According to Example 5 of Japanese Patent Publication No. 54-2912, 2 ml of monoethanolamine was dissolved in 18 ml of water, and 1.93 g of ammonium metavanadate was added thereto and dissolved by stirring. Add 58.5g of anatase-type titanium oxide powder roasted at 600℃ to this liquid and evaporate the water at about 90℃ while stirring, until the water disappears.
Dry at 200°C. After pulverizing this, add about 25 ml of distilled water, wet grinding in a Raikai machine for 30 minutes, and extrude into a size of 6 mm in diameter. The obtained molded product is dried and then fired in a Matsufuru furnace at 500°C for 5 hours. The catalyst thus obtained has a metal atomic ratio of Ti:
V=97.8:2.2 [ _TiO2 : _V2O2 =97.5:2.5 (weight _ratio )]. This was pulverized to a size of 10 to 20 meshes to obtain catalyst (B) (Comparative Example 3) shown in Table 7 below. The denitrification reaction was carried out in the same manner as in Example 1. The results are shown in Table 7 below.

[Table] The rate of denitrification reaction is expressed by the following formula (1). k=1/tln1/1-x...(1) In the formula, k: reaction rate constant (sec ^-1 ) t: contact time (based on reaction temperature) (sec) t=catalyst volume ()/reaction gas supply amount (/sec) = 3600/SV x 273/273 + T (T: reaction temperature °C) x = NOx conversion rate/100 SV = space velocity (NTP equivalent superficial column standard) (hr ^-1 ). The reaction rate constants when using catalysts A and B shown in Table 7 above are shown as k _A and k _B respectively, and the reaction rate constant k when using catalysts A and B
Let V _A and V _B be the catalyst volumes required to obtain _A and k _B , respectively. If k _A and k _B are calculated according to the above formula (1), they will be as shown in Table 7' below, and since V _A and V _B are the reciprocals of k _A and k _B , respectively, the catalyst volume ratio calculated from this (V _B /V _A ) is shown in Table 7'.

[Table] As shown in Table 7' above, catalyst A of the present invention shown in Example 13 has an extremely high reaction rate compared to catalyst B of Comparative Example 3, and as a result, the same reaction rate can be obtained. It is clear that the amount of catalyst required for catalyst B is about 1.3 times that for catalyst A of the present invention.

Claims (1)

[Claims] 1. A catalyst for reducing nitrogen oxides by heating exhaust gas containing nitrogen oxides to a temperature in the range of 150 to 650°C in the presence of ammonia, the catalyst comprising ( Contains A) titanium oxide and (B) vanadium oxide and/or sulfate (However, rutile type and/or
(excluding those in which vanadium oxide is supported on a titanium oxide support with an anatase type crystal structure), and the content of titanium and vanadium is more than 96% and less than 99.9% in terms of their atomic percentages, and the content of titanium and vanadium is more than 96% and less than 99.9%, and A catalyst for reducing nitrogen oxides, characterized in that the content is less than 4% and 0.1% or more.