JPS6133619B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a catalyst for removing nitrogen oxides from exhaust gases, which removes nitrogen oxides from exhaust gases by catalytic reduction in the presence of ammonia. Conventionally, catalysts that catalytically reduce nitrogen oxides, such as NO, NO ₂ , etc. in the exhaust gas discharged from various boilers and decompose them into nitrogen and water in the presence of ammonia, include V, W, Fe, Cu, Sn, Ce, Ti, Co
Catalysts that combine oxides of many metal elements are known. Among these, panadium oxide catalysts are commonly used as catalysts that maintain high activity in low temperature ranges, and catalysts that combine tungsten oxide, cerium oxide, etc. are used as catalysts that maintain high activity in high temperature ranges. . However, the disadvantage of these commonly used catalysts for the removal of nitrogen oxides, which are highly active and sustained, is that when used for a long period of time to remove nitrogen oxides from exhaust gas containing alkali metal salts as dust components and at a temperature of 350°C or higher, Alkali metal salts in gas dust adhere to the catalyst surface, causing gradual deterioration of catalyst activity.If the exhaust gas temperature exceeds 400℃, ammonia as a reducing agent is decomposed by the catalyst active components and nitrogen oxides are produced. On the contrary, the oxidation of SO ₂ contained in the exhaust gas to SO ₃ by active components, i.e.
Conversion of SO ₂ to SO ₃ occurs, and the degree of conversion of SO ₂ to SO ₃ increases with time, and the converted SO ₃
reacts with ammonia to produce a compound of sulfuric acid and ammonia, which adheres to and corrodes boiler equipment such as air heaters and flue ducts. The present invention eliminates these drawbacks of conventional catalysts for removing nitrogen oxides, is highly active over a wide temperature range, and is less susceptible to deterioration due to alkali metal components.
The invention also relates to a catalyst for removing nitrogen oxides which has a low conversion rate of SO ₂ to SO ₃ and its change over time, and which has a support mainly composed of titanium oxide and a weight ratio of panadium oxide and tungsten oxide of 1:5 to 1:5. After loading the active ingredient at a ratio of 1:50 to 2 to 25% by weight of the carrier, 0.5 to 25% of the carrier weight is loaded with cerium oxide.
This is a catalyst for removing nitrogen oxides from exhaust gas coated with 2.5% by weight. Here, a carrier whose main component is titanium oxide is one that has a catalyst carrier function, that is, the carrier base material itself is made of titanium oxide or a substance whose main component is titanium oxide that contains a small amount of impurities such as Zn Sn. Alternatively, it may be a carrier base material composed of a substance other than titanium oxide, such as alumina or mullite, on which a carrier substance containing titanium oxide as a main component is supported. The present invention is not a catalyst in which all of the active components are mixed and supported at the same time. A specific amount of an active ingredient consisting of a specific ratio of vanadium oxide and tungsten oxide is supported on a carrier mainly composed of titanium oxide as defined above, and then a specific amount of cerium oxide is coated thereon. It is a nitrogen oxide removal catalyst that is highly active over a wide temperature range of ℃, has excellent durability, and has a low conversion rate of SO ₂ to SO ₃ and a small change over time. If vanadium oxide and titanium tungsten oxide are used in combination, initial activity is high and
It is known that the conversion rate of SO ₂ to SO ₃ is low. However, if the surface of the active component layer supporting a combination of vanadium oxide and tungsten oxide is coated with cerium oxide, the above-mentioned effects of the catalyst combining vanadium oxide and tungsten oxide will be maintained, and the alkali metal component in the exhaust gas dust will be reduced. It produces novel effects such as preventing deterioration of catalyst activity over time, that is, imparting alkali metal resistance, inhibiting ammonia decomposition at temperatures above 400°C, and suppressing the increase in the conversion rate of SO ₂ to SO ₃ over time. .
In other words, in the case of only vanadium oxide and tungsten oxide, the denitrification rate decreases little up to 400℃, but as the temperature approaches 550℃, ammonia water is rapidly decomposed, and the denitrification rate is 10% to 30% at 400℃ and 550℃. However, the reduction in the denitrification rate of the catalyst of the present invention is 2% or less. Furthermore, the catalyst of the present invention shows that the denitrification rate decreases due to alkali metals in exhaust gas at 550℃ after an 8000-hour durability test.
This significantly improves the 10-20% decrease in denitrification rate seen with conventional catalysts without cerium oxide coating, and at the same time reduces the change over time in the SO ₂ conversion rate to 5-20% compared with conventional catalysts. The increase was suppressed by 6% and remained below 0.5% after an 8000 hour durability test. The configuration of the present invention will be explained in more detail. It is important that the main component of the carrier for adjusting the catalyst is titanium oxide, and the shape of the carrier may be a carrier made by adding a binder to titanium oxide, kneading, and extrusion molding into a certain shape, or alumina, silica, mullite, etc. It may also be a carrier in which titanium oxide is supported on a ceramic base material such as cordierite and fired. Further, the raw material for the carrier needs to have the performance of titanium oxide when acting as a carrier, but the starting material does not need to be an oxidizing agent. Various oxides, sulfates, nitrates, ammonium salts, and the like can be used as active ingredients for preparing the catalyst, such as vanadium oxide, tungsten oxide, and cerium oxide. However, when supporting active ingredients on a carrier mainly composed of titanium oxide,
It is preferable to support the cerium oxide in a dissolved state in water, an organic solvent, an acid, etc. In particular, it is necessary to coat cerium oxide in a solution state. Note that in order to support vanadium oxide and tungsten oxide on a carrier mainly composed of titanium oxide, the active components of vanadium oxide and tungsten oxide are mixed with a carrier mainly composed of titanium oxide, then molded and fired, or
Alternatively, titanium oxide is coated and supported on a ceramic base material such as alumina or mullite, and the active ingredient is supported on the surface and fired, or powdered titanium oxide and active ingredient are mixed on a base material such as alumina or mullite. After that, the active ingredient may be supported and fired, or the active ingredient may be impregnated and supported on a preformed carrier mainly composed of titanium oxide. The weight ratio of vanadium oxide and tungsten oxide, which are active ingredients, is 1:5 to 1:50, and the total amount of these active ingredients supported is 2 to 2 of the carrier weight.
25% by weight is important. That is, when the weight ratio of vanadium oxide and tungsten oxide is 1:5 to 1:50, the activity including at low temperatures is significantly improved compared to when both components are used alone, and the conversion of SO ₂ to SO ₃ is improved. The rate will also be much lower. If the weight ratio of tungsten oxide to vanadium oxide is less than 5, the conversion of SO ₂ to SO ₃ will rapidly increase, which is undesirable and has an adverse effect on boiler equipment as described above. Furthermore, if the weight ratio of tungsten oxide to vanadium oxide is more than 50, the low temperature activity will drop sharply during activation. Therefore, the weight ratio of vanadium oxide to tungsten oxide must be in the range of 1:5 to 1:50. The weight ratio of vanadium oxide to tungsten oxide is 1:5 to 1:50, and the amount of the active ingredient supported on the carrier mainly composed of titanium oxide must be in the range of 2 to 25% by weight of the carrier weight. . If the supported amount is less than 2% by weight, the activity of the catalyst is low and a high denitrification rate cannot be obtained, and if it exceeds 25% by weight, the catalyst becomes expensive and high activity cannot be obtained, making it uneconomical. This is because it is not. In addition, the coating amount of cerium oxide is 0.5 to 2.5 when the active ingredient is supported on the carrier.
The reason for limiting it to % by weight is that if it is less than 0.5% by weight, the alkali metal resistance will be insufficient, and if it exceeds 2.5% by weight, the alkali metal resistance will be excellent, but the specific surface area of the catalyst will be small and the activity will be low. Because it will be. In addition, the supported catalyst of the present invention is prepared by supporting the active ingredient on the carrier, coating it with cerium oxide, and then drying and firing it at a temperature of 300-700°C, preferably 450-700°C.
This is because if the firing temperature is below 300°C, the thermal decomposition of the active ingredients into oxides will not be sufficient, and if the firing temperature is above 700°C, the active ingredients will sinter and the activity will decrease. be. Further, ammonia is added to the exhaust gas by at least 0.5 mole times, preferably about 1 to 2 moles, of nitrogen oxides. The obtained mixed gas is passed over the catalyst at a space velocity of 2,000 to 100,000/hour, preferably 5,000 to 50,000/hour, based on the superficial velocity. The temperature during the reaction is 200-650℃, preferably
The temperature ranges from 250 to 650°C. In addition, as the type of reactor for carrying out using the catalyst of the present invention, basically various types of reactors used for solid catalysts, such as ordinary fixed bed and moving bed, can be used. Next, the present invention will be explained in detail with reference to Examples. A honeycomb carrier with a length of 150 mm consisting of 36 square cells with a side of 6 mm and a wall thickness of 1 mm, which is mainly composed of titanium oxide, is prepared by dissolving vanadium pentoxide and ammonium paratungstate in monoethanolamine and water. It was immersed in a solution adjusted to contain the ratio and weight of vanadium oxide and tungsten oxide listed in the table, taken out, dried with hot air at 120°C for 30 minutes, and then fired at 500°C for 3 hours. Next, cerium nitrate was dissolved in water, immersed in a solution adjusted to cover the amount of cerium oxide listed in Table 1, taken out, and heated with hot air at 120°C for 30 minutes.
After drying for several minutes, the mixture was calcined again at 525° C. for 3 hours to obtain the catalyst of the present invention shown in Table 1. These catalysts have an inner diameter of 80mmφ and a length of 1000mm.
Six identical catalysts were packed in a stainless steel reactor and brought into contact with B heavy oil-fired boiler exhaust gas under the following conditions.
The denitrification rate and the conversion rate of SO ₂ to SO ₃ were measured from 270°C to 500°C. Further, after carrying out an 8000 hour durability test on these catalysts at 350°C, the denitrification rate and the conversion rate of SO ₂ to SO ₃ were measured again at 270°C to 500°C. The results are shown in Table 1. The test gas composition is NO _x ;
150-200ppm, _SOx ; 400-500ppm, _O2 ; 3-
6%, duct; 50mg/ ^Nm3 , gas temperature 350
Durability tests were conducted at ℃, SV value of 10000 Hr ^-1 , and NH ₃ /NO = 1.1. Note that NO _X was measured using a chemical fluorescence reduced pressure type NO/ _NO
ECL-77A). When measuring the conversion rate of SO ₂ to SO ₃ , stop the ammonia injection and after 5 hours have passed, use an NDIR SO ₂ analyzer to detect the SO ₂ concentration at the inlet and outlet, and use the following formula to convert SO 2 to SO ₃ . The conversion rate was determined. SO ₂ conversion rate (%) = Inlet SO ₂ concentration − Outlet SO ₂ concentration / Inlet SO ₂ concentration × 100 For comparison, the weight ratio and supported amount of vanadium oxide and tungsten oxide were other than those specified in the present invention. The same tests as above were conducted on catalysts coated with cerium oxide, coated with cerium oxide in an amount other than that specified in the present invention, and further with the impregnation order of vanadium oxide, tungsten oxide, and cerium oxide reversed. The results are also listed in Table 1. From Table 1, Figure ₁ shows the relationship between the desorption rate and the reaction temperature for the catalyst No. 10 of the present invention and the catalyst No. 1 of the comparative example at the initial stage and after the 8000 hour durability test.
FIG. 2 shows the relationship between conversion rate and reaction temperature. 1st
As is clear from the figure, catalyst example No. 10 of the present invention has a very small decrease in denitrification rate at reaction temperatures of 400°C or higher both at the initial stage and after the durability test, whereas comparative example No. 1 has a It can be seen that the decrease in the denitrification rate at temperatures above ℃ is large both at the initial stage and after the durability test. Furthermore, from FIG. 2, the initial value and the value after the durability test of the SO ₂ conversion rate are shown for the catalyst No. of the present invention:
It can be seen that whereas Comparative Example No. 1 has a small value, the initial value of Comparative Example No. 1 is large and the change over time after the durability test is extremely large.

【table】

【table】

[Table] As stated above, the main component of the present invention is a carrier made of titanium oxide, on which an active ingredient having a weight ratio of vanadium oxide and tungsten oxide of 1:5 to 1:50 is supported in an amount of 2 to 25% by weight of the carrier. After that, add cerium oxide
Using a catalyst coated with a coating of 0.5 to 2.5% by weight not only improves the activity of the catalyst over a wide temperature range, but also prevents poisoning by dust in the exhaust gas, and provides oxidation that converts SO ₂ in the exhaust gas to SO ₃ . It has an extremely excellent effect of suppressing reactions over a long period of time, and is extremely useful as a catalyst for removing nitrogen oxides from various exhaust gases.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram showing the relationship between the reaction temperature and the denitrification rate at the initial stage and after 8000 hours for a specific example of the present catalyst and a comparative catalyst, and Figure 2 is an illustration showing the initial stage of a specific example of the present catalyst and a comparative catalyst. FIG. 3 is an explanatory diagram showing the relationship between reaction temperature and SO ₂ conversion rate after 8000 hours.

Claims (1)

[Claims] 1. In a nitrogen oxide removal catalyst that catalytically reduces nitrogen oxides in exhaust gas in the presence of ammonia in the temperature range of 250 to 650°C, the weight of vanadium oxide and tungsten oxide is added to a carrier mainly composed of titanium oxide. For removing nitrogen oxides from exhaust gas, characterized in that 2 to 25% by weight of the carrier weight of active ingredients with a ratio of 1:5 to 1:50% is supported, and then coated with 0.5 to 2.5% by weight of cerium oxide. catalyst.