JPH08257402A - Catalyst for removing nitrogen oxide and removing method of nitrogen oxide using the same - Google Patents

Abstract

PURPOSE: To provide a NOx removing catalyst reducing catalytically NOx in an exhaust gas by using a reducing agent such as ammonia. CONSTITUTION: In the catalyst for removing nitrogen oxide, a catalyst component obtained by burning a precipitate obtained by precipitating from an aq. soln. containing an aqueous medium-soluble titanium compd., tungsten compd. and cerium compd. is incorporated.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention removes nitrogen oxides from exhaust gas containing nitrogen oxides (hereinafter abbreviated as "denitration").
For details, refer to the method of reducing nitrogen oxides (hereinafter referred to as “NOx”) contained in high temperature exhaust gas discharged from various industrial processes such as gas engines, gas turbines, boilers, diesel engines, etc. as a reducing agent. The present invention relates to a denitration catalyst suitable for catalytic reduction using a solid reducing agent such as ammonia or urea, melamine, cyanuric acid, ammonium carbonate, and ammonium hydrogencarbonate, a method for producing a denitration catalyst, and a denitration method using the catalyst.

The "denitration catalyst" of the present invention means N in exhaust gas.
It means a catalyst for catalytically reducing Ox using the above reducing agent to convert it into harmless nitrogen and water.

[0003]

2. Description of the Related Art At present, as a method for removing NOx in exhaust gas, NOx can be selectively removed even in exhaust gas containing a high concentration of oxygen, and a small amount of reducing agent is used, which is economical. The catalytic reduction method using ammonia as a reducing agent has become the mainstream.

As catalysts used in this catalytic reduction method, many catalysts in which alumina, silica, zeolite, titanium oxide and the like are combined with oxides of vanadium, copper, tungsten, molybdenum, iron and the like have been proposed so far. There is. Among them, the catalyst containing titanium oxide as the main component is not widely influenced by sulfur oxides (SOx) in the exhaust gas and has a low ability to oxidize SO ₂ to SO ₃ in the exhaust gas, so that it is widely used today. It has been put to practical use. The reaction temperature is usually about 250 to 400 ° C.

As a method for preparing the above-mentioned catalyst containing titanium oxide as a main component, for example, an aqueous solution of a soluble tungsten compound is added to a titanium hydroxide precipitate gel obtained by adding aqueous ammonia to an aqueous solution of a soluble titanium compound, After mixing, molding, drying and firing, or in the case of molding, a part or all of the catalyst mixture is preliminarily decomposed by heating to form a mixture of oxides, followed by molding and drying and firing (Japanese Patent Publication No. 52-35342). A method (Japanese Patent Publication No. 1-14808) in which a metatitanic acid precipitate obtained by thermal hydrolysis of a titanium sulfate solution is solized, a tungsten compound is added thereto, the mixture is calcined, and the mixture is molded, dried and calcined is known. There is.

On the other hand, some gas turbine exhaust gases and diesel engine exhaust gases have an exhaust gas temperature exceeding 400 ° C., and in order to remove NOx in such high-temperature exhaust gas, excellent activity is obtained in a relatively wide temperature range. A denitration catalyst showing

However, the catalyst containing titanium oxide and tungsten oxide as the main components described in Japanese Patent Publication No. 52-35342, as described in Example 1 of the same publication, has the following characteristics.
It shows the highest catalytic activity at a temperature of 00 ° C, and has already reached a temperature range where the denitration rate decreases at 500 ° C. In order to increase the denitration rate, it is conceivable to increase the molar ratio of ammonia to NOx, but under the condition where ammonia is excessively present, not only the consumption amount of ammonia increases and economic efficiency decreases, but also There is a risk that a large amount of ammonia will leak into the exhaust gas when the load changes.

That is, the conventionally known denitration catalyst is 400 ° C.
In the case of catalytic reduction of NOx in high temperature exhaust gas that exceeds the limit, the oxidation (or decomposition) reaction of ammonia as a reducing agent is NO.
Since it occurs at the same time as the reduction of x, the denitrification rate is low, and in order to obtain a high denitrification rate, it is necessary to increase the molar ratio of ammonia to NOx, which increases the consumption of ammonia and reduces the economic efficiency. However, there was a problem with durability at high temperatures, and it was never satisfactory as a practical catalyst.

[0009]

The object of the present invention is to exhibit a high denitration activity in a wide temperature range, particularly in a high temperature range of more than 400 ° C., and even under the condition of a low ammonia / NOx molar ratio, and to have excellent durability. Denitrification catalyst,
The present invention provides a method for producing a denitration catalyst and a denitration method using the catalyst.

[0010]

SUMMARY OF THE INVENTION The present inventors have used a catalyst containing a catalyst component obtained by a predetermined preparation method of titanium, tungsten and cerium, whereby a high denitration is achieved even if the temperature of exhaust gas is high. It has been found that the rate occurs, and the present invention has been completed. That is, the present invention is specified as follows.

(1) Nitrogen oxidation characterized by containing a catalyst component obtained by calcining a precipitate obtained by precipitating an aqueous liquid containing a soluble titanium compound, a soluble tungsten compound and a soluble cerium compound in an aqueous medium. A catalyst for removing substances.

(2) Containing a catalyst component obtained by calcinating a precipitate obtained by neutralizing and precipitating an aqueous liquid containing a soluble titanium compound, a soluble tungsten compound and a soluble cerium compound in an aqueous medium with a basic substance. A method for producing a catalyst for removing nitrogen oxides, comprising:

(3) A method for removing nitrogen oxides, which comprises removing the nitrogen oxides from the exhaust gas containing the nitrogen oxides in the presence of ammonia by using the catalyst as described in 1 above. Hereinafter, the present invention will be described in detail.

The type of soluble titanium compound used in the present invention is not particularly limited as long as it is soluble in an aqueous medium, and inorganic titanium compounds such as titanium tetrachloride and titanium sulfate, and titanium oxalate and tetraisopropyl titanate. The organic titanium compound can be used. The type of soluble tungsten compound is also not particularly limited, and for example, ammonium metatungstate, ammonium paratungstate, etc. can be used. Regarding the ratio of the soluble titanium compound and the soluble tungsten compound, when the mixture is dried and calcined, the weight ratio of titanium oxide / tungsten oxide (TiO ₂ / WO ₃ , hereinafter the same) is 20/1 to 1/1, especially 20/1. It is preferable to set it in the range of from 1.5 to 1.5 because the activity of the denitration catalyst obtained will be high.

As the soluble cerium according to the present invention, inorganic cerium such as cerium nitrate, cerium nitrate, cerium carbonate, cerium sulfate and cerium sulfate,
Organic cerium such as cerium acetate can be used. The amount of cerium is preferably 2001 to 5/1 in terms of the weight ratio of titanium oxide and cerium oxide (TiO ₂ / CeO ₂ ). This is because the denitration efficiency of the catalyst falls outside this range.

Water is usually used as the aqueous medium.

In order to precipitate the soluble titanium compound, the soluble tungsten compound and the soluble cerium compound dissolved in the aqueous medium, conventionally known methods such as a thermal hydrolysis method, a thermal decomposition method and a neutralization method using a basic compound are used. It can be used, but a method of adding a basic compound for precipitation is preferably used, and it is more preferable to precipitate titanium, tungsten and cerium. The type of the basic compound is not particularly limited, and for example, ammonia, sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate or the like can be used. Of these, ammonia or an aqueous solution thereof (ammonia water) is preferably used from the viewpoint of the washability and handleability of the precipitate slurry.

Therefore, in the following, the catalyst according to the present invention will be specifically described by taking a method of precipitation using water as an aqueous medium and ammonia water as a basic compound as an example.

First, a soluble titanium compound such as titanium tetrachloride, a soluble tungsten compound such as ammonium metatungstate and a soluble cerium compound such as cerium nitrate are dissolved in water to obtain an acidic titanium-tungsten-cerium-containing aqueous solution. .
Next, the temperature of this aqueous solution is 60 ° C. or lower, preferably 50
Ammonia water was added to the final p while maintaining the range of ~ 0 ° C.
H is added so as to be in the range of 5 to 8, preferably 5 or more and less than 7 to cause precipitation. When the aqueous solution of the tungsten compound is basic, the aqueous solution containing tungsten is added to the aqueous solution containing titanium at the same time as the aqueous ammonia to cause precipitation.

The "final pH" in the present invention means the pH of the precipitate slurry or gel at the time when the precipitation operation is completed.

When the temperature in the precipitation operation exceeds 60 ° C., the activity of the denitration catalyst obtained becomes low, which is not preferable. If the final pH is lower than 5, the activity of the denitration catalyst obtained will be low, and if it is higher than 8, the activity will be low, and in addition, redissolution of tungsten will occur, which is not preferable.

The titanium-tungsten-cerium precipitate obtained by the above precipitation operation is separated from the precipitate slurry, washed well, dried and calcined to obtain titanium-tungsten-cerium oxide. Above separation,
Washing, drying and calcination can be carried out under the conditions generally used for the preparation of this type of catalyst, but the weight ratio titanium oxide / tungsten oxide is 20/1 to 1/1, especially 20/1 to 1.5 / 1, titanium oxide / cerium oxide weight ratio of 200-5 is 400-750 ° C, especially 4
It is preferable to heat and bake in the range of 50 to 700 ° C. to obtain a denitration catalyst composed of titanium-tungsten-cerium oxide having excellent activity and durability. Also, regarding the baking treatment, (a) a method of drying the titanium-tungsten-cerium precipitate and then molding it, and (b) a method of molding the titanium-tungsten-cerium precipitate and then baking, or (c) Any of the above methods (a) and (b) may be used. In the case of the method (b), the heat treatment in the latter stage may cause shrinkage of the molded body, change of the physical properties of the catalyst, and the like. Therefore, the method (a) or the method (c) is preferable, and also in the case of the method (c). It is preferable to carry out the firing of the latter stage at the same temperature as the firing of the former stage or at a temperature lower than that.

Although it is not clear why the denitration catalyst comprising titanium-tungsten-cerium oxide of the present invention has excellent denitration activity and excellent durability, it is possible to carry out precipitation under the above specific conditions. It is inferred that the active components of tungsten oxide and cerium oxide were obtained in a highly dispersed state in titanium oxide.

The denitration catalyst according to the present invention can be used in combination with a composite oxide such as alumina, silica, zirconia, titania or titania-silica, or a zeolite composed of silica / alumina in a specific ratio.

The denitration catalyst according to the present invention can be used as it is by integrally molding or by supporting it on a carrier. As the carrier, alumina, silica, composite oxides such as silica-alumina, titania-silica, bentonite, diatomaceous earth, silicon carbide, titania, zirconia, magnesia, cordierite, mullite, pumice, inorganic fibers, or stainless steel or the like. A metal plate or net can be used. The supporting method is not particularly limited, and for example, a method of impregnating granular silicon carbide with the slurry of the denitration catalyst and supporting it, a method of coating the stainless corrugated plate with the slurry of the denitration catalyst and supporting it can be used.

The shape of the denitration catalyst (including carrier-supported catalyst) according to the present invention is not particularly limited, and it may be honeycomb, plate, cylinder, ribbon, pipe, donut, lattice, column, or wave. It may be plate-shaped or granular.

If the surface area of the catalyst obtained in the present invention is too low, the denitration activity is low, and if it is too high, the decomposition of ammonia is promoted. Therefore, the surface area is 10 to 120 m ² / g, preferably 15 to 1
00m ² / g is selected, also the pore volume is too low activity is low, too when the catalytic strength decreases high 0.1
0.6 cc / g, preferably 0.2 to 0.5 cc / g is selected.

The composition of the exhaust gas used in the present invention is not particularly limited, and the present invention can be used for the treatment of various exhaust gases containing NOx. For example, S
Ox 0 to 3000 ppm, oxygen 1 to 20% by volume, carbon dioxide gas 1 to 15% by volume, water vapor 5 to 15% by volume, soot dust 0.0
01-30 g / Nm ³ and NOx (mainly NO) 2
Used for the treatment of exhaust gas containing about 0 to 10000 ppm. Furthermore, the denitration catalyst of the present invention can also be used for treatment of special exhaust gas such as NOx-containing exhaust gas containing no SOx and NOx-containing exhaust gas containing halogen compounds.

Exhaust gas treatment conditions using the denitration catalyst of the present invention differ depending on the type and properties of the exhaust gas, the required denitration rate, etc., but cannot be specified unconditionally. Good.

As the reducing agent, ammonia or urea which decomposes to produce ammonia, melamine, cyanuric acid, ammonium carbonate, ammonium hydrogencarbonate or the like is used. Ammonia and urea are preferably used from the viewpoint of safety in terms of handling and cost. The form of injection of the reducing agent into the exhaust gas is not particularly limited, but from the viewpoint of dispersibility and handleability of the reducing agent in the exhaust gas, ammonia gas, liquid ammonia, aqueous ammonia, urea aqueous solution, ammonium carbonate aqueous solution, ammonium hydrogencarbonate aqueous solution or the like gas or It is preferably injected as a liquid. When a reducing agent that decomposes to produce ammonia is used, the amount of the reducing agent added is determined by the amount of ammonia produced from the reducing agent. For example, urea is 1/2 mol of ammonia and melamine is 1/3 mol of injection. It becomes the amount.

The amount of ammonia used as a reducing agent is determined by considering the denitration rate, the amount of leaked ammonia, etc.
The NOx (NO conversion) molar ratio can be appropriately selected within a range of 0.3 / 1 to 3/1, preferably 0.3 / 1 to 1.5 / 1. In particular, in the case of the denitration catalyst of the present invention, a high denitration rate can be obtained even when the ammonia / NOx (NO conversion) molar ratio is in the range of 1.5 / 1 or less, and NOx can be efficiently decomposed and removed. For example, in the case of treating exhaust gas from a boiler, since most of NOx contained in this exhaust gas is NO, it is particularly preferable that the ammonia / NOx (NO conversion) molar ratio is around 1, but the required denitrification rate and leak Considering the amount of ammonia and the like, it is appropriately selected within the range of about 2/1 or less.

The reaction temperature is 400 to 700 ° C., preferably 450 to 650 ° C. The space velocity is usually 1000 to 100,000 Hr ~ ¹ , especially 3000
It is preferably in the range of -80,000 Hr- ¹ . The pressure is not particularly limited, but usually 0.01 to 10 kg / cm ²
Is preferred.

The type of reactor used for treating the exhaust gas is not particularly limited, and ordinary fixed bed, moving bed, fluidized bed, etc. reactors can be used.

[0034]

EXAMPLES The present invention will be described in more detail below with reference to examples.

(Example 1) 11.4 kg of titanium tetrachloride (TiCl ₄ ) in 80 liters of water (hereinafter referred to as L)
Is gradually added dropwise under ice cooling and stirring to dissolve, and an aqueous solution of ammonium metatungstate manufactured by Japan Inorganic Chemical Industry Co., Ltd. (containing 50% by weight as tungsten oxide) is added to this aqueous solution.
An aqueous solution prepared by dissolving 2.4 kg and 0.5 kg of ceric nitrate in 1 liter of water was added. While maintaining the temperature of this aqueous solution at about 30 ° C. and stirring well, aqueous ammonia was added until the pH reached 6, and the mixture was allowed to stand for aging for 2 hours.

The titanium-tungsten precipitate slurry thus obtained was filtered, the titanium-tungsten precipitate obtained was washed with water, dried at 150 ° C. for 10 hours, and then calcined at 600 ° C. for 5 hours. A titanium-tungsten-cerium oxide having titanium oxide / tungsten oxide = 4/1 (weight) and titanium oxide / cerium oxide = 24/1 (weight) was obtained.

Using a kneader, an appropriate amount of water was added to 1 kg of the above-mentioned titanium-tungsten-cerium oxide powder, and after well kneading, an extruder was used to obtain a diameter of 4 mm and a length of 5
It was molded into a pellet of mm. Next, after drying the pellets at 60 ° C., the pellets were baked at 600 ° C. for 5 hours under air flow, and titanium oxide / tungsten oxide = 4/1 (weight)
A catalyst (A) containing titanium oxide / cerium oxide = 24/1 (by weight) was prepared.

(Example 2) As a titanium source, an aqueous sulfuric acid solution of titanyl sulfate (250 g / L in terms of TiO ₂ ) was used.
2 L was taken, 80 L of water was added thereto, and an aqueous solution in which 0.5 kg of ceric nitrate was dissolved in 1 liter of water was mixed. While maintaining the temperature of this aqueous solution at about 30 ° C. while stirring well, 0.54 kg of monoethanolamine and water.2.
7 L and mixed with ammonium paratungstate 1.
35 kg was added and the dissolved aqueous solution and aqueous ammonia were gradually added dropwise, aqueous ammonia was added until the pH reached 6, and the mixture was allowed to stand for aging for 2 hours. Less than,
A catalyst (B) having the same composition was prepared in the same manner as in Example 1.

(Examples 3 to 7) Catalysts (C) to (G) were prepared in the same manner as in Example 1 except that the weight ratio of titanium oxide / tungsten oxide and the weight ratio of titanium oxide / cerium oxide were changed. Was prepared. The catalyst composition is shown in Table 1.

[0040]

[Table 1]

(Comparative Example 1) Commercially available anatase type titanium oxide powder (DT-5 manufactured by Lone Poulin) as a titanium source.
0) Using 4.8 kg, 0.54 kg of monoethanolamine and 2.7 L of water were mixed, an aqueous solution prepared by adding 1.35 kg of ammonium paratungstate was added, and an appropriate amount of water was added with a kneader. While mixing well,
Kneaded Hereinafter, in the same manner as in Example 1, the comparative catalyst (a)
Was prepared.

Comparative Example 2 4.8 kg of metatitanic acid slurry was used as TiO ₂ as a titanium source, 0.54 kg of monoethanolamine and 2.7 L of water were mixed, and 1.35 kg of ammonium paratungstate was added. The resulting aqueous solution was added and kneaded while heating in a kneader, the resulting slurry was dried at 150 ° C. for 10 hours and then calcined at 600 ° C. for 5 hours to obtain a powder having the same composition as in Example 1. It was Thereafter, a comparative catalyst (b) was prepared in the same manner as in Example 1.

(Evaluation Example 1) Examples 1 to 7 and Comparative Example 1
The denitration rate of each of the catalysts (A) to (G) and comparative catalysts (a) to (b) obtained in Examples 1 to 2 was determined by the following method.

20 ml of the catalyst was filled in a quartz reaction tube installed in an electric furnace, and a synthesis gas having the following composition was introduced into the catalyst layer under the following conditions. NOx concentration in the gas at the inlet of the reaction tube (NOx concentration at the inlet) and NO in the gas at the outlet of the reaction tube
x concentration (outlet NOx concentration) is defined by Yanagimoto Seisakusho N
It was measured with an Ox meter (ECL-77A type), and the denitration rate was calculated according to the following formula.

Denitration rate (%) = (inlet NOx concentration-outlet N
Ox concentration) / (inlet NOx concentration) (× 100) Synthesis gas composition NOx: 100 ppm, O ₂ : 15%, SO ₂ : ₂₀₀ p
pm, H ₂ O: 10%, N ₂ : residual reaction conditions Gas amount: 3.33 NL / min, space velocity (SV): 1
0000 Hr ~ ¹ , NH ₃ / NOx (NO conversion) molar ratio: 1.0, temperature: described in Table 2 The obtained results are shown in Table 2.

[0046]

[Table 2]

(Evaluation Example 2) With respect to the catalyst (A) obtained in Example 1, the denitration rate was obtained in the same manner as in Evaluation Example 1 except that the NH ₃ / NOx (NO conversion) molar ratio was changed. The amount of ammonia discharged (leak ammonia amount) was measured by the indophenol method according to JIS-K-0099. Table 3 shows the obtained results.

[0048]

[Table 3]

(Example 8) A catalyst (H) having the same composition was prepared in the same manner as in Example 1 except that the temperature at the time of precipitation was changed from about 30 ° C to 55 ° C.

Example 9 A catalyst (I) having the same composition was prepared in the same manner as in Example 1 except that the final pH at the time of precipitation was changed from 6 to 5.

(Example 10) A catalyst (J) having the same composition was prepared in the same manner as in Example 1 except that the final pH at the time of precipitation was changed from 6 to 8.

Comparative Example 3 A comparative catalyst (c) having the same composition was prepared in the same manner as in Example 1 except that the temperature during precipitation was changed from about 30 ° C. to 80 ° C.

Comparative Example 4 A comparative catalyst (d) having the same composition was prepared in the same manner as in Example 1 except that the final pH at the time of precipitation was changed from 6 to 4.

Comparative Example 5 A comparative catalyst (e) having the same composition was prepared in the same manner as in Example 1 except that the final pH at the time of precipitation was changed from 6 to 9.0.

(Evaluation Example 3) The completed catalysts (H) to (J) and comparative catalysts (c) to (e) obtained in Examples 8 to 10 and Comparative Examples 3 to 5 were evaluated in the same manner as in Evaluation Example 1. The denitration rate was calculated. The results obtained are shown in Table 4.

[0056]

[Table 4]

(Examples 11 to 14) In Example 1,
Catalysts (K) to (N) having the same composition were prepared in the same manner as in Example 1 except that the firing temperature of the titanium-tungsten precipitate was changed as shown in Table 5.

(Evaluation Example 4) With respect to the catalysts (K) to (N) obtained in Examples 11 to 14, the denitration rate was obtained in the same manner as in Evaluation Example 1. Table 5 shows the obtained results.

[0059]

[Table 5]

Evaluation Example 6 Catalysts (A), (E) obtained in Examples 1, 5, 9, 13 and Comparative Examples 2, 3
The durability of (I), (M) and comparative catalysts (b), (c) was evaluated by the following method.

Each catalyst was placed under an air flow in an electric furnace adjusted to 550 ° C. and taken out after a lapse of a predetermined time. Less than,
In the same manner as in Evaluation Example 1, the denitration rate at a reaction temperature of 500 ° C. was measured. The obtained results are shown in Table 6.

[0062]

[Table 6]

[0063]

The denitration catalyst of the present invention has a wide temperature range,
In particular, it exhibits high denitration activity in a high temperature range of 400 ° C. or higher and has excellent durability. Therefore, by using the denitration catalyst of the present invention, NOx in high-temperature exhaust gas discharged from various industrial processes other than gas turbines, boilers, gas engines, diesel engines, etc. can be efficiently decomposed and removed.

The NOx removal catalyst of the present invention exhibits excellent NOx removal activity and durability even in a low ammonia / NOx molar ratio in a wide temperature range, particularly in a high temperature range of 400 ° C or higher. Therefore, by using the denitration catalyst of the present invention, while minimizing the consumption of ammonia, that is, economically,
Moreover, NOx in the high temperature exhaust gas can be efficiently decomposed and removed without causing a problem such as a leak of ammonia.

Claims (3)

[Claims] 1. A nitrogen oxide containing a catalyst component obtained by calcining a precipitate obtained by precipitating an aqueous liquid containing a soluble titanium compound, a soluble tungsten compound and a soluble cerium compound in an aqueous medium. Removal catalyst. 2. A catalyst component obtained by calcining a precipitate obtained by neutralizing and precipitating an aqueous liquid containing a soluble titanium compound, a soluble tungsten compound and a soluble cerium compound in an aqueous medium with a basic substance. A method for producing a nitrogen oxide removing catalyst, comprising: 3. A method for removing nitrogen oxides, wherein the exhaust gas containing nitrogen oxides is removed from the exhaust gas by using the catalyst according to claim 1 in the presence of ammonia.