KR20050019708A - Catalyst and method for clarifying exhaust gas - Google Patents

Abstract

Using a proton-type? Zeolite as a catalyst, an excess of oxygen exhaust gas is brought into contact with the catalyst in the presence of methanol and / or dimethyl ether as a reducing agent to reduce and remove nitrogen oxides NO _x in the exhaust gas. Proton type β zeolite, SiO ₂ / Al ₂ O ₃ is preferred that the molar ratio is between 20 and 70. As a result, it has an excellent removing performance and the durability of the NO _X even in exhaust gas containing oxides of sulfur, not even at a relatively low temperature of about the exhaust gas temperature is 300-400 ℃ the denitration performance.

Description

Catalyst for exhaust gas purification and exhaust gas purification method {CATALYST AND METHOD FOR CLARIFYING EXHAUST GAS}

The present invention provides an exhaust gas purification catalyst effective for removing nitrogen oxides contained in various combustion exhaust gases from a boiler, a diesel engine generator, or a diesel engine vehicle, exhaust gas from an industrial facility, and an exhaust gas purification method using such a catalyst. It is about.

Among the various kinds of exhaust gases discharged from factories, power plants or other industrial plants, automobiles and so on, it is contained in the nitrogen oxide (NO _X), such as hydrogen or nitrogen dioxide, nitric oxide. In particular, such NO _X may not only adversely affect the respiratory system of the human body, but also is one of the causes of acid rain that is a problem in global environmental conservation. For this reason, the development of the technique which removes nitrogen oxide efficiently from these various exhaust gases is calculated | required.

As such a method of removing nitrogen oxides, a three-way catalyst method used for exhaust gas treatment of automobiles (gasoline cars), and an ammonia selective catalytic reduction method using ammonia as a reducing agent are known. However, the three-way catalytic method cannot be applied to the exhaust gas containing excess oxygen than the theoretical amount necessary to completely oxidize unburned content such as hydrocarbons and carbon monoxide remaining in the exhaust gas.

On the other hand, as a method of reducing and removing NO _x in an atmosphere in which oxygen is excessively present, an ammonia selective contact reduction method using a catalyst consisting of V ₂ O ₅ —TiO ₂ as a reducing agent is known. However, in such a method, since the odor is strong and harmful ammonia is used, handling is not easy, and a special apparatus for preventing the discharge of unreacted ammonia is required, so the facility is enlarged. For this reason, it is unsuitable for application to a small-scale exhaust gas generation source or a portable generation source, and also unfavorable in economics.

In recent years, the use of unburned hydrocarbons remaining in a dilute combustion exhaust gas of an oxygen excess atmosphere as a reducing agent has been reported to promote the reduction reaction of NO _X in the exhaust gas. Since this report, various catalysts for promoting the reduction reaction of NO _X have been developed, and for example, a catalyst in which alumina or alumina supports a transition metal and the like are used for reduction reduction reaction of NO _X using hydrocarbons as a reducing agent. There are a number of reports that are valid.

An example of a catalyst for reducing and removing nitrogen oxides in an excess of combustion exhaust gas by using such hydrocarbons as a reducing agent, in addition to a catalyst in which a transition metal is supported on alumina or alumina, 0.1-4% by weight of Cu, Fe, Cr, A reduction catalyst consisting of alumina or silica-alumina containing Zn, Ni or V has been reported (see Japanese Patent Laid-Open No. 4-284848).

In addition, when using a catalyst in which Pt or the like is supported on alumina, it has been reported that the reduction reaction of NO _X proceeds even in a low temperature range of about 200-300 ° C (JP-A-4-267946, Japanese Patent Laid-Open No. 5 -68855, Japanese Patent Application Laid-Open No. 5-103949. However, catalysts containing these noble metals have a reduction reaction to harmless N ₂ because excessive combustion of hydrocarbons as reducing agents is promoted excessively or N ₂ O, which is one of the causes of global warming, is generated as a by-product. It had the drawback of being difficult to selectively promote.

And, the catalyst is formed by supporting the alumina or the like, in a hydrocarbon as a reducing agent in an oxygen excessive atmosphere, are reported having to selectively promote the reduction reaction of NO _X (see JP Patent Application Laid-Open No. 4-281844). Since this report, a number of similar methods for reducing and removing NO _x using silver-containing catalysts have been developed and reported (see JP-A-4-354536).

However, none of the methods for purifying exhaust gas using a denitrification catalyst has a problem that the removal performance of NO _X is remarkably lowered in the excess oxygen exhaust gas containing sulfur oxides, and practical durability is insufficient. In the case of the exhaust gas is little in the relatively low temperature of about 300-400 ℃, the lower the removal performance of the NO _X was also a problem.

Further, using a catalyst impregnated or, V, Cr, Mn, Fe, Co, Ni, etc. made of a hydrogenation catalyst a zeolite in the zeolite, the NO _X under the coexistence of an organic compound A method of reducing and removing has been reported, and examples of the zeolite include Y-type zeolites, L-type zeolites, oleite-eryonite mixed crystal zeolites, ferrierite zeolites, and ZSM-5 zeolites. 2139645 specification). In addition, a method of reducing and removing NO _x in the presence of methanol using a proton zeolite has also been reported, and as such zeolite, a Y zeolite, a ZSM-5 zeolite and a mordenite are shown (Japanese Patent No. 2506598). Reference).

However, none of the methods for reduction and removal of NO _x using the specific zeolite catalyst described above have practically obtained sufficient NO _x removal performance, and have not been put to practical use in the present state.

The present invention, in view of the above circumstances, has an excellent removal performance and durability using the NO _X even in exhaust gas containing sulfur oxides, a high denitration performance at relatively low temperature of about the exhaust gas temperature is 300-400 ℃, practicality An object of the present invention is to provide an excellent catalyst for purifying exhaust gases and a method for purifying exhaust gases using the same.

The exhaust gas purifying catalyst according to the present invention for achieving the above object is capable of reducing and removing nitrogen oxides in an excess of exhaust gas in the presence of methanol and / or dimethyl ether, and made of a proton-type? Zeolite. It features.

In addition, the method for purifying exhaust gas according to the present invention, in order to reduce and remove nitrogen oxides in exhaust gas, contacting a proton type? Zeolite catalyst with an excess of oxygen exhaust gas in the presence of methanol and / or dimethyl ether as a reducing agent. It includes steps.

In the exhaust gas purification catalyst and the exhaust gas purification method of the present invention, it is preferable that the SiO ₂ / Al ₂ O ₃ molar ratio of the proton β zeolite is 20-70.

According to the present invention, it is possible to provide an exhaust gas purifying catalyst provided with the removal of the excellent NO _X performance and use durability, by using this catalyst, it is possible to efficiently remove NO _X from exhaust gas of excess oxygen. In addition, the catalyst for purifying exhaust gases of the present invention exhibits high desorption efficiency even for exhaust gases containing a large amount of sulfur oxides, and the denitrification performance is not lowered even at relatively low temperatures of about 300-400 ° C. , Very practically excellent.

Best Mode for Carrying Out the Invention

According to the present invention, when denitrification of nitrogen oxide NO _X in an excess of exhaust gas, a proton β zeolite is used as a catalyst. As zeolites, in addition to β-type, there are many types such as Y-type, L-type, ZSM-5 (MFI) type, and mordenite type. Among them, proton-type β zeolite, especially hydrogenated β zeolite, contains sulfur oxides. It is known to be very effective for the purification of exhaust gas. That is, by using a proton β zeolite, nitrogen oxide in the exhaust gas containing sulfur oxides can be reduced and removed in the presence of methanol and / or dimethyl ether.

In order to make β-zeolite into a proton type, for example, it can be made a proton type by treating with ammonium acetate aqueous solution or the like to ammonium type and then calcining to volatilize ammonia. In addition, a proton type (beta) zeolite can be shape | molded and can be set as the catalyst of various shapes according to a use.

The silica (SiO ₂ ) / alumina (Al ₂ O ₃ ) ratio of the proton-type β zeolite is preferably in the range of 20-70 in terms of molar ratio in terms of denitration performance for removing NO _X. As the molar ratio exceeds 40, the denitration performance tends to be lowered, and in consideration of stability to heat and steam, the molar ratio of SiO ₂ / Al ₂ O ₃ is more preferably in the range of 20-40.

The catalyst composed of the proton β zeolite can be molded into various shapes such as spherical shape, honeycomb shape and pellet shape by a known molding method. What is necessary is just to select these shapes, a magnitude | size, etc. according to the use condition of a catalyst. It is also possible to coat the proton-type β-zeolite with a wash coat method or the like on the surface of the support substrate of the refractory structure having a large number of through holes in the flow direction of the exhaust gas to form a catalyst.

As a method for purifying an oxygen-exhaust exhaust gas containing nitrogen oxide, the exhaust gas may be brought into contact with the above-described proton type β zeolite catalyst in the presence of methanol and / or dimethyl ether. The amount of methanol and / or dimethyl ether coexisting in the exhaust gas as the reducing agent may be appropriately selected depending on the denitration efficiency, running cost, and the like, which are obtained in operation, but in general, the molar ratio to nitrogen oxides in the exhaust gas (in terms of carbon (C)). As about 0.5-5, it is preferable.

Examples of the exhaust gas containing nitrogen oxide to which the present invention should be applied include exhaust gases from various combustion facilities such as boilers, internal combustion engines such as diesel engine cars and stationary diesel engines, and industrial facilities such as acetic acid production facilities. . These exhaust gases generally contain a reducing component called CO, HC (hydrocarbon) and H ₂ , and an oxidation component called NO _X and O _2, but are in excess of the stoichiometry required for the complete redox reaction of the protons. Contains oxygen. When such an excess of exhaust gas is brought into contact with the catalyst of the present invention in the presence of methanol and / or dimethyl ether, NO _X in the exhaust gas is reduced and decomposed into N ₂ and H ₂ O.

The gas space velocity SV in the exhaust gas purification method using the catalyst of the present invention is not particularly limited, but is preferably set to 1,000-100,000 h ⁻¹ . In addition, even at a relatively low temperature of about 300-400 ° C., the exhaust gas has excellent denitrification performance which is approximately equivalent to that at higher temperatures. Moreover, also in the exhaust gas containing sulfur oxide, it has the outstanding denitration performance, and its durability is also excellent.

In the case of treating the exhaust gas by the method of the present invention, unburned methanol, dimethyl ether and incomplete combustion products may be discharged into the exhaust gas depending on the reaction conditions. In such a case, the exhaust gas after passing through the denitration catalyst of the present invention is brought into contact with an oxidation catalyst, for example, a catalyst supported on a noble metal of Pt or Pd system to remove methanol, dimethyl ether and unburned components. Can be.

A commercially available NH ₄ type β zeolite (SiO ₂ / Al ₂ O ₃ molar ratio: 27) was calcined at 450 ° C. for 5 hours to obtain a proton type β zeolite. This was press-molded, and then ground to obtain a particle size of 350-500 µm, which was used as the catalyst 1 of the present invention. A proton β zeolite was obtained in the same manner as above except that commercially available NH ₄ β β zeolite (SiO ₂ / Al ₂ O ₃ molar ratio: 37) was used, and this was used as Catalyst 2 of the present invention.

On the other hand, as a comparative example, the following catalysts C1-C4 were adjusted. That is, the catalyst C1 is a proton β zeolite obtained by calcining a commercially available NH ₄ β β zeolite (SiO ₂ / Al ₂ O ₃ molar ratio: 75) at 450 ° C. for 5 hours. Catalyst C2 is a proton-type mordenite obtained by calcining commercially available NH ₄ type mordenite (SiO ₂ / Al ₂ O ₃ molar ratio: 20) at 450 ° C. for 5 hours. The catalyst C3 is a proton ZSM-5 obtained by calcining a commercially available NH ₄ ZSM-5 (SiO ₂ / Al ₂ O ₃ molar ratio: 27) at 450 ° C. for 5 hours. The catalyst C4 dissolves 1.3 g of cobalt acetate tetrahydrate in 100 g of ion-exchanged water, and 10 g of a proton β zeolite (molar ratio of SiO ₂ / Al ₂ O ₃ : 27) obtained in the same manner as the catalyst 1 in this solution. It disperse | distributed, it hold | maintained at 60 degreeC, stirred for 12 hours, and after that, it filtered and washed with water, dried at 110 degreeC, and baked at 500 degreeC in air for 3 hours, and obtained. It consists of β zeolite carrying Co. Moreover, Co amount in this catalyst C4 was 2.7 weight% of the whole catalyst in conversion of metal.

The catalysts 1 and 2 of the present invention and the catalysts C1-C4 of the comparative example obtained as described above were filled in a stainless steel reaction tube having an inner diameter of 15 mm, respectively, to form a catalyst body, which was mounted in an atmospheric pressure fixed upstream reactor. In this reaction tube, a mixed gas consisting of NO: 1,000 ppm, O ₂ : 10%, methanol: 1500 ppm, H ₂ O: 10%, SO ₂ : 100 ppm, and the balance: N ₂ is used as the model exhaust gas. It was supplied under the condition of 30,000 h ⁻¹ , and the denitration performance of each catalyst was evaluated. At this time, gas temperature was changed to 300 degreeC, 350 degreeC, and 400 degreeC, respectively.

In addition, the composition of model exhaust gas was changed, and the denitration performance of each catalyst was evaluated similarly. That is, 1,500 ppm of methanol added as a reducing agent in the model exhaust gas was changed to 750 ppm of dimethyl ether, and the denitration performance was evaluated using Catalyst 1, Catalyst C2 and Catalyst C3. And the reducing agent was changed to propylene: 500ppm and 1,000ppm, and denitration performance was evaluated using catalyst 1.

For the analysis of the gas composition at the outlet of the reaction tube, the concentration of NO _X was measured by a chemiluminescence type NO _X system, and the concentration of N ₂ O was measured by using a gasmatograph and a thermal conductivity detector equipped with a Porapak Q column. It was. In the case of any catalyst, N ₂ O was hardly recognized in the gas at the outlet of the reaction tube. Denitration rate as the NO _X removal performance of the catalyst was calculated according to the following equation.

Denitrification Rate (%) =

The denitrification rates of the catalysts 1, 2 and C1-C4 are shown in Table 1 below. As can be seen from this result, catalysts 1 and 2 of the present invention are catalysts C1 of the comparative example, even when the exhaust gas is in an excess of oxygen in which a large amount of sulfur oxide is present, even at a relatively low temperature of 300 to 400 ° C. It can be seen that the NO _x removal performance is remarkably superior to -C4. In addition, it can be seen that the catalysts 1 and 2 of the present invention exhibit excellent denitrification performance by using methanol and / or dimethyl ether as the reducing agent. Also in the proton β zeolite, the catalyst C1 having a SiO ₂ / Al ₂ O ₃ molar ratio of 75 has a very small denitrification rate.

TABLE 1

sample catalyst reducing agent Denitrification Rate (%) 300 ℃ 350 ℃ 400 ℃ One Catalyst 1 Methanol 64 88 95 2 Catalyst 2 Methanol 59 81 90 3 Catalyst 1 Dimethyl ether 64 84 93 4* Catalyst C1 Methanol 18 27 35 5 * Catalyst C2 Methanol 22 46 56 6 * Catalyst C3 Methanol 17 32 51 7 * Catalyst C4 Methanol 42 66 67 8* Catalyst C2 Dimethyl ether 24 44 58 9 * Catalyst C3 Dimethyl ether 15 35 52 10 * Catalyst 1 Propylene (500 ppm) 27 40 61 11 * Catalyst 1 Propylene (1000 ppm) 29 45 67

Note: The sample marked with * in the table is a comparative example.

Next, as evaluation of the durability of the catalyst, the above-described evaluation was made using the catalyst 1 of the present invention described above and the catalyst C2-C4 of the comparative example. That is, in each reaction tube configured as described above, as the model exhaust gas, NO: 1,000 ppm, O ₂ : 10%, methanol: 1500 ppm or dimethyl ether: 750 ppm, H ₂ O: 10%, SO ₂ : 1,000 ppm, remainder: a mixed gas consisting of N _2, gas temperature: was fed for 20 hours at conditions of 30,000h ^-1: 350 ℃, space velocity.

After the endurance test, a model exhaust gas was supplied to each reaction tube under the conditions of a gas temperature of 350 ° C. and a space velocity of 30,000 h ^{−1 in} the same manner as the above composition except that the SO ₂ concentration was 100 ppm. In this manner, the denitrification rate was obtained. The obtained results are shown in Table 2 below.

TABLE 2

sample catalyst reducing agent Denitrification Rate (%) Before the endurance test After endurance test 12 Catalyst 1 Methanol 88 83 13 Catalyst 1 Dimethyl ether 84 80 14 * Catalyst C2 Methanol 46 41 15 * Catalyst C3 Methanol 32 30 16 * Catalyst C4 Methanol 66 54 17 * Catalyst C3 Dimethyl ether 35 32

Note: The sample marked with * in the table is a comparative example.

As can be seen from this result, even after 20 hours of endurance tests by exhaust gas containing high concentration of SO ₂ , the catalyst made of the proton β zeolite of the present invention maintains high activity and is excellent in durability. Do. In the catalyst of the comparative example, the proton-type mordenite catalyst C2 having low denitrification performance and the catalyst C3 of the proton-type ZSM-5 exhibited good durability, but the catalyst C4 made of β zeolite carrying Co having excellent denitrification performance, It is understood that it is poor in terms of durability.

Claims (4)

An exhaust gas purification catalyst comprising a proton-type? Zeolite as an exhaust gas purification catalyst for reducing and removing nitrogen oxides in an excess of exhaust gas in the presence of methanol and / or dimethyl ether. The catalyst for purifying exhaust gases according to claim 1, wherein the molar ratio of SiO ₂ / Al ₂ O ₃ of the proton β zeolite is between 20 and 70. Reducing and removing nitrogen oxides in the exhaust gas by bringing an excess of oxygen into the catalyst consisting of a proton-type? Zeolite in the presence of methanol and / or dimethyl ether as the reducing agent. Purification method. The exhaust gas purification method according to claim 3, wherein a molar ratio of SiO ₂ / Al ₂ O ₃ of the catalyst is between 20 and 70.