JP2012192338A - Method of treating exhaust - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of treating exhaust, capable of highly efficiently treating nitrogen oxides present in the exhaust to a high degree, in particular capable of efficiently removing NO remaining therein subsequent to treating the exhaust using a denitrification catalyst that can treat NOand NO, and of decomposing excessive ammonia present in a standard denitrification treatment.SOLUTION: The method of treating exhaust is characterized by treating exhaust comprising nitrogen oxides with a denitrification catalyst subsequent to adding ammonia and/or urea thereinto, thereafter treating it with an ammonia decomposition catalyst, and further treating it with a NO decomposition catalyst.

Description

The present invention relates to an exhaust gas treatment containing nitrogen oxides, and particularly relates to a technology relating to exhaust gas treatment containing nitrous oxide (hereinafter referred to as “N ₂ O”).

Conventionally, many proposals have been made in the treatment of nitrogen oxides contained in exhaust gas. However, in recent years, higher treatment of nitrogen oxides has been desired for the purpose of preventing air pollution and global warming. As a method for treating nitrogen oxide, a method is proposed in which nitrogen oxide is brought into contact with ammonia on a denitration catalyst and decomposed into nitrogen and water (see Patent Document 1). However, this method can treat nitrogen monoxide and nitrogen dioxide (hereinafter collectively referred to as “NOx”), but is not suitable for treating N ₂ O. On the other hand, a method for efficiently removing N ₂ O has also been proposed (see Non-Patent Document 1). However, in this method, when NOx and ammonia coexist in the exhaust gas, the N ₂ O removal rate is increased. It will be a problem that it drops significantly.

Japanese Patent Laid-Open No. 54-1978857

Applied Catalysis B Environmental 75 (2007) 167-174

The present invention, nitrogen dioxide (hereinafter NO ₂₎ a nitrogen oxides contained in the exhaust _gas, nitrogen monoxide (hereinafter NO), a decomposing technique N 2 _O nitrogen, until the water and oxygen, in particular N ₂ O The purpose of this is to decompose efficiently.

  In order to solve the problems according to the present invention, the present inventors have intensively studied and found the following configuration and related the invention. The invention is specified below.

Characterized in that the ammonia and / or urea into the exhaust gas containing nitrogen oxides is added, NOx treated by the denitration catalyst is then treated with ammonia by an ammonia decomposition catalyst, further processes the N _{2 O} by N _{2 O} decomposition catalyst This is an exhaust gas treatment method.

  By using the present invention, it is possible to decompose nitrogen oxides in exhaust gas into nitrogen and water to a high degree as compared with ordinary exhaust gas treatment technology.

FIG. 1 shows the processing method of the present invention.

The present invention relates to an exhaust gas treatment method characterized by adding ammonia and / or urea to exhaust gas containing nitrogen oxides, treating with a denitration catalyst, then treating with an ammonia decomposition catalyst, and further treating with an N ₂ O decomposition catalyst. It is. The present invention will be specifically described below.

The present invention comprises three steps, the first step is a step of treating NOx using a denitration catalyst, and the second step is a step of treating ammonia with an ammonia decomposition catalyst on the downstream side of step 1. The third step is a step of treating N ₂ O with an N ₂ O decomposition catalyst on the downstream side of step 2.

(Process 1)
The purpose of this step is to reduce NOx contained in the exhaust gas. NOx is NO ₂ or NO. The amount of NOx contained in the exhaust gas is 0.001 to 5% (volume basis, hereinafter the same for the gas body), preferably 0.005 to 2%. It should be noted that there is no problem even if other components are included as long as the technique according to the present invention can be carried out. As other components contained in the exhaust gas, there are oxygen, nitrogen, carbon dioxide, carbon monoxide, water, sulfur dioxide, hydrocarbons, organic compounds, ammonia, dust, etc., depending on the exhausted gas. Moreover, it is preferable that the component which has a reducing action in the said component, for example, reducing substances, such as carbon monoxide, a hydrocarbon, and ammonia, is contained.

Ammonia and urea can be used as the gas added to the exhaust gas. The amount of ammonia and / or urea added is 1.0 to 2.0 times, preferably 1.0 to 1.5 times, more preferably 1.0 to 1.5 times the equivalent of NOx that can be decomposed into nitrogen and water. 1.2 times is desirable. If the amount of ammonia and / or urea added is less than 1.0 times the equivalent of NOx that can be decomposed into nitrogen and water, NO ₂ and NO remain. If the addition amount of ammonia and / or urea exceeds twice the equivalent amount capable of decomposing NOx into nitrogen and water, ammonia remains in the exhaust gas after step 2 and the performance of the N ₂ O decomposition catalyst is improved. It will have an adverse effect. An equivalent amount capable of decomposing NOx into nitrogen and water is as follows. That is, when the target substance is NO, ammonia is 1 mol and urea is 0.5 mol with respect to 1 mol of NO. When the target substance is NO ₂ , ammonia is 2 mol and urea is 1 mol with respect to 1 mol of NO ₂ .

  The denitration catalyst may be any catalyst as long as it can efficiently treat NOx. Specifically, it contains molybdenum, vanadium, tungsten, nickel, cobalt, and iron as active components, and is preferably selected from molybdenum, vanadium, and tungsten. It contains one or more elements or compounds thereof as active ingredients. Further, the active ingredient can be used by being supported on a base material made of an oxide of titanium, aluminum, silicon, zirconium, or a composite oxide of two or more elements selected from these.

  As a preparation method of the denitration catalyst, a general preparation method using various metal salts can be used. For example, an impregnation method, a coprecipitation method, a kneading method, an alkoxide method, and the like are used.

  As starting materials for each catalyst component, oxides, hydroxides, inorganic salts, organic salts, and the like of each element are used. Specific examples include ammonium salts, oxalates, sulfates, nitrates, halides, etc. For example, titanium sources include titanyl sulfate, titanium tetrachloride, tetraisopropyl titanate, etc. Silica sol, water glass, silicon tetrachloride and the like can be used. As the vanadium source, ammonium metatungsten vanadate is used, as the tungsten source, ammonium metatungstate, ammonium paratungstate, etc. are used, and as the molybdenum source, ammonium paramolybdate, molybdic acid, etc. are used.

  The denitration catalyst can be used after being formed into a saddle shape, a pellet, a sphere, or a honeycomb shape by extrusion molding, tableting molding, rolling granulation or the like. Further, it is possible to use a general base material in the form of a saddle, pellets, spheres, and honeycombs with a denitration catalyst component. In order to reduce the pressure loss of the exhaust gas, a honeycomb shape is preferable.

(Process 2)
The purpose of this step is to decompose ammonia added upstream of the denitration catalyst and / or ammonia generated from urea. In addition, ammonia contained in the exhaust gas from the beginning can be targeted for decomposition.

  The ammonia decomposition catalyst may have any composition as long as it can decompose ammonia into nitrogen and water. Specifically, platinum, palladium, rhodium, ruthenium, iridium, manganese, copper, silver, cobalt , One or more elements selected from iron and nickel or compounds thereof as active ingredients, preferably one or more elements selected from platinum, palladium, rhodium, iridium and cobalt as active ingredients Is included. Further, the active ingredient can be used by being supported on a base material made of an oxide of titanium, aluminum, silicon, zirconium, or a composite oxide of two or more elements selected from these. Furthermore, it is preferable that the second active component contains one or more elements selected from molybdenum, vanadium, tungsten, nickel, cobalt, and iron, or a compound thereof.

  The ammonia decomposition catalyst can be used in the form of powder, granules, saddles, pellets, spheres, and honeycombs. Alternatively, the ammonia decomposition catalyst can be used on a sphere, saddle or honeycomb catalyst substrate. Can be used. In order to reduce the pressure loss of the exhaust gas, a honeycomb shape is preferable.

  As a preparation method of the ammonia decomposition catalyst, a general preparation method using various metal salts can be used. For example, an impregnation method, a coprecipitation method, a kneading method, an alkoxide method, and the like are used.

  As starting materials for each catalyst component, oxides, hydroxides, inorganic salts, organic salts, and the like of each element are used. Specific examples include ammonium salts, oxalates, sulfates, nitrates and halides. For example, titanyl sulfate, titanium tetrachloride, tetraisopropyl titanate, or the like is used as the titanium supply source, and silica sol, water glass, silicon tetrachloride, or the like can be used as the silicon supply source. As the platinum source, hexachloroplatinic acid hexahydrate, diammine dinitroplatinum nitrate solution, hexaammine platinum chloride solution, tetraammine platinum chloride, tetraammine platinum hydrochloride solution, etc. are used, and as the palladium source, palladium chloride, nitric acid solution are used. Palladium, palladium sulfate, diamminedinitropalladium, tetraamminepalladium chloride, tetraamminepalladium hydrochloride solution, palladium acetate, etc. are used. , Ammonium paratungstate, and the like, and ammonium sources such as ammonium paramolybdate and molybdic acid are used.

  The ammonia decomposition catalyst can be used after being formed into a saddle shape, pellets, spheres, or honeycombs by extrusion molding, tableting molding, rolling granulation, or the like. Further, it is also possible to use a general substrate in the form of saddle, pellet, sphere or honeycomb coated with an ammonia decomposition catalyst component. In order to reduce the pressure loss of the exhaust gas, a honeycomb shape is preferable.

(Process 3)
The purpose of this step is to decompose N ₂ O that could not be processed in steps 1 and 2.

The N ₂ O decomposition catalyst may be one that can efficiently treat N ₂ O, but specifically contains one or more elements selected from cobalt, nickel, and iron or compounds thereof as active components, and more Preferably, it contains at least one element selected from cobalt and nickel or a compound thereof as an active ingredient. Moreover, it is preferable that the promoter component is included with said active component. The promoter component is at least one element selected from alkali metals or alkaline earth metals or a compound thereof. Further, the above components can be used by being carried on a base material made of an oxide of titanium, aluminum, silicon, zirconium, or a composite oxide of two or more elements selected from these. The shape of N 2 _O decomposition catalyst, powder, granules, saddle, pellets, spheres, other that can be used by molding into a honeycomb shape, spherical, saddle, N ₂ O in the honeycomb catalyst base material A cracking catalyst can be coated and used. In order to reduce the pressure loss of the exhaust gas, a honeycomb shape is preferable.

As a method for preparing the N ₂ O decomposition catalyst, a general method using various metal salts can be used. For example, an impregnation method, a coprecipitation method, a kneading method, an alkoxide method, or the like is used.

As starting materials for each catalyst component, oxides, hydroxides, inorganic salts, organic salts, and the like of each element are used. Specific examples include ammonium salts, oxalates, acetates, sulfates, nitrates, halides, and the like. For example, cobalt oxide, cobalt nitrate, cobalt sulfate, cobalt chloride, cobalt acetate, cobalt hydroxide and the like are used as the cobalt supply source. The N ₂ O decomposition catalyst can be used after being formed into a saddle shape, a pellet, a sphere, or a honeycomb shape by extrusion molding, tableting molding, rolling granulation or the like. Further, it is also possible to use a general base material in the form of a saddle, pellets, spheres, or honeycombs by coating the components of the N ₂ O decomposition catalyst. In order to reduce the pressure loss of the exhaust gas, a honeycomb shape is preferable.

  The temperature of the exhaust gas is 100 to 650 ° C, preferably 200 to 600 ° C. If the temperature is lower than 100 ° C., the catalyst processing efficiency is lowered, and if it exceeds 650 ° C., the deterioration of the catalyst is accelerated, which is not preferable.

The space velocity during processing 1,000~1,000,000hr ^-1, preferably 2,000~100,000hr ^-1.

The _{N 2} O decomposition catalyst, the fact that its performance deteriorates when NOx coexist in the exhaust gas, _{N 2} O concentration of NOx contained in the exhaust gas decomposition catalyst inlet 0.001% is less, preferably not more than 0.0005% More preferably, the content is 0.0001%. Therefore, it is desirable that the temperature of the exhaust gas in step 1 is 100 to 650 ° C, preferably 200 to 600 ° C. If the temperature is lower than 100 ° C., the catalyst processing efficiency is lowered, and if it exceeds 650 ° C., the deterioration of the catalyst is accelerated, which is not preferable. The space velocity is 1,000～100,000Hr ^-1 in denitration reaction of Step 1, preferably 2,000～50,000Hr ^-1, more preferably 3,000～20,000Hr ^-1 is desirable. If the space velocity exceeds 100,000 ^-1 is difficult to remove the NOx to 0.001% or less, but space velocity catalyst performance is less than 1,000 hr ^-1 is no large difference device Because it becomes large and inefficient.

Also the fact the _{N 2} O decomposing catalyst to poor performance when coexisting ammonia in the exhaust gas, _{N 2} O concentration of ammonia contained in the exhaust gas decomposition catalyst inlet 0.001% is less, preferably not more than 0.0005% More preferably, the content is 0.0001%. Therefore, it is desirable that the temperature of the exhaust gas in step 2 is 100 to 650 ° C, preferably 200 to 600 ° C. If the temperature is lower than 100 ° C., the catalyst processing efficiency is lowered, and if it exceeds 650 ° C., the deterioration of the catalyst is accelerated, which is not preferable. The space velocity is 1,000～100,000Hr ^-1 in the ammonia decomposition reaction of step 2, preferably 2,000～50,000Hr ^-1, more preferably 3,000～20,000Hr ^-1 is desirable. If the space velocity exceeds 100,000 ^-1 it is difficult to remove ammonia to 0.001% or less, but space velocity catalyst performance is less than 1,000 hr ^-1 is no large difference device Because it becomes large and inefficient.

  In the case where both NOx and ammonia coexist in the exhaust gas, it is desirable that the concentration of each is 0.001% or less, preferably 0.0005% or less, more preferably 0.0001%.

Although the capacity | capacitance between each catalyst which concerns on this invention is influenced by the to-be-processed component in waste gas, 0.1-10 are preferable normally for the capacity | capacitance ratio of a denitration catalyst / ammonia decomposition catalyst. The volume ratio of ammonia decomposition catalyst / N ₂ O decomposition catalyst is preferably 0.1 to 10. The volume ratio of N ₂ O decomposition catalyst / denitration catalyst is preferably 0.1-10.

The present invention will be described in detail below with reference to examples, but the present invention is not limited to the following examples as long as the effects of the present invention are achieved. Hereinafter, it describes about the catalyst used for an Example and a comparative example, and the measuring method of the density | concentration of each gas. Measurement of each gas into account the effect of each catalyst, the NOx and ammonia gas was measured by N _{2 O} decomposition catalyst inlet for N 2 _O was measured at the exit of the N _{2 O} decomposition catalyst.

-Denitration catalyst-
<Preparation of titanium-silicon composite oxide powder>
A mixture of 16 kg of silica sol (containing 30 mass% as SiO ₂ ), 270 kg of 25 mass% aqueous ammonia and 180 L of water is mixed well with 500 L of sulfuric acid solution of titanyl sulfate (70 g / L as TiO ₂ , sulfuric acid concentration 285 g / L). The solution was gradually added dropwise to produce a precipitate. The slurry was aged, filtered and washed, and then dried at 150 ° C. for 10 hours. This was calcined at 500 ° C. for 5 hours and further pulverized using a hammer mill to obtain a titanium-silicon composite oxide (hereinafter referred to as Ti—Si composite oxide).

<Addition of vanadium and tungsten>
A homogeneous solution was prepared by mixing and dissolving 2.0 kg of ammonium metavanadate, 2.6 kg of oxalic acid, and 0.8 kg of monoethanolamine in 3 L of water. The vanadium-containing solution and 2.8 L of a 10% by weight methylamine aqueous solution of ammonium paratungstate (400 g / L as WO ₃ ) together with a molding aid and an appropriate amount of water, 20 kg of the previously prepared Ti-Si composite oxide powder 20 kg In addition, after kneading with a kneader, it was extruded into a honeycomb shape having an outer shape of 80 mm square, a length of 500 mm, an aperture of 2.9 mm, and a wall thickness of 0.4 mm. Then, after drying at 80 degreeC, it baked at 450 degreeC for 3 hours, and the catalyst 1 was obtained.

The composition of the catalyst 1 was (Ti—Si composite oxide): V ₂ O ₅ : WO ₃ = 88: 7: 5 (mass ratio).

-Ammonia decomposition catalyst-
<Preparation of palladium, vanadium, tungsten, titanium-silicon composite oxide>
The previously prepared catalyst 1 was impregnated with a palladium nitrate solution, dried at 80 ° C., and then calcined at 400 ° C. for 2 hours to obtain catalyst 2. The composition of the catalyst 2 was (Ti—Si composite oxide): V ₂ O ₅ : WO ₃ : Pd = 87.6: 7: 5: 0.4 (mass ratio).

-N ₂ O decomposition catalyst-
<Preparation of Co / Cs oxide>
An aqueous solution containing 0.47 g of cesium nitrate was added to 25 g of commercially available cobalt carbonate (manufactured by Nacalai Tesque), and the mixture was stirred and heated at 100 ° C. for 1 hour until moisture sufficiently evaporated with a hot stirrer. After drying at 120 ° C. for 2 hours, the catalyst was obtained by calcination at 400 ° C. for 4 hours (Cs / Co (molar ratio) = 0.01).

-Ammonia concentration measurement method-
The ammonia concentration in the gas was calculated as follows. A certain amount of gas is passed through an absorption bottle containing a 0.5% by mass boric acid solution to absorb the ammonia contained in the gas into the boric acid solution, and the boric acid aqueous solution containing ammonia after the absorption operation is added to pure water. Then, the ammonia concentration was measured by quantifying ammonium ions in the liquid with a Tosoh cation chromatograph.

-Measurement method of NOx concentration-
The NOx concentration in the gas was measured by a chemiluminescence method using a NOx meter manufactured by Nippon Thermo Electron.

-N ₂ O concentration in the assay -
The N ₂ O concentration in the gas was measured with a gas chromatograph (manufactured by Shimadzu Corporation, GC-8A, column: porapakQ).

Example 1
The reaction tube was filled with a denitration catalyst, an ammonia decomposition catalyst, and an N ₂ O decomposition catalyst in this order from the inlet side. As the reaction gas, NOx is 0.05% (500 ppm), N ₂ O is 0.03% (300 ppm), H ₂ O is 10%, O ₂ is 16%, and ammonia (reducing agent) is 0.055% ( 550 ppm), and the rest was N ₂ . Under normal pressure, the reaction gas was passed through a reaction tube filled with a catalyst at 300 ° C., and an evaluation test was performed. SV (hr ⁻¹ ) in each reaction is as follows. The denitration reaction is 5,000, the ammonia decomposition reaction is 7,500, and the N ₂ O decomposition reaction is 10,000.

As a result of the evaluation test, the concentration of NOx contained in the exhaust gas at the N ₂ O decomposition catalyst inlet was 0.00002 (0.2 ppm), and the ammonia concentration was 0.00066% (6.6 ppm). The N ₂ O concentration after the N ₂ O decomposition catalyst exit was 0.00621% (62.1 ppm), and the N ₂ O removal rate was 79.3%.

(Comparative Example 1)
The reaction tube was filled in the order of a denitration catalyst and an N ₂ O decomposition catalyst from the inlet side. As the reaction gas, NOx is 0.05% (500 ppm), N ₂ O is 0.03% (300 ppm), H ₂ O is 10%, O ₂ is 16%, and ammonia (reducing agent) is 0.055% ( 550 ppm), and the rest was N ₂ . Under normal pressure, the reaction gas was passed through a reaction tube filled with a catalyst at 300 ° C., and an evaluation test was performed. SV (hr ⁻¹ ) in each reaction is as follows. The denitration reaction is 5,000, and the N ₂ O decomposition reaction is 10,000.

As a result of the evaluation test, the concentration of ammonia contained in the exhaust gas at the N ₂ O decomposition catalyst inlet was 0.00321% (32.1 ppm). The N ₂ O concentration after the N ₂ O decomposition catalyst exit was 0.0291% (291 ppm), and the N ₂ O removal rate was 3.0%.

  The present invention can be used for the treatment of exhaust gas, and can be used particularly in the field of exhaust gas treatment containing nitrogen oxides.

Claims (6)

An exhaust gas treatment method comprising adding ammonia and / or urea to exhaust gas containing nitrogen oxides, treating with a denitration catalyst, then treating with an ammonia decomposition catalyst, and further treating with an nitrous oxide decomposition catalyst. The amount of ammonia and / or urea added is 1.0 to 2.0 times the equivalent amount capable of decomposing nitrogen monoxide and nitrogen dioxide in the exhaust gas into nitrogen. The exhaust gas treatment method described. The exhaust gas treatment method according to claim 1 or 2, wherein in the exhaust gas treatment method, the total concentration of nitrogen monoxide and nitrogen dioxide contained in the exhaust gas at the nitrous oxide decomposition catalyst inlet is 0.001% by volume or less. The exhaust gas treatment method according to any one of claims 1 to 3, wherein in the exhaust gas treatment method, the concentration of ammonia contained in the exhaust gas at the nitrous oxide decomposition catalyst inlet is 0.001 vol% or less. The exhaust gas treatment method according to claim 1, wherein the active component of the nitrous oxide decomposition catalyst is at least one element selected from the group consisting of cobalt, nickel and iron or a compound thereof. 6. The exhaust gas treatment method according to claim 1, wherein the promoter component of the nitrous oxide decomposition catalyst is at least one element selected from alkali metals and alkaline earth metals or a compound thereof.

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