JPH0483516A - Removal of nitrogen oxide - Google Patents

Abstract

PURPOSE:To efficiently remove nitrogen oxide by decomposing and removing NOx in the presence of 2-4C unsaturated hydrocarbon. CONSTITUTION:Exhaust gas and fuel are introduced into the first catalyst bed composed of alumina or titania and the fuel is subjected to decomposition reaction herein to be converted to hydrocarbon mainly composed of 2-4C unsaturated hydrocarbon. Continuously, the above-mentioned exhaust gas is introduced into the second catalyst bed 7 (e.g. a catalyst wherein Ni or Pt is supported on zeolite or non-aluminosilicate) along with said hydrocarbon and nitrogen oxide in the exhaust gas is decomposed and removed in the presence of said hydrocarbon in an oxidative atmosphere. That is, the exhaust gas is treated under the above-mentioned conditions to make it possible to efficiently remove NOx.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for removing nitrogen oxides, and more specifically, the present invention relates to a method for removing nitrogen oxides, which are emitted from internal combustion engines such as automobiles, such as gasoline engines and diesel engines, and boiler industrial plants. The present invention relates to a method for efficiently removing nitrogen oxides from exhaust gas.

(Prior art) In recent years, nitrogen oxides (hereinafter referred to as NO
It contains harmful ingredients (sometimes referred to as x) and causes air pollution. For this reason, removal of NOx from this exhaust gas is being studied from various perspectives.

Conventionally, for example, in the case of automobile exhaust gas, a three-way catalyst is used to treat the exhaust gas and convert it into hydrocarbons (hereinafter, hydrocarbons in exhaust gas are expressed as rHcJ) and -carbon oxides (CO) as well as N.
A method of removing Ox is used. This method is carried out under conditions where only enough air is introduced to allow complete combustion of the fuel. However, as the ratio of air to fuel (air-fuel ratio = air/fuel) increases, there will be an excess of hydrogen than is necessary to completely burn unburned components such as HC and CO in the exhaust gas. Therefore, under such an oxidizing atmosphere, NOx cannot be reduced and removed by a three-way catalyst.

Furthermore, in the case of diesel engines and boilers among internal combustion engines, a method of removing NOx using a reducing agent such as ammonia, water bladder, or carbon monoxide is used. However, this method has a problem in that special equipment is required to recover and treat unreacted reducing agent.

Recently, as a method for removing NOx, a method using a NOx decomposition catalyst made of crystalline aluminosilicate containing copper ions has been proposed (Japanese Unexamined Patent Publication No. 125250/1982). However, this publication simply states that oxidant monoxide (No) can be decomposed into nitrogen (N2) and oxygen (02), and does not show its effectiveness under actual exhaust gas conditions. There is no mention that unsaturated hydrocarbons are effective in decomposing NOx.

Furthermore, JP-A No. 63-100919 states that when exhaust gas is treated using a copper-containing catalyst in an oxidizing atmosphere in the presence of hydrocarbons, the reaction between NOx and hydrocarbons is preferentially promoted, and NOx becomes more efficient. It is stated that it can be easily removed. The hydrocarbons used in this method are H
It is said that C or a hydrocarbon added externally as required may be used. In addition, as a specific aspect,
The exhaust gas is first contacted with a copper-containing catalyst to remove NOx,
A method is also disclosed in which HC, CO, etc. are then removed by contacting with an oxidation catalyst.

However, there is no mention of specific supply means or methods when adding hydrocarbons.
Furthermore, the conditions disclosed in this publication are conditions in which the concentration ratio of hydrocarbons to NOx is high, and no study has been made under conditions in which the concentration ratio of hydrocarbons to NOx is high in hydrocarbons. Furthermore, the NOx decomposition effect of this method is also low.

Furthermore, Japanese Patent Application Laid-open No. 1-139145 discloses a reduction catalyst in which a transition metal is supported on zeolite on the exhaust gas inflow side, and an oxidation catalyst or a three-way catalyst in which catalyst components are supported on alumina on the exhaust gas outflow side. A purification catalyst is disclosed. Here, the above-mentioned Japanese Patent Application Laid-Open No. 63-100919
Similar to the specific embodiment described in the publication, NOx in the exhaust gas is first removed by a reduction catalyst, and then HC is removed by an oxidation catalyst.
The purpose is to remove lC0.

However, although this publication states that HC is necessary to remove NOx, it does not disclose any specific means or method for supplying the HC, or the amount used.

(Problems to be Solved by the Invention) The present inventors have discovered that an excessive amount of NOx in exhaust gas is
It has already been discovered that NOx can be efficiently removed by treating exhaust gas in an oxidizing atmosphere in the presence of C4 unsaturated hydrocarbons, which is a suitable condition for exhaust gas purification treatment.

However, the HCO content in exhaust gas discharged from internal combustion engines, such as diesel engines, is generally low, and if the exhaust gas is used as it is, it cannot be purified under the above-mentioned suitable conditions. In addition, diesel oil and LPG
contains almost no 02-C-unsaturated hydrocarbons and consists of hydrocarbons and paraffins with a large number of carbon atoms, so if these were added as they were, it would be difficult to carry out purification treatment under the suitable conditions described above. I can't.

Therefore, an object of the present invention is to provide a method for efficiently removing NOx by treating exhaust gas under the above-mentioned suitable conditions.

(Means for Solving the Problems) In view of this situation, the inventors of the present invention have made extensive studies and found that when purifying exhaust gas, the exhaust gas is mixed with commonly used fuel in advance. a catalyst containing alumina or silica, or further Ni, Co
, FeS K, etc., to convert the fuel into hydrocarbons mainly consisting of 02-C4 unsaturated hydrocarbons by decomposition, disproportionation, oxidative dehydrogenation, etc. The present invention was completed based on this knowledge.

In addition, in the present invention, mainly 02-
Reactions that produce hydrocarbons composed of C4 unsaturated hydrocarbons are collectively referred to as "decomposition reactions."

That is, the present invention introduces exhaust gas and fuel into a first catalyst layer, where the fuel is subjected to a decomposition reaction to convert it into hydrocarbons mainly consisting of C2-caF saturated hydrocarbons,
Subsequently, the above-mentioned exhaust gas together with this hydrocarbon is introduced into a second catalyst layer, where the nitrogen oxides in the exhaust gas are decomposed and removed in an oxygen atmosphere and in the presence of the above-mentioned hydrocarbon. Regarding the method.

The above-mentioned oxidizing atmosphere refers to a state in which there is an excess of oxygen compared to the amount of oxygen required to completely oxidize the combustion components such as HC, CO1H2 in the exhaust gas, and carbon (such as arsenic) generated by the decomposition reaction of the above-mentioned fuel. It means something.

The present invention will be explained in detail below.

Examples of the fuel used in the present invention include light oil, kerosene, gasoline, and LPG.

According to the method of the present invention, waste gas discharged from an internal combustion engine such as an automobile, a boiler, an industrial plant, etc. and the above-mentioned fuel are introduced into the first catalyst layer, where the fuel is subjected to a decomposition reaction. The hydrocarbons mainly composed of C2 to C-unsaturated hydrocarbons are converted into hydrocarbons, and the thus obtained hydrocarbons and exhaust gas are introduced into the second catalyst layer, where the NOx in the exhaust gas under an oxidizing atmosphere is The exhaust gas is purified by decomposition and removal in the presence of hydrocarbons.

The catalyst used in the first catalyst layer is one that can convert the fuel into hydrocarbons mainly consisting of 02 to C4 unsaturated hydrocarbons through a decomposition reaction, and at least one selected from alumina, silica, zeolite, titania, and zirconia. A catalyst consisting of one oxide is used. In particular, in addition to the above oxide, NiS Co, , Fe, KS Cr
,■,Mo.

Those carrying at least one metal selected from Bi, Sn, Pt, Pd and Rh are preferably used.

The above-mentioned metal may be supported on the above-mentioned oxide in the form of a water-soluble salt such as a nitrate, sulfate, or carbonate by a conventional method. The supported amount of this metal is 0.1 based on the weight of the above oxide.
It is preferable to set it as 20 weight%.

The above-mentioned 62 to C-unsaturated hydrocarbons mean unsaturated hydrocarbons having 2 to 4 carbon atoms such as ethylene, propylene, butene, butadiene, and the above-mentioned mainly C2 to C-unsaturated hydrocarbons (hereinafter simply referred to as Hydrocarbons consisting of unsaturated hydrocarbons (sometimes referred to as "unsaturated hydrocarbons") mean hydrocarbons containing at least 50 M% (in terms of CH) of such unsaturated hydrocarbons.

If the content of unsaturated hydrocarbons in the hydrocarbons is too low, a large amount of fuel must be used to adjust the ratio of unsaturated hydrocarbons and NOx in the second catalyst layer (to be described later) to a desired range, which is not economical. In addition, a large amount of hydrocarbons other than the above-mentioned unsaturated hydrocarbons will be present, making it difficult to decompose and remove these hydrocarbons in the second catalyst layer, which is undesirable.

The temperature in the first catalyst layer cannot be absolutely specified because it varies depending on the type of catalyst used, but it may be a temperature sufficient to cause the decomposition reaction of the fuel. Usually 3
00-700°C is sufficient.

When the exhaust gas temperature is sufficiently high, the decomposition reaction of the fuel can be carried out using the heat of the exhaust gas as it is without providing any special heating means.

Note that if the temperature of the exhaust gas is low, a heating means such as a heater may be provided upstream of the first catalyst layer.

The volume of the catalyst in the first catalyst layer is determined by the space velocity of the exhaust gas (
SV) is 10000~100000h rl (STP)
It is preferable to do so.

The catalyst used in the second catalyst layer is one that can decompose and remove NOx by contacting exhaust gas in the presence of hydrocarbons as described above, and is made of metal elements such as Cu, Co, Ni, Pt, and Pd. At least one selected type is zeolite,
For example, zeolites of type ZSM-5, Y, ferrierite or mordenite, or non-aluminosilicates such as Ga, Cr, Ge, Ti,
A catalyst supported by ion exchange on a silicate containing La, Zr, Mn, Cu or Fe can be used.

N1, Mn, Fe, Co, N1, Cu, Z
A catalyst made of a composite oxide containing at least two elements selected from n, Sr, Y, Ba, La, and Ce can also be used.

Furthermore, at least one element selected from rare earths such as Ce, La, Sm, and Tb, and Pt, Pd, and Rh.
A catalyst in which at least one element selected from noble metals such as , Ir, etc. is supported on alumina can also be used.

As the metal salt to be ion-exchanged with the zeolite or non-aluminosilicate, nitrates and acetates of each metal are used, but the ion-exchange type is 0.1 to 10% by weight based on the weight of the zeolite or non-aluminosilicate. It is preferable to do so. In order to increase the size of the ion exchange type, zeolite or non-aluminum silicate may be added to a metal salt aqueous solution with ammonia added.

Water-soluble metal salts such as nitrates, acetates, and sulfates are preferably used as sources of each metal element used in the preparation of the composite oxide. There are no particular restrictions on the method for preparing the above-mentioned composite oxide using these metal salts. For example, a mixed solution of metal salts may be evaporated to dryness and then calcined, or an aqueous ammonia solution may be added dropwise to a mixed solution of metal salts. coprecipitation, filtration,
It can be prepared by baking after washing.

As the alumina used for preparing the catalyst in which rare earth elements and noble metal elements are supported on alumina, γ-alumina having a BET surface area of 50 to 200 m 2 /g is preferably used. As sources of rare earth elements, nitrates, sulfates,
Water-soluble salts such as carbonates, and noble metal sources include chloroplatinic acid, dinitrodiammine platinum, palladium chloride,
Water-soluble salts such as palladium nitrate are preferably used.

There are no particular restrictions on the method of supporting rare earth elements and precious metal elements on alumina, such as a method of supporting a rare earth element on alumina and then supporting a noble metal element, a method of supporting a noble metal element on alumina, and then supporting a rare earth element. ,
Alternatively, any method can be used, such as a method of simultaneously adding rare earth elements and noble metal elements to alumina.

Note that the supported amounts of rare earth elements and noble metal elements are preferably 0.1 to 30% by weight and 0.01 to 10% by weight, respectively, based on the weight of alumina.

The ratio of the hydrocarbons mainly consisting of 02-C4 unsaturated hydrocarbons introduced into the second catalyst layer and the exhaust gas is the concentration of 02-C4 unsaturated hydrocarbons (%) relative to the NOx concentration (ppm).
The ratio of CHa conversion) (ppm), that is, C2~Ca unsaturated hydrocarbon concentration (CH, conversion)/NOx concentration is 3:1~
It is preferable to adjust the ratio to be within the range of 50:1.

If the above ratio is less than 3:1, that is, the ratio of unsaturated hydrocarbon to NOx is small, the unsaturated hydrocarbon reacts preferentially with oxygen, making it difficult to react with NOx.
Therefore, the NOx decomposition efficiency decreases. On the other hand, 50:1
In other words, even if the concentration of unsaturated hydrocarbons is increased too much, no commensurate NOx removal effect will be observed; on the contrary, the concentration of unreacted hydrocarbons will increase and will be removed sufficiently by the second catalyst layer. This becomes difficult.

Since the NOx concentration in the exhaust gas used in the present invention is usually about 100 to 3000 ppm, for the reasons mentioned above, the unsaturated hydrocarbon concentration obtained in the second catalyst layer is about 300 to 110000 ppm. It is preferable to

The ratio between the unsaturated hydrocarbon concentration and NOx concentration introduced into the second catalyst layer is determined by considering the composition of the hydrocarbon obtained in the first catalyst layer, etc. of the fuel introduced into the first catalyst layer. It can be easily adjusted within the above range by controlling the amount.

The temperature in the second catalyst layer is also not particularly limited, as long as it is a temperature at which decomposition of NOx in the exhaust gas occurs. Usually, 200 to 700°C is sufficient. Therefore,
If the temperature of the exhaust gas used is high, the hydrocarbons and exhaust gas from the first catalyst layer can be directly introduced into the second catalyst layer to decompose and remove NOx.

Regarding the volume of the catalyst in the second catalyst layer, the space velocity (
SV) is preferably 10,000 to 100,000 hr (STP).

According to the method of the present invention, not only NOx in the exhaust gas is removed, but also HC, Co, and hydrocarbons caused by the decomposition reaction of the fuel in the exhaust gas are almost completely oxidized, so that the exhaust gas can be efficiently purified.

There is no particular restriction on the arrangement of the first catalyst layer and the second catalyst layer, and the first catalyst layer and the second catalyst layer may be arranged in contact with each other, or they may be placed with a certain distance between them. It may also be placed.

Further, there is no particular restriction on the shape of the catalyst used in the first catalyst layer and the second catalyst layer, and the shape may be pellet-like, honeycomb-like, or the like. In consideration of pressure loss, a honeycomb shape is preferable.

(Effects of the Invention) According to the method of the present invention, since NOx is decomposed and removed in the presence of 02-C4 unsaturated hydrocarbons, nitrogen and arsenic compounds can be efficiently removed.

In the method of the present invention, the above-mentioned unsaturated hydrocarbon is first added as a fuel.
Since it is decomposed in a catalyst layer and then supplied, it can be particularly effectively used for purifying exhaust gas with a low content of unsaturated hydrocarbons, such as exhaust gas from a diesel engine.

Since the method of the present invention decomposes and removes NOx in the presence of unsaturated hydrocarbons as described above, it can effectively remove NOx in an oxidizing atmosphere. It can be used particularly effectively to purify exhaust gas that has been combusted under conditions of high oxidation.

(Example) Hereinafter, the present invention will be described in more detail with reference to Examples.

(Preparation of Catalyst) [Catalyst (A)] A catalyst was prepared using a commercially available cordierite honeycomb carrier having about 400 gas flow cells per square inch of cross section.

γ-alumina Ik with BET surface area 100 m2/g
g to iron nitrate [F e (NO3)a・9H20co253
, an aqueous solution containing Og and 77.7 g of potassium nitrate was added and mixed, and the mixture was heated to 2FW at 120°C! Dry for a while, then 500
It was baked at ℃ for 2 hours. An aqueous slurry was prepared by wet-pulverizing the obtained powder using a ball mill. After immersing the honeycomb carrier in this aqueous slurry and taking it out,
Excess slurry in the cells was blown out with compressed air to unclog all cells. Then, the catalyst (A) was obtained by drying at 150° C. for 2 hours.

[Catalyst (B) ZSM-5 zeolite
liza-tion Method”Proceed
ings8th InternationalCong
Ress on Catalysis, Berlin
, +984. Prepared based on Vol. 3°P, 569. The obtained zeolite was determined by X-ray diffraction as ZSM-
It was confirmed that it was type 5.

Add 4 prongs of pure water to 1 kg of the above ZSM-5S
After stirring at 98°C for 2 hours, 0.2 mol/4 at 80°C
72 of copper nitrate aqueous solution was slowly added dropwise. After the dropwise addition was completed, the mixture was heated and stirred at 80° C. for 12 hours to perform ion exchange. The ion-exchanged zeolite was filtered and thoroughly washed until no nitrate ions were detected. This ion-exchanged zeolite was dried at 120°C for 24 hours.

The obtained powder was wet-pulverized in a ball mill, and the catalyst (
Catalyst (B) was obtained in the same manner as A).

[Catalyst (C)] 1 kg of silica with a BET surface area of 500 m2/g
cm Nickel nitrate [N i (NO3) ・6H20
]194,. An aqueous solution containing 7g of cobalt nitrate and 36.3g of cobalt nitrate was added and mixed, dried at 120"C for 2 hours r:J, and then calcined at 500°C for 2 hours.

The obtained powder was wet-pulverized in a ball mill, and the catalyst (
Catalyst (c) was obtained in the same manner as A).

[Catalyst (D) 500 g of Fe-alumina powder obtained in Example 1,
300 g of Y-type zeolite substituted with Buton and 200 g of silica-alumina powder were wet-pulverized in a ball mill.

Catalyst (D) was obtained in the same manner as catalyst (A).

[Catalyst (E)] Cobalt nitrate [Co (N 03) 2 ・6 H
2Oko 125] g,? Lanthanum acid [La (NO3
) 3・6H20] 1490 g and 182 g of strontium nitrate [Sr (NO3)2] were dissolved in 32% pure water, stirred, thoroughly dried at 120°C, and the resulting solid was ground in an agate mortar to give 800 g of strontium nitrate [Sr (NO3)2]. It was baked at ℃ for 5 hours. The obtained powder was confirmed to be a perovskite type oxide by X-ray diffraction. 1 kg of the above perovskite type oxide was wet-pulverized using a ball mill.

Catalyst (E) was obtained in the same manner as catalyst (A).

[Catalyst (F)] 1 kg of the same γ-alumina used in the preparation of catalyst (A)
An aqueous solution containing 511 g of praseodymium nitrate was added and mixed, dried at 120°C for 2 hours, and then baked at 500°C for 2 hours. Dinitrodiammine platinum 10 was added to the obtained powder.
An aqueous solution containing g was added and mixed, dried at 120°C for 2 hours, and then baked at 500°C for 2 hours. This powder was wet-milled using a ball mill.

Catalyst (F) was obtained in the same manner as catalyst (A).

Example The method of the present invention was carried out using the apparatus shown in FIG.

Here, a commercially available direct injection diesel engine (4 cylinders, 2800 cc) was used as the engine 1.

The exhaust gas passed through the exhaust manifold 2 and was introduced from the fuel tank 5 into the first catalyst layer 6 together with the fuel supplied from the fuel introduction part 3 by the fuel pump 4. Above in this first catalyst N6! IC! After feeding the fuel into the decomposition reaction,
A mixture of carbonized water obtained by the decomposition reaction of the fuel and exhaust gas was directly introduced into the second catalyst layer 7. The exhaust gas after NOx was removed in the second catalyst layer 7 was discharged to the outside air through a muffler 8.

9.10.11.12 are all sampling valves, and valve 9 receives the exhaust gas from the engine 1, valve 10 receives the gas after passing through the first catalyst N6, and valve 11 receives the gas after passing through the first catalyst N6. The gas after passing through the layer 7 and the gas introduced into the first catalyst F'6 through the valve 12 are sampled, and the first catalyst layer 6 is filled with catalyst (A), (C) or (D). In addition, a catalyst (B
) (E) or (F) was filled.

Light oil was used as fuel.

The space velocity (SV) of the first catalyst layer 6 and the second catalyst layer 7 are both 85,000 hr'', the inlet temperature of the first catalyst N6 is 450°C, and the inlet temperature of the second catalyst N7 is 400°C.
It was ℃.

The results are shown in Table 1.

Note that the concentration ratio of hydrocarbons or unsaturated hydrocarbons to NOx in the second catalyst layer was adjusted by controlling the amount of fuel introduced. Moreover, unsaturated hydrocarbons were measured by gas chromatography.

In the comparative examples, the first catalyst layer 6 was not filled with any catalyst, that is, no fuel decomposition reaction was performed, or propane was used instead of light oil as the fuel, or hydrocarbons or Exhaust gas was treated in the same manner as in the example except that the amount of fuel supplied was changed so that the concentration ratio of unsaturated hydrocarbons was outside the range of the present invention. The results are shown in Table 1. (Margin below)

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of an exhaust gas treatment device used in Examples and Comparative Examples. 1. Engine, 2. Exhaust manifold, 3
...Fuel introduction part, 4..Fuel pump, 5..Fuel tank, 6..First catalyst layer, 7..Second catalyst layer, 8..Muffler 9.10.11.12..Sampling valve

Claims (6)

[Claims] (1) Exhaust gas and fuel are introduced into the first catalyst layer, where the fuel is subjected to a decomposition reaction to be converted into hydrocarbons mainly consisting of C_2 to C_4 unsaturated hydrocarbons, and then the above-mentioned exhaust gas is A method for removing nitrogen oxides, which comprises introducing nitrogen oxides into a second catalyst layer, and decomposing and removing nitrogen oxides in exhaust gas in an oxidizing atmosphere and in the presence of the hydrocarbons. (2) Claim (1) wherein the catalyst used in the first catalyst layer is a catalyst made of at least one oxide selected from alumina, silica, zeolite, titania, and zirconia.
) The method for removing nitrogen oxides described in ). (3) The catalyst used in the second catalyst layer is Cu, Co, Ni.
The method for removing nitrogen oxides according to claim 1, wherein the catalyst is a catalyst in which at least one metal element selected from , Pt, and Pd is supported on zeolite or non-aluminosilicate. (4) The catalyst used in the second catalyst layer is Mn, Fe, Co
, Ni, Cu, Zn, Sr, Y, Ba, La and Ce
The method for removing nitrogen oxides according to claim 1, wherein the catalyst is a composite oxide containing at least two elements selected from the following. (5) The method for removing nitrogen oxides according to claim 1, wherein the catalyst used in the second catalyst layer is a catalyst in which at least one rare earth element and at least one noble metal element are supported on alumina. (6) C_2 to C_4 hydrocarbon concentration (CH_4 conversion) relative to nitrogen oxide concentration (ppm) in the second catalyst layer (
Claim (1) wherein the ratio of ppm) is 3:1 to 50:1.
The method for removing nitrogen oxides described in .