JPS6349234A - Method for removing nitrogen oxides - Google Patents

Abstract

PURPOSE:To increase the removal rate of nitrogen oxides by constituting a device of a 1st catalyst layer for oxidation and 2nd catalyst layer which consists essentially of zeolite and is provided on the down stream side thereof and controlling the 2nd catalyst layer to the temp. lower than the temp. of the 1st catalyst layer. CONSTITUTION:The 1st catalyst layer for oxidation consisting of a metal or the oxide. halide, etc., thereof and the 2nd catalyst layer which consists essentially of the zeolite and is provided on the down stream side thereof are disposed in series. While the 1st catalyst layer (oxygen oxidation reaction of NO) has an activity peak on the slightly higher temp. side, the 2nd catalyst layer (decomposition and removal reaction of NO2) has the higher activity on the lower temp. side. The temp. of the 1st catalyst layer is, therefore, controlled to 300-400 deg.C and the temp. of the 2nd catalyst layer is controlled to the temp. lower by 50-100 deg.C than the temp. of the 1st catalyst layer. The nitrogen oxides in an exhaust gas are nearly completely removed in the dry state in the presence of the oxygen by the above-mentioned constitution. The temp. control is executable to maintain the temp. difference between the 1st catalyst layer and the 2nd catalyst layer by interposing a space for heat insulation between the layers.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a method for catalytically removing nitrogen oxides contained in combustion exhaust gas or the like in which oxygen coexists without using a reducing agent.

Exhaust gas emitted from conventional combustion equipment contains carbon dioxide (C
It contains nitrogen oxides (NO and CO2: collectively called NOx), water (N20), nitrogen (N2), oxygen (02), etc. This nitrogen oxide, especially No2, is harmful to the human body,
It is said to be a factor that causes various respiratory diseases. However, No is easily oxidized to No2 in the atmosphere at room temperature, and it is necessary to remove all nitrogen oxides in order to prevent the effects on the human body. In order to remove these nitrogen oxides, the following methods have been mainly studied.
←) Combustion Edition” (1978) P, 250-283).

■ Dry method: Ammonia (NH3) and water
(N2), -9-carbonized carbon Z (CO), methane (cH4), and other reducing agents are added to reduce No. to N2.
Through the reaction described below, harmless ρ and Co can be converted into N2 without forming them as by-products.

2No+2H2→N2+2H20 2NO2+ 4 H2→N2+4H202NO+2CO
-bN +2CO2 6NO+4NH-+5N2+6H20 6N02+8NH3→AN2+12H20 ■ Wet method 2 This is a method in which NO and No2 are absorbed and removed in water or an alkaline solution, and the absorbent is an alkaline solution such as NaOH,
An acid solution such as KMnO4 or NaC102, or Fe3O3 or FeCl2, which forms a complex, is used.

Problems to be Solved by the Invention In the above conventional methods, the latter (wet method) requires treatment or regeneration of the solution after absorption, requires large and complicated equipment, and requires small exhaust gas generating equipment. (For example, it is unsuitable for household combustion equipment, etc.). On the other hand, the former (dry method) requires the supply of a reducing agent, and in order to cause an effective reaction, it is necessary to supply an excessive amount of the reducing agent, which may cause leakage of this reducing agent and cause secondary pollution. In addition, with the exception of ammonia, general reducing agents preferentially react with oxygen in exhaust gas, so a huge amount of reducing agent is required to treat combustion exhaust gas with an oxygen concentration of several inches. Furthermore, the apparatus is large in size due to the addition of a reducing agent supply means, making it unsuitable for small-sized exhaust gas generating equipment. Among the former methods, a method has also been proposed in which NOx is directly catalytically decomposed to N2 without using a reducing agent, but NO, which accounts for the majority of NOx, is a fairly stable chemical species, especially in combustion exhaust gas. However, the reaction rate is extremely slow for catalytic cracking, and in the presence of oxygen there is a decomposition inhibiting effect, so no single catalyst has been found that can be used practically for actual exhaust gases.

In response to this, the present inventor previously proposed that NOx in exhaust gas coexisting with oxygen could be removed using a composite catalyst consisting of a No oxidation catalyst in the first layer and zeolite in the second layer, but the optimum temperature condition was Even when used, the removal rate remained at about 90%.

The present invention solves the above-mentioned conventional drawbacks, and eliminates the need for reducing agents and absorption solutions, and allows the removal of N in exhaust gas in the presence of oxygen in a dry process.
The present invention provides a method for removing nitrogen oxides using a catalyst that makes it possible to almost completely remove nitrogen oxides.

Means for Solving the Problems Technical means used in the present invention to solve the above conventional problems include a first catalyst layer for oxidation consisting of a metal or its oxide, halide, etc., and a catalyst layer downstream thereof. In addition to arranging the second catalyst layer containing zeolite as the main component in series, the temperature of the second catalyst layer is controlled to be lower than that of the first catalyst layer, and the conditions for both to exhibit the highest practical activity are satisfied. This improves the removal rate of nitrogen oxides.

Function Exhaust gas discharged from combustion equipment contains oxygen coexisting in an amount ranging from several to a dozen or so, and also contains several tens to hundreds of ppm of nitrogen oxides. Generally, 95% or more of the nitrogen oxides are No, and the remaining 5% or less is NO2. However, No is low temperature (below 300-400℃)
is oxidized by oxygen, and the reaction of 2NO00 → 2N○2 occurs relatively easily and proceeds at a certain rate in the atmosphere without a catalyst, but the decomposition reaction 2NO → N2+02 is extremely slow, and the reaction of platinum (pt ) and vanadium pentoxide■2
Even if a catalyst such as 05) is used, only a decomposition activity of about 30 at 1000'C is obtained (edited by Osaka Science and Technology Center; "Current Status of Development of Nitrogen Compound Control Technology", (April 1976) P. , 91). Furthermore, it has poor reactivity with respect to adsorption and absorption, and becomes metastable, making it difficult to remove.

The present invention first oxidizes metastable NO, which accounts for most of the NOx in the combustion exhaust gas, into highly reactive NO2 using coexisting oxygen in the first catalyst layer, and then decomposes and adsorbs it in the second catalyst layer. We have discovered a composite catalyst that can be used to easily remove No. Silver (A
q), metals such as manganese (Mn), chromium (Cr), palladium (Pd), or diiron trioxide (Fe203)
, copper oxide (CtxO), metal oxides such as zinc oxide (ZnO), copper chloride (CuC12), copper fluoride (CuF
), iron chloride (FeC13), etc., can be used alone or in mixtures, and these metals and metal compounds can be used directly in the form of granules, lumps, foams, or fibers, or can be used directly in the form of porous It was also shown that it is possible to use the zeolite which constitutes the second catalyst layer described below as the carrier. The zeolite used as the second catalyst must have a pore size capable of adsorbing NO□ with a molecular diameter of approximately 3 angstroms, preferably 6 angstroms.
It has been clarified that synthetic zeolites with uniform pore diameters of one or more pore sizes are suitable. In this way, the nitrogen oxide sound in the exhaust gas, which is mostly in the NO state, is oxidized to NO2 in the first catalyst layer, decomposed into N2 and 02 in the second catalyst layer, or partially adsorbed, and the nitrogen oxides are removed. It was something that could be done. However, in this case, since both soot layers were used at the same temperature, the most active Cu Cl 2
/Molecular Sheep 13X-Molecular Sheep 13
X system (reaction temperature = 3QO ℃, space velocity (SV) = 200
Even if 00h-1) was used, the No removal rate in the exhaust gas remained at about 90%.

Looking at the individual temperature characteristics of the first catalyst layer and the second catalyst layer, the first catalyst layer (oxygen oxidation reaction of NO) has an activity peak on the slightly higher temperature side; It became clear that the layer (No2 decomposition/removal reaction) was more active at lower temperatures. Therefore, by creating a temperature difference between the two, making the first catalyst layer high and the second catalyst layer low, a more highly active catalytic action can be exhibited. Although this temperature difference has a slight distribution depending on the type of oxidation catalyst used in the first catalyst layer, since the oxidation activity peak due to the above metals or metal oxides/halides is between 300 and 450 degrees Celsius, it is not practical and extreme. It is difficult to control the temperature to provide a large temperature difference, and it is not necessary to provide sufficient activity in the second catalyst layer, and a temperature difference of 50 to 100° C. can provide a satisfactory activity.

Although it is possible to control the temperature of both independently, if they are provided in the combustion exhaust gas flow path.

By installing the first catalyst layer at a position with appropriate temperature conditions and interposing a heat insulating space at an appropriate distance from the second catalyst layer, it is possible to position the second catalyst layer in a low temperature region with a predetermined temperature difference. This objective can be achieved without the need for additional temperature control means.

Examples Examples of the present invention will be shown below. The reaction gas used was combustion exhaust gas discharged from a kerosene combustor, and its composition was as follows: CO2 = 1o%, 02 = 'y%, NO = 140p.
pm (NO=135ppm, No2=5ppm), and the remainder is N2. t ta2fa'bi [(SV) is 2
The reaction was carried out at 0000 h. The measured value of the product gas is the value after the reaction gas has been allowed to pass for 1 hour and stabilized.

[Example 1] Granular alumina 1oOr supported with oxide @(CuO)
nl (copper oxide content about 81) as the first catalyst layer, commercially available Molecular Sheep 5A (A-type zeolite/Na-Ca
The reaction was carried out using 100 1/16" pellets as the second catalyst layer. As shown in Figure 1, the gas concentration after passing through the first catalyst layer peaked at approximately 450°C, and NO2 increased, while NOx mA(NO+N02) increases from the concentration in the reaction gas (14 oppm) above 500''C. Next, the temperature of the first catalyst layer is fixed at 450'C, so the concentration at the entrance to the second catalyst layer is NO = 66 ppm, No□ = 84
When the exhaust gas was analyzed by changing only the temperature of the second catalyst layer, the result was as shown in Fig. 2. 1st
At 450℃, the same temperature as the catalyst layer, NO2 is approximately 24
The removal rate of ppm residue and NOx is approximately 4Q%. However, when the temperature of the second catalyst layer was lowered to 350'C, No.2 decreased to 6 ppm, and the total amount of No. 2 was also about 60 ppm. If the temperature is lowered further, the effect decreases, and here, the first catalyst layer is heated to 450℃, and the second catalyst layer is heated to 350''C.
If this is done, the highest rate of NO removal will be possible.

[Example 2] The first catalyst layer of Example 1 was changed from copper oxide to copper chloride (CuC12
) (copper chloride content approximately 5.55'), the second soot layer was made of Molecular Sheep 13X (X type zeolite/Na type, 1/16" pellets). The gas concentration after passing through the first catalyst layer was As shown in Figure 3, the peak of oxidation activity appears at 350°C, with No2 = 116 ppm.At the same time, No. 2 increases slightly to 143 ppm, and at higher temperatures, No. 2 increases further. I understand that.

If only the temperature of the second catalyst layer is changed as in the first example while the first catalyst layer is fixed at 350'OK, a peak of No. reduction will appear on the low temperature side as shown in Figure 4. ,2
The NO concentration in the exhaust gas is at its lowest between 50 and 300'Q. Incidentally, with this catalyst composition, when the first and second catalyst layers are kept at the same temperature, the results are as shown in FIG. 5.

[Example 3] The carrier of the first catalyst layer in Example 2 was the same molecular sieve 13X as the second catalyst layer, and copper chloride (CuC12) was supported on 1 oornl of the carrier (the content of copper chloride was about 5.3
S'). Furthermore, the temperature of the second catalyst layer is approximately 100°C lower than the temperature of the first catalyst layer (actually, the first layer and the second layer are separated, and a connecting pipe with heat radiation fins is provided between them).
are doing. The results are shown in FIG. The removal rate of No. process increased further, and around the peak of the temperature of the first catalyst layer of 300'C (that is, the temperature of the second catalyst layer was about 20Q0C), approximately 10q6 of No. It is not detected, making it an unprecedented highly active NO removal catalyst.

Particularly, by using zeolite as the carrier for the first catalyst layer, the N◯ process is simultaneously removed within the first catalyst layer, albeit slightly, and the activity can be improved.

Effects of the Invention As described above, the present invention enables highly active dry No. 8 decomposition without the need for a reducing agent, which was previously unobtainable, in combustion exhaust gas in an atmosphere in which No. 8 is present and oxygen coexists. This provides an extremely effective and easily practical method for removing nitrogen oxides in the prevention of air pollution.

Figure 1 shows the changes in No2 and N○ concentration in the gas with respect to temperature.

Name of agent: Patent attorney Toshio Nakao
Figure 1. Figure Reaction 41゛Antiwa・5〔Shallow (ac, 1 Figure 3 and Core control degree (0)

Claims (3)

[Claims] (1) A first catalyst layer that exhibits an oxidation catalytic action and whose main component is a metal or its oxide or halide, and a second catalyst layer which is provided downstream of the first catalyst layer and whose main component is zeolite. A method for removing nitrogen oxides, comprising: a catalyst comprising: a catalyst; the temperature of the second catalyst layer is controlled to be lower than the temperature of the first catalyst layer; (2) The temperature of the first catalyst layer is set to 300 to 400°C, and the temperature of the second catalyst layer is set to 50 to 10°C higher than that of the first catalyst layer.
The method for removing nitrogen oxides according to claim 1, wherein the temperature is lowered by 0°C. (3) A heat-insulating space is interposed between the first catalyst layer and the second catalyst layer to maintain a temperature difference.
The method for removing nitrogen oxides according to item 1 or 2.