JPS589694B2 - Treatment method for exhaust gas containing nitrogen oxides - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for treating exhaust gas by selectively catalytically reducing nitrogen oxides (hereinafter referred to as NOx) in exhaust gas with ammonia (NH3).

Since NOx in exhaust gas is considered to be a cause of photochemical smog, an effective means for treating it is desired.

Many flue gas denitrification methods have been provided in the past, but among them, the NOx catalytic reduction method using NH3 as a reducing agent is a method that selectively removes NOx from NH3 even if a large amount of oxygen coexists in the flue gas. It is considered to be an extremely advantageous method because NOx is reduced to harmless nitrogen gas and water vapor.

Many catalysts have already been proposed for use in this method, but all of them have a 350 to 450
It requires a reaction temperature of °C, and therefore it is necessary to separately install an exhaust gas heating furnace, a waste heat utilization heat exchanger, etc., which is completely disadvantageous in terms of equipment costs and heat costs.

Therefore, in recent years, there has been a trend to require catalysts that exhibit high activity even in low temperature ranges.

The best example of such catalysts is to knead finely divided zeolite with an aqueous copper salt solution, mold the kneaded product, and sinter the molded product at 400°C, preferably using Mg, Ba,
Examples include catalysts obtained by adding Ca salt (Japanese Patent Application Laid-Open No. 1987-
(See No. 59067).

However, although this catalyst is worthy of praise in that it exhibits considerably high activity even at low temperatures, it is impractical due to its narrow reaction temperature range.

As another example, monovalent or divalent
For example, catalysts obtained by supporting copper salts with high valence (see JP-A No. 51-88470) are excellent in that they can show high activity at low temperatures, but they also have extremely low space velocities. Since it is necessary to carry out the reaction in a similar manner, there are still problems in practical application.

The present invention has been made in view of the above-mentioned circumstances, and provides a method for treating NOx-containing exhaust gas that can carry out a reaction at an appropriate space velocity using a catalyst that exhibits high activity in a wide range of low temperature regions. The purpose is to

That is, the present invention provides an exhaust gas treatment method in which nitrogen oxides in the exhaust gas are catalytically reduced by ammonia.
As a denitrification catalyst, a carrier made of Molecular Sieves Y type synthetic zeolite manufactured by Union Carbide Co., Ltd.
This is a method for treating nitrogen oxide-containing exhaust gas, characterized in that the reaction is carried out at a temperature of 250° C. or lower using a catalyst prepared by immersing it in an aqueous solution and drying it.

In the above, the carrier is molecular sieves Y type (Na20/Al203/3 manufactured by Union Carbide, USA).
~5SiO2.xH20) It is limited to those made of synthetic zeolite, and the reason is as follows.

That is, when using a support made of the same X-type carrier, the resulting catalyst does not exhibit practical denitrification activity, and a catalyst using the same X-type support does not exhibit as high an activity as a catalyst using the same Y-type support.

On the other hand, the Y-type carrier catalyst exhibits even higher activity than the γ-alumina carrier catalyst, which has been known as a highly active catalyst.

The reason why CuCl2.2NH4Cl is used as a catalyst material is as follows.

That is, when using CuBr2 among copper salts, a highly active catalyst can be obtained, but this catalyst has poor thermal stability and
Activity decreases even in the temperature range below 0°C.

300-350 for CuCl2 and CuSO,
Although it exhibits stable activity even at temperatures as low as ℃, the activity level is not sufficient.

In addition, C u C l2 is added to the molecular sieves Y-type carrier.
When 802 is present in the exhaust gas, the activity of the catalyst obtained by supporting 802 is significantly reduced.

Among other metal salts, pecA3tCoCl2, CrCl
In cases such as s, the activity is obviously low, and H2PtCl6
Catalysts supported also do not exhibit sufficient activity.

On the other hand, when CuCl2.2NH4Cl is used, a catalyst is obtained that exhibits high activity over a wide range of low temperatures, and this catalyst also has excellent SO2 resistance.
For example, SO2 such as boiler exhaust gas or desulfurization process exhaust gas
When processing gas with a concentration of 10 to 20 ppm, long-term continuous operation is possible, and there is almost no effect when the concentration is lower than that.

CuCl2.2NH4Cl is supported on the carrier by a conventional method as described below.

That is, the carrier is immersed in a C u C 12 .2NH 4 Cl aqueous solution for several hours, for example, 3 hours, the immersed product is separated from the solution, and then dried at 110 to 120°C.

Separation of the soaked product is carried out by removing the soaked product from the solution and then leaving it to stand, or preferably by performing solid-liquid separation using a centrifuge.

Regarding the concentration of the CuCl2.2NH4CA aqueous solution, when the concentration is 0.05 mol/l or less, as the concentration increases, the prepared catalyst also shows high activity;
．． If it is more than 0.05 mol/l, even if the concentration becomes high,
The activity of the catalyst is constant.

Therefore, the solution concentration does not need to be higher than necessary and is usually 0.5 mol/l or less (the Cu loading rate in this case is about 3 to 4 parts by weight).

Next, the reaction temperature in this invention will be described.

CuCl2・2NH4Cl is CuCl2 containing crystal water
・2NH4Cl・2H20 is commercially available in the form of a hydrate, but based on the results of TG and DTA analyses, this hydrate releases crystal water at 110°C and decomposes at 250°C or higher to produce ammonia and chlorine. and releases C at 500℃
A change to uC7 is observed.

Therefore, it is estimated that the upper limit of the thermal stability of the catalyst supporting C u C 12 .2NH 4 Cl is about 250°C.

This also corresponds to the measurement results of the denitrification activity described below.

That is, when the reaction is carried out at a temperature of 250°C or lower, the denitrification activity does not deteriorate for a long period of time, but once the reaction is carried out at a temperature of 250°C or higher, the activity immediately deteriorates.

Therefore, the upper limit of the temperature at which this catalyst can be used is approximately 250
It is ℃.

Since the exhaust gas treatment method of the present invention is configured as described above, it is possible to obtain a high denitrification rate in a wide range of low temperature ranges.

Examples of the present invention will be shown below along with comparative examples.

Example 1 Molecular sieves Y manufactured by Union Carbide, USA
A carrier obtained by crushing mold-synthesized zeolite (extrusion molded product) to a particle size of 8 to 14 mesh was mixed with Cu at a concentration of 0.5 mol/l.
It was immersed in a Cl2.2NH4Cl aqueous solution at room temperature for 3 hours.

After removing the soaked material from the liquid, it is placed in a centrifuge,
It was then dried at 110-120°C overnight.

The Cu loading rate of the catalyst thus prepared was 3.4% by weight.

Next, 7 ml of this granular catalyst was filled into a quartz flow tube reactor with an inner diameter of 30 mm, the reaction temperature was varied in the range of 100 to 260°C, and the prepared exhaust gas for testing with the composition shown in Table 1 was heated to a space velocity of iooooo. Flow through the reactor at hr-1.

The concentration of NO in the exhaust gas at the reactor outlet was measured using a chemiluminescence analyzer.

In this way, the denitrification rate at each temperature looking for I met.

The results are shown in Figure 1. Note that when the reaction was carried out once at a temperature of 250°C and then the temperature was returned to 100°C, the denitrification rates at the initial and final temperatures of 100°C were the same.

Comparative Examples 1 to 4 Instead of the 0.5 mol/l CuCA2.2NH, CA aqueous solution, the same concentration of CuCl2 aqueous solution (Comparative Example 1)
, CuBr2 aqueous solution (Comparative Example 2), CuS04 aqueous solution (
Comparative Example 3) and a Cu (NOs) 2 aqueous solution (Comparative Example 4) were used to prepare catalysts in the same manner as in Example 1, respectively.

The Cu loading ratio of each catalyst was 6.1 weight ratio (comparative example 1), 4
．． 3 weight ratio (comparative example 2), 5.4 weight ratio (comparative example 3),
The weight ratio is 5.5 (Comparative Example 4).

Next, the denitrification rates of these catalysts were determined in the same manner as in Example 1.

The results are shown in Figure 1. As can be seen from the figure, CuCl
A catalyst supporting 2.2NH4Cl has significantly higher activity even at low temperatures than catalysts supporting other copper salts.

Comparative Example 5 A catalyst was prepared in the same manner as in Example 1, except that Molecular Sieves Type 13X synthetic zeolite manufactured by Union Carbide Company, USA was used instead of Molecular Sieves Type Y.

Next, the denitrification rate of this catalyst was determined by the same operation as in Example 1.

The results are shown in Figure 2. As can be seen from the figure, the catalyst using the molecular sieve Y type carrier has higher activity even at lower temperatures than the 13X type catalyst.

Comparative Examples 6 to 9 Instead of CuCA2-2NH4C# aqueous solution, FeCl3 aqueous solution (Comparative Example 6) with a concentration of 5% by weight, CoC
Catalysts were prepared in the same manner as in Example 1, except for using the 12 aqueous solution (Comparative Example 7) and the CrCl3 aqueous solution (Comparative Example 8).

In addition, the carrier used in Example 1 was 0.1% by weight as Pt.
Immersed in H2PtC76 aqueous solution (Comparative Example 9) for 3 hours,
The soaked product was taken out, centrifuged, dried at 110°C for 1 day, and further calcined at 450°C for 3 hours under air circulation to prepare a catalyst.

Next, the denitrification rates of these catalysts were determined in the same manner as in Example 1.

The results are shown in Figure 2. As can be seen from the figure, Cu
A catalyst supporting C 12.2 N H4 Cl has higher activity even at low temperatures than a catalyst supporting metal salts other than copper salts.

Comparative Example 10 Instead of the CuCl2.2NH4Cl aqueous solution, a 0.5 mol/l CuCl2 aqueous hydrochloric acid solution (however, the hydrochloric acid concentration was 0
．． A catalyst with a Cu loading rate of 6.0% by weight was prepared by carrying out the same operation as in Example 1, except that 01 mol/l) was used.

Next, the denitrification rate of this catalyst was determined by the same operation as in Example 1.

The results are shown in FIG.

As can be seen from the figure, the catalyst obtained using the CuCl2.2NH4Cl aqueous solution has higher activity at lower temperatures than the catalyst obtained using the hydrochloric acid aqueous solution.

Comparative Example 11 Carrier used in Example 1, that is, molecular sieves Y
Granules made of mold-synthesized zeolite were mixed with 2 mol/l of NH
Immerse in 4Cl aqueous solution at 70°C for 1 hour.

This aqueous solution is replaced with a new one, immersion is performed again, and this operation is repeated 10 times.

In this way, Na+ in the carrier was ion-exchanged into NH4+, and the soaked product was thoroughly washed with water to remove excess chlorine, and the soaked product was centrifuged and then dried at 120 to 130° C. for 3 hours.

Using the NH4+ type carrier thus prepared, the same operation as in Comparative Example 1 was carried out to obtain a C u C 12 supported catalyst.

Next, the denitrification rate of this catalyst was determined by the same operation as in Example 1.

The results are shown in Figure 3.

As can be seen from the figure, Cu
Catalyst obtained by supporting C 13 2.2NH4Cl (
In Example 1), C u C was added to the NH, ten-type zeolite carrier.
It has higher activity even at lower temperatures than the catalyst obtained by supporting 132 (Example 11).

Example 2 C with different concentrations in the range 0.01-2 mol/l
Seven types of catalysts (Example 1
(including catalysts) were prepared.

Next, the denitrification rates of these catalysts were determined at a temperature of 130° C. by the same operation as in Example 1.

The results are shown in Figure 4.

As can be seen from the figure, the denitrification rate is CuCl2・2NH4
The denitrification rate shows a high value over a wide range of concentrations of the Cl aqueous solution, and even if the concentration increases above 0.05 mol/l, the denitrification rate shows a constant value.

Example 3 For the catalysts obtained in Example 1, Comparative Example 1, and Comparative Example 11, other operations were carried out using the prepared test exhaust gas containing SO2 shown in Table 2, maintaining the reaction temperature at 200 ° C. The procedure was carried out in the same manner as in Example 1, and each flow time (1 to 2
5 hours) was determined.

The results are shown in Figure 5. As can be seen from the figure, Example 1
The catalyst obtained in Example 1 has better S02 resistance than that of the comparative example.

Furthermore, when a carrier treated with an aqueous NH4C7 solution (Comparative Example 11) is used, the S02 resistance is improved compared to when an untreated carrier is used (Comparative Example 1).
It is not as good as n-supported catalyst.

[Brief explanation of drawings]

Figures 1, 2, and 3 are graphs showing the relationship between reaction temperature and denitrification rate for each catalyst, Figure 4 is a graph showing the relationship between immersion liquid concentration and denitrification rate, and Figure 5 is a graph showing the relationship between gas flow time and denitrification rate. It is a graph showing the relationship between denitrification rates.

Claims (1)

[Claims] 1 In an exhaust gas treatment method in which nitrogen oxides in exhaust gas are catalytically reduced with ammonia, a carrier made of molecular sieves Y type synthetic zeolite is
Using a catalyst prepared by immersing it in an aqueous H4Cl solution and drying it,
A method for treating nitrogen oxide-containing exhaust gas, characterized in that the reaction is carried out at a temperature of 250° C. or lower.