DE2443899B2 - Process for the production of a reduction catalytic converter for the separation of nitrogen oxides from exhaust gases - Google Patents

Description

The separation of nitrogen oxides from exhaust gases from z. B. Boiler systems are becoming more and more urgent for ecological reasons. For example, it has already been proposed, among other things, to separate the nitrogen oxides (which are also generally referred to as "NO" in the following) by oxidizing NO to NO ₂ in the presence of a small amount of oxygen and then adsorbing NO ₂ . DT-OS 24 22 708 describes an adsorbing oxidation catalyst with which the dry gas mixture is brought into contact. The catalyst can be obtained by heating a natural tuff, which consists essentially of SiO ₂ , Al ₂ O ₃ and H ₂ O, contains a total of 1 to 10 percent by weight alkali and alkaline earth oxide and has an X-ray diffraction diagram similar to that in Tables I and below Il represented corresponds to. The existing as well as the NO ₂ formed from the monoxide is then attached to the catalyst serving as an adsorbent, from which it is z. B. split off again by heating, whereupon the oxidation-adsorption process is repeated.

In the case of exhaust gases from boiler systems, however, the method has become nitrogen monoxide (NO) through oxidation and Removing adsorption of the NO2 has been found to be unusable if the exhaust gas has a greater proportion of moisture (about 10 to 15% by volume). The moist gas mixture must then be oxidized dried, otherwise the moisture affects the activity of the catalyst. aside from that the adsorbent must be heated to a relatively high temperature for the desorption of NO2.

Attempts have therefore also been made to reduce the nitrogen oxides contained in gas mixtures by reduction with a gas, for example with H ₂ , CO, hydrocarbons or NHj as the reducing agent. Here, metal oxides such. B. the oxides of Cu, Fe, Cr, V, Mo or W are used.

However, this method has significant disadvantages, since the nitrogen oxides due to the low gas Reduction rate can only be withdrawn very slowly and the catalyst has an all too short lifetime

jü has. In addition, the reaction temperature is very high, and the catalyst is after a short time by the accompanying substances in the exhaust gases, z. B. the sulfur dioxide, poisoned. The process has therefore not yet been used on an industrial scale.

The invention is based on the object of providing a new, selectively acting reduction catalytic converter with the aid of which all nitrogen oxides (NO ₁ ) can be removed from the exhaust gases in an effective and simple manner.

The process according to the invention for producing a catalyst which effects or promotes the reduction of nitrogen oxides in exhaust gases, based on natural _{tuff consisting essentially of SiO 2} , Al ₂ Oj and H ₂ O, which contains 1 to 10% by weight

4> contains alkali and / or alkaline earth oxide, as Carrier substar.z and one or more metals or metal compounds as catalytically active Component, whereby the comminuted carrier substance either with a solution containing the metal or metals contains as cations, impregnated or dry mixed with the metal (s) or metal compounds which solves this problem is characterized in that one is used as metals or metal compounds or several metals from groups Ib, Hb and VIII or chromium or manganese or their salts, oxides or Used hydroxides.

Representative examples of suitable catalyst supports are naturally occurring tuff species, especially in Japan, which essentially consist of SiO ₂ ,

ho Al ₂ Oj and H2O exist and contain a total of 1 to 10 percent by weight of alkali oxide and / or alkaline earth oxide and their X-ray diffraction diagram is shown in Tables I and II. These are the same types of tuff as those according to the DT-OS

b) 24 22 708 are used for the oxidation of NO, im In the present case, however, only serve as a carrier for the actual catalyst.

Table I.

§ Grid spacing relative intensity *) Grid spacing relative

:! Intensity*)

Ι A 10 Uh Ä 10 Uk

I 13.9 ± 0.1 2 3.23 ± 0.03 6

I 9.1 ± 0.1 4 3.10 ± 0.03 0-1

I 6.6 ± 0.1 4 2.90 ± 0.03 3

] 6.5 ± 0.1 2 2.85 ± 0.03 0-2 _:

} 6.1 ± 0.1 2 2.71 ± 0.03 1

I 5.83 ± 0.05 2 2.58 ± 0.03 1

I 4.55 ± 0.05 2 2.53 ± 0.03 2

! 4.30 ± 0.10 0-5 2.49 ± 0.03 0-4

!; 4.26 ± 0.10 0-2 2.47 ± 0.03 0-3

I 4.08 ± 0.10 0-4 2.45 ± 0.03 0-2

1 4.05 ± 0.10 0-6 2.04 ± 0.03 2

$ 4.01 ± 0.05 7 1.96 ± 0.03 1

I 3.85 ± 0.03 2 1.88 ± 0.02 1

1 3.81 ± 0.10 0-4 1.82 ± 0.02 1

I 3.77 ± 0.05 1 1.82 ± 0.02 0-2

I 3.48 ± 0.03 10 1.79 ± 0.02 1

I 3.40 ± 0.03 5 1.53 ± 0.02 1

I 3.35 ± 0.10 0-8

5 *) Note: The values for the position and the specific intensity 10 ■ Uh in the X-ray diffraction spectrum can easily

\ will vary depending on the equipment used to measure humidity and temperature and the nature of the crystals

, ·> Were used.

j Table II

j Grid spacing relative intensity *) Grid spacing relative

Intensity")

• Ä iO · /// ο Ä IO · Uk

9.10 ± 0.1 7 3.18 ± 0.03 4

j 7.99 ± 0.1 4 3.15 + 0.03 4

■; 6.82 ± 0.1 2 2.99 ± 0.03 0-1

i 5.85 ± 0.08 5 2.98 ± 0.03 4

::; 5.29 ± 0.08 2 2.39 ± 0.03 4

ί 5.12 + 0.05 3 2.85 + 0.03 0-2

4.67 ± 0.05 2 2.81 ± 0.03 3

4.30 + 0.10 0-5 2.74 ± 0.03 1

: 4.26 ± 0.10 0-2 2.53 ± 0.02 2

4.08 ± 0.10 0-4 2.49 ± 0.03 0-4

4.05 ± 0.10 0-6 2.47 + 0.03 0-3

: 3.98 ± 0.05 10 2.46 + 0.02 2

3.85 ± 0.05 2 2.45 ± 0.03 0-2

3.81 ± 0.10 0-4 2.02 ± 0.02

3.77 + 0.05 2 1.95 ± 0.02 0.05

3.47 ± 0.03 7 1.87 ± 0.02 0.5

■: 3.34 ± 0.10 0-8 1.81 ± 0.02 0-2

i 3.35 ± 0.03 5 1.72 ± 0.02 0.5

3.22 ± 0.03 4

*) Note: The time for the position and the specific intensity IO ■ IlIn in the X-ray diffraction spectrum can vary slightly, depending on the apparatus used to measure the humidity and temperature and the way in which the crystals were used.

The X-ray diffraction grating and the chemical bo of more than 0.29 mm, preferably from 4 to 6.7 mm,

Composition of the crushed in Tables I and II. Catalyst beds with a content of

listed types of tuff that act as a carrier for the smaller particles have an even better one

Catalysts according to the invention are suitable, correspond to adsorption effect for the nitrogen oxides, since they have a

largely the values of "Mordenite" and a larger surface, but then the »Clinoptilolit« well-known minerals, which also here b5 pressure, which is why it is on a fine-grained carrier

are particularly useful. applied catalyst optionally in the form of

To be used as a carrier for the inventive catalyst granules, tablets or other contact bodies

To be usable, the tuff will have to use a delicacy.

The cation-containing solution used to impregnate the carrier as described above is prepared in such a way that the relevant metal salt of a mineral acid (HCl, HNO ₃ , H ₂ SO ₄ ) is simply dissolved in water. The soaking then takes place at room temperature or in the warm.

Instead, however, you can also dry the powdered carrier in granulated form Metal or its compound, e.g. B. mix its salt, oxide or hydroxide, granulate the mixture and optionally press tablets or other contact bodies therefrom.

The ratio of metal to support is in the range of 0.01-5.0: 100, in particular 0.02-4.0: 100 Parts by weight.

According to a variant of the method according to the invention, the carrier is before the impregnation with the Treated with cation solution or mixing with the metal or the metal compound with a solution, which contains NH4 ions. As can be seen from the examples, this can reduce the effect of the catalyst be improved.

According to a particularly expedient variant of the manufacturing process, the carrier is used several times one after the other with a freshly prepared solution containing metal cations. In In this case you can reduce the nitrogen oxides at a relatively low temperature, about 290 to 380 ° C., the temperature range being wider than when using one according to the invention Catalyst that was produced by soaking it only once with the cation solution.

The catalyst produced according to one of the mentioned variants of the process according to the invention can be used without further aftertreatment, but it is advantageous if it is subjected to a heat treatment at 100 to 900 ^° C., preferably at about 200 to 700 ^° C., before use . hot air is blown onto the catalyst which has passed on a conveyor belt, preferably in a tunnel furnace.

The catalyst obtained according to the invention is applied with the gas mixture containing the nitrogen oxides brought into contact in the usual way, being e.g. B. is arranged as a fluidized bed or fixed bed. The withdrawal of the Nitrogen oxides are based on the fact that they, even if they are only present in low concentrations in the gas, be reduced evenly to nitrogen, which is harmless as a component of the mixture.

To improve the reducing effect, a relatively small amount of a reducing agent such as H2, CHjOH can be added to the gas mixture to be treated. Add NH ₃ , CO, paraffin, olefin or the like, which significantly increases the reaction rate and thus the conversion of nitrogen oxides and has no negative influence whatsoever.

The influence on the reduction rate is also hardly dependent on whether the additional Reducing agent is used in a stoichiometric amount or in excess or deficiency.

The catalyst obtainable according to the invention acts extremely selectively with respect to those present Nitrogen oxides, so that the presence of oxygen or sulfur dioxide (up to several Thousands of ppm) in the gas mixture does not impair the effectiveness of the catalyst.

The reduction can at normal pressure and a temperature of about 200 to 500 C. preferably from 300 to 400 ° C. and, in particular, proceeds at around 320 to 380 ° C. with a desirable result Result.

The throughput of exhaust gas in volumes per volume of catalyst expediently corresponds to the approach:

0.1 to 4ma! 1O ⁴ Ir- ¹ .

The use of the catalyst obtainable according to the invention for separating nitrogen oxides from Exhaust has the following advantages:

1) The reaction rate is very high even in great dilution, so that the nitrogen oxides can be removed extremely quickly;

2) the moisture usually present does not have to be removed from the exhaust gas beforehand, so that the process steps and devices required for this are omitted;

3) the prime cost of the catalyst is low and its life is much longer than that of conventional catalysts, as it is not poisoned by admixtures such as SO2.

The examples serve to explain the invention in more detail.

Example la

jo For the production of the catalyst a in Japan (Toholu and Chugoku districts) Naturally occurring tuff that meets the criteria given in Table I. corresponds to a grain size of 4 to 6.7 mm and the powder for 1 hour at more than 600 "C

j5 heated, after which it was allowed to cool.

The carrier prepared in this way was immersed for two hours in an aqueous solution of CuNOj (0.25-2.0 mol / l H ₂ O) heated to 100 ° C. or sprayed with such a solution and exposed to air at 600 ° C. or The copper catalyst then contains 0.02 to 3.0 parts by weight of CuNOj, calculated as metallic Cu, per 100 parts by weight of carrier.

In the same way, Ag, Zn, Cd, Fe, Cr, Ni, Co, Mn and Pd catalysts were prepared, where an aqueous solution of the corresponding metal nitrate was used in each case.

Example Ib

The carrier produced according to Example la was mixed with a precipitate which had been precipitated from a copper sulfate solution with sodium hydroxide solution, the mixture was _{kneaded while moist with pulverized MnO 2} and then granulated. Heating the granulate to about 100 ° C. gave a catalyst with about 36.5% by weight of metal salts, calculated as metal.

Example Ic

Using a tuff according to Table I, a catalyst was prepared _{according to Example b o la.} In this case, however, the carrier, before it was treated with the cation solution, was immersed in an aqueous solution of NH- (Cl (1 mol / l sol.) At about 30 ° C. and dried at room temperature.
(, 5 The carrier contained 0.02 to 3% by weight copper nitrate.

In the same way with aqueous solutions of the corresponding metal nitrates Ag-, Zn-, Cd-, Fe-, Cr-. Ni. Co-. Mn and Pb catalysts produced.

Ii c i s ρ i c I ld

According to Example Ib, a KnUilysulor was produced, however, the carrier prior to mixing with the metal-containing portion was immersed in an aqueous solution of NH4Cl (1 mol / l sol.) for 15 minutes. The catalyst contained about 36.5% by weight metal salt, calculated as metal.

Example Ic

Example 3

10

(1) A tuff according to Table I was crushed to 6.7 mm and used as a carrier in an aqueous solution of 0.2 mol Fe nitrate per liter immersed and heated in air. The immersion and drying was done several times repeated with fresh Fe nitrate solution.

(2) Instead of the Fe nitrate solution, a solution of 0.25 mol Fe chloride per liter of water was used, but this time the carrier was crushed to a maximum grain size of about 4 mm.

(3) In the same way, using the corresponding metal nitrates, Ag-, Zn-, Cd-, Cu-, Cr-, Ni, Co, Mn and Pd catalysts prepared.

Examples 2a, 2b and 3 illustrate the invention Process when using several metal compounds on a common carrier.

Example 2a

A naturally occurring tuff having an X-ray diffraction pattern in Table I was crushed to a powder of 2.4-3.3 mm particle size, held for 30 minutes at 100 ⁰ C in an aqueous copper nitrate solution (1 mol / liter), and then at 400 ° C has been air dried. The dry powder was then again introduced into an aqueous solution which contained 1 mol / liter of iron nitrate and 0.2 mol / liter of manganese nitrate and ^{treated at 100 °} C. for 60 minutes, after which it was again air-dried.

40

Example 2b

Production takes place according to Example 2a, but chromium nitrate was used instead of manganese nitrate.

Use examples I to IV

The effect of following the examples above Catalysts produced were tested in four series of tests, in which in addition to the carrier substance and the catalyst metal or its compound, the reduction temperature and the composition and the hourly diirchsalznienge of the exhaust gas were varied.

The catalyst to be tested was located in a tube with an internal diameter of 25 mm and a length of 100 or 150 mm, through which the moist exhaust gas was passed.

The reaction temperature was varied between 130 and 400 ° C, average about 300 ° C.

The exhaust gases were composed as follows:
Content of nitrogen oxides (NO ₁ ):

about 500-930 ppm
Content of SO ₂ :

usually at most 100 ppm, in some cases

up to 1300 ppm
Content of O2:

0.5 to (mostly) 4%
Reducing agent content
(in all cases NHj):

170 to 1700 ppm, which is a molar ratio

NO, from about 0.2 to 2.5

Exhaust gas throughput in volumes per volume of catalyst
per hour:

mostly about 12,000, minimum 1,000, maximum 40,000.

The catalyst contained, in parts by weight of metal per 100 parts by weight of carrier:

45

Naturally occurring tuff with an X-ray diffraction pattern as shown in Table I was ground to 2.4-3.3 mm as above. The powder was then kept for 30 minutes at normal temperature in an aqueous ammonium chloride solution (1 mol / liter) and, after this pretreatment, kept for a further 30 minutes at 100 ^° C. in an aqueous copper nitrate solution (1 mol / liter) and air-dried. The dry powder was then placed in an aqueous solution of 1 mol / liter of iron nitrate and 0.2 mol / liter of manganese nitrate, held therein at 100 ° C. for 60 minutes, and air-dried.

M)

Examples 4-6

If you work according to Examples I to 3, used however, as a support a natural tuff, the criteria of which correspond to Table II, is obtained with the metal salts according to the invention are also suitable catalysts.

Test series I:
Test series II:
Test series III:
Test series IV:

0.8Cu
1.8-3.0 Cu
10.0-36.5 Cu + Mn
1.8 Cu + 0.4 Ni

In addition, Fe, Ni (alone), Zn, Cd, Co, Ag and Pd were tried out as catalyst metals.

In the test series I to IV were almost always up to 99% and above of the existing nitrogen oxides are reduced, with the results in rows III and IV particularly were good. Similar results were obtained using the other catalyst metals mentioned achieved, which, however, in individual process variants also behind those with Cu, Cu + Mn or Cu + Ni reached lagged behind.

In two comparative examples in which the tuff carrier alone, without metal, was used as a catalyst (exhaust gas: 509 ppm NO ₁ , 4% O2, no S, addition 500 ppm NHj), only about 30% of the nitrogen oxides were reduced.

The life of the catalyst was consistently more than 300 hours.

In summary, it can be said that with an appropriate choice of the reaction conditions with With the help of the catalysts produced according to the invention, industrial exhaust gases (even if they have relatively high SO2 proportions contain) practically all of the nitrogen oxides can be removed by adding them a reducing agent can be stroked over the catalysts at a relatively high speed. Since the gases are usually hot, an additional heat shield is only necessary in exceptional cases.

Claims (4)

Patent claims: 1. Process for the production of a catalyst which effects or promotes the reduction of nitrogen oxides in Al gases based on natural _{tuff consisting essentially of S1O2, Al2O3 and H 2} O, which contains 1 to 10% by weight of alkali and or or contains alkaline earth oxide as a carrier substance and one or more metals or metal compounds as a catalytically active component, the comminuted carrier substance being either impregnated with a solution containing the metal or metals as cations or dry mixed with the metal or metal compounds , characterized in that one or more metals from groups Ib, Hb and VIII or chromium or manganese or their salts, oxides or hydroxides are used as metals or metal compounds. 2. The method according to claim 1, characterized in that the carrier or before the impregnation Mixing with the catalytically active component impregnated with a solution, the NH4 ions contains. 3. The method according to claim 1 or 2, characterized in that one with the catalytically active component loaded carrier heated to ICO up to 900 ° C. 4. The method according to claim 1, 2 or 3, characterized in that the impregnation of the carrier repeated several times, each time using a freshly prepared solution of metal cations.