DE2443899C3 - 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. 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 oxidation of NO to NO ₂ in the presence of a small amount of oxygen and subsequent adsorption of NO ₂ . In DE-OS 24 22 708 an adsorbing oxidation catalyst is described 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 Os 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 II below shown corresponds. 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 of _{removing nitrogen monoxide (NO) by oxidation and adsorption of the NO 2 has} proven to be unusable if the exhaust gas contains a greater proportion of moisture (around 10 to 15% by volume). The moist gas mixture must then be dried before the oxidation, since otherwise the moisture will impair the activity of the catalyst. In addition, the adsorbent must be heated to a relatively high level for the desorption of NO _2.

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 NH ₁ 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 speed only very slowly e

and the catalyst has an all too short lifetime

ίο 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' reducing catalyst, with the help of which all nitrogen oxides (NO,) can withdraw.

The process according to the invention for producing a catalyst which effects or promotes the reduction of nitrogen oxides in exhaust gases and is based on natural _{tuff consisting essentially of SiO 2} , Al ₂ O ₃ and H ₂ O, which is 1 to 10% by weight Contains alkali and / or alkaline earth oxide as a carrier substance and one or more metals or metal compounds as a catalytically active component, the comminuted carrier substance either impregnated with a solution containing the metal or metals as cations or dry with the metal or metals or metal compounds is mixed, which solves this problem, is characterized in that one or more metals from groups Ib, Hb and VIII or

γ ·, chromium or manganese or their salts, oxides or hydroxides are used.

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

bo Al2O3 and H ₂ O 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

t> 5 24 22 708 can be used for the oxidation of NO, im In the present case, however, only serve as a support for the actual catalyst.

Table I. Grid spacing relative intensity *) Grid spacing relative Intensity*)

Ä 10 - HI ₀ A 10 - J / lo

13.9 ± 0.1 2 3.23 ± 0.03 6

9.1 ± 0.1 4 3.10 ± 0.03 0-1

6.6 ± 0.1 4 230 ± 0.03 3

6.5 ± 0.1 2 2.85 ± 0.03 0-2

6.1 ± 0.1 2 2.71 ± 0.03 1

5.83 ± 0.05 2 2.58 ± 0.03 1

4 ^ 5 ± 0.05 2 2.53 ± 0.03 2

430 ± 0.10 0-5 2.49 ± 0.03 0-4

4.26 ± 0.10 0-2 2.47 ± 0.03 0-3

4.08 ± 0.10 0-4 2.45 ± 0.03 0-2

4.05 ± 0.10 0-6 2.04 ± 0.03 2

4.01 ± 0.05 7 1.96 ± 0.03 1

3.85 ± 0.03 2 1.88 ± 0.02 1

3.81 ± 0.10 0-4 1.82 ± 0.02 1

3.77 ± 0.05 1 1.82 ± 0.02 0-2

3.48 ± 0.03 10 1.79 ± 0.02 1

3.40 ± 0.03 5 1.53 ± 0.02 1 3.35 ± 0.10 0-8

*) Note: The values for the position and the specific intensity IO · I / k 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.

Table II Grid spacing Relative intensity *) Grid spacing Rclalivc Intensity*)

Ä 10 // Α, Ä 10 l / k

9.10 ± 0.1 7 3.18 ± 0.03 4

7.99 ± 0.1 4 3.15 ± 0.03 4

6.82 ± 0.1 2 2.99 ± 0.03 0-1

5.85 ± 0.08 5 2.98 ± 0.03 4

5.29 ± 0.08 2 2.89 ± 0.03 4 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 I 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 0.5 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

3.35 ± 0.03 5 1.72 ± 0.02 0.5 3.22 ± 0.03 4

*) Note: The values for the position and the specific intensity 10 · l / k 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 gratings and the chemical eo of more than 0.29 mm, preferably from 4 to 6.7 mm.

Composition of the crushed in Tables 1 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 65 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 catalyst granules, tablets or other contact bodies according to the invention

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

The cation-containing solution used to impregnate the carrier obtained above is prepared in such a way that the relevant metal salt of a mineral acid (HCl, HNOj, H ₂ SO ₄ ) is simply dissolved in Wisser. Soaking 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. his Sab, oxide or mix hydroxide, granulate the mixture and optionally tablets or others from it Press the contact body.

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 treated with a solution which contains NH ₄ ions before the impregnation with the cation solution or the mixing with the metal or the metal compound. As can be seen from the examples, this can improve the effect of the catalyst.

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 , whereby 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, the gas mixture to be treated can be given a proportionate Add a small amount of a reducing agent such as H2, CH3OH, NH3, CO, paraffin, olefin or the like Reaction speed and thus the conversion of starch oxides significantly increased and no negative whatsoever Has influence.

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 Thousand ppm) in the gas mixture does not interfere with the effectiveness of the (.5 catalyst.

The reduction may be carried out at atmospheric pressure and a temperature of about 200 to 500 ^'1 C, preferably of 300 to 400 "C. And proceeds in particular at about 320-380 ⁰ C with desirable result.

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

0.1 to ⁴ times 10 4 h- '.

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

A naturally occurring one in Japan (Toholu and Chugoku districts) was used to make the catalyst Tuff, which meets the criteria given in Table I, to a grain size of 4 to 6.7 mm crushed and the powder heated to more than 600 ° C for 1 hour, whereupon it was allowed to cool.

The thus prepared support was heated to 100 ⁰ C heated aqueous solution of G1NO3 (0.25-2.0 mol / l H ₂ O) was immersed for two hours in one or sprayed with such a solution and in the air at 600 ⁰ C or dried at room temperature. The copper catalyst then contains 0.02 to 3.0 parts by weight of CuNO ₃ , calculated as metallic Cu, per 100 parts by weight of support.

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

Example Ib

The carrier prepared 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

With a Tuff in accordance with Table I, a catalyst was made in this case according to Example la, however, was the carrier before being treated with the cation solution at about 3O ⁰ C in an aqueous solution of NH ₄ Cl (1 mol / l sol.) immersed and dried at room temperature.

The carrier contained 0.02 to 3% by weight of 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.

BL ¹ is ρ ic I id

A catalyst was sealed according to Example 1h. _{However, the carrier was immersed in an aqueous solution of NH 4} Cl (1 mol / l l.sg.) for 15 minutes before mixing with the metal-containing portion. The catalyst contained about 36.5% by weight of metal salt, calculated as metal.

Example Ic

(1) A tuff according to Table I was crushed to b.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 process according to the invention when several are used Metal connections on a common carrier.

Example 2a

Kin naturally occurring tuff with an X-ray diffraction diagram according to Table I was comminuted to a powder of 2.4-3.3 mm grain size, which was kept 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 placed in an aqueous solution containing 1 mol / liter of iron nitrate and 0.2 mol / liter of manganese nitral and handled at 100 ° C. for 60 minutes. ¹ ·, whereupon it was again air-dried.

Example 2b

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

Example 3

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 introduced into 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 dried in the air.

Examples 4-6

If you work according to Examples 1 to 3, however, a natural tuff is used as the carrier, its If the criteria in Table II meet, the metal salts according to the invention are also good useful catalysts.

Usage examples I to IV

The effect of the catalysts prepared according to the above examples was shown in four comparisons · "> Hen tried, in addition to tier Trageisubsian / and the catalyst all or its compound the Reduction temperature as well as the composition and the hourly diirchsaizquote 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 humid exhaust gas was passed.

The reaction temperature was varied between I 30 and υ 400 "C, average value about 300 C.

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

about 500-930 ppm
,,, Gehall to SOj:

usually a maximum of 100 ppm. in some grooves
up to 1300 ppm
Content of O2:

0.5 to (mostly) 4%

, - content of reducing agent

(in all cases NHi):

170 to 1700 ppm, which is a Moivcrhälinis / u
NO ₁ corresponded to about 0.2 to 2.5

Exhaust gas flow in volumes per volume of catalyst
so per hour:

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

The catalyst contained metal in parts by weight π 100 parts by weight carrier:

Test series I: 0.8 Cu
Test series II: 1.8-3.0 Cu
Test series III: 10.0-36.5 Cu + Mn
Test series IV: 1.8 Cu + 0.4 Ni

In addition, Fe, Ni (alone). Zn. Cd, Co. Ag and Pd tested as Caialysatormetalle.

In the test series I to IV were almost always up to 99% and above of the existing nitrogen oxides reduced. 4) with the results in rows III and IV being special 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 ) ii 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% O ₂ , no S, additive. 500 ppm NH 1), only about 30% of the nitrogen oxides were τ - reduced.

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

In summary, one can say that at

appropriate choice of reaction conditions with

With the aid of the catalysts produced according to the invention, industrial waste gases (even if they _{contain relatively high SO 2} proportions) can practically remove all of the nitrogen oxides by adding a reducing agent with a relatively large amount

b5 lets speed sweep over the catalytic converters.

Since the gases are usually hot, this is only an exception an additional heat input is necessary.

Claims (4)

Patent claims: 1. Process for the production of a catalyst which effects or promotes the reduction of nitrogen oxides in exhaust gases on the basis of natural _{tuff consisting essentially of SiO 2} , Al ₂ O ₃ and H ₂ O, which contains] up to 10% by weight of alkali and / 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 either impregnated with a solution containing the metal or metals as cations or dry with the metal or metals or Metal compounds are mixed, 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 ingredient with a solution impregnated, the NH4 ions contains. 3. The method of claim 1 or 2, characterized in that heating the laden with the catalytically active component on carrier 100 to 900 ⁰ C. 4. The method according to claim 1, 2 or 3, characterized characterized in that the impregnation of the carrier is repeated several times, one fresh each time prepared solution of metal cations is used.