DK169610B1 - Method of Removing Nitric Oxides from Combustion Gases - Google Patents

Abstract

In a process for eliminating nitrogen oxides from combustion exhaust gases which contain arsenic compounds as impurities, by selective catalytic reduction with ammonia, a catalyst is used which essentially contains titanium oxide and catalytically effective quantities of compounds of vanadium, molybdenum, tungsten and zinc. The catalyst contains A) titanium in the form of oxides, B) vanadium in the form of oxides or sulphates, C) molybdenum and/or tungsten in the form of oxides and D) zinc in the form of oxides or sulphates, with the proviso that the metal components A to D are present in the A/B/C/D atomic ratios of 1/0.0022-0.0165/0.021-0.21/0.009-0.028.

Description

The invention relates to a method for selectively removing NO from combustion gases containing arsenic compounds as contaminants by selective catalytic reduction with ammonia.

5

A number of methods are known for removing Ν0 k from waste gases. One knows, for example. methods by which NO reduces

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for nitrogen with a fuel, e.g. natural gas or methane, in the presence of precious metals as catalysts. However, these methods have not achieved any technical significance because also the oxygen present in the waste gases is reacted alongside the oxides of nitrogen, which, in addition to the disadvantage which comprises an undesirably high rise in temperature, also has the further disadvantage which comprises a high fuel consumption, except that the precious metal catalysts are non-poisonous.

For this reason, a selectively working catalytic method has also been developed by which the nitrogen oxides in the presence of non-precious metal catalysts are selectively reduced with ammonia to nitrogen. This method, which has become known as the Selective Catalytic Reduction (SCR) method, has gained growing technical significance.

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By this method, the oxygen present in the waste gases is practically not attacked, and the consumption of ammonia is directed solely at the nitrogen content of the waste gases to be treated.

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Various catalysts are used for this method. JP-A-53-117689 discloses a process for catalytic reduction of NO from combustion exhaust gases, wherein the catalyst consists of titanium, tungsten, vanadium oxide and sulfate as well as zinc or tin oxide.

2 U.S. Patent No. 3,279,884 discloses catalysts containing catalytically active amounts of vanadium, molybdenum and / or tungsten oxides. In addition, DE-PS 1,253,685 also discloses catalysts which may also contain manganese oxide and / or iron oxide.

DE-PS 2,458,888 also discloses for the same purpose catalysts containing 10 1. titanium in the form of oxides and 2. at least one element of the group a) iron and vanadium in the form of oxides and / or sulfates 15 and / or b) molybdenum, tungsten, nickel, cobalt, copper, chromium and uranium in the form of oxides and optionally 20 3. tin in the form of oxides 4. metals of the group silver, beryllium, magnesium, zinc, boron, aluminum, yttrium, the rare earth metals, silicon, niobium, antimony, bismuth and manganese in the form of oxides 25.

These catalysts can be used as full catalysts or can be applied to conventional carriers. The components listed under "4", according to the disclosures in the patent specification, must not exert any detrimental effect on the catalyst properties, i.e. in other words, they also do not increase the catalytic activity. For this reason, they may exist as bicomponents or contaminants in the starting materials of components 1 to 3, 35 without having to be removed in advance.

DE-A-3 438 367 discloses catalysts comprising 3 1. of a binary oxide compound of titanium and silicon and / or of titanium and zirconium and / or of a ternary oxide compound of titanium, zirconium and silicon and 5% by weight of a vanadium oxide and 3. 1-15% by weight of at least one oxide of the elements tungsten, molybdenum, tin and cerium.

DE-AS 24 14 333 further describes catalysts for the SCR method, which catalysts consist of a vanadium catalyst having the general formal V A 0. This formula is the xyz A elements copper, zinc, tin, lead, titanium, phosphorus, chromium, iron, cobalt and nickel, y and x are numbers with a value from 0.5 to 12 and the value of z is V and Ay.

Among the catalyst components described in the 20 references cited above, in particular, for the activity of NO reduction, catalysts have proved suitable which contain the elements vanadium, tungsten, molybdenum and titanium. Particular attention was paid to the development of catalysts which ensure a reduction of Ν0χ which is as high as possible, but which, on the other hand, is as inert as possible towards the oxidation of SO2 to SOg. S02 always occurs in the combustion gases of sulfur-containing oils or coal. 1 2 3 4 5 6

From the previously cited DE-A 34 38 367 it is known that a 2 content of vanadium oxide in the catalyst, on the one hand, is 3 responsible for the high activity of the NO reduction, but 4 on the other hand also favors an undesirable oxidation 5 of SO2 to SOg, which in turn leads to disturbances of 6 due to the formation of ammonium sulfates which settle in the appliance parts and pipelines following the catalytic reaction zone.

4

EP-A-271 919 discloses a further catalyst which, in addition to titanium, vanadium and tungsten or molybdenum, contains zinc in small quantities. The vanadium content is quite high and there is no mention of its effect on arsenic as a catalyst poison.

In the combustion of coal, the combustion waste gases in addition to the combustion products as well as nitrogen oxides and sulfur dioxide also contain additional contaminants, especially 10 contaminants of metals, due to which the catalysts are damaged, which is associated with a reduction in their activity. It has been found that especially arsenic compounds are responsible for these catalyst damage. Combustion gases associated with furnaces with molten ash extract have a higher arsenic content than waste gases associated with dry ash furnaces. Also in waste gases of waste incineration plants varying amounts of arsenic contaminants can occur.

In EP-OS 02 60 614, catalysts used in the SCR method are described in the treatment of waste gases containing arsenic compounds and which should exhibit an increased resistance to arsenic poisoning. These catalysts contain titanium and vanadium as essential components, whereby the weight ratio of titanium oxides to vanadium oxides must be between 99.9: 0.1 to 92.0: 8.0. The vanadium must be enriched at the surface up to a depth of 200 (im) in such a way that its concentration in this range is at least 1.5 times that of the vanadium concentration in the other regions. special precautions are taken when preparing the catalyst.

35 Also in EP-OS 0 256 359 catalysts are disclosed which exhibit an increased poisoning resistance to arsenic-containing waste gases. These catalysts must exhibit two groups 5 of pores, namely, first, pores with a diameter of 1 x 10 Å to less than 1 x 10 Å, whereby the pore volume of the first group must be at least 10% of the total pore volume. In addition to titanium, the catalyst must contain an additional element selected from molybdenum, vanadium, tungsten, manganese, cobalt, copper, iron, chromium, nickel, zinc and tin. In order to provide the large pores in the finished catalyst, in the preparation, solids are incorporated as homogeneously as possible, which is burnt away during calcination. which also impedes the production of catalysts with reproducible properties.

The object of the invention is to provide a process of the kind set forth in the preamble of claim 1, in which use is made of a simple-to-manufacture catalyst which exhibits a high activity and selectivity in NO pollutants, and which also exhibits high poisoning resistance to 20 arsenic compounds even at long periods of residence.

The process according to the invention is characterized in that the composition of the catalyst is as defined in the characterizing part of claim 1.

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From Group C, tungsten is particularly preferred. Preferred titanium: tungsten atomic ratios are 1: 0.023-0.1, in particular 0.023-0.05. 1 2 3 4 5 6

Surprisingly, due to the zinc content of the ka 2 catalyst within the previously stated limits, the poisoning resistance of the catalyst 4 to arsenic compounds succeeds significantly. Essentially, the zinc 5 compounds are present as zinc oxide, but they may also be in the form of a zinc compound which is stable at the catalyst operating temperature, e.g. zinc sulphate.

6

The titanium oxides may be present as TiCl 2 in the rutile form or preferably in the anatase form, but they may also be present as binary oxides, e.g. as an oxide compound of titanium or zircon, or of titanium and silicon, or as ternary oxide compounds of the indicated elements.

There are various possibilities known in the art for preparing the catalysts: One possible embodiment consists in thoroughly mixing a mixture of titanium oxides, tungsten oxides and / or molybdenum oxides and a compound of vanadium such as ammonium hydroxide, water inoxalate and vanadium oxide. and to this is added during the mixing process an aqueous, alcoholic, ethereal or ammonia solution of a compound of the zinc. The softened material can be further processed into the most diverse geometric shapes. For the design, a drying at 50 ° C-150 ° C, especially 60 ° C-130 ° C, and a calcination at 350 ° C-750 ° C, especially 450 ° C-650 ° C, are joined.

Zinc can be introduced in the form of commercial products such as e.g. oxide, nitrate, sulfate, oxalate, tungsten, molybdate, stearate, carbonate, in the catalyst mass.

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The titanium oxide used for the catalysts according to the invention is preferably made of metatitanoic acid and the wool framework is preferably made of ammonium parawolframate. 1 2 3 4 5 6

But it is also possible to proceed from homogeneous solutions 2 of components A, B, C and D, to evaporate them, to dry 3 them and to transfer them to the oxides by calcination at 4 the previously stated temperatures. However, on the basis of the stated solutions 5, the components 6 can also be separated by precipitation, whereby these precipitates are transferred to the oxides by hydrolysis.

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Of course, the catalysts may also contain the oxides known to fulfill the present purpose, such as the oxides of tin, nickel, cobalt, copper, chromium, uranium, cerium and the like.

5

The catalysts may either consist solely of the components specified or they may be applied to a carrier known per se, e.g. alumina in alpha or r form, silicon dioxide, silicic acid or silicates such as mullite or cordierite.

The catalysts can be used in any form, e.g. such as extruded bodies, tablets, but preferably in the form of cell bodies with channels extending reciprocally and in parallel.

The process is carried out at temperatures between 100 and 600 ° C, preferably between 300 and 450 ° C, at room rates between 1000 and 150000 h -1, especially 2000 to 100000 h -1. The ammonia is used in a molar ratio NH 1.4, preferably between 0.7 and 1.1, in particular at a molar ratio <1, to avoid an NH

The process according to the invention is illustrated in more detail by the following examples.

Examples: 150 g of TiO (OH) 2 was thoroughly mixed with 3750 g of concentrated H2SO4 'was heated for 2 hours at 200 ° C and then dissolved in 7.5 liters of H2 O. The residue was filtered off and the filtrate mixed with 25% by weight -% NH 2 solution until the precipitation of the metatitanoic acid is completed, filter and wash thoroughly with distilled water, to the moist product add 136 g of ammonium p-tungstate in 2.0 L H2 O, heated to 90 ° C and stir for 1 hour, then the product is evaporated The TiO 2 / WOg atomic ratio of the 8 evaporated product was 1: 0.036.

To 1200 g of the evaporated product is added a solution of NH 4 V0 3 in monoethanolamine / 1 O and a solution of Zn (NO 3> 2 '6 1 ^ 0 in H 2 O.) Thoroughly mix and add bentonite as a stringing aid. 5 mm diameter strands are dried at 110 ° C and calcined for 6 hours at 600 ° C. The amounts of NH 2 VOg and Zn used (NO 3> 2 '6 1 ^ 0 are shown in the following Table 1).

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The catalysts obtained according to Examples 1 to 4 and Comparative Examples 1 to 4 are, for the purpose of investigating their activity, firstly subjected to a flue gas having the following composition: 1000 Vpm NO, 1000 Vpm NH 2, 1000 Vpm SO2, 5 vols. %, 10% water vapor, residual nitrogen, at a temperature of 380 ° C and a velocity of 66,500 h "1. In each case 1.5 ml of catalyst ground to a grain size of 0.5 to 1 mm.

The test reactor has a diameter of 6 mm. Second, under the same conditions, they are also subjected to a flue gas which further contains 1 Vpm As 2 O 2, and the NO conversion is measured after 7 hours. The results are summarized in Table 2.

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From the results summarized in the table, it can be seen that the catalysts of Example 1 and Comparative Example 1 differ only in that the catalyst from the comparative example contains no 5 zinc. According to the results in Table 2, the N0 reaction of the Zn-free catalyst decreases significantly more strongly under the influence of an As4 <Dg-containing gas than that of the Zn-containing catalyst of Example 1. Analogous results appear in a comparison of the catalysts according to Examples 2 to 3 and the catalyst according to Comparative Example 2.

In Comparative Examples 3 and 4, catalysts departing from the ranges specified in the claims are illustrated both in vanadium and zinc contents. Although, according to Comparative Example 3, the catalyst exhibits good activity at As ^ And-free gases, it decreases strongly after interaction with As ^ And-containing gas. In contrast, the catalyst of Comparative Example 4, in which both the vanadium-20 and the zinc content are above the ranges specified in the claims, already exhibits poor initial activity with As40g-free gases.

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Claims (3)

A process for removing nitrogen oxides from combustion gases containing arsenic compounds as impurities by selective catalytic reduction with ammonia in the presence of a catalyst substantially containing titanium oxide as well as catalytically active amounts of compounds of vanadium and of molybdenum and / or tungsten, characterized in that the catalyst contains A) titanium in the form of oxides B) vanadium in the form of oxides or sulphates C) molybdenum and / or tungsten in the form of oxides and D) further zinc in the form of oxides or sulphates, with the proviso that the metal components AD are present in the 20th atomic ratio A: B: C: D of 1: 0.0022 - 0.0165: 0.021 - 0.21: 0.009 - 0.028. Process according to claim 1, characterized in that the oxide is tungsten oxide, with the proviso that the atomic ratio of the metal components A and C is from 1: 0.023 to 1: 0.1. Process according to claim 2, characterized in that the atomic ratio of the metal components A and C is from 1: 0.023 to 1: 0.05. 35