GB2079172A - Catalyst for removing nitrogen oxides - Google Patents

Abstract

A catalyst for removing nitrogen oxides from exhaust gases, having a large average pore diameter is prepared by kneading catalyst raw material compound(s) containing at least one of Ti, V, Mo, W, Sn, Cr, Mn and Fe with water in a pendular state; adding an inorganic fibrous material and further kneading them; and further adding water and kneading them so as to bring them into a funicular state or a capillary state, followed by conventional process for tableting.

Description

SPECIFICIATION Catalyst for removing nitrogen oxides This invention relates to a catalyst for removing nitrogen oxides (denoted hereinafter as NOx) contained in exhaust gases, particularly a catalyst employed at the time of ammonia reduction of NOx contained in exhaust gases containing sulfur oxides, and a method for preparing the same.

For removing NOx contained in exhaust gases from boilers,metal-heaing furnaces, coke ovens, etc., a denitration process by way of selective catalytic reduction with ammonia has now been widely employed, and as catalysts employed for such selective reduction, platinum group compounds, iron group metal compounds, each oxides of vanadium, molybdenum and tungsten, etc. have so far been proposed. Further, as the types for treating exhaust gases with the catalysts, a granular catalyst packed layer system or a parallel flow system with honeycomb-form catalyst or plate-form catalyst has been employed.

In the case of the packed layer system, for periodically removing deposited dusts on the catalyst layer, a process of subjecting the fixed layer to a soot blow treatment, a process of moving and circulating the catalyst by way of a moving layer, etc. have been employed. For such processes, a catalyst having a high compressive strength and a superior abrasion resistance has been required.

For such a purpose, tableted catalysts have been regarded optimum, since they have merits of constant formability and weight, and high strength. But on the other hand, they have a high density, so it seems that physical properties, particularly pore diameter, pore volume, surface area, etc., required for catalytic reactions, including gas diffusion, are not optimal. Thus, there has been employed a process of -controlling the primary particle diameter of catalyst to raise its voids, or a process of adding a combustible auxiliary such as agar, gelatin, cellulose, etc. to utilize voids formed after burning treatment.

However, the former case has a drawback in that the catalyst composition and the heat treatment conditions are restricted, while the latter case has a drawback in that reduction in the catalyst strength is unavoidable.

Further, for improving the catalyst activity in the denitration reaction, particularly that in the lower temperature region, there has been employed for example a catalyst consisting of four component oxides of Ti, V, W and Sn (abbreviated hereinafter to Ti-V-W-Sn system), which catalyst is superior in the lower temperature activity and also superior in the performance of regeneration treatment for the activity reduction which occurs inevitably at the time of its use at lower temperatures, and it has been necessary for such a cataylst to employ a calcination temperature in the vicinity of 55000, but such a condition could not have been satisfied by the above-mentioned means.

The object of the present invention is to provide a catalyst for removing nitrogen oxides which preferably is free from the above-mentioned drawbacks of the prior art; has a practical catalyst strength; and is improved in the catalyst activity for the denitration reaction, particularly that in the lower temperature region.

For attaining this object, the present inventors have attempted to overcome the drawbacks of the tableted catalyst by incorporating an commercially available inorganic fibrous material such as silica, silica-alumina or an alkali glass into the active ingredients of the catalyst. However, while such an incorporation of inorganic fibrous materials is effective for reducing the density of the catalyst and enhancing the pore diameter, pore volume and surface area of the catalyst, such a mere incorporation of inorganic fibrous material is not sufficient to give superior properties to the tableted catalyst. The present inventors have further made various studies on a method for preparing granules suitable for tableting.As a result, it has been found that the above-mentioned object can be attained by a tableted catalyst prepared by kneading raw materials for the catalyst with water in a pendular state (a half-dry loose state), adding inorganic fibrous materials to the resulting mixture, continuing kneading, further adding water and continuing kneading till the resulting intimate mixture is brought into a funicular state or a capillary state (a sticky state), followed by conventional procedure for tableting.

The present invention resides in: a tableted catalyst useful for removing nitrogen oxides from exhaust gases, prepared by the steps which comprises; kneading at least one catalyst compound raw material containing at least one metal component selected from the group consisting of Ti, V, Mo, W, Sn, Cr, Mn and Fe with the presence of water in a pendular state; then adding an inorganic fibrous material to the mixture and further intimately mixing them together; further adding water to the resulting mixture and intimately mixing them together so that the resulting mixture is brought into a funicular or capillary state; and subjecting the mixture to a conventional procedure of extrusion molding, drying, grinding, sizing and tableting.

Some embodiments of the invention will now be described by way of example.

As the active ingredients of the catalyst of the present embodiments, for reducing the catalyst poisoning brought about by sulfur oxides contained in exhaust gases, those comprising titanium (Ti) as a base and at least one metal component selected from the group consisting of vanadium (V), molybdenum (Mo), tungsten (W), tin (Sn), chromium (Cr), manganese (Mn), ion, etc. added to the base are preferably employed. It goes without saying that, by mixing these active ingredients together, a mere dispersion of elements is not only brought about, but new compounds are formed due to the interaction between the ingredients which occurs at the time of the calcination treatment to yield a synergistic effect. Concrete examples of the muiticomponent systems are Ti-V, Ti-Mo, Ti-W, Ti-Sn, Ti-V Mo, Ti-V-W, Ti-V-Sn, Ti-V-Mo-Sn, Ti-V-W-Sn.The latter three systems among them are superior in the activity in the lower temperature region of 2000to 2500C, and also it is possible to easily recover the activity reduction caused by ammonia added as a reducing agent for the denitration reaction and sulfur oxides contained in exhaust gases, by a heat treatment from 3000 to 4000 C. As the starting raw materials of these ingredients, it is possible to employ their oxides, hydroxides, sulfates, ammonium salts, oxalates, nitrates, chlorides, etc.

As the inorganic fibrous material employed in the present embodiments, any one of commercially available silica, silica-alumina and alkali glasses may be employed, but those having a softenng point of 6000C or higher are preferred. The fiber length has no particular limitation, but those having a fiber diameter of 1 to 10 ym are preferred. Further, the amount of the fibrous material added is preferably 0.1 to 50% based on the weight of the catalyst, and most preferably 5 to 30%. If the amount is less than 0. 1%, no addition effectiveness is observed, while if it exceeds 50%, the catalyst strength after tableting is reduced and also there occurs an activity reduction according to the reduction in the catalyst concentration.

Further, as the lubricant for the tableting, although sodium benzoate, metal salts of boric acid and stearic acid, etc. among those usually employed, are not desirable, if the metal components have a bad effect on the catalyst, generally graphite, oils, starch, etc., and particularly graphite, may be preferably employed.

In the present embodiments, it is important to obtain granules (or powder) mixture suitable for tableting, as mentioned above. As the raw material granules to be tableted, it is required that the granules to be tableted, it is required that the granules are easily fluidized and combined with each other at the time of compression and does not adhere onto the surfaces of pounder and mortar; etc., but these requirements are notably inhibited by the addition of the inorganic fibrous materials. Namely, in the presence of the fibrous material, while the granules become larger in their volume, their fluidity becomes inferior, the amount thereof fed to the mortar varies, and in some case, the fibrous material and the active ingredients separate from each other, resulting in the reduction of the catalyst strength.

Thus, according to the present embodiments, water is added to the mixture of the active ingredients and intimately mixed by a kneader in a pendular state (a half-dry, loose state), followed by adding the fibrous material and continuing intimate mixing. The fibrous material may be in advance cut to a suitable length such as 5 to 50 mm, but longer fibres than the above length are gradually cut by the above-mentioned intimate mixing treatment. Successively water is added and kneading is continued so as to bring the intimate mixture into a funicular to a capillary state (a sticky state). The intimate mixture obtained by the above treatment is made up into granules (or powder) according to a conventional procedure, that is, extrusion molding, drying, grinding, addition of a lubricant and sizing treatment, followed by tableting.

In the present specification, the terms "pendular state", "Funicular state" and "capilary state" are defined as shown in table 1.

TABLE 1

Water content (Wt.%) 0 100% Solid Continous Continous Continous Discontinous Discontinous (Particles) Liquid Discontinous Continous Continous Continous Continous (Water) Gas Continous Continous Continous 0 0 (Air) F-1 F-1 Packing state Pendular Funicular Capillary Slurry As shown in Table 1, a pendular state means that a solid phase and a gas phase in the mixture are continuous or contacted each other, respectively, and a liquid phase is discontinuous forming a ring-like liquid block. A Funicular state means that a solid phase and a liquid phase are continuous, and a gas phase is continuous (F-l) or Discontinuous (F-ll).In the F-ll state, a gas phase becomes discontinuous and air buffles are formed in the mixture. A capillary state means that a gas state disappears (zero in Table 1) and a solid state is discontinuous, while a liquid state is continuous.

According to the present embodiments, water content in the pendular state of the mixture has been found to be in the range of 20% to 30%, preferably 25% to 30% by weight of the catalysts raw materials, and water content in the funicular to capillary state of the mixture be in the range of about 33% to 38% by weight of the catalyst materials.

The present invention will be further concretely described by way of Examples and Comparative examples.

EXAMPLE 1 A titanium oxide slurry (1000 g, TiO2: 30% by weight) prepared from titanyl sulfate and ammonium metavanadate (23 g) were introduced into a hot kneader of 1 I capacity and intimately mixed to obtain a half-dry, loose intimate mixture. The water content of the resulting mixture was 27% by weight of the catalyst materials. To this mixture was added 48 g of a silica-alumina as an inorganic fibrous material (50/50%) having a diameter of 1 ym, followed by intimate mixing for 30 minutes.

Water was then added to bring them into a sticky state, followed by further intimate mixing for one hour. The water content of the resulting intimate mixture was about 35% by weight of the original catalyst materials. The mixture was then extruded into a rod form of 5 mm in diameter, which was next dried at 1 1 OOC, ground and sized into 60 meshes pass. To the resulting material was added 10 g of graphite as a lubricant, followed by blending to prepare granules for tableting, which were then press molded by a tablet machine into a cylindrical form of 6 mm in diameter and 6 mm in height to obtain a tableted catalyst. This catalyst is named "catalyst A". In addition, the tableting pressure was 7,000 kg/cm2 in this Example, when the crushing strength of the catalyst was adjusted to 1 5 kg/granule.

EXAMPLES 2 and 3 A tableted catalyst was prepared in the same manner as in Example 1 except that ammonium molybdate (35 g) or ammonium paratungstate (52 g) was substituted for ammonium metavanadate, and the inorganic fibrous material was added in an amount of 49 g for the former or 52 g for the latter.

The catalysts thus obtained are named "catalyst B" and "catalyst C", respectively.

EXAMPLES 4~6 Tableted catalysts were prepared in the same manner as in Example 1 except that, to the titanium oxide slurry (1.000 g) and ammonium metavanadate (24 g) were added ammonium molybdate (37 g), ammonium paratungstate (55 g) or tin oxalate (43 g) and the inorganic fibrous material (52 g, 55 g or 53 g in the respective cases of the latter three). The catalysts thus prepared are named "catalyst D", "catalyst E" or "catalyst F", respectively.

EXAMPLE 7~10 Tableted catalysts were prepared in the same manner as in Example 1 except that, to the titanium oxide slurry (1,000 g), ammonium metavanadate (20 g), ammonium paratungstate (58 g) and tin oxalate (33 g) was added the inorganic fibrous material in varied amounts of 4 g, 1 9 g, 58 g or 116 g.

The catalysts thus prepared are named "catalyst G", "catalyst H", "catalyst I" or "catalyst J", respectively.

EXAMPLE 11 A tableted catalyst was prepared in the same manner as in Exampled 7~10 except that ammonium molybdate (39 g) was added in place of ammonium paratungstate and the inorganic fibrous material was employed in an amount of 58 g. The catalyst thus obtained is named "catalyst K".

Comparative examples 1~5 The respective catalyst preparations of Example 1 (Ti-V system), Examples 2 and 3 (Ti-Mo and Ti W systems), Examples 7~10 (Ti-V-W-Sn system) and Example 11 (Ti-V-Mo-Sn) were repeated except that no inorganic fibrous material was added in any of these examples. The catalysts thus obtained are named "catalyst L", "catalyst M", "catalyst N", "catalyst 0" and "catalyst P", respectively.

Experimental example 1 Each 100 g were taken from the respective catalysts A to P obtained in Examples 1 1 and comparative examples 1~5 and subjected to a calcination treatment in an electric oven at 5000C for 2 hours to activate the catalysts.

Experimental example 2 The calcination-treated catalysts AP obtained in Experimental example 1 were subjected to measurements of the crushing strength and apparent density of granules. Further, the granules were ground and the diameters of the granules were sized into 10 meshes pass to 20 meshes on to measure their pore volume and average pore diameter.

Experimental example 3 The respective granules of the calcined catalysts AP obtained in Experimental example 1 were ground and their diameters were sized into 10 meshes pass to 20 meshes on, followed by measuring their activity of reduction of nitrogen monooxide with ammonia, i.e. their catalyst activity, under the conditions shown in Table 2. Further, after lapse of 100 hours, they were subjected to regeneration treatment by heating at 3500C for 3 hours to measure the percentage NO removal.

TABLE 2

Experimental Experimental Item Example 3 Example 4 1. Composition of inlet gas NO (ppm) 200 200 502 (ppm) 500 500 SO3 (ppm) 50 2 CO2 (%) 12 12 03 (%) 3 3 H20 (%) 12 12 2. Amount of NH, injected Ratio to NO: Ratio to NO: 1.0 1.0 3. Reaction temperature ("C) 300 250 5. Diameter of reaction tube 30 30 (mm) 6. Amount of catalyst packed 50 50 (ml) 7. Space velocity (hi) l 20,000 10,000 Experimental example 4 This example was carried out in the same manner as in Experimental example 3 under the conditions shown in Table 2 except that the reaction temperature was varied from 3000C to 2500 C.

The results obtained in Experimental examples 2~4 are shown in Table 3 (1) and Table 3 (2).

TABLE 3 (1)

Component Experimental Example 2 Amount Physical properties of 0 silica alumina Crushing Pore Average o E Active added strength volume pore radius a, x Co Co elements (%) (kg) (cc Ig) (A) A Ti V 15 6.5 0.401 730 B Ti-Mo 15 9.0 0.350 590 C Ti-W 15 8.2 0.411 680 D Ti-V-Mo 15 7.5 0.378 620 E Ti-V-W 15 8.1 0.435 690 F F Ti-V-Sn 15 9.2 0.399 630 Cd X G Ti-V-W-Sn 1 4.2 0.299 380 w H Ti-V-W-Sn 5 6.5 0.386 620 Ti-V-W-Sn 15 8.9 0.460 700 J Ti-V-W-Sn 30 12.0 0.475 820 K K Ti-V-Mo-Sn 15 8.8 0.392 630 L Ti-V 0 1.5 0.265 300 . M Ti-Mo 0 2.2 0.201 250 x N N Ti-W 0 2.4 0.212 280 Cd E O Ti-V-W-Sn 0 2.1 0.277 320 0 0 P Ti-V-Mo-Sn 0 2.3 0.195 220 TABLE 3 (2)

1 Experimental Example 3 Experimental Example 4 Percentage NO removal at 300-C ( /0) Percentage WO removal at 250 ( /0) O Just after Just after I reaction After After reaction After After " initiation 100 h regeneration initiation 100 h regeneration A 96.8 93.5 96.7 95.1 92.5 94.8 A 96.8 93.5 96.7 95.1 92.5 94.8 B 74.7 73.5 74.7 85.7 81.5 83.2 C 70.8 69.5 61.0 80.4 76.9 80.0 D 95.5 94.4 95.0 97.3 93.2 96.5 E E 97.2 95.4 97.4 98.1 94.5 97.9 0) E F 98.9 96.2 98.4 95.9 94.4 93.9 x W G 91.0 85.3 90.0 92.8 87.8 91.5 H 95.2 92.1 94.0 94.2 92.7 94.0 l 99.5 98.5 99.2 97.2 96.0 97.3 J 1 98.9 98.0 98.4 98.5 97.9 98.4 K 95.4 93.2 94.7 96.4 94.4 96.0 L 88.0 79.6 75.2 90.3 72.2 84.5 M M 58.0 40.5 51.7 57.2 34.5 44.0 w as N 53.0 36.4 40.0 50.9 33.1 40.6 o O 90.2 80.3 82.2 91.8 80.3 82.2 0 P 89.5 75.6 78.2 88.1 74 9 77.7 As seen from the results of these Tables, the catalysts AK of Examples are improved in the crushing strength and increased in the pore volume as compared with the catalysts LP of Comparative examples. Further, it is also observed that even under the severe conditions of a SO, content of 50 ppm and a reaction temperature of 3000C, the catalysts prepared according to the present invention are high in the activity, small in the activity reduction with the lapse of time and notably improved in the catalyst performance. This applies also to Experimental example 4.

Thus, according to the present invention wherein an inorganic fibrous material is added to the catalyst raw materials and they are stepwise intimately mixed, it is possible to prepare a catalyst for removing nitrogen oxides, which has a large average pore diameter, a superior activity in the lower temperature region and a practical strength.

Claims (5)

1. A tableted catalyst useful for removing nitrogen oxides from exhaust gases, prepared by the steps which comprises: kneading at least one catalyst compound raw material containing at least one metal component selected from the group consisting of titanium, vanadium, molybdenum, tungsten, tin, chromium, manganese and iron with the presence of water in a pendular state; then adding an inorganic fibrous material to the mixture and further intimately mixing them together; further adding water to the resulting mixture and intimately mixing them together so that the resulting mixture is brought into a funicular state or a capillary state; and subjecting the mixture to a procedure of extrusion molding, drying, grinding, sizing and tableting.

2. A catalyst according to claim 1 comprising titanium as its base and at least one metal member selected from the group consisting of vanadium, molybdenum, tungsten, tin, chromium, manganese and iron, and added to the base.

3. A catalyst according to claim 1 wherein said inorganic fibrous material is at least one member selected from the group consisting of silica, silica-alumina, alumina and inorganic glasses.

4. A catalyst according to claim 1 wherein water content of the catalyst mixture is 20% to 30% by weight of the catalyst raw materials in said pendular state, and 33% to 38% by weight of the catalyst raw materials in said funicular to capillary state.

5. A catalyst substantially as hereinbefore described with reference to any one of the examples of the invention.