WO1998052681A1

WO1998052681A1 - Method for removing nitrogen oxides using natural manganese ores

Info

Publication number: WO1998052681A1
Application number: PCT/KR1998/000123
Authority: WO
Inventors: Sung Ho Hong; Hyun Young Chang; Seok Joo Hong; Dong Won Kim; Sung Chang Hong; Tae Sung Park; Dong Sup Doh
Original assignee: Korea Power Engineering Company, Inc.
Priority date: 1997-05-17
Filing date: 1998-05-16
Publication date: 1998-11-26
Also published as: DE19882409T1; DE19882409B3; JP3826167B2; CN1108850C; CN1256640A; JP2002508701A; KR970058770A

Abstract

There is disclosed a method for removing nitrogen oxides of exhaust gas using natural manganese ores. In the method, ammonia is used as a reductant to selectively reduce the nitrogen oxides in the presence of a catalyst prepared from the natural manganese ores. The catalyst allows nitrogen oxides to be completely removed from exhaust gas at a relatively low temperature of 130-250 °C without discharging unreacted ammonia. The catalyst is superior in economical terms in addition to preventing the deleterious effects which occur when discharging ammonia.

Description

METHOD FOR REMOVING NITROGEN OXIDES USING NATURAL MANGANESE ORES

Technical Field

The present invention relates, in general, to a method for removing nitrogen oxides (hereinafter referred to as "NOx") and, more particularly, to the use of natural manganese ores as a catalyst for selectively reducing the NOx contained in exhaust gas.

Background Art Many techniques have been suggested to remove the NOx contained in the exhaust gas from a source, such as a burner, a boiler, etc. Of them, selective catalytic reduction (SCR) techniques are now evaluated to be the most preferable in economic and technical aspects and extensive studies are being made on the topic of technique. In such an SRC technique, NOx, such as nitrogen monoxide and nitrogen dioxide, is reduced to nitrogen and water in the presence of a catalyst with ammonia serving as a reductant, as seen in the following reaction formulas I to IV:

6NO + 4NH₃ - 5N₂ + 6H₂0 (I)

4NO + 4NH₃ + 0₂ - 4N₂ + 6H₂0 (II)

6N0₂ + 8NH₃ - 7N₂ + 12H₂0 (III)

2N0₂ + 4NH₃ + 0₂ - 3N₂ + 6H₂0 (IV)

Whether the SRC techniques are successfully performed or not is dependent on the catalyst.

The catalysts used in the SRC technique have a common feature of being higher in the conversion rate of NOx as the reaction temperature increases. The temperatures at which the conversion rate of the NOx reaches the maximum are different with catalyst type and it is the inherent property of the catalysts, each. At high temperatures, however, ammonia is apt to oxidize by the reaction with the oxygen contained in the exhaust gas, to lose its function as a reductant, as shown in the following reaction formulas V and VI:

4NH₃ + 50₂ - 4N0 + 6H₂0 (V) 4NH₃ + 30₂ - 2N₂ + 6H₂0 (VI)

Various ingredients which may considerably affect the SRC technique, are present in most of the exhaust gas containing NOx. For instance, oxygen, moisture, sulfur oxides have a critical influence on the catalyst activity. In addition, a part of the ammonia which is supplied to remove the NOx may remain unreacted, producing pollution of the environment when it is discharged together with the exhaust gas. In this case, the supply amount of the ammonia must be controlled or the unreacted ammonia must be treated through oxidation before being discharged into the air.

There is a variety in the catalysts for use in SRC. In the case of precious metal catalysts, they are reported to be labile to the poison of sulfur dioxide so that almost all of their catalytic activities are lost within 40 min after reaction initiation (Foley, J.M., Katzer, J.R. and Monogue, W.H. : Ind. Eng. Che . Prod. Res. Dev., 18, 170 (1979)). As for V₂0₅ catalysts, they are generally impregnated in Siθ₂, A1₂0₃, or Ti0₂ and are reported to show far superior selective catalytic reaction effects at about 300 ^CC (Barten, H. , Janssen, F.J.J.G., Van den Kerkhof, F.M.G., Leferink, R. , Vogt, E.T.C., Van Diller, A.J. and Geus, J.W. : "Preparation on Catalysis IV" (B. Delmon, P. Grange, P.A. , Jacobs and G. Poncelet Eds.), Elsevier, Amsterdam, 103 (1987)). It is reported that zeolite catalysts, commonly impregnated in Cr, Fe or Cu salt, exhibit excellent performance in removing NOx at a wide temperature range of up to about 500 °C (Karlesson, H.T. and Rosenberg, H.S.: Ind. Eng. Che . Prod. Res. Dev., 23, (1984)). As in above, extensive effort and arduous labor have been made on the preparation of the catalysts for removing NOx.

For manganese catalysts, U.S. Pat. No. 3,975,498 to Miyazaki, Kazuhide, T. discloses that electrolytic manganese dioxide is utilized to remove NOx by adsorption.

U.S. Pat. No. 4,883,647 discloses the use of manganese nodules in removing at least one of the pollutants contained in exhaust gas. Like natural manganese ores, manganese nodules comprise Fe, Mn, Si, Ca and P. However, manganese nodules are quite different from natural manganese ores in the state of manganese. That is, manganese nodules comprise 15-30 weight% of manganese and a trace amount of Pt, Ni, Co, Cu, Ti, and Pb and manganese is present as crystals while, in manganese ores, manganese is present as oxides. Also, manganese nodules and natural manganese ores are different from each other in occurrence state, production area, manganese content and physical properties. The chemical composition and physical properties of manganese nodules are given as shown in Table 1, below.

TABLE 1

U.S. Pat. No. 4,883,647 to Kainer, H. , Grimm, D. , Schnelle, W. , and Halbach, P. also discloses the use of manganese nodules in removing NOx with ammonia serving as a reductant. This patent presents the data that the conversion rate of NOx is 30-50 % at a temperature range of 250-350 °C. However, the conversion rate is too low while the treatment temperature range is too high.

Disclosure of the invention

The intensive and thorough research on the selective removal of the NOx contained in exhaust gas, repeated by the inventors, resulted in the finding that natural manganese ores show excellent catalytic activity in reducing NOx at low temperatures without further subjecting the ores to difficult and costly processing.

It is therefore an object of the present invention to overcome the above problems encountered in prior arts and to provide a method for removing the NOx contained in exhaust gas, with which the NOx contained in exhaust gas is reduced at relatively low temperatures at an excellent efficiency. It is another object of the present invention to provide a SCR method for removing the NOx contained in exhaust gas, which is economically favorable.

In accordance with the present invention, the above objects could be accomplished by a provision of a method for removing the nitrogen oxides of exhaust gas, in which a selective catalytic reduction technique using ammonia as a reductant is carried out in the presence of a catalyst prepared from natural manganese ores.

Brief Description of the Drawings The above and other objects and aspects of the invention will become apparent from the following description of embodiments with reference to the accompanying drawings in which:

Fig. 1 shows the conversion rate of the NOx contained in exhaust gas with regard to temperature when the exhaust gas is treated with ammonia in the presence of a catalyst prepared from natural manganese ores according to Example I, and shows the discharge amounts of nitrogen dioxide and ammonia in the treated exhaust gas

Fig. 2 shows the NOx conversion rates plotted with regard to the 0₂ concentration of exhaust gas at different temperatures (175 and 200 °C) when using the catalyst prepared in Example I; Fig. 3 shows NOx conversion rates by space velocity

(GHSV) with regard to temperature when using the catalyst prepared in Example I;

Fig. 4 shows the conversion rate of the NOx contained in exhaust gas with regard to temperature when the exhaust gas is treated with ammonia in the presence of a catalyst prepared from natural manganese ores according to Example II, and shows the discharge amount of ammonia in the treated exhaust gas;

Fig. 5 is a plot showing NOx conversion rate changes with temperature according to the concentrations of natural manganese ore components which the catalysts of Example III have; and

Fig. 6 shows the change of NOx conversion rate with regard to NH₃/NO molar ratios when using a catalyst prepared in Example IV.

Best Modes for Carrying Out the Invention

In accordance with the present invention, exhaust gas is deprived of NOx in the presence of natural manganese ores. Serving as a catalyst, the natural manganese ores have an average chemical composition and physical properties, as listed in Tables 2 and 3, below.

TABLE 2 Average Chemical Composition of Natural Manganese Ores

expressed in a total amount of the O^ combined with Mn and Fe because Mn and Fe each coexist as in MnO> , Mn.O,, Mn,0₄, FeOj and Fe,0,ι so that their individual compositions are difficult to describe. Its amounts to 80 wt.% or more when being calculated on the basis of Mn0₂.

It should be noted that the expression "natural manganese ores" as used herein, means the manganese ores which are found in mineral deposits on the earth's surface. As seen in Table 2, natural manganese ores consist mainly of the oxides of Mn, Fe, Ca, Mg, Al and Si with the most abundance in Mn. In natural manganese ores, 80 weight % or more of the Mn oxides are of pyrolusite (Mn0₂) .

TABLE 3 Average Physical Properties of Natural Manganese Ores

The data of Table 2 show that natural manganese ores contain various metal oxides known to catalytically function in SCR in addition to Mn and Fe, so that they can be used as a catalyst for SCR.

A mix gas of NOx, ammonia and oxygen was introduced into a reactor (e.g. fixed bed reactor) in which natural manganese ores were used as a catalyst, and the conversion rate of NOx was observed, demonstrating that it is at a considerably low temperature (about 150 °C) that the natural manganese ores have the maximal conversion rate of NOx and that it is in a considerably wide temperature range (about 130-250 °C) that the natural manganese ores can maintain 90% or higher of their maximal conversion rate. Therefore, the use of the natural manganese ores brings about a significant, economical profit because the exhaust gas needs not be heated to high temperatures in order to carry out the SRC technique. In addition, the wide range of the temperatures at which the natural manganese ores can treat NOx, allows them to be applied to various process conditions.

It is preferable that the concentration ratio of ammonia to NOx ranges from 0.7 to 1.2 in the presence of the catalyst of the present invention. For example, if too low concentration ratio is used, the catalyst's acitivity is expressed in too low efficiencies. On the other hand, if the concentration ratio exceeds 1.2, an increased amount of the catalyst is needed to prevent NH₃ from remaining unreacted and thus, it is economically unfavorable.

In accordance with the present invention, natural manganese ores are pulverized into particles of a homogeneous size in order to enhance their catalytic activity by virtue of the surface area thus increased. The size is determined by the use type of the catalyst. For instance, where natural manganese ores are applied to a honeycomb structure, they are finely powdered to an average size of 1 μm or less. If the average size of the powder is over 1 μm, it is hard to slurrify the powder, which thus makes it almost impossible to coat the powder on the honeycomb structure. Alternatively, the natural manganese ores may be crashed to granular sizes if the resulting granules are proper enough to play the role of catalyst as they are filled in a reactor. In this case, the crashed natural manganese ores are required to be completely dehydrated at, for example, 103 °C in order that side-reactions are prevented while the catalyst works.

A detailed description will be given of the application procedure of natural manganese ores to a honeycomb structure, below.

First, natural manganese ores are finely powdered to an average size of 1 μm or less by using a mill.

Then, the powder is added in distilled water and mixed together, to give a solution. Preferably, the amount of the powder ranges from about 20 to 50 weight % based on the weight of the water. For example, if the amount of the powder is below 20 weight % relative to the weight of the distilled water, a coating work to be performed later, is not finished in a short time. On the other hand, a concentration higher than 50 weight % results in a solution which is too viscous to coat therewith.

Thereafter, the solution is adjusted to pH 6.5-6.8 by an acid with stirring. Illustrative, non-limiting, examples of the acid available include sulfuric acid, hydrochloric acid, nitric acid and acetic acid with preference to nitric acid. A pH value less than 6.5 causes the fine particles to aggregate together and precipitate. On the other hand, if the pH value exceeds 8.5, there occurs an ionic action among the fine particles of the solution, which hinders the solution from being coated.

A binder is added at an amount of about 1-5 weight parts based on 100 weight parts of the solution. The binder may be selected from the group consisting of methoxymethyl cellulose (MC) , polyvinyl alcohol (PVA) , carboxymethyl cellulose (CMC) , polyethylene glycol (PEG) , silica sol, alumina sol and the mixtures thereof.

Next, a honeycomb structure, commercially available, is immersed in the solution for 2-3 hours and dried at room temperature. A further step of drying at 103 °c for 4-6 hours is very helpful in preventing side- reactions while the -resulting honeycomb is used as a catalyst. Subsequently, the honeycomb structure is baked at 350-500 °C for 4-8 hours in an electric furnace, to coat the natural manganese ore powder thereon. A better understanding of the present invention may be obtained in light of the following examples which are set forth to illustrate, but are not to be construed to limit the present invention.

EXAMPLE I

Natural manganese ores were tested for the catalytic activity in reducing NOx. To this end, natural manganese ores were crashed into particles ranging, in size, from 40 to 50 mesh (average particle size 0.359 mm) , followed by filling the particles to a volume of 3 ml in an fixed bed reactor with an inner diameter of 8 mm. NOx was provided at a concentration of 690 ppm with the provision of ammonia at an amount 1.12 times as much as that of the NOx. The catalyst layer allowed gas to pass through itself at a space velocity (GHSV) of 20,000 h^"1.

Referring to Fig. 1, there are graphed the data of Example 1. The data show that the catalyst crashed from the natural manganese ores has a conversion rate of near 100 % from about 150 °C. This testifies manganese oxides serve as a catalyst superior in reducing NOx. Also, the data show that the temperature range at which the conversion rate of the manganese ores is kept at 100 %, is wide, from 150 to 250 °C. In addition, in this temperature range, the excess ammonia was found to be completely oxidized without remaining unreacted. This is believed to result from the low-temperature reduction capacity of manganese oxides and the influence from other metal oxides contained in the ores or the synergy effect therebetween. Therefore, it is not unreasonable that, where natural manganese ores are used as a catalyst for SCR, at ^"least 90 % of NOx can be completely removed at a temperature of 130-250 °C. Natural manganese ores, which are newly recognized as a low- temperature catalyst in accordance with the present invention, are preferably used at a temperature of 130- 220 °C for converting NOx.

Referring to Fig. 2, there is shown the influence of 0; concentration on the catalyst of Example I's conversion rate for NOx at predetermined temperatures (175 and 200 ^CC) . In order to investigate the influence, 430 ppm was given for the concentration of NOx, 1.13-folds higher concentration for ammonia, and 50,000 h^"1 for the space velocity in the catalyst bed. The data of Fig. 2 show that an oxygen concentration as high as or higher than 0.5 % has no influence on the conversion rate. Because the oxygen concentration in exhaust gas is, on average, 1 % or more, the catalyst according to the present invention can put all of its full catalytic capacity in the reduction of NOx, regardless of the oxygen concentration.

With reference to Fig. 3, there are plotted NOx conversion rates by space velocity (GHSV) with regard to temperature. To this end, the concentration of oxygen was 3 % and NOx was provided at a concentration of 430 ppm with the provision of ammonia at an amount of 1.13 times as much as that of the NOx. As shown in the plot, the catalyst according to the present invention expresses high efficiencies at relatively low temperatures even at a space velocity as high as 70,000 h^"1. Therefore, the catalyst according to the present invention is not so much affected by space velocity.

EXAMPLE II

Using a honeycomb structure which was coated with finely powdered natural manganese ores, an SCR technique was carried out to remove NOx. For coating the ^"honeycomb structure with the powder, first, natural manganese ores were pulverized into a fine powder with an average particle size of 1 μm or less. The powder was added to 1000 g of water to give a 30 weight % solution. The solution was adjusted into about pH 7 with nitric acid while stirring the solution, followed by adding 30 g of methyl cellulose (MC) to the solution. A honeycomb structure, preferably made from cordierite, was immersed in the solution for about 3 hours, dried at room temperature and then, at about 103 °C for about 5 hours, and baked at 400 °C for 6 hours in an electric furnace.

Before carrying out the SCR technique, the prepared honeycomb structure was inserted in a conical type honeycomb reactor with a size of 5 cm in diameter. In this experiment, oxygen was provided at a concentration of 3 %, NOx at a concentration of 420 ppm, and ammonia at a concentration 1.10 times as much as that of NOx. The ratio of diameter to height of the honeycomb structure was 0.75.

With reference to Fig. 4, the catalytic activity of the honeycomb structure-supported catalyst according to the present invention is shown in terms of NOx conversion rate and NH₃ discharge. As apparent from the data of Fig. 4, the honeycomb structure-supported catalyst according to the present invention removes NOx in high efficiencies at low temperatures and does not allow ammonia to be discharged.

EXAMPLE III

The procedure of Example II was repeated, except that the natural manganese ores powder was added at an amount of 30 weight %, 40 weight % and 47 weight % to water and that the honeycomb structure used was 13 mm high with a ratio of diameter to height being 0.25. Referring to Fig. 5, the NOx conversion rates according to the concentration of the catalytically active ingredient-containing solution to be coated on the honeycomb structure, are plotted with regard to temperature. As shown in Fig. 5, the natural manganese ore components must be above a certain concentration in the solution in order to maintain the catalytic activity of the honeycomb-supported catalyst high. This says a certain amount of the natural manganese ores' catalytically active ingredients which are coated on the honeycomb structure, are needed in removing NOx at high efficiencies. In fact, the conversion rate was measured to increase 2-3 % every coating round to a certain round number (about 5 rounds) .

EXAMPLE IV

Crashed natural manganese ores with an average particle size of 359 μm were filled in the same fixed bed reactor as that of Example I and this reactor was used to measure the change of NOx conversion rate with regard to NH₃/N0 molar ratios under the conditions that NO was flowed in the reactor at a concentration of 440 ppm with 3 % oxygen and the reaction temperature was 200 °C. The results are given as shown in Fig. 6.

The data of Fig. 6 show that the NOx conversion rate is almost linearly proportional to the molar ratio of NHj/NO until the molar ratio reaches 1:1 and that NH₃ reacts with NO at a molar ratio of almost 1:1. The NOx conversion rate of the catalyst starts to increase slightly at a molar ratio of 0.7 and reaches 100 % at a molar ratio of 1:1. Therefore, an optimal condition concerning the molar ratio of NH₃ to NO ranges from 0.7 to 1.2.

Industrial Applicability

In the presence of a catalyst according to the present invention, prepared from natural manganese ores, as described hereinbefore, an SCR process using ammonia as a reductant allows NOx to be completely removed from exhaust gas at a relatively low temperature of 130-250 °C without discharging unreacted ammonia. Therefore, the catalyst according to the present invention exhibits a superior catalytic activity in converting the NOx of exhaust gas even at a relatively low temperature range and is superior in economical terms as well as prevents the deleterious effects which occur when discharging ammonia.

The present invention has been described in an illustrative manner, and it is to be understood the terminology used is intended to be in the nature of description rather than of limitation. Many modifications and variations of the present invention are possible in light of the above teachings. Therefore, it is to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

Claims

1. A method for removing the nitrogen oxides of exhaust gas, in which a selective catalytic reduction technique using ammonia as a reductant is carried out in the presence of a catalyst prepared from natural manganese ores.

2. The method as set forth in claim 1, wherein said catalyst is a mass of the crashed natural manganese ore particles which are filled in a reactor, or a mass of the natural manganese ore powders with an average size of 1 ╬╝m or less which are coated on a honeycomb structure.

3. The method as set forth in claim 2 , wherein said honeycomb structure is immersed in an aqueous solution which comprises said powders at an amount of about 20-50 weight % and is adjusted into pH 6.5-8.5 with an acid.

4. The method as set forth in claim 1, wherein said selective catalytic reduction technique is carried out at a temperature of 130-250 ┬░C.

5. The method as set forth in claim 1, wherein said ammonia is fed at a molar ratio of 0.7-1.2 relative to the moles of said nitrogen oxides.