KR20050023688A - Hybrid catalyst for removing NOx and method for removing NOx using the same - Google Patents

Abstract

PURPOSE: To provide a NOx removing hybrid catalyst of which unit price is inexpensive, and which has excellent denitrification performance even under an exhaust gas containing sulfur dioxide, and a method for removing NOx using the NOx removing hybrid catalyst. CONSTITUTION: A hybrid catalyst for removing NOx is characterized in that the hybrid catalyst is prepared by mixing a commercial catalyst with natural manganese sulfur oxides obtained by pre-treating natural manganese ore with sulfur dioxide gas or pre-treating the natural manganese ore with a sulfuric acid solution, wherein the commercial catalyst is a vanadium-based catalyst, wherein an optimum activation temperature range of the hybrid catalyst is from 300 to 350 deg.C, wherein a mixing mass ratio of the natural manganese sulfur oxides to the vanadium-based commercial catalyst is 7:3 to 1:9, wherein the hybrid catalyst is a two-stage catalyst in which a natural manganese sulfur oxide catalyst layer and a vanadium-based commercial catalyst layer individually exist, and wherein a weight ratio of the natural manganese sulfur oxides to the vanadium-based commercial catalyst is 8:2 to 1:9. In a method for removing nitrogen oxides(NOx) in exhaust gas by a selective catalyst reduction method, a method for removing NOx comprises the process of reacting ammonia that is a reducing agent with the nitrogen oxides(NOx) using a hybrid catalyst prepared by mixing a commercial catalyst with natural manganese sulfur oxides obtained by pre-treating natural manganese ore with sulfur dioxide gas or pre-treating the natural manganese ore with a sulfuric acid solution.

Description

Hybrid catalyst for removing NOx and method for removing NOx using same {Hybrid catalyst for removing NOx and method for removing NOx using the same}

The present invention relates to a composite catalyst for removing nitrogen oxides, and more particularly, to a composite catalyst for removing nitrogen oxides that can be used for exhaust gas containing sulfur dioxide and a method for removing nitrogen oxides using the same.

      Industrial facilities that use fossil fuels, such as power plants and chemical plants, inevitably produce nitrogen oxides (NOx). These are commonly known as substances formed by the combination of nitrogen and oxygen, which combine with water vapor in the atmosphere to cause acid rain. Not only does it generate water, but it is known to cause global warming. Therefore, countries including Korea are strictly prohibited from releasing more than a certain level of nitrogen oxides from industrial facilities through various laws, and accordingly, methods for removing nitrogen oxides from exhaust gases emitted from combustion systems have been studied.

In order to remove nitrogen oxides, it is necessary to improve combustion conditions such as low oxygen combustion and exhaust gas circulation, but since these methods alone cannot completely remove nitrogen oxides, a process of removing nitrogen oxides by post-treatment of exhaust gases has been proposed. The most representative of such post-treatment techniques is Selective Catalytic Reduction (SCR), which uses ammonia as the reducing agent and converts nitrogen monoxide and nitrogen dioxide into nitrogen and steam by using a catalyst. Titania, alumina, silica, zirconia, etc. are used as the carrier of the catalyst, and oxides such as vanadium, molybdenum, nickel, tungsten, iron, and copper are used as active metals. In general, the optimum temperature range is different depending on the type of catalyst, the supply ratio of nitrogen oxide and ammonia is also different. Representative catalysts used in the selective catalytic reduction method may be V ₂ O ₅ / TiO ₂ form, but the vanadium-based catalyst has a disadvantage that the price is very expensive, a method using natural manganese ore to replace this proposed It became.

Korean Patent No. 275301 discloses a method of removing nitrogen oxides by reacting at 150-250 ° C. using natural manganese ore having ammonia as a reducing agent and β-MnO ₂ pyrrolusite as a main crystal as a catalyst. However, in the case of natural manganese ore, since ammonia is oxidized by the reaction shown in Scheme 1 at about 240 ° C., ammonia is oxidized by itself in addition to participating in the nitrogen oxide removal reaction. As a result, the consumption of ammonia increases, which is disadvantageous.

4NH ₃ + 5O ₂ → 4NO + 6H ₂ O

Accordingly, when the molar ratio of ammonia / nitrogen oxide is lowered to 1.0 or less in order to reduce ammonia consumption, nitrogen oxide is unreacted and desorbed on the surface of the natural manganese ore catalyst, leading to increased production of nitrogen dioxide, resulting in yellow fume. There is a problem. In addition, when sulfur dioxide (SO ₂ ) is present in the exhaust gas, it is immediately converted to MnSO ₄ , and FIG. 4 shows the conversion of nitrogen dioxide according to the temperature of MnSO ₄ . As shown in FIG. 4, since the denitrification performance is very low in the temperature range of 150 ° C. to 250 ° C., the MnSO ₄ may not be used as a catalyst, and for this reason, the MnSO ₄ may be used only when there is no sulfur dioxide in the exhaust gas.

Accordingly, the first technical problem to be achieved by the present invention is to provide a composite catalyst for removing nitrogen oxides, which is inexpensive and has excellent denitrification performance even under exhaust gas containing sulfur dioxide.

The second technical problem to be achieved by the present invention is to provide a method for removing nitrogen oxide using the complex catalyst.

The present invention to achieve the first technical problem,

The present invention provides a composite catalyst for removing nitrogen oxides in which a natural manganese sulfur oxide obtained by pretreating natural manganese ore with sulfur dioxide gas or a sulfuric acid solution and a commercial catalyst are mixed.

According to one embodiment of the present invention, the commercial catalyst is preferably a vanadium-based catalyst.

In addition, the optimum active region of the composite catalyst is preferably 300 to 350 ℃.

In addition, the mixed mass ratio of the natural manganese sulfur oxide and the vanadium-based commercial catalyst is preferably 7: 3 to 1: 9.

According to another embodiment of the present invention, the composite catalyst may be a two-stage catalyst in which the natural manganese sulfur oxide catalyst layer and the vanadium-based commercial catalyst layer are present separately.

The mass ratio of the natural manganese sulfur oxide catalyst layer and the vanadium-based commercial catalyst layer is preferably 8: 2 to 1: 9.

The present invention to achieve the second technical problem,

In the method for removing nitrogen oxide in the exhaust gas by selective catalytic reduction method, using a complex catalyst obtained by pretreatment of natural manganese ore with sulfur dioxide gas or a mixture of natural manganese sulfur oxide and a commercial catalyst obtained by pretreatment with sulfuric acid solution, It provides a method for removing nitrogen oxides, characterized in that for reacting ammonia and nitrogen oxides as a reducing agent.

According to one embodiment of the present invention, the commercial catalyst is preferably a vanadium-based catalyst.

In addition, the optimum active region of the composite catalyst is preferably 300 to 350 ℃.

In addition, the mixed mass ratio of the natural manganese sulfur oxide and the vanadium-based commercial catalyst is preferably 7: 3 to 1: 9.

According to another embodiment of the present invention, the composite catalyst may be a two-stage catalyst in which the natural manganese sulfur oxide catalyst layer and the vanadium-based commercial catalyst layer are present separately.

The mass ratio of the natural manganese sulfur oxide catalyst layer and the vanadium-based commercial catalyst layer is preferably 8: 2 to 1: 9.

Hereinafter, the present invention will be described in more detail.

The composite catalyst according to the present invention is characterized by using a natural manganese sulfur oxide catalyst mixed with a common commercial catalyst, or arranged in the front end of the commercial catalyst layer.

In general, natural manganese ore is a complex metal oxide whose main component is in the form of manganese oxide, but a small amount of metal oxides such as Fe _x O _y , CaO, MgO. Typically NMO is divided as such is the nature of the ore other _{α-MnO 2 (Psilomelane group)} , β-MnO 2 (Pyrolusite) in accordance with the crystal structure of MnO _2, this because of its excellent in the β-MnO ₂ denitration In the present invention, it is preferable to use natural manganese ore having pyrrolusite as the main crystal structure. Tables 1 and 2 show chemical compositions and average physical properties of natural manganese ores analyzed by ICP.

element Mg Ca Al Mn Fe O balance weight% 0.039 0.047 1.881 51.94 4.033 42.009

Density (g / cm ₃ ) 4.122 Surface area (m ₂ / g) 6.231 Void volume (cm ₃ / g) 0.253 Average particle size (㎛) 100 or less

As mentioned above, natural manganese ore can be used directly in the selective reduction reaction at a low temperature of 150 ~ 250 ℃ because of its excellent denitrification capacity, but is rapidly poisoned by sulfur dioxide and loses activity in the above temperature range. However, natural manganese sulfur oxides of the MnSO ₄ form shows excellent denitrification activity in the temperature range of 300 ~ 350 ℃, the temperature range is similar to the operating temperature range of the vanadium-based commercial catalyst, even without the presence of sulfur dioxide It has the advantage of showing constant denitrification performance. Therefore, when the natural manganese sulfur oxide is used in combination with a vanadium-based commercial catalyst, it is economical because it can drastically reduce the amount of the vanadium-based commercial catalyst required for treating the same concentration and the same amount of nitrogen oxide. Since the proportion of the catalyst which is not affected by sulfur dioxide in the amount of the catalyst is increased by that much, the poisoning concern by sulfur dioxide is reduced.

      On the other hand, as another embodiment of the present invention, when the catalyst layer loaded with natural manganese sulfur oxide is used at the front end of the commercial catalyst layer, since the nitrogen oxide is pre-filtered before contacting the vanadium-based commercial catalyst, the latter commercial catalyst The load on the load is reduced, and thus the effect of improving the removal efficiency of the nitrogen oxides is generated.

      Figure 4 shows the denitrification conversion according to the reaction temperature of the natural manganese sulfur oxide catalyst used in the present invention. As can be seen in Figure 4 and natural manganese sulfur oxide has a denitrification conversion of 65% at 300 ~ 350 ℃, this denitrification temperature range is similar to the vanadium-based catalyst. The commercial catalyst used in the present invention is not particularly limited, such as a noble metal catalyst, a metal oxide or a zeolite used in the selective catalytic reduction method, but a vanadium-based catalyst is preferable in that the optimum active temperature range is consistent.

      The mixed mass ratio of the natural manganese sulfur oxide and the vanadium-based commercial catalyst is preferably 7: 3 to 1: 9. When the natural manganese sulfur oxide exceeds the ratio of 7: 3, denitrification performance is not sufficient, and 1: 9. If less, it is not preferable because the effect of natural manganese sulfur oxide mixing is weak.

      According to another embodiment of the present invention, the composite catalyst may be a two-stage catalyst in which the natural manganese sulfur oxide catalyst layer and the vanadium-based commercial catalyst layer are separately present, in which case the preliminary filtering is performed by the natural manganese sulfur oxide. In addition, the load on the vanadium-based commercial catalyst layer is reduced, and the denitrification efficiency as a whole is increased. As such, when the two-stage catalyst layer is used, the mass ratio of the natural manganese sulfur oxide catalyst layer and the vanadium-based commercial catalyst layer is preferably 8: 2 to 1: 9. When the natural manganese sulfur oxide layer exceeds 8: 2, the denitrification performance is poor. It is not preferable because it is not enough.

The method for producing natural manganese sulfur oxide according to the present invention is as follows. The first method is to grind the natural manganese ore using a roll mill and ball mill, sift it to the desired size, place it in a quartz reactor before denitrification, and then mix nitrogen and 2000 ppm SO ₂ gas under similar conditions as denitrification. After 60 minutes of flowing, the natural manganese ore reacts with sulfur dioxide to produce natural manganese sulfur oxides. The second method is a method of preparing a sulfur oxide by pulverizing a natural manganese ore in a sulfuric acid 0.5M 2M solution, which may be prepared by using the bentonite as an inorganic binder after drying the sulfur oxide again.

      The method for removing nitrogen oxides according to the present invention is characterized in that the complex catalyst is used in a fixed bed reactor or in the form of a honeycomb structure to react ammonia, which is a reducing agent, with nitrogen oxides.

1 shows a schematic diagram of a denitrification reaction system having a composite catalyst prepared according to the present invention in a fixed bed. The mixed gas for the denitrification reaction is a mixture of nitrogen oxides, ammonia, oxygen, sulfur dioxide and nitrogen, and each gas is controlled at an appropriate flow rate through an MFC (Mass Flow Controller) to be injected into a fixed bed reactor. Before entering the reactor, a gas preheater was passed and a syringe pump was also connected to the preheater so that steam could be injected into the mixed gas when necessary. The fixed bed was a quartz reactor capable of withstanding high temperatures, and the quartz bed was supported by a support and a denitrifier was placed thereon so that the reaction gas could pass through the fixed bed without missing the denitrifier even at a high temperature. A temperature-controlled heater was installed around the denitrifier to adjust the temperature to the denitrification reaction. After the reaction was completed, the water was completely removed through a cold trap, and NO in the gas was detected using an analyzer using chemical fluorescence.

Figure 2 shows the reactor portion of a fixed bed reaction system using a complex catalyst according to the present invention. The mixed gas containing nitrogen oxide is introduced into the lower portion, and is designed to pass through the vanadium-based catalyst layer first after passing through the natural manganese sulfur oxide catalyst. As the support, quartz wool, quartz beads, heat resistant steel mesh, or the like can be used.

On the other hand, in the case of using the composite catalyst according to the present invention in the honeycomb structure for exhaust gas is the manufacturing method as follows. 20-50% by weight of the fine powder of the natural manganese sulfur oxide is mixed with water (distilled water), followed by stirring to prepare a mixed solution. Next, one or more binders selected from the group consisting of methoxymethyl cellulose (MC), polyvinyl alcohol (PVA), carboxymethyl cellulose (CMC), polyethylene glycol (PEG), silica sol and alumina sol After adding to the mixed solution, the honeycomb structure commercially available in the mixed solution is immersed, dried, and then fired in an electric furnace to prepare a honeycomb structure coated with natural manganese ore sulfur oxide. 3 shows a honeycomb structure in which only the natural manganese sulfur oxide catalyst layer is applied to the front end and the conventional vanadium catalyst layer is applied to the rear end.

      Hereinafter, the present invention will be described in more detail with reference to preferred embodiments of the present invention, but the present invention is not limited thereto.

      Example 1

Preparation of 1- (1) Natural Manganese Sulfur Oxide

Natural manganese ore was pulverized using a roll mill and a ball mill, followed by sieving to obtain fine powder having an average particle size of 10 μm. 5 wt% of bentonite binder was added to the powder, and a small amount of moisture was added to form a dough, which was then injected using an injection machine. The denitrifier injected in the form of a cylinder was calcined at 450 ° C. for 4 hours and then pulverized to a size of 300-500 μm to put 2.4 g in a quartz reactor, and a mixture of nitrogen and 2000 ppm SO _{2 was} mixed with MFC (Mass Flow Controller). Natural manganese ore sulfur oxide was prepared by flowing for 1 hour at a flow rate of 1000 ml / min through.

Fabrication of denitrification system using 1- (2) fixed bed reactor

As a material of the fixed bed reactor, a quartz reactor capable of withstanding high temperatures was used, and the natural manganese ore sulfur oxide and vanadium oxide commercial catalyst prepared above were mixed at a mass ratio of 7: 3, and then the quartz cotton was supported in the middle of the reactor. Then put on it. The mixed gas for denitrification was NO 400 ppm, NH ₃ 400 ppm, oxygen 4%, sulfur dioxide 200 ppm, and the rest was nitrogen. Each gas was controlled at a flow rate of 1000 ml / min through MFC (Mass Flow Controller). And injected through the upper portion of the fixed bed reactor. A heater was installed around the denitrifier to adjust the temperature to 350 ° C., and the reaction gas was removed from the water through a cold trap to detect NO in the gas using an analyzer using chemical fluorescence.

       Example 2

Preparation of 2- (1) Natural Manganese Sulfur Oxide

Natural manganese ore was pulverized using a roll mill and a ball mill, followed by sieving to obtain fine powder having an average particle size of 10 μm. The fine powder was placed in a 2M sulfuric acid aqueous solution for sulfidation for 4 hours, and then put in a 110 ° C. drying furnace for 1 hour to obtain a natural manganese sulfur oxide.

Fabrication of denitrification system using 2- (2) fixed bed reactor

       A denitrification system was prepared in the same manner as in Example 1- (2).

       Example 3

       A denitrification system was prepared in the same manner as in Example 1, except that the mixed mass ratio of the natural manganese sulfur oxide and the vanadium oxide commercial catalyst was 5: 5.

       Example 4

       A denitrification system was manufactured in the same manner as in Example 1, except that the mixed mass ratio of the natural manganese sulfur oxide and the vanadium oxide commercial catalyst was 1: 9.

       Example 5

       After the natural manganese sulfur oxide was put on the quartz wool support, the quartz wool support was again installed on the upper surface, and the vanadium oxide commercial catalyst was placed on the support to prepare a two-stage catalyst layer having a mass ratio of 8: 2. Prepared a denitrification system in the same manner as in Example 1.

       Example 6

       A denitrification system was prepared in the same manner as in Example 5, except that the mass ratio of the natural manganese sulfur oxide layer and the vanadium oxide layer was 7: 3.

       Example 7

       A denitrification system was prepared in the same manner as in Example 5, except that the mass ratio of the natural manganese sulfate layer and the vanadium oxide layer was 5: 5.

       Example 8

A denitrification system was prepared in the same manner as in Example 5, except that the mass ratio of the natural manganese sulfate layer and the vanadium oxide layer was 4: 6.

       Example 9

The natural manganese sulfur oxide was made to a particle size of less than 10㎛ using a grinder and then mixed with about 30 wt% in 1000g of distilled water, and then 30g of methoxymethyl cellulose (MC) was added. A honeycomb structure made of cordierite was placed in the solution for about 3 hours, dried at room temperature, dried at 100 ° C. for 5 hours, and then fired in an electric furnace at 400 ° C. for 6 hours to prepare a honeycomb structure. Next, an experiment was performed by inserting and filling the honeycomb structure prepared above into a 5 cm honeycomb reactor of the cone type.

Comparative Example 1

A denitrification system was prepared in the same manner as in Example 1, except that the mixed mass ratio of natural manganese sulfur oxide and vanadium oxide commercial catalyst was set to 9: 1.

Comparative Example 2

A denitrification system was manufactured in the same manner as in Example 1, except that the mixed mass ratio of natural manganese sulfur oxide and vanadium oxide commercial catalyst was set to 8: 2.

Comparative Example 3

A denitrification system was manufactured in the same manner as in Example 5, except that the mass ratio of natural manganese sulfur oxide layer and vanadium oxide layer was 9: 1.

Test Example 1

Comparison of Denitrification Performance of Mixed Catalyst with Natural Manganese Sulfur Oxide and Vanadium Commercial Catalyst

400 ppm NO, 400 ppm NH ₃ , 200 ppm SO _2, 4% O ₂ , and the rest of the mixture was denitrified using a conventional vanadium catalyst (V ₂ O ₅ / TiO ₂ + WO ₃ + MoO ₃ ) 5 and 6 show a result of comparing the case where the denitrification reaction was performed using the mixed catalyst prepared according to Examples 1 and 9 of the present invention. In the conventional vanadium catalyst, the NO equilibrium concentration was 14 ppm, and in the case of the mixed catalyst, Example 1 maintained 26 ppm and Example 9 maintained 37 ppm. This is also an acceptable range for NOx emissions standards for general power plants since 2005 (less than 70 ppm for liquid fuel plants and less than 80 ppm for solid fuel plants). In the case of the mixed catalyst, it can be seen that excellent denitrification performance is maintained even though the amount of vanadium used is significantly reduced.

     Test Example 2

Comparison of Denitrification Performance by Natural Manganese Sulfur Oxide Manufacturing Method

After denitrification system was constructed using natural manganese sulfur oxide prepared using sulfur dioxide gas in Example 1- (1) and natural manganese sulfur oxide prepared using sulfuric acid in Example 2- (1), NO and NH The concentration ratio of ₃ is 1: 1 and 200 ppm of SO ₂ , 3% of oxygen, and the rest are supplied with a mixed gas containing nitrogen at 100 ml / min, and the denitrification performance is measured at 350 and 1 atm, and is shown in FIG. 7. As shown in FIG. 7, the denitrification performance of natural manganese sulfur oxide was excellent despite the introduction of high concentration of nitrogen oxide of 5000 ppm, and the initial removal rate was excellent in the case of natural manganese sulfur oxide prepared in Example 1- (1). However, it can be seen that the denitrification performance of both is similar at steady state.

      Test Example 3

Comparison of Denitrification Performance by Mixing Ratio of Natural Manganese Sulfur Oxide and Vanadium-based Commercial Catalysts

For the denitrification system according to Examples 1, 3, 4 and Comparative Example 1 2, the denitrification performance was tested using a mixed gas using NO 400 ppm, NH ₃ 400 ppm, oxygen 4%, sulfur dioxide 200 ppm and the rest using nitrogen. Each gas was controlled at a flow rate of 1000 ml / min through a mass flow controller (MFC) and injected through the upper portion of the fixed bed reactor. A heater was installed around the denitrifier to adjust the temperature to 350 ° C. After the reaction was completed, the gas was removed through a cold trap to detect NO in the gas using an analyzer using chemical fluorescence. Is shown in FIG. 8. From the results shown in FIG. 8, when the mixed mass ratio of the natural manganese sulfur oxide and the vanadium-based commercial catalyst exceeds 7: 3, it can be seen that the concentration of NO after denitrification is not more than 150 ppm.

      Test Example 4

      For the denitrification system according to Examples 5 to 8 and Comparative Example 3, the denitrification performance was tested under the same conditions as in Test Example 3, and the results are shown in FIG. 9. As a result, it can be seen that the case of undergoing the two-stage reaction is more excellent denitrification performance than when the natural manganese sulfur oxide and the vanadium-based catalyst are directly mixed.

The composite catalyst according to the present invention can be used not only for automobile exhaust but also for power plants. In particular, since the optimum active temperature range is 300 ° C or higher, it is necessary to raise the temperature again in the case of the exhaust gas which has undergone desulfurization reaction. It is advantageous because it can be adjusted.

As described above, the composite catalyst prepared according to the present invention is very inexpensive due to the low production cost, and still exhibits excellent denitrification performance while reducing the amount of conventional vanadium-based commercial catalysts. In addition, according to the nitrogen oxide removal method according to the present invention, since there is no poisoning phenomenon due to sulfur dioxide in the natural manganese sulfur oxide, as a whole of the composite catalyst, there is less concern about poisoning by sulfur dioxide.

1 is a schematic diagram of a denitrification reaction system having a composite catalyst prepared according to the present invention in a fixed bed.

2 is an enlarged view of a fixed bed reactor used in the denitrification system according to the present invention.

3 is a schematic diagram of a honeycomb structure for use in the denitrification system according to the present invention.

Figure 4 shows the NOx removal rate according to the temperature of the natural manganese sulfur oxide used in the present invention.

5 is a graph comparing the denitrification performance of the conventional vanadium-based catalyst and the composite catalyst prepared according to Example 1 of the present invention.

6 is a graph comparing the denitrification performance of the conventional vanadium catalyst and the composite catalyst prepared according to Example 9 of the present invention.

7 is a graph showing the denitrification performance of the denitrification systems according to Example 1 and Example 2. FIG.

8 is a graph showing the denitrification performance of the denitrification systems according to Examples 1, 3, and 4 and Comparative Examples 1 and 2. FIG.

9 is a graph showing the denitrification performance of the denitrification systems according to Examples 5 to 8 and Comparative Example 3. FIG.

<Description of the symbols for the main parts of the drawings>

1: Commercial catalyst 2: Support

3: natural manganese sulfur oxide catalyst 4: mixed gas

Claims (12)

 A composite catalyst for removing nitrogen oxides comprising a mixture of natural manganese sulfur oxide and a commercial catalyst obtained by pretreatment of natural manganese ore with sulfur dioxide gas or by pretreatment with sulfuric acid solution.        The complex catalyst for removing nitrogen oxides of claim 1, wherein the commercial catalyst is a vanadium-based catalyst.        The composite catalyst for removing nitrogen oxides according to claim 1, wherein the optimum active region of the composite catalyst is 300 to 350 ° C.        The composite catalyst for removing nitrogen oxides according to claim 1, wherein the mixed mass ratio of the natural manganese sulfur oxide and the vanadium-based commercial catalyst is 7: 3 to 1: 9.        The composite catalyst for removing nitrogen oxides of claim 1, wherein the composite catalyst is a two-stage catalyst in which a natural manganese sulfur oxide catalyst layer and a vanadium-based commercial catalyst layer are present separately.        The composite catalyst for removing nitrogen oxides according to claim 5, wherein the mass ratio of the natural manganese sulfur oxide catalyst layer and the vanadium-based commercial catalyst layer is 8: 2 to 1: 9.         In the method for removing nitrogen oxide in the exhaust gas by selective catalytic reduction method, using a complex catalyst obtained by pretreatment of natural manganese ore with sulfur dioxide gas or a mixture of natural manganese sulfur oxide and a commercial catalyst obtained by pretreatment with sulfuric acid solution, A method for removing nitrogen oxides, comprising reacting ammonia, which is a reducing agent, with nitrogen oxides.        8. The method of claim 7, wherein the commercial catalyst is a vanadium-based catalyst.        8. The method for removing nitrogen oxides according to claim 7, wherein the reaction temperature is 300 to 350 deg.        8. The method for removing nitrogen oxides according to claim 7, wherein the mixed mass ratio of the natural manganese sulfur oxide and the vanadium-based commercial catalyst is 7: 3 to 1: 9.        The method of claim 7, wherein the complex catalyst is a two-stage catalyst in which a natural manganese sulfur oxide catalyst layer and a vanadium-based commercial catalyst layer are present separately. The method for removing nitrogen oxides according to claim 10, wherein the mass ratio of the natural manganese sulfur oxide catalyst layer and the vanadium-based commercial catalyst layer is 8: 2 to 1: 9.