JP3029311B2 - Method and apparatus for removing nitrogen oxides from mixed gas - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates, upon obtaining a high purity carbon dioxide from a mixed gas containing at least carbon monoxide and nitrogen oxides, NO contained in the mixed gas, NO nitrogen oxides such as ₂ (hereinafter NOx And a device therefor. More specifically, the present invention relates to a method and an apparatus for reducing, decomposing and removing NOx present in a mixed gas obtained by collecting and concentrating CO ₂ contained in combustion exhaust gas or the like by a catalytic reaction.

[0002]

2. Description of the Related Art In a mixed gas mainly composed of CO ₂ ,
N _2, O ₂ as the other components, CO, NO, there NO ₂ etc. a small amount, also often contain sulfur compounds such as SO ₂ or SO _3, H ₂ S traces. Among these various components, it is particularly difficult to remove NO and NO ₂ , which causes a reduction in the purity of the carbon dioxide product. Conventionally, as a method for removing NOx, (1) a wet cleaning method, (2) an adsorption method, and (3) a high-temperature catalytic method are known. (1) is a method in which a gas is washed with a solution of an oxidizing substance such as a KMnO ₄ solution to oxidize contained NO to NO ₂ , and the generated NO ₂ is absorbed and removed in an absorbing solution. (2) NO is converted to NO ₂ by an adsorbent supporting an oxidizing agent.
It oxidizes to NO and adsorbs and removes NO ₂ . (3) is to pass gas through a catalyst maintained at a high temperature of 200 ° C. or higher to decompose and remove NOx.

[0003]

In the wet cleaning method, the handling of the cleaning liquid is not easy, the operation management is complicated, and the denitration rate is at most about 80%, so that a sufficient effect cannot be obtained. . The adsorption method is easy to handle, but the adsorption capacity is not sufficient and the denitration rate is not high. Therefore, the treatment of a gas containing a high concentration of NOx requires a large amount of adsorbent, which is not economically advantageous. The high-temperature catalyst method has a high degree of risk due to high temperature, and if the treatment is of a large capacity, energy consumption for heating is increased, which is economically disadvantageous. In view of such a situation, the present inventors have variously studied a method for removing NOx, which is an undesirable impurity, in producing CO ₂ with a higher purity than a mixed gas containing at least CO and NOx. The method of 3) for decomposing and removing NOx by the catalyst was further studied.

Generally, in order to decompose and remove NOx in a mixed gas by a catalyst, a high temperature of 200 ° C. or more is required.
At lower temperatures, it is said that NOx decomposition cannot be performed substantially. However, when the decomposition of NOx at a high temperature, not only the energy consumption is increased, because NH ₃ as by-products are produced, depending on the application of CO ₂ is
Another new problem arises.

[0005] JP-A-1-290517 discloses that in producing high-purity liquefied carbon dioxide, a CO ₂ gas containing NO is subjected to a deoxygenation treatment using a catalyst such as platinum before liquefaction to produce NO. There is disclosed a method of suppressing oxidation to NO ₂ and removing NO as NO. Also, JP-A 1-29
No. 0518 discloses that CO ₂ gas containing NO is oxidized with NO ₂ using a noble metal catalyst to oxidize NO before liquefaction.
A pretreatment method of a raw material gas in liquefied carbonic acid production in which NO ₂ is formed and then adsorbed and removed is disclosed. However, these methods are fundamentally different from the decomposition removal of NOx of the present invention.

[0006]

According to the present invention, at least C
A method for removing NOx from a mixed gas containing O and NOx, comprising a step of oxidizing CO using a catalyst in the presence of oxygen and a step of reducing and decomposing NOx using a catalyst in the presence of hydrogen. NOx characterized by comprising two steps
It is intended to provide a removal method and an apparatus used for performing the method.

[0007] If CO is contained in the mixed gas containing NOx, the catalyst is temporarily poisoned by CO during the reduction catalytic reaction of NOx, and the catalytic activity is reduced. In such a case, it is necessary to increase the temperature of the reduction reaction or to remove CO before the reduction decomposition reaction of NOx. However, when the reaction is carried out at a high temperature, it is difficult not only to maintain a stable effect of the catalyst but also to generate NH ₃ which is a side reaction, which is not preferable. Therefore, in the present invention, NOx
CO is removed prior to the reductive decomposition reaction of. In this way, the process is divided into two, CO is selectively oxidized by the catalytic reaction of the former stage, and poisoning of the catalyst of the latter stage is prevented by removing CO, and the NOx reduction by the catalytic reaction at a lower temperature in the latter stage The decomposition reaction can be performed stably and efficiently.

The mixed gas used in the present invention is not particularly limited as long as it is a mixed gas containing CO ₂ as a main component, for example, a gas obtained by collecting and condensing combustion exhaust gas such as coal, oil and natural gas. Rather, the CO ₂ concentration is suitably used in the range of usually 60% or more, preferably 90% or more. If the mixed gas contains sulfur compounds such as SO ₂ or SO ₃ , H ₂ S, etc., these will hinder the catalytic reaction of the present invention, and must be removed in advance.

Although there are various methods for removing CO, from the viewpoint of reducing and decomposing and removing NOx in the next step, CO is selectively oxidized by a catalytic reaction to reduce CO _2.
Is the most desirable method. The catalyst is not particularly limited, but is used for general oxidation reaction,
For example, platinum, palladium, rhodium, ruthenium, gold, silver and the like of a white metal element are mentioned, and platinum and palladium are particularly preferable. There is no problem if a sufficient amount of O ₂ is present in the mixed gas for CO oxidation, but if substantially no O ₂ is contained, the necessary amount of O ₂ is added to the mixed gas. Amount of O _2, compared CO1 molar contained a suitable amount is 0.5 to 1.5 mol. No more O ₂
Addition of, undesirable since it must of H ₂ was added in large amounts in the later stage.

The reductive decomposition of NOx is carried out in the presence of H ₂ using a noble metal catalyst similar to that used for the oxidation of CO. There is no problem when a sufficient amount of H ₂ for reduction is present in the mixed gas, but when there is no sufficient amount of H ₂ for reduction in the mixed gas or when the amount is insufficient,
H ₂ is added and sent to the catalyst tank while maintaining the reducing atmosphere. The amount of H ₂ required for the reduction depends on the amount of O ₂ and NO present in the gas.
It is determined by the amount of x. The amount for O ₂ 0.5
２2 moles, and 3-7 moles relative to NOx. If the amount of H ₂ is too small, the removal of NOx will not proceed efficiently, and if too much is used, the purity of the product CO ₂ will decrease.

Thus, in carrying out the method of the present invention, first, the mixed gas is heated by a heater to 100 ° C. to 200 ° C., which is the temperature required for the CO oxidation reaction.
It is sent to the preceding catalyst tank. At temperatures below 100 ° C,
At a temperature higher than 200 ° C., the oxidation of CO becomes incomplete and energy consumption only increases, and a corresponding effect cannot be expected. As described above, for example, a catalyst carrying a noble metal such as platinum or palladium is used as a catalyst in the former catalyst tank. In particular, a palladium catalyst is preferably used to achieve the purpose at a low temperature. In such a catalyst tank, CO reacts with O ₂ to become CO ₂ , and CO in the mixed gas is removed.

The mixed gas that has exited the preceding catalyst tank is cooled by a cooler to a temperature of 40.degree.
Cool down to ° C. At a temperature lower than 40 ° C., decomposition and removal of NOx become incomplete, and at a temperature higher than 100 ° C., a side reaction may occur, which is not preferable. 40 ° C-100
The mixed gas maintained at ℃ is added with H ₂ and sent to a subsequent catalyst tank in a reducing atmosphere. In the subsequent catalyst tank, NO
x is removed by a reductive decomposition reaction at a relatively low temperature.
At this time, NH ₃ as a by-product is hardly generated, and the denitration rate can be 99% or more. As a result, it becomes possible to obtain high-purity CO ₂ substantially containing no NOx.

Hereinafter, an apparatus used for carrying out the method of the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a flow sheet of one embodiment of an apparatus for performing the method of the present invention. In this device, a mixed gas containing at least CO and NOx is supplied to the compressor 1 at a pressure higher than the atmospheric pressure and 3 kg / c.
m ² or less, O _{2 of} 0.5 to 1.5 times the molar amount of CO was added from the oxygen supply device 7,
After being heated to 00 to 200 ° C., it is passed through the oxidation tank 6. The oxidation tank 6 is filled with a catalyst supporting platinum or palladium, and CO in the mixed gas is oxidized to CO ₂ . After that, the mixed gas exiting the oxidation tank 6 is cooled by the cooler 5 for 4 hours.
It is cooled to 0 to 100 ° C., supplied with H ₂ from the hydrogen supply device 4, and led to the reduction tank 3. The reduction tank 3 is filled with a catalyst supporting platinum or palladium,
x undergoes a reduction reaction and is decomposed into N ₂ and H ₂ O. In the device of the present invention, a compressor, a heater, a cooler,
As the hydrogen supply device, the oxygen supply device, the oxidation tank, and the reduction tank, known ones can be appropriately combined and used depending on the processing amount of the mixed gas and the like.

[0014]

EXAMPLES The present invention will be described in more detail with reference to the following Examples, but it should not be construed that the invention is limited thereto. Examples 1 to 3 A cylindrical cylinder made of stainless steel having an inner diameter of 40 mmφ × 250 mmL was filled with 200 cc of a catalyst supporting palladium on alumina to form a catalyst tank for oxidizing CO, and a catalyst of 200 cc supporting platinum on alumina in a container of the same type.
Using a test device which was filled and used as a NOx reduction / decomposition catalyst tank, a gas having the following composition was used as a raw material, and a gas flow rate of 1 m
^A NOx removal test was performed under the conditions of ³ / hr, a CO oxidation temperature of 150 ° C., a NOx reduction decomposition temperature of 55 ° C., and a predetermined amount of H ₂ added. _{CO 2: 97~99%, N 2} : 1~3%, CO: 10~150ppm, O 2: 150~300ppm, NOx: 24~48ppm results are shown in Table 1.

[Table 1]

Example 4 Using the same apparatus as in Example 1, CO ₂ : 98-99%,
_{N 2: 1~2%, CO:} 200ppm, NOx: 48p
pm as a raw material, and O
Example 3 except that ₂ was added to be 250 ppm.
A NOx removal test was performed in the same manner as described above. The gas concentration at the outlet of the latter catalyst tank is as follows: CO: 1 ppm or less, NOx:
0.1 ppm, NH ₃ : not detected.

Comparative Examples 1 to 3 Using the same apparatus as in Example 1, the same mixed gas as in Example 1 was used, except that the preceding CO oxidation step was omitted.
The reduction and decomposition reaction of NOx was performed in the same manner as described above. The reaction temperature was 55 ° C., 110 ° C., and 150 ° C., and the concentration of NOx and NH ₃ in the gas at the outlet of the catalyst tank after passing the gas through the catalyst tank for 5 minutes was measured for each. The results are shown in Table 2.

[Table 2]

[0017]

According to the method of the present invention, NOx in a mixed gas containing at least CO and NOx is catalytically reduced and removed at a relatively low temperature of 40 ° C. to 100 ° C. using a noble metal catalyst such as platinum or palladium. Is possible,
CO is previously oxidized with a noble metal catalyst such as platinum or palladium to form harmless CO _2, and then NOx is reduced to 4%.
A system that skillfully combines oxidation and reduction, in which catalytic reduction is carried out at a relatively low temperature of 0 to 100 ° C. to remove and decompose, has made it possible to obtain a mixed gas substantially free of NOx.

[Brief description of the drawings]

FIG. 1 is a flow sheet of an apparatus embodying the present invention.

[Explanation of symbols]

1: compressor, 2: heater, 3: reduction tank, 4:
Hydrogen supply device, 5: cooler, 6: oxidation tank, 7: oxygen supply device

────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor: Osamu Oseto, 346-1 Miyanishi, Harima-cho, Kako-gun, Hyogo Sumitomo Seika Co., Ltd. (72) Inventor: Yu Uno 1-346, Miyanishi, Harima-cho, Kako-gun, Hyogo Sumitomo (72) Inventor Fumio Arai 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Kenji Matsuzaki 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Takamasa Takahashi 4-1-1, Kudankita, Chiyoda-ku, Tokyo (Heiei Kudankita Building) Inside Adchemco Co., Ltd. (56) References JP-A 64-80430 (JP, A) JP-A Sho 59 -213425 (JP, A) JP-A-51-63363 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C01B 31/20 B01D 53/86 B01D 53/94

Claims (2)

(57) [Claims] 1. A method for removing nitrogen oxides from a mixed gas mainly composed of CO ₂ containing at least carbon monoxide and nitrogen oxides, wherein the nitrogen oxides are removed at 100 ° C. using a platinum or palladium catalyst in the presence of oxygen. A process of oxidizing carbon monoxide at ~ 200 ° C and a process of reductively decomposing nitrogen oxides at 40 ° C to 100 ° C using a platinum or palladium catalyst in the presence of hydrogen. A method for removing nitrogen oxides to obtain high-purity carbon dioxide. 2. A compressor and a heater for adjusting a mixed gas mainly composed of CO ₂ containing at least carbon monoxide and nitrogen oxide to a predetermined pressure and temperature, and a heater heated to 100 ° C. to 200 ° C. by the heater. An oxygen supply device for supplying oxygen to the mixed gas, an oxidation tank for oxidizing carbon monoxide in the mixed gas supplied with oxygen at 100 ° C. to 200 ° C. with a platinum or palladium catalyst, and a mixed gas exiting the oxidation tank A cooler for cooling to 40 ° C to 100 ° C;
A hydrogen supply device for supplying hydrogen to the cooled mixed gas,
A reduction tank for reducing and decomposing nitrogen oxides in the mixed gas supplied with hydrogen with a platinum or palladium catalyst at 40 ° C to 100 ° C, to remove nitrogen oxides for obtaining high-purity carbon dioxide gas. apparatus.