JP5702172B2 - Activated alumina catalyst and method for removing nitrous oxide - Google Patents

Description

  The present invention relates to an activated alumina catalyst and a nitrous oxide removal method.

Fluidized bed combustion using coal, heavy oil, petroleum coke, biomass, industrial waste or the like as fuel is a low temperature of about 800-850 ° C. Therefore, nitrogen oxides such as nitrogen monoxide (hereinafter sometimes referred to as “NO”) and nitrogen dioxide (hereinafter sometimes referred to as “NO ₂ ”), which are polluted gases produced under high-temperature combustion conditions. The amount of nitrous oxide (hereinafter sometimes referred to as “N ₂ O”) with a high global warming potential is large, which causes global warming. Therefore, there is a need to develop a reduction measure.

On the other hand, a selective reduction reaction using a reducing agent that has been well known as a denitration technique for automobile exhaust gas and combustion furnace exhaust gas can efficiently reduce nitrogen oxides at a low temperature of 500 ° C. or lower. However, since this technique requires an external denitrator, it cannot be applied to an existing fluidized bed combustion furnace that does not have an external denitrator. The selective reduction reaction is a technique in which nitrogen oxides are reduced by a reducing agent such as unburned hydrocarbon or ammonia additive and converted into nitrogen, oxygen, water, and the like. In a fluidized bed combustion furnace, unlike automobile exhaust gas, there is almost no unburned hydrocarbon, and even when a reducing agent such as ammonia is added at the combustion temperature of the fluidized bed combustion furnace, oxidation of the reducing agent proceeds more selectively. Therefore, the nitrogen oxide removal reaction does not proceed. Furthermore, it is generally known that noble metal catalysts such as Pd and Pt are used to remove nitrogen oxides in automobile exhaust gas, and that extremely high catalytic performance is exhibited. However, in a fluidized bed combustion furnace, since the catalyst itself is used as a fluid medium, it is not a good idea to use a high-cost noble metal catalyst.
Therefore, there is a need for a technique that efficiently removes nitrogen compounds using a reducing agent that has no reducing agent or that has low reactivity even in the combustion conditions of a fluidized bed combustion furnace.

As a method of reducing N ₂ O contained in the combustion gas discharged from the combustion furnace, N ₂ O decomposition by a catalyst (see, for example, Patent Documents 1 and 2), N ₂ O adsorption by active coke, auxiliary fuel gas Methods such as partial heating by blowing into a combustion device are known.
Patent Document 1 describes an alumina catalyst that allows the N ₂ O removal reaction to proceed even in an atmosphere in which no reducing agent is present. Patent Document 2 describes a method of decomposing N ₂ O in a combustion furnace by charging activated alumina or a mixture of activated alumina and a metal compound into the combustion furnace.

Japanese Patent Laid-Open No. 6-327973 JP-A-6-123406

However, the direct decomposition reaction for decomposing nitrogen oxides without a reducing agent described in Patent Document 1 has a slow reaction rate and a large amount of catalyst to be used. In order to reduce the cost, it is necessary to further increase the activity of the catalyst and reduce the amount of catalyst used. Further, in the method described in Patent Document 1, since N ₂ O is removed using a denitration catalyst outside the furnace, it is necessary to significantly modify the existing combustion apparatus, which requires a large amount of cost. is there.

On the other hand, in the method described in Patent Document 2, a comparison is made with α-alumina, which has no activity as a normal catalyst, but knowledge of what kind of alumina is effective for N ₂ O decomposition has not been obtained. Absent.
Generally, an alumina catalyst is subjected to thermal aggregation sequentially by being exposed to a high temperature for a long time. As described in Patent Document 2, when α-alumina, which is considered to be produced by thermal aggregation, is used, N ₂ O decomposition performance is not shown at all. Although rapid phase change of alumina does not occur at the combustion gas temperature of the fluidized bed boiler, it is considered that phase change to θ type and α type proceeds if it stays in the boiler for a long time. As is well known, there are trace amounts of many inorganic metals in coal, and the deactivation of the catalyst by these trace elements is expected. In order to compensate for the performance deterioration due to the catalyst deactivation, it is necessary to sequentially replace the alumina catalyst, which is a fluidized medium, in the furnace, increasing the amount and labor of the catalyst and increasing the cost. For this reason, it is desired to reduce the amount of alumina catalyst necessary for N ₂ O removal by improving the performance of the alumina catalyst itself.

  Therefore, an object of the present invention is to provide an activated alumina catalyst and a nitrous oxide removal method capable of efficiently removing nitrous oxide contained in combustion gas at low cost.

As a result of intensive studies to achieve the above object, the present inventors have found that an activated alumina catalyst containing a specific activated alumina and a trace amount of a base metal is highly activated, thereby completing the present invention. It was.
That is, the activated alumina catalyst of the present invention is an activated alumina catalyst that removes nitrous oxide in combustion gas, and includes activated alumina , and the activated alumina includes κ, δ, γ, η, χ, ρ, and any type of boehmite, or is composed of mixtures thereof, further unrealized 1.0 wt% 0.1 wt% of base metal, either Ru (ruthenium), Rh (rhodium), and Ir (iridium) Is not included .
Further, the activated alumina catalyst of the present invention, the base metal, Cu (copper), Fe (iron), and Ru at least any one Tanedea selected from among Ni (nickel).

On the other hand, the nitrous oxide removing method of the present invention is characterized in that the nitrous oxide is removed by bringing the activated alumina catalyst into contact with a combustion gas containing nitrous oxide in a fluidized bed combustion furnace.
And in the nitrous oxide removal method of this invention, it is preferable to control the furnace temperature of the said fluidized bed combustion furnace to 550 degreeC or more and 900 degrees C or less.
Furthermore, in the nitrous oxide removal method of the present invention, the activated alumina catalyst is preferably filled as a fluid medium in the fluidized bed combustion furnace.

  According to the present invention, when the activated alumina catalyst contains specific activated alumina and a base metal, the activated alumina catalyst is highly activated, and nitrous oxide in the combustion gas can be removed with high efficiency. That is, a highly active activated alumina catalyst can be obtained simply by adding an inexpensive base metal to activated alumina, so that the catalyst can be produced at a low cost. Further, since nitrous oxide can be removed with high efficiency, the amount of the activated alumina catalyst used can be suppressed. Furthermore, an existing fluidized bed combustion furnace can be used as the combustion device. Therefore, nitrous oxide contained in the combustion gas can be efficiently removed at low cost.

Schematic of the experimental apparatus for implementing the nitrous oxide removal method of this invention.

Hereinafter, embodiments for carrying out the present invention will be described in detail.
[Activated alumina catalyst]
The activated alumina catalyst of this embodiment contacts N ₂ O contained in the combustion gas generated in the fluidized bed combustion furnace to remove N ₂ O from the combustion gas.
The activated alumina catalyst of the present embodiment contains, as activated alumina, any one of κ, δ, γ, η, χ, ρ, and boehmite type, or a mixture thereof, and further contains 0.1% by mass of a base metal. The content is preferably 20% by mass or less.

The activated alumina preferably has a purity of 95% by mass or more, and more preferably 98% by mass or more. If the purity is less than 95% by mass, the removal performance of N ₂ O may deteriorate. Activated alumina has impurities such as sodium oxide (hereinafter abbreviated as “Na ₂ O”), sulfur trioxide (hereinafter abbreviated as “SO ₃ ”), silicon dioxide, titanium dioxide, iron trioxide (Fe ₂ O). ₃ ) Contains calcium oxide, magnesium oxide, chlorine and the like.

Here, the Na ₂ O content is 0.05 mass% or less than 0.01 mass%. When the content of Na ₂ O is less than 0.01% by mass, it takes time to clean the activated alumina, which may make it difficult to produce activated alumina efficiently. Moreover, even if the content of Na ₂ O is less than 0.01% by mass, the N ₂ O removal performance may not be improved. On the other hand, when the content of Na ₂ O exceeds 0.05 mass%, the removal performance of N ₂ O decreases.

Na ₂ O as an impurity should be reduced by acid cleaning with bauxite, sodium aluminate in the process of obtaining alumina from bauxite, and aluminum itself such as alumina itself with acid (hydrochloric acid, sulfuric acid, nitric acid), etc. Can do.
For example, an alumina-containing slurry (a slurry concentration of 50% by mass or less) in which alumina and dilute hydrochloric acid are mixed is placed in a container equipped with a stirrer, stirred for 1 to 24 hours, and solid content obtained by solid-liquid separation by filtration is sufficient. Wash with water. Washing is stopped when the pH of the water after washing is approximately the same as that of water before washing.

In addition, when bauxite is treated with a strong alkali other than sodium hydroxide, such as calcium hydroxide, calcium aluminate can be obtained, and activated alumina having a very low Na ₂ O content can also be obtained.
Furthermore, aluminum alkoxide may be hydrolyzed as a method for producing activated alumina substantially completely free of Na ₂ O.

The content of SO ₃ as an impurity is 1% by mass or less, and more preferably 0.01% by mass or less.
When the content of SO ₃ exceeds 1% by mass, N ₂ O removal performance decreases. The content of SO ₃ was 0.01% by mass or less is usually the detection limit of the fluorescent X-ray analysis to be used for the detection of SO ₃ (XRF), ensures the _{N 2} O If this less This is because the removal performance is improved.
As in the case of Na ₂ O, considering the alumina cleaning efficiency, the lower limit of SO ₃ is 0.001% by mass, and even if it is less than 0.001% by mass, the N ₂ O removal performance is This is because it is not considered that it will improve dramatically.
It should be noted that impurities other than Na ₂ O and SO ₃ do not significantly reduce N ₂ O removal performance even when the content is large.

Activated alumina is classified into α-alumina, γ-alumina, and boehmite when classified by crystal form. When γ-alumina is classified in detail, it can be divided into seven types: κ, θ, δ, γ, η, χ, and ρ.
Here, when the activated alumina is α-alumina, θ-alumina, or a mixture of α-alumina and θ-alumina, N ₂ O removal performance is low, which is not preferable. Therefore, the activated alumina is preferably composed of any one of κ, δ, γ, η, χ, ρ, and boehmite type, or a mixture thereof.
Further, when the activated alumina is composed of any one of χ, boehmite type, or a mixture thereof, if the content of Na ₂ O is large, the content of Na ₂ O is decreased by 0.01 by washing with hydrochloric acid. It can be easily reduced to less than mass%.

The activated alumina has a pore volume of 0.2 cm ³ / g or more and 1.0 cm ³ / g or less, preferably 0.3 cm ³ / g or more and 0.8 cm ³ / g or less. When the pore volume is 0.2 cm ³ / g or more and 1.0 cm ³ / g or less, activated alumina has a good N ₂ O removal performance even when the fluidized bed combustion furnace is about 550 ° C.

The activated alumina preferably has a specific surface area of 100 m ² / g or more and 300 m ² / g or less. Outside this range, the strong active sites of alumina considered to be acid sites disappear and N ₂ O removal performance decreases, or the catalyst strength decreases too much, making it very difficult to manufacture. .

In the present embodiment, as the base metal, it is preferable to use at least one selected from Cu (copper), Fe (iron), and Ni (nickel), and it is particularly preferable to use Cu.
There are few hydrocarbons with 2 or more carbons useful as reducing agents in coal combustion exhaust gas, but there are reducing agents with low reactivity such as methane and carbon monoxide (hereinafter sometimes referred to as CO). there's a possibility that. The activated alumina catalyst containing any one of Fe, Ni, and Cu has a large N ₂ O removal performance as compared with the case where activated alumina is used alone even when CO having low reactivity is used as a reducing agent. In addition, the activated alumina catalyst containing Cu or Ni has higher N ₂ O removal performance than activated alumina alone under the condition that no reducing agent is used, and particularly the activated alumina catalyst containing Cu has higher N ₂ O removal performance.
The base metal content in the activated alumina catalyst is preferably 0.1% by mass or more and 20% by mass or less, and more preferably 0.5% by mass or more and 5% by mass or less. When the base metal content is less than 0.1% by mass, the activity improvement effect due to the addition of the metal is small, and the performance may not be commensurate with an increase in the catalyst cost of adding the metal. Moreover, since the performance as a catalyst has reached its peak, if it exceeds 20 mass%, there is a possibility that performance improvement according to the increased cost of the metal amount cannot be expected.

  In the present embodiment, the activated alumina catalyst is obtained by loading a base metal on activated alumina, but the loading method is not particularly limited, and a normal impregnation method, coprecipitation method, kneading method, or the like can be used. . Of these, the impregnation method is particularly preferable.

The activated alumina catalyst having the above-described configuration is preferably used in a fluidized bed combustion furnace. The fluidized bed combustion furnace includes a fluidized bed portion filled with an activated alumina catalyst and a fuel as a fluidized medium, and an air inflow portion disposed below the fluidized bed portion. When air flows from the air inflow portion into the fluidized bed portion, the activated alumina catalyst becomes fluidized together with the fuel.
Examples of the fluidized bed combustion furnace include a normal pressure type, a pressure type, a bubbling type, and a circulation type.

[Nitrous oxide removal method]
Next, the nitrous oxide removal method of this embodiment will be described.
First, an activated alumina catalyst and fuel are filled as a fluidized medium in a fluidized bed combustion furnace. After filling, air is introduced from the lower part of the fluidized bed combustion furnace, and the activated alumina catalyst and the fuel are caused to flow to be in a combustion state. At this time, the temperature in the fluidized bed combustion furnace is preferably controlled to 550 ° C. or more and 900 ° C. or less, more preferably 600 ° C. or more and 900 ° C. or less by the temperature control unit of the fluidized bed combustion furnace. In this temperature range, includes the N _{2 O} in the combustion gases, activated alumina catalyst, by contacting the N 2 _O, can be removed N 2 _O. If the combustion temperature is less than 550 ° C, incomplete combustion of the fuel or the combustion energy inherent in the fuel may not be used efficiently, and if it exceeds 900 ° C, a large amount of NOx may be generated. If the temperature is controlled within the above temperature range, the risk of increasing the amount of N ₂ O that is a powerful warming gas increases, but the risk can be remarkably reduced by using the N ₂ O removal method of the present invention. The utility value of becomes very high.

  EXAMPLES Hereinafter, although an Example and a comparative example are given and this invention is demonstrated more concretely, this invention is not limited to the content, such as an Example, at all.

(Preparation of activated alumina catalyst)
In Examples 1 to 5 and Comparative Examples 2 to 3, γ-alumina (AKP-G015, manufactured by Sumitomo Chemical Co., Ltd.) was supported at a predetermined ratio by an ordinary impregnation method, and an activated alumina catalyst was used. Was prepared. As Comparative Example 1, the aforementioned γ-alumina was used alone as an active alumina catalyst.
The impurity amount of γ-alumina was Fe: 4 ppm, Si: 2 ppm, Na: 2 ppm, Mg: 1 ppm, Cu: 1 ppm or less, and the specific surface area was 148 m ² / g.

The preparation of the activated alumina catalyst will be described in detail.
The activated alumina catalyst was prepared by the impregnation (evaporation to dryness) method using the metal salts shown below. The metal salt was dissolved in pure water, impregnated with alumina, and then evaporated under vacuum using an evaporator at room temperature. Thereafter, it was dried at 120 ° C. for 24 hours and then calcined in air at 500 ° C. for 4 hours. The catalyst powder was sized to 30-42 mesh and then calcined in air at 800 ° C. for 4 hours for use in the reaction.
<Metal salt>
Ag (NO ₃ ) manufactured by Kanto Chemical
Co (CH ₃ COO) ₂・ 4H ₂ O Made by Kishida Chemical
Cu (CH ₃ COO) ₂・ H ₂ O Made by Kishida Chemical
Fe (NO ₃ ) ・ 9H ₂ O Kanto Chemical
Ni (CH ₃ COO) ₂ / 4H ₂ O Made by Kanto Chemical

(Configuration of experimental equipment)
As shown in FIG. 1, the experimental apparatus 1 includes gas storage units 11a to 11d that store N ₂ O gas, O ₂ gas, He gas, CO gas, and C ₂ H ₄ gas, respectively, and gas storage units 11a. To 11d, a quartz tube 13 having an inner diameter of 6 mm, to which the gas introduction tube 12 is connected to the bottom, and a gas discharge tube 14 connected to the top of the quartz tube 13. Further, an electric furnace 15 that surrounds the periphery of the quartz tube 13 and heats the activated alumina catalyst is provided at a substantially central portion in the axial direction of the quartz tube 13. Gas flow meters 121a to 121d are arranged in the gas introduction pipes 12 connected to the gas storage parts 11a to 11d, respectively. A gas chromatogram 141 is arranged, and a thermocouple 152 connected to a thermometer 151 is connected to a substantially central portion of the quartz tube 13.

The alumina tube 21 was formed by filling the quartz tube 13 of the experimental apparatus 1 with the activated alumina catalyst according to the example and the comparative example so that the filling length was 21 mm (inner diameter 6.1 mm). The filling amount of the activated alumina catalyst was 0.4 g, and the particle size was 30 to 42 mesh (355 to 500 μm).
A quartz wool layer 22 a was formed on the upper end of the alumina layer 21. Similarly, a quartz wool layer 22 b was formed on the lower end of the alumina layer 21. The alumina layer 21 is held and fixed by these quartz wool layers 22a and 22b.

(experimental method)
Diluted with helium, oxygen concentration 2%, N ₂ O concentration 1000 ppm, and gas introduction pipes via gas flow meters 121a-121d so that the total flow rate of all gases is 130 mL / min (normal temperature, normal pressure) Each gas was supplied from 12 to the bottom of the quartz tube 13. When CO was used as the reducing agent, the CO concentration was 1000 ppm, and when C ₂ H ₄ was used as the reducing agent, the C ₂ H ₄ concentration was 500 ppm. Then, N ₂ O gas was passed through the alumina layer 21 while changing the temperature of the alumina layer 21 with the electric furnace 15. The exhaust gas discharged from the top of the quartz tube 13 to the gas exhaust tube 14 was guided to the gas chromatogram 141, where the N ₂ O concentration in the exhaust gas was analyzed to measure the N ₂ O removal rate for each activated alumina catalyst. . The measurement results are shown in Table 1.

As can be seen from Table 1, in the Cu, Fe, and Ni-containing catalysts from Example 1 to Example 5, the N ₂ O decomposition performance is improved by using a reducing agent as compared with the alumina of Comparative Example 1 alone. On the other hand, it was confirmed that the performance of the Cu-containing catalysts of Example 1 and Example 2 greatly increased under the condition without a reducing agent. It was confirmed that the Fe and Ni-containing catalysts of Examples 3 to 5 were almost the same or slightly improved at 550 ° C. and improved at 600 ° C. or higher.
On the other hand, the Co-containing catalyst of Comparative Example 2 and the Ag-containing catalyst of Comparative Example 3 have lower N ₂ O decomposition performance than the alumina alone of Comparative Example 1 and Examples 1 to 5 regardless of the presence or absence of a reducing agent. That is, according to the present invention, it can be seen that in Examples 1 to 5, N ₂ O can be effectively removed in a fluidized bed combustion furnace regardless of the presence or absence of a reducing agent. In particular, the Cu-containing catalysts of Example 1 and Example 2 are effective.

The present invention can be used in a method for removing N ₂ O using an activated alumina catalyst. In particular, it can be used as a method for removing N ₂ O in combustion gas in a fluidized bed combustion furnace.

DESCRIPTION OF SYMBOLS 1 Experimental apparatus 11a, 11b, 11c, 11d Gas storage part 12 Gas introduction pipe 13 Quartz tube 14 Gas discharge pipe 15 Electric furnace 21 Alumina layer 141 Gas chromatogram

Claims (4)

An activated alumina catalyst for removing nitrous oxide in combustion gas,
Containing activated alumina ,
The activated alumina is composed of any one of κ, δ, γ, η, χ, ρ, and boehmite type, or a mixture thereof.
Further below viewing containing 1.0 wt% 0.1 wt% base metals,
Does not include any of Ru (ruthenium), Rh (rhodium), and Ir (iridium),
The activated alumina catalyst , wherein the base metal is at least one selected from Cu (copper), Fe (iron), and Ni (nickel) . A method for removing nitrous oxide, comprising bringing the activated alumina catalyst according to claim 1 and a combustion gas containing nitrous oxide into contact with each other in a fluidized bed combustion furnace. The nitrous oxide removal method according to claim 2 ,
A method for removing nitrous oxide, wherein the temperature in the fluidized bed combustion furnace is controlled to 550 ° C or higher and 900 ° C or lower. In the nitrous oxide removal method according to claim 2 or 3 ,
The method for removing nitrous oxide, wherein the activated alumina catalyst is filled as a fluidized medium in the fluidized bed combustion furnace.

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