JPH0538432A - Catalyst for exhaust gas recombiner - Google Patents

Abstract

PURPOSE:To prevent the lowering of the strength of a carrier and the deterioration of the capacity of a catalyst caused by the phase transition of gamma-alumina carrier due to operation temp, an operation time or the mixing of impurity in the catalyst wherein palladium is supported on the gamma-alumina carrier for an exhaust gas recombiner. CONSTITUTION:alpha-alumina most stable among aluminas is used as the carrier of a catalyst for a recombiner. By the use of alpha-alumina, the carrier itself generates no phase change and the deterioration of the capacity of the catalyst related to that of the strength of the carrier is not generated and the catalyst keeps stable capacity over a long period of time.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalyst for an exhaust gas recombiner installed in a radioactive gas waste treatment facility of a boiling water nuclear power plant, for example.

[0002]

2. Description of the Related Art The flow of a radioactive gas waste treatment facility of a boiling water nuclear power plant is as shown in FIG. Hydrogen and oxygen gas generated by radiolysis of reactor water in the nuclear reactor 1 together with leaked air from the main condenser 2 and an air extractor 3
Is extracted from the main condenser 2 and diluted by the driving steam of the air extractor 3 to below the explosion noise limit. After that, the temperature of the exhaust gas preheater 4 is adjusted and the hydrogen and oxygen gas become steam by the catalyst in the exhaust gas recombiner 5. The steam is condensed and removed together with the driving steam by the exhaust gas condenser 6. hydrogen,
The radioactive exhaust gas from which oxygen and water vapor have been removed is attenuated by the activated carbon adsorption tower 7 and then released into the atmosphere by the exhaust stack 8. Here, the exhaust gas recombiner 5 will be briefly described. The exhaust gas recombiner 5 is a device that combines hydrogen generated by radiolysis of water with oxygen to convert it into water. The catalyst packed in the exhaust gas recombiner 5 is a γ-group alumina having a large specific surface area among alumina. This is a carrier in which 0.5 wt% of palladium is supported.

A honeycomb catalyst or a metal catalyst is also used as the catalyst to be filled in the exhaust gas recombiner 5. Since the honeycomb-shaped catalyst supports palladium as a catalyst component,
Made of honeycomb ceramics such as cordierite,
Γ-group alumina, which is relatively active among alumina and has a large specific surface area, is used as the ceramic and 0.5% palladium is used.
wt% is carried.

The metal catalyst has a structure in which palladium as a catalyst component is supported, but it is difficult to directly support palladium on a metal as a carrier. Therefore, γ-group alumina, which is relatively active and has a large specific surface area, is coated, and 0.5 wt% of palladium is supported on the alumina.

[0005]

The γ-group alumina (ρ, χ, γ, η, δ, κ, θ) which is generally used as a palladium carrier for an exhaust gas recombiner catalyst has relatively unstable crystallinity. When the carrier itself is heated, it undergoes a phase transition to α-alumina having a stable crystal structure. However, the temperature at which the γ-group alumina transforms into α-alumina is a few thousand degrees (for a short time), and the specifications of the exhaust gas recombiner 5 (operating temperature 300 to
When considering 400 ℃), there is no possibility of phase transition. However, the present inventors have found that, as shown in FIG. 3, the probability that the γ-group alumina undergoes a phase transition to α-alumina increases depending on the operation period of the recombiner, and the strength of the catalyst itself decreases. It was This is because when impurities other than the catalyst are mixed, the temperature at which the γ-group alumina transforms into α-alumina becomes low. Also, considering the specifications of the recombiner, steam is added to the catalyst as a load, and there is a possibility that phase transition will occur depending on the operating time. Further, considering the decrease in the strength of the carrier due to the phase transition of the alumina carrier, there is a sintering phenomenon involving the α transition of the phase of γ-alumina from the cubic close-packed structure to the hexagonal close-packed structure. Therefore, considering that the pore volume and the specific surface area are decreasing, the alumina carrier has a rough structure due to the sintering and increase of the alumina fine particles, and the mechanical strength of the carrier is decreased due to the decrease of the number of fine particle contact points or the contact bonding force. Strength is reduced. Since this decrease in alumina strength eventually progresses to pulverization, there is a possibility that not only the catalyst performance will decrease, but also the pulverized alumina will close the downstream side, and palladium that has fallen off will damage pipes and equipment. There is also a problem related to plant shutdown. From these facts, γ
The use of Group Alumina has a problem that it cannot withstand long-term use.

The present invention has been made to solve the above-mentioned problems, and it is possible to maintain stable performance for a long period of time without causing a phase change of alumina itself, without deterioration of catalyst performance due to strength deterioration. An object of the present invention is to provide a catalyst for exhaust gas recombiners.

[0007]

A first invention is an exhaust gas recombiner catalyst comprising palladium supported on a granular carrier for recombining hydrogen and oxygen in exhaust gas, wherein the carrier is composed of α-alumina. Characterize.

A second invention is a catalyst for an exhaust gas recombiner in which palladium is supported on a honeycomb carrier for recombining hydrogen and oxygen in exhaust gas, wherein the honeycomb carrier is α-alumina on a ceramic surface such as cordierite. It is characterized by comprising.

A third invention is an exhaust gas recombiner catalyst comprising palladium supported on a metal carrier for recombining hydrogen and oxygen in exhaust gas, wherein the metal carrier surface is provided with α-alumina. To do.

[0010]

In the present invention, the most stable α-alumina among alumina is used for supporting the catalyst. When using α-alumina, the difference in specific surface area, which is the biggest difference from γ-alumina (γ: 150 m ² / g, α: 10-20 m ² / g)
There is. However, even when γ-alumina is used as a carrier, the amount of palladium supported on the catalyst for the recombiner is reduced.
The ability to carry 0.5 wt% is sufficient. The use of α-alumina does not cause a phase change of the carrier itself. Further, since the performance of the catalyst does not deteriorate due to strength deterioration, stable performance can be maintained for a long time. Furthermore, since hydrogen and oxygen binding capacity is sufficient, even considering the replacement cost when using a metal catalyst and the cost of increasing the amount of catalyst relative to the binding capacity in the present invention, it is advantageous when considering the life of the plant. Becomes

[0011]

EXAMPLES Examples of the exhaust gas recombiner catalyst according to the present invention will be described with reference to FIG. Since the exhaust gas recombiner of the radioactive gas waste treatment facility of the nuclear power plant has been described in the conventional example, it will be omitted and only the catalyst thereof will be described. The types of this catalyst are granular catalyst, honeycomb catalyst,
There is a metal catalyst. First, the manufacturing method of each catalyst will be described.

Regarding the granular catalyst, after extruding an alumina raw material (hydrate such as boehmite) as a carrier,
It is baked into the alumina form (γ to α etc.) to be used, soaked in a palladium solution serving as a catalyst, and then dried. For the honeycomb catalyst, cordierite (Mg, Al, S
(a mixture such as i) is extruded and then fired to form a honeycomb structure, the surface of which is coated with γ-alumina or the like, which is heated to cause a phase transition to α-alumina, and then dipped in a palladium solution and dried. It was done. As for the metal catalyst, a sponge-like alloy of nickel and chromium (porous material) is molded according to the purpose of use, γ-alumina or the like is coated on the surface, then heated to cause a phase transition to α-alumina, and then a palladium solution. It is pickled and dried. These catalysts, steam flow rate using the apparatus shown in FIG. _{1, H 2, O 2,} N 2 After kept at a predetermined temperature by adjusting the concentration, H ₂ before and after the reaction tube for accommodating the catalyst and The performance of the catalyst was confirmed by measuring the O ₂ concentration by gas chromatography.

That is, reference numeral 11 in FIG. 1 denotes a reaction tube for accommodating a catalyst, and a preheating heater 12, a flow rate-adjusted gas supply system 13, a steam supply line 14 and a boiler 15 are sequentially provided on the upstream side of the reaction tube 11. It is connected. A cooler 16 is connected to the downstream side of the reaction tube 11.

[0014]

[Table 1]

Table 1 shows the loading performance of palladium used as a catalyst by the apparatus shown in FIG. Table 1
As is clear from the above, there is a considerable difference in the recombination ability of hydrogen and oxygen, but there is a hydrogen / oxygen recombination ability.

The values in Table 1 are relative values when γ and α, which are in the form of alumina, are used. Since the contact areas of the granular catalyst, the honeycomb catalyst and the metal catalyst with the fluid in the container are considerably different, If the volume is the same, the performance is as shown in Table 2, and there is a difference in performance depending on the shape and the like.

[0017]

[Table 2]

Next, the difference in action and effect between the above catalysts will be described. The granular catalyst has a size of ~ 8 mm and is used by filling the container in the reaction part. Compared with the honeycomb catalyst and the metal catalyst, there are few restrictions on the shape of the reaction part and it is possible to design according to the usage situation. Become.

The honeycomb catalyst has the largest apparent surface area and the highest performance. When the reaction part is the same container, the contact area with the inflowing gas can be made larger than other areas. Therefore, it is possible to obtain a high processing capacity and also to suppress the flow resistance to the processing fluid to be low. Since a sponge-like metal catalyst is used as the metal catalyst, a large surface area can be obtained with respect to the granular catalyst. Therefore, when the reaction portion is the same, the performance is superior to that of the granular catalyst. Further, since the catalyst carrier is a metal, it has excellent strength against vibration and the like.

As described above, each catalyst is variously selected according to the usage situation and purpose. Generally, α-alumina obtained by firing alumina raw material at a very high temperature (1300 ° C or higher) at once becomes very hard as represented by ruby and sapphire. However, the strength of γ-alumina becomes extremely weak when it is converted to α over a long period of time while being continuously exposed to water vapor under the temperature condition of the exhaust gas recombiner of a nuclear power plant. Therefore, the deterioration of the strength of the carrier causes pulverization, and the deterioration of the strength of the alumina causes the exfoliation from the metal, the ceramics, or the like, and the palladium that controls the recombiner also exfoliates at the same time to deteriorate the catalytic performance.

On the other hand, according to the present invention, if γ-alumina is used in any form, as long as the γ-alumina is used in the exhaust gas recombiner of the nuclear power plant, the α-formation of the alumina form proceeds, In parallel with this, the problem of decreasing the strength of the alumina portion is solved.

[0022]

According to the present invention, the use of α-alumina in the alumina for supporting palladium of the catalyst for the exhaust gas recombiner does not cause a phase change of the carrier itself, and the performance of the catalyst is deteriorated due to the strength deterioration. There is no. Therefore,
Can maintain stable performance for a long time.

[Brief description of drawings]

FIG. 1 is a piping system diagram for performing a performance test of a catalyst according to the present invention.

FIG. 2 is a flow diagram showing a radioactive gas waste treatment facility for explaining a conventional example and the present invention.

3 is a characteristic diagram showing a deterioration tendency of a catalyst in the exhaust gas recombiner in FIG.

[Explanation of symbols]

1 ... Reactor, 2 ... Main condenser, 3 ... Air extractor, 4 ... Exhaust gas preheater, 5 ... Exhaust gas recombiner, 6 ... Exhaust gas condenser, 7
... activated carbon adsorption tower, 8 ... exhaust stack, 9, 10 ... valve, 11 ... reaction tube, 12 ... preheat heater, 13 ... gas supply tube, 14 ... steam supply line, 15 ... boiler, 16 ... cooler.

Claims (3)

[Claims] 1. A catalyst for an exhaust gas recombiner comprising palladium supported on a granular carrier for recombining hydrogen and oxygen in the exhaust gas, wherein the carrier comprises α-alumina. 2. A catalyst for an exhaust gas recombiner comprising palladium supported on a honeycomb carrier for recombining hydrogen and oxygen in exhaust gas, wherein the honeycomb carrier comprises α-alumina on a ceramic surface such as cordierite. A catalyst for an exhaust gas recombiner, which is characterized in that 3. An exhaust gas recombiner catalyst comprising palladium supported on a metal carrier for recombining hydrogen and oxygen in the exhaust gas, wherein the metal carrier surface is provided with α-alumina. Catalyst for combiner.