Method for removing and purifying radioactive methane in hydrogen-containing waste gas in nuclear industry

Abstract

The invention discloses a method for removing and purifying radioactive methane in hydrogen-containing waste gas in the nuclear industry, which comprises the following steps: contacting the hydrogen-containing waste gas in the nuclear industry with a curing agent, treating, removing and purifying the radioactive methane in the hydrogen-containing waste gas, and generating molybdenum carbide; the curing agent is selected from molybdenum oxide and / or molybdenum nitride. The purification process is efficient, safe and green, one-way complete removal of methane can be realized at 750 DEG C, and the used curing agent can be directly treated as radioactive solid waste and can also be regenerated and recycled. The technical bottleneck of a traditional catalytic oxidation method for radioactive methane treatment is broken through, the efficient, safe and green treatment method is provided, the strict requirement for nuclear industry waste gas treatment can be met, good operability and economical efficiency are achieved, and the important industrial application value is achieved.

Description

Technical Field

[0001] The present application relates to a method for removing and purifying radioactive methane in hydrogen-containing waste gas of the nuclear industry, and belongs to the field of nuclear waste treatment. Background Art

[0002] During the operation of nuclear power, the radioactive waste gas produced by the activation reaction of the reactor fuel elements and the core area will partially dissolve in the reactor loop cooling water and escape with the leakage of cooling water or steam to form process waste gas. The life of radioactive nuclides in reactor waste gas is mostly short. Most of the short-lived nuclides in the waste gas can be removed by decay through methods such as pressurized storage decay and activated carbon bed retention. However, the carbon 14 nuclides in the form of methane in the waste gas have a half-life of up to 5730 years, which has become a difficult problem for nuclear industry waste gas treatment. On the other hand, due to the presence of a certain amount of hydrogen (about 11-70%) in the nuclear industry process waste gas, the removal of methane is more difficult. If the methane decomposition method (CH4→2H2+C) is used, the reaction is limited by the reaction equilibrium. It is necessary to exceed 1200°C at least to completely solidify methane into carbon deposits in a single pass. Low-temperature reactions require cyclic treatment and removal of the generated hydrogen. If the catalytic combustion method is used, hydrogen needs to be burned together with methane because hydrogen molecules are more active than inert methane molecules. It is worth noting that the catalytic oxidation method currently used requires a large amount of circulating nitrogen to dilute the hydrogen concentration to no more than 2.5%, and then mix it with added oxygen for catalytic combustion. On the one hand, this increases the gas processing volume and increases the burden on system operation. On the other hand, the use of oxygen increases the potential risk of explosion. Therefore, the above two methods for treating methane often require the existing hydrogen to be removed by separation (membrane separation, adsorption separation, etc.), which greatly increases the process complexity and equipment volume. Therefore, the removal of radioactive methane in hydrogen-containing process waste gas in the nuclear industry urgently needs to develop a simpler, more efficient and safer treatment method. Summary of the invention

[0003] In view of the above problems, the present invention provides a curing agent for selective cracking, solidification and decarbonization of methane in a hydrogen-containing atmosphere and its application in the removal and purification of radioactive methane in hydrogen-containing waste gas.

[0004] According to one aspect of the present application, a method for removing and purifying radioactive methane from hydrogen-containing waste gas in the nuclear industry is provided, characterized in that:

[0005] The following steps are involved:

[0006] The hydrogen-containing waste gas from the nuclear industry is brought into contact with a curing agent, treated, and the radioactive methane therein is removed and purified to generate molybdenum carbide, which has high stability and is not easily released again.

[0007] By catalytically cracking methane, all carbon elements are solidified in solid form and the radioactive carbon 14 methane in hydrogen-containing waste gas is purified.

[0008] The curing agent is selected from molybdenum oxide and / or molybdenum nitride.

[0009] The treatment temperature is 650-850°C.

[0010] The molybdenum oxide is activated;

[0011] The activation is carried out at 500-700° C. in a mixed atmosphere of hydrogen and nitrogen in a volume ratio of 3:1 for 3-15 hours.

[0012] The molybdenum oxide is doped with at least one of Ni, Fe, Co and Ru, with a mass content of 0.1 to 10 wt%.

[0013] The used solidifying agent containing radioactive carbon 14 can be directly packaged and landfilled as a radioactive solid. It can also be recycled after being regenerated by oxidation roasting.

[0014] The regeneration is to calcine the generated molybdenum carbide at 500-700° C. in a mixed atmosphere of oxygen and nitrogen to obtain a curing agent and carbon dioxide;

[0015] In the mixed atmosphere of oxygen and nitrogen, the volume content of oxygen is 5-20%.

[0016] The radioactive carbon dioxide gas produced during the oxidative roasting process in the regeneration process can be absorbed by a calcium hydroxide solution, and the radioactive carbon element is completely precipitated into calcium carbonate. The radioactive calcium carbonate is separated, dried, and then packaged and landfilled.

[0017] The beneficial effects of this application include:

[0018] 1. The curing agent provided by the present invention can achieve the selective removal of radioactive methane in a hydrogen-containing atmosphere at 750°C through the efficient cracking effect of a molybdenum-based catalyst, and the methane one-way removal rate can reach 100%, which has obvious advantages over the high-temperature catalytic carbon deposition method;

[0019] 2. The radioactive methane removal process provided by the present invention is highly safe, does not require the addition of oxygen, and does not require circulating nitrogen to dilute hydrogen, thus avoiding the explosion risk that may be caused by the catalytic combustion method;

[0020] 3. The curing agent provided by the present invention can be recycled and reused, and the oxidation roasting regeneration and hydrogen-nitrogen activation can be operated online, which is suitable for continuous processing;

[0021] 4. The curing agent and process provided by the present invention are highly targeted and suitable for the purification of long-lived radioactive carbon 14 methane in nuclear industrial waste gas. They can achieve highly selective cracking in complex hydrogen-containing atmospheres, breaking through the technical bottleneck of traditional methods. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 The performance of curing agent S4# in Example 4 in methane carbon fixation;

[0023] Figure 2 This is the performance of the methane carbon deposition catalyst D1# in comparative example 1 in the methane decomposition carbon deposition reaction. DETAILED DESCRIPTION

[0024] The present application is described in detail below with reference to embodiments, but the present application is not limited to these embodiments.

[0025] Unless otherwise specified, the raw materials in the examples of this application were purchased through commercial channels.

[0026] Example 1

[0027] Weigh molybdenum oxide powder (purity ≥ 99.9%) and put it into a reaction tube. Pass a mixed gas of hydrogen and nitrogen (the ratio of hydrogen to nitrogen is 3:1), heat to 600°C, and continue to treat at this temperature for 8 hours to obtain an activated molybdenum nitride curing agent.

[0028] Example 2

[0029] Weigh nickel-doped molybdenum oxide powder and put it into a reaction tube. Pass a mixed gas of hydrogen and nitrogen (the ratio of hydrogen to nitrogen is 3:1), heat to 600°C, and continue to treat at this temperature for 8 hours to obtain an activated nickel-doped molybdenum nitride curing agent.

[0030] Example 3

[0031] Molybdenum nitride catalyst (mass 5 g) was loaded into a fixed bed reactor. A methane-containing gas mixture (0.5% methane, 10% hydrogen, and the rest nitrogen) was introduced at a gas flow rate of 10 ml / min. The reactor was heated to 800°C and maintained for 10 hours. The methane concentration in the outlet gas was detected by an online gas chromatograph, and the results showed that methane was completely removed in the first 5 hours and no CO or CO2 was produced. The carbon element was retained in the solid in the form of molybdenum carbide. The solid after the reaction was placed in the air for 1 week. There was no obvious oxidation or decomposition of the carbide, and the stability was good.

[0032] Example 4

[0033] 5 g of the activated nickel-doped molybdenum nitride catalyst prepared in Example 2 was loaded into a fixed bed reactor. A methane-containing gas mixture (0.5% methane, 10% hydrogen, and the remainder nitrogen) was introduced at a gas flow rate of 10 ml / min. The reactor was heated to 750° C. and maintained for 10 hours. Figure 1 The performance of curing agent S4# in Example 4 in methane carbon fixation; the methane concentration in the outlet gas was detected by online gas chromatograph, and the results showed that after the system was stable, the methane conversion rate was greater than 99% in the 10th to 30th hour, and the methane was completely removed in the 15th to 23rd hour without the generation of carbon monoxide or carbon dioxide. The carbon element was retained in the solid in the form of molybdenum carbide. The solid after the reaction was placed in the air for 1 week, and there was no obvious oxidation or decomposition of the carbide, and the stability was good.

[0034] Example 5

[0035] The used catalyst in Example 4 was loaded into a reaction tube, and an oxygen-nitrogen mixed gas (oxygen accounting for 10%) was introduced, and the temperature was gradually raised to 600°C, and the treatment was maintained for 4 hours. The outlet gas composition was detected by an online gas chromatograph, and only carbon dioxide was generated, and no carbon monoxide was detected. The carbon dioxide generated during the oxidation process was introduced into the calcium hydroxide solution to form calcium carbonate precipitation. According to elemental analysis and X-ray diffraction (XRD) analysis, the surface of the regenerated catalyst was completely converted into a molybdenum oxide structure, and there was no residual carbide.

[0036] Comparative Example 1

[0037] 5 g of nickel-alumina catalyst (mass) was loaded into a fixed bed reactor. A methane-containing gas mixture (0.5% methane, 10% hydrogen, and the remainder nitrogen) was introduced at a gas flow rate of 10 ml / min. The reactor was heated to 750°C and maintained for 20 hours. Figure 2 The performance of methane carbon deposition catalyst D1# in the methane decomposition carbon deposition reaction in comparative example 1. The methane concentration in the outlet gas was detected by online gas chromatograph, and the results showed that only 55% of the methane was converted after the system was stabilized, and the remaining 45% remained and could not be completely removed.

[0038] The above are only a few embodiments of the present application and do not constitute any form of limitation to the present application. Although the present application is disclosed as above with preferred embodiments, it is not intended to limit the present application. Any technician familiar with the profession, without departing from the scope of the technical solution of the present application, using the technical content disclosed above to make slight changes or modifications are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. A method for removing and purifying radioactive methane from hydrogen-containing waste gas in nuclear industry, characterized in that: The following steps are involved: The hydrogen-containing waste gas from the nuclear industry is contacted with a solidifying agent, treated, and the radioactive methane therein is removed and purified to generate molybdenum carbide; The curing agent is selected from molybdenum oxide and / or molybdenum nitride.

2. The method according to claim 1, characterized in that The treatment temperature is 650-850°C.

3. The method according to claim 1, characterized in that The molybdenum oxide is activated; The activation is carried out at 500-700° C. in a mixed atmosphere of hydrogen and nitrogen in a volume ratio of 3:1 for 3-15 hours.

4. The method according to claim 1, characterized in that: The molybdenum oxide is doped with at least one of Ni, Fe, Co and Ru, with a mass content of 0.1 to 10 wt%.

5. The method according to claim 1, characterized in that The curing agent can be regenerated; The regeneration is to calcine the generated molybdenum carbide at 500-700° C. in a mixed atmosphere of oxygen and nitrogen to obtain a curing agent and carbon dioxide; In the mixed atmosphere of oxygen and nitrogen, the volume content of oxygen is 5-20%.

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