RU2479877C2 - Method to condition solid organic radioactive wastes - Google Patents

Abstract

FIELD: power engineering.

SUBSTANCE: method includes the following actions: pyrolysis without air access to form a coke and ash residue. Burning of pyrolysis gases produced during pyrolysis in an air flow to form smoke gases, their cooling and cleaning. Afterburning of the coke and ash residue produced during pyrolysis in the air flow. Withdrawal of the produced residue for further burial.

EFFECT: invention makes it possible to increase efficiency.

Description

The invention relates to the field of conditioning of organic radioactive waste (wood, paper, rags, rubber gloves, plastic film, etc.) generated during research and production work in organizations, in order to reduce their volume for subsequent export to the point burial or long-term storage.

A known method of conditioning solid waste contaminated with radioactive components, including the burning of waste in a shaft furnace containing a burner device with the supply of excess air: utility model patent No. 22837 [1]. The disadvantages of this method are:

1. waste is burned due to the operation of a gas burner with additional formation of flue gases, as a result of which the time of efficient operation of the output filters is reduced due to their rapid clogging with soot, the formation of secondary radioactive waste due to it;

2. the temperature in the afterburner is above 800 ° C, as a result of which complete capture of easily volatile radioactive waste, for example cesium radionuclides, is not achieved, and there is also the possibility of uncontrolled release of radioactive aerosols into the atmosphere;

3. the technological process due to the supply of excess air takes place at excess pressure, as a result of which there is the possibility of uncontrolled release of radioactive materials into the atmosphere.

Known methods for the disposal of radioactive waste due to the thermal decomposition of waste in an oxygen-free atmosphere (pyrolysis).

Thermal decomposition (pyrolysis) of waste in an oxygen-free atmosphere allows you to transfer the bulk of the organic substances contained in the waste into a gaseous state, thereby concentrating the bulk of the radioactive components in the solid residue. The afterburning of the coke residue makes it possible to concentrate more than 80% of the initial radioactive waste in the ash and reduces the amount of ash to be disposed of.

Closest to the claimed method for the combination of essential features is a method for conditioning solid organic radioactive waste described in the patent of the Russian Federation No. 2335700 [3]. This method is based on the thermal decomposition of radioactive waste without access to air (pyrolysis).

Solid waste is first placed in a box under vacuum, and then transferred to a loading device. Then they are sent to a thermal decomposition chamber, which is a metal retort, heated externally by flue gases generated from burning external fuel in the burner. The waste is heated to a final temperature of 650-700 ° C without oxygen. During heating, their thermal decomposition (pyrolysis) occurs, as a result of which approximately 60-70% of gaseous combustible gases and 30-40% of coke-ash residue are formed. The gaseous products of pyrolysis due to the draft of the smoke exhauster through the channel enter the furnace. In the furnace they are mixed with the required amount of air, as a result of which they are burned. The resulting flue gases due to the draft of the exhaust fan are successively sucked through the cartridge of the catalytic afterburner, in which the low-temperature oxidation of the residues of organic compounds takes place, in the heat exchanger, where the temperature of the gases is reduced to the level of 150-120 ° С, and then they enter the dust cleaning system. At the stage of dust cleaning, the flue gases are first cleaned of dust in a “dry” cyclone, and then they are washed with liquid in a “wet” scrubber. The cleaned flue gases enter the flue through which they are emitted into the atmosphere. The captured dust is unloaded from the cyclone of the system as it accumulates in a sealed container through a box under vacuum. Disposal of this dust is carried out using special technology. Collection and unloading of sludge from the scrubber system is carried out through the same box in textile or plastic bags. Loaded sludge is sent back to the thermal decomposition chamber. The coke ash residue formed as a result of pyrolysis as it accumulates in the chamber is discharged through the discharge device onto the grate, under which atmospheric air enters to burn these residues due to the exhaust fan draft. The inorganic part of the coke ash residue (ash) remaining after combustion is removed from under the grate by a special transport device through a special box under vacuum. Unloading of ash is carried out in a sealed container, and its disposal is carried out by special technology.

The disadvantages of the method are:

1. its rather complicated implementation associated with a large number of operations;

2. the high temperature of pyrolysis and burning of the coke-ash residue increases the likelihood of entrainment of volatile radionuclides, in particular cesium radionuclides;

3. the formation of a large amount of secondary solid and liquid radioactive waste during flue gas treatment.

The objective of this invention is to simplify the method of conditioning solid organic radioactive waste while reducing costs, reducing the amount of waste to be conditioned, reducing the generation of secondary waste and reducing the likelihood of radionuclide releases into the atmosphere.

The problem is solved in that in a method for conditioning solid organic radioactive waste (wood, paper, rags, rubber gloves, plastic film, etc.), which includes: pyrolysis without air access with the formation of a coke residue, burning pyrolysis gases generated during pyrolysis in an air stream with the formation of flue gases, their cooling and purification, afterburning of the coke ash residue formed during pyrolysis in an air stream, extraction of the formed residue for further burial, n It is clear that the waste is preliminarily impregnated with a solution of cesium sulfate in sulfuric acid, this mass is pressed into standard blocks and loaded into the lower part of the vertically installed thermal decomposition chamber, in which at t = 380-420 ° C they are pyrolyzed without air to form coke residue. The combustion process of pyrolysis gases generated during pyrolysis is carried out at t = 780-820 ° C in the upper zone of the thermal decomposition chamber with air supply to this zone. Flue gas cooling is carried out in an air heat exchanger, flue gas cleaning is carried out first from coarse particles, and then from aerosols. The afterburning of the coke-ash residue formed during pyrolysis is also carried out at t = 380-420 ° C after the termination of the pyrolysis process with the heating of the pyrolysis gas combustion zone turned off and air supply to the lower part of the thermal decomposition chamber zone. Removing the resulting ash residue is carried out from the ash pan located under the lower zone of the thermal decomposition chamber. Cleaning flue gases from coarse particles is carried out on a tarp filter such as stockings, cleaning from fine aerosols is carried out on a filter made of Petryanov’s fabric. Spent coarse and fine filters are re-conditioned in the same installation.

In the inventive method, there are no operations of various overloads, which is the prototype. Pressing the waste into standard blocks before loading them into the thermal decomposition chamber allows several times to reduce their volume, to conduct a series of pyrolysis of several blocks before the operation of burning coke residue, and thereby increase productivity.

By impregnating the waste with a solution of cesium sulfate, the likelihood of entrainment of easily volatile cesium radionuclides is reduced.

At a lower process temperature, the likelihood of entrainment of volatile radionuclides is also reduced. When cleaning flue gases, no liquid radioactive waste is generated.

The inventive method is based on the fact that it was experimentally established that at a heating temperature of the pyrolysis gas combustion zone of 800 ° C, the heating temperature of the lower zone of the thermal decomposition chamber in the range of 380-420 ° C is sufficient for pyrolysis of organic waste and burning of coke residue in a vertical installation thermal decomposition chambers.

Figure 1 presents a diagram of an installation that implements this method, where: 1 - thermal decomposition chamber (CTE); 2 - loading hatch; 3 - manhole cover; 4 - clamping device; 5 - lower zone of thermal decomposition of KTP; 6 - upper zone KTR for afterburning of pyrolysis gases; 7 - an ash pan; 8 - pipe for supplying air to the lower zone of the KTP; 9 - pipe for supplying air to the upper zone; 10 - pipe - heat exchanger for KTR exhaust gases; 11 - filter for rough cleaning of flue gases such as stockings; 12 - fine filter of flue gases from the fabric of Petryanov.

The work of the installation that implements the method.

Compressed into a standard block and then impregnated with a solution of cesium sulfate in sulfuric acid, material to be conditioned is fed into a vertically mounted thermal decomposition chamber 1 through a loading hatch 2.

The cover 3 of the loading hatch is closed using the clamping device 4. The water cooling of the cover is turned on, the required air flow through the pipe 8 in the upper zone of the KTP is set, and then the heating of the upper zone of the 6 KTP is turned on. When it reaches 800 ° C, the lower zone of KTP 5 is heated to a temperature of 400 ° C (heating of the upper and lower zones of KTP is performed using two tubular electric furnaces). At this temperature, pyrolysis and the formation of a coke-ash residue occur.

At the end of the pyrolysis process, the heating of the upper KTP zone is turned off, the air supply to it through the pipe 8 is stopped, and the air through the pipe 9 is supplied with the same flow rate to the lower zone of the KTP 5, and the coke ash residue is burned at t = 400 ° C. The resulting ash residue falls into ash pan 6, from which it is extracted after the end of the process and transferred to the collection container for temporary storage.

The pyrolysis gases formed during pyrolysis enter the upper chamber of KTP 6, in which they are burned and converted into flue gases, which, passing through the air heat exchanger (pipe 10), cool and enter the coarse filter 11, then the fine filter 12.

The coarse and fine filters that have exhausted their service life are air-conditioned using the same installation.

Table 1 shows the experimental data for the conditioning of waste from cotton fabric, confirming the possibility of pyrolysis of organic waste and burning the resulting coke residue at 400 ° C and data on the ablation of radionuclides through a coarse filter.

No. p / p t _pyrolysis , ° C T _burn. ° C Ash residue,% weight. by weight ref. SRW Ablation of Cs-137,% Ablation Am-241,% one 2 3 four 5 6 one. 500 500 1.4 0.3 ≤2 · 10 ^-2 2. 500 500 1.3 0.4 ≤2 · 10 ^-2 3. 400 400 1,5 0.12 ≤1 · 10 ^-2 four. 400 400 1.3 0.09 ≤1 · 10 ^-2 5. 300 300 1.8 0.12 ≤1 · 10 ^-2 6. 300 300 1.3 0.12 ≤1 · 10 ^-2

An example of a specific implementation of the method.

A sample of material weighing 0.8-1.0 kg (cotton fabric, synthetic fabric, filter paper, plastic film, wood, rubber gloves, Petryanov's fabric) with Cs-137 radionuclides deposited on it with A = 1.8 · 10 ⁵ Bq and Am-241 with A = 2.2 · 10 ⁵ Bq was pressed in a cardboard sleeve, impregnated with a 0.01 M solution of Cs ₂ SO ₄ in 4% H ₂ SO ₄ . After that, the package was placed through the loading hatch into a thermal decomposition chamber (KTR) made of stainless steel 12X18H10T, shown in Fig. 1, of a pyrolysis-combustion unit. The loading hatch lid was closed, water cooling of the lid was turned on, the air flow through pipe 8 in the upper zone of the KTP was set equal to 15-18 m ³ / h, after which the heating of the upper zone of the KTP was turned on. When it reached 800 ° C, the lower KTP zone was heated to a temperature of 400 ° C (the upper and lower KTP zones were heated using two tubular electric furnaces). At this temperature, the sample was pyrolyzed for 35-50 minutes.

At the end of the pyrolysis, the heating of the upper KTP zone was turned off, the air supply through it through the pipe 8 was shut off, the air through the pipe 9 was supplied with the same flow rate to the lower KTP zone, and the coke ash residue was burned at t = 400 ° C for 35-50 min.

The resulting ash residue was extracted from the ash pan, its volume was weighed and measured. The ablation of radionuclides through the coarse filter was determined by the results of gamma-spectrometric analysis of the fine filter.

As a result of the operations described above, at an air flow rate of 15-18 m ³ / h and the time of both pyrolysis and burning of the coke ash residue 35-50 min, the ash residue yield from the weight of the starting material was 0.5-2.5% by weight, 0 , 5-4.5% by volume, the entrainment of Cs-137 and Am-241 through the coarse filter was, respectively, 0.1-0.4% and <2 · 10 ^-2 % of the activity of the starting material. If we take into account that according to the literature [3], the penetration coefficient through the Petryanov filter from FPP-25-3.0 material is 0.001%, then the volumetric activity of the flue gases generated during conditioning of low-level radioactive waste and passed through the filtering system will be lower than the permissible volumetric activity for the population [4].

The inventive method was tested in the decontamination workshop of the PNPI RAS. As a result of the verification, all the advantages of the method were confirmed.

Literature

1. RU patent for utility model No. 22837 IPC F23G 5/08.

2. RF patent №2335700 IPC F23G 5/08 - prototype.

3. A.K. Budyka, N.B. Borisov. Fiber filters to control air pollution. M., Publishing House, 2008.

4. Radiation safety standards NRB-99/2009.

Claims (2)

1. A method for conditioning solid organic radioactive waste (RAW), including: carrying out pyrolysis without air access with the formation of a coke oven residue, burning pyrolysis gases generated during pyrolysis in an air stream with flue gases, cooling and purifying them, afterburning the coke oven residue formed during pyrolysis air flow, extracting the resulting radioactive ash residue for further disposal, characterized in that previously solid RW is impregnated with a solution of sulfuric acid cesium in sulfuric acid, they are pressed and fed into the lower part of a vertically mounted cylindrical thermal decomposition chamber, followed by pyrolysis without air access with the formation of a coke-ash residue in the lower zone of the thermal decomposition chamber at T = 380-420 ° C, and the combustion process pyrolysis gases generated during pyrolysis are carried out in the upper zone of the thermal decomposition chamber with air supply to this zone at T = 780-820 ° C, the flue gases are cooled in an air heat exchanger, the flue gases are cleaned first they are removed from coarse particles, and then from aerosols in a dry way using filters, and afterburning of the coke-ash residue formed during pyrolysis is carried out after the pyrolysis process is stopped with the heating of the pyrolysis gas combustion zone turned off at T = 380-420 ° C and when air is supplied to the lower part zone of the thermal decomposition chamber, after which the resulting radioactive ash residue is removed from the ash pan located under the lower zone of the thermal decomposition chamber. 2. The method of conditioning organic radioactive waste (RAW) according to claim 1, characterized in that the flue gas is purified from coarse particles on a tarpaulin filter such as stockings, and the fine aerosols are cleaned on a filter made of Petryanov’s fabric.

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