RU2560095C2 - Method of recycling wastes containing uranium compounds - Google Patents

Abstract

FIELD: atomic physics.

SUBSTANCE: invention relates to recycling organic wastes containing uranium-235 compounds (protective garments, plastic compound, filters etc.). The wastes are crushed, discretely fed into a hopper and then into a first sluice feeder. From the latter, the wastes are continuously fed into a thermal decomposition chamber in an oxygenless atmosphere (pyrolysis) and moved uniformly along the axis of the chamber towards its outlet. Further, the formed solid products are continuously fed into a second hopper, from which the accumulated portion is discretely moved through a sluice valve into a receiving hopper box. The formed gaseous products are uniformly fed into a burner device and flue gases formed from combustion of said products are neutralised in a catalytic afterburner, uniformly cooled in a heat exchanger, washed in a wet scrubber and released into the atmosphere. The accumulation of a critical mass of uranium is prevented by matching the operating mode of the devices which move the wastes.

EFFECT: safer operation of equipment and nuclear safety.

1 dwg

Description

The invention relates to the field of disposal of organo-waste with a content of 235 uranium compounds of more than 5%.

A known method and installation for the processing of radioactive waste, providing for the burning of waste in a shaft furnace [Markelov N.I. and other installation for the processing of radioactive waste. Patent model-22837, July 9, 2002].

The disadvantage of this method is that since the waste is burned due to the operation of the burner with excess air and the supply of flue gases to the material layer, the entire process takes place under excessive pressure and there is the possibility of uncontrolled release of radioactive materials into the atmosphere.

Another disadvantage of this method is the lack of devices that prevent the possibility of radioactive components entering the atmosphere during loading and unloading operations, as well as the mismatch of the design of individual components with nuclear safety requirements.

Closest to the proposed method is a method for the disposal of solid waste contaminated with radioactive components, in which the waste is pyrolyzed in a thermal decomposition chamber, the resulting dusty gaseous products are burned in a furnace, the coke residue is burned on a grate under a thermal decomposition chamber, the resulting flue gases are subjected to complex dust cleaning, and the entire recycling process is carried out under the vacuum created by the smoke exhauster located at the outlet of the sheep [Avramenko AV, Dvoskin G.I. et al. Method for the disposal of waste containing radioactive components. RF patent No. 2335700, dated 10.10.2008].

The disadvantage of this method is the pulsation of the operational parameters of the process due to the uneven cyclical decomposition of waste and, accordingly, the cyclical intake and combustion of gaseous products in the burner with uneven, short-term (volley) evolution of a large amount of heat, which, in turn, requires large surfaces of heat-exchange equipment , which is used inefficiently, since it works in a non-optimal pulsating mode. This process flow is explained as follows.

The subject of disposal is, as a rule, a mixture of wastes contaminated with uranium oxides with the following composition: rubber products - 50%, rags - 35%, plastics - 10%, etc. - 5%. The average calorific value of the mixture is approximately 20-22 MJ / kg. In the process of organic pyrolysis, the main time is heating the material to the temperature of the onset of thermal decomposition, and the pyrolysis process itself occurs very quickly. When a portion of a material is loaded into an already heated reaction device, its decomposition cycle is divided into three time periods: heating — about 35% of the total cycle time, the main pyrolysis process — 20%, and residual processes — 45%. Thus, the main load on the equipment, including heat transfer, accounts for only 20% of the total cycle time. The rest of the time, energy resources, designed to supply a large amount of cooling agent (air) to the heat exchanger, are wasted. This is especially true for the pyrolysis of high-calorie material, when there is a rapid release of a large amount of heat within a few minutes. To remove a large amount of heat in a limited time, the heat exchange equipment must have a large surface, which, based on nuclear safety requirements, limiting the size of the equipment nodes, has to be assembled from a large number of small nodes, which is not technologically and uneconomical. It is impossible to equalize the pulsation of the process by adding a new portion of material to the reaction device because of the danger of a critical mass of uranium 235 accumulating in it.

Another disadvantage of this method is that when burning coke residue on the grate, the accumulation of ash can also lead to the formation of a critical mass of uranium 235, since the bulk of the radioactive components remain in the ash.

Another disadvantage of this method is that when loading another portion of the waste, air from the environment may enter the thermal decomposition chamber.

The technical result to which this invention is directed is to reduce the metal consumption of heat exchange equipment and save energy by ensuring the uniformity of processes: - pyrolysis of waste, - combustion of gaseous products of pyrolysis and cooling of combustion products, ensuring the impossibility of air entering the thermal decomposition chamber and ensuring the impossibility of accumulation critical mass of uranium 235 in individual nodes of the installation.

The technical result is achieved by the fact that a portion of the waste is pre-crushed in a grinder and moved by a moving device to the first storage hopper, from where a portion of the waste is discretely transferred to the first airlock feeder, from which the waste is continuously removed into a metal thermal decomposition chamber heated by an external heater and moved uniformly by the moving device along the axis of the chamber to its exit, from which the formed solid products are withdrawn continuously into the second storage hopper, from which the accumulated portion is discretely transferred to the second airlock feeder and then to the receiving hopper box, the resulting gaseous products are fed continuously to the burner, the flue gases formed after their combustion are neutralized in a catalytic afterburner, cooled in a heat exchanger, cleaned in a cyclone, washed in a wet scrubber and removed from the process by a smoke exhaust, and the possibility of accumulating a critical mass of uranium - 235 during the technological process is eliminated by coordinating the operating mode of the devices, waste. The possibility of air entering the thermal decomposition chamber is eliminated due to the accumulation of waste layers in the device moving the waste into the first storage hopper and in the first lock feeder.

The essence of the proposed technical solution is illustrated by the drawing, where figure 1 shows a diagram of a device that implements the process of thermal decomposition of waste containing uranium 235.

The proposed method is carried out using the device shown in Fig. 1, including a chopper 1 with a moving device 2, a first storage hopper 3, a first lock feeder 4, a thermal decomposition chamber 5, with an external heater 6, a moving device 7, a second storage hopper 8, second lock feeder 9, hopper box 10, burner 11, catalytic afterburner 12, heat exchanger 13, cyclone 14, wet scrubber 15, exhaust fan 16.

The method is as follows.

A portion of the waste mixture weighing no more than 2.5 kg is loaded into the chopper - 1 and the resulting crumb by the transfer device - 2 is transferred to the first storage hopper - 3. After the transfer of the waste portion to the hopper - 3 is completed, the transfer device - 2 is stopped, the upper shutter of the first lock is opened feeder - 4, move a portion of waste into it, close the upper shutter and open the lower shutter of the gateway feeder - 4. Waste is collected by a continuously operating moving device - 7 and is fed into the internal volume of the thermal chamber application - 5 preheated by an external heater - 6. As the waste moves along the axis of the heated thermal decomposition chamber - 5 to its outlet, the waste gradually heats up, pyrolyzes and turns into gaseous and solid products. At the exit from the thermal decomposition chamber - 5, solid pyrolysis products (semi-coke) are uniformly discharged into the second storage hopper - 8, and gaseous products uniformly enter the burner device - 9. By the time of the first emptying of the first lock feeder - 4, in the first storage hopper - 3 prepare a new portion of waste. The lower gate of the gateway feeder - 4 is closed, the upper gate is opened, and the operation of moving waste from the storage hopper - 3 to the gateway feeder - 4 is repeated.

After filling the second storage hopper - 8 with the specified weight of the semi-coke, it is loaded into the gateway feeder - 10, open its lower shutter and dump a portion of the semi-coke into the hopper-box - 11. After the combustion of gaseous products in the burner - 9, the resulting flue gases enter the catalytic afterburner - 12, where the catalytic afterburning of organic residues takes place, and then into a shell and tube, air heat exchanger - 13. The flue gases cooled in the heat exchanger - 13 are cleaned of dust in a cyclone - 14, washed in a “wet” spray Bber - 15 and through the smoke exhaust - 16 are removed from the process.

The presence of layers of waste in the device moving the waste into the first storage hopper - 3 and in the first lock feeder - 4 eliminates the possibility of air entering from the external environment into the thermal decomposition chamber - 5 and, conversely, gaseous pyrolysis products from the thermal decomposition chamber - 5 into the external environment.

Continuous waste collection from the sluice feeder - 4 ensures their uniform entry into the thermal decomposition chamber - 5 and, as a result, almost the same steady-state mode of heating and pyrolysis of the waste over the temperature zones of the thermal decomposition chamber - 5, which in turn ensures a uniform supply of gaseous products pyrolysis to the burner - 9 and then to the heat exchanger - 13.

The uniform supply of flue gases to the heat exchanger - 13 allows them to be cooled in a heat exchanger with a calculated heat transfer surface, which avoids the need to use several of these heat exchangers at once to remove a large amount of heat in a short period of time, which results in a decrease in the total metal consumption of the installation.

Periodic shutdown of the moving device - 2 with a continuously operating moving device - 7 avoids the accumulation of a critical mass of uranium 235 in the units of the installation.

Example

In order to compare the value of the heat transfer surface necessary to remove the heat generated during the combustion of gaseous pyrolysis products, comparative experiments were conducted on the thermal decomposition of waste containing organic matter: - with a single load of a portion of the waste and - with a uniform supply of the same portion of the waste to the thermal decomposition chamber (KTP) .

Initial data: Mass of utilized waste - 4 kg / h; the maximum allowable diameter of the heat exchanger tubes is 110 mm; heat transfer coefficient (gas-gas) - α = 20, kcal / kg * m ² * hour * city C; temperature gradient - Δt = 300, ° C; the calorific value of gaseous pyrolysis products is 4500 kcal / kg.

1. One-time loading - 4 kg. The total process time is 20 minutes, including: heating the waste before the pyrolysis process begins - 7 minutes (35% of the total time), the active period of the process, during which 80% of the waste decomposes - 4 minutes (20% of the time); completion of the process, during which 20% of the waste material is heated and decomposed (some components, wet rags, etc.) - 7 minutes (45% of the time).

In the active period of the pyrolysis process (4 minutes), 2.24 kg of gaseous products are released. In terms of hourly productivity - 33.6 kg / h. The rate of heat input to the heat exchanger is Q = 33.6 * 4500 = 151200 kcal / h.

Necessary heat exchange surface

Required total heat exchanger pipe length

2. Uniform supply of waste into the thermal decomposition chamber - 4 kg / h. During this time, 2.8 kg of gaseous products will evenly stand out. The amount of heat that these products contain and which must be removed in the heat exchanger is Q = 2.8 * 4500 = 12600 kcal / h.

Necessary heat exchange surface

Required total heat exchanger pipe length

To remove heat with a uniform flow of flue gases into the heat exchanger, it is necessary to supply 326 n.m ^{3 of} air for an hour.

With an uneven intake of flue gases, the required air flow during the volley heat release is 3900 nm ³ / hour. Although this flow rate is only necessary for 20% of the total cycle time, in order to avoid complex problems with a constant change in the operation mode of the blower, the flow rate does not change during the entire cycle, which results in unnecessary energy consumption.

Thus, the combination of these essential features ensures a reduction in the metal consumption of the heat exchange equipment and saves energy resources due to the uniformity of the cooling of the combustion products, makes it impossible for air to enter the thermal decomposition chamber due to the accumulation of waste layers in the conveying device and the lock feeder and makes it impossible to accumulate a critical mass of uranium 235 due to the coordination of the mode of operation of devices that move waste.

Claims (1)

A method of disposing of waste containing uranium compounds, including loading the waste into a thermal decomposition chamber, heating and decomposing organics (pyrolysis) to form gaseous and solid products, burning gaseous products, rendering them harmless, cooling and dusting the flue gases, characterized in that the waste is pre-ground and fed discretely to the storage hopper, from which a portion of the waste is discretely transferred to the airlock feeder, from which the waste is continuously taken into metal heated by an external heater the thermal decomposition chamber and move uniformly along the axis of the chamber to its exit, from which the formed solid products are continuously discharged to the storage hopper, from which the accumulated portion is discretely transferred to the lock gate and then to the receiving hopper box, the resulting gaseous products are fed continuously to the burner the flue gases formed after their combustion neutralize, evenly cool in the heat exchanger, clean and discharge into the atmosphere, the possibility of air entering the chamber Decomposition is eliminated due to the formation of layers of waste in the device moving the waste from the shredder to the storage hopper and in the hopper feeder, and the possibility of accumulating a critical mass of uranium during the process is eliminated by coordinating the operating mode of the devices moving the waste.