FR2556608A1 - Filter elements for hot gases and process for their use - Google Patents

Abstract

Device for filtering hot gases, especially gases containing a radioactive contamination, characterised in that a metal filter candle core 24 which is pierced with suitable holes is employed and in that a refractory oxide powder is deposited, in situ, on this core. The invention also relates to a process for using the said device, characterised especially in that the hot gases are at a temperature of approximately 300-400 DEG C.

Description

filter elements for hot gases and tower process
their usage.

 The nuclear industry produces waste, many of which is contaminated, requiring the use of precautions not used in other industries.

 In particular, a large tonnage of materials and supplies with very low contamination cannot therefore be landfilled. We can cite rags, plastic parts, filters, papers, gowns among these products.

To limit the volume of storage without dissemination of radioactivity, it has long been thought to incinerate all organic products such as those listed above
However, an incineration produces hot gases, fumes, dust and ash.

 Part of the radioactive contamination remains in the ashes, but a significant part leaves, in solid form, entrained by the hot gases.

 For particles larger than 0.3 microns, a filtration efficiency of 99.99E must be obtained before discharge to the outside, which makes it desirable to exceed 97 efficiency at the first filtration of hot gases (temperature can reach 750oC).

French Patent No. 1,498,522 describes the
possibility of filtering hot gases by use
a porous candle containing a mineral fiber
(deposited by spraying).

 The description contained in this patent in no way indicates the nature of the porous material or the nature of the fibrous material.

Note, however, that the filter substrate is placed inside a porous tube.

The length of this tube and its small diameter make it difficult to unclog in an active environment.

 French Patent No. 2,397,703 describes a filter candle composed of a ceramic playing both the role of support and filter medium.

When the spark plug is clogged, it must be replaced completely. When the spark plug breaks (in particular by thermal shock), it must also be replaced in its entirety. We are then led to store as active waste the whole candle (whole or in pieces) plus the dust fixed on it.

The present invention relates to a device for filtering hot gases from a waste incinerator, said device subsequently being called a filter candle or candle if there is no ambiguity,
According to the present invention, the candle consists of a metal tube pierced with holes acting as a support for the filtering medium, the latter being composed of a powder of fine particle size bonded to the surface of the tube by simple adhesion.

 A feature of the present invention is to use as a metal tube pierced with holes, a perforated sheet rolled and welded (or stapled) or a sintered metal tube.

 The present invention also relates to a process for the recovery of dust contained in hot gases, in particular hot gases originating from the combustion of xadioactively contaminated materials, said process being characterized in that an oxide powder is deposited, in situ refractory on the support of filter candles and that the gases to be purified are passed, while they are still preferably at a temperature of about 300 to 4000 ° C., on the filter candles thus produced.

 To follow the detailed description of the device which is the subject of the present invention, reference will be made to the figures accompanying the text.

 Figure 1 shows the general organization of an incinerator to show the space occupied by the filtration device.

 Figure 2 shows a view of the filtration device proper and more specifically, of a filter candle.

 Figures 3 and 4 show the shape detail of the collar of a candle.

 Figures 5 and 6 show the fingers of heated gloves placed in the enclosure.

 In FIG. 1, we see at 1, the loading well of an incinerator comprising the inclined grid 2 where the waste to be burned is placed.

Under the grate, a tube 4 supplies the air serving both to ensure combustion and to cool the grates. An ashtray 3 collects the ashes.

 The part overhanging the grate constitutes the combustion chamber 6 with a refractory brick wall 5. Immediately adjacent to and communicating with the combustion chamber through the upper part is the afterburner chamber 7 filled with refractory aggregates 8.

At the base part of said post-combustion chamber, a pipe 9 (possibly cooled) leads the gases to the filtration chamber 11, which is cut in leaktight manner by a horizontal partition 12 provided with holes in which the candles 13 are fitted. .

The tubing 9 opens into the filtration chamber, either above the partition 12, in this case, it is represented by the reference 101, or below the partition 12 and in this case, it is represented by the item 102.

 The filtration chamber has a series of glove fingers, either horizontal, point 14, or vertical, point 15.

 In these fingers are placed electrical resistors. Figures 5 and 6 show them in detail.

The filtered gases leave the enclosure 11, either through the upper tube 17, or through the lower tube 16.

It will be noted that the pipes correspond: - if gases enter through the high pipe 101, they
stand out after crossing candles 13 by
the low tubing 16.

- if the gases enter through the lower tube 102,
they come out after crossing candles 13
through the high tubing 17.

 The pipes 01, 102, 16 and 17 are provided with valves 18 making it possible to isolate the enclosure, to inject air into it or to create a depression there.

 the enclosure further comprises a hatch 19 for emptying the ashtray 103 and an upper intervention hatch 104.

 In FIG. 1, to simplify the drawing, only two filter candles 13 have been shown, two horizontal glove fingers 14 and a vertical glove finger 15. The complete apparatus will in fact include at least six candles and at least four fingers.

 The filter candle proper, shown in Figure 2 comprises a metallic core of cylindrical shape 21, having at one end a flange 23 and at the other end a bottom 22. The diameter of the holes drilled in the horizontal partition 12 (of the Figure 1) is such that the metal core slides easily in the hole and that the flange holds the spark plug in place. It is advantageous to design the flange as a solid metal piece having a sufficient weight for the candle to be held in place.

 On the lateral surface of the candle is placed a fine powder, either external 24 or internal 25.

 The powder is placed outside when the direction of gas flow is from 101 to 16.

The powder is placed inside when the direction of gas circulation is from 102 to 17.

 The detail of the flange and the hole in the horizontal partition is better shown in FIGS. 3 and 4. The flange 31 rests on the horizontal partition 32 (corresponding to the reference 12 in FIG. 1), but to facilitate guiding the spark plug, the hole in the partition 32 is provided with a flange 33.

 The play between this rim and the metallic core 34 is greatly exaggerated in the figure to aid understanding.

 Another embodiment of the flange is shown in Figure 4: in order to improve the seal between the partition 43 (corresponding to the reference 12 of Figure 1) and the flange 41, the hole is provided with a flange 44 directed upwards and entering a groove 42 machined in the flange 41.

 In the same way as previously, the outside diameter of the core 45 is such that the spark plug penetrates with a limited clearance in the hole limited by the rim 44.

 FIG. 5 represents a vertical thermowell such as the reference finger 15 of FIG. 1.

 This figure allows em at the same time to better see the side wall of the filtration chamber.

This wall is double - on the inner side 56, it is composed of a
thick refractory plate 50 (vertical wall) or
58 (background). A suitable material for this
realization is a refractory steel such as
NS 30, - on the outside, a fibrous insulator 55 such as
Fiber Frax (manufactured by Carborundum Cy.) and
a thin sheet 57 completes the envelope.

 No double wall was planned on the bottom.

 The thermowells consist of a sheath 51 placed between the wall 50 and the insulator 55.

A flange 54 allows a tight fixing (seal not shown) on the bottom 58.

 Inside the finger, an electrical resistor 53 is connected by the wires 52 to a regulated electrical supply which is not shown.

 The sheath of these thimbles can advantageously be made of a refractory material such as vitreous silica, alumina or sintered carborundum.

 FIG. 6 represents a horizontal section of the enclosure at the level of the axis of a horizontal thermowell.

 There is the inner wall of refractory steel 61 on which is welded a metal tube 64 closed at one end. Said tube passes through the fibrous insulation 62 and the outer sheet 63 to which it is fixed by a flange 65.

 We also see in this figure, the filter candles 66, which shows that the thermowell passes between the candles.

 An electrical resistor 68 connected by the wires 69 to an external power supply (not shown) is placed in the tube 64. It may be advantageous to arrange the assembly so that the heating part is inside the enclosure. 61.

 The filter candles are formed on site according to the operating procedure defined here. The metal cores are in place in the holes of the horizontal partition.

1st case
Filtering media outside the metal friend, there is an enclosure comprising pipes 102 and 17.

A quantity of alumina powder is introduced through the hatch 19 (if it is desired to produce an alumina filtering medium), through the tube 17, a vacuum is created while blowing air into the alumina through the tube 102 to suspend it in the air.

Under the effect of depression, the grains stick to the outside of the metallic core.

There is a self-regulating effect, because the suction is stronger at the level of the holes not yet covered, which brings powder there. When all the alumina is fixed, we never stop blowing air and all the candles are ready for filtration. It is best to choose a powder with a fine enough particle size for this suspension to be done easily
Grains smaller than 125 microns are very suitable.

2nd case:
Filtering media inside the candle, there is an enclosure comprising the pipes 101 and 16.

Through the hatch 104, an annex tube is introduced blowing a powder-air mixture, then, the hatch is closed, blows air through the tube 101 and sucks in through the tube 16.

For the same reason as in the previous case, a continuous layer is created over the entire lateral surface, without any preferential hole or path
Thermowells with electrical resistance are provided in the filtration chamber, because tests have shown that the powder making up the filtering medium tends to absorb moisture from the air (and combustion gases) at ordinary temperature. , while maintaining the enclosure at 300 or 400cl., the internal atmosphere was perfectly dry and the filtering medium kept its permeability. On the contrary, the fixation of water on the powder forms a pasty block of very reduced permeability.

 A very important additional advantage is due to this temperature maintenance: there is no condensation of water (containing many corrosive products) in the enclosure where a lifetime of this material is practically unlimited.

 The difference in corrosion is particularly marked when incinerating plastics containing chlorine or fluorine and especially polyvinyl chloride and polytetrafluoroethylene.

 For the proper conservation of the installation, the enclosure is kept warm at all times, even during periods of shutdown of the incinerator.

Examples of realization of the metallic train:
Example 1
We start with an expanded sheet of Inconel 601
dimensions 900 mm x 150 mm, thickness 0.4 mm.

The diamond-shaped holes measuring approximately
0.5 x 2.5 mm.

We roll this sheet to form a tube of
diameter 42 mm and which is closed with a few
welding or stapling points. We fix a
bottom on this tube and then fixed by welding
the tube on a massive flange.

Example 2
We start from a metal tube obtained by sintering
a powder size of Inconel 601
less than 50 microns. The finished tube is
dimension: thickness 5 mm, length 860 mm,
diameter 45 mm. We also provide it with a
massive flange fixed by welding and a
background.

Instead of the Inconel 601, various metals or alloys chosen to resist temperature and corrosion can be used, in particular a
Hastelloy, or Tantalum.

 The metallic core of expanded metal only serves to support the filtering medium and plays no role in stopping particles. With an alumina powder of particle size less than 25 microns, 98% of the particles measuring more than 0.3 microns are stopped. It is clear that it is not the 0.5 x 2.5 mm holes of the expanded metal which play the filtering role. On the other hand, the sintered metal core could itself play a certain filtering role.

After a certain operating time, the filter medium is clogged and must be changed if:
Powder placed outside the candle.

The valve 18 of the pipe 102 is closed and through the pipe 17, a strong pressure shot is sent to drop the powder towards the lower part 103 where it is recovered.

It will be noted that the polluted powder has remained in the lower zone, where the polluted gases arrive and that the upper part, above the partition 12 is perfectly protected.

2nd case:
Powder placed inside the candle.

Through the access door 104, a pneumatic nozzle is introduced to suck the powder into each candle, it is completed by blowing air through the tube 16
For a very high gas flow, for example, linked to a dilution of too hot gases by cold air, it is advantageous to work with the powder inside. Otherwise, the powder position outside is easier to use, in particular for unclogging.

All refractory oxides are suitable as long as they are not attacked by hot gases often containing chlorine or fluorine.
We will therefore use alumina or zirconia in this category. It is also easy to use silicates or alumino-silicates such as mullite or talc. We can obviously mix the previous constituents to make the filter media.

Claims (5)

1. A device for filtering hot gases from an incinerator by passing said hot gases through at least one filter candle consisting of a pierced core and a non-agglomerated filter medium, characterized in that said core is metallic and is pierced with holes whose average dimensions are between 0.5 and 2.5 mm, or which are obtained by sintering a powder. 2. Device according to claim 1, characterized in that said non-agglomerated filtering medium consists of a powder of these refractory materials such as alumina, zirconia, aluminosilicates or mixtures of said materials, the average particle size of which is between 0.025 mm and 0.125 mm, said powder being deposited in situ on said core.  3. Device according to one of claims 1 and 2, characterized in that the metal tube consists of a sheet of expanded metal, rolled and welded. 4. Method for carrying out filtration of hot gases from the incineration of materials and supplies having radioactive contamination, characterized in that said hot gases are sent through at least one device according to one of claims 1 to 3. 5. Method according to claim 4, characterized in that said filtration is carried out while said gases are at a temperature of 300 to 4000C.