DE8809641U1 - Catalytic igniter for igniting gas mixtures containing hydrogen and oxygen - Google Patents

Description

Description:

The invention relates to a catalytic igniter for igniting gas mixtures containing hydrogen and oxygen with the features specified in the preamble of claim 1. Such igniters are used, as described in DE-PS 3 004 677, in particular for reducing the hydrogen enclosed in the containment vessel of a water-cooled nuclear reactor system. According to DE-PS 3 004 677, several such catalytic igniters are distributed in the containment vessel, which are intended to flare off the hydrogen that may be produced in the event of an accident, i.e. to burn it in open combustion to form water as soon as an ignitable mixture of hydrogen and oxygen is present in the containment vessel.

A device for removing hydrogen from the atmosphere in the containment vessel of a nuclear reactor plant is already known from DE-OS 36 04 416, which has a storage vessel closed by bursting disks, which contains a solid-state storage device suitable for absorbing hydrogen. The solid-state storage device consists in particular of carrier bodies and foils made of titanium, vanadium, zirconium, niobium or tantalum, which are layered to form a foil package and have a protective layer of palladium to protect against surface oxidation. The effectiveness of this device is based partly on the absorption of hydrogen and partly on the catalytic combustion of hydrogen. The disadvantages here are the limited

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Storage capacity and the risk that the catalyst absorbs catalyst poisons and thus becomes ineffective.

The present invention is based on the object of creating an igniter for hydrogen-oxygen mixtures which is as simple, long-lasting and reliably igniting as possible and which is particularly suitable for use in safety containers ^of water-cooled nuclear reactors.

_; 10 This task is solved by a detonator with the

■ features specified in claim 1. Advantageous further developments of the invention are the subject of the subclaims.

: The igniter according to the invention dispenses with any water

■■ 15 material storage and acts solely as an igniter. To

■ to protect against catalyst poisons, it is located in a

j sealed housing, the locks of which can only be opened when

I exceeding a given critical pressure and/or a

open automatically at a given critical temperature.

l 20 During normal operation, the catalyst in the igniter is therefore not exposed to the atmosphere in the containment vessel. However, if an incident occurs that leads to the igniter opening, the gas, which may contain hydrogen and oxygen, is fed to the catalyst via a filter that can retain catalyst poisons such as iodine, dust, alkali and alkaline earth compounds, water, oil mist, etc. Preferably,

&zgr; is the filter made of a coarse filter, for example from a

Ceramic or metal fleece to retain dust and other

particles, as well as a fine filter, in particular a ceramic carrier or metal carrier, which is coated on the surface with a chemically active substance that adsorbs the expected catalyst poison. A porous ceramic body that contains silver nitrate and extends in the form of a disk over the entire cross-section of the housing is suitable as a fine filter. The silver nitrate can be introduced into the ceramic body by immersing the porous ceramic body in a silver nitrate solution and then drying it.

According to the invention, the catalyst body used is not a conventional ceramic honeycomb body, but a metallic carrier, in particular in the form of a corrugated or embossed sheet or a corrugated or embossed metal foil, which is rolled up into a coil and inserted into the housing in such a way that the main flow direction running from the inlet to the outlet opening is also the direction of the axis of the coil. Compared to a ceramic catalyst body, such a metallic catalyst body has the advantage that it can be made much more stable, which is of great importance in view of the fact that it is to be part of a safety device for decades. In addition, a metallic catalyst body has the advantage over a porous, ceramic catalyst body that it does not absorb water vapor, which greatly reduces the catalytic effectiveness of a catalyst constructed with a ceramic carrier.

Not only do these catalyst bodies not absorb water, but the build-up of water on the surface can also be prevented by providing the metallic catalyst bodies with a water-repellent coating. A suitable coating for this purpose is made of silicon dioxide. The catalytically active metal can also be anchored in and on the silicon dioxide layer. Palladium and platinum are particularly suitable as catalytically active metals.

An ignitable gas mixture can arise if the hydrogen content in the atmosphere exceeds approx. 4 vol.%. Even before this ignitability limit is reached, hydrogen is catalytically burned on the catalyst. Even if the ignitability limit is exceeded, there is no open combustion of hydrogen on the catalyst, but only catalytic combustion, because the catalytic combustion takes place faster than the transport of material to the catalyst, so that a depletion of hydrogen occurs between the filter and the catalyst. To counteract this, the catalyst body of the igniter according to the invention has metallic extensions that extend through the filter into the space between the filter and the inlet opening of the housing. The catalyst body is heated by the catalytic combustion and its extensions are also heated by heat conduction to such an extent that the hydrogen is released from them in the space in front of the filter, where the hydrogen content is higher than behind the filter.

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ignites and burns openly but in a controlled manner. The heating of these extensions can be further promoted by coating them with catalytically active metal. The extensions can be thin, straight-running wires with a not too high heat capacity or narrow strips of sheet metal. The extensions can be soldered or welded to the catalyst body. This ensures that the heat transfer from the catalyst body to the extensions is optimal and permanent. If a ceramic catalyst body is used, the firm and reliable connection to the metal extensions required for good heat transfer would be difficult to achieve and, above all, not guaranteed for decades. Another option is to manufacture the metal catalyst bodies in one piece with their extensions. If nets or metal fiber fabrics are used, the extensions can be woven into the nets in the form of wires or into the metal fabrics.

To avoid unnecessary heat transfer between the catalyst body and its extensions on one side and the housing on the other side, there is preferably a gap between them.

The closures at the ends of the housing ensure that the catalyst effect is not reduced by the ambient atmosphere before ignition is required. The closures can be membranes made of a

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They can be plastic that melts at high temperatures. They allow access to the catalyst if an increased temperature occurs in the event of an accident that could produce hydrogen. Another possibility is to use bursting membranes as closures that are torn by the excess pressure that occurs in the event of an accident.

The closures can also advantageously be designed in the form of bimetallic sheets, which deform when the temperature increases and thus allow access to the catalyst.

Embodiments of the invention are shown in the accompanying drawings.

Figure 1 shows a detonator in a perspective view, partly in section,

Figure 2 shows another embodiment of the catalyst body for use in an igniter as shown in Figure 1, and

Figure 3 shows another embodiment of a catalyst body, which could also be used in an igniter as shown in Fig. 25

The igniter shown in Figure 1 comprises a metal tube 1 which runs vertically in the operating position as a housing. This can be a steel tube with a diameter of 200 mm and

a length of 600 ran and a wall thickness of about 3 mm. These details show the typical size.

In the upper area of the tube 1, a catalyst body 2 is arranged, which has a slightly smaller diameter than the inner diameter of the tube 1. Its length is typically 150 mm. The distance between the inner wall of the tube 1 and the catalyst body 2 is, for example, 15 mm. The catalyst body 2 is fixed to the inner wall of the tube with projections 3, which ensure that the heat transfer between the catalyst body and the tube 1 is as low as possible. The projections preferably cause point-like contact with the catalyst body.

The catalyst body 2 consists of a corrugated metal foil which is rolled up into a coil. Foils of 40 to 50 µm thick made of aluminum-containing, high-temperature-stable steel are suitable. The foils are expediently selected and rolled up so that they have a cell density of 10 to 80 cells per cm ^of cross-sectional area in the cross section. The foil is coated with aluminum oxide or silicon oxide in a thickness of 10 to 20 µm and contains platinum or palladium in the surface layer of this coating in a concentration of 0.1 to 10 g per liter based on the volume of the catalyst body.

A filter 7 is provided under the catalyst body 2, which fills the pipe cross-section but is only about 30 mm high. It

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is used for the chemical neutralization of catalyst poisons, in particular iodine, and contains silver nitrate for this purpose. The filter 7 can be a ceramic body soaked in silver nitrate or a disk made of pressed fiber material (wire wool or asbestos fabric). It is important that the flow resistance is relatively small.

In a section of the catalyst body 2, it is shown that a wire 9 is attached, in particular welded, to the wall of one of the channels 4 that lead through the catalyst body. As can be seen, several such wires, distributed over the pipe cross-section, protrude downwards through the filter 7 into the space 12 between the filter and the lower closure 16 of the pipe 1. The wire ends 11 are heated by thermal conduction during catalytic combustion on the catalyst body 2 and can ignite hydrogen flowing into the free space 12 if the hydrogen concentration is greater than 4 vol.%.

Closures 14 and 16 are provided at the ends of the tube 1, which open depending on the temperature and/or pressure. In the example shown (Fig. 1), bimetallic strips 15 running parallel to one another are provided in four segments, which bulge outwards when the temperature changes, thus enabling a gas flow that is fuelled by the heating of gas in the tube 1.

The igniter works completely without external energy sources. It is maintenance-free and can therefore be installed anywhere inside the containment shell of a water-cooled nuclear reactor, in particular

of a pressurized water reactor. In the event of an accident, the increased pressures and temperatures that then occur cause the closures 14 and 16 on the pipe 1 to open. The medium in the safety casing therefore has access to the inside of the pipe. The hydrogen present in this medium is catalytically converted to water with the oxygen that is also present in the area of the catalyst body 2. During this exothermic reaction, the catalyst body 2 heats up. This sets in motion a gas flow that leads from bottom to top through the pipe 1. This supplies further gas to the catalyst body 2, the hydrogen content of which is further reduced by catalytic conversion.

If the hydrogen content exceeds the ignition limit of 4 vol.%, the wires 9 heated with the catalyst body lead to ignition in the free space 12, so that open combustion takes place there and only unburned hydrogen components have to be converted in the area of the catalyst body itself. The use of a housing (tube 1) ensures that the aforementioned combustion as well as the catalytic heating take place in a shielded space and remain under control.

In the embodiment according to Figure 2, the catalyst body is also a winding of a metal foil, which, however, instead of welded wires, has integral extensions 9* which, like the actual catalyst body, can carry catalytically active metal.

In the embodiment according to Figure 3, the catalyst body 2 ¹¹ consists of wire nets 17 with a circular cross-section stacked on top of each other.
running wires 9 ¹ ' are braided in, which, like the wires 9 in the example according to Figure 1, serve for ignition in the free space 12.

Claims (10)

DR. RUDOLF BAUER · blPÜ.-IPtai/-HELMUT HUBBUCH DIPL.-PHYS. ULRICH TWELMEIER WESTLlCKE 29-31 (AM LEOPOLDPLATZ) 7530 PFORZHEIM (VVEST-GERMANVl ((O7231I 102290/70 ^< $PATMARK TELEX 783929 patma 25.07.1988 III/Be DODUCO GMBH. + Co. Dr. Eugen Dürrwächter, D-7530 Pforzheim Catalytic igniter for igniting gas mixtures containing hydrogen and oxygen Expectations: 1. Catalytic igniter for igniting gas mixtures containing hydrogen and oxygen, with an elongated housing having an inlet opening and an outlet opening opposite it, which are provided with pressure-dependent and/or temperature-dependent opening closures, with a metallic catalyst body arranged in the housing between the inlet opening and the outlet opening, which has continuous channels in the main flow direction from the inlet to the outlet opening and a hydrogen ■■ 111) 1) « « 1 1»·· » 1 · ■ * combustion-catalyzing metal, in particular platinum and/or palladium, "characterized in that a filter (7) for retaining catalyst poisons is provided between the catalyst body (2) and the inlet opening (21) of the housing (1), and in that metallic extensions (9, ^9'1 , ^9'1 ) extend from the catalyst body (2) through the filter (7) into the space (12) located between the filter (7) and the closure (16) closing the inlet opening (21). 2. Igniter according to claim 1, characterized in that the extensions (9, 9") are wires. 3. Igniter according to claim 1 or 2, characterized in that the catalyst body (2) consists of corrugated or embossed sheet metal or metal foil and is rolled up into a coil. 4. Igniter according to claim 2, characterized in that the catalyst body (2 ¹ ·) is formed by a stack of wire nets (17) and that the wires (9 ¹⁵ ) leading through the filter (7) are woven in at individual crossing points of the wire nets (17). 25 5. Igniter according to one of the preceding claims, characterized in that the housing (1) is a tube. 6. Igniter according to one of the preceding claims, characterized in that the catalyst body (2) and its Extensions (9, 9 ¹ , 9 ¹¹ ) are arranged at a distance from the housing wall.
5 7. Igniter according to one of the preceding claims, characterized in that the catalyst body (2) has a ceramic coating in which the catalytically active metal is anchored.
10 8. Igniter according to one of the preceding claims, characterized in that the extensions (9, 9', 9*') carry catalytically active metal. 9. Igniter according to one of the preceding claims, characterized in that the catalyst body (2) has a water-repellent coating. 10. Igniter according to claim 7 and 9, characterized in that the catalyst body (2) is coated with silicon dioxide.