Ignition system for igniting combustible gas mixtures

Abstract

An autonomously functioning ignition system, even though it is simple in design, allows for the reliable ignition of combustible gas mixtures that are only slightly above the ignition limit. The ignition system for igniting combustible gas mixtures, particularly in a containment structure of a nuclear facility, includes an electric ignition element and a thermoelectric generator that forms a source of current for the ignition element. A catalytic recombiner for the gas mixture, which is configured as a flow channel for the gas mixture, forms a heat source for the thermoelectric generator.

Claims

1. An ignition system for igniting a combustible gas mixture containing hydrogen and oxygen in a containment of a nuclear installation, the ignition system comprising: an electric ignition element being a heating wire or a glow plug; a thermoelectric generator having a plurality of N-doped and P-doped semi-conductor elements connected in series to form a power source for said ignition element; a catalytic recombiner for the gas mixture forming a heat source for said thermoelectric generator; wherein said recombiner is constructed in a flow channel for the gas mixture, and said thermoelectric generator is arranged outside the flow channel for the gas mixture.

2. The ignition system according to claim 1, wherein the flow channel is configured for conducting a natural draught of the gas mixture.

3. The ignition system according to claim 1, further comprising a heat pipe for discharging heat from said thermoelectric generator.

4. The ignition system according to claim 1, wherein said recombiner comprises ignition wires to be activated by way of reaction heat for igniting the gas mixture.

5. A nuclear installation, comprising a containment and an ignition system according to claim 1 disposed in said containment.

6. The nuclear installation according to claim 5 configured as a nuclear power station.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0042] Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown a schematic overview of an ignition system 2 for igniting a combustible gas mixture which can be formed in particular in a containment 4 of a nuclear installation, specifically a nuclear power station 6, in the event of a severe accident.

[0043] The ignition system 2 is constructed for autonomous and passive operation and comprises as a significant element a thermoelectric generator (TEG) 8 which in accordance with the Seebeck effect converts a temperature difference or a heat flux between a relatively cold side 10 and a comparatively hot side 12 directly into thermoelectric voltage. The thermoelectric generator 8 comprises for this purpose, for example, a plurality of N-doped and P-doped semiconductor elements 14 which are electrically connected in series and which with junctions 16 located therebetween being formed are arranged between the cold side 10 and the hot side 12. The junctions 16 are in this instance insulated by means of a highly thermally conductive electrical insulation element, for instance, a ceramic layer 18, with respect to the electrically conductive components of the environment.

[0044] There are located on the first and on the last semi-conductor element 14 of the series connection electrical connections 20 for tapping the thermoelectric voltage. An ignition current circuit 22 which is connected to the connections 20 contains an ignition element 24 which is arranged with some spacing from the thermoelectric generator 8 in the form of a heating wire, a glow plug or a spark igniter. With a sufficiently high thermoelectric voltage or sufficiently strong current flow (direct current, DC) in the ignition current circuit 22, the ignition element 24 is activated and brings about an ignition in the surrounding gas mixture, assuming that ignitable concentrations are present.

[0045] For a high level of effectiveness of the thermoelectric generator 8, the cold side 10 thereof is thermally coupled to a heat sink 26, while the hot side 12 is thermally coupled to a heat source 28. A consistently high temperature difference is thereby ensured.

[0046] The heat sink 26 may be located with some spacing from the thermoelectric generator 8, wherein the thermal coupling and heat discharge (discharged heat flux dQ/dt) in this instance are preferably brought about by passive heat pipes 30. The heat source 28 in contrast is preferably located in direct thermal contact with the thermoelectric generator 8 to the greatest possible extent, where applicable with a thermal conductor 32 located therebetween as a connection element.

[0047] In a preferred embodiment, the heat source 28 is formed by means of a passive autocatalytic recombiner (PAR) 34 which, for example, recombines hydrogen and oxygen in a surrounding or passing gas mixture in a flameless manner to form water vapor and which becomes heated as a result of the exothermic reaction (supplied heat flux dQ/dt). Alternatively or additionally, for example, a catalytic conversion of carbon monoxide and oxygen to form carbon dioxide may be provided.

[0048] A first variant of a specific embodiment of this concept is illustrated in FIGS. 2 to 4. The different components of the ignition system 2 are in this instance combined in a compact arrangement to form a common ignition unit 36.

[0049] In the center of the ignition unit 36 is a flow channel 40 which is delimited by a cylindrical pipe wall 38 and which is open in each case at the end side. In the usual installation or assembly situation, the flow channel 40 is oriented vertically and a gas mixture is able to flow through it as a result of the chimney effect produced generally from the bottom to the top. The flow channel 40 is sub-divided by means of cylindrical intermediate walls 42 which are arranged concentrically relative to the center axis into a plurality of annular chambers 44 which can be flowed through in parallel. The intermediate walls 42 and where applicable also the external pipe wall 38 contain in the sense described above catalytically effective surfaces, coatings or zones for the catalysis of the recombination reaction in the gas flow which is directed past, in particular of hydrogen with oxygen.

[0050] The geometry of the flow channel 40 and the sub-divisions thereof may in particular be constructed as described in the above-mentioned European published patent application EP 0 596 964 A1. This applies accordingly to the arrangement of optionally provided ignition wires (see further below). The embodiment of the catalysts is preferably carried out by coating a carrier sheet as described in European patent EP 0 923 707 B1 with a palladium strip for the rapid initiation of the catalytic reaction. The disclosures of the two publications are hereby expressly incorporated into the present text.

[0051] The heat released during the exothermic recombination reaction serves to heat at least one thermoelectric generator 8 at the hot side 12 thereof. In the present case, the flow channel 40 is surrounded by an in particular square housing 46 which is quadrilateral in cross-section, wherein the intermediate spaces between the outer side of the pipe wall 38 and the inner side of the housing 46 are filled with a highly thermally conductive filling material 48. For example, thermoelectric generators 8 are mounted at three of the four lateral outer faces of the parallelepipedal housing 46. Each of the thermoelectric generators 8 has in the example shown here a flat plate-like outline, wherein the inner base face which is active as the hot side 12 faces the filling material 48 and is connected thereto in a highly thermally conductive manner. To this end, the housing 46 is provided with suitable apertures. The outer base face which acts as a cold side 10 faces away from the housing 46.

[0052] In order to improve the cooling and the heat discharge from the cold side 10 of the respective thermoelectric generator 8, one or more heat pipes 30 are thermally coupled to the cold side 10, in this instance by means of a highly thermally conductive connection plate 52, for example, of aluminum (with respect to the heat pipe 30, the cold side 10 of the thermoelectric generator 8 is intended to be understood to be a heat source). The respective heat pipe 30 has a U-shaped structure, wherein one of the two legs 54 is in abutment with the connection plate 52 or directly with the cold side 10 of the thermoelectric generator 8, and the other leg 56 leads away from the housing 46 or as shown here extends with some spacing parallel therewith. The respective heat pipe 30 contains a heat transport medium which preferably circulates in a natural circuit between the relatively hot leg 54 which acts as an evaporator for the heat transport medium and the relatively cold leg 56 which acts as a capacitor. On the latter there is a heat discharge to the environment. A grid which surrounds the heat pipes 30 of thermally conductive sheets 58 (cooling ribs) optimizes the heat transfer to the surrounding atmosphere.

[0053] In place of heat pipes which are driven by gravitational force and/or driven by means of capillary action with a two-phase cooling circuit, it is also possible to use heat pipes which have a single-phase cooling circuit (a pure thermosiphon) but which are not so effective with regard to the heat transport.

[0054] Carrier arms 60 which are mounted laterally on the housing 46 and which protrude outward retain with some spacing from the housing 46 an ignition element 24, for example, in the form of a heating wire, a glow plug or a spark igniter. The conductors or cables of the associated electrical connection line advantageously extend within the carrier arms 60 which are constructed as casing pipes. The thermoelectric generators 8 may be connected electrically in series or in parallel depending on requirements with respect to the thermoelectric voltage produced by them.

[0055] As soon as a gas mixture which is capable of recombination flows through the flow channel 40, the exothermic recombination combination begins in the catalytically active zones of the recombiner 34 and begins the production of the thermoelectric voltage in the thermoelectric generator 8. The required ignition energy for igniting the gas mixture can consequently already be produced before reaching the lower ignition limit so that the ignition by the ignition element 24 can then be carried out directly after this limit has been exceeded.

[0056] Typically, the thermoelectric generators 8 produce an electric power of from 5 to 200 W, by means of which an ignition wire/heating coil of the ignition element 24 is brought to ignition temperature (typically >500 C.) or alternatively a spark igniter is supplied.

[0057] Furthermore, the catalytic recombiner 34 may also be active itself as an igniter for the gas mixture. This is particularly the case when, as a result of (large volume, global) transient gas displacement processes within the environment or as a result of extremely rapid releases of ignitable gas mixtures, there is produced a sudden massive flow which heats the surface of the catalytically active zones to high temperatures (typically >500 C.). In order to support this alternative ignition mechanism, ignition wires 62 may be present and are connected at one end to the catalytically active zones and are heated by them to ignition temperature and at the other end protrude into the flow channel 40. With respect to the details in relation to this ignition mechanism, reference may be made to the above-mentioned publication EP 0 596 964 A1.

[0058] The variant of the ignition system 2 illustrated in FIG. 5 differs from the previously described variant only in terms of the geometry of the flow channel 40 which in this instance has a rectangular cross-section and is sub-divided by a plurality of flat plates 64 with a catalytic coating or catalytic zones into parallelepipedal part-channels.

[0059] Such a rectangular geometry is also present in the variant illustrated in FIGS. 6 and 7. A convection housing 66 which protrudes beyond the housing 46 with an outlet 68 which is arranged at the upper end supports the desired natural draught of the chimney-like arrangement (a similar chimney pot, but not quite so high, can also be seen in FIGS. 3 and 4).

[0060] In addition to the first flow channel 40 which is active as a recombiner 34 and in which the flowing gas mixture is reduced or depleted in terms of the concentration. of the ignitable components thereof, there is located in parallel arrangement a second flow channel 70 (convection channel) which is separated in terms of flow from the first flow channel 40 without recombiner elements in which no depletion of the convection flow is carried out. Through apertures of the intermediate wall 72, ignition wires 62 of the first flow channel 40 are guided into the second flow channel 70. These ignition wires 62 are heated by the catalytic zones in the first flow channel 40 and direct, in the context of the earliest possible ignition, the ignition energy into the second flow channel 70, where the gas flow can be more readily ignited as a result of the lacking depletion.

[0061] The following is a summary list of reference numerals and the corresponding structure used in the above description of the invention:

[0062] 2 Ignition system

[0063] 4 Containment

[0064] 6 Nuclear power station

[0065] 8 Thermoelectric generator

[0066] 10 Cold side

[0067] 12 Hot side

[0068] 14 Semi-conductor element

[0069] 16 Junction

[0070] 18 Ceramic layer

[0071] 20 Connection

[0072] 22 Ignition power circuit

[0073] 24 Ignition element

[0074] 26 Heat sink

[0075] 28 Heat source

[0076] 30 Heat pipe

[0077] 32 Thermal conductor

[0078] 34 Catalytic recombiner

[0079] 36 Ignition unit

[0080] 38 Pipe wall

[0081] 40 Flow channel

[0082] 42 Intermediate wall

[0083] 44 Annular chamber

[0084] 46 Housing

[0085] 48 Filling material

[0086] 52 Connection plate

[0087] 54 Leg

[0088] 56 Leg

[0089] 58 Thermally conductive sheet

[0090] 60 Carrier arm

[0091] 62 Ignition wire

[0092] 64 Plate

[0093] 66 Convection housing

[0094] 68 Outlet

[0095] 70 Flow channel

[0096] 72 Intermediate wall