CA2709226C - Recombiner element - Google Patents

Abstract

The invention relates to a recombiner element (4) comprising a plurality of catalyst elements (8) that are arranged in a common housing (6) and that trigger a recombination reaction with oxygen when hydrogen is carried along in a feed gas flow, the housing (6) surrounding the catalyst elements (8), which are arranged therein, in a funnel-type way in such a manner that the heat released by the recombination reaction supports the gas flow inside the housing (6) by a convection effect, which recombiner element safeguards a reliable removal of the hydrogen from the gas mixture with an especially high degree of operational safety even under comparatively extreme conditions or scenarios of said type.
According to the invention, at least one of the catalyst elements (8) arranged inside the housing (6) has a predetermined ignition zone (20) in which a surface temperature of more than 560°C is produced in the convective operation at ambient conditions of approximately 1 bar and 100°C at a hydrogen concentration in the feed gas flow of more than 5% by volume.

Description

Description Recombiner Element The invention relates to a recombiner element, in particular for use in a security system for a nuclear plant, comprising a plurality of catalyst elements that are arranged in a common housing and that trigger a recombination reaction with oxygen when hydrogen is carried along in a feed gas flow, the housing surrounding the catalyst elements, which are arranged therein, in a funnel-type way in such a manner that the heat released by the recombination reaction supports the gas flow inside the housing by a convection effect.
In a nuclear plant, in particular in a nuclear power plant, the formation and release of hydrogen gas and carbon monoxide within the safety container or containment surrounding the reactor core must be expected when there are breakdowns or accidental situations in which, for example, an oxidation of zirconium can occur due to nuclear heating. In particular after a coolant loss problem, large quantities of hydrogen can be released thereby. As a result, explosive gas mixtures can be produced inside the contairiment.
Without counter measures, the accumulation of hydrogen in the containment atmosphere is thereby possible to the extent that the integrity of the safety container could be endangered when there is a- random ignition due to the combustion of a large quantity of hydrogen.
To prevent the formation of explosive gas mixtures of this type in the containment of a nuclear power plant, various devices or procedures are being discussed.
These include, for example, devices such as catalytic recombiners, catalytic and/or electrically operated ignition devices or the combination of both of the aforementioned devices and methods for a permanent intertization of the containment.
When using an ignition system to remove the hydrogen from the atmosphere of the containment, a reliable recombination of the hydrogen with oxygen should be obtained by means of a controlled combustion. A
significant pressure build-up as a result of a virulent hydrogen combustion should thereby be safely avoided. An ignition system of this type is thereby usually designed in such a way that a reliable ignition of the hydrogen is also already ensured when the lower ignition threshold of a gas mixture is exceeded, i.e. in a gas mixture having comparatively low hydrogen concentration, or when the inertization threshold of about 55% by volume vapor is fallen below and also at high hydrogen concentrations.
An ignition system known from EP 289 907 B1 for the controlled ignition of a hydrogen-containing gas mixture comprises a spark igniter which can be fed via an integrated energy storage. The ignition system is thereby provided with an energy storage designed to be autark, so that no feed lines are required. In this case, in particular, a drycell battery is provided as energy storage.
However, this ignition system is only suitable for a limited service life due to the capacity of the integrated energy storage.
Furthermore, this results in fundamentally preventing a flameless catalytic oxidation due to premature ignition in the concentration range of e.g. 5 to about 8% by volume. Therefore, an advantageous flameless catalytic degradation at higher concentrations and the simultaneous creation of high temperature regions (> 600 - 900 C) is excluded.
The range of the flameless catalysis is thus practically reduced to the non-ignitable range and a premature single ignition with quick gas displacement processes is already triggered at slight variations in concentration without enabling an effective counter ignition to attain short flame acceleration means due to the lacking high-temperature regions. Furthermore, when there is a premature excitation of the spark igniter during a breakdown with subsequent hydrogen release, a controlled ignition of the hydrogen is only possible to a limited extent. In addition, this ignition system also does not react until after an ignition delay period has expired on the release of hydrogen. Also, a long-term operation of the ignition system, which would be required to cover all feasible breakdown scenarios, would only be possible with restrictions.
Furthermore, a precautionary excitation of the ignition system already in the forefield of an anticipated breakdown from an external station, as for example from the control tower of a power plant, is not possible.
Moreover, in security systems based exclusively on the use of ignition processes for hydrogen, e.g. in the form of spark-plug systems, there is the additional restriction that no hydrogen decomposition whatsoever can be conducted in vapor-inert situations.
Accordingly, in systems of this type, hydrogen accumulating in the safety container can not be completely combusted until after corresponding vapor condensation. When there is hydrogen accumulation in vapor, this can lead to comparatively high hydrogen quantities or concentrations which are then burned in a comparatively short time due to the ignition, so that uncontrolled reaction patterns can arise. In addition, in systems based exclusively on ignition, it must be taken into consideration that the ignition could be omitted completely in so-called "station-black-out" scenarios, i.e. scenarios with complete loss of the power supply within the containment.
Alternatively or in addition, therefore, so-called passive autocatalytic recombiners can be arranged in the safety container or containment of a nuclear plant within the scope of a security system. These usually comprise suitable catalyst elements which, in a catalytic manner, trigger a recombination reaction with oxygen when hydrogen is carried along in a feed gas flow. The catalyst elements are thereby usually provided with a surrounding housing, wherein the housing is designed in the manner of a funnel such that a convection flow occurs automatically within the housing due to the funnel effect, so that the gas mixture is conveyed reliably along the respective catalyst element and the catalytic recombination reaction can be maintained in this way. The actual catalytic elements are hereby arranged primarily vertically and to a great extent parallel within the respective catalytic recombiner element in order to produce and promote the upward lift between the elements.
When hydrogen occurs in the gas mixture of the containment, these devices usually start automatically and oxidize the hydrogen with oxygen contained in the atmosphere, so that an effective hydrogen degradation can be obtained without ignition, in particular also in the presence of vapor-inert conditions or gas mixtures slightly above the ignition threshold.

However, in postulated breakdowns scenarios with high hydrogen release rates and simultaneously low vapor concentrations in the safety container, locally or globally critical concentrations and amounts of resultant hydrogen can also be obtained in systems of this type. As the ignition on such recombiners has to date only been observed at random under varying atmospheric conditions such as hydrogen concentrations, vapor components, etc., devices of this type do not provide a reliable prevention of undesired ignitions nor a guarantee of the ignition function. Furthermore, time delays of up to 30 min were reported in studies to obtain the maximum reaction temperatures on such catalyst units. Steps to completely prevent catalyst ignitions, such as e.g. by reduced coating thicknesses or diffusion-inhibiting coatings, etc. did not lead to the safe exclusion of undesired ignitions in the higher concentration range. Even if this were successfully shown, random ignitions can not, basically, be generally excluded due to feasible other unstable ignition sources in the containment.
Therefore, for the safe design of a containment when using catalytic recombiners, the maximum concentration in the safety container resulting from an excessive hydrogen feed is determined in each case and subjected to an ignition under these conditions.
In ignition scenarios of this type, the formation of quick deflagrations up to possibly deflagration/detonation transitions are to be expected. In order to be able to suitably compensate the considerable loads and differential pressures of up to several bar theoretically occurring thereby with the structural design of the containment, the corresponding structures of the containment and the fittings provided therein are usually designed equally massive.
Therefore, a modified design of a security system would be desirable in which too great a concentration of hydrogen in the atmosphere under the aforementioned conditions were to be excluded from the start and the aforementioned ignition or detonation scenarios could thus be safely prevented.
In order to meet endeavours of this type, combined systems can also be provided which comprise both igniters and catalytic recombiners.
A combined catalyst/ignition system for the recombination of hydrogen in a gas mixture is known, for example, from EP 596 964 Bl.
In this system, during the catalytic recombination of hydrogen, the heat obtained on a catalyst body is conveyed to an ignition device and used there to ignite non-depleted hydrogen-containing gases. In a combined catalyst/ignition system of this type, however, the ignition of the hydrogen does not occur until after an ignition delay period after the release of the hydrogen has terminated. That is, a certain amount of time is required after the first release of the hydrogen until the catalyst body, including the adjoining ignition device, has warmed up sufficiently to enable an ignition of the hydrogen. The result of this time delay is that, in quick gas displacement processes, the ignition of the hydrogen does not start until there are comparatively high hydrogen concentrations.
After the entire system has warmed up, however, a premature ignition-already occurs after the lower ignition threshold has been exceeded on the non-catalytic parts. This results in basically preventing a flameless catalytic oxidation due to premature ignition in the concentration range of e.g. 5% by volume to about 10% by volume.
A flameless catalytic degradation at higher concentrations and the simultaneous creation of high-temperature regions is excluded in this way.
Consequently, the range of the flameless catalysis is practically reduced to the non-ignitable range and premature single ignitions with quick gas displacement processes are already triggered at slight differences in concentration without enabling an effective counter ignition to obtain short flame-acceleration paths due to the missing high-temperature regions.
In other combined systems with catalytic recombiners and with a plurality of autonomous spark igniters in which the ignition is introduced independent of the catalytic recombination in an ignition device, a comparatively large expenditure is to be expected due to adjusting the systems to one another in a corresponding manner and, in particular, the handling of an unfavorable consequence is problematic when there is an incorrect ignition. In principle, in this case also, it is true that premature single ignitions are triggered with corresponding gas-displacement processes without enabling an effective counter ignition to ensure short flame-acceleration paths due to the missing high-temperature potentials.
Therefore, some embodiments of the invention may provide a recombiner element of the aforementioned type, in particular for use in a security system in a nuclear plant, with which a reliable removal of hydrogen from the gas mixture with especially high operational safety is assured, also under comparatively extreme conditions or scenarios of the stated type.
According to an embodiment of the invention, at least one of the catalyst elements arranged inside the housing has a predetermined ignition zone in which a surface temperature of more than 560 C is produced in the convective operation at ambient conditions of approximately 1 bar and 100 C at a hydrogen concentration in the feed gas flow of more than 5% by volume.
According to an embodiment of the invention, there is provided a recombiner element comprising a plurality of catalyst elements that are arranged in a common housing and that trigger a recombination reaction with oxygen when hydrogen is carried along in a feed gas flow, the housing surrounding the catalyst elements, which are arranged therein, in a funnel-like way in such a manner that the heat released by the recombination reaction supports the gas flow inside the housing by a convection effect, and wherein at least one of the catalyst elements arranged inside the housing has a predetermined ignition zone, wherein the at least one of the catalyst elements is configured in such a manner that a surface temperature, lying above the ignition temperature present under these conditions, of 560 C is produced at the predetermined ignition zone in the convective operation at ambient conditions of 1 bar and 100 C and at a hydrogen concentration in the feed gas flow of more than 5% by volume, and wherein means for flow focussing are connected upstream of the catalyst element on the gas-flow side, the means for flow focussing comprising baffle plates, directional baffles, vortex generators, or turbulence generators, supporting a selective feeding of incoming pressure pulses to the predetermined ignition zone.
The invention is based on the consideration that a reliable removal of hydrogen under the aforementioned, perhaps extreme 8a conditions can be attained while reliably preventing the formation of critical concentrations and with consequential exclusion of detonation scenarios by completing a system based essentially on a catalytic recombination in an especially suitable manner by ignitions which are introduced in a controlled manner. For this purpose, to maintain especially high work safety standards and also to control "black-out"
scenarios completely or at least partially, the ignition system should be designed largely passively. A specific completion of this type of a system based on catalytic recombiners by suitable ignition mechanisms can be obtained by using the heat released during the catalytic recombination locally in the region of the catalyst elements to introduce ignitions in a controlled manner.
In particular, the system should thereby be designed in its entirety in such a way that a catalytic hydrogen degradation, in particular already at non-critical concentrations of e.g. 6 to about 8% by volume hydrogen, be introduced prematurely, also in dry scenarios with moderate hydrogen release and comparatively low vapor components. This flameless recombiner operation should be expanded at higher vapor concentrations of e.g. > 30% by volume up to about > 8% by volume hydrogen concentration, at > 40% by volume, = CA 02709226 2010-06-14 preferably up to about 10% by volume and more hydrogen concentration.
It is hereby attained that no ignition at all occurs in a number of scenarios. Only in extreme scenarios, in particular with the occurrence of relevant hydrogen quantities having concentrations above about 8% by volume in each case, however, with hydrogen concentrations of more than 10% by volume, a further increase in concentrations should, as a precaution, be prevented and an ignition automatically triggered in a controlled manner in the various spatial areas of the safety container.
To reliably ensure this while preventing an ignition delay time that is deemed too high, it is now provided to use the flowing and ignition behaviour of a hydrogen-enriched gas flow in the area of the respective catalytic elements in a controlled manner by means of a suitable struatural positioning and dimensioning of the catalyst elements and the housing surrounding them and a suitable structural design, in particular with respect to the predetermination of the flow path and dimensioning of the components provided therefor.
The incoming hydrogen in the starting phase of the reaction, even at low temperatures, is thereby converted by a catalytically especially effective system on the preferably small masses, a quick temperature increase is attained and thus the catalytic reaction is further accelerated and in this way a surrounding boundary layer built up.
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It is thereby understood that, in a catalytic recombiner of the aforementioned type in which, for example due to funnel effects or the like, the gas flow is conveyed along the catalytic elements at a specific flow rate and in this way the recombination reaction is started and maintained, in the state of equilibrium of the = CA 02709226 2010-06-14 catalyst, i.e. in particular in the working condition of the natural convection, preferably in the laminar flow region, now with quickly running recombination reaction, a thicker border layer can be built up and in this way a depletion of the hydrogen component in the gas flow directly adjacent to the catalytically active surfaces takes place. This is a consequence of the very quick kinetic recombination reaction, now in the equilibrium state of the catalyst at the increased temperatures, on the catalyst and the simultaneously limiting gas diffusion processes which leads directly in the peripheral region of the catalytically active surface to the gas flowing past due to the conversion of the hydrogen with oxygen carried along to a local depletion of the hydrogen and oxygen component in direct vicinity of the catalyst and quasi to the construction of a protective layer.
In this way, the device is dimensioned in a suitable manner such that, in particular in the concentration increase phase, already in not yet ignitable mixtures, a quick and uniform heating in the predetermined ignition zone region takes place, the kinetics of the catalytic reaction is then accelerated accordingly and a complete concentration/protective layer placed effectively in this way about .the high-temperature predetermined ignition zone.
That is, the heat produced by the catalytic recombination reaction which heats the catalyst accordingly can therefore only lead to an ignition of the circulating gas flow in an equilibrium state of this type when there is a hydrogen component of the gas mixture which is still sufficient for the ignition having regard to the temperature prevailing in the catalyst element, even in the depleted zone. In controlled introduction of ignitions, this can already be used during the natural convection operation for especially short ignition delay times by setting or maintaining temperatures above the ignition temperature of hydrogen, i.e. above about 560 C, by suitable structural design of the components in areas provided therefore, i.e. of a predetermined ignition zone.
The ignition in an area of this type with overcritical conditions takes place in this case without appreciable delay time, as soon as not significantly depleted and thus ignitable gas mixture does not reach sufficiently close to the respective area.
At higher temperatures, it should further be considered that the ignition region still expands at the lower and upper ignition threshold of a hydrogen mixture and, accordingly, a slight trend to a premature ignition is observed.
In particular, an ignition can be triggered by gas displacement processes, as a result of which an ignitable gas mixture reaches into the direct surroundings of the respective predetermined ignition zone. By setting the aforementioned conditions in the predetermined ignition zone in the manner of an "overcritical"
state, the system in this area thus reacts comparatively quickly and sensitively to gas displacement processes of any type, so that ignitions are already triggered quickly and safely in the forefield of anticipated large-area breakdowns.
The intentional setting of the surface temperature in the predetermined ignition zone can thereby take place in particular by suitable structural design of the respective catalyst elements and the adjacent components. In particular, the heat released by the recombination reaction in accordance with the design during the convection operation and heating the catalyst elements, as well as the corresponding heat dissipation, in particular in the form of radiant heat, and the heat conduction over directly coupled components can be taken into consideration.
The adjustable temperature can thereby be appropriately influenced, in particular, by suitable geometry and dimensioning selection of the respective components with respect to the respective heat conductivity. If necessary, the system could thereby be provided with an additional heating in the area of the predetermined ignition zone for the reliable setting of the aforementioned surface temperature.
However, for an especially high operational safety, advantageously, the system is designed as a completely passive system in which the adjusting temperature is essentially given by the heat released during the recombination reaction and the corresponding conveyance into the area of the predetermined ignition zone.
Advantageously, the system is thereby designed in such a way that a surface temperature of between 600 C and 900 C is set with the predetermined ignition zone in the convection operation at ambient conditions of about 1 bar and 100 C at a hydrogen concentration in the feed gas flow of more than 5% by volume.
To ensure sufficiently high temperatures inside the housing, at least three, preferably at least ten, catalyst elements are arranged inside the housing.
To generate a reliable recombination reaction, the or each catalyst element is preferably appropriatebly designed.
In particular, the or each catalyst element, respectively, has a catalytically active zone composed of porous material, preferably of A1203, whereby, in a further preferred embodiment, the porous material as a ceramic wash coat pore system, optionally in addition with a suitable etch-coat adhesive layer, for obtaining adequate abrasion resistance, is applied to a suitable carrier, preferably to a thin metal carrier with an Al component. In this way, an enlargement of the inner surface promoting the catalytic effect can be obtained, preferably by more than the factor 1000, especially advantageously by more than the factor 10000.
In a further advantageous embodiment, the porous material of the catalytically active zone is doped with catalyst material, preferably with Pt and/or Pd, whereby the Pt and/or Pd distribution is also placed into the deeper lying areas of the pore system, advantageously to prevent a deactivation by catalyst poisons or the like.
For an especially advantageous catalytic effect, an overdoting of 2 to 10, in particular of up to 25 g/m2, is advantageously provided for the noble metal concentration. The doting is advantageously higher in the area of the predetermined ignition zone than in the remaining catalytically active area.
The catalytically active materials, in particular platinum and/or palladium, can be doted on closed metal carriers, on perforated carriers or also on ceramic carriers, e.g. balls or pellets, and applied as bulk within suitable metal frame-support structures.
The coating density with the catalytically active noble metals can also be kept local and, in particular, variable over the flow height, so that, among other things, location and height of the adjusting surface temperature and thus also the position of the predetermined ignition zone can be influenced with respect to the recombination reaction.
Advantageously, for example, the predetermined ignition zone in the inlet area of the catalyst element can be localized in this manner. For example, an increased enrichment of catalyst material can take place hereby in the area of the predetermined ignition zone, especially advantageously from the two catalyst elements Pt and Pd.
Advantageously, the catalyst elements are arranged mainly in the lower area of the recombiner element, so that the funnel effect and thus the convection flow inside the housing can be especially supported by the heat resulting from the recombination process.
Advantageously and for especially advantageous flow conditions, the recombiner element has a free cross sectional area in its inlet area, i.e. a portion of the cross sectional area on the entire cross sectional area which can be freely flowed through by the gas flow, of more than 40%, preferably more than 90%.
With a construction in thin-foil technology, an especially advantageous free cross sectional area of up to 98% is possible.
Furthermore, in an advantageous embodiment, the device is dimensioned with low catalyst masses and only reduced heating of the non-catalytic housing parts not lying in the catalytic areas such that, in particular in the concentration increase phase or in not yet ignitable mixtures, already a quick uniform heating takes place, the catalytic reaction is then accelerated accordingly and a complete concentration protective layer can in this way be effectively placed about the high-temperature area of the predetermined ignition zone in the inlet area.

Advantageously, a ratio of shaft depth to shaft height of about 1:3 to 1:5 is provided to promote the funnel effect and thus for especially stable flow conditions.
The overall height of the recombiner element can thereby be, for example, 0.3 m to 3 m.
The catalyst elements of the recombiner element are preferably designed as catalyst plates, said catalyst plates are preferably orientated predominantly vertically to support the flow.
The catalyst elements are preferably spaced from about 0.5 cm to 3 cm from one another and arranged at about the same height as the lower edge of the housing, however, advantageously with their lower edge at about or up to 10 cm (higher than the lower edge of the housing). The spaces between individual catalyst elements may also be varied, so that, with less space, a higher degree of reaction and thus locally higher temperatures are obtained. This parameter can thus also be referred to for the temperature guide and presetting the predetermined ignition zone. Alternatively or in addition, by locally increased installation density of the catalytically active surfaces, a so-called hot spot, i.e. a zone of increased temperature, can be created to define the predetermined ignition zone.
For an especially reliable ignition introduction, the catalyst plates are advantageously configured as thin elements or in thin-foil construction with a wall thickness of less than 1 mm, preferably less than 0.2 mm, partially even clearly less. Due to the accompanying comparatively low local thermal inertness, in particular in transient processes, an especially spontaneous catalytic mixture depletion and local temperature increase can be obtained thereby, so that especially in transient changes in mixture, the desired ignition function can be especially effectively ensured.
By using thin-foil technology and a suitable catalytic structure, it can be ensured that, e.g. in transient concentration changes in the range of e.g. above 3% by volume hydrogen, a surface temperature increase rate sets in on the catalyst element of >
500C/% H2, within 30s, preferably within <10s. Furthermore, the catalyst elements can hereby also be designed and arranged in the predetermined ignition zone such that, with transient feed of hydrogen of almost 0% by volume to spontaneously > 6% by volume, preferably > 8% by volume, the build-up of a completely effective concentration protective layer does not take place partially and for a short time and that, to be on the safe side, a premature ignition takes place.
This catalytic hydrogen degradation can preferably take place at hydrogen concentrations of >8 to 10% by volume, at correspondingly high vapor-0O2 content of e.g. about 40% by volume, at >50 - 55 vapor CO2 content, also with hydrogen concentrations of >10% by volume. The dual function of the device is also shown in these vapor-inert areas, at about >55% by volume vapor-0O2 content, as advantageous, since hydrogen can already be seriously degraded by the flameless oxidation and at the same time the creation of correspondingly high temperature potentials, of e.g. > 600 C, in particular however at the upper more critical ignition threshold due to the ignition conditions, also temperature potentials of up to 900 C. Due to these high temperatures, the increased heat dissipation resulting from the high vapor content can be compensated in the ignition zone and the safe ignition can also take place under these conditions.
By combining this specific recombination device with the operation of a containment spray system, an intensive mixture of the atmosphere in the vapor-inert area can take place by spraying and the generated recombiner convection flows are produced, and a reduction of the hydrogen content can be simultaneously obtained.
In particular, possible critical high-concentration clouds with relevant potential for the flame acceleration can be mixed briefly with the remaining atmosphere and, furthermore, the various high-temperature predetermined ignition zones can be brought to a more uniform level.
In this case, especially pronounced high temperatures can be set in the predetermined ignition zones of 700 C, preferably 800 C. Furthermore, The temperatures of the high-temperature predetermined ignition zones can be determined directly and representatively by suitable sensors and the hydrogen control strategy, i.e. in particular the activation and/or actuation of the containment spray system, can be adjusted accordingly on the basis of this information. Furthermore, the device delivers very reliable spontaneous ignitions through the pronounced high temperature zone, even when high gas velocities of e.g. < 50 m/s and more occur.
The resultant cooling effect, caused by the cooler ambient atmosphere massively flowing in, can most likely be compensated by the present temperature adjustment of the substances inside the device.
In an especially advantageous embodiment, a catalytically non-active area, arranged downstream of a catalytically active zone on the gas current side is provided as predetermined ignition zone.

The heat resulting in the catalytically active area is thereby conveyed into the downstream non-active area in a controlled manner. This is based on the consideration that there is already a depleted gas flow in the flow-off area of the catalytically active surface areas due to the preceding recombination reaction. Only in the event that comparatively large gas quantities will occur shortly, i.e. for example due to gas displacement processes inside the safety container, does an ignitable gas mixture reach into these zones, so that an ignition trigger, especially on an "as required basis", is ensured.
The recombiner device is hereby advantageously designed in such a way that e.g. a spontaneous doubling of the gas velocity in the predetermined ignition zone area, preferably at >5 m/s, at partial catalyst zone temperatures of >560 C (> preferably >600 C) and in this way the activation of the ignition function is obtained in a controlled manner.
In an additional or alternative advantageous embodiment of the recombiner element, means for flow focussing are arranged upstream to the catalyst element or elements on the gas current side. In this way, it can be ensured that external gas displacement processes in the safety casing result suitably focussed and amplified in a feed gas flow on the catalyst elements or, in particular, in the area of the predetermined ignition zone, so that especially in situations of this type the depletion zone in the gas flow is broken open in the area of the predetermined ignition zone and the ignition reliably triggered. The focussing can thereby be obtained or supported by suitable baffles or other diversion means, by means for generating turbulence, by spiral confusers and/or by cross sectional reductions. In particular, devices of this can be arranged in all main directions in the lower part of the housing, vertically or horizontally on the housing or also integrated in the catalyst elements. Furthermore, the catalyst device can be connected with a fully or partially closed tube or channel system. By delivering a pressure pulse, the gas rates can be increased in a specific manner in the area of the predetermined ignition zone via a tube element of this type, for example also combined with an ejector for drawing in ambient air, and the ignition triggered deliberately.
Preferably, the recombiner element is installed in a security system of a nuclear plant.
In particular, the advantages sought with the invention lie in that, by providing a predetermined ignition zone with a surface temperature which is overcritical in the convection operation, i.e.
above the ignition temperature for hydrogen, specifically the realization that a boundary layer with depleted hydrogen content forms in vicinity of the catalyst which can be used to ensure especially reliable and quick ignition processes. An ignition can be triggered quickly and reliably in a system of this type especially when the depletion layer is broken open due to breakdown conditions.
In particular, this is the case when an incoming pressure pulse or gas displacement process generates high gas flow rates in the inlet area of the recombiner element or in the area of the predetermined ignition -zone such that the gas layer found in the direct vicinity of the catalytic surface with depleted or reduced hydrogen content is broken open. As a result, no or only a little depleted gas can have direct contact with the comparatively hot surfaces of the catalytic element, so that an ignition is reliably triggered in this spatial area.

Consequently, the recombiner element is especially useful in security systems in which voice-over effects between individual recombiners in which an ignition in a recombiner is triggered by the flame front arriving from another recombiner, with respect to which instabilities associated therewith are to be prevented. Due to the fact that, during slow deflagrations, the pressure waves produced thereby with a comparatively longer period of oscillation and lower amplitude of the corresponding flame front precede, the ignition in the recombiner is triggered by the gas displacement processes produced by this before the flame front arrives. The massive combustible gas feed thus leads to an overfeeding of the local recombiner device and to a minimization of the concentration depletion in the boundary layer area at the heating surface and on the phase borders to a contact surface failure, so that, in addition, further convective flows are produced and a safe ignition made possible. As a result, a safety ignition of critical areas is ensured prior to a further increase in concentration, whereby, in the manner of a domino effect or a domino ignition proceeding from a first recombiner device, ignitions in adjacent or recombiner devices adjoining on the flow side are safely triggered. Voice-over effects and uncontrolled flow conditions can be safely prevented as a result, so that loads that have to be put up with are minimized.
Accordingly, the system in its entirety can be designed with an emphasis on the catalytic function of the hydrogen degradation, wherein a hydrogen degradation can take place only catalytically in comparatively many scenarios, i.e. in particular at concentrations of less than 8 - 10% by volume and corresponding vapor concentrations without ignitions. At higher concentrations, ignitions and combustion processes take place primarily in the concentration range or reaction range of slow deflagrations, whereby safe ignition processes can be introduced in adjacent devices due to the gas displacement processes preceding the combustion waves or flame fronts at intervals.
An embodiment of the invention will be described in greater detail with reference to the drawings, showing:
Fig. 1 a security system for recombination of hydrogen and oxygen in a gas mixture, Fig. 2 a catalytic recombiner in longitudinal section, and Fig. 3 the recombiner according to Fig. 2 in a side view.
The same parts are provided with the same reference numbers in all figures.
The security system 1 according to Fig. 1 is provided for the recombination of hydrogen in a gas mixture, namely in the containment atmosphere of a safety container 2 of a nuclear plant, shown in extracts in Fig. 1. To this end, the security system 1 comprises a plurality of catalytic recombiner elements 4 arranged inside the safety container 2; each of said recombiner elements catalytically triggering a recombination reaction of hydrogen carried along in a feed gas flow with oxygen contained in the containment atmosphere.
For this purpose, each of the thermal recombiner elements 4, as can be seen in the enlarged illustration in Fig. 2, comprises a plurality of catalyst elements 8 arranged in a housing 6. In Fig.
2, for the sake of clarity, four catalyst elements 8 are visible, however, in the example of the embodiment, in particular ten or more catalyst elements 8 are arranged in a common housing 6. The catalyst elements 8 each have a surface provided with a suitably selected material, for example palladium and/or platinum, said surface triggering a catalytic recombination reaction with oxygen contained in the atmospheric gas in an adjacent gas mixture in the event that this gas mixture contains significant hydrogen components of e.g. several % by volume. The hydrogen with the oxygen undergoes an exothermic reaction thereby while forming water. The catalyst elements 8 for their part are heated by this exothermic reaction, so that, due to the temperature drop resulting therefrom, a convection flow from the bottom to the top is produced in the surrounding gas chamber.
To promote this convection flow by the so-called funnel effect, the housing 6 of the respective recombiner element 4 surrounding the catalyst elements 8 is appropriately designed, in particular in a funnel-like manner, and, to further facilitate the convection flow resulting therefrom, the catalyst elements 8 are designed essentially plate-like and arranged parallel to one another.
Furthermore, the recombiner element 4 has an overall height of about 3 m and a ratio of shaft- depth and shaft height of 1:3 to 1:5. In the inlet area 10 for the gas flow, the recombiner element 4 has, in addition, a portion of freely flowable cross sectional areas, i.e. not hindered by the fittings, of about 90%. In its entirety, the recombiner element 4 formed from these components thus has structural properties which automatically start a catalytic recombination process in the presence of hydrogen in the atmospheric gas of the safety container 2 and maintain it by the supportive effect of the convection flow due to the funnel effect, and cause a further mixture of the atmosphere until a sufficient degradation of the hydrogen has taken place.
The catalyst element 8 has a catalytically active zone 12 consisting of a porous material in each case, in particular of A1203. The porous material is thereby applied to a suitable thin metal carrier having an Al component as a ceramic wash coat pore system and, in addition, with a suitable etch-coat adhesive layer to attain sufficient abrasion resistance.
This ensures an enlargement of the inner surface, which promotes the catalytic effect, by more than the factor 10000. The porous material of the catalytically active zone 12 is thereby doted with catalytic material, in particular with Pt and/or Pd, wherein the Pt and/or Pd distribution is placed largely homogeneously, also into the deeper lying areas of the pore system, to prevent deactivation by catalyst poisons or the like.
For an especially advantageous catalytic effect, an overdoting of up to 25 g/m2 is provided for the noble metal concentration.
The security system 1 is designed in its entirety to ensure a safe and reliable recombination of the hydrogen produced thereby, possibly in the atmosphere of the safety container 2, in a plurality of possible breakdown scenarios, including also comparatively improbable extreme breakdown conditions. For this purpose, the security system 1 is designed for the degradation of hydrogen with the emphasis on catalytic recombination, whereby, if necessary, and in particular locally limited, an ignition of an ignitable gas mixture should also take place. To this end, the catalytic recombiner elements 4 are predominantly designed, with respect to type, positioning and dimensioning of their components, such that no ignition takes place in gas mixtures having a hydrogen concentration of up to about 6% by volume or, if required, also up to about 8% by volume, at higher vapor concentrations of up to >10%
by volume, but that the hydrogen degradation takes place on the surface of the catalyst elements 8 by the catalytically triggered recombination reaction.
On the other hand, for higher hydrogen concentrations, it is additionally provided that the catalyst elements 8 are heated by the thermal energy released by the catalytic recombination reaction such that their temperature is in the manner of so-called "hot spots" at predetermined ignition zones 20 provided therefor and which preferably lie directly in the inlet area of the catalyst, above the ignition temperature of the gas mixture and thus supports an ignition of the gas mixture in the manner of a passive system automatically triggering the recombination process. The individual components of the system are thereby designed by arrangement, structure and dimensioning of the fittings within the housing 6, in particular the catalyst elements 8, taking into consideration the heat released during the recombination reaction and the dissipation of heat due to radiant heat or also- in the form of heat conduction over the individual components of the system such that a surface temperature is produced in the predetermined ignition zone 20 of between 600 C and 900 C, i.e. of more than the ignition temperature of hydrogen of about 560 C, under reference conditions in the convective operation of the respective recombiner element 4 at 25 =
ambient conditions of about 1 bar and 100 C of a hydrogen concentration in the feed gas flow to the catalyst elements 8 of more than 5% by volume.
In this design of the recombiner elements 4, the realization is taken into account that each of the catalyst elements 8, which can also in part be partially surrounded by metal, are flowed about in the catalytic recombination operation, i.e. in the presence of natural convection, by the gas flow requiring treatment, a depletion of the hydrogen constituent in the gas flow taking place in direct vicinity of the catalytic surfaces of the catalyst elements 8 due to the ending recombination reaction. In the state of the natural convection, the catalyst elements 8 are thus contacted directly by the depleted gas in the manner of a layered gas flow, whereby non-depleted gas with a correspondingly increased hydrogen content is present in further remote spatial areas.
Therefore, in this state of the natural convection, the ignition effect which the heated catalyst material can exert on the surrounding gas flow is reduced by the depleted gas layer.
This effect is used in the recombiner element 4 to operate the predetermined ignition points 20, if necessary, i.e. in particular in the state of the natural convection, in the manner of an overcritical modus in which a surface temperature that is actually above the ignition threshold prevails. In a state of this type, the system is thus comparatively sensitive to breakdowns or the flow conditions, where, in the event that the depleted gas layer which contacts the overheated surface parts is broken open and non-depleted gas can reach these surface parts, an ignition is spontaneously triggered due to the increased temperature. Thus, with this design of the system, the triggering of an ignition can be obtained spontaneously and with negligible ignition delay time when there are pressure pulses or gas displacement processes in flow conditions in the direct vicinity of the catalyst elements 8.
Thus, in particular an automatic and passive ignition can be assured in the event that pressure pulses occur inside the safety container 2, so that a reliable ignition can already be triggered in the forefield of possibly imminent breakdowns or the like.
To further increase the sensitivity of the system to pressure pulses or gas displacement processes or the like and thus further increase the reliability and safety of the ignitions to be introduced, suitable means for flow focussing can be connected upstream in individual catalyst elements 8 which promote or increase the controlled conveyance of incoming pressure pulses or gas flows to the predetermined ignition zones 20. As can be seen in the illustration in Fig. 3, baffles 22, diversion plates 24, whirling or turbulence generators 26 or other suitable means of confusers are provided for the catalyst elements 8 arranged in the housing as suitable means for the flow conductance, an incoming pressure can be guided in a controlled manner to the spatial area in the vicinity of the predetermined ignition zone.

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Reference Numbers 1 Security System 2 Safety Container 4 Recombiner Element 6 Housing 8 Catalyst Element Inlet Area 12 Active Zone Predetermined Ignition Zone 26 Turbulence Generator =

Claims (19)

1. A recombiner element comprising a plurality of catalyst elements that are arranged in a common housing and that trigger a recombination reaction with oxygen when hydrogen is carried along in a feed gas flow, the housing surrounding the catalyst elements, which are arranged therein, in a funnel-like way in such a manner that the heat released by the recombination reaction supports the gas flow inside the housing by a convection effect, and wherein at least one of the catalyst elements arranged inside the housing has a predetermined ignition zone, wherein the at least one of the catalyst elements is configured in such a manner that a surface temperature, lying above the ignition temperature present under these conditions, of 560°C is produced at the predetermined ignition zone in the convective operation at ambient conditions of 1 bar and 100°C
and at a hydrogen concentration in the feed gas flow of more than 5% by volume, and wherein means for flow focussing are connected upstream of the catalyst element on the gas-flow side, the means for flow focussing comprising baffle plates, directional baffles, vortex generators, or turbulence generators, supporting a selective feeding of incoming pressure pulses to the predetermined ignition zone.

2. The recombiner element according to claim 1, wherein a surface temperature of between 600°C and 900°C is produced at the predetermined ignition zone in the convective operation at ambient conditions of 1 bar and 100°C and at a hydrogen concentration in the feed gas flow of more than 5% by volume.

3. The recombiner element according to claim 1 or 2, wherein at least three catalyst elements are arranged inside the housing.

4. The recombiner element according to claim 3, wherein at least ten catalyst elements are arranged inside the housing.

5. The recombiner element according to any one of claims 1 to 4, wherein each of the plurality of catalyst elements has a catalytically active zone comprised of porous material.

6. The recombiner element according to claim 5, wherein the porous material comprises Al2O3.

7. The recombiner element according to claim 5 or 6, wherein an enrichment of catalyst material that is increased in comparison to the catalytically active zone is provided in the area of the predetermined ignition zone.

8. The recombiner element according to any one of claims 5 to 7, wherein the porous material of the catalytically active zone is doted with catalytic material.

9. The recombiner element according to claim 8, wherein the catalytic material comprises at least one of Pt and Pd.

10. The recombiner element according to any one of claims 1 to 9, comprising a portion of a cross sectional area of an inlet area through which the gas flow flows unhindered by fittings, said portion comprising more than 40% of the total cross sectional area of the inlet area.

11. The recombiner element according to claim 10, said portion comprising more than 90% of the total cross sectional area of the inlet area.

12. The recombiner element according to any one of claims 1 to 11, whose catalyst elements are designed as catalyst plates.

13. The recombiner element according to claim 12, wherein the catalyst plates are configured as thin elements having a wall thickness of less than 1 mm.

14. The recombiner element according to claim 13, wherein the wall thickness is less than 0.2 mm.

15. The recombiner element according to any one of claims 1 to 14, wherein a catalytically non-active area that is connected downstream of the catalytically active zone on the gas flow side is provided as predetermined ignition zone.

16. The recombiner element according to any one of claims 1 to 15, wherein the catalyst element mass in a feed area is dimensioned and arranged in such a manner that a surface temperature increase rate on the catalyst element of > 50°C per % hydrogen within 30s is produced during transient concentration changes.

17. The recombiner element according to any one of claims 1 to 15, wherein the catalyst element mass in a feed area is dimensioned and arranged in such a manner that a surface temperature increase rate on the catalyst element of > 50°C per % hydrogen within 10s is produced during transient concentration changes.

18. The recombiner element according to any one of claims 1 to 15, wherein the catalyst element mass in a feed area is dimensioned and arranged in such a manner that a surface temperature increase rate on the catalyst element of > 50°C per % hydrogen within 30s is produced during transient concentration changes above 3 hydrogen % by volume.

19. The recombiner element according to any one of claims 1 to 15, wherein the catalyst element mass in a feed area is dimensioned and arranged in such a manner that a surface temperature increase rate on the catalyst element of > 50°C per % hydrogen within 10s is produced during transient concentration changes above 3 hydrogen % by volume.