CA2420067C - Recombiner with a stabilized reaction temperature - Google Patents

Abstract

Device for recombining hydrogen, with at least one catalyst element, comprising at least one heat pipe that carries an operating medium and is connected with the catalyst element in a thermally-conductive manner, and a refrigerating device for refrigerati ng the coolant connected to the heat pipe, by which is solved the technical problem of reacting in a controlled manner both small and large quantities of hydrogen with the atmospheric oxygen present in the safety containments in a wide range of concentrations, and dissipating the heat of reaction generated to such a degree that the respective ignition temperatures are not reached. To this end, a first section of the heating tu be is filled with liquid coolant and the first section is connected to the catalyst element, a nd a second section of the heat pipe is filled with a vapor cushion containing an operating medium in vapor form and the refrigerating device is located in the second section.

Description

Recombiner with a stabilized reaction temperature The invention refers to a device to recombine hydrogen comprising at least one catalyst element, with at least one heat pipe carrying an operating medium and connected with the catalyst element in a thermally conducting manner, and a refrigerating device connected to the heat pipe for cooling the operating medium.

Hydrogen or other flammable gases may be released not only in normal operation, such as charging of high-performance accumulators for storing current, but also in accidental situations such as leakage from storage containers, in core meltdowns of nuclear reactors, etc., and depending on the hydrogen concentration, react exothermically with atmospheric oxygen by deflagration or detonation. This hazard can be countered with the use of catalytically acting recombiners, so long as they do not themselves become ignition sources from overheating as a result of the release of energy and thus set off an explosion. The following considerations are discussed by way of example for a nuclear engineering application. They can be transferred analogously to other hydrogen applications, for instance to leakages from hydrogen-fuelled vehicles in garages or parking structures.

Large quantities of hydrogen, which accumulate in the reactor safety containments, are generated during critical breakdowns in water-cooled (LWR) nuclear reactors as a result of the reduction of steam with the metallic fuel elements and cladding tubes.
The maximum hydrogen quantities may reach about 20,000 m"3 with both pressurized and boiling water reactors. Owing to the atmospheric oxygen found in the containments, there is a risk of formation of flammable mixtures whose uncontrolled ignition would produce severe dynamic pressure stresses on the containment walls. Moreover, steam and hydrogen would lead to continuous increases in the pressure and temperature of the accident atmosphere. Overpressure could make them the driving force for leakages from the containment vessels, with the danger of a release of radiotoxic substances.
Precautionary safety nieasures exist, such as inerting the gas volumes with nitrogen, as is planned or carried out in the case of boiling water reactors (BWRs). Measures discussed and to some extent put into practice for pressurized water reactors (PWRs) relate to catalytic recombiners. They would be used to recombine the hydrogen generated exothermically and catalytically, both inside and outside the exothermic ignition limits, that is, by converting it into water vapor with the generation of considerable amounts of heat. This process prevents overheating of this mainly uncooled apparatus in order to avoid uncontrolled ignition and the associated damage to the facilities and the environment.

Both thermal and catalytic recombiners, exothermically recombining the hydrogen with atmospheric oxygen to form steam, have been developed for the elimination of hydrogen released in normal operation or during accidents. The preference is for catalytic systems which operate passively, that is, they are self-initiating, and without external energy sources, so that their availability in case of accident is guaranteed. At the present time there are two types of recombiners, which have demonstrated their usefulness in extensive testing, including in response to potential catalyst poisons. Both conceptions, including their dimensions and test results, have been described in the literature in detail.
Substrates used have included not only metal plates or foil, but also highly porous granulate, to which platinum or palladium is applied as catalyst. Several foil and granulate packets - the granulate is held together in packets by wire netting -are arranged vertically and parallel to each other in a sheet metal housing. The hydrogen/air mixture enters the bottom of the housing. It comes to reaction on the catalytically coated surfaces. The mixture and the reaction products flow over the surface as a result of the thermal uplift. Thus, a fan is not required. Information from the reactor industry indicates that the hydrogen problem can be solved with a quantity of from 40 to 100 catalytic recombiners. They are disposed in the containment in such a way that their heat output follows as closely as possible the direction of large-scale convection movements were a major accident to occur.

A disadvantage is the fact that, before entering the recombiners, neither a targeted premixture nor a cooling of the atmosphere is possible. The reactants hydrogen and oxygen are fed into the recombiners as they arise or as they are locally present. This does not eliminate the possibility of short-term or long-lasting overheating. The maximum degradation rates and heat outputs of existing systems are limited by the flow over the surfaces and the low material and heat transition coefficient. In addition, the potential for heat storage is low.

Despite the extensive fi-ee surface areas of the recombiner using porous granulate as substrate, their effectiveness is clearly less than that of other systems developed and for which plates or foil are used. In its passage over the coated substrate the gas/air combustion mixture does not reach all the catalytic centers, since the reaction takes place solely on the surfaces of the packages, and thus the temperature relationships remain unclear. The reaction is incomplete in both systems.

The removal of the heat of reaction from the system is essentially problematic, owing to defective cooling. It takes place almost exclusively by convection from the fixed surface to the gasses flowing past and by radiation of heat to adjacent structures.
Too high hydrogen concentrations can lead to short-term or permanent overheating of the coated substrate, so that ignition temperature is reached or exceeded, which can result in homogenous gas-phase reactions with deflagration or detonation. Yet another disadvantage is the additional heating of the environment and the containment, with the resultant pressure buildup.

As has been shown, the reaction within the recombiners is diffusion controlled and not, as is still often assumed, surface controlled. That means that the pace at which the reactants are transported to the catalytically acting surfaces and the removal of the products of reaction from those surfaces is of critical significance for the speed of the recombination process. This allows us to conclude that the design of a recombiner should primarily aim at optimizing the transport process rather than optimizing housing design and heat dissipation.

The technical problem is thus how to carry out a controlled reaction of both small and large quantities of hydrogen with the atmospheric oxygen present in the containments, in a broad range of concentrations, and how to dissipate the heat of reaction generated thereby to such a degree that the ignition temperature for the mixture present is not reached.

In addition, however, the catalytically acting surfaces must not be cooled down so far that the catalytic reaction ceases to run or the steam condenses, with excessive moisture penetration of the catalytic substrate. Rather, the reaction temperatures must be high enough that a natural convection to bring in and remove the reaction partners is set in motion and maintained. The device should additionally be capable of operating without exceeding ignition temperature, even with forced-draft recombiners with increased materials transport and the resultant higher rates of hydrogen dissipation.
And finally, for reasons of safety and reliability, the device should be independent of external energy sources and satisfy the principles of passive safety systems.

The technical problem set out above is solved by the invention by a device according to Claim 1, in which a first section of the heat pipe is filled with liquid coolant, the first section being connected with the catalyst element, and a second section of the heat pipe is filled with a vapor cushion containing an operating medium in vapor form, and the refrigerating device is located in the second section.

According to the invention, the working temperature of the heat pipe is selected, along with the operating medium and working pressure, so that on the one hand the ambient temperature allows a rapid pre-heating of the catalyst to a suitable start temperature, and, on the other, any exceeding of the ignition temperature can be ruled out.
Because the operating medium has not yet reached its pressure-dependent evaporation temperature, only minimal heat exchange takes place through the gas cushion, so that the temperature at the catalyst elements quickly rises to the operating temperatures. If, however, the temperature of the liquid operating medium is in the boiling temperature range, there occurs an increased evaporation while the temperature of the operating medium remains essentially the same at a given working pressure. The result is thus an effective cooling of the catalyst elements at operating temperature.

The invention accordingly describes a device for elimination of hydrogen released into non-inerted spaces in operations or in accidents. In so doing, it averts the danger of overheating the catalyst leading to an ignition of the gas mixture. At the same time, excessive cooling during the warm-up to the operating temperature can be largely prevented. This would lead to inadequate reaction performance. Inside the device, the hydrogen, in the presence of the existing atmospheric oxygen, is recombined into water vapor by a catalytic process. To prevent ignition and additional heating of the surrounding atmosphere, the heat of reaction will be drawn off to the operating medium by a heat pipe. In this way, and by suitable conformation of the catalysts, its maximum temperature will be limited and stabilized. The catalytic devices fitted with heat pipes can also be advantageously used in recombiner units with natural convection as well as in turbo-compressor units with forced convection.

Certain exemplary embodiments can provide for a device for recombining hydrogen comprising: at least one catalyst element; at least one heat pipe carrying an operating medium and connected with the at least one catalyst element in a thermally conductive manner; and a cooling device for refrigerating the coolant connected to the heat pipe, characterized in that a first section of the heat pipe is filled with liquid coolant; the first section is connected with the at least one catalyst element; a second section of the heat pipe is filled with a vapor cushion containing the operating medium in vapor form; and the cooling device is located in the second section.

5a Further details and advantages of the device according to the invention are the object of auxiliary claims 2 to 9 as well as the following description of typical embodiments, in which reference is made to the accompanying drawings. The drawings show:

Fig. 1 in a schematic view, a reactor containment with a device known to art in the left half and a device according to the invention in the right half;

Fig. 2 a schematic representation of a device according to the invention;

Fig. 3 a first embodiment of a catalytically coated plate of the device according to the invention;

Fig. 4 a second embodiment of a catalytically coated plate of the device according to the invention;

Fig. 5 an arrangement of a device according to the invention in a housing with additional components for cooling the gas mixture.

Fig. I serves to make clear the solution of the problem. It shows schematically the safety containment of a reactor installation as well as some other components. In the reactor pressure container 1, following a breakdown, hydrogen is generated from the reaction of the steam with the fuel rod cladding. A mixture of steam, air, hydrogen and aerosols passes first into the containment cell 2 and from there through existing passthroughs 3 into the safety containment or into containment 4. There it rises owing to the difference in density between the high temperature of the mixture and the low density of the hydrogen, where it may also be undesirably enriched by stratification. As a result of heat dissipation to the cooler external walls of the containment, it begins to move downward because of its higher density.

Shown schematically in the left half of Fig. 1 is also a recombination device 5 in plate or foil configuration. The plates or foil coated on both sides with catalyst material are arranged parallel to each other in a metal housing. On the coated surfaces, the hydrogen reacts with the oxygen. Steam and heat are produced. The heat of reaction generated propels the gas upwards between the substrates. Additional components, such as a refrigerating unit or blower, are not shown here. If the substrate is oversupplied with hydrogen-rich gas, ignition of the mixture on the catalytically acting surfaces is not to be ruled out.

In the right side of Fig. 1 we see a typical embodiment of the device according to the invention. It also has a housing 6; this is however not required for the implementation of the invention and is thus optional. In it are arranged heat pipe 7 and, to enlarge the surfaces, ribs, or catalyst elements here with plates or foil 8 coated with catalyst material as in known recombiners. The hydrogen-rich mixture (arrow 9) enters at the bottom of the device; it leaves the device depleted (arrow 10).

The heat of reaction created on the substrate is for the most part carried off as a result of thermal conductance in the operating medium 11 of the heat pipe 7 as shown in a simplified embodinient. A smaller proportion is also given off by means of convection to the gas flowing across the plates or foil 8 as well as by heat radiation to neighboring plates or foil 8 or to the housing wall 6. The substrates are in this example provided as ribs which produce higher heat and material transfer coefficients and thus contribute to improve heat transfer (cooling).

The operating medium I I vaporizes at a specific temperature which is derived from the vapor pressure curve as a function of operating medium 11 and the pre-set working pressure. The heat taken up is transferred by the vapor through the gas cushion 12 located above it, in an upwards flow (arrow 13) to the refrigerating device 14. The operating medium condenses and flows back again (arrow 15). Alternatively there are also heat pipes which return the operating medium (passively) to the evaporator, making use of other physical processes such as capillary action.

The particular characteristic of this type of heat pipe 7, specifically, that it transports only very little heat until evaporation sets in - as opposed to a permanent cooling -, leads to a rapid heating of the catalytic substrate on the plates or foil 8. In this way, the starting behavior of the recombination reaction is improved. Accordingly, a highly effective heat transport kicks in at a defined temperature, which reliably excludes the possibility of further overheating.

Shown enlarged in Fig. 2 are the heat pipe 7 together with the catalytically acting substrate on the plates or foil 8 as they are arranged in known recombiners.
Size, shape and intervals of the plates or foil 8 in relation to each other will be selected for the highest possible hydrogen conversion.

The choice of the best-suited operating medium 11 allows reaction temperatures to be set on the substrate that always lie below ignition temperature. If, for instance, water is used as the operating medium and if the working temperature of the overhead gas cushion 12 is 1 bar, the temperature on the plates or foi18 will, given the vapor pressure curve, be only slightly above 100 C. Increasing or reducing the working pressure would allow correspondingly higher or lower reaction temperatures. Too low temperatures on the plates or foil 8 would however reduce the uplift and thus the carriage of the reactants to the plates or foil 8 coated with catalyst material, or the transport away from the plates or foil 8 would be reduced owing to the lower speeds, and consequently the material transfer coefficient and thus hydrogen conversion would be reduced. Therefore, the selection of operating medium and the pressure of the gas atmosphere - generally air, or, to prevent corrosion, nitrogen - takes on special significance. Heating oils or liquid metals, for example, would result in higher reaction temperatures on the plates or foil 8 and thus improve uplift. That means that, as a rule, the gas depleted on plates or foil 8 (arrow 16) would generally leave the device against gravity.

Fig. 3 shows a simple embodiment of a plate or foil 8 coated with precious metal as is used for known recombiners. Other forms such as rectangles, polygons, circles, ellipses, etc., are possible. The cutouts 24a, 24b and 24c for the tube of the heat pipe 7 are arranged centrally or excentrically. Excentric arrangements permit the adaptation of plate geometry and coatings to conditions, and consequently_ improve heat dissipation. The -plates or foil can be extended in the direction of flow that two or more passthroughs to accommodate the previously discussed heat pipes (supply and return lines) are possible.

Fig. 4 shows an embodiment for a catalytically coated plate or foil 8. In this example, the flow over plate 8 is from bottom to top (arrow 9). The plate or foil 8 displays an opening 24a for the pipe 7 and can be coated on both sides or on one side only, in which case the coating is indicated as 8a. The forward area 8b of plate or foi18 can even remain uncoated. This is because the flow over the leading edge of a plate or foil 8 would produce too high material transfer coefficients owing to the absence of a boundary layer or a thin boundary layer, which could even lead to ignition with the right composition of gases.

In principle, the contour of the side exposed to the flow can be modified in relation to the heat pipe 7 in such a way that the heat conductance path on the exposed side is sufficiently short that overheating is excluded. In this way it would be ensured that the predominant portion of the heat of reaction generated there (arrow 17) would be conducted by the plates or foil 8 into the working medium by means of thermal conductance. Corners or angles fi-om which the heat cannot be adequately conducted are thus avoided. On the exposed side 8c the end of the plate or foil 8 can also be used only to cool the departing gases.

The ribs or substrate thicknesses are to be adjusted to requirements. Foil that is too thin has inadequate heat storage capability on account of the insufficient thickness of its walls.
The correct substrate thickness and the right choice of thermal conductance of the material will reliably prevent ignition. The part-coatings described above form an additional design parameter.

The catalytic substrate and the selection of operating medium can be chosen in such a way that the uplift ensures that the accident atmosphere is fed through and that the portion of the heat of reaction not conducted into the operating medium 18 is as low as possible. The depleted hot mixture (arrow 16) leaves the plate or foil 8 at the top.

Fig. 5 shows by way of example an arrangement of the device described above in reactor containment 4. The device 19 is exposed to the flow of the hydrogen-rich containment atmosphere (arrow 9) and is located in a position where stratification and concentration of the hydrogen may occur. In the device, a portion of the hydrogen is degraded in the described manner so that it will not ignite at any point. The degraded mixture (arrow 20) then flows through the pipes of a building refrigeration unit 21, in which a part of the heat generated during the accident and of the heat of reaction still present is given off to a coolant such as water. This heat is transferred to basins of water 22 located higher up outside the safety containment, typically also by means of the heat pipe principle. As a result of the heat loss to the tubes of the refrigerating unit 21 there is an increase in the density of the atmosphere in the device and thus in its downdraft, which increases the velocity of flow at its entry into the device and thus also leads to an increased transfer of material. This results in higher rates of hydrogen conversion and better cooling of the total accident atmosphere with a proportional drop in pressure. The mixture leaves the entire unit at the bottom (arrow 23).

This fundamental principle can also be used for the recombiners mentioned earlier as using porous catalytic structures, if the granulates and the nets holding them are installed for instance between cooled plates and the interval between them so chosen that the catalytic substrate undergoes sufficient cooling.

The essential characteristics and functions of the catalytically acting device according to the invention described in the foregoing, and their preferred embodiments can be summarized as follows. What it relates to is a device a) in which substrates (plates or foil) coated with catalyst material or with porous catalyst substrates arranged between cooling plates and cooling tubes, similar to known recombiners, act as base bodies for the degradation of hydrogen, b) in which the shape of the plates or foil is adjusted to the flow and cooling conditions, c) in wllich the plates or foil can be used as ribs to increase the surface area and thus the heat and material transfer coefficients, d) in which the plates or foil are connected with a heat pipe to prevent ignition, e) in which the plates or foil are arranged centrally or excentrically in relation to the heat pipe, f) whose plates or foil are wholly or partially coated, so that the greatest possible portion of the heat of reaction is transferred to the operating medium as a result of solid conductance and does not reach the flow of the mixture, g) in which the temperature of the plates or foil and thus the start behavior of the catalytic reaction is regulated by the selection of the operating medium and the working pressure of the gas cushion located above it, h) the refrigeration units can be arranged in front or behind for the purposes of increasing the velocity of the flow and the heat and material transfer coefficients, i) which can preferably be connected with self-actuating passive or even with actively operated turbo-compressor units to increase flow velocity and heat and material transition coefficients, j) whose operating temperature can alternatively be set at a higher or lower level by exploiting uplift or downdraft, k) whose maximum temperature can alternatively be limited by secondary heat exchanger plates with air, gas or liquid cooling.

Claims (9)

1. A device for recombining hydrogen comprising:
at least one catalyst element;
at least one heat pipe carrying an operating medium and connected with the at least one catalyst element in a thermally conductive manner; and a cooling device for refrigerating the coolant connected to the heat pipe, characterized in that a first section of the heat pipe is filled with liquid coolant;
the first section is connected with the at least one catalyst element;
a second section of the heat pipe is filled with a vapor cushion containing the operating medium in vapor form; and the cooling device is located in the second section.

2. The device according to Claim 1, characterized in that the second section has a working pressure having a pre-determined value.

3. The device according to Claim 1 or 2, characterized in that the operating medium is water, heating oil or metal.

4. The device according to any one of Claims 1 to 3, characterized in that the heat pipe is connected with the at least one catalyst element on the feed end and the return end of the first section.

5. The device according to any one of Claims 1 to 4, characterized in that each of the at least one catalyst element comprises a passthrough that is centrally located for connection with the heat pipe.

6. The device according to any one of Claims 1 to 4, characterized in that each of the at least one catalyst element comprises a passthrough that is excentrically located for connection with the heat pipe.

7. The device according to Claim 5 or 6, characterized in that each of the at least one catalyst element comprises inflow heat conductance paths that are essentially equally long as far as the passthrough.

8. The device according to Claim 7, characterized in that each catalyst element has a contour to confer each catalyst element with an essentially round or elliptical shape on its inflow side.

9. The device according to any one of Claims 1 to 8, characterized in that each catalyst element and at least part of the first section of the heat pipe are located in a housing.