Patent Number: 052308596
Section: summary

The invention relates to a process for removing flammable gas mixtures in a gas space. The invention also relates to devices for carrying out such processes. In some processes, for example of a chemical nature, the formation of flammable gas mixtures is unavoidable. Explosions of flammable gas mixtures can cause considerable damage to the plants containing these gas mixtures. The removal of a hydrogen/air mixture after a core-melting accident in a nuclear power station (NPS) serves as an application example for describing the invention. In the event of core-melting, large quantities of hydrogen are released within a short time in the containment vessel (CV) of the NPS due to metal/water reactions, which hydrogen must be removed immediately from the gas space of the CV, so that no uncontrolled reactions of the hydrogen (H.sub.2) with the oxygen (O.sub.2) of the CV air are ignited; otherwise, it would not be possible to exclude premature CV failure in such a case in the event of an H.sub.2 /O.sub.2 deflagration or detonation. It has been proposed to use open ignition sources for removing such a flammable hydrogen/air mixture. Battery-operated spark gaps of catalytic ignition devices are on offer //Siemens: Hydrogen ignition device, Order No. A 19100-U822-A107 May 1988// which initiate a deflagration or detonation in an ignitable hydrogen/air mixture in the containment vessel of a nuclear power station, for example in the event of a core-melting accident. Further processes for the recombination of H.sub.2 and O.sub.2 have been known for a long time, for example: continuous flow processes--The explosive mixture is passed through pipes into a reaction chamber and recombined therein, for example thermally or catalytically. With a free volume of about 70,000 m.sup.3 of the CV of a German light-water reactor, the continuous flow process proves to be unsuitable because of the extremely long time required; this includes Swiss Patent Specification 514,217 which describes the formation of a combustion chamber in a line carrying an exit gas flow by means of porous isolation devices. PA1 Multiple, local recombination processes--Predominantly catalytic recombinators (contact catalysts) have been proposed. Catalytic processes react very sensitively to traces of catalyst poisons. If catalytic surfaces act as recombinators, they heat up in the case of sufficient H.sub.2 /O.sub.2 availability to temperatures&gt;ignition temperature of the explosive H.sub.2 /O.sub.2 mixture //loc. cit. Siemens, May 1988// and initiate a deflagration or detonation in the CV. PA1 electric sparks via spark gaps PA1 hot surfaces on glow igniters (glow plugs) PA1 naked flames and/or PA1 catalytic surfaces. PA1 miners' safety lamps with a naked flame according to Davy--miners' safety lamps as a light source with a naked flame have proved themselves for a long time as explosion-proof (protected) light source in mines even in the case of explosive methane/air mixtures. If more than 1% of methane is present in the mine air, a bluish cap, which spreads with increasing methane content, appears above the luminous flame. The design of the miners' safety lamp is as a light source with the naked flame; it is unsuitable for the removal of flammable gases since, for example, explosion-like combustion processes cannot occur in the interior of the miners' safety lamp, because the distances between the flame and the wall are too small. PA1 Sparking plugs or glow plugs in internal combustion engines--Sparking plugs or glow plugs have proved themselves for decades as suitable ignition sources for explosive gas mixtures in internal combustion engines. Electric sparks ignite hydrogen/air mixtures with a hydrogen content from 4 to 75% by volume. PA1 Catalytic igniters--Platinum surfaces have proved themselves as igniters for hydrogen. PA1 Naked flames--Naked flames have been known for a long time as igniters in gas burners. It is also known that flammable gas mixtures present in a container deflagrate below the detonation limit if the ignition occurs at one place in the container. It is also known, however, that, as a result of almost simultaneous ignition of a flammable gas mixture likewise below the detonation limit at various points in a container, detonation-like effects, which do not occur in the case of local ignition at only one point in the container under otherwise the same overall conditions, are already reached as a result of a kind of explosion jet. Even without detonation-like effects, a hydrogen deflagration in the CV of an NPS can lead, as a result of the associated release of heat energy, to the build-up of a pressure of&gt;10 bar, which exceeds the failure pressure of the CV of 9.5 bar. The question whether detonations--even if they may be locally limited--in the case of serious nuclear power station accidents, such as e.g. core-melting accidents, should be accepted in the CV, if the CV represents the last intact barrier towards the environment, has not yet been checked by large-space experiments in a sensible volume ratio to the CV, but only by means of computer calculations for simulated accident sequences. All such computer codes require the fixing of defined overall conditions which must completely cover and describe the serious accidents, for which the nuclear power stations are not designed. This leaves open the question whether and to what extent potential core-melting accidents may perhaps deviate from the preconceived ideas of the users of such simulation codes with respect to the sequence of events. It has been proposed for the removal of hydrogen in a boiling-water reactor to arrange in a line carrying exit gas (inter alia H.sub.2 and O.sub.2) a first porous isolation element, an ignition source and a second porous isolation element, in order to burn H.sub.2. The porous isolation elements arranged transversely to the line exert a considerable flow resistance to the flow of a flammable gas mixture, which resistance is overcome by a corresponding differential pressure. The flammable gas mixture is continuously burned after a single ignition in the combustion chamber formed between the first and second porous isolation element, since the first porous isolation element acts like a gas burner, the combustion chamber being selected to be sufficiently narrow for avoiding damage which can arise due to an explosive ignition of the flammable gases; in fact, a rapid pressure relief through the porous isolation elements is not possible because of the nature of pores. The invention is based on the object of allowing an explosion-proof, short-period removal of a flammable gas mixture even in a relatively large gas space. This object is achieved by burning or recombining the flammable gas mixture in numerous part volumes which are divided off from the remaining gas space by grilles. The combustion in the interior of the part volumes is effected by ignition sources such as The invention utilizes a suitable combination of devices known for a long time: The process proposed here, predominantly with the devices tested for a long time in mining and in internal combustion engines, is described by reference to the example mentioned: a grille divides off a part volume from the remaining gas space. The grille is a wire netting made of metal, preferably of stainless steel. The grille shows only a small flow resistance to gases and vapors. The flammable gas mixture is present without a differential pressure in the part volume in the same concentration as in the remaining gas space. If an ignition with explosion-like combustion occurs in the part volume, the grille prevents spreading of the explosion-like combustion into the remaining gas space, if the mesh width of the grille is selected sufficiently small, depending on the flammable gas mixture present. For hydrogen/air mixtures, a grille mesh width smaller than or equal to 0.2 mm and greater than or equal to 0.05 mm has proved suitable. After the explosion-like combustion has taken place, vaporous or gaseous combustion products can flow out of the part volume virtually with hardly any hindrance. Due to the heat of combustion released, the combustion products are hotter than the surrounding atmosphere of the flammable gas mixture and generate a thermal upflow, so that further, cooler unburned gas mixture flows into the part volume and can again be ignited. As a consequence thereof, it proves to be advantageous to choose the geometrical shape of the part volume with a large surface area and a small height. In the case of an unduly small distance between the point of ignition and the next point of the grille, however, no explosion-like combustion in the part volume, but a continuous combustion in the interior of the part volume takes place. In this case, the part volume acts as a combustion chamber of a continuous combustion, as described, for example, in Swiss Patent Specification 514,217. The porous isolation elements described therein for dividing off the combustion chamber in an exit gas line are unsuitable for an explosion-like combustion, since sufficiently fast flowing out of the combustion products through the pores of the isolation elements cannot take place. It is therefore stated therein on page 6, lines 26 et seq.: "In the case that the flame should temporarily go out and must then be re-ignited, the space between the isolation elements 52 and 53 is sufficiently narrow for avoiding damage which can be caused by explosive ignition of the gases between the isolation elements." In contrast to the porous isolation elements proposed therein, a grille of suitable mesh width as a divider from the remaining gas space allows an explosion-like combustion of the gas mixture in a part volume which is substantially greater than that achievable by porous isolation elements. According to the invention, the smallest distance between the point of ignition and the grille is therefore selected such that only an explosion-like combustion can occur in the part volume. A continuous combustion would not be controllable from the outside and might lead to overheating of the grille and hence to an ignition in the remaining gas space; this might trigger an explosion outside the part volume, which is precisely what must be prevented.