Patent Publication Number: US-2005136305-A1

Title: Fuel cell system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      This application claims the benefit of priority under 35 U.S.C. § 119 of German patent application DE 103 60 458.8 filed Dec. 22, 2003 the entire contents of which are incorporated herein by reference.  
     FIELD OF THE INVENTION  
      The present invention pertains to a fuel cell system.  
     BACKGROUND OF THE INVENTION  
      Fuel cell systems are frequently used in vehicles as so-called auxiliary power sources in order to make it possible to supply users, with electric energy. Such a need is increasingly encountered in vehicles. It is of significance here that such energy uses are frequently to be operated not only when a drive unit designed, e.g., as an internal combustion engine, is also put into operation and thus can also be used to generate electric energy via a generator arrangement. It may be necessary, for example, to activate various systems for preheating a vehicle even before the vehicle is put into use. Besides an auxiliary heater operated with fuel, these systems may also comprise heaters that are to be operated electrically, for example, seat heaters, outside mirror heaters, windshield heaters as well as even delivery means, by which a medium to be heated, for example, the cooling water of a drive unit or the air to be introduced into the interior space of the vehicle, can be delivered through a heat exchanger provided in the area of an auxiliary heater.  
      There are various requirements in such systems. On the one hand, it shall be ensured during the start phase that the different areas of the system, especially also the fuel cell, will as quickly as possible reach a suitable operating temperature as rapidly as possible. This ensures that the overall system can operate at a high efficiency. On the other hand, the starting materials used to generate energy, i.e., consequently a gaseous medium that is to be introduced into a fuel cell and contains hydrogen, shall be utilized as efficiently as possible in order to increase the efficiency in this respect as well and, of course, to lower the operating costs of the overall system.  
     SUMMARY OF THE INVENTION  
      The object of the present invention is to provide a fuel cell system that can be used in motor vehicles and is able to operate at increased efficiency.  
      This object is accomplished according to the present invention by a fuel cell system comprising a fuel cell, a burner, which can be operated optionally with fuel and/or fuel cell waste gas, a heat exchanger arrangement for transferring heat generated in the burner to air to be fed into the fuel cell and/or a hydrogen-containing gas to be fed into the fuel cell.  
      Due to its variability concerning the material to be used for the combustion, the burner provided in the fuel cell system according to the present invention can be activated during different phases of operation in order to make possible in this manner the greatest possible utilization of the energy present in the system or the starting materials being used to generate energy. Thus, by operating the burner with fuel, i.e., for example, with liquid fuel or optionally also with a gaseous fuel, it can be ensured during the start phase, in which, for example, the fuel cell itself cannot yet be used to generate electric energy, that the fuel cell itself will be heated by preheating the air flowing through the fuel cell during this phase and is thus preconditioned to reduce the duration of the start phase. If the fuel cell is put into operation, the fuel cell waste gas leaving the fuel cell, which still contains a considerable percentage of hydrogen not reacted to generate electricity, can be alternatively or additionally introduced into the burner and burned there, e.g., together with the fuel cell waste air which is likewise leaving the fuel cell and was also preheated. The heat thus generated can in turn be transferred to the air to be introduced into the fuel cell and optionally also to the hydrogen-containing gaseous medium to be introduced into the fuel cell in order to make it possible to obtain an improved operating characteristic of the fuel cell itself. Furthermore, it is, of course, possible to utilize the heat generated in the burner during the combustion, which is not transferred to the media to be introduced into the fuel cell, in another heat exchanger arrangement to heat, for example, the air to be introduced into the interior space of a vehicle or even to heat the cooling medium present in the cooling circulation of an internal combustion engine in order to also make it possible in this manner to achieve preconditioning in the area of the internal combustion engine.  
      Provisions may be made in the system according to the present invention for the burner to comprise a combustion chamber with a feed line for fuel, a feed line for fuel cell waste gas and a feed line for combustion air. It is, furthermore, advantageous, in this case for the feed of fuel and the feed of fuel cell waste gas to take place via a common feed line, which feeds the fuel and the fuel cell waste gas, respectively, in the bottom area in the direction of the combustion chamber.  
      To keep the design of the system according to the present invention as simple as possible, it is proposed, furthermore, that the feed of fuel and the feed of fuel cell waste gas take place via a common feed line, which feeds the fuel and the fuel cell waste gas, respectively, in the bottom area in the direction of the combustion chamber. To separate the paths of introduction of the fuel, it may be advantageous, especially if a liquid medium, i.e., for example, fossil fuel, such as gasoline or diesel or biodiesel is used, to feed fuel and fuel cell waste gas via separate feed lines, which feed the fuel and the fuel cell waste gas, respectively, in the bottom area in the direction of the combustion chamber.  
      Very good mixing of the combustion air to be introduced into the combustion chamber with the fuel or fuel cell waste gas, which is likewise to be introduced into the combustion chamber or is already present there, can be achieved by feeding fuel and fuel cell waste gas via separate feed lines, which feed the fuel and the fuel cell waste gas, respectively, in the bottom area in the direction of the combustion chamber. The efficiency of the combustion can be further improved by optimized mixing by providing a premixing chamber, in which at least part of the fuel cell waste gas and at least part of the combustion air are mixed before being fed into the combustion chamber.  
      To bring especially liquid fuel to a state in which it forms an ignitable and combustible mixture, it is proposed that a porous evaporator medium, preferably one with a heating means, be provided at least in the bottom area of the combustion chamber, via which evaporator medium at least the fuel is fed into the combustion chamber.  
      It is proposed in an alternative embodiment that the burner have an atomizer arrangement with a feed line for liquid fuel, a feed line for combustion air, which is also used to atomize the fuel, and a feed line for fuel cell waste gas.  
      Provisions may be made in this connection for the atomizer arrangement to have an outer swirl flow space and an inner swirl flow space to generate an outer swirl flow and an inner swirl flow leading to an atomization lip and for the feed line for fuel cell waste gas to comprise a feed line through which the fuel cell waste gas is introduced into the area of the inner swirl flow and/or the outer swirl flow.  
      Provisions may, furthermore, be made in an embodiment that has a simple design and yet ensures efficient mixing for the feed line to pass through a flow guide element, which defines the inner swirl space.  
      The present invention will be described in detail below on the basis of the attached drawings based on preferred embodiments. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a block diagram of a fuel cell system according to the present invention;  
       FIG. 2  is a partial sectional view of a burner that can be used in the system according to  FIG. 1 ;  
       FIG. 3  is a view corresponding to  FIG. 2  of an alternative embodiment of the burner;  
       FIG. 4  is another view corresponding to  FIG. 2  of an alternative embodiment of a burner;  
       FIG. 5  is another view corresponding to  FIG. 2  of an alternative embodiment of a burner;  
       FIG. 6  is another view corresponding to  FIG. 2  of an alternative embodiment of a burner;  
       FIG. 7  is another view corresponding to  FIG. 2  of an alternative embodiment of a burner;  
       FIG. 8  is a simplified axial sectional view of the burner shown in  FIG. 7 ; and  
       FIG. 9  is a partial sectional view of an atomizer arrangement for a burner that can be used in the system according to  FIG. 1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      A fuel cell system according to the present invention is generally designated by  10  in  FIG. 1 . This fuel cell system  10  has as its essential component a fuel cell  12 , into which a hydrogen-containing gas is introduced, as is indicated by an arrow  14 , and, as is indicated by arrow  16 , air, i.e., oxygen, is introduced. The hydrogen contained in the hydrogen-containing gas is reacted in the fuel cell  12  with the oxygen contained in the air to generate electric energy. As is indicated by an arrow  18 , a hydrogen-depleted fuel cell waste gas and, as is indicated by an arrow  20 , an oxygen-depleted fuel cell waste air will then leave the fuel cell  12 .  
      The hydrogen-containing gas to be fed into the fuel cell  12  is prepared in a reformer  22  in the fuel cell system being shown. This reformer  22  is fed by a fuel supply means  24  with fuel, generally liquid fuel, e.g., gasoline, diesel fuel or another hydrocarbon. Furthermore, air is fed into the reformer  22  by an air supply means  26 , and, as is indicated by an arrow  28 , this air flows through a heat exchanger  30  before being fed into the reformer  22  and can take up heat in the process during the reforming operation from the hydrogen-containing gas leaving the reformer  22 , which is generally also called reformate. The air thus enters the reformer  22  in an already preheated state.  
      It shall be pointed out here that the fuel supply means  24  and the air supply means  26  may have suitable delivery members, e.g., pumps or blowers in order to obtain the desired fuel and air flows.  
      The hydrogen-containing gas leaving the reformer  22  via the heat exchanger  30 , i.e., reformate, will then flow in the direction of the fuel cell  12  and can be sent through an additional heat exchanger  32  before being fed into the fuel cell. The air to be delivered in the direction of the fuel cell  12  by the air supply means  26 , which supplies not only the reformer  22  but also the fuel cell  12  with the air necessary for carrying out the desired reaction, also flows through this heat exchanger  32  and can take up heat therein. The air to be introduced into the fuel cell  12  thus enters the fuel cell  12  already in a state in which the air has a temperature that is suitable for permitting the desired reaction to take place. Furthermore, by heating this air in a phase in which the fuel cell  12  is not yet being operated to produce electric energy, this air can be used to precondition, i.e., preheat the fuel cell  12 .  
      To make it possible to make available the heat to be transferred in the heat exchanger  32  to the air and optionally to the hydrogen-containing gas, a burner  34  is arranged upstream of the heat exchanger  32 . This burner  34  is designed to burn various combustible media with oxygen in order to then send the hot combustion waste gases in the direction of the heat exchanger  32  and to preheat the gaseous media mentioned, namely, air and reformate, in the process. It can be recognized from  FIG. 1  that a combustible medium can be fed to the burner  34  by both the fuel supply means  24  in the form of the fuel being delivered by same and to the fuel cell  12  in the form of the fuel cell waste gas. Furthermore, the fuel cell waste air  20 , which still contains oxygen, is fed to the burner  34 , so that not only a combustible medium, but also the oxygen necessary for the oxidation is made available for the combustion in the burner  34 .  
      The heat exchanger  32  is followed by another burner  36  with a heat exchanger  38  associated therewith. Fuel can be fed into the burner  36  by the fuel supply means  24 . Furthermore, the waste gas or gaseous medium leaving the burner  34 , which still contains at least a percentage of oxygen that can be utilized for combustion in the burner  36 , can be fed to the burner  36  via the burner  34  and the heat exchanger  32 . A medium, which is to be heated, can be fed to the heat exchanger  38 , as is indicated by an arrow  40 , to take up heat. This medium, which is to be heated, may be, for example, the air, which is to be introduced into the interior space of a vehicle and is to be heated in advance, or, as an alternative or in addition, it may, of course, also be the cooling medium, which may circulate in a cooling circuit of a drive unit.  
      Before explaining various embodiments of the burner  34  in detail below, the operation of the fuel cell system  10  according to the present invention will be described with reference to  FIG. 1 .  
      It shall first be assumed that a vehicle equipped with such a system  10  is not yet put into operation, but that, for example, the vehicle is nevertheless to be preheated or preconditioned. The various areas of the system are still cold during this phase. The burner  34  is therefore activated at first, doing so by feeding fuel from the fuel supply means  24  and by feeding combustion air from the air supply means  26 . This combustion air will then flow over the heat exchanger  32 , through the fuel cell  12  and as a “fuel cell waste air” into the burner  34 . The hot combustion waste gases produced in the burner  34  are sent in the direction of the heat exchanger  32 . They heat the air being delivered by the air supply means  26  in the direction of the fuel cell  12  and the burner  34  in the process. This heated air will in turn heat the fuel cell  12  from the inside and thus prepare same already for the operation to generate electric energy. After flowing through the heat exchanger  32 , the combustion waste gases of the burner  34 , which have already cooled somewhat, flow in the direction of the burner  36 . If the interior space of the vehicle is also to be preheated or a drive unit is optionally to be preheated during this phase, fuel may additionally also be fed to the burner  36  from the fuel supply means  24  in order to burn the oxygen still contained in the combustion waste gases of the burner  34  with this fuel and to subsequently transfer the heat formed in the process to the medium to be heated in the heat exchanger  38 . The air supply means  26  delivers so much oxygen in the direction of the burner  34  for this purpose that the waste gases leaving this burner  34  will still transport a sufficient amount of oxygen. It is consequently unnecessary to provide an additional air supply line for the burner  36 . If the temperature of the waste gases leaving the heat exchanger  32  is still high enough to bring the medium to be heated in the heat exchanger  38  to the desired temperature, it would not be necessary during this phase of the operation to additionally activate the burner  36 . It would also be possible in this case, in principle, if the burner  34  or the heat exchanger  38  is dimensioned correspondingly, to omit the burner  36  altogether and to use the heat, which is also to be transported in the combustion waste gases  34  to the heat exchanger  32 , to bring the medium to be heated to the desired temperature.  
      Since the on-board electrical system is under a very high load during this phase of operation, especially at comparatively low outside temperatures, and the on-board electrical system is represented, in general, by a battery during this phase, it is advantageous to also operate or put into operation the fuel cell  12  to generate electric energy. Therefore, the reformer  22  is also activated, doing so by supplying hydrocarbon and air that is necessary for the reforming. The reformer  22  may now also be preheated in order to bring it to the desired operating temperature very rapidly. This preheating could optionally or alternatively also be carried out by means of a separate heating means, for example, a heating means that can be operated electrically.  
      A gas containing hydrogen is produced during the reforming operation that is now taking place in the reformer  22 , and this gas will flow at a very high temperature in the direction of the heat exchanger  30  and, as was mentioned already, preheat the air to be fed into the reformer. The reformate that will now leave the heat exchanger  30  can be sent either directly in the direction of the fuel cell  12  or, as is indicated in  FIG. 1 , it will flow through the heat exchanger  32  and take up some more heat in the process. If preheated air is now introduced into the fuel cell  12  together with the likewise very hot reformate, the reaction of hydrogen being transported in the hydrogen-containing gas with the oxygen contained in the air to yield water can take place in the fuel cell  12  while electric energy is generated. This electric energy can in turn be used, for example, to operate heating means optionally present in the different burners  34 ,  36  and also in the reformer  22  and optionally also other users of electric energy that are to be operated in a vehicle during this phase of operation. Thus, the entire system will not load the on-board electrical system or the battery present therein any longer.  
      Once the fuel cell  12  is put into operation or a reformate containing hydrogen is fed by the reformer  22  in the direction of the fuel cell  12 , a fuel cell waste gas, which still contains hydrogen, will leave the fuel cell  12  even if it has been put into operation. Since this hydrogen can be burned together with the oxygen still contained in the fuel cell waste air in the burner  34 , it is no longer necessary during this phase to supply the burner  34  with fuel from the fuel supply means  24 .  
      The heat balance is affected in the system described above and shown in  FIG. 1  essentially by the amount of air delivered by the air supply means  26  in the direction of the fuel cell  12 . The larger the amount of air, the more oxygen is also available in the burner  34  when the fuel cell  12  is in operation, and more oxygen is consequently also available in the burner  36 . Furthermore, the consequence of the supply of a larger amount of air is that when the fuel cell  12  has been put into operation, a sufficient amount of heat can be removed from the fuel cell  12 , and it is also ensured at the same time by preheating the air to be introduced into the fuel cell  12  that a load on the fuel cell  12 , which occurs due to excessive differences in temperature, can be avoided. To further affect the heat behavior especially of the fuel cell, it may be necessary, as is also indicated by a line connection  42  drawn in broken line, to feed air into the burner  34  directly from the air supply means  26 . This air or the oxygen contained therein will not be necessary to allow the combustion to take place in the burner  34 . The oxygen still contained in the fuel cell waste air is sufficient for this. The consequence of this supply of additional air is rather that the very hot waste gases of the burner  34  will be cooled somewhat, so that the air, which is to be fed into the fuel cell  12  and which flows for this purpose beforehand through the heat exchanger  32 , can enter the fuel cell  12  at a suitable temperature.  
      Various embodiments of the burner  34  and of the system components of this burner  34  will be described in detail below. The burner  34  shown in  FIG. 2  comprises a burner housing, generally designated by  44 , with a circumferential wall area  46  and a bottom wall area  48 . The circumferential wall area  46  and the bottom wall area  48  define a combustion chamber  50 , which is open via a flame baffle or a flame retention baffle  52  to a volume area  54  leading in the direction of the heat exchanger  32 . In its section near the bottom wall area  48 , the circumferential wall area  46  is surrounded by an annular space  56 . This annular space  5  is in connection with the combustion chamber  50  via a plurality of openings  58 . As is indicated by arrows P 1 , combustion air can thus flow into the combustion chamber  50  radially from the outside. The bottom wall area  48  may also have a plurality of such openings, which are not shown in  FIG. 2  and through which at least a portion of the combustion air can enter the combustion chamber  50 . As was already described above with reference to  FIG. 1 , this combustion air is fed essentially in the form of the fuel cell waste air (arrow  20  in  FIG. 1 ) in the direction of burner  34 .  
      The combustion chamber  50  is defined in the direction of the bottom wall area  48  by a porous evaporator medium  60 , which covers the bottom wall area  48  essentially completely. This porous evaporator medium  60  may be a braiding, a knitted fabric, a foam ceramic or another material provided with fine pores, in which liquid fuel can be distributed by capillary action. A heating means  62 , which can be operated electrically, may be positioned between the porous evaporator medium  60  and the bottom wall area  48  in order to heat the porous evaporator medium  60  and to support the evaporation of the fuel being distributed therein in the direction of the combustion chamber  50 .  
      A feed line  64  opens into the bottom wall area  48  or the combustion chamber  50  in a central area. Both the fuel, which is being fed by the fuel supply means  24  and is generally liquid, and the fuel cell waste gas, i.e., a gas still containing hydrogen, can be fed in via this feed line  64  in this embodiment. A valve arrangement  66 , which is shown only schematically, is provided to make it possible here to choose between the different fuels. This valve arrangement comprises a valve slide  68 , which connects either the line  18  coming from the fuel cell  12  with the feed line  64  or the line  70  coming from the fuel supply means  24  with the feed line  64 , or does not connect any of these lines  18 ,  70  with the feed line  64 , depending on the positioning in one of the three possible switching positions. If the line  70  is connected with the feed line  64 , liquid fuel is consequently introduced into the porous evaporator medium  60 . This fuel is distributed by capillary action over the entire porous evaporator medium and will also evaporate on the side of the porous evaporator medium facing the combustion chamber  50  in the direction of the combustion chamber  50  because of the heating action of the heating means  62 . An ignitable and combustible mixture is generated now with the air, which is likewise introduced into the combustion chamber  50 , i.e., the fuel cell waste air, which contains more or less oxygen depending on the operating state of the fuel cell  12 . An igniting member  72 , for example, a glow type igniting pin, is arranged at a closely spaced location above the porous evaporator medium  60  to start this combustion. Once the ignition has taken place, the hot combustion waste gases flow in the direction of the heat exchanger  32 , as is indicated by the arrows P 2 , and will also flow past, e.g., a temperature sensor  74 , which can thus determine, by sensing the temperature of the gases leaving the combustion chamber  50 , whether ignition has already taken place or not.  
      If hydrogen-containing gas is available for the combustion via the line  18 , the valve is reversed and the line  18  is now connected with the feed line  64 . The hydrogen-containing gas will then flow through this feed line  64  and the bottom wall area  48  and enter the combustion chamber  50  in the local area, in which the feed line  64  is open toward the porous evaporator medium  60 , it will be mixed there with the air available for the combustion and likewise produce an ignitable and combustible mixture.  
      A variant of such a burner is shown in  FIG. 3 . It can be recognized here that while the design is basically the same, there is a difference only in the area of the feed of the different media used for the combustion. Furthermore, the feed line  64  is present, via which liquid fuel is now introduced into the porous evaporator medium  60 , which is fed, for example, from the line  70 . Furthermore, a feed line  76  is provided, which surrounds the feed line  64  at least in the end section that is close to the bottom area  48  and is now in connection with the line  18  and can supply the hydrogen-containing gas in the direction of the combustion chamber  50 . Consequently, since separate feed lines  64  and  76  are available here for the fuel, on the one hand, and the fuel cell waste gas, on the other hand, a valve mechanism  66 , as is shown in  FIG. 2 , can be omitted, in principle. Furthermore, it is possible in this variant, by providing a separate feed line for the fuel cell waste gas, to feed this fuel cell waste gas into the combustion chamber  50  in an optimized manner, i.e., without being dependent on the site of the fuel feed via the feed line  64 . Thus, the coaxial feed shown in  FIG. 3  is not absolutely necessary. The fuel cell waste gas can rather be fed in, distributed over the bottom wall area  48 , via a plurality of introduction points in the direction of the combustion chamber  50 . Improved mixing of the fuel cell waste gas with the fuel cell waste air or the combustion air can thus be achieved.  
      An embodiment, in which such mixing can be further improved, is shown in  FIG. 4 . It is recognized here that an intermediate space, which provides a premixing chamber  78 , is formed between the bottom wall area  48  and the porous evaporator medium  60 . At least part of the air used for the combustion, i.e., the fuel cell waste air, enters this premixing chamber  78  through openings  80  provided in the bottom wall area  48 . Furthermore, the feed line  76  is open toward this premixing chamber  78  in the area in which it is connected to the housing  44 . The fuel cell waste gas thus mixes with the fuel cell waste air before being fed into the combustion chamber  50  via the porous evaporator medium  60 . Since no liquid fuel is fed into the porous evaporator medium  60  via the feed line  64  during the phase during which the fuel cell waste gas is used for the combustion, the pores of this evaporator medium are also not blocked by liquid, so that the mixture produced in the premixing chamber  78  in a bottom area of the combustion chamber  50  or the housing  44 , which area is generally designated by  82 , can enter the combustion chamber  50  without greater resistance and can burn there. The porous evaporator medium  60  also forms a flame retention baffle in this exemplary embodiment or in this phase of the operation, which ensures that no combustion will take place in the premixing chamber  78  itself.  
      A variant of this embodiment is shown in  FIG. 5 . It can be recognized here that the feed line  76  extends into the area of the premixing chamber  78  and has a plurality of radially outwardly leading openings  84  there. This can also contribute to improved mixing of the fuel cell waste gas with the fuel cell waste air.  
      It shall be pointed out that a heating means, e.g., one in the form of a heating coil, may, of course, be associated with the porous evaporator medium  60  in the embodiment variants according to  FIGS. 4 and 5  as well, in order to improve the evaporation of the liquid fuel in the direction of the combustion chamber  50  when such a liquid fuel is fed in via the feed line  64 .  
      Another embodiment of such a burner is shown in  FIG. 6 .  
      The housing  44  with the circumferential wall area  46  and the bottom wall area  48  can be recognized here as well. Adjoining this bottom wall area  48  and in the bottom area  82 , an insulating material layer  86  is provided, which is then followed by the heating means  62  as well as the porous evaporator medium  60 . The feed line  76  for the fuel cell waste gas extends in the central area of this bottom area  82 . This feed line  76  extends through the bottom wall area  48  and also the porous evaporator medium  60  and is then open toward the combustion chamber  50  via one opening or a plurality of openings. The fuel is fed here via the feed line  64  led in radially. The advantage of this embodiment is that the fuel cell waste gas can be fed regardless of whether liquid fuel is still present in the porous evaporator medium and whether such fuel consequently covers the pores of the evaporator medium. This also guarantees easier introduction of the fuel cell waste gas in the bottom area  82  and consequently improved mixing with the combustion air or fuel cell waste air, which is to be introduced again radially from the outside, but optionally also through openings in the bottom wall area  48 .  
      Another alternative embodiment is shown in  FIGS. 7 and 8 . It can be recognized that the design corresponds basically to the burner shown in  FIG. 2 . However, the feed line  64  now opens into the porous evaporator medium  60  radially from the outside and supplies fuel to the evaporator medium radially from the outside. Since, as is illustrated in  FIG. 7 , the fuel is fed in radially from the outside and preferably from the top, it can be distributed very uniformly in the porous evaporator medium, utilizing the force of gravity, which forces it to move downward.  
      The bottom wall area  48  is covered by a housing  200  on its side facing away from the combustion chamber  50 . As is illustrated in  FIG. 8 , a gas feed line  202  opens essentially tangentially into this housing  200 . This gas feed line  202  may be in connection, directly or optionally via a valve arrangement, with the line  18  recognizable in  FIG. 1 , via which the anode waste gas, i.e., a gas still containing residual hydrogen, can be fed. Openings providing a nozzle-like passage opening  204  each are provided in a central area in the bottom wall area  48 , the heating means  62  and the porous evaporator medium  60  each. This nozzle-like passage opening  204  has a cross section expanding in the direction of the combustion chamber and forms a swirl nozzle for the gas flowing tangentially into the housing  200  from the line  202 . As is indicated in  FIG. 8 , a swirl-like turbulence of the gas introduced into the combustion chamber  50  is generated during this flow of the gas leaving the fuel cell via the line  18  via a swirl chamber  206  formed in the housing  200  and the nozzle-like opening  204 , as a result of which extraordinarily good mixing, e.g., with the combustion air being introduced radially from the outside, is brought about. To prevent backflash of the flame from the combustion chamber  50 , a flame barrier, for example, one in the form of a grid or another structure provided with openings, may be provided in the area of the passage opening  204  and even in the area in which the line  202  opens intro the chamber  206 .  
      Valve devices may be associated separately with the feed line  64  as well as the gas line  202  in order to make it possible to close and open these lines as desired and thus to introduce the fuel cell waste gas or the fuel into the combustion chamber  50  as desired.  
      An alternative embodiment of a burner  34  to be used in the system according to  FIG. 1  is shown in  FIG. 9 . While the burners described above with reference to  FIGS. 2 through 8  operate according to the principle of an evaporator burner in case of use of a liquid fuel, i.e., the mixture of fuel and combustion air is provided by evaporating the fuel, a so-called atomizer arrangement  90  is provided in the burner  34  being shown in  FIG. 9 . An initially liquid fuel is atomized by this atomization device  90  into very fine fuel particles and fed together with the combustion air into the combustion chamber  50  provided in the housing  44 . This atomizer arrangement  90  also forms essentially the bottom area  82  of the combustion chamber  50  or of the housing  44 .  
      It can be recognized from  FIG. 9  that this atomizer arrangement  90  comprises a nozzle body which is generally designated by  92  and has an essentially pot-shaped design. Three insert parts  94 ,  96 ,  98  are inserted into this nozzle body  92  in an axially staggered manner, axial being related to a longitudinal axis of the burner  34 . The insert part  94  is arranged here seated on a bottom  100  of the nozzle body  92  and has a shape that tapers concavely in the direction of the axis A and is preferably rotationally symmetrical with this axis A.  
      The second insert part  96  is arranged at a spaced location from the first insert part  94  and has an approximately cylindrical section terminating in an atomizer lip  102  in its radially inner area. An inner swirl flow space  104 , in which a plurality of flow deflecting elements  106  ensure that the combustion air, introduced from a radially outer, annular space  108 , i.e., again the fuel cell waste air here, will also receive a circumferential flow component during the flow radially from the outside to radially to the inside and is thus sent in the direction of the atomizer lip  102  in the form of a flow provided with a swirl around the axis A, is formed between the first insert part  94  and the second insert part  96 .  
      The third insert element  98  is arranged axially after the second insert part  96  and at a spaced location from same. An intermediate space, which defines an outer swirl flow space  110 , is also provided between these two insert parts  96 ,  98 . A plurality of flow deflecting elements  112  are provided here as well. These ensure that the air arriving radially from the outside will likewise contain a circumferential component, so that a flow provided with a swirl will also flow from the outside in the direction of the atomizer lip  102 .  
      A groove-like hollow  114  is provided in the second insert part  96  on its side defining the inner swirl flow space  104 . The liquid fuel to be atomized is introduced into this hollow and distributed in the circumferential direction by a fuel distributing element indicated only schematically. Thus, this fuel reaches the surface area of the second insert part  96  that is swept by the inner swirl flow and is transported in the direction of the atomizer lip  102  under the delivery action of the inner swirl flow. If the fuel, which is initially still liquid and forms a thin coating film on the second insert part  96 , then reaches the atomizer lip  102 , it is disintegrated into very fine particles by the two swirl flows converging there, and these particles can then be burned in the combustion chamber  50  with the combustion air.  
      It shall be pointed out that the design of such an atomizer arrangement  90  is basically known from DE 102 05 573 A1. Concerning the further design details of such an atomizer arrangement  90 , reference is therefore made to that document, whose contents are hereby included here by reference to the disclosure content.  
      The feed line  76 , which passes through the bottom  100  of the nozzle body  92  and also the first insert part  94  and protrudes into the volume area that is also surrounded by the essentially cylindrical area of the second insert part  96 , which said essentially cylindrical area leads to the atomizer lip  102 , can be further recognized from  FIG. 9 . The fuel cell waste gas fed in via this feed line  76  consequently enters from the feed line  76  a volume area in which the combustion air flows as well, here in the form of the inner swirl flow. Due to the swirl of this inner swirl flow, the fuel cell waste gas will be mixed with this inner swirl flow immediately after the discharge from the feed line  76  and, shortly thereafter, also with the outer swirl flow, and it will then enter the combustion chamber  50  in this mixture for combustion.  
      Consequently,  FIG. 9  also shows a burner  34 , which can be operated with fuel, i.e., preferably liquid fuel in this case, or with the fuel cell waste gas as desired. The fuel cell waste air can then be utilized as combustion air in this case as well, and it can also be used at the same time to atomize liquid fuel for the combustion if a liquid fuel is used.  
      It shall be pointed out that not only liquid fuel can be used to operate the burner  34  in all the burners  34  described above in the operating state in which, for example, the hydrogen-containing fuel cell waste gas is not yet available. If the fuel supply means  24  is designed correspondingly, it is, of course, also possible to use a gaseous fuel, e.g., natural gas, as a starting material to be burned with the fuel cell waste air.  
      While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.