Operating procedures for a fuel cell system

Abstract

Method for starting a fuel cell system (1), - wherein the fuel cell system (1) comprises a fuel cell (2), a reformer (33) and an auxiliary burner (20), - in which fuel cell air is preheated with the auxiliary burner (20) and supplied to a cathode side (8) of the fuel cell (2), - in which residual gas is circulated from an anode side (6) of the fuel cell (2) to the reformer (33) and from the reformer (33) to the anode side (6), - in which, upon reaching a predetermined anode limit temperature, the reformer (33) is operated in a reformer operating state in order to convert any oxygen contained in the residual gas continuing to circulate between the anode side (6) and the reformer (33), - in which the reformer (33) generates reformate gas in the reformer operating state, wherein the reformate gas is reacted in a residual gas burner (3) together with the fuel cell air discharged from the cathode side (8), - in which the reformer (33) is switched off again when a predetermined further anode limit temperature is reached and the oxygen-free residual gas continues to circulate between the anode side (6) and the reformer (33), - in which the reformer (33) is switched on again upon reaching a predetermined further anode limit temperature and is immediately operated in reformer operating mode, - in which the auxiliary burner (20) is deactivated as soon as a predetermined anode operating temperature is reached, - in which the fuel cell (2) is activated when the anode operating temperature is reached.

Description

[0001] The present invention relates to a method for operating a fuel cell system, in particular for use in a motor vehicle or also for stationary applications, e.g. as an additional supply of electricity and heat, for example in households or industrial areas.

[0002] A fuel cell system, which can be installed in a motor vehicle or other mobile or stationary application as the sole or supplementary source of electrical power, typically comprises a fuel cell, usually formed by a stack of individual fuel cell elements, which reacts an anode gas with a cathode gas to generate electricity. A waste gas burner may be downstream of the fuel cell, which reacts the fuel cell's exhaust gases, i.e., anode and cathode gases, producing burner exhaust gas. The fuel cell system may also be equipped with a reformer to generate reformate gas, which can be supplied to the fuel cell as anode gas. Under nominal operating conditions, the fuel cell operates exothermically. Similarly, the reformer, if it operates with partial catalytic oxidation of the fuel, also operates exothermically under nominal conditions.Such fuel cell systems with fuel cell, reformer and residual gas burner are known, for example, from DE 10 2008 018 152 A1, US 2005 / 0 249 991 A1, US 2002 / 0 068 204 A1 and US 2002 / 0 045 078 A1.

[0003] From DE 101 45 441 A1 a fuel cell system is known which has a fuel cell, a reformer and an additional burner which is connected upstream of the reformer.

[0004] A solid oxide fuel cell with an integrated heat exchanger is known from DE 603 ​​02 618 T2.

[0005] During a cold start of the fuel cell system, when the individual components are brought up from ambient temperature, heat must be supplied to the fuel cell to reach its operating temperature. Similarly, heat must be supplied to a reformer operating with a catalyst to reach its operating temperature. During this start-up process, the fuel cell system has a poor energy efficiency and comparatively high pollutant emissions. To minimize the start-up time, it is possible to design the residual gas burner and the reformer specifically for this purpose, such that they generate as much heat as possible during the cold start operation, which can then be used to warm the system components. However, such a design for the start-up process inevitably leads to an incorrect design or...Oversizing for the nominal operating condition. This particularly worsens the energy efficiency of the fuel cell system during nominal operation.

[0006] The subsequently published DE 10 2009 030 236 A1 discloses a fuel cell system with fuel cell, reformer, afterburner and auxiliary burner, wherein the auxiliary burner preheats the cathode air during a cold start of the fuel cell system in order to heat up the fuel cell.

[0007] The present invention addresses the problem of providing an improved embodiment for an operating method of such a fuel cell system, which is characterized in particular by a material-friendly yet comparatively fast-acting procedure and / or by an increased efficiency of the fuel cell system.

[0008] This problem is solved according to the invention by the subject matter of the independent claim. Advantageous embodiments are the subject matter of the dependent claims.

[0009] The invention is based on the general concept of equipping a fuel cell system with a reformer with an additional burner, which makes it possible to preheat the air supplied to the fuel cell during cold start operation. The fuel cell air preheated by the additional burner is then supplied to a cathode side of the fuel cell. This allows the fuel cell to be heated to its operating temperature relatively quickly, thus shortening the warm-up phase. Simultaneously, the operating method according to the invention proposes for the cold start of the fuel cell system to circulate residual gas contained in gas-carrying components of the fuel cell system from an anode side of the fuel cell to the reformer and from the reformer to the anode side, particularly as long as the anode or anode side of the fuel cell is below a certain anode temperature limit.In other words, in one section of the fuel cell system, residual gas is circulated between the reformer and the anode side of the fuel cell. Since the fuel cell air, preheated by the auxiliary burner, heats the cathode side of the fuel cell, this automatically results in the anode side also being heated, thus transferring heat to the circulating residual gas. This circulating residual gas carries the heat to the reformer, preheating it and, in particular, its catalyst.

[0010] The start-up procedure presented here uses the auxiliary burner to simultaneously preheat the fuel cell and the reformer. This allows the reformer to be ready for use more quickly, thus shortening the overall start-up procedure, while at the same time ensuring a material-friendly approach to prevent damage to the individual components due to excessive thermal stress.

[0011] By using the auxiliary burner, for example, a residual gas burner can be designed for nominal operation of the fuel cell, since the auxiliary burner can be switched off at the end of the cold start operation. Consequently, the efficiency of the fuel cell system is improved during nominal operation.

[0012] According to the invention, before reaching a predetermined (first) anode limit temperature, which may be, for example, around 250°C, the reformer is operated in a reformer operating state. Such reformer operation can be achieved at a sufficiently high temperature, for example, by temporarily supplying fuel and reformer air to the reformer at a suitable air-fuel ratio. In this way, any oxygen contained in the residual gas that continues to circulate can be converted or consumed. It is important that during this temporary reformer operating state, the residual gas continues to circulate between the anode and the reformer. This ensures that all the oxygen contained in the residual gas is reliably consumed.This temporary reformer operating state is implemented to allow the residual gas to continue circulating even at rising temperatures, without damaging the fuel cell anode. At higher temperatures, for example above 300°C, the risk of permanent anode damage from contact with oxygen increases significantly.

[0013] If a hot start of the reformer with immediate reformer operation is not possible, a cold start must be performed, during which the reformer is initially operated in a burner mode. According to a further development of the starting procedure presented here, the reformer can thus be operated in a burner mode below a predetermined catalyst temperature. Reformer air is supplied to the reformer, and the reformer exhaust gas generated within the reformer is discharged via an exhaust line. The reformer then serves as an additional heat source, namely as an additional burner to heat the catalyst. Once the catalyst temperature limit, which can range between 350°C and 900°C, is reached, the reformer can be switched to its normal operation.

[0014] As long as the temperature at the anode side remains below a reoxidation limit, which might be around 300°C, for example, the gas coming from the reformer can be routed through the anode side. Optionally, the gas coming from the reformer can be routed to the exhaust pipe, bypassing the anode side, thus preventing contact between the anode and the oxygen carried in the gas coming from the reformer.

[0015] Regardless of whether the reformer exhaust flows through or bypasses the anode side, the reformer exhaust can be used to preheat fuel cell air.

[0016] Once the reformer's catalyst reaches its predetermined operating temperature, for example 900°C, the reformer can operate particularly efficiently in its reformer mode. The reformate gas typically contains no oxygen and can be passed through the anode side, further heating the fuel cell. Additionally, the reformate gas can be combusted in a residual gas burner together with the fuel cell air drawn from the cathode side, releasing further heat that can be used to preheat the fuel cell air.

[0017] According to the invention, the auxiliary burner is deactivated as soon as the residual gas burner takes over the preheating of the fuel cell air or as soon as a predetermined (second) anode limit temperature or anode operating temperature is reached.

[0018] According to the invention, it is further provided that the reformer is switched off again when a predetermined third anode limit temperature is reached, and the now oxygen-free residual gas continues to circulate between the anode side and the reformer. This third anode limit temperature is significantly below the second anode limit temperature or below the anode operating temperature. However, the third anode limit temperature is also above the first anode limit temperature. Below the anode operating temperature, which can be, for example, 650°C, there is a risk of soot formation or soot deposits on the anode of the fuel cell. By switching off the reformer, this risk can be significantly reduced, as the temperature range critical for soot formation is bypassed.

[0019] According to the invention, when a predetermined fourth anode limit temperature is reached, the reformer is switched on again and then immediately operated in reformer mode. The fourth anode limit temperature is always higher than the third anode limit temperature. The third anode limit temperature can, for example, be around 350°C. The fourth anode limit temperature can be around 650°C. It can therefore be chosen to be the same as the aforementioned second anode limit temperature or as the anode operating temperature. Switching the reformer back on when the fourth anode limit temperature is reached enables a warm start of the reformer, i.e., immediate operation of the reformer in reformer mode. At the comparatively high temperatures now present, the risk of soot formation or soot deposits on the anode is significantly reduced.

[0020] Once the anode side or the fuel cell reaches a minimum temperature, the fuel cell will be put into operation. The start-up procedure is then complete.

[0021] According to another advantageous embodiment, to regulate the temperature of the fuel cell, air can be introduced from a bypass air line, which bypasses a first heat exchanger arranged in a fuel cell air line, via a bypass line, which bypasses a second heat exchanger arranged in the bypass air line, into the fuel cell air line downstream of the first heat exchanger. The first heat exchanger can interact with the exhaust gas stream of the residual gas burner to heat the fuel cell air. The second heat exchanger can interact with the auxiliary burner to preheat the fuel cell air with the hot auxiliary burner exhaust gas. If it is necessary to regulate the temperature of the fuel cell, e.g.,To reduce or limit the temperature of the electrolyte, cathode, or anode to prevent overheating of the respective fuel cell component, it is now possible to supply cooling air drawn from the environment to the cathode side of the fuel cell, bypassing both heat exchangers. This is made possible by the bypass line, which connects the bypass air line to the fuel cell air line between the two heat exchangers.

[0022] Further important features and advantages of the invention will become apparent from the dependent claims, the drawings and the associated description of the figures based on the drawings.

[0023] It is understood that the features mentioned above and those to be explained below can be used not only in the combinations specified, but also in other combinations or individually, without leaving the scope of the present invention as defined by the claims.

[0024] Preferred embodiments of the invention are shown in the drawings and are explained in more detail in the following description, wherein identical reference numerals refer to identical or similar or functionally identical components.

[0025] They show, schematically: Fig. 1. A highly simplified, circuit diagram-like schematic representation of a fuel cell system, Fig. 2 a representation as in Fig. 1, however, under a different operating condition, Fig. 3 a representation as in the Fig. 1 and Fig. 2, however in a different embodiment.

[0026] According to the Fig. 1, Fig. 2 to Fig. The system comprises a fuel cell system 1, which can be arranged in a motor vehicle or in any other mobile or stationary application as the sole or as an additional electrical energy source, a fuel cell 2, and a waste gas burner 3. The fuel cell system 1 can alternatively also be used for stationary applications. During operation, the fuel cell 2 generates electrical current from anode gas and cathode gas, which can be tapped via electrodes 4. The fuel cell 2 is preferably designed as a SOFC fuel cell. During operation, the waste gas burner 3 reacts anode exhaust gas with cathode exhaust gas, producing burner exhaust gas. This reaction can take place with an open flame. A catalytic reaction is also conceivable.

[0027] An anode exhaust line 5 connects an anode side 6 of the fuel cell 2 to the waste gas burner 3. A cathode exhaust line 7 connects a cathode side 8 of the fuel cell 2 to the waste gas burner 3. The fuel cell exhaust gases are then converted in a combustion chamber 9 of the waste gas burner 3. The waste gas burner 3 can form a structurally integrated unit with the fuel cell 2. The anode exhaust line 5 and the cathode exhaust line 7 are then internal conduits or pathways.

[0028] In the fuel cell 2, an electrolyte 10 separates the anode side 6 from the cathode side 8. Anode gas is supplied to the anode side 6 of the fuel cell 2 via a reformate gas line 11 or an anode gas line 11.

[0029] Cathode gas is supplied to the cathode side 8 of the fuel cell 2 via a fuel cell air line 12. The cathode gas is preferably air. A burner exhaust line 13 carries the burner exhaust gas generated by the residual gas burner 3 away from the residual gas burner 3 or from its combustion chamber 9. A first heat exchanger 14 is integrated into this burner exhaust line 13 and is also integrated into the fuel cell air line 12. The first heat exchanger 14 creates a media-separated heat transfer coupling between the fuel cell air line 12 and the burner exhaust line 13. The first heat exchanger 14 can be structurally integrated into the residual gas burner 3.

[0030] In this example, the fuel cell system 1 is equipped with a fuel cell module 15, which comprises the fuel cell 2, the residual gas burner 3, and the first heat exchanger 14. Furthermore, this fuel cell module 15 is equipped with a thermally insulating casing 16, which encloses the components of the fuel cell module 15.

[0031] The fuel cell system 1 is also equipped with an air supply unit 17, which can be, for example, a blower, a compressor, an electrically driven turbocharger, or a pump. During operation, this air supply unit 17 supplies air as cathode gas to the fuel cell 2 via the fuel cell air line 12. The air supply unit 17 is part of an air supply module 18, which has its own thermally and / or acoustically insulating enclosure 19 in which the air supply unit 17 is located. The air supply unit 17 can preferably be equipped with a filter device 71 to filter out particles and / or aerosols from the supplied air.

[0032] The fuel cell system 1 is also equipped with an auxiliary burner 20, which is configured to convert air with a fuel into auxiliary burner exhaust gas during operation. This auxiliary burner exhaust gas is discharged from the auxiliary burner 20, or from a combustion chamber 22 of the auxiliary burner 20, via an auxiliary burner exhaust line 21, or simply auxiliary exhaust line 21. The auxiliary exhaust line 21 preferably includes a shut-off device 67 for decoupling the auxiliary burner 20 during normal operation of the fuel cell system 1, when the auxiliary burner 20 is switched off. The shut-off device 67 then acts as a non-return valve. A second heat exchanger 23 is integrated into this auxiliary exhaust line 21. Furthermore, the second heat exchanger 23 is integrated into a bypass air line 24. The second heat exchanger 23 thus creates a media-separated, heat-transferring coupling between the additional exhaust gas line 21 and the bypass air line 24.The second heat exchanger 23 can be structurally integrated into the auxiliary burner 20.

[0033] The bypass air line 24 bypasses the first heat exchanger 14 on the air side. For this purpose, the bypass air line 24 is connected on the inlet side via a sampling point 25 between the air conveying device 17 and the first heat exchanger 14 to the fuel cell air line 12. On the outlet side, the bypass air line 24 is connected on the outlet side via an inlet point 26 between the first heat exchanger 14 and the fuel cell 2 to the fuel cell air line 12. A first section of the fuel cell air line 12, which leads from the air conveying device 17 to the inlet point 26, is referred to below as 12', while a second section of the fuel cell air line 12, leading from the inlet point 26 to the fuel cell 2 or to the cathode side 8, is referred to below as 12"".

[0034] According to the advantageous embodiments shown here, a bypass line 72 can optionally be provided, which connects a draw-off point 73 of the bypass air line 24, arranged upstream of the second heat exchanger 23, with the inlet point 26, i.e., with the fuel cell supply air line 12. This bypass line 72 thus enables the second heat exchanger 23 to be bypassed within the bypass air line 24. A first section of the bypass air line 24, which leads from the draw-off point 25 to the further draw-off point 73, is hereinafter referred to as 24', while a second section of the bypass air line 24, leading from the further draw-off point 73 to the inlet point 26, is hereinafter referred to as 24". A further valve 74 can be provided for controlling the bypass line 72; in this example, it is expediently arranged at the further draw-off point 73.

[0035] In normal operation of the fuel cell system 1, i.e., with the auxiliary burner 20 switched off, the fuel cell air is preheated exclusively via the first heat exchanger 14. In certain operating situations, it may be necessary to prevent a further temperature increase of the fuel cell 2 or to cool the fuel cell 2. This may be necessary, for example, to protect a component of the fuel cell 2, such as the electrolyte 10, from overheating. The temperature of the fuel cell 2 can be regulated by supplying cold ambient air to the fuel cell air to reduce its temperature. The cold ambient air can be supplied to the second section 12" of the fuel cell air line 12 via the bypass air line 24, bypassing the first heat exchanger 14. However, if, for example,During start-up operation, with the auxiliary burner 20 still active, the second heat exchanger 23 located in the Bapyss air duct 24 must also be bypassed to achieve cooling of the fuel cell air. The bypass duct 72 is used for this purpose. The cooling air then flows through the first section 24' of the bypass air duct 24 to the bypass duct 72 and from the bypass duct 72 into the second section 12' of the fuel cell air duct 12. The cooling air thus bypasses both the first heat exchanger 14 and the second heat exchanger 23.

[0036] The auxiliary burner 20 is supplied with air via an auxiliary air supply unit 27 and a corresponding air supply line 28. The auxiliary supply unit 27 can preferably be equipped with a filter unit 75 to filter out particles and / or aerosols from the supplied air. The air for the auxiliary burner 20 is preferably drawn from an environment 52 of the fuel cell system. The auxiliary burner 20 is supplied with fuel by means of a fuel supply unit 29 via a corresponding fuel line 30. The fuel can be, for example, any hydrocarbon. However, a fuel that is also used to operate an internal combustion engine of the vehicle equipped with the fuel cell system 1 is preferred. In particular, the fuel is therefore diesel, biodiesel, or heating oil.Likewise, gasoline or natural gas or any biofuel, as well as synthetic hydrocarbons, are conceivable. Consequently, the fuel line 30 is expediently connected to a fuel tank 53 of the vehicle, which is not shown in detail here.

[0037] The auxiliary burner 20 and the second heat exchanger 23 are components of an auxiliary burner module 31, which has its own thermally insulating casing 32 in which the auxiliary burner 20 and the second heat exchanger 23 are arranged. In this example, the auxiliary air supply unit 27 and the fuel supply unit 29 of the auxiliary burner 20 are also components of the auxiliary burner module 31. However, these components are located outside the associated casing 32.

[0038] In the example shown, the fuel cell system 1 is also equipped with a reformer 33, which, during operation, reacts air with a fuel substoichiometrically, i.e., at an air-fuel ratio < 1, thereby producing hydrogen- and carbon monoxide-containing reformate gas. This reformate gas is supplied to the anode side 6 of the fuel cell 2 via the reformate gas line 11 as anode gas. A reformer air line 34 is provided to supply the reformer 33 with reformer air; this line is also fed by the air supply unit 17. In addition, in the embodiment shown here, a further supply unit 35 is arranged in the reformer air line 34 downstream of the air supply unit 17; this is referred to below as the reformer air supply unit 35. With the aid of this reformer air supply unit 35, the air supplied to the reformer 33 can be brought to an increased pressure level.Additionally, this reformer air conveying device 35 can be designed as a hot gas conveying device. For example, it can be designed like a blower, compressor, electric turbocharger, or pump.

[0039] To supply the reformer 33 with fuel, a reformer fuel supply 36 is provided, which feeds a suitable fuel to the reformer 33 via a corresponding fuel line 37. This fuel can again be any hydrocarbon. Preferably, the fuel that is also supplied to the internal combustion engine of the vehicle equipped with the fuel cell system 1 is used. Accordingly, the fuel line 37 provided for supplying the reformer 33 is also expediently connected to the tank 53 of the vehicle.

[0040] The reformer 33 contains a combustion chamber 38 or mixing chamber 38. The reformer 33 also contains a catalyst 40, with the help of which the reformate gas can be produced by means of partial oxidation.

[0041] The reformer 33 is part of a reformer module 41, which has a separate or independent thermally insulating and / or gas-tight enclosure 42 in which the reformer 33 is arranged. In this example, the reformer fuel conveying device 36 belongs to the reformer module 41. However, said conveying device 36 is arranged outside the enclosure 42 of the reformer module 41.

[0042] The burner exhaust line 13, or exhaust line 13 for short, contains an oxidation catalyst 43 for exhaust aftertreatment downstream of the first heat exchanger 14. A heating heat exchanger 44 can also be integrated into the exhaust line 13, which, during operation, can heat a fluid flow 45, indicated by an arrow. This fluid flow 45 can be an air flow that can be supplied to a vehicle interior (not shown here). Alternatively, the fluid flow 45 can also be a coolant from a cooling circuit, whereby the cooling circuit contains a heat exchanger for heating an air flow that can then, for example, be supplied to a vehicle interior. The heating heat exchanger 44 is advantageously arranged downstream of the oxidation catalyst 43. This allows the heat potentially released in the oxidation catalyst 43 during the conversion of pollutants to be used for heating the vehicle interior.

[0043] The outlet 25, where the bypass air line 24 branches off from the fuel cell air line 12, is expediently designed as a valve or arranged on a valve 46. This valve 46 allows, for example, virtually any distribution of the airflow supplied by the air delivery device 17 to the section of the fuel cell air line 12 passing through the first heat exchanger 14 and to the bypass air line 24. The valve 46 is expediently part of a valve assembly 47, which, via a distribution manifold 48, distributes the air supplied on the pressure side by the air delivery device 17 to the fuel cell air line 12 and to the reformer air line 34. A further valve 49, which can also be part of the valve assembly 47, can be provided to control the amount of air supplied to the reformer 33. Furthermore, the example includes a cooling gas line or cooling air line 50, through which cooling air can be supplied to the residual gas burner 3.The cooling air line 50 is controllable by a valve 51, which in this example also belongs to the valve assembly 47. The air conveying device 17 also draws air from the environment 52 of the fuel cell system 1 via a suction line 53. In this example, the valve assembly 47 is also part of the air supply module 18 and is arranged within the associated housing 19.

[0044] The valves of the valve assembly 47 and the air supply units 17, 35 are preferably temperature-controlled or temperature-regulated. For example, the valve 49, the supply unit 17, and the reformer air supply unit 35 are regulated depending on the temperature of the mixing chamber 38 and / or depending on the temperature of the catalyst 40. The valve 51 and the air supply unit 17 can, for example, be regulated depending on the temperature of the combustion chamber 9. The valve 46 and the air supply unit 17 can, for example, be regulated depending on the temperature of the cathode side 8. The air supply unit 35 can, for example, be regulated depending on the temperature of the mixing chamber 38 and / or depending on the temperature of the catalyst 40.

[0045] The electrical current generated by the fuel cell system 1 is used to supply electrical consumers 54 with electrical current or electrical energy. The [missing information] symbolizes the [missing information]. Fig. 1, Fig. 2 to Fig. Figure 3 shows an electrical consumer 54, encompassing all electrical consumers that can be supplied with electrical energy by the fuel cell system 1. These include, on the one hand, external consumers, i.e., electrical consumers 54 of the vehicle, such as an air conditioner, a refrigerator, a coffee machine, a television, etc. On the other hand, they also include all internal consumers, i.e., all electrical consumers 54 of the fuel cell system 1. Electrical consumers 54 of the fuel cell system 1 include, for example, the conveying devices 17, 27, 29, 35, 36, the valves 46, 49, 51, 67, 74, 76, and ignition devices, such as glow plugs and spark plugs, with which a combustion reaction can be initiated in the residual gas burner 3, in the auxiliary burner 20, and in the reformer 33.Likewise, a control device 55, with the help of which the individual components of the fuel cell system 1 can be operated, can represent an electrical consumer 54 of the fuel cell system 1.

[0046] According to the preferred embodiment shown here, the fuel cell system 1 can also include an electrical energy storage device 56, which is designed, for example, as a battery or accumulator. Capacitors are also conceivable as electrical energy storage devices 56. The energy storage device 56 serves to supply electrical loads of the fuel cell system 1. Furthermore, at least one voltage converter 57, or simply converter 57, can be provided, with the aid of which a voltage transformation is carried out. For example, a DC / DC converter can be provided. Additionally or alternatively, depending on the application, a DC / AC converter can also be provided. The respective converter 57 transforms the voltage between a voltage level of the fuel cell 2 on the one hand and a voltage level of the electrical loads 54 of the fuel cell system 1 and / or the energy storage device 56 on the other.The energy storage device 56 and optionally also the converter 57 can be housed in an energy storage module 58, which, for example, has its own enclosure 59. The enclosure 59 can also be designed as a separate, thermally and / or electromagnetically insulating shell, which can also be referred to as the enclosure 59 in the following. Particularly in stationary applications, a DC / AC converter as described above can be provided, which can then be arranged, for example, between the energy storage device 56 and the respective AC load 54.

[0047] In the embodiments shown here, the auxiliary exhaust line 21 is connected to the exhaust line 13 via an inlet 60, downstream of the first heat exchanger 14. This inlet 60 is advantageously positioned upstream of the oxidation catalyst 43. This allows the residual heat from the auxiliary burner exhaust to be used to heat the oxidation catalyst 43. Simultaneously, the residual heat from the auxiliary burner exhaust can be used to heat the heating heat exchanger 44.

[0048] During the Fig. 1, Fig. 2 to Fig. In the three embodiments shown, the auxiliary exhaust line 21 leads directly from the auxiliary burner 20 to the exhaust line 13. Alternatively, in another embodiment, the auxiliary exhaust line 21 may have a reformer branch that is heat-transferred to a housing of the reformer 33. For example, this reformer housing can form a shell through which the auxiliary burner exhaust gas flows, thus heating the reformer 33 from the outside. This branch of the auxiliary exhaust line 21 advantageously diverges from the auxiliary exhaust line 21 via a valve, which allows for virtually any distribution of the auxiliary exhaust gas flow between the branch and the section of the auxiliary exhaust line 21 leading directly to the exhaust line 13.

[0049] Additionally or alternatively, said branch can be heat-transferred coupled to an end plate of fuel cell 2. Likewise, two branches can be provided to heat the reformer 33 and the end plate of fuel cell 2 independently of each other with the additional exhaust gas from the auxiliary burner 20.

[0050] The fuel cell 2 typically has a stacked structure, in which a large number of plate-shaped fuel cell elements are stacked on top of each other, thus forming a fuel cell stack. The fuel cell stack is terminated at its ends by two end plates: the aforementioned end plate and a further end plate. In this example, the further end plate has an anode gas connection 61, to which the anode gas line 11 or reformate gas line 11 is connected; a cathode gas inlet 62, to which the cathode gas line 12 or fuel cell air line 12 is connected; an anode exhaust outlet 63, to which the anode exhaust line 5 is connected; and a cathode exhaust outlet 64, to which the cathode exhaust line 7 is connected. Since all reactant connections are thus located on this further end plate, it can also be referred to as the terminal plate.In contrast, the other end plate merely forms a termination of the fuel cell stack, so it can also be referred to as the end plate.

[0051] In another embodiment, a further shell, which is particularly gas-tight, can be arranged within the thermally insulating casing 16 of the fuel cell module 15. This inner shell can also have a thermal insulating effect. It is also conceivable to design the outer shell 16 to be gas-tight. Furthermore, one shell may suffice if it is both thermally insulating and gas-tight. In particular, it is now possible to connect the aforementioned branch of the auxiliary exhaust line 21 to an interior space of the fuel cell module 15 enclosed by the inner shell. The branch enters this interior space at an inlet point and exits it again at a remote outlet point. This allows the fuel cell module 15 to be heated with the auxiliary burner exhaust gas. In particular, this can be combined with the heating of the fuel cell 2. For example,The exhaust gas from the auxiliary burner can initially be routed via the branch not shown to the end plate and from there exit into the interior, in order to be discharged from the interior again via the exit point.

[0052] In the preferred embodiments shown here, the fuel cell system 1 is further equipped with a recirculation line which is connected on the inlet side to the anode exhaust line 5 and on the outlet side via an inlet 66 to the reformer air line 34, upstream of the reformer air supply unit 35. Since the recirculated anode exhaust gas can have comparatively high temperatures during operation of the fuel cell system 1, the reformer air supply unit 35 is advantageously designed to be supplied with hot gases, which may also be toxic and / or explosive.

[0053] In the example, the valve assembly 47 is designed to divide the air drawn in by the air conveying device 17 on the pressure side to the fuel cell air line 12, to the bypass air line 24, to the cooling air line 50 and to the reformer air line 34.

[0054] In another embodiment not shown, the air supply device 17 can also be used to supply air to the auxiliary burner 20 via the valve device 47. For this purpose, the auxiliary burner air line 28 can be connected to a distribution manifold 48 via a further valve. Alternatively, the auxiliary air supply device 27 can also be omitted from the auxiliary burner air line 28.

[0055] The control unit 55 is suitably designed and programmed to perform the following operating procedure. For this purpose, it can be connected to an unspecified sensor system, which may include several sensors for temperature T, pressure p, voltage U, current I, and electrical power P. el can exhibit.

[0056] During a start-up process of the fuel cell system 1, particularly during a cold start, the auxiliary burner exhaust gas is used to preheat the fuel cell air, which is supplied through the bypass air line 24. Simultaneously, the auxiliary burner exhaust gas can be used to preheat the oxidation catalyst 43 during the start-up process. In the embodiment described above, the auxiliary burner exhaust gas can also be used to preheat the reformer 33. Alternatively or additionally, according to the embodiment described above, the auxiliary burner exhaust gas can also be used to preheat the end plate of the fuel cell 2. Alternatively or additionally, according to the other embodiment described above, the auxiliary burner exhaust gas can also be used to preheat the entire fuel cell module 15. Once the start-up process is complete, the auxiliary burner 20 can be switched off.In particular, the auxiliary burner 20 is switched off during nominal operation of the fuel cell system 1.

[0057] In addition to preheating the fuel cell air with the aid of the auxiliary burner 20, a residual gas circulation can be realized in a circulation circuit 68 during a cold start of the fuel cell system 1, in which the reformer 33 in particular is also at ambient temperature. Fig. 1 is indicated by a dashed line. In this circulation circuit 68, residual gas is conveyed in a closed loop from the anode side 6 to the reformer 33 and from the reformer 33 back to the anode side 6. For this purpose, the conveying device 35 is activated while, at the same time, the valve 49 is closed to prevent the intake of ambient air 52. Consequently, the conveying device 35 draws residual gas from the recirculation line 65 and thus from the anode side 6 via the anode exhaust line 5. On the pressure side, the conveying device 35 conveys this residual gas via the reformer air line 34 to the reformer 33. From the reformer 33, this residual gas then returns to the anode side 6 of the fuel cell 2 via the reformer gas line 11.

[0058] To regulate the residual gas circulation, a further valve 76, expediently designed as a hot gas valve, can be provided, which is arranged in the recirculation line 65 between the inlet point 66 and the anode exhaust line 5. This further valve 76 is expediently located within the casing 19, so that it forms part of the air supply module 18.

[0059] In this context, the term "residual gas" refers to the gas contained within circulation loop 68, i.e., within the affected components and pipe sections. This includes, in particular, residual air and residual anode gas or reformate gas.

[0060] During this start-up procedure, the air supply unit 17 delivers fuel cell air from the intake point 25 via the bypass air line 24 through the second heat exchanger 23 to the inlet point 26 and from there via the second section 12" of the fuel cell air line 12 to the cathode side 8 of the fuel cell 2. The fuel cell air then flows via the cathode exhaust line 7 through the residual gas burner 3 and via the burner exhaust line 13 through the first heat exchanger 14. Here, the fuel cell air, heated in the second heat exchanger 23, can transfer a relatively large amount of heat to the cathode, i.e., to the electrolyte 10. Simultaneously, the anode is also heated by heat transfer, so that the circulating residual gas, which passes through the anode side 6, can absorb heat and carry it to the reformer 33. In this way, the reformer 33 can also be heated.In particular, the catalyst 40 of the reformer 33 can be preheated in this way.

[0061] Depending on predetermined boundary conditions, the reformer 33 can now be operated, at least temporarily, as a burner. For example, it is important to prevent the anode side 6 from coming into contact with oxygen above a predetermined first anode limit temperature. Accordingly, before reaching this predetermined first anode limit temperature, which may be around 250°C, the reformer 33 is switched, at least temporarily, to a reformer operating state. During this reformer operation, any oxygen present in the residual gas is consumed. The circulation circuit 68 remains active during this reformer operation of the reformer 33, so that any oxygen still contained in the residual gas is consumed in this reformer operating state. As soon as no more oxygen is contained in the circulating residual gas, the reformer 33 can be switched off again, while the circulation operation continues.Now, temperatures above the first anode limit temperature can also be operated without the risk of damage to the electrolyte 10 on the anode side due to oxygen in the residual gas.

[0062] Alternatively, it is also possible to continue operating the reformer 33 in reformer mode to use it as an additional heat source for heating the fuel cell 2. However, this increases the risk of soot formation and deposits on the anode side 6 of the fuel cell 2, since comparatively low temperatures still exist, at least at the anode or on the anode side 6 of the electrolyte 10. To avoid such soot deposits, it may therefore be advantageous, as described above, to temporarily switch off the reformer 33 again in order to continue heating the fuel cell 2 via the auxiliary burner 20 until the temperature range critical for soot deposits has been exceeded.

[0063] In order to operate the reformer 33 in reformer mode, its catalyst 40 must have a predetermined catalyst limit temperature, which is at least 350°C and can be a maximum of approximately 900°C. If the catalyst temperature is above this limit temperature, a warm start of the reformer 33 is possible, meaning the reformer 33 can be operated immediately in its reformer mode. However, if the catalyst temperature is below this limit temperature, a cold start must be performed. During a cold start, the reformer 33 is initially operated in a burner mode until its catalyst reaches the limit temperature. The reformer 33 can then be switched to the reformer mode. In the burner mode, the reformer 33 is supplied with reformer air and fuel in a strongly superstoichiometric ratio.For reformer operation, a significantly substoichiometric air-fuel ratio is set. To switch from burner operation to reformer operation, the media supply to reformer 33, i.e., the supply of reformer air and fuel, can be briefly interrupted.

[0064] As soon as the reformer 33 is operated in burner mode or in reformer mode, reformer air is also supplied via the corresponding actuation of valve 49. Simultaneously, gas coming from the reformer, i.e., reformer exhaust gas or reformate gas, is routed to the residual gas burner 3 via the anode exhaust line 5, so that a combustion reaction can take place there with the aid of the cathode air. The resulting burner exhaust gas can be used in the first heat exchanger 14 to heat cathode air, which is then routed via the cathode gas line 12. In particular, it is possible to heat the air intended for the cathode side 8 via both the cathode air line 12 and the bypass air line 24.

[0065] In this case, the situation is as follows: Fig. 2. Here, the hot reformate gas is routed via reformate line 11 to the anode side 6, where it can transfer heat to the fuel cell 2. The reformate gas then continues via anode exhaust line 5 through the residual gas burner 3 and via burner exhaust line 13 through the first heat exchanger 14. From this point on, heat can also be transferred to the fuel cell air via the first heat exchanger 14 to preheat it. Accordingly, by appropriately controlling the valve 46, a partial flow of the fuel cell air can also be routed through the first section 12' of the fuel cell air line 12. Proportionally, the output of the auxiliary burner 20 or the airflow through the bypass line 24 can be reduced.

[0066] Fig. Figure 3 shows an alternative embodiment in which an additional bypass line 69 is provided, branching off from the reformer gas line 11 and bypassing the anode side 6 of the fuel cell 2. This makes it possible to maintain the burner operating state for a longer period, for example, to heat the reformer 33 gently without the risk of damage to the anode from residual oxygen. In this example, this bypass line 69 is connected to the anode exhaust line 5, so that reformer exhaust gas upstream of the residual gas burner 3 is reintroduced into the original path. The bypass line 69 can be controlled by a corresponding valve 70. Advantageously, the bypass line 69 is designed such that its flow resistance is lower than the flow resistance of the anode side 6 of the fuel cell 2.With valve 70 open, the reformer exhaust gas follows the path of least resistance and flows not through the anode side 6, but through the bypass line 69. In this configuration, the reformer 33 can easily be operated superstoichiometrically in its burner operating state, since contact between the anode side 6 and residual oxygen in the reformer exhaust gas is not to be expected. This virtually arbitrary superstoichiometric operating mode of the reformer 33 simplifies the burner operation of the reformer 33, particularly for maintaining lower temperatures.

[0067] Once a catalyst operating temperature or activation temperature of catalyst 40 is reached, the operating mode of reformer 33 can be switched from burner operation to reformer operation. This can be achieved, for example, by reducing the supply of reformer air to reformer 33 and / or increasing the fuel supply to switch from a superstoichiometric to a substoichiometric operating state, in which only partial oxidation is possible in catalyst 40, enabling the production of the desired reformate gas. During this reformer operation, the reformate gas typically contains no oxygen, so it is again possible to pass the reformate gas through the anode side 6 to further heat the fuel cell 2. A comparatively large amount of thermal energy is chemically stored in the reformate gas, namely in the form of highly reactive hydrogen and carbon monoxide.In the residual gas burner 3, the reformate gas can now react with the cathode air, which is also discharged from the cathode side 8 through the residual gas burner 3. As a result, very hot burner exhaust gas can be generated, which is discharged via the burner exhaust line 13. In the first heat exchanger 14, intensive heat transfer to the fuel cell air can now be achieved. Consequently, the fuel cell air can be preheated using the waste heat from the burner exhaust gas.

[0068] Once the residual gas burner 3 is activated to fully take over the preheating of the fuel cell air, the auxiliary burner 3 is no longer needed and can therefore be deactivated. Alternatively, the auxiliary burner 20 can also be deactivated only when the fuel cell 2 has reached its minimum operating temperature. However, since the residual gas burner 3 can generate a significant amount of heat, sufficient heating of the fuel cell 2 can essentially be achieved solely via the fuel cell air line 12, so the bypass air line 24 can be deactivated.

[0069] To prevent a temperature shock at the second heat exchanger 23 when the auxiliary burner 20 is switched off, it can be advantageous to first activate the bypass line 72 for the shutdown process. This allows the fuel cell air to flow from the outlet 25 via the first section 24' of the bypass air line 24 to the further outlet 73, then through the bypass line 72 to the inlet 26, and then via the second section 12' of the fuel cell air line 12 to the cathode side 18. The second section 24" of the bypass air line 24 is then deactivated, so that the second heat exchanger 23 is no longer supplied with cold ambient air. This allows the auxiliary burner 20 to be switched off in a way that is gentle on the components.

[0070] The residual gas burner 3 is operated in such a way that its burner exhaust gas does not exceed a predetermined heat exchanger limit temperature. This heat exchanger limit temperature must be maintained for the first heat exchanger 14 located in the burner exhaust line 13 in order to prevent overheating of this first heat exchanger 14.

[0071] Provided, as in the case of the Fig. Since the bypass line 69 for bypassing the anode side 6 is provided in the embodiment shown in Figure 3, bypassing the anode side 6 can also be advantageous for the reformer operation of the reformer 33. For example, during a phase of the start-up procedure in which the fuel cell 2 is relatively cold with respect to the reformate gas, it can be advantageous to bypass the fuel cell 2 in order to avoid a thermal shock to the electrolyte 10 caused by exposure to the very hot reformate gas.

[0072] The heating of the electrolyte 11 then continues via the preheated fuel cell air, i.e., via the cathode side 8. As soon as the temperature difference reaches a tolerable level, the hot reformate gas can then be passed through the anode side 6 to complete the heating of the fuel cell 2. Once the fuel cell 2 has reached its electrolyte operating temperature 10, it can be activated.

[0073] Provided that the fuel cell system 1 is started when the reformer 33 is still warm, i.e., its catalyst 40 has reached its minimum temperature or its activation temperature, it is also possible to use the [material / component] in the Fig. 1 circulation operation shown, in which the circulation circuit 68 is realized, to skip in order to start the reformer 33 directly in the burner operating state or, if its catalyst 40 has a sufficient temperature, directly in the reformer operating state.

[0074] During the start-up process, as explained, the reformer 33 can be started as a burner, which is achieved in particular by a superstoichiometric air supply. After reaching the activation temperature of the catalyst 40 of the reformer 33, a transition from burner operation to reformer operation can then take place. For this purpose, the air-fuel ratio is adjusted to a suitable substoichiometric value. During the start-up process of the fuel cell system 1, the reformer 33 begins with a comparatively low reformer output, which corresponds, for example, to about one-third of the reformer output during nominal operation. The air-fuel ratio in the reformer is initially still relatively high in order to keep the proportions of carbon monoxide and residual hydrocarbons in the reformate gas low.Once the fuel cell temperature has reached a minimum operating temperature, the reformer power is gradually increased while simultaneously reducing the air ratio to increase the proportion of hydrogen and carbon monoxide in the reformate gas.

[0075] The reformer 33 can be configured as a self-starting reformer 33, for which purpose it is equipped with a suitable ignition device (not shown here). It can be operated as a burner to reach the activation temperature of its catalyst 40. During reformer operation, a reformate gas with a high hydrogen and carbon monoxide content can be generated in the reformer 33 or at its catalyst 40 by means of a substoichiometric fuel-air ratio.

[0076] With the auxiliary burner 20 switched off, the bypass air duct 24 can be used for temperature control of the fuel cell 2. Since the bypass air duct 24 bypasses the first heat exchanger 14, the air transported in it is comparatively cold, at least relative to the air passing through the first heat exchanger 14. This allows a defined air temperature to be set for the cathode side 8 of the fuel cell 2 for control purposes.

[0077] Particularly when the fuel cell system 1 is switched off, the auxiliary burner 20 can be used, for example, to implement a parking heater operation. The hot additional exhaust gases generated by the auxiliary burner 20 heat the heating heat exchanger 44 and thus enable the heating of the airflow 45.

[0078] The energy storage device 56 can be used, on the one hand, to operate the electrical consumers 54 of the fuel cell system 1, in particular the various conveying devices for media supply, ignition devices, heating elements, and the control unit 55. This may be necessary, for example, for the start-up process, as long as the fuel cell 2 itself is not generating any electrical current. On the other hand, electrical energy generated by the fuel cell system 1 can be fed into the energy storage device 56. For example, the energy storage device 56 can thus be easily charged. Should the dynamics of the external electrical consumers, such as an electrically operated compressor of an air conditioning system, exhibit a faster response than the dynamics of the fuel cell system 1, the energy storage device 56 can also serve as a buffer system. This buffer system could, on the one hand, provide the additional electrical energy required by the respective external consumer 54.On the other hand, the buffer system could absorb excess electrical energy from the fuel cell system 1 in order to avoid having to perform an emergency stop for the fuel cell system 1 in the event of a so-called "load shedding", i.e., an abrupt shutdown of larger electrical consumers 54.

[0079] In another embodiment, the thermally insulating shells 16, 42, and 32 can be coupled and / or attached to one another. The aim is to maintain a temperature level that is as uniform as possible in the aforementioned shells 16, 32, and 42, or in the associated modules 15, 31, and 41.

[0080] In the embodiments of the Fig. 1, Fig. 2 to Fig.Figure 3 also indicates a box 39, in which the entire fuel cell system 1 is housed. This box 39 can form a common enclosure for the components of system 1, which simplifies the assembly of the system in the respective mobile or stationary application. For example, system 1 can be integrated into a vehicle in this box 39 or, in a stationary application, mounted on a support, a wall, or a base.

[0081] The valves used 46, 49, 51, 70, 74, 76 can be designed as switching valves or control valves or as regulating valves.

[0082] The start-up procedure of the fuel cell system 1 can be summarized as follows: During a cold start of the fuel cell system 1, the auxiliary burner 20 is used to preheat the fuel cell air, while at the same time preheating is achieved via recirculation according to the recirculation circuit 68 by circulating residual gas between anode side 6 and reformer 33.

[0083] Since the residual gas conveyed in circulation circuit 68 may contain air or oxygen gas, an initial anode limit temperature is monitored. This temperature may be, for example, around 250°C, but in any case, it is below the reoxidation temperature of the respective anode material used. This reoxidation temperature may be, for example, around 300°C. Once the initial anode limit temperature is reached, e.g., around 250°C, the reformer 33 is started. Depending on the current catalyst temperature, two start procedures are available: a warm start when the catalyst temperature is sufficiently high, and a cold start when the catalyst temperature is insufficient. The catalyst limit temperature to be observed in this case may be, for example, around 350°C.

[0084] If a cold start is required for the reformer 33, reformer air and fuel are supplied superstoichiometrically, ignited, and reacted, producing oxygenated reformer exhaust gas that rapidly brings the catalyst 40 to a minimum operating temperature, enabling a switch to reformer operation. As described, this reformer exhaust gas can be routed through the anode side 6, as this is only a comparatively small volume flow. Alternatively, if a bypass line 69 is available, the anode side 6 can be bypassed in this case to prevent the electrolyte 10 from coming into contact with oxygen on the anode side.

[0085] Once the catalyst 40 has reached the desired catalyst limit temperature, the operation can be switched from burner mode to reformer mode. In reformer mode, reformer air and fuel are supplied to the reformer 33 at substoichiometric rates, thereby generating the desired reformate gas, preferably by means of partial oxidation. If a sufficiently high catalyst temperature is already present from the outset, the reformer 33 can be started warm, i.e., operated directly in reformer mode.

[0086] During reformer operation, any residual gas recirculation, which may contain oxygen, can be consumed until no more oxygen is recirculated. The risk of anode damage from contact with oxygen is then eliminated.

[0087] Two different options now exist for the further operation of fuel cell system 1 and for the continuation of the start-up procedure. Firstly, the reformer 33 can continue to operate in reformer mode to achieve the desired heating of fuel cell 2 as quickly as possible. However, due to the still relatively low temperature of the electrolyte 10, there is a risk of soot deposits forming on the anode side of the electrolyte 10. Alternatively, the reformer 33 can be temporarily switched off again to ensure that the further heating of fuel cell 2 is achieved solely by the auxiliary burner 20. During this time, the recirculation of residual gas between anode side 6 and reformer 33 can continue. Since only oxygen-free residual gas is then circulated, there is no risk to the anode.During this process, the high temperature level in the reformer 33 can be maintained simultaneously in order to enable a warm start of the reformer 33 at a later time.

[0088] For example, the reformer 33 can be switched off again when a predetermined further or third anode limit temperature is reached. This third anode limit temperature can be, for example, around 350°C or around 400°C. It is expediently chosen to be high enough to ensure an oxygen-free residual gas. Alternatively, the time to switch off the reformer 33 can also be determined by means of a lambda probe that measures the oxygen content in the residual gas.

[0089] Once fuel cell 2 has heated up sufficiently, a fourth anode limit temperature can be reached, which might be, for example, 650°C. Upon reaching this fourth anode limit temperature, reformer 33 can be switched on again, enabling a warm start. This is of particular interest because a warm start prevents excess air and therefore excess oxygen in the gas coming from reformer 33.

[0090] As soon as fuel cell 2 reaches its operating temperature, or as soon as its anode reaches its anode operating temperature, fuel cell 2 can be put into operation to deliver electricity. This anode operating temperature simultaneously constitutes a second anode limit temperature, at which point the auxiliary burner 20 is switched off. The anode operating temperature can also be advantageously set at approximately 650°C, so that ultimately the second anode limit temperature, the fourth anode limit temperature, and the anode operating temperature can be the same.

[0091] In a completely alternative embodiment, it is also possible to start the fuel cell system 1 by using only the auxiliary burner 20 to preheat the fuel cell air from the outset, in order to reach the anode operating temperature. With this alternative approach, there is no circulation of residual gas between the anode side 6 and the reformer 33. Once the anode operating temperature is reached, the reformer 33 is then put into operation. Since it is comparatively cold without recirculation, a cold start must be performed, so that the reformer 33 can first be operated as a burner and only then as a reformer. This approach accepts a brief contact of the hot anode with oxygen, which is temporarily tolerable with certain electrolytes 10.The advantage of this embodiment is that the risk of soot deposits on the electrolyte 10 is greatly reduced, since the critical temperature range for this has already been exceeded.

Claims

[1] Method for starting a fuel cell system (1), - wherein the fuel cell system (1) comprises a fuel cell (2), a reformer (33) and an auxiliary burner (20), - in which fuel cell air is preheated with the auxiliary burner (20) and supplied to a cathode side (8) of the fuel cell (2), - in which residual gas is circulated from an anode side (6) of the fuel cell (2) to the reformer (33) and from the reformer (33) to the anode side (6), - in which, upon reaching a predetermined anode limit temperature, the reformer (33) is operated in a reformer operating state in order to convert any oxygen contained in the residual gas continuing to circulate between the anode side (6) and the reformer (33), - in which the reformer (33) generates reformate gas in the reformer operating state, wherein the reformate gas is reacted in a residual gas burner (3) together with the fuel cell air discharged from the cathode side (8), - in which the reformer (33) is switched off again when a predetermined further anode limit temperature is reached and the oxygen-free residual gas continues to circulate between the anode side (6) and the reformer (33), - in which the reformer (33) is switched on again upon reaching a predetermined further anode limit temperature and is immediately operated in reformer operating mode, - in which the auxiliary burner (20) is deactivated as soon as a predetermined anode operating temperature is reached, - in which the fuel cell (2) is activated when the anode operating temperature is reached. [2] Method according to claim 1, characterized by , that - below a predetermined catalyst limit temperature of a catalyst (40) of the reformer (33) the reformer (33) is operated in a burner operating condition, - when the catalyst limit temperature is reached, the operation of the reformer (33) is switched to the reformer operating state. [3] Method according to claim 1 or 2, characterized by , - that gas coming from the reformer (33) bypasses the anode side (6) and reaches the exhaust pipe (13), or - that gas coming from the reformer (33) passes through the anode side (6) to the exhaust pipe (13), - regardless of whether the gas coming from the reformer (33) flows through or bypasses the anode side (6), it may in particular be provided that the gas coming from the reformer (33) is used to preheat fuel cell air. [4] Method according to any one of the preceding claims, characterized by, that the residual gas burner (3) is operated in such a way that the burner exhaust gas does not exceed a heat exchanger limit temperature of a heat exchanger (14) provided for heat transfer between burner exhaust gas and fuel cell air. [5] Method according to any one of claims 1 to 4, characterized by , that to regulate the temperature of the fuel cell (2) air is introduced from a bypass air line (24), which bypasses a first heat exchanger (14) arranged in a fuel cell air line (12), via a bypass line (72), which bypasses a second heat exchanger (23) arranged in the bypass air line (24), into the fuel cell air line (12) downstream of the first heat exchanger (14).

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