Method for heating a fuel cell system and a fuel cell system

Abstract

The invention relates to a method for heating a fuel cell system (1a; 1b; 1c; 1d), the fuel cell system having at least one fuel cell stack (2) comprising an anode part (3) and a cathode part (4) and a reformer (5) for steam reforming when using a fuel upstream of the anode part (3), wherein the reformer (5) comprises a nickel-based catalyst, the method comprising the following steps: starting a heating process for heating the fuel cell system (1a; 1b; 1c; 1d) by means of a heating device (6); guiding a carbonaceous fluid and water vapor through the nickel-based catalyst of the reformer (5) during the heating process. The invention also relates to a fuel cell system (1a; 1b; 1c; 1d) designed to carry out the method according to the invention.

Description

Technical Field

[0001] The invention relates to a method for heating a fuel cell system, in particular a SOFC system, and to an associated fuel cell system. Background Art

[0002] Before the SOFC system can be operated at full power, it must be placed at operating temperature. This occurs during the heating or startup of the SOFC system. A known SOFC system has at least one fuel cell stack including an anode portion and a cathode portion and a reformer upstream of at least one fuel cell stack. It is also known that the reformer is equipped with a nickel-based catalyst for steam reforming or that the reformer is constructed in the form of a nickel reformer. In order to prevent nickel from being oxidized during the heating operation, the nickel-based catalyst of the known system is surrounded by a purge gas or protective gas, for example in the form of a carbon-containing gas. That is, at the nickel-based catalyst, a corresponding reducing environment is formed during the heating operation. In order to avoid carbon deposition, the carbon-containing gas can be additionally supplied with an oxygen-containing gas. As a result, the carbon in the carbon-containing gas can be oxidized and sent out of the reformer in the form of carbon dioxide. That is, by supplying an oxygen-containing gas during the heating operation, the carbon in the carbon-containing gas can be combined. Carbon deposition can be avoided accordingly.

[0003] However, it has been shown that when using nickel-based catalysts, additional measures are required in order to keep the nickel in a reduced state and not to oxidize it during heating operation. Summary of the invention

[0004] The object of the present invention is to at least partially address the aforementioned problems. In particular, the object of the present invention is to provide a method for heating a fuel cell system having a nickel-based catalyst arranged in a reformer, by which the nickel in the system is kept in a reduced state and not oxidized during the heating operation of the fuel cell system in a simple manner. In addition, the object is to provide a fuel cell system designed accordingly for carrying out such a method.

[0005] The aforementioned object is achieved by the claims. In particular, the aforementioned object is achieved by a method according to claim 1 and a fuel cell system according to claim 10. Further advantages of the invention result from the dependent claims, the description and the figures. Here, the features and details described in relation to the method are obviously also applicable in relation to the fuel cell system according to the invention and vice versa, so that the disclosures of the individual inventive aspects are always referred to or can be referred to one another.

[0006] According to a first aspect of the invention, a method for heating a fuel cell system is provided, the fuel cell system having at least one fuel cell stack including an anode part and a cathode part and a reformer upstream of the anode part for steam reforming when using a fuel, wherein the reformer comprises a nickel-based catalyst. The method comprises the following steps:

[0007] - using the heating device to start a heating process for heating the fuel cell system,

[0008] - conducting the carbonaceous fluid and the water vapor through the nickel-based catalyst of the reformer during the heating process.

[0009] During the heating process, oxygen is supplied to the nickel-based catalyst via water vapor. However, unlike the methods known in the prior art, the oxygen in the water vapor is provided in a combined form. This prevents direct combination of oxygen with reduced nickel.

[0010] It has been found within the scope of the invention that the combined oxygen in the water vapor is as suitable as the uncompounded oxygen for binding to the carbon present and preventing corresponding deposition, and that the excess hydrogen serves to keep the nickel in a reduced state and not oxidize it. To avoid carbon deposition, the water vapor is supplied in a predetermined water vapor / carbon ratio, wherein the water content can be selected to be slightly higher than the water content in the equilibrium state in order to provide a certain safety.

[0011] The present invention preferably relates to a method for heating a SOFC system, in particular a stationary SOFC system. The method is preferably operated using a nickel-based catalyst containing about 30 to 70% by weight, preferably about 50% by weight, of nickel. That is, a nickel-based catalyst is a catalyst with a relatively high nickel content.

[0012] The heating process may be a process of heating the fuel cell system, in particular during a fuel cell system startup process. The heating process is preferably carried out until the reformer is heated to a predeterminable operating temperature, for example in the range between 400° C. and 600° C., in particular in the range between 500° C. and 550° C.

[0013] From a temperature of about 50° C., nickel oxidation will occur in the reformer in the presence of oxygen. This is not desirable. Oxidized nickel should be reduced again during operation of the fuel cell system so that steam reforming can lead to an equilibrium composition. That is, the entire catalyst should be reduced again. This can take a long time and the structure of the catalyst can be damaged in the process. In order to keep nickel oxidation at a minimum and reversible level, the reformer or the nickel-based catalyst of the reformer is supplied with a carbon-containing fluid according to the invention, whereby the oxygen at the catalyst can be displaced.

[0014] The fuel is in particular a fuel containing hydrocarbons. The heating device may have a plurality of heating means at different locations within the fuel cell system.

[0015] The carbon-containing fluid and water vapor are guided through the nickel-based catalyst of the reformer by a fluid conveying device. The fluid conveying device may have a pump and / or a fan for conveying or guiding a liquid fluid and / or a gaseous fluid.

[0016] According to another embodiment of the present invention, it is feasible in the method to guide water vapor through the nickel-based catalyst in the fuel cell system from a predeterminable threshold temperature. The threshold temperature is preferably pre-set according to the environment of the fuel cell system or according to environmental parameters and / or operating conditions of the fuel cell system. The purpose is to keep the water vapor in a gaseous state as much as possible to avoid water deposition. At ambient pressure and also in general ambient conditions and / or operating conditions, the value of the threshold temperature can therefore be determined to be at least 100° C. Preferably, the value of the threshold temperature is pre-set between about 100° C. and about 110° C. In order to be able to utilize the beneficial effects of water vapor as early as possible or for a long time, the temperature should not be selected too high. It has been shown in experiments within the scope of the present invention that small amounts of water deposition can be tolerated because the fuel cell system can locally vaporize it during the heating process without causing damage.

[0017] It is also possible to use a fuel, in particular methane, natural gas or LPG, as the carbon-containing fluid in the method according to the invention. That is, the same fuel is used as in the subsequent normal operation of the fuel cell system, i.e. in the operating mode in which the fuel cell system generates electric current. By using the same fuel for the start-up operation and the normal operation, the fuel cell system can be operated efficiently. A separate and / or additional fuel storage and / or fuel source can be omitted. In order to omit the evaporation step, preferably only gaseous fuel is used.

[0018] In another embodiment of the invention, it is possible to use a fuel as the carbon-containing fluid in the heating process, the amount of which is between 5% and 20% of the amount of fuel used in normal operation of the fuel cell system during steam reforming. This means that the amount of fuel used is kept relatively low compared to normal operation. This prevents the at least one fuel cell stack downstream of the reformer from cooling down due to endothermic reforming and accordingly slows down the heating process.

[0019] Furthermore, in the method of the invention, the heating device may have a heat exchanger at the reformer and an afterburner for burning cathode exhaust gas and / or anode exhaust gas from the at least one fuel cell stack, wherein the hot side of the heat exchanger is supplied with afterburner exhaust gas during the heating process downstream of the afterburner in order to heat the reformer. In the case of using afterburner exhaust gas, the reformer can be heated particularly efficiently. The afterburner is arranged downstream of the at least one fuel cell stack and is in fluid communication with the fluid outlet of the anode part and the fluid outlet of the cathode part, respectively.

[0020] It is also possible in the method of the present invention that the water vapor and the carbon-containing fluid in the form of a fuel are guided to the reformer as a fuel / water vapor mixture via an anode gas supply line upstream of the reformer. That is, the water vapor and the fuel are provided as a fuel-water vapor mixture upstream of the reformer, especially upstream of a fluid delivery device also arranged upstream of the reformer, and from there are sent to the reformer via the anode gas supply line. This can provide a particularly space-saving and weight-reduced process fluid pipeline system for the fuel cell system. The anode gas supply line refers to the following fluid pipeline, through which the fuel or fuel mixture to be reformed in the normal operation of the fuel cell system is guided to the reformer. The anode gas supply line is arranged upstream of the optional fluid delivery device, downstream of the fluid delivery device, and accordingly upstream of the reformer.

[0021] According to another design variation of the present invention, a fluid conveying device as described above for conveying fuel to the reformer can be provided in the method upstream of the reformer in the anode gas supply line, and in addition to the anode gas supply line, a water vapor supply line for conveying water vapor into the anode gas supply line is also provided, wherein the water vapor is guided through the water vapor supply line during the heating process, and is fed into the anode gas supply line downstream of the fluid conveying device and mixed with the fuel. Since the water vapor is only supplied to the anode gas supply line downstream of the fluid conveying device through a separate water vapor supply line, the fluid conveying device only needs to be designed for conveying or conveying the preferably gaseous fuel, thereby saving costs here. In order to convey water vapor, other fluid conveying devices suitable for this purpose can be provided in the water vapor supply line. The other fluid conveying devices only need to be started during the start of operation or during the heating process, thereby obtaining a relatively long durability of the other fluid conveying devices with respect to the service life of the fuel cell system.

[0022] Furthermore, in the method according to the invention, it is possible to provide a fluid conveying device for conveying fuel to the reformer in the anode gas supply line upstream of the reformer, and to provide a water vapor supply line for conveying water vapor into the anode gas supply line in addition to the anode gas supply line, wherein the water vapor is guided through the water vapor supply line during the heating process and introduced into the anode gas supply line upstream of the fluid conveying device and mixed with the fuel. By introducing the water vapor into the anode gas supply line upstream of the fluid conveying device, in particular immediately upstream of the fluid conveying device, fluid dynamics advantages can be achieved with a simple and compact fuel cell system design.

[0023] In addition, in the method of the present invention, a fluid conveying device for conveying fuel to the reformer can be provided in the anode gas supply pipeline upstream of the reformer, and a water vapor supply pipeline for conveying water vapor to the anode gas supply pipeline is provided in addition to the anode gas supply pipeline, wherein the hot side of the exhaust gas heat exchanger is provided in the water vapor supply pipeline, and the water vapor is guided through the water vapor supply pipeline and the hot side of the exhaust gas heat exchanger during the heating process, and is input into the anode gas supply pipeline downstream or upstream of the fluid conveying device and mixed with the fuel. As a result, the water vapor can be effectively preheated during the heating process. The exhaust gas heat exchanger or the hot side of the exhaust gas heat exchanger can be arranged downstream of the hot side of the reformer heat exchanger (or the heat exchanger at the reformer). The exhaust gas heat exchanger can also be arranged downstream of the hot side of the cathode gas heat exchanger, which is especially arranged downstream of the post-burner in the cathode gas supply pipeline. That is, the exhaust gas heat exchanger is a heat exchanger that is originally used or required in the fuel cell system.

[0024] Furthermore, according to another aspect of the present invention, a fuel cell system is provided, which is designed to perform the method as described above. The fuel cell system has at least one fuel cell stack including an anode part and a cathode part and a reformer for reforming fuel steam upstream of the anode part, wherein the reformer includes a nickel-based catalyst.

[0025] Therefore, the fuel cell system of the present invention brings the same advantages as explicitly described with respect to the method of the present invention. As described above, the heating device can have a heat exchanger at the reformer and an afterburner for burning cathode exhaust gas and / or anode exhaust gas from at least one fuel cell stack. Upstream of the reformer, a fluid delivery device for delivering fuel to the reformer can be arranged in the anode gas supply line. In addition to the anode gas supply line, a water vapor supply line for delivering water vapor to the anode gas supply line can also be provided. A hot side of an exhaust gas heat exchanger is provided in the water vapor supply line, wherein the exhaust gas heat exchanger is arranged downstream of the hot side of the cathode gas heat exchanger, and the cathode gas heat exchanger is particularly arranged downstream of the afterburner in the cathode gas supply line. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Further measures for improving the invention are provided by the following description of various exemplary embodiments of the invention, which are schematically shown in the figures. All features and / or advantages from the claims, the description or the figures, including structural details and spatial arrangements, may be important to the invention both individually and in different combinations, as schematically shown in the figures:

[0027] Figure 1 A block diagram for explaining a fuel cell system according to a first embodiment of the present invention is shown.

[0028] Figure 2A block diagram for explaining a fuel cell system according to a second embodiment of the present invention is shown.

[0029] Figure 3 A block diagram for explaining a fuel cell system according to a third embodiment of the present invention is shown.

[0030] Figure 4 A block diagram for explaining a fuel cell system according to a fourth embodiment of the present invention is shown.

[0031] Figure 5 A diagram is shown for explaining the method according to the invention for heating a fuel cell system.

[0032] Parts with the same function and working method are Figure 1-4 The same reference numerals are used throughout. DETAILED DESCRIPTION

[0033] exist Figure 1 A fuel cell system 1a according to a first embodiment is schematically shown in the figure. The fuel cell system 1a has a fuel cell stack 2 including an anode part 3 and a cathode part 4, and a reformer 5 for reforming fuel steam upstream of the anode part 3. The reformer 5 includes a nickel-based catalyst. In order to adjust the temperature of the fuel cell system 1a, a heating device 6 is provided. The heating device 6 includes a heat exchanger at the reformer 5, a post-burner 7 for burning cathode exhaust gas and / or anode exhaust gas from the fuel cell stack 2, and a cathode gas heat exchanger 12 in the cathode gas supply line 13. In addition, the heating device 6 can also be equipped with other heating elements and functional components, but this is not described in detail here.

[0034] The cathode gas supply line 12 is provided for conveying cathode supply gas to the cathode part 4. The cathode supply gas may in particular refer to air or other oxygen-containing fluids. An anode gas supply line 8 is provided upstream of the anode part 3. Through the anode gas supply line, the anode gas or the process fluid for the anode part can be sent to the reformer and / or the anode part. That is, through the anode gas supply line, not only the gas but also other process fluids for the anode part are guided.

[0035] The afterburner 7 is arranged downstream of the fuel cell stack 2 and upstream of the hot side of the cathode heat exchanger 12. A fluid delivery device 9 for delivering fuel to the reformer 5 is provided in the anode gas supply line 8 upstream of the reformer 5. The fuel cell system 1a also has a recirculation path 14, through which the fuel cell exhaust gas, in particular the anode exhaust gas from the anode part 3, can be reused downstream of the fuel cell stack 2, or it can be supplied to the anode part again.

[0036] exist Figure 2 FIG. 2 shows a fuel cell system 1b according to a second embodiment. Figure 2The fuel cell system 1b shown basically corresponds to Figure 1 The fuel cell system 1 a shown has, in addition to the anode gas supply line 8 , a water vapor supply line 10 for supplying water vapor into the anode gas supply line 8 downstream of the fluid conveying device 9 .

[0037] Figure 3 A fuel cell system 1c according to a third embodiment is shown. Figure 3 The fuel cell system 1b shown basically corresponds to Figure 1 The fuel cell system 1 a shown has, in addition to the anode gas supply line 8 , a water vapor supply line 10 for conveying water vapor into the anode gas supply line 8 upstream of the fluid conveying device 9 .

[0038] Figure 4 A fuel cell system 1d according to a fourth embodiment is shown. Figure 4 The fuel cell system 1b shown basically corresponds to Figure 1 The fuel cell system 1a shown here is provided with a water vapor supply line 10 for conveying water vapor into the anode gas supply line 8 in addition to the anode gas supply line 8, and a hot side of an exhaust gas heat exchanger 11 is provided in the water vapor supply line 10. Thus, the water vapor can be guided through the water vapor supply line 10 and the hot side of the exhaust gas heat exchanger 11 during the heating process, and then fed into the anode gas supply line 8 and mixed with the fuel.

[0039] Reference Figure 5 Then, a method for heating such as Figure 4 Method for the fuel cell system 1d shown. For this purpose, a heating process for heating the fuel cell system 1d is first started with the aid of a heating device 6. Starting from a temperature T1 of approximately 50° C. corresponding to the time Z1, the reformer 5 is supplied with methane via the anode gas supply line 8, the amount of fuel supplied being equal to approximately 10% of the amount of fuel used in normal operation of the fuel cell system 1d during steam reforming. As soon as a threshold temperature T2 of more than 100° C. is reached in the fuel cell system at the time Z2, the reformer 5 or the nickel-based catalyst located therein is supplied with water vapor via the water vapor supply line 10, the hot side of the exhaust gas heat exchanger 11 and subsequently the anode gas supply line 8. Carbon formation and corresponding deposition may begin at 200° C., but this is prevented by the supply of water vapor. As soon as the fuel cell system 1d has reached the operating point P, at which a temperature of approximately 520° C. is reached, the heating process is terminated.

[0040] When no more current should be generated by the fuel cell system 1d, the fuel cell system 1d is shut down. To this end, the electricity users (not shown) of the fuel cell system 1d are first shut down, whereby the current becomes zero. Then, the fuel cell system 1d is air-cooled. The fuel supply can be throttled, for example, to about 10% of the rated power. In addition, water can be carbon-free input after reaching equilibrium conditions. Then, the air is adjusted to as low a temperature as possible in the afterburner 7, where complete oxidation of the gaseous reformate can also be achieved. As a result, the fuel cell system 1d becomes cold. Once the temperature of the reformer reaches a target temperature of about 200°C, the fuel cell exhaust gas recirculation performed during the shutdown process is stopped and the recirculation path 14 is purged with fuel gas to obtain water and carbon monoxide from the fuel cell system and oxidize it. In addition, the generation of nickel oxide and the condensation of water vapor in the reformer 5 are prevented or at least minimized. Then, the afterburner 7 is deactivated, and the fuel cell system 1d is actively cooled or naturally cooled completely with air. The reformer can thus be protected from damage and the required catalyst reduction can be omitted or shortened the next time the fuel cell system 1d is put into operation. The small amount of carbon that may be formed here is tolerable because it is regenerated the next time the fuel cell system 1d is put into operation.

[0041] The invention allows other design principles besides the embodiment shown. That is, the invention should not be considered as being limited to the embodiments explained with reference to the drawings.

[0042] Reference numerals list

[0043] 1a-1d Fuel Cell System

[0044] 2 Fuel cell stack

[0045] 3 Anode

[0046] 4 Cathode

[0047] 5 Reformer

[0048] 6 Heating device

[0049] 7 Afterburner

[0050] 8 Anode gas supply pipeline

[0051] 9 Fluid transport device

[0052] 10 Water vapor supply pipeline

[0053] 11 Exhaust gas heat exchanger

[0054] 12 Cathode gas heat exchanger

[0055] 13 Cathode gas supply pipeline

[0056] 14 Recirculation Path

Claims

1. A method for heating a fuel cell system (1a; 1b; 1c; 1d), the fuel cell system having at least one fuel cell stack (2) comprising an anode part (3) and a cathode part (4) and a reformer (5) for steam reforming using a fuel upstream of the anode part (3), wherein: The reformer (5) comprises a nickel-based catalyst, and the method comprises the following steps: - starting a heating process for heating the fuel cell system (1a; 1b; 1c; 1d) by means of a heating device (6), A carbonaceous fluid is provided through an anode gas supply line (8), upstream of the fluid delivery device (9), water vapor is delivered from a water vapor supply pipeline (10) into the anode gas supply pipeline (8) to form a fuel / water vapor mixture with the carbonaceous fluid in the anode gas supply pipeline (8); - in order to keep nickel oxidation of the nickel-based catalyst at a minimum and reversible level, during the heating process, the fuel / steam mixture is guided from the fluid conveying device (9) through the anode gas supply line (8) through the nickel-based catalyst of the reformer (5), The amount of fuel used as the carbon-containing fluid in the heating process is between 5% and 20% of the amount of fuel used in normal operation of the fuel cell system (1a; 1b; 1c; 1d) during steam reforming.

2. The method according to claim 1, characterized in that: Above a predefinable threshold temperature, the water vapor is conducted through the nickel-based catalytic converter within the fuel cell system (1a; 1b; 1c; 1d).

3. The method according to claim 1 or 2, characterized in that: Fuel is used as the carbonaceous fluid.

4. The method according to claim 3, characterized in that: As the carbonaceous fluid, the fuel used is methane, natural gas or LPG.

5. The method according to claim 1 or 2, characterized in that: The heating device (6) has a heat exchanger at the reformer (5) and an afterburner (7) for burning cathode exhaust gas and / or anode exhaust gas from the at least one fuel cell stack (2), wherein the hot side of the heat exchanger is supplied with afterburner exhaust gas downstream of the afterburner (7) during the heating process to heat the reformer (5).

6. The method according to claim 1, wherein A fluid delivery device (9) for delivering fuel to the reformer (5) is provided in the anode gas supply pipeline (8) upstream of the reformer (5), wherein the water vapor is guided through the water vapor supply pipeline (10) during the heating process and is input into the anode gas supply pipeline (8) downstream of the fluid delivery device (9) and mixed with the fuel.

7. The method according to claim 1, characterized in that A fluid delivery device (9) for delivering fuel to the reformer (5) is provided in the anode gas supply pipeline (8) upstream of the reformer (5), wherein water vapor is guided through the water vapor supply pipeline (10) during the heating process and is input into the anode gas supply pipeline (8) upstream of the fluid delivery device (9) and mixed with the fuel.

8. The method according to claim 1, characterized in that A fluid delivery device (9) for delivering fuel to the reformer (5) is provided in the anode gas supply pipeline (8) upstream of the reformer (5), wherein the hot side of the exhaust gas heat exchanger (11) is provided in the water vapor supply pipeline (10), and the water vapor is guided through the water vapor supply pipeline (10) and the hot side of the exhaust gas heat exchanger (11) during the heating process, and is input into the anode gas supply pipeline (8) downstream or upstream of the fluid delivery device (9) and mixed with the fuel.

9. A fuel cell system (1a; 1b; 1c; 1d) designed to perform the method according to any one of claims 1 to 8, the fuel cell system having at least one fuel cell stack (2) comprising an anode part (3) and a cathode part (4) and a reformer (5) for reforming fuel steam upstream of the anode part (3), wherein: The reformer (5) comprises a nickel-based catalyst.

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