Fuel cell exhaust system

Abstract

A fuel cell exhaust system comprises an exhaust system inlet area (24) to be connected to an outlet area (20) of a fuel cell (12) for receiving fuel cell exhaust gas, a mixing unit (30) with a mixing chamber (32) for receiving fuel cell exhaust gas downstream of the exhaust system inlet area (24), an additive supply assembly (34) for supplying additive (A) promoting the separation of water from the fuel cell exhaust gas to the mixing chamber (32), and a separation unit (46) with a separation chamber (48) for receiving water separated from the fuel cell exhaust gas downstream of the mixing unit (30).

Description

The present invention relates to a fuel cell exhaust system for a fuel cell system used, for example, in a vehicle for the generation of electrical energy. When electrical energy is generated in a vehicle using a fuel cell, fuel cell exhaust gas highly enriched with water or water vapor is produced. When such fuel cell exhaust gas is released into the environment at relatively low ambient temperatures, fog forms upon contact with the cold ambient air due to condensation of the water. To prevent this fog formation, it is known to use condensers in the exhaust systems of fuel cell systems. These condensers cool the fuel cell exhaust gas within the exhaust system, thus contributing to the condensation of at least a portion of the water or water vapor contained in the fuel cell exhaust gas. The efficiency of such condensers is highly dependent on the thermal conditions in the vicinity of a fuel cell exhaust system and, in particular, on the availability of a suitable medium for cooling the fuel cell exhaust gas. The object of the present invention is to provide a fuel cell exhaust system in which water can be efficiently separated from the fuel cell exhaust gas essentially independently of thermal conditions. According to the invention, this problem is solved by a fuel cell exhaust system comprising: - an exhaust system inlet area to be connected to an outlet area of ​​a fuel cell for receiving fuel cell exhaust gas, - downstream of the exhaust system inlet area a mixing unit with a mixing chamber for receiving fuel cell exhaust gas, - an additive supply assembly for supplying an additive promoting the separation of water from the fuel cell exhaust gas to the mixing chamber, - downstream of the mixing unit a separation unit with a separation chamber for receiving water separated from the fuel cell exhaust gas. By using an additive that forms condensation nuclei for water contained in the fuel cell exhaust gas when introduced, water contained in the fuel cell exhaust gas can be efficiently separated from the exhaust gas, regardless of the thermal conditions in the area of ​​the fuel cell exhaust system or whether a sufficient cooling medium is available or can be used to cool the fuel cell exhaust gas and thus condense the water. The resulting water-reduced fuel cell exhaust gas can then be released into the environment with virtually no mist formation. In order to be able to shut off the fuel cell exhaust system during operating phases in which fuel cell exhaust gas should not be released into the environment, the exhaust system inlet area can include an exhaust system shut-off valve for selectively shutting off the fuel cell exhaust system and releasing the fuel cell exhaust system for the flow of fuel cell exhaust gas. For efficient additive feeding operation, the additive supply assembly may preferably comprise the following assemblies: - an additive storage tank for storing additive, - an additive conveying device for conveying additive from the additive storage tank to the mixing chamber, - a storage tank shut-off valve for selectively shutting off the storage tank and releasing the storage tank for the dispensing of additive. Furthermore, the additive feed assembly can include an additive dispensing device for dispensing additive into the mixing chamber, so that, for example, the additive can be introduced into the mixing chamber in a configuration and dispensing direction efficient for mixing with the fuel cell exhaust gas. The additive can comprise a liquid, preferably a salt solution. Such hygroscopic salt solutions, especially when supersaturated or highly concentrated, lead to the efficient precipitation of water contained in the fuel cell exhaust gas in gaseous form, i.e., particularly as water vapor. The additive can, for example, comprise a salt solution: - a preferably aqueous NaCl solution, or / and - a preferably aqueous LiCl solution, or / and - a preferably aqueous LiBr solution, or / and - a silver iodide solution, wherein, due to the fact that silver iodide is not water-soluble, acetone, for example, can be used as a solvent. Alternatively or additionally, the additive can include solid particles to provide condensation nuclei. The solid particles can include, for example, fine dust or soot particles, or similar materials, which, upon condensation of water, are separated from the fuel cell exhaust along with the condensed water and thus are not released into the environment. Highly hygroscopic powder materials, such as zeolite powder, can also be used. For efficient mixing of the additive with the fuel cell exhaust gas, the additive delivery device can include a nozzle for releasing an additive mist, i.e., a finely distributed cloud of tiny liquid droplets or solid particles, into the mixing chamber. The invention further relates to a fuel cell system comprising at least one fuel cell and a fuel cell exhaust system constructed according to the invention, which is connected to an outlet area of ​​the at least one fuel cell for receiving fuel cell exhaust gas. The present invention is described in detail below with reference to the accompanying Fig. 1, which shows in principle a fuel cell system comprising a fuel cell and a fuel cell exhaust system. Figure 1 shows a schematic representation of a fuel cell system 10, used, for example, to generate electrical energy in a vehicle. The fuel cell system 10 comprises a fuel cell 12, configured, for example, as a fuel cell stack, with an anode region 14 and a cathode region 16. An anode gas containing molecular hydrogen is supplied to the anode region 14 at an anode inlet region 18. An oxygen-containing cathode gas, for example, air, is supplied to the cathode region 16 at a cathode inlet region 19. Cathode gas generated during fuel cell operation leaves the fuel cell 12 or the cathode area 16 via a cathode outlet area 20 as fuel cell gas or as a significant component of the fuel cell gas. In various operating states, for example during a purge process, anode gas can be mixed with the cathode gas as a component of the fuel cell gas at an anode outlet area 22, or it can be expelled as the sole component of the fuel cell gas or fed back into the fuel cell operation. The fuel cell exhaust gas emitted by the fuel cell 12 during fuel cell operation enters an exhaust system inlet area 24 of a fuel cell exhaust system, generally designated 26, which is connected to the outlet area 20. An exhaust system shut-off valve 28 can be assigned to the exhaust system inlet area 24 to selectively open the fuel cell exhaust system 26 to receive fuel cell exhaust gas or to shut it off against the entry of fuel cell exhaust gas. The exhaust system shut-off valve 28 can also be used, in particular, to shut off the cathode outlet area 20 against the release of cathode exhaust gas.It should be noted that, for example, a shut-off valve may also be assigned to the anode outlet area 22 in order to be able to shut off the anode area 14 against the escape of anode exhaust gas, especially when the exhaust system shut-off valve 28 releases the fuel cell exhaust system 26 to receive cathode exhaust gas from the cathode outlet area 20, which then essentially provides the fuel cell exhaust gas. The fuel cell exhaust system 26 comprises a mixing unit 30 downstream of the exhaust system shut-off valve 28, with a mixing chamber 32 formed, for example, in a housing. The fuel cell exhaust gas drawn into the fuel cell exhaust system 26 when the exhaust system shut-off valve 28 is switched to its enabling position enters the mixing chamber 32. It is preferably provided that the entire flow of fuel cell exhaust gas is introduced into the mixing chamber 32. The fuel cell exhaust system 26 further comprises an additive feed assembly 34. The additive feed assembly 34 includes an additive storage unit 36 ​​in which, for example, a liquid additive A is stored. Additive A can be introduced from the additive storage unit 36 ​​into the mixing chamber 32 of the mixing unit 30 via an additive line 38. For this purpose, the additive supply assembly 34 includes an additive conveying device 40, for example designed as a pump. In order to prevent additive leakage towards the mixing chamber 32, for example when the fuel cell 12 is not in operation and accordingly no additive A is to be fed into the mixing chamber 32, a storage shut-off valve 42 is assigned to the additive line 38, by which the additive line 38 can be selectively shut off or released for the delivery of additive A into the mixing chamber 32, in particular when the additive conveying device 40 is in operation. For efficient mixing of additive A and the fuel cell exhaust gas introduced into the mixing chamber 32, the additive supply assembly 34 can include an additive dispensing device 44, for example, in the form of a nozzle or atomizing nozzle. Particularly when additive A is supplied as a liquid, a spray mist S of additive A can thus be generated, comprising a multitude of very fine droplets of additive A that can interact with the fuel cell exhaust gas to separate water or water vapor contained therein. After the additive A has been mixed with the fuel cell exhaust gas in the mixing chamber 32, the mixture thus formed can flow to a separation unit 46 located downstream of the mixing unit 30. The separation unit 46 comprises, for example, a separation chamber 48 within a housing, in which, for example, due to flow stabilization in the fuel cell exhaust gas, water still carried along and already precipitated in liquid form through interaction with the additive A is separated from the fuel cell exhaust gas and can collect, for example, in a collection chamber 50 in a lower area of ​​the separation unit 46 by gravity. Water separated from the fuel cell exhaust gas and collected in the separator unit 46 can be discharged via a water discharge line 54 connected to an outlet area 52 of the separator unit 46. This water can then be discharged, for example, in liquid form to the environment or into a water tank, for instance, to allow such water separated from the fuel cell exhaust gas to be fed back into the fuel system. A separator unit shut-off valve 56 can be associated with the water discharge line 54 to discharge water only when required or desired. For efficient water removal from fuel cell exhaust, the additive can be in the form of a hygroscopic salt solution. For example, an aqueous NaCl solution, an aqueous LiCl solution, or an aqueous LiBr solution can be used. The use of a silver iodide solution is also possible; however, since silver iodide is not soluble in water, acetone, for example, can be used as a solvent. In an alternative configuration, the additive can comprise solid particles. Even the finest dust or soot particles, as can be observed, for example, in the exhaust jets of jet engines, form condensation nuclei on which water transported in vapor form (i.e., in a gaseous state) in the fuel cell exhaust gas can condense. Such solid particles can, for example, be blown from a storage container into the mixing chamber 32 through a nozzle of the additive dispensing device 44 by means of a conveying device, such as a blower. In the separation unit 46, these solid particles forming condensation nuclei, together with the water condensed on them, are separated from the fuel cell exhaust gas, so that such solid particles are not emitted into the environment with the water-depleted fuel cell exhaust gas.It is also possible to use highly hygroscopic powder materials, such as zeolite powder, to separate water carried in the fuel cell exhaust gas. In the fuel cell exhaust system 26 constructed according to the invention, the mixing unit 30 and the separation unit 46 can, for example, be structurally combined and housed, for instance, in a common casing. Such a casing can contain successive chambers in the direction of flow, with an upstream chamber essentially providing the mixing chamber 32 and a downstream chamber essentially providing the separation chamber 48. In a further alternative embodiment with a structurally combined mixing unit 30 and separation unit 46, the mixing chamber 32 and the separation chamber 48 can also be configured as successive volume regions of one and the same chamber in the direction of flow. Finally, it should be noted that the fuel cell exhaust system may include additional measures for separating water from the fuel cell exhaust. For example, the fuel cell exhaust system may incorporate one or more condensers. This allows for the elimination of additives when, due to favorable thermal conditions in the vicinity of the fuel cell exhaust system, efficient water removal from the fuel cell exhaust is possible solely through condensation in one or more such condensers. In such cases, additives are primarily used when efficient removal of water from the fuel cell exhaust is not possible due to thermal conditions.

Claims

Fuel cell exhaust system, comprising: - an exhaust system inlet area (24) to be connected to an outlet area (20) of a fuel cell (12) for receiving fuel cell exhaust gas, - downstream of the exhaust system inlet area (24) a mixing unit (30) with a mixing chamber (32) for receiving fuel cell exhaust gas, - an additive supply assembly (34) for supplying additive (A) promoting the separation of water from the fuel cell exhaust gas to the mixing chamber (32), - downstream of the mixing unit (30) a separation unit (46) with a separation chamber (48) for receiving water separated from the fuel cell exhaust gas. Fuel cell exhaust system according to claim 1, characterized in that the exhaust system inlet area (24) comprises an exhaust system shut-off valve (28) for selectively shutting off the fuel cell exhaust system (26) against and releasing the fuel cell exhaust system (26) for the flow of fuel cell exhaust gas. Fuel cell exhaust system according to claim 1 or 2, characterized in that the additive supply assembly (34) comprises: - an additive storage unit (36) for storing additive (A), - an additive conveying device (40) for conveying additive (A) from the additive storage unit (36) to the mixing chamber (32), - a storage shut-off valve (42) for selectively shutting off the additive storage unit (36) against and releasing the additive storage unit (36) for the delivery of additive (A). Fuel cell exhaust system according to one of claims 1 - 3, characterized in that the additive supply assembly (34) comprises an additive dispensing device (44) for dispensing additive (A) into the mixing chamber (32). Fuel cell exhaust system according to one of claims 1 - 4, characterized in that the additive (A) comprises a liquid. Fuel cell exhaust system according to claim 5, characterized in that the additive (A) comprises a salt solution. Fuel cell exhaust system according to one of claims 1 - 6, characterized in that the additive (A) comprises: - a NaCl solution, or / and - a LiCl solution, or / and - a LiBr solution, or / and - a silver iodide solution. Fuel cell exhaust system according to claim 4, characterized in that the additive (A) comprises solid particles. Fuel cell exhaust system according to claim 8, characterized in that the solid particles comprise: - dust particles, or / and - soot particles, or / and - zeolite particles. Fuel cell exhaust system according to one of claims 5 - 9, insofar as related back to claim 4, characterized in that the additive dispensing device (44) comprises a nozzle for dispensing an additive mist into the mixing chamber (32). Fuel cell system comprising at least one fuel cell (12) and a fuel cell exhaust system (26) connected to an outlet area (20) of the at least one fuel cell (12) for receiving fuel cell exhaust gas according to one of claims 1 - 10.

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