DE102014007013A1 - Process for heating a catalyst - Google Patents

Abstract

The invention relates to a method for heating an element (10) with catalytically active material in an exhaust gas stream of a fuel cell (2), which contains cathode and anode side exhaust gases, with a targeted hydrogen excess is passed to the element (10) with the catalytically active material , The inventive method is characterized in that for this purpose the fuel cell (2) is operated with a hydrogen excess on its anode side.

Description

The invention relates to a method for heating an element with a catalytically active material in an exhaust stream of a fuel cell according to the closer defined in the preamble of claim 1 and 3.

Elements with catalytically active materials are used very frequently in the exhaust gas of fuel cell systems. In particular, they may have the task of correspondingly converting residual hydrogen in the exhaust gases from the anode side with residual oxygen in the exhaust gases from the cathode side in order to prevent hydrogen emissions from entering the surroundings of the fuel cell system. In some applications, a turbine is also provided as an added benefit, which uses energy from the hot exhaust gases of this catalytic afterburning to convert the heat energy, in particular into electrical energy or drive energy for an air conveyor. In this context can be exemplified on the DE 10 2011 108 598 A1 to get expelled. Such a system is described there in two different embodiments. In one embodiment variant, only exhaust gas from the fuel cell is converted, in another embodiment, optional fuel from a fuel reservoir for the fuel cell in the region of the element with the catalytically active material can be additionally supplied. This makes it possible to heat the element with the catalytically active material in the event of a cold start.

Another application for elements with catalytically active material in the exhaust gas lines of fuel cell systems is that in fuel cell systems possible hydrogen emissions pose a potential danger, since from a critical hydrogen concentration of more than 4% in the air ignitable mixtures can arise. For this reason, hydrogen sensors are very often arranged in the exhaust gases of a fuel cell system, for example, to reliably detect unwanted hydrogen emissions due to leakage, failure of the membranes in the fuel cell or the like, so as to trigger a corresponding warning or alarm and the fuel cell, if necessary off.

Since conventionally constructed hydrogen sensors are often very expensive and very unreliable in their functionality, in particular for vehicle applications with strong changes in temperature and a very heavy load of the sensors with humidity, also elements with catalytically active material can be used together with temperature sensors, to detect a critical hydrogen concentration. By way of example, with regard to such a construction, reference is made to the combination of a temperature sensor with catalytic coating and an uncoated temperature sensor, which is thus known from US Pat EP 1990858 B1 are known.

From the not pre-published older DE 10 2012 018 873.0 It is also known to equip a catalyst with a fuel cell system, as known per se and described above, for example, with a temperature sensor in order to use this alone or in addition to the reaction of residual hydrogen as a concentration sensor for hydrogen.

All these elements with catalytically active materials in the exhaust gas of a fuel cell have, and this is particularly true for vehicle applications where a start must be made quickly and reliably under adverse, such as humid and / or very cold environmental conditions, the problem that they, until the implementation of hydrogen starts, must have reached a certain ignition temperature and dryness. Only then, on the one hand, can the reliable conversion of residual hydrogen and, on the other, reliable use as a concentration sensor for critical hydrogen concentrations become possible. In catalysts for afterburning of residual hydrogen are, similar to catalysts in conventional vehicle technology, electric heaters known. However, these burden in the cold start case, the electric starter batteries in addition and have a comparatively poor efficiency, since the electrical energy must always be generated first and then cached with a storage efficiency of the energy storage before it is used again.

The object of the present invention is now to provide an improved method for heating an element with catalytically active material in the exhaust gas of a fuel cell, which avoids the disadvantages mentioned and moreover can provide an added benefit.

According to the invention this object is achieved by a method having the features in the characterizing part of claim 1 and / or in the characterizing part of claim 3. Advantageous embodiments of the methods are specified in the dependent subclaims.

The first method according to the invention, which is described by the features in claim 1, uses a conventional fuel cell system, in which the element with the catalytically active material is provided, in particular for the conversion of residual hydrogen, to emissions to the Prevent environment. For heating or heating of the element with catalytically active material, it then suffices, as the inventors have recognized, when the fuel cell is operated with a corresponding excess of hydrogen. On the one hand, this hydrogen ensures good flow through the fuel cell itself, so that any moisture that may still be present in the fuel cell is discharged very well. As a result, comparatively large amounts of hydrogen then enter the exhaust gas of the fuel cell, which initially appears to be a disadvantage, at first. However, it is very easily possible by the higher amount of hydrogen in the exhaust gas to ignite a reaction of hydrogen and oxygen at the element with the catalytically active material. This can be done in particular passive, but may optionally also be supported by an ignition device such as a spark plug or the like. By over the conventional operation clearly surplus of hydrogen, so in particular set concentrations of more than two percent by volume in the common exhaust gases from the cathode compartment and anode compartment, a reliable ignition is achieved and by the resulting in the reaction of hydrogen with the residual oxygen in the cathode exhaust air Reaction heat is the element with the catalytically active material safely and reliably heated. After this has reached the desired temperature, can be changed to an operation of the fuel cell with a normal amount of hydrogen. After the element with the catalytically active material has reached the operating temperature, the implementation of very small amounts of residual hydrogen in the exhaust gases can be done easily on the catalytically active material.

This preheating can be done not only in an element with catalytically active material, which is used to convert residual hydrogen, but also in an element with catalytically active material, which is used together with a temperature sensor as a hydrogen concentration sensor. This is provided in accordance with an advantageous development of the method. This makes it possible to heat the hydrogen concentration sensor and to free it of potential moisture and / or ice, in order to ensure reliable functionality after the successful heating in each case.

Of course, it is of course fundamentally desirable to have the functionality of a hydrogen concentration sensor relevant to safety already available if, for example, the fuel cell itself is not yet in operation or is supplied with hydrogen. In the alternative parallel embodiment of the method, it can therefore be provided that the element with the catalytically active material is provided together with a temperature sensor as a hydrogen concentration sensor, wherein the excess of hydrogen is guided in a bypass line to the fuel cell to the element. Such a line, which leads directly from the fuel or hydrogen storage to the element, thus enabling heating and thus ultimately reliable startup of the element used as a hydrogen sensor with the catalytically active material parallel to or before the start of the actual fuel cell, so that the Safety can be further increased thereby.

In a further very favorable embodiment of the two methods described, it is further provided that the excess of hydrogen is adjusted so that sets a hydrogen concentration of two to four percent by volume of the element with the catalytically active material. In particular, such a hydrogen concentration with about two to four percent by volume of hydrogen in the gas mixture arriving at the element is of particular advantage for the processes according to the invention. From a concentration of about two percent by volume of hydrogen in the gases takes place with sufficient presence of oxygen, which is always ensured in a flow through the cathode of the fuel cell in general, a safe and reliable ignition in the range of the catalytically active material. Thus, even under adverse conditions, the heating of the catalytic active material can be done safely and reliably. Ideally, however, the hydrogen concentration during this time remains below four percent by volume for 3 seconds, since in this case, for example, a flow through the cathode without simultaneous hydrogen metering into the fuel cell is assumed to be the maximum oxygen concentration can form ignitable mixtures of hydrogen and oxygen. In addition to this maintenance of concentrations over a period of time of more than 3 seconds of a maximum of four percent by volume, however, the concentration itself may fluctuate correspondingly within these 3 seconds, whereby concentration peaks of up to eight percent by volume may be achieved. Thus, it is possible to initiate short concentration peaks of almost eight percent by volume of hydrogen, so as to allow, for example, even under very unfavorable conditions, a basic conversion of hydrogen to the catalyst, so its safe and reliable ignition. However, if the concentration is kept below this four volume percent of hydrogen over time, then it can be ruled out in each case that without the help of a catalyst ignitable and thus potentially safety-critical mixtures are formed and may possibly enter the environment.

In a further extremely advantageous embodiment of the method according to the invention, it can now also be provided that the excess of hydrogen is used to check and / or to calibrate the temperature sensor, if present. This is especially true in the case that hydrogen is passed through the fuel cell or preferably around the fuel cell to the element with the catalytically active material without the hydrogen in the fuel cell itself being consumed. As a result, a reliable prediction of the hydrogen concentration arriving at the element with the catalytically active material can be achieved comparatively easily with typically known air mass flow through the cathode side or from experience of sufficient oxygen and hydrogen, for example in a bypass line around the fuel cell to be hit. Thus, the function check and / or calibration of the temperature sensor of the hydrogen concentration sensor is always possible during or in particular before the system is put into operation. This considerably increases the safety of the system, since such a calibration of the hydrogen concentration sensor carried out before or during operation of, for example, a system in a vehicle leads to a considerably higher safety, since both the fundamental function of the hydrogen concentration sensor can be determined and its calibration readjusted. This takes place as a function of the aging which has already occurred at the element with the catalytically active material, for example a reduction of the catalytically active material due to thermal effects, rupture or spalling of coatings or the like. This ultimately leads to an increase in security.

Even during operation of a fuel cell, such a calibration can be performed again and again. This is especially true when the anode side of the fuel cell is operated with a so-called anode circuit. In such an anode circuit, the exhaust gas is recirculated from the anode in the circuit, for example via a gas jet pump or a recirculation fan to the input of the anode compartment and this fed again mixed typically with fresh hydrogen. In this anode cycle then inert gas accumulates over time, so that the exhaust gases are discharged, for example, from time to time from this anode circuit. In the times in which no exhaust gas is discharged from the anode circuit, only the exhaust air from the cathode arrives at this structure in the region of the hydrogen sensor. In the event that this exhaust air contains no hydrogen emissions, which would indicate a leakage within the fuel cell, the hydrogen concentration sensor does not measure hydrogen in this situation. Once such a measurement result has been established, the bypass line around the fuel cell can be briefly opened in order to lead a corresponding amount of hydrogen directly to the element with the catalytically active material. The now occurring temperature increase can then be used on the one hand for functional testing of the hydrogen concentration sensor and on the other hand for its calibration. This can be done during normal operation, and indeed whenever no exhaust gases from the anode circuit enter the region of the exhaust pipe. As a result, the security of the system is further increased, since a review of the functionality and calibration of the hydrogen concentration sensor so easily and efficiently during operation and thus can be made frequently to ensure high reliability of the function of the hydrogen sensor.

Further advantageous embodiments of the method according to the invention also result from the exemplary embodiments, which are described in more detail below with reference to the figures.

Showing:

1 a possible embodiment of a fuel cell system for carrying out the method according to the invention; and

2 an alternative embodiment of a fuel cell system, which is also suitable for carrying out the method according to the invention.

In the presentation of the 1 is very simplistic a fuel cell system 1 illustrated how it can be used for example in a vehicle for generating electrical drive power. The fuel cell system 1 includes a fuel cell 2 , which is typically constructed as a stack of single cells, as a so-called fuel cell stack. In the exemplary embodiment shown here, as is typical for vehicle applications, the fuel cell 2 be constructed as a PEM fuel cell. An example is in the fuel cell 2 a cathode compartment 3 and an anode room 4 shown. The cathode compartment 3 Air becomes an oxygen supplier via an air conveyor 5 fed. This air is supplied via optional gas / gas humidifier 6 in which it is moistened, in the anode compartment 3 , Exhaust air from the anode compartment 3 , which with the most of the resulting product water is loaded, in turn passes through the gas / gas humidifier 6 and moisten the dry supply air in it. The exhaust air then passes through an exhaust duct 7 in an exhaust pipe 8th in which a water separator 9 and an element 10 is arranged with catalytically active material. The optional water separator 9 ensures that no or very little liquid in liquid form in the area of the element 10 get with the catalytically active material. The separated water becomes the exhaust pipe 8th in the direction of flow after the element 10 fed again, what a drainage line 11 is provided.

The anode compartment 4 the fuel cell 2 becomes hydrogen from a compressed gas storage 12 via a pressure regulating and metering device 13 fed. The hydrogen passes through a gas jet pump 14 , whose functionality will be discussed later, in the anode compartment 4 , Residual gases, in particular unconsumed hydrogen and inert gases which diffuse through the membranes of the fuel cell and inert radicals which were contained in the stored hydrogen, arrive together with the smaller one in the region of the anode space 4 resulting part of the product water via a recirculation line 15 back to the gas jet pump 14 , By vacuum and pulse exchange effects of this recirculated exhaust gas from the recirculation line 15 aspirated and mixed with fresh hydrogen to the anode compartment 4 fed again. This structure is also called anode circuit 16 and is well known and commonplace. To the water and the inert gases, which with time in the anode cycle 16 accumulate at least from time to time to be able to drain is in the embodiment of the anode circuit shown here 16 a water separator 17 provided, which via a drain line 18 with the exhaust pipe 8th connected is. In order to control the discharge of water and gas, the so-called drain and purge, is a blow-off valve 19 , a so-called purge valve, provided. When this is opened, first water and then gas via the drain line 18 in the exhaust pipe 8th , It would equally well be possible to separate the discharge of water and the blowing off of the gases into two lines and valves, which is also frequently provided in the prior art.

In the presentation of the 2 an alternative structure is shown, which largely corresponds to the structure just described. The only difference is that a fuel bypass line 20 with a corresponding valve 21 is provided, via which hydrogen directly, so at the fuel cell 2 over, into the exhaust pipe 8th can be directed, in the flow direction in front of the element 10 with the catalytically active material.

As the element with the catalytically active material, for example, for the implementation of residual hydrogen in the exhaust pipe 8th is to be used, and / or in combination with an optional temperature sensor 22 As a hydrogen concentration sensor, its full functionality is during operation of the fuel cell system 1 crucial. Now, however, it can happen that the item 10 with the catalytically active material at the start of the fuel cell system 1 is covered with ice and / or water, so the element 10 must first be heated with the catalytically active material before safe functionality is ensured. According to the inventive method, this can now be done so that the fuel cell system 1 is operated with a hydrogen excess, so that a correspondingly high amount of hydrogen on the element 10 arrives and this ignites accordingly and heats up by the heat of reaction of hydrogen and oxygen.

This can, for example, in the representation of the fuel cell system 1 according to 1 done so that the fuel cell system 1 is started, for what by the air conveyor 5 Air through the cathode compartment 3 is encouraged. At the same time or with a time interval before or after the start of the air supply, the hydrogen supply of the anode compartment 4 recorded, in this case with open purge valve 19 , Then the unreacted hydrogen reaches the element together with the optionally depleted in oxygen exhaust air 10 with the catalytically active material. If now a corresponding high amount of hydrogen is used, that is to say an excess of hydrogen relative to the amount of hydrogen used to generate the desired electrical power of the fuel cell 2 would be necessary, then, by the starting from a hydrogen concentration of about two percent by volume in the exhaust gas very reliably forming catalytic reaction, the element 10 be heated. Once this works reliably, such as the optional temperature sensor 22 can be determined or what is typically the case from experience after a certain period of time, can then in the regular per se known operation of the fuel cell system 1 change.

In the embodiment of the fuel cell system 1 according to 2 In addition or as an alternative, there is also the option of hydrogen while the air supply to the fuel cell is running 3 via the hydrogen bypass line 20 in the area of the element 10 with the catalytically active material to promote, so without the fuel cell 2 Even with excess hydrogen to operate or even without the fuel cell 2 already have to supply with hydrogen that element 10 heat. This is particularly advantageous when it comes to the element 10 is a hydrogen concentration sensor or a catalyst used as such. This allows, before the hydrogen supply to the fuel cell 2 itself is recorded, the operating temperature of the element 10 be achieved, which contributes to an improvement in functionality and thus ultimately to increase security.

In both cases, but especially in the embodiment according to 2 Moreover, it is possible when using the item 10 with the catalytically active material together with the temperature sensor 22 as a hydrogen concentration sensor to do a self-diagnosis of the same. The functionality can be tested by selectively adding a quantity of hydrogen to the element 10 is then checked to see if this leads to the desired temperature swing. Is it possible to adjust the amount of hydrogen, for example via the hydrogen bypass line when the anode side of the fuel cell is not yet supplied, or during operation when the purge valve is closed 19 , in addition to the basic functional test of the self-diagnosis, a calibration can also be carried out. For example, if an amount of hydrogen of the order of three percent by volume comes to the element 10 , this would have to cause a temperature increase by a predetermined temperature value after a certain experience. This can be checked accordingly. Be different hydrogen concentrations to the element 10 In addition, a calibration can be made directly to these hydrogen concentrations, so that, for example, aging effects occurring in the area of the element during operation 10 be recognized with the catalytically active material and adaptively taken into account, so that regardless of the aging state of the element 10 always a reliable measurement of a hydrogen concentration when using the element 10 as a hydrogen concentration sensor is possible.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102011108598 A1 [0002]
• EP 1990858 B1 [0004]
• DE 102012018873 [0005]

Claims (7)

Method for heating an element ( 10 ) with catalytically active material in an exhaust gas stream of a fuel cell ( 2 ), which contains cathode and anode-side exhaust gases, specifically an excess of hydrogen to the element ( 10 ) is conducted with the catalytically active material, characterized in that for this purpose the fuel cell ( 2 ) is operated with an excess of hydrogen on its anode side. Method according to claim 1, characterized in that the element ( 10 ) with the catalytically active material together with a temperature sensor ( 22 ) is used as the hydrogen concentration sensor. Method for heating an element ( 10 ) with catalytically active material in an exhaust gas stream of a fuel cell ( 2 ), which contains cathode and anode-side exhaust gases, specifically an excess of hydrogen to the element ( 10 ) is conducted with the catalytically active material, characterized in that the element ( 10 ) with the catalytically active material together with a temperature sensor ( 22 ) is used as a hydrogen concentration sensor, wherein the excess of hydrogen in the bypass around the fuel cell ( 2 ) to the element ( 10 ) to be led. A method according to claim 1, 2 or 3, characterized in that the excess of hydrogen is chosen so that a hydrogen concentration of two to four percent by volume in the exhaust gas stream at the element ( 10 ) arrives with the catalytically active material. Method according to one of claims 2 to 4, characterized in that the excess of hydrogen for functional testing and / or calibration of the temperature sensor ( 22 ) is used. A method according to claim 5, characterized in that the functional test and / or calibration of the temperature sensor ( 22 ) before and / or during operation of the fuel cell ( 2 ) takes place only when no hydrogen is passing through the anode space to the element ( 10 ) flows with the catalytically active material. A method according to claim 5 or 6, characterized in that in a fuel cell ( 2 ) with anode circuit ( 16 ) with a blow-off valve ( 19 ) the excess of hydrogen for functional testing and / or calibration of the temperature sensor ( 22 ) takes place before or during operation only when no exhaust gas is discharged from the anode circuit.

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