Burner and fuel cell system equipped therewith

The invention relates to a burner, in particular a residual gas burner for a fuel cell system. The burner includes a combustion chamber which is bordered by a supply wall and by a heat exchanger and which is encompassed at the sides by a burner wall. The heat exchanger is a cross-current heat exchanger having a primary path and a secondary path. The supply wall has a burner zone with oxidizer openings for oxidizer gas and with combustion gas openings for combustion gas and a bypass zone with bypass openings for bypass gas. The bypass zone is arranged in a section of the supply wall which is allocated to an area of the heat exchanger adjacent to the primary path and to the secondary path at the inlet end, so that the bypass gas or a bypass gas-burner exhaust gas mixture acts upon this area.

FIELD OF THE INVENTION

The present invention relates to a burner, in particular a residual gas burner for a fuel cell system. The invention also relates to a fuel cell system equipped with such a burner.

BACKGROUND OF THE INVENTION

DE 10 2004 033 545 A1 discloses a burner using a fuel cell which is used in a fuel cell system for combustion of residual gases, i.e., anode exhaust gas and cathode exhaust gas. The burner comprises a combustion chamber bordered at the inlet end by a supply wall and at the output end by a heat exchanger. The combustion chamber is also encompassed laterally by a burner wall. The supply wall has a burner zone with oxidizer gas openings to supply the combustion chamber with oxidizer gas and with combustion gas openings to supply the combustion chamber with combustion gas. With the known burner, an inside wall encompassing the combustion chamber laterally is arranged inside the burner wall and at a distance therefrom to provide thermal insulation for the combustion chamber. This creates an annular cooling space, which is supplied with gaseous coolant, between the inside wall and the burner wall on the outside. The gaseous coolant is supplied through openings in the supply wall. Oxidizer gas supplied to the oxidizer gas openings of the burner zone is preferably used as the gaseous coolant.

SUMMARY OF THE INVENTION

The present invention relates to the problem of providing an improved embodiment for a burner and/or a fuel cell system equipped therewith such that it is characterized in particular by a reduced thermal load on the heat exchanger.

The invention proposes the use of a cross-current heat exchanger for bordering the combustion chamber, in which a primary path may be provided through which burner exhaust gas may flow as the primary medium in a primary direction and in which a secondary path of a secondary medium is coupled to the primary path in such a way so as to provide heat transfer in a secondary direction oriented across the primary direction. With the help of the cross-current design, the heat exchanger may be designed as a high-temperature heat exchanger. In this way, the heat exchanger is especially suitable for bordering the combustion chamber in the primary direction, i.e., in the direction of flow of the burner exhaust gases. Such a cross-current heat exchanger may be exposed to extremely high thermal loads during operation of the burner, because the hot, uncooled primary medium enters at the inlet end of the primary path, whereas the comparatively cold secondary medium, which is therefore used for cooling, enters at the inlet end of the secondary path. In this area of the cross-current heat exchanger which is adjacent to the inlet end of the primary path as well as to the inlet end of the secondary path, the maximum temperature difference thus occurs within the heat exchanger. This is thus associated with an extreme thermal load on the heat exchanger in this area. To reduce this thermal burden on the heat exchanger, the present invention proposes providing a bypass zone having bypass openings in the supply wall, such that these bypass openings serve to supply bypass gas to the combustion chamber. This bypass zone is arranged inside the supply wall according to the present invention, such that the critical area of the heat exchanger, which was described previously and is subject to such high thermal loads, is acted upon by comparatively cold bypass gas, i.e., cooled burner exhaust gas. In this way, the thermal burden on the heat exchanger is reduced. This invention is based on the general idea that to reduce the thermal load on the heat exchanger, it is sufficient to lower the gas temperature in the area of the heat exchanger adjacent to the inlet end of the primary path on the one end and to the inlet end of the secondary path on the other end. The present invention deviates from the usual design in this regard, in that for the primary path as well as the secondary path, the goal is most homogeneous possible temperature distribution in the flow cross section of the primary medium and/or the secondary medium at the inlet end in order to achieve the highest possible efficiency in heat transfer between the primary medium and the secondary medium. In contrast with that, the inventive design is based on the temperature gradient and reduces the thermal burden on the heat exchanger. The resulting unavoidable loss of efficiency in heat transfer is acceptable and is more than balanced by the prolonged lifetime of the heat exchanger achieved with the help of this measure.

In an exemplary embodiment, the bypass zone may be arranged exclusively in the area of the supply wall allocated to the inlet ends of the primary and secondary paths. This embodiment is based on the consideration that for the desired reduction in thermal burden on the heat exchanger, it is sufficient for bypass gas and/or cooled burner exhaust to act on the heat exchanger only in the area where the greatest temperature gradient occurs. The implementation of the respective bypass zone therefore becomes comparatively inexpensive.

It is self-evident that the features mentioned above and those yet to be explained below may be used not only in the particular combination given but also in other combinations or alone without going beyond the scope of the present invention.

Exemplary embodiments of the invention are depicted in the drawings and explained in greater detail in the following description, where the same reference numerals are used to refer to the same or similar or functionally identical components.

DETAILED DESCRIPTION OF THE INVENTION

According toFIGS. 1 and 2, a fuel cell system1, which is shown only partially here, includes a fuel cell2, a burner3and a heat exchanger4. The fuel cell2serves in the usual way to produce electricity from an oxidizer gas, in particular air or pure oxygen and from a combustion gas, preferably containing hydrogen.

The fuel cell system1may be arranged in a motor vehicle. For example, the fuel cell system1may serve as an additional electric power supply system in the vehicle, which operates independently of the engine in particular. Likewise, it is fundamentally possible to replace the generator of the vehicle with the help of such a fuel cell system1.

For generating combustion gas, the fuel cell system1may also be equipped with a reformer (not shown here) which produces the combustion gas, e.g., by partial oxidation from a hydrocarbon fuel and from an oxygen-containing oxidizer. The combustion gas is supplied to the fuel cell2via an anode input5. The fuel cell2also receives the oxidizer gas via a cathode inlet6. Then the electrochemical generation of electricity takes place in the usual way in the fuel cell2, converting carbon monoxide and hydrogen into water and carbon monoxide on the anode side with the help of oxygen from the cathode side. The fuel cell2is preferably a solid oxide fuel cell (SOFC), which may be designed as a high-temperature fuel cell in particular.

Anode exhaust gas emerges from an anode outlet7of the fuel cell. Since the conversion of the combustion gas in the fuel cell process is usually incomplete, the anode exhaust gas also still contains reactive hydrogen. The anode gas is thus a combustion gas. Cathode exhaust gas emerges from a cathode outlet8of the fuel cell on the cathode side. Because of the incomplete conversion process, the cathode exhaust still contains unconverted oxygen, so the cathode exhaust is still an oxidizer gas.

A fuel cell2usually consists of a stack of ceramic plates which form the anode side of the fuel cell2on the one hand, while on the other hand also forming the cathode side. This stack of plates is sealed with end plate9on at least one end of the fuel cell2, with the anode outlet7and the cathode outlet8situated there as well. Likewise, it is fundamentally possible to integrate the anode inlet5and the cathode inlet6into this end plate9, although that is not shown here for the sake of simplicity. With the configuration of the fuel cell system1and/or the burner3shown here, the end plate9of the fuel cell2forms a supply wall10of the burner3. In other words, the supply wall10forms the end plate9of the fuel cell2. This yields a simplified line guidance for the combustion gas (anode gas) that is supplied to the burner3and the oxidizer gas (cathode gas) that is supplied to the burner3. At the same time, an extremely compact design of the fuel cell system1is achieved.

The burner3has a combustion chamber11which is bordered at the inlet end by the supply wall10. At the outlet end, the combustion chamber11is bordered by the heat exchanger4. To this extent, the heat exchanger4may also be interpreted as a component of the burner3. The combustion chamber11is enclosed at the sides by a peripheral burner wall12. The supply wall10has a burner zone13and has a bypass zone14on its side facing the combustion chamber11. Several oxidizer gas openings15and several combustion gas openings16are formed in the supply wall10in the burner zone13. The oxidizer gas openings15serve to supply oxidizer gas to the combustion chamber11. Accordingly, the oxidizer gas openings15are connected to the cathode outlet8via an oxidizer gas line system17integrated into the supply wall10. The combustion gas openings16serve to supply combustion gas to the combustion chamber11. Accordingly, the combustion gas openings16are connected to the anode outlet7via a corresponding combustion gas line system18designed in the supply wall10.

The supply wall10is provided with a plurality of bypass openings19in the bypass zone14, so that a bypass gas can flow through these openings into the combustion chamber11. In general, the bypass gas is a gas that bypasses the combustion process laterally in the combustion chamber11. The bypass gas is relatively cool in comparison with the burner exhaust gas. Essentially any inert gas is a suitable bypass gas. However, an oxidizer gas may also be used as a bypass gas. An exemplary embodiment uses the oxidizer gas that is available anyway, namely the cathode exhaust gas, as the bypass gas. Accordingly, the bypass openings19are formed by oxidizer gas openings15and are connected to the oxidizer gas line system17. Essentially, however, an external cooling gas supply is also conceivable, e.g., via a cooling gas line32.

The heat exchanger4is designed as a cross-current heat exchanger. Accordingly, the heat exchanger4has a primary path20, which is indicated here by vertical arrows, and a secondary path21, which is indicated here by horizontal arrows. The two paths20,21are linked together to allow heat exchange. In the primary path20, a primary medium is passed through the heat exchanger4in a primary direction22indicated by an arrow. In the secondary path21, a secondary medium is passed through a secondary direction23indicated by arrows. The heat transfer coupling between the paths20and21leads to a transfer of heat between the media. It is characteristic of the cross-current heat exchanger4that the primary direction22runs essentially perpendicular to the secondary direction23.

Since the heat exchanger4borders the combustion chamber11at the output end, the primary medium is formed by the burner exhaust. At the same time, with the selected arrangement, an inlet end24of the primary path20runs across, i.e., perpendicular to the primary direction22. Likewise, the supply wall10here extends essentially across the primary direction22. Furthermore, the supply wall10here is designed to be planar, so that the supply wall10runs parallel to the inlet end24of the heat exchanger4. The openings15,16and19are preferably arranged in the supply wall10, so that during operation of the burner3, the respective gas can flow into the combustion chamber11in the primary direction22. The supply wall10and the heat exchanger4are arranged a distance apart in the primary direction22and form the combustion chamber11between them. The heat exchanger4has an area25, which is indicated by the curly brackets and is arranged at the inlet end with respect to the primary path20and also with respect to the secondary path21. This area25receives the secondary medium before the heat transfer on the part of the secondary path25and receives the burner exhaust before the heat transfer on the part of the primary path20. Subsequently the maximum temperature difference between the burner exhaust and the secondary medium prevails in this area25.

According to an embodiment of this invention, the bypass zone14is arranged inside the supply wall10in a section allocated to said area25of the heat exchanger4. Subsequently, during operation of the burner3, bypass gas or a mixture of bypass gas and burner exhaust may act on the area25at the primary end. Consequently, the area25is exposed to a reduced temperature on the side of the primary path, so that the temperature gradient between the primary end and the secondary end is lowered in this area25of the heat exchanger4. Thermal stresses associated with the temperature gradient can be reduced in this way.

The bypass zone14may be situated exclusively in the section of the supply wall10allocated to the area25. In this way, the total quantity of bypass gas available is concentrated and sent to the area25to achieve a maximal reduction in the temperature gradient.

In the embodiment of the supply wall10with the burner zone13and the bypass zone14, a reaction space26bordered by the burner zone13and a bypass space27bordered by the bypass zone14are formed in the combustion chamber11. In the embodiments inFIGS. 1 and 3, the reaction space26develops openly into the bypass space27. An imaginary boundary between the burner zone13and the bypass zone14and/or between the reaction space26and the bypass space27is represented by an interrupted line labeled as28. With this open transition between the reaction space26and the bypass space27, there is a certain mixing of burner exhaust and bypass gas during operation of the burner3. At the same time, a portion of the bypass gas may participate in the combustion process in the reaction space26inasmuch as this is oxidizer gas. In this embodiment, at least one row of bypass openings consisting exclusively of bypass openings19is arranged inside the bypass zone14. A row33of oxidizer openings consisting exclusively of oxidizer gas openings15is arranged in proximity to the row29of bypass openings. The borderline28runs between the row33of oxidizer openings and the row29of bypass openings, and the burner zone13and the bypass zone are adjacent to one another. Oxidizer gas emerging from these openings15of the row33of oxidizer openings participates in the combustion reaction in the reaction space26and/or can be mixed with bypass gas. The oxidizer gas flow from this row33of oxidizer openings leads to a shield of the oxidizer gas flow emerging from the other bypass hole row29, so that the flow passes through the bypass space27comparatively unhindered and can act upon the area25.

With the embodiment shown inFIGS. 2 and 4, a partition30is provided in the combustion chamber11. This partition30separates the reaction space26from the bypass space27. The partition30contacts the supply wall10while it may be a distance away from the inlet end24of the heat exchanger4. The oxidizer gas flow emerging from the bypass openings19is largely separated by the partition30from the combustion process of the reaction chamber11, so the bypass flow can act on the area25essentially unhindered.

According toFIGS. 3 and 4, the oxidizer gas openings15and the combustion gas openings16may be arranged so they alternate regularly with one another inside the burner zone13, whereby the combustion gas openings16may be enclosed by and/or adjacent to oxidizer gas openings15on all sides. Exclusively, bypass openings19are arranged in the bypass zone14, i.e., in particular there are no combustion gas openings16. It is clear here that the bypass openings19are oxidizer gas openings15as soon as oxidizer gas is used as bypass gas. In addition, the combustion gas openings16are arranged exclusively in the burner zone13.

According toFIGS. 1 and 2, the heat exchanger4may be used to preheat the oxidizer gas supplied to the fuel cell2. Accordingly, the secondary path21is connected at the outlet end to the cathode inlet6of the fuel cell2via an oxidizer line31.

During starting operation of the fuel cell system1, the fuel cell2must be raised to an operating temperature above which the fuel cell process can take place. During this warm-up phase, the combustion gas supplied to the anode flows through the fuel cell2and the oxidizer gas supplied to the cathode flows through the fuel cell2, both of them more or less without reacting. At the same time, there is immediately an intense combustion reaction in the burner3with a great release of heat to the heat exchanger4. During the heating phase, the cathode exhaust gas, i.e., the oxidizer gas, enters the bypass space27as bypass gas more or less at the ambient temperature in the area of the bypass zone14and leads to intense cooling of the inlet area25. The ambient temperature may be 20° C., for example, while the combustion exhaust gases may already have a temperature between 900° and 1000° C. shortly after starting operation of the fuel cell system1. Only with progressive heating of the fuel cell2is there a corresponding increase in temperature in the cathode exhaust. If an intense cooling of the inlet area25is also necessary for normal operation of the fuel cell system1, then a suitable cooling gas, e.g., the oxidizer gas bypassing the heat exchanger4, may additionally be supplied to the bypass openings19via the cooling gas line that is optionally provided.