GB2478188A

GB2478188A - Fuel cell system with a reformer in an improved arrangement

Info

Publication number: GB2478188A
Application number: GB1103001A
Authority: GB
Inventors: Frank Baumann; Wolfgang Friede; Uwe Limbeck
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2010-02-24
Filing date: 2011-02-21
Publication date: 2011-08-31
Anticipated expiration: 2031-02-21
Also published as: DE102010002276A1; GB201103001D0; NL2006281A; NL2006281C2; GB2478188B

Abstract

The invention relates to a fuel cell system (1), comprising at least one SOFC fuel cell of a tubular configuration (10), wherein at least one reformer (11) is provided for supplying reducing agent (12) that is needed for operation of the fuel cell. For this purpose, according to the invention the reformer (11) is disposed in the tubular fuel cell (10).

Description

Fuel cell system with a reformer in an improvedmt The present invention relates to a fuel cell system, comprising at least one SOFC fuel cell of a tubular configuration, wherein at least one reformer is provided for supplying reducing agent that is needed for operation of the fuel cell. The fuel cell is formed in particular by a tubular electrolyte body or a body comprising an anode-or cathode substrate.

Background art

Fuel cell systems of the type relevant here comprise SOFC fuel cells, wherein SOFC describes a fuel cell with a ceramic electrolyte (SOFC solid oxide fuel cell), and wherein the fuel cell is of a tubular configuration, wherein the tubular shape is not limited to a round cross section and describes only an elongate extent that may be both closed at the end or open to allow the reducing agent to be conveyed through the fuel cell. Disposed on the inner side of the fuel cell is an anode material that may interact with the reducing agent. On the outer side, separated from the anode material by the ceramic electrolyte, the fuel cell comprises a cathode material, around which oxygen, preferably air, flows. Each of the three layers (anode material, electrolyte or cathode material) or alternatively some other temperature_resistant gas-permeable material may take the form of a carrier material and guarantee the mechanical stability, while the other two layers are applied as thinner layers.

Because of its potential for lowering the C02 emission for the supplying of power and heat, combined heat and power generation is playing an increasingly important role in the energy market. Fuel cell systems based on ceramic cells are now known, which are operated at a high temperature of 650°C to 1000°C, with very high electrical efficiencies being achieved. Fuel cell systems consist of one or preferably more fuel cells, in which methane, hydrogen and carbon monoxide react with oxygen to form carbon dioxide and water, while simultaneously producing power and heat.

In this case, natural gas or some other fuel such as for example methanol or kerosine is supplied to the anode side and is converted by catalytic prereforming depending on the system concept wholly or partially into hydrogen. For this purpose a reformer is used, wherein the reformer process is preferably endothermic and may consequently be operated only by supplying energy. For the reforming, which preferably takes the form of steam reforming, the fuel gas has to be mixed with water vapour optionally in the fuel cell system before entering a prereformer.

Tubular and hence pipe-shaped fuel cells with a differently shaped cross section, preferably round or oval, are to be distinguished from planar fuel cells, which are assembled in the form of a stack and are consequently formed by a plurality of functional layers that are stacked one on top of the other. The form of construction of the tubular fuel cell is the criterion for demarcation from the planar fuel cell, wherein the tubular shape is predetermined by the electrolyte body or the body comprising anode-or cathode substrate. The tubular fuel cells are mostly mounted on a basic body, via which both the fuel gas supply and the removal of the waste gas are conducted.

A further distinction is made between tubular fuel cells that are open at both ends and fuel cells that have a closed end. In the latter type, the fuel gas and/or the hydrogen is fed into the fuel cell through a lance.

An essential weak point of fuel cell systems are seals that are needed between the basic body and the fuel cell or for example between the lance and the basic body. The seals separate the air side from the fuel gas side, wherein the materials required for the seal have to be stable at high temperatures. The temperatures of up to iooo°c necessitate glass seals, which however given very long operating periods may fail as a result of therrnomeChaflical stresses.

From DE 10 2007 015 079 Al a fuel cell system is known, wherein it is proposed to dispose the reformer in such a way that the removal of heat after the fuel cell process of chemical-electrical energy conversion is provided in such a way that the heat may be supplied to the reformer for the reforming process. The integration of the heat removal for the reforming or gasification process for supplying hydrogen or the integration of the reforming or gasification process itself including the heat transfer to an SOFC stack and the integration of the heat removal, for example to an evaporator for supplying steam for the reforming reaction in an SOFC stack, is in this case to extend constructionally in such a distributed manner over the stack that a fairly uniform temperature distribution is achieved and these components form a compact structure, to the block of which the countercurrent heat exchangers for heating up the air and the fuel and for cooling down the waste gases of anode and cathode are connected by their hot ends. The advantageous arrangement of the reformer is represented for a fuel cell of a planar configuration, wherein the reformer is integrated in the fuel cell stack so that the process heat may heat the reformer. The advantageous arrangement of the reformer is however limited to fuel cells of a planar configuration.

The object of the present invention is therefore to provide a fuel cell system comprising at least one tubular SOFC fuel cell, in which the reformer is disposed in such an advantageous way that the process heat of the energy conversion in the fuel cell may be utilized to introduce heat into the reformer. In particular, the object of the present invention is to provide an advantageous temperature distribution in the fuel cell, in particular to provide a fuel cell system that presents minimal thermal loading of necessary seals and a minimal temperature gradient over the cell surface.

proceeding from a fuel cell system according to the preamble of claim 1 this object. is achieved in conjunction with the characterizing features. Advantageous developments of the invention are indicated in the dependent claims.

Disclosure of the invention

The invention includes the technical teaching that the reformer is disposed in the tubular fuel cell.

The invention is in this case based on the idea of disposing the reformer directly in the hot region inside the tubular fuel cell in such a way that the process heat of the fuel cell system may act directly upon the reformer in order to run and/or support the endothermic reformer reaction. As the fuel cell has a tubular, substantially pipe-like shape, according to the present invention the reformer is adapted to the pipe-like shape of the fuel cell, for example the reformer is configured in the shape of a slender cylinder. This may be disposed in the fuel cell advantageously in such a way that hot waste gas produced by the fuel cell process flows around the reformer in such a way that a transfer of heat from the hot waste gas to the reformer may occur.

According to an vantageOuS form of implementation of the present invention, the fuel cell may be configured as a body that is closed at one end, wherein a lance that extends through the tubular cell is provided and wherein the reformer is disposed in the lance. The lance forms a tubular formation that extends through the fuel cell substantially over the entire length. The reducing agent is conveyed through the lance to a point in front of the closed end of the fuel cell, so that it exits from the lance there. The reducing agent then flows back through the annular gap between the lance and the fuel cell. In so doing, the reducing agent flows around an anode disposed at the inner side of the fuel cell, while at the same time air flows around a cathode disposed at the outer side of the fuel cell. The reducing agent in the present case describes a mixture of i.a. methane, hydrogen and carbon monoxide, which is converted by the fuel cell process at least partially into water and carbon dioxide, which leaves the fuel cell in the form of hot waste gas. As the hot waste gas flows in an envelope-shaped manner around the lance over wide regions, the heat from the hot waste gas may heat the reformer. By virtue of the arrangement in the interior of the cell, heat is moreover transferred by radiation from the fuel cell to the reformer.

A further possible form of implementation moreover arises.

The gas may flow in the opposite direction, i.e. past the anode and back through the lance. In this case, the reformer is disposed between fuel cell tube and lance. The heat transfer occurs from the lance to the reformer, optionally via ribs that are connected to the lance and are coated with reformer catalyst or surrounded by reformer catalyst. In this way too, the gas flowing back to the basic body may be cooled down.

According to a further advantageous form of implementation a basic body is provided on which the at least one, preferably closed fuel cell with the internally situated lance is disposed, wherein the reformer is disposed in the region of the lance that adjoins the basic body. The lance is in this case configured in a way that enables a transfer of heat through the wall of the lance. If the reducing agent and the waste gas formed from the fuel cell process flows in an envelope-shaped manner around the lance, then the fuel cell presents the highest temperatures approximatelY in the region of the mechanical seating on the basic body. If the reformer is disposed in this region, a cooling effect arises as a result of the endotherrniC reformer reaction in the reformer, thereby enabling a uniform temperature distribution and consequently a lower thermal loading of the seals between the fuel cell and the basic body as well as between the lance and the basic body. The service life of a fuel cell may therefore be substantially increased if the seals, for example seals made of a glass solder, are exposed to lower thermal loads.

In an advantageous manner a heat transfer may occur between the reducing agent and/or the waste gas resulting therefrom that is conveyed past the anode and the reformer. The heat transfer in this case comprises a transfer of heat from the outer side to the inner side of the preferably tubular lance, so that in particular heat may be transferred from the reducing agent and/or from the waste gas to and/or into the reformer. The lance may be manufactured from a highly hat-condUctiflg material, for example from a metal material. The lance in this case has to be able to withstand the high operating temperatures of the fuel cell, wherein ceramic materials having good heat conduction properties may also be used to manufacture the lance. The wall of the lance may moreover be made very thin in order further to improve the introduction of heat from the reducing agent and/or from the waste gas into the reformer.

To the inner side of the fuel cell an anode and to the outer side of the fuel cell a cathode may be applied, wherein the reducing agent and/or the waste gas conveyed past the anode flows in an envelope-shaped manner around the reformer and preferably runs into an outlet channel in the basic body. The reformer may consequently be disposed adjacent to the outlet channel so that in particular the seal between the lance and the basic body and moreover the seal between the fuel cell and the basic body are exposed to lower temperatures because in the reformer an endothermic reaction proceeds, thereby producing a cooling effect. The outlet channel may be disposed adjacent to the supply channel in the basic body so that the reducing agent is supplied to the fuel cell through the supply channel, wherein the waste gas derived from the fuel cell may be removed through the outlet channel. If the outlet channel and the supply channel in the basic body are disposed adjacent to one another such that the two channels are separated for example only by a wall, then heat may transfer from the outlet channel to the supply channel.

The result is that through the wall in the basic body heat is transferred from the waste gas, which flows through the outlet channel, to the fuel gas, which may be for example methane or methanol and is supplied to the reformer. To improve the heat transfer, the wall may be confiured in the manner of a heat exchanger wall for example with an enlarged heat transfer surface. consequently, the wall of the lance may also advantageously be configured in the manner of a heat exchanger wall. Furthermore, the outlet channel and the. supply channel may each be disposed as fluid-carrying formations, for example in the form of meander-shaped or linear cooling coils, adjacent to one another in order further to optimize the transfer of heat from the waste gas to the fuel gas.

According to yet a further advantage the reformer may be designed with an reformer activity that differs from region to region and is adapted gradually in flow direction of the fuel gas, preferably in such a way that the reformer activity increases from the approach flow side to the discharge side of the reformer. Thus, despite a changing gas composition a uniform reforming may be produced over the region, over which the elongate reformer preferably disposed in the lance extends.

If the fuel gas is not completely converted to hydrogen and carbon monoxide in the prereformer then for example methane reaches the anode of the fuel cell and is reformed here, this being referred to as internal reforming. Unlike the exothermic electrochemical reaction of the fuel cell that occurs distributed over the entire cell surface, the endothermic internal reforming occurs substantially in the vicinity of the approach flow side of the anode As a result, the fuel cell becomes colder at the approach flow side of the anode than at the discharge side of the anode.

In a further advantage, by virtue of the purposeful gradual development of the reformer activity it may be ensured that a constant temperature arises over the entire length of the fuel cell. This leads to a more uniform distribution of the electrochemical reaction over the cell and hence to an improved utilization of the surface as well as to reduced loading by thermomechaflical stresses. As the internal reforming at the anode is at its greatest where the gas meets the anode, it may be meaningful for this purpose to allow the reformer activity to decrease from the approach flow side of the reformer to the discharge side thereof.

Preferred embodiment of the invention Further measures that improve the invention are described

in detail below together with the description of a

preferred embodiment of the invention with reference to a single figure. This shows: Figure a diagrammatic representation of an embodiment of a fuel cell system having the features of the io present invention.

The figure shows a diagrammatic view of an embodiment of a fuel cell system 1 according to the present invention. The fuel cell system 1 is configured by way of example with one fuel cell, which is represented in the form of an electrolyte body 10 that is disposed on a basic body 27.

In principle a plurality of fuel cells, for example with electrolyte bodies 10 arranged parallel alongside one another, may be configured, which are disposed jointly on the basic body 27.

As the electrolyte body 10 is configured as an electrolyte body 10 that is closed at the end, a reducing agent 12, in particular hydrogen, is fed into the electrolyte body 10 through a lance 15. The reducing agent 12 flows through the lance 15 and exits at the end thereof. The reducing agent 12 then flows in an envelope-shaped manner around the lance 15 and consequently flows around the inner side of the electrolyte body 10. This side is encumbered with an anode 16 so that the reducing agent 12 may enter into interaction with the anode 16. On the outer side of the electrolyte body 10 a cathode 17 is fitted, so that as a result of an air conduction 26 the cathode 17 may enter into interaction with oxygen from the air.

The illustrated fuel cell is configured merely by way of example with a closed-end electrolyte body 10, with the result that this electrolyte body may alternativelY be fashioned as an electrolyte body that is open at both ends, in which case the reducing agent 12 enters at a first end and exits in the form of waste gas through a second end.

ccording to the present embodiment the inflow of fuel gas 13 and the outflow of waste gas 24 is carried out via the basic body 27, which has corresponding channels. The inflow of fuel gas 13 is represented by a first arrow, while the removal of the waste gas 24 is indicated by a further arrow. The fuel gas 13 is moreover mixed at least with water vapour in order to enable the steam reforming reaction: for methane for example OH4 + 1-120 -> CO + 3H2. In the present case, by the fuel gas 13 a reactive mixture is described.

iccording to the invention a reformer 11 is represented inside the electrolyte body 10 and in particular inside the lance 15. The reformer 11 is used to convert the fuel gas 13 into a reducing agent 12, for example to convert natural gas, methane or methanol with the aid of water vapour into hydrogen and carbon monoxide. By virtue of the arrangement of the reformer 11 in the lance 15, the heat present in the waste gas 24 may be dissipated through the wall of the lance 15 to the reformer 11. This transfer of heat is characterized by arrows 19, wherein the reformer 11 is disposed in the region of the lance 15 that adjoins the basic body 27. Between the lance 15 and the basic body 27 as well as between the electrolyte body 10 and the basic body 27 seals 25 are diagrammaticallY indicated, which may for example comprise a glass solder, with the result that lower operating temperatures are advantageous for such seals 25 particularly with regard to endurance. The reformer converts the fuel gas 13, which is fed through the supply channel 18, to a reducing agent 12, which may be specified as hydrogen and carbon monoxide (or: hydrogen-rich gas) . The reformer process is endothermic, with the result that the heat in the waste gas 24 is absorbed by the reformer 11 through the wall of the lance 15 in accordance with the indicated heat transfer 19. 1-\s this produces a cooling effect, the seals 25 are exposed to lower temperatures.

The supply channel 18 and the outlet channel 21 are diagrammatically shown adjacent to one another and extend through the basic body 27. By virtue of the mutually adjacent arrangement of the two channels 18 and 21, these are separated from one another by the wall 22. As a result of the high temperature in the waste gas 24 and the lower temperature in the fuel gas 13 a heat transfer 23 from the waste gas 24 to the fuel gas 13 may occur. The effect thereby achieved is that the fuel gas 13 is already at a raised temperature when it enters the reformer 11, so that on the whole the efficiency of the fuel cell system 1 may be further increased, wherein the heating of the fuel gas 13 may be carried out also only to a limited extent in order not to reduce the cooling effect.

The present invention in terms of its configuration is not limited to the previously indicated preferred embodiment.

Rather, a number of variants are conceivable, which make use of the represented solution also in configurations of a basically different nature. All of the features and/or advantages, including constructional details, spatial arrangements and method steps, that emerge from the claims, the description or the drawings may be both individually and in the diverse combinations essential for the invention. In particulars a fuel cell system 1 comprising electrolyte bodies 10 that are open at both ends may be provided. In this case, the reducing agent 12 flows without lance 15 through the electrolyte body 10 past the anode 16. The reformer may be adapted in diameter to the inside diameter of the electrolyte body 10, so that initially the fuel gas 13 enters the tubular electrolyte body 10 and is converted to the reducing agent 12. In order to heat the reformer 11, hot waste gas, which also contains water vapour as a reaction product, may be added to the reformer 11 by means of a waste gas feedback, so-called recirculation. In particular, during the fuel cell process a 100% conversion of reducing agents 12 to waste gas 24 does not occur. In the waste gas 24 thereis basically a residual component of reducing agent 12 that may be recycled to the fuel cell process. A particular advantage is achieved in any case if the reformer or at least regions thereof are disposed adjacent to the seals 25.