AU678482B2

AU678482B2 - High-temperature fuel cell installation

Info

Publication number: AU678482B2
Application number: AU68406/94A
Authority: AU
Inventors: Wolfgang Drenckhahn; Hans-Peter Schabert; Horst Vollmar
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1993-06-14
Filing date: 1994-06-01
Publication date: 1997-05-29
Anticipated expiration: 2014-06-01
Also published as: DE59401615D1; DE4319411A1; EP0704109B1; JPH08506691A; CA2165085C; DK0704109T3; WO1994029922A1; CA2165085A1; AU6840694A; EP0704109A1; ATE147893T1

Abstract

In order to obtain a particularly simple structure for a high-temperature fuel cell installation (1) and a very low power requirement for the compressor (70) on the air side, according to the invention at least one high-temperature fuel cell stack (4 14) in a container (2) forms a gastight separator or a component thereof in the container (2) between an air inlet chamber (28) into which the air inlets from the high-temperature fuel cells open and an air outlet chamber (16) into which the air outlets from the hight-temperature fuel cell open. There is preferably at least one point in this separator at which the air (22) in the air outlet chamber (16) can be at least partially recirculated by means of the air (16) flowing into the air inlet chamber (16). Thus the compressor compresses cold, relatively dense air and the hot air (22) can be recirculated in the container (2) without the addition of moving parts. The invention is essentially applicable to all high-temperature fuel cell installations.

Description

GR 93 P 3301 P 1 Description High-temperature fuel cell system The invention relates to a high-temperature fuel cell system having a container and having at least one high-temperature fuel cell stack arranged in the container.

A fuel cell stack comprises a plurality of planar solid-electrolyte high-temperature fuel cells, fixed on one another and electrically connected in series. In this case one bipolar plate is built in respectively between directly neighboring cells, which plate electrically conductively connects the cathode of the one cell to the anode of the cell neighboring it, guarantees gas distribution and represents a supporting structural element.

A process which essentially represents a reversal of the electrolysis takes place in the fuel cell. The reaction partners of the combustion reaction, namely the fuel, generally hydrogen, and the oxygen carrier, generally air, are supplied separately. In a hightemperature fuel cell, the supply lines carrying fuel and oxygen are separated from one another in gas-tight fashion by a ceramic solid electrolyte which is provided with electrodes on both sides. During operation, electrons are given out at the electrode on the fuel side of the solid electrolyte, namely the anode, and electrons are received at the electrode on the oxygen side of the solid ulectrolyte, namely the cathode. A potential difference, the open-circuit voltage, is set up at the two electrodes of the solid electrolyte. The solid electrolyte has the function of separating the reactants, of transporting the charges in the form of ions and, simultaneously, of preventing an electronic short-circuit between the two electrodes of the solid electrolyte. For this purpose, it must have a low electronic conductivity RA4 7S- 83,

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GR 93 P 3301 P 2 together with a high ionic conductivity.

Such high-temperature fuel cells are suitable, as a result of the relatively high operating temperature (it is in the range from 8000 to 11000C) in contrast to lowtemperature fuel cells, for converting hydrocarbons such as, for example, natural gas or propane storable in liquid form, in addition to hydrogen gas. High power densities which, as an order of magnitude, are in the range of many hundreds of mW per cm 2 of cell surface area, can be reached with high-temperature fuel cells.

The individual high-temperature fuel cell produces an open-circuit voltage of somewhat more than one volt.

Further details of high-temperature fuel cells can be.

found in the "Fuel Cell Handbook" by Appleby and Foulkes, New York, 1989.

The way in which high-temperature fuel cells can be used, for example in combined heat and power plants, can also be found in the article "Technische und wirtschaftliche Aspekte des Brennstoffzellen-Einsatzes in Kraft-Whrme-Kopplungs-Anlagen" [Technical and Economic Aspects of Fuel Cell use in Combined Heat and Power Plants" by Drenckhahn, Lezuo and Reiter in VGB Kraftwerkstechnik, Volume 71, 1991, Issue 4.

In a high-temperature fuel cell system, one or more stacks of high-temperature fuel cells are usually built into a container. The fuel and the oxygen carrier, usually air, are supplied in heated and slightly compressed form via external supply lines to the anodes and cathodes, respectively, of the high-temperature fuel cells. The fuel supply is in this case usually designed in such a way that approximately 80 of the fuel is consumed in the high-temperature fuel cells and the remaining 20 of the fuel is discharged together with the product water formed from hydrogen and oxygen ions in the reaction via pipelines. On the fuel side, the gas mixture discharged from the high-temperature fuel cells is not recirculated but instead catalytically postcombusted, the -r I I GR 93 P 3301 P 3 liberated energy being used to preheat the reactants and/or to produce steam.

On the cathode side the air volume flow is greater by approximately a factor of 8 compared to the fuel volume flow. In order not to lose, or only partially to lose, the heat content of the exit-air mixture leaving the high-temperature fuel cells in the container, it is customary to discharge the exit-air mixture on the cathode side from the container at least partially via pipelines, to recompress it and to feed it back again into the container via supply lines. In this case, however, a series of disadvantages occur: in the case of this hitherto known so-called "monobloc design" (cf. Fuji Electric Review, Vol. 38, No. 2, page 58, and MBB in "Handelsblatt" of 06.12.1990), very large pressure drops are produced on the distributor side and the manifold side, which is to say in the fuel-cell inlets or outlets on the air side, and these pressure drops can only be compensated for with a compressor having a relatively high power demand. These pressure drops are usually above approximately 50 mbar.

In particular in the case of high total electrical powers of the high-temperature fuel cell system it is easy to recognize that considerable problems exist on the cathode side due to the multiplicity of supply lines and discharge lines and due to the gas compressor. This gas compressor must compress a hot, oxygen-containing exit gas on the cathode side, which causes particularly high maintenance expenditure, in particular for the moving parts of the compressor. In order to avoid this disadvantage, DE-A 40 21 097 discloses first cooling the exit gas on the cathode side to below approximately 650 0 C, and then compressing and subsequently reheating it. Disadvant-ageously, this configuration makes the use of additional heat exchangers and the introduction of additional quantities of heat necessary. In addition the flexurally non-rigid routing and the fitting together of this sl~ multiplicity of individual pipes on the supply and discharge sides of the cathodes are difficult.

The object of the invention is to ameliorate one or more of the abovementioned disadvantages.

According to one aspect of the invention there is disclosed a high-temperature fuel cell system having a container and at least one high-temperature fuel cell stack arranged in the container, wherein the high-temperature fuel cell stack is a partition or forms part of a partition which separates an air entry space, into which the air inlets of the high-temperature fuel cells open, and an air exit space, into which the air outlets of the high-temperature fuel cells open, from one another in the container, at least one location being provided in the partition, at which the air situated in the air exit space can be recirculated at least partially into the air entry space by means of the air flowing into the air entry space.

The result of this is that, on the cathode side, the oxygen carrier, usually air, is introduced into only one space, the air entry space, and is discharged from only one space, the air exit space. This leads to a particularly low pressure drop when 'distributing and collecting the oxygen carrier.

o* In a particularly advantageous development of the invention, it is possible that at least one location is provided in the partition, at which the air situated in the air exit space can be recirculated at least partially into the air entry space by means of the air flowing into the air entry space. The result of this is that the air flowing out from the high-temperature fuel cells is guided into the air exit space common to all the air outlets and (without the hitherto customary extraction from the container with subsequent compression outside the container) at least partially fed back into the air entry space.

In this case the hot [N:\IibCcIOO800:BFD ~II I GR 93 P 3301 P 5 air situated in the air exit space is at least partially recirculated by means of the flow movement of the somewhat cooler air flowing into the air entry space, as a result of which the air flowing into the air inlets of the high-temperature fuel cells already approximately has a preferred temperature for operating the hightemperature fuel cells.

A particularly advantageous structure of the high-temperature fuel cell system results if a plurality of high-temperature fuel cell stacks are arranged directly next to one another in a ring. In this case, "in a ring" also means that a plurality of stacks are arranged in the form of a polygon. In this way the air entry space and the air outlet space in the container are particularly easy to separate. In this case the so-called central space enclosed by the high-temperature fuel cell stacks can be the air exit space and, accordingly, the so-called ring space lying outside the ring of the hightemperature fuel cell stacks can be the air entry space, and vice versa.

Since the pressure drop in the case of the hightemperature fuel cell system configuration according to the invention is only relatively low on the air side, it is expedient if labyrinth chicanes are used as partitioning and/or sealing means. In this way the air entry space can be easily sealed off and separated from the air exit space, even between the high-temperature fuel cell stacks arranged in a ring.

An air jet pump (ejector) for recirculating the air situated in the air exit space can be used as a particularly simple air-recirculation means that requires no maintenance.

If air compressed in the cold state and cubsequently heated can be fed to the air jet pump, I I CCs~R--- -6a particularly low power demand for the air compressor results, because the compressor compresses relatively cold air with relatively high density, before the air is preheated, which, as is known, leads to a decrease in the air density.

Exemplary embodiments of the invention are explained in more detail with the aid of six figures, in which: Figure 1 shows a longitudinal section through a schematically represented high-temperature fuel cell system; Figure 2 shows a section on the line II-II in the high-temperature fuel cell system according to Figure 1; Figure 3 shows an enlargement of the detail III shown in Figure 2; Figure 4 shows the high-temperature fuel cell system according to Figure 1 integrated in a combined heat and power plant; Figure 5 shows an enlargement of the detail III shown in Figure 2, with a hightemperature fuel cell stack constructed of partial stacks; and Figure 6 shows a schematic representation of the high-temperature fuel cell stack, constructed of partial stacks, shown in Figure In Figures 1 to 6, the same parts have the same references.

oo.o Figure 1 shows a longitudinal section through a high-temperature fuel cell 00 system 1. In this system 1, six high-temperature fuel cell stacks 4 to 14 are arranged 20 directly adjoining one another in a ring, inside a cylindrical reactor container 2 (cf. also 0@ 0 0 Figure Each high-temperature fuel cell stack 4 to 14 consists of 416 planes with high-temperature fuel cells in each plane, so that, with an average power of approximately 2 Watt per fuel cell and with 49 920 fuel cells, an average electrical ooo oi I R I. -O

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-in~N tN:\~bccIooBoo~so 0 rl I -7power of the high-temperature fuel cells system 1 equal to approximately 100 kW results. In this exemplary embodiment the air mixture 22 flows from an air entry space, the so-called annular space 28 through the high temperature fuel cells (4-14) and any air not consumed thereby flows to the ring space 28, the so-called air exit space.

In this embodiment the air exit space 16 is separated from and sealed off from the air entry space 28 by the fuel cell stacks (4-14) arranged in a ring as well as by an exit air pipe 20. There are further provided labyrinth chicanes 18 and labyrinth chicanes 24 for sealing off the central space 16 from the annular space 28. In this embodiment the openings 30 in the flow guides 26 draw a part of the air 22 situated in the central space 16 back into the annular space 28. An air jet pump 32 to which preheated compressed air 36 is supplied via an air supply line 34 is arranged centrally in the central space 16, below the high-temperature fuel cell stacks 4 to 14. In this case the nozzles of the air supply tubes 38 of the air jet pump 32 project into the flow guide pipes 26 which serve as suction tubes.

One fuel supply line 40 and one exit gas line 42 are in each case connected to each stack 4 to 14 in the upper part of the high-temperature fuel cell stacks 4 to 14. A gas mixture 44 consisting of previously compressed and heated hydrogen gas, obtained *o*o 1 from the reformation of natural gas, still unreformed natural gas and water is fed via -the fuel supply line 40 to the stacks 4 to 14. An exit gas 46 flowing out of the stacks 4 S 20 to 14 and consisting of unconsumed hydrogen gas and the product water formed in the

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combustion reaction is discharged via the exit gas line 42.

During operation of the high-temperature fuel cell system 1, with a power of approximately 100 kW selected in the exemplary embodiment, air 36 heated to approximately 700'C is supplied via the air supply line 34 with a mass flow of l approximately 60 g per second, which corresponds to a volume flow of approximately 210 liters per second. By means of the air jet pump 32, RA t 'T 0< JN:VXibccJ0O8OO;8FD c\J -o I I I I I GR 93 P 3301 P 8 the air 36 is injected into the-annular space 28 via the air supply tubes 38. In this case the air 36 injected into the annular space 28 draws with it a part of the air 22 situated in the central space 16 and at a temperature of approximately 1000*C, so that the air temperature in the annular space 28 is approximately equal to 9000C, and the mass flow is approximately equal to 180 g per second, which corresponds to a voralume flow of approximately 650 liters per second. By corresponding flow guiding in the individual high- temperature fuel cells, which is explained further below in Figure 3, the pressure difference between the annular space 28 and the central space 16 is limited to only approximately 5 mbar. This low pressure drop makes it possible to use simple labyrinth chicanes 18, 24 for sealing off the central space 16 from the annular space 28. Since the air 36 introduced into the air jet pump 32 has already been compressed in the cold state, the power demand of the compressor required for this is so low that a total leakage cross section of, in the exemplary embodiment, approximately 60 cm 2 is inconsequential, and in particular constitutes only approximately 2 of the total cross section of the air guide channels in the bipolar plates, not further represented, of the hightemperature fuel cells not further represented in Figure i.

The molecular oxygen in the air/exit-air mixture 22, 36, at a temperature of approximately 9000C, which flows into the fuel cells is converted at the cathodes of the high-temperature fuel cells into oxygen ions. The electrons required for this are liberated at the anodes of the high-temperature fuel cells by oxidation of the hydrogen gas which is contained in the gas mixture 44 and has, on average, a total volume flow of 80 liters per second. The electrons liberated at the anodes flow to the cathodes via an external circuit, not further represented here, the oxygen ions flowing through an electrolyte that conducts oxygen ions, which is arranged between the anode and the cathode, and form water on the anode side with 7 .0 161, /V e GR 93 P 3301 P 8a the hydrogen ions. This product water is discharged, together with unconsumed hydrogen gas, as anode exit gas 46 out of the container 2 via the exit gas line 42.

4, \j~rC) lrl I GR 93 P 3301 P Since the anode gas 46 is fed in pipes with relatively small cross section the pressure drop on the anode side is approximately 50 mbar. This pressure drop is, however, not considerable since the gas mixture 44 is, after approximately 80 of the fuel gas is used up, not recirculated back into the fuel cells, but instead subsequently combusted, which is further explained below with reference to Figure 4.

The atmospheric oxygen not consumed in the fuel cells flows, together with the inert components of the air 36, as exit air 22 into the central space 16. As already described, a part of this exit air 22, namely approximately 120 g per second, is recirculated into the annular space 28 by means of the air jet pump 32. The remaining exit air 22 is discharged with a mass flow of g per second via the exit air pipe 20, subsequently combined with the anode exit gas 46 and combusted.

The section, represented in Figure 2, on the line II-I in Figure 1 again clarifies the way in which the fuel cell stacks 4 to 14 form part of a partition which separates the central space 16 from the annular space 28.

In this case the number of fuel cell stacks 4 to 14, directly adjoining one another in gas-tight fashion, can be freely selected within wide limits as a function of the desired power of the high-temperature fuel cell system 1.

Figure 3 represents on an enlarged scale the detail III sketched in in Figure 2. This detail shows in schematic representation, by way of example, the structure of a plane 50, consisting of 20 hightemperature fuel cells 50a to 50t with a size of approximately 5 x 5 mm each. The high-temperature fuel cells 50a to 50t are arranged, in matrix fashion, in four rows and five columns. On the cathode side, which is to say on the air side, flow takes place in the plane through four parallel channels, each having five fuel cells connected in series. Specifically, these are the channels for the high-temperature fuel cells 50a to to 50j, 50k to 500 1 I I I -~P1 rl~ GR 93 P 3301 P 10 and 50p to 50t. On the anode side, which is to say on the hydrogen gas side, the gas mixture 44 is guided in crossed cocurrent flow with respect to the exit-air/air mixture 22, 36, and specifically, in sequence, through the high-temperature fuel cells 50a, f, k, p, q, 1, g, b, c, h, m, r, s, n, i, d, e, j, o, t. It is however also equally conceivable to guide the reactants in crossed countercurrent flow, which would mean that, for example, the gas mixture 44 would flow in exactly the opposite direcLion from that represented in Figure 3.

Advantageously, the composition of the anodes and cathodes, or the way in which they are coated with catalysts, may be different on high-temperature fuel.

cells connected in series in the flow direction, so that internal reforming of the natural gas present in the gas mixture 44 does not take place too suddenly and with excessive local overcooling, with the result that thermal stresses can be avoided in the individual planes.

Specifically, this may mean that, for example, the concentration of '.talysts on the surface of the anode increases in the direction of flow of the gas mixture 44.

With the aid of Figure 3 it is once again explicitly shown that the air channels, not here further represented, in the bipolar plates start in the annular space 28 and end in the central space 16 of a cylindrical arrangement (cf. the cylindrical reactor container In this way, the pressure drop when distributing the exitair/air mixture 22, 36 and when collecting the exit air 22 is in each case very small. As a result, the power demand for the air compressor is particularly low, which is in contrast to the hitherto customary compressor powers of high-temperature fuel cell systems in which the exit air 22 is discharged from and the air 36 is fed to the high-temperature fuel cell stacks via a multiplicity of pipes.

Figure 4 schematically represents the way in which the high-temperature fuel cell system 1 according to Figures 1 to 3 is integrated into a combined heat and /y T R< power plant 7,

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I I ~e Is GR 93 P 3301 P 11 Th, description of Figure 4 essentially deals with an air supply section 62, a fuel supply section 64, an exit air section 66 and an exit gas section 68. The arrows drawn in on Figure 4 in the flow sections 62 to 68 indicate the flow direction of the respective flow medium.

In the air supply section 62, an induced-draught fan 70, the secondary side of the first air preheater 72, the secondary side of the second air preheater 74 and the air jet pump 32 are, in sequence, built in. In the fuel supply section 64, starting from a natural gas store 76, an induced-draught fan 78 and the secondary side of a prereformer 80 are built in. The exit air section 66 begins at the central space 16 and extends via the.

primary side of the prereformer 80 to a burner 82. The exit gas section 68 starting from the high-temperature fuel cell stacks 4, 6, opens directly into the burner 82.

From the burner 82, the exit gas section 68 and the exit air section 66 extend together, in sequence, through the primary s .a of the second air preheater 74, the primary side of a steam gene'ator 84, the primary side of a first air preheater 72 and, finally, into a chimney 86.

Starting from the secondary side of the steam generator 84, a steam supply line 90 opens, via a valve 88, into the fuel supply section 64, specifically, in the flow direction of the natural gas, between the induced-draugh'; fan 78 and the secondary side of the prereformer 80. In addition, a steam output coupling device 92 which leads to a power-generation turbine, not further represented here, is furthermore.connected to the steam supply line During operation of the combined heat and power plant 60 having a high-temperature fuel cell system 1 with an electrical power of approximately 100 kW, air at a temperature of approximately 7000 f4n*rua is fed with a mass flow of approximately.60 g per second, virtually without the use of pressure, to the air jet pump 32. In this case the air was delivered via the air supply -I the secondary sides of the first and second air I j

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srL:'r Trl GR 93 P 3301 P 12 heaters 72, 74, heated to said tempera 1 3. In addition, natural gas which is at a temperature of approximately 10000 4**f=ajrs and is withdrawn from the natural gas store 76 is supplied to the high-temperature fuel cell system 1 with the aid of the induced-draught fan 78. The temperature of the natural gas is set on the secondary side of the prereformer 80. Here, approximately one half of the natural gas is also prereformed. By introducing steam into [sic] the natural gas via the steam supply line 90 and the valve 88 any foration of soot as a result of the reforming of the natural gas in the preformer 80 and because of the high temperatures is avoided.

In the high-temperature fuel cell system 1, the already described combustion reaction then takes place with consumption of atmospheric oxygen and hydrogen. In this case the mass flow on the air side in the annular space 28 is approximately equal to 180 g per second.

Approximately 120 g per second of the air 16 situated in the central space 16 is recirculated with the aid of the air jet pump 32 into the annular space 28 and thereby into the high-temperature fuel cell stacks 4, 6.

Approximately 80 of the natural gas is consumed in the high-temperature fuel cell system 1 and introduced into the burner 82 via the exit gas section 68. The air still situated in the central space 16 is likewise introduced into the burner 82 via the exit air section 66 and via the primary side of the prereformer 80, the heat content of the exit air being advantageously used for prereforming of the natural gas.

In the burner 82, the hydrogen molecules and carbon molecules still contained in the gas mixture 44 are combusted together with the oxygen still contained in the exit air 22. The heat content of the burner exit gas in the exit-air/exit-gas section 66, 68 is first partially transferred in the second air preheater 74 to the supplied air for the purpose of preheating, then used to generate steam in the steam generator 84, and subsequently used in the first air preheater for initial

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GR 93 P 3301 P 12a temperature elevation of the air supplied to the hightemperature fuel cell system 1.

OV: GR 93 P 3301 P 13 The extensively cooled burner exit gas is subsequently guided into the atmosphere via the chimney 86.

Figure 5 again represents the detail represented in Figure 3. In contrast to Figure 3 however, the fuel cell stack 14 is replaced by a fuel cell stack 94 which consists of 10 partial stacks 94a to 94j arranged one on top of the other (cf. Figure 16 high-temperature fuel cells 96a to 96p are now arranged in a plane 96 of the high-temperature fuel cell stack 94. As in the case of the fuel cell stacks 50 to 50t in Figure 3, the exitair/air mixture 22, 36 and the gas mixture 44 that essentially contains hydrogen flow through these hightemperature fuel cells 96 to 96p in crossed cocurrent flow. On the cathode side, which is to say on the air side, flow takes place in the plane 96 through four parallel channels, each having four high-temperature fuel cells connected in series. Specifically, these are the channels for the high-temperature fuel cells 96a to 96d, 96e to 96h, 96i to 961 and 96m to 96p. On the anode side, which is to say on the hydrogen gas side, the gas mixture 44 is guided in crossed cocurrent flow with respect to the exit-air/air mixture 22, 36, and specifically, in sequence, through the high-temperature fuel cells 96a, e, i, m, n, j, f, b, c, g, k, o, p, 1, h, d.

This structure of the plane 96 makes it possible to guide the fuel supply line 40 and the exit gas line 42 on the same side of the partial stack 94a. As Figure 6 illustrates, the partial stacks 94a to 94j are alternately connected to the fuel supply line 40 and the exit gas line 42 on opposite sides. In this way it becomes particularly simple to remove a defective partial stack from the high-temperature fuel cell stack 94.

In addition, it is considerably simpler to produce a relatively small partial stack 94a to 94j than to produce a

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GR 93 P 3301 P 14 single large stack because, in particular during welding of the individual fuel cells to form a fixed -tack, nonnegligible gravitational force effects act as a result of the weight of the high-temperature fuel cells stacked one on top of the other. Operation of a high-temperature fuel cell system 1 with the partial stacks shown in Figures and 6 is also more secure compared to the fuel cell stacks 4 to 14 consisting of a single unit because, in the event of leaks, local burning of oxygen and hydrogen remains limited to the relatively small region of a partial stack 94a to 94j.

The reactor variant with a power of 100 kW, represented in the exemplary embodiments, can be increased straightforwardly, even with operation not driven by pressure, which is to say at atmospheric pressure, up to 400 to 600 kW. For this purpose, for example, the number of high-temperature fuel cell stacks 4 to 14, 94 arranged in a ring can be doubled from the six stacks in the exemplary embodiment to twelve stacks.

In addition, a plurality of reactor containers 2 can be arranged one on top of the other.

Claims

1. A high-temperature fuel cell system having a container and at least one high-temperature fuel cell stack arranged in the container, wherein the high- temperature fuel cell stack is a partition or forms part of a partition which separates an air entry space, into which the air inlets of the high-temperature fuel cells open, and an air exit space, into which the air outlets of the high-temperature fuel cells open, from one another in the container, at least one location being provided in the partition, at which the air situated in the air exit space can be recirculated at least partially into the air entry space by means of the air flowing into the air entry space.

2. The high-temperature fuel cell system as claimed in claim 1, characterized in that a plurality of high-temperature fuel cell stacks are arranged directly next to one another in a ring.

3. The high-temperature fuel cell system as claimed in one of claims 1 or 2, characterized in that labyrinth chicanes are used as partitioning and/or sealing 15 means.

The high-temperature fuel cell system as claimed in one of claims 2 to 3, characterized by an air jet pump for recirculating the air situated in the air exit space.

5. The high-temperature fuel cell system as claimed in claim 4, characterized in that the air jet pump can be operated by means of air compressed in the cold state and subsequently heated.

6. The high-temperature fuel cell system as claimed in claim 4 or characterized in that the air jet pump is arranged centrally in the air exit space, blowing in the direction of the air entry space.

7. The high-temperature fuel cell system as claimed in one of claims 1 to 6, characterized in that, within a high-temperature fuel cell stack a plurality of high- temperature fuel cells are arranged in a plane perpendicularly to the stacking direction and a fuel gas and air flow through them in crossed cocurrent or countercurrent flow. IN:\IIbcCiOOSOO:FD i -16-

8. The high-temperature fuel cell system as claimed in one of claims 1 to 7, characterized in that a high-temperature fuel cell stack is subdivided into a plurality of partial stacks arranged one on top of the other.

9. The high-temperature fuel cell system as claimed in one of claims 1 to 8, characterized in that the cathode and anode composition of the high-temperature fuel cells or the way in which they are coated with catalytic material is different in the case of high-temperature fuel cells connected in series in the flow direction.

The high-temperature fuel cell system as claimed in one of claims 1 to 9, characterized in that, by means of the heat content of the air not recirculated out of 1 o the air exit space into the air entry space, the fuel gas can be pre-reformed or partially reformed according to requirements.

11. A high-temperature fuel cell system substantially as described herein with reference to the accompanying drawings. DATED this Tenth Day of February 1997 15 Siemens Aktiengesellschaft "Patent Attorneys for the Applicant SPRUSON FERGUSON o a *o ooo. o*ooo* INAI:\libcOO800BFD I I-I I