Note: Descriptions are shown in the official language in which they were submitted.
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2165085
_ GR 93 P 3301 P ~jtE~, pt~YtN'TH1S A~.~~NDED
- TRANSLATION
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 procass 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 high-
temperature 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 electrolyte, 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
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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 800° to 1100°C) in contrast to low-
s temperature 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 cm2 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 and
wirtschaftliche Aspekte des Brennstoffzellen-Einsatzes in
Kraft-Warme-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 post-
combusted, the
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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 MHB 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°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
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multiplicity of individual pipes on the supply and discharge
sides of the cathodes are difficult. The object of the
invention is therefore to provide a high-temperature fuel cell
system in which the fuel and oxygen carriers are guided with a
particularly low pressure drop in the high-temperature fuel
cell system.
This object is achieved according to the invention
in that a high-temperature fuel cell system comprises a
container (2) and at least one high-temperature fuel cell
stack (4-14, 94) arranged in the container (2), wherein the
high-temperature fuel cell stack (4-14, 94) is a partition or
forms part of a partition which separates an air entry space
(28), into which air inlets of the high-temperature fuel cells
open, and an air exit space (16), into which air outlets of
the high-temperature fuel cells open, from one another in the
container (2), at least one location being provided in the
partition, at which air (22) situated in the air exit space
(16) can be recirculated at least partially into the air entry
space (28) by means of the air (36) flowing into the air entry
space ( 2 8 ) .
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 at least partially fed
back into the air entry space. In this case the hot
20365-3537
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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 high-
temperature 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 high
temperature fuel cell stacks can be the air entry space,
and vice versa.
Since the pressure drop in the case of the high-
temperature 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
subsequently preheated can be fed to the air jet pump,
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a 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.
Further advantageous developments of the
invention can be found in the rest of the subclaims.
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-temperaturefuel
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 sketched
in 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 sketched
in in Figure 2, with a high-temperature 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 5.
In Figures 1 to 6, the same parts have the same
references.
Figure 1 shows a longitudinal section through a
high-temperature fuel cell system 1. In this system 1,
six high-temperature fuel cell stacks 4 to 14 are
arranged directly adjoining one another in a ring, inside
a cylindrical reactor container 2 (cf. also Figure 2) .
Each high-temperature fuel cell stack 4 to 14 consists of
416 planes with 20 high-temperature fuel cells in each
plane, so
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that, with an average power of approximately 2 Watt per
fuel cell and with 49 920 fuel cells, an average
electrical power of the high-temperature fuel cell
system 1 equal to approximately 100 kW results. In this
exemplary embodiment, an air exit space, the so-called
central space 16, is separated from and sealed off from
an air entry space, the so-called annular space 28, by
the fuel cell stacks 4 to 14 arranged in a ring as well
as by an exit air pipe 20, for the exit air 22 on the
cathode side, provided with labyrinth chicanes 18, and
with other labyrinth chicanes 24 and flow guide pipes 26.
Openings 30 in the flow guide pipes 26 are not included
in this . An air j et 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 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 to 14 and
consisting of unconsumed hydrogen gas and the product
water formed in the 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 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,
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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 900°C, and
the mass flow is approximately equal to 180 g per second,
which corresponds to a volume flow of approximately
650 liters per second. Hy 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 cm2 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 high-
temperature fuel cells not further represented in Figure
1.
The molecular oxygen in the air/exit-air mixture
22, 36, at a temperature of approximately 900°C, 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
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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.
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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
60 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-II 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 high-
temperature 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 50
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 50e,
50f to 50j, 50k to 500
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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, l, g, b,
c, h, m, r, s, n, i, d, e, j, o, t. It is however also
equally conceivable to guide the reactants a.n crossed
countercurrent flow, which would mean that, for example,
the gas mixture 44 would flow in exactly the opposite
direction 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 catalysts 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 2). In
this way, the pressure drop when distributing the exit-
air/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
power plant 60.
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The 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 side of the second air preheater 74, the primary
side of a steam generator 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-draught
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
90.
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 700°C 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 section with the
aid of the induced-draught fan and, on the secondary
sides of the first and second air
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heaters 72, 74, heated to said temperature. In addition,
natural gas which is at a temperature of approximately
1000°C 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. l3ere, approximately one half of the
natural gas is also prereformed. By introducing steam in
the natural gas via the steam supply line 90 and the
valve 88 any formation 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 0 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 pre-
reforming 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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temperature elevation of the air supplied to the high-
temperature fuel cell system 1.
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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 6). 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 exit-
air/air mixture 22, 36 and the gas mixture 44 that
essentially contains hydrogen flow through these high-
temperature 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 flaw 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 lane 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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single large stack because, in particular during welding
of the individual fuel cells to form a fixed stack, non-
negligible 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 5
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.
