Note: Descriptions are shown in the official language in which they were submitted.
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GR 94 P 3632 P ~L~, P~ T~3~ R~ ;;~L-
R~ hTlfi~
Description
.,
Fuel-cell syste~ and method for operating a fuel-cell
system
The invention relates to a fuel-cell ~ystem, in
particular a hi~h-temperature fuel-cell system!
comprising at least one fuel-cell block having an anode
section and a cathode section, and also to a method for
operating such a fuel-cell system.
A fuel cell contain an anode and a cathode which
are separated by an ;mme~;ately adjacent, ion-conducting
electrolyte. Said eLectrolyte may be composed of an ion-
conducting liguid or of a polymer membrane or, a~ in the
case of a high-temperature fuel cell, of a solid body,
~uch as, for example" of zirconium oxide contA;n;ng small
additions of yttrium oxide. The electrolyte of a high-
temperature fuel cell is oxygen-ion-conducting at
operating temperatures of the high-temperature fuel cell
of about 1000~C. The fuel, generally hydrogen, is fed to
the anode and the oxygen or the combustion air to the
cathode by suitable duct systems and the water produced
in the reaction of h~drogen and oxyye~l is discharged from
the fuel cell with the anode offgas or the cathode
offgas, dep~n~;ng on the type of fuel cell. A fuel cell
can convert the fuel into electrical energy with a higher
efficiency and lower environmental pollution than
conventional internal-combustion engines hitherto known,
whose efficiency i8 limited by the so-called Carnot
proces~, are capable of doing this.
In the case of development project~ currently
rllnn; ng, attempts are additionally also being made to
utilize the heat produced during the operation of fuel
cells, in particular during the operation of high-
temperature fuel cells. Thus, the development of a high-
temperature fuel-ce:Ll power station proceeds as a rule
from the combination
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of high-temperature fuel cells with a gas turbine and,
optionally, a ~team turbine connected downstream of the
ga~ turbine, the high-temperature fuel cell t~k;~ over
the function of the combustion chamber of the gas
turbine.
In particular, in the devices disclosed in the
p a p e r b y D r . E . E r d l e e n t i t l e d
"Hochtemperaturbrenn~toffzelle SOFC-Stand der Forschung
fur eine neue Technik zur Stromerzeugung" ("High-
temperature SOFC fuel cell - state of research aimed at
a new technology for current generation" in VDI Berichte
No. 1029, 1993 and in German Offenlegungsschrift DE 40 21
097 A1 for operating a high-temperature fuel cell, a
bifurcation is provided in each case for the cathode
offgas produced on the cathode side, at which bifurcation
a portion of the cathode offgas is fed to a combustion
ch~m~er and at which bifurcation a ~econd portion of the
cathode offgas is conveyed via a recuperative heat
exchanger to the temperature-lowering system and then
mixed with an inflow of cooler air. The inflow of cooler
air is fed back into the cathode gas spaces together with
the cooled subflow of the cathode offgas via a compressor
and the same recuperative heat ~ch~nger. The non-
recirculated portion of the cathode offgas is combusted
in a burner with the anode offgaA. The flue gas of this
combustion process is normally fed to a gas turbine.
This arrangement is ~uite sensible thermodynamically, but
has the disadvantage that the cathode offgas at about
1000~C has to be bifurcated. Consequently, a high
expenditure is necessary in regard to the pipe system and
the connecting and weld~ng procedure.
A further disadvantage of this arrangement is
that the amount of recirculated cathode offgas must not
exceed a certain proportion because the ~uantity of heat
might otherwise no longer be ade~uate to preheat the
cathode gaA to be fed to the fuel cell.
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The object of the invention i~ therefore to
provide a fuel-cell system and a method for operating it,
in which, in a particularly ~imple manner, the problem
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of removing the cathcde offgas and of heat utilization of
the cathode offgas is solved.
In regard to the fuel-cell sys~-em, thi ob~ect i8
achieved according to the invention in that, proceeding
from the cathode sect:ion, an offgas pipe i8 provided for
the entire cathode offgas, which offgas pipe runs via a
heat ~ch~nger to a bifurcation comprising two branch
pipes, the first branch pipe opening into the cathode
section via an air addition point and the heat ~ch~nger,
and a second branch pipe opening preferably into heat
utilization means, in particular via temperature-
increasing means in the heat utilization means.
In regard to the method, this object is achieved
according to the invention in that the entire cathode
offgas originating from the cathode section is cooled and
divided up into at least two cathode offgas subflows, a
first cathode offgas subflow being supplemented with air,
heated and fed into the cathode ~ection of the fuel-cell
block and the heat of the second cathode offgas subflow
preferably being utilized.
In this way it is pos ible to avoid a bifurcation
of the hot cathode offgas from the cathode gas spaces.
At the same time, the heat removed from the cathode
offgas in the heat ~ch~nger is fed back again to the gas
mixture flowing into the cathode gas spaces, a reduction
in temperature, which has a beneficial effect on the
specification of a compressor, being achieved by feeding
in fresh air and an increase in the oxygen content of the
gas mixture entering the cathode gas spaces. To explain
what is meant by feeding via a heat exchanger to a
bifurcation, it may be noted that a splitting-up of the
cathode offgas into at least two subflows can al~o
already be provided in or at the heat ~ch~nger in the
case of a suitably low cathode offgas temperature.
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This arrangement fur~her~ore ensures that the
quantity of heat available in the heat ~chAnger for
preheating the cathode ga5 to be fed into the fuel cell
is adequate for all the cathode offgas divisions.
An advantageous embodiment may provide that a
compressor is disposed between the heat ~rh~n~er and the
air addition point. The temperature of the cathode
offga~ subflow is reduced because of the addition of air,
with the result that an inexpensive air compressor of
particularly simple design, in particular an induced
draught fan, can be used. Alternatively, provision can
also be made, proceeding from the bifurcation in the
first branch pipe in the said sequence, to dispose a
first further heat ~ch~nger and a compressor upstream of
the air addition point, the air being feedable via the
first further heat e~chAnger to the addition point. In
this case, the temperature of the cathode offgas subflow
is reduced by the first further heat ~ch~nger, 80 that
a compressor mentioned above can again be provided. The
air fed to the addition point is heated by the heat
removed from the cathode offgas subflow and can be fed to
the cathode offgas subflow upstream of the heat
exchanger.
In a particularly advantageous implementation of
the invention, the temperature-increasing means may be a
second further heat e~ch~nger, the ~econd branch pipe
being routed further from the second further heat
~ch~nger into the i~let of a turbine and a flue gas pipe
being provided which opens into the outlet of the turbine
via the second further heat exch~nger. In this way, it
is pos~ible to deliver heat supplied via the flue gas
pipe to the second cathode offgas subflow. In this way,
a gas mixture is injected into the turbine inlet at a
relatively high flowrate and relatively high temperature,
with the result that, when the hot gas mixture, which is
still under pres~ure at the cathode outlet, i~ expanded,
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particularly high power efficiency is achieved in the gas
turbine.
In an equally advan'ageou~ manner, the
temperature-increasing means may alternatively be a
combustion chamber to which there is connected on the
inlet side, in addition to the second branch pipe, a feed
pipe for a gas mixture originating from the anode section
of the fuel-cell block and an air feed pipe, and, on the
outlet side, a pip,e connected to the inlet of the
turbine. In this way, the second cathode offgas subflow
is heated by means of the combustion of a suitable gas
mixture, in this case the offgas and air originating from
the anode section, which mani~ests itsel~ in a relatively
high inlet temperature at the turbine. A gas mixture
originating from the anode section of the fuel-cell block
is understood as me~n;ng~ inter alia, the anode offgas
itself, but also an anode offgas additionally reduced by
fuel (hydrogen) or even an anode offgas, a so-called
anode residual gas, additionally reduced by fuel and
carbon dioxide.
In addition, it is conceivable to combust a
combustible gas mixture, which may additionally be
available, together with the anode offgas. Such a gaR
mixture may be produced, for example, in the case of fuel
reforming or coal ga~ification.
Further advantageous implementations of the
invention are to be found in the r~m~;n;ng ~ubclaims.
Exemplary e~bodiments of the invention are
explained in greater detail by reference to a drawing.
In the drawing:
FIG. 1 shows the process flow chart of a high-
temperature fuel-cell system with downstream gas
turbine;
FIG. 2 shows the process flow chart of a fuel-cell
sy~tem modified with respect to Figure 1; and
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FIG. 3 shows the proce~s flow chart of a further high-
temperature fuel-cell ~ystem slightly modified
with re~pect to Fi~lre 2.
In Figures 1 to 3, identical parts have identical
reference symbols.
The process flow chart shown in Figure 1 of a
high-temperature fuel-cell system 2 shows in a
diagrammatic representation a high-temperature fuel-cell
block 4 which is divided up into an anode section 6
having anode gas spaces, not shown further, and a cathode
section 8, having cathode gas spaces, not shown further.
In the exemplary embodiment, the high-temperature fuel-
cell block 4 is made up of a multiplicity of planarly
constructed high-te~perature fuel cells, not shown
further, and has an electrical power of 40 megawatts.
Connected to the fuel-cell block 4 is a power inverter 10
which converts the direct current generated by the fuel-
cell block 4 into alternating current for a power
network, not shown further here.
A steam-cont~;n;ng~ hydrogen-cont~;n;ng and/or
carbon-mono~;de-cont~;n;ng fuel gas 14, which i~ heated
beforehand to about 900~C in an anode-side recuperative
heat ~h~nger 16, is fed to the anode side 6 via a fuel
feed pipe 12. A hydrogen-depleted and/or carbon-
mo~ox;de-depleted anode offgas 20 at a temperature of
about 1000~C is discharged from the anode section 6 via
an anode offgas pipe 18. The anode offgas 20 flows via
the recuperative heat PYch~nger 16 and gives up most of
its heat therein to the fuel gas 14 flowing into the
anode section 6. The anode offgas 20 is fed directly in
the exemplary embodiment to a co~mbustion chamber 22, in
which the residual hydrogen contained in the anode offgas
20 is combusted with air supplied via a compressor 24,
which is disposed in an air feed pipe 25.
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The flue gas 26 prod~ced in the combustion chamber 22 is
fed via a flue gas pipe 28 to a first further heat
exchanger 30, in which heat i~ removed from the flue gas
26. Downstream of the first further heat ~h~nger 30,
the flue gas pipe 28 opens into a turbine outlet pipe 32
connected to the turbine outlet 31. On entering the
turbine outlet pipe 32, the flue gas 28 is therefore at
about the temperature of the gas leaving the turbine 34.
Connected to the cathode section 8 on the outlet
side i8 a cathode offgas pipe 3 6, via which a cathode
offgas 38 at about 1000~C i~ fed via a cathode-side
recuperative heat P~ch~nger 40 to a bifurcation 42.
Proceeding from said bifurcation 42 is a first and a
second cathode offgas branch pipe 44 and 46,
respectively, for a first and a second cathode offgas
sub~low 48 and 50, respectively. The first cathode
offgas branch pipe 44 i8 fed from the bifurcation via an
air addition point 52, a circulating fan 54 and the heat
~chAnger 40 into the cathode gas spaces, not shown
further, of the cathode section 8. Comparatively cool
air is fed to the air addition point 52 via an air feed
pipe 56 and a compressor 58 disposed therein. This
results in a temperature reduction in the air-enriched
first cathode offgas subflow 48. The circulating fan 54
can therefore be operated at operating temperatures below
600~C, which affects the cost and the design of the
circulating fan 54 advantageously. The first cathode
offgas subflow 48 forms about 50 to 90%, preferably about
60 to 80%, of the cathode offgas 38 supplied to the
bifurcation 42. In the heat exchanger 40, the first
cathode offgas subflow 48 is heated by means of the heat
given up by the cathode offgas 38 to about 850 to 900~C.
The portion of the cathode offgas 38 left over at
the bifurcation 42 is fed as second cathode offgas
subflow 50 to the inlet 60 of the turbine 34 via the heat
exchanger 30. The second cathode offgas subflow 50
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is heated by meanR of the heat given up by the flue gas
26 in the heat ~ nger 30 to a particularly high
tur~ine inlet te~perature in order to achieve as high an
output as possible during the expansion of the second
cathode o~fgas subflow 50 in the turbine 34. The gas
mixture di~charging ~rom the turbine 34 can e~cape into
the open air via a throttle valve 62 if the valve 62 is
in the open position or, alternatively, if the valve
poRition is completely closed or slightly throttled, can
be fed into a steam generator 64 and from there into the
open air. The steam generator 64, which is supplied with
water 66, supplies process steam 68, which can be
utilized in a steam turbine, not shown further here.
Some of the proce~s steam 68 may also be in~ected into
the fuel gas 14, where it serves to reform a carbon-
cont~;n;n~ fuel gas. Given a steam excess in the fuel
gas,~~soot ~ormation, which normally occurs in the
reforming of natural gas to form hydrogen and methane,
can largely be avoided.
It should be repeated yet again that the process
flow chart explained above is notable for four particular
advantages. Fir~tly, the hot cathode offgas 38
discharging from the cathode section 8 is first
bifurcated in the cathode-~ide heat e~ch~nger 40 after it
has been cooled. Secondly, the air addition point 52 i8
disposed upstream of the circulating fan 54 in the flow
direction of the first cathode offgas subflow 48, with
the re~ult that the temperature of the first cathode
offgas subflow 48 flowing into the circulating fan 54 i5
substantially reduced by the addition of the
comparatively cool, compressed air. Thirdly, the
temperature of the ~econd cathode offgas subflow 50 is
raised considerably in the first further heat exchanger
30 as a result of the utilization of the heat content of
the flue gas 26, which corresponds at the ~ame time to a
higher gas inlet temperature at the inlet 60 of the
turbine 34. Fourthly, an adequate preheating o~ the
cathode offgas subflow 48 i~ always ensured, regardles~
of the division of the cathode offgas 38 at the
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bifurcation 42.
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The drive pipe [~ic] generated by means of the
turbine 34 is utilized in the exemplary embodiment to
drive the air compre~sor 58, a generator 70 and the air
compre~sor 24. The abovementioned components are
disposed on a common shaft 72. In this connection, the
generator can advantageously also be operated as a motor
for starting up the turbine 34.
Figure 2 shows a fuel-cell system 74 which is
slightly modified compared with Figure 1. Said fuel-cell
sygtem 74 differs from the system shown in Figure 1
~olely in a modification of the fresh-air feed to the
addition point 52 in the first cathode offgas subflow 48.
In the flow direction of the fir~t cathode offgas subflow
48 after the bifurcation 42, there is now disposed,
firstly, a second further heat exchanger 76, then the
circulating fan 54 and the addition point 52 subsequent
thereto. The air supplied by means of the air compressor
5~3 is now heated in the second further heat exchanger 76,
which results in the intended and advantageou~
temperature reduction of the first cathode offgas subflow
48. In this exemplary embodiment, too, the circulating
fan 54 therefore has to pump the first cathode offgas
subflow 48, which is at a comparatively low temperature.
The air then fed to the addition point 52 and already
heated flows from that point together with the first
cathode offgas subflow 48 to the cathode-side heat
exchanger 40.
Figure 3 shows a fuel-cell system 78 which is
modified with respect to Figure 2. The modifications
relate here to the ~econd cathode offgas subflow 50 and
also to the feed of the anode offgas 20. Proceeding from
the bifurcation 42, the second branch pipe 46 for the
second cathode offgas subflow 50 ig now fed via a
combustion chamber 80 to the inlet 60 of the turbine 34.
The anode offgas pipe 18 is now fed via an anode offgas
compressor 82 into the combustion chamber 80. In
addition, there br~nch.os off from the air feed pipe 56 a
branch
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pipe 84, which also opens into the combustion ch~mher 80.
In the exemplary emhodiment, the combustion chamber 80
serves as tamper~tur~-increa~ing means. The heat
liberated during the combustion of the anode offgas 20
with the air and the second cathode offgas subflow 50
results in the second cathode offgas subflow 50, which
flows into the inlet 60 of the turbine 34, having a
comparatively high inlet temperature and a co~paratively
hi~h mass flow, which has an advantageous effect on the
output achievable with the turbine 34. At the same time,
the offgas mass flow, and consequently also the offgaR
lo~ses, can be reduced with respect to the arrangement
according to Figure 2. The compressor 82, which brings
the anode offgas 20 to the pressure o~ the cathode offgas
:Elow 38, is driven in the exemplary embodiment by means
of the Rhaft 72 of the turbine 34.
~~ In an alternative not shown further here, the
anode offgas 20 can be additionally subjected to a gas
~eparation upstream of the compression in the compressor
82. In said gas separation, inert constituents in the
anode offgas 20, in particular carbon dioxide, can be
removed. Although this results, on the one hand, in the
reduction in the ma~s flow of the anode offgas 20, it
results, on the other hand, in an increase in the
temperature in the combustion chamber 80 because inert
gas constituents, su.ch as, for example, carbon dioxide,
no longer have to be heated in the combustion chamber 80.
