Note: Descriptions are shown in the official language in which they were submitted.
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INTEGRATED MODULE FOR SOLID OXIDE FUEL CELL SYSTEMS
BACKGROUND OF THE INVENTION
The present invention relates to an integrated module for solid oxide fuel
cell ("SOFC")
systems comprising an afterburner, a heat exchanger and a fuel processor.
As many remote power customers use natural gas or propane, these are obvious
choices
for SOFC fuel in remote power generation systems. As Well, many other
applications
exist for fuel cell systems such as residential cogeneration and automotive
uses. SOFCs
have the advantage of easily being able to use hydrocarbon fuels through fuel
processing
methods including steam reforming, partial oxidation and autothermal
reforming. As fuel
processing of hydrocarbons occurs at or near SOFC operating temperatures,
thermal
integration of both the fuel processor and stack is desired.
Steam reforming is a method that realizes a high overall system efficiency and
provides
the stack with a hydrogen-rich fuel. Therefore, it is desirable to provide a
SOFC system
that uses steam reforming of a hydrocarbon fuel.
2s It would be advantageous if a module for use with a SOFC system would
effectively: 1)
completely oxidize the fuel remaining in the SOFC stack anode exhaust gas
using the
stack cathode exhaust gas or other air, and 2) directly utilize the heat
produced by
oxidation of the anode exhaust gases to preheat and prereform all or a desired
portion of
the hydrocarbon/water fuel mixture being fed to the SOFC stack, using a
suitable
catalyst, and 3) directly heat the incoming (to SOFC stack) cathode air.
SUMMARY OF THE INVENTION
An integrated module for use in a solid oxide fuel cell ("SOFC") system is
disclosed
which combines several functions into one unit. In one embodiment, the
integrated
module oxidizes the fuel cell stack anode exhaust using the stack cathode
exhaust or
other air, preheats and prereforms (processes a percentage of, or completely)
the
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incoming hydrocarbonlwater mixture using a suitable catalyst to provide a
hydrogen and
carbon monoxide rich stream for the fuel cell anode, and also further heats
the air
destined for the SOFC cathode.
In one aspect, the present invention comprises a SOFC system including a fuel
cell
to having a fuel intake, an air intake, a cathode exhaust and an anode
exhaust, and
comprising an integrated module comprising an afterburner, a fuel processor
and a heat
exchanger, wherein:
(a) said afterburner comprises an intake connected to the anode
exhaust, or anode and cathode exhausts, and an igniter;
15 (b) said heat exchanger comprises an intake connected to an air supply
and an exhaust connected to the air intake of the SOFC wherein the
heat exchanger is thermally coupled to the afterburner; and
(c) said fuel processor comprises an intake connected to a fuellwater
supply, a fuel reforming catalyst, and an exhaust connected to the
20 fuel intake of the SOFC wherein the fuel processor is thermally
coupled to the heat exchanger and/or the afterburner.
The afterburner burns the unused fuel in the SOFC stack exhaust. The heat
produced by
the afterburner is exchanged by the heat exchanger to preheat the air stream
into the
25 SOFC stack. The fixellwater stream is also preheated and prereformed in the
fuel
processor which also uses heat from the afterburner. The fuel processor
comprises an
effective catalyst so that steam reformation of the hydrocarbon fuel may take
place as it
passes through the fuel processor.
30 In one embodiment the afterburner is comprised of a tubular combustion
chamber and an
igniter. The combustion chamber is encircled by a high temperature heat air
exchanger
for transferring the combustion heat to the incoming air and fuel/water
mixture. The heat
exchanger is itself encircled by the fuel processor. Therefore, the integrated
module may
be comprised of a tubular core, a middle shell which contains the heat
exchanger and an
35 outer shell which contains the fuel processor.
BRTEF DESCRIPTION OF THE DRAWINGS
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Figure 1 is a schematic of a process of the present invention.
Figure 2 is a schematic depiction of an apparatus of the present invention.
to Figure 3 is a view of the afterburner and heat exchanger. Figure 3A is a
view of a burner
assembly.
Figure 4 is a view of the fuel processor.
Figure 5 is a view of the integrated module complete with. an outer-shell.
Figure 6 is a bottom plan view of the apparatus.
DETAILED DESCRIPTION
The present invention provides for an integrated module for use within a SOFC.
When
describing the present invention, all terms not defined herein have their
common art-
recognized meanings.
In one embodiment, the cathode and anode exhausts from the fuel cell stack
passes
through the centre of the module (10) in a generally tubular conduit (12),
referred to
herein as the afterburner. The anode exhaust is directed directly into the
afterburner (12)
through a burner assembly (14). The cathode exhaust enters through a port (17)
and
manifold (1~) as shown in Figure 1, and is directed into the afterburner (12)
adjacent the
3o burner (14). The anode exhaust contains unoxidized fuel species such as a
hydrocarbon
such as methane, carbon monoxide or hydrogen. The cathode exhaust is typically
primarily air (oxygen depleted). It is preferred to use the cathode exhaust in
the
afterburner as that air stream is heated in the SOFC, however, it is not
essential and a
separate air source may be used, or none at all if the anode exhaust itself
contains enough
oxygen to permit complete combustion of the remaining fuel in the afterburner.
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The top end of the afterburner (12) includes a mounting flange (16) which
defines
openings for the afterburner intake, and heat exchanger exhaust and fuel
processor
(reformate) exhaust. As shown in Figure 3A, the burner (14) is mounted to a
similar
flange (15) which allows insertion of the burner tube (14) into the
afterburner (12).
An igniter (20) is inserted at the far end of the afterburner (12) which is
used for igniting
the afterburner (12) on cold system starts. The igniter (20) may be in
operation only to
initiate combustion, and then can be turned off. The igniter may be a pilot
flame, an
electronic spark device or other ignition means.
In one embodiment, the fuel burner tube (14) is contained in the afterburner
(12) to
control mixing of anode and cathode exhausts as depicted in Figure 5. Other
combustion
technologies such as sintered metal or porous ceramic nozzles, or other well
known
combustion/burner means can be utilized. Controlled mixing is required during
normal
operation as the afterburner operating temperatures may exceed the auto-
ignition
2o temperature of the fuel species present in the anode exhaust. The burner
tube (14)
contains small holes for the fuel gas mixture to escape and also acts as a
burner flame
support.
Surrounding the afterburner (12) are the heat receiving portions of the
integrated module
(10). In one embodiment, a high temperature air heat exchanger (24) encircles
the
afterburner (12) as is shown in Figure 3. The heat exchanger (24) transfers
heat energy
from the afterburner (12) to the air stream destined for the cathode in the
SOFC. Air
enters the heat exchanger through a port (26) and exits through tubes (28) and
port (30).
A coupling (32) where a thermocouple or gas sampling means may be attached may
be
3o provided. In one embodiment, the interior of the heat exchanger (24) is
baffled to route
the air through a tortuous path, increasing the potential for heat transfer to
the air. As
shown in Figure 3, the baffle (34) may take the form of a continuous spiral
baffle which
routes the air through the heat exchanger is a substantially helical fashion.
As is
apparent, the baffle also serves to conduct heat into the heat exchanger from
the
afterburner. The baffling may take any configuration which serves~to route air
through a
tortuous path and to conduct heat.
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The heat exchanger is contained within a middle shell (40) as shown u1 the
Figures,
which also serves as the inner wall of the fuel processor (42). The fuel
processor (42)
may also be referred to as a prereformer. A hydrocarbon, preferably natural
gas, is
combined with water and is passed through the fuel processor (42) which
includes a
suitable steam reforming catalyst such as a nickel/alumina catalyst. Suitable
catalysts for
to steam reformation of methane are well known in the art. The hydrocarbon and
steam
react in an endothermic reaction to produce hydrogen and carbon monoxide. The
thermal
energy released from the afterburner (12) is used to drive this endothermic
reaction. In
one embodiment, the catalyst is in pellet form contained within the fuel
processor
chamber (42) contained by a perforated baffle plate (44). The
hydrocarbon/water feed
enters through a port (46) and the reformate exits through tube (4~) and port
(50). The
fuel processor (42) may also include a plurality of fuel processor fins (54)
which radiate
outwards from the shell (40) and which serve to contain the catalyst and to
conduct heat
into the fuel processor (42). The fuel processor is contained within an outer-
shell (52)
which is the outer shell for the module (10).
In the embodiment shown and described, the afterburner, heat exchanger and
fuel
processor are concentric cylinders. However, in alternative embodiments, the
three
elements may take different shapes or configurations. What is necessary is
that the
afterburner receive and burn fuel from the SOFC exhaust and provide heat to
the heat
exchanger and fuel processor.
The integrated module (10) may also function in conjunction with a low
temperature heat
exchanger (not shown) that preheats the incoming raw air and fuel/water
mixture. The
configuration of the low temperature heat exchanger and the integrated module
is shown
schematically in Figure 1. The fuel oil water may be combined before entering
heat
exchanger or afterwards, before entering the fuel processor (42). The
preheating of the
air and fuel is preferably done in stages so as to avoid large thermal
stresses upon the heat
exchangers.
The integrated module is thermally integrated with the SOFC stack as is
described herein
and as will be apparent to one skilled in the art. The described integrated
module (10)
offers a unique functional thermal system during heatup, normal operation, and
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transients. During transients of the power load on the fuel cell stack, and
changes in air
and fuel flow rates, the integrated module (10) offers excellent response. For
example, if
the stack electrical load is decreased, the heat generated in the stack (the
waste heat
portion of the fuel cell reaction) also decreases. However, the afterburner
(12) in the
integrated module (10) responds automatically due to the change in incoming
fuel
to composition, and increases in temperature. The increase in afterburner
temperature
increases the temperature of the air and fuel fed to the stack, thus
maintaining a relatively
constant stack temperature.
During startup, where both the SOFC stack and integrated module are cold, the
ftxel
passes through the fuel processor (42) without being reformed and the SOFC
without
being oxidized. Accordingly, the afterburner receives substantially all of the
ftiel and
thus operates at the upper end of its temperature range. As a result of the
elevated
afterburner temperature, the air feed to the SOFC stack heats rapidly and the
fuel
processor heats up as well. When the fuel processor reaches a temperature
sufficient to
2o support the endothermic steam reformation reaction, the hydrocarbon is
converted to
hydrogen and carbon monoxide. The hydrogen and carbon monoxide are fuel
species
utilizable in the SOFC. When the SOFC heats up to operating temperature, the
amount
of fuel which reaches the afterburner is reduced, reducing the temperature of
the
afterburner and therefore the temperature of the air and fuel being fed to the
stack.
At any time when the stacks are not producing electrical power, such as during
startup,
the afterburner (12) is the sole or main source of heat to bring the fuel cell
stack (and thus
complete system due to its thermal integration) up to operating temperature.
However,
additional system burners can be added to provide a faster warm up from a cold
start, or
3o provide more rapid changes from one operating temperature to another. In
normal
continuous operation, the afterburner consumes hydrogen, carbon monoxide and
any
hydrocarbon fuel not consumed by the fuel cell. In the current embodiment,
during heat
up, standby and normal operation, the mixture fed through the afterburner (12)
is the
exhaust from the fuel cell stacks. When the fuel processor (42) and stack are
in a
temperature range when fuel reforming is possible, such as in normal
operation, hydrogen
and carbon monoxide are the major fuel species found in the fuel (anode
exhaust to
afterburner) together with a small amount of raw fuel such as methane. The
burner (14)
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may be optimised for this mixture, while still being able to burn natural gas
(or other raw
fuel) and air mixtures during a cold start.
As a by-product of the electrochemical reaction to generate electricity in the
fuel cells,
hydrogen and carbon monoxide formed in the fuel processor (42) are converted
to water
to and C02. The water is in vapour form as it exhausts from the fuel cell
stack due to the
high temperature and passes through the afterburner (12) as superheated steam.
In the
afterburner (12), substantially complete oxidation of all fuel species occurs,
resulting in a
high temperature exhaust stream only containing water vapour, carbon dioxide
and
nitrogen, and usually excess oxygen. The afterburner feed gas (anode and
cathode stack
exhausts combined) is preferably fuel lean to stoichiometric to reduce the
possibility of
unoxidized fuel leaving the system. Typical air stoichiometries for the
combustion
reaction in the afterburner are about 1.0 to about 3Ø After combustion, the
afterburner
combustion products are exposed to the high temperature heat exchanger (24) in
the
integrated module (10), and the low temperature heat exchanger located outside
2o integrated module, where the combustion products give up a substantial
portion of their
heat to the incoming fuel and air flows, and then is exhausted to the
atmosphere or
another heat recovery system. As for the fuel stream, water is inj ected in a
preheated
hydrocarbon fuel gas prior to it entering into the prereformer (42) through
the fuel/water
inlet (46). In another embodiment,, air or oxygen can also be added to the
hydrocarbon/water mixture passing through the fuel processor portion of the
integrated
module (10) to realize autothermal reforming through partial oxidation of the
hydrocarbon within the fuel processor. When in the fuel processor (42), the
hydrocarbon
fuel / water mixture reacts, converting the incoming gases to a hydrogen and
carbon
monoxide rich stream when heat is supplied from the afterburner (20). Normally
this is
3o done with a steam to carbon ratio of 1.3:1 to 3.0:1.0 to ensure that solid
carbon is not
formed when the hydrocarbon / water mixture is heated. After conversion by
steam
reforming, the hot gas composition is generally dictated by the gas
temperature and
related thermodynamic gas equilibrium.
If pure hydrogen is available as a fuel, instead of a hydrocarbon, the fuel
processor may
be converted to a hydrogen pre-heater by simply removing the catalyst. In such
hydrogen
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systems, the requirement for water, or air, to be added to the fuel stream is
unlikely, but
will not necessarily have to be removed.
As will be apparent to those skilled in the art, various modifications,
adaptations and
variations of the foregoing specific disclosure can be made without departing
from the
to scope of the invention claimed herein.
s
