Note: Descriptions are shown in the official language in which they were submitted.
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AN ELECTROCHEMICAL FUEL CELL GENERATOR HAVING
AN INTERNAL AND LEAK TIGHT HYDROCARBON FUEL REFORMER
~~vernment Contract:
The Government of the United States of America has rights in this invention
pursuant to Contract No. DE-FC21-91MC28055, awarded by the United States
Department of Energy
2. Field ~ the Invention:
The invention relates to the field of electrochemical generators and
configurations thereof comprised of solid electrolyte fuel cells which
generate
electricity from air and fuel gas for electrical power stations. The invention
more
particularly relates to the field of high temperature) solid oxide electrolyte
fuel cell
generators and configurations thereof containing internal hydrocarbon fuel gas
reformers which precondition hydrocarbon feed fuels prior to electrochemically
processing is the fuel cell stack of the generator. Even more particularly )
the invention
relates to the field of internal hydrocarbon reformers and improved
configurations
thereof which are used inside the fuel cell stack of high temperature ) solid
oxide
electrolyte fuel cell generators and which perform a dual function as a
hydrocarbon
reformer anc 'uel ceU stack divider. The invention especially provides a
combination
internal hydrocarbon fuel reformer and fuel cell stack divider configuration
with a gas
barrier to reduce fuel gas leakage therethrough and enhance structural
integrity thereof.
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3. Back~~round of the Invention:
High temperature, solid oxide electrolyte fuel cells and mufti-cell generators
and
configurations thereof are well known) and taught, for example, in U.S. Patent
Nos.
4,395,468 (Isenberg) and 4,490,4.4.4 (Isenberg). The solid oxide fuel cell
generator is
designed to convert chemical fuel derived from hydrocarbons into direct
current (DC)
electricity. The solid oxide fuel cell generator is conventionally operated at
temperatures between about 600 ~ C and 1, 200 ~ C ) more particularly about
800 ~ C to
1,050~C) to render the solid oxide electrolyte sufficiently electrically
conductive for
electrochemical reactions which generate electricity.
In such mufti-cell generators) a plurality of electrically connected tubular
solid
oxide fuel cells are placed in a generator chamber defined by an alumina board
housing, otherwise known as a fuel cell stack, and are exposed to a supply of
gaseous
oxidant and reformed gaseous hydrocarbon feed fuel. In larger mufti-cell
generators,
divider boards of insulation material such as alumina boards are placed
between either
individual fuel cells or a plurality of fuel cells, otherwise known as cell
bundles) for
thermal and electrical insulation) and also for internal structural support of
the
generator, as taught in U.S. Patent Nos. 4,876,163 (Reichner) and 4,808,491
(Reichner). The divider boards are typically used to separate rows of cell
bundles)
which cell bundles typically contain from 12 to 36 fuel cells.
Mufti-cell generators feature a plurality of parallel) elongated tubular solid
oxide
fuel cells arraaged in cell bundles. Each tubular solid oxide fuel cell is
made of an
inner porous air electrode of, for example, strontium-doped lanthanum
manganite. A
dense) gas-tight solid oxide electrolyte of, for example, yttria stabilized
zirconia covers
the air electrode, except in a strip along the entire active cell length. This
strip of
exposed length is covered by an interconnect of dense, gas-tight layer of, for
example,
magnesium-doped lanthanum chromite which serves as the electric contacting
area to
an adjacent fuel cell or to a power contact. A porous fuel electrode of, for
example)
nickel-zirconia cermet covers the solid oxide electrolyte except in the
vicinity of the
interconnect. Spent fuel is combusted with spent oxidant in a separate
combustion
chamber and exits the generator as hot exhaust gas.
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In these high temperature, solid oxide multi-cell generators, air and fuel are
combined to form heat and electricity through electrochemical reactions. The
fuel can
be derived from fossil fuels such as coal derived fuel gas, natural gas, or
distillate fuel.
Each solid oxide fuel cell readily conducts oxygen ions from the air electrode
(cathode)
of the fuel cell in contact with the air) where the oxygen ions are formed,
through the
solid oxide electrolyte of the fuel cell between the air electrode and fuel
electrode to
the fuel electrode (anode). The oxygen ions then react with carbon monoxide
(CO)
and/or hydrogen (H~ derived from a reformed hydrocarbon fuel gas to deliver
electrons and produce electricity.
However, the direct use of hydrocarbon fuels as a fuel to the fuel cells of
the
generator) such as methane, ethane, mixtures of hydrocarbons such as natural
gas
(mostly methane plus ethane, propane) butane, and nitrogen), vaporized
petroleum
distillates such as naphtha, or alcohols such as ethyl alcohol) is
undesirable. These
hydrocarbons form undesirable carbon deposits and soot on the fuel cells and
other
components of the generator if used directly as the fuel gas. This can reduce
the
efficiency of the fuel cells and can interfere with proper generator
operations. For
instance) carbon deposition on the fuel cells may block gas transport paths in
the
porous electrodes and provide electrical short-circuit paths between the
electrodes.
Carbon deposition on other generator components such as insulation materials
may
reduce insulation effectiveness and provide electrical short-circuit paths
between fuel
cell bundles through separating insulation.
Accordingly) the fuels that have been supplied in the fuel cell generator
generally have been limited to carbon monoxide (CO) and hydrogen (H~. The
carbon
monoxide and hydrogen fuels can be obtained by reforming hydrocarbon fuel
gases.
Reforming is a process in which the reformable hydrocarbon fuel is combined
with
water vapor andl or carbon dioxide to produce carbon monoxide and hydrogen.
For
example, the reforming of methane using water and carbon dioxide is given by
Equatioas (1) and (2).
CH, + HZO ~ CO + 3H2 ( 1 )
CH, + COZ ~ 2C0 + 2H~ (2)
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Therefore) in order to e~cicntly use the hydrocarbon fuels without the
detrimental effects in the fuel cell generator, fresh hydrocarbon feed fuel
gases are
combined with water vapor and/or carbon dioxide) typically obtained from
rtcirculated
spent fuel gas) to form a reformable fuel mixture. This reformable fuel
mixture is then
reformed) that is) converted to carbon monoxide and hydrogen, through the use
of a
reforming catalyst) usually platinum or nickel compounds and usually supported
oa
alumi.na in the form of pellets or boards. The reformed fuel is then used as
the fuel
gas to the solid oxide fuel cells in the fuel cell stack of the generator.
Since hydrocarbon reforming is endothermic (i.e., requires a supply of heat))
heat must be supplied to the reaction. Reforming hydrocarbon fuel outside of
the fuel
cell generator is undesirable as it results in a loss of energy as heat in the
reformer and
in connecting conduits between the generator and the reformer) makes the
generator
system more complicated (i.e., requires heat exchangers) pumps) and more
space) and
is more expensive overall. U.S. Pat. No. 4) I28,700 illustrates reforming of
fuel
outside of the fuel call generator. Attempts have betn made to reform
hydrocarbon
fuel inside the generator which is desirable) especially since reforming is
best
performed at temperatures of about 900~C to 1,000~C which is close to that of
the
solid oxide fuel cell operation of generally about 600~C to 1,200~C and more
particularly about 800~C to 1,050~C.
In U. S. Pat. No. 4,374) 1 &4 (Somers . )) an attempt to solve this problem
was made by intrrnal reforming on a deliberaueiy constructed inactive end of
each
~lar fuel cell. This process relieved) somewhat) the excessive thermal
gradients is
ttse fuel ctll stack. This) however) cut down dramatically on active fuel cell
area
within ttse fuel cell stack. In U.S. Pat. No. 4,729,93I (Grimbie), internal
reforming
is done using a catalytic packing such as finely divided aicke! or platinum
for
reformation of hydrocarbon fuels, which is piactd in a catalyst chamber
adjactnt to the
fuel cell generator chamber and external to the fuel ctll stack. In this
arrangement, a
hydrocarbon fuel gas is fed into a nozzle and is mixed with recirculat~d spent
fuel gas
containing water vapor and carbon dioxide) and this reformable gas mixture is
drawn
along side the generator chamber in heat transfer communication therewith into
the
catalytic packing where it is reformed, and then the reformed gas is passed
into, the fuel
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plenum to the fuel cells in the generator chamber. In U.S. Pat. No. 4,808,491
(Reichner)) internal reforming is done using the hot exhaust gas of the
generator as the
heat source for reforming) which exhaust gas is passed in heat transfer
communication
. with a reformer catalyst bed external to the fuel cell stack but directly
underneath the
S closed ends of the fuel cells.
With the methods described previously for internal reforming, it is still
difficult
to transfer heat necessary for the endothermic reforming reaction without the
creation
of excessive temperature gradients within the fuel cell stack and in the
reformer. Air
flow to the fuel cells consequently must be increased beyond that required for
electrochemical reaction with the fuel, to prevent excessive temperature
gradients. One
attempt to solve this heat transfer problem is in U.S. Pat. No. 4,983,471
(Reichner,
et al.) in which a reformable fuel mixture channel is passed through the axial
length
of the fuel cell stack. A reformable fuel mixture of combined recirculated hot
spent
fuel gas and fresh hydrocarbon fuel to be reformed is passed in the channel
and
through entry ports into the fuel cell stack along the length of the fuel
cells with an
axis transverse to the fuel cells. In the fuel cell stack, the refortnable
mixture contacts
a reforming material which is distributed along the length of the fuel cells
within the
fuel cell stack) such that the mixture can pass transversely through the
reforming
material such as nickel impregnated on porous partition boards prior to
contacting the
fuel cells.
Another attempt to solve this problem is in U.S. Pat. No. 5,082,75I
(Reichner),
in which internal reforming in the solid oxide fuel cell generator is
performed on
individual fuel cell reformer-dividers or cell bundle reformer-dividers within
the fuel
cell stack. In this design, the axially elongated tubular fuel cells or cell
bundles in the
fuel cell stack are separated by elongated dividers which can be made of
porous
alumina insulation material, such as alumina boards, which are coated or
impregnated
with reforming catalyst. The reformer-divider boards serve a dual purpose of
separating individual fuel cells or cell bundles for internal structural
support and
reforming reformable hydrocarbon fuel mixtures in order to be fed as a fuel to
the solid
oxide fuel cells of the generator. In Reichner '751 ) the reformer-dividers
are elongated
and positioned between the fuel cell bundles) to separate and form a wall
between the
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cell bundles. The reformer-dividers are further hollowed along a selected
portion of
their length and impregnated with reforming catalyst, to form a reforming
channel
therein having solid elongated partition walls exposed to the cell bundles. A
reformable fuel mixture inlet into the reforming channel is provided as well
as a
reformed fuel gas exit, to allow passage of reformed fuel to the fuel inlet
plenum
beneath the solid oxide fuel cells. In this internal reformer configuration)
reformer
heat consumption is distributed along the axial length of the fuel cells at
multiple
locations between fuel bundles. Thus, the area for heat exchange is greatly
increased
and the excess heat to be removed by excess air flow is significantly reduced.
This
configuration also uses space which is already present between the fuel cells
and does
not reduce the active area of the fuel cells.
In operation of the Reichner '751 configuration, unreformed hydrocarbon fuel
leakage occurs through the reformer-divider boards into the fuel cell stack.
This
remains a substantial problem to the efficiency of this internal reformer-
divider
configuration. Lxakage is due to the fact that these reformer-divider boards
are
fabricated from porous (low density) alumina insulation materials. Lxakage of
uirreformed fuel into the fuel cell stack can result in carbon deposition on
the fuel cells
and other generator components which is undesirable. Unsuccessful attempts
have been
made to prevent unreformed fuel leakage through the porous alumina board
material
by densifying or plasma spraying a brittle ceramic coating on the exterior
surface of
the boards to prevent leakage. This, however, does not significantly cut down
the
leakage because of cracking of the external coating as well as the alumina
boards.
Cracking occurs as a result of thermal stresses arising from large temperature
gradients
on the face of boards) and by bowing caused from temperature gradients through
the
board.
What is needed is an internal hydrocarbon reformer for a high temperature,
solid oxide fuel cell generator which performs both hydrocarbon fuel reforming
within
the fuel cell stack of the generator and separation of the fuel cells or cell
bundles for
support but without unreformed fuel leakage associated with prior designs.
The present invention provides better configurations for internal reformer-
dividers within the fuel cell stack of an electrochemical fuel ceU generator
which serve
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the dual purpose of reformer for hydrocarbon feed fuels and separator for fuel
cell
stacks. In this invention, the inventors have solved the problems of) inter
alia)
hydrocarbon fuel leakage through the reformer-divider boards, structural
integrity of
the boards when subject to thermal expansion) and manufacturabiliry of the
boards.
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4. Summary of the Invention:
It is an object of the invention to provide an electrochemical fuel cell
generator
apparatus with an internal hydrocarbon reformer-divider configuration inside
the fuel
cell stack which eliminates the problems associated with prior designs.
It is another object of the invention to provide an internal hydrocarbon
reformer-divider configuration in an electrochemical fuel cell generator
apparatus
having a significant reduction in unreformed hydrocarbon fuel leakage through
the
reformer-divider boards to the fuel cells in the fuel cell stack of the
generator.
It is yet another object of the invention to provide an internal hydrocarbon
reformer-divider configuration inside the fuel cell stack of a high
temperature) solid
oxide electrolyte fuel cell generator, where the reformer-divider acts as a
combination
reformer for hydrocarbon fuel gas and separator for the individual fuel cells
or cell
bundles, and further where the reformer is provided with a gas barrier means
to
prevent unreformed fuel leakage into the fuel cell stack and without
inhibiting thermal
expansion due to temperature gradients between the fuel cell stack and
reformer.
It is an advantage of the invention to significantly reduce unreformed fuel
leakage from the reformer-divider configuration to the fuel cells.
It is another advantage of the invention to allow for thermal expansion within
the reformer-divider configuration to improve structural integrity of the
reformer-
divider.
Accordingly, the invention resides in an electrochemical generator
configuration, such as a high temperature, solid oxide electrolyte fuel cell
generator
configuration characterized by: a generator chamber containing a fuel cell
assembly
comprising one or more fuel cell bundles, each cell bundle containing a
plurality of
electrically connected) axially elongated fuel cells, each fue! cell
containing an outer
fuel electrode, an inner air electrode) and solid oxide electrolyte
therebetween; a fresh
gaseous feed hydrocarbon fuel inlet into the generator chamber for fuel to
pass over
the outside of the outer fuel electrode; a gaseous feed oxidant inlet into the
generator
chamber for oxidant to pass inside the inner air electrode; at least one
gaseous spent
fuel exit channel where the spent fuel containing water vapor and/or carbon
dioxide
from the generator chamber is mixed with the fresh hydrocarbon feed fuel
inlet; a
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combustion chamber from the generator chamber for combustion of spent fuel and
spent oxidant; and, at least one combusted gas exhaust channel from the
combustion
chamber; the generator further characterized by: having one or more elongated
dividers
that are axially positioned between the fuel cells or fuel cell bundles along
the axial
length of the fuel cells or cell bundles to provide partitions between cells
andlor cell
bundles; and the generator even further characterized by: including in at
least one of
the one or more elongated dividers) a hollow channel along a selected portion
of its
length) the hollowed elongated divider having solid elongated walls, a
reformable fuel
mixture inlet for passage of reformable fuel mixture to be reformed in the
hollow
channel) a reformed fuel exit for passage of reformed fuel to the fuel cells,
and further
containing a catalytic reforming material in the hollowed channel, the
dividers also
containing a means effective to prevent unreformed gas leakage of the
refotmable fuel
mixture from the dividers to the fuel cells without inhibiting thermal
expansion of the
dividers.
More specifically, the invention resides in an internal reformer within the
fuel
cell stack of a high temperature, solid oxide fuel cell generator, the
reformer
characterized by having one or more reformer-divider boards passing through
the axial
length of the fuel cell stack and positioned in between one or more axially
elongated
fuel cell bundles or axially elongated fuel cells to separate the cell bundles
or fuel cells,
where the one or more reformer boards are hollowed and impregnated with
catalytic
reforming material in the hollowed area having solid exterior walls, a
reformabIe fuel
mixture inlet to the hollowed area and reformed fuel outlet from the hollowed
area to
the fuel cells, and also including a means effective to prevent unreformed gas
leakage
of the reformable fuel mixture through the exterior walls of the reformer-
divider
boards, characterized by surrounding the walls of the hollowed reformer boards
except
at the reformabie fuel mixture inlet with a metallic foil layer of nickel
foil, Inconel
foil, or other suitable nickel based alloy foil) to provide a barrier against
unreformed
fuel gas diffusion to the fuel cells, and further surrounding the metallic
foil layer
except at the reformable fuel mixture inlet with a housing made from the same
material
as the reformer boards to prevent short circuit of the generator.
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5. Brief Description of the Drawings:
There are shown in the drawings certain exemplary embodiments of the
invention as presently preferred. It should be understood that the invention
is not
limited to the embodiments disclosed, and is capable of variation within the
spirit and
scope of the appended claims. In the drawings,
FIGURE 1 is a side view in section of one embodiment of an electrochemical
generator apparatus of the invention, showing two fuel cell bundles, each
bundle
containing a plurality of fuel cells) and further showing the bundles being
separated by
a reformer-divider board having a gas barrier substantially impervious to fuel
leakage;
FIGURE 2 is a enlarged side view in section of a portion of an electrochemical
generator apparatus of the invention) showing two fuel cell bundles) each
bundle
containing a plurality of fuel cells) and further showing the bundles being
separated by
a reformer-divider board having a gas barrier impervious to fuel leakage;
FIGURE 3 is a side view of an internal reformer-divider board of the invention
positionable within the fuel cell stack between fuel cells or cell bundles and
having a
gas barrier cutaway at a portion to show the mufti-part configuration thereof;
FIGURE 4 is a bottom view of the reformer-divider board of FIGURE 3 along
line 4-4;
FIGURE 5 is a top view of the reformer-divider board of FIGURE 3 along line
5-5;
FIGURE 6 is a side view in section of the reformer-divider board of FIGURE
3 along line 6-6;
FIGURE 7 is side view of an alternate embodiment of an internal reformer-
divider board of the invention.positionable within the fuel cell stack between
fuel cells
or cell bundles and having a gas barrier cutaway at a portion to show the
mufti-part
configuration;
FIGURE 8 is a bottom view of the reformer-divider board of FIGURE 7 along
line 8-8;
FIGURE 9 is another alternative embodiment of an internal reformer-divider
board of the invention positionable within the fuel cell stack between fuel
cells or cell
bundles and having a gas barrier; and)
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FIGURE 10 is another alternative embodiment of an intet~nal refo~rner-divider
board of the invention positionable within the fuel cell stack between fuel
cells or cell
bundles and having a gas barrier.
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6. Detailed Description of the Preferred Embodiments of the Invention:
Referring now to FIGURE 1, an exemplary electrochemical generator apparatus
(10), for example, a high temperature) solid oxide eiectrolyte fuel cell
(SOFC)
generator) is shown. An outer housing ( 12) surrounds the entire
electrochemical
apparatus. The outer housing can be made of metal such as steel. An inner
housing
( 14) surrounds a plurality of chambers including a generator chamber ( 16)
and a
combustion chamber ( 18). The inner housing ( 14) can be made of high
temperature
and oxidation resistant metal such as Inconei or similar material. Thermal
insulation
(20) generally fines the inside of the outer housing (12) and also surrounds
a11 of the
chambers. The insulation (20) can be made of low density, alumina material)
such as
alumina felt or alumina insulation boards.
The generator chamber ( 16) (also referred to herein as the "fuel cell stack")
contains one or more cell bundles) and, as shown, contains two cell bundles
(22) and
(24), each bundle containing a plurality of parallel, axially elongated,
preferably
tubular, electrochemical cells (26), for example, high temperature, solid
oxide
electrolyte fuel cells (SOFCs). Each fuel cell (26) contains an outer, porous
fuel
electrode (otherwise referred to herein as the "anode") (28) covering its
axially
elongated exterior surface, an inner, porous air electrode (otherwise referred
to herein
as the "cathode") (30) covering its axially elongated interior surface, and a
dense) gas-
tight solid oxide electrolyte (32) sandwiched between the fuel electrode (28)
and the air
electrode (30). The inner air electrode (30) can be a doped ceramic of the
perovskite
family, for example, strontium-doped lanthanum manganite (LaMn03), the solid
oxide
electrolyte (32) can be a dense, gas-tight yttria- or scandia-stabilized
zirconia (Ztfl~,
and the outer fuel electrode (28) can be a porous nickel-zirconia cermet.
The inner air electrode (30) can be supported on an optional porous ceramic
support tube (not shown) of calcia-stabilized zirconia. Both the outer fuel
eiectrode
(28) and solid electrolyte (32) are typically discontinuous in a selected
segment along
the axial length of the inner air electrode (not shown) to allow for inclusion
of an
interconnect (not shown) on the air electrode (30) to provide means to
electrically
connect adjacent fuel cells (26). The interconnect can be a magnesium-doped
lanthanum chromite (LaCrO~) and can also include a top cover (not shown) of
nickel-
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zirconia cermet. The electrical interconnection via the interconnect of
adjacent fuel
cells (26) is enhanced by a porous metal felt (34), for example, a nickel
fiber felt)
positioned along the active lengths of the fuel cells) preferably through
direct contact
of the outer surfaces.
In the drawings) each fuel cell (26) is shown as being tubular) hut of course
other configurations are possible, such as planar-shaped fuel cell
configurations. It
should also be recognized that the relative positioning of the air electrode
(30) and fuel
electrode (28) can be inverted as well, so long as the fuel electrode remains
in contact
with fuel and the air electrode remains in contact with air or oxygen.
Penetrating the outer housing ( 12) and thermal insulation (20) are ports for
electrical leads (not shown) as well as ports for the electrochemical
reactants of fuel
and oxidant. A fresh hydrocarbon fuel reactant inlet port (36) is shown, the
hydrocarbon feed fuel, being shown as (F7, passing therein, which feed fuel (~
typically is unreformed natural gas comprising in most part methane. The
hydrocarbon
feed fuel (~ is directed to pass through a series of conduits for fuel
conditioning such
as in a reforming channel prior to passing over the outer fuel electrode as
more fully
described hereinbelow. Also shown is a fresh oxidant reactant Met port (38))
the
oxidant, being shown as (O), passing therein, which oxidant (O) typically is
air or
oxygen. The oxidant (O) is directed to pass through a series of conduits prior
to
passing over the interior air electrode as more fully described hereinbelow)
The generator chamber ( 16) extends between a fuel distribution plate (40) and
a porous barrier (42). The porous barrier (42) is designed to allow partially
reacted
or spent fuel gas, being shown as (Sl;~) to exit the generator chamber (16)
after passing
over the exterior of the fuel cells for combustion with partially reacted or
spent
oxidant) being shown as (SO)) after passing through the interior of the fuel
cells in the
combustion chamber ( 18)) to form hot combusted exhaust gas, being shown as
(E))
which passes through a combusted exhaust channel (44) and into the atmosphere.
The
combusted exhaust channel can be made of high temperature and oxidation
resistant
metal such as Inconel. A portion of the spent fuel (SF~ containing water vapor
andlor
carbon dioxide not being directed into the combustion chamber ( 18), can be
directed
in a spent fuel recirculation channel (46), to combine with fresh hydrocarbon
feed fuel
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(F) at a mixing chamber (48) to form a reformable fuel mixture (RF?~ in order
to
supply the oxygen species needed for fuel reformation reactions and to
facilitate
hydrocarbon reforming of the unreformed feed fuel (F). The spent fuel
recirculation
channel can be made of high temperature and oxidation resistant metal such as
Inconel.
Spaced between the fuel distribution plate (40) are generator chamber entry
ports (50)
for entry of reformed fuel gas into the fuel cell stack ( 16) to the fuel
electrode (28) of
the fuel cells (26).
The elongated) preferably tubular) fuel cells (26) extend in the generator
chamtxr between the fuel distribution place (40) and the porous barrier (42).
Each fuel
cell (26) has an open end (52) in the combustion chamber ( 18) and a closed
end (54)
in the generator chamber (16) tzar the fuel distribution plate (40). The open
ends (52)
of the fuel cells (26) contain oxidant feed conduits (56), such as oxidant
riser tubes
positioned therein.
In such a fuel cell generator arrangement (10), the inventors have discovered
a new way to reform the fresh gaseous hydrocarbon feed fuel (F~ prior to
contacting
the fuel electrode (28) of the fuel cells (26) to reformed fuel (R~ inside the
fuel cell
stack (16) using improved reformer-separator (also referred to herein as
"reformer-
divider") configurations without appreciable unreformed fuel (>7 leakage into
the fuel
cell stack prior to fuel reformation The reformer-dividors of the invention
arc used
to both divide and support the fuel cells (2~ or cell bundles (22) 24) within
the fuel
cell stack (1~) and also support other generator components) while also
substantially
eliminating utbdesirable unreformed fuel gas (F~ leakage into the fuel cell
stack ( 16) to
contact the fuel cells (2~ due to porosity of the reformer-divider boards. The
reformcr-
divider configurations also substantially eliminate undesirable structural
degradation of
the reformer~ivider due to thermal expansion of the boards.
In once arrangement of the invention, a plurality of axially elongated,
preferably
tubular) fuel cells (26), forming cell bundles (22) and (24), are separated by
elongated
dividers ('~0~ extending between the porous barrier (42) and the fuel
distribution board
(40) and which are used to divide the fuel ctll stack (1~. These dividers (58)
can be
trade of solid pieczs of porous alumina boards of suitable thickness and are
posiaorKd
within the fuel cell stack ( 16) to separate the stack and provide internal
structural
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integrity to the generator. In the present invention) at least one ocher
divider will be
a dual purpose reformer-divider (60) as shown.
The reformer-divider (60) is positioned within the fuel cell stack between
individual fuel cells {26) or cell bundles (22) and (24), extending between
the porous
barrier (42) and fuel distribution plate (40), to provide a means for
reforming the fresh
hydrocarbon feed fuel (~ to reformed fuel {RF) prior to passing over the outer
fuel
electrodes (28) of the fuel cells (26). The reformer-divider (60) can be made
of a
porous alumina board. The reformer-divider (b0), unlike conventional reformer-
dividers located within the fuel cell stack) as taught in U.S. Pat. No.
5,082,751
(Reichner)) also contains an effective means to prevent unreformed fuel gas
(F) from
leaking through the external walls of the porous alumina boards and thereby
passing
over the outer fuel electrodes (28) prior to being reformed to reformed fuel
(RF). A
gas diffusion barrier means as more fully described hereinbelow is provided in
the
reformer-divider boards (60) to eliminate gas leakage due to diffusion and is
also
arranged therein to eliminate structural degradation of the reformer-divider
boards (60)
due to thermal expansion.
Referring now to FIGURE 2) each of the reformer-dividers (60) include an
inner board (62) having a hollowed inner channel (64) surrounded by solid
elongated
walls (66), the inner channel having an opened end or entrance (68) for the
gaseous
reformable fuel mixture (RFM) (e.g., natural gas combined with spent fuel)
near the
closed ends (54) of the fuel cells (26) and a closed end (70) near the opened
ends (52)
of the fuel cells (26). The inner channel (64) can, for example) be formed by
tubes
(72) or by a partition (74), both of which will allow the reformable fuel
mixture (RFIv1?
to pass inside the reformer-divider (60) to the closed top end (70) of the
reformer-
divider (60) and then back to exit through the bottom opened end (68) as
reformed fuel
(RF) to the fuel cells (26) in the fuel cell stack ( 16), via a fuel
distribution plenum (88)
and fuel ports (50) near the closed ends (54) of the fuel cells (26). The
reformer-
divider (60) of the invention also includes a gas barrier (76) means
substantially
surrounding the solid elongated walls (66) except at the opened entrance (68)
and an
outer board housing (78) substantially surrounding the reformer-divider (50)
and gas
barrier (76) assembly except at the opened entrance (68).
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As shown in the drawing) when tubes (72) are used the reformable fuel mixture
(RFM) passes from entrance (68), through the inside of the riser tube (72), to
the top
of the tube where the reformable fuel mixture exits near the closed end (70))
reverses
flow, and passes back down the reformer-divider (60) to exit as reformed fuel
(RF) at
entrance (68). When a single partition (74) is used) the refolmabIe fuel
mixture (RFM)
passes from entrance (68), along one side of the partition, in the channel
formed by the
partition and the divider walls, to the top of the partition, over the top of
the partition)
where the reformable fuel mixture (RFM) reverses flow and passes back down the
reformer-divider (60) in the channel formed by the partition and the other
divider wall.
Catalytic reforming material will be disposed either within the cross-section
of
the reformer-divider (60), for example, as a coating on or within the inner
side walls
(80) of the hollow reformer-divider, or as a packing within the chamber area
(82)
between reformable fuel riser tube and the side walls of the reformer-divider;
or in the
feed side (84) and/or return side (86) formed by partition (74)) that is, on
one side or
both sides of partition. The catalytic reforming material will (80, 82) 84 or
86) contain
a catalyst effective to reform hydrocarbon feed fuel (F}) and if used as a bed
in
portions (82) 84, or 86) it should not be Backed so tightly as to excessively
restrict gas
flow. The reforming material will preferably contain at least one of platinum
and
nickel, and will most preferably contain nickel. The reforming material can be
in the
form of a film, a coating, metal fibers, high surface area pellets or
particles by
themselves or with alumina filaments, as a coating on alumina filaments, and
the like
and can also contain effective amounts of additives that will help to control
carbon
deposition.
This reforming is a process in which the reformabIe hydrocarbon fuel (F) is
combined with water vapor (steam) and/or carbon dioxide, preferably from the
spent
fuel (SF)) to provide a reformable fuel mixture (RFM) which when contacted
with
catalytic reforming material as to the hydrocarbon fuel) will produce carbon
monoxide
(CO) and hydrogen (H~ in a heat environment) preferably about 900~C. For
example)
the reforming of methane and ethane (natural gas) is given by Equations (1)-
(4):
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CH, + HZO '~ CO + 3H2 (I)
CH, + COZ "~' 2C0 + 2HZ (2)
CZIi6 + ~ 2CO + 5H2 (3)
2H0
CZHb + ~ 4C0 + 3H2 (4)
2C02
Excess water is generally provided in the reformable fuel mixture (RFM) to
reduce the
tendency for carbon deposition. After reformation) the reformed fuel (RF7
passes
outside of the reformer-divider (60) to contact the fuel cells (26), as
through
distribution plenum (88) fuel stack entry ports (SO) formed through the fuel
distribution
piste (40) near the bottom closed end (54) of the fuel cells (26).
In a preferred example) the channels will be a series of hollow ceramic or
high
temperature-resistant metal (such as Inconel) tubes (72) within hollowed out
alumina
partition boards ) and particles of nickel acting as reforming catalyst will
be contained
within the chamber area between the tubes and the inside alumina board walls.
Reformed fuel (R~ enters the generator chamber ( 16) through the ports (50)
near the closed end (54) of the fuel cells {26) and flows over the periphery
of the cells
contacting the fuel electrodes (28). The reformed fuel (R~ electrochemically
reacts
with the oxidant (O)) e.g.) air, passing through the solid electrolyte (32)
from the air
electrode (30), and reaches the porous barrier (42) in depleted form as spent
fuel (SF).
The hot) depleted or spent fuel (SF) passes through the barrier (42)) into the
preheating
combustion chamber ( I 8)) where it reacts directly with the oxygen depleted
air or spent
oxidant (SO) returning from the inside of the fuel cells. The sensible heat in
the
depleted fuel and air) as well as the heat of the reaction) are utilized to
preheat the
entering oxidant. The products of the direct fuel-air interaction are then
discharged
from the preheating chamber) and the heat energy contained in the products can
be
advantageously utilized) for example) to preheat incoming reactants in
conventional
metallic heat exchangers.
Each fuel cell (26) contained in the fuel cell stack ( 16) is supplied with
both a
fuel gas, e.g., HZ, CO, CH,, natural gas) etc.) and an oxidant, e.g., air or
oxygen, at
temperatures of about 800~C to 1,200~C. The oxidant electrochemically oxidizes
the
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fuel through a series of electrochemical reactions in the fuel cell which
produce direct
current (DC) electrical energy, heat and water vapor (steam) as well as
depleted i.e.,
partially reacted) or spent fuel and spent oxidant as by products. Each fuel
cell
typically generates a rather small open circuit voltage of less than 1 volt,
and
accordingly) multiple fuel cells are electrically connected, at least in
series, preferably
in a series-parallel rectangular array) in order to generate a higher output
voltage. For
a detailed description of the materials and construction of an exemplary fuel
cell, fuel
cell generators, electrical interconnections and configurations thereof,
reference can be
made to U.S. Pat. Nos. 4,395,468 (Isenberg)) 4,490,444 (Isenberg) and
4,751,152
(Zymboly), which are incorporated by reference herein in their entireties.
Oxidant (O) is fed through oxidant inlet port (38) through the feed conduits
(56)
inserted into the open ends (52) of the fuel cells to contact the inner air
electrode (30))
and a reformed fuel, being shown as (RF), such as hydrogen (H~ and carbon
monoxide
(CO)) is passed over the outside of the fuel cells to contact the exterior
fuel electrode
(28). At the fuel cell operating temperatures of about 600~C to 1,200~C) more
approximately 800~C to 1,050~C) oxygen ions produced at the air electrode {30)
and
solid oxide electrolyte (32) interface pass through the electrolyte (32) to
combine with
the reformed fuel (RF) at the fuel electrode (28) and solid oxide electrolyte
(32)
interface. The reformed fuel (RF} is electrochemically oxidized releasing
electrons
which flow through an external load circuit to the air electrode to generate a
flow of
electrical current.
The electrochemical reaction of the oxidant (O) with the reformed fuel (RFC
thus produces a potential difference across the external load circuit which
maintains a
continuous electron and oxygen ion flow in a closed circuit, whereby useful
electrical
power can be derived. The electrochemical reactions which occur in operation
where
the reformed fuel gas is either hydrogen gas or carbon monoxide gas can be
shown as
Equations (5), (6) and (7).
Air Electrode: 02 + 4e' r 20~' (5)
Fuel Electrode: O~' +H~ ~~ HZO + 2e' (6)
0I' + CO ~ COi + 2e' (7)
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Referring now to FIGURES 3-6, more particularly, an exemplary reformer-
divider (60) with a gas diffusion barrier, as shown, is preferably a three-pan
configuration. The reformer-divider (60) comprises an inner board (62) which
contains
a hollow reforming channel (64) for reforming the reformable fuel mixture
(RFM), a
gas-tight barrier (76) generally surrounding the external walls (b6) of the
inner board
for preventing unreformed fuel (~ leakage into the fuel cell stack prior to
reforming,
and an outer board (78) for housing the inner board-gas barrier subassembly.
The
inner reformer board (62) is provided with a hollow interior reforming cavity
(64)
containing reforming catalyst (80). The inner board (62) is also provided with
an
opened end or entrance (68) for the reformable fuel mixture (RF11~ to be
positioned
near the closed ends (54} of the fuel cells (26) and a closed end (70) to be
positioned
near the opened ends (52) of the fuel cells (26). Positioned within the
refortnable fuel
mixture entrance (68) are fue! riser tubes (72) which extend within the hollow
cavity
(64) to near the closed end (70) which allow the reformable fuel mixture (RFM)
to pass
inside the inner board {62) to near the closed end top (70) of the inner board
near the
opened ends of the fuel cells, then back down the inner board cavity (64) in
contact
with the internal walls of the inner board impregnated or coated with
reforming catalyst
(80), and then exits as reformed fuel (RF) to the fuel cells (26).
The inner board (62) is sheathed in a gas-tight barrier (76). The gas-tight
barrier (76) can be made of nickel foil) Inconel foil, or other suitable
nickel based alloy
foil and the like. The gas-tight barrier (76) essentially envelopes the inner
board (62)
except at the opened end entranct (68) for the refotmable fuel mixture (RF'1~.
The
gas-tight barrier is used to block unreformed fuel gas leakage (F) passing
down the
interior of the inner board into the fuel cell stack. The gas-tight barrier
(76) is also
surrounded by an outer board (78) which envelopes the gas-tight barrier and
reformer
board subassembly) except at the opened end reformable fuel mixture entrance
(68).
Each reformer-divider (60) thus has an inner reformer (62} with an entrance
(68) for
a reformable hydrocarbon feed fuel mixture (RF'M), for example) natural gas
mixed
with spent fuel, a reforming channel within the inner board containing
reforming
catalyst (80), a reformed fuel exit (88) to direct the reformed fuel (RF) to
the fuel entry
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ports (50) to the fuel cells, a gas barrier means (76) and an external
separator board
(78) exposed to the fuel cells (26).
The inner reformer board (62) which acts as the reformer) can be made of
porous alumina insulation board ) preferably generally rectangular in shape,
and of
sufficient thickness to have a hollow channel formed within the board,
preferably a
rectangular channel, extending from an open end (68) of the board near the
closed ends
(54) of the fuel cells (26)) along the axial length of the fuel cells) to a
closed end (70)
near the open ends (52) of the fuel cells (26). The interior cavity (64) of
the inner
board is preferably coated or impregnated with catalytic material (80) for
reforming
such as nickel or platinum, which provides the reforming surface for the
hydrocarbon
feed fuel refornlable mixture. A more detailed description of catalytic
reforming
material and methods of impregnating catalytic material on alumina boards can
be
found in U.S. Pat. No. 4,898,792 (Singh, et al.), which is hereby incorporated
by
reference herein in its entirety.
I S The inner reformer board (62) is generally surrounded by a gas-tight
barrier or
sheath (76) which provides a gas diffusion barrier between the outer walls
(66) of the
inner board and the fuel cells (26) within the fuel cell stack (16). The gas-
barrier (76)
can be made of a metallic foil which is wrapped around the outer walls (66) of
the
inner reformer board (62) except at the open end (68) of the reforming cavity.
The
metallic foil can be nickel or Inconel foil or the like. The metallic foil
serves as the
barrier to prevent any significant leakage through the inner board surfaces to
the fuel
cells.
The outer divider board (78) caa also be trade of porous alumina insulation
board) preferably generally) rectangular in shape. The outer board (78) is
approximately the width and height of the fuel cell stack and of sufficient
thickness to
have formed therein a hollow channel, preferably a rectangular channel, to
house the
inner board (62) and gas-barrier metallic foil (76) subassembly. The outer
board (78)
is supported below the fuel distribution board (40) beneath the fuel cell
stack (16). The
outer board preferably contains a hollow channel (90) for the inner board (62)
and gas-
barrier metallic foil (76) subassembly and with a sufficient clearance (92) to
accommodate thermal expansion relative to the inner (62) and outer (78)
alumina
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boards, thereby significantly reducing structural damage occurring to the
boards as a
result of thermal stresses occurring during generator and reformer operations.
The gas-
barrier metallic foil envelope (76) can thus locally deform to accommodate
thermal
expansion relative to the alumina boards. The outer board (78) also provides
the
balance of the required total wall thickness to give the correct heat transfer
from the
fuel cell stack { 16) to the reforming channel (64) and to the reformable gas
mixture
(RFM), as well as providing electrical insulation between the separated fuel
cells or
cell bundles inside the fuel cell stack and metallic foil to prevent short-
circuiting of the
fuel cells and the generator.
The reformer-divider board (60) containing a gas-barrier layer) such as a
metallic foil barrier) as a means to prevent gas leakage of the unreformed
fuel mixture
through the reformer-divider walls provides many advantages to the high
temperature,
solid oxide electrolyte fuel cell generators with an internal hydrocarbon fuel
reformer
located within the fuel cell stack between fuel cells or cell bundles. Heat
transfer
necessary for the endothermic reforming reaction is provided without the
creation of
excessive temperature gradients within the fuel cell stack and the reformer as
compared
to other internal hydrocarbon reformers not located within the fuel cell
stack.
Accordingly, it is not necessary to increase air flow to the fuel cells to
prevent
excessive temperature gradients ) thereby desirably reducing pumping power
requirements. This can also alleviate excessive thermal stresses on the
reformer-
divider, and improve its structural integrity during long term generator
operations. In
addition, unreformed gas leakage of the reformabie gas mixture through the
reformer-
divider boards are significantly reduced) thereby preventing carbon or soot
formation
on the fuel cells and other generator components during long term generator
operations.
Reduction in soot formation prevents undesirable blocking of gas transport
paths and
prevents the creation of electrical short circuit paths. Furthermore) the
reformer-
divider board is provided with a much greater ability to withstand local
temperature
gradients along the face of the board and bowing of the board caused from
thermal
stresses arising from temperature gradients through the board from face to
face.
The outer reformer-divider boards (78) used in, for example) a 100 kilowatt
high temperature, solid oxide fuel cell generator are rectangular in shape
with
SUBSTITUTE SHEET (RULE 26)
CA 02266777 1999-03-19
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approximately the width and height of the fuel cell stack typically about 60
inches long
and about 34 inches wide and about 1.75 inches thick. The inner reformer-
divider
boards (62) and the metallic foil gas barrier sheath (76) are provided in a
slightly
smaller overall width and height such that the inner board and gas barrier
combination
can be completely covered by the outer board. The inner board (62) is
typically about
59 inches long and about 32 inches wide and about 1 inch thick, or segmented
into
several pieces with the same overall dimensions. The metallic foil (76)
separating the
inner board from the outer board is approximately 0.001 - 0.005 inch (1.0 mil.
to S.0
mil. ) thick. The clearance (92) between the inner board-gas-barrier
subassembly and
the outer board is approximately 0.050 inch (50 mil.) thick. A rectangular
hollow
channel (64) forming a reforming channel extends from an open end (68) along
the
length of the inner board and terminates at a closed end (70) a distance along
the
internal length of the board away from the opposite end and is approximately
slightly
less than the length of the inner board. The hollow channel ( 6~ is
impregnated with
a reforming catalyst material for reforming the unreformed hydrocarbon fuel by
conventional txhniques and provides the reforming surface for the foci.
A variety of methods can be used to fabricate the reformer-divider board
assemblies (60): In one example, the inner (62) and outer (78) reformer-
divider boards
are alumina insulation boards which are split along the length into halves,
exposing the
inner surface of the boards. A rectangular channel is machined into each half
along
the internal surface from one end to near the other end to form an interior
channel (64)
and (90) having an open end and a closed end) respectively. The inr~r board
halves
can be held together with Inconel straps that arc set into recesses machined
along the
width of the exterior surface of the inner board and welded in place. Since
the metallic
foil is used to contain the reformablc gas mixture within the reforming
channel, it is
not necessary to have leak-tight joints between inner board halves. This can
eliminate
time consuming and unreliable cementing to provide leak tight seals for the
inner board
assembly. The metallic foil layer can be fabricated from nickel or Inconel
sheet which
is folded around the inner board assembly and then welded along the side
seams. The
metallic foil can be reinforced at the weld lime with two layers of aickel
strips to
provide adequate material for welding. The reinforcement can be spot welded to
hold
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the pieces together during handling, and then TIG welded to provide a gas-
tight seam.
It is also possible that the seams can also be resistance, electron beam) or
laser welded.
The outer board assembly can be cemented together, or preferably held together
with
insulated or ceramic clips along the edges.
By way of example of operation of the refotTner-divider (60) of the invention
during generator ( 10) operation) a gaseous feed oxidant (O), such as air) is
fed through
oxidant feed inlet (38), and enters the oxidant feed conduits (56) at a
temperature of
about 500 ~ C to 700 ~ C and a pressure slightly above atmospheric. The
oxidant feed
(O) can optionally be heated by conventional means prior to penetrating the
housing
( 12), such as by a heat exchanger coupled to a blower (not shown). The
oxidant (O),
within the conduits (56)) is passed in heat transfer relation through the
combustion
chamber (18), where the oxidant (O) is further heated to a temperature of
about 800~C
to 900 ~ C by the sensible heat released by the combusted exhaust gas (E) .
The oxidant
(O) then flows through the length of the oxidant circuit, through the oxidant
conduits
which extend down the inside length of the fuel cells (26)) being further
heated to
approximately 1,000~C, by virtue of absorbing mast of the heat generated
during the
electrochemical reaction. A smaller fraction of the heat is absorbed by the
fuel.
The oxidant (O) is then discharged into the closed ends (54) at the bottom of
the fuel cells (26) to contact the inner air electrodes (30) along the active
length of the
fuel cells. The oxidant (O) released within the fuel cells (26) then reverses
direction)
and electrochemically reacts at the inner air electrode (30) along the inside
active
length of the fuel cells, being depleted in oxygen as it approaches the opened
ends (52)
of the fuel cells. The depleted or spent oxidant (SO) is then discharged into
the
combustion chamber ( 18) through the opened cell ends (52). The spent oxidant
(SO)
combusts with depleted or spent fuel (SF~ ) where part of the total depleted
fuel (SF~
passes through porous barrier (42) to form hot combusted exhaust gas (E))
which exits
the generator through combusted exhaust gas exit channel (44). The combusted
exhaust
(E) gas can be directed to pass in heat transfer relation with other generator
components (not shown)) for example) the wall of the reforming chamber) prior
to
exiting the generator to provide an additional heat source.
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In this invention) a gaseous hydrocarbon feed fuel (F~ that has yet to be
reformed ) such as gaseous hydrocarbon, including methane, ethane, propane and
the
like, vaporized petroleum fractions such as naphtha, and alcohols such as
ethyl alcohol,
and preferably natural gas) that is, a mixture of approximately 85% methane,
and 10%
ethane with a balance of propane, butane and tlitrogen, can be used. These
reformable
fuel mediums are fed into the fresh hydrocarbon feed fuel inlet (36) as
unreformed feed
fuel (F) . The gaseous hydrocarbon feed fuel (F) is combined with water vapor
and/or
carbon dioxide to form a refotmable fuel mixture (RFI~ at the mixing chamber
(48).
In the generator, the water vapor and/or carbon dioxide can be supplied to the
feed fuel
gas from the spent fuel (S~. As shown, a major portion of the hot depleted or
spent
fuel (SF) formed along the axial length of the outer fuel electrode (28) is
directed to
a spent fuel gas recirculation channel (46). As mentioned above, the other
portion of
the spent fuel (SF) passes into the combustion chamber ( 18), to combust with
spent
oxidant, and to preheat the fresh oxidant feed (O). The spent fuel
recirculation channel
(4b) passes from the generator chamber ( 16) to feed into and combine with the
fresh
hydrocarbon feed fuel (F) at a mixer (48) such as an ejector, jet pump)
aspirator. This
allows recirculation of a portion of the spent fuel containing at least water
vapor and/or
carbon dioxide) to provide the oxygen species required for reforming) and also
if
desired excess oxygen species for reforming without significant hydrocarbon
cracking.
The combined spent fuel and fresh hydrocarbon feed fuel provides a reformable
fuel
mixture (RFM) for reforming en route to the fuel cell stack { 16) containing
the fuel
cells (26).
In this invention, the refortnable fuel mixture (RFM} passes through reforming
chambers (60) located inside the fuel cell stack between individual fuel cells
(26) cell
bundles (22) 24). During operation of the reformer-divider (60), when fuel
riser tubes
(72) are used) the reformable fuel mixture (RFIvi) passes from tube entrance
through
the inside of the riser tube, to the top of the riser tube where the
reformable fuel
mixture exits near the closed end (70) of the reformer-divider (60)) reverses
flow, and
passes back down the interior channel (64) of the reformer-divider, while
making
contact with the internal walls of the inner board which are impregnated with
reforming
catalyst. The reformable fuel mixture is thereby reformed along the inside
active
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length of the inner reformer boards. When a single partition (74) is used the
reformable fuel mixture (RFM) passes from entrance, along one side of the
partition,
in the channel formed by the partition and divider walls) to the top of the
partition,
over the top of the partition) where the reformable fuel mixture reverses flow
and
passes back down the reformer-divider in the channel formed by the partition
and other
divider wall and is reformed.
The reformed fuel mixture after passing through the reforming material in the
reformer~ivider board (60) passes as reformed fuel (RF) through a series of
ports (50)
in the fuel distribution plenum (88) which connects the reformer-divider (60)
to the
generator chamber ( 16). The reformed fuel (~' ) passes into the generator
chamber and
over the outer fuel electrodes (28) of the fuel cells. The reformed fuel (RF)
released
over the fuel elxtrodes (28) of the fuel ctlls (26) electrochemically reacts
at the outer
fuel electrode (28) along the outside active length to the fuel oils (26),
being depleted
in fuel as it approaches the porous barrier (42) and spent fuel ttcirculation
channel
(46). The depleted or spent fuel (SF) is then discharged into the combustion
chamber
(18) through the porous barrier (42) and also into the spent recirculation
channel (46)
as previously mentioned.
The overall electrochemical reactions of the generator operating at a
temperature
o f about 800 ~ C to 1, 200~ C, typically 1, 000 ~ C convert reformed fuel gas
(RF]) such
as hydrogen (H~ and carbon monoxide (CO) to direct current (DG~ electricity)
heat
and water vapor. The oxidant (O) passing inside the fuel cell is
electrochemically
reduced at the air electrode-elxtrolyte interface. The electrons for the
reduction of
oxidant are supplied by the air electrode. The oxygen ions formed become part
of the
solid oxide electrolyte crystal structure and migrate through the electrolyte
to the
electrolyte-fuel electrode interface. Fuel passing over the outside of the
fuel cells is
electrochemically oxidized at the electrolyte-fuel electrode interface. The
oxidized fuel
released is carried away. The electrons released are directed to flow through
an
external circuit to the air elxtrode, thus generating a flow of DC electrical
current.
For) more complete dexription of electrochemical operations of a high
temperature,
solid oxide fuel cell generator, reference can be made to U.S. Pat. No. Re.
28,792
(Ruka), incorporated by reference herein in its entirety.
A~~1DED SHED
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In a second embodiment of the invention as shown in FIGURES 7 and 8) the
reformer-divider board ( 100) is provided as a plurality of axial segments (
102) stacked
on top of each other to form the reformer-divider of desired size. Each axial
segment
( 102) is provided with a hollow inner board ( 104) impregnated with reforming
catalyst
S ( 106) ) a gas-tight barrier ( 108) , and a hollow outer board ( 110). The
gas-tight barrier
layer, e. g. metallic foil, by removing the leak tightness requirement from
the alumina
board assemblies, makes it possible to fabricate the board assemblies in
several axial
sections ) if desired. This would then make it possible to machine the
internal pockets
of the inner and outer board with conventional tooling without splitting the
boards into
halves, thereby eliminating the need for holding the board halves together.
These
subsections ( 102) can be approximately 12 to 20 inches in height and
approximately 24
inches in width) could then be stacked to give any height to the reformer-
divider as
required. The subsections could be held in place and in alignment with ceramic
tie
rods ( 112) through the length of the combined subsections at positions
adjacent to the
internal channel for reforming.
In a third embodiment of the refotiner-divider as shown in FIGURE 9, an
alternative gas barrier sheath arrangement is shown. In this embodiment, the
reformer-
divider board (200) is rectangular in shape and is axially segmented along its
length
into a plurality of subsections (202). Each subsection is hollowed (204) along
the
length of its interior surface having two open ends (206) and (208)) thereby
forming
a rectangular tube. The hollowed area (Z04) is impregnated with reforming
catalyst
(210). A gas-barrier means (212) such as metallic sheath comprising two halves
is
provided to act as a gas leakage barrier. The metallic sheath (212) can be
made of
high temperature resistant metal such as Inconel. Into each half of the
metallic sheath
(212), the axially segmented subsections (202) of reformer-divider boards
(200) are
placed, stacked one on top of each other to obtain the desired height. An
Inconel
separator (214) is provided into one of the half assemblies comprising the
stacked
reformer-divider and metallic sheath. The two half assemblies are connected
with
rectangular bellows (216). The bellows are located at the same elevation as
the Inconel
separator (214) and serve to accommodate differential growth of the Inconel
envelope
relative to the alumina insulation board. The bellows preferably will deflect
into reliefs
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(not shown) machined into the separator. In addition, the bellows may provide
a
stiffening effect for the envelope at its midplane to aid in controlling
possible thermal
distortion. The outside of the metallic sheath is insulated with alumina paper
(218) to
avoid short circuiting of the fuel cell stack.
In a fourth embodiment of the reformer-divider as shown in FIGURE i0, an
alternative gas barrier arrangement is shown. In this embodiment, the
refornner-divider
board (300) is rectangular in shape and is axially segmented along its length
into a
plurality of subsections (302). Each subsection (302) is hollowed along the
length of
its interior surface to form a hollow channel (304). The hollow channel (304)
is
impregnated with reforming catalyst (306). The axial segments (302) are then
stacked
inside a gas-barrier means (308) such as a metallic sheath. The metallic
sheath (308)
can be made of high temperature resistant metal such as Inconel. If necessary,
to
stiffen the sides and reduce any buckling tendency, tie wires or rods (310)
can be
passed through the metallic envelope (308) and reformer-divider boards along
its cross-
sectional length and then welded to the outside face of the metallic envelope.
If tie
wires or rods (310) are used) the reformer-divider boards would be provided
with slots
(312) to allow for movement of the tie members due to relative thermal growth.
When
installed in the fuel cell stack of a generator, the growth of the envelope
can be
permitted at the bottom where the lower edge of the envelope could either grow
into
a crevice left between the fuel distribution board (40) and the reformer-
divider
assembly, or grow until the lower edge is flush with the top of the cell
closed end
positioning board. In either case) any fuel leakage to thermal expansion would
be
small and confined to the bottom of the fuel cell stack. The Inconel envelope
can be
insulated with alumina paper (3I4) or other insulation material such as
sprayed ceramic
to prevent short-circuiting of the fuel cell stack.
In both of the last two alternative embodiments, the problem of fuel leakage
or
diffusion through the porous alumina boards is signiflcaatly reduced) since
the metallic
sheath envelope will contain the fuel.
The invention will now further be clarified by a consideration of the
following
example, which is intended to be purely exemplary of the reformer-divider
configuration and operation of the invention.
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EXAMPLE
A reformer-divider board which will be used in a 100 kW SOFC generator was
constructed in accordance with the reformer-divider design shown in FIGURES 1-
b for
the purpose of evaluating mechanical integrity, reforming capability, and ease
of
manufacture. As such, the reformer-divider board was comprised of an inner
board
assembly made from machined alumina board material, a nickel foil liner of
about
0.005 inch thickness which was placed around the finished inner board
assembly, and
an outer board assembly from machined alumina board material which was placed
around the inner board and metallic foil subassembly. The inner board was
machined
to form the gas pocket, although this pocket could be formed by other means)
such as
by attaching strips of material to the edges of a flat sheet. The catalyst was
loaded into
the inner board assembly by soaking the boards in catalyst bearing solution.
The foil
liner was formed around a mandrel) and the seams were resistance welded to
form a
gas tight seal. Subsequent pre-test leak checks at ten times the expected
operating
pressure showed that there were no leaks at the seams. The outer board
assembly was
machined in two pieces and assembled over the inner board assembly.
The assembled reformer-divider board was placed in a reformer-divider board
test rig which simulated the fuel cell stack environment. Methane rich fuel
was
supplied to the board over an extended period of time (1,500 hours} and
periodic
measurements of exit fuel gas composition were taken. Fuel flowrates were
varied to
represent the operating points expected for a 100 kW generator.
The tests revealed that the overall reforming percentage was acceptable and
superior to that achieved with present external reformers used in smaller
generator
designs. Reforming percentages varied from about 85 % to 98 % with higher
percentage
occurring for the low flow cases (minimum power simulation). Visual post test
inspection revealed no oxide or corrosion on the surface of the foil liner and
no
apparent mechanical creep. The foil liner was checked following the test and
found
to still remain leak tight. Material samples were taken and analyzed to
examine the
microstructure of the foil. Based on the results obtained, the reformer-
divider design
is targeted for application in the SOFC generators of the present and future.
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This invention disclosure incorporates by reference herein a11 of the
hereinabove
mentioned U.S. patents in their entireties.
The invention having been disclosed in connection with the foregoing
embodiments and examples, additional variations will now be apparent to
persons
skilled in the art. The invention is not intended to be limited to the
embodiments and
examples specifically mentioned, and accordingly reference should be made to
the
appended claims rather than the foregoing discussion, to assess the spirit and
scope of
the invention in which exclusive rights are claimed.
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