Note: Descriptions are shown in the official language in which they were submitted.
50 ~039
A FI~L CELI, G:ENERATOR AND MET~IOD
OF OPEP~ATING SAME
om eR~7~r CO~Ao~ c uulaE
me ln~entiorl disclc~sed herein was made OI' COIl-
celved in the cour~e of or u~der a contract with the Unlted
St~te~ Gove~nment identifled as MoO DE~ 0379E771305.
CROS~13FERENCE TO REI~TE D APPI,ICATION
This applicat$on is rel~ted to C~nadi~ appllca-
tion Serial No. 3l35,893 ~iled September 15, 1981 in the
name o~ A. O. ~s~nberg and entitled "Fuel Cell GeIlera~
tor~ e rel~ted applicatlon may be reîerred to îor
additional lnformation c~n an exemp~ary :fuel cell generatcr
o~ the type for which the disclo~ed inv~ntion is applicable.
T~r~D ~ m~ ol ~Z INV~ ON
: Field o~ the ~nvention:
____
This ln~ention relates to soli.d electrolyte ~uel
cell generation sy~tem~, and more partlc~larly provides a
method ~or processing reactants ~or such systems~
E~5~ t~ r Arto
High temperature solid electrcllyte ~lel cells
con~er~, through a usually exothermlc el.ectrochemical re
action, ~h~mlcal energy into dirert current electrical
energy? typically at ~emperatures abo~e 700~C. The re~c~
tion take~ place at the electrode~electrolyte ~nterfaces
where the electrolyte i~ sandwiched be^tween an anode a~d Q
~: '
2 50,039
catho~e. The reaction involves a relatively pure fue].,
for example a mixture of hydrogen and carbon monoxide, and
an oxidant such as oxygen or air. Where hydrogen and
carbon monoxide fuel is utilized in a fuel cell based
, 5 system, it is typically provided from a reformer, upstream
I of a fuel cell stack. The reformer reacts, for example,
hydrocarbons, natural gas, or alcohols with steam in an
endothermic process to produce a fuel suitable for the
fuel cells, such as hydrogen and carbon monoxide mixtures.
The exothermic reaction in the fuel cells and
i heat released by other cell losses requires that a sub-
I stantial cooling means be utilized. For example, rela-
tively large amounts of cooling air are passed adjacent
selected cell components. This may detract from overall
15 system efficiency. Similarly, the endotharmic reaction at
I the reformer reguires substantial heat input, which also
may detract from overall efficiency.
Many reformers operate with catalysts that are
in limited supply, such as platinum. It is known that
20 reformation can take place in that presence of less exotic
materials, such as nickel. Also known is the use of
cobalt for this purpose. Reformers upstream of the fuel
cells have utilized particulated, or high surface area
catalysts. Certain fuel cell configurations have been
25 considered for so called indirect fuel cell systems. For
example, Figure 2.2 of a text entitled Fuel Cells For
Public Utility And Industrial Power, Noyes ~ata Corp.,
1977, and the accompanying description, refer to a molten
carbonate electrolyte cell as a promising candidate for
30 utilizing a hydrocarbon fuel reformed to hydrogen and C0
at a nickel anode. The description also refers to an
indirect solid-electrolyte fuel cell system wherein excess
heat from the fuel cell system reaction at the cell is
''4 utilized as input~to a coal gasification process. U.S.
Patent No. 3,462,306/also ~éscribes a liguid electrolyte
fuel cell system having a nickel electrode which converts
a mixture of methane (CH4~ and water vapor into carbon
dioxide and:hydrogen.
3 50,03g
Technical problems have, however, prevented such
contemplated designs from achieving a system which is
workable in practice. Concerns are typically raised
relating to materials compatability and stability, reac-
tant and product transport to and from the fuel cells, andparticularly overall system efficiency.
It is thus desirable to provide a method of
operating a fuel cell based system which offers increased
system efficiency and alleviates other deficiencies of
existing designs.
SUMMARY OF THE INVENTION
( This invention provides a method of operating a
fuel cell based system, with in-situ reformation of a re-
actant gas, which achieves good system eficiency and low-
ers cell cooling reguirements. In essence the preferred
method symbiotically combines an exothermic electrochemi-
cal generation reaction and an endothermic reformation
reaction. In preferred form a solid electrolyte so-called
sealless fuel cell generator, of a type described more
fully in the cross referenced application, includes a
housing surrounding a plurality of chambers. A porous
barrier separates a generator chamber and a preheating,
combustion product chamber.
Elongated, annular fuel cells having an electro~
chemically active length within the generator chamber have
an anode including a material which functions in part as a
catalytic high temperature reformer for gaseous reactant
mediums such as hydrocarbons, natural gas and alcohols.
The cells also have an electrGchemically inactive length
which also provides reformation. The anode is preferably
the outer electrode of tubular shaped fuel cells, and
includes an outer surface of porous nickel or cobalt.
Cobalt is particularly beneficial where the reformable
medium contains sulphur compounds.
An oxidant, such as air, flows within the fuel
cells, adjacent the cathode, and subsequen~ly, in depleted
form, into the combustion product chamber. The reformable
4 50,039
gaseous reactant medium is mixed with steam, to form a
mixture at a temperature in the range of 1200F. The
mixture is then fed to a plenum within the generator, and
from the plenum through a distributor plate to the chamber
containing the active len~th of the fuel cells, in a
manner such that the mixture surrounds the outer surface
of the anodes and the inactive length.
Some of the mixture is reformed alony the inac-
tive length and the mixture then diffuses to the anodes
where it is catalyzed with the steam to, for example,
hydrogen and CO. Oxygen is transported from the cathodes,
I through the solid electrolyte, also to the anodes, where
it combines with the reaction products of the reformation
reaction, such as hydrogen and carbon monoxide.
15 The excess reactants and chemical products of
these reactions are ultimately transferred to the preheat-
ing combustion chamber, through the porous material, where
they are combusted and function to preheat incoming oxi-
dant.
BRIEF ESCRIPTION OF THE DRAWINGS
The advantages, nature and additional features
of the invention will become more apparent from the fol-
lowing description, taken in connection with the accom-
panying drawing, in which:
Figure 1 is a broken perspective view of an
exemplary fuel cell generator of the type to which the
invention is applicable;
Figure 2 is a view, partially in section, of the
generator shown in Figure 1;
Fi~ure 3 is a view, partially in section, of
another exemplary generator;
Figure 4 is a block diagram schematically illu-
strating one reactant feed method in accordance with the
invention;
Figure 5 is a cross-sectional view through an
active length of a fuel cell; and
50,039
Figure 6 is a cross-sectional view through an
inactive length of a fuel cell.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to Figures 1 through 3, there is
shown a fuel cell generator 10 including a gas-tight
housing 12. The housing :L2 surrounds a plurality of
chambers, including a generating chamber 14 and a com-
bustion product or preheatirlg chamber 16. Also provided
is a reformable reactant mixing and distribution chamber
19. An oxidant inlet chamber 18 can also be contained
within the housing 12. Alt:ernatively, other means for
manifolding an oxidant into conduits 20 can he utilized.
The housing 12 is preferably comprised of steel, and lined
throughout with a thermal insulation 22 such as low density
alumina insulation. Psnetrating the housing 12 and insu-
lation 22 is a reformable medium inlet port 25, an air
inlet port 26, and a combustion product outlet port 28, as
well as ports for electrical leads.
The generating chamber 14 extends between a wall
30 of the housing 12 and a porous barrier 32. The pre-
heating chamber 16 extends between the porous barrier 32
and a tube support structure such as a tube sheet 34. The
oxidant inlet chamber 18 extends between the tube sheet 34
and an end wall 36 o~ the housing 12. The mixing and
distribution chamber 19 extends between an end wall 31 and
the wall 30. The wall 30 is porous to the reformable
gaseous medium entering the chamber 19, and preferably
includes perforations 21 which ensure a preselected dis-
tribution of the reformable gaseous medium into the gener-
ating chamber 14.
The dividiny barriers can include various struc-
tural types, and additional support and flow baffles can
be incorporated. The shown porous barrier 32 and the tube
sheet 34 need not be sealed structures. The porous bar-
rier 32, in particular, is designed to allow flow betweenthe generating chamber 14, operating at an approximate
pressure slightly above atmospheric, and the preheating
6 50~ 039
chamber 16, operating at a slightly lower pressure, as
indicated by arrow 38~ While the generator 10 is shown in
a hori~ontal orientation in Figure 1, it can be operated
in a vertical or other position.
High temperature, elongated, solid oxide elec-
j trolyte annular fuel cells 40 extend between the preheat-
ing chamber 16 and the generating chamber 14. The cells
have open ends 42 in the preheating chamber 16, and closed
ends at an inactive length 44 in the generating chamber
14. The fuel cells are preferably tubular, including a
solid oxide electrolyte sandwiched between two electrodes,
( supported on a tubular porous support. Each cell includes
an electrochemically active length 46 and an inactive
length 48. Another electrochemically inactive length is
at the closed end 44, and can extend a substantial portion
of the entire length of the cells. The active length is
contained within the generating chamber 14. The length 4B
is electrochemically inactive. The inactive length 44 is
comprised of a material which, at elevated temperatures,
above approximately 500C, has the capability to reform a
gaseous medium to products such as hydrogen or carbon
monoxide. Such products are the fuel for the electrochem-
ical reaction at the active cells. The inactive length of
the cell tubes, particularly length 44, may be fabricated
bare of an underlying air electrode (cathode), or bare of
an electrical contact to the cathode, yet it carries a
dense electrolyte layer and the porous layer, representing
the fuel electrode in the active cell area, on top of the
electrolyte and thus in contact with the reformable gas-
eous medium~
Eigure 5 shows a cross section through theactive length 46. Serially from the interior area radial-
ly outward, the cross section includes a porous tubular
support 45, a porous electrode 47 such as a cathode, the
solid electrolyte 49, and a second porous electrode 51
such as the anode. Similarly, Figure 6 shows a cross
section through the inactive length 44. Serially from the
.
7 50,039
interior area radially outwar~, the cross section includes
the porous insulator support 45, the solid electrolyte
49', and the second porous electrode 51. Here the solid
electrolyte 49' has replaced the volume occupied by the
5 porous electrode 47 and the electrolyte 49 in the active
I length. The broken area shown in Figure 5 is the location
where series electrical interconnection to an adjacent
cell can be made.
Each individual cell generates approximately one
10 volt, and a plurality are electrically interconnected,
preferably in a series-parallel rectangular array. For
( descriptive purposes, the arrangement can be described as
including rows 50 and columns 52. Each cell in a row 50
is electrically connected along its active length 46 to
15 the next adjacent cell, preferably through direct contact
of their outer peripheries. For the preferred configur-
ation shown in Figure 1, where the reformable medium flows
about each cell and an oxidant, such as air, flows within
each cell, the anode is the outer electrode of each cell
20 and the cathode is on the inside. Thus, cell-to-cell con-
tact within a row is in parallel, among adjacent anodes.
Each cell in a column 52 is electrically inter-
connected in series to the next adjacent cPll 40. In the
preferred configuration, this interconnection is made from
25 the inner cathode of one cell to the outer anode of the
next consecutive cell, through an interconnect 54.
In the configuration shown in Figure 1 and 2,
the anode is the outer electrode and the cathode is the
inner electrode. In the configuration shown in Figure 3,
30 discussed more fully hereinafter, the cathode is the outer
electrode and the anode is the inner electrode. In the
configuration of Figures 1 and 2, the anode, or a porous
surface surrounding the anode, is comprised, similar to
the inactive length, of a material which, at elevated
35 operating temperatures, is capable of catalytically re-
forming a gaseous medium to a uel for the cells, such as
hydrogen or carbon monoxide. Preferred materials are
8 50,039
nickel, n.ickel containing compounds, cobalt and cobalt
containing compounds. Other catalytic materials can also
be utilized.
The direct current electrical energy generated
by the cells is preferably collected by a first current
collector, such as a conductive metal plate 56 or felt
pad, positioned in electrical contact with each cell 40 in
the first row 50, and a s:imilar second collector (not
shown), positioned in contact with the last row. Electri-
cal leads 58 are accordingly provided to the current
collectors.
i The conduits 20 are preferably loosely supported
at one end in the tube sheet 34. The tube sheet 34 is
preferably stainless steel, with bores that fit loosely
about the conduits 20 to allow free thermal expansion.
The conduits 20 are preferably comprised of alumina, and
the tube sheet is covered with an insulation 62 such as
low density alumina. Oxidant may leak across the tube
sheet 34.
The conduits 20 extend from the tube sheet 34
into the open end 42 of the fuel cells 40, a single con-
duit 20 corresponding to a single fuel cell. Each conduit
20 extends to the active length 46 of the fuel cell, and
preferably close to the closed end of the cell. Each
conduit is provided with a means for discharging a reac-
tant medium into the fuel cell 40.
The porous barrier 32, which allows a throughput
of depleted fuel, is preferably a porous ceramic baffle,
such as one comprised of fibrous alumina felt, or ceramic
plate segments with porous inserts such as ceramic wool
plugs, surrounding each fuel cell 40.
During operation an oxidant such as air enters
the inlet chamber 18 through inlet port 26. The chamber
18 functions as an inlet manifold for the individual
conduits 20. Air enters the conduits at a temperature of
approximately 500 700C, and a pressure above atmospheric,
being initially heated prior to en-tering the housing by
9 50,039
conventional means. The air flows within the conduits,
through the preheating chamber 16, where it is further
heated to a temperature of approximately 900C. The air
flows through the length of the conduit, beiny further
heated to approximately 1000C, and is discharged into the
fuel cell 40. The air within the fuel cell electrochemi-
cally reacts at the fuel cell cathode along the active
length 46, depleting somewhat in oxygen content as it
approaches the open end 42 of the cell. The depleted air
is discharged into the combustion product or preheating
chamber 16.
As shown schematically in Figure 4, there is
provided means for providing a medium reformable to a fuel
useful to the electrochemical reaction at the fuel cells,
such as a container 47 and conduit 49. Additionally uti-
lized are means for providing another reactant useful in a
reformation process, such as steam, including a heater 51
and conduit 53. The reformable medium is preferably gas-
eous, and can include hydrocarbons such as methane (CH4),
~0 alcohols such as ethyl alcohol (C2H40H), and compositions
such as natural gas. The steam and reormable medium are
integrated in means for mixing such as mixer 55, and flow
through a conduit 57 to the distribution chamber 19. The
steam and reformable medium are preferably heated to a
temperature of approximately 1200F prior to entering the
distribution chamber 19.
From chamber 19 the steam-reformable medium
mixture flows through wall 30 into the generating chamber
14 and about the fuel cells. The mixture first contacts
the inactive length 44 of the cells, and reformation of
part of the mixture to a useable fuel occurs. The fuel,
other reformation products and the reformable mixture flow
to the active length, where further reformation takes
place, as does the electrochemical power generation reac-
tion.
The mediums diffuse to the anode surface adja-
cent the electrolyte, a portion being catalyzed with steam
50,039
to H~ and C0. Also at the anode surface, oxygen has been
transported across the cell electrolyte to the anodes,
where it combines with mosk of the reformed products H2
and C0. It may also combine directly with some of the
- 5 unreformed medium, such as methane, which is present at
the anode. The combustion products arisinq from the oxy-
gen reaction, 2H2 + 2 = 2H20; 2C0 + 0~ = C0; CH4 ~ 202 =
C2 + 2H20, diffuse into the generating chamber between
the fuel electrodes where they are transporked to the
exhaust end of the generat:ing chamber. There they are
exhausted along with unreacted ~2~ C0 and trace CH4,
present at the exhaust end, through the porous barrier 32
and into the combustion product chamber 16. The porous
barrier with a slight pressure drop across it preferably
precludes oxygen from the air exhaust backflowing into the
generating chamber. Similarly, the air, functioning as
both a process reactant and a cooling medium, enters the
oxidant inlet chamber 18, en-ters the conduits Z0 and flows
into the cells where oxygen from the air is picked up by
the air electrode (cathode) and is transferred to the
electrolyte. The air exhaust, partially depleted of 2~
~ exhausts through the porous barrier and enters the com-
! bustion zone, where it combusts with H2, C0, and CH4
present in the fuel exhaust, further preheating the air
flowing through the conduits 20.
An exemplary system is based upon the following
parameters: a methane feed rate for 600 millivolts, 400
amps per ft , for an electrode area of 30 cm. x ~ x 1.5
cm = 0.13 ft2, and for an electrical output of 11,852 Btu
30 per lb of CH4, is 0.91 x lO 2 lb per hr. For the flow
rate of 0.91 x 10 2 lb per hr. of methane, a mean velocity
of 0.01 ft per sec t43 ft per hr.) is attained in an
approximately l/8" annulus of the fuel-cell anode, and the
ReSc number is 0.12. The tube length for 9~.9% conversion
35 of met~ane would be less than 1.0" of a 12'1 fuel cell
tube. This is based on the following mass transfer calcu-
lation, I and II.
7 ~ ~
11 50,039
I. Laminar convection coefficients for fully
established flow in a tube are given by
St = 3.65/Re Sc
St(mass transfer St:anton number) ~ u-
Re(Reynolds number) = u~D
Sc(Schmidt number) = D~
h(mass transfer convection coefficie~t, ft/sec.)
u(mean flow velocity in tube, ft/sec.)
D(tube diameter, ft); D12 (diffusion constant,
10 ft2/sec )
~ (kinematic vi scosi ty, ft2/sec.)
The Stanton number correlation applies to fully
developed flow in tubes; since the Stanton number is high-
er in the entrance region of a tube, the use of 3.65/ReSc
i5 conservative and implies an upper bound to the calcula-
tion of the tube length re~uired for 100% reforming o CH4
to H2 C0.
Analyzing methane concentration (cross-sectional
mean value, p, lb per ft3) along the length of a tube
within which a reforming reaction is taking place, it is
found that if all the methane reaching the wall is cata
lyzed to H2 and C0, the mean methane concentration over
any cross-section along the length of the tube can be
determined to be
p(CH4 at x) = pO (CH4 at x = ~) exp { Du x }.
The ReSc numbers can be evaluated from uD/D12,
where D12 is the diffusion coefficient of methane in
steam. (This follows from uD/~ x ~/D12). Estimates of
D12, obtained from standard procedures are 0.23 x 10 2 ft2
per sec. The value of x at p/pO = 0.001, is less than 1".
.
3 ~
12 50,039
II. The following data apply to an exemplary
commercial natural-gas/steam reformer using steam-hydro-
carbon reforming catalyst.
Type of Plant, Syn gas
Feedstock, Natural gas
I Catalyst Volume, ~200 ft3
I Catalyst Size, 3/4 x 3/4 rings (inches)
No. Tubes, ~48
Tube I.D. 5"
( 10 Data relating primary reformer space velocities
to tube diameter are available from known reformer data.
A 5" tube has a space velocity of 1200 SCFH2 per hr ft3 of
catalyst volume, and the 3/4" x 3/4" rings have an esti-
mated surface area per unit volume of 74 ft2/ft3, and this
converts the space velocity to 16.2 SCFH~ per hr ft2 of
catalyst area. The fuel electrode area of a fuel cell is
about 0.13 ft2; thus the convected space velocity applied
to this area would produce 10 2 lb H~ per hr. The methane
feed to the exemplary fuel cell will produce only 0.3 x
lO ~ lb of H2 per hr.~ so the wall catalysis is three
times greater than needed. Thus, the ability of the
- exposed catalytic surface along the active length of the
fuel cells to reform methane is three times greater than
the feed rate of methane.
Similarly, an alcohol based system can be uti-
lized. For example, ethyl alcohol and steam will reform
at a nickel or cobalt surface generally according to
C2H40H + H20 ~ 2C0 ~ 3~iH2. This reaction will require an
amount of steam in excess of the stoichiometric amount to
ensure that coking does not occur.
It will now be apparent that similar processes
can be carried out with the system confi~urativn of Figure
3. In either configuration, the combustion product pre-
heating chamber 16 serv~s to preheat the reactants, through
combustion as well as sensible heat, and thus assist the
13 50,039
endothermic reforming reaction. The cells and catalytic
reformable surfaces are further heated by the exothermic
electrochemical reaction occurring across the electrolyte.
Referring now to Figure 3, the fuel anode is on
the inside of the annular fuel cells and the oxidant
¦ cathode is on the outside. The controlled leakage or
- seal-less arrangement described above is utilized. In
¦ Figure 3, four primary chambers are shown within the
insulated sealed housing 112, including an oxidant inlet
10 chamber 118, a generating chamber 114, a combustion pro-
duct chamber 116, and a reformable medium manifold inlet
! and distribution chamber 117.
Oxidant preheating conduits 120 are mounted in a
tube sheet 134, and fuel preheating conduits 121 are
mounted in a second tube sheet 135. The mountings, in-
I cluding insulation, can be similar to that described with
reference to Figures 1 and 2, allowing thermal expansion
and leakage. The uel conduits 121 extend into annular
fuel cells 140, and the air conduits 120 are interspersed
among the cells. The air conduits 121 can be arranged asrows in-terspersed among selected groupings of cells, for
example, three columns of cells can be interconnected in
series-parallel as previously described, and electrically
segregated from another grouping of three columns by a
column of air conduits. In this case, peripheral elec-
trical collector plates would be associated with each
grouping of three columns. Alternatively, tha cells can
be interconnected among one another, with air conduits
placed about the periphery of the entire set of inter-
connected cells. Additionally, if the cells are of largediameter relative to the diameter of the air conduits, the
air conduits can be positioned in the gap between a group-
ing of, for example, four cells in a square array.
During operation, preheated oxidant, such as
air, enters the oxidant inlet chamber 118, and is mani-
folded to the conduits 120. The air traverses the con-
duits 120, being further preheated, and is discharyed into
14 50,039
generating chamber 114, where it flows about the fuel
cells 140 and the electrochemical reaction takes place.
The cells include an active length. Depleted air then
flows through a porous barrier 132 and into the combustion
product chamber 116, for direct combustion with deplsted
fuel. A fee~back duct interconnecting the higher pressure
generating chamber 114 and I:he lower pressure combustion
chamber 116 can also be utilized.
Preheated reformable medium enters inlet cha~oer
117 and flows through conduits 121, being further pre-
heated. The medium is then discharged into the fuel cells
140 and flows in the reverse direction, being reformed to
a fuel usable in the subsequent electrochemical reaction.
Depleted fuel along with remaining products, such as
unreformed gaseous medium, is then discharged into the
combustion product cham~er 116. In the combustion product
chamber 116, the depleted fuel, depleted oxidant, reform-
able medium which may flow through the tube sheet 135,
oxidant which may flow through the barrier 132, and other
products such as excess water vapor, directly react to
combust and generate heat. The heat of this reaction,
along with the sensible heat contained in the depleted
products, preheat the reformable medium entering through
conduits 121. Excess energy discharged with the combus-
tion products through outlet 128 can be advantageouslyutilized downstream of the generator.
The described exemplary reformation-generation
systems have a reformer heat absorption of about 6000
Btu's per lb of, for example, methane fed to the gener-
ator. If an external reformer design is used and the 6000Btu's is usually supplied by combustion of the anode
exhaust gas, about 20-25% of the fuel value fed to the
fuel cell is used to reform the ~as to H2 and C0. In the
integrated fuel cell/reformer design the 6000 Btu's are
supplied by the heat generated in the fuel cell, so that
substantial:ly all Qf the reformed gas, as much as 95% or
more, can be used to generate electricity. Thus, the
50,039
system electrical efficiency will be increased in the
range of 25%. In addition, the 6000 Btu reforming reac-
tion will reduce the cathode cooling air used by about
40%.
Since numerous changes may be made in the above-
describecd arrangement without daparting from the spirit
and scope thereof, it is intended that all matter con-
tained in the foregoing be interpreted as illustrative,
and not in a limiting sense.
