Fuel cell aluminum production

A process and apparatus are disclosed for electrolytically smelting alumina to produce aluminum metal, including providing a combination solid oxide fuel cell and electrolytic smelting cell for the production of aluminum from refined alumina positioned near tile solid oxide fuel cell. In one aspect, an alumina ore refinery for producing the refined alumina is positioned near the solid oxide fuel cell, and refined alumina is passed at a temperature of at least 900.degree. C. directly from the alumina ore refinery to the electrolytic smelting cell. In one aspect, the solid oxide fuel cell incorporates a planar construction having a solid state cathode material of lanthanum strontium manganate, a solid electrolyte of yttria stabilized zirconia, and a nickel/yttria stabilized zirconia cermet anode.

BACKGROUND OF THE INVENTION 
1. Technical Field 
This invention relates to a process for the electrolytic production of 
aluminum. In one aspect, this invention relates to a novel process for the 
electrolytic production of aluminum using a fuel cell. 
2. Background 
Aluminum metal is produced commercially by the electrolytic smelting of 
alumina. Bauxite alumina ore is refined in a Bayer plant, and the refined 
alumina is smelted in an electrolytic bath of molten salts in a 
Hall-Heroult cell plant. These molten salts include cryolite 
(3NaF--AlF.sub.3) and minor additions of salts such as LiF and CaF.sub.2. 
Actual Hall-Heroult cell bath contains less NaF than the specific 
composition formula of 3NaF-AlF.sub.3 and varies somewhat from plant to 
plant. Relatively pure refined alumina is reduced in the molten salts in 
the Hall-Heroult electrolytic cell, named Hall-Heroult after the earliest 
independent inventors of the electrolytic process for producing aluminum. 
The electrolytic smelting reaction is carried out in the Hall-Heroult cell 
process in an aluminum reduction pot in which alumina is dissolved in the 
molten salt bath. The alumina in solution in the molten salt is 
electrolyzed to form metallic aluminum. Aluminum metal produced in the 
reaction is heavier than the electrolyte and forms a molten layer at the 
bottom of the reduction pot which serves as the cathode of the cell. 
Carbon anodes extend into the bath, and oxide ions react electrochemically 
with the carbon to produce carbon dioxide, which is liberated at the 
anode. 
The cost of making aluminum has not changed very much in the past few 
decades. Those changes that have taken place have been incremental, 
evolutionary changes in the fundamental Hall-Heroult process. 
INTRODUCTION TO THE INVENTION 
Most attempts to improve the energy efficiency of the aluminum production 
smelting cells have worked on reducing the IR drops in the cells. However, 
programs for reducing the IR drops in the Hall-Heroult cells run into the 
thermodynamic barrier of having to create heat just to maintain the 
temperature of the cell. The "Inert Anode Cell," also called the "Energy 
Efficient Cell," is an alternative aluminum reduction cell design 
developed and built by Aluminum Company of America for its electrical 
efficiency. The Inert Anode Cell also runes into the thermodynamic barrier 
of having to create heat just to maintain the temperature of the cell. 
Aluminum smelting in a Hall-Heroult process cell contributes significantly 
to the consumption of electrical energy, most of which comes from the 
burning of fossil fuels in the United States. 
A process is needed to produce aluminum in commercial quantities 
efficiently and economically, to conserve electrical energy, and to reduce 
the emissions of carbon dioxide released to the atmosphere. 
Fuel cells are energy conversion devices. A fuel cell is a device that 
converts chemical energy to electricity and thermal energy. A fuel cell 
operating on hydrogen from reformed hydrocarbon fuel combines hydrogen and 
oxygen from air to produce water as the overall chemical reaction in the 
fuel cell. Electrochemical reactions at the two electrodes distinguish the 
fuel cell from simple combustion. 
The fuel cell components are the anode where fuel is oxidized, the cathode 
where oxygen is reduced, and the electrolyte. The electrolyte may be a 
liquid or a solid and separates the reactants to isolate the anode and 
cathode electronically. Tile electrolyte forms an ionic bridge between the 
two electrodes. Multiple cells are stacked in series to achieve higher 
voltages. Fuel cell reactants can be supplied continuously to produce 
electricity indefinitely. 
The net reaction in a fuel cell power plant is the combination of the fuel 
and oxidant to produce water and carbon dioxide. Unlike a process for 
burning fuel to liberate heat, a fuel cell converts chemical energy 
directly into electrical energy and is not limited in efficiency by the 
Carnot cycle. 
A hydrogen/oxygen fuel cell at room temperature can achieve thermodynamic 
efficiencies of 95%. By contrast, a steam or gas turbine power plant being 
limited by the Carnot cycle, achieves less than a 40% energy conversion 
efficiency. 
Fuel cells were used by NASA in the Gemini and Apollo space programs aid 
still are used today in the Space Shuttle. Alkaline H.sub.2 /O.sub.2 fuel 
cells were selected for low weight considerations at high energy density. 
The alkaline H.sub.2 /O.sub.2 fuel cells which powered the Gemini/Apollo 
space capsules and the Space Shuttle cannot use air as the oxidant since 
too much atmospheric CO.sub.2 would be absorbed. Phosphoric acid based 
fuel cells have no such limitation and have been developed as local 
electric power generators. The U.S. armed forces uses them as mobile 
electricity sources and promotes their use in civilian applications. 
ONSI, a private fuel cell manufacturer, is selling 200 kW units which run 
off of natural gas. Over 100 such units have been delivered. However, 
these fuel cells use expensive platinum catalysts. They also require a 
reformer to convert more commonly available hydrocarbon fuels into 
hydrogen. 
Molten carbonate fuel cells (MCFC) use an electrolyte of alkali metal 
carbonates and circumvent kinetic limitations by operating at higher 
temperatures (650.degree. C.). Less expensive nickel catalysts can be 
used. However, a reformer is needed to use standard hydrocarbon fuels. 
Demonstration scale MCFCs have been run and soon will be available 
commercially. 
Greatly advantageous would be a process to produce aluminum which would 
reduce the electrical energy required to produce the aluminum metal on a 
per pound basis, and further which would reduce the overall emissions of 
carbon dioxide into the atmosphere. 
It is an object of the present invention to provide a novel process for the 
electrolytic smelting of alumina to produce aluminum metal. 
It is an object of the present invention to provide a novel process for the 
electrolytic smelting of alumina to produce aluminum metal efficiently and 
cleanly. 
It is an object of the present invention to provide a novel process for the 
electrolytic smelting of alumina to produce aluminum metal which would 
reduce the electrical energy required to produce the aluminum metal on a 
per pound basis. 
It is an object of the present invention to provide a novel process for the 
electrolytic smelting of alumina to produce aluminum metal which would 
reduce overall emissions of carbon dioxide into the atmosphere. 
It is an object of the present invention to provide a novel process for the 
electrolytic smelting of alumina to produce aluminum metal which overcomes 
the problems associated with the heat consuming nature of aluminum 
smelting. 
It is an object of the present invention to provide a novel process for the 
electrolytic smelting of alumina to produce aluminum metal which does not 
require connection to all external electric power grid or the associated 
electrical equipment for connection to an external electric power grid. 
These and other objects of the present invention will be described in the 
detailed description of the invention which follows. These and other 
objects of the present invention will become apparent to those skilled in 
the art from a careful review of the detailed description and by reference 
to the figures of the drawings. 
SUMMARY OF THE INVENTION 
The process and apparatus of the present invention provide method and means 
for electrolytically smelting alumina to produce aluminum metal, including 
providing a combination solid oxide fuel cell and electrolytic smelting 
cell positioned near the solid oxide fuel cell or in contact with the 
solid oxide fuel cell. In one embodiment, the electrolytic smelting cell 
is positioned within no more than two centimeters distance from the solid 
oxide fuel cell. In one aspect, the present invention includes providing 
an alumina ore refinery for producing the refined alumina, wherein the 
refinery is positioned near the solid oxide fuel cell, and passing the 
refined alumina at a temperature of at least 900.degree. C. directly from 
the alumina refinery to the electrolytic smelting cell. In one aspect, the 
present invention incorporates a solid oxide fuel cell of a planar 
construction having a solid state cathode material of lanthanum strontium 
manganate, a solid electrolyte of yttria stabilized zirconia, and a 
nickel/yttria stabilized zirconia cermet anode.

DETAILED DESCRIPTION 
The present invention provides a novel fuel cell powered process for the 
electrolytic smelting of aluminum. Several fuel cells are combined in 
series with an aluminum electrolytic smelting cell. The fuel cells are 
combined in such a way that the electrical power consumed by the aluminum 
electrolytic smelting cell is supplied by the fuel cells. 
In one aspect, the fuel cells preferably are placed in close physical 
contact with the aluminum electrolytic smelting cell. Although the fuel 
cells can be placed in a position physically separate from the aluminum 
electrolytic smelting cell, it has been found that special advantages 
accrue from placing the fuel cells in close physical contact with the 
aluminum electrolytic smelting cell. Such close physical contact 
facilitates heat exchange and reduces electrical resistance. 
In one embodiment, the electrolytic smelting cell is positioned within no 
more than six centimeters distance from the solid oxide fuel cell, 
preferably no more than two centimeters distance from the solid oxide fuel 
cell, and, most preferably, the electrolytic smelting cell is positioned 
within direct physical contact of the solid oxide fuel cell. 
Such close physical contact provides significant heat conduction and lower 
electrical resistance. 
In one aspect, no external electrical power needs to be supplied to the 
cell. All the energy needed to smelt aluminum can come from the fuel and 
air or oxygen supplied to the fuel cell. 
The fuel to be supplied to run the cell can be natural gas, hydrogen, coal 
gas, synthesis gas, or other such materials. 
The oxidizer can be air or oxygen gas derived from smelting or an external 
oxygen plant. 
Waste heat derived from the fuel cell can supply the reversible heat 
consumed by the smelting cell. The waste heat utilization provides for an 
efficient smelting cell design. Additional waste heat is used to reform 
methane, exchange heat with incoming gas streams, or heat an external 
process such as the refining of alumina. 
Energy savings are achieved by eliminating electrical leads, eliminating 
the need to conduct electrical current in electrodes parallel to the plane 
of the electrodes. shrinking the thickness of electrodes and electrolytes, 
increasing the electrode surface area per unit volume, and reducing 
current density. 
The aluminum production electrolytic anode technology incorporated in the 
process of the present invention includes an inert anode, a solid oxide 
anode, all anode depolarized with a reducing gas, or a consumable carbon 
anode. 
Preferably, the fuel cells are solid oxide fuel cells (SOFC). Solid oxide 
fuel cells (SOFC) as used in the process of the present invention for 
producing aluminum incorporate solid electrolytes which conduct oxide ions 
and operate around 700-1000.degree. C. The solid electrolyte in SOFCs 
preferably is yttria stabilized zirconia. There is no liquid electrolyte 
to leak. The kinetics are fast, and current densities are high, even 
without exotic catalysts. SOFCs can be fed simple hydrocarbons, such as 
methane, in the presence of steam. They can reform simple hydrocarbons 
in-situ to H.sub.2 and CO, thus eliminating the need for a separate 
reformer. The SOFC when used in the process of the present invention can 
run continuously for many years. 
In the combined fuel cell/smelting cell of the present invention, the fuel 
cell and the smelting cell provide enhanced efficiencies resulting from a 
close physical contact with each other inside a complete process unit. The 
fuel cell and smeltil--cell are close to the same temperature and can 
transfer heat back and forth. Their close proximity to each other permits 
other energy efficiencies that can not be achieved with separated cells. 
The apparatus of the present invention for reducing a metal salt to a metal 
includes an electrolysis cell having an electrolysis anode, an 
electrolysis cathode, and a metal salt in a chamber between the 
electrolysis anode and cathode; a fuel cell having at fuel cell cathode 
adjacent the electrolysis anode and connected electrically therewith, a 
fuel cell anode, a solid conductor of anions between the fuel cell cathode 
and anode, and a fuel compartment adjacent the fuel cell anode; and 
conductor means for electrically connecting the electrolysis cathode and 
the fuel cell anode. 
The process of the present invention for electrolytically reducing a metal 
salt to a metal in the apparatus of the apparatus of the present invention 
includes (1) reacting a fuel with anions adjacent the fuel cell anode; (2) 
electrolyzing the metal salt in the electrolysis cell, thereby to produce 
a metal adjacent the electrolysis cathode and anions adjacent the 
electrolysis anode; and utilizing the anions of step (2) in the reaction 
of step (1). 
The process of the present invention further includes providing conductive 
heat transfer back and forth from the electrolysis cell and the fuel cell. 
The reduced metal produced by the process of the present invention includes 
aluminum, magnesium, silicon, titanium, lithium, lead, zinc, or zirconium. 
The metal salt reduced by the process of the present invention includes 
aluminum oxide, magnesium oxide, silicon oxide, titanium oxide, lithium 
oxide, lead oxide, zinc oxide, zirconium oxide, or aluminum chloride. 
Referring now to FIG. 1, a combination electrolytic/fuel cell 10 is shown 
having an solid oxide fuel cell (SOFC) component 12 combined with an inert 
anode smelting cell component 14. 
The SOFC component 12 includes three SOFC cells 16 in series to build up 
enough voltage to drive one smelting cell 18. The SOFC component shown in 
FIG. 1 contains only three cells for convenience of illustration. The SOFC 
component preferably will include more than three cells. Each SOFC cell 16 
has a cathode compartment 20 containing air 22 and a solid state cathode 
material 24, e.g., such as lanthanum strontium manganate (LSM), where 
oxygen in the air 22 is reduced to oxide ions 26, as shown in Equation 1. 
Alternatively, the compartment 20 may contain oxygen. 
EQU O.sub.2 +4e.sup.- 2O.sup.2- (Eq. 1) 
Oxide ions 26 electro-migrate through the cathode 24 and a solid 
electrolyte 28, e.g., such as yttria stabilized zirconia (YSZ) to an anode 
30, e.g., such as Ni/YSZ cermet. At the anode 30, oxide ions 26 are 
oxidized in the presence of a hydrogen and carbon monoxide mixture 32 and 
release electrons, as shown in Equations 2 and 3. Other Suitable reducing 
agents may replace all or part of the hydrogen and carbon monoxide 
mixture. 
EQU O.sup.2- +H.sub.2 .fwdarw.H.sub.2 O+2e.sup.- (Eq. 2) 
EQU O.sup.2- +CO.fwdarw.CO.sub.2 +2e.sup.- (Eq. 3) 
Methane-steam mixtures can be used as the fuel, and at the temperatures of 
operation of cell 16, i.e., 900-1000.degree. C., the methane-steam mixture 
can reform to produce hydrogen and carbon monoxide, as shown in Equation 
4. 
EQU CH.sub.4 +H.sub.2 O.fwdarw.CO+3H.sub.2 (Eq. 4) 
The carbon monoxide and hydrogen then can oxidize oxide ions 26 at the 
anodes 30. 
The three fuel cells 16 in series develop enough voltage to drive the one 
inert anode cell 18. Aluminum ions dissolved in the molten bath 33 are 
reduced at a TiB.sub.2 inert cathode 34 to produce aluminum metal. Oxygen 
36 is evolved at the inert anode 38. If the inert anode 38 is porous, 
oxygen gas 36 then can flow to the cathode 24 of the fuel cell 16 where it 
is reduced. Alternately, the inert anode 38 can itself be an oxide 
conductor and conduct oxide ions through the electrolyte 28 to the fuel 
cell anode 30. 
The overall reaction is shown in Equation 5. 
EQU 12O.sub.2 +9CCH.sub.4 +4Al.sub.2 O.sub.3 .fwdarw.+8Al+9CO.sub.2 +18H.sub.2 
O(Eq. 5) 
Referring now to FIG. 2, a combined cell 40 operates in conjunction with a 
Hall-Heroult cell 42. An anode of carbon 44 is oxidized to CO.sub.2 gas 
46. Aluminum metal produced in the cell is deposited on the cathode 47. A 
sliding electrical contact 48 is made to a cathode 50 of a fuel cell 52, 
but no air can be permitted to contact the carbon anode 44 and cause air 
burning. As carbon is consumed, it can be replenished by sliding fresh 
carbon in its place. 
The electrical connections 48 and 54 for adjacent cells 56 are shown in the 
schematic of FIG. 2 as leading out of the cell stack for illustration 
purposes. 
Referring now to FIG. 3, in actual operation, electrical connections are 
constructed directly in the stack as shown in electrical connections 60 
constructed directly in the stack. The electrical connections also serve 
as separators to keep the fuel and oxidizer streams from contacting each 
other. Hydrogen 62 is shown adjacent the anode 30, and oxygen 64 is 
adjacent cathode 24. 
In the case of electrical connections 60 constructed directly in the stack, 
the voltage drop attributable to electrical conduction is minimized 
laterally through cathodes 24 and anodes 30 and in lieu of exterior 
electrical connections. Cathodes 24 and anodes 30 are not required to be 
made thick, just to allow adequate lateral electrical conduction, as is 
the case with conventional Hall-Heroult cell and inert-anode designs. In 
the combined process of the present invention, cathodes 24 and anodes 30 
are made thin, thereby to conserve the cost of materials while at the same 
time reducing IR drop. In the combined process of the present invention, 
thinner cells provide lower material costs, greater metal production per 
unit volume, and more efficient operations. 
Referring now to FIG. 4, a modular SOFC "Power Pack" is shown. The modular 
SOFC "Power Pack" is sealed to prevent bath penetration, in powering a 
consumable carbon anode cell such as a Hall-Heroult cell. The modular SOFC 
unit can be removed from the smelting cell as the cell is decommissioned, 
and the modular SOFC unit then can be reused. Air 72 and methane 74 are 
fed to the fuel cell 70, and POC 78 is withdrawn. POC 78 represents 
products of combustion including CO.sub.2, H.sub.2 O and N.sub.2. Smelting 
takes place between carbon anode 78 and titanium diboride cathode 80 to 
form molten aluminum metal 82. 
The combined fuel cell/electrolytic cell of the present invention 
significantly reduces the energy cost of smelting aluminum. The 
combination cell can be designed with thin gaps and high electrode surface 
area to reduce the IR drop in the cell. 
In the combined fuel cell/electrolytic cell of the present invention, much 
of the capital cost involved with power generation, transmission, 
transforming, rectification, and distribution is eliminated. 
In the combined fuel cell/electrolytic cell of the present invention, the 
amount of aluminum smelted per unit volume is increased. The higher amount 
of aluminum smelted per unit volume reduces capital costs. 
Tile combined fuel cell/electrolytic smelting cell of the present invention 
is run in a continuous operation. The continuous operation reduces labor 
costs. 
In the combined fuel cell/electrolytic cell of the present invention, high 
temperature waste heat is reused in processes, upstream and downstream 
from the smelting cell. 
The higher energy efficiency of the combined fuel cell/electrolytic cell of 
the present invention significantly reduces the amount of carbon dioxide 
released to the atmosphere. The amount of carbon dioxide generated in the 
combined fuel cell electrolytic cell of the present invention to produce a 
pound of aluminum is significantly lower than the amount of carbon dioxide 
generated to produce a pound of aluminum in all process generating 
electricity from coal combined with a conventional refinery and 
Hall-Heroult cell. 
In the aluminum smelting process of the present invention powered by solid 
oxide fuel cells, waste heat from the fuel cells is put to use in other 
parts of the plant. Energy is saved by utilizing the highly efficient fuel 
cells. 
In one aspect, the solid oxide fuel cells integrated closely with the 
smelting cells forms a complete unit with no exterior electrical 
connections. All the electrochemistry is internal to the unit. Alumina, 
fuel, and air are fed to the unit. Exhaust gases and aluminum metal come 
out. The thermodynamics and design of this integrated cell of the present 
invention allow for significant energy efficiencies. 
In one aspect, an alumina ore refilling plant is integrated with the 
smelting plant. The waste heat from the fuel cells supplies part or all 
the heat requirements of the alumina ore refinery, reducing the cost of 
the refined alumina. 
The theoretical maximum thermal efficiency of a fuel cell, based on the 
heating value of the fuel, drops with increasing temperature. More waste 
heat must be evolved as temperature rises. 
In the combination fuel cell/smelting cell of the present invention, 
however, high temperature waste heat has been found to be put to good use. 
The operating temperature of the solid oxide fuel cell (about 100.degree. 
C.) in the aluminum smelting process of the present invention powered by 
solid oxide fuel cells is in the range of the operating temperature of a 
Hall-Heroult cell (960.degree. C.) and also of the fluidized bed calciner 
(900.degree.-950.degree. C.). 
In the aluminum smelting process of the present invention powered by solid 
oxide fuel cells, electrical power is transmitted directly from the DC of 
the fuel cells to the DC of the smelting pot. The power conditioning 
equipment needed to convert the fuel cell DC to high voltage AC current is 
eliminated, as well as the transformers and rectifiers used to convert AC 
to the DC current used by the pot-line. 
In an integrated plant for the alumina refining and aluminum smelting 
process of the present invention powered by solid oxide fuel cells, the 
hot exhaust gases are used to calcine alumina, to provide steam to the 
Bayer plant, and to fire furnaces for metal processing. 
In the combined fuel cell/smelting cell of the present invention, alumina, 
fuel, and oxygen come in. Exhaust gas and aluminum metal come out. The 
system either consumes or produces electrical energy. Heat is rejected to 
its surroundings or absorbed from its surroundings. In a manner of 
analysis, a Hall-Heroult cell is a combination fuel cell/smelting cell. 
Fuel is fed to it in the form of carbon, but it also consumes electricity 
and produces aluminum. 
Reversible systems are ideal systems so perfect and frictionless that they 
are at the maximum efficiency allowed by the second law of thermodynamics. 
The reversible efficiencies of various electrochemical cells can be viewed 
as a function of reversible heat flow. 
Both the Hall-Heroult and Inert Anode smelting cells are endothermic. Under 
ideal or reversible conditions, they must absorb heat from outside sources 
to maintain all entropy balance and therefore satisfy the second law of 
thermodynamics. If they do not absorb external heat, the cell will cool 
down. 
Fortunately, from the standpoint of maintaining cell temperature, the 
present day Hall-Heroult cell is energy inefficient. Enough electrical 
power is destroyed through IR drops and other mechanisms not only to 
supply the reversible heat, but also to create all excess of heat which 
must be removed from the cell. 
The efficiency of smelting cells could improve, for example, by reducing 
the anode-cathode distance, by reducing the electrical resistance of the 
anode and cathode blocks, or by increasing the electrode surface area 
while reducing the current density. However, the cell's heat loses also 
must be cut, through better insulation and more heat recovery from anode 
gases. Eventually, the cell is insulated perfectly, and the heat created 
by cell inefficiencies matches the reversible heat absorbed by the system. 
This represents a practical upper limit on the efficiency of a smelting 
cell. Of course, one could supply the reversible heat from another source, 
such as a gas flame, but practically, it is simpler to destroy electrical 
energy. 
The hydrogen fuel cell represents the other extreme. According to the 
second law of thermodynamics, under reversible conditions it must reject 
heat to the environment. Therefore, out of all the chemical energy made 
available to it, some must be lost as heat, the remainder could 
theoretically be converted to electricity. 
The reversible heat loss of the hydrogen fuel cell goes up with 
temperature. SOFCs must compete with other fuel cell concepts which 
operate at lower temperatures and have higher theoretical efficiencies. In 
practice, however, efficiencies for all present fuel cells are in the 
40-60% range. Overall efficiencies of SOFCs are boosted by utilizing the 
high temperature waste heat, such as generating steam or running a 
turbine. 
The theoretical maximum efficiency of a methane burning fuel cell is 
virtually 100%. It may be viewed as a methane reformer operating in series 
with a hydrogen fuel cell. While the hydrogen fuel cell is exothermic, 
methane reforming is endothermic. The reversible heat given off by the 
hydrogen fuel cell can be reversibly absorbed by the reforming reaction. 
Very little net reversible heat is given off making the maximum 
theoretical efficiency very high. 
The methane fuel cell cannot be expected to be perfectly efficient. Some 
electrical energy will be destroyed and turned into heat which must flow 
from the system. Some fuel gas will leak through pores in the electrolyte 
and react with oxygen to produce heat. The waste heat can supply 
reversible heat to the smelting cell, by putting both cells in intimate 
contact. Other efficiencies appear in both the fuel cell and smelting cell 
as a result of this combination. 
Real combined-cell characteristics involve voltages obtained by combining 
fuel cells in series to create a voltage high enough to drive a smelting 
cell, and specified current density as viewed in the perspective of real 
polarization curves of the cells. 
Referring now to FIG. 5 as an example, experimental polarization curves are 
shown for two different SOFCs A and B multiplied by three to account for 
three cells being connected in series. The two SOFC cells A and B differ 
because of different operating conditions and cell design. Most SOFC 
polarization curves fall between these two cases. 
The theoretical inert anode polarization curves C and D were obtained using 
the equilibrium voltage plus voltage drop through the anode-cathode 
distance, ACD, for ionic conductivity plus the over-potential needed to 
drive the anode and cathode reactions. The voltage drops through the anode 
and cathode materials were discounted. The TiB.sub.2 cathode is very 
conductive. The inert anode itself call be made conductive through metal 
additions. Alternatively, discounting anode wear, the anode can be made 
thin, thus reducing the voltage drop through it. The anode cell voltage is 
actually negative in relation to the fuel cell voltage. 
No external voltage is being applied. So the sum of the voltages of the 
smelting cell and the three SOFCs must be zero. The current through the 
SOFCs and the smelting cell is equal. The operating point is at the 
junction where the SOFC and smelting curves cross in FIG. 5. 
In the combined fuel cell/smelting cell of this example, the operational 
current density ranges from 0.14 to 0.28 A/cm.sup.2. In an illustrative 
embodiment, three SOFCs power one smelting cell. More SOFCs generate a 
greater voltage and permit operation of the smelting cell at greater 
current densities and greater aluminum throughput. Any number of SOFCs can 
be used in series provided enough voltage is generated to drive at least 
the thermodynamic equilibrium voltage of the smelting cell. The 
operational current density for Hall-Heroult cells typically is 1 
A/cm.sup.2. The operational current density for the Inert Anode cell 
typically is 0.3 A/cm.sup.2. Loss of metal production per unit electrode 
area is made up by increasing the surface area of the electrodes per unit 
volume in the combined fuel cell/smelting cell of the present invention. 
The thickness of a single SOFC typically is 150 microns. The gas space and 
separator between cells is on the order of a few millimeters. At a smaller 
dimension for the size of the smelting cell in the combined fuel 
cell/smelting cell of the present invention, the production of aluminum 
per unit volume is high even with the lower current density. 
In the combined fuel cell/smelting cell of the present invention, the fuel 
cell and the smelting cell provide enhanced efficiencies resulting from a 
close physical contact with each other inside a complete process unit. The 
fuel cell and smelting cell are close to the same temperature and can 
transfer heat between each other. Their close proximity to each other 
permits other energy efficiencies that can not be achieved with separated 
cells. In an illustrative embodiment, a slab of electrode material or a 
region of electrolyte has length l, width w, and thickness t. For a 
resistivity, .rho., of such a region, the resistance R for electrical 
current in the perpendicular direction is shown in Equation 6. 
EQU R=.rho.(1.multidot.w/t) (Eq. 6) 
The resistance R for electrical current in the parallel direction is shown 
in Equation 7. 
EQU R=.rho.(w.multidot.t/1) (Eq. 7) 
As the thickness t decreases, the resistance for taking current in or out 
of the electrode in the parallel direction becomes inversely greater. The 
resistance associated perpendicular current becomes proportionately less. 
In the combination apparatus and process of the present invention, a path 
is provided for the progressive miniaturization of both the smelting cell 
and fuel cell components without the exorbitant resistive losses 
associated with conventional parallel circuitry. In the combination 
apparatus and process of the present invention, the electrode and 
electrolyte resistances decrease with miniaturization because of the close 
physical contact. 
In the combined fuel cell/smelting cell of the present invention, capital 
other auxiliary equipment. Power generated in the fuel cell is greater 
because of lower voltage loss in electrical leads and lateral conduction 
attributable to the efficient cell design. 
In the combined fuel cell/smelting cell of the present invention, the 
process integration makes use of the copious amounts of high temperature 
heat evolved from the solid oxide fuel cells. The integrated fuel 
cell/refinery/smelting cell of the present invention integrates a refinery 
with a smelting plant. 
Low cost refineries are located close to bauxite mines to minimize the cost 
of transporting bauxite ore. Low cost smelting plants are situated close 
to sources of cheap electricity, usually hydroelectricity. Few sites have 
both bauxite ore and inexpensive electrical power. So, the alumina 
refining and smelting plants for the production of aluminum have been 
separate entities with the requirement to transport alumina between them, 
from the ore refinery to the smelting plants. 
In the combined fuel cell/smelting cell of the present invention, several 
international locations are known to have both bauxite ore and inexpensive 
natural gas or fossil fuels. The transportation costs of alumina are 
eliminated, e.g., at a reduction of about $0.05/lb on the price of 
aluminum, at $50/tonne transportation cost. 
Waste heat from the SOFCs used to drive the smelter is used to heat the 
refinery as well. 
The integrated process of the present invention uses the efficient DC 
electrical energy generated by the SOFC and the heat transfer which 
supplies the endothermic heat to the smelting cell. In the case of an 
inert anode cell, hot oxygen is supplied to one or more of the driving 
fuel cells. 
The integrated process of the present invention further uses hot gases from 
the fuel cell to fire the fluidized bed calciner of the refining plant. 
The alumina produced in the calciners at 900-950.degree. C. is normally 
cooled down, stock piled, and shipped to the smelter where it is 
stockpiled again and finally fed to the Hall-Heroult cells at close to 
ambient temperature. In an integrated plant of the present invention, the 
alumina produced in the calciners at 900-950.degree. C. is fed as hot 
alumina directly to the smelting cells in a fluidized state. If desired, 
inert gas washes, i.e., depleted SOFC air stream, is used to remove water 
vapor. The hot alumina provides energy to the endothermic smelting cells. 
In an integrated plant of the present invention, a continuous process 
requires no storage of inventory of materials between stages. The hot 
alumina provides energy to the endothermic smelting cells. 
The calciner in the integrated process of the present invention preferably 
uses a portion of its waste heat to produce steam to drive the aqueous or 
"wet" side of the refining process, such as steam to heat the digesters 
and run the evaporators. Therefore the calcination heat is used twice, 
once for calcination and then for steam generation. 
An important part of analyzing the effect of any developmental project or 
action item on net carbon dioxide emissions is not only to examine the 
emissions at the smelter but also the emissions at the power source and 
refinery. A smelter getting its power from hydroelectricity does not mean 
no CO, emissions from burning coal. In a saturated hydroelectricity 
market, as in the U.S., a smelter switching from buying power from a coal 
plant to a hydroelectric source only means that someone else will buy 
power from the coal fired plant. The net effect on carbon dioxide 
emissions is zero. 
However, a smelter cutting in half the electrical energy in a pound of 
aluminum produces a net reduction in CO.sub.2 emissions. If the power 
source were hydroelectric, the smelter could cut its electric demand in 
half. Some one else would buy that hydroelectric power, rather than buying 
power from a coal plant. Somewhere a coal plant would be idled, reducing 
the production of carbon dioxide. 
For the Hall-Heroult cell with conventional refining and a coal fired power 
plant, the total carbon dioxide production is 16.74 lb CO.sub.2 /lb Al. 
For a combined smelting/fuel cell of the present invention integrated with 
a refining plant, the total carbon dioxide production is 2.32 lb CO.sub.2 
/lb Al. At a factor of 8 difference in CO.sub.2 emissions there is a 
factor of about two effect by using, methane as a fuel which produces more 
water vapor and less CO.sub.2, a factor of two reduction in the energy 
needed to smelt aluminum because of less IR drop, and a factor of two 
difference in tile conversion of energy by using an efficient fuel cell 
versus a heat engine. 
Accordingly, a several fold reduction in the CO.sub.2 evolved during the 
production of aluminum is provided even using a fossil fuel burning power 
source. 
The two main types of SOFCs sure tubular and planar. Tubular cells solve a 
sealing problem but lose power because of contact resistance. Energy 
density per unit volume also is low. 
Planar cells have less IR drop and greater energy density, but sealing the 
cells at high temperature and with thermal cycling is an issue. Planar 
SOFCs interface more easily with planar smelting cells. 
Lower operating temperatures (700-900.degree. C.) reduce the rate of 
degradation and corrosion of materials as well as permit the use of metal 
interconnects and seals. Lower operating temperatures reduce material 
costs and fix the leakage problem. However, at lower temperatures, the IR 
drop through the electrolyte becomes prohibitive. 
Some designs use an external reformer and run the SOFC from hydrogen. 
Others use internal reforming. Internal reforming balances the 
exo-thermicity of the hydrogen reaction with the endo-thermicity of the 
reforming reaction. Internal reforming runs the risk of carbon fouling, 
but this is avoided by using excess water vapor. 
High pressure improves cell performance, reduces concentration 
polarization, improves fuel utilization, and permits a smaller 
interconnect space. In using the exhaust gases to heat calcination, the 
cell is pressurized to drive the process flow. High pressure requires a 
compressor and the energy to drive it. 
In the smelting process of tile present invention, the inert anode is 
preferable to sacrificial carbon anodes because it holds geometric 
tolerances, thereby permitting smaller anode current densities (ACD) and 
better electrical efficiencies. The hot oxygen evolved on the anode can be 
used to supply one of the fuel cell cathodes. If no bubbles are evolved, 
as with a porous or oxide conducting anode, the ACD can be made thinner 
still. 
Lower temperatures in the smelting cell reduce the wear on cell materials 
and permit greater use of common metals. The wear rate on an inert anode 
would be less at low temperatures. Sodium activity is less at lower 
temperatures, thereby prolonging cell life. However, the solubility and 
rate of dissolution of alumina is less at lower temperatures, and is 
ameliorated by using external alumina dissolution. 
In the process of the present invention, the inert anode can be porous to 
allow the penetration of oxygen gas to the adjacent fuel cell cathode. The 
pores are treated with CVD BN Chemical Vapor Deposited Boron Nitride to 
prevent bath penetration. Alternately, the inert anode is constructed of 
an oxide conductor, e.g., such as ceria or zirconia, at certain 
temperatures, which conducts oxide ion's directly to the SOFC electrolyte. 
At smelting cell current efficiencies less than 100%, additional air is 
fed to the adjacent SOFC cathode. In the case that HF is observed to 
degrade the inert anode as well as the fuel cell, a completely sealed SOFC 
is used. 
A high 'surface area cell increases the energy efficiency of a smelting 
cell. By increasing the surface area per unit volume, the current density 
is reduced while increasing or maintaining aluminum production. At power 
loss, I.sup.2 R, current density reductions have a marked effect on energy 
losses. Power loss has tile limitation of endothermic heat loss problems. 
The combined fuel cells/smelting cells of the present invention counter 
this effect and make high surface area cells practical. 
External alumina dissolution, involving alumina dissolved in Hall-Heroult 
cell bath in a separate vessel provides saturation and precludes large 
suspended particles from entering the smelting cell. Inert anodes require 
alumina saturated bath to reduce wear rate, but solubility control can be 
hard to maintain in simple cells where the alumina is added directly. 
External alumina dissolution with active recirculation of electrolyte 
attain's this control. Thin ACDs can not be mainitainied with suspended 
alumina particles in the electrolyte which could lead to clogging or 
mucking of the metal pad. Miniaturization could not be achieved without 
external alumina dissolution. 
Process steam from calcination provides added energy efficiency realized by 
converting the waste heat of the calcination process to steam. 
Conventional processes have not surmounted the economic barrier of the 
cost of rebuilding the hot end of the calciner. In one aspect, the process 
of the present invention redesigns the calciner hot-end to handle heating 
with SOFC off-gases and hot alumina feed. Process steam from calcination 
is added as an extra bonus. 
The energy efficiency of the SOFC driven smelting process of the present 
invention provides significant efficiencies by and through utilizing the 
fuel cell waste heat to heat the smelting cell, to heat the incoming, gas 
streams, to drive methane reforming, and to supply heat to the alumina 
refining process. 
Fuel cell and smelting cell efficiencies of the present invention are 
improved by reducing electrical lead resistance, by decreasing the 
thickness of electrolyte and electrodes, by reducing the current density, 
and by simultaneously increasing the electrode area per unit volume. 
In the process of the present invention, net CO.sub.2 reduction is 86% of 
that of aluminum production by the conventional Hall-Heroult Cell. Most of 
the net CO.sub.2 reduction is in the form of idled fossil fuel plants as a 
result of increased efficiency. Some of that is attributable to burning 
methane, which produces more water vapor, as opposed to coal. Minor 
amounts are attributable to supplying heat to the refinery and eliminating 
CO.sub.2 from Hall-Heroult cells with inert anodes. 
Thus, it can be seen that the present invention accomplishes all of the 
stated objectives. 
Although the invention has been illustrated by the preceding detailed 
description, it is not intended to be construed as being limited to tile 
specific preferred embodiments employed therein. 
Whereas particular embodiments of the invention have been described herein 
above, for purposes of illustration, it will be evident to those skilled 
in the art that numerous variations of the details may be made without 
departing from the invention as defined in the appended claims.