long-term energyand environmentalfuture of the United
States is much in the headlines these days. Helping to make the
prospects brighter is a team of Lawrence Livermore scientists working
to develop a method for producing electricity that is safe, relatively
simple, remarkably efficient, and kind to the environment.
Called direct carbon conversion,
the process has been demonstrated convincingly in the laboratory
over the past year. The electrochemical process converts carbon
particles, obtained from different fossil fuels, directly into electricity
without the need for such traditional equipment as steam-reforming
reactors, boilers, and turbines.
The breakthrough Lawrence
Livermore method, the result of a two-year study funded by the Laboratory
Directed Research and Development Program, pushes the efficiency
of using fossil fuels for generating electricity far closer to theoretical
limits than ever before. If adopted on a large scale, direct carbon
conversion would help to conserve precious fossil resources by allowing
more power to be harnessed from the same amount of fuel. It would
also improve the environment by substantially decreasing the amount
of pollutants emitted into the atmosphere per kilowatt-hour of electrical
energy that is generated. Perhaps most important, it would decrease
emissions of carbon dioxide, which are largely responsible for global
What if we could nearly
double the energy conversion efficiency of fossil fuels in electric
power generation over the conversion efficiency of todays
coal-fired power plantswhich is about 40 percentand
thereby cut the carbon dioxide emissions per kilowatt almost in
half? asks lead researcher John Cooper, scientific capability
leader for electrochemistry and corrosion in Lawrence Livermores
Chemistry and Materials Science Directorate. And what if we
could produce a pure carbon dioxide byproduct for sequestration
or industrial use at no additional cost of separation while avoiding
the air pollution problems associated with combustion?
Cooper explains that direct
carbon conversion requires a unique kind of fuel cell. A fuel cell
is an electrochemical device that efficiently converts a fuels
chemical energy directly to electrical energy without burning the
fuel. However, instead of using gaseous fuels, as is typically done,
the new technology uses aggregates of extremely fine (10- to 1,000-nanometer-diameter)
carbon particles distributed in a mixture of molten lithium, sodium,
or potassium carbonate at a temperature of 750 to 850°C. The
overall cell reaction is carbon and oxygen (from ambient air) forming
carbon dioxide and electricity.
The reaction yields 80 percent
of the carbonoxygen combustion energy as electricity. It provides
up to 1 kilowatt of power per square meter of cell surface areaa
rate sufficiently high for practical applications. Yet no burning
of the carbon takes place.
(C) and oxygen (O2) can react in a high-temperature
fuel cell with the carbon, delivering electrons (e) to an external
circuit that can power a motor. The net electrochemical reaction
carbon and oxygen forming carbon dioxideis the same as
the chemical reaction for carbon combustion, but it allows greater
efficiency for electricity production. The pure carbon dioxide
(CO2) product can be sequestered in
an underground reservoir or used to displace underground deposits
of oil and gas.
Water to Boil
not burning fossil fuels to boil water to drive turbines and dynamos
to generate electricity, says Lawrence Livermore electrochemist
Nerine Cherepy, who has been researching the breakthrough concept.
This is a simpler, more efficient, and more environmentally
friendly process that obtains the greatest possible fraction of
energy from the starting fossil fuel with little waste heat.
The thermodynamic efficiency
of the direct carbon conversion cell exceeds the 70-percent requirement
of the next-generation fuel cell envisioned by the Department of
Energy. In contrast, conventional coal- and natural-gas-fired power
plants are typically between 35- and 40-percent efficient. Combined-cycle
pilot plants that burn natural gas in multistage turbines now operate
at 57-percent efficiency, based on the higher heating value of the
fuel. (Higher heating value, or HHV, is the total amount of heat
released when a fuel is burned completely and the products are returned
to their natural, room-temperature states.) High-temperature fuel
cell hybrid systems (fuel cells combined with turbines), such as
a technology developed by Westinghouse, are expected to operate
on natural gas at 60-percent HHV.
Direct carbon conversion
can use fuel derived from many different sources, including coal,
lignite, petroleum, natural gas, and even biomass (peat, rice hulls,
corn husks). Cooper notes that 90 percent of Earths electric
energy comes from the burning of fossil fuels. Half of our fossil-fuel
resources is coal, and 80 percent of the coal belongs to the United
States and Canada, the former Soviet Union, and China. Coal-fired
plants produce 55 percent of U.S. electricityas well as large
amounts of pollutants. As a result, the vast energy reserves of
coal remain underused. Direct carbon conversion has the potential
to be the long-sought clean coal technology.
carbonair fuel cell gives off a pure stream of carbon dioxide
that can be captured without incurring additional costs of collection
and separation from smokestack exhausts. The stream of carbon dioxide,
already only a fraction of current processes, can be sequestered
or used for oil and gas recovery through existing pipelines. (Lawrence
Livermore environmental scientists are studying the sequestering
of carbon dioxide in geologic formations as part of a Department
of Energy effort. See S&TR, December
2000, A Solution
for Carbon Dioxide Overload.)
decomposition method used to turn hydrocarbons into hydrogen and
tiny pure carbon particles used in direct carbon conversionconsumes
less energy and requires less capital than the electrolysis or steam-reforming
processes required to produce hydrogen-rich fuels. Pyrolysis produces
billions of kilograms of carbon blacks annually in the U.S. Carbon
black is a disordered form of carbon produced by thermal or oxidative
decomposition of hydrocarbons and is used to manufacture many different
products, including tires, inks, and plastic fillers.
kinds of fuel cells using phosphoric acid, molten carbonate,
and solid oxide yield electricity from methane fuel at efficiencies
in the range of 35 to 55 percent of higher heating value. Using
waste heat from the fuel cell in turbines (hybrids) can increase
the total efficiency even further. The thermodynamic efficiency
of the direct carbon conversion cell already exceeds the 70-percent
efficiency goal of the 21st century fuel cell envisioned by
the Department of Energy.
Electricity direct from coal
is one of the earliest dreams of electrochemical science. The first
attempts date from the late 19th century, when Boston entrepreneur
William Jacques fashioned a coal fuel battery that used coke electrodes
in a molten sodium hydroxide electrolyte. Because the molten electrolyte
became exhausted, Jacquess invention operated as an exhaustible
battery, not as a fuel cell, despite impressive demonstrations on
the kilowatt scale. Other problems included a buildup of ash entrained
with the fuel, the cost of making the carbon anodes, and the difficulty
of distributing carbon fuel electrodes to the many cells. Efforts
to develop practical carbon-based fuel cells during the 20th century,
such as those tested at the Stanford Research Institute in the 1980s,
were also hindered by the buildup of ash and by the costs and difficulties
of carbon electrode manufacture.
The Lawrence Livermore approach
circumvents the historic barriers to a coal fuel cell by using extremely
fine, virtually ash-free, turbostratic carbon particles
that contain small amounts of ash and have a high degree of structural
disorder on the nanometer scale. The team found that turbostratic
carbon particles, when mixed with molten carbonate to form a slurry,
operate like rigid electrodes when the melt is brought into contact
with an inert metallic screen. Exactly how the carbon particle delivers
energy to the screen is under investigation, but reactive chemicals
in the melt produced by the carbon are likely intermediates.
Also, the team found that
carbon particles can be distributed pneumatically to individual
cells by a small amount of carbon dioxide fed back to the cell from
the continuously produced carbon dioxide stream. (The pneumatic
transport of carbon particles through complex equipment is a widespread
The carbon particles and
oxygen (ambient air) are introduced as fuel and oxidizer, respectively.
The slurry formed by mixing carbon particles with molten carbonate
constitutes the anode. The anode reaction is carbon and carbonate
ions forming carbon dioxide and electrons. At the cathode, which
is similar to that used in other high-temperature fuel cells, oxygen,
carbon dioxide, and electrons from the anode form carbonate ions.
A porous ceramic separator holds the melt in place and allows the
carbonate ions to migrate between the two compartments.
The technology has been demonstrated
in a number of small, experimental cells with reaction areas of
about 3 to 60 square centimeters. The cells feature different designs
and different materials, including stainless steel, ceramic, and
sometimes graphite. Each cell type features tubes for gases to enter
and exit the cell, thermocouples (for measuring temperature), and
a reference electrode. Temperature is maintained by a computer-controlled
furnace. The computer also acquires continuous data on current and
In repeated tests, the cells
deliver up to 0.1 watt continuously per square centimeter and are
80-percent efficient at 80 milliwatts per centimeter. Recently,
using a new cell design that automatically regulates the amount
of molten salt, the team has operated cells for days, simply by
adding more carbon fuel.
principle, any fossil fuel or biomass can be converted to electric
power using direct carbon conversion. For natural gas and oil,
pyrolysis (thermal decomposition) yields hydrogen and carbon.
For dirtier resources (coals, biomass), the carbon may have
to be extracted by reaction with hydrogen, followed by pyrolysis.
carbonoxygen reaction is attractive in two unique ways, says
Cooper. First, almost no entropy change occurs in the overall cell
reaction. (Entropy is a measure of the disorder in a system. A significant
entropy decrease would mean that the cell produces a great deal
of waste heat.) Because the entropy change is close to zero for
the carbonoxygen reaction, 100 percent of the heat energy
of combustion of the carbon can instead be converted by the cell
into electrical energy under ideal conditions.
the driving force for energy production, called electromotive force
or maximum voltage, does not degrade as the carbon is progressively
consumed to make power and carbon dioxide, so the voltage remains
constant.That means that in making a single pass through the cell,
all the carbon is consumed at a maximum yet constant voltage.
Realistically, we can get out a maximum of about 80- to 85-percent
efficiency, based on the heating value of the carbon, when the cell
is operated at a practical rate, which is about 100 milliamperes
per square centimeter, says Cooper. The losses are primarily
those associated with the sluggishness of electrode reactions and
the electrical resistance of the cell. It was the two thermodynamic
propertieszero entropy change and constant electromotive forcethat
first drew our attention to carbon as an attractive electrochemical
fuel. In contrast, the entropy decrease for the hydrogenoxygen
reaction in high-temperature fuel cells limits conversion efficiency
to 70 percent of the fuels HHV, while electrical efficiencies
(about 80 percent) and practical fuel use (about 80 percent) further
reduce the total efficiency to below 50 percent. (See the box below.)
The part of a fuels combustion energy that is not converted
to electric power appears as heat. Some of this heat could be used
to generate steam and drive a turbine generator, as in hybrid systems.
But the additional cost and complexity must be weighed against the
comparatively small additional savings in fuel.
cells in the laboratory are used to react carbon and atmospheric
oxygen. These cells consist of a metal anode current collector,
a ceramic matrix for holding the melt, and a metal screen for
reacting the oxygen from the air.
Cherepy and senior scientific associate Roger Krueger have tested
a number of pure carbons that differ principally in the degree and
nature of disorder on the nanoscale. They have correlated significant
differences in the carbon fuels three-dimensional atomic structures
with their electrochemical reactivities. The more disordered the
carbon atoms, the more easily they yield electrons. Cherepy and
Krueger are paying particular attention to turbostratic carbons,
which feature planes of atoms arranged at different angles and with
lots of defects at the edges that make the atoms more accessible
for chemical reactions. (Graphite, in contrast, has a more ordered
structure and is less reactive by a factor of about 1,000.)
candidate turbostratic carbons exhibiting discharge rates of more
than 20 milliamperes per square centimeter at 0.8 volts have been
analyzed in greater detail with transmission electron microscopy
and x-ray diffraction. Researchers in Livermores Chemistry
and Materials Science Directorate conducted some of these characterization
tests, and other characterizations have been done by Kim Kinoshita
and coworkers at Lawrence Berkeley National Laboratory.
Cherepys investigation has focused largely on carbon blacks
because they have the highest electrochemical reactivity of any
carbon fuel yet tested. Made from a variety of sources, carbon blacks
are the basis of a large commercial industry. Four and a half billion
kilograms per year of carbon black (all turbostratic to various
degrees) are produced annually for automobile tires, pigments, plastics
fillers, wire insulation, and other products. Although most carbon
blacks contain about 0.02 to 0.05 percent residual ash, it should
have no effect on system performance, cost, or cell lifespan because
the rate of ash accumulation would be slow. (Carbon with 0.02-percent
ash would clog the cell after about 50 years, five times the life
expectancy of cell hardware.)
carbon blacks, a range of reactivities has been measured. For example,
one carbon material had a peak power density of about 8 milliwatts
per square centimeter while a second material measured almost 50
milliwatts per square centimeter. A third, the best material tested,
yields energy at about 100 milliwatts per square centimeter and
100 milliamperes at 0.8 volts, sufficient for many fuel cell or
differences in microstructure and nanostructure were found in electron
micrographs of the three samples, although all are nearly pure carbon
and look like black dusts. X-ray diffraction measurements showed
that all had much greater spacing between layers of carbon atoms
than does graphite. The x-ray data also revealed only small areas
of crystallinity compared to graphite. Finally, the more reactive
carbons have higher surface area and were found to oxidize more
rapidly when exposed to high temperatures in air.
notes that the team is working to achieve a better understanding
of the relationship between the nanostructure
of carbons and their electrochemical reactivity in molten salts.
A related goal is being able to predict carbon nanostructure from
the conditions of pyrolysis and the nature of the starting materials
undergoing pyrolysis. Success here is critical to the economic
attractiveness of the process and its ability to draw upon any fossil
fuel resource, he says.
structure of the carbon material is the key to widely different
electrochemical reactivities. The two carbon blacks in (a) and
(b), revealed in photomicrographs at two magnifications, were
produced by pyrolysis at different temperatures and started
out as different fossil fuels. In (c), the degree of disorder
of the carbon increases as the temperature of formation decreases
from 2,000°C down to about 700 to 1,000°C.
Class of Fuels, Many Sources
significant advantage of direct carbon conversion is that practically
any fossil fuel, including coal, lignite, biomass, natural gas,
and petroleum, can produce turbostratic carbons. One method, pyrolysis,
uses moderate temperatures (800 to 1,200°C) to produce a stream
of elemental carbon particles and a stream of hydrogen gas from
a pure hydrocarbon. The byproduct hydrogen gas can be sold for a
number of uses, including chemical synthesis, combustion, and powering
fuel cells. The pyrolysis step consumes 5 to 10 percent of the starting
fuel value (1 to 2 percent is lost because of process inefficiencies).
Some fossil fuels, such as coal and biomass, first require treatment
with hydrogen under high pressure to produce a hydrocarbon that
can then be pyrolyzed into carbon fuel and recyclable hydrogen.
This treatment is called hydropyrolysis and has many variants.
of the most intriguing options is using coal as a carbon source
because of the nations (and the worlds) vast resources
of coal and the difficulty in using coal as a clean energy source.
Because of most coals high sulfur and ash content, it must
undergo hydropyrolysis or some other means of purification.
carbon blacks are equally pure, are made by pyrolysis, and
cost about the same, but they differ significantly in structure
on the nanometer scale. As a result, their electrochemical
reactivities are quite different. The two graphs depict the
three carbons voltage and power
two different functions of electrical currentas tested
in a direct carbon conversion cell. Power densities (bottom)
show carbon-3 reacting at a rate 10 times greater than carbon-1,
providing about 100 milliwatts per square centimeter at 850°C.
(Graphite, by comparison, is about 1,000 times less reactive
carbon from petroleum coke could be highly advantageous for the
carbon conversion cell, says Cooper, because it would likely be
the least expensive source of carbon fuel. Some 2 to 8 percent of
all petroleum refining ends up as petroleum coke, an inexpensive
waste product that is naturally turbostratic and could be modified
and used for direct carbon conversion. The amount of coke produced
will increase as lighter crude resources become exhausted. Because
coke commonly contains 0.25- to 5-percent sulfur, direct carbon
conversion cells would require either coke refining or the use of
graphite conductors in the carbonair cell to prevent sulfur
natural gas, Cooper envisions small (100-kilowatt), transportable
power stations that could be run from any natural gas pipeline.
Such small power stations would be ideal in natural gas production
fields; when a field becomes exhausted, the cell would be moved
to a new location. Natural gas would be filtered and pyrolyzed at
the wellhead. The resulting turbostratic carbon would go immediately
to a direct carbon conversion cell, the hydrogen to a fuel cell,
and hot carbon dioxide from the carbon cell used to displace more
Direct carbon conversion might also make use of a significantly
underused family of fuels that includes biomass, lignite, peat,
and others. Some of this material, such as rice hulls, straw, and
corn stalks, is simply burned in the field after harvest. Antipollution
regulations are increasingly making such burning unlawful. Instead,
such material could be charred, and the carbon component extracted
analyzing the various fuel options, the team, together with Meyer
Steinberg from Brookhaven National Laboratory, has calculated the
total HHV efficiencies for electric power generation through five
different routes to the production of turbostratic carbons, including
petroleum coke, refinery products, natural gas, and lignite coal.
The findings were 80 percent for direct petroleum coke, 67 to 75
percent for natural gas (methane), 72 percent for heavy oil, and
68 percent for lignite.
cells use hydrogen, simple hydrocarbons such as methane,
or carbon to produce electricity electrochemically rather
than by burning them as fuels. Electrochemical
means of providing electricity are generally much more
efficient than burning fossil fuels in power plants
to drive boilers and dynamos. The theoretical efficiencies
of hydrogen or methane fuel cells top out at 69 percent
90 percent, respectively, compared to 40-percent efficiency
for typical power plants. The carbonoxygen reaction
that drives a direct carbon conversion fuel cell is
unique: theoretically, all the potential combustion
heat can be converted to electric power.
Methane and hydrogen
fuel cells have other disadvantages. For one, the fuels
are continuously diluted by their own reaction products
as they are consumed.
The voltage drops to ever-lower values, and as a result,
not all of the fuel can be consumed. For carbon, no
such dilution occurs, and all of the incoming fuel can
make electricity at about the same rate and voltage.
Hydrogen, methane, and carbon fuel cells have practical
voltage efficiency, that is, they operate at 80 percent
of the maximum cell voltage.
The total electrical
efficiency of a fuel cell is the product of three factors:
theoretical efficiency, the fraction of fuel used, and
the voltage efficiency. Carbon has a high total efficiency
because of the favorable thermodynamics of the carbonoxygen
reaction. The actual efficiencies of the hydrogen and
methane cells achieved in pilot plants are listed in
the table below.
Of course, different
kinds and amounts of energy are used in making these
fuels. Methane needs only to be extracted from natural
gasa low-cost technology. Hydrogen can be produced
from nuclear and renewable energies without any production
of carbon dioxide. Carbon can be derived at a low energy
cost from nearly any fossil fuel.
of efficiencies of fuel cells
of fuel used in
available at practical rate
(higher heating value)
(Operating temperature of 750°C. Energy cost of
fuel synthesis is excluded.)
Keep Things Interesting
important aspect of the research effort is estimating costs for
electrical production and for cell components. Petroleum coke is
by far the least expensive source of fuel (costing as little as
5 cent per kilogram) because it is the byproduct of the oil refining
industry. In the carbon black industry, the pyrolysis step costs
about 20 cents per kilogram of carbon produced and thus would contribute
about 3 cents to the cost per kilowatt-hour of electricity generated
using carbon-black fuel.
this time, cost estimates are difficult to make. A final design
for the hardware has not been settled on, and increases in power
density are expected that inversely affect hardware size and cost.
Nevertheless, the cost of the most expensive part of the cellthe
commercial ceramic matrix holding the electrolyte and electrodesis
about $200 per square meter (that is, about $200 per kilowatt at
1 kilowatt per square meter). By comparison, modern gas turbine
plants generate power at about $350 per kilowatt. Currently, the
cost of cell hardware is low enough to be interesting.
sheer simplicity of the cell contributes to keeping costs down.
The cells fundamental thermodynamic properties mean almost
no waste heat and full fuel consumption. Also, because the carbon
conversion process produces pure carbon dioxide ready for sequestration
or industrial use, cell design does not need costly components to
collect and scrub the carbon dioxide before storage or use.
cell components and fuel are nontoxic and relatively hazard-free.
In particular, because the carbonmolten salt slurry does not
explode if inadvertently brought into contact with air, no explosion-prevention
safeguards need to be engineered into the cells.
advanced cell design scales up the dimensions of tested cells
to the 1,000-square-centimeter level. A maximum of 100 watts
is expected from this design.
Cherepy and John Cooper assemble an experimental carbon conversion
Points to Understanding Carbon Conversion and Its
single solution exists to meet 21st century energy
and environmental needs. Electrification of highway
vehicles, conservation, advanced turbines, electrochemical
conversion of fuels (as with direct carbon conversion),
nuclear power, and renewable energy are all likely
to be important.
is critically important to develop technologies that
generate electric power much closer to theoretical
limitsthe best large-scale commercial technologies
are only halfway there.
carbon conversion generates electricity from reacting
carbon and oxygen in a fuel cell and makes a pure
carbon dioxide product available for industrial use
fossil energy as carbon in a carbon fuel cell produces
little waste heat and consumes all the fuel in a single
pass, thereby bringing total efficiencies of 70 to
80 percent into reach.
for the Grid
is thinking ahead to the day when direct carbon conversion units
could be used to generate electric power for the grid. Such a power
plant would be large, but not gigantic.
3-gigawatt direct-carbon-conversion power plant, big enough to continuously
supply some 3 million homes with about 1 kilowatt each, would only
be the size of a large, two-story office building.
achieve commercial adoption, however, requires greater understanding
of the underlying science, especially
the three-way relationship between conditions of pyrolysis, the
resulting carbon nanostructure, and the electrochemical reactivity.
While pyrolysis of natural gas and oil products to make turbostratic
carbons is well known and widely practiced, the extraction of carbon
from coal is less developed. The extraction of carbon from
coal, for example, by hydropyrolysis, needs to be developed if this
approach is to aid the conversion of 50 percent of Earths
fossil fuels, says Cooper.
team is planning to scale up a demonstration unit from the 3-watt
experimental cell to a stackable, 100-watt engineering module with
1,000 square centimeters of active area. The large-scale experiments
should reveal any materials and operational problems on a practical
scale, especially during extended tests.
the team is testing more carbon blacks from commercial suppliers
and turbostratic carbon fuels from new sources, such as petroleum
cokes and coals. The tests with coal will be particularly important
because of its large-scale reserves.
points to the complex task of providing energy while controlling
greenhouse gases, particularly carbon dioxide. The solution
is beyond the scope of power production technology alone,
he says, noting that electrical energy production currently accounts
for just one-sixth of the total output of carbon dioxide. Advanced
combustion, fuel cells, nuclear and renewable energy, and conservation
may all combine to help the situation in a way that cannot presently
Livermore team considers it vitally important to develop a simple
fuel cell technology that greatly increases the yield of electric
energy from each unit of fossil fuel, uses fuels derived efficiently
from almost any fossil fuel, significantly decreases the carbon
dioxide released into the atmosphere, and makes it easy to capture
the carbon dioxide for sequestration or other use.
were just beginning to hear about direct carbon conversion.
biomass, carbon black, carbon dioxide, coal, direct carbon conversion,
fuel cell, global warming, hydrogen fuel cell, hydropyrolysis, natural
gas, petroleum coke, pyrolysis, turbostratic carbon.
information contact John Cooper (925) 423-6649 (firstname.lastname@example.org).
received his B.A. in chemistry from Pomona College in 1968 and
his Ph.D. in chemistry from the University of California at
Berkeley in 1975. For 25 years, he has specialized in electrochemical
science and engineering, with particular emphasis on fuel cells,
fuel batteries, and power generation using reactive metals,
zinc, aluminum, or elemental carbon with air-depolarized cathodes.
He led the DOE National Program to develop novel metalair
fuel batteries for electric vehicle propulsion. He assembled
and led a team to develop advanced processes for the growth
and production of optical crystals for lasers. In addition,
he has led projects to develop advanced processes, such as molten-salt
oxidation, for treating mixed waste and military waste.
Cooper is scientific capability leader for electrochemistry
and corrosion in the Chemistry and Materials Science Directorate.
He is also the technical director of a private-sector collaboration
to develop zincair fuel cells and batteries and is the
inventordirector of projects to develop practical high-efficiency
carbonoxygen cells for mitigating the greenhouse gas emissions
associated with electric power generation.