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An Overview of Carbon Sequestration |
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Carbon sequestration is the capture,
from power plants and other facilities, and storage of
carbon dioxide (CO2) and
other greenhouse gases that would otherwise be emitted
to the atmosphere. The gases can be captured at the
point of emission and can be stored in underground
reservoirs (geological sequestration), injected in deep
oceans (ocean sequestration), or converted to rock-like
solid materials (advanced concepts). |
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Fossil fuels will remain the
mainstay of energy production well into the 21st
century. Availability of these fuels to provide clean,
affordable energy is essential for global prosperity and
security. However, unless energy systems significantly
reduce the carbon emissions to the atmosphere, increased
atmospheric concentrations of CO2 due to carbon emissions are
expected.
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To stabilize and ultimately
reduce concentrations of CO2 it will be necessary to
capture, separate, and store or reuse carbon dioxide.
Carbon sequestration, along with reduced carbon content
of fuels and improved efficiency of energy production
and use, must play a major role if the world is to
continue using fossil fuels as a key energy source.
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Capture and Separation |
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Before CO2 gas can be sequestered from
power plants or industrial sources, it must be captured
as a relatively pure gas. CO2 is routinely separated and
captured as a by-product from industrial processes such
as synthetic ammonia production, hydrogen production,
and limestone calcination. However, existing capture
technologies are not cost-effective for widespread
CO2 sequestration.
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Carbon dioxide capture is
generally estimated to represent three-fourths of the
total cost of a carbon capture, storage, transport, and
sequestration system. Evolutionary improvements in
existing CO 2 capture
systems and revolutionary new capture and sequestration
concepts will be needed to bring carbon capture costs
down. The most likely options currently identifiable for
CO 2 separation and
capture include the following:
- Absorption (chemical and physical)
- Adsorption (physical and chemical)
- Low-temperature distillation
- Gas separation membranes
- Mineralization and biomineralization
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Opportunities for significant cost
reductions exist. Several innovative schemes have been
proposed that could significantly reduce CO2 capture costs, when compared
to conventional processes. "One box" concepts that
combine CO2 capture with
reduction of criteria-pollutant emissions need to be
explored. |
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Geological Sequestration |
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Carbon dioxide sequestration in
geologic formations includes use of site such as
depleted oil and gas reservoirs, shale formations with
high organic content, unmineable coal seams, and
underground saline formations. |
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Depleted
Oil and Gas Reservoirs In some cases,
production from an oil or natural gas reservoir can be
enhanced by pumping CO2
into the reservoir to push out the product, a process
called enhanced oil recovery. Enhanced oil recovery
(EOR) represents an opportunity to sequester carbon at
low net cost, due to the revenues from the recovered
oil/gas. |
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In an EOR application, the integrity
of the CO2 that remains
in the reservoir is well-understood and very high, as
long as the original pressure of the reservoir is not
exceeded. The scope of this EOR application is currently
economically limited to point sources of CO2 emissions that are near an
oil or natural gas reservoir. |
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Unmineable
Coal Seams Coal beds typically contain
large amounts of methane-rich gas that is adsorbed onto
the surface of the coal. The current practice for
recovering coal bed methane is to depressurize the bed,
usually by pumping water out of the reservoir. An
alternative approach is to inject carbon dioxide gas
into the bed. Tests have shown that CO2 is roughly twice as adsorbing
on coal as methane, giving it the potential to
efficiently displace methane and remain sequestered in
the bed. CO2 recovery of
coal bed methane has been demonstrated in limited field
tests, but much more work is necessary to understand and
optimize the process.
Similar to the by-product
value gained from enhanced oil recovery, the recovered
methane provides a value-added revenue stream to the
carbon sequestration process, creating a low net cost
option. Much of the world's coal is unmineable due to
seam thickness, depth, and structural integrity. Another
promising aspect of CO2
sequestration in coal beds is that many of the large
unmineable coal seams are near electricity-generating
facilities that are large point sources of CO2 gas. Thus, limited pipeline
transport of CO2 gas
would be required. Integration of coal bed methane with
a coal-fired electricity generating system can provide
an option for additional power generation with low
emissions. |
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Underground Saline
Formations Sequestration of CO2 in deep saline formations
does not produce value-added by-products, but it has
other advantages. First, the estimated global carbon
storage capacity of saline formations is large, making
them a viable long-term solution. |
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Second, most existing large CO2 point sources are within easy
access to a saline formation injection point. Therefore
sequestration in saline formations may be compatible
with a strategy of transforming large portions of the
existing energy and industrial facilities to near-zero
carbon emissions. |
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Assuring the environmental
acceptability and safety of CO2 storage in saline formations
is a key component of this program element. Determining
that CO2 will not escape
from formations and either migrate up to the earth's
surface or contaminate drinking water supplies is a key
aspect of sequestration research. Although much work is
needed to better understand and characterize
sequestration of CO2 in
deep saline formations, a significant baseline of
information and experience exists. For example, as part
of enhanced oil recovery operations, the oil industry
routinely injects brines from the recovered oil into
saline reservoirs. |
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Since 1996, the Norwegian oil
company, Statoil, is has been injecting approximately
one million tons per year of recovered CO2 into the Utsira Sand, a
saline formation under the North Sea associated with the
Sleipner West Heimdel gas reservoir. The amount being
sequestered is equivalent to the output of a
150-megawatt coal-fired power plant. This is the only
commercial CO2 geological
sequestration facility in the world using a saline
reservoir to store CO2.
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Ocean Sequestration |
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CO2 is
soluble in ocean water, and oceans both absorb and emit
huge amounts of CO2 into
the atmosphere through natural processes. It is widely
believed that the oceans will eventually absorb most of
the CO2 in the atmosphere. However, the kinetics of
ocean uptake are unacceptably slow. The program will
explore options for speeding up the natural processes by
which the oceans absorb CO2 and for injecting CO2 directly into the deep ocean.
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The concept of ocean sequestration is
in a much earlier stage of development than other
sequestration approaches. Ocean sequestration has huge
potential as a carbon storage sink, but the current
level of scientific understanding to support ocean
sequestration as a major sequestration option is not
currently available. |
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Enhancement of Natural Carbon
Sequestration CO2 absorption in the ocean
involves adding combinations of micronutrients and
macronutrients to those ocean surface waters deficient
in such nutrients. The objective is to stimulate the
growth of phytoplankton, which are expected to consume
greater amounts of carbon dioxide. When carbon is thus
removed from the ocean surface waters, it is ultimately
replaced by CO2 drawn
from the atmosphere. The extent to which the carbon from
this increased biological activity is sequestered is
unknown at this point, and would require additional
research. Any R&D on natural enhancement would also
require complete examination of potential environmental
issues. |
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Direct
Injection of CO2 Technology exists
for the direct injection of CO2 into deep areas of the ocean;
however, the knowledge base is not adequate to optimize
the costs, determine the effectiveness of the
sequestration, and understand the potential changes in
the biogeochemical cycles of the ocean.
To assure
environmental acceptability, developing a better
understanding of the ecological impacts of both ocean
fertilization and direct injection of CO2 into the deep ocean is a
primary focus of this program element. It is known that
small changes in biogeochemical cycles may have large
consequences, many of which are secondary and difficult
to predict. Of particular concern to some is the
potential effect of CO2
on the acidity of ocean water. |
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Terrestrial Sequestration |
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Carbon sequestration in terrestrial
ecosystems is either the net removal of CO2 from the atmosphere, or the
prevention of CO2
emissions from the terrestrial ecosystems into the
atmosphere. |
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Enhancing the natural processes that
remove CO2 from the
atmosphere is thought to be one of the most
cost-effective means of reducing atmospheric levels of
CO2, and forestation and
deforestation abatement efforts are already under way.
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The terrestrial biosphere is
estimated to sequester large amounts of carbon
(approximately 2 billion metric tons of carbon per
year). R&D in this program area seeks to increase
this rate while properly considering all the ecological,
social, and economic implications. There are two
fundamental approaches to sequestering carbon in
terrestrial ecosystems: (1) protection of ecosystems
that store carbon so that sequestration can be
maintained or increased; and (2) enhancement of the
ability of ecosystems to increase carbon sequestration
beyond current conditions. |
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Research is focused on integrating
measures for improving the full life-cycle carbon uptake
of terrestrial ecosystems, including farmland and
forests, with fossil fuel production and use. The
following ecosystems offer significant opportunity for
carbon sequestration: |
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- Forest lands
The focus includes below-ground carbon and long-term
management and utilization of standing stocks,
understory, ground cover, and litter.
- Agricultural
lands The focus includes crop lands,
grasslands, and range lands, with emphasis on
increasing long-lived soil carbon.
- Biomass
croplands As a complement to ongoing efforts
related to biofuels, the focus is on long-term
increases in soil carbon.
- Deserts and degraded
lands Restoration of degraded lands offers
significant benefits and carbon sequestration
potential in both below-and above-ground systems.
- Boreal wetlands and
peatlands The focus includes management of
soil carbon pools and perhaps limited conversion to
forest or grassland vegetation where ecologically
acceptable.
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Advanced Concepts |
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Advanced
Chemical and Biological Approaches
Recycling or reuse of CO2
from energy systems would be an attractive alternative
to storage of CO2. The
goal is to reduce the cost and energy required to
chemically and/or biologically convert CO2 into either commercial
products that are inert and long-lived or stable solid
compounds. |
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Two promising chemical pathways are
magnesium carbonate and CO2 clathrate, an ice-like
material. Both provide quantum increases in volume
density compared to gaseous CO2. As an example of the
potential of chemical pathways, the entire global
emissions of carbon in 1990 could be contained as
magnesium carbonate in a space 10 kilometers by 10
kilometers by 150 meters. |
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Concerning biological systems,
incremental enhancements to the carbon uptake of
photosynthetic systems could have a significant positive
effect. Also, harnessing naturally occurring,
non-photosynthetic microbiological processes capable of
converting CO2 into
useful forms, such as methane and acetate, could
represent a technology breakthrough. An important
advantage of biological systems is that they do not
require pure CO2 and do
not incur costs for separation, capture, and compression
of CO2 gas. |
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Research will seek to develop
novel and advanced concepts for capture, reuse, and
storage of CO2 from
energy production and utilization systems based on, but
not limited to:
- Biological systems;
- Advanced catalysts for CO2 or CO conversion;
- Novel solvents, sorbents, membranes and thin
films for gas separation;
- Engineered photosynthesis systems;
- Non-photosynthetic mechanisms for CO2 fixation (methanogenesis
and acetogenesis);
- Genetic manipulation of agriculture and forests
to enhance CO2
sequestering potential;
- Advanced decarbonization systems; and
- Biomimetic systems.
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