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AAAS Policy Brief: Carbon Capture & Storage
Issue Summary | Resources
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Introduction
Carbon Capture
Carbon Storage: Geologic Formations
Carbon Storage: Indirect Sequestration
Policy Approaches
Introduction
Coal is found on all seven continents and its widespread availability makes
it a preferred energy source for many developed and emerging economies.
For example, coal power plants supply nearly 50 percent of the electricity in the
United States. China and India, both of which rely heavily on coal for electricity production, are expected to account for almost 80 percent of the increase in world demand for coal by 2030 with China alone expected to nearly triple its consumption.1
Coal combustion results in the release of carbon dioxide, which affects
the Earth's climate. U.S. coal plants emit 1.5 billion tons of carbon
dioxide (CO2) per year, about 26 percent of total CO2 emissions
in the United States. 2
Due to growing concerns about energy security and climate change, research, development
and legislation have emerged that focus on capturing and storing ("sequestering")
emissions from coal combustion. Capturing CO2 and then storing it is one
potential tool to reduce greenhouse gas emissions while allowing continued
use of fossil fuels for energy production. A 2007 study by the U.S. Department of Energy found that geoloic formations in the U.S. and Canada could store up to a few centuries worth of U.S. emissions, demonstrating the tremendous potential of this technology.3
However, while carbon capture and storage (CCS) is currently possible, it cannot yet be implemented
at a scale to significantly contribute to climate change mitigation. A number
of research and small-scale implementation projects are developing and
testing various types of capture and sequestration in an attempt to make
CCS an economically-viable technology. Key considerations for evaluating
types of sequestration and their efficacy include ecological impact, cost,
timescale, and amount of carbon sequestered. Further research and development
is necessary, particularly large-scale tests of sequestration.
The federal government has begun to develop regulations for this emerging field. In July 2008, the Environmental Protection Agency (EPA) released for public comment proposed regulations to govern the sequestration of carbon dioxide. The rule would create a new class of injection wells under the authority of the Safe Drinking Water Act's Underground Injection Control program.
Though no bills focused on carbon capture and storage passed during the 110th session of Congress, the Emergency Economic Stabilization Act (H.R. 1424) contained incentives for CCS demonstration projects. The bill would amend the tax code to provide tax credits for storage of carbon dioxide for qualifying facilities. Back to top
Carbon Capture
The first step in any type of direct carbon sequestration is carbon capture.
Carbon capture has been conducted on a small scale in the private sector
for several decades, as CO2 is a commodity that can be sold for various
chemical manufacturing processes. However, the majority of CO2 currently
captured is from sources that produce relatively pure CO2, such as oil
and gas refineries or ethanol plants, where CO2 composes up to 99 percent
of the exhaust stream.
In contrast, the exhaust from coal power plants is generally less than
15 percent CO2 and capture is therefore much more difficult and costly.
Most analyses indicate that the majority of the costs of carbon capture
and sequestration will come from the carbon capture process. Thus, research
and development (R&D) is focusing on increasing the capacity for carbon
capture and lowering the cost.
There are a variety of methods being used and developed for carbon capture,
but most follow the same general principles: a combination of solvent
reactions and shifting pressure and temperatures is used to extract CO2
from the exhaust of a coal power plant. New methods being explored include
various types of physical and chemical adsorption and absorption, including
polymeric membranes and carbon fiber molecular sieves; low-temperature
distillation; mineralization and biomineralization.4
Since CCS would impact both the cost and efficiency of a coal plant, it is expected that carbon capture will be most efficient
and least costly if a plant is designed with capture in mind from its
inception, rather than trying to "add-on" or retrofit carbon
capture technology to existing plants. Because of the pressures and temperatures
needed for carbon capture, a plant that is designed for integrated carbon
capture will be fundamentally different than one built without it.
The type of power plant and type of coal will impact the design of the
carbon capture system.5 Two main types of
plants are in use today:
•Pulverized
coal power plants:
The traditional and most common type of coal power plant is a pulverized
coal plant. The efficiencies of these systems depend on design, operating
parameters, and coal type and range from 33 to 40 percent, with further
advances expected to create plants with efficiencies as high as 46 percent.
Implementing carbon capture on a subcritical coal plant is expected to drop
the efficiency by approximately 9 percent. For a supercritical coal plant,
implementing carbon capture is expected to increase the cost of electricity
61 percent from 4.78 cents per kilowatt hour (¢/kWh) to 7.69 ¢/kWh.
6
• IGCC
power plants:
A newer type of coal power plant is the Integrated Coal Gasification Combined
Cycle (IGCC) plant. IGCC plants incorporate a component
that gasifies the coal, creating a "syngas" that is then combusted
in a turbine. IGCC plants are generally more efficient than pulverized coal
plants, and it is easier to remove some emissions such as sulfur dioxide
(SO2) and nitrogen oxides (NOX). IGCC plants are more costly to
build, and there is little industry experience with their reliability since
they are all relatively new plants. Implementing carbon capture at IGCC
plants is expected to lower efficiency from 38 percent to about 31 percent,
and raise the cost of electricity 27 percent, from 5.13 ¢/kWh to 6.52
¢/kWh. 7
Much of the discussion about carbon capture has focused on applying this
technology to new IGCC plants. However, neither IGCC nor pulverized coal
plants currently incorporate carbon capture. Research and development
efforts are working towards industry-scale, economic methods for carbon
capture at both types of plants. Because the two types of plants
have very different combustion systems, the most effective method of carbon
capture will likely be different for each.
A new technology was recently announced that may provide a way to capture
carbon from ambient air rather than from point sources such as power plants.
A demonstration project was able to collect small amounts of CO2
and further development is working towards larger scale deployment.8
Regardless of the method of carbon capture, the captured CO2 must then
be compressed and transported to a sequestration site. Because of the
oil industry, a significant pipeline network exists in the southwest part
of the United States, but little infrastructure exists elsewhere. The
cost of compressing and transporting CO2 and the necessary infrastructure
must be considered in CCS development plans. Back to top
Carbon Storage
Geologic Sequestration
Once CO2 has been captured, compressed, and transported to a sequestration
site it can be pumped deep underground and injected into a variety of
geologic formations for storage. The variation between types of geologic
formations is significant, and even within a particular type the variation
between sites can be considerable. Characteristics to consider in evaluating
sequestration sites include:
- Depth of the formation, which influences pressure and temperature,
both of which have a significant effect on the potential stability of
the CO2
- Porosity of the surrounding material
- Presence of potential vents, such as old oil wells
- Nearness to freshwater aquifers
- Potential volume for CO2 storage
- Proximity to sources of CO2
The expected storage time for direct sequestration is on the order of
millennia. Because of the long time period involved, even an annual leak
rate of <1 percent could make the sequestration ineffective for climate
change mitigation. Large-scale sequestration tests have not yet been conducted
over long enough time periods or with sufficient monitoring to have a
solid understanding of the potential for leaks after injection, though
research is ongoing.
Few environmental impacts of geologic sequestration have been identified,
but studies are ongoing. In particular, the possible effects of CO2 sequestration
on groundwater are not yet well documented and the subject of research
projects.
Types of geologic formations that have been considered, and in some cases
tested, for carbon sequestration include:
Oil and gas fields
Most industry experience injecting CO2 below ground is in oil and gas
fields. The oil industry has been using a process called Enhanced Oil
Recovery (EOR) for several decades wherein they inject CO2 into depleted
oil fields to increase their yields. However, EOR usually only employs
injection on the scale of 1000's of tons of CO2 per year for each site, while a typical
coal power plant produces 2-4 million tons of CO2 per year. Because of
the difference in scale between EOR and the hopes for greenhouse gas emission
mitigation sequestration, the technology is not directly transferable.
Such an increase will likely require the development of innovative technologies,
rather than merely an amplification of current methods.
A significant international sequestration project has been undertaken
at the Weyburn oil field in Canada that attempts CO2 injection for EOR
on a larger scale. The initial project demonstrated both the potential
for CO2 transport and substantial storage in oil fields, with
the economic benefit of significantly increased oil output. Additional
studies that aim to improve injection efficiencies and CO2
monitoring continue at the site. Overall, the site is expected to store 20 million tons of carbon CO2 over the project lifetime.9
Submarine sediments
A potential benefit of injecting CO2 into rocks beneath the
seafloor is that injected CO2 may be more stable in these submarine
sediments than terrestrial formations. Some scientists have found that
a depth range exists in submarine sediments where injected CO2
will be denser than the pore fluid surrounding it, which means that the
CO2 will be gravitationally stable and will not rise up. In most terrestrial
sequestration scenarios the injected CO2 will be less dense
than any surrounding fluid, and will therefore rise unless it is stopped
by some non-porous caprock material. Though the capacity for submarine
geologic sequestration is extremely vast, transportation of the CO2
from capture sites is likely to be the main constraint.10
The largest volume, longest running sequestration project to date uses
submarine sequestration at the Sleipner natural gas field in the North
Sea off the coast of Norway. Since 1996, the Statoil company has been
sequestering 1 million tons of CO2 a year from its natural gas refinery
into sediments deep beneath the seafloor. This has been economically profitable
for Statoil, who must pay for released into the
atmosphere under Norway's emission tax program. 11
Saline aquifers
Sometimes large, deep formations of porous rock (such as sandstone and
limestone) contain large amounts of briny water in their pore space. These
formations are known as saline aquifers, and the water they contain can
be displaced by CO2, providing a storage site.
In 2004 an experimental sequestration project was undertaken in a saline
aquifer near Frio, Texas that has closely monitored the injected CO2.
Results thus far closely matched predictions from detailed models of the
system and indicate that saline aquifers are a viable option for geologic
CO2 sequestration. 12
Unmineable coal seams
Carbon dioxide can be sequestered by pumping it into unmineable coal seams,
where it adsorbs to the surface of the coal. A major benefit of this method
is that the injected CO2 will displace methane that was adsorbed to the
coal, and this methane can be collected at the surface and sold, offsetting some of the costs of sequestration.
Consol Energy began a project in 2001 to pioneer carbon sequestration
in unmineable coal seams. The project, supported by the Department of Energy, is still
in the first phase, wherein 50-60 percent of the methane is being degassed
before CO2 sequestration is begun. Not only could unmineable coal seams store up to 156 billion metric tons of emissions, but the IPCC has determined that well-selected and managed projects could store 99 percent of injected CO2 for up to 1,000 years.13
Back to top
Carbon Storage
Indirect sequestration:
Carbon can be sequestered "indirectly" by inducing
the marine or terrestrial biosphere to take up more CO2. These methods
are called indirect because they rely on influencing natural processes
that lead to carbon sequestration, but not executing the carbon sequestration
itself. Therefore, one of the main challenges of indirect sequestration
is the limited degree of control over the biologic systems being manipulated.
Marine and terrestrial indirect sequestration are expected to provide
less permanent carbon storage than direct sequestration, on the order
of decades or at most centuries, because of the relatively rapid turnover
time of the systems.
Marine Sequestration ("Ocean fertilization")
Marine indirect sequestration is based on the idea that increased
CO2 uptake could be induced by spurring the growth of marine
phytoplankton (algae and other organisms) in the ocean surface. Studies
have shown that this can be done in some regions by "fertilizing"
the oceans surface with iron, and it is possible that regions with different
nutrient limitations could respond to other nutrient enrichments. When
phytoplankton growth increases, more CO2 is drawn out of the
atmosphere, and the hope is that once this biomass has accumulated in
the surface ocean a significant fraction of it will sink to the ocean
floor (a naturally occurring process) -- effectively sequestering CO2
from the atmosphere in the ocean. However, only a fraction of the phytoplankton
that grows will eventually sink to the ocean floor, meaning that much
of the CO2 which is drawn out of the atmosphere will be returned
in a matter of days to years as the phytoplankton die, decompose, or are
ingested by other near-surface organisms.
Inducing large blooms of phytoplankton could have ecological consequences
for the ocean that are not yet well understood, such as the growth of
algal species and the depletion of oxygen. The scientific advisory group
for the parties of the London Convention, the treaty governing ocean dumping,
has issued a public statement on the topic that "knowledge about
the effectiveness and potential environmental impacts of ocean iron fertilization
currently is insufficient to justify large-scale operations."
Terrestrial Sequestration:
Most methods of indirect sequestration in the terrestrial biospheres work
on the concept of inducing enhanced uptake of CO2 by increasing the growth
of land plants through planting trees, mitigating deforestation, or adjusting
forest management practices. Under the Kyoto Protocol, parties may offset their emissions by increasing the amount of greenhouse gases removed from the atmosphere by certain terrestrial sequestration activities. Key issues regarding offsets relate to determining if they are real, additional, permanent, and independently verifiable.
Because trees take in the most carbon when they are young and rapidly
growing, there has been some discussion of replacing old growth forests
with young new trees; however, cutting down old growth trees releases
CO2, and therefore offsets much of the carbon benefits of the young new
trees.
In addition to plants, the soil can be a significant carbon storage reservoir,
and therefore land use practices can have a significant impact on the
carbon budget of the terrestrial biosphere. Forestry and agriculture practices
can be adjusted to increase soil carbon uptake, and reintroduction of
wetlands has also been shown to be a very effective way to draw down carbon.
Terrestrial indirect sequestration presents less of an ecological concern
than marine sequestration, and many practices that would enhance terrestrial
uptake of CO2, such as planting trees, preventing deforestation, and reintroducing
wetlands, are ecologically beneficial apart from their impact on climate
change, though the change in land use may have effects on local industries.
However, most terrestrial sequestration methods are relatively short-lived
compared to direct geologic sequestration. Back to top
Policy Approaches
The Department of Energy has led federal CCS R&D efforts. DOE's flagship project was FutureGen, a proposed 275 Megawatt IGCC
power plant that would incorporate both CCS and hydrogen production. 14 In January 2008, DOE announced that it would not proceed with the original plan for FutureGen, instead focusing on multiple, smaller commercial-scale demonstration plants. The Obama Administration appeared to have reserved that decision by awarding over $1 billion in stimulus funds as a “provisional agreement” to revive the project in June 2009.
In addition, DOE's Regional Carbon Sequestration Partnerships is a network of seven Regional Carbon Sequestration Partnerships (RCSPs) to help develop the technology, infrastructure, and regulations to implement large-scale CO2 sequestration in different regions and geologic formations.
Several bills have been introduced in Congress that aim to stimulate
research, development, and deployment of CCS technology, although only two have recently been passed. The Energy Independence and Security Act (EISA) of 2007 (P.L. 110-140 and H.R. 6), signed into law on December 19, 2007, directs the Department of the Interior (DOI) to develop a methodology for determining capacity for geologic storage and biologic sequestration of CO2, create an inventory of stored CO2, and to submit a regulatory framework for sequestering and geologically storing CO2 on public lands. The law also requires the Department of Energy (DOE), in concert with the National Academy of Sciences, to perform a review of its CCS R&D programs, develop graduate degree programs on geologic sequestration science, and provide grant for university-based R&D on carbon sequestration for various types of coal. Finally, the law requires the Environmental Protection Agency (EPA) to determine the environmental impacts of carbon sequestration on public health and safety and the environment.15
The EPA submitted its proposed rule for regulation of underground injection of CO2 under the Safe Water Drinking Act on July 25, 2008. The U.S. Geological Survey (USGS), a DOI agency, has cooperated with DOE on CCS research that produced the Carbon Sequestration Atlas of the U.S. and Canada.16 In March 2009, USGS released a plan for assessing potential locations for underground storage. The assessment will focus on geologic formations that are "technically accessible" with today’s drilling and injection technologies and can hold liquefied carbon for over 10,000 years. 17 On June 3, U.S. Secretary of the Interior Ken Salazar released a report containing recommendations for carbon capture and sequestration (CCS) on U.S. public lands that contains information about potential sites and a plan for leasing lands, environmental protection, and federal liability
In addition to the EISA of 2007, the Emergency Economic Stabilization Act (H.R. 1424), which was signed into law on October 3, 2008 (P.L. 110-343), contains incentives for CCS demonstration projects. The bill also amended the tax code to provide tax credits for storage of carbon dioxide for qualifying facilities.
The American Recovery and Reinvestment Act (HR 1) also contained several measures for advancing CCS technology, totaling $2.4 billion in related funding. This included funding for general fossil energy R&D, CCS technology demonstration, industrial carbon and efficiency projects, geologic site characterization, and training and research grants.
On May 7, a bipartisan group of Senate Energy and Natural Resources Committee members, including Chairman Jeff Bingaman (NM), introduced the Department of Energy Carbon Capture and Sequestration Program Amendments Act of 2009 (S 1013). The bill, which would allow the Energy Secretary to assume control of long-term carbon storage sites, lays out additional funding for ten large-scale CCS demonstration projects to be sponsored by DOE, establishes requirements for science-based maintenance and monitoring of geologic sites, and creates a grant program for state personnel training. A hearing on the bill was held on May 14.
On July 28, 2009, the House Select Committee on Energy Independence and Global Warming held a hearing to discuss new, cutting-edge carbon capture technology. Witnesses spoke of selling sequestered CO2 to oil companies for enhanced-oil-recovery, sequestering CO2 in concrete by combining it with sea water, and deep sea geologic sequestration.
For updates on congressional action on CCS, see the CSTC
newsletter.
Endnotes
- International Energy Outlook 2008, Energy Information Administration
- Future of
Coal, MIT
- Carbon Sequestration Atlas of the United States and Canada, NETL 2007
- DoE Carbon Sequestration
website
- American Coal Foundation
- Future of Coal,
MIT
- Future of Coal,
MIT
- Physorg.com
- The Potential for Carbon Sequestration in the United States, CBO 2007
- House, Schrag, et al Permanent carbon dioxide storage
in deep-sea sediments PNAS 2006
- Statoil
- Bureau of Economic Geology presentations
- The Potential for Carbon Sequestration in the United States, CBO 2007
- DOE FutureGen
- Energy Independence and Security Act of 2007: Summary of Major Provisions, CRS 2007
- Statement of Robert C. Burruss, Research Geologist at USGS, before the House Subcommittee on Environment and Hazardous Materials, July 24, 2008.
- Burruss, Robert, et. al.Development of a Probabilistic Assessment Methodology for Evaluation of Carbon Dioxide Storage, USGS. USGS, March 4, 2009.
Updated July 30, 2009
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