Programs: Science and Policy
AAAS Policy Brief: Carbon Capture & Storage
Coal is found on all seven continents and its widespread availability makes it a preferred energy source for many developed and emerging economies. 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; China alone is 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 are focusing on ways to capture and store (sequester) emissions from coal combustion. Capturing and storing CO2 is one way to reduce greenhouse gas emissions while continuing to use fossil fuels for energy production. A 2007 study by the U.S. Department of Energy (DOE) found that geologic formations in the U.S. and Canada could store up to a few centuries worth of U.S. emissions.3
While carbon capture and storage (CCS) is possible, it cannot yet be implemented at a scale large enough to slow climate change. A number of research and small-scale implementation projects are developing and testing various mechanisms for CSS in an attempt to make it an economically-viable technology. Key considerations for evaluating types of CSS include ecological impacts, cost, timescale, and amount of carbon sequestered.
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 focusing 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.
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 more difficult and more costly. Most analyses indicate that carbon capture is the most expensive part of the CSS process.
There are a variety of methods used for carbon capture, but most follow the same general principles: a combination of solvent reactions, and changing pressures 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, rather than trying to retrofit carbon capture technology to existing plants.
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 it is difficult to assess their reliability since they are 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. R&D 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 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 researchers are 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.
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 for 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 been conducted over long enough time periods or with sufficient monitoring to have a solid understanding of the potential for leaks after injection (and the effects of leaks on groundwater), though research is ongoing.
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 in which 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, which means that the technology is not directly transferable.
A significant international sequestration project that attempts CO2 injection for EOR on a larger scale is under way at the Weyburn oil field in Canada. The initial project demonstrated the potential for CO2 transport and storage, and increased oil output. Overall, the site is expected to store 20 million tons of CO2 over the lifetime of the project.9
It is possible that CO2 may be more stable when injected into the sea floor than it is when injected into 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 per year from its natural gas refinery
into sediments deep beneath the seafloor. This has been economically profitable
for Statoil, who must pay for CO2 released into the
atmosphere under Norway's emission tax program. 11
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.
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 DOE, is still
in the first phase; 50-60 percent of the methane is being degassed
before CO2 sequestration begins. Not only could unmineable coal seams store up to 156 billion metric tons of emissions, but the Intergovernmental Panel on Climate Change has determined that well-selected and well-managed projects could store 99 percent of injected CO2 for up to 1,000 years.13
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, without actually executing the sequestration. One of the main challenges of indirect sequestration is the limited degree of control over the biological 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's surface. Studies have shown that this can be done in some regions by "fertilizing" the ocean's 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 water, a significant fraction of it will sink to the ocean floor (a naturally occurring process) -- effectively sequestering CO2 from the atmosphere into the ocean. However, only a fraction of the phytoplankton that grows will eventually sink to the ocean floor, meaning that much of the CO2 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."
Most methods of terrestrial indirect sequestration involve 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.
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.
Soil can also store carbon; therefore, land use practices can have a major impact on the carbon budget of the terrestrial biosphere. Forestry and agricultural 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.
The DOE leads federal CCS R&D efforts. On August 5 2010, the DOE and the Obama Administration awarded $1 billion in stimulus funds to retrofit an existing coal-fired plant in Meredosia, IL with oxy-fuel technology. This is the second incarnation of the FutureGen project, a proposed 275 Megawatt IGCC power plant with CCS and hydrogen production canceled by the Bush Administration in 2008.14 In a surprise move, local leaders announced that involvement in the revised FutureGen project was not in the county’s best interest. The DOE is expected to search for other locations for the project.
In addition, the 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 submit a regulatory framework for sequestering and storing CO2 on public lands. The law also requires the 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 grants for university-based R&D on carbon sequestration for various types of coal. Finally, the law requires the 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 the DOE on CCS research that produced the Carbon Sequestration Atlas of the U.S. and Canada.16 In March 2009, the 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 CCS on U.S. public lands.
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), contained 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 (H.R. 1) also contained several measures for advancing CCS technology. The bill granted over $2 billion in 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, 2009, a bipartisan group of Senate Energy and Natural Resources Committee members, including Chairman Jeff Bingaman (NM), introduced the DOE Carbon Capture and Sequestration Program Amendments Act of 2009 (S. 1013). The bill, which would have allowed the Energy Secretary to assume control of long-term carbon storage sites and allocated additional funding for ten large-scale CCS demonstration projects to be sponsored by DOE, died in Committee.
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.
The Obama Administration has also expressed its support for CCS research and implementation. On February 3, 2010, the Administration announced the formation of an interagency task force dedicated to tackling the challenge of implementing large-scale CCS within 10 years. The task force report, released on August 12, 2010, outlined financial, economic, technological, legal, and institutional obstacles to CCS deployment. Recommendations include the adoption of a carbon price, means of improving federal coordination, and an analysis of liabilities associated with CCS.
For updates on congressional action on CCS, see the CSTC
- 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
- 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
- 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.
- Presidential Memorandum-A Comprehensive Federal Strategy on Carbon Capture and Storage, February 3, 2010.
Updated August 13, 2010