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AAAS Center for Science, Technology and Congress

AAAS Policy Brief: Carbon Capture & Storage

Issue Summary | Resources

Introduction
Carbon Capture
Carbon Storage: Geologic Formations
Carbon Storage: Indirect Sequestration
Policy Approaches
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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 50 percent of the electricity in the United States. China uses more coal than the United States, the European Union and Japan combined and has seen record increases in annual use in recent years.

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. ¹

Due to growing concerns about energy security and climate change, research, development and legislation have focused 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.

Carbon capture and storage (CCS) is currently possible, but not on 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, to address these issues.

No federal guidelines or regulations on carbon sequestration exist. In October 2007, the Environmental Protection Agency announced that it would propose new regulations on carbon capture and storage for public comment by the summer of 2008. EPA has authority for underground injection of CO₂ under the Safe Drinking Water Act (SDWA) Underground Injection Control (UIC) program.

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. ²

In general, 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.³ 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. ⁴

• IGCC power plants:
A newer type of coal power plant is the Integrated Coal Gasification Combined Cycle (IGCC) plant. There are at least nine IGCC plants operational in the world, three of them in the United States. 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 (NO_X). 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. ⁵

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.⁶

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.

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 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, 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 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 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.⁷

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.⁸

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 $55/ton for CO₂ released into the atmosphere under Norway's emission tax program. ⁹

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. ¹⁰

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.

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.

Policy Approaches

The Department of Energy has led federal CCS R&D efforts, with the following goals:

Have a portfolio of carbon capture and sequestration technologies prepared for implementation by 2012
Develop 90 percent capture capability, 99 percent sequestration permanence
Cost of power less than or equal to 2 percent increase from non-CCS sources
Cost of CCS $10 or less per ton of CO₂

DOE's flagship project is FutureGen, a proposed 275 Megawatt IGCC power plant that will incorporate both CCS and hydrogen production. The power plant is expected to be completed by 2012, and be fully operational by about 2025. ¹¹ In addition, DOE's Regional Carbon Sequestration Partnerships awarded the first three large-scale carbon sequestration projects in the United States in October 2007. The three projects - Plains Carbon Dioxide Reduction Partnership; Southeast Regional Carbon Sequestration Partnership; and Southwest Regional Partnership for Carbon Sequestration - will conduct large volume tests for the storage of one million or more tons of carbon dioxide (CO2) in deep saline reservoirs.

Several bills have been introduced in Congress that aim to stimulate research, development, and deployment of CCS technology. These measures include increased funding for DOE on CCS technology, evaluations of potential geologic storage formations throughout the nation, and calls for the implementation of large-scale carbon sequestration tests in a variety of geologic formations. Many of these measures have been folded into the energy packages passed by the House and Senate, though none have yet been signed into law.

For updates on congressional action on CCS, see the CSTC newsletter.