Spinning Globe     Carbon Sequestration Leadership Forum
  An Overview of Carbon Sequestration  
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).

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.

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.
Capture and Separation

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.

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 CO2 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 CO2 separation and capture include the following:
  • Absorption (chemical and physical)
  • Adsorption (physical and chemical)
  • Low-temperature distillation
  • Gas separation membranes
  • Mineralization and biomineralization
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.
Geological Sequestration
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.
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.
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.
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.
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.
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.
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.
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.
Ocean Sequestration
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.
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.
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.
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.
Terrestrial Sequestration
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.
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.
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.
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:
  • 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.
Advanced Concepts
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.
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.
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.
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.