Carbon Capture and Storage
With overwhelming consensus among scientists that climate change is occurring (at the fastest rate ever recorded), and about why it is occurring (primarily greenhouse gas emissions from human activity) many engineers and other experts have turned their attention to the development of practical solutions to address the problem. One emergent, technology-based approach is carbon capture and storage (also called carbon capture, carbon sequestration, or CCS), the process of physically capturing carbon dioxide (CO2) from flue emissions—the “tailpipes” of fossil-fuel plants—and storing it underground so it never reaches the atmosphere.
CCS is heralded by some as a win-win—it allows for the continued use of relatively cheap and abundant fossil fuels while mitigating the atmospheric impact of by-product CO2, a major greenhouse gas. But others vigorously debate the long-term utility of CCS, arguing that it is technically impractical or that it could prolong society’s unsustainable reliance on fossil fuels. Even as a growing number of fossil-fuel plants are beginning to pilot this approach, economists and policy makers are grappling with its potential costs and benefits, and scientists are working to refine understanding of its implications for the environment, now and in the future.
Why do CO2 levels in the atmosphere matter?
Certain gases in the atmosphere, including CO2, contribute to a “greenhouse” effect on Earth by absorbing infrared radiation from the sun and trapping its energy in the form of heat.
Normal levels of greenhouse gases in the atmosphere help keep the planet hospitable for life, but the presence of too much greenhouse gas leads to overheating.
Rising levels of atmospheric CO2 increase the greenhouse effect beyond what is normal and cause global warming, a key contributor to climate change.
Roughly 81 percent of the greenhouse gas added to the atmosphere from human activity is CO2.
Three ways to capture CO2
The goal of carbon capture and storage is to remove CO2 from the emissions given off by industrial power plants and factories—which typically release it into the atmosphere along with other combustion byproducts via flues or smoke stacks. There are three main approaches to isolate and capture CO2.
Pre-combustion: This approach involves separating out CO2 before combustion is completed. It is advantageous because it requires minimal additional chemical material, but it can be difficult to retrofit older plants for this approach.
- Fossil fuels are gasified (partially combusted) with very small amounts of oxygen at a high pressure.
- This produces a gaseous mixture of carbon monoxide (CO) and hydrogen (H2) (among other gases), to which steam (H2O) is added.
- By lowering the temperature of the mixture, a reaction is triggered to create CO2 and H2
- The CO2 is removed from the H2 mixture and pressurized for transport.
- Post-combustion: This method involves separating CO2
from the rest of combustion emissions just before they are released into the atmosphere. Post-combustion works well as a retrofit for existing plants. It can work in a few different ways:
- Absorption into a liquid: Before exiting into the atmosphere, flue gas is passed through a liquid absorbent (such as ammonia, an amine-based solvent, or a solution of potassium carbonate and other catalysts) that is later heated to release or “scrub out” the captured CO2. Post-combustion capture with amine scrubbing accounts for the vast majority of CO2 capture technologies currently in use.
- Adsorption onto a solid: A solid surface adsorbs CO2 from emissions before they exit the plant. Then, by either decreasing the pressure or increasing the temperature, the isolated CO2 is released.
- Through a membrane: Flue gas is filtered through a permeating membrane that isolates CO2 so it can be collected.
- Oxyfuel combustion: While this is one of the most efficient approaches for capturing CO2 from emissions mixtures, it is also much costlier than other options in terms of both capital and operational costs. Here’s how it works:
- High-concentration oxygen (O2) is used in the combustion process, resulting in high-purity CO2 and steam as by-products.
- The CO2 is pressurized and prepared for transport, and the steam can be used to power turbines to create electricity.
- Other emerging concepts: Researchers are working to develop even more economically viable and efficient technologies for carbon capture, including:
- Chemical looping (a combustion reaction that isolates the burning fuel from the air),
- Calcium looping (where calcium carbonate and calcium oxide are used to extract CO2 from flue gases),
- Silicon-carbon nanotubes (which can be used to separate CO2 efficiently from flue gas at room temperature),
- As well as multiple new membrane technologies.
How CO2 is transported
- Once captured, CO2 is compressed and prepared for transport to a storage location.
- Depending on the geological nature of the storage location, CO2 can be kept as a compressed liquid or supercritical liquid (meaning that it has the viscosity of a gas but the density of a liquid).
- Most commonly, CO2 is transported via pipeline to a suitable storage location.
Where CO2 is stored – in the ground or in the ocean
- An ideal geological storage location can hold hundreds of thousands of tons of CO2 (for comparison, the average car emits 46 metric tons of CO2 over 10 years).
- In order to be securely stored underground, CO2 must be pressurized to 100 bar or more—a pressure equivalent to what is found one kilometer below the ocean—and then injected (either as a liquid or supercritical liquid) to a minimum depth of 800 to 1000 meters (m) below the Earth’s surface.
- At this depth, and when properly sealed under a cap rock, CO2 can remain trapped for up to thousands of years.
- Oil and gas reservoirs are promising options for geological storage because they are well understood in the fossil fuel industry and can be quickly adapted to accommodate CO2 stores. This option also carries benefits for oil or gas companies because injecting CO2 into a reservoir can enhance recovery of oil and gas.
- Deep saline formations are underground porous rock formations, prevalent in the United States and around the world, that are filled with salt water to which CO2 an be added for long-term storage.
- Basalt formations and shale basins are also potential storage options, but more research is needed to understand their suitability.
- Coal beds (those too deep or too thin to be mined) are widely seen as less promising for now because little is known about their storage capacities.
- Beneath the ocean floor. This type of storage is similar to geological storage options, except the storage formations are located beneath the sea bed.
- On the ocean floor. Liquid CO2, if injected at a sufficient depth, is denser than sea water and will sink to and stay on the ocean floor, forming CO2 “lakes.”
- In the sediment on the ocean floor. Recent studies have found that CO2 an be injected into sediment on the ocean floor, taking advantage of “self-sealing” mechanisms that can keep CO2 from escaping.
Benefits and drawbacks of carbon capture
- Compared to other carbon capture methods—such as capturing CO2 directly from the atmosphere, where CO2 concentrations are lower than near combustion sources—carbon capture and storage has a lower cost, is viable for removing much larger amounts of CO2, and has the potential to significantly reduce future CO2 emissions into the atmosphere.
- Slowing CO2 emissions could buy time until renewable energy sources are more prevalent, especially if used in combination with other climate-change-mitigating technologies. For this reason, CCS is viewed by some as an ideal approach to support the transition from fossil fuels toward renewable energy.
- In 2013, the U.S. Geologic Survey found that the United States has a technical storage capacity of three trillion metric tons of CO2 —500 times the total CO2 output of the United States in 2011. (Note, however, that this is only an estimate and does not take into account affordability.)
- Geological storage of CO2 capitalizes on a robust scientific knowledge base in the oil and gas industries about underground geological features.
- Geologically stored CO2 can be used to extract geothermal heat from the same locations in which it is stored, meaning that captured CO2 could be used to produce renewable geothermal energy.
- The most easily accessible and economically viable storage reservoirs together have limited storage capacity; additional capacity exists but is more expensive to tap.
- Although models suggest long-term storage is feasible, it is not certain how storage reservoirs will perform over time. Reliable mechanisms need to be developed to monitor and maintain reservoirs for the future.
- There may exist economic or social “limits” to CO2 storage, as it may not remain economical or socially acceptable to use all the available storage space.
- The injection of large amounts of CO2 into underground formations may lead to small, localized earthquakes, which could compromise the integrity of sealed underground reservoirs and present real or perceived risks for residents of nearby communities.
- Ocean storage is less well studied than other storage options and has the potential to inflict environmental harm. When CO2 dissolves in water, carbonic acid is formed, increasing the ocean’s acidity. Ocean acidification has been shown to negatively impact many marine organisms including oysters, clams, sea urchins, and coral.
- Some environmental scientists are concerned that investing in CCS technologies will reduce the sense of urgency to address climate change and lead to delays in the development of sustainable, renewable energy sources.
Where is carbon capture currently used?
- The world’s first carbon capture and storage plant was the Weyburn-Midale Project in Canada, built in 2008.
- In 2016, the Petra Nova plant in Texas, which captures CO2 using post-combustion carbon capture technology, became the first operational plant to use CCS technologies in the United States.
- As of mid-2018, 17 facilities in the world were using CCS technologies—nine of which are in the United States, mainly in Texas, Oklahoma, Wyoming, and Illinois—and a total of 37 facilities worldwide were using or are planning to use CCS technologies, including 15 newly created between 2014 and 2018.
The future of carbon capture and storage
- In its 2014 Fifth Assessment Report, the Intergovernmental Panel on Climate Change concluded that carbon capture and storage should be an important component of strategies to reduce global warming.
- Current research is focused on developing new carbon capture methods, understanding and decreasing potential environmental impacts, and exploring synergies with renewable energy research and development.
- Ultimately, the uptake of CCS will depend on economic factors, such as improved efficiencies and cost reductions; policy decisions, such as the introduction of tax credits or other incentives to quickly increase capacity; and focused scientific efforts to anticipate and address potential safety and environmental concerns.
key references for those who want to dig deeper
- In 2017, the U.S. Global Change Research Program released an updated Climate Science Special Report as a part of the Fourth U.S. National Climate Assessment. The report highlights the impacts of climate change across the United States and presents science-based perspectives on climate change mitigation strategies, all of which are important for motivating the development of CCS.
- A 2018 Congressional Research Service report, Carbon Capture and Sequestration (CCS) in the United States, describes the future of carbon capture and storage technologies in the United States, and related policies and incentives.
- The Intergovernmental Panel on Climate Change (IPCC) is a scientific and intergovernmental agency that provides objective science reports on climate change. The mitigation-focused section of the IPCC’s 5th Assessment Report, published in 2014, includes details on multiple CCS technologies, their advantages and disadvantages, as well as possibilities for their future use.
- A 2012 study, in the Proceedings of the National Academy of Sciences found that the United States has the potential to store CO2 and stabilize emissions at current levels for at least a century. The U.S. Geologic Survey published a similar national assessment, in 2013 of geologic CO2 storage resources in the United States.
- Multiple studies have used different methods to study the impact of CCS, concluding that it can significantly reduce the CO2 emissions of industrial plants. One 2011 study, published in the International Journal of Greenhouse Gas Control found that CO2 emissions were reduced by 64 - 78 percent, depending on the CCS technology used.
- Multiple studies published in 2011, 2012, and 2013 in the International Journal of Greenhouse Gas Control describe the potential negative environmental impacts of CCS, including increased eutrophication and ocean acidification.
- The risk of CO2 leakage and its possible ramifications are still being explored. The IPCC’s Special Report on Carbon Dioxide Capture and Storage, published in 2005, provides detailed description of leakage potential from CO2 storage locations. A 2017 study in Climatic Change, Leakage Risks of Geologic CO2 Storage and the Impacts on the Global Energy System and Climate Mitigation, determined that even if CO2 storage facilities do leak, the financial losses involved are not likely to be prohibitive. Additional studies ( (International Journal of Greenhouse Gas Control, 2015; Environmental Science & Technology, 2016) have found similar results.
- A 2013 National Academies of Science, Engineering, and Medicine report, Induced Seismicity Potential in Energy Technologies, concludes that it is possible for underground CO2 storage to induce earthquakes, resulting in possible leakage of CO2 into the atmosphere.
- A 2017 study in Renewable and Sustainable Energy Reviews, Pollution to solution: Capture and sequestration of carbon dioxide (CO2) and its utilization as a renewable energy source for a sustainable future, summarizes how CCS technologies can be used in conjunction with other renewable energy solutions to reduce carbon emissions and encourage sustainable energy practices.
- A 2015 study published in Applied Energy also reports that CCS can be combined with geothermal energy to generate electricity.
- The irreversible momentum of clean energy, published in Science in 2017 by former President Barack Obama, summarizes the ways in which some American companies have taken steps to meet the commitments outlined in the 2016 Paris Agreement (from which the United States subsequently excepted itself).
- In 2018, the United States increased tax credit incentives for companies that use CCS (Carbon Dioxide Sequestration Credit ) offering $50 for every metric ton of CO2 stored and $35 for every ton re-utilized.
- As the interest in and demand for CCS technology grows around the world, various countries have worked to develop CCS technologies independently of one other. A 2016 paper, Learning through a portfolio of carbon capture and storage demonstration projects, argues that this is counter-productive; and countries should instead develop a coherent program to expedite development. This is the expressed goal of the Global CCS Institute, whose members include countries and large corporations.