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.

• 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.

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.

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.

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.

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.

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.

• 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.

• 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.

• 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.

• 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.

• 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.