What Is Carbon Capture and Storage (CCS)?

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Carbon capture and storage (CCS) is the process of directly capturing carbon dioxide (CO2) gas from coal-fired power plants or other industrial processes. Its primary goal is to keep CO2 from entering the Earth’s atmosphere and further exacerbating the effects of excess greenhouse gases. The captured CO2 is transported and stored in underground geologic formations. 

There are three types of CCS: pre-combustion capture, post-combustion capture, and oxyfuel combustion. Each process utilizes a very different approach to reduce the amount of CO2 that comes from the burning of fossil fuels.  

What Is Carbon, Exactly?

Carbon dioxide (CO2) is a colorless, odorless gas under normal atmospheric conditions. It is produced by the respiration of animals, fungi, and microorganisms, and used by most photosynthetic organisms to create oxygen. It is also produced by the combustion of fossil fuels such as coal and natural gas.

CO2 is the most abundant greenhouse gas in the Earth’s atmosphere after water vapor. Its ability to trap heat helps regulate temperatures and make the planet inhabitable. However, human activities such as fossil fuel burning have released too much of the greenhouse gas. Excess levels of CO2 are the main driver of global warming.

The International Energy Agency, which collects energy data from around the world, estimates that CO2 capture capacity has the potential to reach 130 million tons of CO2 per year if plans for new CCS technology move forward. As of 2021, there are more than 30 new CCS facilities planned for the United States, Europe, Australia, China, Korea, the Middle East, and New Zealand.

How Does CSS Work?

Carbon Capture Technology illustration

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There are three pathways to achieve carbon capture at point sources such as power plants. Because approximately one-third of all human-produced CO2 emissions come from these plants, there is a large amount of research and development going into making these processes more efficient.

Each type of CCS system uses different techniques to achieve the goal of reducing atmospheric CO2, but all must follow three basic steps: carbon capture, transportation, and storage.

Carbon Capture

The first and most widely used type of carbon capture is post-combustion. In this process, fuel and air combine in a power plant to heat water in a boiler. The steam that is produced turns turbines that create power. As the flue gas leaves the boiler, CO2 is separated from the other components of the gas. Some of these components were already part of the air used for combustion, and some are products of the combustion itself. 

There are currently three main ways to separate CO2 from flue gas in post-combustion capture. In solvent-based capture, the CO2 is absorbed into a liquid carrier like an amine solution. The absorption liquid is then heated or depressurized in order to release the CO2 from the liquid. The liquid is then reused, while the CO2 is compressed and cooled in liquid form so that it can be transported and stored. 

Using a solid sorbent to capture CO2 involves the physical or chemical adsorption of the gas. The solid sorbent is then separated from the CO2 by decreasing pressure or increasing the temperature. Like in solvent-based capture, the CO2 that is isolated in sorbent-based capture is compressed. 

In membrane-based CO2 capture, flue gas is cooled and compressed and then fed through membranes made from permeable or semipermeable materials. Pulled by vacuum pumps, the flue gas flows through the membranes which physically separate the CO2 from the other components of the flue gas. 

Pre-combustion CO2 capture takes a carbon-based fuel and reacts it with steam and oxygen gas (O2) to create a gaseous fuel known as synthesis gas (syngas). The CO2 is then removed from the syngas using the same methods as post-combustion capture. 

Nitrogen removal from the air that feeds the fossil fuel combustion is the first step in the process of oxyfuel combustion. What is left is almost pure O2, which is used to combust the fuel. CO2 is then removed from the flue gas using the same methods as post-combustion capture. 

Transportation

After CO2 is captured and compressed into liquid form, it must be transported to a site for underground injection. This permanent storage, or sequestration, into depleted oil and gas fields, coal seams, or saline formations, is necessary to safely and securely lock away the CO2. Transportation is most commonly done by pipeline, but for smaller projects, trucks, trains, and ships may be used. 

Storage

CO2 storage must happen in specific geologic formations to be successful. The U.S. Department of Energy is studying five types of formations to see if they are safe, sustainable, and affordable ways to permanently store CO2 underground. These formations include coal seams that cannot be mined, oil and natural gas reservoirs, basalt formations, saline formations, and organic-rich shales. CO2 must be made into a supercritical fluid, meaning it must be heated and pressurized to certain specifications, in order to be stored. This supercritical state allows it to take up much less space than if it were stored at normal temperatures and pressure. The CO2 is then injected by a deep pipe where it becomes trapped in rock layers. 

There are currently several commercial-scale CO2 storage facilities around the world. The Sleipner CO2 Storage Site in Norway and the Weyburn-Midale CO2 Project have successfully been injecting over 1 million metric tons of CO2 for many years. There are also active storage efforts happening in Europe, China, and Australia.

CCS Examples

The first commercial CO2 storage project was built in 1996 in the North Sea off Norway. The Sleipner CO2 gas processing and capture unit removes CO2 from the natural gas that is produced in the Sleipner West field and then injects it back into a 600-foot thick sandstone formation. Since the beginning of the project, over 15 million tons of CO2 have been injected into the Utsira Formation, which may ultimately be able to hold 600 billion tons of CO2. The most recent cost of CO2 injection at the site was around $17 per ton of CO2.  

In Canada, scientists estimate that the Weyburn-Midale CO2 Monitoring and Storage Project will be able to store more than 40 million tons of CO2 in the two oil fields where it is located in Saskatchewan. Every year, approximately 2.8 million tons of CO2 are added to the two reservoirs. The most recent cost of CO2 injection at the site was $20 per ton of CO2.

CCS Pros and Cons

Pros:

Cons:

  • The biggest barrier to implementing efficient CCS is the cost of separating, transporting, and storing the CO2. 
  • Long-term storage capacity for the CO2 removed by CCS is estimated to be less than what is needed.
  • The ability to match sources of CO2 to storage sites is highly uncertain. 
  • Leakage of CO2 from storage sites could cause great environmental harm.