How CCS Works

1: Capture

Carbon capture is the separation of CO₂ from the other gases produced when fossil fuel is burnt for power generation and in other industrial processes.

2: Transport

Once separated, the CO₂ is compressed and transported to a suitable site for geological storage.

3: Storage

At its storage site, CO₂ is injected into deep underground rock formations, often at depths of one kilometre or more.

1: Capturing the carbon dioxide

Separating CO₂ with solvent.

Separating CO₂ with a membrane.

Capturing CO₂ emissions from industrial processes is easiest at large industrial plants where CO₂-rich flue gas can be captured at the facility.
The separation of CO₂ is already performed in a number of industries as part of the standard industrial process. For example, in natural gas production, CO₂  needs to be separated from the natural gas during processing. Similarly, in industrial plants that produce ammonia or hydrogen, CO₂ is removed as part of the process.
In other industries, such as in steel mills and cement plants, capture processes have not yet been demonstrated at a large enough scale, but in each case an existing capture method can be tailored to suit the particular production process. For instance, collection of CO₂ from cement plants uses post-combustion capture, and collection from modified steel manufacturing processes uses a type of oxy-fuel combustion.
As the largest contribution to CO₂ emissions is from the burning of fossil fuel, particularly in producing electricity, three main processes are being developed to capture CO₂ from power plants that use coal or gas. These are:

• post-combustion capture;
• pre-combustion capture; and
• oxy-fuel combustion capture.

Each of these processes involves separating the CO₂ from the other waste gases. The main methods being developed are absorption using:

• a liquid solvent;
• a solid material; or
• membranes that can allow some gases through and not others.

2: Transporting the carbon dioxide

Once separated from other components of the flue gas, the CO₂ is compressed to make it easier to transport and store. It is then transported to a suitable storage site. Today, CO₂ is already being transported by pipeline, by ship and by road tanker — primarily for use in industry or to recover more oil and gas from oil and gas fields. The scale of transportation required for widespread deployment of CCS is far more significant than this small-scale transport, and will involve the transportation of dense, concentrated CO₂.

3: Storing the carbon dioxide

Geological storage options for CO₂.

The final stage of the CCS process sees the CO₂ injected into deep underground rock formations, often at depths of one kilometre or more. At this depth, the temperature and pressure keep the CO₂ as a dense fluid. The CO₂ slowly moves through the porous rock, filling the tiny spaces known as pore space.
Appropriate storage sites include depleted oil fields, depleted gas fields or rocks which contain fresh or saline water (saline formations). These storage sites generally have an impermeable rock (also known as a 'seal') above them. The seal and other geological features prevent the CO₂ from returning to the surface.
These sites have securely contained fluids and gases for millions of years, and with careful selection, they can securely store CO₂ for just as long.
Once injected, a range of sensing technologies are used to monitor the CO₂'s movement within the rock formations. Monitoring, reporting and verification processes are important to assure the public and regulators that the CO₂ is safely stored.
Finding appropriate storage sites requires the collection of a great deal of data, and takes significant time and effort. Many economies around the world have active programs to identify storage sites for CO₂, including the USA, Canada, China, South Africa, Australia and Europe.