CO₂ Capture and Storage

3. How do CO₂ capture technologies work?

• 3.1 What capture technologies are currently available?
• 3.2 What are the costs of CO₂ capture?

3.1 What capture technologies are currently available?

To capture carbon dioxide (CO₂) it is first separated from other gases resulting from combustion or processing. It is then compressed and purified to make it easier to transport and store.

Carbon dioxide resulting from combustion, particularly in the electricity sector, can be captured using one of three systems:

 

System	Status	Net amount of CO2 “avoided”
Post-combustion	In commercial use for decades in other, related applications	80-90%
Pre-combustion
Oxyfuel combustion	Demonstration phase	90%

In a post-combustion system, the flue gas produced by combustion of the fuel with air only contains a small fraction of CO₂. It is captured by injecting the flue gases in a liquid that selectively absorbs the CO₂ (such as a cooled or compressed organic solvent). Nearly pure CO₂ can then be released from the liquid, typically by heating it or releasing the pressure. Similar separation processes are already used on a large scale to remove CO₂ from natural gas.

In a pre-combustion system, the primary fuel is first converted into gas by heating it with steam and air or oxygen. This conversion produces a gas containing mainly hydrogen and CO₂, which can be quite easily separated out. The hydrogen can then be used for energy or heat production.

Oxyfuel combustion uses pure oxygen to burn the fuel instead of using air which only contains 20% of oxygen and a lot of nitrogen. It results in a gas mixture containing mainly water vapour and CO₂. The water vapour is then easily removed from the CO₂ by cooling and compressing the gas stream. However, for this process one must first separate oxygen from the air, which is fairly complex process.

Similar capture systems are already used in several industrial processes, such as hydrogen or urea production, and coal gasification. More...

3.2 What are the costs of CO₂ capture?

Capture systems reduce the CO₂ emissions from combustion plants by about 80 to 90%. These figures take into account the fact that capture systems require additional energy (see Figure TS.11).

For new fossil fuel power plants, CO₂ capture can increase the cost of electricity production by 35 to 85% depending on different assumptions in plant design, operation and financing. This represents 0.01 to 0.03 US$ per kWh of electricity produced.

 

	Cost in US$/kWh
New fossil fuel plants without capture	0.03 – 0.06
New fossil fuel plants with capture	0.04 – 0.09
Capture alone	0.01 – 0.03

Capture costs can also be expressed in US$ per net tonne of CO₂ captured. This unit cost varies greatly for different types of combustion plants and for industrial processes. The unit cost of capture is generally lower where a relatively pure CO₂ stream is produced, such as in natural gas processing, hydrogen production, and ammonia production. More...