The capture

This stage of the process involves separating the CO2 from the smoke and fumes (mainly water vapor and nitrogen) given off during combustion. This is by far the most costly part of the process (about two-thirds of overall cost).

But this stage is necessary, for two main reasons:

• the smoke and fumes from combustion usually contain between 3% and 15% CO2. Separating the CO2 reduces the quantity of gas to be compressed and stored, and therefore reduces costs;
  
• geological storage sites must meet certain criteria, and only a limited number are suitable for CO2, so reducing gas volume by separating the CO2 optimises the available storage capacity.

CO2 separation (or recovery) is already carried out as part of some industrial processes, such as the production of ammonia and hydrogen (in refineries and fertilizer plants). It also occurs during natural gas treatment processes, because natural gas reservoirs contain a mixture of gas and CO2. After the gas is extracted from the reservoir the two are separated, because commercial gas must have a very low CO2 content.

But recovering the CO2 from combustion smoke and fumes poses technical challenges, mainly to do with the change of scale and an improvement in process energy efficiency. These are the main focus of current research.

The high capital cost of implementing capture technologies means that they are best suited to plants that generate a high volume of concentrated CO2 emissions, such as power stations or energy-intensive industrial sites like refineries, cement factories, petrochemical plants, steelworks and fertilizer factories.

Depending on the type of plant, CO2 capture may use one of three different technologies: postcombustion, precombustion or oxycombustion. Find out more >> Find out more: CO2 capture methods

These three methods have quite different characteristics (cost, energy consumption, ease of implementation, etc.) and are currently at different stages of development and mastery. All three have potential for positive technical progress and they are all the focus of ongoing research programs.