Capturing carbon dioxide (CO2) is the first step in carbon capture and storage (CCS), a suite
of technologies that prevents large quantities of CO2 from being released into the atmosphere.
Carbon capture can be applied to large-scale emissions processes, including coal and gas-fired power generation, natural gas processing and fertiliser production, as well as the manufacture of industrial materials such as cement, iron and steel and pulp and paper. The application of carbon capture technologies to these processes can play a major role in reducing the world’s greenhouse gas emissions. Carbon separation/capture technologies have been operational at large-scale in the natural gas and fertiliser industries for decades and have recently become operational in the power sector.
How is CO2 captured?
Energy from fossil fuels such as coal, oil and natural gas is released in the combustion (burning) and conversion process, which also results in the emission of CO2 as a by-product.
In systems where the coal is pulverised to a powder, which makes up the vast majority of coal-based power plants through North America, Europe and China, the CO2 must be separated at diluted concentrations from the balance of the combustion flue gases. In other systems, such as coal gasification (where coal is converted to chemicals, natural gas or liquids), the CO2 can be more easily separated.
There are three basic types of CO2 capture: pre-combustion, post-combustion and oxyfuel with post-combustion.
Pre-combustion capture processes convert fuel into a gaseous mixture of hydrogen and CO2. The hydrogen is separated and can be burnt without producing any CO2. The CO2 can then be compressed for transport and storage. The fuel conversion steps required for pre-combustion are more complex than the processes involved in post-combustion, making the technology more difficult to apply to existing power plants.
Pre-combustion capture is used in industrial processes (such as natural gas processing) while its application in power generation will be via new build projects (such as the Kemper County Energy Facility now under construction).
Post-combustion processes separate CO2 from combustion exhaust gases. CO2 can be captured using a liquid solvent or other separation methods. In an absorption-based approach, once absorbed by the solvent, the CO2 is released by heating to form a high purity CO2 stream. This technology is widely used to capture CO2 for use in the food and beverage industry.
Oxyfuel combustion processes use oxygen rather than air for combustion of fuel. This produces exhaust gas that is mainly water vapour and CO2 that can be easily separated to produce a high purity CO2 stream.