The main contributor of CO2 emissions into the atmosphere are fossil-fuel burning power plants. Emissions were estimated at 28.1 billion metric tonnes in 2005 and projected emissions are 34.3 billion metric tonnes for 2015 (according to the International Energy Outlook, 2008). Other sources include: automobile engines, industrial and resource extraction processes.
Image courtesy of CO2CRC, sourced from the IPCC Special Report on Carbon Dioxide Capture and Storage, 2005
CCS is the technology used to prevent and reduce CO2 atmospheric levels as an attempt to mitigate global warming. CCS comprises of three main components: capture, transport and storage.
Capture: Involves separating CO2 from other gases in the exhaust stream. It can be applied to a variety of sources using a variety of techniques. At power plants, separation technologies can be used to capture CO2 after combustion (post-combustion capture) or to decarbonise the fuel before combustion (pre-combustion capture). Another alternative is to burn the fuel in pure oxygen, rather than air, resulting in more complete combustion giving an almost pure CO2 emission (~ 90%) which could be easily separated (oxy-fuel combustion capture).
Transport: The next step in the CCS process is to transport the captured CO2 to a suitable site for final storage located at a distance from the CO2 source. This is usually done by pipeline with CO2 compressed to a supercritical state (temperature and pressure at which it behaves as both liquid and gas).
Storage: Various options are possible for final
storage of CO2. At present, injection into underground geological formations
is the most promising and developed method. There are three main proposed
underground storage sites: depleted oil and gas reservoirs, deep saline
aquifers and deep unmineable coal seams. Injection into the deep ocean has
also been proposed but is associated with many more risks and has not been
adequately researched. Industrial fixation in inorganic carbonates (mineral
carbonation) could have an increasingly important role in future CCS operations
particularly if waste materials are put to use. Some industrial processes
might also utilise and store small amounts of captured CO2 in manufactured