CO2 accounts for a significant amount of the gases emitted from fossil fuel conversion for power generation, for example, making up 3-15% of the gas (known as flue gas) that results from fossil fuel combustion for power generation.
CO2 must be separated, or captured, from the flue gas before it can be geologically stored. There are three main methods for capturing CO2 from fossil fuel combustion. Post-combustion captures CO2 directly from flue gases exiting standard combustion processes, typically with the use of solvents, sorbents, membranes or cryogenics, pre-combustion capture produces a synthetic gas from fossil fuels which have been stripped of carbon prior to combustion, leaving hydrogen to burn, and oxyfuel combustion burns coal or gas in pure oxygen to yield only CO2 and water.
The captured CO2 must be as pure as possible – the presence of impurities, such as nitrogen and sulphur oxides, in the captured CO2 can increase the costs of transportation and compression and can lead to more uncertain behaviour in the geological storage reservoir.