coal fields or other deep seams. The CO2 remains underground or is channelled into disused
sandstone reservoirs. However, the sale of oil and natural gas is profitable, so the $17-per-ton
sequestration cost is easily borne. There is also a plan for the injection of CO2 into saline
aquifers, 1,000 metres beneath the seabed, to prevent its release into the atmosphere.
Carbon capture
While CO2 storage has been accomplished, its capture from power plants remains largely
hypothetical, although CCS plants throughout Western Europe and North America are on the
drawing board.
There are three main forms of CCS: pre-combustion, post-combustion, and oxy-firing. In a
2012 paper from the US Congressional Budget Office (CBO), post-combustion capture was
viewed most favourably since existing power plants can be retrofitted with it, whereas pre-
combustion and oxy-firing mean the construction of entirely new plants. However, pre-
combustion and oxy-firing remove more CO2 than post-combustion and generate more
electricity.
Post-combustion capture means CO2 is separated from gas after coal is burnt but before
electricity is generated, while in oxy-firing, coal is combusted in pure oxygen. In pre-
combustion, as in an Integrated Gasification Combined Cycle system (IGCC), oxygen, coal, and
water ae burnt together to produce a synthetic gas called Syngas – mainly hydrogen – which
drives two sets of turbines, firstly gas-driven ones, then, as the cooling Syngas travel through
water, steam-driven ones. Emissions from this process contain around ten percent of the CO2
that burning coal produces.
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