CARBON CAPTURE AND STORAGE
Carbon
capture and storage (CCS) has been put forward as one way of reducing emissions
of the greenhouse gas carbon dioxide. Carbon dioxide from the use of fossil
fuels is stored underground in closed geological formations such as aquifers or
depleted oil and gas wells. A number of schemes have been announced and in at
least one case CCS is already being carried out on a large scale. Because these
schemes are in early stages their long-term viability is not yet established.
There are some doubts about the desirability of CCS. For example, Greenpeace,
while it does not object to CCS in principle, feels that it is diverting our
attention from the superior goal of renewable energy. It feels that the
subsidies, which will be spent on CCS, would be better spent on renewables.
Many
schemes are proposed in a number of countries. For example, a process patented
by BP and GE, is proposed in which natural gas, extracted with the assistance
of carbon dioxide, is brought ashore and converted into hydrogen and CO2.
The hydrogen is then burnt to produce electricity. The CO2 is
returned to the gas field to extract more gas. Eventually the gas field will
consist mainly of CO2 with a small amount of residual natural gas
and will be sealed. There was a plan to carry out this scheme in the North Sea,
but this has now been abandoned. However there are a number of possible sites
for the scheme worldwide and some of these will be pursued.
The
most advanced scheme is at the Sleipner field in the North Sea between Norway.
and Scotland, which has been running since 1996. This is shown schematically
below. Natural gas, which contains a high proportion of CO2, is
extracted from a gas field and brought up to a rig, where the CO2 is
separated from the rest of the gas. The CO2 is then compressed and
stored in an aquifer, which lies, enclosed in impervious rock between the gas
field and the surface. The form the CO2 takes in the aquifer is
being studied and the storage is monitored for stability. Of course, you could
argue that this project is limited because it does not store CO2
emissions from combustion, but merely does not extract the CO2 from
the gas field. Nevertheless, it is an important stage in the development of
CCS.
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