Capture and storageof carbondioxide 289Capture and storage of carbon dioxide
An alternative to moving away from fossil fuel sources of energy is to
prevent the carbon dioxide from fossil fuel burning from entering the
atmosphere. This can be done either by removing it from the flue gases
in a power station, or the fossil fuel feedstock could, in a gasification
plant, be converted through the use of steam,^24 to carbon dioxide and
hydrogen. The carbon dioxide is then relatively easy to remove and the
hydrogen can be used as a versatile fuel. The latter option will become
more attractive when the technical andlogistic problems of the large-
scale use of hydrogen in fuel cells to generate electricity have been
overcome – we mention this again later in the chapter.
Various options are possible for the disposal (or sequestration) of
the very large amounts of carbon dioxide which result. For instance,
the carbon dioxide can be pumped into spent oil or gas wells, into deep
saline reservoirs or into unminable coal seams.^25 Other suggestions have
also been made such as pumping it into the deep ocean, but these are
more speculative and need careful researchand assessment before they
can be realistically put forward. In the most favourable circumstances
(for instance when power stations are close to oil or gas fields and when
the extraction cost is relatively small), the cost of removal, although
significant, is only a small fraction of the total energy cost. For instance,
in Norway where there is carbon tax of $US l5 per tonne of carbon, a
company is finding it economic to pump over one million tonnes peryear
of carbon dioxide removed from a natural gas stream into storage under
the North Sea. In other circumstances estimates of the cost are larger
(perhaps up to 100% on top of the energy cost) – the cost of extraction
being generally larger than the cost of storage.
The technology of carbon capture and storage could enable contin-
uing use of fossil fuels without the deleterious effects of carbon dioxide
emissions. The global potential for underground carbon dioxide storage
is large. For instance, it has been estimated that over 200 Gt of carbon
as carbon dioxide could be stored in geological reservoirs in north west
Europe alone. How much it is used will depend more on the cost than
on the availability of suitable storage sites.
Renewable energy
To put our energy use in context it is interesting to realise that the energy
incident on the Earth from the Sun amounts to about 180 thousand million
million watts (or 180 000 terawatts, 1 TW= 1012 W). This is about
14 000 times the world’s average energy use of about 13 million million
watts (13 TW). As much energy arrives at the Earth from the Sun in