CO Capture and Storage - Energy and Climate: How to Achieve a Successful Energy Transition - page 106

- Reacting CO 2 with a basic rock, either after grinding the rock in
order to obtain a mineral powder ( ex situ mineralization ), or within

).
- Producing a material or a chemical production through a reaction
with CO 2 . In such a case, the carbon is

the geological formation itself ( in situ mineralization

recycled .

These different options will be examined in this chapter. Whilst they
seemed somewhat far off options a few years ago, at least some of them
have now become an industrial reality.

Carbon sinks

In a broad sense, any natural reservoir able to absorbCO 2 is a carbon sink.
The main natural carbon sinks are oceans and forests. Photosynthesis is
the main carbon sequestration mechanism, but CO 2 can be also absorbed
in the oceans by dissolution. Carbon dioxide exchanges between the
atmosphere, vegetation and ocean surfaces are very large:

- 60Gt of carbon is exchanged each year between vegetation and the
atmosphere;
- 90Gt of carbon is exchanged between ocean surfaces and the
atmosphere;
- 40-50Gt of carbon is exchanged between ocean surfaces andmarine
biomass.

The quantity of carbon emitted each year in the atmosphere as a result of
fossil fuel consumption, i.e. 7Gt/year, might appear small when com-
pared with these exchanges. In fact, this additional quantity of carbon
modifies a fragile balance, producing therefore a continuous shift in the
CO 2 concentration in the atmosphere.
A first option for sequestering CO 2 consists of producing biomass to
obtain a carbon sink, for instance by growing forests which will absorb
carbon present in the atmosphere. It is necessary to growbiomass on large
surface areas. Biomass thus produced can represent typically around
10 tons of carbon captured per hectare per year, amounting to 37 tons
of CO 2 . The implementation of such carbon sinks has become an eco-
nomically viable option following the Kyoto Protocol, as carbon sinks can
be taken into account in the carbon trade mechanisms.
Nevertheless, it must be underlined that the amounts of carbon
thus sequestered are limited, as it is necessary to grow biomass over