Forestry is another option for carbon dioxide transformation on land and it is estimated that this alternative may be able to provide 1.2 GtC/year of storage capacity (Winjum et.al. (1992)).
The oceans are under-saturated with respect to carbon dioxide and it is estimated that the storage capacity of the oceans may be in the order of 1000 GtC (Cole et, al. (1993), Herzog et, al. (1997))
Sequestration and disposal of CO2 m the ocean is one option of reducing the greenhouse effect related to disposal of carbon dioxide to the air. There are two different approaches of getting pure carbon dioxide into the sea. Carbon dioxide can either be premixed with seawater before depositing or injected through a system where the mixing is part of the CO2 injection system, like the system proposed by Saito et. al. (1995) In this system carbon dioxide is dissolved in the rising part of an upward tube. The carbon dioxide enriched seawater on the other end of the tube will sink due to gravity since this seawater will be heavier than the surrounding seawater. The other approach is to pump liquid carbon dioxide to the actual deposition site. Distribution scenarios of these two approaches with the ocean streams will be completely different. Several simulation studies (Bacastow & Dewhave (1996), Drange & Haugan (1992), Drange et. al. (1993), Haugan & Drange (1992), Haugan & Drange (1996), Marchetti (1997)) contributed in increasing the understanding of the distribution of the deposited carbon dioxide with the ocean streams. But there is still large uncertainties related to the real nature of the problem. Kobayashi (2000) propose the use of mixing vessel that confines a contact area between gas and liquid at around 1000 meter. The gas is compressed and transported by pipeline to the vessel, where a fan ensures rapid renewal of gas interface. The liquid side of the interface is continuously stirred by a pump system. The paper discuss the extra dissolution rate due to the mixing at the interface but it is not clear whether this is thermodynamically equilibrium dissolved rates or a combination of physically distributed gas and dissolved gas. All model calculations are performed on the assumption that the carbon dioxide is uniformly distributed. Mixing of carbon dioxide and seawater onshore before deposition might be more efficient due to the extra residence time for transport that will contribute in converting a larger portion of the physically distributed carbon dioxide into dissolved carbon dioxide. In summary these approaches are comparable to strategies of intermediate deposition and it is still not verified how large portions of the deposited carbon dioxide that will sink to large depths relative to the transport induced by ocean streams.
Carbon dioxide deposited at depths beyond 2800 meters will be in the form of liquid carbon dioxide and storage beyond these depths are considered as the best ocean storage option, with estimated re-circulation rates in the order of 1000 - 2000 years (Herzog et. al. (1997)). For this reason much of the focus on storage of carbon dioxide in the sea is related to these depths. The question is how to get the carbon dioxide down to these depths. Pipelines from an onshore plant would be one option although this would require some new technology for these large depths. The cost related to such installations is also substantial. For this reason most of the focus during the last decade has been on the development of technologies that can make use of natural gravity in parts of the transport of the carbon dioxide to large depths.
One option would be to drop the carbon dioxide into the sea in forms of dry ice blocks. Kobayashi & Sato (1995) discuss this option and find that if that cubic blocks of 1 m3 solid carbon dioxide is sent from surface water then approximately half of the carbon dioxide melted before the blocks reaches depths beyond 3000 meter. This option will of course be expensive due to the cost of liquidification and subsequent freezing of the carbon dioxide.
Solid carbon dioxide hydrate is another option. At 4 degree Celsius carbon dioxide forms hydrate with seawater at around 20 bar. This means that if carbon dioxide is deposited as hydrate at these depths at least some of the carbon dioxide will sink to larger depths since carbon dioxide hydrate is heavier than seawater. Perfect filling of all large cavities in structure I will give a density of carbon dioxide hydrate equal to 1126 kg/m3. Laboratory measurement (Aya et. al. (1998)) indicate that produced hydrate may have density as low as 1.109 kg/m3 (270 bar, 3 degree Celsius). This is still above seawater density at the same conditions (1032 kg/m3). Yamasaki (2000) discuss one specific approach for formation of CO2 hydrate in a sub-sea vessel.
