TECHNOLOGY FOR DEEP OCEAN DEPOSITION OF CO2
Department of Physics, University of Bergen
Allegt. 55, 5007 Bergen, Norway, e-mail: email@example.com
In order to reduce the greenhouse effect several possible options for storage of carbon dioxide is yet being explored. The carbon content in the oceans could be increased by a factor in the order of six before thermodynamic equilibrium with the atmosphere is achieved. Storage of carbon dioxide on large depths (3000 m or deeper) is the ocean storage option that has the largest time scale for transport of carbon back to the surface water again. This is therefore the most attractive option. Construction of facilities for transport of carbon dioxide to large depths by pipeline will be expensive. Several concepts that can make use of gravity for parts of this transport have been proposed. These include sending the carbon dioxide as block of dry ice, dilution in seawater to create density gradients, production of hydrate and deposition of large cold droplets of liquid carbon dioxide at intermediate depths. Thermodynamic de-stabilisation of these different phases, and the corresponding kinetic rates depends on the free energy differences between the different phases. The background content of carbon in the oceans is generally too low to give hydrate stability. Hydrate particles will therefore melt as they sink towards the bottom. Melting rate will depend on the size of the hydrate particles but estimates indicates a very rapid melting due to very chemical potential of diluted (background concentration) C02 in seawater. Dry ice will melt as a function of heating from the seawater. In addition the chemical potential of carbon dioxide in the seawater at background concentration is lower than that of dry ice, which gives an additional driving force for the melting process. Ice formation will reduce this effect, similar to the idea behind the COSMOS approach. The production of dry ice still makes this a very expensive approach. In the COSMOS concept the idea is to send cold (-55 degree Celsius) liquid droplets from 500 m depth through a pipeline. This will result in ice on the droplet surface. Estimates indicates that this process might be too slow for the critical phase of the deposition stage. The situation might be improved if some of the carbon dioxide is converted to hydrate prior to deposition. If ice is formed and the system is mechanically stable the COSMOS approach is the thermodynamically most stable sending approach. Kinetic estimates indicate an fairly slow melting process even at 7 degrees Celsius.
The yearly emissions of carbon to the atmosphere from fossil fuels is in the order of 6 Gt/year (Cole et.al. (1993), Herzog et.al. (1997)). In order to reduce the greenhouse effect several options for storage of carbon dioxide are being considered.
One option is the storage of carbon dioxide in disused oil and gas fields. The estimated worldwide capacity with this option is estimated to 180 GtC (Herzog et. al. (1997)). The distribution of these storage possibilities is however not evenly distributed around the world. Carbon dioxide may also be used as part of an EOR strategy. An example of this can be found in Sleipner Vest where the separated carbon dioxide is re-injected into the Utsira formation.
Another possibility is aquifers, which in Europe alone may represent a capacity of 200 GtC. Transformation of carbon dioxide to calcium- and magnesium-carbonate represent possible storage options on land. The cost of this is still somewhat uncertain and depends on the availability of mineral oxides.