For further information, contact Nicolas Murray, SAI/ME JRC EC,nicholas.murray@jrc.it
Last changed 6th Novemeber 1996
Proposed International Patent Application Claiming Priority from
UK Patent Applicatiom No. 9403037.6 SOLID CARBON DIOXIDE PENETRATOR
in the name of The European Atomic Energy Community (EURATOM).
International filing date 13 Febuary 1995.
1. Problem to be resolved,
The work of Marchetti and subsequent studies suggested the need to inject the gas at great depths or in sinking currents to avoid rapid outgassing to the atmosphere. More recent investigations by Haugen and Drange (Nature, Vol 357, 318-320, 1992) have indicated that even shallow injection could be envisaged by relying on the increase in water density that would result from carbon dioxide dissolution in seawater to transport the dissolved gas to greater depths. These authors propose that if carbon dioxide is injected near the shore (at depths in the range of 200-400m), gravity currents will carry the dense carbon dioxide waters along the bottom slope towards deep water.
Both the disposal scenarios proposed depend on the injection of carbon dioxide at the required depth, either directly into the sea as a liquid-gas mixture, or by pumping artificially enriched seawater containing the carbon dioxide at elevated pressures. The use of such techniques would clearly depend on the development of a highly efficient means of separating the gas on the industrial scale and demonstrating the economics of such a disposal method. Haugen and Drange argue that shallow injection in coastal regions will be less expensive in terms of energy and capital costs than deep-ocean disposal.
The choice of the marine environment as a disposal medium is based on the observation that the ocean waters and sediments play an important role in the global carbon cycle and are known to be major sink and source of natural carbon. The role of sedimentation ensures that the marine environment is an overall sink through carbonate sequestration. However, although seawater itself contains large quantities of carbon dioxide as carbonate and bicarbonate ions and as dissolved carbon dioxide gas, the system is in dynamic exchange with atmospheric carbon dioxide and depending on temperature and salinity can act as a sink or a source for this gas.
It is exactly this point that makes the disposal of carbon dioxide by dissolution in either shallow or deep waters uncertain from a technical point of view, irrespective of any question of potential biological impacts, economics or social acceptability of such a disposal method. Although the capacity of ocean waters is very large to dilute and dissolve carbon dioxide and natural processes (biological and chemical) will tend to sequester it as sedimentary material over long time (geological) scales, short term physical and biological oceanographic processes (decades to hundreds of years) such as changes in deep water formation or primary production may be capable of returning it to the atmosphere very rapidly.
A number of chemical scrubbing processes are used to remove major contaminants produced from coal combustion such as fly ash solids, sulphur compounds and other materials, which are then either treated or dumped at waste disposal sites on land. At present atmospheric dilution is the only method used for the waste gaseous carbon dioxide. International research, under the aegis of the IEA Greenhouse Gas Programme, is being directed to assess the environmental feasibility of using the oceans (as well as a certain number of terrestrial options) as a disposal medium directly, as described above.
Solid carbon dioxide free fall penetrator. In order to get round waste disposal based, as at present, on atmospheric dilution or the uncertainty related to ocean dispersion by dilution as discussed above, the use of a solid carbon dioxide free fall penetrator is proposed to permanently dispose of separated anthropogenic carbon dioxide.
A solution for permanently sequestering carbon dioxide may be proposed on the basis of the observation of the occurrence of carbonate and silicate rich sediments which are ubiquitous in the sedimentary formations of the ocean, and which form a natural stable sink of carbon once they have been laid down. Deep ocean areas such as trenches (depths > 6-8km) and abyssal plains (depths > 3-4km) are among the most inaccessible areas on Earth and unlikely to be disrupted even by rapid climatic changes.
The technique makes use of the fact that carbon dioxide can be obtained as a solid at atmospheric pressure by cooling to -78.5 degrees centigrade. The overall specific gravity is approximately one and a half times (1.56) that of seawater and would thus fall through the water column if produced as a solid (Nakashiki et al. CRIEPI report EU91003, pp 19, 1991). Such pieces would not enter into any underlying sediments and would slowly dissolve on the sea floor with the risk of the waters eventually returning to the surface. However, if the solid was shaped as a torpedo and then left to fall through the water column it would penetrate soft sediments. In selected deep ocean carbonate rich sediments (at depths ranging from 1-4 km) pore water movement can be shown to be negligible thus effectively permanently isolating the carbon dioxide from the overlying ocean waters. At the in-situ pressures and temperatures (approx. 100-400 atmospheres, 2C) the carbon dioxide would eventually form a stable clathrate type compound with the pore water of clay or carbonate sediments and be permanently sequestered by the surrounding geological media.
A number of detailed studies have been undertaken investigating penetrometer design (Freeman et al, Nature, Vol 310, 130-133, 1984, Murray and Visintini, IEEE Journal of Oceanic Engineering, Vol OE-10, 38-49, 1985). The reference cited above shows the large range of parameters that need to be considered. A prototype submarine vehicle intended to measure data at the deep ocean sea- bottom was constructed by Murray and Jamet using a slow penetrator technique. A European patent was granted on the 24.4.1990 (Definitive Number 0362991) and has further demonstrated the extent of possible applications of this technology.
It needs to be clearly stated that under present International leglislation (United Nations Convention on the Law of The Sea[UNCLOS], London Dumping Convention) such a disposal option would not be permited. Furthur, any proposed use of deep sub-seabed geological formations is one that would need a global social consensus which would be unlikely without unequivical demonstration as to its long term safety. Whether in view of the volumes of waste material potentially involved such a solution would be economically viable is clearly at present difficult to assess. It will depend on political and industrial conditions that might occur if global climatic change were to be demonstrated in an clear and unequivocal manner and serious attempts to decrease carbon dioxide emissions were considered necessary.
From a purely technical point of view the proposed solution should, if proven feasible and safe, effectively isolate the carbon dioxide from the overlying ocean waters using the natural barrier properties of the surrounding stable soft clay or carbonate rich sediments. Thus the risk of the eventual return of carbon dioxide to the atmosphere through upwelling of ocean waters can be effectively excluded. Disposal would thus be permanent. A further advantage could be the creation of a waste disposal industry using this type of technology and financed on the polluter-payer principle. The technology appears to be easily within present day engineering capabilities.
A detailed description of the penetrator option and a conceptual design of an industrial disposal system are developed in the following articles:
Murray, C.N., Visintini, L., Bidoglio, G., Henry, B.
Permanent storage of carbon dioxide in the marine environment: the solid CO2 penetrator.
Presented in Int. Conf: Greenhouse Gases: Mitigation Options. London, UK, 22-25 Aug.1995.
Guevel, P. Fruman, D.H., Murray, C.N.
Conceptual design of an integrated solid CO2 penetrator marine disposal system.
Presented in Int. Conf: Greenhouse Gases: Mitigation Options. London, UK, 22-25 Aug.1995.