Embedding of solid carbon dioxide in sea floor sediment

Details Embedding of solid carbon dioxide in sea floor sediment Embedding of solid carbon dioxide in sea floor sediment

1996-10-16

2001-02-20

Lillis, Eileen D. (Department: 3673)

Hazardous or toxic waste destruction or containment

Containment

Geologic, marine, or extraterrestrial storage and containment

C405S128350

Reexamination Certificate

active

06190301

ABSTRACT:

The present invention relates to a process and vehicle for the disposal of gaseous carbon dioxide. Gaseous carbon dioxide is a naturally produced gas which is present in the atmosphere; it is also a biproduct of industrial activity (anthropogenic carbon dioxide). The process comprises solidifying atmospheric carbon dioxide and, at least partially, embedding the solid carbon dioxide in open water floor sediment. The vehicle comprises solidified atmospheric carbon dioxide in a form which when allowed to free fall through open water at least partially embeds itself in deep geological sedimentary formations.
Carbon dioxide is considered to be one of the principal greenhouse gases contributing to global warming. A number of solutions have recently been proposed for the separation of carbon dioxide from the atmosphere in order to counterbalance the increasing atmospheric concentrations of carbon dioxide: Orr.J.C., Nature, Vol.357,283-284 (1992), Nakicenovic, N. & John, A., Energy Vol. 16,No.11/12, 1347-1377 (1991), De Baar, H. J. W., Energy Convers. Mgmt. Vol.33, NO.5-8, 635-642 (1992) and Wilson, T. R. S., Energy Convers. Mgmt. Vol.33, No.5-8, 627-633 1992).
Use of the deep oceans has been proposed as a possible suitable disposal medium for excess atmospheric carbon dioxide, [Marchetti, C., Climate Change 1, 59-68 (1977)]. Investigations have underlined the need to dispose of carbon dioxide at great depths in oceans, for example depths greater than 3 km and to use sinking currents to avoid rapid release of the carbon dioxide to the atmosphere [Herzog, H., Golomb, D. & Zemba, S. Env.Prog. 10, 64-74 (1991)), and Bacastow, R. & Stegen, G. R. in Proc. Oceans '91 Vol.3, 1654-1657 [IEEE, Honolulu, Hawaii, (1991)].
More recent investigations have indicated that even shallow injection of carbon dioxide 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 [Haugen, P. M. & Drange, H., Nature, Vol.357,318-320 (1992)]. It has been proposed that if carbon dioxide is injected near the shore (at depths in the range of 200-400 m), gravity currents may carry the dense carbon dioxide laden waters along the bottom slope towards deep water. Both the disposal scenarios described above 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. It has been argued that shallow injection of carbon dioxide in coastal regions is less expensive in terms of energy and capital costs than deep-ocean disposal [Haugen, P. M. & Drange, H. Nature, Vol.357, 318-320 (1992)].
The choice of the marine environment as a disposal medium for carbon dioxide is based on the observation that ocean waters and sediments play an important role in the global carbon cycle and are known to be major sinks and sources of natural carbon. Sedimentation ensures that the marine environment is an overall carbon sink through carbonate sequestration. 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 the marine environment can act as a sink or a source for this gas.
This reasoning makes the disposal of carbon dioxide by dissolution in either shallow or deep waters uncertain, irrespective of potential biological impact, economics or social acceptability. Although the capacity of ocean waters to dilute and dissolve carbon dioxide is very large and natural processes will tend to sequester it as sedimentary material over long time scales due to geological processes (millions of years), changes in physical and biological oceanographic processes such as deep water formation or primary production may be capable of releasing it to the atmosphere very rapidly (decades to hundreds of years). The techniques proposed so far will thus only tend to displace the problem into the relatively near future without ensuring a permanent and/or long-term disposal solution [Hoffert, M. L., Wey, Y -C., Callegari, A. J., Broecker W. S., Climate Change, 2, 53-68 (1979)].
Deep ocean areas such as trenches (at depths greater than 6-8 km) and abyssal plains (at depths greater than 3-5 km) are among the most inaccessible on Earth and are unlikely to be disrupted even by potential rapid climatic changes. Deep sea pelagic sediments are quantitatively made up of two main types of sediments: deep-sea clays and carbonates. The former consist predominantly of fine grained land derived (lithogenous) particles which have been deposited at slow rates (less than 10 mm per 1000 years), but may also contain authigenic (hydroqenous) components such as ferromanganese phases which are rich in selected trace elements (Chester, R., and Aston,S. R., The Chemistry of Deep-sea sediments; In Chemical Oceanography, Eds Riley, J. P., and Chester, R. Vol. 6,34, 281-383, (1976)].
Deep-sea carbonates on the other hand have a rapid rate of deposition (greater than 10 mm per 100 years) and contain a significant proportion of calcareous shell debris which is usually relatively impoverished in trace elements other than strontium, [Berger, W., H., Biogenous Deep-sea sediments: production, preservation and interpretation, Chemical Oceanography, Eds Riley, J. P., and Chester, R. Vol.5, 29, 265-372, (1976)]. The pelagic environment embraces a little over half of the Earth's surface (268 of 510. 10
−6
km
2
), of which about half the deep sea floor is covered by calcareous oozes and is thus the most widespread crustal covering material (approximately 25%) of the planet's surface,[Berger, W., H., Biogenous Deep-sea sediments: production, preservation and interpretation. In Chemical Oceanography, Eds Riley, J. P., and Chester, R. Vol.5, 29, 265-372, (1976)].
Accordingly, the present invention provides a process for disposal of gaseous carbon dioxide comprising the steps of:
solidifying carbon dioxide; and
at least partially embedding the solid carbon dioxide in open water floor sediment.
The open water is usually the sea. The carbon dioxide may be sequestered into the sedimentary sea floor.
Preferably the sea floor sediment is the carbonate rich sediment unbiquitous in the sedimentary formations of the ocean which occur at depths less than the carbonate compensation depth (CCD) at around 4 km deep and which form a natural stable sink for carbon once they have been laid down. An alternative preferred sea floor sediment is the soft clay-type sediment at 4 km or deeper which also acts as an effective barrier to the release of carbon dioxide to the atmosphere.
The present invention depends on the fact that carbon dioxide can be obtained as a solid by cooling to −78.5° C. Its overall specific gravity when frozen is approximately one and a half times (1.56) that of seawater.
The present invention also provides a vehicle for disposal of carbon dioxide comprising solidified carbon dioxide in a form the physical dimensions of which allow it to at least partially embed itself in the open water sedimentary floor when allowed to free fall through open water. Hereinafter such a vehicle is described as a solid carbon dioxide penetrator. A preferred embodiment comprises the frozen carbon dioxide in the form of a hydrodynamic shape, for example a torpedo shape. In such a preferred embodiment, the solid carbon dioxide when allowed to fall through open water will penetrate soft sediments on the sea floor.
Preferably the carbon dioxide is anthropogenic waste gas removed directly from its source before diffusion into the atmosphere.
Partial embedment of the solid carbon dioxide penetrator occurs when the penetrator does not embed itself completely in the open water floor. This occurs in the case of small penetrators weighing 5 ton or less, having a densit

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