Environmental Engineering Reference
In-Depth Information
Continuing emissions of CO
2
till depletion
of fossil fuels in about 250 years
Sequestration of incremental amounts
of CO
2
in the deep ocean
0
500
1000
Time (
y
)
1500
2000
Figure 10.11
Qualitative illustration of the effect of ocean sequestration on atmospheric concentrations of
CO
2
. Note that in about 1000 years a new level of CO
2
concentration will be reached regardless of ocean
disposal. (Adapted from Wilson, T. R. S., 1992.
Energy Convers. Manage.,
33,
627-633.)
If injected at 500 m or less, liquid CO
2
would immediately flash into gaseous CO
2
and bubble up
to the surface. Between 500 and 3000 m the liquid CO
2
injected from a diffuser at the end of a
pipe would disintegrate into droplets of various diameters, depending on hydrostatic pressure of
the liquid CO
2
and the release orifices' diameter. Because at these depths the density of liquid CO
2
is less than that of seawater, the droplets would ascend due to buoyancy and may reach the 500-m
level, where they would flash into gaseous CO
2
. It is estimated that a minimum injection depth of
1000 m and a maximum initial CO
2
droplet diameter of 1 cm would be necessary for complete
dissolution in seawater of the ascending CO
2
droplets before they reach a depth of 500 m, where
they would flash into gaseous bubbles. Beneath about 3000 m, liquid CO
2
becomes denser than
seawater, so the injected droplets would sink to the ocean bottom.
Another feature of CO
2
is that it forms a clathrate, also called hydrate. At a hydrostatic pressure
of about 50 atm (corresponding to a depth of about 500 m) and temperatures below 10
◦
C, a solid
CO
2
hydrate is formed, the crystalline structure of which contains one CO
2
molecule surrounded
by 5-7 H
2
O molecules tied to CO
2
by hydrogen bonds. Laboratory and pilot-scale releases of CO
2
in the deep ocean confirm the formation of CO
2
hydrate. At this time it is not clear whether hydrate
formation is good or bad for CO
2
sequestration. If the hydrate crystals were heavier than seawater,
they would sink to greater depth from the release point, thereby increasing the sequestration period.
If the hydrate crystals were to occlude liquid or gaseous CO
2
, they would ascend by buoyancy and
would hinder the dissolution of the released CO
2
in seawater. The hydrates may also clog up the
release pipe and diffuser. Further pilot-scale releases are planned at various depths to ascertain the
optimum depth and release configuration before large-scale injection of CO
2
in the deep ocean is
to be implemented.
The laying of large-diameter pipes on the continental shelf reaching to 1000-m depth is a
formidable task, but pipes have been laid to off-shore oil wells at such depths in the Gulf of
Mexico, the North Sea, and elsewhere. The cost of such pipelines is estimated between $1 and $2
million per kilometer length. Therefore, it is desirable to reach depths of 1000 m or more as close
