Experiment in deep-sea
disposal of carbon dioxide uncovers unexpected glitch
BY LISA TREI
Scientists exploring ways
to reduce emissions of greenhouse gases have discovered
that experiments involving deep-sea disposal of carbon
dioxide raise new, unexpected questions.
The research is part of a
broad, ongoing effort by scientists to find new methods
to dispose of excess carbon dioxide generated by the
burning of fossil fuels creating so-called
"greenhouse gases" that contribute to global
warming. Other potential disposal methods include
injecting CO2 into underground oil and gas reservoirs,
coal seams and deep, salty aquifers.
When a research team from
the Monterey Bay Aquarium Research Institute (MBARI) used
a deep-sea remotely operated vehicle to release CO2 into
the ocean, they discovered that it combines with water to
form a compound known as a hydrate, expanding four times
in volume.
Although the solid hydrate
will eventually dissolve into the seawater, researchers
say it is not known how long that will take, what effect
high concentrations of the hydrate will have on the
deep-sea environment, and how marine life will respond.
The findings are described
in "Direct Experiments on the Ocean Disposal of
Fossil Fuel CO2," that was published May 7 in Science.
Ocean chemist Peter G. Brewer and two colleagues from
MBARI collaborated with Franklin M. Orr Jr., dean of the
School of Earth Sciences, to carry out the experiments
near Moss Landing.
While the scientific
community has discussed the possibility of deep-sea
disposal of carbon dioxide, Orr says that no one had
tested the hypothesis until the MBARI experiments took
place.
"Ocean disposal is
potentially interesting because the ocean already has
huge quantities of CO2 dissolved in it," says Orr,
the Keleen and Carlton Beal Professor of Petroleum
Engineering. "It's just a natural part of the carbon
cycle. Also, 85 percent of the CO2 that goes up into the
atmosphere eventually finds its way into the ocean. The
idea is to short-circuit that by putting the stuff in the
ocean directly."
Initial experiments
injected carbon dioxide at 349 meters, 430 meters and 905
meters while MBARI's remotely operated vehicle (ROV), Ventana,
recorded the results.
"At those depths when
you release CO2, you get a bubble and it grows a rind of
solid hydrate around it," says Orr. "Then
things just sit there because the hydrate rind restricts
contact between CO2 and water." At depths greater
than 2,600 meters, liquid CO2 is denser than seawater.
With an experiment that took place at 3,627 meters,
researchers also expected a stable "skin" of
hydrate to form on top of the liquid carbon dioxide,
similar to a pond covered by ice in winter.
But the video camera
installed on the institute's deep-sea ROV, Tiburon,
recorded surprising results that the scientists observed
as they unfolded. Tiburon injected a 7-liter
beaker half full with liquid carbon dioxide. Within an
hour, the liquid CO2 increased in volume, flowing over
the top of the beaker onto the sea floor. "None of
us expected to see the CO2 expanding and pouring over the
sides," says Orr. "That was truly
unexpected."
In retrospect, Orr says,
what really happened was that each CO2 molecule
"stuck" to about six water molecules to build
hydrate particles, which then fell into the liquid carbon
dioxide in the beaker. Although the CO2 appeared to be
expanding, it was the water molecules that actually
displaced it and pushed it over the sides of the beaker.
This happened much faster at 3,627 meters than the
shallower depths because the carbon dioxide hydrate
separated spontaneously from the interface between the
CO2 and water.
"What this says is
that the idea of having a lake of CO2 [covered by a layer
of hydrate] is not really right," says Orr.
"Instead it's going to convert pretty fast into a
hydrate." This, in turn, asks the question,
"What happens to that?" he says, "which is
important because the ocean is not saturated with CO2.
The hydrate [must] dissolve, but how fast is not
known."
The scientists now plan to
conduct more experiments and will collaborate with
ecologists to study the possible effects of liquid carbon
dioxide on deep-sea organisms.
In addition, before
deep-sea disposal can be considered viable, researchers
must look at the costs associated with separating CO2
from other gases produced by, for example, power plants,
and the related costs linked to compression,
transportation and delivering CO2 to the right depth in
the ocean.
On a political level, Orr
says, society is not yet prepared to make decisions
concerning the disposal of excess carbon dioxide.
"So, the prudent thing to do is to have the research
base in place so that policy makers, when they're ready
to deal with this question, actually can do so," he
says. "I think our job is to make sure we understand
what would happen if we decided to go forward [with this]
because the last thing we want to do is trade one set of
problems for a worse set." SR
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