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."