Scientific American  [Printer-friendly version]
January 1, 1998


Could pumping carbon dioxide into the ground forestall global warming?

By David Schneider

In December world leaders gathered in Kyoto, Japan, to grapple with
the growing threat of global warming caused by the burning of fossil
fuels. To combat the surge in greenhouse gases--chiefly carbon
dioxide--researchers and policymakers have called for energy
conservation, taxes on carbon emissions and the swift development of
renewable energy sources, such as wind and solar power. Still, with
nuclear energy out of favor and no easy replacement for fossil fuels
on the horizon, the rise in atmospheric carbon dioxide might appear
unstoppable. But a growing number of scientists are pointing out that
another means of combating greenhouse warming may be at hand, one that
deals with the problem rather directly: put the carbon back where it
came from, into the earth.

The idea of somehow "sequestering" carbon is not new. One method is
simply to grow more trees, which take carbon from the atmosphere and
convert it to woody matter. Although the extent of plantings would
have to be enormous, William R. Moomaw, a physical chemist at Tufts
University, estimates that 10 to 15 percent of the carbon dioxide
problem could be solved in this way.

Other scientists, engineers and energy planners advocate placing the
carbon where it does not contact the atmosphere at all. Howard J.
Herzog of the Massachusetts Institute of Technology, for instance,
proposes pumping carbon dioxide into the deep ocean. Although that
tactic might be viewed as exchanging one form of pollution for
another, there are good reasons to consider making the trade. The
ocean contains at least 50 times more carbon than the atmosphere does,
so adding the carbon dioxide from the burning of fossil fuels to the
sea would have a proportionally smaller effect.

Advocates of this fix also point out that much of the carbon dioxide
now released finds its way into the ocean anyway, disturbing the
chemistry of the surface waters. Purposefully placing it at greater
depth should do less harm, because hundreds of years would elapse
before the dissolved carbon dioxide mixed back toward the surface, a
delay that would buffer the otherwise sudden rise to worrisome levels.
Herzog and others will soon perform tests, perhaps off Hawaii, to
investigate how piping carbon dioxide into the deep ocean affects that

Rather than sequestering carbon dioxide in the sea, other researchers
argue the carbon should be returned to the ground. Many natural gas
deposits already contain huge quantities of carbon dioxide. So it is
unlikely that pumping in more would harm the subterranean environment.
And petroleum engineers are already well versed in the mechanics of
this operation. For years oil companies have taken carbon dioxide from
underground deposits and injected it into deep-seated formations to
aid in flushing oil from dwindling reservoirs. Although such efforts
to enhance recovery normally cycle the carbon dioxide back to the
surface, one could, presumably, permanently park the carbon dioxide in
suitable formations (for example, depleted natural gas fields).

Some petroleum companies are banking on that premise. For example, the
largest Norwegian oil concern, Statoil, is now completing an offshore
facility to separate carbon dioxide from the natural gas it extracts
from one field under the North Sea. Making up 9 percent of the gas
there, this carbon dioxide constitutes an irksome contaminant. Rather
than vent the unwanted gas, Statoil will return it to a nearby
underground formation and avoid having to pay the Norwegian carbon tax
on its release.

Even more dramatic plans are in the works for a huge natural gas field
near the Indonesian island of Natuna. Because nearly three quarters of
the gas in that deposit is carbon dioxide, the developers (Mobil,
Exxon and the Indonesian state oil company) have decided that they
will put this greenhouse gas immediately back underground. Otherwise,
exploiting the Natuna field would add about one half of 1 percent to
the carbon dioxide produced globally by the combustion of fossil
fuels--an enormous contribution for a single source.

But perhaps the prime example that could serve as the template for
combating global warming with sequestration comes from the Great
Plains Gasification Plant. That North Dakota facility, a spin-off of
the U.S. government's former synthetic fuels program, now converts
coal to gas (methane), a fuel considered relatively benign because it
contains less carbon per unit of energy. Carbon that was originally in
the coal will soon be piped over the border to Canada as compressed
carbon dioxide, to be used for enhanced oil recovery in Saskatchewan's
Weyburn Field.

Such separation of carbon from coal and injection as carbon dioxide
into the ground may prove especially relevant to developing nations,
such as India and China, which will surely want to exploit their large
coal reserves into the next century. China alone has more than 10
percent of the world's supply. But using such deposits need not
transfer all that fossil carbon to the atmosphere if these countries
convert the coal to cleaner fuels (methane or methanol) and sequester
the leftover carbon dioxide.

Eventually, these and other countries could stop releasing carbon
entirely. One idea, first advanced by Dutch workers in 1989, would be
applicable to so-called integrated coal-gasification combined-cycle
power plants. Wim C. Turkenburg of Utrecht University explains what he
and his colleagues proposed: Oxygen added to the coal would form an
intermediate gas mixture that would then be converted to hydrogen and
carbon dioxide at high pressure by reacting it with water vapor. The
hydrogen could be burned to generate electricity, and the carbon
dioxide could be separated and sequestered underground. Turkenburg
says that "the increase in production costs would be about 30
percent," whereas previous estimates for removing carbon dioxide from
the flue gases of a conventional power plant had promised to double
the price of electricity.

Robert H. Williams of Princeton University's Center for Energy and
Environmental Studies was particularly struck by the Dutch idea: "In
effect what they were doing was making hydrogen out of coal."
Williams, who in 1989 had just written a book about producing hydrogen
from solar energy, still looks forward to a hydrogen-based economy,
but his thinking about the prospects for generating this fuel has
since shifted. "For most of the next century, I think that hydrogen
will be produced from carbonaceous feedstocks," Williams opines.

Producing hydrogen in that way is, in fact, going on today -- and on a
large scale. About 5 percent of the natural gas in the U.S. is
routinely converted to hydrogen for use by petrochemical industries or
for making fertilizer. Such production could presumably expand
rapidly, were hydrogen ever desired to run fuel-cell-powered vehicles
or electrical generating stations.

The prospects for "decarbonizing" fossil fuels are certainly
promising. But the difficulties in handling large quantities of carbon
dioxide safely (the gas, though nontoxic, can cause asphyxiation) and
the costs of separation and sequestration will be difficult to judge
until further projects test the practicality and economics of this
approach. One attempt to do so may begin as early as 2001 in Norway,
where a tax of $53 per ton of carbon dioxide released provides good
incentive to pursue alternatives.

Such efforts, which would need to involve the oil and petrochemical
industries in planning and execution, will surely blur the lines
usually drawn in debates about how best to address increasing carbon
dioxide and the threat of global warming. So it may take people on all
sides of the issue a while to get comfortable with the notion that
fossil fuels, if exploited properly, could continue to service society
without threatening to change the climate.