Scientific American, January 1, 1998

BURYING THE PROBLEM

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 realm.

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.