"Carbon Underground," Feature Article, February 2003

carbon underground

Much of the world points a finger at CO₂. Now it needs a place to hide.

by Jeffrey Winters, Associate Editor

Robert Kane could well have been holding the salvation of the Earth's atmosphere in his hands. It usually sits on a shelf behind his desk, but to illustrate a point, Kane, an environmental scientist at the Department of Energy's Office of Fossil Energy, picked it up. "I have in my hand a brick that is comprised of, well, a bunch of things—but by weight, it's 20 percent carbon dioxide," Kane said. "It's permanent storage for CO₂, and it has a lot of potential."

Right now, a CO₂ brick is far too expensive to be economically practical, but it is one example of the lengths researchers are going to find a way to remove carbon dioxide from the atmosphere and lock it away for good. Carbon sequestration, as it's called, is quickly becoming a cornerstone of the Bush administration's approach to dealing with the issues surrounding global climate change and the influx of greenhouse gases in the atmosphere. Last November, for example, Energy Secretary Spencer Abraham announced the establishment of a network of public-private partnerships to develop a means of permanently storing carbon dioxide from coal-burning power plants.

It's an approach that drives most environmentalists nuts. Many see carbon sequestration research as a diversion from the unfinished task of developing inexpensive, renewable zero-emission energy sources. But even the most optimistic long-term forecasts of energy use suggest that we're not going to abandon burning carbon fuels—not any time soon, maybe not ever.

If governments want to avoid increasing atmospheric CO₂ levels—and many, especially in the European Union, have publicly declared their intent to reduce carbon emissions as a step toward averting catastrophic climate change—then finding cheap, permanent carbon sinks becomes imperative.
Fortunately, there are lots of good places to hide the extra carbon.

Of course, the fact that a preponderance of researchers and governments are convinced that certain gases—CO₂ chiefly, but also methane and nitrogen oxides—are triggering changes in the global climate doesn't make it so. Others have argued that the climate is changing due to naturally occurring factors, or that it is not changing at all.

A Gathering Storm

Nonetheless, geologists have established that atmospheric carbon dioxide levels have risen over the past century. Measurements of air bubbles trapped in glacial ice show that the CO₂ concentration in the atmosphere was 280 parts per million before the Industrial Revolution; current readings are now more than 360 ppm.

And it seems incontrovertible that CO₂ levels are set to rise even more during the first half of the 21st century. The Intergovernmental Panel on Climate Change has projected energy use to 2050 and beyond. The group's baseline model—a "business as usual" scenario that accounts for an ever-increasing worldwide power demand and predicts no radical move away from fossil fuels—suggests that concentrations of CO₂ could top 500 ppm by mid-century and reach 700 ppm by 2100.

The problem is rising power demands. It's been calculated that in order to create just one dollar of additional gross domestic product, more than 4 kilowatt-hours of power must be consumed. Want to raise average incomes in India or China by $1,000? You'll have to develop another 500 GW. Even taking into account long-term gains in energy efficiency, trends in population and economic development suggest that worldwide primary energy demand will increase from 400 quadrillion BTU, or "quads," at present to some 1,000 quads by 2050. The U.S. alone will require some 260 quads—a 260 percent increase over current consumption.

For sure, renewable energy sources, such as geothermal and biomass, are expected to make up an increasingly large share of total energy production over the next few decades. Solar and wind power have received a great deal of interest, since they are both nonpolluting and potentially inexpensive. Already, wind power is competitive with fossil fuel in some markets and the price per watt for solar cells is dropping rapidly.

Building with gas: These bricks, fabricated at the Office of Fossil Energy's Albany Research Center, are about one-fifth carbon dioxide by weight.

But in an argument that inverts the environmental arguments of the last 30 years, some researchers now question whether renewables are plentiful enough to supplant fossil fuels. One of the leaders in this reassessment is Columbia University professor Klaus Lackner, who says the best way to look at the problem is to examine the energy passing through a typical square meter. Lackner estimates that the average flux of solar energy in desert climates, allowing for day and night and weather, is 200 W; the energy of a stiff wind passing through the same square meter is 600 W.

If, instead of extracting the energy of the wind, you removed all the CO₂ from a cubic meter of air, Lackner said you could generate some 10,000 W of power through burning fossil fuels before fully replacing the carbon in that volume of air. "The energy represented by carbon dioxide in the air is far more concentrated than the kinetic energy harnessed by a windmill," Lackner said.

That's when the idea of carbon sequestration starts to gain some traction. To people like Lackner, capturing that carbon and locking it away erases the negative impact the fossil fuels have on the atmosphere. A coal-fired power plant that emits no sulfur, NO_x, or CO₂ can be thought of as environmentally benign as a bank of photovoltaic cells or an array of wind turbines. And it can provide far more energy.

Two problems stand in the way of that decarbonized future. The cost of extracting CO₂ from an exhaust stream or the atmosphere is, at present, prohibitive. Kane pegs it at around $100 per ton of carbon—about 7.5 cents per kilowatt-hour of power from a conventional coal plant or $1 per gallon of gasoline. "But we're getting a lot of preliminary experimental information that shows the cost of capture is coming down significantly, to as little as $20 a ton of carbon," Kane adds. The DOE has a target of $10 per ton, less than the cost of extracting CO₂ from natural reservoirs.

But once you've captured the CO₂, you then have to put it somewhere.

No Free Lunch

One approach that's been popular among policy makers is that of natural carbon sinks: relying on the carbon-fixing activity of plants to remove CO₂ from the atmosphere. One of the reasons for the United States' refusal to accept the Kyoto Protocol on climate change was the Bush administration's insistence on counting tree and plant growth against industrial CO₂ emissions.

Letting trees sop up excess carbon from the atmosphere is an attractive idea: It requires no carbon capturing—in fact, little action beyond planting some seeds—and no infrastructure aside from fields and forests. But the ability of plants to become meaningful carbon sinks is fiercely debated. In December, a team of Stanford researchers published a study that suggested expected consequences of climate change, such as higher temperatures and increased nitrogen deposition in the soil, will reduce the rate at which plants can capture carbon. If this and other studies are correct, there will be no free lunch for CO₂.

Another much-discussed sequestration scheme involves capturing CO₂ and pumping it to the bottom of the ocean, where the crushing pressure will keep it in liquid form, pinned to the sea floor for decades. The ocean already absorbs more than two gigatons of carbon a year. And researchers estimate that the deep ocean can hold between 1,000 and 27,000 gigatons more. But adding that much CO₂ might eventually change the ocean's pH balance, making the waters too acidic for many types of sea life. And, in terms of CO₂, the atmosphere and the ocean are in balance, meaning that as the level drops in the atmosphere (as it surely must, someday, if these steps are taken) the ocean will become a net source of carbon. That prospect makes sinking carbon in the ocean a temporary fix at best, and not a terribly good one.

Burying Carbon

From a platform in the North Sea, a carbon sequestration project has been up and running since 1996, pumping CO₂ into what looks like a more-or-less permanent storage site. The project, run by the Norwegian petroleum giant Statoil, pulls CO₂ from a stream of natural gas pumped from offshore fields. The CO₂ is then injected down into an aquifer some 3,000 feet below the surface.

Statoil's project in the Sleipner West gas field stores some 2,800 tons of CO₂ a day—about the rate produced by a 140-MW coal power plant. The aquifer is more than 800 feet thick and extends for hundreds of miles. The project manager, Tore A. Torp, contends, "The entire carbon dioxide emissions from all the power stations in Europe for 600 years could be deposited in this structure."

The case for replicating Sleipner's success on a larger scale is compelling. Seams of gas—such as CO₂ and methane—are found in all sorts of geological formations, captured or produced through natural processes. (There would be no natural gas industry, after all, without natural gases.) Under layers of impermeable clay or shale, these gases can lie trapped for millions of years. In more porous strata, however, gas can burble back to the surface.

Carbon eraser: The saline aquifer beneath this Statoil platform in the North Sea has the potential to store the carbon dioxide emissions from every European coal-fired power plant for the next 600 years.

Scientists are actively tracking gas leakage from various types of geologic formations. For now, the gas injected by Statoil seems to be staying put. "The question is how long is long enough," said Sally Benson, director of the Earth Sciences Division at Lawrence Berkeley National Laboratory. Benson is leading a government effort to study geological carbon sequestration options. "If you have leaking at a rate of 0.1 percent or 0.01 percent a year, then I think sequestration will be very effective."

The project at Sleipner aside, CO₂ is already being bought on the open market and being pumped into underground reservoirs, although on a modest scale. Producers have known for some time that injecting CO₂ into an oil reservoir will help push oil toward the production well, extending the potential recovery from a mature well by 10 to 15 percent. In one operation, PanCanadian Resources is injecting some 5,000 tons a day of CO₂ piped from a coal gasification plant in North Dakota into the Weyburn oil field in Saskatchewan; this will extend the productive life of the field by 25 years.

CO₂ can also be used to extract methane from coal seams. Long the bane of miners, methane is often pumped out of coal beds, but the gas molecules prefer to stick to the surface of the rocks. Those rocks turn out to be more attracted to CO₂ than to methane, so if CO₂ is pumped into the coal seam, the rocks release the methane, creating a larger pool of recoverable gas.

Oil wells and deep coal seams provide huge potential reservoirs for sequestering carbon dioxide. Geologists estimate that as much as 500 billion tons of carbon—about two-thirds of all the carbon in the atmosphere today—can be locked away in such sites.

Unfortunately, that still isn't enough. To keep atmospheric CO₂ at current levels—that is, about 30 percent higher than the pre-industrial rate—a much larger reservoir will be needed. Saline aquifers trapped in formations more than a mile below the surface offer perhaps the best sites for permanent carbon storage. The capacity is huge—500 billion tons in the United States alone. Taken together, Benson said, geologic formations have the potential to store every gram of projected CO₂ emissions for the next 100 years.

Making Rocks

Geologists know of another, natural mechanism for getting rid of excess carbon from the atmosphere. CO₂ and water form carbonic acid, which then reacts with certain minerals, such as magnesium-rich serpentine, to create quartz and the kinds of rocks called carbonates. Of course, these carbonates formed through the weathering of precursor minerals over the course of millions of years.

But researchers have explored ways to speed things up a bit. "We're accelerating the rate at which the rocks weather," said Richard Walters, associate director of the Office of Fossil Energy's Albany Research Center in Oregon. "What normally takes geologic time to convert, we're trying to convert in engineering time." If CO₂ can be locked up into carbonates in a matter of minutes, the process might provide a lasting means of sopping up excess carbon.

The progress made at Albany and other labs is impressive. They've found, for instance, that by heating the serpentine, modifying the carbonic acid with bicarbonate and salt, and increasing the CO₂ pressure, 80 percent of the magnesium silicate can convert to a carbonate in about 30 minutes. But much more work needs to be done, especially in finding alternative reactions that don't require adding heat to the system.

The question remains: What do you do with the tons of carbonate churned out every day? You could bury some of it back in the pit where the serpentine was mined, but the volume of the carbonate exceeds that of the stock mineral. Researchers at Albany have been working on that problem, too. The brick that Kane has in his office? They made it. "It's a nice demonstration piece," Walters said. But it's far from a practical answer: The brick was made from magnesite particles bound up by common wood glue.

"Still, given the amount of material we could be generating," Walters said, "you want to do something with it."

Surely, someone will come up with a way to make something useful out of all that carbonate. If not bricks or building blocks, then fertilizer or fireproofing. After all, our treating an industrial byproduct—CO₂—as worthless waste is why carbon sequestration has become a research topic in the first place.