BURNING THE FUTURE: Coal is so cheap and plentiful that it will likely continue to play a large role in electricity generation, but in order to combat climate change the carbon dioxide produced will need to be sequestered. Image:
The world emitted 25 billion metric tons of carbon dioxide (CO2) in 2003—more than one third, 9.3 billion metric tons, came from burning coal. The dirty rock provides half of the electricity in the U.S. and its role (or the nation's dependence on it) is likely to grow, according to a new report from the Massachusetts Institute of Technology. "It's cheap, there's lots of it and there's lots of it in places with high demand, namely the U.S., China and India," says co-author and M.I.T. physicist Ernest Moniz. "Sequestration," he adds, "is a key enabling technology for coal use in a carbon-constrained world."
Sequestration, as envisioned in the report, involves capturing the CO2 from coal-fired power plants, compressing it into a liquid and injecting it deep beneath the earth into old oil fields or saline aquifers. There, according to geologists, the CO2 would be trapped by sealing cap rock to prevent it from seeping back to the surface and into the air. It is relatively cheap to get it there, the report says. The difficulty is capturing it at the power plant without sapping too much energy or pushing electric costs up too high. For example, one 500-megawatt coal-fired power plant (there are the equivalent of 500 of these in the U.S. and China is building the equivalent of two of them each week) produces three million tons of CO2 annually. Adding carbon capture technology to that plant sucks up 40 percent of the power it can produce and adds at least 2.7 cents to the retail price of that electricity.
"If you capture most of the CO2 and sequester it for the 50-year life of the plant, you're talking about one billion barrels of supercritical CO2," Moniz says. "That's a pretty big reservoir."
To date, the largest sequestration project—the Sleipner gas field in the North Sea—slurps up one million tons of CO2 per year (11 million or so since inception) and relies on sonar to detect any major leaks. "So far, so good," says Howard Herzog, principal research engineer at M.I.T.'s Laboratory for Energy and the Environment. "The problem with Sleipner is it's not as instrumented as we would like."
In other words, it does not have the kind of in-place monitoring systems critical to understanding the true workings of liquid CO2 stored underground.
Nor is it big enough to help understand what would happen if even larger amounts of supercritical CO2 were pumped underground. In fact, it would take 3,600 projects of Sleipner's scale—which is the largest such project underway—to reduce current carbon dioxide emissions from coal by less than half, the report says. But even the small projects are already turning up surprises, such as the relative permeability of various rocks and the ability of CO2 to mix with saline and form carbonic acid, which eats away surrounding rock. And, of course, no one knows exactly how long the carbon dioxide could be contained. "The long-term, chemical fate of CO2 remains to be understood," Moniz notes. "It's like a mortgage. It gets us out of the problem in the 21st century, spreading it out over a longer time and not breaking the budget."
U.S. companies have already planned several demonstrations of such carbon capture and sequestration. Among the 25 projects authorized by the federal Department of Energy, First Energy plans to install a new carbon capture technology on its R. E. Burger power plant in Ohio and then partner with engineering firm Battelle to test pumping it 7,000 feet beneath the surface. But none of these projects is of yet sufficiently large scale and that, the report's authors argue, is because there is no cost associated with emitting CO2. "Right now, it's free to vent carbon dioxide to the atmosphere," says S. Julio Friedmann, a geologist and head of the carbon management program at Lawrence Livermore National Laboratory.