Cement plants? They spit out a lot of carbon dioxide. So do fossil-fuel power plants, for that matter. But now it turns out that maybe if you combine the two, you might be able to sequester most of that carbon. Although it's a big "maybe." Scientific American's David Biello reports that a California-based company called Calera is looking at a process to run the flue gas that wafts up out of the smokestacks in a natural-gas plant through ordinary seawater, which could capture up to 90 percent of the carbon dioxide and make, well, chalk that could potentially be used to make cement:
The Calera process essentially mimics marine cement, which is produced by coral when making their shells and reefs, taking the calcium and magnesium in seawater and using it to form carbonates at normal temperatures and pressures. "We are turning CO2 into carbonic acid and then making carbonate," Constantz says. "All we need is water and pollution."
The company employs spray dryers that utilize the heat in the flue gas to dry the slurry that results from mixing the water and pollution. "A gas-fired power plant is basically like attaching a jet engine to the ground," Constantz notes. "We use the waste heat of the flue gas. They're just shooting it up into the atmosphere anyway."
Odds are good that a carbon-constrained world is going to have to make heavy use of various forms of sequestration. But carbon capture and storage for coal plants is still very much unproven; it's possible to scrub the carbon from flue gas and then inject it down into, say, used oil fields, or saline formations, or unminable coal seams, but no one's absolutely sure that the carbon won't leak back out into the air—or, worse, into nearby groundwater. And sequestering carbon deep down in the ocean causes increased acidification, which can wreak havoc on aquatic ecosystems. (I wonder if that might be a problem here, too—what happens to the seawater you use?)
But if there were a cost-effective and safe way of storing those carbon emissions in cement, well, that would be a good deal more workable—and maybe even profitable. China, after all, gobbled up 800 million metric tons of Portland cement last year. And, according to Calera, the process could work for coal plants, too. One major hurdle, it seems, will be first testing the cement, and then, at some point, going down the long, hard road of convincing the construction industry that it's safe enough to use.