THE VINE AUGUST 12, 2008
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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.
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4 comments
Wet scrubbers have been turning lime into gypsum, while capturing sulfur oxides, for a long time now. The same is technically feasible with carbon dioxide, except that there is at least 100 times the amount of it to scrub, so more than 100 times more expensive to operate and 15-20 times more expensive to build.
- r-ennis
August 12, 2008 at 2:08pm
This is a sort of Chemistry 101 solution to carbon dioxide sequestration. You could demonstrate the major components in a high school lab - just take some water, bubble C02 through it to satuaration, then heat the resulting carbonated water, and Calcium Carbonate will precipitate (I strong doubt they are realy "drying" the sea water, since that would give them mostly salt, with a little cement mixed in - the dissolved solids in sewater are about 80% common salt, and only about 1% Calcium).
But there is a big rub: when you dissolve the CO2 you're making carbonate ions (HCO3 minus), and hydrogen ions (H plus). The latter are what makes water acidic. So, you dump the seawater back more acidic than you took it in. But that's ok, right?
Well - not really. Nature has been doing this experiment for us ever since we started raising the CO2 concentration in the atmosphere. CO2 dissolves into seawater at the ocean-atmosphere interface, aided by storms and wave induced mixing, and thus acidifying the ocean.. If this weren't happening, in fact, we'd be much further along the CO2 induced warming curve than we are because atmosphere CO2 levels would be about 60ppm higher than they are.
Those H plus ions have two implications for the sequestration scheme: First, they are a real environmental liability. Seawater acidification is deadly to corals and other organisms that secrete calcium carbonate shells, because it makes it harder to create and maintain their skeletons. See oceanacidification.wordpress.com/.../isrs-coral-reefs-and-ocean-acidification-briefing-paper for an intro, if you're interested. Long term, ocean acidification and the resulting ecological changes may be one of the more significant climate impacts of increased atmospheric CO2.
Which leads to the second problem: just as it will be more difficult for polyps to make coral as the ocean acidifies, it will be increasingly difficult for the "cement" plant to dissolve CO2 in increasingly acidic seawater.
The second problem may not be significant (at least until after the ocean is effectively dead) - I haven't done the required calculation to guess, but the first is exactly as significant as the degree to which we successfully sequester CO2 by this method. Sequester an insignificant portion of our emissions, and it probably doesn't matter. Actually make a difference in our emissions, though, and the effect is very important.
None of this means the idea should be DOA, but it needs to be carefully considered in a larger context. It's not as simple as "mix pollution and seawater and pour your new coral patio" and everyone will be better off.
- sdemuth
August 12, 2008 at 5:28pm
sdemuth makes some good points. The procedure looks promising so I hope the chemists at Caldera have done sdemuth's calculations. After all, they should be Chem 101 calculations. Thanks sdemuth for the heads up. Let's hope Caldera has done your math too.
- jet
August 12, 2008 at 6:06pm
Thanks for the science lesson, sdemuth.
...no seriously! I feel smarter, as I always do when reading The Vine and its comments.
- bigfish
August 13, 2008 at 12:05pm