Michael Kanellos | April 2, 2009 at 12:41 PM 2 Comments

Are Microorganisms the Missing Ingredient in Green Cement Controversy?

First, hats off to John Carey of BusinessWeek for breaking the story on the controversy between Calera, the green cement company funded by Khosla Ventures, and Ken Caldeira, a well-regarded and notable climate scientist with the Department of Global Ecology at the Carnegie Institution of Washington. The debate is thus: Calera says it can make cement out of seawater and carbon dioxide. The process would curb energy use because cement production is incredibly energy intensive and sequesters carbon dioxide. Caldeira says the vague statements don't add up. Instead, Calera may just be taking calcium carbonate, the principal ingredient in cement, putting it through chemical reactions with other materials like magnesium hydroxide to get magnesium oxide, calcium and carbon dioxide. The company then likely remixes to get calcium carbonate. "They are just putting back what they started out with," he said. "I think all they are doing is taking alkaline minerals and returning them back to be carbonate materials." Calera, Caldeira noted, built its prototype plant at a magnesium oxide factory. MIxing seawater, carbon dioxide and heat, he added, typically ends up as water vapor, salt and carbon dioxide. Sources, however, told me a while back that Calera creates its carbonates through metabolic engineering, i.e., microrganisms. The organisms consume carbon dioxide and are able to produce, directly or indirectly, carbonates. Founder Brent Constantz has said in interviews that the process mimics marine cement, which is produced by coral extracting calcium and magnesium from seawater. Calera, in other words, might have figured out a way to harness this process for industrial purposes. Like coral and other marine animals, the mineralization process can occur at somewhat normal temperatures and pressures. Constantz is also an expert in biomineralization. Metabolic engineering is in its infancy but drawing considerable interest. Algae companies are growing algae through carbon dioxide. Abalone shells are made out of calcium carbonate: the abalone accomplishes by secreting specific proteins. MIT researchers earlier today showed off a battery cathode that is coated with carbon nanotubes. Startup Climos has raised money to experiment with ways to fix carbon dioxide out of the atmosphere with plankton. Is this the answer to the controversy? Possibly. Or not. Turning carbon dioxide into cement would still require massive feeder ponds, so Calera may not be scalable. There is also no confirmed evidence that this is what they are doing. And most metabolic engineering companies are in lab experiments with their magic microbes at the moment. The company doesn't comment in much detail on its process. Calera may also be exaggerating its effect on greenhouse gases. Its process might produce less carbon dioxide than standard cement, but may not cause a net reduction in atmospheric CO2. Researchers are also looking at other ways to produce similar minerals. At Harvard, researchers are examining whether hydrochloric acid, produced from hydrogen harvested from the "green" electrolysis of water, drizzled over silicate rocks could work, or grinding silicate materials. (Startup Carbon Sciences turns carbon dioxide into carbonates with heat. The company, however, readily admits it is a carbon sequestration play that will rely on greenhouse gas regulations.) UPDATE: A patent application surfaced in January with Constantz name on it that indicates that the company may just be working with ordinary chemical processes after all. See here. One section reads:

[0049]In normal sea water 93% of the dissolved CO.sub.2 is in the form of bicarbonate ions (HCO.sub.3.sup.-) and 6% is in the form of carbonate ions (CO.sub.3.sup.-2). When calcium carbonate precipitates from normal sea water, CO.sub.2 is released. Above pH 10.33, greater than 90% of the carbonate is in the form of carbonate ion, and no CO.sub.2 is released during the precipitation of calcium carbonate. While the pH of the water employed in methods may range from 5 to 14 during a given precipitation process, in certain embodiments the pH is raised to alkaline levels in order to drive the precipitation of carbonate compounds, as well as other compounds, e.g., hydroxide compounds, as desired. In certain of these embodiments, the pH is raised to a level which minimizes if not eliminates CO.sub.2 production during precipitation, causing dissolved CO.sub.2, e.g., in the form of carbonate and bicarbonate, to be trapped in the carbonate compound precipitate. In these embodiments, the pH may be raised to 10 or higher, such as 11 or higher.

It's a long application and most of it goes over my head, but it sounds like they are manipulating the pH balance to maximize carbonates and minimize off-gassing. Would something like this work? Or would would you just have with salt and CO2? Calera has not returned calls for comment.