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Shells egg on hydrogen output

ChemicalProcessing.com
10/30/2007

Egg shells may help improve hydrogen production via the water-gas-shift reaction, believe researchers at Ohio State University, Columbus. The shells can serve as a low-cost source of calcium carbonate in the calcium looping process patented by the university, notes L. S. Fan, a professor of chemical and biomolecular engineering. A pilot plant with $2.5 million in funding from the U.S. Department of Energy should start up by the end of the year, Fan expects.

In the looping scheme, calcium carbonate is converted to calcium oxide by calcining. The CaO then is used in the reactor to remove CO₂ by reforming CaCO₃, thus shifting the reaction equilibrium to more H₂ production. The reaction takes place at more than 500°C and about 20 atmospheres, but doesn’t require a catalyst and needs far less water than conventional routes, only about the stoichiometric amount, says Fan.

Carbon dioxide is reduced to ppb levels, he notes, adding that other contaminants also are removed: H₂S to less than 1 ppm, and COS and HCl to ppm-levels. The carbonate then is removed and recalcined to CaO, which is reused; the CO₂ recovered is sequestered ultimately.

While the calcium looping process can use limestone and other sources of calcium carbonate, egg shells are attractive because they now are discarded and so should be available at low or no cost, notes Fan. In addition, the reactivity of the CaO made from egg shells equals that of the best material now available, so-called engineered limestone developed at Ohio State, he says.

Fragments of egg shell are placed in an acetic acid solution, which frees the membrane inside the shell and also conditions the shell for the right porosity for CO₂ removal, explains Fan. Because the membrane is lighter than the shell, it floats and can be physically separated. The valuable collagen content of the membrane then can be recovered by conventional means, he notes. Meanwhile, the shell, which is around 95% calcium carbonate, is ground to 200–300-micron size particles for calcining.

A different application for the calcium-looping process, for CO₂ removal during coal combustion, is further along; a pilot plant has been operating for about a year.