Solar cement — Solar-driven electrolysis for making lime and no CO2 emission

Energy gurus often talk about reducing CO2 emissions. Why not be more aggressive and talk about eliminating CO2 emissions? And, why not start with a heavy CO2-producing industry, such as cement?
Some estimate that cement production generates five to six percent of all anthropogenic (human-generated) CO2 emission. There is an almost one-to-one correspondence of CO2 generated to cement made — 10 kg of cement generates 9 kg of CO2. The global annual consumption of cement is more than 3 x 1012 kg , and 90 percent of that is a lot of CO2. That translates to about 3,300 million tons of cement and just under 3,000 million tons of CO2.
Indeed, cement researchers often ponder how to to significantly reduce the emissions problem, and many strides have been made in partnership with large cement makers.
The cement-making process generates CO2 from the decomposition reaction of calcium carbonate to calcium oxide (lime) and from the combustion of fossil fuels to fire kiln reactors (to about 900˚C). Ninety percent or so of the total energy needed to make concrete is used just to make the cement.
Decomposition is a brute force approach to making lime — heat the stuff until it gives up its bonds and falls apart. Professor Stuart Licht at George Washington University is a STEP ahead, though, and has demonstrated the feasibility of making lime by electrolysis with a process he calls Solar Thermal Electrochemical Production. In a paper published this month in Chemical Communications, (DOI: 10.1039/C2CC31341C), he describes a solar-driven process that exploits “a new thermal chemistry, based on anomalies in oxide solubilities, to generate CaO, without CO2 emission.”
In the process, molten carbonates heated by solar energy are electrolyzed and form oxides, which in the presence of calcium carbonate precipitate as lime. The solubility of calcium carbonate is high in molten carbonates at high temperatures (in the 750-950˚C range). However, the solubility of calcium oxide in molten carbonates is low, up to 100 times lower than calcium carbonate.
The team experimented with two kinds of electrolyte, a eutectic mix of carbonates and pure lithium carbonate. The paper explains how lime forms in the electrolytic cell, “when molten carbonates undergo electrolysis to form oxides, added calcium carbonate will precipitate the desired CaO product for extraction, and the added carbonate replenishes the electrolyte for continued, ongoing CaO production.”
At temperatures below 800˚C, the calcium carbonate electrolyzes to CaO, C and O2. Above 800˚C, the reaction products are CaO, CO and O2. (CO is a commercially valuable compound.) No CO2 is produced in either temperature regime.
Electrolysis of carbonates is endothermic, which means much of the thermal energy required to drive the process can be provided by solar energy. And, if all of the heat is provided by solar energy, no fossil fuels are burned and no CO2 is generated by the process itself.
The resulting calcium oxide is high density and appears to be easy to harvest as it “forms a slurry at the bottom of the vessel where it may be removed by tap in the same manner in which molten iron is removed from conventional iron production kilns.”
The authors realize that scaling-up to industrial production levels and incorporation into production systems will be challenging. But, this is familiar territory for industries and engineers. There is precedent, too, for industrial-scale electrolysis processes. Electrolysis is the basis of the Hall-Heroult process (pdf) for extracting aluminum from bauxite. Similar to Licht’s experiment, the key step is to dissolve alumina in a molten salt, in this case, sodium aluminum fluoride.
Cementiers will be interested to know that the American Ceramic Society’s Cements Division is holding its Third Advances in Cement-based Materials meeting in June. The theme is “Characterization, Processing, Modeling and Sensing.” The plenary speaker will be Edward Garboczi from NIST whose talk is titled, “Computational Materials Science of Concrete: Past-Present-Future.”
Edited By Eileen De Guire • April 12, 2012 (After

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