|Conventional thermal decomposition production of lime (left) versus STEP direct solar conversion of calcium carbonate to calcium oxide (right). Click to enlarge.|
A team at George Washington University has demonstrated a new solar process that can produce lime (CaO) for cement without any emission of carbon dioxide, and at lower projected cost than the existing cement industry process. Production of cement accounts for 5-6% of all anthropogenic CO2 emissions, generating 9 kg of the greenhouse gas for each 10 kg of cement produced, notes Dr. Stuart Licht and his colleagues in a paper on their process accepted for publication in the RSC journal Chemical Communications. The majority (about 60%) of those CO2 emissions result from the production of lime.
The Solar Thermal Electrochemical Production of CaO without CO2 (STEP Cement) process is based on the STEP theory of an efficient solar chemical process, based on a synergy of solar thermal and endothermic electrolyses, introduced by Licht and his colleagues in 2009. (Earlier post, earlier post.)
The majority of CO2 emissions occurs during the decarbonation of limestone (CaCO3) to lime (CaO)...and the remainder (30 to 40%) from burning fossil fuels, such as coal, to heat the kiln reactors to ~900°C.
...Here we show a new thermal chemistry, based on anomalies in oxide solubilites, to generate CaO, without CO2 emission, in a high throughput, cost effective, environment conducive to the formation of cement. The aqueous solubility of CaCO3 (6x10-5 m, where molal ≡ moles per kg solvent) is 3 orders of magnitude less than the 2x10-2 m solubility of calcium oxide, dissolving as calcium hydroxide. Surprisingly, this situation is reversed at high temperatures in molten carbonates, which allows the endothermic, electrolytic one pot synthesis, and precipitation of CaO. Conducive to our new solar process, electrolysis of molten carbonates forms oxides, which precipitate as calcium oxide when mixed with calcium carbonate. Thus no CO2 is formed, to eliminate cement’s greenhouse gas contribution to anthropogenic climate change.—Licht et al.
STEP Cement uses solar thermal energy to drive calcium oxide production without any emission of CO2 in a one pot synthesis; solar thermal energy is used both for the enthalpy of calcium oxide formation from calcium carbonate and to decrease the required electrolysis potential.
In the process, limestone undergoes low energy electrolysis to produce (i) CaO; (ii) O2 and (iii) reduced carbonate without carbon dioxide emission.
Molten carbonate electrolytic synthesis operates in the reverse mode of molten carbonate fuel cells (MCFC); rather than injecting fuel to produce electricity as a product as in the MCFC, electrical energy is supplied and energetic chemical products are generated. Carbonate electrolysis is endothermic, which provides the opportunity to add a significant portion of the required energy to drive the process as solar thermal heat. When the requisite low energy of the solar-heated electrolysis is generated by a non-fossil fuel electricity source, the process is fully carbon dioxide free.
In their STEP electrolysis experiments, Licht et al. used three electrolyses in series, with lithium carbonate using thin planar nickel and steel electrodes, as detailed in the Electronic Supplementary Material (ESI) for the paper.
The STEP Cement process, the authors note, also cogenerates a more valuable product than cement: either CO or carbon. The CO is produced at below current market values; the low cost of the cogenerated product is due to the endothermic, reactive nature of the available hot carbonate from the limestone, which as they demonstrated in the study, is easily reduced at high activity/low energy in the molten state to carbon or carbon monoxide. CO is an energetic industrial reagent used to produce fuels, purify nickel, and to form plastics and other hydrocarbons.
As a result, the authors suggest, STEP Cement can produce lime at less cost than that of conventional industry cement processes; the projected cost of the produced calcium oxide is decreased by the value of the byproduct, either solid carbon or CO.
This study presents a new chemistry of energy efficient, CO2-free lime production, and the challenge of system engineering and scale-up awaits. It should be noted that the carbonate product is readily removed (dropping cleanly from the extracted steel wire cathode when it is uncoiled, or at higher temperature as a simple evolved gas (CO)), oxygen evolution is confined to the vicinity of the anode, and the high density calcium oxide product is not reactive (does not decompose) in the molten carbonate and 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.—Licht et al.