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GWU team develops cost-effective solar process to produce lime for cement without CO2 emission

Solar cement graphic SLicht
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.


  • Stuart Licht, Hongjun Wu, Chaminda Hettige, Baohui Wang, Joseph Asercion, Jason Lau and Jessica Stuart (2012) STEP Cement: Solar Thermal Electrochemical Production of CaO without CO2 emission. Chem. Commun., 2012, Accepted Manuscript doi: 10.1039/C2CC31341C

  • Electronic Supplementary Material



This is awesome. They use solar thermal obviously, but could also use PV solar for the electrolysis. I have always thought that chemical processes might be the best application for solar. If the cost of electrolysis cells are low enough, essentially all of the produced thermal and PV solar energy can be harvested. Again, the cost of the electrolysis cells would have to be low and modular such that as more or less solar energy were produced you could bring in to activity more production cells. You would match the number of electrolysis cells to the maximum solar energy attainable in your array. You would typically be using something less than the total electrolysis cells, but always be accumulating as much solar as possible for the exact condition that occurs.


Yes that is remarkable and smart, I love simple ideas that can yield big reward, making cement consumes a lot of energy and emit a lot of CO2, but cement is ubiquitous, here they reduce power consumption, nullify CO2 emission and yield by product of value like CO or graphite (that can be used as soil fertilizer)

truly awesome, wish it can come true, cement industry is extremely conservative. On the other side the cost of fossil energu is a problem for cement industry


If this turns out to be genuinely more cost-effective than current processes, there won't be any problem with the cement industry adopting it - as people here like to point out, profit is king in this world.

The danger, of course, is that it turns out not to be more cost-effective, or has other problems (quality of product, or reliability of the process), but do-gooders force its use legislatively.


Good points Matthews.

Cement factories have to find ways to lower their cost while increasing sale price. That's the way to increase their profit margin.

Big Oil has found two ways to get higher oil sale price and increase their profit margin: 1) Convince Iran to stop exporting Oil to increase world price to $150/barrel. 2) Convince Iran to block the Strait of Hormuz to increase world oil price to $200/barrel.

No. 1) has started with cuts to EU and USA and price has gone over $100/barrel. Cuts to Japan, China and India would take price up to $150/barrel.

No. 2) has been floating around for a while and just talking about it over and over again will have a snowball effect.

Shale and Tar sands extraction people and many others are smiling.


I've seen a number of low/zero/negative CO2 cement ideas over the years, everything from geopolymers to magnesium oxide cements. Those are what they need to compare their "STEP Cement" to.

BTW, we can also now replace the other high embodied CO2 building material used in construction, steel, with a lower CO2 alternative - Glulam;


I suspect that one issue might be with impurities in the carbonate source. Many cement plants are large emmitters of mercury and other metals, which may be contaminants in the electrolysis cell. On the other hand it actually presents an opportunity to develope a hot liquid mercury capture technology rather than trying to capture mercury in a dilute gaseous state.


The embodied energy of concrete is actually very low, just 1.3 MJ/kg, that's even lower than what it takes to produce milled lumber. The real problem is that the energy is in the form of high process thermal where as lumber is milled by electric motors. There also the problem of how much of it we use, we use it in all our infrastructure and it's strength to weight ratio requires that we use more of it than anything else in the same application.

Jay Kalend

I recall a similar development where Li carbonate is used to smelt iron and recover ckoke, with no roasting, only electrolysis, and at a lower temperature/free energy barrier than chemistry data specifies. Metals is where its at, since some carbon is used, electrification has a higher payoff, and the dengue has CaCO3 in it which has some value scrubbing out impurities like silicon. The process in the article has the flaw with what if you were dealing with impurities? You have to make clinker before you make cement, and that is a very energetic process to remove nasty stuff like thallium. So are we dealing with a one step process to eliminate kiln drying and air cyclone filters? If so, that already saves energy.


As I recall, lime cement absorbs CO2 from the air to become carbonate again, so it is carbon-neutral over the long term.

The melting point of LiCO3 is within the range of molten-salt reactors, and the electrolysis of carbonates is a reasonable dump load for off-peak power.  If the carbon is recovered as either solid or CO, it can be converted to fuels or chemicals.  This could be a winner for the environment.


Yes that's true. But lime (CaO) only absorbs as much CO2 as was driven off from the limestone (CaCO3). It would not absorb any of the CO2 emitted from the burning of fossil fuels that's otherwise needed to generate the heat.

This STEP process could actually be carbon negative if the lime cement absorbs CO2 from the air to become carbonate again & the carbon/CO is sequestered or at least used to offset another process.

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