Zero Carbon Cement Production with Solar Thermal

April 10th, 2012 by Susan Kraemer

In a study published in a recent issue of Chemical Communications, a team of researchers from Virginia’s George Washington University explain a revolutionary way to make lime cement that releases zero CO 2 emissions – and costs less too.

After coal-powered electricity, cement manufacture is the next biggest emitter of greenhouse gases, because cement is ubiquitous in modern life.

It is needed for virtually all skyscrapers, bridges and freeway overpasses and many other buildings and structures including nuclear power plants. The world consumes about 3 trillion kg of cement annually.

Pound for pound, kilogram for kilogram, ton for ton, every 10 units of cement will release 9 units of CO 2. So it is a huge problem for the increasingly unstable climate we are creating for ourselves.

Of the two ways that making cement releases carbon dioxide, separating the lime from the limestone (decarbonation, or removing the carbon atom and two oxygen atoms in limestone (CaCO 3 ) to obtain lime (CaO) with CO 2 ) accounts for 70% of the emissions.

The other 30% is because it takes a lot of heat to heat the kiln reactors, burning fossil fuels.

Solar thermal power would be used. And not just to heat the limestone – but also to help in electrolysis. This would produce a different chemical reaction without a carbon dioxide byproduct.

In electrolysis, a current applied to the limestone changes the chemical reaction so that instead of separating into lime and CO 2 , the limestone separates into lime and some other combination of carbon and oxygen atoms, depending on the temperature of the reaction.

When electrolyzed below 800°C, the molten limestone forms lime, C, and O 2 . When electrolyzed above 800°C, the product is lime, CO, and ½O 2 .

Instead of a CO 2 byproduct, their reactions produce useful industrial chemicals. Their carbon monoxide byproduct (in the higher temperature reaction) can be used to make fuels, purify nickel, and form plastics and other hydrocarbons.

This makes it cheaper than current lime production which costs $70 a ton, because the CO can be sold.

The researchers’ rough analysis shows that the total cost of the limestone material, solar heat and electricity is $173 per ton of lime and 0.786 tons of carbon monoxide (0.786 tons of carbon monoxide are produced for every ton of lime).

The market value of carbon monoxide is $600 per ton, or $471 per 0.786 tons. So after selling the carbon monoxide, the cost of the lime production is actually a negative number. $173 – $471 = minus $298 per ton.

No carbon emissions. Cheap. And even better, it has wide applications.

Nearly all of the other heaviest emitters could similarly be stripped of their greenhouse gas problem with this technology, the scientists say.

(Among other industries, these industrial processes include purifying iron and aluminum, making glass, paper, sugar, and agriculture, cleaning smoke stacks, softening water, and removing phosphates from sewage.)

The next step would be is simply scaling up the fairly straightforward process for commercialization. “Although the process itself is entirely new” coauthor Stuart Licht, a chemistry professor at George Washington University, told Phys.org.”the individual components (solar towers, 24/7 operation storing solar energy with molten salts) are already in place. Solar energy can be used to efficiently make products without carbon dioxide, and at solar energy efficiencies higher than in photovoltaics.”

The timing is perfect: a burgeoning Asia is about to build the new mega cities of the 21st century. And super hot solar thermal heat is ready: Halotechnics Molten Glass Thermal Storage Could Mean 6 Cent Solar.

In the electrolysis process alone, even without solar power, but using fossil heat source, “worst case scenario” says Licht, “the products are lime, graphite and oxygen; there is still no CO 2 product, but CO 2 would be used in the energy to drive the process.”

Stuart Licht, et al. “STEP Cement: Solar Thermal Electrochemical Production of CaO without CO2 emission.” Chem. Commun., DOI: 10.1039/C2CC31341C









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