Solar thermal process produces cement with no carbon dioxide emissions

Apr 10, 2012 by Lisa Zyga report
In the conventional production of lime from limestone, fossil fuels are burned during the decarbonation process, resulting in a carbon dioxide byproduct. In the STEP process, solar thermal energy is used to heat the limestone as well as assist in electrolysis, producing a different chemical reaction with no carbon dioxide byproduct. Image credit: Licht, et al. ©2012 The Royal Society of Chemistry

(Phys.org) -- While the largest contributor to anthropogenic greenhouse gas emissions is the power industry, the second largest is the more often overlooked cement industry, which accounts for 5-6% of all anthropogenic CO2 emissions. For every 10 kg of cement produced, the cement industry releases a full 9 kg of CO2. Since the world consumes about 3 trillion kg of cement annually, this sector has one of the highest potentials for CO2 emission reductions. But while processes are being explored to sequester the CO2 from cement production, so far no process can completely eliminate it.

Jumping on this opportunity for improvement, a team of researchers from George Washington University in Ashburn, Virginia, has developed a method for that releases zero CO2 emissions. In addition, the scientists estimate that the new production process will be cheaper than the existing process used in the industry.

In their study published in a recent issue of , the scientists describe the process as the Solar Thermal Electrochemical Production of cement, or STEP cement. (The team previously used a similar STEP process for carbon capture, with the potential for decreasing CO2 levels in the atmosphere to pre-industrial levels.)

As the scientists explain, 60-70% of CO2 emissions during cement production occurs during the conversion of into lime. This conversion involves decarbonation, or removing the carbon atom and two oxygen atoms in limestone (CaCO3) to obtain lime (CaO) with CO2 as the . The remainder of the emissions comes from , such as coal, to heat the kiln reactors that produce the heat required for this decarbonation process.

The STEP process addresses both issues, starting by replacing the fossil fuel with solar thermal energy. The solar heat is not only applied directly to melt the limestone, it also provides heat to assist in the electrolysis of the limestone. In electrolysis, a current applied to the limestone changes the chemical reaction so that instead of separating into lime and CO2, 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 O2. When electrolyzed above 800°C, the product is lime, CO, and ½O2.

“Electrolysis changes the product of the reaction of the limestone as it is converted to lime,” coauthor Stuart Licht, a chemistry professor at George Washington University, told Phys.org. “Rather than producing carbon dioxide, it reduces the carbon dioxide (adds electrons) and produces only oxygen and graphite (which can be readily stored as solid carbon) or CO for fuels, plastics or pharmaceuticals. This is accomplished at low energy and high throughput.”

When separated, the carbon and no longer pose the threat to the atmosphere that they do as CO2. As Licht explained, the carbon monoxide byproduct in the higher temperature reaction can be used in other industries, such as to produce fuels, purify nickel, and form plastics and other hydrocarbons. Plus, the carbon monoxide is produced significantly below market value by this solar thermal electrolytic process. The main product, lime, doesn't react with the other byproducts, but instead forms a slurry at the bottom of the vessel where it can easily be removed.

“This study presents a low-energy, entirely new synthetic route to form CaO without any carbon dioxide emission, and is based on unexpected solubility behavior in molten salts,” Licht said. “This synthesis can be accomplished without solar energy, and without our new STEP process, but is particularly attractive when combined with this new solar process. Alternatively, the new synthesis could be used by industry to produce cement using any non-solar renewable or nuclear energy without any CO2 release, or greatly decrease CO2 if were used to drive the new cement production (in the latter, worst-case scenario, the products are lime, graphite and oxygen; there is still no CO2 product, but CO2 would be used in the energy to drive the process).”

According to the researchers, the STEP process can be performed at a lower projected cost than the existing process. In fact, when accounting for the value of the carbon monoxide byproduct, the cost of the lime production is actually negative. The researchers' rough analysis shows that the total cost of the limestone material, , 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 , the cost of the lime production is $173 - $471 = -$298 per ton. For comparison, the cost to produce lime in the conventional way is about $70 per ton. The researchers emphasize that this analysis is not comprehensive, but it indicates the cost benefit of STEP cement, not even considering the value of eliminating CO2 emissions.

The scientists add that the STEP process could be extended beyond cement production to other applications that convert limestone to lime, such as purifying iron and aluminum; producing glass, paper, sugar, and agriculture; cleaning smoke stacks; softening water; and removing phosphates from sewage.

The next challenge for the researchers lies in scaling up the process for commercialization. They note that Gemasolar, a large-scale solar thermal plant, is already in operation. Other solar thermal plants are following, with electricity costs expected to decrease. To maintain constant operation, molten salt storage of the thermal energy can allow production to continue even during fluctuations in sunlight and at night. Another issue may be finding enough lithium carbonate for the electrolyte, although the metal is not consumed in the STEP process and so is not a recurring cost.

“We plan to scale up the outdoor STEP cement prototype, and in general want to increase the portfolio of useful chemicals made by our new solar process,” Licht said. “The goals are to replace today's fossil fuel economy with a renewable chemical economy. Scale-up is the challenge. Although the is entirely new, 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.”

Explore further: New material could enable new facial reconstruction treatment

More information: 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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Cluebat from Exodar
1.8 / 5 (10) Apr 10, 2012
I'm gonna go way out on a limb here and guess that this stroke of genius is impractical and unprofitable in the near to far term.

Doable? Yes.

If we were to convert all of our manufacturing to this new process, I further predict that we will be importing all future cement from North Korea, etc.

Progress!
GSwift7
2 / 5 (6) Apr 10, 2012
Well, you're probably right, but I don't know enough about it to say for sure. Just based on general rule of thumb though, it's usually not cost-effective to add steps in a process manufacturing sequence.

When they say the "value" of carbon monoxide, that's a bit misleading. When CO is needed, it is produced on-site cheaper than this "price". Thier own calculations show that it's cheaper to produce it on-site, with their own process, than it would be to buy it "at market value". You would have ZERO market for bottled CO if you tried to implement this broadly, as well. The market for bottled CO is already rather small and specialized, so you would over-satisfy demand very quickly.

Also, it is worth noting that the chemical process from limestone to quicklime to cured cement is essentially carbon neutral. Cement takes carbon back in when it cures.

Continued:
Mungaman
1.5 / 5 (8) Apr 10, 2012
So true Cluebat. Like the part where it was predicted to be cheaper than conventional means. Like my Mom always said, "Don't look for cheaper, more environmentally friendly ways to do anything because all techniques currently used to do anything are the best". Good ol' Mom.
GSwift7
1.3 / 5 (4) Apr 10, 2012
I have a question that maybe someone here can help with.

The article is talking about making the quicklime at temperatures above or below 800 C, but they need to heat the limestone to over 1300 C to melt it anyway. If they are talking about no longer needing to melt the limestone, then I'm sure they would say that, since it would be the biggest cost saving you could get here.

Another point: The state of the art in cement kilns are portable, trailer mounted units that can be relatively easily assembled in the field. They can take them to wherever the limestone is being quaried and make the cement on-site to eliminate the transportation of the heavy raw material. The above system needs to be competitive with that too.

Last point: Cement raw material usually has heavy metal contamination. I have no idea what effect the electrolosys process would have on those byproducts, but it would need to be considered.
Kinedryl
1 / 5 (4) Apr 10, 2012
It's a economical nonsense from both practical, both theoretical perspective.
Husky
5 / 5 (4) Apr 10, 2012
A good place place would be Koewait, Saudi Arabia or another oil producing state with a major port, abundant sunshine, underutillised dessertland and refinery infrastrucure in place. There, the sun probably could drives adequate production and the production cycle could be closed by pumping the Carbon Monoxide directly to the oil refineries while the raw cement could be shipped from the port.

Another bussinessmodel could be build around nuclear reactors with high thermal output to supply both high quality heat and juice for the electrolysis.
Husky
5 / 5 (2) Apr 10, 2012
you know what, just mix the lime in a molten salt reactor and let the neutron bake the cement for you, only halve joking here, maybe some synergy/savings can be created as both technologies involve equipment for handling molten salt streams.
GSwift7
1 / 5 (1) Apr 10, 2012
to Husky:

It's not practical to ship the limestone raw materials or the finished product any farther than you need to. On average, you only get about 20% the weight of finished product versus the weight of raw material input (limestone). You really, really, really want to convert the limestone to cement right where it is being quarried. You also want to quarry the limestone as close to the point of use as possible. For example, the Kansas highway department has mobile cement processing machines. It comes on several trailers that hook together like "Transformers" robots. They quarry the limestone right next to the work site so there's almost zero transportation involved. That's not possible everywhere, but since the Midwest US is one big limestone deposite, it works there.
antialias_physorg
5 / 5 (2) Apr 10, 2012
In addition, the scientists estimate that the new production process will be cheaper than the existing process used in the cement industry.

Woha. Cement is one of the cheapest products available. If they can beat that by price then they are on to something. But if you figure in taxation on CO2 then it might be economically feasible.

Unfortunately cement production requires loads of energy. Getting enough solar thermal may require a lot of space close to cement plants (and I'm not sure that such space will be available everywhere). Adding to that that such factories are "high dust" environments solar thermal or PV may not mix in well.

Wind/biogas may be a better mix for achieving the same effect.
GSwift7
1 / 5 (3) Apr 10, 2012
to antialias:

Unfortunately cement production requires loads of energy. Getting enough solar thermal may require a lot of space close to cement plants (and I'm not sure that such space will be available everywhere). Adding to that that such factories are "high dust" environments solar thermal or PV may not mix in well


That was my initial knee-jerk reaction as well. Then I realized that you can generate the power just about anywhere in the region and hook up to the grid. That led me to the realization that the issue of power generation is really a seperate issue here. You don't have to have the power system attached to the kiln. That, in turn, leads to the fact that the people above are merely doing solar power advocacy. In other words, this was a public relations piece, rather than a serious science announcement.

If you look up cement on wiki, there are more ecologically sound ways to make cement. The heavy metals are the biggest problem though; not CO2.
antialias_physorg
5 / 5 (1) Apr 10, 2012
Then I realized that you can generate the power just about anywhere in the region and hook up to the grid

For the electricity: sure. But not for the heat required - as described in the article
Though that heat could be produced via electric heater. But then the efficiency is way lower - i.e. the cost is higher.
Eikka
not rated yet Apr 10, 2012
But then the efficiency is way lower - i.e. the cost is higher.


How is the efficiency lower? Electric heaters are pretty much the most efficient thing you can do with electricity, at least directly, because heat pumps don't work so well up to hundreds of degrees.

A windmill powering an electric heater experiences only the grid losses, which are similiar to what a gas powered furnace would lose through the smokestack.
antialias_physorg
5 / 5 (3) Apr 10, 2012
How is the efficiency lower?

Making electricity from solar thermal (or PV) elsewhere in order to send it over a wire and then turn it back into heat is certainly lower in efficiency than using the solar thermal energy right at the site without converting to electricity and back.
hikenboot
not rated yet Apr 10, 2012
There was another article that appeared on this site about a year ago about a process to produce CO2 free cement that actually absorbed CO2 in the process and would last something like 3 times longer and was like 10 times stronger...wish I could find the article but I can't....
Eikka
not rated yet Apr 10, 2012
Making electricity from solar thermal (or PV) elsewhere in order to send it over a wire and then turn it back into heat is certainly lower in efficiency than using the solar thermal energy right at the site without converting to electricity and back.


If you can get solar thermal energy "right at the site".

If you need a square mile of mirrors to run the plant, you lose all that efficiency to transporting the materials in and out, whereas with a grid connected device you can basically pull it up where you have a wire to it.

Think about it. Would you haul cement from Portugal to Norway, or simply make it in Norway, from the locally available materials?
GSwift7
2.5 / 5 (2) Apr 10, 2012
The standard way to do it is with a natural gas fire, with a rotary kiln. There's a German company that went operational with a working prototype system last year. They claim 50% less energy used per ton of cement. It's a gas system with an improved heat exchange and preheater system. Interestingly, they say the giant fan for the pre-heat system is the most energy-hungry part of the system. Apparently that's commonly the least efficient part of the system.

People have been making cement for around 2000 years. I think it's amazing that someone could still make that big an improvement on the technique. Of course, environmentalism has never been part of the equation until recent times. There's always a trade-off. Like with a catalytic converter on your car. It cuts down on some of the nasty chemicals in the exhaust, but it also cuts down on your mileage, so you're using more gas.
antialias_physorg
not rated yet Apr 10, 2012

If you need a square mile of mirrors to run the plant, you lose all that efficiency to transporting the materials in and out, whereas with a grid connected device you can basically pull it up where you have a wire to it.

Did you even read the article?
Eikka
not rated yet Apr 10, 2012
Did you even read the article?


Yes.

What's your point?

You're obviously not going to be using solar thermal energy any more north than Italy for obvious reasons of availability. Unless you want to be out of business six months a year, so all your manufacturing is actually a long way away from your customers which makes it costly and inefficient.

http://greenterra...ants.jpg
Howhot
5 / 5 (1) Apr 10, 2012
I for one welcome the new sun worshiping cement overlords. :-). It's always interesting to read of new engineering ideas. As an environmentalist type, I hope it is a successful technology that becomes mainstream practice.
antialias_physorg
not rated yet Apr 11, 2012
Yes.

What's your point?

The point is that the article specifically mentions the use of heat generated by solar thermal collectors to preheat the kiln.

GSwift7
1 / 5 (1) Apr 11, 2012
antialias:

The point is that the article specifically mentions the use of heat generated by solar thermal collectors to preheat the kiln


A minor point, but technically, they are pre-heating the raw material before it goes into the kiln, just like the German company I mentioned above is doing. The biggest challenge is overcoming the pressure drop between the heat exchanger and the pre-heating chambers. If you don't force in enough hot air and keep the pressure up, then the air cools down from expansion. I did some reading on it yesterday, and that's what I found (in summary). That's why the blower system for the pre-heater is such an energy hog, not really because of the heat.

As others have stated above, I still don't see how you could site a large solar collection array right next to a limestone quary. Finding sites suitable for both the array and the quary, while still being good for transportation would be difficult. It would greatly limit the use of this.
henkel024
not rated yet Apr 14, 2012