Unusual reaction eschews high temperatures and water to lock carbon dioxide away

August 8, 2012, Pacific Northwest National Laboratory
High-resolution scanning electron microscopic images of the reacted forsterite indicating the formation of magnesite (cubes).

(Phys.org) -- When it comes to reducing the impact of the energy we use to cool our homes and power our computers, one option is to remove gaseous carbon dioxide (CO2), pump it into underground reservoirs, and have it become part of the mineral formations. If the CO2 doesn't react, it remains in a state that could be released by drilling or earthquakes, defeating the purpose of sequestering the carbon away from the atmosphere. Keeping the CO2 trapped by transforming into minerals, called carbonation reactions, take place much more readily at high temperatures. But, scientists at Pacific Northwest National Laboratory discovered a reaction that breaks the rules. At relatively low temperatures and while recycling the water it needs, this reaction transforms CO2 into the mineral magnesite.

Our nation needs electricity for industrial and personal use; the electricity that comes from that burn coal and other pumps CO2 into the atmosphere. These emissions contribute to changes in , affecting crops, water supplies, and air quality. Capturing and storing CO2 could mitigate its effects. By understanding the fundamental reactions, scientists can inform policymakers and others about different options. Industries that form minerals could also benefit from the team's discovery, as it offers a pathway to make valuable materials with lower .

This study drew upon the team's desire to recycle water during carbonation to increase the amount of mineral carbonation. Water drives the reactions, but it can also be consumed in the process.

"If the water was re-liberated, you could achieve significant with just a small amount of water," said Dr. Kevin Rosso, a and PNNL Fellow who worked on this study.

To recycle the water, the scientists took on a fairly difficult challenge: create minerals that are not bound to in an environment where the minerals need water to form. Specifically, they wanted water-free or anhydrous magnesite. The water-packed or hydrous form is very water soluble and hence is not effective at sequestering CO2 in mineral form. The anhydrous form is much better for carbon storage. Conventional views state that creating the anhydrous form requires , which often do not occur in carbon storage reservoirs.

The team began with highly reactive nanometer-sized particles of forsterite, MgSiO4. Forsterite-rich minerals are the most abundant minerals in the Earth's mantle for the first 250 miles or so. Using a host of instruments at EMSL, and unique capabilities developed within other parts of PNNL, the team studied the reaction as it happened and after it was done. They ran experiments at 95°F and 122°F.

They introduced water-saturated supercritical CO2 to the particles. This form of CO2 behaves like a liquid and a gas; it is easier to pump than the gaseous form. They examined the forsterite surface using scanning electron microscopy and characterized the molecules formed using confocal Raman spectroscopy, nuclear magnetic resonance spectroscopy, and energy-dispersive x-ray spectroscopy. In addition, the team confirmed their results using a reaction cell approach and the in situ x-ray diffraction spectrometer at PNNL.

"We had to present overwhelming evidence that this reaction had occurred at these low temperatures," said Felmy. "Nobody has been able to form this phase from aqueous solutions at these low temperatures, but we have."

The results show that within 3 to 4 days, the forsterite, water, and CO2 react to form a mixture of two magnesium-based minerals: nesquehonite, which contains water, and anhydrous magnesite, which does not. Water is continually used and released in the process, with the water driving the reaction. Over 14 days, magnesite and a highly porous amorphous silica phase are produced. After two weeks of reacting, the forsterite is transformed into the magnesite.

The team at PNNL is defining the molecular mechanism that forms the magnesite. The team is determining the precursors to the reaction, specifically the nucleation clusters.

Explore further: Plump up the clay: Carbon dioxide moves into and expands a common mineral in carbon sequestration caprocks

More information: AR Felmy, O Qafoku, BW Arey, JZ Hu, MY Hu, HT Schaef, ES Ilton, NJ Hess, CI Pearce, J Feng, and KM Rosso.  2012.  "Reaction of Water-Saturated Supercritical CO2 with Forsterite: Evidence for Magnesite Formation at Low Temperatures." Geochimica et Cosmochimica Acta 91:271-282. DOI:10.1016/j.gca.2012.05.026

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not rated yet Aug 08, 2012
It requires supercritical CO2. So trade the need for high temperatures with a need for high pressures to achieve this?
5 / 5 (2) Aug 08, 2012
Soy: The critical point for CO2 is only 1070 psi and 80F. That means it is less pressure than a natural gas pipeline. It would have to be at about 2000 psi just to be piped to the place where it is sequestered. It is not like water with a higher critical pressure.
5 / 5 (2) Aug 08, 2012
Sorry, I wasn't trying to criticize the idea or suggest that the pressures were too great. I think it's a fantastic idea; more to the point, I think it will lead to more fantastic ideas. =)

I was just trying to understand the nature of this reaction. Is it something that can be achieved at higher temperatures/lower pressures as well? Or is the pressure and supercriticality of the CO2 intrinsic?
1 / 5 (6) Aug 08, 2012
This whole global warming thing has achieved, it seems to me, a consistent plateau of quasi religious hysteria where Doubters are routinely excommunicated and the Faithful are rewarded with grants and publications. I believe that this state of affairs should be troubling even to those who are convinced of the truth of the anthropomorphic nature of most of the reported climate change. Scientific authority is fundamental dependent the on integrity of scientists and the openness and fairness of the scientific community's consideration of the relevant data. The seeming corruption of these elements by the overriding pressure to save both the world and ones career by playing God with the truth is, it seems to me, reprehensible and potentially disastrous for both the scientific project and our society.

Remember the boy who cried wolf.
not rated yet Aug 08, 2012
Sorry, I wasn't trying to criticize the idea or suggest that the pressures were too great. I think it's a fantastic idea; more to the point, I think it will lead to more fantastic ideas. =)

I was just trying to understand the nature of this reaction. Is it something that can be achieved at higher temperatures/lower pressures as well? Or is the pressure and supercriticality of the CO2 intrinsic?

A sliding-scale tradeoff between temp/pressure. 80F/1070psi is likely the most energy efficient balance between them, at least at this stage of development.

not rated yet Aug 08, 2012
Soy: Sorry I misinterpreted the note. To answer the question, there have been a number of studies of injected CO2 that indicate this takes place insitu at the temperature and pressure of the formation the CO2 is injected into. Remmeber that the temperature and pressure go up as you go down into the Earth. You will probably need more than the critical pressure and will certainly be higher than the critical temperature just to pump the CO2 down that far. As for the economics, it is relatively simple to pump liquid CO2 so it is not bad at all and comperable to natural gas.

Now I will wait for one of the whackos to come back and ask if they are driving SUVs underground to raise the temperature like that.
1 / 5 (2) Aug 08, 2012
Actually, this mineralization of CO2 is a bad idea. What if we or future folks need this gas? It can get really cold too as we are living in an ice age during an interglacial period. So perhaps we shouldn't damn all of that evil carbon dioxide to some hard-to-reverse subterranean chemistry.

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