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Researchers Develop New Method for Ocean Sequestration of Carbon Dioxide Through Accelerated Weathering of Volcanic Rocks

7 November 2007

Researchers from Harvard and Penn State have developed a new method to enhance removal of carbon dioxide from the atmosphere and place it in the Earth’s oceans for storage. The work is described in a paper in the journal Environmental Science and Technology.

The process is a manipulation of the natural weathering of volcanic silicate rocks. Unlike other proposed ocean sequestration processes, the new technology does not make the oceans more acid and may be beneficial to coral reefs.

The technology involves selectively removing acid from the ocean in a way that might enable us to turn back the clock on global warming. Essentially, our technology dramatically accelerates a cleaning process that Nature herself uses for greenhouse gas accumulation.

—Kurt Zenz House, graduate student in Earth and planetary sciences, Harvard University.

In natural silicate weathering, carbon dioxide from the atmosphere dissolves in fresh water and forms weak carbonic acid. As the water percolates through the soil and rocks, the carbonic acid converts to a solution of alkaline carbonate salts. This water eventually flows into the ocean and increases its alkalinity. An alkaline ocean can hold dissolved carbon, while an acidic one will release the carbon back into the atmosphere. The more weathering, the more carbon is transferred to the ocean where some of it eventually becomes part of the sea bottom sediments.

The engineered weathering process swaps the weak carbonic acid with stronger hydrochloric acid and thus accelerates the pace to industrial rates. HCl is electrochemically removed from the ocean and neutralized through reaction with silicate rocks. The increase in ocean alkalinity resulting from the removal of HCl causes atmospheric CO2 to dissolve into the ocean where it will be stored primarily as HCO3- without further acidifying the ocean.

In the thermodynamic limit—and with the appropriate silicate rocks—the overall reaction is spontaneous. A range of efficiency scenarios indicates that the process should require 100–400 kJ of work per mol of CO2 captured and stored for relevant timescales. The process can be powered from stranded energy sources too remote to be useful for the direct needs of population centers.

According to House, this would allow removal of excess carbon dioxide from the atmosphere in a matter of decades rather than millennia.

Besides removing the greenhouse gas carbon dioxide from the atmosphere, this technique would counteract the continuing acidification of the oceans that threatens coral reefs and their biological communities. The technique is adaptable to operation in remote areas on geothermal or natural gas and is global rather than local. Unlike carbon dioxide scrubbers on power plants, the process can as easily remove naturally generated carbon dioxide as that produced from burning fossil fuel for power.

The researchers, Kurt House; Daniel P. Schrag, director, Harvard University Center for the Environment and professor of Earth and planetary sciences; Michael J. Aziz, the Gordon McKay professor of material sciences, all at Harvard University and Kurt House's brother, Christopher H. House, associate professor of geosciences, Penn State, caution that while they believe their scheme for reducing global warming is achievable, implementation would be ambitious, costly and would carry some environmental risks that require further study. The process would involve building dozens of facilities similar to large chlorine gas industrial plants, on volcanic rock coasts.

The Link Energy Foundation, Merck Fund of the New York Community Trust, US DOE and NASA supported this work.

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November 7, 2007 in Carbon Capture and Storage (CCS), Climate Change | Permalink | Comments (14) | TrackBack (0)

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Iron particles sound a lot easier and cheaper.

Interesting idea, but it would surely have to be applied at a gargantuan scale to yield the desired effect. Unfortunately, the most readily available form fossil energy that is currently being wasted - flare gas - is typically not collocated with sources of volcanic rock. The logistics for either would be quite expensive.

Renewable energy sources (solar, wind, waves). IMHO, nuclear is a non-starter as long as there is no operational permanent repository for the waste. Geothermal power might be an option in a number of places, e.g. Iceland, New Zealand, Sicily, Chile, Hawaii's Big Island etc. However, the alkaline run-off from the process needs strong surface ocean currents to achieve the dilution required to avoid harming ecosystems/fisheries and promote CO2 uptake over a wide area. Salinity in the surface layer should also not change too much.

A related question is what you're going to do with all the metallic sodium co-produced with the chlorine gas in the hydrolysis step. NaOH is a strong lye in its own right, which would help raise pH levels. A useful byproduct of this energy-intensive process may be fresh water (depending on desalination level).

Another issue is that unlike carbonates, chlorine salts tend to dissolve in water. Does that mean the solid products of this process would have to be stored permanently on land? The paper does not say.

Sounds great but I'd be a little worried about a few things:

1) Law of Unintended Consequences - what happens when we artificially raise the HCO3(-) level in the world's oceans? I would find it hard to believe that it doesn't have some effect on plankton, algae, and on down the food chain.

2) 100-400kJ or work / mol CO2 - This doesn't really matter if one is using a solar panel that no one else can use. But if one gets that energy from a natural gas fired power plant it is a completely different story.
My initial calculations suggest that a 60% efficient NG fired power plant would produce 0.18 and 0.74 Moles of CO2 for that range of energy (100 and 400 kJ, respectively). And 60% is generous and not taking into account transmission losses. At 400kJ / CO2 it would take a 45% efficient generator and electrolysis system to break even in terms of carbon. Is that feasible? Is that realistic? I don't know.

3) Emissions for the life cycle??

I don't mean to be a pessimist. I really don't. Call me crazy but I want to make sure whatever solution(s) we proceed with are well thought-out, practical, and don't actually make the situation worse for some other reason.

Does anyone have the full text of this article?

100 - 400KJ / mole of CO2? That's 4.4 - 17.6 MJ to remove 1kg of CO2 - or about 1.2 - 5KWhrs.

Where would this come from? Coal produces 1kg of CO2 for 1KWhr. So if the electricity comes from coal, the carbon return is negative.

If it comes from any other source, the other source would have been better used displacing a coal fired power station. So this idea could only work in Iceland.

Doesn't raising the amount of dissolved CO2 in the oceans raise the acidity of the water (which is already a big problem)?

Perhaps we could "nuke" the oceans? Plenty of heat there for desalination.

Sounds like it could create a pathway for chlorinated organic compounds.

Putting a price on CO2, say $40 a ton or 4c per kwh is essential to evaluating these proposals.

How much energy would it take to pulverize silicate rock--or even chalk--and just spread that across the ocean? The reactions would be slower than with concentrated hydrochloric acid, but if it prings in the costs under $50 a ton of CO2, then it would be more feasible than this.

I've thought a bit about this kind of thing in the past. The idea is to electrochemically separate acid and base in a salt solution. This is done today by a process called electrodialysis with bipolar membranes (EDBM). The bipolar membrane is a pair of membranes bonded together, one of which conducts negative ions, the other positive ions. Water is split at the junction and the H+ and OH- ions are transported away through the two membranes (with an applied electric field providing the energy to separate them).

I had wondered if adding CO2 could reduce the energy needed, since the reaction:

H2CO3 --> H+ + HCO3-

requires considerably less energy than the splitting of water. Perhaps a bipolar membrane incorporating the enzyme carbonic anhydrase (which catalyzes the conversion of CO2 and water to/from carbonic acid) at the interface could be made to work.

@ TR -

we'd be crazy to use fresh sources of NG for this, but flare gas isn't one of them. It's usually so-called associated gas that is already being wasted in the context of oil drilling today. That's because it would be too expensive to bring it to market, the reservoir geology does not lend itself to re-injection or there are political obstacles to installing modern technology.

The global volume is estimated - based on satellite data - at some 150 billion cubic meters a year. Much of this is going on in Siberia, the Persian Gulf and Equatorial Guinea. For reference, the amount is approx. 20 days' worth of global gas consumption.

@ jlw -

crushing the rock mechanically would take a lot of energy in its own right. Besides, the chemistry at the bottom of the ocean is very different from that at the surface. If your idea were to work at all, it would take many 1000s of years - too slow to make any difference.

My first thought on reading this was to think that Dr. Evil had moved into the climate change business. But the academic affiliation of the authors makes me not brush it off quite so quickly. I see a few major practical problems - basically, who is going to pay for this and why? The only good answer I can come up with is some form of global trading of carbon offsets or their reverse, pollution allowances. I like the idea of a carbon sink, but I don't like the idea of putting industrial HCl into our oceans for that purpose. This system would seem vastly more practical if we come up with some kind of clean, nearly-free energy - whether that is more efficient and cheaper solar, or nuclear fusion, or something else besides, I don't know. It doesn't sound all that practical with our current energy sources, notwithstanding the potential to power some of it with flare gas that would otherwise be wasted.

If we had clean nearly-free energy then this system would not be needed at all: we'd just use that magical power source, stop dumping 25 billion tonnes of CO2 into the air annually, and the problem would correct itself in short order. (The observed rate of increase of atmospheric CO2 implies that the excess is about four billion tonnes annually, so natural removal mechanisms are almost able to keep up. If correct this would be very good news because it means we only have to reduce our fossil fuel use by about 20% to end AGW.)

Zach, they wouldn't put HCl into the sea, but take it out of the sea. (seasalt + H2O --> NaOH + HCl. Take out the HCl, and leave the NaOH in the sea)

1 litre of gasoline produces 2.3kg of CO2, so at a best price of 1.2kWh/kg (see Alex)it would take 2.76 kWh to take out the amount of CO2 produced by 1 litre of gasoline. At the current price of nuclear electricity, that would make european fuel only 20% more expensive. Since (even cheap) new cars are more than 20% more fuel-efficient than old ones, it is very affordable to drive carbon-neutral with regular cars.
Hibrid and surely electric cars are a better solution, but this is an affordable alternative.
Even dirty coal-to-liquid fuel could be completely carbon-neutral AND cheap this way.

Paul, I agree that adding carbonic acid to the process could make the reaction more efficient, but once you have pure CO2, it would be even more efficient to just pump it underground.

Evidently, this process should not be powered with regular fossil fuel, but it's very important to develop this further, so we have the ability to take the CO2 out of the atmosphere in the (near) future.

Even now, 1.2kWh/kg = 1200 kWh/ton = around 130 $/ton at actual electricity prices.
If there is a working carbon-trading scheme, it could be very lucrative to build large nukes or wind-power fields at an adequate location to produce tens of gigawatts to sell the carbon-credits to airliners and others.

As the price of wind-power decreases continuously, and the power-output increases (for instance large-scale 5MW-turbine fields were Sci-Fi a few years ago, and working today (www.C-power.be)) it may be very-large scale soon. If you can sell your power as carbon-credits at night and as expensive electricity in day-time, this could be an attractive industry.
One 5MW-turbine could neutralise 5000/1.2 = 4.1 tons/CO2 per hour (when the wind blows).
At an average of 50% efficiency, one such turbine could neutralise 18000 tons of CO2, which is the amount emitted by a few thousand cars.
It's technically feasable, and economically affordable.

Another interesting feature of large-scale deposition of HCl, is that HCl + metal --> metal-chloride + H2.
So, at suitable locations, it may be possible to process the stones and recapture some of the 'lost' energy as H2.

This is a very interesting evolution

Another interesting feature of large-scale deposition of HCl, is that HCl + metal --> metal-chloride + H2.

But there is very little free metal available to us in nature, and almost all the human-produced free metal came from reduction of oxide or sulfide ores. So this would just be a way of recovering energy that had already been invested, and probably would be inferior to recycling the scrap metal directly.

What you could do is exploit further oxidation of already partially oxidized metal ions. The silicates that would be used for acid neutralization contain Fe(+2); the ferrous salts could be further oxidized, perhaps in some sort of fuel cell, utimately yielding ferric oxides. Electrochemical reduction of the iron back to metal could also be done, avoiding the need to smelt iron (and possibly allowing separation of other useful metals, such as nickel or chromium; the nickel alone would swamp existing markets if all current continuing CO2 emissions were compensated for by this scheme.)

I agree that adding carbonic acid to the process could make the reaction more efficient, but once you have pure CO2, it would be even more efficient to just pump it underground.

Well, this could also double as a means of extracting CO2 from flue gas (dissolving it into a salt solution, then converting to bicarbonate in the ED stack), and would enable you to avoid compression (admittedly not that expensive) so perhaps the economics could be made to work out. Also, it would avoid lingering concerns about underground CO2 escaping -- the bicarbonate is much more stable.

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