RealClimate logo

Air Capture

Filed under: — group @ 30 March 2008 - (Español)

Guest Commentary by Frank Zeman

[This is one of an occasional series on the science of mitigation/adaptation/geo-engineering that we hope to continue. Since this isn’t our core expertise, we’d especially appreciate balanced contributions from other scientists.]

One of the central challenges of controlling anthropogenic climate change is developing technologies that deal with emissions from small, dispersed sources such as automobiles and residential houses. Capturing these emissions is more difficult as they are too small to support infrastructure, such as pipelines, and may be mobile, as with cars. For these reasons, proposed solutions, such as switching to using hydrogen or electricity as a fuel, rely on the carbon-free generation of electricity or hydrogen. That implies that the fuel must be made either by renewable generation (wind, solar, geothermal etc.), nuclear or by facilities that capture the carbon dioxide and store it (CCS).

There is however an alternative that gets some occasional attention: Air Capture (for instance, here or here). The idea would be to let people emit the carbon dioxide at the source but then capture it directly from the atmosphere at a separate facility.

The removal of carbon dioxide directly from the atmosphere is a natural phenomenon that occurs in the surface ocean or during photosynthesis. Ocean absorption is a result of both the higher concentration of CO2 in the atmosphere and the alkaline nature of seawater (Note that this absorption that is leading to the “other” CO2 problem, ocean acidification – which may prove detrimental to coral reefs and other organisms that use carbonate). Land-based air capture is an effort to enhance this mechanism at an industrial scale so that CO2 can be removed from the atmosphere under controlled conditions. Given that it is performed under controlled conditions, we can use more alkaline solutions to improve the rate of capture without adversely affecting the biosphere.

Industrial air capture is based on the absorption of CO2 using alkali earth metals such as sodium or potassium. The process is a variant of the Kraft Process used in most pulp and paper mills and as such, benefits from a long industrial history. The CO2 is absorbed into solution, transferred to lime via a process called causticization and released in a kiln. With some modifications to the existing processes, mainly an oxygen-fired kiln, the end result is a concentrated stream of CO2 ready for storage or use in fuels. An alternative to this thermo-chemical process is an electrical one in which an electrical voltage is applied across the carbonate solution to release the CO2. While simpler, the electrical process consumes more energy as it splits water at the same time. It also depends on electricity and so unless the electricity is renewable or nuclear, will result in the storage of more CO2 than the chemical process.

If the technology is well established and, aside from the oxygen combustion of lime, dates back over 50 years, what stops it from being used and what might change in the future?

The main barrier is the efficiency of the energy requirements during the reducing process. Air capture requires energy to move the air, manufacture the absorbing solutions and solids as well as to produce the oxygen, fuel and make up chemicals. All of these items will result in additional CO2 emissions, which reduce the efficiency and therefore the benefits. The second important consideration, and maybe the dominant one, is cost. Air capture has to be more economical than the proposed alternatives (hydrogen, electricity, renewables, greater efficiency etc.). It should be stated clearly that air capture is not a viable alternative to capture at large, point source emitters such as power plants since it will always be more efficient to capture and store carbon dioxide from more concentrated streams. So while there are any non-CCS fossil fuel plants, Air Capture is a non-starter.

But recent suugestions have re-thought air capture as a thermal process. The early incarnations of air capture used electricity as the energy source and therefore depended on carbon-free sources. A thermal Air Capture system uses heat that can be generated on-site, reducing the inefficiencies associated with producing electricity, but of course it still needs a source of (carbon-free) heat. Notably, this implies that air capture could reduce greenhouse gas emissions independently of developments in the power generation or transportation sector. Preliminary experimentation has shown that causticization can occur at ambient temperatures and that conventional vacuum filtration is sufficient to avoid large evaporation penalties. These developments reduce the total energy required for the process by about half compared to the conventional method and thereby reduce the amount of CO2 that would need to be sent to storage.

However, the cost of air capture is still basically unknown. Estimates have varied wildly and real numbers will only come from pilot projects over the next few years. In some sense, that puts this technology on par with the hydrogen economy with expansion potentially starting sometime after 2015. For now there are far easier (efficiency) and cheaper (power plants) ways of reducing emissions of CO2 and so air capture is not a replacement for other efforts to reduce emissions. But in the long run, all carbon sources will require mitigation – including the transportation sector – and at that time air capture could be the most cost effective option for some sources. It is not any kind of panacea though.

238 Responses to “Air Capture”

  1. 51

    This is great!

    I’m thinking we could do 2 things — stop emitting GHGs as much as possible (efficiency, conservation, EVs plugged into solar/wind power), then also suck out even more CO2 from the air than what we’re emitting through fossil fuel burning. That way we might be able to bring the ppm down to a better number. However, I guess it would also be taking out our current CO2 from respiration, etc, since much the previously emitted CO2 (that didn’t get absorbed soon after emission) is way high up in the atmosphere where such air-capture systems won’t work. Would the plants be OK with that?

  2. 52
    John Lang says:

    The most successful CO2 sequestration project to date is the Weyburn Canada Encana enhanced oil recovery project.

    This project has been/will sink 26 megatonnes of CO2 and enhance oil recovery by 155 million barrels. The project is being carefully studied as it rolls-out and several papers have been published in journals which prove the CO2 will be sequestered for thousands of years at least.

    This is the kind of project which should be pursued first. The economics are so good for the oil company Encana that they are paying a rumoured $100 per tonne (7 times the current market rate) for the CO2 produced in a North Dakota coal gasification plant.

    A new coal-fired power plant is planned nearby (construction to start this year) which will also capture the CO2 and, presumably, use a similar process in the oil fields in the area.

    So, it can be done. People just need to keep the economics and cost in mind. In this case, a side-benefit of increased oil production more than pays for the cost of the project. Of course, the geology has to also be conducive to permanently storing the CO2.

  3. 53

    Ray Ladbury said,

    1)CO2 emissions from a point source are concentrated, so much of the work is already done for you;

    ‘Vagueofgodalming’ mentioned exothermic weathering, and linked a Wikipedia page with varying content. Its assertion that olivine’s iron-free end member is forsterite, Mg2SiO4, sounded reasonable.

    Although pulverized and strewn forsterite has not had the work of CO2 concentration done for it, the data say it is ready to do it. The work of concentration is about 20 kJ/(mol CO2), and forsterite’s reaction with CO2 yields 33 kJ/(mol CO2). (That is the minus-delta=’G’.)

    So it is not surprising that, as he or the article said, pulverized olivine weathers quickly. But the reaction cannot generate thermal power; it would run out of minus-delta-‘G’ around 200 Celsius, and be very sluggish indeed some tens of K below that.

    2)distributed sources preclude an efficient centralized plant;

    No, they don’t, because CO2 can come to such a plant from the whole atmosphere.

    3)No one has even shown that carbon capture and storage could be made to work

    Serpentinite-containing mine tailings have in fact shown this, without the mine operators’ intending it. As above said, they took some CO2 from every puff that every tailpipe has puffed.

  4. 54
    BRIAN M FLYNN says:

    You say:
    “However, the cost of air capture is still basically unknown.”
    and, in comments:
    “The tradeoff is the cost of capturing and converting the CO2 to fuel versus transporting and storing H2 along with new re-fueling stations and vehicles.”

    I suggest you may want to view CO2 capture and conversion to sulfur free, carbon neutral synthetic gas production and organic chemicals(see e.g., Green Freedom™ at:
    The design (for now) is to use existing nuclear power with existing nuclear plant cooling towers as the point of carbon capture thereby to keep infrastructure costs low. Proponents claim that the price at the pump for such gas (with allowance for profit) would have to be about $4.60 per gallon, but with certain technological improvements, the price could be reduced to $3.40 per gallon – where we are presently. It appears worth piloting now.

  5. 55
    David B. Benson says:

    Biochar makes a most useful soil amendment. But about half of the carbon returns to the active carbon cycle with a few years. Moreover, the storage time of the remainder is unknown. The linked report summarizes current understandings regarding biochar:

    Applying biochar to improve the soil is a great idea. But I don’t look to this as a long term solution for removing the excess CO2 from the air.

  6. 56
    Jim Galasyn says:

    Re Gene’s comment in 30:

    Rising sea level and warming sea water will result in an increase in organisms that convert CO2 and various other minerals into carbonates.

    This is probably not the case for a high-CO2 world, because the oceans become more acidic; eventually calcareous creatures can’t make calcium carbonate and their numbers drop steeply. This effect has been clearly observed in ocean sediments from the PETM.

    I don’t expect that carbonate-producing organisms, alone, are a solution, but what can we do to encourage this natural carbon sequestration process, instead of squandering the opportunity in a futile attempt to hold back the rising seas?

    I’m rather fond of the BioRock process.

  7. 57
    Jim Galasyn says:

    I’ve often wondering if it would be possible to separate CO2 electrostatically: suck air into a chamber and zap it with a laser that’s tuned to the right frequency to ionize only the CO2. My very primitive reference design is here. Obviously, such a unit should be solar powered.

    There’s also been some very interesting work on smart membranes that allow only CO2 molecules to pass through them.

  8. 58
    Chris Dudley says:


    #50. What pressure are you taking as the final pressure for your estimate of 350 kJ/mol? At 17 atmospheres final pressure I calculate that using a zeolite based system similar to what is used on the space station the energy requirement is about 34 kJ/mole. 17 atmospheres is a good pressure for Fischer-Tropsch. If you see an error here, I’d appreciate hearing about it.



  9. 59
    Kevin Leahy says:

    Some of these perhaps represent new ideas. However, this topic has already received quite a lot of attention for some time.

    Until or unless some breakthroughs occur, it looks as though the most economic (i.e., lowest costs) solution is to capture the CO2 off of an integrated coal gasification unit. The remaining hydrogen is then combusted in a combined cycle electric generating plant. Oxyfueled plants may also work, resulting in an exhaust stream that is almost pure CO2. The electricity can fuel your transport fleet (except aviation, which is used only for long distances). For this, we need better electric storage.

    The CO2 off the IGCC is already at very high pressure, so you have less cost to compress it for the geologic storage. And in regard to the risk — we are talking about 1 to 2 miles underground. The risk is in a slow leak, not a catastrophic release. This is more of a financial issue for those undertaking the work (they avoid a cost under a cap and trade or carbon tax regime — if you leak, you pay). Oil people laugh when people say “if it works” as they refer back to the fact that the proper geology keeps natural gas underground just fine.

    Finally, to remove CO2 from the atmosphere, if it comes to that, the current thinking is that it may be more cost effective to harvest biomass and run it through above gasifier, with the CO2 stripped out and geologically stored. A twist on this theme, some are talking about using algae as the biomass source. Others are considering the economics of growing it in an enriched CO2 environment as found in the flue gas of a conventional power plant, then gasify and store underground.


  10. 60
    Phil. Felton says:

    Re #33
    We should not ignore NO2 levels.

    Indeed we shouldn’t, however what is being referred to is N2O!

  11. 61

    In regard to operating carbon sequestration systems, there is project in the weyburn, saskatchewan oil field, installed around 2000, which takes CO-2 piped from a North Dakota coal gasification plant, and forces the CO-2 into the oil field area, (thereby enabling recovery of more oil). Google: CO-2 sequestration + weyburn saskatchewan. This installation appears to operating successfully, giving us some hope in this method of CO-2 storage.

  12. 62
    Adrian says:

    Hey thanks for the great blog, I love this stuff. I don’t usually do much for Earth Day but with everyone going green these days, I thought I’d try to do my part.

    I am trying to find easy, simple things I can do to help stop global warming (I don’t plan on buying a hybrid). Has anyone seen that is promoting their Earth Day (month) challenge, with the goal to get 1 million people to take their carbon footprint test in April? I took the test, it was easy and only took me about 2 minutes and I am planning on lowering my score with some of their tips.

    I am looking for more easy fun stuff to do. If you know of any other sites worth my time let me know.

  13. 63
    JCH says:

    One of the papers I read says CO2 stored at depth essentially cycles out of the ocean in 900 years. Is that correct? Has anybody considered “containerizing” liquid CO2/dry ice at the bottom of the ocean.

  14. 64
    Rod B says:

    I’m a little behind (aren’t all us skeptics??), but am I alone in thinking TEQS scheme is a pie-in-the-sky pipedream (to double up my metaphors). From a practical view? Instead of starting with some technology base, it seems we just allocate carbon usage to people, reduce those allocations each year, and just let the poor saps figure it out.

  15. 65
    C L says:

    Can anyone tell me whether thorium based nuclear energy, as suggested in this article, is a credible alternative to conventional nuclear power, please ?

  16. 66
    Chris Dudley says:

    # 30. Gene: The present rate of sea level rise is probably good for encouraging reef growth since the reefs like to maintain a certain depth to have adequate sunlight. A rising sea level gives them room for growth. However, current land use practices discourage reef and mollusk growth because silt fouls the water and nitrogen runoff encourages so much algae growth that the decay of that material creates anoxic conditions which destroy aquatic ecosystems. Corals also seem to be sensitive to the increase in water temperature and will eventually run into trouble owing to ocean acidification. The rate of sea level rise we might expect towards the end of the century, a centimeter per year or so, will be too fast for coral growth to maintain depth so there is a balance. Now would be the time to take advantage of a favorable sea level rise rate. To do this we would need to sharply reduce soil erosion and nitrogen runoff. There are a number of agricultural practices that can help with this and one of the most exciting is the way that biochar can hold nitrogen and help to keep it in the soil and out of the water, though conventional organic methods of agriculture are already available to help.

    In terms of issues with water temperature changes reducing reef growth, the establishment of new reef in a manner that anticipates expected changes might make sense. Also, introducing warmer water corals to reefs that are warming may help. To get a lot of sequestration from reefs we would need to increase their surface area in any case. I make some estimates here. Increasing the current surface area of coral by a factor of 15 appears to sequester all of our lingering emissions to date.

  17. 67
    Ike Solem says:

    We’ll never be able to halt the continuing increase in atmospheric CO2 unless we stop using fossil fuels. If we did come up with an energy source sufficient to capture and stabilize all the CO2 emitted by the burning of fossil fuels, we wouldn’t need fossil fuels at all.

    The notion that one can burn coal, capture the CO2 and store the CO2 in a stable form approaches a perpetual motion machine strategy. Estimates vary, but to capture all the carbon coming out of a coal-fired power plant would require a minimum of 30% of the energy generated, likely much higher. When a solar PV system or wind turbine operates, in contrast, no emissions are generated at all.

    The multibillion DOE dollar project which was supposed to demonstrate coal sequestration was dubbed “FutureGen” and has been abandoned, despite all the hoopla. It proved to be incredibly expensive as well as ineffective, and found no investors.

    There are alternatives, however – the conversion of photosynthetic biomass to charcoal and burial in the soil apparently results in storage of about 20% of the original carbon in the biomass, and produced biofuels as well. However, there’s no way to get around the need to halt the use of fossil fuels, starting with coal.

  18. 68
    Pat Neuman says:

    From what we see happening now, sea level rise towards the end of the century will
    be substantially faster than “a centimeter per year or so”, and will come in spurts.

  19. 69
    Daniel Nall says:

    As an energy conservation engineer, my hopes for widespread implementation of this technology are very dim. It is highly capital intensive and its only product is carbon mitigation. A more likely carbon sequestration technology is the pyrolysis of organic and agricultural waste to produce gas that can be burned for electric energy and substantial amounts of carbon char that can be mixed with soil as a component of fertilizer. The process produces somewhat more energy than is required for the heat of pyrolysis, so that additional saleable products, power and powdered carbon are produced in addition to carbon credits.

    I think the best hope for large scale carbon reduction is to convert as much energy-consuming activity as possible to electricity and concentrate on low carbon power production. If a reasonably power dense electric storage medium could be found, this strategy could even be applied to mobile energy users such as vehicles. In this scenario, hydrogen can be viewed as a transport and storage medium to be evaluated according to its efficacy in those functions. As someone has said, ” the only problems with the hydrogen economy are that hydrogen is difficult to transport, difficult to store and we don’t have any of it.”

  20. 70
    DBrown says:

    This is a joke? No person with any knowledge of statistical thermo really thinks that we can remove such a tiny trace gas (CO2) from our vast atmosphere insuch a way that any real measurable impact on the atmosphere would be achieved?
    Lets look at a few minor numbers: Volume of main atmoshere:1,071,821 km3 (for about 20 km thick sphere over Earth surface (really more but lets use a minor number.) Assume we have a truly amazing processing system: 1 km3 total volume (over ‘n’ process plants) that can operate at the rate of 1 km3 per day is completely done; CO2 is 0.038%; so, lets assume we will remove ‘half’), then 535,910 km3 would need to be processed to get this value.
    then in only 8,213 years, the first run would be complete (this fails to take into account that the released air mixes with the current untreated air which would vastly lengthen the required time).
    This idea is so ridiculous that I cannot understand why anyone would discuss this absurdity in this column. This undermines a very good and serious site.

  21. 71
    Jim Galasyn says:

    Re JCH’s question in 63 about carbon sequestration at the ocean floor:

    I saw Tim Flannery speak a couple of years ago, and he mentioned that experiments had been done along these lines, but the effect on marine life was disastrous. Asphyxiating deep-ocean corals and associated ecosystems with billions of tons of liquid CO2 would be a treatment that’s as bad as the disease.

  22. 72

    C L #65: Thorium sounds great compared with uranium. I suspect it was not developed because the precursor industries were already there for weapons production, favouring uranium. Also, the fuel production process is complex. I remain sceptical of anything that looks good as a bunch of equations and pilot studies until it has been worked through to a production-scale plant. Even if all the claims pan out, if will be decades before thorium-based power could make a significant difference. In short, even if it is a good idea, it will not be possible to scale up fast enough to make the sort of difference we need.

    Some good comments on the Cosmos web site where you found the article.

    In the looks too good to be true department, has anyone seen this? — claims to have a new fusion process which doesn’t require radioactive inputs, doesn’t create radioactive outputs, and can be used to generate electricity directly without the usual lossy heat to steam to rotation to electricity steps.

  23. 73

    Re 70

    Though my interest in geoengineering lies elsewhere,the joke may be on DBrown, in reckoning a throughput much higher than a cubic kilometer of air a day “so ridiculous that I cannot understand why anyone would discuss this absurdity in this column.”

    As 1 Km3 weighs roughly 10 exp 9 kg, processing such a volume works out to ~ 150 grams of air per capita, per diem.

    Since we breathe roughly an order of magnitude more than that, processing a far larger mass of air seems within the bounds of energetic and economic possibility.

  24. 74

    65 CL: Yes, Thorium can be “bred” into uranium 233 by putting it into a reactor. This is just like making plutonium from uranium 238. Uranium 233 is fissionable just like U235. Since there is more that twice as much thorium as uranium in the world, breeding thorium into U233 multiplies the available nuclear fuel by several hundred times. Likewise for breeding plutonium from U238 and using plutonium as reactor fuel. Since only 0.7% of uranium is U235, the rest of the uranium is wasted unless breeding and reprocessing are allowed. Thorium as a source of fissionable uranium is indeed an excellent idea. The problem is paranoid people. I don’t know why so many people are irrationally fearful of all things nuclear, but I think that coal company propaganda has something to do with it. By the way, coal contains so much uranium and thorium that more energy by hundreds of times goes up the smokestack and into the cinders than you get by burning coal. Nuclear power is far safer than coal power.

    Could somebody write down the chemical equations for how CO2 becomes stuck in coal and other things, please. Are these processes reversible? How much do we have to worry about asphyxiating because of a CO2 leak if we live near a sequestration site of type X? CO2 from Lake Nyos killed 1,700 people. How much of this is driven by the coal mine lobby? 100%?

    Would you really want to make CO2 back into a hydrocarbon fuel, or would it be easier to convert a car engine to run on ammonia? Hydrazine is out because hydrazine is a monopropellant/explosive. There must be more economical ways to propel cars than making hydrocarbon fuels out of CO2. What about batteries, perhaps with overhead wires to do quick recharges while driving? Airplanes can use hydrogen because large tanks or cryogenic tanks would be less of a problem for aircraft. Trains can use third rail electricity.

    70 DBrown: Thank you. I agree that taking CO2 out of the air by an industrial machine strikes me as preposterous. And making CO2 back into fuel seems equally preposterous. If we had an infinite supply of free energy, we could do it; but it would be done with tax money and therefore cut from the budget before it got into the budget. There must be easier ways to do what needs to be done if we can overcome the fossil fuel lobby in Washington. Coal alone is a $100 Billion industry in the US alone.

    29 Jim Bullis: How much does all the plumbing cost? How do you get people who live near the source of waste heat to go for all of that stuff? It would work if the place that generates the waste heat is also the place that uses it. It also worked in the old Soviet Union where the town was built to tend the breeder reactor. There is less central planning here, and maybe you need central planning to make waste heat useful much of the time. Sometimes it works.

  25. 75
    GlenFergus says:

    OT: Whereto HadCRUT/NCDC/GISS?

    Time may be fast approaching when we need another post from you guys on the various global temperature series. All that sound and fury of the “warming stopped in ’98” crowd could rate as something-nothing if the current blip persists awhile. Monthly HadCRUT is now back to 94-96 levels.

    Presumably the Pacific just burped some cold stuff. ENSO, sure, but might something else be happening? What would an IPO/PDO phase shift do? That one was mostly warm through ’46, cool through ’76, then warm again since – an occasionally discussed correlation. What if we’re building another temperature saw tooth?

    [See recent PDO index chart.]

  26. 76
    Robert says:

    As a chemical engineer, I must agree with previous comments that air capture of CO2 is impractical compared to other carbon capture alternatives. The energy required to move the volumes of air through an absorber and to regenerate the CO2-absorbing solvent would be cost prohibitive. And the capital cost would be mind boggling. I am sorry to say that the amount of misinformation here is very high, and quite naive.

    A better solution is to gasify biomass to synthesis gas (carbon monoxide and hydrogen), shift the CO with water to CO2 and hydrogen, and capture the CO2 while the shifted synthesis gas is at pressure and concentrated. At least you get some energy from the biomass while storing carbon in either saline aquifers or depleted oil fields.

    Of course, one would only capture CO2 if you bought into AGW, which at this point, I do not. Fire away!

  27. 77
    Erich J. Knight says:

    The co-benifits of Biochar must also be considered beyond Bio-fuel gain; 3X fertility,17% better water use, fungi and wee-beastie microbes to worms, are sequestered carbon adding to that of the Biochar which in Terra Preta soils has C13 tested to 7000 years.

    Dr. Lukas reports 10X N2O soil emission reductions:

    Beyond Zero Emissions interviews Dr Lukas Van Zweitan senior research scientist of the NSW Department of Primary Industries (DPI). Who is working hand-on with soil research focusing on Bio Char (Terra Preta de Indio / Agri Char)

    “we’ve found with some of the biochars in that we’ve had very, very significant reductions in nitrous oxide emissions from the soil; between five- and ten-fold reductions in nitrous oxide emissions.”


  28. 78
    Chris Dudley says:

    Pat: Re: #68. Yes. That was an error. I should have said 10 cm per year or so.

    For a coral growth rate of 10 gm/m2/day and an estimated density of 1.9 gm/cm3 we get an estimated growth rate of 2 millimeters/year, just a little slower than the current rate of sea level rise. On the other hand, the Great Barrier Reef apparently kept pace with an averege rate of sea level rise of about 8 millimeter/year between 13000 and 6000 years ago and individual corals can grow vertically 10 cm/year.
    So, perhaps the growth rate estimate I’m using is one that is limited by the present rate of sea level rise rather than what is chemically possible. Our thinking about the rate of ice sheet disintegration has been similarly limited so this is a possibility.

    In any case, protecting reef seems like good climate policy and most aspects of protecting reef are also good land use policy and fisheries management policy.

  29. 79
    Jim Galasyn says:

    Re 70:

    [C]an [we] remove such a tiny trace gas (CO2) from our vast atmosphere in such a way that any real measurable impact on the atmosphere would be achieved?

    For what it’s worth, Lackner thinks so:

    A device with an opening of one square meter can extract about 10 tons of carbon dioxide from the atmosphere each year. If a single device were to measure 10 meters by 10 meters it could extract 1,000 tons each year. On this scale, one million devices would be required to remove one billion tons of carbon dioxide from the atmosphere.

  30. 80
    Beth S says:

    Air Capture of CO2….ummm… that’s what trees do if I’m not mistaken, take CO2 + light + H2O = cellulose. You can just cut down the tree and drop it in a nice oxygen-poor swamp or deep bay where it won’t rot, and voila! CO2 captured. THat’s how the coal and oil got there in the first place. So… why go to all this bother of this “air capture” thing when it’s already being done all over the place? Is there something I’m missing here?

  31. 81
    pwchase says:

    “emissions from small dispersed sources” Seems as if that is similar to small dispersed units that need carbon dioxide for food. Trees, grass, cereal grains…..

  32. 82
    Martin Vermeer says:

    Re #70 DBrown: ridiculous, eh? But plants (the green variety) are doing it all the time. At a capacity dwarfing your calculation.
    Challenging? Yes. But “impossible” is a term to be used very judiciously. You remind me of the scientist AD 1900 writing that “objects heavier than air will never fly” while a bird was flying past his window :-)

  33. 83
    Ray Ladbury says:

    #53 G.R.L. Cowan, I’m sorry. I should have been more specific. When I said that emissions from diffuse sources precluded a centralized efficient plant, I was speaking in comparison to point sources. Concentrating CO2 from a concentration of 380 ppmv in air to anything like the concentrations needed in solution to make this effective will not be an efficient process. I do not dismiss the technique out of hand, just because the problem of non-point source emissions is so difficult to solve, that we may well be desperate enough to buy time that we’ll try and make this work. However, I think you’ll agree that this does not constitute “low-hanging fruit”.

  34. 84
    JSL says:

    Great job Frank! The most concise overview of air capture yet.

  35. 85

    Re #70 Where DBrown writes “No person with any knowledge of statistical thermo really thinks that we can remove such a tiny trace gas (CO2) from our vast atmosphere in such a way that any real measurable impact on the atmosphere would be achieved?”

    I think that is true if you are talking about engineering, but Nature is pretty efficient at absorbing the small concentrations of CO2 using photosynthesis. If we grew more plants and stored the carbon rich material they produce we could reduce the atmospheric CO2 that way.

    The problem is that we have to store the produce in an anoxic environment to prevent it reverting to carbon dioxide. This could be achieved by sinking it in water or mud. We could even start now by burying cardboard, other packaging materials and wood rather than burning them wastefully on rubbish dumps.

    If there is a problem with finding burial places, we could store them above ground in deserts, or re-grow trees in temperate regions. Trees have the advantage that they store the carbon, and that they absorb carbon over a much greater height than grasses or other crops.

  36. 86

    Great news — just got this today from Environmental Action:

    I’m really excited to announce that, starting today, Environmental Action and ExxonMobil have teamed up as part of a new direction for our work. Thanks to a generous gift from ExxonMobil, we’ve come to realize that global warming isn’t the worst thing in the world, and in fact there are a lot of upsides to climate change.

    For starters, no more winter coats! And, if you invested in property out in Utah, you’ll soon own prime beach-front real estate. And that’s just the start. But, we’re not doing it alone – we want to hear from you. Should we put pressure on the auto industry to make less efficient cars so we can speed up the process? There are a lot of ideas out there, and we want to hear yours. Just click the link below to tell us what you think of this exciting new development.

    I know I’ve slagged on ExxonMobil before, but that was before they made their very generous gift. And the upshot of that is, I’ll never have to ask you for money again! Thanks to their support, we can focus more of our energy on the important things – things like helping make oil drilling safer by getting rid of those pesky fines and regulations about oil spills.

    We’re particularly excited about our new efforts to help open up the Arctic National Wildlife Refuge for oil and gas development, and implement the ‘polar bear resettlement plan’ that will get polar bears out of the way of the rigs and wells.

    But we want to know what you think? What else can we do for the environment with our new partnership. Let us know!

    And thanks ExxonMobil, for making this all possible.


    Dan Stafford
    Environmental Action Organizer

    P.S. Thanks again for your support. Please feel free to share this e-mail with your family and friends.

    Then, of course, you go to the link, and see: HAPPY APRIL FOOLS DAY!

  37. 87
    Alex Tolley says:

    I am flummoxed as to why a crude technological solution is being proposed when we have a biological solution – plants. Far better to encourage the growth of fast growing plants and sequester their biomass. The solution is cheap, self reproducing and looks quite nice too. Building large facilities that mimic SciFi terraforming ideas is not likely to be an economic approach. If you want to spend money of research, spend it on biotechnological ways of improving photosynthetic efficiency.

  38. 88
    Phil Scadden says:

    #30. Round here, we naively discussed an idea that increased shellbed production could be used as a carbon offset but people on this site kindly showed me the error in this.

    When you precipitate CaCO3, then yes, you bind a CO2 molecule, but you alter seawater chemistry so it hold less CO2 (pH buffering) and so CO2
    is also released.

    The rate that reef/shell precipitation truly binds CO2 is dependent on a Ca flux from weathering into the ocean. If you could find a way to speed that process then yes.

  39. 89
    Eli Rabett says:

    #65, for many years the Canadians operated thorium reactors (CANDU). It was great if you like hydrogen sulfide (needed to separate D2O from water, so yes, it can be done.

  40. 90
    Lawrence Brown says:

    The President has announced that he’s going to refocus US energy policy on conservation, efficiency and renewables, embrace the principles of Kyoto and place a cap on carbon emissions.
    Also Happy April Fool’s Day!

  41. 91
    Paul Dietz says:

    The bottom line is that there is a dilution factor beyond which a point of economic “no-return” is surpassed for which it doesn’t make sense (e.g., low-grade uranium ore, uranium in sea water, low-grade iron ore, or in this case, CO2 in air), to expend energy to recover it. This is without even mentioning the embodied energy in the equipment used to carry out the process.

    The thermodynamic minimum energy required for the extraction goes only as the log of the dilution, so your claim is rather dubious. Indeed, for uranium extraction from seawater on adsorbing polymers the energy needed to liberate the U from the saturated polymers is only a small fraction of the energy embodied in the U itself (even just counting the fission energy of the 235U, not the 238U).

    This is a joke? No person with any knowledge of statistical thermo really thinks that we can remove such a tiny trace gas (CO2) from our vast atmosphere insuch a way that any real measurable impact on the atmosphere would be achieved?

    It may not be economically feasible, but vague and incorrect appeals to the laws of thermodynamics do nothing but demonstrate you haven’t though enough about the issue. Indeed, simply observing that nature itself will exothermically react the CO2 with minerals over long enough periods of time shows that thermodynamics alone is not the showstopper.

    BTW, the volumetric concentration of inorganic carbon in seawater is greater than that in air, so it may make sense to remove the inorganic carbon from there instead (allowing the oceans to continue to draw down atmospheric CO2 without saturating or getting too acidic.)

  42. 92

    re My comment on 70:

    should read ” 150 kilograms per capita- processing more than twice our weight in air each day seems within civilization’s economic bounds “

  43. 93
    Jim Galasyn says:

    RE the question CL’s about thorium in 65, I don’t know, but there’s a new subthread at The Oil Drum, with the usual spirited debate.

  44. 94
    Kiashu says:

    Martin at comment #40, liquid CO2‘s density depends on pressure and temperature, but is usually much less than the solid form. In my informal article on it, I note that already to get oil out people pressure the CO2 into a supercritical fluid; this has roughly 1/234th the volume of the gas at standard temperature and pressure.

    A further point is that this is not a trivial or zero-energy process; if you captured the CO2 from a coal-fired power station, it’d take fully one-third the power it produced to be able to liquefy the CO2. So if all our coal-fired plants had CCS, we’d have to either reduce our power consumption from them by 33%, or else have 50% more generation capacity.

    If we’re able to just reduce our power consumption by a third, then surely we should go ahead and do that? We’d then reduce our emissions by a third, too. That then buys us some time to actually make CCS work, to put in renewables and so on. Of course, if we decide that reducing consumption is impossible, then we must build more power plants to make electricity to be able to clean up the mess from the existing power plants… Seems like a bit of a merry-go-round.

    The numbers become much better for natural gas and the like. But coal-fired is the most common type of fossil fuel burning power station in the world, and of coal, oil and natural gas, we expect coal to peak last, so it seems fair to take the coal generation as the baseline.

    It’s a crock. Burn less stuff.

  45. 95
    Thomas says:

    Go back to comment #53. Basically what is needed is to simply crush a bunch of rocks of the right kind, and spread then out to a depth shallow enough that the CO2 is absorbed within a few decades. The problem is that the quantity would need to be similar to the volume of oil we use per year (3-4 km**3), I don’t know how feasible this is, clearly it would mean disturbance of many square KM**2 per year of desert. The main energy cost are the explosives for breaking up the rock, and to push it around. Chemistry, and time do the rest for free.

    Of course as in the exercise to design a solar powered home, the most cost effective thing to do is to first pursue a high degree of energy efficiency. In our case that can be supplemented by low carbon energy (nuclear and/or renewables).

    I suspect we have a modest amount of cheap CO2 absorption capacity, such as biochar, and possibly ocean fertilization, but this low hanging fruit is unlikely to be of sufficient volume to constitute more than a modest wedge or two of the needed amount.

    A note on burying CO2 for enhanced oil recovery. That Canadian experiment claims to get 6barrels of oil per ton of CO2. How much CO2 does 6barrels (approx 250gallons) of oil contribute when it burns. Oil field burial for EOR is really a way to reuse the CO2 in the form of fuel.

  46. 96
    Harold Ford says:

    I do feel like a broken record: I’m sure man can mass collect CO2, however plants are much more efficient at the capture. In fact, if a plant grows it is getting its extra mass from CO2. Plant growth equals capture of CO2, simple. Find a weed or grass that does not deplete the soil and that grows quickly, trim it often (I’ve also thought of using trees, trees are good for up to 30 years, after which they should be cut down and replanted). Place the trimmings in a nearby container to store the carbon. It is cost effective, you don’t have to move it very far, it takes a little work and the carbon is contained until the trimmings rot away to nothing. The problem with carbon is it oxidates creating natural gas, so burying it is a simple solution, or use it as fertilizer. The idea of making a process that is more costly smacks of big business: you may make a large sum of money, but can you eat it? I suppose you could burn for heat.

  47. 97
    Nick Gotts says:

    RE #74 [Edward Greisch] “Thorium can be “bred” into uranium 233 by putting it into a reactor. This is just like making plutonium from uranium 238. Uranium 233 is fissionable just like U235. Since there is more that twice as much thorium as uranium in the world, breeding thorium into U233 multiplies the available nuclear fuel by several hundred times. Likewise for breeding plutonium from U238 and using plutonium as reactor fuel. Since only 0.7% of uranium is U235, the rest of the uranium is wasted unless breeding and reprocessing are allowed. Thorium as a source of fissionable uranium is indeed an excellent idea. The problem is paranoid people.”

    No, the problem is the close connection between nuclear power and nuclear weapons. Plutonium and U233 are both excellent nuclear weapons material – or if you don’t have the technology for that, just add a little to a conventional bomb and let it off in a city centre. Not many deaths, but one hell of a clean-up bill. Yes, if none of the alternatives work out, we’ll have to go to nuclear energy on a large scale. But pretending there are no serious problems with it is just foolishness. Sorry to be a bore about this, but as long as Edward Greisch and his ilk keep putting out this “paranoid people” line, without tackling the weapons connection, I’ll keep responding.

  48. 98
    Ray Ladbury says:

    For all those who say this is impossible as well as all those contending that it is easy–you really should look into this matter before commenting. There are no physical laws being violated. In fact, what Frank is suggesting is just accelerating the geologic mechanisms that take CO2 out of the air. This is a more permanent solution than, say, biochar and other related schemes, since it can be difficult to store the carbon in a way that is stable. Coal beds are about the most efficient storage solution, but we seem bent on digging those up and releasing the carbon.
    At the same time this is a very difficult problem–getting any acceleration requires concentrating the CO2 in some way, and that’s bound to cost energy. Even so, the problem of diffuse sources of pollution–any pollution of water, air, land…–is a very difficult one. I do not think we can dismiss this solution out of hand. As Frank implies, we need to go after the low-hanging fruit first–conservation, renewable energy, etc. I do not think that this will be enough to get us out of the soup, and we will have to then look at more difficult options. It is certainly not too early to start thinking about these. If you haven’t sat down and done the math, don’t jump to the conclusion that you’ve solved the problem.

  49. 99
    C L says:

    Thankyou for several helpful comments re thorium reactors.
    I notice some folks put faith in planting trees to take up CO2. Nice idea, eh. They are often so very beautiful, contribute to the hydrological cycle, provide habitat, fruits, nuts, timber,etc. But as a solution ? We can’t even look after the forests we already have, nor reconstruct the complex oldgrowth ecosystems that are the most efficient at utilising particular areas. You plant your new forest. The climate changes. The trees will die…,8816,1725975,00.html

  50. 100
    Paul Dietz says:


    A further point is that this is not a trivial or zero-energy process; if you captured the CO2 from a coal-fired power station, it’d take fully one-third the power it produced to be able to liquefy the CO2.

    You are being misleading. This is the total energy cost using current off the shelf CO2 flue gas separation technology. This includes the stripping the CO2 from the flue gas, not merely compressing and liquefying the separated CO2. Other technologies (such as IGCC, or ammonia-based flue gas CO2 separation) promise considerably lower energy overhead.