Guest Commentary by Frank Zeman
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.
Well, there’s a lot, a whole lot, online about the idea.
Let’s see if this will be allowed by the paranoid spam filter:
http://www.netl.doe.gov/technologies/carbon_seq/refshelf/presentations/mineralseq.PDF
Status of Research Effort
Preliminary evaluation of reaction paths:
−Aqueous HCL process employing mineral-derived Mg(OH)2+ H2O (found expensive)
−MgCl2 Molten Salt process (found unreactive)
−Direct Carbonation using supercritical CO2 and water at elevated pressures and temperatures
(found promising)
———- end excerpt———–
That’s from 8 years ago.
#191 Dylan – you’re right that CO2 levels would likely be elevated in a tunnel and if you have ventilation shafts then you have a nice access point. The advantage of air capture is you can site it where you want, which avoids trying to get pipelines through urban areas etc. The slight elevation in CO2 would only help if the CO2 transport was not significantly harder.
#193 Andy – absolutely, it’s much easier to not create the problem then have to solve it after the fact. If you look at Pacala and Socolow’s wedges, you’ll notice efficiency as a big player.
as for mineral sequestration. There is work at columbia and my understanding is all of it occurs at high CO2 pressures (20 atm) so any atmospheric usage, like spreading in the desert, might have kinetic barriers. On the other hand, if it does the job in 100 years that’s fine as long as it’s not covered by blowing sand.
Frank Zeman said,
And what would happen, Frank, if olivine grains were covered by blowing sand? No doubt you did not miss, nor find fault with, my above figuring that the slowest-acting olivine variant would, in 100-micron-screened grains, be consumed in four years.
Are you figuring that 30 cm of overlying sand would add 96 years of diffusion time?
Ok, so maybe I’m not the brightest bulb here but there has to a product to this: CaMgSiO4 + CO2. I’m only guessing at it but I hope the product would be Dolomite CaMg(CO3)2 and sand. There are several other products it could be and hopefully it wouldn’t be Calcite or Magnesite. If the process only produces dolomite and sand then I’m a fan of this idea otherwise what exactly is the end product?
More here, discussion with links:
http://lablemminglounge.blogspot.com/2007/06/carbon-sequestration-in-mine-tailings.html
Watch out for the asbestos fibers, better not put anything containing that in fine particles where the wind blows. Mine tailings have a lot of issues, but clearly people are working on this notion eagerly. Any opportunity to turn a hazardous waste problem into a carbon dioxide sink is going to be attractive.
To GRL Gowan:
My comment was more general that your detailed calculations in post 151. The 100 years referred to a generic time scale to solve the CO2 problem as the technology of spreading magnesium silicates in the desert can be termed a passive technology. The 4 years you refer to is called “weathering”. Does this refer to size reduction or dissolution? In your early quote regarding mine tailings you mention that the reaction is aqueous, which would not happen in a desert. As for the 30 cm, I’m not sure where you got that number but yes the gist of the comment was that if the magnesium rock is buried then it will react more slowly. My only trip to the Morrocan desert included tales of 30ft high dunes moving around. I’m sure there are other deserts that are more flat (Altacama). It was a more general comment. In what I’ve seen of mineral sequestration, the initial effort is usually to get the magnesium ion into solution as gas solid reactions are much slower. Again, everything I’ve seen has involved CO2 pressures of 20 atm not 400 ppm. Doesn’t mean it won’t work though. Just thoughts.
peace
Frank
Found this pdf article dated 2001 which covers this topic that I found meaningful:
http://eny.hut.fi/library/publications/tkk-eny/TKK-ENY-3_print.pdf
The article mentions that olivine reacts slowly with CO2 in an exothermic reaction. It also mentions that a few things were done to speed up the reaction at Los Alamos so that scrubbing of a coal plant’s CO2 could occur. Turns out that what they did increased efficiency by 25% from the norm (whatever the norm is). Seems like simply increasing the amount and surface area of Olivine would do the trick for scaling the amount of CO2 reacted per unit time. Second trick would be to find enough Olivine, someone mentioned Washington State. The final trick would be sequestering the Magnesium Carbonate, as leaving it out to be weathered means that it could release the CO2 through contact with acid, getting blown into the ocean wouldn’t help.
Harold Ford said,
But neither would it hurt. Magnesite is stable. The reaction of atmospheric CO2 and olivine — whose occurrence you can find discussed under the name “dunite” — goes only one way.
So we haven’t found a perpetual motion mechanism, but at least we can deal with CO2.
Talking of ‘Air Capture’, wouldn’t it be economical and chemically efficient to capture concentrated CO2 from stacks by having the stack gases pass through a disk-shaped sponge or other porous matrix impregnated with calcium oxide/iron oxide and a catalyst? The resulting CaCo3/iron carbonate ‘disk-shaped bricks’ could be easily disposed off, or if rigid enough, used in selected constructions. (Can we count on PM2.5 to catalyze the reaction?!!!)
> , but at least we can deal with CO2.
But nobody’s interested in doing this. Is it because of the asbestos problem, do you think?
Here’s a process that adds CO2 to produce soluble magnesium bicarbonate:
http://dx.doi.org/10.1016/j.hydromet.2008.01.011
Leading question. No asbestos problem has been shown to exist with olivine. There may be one with serpentine, if asbestos-free deposits of it are for some reason hard to find.
Appeal to authority, and falsehood. I’m interested and so is the professor-doctor I mentioned. Prof.dr. Olaf Schuiling.
It might be rather boring. What if no further research were nee–
were nee–
What if further research were not, not, ah, … well, you probably know what I’m getting at. You try and say it.
I’m just asking why I can’t find anything new about the idea, it seems to have surfaced some years back and been evaluated and dropped. Where are you seeing interest?
The observed rate of increase of atmospheric CO2 indicates that natural sinks of CO2 cannot absorb more than about 20 billion tonnes of human-created CO2 annually. The world’s current rate of use of fossil fuels creates some 30 billion tonnes of CO2 annually, or an excess of roughly ten billion tonnes of CO2 annually, equivalent to 5000 cubic kilometers of gas (1200 cubic miles) of gas or 80 cubic kilometers (20 cubic miles) of liquid at 60 bar (nearly 60 atmospheres) pressure. Assuming no change in the use of fossil fuels, halting global warming would require capturing, handling and storing that much CO2 every year, forever. The sheer scale of this effort looks infeasible, and it may be physically impossible.
Assuming the low end of the costs anticipated by the Department of Energy for its sequestration demonstration projects ($40 to $100 per tonne of CO2 sequestered), attempting this apparent impossibility would cost four hundred billion dollars per year. But notice that this is equivalent to nearly $150 per tonne of carbon sequestered. A fossil carbon tax of $50 per tonne with the proceeds invested in non-fossil energy sources would be far less expensive and more easily implemented, and would start reducing CO2 emissions more quickly than could any feasible sequestration effort.
Note that none of the above invalidates the feasibility of the work by LANL on capturing atmospheric CO2 to manufacture artificial carbon-neutral liquid hydrocarbons for vehicle fuels:
http://www.lanl.gov/news/index.php/fuseaction/home.story/story_id/12554
The goal of that work is to eliminate the need for petroleum and coal-to-liquids (CTL) fuels, both of which contribute to global warming.
Hank Roberts said,
Two ideas have been mentioned: reacting mineral silicates in some kind of pot, which as you say would be expensive, and reacting them outdoors.
Have you seen any dropping of the latter idea?
Richard Schumacher said,
I don’t know if Schumacher will resist the idea that it’s a lot easier if the capture occurs at widely dispersed dustmotes’ surfaces, and nothing need subsequently be done with the captured CO2.
But a lot of people in this thread seem to wish to overstate the difficulties. At least the thermodynamic bluffers seem to have quit. For the moment. Maybe they slid away on a conjugated sub-entropic gradient.
Nope, I’ve seen no recent mention of either idea except yours. That’s why I was asking where you’re seeing it.
#209 – That has been considered and is termed capture with solid sorbents. If you want more information look up anything by a spanish researcher called Juan Carlos Abanades. You are right to mention capture from power plants and other large stationary sources that will be likely easier and cheaper than Air Capture. The standard technology is wet scrubbing using a chemical called MEA. MEA is considered better than lime (calcium oxide) because it consumes 40% less energy (140 kJ/mole CO2 for MEA versus 200 kJ/mole for lime) and needs energy at much lower temperatures (200oC for MEA versus 900oC for lime). It’s important to remember that you can’t dispose of the product (CaCO3) because you had to make it in the first place. Lime is made from limestone so if you dispose of it as limestone again then you’ve lost something because “you can’t get something for nothing”.
#213 The natural capacity you are referring to is the ocean. There is a price for letting the ocean absorb all our CO2 and that is acidification. That’s out of my area but I have not heard good things about it. The oceans are already being challenged so it’d may be nice to reduce the CO2 they have to absorb. As for the amount of material, yes its big and really we should be aiming at getting maybe 10 billion tonnes of CO2 using CCS with as big a portion from efficiency and renewables. See wedges.
#214 Again, I’ve not said that reaction between atmospheric CO2 and magnesium silicates is not possible but you have to provide evidence that the time scale, naturally geologic, can be accelerated to less than 100 years. I’ve not seen any experiments done with atmospheric CO2 pressure. Thermodynamics tells you where it will eventually end up but nothing about how fast it gets there.
Atmospheric CO2 pressure and olivine are both ubiquitous, so nature can hardly have refrained from doing the experiments. Earlier when I searched, IIRC, on (olivine “weathering rate”) I found that the rate has been measured. Hmm, I see I gave that Velbel link before, but with a comma that made it not work.
Zeman asked earlier, “The 4 years you refer to is called “weathering”. Does this refer to size reduction or dissolution?”
Velbel’s Figure 1 gives it in terms of moles per square-centimetre-second, which can be converted, as I did in comment 151, into a recession rate; this does not refer to, and is independent of, size reduction, aka comminution.
Comminution is a job, not for the weather, but for us. It costs about five percent as much energy as was earlier yielded when the CO2 we’re after was released, if a coal fire in a power plant released it.
For magnesium orthosilicate, the slowest-weathering variant of olivine, Figure 1 gives 7.9e-13 moles per square-centimetre-second. Comminution will allow a mole to be eaten in significantly less than a century, i.e., to have a weathering rate significantly above 3.2e-10 moles per second, through having significantly more than (3.2e-10/7.9e-13), significantly more than 400, square centimetres of surface.
I guess I’m still missing something. Does the rate of weathering refer to the release of Mg2SiO4 into the environment or does it refer to the conversion of Mg2SiO4 to 2MgO and SiO2? The latter step is the reason acids and other chemical weathering techniques are used in mineral sequestration research.
I agree that mother nature is doing the atmospheric experiments for us. The lifetime of CO2 in the atmosphere is 200 years which suggests that that is the relevant kinetic time frame for natural processes. The objective of mineral sequestration is to reduce this time frame by two orders of magnitude. So if you grind it up (agree that grinding is about 5%) and spread it (another 5%?) then what happens? If the next reaction is Mg2SiO4 + 2CO2 > 2MgCO3 + SiO2 then my hunch it’s still too slow.
Using the rate of 8e-13 mol/cm2/s, which can be converted to 1e-5 tonne/m2/yr, we can calculate that about 0.1 km2 are needed to absorb 1 tonne of CO2 in a year. So in order to handle 7% of the global problem (1 wedge) we need to cover 200 million km2 of earth considering a monolayer of material. I assume that a monolayer is not necessary but don’t have any numbers for the allowable depth. Given that the land area of the US is 9 million km2 that’s a lot of area.
200 million square kilometres is more land than there is. If depositing the monolayer were possible, 200 million km^2 of sea surface might be a good place for it, although it would break into particles, and if these settled into the depths too quickly, they would decarbonate places where our CO2 hasn’t yet reached, and not the places where it has.
Rather than a monolayer, what I had in mind was more along the lines of a crusher that, per mole of targeted CO2, uses 20 primary kJ to crush rock and another 20 primary kJ to power a conveyer that throws the powder vertically upward with an initial kinetic energy of 7 kJ.
This is enough, if it’s a broad stream that at first feels little air resistance, to lift it ~10 km. The proper stream would be narrow enough that the air it was flying through would have picked it apart by about the same time it would be coming to a halt, and it could then ride a trade wind while it settled.
Enough initial kinetic energy to climb 10 km implies initially supersonic speed. Probably it’s better to try for only 5 km, then, and spend 10 primary kJ.
Does anyone know, or can anyone find, a Bond work index for dunite or olivine? I could not, so am continuing to use 25 kWh/tonne, which is probably high.
The Bond work index is the work of comminution from large pieces down to 100-micron ones. The latter sink about 1 m/s in air.
Doubling the energy invested in comminution from 20 kJ to 40 kJ — per mole of targeted CO2 — reduces the particle size fourfold, down to 25 microns, and reduces their sink rate 16-fold. This is probably worth doing, even though it raises the invested energy to 50 kJ. (It also reduces the particles’ lifetimes with respect to reacting with atmospheric CO2 from four years down to one year.)
The mentioned 200 million km^2 of surface area is that 10^23 spherical 25-micron particles, volume 833 million m^3.
Can these be strewn widely enough to mix with ten times their volume of naturally blowing dust, and still make a fairly thin layer? 9 km^3 of mixture, if it is to be a 1-cm layer, will cover 0.9 million km^2.
CO2 has to percolate the 1-cm layer, but I guess a year is plenty of time for it to do that, and therefore I believe this is a better arrangement than the 200-million-km^2 monolayer.
That’s a neat way to spread it. I think the grinding number is right, the cement industry is a bit lower at 16 kWh per tonne. I guess it should be easy to try that and measure the CO2 uptake.
Olivine to remove CO2 from the atmosphere.
I have probably been the first (1986)to use olivine to neutralize waste acids. Olivine is the most common silicate in the Earth, and it is available in large massifs in many countries, where it can easily be mined in open pit mines. Rate of weathering depends mainly on grain size, temperature and pH. Weathering of basic silicate rocks (olivine is the most proiminent basic silicate) removes between 2 and 2.5 Gt of CO2 each year from the atmosphere. That makes weathering by far the most important mechanism for capturing CO2, with burial of organic carbon a distant second. In order to reach a new balance, we must speed up the weathering of olivine by a factor of 10. This can be done by opening new olivine mines in tropical countries, where weathering is fastest. There the olivine rock must be ground and spread in the wider surroundings of these mines. These new olivine mines will be larger than existing olivine mines, so the economy of scale will drive the price down. Transportation cost will be low, because the material will not be transported more than a few hundred kilometer from each mine. Calculations show that the price will be around 10 to 15 US$ per ton of captured CO2, say 5 to 10 times cheaper than other proposals like carbon capture and storage. Moreover, this approach will bring new employment to developing countries.
The reaction is often misunderstood. The products are not solids, but dissolved Mg- and bicarbonate ions, that will find their way by passing through groundwaters and rivers. They will help the oceans to counteract the ongoing acidification. We are developing now a simple technology to keep fine-grained olivine floating on the surface of the sea. A large field test with olivine has started this week in the Netherlands. The theory behind the enhanced weathering concept can be found in:Schuiling, R.D.and Krijgsman (2006) Enhanced weathering; an effective and cheap tool to sequester CO2 . Climatic Change, 74, nrs 1-3, p.349-354. A proposal along these lines has been submitted to Virgin Earth Challenge, for the best idea to remove 1 bliion tons of CO2 from the atmosphere.
This is on the broader topic of dealing with GHG emissions from “small, dispersed sources” — here’s a source being warned of. I don’t know anything about the website, it says it’s an independent journalism source.
Question (not addressed in the article I found) — are refrigerated shipping containers still being made using new CFCs, being made new in the countries that have longterm exemptions? Or are those complained of only old contaniners still in use from before the phaseout?
Story found here:
http://www.energymeetsclimate.com/article_detail.php?html_hdnarticleid=300
Excerpt follows:
Published Date : 09-20-07
Reefers: Not Smokin’ but Leaking GHGs
Old Refrigerator Shipping Containers Called Major Source of Greenhouse Gases
“… a loophole. According to the California Air Resources Board, that loophole allows shippers to discard a hundred thousand worn out reefers [refrigerated ocean shipping containers, the huge metal boxes you see on ships] a year in Wilmington and other communities under the guise of long-term storage. By so doing, they are evading requirements to properly recycle or dispose of the harmful chemicals.
…
“…[t]he California Air Resources Board [has been asked by citizens’ groups] to require shippers to drain the chemicals … [CARB plans] to look at the problem in the context of a revised list of early action measures to control greenhouse gases under California’s climate protection law, AB 32. In its revised list, the board is planning to study the potential problem….”
—–end excerpt—–
This kind of GHG isn’t susceptible to “air capture” at all, and is a serious forcing.
I’m glad Dr. Schuiling has made an appearance. A photograph that pleasantly shows a dunite massif is here: alpinenz.com/images/tn/Richards-tn.jpg.
It is said to show the massif’s “east contact”, by which I gather it means where it contacts non-olivine terrain. The boundary can be very plainly seen: olivine is ferric orange, non-olivine is plant green. Olivine massifs tend to be low in Ca, Na, K, P, and B, and therefore, as someone, probably Dr. B.M. Gunn, says here,
Sorry to be picky, but the link to the 1999 Velbel paper in AJS given in comment 151 no longer works – try:
http://earth.geology.yale.edu/~ajs/1999/07-09.1999.07Velbel.pdf
Re #221
If the process adds HCO3⁻ to the ocean it’s the same as adding CO2 so the ocean acidification will be not be reduced and ultimately might not reduce the atmospheric [CO2] either.
Phil. Felton said,
Surely it’s closer to adding CO2 plus equimolar hydroxide ion than it is to just adding CO2.
Re #226
No, if you analyse the competing chemical equilibria you get the following:
pCO2=K2 [HCO3⁻]^2/(K0 K1 [CO3⁻⁻])
Therefore adding an additional 1% of bicarbonate ion is the same as adding 2% of CO2 to the atmosphere.
Phil. Felton said,
The fractional energy investments above figured have been based on MgO in olivine going only to MgCO3.
From Felton’s remark I gather that a mole of MgCO3 can’t be completely effective at pulling down another mole of CO2 this way: MgCO3 + H2O + CO2 —> 2HCO3- + Mg++.
Is the below a fair summation of what must happen, then?
MgCO3 + x CO2 + (1-x)H3O+
—ocean—>
(1+x) HCO3- + (Mg++) + (1-2x) H2O
In words, that extra mole of CO2’s removal would be a bonus, and perhaps we don’t get it, but to the extent we don’t get it, we get a reduction in ocean acidity instead. Do you agree?
An interesting commercial it claims that a process exists that can combine CO2 with Calcium Silicate + H2O + 15min and get limestone:
http://www.businessgreen.com/business-green/analysis/2208306/capture-carbon-add-calcium
Ty GRL (#208 Magnesite is stable) just had to verify it. I am remembering an article of some experiment concerning the running of (CO2 free) water over said Magnesite + Heat causing it to change into Brucite:
http://www.minsocam.org/ammin/AM47/AM47_1456.pdf
Unlikely as that may seem to pose a problem as Magnesite is part of the oceans’ long term carbon cycle, could it pose a problem on some ecological order, assuming large quantities of Magnesite exposed to air randomly assaulted by weathering over large quantities of time? I pose this only because man has gone through countless “Ah hah!” moments only to find more problems caused by the solutions implemented thus causing a confused (standard?) “Ah hah?” response.
Harold Ford said,
The paper tells how much heat:
I gather from Felton’s silence that he does agree, but does not wish to seem agreeable.
This idea has been around awhile and knowing theoretically what would happen if we did use olivine would help decide whether or not to use it in this way. Could a computer simulation be set up or an experiment be done? As I recall there is a large building somewhere in the western US deserts that was supposed to simulate a livable Earth like environment, a failed experiment that was a stepping stone to live on Mars (vegetation thrived but animals died, seems there is a micro-organism in the the desert that devours oxygen). The last I heard of it it was being used to conduct weather experiments. It’s name is… I forget but it does exist and it does not sound like it would be too much trouble to convince someone to use it.
Could an experiment be done? According to comment 221,
Harold, you’re thinking of Biosphere II. Not likely usable.
Re #228
“In words, that extra mole of CO2’s removal would be a bonus, and perhaps we don’t get it, but to the extent we don’t get it, we get a reduction in ocean acidity instead. Do you agree?”
No the relationship between CO2 and [HCO3⁻] is given by the equation I gave above, it doesn’t matter how the CO2 or HCO3⁻ got there:
pCO2=K2 [HCO3⁻]^2/(K0 K1 [CO3⁻⁻])
“I gather from Felton’s silence that he does agree, but does not wish to seem agreeable.”
A very dangerous assumption, in this case the silence was due to travelling and not seeing the post until today!
From #221 (ty GRL) Dr. Dirk Schuiling, it is a process of improving the ability of weathering to remove CO2 but olivine does not react with the gaseous form of CO2 w/o H20 *hits head*.
(from #221)
A proposal along these lines has been submitted to Virgin Earth Challenge, for the best idea to remove 1 bliion tons of CO2 from the atmosphere.
#
Ok, my hair brained scheming has come up with a plan for that challenge and that has to do with plants (of course). But not just any plants, plants that can absorb CO2 directly from the air, just add water. Rather than waiting for rain, these plants would have a water supply close by and continuously devour CO2. What plants? not sure, it would seem to me that a shallow rooted plant that does not deplete the soil of mineral content would be the choice. A plant that can be used to make clothing, sail boat sails, rope, medicene, building material and/or food would be good. However we cannot save the world with an illegal substance, that would be wrong.
Ok, jokes aside. Let’s say we have a plot of land that is 100 m^2. On this land we put hypothetical plants that gain weight at a rate of 4 grams a day per plant and that it takes 0.1 m^2 to support one plant therefore 1000 plants. Baring the explanation of upkeep and harvesting, the total that these plants absorb is 4kg per day 6CO2+6H2O = C6H12O6+6O2 for photosynthesis means that (264/180*4kg) 5.867kg of CO2 was absorbed from the air. Now the challenge is to use the same amount of land using olivine (or other mineral) to estimate the amount of CO2 absorbed through weathering. Using Washington States annual rainfall gestimate 900ml over an area of 100m^2 gives a 90m^3 amount of rainfall for Seattle comes to a weight of (1000kg/m^3) 90000kg of water. Not sure but lets use 2 parts CO2 for every 1000 parts water or 180kg of CO2 possible for one year… 5.867kg/day * 365 days = 2141kg CO2/year for the hypothetical plant while 180kg for the hypothetical 2000 ppm CO2 to H2O rainfall over olivine. It would seem the plants win at 2000 ppm and would be equal at 23789 ppm?
Thanks Hank (#233) for the Biosphere II info.
Now, you would like to sequester the CO2 after combustion. While you a right in saying that we know “how” to do this, it is somewhat energy intensive because an absorbent or adsorbent is generally used to separate CO2 fromt the flue gases, which later has to be regenerated using even more energy. Enough so that the 1 Btu margin you got from growing the corn and making ethanol out of it would essentially be annhialated. IMO, you would be better off partially oxidizing the cellulose, producing “char” for sequestration, and burning the modest amount of off gas for energy. You would have to count enriching the soil as part of the “benefit” in order to justify doing it this way.
Re 221 – “Weathering of basic silicate rocks (olivine is the most proiminent basic silicate) removes between 2 and 2.5 Gt of CO2 each year from the atmosphere.”
Actually I think that may be a factor of 3 or so smaller.