Hi, the process is quite energy inefficient and it doesn't tackle the problem at the source. Other scientists have already replied to Zeman and said it might be smarter to get rid of fossil fuels, use biomass + CCS instead and utilize the carbon-negative electricity (in transport):

Scientists propose artificial trees to scrub CO2 out of the atmosphere - but the real thing could be smarter

Jonas.

"Subsequently, the calcium carbonate precipitate is filtered from solution and thermally decomposed to produce gaseous CO2."

I don't get the point of this step; they have already sequestered the carbon in the CaCO3 (chalk/limestone), what's the point of turning it into a gas again? eliminating this step would also make the process even more energy-efficient. you could dump the chalk wherever as long as it doesn't get involved in the water cycle, say a desert.

Sounds interesting but, in my opinion it has some much more excting competition, namely pyrolisys and burying the carbon in soils. Not only is it energy positive but also benefits agriculture. Most interestingly has the potential to sequester more carbon the we currently output. see http://www.technologyreview.com/Energy/18589/

"http://www.technologyreview.com/Energy/18589/"

if char is such a boon to agriculture, why is it not being used already on a large scale?

I don't get the point of this step; they have already sequestered the carbon in the CaCO3 (chalk/limestone), what's the point of turning it into a gas again?

So, where are you going to get the calcium hydroxide from?

Paul,

The CaO to CaCO3 and then back to CaO and CO2 reactions are just intermediate steps. If you leave it as CaCO3 you have no net reduction in CO2. (You had to roast CaCO3 to get the CaO to start with. CaO does not occur in nature. This preparatory step is not shown.)

I did a back of the envelope on his listed energy requirements: If you assume a cost of $0.05/kwh, a reasonable cost for steady state nuclear or hydroelectric power, it works out to $100/ton of CO2. Disposal costs would have to be added to this value. As you would expect, this is considerably higher than estimates for CCS from combustion streams.

The author's point is well taken, even if we put CCS on all the stationary sources of CO2, we still emit too much CO2. This is an approach to the rest of the emissions and worthy of development/trial funding.

Coal and oil are excellent natural forms of carbon sequestration. Developing solar and wind energy to replace fossil fuels is the most viable and sustainable form of carbon sequestration.

Meanwhile, plants will continue to remove more CO2 from the air, and if we turn these waste biomass into charcoal and bury it into the ground, then there will be a net reduction in atmospheric CO2.

We already have a perfectly good way of sequestering highly dilute CO2. It's called photosynthesis. Someone suggested on this forum that we should grow algae and dump it in spent natural gas reservoirs.

Alternatively, you could use it as food, feed or fuel and leave more of the carbon that's already sequestered in the ground.

They are called trees.

Energy efficient buildings and cities, not just light bulbs, solar, wind, geothermal and nuclear, biodiesel for commercial transport, electric vehicles for consumers, quit cutting trees down for agriculture or fuel, and plant new ones.

On the way there dump our coal plants co2 into the ground, build plug-in hybrids, and most importantly convince our own government that we shouldn't wait until next year or wait until China does the same. Along those lines, don't argue that the market should solve the problem because it's not going to soon enough and while we continue to debate it we will continue to dig ourselves in a hole while hundreds will suffer asthma attacks.

/rant

Does it only scrub out CO2? Or all airborne particles like pollen and smog?

Something like this may be the only hope if we've already pushed the climate too far to wait for gradual legislated carbon reductions and source sequestration methods.

If there's a gas tax coming, this should be the type of thing it pays for.

The CaO to CaCO3 and then back to CaO and CO2 reactions are just intermediate steps.

Of course. My question was rhetorical.

We already have a perfectly good way of sequestering highly dilute CO2. It's called photosynthesis.

The areas required would be enormous, unfortunately, due to the low efficiency.

Here's my approach to a cost comparison; if CO2 has a molar mass of 44 grams then a tonne (megagram) is about 23,000 mol. Now .35mJ is 1.26 kwh let's say worth 15c retail of nuclear electricity. This capture process works out about $3500 per ton, way too expensive. Carbon tax enthusiasts are talking about $40 per ton and tree offset hucksters reckon about $5. Check my calcs.

Aussie,

I think your numbers are off. 350kJ = .35MJ, there are 3.6 MJ per kWh, so that would be .35/3.6 ~ .1 kWh/mole (rather than 1.26kWh). Assuming $.10/kWh in nuclear prices, this gives you a cost of $.01/mol, and at 23,000 mol per tonne, $230 per tonne.

Still more than the $40 or $5 figures, but if you can get the price of electricity down to $.05/kWh then you are at $115/tonne which might be feasible.

Iron sulphate is cheaper and efficient.

http://en.wikipedia.org/wiki/Iron_fertilization

Thanx FK I shoulda divided. The SI time unit is the second but we keep using those pesky multiples of 3600 seconds otherwise known as hours.

because of the very high dilution of CO2 in the atmosphere, very high volumes of air need to be scrubbed.
It would be interesting to combine the scrubbing proces with the energy tower
(http://en.wikipedia.org/wiki/Energy_tower_(downdraft))

The energy tower could provide the energy, while the very high air volumes could provide the CO2.

@ Paul Dietz -

you're right if you're thinking of slow-growing land-based higher plants in temperate latitudes. Farming fast-growing single-celled algae out on the open ocean in the tropics would address the acreage problem, though it introduces others - such as containment, impact on marine ecosystems etc.

Another approach is to concentrate the sunlight onto closed bioreactors to prevent evaporation in the desert/badlands locations these would be located in. Uninhabited islands would be another option. The algae species would have to be hardy, because even with smart glass (formulations that are opaque in the infrared) and forced cooling, there would still be the issue of very intense visible light to contend with. On the upside, the collected solar heat may be used to produce electrictity via a Kalina cycle or else, thermally desalinated water. Water and carbon dioxide comprise the bulk inputs; trace minerals would also have to be provided.

Depending on the species chosen, the resulting biomass can be used for food, feed, fuel (allowing more fossil fuel to be left in the ground) or purely as an active carbon sink. In the latter case, the dried and compacted biomass would be vacuum-sealed in plastic containers and buried on land in abandoned open-cast mines or, dropped to the bottom of the sea and covered with a layer of mud.

Another approach is to concentrate the sunlight onto closed bioreactors to prevent evaporation in the desert/badlands locations these would be located in.

How does this remove CO2 from the atmosphere? The bioreactors are closed! If you propose removing CO2 from the air for subsequent injection into the bioreactors, then you've just tacked on an unnecessary secondary step (growing the algae) to a scheme that already is removing CO2 from the air by some other means.

The next step is to electrolyze the CO2 into CO and oxygen, combine the CO with H2 electrolyzed from water to make synthesis gas, and then use that to manufacture liquid hydrocarbons. Use carbon-neutral energy sources to drive all these reactions and you have an endless supply of vehicle fuels with no net greenhouse effect and no need to put a hundred million hectares under the plow. Are you listening, Richard Branson? (It's a pity that there's no capture mechanism which directly results in CO, but oh well...)

"Another approach is to concentrate the sunlight onto closed bioreactors to prevent evaporation in the desert/badlands locations these would be located in."

the Aquatic Species Program already tried this, the costs are completely prohibitive on any reasonable timescale.

Wouldn't be wiser to correct the problem at the source and stop producing CO2?

gavin walsh wrote:

"if char is such a boon to agriculture, why is it not being used already on a large scale?"

It has been and still is considered un-economical, other imputs are cheaper but if the farmer was paid a little bit for the carbon requestered it would become economical.

The first reaction uses NaOH. If it could be made out of NaCl efficiently enough, I would propose a simpler method : Just spray the NaOH in the oceans. The H2CO3 in the ocean would be 'sequestered', since NaHCO3 can't be returned to CO2. As CO2 in the air and ocean are in a dynamic equilibrium with a transfert of about 90 billion tons of CO2/year, it could easily draw the CO2 out of the air.
The HCl produced in the NaOH-production could probably be used to produce hydrogen out of low-grade ore (metal + HCl --> metal-chloride + H2). It could probably just be pumped into empty mines, and the H2 could be recovered.
The tricky part will certainly be to produce cheap and carbon-free NaOH out of seawater.
propositions ?

The HCl produced in the NaOH-production could probably be used to produce hydrogen out of low-grade ore (metal + HCl --> metal-chloride + H2).

Ores with free metals that would allow that reaction to occur are extremely rare. Almost all ores are oxides, sulfides or other such compounds.

The real way to dispose of the acid would be by reaction with olivine or serpentine, which (in strong acids) rapidly dissolve to form silica and magnesium salts. This would go much faster than the reaction with carbonic acid solutions, which have higher pH.

BTW, separation of salt solution into acid and base can potentially be done by electrodialysis with bipolar membranes (EDBM), although I don't know if this would be economical in this case.