A news article in a new science journal Nature Reports: Climate Change, Volume 1, page 92, December 2007 reports thusly:
Geophys. Res. Lett. 34 L19703 (2007)
Removal of carbon dioxide directly from the atmosphere is essential for combating climate change suggests a new study. Most efforts to mitigate global warming focus on reducing emissions of greenhouse gases, notably CO2. But that in itself will not be enough, say scientists...
......Even if the CO2 emissions were stabilized at a tenth of current levels, temperatures would rise by 2C eventually. This suggests that nothing short of active removal of CO2 from the atmosphere is necessary to avoid unmanageable warming.
This sort of thinking has always been in the back of my mind since I note that living systems do precisely this. Thus the feasibility of doing this is demonstrable by example, much as the possibility of heavier than air flight was demonstrable by biological example, insects and birds, for many thousands of years before it became technologically accessible.
In fact, the removal of carbon dioxide from air has been technologically accessible for many years, as anyone who has ever tried to standardize a solution of barium hydroxide realizes. If you make a solution of barium hydroxide (or for that matter, a solution of slaked lime, calcium hydroxide) and leave it exposed to air, the initially clear solution will slowly cloud and eventually a white precipitate will form. The insoluble material formed is a carbonate which represents carbon dioxide captured from the air.
(This reaction, by the way, is precisely the reaction that takes place in concrete when it cures.)
So then, what's the problem?
Um, energy, that's what. In fact, limestone is a carbonate and one needs to heat it to make what is called "slaked lime", calcium oxide. When calcium oxide absorbs a molecule of water, it becomes calcium dihydroxide, and when this calcium hydroxide, which can then absorb a molecule of carbon dioxide (regenerating a water molecule) to make calcium carbonate, an insoluble material that is only decomposed by acid. All of the earth's limestone (and all of its marble as well) originated precisely from this kind or reaction.
The separation of carbon dioxide from air requires energy (to overcome the inherent entropy of mixing).
It would be obvious, I think, to maybe everyone but the membership of Greenpeace, perhaps, that if the energy in question is generated by the use of dangerours fossil fuels, say dangerous natural gas, the process will be less than perfectly efficient and would actually end up releasing more carbon dioxide than it captured. To make sense, the energy must come either from nuclear energy (geothermal or fission) or solar energy (wind, solar PV, etc, etc, blah, blah, blah.) Note that biomass - as is being proved in other cases - would be useless here.
Of course, in the biological case, the source of the energy - with the exceptions of some life forms at geothermal vents that are powered by nuclear energy - is the sun. The process is of capturing energy and converting it to biomass - which in an oxygen atmosphere represents potential (stored) energy - is not particularly efficient by the way. Most of the energy captured by chlorophyll is in fact wasted as heat. In fact more energy is wasted as heat in photosynthesis than is wasted by a dangerous fossil fuel plant - like a coal, oil, or dangerous natural gas plant. Only a small segment of the radiant energy absorbed by chlorophyll is represented by the conversion to chemical potential energy - sugars and other biomolecules.
This is a hot topic in the scientific literature - with rather nasty exchanges about "negative entropy" and the second law of thermodynamics - a law that Greenpeace and some members of Congress, as well as a whole host of pretty crazy inventors and cranks - believes is subject to repeal by popular vote.
Be that as it may, the overall efficiency of photosynthesis as energy converted to chemical energy is less than 5%, or about half of the best solar PV cells - which do not, in any case, produce chemical energy. This is not really a new concept, in spite what you might read on the Greenpeace website. Back in 1976 in Naturwissenschaften 63, 491 498 (1976) Moesta wrote:
Since 1973, there has been an abundance of proposals concerned with the use of solar energy. Most of them have not yet reached the state of publication, nevertheless, the public is becoming more interested in the matter. This article will not try to give a complete review, but will give an outline which covers the typical problems and ideas. Since the use of solar energy is much more an economical problem than a scientific one, it is useful to remember a few facts at the beginning...
...All plant growth on earth receives its energy from the sun. So, photosynthesis is an annually renewable source for both material and energy. The efficiency of normal agriculture - the energy content of the harvest compared to the influx of solar energy-rarely exceeds 0.5% on rich soils. But, on the other hand, certain algae have reached 4 to 10%. Anaerobic bacteria may produce about 265 m 3 methane from 1 ton of organic waste with an energy content of 9,450 kcal/m3 [11-14]. The conversion of carbohydrates by fermentation to hydrocarbons may become economical again...
The year 1976, by the way, is the same year that the scientifically illiterate mystic Amory Lovins - today paid millions of millions of dollars by corporations seeking to Greenwash themselves - began publishing scientifically illiterate speculations on how we were going to save the planet with readily available "renewable" energy systems thus denying a need for nuclear energy.
More recent publications on the thermodynamic efficiency of photosynthesis are found in Biophysical Chemistry 90 2001.249-253 and many other places.
Capturing enough biomass to replace dangerous fossil fuels is something of an environmentally, economically, and scientifically dead issue in my view, but that's just my opinion, mostly because of the related and much ignored issue of water.
But is there a mimetic way to capture carbon dioxide from the air?
Well yes, as I indicated in the little blurb on barium hydroxide above. A related (and much cheaper approach involves the substition of the calcium analogue, as stated.
As it happens the thermodynamic efficiency of this process isn't spectacular, though. The matter is detailed in a recent paper in Environ. Sci. Tech. Surprisingly - given that we live in the Golden Age of Chemistry, the chemistry reported here is much older than I am - and I'm not teenager. The paper in question is Zeman, Environ. Sci. Technol. 2007, 41, 7558-7563.
The abstract is here.
Some excerpts including silly "sequestration" talk:
Research into Carbon Capture and Storage (CCS) has recently been summarized in the Intergovernmental Panel on Climate Change (IPCC) Special Report on CCS (1). The report describes various technologies focusedonemitters producing at least 0.1 Mt per year of CO2. All totaled, these sources produce 13.6 Gt of CO2 annually while global emissions are estimated at 25.7 Gt of CO2 (2). The nominal 90% capture rate of most CCS technologies suggests that more than 50% of global emissions would remain unabated even if these were fully deployed. The remaining emissions, from dispersed and often mobile sources, are not amenable to "end-of-pipe" CCS technologies and require other mitigation techniques.
There are no permanent repositories for the dangerous fossil waste carbon dioxide; none are planned; none are sited; and none will prevent dangerous fossil fuel waste from remaining dangerous for eternity.
That said, carbon dioxide is useful as an energy carrier, which is precisely the way plants use it, by the way.
Zeman and Lackner (9) have previously outlined a specific method of Air Capture, shown schematically in Figure 1. The previous work developed the scientific basis for this particular air capture process by establishing its thermodynamic feasibility. The process is summarized in the following paragraphs. First, CO2 is absorbed by an alkaline NaOH solution to produce dissolved sodium carbonate...
...The carbonate ion is removed from the solution by reaction with calcium hydroxide (Ca(OH)2), which results in the precipitation of calcite(CaCO3). The causticization reaction is a mildly exothermic, aqueous reaction that occurs in an emulsion of calcium hydroxide...
...Causticization is performed ubiquitously in the pulp and paper industry and readily transfers 94% of the carbonate ions from the sodium to the calcium cation (10). Subsequently, the calcium carbonate precipitate is filtered from solution and thermally decomposed to produce gaseousCO2. The calcination reaction is the only endothermic reaction in the process...
Oh good. The "only" endothermic reaction.
...An important consideration is the overall energy balance of the process. The sum of all the reaction enthalpies listed in Table 1 is zero. The enthalpy changes are given at standard conditions, but in the process outlined, the CO2 enters and leaves at different pressures. As a thermodynamic minimum, the energy cost is -RT ln (P2/P1) where P1 is the partial pressure of the input stream and P2...
Thermodynamics? We don't need no stinking thermodynamics. Greenpeace is lobbying for the repeal of the second law of thermodynamics, and it's a good thing too, since the second law has been widely criticized going back to the days before Ludwig Boltzmann killed himself.
Thank goodness though, that higher concentrations of carbon dioxide make the process more economic:
The first item is Air Contacting, which refers to the energy consumed by the pumps and blowers in the absorption tower. The system proposed by Baciocchi et al. is different in that they assume 500 ppm CO2 concentrations. The higher ambient concentration results in a lower throughput for a similar capture rate.
The energy for this process will be delivered by Jesus when he returns once again from the dead, something that has been widely predicted for about 2000 years now.
How does 500 pm correspond to the situation obtained now? Oh, it's only 20% than current levels. Don't worry, we're working on it.
The net
amount of CO2 sent to storage by such a facility, disregarding leakage, will depend on the source of the electricity. Renewable energy, such as wind, could be used and the negligible emissions would be balanced against higher cost and intermittency. The emissions from fossil fuel electricity plants can be estimated by using the representative values contained in Table TS.3 of the IPCC Special Report (1). The listed value for a natural gas combined cycle plant is 0.367 kg CO2/kWh, which can be converted to 2.3 mmol/kJe. Similarly, a conventional pulverized coal plant produces 0.762 kg CO2/kWh or 4.8mmolCO2/kJe. The emissions for any combination can be obtained by multiplying the fraction of generation capacity by the representative emissions factor.
I am very fond of perpetual motion machines myself.
Speaking of renewable energy, how many times have you seen the words "renewable energy" used in a sentence of the last 50 years that did not contain conditional tenses?
Just wondering.
You never know. I could see such a sentence in my lifetime, but most of the sentence I see consist of statements like "if we just covered every square inch of North Dakota with windmills, we could provide all the energy needs of Earth, Mars, Venus and parts of Mercury.
Can you say "nuclear?"
No?
You can't?
Why am I not surprised?
And so what is the thermodynamic efficiency of this carbon capture process?
According to the paper, about 6%.
This is somewhat better than chlorophyll, apparently, although it would appear that chlorophyll is falling behind in the carbon capture business and could use some help.
One hears these great things about chlorophyll by the way, which is effectively, if one looks at it carefully, a wonderful "hydrogen from solar energy" device. For instance, I have heard that if we just could cover every square centimeter of Iowa with corn plants and distilleries, we could get get Iowa to stop importing gasoline.