Compared with Calcium Carbonate (limestone, marble, chalk, etc), for CO2 storage hydrocarbons like oil and coal are a sideshow. Cite. Limestone and marble store 15-20,000 times as much. Beware acid rain!
Now, the population explosion has occasioned a building explosion. The production of cement involves the release of CO2, to name one CO2-emitting process. The Guardian says that by 2050 it will be 5 gigatonnes per year or 5% of emissions, but that’s from both the reduction of CaCO3 and the burning of the coal to make it, so how does that break down? But that’s cement alone. What indeed about acid rain? Other processes? Beyond cement, I’m drawing blanks.
So I’m wondering how much else our use of Calcium Carbonate contributes to atmospheric CO2?
And how easy would it be to reverse that process, either by farming creatures which produce Calcium Carbonate (intensive snail farming, anyone?) or by processing Plagioclases directly into limestone?
And please, let’s keep this GQ-only.
Edit: and, of course, like a twit, I’ve posted this in GD. :smack::smack::smack:
It’s obvious that limestone and marble store more carbon than coal - you can tell that just by hefting a piece of each in your hand. But chemical erosion of limestone and marble doesn’t produce anything like a comparable amount of CO2 annually, compared to the burning of coal and oil. (And physical erosion produces none AFAIK.) Plus, where does acid rain come from? Coal burning power stations are a significant source.
Snails and feldspar won’t really gain you much of a return, but coccoliths are more promising. The idea is that by dumping iron into the ocean you “farm carboniferous phytoplankton” which captures carbon from the air. Of course, you also change a delicate ecosystem more radically than climate change ever would and there are possible dangerous side-effects such as releasing ocean methane, so I’d personally advocate a vast, global forest-planting programme instead (and if such radical, global action was ever deemed necessary by consevative politicians, we’d probably be pretty much hosed already). However, it’s certainly a possible last resort to keep in mind if worst-case runaway climate change ever looked inevitable.
Erosion of limestone does release CO2, and whilst the rate is increased by higher rain acidity, it occurs naturally anyway. Generally however, physical erosion processes are a net absorber of the greenhouse gas.
Rather than attempting to mitigate the problems caused by Portland cement, an easier and more successful approach would be to switch to magnesium oxide, chloride or phosphate based substitutes. These can be materially as strong, or many times stronger, and economically competitive. They have been available and used for many centuries, are not in material short supply, and are beginning to be recognised for their environmental advantages. They could even be used to sequester the CO2 themselves.
Sequestering atmospheric CO2 into a physical form, such as CaCO3, is currently the subject of widespread research and Wikipedia provides a more comprehensive summary than I can.
Plagioclases cannot be directly processed into limestones, nor would there be any value in doing so.
Plagioclase is a type of feldspar, which are themselves are significant type of rock forming silicate mineral. These are relatively limited within the Earth’s mantle, but are collectively the most abundant mineral group in the crust, and are found in types of igneous, metamorphic and sedimentary rocks. They are chemically diverse due to the ability of Al to substitute of Si within their crystal framework, varying between 1:3 and 2:2. This allows for cations such as K+, Na+, Rb+, Cs+, Ca2+, Ba2+ and Sr2+ to find space in the crystal arrangement. The most important of these cations are K+, Ca2+ and Na+.
Plagioclase is what geologists like to call a ‘solid solution’ mineral. It does not have a fixed stoichiometry, but exists anywhere between the calcium and sodium ‘end members’ of the group of feldspars.
In order to process plagioclase into limestone ex situ, and sequester CO2, the least worst case, with the highest CaCO3 yield, would involve the most calcium rich end member, CaAl2Si2O8, called anorthite. This reacts slowly with CO2 and water along the lines of,
CaAl2Si2O8 + CO2 + H2O -> CaCO3 + Al2Si2O5(OH)4
Technically this is thermodynamically favourable (so my choice of the word ‘directly’ was arguably inappropriate), but it is not something yet credible in terms of industrial CO2 mineralisation, beyond what is accomplished in nature. There are three current challenges to this approach; the massive scale required, the need to accelerate the rate of the above reaction to make it efficient and the massive energy costs involved.
Under ideal conditions 23.1 ton of pure anorthite would be required to sequester a ton of CO2. By comparison other minerals such as forsterite (Mg2SiO4) could achieve the equivalent several times more efficiently. A mere 6.4 million tons of pure forsterite would required to to sequester the 4 million tons of CO2 emitted from an average coal fired power station. To provide some perspective, or not, this creates 2.6 million cubic metres of magnesite end product. Note however that plagioclase, or equally forsterite, do not occur in rocks by themselves, but with other minerals which would compete for the available metal cations during their breakdown. The actual volume required in the reality, rather than the ideal, is many times greater.
The rate at which anorthite breaks down in nature, by weathering, is in geological standards relatively Usain Bolt-like, depending on the prevailing conditions. However in terms of a human reference frame it is very slow. The initial breakdown of the silicate structure is the impediment to this reaction. To improve this, options include hastening the process by heating or by adding an acid, or both, and grinding the silicate first.
So once we’ve added up the energy costs of mining the massive volumes of non ideal material, transporting it to a suitable industrial complex and the construction thereof, grinding it, heating it, adding acid, or both, and then finding somewhere to store it where it won’t readily revert back to CO2, it begins to become apparent that their might not be the best solution to the problem. Total sequestration of current annual emissions on a simple cost basis, and ignoring the technical near impossibility and emissions it itself would produce, would speculatively cost in the order of >$1,500,000,000,000. My spidey-senses tell me that a few anti-AGW might baulk at this.
Alternatively we might attempt marginally accelerated, in situ weathering of feldspars and other suitable minerals. Given 100,000 years and a suitable sandstone, you may produce as much as 90kg of carbonate mineral in a cubic metre. More promisingly however is the possibility of sequestering CO2 into mafic and ultramafic rocks, like basalt. These are rich in divalent cations capable of forming carbonate minerals to a significant extent within 10-100 years, and are not stymied by an excess of silica. This is not itself without technical difficulties, but presents one of the best options currently available in this field. It is the subject of ongoing research and is being realistically tested by the CarbFix Project, at Hellisheidi, Iceland.
AIUI, cement reabsorbs CO[sub]2[/sub] as it cures. That was the main problem in Biosphere II: the cement was taking away the oxygen by this process. They ended up having to introduce more oxygen to make up for it.
Now whether it reabsorbs as much CO[sub]2[/sub] as was driven off in its manufacture, I don’t know. I would guess that it’s about the same amount. But the reabsorption won’t make up for the carbon dioxide from the fuel used to make it, of course.
Googling to find the answer to how much CO[sub]2[/sub] is absorbed by cement, I find that someone’s developed a cement that absorbs more than it gave off: Novacem
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