Over time the CO2 can react with rocks such as magnesium or calcium silicates to form the carbonate. This is the natural way CO2 has been locked up in ancient times.
Magnesium silicates occur in peridotite, serpentinite, basalt
|MgSiO3 + CO2 ---> MgCO3 + SiO2 (exothermic)|
In Oman, an area of exposed rock called peridotite is naturally absorbing 10,000 to 100,000 tons of carbon a year.
This process of locking up carbon in the rocks could be speeded 100,000 times or more by boring down and injecting heated water containing pressurized CO2.
Oman alone could probably absorb some 4 billion tons of atmospheric carbon a year—a substantial part of the 30 billion sent into the atmosphere by humans, mainly through burning of fuels. With large amounts of new solids forming underground, cracking and expansion would generate micro-earthquakes—but not enough to be readily perceptible to humans,
A peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene.
Olivine is a magnesium orthosilicate containing some iron with the variable formula (Mg,Fe)2SiO4;
Pyroxenes are chain silicates having the variable formula (Ca,Na,Fe+2,Mg)(Cr,Al,Fe+3,Mg,Mn,Ti,Va)Si2O6 comprising a large number of different minerals.
Peridotite comprises most or all of the rock in the mantle, which undergirds earth’s crust. It starts some 20 kilometers or more down, but occasionally pieces are exhumed when tectonic plates collide and push the mantle rock to the surface, as in Oman.
As well as in Oman, Peridotite is also found on the Pacific islands of Papua New Guinea and Caledonia, and along the coasts of Greece and the former Yugoslavia; smaller deposits occur in the western United States and many other places. Wikipedia
Mineral Carbonation International (MCi) is a joint venture between The Uni of Newcastle, Orica, and GreenMag Group. They have built a pilot plant to develop a process for using CO2 emissions to carbonate the mineral serpentinite.
Basic rock such as serpentinite is mined, crushed, heated and then mixed with water and pressurised with CO2 to speed up the carbonation reaction.
The products are magnesium carbonate and silica which can produce bricks and pavers, etc.
The process is exothermic so it can reduce the amount of energy used for carbon capture and storage. It is the most economical of the methods developed at present.
The process economics depends on the price of carbon.
To fix a tonne of CO2 requires about 1.6 to 3.7 tonnes of rock. This means 5-10 T of rock /T of coal.
There is more than enough mineral to store all the CO2 emissions from available fossil fuels for centuries to come. There is no other known economic use for the serpentinite, and it occurs in land of low agricultural value.
Metals such as iron, nickel and chrome can be extracted in this process.
Direct injection into serpentinite
One idea is to drill, hydraulically fracture, inject the CO2, then heat to get the carbonation reaction going. The reaction is exothermic and should keep going if fed with CO2. Very little research has been done on this.
Serpentinite sequestration is favored because of the non-toxic and stable nature of magnesium carbonate. The ideal reactions involve the magnesium end member components of the olivine(reaction 1) or serpentine (reaction 2), the latter derived from earlier olivine by hydration and silicification (reaction 3). The presence of iron in the olivine or serpentine reduces the efficiency of sequestration, since the iron components of these minerals break down to iron oxide and silica (reaction 4).
Olivine is is a magnesium iron silicate with the formula (Mg+2, Fe+2)2SiO4. The amount of Mg and Fe vary.
Mg2SiO4 + 2CO2 → 2MgCO3 + SiO2 + H2O
Mg3[Si2O5(OH)4] + 3CO2 → 3MgCO3 + 2SiO2 + 2H2O
3Mg2SiO4 + 2SiO2 + 4H2O → 2Mg3[Si2O5(OH)4
3Fe2SiO4 + 2H2O → 2Fe3O4 + 3SiO2 + 2H2
Reacting with Basalt
In some places such as India there are large lava flows. Where porous basalt is covered with dense glassy basalt, then there is the opportunity to store CO2.
In a relatively short time the CO2 reacts with the minerals to produce carbonates, thereby locking it up permanently. This is known as mineral carbonisation.
Work has been carried out in Iceland.
Reacting with Serpentinite
Serpentinite is a green greasy rock with low porosity, but it reacts easily with CO2 to form carbonates. See mineral carbonisation.
It is common very deep underground, or in complex uplifted formations. There are some shallow formations in Eastern Australia that may be suitable.
It may be possible to:
Again, not much research into this.
Over geological time, CO2 has combined with rocks to form carbonates in the process we know as weathering. This can be done industrially, but it would need about 40% of the coal stations energy.
One bright idea is add the CO2 to a mineral to make a cement.
Bauxite processing waste
CO2 can be reacted with mineral residues such as red muds produced by alumina refineries.
Using CO2 to manufacture fuel, chemicals and materials
IPPC report introduction
An IPPC report is not as optimistic:
Magnesium and calcium silicate deposits are sufficient to fix the CO2 that could be produced from the combustion of all fossil fuels resources.
From a thermodynamic viewpoint,inorganic carbonates represent a lower energy state than CO2; hence the carbonation reaction is exothermic and can theoretically yield energy. However, the kinetics of natural mineral carbonation is slow; hence all currently implemented processes require energy intensive preparation of the solid reactants to achieve affordable conversion rates and/or additives that must be regenerated and recycled using external energy sources.
The best case studied so far is the wet carbonation of the natural silicate olivine, which costs between 50 and 100 US$/t CO2 stored and translates into a 30-50% energy penalty on the original power plant. When accounting for the 10-40% energy penalty in the capture plant as well, a full CCS system with mineral carbonation would need 60-180% more energy than a power plant with equivalent output without CCS.
The industrial use of CO2 as a gas or a liquid or as feedstock for the production of chemicals could contribute to keeping captured CO2 out of the atmosphere but the lifetime of the chemicals produced is too short with respect to the scale of interest in CO2 storage. Therefore, the contribution of industrial uses of captured CO2 to the mitigation of climate change is expected to be small.
Source: IPPC report