25 January 2009

Carbon Sequestration by Mineral Carbonation

Posted by John Freeland

Given the evidence supporting the view that burning fossil fuels is contributing to global warming and a potential dangerous climate perturbation, there’s considerable interest in carbon storage. So-called Carbon Capture and Sequestration (CCS) could be done many ways such as increasing soil organic matter and planting more trees, ocean storage, burial in deep geologic formations, and mineral carbonation.

Howard Herzog of the MIT Laboratory for Energy and the Environment has this assessment of prospects for CCS by way of carbonation, i.e., formation of carbonate minerals.

Thermodynamically, the plan makes sense as the formation of both calcite and magnasite releases heat (exothermic) as shown here:

CaO + CO2 =  CaCO3 + 179 kJ/mole
MgO + CO2 = MgCO3 + 118 kJ/mole.

The article claims “calcium and magnesium are rarely available as binary oxides.” Instead, carbon dioxide could be reacted with dissolved magnesium silicates, forsterite and serpentinite, in an anion exchange reaction to form magnesite (MgCO3), like this:

½Mg2SiO4 (Forsterite) + CO2 = MgCO3 (Magnesite) + ½SiO2 + 95kJ/mole

1/3Mg3Si2O5(OH)4 (Serpentine) + CO2 = MgCO3 (Magnesite) + 2/3SiO2 + 2/3H2O + 64kJ/mole

Again, since the reactions are exothermic, the reactions should proceed to the right, the lower energy states. A big andvantage of this process, if feasible, is permanent carbon storage. Other proposed CCS techniqes may leak back to the atmosphere.

There are problems with mineral carbonation, though, involving solubility of the reactants and kinetics. It turns out the forsterite and serpentine need to be dissolved in acid or molten salts, which are both potentially messy and expensive operations. Or, the rock can be ground into a fine powder and dissolved in a hot water solution. The rock grinding consumes a lot of energy.

Such a scenario would require setting up the mineral carbonation factory at a large serpentinite mine where such rocks exist – Quebec, for example. The process would basically dig up one kind of rock, make a carbonated rock, and stuff the new rock back in the hole. The biggest obstacle to making it work, according to Herzog, seems to be finding an economical way to speed up the chemical kinetics.

CCS is a brand new technology and there are no doubt many ready to set up shop and get there hands on some of the big federal economic stimulus money. We’ll need to watch out for snake oil salesman. Herzog does not enthusiastically endorse the process described above but, rather, recommends a “portfolio approach” consisting of a diversified package of strategies.

Like all good researchers, he recommends more study.

Herzog’s article quotes significantly from the following:

Yegulalp T.M., K.S. Lackner and H.J. Ziock, “A Review of Emerging Technologies for Sustainable Use of Coal for Power Generation,” presented at Sixth International Symposium on Environmental Issues and Waste Management in Energy and Mineral Production, Calgary, Alberta, Canada, May 30-June 2 2000).