In the next issueCarbon Capture Journal news & articles Visit our social network site
Editorial Calendar RSS news feed Buy a subscription
Sign up to EMail newsletter Download past issues About Carbon Capture Journal Who reads Carbon Capture Journal
Advertise in the magazine Links to carbon capture and storage websites Events Diary Contact Us Sister magazine - The Hydrogen Journal Sister magazine - Digital Energy Journal Write for us
Could a billion tonnes of CO2 a year be stored in carbonates?
Storage, Nov 05 2008 (Carbon Capture Journal)
- In a paper published in the Proceedings of the National Academy of Sciences, Peter B. Kelemen and Jürg Matter from Columbia University describe how the natural process of carbonation of a mineral, peridotite, could be enhanced to capture and store billion of tonnes of CO2 every year from the atmosphere.
Peridotite is a naturally occuring rock formation present in Oman, as well as other locations around the world including Papua New Guinea, New Caledonia and along the east coast of the Adriatic Sea.
The researchers found that hundreds of thousands of tonnes of CO2 are naturally stored in Oman alone every year, and suggest ways in which this process could be enhanced to store more than a billion tonnes.
“Peridotite carbonation can be accelerated via drilling, hydraulic fracture, input of purified CO2 at elevated pressure, and, in particular, increased temperature at depth,” says Professor Kelemen.
“In fact, after an initial heating step, CO2 pumped at 25 or 30 °C can be heated by exothermic carbonation reactions that sustain high temperature and rapid reaction rates at depth with little expenditure of energy.”
This means the reactions would be self-sustaining, and could potentially store all the CO2 emissions from power stations around the world.
Enhancing the capture process
The rock formations in Oman store CO2 as magnesium and calcium carbonate and dolomite (a mineral composed of calcium, magnesium, carbon and oxygen) in a network of underground veins.
Olivine ((Mg,Fe)2SiO4) one of the main constituents of peridotite, reacts with groundwater containing dissolved CO2, forming carbonates that increase the volume of the rock by up to 44%, causing fractures to appear.
The fractures allow more water to penetrate and increase the speed of the reaction.
The authors propose that fracturing techniques used in the oil and gas industry to allow oil to flow more easily could be used to increase the volume of rock exposed to CO2 and allow more of the gas to react.
Another key factor that affects the rate of carbonation is the temperature of the rock. The authors calculate that the optimal temperature for carbonate formation is 185 °C.
After fracturing, the rock could be heated to this temperature by injecting hot fluid. This could increase the rate of carbonation by up to a million times, the authors say.
As dissolved CO2 in surface water cannot be supplied rapidly enough to keep pace with the enhanced carbonation rates, a pure stream of CO2 or a CO2 rich mixture of fluid would then be injected.
As the carbonation reaction is exothermic, the scientist calculate that an initial heating step is all that would be required to maintain the higher rates of CO2 capture at depth.
This method could store over a billion tonnes of CO2 in a cubic kilometre of rock, the authors say.
Capturing CO2 from seawater
An alternative process could avoid prolonged pumping of fluid and the use of purified CO2.
In Oman, New Caledonia, and Papua New Guinea, peridotite is present beneath a thin veneer of sediment offshore, beneath the sea bed. Here, peridotite could be drilled and fractured, and a volume could be heated using the method described above.
Little heating would again be required, as the temperature at the bottom of a 5km borehole is already 100 °C.
Seawater could then be pumped into the well, where it would heat up and the dissolved CO2 would react with the peridotite raising the temperature further.
The seawater could then rise back to the surface through another well, several kilometres from the first, through convection.
The CO2 depleted seawater would absorb more CO2 from the atmosphere, reducing overall world concentrations of the gas.
The authors calculate that this method could store only around ten thousand tonnes of CO2 in a cubic kilometre of rock, due to the limited concentration of CO2 in seawater, but at relatively little cost.
Conclusion
The authors have described a process that provides a way to permanently store CO2 in mineral deposits available in several locations around the world.
One method involves transport and injection of concentrated CO2, in a similar way to currently proposed plans to store CO2 in aquifers or depleted oil and gas fields.
The other method could provide a way for CO2 from the atmosphere to be extracted and stored without the energy penalty of capturing from point sources such as power stations.
Although this method would store considerably less CO2 by volume of rock, it would be relatively cheap to implement, and since the rock does not have to be heated, could be employed on a very large scale.
Further studies needed
According to the paper, the reactions studied are virtually impossible to replicate in a lab.
More elaborate models combined with field tests will be required to evaluate and optimise the method.
For example, it is difficult to predict the consequences of hydraulic fracturing of peridotite, plus cracking associated with heating, hydration, and carbonation, in terms of permeability and reactive volume fraction, say the authors.
“Large-scale field tests should be conducted, because the proposed method of enhanced natural CO2 sequestration provides a promising potential alternative to storage of supercritical CO2 fluid in underground pore space,” says Professor Keleman.
