Mineral Basalt Sequestration
In situ mineral carbon dioxide sequestration:
In situ mineral carbon dioxide sequestration is a sequestration process in which highly pressurized (super-critical) carbon dioxide (CO2) is pumped into basaltic rock formations in the earth. These basaltic rock formations are rich in mineral silicates that react with CO2 to form mineral carbonates or bicarbonates. This is an exothermic and, thus, thermodynamically favorable reaction that runs deep below ground, sequestering CO2 in a mineral form. (Lackner, 2003).
- Mineral sequestration mimics a natural chemical process that occurs slowly during erosion and weathering of rocks. Calcium carbonate and magnesium carbonate are very stable and harmless compounds that can be easily and safely stored, thus providing a good method of carbon sequestration. Mineral silicates, such as olivine and serpentine, react with pure carbon dioxide to form carbonates, such as calcium carbonate and magnesium carbonate, elements that make up limestone (Wenzhi Li, Wen Li, Baoqing Li, Zongqing Bai).
- Basaltic rock is formed by volcanic activity, so it has many different layers due to the lava cooling process. Some layers are very porous and can hold and react with CO2 while the layers above and below are much impermeable, safely trapping the unconverted CO2 deep beneath the ground (Big Sky).
- CO2 has almost no chance of escaping back into the atmosphere because it is stored in other compounds (Lackner,2003).
- In situ storage is more expensive than most other sequestration methods.
Potential Storage Sites:
- In situ storage is currently being implemented in the US Northwest and Iceland and may also be implemented in India, Siberia, and below the oceans, where large basaltic formations are present.
- Worldwide data is limited, but case studies have been carried out in the US and Iceland.
Two such projects are the “Big Sky Carbon Sequestration Partnership,” headed by a team including researchers from Idaho National Laboratory, the University of Idaho, Boise State University, Idaho State University, (Hickey), and “Carbfix,” a project in Iceland run by a team from the University of Iceland, Columbia University, Centre national de la recherche scientifique, and Reykjavik Energy (CarbFix).
Promising research and data have emerged from both sites, especially from the “Big Sky Carbon Sequestration Partnership.”
This site spans six states, Idaho, Montana, Wyoming, South Dakota and eastern Washington and Oregon, spanning an area of about 85 thousand square miles that has the potential to store over 100 Gt of CO2total (Hickey). Currently only a pilot program is running in Walla Walla county, Washington. After five years of developing the method, surveying the site, and testing the seismic stability of the rocks, the project started its first drill in January 2009 (Hillhouse). According to the Big Sky Project website, 1,000 tonnes of CO2 have successfully been injected this year and will be monitored for the next year. Upon success of this pilot project, Big Sky plans to implement a commercial-scale test, on the scale of 1 million tonnes of CO2 sequestered.
- General cost estimates are rough, since costs are highly specific to each site.
- Klaus S. Lackner in his review article gives an estimate of less than $10 per tonne of CO2, but his estimate neglects the cost of transporting CO2; it only covers mining and preparation costs. The CarbFix project's pilot project has cost $11 million and has sequestered 2,000 tons of CO2 in 9 months (The Earth Institute at Columbia University, 2009). This is far above the cost estimate given by Lackner, but pilot programs will cost much more than industrial implementation because of the experience gained from the pilot programs.
This technology is already being implemented and closely monitored in the US and Iceland.
The success of these projects suggests the ability for full industrial implementation within the next 15 years.
Ex Situ Mineral Sequestration:
Ex situ mineral sequestration, is a process similar to the in situ process, but in ex situ, the rock may be transported to a laboratory. Ex situ mineral sequestration is the reaction of carbon dioxide (CO2) with minerals to form stable carbonates above ground. It involves laboratory techniques that catalyze the dissolution of minerals to release cations, which may then react with dissolved CO2 to form geologically stable carbonates. The two main types of rock with which the CO2 interacts are forsterite rock, which is low in magnesium concentration, and serpentine rock, which is high in magnesium concentration. Each of these reacts with the CO2 differently (see equation A) (Guthrie et al, 2002).
equation A) Forsterite Rock: 1/2Mg2SiO4(s) + CO2(g) → MgCO3(s) + 1/2SiO2(s) -95 kJ/mole
Serpentine Rock: 1/3Mg3Si2O5(OH)4(s) + CO2(g) → MgCO3(s) + 2/3SiO2(s) + 2/3H2O(l) -64kJ/mole
Rocks high in magnesium concentration, specifically ultramafic serpentinized rocks, are optimal for mineral sequestration because magnesium is more reactive than other alkaline metals present in rocks (Herzog et al, 2002).
- Because carbonates exist at a more stable energy state than does CO2, mineral sequestration reactions are thermodynamically favorable, produce energy (exothermic), and do not require any energy input to proceed (spontaneous). In theory, the energy released from these reactions can be harnessed to speed up subsequent carbonation reactions or even to generate power (Goldberg et al, 2000). The high stability of the carbonates means that the CO2 is unlikely to escape (Herzog et al, 2002).
- Ultramafic rock deposits, which are used for ex situ mineral sequestration, are found across the globe (Carbon Sequestration Atlas, 2008).
- Peridotite, the most abundant mineral in the upper mantle, is highly reactive with CO2, and, thus, could quickly form carbonates in the laboratory (Sciencedaily, 2008).
- The costs of mineral sequestration may be offset by the profits made from the products, the mineral carbonates, as they may be sold in the form of cement (Herzog et al, 2002).
- While the thermodynamics are favorable, currently, the forward reaction rate is too slow to be economically viable, as it takes thousands of years to occur in nature. To speed up the process, hydrochloric acid (HCl) may be used to dissolve the serpentine rock, though this process is expensive (Herzog et al, 2002). Another method attempts to dissolve the rock in an aqueous medium without the mediation of an acid, but this method requires pre-treatment of the rock that is also not economically viable due to its energy consumption (Lackner et al, 1997).
- Current and future research must continue in order to find an energy-efficient balance between the catalysis of the carbonation process and the necessary energy input.
- Transportation and storage rocks used in the ex situ process may be expensive and potentially could lead to an increase of vehicle emissions (Herzog et al, 2002).
- Mining operations required for mineral sequestration will pose negative environmental impacts.
Potential Storage Sites:
- The United States contains 6000 square miles of ultramafic rock that reside at or near the Earth’s surface, which is enough to sequester 125 years worth of current global emissions (Carbon Sequestration Atlas 2008).
- More than 1012 tonnes of global rock deposits have potential for mineral sequestration, though not all of this space could be utilized for storing carbon dioxide (Herzog et al, 2002).
- Currently capacity estimates are only speculative.
- The costs of mineral sequestration are solely speculative, as no large-scale projects have been previously implemented (Herzog et al, 2002).
- Current estimates put mineral sequestration at about $70 per ton of carbon dioxide sequestered (Herzog et al, 2002). However, if future research yields the creation of a new laboratory technique that reduces pre-treatment costs and overcomes separation problems, the costs can theoretically be reduced to $30 per ton of carbon dioxide sequestered (Herzog et al, 2002).
- Total estimated cost per tonne of CO2 sequestered is about $120 (Herzog et al, 2002).