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Overview

Reducing CO2 in the atmosphere that is emitted from fossil fuel-fired power plants will require a range of carbon management and sequestration options. Two emerging carbon storage methods are mineral CO2 sequestration (also known as mineral carbonation) and accelerated weathering of limestone (AWL). One responsibility of the USGS in CO2 sequestration studies is to provide critical geologic information to underpin the chemical and industrial research and development.

One of the first tasks to address the viability of both of these approaches is to estimate the available mineral reserves. For this initial estimation, the USGS has produced national-scale geologic maps that show magnesium-rich ultramafic and calcium-rich limestone rocks. These maps were produced in cooperation with researchers from the Earth Institute at Columbia University (http://www.energy.columbia.edu/), the University of California, Santa Cruz, and the Carnegie Institution of Washington. Both national-scale maps detail the geographical distribution and extent of rocks suitable for mineral carbon sequestration and accelerated weathering of limestone.

Maps of US power plant distance to limestone and suitable ultramafic rocks.
Left map shows distance from coastal power plants and cement plants to potential sources of limestone. Right map shows ultramafic rocks suitable for mineral CO2 sequestration.

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Mineral Carbonation Using Ultramafic Rocks

Mineral carbonation converts gaseous CO2 to a geologically stable solid by combining CO2 with magnesium or calcium oxide to form mineral carbonates such as magnesite or calcite. These mineral carbonates are stable over geologic time (millions of years), providing a site for permanent, safe storage of CO2. The technology originated in the 1990s; the initial idea was credited to W. Seifritz and its initial development was by Klaus Lackner and others at Los Alamos National Laboratory.

Mapping magnesium-rich ultramafic rocks in the conterminous United States

Host rocks containing suitable Mg oxide-bearing minerals for mineral CO2 sequestration are ultramafic rocks, which are enriched in magnesium-silicates such as olivine and serpentine minerals. Researchers at the U.S. Geological Survey and Columbia University have developed a digital geologic database of ultramafic rocks in the conterminous United States that were deemed suitable for mineral CO2 sequestration. Data were compiled from varied-scale geologic maps showing magnesium-silicate ultramafic rocks. However, not all ultramafic rock lithologies were deemed magnesium-rich enough to be suitable for CO2 sequestration. Only ultramafic rocks that are estimated to contain magnesium minerals greater than 18 weight percent are included in the geologic compilation. Specific lithologies that met these criteria are peridotite, dunite, serpentinite and picrite (an olivine-rich volcanic basalt that is rare in the United States). Ultramafic rock types omitted are amphibolite, pyroxenite, and hornblendite.

Photos of ultramafic rock types.

Using geophysical data to extend the geologic map of ultramafic rocks and to better characterize their mineralogy

Serpentinite and peridotite rocks have very high magnetic susceptibilities compared to other common igneous and metamorphic rock types. This makes them especially attractive targets for aeromagnetic surveying.

The USGS has extended the geologic mapping of the ultramafic rocks to include geophysical studies related to mapping their volume, area, and mineralogy. The geophysical focus of this research is twofold. First, we investigate how airborne magnetic data can be used to map the shallow subsurface geometry of ultramafic rocks to estimate the volume of rock material available for mineral CO2 sequestration. Secondly, we explore, on a regional to outcrop scale, how magnetic mineralogy, as expressed in magnetic anomalies, may vary with magnesium minerals, which are the primary minerals of interest for CO2 sequestration.

Examples of aeromagnetic anomaly interpretive filters.
(a) Airborne magnetic anomaly caused by exposed and buried ultramafic rock; (b) interpretive "terrace" filter applied to magnetic data to enhance the edges of the buried rock and; (c) estimation of geometry and area of exposed and buried ultramafic rock available for mineral CO2 sequestration.

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Accelerated Weathering of Limestone

The USGS, in collaboration with the Institute of Marine Sciences (University of California, Santa Cruz) and the Department of Global Ecology (Carnegie Institution of Washington, Stanford, CA) is investigating a method of capturing and sequestering CO2 from the stacks of fossil-fuel-fired power plants. The method, referred to as the Accelerated Weathering of Limestone (AWL), hydrates waste CO2 with seawater to produce a carbonic acid solution, which in turn is reacted and neutralized with limestone, thus converting the original CO2 gas to dissolved calcium bicarbonate. Access to an inexpensive source of limestone is essential, and the USGS is the lead agency providing information on the availability and properties of carbonate rocks for use in AWL.

Photo of aggregate quarry.
Aggregate operations are well equipped to handle the large amounts of material required for AWL.

Details of Accelerated Weathering of Limestone

Accelerated weathering of limestone (AWL) is proposed as a low-tech method to capture and sequester CO2 from fossil fuel-fired power plants and other point-sources such as cement manufacturing. AWL combines captured CO2 with water and calcium carbonate to produce wastewater rich in bicarbonate ions. This slurry can be released in the ocean with little to no negative environmental impact. Host rocks containing Ca oxide-bearing minerals are carbonate-rich rocks such as limestone. AWL requires about 2.3 tonnes of calcium carbonate to react one tonne of CO2. The large amounts of limestone fines produced during the processing of crushed limestone may be useful in CO2 sequestration.

Accelerated weathering of limestone process diagram.
An example of a possible carbonate dissolution reactor design, from Rau and Calderia (1999). A CO2 -rich gas stream (1) enters the reactor vessel (5) by one or more entryways (e.g., 2, 3, and/or 4). The gas stream then passes over or through a wetted, porous bed of limestone particles within the reactor. This carbonate mass is sprayed (6) and wetted with, and partially submerged in, a water/carbonic acid solution which is unsaturated with respect to bicarbonate ion. This arrangement exposes the incoming gas to a large surface area of water/solution in the form of droplets and wetted carbonate particle surfaces in (5), facilitating hydration of the entering CO2 to form a carbonic acid solution within the reactor. CO2-depleted gas then exits the reactor (7). The carbonic acid solution formed reacts with the carbonate to form calcium and bicarbonate ions in solution, which is either recirculated or bled from the reactor and replaced with unreacted water within the reactor at a rate that maximizes benefit/cost.

Feasibility requires access to an inexpensive source of limestone and to seawater, thus limiting AWL facilities to within about 10 km of the coastline. The majority of U.S. coastal power-generating facilities are within economical transport distance of limestone resources. AWL presents opportunities for collaborative efforts among the crushed stone industry, electrical utilities, cement manufactures, and research scientists.

Point source CO2 emitters map.
Coastal CO2 emitters map.

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Products

McCafferty, A.E., Langer, W.H., and Van Gosen, B.S., 2016, U.S. Geological Survey Cooperative Research on Carbon Dioxide Sequestration Using Ultramafic and Carbonate Rocks, in Smith, K.S., Phillips, J.D., McCafferty, A.E., and Clark, R.N., eds., Developing integrated methods to address complex resource and environmental issues: U.S. Geological Survey Circular 1413, p. 54-56., http://dx.doi.org/10.3133/cir1413.

Mapping the Mineral Resource Base for Mineral Carbon-Dioxide Sequestration in the Conterminous United States — S.C. Krevor, C.R. Graves, B.S. Van Gosen, and A.E. McCafferty, 2009.

Langer, W.H., San Juan, C.A., Rau, G.H., and Caleira, K., 2009, Accelerated weathering of limestone for CO2 mitigation: Opportunities for the stone and cement industries: Mining Engineering Magazine, February 2009, p. 27-32. View article [PDF file, 804 KB, article provided courtesy of SME]

Geophysical delineation of Mg-rich ultramafic rocks for mineral carbon sequestration — A.E. McCafferty, B.S. Van Gosen, S.C. Krevor, and C.R. Graves, 2009. (powerpoint presentation)

Geologists Map Rocks to Soak CO2 From Air — Earth Institute News (news release)

Mineral Carbon Sequestration Research at Columbia University

In situ carbonation of peridotite for CO2 storage: Lamont-Doherty Earth Observatory, Columbia University — Peter B. Kelemen and Jürg Matter, 2008.

International Conferences on Mineral Carbonation

Acclerated Carbonation for Environmental and Materials Engineering Conference Turku, Finland Nov. 29-Dec. 1, 2010

Geological Carbon Capture & Storage in Mafic and Ultramafic Rocks, January 2011.

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Research Personnel

Mineral Carbonation Using Ultramafic Host Rocks

Accelerated Weathering of Limestone

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Project Contacts

Anne McCafferty
Phone: 303-236-1397
Email: anne@usgs.gov
Crustal Geophysics and Geochemistry Science Center

Bradley Van Gosen
Phone: 303-236-1566
Email: bvangose@usgs.gov
Central Mineral and Environmental Resources Science Center

The use of firm, trade, and brand names is for identification purposes only and does not constitute endorsement by the U.S. government.

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