Wednesday, October 6, 2010

Recapturing carbon dioxide from the air

Would-be practitioners of "climate engineering," once called "geoengineering," are stumbling across a minefield of problems. Early investigations assumed the key issue would be finding ways to block sunlight from reaching the lower atmosphere. [1] [2] Reducing solar inputs could retard and possibly reverse warming trends. However, more recent investigations, using weather modeling and weather records after large volcanic eruptions, show that blocking sunlight reduces rainfall. [3] Land areas already at low rainfall could become deserts.

Discovery of major hazards from blocking sunlight sparked renewed interest in recapturing carbon dioxide already released by human activity, a difficult prospect that would face three key challenges:
(1) a very large amount of carbon dioxide to be recaptured
(2) low concentration of carbon dioxide in the atmosphere
(3) lack of storage capacity for gaseous carbon dioxide

The total carbon dioxide released to the atmosphere from human activities has been about 800 billion tons since the early 1800s. [4] [5] [6] A large majority came from burning coal. Since 2000, the total has been increasing an average of 2.0 percent per year. [4] Those observations set practical requirements for any proposals to recapture carbon dioxide. A realistic approach must extract and permanently store several hundred billion tons.

Carbon dioxide penetrates some solid materials, including many rubbers and plastics, more than other gases do, making possible separation by a solid, semi-permeable membrane. Other potential membrane separations depend on molecular size differences and porous membranes. [7] Commercial membrane systems for removing carbon dioxide from pressurized gas streams are available. As applied to recapture from the atmosphere, key limitations are the need to compress very large air volumes, the need for a cascade of separation stages because of the low carbon dioxide concentration, and the low permeabilities and flow kinetics of known membranes. Recapture systems using membranes would be enormous, energy-intensive and costly; practical systems have not yet been demonstrated.

Absorption of carbon dioxide by strong alkali is a well known separation technique, widely applied in equipment that maintains sealed environments. It has been demonstrated for recovery of carbon dioxide at atmospheric concentrations, and a closed-cycle process is known. [8] The last process stage is calcining lime at very high temperatures, as used to make cement. It yields concentrated carbon dioxide but takes large amounts of energy, about 2 MWh per ton of carbon dioxide. [9] To recapture the atmospheric inventory of carbon dioxide released by human activities would require a total of about 2 billion GWh. Supplying that energy from fossil fuels would work at cross-purposes. Supplying it from nuclear power would take 5,000 large, 1 GWe reactors about 50 years. Costs would approach US$100 trillion for energy, [10] plus costs of chemical processing steps.

Those and other methods of recapturing carbon dioxide would be useless alone, because there would be no place to put all the carbon dioxide. Even when compressed to a liquid, carbon dioxide occupies about six times the volume of the same amount of carbon as coal. In all the world's mines and wells combined, there is no space for more than a small fraction of the carbon dioxide that human activity has released. Most mines and wells have rock fissures that allow carbon dioxide to seep back to the atmosphere. Permanent storage requires combining carbon dioxide into a stable, solid mineral. While not difficult, most mineralization processes start with a substrate that was made from a carbonate, by driving off carbon dioxide. Obviously that will not do.

Rock formations containing substantial fractions of calcium and magnesium oxides readily combine with carbon dioxide, making carbonates. The formations useful as substrates are geologically young, because when weathered alkaline earth oxides have been consumed. Suitable formations are uncommon and variable in reactivity. [11] Rock formations containing substantial calcium silicate as wollastonite or magnesium silicate as forsterite are also potential substrates. [12] [13] Suitable formations are more common than those containing alkaline earth oxides but are also variable in reactivity. Reaction rates are slower than those of alkaline earth oxides, taking many hours to reach only modest yields.

Silicate carbonation is enhanced when rock substrates are pretreated at very high temperatures and ground to very fine dust. About three-fourths of silicate will then carbonate with a half-hour exposure. The process is energy-intensive; costs have been estimated at about US$70 per ton of carbon dioxide. [14] An unanswered question is emission of carbon dioxide from high-temperature pretreatment. Using the process to store the inventory of carbon dioxide released by human activities, costs would approach US$50 trillion.

Known technologies can recapture carbon dioxide from the air and store it permanently. However, potential costs of applying them to recapture all carbon dioxide released by human activities probably approach US$200 trillion. With total emissions of carbon dioxide increasing at two percent per year, incremental costs to recapture from the atmosphere the current carbon dioxide emissions probably approach US$4 trillion per year. Such an amount could be viewed as remediation of potential environmental damage. It could be raised by taxing carbon dioxide emissions at about US$900 per ton of carbon. That is far more than US$1 to US$50 tax rates factoring in recent political controversies.

Taxing carbon dioxide emissions enough to pay for their recapture from the air and their permanent storage, using those known technologies, would probably raise U.S. retail gasoline prices from about US$3 to about US$6 per gallon. It would probably raise wholesale U.S. prices for coal-fired electricity from about US$.05 to about US$.60 per KWh. Ending further carbon dioxide additions and recapturing all the releases from human activity, carried out over 50 years, would probably cost about 12 percent of the gross world product, estimated from the current level. Less expensive technologies may eventually be developed. For example, carbon dioxide capture from power-plant flue gases is being tested and will probably be less costly. Catalysts that reduce energy consumption have been conjectured, but so far no economically effective catalyst has been found. Substantial improvements are not likely to be found quickly in such longstanding, well known areas of technology.

[1] James E. Hansen and Andrew A. Lacis, Sun and dust versus greenhouse gases, Nature 346:713-719, August 23, 1990.

[2] Edward M. Teller, Lowell Wood and Roderick Hyde, Prospects for physics-based modulation of global change, UCR Livermore National Laboratory Report UCRL-JC-128715, August 15, 1997, available at

[3] Gabriele C. Hegerl and Susan Solomon, Risks of climate engineering, Science 325:955-956, August 21, 2009.

[4] Pieter Tans, Mauna Loa carbon dioxide records, U.S. National Oceanic and Atmospheric Administration, Earth Systems Research Laboratory, 2010, available through

[5] D.M. Etheridge, L.P. Steele, R.L. Langenfelds, R.J. Francey, J-M. Barnola and V.I. Morgan, Historical CO2 record derived from ice cores, Australia Commonwealth Scientific and Industrial Research Organization, Division of Atmospheric Research, 1998, available at

[6] Kevin E. Trenberth and Lesley Smith, The mass of the atmosphere, Journal of Climate 18(6):864-875, 2005.

[7] Colin A. Scholes, Sandra E. Kentish and Geoff W. Stevens, Carbon dioxide separation through polymeric membrane systems for flue gas applications, Recent Patents on Chemical Engineering 1:52-66, 2008, available at

[8] Gregory V. Lowry, Joshuah Stolaroff and David Keith, CO2 extraction from ambient air using alkali-metal hydroxide solutions, American Chemical Society, Division of Fuel Chemistry Proceedings 49(1):362-363, 2004, available at

[9] Wicky Moffat and M.R.W. Walmsley, Understanding lime calcination kinetics for energy cost reduction, Australian Pulp and Paper Industry Technical Association Proceedings 59:487-494, 2005, available at

[10] Craig A. Severance, Business risks and costs of new nuclear power, Electricity Journal 22(4):112-120, 2009, draft version available at

[11] N. Koukouzas, V. Gemeni and H.J. Ziock, Sequestration of CO2 in magnesium silicates, International Journal of Mineral Processing 93:179-186, 2009, available at

[12] Sebastian Teir, Sanni Eloneva and Ron Zevenhoven, Production of precipitated calcium carbonate from calcium silicates and carbon dioxide, Energy Conversion and Management 46(18):2954-2979, 2005.

[13] George D. Guthrie, Jr., J. William Carey, Deborah Bergfeld, Darrin Byler, Steve Chipera, Hans-Joachim Ziock and Klaus Lackner, Geochemical aspects of the carbonation of magnesium silicates, Los Alamos National Laboratory, National Conference on Carbon Sequestration, Washington D.C., May 14-17, 2001, available at

[14] W.K. O'Connor, D C. Dahlin, G. E. Rush, C. L. Dahlin and W. K. Collins, Carbon dioxide sequestration by direct mineral carbonation, Minerals and Metallurgical Processing 19(2):95-101, 2002.