[cdn-nucl-l] A Guide to CO2 Sequestration

["Adam McLean" <adam.mclean@utoronto.ca>]

Posted in Science Magazine, Volume 300, Number 5626, Issue of 13 Jun 2003,
pp. 1677-1678 and at:
http://www.sciencemag.org/cgi/content/full/300/5626/1677
Some excellent data on CO2.

Adam

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A Guide to CO2 Sequestration
Klaus S. Lackner*

Climate change concerns may soon force drastic reductions in CO2 emissions.
In response to this challenge, it may prove necessary to render fossil fuels
environmentally acceptable by capturing and sequestering CO2 until other
inexpensive, clean, and plentiful technologies are available.

Today's fossil fuel resources exceed 5000 gigatons of carbon (GtC) (1),
compared with world consumption of 6 GtC/year, assuring ample transition
time. However, by 2050, the goal of stabilizing the atmospheric CO2
concentration while maintaining healthy economic growth may require
"carbon-neutral" energy in excess of today's total energy consumption (2).
Lowering world CO2 emissions to 2 GtC/year would shrink the per capita
emission allowance of a projected world population of 10 billion people to
3% of today's per capita emission in the United States.

If sequestration is to achieve this goal, it must operate on a multiterawatt
scale while sequestering almost all produced CO2. It must also be safe,
environmentally acceptable, and stable. For small stored quantities, storage
time requirements can be minimal (3). But as storage space fills up,
lifetime constraints due to aggregate leakage emissions would tighten, until
storage times for the entire carbon stock would reach tens of thousands of
years. If carbon emissions are reduced mainly through sequestration, then
total carbon storage in the 21st century will likely exceed 600 GtC. Because
leaking just 2 GtC/year could force future generations into carbon
restriction or recapture programs, even initial storage times should be
measured in centuries.

Storage time and capacity constraints render many sequestration
methods--such as biomass sequestration and CO2 utilization--irrelevant or
marginal for balancing the carbon budget of the 21st century. Even the
ocean's capacity for absorbing carbonic acid is limited relative to fossil
carbon resources (4). Moreover, with natural ocean turnover times of
centuries, storage times are comparatively short. Generally, sequestration
in environmentally active carbon pools (such as the oceans) seems ill
advised because it may trade one environmental problem for another.

Underground injection is probably the easiest route to sequestration. It is
a proven technology suitable for large-scale sequestration (5). Injecting
CO2 into reservoirs in which it displaces and mobilizes oil or gas could
create economic gains that partly offset sequestration costs. In Texas, this
approach already consumes ~20 million tons/year of CO2 at a price of $10 to
$15 per ton of CO2. However, this is not sequestration, because most of the
CO2 is extracted from underground wells.

Oil and gas sites have limited capacity (see the figure). Once they fill up,
saline aquifers may be used, as demonstrated under the North Sea where the
Norwegian energy company Statoil has sequestered CO2 removed from natural
gas (6, 7). Ubiquitous saline reservoirs imply huge storage capacities.
However, because of uncertainties in storage lifetimes, seismic instability,
and potential migration of buoyant CO2, long-term integrity must be
established for each site.

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Estimated storage capacities and times for various sequestration methods.
The "fossil carbon" range includes at its upper end methane hydrates from
the ocean floor. The "oxygen limit" is the amount of fossil carbon that
would use up all oxygen available in air for its combustion. Carbon
consumption for the 21st century ranges from 600 Gt (current consumption
held constant) to 2400 Gt. "Ocean acidic" and "ocean neutral" are the
ocean's uptake capacities for carbonic acid and neutralized carbonic acid,
respectively. The upper limits of capacity or lifetime for underground
injection and mineral carbonates are not well constrained. EOR stands for
enhanced oil recovery. 

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A more expensive but safer and more permanent method of CO2 disposal is the
neutralization of carbonic acid to form carbonates or bicarbonates (4).
Neutralization-based sequestration accelerates natural weathering processes
that are exothermic and thermodynamically favored, and results in stable
products that are common in nature. Mineral deposits larger than fossil
resources ensure essentially unlimited supplies of base ions (mainly
magnesium and calcium, but also sodium and potassium).

The least expensive way to neutralize CO2 may be its injection into alkaline
mineral strata. CO2 would gradually dissolve into the pore water. Because it
is acidic, it would leach mineral base from the rock, resulting in
carbonates or bicarbonates that eliminate all concerns over long-term
leakage. Neutralizing carbonic acid with carbonates as a base would create
aqueous bicarbonate solutions. Unless injected underground, they would
likely find their way into the ocean, which fortunately could accept far
larger amounts of bicarbonates than of carbonic acid.

A better option than forming water-soluble bicarbonates would be the
formation of insoluble carbonates that could be stored at the location of
the mineral base, confining environmental impact to a specific site. To this
end, serpentine or olivine rocks rich in magnesium silicates can be mined,
crushed, milled, and reacted with CO2. Estimated mining and mineral
preparation costs of less than $10 per ton of CO2 seem acceptable, adding
0.5 to 1¢ to a kilowatt-hour of electricity.

Improved methods for accelerating carbonation are, however, still needed.
The current best approach--carbonation of heat-treated peridotite or
serpentine in an aqueous reaction--is too costly. Elimination of the
energy-intensive heat treatment could render the process economically and
energetically feasible. Above-ground mineral sequestration has the capacity
of binding all CO2 that could ever be generated and limiting the
environmental impact, including terrain changes, to relatively confined
areas.

Most sequestration methods require concentrated CO2, which is best captured
at large plants that generate clean, carbon-free energy carriers such as
electricity and hydrogen. Retrofitting existing plants appears too
expensive; new plants designed for CO2 capture are more promising (8).
Complete CO2 capture opens the door to radically new power plant designs
that eliminate all flue gas emissions, not only CO2. Oxygen-blown
gasification could approach this goal today. More advanced designs could
even remove the efficiency penalty associated with CO2 capture.

For example, sending gasification products of coal together with steam
through a fluidized bed of lime would shift oxygen from water to carbon.
Capture of CO2 on lime would promote hydrogen production and provide
necessary heat. Half of the hydrogen-rich output would be used to gasify
coal; the other half would be oxidized in a high-temperature solid-oxide
fuel cell. The water-rich spent fuel gas would be returned to the lime bed
to repeat the cycle. Only excess water, ash, and impurities captured in
various cleanup steps would leave the plant. Once the lime becomes fully
carbonated limestone, CO2 would be produced in a concentrated stream while
the limestone is converted back to lime with waste heat from the fuel cell.
Careful heat management could drive power plant efficiency to 70% (9) (for
comparison, conventional coal-fired power plants are in the 30 to 35% range;
modern gas-fired power plants can approach 50%).

CO2 is three times as heavy as fuel and therefore cannot be stored in cars
or airplanes. CO2 from these sources will have to be released into the
atmosphere and recaptured later. Currently, photosynthesis is the only
practical form of air capture. Capture from air flowing over chemical
sorbents--such as strong alkali solutions or activated carbon
substrates--appears feasible but needs to be demonstrated (10). Wind is an
efficient carrier of CO2. The size of less than 1% that of capture apparatus
would be windmills that displace equal CO2 emissions, suggesting that they
could be quite cheap to build (11). The additional cost of sorbent recycling
should also be affordable (12).

Because the atmosphere mixes rapidly, extraction at any site, however
remote, could compensate for emissions from anywhere else. By decoupling
power generation from sequestration, air capture would allow the existing
fossil fuel-based energy infrastructure to live out its useful life; it
would open remote disposal sites and even allow for the eventual reduction
of atmospheric CO2 concentration.

Cost predictions for sequestration are uncertain, but $30 per ton of CO2
(equivalent to $13 per barrel of oil or 25¢ per gallon of gas) appears
achievable in the long term. Initially, niche markets (for example, in
enhanced oil recovery) would keep disposal costs low, with capture at
retrofitted power plants dominating costs. Over time, new power plant
designs could reduce capture costs, but the costs of disposal would rise as
cheap sites fill up and demands on permanence and safety tighten. Some
applications--for example, in vehicles and airplanes--could accommodate the
higher price of CO2 capture from air, eliminating CO2 transport and opening
up remote disposal sites.

Today's urgent need for substantive CO2 emission reductions could be
satisfied more cheaply by available sequestration technology than by an
immediate transition to nuclear, wind or solar energy. Further development
of sequestration would assure plentiful, low-cost energy for the century,
giving better alternatives ample time to mature.

References and Notes

H.-H. Rogner, Annu. Rev. Energy Environ. 22, 217 (1997) [Abstract]. 
M. I. Hoffert et al., Science 298, 981 (2002). 
The reason is that leakage rates are proportional to storage size and
inversely proportional to storage lifetime. 
K. S. Lackner, Annu. Rev. Energy Environ. 27, 193 (2002) [Abstract]. 
S. Holloway, Annu. Rev. Energy Environ. 26, 145 (2001) [Abstract]. 
H. Herzog, B. Eliasson, O. Kaarstad, Sci. Am. (February 2000), p. 72
[GEOREF]. 
R. A. Chadwick et al., paper presented at the Sixth International Conference
on Greenhouse Gas Technology (GHGT-6), Kyoto, Japan, 1 to 4 October 2002
[Conference]. 
H. J. Herzog, E. M. Drake, Annu. Rev. Energy Environ. 21, 145 (1996)
[Abstract]. 
T. M. Yegulalp, K. S. Lackner, H.-J. Ziock, Int. J. Surf. Mining Reclam.
Environ. 15, 52 (2001) [GEOREF]. 
K. S. Lackner, H.-J. Ziock, P. Grimes, in Proceedings of the 24th
International Conference on Coal Utilization & Fuel Systems, B. Sakkestad,
Ed. (Coal Technology Association, Clearwater, FL, 1999), pp. 885-896. 
At a wind speed of 6 m/s, the U.S. per capita emission of 22 tons/year of
CO2 flows through an opening of 0.2 m2. Through the same opening blow 21 W
of wind power or 0.2% of the U.S. per capita primary power consumption. 
F. S. Zeman, paper presented at the 2nd Annual Conference on Carbon
Sequestration, Alexandria, VA, 5 to 8 May 2003 [Conference]. 

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The author is in the Department of Earth and Environmental Engineering,
Columbia University, New York, NY 10027, USA. E-mail: kl2010@columbia.edu
10.1126/science.1079033

Volume 300, Number 5626, Issue of 13 Jun 2003, pp. 1677-1678. 
Copyright © 2003 by The American Association for the Advancement of Science.
All rights reserved.