PerspectiveClimate Change

# A Guide to CO2 Sequestration

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Science  13 Jun 2003:
Vol. 300, Issue 5626, pp. 1677-1678
DOI: 10.1126/science.1079033

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.

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.

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