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Putting a Lid on Carbon Dioxide

December 20, 2004 | A version of this story appeared in Volume 82, Issue 51

Statoil has been injecting CO2 into a deep saline aquifer beneath the floor of the North Sea since 1996.
Statoil has been injecting CO2 into a deep saline aquifer beneath the floor of the North Sea since 1996.

It sounds so painless: concentrate, capture, pressurize, and inject carbon dioxide deep into Earth's geologic formations and avoid, or at least delay, the likely damage of global warming.

There, the greenhouse gas would rest forever, providing the U.S. with time to develop a long-term solution to overcome the climate-change impact of anthropogenic greenhouse gas emissions.

That is the optimistic scenario upon which nearly all current federal global warming research is based. The U.S. hopes, in the words of the State Department's senior climate negotiator, Harlan L. Watson, to develop "transformational technologies" that will lead to an energy future free of climate-change fears.

Carbon sequestration is being studied at the Weyburn oil field in Saskatchewan, Canada, where CO2is injected to recover oil from a depleted reservoir.
Carbon sequestration is being studied at the Weyburn oil field in Saskatchewan, Canada, where CO2is injected to recover oil from a depleted reservoir.

A focus on CO2 is the logical first place to look for greenhouse gas reductions. Although it is but one of many greenhouse gases, CO2 makes up more than 70% of the world's greenhouse gas emissions. The U.S. is by far the largest greenhouse gas emitter, and CO2 emissions are 81% of its mix of greenhouse gases.

Nearly all anthropogenic CO2 emissions are from the burning of fossil fuels, and for the U.S., the largest source is electricity generation, which produces 39% of the country's CO2 emissions. Cars, trucks, and other transportation-related sources are a close second, however.

Although CO2 emissions could be quickly and dramatically trimmed by increasing vehicle efficiency, that path is unpopular with U.S. politicians, automakers, and many American drivers. Instead, government planners are targeting electricity generation, particularly coal-fired power plants that alone are responsible for some 30% of all U.S. CO2 emissions.

The U.S. is coal-dependent and coal-rich. It gets 52% of its electricity from coal and has enough coal to last at least 250 years. Coal and other fossil fuels are likely to be a big part of the nation's energy mix well into the middle of the century, argues Scott Klara, technology manager for sequestration at DOE's National Energy Technology Laboratory in Pittsburgh.

When asked about coal's future, Klara says: "Economics rules. Coal is literally cheaper than dirt," he says, "about a penny or two a pound. I doubt you can get topsoil for that."

Until the cost of nonfossil, renewable energy gets to the point that it can replace fossil fuels, Klara says, "economics of fossil fuels will rule." He emphasizes, however, that he is not making a prediction about fossil-fuel use into the far distant future. Future CO2-free energy needs, he says, will include nuclear and renewable options. The U.S. funds R&D into solar energy at about $80 million per year, wind at $40 million, and nuclear at around $100 million. These amounts, however, are a shadow of President George W. Bush's clean-coal-related research target of $447 million a year.

THE GOVERNMENT has also balked at tax and other incentive programs to ease renewables' expansion into the marketplace. States do offer incentive programs, primarily for solar, to encourage consumers to buy renewable energy. But there is nothing in the U.S. like Japanese and some Western European national programs that subsidize the purchase of renewable energy that will help these countries meet the Kyoto protocol requirements to reduce CO2 emissions.

In large part, the U.S.'s response to global warming rests on DOE's coal technology program and its research to increase efficiency of the nation's aging coal-fired power plants and to find ways to capture and sequester their CO2 emissions.

DOE began its carbon sequestration R&D program in 1997 with some $1 million a year. Today, the program has grown to about $40 million annually. It also benefits from a near equal amount of private R&D spending by industry, primarily oil and electric companies. This level, Klara notes, is coupled with the Administration's "coal research initiative," which includes the President's $1 billion, 10-year FutureGen R&D plan to build a 275-MW power plant using integrated gasification combined cycle (IGCC) technology.

IGCC is more efficient than conventional pulverized coal technologies. The technology may be key to any system to capture and sequester CO2, since it allows CO2 to be concentrated and more easily removed from the plant. It also can be used to produce hydrogen, another critical element in the Bush energy plan (C&EN, Feb. 23, page 20).

Indeed, hydrogen production for energy, sequestration of carbon dioxide, and increased power-plant efficiency have been woven together into the Administration's plan to continue to burn coal in a world facing global warming.

But the scale needed for sequestration is daunting. To sequester the world's CO2 emissions for the 21st century, Klara says, would eat up a Great Lake or two in underground volume. He stresses, however, the world's potential storage capacity exceeds this demand.

There are doubters. "The question becomes what fraction of aquifers are safe and permanent," says Klaus S. Lackner, a Columbia University geophysicist. "There are plenty that are unquestionably safe. But there are a lot of debatable aquifers counted sometimes." He argues that no one will really know if capacity is sufficient until the storage space runs out.

The second problem to be investigated is whether geologic formations will leak over time. Klara says DOE is beginning detailed monitoring of its trial sequestration programs. Research is focused on ensuring permanent storage, but he adds that without sequestration, "we are leaking 100%."

DOE's immediate goal is to have new capture, sequestration, and monitoring technologies fully developed for widespread deployment by 2012–15, Klara says, and by 2020 to have developed the infrastructure--pipelines, regulations, and willing buyers and sellers--to allow injection of large amounts of CO2. Infrastructure is important, Klara notes, adding that most Americans don't know what sequestration is. DOE is working with some 150 industry and state and local government partners in 40 states to develop a system to oversee regulating, buying, selling, and moving CO2.

An entirely new waste-management business will eventually be created to handle CO2, predicts Robert Socolow, who codirects the Carbon Mitigation Initiative at Princeton University. The industry will take CO2 from the plant gate, certify that it has not been vented to the atmosphere, and dispose of it through sequestration.

SEQUESTRATION is only one part of the government program, but much hinges on its success and whether research can ensure that underground geologic formations will be sufficient to keep CO2 where it is put. DOE has some six U.S. field tests of geologic sequestration under way.

"Sequestration can and already does work," says Jonathan Pershing, director of climate, energy, and pollution programs at the World Resources Institute. "There are many examples where we capture and pump CO2 back into oil wells, for instance, and it has shown itself to be successful and profitable. But these operations are small.

"However, we do understand how to capture CO2, how to get it around in pipelines, and how it fits in geologic reservoirs. But there are questions in all these systems that must be answered."

Pershing also notes that many older coal-fired units are paid for and highly profitable for operators to just keep running with few environmental controls. He warns that it may be difficult to encourage owners to install new capture technologies or to invest in IGCC. "There needs to be an aggressive federal policy to encourage this. It could be a carbon tax, financial incentives, or subsidies, but we need something."

He also criticizes the federal program for not pushing efficiencies harder. "We could get faster and bigger reductions with increased efficiency in the U.S. We are so inefficient."

The government's path is to lower costs so companies will be willing to add new technologies, not to make the firms do so through regulations. Currently, DOE estimates capture and sequestration would cost $100 or more per ton of avoided CO2 emissions. DOE wants to get that number down to $10 a ton by 2012.

"We want to make sure the capture technologies will not increase the cost of electricity by more than 10%," Klara says. He estimates that currently a new IGCC plant will increase cost of electricity by 30% or more, and retrofitting an old plant to use pulverized coal could increase the cost by 100%.

All these cuts and timelines have been factored into a DOE strategic plan, with the goal of providing a portfolio of technologies to reduce U.S. CO2 emissions to 2001 levels by 2050. In 2001, the U.S. emitted some 6.2 billion tons of CO2 to the atmosphere, and to reach 2001 levels by 2050, DOE projects that 45 years in the future, 5.3 billion tons of CO2 emissions must somehow be captured or eliminated each year.

DOE's plan makes sequestration responsible for two-thirds of this reduction, with the rest to come from energy-sector efficiency gains, mostly at power plants.

Even this rosy scenario would only stabilize CO2 emissions at 2001 levels by 2050, critics note. It would take far deeper cuts to return CO2 atmospheric concentrations to 2001 levels by 2050, considering how much would be emitted between 2001 and 2050 as the plan is implemented.

The Bush Administration does have a voluntary program to try to cut CO2 emissions as a percent of gross domestic product by 18% over the next decade through national gains in efficiency. But this is unlikely to cut actual emissions. DOE's figures show that the U.S. achieved a 30% reduction of CO2 as a percent of GDP between 1980 and 2000, but actual CO2 emissions increased 15% as economic growth far outpaced efficiency during this period.

That aside, DOE and utilities policymakers and researchers are counting heavily on carbon sequestration to usher in most of the 5.3 billion-ton carbon dioxide reduction goal. So, how will CO2 be captured, and where will it be put?

Capturing CO2 is the "big ticket" item, Klara says. DOE estimates that gathering CO2 would draw 75% of the cost of a complete removal, transportation, and storage or disposal system for CO2.

CO2 is a commodity chemical and is routinely collected in industrial processes such as synthetic ammonia production, hydrogen production, and limestone calcination. It is also removed during processing of raw natural gas. However, these removal technologies are far too expensive to use to capture CO2 from a coal-fired power plant's flue gas.

CO2 is currently recovered from combustion gases by using aqueous amine absorbers and cryogenic coolers at a cost of up to $100 per ton of carbon. DOE estimates that applying existing capture technologies to large-scale, coal-fired units would increase the cost of electricity by at least 2.5 to 4 cents or more per kilowatt hour, at least doubling the price for most U.S. electricity.

Largely, the problem is that CO2 is emitted by conventional air-fired pulverized coal combustion systems at or near atmospheric pressure and in concentrations within 3 to 15%. Collection of CO2 at the lower pressure and concentrations requires energy-intensive concentration methods and means at least 40% more coal would be consumed just to collect CO2.

Consequently, DOE is researching a range of technologies that could be added to the nation's fleet of old coal plants or used for new conventional plants, as well as new technologies for gasification. R&D approaches involve using liquid and solid chemical absorbents, physical sorbents, solvents, membranes, hydrates, and combinations of these approaches to collect CO2.

Klara highlights a technology in which pulverized coal is burned in a pure oxygen environment, rather than in air. This eliminates most nitrogen from the plant's input and from its emissions. The result is a flue gas stream that is mostly CO2 and water. The oxygen process would avoid the need to scrub flue gas to remove CO2 or nitrogen, getting rid of two troublesome emissions at once.

A more sweeping approach that DOE and utilities are exploring is IGCC technologies. DOE has constructed two demonstration IGCC plants and proposed the FutureGen plant, which is based on gasification technology. Two utilities have recently signaled their intention to build large-scale IGCC plants by 2010 due to the plants' ability to increase efficiency, concentrate and capture CO2, and emit lower levels of conventional air pollutants (C&EN, Sept. 20, page 36).

With IGCC, coal is thermally broken down in the absence of oxygen to form a hydrogen-rich "synthesis gas." The synthesis gas contains 40 to 60% CO2 at pressures of several hundred psi, making it easier to capture CO2 before combustion. Current systems use a liquid glycol solvent technology to collect CO2. Under examination are several other capture technologies that use a mix of various sorbents and membranes, Klara notes.


IGCC plants are roughly 20 to 30% more expensive than conventional coal-fired power plants, and the Administration has offered no regulatory or economic incentives to encourage their construction. The two utilities that are planning trial facilities were pushed to do so by their shareholders. Pershing thinks the U.S. should subsidize IGCC construction.

Looking at sequestration, Klara notes, the goal must be to isolate CO2 from the atmosphere for hundreds to thousands of years. Sequestration approaches include increasing the normal uptake of carbon in soil and forests, placing more in the sea, or putting it underground.

Terrestrial sinks lack the capacity to make much of a dent in U.S. anthropogenic CO2 emissions, he says. The ocean, Klara adds, is not seen as a feasible option now, although it has a huge capacity. "We are just now trying to understand what happens in the natural ocean uptake of CO2 and what the implications are of placing more CO2 deep in the sea."

"GEOLOGIC SEQUESTRATION is the option with enough capacity to sequester what is needed in this century," he says. Sequestration is not a new technology, he notes. Many U.S. oil companies inject CO2 to enhance oil recovery. Some 32 million tons was injected last year, although not to sequester CO2.

The department and a host of academic and corporate partners are looking at three primary injection possibilities: depleted or partially depleted oil and gas reservoirs, unmineable coal seams, and saline formations. There are two large-scale applications of CO2 injection, which are being closely watched and monitored.

Off the coast of Norway, Statoil, a Norwegian oil and gas company, has annually pumped since 1996 more than 2 million tons of CO2 into a saltwater aquifer beneath the seafloor. The aquifer has a maximum capacity of 600 billion tons of CO2, the company estimates, enough capacity to store more than 400 years of CO2 emissions for all of Europe, Klara says. Statoil is now injecting about 1 million tons per year, equal to the annual CO2 emissions of a small, 150-MW conventional coal-fired power plant.

The CO2 is separated from natural gas extracted at the site. Rather than vent it, Statoil determined injection was an environmentally sound way to avoid Norway's carbon emissions tax.

Saline porous rocks saturated with brine hold enormous domestic capacity, Klara notes, and preliminary studies show they may be the most permanent of the options as CO2 continues to dissolve in water and precipitate into a carbonate structure. DOE says the rocks may hold some 500 billion tons of volume in the U.S.

In western Canada, a consortium of companies and governments has been studying since 2000 the injection of compressed CO2 into the Weyburn oil field in Saskatchewan to recover oil deposits. The four-year-old program pipes CO2 325 km from a U.S. synthetic fuels gas plant in North Dakota to the oil field.

EnCana, the Canadian oil and gas company that operates the field, expects to store 22 million tons of CO2 there and to produce 130 million barrels of oil over 20 years.

Pressurized supercritical CO2 is a strong hydrocarbon solvent, Klara notes, that can easily recover oil or gas. In the U.S., oil and gas formations could store some 150 billion tons of CO2, or 30 years' worth of U.S. emissions, DOE estimates.

The third disposal option Klara points to is using CO2 to recover methane in unmineable coal seams. In this case, CO2 input can exceed methane output, because two or three molecules of CO2 are adsorbed into the coal seams for each molecule of methane released. Although it is a good CO2 sink, the coal can expand with the added molecules, blocking CO2 from entering the seam and recovering the methane. This and other problems are being examined, and DOE estimates some 90 billion tons of CO2 could be held in domestic coal seams.

CO2 sequestration has great potential and offers a familiar option to get rid of CO2. If it is shown to work on such a grand scale, advocates say it may be the simplest way to reduce atmospheric CO2 without upsetting the U.S. economy.


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