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How To Reduce Greehouse Gases

NAE meeting considers range of technologies and measures, including carbon sequestration

by Bette Hileman
published in Chemical & Engineering News (May 27, 2002)

Not all meetings on global climate change are focused on the science of global warming. Many actually address how to slow the buildup of greehouse gases in the atmosphere. Such a symposium was held last month by the National Academy of Engineering. It discussed current and emerging technologies that can be applied to carbon dioxide reduction and government and state policies that would help this effort. In addition, speakers spent a great deal of time discussing which energy futures would both be economical and produce little or no net CO2.

One aim of the meeting was to develop a set of recommendations that may eventually influence federal research programs and help shape future government policies for reducing greenhouse gases.

Last year, the Bush Administration rejected the Kyoto protocol that would have set specific goals for CO2 reduction by the U.S. At the same time, the Administration made a pledge to intensify the search for technologies to reduce or control the buildup of CO2 in the atmosphere over the long term. The technologies under consideration range from renewable energy sources that produce little or no CO2 to sequestering the CO2 from fossil fuels in biomass, geologic formations, or the deep ocean. Separating and sequestering CO2 from stack emissions could allow the use of fossil energy to continue while making the transition to nonfossil energy sources.

Robert H. Socolow, professor of aerospace and mechanical engineering at Princeston University, opened the meeting by observing that for every 2.1 billion tons of CO2 (measured as carbon) that humans add to the atmosphere, the concentration will go up 1 ppm. "We are now transferring between 6 billion and 7 billion metric tons of carbon per year from below ground to the atmosphere, but the increase in the concentration of CO2 is about 1.5 ppm per year," he explained. This means that about half the CO2 emitted is finding its way into terrestrial and ocean sinks, he said.

Coal is very likely to remain a major source of energy over the coming century because it is cheap and abundant relative to oil and natural gas, Socolow said. China and India get most of their energy from coal, and the U.S. is expected to continue using large amounts of coal during this century, he explained. Therefore, if a near-zero carbon emissions option for coal were feasible, this would go a long way toward solving the greenhouse gas problem - at least over the next 100 years, he said.

One concept that could contribute to this goal is what Socolow calls a "carbon refinery". This would be a large facility that could use coal, natural gas, biomass, or petroleum to produce a variety of fuels and chemicals as well export electricity - and it would capture the CO2 it emits and place it in geologic formations or on the ocean floor, he said. Over time, as hydrogen-consuming fuel cells begin to be used on a large scale, a greater portion of the carbon refinery products would be hydrogen.

Rules for federal permitting of CO2 storage sites should be worked out now, Socolow said, adding that Environmental Protection Agency rules for the underground injection of hazardous wastes provide an inadequate precedent for such regulations. A permit for hazardous waste injection requires, for example, almost no monitoring after injection to see if the models are right.

Gardiner Hill, the CO2 program manager for BP, described a CO2 capture project to use CO2 to recover natural gas and oil and to store CO2 in depleted oil and gas wells or in coal mines that cannot be mined economically. The U.S., Norway, and the European Union, as well as eight companies - BP, ChevronTexaco, ENI, the Royal Dutch/Shell Group, Statoil, Norsk Hydro, Suncor Energy, and PanCanadian- are participating in the project. It is reasonable for oil and gas companies to invest in this project, Hill said, becasue "it builds on many years of experience successfully managing geologic reservoirs and storage of fluids and gases."

The goal of the $25 million project is to achieve major reductions in the cost of CO2 capture and storage and to demonstrate to stakeholders "that CO2 storage is safe, measurable, and verifiable," Hill said. The participating companies aim to have "at least one large-scale application in operation by 2010," he said. If CO2 sequestration is to be useful in the long term, he explained, it must be large-scale because many industrial operations emit more than 20,000 tons of CO2 per day.

Enormous amounts of CO2 could potentially be stored in geologic formations, Hill said. An estimated 40 billion to 50 billion tons could be stored in depleted oil reservoirs, 80 billion to 100 billion tons of CO2 could be stored in U.S. gas reservoirs, and 15 billion to 20 billion tons could be sequestered in unminable coal beds.

However, before CO2 can be sequestered on a large scale, key technical questions need to be answered, Hill said. Where are suitable formations with the right properties to provide CO2 traps? What is the geochemical and geotechnical response of faults and cap rocks to large quantities of CO2? "In trying to solve one environmental problem, we don't want to create a new problem," he said.

Geologic formations can definitely store high-pressure gases for geologic times, said Franklin M. (Lynn) Orr Jr., petroleum engineering professor and dean of the School of Earth Science at Stanford University. Oil and gas reservoirs store carbon in the form of methane and petroleum on timescales of millions of years, he said, and the technologies for CO2 injection into oil and gas reservoirs are well established.

Currently, Orr said, the use of CO2 for enhanced oil recovery is limited only by supplies of natural CO2 in the ground. In the U.S., 66 projects are using natural CO2 for oil recovery, producing about 180,000 barrels per day.

Another way to sequester CO2 is to use it to displace coal-bed methane - the methane that is absorbed on coal surfaces in mines, Orr said. "You can absorb about twice as much CO2 as methane in coal beds," he explained. However, again, some questions need to be answered before this can be done on a large scale, he said. Techniques are needed to select seams with good lateral permeability, and the long-term integrity of CO2 sequestration in coal beds needs to be investigated.

"There are also interesting geochemical questions about the long-term fate of CO2 in aquifers," Orr said. One is the possible reaction chemistry of low-pH brines with aquifer minerals.

It is easy to meter how much CO2 goes into a saline aquifer, Orr said, but it is more difficult to tell where it goes after that. Failure of a large injection well, for instance, could produce a dense low-lying cloud of CO2 - an asphyxiant.

REFORESTATION, saving old-growth forests, and improved land management are good for humankind but have little long-term impact on the CO2 content of the atmosphere, said Dale R. Simbeck, vice president of technology for SFA Pacific, a consulting firm. Much of the carbon tied up in forests and soils moves back into circulation again in 30 to 60 years, as forests are harvested and as soil carbon is oxidized.

David W. Keith, assistant professor of engineering and public policy at Carnegie Mellon University, argued that the total cost of using fossil fuels for electricity production and sequestering the CO2 in geologic formations is now in more or less the same ballpark as electricity produced by renewable means. "We often hear that the cost of solving the climate problem will be very high," he said, "but I am skeptical."

The Department of Energy is aiming to bring down the cost of carbon sequestration to $10 per ton of carbon, from the current estimated cost of$100 per ton. "But is $100 per ton of carbon a big number?" Keith asked. This cost would increase the producer price of electricity by a few cents per kilowatt-hour. That is certainly a significant rise, he said, but not in the context of varying electricity prices over the past few decades, which have been "all over the map." If the U.S. abated half of all its CO2 emissions (or 0.75 billion tons per year) at $100 per ton of carbon, the total cost would be $75 billion per year, Keith said. This amounts to 0.75% of the gross domestic product. In contrast, the current bill for all environmental controls is 1 to 2% of the gross domestic product, or $100 billion to $200 billion, he said.

Keith claimed that the costs of wind energy, nuclear energy, and fossil energy are similar to the cost of electricity produced with carbon sequestration - on the order of 5 to 7 cents per kWh. But they all pose risks, he said. Renewables can threaten wilderness, if large quantities of biomass are used for energy or windmills or solar panels occupy large expanses of land; nuclear power poses the risk of weapons of proliferation; and burning fossil fuels with carbon sequestration poses the risk that CO2 will leak from storage sites.

Renewable energy systems are becoming more efficient, reliable, and affordable, said Robert K. Dixon, deputy assistant secretary in DOE's Office of Power Technologies. He said that electricity from photovoltaics now costs about 20 cents per kWh and is expected to drop to 2 to 4 cents by 2007. The cost of energy from biomass gasification declined from 8 cents per kWh in 1995 to 7 cents in 2000, and is expected to be 6 cents in 2010."We have plenty of wood waste, poultry manure, and hog waste that can be used as biomass fuels," he said. "Commercial applications of renewable energy are expanding in the U.S. and around the world."

In contrast, Howard J. Herzog, a research engineer at the Massachusetts Institute of Technology Laboratory for Energy & the Environment, is much less optimistic about renewable energy. Fossil fuels are here to stay for 50 years if not 100, he said. "They have an 85% market share, and that market share is rising," he explained. Therefore, "we need technologies to deal with them," he said.

"Carbon sequestration technologies exist today and are a robust solution for a wide range of energy scenarios," Herzog continued. Norway's Statoil is putting CO2 from its North Sea Sleipner oil and gas field into a saline aquifer under the ocean. By law, the CO2, which constitutes 9% of the natural gas, must be separated before the gas is sold.

Basically, there are four reservoirs for carbon sequestration: saline aquifers, coal mines, oil and gas wells, and the ocean, Herzog said. Right now, about one-third, or 2 billion tons, of yearly CO2 emissions go into the surface ocean inadvertently, he said. "We cannot put the ocean off-limits as a sequestration reservoir, but we need a lot of research to see the effects of putting carbon in the ocean," he added. "I don't feel we are at a point where we can choose the right mix of energy sources and technologies to reduce greenhouse gases."

Klaus Lackner, professor of geophysics at Columbia University, described a "simple, cost-effective method" for extracting CO2 directly from the air rather than from flue gases. This technique, which he devised in collaboration with researchers from Los Alamos National Laboratory, would allow the CO2 from numerous small sources to be captured.

In Lackner's technique, air is passed over an extraction agent - a solution of calcium hydroxide, for example. The CO2 in the air reacts with the Ca(OH)2 and is converted to calcium carbonate (limestone), which falls to the bottom of the extractor.

The CaCO3 is then heated to produce a pure stream of CO2 and CaO, which is returned to the extractor, Lackner said. The recovered CO2 could be sold for enhanced oil recovery or displacement of coal-bed methane or it could be sequestered in a geologic formation, he said. The extractor captures CO2 from wind as it blows over the device.

Lowell Wood, a researcher at the Lawrence Livermore National Laboratory and a visiting fellow of the Hoover Institution at Stanford University, described a radically different approach to mitigating the warming effects of increasing concentrations of CO2. He would allow atmospheric levels of CO2 and other greenhouse gases to rise indefinitely and counteract their radiative effects by placing some form of reflective material in the stratosphere. For example, using sulfur aerosols whose diameters are "several-fold smaller" than the wavelengths of light would "selectively" scatter back into space the largely deleterious ultraviolet component of sunlight," while diminishing the light that plants usefor photosynthesis imperceptibly, he explained. The cost of using such aerosols would be about $1 billion per year, he said.

Alternatively, Wood said, small, thin, metallic-walled super-pressure ballons could be placed in the stratosphere at a cost of only $200 million per year. With either method, enough matter would need to be injected to substantially scatter incoming solar radiation over 1 million km2 of Earth's surface, he explained.

Several attendees strongly objected to Wood's approach. Lackner pointed out that if the concentration of atmospheric CO2 is allowed to rise from its current level of about 370 ppm to 550-600 ppm - as is expected by 2100 if controls are not put in place - the growth rates of corals would decline 30-40%. CO2 dissolves in the surface ocean, increasing the level of carbonate ion and decreasing the level of carbonate ion. Carbonate is required for coral growth.

The recommendations from the NAE meeting are not yet available. However they turn out, DOE's research on carbon sequestration is likely to remain one of the fastest growing areas of investigation in the department. DOE officials say sequestration is needed because renewable or efficient technologies alone cannot stabilize concentrations of CO2. In 2001, DOE's budget for sequestration was $18 million, and it increased to $32 million in 2002. For fiscal 2003, the department has requested $54 million. And taking the NAE meeting as an indication, the department has plenty of reasearch areas to explore.

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