The Pros and Cons of Carbon Dioxide Dumping

Global warming concerns have stimulated a search for carbon sequestration technologies.

Last month, at the international climate change conference in Kyoto, Japan, progress toward establishing uniform targets and timetables for curbing CO₂ emissions was frustratingly slow. However, with the reality of increasing fossil fuel use, most attendees agreed that research aimed at developing and deploying promising technologies that can prevent release of CO₂ to the atmosphere must be aggressively pursued.

Capture and disposal of CO₂ are actively being sought as a means to avoid release of the greenhouse gas to the atmosphere. At the Sleipner West gas field in the North Sea, Norway's state-owned oil company, Statoil, is conducting the world's first carbon sequestration project that has as its main objective protection of the atmosphere (1). CO₂, a natural gas contaminant, is being recovered and pumped into a huge aquifer beneath the sea floor. Another project is planned off Hawaii's Kona coast, where a multinational research team will soon perform field trials, testing options for direct ocean disposal of CO₂. At the Natuna gas field, north of Borneo, another project is planned that will annually sequester millions of tons of CO₂ in aquifers under the sea floor. Land-based options for capture and disposal of CO₂, such as reuse for enhanced oil recovery operations and capture of fossil-fueled power plant emissions, are at commercial stages of development.

Intrigued by research possibilities for CO₂ capture and disposal, Henry C. Kelly of the White House Office of Science and Technology Policy said, "There are only a limited number of options we have to reduce carbon dioxide emissions, and this is definitely worth looking into."

Critics of CO₂ disposal, however, question the safety, cost, and even the need for such an approach. Although Michael Oppenheimer, chief scientist of the Environmental Defense Fund, agrees with Kelly, stating, "We should continue to support research on CO₂ capture and disposal," he is uncertain whether such technologies are environmentally and economically sound. "I don't see a large-scale application of these techniques in the short term," he said. Darren Goetze, a biophysicist at the Union of Concerned Scientists in Washington, D.C., said, "It is very expensive and completely unnecessary. There are much more readily available options like renewable energies. What we need to be focusing on is the reduction of the use of fossil fuels."

Ocean storage is a more disputed option than underground disposal into deep aquifers. Dan Lashof of the Natural Resources Defense Council is concerned about potential impacts on marine organisms that may result from changes in the pH of ocean water in the presence of increased levels of CO₂. Reacting to various schemes for sequestering carbon in the oceans, George Woodwell, director of the Woods Hole Research Center in Massachusetts, said, "We should stop putting carbon dioxide in the air instead of fiddling around with trying to make the oceans absorb more."

Many researchers are convinced that with the impetus of increasingly stringent carbon restrictions, CO₂ capture and disposal technology will play an important role in the future. They also believe that the technology can be improved to control costs. "Off-the-shelf techniques are far from being optimized," said Amit Chakma, dean of engineering at the University of Regina in Saskatchewan, Canada.

CO₂ disposal has been performed for reasons not related to global warming concerns. For example, natural underground CO₂ fields in the United States have been tapped to aid oil companies in oil recovery operations. The gas is piped to on-shore oil fields, where it is injected into underground reservoirs and replaces recovered oil. Sam Holloway, a geologist at the British Geological Survey in Nottingham, observed, "This has been an everyday operation for years. These operations are not considered to involve any undue risks to man or the natural environment." However, he agrees with Lashof that more research needs to be done on CO₂ migration paths in geologic formations.

According to Chakma, CO₂ disposal into underground formations, such as depleted oil and gas reservoirs, is not unproven technology. Several commercially viable projects for enhanced oil recovery have been operating throughout the United States for many years. Chakma estimates that the storage capacity of oil and gas reservoirs already depleted in the United States is sufficient to store up to 2870 million metric tons of CO₂.

Techniques available for capture and disposal of CO2 are expensive to use, however. According to Howard Herzog, principal research engineer at the Massachusetts Institute of Technology (MIT) Energy Laboratory, costs can range from $30 to $90 to avoid the release of 1 ton of CO₂ to the atmosphere (2).

Seabed disposal in the North Sea
The largest CO₂ seabed disposal project, initiated in September 1996 (1), is Statoil. It has been pumping CO₂ into a 32,000-square-kilometer (km²) aquifer. This porous, salt water-filled sandstone formation lies 800 m below the floor of the North Sea. The amount of CO₂ injected into the aquifer - 1 million tons per year - represents about 3% of Norway's total emissions. Natural gas extracted from the field reservoir contains 9.5% CO₂, which must be reduced to about 2.5%, the specified CO₂ content limit for commercial natural gas. Waste CO₂ would normally be released into the atmosphere, but because the contaminant already has to be removed from collected natural gas to meet commercial fuel specifications, additional costs for pumping CO₂ into the aquifer are relatively small. Total commercial gas production costs are raised by about 1%, a small price to pay compared with carbon taxes that would be levied on any uncontrolled CO₂ emissions.

Off the coast of Norway, CO₂ is being separated from natural gas and pumped into a huge aquifer 800 meters below the ocean floor. Other companies are considering this type of disposal to prevent climate damage. (Courtesy Statoil)

In Norway, a carbon tax has been in effect since 1991. According to Olav Falk-Pedersen of Kvaerner Water Systems in Sandefjord, Norway, this carbon tax "motivated the Norwegian oil and gas companies to study new methods and technologies for reduction of the total CO₂ emissions." The carbon tax internalizes estimated costs of climate damages associated with atmospheric release of CO₂ and is equivalent to $55/ton of emissions. In Alberta, Canada, 20 land-based, acid-gas (consisting of CO₂ and H₂S) reinjection projects, the first started in 1989, are in operation. However, according to Chakma, "No one is contemplating reinjection of CO₂-only gases in Alberta, because it is not required by law."

Erik Lindeberg, senior scientist of the Continental Shelf Institute Petroleum Research group in Trondheim, Norway, considers off-shore underground disposal of CO₂ from gas fields a permanent and safe solution. In a 1993-95 study that he carried out for the European Union, Lindeberg estimated the capacity of North Sea aquifers alone would be large enough to take up all CO₂ emissions produced at European Union power plants for the next several hundred years (3).

Lindeberg said his model calculations show that it will take more than 10,000 years before "even minor amounts of CO₂" escape from the aquifer and make their way up to the sea floor-ocean interface. In the event of an undetected fracture 8000 m from the injection well, Lindeberg estimates that it will take approximately 500 years before stored CO₂ reaches the fracture. In a worst-case scenario, 20% of the sequestered CO₂ might then escape during the following 1500 years. Thomas Palm, an employee of the Bellona Foundation, a Norwegian environmental organization, agreed and said, "If you accept that natural gas is produced and used, then disposing of CO₂ is the best way to control emissions."

Statoil plans to expand the use of disposal technologies as part of its program to curtail emissions of CO₂. In September, Statoil announced a new program for cutting CO₂ emissions by 30% over a 10-year period. In part, this will be accomplished by capture and disposal of CO₂ emissions from land-based plants used for natural gas processing and compression.

CO₂ disposal is also being considered by a consortium, including Exxon and the Indonesian state-run oil company, Pertamina, at the Natuna off-shore gas field. CO₂ will be injected into an aquifer located deep beneath the South China Sea. The Natuna gas field, 375 miles east of Singapore, is one of the largest in the world. If the project is implemented, 100 million tons of CO₂ would be disposed of annually.

Expensive power plant CO₂ removal
In addition to imposing carbon taxes, Norway is the first country to require CO₂ capture and disposal from land-based, gas-fired power plants. Naturkraft, which wants to build Norway's first two gas plants, recently announced plans to develop a CO₂ capture technology. However, unlike the situation at Statoil, where project economics favor injecting separated CO₂ into deep, off-shore aquifers, emissions capture from power plants can be expensive.

When oil prices were high in the late 1970s and early 1980s, CO₂ was routinely captured from U.S. power plants for use in enhanced oil recovery operations. Most of these operations were shut down when prices dropped in the mid-1980s. Some CO₂ is still recovered from power plant emissions for use in food industry market applications and for soda-ash production. Recovery involves the use of chemical absorption processes that consume significant amounts of energy. "In the power sector, carbon dioxide removal may increase electricity production costs by 30-100%," estimated Wim C. Turkenburg of the Department of Science, Technology, and Society at Utrecht University in The Netherlands.

Other CO₂ recovery processes have been considered, including membrane separation, cryogenic fractionation, and adsorption using molecular sieves; but all are even less energy efficient and more expensive than chemical absorption. As a result, power companies tend not to be enthusiastic about CO₂ disposal techniques. James Smithson, an employee of the Illinois Power Company, said, "None of the different strategies seems to be cost-effective." Robert Kane, the program manager of the Office of Planning and Environmental Analysis Global Climate Change program at the U.S. Department of Energy (DOE), is more optimistic. "While costs and energy requirements for current capture processes are high, the opportunities for significant reductions exist, since researchers have only begun to address these needs," he said. Current DOE funding for research in this area is $1.6 million annually.

According to Perry Bergman, an engineer with the DOE Power and Engineering Systems Division, the storage potential of deep aquifers in the United States is between 5 billion tons and 500 billion tons of CO₂. Annual U.S. power plant CO₂ emission is about 1.7 billion tons. Bergman believes that 65% of CO₂ captured from U.S. power plants could possibly be injected directly into deep aquifers without any need for transport through long pipelines.

Safety issues need to be considered, say critics who cite the natural disaster that occurred in 1986 at the volcanic crater lake, Nyos, in Cameroon. A large-scale release of CO₂ killed more than 1500 people and all animal life up to 14 km from the crater site. Although the release of CO₂ from Lake Nyos bears little relation to scenarios involving injection of CO₂ into an aquifer - the mechanisms completely differ - the illustration serves to flag dangers of potential large-scale CO₂ releases and the need to anticipate possible flow rates in plausible scenarios. Liability questions would have to be solved before power companies would embark on large-scale disposal, said Herzog.

Direct ocean disposal scrutinized
Direct ocean disposal presents a possibly safer alternative, at least to humans. Large and natural potential CO₂ reservoirs, oceans are considered as sites for direct disposal, but only theoretical studies of ocean disposal have been performed.

"The amount of carbon in the oceans is 60 times greater than in the atmosphere. That means even if you inject flue gases from the world's power plants, the overall concentration in the oceans would barely change," explained Herzog. Model calculations show that most CO₂ emitted today will eventually end up in the oceans anyway. However, the ocean's absorption of gas occurs very slowly. "It takes about 1000 years, too long to even out the quick increase of carbon dioxide in the air," explained Eric Adams, a senior research engineer at the MIT Energy Laboratory. However, by pumping the gas directly into the sea, the natural process could be accelerated, mitigating the rise of CO₂ in the atmosphere.

Several different disposal techniques have been studied, including injection of CO₂ by pipeline and releasing it from a ship, either as dry ice blocks or as a liquid poured from a towed pipe (see Figure 1) (4). The most significant anticipated environmental impact is lowering the pH in surrounding waters. Although skeptical of the approach in general, Woodwell is not particularly concerned about possible pH changes. "The oceans are pretty well buffered," he said.

FIGURE 1

Options for direct ocean disposal of CO₂

At the Energy Laboratory, David Auerbach and Jennifer Caulfield simulated CO₂ injection and estimated possible marine-life impacts. They showed that the ocean's natural pH of 8 can be lowered to as much as pH 4 near the injection point. Alternative designs of injection devices, which disperse CO₂ as it dissolves, can be tested to ensure that biological impacts resulting from changes in ocean pH are minimized.

The first direct ocean disposal field experiment is scheduled to take place in 2000, possibly at a site along the Kona coast of Hawaii. The project will be conducted in collaboration with MIT, the Japanese Research Institute of Innovative Technology for the Earth, and the Norwegian Institute for Water Research.

TABLE 1. Costs of different capture and disposal options

Dumping CO₂ into the ocean also faces international and national legal and regulatory hurdles. Judith Kildow, a professor for ocean policy at MIT, believes that the London Dumping Convention most likely would have to be amended if CO₂ were placed in the oceans, even though it does not explicitly deal with CO₂.

She added that in 1972, the United States passed the Ocean Dumping Act, part of the Marine Protection, Research and Sanctuaries Act. This legislation forbids off-shore disposal of any kind without special permits. Bill Eng of EPA indicated that a section of Title 40 of the U.S. Code of Federal Regulations requires parties considering ocean dumping of CO₂ "to determine whether the discharge causes unreasonable degradation of the marine environment." Yet to define "unreasonable degradation," further studies of the effect of pH change on the marine environment must be undertaken.

Many scientists and environmentalists remain skeptical about ocean disposal. "How do we know that the ocean really is a permanent sink for CO₂?" asked Kelly Sims, science policy director of Ozone Action in Washington, D.C. Voicing related concerns, Ronald Prinn, director of the Center for Global Change Sciences at MIT, said, "I would like to see more study on the stability of ocean currents." Mick Follows, an MIT ocean modeler, agreed. "I am skeptical about CO₂ disposal. I am not convinced that a rapid injection would be safe for the ocean environment. How soon the injected CO₂ comes back into contact with the atmosphere depends on where you put it. That requires a good understanding of ocean circulation and mixing processes. Yet many aspects of this are not known, especially the [time-dependent] variability of ocean currents," he said.

Herzog believes that within the next 10 years, capture and sequestration will be viable only in niche applications, such as oil and gas operations at the Sleipner West field, or for fulfilling a commercial need for CO₂, such as in enhanced oil recovery operations. However, over the next 30 years, he believes, "We may see most new power plants include CO₂ control technology, just as today's power plants control SO₂, NO_x, and particulate."

CO₂ emissions produced at fossil-fueled power plants can be captured using commercially available absorption technology such as this CO₂ recovery plant in Shady Point, Okla. Widespread near-term use is uncertain because this technology can be expensive. (Courtesy ABB Lummus Global/AES)

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