Disposing of CO, fkom Fossil-Fueled Power Plants Reportfrom a Recent U.K. Meeting
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ost governments now accept t h a t emissions of carbon dioxide will have to be controlled in order to combat global warming. But growing volumes of scientific research-much of it funded by the fossil fuel industry-are highlighting ways in which CO, could be captured from power plant flues, perhaps allowing fossil fuel burning to continue unchecked for centuries. Attendees of a recent conference held in Oxford, England, heard how disposing of CO, by dissolving it in oceans, aquifers, and exhausted oil wells may become a viable option. The conference, staged by the International Energy Agency (IEA), brought together research teams from North America, Europe, and Japan. In 1991 the IEA launched the $2.2 million Greenhouse Gas R&D Program to study techniques for capturing CO, from power plant flues and for disposing of, or utilizing, the product. Based in Cheltenham, England, at the Coal Research Establishment, the program has become a focal point for much of the international research effort on CO, mitigation techniques. This program is dwarfed, however, by Japan’s government-funded Research Institute for Innovative Technology for the Earth, which is investing considerable funds in CO, mitigation. The IEA program, now half way through its three-year term, is funded by 13 countries, including the United States, Japan, and the European Community Commission. In June a meeting was held in Canada to discuss a follow-up program to continue the research effort for three more years. Prkcis articles are reports of meetings of unusual significance, international or national developments of environmental importance, significant public policy developments, and related items. 1282 Environ. Sci. Technol., Vol. 27,No. 7, 1993
BY JULIAN ROSE One of the four areas identified for further study is beneficial uses of waste CO, captured from power plants. CO, is already used as a feedstock for some chemical manufacturing processes, and the program’s aim is to develop industrial chemistry so that CO, can be used to make organic chemicals. But another major component of the second phase of the program will be ocean disposal techniques.
Ocean disposal Ocean disposal is not a permanent solution to the fossil fuelgenerated greenhouse effect. After a few hundred years, the amount of CO, left in the atmosphere is the same whether the gas is released into the air or injected into the oceans: The equilibrium between CO, in the oceans and in the atmosphere is defined by physical laws beyond human control. However, ocean disposal may allow fossil fuels to be burned at high rates for a few generations while alternative energy sources are developed. Of all the disposal options, the oceans represent by far the largest available CO, sink. Figures projected by the IEA program identify substantial CO, storage capacity in aquifers (87 gigatons), exhausted oil and gas wells (125 gigatons], and enhanced oil recovery (4 gigatons). But these figures are dwarfed by the oceans’ 20,000,000-gigaton capacity for absorbing CO,. The deeper layers of the ocean contain < 0.1 kg/m3 CO,, whereas the solubility is roughly 40 kg/m3. In 1990 the United Nations’ Intergovernmental Panel on Climate Change put the dissolved inorganic carbon content of the oceans at 38,000 gigatons; anthropogenic CO, emissions are 6 gigatons per year (measured as carbon). “If all of that carbon were disposed of in the ocean, the yearly in-
crease would be 0.016%,” Dan Golomb of the University of Massachusetts at Lowell told the Oxford conference. However, only 30% of these emissions are from large-scale sources. At ocean depths below 500 m, where the temperature is about 10 “C and the pressure 50 atm, CO, becomes liquid. And at 3000 m it becomes denser than water and sinks toward the ocean floor until it dissolves. “This led early investigators to suggest that CO, be released below 3000 m, so it would form a permanent lake on the bottom of the ocean,” Golomb said. “However, liquid CO, cannot be conveyed to such depths with current pipelaying technology.” Instead, research is now focusing on techniques for introducing CO, at intermediate depths. One option is to dissolve the CO, in seawater before it is injected into the ocean. A pipeline from a coastal power plant could take the dissolved CO, to a depth of about 200 m; the CO, would then descend because of its higher density, forming a plume deep in the sea. Anne Britt Sando and colleagues, from the Nansen Environmental and Remote Sensing Center in Norway, have modeled the behavior of such a CO, plume using a diffusion equation. They considered a 2000-MW gas-fired plant operating for 100years with all the CO, it generated delivered into the ocean. The ocean’s ability to retain CO, is impressive: Sando told the conference that the total release of CO, into the atmosphere, 200 years after the plant ceased operation, would be just 4% of the total CO, produced, assuming average north Atlantic and north Pacific waters. Differing salinity and temperature profiles, however, mean that in the Norwegian Sea 14% of the CO, would reach the atmosphere after 200 years. Another analysis of the residence time of CO, added to the ocean was
0013-936)(/93/0927-1282$04.00/0 0 1993 American Chemical Society
presented by Gilbert Stegen of Science Applications International Corporation (Bellevue, WA). Working with colleagues at the Scripps Institute of Oceanography in San Diego, CA, Stegen uses threedimensional ocean general circulation models to make detailed predictions. The models take into account many biological and chemical aspects of the carbon cycle. As the scientists in Norway did, Stegen modeled the effect of running a single power plant for 100 years-“since fossil fuel reserves are finite, the power plant was assumed to have a 100-year lifetime.” The model was based on the assumption that CO, was captured and injected into the ocean at various depths. Stegen’s team found that, as expected, the long-term increase in atmospheric CO, concentration was determined only by the equilibrium between the ocean and the atmosphere. Four hundred years after the closing of the power plant, only about 13% of the CO, emitted would be released to the atmosphere; the rest would be absorbed in the ocean. But for a power plant releasing directly to the atmosphere, the peak atmospheric concentration-the key factor affecting the scale of global warming-is significantly more than double this level. By injecting the CO, into the ocean at about 600 m, the size of the peak atmospheric concentration can be halved. And the deeper the CO, is injected, the longer the gas takes to return to the atmosphere and the smaller the peak concentration. The results also showed that the North Atlantic would be less effective at sequestering CO, than would the North Pacific. And, unhelpfully for industrialized nations, the Equatorial Pacific is the most effective disposal site.
can drop as low as 3.5 when seawater is in equilibrium with liquefied CO,. Hydrate particles will also be formed by the added CO,, which may interfere with feeding habits and bury organisms on the ocean floor. The other principal drawback is the enormous cost of ocean disposal. The cost of capturing CO, from power plants is estimated to be between $80 and $300 per ton of carbon. The IEA program has settled on a figure of $130 per ton and says this would add 50% to the generating costs of a coal-fired plant. Ocean disposal costs will be considered in the program’s next phase. A recent study by the engineering contractor Bechtel, commissioned
Consequences of ocean disposal A major factor affecting ocean disposal is the potential environmental impact of locally increased CO, concentrations. Golomb said pilotscale releases should be undertaken to find some answers. “The public must be convinced that such releases carry very little risk to the environment, or at least a smaller risk than discharging CO, into the atmosphere,’’ he said. Loading the oceans with CO, will acidify the water. Seawater is normally slightly alkaline, about pH 8. A recent study found that the pH
Alternatives One alternative to ocean disposal would be to dissolve the gas in deep subterranean aquifers, which have capacity for an estimated 8 7 gigatons of carbon. Feasibility studies have been undertaken in the Netherlands that include the identification of suitable aquifers, estimates of CO, storage capacity, and a discussion of the environmental risks. Potential problems include damage to the rock caused by acidification, groundwater pollution, and ground destabilization. Another option is to store CO, in
Of all the disposal opti ons, the oceans represent by far the largest available CO, sink. by British Coal, looked at the costs of piping liquefied CO, from a power plant in the eastern U.K. and delivering it to sites in a North Sea gas field and the Atlantic Ocean. It c o n c l u d e d that p i p e l i n e costs would be up to $1.1million per kilometer. The total added costs, including CO, capture in the power plant and its disposal in the ocean, would be $30-$180 per ton of avoided CO, emissions, the study concluded.
exhausted oil and gas wells. Storage of natural gas in underground reservoirs in order to meet peak demand is already practiced, and CO, is used to enhance oil recovery from active wells. “Disposal of CO, in exhausted gas wells is probably the simplest and the most environmentally acceptable disposal option and for many countries represents the cheapest option,” Bill Ormerod of the IEA program told the conference. Costs have been estimated at less than $32 per ton of carbon. About two-thirds of the carbon generated by combusting oil and gas extracted from reservoirs could be returned to them as CO,, Ormerod said. Total capacity could be 125 gigatons. CO, used in enhanced oil recovery could offer another disposal route, but at no net cost and with the appeal of returning carbon to the reservoirs from which it was extracted. This technique is already practiced i n some parts of the United States using CO, obtained from cheap sources such as underground reservoirs, natural gas processing plants, or fertilizer plants. Unfortunately, the capacity for using CO, in enhanced oil recovery is thought to be only 4 gigatons. A $600,000 study of the use of captured CO, in enhanced oil recovery has been completed by the Alberta Oil Sands Technology and Research Authority. The study targeted the capture of 50,000 tons per day of CO, for use in oil fields in Alberta a n d Saskatchewan. This amount of CO, is equal to 1 2 % of the current emissions from both provinces and is less than half of the amount of CO, used in the United States for oil recovery. The most bizarre of the proposed techniques mentioned at the conference was to construct massive spheres of solid CO,. The spheres, 400 m in diameter with 2 m of insulation, would take 4000 years to sublime completely and would have a similar effect on damping atmospheric CO, concentration as ocean disposal would. However, if fossil fuels are to remain in largescale use, a 500-MW coal-fired power plant, for example, would need one of these giant spheres constructed each decade.
Julian Rose is a London-based freelance writer specializing in environm en tal an d technological issues. He was previously editor of Environmental News. Environ. Sci. Technol., Vol. 27, No. 7, 1993 1283
