In the next year or two, the Kona Coast of the island of Hawai’i will be the site of an experiment intended to help scientists gauge the feasibility of a technique called ocean sequestration of carbon. Among many policy makers and scientists addressing the question of global warming, this approach is regarded as one of the most promising means of reducing levels of carbon dioxide in the Earth’s atmosphere.
Sequestering carbon involves first removing carbon dioxide either from the atmosphere or from industry smokestacks or other industrial processes. Then, the carbon is placed in a medium where it will be prevented from entering the atmosphere for an indefinite period. The hope is that by sequestering carbon emissions, the burden of atmospheric carbon dioxide will be reduced or, at the very least, kept from increasing to levels that would have a severe impact on Earth’s climate.
While curbs in industrial processes and fossil-fuel consumption would achieve results better than any of those that can be achieved through sequestration, the idea behind sequestering carbon is that a technological fix to the problem might have a greater chance of success than efforts focused on conservation or development of renewable energy resources.
Several media are thought to offer long-term storage prospects for carbon dioxide. Already, Norway is experimenting with injections of C02 into depleted oil fields off its coast. Other approaches involve increasing the amount of carbon held in soil through special agricultural techniques, injecting carbon into geological formations (such as abandoned coal mines and saline aquifers), developing bioengineered organisms to eat C02, fertilizing the ocean with iron to spur growth of phytoplankton, and – this is where the Hawai’i experiment comes into the picture – increasing the levels of carbon dioxide held in the ocean.
All these approaches are being investigated in a series of experiments authorized under the Kyoto protocol of 1997, which was intended to address ways of mitigating long-term climate change.
The Keahole Project
The carbon sequestration trial to be conducted in Hawai’i will be based out of the facilities of the Natural Energy Laboratory of Hawai’i Authority (NELHA) at Keahole Point, on the Big Island. In charge of the experiments is the Honolulu-based Pacific International Center for High Technology Research (PICHTR), a private non-profit research entity. Collaborating with PICHTR on the project is the University of Hawai’i’s Hawai’i Natural Energy Institute (HNEI). According to PICHTR, an application to conduct the experiments will be made to NELHA within the next few months.
The experiment involves injecting carbon dioxide into seawater at a depth of about 3,000 feet at different flow rates. The C02 will be forced through insulated 1- to 2-inch pipeline from the shoreline at Keahole to the injection depth, where it will be released into the seawater through a perforated nozzle.
The carbon dioxide will be at the same temperature and pressure as the surrounding sea water, according to Stephen Masutani of the Hawai’i Natural Energy Institute. While carbon dioxide exists as a gas at the Earth’s surface, at the pressures found at ocean depths of about 1,500 feet and greater, carbon dioxide becomes liquid.
The actual experiment will run no more than 40 hours over a two-week period in the summer of either 2000 or 2001, Masutani told Environment Hawai`i. The carbon dioxide used in the tests will not be removed from any industrial process, but instead will simply be purchased from a local supplier, Masutani said.
Impacts?
Anticipated environmental impacts from the experiment are minimal, say the experiment’s designers. According to information on the web site developed by the project scientists ([url]www.co2experiment.org[/url]), the experiment will not affect fishing in the area. Only during deployment and removal of the small delivery pipeline will the ocean area next to Keahole Point be restricted, for no more than a few days,” the website authors say.
While the small scale and short time frame of the Keahole experiment are unlikely to produce lasting adverse environmental effects, the impact of deep-ocean carbon sequestration on a large scale, such as that required to mitigate any global warming scenario, is largely unknown.
However, a Department of Energy publication, “Carbon Sequestration: State of the Science” (February 1999 draft), suggests some of the potential problems that may arise with deep-ocean projects. Before such projects are undertaken on a large scale, it reports, environmental impacts near the injection point must be detailed, and the long-term, broad-scale impacts on the function of the ocean ecosystem must be understood.”
“The most significant environmental impact,” the report continues, “is expected to be associated with lowered pH [increased acidity] as a result of the reaction of CO2 with seawater. Non-swimming marine organisms residing at depths of about 1000 m or greater [the approximate depth of the Keahole experiment] are most likely to be affected adversely by more acidic seawater; the magnitude of the impact will depend on both the level of pH change and the duration of exposure. The microbial community would also be affected, causing unknown impacts on biogeochemical processes that play a crucial role in the ocean carbon cycle.”
The ocean carbon cycle refers to the process by which carbon circulates from the ocean surface to the ocean floor. Every year, about a third of the carbon dioxide generated by human activity is absorbed by the ocean; without human intervention, about 90 percent of the C02 now in the atmosphere would be absorbed by the ocean over the next 1000 years. Scientists estimate that the ocean already holds some 40,000 gigatons (40,000 billion tons) of carbon dioxide. And, according to the “Carbon Sequestration” report, “the amount of carbon that would cause a doubling of the atmospheric concentration would change the deep ocean concentration by less than 2 percent.”
But natural cycling of carbon will not occur on a time scale to prevent the worst anticipated effects of global warming as a result of human activities that pump unnaturally high levels of carbon dioxide into the atmosphere. Hence, the international plan to address global warming that was developed at Kyoto, Japan, in 1997, includes research into carbon sequestration. According to the Department of Energy, “the goal [of research] is to have the potential to sequester a significant fraction of 1 GtC [gigaton of carbon] a year in 2025 and 4GtC/ year in 2050.”
The ocean can absorb vast quantities of carbon dioxide without severe changes in its pH. The DOE report provides an example calculation of what would occur to the ocean s pH if 1,300 gigatons of carbon were added. Ocean pH levels would decrease 0.3. This pH change is similar to the change that will occur in the surface ocean as a result of doubling the pre-industrial amount of atmospheric C02. The change in surface seawater pH today, from that of preindustrial times, is already 0.1.
Yet even scientists involved in studying the potential promise of carbon sequestration voice caution. As the DOE report notes, “Many people are wary of ocean sequestration, including some authors of this chapter, because it is known that small changes in biogeochemical cycles may have large consequences, many of which are secondary and difficult to predict… At present, we do not have enough information to estimate how much carbon can be sequestered without perturbing marine ecosystem structure and function; obtaining this information is one of the goals of the proposed research.”
The Keahole experiment, Masutani says, is not to prove the feasibility of carbon sequestration to show whether it can be cost-effective, but only to help scientists understand what happens when carbon dioxide is pumped into the deep ocean.
To be sure, the behavior of C02 at great depths is unusual – at least, with respect to its behavior in gaseous form. As scientists from the Monterey Bay Aquarium Research Institute discovered when performing somewhat similar experiments off the coast of California, carbon dioxide becomes an ice-like solid when it mixes with seawater. Specifically, water molecules form a lattice like structure, called a hydrate, around molecules of carbon dioxide on the surface of the carbon dioxide. The chemical reaction that generates hydrates produces substantial heat, and also causes salt and other minerals to precipitate out of the seawater (there is no room at the hydrate inn for minerals in solution).
In theory, the hydrates are denser than seawater. In practice, however, they rise through the water column, since not every “room” in the hydrate inn is occupied.
The formation of hydrates is the subject of much ongoing research. Some scientists have noted that the volume of hydrates is much larger than what the theory projected – the result, they speculate of “a large number of vacancies” in the lattice cages formed by the water molecules.
Some scientists had hoped that injection of carbon dioxide at great ocean depths would result in the formation of permanent hydrates, which would keep the carbon dioxide locked up forever. However, hydrates have been discovered to be transient, forming, disappearing, and re-forming quickly on the surface of liquid carbon dioxide. While the dissolution of carbon dioxide in seawater may be slowed by this process, eventually the C02 dissolves or interacts with water to form bicarbonate (HCO3) and carbonate (C03) ions and carbonic acid (H2C03). In this manner the acidity of seawater is increased by the addition of carbon dioxide, as the DOE report notes.
The carbon sequestration scenarios generally suppose that C02 will be removed from natural gas used as fuel in power plants sited along coasts with quick access to ocean depths or possibly removed from the flue gases of plants that burn coal or other fossil fuels. Masutani and other researchers from Hawai’i, however, have noted that “for C02 direct removal measures to he effective, a fraction of anthropogenic carbon dioxide in excess of the power industry contribution (around 30 percent) will have to be processed.”1 Just how much will the removal of C02 from the exhaust gases of these plants cost?
Several efforts have been made to calculate the added cost to producers and consumers of electricity from plants where carbon is removed. In 1992, researchers at PICHTR, including Masutani, along with CM. Kinoshita of the Hawai’i Natural Energy Institute, found that removing carbon from a 562 megawatt natural-gas fired power plant would raise the cost of electricity about 28.4 percent. The price to consumers would rise about 1.66 cents per kilowatt hour, they determined.2
That estimate is low in comparison to what is reported in the Department of Energy report on carbon sequestration. There, two more recent studies are cited. One, conducted for the International Energy Agency in 1998, suggests increased kilowatt-hour costs of from 2.5 cents to 21.5 cents. The other, done by H J. Heizog, determined that the increase would be from 2 to 3 cents per kilowatt hour.
On one point, there would seem to be little disagreement among scientists: carbon sequestration will be expensive. “Reducing consumption of fossil fuels really does appear to be the most effective means to forestall the peak values of atmospheric C02,” Masutani told Environment Hawai’i. Whether or not people will have the resolve to make the necessary sacrifices remains to be seen.
1. G.C. Nihous, S.M. Masutani, L.A. Vega, C.M. Kinoshita, “Projected Impact of Deep Ocan Carbon Dioxide Discharge on Atmostpheric CO2 Concentrations,” Climate Change, June 1994, p. 243.
2. Y. Mori, S.M. Masutani, G.C. Nihous, L.A. Vega, and C.M. Kinoshita, “Pre-Combustion Removal of Carbon Dioxide from Natural Gas Power Plants and the Transition to Hydrogen Energy Systems,” Journal of Energy Resources Technology, September 1992.
Volume 10, Number 2 August 1999