Two scientists at Lawrence Livermore National Laboratory believe they’ve devised a way to use the ocean for carbon dioxide sequestration that is different from the method currently envisioned. The proposed approach has received favorable reviews from marine scientists – including some ocean sequestration critics.
“Until recently, many scientists have considered the ocean the best place to sequester carbon dioxide,” says energy journalist Lisa Kosanovic, who has written an article on the new process, developed by Livermore’s Greg Rau and Ken Caldeira.
Kosanovic quotes Howard Herzog, a principal research engineer at the Massachusetts Institute of Technology. “Everybody loves to talk about the ocean,” says Herzog, who has written extensively on the subject. “But if you look at research money now, there’s more emphasis on geologic sequestration.”
Sequestration is the process of capturing carbon dioxide (CO2) produced when fossil fuels are burned and depositing it in natural storage spaces so that it does not build up in the atmosphere.
The ocean has been the favored site because it is large and can hold more than other natural reservoirs, such as geologic formations. But opposition to ocean sequestration has increased steadily because of its potential for environmental damage. Ocean sequestration entails introducing CO2 more than 650 feet below the surface, usually via a pipeline.
Kosanovic cites the October 12, 2001 issue of Science, which featured two articles warning of the dangers of ocean sequestration. One argued that direct injection of CO2 into the ocean could harm deep-sea animals. The other predicted that widespread fertilization of phytoplankton – organisms that live on the ocean surface and convert CO2 to organic carbon – thereby enabling the ocean to absorb CO2 from the atmosphere – would certainly alter the cycles of the earth and ocean, and might even worsen global warming.
“The known consequences and uncertainties of ocean fertilization already far outweigh hypothetical benefits,” wrote Paul Falkowski of Rutgers University and Sallie Chisholm of the Massachusetts Institute of Technology.
The Lawrence Livermore technique, called the Rau-Caldeira method, “seems to be better” for the environment than the other methods of ocean sequestration, according to Falkowski.
The method mimics the earth’s natural carbon cycle, but accomplishes in one day what would take about 6,000 years. In the carbon cycle, CO2 in the atmosphere is dissolved in the surface of the ocean, and forms a weak acid that reacts with mineral carbonate (rocks, for example) to produce bicarbonate ions in solution (HC O3-). Over several hundred years, the carbon is transferred from surface water to the deep ocean, and in hundreds of thousands of years it is buried in the ocean floor as carbonate sediment.
The Rau-Caldeira method would bypass nature’s first 6,000 years and produce bicarbonate-rich seawater right at the power plant. The reaction would take place in a large vessel, where stack gas would flow through a pile of crushed limestone that is continuously wetted with seawater.
The resulting effluent would still contain some CO2, but would be rich in bicarbonate ions. In order to prevent the CO2 from degassing (returning to a gas and escaping), the effluent would either have to be injected into the ocean, or degassed under controlled conditions. The degassed CO2 would then be recycled into the reactor vessel.
Rau and Caldeira estimate an energy penalty of 5 percent to cover the cost of crushing limestone and pumping the necessary water (4.5 million gallons per MW). But their pumping estimate doesn’t take into consideration pressure drops and other losses, and it also does not take into consideration the energy that would be required to inject the effluent into the ocean.
Rau concedes that he and Caldeira lack engineering expertise and cannot answer important questions about their system (e.g., residence time and conditions inside the reactor). This deficiency, coupled with the method’s large water requirements, has prevented them from getting research money from the Department of Energy, he says.
But Rau and Caldeira are undaunted. Though large water requirements mean high water costs, they say the method is still cost-effective for coastal power plants ($12 to $40 to sequester 1 metric ton of CO2, compared to $90 to $180 for direct injection). And Rau says once they convince the DOE that their work is important, they will get the money to hire engineers who can fill in the blanks. “We think there’s just a lack of understanding,” says Rau. Even if he’s right, they’ll still have to answer environmental questions.
Brad Seibel, a postdoctoral researcher at the Monterey Bay Aquarium Research Institute in California and co-author of the Science article about direct injection, says the Rau-Caldeira method could still harm deep-sea animals because it would increase the partial pressure of carbon dioxide in the ocean just as direct injection would. Since deep-sea animals replenish nutrients in shallow water, this would harm fisheries and tourism.
“We’re tinkering with enormous processes that we barely understand,” says Seibel. “We would be much better off doing everything within our power to reduce consumption of fossil fuels in the first place.”