Engineering design of the first industrial-scale project carbon sequestration project in Scotland, to be run by BP, ConocoPhilips, Shell and Scottish and Southern Energy.
Credit: British Petroleum
SYDNEY, 8 August 2006 - Carbon dioxide emissions from coal burning, a large contributor to global warming, could one day be safely stored in deep-sea sediments, researchers argue.
According to a Harvard University report published this week in the U.S. journal, Proceedings of the National Academy of Sciences, deep-sea sediments could provide a virtually unlimited and permanent reservoir to store excess carbon dioxide (CO2).
The total annual CO2 emissions for the United States could be stored in an area beneath the sea floor of just 80 square kilometres, according to lead authors Kurt Zenz House and Daniel P. Schrag at Harvard.
"Supplying the energy demanded by world economic growth without affecting the Earth's climate is one of the most pressing technical and economic challenges of our time," they said.
"If fossil fuels, particularly coal, remain the dominant energy source of the 21st century, then stabilising the concentration of atmospheric CO2 will require developing the capability to capture CO2 from the combustion of fossil fuels and store it safely away from the atmosphere," they concluded.
Under industry proposals for geosequestration (as CO2 burial is known), specially designed plants attached to power stations would capture emissions as they are generated and send them to the seabed through a system of pipes.
The CO2 would then be injected into 300 metre thick sediment in water of no less than 3,000 metres in depth, where it should permanently settle regardless of geomechanical perturbations, the report said.
"At the high pressures and low temperatures common in deep-sea sediments, CO2 resides in its liquid phase and can be denser than the overlying pore fluid, causing the injected CO2 to be gravitationally stable," the authors argued.
The CO2 will slowly dissolve into the pore fluid of the sediment and sink. Further transport can then only be accomplished by molecular diffusion over millions of years, the researchers said.
Several other ideas have also been proposed for the sequestering of captured CO2, such as storing it in terrestrial geologic formations, or injecting it directly into the open ocean.
However, terrestrial reservoirs would have to be capped with impermeable layers of rock and monitored for thousands of years, with the buoyant CO2 migrating upward through any available conduit or leak, they said.
Meanwhile, injecting CO2 directly into the ocean could have an adverse effect on marine ecosystems. In addition, ocean currents would mix the injected CO2, causing a large fraction to eventually be released into the atmosphere.
CO2 injected into deep-sea sediments, on the other hand, would be gravitationally stable, and unlikely to escape, the authors argued. If field experiments confirm that the system performs as expected, CO2 could be stored in sediments without any investment in monitoring or verification technology.
Plants that capture and sequester emissions have already been proposed for Scotland, California and the Australian state of Queensland, with the Scottish plant likely to be operational by as soon as 2010.
But injecting CO2 into deep-sea sediments is still a long way off, with mechanical difficulties likely to be uncovered with further study and experimentation, the report said.
Also, as a volume of pore water roughly equal to the volume of injected CO2 would be forced up into the ocean from sediments, the implications of this will have to be considered, the authors cautioned.
