One of the ways to reduce greenhouse gas emissions is to capture carbon dioxide at its source, when it is emitted from power plants for example, and to store it in other places, such as depleted oil and gas reservoirs or even the ocean after liquefaction. But, according to Youxue Zhang, a professor at the University of Michigan, there are pitfalls in this last plan. If the carbon dioxide is not injected deep enough, it can come back to the surface and return to the atmosphere, which is obviously not the desired goal. But, even worse, the liquid-to-gas conversion could happen too suddenly, which could cause a potentially dangerous eruption. So Zhang has developed a model which shows that liquid CO2 would have to be injected to a depth of between 800 and 3,000 meters to keep it from escaping from the ocean.
Here is the description of Zhang's model.
Small-scale ocean experiments have been done to investigate how the carbon dioxide (CO2) actually would behave, but such experiments are too costly and time consuming to carry out under a wide range of ocean conditions.
However, a new theoretical model developed by University of Michigan researcher Youxue Zhang can be used to explore the fate of CO2 injected into oceans under various temperature and pressure conditions. Zhang's model shows that liquid CO2 would have to be injected to a depth of at least 800 meters (about a half mile) and possibly as much as 3,000 meters (nearly two miles) to keep it from escaping.
In fact, as soon as liquid CO2 reaches a depth of 300 meters, it can become a gas. One solution is to store the gas in droplets as small as possible.
"Droplets injected to a depth of 800 meters will rise, but if they are small enough they should dissolve completely before reaching the liquid-gas transition depth—assuming everything works perfectly," said Zhang, a professor of geological sciences.
"An even safer injection scheme would be to inject into a depth of more than 3,000 meters, where CO2 liquid is denser than seawater and would sink and dissolve," Zhang said.
The idea of injecting liquid CO2 in depleted oil reservoirs or deep inside the ocean is not new and has been investigated for several years now by organizations such as the Carbon Capture and Sequestration Initiative at the MIT.
For example, below is a diagram showing the dispersion of a simulated CO2 injection at 1750 m depth off the East coast of the United States (Credit: Howard J. Herzog, Ken Caldeira, and Eric Adams). "The contours represent the concentration of the injected carbon relative to the concentration in the grid cell containing the injection point. Results are for 20 years after the start of injection and at the injection depth, showing the CO2 is advected southward with the countercurrent running beneath the Gulf Stream."
This diagram has been extracted from Carbon Sequestration via Direct Injection by Howard J. Herzog, Ken Caldeira, and Eric Adams, in Encyclopedia of Ocean Sciences, eds. J.H. Steele, S.A. Thorpe, and K.K. Turekian, Vol. 1, pp 408 - 414, Academic Press, London, UK, September (2001).
Youxue Zhang's research work has been published by Environmental Science & Technology underr the name "Fate of Rising CO2 Droplets in Seawater" (Vol. 39, Issue 19, Pages 7719 - 7724, October 1, 2005). Here is a link to the abstract.
It seems to me that many more studies will be necessary before massively injecting CO2 into the ocean -- especially if the process is inefficient and dangerous for humans and the environment.
Sources: University of Michigan news release, November 3, 2005; and various web sites
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