By Bill Chameides
If you see a future for coal, it probably includes carbon capture and storage (CCS).
A lot of questions and imponderables face us as we look toward a future low-carbon economy. One relates to the future of the fossil fuel known as "King Coal" -- the stuff that has fueled the economy of Appalachia, in much the same way King Cotton in its heyday ruled the South's. (More on King Coal here and here.)
|CCS: Shorthand for carbon capture and storage process, also referred to as carbon capture and sequestration. (See map of potential geologic storage sites.)
|'Clean' Coal: Coal that uses the CCS process to generate energy. It is called "clean" because very little CO2 is emitted. Some argue that this is a misnomer because even with CCS, coal is far from "clean" (e.g., there are environmental issues associated with coal mining, coal ash storage, and emissions of air pollutants).|
|Saline Aquifer: A likely place to store CO2 because:|
There's lots of coal in the ground for the taking, but it is, if I may come clean about the stuff, the dirtiest of the fossil fuels. Burning coal generally leads to about 35 percent more carbon dioxide (CO2) emissions than burning petroleum and about twice as much (CO2) emissions as burning natural gas. That would suggest that coal does not figure into our low-carbon future.
But there are lots of contrarians. They argue that we just cannot do without coal and that If we use CCS (that is, if we capture the CO2 from the effluent of burning coal and then store the CO2, say, deep below the Earth's surface), coal suddenly becomes a low-carbon source of energy.
A Potential Game Changer
CCS is potentially another one of those game changers. Without CCS, an abundant and relatively cheap source of energy is off the table. And with coal off the table, powerful political groups -- the coal industry and coal workers -- are suddenly threatened by the march to a low-carbon economy. So, not surprisingly, there's a big push for CCS with the government providing incentives, subsidies, and research and development dollars.
Source: Carbon Dioxide Capture and Storage, p. 199 (IPCC, 2005; Cambridge University Press, UK. pp 431.)
But alas, as is often the case, there are some nagging problems that cloud up the "clean" coal crystal ball:
- there's the issue of the other (in many cases significant) environmental insults that "clean" coal can engender (see sidebar);
- there are NIMBY issues: it is likely that lots of folks will favor CCS in principle, but oppose it when storage locations are proposed for their neighborhood; and finally
- there's the question of permanence -- will the CO2 leak out, and if it does, what happens then?
It's that last issue that was addressed in a new paper by Mark Little and Rob Jackson, my colleagues at Duke, and published last week in the journal Environmental Science & Technology.
Little and Jackson investigated what would happen if CO2 placed in a saline aquifer escaped into overlying groundwater. The premise for their study was the following:
- While saline aquifers are excellent places to trap CO2, they are not perfect [pdf]. Some escape of CO2 is probably inevitable (see graphic below); and
- Some of that escaped CO2 would likely find its way into overlying freshwater aquifers used for drinking water, where it may make that drinking water ... well, undrinkable.
Will it? To answer that question, the authors gathered sediment samples from freshwater aquifers that lie above saline aquifers that could be used as CO2 storage sites. They then exposed these samples to water solutions with enhanced concentrations of dissolved CO2. The authors found that the samples with enhanced CO2 showed increased acidity, as would be expected because dissolved CO2 is a weak acid (i.e., carbonic acid).
But they also found that the enhanced acidity led to dissolution of metals such as uranium, cobalt, cadmium and iron. In some cases the concentrations of metals increased by two orders of magnitude and in some cases the increased concentrations exceeded the primary or secondary safe drinking-water standards, meaning these concentrations could pose a health risk and/or cause cosmetic effects (like skin discoloration) or aesthetic effects (like taste or odor).
The authors do not suggest that CCS should be abandoned as a result of their findings -- just that there should be careful monitoring of groundwater in areas where CO2 is being stored. Such monitoring would provide an "early detection of ... leaks" that could presumably lead to remedial action to prevent serious drinking water contamination. Quite reasonable but I suspect that Little and Jackson's results will provide fodder for the NIMBY opponents of CCS when the coal industry comes knocking on their door looking for storage sites. Like I said: the future of "clean" coal is murky.
Source: Carbon Dioxide Capture and Storage, p. 243 (IPCC, 2005; Cambridge University Press, UK. pp 431.)