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Carbon Capture and Storage: A Fresh Look at Storage and Other Issues

New research concludes we've got space to store carbon, but other issues remain.
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New research concludes we've got space to store carbon, but other issues remain.

The End of Coal-Fired Electricity?

By some reports (see here and here), the U.S. Environmental Protection Agency's proposed new carbon pollution rule heralds in the new age of natural gas while sounding the death knell for coal-fired power plants. After all, the rule sets limits on emissions of carbon dioxide (CO2) from new power plants in line with emissions rates from modern, combined-cycle natural gas plants but far lower than anything today's standard coal-fired plant can achieve. Among the many critics of the rule was the National Mining Association's chief executive and president, Hal Quinn, who stated that the proposed "standards would deliberately push America to abandon coal, its most abundant and reliable energy source in favor of costlier fuels."

Certainly the new rule does not augur well for new coal-fired electrical generation in the United States, but, regardless of what you might think of the fuel, it by no means shuts the door on coal. That door is open just a bit, and one way the coal industry can squeeze through it is to promise to join up with the technology that's known as carbon capture and storage (CCS) down the line. In fact, six of the 15 coal-fired plants currently holding permits have already walked into the opening afforded them in the rule by incorporating into their business plans the means to capture the CO2 from burning coal and storing that CO2 (most likely deep below the Earth's surface) to prevent it from being emitted into the atmosphere. Once the carbon is captured and stored, burning coal looks a lot more like a low-carbon energy source.

The question is: Will CCS be able to come along? That question can be parsed in a lot of ways, but first and foremost technical issues around storage must be resolved.

Delving into the Storage Piece of CCS

The action part of CCS is two-fold -- capture and storage -- and the technology piece for the capture half (in which CO2 is separated from the fuel either before or after combustion) seems to be pretty well in hand. (See here and here.) For the storage part you need to figure out good locations to put the CO2 where it will stay sequestered for good, and you've got to be sure there's enough space in those places for all the CO2 destined for storage.

For the most part the question of where seems pretty well settled: the most accessible, safe and reliable place for the CO2 is deep underground in a favorable geologic formation. And of the formations that are being considered, a top candidate is deep saline aquifers (permeable formations saturated with a brine) with some sort of cap rock that separates it from potable groundwater. CO2 is readily dissolvable in brines, and brines mixed with CO2 tend to be more dense than the surrounding brine making it less likely to migrate upward out of the aquifer.

But what's not so clearly settled is the CO2 storage capacity of these aquifers. Are they large enough to absorb all the CO2 we might produce in the coming decades? Investigators who have looked at this question have come up with wildly different conclusions. Capacity estimates for saline aquifer storage in the United States range from as little as five billion metric tons of CO2 to as much as 20,000 billion metric tons. (For comparison, the United States currently produces about 5.7 billion metric tons of CO2 per year of which about 2.2 billion metric tons comes from electric power plants.)

That's a pretty large range if you're planning long-terms investments in new energy infrastructure. If the upper estimate is right, it would mean we've got enough storage to last us about 10,000 years -- for all intents and purposes forever. But the lower estimate would only give us a couple of years, making CCS a poor investment indeed.

A paper published last week in the Proceedings of the National Academy of Sciences by Michael Szulczewski of the Massachusetts Institute of Technology and colleagues attempts to provide a more definitive answer to the storage capacity question.

Sussing Out Saline Aquifer Storage

Szulczewski et al looked at the storage capacity of 11 large saline aquifers in the United States. Their approach, more sophisticated than most previous investigators', used a fairly detailed fluid dynamics model to simulate the injection, flow, and subsequent trapping of the CO2 in each of the 11 aquifers. They found that there are two critical factors that determine an aquifer's viability to store CO2. One, not surprisingly, is the aquifer's overall capacity. But also important is the rate at which the aquifer can take up the CO2 without belching some of it back out.

If the combination of factors limits uptake of CO2 such that it is not able to keep pace with the rate of CO2 produced by our fleet of power plants production, CCS becomes of limited utility even if there's lots of remaining storage capacity.

Michael Szulczewski et al examined 11 aquifers, chosen for their large size and good characterization, to assess their storage capacity for CO2. This map shows the locations of the aquifers and their storage capacities for an injection period of 100 years. (From "Lifetime of Carbon Capture and Storage As a Climate-Change Mitigation Technology," PNAS, April 3, 2012, Vol. 109 No. 14 5185-5189)

The results of the Szulczewski et al analysis are pretty encouraging if you're rooting for coal. They estimate that the capacity of the 11 saline aquifers they considered is adequate to stabilize emissions at current rates for the next 100 years.

Of course, before popping the corks, the coal folks have other issues to worry about when it comes to CCS. One is certainly societal -- I can imagine there are lots of people who'll think that CCS is a fine idea until they find out that the CO2 is going to be stored in an aquifer that sits below their house.

And then there's the economics. The CCS process doesn't come free, and adding it to the back end of coal-fired power plants can significantly add to their costs. The National Academy of Science's report on America's energy future estimates that one would need a price on carbon emissions of well in excess of $50 per ton to make a coal plant with CCS cost competitive with a combined cycle natural gas plant without CCS. Carbon emissions in the European Union are currently trading in the $9-10 per metric ton range. You'll need a considerably higher price to get CCS to market.

Here's a who'd-a-thunk thought: the coal industry ends up being the constituency pushing for a higher price on carbon.

Crossposted with TheGreenGrok.