Carbon capture and storage (CCS) has mostly received attention as a way to limit the impact of burning coal, which emits the most CO 2 of all fossil fuels. Using this approach, CO 2 would be captured from the exhaust stream of a power plant and pumped into a geological formation that would store it for centuries, preventing it from influencing the climate or ocean chemistry. As emissions continue to rise, CCS has attracted attention as a way to emit now and fix the problem later. The same technology that isolates CO 2 from the exhaust stream of a power plant could also conceivably pull it right out of the atmosphere.

But how much does it cost? Some initial estimates put the cost of CCS from the atmosphere at under $200 per ton. But a new analysis in PNAS suggests these estimates are misguided—we already know how much it costs to obtain trace chemicals from a mixture, and it's a whole lot more expensive than that.

The PNAS authors focus on two things: a thermodynamic analysis of CCS, and an empirical evaluation based on our experience with industrial processes for isolating trace gasses.

The thermodynamic analysis is easy to understand. We can calculate the energetic cost of taking gas with our current level of CO 2 (about 400 parts-per-million) and cutting that level in half, while concentrating what we've removed. It's only about 20 kiloJoules for each mole of CO 2 .

But nothing ever actually approaches that sort of perfect efficiency. For example, the authors note that NO x emissions are removed from the exhaust gasses of power plants through a chemical reaction that's energetically favorable. But the energetic costs of setting up the reaction components and moving them and the exhaust gasses through the system is huge. As a result, nearly 500kJ of energy is expended for each mole of NO x removed.

Unfortunately, thermodynamics can only set a lower limit on the amount of energy (and thus cost) that CCS will require. It doesn't provide any guidance as to how much more the actual cost will be and, right now, we've only got information from small-scale research systems.

But there are non-thermodynamic methods of estimating these costs. The authors point out that, back in 1959, someone named Thomas K. Sherwood analyzed the empirical relationship between the market price of a metal and the metal's concentration in the ore that we obtain it from. Since then, Sherwood's methods have been adapted for things like isolating pollutants from a waste stream, obtaining valuable organic compounds, and (most importantly) obtaining a single gas from a complex mixture.

Here, things look pretty good when it comes to going after the exhaust from power plants. The high concentration of CO 2 means that it would only cost about $10/ton for coal-fired planets and $25/ton for natural gas. The atmospheric concentration, however, is over 100 times lower, which shoots the cost up to $2,500/ton. Even adding in various optimistic assumptions doesn't bring the cost down below $1,000/ton.

At those levels, CCS from the atmosphere just doesn't make sense. "Unless a technological breakthrough that departs from humankind’s accumulated experience with dilute gas separation can be shown to 'break' the Sherwood plot and the second-law efficiency plot," the authors conclude, "direct air capture is unlikely to be cost competitive with CO 2 capture at power plants and other large point sources."

The authors have identified one potential way to "cheat." Biomass can sometimes be obtained cheaply, and could be burned in place of fossil fuels in a plant with CCS equipment. That way, biology does the hard part of extracting carbon from the atmosphere, which we then capture when it's economical, as part of a high-concentration exhaust stream. Right now, however, we haven't done biomass power on the sorts of scales needed to really know if this is viable.

In any case, other researchers have come up with numbers that are substantially lower than these for atmospheric CCS. Hopefully, they'll put together a response to this paper.

PNAS, 2011. DOI: 10.1073/pnas.1012253108 (About DOIs).