Numerous studies have shown that the cost of removing one substance from a mixture depends on its initial concentration, so the much lower concentration of CO2 in outside air makes its removal from air much more costly than from exhaust gases. After a detailed comparison, the MIT-led team concluded that the cost of such removal is likely to be more than $1,000 per ton of CO2 avoided, compared to $50 to $100 per ton for current powerplant scrubbers.
It turns out that “many of those advocating air-capture deployment and research are really lowballing the cost,” Herzog says. When the underlying chemistry and mechanics are analyzed, their numbers don’t hold up, he says. Compared with removing carbon dioxide from the emissions at a powerplant — technology that exists and can be measured — removing it from the outside air means processing about 300 times more air per ton of CO2 removed, because that’s the difference in CO2 concentration.
While the study found that such technologies are unlikely to have a place over the next few decades, it did see one possible area where a particular variation of such systems might make sense, at least to a limited extent: planting trees or other plants to extract carbon dioxide from air, then burning them to produce electricity while scrubbing the carbon dioxide at that powerplant. That type of process would take advantage of plants’ natural ability to carry out the initial extraction from the air, and would be renewable because the plants could be harvested and then replaced. The total cost of such a system could be a few hundred dollars per ton of carbon removed — which is not competitive at today’s prices, but might be in the future.
Even then, it would be more expensive than cleaning up fossil-fuel emissions right at the powerplants, Herzog says: “What makes us think, if we’re not willing to do these cheaper alternatives today, that future generations will do these much more expensive things?”