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That's the sound of clean coal. Well, cleaner coal. A relatively¹ small unit attached to the smokestack at the Mountaineer Power Plant in West Virginia is capturing some 1.5 percent of the carbon dioxide the coal-fired plant would otherwise belch² into the sky. 

The loud thrum comes from the whirring of fans that cool the flue gas and the jostling of an agitator³ that keeps things moving in the tower where the reaction to actually capture the CO2 takes place. There’s also the chug of the compressor, which turns the odorless, colorless greenhouse gas into a milky⁴ liquid at 1,400 pounds per square inch (psi). 

After that it's off to the storage wells where the fluid CO2 is further compressed to more than 2,000 psi and pumped a mile and a half underground where it's injected into the pores between grains of rock in a layer of sandstone laid down some 440 million years ago. 

So far so good for the Mountaineer , which cost the utility that owns it more than $70 million to build. But questions remain. What can be done to clean up coal's other problems, like toxic⁵ ash residue⁶ or the removal of actual mountaintops? Can carbon capture and storage be scaled up to the size necessary？ to capture a significant fraction of the world's greenhouse gas emissions⁷ from fossil fuel burning?What will that cost? And will all that CO2 stay put?

After all, as the mayor of the Ohio town across the river from Mountaineer told me: carbon capture and storage sounds all well and good... until something happens.