Just a reminder: there's no such thing as clean coal.

During last night's presidential debate, Barack Obama and Mitt Romney each reiterated their energy policies. And they both included clean coal as an aspect of their plans. But there's a catch: clean coal is more theoretical than it is practical.

The term clean coal can have a couple of meanings. It can refer to technologies that may make the process of burning coal for electricity more efficient; it can also refer to scrubber technology that would lower the amount of some pollutants. Those technologies don't reduce problematic emissions to anything near clean, though.

But clean coal is often used as another way of referencing carbon capture and sequestration technology: the coal is burned and the carbon emissions are captured and stored, possibly in underground rock formations. The technology to capture the carbon exists, but it is extremely expensive and has not been used outside of experimental demonstration projects.

In July, Politico published an article about the problems facing clean coal. One issue it highlights: there's no incentive for companies to implement the technologies without climate legislation. It also says that billions of dollars in federal investment would be needed to deploy the technology. The article is worth a read.

But there's another simple reason about why coal can't be considered clean, even if the polluting emissions from burning it are somehow controlled. Coal mining is inherently dirty and often destructive, and generates polluting byproducts.