I remember a big discussion on this topic somewhere but I couldn't find it exactly. The term clean coal has always kind of seemed an oxymoron to me but I knew I did not know enough about the science behind it all to give an educated opinion.

I found this article however and it sparked again some interest in me.

In reality, we can't really talk about clean coal — it doesn't exist. Though the coal industry is right to point out that it has improved filters on coal plants, sending less traditional pollutants like sulfur dioxide and mercury into the air, the toxic waste that remains behind is only growing.