First of all, let's address the elephant present in every room that the phrase 'clean coal' has ever been uttered in: Yes, it's an oxymoron. No matter how successfully technological advances may allow us to sequester coal and transport it to the bowels of the earth, mining, moving, and processing the stuff remains a dirty, dirty job.
Extracting coal is ugly business. These days, it often means engaging in mountaintop removal mining, which involves dynamiting pristine mountains to expose the coal seems within. The process results in contaminated waterways and the release of dangerous chemicals, along with devastated natural environments. But even non-MTR coal mining operations can contaminate groundwater with hazardous chemicals and leech pollution into surrounding habitats. Finally, coal must be shipped to the power plants where it will be burned. This can mean coal trains, truck convoys, or emissions-belching transocean frigates -- all of which pollutes in its own right.
And all this is to say nothing of the 100,000 plus American coal miners that were killed on the job over the last 100 years or so.
Point is, mining coal will never be clean. There's simply no way to extract and ship the stuff in a way that could be considered a long-term solution to pollution.