How clean is "clean coal"? Well, if by "clean coal" you mean anything other than carbon capture and storage (CCS), the answer is "dirtier than everything else except older, even worse coal."
As this nice summary of Credit Suisse research at Macrobusiness points out, even "advanced ultra super critical" technologies involve more twice the CO2 of gas combined cycle. Of course, other gas combined heat and power solutions can do better yet!
And carbon capture and storage is not advancing toward commercialization and seems to have a number of significant issues.
And it is absolute folly to invest in new ultra super-critical plants today, since they only achieve very modest CO2 reductions over old plans, and they will be around for the next 50 years or more once built.
Heck, even the Kentucky "coal mining museum" is adding solar.
So remember: coal belongs IN the museum: and solar belongs ON the roof!