The oxymoronic term "clean coal" refers to carbon-capture-and-storage (CCS) technology [0], touted by the fossil fuel industry as a way to reduce greenhouse gas emissions and continue employing coal workers.

Thus far, it is incredibly expensive, at a time when solar and wind generation is cost-competitive with fossil-fuel plants which don't employ CCS. It is simply a dead end. You can generate more renewable energy, and store it, for far less than it takes to equip and operate CCS in conjunction with a fossil-fuel-fired plant. Only direct government subsidy makes it viable for a vanishingly small amount of GHG emissions.

[0]: https://en.wikipedia.org/wiki/Carbon_capture_and_storage