clean coal mostly refers to carbon capture and storage (ccs), which is supposed to trap C02 from coal plants and store it underground.
in practice, it's expensive, rarely works at scale, and has a bad track record.
the US government spent over $7 billion on ccs projects from 2010 to 2017. according to a government accountability office (gao) report, most of that money went to projects that were canceled or never built.
the kemper plant in mississippi was supposed to be a showcase for clean coal. it burned through $7.5 billion and never worked as intended. the carbon capture system was scrapped in 2017.
even when ccs works, its inefficient. it can use up 20/30% of the energy a plant produces, which makes coal more expensive and less viable compared to renewables
coal still causes other pollution. mercury, sulfur dioxide, nitrogen oxides, particulate matter. even with modern scrubbers. and mining coal still wrecks land, contaminates water, and causes long-term health problems.
the levelized cost of energy from renewables is now lower than coal almost everywhere. even nuclear is far cleaner andsafer over time.