How do regular coal and clean coal compare?
Clean coal just means that it has few impurities that lower the value of the coal.
Coal naturally has a wide compositional variability to it, depending on its rank, original setting, and other geological parameters. For example, a coal with a lot of ash in it (ash is original clays/silts that got trapped in the plant before it became coal) this makes for poorer quality coal with less heating value,. Also, if coal has a lot of pyrite (sulphur) in it, this makes it a highly polluting coal when it is burned - gives off acid rain producing chemicals. High quality clean coal has low ash, high BTU energy content and low sulphur content and hence commands the best price.
Clean coal is used in two contexts; 1) as above for a coal with few impurities,, and 2) in relation to clean coal technologies that try to scrub the gases as the coal is burned to make it "cleaner". However, clean coal technologies have not lived up to their promise or hype, as they cost billions to install and industry is reluctant to do this unless required to by government.
Right now, natural gas is way cheaper and cleaner than coal (even clean coal or clean coal tech), and most utilities in North America are switching from coal to natural gas.