A process called "gasification" allows many of the impurities in coal to be removed before it is combusted to generate electricity
Instead of directly burning coal, like traditional plants, the coal is gasified. As a gas, almost all pollutant-forming impurities can be removed. The coal gas can be made clean and burned in a gas turbine-generator. The turbine exhaust is then used to power a steam turbine-generator. This combination allows coal to be used more efficiently than ever before.
Source: Department of Energy

Gasification allows the coal to burn more efficiently and obtain more energy than traditional plants when using the same amount of coal
Gasification plants built near Tampa, Florida, and West Terre Haute, Indiana, are the cleanest, most efficient coal plants in the world.
Source: Department of Energy