"Cleaning" coal does not mean washing the black soot material by means of washing.
This is a noun in chemical and fuel engineering-related fields, and the phrase refers to technologies that mitigate or avoid pollutants or carbon dioxide emissions from the conversion of coal into electricity.
Low cost, easy to transport, and widely available, coal is the main fuel for electricity generation in many other countries around the world.
The problem is that burning coal per unit of electricity emits more carbon dioxide into the atmosphere than other fossil fuels, contributing to global warming.
In order to burn coal more cleanly, a number of technologies have been developed (some of which have already been put into commercial use). These include:
The use of new high-temperature and high-strength alloys to increase the boiler pressure and temperature of coal-fired power plants, and increase the efficiency by more than 25%, thereby reducing coal use and pollutant emissions per unit of power generation
Converts coal into "syngas", consisting mainly of carbon monoxide and hydrogen, to remove pollutants before being used in gas turbines;
Carbon dioxide is captured from fossil fuel combustion or gasification, compressed and stored deep underground.
Well-funded research, development, and successfully deployed demonstrations are critical to the timely success of clean coal technologies.