what is dirty and clean coal?

"Clean coal" is a term describing steps taken to reduce the pollution emitted when coal is burned for power. They wash it for one to eliminate the particulate dust at the mine site and power plant. But this is not new technology. The pulverize the coal into a super fine powder and inject it into the burner. This increases the surface area exposed to oxygen and leads to a more efficient higher temperature burn. More heat per unit of fuel, more steam, more power. Chimney filters eliminate the soot, particulates and handle nitrates, sulfur, lead, mercury to a better degree.

Long ago, I heard about a chemical way to make coal greener by removing some carbon. I don't remember the details.

The government won't allow it. Obama promised that he would kill coal and increase energy costs. Well here we are.