When we talk about “clean” coal, it is a relative term. All coal produces heavy pollutants and atmospheric carbon by the ton when it burns in a power plant. “Clean” coal produces more atmospheric carbon and less heavy pollutants because of the way it has been washed before burning. The trouble, of course, is what happens to the water and detergents that are left over from the washing process. This is an unsolved problem every bit as vexing as nuclear waste.
The by-product of washing coal after it is mined is an acid and highly toxic solution called coal slurry, though it is better described as sludge after some of the water has evaporated away. Mining companies fill whole valleys with lakes of coal slurry, held behind dams that aren’t built to the usual engineering standards of an earthen dam. The inevitable result is a long series of well-documented disasters as these dams break. Usually, of course, the dams hold, and the sludge eventually dries and forms a relatively stable, though still highly toxic, solid.
It is a horrendous answer to the problem of coal sludge, but it is the best we know of. Coal slurry is too acid to be held in tanks, acid enough to eventually destroy any tank used to contain it. Sometimes coal slurry is dumped back into coal mines, where it seeps into the ground water and may destroy all underground water supplies in the local area. That is a problem so costly that regulators are talking about completely banning this approach. Known industrial techniques for drying coal slurry release much of the acid and toxic minerals into the atmosphere, largely defeating the purpose of washing coal in the first place.
All known techniques to handle coal sludge safely are hugely expensive. There are similar problems with the other local by-product of coal, the coal ash that results when coal is burned. Coal ash holds most of the same poisonous materials found in coal sludge (though in quite different proportions). Coal ash should be buried, but often is dumped into the same reservoirs that hold coal sludge. If we took the trouble to handle coal sludge and coal ash safely, coal would no longer be a “cheap” fuel, but would be more expensive than oil as a source of electricity. That’s why we live with the risks associated with artificial lakes filled with the toxic by-products of coal.
In truth, coal-based electricity has been more expensive than oil all along. It is less expensive to electricity users only because many of the costs have been shifted onto the community at large. In the long run, we have no choice but to make the transition from coal to less costly sources of energy.