Straight Dope: Is “clean coal” really clean?

This week's question on Straight Dope:

I've been reading about clean coal technology — in particular, coal gasification and its use in power production. Everyone touts the clean emissions consisting of CO², which gets sequestered underground, and water vapor. But what about the other by-products, such as sulfur, nitrogen oxide and ash? What about trace metals such as mercury, lead and uranium? Remember, coal gasification isn't new technology — gas companies are still cleaning up the sites of old "town gas" plants contaminated with dangerous chemicals such as xylene, toluene and benzene. What becomes of all this stuff? —Paul O'Brien, Chicago

Read Cecil Adams' answer on the full post: Is clean coal really clean?

This article appears in Jul 1-7, 2010.