Most of what shows is steam. The dark, sooty material called fly ash that once went up the stack is now removed by filters or by devices called precipitators. With precipitators, the flue gas is passed through an electric field. The ash particles become negatively charged and are attracted to positively charged collecting plates and later removed for disposal. This method eliminates 99.5 percent of the offending material.
Coal contains sulfur, which combines with oxygen when the coal is burned to produce sulfur oxides.
The effect of sulfur oxides on the environment has been the topic of significant debate for a number of years. Beginning in the 1970s, coal producers and major coal consumer initiated a number of efforts to reduce the amount of sulfur compounds emitted into the environment. In another, sulfur and sulfuric acid are produced as by-products. Still others can produce a dry by-product. Less than two decades after the Clean Air Act was passed, the sulfur content of coal purchased and burned by electric utilities had decreased 37 percent.