Coal pollution mitigation emissions of sulfur dioxide or oxides of nitrogen associated with the utilization of coal in the generation of electricity." Before being adopted in this fashion, historically "clean coal" was used to refer to clean-burning coal with low levels of impurities, though this term faded after rates of domestic coal usage dropped. The term appeared in a speech to mine workers in 1918, in context indicating coal that was "free of dirt and impurities." In the early 20th century, prior to World War II, clean coal (also called "smokeless coal") generally referred to anthracite and high-grade bituminous coal, used for cooking
Pittsburgh coal seam that was piled with coke inside iron furnaces. It has a high proportion of carbon, which accelerates combustion, and a low proportion of impurities, including ash and moisture, which retards combustion. Pittsburgh coal also has a low proportion of sulfur, which is critical to making high-quality iron. In addition, the Pittsburgh seam was located close to Pittsburgh, then the center of the growing American iron and steel industry. Coke had to be transported by water or rail to iron furnaces, and the Pittsburgh seam's proximity to iron furnaces gave the bed another competitive advantage over more distant coal seams that