Lignite generates less heat per ton than higher grades of coal because of its high moisture content and, like peat, it is a dirty fuel when burned. At higher temperatures, the sub-bituminous converts to bituminous grade coal, which contains lower moisture content but approximately the same volatile matter. At yet higher temperatures, almost all the moisture and volatile matter is driven off, converting the bituminous grade of coal to anthracite. Anthracite burns with a clean, hot fire because it is almost pure carbon, low in moisture and low in volatile matter. Wetlands trap heavy metals which chemically bond to sulfur as generally insoluble sulfides.
Sulfur dioxide reacts with water in the atmosphere to form sulfuric acid.
Sulfuric acid is one of the most common corrosive substances. Coal produces more CO 2 than any other hydrocarbon per energy unit. Strip-mining for coal, while providing us with an important resource, has contributed to devastating environmental damage. Strip-mining is practiced when coal seams lie close to the Earth's surface. The layers of Earth overlying the coal are removed using standard construction equipment until the coal is reached.