Coal is a sedentary rock that varies in composition. It forms as organic material compressed under the layers of soil and more organic material above it.
Coal is a fossil fuel used in producing steel and generating electricity. Due to the differences in coal, engineers design power plants that use it for a fuel to operate using the available feedstock while also reducing emissions, including sulfur and mercury. Harder forms of coal generally provide more energy than softer forms. Anthracite, the top-rated form of coal, provides 30 Millijoule per kilogram of energy. In comparison, lignite, also known as brown coal, provides only 5.5 to 14.3 Millijoule per kilogram.