What is coal?
Coal, like oil and gas, is a fossil fuel. It comes in the form of a black sedimentary rock. Its formation is a little different from that of oil and gas, however.
Oil and natural gas formed from the organic remains of marine organisms. In contrast, coal formed from peat, which in turn formed from plants (mainly those of the Carboniferous period). Over the course of millions of years, peat buried under layers of sediment dried out and was altered chemically by heat and pressure.
There are various forms of coal with more or less carbon. Bituminous or soft coal has the highest energy content, followed closely by anthracite or hard coal. Lignite and sub-bituminous coal are intermediate between hard coal and peat.