Coal is a nonrenewable energy source because it takes millions of years to form. It is a fossil fuel created from plant remains∕vegetation that once lived about 100–400 million years ago when part of the world was covered with huge swampy forests. Coal is a combustible, sedimentary, organic rock, composed mainly of carbon, hydrogen, and oxygen formed from vegetation, which has been consolidated between other rock strata, and altered by the combined effects of pressure and heat over millions of years to form coal seams.
The principal chemical constituents of coal are carbon, hydrogen, oxygen, nitrogen, and sulfur. Coal may also contain incombustible mineral matter and trace amounts of metallic elements, oxides, and rare gases. The properties of a given coal deposit vary depending on a variety of site-specific factors, including the type of vegetative matter from which the coal formed, the age of the deposit, and the conditions under which the coal formed.
Coal is however the most abundant fuel in the fossil family and has been in use mostly for heating purposes since the caveman age. Archaeologists have also found evidence that the Romans in England used it in the second and third centuries (AD 100–200).
