Coal is a combustible black or brownish-black sedimentary rock, primarily composed of carbon, hydrogen, oxygen, and sulfur. It forms via the geological process of coalification, transforming plant matter under heat and pressure over millions of years. As a complex mixture rather than a single chemical compound, it possesses no unified molecular formula; however, its energy content is defined by calorific value, typically ranging from 24 to 35 MJ/kg depending on rank.
Strict safety protocols are required due to risks of spontaneous combustion and explosive dust accumulation. Storage must be in cool, ventilated areas away from oxidizers. Occupational exposure limits apply to coal dust to prevent pneumoconiosis (black lung disease). Environmental regulations mandate emissions controls for SOx, NOx, and particulate matter during processing.