Coal is formed from dead plant material that accumulates in swampy or wetland environments where it is protected from oxidation by water or mud. This plant material first forms peat, a spongy, fibrous organic substance. Over millions of years, as layers of sediments bury the peat, heat and pressure increase, causing physical and chemical changes that remove water, methane, and carbon dioxide, enriching the carbon content. This process, called coalification, transforms peat into various types of coal—lignite, sub-bituminous, bituminous, and anthracite—depending on the temperature, pressure, and time of burial.
Stages of Coal Formation
- Peat : Accumulated dead plant material in oxygen-poor wetland conditions, partially decomposed.
- Lignite (Brown Coal) : Formed when peat is buried under sediments and subjected to mild heat and pressure.
- Bituminous Coal : Further compaction and heating transform lignite into this denser, higher carbon coal, with negligible plant remains visible.
- Anthracite (Hard Coal) : The highest rank of coal formed under the highest heat and pressure, with the greatest carbon content and energy value.
Chemical Changes During Coalification
The main chemical changes include loss of water (dehydration), carbon dioxide (decarboxylation), and methane (demethanation), leading to increased carbon concentration and formation of aromatic carbon rings in the coal structure. These processes increase the coal's energy content and alter its physical properties.
Thus, coal formation is a gradual geological process that converts accumulated plant matter into a carbon-rich fuel over millions of years through burial, heat, and pressure. The type and quality of coal depend on the conditions under which coalification took place.
