Stages of Coal Formation

Understanding the different types of coal involves first developing a basic understanding of how coal forms within the earth. The types of coal are differentiated by specific properties which result from having undergone varying degrees of heat and pressure over different lengths of time during formation. Coal takes millions of years to develop, and is derived from ancient plant matter that has been subjected to intense heat and pressure that affected physical and chemical alterations. The process through which coal forms from such ancient plants is known as coalification.

In general, coal forms from the remains of plants which died in prehistoric swamps and wetlands. Organic plant matter at various stages of decay form peat, which, under certain pressure, time and heat conditions experiences slow rates of bacterial decay and eventually goes on to form coal. As peat is buried by sediment and becomes compressed, it slowly releases water and other elements contained within it, resulting in an increasingly compact and carbon rich substance. The natural process converting plant matter to peat may go through different stages, first forming lignite, then sub bituminous coal, bituminous coal and eventually anthracite coal.

When we mine coal from the earth, it may be at any of the above mentioned stages of formation, which contain increasing levels of carbon. Below we examine some of the characteristics of each rank:

-Lignite: Lignite coal is the lowest ranked coal for its heat energy producing capability and carbon content. A soft, brownish coal, lignite contains the highest levels of moisture of the coal types and is known for its crumbly texture. Lignite deposits are the youngest of the coal types and have undergone the lowest intensity of heat and pressure.

-Sub Bituminous: With higher carbon content and thus a higher heating value than lignite, sub bituminous and bituminous coal collectively make up the majority of coal produced in the United States. Sub bituminous coal contains less water and is therefore harder than lignite, facilitating easier transportation and storage. The relatively low sulfur content in sub bituminous coal makes it an attractive option for power plants.

-Bituminous: Estimated to be between 100 and 300 million years old, bituminous coal makes up the majority of coal mined in the U.S. Bituminous coal is heavily utilized for energy in power plants and for the production of steel.

-Anthracite: Having the highest concentration of carbon, anthracite is also the hardest coal. Anthracite coal is known to be the cleanest burning coal, but is one of the rarest, being found mainly in limited reserves in Pennsylvania.