The coal formation process involves the burial of peat, which is made of partly decayed plant materials, deep underground. The heat and pressure of burial alters the texture and increases the carbon content of the peat, which transforms it into coal, a type of sedimentary rock. This process takes millions of years.
Types, or “ranks,” of coal are determined by carbon content. There are four types of coal, ordered from highest to lowest rank below.
Anthracite, or “hard coal,” contains the highest amount of carbon out of all coal ranks (86%-97%). It has a brittle texture and lustrous sheen. It is used mostly in industrial settings and the metals industry due to its high heat value.
Bituminous coal, often called “soft coal,” has slightly lower carbon content than anthracite (45%-86%). The wide range of carbon content in bituminous coal warrants use for both electricity and steel production. It can be identified through its shiny luster and layered texture.
Subbituminous coal does not have the same shiny luster as higher ranked coals. As the name suggests, it has lower carbon content than bituminous coal (35%-45%) and is primarily used for electricity generation.
Lignite is often called “brown coal” because it is lighter in color than the higher ranks of coal. It has the lowest carbon content out of all the coal ranks (25%-35%) and it has a high moisture content and crumbly texture. It is mainly used in electricity generation(Keane, n.d.).
What’s a raw material in your industry that has surprisingly diverse variations? Let’s discuss the subtle (or not-so-subtle) differences that matter!
References:
Keane, C. (n.d.). What are the different types of coal? American Geosciences Institute. Retrieved May 28, 2025, from https://profession.americangeosciences.org/society/intersections/faq/what-are-the-different-types-of-coal/
