rank

Coal classification, or rank, refers to the degree of coalification a coal has undergone. This process is influenced by burial depth and temperature over time.1

The progression of coal rank starts with peat, which transforms into lignite, a soft, low-rank coal. Further increases in temperature convert lignite into subbituminous coal, then bituminous coal, and finally, anthracite, the highest rank of coal. As rank increases, so does the fixed carbon content, while moisture and volatile materials decrease. The calorific (heat) value generally rises from lignite through bituminous coal.1

• Lignite: The first product of coalification, intermediate between peat and subbituminous coal.2
• Subbituminous coal: Intermediate between lignite and bituminous coal, generally dark brown to black.3
• Bituminous coal: The most abundant form, dark brown to black and often banded.4
• Anthracite: The most metamorphosed form, black with a metallic luster, and the least plentiful. It is also known as stone coal.56