How do heat and burial transform lignite into higher-rank coal?

The transformation of lignite into higher-rank coal, such as subbituminous, bituminous, and anthracite, is a process known as coalification. While burial is associated with this change, it is primarily the increase in temperature that drives the process, rather than the pressure from the overlying sedimentary cover.12

As coal is buried deeper, it is subjected to higher temperatures due to the Earth's geothermal gradient. This heat triggers chemical changes that break down the original plant-derived organic compounds, such as cellulose and lignin. During this transformation, volatile phases rich in hydrogen and oxygen—including water, carbon dioxide, and methane—are expelled from the coal mass.13

As these volatiles escape, the coal becomes progressively enriched in carbon, and the remaining material undergoes structural changes, increasing the connections between aromatic nuclei. Consequently, the coal’s rank increases, characterized by a higher fixed carbon content and a greater calorific value. Conversely, increasing overburden pressure is not considered a driver of coalification and may even hinder the process by trapping volatile compounds.321