How does the heating value of coal change as coal rank increases?

The heating value of coal—also known as its calorific value—is the amount of chemical energy released as thermal energy during combustion. This value is directly linked to the coal's rank, which represents the degree of coalification—the process by which organic material is transformed into coal through increasing temperature and burial depth over time.1234

In general, the calorific value of coal increases as its rank rises, progressing from lignite through subbituminous coal and into bituminous coal. This trend occurs because the coalification process involves an increase in fixed carbon content and a decrease in moisture and volatile matter, both of which influence the energy density of the fuel.35

Because of this relationship, calorific value is used as a primary metric for classifying coals of lower rank, while higher-rank coals (such as those above high-volatile bituminous) are typically classified by their fixed-carbon content.15