How do coal scientists use fixed carbon content to classify coal?

Scientists classify coal by rank—a measure of the degree of coalification—using fixed carbon content as a primary indicator. Fixed carbon is defined as the solid combustible residue that remains after a coal sample is heated and its volatile matter is expelled.123

In a proximate analysis, this value is determined by subtracting the percentages of moisture, ash, and volatile matter from the total sample weight. As coal rank increases—moving from lignite through subbituminous and bituminous coal to anthracite—the fixed carbon content generally increases, while moisture and volatile material decrease.342

ASTM International utilizes these fixed-carbon limits, alongside volatile matter and calorific value, to assign specific ranks to coal. Because fixed carbon represents the solid material that does not vaporize when heated, a higher content typically indicates that the coal will require a longer time to undergo combustion.43