How is fixed carbon calculated in a coal analysis?

In a proximate analysis of coal, fixed carbon is defined as the solid, combustible material—excluding ash—that does not vaporize when the coal is heated in the absence of air.1

It is calculated by subtracting the sum of the percentages of moisture, ash, and volatile matter from 100 percent.12

$$\text{Fixed Carbon (\%)}=100\%-(\text{Moisture\%}+\text{Ash\%}+\text{Volatile Matter\%})$$

Because fixed carbon represents the residue remaining after volatile matter has been expelled, a high fixed-carbon content indicates that the coal will require a longer time to complete the combustion process, as gas-solid combustion reactions occur more slowly than gas-gas reactions.2