Fixed Carbon in Coal (Part of Proximate Analysis)
Fixed carbon is a measure of the amount of non-volatile carbon remaining in a coal sample. It is a calculated value determined from other parameters measured in a proximate analysis, rather than through direct measurement (ASTM method D3172-07a; American Society for Testing and Materials, 2013, p. 492-493). Fixed carbon is the calculated percentage of material that was lost during the testing for moisture, volatile matter, and ash:
Weight % fixed carbon = 100 – weight % moisture + weight % volatile matter + weight % ash
Fixed carbon from proximate analysis is a different value than total carbon from ultimate analysis. Total carbon includes some organic carbon that escapes as volatile matter emissions during combustion. Fixed-carbon content increases with rank, and is used to define ranks above medium-volatile bituminous coal. Fixed carbon has the opposite trend of volatile matter with increasing rank because increases in the amount of volatile matter driven off of coal increase the relative amount of carbon (Stach and others, 1982). Fixed-carbon content is also an important criteria for estimating the amount of coke that can be distilled from a coal (Zimmerman, 1979). Coke is a high-carbon product used in steel production.
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