Volatile Matter (Part of Proximate Analysis)
Volatile matter is one of the most common parameters measured in coal. It is part of a standard proximate analysis. Volatile matter is essentially a measure of the nonwater gases formed from a coal sample during heating. It is measured as the weight percent of gas (emissions) from a coal sample that is released during heating to 950 C° in an oxygen-free environment , except for moisture (which will evaporate as water vapor), at a standardized temperature. Volatile matter is measured directly in the automated proximate analyzer. Results are presented in weight percent.
Prior to testing, moisture content is driven off and recorded as the residual moisture content. Volatile matter testing occurs after the sample is dry. In the test, a coal sample is pulverized to -60 mesh (less than or equal to 250 microns particle size), weighed, and subjected to high heat in an atmosphere of pure nitrogen gas, and then weighed again. The difference is the weight percent lost as emissions during combustion, which should be the volatile matter in the sample (ASTM method D3175-11; American Society for Testing and Materials, 2013, p. 504–509).
volatile matter = weight loss % of sample =
(weight of preheated sample – weight of sample after heating) /
weight of precombustion sample
Most volatile matter comes from the organic components in coal, but some elements from minerals in coal, can also combine to form volatile oxides. The most common volatile matter in coal is water, carbon dioxide, and sulfur dioxide.
Volatile matter is directly related to coal rank; as rank increases, volatile matter content decreases (Stach and others, 1982). Volatile matter has the opposite trend of fixed carbon with rank. As volatile matter is driven from the coal matrix with increasing rank, the relative carbon percentage tends to increase. Both volatile matter and fixed carbon are used to define coal rank in high-volatile, medium-volatile bituminous, and higher-rank coals in the U.S. classification system (ASTM method D388-12; American Society for Testing and Materials, 2013, p. 390–396). Along with moisture and ash contents, the percentage of volatile matter in a coal sample is needed to calculate the fixed-carbon content of a coal.
Many electric utilities prefer coals within a narrow range of volatile matter (25 to 35 percent) for optimal flame stability in the boiler (Thomas, 1992). High-volatile matter may also be associated with spontaneous combustion, especially in low-rank coals (Fierro and others, 1999). Hence, knowledge of volatile matter is important for safety, handling, and transporting coal.
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