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What is the proximate analysis of coal?

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Proximate analysis reveals the quality and precise chemical composition of a coal sample. This analysis examines four factors: moisture, volatile compounds, ash content and fixed carbon. Information revealed during proximate analysis impacts coal prices and determines the most efficient use for each shipment.

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Proximate analysis measures four types of moisture. Surface moisture surrounds and coats the coal but is not part of its structure. Hydroscopic moisture is similar but is found only in microscopic passageways inside the coal sample. Decompositional moisture is also found within coal samples but belongs to incorporated organic material rather than to the coal itself. The fourth type of moisture is mineral moisture, which reveals the presence of clay and other wet minerals.

The second component of proximate coal analysis tests the quantity of volatile compounds in the sample. These are flammable and potentially dangerous. The portion of proximate analysis dedicated to them must meet strict legal and scientific guidelines established to ensure accuracy. The United States' guidelines call for placing the coal sample in a platinum crucible and heating it to 1,740 F. Other countries use different guidelines.

Volatile compound analysis destroys some of the sample's carbon content. The carbon that remains is called fixed carbon and is represented as a fraction or percentage of the original sample's weight. After recording this measurement, the tester burns the coal. The remaining material is ash. As with fixed carbon, a sample's ash content is expressed as a percentage of the weight of the original coal sample.

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