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To most people, coal is coal — black lumps of dusty, combustible material that double as Christmas presents for naughty children. To experts, however, every piece of coal is different, with a wide range of properties and qualities. Proper assessment of these traits is vital to ensuring a particular supply of coal is used in the most effective way, and coal analysis is the sophisticated, scientific way that is done.
Somewhat like diamonds, there is a system of classification for coal that depends on various factors. Unlike diamonds, which are graded on criteria like clarity, cut, and color, coal analysis is done in one of two ways. Proximate analysis is based on factors such as moisture presence, volatility, and carbon content. These are derived from the four main ingredients in a piece of coal, which are moisture, volatile matter, ash, and carbon. Ultimate analysis, by contrast, measures coal's constituent elements — oxygen (O), carbon (C), sulfur (S), and hydrogen (H), among others.
According to the results of either of the two types of coal analysis, a sample may be classified as lignite, bituminous, anthracite, or graphite. Each of these types has a different specific purpose in the coal industry. Lignite is alternatively known as brown coal, and bituminous is the classic black coal most people are familiar with.
The various parts of a proximate analysis can be undertaken in a lab, and involve a variety of tests and measurements. Coming from underground, coal is wet when it is mined. Groundwater and other liquids add to the moisture level within coal, which is known as inherent moisture. Coal analysis attempts to gauge how much inherent moisture is in a particular sample. As logic would dictate, the less moisture in a piece of coal, the better.
The volatility of coal is measured by the proportion of volatile matter, which includes various types of hydrocarbons and sulfur, in a sample. This measure basically indicates how completely a piece of coal burns when air is not present. It is tested by heating a sample to upwards of 1740°F (950 C).
Subtracting what remains after a volatility test from the original mass of a sample also provides a measures of what is known as fixed carbon content. This generally makes up about half the overall mass of a given sample of coal. The measure of ash in a coal analysis simply determines how much material remains after burning. Since virtually all the carbon, sulfur, and moisture is burned off when ignited, the ash that remains is only a small percentage of the original amount of coal.
Ultimate analysis also involves various lab tests. Increasingly though, a technique known as Laser Induced Breakdown Spectroscopy (LIBS) offers a more problem-free approach to assessing the elemental makeup of coal. Other laser techniques have also been adopted in 21st-century coal analysis that eliminate the need for the traditional battery approach to testing, while improving accuracy.
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