Basic Coal Analysis

Prior to discussing the rank, type, grade, and classification systems of coal, a brief description of basic coal analyses, upon which classification schemes are based, is provided. These analyses do not yield any information on coal structure but do provide important information on coal behavior and are used in the marketing of coals. Three analyses are used in classifying coal, two of which are chemical analyses and one is a calorific determination. The chemical analyses include proximate and ultimate analysis. The proximate analysis gives the relative amounts of moisture, volatile matter, ash (i.e., inorganic material left after all the combustible matter has been burned off), and, indirectly, the fixed carbon content of the coal. The ultimate analysis gives the amounts of carbon, hydrogen, nitrogen, sulfur, and oxygen comprising the coal. Oxygen is typically determined by difference—that is, by subtracting the total percentages of carbon, hydrogen, nitrogen, and sulfur from 100—because of the complexity in determining oxygen directly; however, this technique accumulates all the errors that occur when determining the other elements into the calculated value for oxygen. The third important analysis, the calorific value, also known as heating value, is a measure of the amount of energy that a given quantity of coal will produce when burned.

Because moisture and mineral matter (or ash) are extraneous to the coal substance, analytical data can be expressed on several different bases to reflect the composition of as-received, air-dried, or fully water-saturated coal or the composition of dry, ash-free (daf), or dry, mineral-matter-free (dmmf) coal. The most commonly used bases in the various classification schemes are shown in Figure 1-3 [13]. The most commonly used bases can be described as follows [1]:

• As-received—Data are expressed as percentages of the coal with the moisture. This category is also sometimes referred to as as-fired and is commonly used by the combustion engineer to monitor operations and for performing calculations as it is the whole coal that is being utilized;

• Dry basis (db)—Data are expressed as percentages of the coal after the moisture has been removed;

• Dry, ash-free (daf)—Data are expressed as percentages of the coal with the moisture and ash removed;

• Dry, mineral-matter-free (dmmf)—The coal is assumed to be free of both moisture and mineral matter, and the data are a measure of only the organic portion of the coal;

total moisture mineral matter pure coal surface moisture inherent moisture ash volatile mineral matter volatile organic matter volatile matter fixed carbon m

FIGURE 1-3. Relationship of different analytical bases to coal components. (From Ward, C. R., Ed., Coal Geology and Coal Technology, Blackwell Scientific, Melbourne, 1984, p. 66. With permission.)

• Moist, ash-free (maf)—The coal is assumed to be free of ash but still contains moisture;

• Moist, mineral-matter-free (mmmf)—The coal is assumed to be free of mineral matter but still contains moisture.

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