The calorific value of coal is measured for almost all coal samples. It is a measure of the heating ability of a coal and is needed to estimate the amount of coal needed to produce a desired amount of heat. Calorific values are also used to define coal rank in low- and medium-rank coals below the rank of medium-volatile bituminous in the U.S. classification system.
Calorific value is a measure of the amount of energy produced from a unit weight of coal when it is combusted in oxygen. A measured sample of coal is completely combusted in a bomb calorimeter, which is a device for measuring heat (ASTM method D5865-12; American Society for Testing and Materials, 2013, p. 648–666).
In standard practice, the calorimeter is calibrated by combusting 1 gram of benzoic acid, which has a known calorific value. Next, approximately 1 gram of -60 mesh (less than or equal to a particle size of 250 microns) coal is placed in a metal sample cup. The cup is then placed into a holding slot between two electrodes extending from the lid of a stainless steel container. A thin, metal wire fuse is attached to the electrodes and forms a loop, into the coal sample. The cup and electrode are then placed into the container, termed a "bomb." The bomb (containing the sample) is then transferred to a water bath and an electrical current is used to spark the sample, which ignites in the bomb. Ignition of the sample, in turn, heats the water bath. The change in water-bath temperature is used to determine the calorific value of the sample.
Calorific value can be measured in standard English units (Btu/lb) or in metric units (kilojoules/kg or megajoules/kg). One Btu is equivalent to the amount of work (directional force) needed to raise the temperature of 1 pound of water by 1° Fahrenheit. Metric equivalent units can be calculated from English units using the formula:
MJ/kg = (Btu/lb) X 0.002326
For example, a coal with a calorific value of 14,000 Btu/lb has a metric calorific value of 32.56 MJ/kg.
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