Ash Yield in Coal (Proximate Analysis)
Ash yield is one of the most common parameters measured in coal. Ash yield is measured to determine how much material remains (called ash residue) after a coal is combusted. Ash yield is measured directly in an automated proximate analyzer or ashing furnace. Results are presented in weight percent.
For this test, a coal sample is ground to a set size (-60 mesh, less than or equal to 250 micron particle size), weighed, and then combusted at temperatures between 500 and 750°C for 1 hour. The weight of the material that remains after combustion is then compared to the original weight of the sample to measure the percent ash yield (ASTM method D3174-12; American Society for Testing and Materials, 2013, p. 498–503):
Ash yield (weight %) = (weight of ash remaining/weight of precombustion sample) X 100 weight %
The ash yield or residue from a coal sample is largely composed of the mineral matter in the coal, but does not equal the original mineral matter content of the coal, because some of the minerals or mineral phases in the original coal sample (e.g., sulfides and carbonates) are destroyed or converted to other mineral compounds during combustion.
Ash yield is an important parameter for steam coals and for coals used in industrial furnaces. Increasing ash yield corresponds to lower heating (calorific) value. Most steam coals used for electricity require less than 20 percent ash content (air-dried), and less than 10 percent ash is required in most utility contracts. Because ash represents that part of a coal that will not combust in the boiler, the ash yield provides information on how much material will be left after combustion, which will need to be disposed of.
In the steel industry, low-ash contents are preferred for coking coals. High volatile matter in coals produces lower coke yields. Coals with volatile matter of 20 to 35 percent (on an air-dried basis) are preferred for coking coals (Thomas, 1992). Coals with very low ash yields (less than 2 percent) are sometimes used to make silicon metal and as a feedstock for chemicals.
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