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Chapter 7 | Calculating Coal Analyses from As-Determined Values to Different Bases
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The results of a coal analysis may be reported on any of several bases, differing from each other in the manner by which moisture and ash are treated. Except for data reported on a dry basis, it is essential that an appropriate moisture content be given in the data report. This would avoid ambiguity and provide a means for conversion of data to other bases. These bases are defined in ASTM D121  and include the following:• As-determined basis (ad): The basis for analytical data obtained from an analysis sample of coal or coke after conditioning and preparation to USA Standard No. 60 (250 μm) sieve in accordance with ASTM D2013. As-determined data represent the numerical values obtained at the particular moisture level in the analysis sample at the time of the analysis.• As-received basis (ar): The basis for analytical data calculated to the moisture condition of the sample as it arrived at the laboratory and before any processing or conditioning. If the sample has been maintained in a sealed state so there has been no gain or loss, then the as-received basis is equivalent to the moisture basis as sampled.• Dry basis (d): The basis for analytical data calculated to a theoretical basis of no moisture associated with the sample. The numerical value as established in ASTM D3173 or D7582 is used for converting the as-determined data to a dry basis.• Dry, ash-free basis (daf): The basis for data calculated to a theoretical basis of no moisture or ash associated with the sample. Numerical values as established by ASTM D3173 and D3174, or D7582, are used for converting the as-determined data to a moisture- and ash-free basis.• Equilibrium moisture basis: The basis for data calculated to the moisture level established as the equilibrium moisture. Numerical values as established by ASTM D1412 are used for the calculation to an equilibrium moisture basis.It should be noted that if a coal sample is analyzed over a period of time, or in different laboratories with different temperature and humidity conditions, there will be more than one as-determined moisture value. Ideally, a split for moisture should be weighed at the same time and under the same conditions for each separate analysis. Also, it should be noted that samples previously processed, including grinding and air-drying, are not as-received samples even if they arrived at the laboratory in that condition. They are as-determined samples and a new moisture value should be determined.