The impact of errors associated with the sampling and analyses of coal are often underestimated. This review looks at the impact of the stated error of dry ash when using general purpose ASTM coal sampling. ASTM D 2234 is the “Standard Practice for Collection of a Gross Sample of Coal” and it states that using the general purpose method, which most of us use will produce a dry ash value +/- 1/10 of the value obtained. Ten percent ash coal will report between 9 and 11 percent ash, 95% of the time. This is when we take the 35 increments of 3 Kg each for each 908 metric ton lot. There is no mention of the precision of moisture values, but many believe that it is at least that of dry ash. This is a real issue with coals being purchased in the 20 to 40% moisture range. A coal with thirty percent moisture will be in the 27 to 33% moisture range 95% of the time. These variations or potential errors directly influence the calorific or heating value of a coal.
This means that the potential errors cause by coal sampling can exceed that of the CV measurement. The CV potential error listed for the same 60 mesh coal sample is 56 Kcal/Kg for different operators and machines. Sampling errors can influence the measured CV hundreds of Kcals/Kg depending on the coal type.
The chart below uses the following coals to look at sampling error and the 56 Kcal/Kg lab error on the price of coal. As the ash and moisture go up, the error increases.
Coal Used all priced at $50 per ton ( I wish)
CV Kcal/Kg Moisture dry Ash Average (and Price) CV Error Potential
7,333 5 7 1.7 %
6,555 12 8 2.8 %
6,000 18 10 3.8 %
5,000 27 4 4.8 %
4,667 30 6 5.6 %
3,800 35 10 7.3 %
Chart 1. Showing as the diluents moisture and ash increase, the error in CV measured is impacted.
This is analyses of error impacts strongly suggests that buyers of low rank, low CV coal have substantially more risk in analyses error than high rank bituminous coal buyers have.
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