Volume 6, Issue 2 (February 2009)
Determination of Appropriate Sample Depth during Air-Drying of Low Rank Coals Reduced to 2.36 mm (No. 8) Sieve Size
The current ASTM International standard for measuring Total Moisture in Coal (D3302-05) prescribes a multi-step process beginning with air-drying of the sample at a depth of one inch or less, but it has been noted in the literature that the final moisture results for low rank coals can be affected by the exact depth at which they are air-dried. The ASTM D05.21 Task Group on revisions to this method has conducted a number of studies on subbituminous C and lignite coals to assess whether the air-drying depth should be further restricted. Fresh samples were collected, prepared to minus 2.36 mm (No. 8) sieve size, then air-dried at various depths and analyzed at laboratories familiar with the particular coal(s) used. Compared to samples air-dried at more shallow depths, samples air-dried at depths of one inch showed lower results in total moisture by as much as -0.32 percentage points, with commensurate increases in calorific value of as much as +21 Btu/lb on the as-received basis. When air-drying was conducted at very shallow depths of 1/4 inch or less, statistically significant changes in MAF Btu/lb were identified. The likely explanation for the low moisture results is that the one inch depth does not allow buried coal particles to dry sufficiently, and moisture is then lost (but not accounted for) during subsequent pulverizing and riffling. It is thought that changes in MAF results are due to notable sample degradation occurring at shallow depths where many more particles are exposed to the atmosphere and significant oxidation takes place. Thus, varying the sample depth during air-drying can lead to inconsistent results for total moisture (and therefore as-received Btu/lb) as well as MAF Btu/lb. This paper concludes that to minimize these inconsistencies, air-drying of subC and lignite coals should be restricted to sample depths of about 1/2 inch.