Selective maceral enrichment during grinding and effect of particle size on coal devolatilisation yields

The effect of particle size on rapid-heating volatile yields from a range of particle size cuts prepared from three different coals have been measured using a wire-mesh apparatus. Samples were pyrolysed in helium at heating rates of 1000 K s(-1) to 950 degrees C with 10 s hold time. Particle size cuts over the range 38-150 mu m were prepared by two methods: sieving from a ground coal and regrinding the largest-sieved size cut. The latter approach was employed in a (partially successful) effort to minimise differences in size-cut compositions. In general, total daf volatile yields were observed to increase slightly with decreasing particle size, but for two of the coals, this effect was not monotonic over the range 38-150 mu m and was also dependent on the sample preparation method. Maceral analysis showed significant differences in all cases; however, with smaller particle-size cuts generally enriched in liptinite. To examine the effect of particle size without interference from maceral enrichment, a vitrain was used to prepare size cuts over the range 38-300 mu m. Volatile yields from these samples were virtually identical. The overall conclusion from this study is that mass transfer effects due to different particle sizes are probably less significant than unavoidable maceral enrichment effects during sample preparation. For rapid-heating volatile release tests (e.g., drop tube furnace, wire-mesh apparatus) requiring a sized coal sample, a comparison of TGA (thermogravimetric analysis) volatile yield measurements have been shown to be a reasonable indicator of whether or not the size cut has a similar composition to the whole coal. (C) 1998 Elsevier Science B.V. All rights reserved.