Production and characterization of ash-free coal from low-rank Canadian coal by solvent extraction

In this work, ash-free coal (AFC) was extracted from low-rank Canadian coals with mean maximum vitrinite reflectance (MMVR) in the range of 0.38-0.69 using non-polar organic solvent, organic solvent combination (polar-nonpolar solvent mixture), and with and without hydro-treated heavy aromatic hydrocarbon solvents from coal-tar industry to study the effect of type of coal and solvent type on the production yield of AFC. High temperature solvent extraction was carried out in 0.5 L autoclave in the temperature range of 473 to 723 K. It was observed that 1-methylnaphthalene (1-MN), a non-polar solvent did not give any significant difference in yields [~ 30% (daf)]. However, an extraction yield of 73% (daf) AFC was achieved using hydrotreated aromatic hydrocarbons at 673 K. The performance of extraction yields was correlated by the vitrinite content and MMVR of the coal and it was observed that higher proportions of vitrinite and a lower MMVR value of coal produced higher extraction yield. Proximate and ultimate analysis, FTIR, ICP-MS, 13C CP/MAS NMR, thermogravimetric analysis and particle size distribution were used to characterize AFC. The heating value of the AFC was estimated to be in the range of 36-37 MJ/kg and a substantial decrease of sulfur content (ca. 12.5-61.1%) is also observed in AFC. AFC showed a narrower particle size distribution with a d50 of 7.0 μm.