Extraction of Taiheiyo coal with supercritical water–HCOOH mixture

Taiheiyo coals were extracted with supercritical toluene (SC-toluene) (653 K, 20 MPa), supercritical water (SCW) (653 K, 35 MPa) and formic acid (HCOOH)–SCW mixed solvents (653 K, 35 MPa) using a semi-batch type system. The results clearly indicate that the coal conversion (=1−weight of residual coal (daf)/weight of loaded coal (daf)) and the liquid yield (=weight of liquid extracts/weight of loaded coal) in HCOOH–SCW were higher than those in SCW and SC-toluene. A considerable portion of coal is thus converted into light oils probably through hydrolysis and hydrogenation in HCOOH–SCW. We conducted another series of experiments for direct in situ observation of coal conversion in SCW and HCOOH–SCW using a diamond anvil cell (DAC). In HCOOH–SCW, the effective and rapid hydrogenation of coal with HCOOH occurs mainly at the early stage of the reaction.