Irreversible structural changes in coals during heating

A differential scanning calorimetry (DSC) study on the extraction residues of coals giving different extraction yields (which were prepared by changin g the solvent composition of a carbon disulfide-N-methyl-2-pyrrolidinone mi xed solvent extraction) was carried out. For the residues from Upper Freepo rt coal (APCS-1) with extraction yields lower than 30 wt % (daf), an endoth ermic peak similar to that given by the raw coal was observed around 350 de grees C. This endothermic peak disappeared on the second and third scans, i ndicating that the peak is due to irreversible structural changes in coals. For the residues from the high extraction yield experiments, the peak was not observed up to 400 degrees C, even during the first scan. The reason fo r these endothermic peaks was discussed from the relationship among the ext raction, swelling, and structural changes of coals.