Impacts of Mild Pyrolysis and Solvent Extraction on Coking Coal Thermoplasticity

The thermoplastic development of a coking coal was studied before and after treatment to study the impacts on thermoswelling and volatiles release. The treatment process consisted of two consecutive steps, namely mild pyrolysis and solvent extraction. The intention was to characterize the thermally generated and solvent extractable material produced immediately prior to swelling onset and to study the impact of its removal. The heating step removed ∼5% of the coal volatiles and reduced its swelling extent from 83% to 25%. Subsequent solvent extraction with tetrahydrofuran (THF) on the heated coal removed up to 21% of material on a dry basis. The residue after extraction showed little swelling extent and significantly altered volatiles release profile on heating, indicating the role of extracted materials in thermoplastic development and tar formation. The molecular weight distributions of volatile tars collected after pyrolysis shared a similar molecular weight distribution spanning between 200 and 600 Da and peaking at ∼347 Da. By comparison, the molecular weight distribution of the THF extract peaked at ∼472 Da and extended to ∼3000 Da. In addition, it consisted of two classes of compounds: one covered the molecular weight range of tars with repeating structures every 12–14 Da up to 600 Da and the other contained 24 Da reoccurring units at molecular weight above 600 Da.