New applications of differential scanning calorimetry and solvent swelling for studies of coal structure: Prepyrolysis structural relaxation

Abstract

Differential scanning calorimetry and solvent swelling have been applied for identifying physical transition temperatures in the macromolecular structure of coals. The transition processes seem to be associated with physical relaxation of the coal structure and are irreversible processes. In Pittsburgh no. 8 coal, one physical transition was noted at 250–300 °C (at 8 °C min −1) without any significant accompanying weight loss. Illinois no. 6 coal does not exhibit such a relaxation process at these temperatures. Coals of higher rank than high volatile bituminous i.e. Upper Freeport and Pocahontas no. 3 coals, exhibit structural relaxation just before the major thermal decomposition process and a sharp increase in solvent swellability accompanies this relaxation. In the case of both the Pittsburgh no. 8 and the Upper Freeport coal, structural relaxations at around 360 °C seem to coincide with release of ‘guest molecules’. In addition, significant shifts in transition temperatures have been observed with heating rate. In coals of higher rank than high volatile bituminous, pyridine exposure prior to thermal treatment does not influence the transition temperatures, indicating that heat is more effective than pyridine in relaxing the macromolecular structure, as suggested by Sakurovs et al. On the other hand, CS 2 pyridine mixtures are effective at relaxing the structure of the Upper Freeport coal, even at room temperature.