Changes in Coal Aggregate Structure by Heat Treatment and Their Coal Rank Dependency

Abstract

Several Argonne coal samples were heat-treated at variable heating rates of 10 to 1000 °C/ min in the temperature range of 250 to 450 °C, to clarify changes in secondary aggregated structure of the coals revealed by step scan XRD and solvent swelling measurements. Swelling ratio of the Wyodak(WY) coals with DMF once increased by the heat treatment at 250 °C, and then decreased above 300 °C, suggesting that the coal aggregate structure may be loosened by the removals of carboxylic and phenolic groups in coal macromolecules at around 250 °C. The step scan XRD profiles of the coal showed a slight enlargement of the peak at 20°, that is the so-called γ-band, reflecting from such loosening of coal aggregate structure revealed by the removal of oxygen functional groups through the heat treatment. In the case of Beulah Zap(BZ) coal, the XRD profile was not reflected in the solvent swelling, probably due to its higher content of ion-exchangeable cations, which show some hardness against solvent swelling. For the higher ranking coals of Pocahontas(POC), the solvent swelling ratio did not change so much by the heat treatment, reflecting its insensibility to the low-temperature heat treatment. The swelling ratio of Upper Freeport(UF) coal was not able to be measured because of the high fusibility of the coal particles above 350°C. The XRD profile of WY coal was in good agreement with the solvent swelling behavior. It is noted that the peak at 26° was intensified by the heating treatment of UF coal above 350 °C, suggesting its enhanced fusibility by rapid heating. The higher heating rate influenced more significantly such ordering of the coal aggregate structure, making the coals fusible at relatively lower temperatures below 400 °C. Such effects of the heat treatment are discussed in terms of the changes in the coal aggregate structures revealed by XRD and solvent swelling which are both strongly subjected to the coal rank.