Does Illinois bituminous coal manifest glass transition: DMA, DSC, and FTIR results

Abstract

It has been argued that coal, especially deep, unmineable seams, holds the potential of sequestering carbon dioxide (CO2) while recovering fuel gases. In the recent past, it has been proposed that various ranked coals display glass transition or second order transition at 100°C⩽T⩽200°C under nitrogen environment, and this transition temperature shifts to lower temperatures when coals were pressurized with relatively low pressure (⩽3MPa) CO2. If glass transition does exist in coal, then on pressurizing the deep, unmineable coal seams, where pressures are expected to be higher than 3MPa of CO2, the presence of glass transition could potentially pose reservoir stability concerns. We undertook differential scanning calorimetric (DSC) and dynamic mechanical analysis (DMA) measurements on Murphysboro seam coal (Illinois bituminous) at 30°C⩽T⩽250°C under N2 gas environment to ascertain whether this coal even has a glass transition. These experiments were further complemented by conducting in situ diffuse reflectance infrared spectroscopic measurements (DRIFT). It is worth mentioning that the DMA technique is much more sensitive in detecting glass transition than the DSC technique, and previously reported glass transitions were ascertained by the DSC method. Though we did observe thermal events, ascribed to the evaporation of bulk and hydrogen-bonded water at 90°C⩽T⩽190°C, we did not observe any glass or second order transition for Murphysboro coal at 30°C⩽T⩽250°C. The storage modulus (E′) of the coal steadily decreased as the temperature increased from 30°C with E′ showing two minima at 158°C and 221°C. These minima could be attributed to the evaporation of CO2 from the coal produced during self oxidation rather than any thermodynamic event.