Variations in the mechanical behavior of Illinois bituminous coals

Unmineable coal beds are being considered as one of the geological sequestration options for storing carbon dioxide (CO2). The storage mechanisms and potential risks associated with the effects of CO2 on the coal structure are not yet understood and must be evaluated. The mechanical properties of the coal are expected to play an important role in the coal seams’ stability, especially under external perturbations. Typically, the mechanical characteristics of coal are investigated as a bulk material, which averages the effects of various structural inhomogeneities as well as of face and butt cleat fractures present in the coal. In this paper, we attempt to establish baseline mechanical characteristics of Illinois bituminous coals while minimizing the fracture effects. Rectangular coal strips (length <20 mm), which showed no visible macro-defects, from two different Illinois bituminous coal seams, were subjected to three-point bending tests. Our results suggest there are significant variations in the flexural modulus (ranging from 0.7 GPa to 3.4 GPa) of the coal samples even though the coal rectangular strips originated from the same coal chunk. Vibrational spectroscopic analysis on the samples, which underwent mechanical testing, indicates a correlation between the flexural strength and modulus with the intensity of aliphatic groups. However, the mineral content of the coal seems not to influence the mechanical behavior of Illinois bituminous coals.