Impact of temperature-stress synergistic effects on coal structural alignment

The potential synergistic effect of temperature-stress ( T = 25–1000 °C, P = 0–300 MPa) on coal coalification and graphitization has been reported since 1960 s. However, its contribution to coal structural alignment, which will likely enhance graphitization, remains unclear. Hence, we have performed both single-factor ( T or P ) and dual-factor ( T and P ) experiments alongside structural characterization (Raman and high-resolution transmission electron microscopy, HRTEM) to unravel the impact of temperature-stress synergistic effects on structural alignment of basic structural unit (BSU) using high-volatile bituminous coal as starting point. Compared with raw coal, the average orientation of polycyclic aromatic hydrocarbon (PAHs) for the synergistically-altered coals is enhanced by 31 %, higher than the sum of thermally- (6 %) and stressed-altered coals (21 %), indicative of significant temperature- stress synergistic effects. Greater structural alignment of PAH length, stacking, and curvature were also achieved during T - P synergistic effects than individual thermal- or stress-related coalification. However, their dominated factor varies. The PAH size and stacking were enhanced by heating. In contrast, the PAH orientation enhanced more by shear-stress, rather than heating. The I _D/ I _G (area ratio of D peak to G peak in Raman spectrum) increases at temperatures <1686 °C, then decreases with higher temperature. The T - P synergistic effect increased the graphitization degree, however, stress in the absence of heating mainly induced additional structural defects. Hence, temperature-stress synergistic effect is expected to favor coal structural alignment in areas affected by both tectonic stress and contact metamorphism.