Optical properties of vitrinite carbonized at different pressures

The effect of pressure on the optical properties of cokes from a medium volatile bituminous coal (carbon = 87.9 wt% daf), some carbonized at atmospheric pressure and others under hydraulic pressure (21–310 MPa), over temperatures ranging from 350 to 600 °C at 50 °C intervals, has been studied. The cokes formed at atmospheric pressure developed fine grained mosaics, while medium-flow type mosaics formed in coke carbonized under hydraulic pressure. The thermal decomposition stage began at lower temperatures with increasing hydraulic pressure, resulting in a prolonged devolatilization phase for coke formed at a pressure of 21 MPa. Hence the fluidity of samples carbonized under pressure decreases with increasing hydraulic pressure. Pressure promotes the optical anisotropy apparent from the level of bireflectance. The reflectance of coke formed at atmospheric pressure is higher than that of cokes carbonized under hydraulic pressure, perhaps due to the inhibitory effect of entrapped volatile matter during carbonization under hydraulic pressure. The morphological features of vitirinite carbonized under pressure resemble those of coals naturally affected by heat.