Thermo-swelling Behavior of Australian Coking Coals from Different Basins: Relating to Rank and Maceral Compositions

Eight Australian coking coal samples (R-vMax from 0.8 to 1.6) from six different basins were employed to investigate the effect of coal maceral compositions and rank on thermo-swelling behavior. Coal pyrolysis experiments were conducted at a heating rate of 5 C/min from room temperature (25 degrees C) to 1000 degrees C with a top coal particle size of 212 mu m. Thermoplasticity of coal was evaluated on the basis of the change of permeability of coal pellets, plastic range, swelling factor (defined by maximum swelling %/vitrinite %), and heat of exothermic reactions during the primary devolatilization. The endothermic and exothermic processes were identified by the estimate of the apparent specific heat using the computer-aided thermal analysis (CATA) technique, while swelling and permeability of gas flowing through coal pellets were simultaneously investigated with extended volumetric measurement downstream and pressure sensor upstream, respectively. Volatile evolution profiles of heating coal samples with temperature were obtained using thermogravimetric analysis (TGA). Overall, the low-rank coals with high vitrinite contents showed larger exothermic heats, higher volatile evolution rates, and higher swelling factors than the high-rank coals with low vitrinite contents. These lower rank coals also initiated a decrease in permeability at lower temperatures, showed large plastic ranges, and resulted in a larger pore size of semi-coke. When we account for total fusible maceral (vitrinite plus fusible semi-inert) rather than vitrinite content alone, the trend of the swelling factor decreasing with the coal rank became clearer. However, it was found that coals from basin III showed a significantly lower swelling factor than the comparative rank coals and the heat of exothermic reactions showed a similar trend, despite these coals having a similar volatile evolution rate with the comparative rank coals. The reasons for this have been discussed on the basis of the thermoplasticity, which suggests that not only the rank and maceral compositions but also the origin and chemistry of the coal may affect thermoplasticity of coking coal.