Effect of water soaking and air drying on the thermal effect and heat transfer characteristics of coal oxidation at the low-temperature oxidation stage

The microstructure, thermal behaviour (30 °C-300 °C) and thermophysical properties (30 °C-300 °C) of raw and water-soaked air-dried coal samples were analysed through scanning electron microscopy, differential scanning calorimetry and laser thermal conductivity meter to study the effect of water soaking and air drying on the thermal effect and heat transfer characteristics of coal at the low-temperature oxidation stage. Experimental results show that the action of water soaking and air drying changes the apparent morphology of coal, increasing the density of surface pore distribution and number of all pore types to different degrees. Under the same temperature conditions, water-soaked air-dried coal has lower heat absorption and heat release than raw coal. Moreover, the time required by water-soaked air-dried coal to reach thermal equilibrium is 32.4 s faster than that needed by raw coal. The activation energy of water-soaked air-dried coal calculated by using the KAS and FWO methods is reduced to about 83% of that of raw coal. This result indicates that water soaking and air drying increase the tendency of coal to self-ignite. The thermal diffusivity of water-soaked air-dried coal is higher than that of raw coal and shows a maximum difference of 0.005 mm2/g at 60 °C. The specific heat capacity and thermal conductivity of water-soaked air-dried coal are lower than those of raw coal and show the maximum differences of 0.19 J g/K and 0.18 W/m·K at 210 °C and 300 °C, respectively. Although the spontaneous combustion of water-soaked air-dried coal spreads at a faster rate than that of raw coal, the hazards following the spontaneous combustion is less than that of raw coal. This research contributes to understanding the effect of water soaking and air drying behaviour on coal spontaneous combustion characteristics.