Effect of calcium chloride on the self-ignition behaviours of coal using hot-plate test

Abstract The effect of calcium chloride (CaCl2) on the self-ignition behaviours of coal was studied under air atmosphere in this paper. Aqueous CaCl2 solutions (50, 100, 200, and 300 g/L) was used to treat a non-caking coal sample with high self-ignition potential, and the minimum auto-ignition temperature (MAIT) of the coal dust layer was determined using hot plate test to affirm thermal runaway. Both thermogravimetric analyzer and differential scanning calorimetry (TGA-DSC) were applied to investigate the decomposition of the samples. Results showed that CaCl2 generated a coating film with dense slurry-like envelope on the particle surface, which took an inhibition effect at low thermal conditions (before crossing point) because of heat absorption of moisture evaporation and oxygen diffusion resistance of pore structure blockage. The MAIT of the CaCl2-treated coal sample was increased from 210 to 240 °C, with a max elevation of 30 °C in comparison to H2O-Coal sample. However, opposite effect was also found at higher temperatures (after the crossing point). For 50 g/L CaCl2-Coal sample on hot surface of 240 °C, the calculated inhibiting rate at 10 and 50 min was 54.8% and −115.6%, respectively, suggesting that the catalysis effect at the later stage after ignition was over two times than the inhibiting effect at the initial self-ignition process. The Ca2+ in the form of its oxides (CaO and CaO2) at higher thermal conditions promoted the diffusion of oxygen to the coal surface and expanded the main combustion zone, suggesting a catalytic role in the coal combustion.