Experimental research on the characteristics of the secondary oxidation heat of different unloaded coals

The oxidation heat characteristics of coal are key factors affecting the spontaneous combustion of coal. To study the thermal characteristics of the secondary oxidation of unloaded coal, a C600 microcalorimeter was used to test the unloaded coal under different stress conditions (0 MPa, 3 MPa, 6 MPa, 9 MPa, 12 MPa, 15 MPa) at 30–300 °C. The initial heat release temperature, heat release, and activation energy of coal were analyzed. The results shows that the initial heat release temperature of unloaded coal shows a trend of first increasing and then decreasing with increasing stress, and as the degree of oxidation increases, the maximum value of the initial heat release temperature point moves to a lower value. As the stress increases, the amount of heat release shows a fluctuation that first decreases and then increases, and as the degree of oxidation increases, the minimum heat release moves forward from 12 MPa for primary oxidation to 6 MPa for secondary oxidation. Similarly, the activation energy first increases and then decreases with increasing stress, and the maximum activation energy increases with increasing degree of oxidation, which is consistent with the trends of heat release. As the degree of oxidation increases, the overall performance of unloaded coal with increasing stress shows that the preoxidized coal sample is more susceptible to oxidation when it is reoxidized.