Studies on coal pyrolysis via a thermogravimetric analyzer (TGA) are typically performed under a variety of temperature increases beginning at ambient temperature. A main limitation in these procedures is that the reaction temperatures (generally lower than 1000 degrees C) are typically lower than those met in pulverized coal boilers and ash flow temperatures (AFTs; generally higher than 1100 degrees C). In this paper, five Chinese coals are clarified to low- or mid-AFT coals as their AFTs increase. They were then prepared in TGA at several temperatures below or above their AFTs under N-2 atmosphere for approximately 30 min after a heating procedure at a rate of 80 degrees C/min. The coal and residual char samples were collected and analyzed via X-ray diffraction and compared to the chemical thermodynamic calculation. The secondary weight loss appeared between 1200 and 1450 degrees C in the TG curves of four mid-AFT coals but not the low-AFT coal. The results showed that this secondary weight loss was caused by the carbothermal reduction and carbothermal reduction nitridation reaction between minerals and carbon, which led to the appearance of silicon carbide, aluminum nitride, and silicon nitride. The low concentrations of SiO2 and Al2O3 and high concentrations of Fe2O3 and CaO inhibited the secondary weight loss process in high-temperature coal pyrolysis. The reason why the secondary weight loss was not present in low -AFT coal was possibly due to low-melting eutectic noncrystal matters consisting of calcium oxide and ferrum oxide. These fused low-melting eutectic noncrystal matters stopped the carbothermal reaction at high pyrolysis temperatures and inhibited the production of SiC, AIN, and Si3N4.