Evolution of pore structure and changes in oxidation characteristics of coal after heating freezing treatment

To study the spontaneous combustion characteristics of preheated coal treated by liquid nitrogen (LN2), surface structure test, thermal analysis experiment and chemical characteristic analysis experiment were used to study the change characteristics of surface structure, oxidation characteristic and chemical structure of coal under the action of LN2 quenching. The results showed that with the increase of preheating temperature, the fractal dimension, specific surface area, pore volume and average pore diameter of the LN2 treated coal (quenched-coal) increase. With the increase of preheating temperature, under the oxidation condition, the CO and CO2 produced at ST of 230 ℃ are 1.19 times and 1.21 times higher than those at ST of 70 ℃. The CO/CO2 emission of quenched-coal is significantly increased, and the secondary oxidation capacity is stronger. Due to the more developed pore cracks of coal under pyrolysis and quenching conditions, the natural characteristics of coal are stronger. The oxygen adsorption capacity and apparent activation energy of pyrolysis coal in the stageⅠ affected by LN2 are stronger. Oxidized coal shows stronger oxidation and heat release ability in the stageⅡ under the action of LN2, resulting in higher apparent activation energy in this endothermic stage. The addition of LN2 blocked the oxidation reaction of preheated coal and sealed the structure of -C = O, –COOH and –OH. The bridge bond –CH2- structure of pyrolysis coal is further destroyed, and the ratio of –CH2/–CH3 is reduced. The increase of alkyl free radicals in pyrolysis coal and alkyl oxygen free radicals in oxidation coal accelerates the spontaneous combustion oxidation of coal. The research results have important guiding significance for the occurrence and prevention of coal reignition disaster after LN2 fire quenching.