Study on the characteristics and microscopic mechanism of coal spontaneous combustion based on programmed heating experiment

For the situation that coal is still capable of oxidative spontaneous combustion under different oxygen conditions, we choose three coals with different metamorphic degrees: lignite, gas coal and fatty coal, selected as experimental samples to carry out programmed temperature gas chromatography(TPGC) experiments and molecular dynamics simulations based on ReaxFF force fields at oxygen concentrations of 3%, 5%, 10%, 14% and 18%, to investigate the characteristics of changes in the oxygen influence of groups under oxygen-poor conditions in coals with different degrees of metamorphosis according to the production patterns and pathways of CO and CO2, etc. The results show that coal is easily affected by temperature and oxygen to produce indicator gases such as CO and CO2, and when the indicator gas tends to increase exponentially, the lower the degree of coal deterioration and the lower the temperature at which the indicator gas is produced at the same oxygen concentration. The simulations show that the reaction of coal with oxygen under oxygen-poor conditions is likely to be an active intermediate fragment reaction between oxygen and coal affected by temperature, rather than a direct attack on the main structure of the coal. The coal structural groups are not directly altered when the coal is subjected to temperature, but undergo structural adjustments for bond twisting, breaking and atomic transformations to produce reactive intermediate fragments suitable for reaction with oxygen. The higher the degree of metamorphosis, the more stable the molecular structure of the coal and the less likely it is to be affected by temperature to produce reactive intermediate fragments, and when oxygen is introduced, the rate of reaction of oxygen with the reactive intermediate is lower than the rate at which the coal is affected by temperature to produce the reactive intermediate, therefore, oxygen under depleted conditions is generally reacted with reactive intermediates to produce indicator gases such as water, CO, rather than with the main structure of the coal among them. A combination of experimental and simulation analysis reveals that the amount of water and oxygen containing gases increases significantly at 3%-5% oxygen concentration for lignite, 10%-14% oxygen concentration for gas coal and 14%-18% oxygen concentration for fat coal compared to the other stages, indicating that these three coals with different degrees of deterioration are more affected by oxygen in this oxygen-poor concentration range and accelerated the process of coal oxidation reaction. © 2024 China Coal Society. All rights reserved.