Index gases generation law of different rank coal molecules based on ReaxFF molecular dynamics

To obtain the index gases generation characteristics and differences of coal from different ranks during spontaneous oxidation, five coal samples representing four coal ranks were selected for research. Molecular models of coals were established, and molecular dynamics simulations using the ReaxFF force field, combined with heating experiments on coal samples, were conducted to derive the macroscopic and microscopic generation rules of the index gases C2H4, C2H2, CO, and CO2.Research indicated that during the spontaneous combustion of coal, C2H4 and C2H2 depend primarily on the amount of aliphatic side chains, while CO and CO2 depend primarily on the carbon atoms in the coal molecule. When oxidative spontaneous combustion reaches the later stage, C2H4, C2H2, and CO are converted to CO2, the complete product of combustion. The initial temperatures and the peak values of index gases are in order of coal rank. The order of appearance of the different index gases is CO2, CO, C2H4, and C2H2. The research plays a pivotal role in determining the appropriate oxidation-induced spontaneous combustion index gases within goafs, thereby providing a theoretical basis for the subsequent development of comprehensive mine fire prevention and extinguishment strategies.