Atomic-scale experimental study of oxidation mechanism of low-metamorphic coal at room temperature

The oxidation of coal at room temperature provides the initial heat for the occurrence of coal spontaneous combustion (CSC). The study of the oxidation mechanism of coal at room temperature can provide a theoretical basis for controlling the occurrence of CSC. In order to distinguish the oxygen atoms in natural coal and air (¹⁶O abundance of 99.756%), the coal samples were oxidized at normal temperature under different conditions in dry air and ¹⁸O₂ atmosphere. The rule of change of oxygen-containing gas molecules (O₂, CO and CO₂) outside the coal was analyzed by the homemade room-temperature cyclic oxidation and multi-component gas real-time on-line monitoring system. The migration characteristics of oxygen atoms in coal were analyzed by Fourier transform infrared spectroscopy. The oxygen isotope content of oxygen-containing functional groups in raw coal and gas generated by coal oxidation in dry air was compared by isotope ratio mass spectrometer, and the ¹⁸O isotope accumulation fractionation phenomenon of oxidation products was analyzed. Finally, the oxidation mechanism of low-metamorphic coal at room temperature is discussed by the correlation between the oxygen molecules outside coal and the migration of oxygen atoms inside coal through the distribution of oxygen isotopes of CO and CO₂ labeled by ¹⁸O. The results showed that the rate of oxygen consumption was determined by the oxygen concentration and that there was a quadratic relationship between the two during the oxidation of low-metamorphic coals at room temperature. The number of reactive groups reacting with oxygen in the coal is sufficient and the type has not changed significantly. The number of reactive groups reacting with oxygen in the coal was sufficient and the type did not change significantly. The CO concentration increased proportionally to the oxidation time and then increased slowly. The concentration of CO increased as a proportional function of oxidation time and then increased slowly. The CO₂ concentration had a primary function with the oxidation time. CO and CO₂ release rates were influenced by a combination of chemical reactions and desorption-diffusion processes. By analyzing the relationship between the oxygen consumption pattern and functional groups, it was found that the aliphatic structure in coal is the key active group that affects the rate of oxygen consumption. The aliphatic structure reacts with oxygen to provide the initial heat for CSC. The formation of CO and CO₂ from the room temperature oxidation of low-metamorphic coals was the result of the synergistic action of oxygen-containing functional groups and the oxidation of aliphatic structures. Oxygen isotope fractionation does not occur in the experiment of coal oxidation at room temperature using self-made cyclic oxidation system, and oxygen isotope tracer experiment is feasible. More than 88% of CO originates from H-recapture reactions of aldehyde and hydroxyl groups with hydroxyl radicals, and more than 97% of CO₂ originates from H-recapture reactions of carboxyl groups with hydroxyl radicals. The use of targeted inhibitors to reduce the activity of aliphatic structure and the concentration of hydroxyl radical can effectively inhibit the CSC and solve the problem of exceeding the upper limit of CO.