Mechanochemical evolution of coal microscopic groups: A new pathway for mechanical forces acting on coal spontaneous combustion.

Coal spontaneous combustion (CSC) remains a significant threat to regional ecological environments. As coal mining operations extend deeper into the earth, the increasingly complex mechanical force conditions in deep-seated mines escalate the potential risk of CSC. Mechanical forces such as ground stress and mechanical cutting are traditionally believed to be linked to CSC through the following pathway: mechanical forces act → mechanical energy is input → mechanical crushing and pulverization occur → coal-oxygen contact area increases → CSC accelerates. Noteworthily, these forces do more than just physically break coal; they also trigger a mechanochemical effect (MCE) that alters coal's microscopic chemistry. However, an independent evaluation of its influence on CSC was lacking. This study characterized coal's microscopic chemical group responses to the MCE. It was found that the MCE led to the degradation of aliphatic side chains while enhancing the polycondensation of aromatic ring structures, indicating a synergistic effect. Additionally, an increase in oxygen-containing functional groups, such as alkyl/aryl ethers, suggested enhanced interactions of the coal microscopic groups with oxygen due to mechanical forces. Based on these findings, an MCE-modified coal macromolecular model was developed and molecular quantum mechanical calculations were conducted. The results indicated that the MCE boosted coal macromolecule reactivity, thus facilitating easier activation. These conclusions were validated through modern thermal analysis tests. Finally, this study proposed a new pathway of mechanical forces acting on CSC: mechanical forces act → mechanical energy is input → the MCE occurs → evolutions of the microscopic groups within coal are induced → Activity of coal molecules is enhanced → CSC accelerates.

Characteristics for Oxygen-Lean Combustion and Residual Thermodynamics in Coalfield-Fire Zones within Axial Pressure.

Coalfield fires during coal mining have become a major problem in the world today. To effectively prevent such disasters, we established an experimental platform to measure the spontaneous combustion characteristics of large-scale pressurized coal; thermal analysis experiments and microscopic analysis of briquettes under different axial pressures were carried out. It can be seen from the results that when the axial pressure is 4 MPa, the heating rate of the oxidative combustion of coal samples is accelerated, the crossing point temperature is lower (reduced by 71.09 °C), the activation energy is reduced (the second stage is decreased by 21.3 kJ/mol), and the oxidative combustion is more intense. Simultaneously, the porosity evolution process of briquettes under different axial pressures is simulated. Through calculation, it can be seen that the porosity and thermal conductivity show a linear increasing trend. The basis for the increase in the internal oxygen supply channels and increase in oxygen consumption when the axial pressure is 4 MPa is given. Through thermogravimetric-differential scanning calorimetry analysis, it is found that the maximum mass loss rate and maximum mass growth rate of residual coal after combustion under an axial pressure of 4 MPa are low, the residual rate after combustion is large, and the flammability rate is low when reoxidized, while complete combustion oxidation releases more heat. The application of axial pressure will change the combustion characteristics of briquettes, and the promotion effect is more obvious at 4 MPa. Analyzing the laws of the coal-oxygen composite reaction under different axial pressures provides theoretical guidance for the prevention and control of multistress coupling fields in coalfield-fire areas.

Synergistic inhibition effect on the self-acceleration characteristics in the initial stage of methane/air explosion by CO2 and ultrafine water mist.

Cellular instability is responsible for the self-acceleration of a flame, and such acceleration might cause considerable damage. This paper presents an experimental study on the inhibition effect of CO2 and an ultrafine water mist on the self-acceleration characteristics of a spherical flame in the initial stage of a 9.5% methane/air explosion in a constant volume combustion bomb. Results showed that insufficient water mist enhanced the self-acceleration of the spherical flame and the intensity of the explosion; nevertheless, the synergistic inhibition effect of CO2 and ultrafine water mist prevented enhancement of the explosion and significantly mitigated the self-acceleration of spherical flames, which observably delayed the appearance time of a cellular flame, and reduced the flame propagation speed, overpressure and the mean rate of pressure rise, indicating that suppression of flame self-acceleration could effectively mitigate the damage from a methane/air explosion. The reason for the synergistic effect was a result of a combination of physical suppression and chemical suppression: due to the preferential diffusion dilution effect of CO2, the initial flame speed was reduced, and the flame became thicker, which increased the evaporation time and quantity of droplets around the flame front, accordingly enhancing the cooling effect on the flame front. The increased flame thickness could withstand greater disturbance and inhibit the formation and development of a cellular flame. Meanwhile, CO2 and H2O can also reduce the concentration of active radicals (O, H and OH) and reduce the reaction rate and combustion rate of a methane/air explosion.

Methane explosion suppression characteristics based on the NaHCO3/red-mud composite powders with core-shell structure.

The NaHCO3/red-mud (RM) composite powders were successfully prepared by the solvent-anti-solvent method for methane explosion suppression. The RM was used as a carrier, and the NaHCO3 was used as a loaded inhibitor. The NaHCO3/RM composite powders showed a special core-shell structure and excellent endothermic performance. The suppression properties of NaHCO3/RM composite for 9.5% CH4 explosion were tested in a 20L spherical explosion vessel and a 5L Perspex duct. The results showed that the NaHCO3/RM composite powders displayed a much better suppression property than the pure RM or NaHCO3 powders. The loading amount of NaHCO3 has an intensive influence on the inhibition property of NaHCO3/RM composite powders. The best loaded content of NaHCO3 is 35%. It exhibited significant inhibitory effect that the explosion max-pressure declined 44.9%, the max-pressure rise rate declined 96.3% and the pressure peak time delayed 366.7%, respectively.