Influence of Long-Term Immersion in Water at Different Temperatures on Spontaneous Combustion Characteristics of Coal.

The effect of hydrothermal coupling on the risk of spontaneous combustion of coal (SCC) soaked at different water temperatures was studied. The results showed that the porosity and permeability of coal increased significantly after immersion in water at different temperatures. The increase at a water temperature of 30 °C was the largest, the porosity was 22.77% higher, and the permeability was 3.44 times that of raw coal. The relative content of oxygen-containing functional groups, aliphatic functional groups, and hydroxyl functional groups reached 1.8-4.5, 1.1-4.4, and 0.7-1.48 times that of raw coal after soaking in water at different temperatures. The increase ratio of functional groups is the largest under the water temperature of 30 °C. The activation energy of coal samples soaked at water temperatures of 30 and 45 °C decreased by 9.4 and 2.9%, respectively. Under the condition that the coal sample temperature was lower than 110 °C, the highest oxygen consumption rate and heat release rate were coal samples soaked at water temperatures of 30 °C, and the lowest was raw coal. When the coal temperature was higher than 170 °C, all coal samples soaked in different water temperatures are higher than that of raw coal, and the water temperature of 30 °C is the highest. The risk of spontaneous combustion of coal is highest after long-term soaking at 30 °C water.

Experimental Study on Flammability Limits Behavior of Methane, Ethane, and Propane with Dilution of Nitrogen.

The dilution inerting process of multi-component flammable gaseous mixtures is an important emergency disposal technology that has been widely applied in the explosion-proof field of flammability gases (vapors). In this study, we examined the flammability limits (LFLs and UFLs) of mono and binary alkane mixtures of methane, ethane, and propane when nitrogen is used for dilution inerting. The HY12474B explosion limit test device was used to determine the flammability limits, and the obtained data were compared with the literature data and Chatelier's law. Additionally, the sensitivity coefficient of the chemical reaction chain for LFLs and UFLs of the binary alkane mixtures was analyzed. The minimum inerting concentration (MIC) of methane was found to be sequentially higher than that of ethane and propane when using nitrogen for dilution inerting, and the MIC of the binary alkane mixtures follows the rule of methane/ethane > methane/propane > ethane/propane. Chemical kinetics calculation revealed that the maximum positive sensitivity coefficient of methane/ethane, methane/propane, and ethane/propane are both R5 H + O2 ↔ O + OH, and the reaction with the maximum negative sensitivity coefficients are both R34 H + O2(+M) ↔ HO2(+M) and R43 CH3 + H(+M) ↔ CH4(+M), respectively. The limiting oxygen concentration (LOC) for both mono alkane and binary alkane mixtures ranged between 10 and 13%. The region of the triangular flammability diagram for methane and ethane was greater than the regions for methane/ethane and methane/propane. In contrast, propane had a smaller region compared to other mono alkane or binary alkane mixtures.

Low-Field NMR Experimental Study on the Effect of Confining Pressure on the Porous Structure and Connectivity of High-Rank Coal.

To study the influence of different confining pressures on the pore structure and connectivity of high-rank coal, the high-rank raw coal of the Shanxi Xinjing Mine No. 9 coal seam was studied. A low-field nuclear magnetic resonance (LNMR) test system and a vacuum pressurized water saturation system were used to analyze the samples. The T 2 spectra of samples, saturated with water under different confining pressures and containing residual water after centrifugation, were tested. The coal sample pore size distributions, permeabilities, free fluid values, bound fluid values, and other parameters were obtained, and a calculation model of the coal pore connectivity ratio was established. The results were as follows. When the saturated pressures were 5, 10, 15, 20, 25, and 30 MPa, the pore diameters of the coal samples were mainly concentrated in the ranges of 0.00023-0.069 and 1.29-24.09 µm. Among them, micropores (<10 nm) and small pores (10 < 100 nm) account for the main part, mesopores (100 < 1000 nm) were underdeveloped, and relatively few macropores (>1000 nm) and fissures developed. As the confining pressure increased, the coal porosity and connectivity showed a trend of decreasing, then increasing, and finally remaining basically unchanged. The total pore connectivity rates of the coal samples were 37.0-62.6%. The interconnection rates of the micropores, small holes, mesopores, and macropores are 2.90-34.55, 89.09-99.03, 97.09-100, and 100%, respectively. The total pore connectivity followed an exponential functional relationship with permeability, and the critical confining pressure of high-rank coal was 25 MPa. These results provide a scientific basis for the high-pressure water injection of high-rank coal seams.

Experimental Study on Spontaneous Combustion Characteristics of Large Coal Particles after Soaking.

The spontaneous combustion of coal is affected by many factors, among which the influence of water is significant and complicated. To explore the influence of water on the spontaneous combustion characteristics of goaf residual coal, coal samples with similar particle size distributions to those of goaf residual coal were prepared. After the coal samples were immersed in water for 7-21 days and the external flowing water was drained, spontaneous combustion experiments were carried out using a temperature-programmed method. The results showed that soaking in water could promote and inhibit the spontaneous oxidative combustion of large coal particles in different temperature ranges. When the coal temperature was below 50 °C, water immersion had a significant inhibition effect on coal oxidation and spontaneous combustion. When the temperature of coal was 50-110 °C, soaking in water for 7 days could promote the oxidation and spontaneous combustion of coal. However, soaking for 14 and 21 days had a significant inhibition effect in this temperature range. When the coal temperature was higher than 110 °C, water immersion had a significant inhibition effect on the coal. Moreover, a prolonged immersion time significantly enhanced the inhibition effect. When the immersion time was less than 21 days, the spontaneous combustion of large coal particles by short-term soaking was mainly inhibited.

Rapid Contrastive Experimental Study on the Adiabatic Spontaneous Combustion Period of Loose Lignite.

The adiabatic spontaneous combustion period of coal is an important index for the macroscopic characterization of coal spontaneous combustion, and it is affected by many internal and external factors. There are several methods to study it, but there are various shortcomings to these methods. Some require too much time, while others have too many interfering factors. To quickly obtain the accurate adiabatic spontaneous combustion period of coal, a rapid contrastive experimental method was designed. In this method, the coal samples of the experimental and control groups were the same, and air and nitrogen were used as control atmospheres. A theoretical calculation method for the adiabatic spontaneous combustion period based on this method is proposed. The experimental results showed that during the temperature-programmed coal spontaneous combustion experiment, the increase in the coal temperature was due to physical and chemical heating. Physical heating is the heating effect of the temperature-programmed furnace body and the heated gas on the coal sample. Chemical heating includes oxidative exothermic heating promoted by physical and adiabatic oxidation heating. The adiabatic oxidative heat release can be determined by the oxidation heat release in the air atmosphere minus the oxidation heat release corresponding to the coal sample temperature in the nitrogen environment at the same period. The adiabatic spontaneous combustion period of coal can be calculated from the adiabatic oxidation heat release. Our results provide a rapid contrastive experimental method to quickly obtain the accurate adiabatic spontaneous combustion period of coal.

Experimental Study on the Variation of Surface Widths of Lignite Desiccation Cracks during Low-Temperature Drying.

Significant volume shrinkage and drying cracking of high-water-content lignite will occur during low-temperature drying. To determine the variation behaviors of the drying shrinkage rate and desiccation crack surface width in the process of low-temperature drying, low-temperature and low-humidity drying experiments were conducted, and the variations of the surface widths of the desiccation crack with time and water content of old lignite were examined. The results showed that the slow drying of lignite at low temperatures caused significant volume shrinkage and desiccation crack formation, and the occurrence and development of desiccation cracks were highly nonuniform. Three stages of the variation of surface widths of the desiccation cracks were observed with the water content decrease: an initial rapid increase stage, a second slow decrease stage, and a final stable stage, and the average width of the desiccation cracks increased in a Gaussian function. The higher the evaporation rate and volume drying shrinkage rate, the lower the surface width of the desiccation cracks under low-temperature drying conditions. To achieve safe and green mining, storage, transportation, processing, and utilization of lignite, the moisture content of old lignite should be controlled to be above 13-15%.

Regularity of Mine Gas Flow Disaster Induced by Gas Natural Ventilation Pressure after Coal and Gas Outbursts.

The gas pressure generated during a coal and gas outburst is an important factor affecting the stability of mine ventilation systems. The gas released from an outburst flows and diffuses in the mine, leading to an uneven distribution of the air in the mine ventilation system and the formation of natural wind pressure. Because of the effect of the gas pressure, the mine ventilation system becomes disordered, leading to a counter flow in the roadway. This increases the complexity of the gas movement, extends the influence range of the gas, enlarges the disaster area in the mine, and exacerbates the destructiveness. In this study, the TF1M simulation program was applied to simulate a coal and gas outburst accident that occurred in the 1747 heading roadway of the Sizhuang Coal Mine, in a bid to reproduce the entire process of the diffusion flow of the counter current and outburst gas in the mine roadway. Moreover, the dynamic influence of the gas pressure on the entire ventilation network was analyzed. An experimental device was set up to test the relationship between the natural wind pressure and the height difference of the roadway and density of the gas flow, and the formation and mechanism of the natural wind pressure were explored. By analyzing the experimental and numerical results, the variation law of the air flow and the law of gas movement under the influence of gas pressure were summarized. The movement law and influencing factors of the gas during the mine outburst period were studied, the influence range of the gas was determined, the distribution law of the gas concentration after the outburst was obtained, and further expansion of the gas was prevented. This study has theoretical and practical significance in enhancing our understanding of the development process of mine gas disasters, which can help establish effective emergency response strategies and reasonably implement postdisaster relief measures.

Determination of the Spontaneous Combustion Hazardous Zone and Analysis of Influencing Factors in Bedding Boreholes of a Deep Coal Seam.

Accurately determining the spontaneous combustion zone of coal around the borehole plays an important role in preventing borehole accidents. To solve the problem of dividing the hazardous zone of spontaneous combustion in boreholes, a fully coupled model of the gas flow, coal oxidation reaction, and energy transportation is developed in this study. Taking the drainage borehole of the 24130 working face in the No. 10 Coal Mine of the Pingdingshan mining area as an example, the proposed model is used to simulate the seepage velocity field, oxygen concentration field, and temperature field of the coal around the borehole. The simulation results are found to be consistent with the field test results. Based on the simulation results, the coal around the borehole is divided into two areas in the axial direction of the borehole. The intersection of the seepage velocity u ≤ 0.004 m/s and oxygen concentration 7% ≤ C(O2) ≤ 21% are considered the "hazardous zone", and the union of the oxygen concentration C(O2) < 7% and seepage velocity u > 0.004 m/s are considered the "safety zone". The influences of various factors inducing spontaneous combustion of coal around the borehole on the hazardous zone are revealed by analyzing the drainage negative pressure, sealing length, and roadway temperature. The results show that reducing the drainage negative pressure and increasing the sealing length can effectively restrain the spontaneous combustion of the borehole and can also help reduce the scope of the hazardous zone of the borehole. Finally, a reasonable arrangement of the predrainage period in the appropriate season can also effectively inhibit the spontaneous combustion of coal around boreholes.