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In order to explore the distribution characteristics of “three zones” of spontaneous combustion in goaf under different ventilation modes, based on the engineering background of “two into one” ventilation mode at 1532 (1) working face of Gubei Mine, and based on airflow control equation, concentration component equation and energy equation, by using the Fluent numerical simulation method, the multi-physical coupling numerical model of coal spontaneous combustion and the real-time dynamic advance model of working face are established, and the distribution characteristics of “three zones” of spontaneous combustion in goaf by the “110 construction method” under three ventilation modes are simulated. The results show that with the continuous advance of the working face, under the conditions of ventilation mode 1 (air intake from the machine side and gob-side entry retaining, return air along the return air lane) and ventilation mode 2 (air intake from the machine side and along the return lane, and return air in gob-side entry retaining), the oxidation spontaneous combustion belt in the goaf is evenly distributed in the side of the belt machine, the middle of the working face and the side of the lane, and the oxygen concentration distribution is not much different. About 50 days after mining, the highest temperature appeared in the deep side of the transportation lane, and the temperature at this position did not increase with the continuous advance of the working face. Under the condition of ventilation mode 3 (air intake in gob-side entry retaining and return air lane, and air return at the side of the machine lane), the oxidation spontaneous combustion belt at the side of the goaf retaining roadway becomes 2-4 times wider, the middle of the working face and the side of the belt conveyor become obviously narrow, the oxygen volume fraction at the side of the belt conveyor in the goaf is lower, the oxygen concentration at the side of the belt conveyor is higher, and the air leakage increases significantly. The high temperature location of goaf mainly appears in the side of the stop line of 1532(1) working face, and the temperature of goaf here is the highest after 50 days of working face. In summary, combined with the actual situation of the site, the width and location of the oxidation spontaneous combustion belt are analyzed, and the ventilation mode 3 is finally determined to be more conducive to preventing the occurrence of coal spontaneous combustion, which provides theoretical guidance for the prevention of coal spontaneous combustion in mines.

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In order to study the effect of oxygen volume fraction on the spontaneous combustion of coal oxidation at low temperature, the non-adhering coal from a mine in Inner Mongolia was taken as the research object. The low-temperature oxidation experiments of coal were carried out under the conditions of oxygen volume fraction of 5%, 15% and 21% by temperature-programmed experiment device and the production rules of CO, CO2 and C2H4 gas were obtained. The oxygen consumption rate, gas production rate, heat release intensity and apparent activation energy of coal under different oxygen volume fractions were calculated, and the relationship between oxygen volume fraction and activation energy at three temperature stages was analyzed. The results show that the volume fraction of CO, CO2 and C2H4 increases with the increase of oxygen volume fraction, and increases exponentially with the increase of temperature. The change trend of oxygen consumption rate, gas production rate and heat release intensity is the same, and they all increase with the increase of oxygen volume fraction. The apparent activation energy decreases with the increase of oxygen volume fraction in the first and third stages of oxidation reaction. The apparent activation energy increases with the increase of oxygen volume fraction in the second stage, but the activation energy reaches the maximum when the oxygen volume fraction is 5%. It shows that reducing the volume fraction of oxygen can inhibit coal spontaneous combustion at low temperature.

In order to study the influence of different oxygen concentrations on low-temperature oxidation and spontaneous combustion of sulfur-containing coal, coal samples of No. 14 and No. 15 coal seams of Shanxi Shigang Coal Industry Co., Ltd. were selected as test samples to carry out temperature-programming experiments of tube furnaces under different oxygen concentrations. There are 6 groups in total, the oxygen concentrations of No. 14 and No. 15 coal samples are 21.0% and 21.0%, 12.2% and 12.1%, 10.2% and 10.1%, 7.1% and 7.2%, 5.0% and 5.1%, 2.9% and 2.8%, respectively. And the gas composition was analyzed by parallel gas chromatograph. By calculating the oxygen consumption rate and heat release intensity at different oxygen concentrations and temperatures, the corresponding relationship between the oxygen consumption rate, heat release intensity and temperature was analyzed, and the oxygen consumption rate and heat release intensity of No. 14 coal sample and No. 15 coal sample were fitted when the oxygen concentration was 21%. The apparent activation energy of low temperature oxidation of No. 14 and No. 15 coal samples under different oxygen concentrations was calculated, the relationship between oxygen concentration and apparent activation energy was analyzed. According to the critical temperature of spontaneous combustion oxidation and the change characteristics of apparent activation energy, the critical oxygen concentration of No. 14 and No. 15 coal samples was obtained. In the test range(oxygen concentration 2.8%−21%, temperature 30−300 ℃), the results show that increasing the oxygen concentration is helpful for the oxidation development of No. 14 and No. 15 coal samples, while reducing the oxygen concentration will inhibit its development. The spontaneous combustion tendency of No. 14 coal sample is greater than that of No. 15 coal sample, and the critical oxygen concentration of spontaneous combustion of No. 14 and No. 15 coal seams is 5.0% and 7.2% respectively. When the oxygen concentration is constant, the CO release volume, oxygen consumption rate and heat release intensity all increase with the increase in temperature, which conforms to the exponential function. The CO release volume, oxygen consumption rate and heat release intensity of No. 14 coal sample at each temperature point of the same group are larger than that of No. 15 coal sample, while the apparent activation energy and critical temperature of spontaneous combustion are smaller than that of No. 15 coal sample. When the temperature is constant, the CO release, oxygen consumption rate and heat release intensity decrease with the decrease of oxygen concentration, while the apparent activation energy and critical temperature of spontaneous combustion increase. The fitting curve of oxygen consumption rate and heat release intensity with temperature and the critical oxygen concentration value of spontaneous combustion can provide important parameters for numerical simulation of spontaneous combustion characteristics of coal in goaf and classification of spontaneous combustion risk area.

Coal spontaneous combustion in underground mine goaf has a great impact on coal mining. The temperature-programmed experiment is a commonly used and effective method for studying the characteristics of coal spontaneous combustion. Aiming at the problem that the numerical simulation of coal spontaneous combustion characteristics under the condition of external air supply and temperature control in a temperature-programmed experiment is insufficient, a multi-physical field coupling numerical model of coal spontaneous combustion in the temperature-programmed experiment is established. The variation characteristics of coal temperature, oxygen, and oxidation products under external air supply and temperature control were studied. The results show that the numerical simulation results are consistent with the experimental results. With the increase in temperature, the volume fractions of oxygen and carbon dioxide decrease and increase, respectively. As the air supply volume increases, the oxygen volume fraction at the outlet increases, and the peak value of the oxygen volume fraction change rate exhibits a "hysteresis" feature, and the time corresponding to the peak value increases. Moreover, the temperature change rate increases. With the increase in the heating rate, the peak value of the oxygen volume fraction change rate increases and shows an "early appearance" characteristic, at the same time, the maximum coal temperature displays a linear increase trend. [ABSTRACT FROM AUTHOR]

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To study the effect of oxygen volume fraction on the explosion characteristics of silo coal, an experiment was conducted on the explosion parameters(maximum pressure pmax, maximum pressure rise rate (dp/dt)max , and explosion index Kst ) of mixed coal in silos with two calorific values and oxygen volume fractions of 21%, 15%, and 10% using a 20 L spherical explosion device. The effect of oxygen volume fraction and calorific value on explosion characteristics was analyzed. The results indicate that: first, when the concentration of coal dust increases, the explosive characteristics of mixed coal with three distinct oxygen concentrations increase first and eventually fall; when the oxygen volume fraction decreases from 21% to 10%, the explosion parameters of mixed coal decrease significantly, and the pmax peaks of mixed coal with two calorific values decrease by 38.9% and 44.8%, respectively, compared with that in air, and(dp/dt)max dropped by 68.5% and 81.1%, respectively; the explosion risk decreased from high and moderate to weak; second, as the oxygen content decreases, the coal dust concentration of the pmax and(dp/dt)max peaks of mixed coal changes. The lower the oxygen concentration, the lower the concentration of pmax and (dp/dt)max peaks of coal dust. Finally, the calorific value has a substantial impact on the explosion properties of mixed coal. The higher the calorific value, the larger the explosion risk, and under conditions of low oxygen and higher coal dust concentration, the coal with a high calorific value has a smaller explosion risk than coal with a low calorific value.

In order to effectively prevent and control coal spontaneous combustion disasters in Tangjiahui Mine, thermogravimetric experiments were used to analyze the thermal reaction kinetics characteristics of coal samples at different heating rates and different oxygen volume fractions. The results showed that at the same oxygen volume fraction, the apparent activation energies of coal at heating rates of 2 ℃/min, 5 ℃/min, 10 ℃/min were 150.27 kJ/mol, 131.97 kJ/mol, 106.86 kJ/mol, respectively; in addition, at the same heating rate, the apparent activation energies of coals with oxygen volume fraction of 6%, 9%, 12%, 15%, 18%, 21% are 106.86, 120.61, 127.31, 131.58, 135.26 and 136.82 kJ/mol, respectively. The apparent activation energies decrease with the increase of heating rate and oxygen volume fraction, and the change law is basically in line with the increase of logarithmic function. The calculation results of activation energy kinetics reveal that the degree of coal-oxygen composite reaction is different under different heating rates and different oxygen volume fractions.

Coal spontaneous combustion is one of the important disasters affecting the development of coal-related industries. To understand the characteristics of various parameters of coal spontaneous combustion process in detail, a combination of experiment and numerical simulation was used to study the coupling characteristics of multiple physical parameters of coal low-temperature oxidation, the reliability of the numerical model is verified by the experiment. The evolution law of coal spontaneous combustion under different oxygen conditions is further explored, the results show that the established numerical model can well characterize the multi-parameter coupling characteristics of coal spontaneous combustion. The oxygen volume fraction decreases with the increase of coal temperature, and the CO and CO2 volume fractions increase with the increase of coal temperature. The gas consumption and production characteristics of coal oxidation reaction show hysteresis characteristics. The CO and CO2 produced by the coal oxidation reaction under different oxygen conditions increase with the increase of the oxygen volume fraction, and the change within 8 h has a linear relationship with the oxygen volume fraction. The distribution of oxygen inside the coal body presents a “stratified” feature. Taking 2 h as the critical time, the minimum temperature and the average temperature of the coal sample show different increasing trends. As the oxygen volume fraction decreases, the low temperature area in the coal body expands to the gas inlet area.

To understand the characteristics of coal low-temperature oxidation and its physical process in more detail, temperature-programmed experiments are used to study the evolution of key parameters of the low-temperature oxidation process of different coal sample sizes. The numerical simulation method is used to further explain the detailed characteristics of the multi-physical field evolution of the coal low-temperature oxidation process. The correctness of the numerical simulation was verified based on the experimental results. Based on the combination of experiment and numerical simulation, the characteristics of temperature, mass fractions of reactants and products, and fluid flow of the coal samples in the temperature-programmed experimental chamber were explored. The result shows that the numerical simulation can well characterize the low-temperature oxidation process of coal, and show the change characteristics of the time series of various physical fields in detail. According to the gas emission and oxygen consumption of coal oxidation process, low-temperature coal oxidation can be divided into two stages, with 70 ℃ as the critical value. With the development of low-temperature coal oxidation, the frontier of coal oxidation reaction is moving to the region of high oxygen volume fraction.

To explore the hazard zone of coal spontaneous combustion in the adiacent goaf at different stages of the roadway driving face, the 2202 working face and the 2201 adjacent goaf of Yingpanhao Coal Mine were taken as examples. The large-scale experiment was conducted to analyze the coal spontaneous combustion limit parameters of the 2201 goaf. The conditions for identifying the hazard zone of coal spontaneous combustion in the adjacent goaf were proposed. Monitoring the gas concentration and temperature in 2201 adjcent goaf by the drilling holes through coal pillar, and the numerical simulation was conducted to study the oxygen concentration distribution in 2201 adjcent goaf, and the hazard zone of coal spontaneous combustion in the adjacent goaf was obtained. The results show that during the mining period of the 2202 working face, the coal pillar was broken by the stress concentration, and the oxygen volume fraction of the adjacent goaf was in the range of 10.1% to 13.8%. The oxygen volume fraction of the adjacent goaf before the stop mining period was between 10.3% and 15%. During the mining period, a narrow area on the coal pillar side with width 55 m from the front 45 m and behind 119 m of 2202 working face in the adjacent goaf was the coal spontaneous combustion hazard zone. Before the stop mining period, a narrow area on the coal pillar side with width 42 m from the front 63 m and behind 107 m of 2202 working face in the adjcent goaf was in an oxidizing environment.

Oxygen sensors are prepared by Pt paddle bonding method with dense diffusion barrier of short-circuited yttria-stabilized zirconia (YSZ) solid electrolyte. Different from the previous normal diffusion and Knudsen diffusion mechanisms, the limiting current equation of this type oxygen sensor is derived based on the diffusion of oxygen ions in the dense diffusion barrier. The effects of oxygen volume fraction, temperature and diffusion layer thickness on the limiting current density are investigated. The relationship between the physical quantities in the experiment is consistent with the derived equation. [ABSTRACT FROM AUTHOR]

Liquid nitrogen has excellent ability of cooling and asphyxiating, which is suitable for fire extinguishing in urban underground utility tunnel. In order to obtain the influence of injection method on the fire extinguishing efficiency, a medium-sized square utility tunnel was built. Fire extinguishing experiments with different liquid nitrogen injection methods and distances were carried out. The distribution and the decrease law of oxygen volume fraction and temperature are analyzed. The result shows that the time to extinguish the fire and enter the controllable stage are greatly reduced under both conditions. Among them, the fire extinguishing efficiency of vertical injection is higher. Liquid nitrogen accumulates near the injection port first, which has a great cooling effect on this area, and acts on the fire in two stages. As the distance increases, the cooling effect is weakened and the oxygen volume fraction increases stepwise. Liquid nitrogen shows continuous cooling and asphyxiating effect during horizontal injection. It moves fast in this mode, so the difference in oxygen volume fraction is small. The initial cooling effect of vertical liquid nitrogen injection is better than that of the horizontal injection. However, the temperature at the end recovers earlier after fire extinguishing in the latter condition.

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• Coal spontaneous combustion be predicted by the law of migration in temperatures. • High-temperature region in the goaf was mainly concentrated on the air inlet side. • With increasing advancing speed, the high-temperature point gradually decreases. • Higher the air supply rate, the higher the high-temperature point in the goaf. In regard to coal spontaneous combustion (CSC) in the goaf, it is difficult to determine the high temperature regions due to its hidden fire source, while effective prevention and control of CSC in the goaf have become increasingly difficult. COMSOL Multiphysics was adopted to conduct theoretical calculations and numerical simulation analysis of the dynamic evolution of high temperature regions in CSC process. The results indicated that the general trend of the high-temperature regions in the experimental furnace involved movement from the middle and upper parts to the lower air inlet. The high-temperature regions in the goaf was mainly concentrated on the air inlet side. Both the advancing speed and air supply rate at the working face impacted the dynamic evolution of the high-temperature regions. With the increase of the advancing speed, the heat and mass transferred process in the goaf became more and more intense, so that the caving coal and rock entered the suffocation zones after a short period of oxidation and temperature rise. Moreover, the temperature difference between the suffocation zones and the deep goaf was small, so there was some uncertainty in the high-temperature area. With the increase of air supply, the heating rate of high-temperature point increased, and the material migration speeds in the goaf also increased, which made the high-temperature regions move towards the deep part of the goaf, which was more unfavorable to the prevention and control of CSC. [ABSTRACT FROM AUTHOR]