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. 2023 Jan 9;8(2):2685–2697. doi: 10.1021/acsomega.2c07236

Hazardous Area Reconstruction and Law Analysis of Coal Spontaneous Combustion and Gas Coupling Disasters in Goaf Based on DEM-CFD

Jiaxing Zou †, Rui Zhang ‡, Fengyuan Zhou †, Xiaoqiang Zhang †,*
PMCID: PMC9850464  PMID: 36687050

Abstract

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Coal spontaneous combustion and gas coupling disasters are the highest percentage of serious accidents in coal mines, causing the most serious disasters. China is one of the countries with the most serious coal spontaneous combustion and gas coupling disasters in goaf, and it is of great significance to explore the evolution law of coal spontaneous combustion and gas coupling disasters in goaf for disaster prevention and control. To study the three-dimensional spatial characteristics of the hazardous area of the coupling coal spontaneous combustion and gas disaster in goaf, a discrete element method-computational fluid dynamics (DEM-CFD)-based hazardous area reconstruction method was proposed. Taking a fully mechanized caving face of a coal mine in Shandong, China, as an example, first, the working face mining model was established by PFC3D, and the porosity of different horizontal and vertical positions of goaf after mining was extracted. Second, the porosity extracted was imported into computational fluid simulation software FLUENT by UDF. Finally, the distribution laws of oxygen and gas concentration during the real goaf mining process were simulated and analyzed. The results showed that the oxygen concentration in the intake roadway at a depth of 50.7 m in goaf decreased to 12%, and the gas concentration at a depth of 42.0 m in goaf increased to 16%. The oxygen concentration in the return airway roadway was reduced to 12% at 17.3 m depth in goaf, and the gas concentration increased to 16% at 6 m in the direction of goaf. The gas concentration was higher at the return air corner. The three-dimensional shapes of the hazardous area in goaf were constructed to satisfy the coupling of O2 concentration field, CH4 explosion limit concentration field, and fracture field and so were the laws of hazardous area analyzed qualitatively and quantitatively. It has important research significance for the rapid identification and determination of the coal spontaneous combustion and gas coupling disasters hazardous area.

1. Introduction

In China, the energy shortage problem is still serious, and coal is still the main part supporting the country’s energy structure.1−4 Coal spontaneous combustion and gas coupling disasters are the highest percentage of serious accidents in coal mines and the most serious disasters caused by them, which seriously threaten the environment and the safety of workers’ lives.5,6 Air leakage will cause spontaneous combustion of residual coal in goaf and affect gas migration in goaf. When the combustion temperature of residual coal increases to the ignition temperature of gas, gas can be ignited and developed into gas explosion.7,54 According to statistics, gas explosion accidents caused by spontaneous combustion of residual coal have occurred in China, India, Poland, Australia, and other countries around the world, causing huge casualties and economic losses.8−11 In recent years, with the exhaustion of shallow coal resources, deep coal mining has become the new normal of coal resources mining in China.12−14 As the mining depth gradually deepens, the observable in situ stress, ground temperature, gas content, and gas pressure also increase, which makes the coupling disasters represented by coal spontaneous combustion and gas in fully mechanized caving goaf more frequent and serious.15−17 Therefore, the prevention and control of such coupling disasters as well as the early warning researches are of great importance to ensure the safety of the coal mining process.

The oxidative self-heating of coal is a complex physicochemical reaction chain process that simultaneously links the flow and transport of gases in goaf, the transfer and transmission of energy, and the reaction kinetics.18,19 The spontaneous combustion of residual coal provides an ideal ignition source for gas explosion in goaf, so the dynamic gas concentration change around the spontaneous combustion area of residual coal is of great significance to evaluate the different risks of gas explosion. Coal spontaneous combustion and gas coupling disasters are a result of coupling the multiscale, multitime, and multiphysical process.20−23 If only from the single disaster prevention and control of residual coal spontaneous combustion disaster or gas disaster, it is easy to create an imbalance, causing more serious disasters.24−26 At present, domestic and foreign scholars are studying and analyzing the mechanism of coupling the two disasters to cause them: for example, Qin et al. used experimental studies to obtain the explosion concentration range and explosion hazard of the mixture of CH4 and CO and theoretically analyzed the possible areas of coal spontaneous combustion detonation gas and the process of occurrence;27 Zhang et al. put forward the dynamic isolation technology and carried out the industrial test in the field. The test showed that the new dynamic isolation technology reduces the scope of the scattered tropical zone along the two troughs in goaf, the width of the oxidation zone was shortened, and the gas concentration at the corner of the return air was significantly reduced, which reduces the risk of coal spontaneous combustion and gas coupling disasters;28 Li et al. proposed a method for quantitatively evaluating the risk of gas explosion in underground coal mines using Bayesian networks and verified the feasibility of this method using the example of Babao coal mine in China;29 and Tutak et al. reduced the possibility of coal spontaneous combustion and gas coupling disasters by changing the ventilation mode of different mining processes.30

Coal spontaneous combustion and gas coupling disasters have the characteristics of complexity, concealment, dynamics and coupling, etc.31−33 The prevention and control of the coupling disasters are characterized by high risk, difficulty to identify, and difficulty to warn and manage.34−36 Therefore, rapid and accurate prediction of the coupling hazardous area is the key to coupling disasters management and prevention. Hazardous area of coupling disasters is a region coupled by multifield space–time evolution such as the fracture field, oxygen concentration field, residual coal distribution field, and gas seepage field.37−40 Due to the unique multifield coupling mechanism, various factors interact with each other, which make the dynamic description of the hazardous area of coupling disasters more complicated and difficult. In recent years, scholars have done a lot of research on the determination of the coupling hazardous area and have achieved considerable results. For example, a coupled model involving the interaction of oxygen transport, coal spontaneous combustion, and gas transport was developed by Li et al. to determine the maximum degree of coupling hazard.41 Based on the measurement and analysis of the oxygen concentration and gas concentration in the field, Cheng et al. determined the plane range of the symbiotic disaster hazardous area at different heights of goaf by superposition. Using the three-dimensional spatial interpolation technology, the three-dimensional distribution of the disaster hazardous area of spontaneous combustion and gas coupling in the fully mechanized caving goaf was constructed.42 Yu et al. obtained the gas concentration and temperature data of the fully mechanized caving goaf through field measurements and constructed the plane distribution of the symbiotic disaster hazardous area in goaf without considering the influence of other spontaneous combustion gases.43 Ma et al. studied the law of gas migration caused by coal spontaneous combustion by designing the experimental platform of longwall goaf and determined the area where symbiotic disasters may occur.44 In general, all of the abovementioned scholars’ researches have laid a solid foundation for the management and early warning of coupling disasters. However, since goaf is a porous medium space, the porosity and permeability are transient, and previous studies often ignore the mining process of the working face of real coal mining. In addition, there is a lack of quantitative analysis of the potential and existing area for the coupling disasters hazardous area.

In this study, a three-dimensional reconstruction method for the coupling disasters hazardous area was proposed, engineering the background of a fully mechanized caving face in Shandong, China. First, the excavation model of the working face was established by PFC3D, and the porosity at different horizontal and vertical positions of the fully mechanized caving goaf after excavation was extracted. Second, the porosity was imported into the computational fluid simulation software FLUENT through the UDF, and the distribution law of oxygen and gas concentration in the fully mechanized caving goaf during the real mining process was simulated and analyzed. Based on the coupling hazardous area determination theory, the three-dimensional form of the coupling hazardous area was finally constructed. The reconstruction method can not only determine the location of the occurrence of the coupling disasters in the coal mining process so that effective prevention and control measures can be taken in advance but also quickly identify the coupling disasters hazardous area to ensure the safety of the working face mining front. The research results were of practical significance to improve the safety risk control of the coal mining process.

2. Methodology

The study was conducted under the engineering background of a coal mine in Shandong, China, whose geographical location and geological profile are shown in Figure 1. The average thickness of coal seams was 3.36 m, accounting for 71% of the recoverable coal seam. The roof of coal seam was mostly mudstone and siltstone strata, and some sections were medium-coarse sandstone strata. The coal seam floor was mainly mudstone and siltstone strata, and the local was medium-fine sandstone strata. The fully mechanized caving face in this study was a typical high gas emission face. There was a risk of spontaneous combustion in the coal seam. The shortest spontaneous combustion period of the coal seam was 24 days. The absolute gas emission at the beginning of the working face advancing process was 1.53 m3/min. The strike length of the working face was 1265 m, the inclination length was 100 m, the buried depth was more than −900 m, the dip angle of the coal seam was 4–6°, and the total thickness of the mining coal seam was 2.5–7.3 m, with an average of 6 m.

Figure 1.

Figure 1

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Location and geological profile of coal mine.

2.1. DEM Model

2.1.1. Inversion of Micro-Macroscopic Parameters

Discrete element method (DEM) was to discrete the analysis object (including modeling, structure, etc.) into a certain number of spherical disks or blocks so as to achieve a better reflection of the real fracture and the variation of pore space between particles.45,46 The commonly used DEM numerical simulation softwares were PFC, 3DEC, and so on. Because PFC numerical simulation software has high simulation accuracy and can observe the fracture process of blocks in the simulation process, it was more favored by researchers. In PFC numerical simulation, the microscopic parameters of particles characterize the macroscopic mechanical properties of rocks. Therefore, the correct particle macroscopic parameters were particularly important. At present, scholars usually used PFC2D software to conduct tensile and compressive strength test of rock mass and then obtained microparameters such as normal stiffness and tangential stiffness through empirical formula inversion calculation.47−49 Based on the geological conditions of a fully mechanized caving face, the micro-macroscopic parameters of each rock layer and coal seam of the model were obtained as shown in Table 1.

Table 1. Micro-Macroscopic Parameters of Strata.
    macroscopic parameters
     microscopic parameters
number lithology Poisson’s ratio elastic modulus (MPa) tensile strength (MPa) cohesion (MPa) internal friction angle (deg) Krat Emod (GPa) Kn (GPa) Ks (GPa) Pb – kn (GPa) Pb – ks (GPa)
J1 fine sandstone 0.21 24.8 3.06 5.77 38.8 1.6 16.9 33.8 21.3 21.1 13.2
J2 coarse sandstone 0.18 15.4 2.58 4.35 35.3 1.3 10.1 20.2 14.5 12.6 9.1
J3 coarse siltstone 0.23 18.2 4.66 11.05 36.0 1.7 12.7 25.4 14.3 15.8 9.0
J4 sandy mudstone 0.15 7.4 1.42 7.60 36.4 1.2 4.6 9.2 7.6 5.7 4.7
J5 fine sandstone 0.14 17.4 3.06 5.77 38.8 1.1 10.8 21.6 18.8 13.5 11.8
J6 coarse siltstone 0.23 18.2 4.66 11.05 36.0 1.7 12.7 25.4 14.3 15.8 9.0
J7 coarse sandstone 0.18 15.4 2.58 4.35 34.3 1.3 10.1 20.2 14.5 12.6 9.0
J8 fine sandstone 0.21 24.8 3.06 5.77 38.8 1.6 16.9 33.8 21.3 21.1 13.2
J9 sandy mudstone 0.15 7.2 1.36 6.32 35.4 1.2 4.5 9.0 7.4 5.6 4.6
J10 coarse siltstone 0.23 18.2 4.66 11.05 36.0 1.7 12.7 25.4 14.3 15.8 9.0
J11 coal 0.27 6.9 1.33 2.42 42.6 2.1 5.1 10.2 4.7 6.3 2.9
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2.1.2. PFC Model and Boundary Conditions

According to the actual geological conditions of the fully mechanized caving face of the coal mine, the excavation model was established under the condition of ignoring the factors such as rock joint fissures and interrock faults, and only considering the development of overburden fissures caused by excavation, as shown in Figure 2. In the rectangular coordinate system, taking the corner position of the return airway roadway as the origin, the inclination direction of the working face was X direction, the length direction of goaf was Y direction, and the vertical direction of the working face was Z direction in which the model was 100 m long, 100 m wide, and 40 m high, with a minimum particle size of 0.5 m, a particle size ratio of 1.66, and a porosity of 0.05. The top boundary of the model was a free interface; the front, back, left, and right boundaries restrict horizontal movement, while allowing movement along the vertical direction, and the bottom boundary restricts movement in the vertical direction. Since the actual average dip angle of the coal seam was 5°, it was regarded as a near-horizontal coal seam. The model had a total of 11 layers from top to bottom, and different particle colors were used to represent different lithologies. Among them, the 11th layer was the coal seam, and the micro-macroscopic parameters obtained by inversion were assigned to each layer to achieve the authenticity of its simulation.

Figure 2.

Figure 2

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DEM physical models.

2.2. CFD Model

2.2.1. CFD Simulation Method

Computational fluid dynamics (CFD) was a branch of fluid mechanics, which was widely used in mining engineering due to its high efficiency, high precision, and high stability. The essence of using CFD software to simulate the gas flow in goaf was to solve the Navier–Stokes equation, and the finite-element FLUENT software was used as the basis for simulation to obtain the flow characteristics of the fluid. At the same time, the gas flow in goaf must follow the law of conservation of mass and energy, and because it contains the mixing and mass transfer of different components, it must also follow the law of conservation of components.50,51 In the simulation, goaf was regarded as a porous medium. The movement of gas in coal seam and goaf can be regarded as laminar flow. The movement of gas in the roadway should be regarded as turbulence because the Reynolds number Re, which measures the flow characteristics were large and there are corners in the roadway. The RNG k−ε model was used to simulate the turbulent flow of gas in goaf.

Equation of component transport
2.2.1. 1

where ci is the mass fraction of gas component i, %; Ji is the diffusion flux of component i, kg/(m2·s); and Smi is the quality source of component i, kg/(m3·s).

RNG k−ε turbulence model52,53

2.2.1. 2
2.2.1. 3

where Gk is the turbulent kinetic energy produced by the average velocity gradient; Gb is the turbulent kinetic energy produced by the buoyancy effect; YM is the effect of pulsating expansion of compressible turbulence on the dissipation rate; αε and αk are the reciprocals of the effective turbulent Prandtl numbers of turbulent kinetic energy k and dissipation rate ε; μt = ρCμk2/μ, C1ε, C2ε, C3ε, and Cμ are all constant terms; R is the universal gas constant; μt is the turbulence viscosity coefficient; μ is the air flow viscosity; and Sk and Sε are user-defined source terms.

How to import porosity obtained by DEM simulation into CFD software was the key to simulate the real flow field in goaf. In addition, the porosity data in PFC3D were unit parameters and did not need interpolation or interpolation changes. Therefore, the porosity image can be rasterized by digital elevation technology to extract the corresponding porosity data. By compiling the data transfer program, the unidirectional conduction of data between PFC3D and FLUENT was realized, and the FLUENT model with the same proportion as PFC3D was established. The calculation was based on FLUENT software, and the porosity data simulated by PFC3D were compiled into the UDF file. The flow of porosity data is shown in Figure 3.

Figure 3.

Figure 3

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Flow graphs of porosity data.

2.2.2. Model Assumptions

Considering the complex fluid distribution in goaf, the following model assumptions were made by simplifying the simulation calculation. (1) goaf was regarded as an isotropic porous medium; (2) the change of air leakage intensity in goaf conforms to Darcy’s law, the temperature change was not considered in the simulation process, and the gas and air in goaf were regarded as incompressible gas; (3) the equation of state of ideal gas was satisfied; and (4) the air flowing into goaf was only composed of 21% oxygen and 79% nitrogen without other gases.

2.2.3. Model Establishment and Boundary Conditions

According to the actual situation of the fully mechanized caving face, both sides of the working face were solid coal and were not affected by the adjacent working face, U-type ventilation was adopted. The physical model of goaf was established using the DesignModeler module of the finite-element FLUENT simulation software, as shown in Figure 4. The size of the working face was 100 m (length) × 7.5 m (width) × 3.0 m (height), the size of intake and return airways was 10 m (length) × 4.0 m (width) × 3.0 m (height), the length of the strike direction of goaf was 100 m, the length along the dip direction was 100 m, and the height was 40 m. To simulate the actual situation of goaf more realistically, the gas was uniformly ejected from the coal seam floor from the source term into goaf. The solution was to select the second-order upwind format in SIMPLEC and iterate 1500 times to achieve the minimum residual. The physical model parameters and boundary conditions are shown in Table 2.

Figure 4.

Figure 4

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CFD physical models.

Table 2. Parameter and Boundary Condition Setting of the CFD Model.
model parameters simulation conditions
solution method SIMPLEC
boundary conditions of the inlet velocity inlet, v = 2.1 m/s
boundary conditions of the outlet free outflow
inclination condition 0°
system ventilation method U-type ventilation
solid walls around the working face defined as a nonslip boundary condition
wall settings thermal insulated surface
gas component in goaf 100% CH4
iteration times 1500
gas emission source term 3 × 10–6 kg·m–3·s–1, escape evenly from the floor
porosity of porous media import PFC porosity data using UDF function
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Using the built-in mesh function of FLUENT to divide the structured mesh, the grid type was hexahedral grid. It should be noted that the mesh should have higher mesh quality, and the average mesh quality should be more than 0.8. Therefore, the quality statistics of the divided grids were carried out. According to statistics, the number of grid nodes were 977 013, and the total number of units were 227 420. The grid quality of most grids was above 0.95, and the minimum grid quality was 0.9. In general, the average grid quality was far more than 0.8, which can meet the requirements of this simulation.

2.3. Division Theory of Hazardous Area of Coupling Disasters

After the spontaneous combustion of residual coal in goaf, the high-temperature region of coal spontaneous combustion continues to migrate, resulting in the dynamic change of the gas explosion limit in goaf affected by the gas composition and temperature. When the high-temperature region and the gas explosion risk region overlap each other, it was easy to cause the gas explosion phenomenon. The coal spontaneous combustion and gas coupling disasters in goaf were a result of the coupling effect of four fields: goaf fracture field, residual coal distribution field, O2 concentration field, and CH4 concentration field.28 The mathematical expression of the hazardous area of coal spontaneous combustion and gas coupling disasters is

2.3. 4

where Se(τ, x, y, z) is the hazardous area of gas and coal spontaneous combustion disasters; SO2(τ, x, y, z) is the oxygen concentration region of disasters; SH(τ, x, y, z) is the distribution area of residual coal; SCH4(τ, x, y, z) is the flammable or explosive concentration region of gas; and S1(τ, x, y, z) is the fracture field area of goaf.

If the coal spontaneous combustion in goaf induces gas explosion and the explosion can propagate, the oxygen concentration in goaf must exceed the minimum oxygen concentration required for gas explosion, and the gas concentration is within the explosion limit

2.3. 5

where S1c(τ, x, y, z) and S1d(τ, x, y, z) are the upper and lower limits of porosity corresponding to gas explosion in the fracture field; CO2c is the lower limit of oxygen concentration in gas explosion, ppm; and Cc and Cd are the upper and lower limits of the concentration of gas explosion, ppm.

3. Results and Discussion

3.1. PFC3D Simulation Results

Porosity is an important parameter for the study of cracks under mining action. Under the influence of coal seam mining, it changes with the movement of overlying strata and has the characteristics of randomness, nonuniformity, and changes with the depth of coal seam mining. It will directly affect the generation and development of cracks and provide a channel for the migration of gas in goaf. Therefore, in the simulation of the fully mechanized caving face excavation process, the dynamic tracking excavation model of the roof ruptures and collapses. To describe the evolution process of roof pore fracture initiation → development → failure under mining action, the overburden fracture changes and porosity changes of goaf mining to 20 m (initial mining stage), 40 m (initial pressure stage), and 60 m (normal mining stage) were selected for the analysis.

When the working face advances to 20 m from the cut hole, the roof caving and subsidence map is shown in Figure 5a. It can be seen from Figure 5a that at this time, the roof grazing rock caving in goaf has not yet developed completely. The roof subsidence in the middle of goaf was the largest, and the fractures were fully developed and symmetrically distributed. In addition, there was a separation layer across the whole goaf in the middle of goaf, and the subsidence between the upper and lower parts of the separation layer was quite different, which leads to a different fracture development. It can be seen from Figure 5c that with further advancement of the working face to 40 m, the development degree and height of the separation layer also increased. At the position of the separation layer, the rock strata had obvious displacement deformation, and the deformation and failure exceed the tensile strength of the rock strata, which makes the fracture change area further increase. The roof erosion rock had a large area of collapse, and the first periodic pressure occurs. The area with high deformation of roof erosion rock is in the “arch” distribution. After the first periodic pressure, the working face enters the normal mining stage. When the working face is mined to 60 m, as shown in Figure 5e, it can be seen from Figure 5e that the roof erosion rock had been fully collapsed and was basically not affected by the mining action in front of the working face.

Figure 5.

Figure 5

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PFC3D rock collapse and porosity diagram.

With the increase of working face advancing distance, the porosity of goaf also changes due to the collapse of the roof rock. When the working face was mined to 20 m, the porosity is shown in Figure 5b. It can be seen from Figure 5b that in goaf at the position of 20 m in the working face, the gangue was still not fully compacted, and some roofs still played a supporting role. At this time, the porosity of the whole goaf was large, and it gradually decreased from the bottom to the top. It can be seen that the porosity of the overlying strata was distributed in a pyramid shape. When the working face was mined to 40 m, the porosity is shown in Figure 5d. When the working face was pushed to 40 m, the 40 m position was still within the influence range of the periodic pressure of the working face. At this time, the collapse of the overlying strata in goaf was not stable, and the porosity decreases gradually from bottom to top. The porosity in the middle of the working face was small, and the porosity at both ends was relatively large. As the working face continues to advance, the porosity graph is shown in Figure 5f. Further compaction occurs in the middle of goaf, and the porosity in the middle decreases. The porosity in the middle of goaf was related to the accumulation characteristics of broken coal and rock mass and has strong randomness, but the fluctuation range was only between 0.2 and 0.3. At both ends of the working face, due to the noncoal caving along the trough and the support of the coal walls on both sides, the impact of caving compaction was smaller and the porosity was larger.

Under the influence of mining action, goaf has the characteristics of time variation and instantaneity, and the porosity of different mining periods and different mining positions of goaf was changing. In this study, taking the example of complete mining of the fully mechanized caving face, the porosity of different goaf section heights is extracted. Due to the limitation of the length of the article, the porosity change diagram of 0 m goaf height was taken as an example, as shown in Figure 6.

Figure 6.

Figure 6

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Diagram of porosity change at 0 m height in goaf.

3.2. Analysis of CFD Simulation Results

3.2.1. Analysis of Oxygen Migration Law in Goaf

The three-dimensional distribution of oxygen concentration in goaf was obtained by CFD numerical simulation, as shown in Figure 7. As can be seen from Figure 7, the oxygen concentration near the working face and the intake roadway was high, and the oxygen concentration in the intake roadway was significantly higher than that in the return airway roadway at the same depth. This may be because the air leakage intensity of the working face and the intake roadway was relatively large, and the reaction between coal and oxygen was less in unit time. However, with the increase of the depth of goaf, the air leakage was weakened, and the reaction time between coal and oxygen was increased, and the oxygen concentration was gradually reduced. Along the direction of goaf, oxygen concentration decreases with the increase of goaf depth. In the tendency direction, the range of oxidation zone gradually decreases, reaching the minimum at the return airway roadway, about 7 m wide. In the vertical direction, the closer the oxygen was to the top of goaf, the smaller the oxygen range, and the lower the oxygen concentration. This may be because the continuous momentum loss of oxygen in the porous medium flowing into the return airway roadway makes the oxygen diffusion rate in the intake roadway greater than that in the return airway roadway.

Figure 7.

Figure 7

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Three-dimensional distribution of oxygen in goaf.

To quickly identify the coal spontaneous combustion and gas coupling disasters hazardous area, the oxygen concentration data was extracted at 1 m from the coal seam floor, as shown in Figure 8, and because this area was close to the intake roadway, the air leakage was large, and the porosity distribution near the roadway was different. Second, to describe the change of flow field in goaf more intuitively, three linear measuring lines were arranged, which had 20 measuring points. Measuring line 1 was 1 m away from the outside of the intake roadway, measuring line 2 was 50 m away from the outside of the intake roadway, and measuring line 3 was 1 m away from the outside of the return airway roadway.

Figure 8.

Figure 8

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Contours of oxygen concentration distribution (z = 1 m).

It is generally believed that when coal spontaneous combustion and gas coupling disasters occur in goaf, the oxygen concentration range was 12–18%. Therefore, the oxygen concentration data at the three measuring lines were extracted, as shown in Figure 9. In addition, the oxygen concentration of the three measuring lines was consistent with the distribution law of high concentration in the shallow part of goaf and low concentration in the deep part. The oxygen concentration in the intake airway (1#Line) decreased to 18% at 40.5 m depth in goaf for the first time, and to 12% at 50.7 m depth in goaf. The oxygen concentration of 2#Line was reduced to 18% at 26.3 m depth and 12% at 35.8 m depth. In the return airway roadway of goaf (3#Line), the oxygen concentration was as low as 18% at 7.8 m depth in goaf and reduced to 12% at 17.3 m depth in goaf. Due to the continuous decrease of air leakage in goaf, after 73 m depth in goaf, the oxygen concentration at the three measuring lines was very low, close to zero.

Figure 9.

Figure 9

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Oxygen concentrations at three measuring lines.

3.2.2. Analysis of Gas Migration Law in Goaf

According to the numerical simulation results, the three-dimensional distribution of gas concentration in goaf was obtained, as shown in Figure 10. According to the analysis of Figure 10, along the direction of goaf, the gas concentration near return airway roadway, intake roadway, and working face decreases with the increase of the working face advancing distance. The reason for this situation may be that the air leakage intensity in the shallow part of goaf was large, and the gas was not easy to accumulate. On the contrary, the air leakage intensity gradually decreases and the gas accumulates with the working face advancing in the deep part of goaf. The area more than 60 m away from the working face was due to the subsidence of the deep collapse rock in goaf and was basically in a compaction state. The porosity was small, the permeability was poor, the gas release rate was low, and the gas concentration increases. Along the inclined direction of goaf, the low gas concentration area of the intake roadway was significantly higher than that of the return airway roadway. With the increase of the cross-sectional height, the low gas concentration area gradually decreases. The reason for this phenomenon was that when the air flow of the intake roadway passes through the working face, due to the instantaneous change of the flow direction, a small amount of air flow enters goaf, resulting in dilution. The gas concentration decreases, and the air leakage flow enters goaf from the lower corner of the intake roadway. Therefore, the closer to the lower corner of the working face, the more obvious the gas dilution phenomenon. The velocity of air leakage flow decreases under the effect of resistance generated by permeability, and finally the air flow dissipates somewhere in the deep part of goaf, and the gas concentration does not change. In addition, because the gas in goaf receives the lifting effect of gravity, the gas generated by the residual coal in the floor gradually increases to the fracture zone, so the gas concentration increases.

Figure 10.

Figure 10

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Three-dimensional distribution of gas in goaf.

The coal spontaneous combustion and gas coupling disasters will cause gas explosion. The upper limit of gas concentration that produces gas explosion is 16%, and the lower limit of gas concentration is 5%; that is, the gas explosion disaster is possible only within this range of gas concentration. To determine the gas explosion concentration range, the gas concentration data at 1 m from the floor were extracted, as shown in Figure 11, and three measuring lines were also deployed. The gas concentration data of the three lines were extracted, as shown in Figure 12.

Figure 11.

Figure 11

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Contours of gas concentration distribution (z = 1 m).

Figure 12.

Figure 12

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Gas concentrations at three measuring lines.

It can be seen from Figure 12 that the gas concentration was distributed regularly with low concentration in the shallow part of goaf and high concentration in the deep part. The gas concentration in the intake roadway (1#Line) increased to 5% at 37.8 m depth in goaf for the first time, and 16% at 42.0 m depth in goaf. The gas concentration of 2#Line increased to 5% at 21.1 m depth in goaf and 16% at 28.2 m depth in goaf. The gas concentration of return airway roadway (3#Line) increased to 5% at 3 m direction of goaf and 16% at 6 m direction of goaf. After 63 m depth in goaf, the gas concentration at the three measuring lines was very high.

4. Analysis on Reconstruction Law of Hazardous Area

4.1. DEM-CFD Coupling Process

The basic idea of DEM-CFD coupling was to use DEM to calculate the porosity of the goaf system, import the custom function into CFD software through UDF compilation, and then use CFD technology to solve the flow field of the fluid. Both of them must transfer mass, momentum, and energy with a certain model, and finally realize DEM-CFD coupling. The porosity profile images of different goafs at different spatial locations were extracted, and the images were enhanced and denoised to enhance the quality of the graphics. Then, the images were rasterized, and the porosity data were extracted. The rasterized images were stacked according to the spatial position relationship, and the models are indicated for restoration, smoothing, etc. using MATLAB software. The digital simulation model of goaf porosity was obtained. Then, the UDF was imported into FLUENT software. The physical model with equal proportion of PFC3D was established in FLUENT software, and the corresponding initial parameters and boundary conditions were set. The migration distribution law of three-dimensional gas in goaf was obtained through FLUENT calculation. The coupling process is shown in Figure 13.

Figure 13.

Figure 13

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DEM-CFD coupling process.

4.2. Division of Hazardous Area of Coal Spontaneous Combustion and Gas Coupling Disasters

Goaf is a porous medium space with the characteristics of multidirectional anisotropy and heterogeneity, which makes the coupling disasters of coal spontaneous combustion and gas have the characteristics of complexity, concealment, dynamics, and coupling. Therefore, the hazardous area of coal spontaneous combustion and gas coupling disasters changes with the advancing speed of working face, the collapse of goaf, and the air leakage of working face. At the same time, the hazardous area for coupling disasters at different mining periods was also different. Due to the limitation of conditions, this study took the mined-out area of a fully mechanized caving face after full mining as the reconstruction object and constructed the three-dimensional shape of the hazardous area of coal spontaneous combustion and gas coupling disasters under ideal conditions.

It is generally believed that coal spontaneous combustion and gas coupling disasters are prone to occur in the area of oxygen concentration (12–18%) in goaf, which belongs to the oxidation zone area. If the gas mixed gas in this area is in the range of gas explosion (5–16%), the accumulated heat can meet the ignition point of gas explosion so as to produce gas explosion disaster. Therefore, in this study, the upper limit of oxygen concentration threshold of coal spontaneous combustion and gas coupling disasters is 18%, and the lower limit is 12%. The upper limit of gas concentration explosion threshold of coal spontaneous combustion and gas coupling disasters is 16%, and the lower limit of explosion threshold is 5%.

To analyze the three-dimensional distribution characteristics of the gas explosion hazardous area in goaf, based on the three-dimensional distribution law of gas in the mining area obtained by CFD numerical simulation, the contours of oxygen concentration and gas concentration distribution at different heights were extracted, and the oxygen concentration range (12% < O2 < 18%) and gas concentration range (5% < CH4 < 16%) in the mining area were used as the boundary of the hazardous area, and cross-sectional maps of the hazardous area under the heights of z = 1, 15, 30, and 40 m were obtained as shown in Figure 14. The yellow area was the gas concentration range to meet the gas explosion, the cyan area was the oxygen concentration range to meet the gas explosion, and the red area was the hazardous area to meet the gas explosion.

Figure 14.

Figure 14

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Reconstruction of hazardous area with different section heights.

Figure 14 can be clearly seen when z = 1 m, and the hazardous area at the corner of the return airway roadway was close to the working face, but with the increase of goaf height, the hazardous area gradually deviates from the working face, extends to the depth of goaf, and extends to the maximum value of 19.8 m when the goaf section height was 40 m. The hazardous area at the intake roadway shows a change pattern of first decreasing and then increasing along the deeper part of the goaf area. Along the strike direction of goaf, the hazardous area as a whole tends to extend to the deep part of goaf with the increase of the section height. However, the extension rate of the hazardous area in the upper part of goaf was obviously higher than that in the lower part of goaf. This is due to the effect of gravity increasing and floating and air viscous resistance so that the gas increase rate in the upper part of goaf is higher than that in the lower part of goaf, and the oxygen decrease rate in the upper part of goaf is lower than that in the lower part of goaf. Therefore, the mixed gas in the upper goaf was more likely to meet the conditions of gas explosion, and the hazardous area extends more quickly.

4.3. Reconstruction of Coal Spontaneous Combustion and the Gas Coupling Disasters Hazardous Area

By extracting the two-dimensional plan view of the hazardous area under different section heights and superimposing them and then modeling them in equal scale, a three-dimensional hazardous area view was constructed, as shown in Figure 15. The superimposed area in the figure (red shaded part) was the coal spontaneous combustion and the gas coupling disasters hazardous area. From the analysis in Figure 15, it can be seen that the coupling disasters hazardous area runs through the whole goaf tendency, which is mainly determined by the roof caving characteristics that determine the fracture propagation direction. And the hazardous area was roughly in the shape of a long strip inclined to the working face, gradually to the deep part of the mining area along the extension. With the increase of the height of goaf, the coupling hazardous area got closer and closer to the working face, and the hazardous area was roughly in the shape of an inverted “C”. To more clearly observe the spatial evolution law of hazardous area and provide a theoretical basis for the rapid identification and prevention of coal spontaneous combustion and gas coupling disasters, the furthest and nearest distances between the hazardous area and working faces with different cross-sectional heights were extracted, as shown in Figure 16.

Figure 15.

Figure 15

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Reconstructed 3D hazardous area stereogram.

Figure 16.

Figure 16

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Distance between the reconstruction hazardous area and working face at different heights.

According to the analysis in Figure 16, under different goaf heights, the distance between the hazardous area and the working face showed the same change trend. In addition, the width of the hazardous area first decreases and then increases with the increase of the goaf height. Under the same goaf height, the growth rate of the upper width of goaf was significantly higher than that of the middle and lower parts of goaf, which also conforms to the rule that the extension rate of the return airway roadway to goaf was greater than that of the intake roadway. The nearest and farthest distances of the hazardous area from the working face at z = 1 m were 1.7 and 41.9 m; at z = 15 m were 10.4 and 31.4 m; at z = 30 m were 14.6 and 44.0 m; and at z = 40 m were 15.7 and 45.6 m. The nearest and farthest distances from the working face at z = 40 m are 15.7 and 45.6 m, respectively. It can be found that the nearest and farthest distances from the hazardous area to the working face were respectively distributed near the return airway roadway and the intake roadway. It is worth noting that the decrease rate of Figure 16c,d abruptly changes at the inclined distance of 90 m in goaf, which may be caused by the uneven division of structural grid in this area when FLUENT simulation software was used to solve the problem.

5. Conclusions

In this study, based on the two numerical simulation software of DEM and CFD, the laws of overburden change and gas migration in fully mechanized caving goaf were truly reflected, and the spatial forms of the coupling disasters hazardous area were quantitatively constructed. The following conclusions were drawn.

  • (1)

    Based on the discrete element software PFC3D, the variation of overburden rock caving and porosity in goaf was simulated. The porosity data of the fully mechanized caving face after all mining were extracted, and the porosity data were imported into FLUENT software using UDF custom function to obtain the fluid distribution law of goaf under real conditions.

  • (2)

    The three-dimensional time–spatial revolution law of oxygen and gas in fully mechanized caving goaf was obtained by FLUENT simulation. The distribution of the flow field at different locations in goaf was not consistent, the oxygen concentration was higher in the intake roadway and near the working face, and along the direction of goaf, the oxygen concentration gradually decreases with the increase of the depth of goaf, showing asymmetry. As the distance from the working face increases, the gas concentration in goaf gradually increases, and the gas concentration gradually increases from the intake side along the direction of the tendency of goaf, and the gas concentration near the corner of the upper return wind was higher.

  • (3)

    Based on the contour plots of oxygen concentration and gas concentration in different areas of goaf, combined with the distribution characteristics of fracture collapse and coal residues in goaf, the three-dimensional reconstruction method was used to obtain a three-dimensional spatial area of coal spontaneous combustion and gas coupling disasters in the fully mechanized caving goaf at a certain mining stage. From the results, the coupling hazardous area was characterized by spatial asymmetry, inhomogeneity, etc. The coupling hazardous area was relatively close to the return airway roadway side and dips forward from the bottom plate upward to the working face direction and gradually shrinks.

  • (4)

    Due to the multidirectional nonhomogeneity and nonuniformity of goaf, the porosity changes with the mining of the working face, also resulting in the change in the location of the coupling hazardous area, and the coupling hazardous area was not invariable. The reconstruction method of the coupling hazardous area proposed in the study can facilitate the rapid reconstruction of the spatial shape of the coupling hazardous area in different mining stages and can provide theoretical guidance to improve the targeting and effectiveness of this kind of disaster prevention and control under the consideration of certain safety thresholds. However, to accurately characterize the spatial dynamics of the coupling hazard, many factors have to be considered, such as air leakage rate, thermal storage environment, and coal adsorption. In addition, the study of the construction of the coupling hazardous area for different mining periods, different propulsion speeds, and different wind supply is an important direction worthy of indepth study in the future.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (No. 51804270), the Excellent youth project of Hunan Provincial Department of Education (No.22B0169), the Natural Science Foundation of Hunan Province (No. 2020JJ5547), and the Research Fund of Hunan Provincial Department of Education (No. 19C1745).

Author Contributions

J.Z.: writing—original draft, validation, and investigation; R.Z.: supervision and investigation; F.Z.: investigation; X.Z.: writing—review and editing, methodology, and funding acquisition.

The authors declare no competing financial interest.

References

  1. Tao S.; Pan Z.; Chen S.; Tang S. Coal seam porosity and fracture heterogeneity of marcolithotypes in the Fanzhuang Block, southern Qinshui Basin, China. J. Nat. Gas Sci. Eng. 2019, 66, 148–158. 10.1016/j.jngse.2019.03.030. [DOI] [Google Scholar]
  2. Wang S.; Li X.; Qin Q. Study on Surrounding Rock Control and Support Stability of Ultra-Large Height Mining Face. Energies 2022, 15, 6811. 10.3390/en15186811. [DOI] [Google Scholar]
  3. Xu C.; Nie W.; Liu Z.; Peng H.; Yang S.; Liu Q. Multi–factor numerical simulation study on spray dust suppression device in coal mining process. Energy 2019, 182, 544–558. 10.1016/j.energy.2019.05.201. [DOI] [Google Scholar]
  4. Zhang X.; Zhou F.; Zou J. Numerical Simulation of Gas Extraction in Coal Seam Strengthened by Static Blasting. Sustainability 2022, 14, 12484. 10.3390/su141912484. [DOI] [Google Scholar]
  5. Zhang X.; Pan Y. Preparation, Properties and Application of Gel Materials for Coal Gangue Control. Energies 2022, 15, 557. 10.3390/en15020557. [DOI] [Google Scholar]
  6. Zhai C.; Xiang X.; Xu H.; Wu S. The characteristics and main influencing factors affecting coal and gas outbursts in Chinese Pingdingshan mining region. Nat. Hazards 2016, 82, 507–530. 10.1007/s11069-016-2195-2. [DOI] [Google Scholar]
  7. Zhang L.; Wang H.; Chen C.; Wang P.; Xu L. Experimental study to assess the explosion hazard of CH4/coal dust mixtures induced by high-temperature source surface. Process Saf. Environ. Prot. 2021, 154, 60–71. 10.1016/j.psep.2021.08.005. [DOI] [Google Scholar]
  8. Xie C.; Xiong G.; Chen Z. Using CFD to Simulate the Concentration of Polluting and Harmful Gases in the Roadway of Non-Metallic Mines Reveals Its Migration Law. Sustainability 2022, 14, 13349. 10.3390/su142013349. [DOI] [Google Scholar]
  9. Deng J.; He Y.; Zhai X.; Wang K. Cause analysis of gas explosion accident for Babao mine based on cybernetics. Saf. Coal Mines 2015, 46, 234–237. 10.13347/j.cnki.mkaq.2015.11.065. [DOI] [Google Scholar]
  10. Harris D.; Kirsch P.; Shi S.; Li J. Comparative Analysis of Coal Fatalities in Australia, South Africa, India, China and USA, 2006–2010. Coal Operators’ Conference 2014, 399–407. [Google Scholar]
  11. Cheng J.; Cheng W.; Zhang S. Methods to determine the mine gas explosibility – An overview. Process Saf. Environ. Prot. 2012, 25, 425–435. 10.1016/j.jlp.2011.12.001. [DOI] [Google Scholar]
  12. Tutak M.; Brodny J. Determination of Particular Endogenous Fires Hazard Zones in Goaf with Caving of Longwall. IOP Conf. Ser.: Earth Environ. Sci. 2017, 95, 042026 10.1088/1755-1315/95/4/042026. [DOI] [Google Scholar]
  13. Wagner H. Deep Mining: A Rock Engineering Challenge. Rock Mech. Rock Eng. 2019, 52, 1417–1446. 10.1007/s00603-019-01799-4. [DOI] [Google Scholar]
  14. Wang X.; Liu W.; Jiang X.; Zhang Q.; Wei Y. Evolution Characteristics of Overburden Instability and Failure under Deep Complex Mining Conditions. Geofluids 2022, 2022, 6418082 10.1155/2022/6418082. [DOI] [Google Scholar]
  15. Liu H.; Zhang B.; Li X.; Liu C.; Wang C.; Wang F.; Chen D. Research on roof damage mechanism and control technology of gob-side entry retaining under close distance gob. Eng. Failure Anal. 2022, 138, 106331 10.1016/j.engfailanal.2022.106331. [DOI] [Google Scholar]
  16. Aitao Z.; Wang K. A new gas extraction technique for high-gas multi-seam mining: a case study in Yangquan Coalfield, China. Environ. Earth Sci. 2018, 77, 150. 10.1007/s12665-018-7291-z. [DOI] [Google Scholar]
  17. Ma Y.-k.; Nie B.; He X.; Li X.; Meng J.; Song D. Mechanism investigation on coal and gas outburst: An overview. Int. J. Miner., Metall. Mater. 2020, 27, 872–887. 10.1007/s12613-019-1956-9. [DOI] [Google Scholar]
  18. Li D.; Cheng Y. Evaluation of Gas Control Ability of a Coal and Gas Outburst Mine. Energy Sources, Part A 2014, 36, 2401–2409. 10.1080/15567036.2011.567241. [DOI] [Google Scholar]
  19. Chen L.; Qi X.; Zhang Y.; Xin H.; Liang Z. Reaction activity and mechanism of R3-CH structure oxidation in coal self-heating. Fuel 2021, 290, 119797 10.1016/j.fuel.2020.119797. [DOI] [Google Scholar]
  20. Wang C.-P.; Deng Y.; Xiao Y.; Deng J.; Shu C.; Jiang Z. Gas-heat characteristics and oxidation kinetics of coal spontaneous combustion in heating and decaying processes. Energy 2022, 250, 123810 10.1016/j.energy.2022.123810. [DOI] [Google Scholar]
  21. Xia T.; Zhou F.; Wang X.; Zhang Y.; Li Y.; Kang J.; Liu J. Controlling factors of symbiotic disaster between coal gas and spontaneous combustion in longwall mining gobs. Fuel 2016, 182, 886–896. 10.1016/j.fuel.2016.05.090. [DOI] [Google Scholar]
  22. Zhou F. Study on the coexistence of gas and coal spontaneous combustion (I): disaster mechanism. J. China Coal Soc. 2012, 37, 843–849. 10.13225/j.cnki.jccs.2012.05.027. [DOI] [Google Scholar]
  23. Liu J.; Chen Z.; Elsworth D.; Qu H.; Chen D. Interactions of multiple processes during CBM extraction: A critical review. Int. J. Coal Geol. 2011, 87, 175–189. 10.1016/j.coal.2011.06.004. [DOI] [Google Scholar]
  24. Xia T.; Zhou F.; Liu J.; Kang J.; Gao F. A fully coupled hydro-thermo-mechanical model for the spontaneous combustion of underground coal seams. Fuel 2014, 125, 106–115. 10.1016/j.fuel.2014.02.023. [DOI] [Google Scholar]
  25. Chu T.; Li P.; Chen Y. Risk assessment of gas control and spontaneous combustion of coal under gas drainage of an upper tunnel. Int. J. Min. Sci. Technol. 2019, 29, 491–498. 10.1016/j.ijmst.2018.05.002. [DOI] [Google Scholar]
  26. Gieras M.; Klemens R.; Rarata G.; Wolanski P. Determination of explosion parameters of methane-air mixtures in the chamber of 40 dm3 at normal and elevated temperature. J. Loss Prev. Process Ind. 2006, 19, 263–270. 10.1016/j.jlp.2005.05.004. [DOI] [Google Scholar]
  27. Ye J.; Tao S.; Zhao S.; Li S.; Chen S.; Cui Y. Characteristics of methane adsorption/desorption heat and energy with respect to coal rank. J. Nat. Gas Sci. Eng. 2022, 99, 104445 10.1016/j.jngse.2022.104445. [DOI] [Google Scholar]
  28. Qin B.; Zhang L.; Wang D.; Yao Y. Mechanism and restraining technology on spontaneous combustion of coal detonating gas in goaf. J. China Coal Soc. 2009, 34, 1655–1659. [Google Scholar]
  29. Zhang X.; Zou J. Research on collaborative control technology of coal spontaneous combustion and gas coupling disaster in goaf based on dynamic isolation. Fuel 2022, 321, 124123 10.1016/j.fuel.2022.124123. [DOI] [Google Scholar]
  30. Li M.; Wang H.; Wang D.; Shao Z.; He S. Risk assessment of gas explosion in coal mines based on fuzzy AHP and bayesian network. Process Saf. Environ. Prot. 2020, 135, 207–218. 10.1016/j.psep.2020.01.003. [DOI] [Google Scholar]
  31. Tutak M.; Brodny J.; Szurgacz D.; Sobik L.; Zhironkin S. The Impact of the Ventilation System on the Methane Release Hazard and Spontaneous Combustion of Coal in the Area of Exploitation—A Case Study. Energies 2020, 13, 4891. 10.3390/en13184891. [DOI] [Google Scholar]
  32. Du F.; Wang K.; Guo Y.; Wang G.; Wang L.; Wang Y. The mechanism of rockburst-outburst coupling disaster considering the coal-rock combination: An experiment study. Geomech. Eng. 2020, 22, 255–264. 10.12989/gae.2020.22.3.255. [DOI] [Google Scholar]
  33. Yin G.; Jiang C.; Xu J. Experimental study of Solid-liquid-gas coupling seepage of containing-gas coal. Disaster Adv. 2012, 4, 103–108. [Google Scholar]
  34. Du F.; Guo Y.; Wang L.; Xu C.; Zhao A.; Wang G. EDEM-FLUENT coupled simulation of coal-gas outburst two-phase flow. Energy Explor. Exploit. 2021, 39, 1786–1802. 10.1177/01445987211023853. [DOI] [Google Scholar]
  35. Qin Y.; Song Y.; Liu W.; Wei J.; Lv Q. Assessment of low-temperature oxidation characteristics of coal based on standard oxygen consumption rate. Process Saf. Environ. Prot. 2020, 135, 342–349. 10.1016/j.psep.2019.12.039. [DOI] [Google Scholar]
  36. Peng S.; Xu J.; Yang H.; Liu D. Experimental study on the influence mechanism of gas seepage on coal and gas outburst disaster. Saf. Sci. 2012, 50, 816–821. 10.1016/j.ssci.2011.08.027. [DOI] [Google Scholar]
  37. Chen S.; Tang D.; Tao S.; Liu P.; Mathews J. Implications of the in situ stress distribution for coalbed methane zonation and hydraulic fracturing in multiple seams, western Guizhou, China. J. Pet. Sci. Eng. 2021, 204, 108755 10.1016/j.petrol.2021.108755. [DOI] [Google Scholar]
  38. Zhu Z.; Feng T.; Yuan Z.; Xie D.; Chen W. Solid-gas coupling model for coal-rock mass deformation and pressure relief gas flow in protection layer mining. Adv. Civ. Eng. 2018, 2018, 5162628 10.1155/2018/5162628. [DOI] [Google Scholar]
  39. Gao J.; Hu Q.; Gao Y.; Li M.; Sun D. Gas Expansion Energy Model and Numerical Simulation of Outburst Coal Seam under Multifield Coupling. Geofluids 2021, 2021, 5552108 10.1155/2021/5552108. [DOI] [Google Scholar]
  40. Fan J.; Liu P.; Li J.; Jiang D. A coupled methane/air flow model for coal gas drainage: Model development and finite-difference solution. Process Saf. Environ. Prot. 2020, 141, 288–304. 10.1016/j.psep.2020.05.015. [DOI] [Google Scholar]
  41. Xue S.; Wang Y.; Xie J.; Wang G. A coupled approach to simulate initiation of outbursts of coal and gas - Model development. Int. J. Coal Geol. 2011, 86, 222–230. 10.1016/j.coal.2011.02.006. [DOI] [Google Scholar]
  42. Li Z.; Xu Y.; Liu H.; Zhai X.; Zhao S.; Yu Z. Numerical analysis on the potential danger zone of compound hazard in gob under mining condition. Process Saf. Environ. Prot. 2021, 147, 1125–1134. 10.1016/j.psep.2021.01.043. [DOI] [Google Scholar]
  43. Cheng W.; Zhang X.; Wang G.; Yang X.; Lin X. Reconstruction technology of gas and coal spontaneous combustion coupled hazard in fully mechanized caving goaf. J. China Coal Soc. 2016, 41, 662–671. 10.13225/j.cnki.jccs.2015.0682. [DOI] [Google Scholar]
  44. Yu M.; Chang X.; Jia H.; Lu L. Analysis of spontaneous combustion “three -zone” in goaf based on Matlab. J. China Coal Soc. 2010, 35, 600–604. 10.13225/j.cnki.jccs.2010.04.024. [DOI] [Google Scholar]
  45. Ma D.; Qin B.; Gao Y.; Jiang J.; Feng B. An experimental study on the methane migration induced by spontaneous combustion of coal in longwall gobs. Process Saf. Environ. Prot. 2021, 147, 292–299. 10.1016/j.psep.2020.09.046. [DOI] [Google Scholar]
  46. Fan C.; Xu H.; Wang G.; Wang J.; Liu Z.; Cheng Q. Determination of roof horizontal long drilling hole layout layer by dynamic porosity evolution law of coal and rock. Powder Technol. 2021, 394, 970–985. 10.1016/j.powtec.2021.09.002. [DOI] [Google Scholar]
  47. Zhang X.; Wang S.; Li X.; Meng J.; Li Z.; Zhang L.; Pu D.; Wang L. Research on Theory and Technology of Floor Heave Control in Semicoal Rock Roadway: Taking Longhu Coal Mine in Qitaihe Mining Area as an Example. Lithosphere 2022, 2022, 3810988 10.2113/2022/3810988. [DOI] [Google Scholar]
  48. Wang T.; Zhou W.; Chen J.; Xiao X.; Li Y.; Zhao X. Simulation of hydraulic fracturing using particle flow method and application in a coal mine. Int. J. Coal Geol. 2014, 121, 1–13. 10.1016/j.coal.2013.10.012. [DOI] [Google Scholar]
  49. Du W.; Zhang Y.; Meng X.; Zhang X.; Li W. Deformation and seepage characteristics of gas-containing coal under true triaxial stress. Arab. J. Geosci. 2018, 11, 190. 10.1007/s12517-018-3543-1. [DOI] [Google Scholar]
  50. Li W.; Wang G.; Du W.; Wang P.; Chen J.; Sun W. Development and application of a true triaxial gas-solid coupling testing system for coal seepage. Rock Soil Mech. 2016, 37, 2109–2118. 10.16285/j.rsm.2016.07.036. [DOI] [Google Scholar]
  51. Iqbal N.; Rauh C. Coupling of discrete element model (DEM) with computational fluid mechanics (CFD): A validation study. Appl. Math. Comput. 2016, 277, 154–163. 10.1016/j.amc.2015.12.037. [DOI] [Google Scholar]
  52. Yang Y.; Li Z.; Tang Y.; Liu Z.; Ji H. Fine coal covering for preventing spontaneous combustion of coal pile. Nat. Hazards 2014, 74, 603–622. 10.1007/s11069-014-1203-7. [DOI] [Google Scholar]
  53. Liu Y.; Hinrichsen O. Computers & fluids study on CFD – PBM turbulence closures based on k-e and Reynolds stress models for heterogeneous bubble column flows. Comput. Fluids 2014, 105, 91–100. 10.1016/j.compfluid.2014.09.023. [DOI] [Google Scholar]
  54. Selma B.; Bannari R.; Proulx P. A full integration of a dispersion and interface closures in the standard k-ε model of turbulence. Chem. Eng. Sci. 2010, 65, 5417–5428. 10.1016/j.ces.2010.06.020. [DOI] [Google Scholar]

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