Study on the Cause of Hypoxia in the Corner of Return Air of Shallow Buried Flammable Coal Seam Group Mining Face and the Coordinated Prevention and Control of Coal Spontaneous Combustion

Abstract

Coal is the main energy source in China, and spontaneous combustion of coal is one of the main disasters in coal mines. Fully mechanized caving top coal mining technology is widely used in coal mines. Under the comprehensive action of multiple factors, gas generated by coal oxidation and low oxygen gas in goaf is easy to rush to working face, resulting in lower oxygen concentration in upper corner and exceeding harmful gas concentration. It seriously threatens the health of underground workers and the normal operation of production. If the oxygen concentration in the upper corner is changed, it is bound to cause the change of the oxygen concentration in the goaf and aggravate the threat of spontaneous combustion of coal in the goaf. In order to solve the double threat of low oxygen in upper corner and coal spontaneous combustion in goaf, based on Coulomb model, this paper uses command flow to conduct numerical simulation tests on the failure law of overlying strata, and obtains the influence of mining of the lower 1 coal seam and the lower 5 coal seam on the overlying strata subsidence and surface penetration. Based on the comprehensive consideration of atmospheric pressure, ore pressure activity, external air leakage and other factors, the mixed model of the source and emission of low oxygen gas in goaf was established, and the formation mechanism of low oxygen problem in the corner of return air of working face was defined. The use of pressure air belt to deal with low oxygen in return air corner is proposed creatively. The comprehensive fire prevention measures such as corner plugging, nitrogen injection and grouting are used to inhibit the oxidation of the left coal and the influx of low oxygen gas to the corner in the goaf, so as to achieve the goal of collaborative prevention and control of low oxygen in the corner of return air and coal spontaneous combustion. The research results have important reference significance for the mine with the risk of spontaneous combustion or the problem of low oxygen in the return air corner.

1. Introduction

The fully mechanized top coal caving technology is widely used in our country recently. This technology can greatly improve the production efficiency of coal mine, but there are many problems such as waste coal and serious air leakage, which make the spontaneous combustion of coal in goaf more and more frequent. According to statistics, nearly one hundred state-owned key coal mines are closed every year due to fire, and the amount of frozen coal is over 10 million tons. The closure of the working face and the freezing of resources make the reasonable development and deployment and mining sequence often broken, which brings huge economic losses and great accident risks to the mine. At the same time, under the comprehensive action of multiple factors, gas (H₂S, CO, etc.) produced by coal oxidation and low-oxygen gas in goaf are easy to rush to the working face, resulting in lower oxygen concentration in the upper corner and excessive concentration of harmful gas, which seriously threatens the health of underground workers and the normal operation.

Research Status of Hypoxia Problem in Working Face

Scholars Kai Guo [1] and Fang Baoming [2] found that the reason for the formation of hypoxia was that the coal mine was a low-gas mine, and the coal seam gas was deposited in the CO₂-N₂ zone, resulting in the formation of high nitrogen and low oxygen in the goaf. When a large amount of nitrogen was emitted, low oxygen in the working face was formed. Xinhai Zhang et al. [3] determined the oxygen consumption rate and the mechanism of releasing harmful gases of CO and CO₂ from goaf remnants. Yang Junzhe [4] compared the change curve of oxygen concentration in the corner of return air of working face with atmospheric pressure and temperature, and concluded that oxygen concentration in the corner of return air decreased with the decrease of atmospheric pressure. Zhang Lihui [5], under the negative pressure “U” type ventilation condition of working face with air leakage cracks, caused a large amount of low-oxygen gas already formed in the goaf to rush to the working face for air return through some air leakage cracks. Wenyong Lu et al. [6] used SF₆ tracer gas air leakage detection method to find that the gas in the overlying goaf migrated to the goaf of the coal seam through some fissures, resulting in increased content of low-oxygen gas in the coal seam and easy to gouge out of the working face. Xiaowei Zhang et al. [7] found in the field test that carbon dioxide emission in some coal seams could easily lead to the phenomenon of low oxygen in the working face in a short time. Husheng Liu et al. [8] analyzed the changes of atmospheric pressure and oxygen concentration in the working face of coal mine in a year, and found out the “breathing” characteristics of goaf by comparing the days with the big daily pressure difference. Guoming Chi et al. [9] found that the factors influencing the hypoxia of working face include the surface fractures, the geological conditions of coal seam, the influence caused by the negative pressure ventilation, the influence of the large goaf area and the influence of atmospheric pressure.

By judging the main sources of harmful gases in the coal face, many scholars and coal mine workers have also adopted a series of technical measures to prevent hypoxia. Domestic scholars Guoming Chi. Yuerong Jian and others mainly conducted research based on field work experience and field data collection, and analyzed and proposed some convenient hypoxia prevention and control methods, which were summarized: Surface fissures should be filled in time to reduce surface air leakage [10]. Air curtain is hung at the corner of the inlet and return air lane to prevent the gas emission at the corner of the return air [2,6]. The closed thickness of the goaf should be improved to reduce air leakage in the closed wall [11,12]. The problem of hypoxia and the occurrence of harmful gases are homologous, and the prevention and control of harmful gases can be referred to for inadequate measures. Chunqiao Wang et al. [13] studied the distribution of air leakage before and after the uniform pressure by means of numerical simulation, and the uniform pressure method can effectively solve the problem of excessive CO accumulation in the corner of the working surface. Furong Du et al. [14] analyzed the reasons for the excess of CO and CH₄ at the corner Angle and return air flow on the working face, and took measures of air increase on the working face, local air supply in the upper corner and grouting to reduce the concentration of CO and CH₄. Xinyu Wang [15] tested the CO content in the original coal seam under the background of the CO overrun in the corner of return air in Shendong mining area, studied the CO migration law in the super-large goaf, analyzed the source of CO in the corner of return air, and proposed the technical measures to control the CO overrun in the corner of return air. Yuguo Wu [16] established safety and spontaneous combustion warning concentration prediction models of harmful gases such as CO in return air corner of working face, and proposed fire prevention management regulations in mining areas. Youfa Wang [17] analyzed the over-limit situation of harmful gas in the corner of the fully mechanized mining face of Hejiata Coal Mine, pointed out the sources and causes of harmful gas accumulation, and proposed such treatment measures as spraying inhibitor, hanging windshield curtain, nitrogen injection and yellow mud grouting. Qinghe Qin et al. [18,19,20] combined the equalized pressure ventilation technology operated jointly, such as the equalized pressure fan, the equalized pressure air door and the regulating air curtain, to reduce the pressure difference at both ends, make the working face in a relatively positive pressure ventilation state, inhibit the emission of harmful gases, and contribute to the prevention and control of hypoxia hazards.

2. Research Method

2.1. Background of Coal Mine

The coal seam thickness of 5106 fully mechanized caving face in the test mine is 7.0–10.4 m, with an average of 10 m, which belongs to extremely thick coal seam. Working face surface elevation range: +1868–+1934 m, working face elevation +1552–+1688 m, coal seam depth of 180–382 m, the average depth of 281 m. Adopting backward integrated mechanized top coal caving mining and U-type ventilation, the design mining length is 260 m, mining height is 5.8 m, and roof height is 6 m. 5 coal seams mined are class I prone to spontaneous combustion, with the shortest spontaneous combustion period of 57 days. The north of 5106 working face is the goaf of 5104 working face; It is overlaid with mined-out area 1110 and 1112 of lower 1 coal. The spacing between the lower 1 coal seam and the lower 5 coal seam is about 71 m. The location of the well and the relationship between adjacent mining are shown in Figure 1.

In the mining of shallow coal seam in the test mine, the direct roof of the goaf all caved out, forming broken rocks piled up in the goaf, and the old roof caved into large rocks. The upper strata fractured and settled to create a rift that penetrated the formation to the surface. Because the coal seam of Okhobrak coal mine was buried shallowly, the upper strata of the coal seam collapsed due to coal mining, and the rock fracture formed four cracks through the strata to the surface, and regular cracks can be seen on the ground. The loose deposits on the surface of the test mine are mainly sandy soil, which has good air permeability. In addition, due to the dry climate, the gaps formed cannot be closed in time, so the gaps formed become a good air leakage channel. In the mining process of working face, it is faced with the double hidden dangers of spontaneous combustion of left coal in goaf and excessive concentration of low oxygen gas in the corner of return air (as shown in the Figure 2), which is easy to cause mine safety accidents and affect the normal production of the mine.

2.2. Overburden Failure Model of Shallow Coal Seam

(1)

Monr-Coulomb model

According to the material type and physical and mechanical properties of overlying strata in gob, Monr-Coulomb model is used to calculate and analyze the failure state of overlying strata in coal seam. The implementation of this model uses principal stresses d₁, d₂, d₃ and out-of-plane stresses dz. Principal stresses and principal directions are calculated from the stress tensor component (compressive stress is negative). The principal strain increment D_e1, D_e2 and D_e3 corresponding to d₁, d₂ and d₃ can be decomposed into

$$D{e}_i=D{e}_i^e+D{e}_i^p$$

where: the superscript e and p refer to the elastic and plastic parts respectively, i = 1, 2, 3, and the plastic component is non-zero only in the plastic flow stage. The increment expression of principal stress and principal strain of Hooke’s law is:

$$\begin{array}{l}D{d}_1={\alpha }_{1}D{e}_1^e+{\alpha }_2\left(D{e}_2^e+D{e}_3^e\right)\\ D{d}_2={\alpha }_{1}D{e}_2^e+{\alpha }_2\left(D{e}_1^e+D{e}_3^e\right)\\ D{d}_3={\alpha }_{1}D{e}_3^e+{\alpha }_2\left(D{e}_1^e+D{e}_2^e\right)\end{array}$$

where: α₁ = K + (4/3)G and α₂ = K − (2/3)G. K is the volume modulus and G is the shear modulus. Monr-Coulomb model describes the yield criterion as follows:

$$f_s={\sigma }_1-{\sigma }_2\frac{1+\sin \phi }{1-\sin \phi }-2c\sqrt{\frac{1+\sin \phi }{1-\sin \phi }}$$

where: σ₁ is the maximum principal stress, MPa; σ₂ maximum principal stress, MPa; c is cohesion, MPa; φ is the internal friction Angle, (°). When f_s > 0, shear failure occurs.

(2)

Physical model establishment

In coal seam mining, command flow is used to carry out numerical simulation test to realize step by step coal seam mining process. The sequence of opening and digging can be described by the following physical model according to the exploitation and mining situation.

(1)

The total size of the established model (length × width × height) is: 2000 m × 1000 m × 329 m:

(2)

The step-by-step excavation process of the lower 1 coal seam and the lower 5 coal seam (as shown in Figure 3) is as follows:

(i)

0 m × 80 m × 3.8 (10) m;

(ii)

80 m × 160 m × 3.8 (10) m;

(iii)

160 m × 240 m × 3.8 (10) m;

(iv)

240 m × 320 m × 3.8 (10) m;

(v)

320 m × 500 m × 3.8 (10) m;

(vi)

500 m × 1100 m × 3.8 (10) m.

(3)

Boundary condition setting

The boundary conditions of the model are set as follows: the origin of the model is set in the center of the working face, the lower and surrounding surfaces of the model are fixed boundaries, the initial velocity and displacement are zero, and the upper surface is set as the free moving boundary. In order to observe the variation law of overlying subsidence and surface subsidence caused by mining, monitoring points were set at z = 102,329 m, and the monitoring points were arranged along the strike (x = 0) on the model center line of the working face to monitor the movement of overlying subsidence and surface subsidence.

2.3. Test of Seepage Law in Surface Fractures

(1)

Principle of continuous constant release of tracer gas to detect mine air leakage

The theory of tracer gas measurement of air leakage is based on the law of conservation of mass and the theory of turbulent mass transfer. A constant amount of tracer gas is released continuously in the roadway to be investigated, and gas samples are collected along the direction of the airflow and at a certain distance from the release point. The concentration of gas samples is analyzed in the laboratory, and the air volume of the sampling point is calculated according to the concentration of gas samples and the release amount. The following figure shows the schematic diagram of air leakage measurement by single point and multi-point release. In the Figure 4 and Figure 5 are tracer gas release points with the same amount of gas release. L1–L2 are air leakage sections, and points 1 and 2 are sampling points.

The air volume at 1 point is Q₁, the air volume at 2 point is Q₂, and the air leakage volume is ΔQ, then $Q_2=Q_1+\Delta Q$; The tracer gas concentration at point 1 is C₁, and that at point 2 is C₂:

$$Q_1=\frac{q}{C_1}, Q_2=\frac{q}{C_2}$$

Let the air leakage rate be L and the tracer gas release quantity be q. The definition of air leakage rate is as follows:

$$\mathrm{L} =\frac{Q_2-Q_1}{Q_1}=\frac{\frac{q}{C_2}-\frac{q}{C_1}}{\frac{q}{C_1}}=\frac{C_1}{C_2}-1$$

According to the gas concentration and release of 1 and 2 points to calculate the air volume at two points, and then calculate the air leakage rate.

(2)

Release device and analytical instrument

The use of SF₆ tracer gas specification: SF₆, the purity of 99.999%, cylinder volume 10 L, inflation pressure about 1.8 Mpa. With pressure gauge and flow meter, flow range 0~1 L/min adjustable.

The SF₆ cylinder and continuous release device are shown in Figure 6:

Gas sample collection device used: 20 sampling bags, one SF₆ chromatographic analyzer, detection accuracy 0.1 PPb (10⁻¹⁰).

In order to analyze whether the experimental results are consistent with the actual conditions, two wind gauges of medium and low speed are prepared as far as possible (strict calibration is required before use), mechanical stopwatch, tape measure and tape measure.

(3)

Measuring point arrangement

This underground air leakage detection mainly analyzes the accurate sources of low oxygen gas in the corner of 5106 working face, which may include low oxygen gas emission from the goaf of 5106 working face, low oxygen gas emission from the goaf of 5104 working face, low oxygen gas emission from the goaf of 1110 and 1112 overlying goaf of 5106 working face, and the influence of nitrogen injection. In this test, low-oxygen gas emission in goaf of 5106 working face, low-oxygen gas emission in goaf of 5104 working face and low-oxygen gas emission in goaf of 1112 overlying goaf on 5106 working face were detected.

(4)

Air leakage detection of 5106 working face roadway

Determination of air leakage in goaf of 5106 working face The release point is set at 135 m away from the working face from the air inlet lane and 40 m away from the release point. The measuring point at the coal wall of the inlet and return air roadway is set 15 m away from the coal wall, and 4 measuring points are arranged in the inclined direction of the working face. They are respectively located at 35#, 69#, 133# bracket and the upper corner of the return air, The bracket spacing of 1.75 m is respectively 61.25 m, 120.75 m and 232.75 m from the inlet air trough. Figure 7 shows the layout of release points and measuring points on the working face.

(5)

Check whether the goaf of 5104 and 5106 working face is connected

The release point of the inspection on whether the goaf of 5104 is connected to the goaf of 5106 working face is selected at the closed part of the drainage lane connected to the goaf of 5106 return air lane. The detection point is set in the upper corner of the return air of 5106 working face and 15 m away from the coal wall in the return air lane of 5106. Figure 8 shows the layout of release points and measuring points.

(6)

Check whether the goaf of 1112 and 5106 working face is connected

The release point of the inspection on whether the goaf of 1112 is connected with the goaf of 5106 working face is selected at the detection borehole of 5106 air inlet lane. Nitrogen is used as carrier gas when the tracer gas is released to accelerate the flow speed of the tracer gas in the borehole and the goaf 1112. The detection point is set in the upper corner of the return air of 5106 working face and 15 m away from the coal wall in the return air lane of 5106. Figure 9 shows the layout of release points and measuring points.

(7)

tracer gas sampling distance

Spacing of tracer gas release point and sampling point [MT/T845-1999 Technical Specification for Detecting Air Leakage with SF₆ Tracer Gas in Coal Mine Roadway]:

L ≥ 32 S/U

where:

• L—Distance between tracer gas release point and sampling point, m;
• S—Shaft section area, m²;
• U—Length of perimeter of shaft lane, m;

The transportation channeling is 5106 working face belt conveyor and air inlet channel, and the roadway section is rectangular. Net section of roadway: width × height = 5.1 m × 3.2 m = 16.32 m². Road way circumference U = 2 × (5.1 + 3.2) = 16.6 m.

L ≥ 32 × 16.32/16.6 = 31.5 m

Therefore, the spacing of sampling points in the inlet lane of 5106 working face is set as 40 m.

(8)

tracer gas release calculation

When the tracer gas is used for air leakage measurement, the release amount of tracer gas should be determined on the premise of ensuring the detection accuracy and the emission as little as possible. The detection accuracy of chromatographic analyzer is 0.1 ppb, that is, 10⁻¹⁰, the detection range is 0.0001–1000 ppm, and the optimal detection value is 0.1 ppm.

$$q=KCQ\times {10}^6$$

where:

• $q$—Estimated quantitative release of tracer gas, mL/min;
• K—Coefficient, value 5;
• $Q$—The air volume of roadway is 1500 m³/min at 5106 working face;
• $C$—The minimum concentration of SF₆ tracer gas in the predetermined airflow is 10⁻⁷.

The air volume of 5106 working face is 1500 m³/min. The optimal detection minimum concentration of SF₆ tracer gas in the predetermined air flow is 0.1 ppm, and the calculated release volume is 750 mL/min. The actual release amount of SF₆ tracer gas in the sampling process was calibrated to 750 mL/min by float flowmeter, and each release point was quantitatively released for 30 min continuously.

3. Results and Discussion

3.1. Study on Overburden Failure Law of Mining Shallow Buried Coal Seam Group

The distribution of plastic zone in the numerical simulation results can reflect the failure of rock mass in the roof and floor after coal seam mining. The thickness of the lower 5 coal seam in Okhobrak mine is 10 m, which belongs to the mining of extremely high coal seam. The higher the mining height, the more intense the overburden activity, the more serious the mining deformation and failure.

The lower 1 coal seam of Okhobulak coal mine is about 227 m from the ground, the thickness of the sedimentary layer is about 194 m, and the thickness of the bearing bedrock is 33 m, which controls the subsidence movement of the overlying strata. The geological characteristics of shallow burial depth and thin bedrock make the overlying strata affected by mining obviously, and the fracture/mining ratio is large, and the ground subsidence forms a basin area. As shown in Figure 10(1) and Figure 11(1), overburden failure occurs when the working face advances 80 m. The height of the crack can reach 44 m, and the width of the crack does not change much. As shown in Figure 10(2) and Figure 11(2), when the working face advances 160 m, the overburden failure condition increases to 178 m in height and little changes in width, and the ground subsidence worsens the damage around.

With the increase of the advancing distance of the working face, the influence of mining gradually extends to the advancing direction and the surface direction, until the overburden completely collapses, the overburden broken area has reached the ground, the cracks are connected with the surface, and the basin height left by the ground subsidence reaches the maximum. As shown in Figure 10 and Figure 11(3), when the working face advances 240 m, the overburden failure occurs. The height of the fracture has developed to the ground and it is connected with the basin-shaped breaking ring of the ground.

With the continuous excavation of coal face, mining-induced fissure only develops along the advancing direction, and the basin shape left by ground subsidence only increases in length and height. As shown in Figure 10 and Figure 11(4), overburden failure at 400 m advance of the working face, the goaf of lower 1 coal and the ground are connected through mining-induced fractures, and the basin-shaped fracture area left by the ground subsidence gradually expands along the advance direction of the working face, and the ground subsidence is intensified. As shown in Figure 10 and Figure 11(4), overburden failure occurs when the working face advances by 1100 m. The mining affected area only develops along the advancing direction, while the basin-shaped broken area left by the ground subsidence gradually expands along the advancing direction of the working face.

At the same time, with the stress release after the mining of 300 m, the overburden completely collapsed, in the range of 300 m to 300 m before the final mining line, the ground stress gradually recovered, there is an arch of the historical failure area of the re-compaction, and between the initial mining line to the compaction zone, the compaction zone to the final mining line left a crack “O” ring;

The lower 5 coal mining face has the characteristics of large mining height and fast advancing of the working face, which makes the overburden movement intense and the fracture development degree higher. The rock layer destroyed after the mining of the upper and lower 1 coal seam will be damaged again under the influence of the mining of the lower 5 coal seam, and the damage degree will increase greatly. As shown in Figure 10 and Figure 11, the height of fracture development has reached the goaf of lower 1 coal seam after 320 m mining in lower 5 coal seam, and there is a certain amount of connected fracture channels. As shown in Figure 10 and Figure 11(8), when the coal seam is mined to 500 m, the development height of overlying fractures has covered the goaf of the lower 1 coal and continues to develop towards the surface. As shown in Figure 10 and Figure 11, with the continuation of mining in the lower 5 coal seam, overburden movement intensified, and the failure range communicated with the upper goaf and the ground.

The uncoordinated subsidence movement of the rock strata will produce bed separation cracks among each other, which can well describe the connection between the rock strata. Floor heave is obvious in the initial stage of mining, as shown in Figure 10 and Figure 11(1)–(4), the overlying strata will produce bed separation, and there is a certain amount of compression around the mining space. With the increase of the mining distance, The overlying layer separation increased more and more obviously, and gradually developed to the surface along the vertical direction. As shown in Figure 10 and Figure 11(5),(6), after mining is over 500 m, until the final mining line, the development of bed separation is the largest in the support area on both sides of the mining space. Compaction occurs in the middle, and bed separation is developed directly to the surface, forming a fractured channel.

In a word, with the advance of the mining of the lower 1 coal seam, the overburden failure scope gradually develops to the surface. The shape of the overburden failure scope of the lower 1 coal seam mining is similar to the saddle shape, and the overburden failure height above the goaf boundary is larger and connected to the surface. In the numerical simulation, the height of mining-induced fracture in the overburden of the lower 5 coal seam is much higher than the strata thickness between the lower 113 coal seam and the lower 5 coal seam, so as to be connected with the goaf of the lower 1 coal overburden. Finally, the composite communication between the goaf of the lower 5 coal seam and the goaf of the lower 1 coal seam and the surface is formed. Under the action of the negative pressure in the mine, the ground air will enter the working face along the fractured channel. Meanwhile, the air leaking from the ground passes through the geological weathering zone, carrying CO₂, N₂ and the oxidized CO of the overlying coal seam and the free CH₄ to the working face, and dilute the fresh air flow in the working face. This is also the reason for the low oxygen concentration in the corner of the working face return air.

3.2. Study on Air Leakage Law of Shallow Buried Coal Seam

(1)

On 7 June 2021, SF₆ was released from the observation hole of the sealed wall of contact lane 5106 and 5104, with a discharge flow of 800 mL/min and a release time of 3.5 h. On the 12th of the day: After 30 min of release, SF₆ gas was received at the corner of return air and 15 m away from the corner of 5106 working face. Through gas sample analysis, the maximum SF₆ concentration was 0.00345 ppm, and the SF₆ background concentration measured in the inlet lane was 0.00565 ppm. It can be considered that there is no air leakage between the goaf 5104 and the working face 5106.

(2)

SF₆ was released in the 5106 transport trough on 8 June 2021, with a discharge flow of 800 mL/min and a release time of 1 h. The release begins at 13:57 and ends at 15:07. After 15 min of release, samples were taken at each measuring point successively, and analyzed by SF6 special chromatograph. It was found that the air leakage between the inlet and return air lanes was 20 m³/min.

(3)

On 8 June 2021, SF₆ was released at the detection hole of 5106 transportation lane and 1112 goaf with a discharge flow of 25 L/min and a release time of 1.5 h. In order to accelerate the diffusion of SF₆ gas in the 1112 goaf after the release, nitrogen was injected into the detection hole for 30 min. SF₆ gas was measured at the corner of the return air with SF₆ portable instrument and the concentration signal was the strongest. The gas was taken when the SF₆ gas was detected by SF₆ portable instrument and the concentration signal was the strongest. It is 380 times of the SF₆ background concentration measured in the inlet roadway, which is 0.00565 ppm. It can be determined that there is air leakage between the goaf 1112 and the working face 5106. In addition, through field observation, there is an obvious gas outpouring phenomenon in the detection hole, and the pressure in the hole is 240 pa higher than that in the roadway, which also proves that the 5106 working face is connected with the 1112 goaf after mining.

(4)

In order to verify whether there is air leakage between the ground crack and the 5106 working face, SF₆ was released at the ground crack on 15 June 2021, and the SF6 portable instrument was used for simultaneous detection at the return air corner of the 5106 working face. The release began at 11:05 on the same day with a discharge flow of 10 L/min. SF₆ gas was received at the return air corner of 5106 working face at 11:29 after 24 min of release, and the concentration signal was very strong. Therefore, it could be concluded that there was air leakage between the ground crack and 5106 working face.

3.3. Formation Mechanism of Low Oxygen in Return Air Corner of Working Face

The results show that the low oxygen in the return air corner of 5106 working face is caused by multiple factors, as shown in Figure 12. The sources of low oxygen gas and the low oxygen problem in the corner of return air have the following reasons: (1) The geological report of the well field and the observation of goaf gas in working face 5106 show that the gas in the lower 5 coal seam is in the CO₂-N₂ zone, and the main components of the gas in the goaf are N₂ and CO₂; (2) Under the influence of mining, the goaf of working face 5106 and its overlying goaf 1112 formed a crack in the communication ground. Under the action of atmospheric pressure, the low oxygen body in the goaf 1112 will accelerate to discharge to the goaf 5106. (3) In the process of mining, continuous and open nitrogen injection is adopted to prevent fire in the goaf. With the continuous nitrogen injection, there is a large amount of nitrogen with high concentration in the goaf; (4) Under the action of air flow and periodic pressure, low oxygen gas in the goaf moves towards the working face and gathers in the upper corner, resulting in low oxygen phenomenon.

3.4. Coordinated Prevention and Control Technology System of Low Oxygen in Return Air Corner and Coal Spontaneous Combustion

The reason for the low oxygen in the corner of return air is that the surface air leakage will continuously transport the low oxygen gas to the working face through the goaf, resulting in the low oxygen concentration in the corner of the working face. But low oxygen gas in preventing coal spontaneous combustion in goaf has played a positive role in Influence of methane on the prediction index gases of coal spontaneous combustion: A case study in Xishan coalfield, China. If the oxygen concentration in the goaf is blindly increased, the problem of low oxygen in the corner of return air can be solved, but this will cause a sharp increase in the risk of spontaneous combustion of coal in the goaf. In order to solve this contradiction, this paper creatively uses the pressure air belt to deal with the low oxygen in the corner of the return air. At the same time, comprehensive fire prevention measures such as plugging in the corner, nitrogen injection and grouting are used to restrain the oxidation of the left coal in the goaf and the low oxygen in the corner of the return air and the coal spontaneous combustion.

(1)

Build a pressure zone to increase oxygen concentration in the upper cornerw

In order to solve the problem of local low oxygen in the return air corner, the concept of pressure air belt was proposed from the perspective of increasing fresh air, that is, by introducing compressed air in a certain area (such as the return air corner) to form a three-dimensional pressure air strip. This method can solve the problem of local hypoxia caused by eddy current phenomenon in the area where low oxygen gas is concentrated out (such as the corner of return air). The specific method is as follows: Use the compressed air supplied to the mine by the ground air compressor, install the end control device at the end of the pressure air pipe (as shown in Figure 13), and use the high-pressure hose to guide the downhole pressure air to form a pressure air belt in the upper corner (as shown in Figure 14), so as to increase the oxygen concentration in the upper corner. Arrange four vertical pressure air ducts in the upper corner (as shown in Figure 15), and set 20 to 30 pressure air outlet holes on each pressure air duct to form a pressure air belt in the upper corner, thereby increasing the oxygen concentration in the upper corner.

On 4 June 2021, the equipment of return air corner pressure belt was processed and debugged. According to the field test, the oxygen concentration changes in the return air corner before and after the pressure air belt device is added, as shown in Figure 16. As shown in Figure 16, the oxygen concentration in the return air corner before the pressure air belt device is added is lower than 18% and the lowest is 16.8%. After adding pressure air belt equipment, the oxygen concentration in the corner of return air increased rapidly and stabilized above 19%. By adopting the upper and lower corner plugging and increasing the pressure air equipment, the oxygen concentration in the corner of the return air increased by 2~3% and stabilized at more than 19%, achieving the effect of low oxygen treatment.

(2)

The upper and lower corner of the wind wall

The upper and lower corners of the working face are spaced at a certain distance, and the wind wall is constructed at the same time to reduce the air leakage facing the goaf of the coal seam.

Before the coal bag wall is built in the lower corner of the 5106 working surface, rapid oxidation of leftover coal appears in the goaf at the inlet side. According to the goaf measuring point (J3) in the air inlet roadway, the change of carbon monoxide concentration in the goaf monitored is shown in Figure 17. On 20 May 2021, a coal bag wall was built on the working surface and lower corner of 5106 to reduce air leakage in the goaf, and the surface of the coal bag wall was sprayed (as shown in Figure 18).

As can be seen from Figure 16, carbon monoxide was detected from 12 May before the construction of the coal bag wall, with a concentration of 15 ppm. Subsequently, the concentration of carbon monoxide increased rapidly and the rate of increase gradually accelerated. On 20 May, after the construction of the coal bag wall at the lower corner of the 5106 work surface, the concentration of carbon monoxide reached its maximum and then rapidly decreased until it dropped to zero. It shows that after the coal bag wall is built in the upper and lower corner, the goaf pressure is improved by increasing the resistance at both ends of the working face, which plays an effect of plugging the air leakage and inhibits the oxidation of the coal left in the goaf.

(3)

Analysis of oxidation prevention and control effect of goaf residual coal

As can be seen from Figure 19, the oxidation of coal left in goaf has been effectively controlled by taking measures such as nitrogen injection, grouting, construction of pressure air belt and plugging of air leakage in upper and lower corners. Nitrogen injection is carried out in the buried pipe on the air inlet side of 5106 working face, and the nitrogen injection amount is 864 m³/h. Grouting is carried out in the buried pipe on the return air side of 5106 working face, and the grouting amount is 67.39 m³/h. The concentration of carbon monoxide in goaf has been maintained at a relatively low level, while the O₂ concentration in the return air corner has been stable at more than 19%. The risk of natural combustion and low oxygen in the corner of return air in the goaf of 5106 working face are improved.

4. Conclusions

(1)

Based on the Monr-Coulomb model, numerical simulation tests were carried out on the failure law of overlying strata by using the command flow, and the influence of mining of the lower 1 and 5 coal seams on the overlying strata subsidence and surface penetration were obtained.

(2)

Based on the comprehensive consideration of atmospheric pressure, ore pressure activity, external air leakage and other factors, the mixed model of the source and emission of low oxygen gas in goaf was established, and the formation mechanism of low oxygen problem in the corner of return air of working face was defined.

(3)

The correlation between low oxygen in return air corner and coal spontaneous combustion was made clear. While creatively using pressure air belt to deal with low oxygen in return air corner, comprehensive fire prevention measures such as plugging, nitrogen injection and grouting were used to inhibit the oxidation of left coal in goaf and the influx of low oxygen gas to the corner, so as to achieve the goal of collaborative prevention and control of low oxygen in return air corner and coal spontaneous combustion.

(4)

The research results have been modified to inhibit the oxidation of residual coal and the inflow of hypoxic gas into the corner corner in the goaf, achieving the goal of the coordination prevention of hypoxic gas and spontaneous combustion of coal in the corner corner of return air. The research results have important reference significance for mine with spontaneous ignition risk or low oxygen in return air corner.