Fracture Distribution Characteristics in Goaf and Prevention and Control of Spontaneous Combustion of Remained Coal under the Influence of Gob-Side Entry Retaining Roadway

Abstract

Based on the ventilation characteristics of the gob-side entry retaining face, a mathematical model of spontaneous combustion in the gob-side entry retaining face is established. From the overburden caving of the goaf along the goaf retaining roadway, the development characteristics of rock strata and residual coal fissures in the goaf are summarized and analyzed. In addition, by using numerical simulation software, the effects of normal mining period, goaf retaining roadway as return air roadway, air leakage prevention, and nitrogen injection measures in goaf on spontaneous combustion in goaf are studied, and the distribution characteristics of flow field, oxygen concentration field and temperature field in goaf are obtained. The results show that the mining of the Geng 20 working face has a significant impact on the Geng 19 coal seam. The Geng 19 coal seam is in the range of fracture zone, and the fracture is well developed. Furthermore, the permeability coefficient of the Geng 19 coal seam increases sharply, air leakage in the goaf is increased along the goaf retaining roadway, and the range of oxygen concentration is enlarged, which results in a temperature rise in the goaf. Therefore, air leakage measures are proposed along the goaf to inject 900 m³/h of nitrogen into the goaf, which can prevent the spontaneous combustion of coal left over in the goaf. In addition, according to the characteristics of fracture development and numerical simulation of spontaneous combustion in goaf under the goaf retaining roadway, the hierarchical prevention, control, and fire extinguishing technology system of the goaf retaining roadway is constructed.

1. Introduction

Retaining roadways without pillars along the goaf is currently a common mining technology used in domestic coal mining [1,2], which, in essence, is to preserve the mining roadways along the goaf for the next section. The roadway along the goaf inevitably contacts the goaf, which causes the air in the roadway to enter the goaf and mix with various gases in the goaf. In special circumstances, when the heat released from the oxidized coal in the goaf has not been evacuated in time, fire or even an explosion may occur with an increase in temperature. The spontaneous combustion of leftover coal in the goaf is one of the main disasters in coal mines, and its prevention and control are the focus of mine safety [3,4].

Aiming at the problem of spontaneous coal combustion, many scholars in China and abroad have done a lot of research work and achieved fruitful results. Yang Shengqiang [5], Zhang Donghai [6], and Yu Minggao [7] analyzed the characteristics of coal bodies in high-column areas of coal lanes and numerically simulated the distribution of the flow field and temperature field to establish natural coal bodies in high-column areas. A pyromantic mathematical model has been set to confirm their conclusion. Ren Wanxing [8] and Zhu Hongqing [9] constructed a model to theoretically study the heating law and the effect of nitrogen injection and fire prevention during the development of spontaneous combustion in high coal caving areas. Liu Yingxue et al. [10] studied and analyzed the mechanism of the yellow mud grouting method to prevent spontaneous combustion of coal remaining in the goaf according to the theoretical viewpoints of combustion and thermodynamics and applied it to practice. Qin Botao et al. [11] analyzed the participation mechanism of spontaneous combustion of gas in the goaf to ignite the gas and proposed prevention measures for nitrogen-containing three-phase foam. Qin Yueping [12,13] and Liu Wei [14] studied the influence of coal particle size on the oxidation rate based on the theory of coal–oxygen compounding. Accelerating the advancement speed of the working face significantly reduced the risk of natural ignition in the goaf. Wen Hu [15], Luo Xinrong [16], Li Yanbin [17], and Chu Tingxiang [18] analyzed the characteristics of natural ignition in the goaf of the fully mechanized caving face and put forward the corresponding prevention technology. Wang Yuehong [4] determined the optimal process parameters for nitrogen injection and fire suppression in the goaf by simulating the change law of the temperature field and oxygen concentration field in the goaf under different nitrogen injection process parameters. Ouyang Xiwen [19] proposed the application of a mobile yellow grouting station and gel yellow mud fire prevention technology on the spot by analyzing the law of natural fire in coal seams. Jia Baoshan [20] considered the dilution effect of oxygen on the residual coal and gas emission from the goaf to dilute the oxygen and emphasized the necessity of the application of plugging in the roadway retention. Li Zongxiang [21] studied the air leakage flow pattern, oxygen consumption, and temperature rise characteristics of the coal body around the roadway along with the gob in a fully mechanized sublevel caving road. The finite element method was used to simulate the spontaneous combustion and temperature rise process of the coal body. Wang Yinhui [22] carried out numerical simulation research on the distribution law of the spontaneous combustion danger zone of the residual coal in the goaf and along the goaf. Yu Minggao [23] studied and analyzed the characteristics of air leakage in the goaf along the goaf retention lane under gas drainage and divided its spontaneous combustion danger area. Wen Hu [24] studied the seepage law, the distribution of the oxidation zone, and the key parameters affecting the range of the oxidation zone to prevent the spontaneous combustion of coal in the goaf and along the goaf and established a multiphasic field for the spontaneous combustion of the goaf coupling dynamic model.

The above research mainly focused on the causes and numerical simulation of spontaneous combustion of coal under different geological conditions, the causes of spontaneous combustion of coal remaining in the goaf, and preventive measures. However, there are few related kinds of research about the natural ignition of the goaf in the high-gas spontaneous combustion seam under the condition of retaining the road along the goaf. At present, there is a lack of understanding of the law of natural ignition in the goaf under this condition.

2. Theoretical Analysis of Overburden Crack Development Zone in Goaf

This article uses the Geng 20 working face of the No. 2 coal mine in Pingdingshan as the background. The coal seam inclination angle of the Geng 20-21100 working face is 6°, and the coal thickness is 2.1 m, on average. The inlet side is solid coal. There are high-level boreholes for gas drainage in the goaf of the upper corner. The absolute gas emission is 1–7 m³/min, the air volume on the working surface is 30.7 m³/s, the wind speed is 2.19 m/s, and the working surface temperature is 29.2 °C. The machine lane of Geng 20-21100 working face adopts the technology of cutting the top, pressure relief, and emptying along the road to retain it as the wind tunnel of 21,120 working face. According to the oxidation kinetics determination method of coal spontaneous combustion tendency, the identification results of the spontaneous combustion danger level of Geng 19 and Geng 20 coal seams are all Grade II. The natural firing law of the goaf in spontaneous combustion seam under this condition is studied, and effective prevention and control measures are discussed and analyzed to provide a reference for the natural advance prevention of the remaining coal in the goaf under similar conditions.

The average coal thickness of the Geng 20 coal seam is 2.1 m. The roof of the Geng 20 coal seam belongs to medium-hard rock strata. The height of the fracture zone is calculated according to the empirical formula. The crack zone height [25]:

$$H_l=\frac{100\mathrm{h}}{1.6\mathrm{h}+3.6}\pm 5.6$$

(1)

where H_l is the height of the fracture zone, m; h is the thickness of the coal seam, which is 2.1 m.

The height H_l of the crack zone was calculated to be 30.2 m ± 5.6 m (m). Therefore, the gas-conducting zone height of the goaf roof in the Geng 20 working face is 24.6–37.8 m.

After mining, the height of the fracture zone in the Geng 20 coal seam is about 30.2 m. According to the comprehensive histogram of the Geng 20 working face (Figure 1) and the theoretical calculation of the fracture zone, the Geng 19 coal seam is about 6.8 m above the roof of the Geng 20 coal seam, and the Geng 19 coal seam is in the range of the fissure zone, and the fractures are fully developed. The coal seam is greatly affected by the mining of the Geng 20 working face, and the permeability of the Geng 19 coal seam will also be significantly increased.

The cracks in the overlying strata along the goaf are the main channels for the coal seam to contact with oxygen and release heat and gas after spontaneous combustion. After the key layer was broken for the first time, the key layer was gradually compacted in the middle of the goaf, and a separation zone was still maintained on both sides of the goaf. When collapsed, the roof rock layer of the alleyway is suspended, there are a lot of cracks and holes, and it can exist for a relatively long time. There is negative pressure due to the existence of air leakage in the goaf. The rock layer cracks leak into the goaf, thus causing the airflow in the roadway along the goaf to pass through the retaining roadway, and then the coal that remains in the goaf will be subject to spontaneous combustion.

After the working face is mined, there are two kinds of fractures, delamination fractures and vertical fractures, in the overlying strata above the goaf. There are two main types of cracks in the overburdened rock above the goaf, cracks along the layer and interlayer cracks. On the one hand, the coal seam expands and deforms along with the layer of cracks, and air enters the delamination zone along this crack. On the other hand, the interlayer cracks form the upper and lower layers where the gas, water, and air flow between rock and coal seams. According to theoretical calculations, the maximum crack height of the Geng 20 coal seam can reach 37.8 m. The Geng 19 coal seam is about 6.8 m above the top plate of the Geng 20 coal seam. The cracks are fully developed, and the wind flow in the goaf enters the Geng 19 coal seam through the fissure zone. According to the analysis, there is an air leak from a crack along the roof of the goaf and an air leak from a crack in the overburden. The floating coal in the goaf is in fresh airflow, which is not conducive to the prevention of spontaneous coal combustion.

3. Mathematical Model of the Flow Field in Goaf along the Retaining Roadway

A large number of cracks that are affected by mining appear in the coal body, which greatly increases the internal surface of the coal. According to the theory of coal–oxygen compound [13], the physical surface, chemical adsorption, and chemicals react and release heat. If the heat is not released in time, it causes heat accumulation, increasing the temperature of the coal body, the reaction between coal and oxygen is accelerated, and more heat is generated. The temperature of the body continues to increase, and eventually, the coal temperature reaches the ignition temperature, and the coal spontaneously ignites.

Spontaneous combustion in the goaf is the result of interaction among the pressure field, oxygen concentration, and temperature [13]. The range of the goaf is expanded with the continuous mining of the working face. According to the principle of the moving coordinate system proposed by scholars [13,14], the origin is at the air inlet of the working face, and the X-axis is the direction of deepening in the goaf, the Y-axis is oriented upward along the working face of the stope, as shown in Figure 2. According to the law of conservation of mass, the law of conservation of energy, non-Darcy’s law, Fick’s law, and Fourier’s law, the equations of the flow field, oxygen concentration field, and temperature field in the goaf are established, and a mathematical model of the natural firing law in the goaf is obtained. Since the air seepage velocity in the goaf is much higher than the advancing speed of the working face, the moving coordinates have a greater impact on the temperature field of the collapsed coal rock, while the effects on other fields can be ignored.

The range of the mined-out area is much larger than the height of the collapsed coal rock. The mined-out area can be reduced to a two-dimensional plane for research. According to the law of conservation of mass and the Forchheimer type non-Darcy equation, the flow field equation of the goaf is established [13]:

$${\displaystyle \underset{\mathrm{\Gamma }}{\oint }\rho \left[\frac{g}{2K_x\beta }\left(-1+\sqrt{1-\frac{4K_x^2\beta }{{\rho }_g{g}^2}\frac{\partial p}{\partial x}}\right)dy-\frac{g}{2K_y\beta }\left(-1+\sqrt{1-\frac{4K_y^2\beta }{{\rho }_g{g}^2}\left(\frac{\partial p}{\partial y}+\rho g\sin \alpha \right)}\right)dx\right]}=0$$

(2)

According to Figure 2, the boundary of the mined-out areas Γ1 and Γ2 are the boundary conditions of wind pressure; the boundaries of Γ3 and Γ4 are the boundary conditions of air volume, and the leakage volume is 0. Its boundary conditions are as follows [14]:

$$\{\begin{array}{l}p{|}_{{\mathrm{\Gamma }}_1,{\mathrm{\Gamma }}_2}=p\left(x,y\right){|}_{\left(x,y\right)\in {\mathrm{\Gamma }}_1,{\mathrm{\Gamma }}_2}\\ \left(\frac{\partial p}{\partial y}+\rho g\sin \alpha \right){|}_{{\mathrm{\Gamma }}_3}=0\\ \frac{\partial p}{\partial x}{|}_{{\mathrm{\Gamma }}_4}=0\end{array}$$

(3)

where ρ is the density of the gas in the collapsed coal rock, kg/m³; K_x and K_y are the permeability coefficients in the x and y directions, m/s. The permeability of the air in the goaf in the x and y directions is the same. Therefore, K_x = K_y; p is the sum of static pressure, rapid pressure, and potential energy, Pa; α is the inclination of the coal seam, °; g is the acceleration of gravity, m/s².

3.1. Equation of Oxygen Concentration Field in Goaf

According to the theory of the coal–oxygen complex, the oxidation of oxygen on the surface of the broken coal body by the oxygen in the air is divided into the physical adsorption stage, chemical adsorption stage, and chemical reaction stage, and each stage will release heat, but the physical adsorption stage has the least heat release and chemical reaction. The oxidation and exothermic process of coal satisfy the law of conservation of mass, while the diffusion of oxygen in other gas components meets Fick’s law. Therefore, according to the law of conservation of mass and Fick’s law, the equation of the oxygen concentration field in the goaf is established [26]:

$$\begin{array}{l}{\displaystyle \underset{\mathrm{\Gamma }}{\oint }{c}_{o_2}\left[\frac{g}{2K_x\beta }\left(-1+\sqrt{1-\frac{4K_x^2}{{\rho }_{o_2}{g}^2}\frac{\partial p}{\partial x}}\right)dy-\frac{g}{2K_y\beta }\left(-1+\sqrt{1-\frac{4K_y^2}{{\rho }_{o_2}{g}^2}\left(\frac{\partial p}{\partial y}+{\rho }_{o_2}g\sin \alpha \right)}\right)dy\right]}\\ -k_{o_2}{\displaystyle \underset{\mathrm{\Gamma }}{\oint }nv\left(\frac{\partial c_{o_2}}{\partial x}dy-\frac{\partial c_{o_2}}{\partial y}dx\right)}+{\displaystyle \underset{F}{\iint }u\left(t\right)dS=0}\end{array}$$

(4)

It can be seen from Figure 2 that ① the upper and lower boundaries of Γ₁ and Γ₂ belong to the wind pressure boundary conditions; ② there are no air leaks at the boundaries of Γ₁ and Γ₃ and Γ₄. According to the airflow boundary conditions, the oxygen diffusion flux is 0. The boundary conditions are as follows [14]:

$$\{\begin{array}{l}c{|}_{{\mathrm{\Gamma }}_{1\mathrm{上}},{\mathrm{\Gamma }}_2}=c\left(x,y\right){|}_{\left(x,y\right)\in {\mathrm{\Gamma }}_{1\mathrm{上}},{\mathrm{\Gamma }}_2}\\ \frac{dc}{dy}{|}_{{\mathrm{\Gamma }}_3}=0\\ \frac{dc}{dx}{|}_{{\mathrm{\Gamma }}_{1\mathrm{下}},{\mathrm{\Gamma }}_4}=0\end{array}$$

(5)

In the formula, c_o2 is the concentration of oxygen, mol/m³; k_o2 is the diffusion coefficient of oxygen, m²/s; n is the porosity of the collapsed coal rock, %; u(t) is the oxygen consumption rate in the falling coal rock, mol/(s·m³); v is the seepage velocity of the gas at a certain point in the goaf, m/s.

3.2. Equation of CO Concentration Field in Goaf

On the mining face, the residual coal in the goaf area will be affected by air leakage, which will cause the oxidation reaction, and a large amount of CO gas will appear. The percolation of CO gas in the goaf area is consistent with the theory of porous media. Based on the theory of mass transfer in porous media, Fick’s law, and the law of conservation of mass, the equation of the CO concentration field in the goaf is established as [13]:

$$\begin{array}{l}{\displaystyle \underset{\mathrm{\Gamma }}{\oint }{c}_{co}\left[\frac{g}{2K_x\beta }\left(-1+\sqrt{1-\frac{4K_x^2}{{\rho }_{co}{g}^2}\frac{\partial p}{\partial x}}\right)dy-\frac{g}{2K_y\beta }\left(-1+\sqrt{1-\frac{4K_y^2}{{\rho }_{co}{g}^2}\left(\frac{\partial p}{\partial y}+{\rho }_{co}g\sin \alpha \right)}\right)dy\right]}\\ -k_{co}{\displaystyle \underset{\mathrm{\Gamma }}{\oint }nv\left(\frac{\partial c_{co}}{\partial x}dy-\frac{\partial c_{co}}{\partial y}dx\right)}+{\displaystyle \underset{F}{\iint }{u}^{\prime }\left(t\right)dS=0}\end{array}$$

(6)

3.3. Equation of Temperature Field in Goaf

Many factors affect the heat conduction in the goaf; the distribution of the falling coal rocks, the temperature of the roof and floor, and the thermal conductivity have the greatest influence. The heat conduction methods in the goaf are mainly heat conduction, convective heat transfer, and radiant heat transfer. According to the law of conservation of energy and Fourier’s law, the temperature field equation of falling coal rock (Equation (5)) and the gas temperature equation of falling coal pores (Equation (6)) are established [13,14]:

$${\displaystyle \underset{\mathrm{\Gamma }}{\oint }{\lambda }_s\left(1-n\right)\left(\frac{\partial t_s}{\partial x}dy-\frac{\partial t_s}{\partial y}dx\right)+{\displaystyle \underset{F}{\iint }q\left(t\right)dS-{\displaystyle \underset{F}{\iint }{K}_{e}a\left(t_s-t_g\right)dS}}}-{\displaystyle \underset{\mathrm{\Gamma }}{\oint }\left(1-n\right)}{c}_s{\rho }_s{v}_0{t}_{s}dy=0$$

(7)

$${\displaystyle \underset{\mathrm{\Gamma }}{\oint }{\lambda }_{g}n\left(\frac{\partial t_g}{\partial x}dy-\frac{\partial t_g}{\partial y}dx\right)}-c_g{\displaystyle \underset{\mathrm{\Gamma }}{\oint }{\rho }_g}{t}_g\left(v_{x}dy-v_{y}dx\right)+{\displaystyle \underset{F}{\iint }{K}_{e}a\left(t_s-t_g\right)}dS=0$$

(8)

It can be known from Figure 2 that ① the bottom of the goaf Γ1 and the boundary of Γ2 are treated according to the wind pressure boundary conditions; ② the top of the goaf Γ1, Γ4, Γ5, Γ6, Γ7 are adiabatic boundaries, that is, the heat flux is 0. The boundary conditions are [14]:

$$\{\begin{array}{l}t{|}_{{\mathrm{\Gamma }}_{1\mathrm{下}},{\mathrm{\Gamma }}_2}=t\left(x,y\right){|}_{\left(x,y\right)\in {\mathrm{\Gamma }}_{1\mathrm{下}},{\mathrm{\Gamma }}_2}\\ \frac{dt}{dy}{|}_{{\mathrm{\Gamma }}_{1\mathrm{上}},{\mathrm{\Gamma }}_4,{\mathrm{\Gamma }}_5,{\mathrm{\Gamma }}_6}=0\\ \frac{dt}{dx}{|}_{{\mathrm{\Gamma }}_7}=0\end{array}$$

(9)

In the formula, λ_s and λ_g are the thermal conductivity of the solid particles of the collapsed coal rock and the thermal conductivity of the gas in the pores, J/(m·s·K); t_s and t_g are the temperatures of the solid particles and the collapse of the coal rock, respectively. Gas temperature in pores, K; K_e is the convective heat transfer coefficient of solid particles and gas, J/(m²·s·K); S is the specific surface area of the unit body, m²/kg; ρ_s and ρ_g are the density of gas between pores, kg/m³; c_s, c_g are the specific heat capacity of solid particles, and specific heat of gas between pores, J/(kg·K); v₀ is the advancing speed of the working surface, m/s; q(t) is the exothermic heat of solid particles per unit volume and unit time at temperature t, J/(mol·s), and its value is the chemical adsorption heat of coal at temperature t and the reaction to produce CO, CO₂ and other gases. Sum of exothermic heat; a is the specific surface area of the unit, a = Sn/ΔxΔyΔz, 1/m.

4. Numerical Simulation of the Spontaneous Combustion and Ignition of Coal Left in the Goaf along the Retaining Roadway

To study and analyze the characteristics of spontaneous combustion in the goaf along the retaining roadway, taking the Geng 20 working face of the No. 2 coal mine in Pingdingshan as the background, COMSOL was used to spontaneously ignite the goaf along the retaining roadway under different conditions (see Figure 3). The law is numerically simulated. Whether anti-leakage measures are adopted at the side of the goaf, the length of the road along the goaf, the amount of nitrogen injected into the goaf, during the normal mining of the working face, the high-pit borehole drainage of gas in the goaf corner, retaining the lane as the return air lane affects the oxidation characteristics of the remaining coal in the goaf. The numerical simulation data are shown in

Table 1. Simulated data affecting oxidation characteristics of goaf coal when different measures are taken along goaf retaining roadway.

Working Conditions	Oxidation Zone Range				High-Temperature Area
					Maximum Temperature			CO Content
	From Air-Return Roadway Side/m	From Transportation Roadway Side/m	From the Working Face/m	Width/m	Max/°C	From the Working Face/m	From Retained Roadway/m	Max/‰	From the Working Face/m	From Retained Roadway/m
a	30	54	36	25–32	56.3	75.8	87	0.95	84.5	113
b	26	40	58	28–31	52.8	80.8	91	0.749	87.5	111
c	30	56	63	30–35	52.9	69	1.3	0.920	38	123
d	23	38	26	31–36	65	76	84	1.18	85	110.7
e	20	32	46	22–36	55	82.5	88	0.807	87	113
f	32	34	55	35–70	47.8	57	102	0.51	30	128
g	34	34	54	34–69	47.8	30	128	0.457	30	128
h	32	34	45	38–69	44.6	29	133	0.386	29	133
i	40	64	31	28–34	47.4	77	103	0.507	46	108
j	39	64	29	24–36	53.6	84	119	0.672	53	124
k	30	69	33	34–41	46.9	78	80	0.505	50	75

a. No measures and retain 200 m roadway; b. Take measures and retain 200 m roadway; c. Local ventilation measures and retain 200 m roadway; d. No measures and retain 500 m roadway; e. Take measures and retain 500 m roadway; f. Injecting nitrogen (600 m³/h) into goaf; g. Injecting nitrogen (900 m³/h) into goaf; h. Injecting nitrogen (1500 m³/h) into goaf; i. Normal mining of working face; j. High-level borehole gas drainage; k. Retain roadway for air return.

.

4.1. Influence of the Length of Staying Lanes on the Oxidation Characteristics of Residual Coal in Goaf

Figure 4 shows the simulation results of the goaf oxidation characteristics when the roadway is 200 m, along with no anti-leakage measures. Figure 5 and Figure 6 show the simulation results of the goaf oxidation characteristics when the roadway is 200 m and 500 m, respectively. In addition to measures such as a built-in flexible wind-retaining cloth, a fixed external steel cable, and closing the plate every 50 m during the mining period in order to ensure the normal entry of personnel in the process of roadway retaining, a local fan was set at 20–30 m from the working face, and the air supply was conducted through the air duct with an air supply volume of 400 m³/min.

From Figure 4, Figure 5 and Figure 6, it can be seen that due to the local ventilation on the side of the remaining lane, the air supply from the local fan has almost completely leaked into the goaf. Therefore, the airflow at the end of the local fan is relatively large, and the air leakage gradually converges to the side of the work surface. The inlet side is mainly the return air. Affected by the air supply end, the range of the oxidation zone is at the air intake end, which is biased towards the inside of the goaf. The oxygen concentration in this area is high, the oxidation heat is high, and the heat storage conditions are good. It is an ideal area for forming high-temperature points. If the mining face reduces the mining speed, it will easily form a high-temperature area, which will have a greater impact on the safety of the mining face. The advancement speed of the working face is the main influencing factor of spontaneous ignition in the goaf, and its size affects the ignition source temperature of the goaf, so the daily advance is set to 3 m/d during the simulation.

From Table 1 and Figure 4, the range of the oxidation zone is 8~18% in the goaf when the leakage prevention measures are not taken for 200 m in the roadway is 25–32 m, and the highest temperature in the high-temperature region is about 58.3 °C, which is very close to the coal spontaneous combustion threshold. Temperature (the critical temperature for spontaneous combustion of coal is 60–70 °C), and the maximum concentration of CO in this area is 0.95‰, and the oxidation of the remaining coal in the goaf is intensified. At this time, the goaf may be in danger of spontaneous ignition. From Table 1 and Figure 5, it can be known that the range of the oxidation zone is 8~18% in the mined-out area is 30~35 m when the air leakage prevention measures are taken for 200 m of the alleyway, and the maximum temperature in the high-temperature area is about 52.9 °C. When the width of the oxidation zone increases, the maximum temperature decreases significantly, and the high-temperature point shifts to the lower right side of the return air side; the maximum CO concentration value in this area is 0.92‰, which is affected by local ventilation and causes the work surface to move down.

From Table 1 and Figure 6, the range of the oxidation zone is 8~18% of the goaf when the anti-leakage measures are adopted in the retention lane 500 is 22~36 m, and the highest temperature in the high-temperature area is 55 °C. During the air leakage measure, the range of the oxidation zone does not change much; the temperature is relatively lowered by 10 °C, and the location of the high-temperature point is slightly moved inside the goaf, but the location is not large. The maximum CO concentration value in this area is 0.807‰; the position change is not obvious, but it has an obvious inhibitory effect on residual coal oxidation, and the oxidation degree of residual coal is weakened. However, the width and location of the oxidation zone were not significantly changed compared to when the leak prevention measures were taken when the lane was kept at 200 m. The maximum temperature point in the high-temperature area increased by 2.2 °C, and the CO concentration also increased slightly. The influence of the change of the oxidized zone in the goaf is small, and due to the larger air leakage range, the high-temperature region and the degree of oxidation slightly increase, but the increase is not large.

4.2. Effect of Nitrogen Injection on Oxidation Characteristics of Remaining Coal in Goaf

To reduce the oxidation process of leftover coal in the goaf affected by the roadway along the goaf, under the existing conditions, nitrogen injection is performed from the side of the roadway along the goaf. There are two nitrogen injection ports. In the case of windshield and board closing, the numerical simulation is performed when the nitrogen injection amount is 600 m³/h, 900 m³/h, and 1500 m³/h at the side of the roadway. Here, the mined-out area is selected when the nitrogen injection amount is 900 m³/h. The simulation results of the oxidation characteristics of the residual coal are shown in Figure 7.

Figure 7 is the simulation result under the conditions of nitrogen injection in the goaf when the relevant plugging measures are taken, and the air is supplied locally to the blower. According to Table 1, it can be seen that the width and range of the oxidation zone after nitrogen injection are larger than those under other working conditions. The simulation results showed that the range of the oxidation zone was significantly expanded, the temperature at the high temperature was significantly lowered, and the oxidation of the remaining coal was significantly inhibited, which effectively inhibited the oxidation process in the goaf, and the three zones in the goaf had a greater impact and reduced the amount of CO generated.

Relevant air leakage prevention measures are taken at 200 m of the alleyway, the local fan is 20–30 m behind the working surface, and the air supply is 400 m³/min. Nitrogen is injected into the goaf in the alleyway, and the oxidation characteristics of the goaf are simulated (Figure 8). Because the working face is a high gas coal seam, high-level drilling and drainage of gas are carried out in the goaf of Shangyujiao. The drainage volume is 45 m³/min, the negative pressure is 16 kPa, and the gas content is 15%. Other conditions remain unchanged. The simulation results of the oxidation characteristics of the residual coal are shown in Figure 9.

4.3. Impact of the Oxidation Process of the Coal Remaining in the Goaf during the Mining

From Table 1 and Figure 8, during the normal mining of the working face, the range of the oxidation zone (8–18%) in the goaf is 28~34 m. The highest temperature in the high-temperature region is 47.4 °C, and the maximum CO concentration value in the high-temperature region is 507 ppm. It can be seen in Table 1 and Figure 9 that during normal mining, when the high-level boreholes are performing gas drainage, the range of the oxidized zone (8–18%) in the goaf is 24 to 36 m; the high-level holes have a certain degree of gas drainage. The negative pressure zone causes an increase in air leakage, which leads to the expansion of the mined zone oxidation zone. The highest temperature in the high-temperature zone is 53.6 °C, indicating that the increase in air leakage in the mined zone under the influence of nitrogen injection led to the accelerated oxidation of the remaining coal. When there is no high-level borehole drainage, its position moves slightly inside the goaf, and the dangerous area is more concealed; the maximum CO concentration value in this area reaches 0.672‰, and the oxidation of residual coal is accelerated by the influence of gas drainage.

4.4. Oxidation Characteristics of Remained Coal in Goaf When Retaining Lane along Goaf

When the mining of the working face along the goaf is completed, when the original working roadway is used as the returning roadway of the next working face, the ventilation volume is set to 1850 m³/min, and other conditions are unchanged. The simulation of the spontaneous combustion characteristics of the coal remaining in the goaf is shown in Figure 10.

From Figure 10 and Table 1, when the retaining lane is used as the return air lane of the next working face, the length of the retaining lane is 200 m, and the relevant plugging measures are taken, the range of the oxidation zone (8–18%) in the goaf is 34~41 m, the highest temperature in the high-temperature area is 46.9 °C. Under the influence of nitrogen injection, the oxidation process of the residual coal in the goaf is greatly affected. The oxidation range of the residual coal in the goaf is expanded, and the maximum CO concentration value in the high-temperature region is 0.505‰.

5. Prevention and Control System for Spontaneous Combustion of Leftover Coal in Goaf along Goaf

In the Pingmei No. 2 coal mine, the Geng 20 coal seam and the overburden seam Geng 19 are both high gas spontaneous combustion coal seams. Due to the existence of roads along the goaf, it is easy to cause aggravated air leakage in the goaf, which is not conducive to the spontaneous combustion prevention of broken coal in the goaf. At the same time, due to the strong tendency of coal seam spontaneous combustion, the spontaneous combustion of the leftover coal in the goaf will adversely affect the safe mining of the next working face; it is urgent to increase the spontaneous combustion prevention of the goaf along the side of the goaf.

Based on the technical characteristics of coal seam occurrence in the Pingmei No. 2 mine and the retaining lane along the goaf as the next working face, combined with the results of the experimental study on the coal spontaneous combustion program, the mining area along the side of the remaining goaf is the 20th mining. The empty area has a high recovery rate for the Geng 20 coal seam. Therefore, the remaining coal in the goaf is mainly the overburden seam Geng 19 coal. Therefore, for the remaining coal, spontaneous combustion indicator gas along the goaf side of the goaf, mainly the CO and C₂H₄, supplemented by C₂H₂.

Through low-temperature oxidation experiments of coal, it is found that the oxidizing gas and temperature have significant characteristics. Based on the stage characteristics of spontaneous coal combustion and the sudden increase in the gas generation rate, the critical temperature of coal spontaneous combustion in this goaf is 80 °C, and this temperature is used as the spontaneous combustion temperature. This is a critical characteristic temperature, as the oxidation process continues to occur when the gas is at about 125 °C, gas CO increases suddenly, and this phenomenon is used as the self-heating acceleration characteristic temperature. According to the occurrence and rate of the iconic gas, the process of coal spontaneous combustion and oxidation in the goaf is determined, and according to the characteristics of the stage of spontaneous coal combustion, coordinated fire prevention and technical measures are determined, as shown in

Table 2. Synergistic prevention and control technology of coal spontaneous combustion in different situations.

Stage	CO	C2H4	C2H2	Cooperative Fire Prevention and Extinguishing Technology
~70 °C	√	×	×	Conventional plugging and injecting nitrogen
70~125 °C	√	√	×	Nitrogen injection and vapor mist resistance
125 °C~	√ (Sudden increase)	√ (135 °C)	√ (>240 °C)	Injecting nitrogen and liquid CO2 to goaf

.

Combined with the stage characteristics of low-temperature oxidation of coal remaining in the mined-out area along the goaf side of the Geng 20 working face and the current status of fire prevention and extinguishing technology, coordinated measures for fire prevention and extinguishing at different stages were determined. Regularly, the detection and protection of the air leakage on the left side of the goaf can be strengthened, secondary spraying can be performed in time for the places where the spraying affects cracks, and the nitrogen injection can be performed intermittently. When signs of spontaneous combustion occur, that is, the concentration of CO starts to rise sharply in stages, indicating that the temperature exceeds 70 °C. A continuous nitrogen injection can be used to perform nitrogen injection in the goaf, and at the same time, cannulation along the upper section, the vapor mist resistance operation is performed inside the goaf. When the signs of spontaneous combustion of the coal deteriorate, that is, when C₂H₄ gas is found, it indicates that the temperature exceeds 135 °C. At this time, the high-temperature area of the goaf can be cooled by continuous nitrogen injection and liquid CO₂ infusion.

6. Summary

• Summarizing and analyzing the development of cracks in the overburden strata along the goaf, the development of the rock strata and the remaining coal, the mining of the Geng 20 working face has a significant impact on the Geng 19 coal seam, and the Geng 19 coal seam is in the fracture zone, and the fissures are fully developed, the permeability coefficient of Geng 19 coal seam increases sharply, which is not conducive to the prevention and control of spontaneous ignition in the goaf.
• Mathematical modeling of the flow field, oxygen concentration field, and temperature field of the goaf under the influence of the goaf retention lane. The goaf retention is constructed according to the law of conservation of mass, Darcy flow equation, Fick’s law, Fourier’s law, etc. Mathematical model of spontaneous ignition in goaf under roadway, and numerical simulation of spontaneous ignition in goaf along goaf under different conditions.
• The oxidation characteristics of the residual coal in the goaf along the goaf under different conditions are studied. The results show that along the goaf causes an increase in air leakage in the goaf, an increase in the oxidation range of the goaf, and an increased risk of spontaneous combustion. Affected by the local ventilator along the goaf, the width of the oxidation zone in the goaf increases, the range of the high-temperature area expands, and the degree of oxidation deepens; nitrogen is injected into the goaf, and the highest temperature in the high-temperature area of the goaf increases with the amount of nitrogen injected. Significantly decreased, CO concentration decreased significantly, indicating that nitrogen injection has a significant inhibitory effect on the residual coal oxidation in the goaf; high-level borehole drainage of gas has caused a certain degree of negative pressure in the goaf, an expansion of the oxidation zone range, and an expansion of the high-temperature area. Increase the high temperature and increase the amount of CO. When the empty road is used as the return roadway of the next working face, the width of the goaf increases, the danger zone of spontaneous combustion expands, and the temperature at the high temperature increases significantly, and it can be obtained under the influence of nitrogen injection.
• Based on the stage characteristics and critical temperature of the spontaneous combustion of the residual coal in the goaf along the goaf side of the 20th coal seam, the conventional fire prevention technology, the initial stage of the spontaneous combustion symptoms, and the cooperative fire prevention and treatment technology during the severe stage of the spontaneous combustion were established, respectively. Along with this, the structure and technology system for the prevention, control, and prevention of fire in the lanes.