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Prevention Technology of Coal Spontaneous Combustion Induced by Gas Drainage in Deep Coal Seam Mining

Prevention Technology of Coal Spontaneous Combustion Induced by Gas Drainage in Deep Coal Seam... fire Article Prevention Technology of Coal Spontaneous Combustion Induced by Gas Drainage in Deep Coal Seam Mining 1 2 3 , Jiahui Li , Youxin Zhao and Jinyu Du * School of Civil Engineering, North China University of Science and Technology, Langfang 123000, China; lijiahui@ncist.edu.cn Department of the Emergency Research Institute, China Coal Research Institute CCRI, Beijing 100013, China; zhaoyouxin@mail.ccri.ccteg.cn Law School, Beijing Technology and Business University, Beijing 100048, China * Correspondence: 20170901@btbu.edu.cn Abstract: Due to high gas content and a low permeability coefficient in deep coal seam mining, the spontaneous combustion of coal around the wellbore can easily occur, leading to difficulties in extracting gas during the mining process. To determine the dangerous area around the borehole and conduct advanced prevention and control measures are the keys to preventing spontaneous combustion in boreholes. However, the dangerous area around the borehole is not clear, the sealing parameters lack scientific basis, and the key prevention and control measures are not clear, which have caused great harm to coal mines. This study took the 24,130 working face of Pingdingshan No. 10 Mine as an example, using numerical simulation, theoretical analysis, and field tests to classify the risks of studying the surrounding area of the wellbore. The dangerous area variations under different lengths of shotcrete in the roadway were analyzed, the optimal plugging parameters were studied, and the current “two plugs and one injection” plugging device was optimized. Based on the oxygen concentration and air leakage rate, a method was proposed to divide the dangerous area of fissure coal spontaneous combustion around the borehole induced by gas extraction. The dangerous area Citation: Li, J.; Zhao, Y.; Du, J. of spontaneous combustion around the borehole was defined as having an oxygen concentration Prevention Technology of Coal Spontaneous Combustion Induced by larger than 7% and an air leakage rate less than 0.004 m/s. The comprehensive control measures of Gas Drainage in Deep Coal Seam the grouting length at 2–4 m, hole-sealing parameter at 20-13 (hole-sealing depth 20 m, hole-sealing Mining. Fire 2022, 5, 65. https:// length 13 m) and the “two plugs, one injection and one row” device were determined. doi.org/10.3390/fire5030065 Keywords: coal spontaneous combustion; hazard zone; roadway shotcrete; optimization of sealing Academic Editors: Haiyan Wang, parameters; two plugs; one injection and one row Feng Li, Huiyong Niu, Minbo Zhang and Xuyao Qi Received: 29 March 2022 Accepted: 10 May 2022 1. Introduction Published: 12 May 2022 As of May 2021, there were 4536 coal mines in China; among them 62 mines with a Publisher’s Note: MDPI stays neutral depth of more than one kilometer have an average depth of about 1092 m, involving an with regard to jurisdictional claims in approved production capacity of about 148 million tons. Studies have shown that coal published maps and institutional affil- mining in China is extending to deep coal mining at an average speed of 8–12 m/a. It is iations. foreseeable that the proportion of deep coal mines will become larger and larger. The deep mining of coal resources will become the norm [1–6]. The occurrence environment of deep coal seam mining has the characteristics of “three high and one low”: high ground stress, high ground temperature, high gas pressure, Copyright: © 2022 by the authors. and low permeability. Therefore, mining efforts have been increased by increasing the Licensee MDPI, Basel, Switzerland. drainage diameter, strengthening drilling, increasing negative pressure, and prolonging This article is an open access article drainage time [7–9]. At the same time, the difference between deep coal seam mining and distributed under the terms and shallow coal seam mining is that the environment in which deep coal seams are located is a conditions of the Creative Commons complex mechanical environment of “three highs and one disturbance”. The comprehensive Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ environmental factors around the gas drainage boreholes can easily cause spontaneous 4.0/). combustion of the surrounding broken coal bodies. Mine fires have always been a major Fire 2022, 5, 65. https://doi.org/10.3390/fire5030065 https://www.mdpi.com/journal/fire Fire 2022, 5, x FOR PEER REVIEW 2 of 18 Fire 2022, 5, 65 2 of 18 been a major disaster that frequently occur in coal mine production and have serious con- disaster that frequently occur in coal mine production and have serious consequences. sequences. During 2016–2020, roof and gas accidents are still the main major accidents During 2016–2020, roof and gas accidents are still the main major accidents with the largest with the largest number of gas accidents and fatalities in coal mines in the country, ac- number of gas accidents and fatalities in coal mines in the country, accounting for 18% and counting for 18% and 62% of the total, respectively, as shown in Figure 1. 62% of the total, respectively, as shown in Figure 1. Figure 1. The number of deaths in various large accidents in coal mines. Figure 1. The number of deaths in various large accidents in coal mines. The spontaneous combustion of coal bodies induced by gas drainage in deep coal seams This e dif spon ficult tane to find ous and com contr busti ol.on Determining of coal bo the dies danger indous uced area by will gas bedr the aikey nag to e in deep coal preventing spontaneous combustion in boreholes. Wang [10] proposed the use of infrared seams is difficult to find and control. Determining the dangerous area will be the key to detection technology to measure the spontaneous ignition source range within a range of preventing spontaneous combustion in boreholes. Wang [10] proposed the use of infrared less than 10 m from the coal roadway. Wang [11] used a combination of theoretical analysis detection technology to measure the spontaneous ignition source range within a range of and engineering applications to establish a complete mathematical model, combined with less than 10 m from the coal roadway. Wang [11]used a combination of theoretical analysis a fuzzy comprehensive evaluation model of the loose coal particle size and the fuzzy judgment model of the spontaneous combustion hazard area of the roadway. Liang [12] and engineering applications to establish a complete mathematical model, combined with used the gas concentration at some points to perform inversion using theoretical formulas a fuzzy comprehensive evaluation model of the loose coal particle size and the fuzzy judg- and equations to obtain accurate dispersion coefficients and permeability coefficients, and ment model of the spontaneous combustion hazard area of the roadway. Liang [12] used use them as a basis to predict the location of the fire source. Qi [13] determined that the the gas concentration at some points to perform inversion using theoretical formulas and fire position of the gas drainage borehole was on the inner side of the blocked section equa based tion ons wind to ob speed, tain accura and conduct te dis edpe engineering rsion coeff experiments icients an to d verify perme each. abil Zhou ity coe [14fficien ,15] ts, and use established a borehole spontaneous combustion model and proposed a quantitative eval- them as a basis to predict the location of the fire source. Qi [13] determined that the fire uation index for gas drainage. Huang [16] studied the impact of periodical weighting by position of the gas drainage borehole was on the inner side of the blocked section based introducing weighting intervals into the coupled model of coal self-heating in the gob. on wind speed, and conducted engineering experiments to verify each. Zhou [14,15] es- Liu [17] mastered the air leakage law for the coal pillars on both sides and the gob, and tablished a borehole spontaneous combustion model and proposed a quantitative evalu- determined the distribution law of the dangerous area of spontaneous combustion in the gob. Huang [18] completed a risk evaluation and located the spontaneous combustion in a ation index for gas drainage. Huang [16] studied the impact of periodical weighting by fully mechanized gob. introducing weighting intervals into the coupled model of coal self-heating in the gob. Liu The sealing depth and sealing length directly affect the sealing effect of the drilling. [17]mastered the air leakage law for the coal pillars on both sides and the gob, and deter- Liu [19] and others established the loose circle model of the surrounding rock of the rectan- mined the distribution law of the dangerous area of spontaneous combustion in the gob. gular roadway, studied the shape of the loose circle of the rectangular coal roadway and its Hu formation ang [18] and codevelopment mpleted a ri mechanism, sk evaluation determined and located the loose th cir e cle spon theory taneou formula s com forbustion the in a fully reasonable sealing depth of gas drainage, and established a theoretical basis. Wang [20] mechanized gob. studied the reasonable sealing depth of bedding boreholes using the cuttings method, and The sealing depth and sealing length directly affect the sealing effect of the drilling. theoretical analysis showed that the reasonable sealing depth is between the boundary of Liu [19] and others established the loose circle model of the surrounding rock of the rec- the broken zone and the plastic zone of the surrounding rock. Xu [21] proposed a roadway tan excavation gular ro model adway, about studie the int d ermediate the shape principal of the str lo ess ose and circl shear e of expansion the recta ofngular coal and coal roadway rock and proposed a change of coal stress around the borehole and the expression of the and its formation and development mechanism, determined the loose circle theory for- borehole-sealing depth. Hao [22] used a fluid-solid coupling model to discuss the extraction mula for the reasonable sealing depth of gas drainage, and established a theoretical basis. Wang [20] studied the reasonable sealing depth of bedding boreholes using the cuttings method, and theoretical analysis showed that the reasonable sealing depth is between the boundary of the broken zone and the plastic zone of the surrounding rock. Xu [21] pro- posed a roadway excavation model about the intermediate principal stress and shear ex- pansion of coal and rock and proposed a change of coal stress around the borehole and the expression of the borehole-sealing depth. Hao [22] used a fluid-solid coupling model Fire 2022, 5, 65 3 of 18 time, seal length, and the influence of air leakage on gas concentration. Wang [23] obtained a minimum seal length of 12 m from the ideal elastic-plastic model. The plugging device is also an important part that affects the spontaneous combustion of coal around the borehole [24]. The plugging method commonly used in domestic coal mines is called “two plugging and one injection” [25–28]. The plugging device has the advantages of simple downhole plugging operation, fast plugging speed, and grouting enabled to penetrate deep into the fractures. However, because the “two plugs and one injection” device’s two-capsule bags form a closed space during the grouting process, the internal gas pressure is too large to hinder the grouting of the grouting pump. When the internal and external pressures reach equilibrium, the gas in the grouting section cannot be completely discharged, and the slurry is not solidified, resulting in a reduction in the effective plugging length of the borehole. In view of the various disasters caused by deep coal seams in the process of gas drainage, a single water injection method cannot fundamentally solve the problem of coal spontaneous combustion around the borehole. The study uses theoretical analysis, numerical simulation, and engineering applications to determine how to prevent the danger of coal spontaneous combustion around the borehole. Specifically, dividing the area, analyzing the influence of roadway shotcrete on the dangerous area around the gas drainage borehole, improving the “two plugs and one injection” plugging device, optimizing the plugging parameters, and scientifically predicting and adopting reasonable prevention and control measures to avoid disasters were determined. Safe and efficient gas drainage in deep coal seams is of great important for engineering practice. 2. Division of Dangerous Areas of Spontaneous Combustion in Coal Boreholes Induced by Gas Drainage 2.1. Analysis of Coal Spontaneous Combustion Area around Gas Drainage Borehole A reasonable division of the dangerous areas around the gas drainage boreholes is the key to prevention, governance, and reform. In previous work [29–31], the authors conducted a detailed study on the cause, process, and influence of the natural combustion of the coal body around the borehole during gas drainage. However, at present, the dangerous area of coal spontaneous combustion around the gas drainage borehole is unclear and lacks theoretical basis. As shown in Figure 2, the oxygen concentration of coal near the outer part of the plugging section is greater than the critical oxygen concentration of coal spontaneous combustion and the air flow speed is less than the critical wind speed of coal spontaneous combustion because the oxidized coal around the borehole is connected to the roadway, so an oxidation reaction occurs. The heat generated by the oxidation reaction is less than the heat dissipated at the place where the borehole and roadway are connected by gas extraction, and this area is a scattered zone. Beyond the scattered zone, along the direction of drilling, the oxygen volume fraction in this area decreases and the air flow velocity increases. The heat generated by coal oxidation is greater than the heat dissipated, and this area is the oxidation zone. Beyond the oxidation zone, along the direction of the drilling hole, oxygen concentration is reduced, airflow velocity increases, and heat dissipation is far greater than the heat release from oxidation. This area is the oxidation zone where the borehole fires spontaneously; inside the oxidation zone, the oxygen concentration is reduced, the wind velocity is high, heat accumulation in the area is difficult, and it is a suffocation zone. However, when the energy accumulated in the coal body around the borehole reaches a certain amount, the heat dissipation zone outside the plugging section has a low wind speed and high oxygen concentration, and the heat generated is far less than the heat dissipation. At this point, the heat dissipation zone would also be reduced and become a dangerous area for the spontaneous combustion of coal. Fire 2022, 5, x FOR PEER REVIEW 4 of 18 Fire 2022, 5, 65 4 of 18 Figure 2. Diagram of the dangerous area around borehole. Figure 2. Diagram of the dangerous area around borehole. 2.2. Critical Oxygen Concentration and Critical Air Flow Velocity Determination 2.2. Critical Oxygen Concentration and Critical Air Flow Velocity Determination Coal spontaneous combustion occurs under three conditions: oxygen concentration, wind speed, and time [32–35]. Only when the ventilation and oxygen supply conditions Coal spontaneous combustion occurs under three conditions: oxygen concentration, are continuous and stable for a period of time, enough to combine the coal with sufficient wind speed, and time [32–35]. Only when the ventilation and oxygen supply conditions oxygen, the oxidation process is fully carried out and the coal body can spontaneously are continuous and stable for a period of time, enough to combine the coal with sufficient ignite when a certain amount of heat is accumulated [36,37]. Therefore, choosing the oxygen, the oxidation process is fully carried out and the coal body can spontaneously appropriate critical oxygen concentration and airflow velocity is the key to classifying the spontaneous combustion of coal around the gas drainage hole. In order to obtain the ignite when a certain amount of heat is accumulated [36,37]. Therefore, choosing the ap- critical oxygen concentration of coal spontaneous combustion, this study took the original propriate critical oxygen concentration and airflow velocity is the key to classifying the coal sample from the 24,130 working face of Pingdingshan No. 1 Mine as an example, spontaneous combustion of coal around the gas drainage hole. In order to obtain the crit- and conducted low-temperature oxidation experiments under the conditions of oxygen ical oxygen concentration of coal spontaneous combustion, this study took the original concentrations of 20.9%, 10%, and 8%. The relationship between the heat release and coal coal sample from the 24,130 working face of Pingdingshan No. 1 Mine as an example, and temperature and the specific experimental results are shown in Figure 3. When oxygen concentration is 10.0%, the growth rate of CO and C H output is conducted low-temperature oxidation experiments under the 2co 4nditions of oxygen con- inhibited, but there is still a large growth rate, indicating that when oxygen concentration centrations of 20.9%, 10%, and 8%. The relationship between the heat release and coal reaches 10.0%, the spontaneous combustion of coal has been inhibited, but it is not enough temperature and the specific experimental results are shown in Figure 3. to completely prevent the oxidation of coal. The inhibition of the coal oxidation process is stronger when the oxygen concentration continues to decrease to 8.0%. However, the first generation concentration of each gas behaves roughly the same. The change law of coal CO volume fraction—coal temperature C H volume fraction—coal temperature temperature reflects the degree of oxidation reaction during the spontaneous combustion 2 4 of coal and the change curve is shown in Figure 3. It can be seen from Figure 3 that when the oxygen concentration is 20.9%, the oxidation process of coal has entered the intense Oxygen concentration 8% oxidation stage; when the oxygen concentration is 10.0%, the coal sample also has an Oxygen concentration 8% Oxygen concentration 10% Oxygen concentration 10% obvious violent oxidation process, but the violent combustion stage of the spontaneous Oxygen concentration 20.9% Oxygen concentration 20.9% combustion oxidation of the coal sample is obviously inhibited compared to the air con- dition; the coal temperature at 8.0% oxygen concentration is significantly lower than the coal temperature at 10.0% oxygen concentration, and the degree of oxidation is further suppressed. In summary, it can be obtained that the critical oxygen concentration of the experimental coal sample for spontaneous combustion is determined to be 8.0%. 500 20 0 100 200 300 400 0 100 200 300 400 The temperature of the coal(℃) The temperature of the coal(℃) (a) CO volume fraction—coal temperature (b) C2H4 volume fraction—coal temperature -6 CO volume fraction/10 -6 C H volume fraction/10 2 4 Fire 2022, 5, x FOR PEER REVIEW 4 of 18 Figure 2. Diagram of the dangerous area around borehole. 2.2. Critical Oxygen Concentration and Critical Air Flow Velocity Determination Coal spontaneous combustion occurs under three conditions: oxygen concentration, wind speed, and time [32–35]. Only when the ventilation and oxygen supply conditions are continuous and stable for a period of time, enough to combine the coal with sufficient oxygen, the oxidation process is fully carried out and the coal body can spontaneously ignite when a certain amount of heat is accumulated [36,37]. Therefore, choosing the ap- propriate critical oxygen concentration and airflow velocity is the key to classifying the spontaneous combustion of coal around the gas drainage hole. In order to obtain the crit- ical oxygen concentration of coal spontaneous combustion, this study took the original coal sample from the 24,130 working face of Pingdingshan No. 1 Mine as an example, and conducted low-temperature oxidation experiments under the conditions of oxygen con- Fire 2022, 5, 65 5 of 18 centrations of 20.9%, 10%, and 8%. The relationship between the heat release and coal temperature and the specific experimental results are shown in Figure 3. CO volume fraction—coal temperature C H volume fraction—coal temperature 2 4 Oxygen concentration 8% Oxygen concentration 8% Oxygen concentration 10% 2000 Oxygen concentration 10% Oxygen concentration 20.9% Oxygen concentration 20.9% 500 20 Fire 2022, 5, x FOR PEER REVIEW 5 of 18 0 100 200 300 400 0 100 200 300 400 The temperature of the coal(℃) The temperature of the coal(℃) (a) CO volume fraction—coal temperature (b) C2H4 volume fraction—coal temperature Temperature rise curve of experimental coal samples Temperature in the furnace 20.9% coal body temperature 10% coal body temperature 8% coal body temperature 0 50 100 150 200 250 300 350 time/min (c) Temperature rise curve of experimental coal samples Figure 3. The law of coal temperature, CO, and C2H4 concentration changing with time. Figure 3. The law of coal temperature, CO, and C H concentration changing with time. 2 4 When oxygen concentration is 10.0%, the growth rate of CO and C2H4 output is in- Air flow velocity is the main driving force for the spontaneous combustion and heat hibited, but there is still a large growth rate, indicating that when oxygen concentration dissipation of coal around the borehole. When the airflow velocity of the gas around the reaches 10.0%, the spontaneous combustion of coal has been inhibited, but it is not enough borehole exceeds the critical velocity, even when oxygen is sufficient, it is difficult for heat to completely prevent the oxidation of coal. The inhibition of the coal oxidation process is to accumulate in this area. When the velocity is less than the critical velocity in this area, the stron amount ger whe of heat n the oxyg dissipation en concentr is less, and ation the contin spontaneous ues to decrea combustion se to 8.0%. of coal How may eveoccur r, the fir after st generation concentration of each gas behaves roughly the same. The change law of coal long-term oxidation heat release. Therefore, the critical air flow velocity is an important temperature reflects the degree of oxidation reaction during the spontaneous combustion indicator to measure the spontaneous combustion of the coal around the borehole. Due of coal and to the complex the change c environment urve is shown of the coal in Fig body ure near 3. It c theabor n be seen fro ehole, them wind Figu flow re 3 th velocity at when is the ox difficult ygen to measur concentr e, at soion thisis paper 20.9%, refers the oxida to multiple tion pro works cess o (W f ang coal h Ha et s entered al., 2002) th to e in obtain tense a critical wind flow velocity of 0.004 m/s under the condition of sufficient oxygen. oxidation stage; when the oxygen concentration is 10.0%, the coal sample also has an ob- vious violent oxidation process, but the violent combustion stage of the spontaneous com- 2.3. Determination of the Hazardous Area bustion oxidation of the coal sample is obviously inhibited compared to the air condition; In the early stage of gas drainage, the heat dissipation zone is small and there is no the coal temperature at 8.0% oxygen concentration is significantly lower than the coal tem- risk of spontaneous ignition. In the later stage of gas drainage, the heat dissipation zone perature at 10.0% oxygen concentration, and the degree of oxidation is further suppressed. has the risk of coal spontaneous combustion. The area with spontaneous combustion is the In summary, it can be obtained that the critical oxygen concentration of the experimental coal sample for spontaneous combustion is determined to be 8.0%. Air flow velocity is the main driving force for the spontaneous combustion and heat dissipation of coal around the borehole. When the airflow velocity of the gas around the borehole exceeds the critical velocity, even when oxygen is sufficient, it is difficult for heat to accumulate in this area. When the velocity is less than the critical velocity in this area, the amount of heat dissipation is less, and the spontaneous combustion of coal may occur after long-term oxidation heat release. Therefore, the critical air flow velocity is an im- portant indicator to measure the spontaneous combustion of the coal around the borehole. Due to the complex environment of the coal body near the borehole, the wind flow veloc- ity is difficult to measure, so this paper refers to multiple works (Wang H et al., 2002) to obtain a critical wind flow velocity of 0.004 m/s under the condition of sufficient oxygen. -6 CO volume fraction/10 temperature/℃ -6 C H volume fraction/10 2 4 Fire 2022, 5, 65 6 of 18 “dangerous area” and the suffocation zone is the “safe area”. The safe area around the gas drainage borehole is characterized by a combination of an oxygen concentration less than 8% and wind velocity greater than 0.004, and the remaining areas are collectively referred to as “dangerous areas”. 3. Key Technologies for Prevention and Control of Coal Spontaneous Combustion around Boreholes In areas with serious borehole fires, measures are taken to stop drainage and inject water into the coal body area, but these measures can only temporarily control the spread of fire. Therefore, this thesis mainly discusses the prevention and control of spontaneous combustion in boreholes by using roadway shotcrete, optimizing hole-sealing parameters, and improving the “hole-sealing device”. 3.1. Optimization of Roadway Shotcrete Length Parameters Roadway shotcrete is the first line of defense that isolates the working face and the roadway, seals the cracks in the coal wall and the pores of the coal body itself, and reduces the interaction between the coal body and the air in the roadway. The length of roadway shotcrete affects the scope of the dangerous area around the borehole, so the method of combining numerical simulation and engineering is used to study the law of the influence of shotcrete on the oxygen concentration, fluid velocity, and the scope of the dangerous area around the borehole. According to the spontaneous combustion of the coal seam (Jia et al., 2021) in the 24,130 working face of Pingdingshan No.10 Mine, the numerical simulation set the plugging depth at 15 m, the plugging length at 8 m, the initial ground temperature of the coal seam at 48 C, and the negative pressure of drainage at 23 kPa. For the model’s parameters and boundary conditions, refer to Jia et al., 2021 and Zhao et al., 2021. 3.1.1. Analysis of Oxygen Concentration and Wind Velocity Figure 4 is a cloud map of oxygen concentration and air flow velocity for different roadway shotcrete lengths during gas drainage for 3 days. Figure 4 shows that with the increase of roadway shotcrete length, the air flow velocity around the borehole shows almost no change at all and the area with a larger oxygen concentration changes a little. Oxygen is one of the factors that induces the spontaneous combustion of coal. Reducing the oxygen concentration around the borehole can effectively prevent gas drainage from inducing the spontaneous combustion of coal. The longer the distance of roadway shotcrete is, the greater the safe range of oxygen concentration around the borehole. It can be seen that when the roadway does not take roadway shotcrete measures, the characteristics of the wide range of oxygen concentration and the large oxygen concentration can easily cause the spontaneous combustion of coal in the fissures around the borehole during gas drainage. Because the roadway shotcrete has little effect on the air flow velocity, it can be obtained, from the perspective of oxygen concentration, that the roadway shotcrete can effectively prevent oxygen from entering the fractured coal around the borehole, thereby preventing the oxidation and exothermic reaction of the coal and reducing the possibility of coal spontaneous combustion. Analysis of Roadway Shotcrete in the Dangerous Area around the Borehole Figure 5 shows that, without taking any measures, the longitudinal length of the dangerous area around the borehole is close to 14 m, in the shape of “hands clasped together”, and the dangerous area is very large. When the shotcrete length of the roadway is 2 m, 4 m, and 6 m, the dangerous area of the fractured coal body around the borehole is reduced, and the range of the dangerous area near the roadway does not change much. When comparing different roadway shotcrete lengths, the hazardous area changes a little. Comprehensive analysis of the oxygen concentration, the fractured coal body around the borehole, and the hazardous area range, shows that the best roadway shotcrete length for a single borehole is 2 m. Fire 2022, 5, x FOR PEER REVIEW 7 of 18 No measures Roadway shotcrete 2 m Fire 2022, 5, x FOR PEER REVIEW 7 of 18 Fire 2022, 5, 65 7 of 18 No measures Roadway shotcrete 2 m Roadway shotcrete 6 m Roadway shotcrete 4 m Figure 4. The cloud map of oxygen concentration for different roadway grouting lengths was ob- tained. Roadway shotcrete 6 m Roadway shotcrete 4 m (3) Analysis of roadway shotcrete in the dangerous area around the borehole Figure 5 shows that, without taking any measures, the longitudinal length of the dan- gerous area around the borehole is close to 14 m, in the shape of “hands clasped together”, and the dangerous area is very large. When the shotcrete length of the roadway is 2 m, 4 m, and 6 m, the dangerous area of the fractured coal body around the borehole is reduced, and the range of the dangerous area near the roadway does not change much. When com- paring different roadway shotcrete lengths, the hazardous area changes a little. Compre- hensive analysis of the oxygen concentration, the fractured coal body around the borehole, and the hazardous area range, shows that the best roadway shotcrete length for a single Figure Figure 4.4. The The cc lo lo ud ud m map ap o of f o ox xy yg gen en c o co nncentration centration forfor di fdi fer ff eerent nt road roadway way groug tirou ng lti eng ngt h le sng wth ass ob was tain ob- ed. borehole is 2 m. tained. (3) Analysis of roadway shotcrete in the dangerous area around the borehole No measures Roadway shotcrete 2 m Figure 5 shows that, without taking any measures, the longitudinal length of the dan- gerous area around the borehole is close to 14 m, in the shape of “hands clasped together”, and the dangerous area is very large. When the shotcrete length of the roadway is 2 m, 4 m, and 6 m, the dangerous area of the fractured coal body around the borehole is reduced, and the range of the dangerous area near the roadway does not change much. When com- paring different roadway shotcrete lengths, the hazardous area changes a little. Compre- hensive analysis of the oxygen concentration, the fractured coal body around the borehole, Fire 2022, 5, x FOR PEER REVIEW 8 of 18 and the hazardous area range, shows that the best roadway shotcrete length for a single borehole is 2 m. Roadway shotcrete 6 m Roadway shotcrete 4 m No measures Roadway shotcrete 2 m Figure 5. Changes of hazard areas under different roadway grouting lengths when the gas was ex- Figure 5. Changes of hazard areas under different roadway grouting lengths when the gas was tracted for 3 d. extracted for 3 d. 3.1.2. Experimental Research on Shotcrete Engineering of Roadway In the 24,130 working face, the tunnels around the experimental boreholes were cleaned by shotcrete, the spraying was even, and there was no “skirt wearing, barefoot” phenomenon; the spray thickness was 50 mm, and the experimental boreholes with shot- crete lengths of 0 m, 2 m, 4 m, and 6 m were set. Monitoring the CO concentration in the gas drainage borehole reflects the effect of roadway shotcrete. Figure 6 below shows the shotcrete on site and Figure 7 shows the monitoring result curve of the shotcrete length of different roadways. Figure 6. Site map. Fire 2022, 5, x FOR PEER REVIEW 8 of 18 Roadway shotcrete 6 m Roadway shotcrete 4 m Figure 5. Changes of hazard areas under different roadway grouting lengths when the gas was ex- Fire 2022, 5, 65 tracted for 3 d. 8 of 18 3.1.2. Experimental Research on Shotcrete Engineering of Roadway 3.1.2. Experimental Research on Shotcrete Engineering of Roadway In the 24,130 working face, the tunnels around the experimental boreholes were In the 24,130 working face, the tunnels around the experimental boreholes were cleaned by shotcrete, the spraying was even, and there was no “skirt wearing, barefoot” cleaned by shotcrete, the spraying was even, and there was no “skirt wearing, barefoot” phe- phenomenon; the spray thickness was 50 mm, and the experimental boreholes with shot- nomenon; the spray thickness was 50 mm, and the experimental boreholes with shotcrete crete lengths of 0 m, 2 m, 4 m, and 6 m were set. Monitoring the CO concentration in the lengths of 0 m, 2 m, 4 m, and 6 m were set. Monitoring the CO concentration in the gas drainage borehole reflects the effect of roadway shotcrete. Figure 6 below shows the gas drainage borehole reflects the effect of roadway shotcrete. Figure 6 below shows the shotcrete on site and Figure 7 shows the monitoring result curve of the shotcrete length of shotcrete on site and Figure 7 shows the monitoring result curve of the shotcrete length of different roadways. different roadways. Figure 6. Site map. Figure 6. Site map. The Figure 7 shows that when no measures are taken on the roadway, the CO con- centration rises more obviously within 50 days, and the maximum can reach 430 ppm. When the roadway shotcrete length is 2, 4, and 6 m, the CO concentration around the borehole increases with the gas drainage. The increase in time continues, but compared to the 0 m roadway shotcrete, the rise tends to be slower. The average gas concentrations within 50 days of drilling gas drainage were 266, 210, 179, and 170 ppm for each shotcrete length, and the decline rate slowed down. Therefore, the length of the shotcrete for a single borehole is between 2 m and 4 m. 3.2. Optimization of Drilling and Sealing Parameters for Gas Drainage One of the main reasons for spontaneous fire in gas drainage boreholes is the poor plugging effect of boreholes. The main factors that affect the plugging effect of boreholes are the length of the hole, the depth of the hole, the grouting material, and whether the plugging is standardized. To specific coal mines, reasonable plugging length and plugging depth are the most important, direct, and effective method to improve the plugging effect, which can not only greatly improve the efficiency of gas drainage, but also prevent spontaneous combustion in deep coal seam drainage boreholes. Fire 2022, 5, x FOR PEER REVIEW 9 of 18 Fire 2022, 5, 65 9 of 18 No measures Roadway shotcrete length 2m Roadway shotcrete length 4m Roadway shotcrete length 6m 0 10 20 30 40 50 Gas drainage time/d Figure 7. Results of testing. Figure 7. Results of testing. 3.2.1. Experimental Research on the Stress Distribution around the Roadway Based on the The Figure 7 shows that when no measures are taken on the roadway, the CO con- Drill Cuttings Method centration rises more obviously within 50 days, and the maximum can reach 430 ppm. The drill cuttings method is one of the effective methods used to test the stress dis- When the roadway shotcrete length is 2, 4, and 6 m, the CO concentration around the tribution of the coal seam (surrounding rock). In field applications, pneumatic hand-held bo drilling rehole rigs inc ar rea e gener ses wi ally th used the gas to drill drai holes nage. perpendicular The increase toin the time coalcon wall. tinue Thes, str bu ess t co distri- mpared to bution characteristics of the coal body can be analyzed by the changing law of the amount the 0 m roadway shotcrete, the rise tends to be slower. The average gas concentrations of drill cuttings discharged from drilling and the accompanying dynamic phenomena. within 50 days of drilling gas drainage were 266, 210, 179, and 170 ppm for each shotcrete The yield of drill cuttings changes with the change of stress in the coal body charac- length, and the decline rate slowed down. Therefore, the length of the shotcrete for a single terized by a certain functional relationship. As the stress of the coal body increases, the borehole is between 2 m and 4 m. elastoplastic deformation increases and the yield of drill cuttings also increases. The drill cuttings method is used to study the characteristics of the stress distribution of the roadside 3.2. Optimization of Drilling and Sealing Parameters for Gas Drainage coal body. The drill cuttings method studies the relationship between the pressure on the coal body and the yield of drill cuttings. One of the main reasons for spontaneous fire in gas drainage boreholes is the poor It can be seen from the formula that the yield of drill cuttings is positively corre- plugging effect of boreholes. The main factors that affect the plugging effect of boreholes lated with the pressure of the surrounding rock. When the stress concentration increases, are the length of the hole, the depth of the hole, the grouting material, and whether the the quality of drill cuttings per unit length increases. Therefore, the stress state of the plugging is standardized. To specific coal mines, reasonable plugging length and plug- surrounding rock can be judged by the change in the yield of drill cuttings. ging depth are the most important, direct, and effective method to improve the plugging effect, which can not only greatly improve the efficiency of gas drainage, but also prevent 3.2.2. Determination of Sealing Depth spontaneous combustion in deep coal seam drainage boreholes. At the lower part of the 24,130 wind tunnel near the stop line, 594 mm drill holes were drilled into the coal tunnel along the layer, and drill cuttings were taken to test the stress 3.2.1. Experimental Research on the Stress Distribution around the Roadway Based on distribution around the tunnel. Two 10 kg and 50 kg spring scales and two plastic buckets were used for on-site crumb fetching and weighing, as well as one square woven bag with the Drill Cuttings Method a side length of about 1 m. The field test is shown in Figure 8. The drill cuttings method is one of the effective methods used to test the stress dis- The yield of drill cuttings from the 94 mm drill hole were plotted on a curve as shown tribution of the coal seam (surrounding rock). In field applications, pneumatic hand-held in Figure 9. Figure 9 shows that the yield of cuttings in the first few meters of drilling drilling rigs are generally used to drill holes perpendicular to the coal wall. The stress does not change much, with only slightly increases. As the drilling depth increases, the distribution characteristics of the coal body can be analyzed by the changing law of the yield of drill cuttings first rises sharply, then remains stable, then decreases, and finally amount of drill cuttings discharged from drilling and the accompanying dynamic phe- stabilizes. Among them, 2#, 3#, 4# drill cuttings have a similar trend with the drilling distance. The 2# drilling hole enters the area where the yield of cuttings drops after the nomena. The yield of drill cuttings changes with the change of stress in the coal body charac- terized by a certain functional relationship. As the stress of the coal body increases, the elastoplastic deformation increases and the yield of drill cuttings also increases. The drill CO volume fraction/ppm Fire 2022, 5, x FOR PEER REVIEW 10 of 18 Fire 2022, 5, 65 10 of 18 cuttings method is used to study the characteristics of the stress distribution of the road- yield of cuttings reaches the maximum. The 4# drilling hole has the largest yield of cuttings, side coal body. The drill cuttings method studies the relationship between the pressure indicating that the stress here is the largest. Combining the yield of drill cuttings in each drill hole, it can be seen that the yield of drill cuttings in the range of 0~7.5 m is low, with on the coal body and the yield of drill cuttings. only small fluctuations. At this time, the yield of drill cuttings should only be the quality of It can be seen from the formula that the yield of drill cuttings is positively correlated the drilled coal body; drilling in the range of 8~17 m, the yield of cuttings increases sharply with the pressure of the surrounding rock. When the stress concentration increases, the and remains high in the range of 17~23 m. It decreases sharply in the range of 23~28 m quality of drill cuttings per unit length increases. Therefore, the stress state of the sur- and tends to be stable after about 28 m. At the same time, in the range of 8~28 m, there rounding rock can be judged by the change in the yield of drill cuttings. were different numbers of coal cannons during the drilling process and the sound was loud or quiet, accompanied by the phenomenon of spray holes, the particle size of coal 3.2.2. Determination of Sealing Depth cuttings becoming larger, and the maximum yield of drill cuttings appearing in the drilling hole at a distance of 20 m before and after. Combined with the on-site measurement of the At the lower part of the 24,130 wind tunnel near the stop line, 594 mm drill holes stress distribution of the surrounding rock using the cuttings measurement method and were drilled into the coal tunnel along the layer, and drill cuttings were taken to test the the theoretical analysis of the loose zone, it can be concluded that 0~7 m is the broken zone, stress distribution around the tunnel. Two 10 kg and 50 kg spring scales and two plastic 8~19 m is the plastic zone, and 20~28 m is the elastic zone. The maximum yield of drill buckets were used for on-site crumb fetching and weighing, as well as one square woven cuttings appear around 20 m and the maximum stress appears at about 20 m away from bag with a side length of about 1 m. The field test is shown in Figure 8. the roadway. Figure Figure 8.8. Field Field pictur pictur es.es. 3.2.3. Determination of Sealing Length The yield of drill cuttings from the 94 mm drill hole were plotted on a curve as shown According to the actual situation of the sealing parameters of Pingdingshan No.10 in Figure 9. Figure 9 shows that the yield of cuttings in the first few meters of drilling does Mine, the sealing length is set to 8 m, 10 m, 13 m, and 15 m. For other physical parameters not change much, with only slightly increases. As the drilling depth increases, the yield and boundary conditions, please refer to the paper by Jia et al., 2021; Zhao et al., 2021. The of drill cuttings first rises sharply, then remains stable, then decreases, and finally stabi- coal temperature cloud map at 90 days around the borehole is shown in Figure 10. lizes. Among them, 2#, 3#, 4# drill cuttings have a similar trend with the drilling distance. Figure 10 shows the plugging depth of 20 m; the plugging lengths are 8 m, 10 m, 13 m, The 2# drilling hole enters the area where the yield of cuttings drops after the yield of and 15 m, the gas drainage is 90 d, and there is coal temperature distribution around the bor cutti ehole. ngs It rea can ches be seen the m that axim when um. The the sealing 4# drilli length ng ho is 8le h m, 10 as m, the 13 large m, and st y15 ield o m, the f cu high ttings, indi- temperature of the coal body around the borehole is 83.4 C, 65.5 C, 53.9 C, and 48 C, cating that the stress here is the largest. Combining the yield of drill cuttings in each drill respectively, and the oxygen concentration around the borehole is greater than 8%. The area hole, it can be seen that the yield of drill cuttings in the range of 0~7.5 m is low, with only gradually decreases as the sealing length increases. From the perspective of temperature small fluctuations. At this time, the yield of drill cuttings should only be the quality of the analysis, when the gas drainage is within 90 d, the sealing hole length should be no less drilled coal body; drilling in the range of 8~17 m, the yield of cuttings increases sharply than 10 m; when the gas drainage is greater than 90 d, the sealing hole length should be and remains high in the range of 17~23 m. It decreases sharply in the range of 23~28 m greater than 13 m. However, according to the gas drainage rules, it can be obtained that the and tends to be stable after about 28 m. At the same time, in the range of 8~28 m, there working face gas drainage should last longer than 6 months, so the best sealing length is were different numbers of coal cannons during the drilling process and the sound was 13 m. From the perspective of oxygen concentration, although the area where the oxygen loud or quiet, accompanied by the phenomenon of spray holes, the particle size of coal cuttings becoming larger, and the maximum yield of drill cuttings appearing in the drill- ing hole at a distance of 20 m before and after. Combined with the on-site measurement of the stress distribution of the surrounding rock using the cuttings measurement method and the theoretical analysis of the loose zone, it can be concluded that 0~7 m is the broken Fire 2022, 5, 65 11 of 18 Fire 2022, 5, x FOR PEER REVIEW 11 of 18 concentration is greater than 8% is the smallest when the sealing length is 15 m, considering that the sealing length is too long, it also increases the difficulty and cost of sealing and zone, 8~19 m is the plastic zone, and 20~28 m is the elastic zone. The maximum yield of when the sealing length is 13 m, the area where the oxygen concentration is greater than drill cuttings appear around 20 m and the maximum stress appears at about 20 m away 8% is still small. Combining temperature changes and oxygen concentration changes, the from optimal the ro sealing adway. hole length is 13 m, which can prevent the spontaneous combustion of coal seams and ensure efficient gas drainage. 1# Maximum stress Turning point 2# 3# 4# 5# 0 5 10 15 20 25 30 Drilling distance/m Figure 9. Variation of drilling yield with drilling depth. Figure 9. Variation of drilling yield with drilling depth. The test was carried out on the lower edge of the 24,130 wind tunnel at a distance of 3.2.3. D 500 m fr et om ermi the nation o cut hole. f Sea Theling test L involved ength parameters of (sealing depth-sealing length) 15-13, 18-13, 20-8, 20-10, and 20-13. Concerning drilling hole parameters, each group of According to the actual situation of the sealing parameters of Pingdingshan No.10 drilling spacing is 2 m, the hole diameter is 94 mm, and the drilling depth is 90 m. There Mine, the sealing length is set to 8 m, 10 m, 13 m, and 15 m. For other physical parameters were five holes in each group of boreholes. Each borehole in each group was connected and boundary conditions, please refer to the paper by Jia et al., 2021; Zhao et al., 2021. The to the branch pipe and then connected to the main pipe. Each group of boreholes had coal temperature cloud map at 90 days around the borehole is shown in Figure 10. a monitoring hole to monitor the average gas of each group of boreholes. To extract the volume fraction, the average gas extraction volume fraction of each group of extraction boreholes was monitored once a day and recorded and the six groups of extraction boreholes were continuously monitored for 60 days. The data are plotted in Figure 11. Sealing length 8 m Sealing length 10 m Comparing the sealing parameters of 20-8, 20-10 and 20-13, the average gas extraction volume fraction of boreholes with a sealing parameter of 20-8 is lower and the average gas concentration is 41%; when the sealing parameter is 20-10, the average gas drainage concentration accumulated within 50 days is 56.4% and when the drainage parameters are 20-13, the average gas drainage concentration reaches 70.8%. All comparative analysis shows that when the sealing depth is 20 m, the best gas drainage length of the sealing hole is 13 m. From the perspective of gas drainage concentration, it can prevent spontaneous combustion of the borehole. Comparing the three groups of holes with sealing parameters 15-13, 18-13, and 20-13, the average gas concentration of the drainage holes is 56%, 62.4%, and 70.8%, respectively, and the sealing length is the same, the deeper the sealing depth, Drilling cuttings per meter/(kg/m) Fire 2022, 5, x FOR PEER REVIEW 11 of 18 zone, 8~19 m is the plastic zone, and 20~28 m is the elastic zone. The maximum yield of drill cuttings appear around 20 m and the maximum stress appears at about 20 m away from the roadway. 1# Maximum stress Turning point 2# 3# 4# 5# Fire 2022, 5, 65 12 of 18 the higher the gas drainage concentration. When the 50 d plugging depth is 20 m, the average gas drainage concentration reaches 70.8%, indicating that the plugging effect is better and 0 there is almost 5 no air 10 leakage.15 The experimental 20 analysis 25 of the 30drill cuttings method results in the peak stress of 24,130 working face. The point is 20 m and the sealing Drilling distance/m depth is increased before the stress peak, which can improve the extraction effect. Based on the above analysis, the plugging depth is 20 m and the inner end of the Figure 9. Variation of drilling yield with drilling depth. plugging section of the plugging position is just at the peak stress point, which will not cause a blind area for drainage. When the plugging length is 13 m, which can prevent 3.2.3. Determination of Sealing Length the natural fire of the coal seam and ensure the efficient gas drainage pick. Therefore, According to the actual situation of the sealing parameters of Pingdingshan No.10 in order to increase the extraction efficiency and avoid the spontaneous combustion of Mine, the sealing length is set to 8 m, 10 m, 13 m, and 15 m. For other physical parameters the fractured coal around the borehole, comprehensive numerical simulation and field and boundary conditions, please refer to the paper by Jia et al., 2021; Zhao et al., 2021. The engineering experiment analysis can determine the optimal sealing parameters of the coal coal temperature cloud map at 90 days around the borehole is shown in Figure 10. seam at the 24,130 working face as 20-13. Sealing length 8 m Sealing length 10 m Fire 2022, 5, x FOR PEER REVIEW 12 of 18 Sealing length 13 m Sealing length15 m Figure 10. The temperature cloud map of the coal body around the borehole. Figure 10. The temperature cloud map of the coal body around the borehole. 3.3.Fi New gure Pr 1evention 0 shows Technology the pluggin of “T g wo dep Plugs, th of One 20 m; Injection the plu and ggiOne ng len Row” gths are 8 m, 10 m, 13 m, and 15 m, the gas drainage is 90 d, and there is coal temperature distribution around The most commonly used downhole “two plugs and one injection” hole plugging the borehole. It can be seen that when the sealing length is 8 m, 10 m, 13 m, and 15 m, the device is used to observe the pressure with a grouting pressure gauge, which does not hig indicate h temp that erature the bor of the co ehole isal filled body arou with slurry nd t.he boreh The gas in ole is the 83.4 borehole °C, 6 cannot 5.5 °C, be 53. completely 9 °C, and 48 discharged, and the slurry can easily gather around the borehole after solidification, causing °C, respectively, and the oxygen concentration around the borehole is greater than 8%. the formation of gaps and gas segments. The area gradually decreases as the sealing length increases. From the perspective of tem- perature analysis, when the gas drainage is within 90 d, the sealing hole length should be 3.3.1. Development of “Two Plugs, One Injection and One Row” Device no less than 10 m; when the gas drainage is greater than 90 d, the sealing hole length The “two plugs, one injection and one row” plugging device is composed of three should be greater than 13 m. However, according to the gas drainage rules, it can be ob- main parts: inside and outside sealing pockets (two plugs), grouting pipe (one injection), tained that the working face gas drainage should last longer than 6 months, so the best and grouting pipe (one row); it is made with PVC-U special glue. After connecting the sealing length is 13 m. From the perspective of oxygen concentration, although the area suction tube with glue, the outer sealing bag and the inner sealing bag were tied in a fixed where the oxygen concentration is greater than 8% is the smallest when the sealing length is 15 m, considering that the sealing length is too long, it also increases the difficulty and cost of sealing and when the sealing length is 13 m, the area where the oxygen concentra- tion is greater than 8% is still small. Combining temperature changes and oxygen concen- tration changes, the optimal sealing hole length is 13 m, which can prevent the spontane- ous combustion of coal seams and ensure efficient gas drainage. The test was carried out on the lower edge of the 24,130 wind tunnel at a distance of 500 m from the cut hole. The test involved parameters of (sealing depth-sealing length) 15-13, 18-13, 20-8, 20-10, and 20-13. Concerning drilling hole parameters, each group of drilling spacing is 2 m, the hole diameter is 94 mm, and the drilling depth is 90 m. There were five holes in each group of boreholes. Each borehole in each group was connected to the branch pipe and then connected to the main pipe. Each group of boreholes had a mon- itoring hole to monitor the average gas of each group of boreholes. To extract the volume fraction, the average gas extraction volume fraction of each group of extraction boreholes was monitored once a day and recorded and the six groups of extraction boreholes were continuously monitored for 60 days. The data are plotted in Figure 11. Drilling cuttings per meter/(kg/m) Fire 2022, 5, 65 13 of 18 position so that the suction pipe passes through the outer and inner sealing bags and the grouting pipe passes through the outer sealing bag. The bag and the inner sealing bag are parallel to the axis of the extraction pipe; the grouting pipe is located in the pipeline inside the outer sealing bag and the inner sealing bag. A one-way valve is provided in the pipeline and the pipe wall. There is a grout outlet on the grouting pipe so that the grout flows into the outer and inner sealing bags after the grout passes through the one-way valve in the grouting pipe; when the upward drilling hole sealer is arranged, the adjustment row, the distance from the position of the slurry pipe to the inner sealing bag, is 5~10 cm and the position of the blasting valve to the mouth of the slurry pipe is 20~30 cm, as shown in Figure 12a; when the downward drilling and sealing device is arranged, the distance between the position of the discharge pipe and the outer sealing bag to 5~10 cm were Fire 2022, 5, x FOR PEER REVIEW 13 of 18 adjusted. The position of the blasting valve to the mouth of the discharge pipe is 20~30 cm, as shown in Figure 12b. The grouting process operates as shown in Figure 13. Sealing parameters(20-8) Fire 2022, 5, x FOR PEER REVIEW 14 of 18 Sealing parameters(20-10) Sealing parameters(15-13) Sealing parameters(20-13) 3.3.1. Development of “Two Plugs, One Injection and One Row” Device Sealing parameters(18-13) The “two plugs, one injection and one row” plugging device is composed of three main parts: inside and outside sealing pockets (two plugs), grouting pipe (one injection), and grouting pipe (one row); it is made with PVC-U special glue. After connecting the suction tube with glue, the outer sealing bag and the inner sealing bag were tied in a fixed position so that the suction pipe passes through the outer and inner sealing bags and the grouting pipe passes through the outer sealing bag. The bag and the inner sealing bag are parallel to the axis of the extraction pipe; the grouting pipe is located in the pipeline inside the outer sealing bag and the inner sealing bag. A one-way valve is provided in the pipe- line and the pipe wall. There is a grout outlet on the grouting pipe so that the grout flows into the outer and inner sealing bags after the grout passes through the one-way valve in the grouting pipe; when the upward drilling hole sealer is arranged, the adjustment row, the distance from the position of the slurry pipe to the inner sealing bag, is 5~10 cm and the position of the blasting valve to the mouth of the slurry pipe is 20~30 cm, as shown in Figure 12a; when the downward drilling and sealing device is arranged, the distance be- 0 1020 30 4050 60 tween the position of the discharge pipe and the outer sealing bag to 5~10 cm were ad- Gas drainage time/d justed. The position of the blasting valve to the mouth of the discharge pipe is 20~30 cm, Figure 11. Average volume fraction of gas extraction from boreholes. Figure as shown 11. A in Figur verage volume e 12b. Th fraction e groof uting pr gas extraction ocess op frer om ate bor s as s eholes. hown in Figure 13. Comparing the sealing parameters of 20-8, 20-10 and 20-13, the average gas extraction volume fraction of boreholes with a sealing parameter of 20-8 is lower and the average gas concentration is 41%; when the sealing parameter is 20-10, the average gas drainage 7 5 7 concentration accumulated within 50 days is 56.4% and when the drainage parameters are 20-13, the average gas drainage concentration reaches 70.8%. All comparative analysis shows that when the sealing depth is 20 m, the best gas drainage length of the sealing hole 3 7 is 13 m. From the perspective of gas drainage concentration, it can prevent spontaneous 7 7 combustion of the borehole. Comparing the three groups of holes with sealing parameters 15-13, 18-13, and 20-13, the average gas concentration of the drainage holes is 56%, 62.4%, and 70.8%, respectively, and the sealing length is the same, the deeper the sealing depth, 8 7 1 2 the higher the gas drainage concentration. When the 50 d plugging depth is 20 m, the (a) Upward gas drainage borehole (b) Downward gas drainage borehole average gas drainage concentration reaches 70.8%, indicating that the plugging effect is better and there is almost no air leakage. The experimental analysis of the drill cuttings Figure 12. Schematic diagram of structures of hole packers for upward boreholes. 1 Extraction pipe; Figure 12. Schematic diagram of structures of hole packers for upward boreholes. 1 Extraction pipe; method r 2 grouting esu pip lts e; in 3 the pe grouting ak stre pipe; ss o 4 m f 24, ine 1dri 30 wor lling;k 5ing extern facal e. The po blocking int is bag; 20 m 6 inner and blo the cking sealin bag; g7 2 grouting pipe; 3 grouting pipe; 4 mine drilling; 5 external blocking bag; 6 inner blocking bag; metal clamp; 8 one-way valve; 9 burst valve. depth is increased before the stress peak, which can improve the extraction effect. 7 metal clamp; 8 one-way valve; 9 burst valve. Based on the above analysis, the plugging depth is 20 m and the inner end of the plugging section of the plugging position is just at the peak stress point, which will not cause a blind area for drainage. When the plugging length is 13 m, which can prevent the natural fire of the coal seam and ensure the efficient gas drainage pick. Therefore, in order to increase the extraction efficiency and avoid the spontaneous combustion of the frac- tured coal around the borehole, comprehensive numerical simulation and field engineer- ing experiment analysis can determine the optimal sealing parameters of the coal seam at the 24,130 working face as 20-13. 3.3. New Prevention Technology of “Two Plugs, One Injection and One Row” The most commonly used downhole “two plugs and one injection” hole plugging device is used to observe the pressure with a grouting pressure gauge, which does not indicate that the borehole is filled with slurry. The gas in the borehole cannot be com- pletely discharged, and the slurry can easily gather around the borehole after solidifica- tion, causing the formation of gaps and gas segments. Figure 13. The entity of grouting. Gas drainage concentration/% Fire 2022, 5, x FOR PEER REVIEW 14 of 18 3.3.1. Development of “Two Plugs, One Injection and One Row” Device The “two plugs, one injection and one row” plugging device is composed of three main parts: inside and outside sealing pockets (two plugs), grouting pipe (one injection), and grouting pipe (one row); it is made with PVC-U special glue. After connecting the suction tube with glue, the outer sealing bag and the inner sealing bag were tied in a fixed position so that the suction pipe passes through the outer and inner sealing bags and the grouting pipe passes through the outer sealing bag. The bag and the inner sealing bag are parallel to the axis of the extraction pipe; the grouting pipe is located in the pipeline inside the outer sealing bag and the inner sealing bag. A one-way valve is provided in the pipe- line and the pipe wall. There is a grout outlet on the grouting pipe so that the grout flows into the outer and inner sealing bags after the grout passes through the one-way valve in the grouting pipe; when the upward drilling hole sealer is arranged, the adjustment row, the distance from the position of the slurry pipe to the inner sealing bag, is 5~10 cm and the position of the blasting valve to the mouth of the slurry pipe is 20~30 cm, as shown in Figure 12a; when the downward drilling and sealing device is arranged, the distance be- tween the position of the discharge pipe and the outer sealing bag to 5~10 cm were ad- justed. The position of the blasting valve to the mouth of the discharge pipe is 20~30 cm, as shown in Figure 12b. The grouting process operates as shown in Figure 13. 7 5 3 7 7 7 1 2 (a) Upward gas drainage borehole (b) Downward gas drainage borehole Figure 12. Schematic diagram of structures of hole packers for upward boreholes. 1 Extraction pipe; Fire 2022, 5, 65 14 of 18 2 grouting pipe; 3 grouting pipe; 4 mine drilling; 5 external blocking bag; 6 inner blocking bag; 7 metal clamp; 8 one-way valve; 9 burst valve. Fire 2022, 5, x FOR PEER REVIEW 15 of 18 3.3.2. Analysis of the Effect of the “Two Plugs, One Injection and One Row” Figure Figure 13. 13. The The entity entity of grouting. of grouting. Plugging Device 3.3.2. Analysis of the Effect of the “Two Plugs, One Injection and One Row” (1) Drilling and cutting analysis Plugging Device The simulated drilling adopts a seamless steel pipe (specification is 12 m/piece, 100 (1) Drilling and cutting analysis mm diameter), the extraction pipe adopts a polyvinyl chloride pipe (specification is 3 The simulated drilling adopts a seamless steel pipe (specification is 12 m/piece, m/piece, plug-in type, 50 mm bore diameter), and the hole sealer is designed to be 8 m 100 mm diameter), the extraction pipe adopts a polyvinyl chloride pipe (specification is long and 2 pcs. The length of the capsule band is 800 mm. Two sets of airtightness com- 3 m/piece, plug-in type, 50 mm bore diameter), and the hole sealer is designed to be 8 m long and 2 pcs. The length of the capsule band is 800 mm. Two sets of airtightness parison tests were designed, using “two plugs and one injection” for up- and downholes comparison tests were designed, using “two plugs and one injection” for up- and down- and “two plugs, one injection and one row” for up- and downholes. The cutting machine holes and “two plugs, one injection and one row” for up- and downholes. The cutting separately cut two simulated drill holes near the capsule belt near the exhaust pipe. machine separately cut two simulated drill holes near the capsule belt near the exhaust pipe. Whether the grouting is full is judged by whether the section is compact and complete, Whether the grouting is full is judged by whether the section is compact and complete, and and the cross-section the al vie cross-sectional ws are show views n iar n F e shown igures 1 in Figur 4 and es 15. 14 and 15. Outer capsule bag Inner capsule bag Figure 14. Borehole profile of boreholes for the “Two plugs and one injection” device. Figure 14. Borehole profile of boreholes for the “Two plugs and one injection” device. Inner capsule bag Outer capsule bag Figure 15. Borehole profile of boreholes for “Two plugs, one injection and one row”. Figure 14 shows the part of the traditional “two plugs and one injection” device that is not filled with grout. This part is in the shape of a three-quarter circle. The reason is partly because of cutting jitter and partly because the upward and downward drilling angles are 37°. Figure 15 shows a cross-sectional view of the “two plugs, one injection and one row” device. The bottom and upward borehole grouting part is dense, the grout is completely filled with the plugging section, and there is no air leakage channel, illustrat- ing that the grouting effect of the new process of “two plugs, one injection and one row” is obviously better than the traditional method of “two plugs and one bet”. (2) Analysis of engineering test results Fire 2022, 5, x FOR PEER REVIEW 15 of 18 3.3.2. Analysis of the Effect of the “Two Plugs, One Injection and One Row” Plugging Device (1) Drilling and cutting analysis The simulated drilling adopts a seamless steel pipe (specification is 12 m/piece, 100 mm diameter), the extraction pipe adopts a polyvinyl chloride pipe (specification is 3 m/piece, plug-in type, 50 mm bore diameter), and the hole sealer is designed to be 8 m long and 2 pcs. The length of the capsule band is 800 mm. Two sets of airtightness com- parison tests were designed, using “two plugs and one injection” for up- and downholes and “two plugs, one injection and one row” for up- and downholes. The cutting machine separately cut two simulated drill holes near the capsule belt near the exhaust pipe. Whether the grouting is full is judged by whether the section is compact and complete, and the cross-sectional views are shown in Figures 14 and 15. Outer capsule bag Inner capsule bag Fire 2022, 5, 65 15 of 18 Figure 14. Borehole profile of boreholes for the “Two plugs and one injection” device. Inner capsule bag Outer capsule bag Figure 15. Borehole profil Figure e of 15. boreholes Borehole profile for of “Two boreholes plug for s, “T on wo e injec plugs, tio one n and injection one row” and one.r ow”. Figure 14 shows the part of the traditional “two plugs and one injection” device that is Figure 14 shows the part of the traditional “two plugs and one injection” device that not filled with grout. This part is in the shape of a three-quarter circle. The reason is partly is not filled with grout. This part is in the shape of a three-quarter circle. The reason is because of cutting jitter and partly because the upward and downward drilling angles are 37 . Figure 15 shows a cross-sectional view of the “two plugs, one injection and one row” partly because of cutting jitter and partly because the upward and downward drilling device. The bottom and upward borehole grouting part is dense, the grout is completely angles are 37°. Figure 15 shows a cross-sectional view of the “two plugs, one injection and filled with the plugging section, and there is no air leakage channel, illustrating that the one row” device. The bottom and upward borehole grouting part is dense, the grout is grouting effect of the new process of “two plugs, one injection and one row” is obviously better than the traditional method of “two plugs and one bet”. completely filled with the plugging section, and there is no air leakage channel, illustrat- (2) Analysis of engineering test results ing that the grouting effect of the new process of “two plugs, one injection and one row” The lower side of 24,130 machine lane is constructed with 10 holes, the sealing depth is is obviously better than the traditional method of “two plugs and one bet”. 20 m, and the sealing length is 8 m. The downward hole was sealed along the layer and the (2) Analysis of engineering test results inclination angle was 25 with all adopting the technology of full screen mesh pipe. Hole 1–5# is grouted and sealed according to the new process of “two plugs, one injection and 0 0 one row” and hole 1 –5 # was sealed according to the existing two plugs and one injection. After the sealing was completed, 10 pairs of two boreholes were connected to the extraction pipe, and the gas concentration was measured. Figure 16 below shows the average gas concentration of two boreholes with different sealing methods in 90 days. Through the similar simulation cutting experiment of the uphole plugging device, it can be obtained that the traditional “two plugs and one injection” has a part that is not filled with grout. This part is in the shape of a three-quarter circle. The “two plugs, one injection and one row” borehole grouting is partly dense; the grouting effect of the new “two plugs, one injection and one row” process is significantly better than the traditional “two plugs and one injection” method”. Through underground engineering tests, it can be obtained that the average gas concentration of “two plugs, one injection and one row” within 90 days is 75%, and the gas concentration of traditional sealing technology is 55%, which is an average increase of 36.4% compared with traditional technology. Fire 2022, 5, x FOR PEER REVIEW 16 of 18 The lower side of 24,130 machine lane is constructed with 10 holes, the sealing depth is 20 m, and the sealing length is 8 m. The downward hole was sealed along the layer and the inclination angle was 25° with all adopting the technology of full screen mesh pipe. Hole 1–5# is grouted and sealed according to the new process of “two plugs, one injection and one row” and hole 1′–5′# was sealed according to the existing two plugs and one in- jection. After the sealing was completed, 10 pairs of two boreholes were connected to the extraction pipe, and the gas concentration was measured. Figure 16 below shows the av- Fire 2022, 5, 65 16 of 18 erage gas concentration of two boreholes with different sealing methods in 90 days. "Two plugs, one injection and one row" hole sealing "Two plugs and one injection" hole sealing 0 10 20 30 40 50 60 70 80 90 100 Gas drainage time/d Figure 16. Comparison of gas volume fraction of two sealing methods. Figure 16. Comparison of gas volume fraction of two sealing methods. 4. Conclusions Through the similar simulation cutting experiment of the uphole plugging device, it In view of the problem that it is difficult to control the spontaneous combustion of coal around the borehole induced by gas drainage, the judgment criteria for a dangerous can be obtained that the traditional “two plugs and one injection” has a part that is not area of spontaneous combustion of fractured coal around the borehole were theoretically filled with grout. This part is in the shape of a three-quarter circle. The “two plugs, one determined. The combined methods of experiment and simulation were presented in injection and one row” borehole grouting is partly dense; the grouting effect of the new this paper. The comprehensive prevention and control technologies of shotcrete, sealing “two plugs, one injection and one row” process is significantly better than the traditional parameters, and the “two plugs, one injection and one row” device were introduced, “t rwo esulting plug ins the anfollowing d one injec conclusions: tion” method”. Through underground engineering tests, it can (1) The critical oxygen concentration of coal spontaneous combustion was experimen- be obtained that the average gas concentration of “two plugs, one injection and one row” tally studied. A method of comprehensively dividing the spontaneous combustion risk within 90 days is 75%, and the gas concentration of traditional sealing technology is 55%, area of fractured coal around the borehole induced by gas drainage was proposed based which is an average increase of 36.4% compared with traditional technology. on the oxygen concentration and air leakage speed. The spontaneous combustion risk area was divided by the oxygen concentration of coal around the borehole at8% and the 4.air Co leakage nclusions speed of 0.004 m/s. Scientific division of “dangerous areas of drilling holes” can enhance the pertinence of fire-fighting measures, improve the effect of fire-fighting In view of the problem that it is difficult to control the spontaneous combustion of projects, and effectively prevent spontaneous fire accidents. Production has very important coal around the borehole induced by gas drainage, the judgment criteria for a dangerous practical significance. area o (2) f spon In view taneou of thes pr com oblem bus that tion it is of dif fra ficult ctuto recontr d coal a ol the round t spontaneous he bocombustion rehole weof re theoretically coal around the borehole induced by gas drainage, the optimal roadway shotcrete length determined. The combined methods of experiment and simulation were presented in this was determined to be 2–4 m using the numerical simulation method. The sealing hole depth paper. The comprehensive prevention and control technologies of shotcrete, sealing pa- of 20 m and length of 13 m were determined by combining theory and experiment, which rameters, and the “two plugs, one injection and one row” device were introduced, result- provides a scientific basis for preventing and controlling the spontaneous combustion of ing in the following conclusions: coal around the borehole. (3) On the basis of the “two plugs and one injection” process, the “two plugs, one (1) The critical oxygen concentration of coal spontaneous combustion was experi- injection and one row of hole sealers” method was developed. The simulated drilling mentally studied. A method of comprehensively dividing the spontaneous combustion section was cut. The new process has a compact structure in the pocket part. The traditional risk area of fractured coal around the borehole induced by gas drainage was proposed process has a three-quarter circular blank section. The gas concentration of the traditional based on the oxygen concentration and air leakage speed. The spontaneous combustion Gas drainage volume fraction/% Fire 2022, 5, 65 17 of 18 sealing process is 55% and the average gas concentration of the “two plugs, one injection and one row” method that accumulated within 90 days is 75%, which shows an average increase of 36.4% compared to the traditional process. The new “two plugs, one injection and one row” device is an effective method for improving gas drainage efficiency and preventing spontaneous combustion of gas drainage boreholes. Author Contributions: Conceptualization, J.L. and Y.Z.; Methodology, J.D.; software, J.D. and Y.Z.; validation, J.L., J.L. and J.D.; formal analysis, Y.Z.; investigation, Y.Z.; resources, Y.Z.; data curation, J.L.; writing—original draft preparation, J.L.; writing—review and editing, J.L. and Y.Z.; visualization, J.D. and Y.Z.; supervision, J.D. and Y.Z.; project administration, J.D., J.L. and Y.Z.; funding acquisition, J.L. and J.D. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the Fundamental Research Funds for the Central Universities of China (No.3142020020), China Coal Technology & Engineering Group Co., Ltd. (2019-2-ZD003), the National Natural Science Foundation Youth Project of China (grant number 51804161), the National Natural Science Foundation of China (grant number 52074156), the Langfang City Science and Technology Support Plan Project (grant number 2020011017), and the China Postdoctoral Science Foundation (2020M680490). Data Availability Statement: The data used to support the findings of this study are enclosed with the article. Acknowledgments: The authors would like to thank all the reviewers who participated in the review and Rodrigo Cabanero for linguistic assistance during the preparation of this manuscript. Conflicts of Interest: The authors declare no conflict of interest. References 1. Chen, L.; Chen, L.; Wu, S.; Guo, L.J. Study on the integrated planning of deep mining considering rock burst prediction. IOP Conf. Ser. Earth Environ. Sci. 2020, 570, 042047. [CrossRef] 2. Xie, H.P.; Zhou, H.W.; Xue, D.J.; Wang, H.W.; Zhang, R.; Gao, F. 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Prevention Technology of Coal Spontaneous Combustion Induced by Gas Drainage in Deep Coal Seam Mining

Fire , Volume 5 (3) – May 12, 2022

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Abstract

fire Article Prevention Technology of Coal Spontaneous Combustion Induced by Gas Drainage in Deep Coal Seam Mining 1 2 3 , Jiahui Li , Youxin Zhao and Jinyu Du * School of Civil Engineering, North China University of Science and Technology, Langfang 123000, China; lijiahui@ncist.edu.cn Department of the Emergency Research Institute, China Coal Research Institute CCRI, Beijing 100013, China; zhaoyouxin@mail.ccri.ccteg.cn Law School, Beijing Technology and Business University, Beijing 100048, China * Correspondence: 20170901@btbu.edu.cn Abstract: Due to high gas content and a low permeability coefficient in deep coal seam mining, the spontaneous combustion of coal around the wellbore can easily occur, leading to difficulties in extracting gas during the mining process. To determine the dangerous area around the borehole and conduct advanced prevention and control measures are the keys to preventing spontaneous combustion in boreholes. However, the dangerous area around the borehole is not clear, the sealing parameters lack scientific basis, and the key prevention and control measures are not clear, which have caused great harm to coal mines. This study took the 24,130 working face of Pingdingshan No. 10 Mine as an example, using numerical simulation, theoretical analysis, and field tests to classify the risks of studying the surrounding area of the wellbore. The dangerous area variations under different lengths of shotcrete in the roadway were analyzed, the optimal plugging parameters were studied, and the current “two plugs and one injection” plugging device was optimized. Based on the oxygen concentration and air leakage rate, a method was proposed to divide the dangerous area of fissure coal spontaneous combustion around the borehole induced by gas extraction. The dangerous area Citation: Li, J.; Zhao, Y.; Du, J. of spontaneous combustion around the borehole was defined as having an oxygen concentration Prevention Technology of Coal Spontaneous Combustion Induced by larger than 7% and an air leakage rate less than 0.004 m/s. The comprehensive control measures of Gas Drainage in Deep Coal Seam the grouting length at 2–4 m, hole-sealing parameter at 20-13 (hole-sealing depth 20 m, hole-sealing Mining. Fire 2022, 5, 65. https:// length 13 m) and the “two plugs, one injection and one row” device were determined. doi.org/10.3390/fire5030065 Keywords: coal spontaneous combustion; hazard zone; roadway shotcrete; optimization of sealing Academic Editors: Haiyan Wang, parameters; two plugs; one injection and one row Feng Li, Huiyong Niu, Minbo Zhang and Xuyao Qi Received: 29 March 2022 Accepted: 10 May 2022 1. Introduction Published: 12 May 2022 As of May 2021, there were 4536 coal mines in China; among them 62 mines with a Publisher’s Note: MDPI stays neutral depth of more than one kilometer have an average depth of about 1092 m, involving an with regard to jurisdictional claims in approved production capacity of about 148 million tons. Studies have shown that coal published maps and institutional affil- mining in China is extending to deep coal mining at an average speed of 8–12 m/a. It is iations. foreseeable that the proportion of deep coal mines will become larger and larger. The deep mining of coal resources will become the norm [1–6]. The occurrence environment of deep coal seam mining has the characteristics of “three high and one low”: high ground stress, high ground temperature, high gas pressure, Copyright: © 2022 by the authors. and low permeability. Therefore, mining efforts have been increased by increasing the Licensee MDPI, Basel, Switzerland. drainage diameter, strengthening drilling, increasing negative pressure, and prolonging This article is an open access article drainage time [7–9]. At the same time, the difference between deep coal seam mining and distributed under the terms and shallow coal seam mining is that the environment in which deep coal seams are located is a conditions of the Creative Commons complex mechanical environment of “three highs and one disturbance”. The comprehensive Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ environmental factors around the gas drainage boreholes can easily cause spontaneous 4.0/). combustion of the surrounding broken coal bodies. Mine fires have always been a major Fire 2022, 5, 65. https://doi.org/10.3390/fire5030065 https://www.mdpi.com/journal/fire Fire 2022, 5, x FOR PEER REVIEW 2 of 18 Fire 2022, 5, 65 2 of 18 been a major disaster that frequently occur in coal mine production and have serious con- disaster that frequently occur in coal mine production and have serious consequences. sequences. During 2016–2020, roof and gas accidents are still the main major accidents During 2016–2020, roof and gas accidents are still the main major accidents with the largest with the largest number of gas accidents and fatalities in coal mines in the country, ac- number of gas accidents and fatalities in coal mines in the country, accounting for 18% and counting for 18% and 62% of the total, respectively, as shown in Figure 1. 62% of the total, respectively, as shown in Figure 1. Figure 1. The number of deaths in various large accidents in coal mines. Figure 1. The number of deaths in various large accidents in coal mines. The spontaneous combustion of coal bodies induced by gas drainage in deep coal seams This e dif spon ficult tane to find ous and com contr busti ol.on Determining of coal bo the dies danger indous uced area by will gas bedr the aikey nag to e in deep coal preventing spontaneous combustion in boreholes. Wang [10] proposed the use of infrared seams is difficult to find and control. Determining the dangerous area will be the key to detection technology to measure the spontaneous ignition source range within a range of preventing spontaneous combustion in boreholes. Wang [10] proposed the use of infrared less than 10 m from the coal roadway. Wang [11] used a combination of theoretical analysis detection technology to measure the spontaneous ignition source range within a range of and engineering applications to establish a complete mathematical model, combined with less than 10 m from the coal roadway. Wang [11]used a combination of theoretical analysis a fuzzy comprehensive evaluation model of the loose coal particle size and the fuzzy judgment model of the spontaneous combustion hazard area of the roadway. Liang [12] and engineering applications to establish a complete mathematical model, combined with used the gas concentration at some points to perform inversion using theoretical formulas a fuzzy comprehensive evaluation model of the loose coal particle size and the fuzzy judg- and equations to obtain accurate dispersion coefficients and permeability coefficients, and ment model of the spontaneous combustion hazard area of the roadway. Liang [12] used use them as a basis to predict the location of the fire source. Qi [13] determined that the the gas concentration at some points to perform inversion using theoretical formulas and fire position of the gas drainage borehole was on the inner side of the blocked section equa based tion ons wind to ob speed, tain accura and conduct te dis edpe engineering rsion coeff experiments icients an to d verify perme each. abil Zhou ity coe [14fficien ,15] ts, and use established a borehole spontaneous combustion model and proposed a quantitative eval- them as a basis to predict the location of the fire source. Qi [13] determined that the fire uation index for gas drainage. Huang [16] studied the impact of periodical weighting by position of the gas drainage borehole was on the inner side of the blocked section based introducing weighting intervals into the coupled model of coal self-heating in the gob. on wind speed, and conducted engineering experiments to verify each. Zhou [14,15] es- Liu [17] mastered the air leakage law for the coal pillars on both sides and the gob, and tablished a borehole spontaneous combustion model and proposed a quantitative evalu- determined the distribution law of the dangerous area of spontaneous combustion in the gob. Huang [18] completed a risk evaluation and located the spontaneous combustion in a ation index for gas drainage. Huang [16] studied the impact of periodical weighting by fully mechanized gob. introducing weighting intervals into the coupled model of coal self-heating in the gob. Liu The sealing depth and sealing length directly affect the sealing effect of the drilling. [17]mastered the air leakage law for the coal pillars on both sides and the gob, and deter- Liu [19] and others established the loose circle model of the surrounding rock of the rectan- mined the distribution law of the dangerous area of spontaneous combustion in the gob. gular roadway, studied the shape of the loose circle of the rectangular coal roadway and its Hu formation ang [18] and codevelopment mpleted a ri mechanism, sk evaluation determined and located the loose th cir e cle spon theory taneou formula s com forbustion the in a fully reasonable sealing depth of gas drainage, and established a theoretical basis. Wang [20] mechanized gob. studied the reasonable sealing depth of bedding boreholes using the cuttings method, and The sealing depth and sealing length directly affect the sealing effect of the drilling. theoretical analysis showed that the reasonable sealing depth is between the boundary of Liu [19] and others established the loose circle model of the surrounding rock of the rec- the broken zone and the plastic zone of the surrounding rock. Xu [21] proposed a roadway tan excavation gular ro model adway, about studie the int d ermediate the shape principal of the str lo ess ose and circl shear e of expansion the recta ofngular coal and coal roadway rock and proposed a change of coal stress around the borehole and the expression of the and its formation and development mechanism, determined the loose circle theory for- borehole-sealing depth. Hao [22] used a fluid-solid coupling model to discuss the extraction mula for the reasonable sealing depth of gas drainage, and established a theoretical basis. Wang [20] studied the reasonable sealing depth of bedding boreholes using the cuttings method, and theoretical analysis showed that the reasonable sealing depth is between the boundary of the broken zone and the plastic zone of the surrounding rock. Xu [21] pro- posed a roadway excavation model about the intermediate principal stress and shear ex- pansion of coal and rock and proposed a change of coal stress around the borehole and the expression of the borehole-sealing depth. Hao [22] used a fluid-solid coupling model Fire 2022, 5, 65 3 of 18 time, seal length, and the influence of air leakage on gas concentration. Wang [23] obtained a minimum seal length of 12 m from the ideal elastic-plastic model. The plugging device is also an important part that affects the spontaneous combustion of coal around the borehole [24]. The plugging method commonly used in domestic coal mines is called “two plugging and one injection” [25–28]. The plugging device has the advantages of simple downhole plugging operation, fast plugging speed, and grouting enabled to penetrate deep into the fractures. However, because the “two plugs and one injection” device’s two-capsule bags form a closed space during the grouting process, the internal gas pressure is too large to hinder the grouting of the grouting pump. When the internal and external pressures reach equilibrium, the gas in the grouting section cannot be completely discharged, and the slurry is not solidified, resulting in a reduction in the effective plugging length of the borehole. In view of the various disasters caused by deep coal seams in the process of gas drainage, a single water injection method cannot fundamentally solve the problem of coal spontaneous combustion around the borehole. The study uses theoretical analysis, numerical simulation, and engineering applications to determine how to prevent the danger of coal spontaneous combustion around the borehole. Specifically, dividing the area, analyzing the influence of roadway shotcrete on the dangerous area around the gas drainage borehole, improving the “two plugs and one injection” plugging device, optimizing the plugging parameters, and scientifically predicting and adopting reasonable prevention and control measures to avoid disasters were determined. Safe and efficient gas drainage in deep coal seams is of great important for engineering practice. 2. Division of Dangerous Areas of Spontaneous Combustion in Coal Boreholes Induced by Gas Drainage 2.1. Analysis of Coal Spontaneous Combustion Area around Gas Drainage Borehole A reasonable division of the dangerous areas around the gas drainage boreholes is the key to prevention, governance, and reform. In previous work [29–31], the authors conducted a detailed study on the cause, process, and influence of the natural combustion of the coal body around the borehole during gas drainage. However, at present, the dangerous area of coal spontaneous combustion around the gas drainage borehole is unclear and lacks theoretical basis. As shown in Figure 2, the oxygen concentration of coal near the outer part of the plugging section is greater than the critical oxygen concentration of coal spontaneous combustion and the air flow speed is less than the critical wind speed of coal spontaneous combustion because the oxidized coal around the borehole is connected to the roadway, so an oxidation reaction occurs. The heat generated by the oxidation reaction is less than the heat dissipated at the place where the borehole and roadway are connected by gas extraction, and this area is a scattered zone. Beyond the scattered zone, along the direction of drilling, the oxygen volume fraction in this area decreases and the air flow velocity increases. The heat generated by coal oxidation is greater than the heat dissipated, and this area is the oxidation zone. Beyond the oxidation zone, along the direction of the drilling hole, oxygen concentration is reduced, airflow velocity increases, and heat dissipation is far greater than the heat release from oxidation. This area is the oxidation zone where the borehole fires spontaneously; inside the oxidation zone, the oxygen concentration is reduced, the wind velocity is high, heat accumulation in the area is difficult, and it is a suffocation zone. However, when the energy accumulated in the coal body around the borehole reaches a certain amount, the heat dissipation zone outside the plugging section has a low wind speed and high oxygen concentration, and the heat generated is far less than the heat dissipation. At this point, the heat dissipation zone would also be reduced and become a dangerous area for the spontaneous combustion of coal. Fire 2022, 5, x FOR PEER REVIEW 4 of 18 Fire 2022, 5, 65 4 of 18 Figure 2. Diagram of the dangerous area around borehole. Figure 2. Diagram of the dangerous area around borehole. 2.2. Critical Oxygen Concentration and Critical Air Flow Velocity Determination 2.2. Critical Oxygen Concentration and Critical Air Flow Velocity Determination Coal spontaneous combustion occurs under three conditions: oxygen concentration, wind speed, and time [32–35]. Only when the ventilation and oxygen supply conditions Coal spontaneous combustion occurs under three conditions: oxygen concentration, are continuous and stable for a period of time, enough to combine the coal with sufficient wind speed, and time [32–35]. Only when the ventilation and oxygen supply conditions oxygen, the oxidation process is fully carried out and the coal body can spontaneously are continuous and stable for a period of time, enough to combine the coal with sufficient ignite when a certain amount of heat is accumulated [36,37]. Therefore, choosing the oxygen, the oxidation process is fully carried out and the coal body can spontaneously appropriate critical oxygen concentration and airflow velocity is the key to classifying the spontaneous combustion of coal around the gas drainage hole. In order to obtain the ignite when a certain amount of heat is accumulated [36,37]. Therefore, choosing the ap- critical oxygen concentration of coal spontaneous combustion, this study took the original propriate critical oxygen concentration and airflow velocity is the key to classifying the coal sample from the 24,130 working face of Pingdingshan No. 1 Mine as an example, spontaneous combustion of coal around the gas drainage hole. In order to obtain the crit- and conducted low-temperature oxidation experiments under the conditions of oxygen ical oxygen concentration of coal spontaneous combustion, this study took the original concentrations of 20.9%, 10%, and 8%. The relationship between the heat release and coal coal sample from the 24,130 working face of Pingdingshan No. 1 Mine as an example, and temperature and the specific experimental results are shown in Figure 3. When oxygen concentration is 10.0%, the growth rate of CO and C H output is conducted low-temperature oxidation experiments under the 2co 4nditions of oxygen con- inhibited, but there is still a large growth rate, indicating that when oxygen concentration centrations of 20.9%, 10%, and 8%. The relationship between the heat release and coal reaches 10.0%, the spontaneous combustion of coal has been inhibited, but it is not enough temperature and the specific experimental results are shown in Figure 3. to completely prevent the oxidation of coal. The inhibition of the coal oxidation process is stronger when the oxygen concentration continues to decrease to 8.0%. However, the first generation concentration of each gas behaves roughly the same. The change law of coal CO volume fraction—coal temperature C H volume fraction—coal temperature temperature reflects the degree of oxidation reaction during the spontaneous combustion 2 4 of coal and the change curve is shown in Figure 3. It can be seen from Figure 3 that when the oxygen concentration is 20.9%, the oxidation process of coal has entered the intense Oxygen concentration 8% oxidation stage; when the oxygen concentration is 10.0%, the coal sample also has an Oxygen concentration 8% Oxygen concentration 10% Oxygen concentration 10% obvious violent oxidation process, but the violent combustion stage of the spontaneous Oxygen concentration 20.9% Oxygen concentration 20.9% combustion oxidation of the coal sample is obviously inhibited compared to the air con- dition; the coal temperature at 8.0% oxygen concentration is significantly lower than the coal temperature at 10.0% oxygen concentration, and the degree of oxidation is further suppressed. In summary, it can be obtained that the critical oxygen concentration of the experimental coal sample for spontaneous combustion is determined to be 8.0%. 500 20 0 100 200 300 400 0 100 200 300 400 The temperature of the coal(℃) The temperature of the coal(℃) (a) CO volume fraction—coal temperature (b) C2H4 volume fraction—coal temperature -6 CO volume fraction/10 -6 C H volume fraction/10 2 4 Fire 2022, 5, x FOR PEER REVIEW 4 of 18 Figure 2. Diagram of the dangerous area around borehole. 2.2. Critical Oxygen Concentration and Critical Air Flow Velocity Determination Coal spontaneous combustion occurs under three conditions: oxygen concentration, wind speed, and time [32–35]. Only when the ventilation and oxygen supply conditions are continuous and stable for a period of time, enough to combine the coal with sufficient oxygen, the oxidation process is fully carried out and the coal body can spontaneously ignite when a certain amount of heat is accumulated [36,37]. Therefore, choosing the ap- propriate critical oxygen concentration and airflow velocity is the key to classifying the spontaneous combustion of coal around the gas drainage hole. In order to obtain the crit- ical oxygen concentration of coal spontaneous combustion, this study took the original coal sample from the 24,130 working face of Pingdingshan No. 1 Mine as an example, and conducted low-temperature oxidation experiments under the conditions of oxygen con- Fire 2022, 5, 65 5 of 18 centrations of 20.9%, 10%, and 8%. The relationship between the heat release and coal temperature and the specific experimental results are shown in Figure 3. CO volume fraction—coal temperature C H volume fraction—coal temperature 2 4 Oxygen concentration 8% Oxygen concentration 8% Oxygen concentration 10% 2000 Oxygen concentration 10% Oxygen concentration 20.9% Oxygen concentration 20.9% 500 20 Fire 2022, 5, x FOR PEER REVIEW 5 of 18 0 100 200 300 400 0 100 200 300 400 The temperature of the coal(℃) The temperature of the coal(℃) (a) CO volume fraction—coal temperature (b) C2H4 volume fraction—coal temperature Temperature rise curve of experimental coal samples Temperature in the furnace 20.9% coal body temperature 10% coal body temperature 8% coal body temperature 0 50 100 150 200 250 300 350 time/min (c) Temperature rise curve of experimental coal samples Figure 3. The law of coal temperature, CO, and C2H4 concentration changing with time. Figure 3. The law of coal temperature, CO, and C H concentration changing with time. 2 4 When oxygen concentration is 10.0%, the growth rate of CO and C2H4 output is in- Air flow velocity is the main driving force for the spontaneous combustion and heat hibited, but there is still a large growth rate, indicating that when oxygen concentration dissipation of coal around the borehole. When the airflow velocity of the gas around the reaches 10.0%, the spontaneous combustion of coal has been inhibited, but it is not enough borehole exceeds the critical velocity, even when oxygen is sufficient, it is difficult for heat to completely prevent the oxidation of coal. The inhibition of the coal oxidation process is to accumulate in this area. When the velocity is less than the critical velocity in this area, the stron amount ger whe of heat n the oxyg dissipation en concentr is less, and ation the contin spontaneous ues to decrea combustion se to 8.0%. of coal How may eveoccur r, the fir after st generation concentration of each gas behaves roughly the same. The change law of coal long-term oxidation heat release. Therefore, the critical air flow velocity is an important temperature reflects the degree of oxidation reaction during the spontaneous combustion indicator to measure the spontaneous combustion of the coal around the borehole. Due of coal and to the complex the change c environment urve is shown of the coal in Fig body ure near 3. It c theabor n be seen fro ehole, them wind Figu flow re 3 th velocity at when is the ox difficult ygen to measur concentr e, at soion thisis paper 20.9%, refers the oxida to multiple tion pro works cess o (W f ang coal h Ha et s entered al., 2002) th to e in obtain tense a critical wind flow velocity of 0.004 m/s under the condition of sufficient oxygen. oxidation stage; when the oxygen concentration is 10.0%, the coal sample also has an ob- vious violent oxidation process, but the violent combustion stage of the spontaneous com- 2.3. Determination of the Hazardous Area bustion oxidation of the coal sample is obviously inhibited compared to the air condition; In the early stage of gas drainage, the heat dissipation zone is small and there is no the coal temperature at 8.0% oxygen concentration is significantly lower than the coal tem- risk of spontaneous ignition. In the later stage of gas drainage, the heat dissipation zone perature at 10.0% oxygen concentration, and the degree of oxidation is further suppressed. has the risk of coal spontaneous combustion. The area with spontaneous combustion is the In summary, it can be obtained that the critical oxygen concentration of the experimental coal sample for spontaneous combustion is determined to be 8.0%. Air flow velocity is the main driving force for the spontaneous combustion and heat dissipation of coal around the borehole. When the airflow velocity of the gas around the borehole exceeds the critical velocity, even when oxygen is sufficient, it is difficult for heat to accumulate in this area. When the velocity is less than the critical velocity in this area, the amount of heat dissipation is less, and the spontaneous combustion of coal may occur after long-term oxidation heat release. Therefore, the critical air flow velocity is an im- portant indicator to measure the spontaneous combustion of the coal around the borehole. Due to the complex environment of the coal body near the borehole, the wind flow veloc- ity is difficult to measure, so this paper refers to multiple works (Wang H et al., 2002) to obtain a critical wind flow velocity of 0.004 m/s under the condition of sufficient oxygen. -6 CO volume fraction/10 temperature/℃ -6 C H volume fraction/10 2 4 Fire 2022, 5, 65 6 of 18 “dangerous area” and the suffocation zone is the “safe area”. The safe area around the gas drainage borehole is characterized by a combination of an oxygen concentration less than 8% and wind velocity greater than 0.004, and the remaining areas are collectively referred to as “dangerous areas”. 3. Key Technologies for Prevention and Control of Coal Spontaneous Combustion around Boreholes In areas with serious borehole fires, measures are taken to stop drainage and inject water into the coal body area, but these measures can only temporarily control the spread of fire. Therefore, this thesis mainly discusses the prevention and control of spontaneous combustion in boreholes by using roadway shotcrete, optimizing hole-sealing parameters, and improving the “hole-sealing device”. 3.1. Optimization of Roadway Shotcrete Length Parameters Roadway shotcrete is the first line of defense that isolates the working face and the roadway, seals the cracks in the coal wall and the pores of the coal body itself, and reduces the interaction between the coal body and the air in the roadway. The length of roadway shotcrete affects the scope of the dangerous area around the borehole, so the method of combining numerical simulation and engineering is used to study the law of the influence of shotcrete on the oxygen concentration, fluid velocity, and the scope of the dangerous area around the borehole. According to the spontaneous combustion of the coal seam (Jia et al., 2021) in the 24,130 working face of Pingdingshan No.10 Mine, the numerical simulation set the plugging depth at 15 m, the plugging length at 8 m, the initial ground temperature of the coal seam at 48 C, and the negative pressure of drainage at 23 kPa. For the model’s parameters and boundary conditions, refer to Jia et al., 2021 and Zhao et al., 2021. 3.1.1. Analysis of Oxygen Concentration and Wind Velocity Figure 4 is a cloud map of oxygen concentration and air flow velocity for different roadway shotcrete lengths during gas drainage for 3 days. Figure 4 shows that with the increase of roadway shotcrete length, the air flow velocity around the borehole shows almost no change at all and the area with a larger oxygen concentration changes a little. Oxygen is one of the factors that induces the spontaneous combustion of coal. Reducing the oxygen concentration around the borehole can effectively prevent gas drainage from inducing the spontaneous combustion of coal. The longer the distance of roadway shotcrete is, the greater the safe range of oxygen concentration around the borehole. It can be seen that when the roadway does not take roadway shotcrete measures, the characteristics of the wide range of oxygen concentration and the large oxygen concentration can easily cause the spontaneous combustion of coal in the fissures around the borehole during gas drainage. Because the roadway shotcrete has little effect on the air flow velocity, it can be obtained, from the perspective of oxygen concentration, that the roadway shotcrete can effectively prevent oxygen from entering the fractured coal around the borehole, thereby preventing the oxidation and exothermic reaction of the coal and reducing the possibility of coal spontaneous combustion. Analysis of Roadway Shotcrete in the Dangerous Area around the Borehole Figure 5 shows that, without taking any measures, the longitudinal length of the dangerous area around the borehole is close to 14 m, in the shape of “hands clasped together”, and the dangerous area is very large. When the shotcrete length of the roadway is 2 m, 4 m, and 6 m, the dangerous area of the fractured coal body around the borehole is reduced, and the range of the dangerous area near the roadway does not change much. When comparing different roadway shotcrete lengths, the hazardous area changes a little. Comprehensive analysis of the oxygen concentration, the fractured coal body around the borehole, and the hazardous area range, shows that the best roadway shotcrete length for a single borehole is 2 m. Fire 2022, 5, x FOR PEER REVIEW 7 of 18 No measures Roadway shotcrete 2 m Fire 2022, 5, x FOR PEER REVIEW 7 of 18 Fire 2022, 5, 65 7 of 18 No measures Roadway shotcrete 2 m Roadway shotcrete 6 m Roadway shotcrete 4 m Figure 4. The cloud map of oxygen concentration for different roadway grouting lengths was ob- tained. Roadway shotcrete 6 m Roadway shotcrete 4 m (3) Analysis of roadway shotcrete in the dangerous area around the borehole Figure 5 shows that, without taking any measures, the longitudinal length of the dan- gerous area around the borehole is close to 14 m, in the shape of “hands clasped together”, and the dangerous area is very large. When the shotcrete length of the roadway is 2 m, 4 m, and 6 m, the dangerous area of the fractured coal body around the borehole is reduced, and the range of the dangerous area near the roadway does not change much. When com- paring different roadway shotcrete lengths, the hazardous area changes a little. Compre- hensive analysis of the oxygen concentration, the fractured coal body around the borehole, and the hazardous area range, shows that the best roadway shotcrete length for a single Figure Figure 4.4. The The cc lo lo ud ud m map ap o of f o ox xy yg gen en c o co nncentration centration forfor di fdi fer ff eerent nt road roadway way groug tirou ng lti eng ngt h le sng wth ass ob was tain ob- ed. borehole is 2 m. tained. (3) Analysis of roadway shotcrete in the dangerous area around the borehole No measures Roadway shotcrete 2 m Figure 5 shows that, without taking any measures, the longitudinal length of the dan- gerous area around the borehole is close to 14 m, in the shape of “hands clasped together”, and the dangerous area is very large. When the shotcrete length of the roadway is 2 m, 4 m, and 6 m, the dangerous area of the fractured coal body around the borehole is reduced, and the range of the dangerous area near the roadway does not change much. When com- paring different roadway shotcrete lengths, the hazardous area changes a little. Compre- hensive analysis of the oxygen concentration, the fractured coal body around the borehole, Fire 2022, 5, x FOR PEER REVIEW 8 of 18 and the hazardous area range, shows that the best roadway shotcrete length for a single borehole is 2 m. Roadway shotcrete 6 m Roadway shotcrete 4 m No measures Roadway shotcrete 2 m Figure 5. Changes of hazard areas under different roadway grouting lengths when the gas was ex- Figure 5. Changes of hazard areas under different roadway grouting lengths when the gas was tracted for 3 d. extracted for 3 d. 3.1.2. Experimental Research on Shotcrete Engineering of Roadway In the 24,130 working face, the tunnels around the experimental boreholes were cleaned by shotcrete, the spraying was even, and there was no “skirt wearing, barefoot” phenomenon; the spray thickness was 50 mm, and the experimental boreholes with shot- crete lengths of 0 m, 2 m, 4 m, and 6 m were set. Monitoring the CO concentration in the gas drainage borehole reflects the effect of roadway shotcrete. Figure 6 below shows the shotcrete on site and Figure 7 shows the monitoring result curve of the shotcrete length of different roadways. Figure 6. Site map. Fire 2022, 5, x FOR PEER REVIEW 8 of 18 Roadway shotcrete 6 m Roadway shotcrete 4 m Figure 5. Changes of hazard areas under different roadway grouting lengths when the gas was ex- Fire 2022, 5, 65 tracted for 3 d. 8 of 18 3.1.2. Experimental Research on Shotcrete Engineering of Roadway 3.1.2. Experimental Research on Shotcrete Engineering of Roadway In the 24,130 working face, the tunnels around the experimental boreholes were In the 24,130 working face, the tunnels around the experimental boreholes were cleaned by shotcrete, the spraying was even, and there was no “skirt wearing, barefoot” cleaned by shotcrete, the spraying was even, and there was no “skirt wearing, barefoot” phe- phenomenon; the spray thickness was 50 mm, and the experimental boreholes with shot- nomenon; the spray thickness was 50 mm, and the experimental boreholes with shotcrete crete lengths of 0 m, 2 m, 4 m, and 6 m were set. Monitoring the CO concentration in the lengths of 0 m, 2 m, 4 m, and 6 m were set. Monitoring the CO concentration in the gas drainage borehole reflects the effect of roadway shotcrete. Figure 6 below shows the gas drainage borehole reflects the effect of roadway shotcrete. Figure 6 below shows the shotcrete on site and Figure 7 shows the monitoring result curve of the shotcrete length of shotcrete on site and Figure 7 shows the monitoring result curve of the shotcrete length of different roadways. different roadways. Figure 6. Site map. Figure 6. Site map. The Figure 7 shows that when no measures are taken on the roadway, the CO con- centration rises more obviously within 50 days, and the maximum can reach 430 ppm. When the roadway shotcrete length is 2, 4, and 6 m, the CO concentration around the borehole increases with the gas drainage. The increase in time continues, but compared to the 0 m roadway shotcrete, the rise tends to be slower. The average gas concentrations within 50 days of drilling gas drainage were 266, 210, 179, and 170 ppm for each shotcrete length, and the decline rate slowed down. Therefore, the length of the shotcrete for a single borehole is between 2 m and 4 m. 3.2. Optimization of Drilling and Sealing Parameters for Gas Drainage One of the main reasons for spontaneous fire in gas drainage boreholes is the poor plugging effect of boreholes. The main factors that affect the plugging effect of boreholes are the length of the hole, the depth of the hole, the grouting material, and whether the plugging is standardized. To specific coal mines, reasonable plugging length and plugging depth are the most important, direct, and effective method to improve the plugging effect, which can not only greatly improve the efficiency of gas drainage, but also prevent spontaneous combustion in deep coal seam drainage boreholes. Fire 2022, 5, x FOR PEER REVIEW 9 of 18 Fire 2022, 5, 65 9 of 18 No measures Roadway shotcrete length 2m Roadway shotcrete length 4m Roadway shotcrete length 6m 0 10 20 30 40 50 Gas drainage time/d Figure 7. Results of testing. Figure 7. Results of testing. 3.2.1. Experimental Research on the Stress Distribution around the Roadway Based on the The Figure 7 shows that when no measures are taken on the roadway, the CO con- Drill Cuttings Method centration rises more obviously within 50 days, and the maximum can reach 430 ppm. The drill cuttings method is one of the effective methods used to test the stress dis- When the roadway shotcrete length is 2, 4, and 6 m, the CO concentration around the tribution of the coal seam (surrounding rock). In field applications, pneumatic hand-held bo drilling rehole rigs inc ar rea e gener ses wi ally th used the gas to drill drai holes nage. perpendicular The increase toin the time coalcon wall. tinue Thes, str bu ess t co distri- mpared to bution characteristics of the coal body can be analyzed by the changing law of the amount the 0 m roadway shotcrete, the rise tends to be slower. The average gas concentrations of drill cuttings discharged from drilling and the accompanying dynamic phenomena. within 50 days of drilling gas drainage were 266, 210, 179, and 170 ppm for each shotcrete The yield of drill cuttings changes with the change of stress in the coal body charac- length, and the decline rate slowed down. Therefore, the length of the shotcrete for a single terized by a certain functional relationship. As the stress of the coal body increases, the borehole is between 2 m and 4 m. elastoplastic deformation increases and the yield of drill cuttings also increases. The drill cuttings method is used to study the characteristics of the stress distribution of the roadside 3.2. Optimization of Drilling and Sealing Parameters for Gas Drainage coal body. The drill cuttings method studies the relationship between the pressure on the coal body and the yield of drill cuttings. One of the main reasons for spontaneous fire in gas drainage boreholes is the poor It can be seen from the formula that the yield of drill cuttings is positively corre- plugging effect of boreholes. The main factors that affect the plugging effect of boreholes lated with the pressure of the surrounding rock. When the stress concentration increases, are the length of the hole, the depth of the hole, the grouting material, and whether the the quality of drill cuttings per unit length increases. Therefore, the stress state of the plugging is standardized. To specific coal mines, reasonable plugging length and plug- surrounding rock can be judged by the change in the yield of drill cuttings. ging depth are the most important, direct, and effective method to improve the plugging effect, which can not only greatly improve the efficiency of gas drainage, but also prevent 3.2.2. Determination of Sealing Depth spontaneous combustion in deep coal seam drainage boreholes. At the lower part of the 24,130 wind tunnel near the stop line, 594 mm drill holes were drilled into the coal tunnel along the layer, and drill cuttings were taken to test the stress 3.2.1. Experimental Research on the Stress Distribution around the Roadway Based on distribution around the tunnel. Two 10 kg and 50 kg spring scales and two plastic buckets were used for on-site crumb fetching and weighing, as well as one square woven bag with the Drill Cuttings Method a side length of about 1 m. The field test is shown in Figure 8. The drill cuttings method is one of the effective methods used to test the stress dis- The yield of drill cuttings from the 94 mm drill hole were plotted on a curve as shown tribution of the coal seam (surrounding rock). In field applications, pneumatic hand-held in Figure 9. Figure 9 shows that the yield of cuttings in the first few meters of drilling drilling rigs are generally used to drill holes perpendicular to the coal wall. The stress does not change much, with only slightly increases. As the drilling depth increases, the distribution characteristics of the coal body can be analyzed by the changing law of the yield of drill cuttings first rises sharply, then remains stable, then decreases, and finally amount of drill cuttings discharged from drilling and the accompanying dynamic phe- stabilizes. Among them, 2#, 3#, 4# drill cuttings have a similar trend with the drilling distance. The 2# drilling hole enters the area where the yield of cuttings drops after the nomena. The yield of drill cuttings changes with the change of stress in the coal body charac- terized by a certain functional relationship. As the stress of the coal body increases, the elastoplastic deformation increases and the yield of drill cuttings also increases. The drill CO volume fraction/ppm Fire 2022, 5, x FOR PEER REVIEW 10 of 18 Fire 2022, 5, 65 10 of 18 cuttings method is used to study the characteristics of the stress distribution of the road- yield of cuttings reaches the maximum. The 4# drilling hole has the largest yield of cuttings, side coal body. The drill cuttings method studies the relationship between the pressure indicating that the stress here is the largest. Combining the yield of drill cuttings in each drill hole, it can be seen that the yield of drill cuttings in the range of 0~7.5 m is low, with on the coal body and the yield of drill cuttings. only small fluctuations. At this time, the yield of drill cuttings should only be the quality of It can be seen from the formula that the yield of drill cuttings is positively correlated the drilled coal body; drilling in the range of 8~17 m, the yield of cuttings increases sharply with the pressure of the surrounding rock. When the stress concentration increases, the and remains high in the range of 17~23 m. It decreases sharply in the range of 23~28 m quality of drill cuttings per unit length increases. Therefore, the stress state of the sur- and tends to be stable after about 28 m. At the same time, in the range of 8~28 m, there rounding rock can be judged by the change in the yield of drill cuttings. were different numbers of coal cannons during the drilling process and the sound was loud or quiet, accompanied by the phenomenon of spray holes, the particle size of coal 3.2.2. Determination of Sealing Depth cuttings becoming larger, and the maximum yield of drill cuttings appearing in the drilling hole at a distance of 20 m before and after. Combined with the on-site measurement of the At the lower part of the 24,130 wind tunnel near the stop line, 594 mm drill holes stress distribution of the surrounding rock using the cuttings measurement method and were drilled into the coal tunnel along the layer, and drill cuttings were taken to test the the theoretical analysis of the loose zone, it can be concluded that 0~7 m is the broken zone, stress distribution around the tunnel. Two 10 kg and 50 kg spring scales and two plastic 8~19 m is the plastic zone, and 20~28 m is the elastic zone. The maximum yield of drill buckets were used for on-site crumb fetching and weighing, as well as one square woven cuttings appear around 20 m and the maximum stress appears at about 20 m away from bag with a side length of about 1 m. The field test is shown in Figure 8. the roadway. Figure Figure 8.8. Field Field pictur pictur es.es. 3.2.3. Determination of Sealing Length The yield of drill cuttings from the 94 mm drill hole were plotted on a curve as shown According to the actual situation of the sealing parameters of Pingdingshan No.10 in Figure 9. Figure 9 shows that the yield of cuttings in the first few meters of drilling does Mine, the sealing length is set to 8 m, 10 m, 13 m, and 15 m. For other physical parameters not change much, with only slightly increases. As the drilling depth increases, the yield and boundary conditions, please refer to the paper by Jia et al., 2021; Zhao et al., 2021. The of drill cuttings first rises sharply, then remains stable, then decreases, and finally stabi- coal temperature cloud map at 90 days around the borehole is shown in Figure 10. lizes. Among them, 2#, 3#, 4# drill cuttings have a similar trend with the drilling distance. Figure 10 shows the plugging depth of 20 m; the plugging lengths are 8 m, 10 m, 13 m, The 2# drilling hole enters the area where the yield of cuttings drops after the yield of and 15 m, the gas drainage is 90 d, and there is coal temperature distribution around the bor cutti ehole. ngs It rea can ches be seen the m that axim when um. The the sealing 4# drilli length ng ho is 8le h m, 10 as m, the 13 large m, and st y15 ield o m, the f cu high ttings, indi- temperature of the coal body around the borehole is 83.4 C, 65.5 C, 53.9 C, and 48 C, cating that the stress here is the largest. Combining the yield of drill cuttings in each drill respectively, and the oxygen concentration around the borehole is greater than 8%. The area hole, it can be seen that the yield of drill cuttings in the range of 0~7.5 m is low, with only gradually decreases as the sealing length increases. From the perspective of temperature small fluctuations. At this time, the yield of drill cuttings should only be the quality of the analysis, when the gas drainage is within 90 d, the sealing hole length should be no less drilled coal body; drilling in the range of 8~17 m, the yield of cuttings increases sharply than 10 m; when the gas drainage is greater than 90 d, the sealing hole length should be and remains high in the range of 17~23 m. It decreases sharply in the range of 23~28 m greater than 13 m. However, according to the gas drainage rules, it can be obtained that the and tends to be stable after about 28 m. At the same time, in the range of 8~28 m, there working face gas drainage should last longer than 6 months, so the best sealing length is were different numbers of coal cannons during the drilling process and the sound was 13 m. From the perspective of oxygen concentration, although the area where the oxygen loud or quiet, accompanied by the phenomenon of spray holes, the particle size of coal cuttings becoming larger, and the maximum yield of drill cuttings appearing in the drill- ing hole at a distance of 20 m before and after. Combined with the on-site measurement of the stress distribution of the surrounding rock using the cuttings measurement method and the theoretical analysis of the loose zone, it can be concluded that 0~7 m is the broken Fire 2022, 5, 65 11 of 18 Fire 2022, 5, x FOR PEER REVIEW 11 of 18 concentration is greater than 8% is the smallest when the sealing length is 15 m, considering that the sealing length is too long, it also increases the difficulty and cost of sealing and zone, 8~19 m is the plastic zone, and 20~28 m is the elastic zone. The maximum yield of when the sealing length is 13 m, the area where the oxygen concentration is greater than drill cuttings appear around 20 m and the maximum stress appears at about 20 m away 8% is still small. Combining temperature changes and oxygen concentration changes, the from optimal the ro sealing adway. hole length is 13 m, which can prevent the spontaneous combustion of coal seams and ensure efficient gas drainage. 1# Maximum stress Turning point 2# 3# 4# 5# 0 5 10 15 20 25 30 Drilling distance/m Figure 9. Variation of drilling yield with drilling depth. Figure 9. Variation of drilling yield with drilling depth. The test was carried out on the lower edge of the 24,130 wind tunnel at a distance of 3.2.3. D 500 m fr et om ermi the nation o cut hole. f Sea Theling test L involved ength parameters of (sealing depth-sealing length) 15-13, 18-13, 20-8, 20-10, and 20-13. Concerning drilling hole parameters, each group of According to the actual situation of the sealing parameters of Pingdingshan No.10 drilling spacing is 2 m, the hole diameter is 94 mm, and the drilling depth is 90 m. There Mine, the sealing length is set to 8 m, 10 m, 13 m, and 15 m. For other physical parameters were five holes in each group of boreholes. Each borehole in each group was connected and boundary conditions, please refer to the paper by Jia et al., 2021; Zhao et al., 2021. The to the branch pipe and then connected to the main pipe. Each group of boreholes had coal temperature cloud map at 90 days around the borehole is shown in Figure 10. a monitoring hole to monitor the average gas of each group of boreholes. To extract the volume fraction, the average gas extraction volume fraction of each group of extraction boreholes was monitored once a day and recorded and the six groups of extraction boreholes were continuously monitored for 60 days. The data are plotted in Figure 11. Sealing length 8 m Sealing length 10 m Comparing the sealing parameters of 20-8, 20-10 and 20-13, the average gas extraction volume fraction of boreholes with a sealing parameter of 20-8 is lower and the average gas concentration is 41%; when the sealing parameter is 20-10, the average gas drainage concentration accumulated within 50 days is 56.4% and when the drainage parameters are 20-13, the average gas drainage concentration reaches 70.8%. All comparative analysis shows that when the sealing depth is 20 m, the best gas drainage length of the sealing hole is 13 m. From the perspective of gas drainage concentration, it can prevent spontaneous combustion of the borehole. Comparing the three groups of holes with sealing parameters 15-13, 18-13, and 20-13, the average gas concentration of the drainage holes is 56%, 62.4%, and 70.8%, respectively, and the sealing length is the same, the deeper the sealing depth, Drilling cuttings per meter/(kg/m) Fire 2022, 5, x FOR PEER REVIEW 11 of 18 zone, 8~19 m is the plastic zone, and 20~28 m is the elastic zone. The maximum yield of drill cuttings appear around 20 m and the maximum stress appears at about 20 m away from the roadway. 1# Maximum stress Turning point 2# 3# 4# 5# Fire 2022, 5, 65 12 of 18 the higher the gas drainage concentration. When the 50 d plugging depth is 20 m, the average gas drainage concentration reaches 70.8%, indicating that the plugging effect is better and 0 there is almost 5 no air 10 leakage.15 The experimental 20 analysis 25 of the 30drill cuttings method results in the peak stress of 24,130 working face. The point is 20 m and the sealing Drilling distance/m depth is increased before the stress peak, which can improve the extraction effect. Based on the above analysis, the plugging depth is 20 m and the inner end of the Figure 9. Variation of drilling yield with drilling depth. plugging section of the plugging position is just at the peak stress point, which will not cause a blind area for drainage. When the plugging length is 13 m, which can prevent 3.2.3. Determination of Sealing Length the natural fire of the coal seam and ensure the efficient gas drainage pick. Therefore, According to the actual situation of the sealing parameters of Pingdingshan No.10 in order to increase the extraction efficiency and avoid the spontaneous combustion of Mine, the sealing length is set to 8 m, 10 m, 13 m, and 15 m. For other physical parameters the fractured coal around the borehole, comprehensive numerical simulation and field and boundary conditions, please refer to the paper by Jia et al., 2021; Zhao et al., 2021. The engineering experiment analysis can determine the optimal sealing parameters of the coal coal temperature cloud map at 90 days around the borehole is shown in Figure 10. seam at the 24,130 working face as 20-13. Sealing length 8 m Sealing length 10 m Fire 2022, 5, x FOR PEER REVIEW 12 of 18 Sealing length 13 m Sealing length15 m Figure 10. The temperature cloud map of the coal body around the borehole. Figure 10. The temperature cloud map of the coal body around the borehole. 3.3.Fi New gure Pr 1evention 0 shows Technology the pluggin of “T g wo dep Plugs, th of One 20 m; Injection the plu and ggiOne ng len Row” gths are 8 m, 10 m, 13 m, and 15 m, the gas drainage is 90 d, and there is coal temperature distribution around The most commonly used downhole “two plugs and one injection” hole plugging the borehole. It can be seen that when the sealing length is 8 m, 10 m, 13 m, and 15 m, the device is used to observe the pressure with a grouting pressure gauge, which does not hig indicate h temp that erature the bor of the co ehole isal filled body arou with slurry nd t.he boreh The gas in ole is the 83.4 borehole °C, 6 cannot 5.5 °C, be 53. completely 9 °C, and 48 discharged, and the slurry can easily gather around the borehole after solidification, causing °C, respectively, and the oxygen concentration around the borehole is greater than 8%. the formation of gaps and gas segments. The area gradually decreases as the sealing length increases. From the perspective of tem- perature analysis, when the gas drainage is within 90 d, the sealing hole length should be 3.3.1. Development of “Two Plugs, One Injection and One Row” Device no less than 10 m; when the gas drainage is greater than 90 d, the sealing hole length The “two plugs, one injection and one row” plugging device is composed of three should be greater than 13 m. However, according to the gas drainage rules, it can be ob- main parts: inside and outside sealing pockets (two plugs), grouting pipe (one injection), tained that the working face gas drainage should last longer than 6 months, so the best and grouting pipe (one row); it is made with PVC-U special glue. After connecting the sealing length is 13 m. From the perspective of oxygen concentration, although the area suction tube with glue, the outer sealing bag and the inner sealing bag were tied in a fixed where the oxygen concentration is greater than 8% is the smallest when the sealing length is 15 m, considering that the sealing length is too long, it also increases the difficulty and cost of sealing and when the sealing length is 13 m, the area where the oxygen concentra- tion is greater than 8% is still small. Combining temperature changes and oxygen concen- tration changes, the optimal sealing hole length is 13 m, which can prevent the spontane- ous combustion of coal seams and ensure efficient gas drainage. The test was carried out on the lower edge of the 24,130 wind tunnel at a distance of 500 m from the cut hole. The test involved parameters of (sealing depth-sealing length) 15-13, 18-13, 20-8, 20-10, and 20-13. Concerning drilling hole parameters, each group of drilling spacing is 2 m, the hole diameter is 94 mm, and the drilling depth is 90 m. There were five holes in each group of boreholes. Each borehole in each group was connected to the branch pipe and then connected to the main pipe. Each group of boreholes had a mon- itoring hole to monitor the average gas of each group of boreholes. To extract the volume fraction, the average gas extraction volume fraction of each group of extraction boreholes was monitored once a day and recorded and the six groups of extraction boreholes were continuously monitored for 60 days. The data are plotted in Figure 11. Drilling cuttings per meter/(kg/m) Fire 2022, 5, 65 13 of 18 position so that the suction pipe passes through the outer and inner sealing bags and the grouting pipe passes through the outer sealing bag. The bag and the inner sealing bag are parallel to the axis of the extraction pipe; the grouting pipe is located in the pipeline inside the outer sealing bag and the inner sealing bag. A one-way valve is provided in the pipeline and the pipe wall. There is a grout outlet on the grouting pipe so that the grout flows into the outer and inner sealing bags after the grout passes through the one-way valve in the grouting pipe; when the upward drilling hole sealer is arranged, the adjustment row, the distance from the position of the slurry pipe to the inner sealing bag, is 5~10 cm and the position of the blasting valve to the mouth of the slurry pipe is 20~30 cm, as shown in Figure 12a; when the downward drilling and sealing device is arranged, the distance between the position of the discharge pipe and the outer sealing bag to 5~10 cm were Fire 2022, 5, x FOR PEER REVIEW 13 of 18 adjusted. The position of the blasting valve to the mouth of the discharge pipe is 20~30 cm, as shown in Figure 12b. The grouting process operates as shown in Figure 13. Sealing parameters(20-8) Fire 2022, 5, x FOR PEER REVIEW 14 of 18 Sealing parameters(20-10) Sealing parameters(15-13) Sealing parameters(20-13) 3.3.1. Development of “Two Plugs, One Injection and One Row” Device Sealing parameters(18-13) The “two plugs, one injection and one row” plugging device is composed of three main parts: inside and outside sealing pockets (two plugs), grouting pipe (one injection), and grouting pipe (one row); it is made with PVC-U special glue. After connecting the suction tube with glue, the outer sealing bag and the inner sealing bag were tied in a fixed position so that the suction pipe passes through the outer and inner sealing bags and the grouting pipe passes through the outer sealing bag. The bag and the inner sealing bag are parallel to the axis of the extraction pipe; the grouting pipe is located in the pipeline inside the outer sealing bag and the inner sealing bag. A one-way valve is provided in the pipe- line and the pipe wall. There is a grout outlet on the grouting pipe so that the grout flows into the outer and inner sealing bags after the grout passes through the one-way valve in the grouting pipe; when the upward drilling hole sealer is arranged, the adjustment row, the distance from the position of the slurry pipe to the inner sealing bag, is 5~10 cm and the position of the blasting valve to the mouth of the slurry pipe is 20~30 cm, as shown in Figure 12a; when the downward drilling and sealing device is arranged, the distance be- 0 1020 30 4050 60 tween the position of the discharge pipe and the outer sealing bag to 5~10 cm were ad- Gas drainage time/d justed. The position of the blasting valve to the mouth of the discharge pipe is 20~30 cm, Figure 11. Average volume fraction of gas extraction from boreholes. Figure as shown 11. A in Figur verage volume e 12b. Th fraction e groof uting pr gas extraction ocess op frer om ate bor s as s eholes. hown in Figure 13. Comparing the sealing parameters of 20-8, 20-10 and 20-13, the average gas extraction volume fraction of boreholes with a sealing parameter of 20-8 is lower and the average gas concentration is 41%; when the sealing parameter is 20-10, the average gas drainage 7 5 7 concentration accumulated within 50 days is 56.4% and when the drainage parameters are 20-13, the average gas drainage concentration reaches 70.8%. All comparative analysis shows that when the sealing depth is 20 m, the best gas drainage length of the sealing hole 3 7 is 13 m. From the perspective of gas drainage concentration, it can prevent spontaneous 7 7 combustion of the borehole. Comparing the three groups of holes with sealing parameters 15-13, 18-13, and 20-13, the average gas concentration of the drainage holes is 56%, 62.4%, and 70.8%, respectively, and the sealing length is the same, the deeper the sealing depth, 8 7 1 2 the higher the gas drainage concentration. When the 50 d plugging depth is 20 m, the (a) Upward gas drainage borehole (b) Downward gas drainage borehole average gas drainage concentration reaches 70.8%, indicating that the plugging effect is better and there is almost no air leakage. The experimental analysis of the drill cuttings Figure 12. Schematic diagram of structures of hole packers for upward boreholes. 1 Extraction pipe; Figure 12. Schematic diagram of structures of hole packers for upward boreholes. 1 Extraction pipe; method r 2 grouting esu pip lts e; in 3 the pe grouting ak stre pipe; ss o 4 m f 24, ine 1dri 30 wor lling;k 5ing extern facal e. The po blocking int is bag; 20 m 6 inner and blo the cking sealin bag; g7 2 grouting pipe; 3 grouting pipe; 4 mine drilling; 5 external blocking bag; 6 inner blocking bag; metal clamp; 8 one-way valve; 9 burst valve. depth is increased before the stress peak, which can improve the extraction effect. 7 metal clamp; 8 one-way valve; 9 burst valve. Based on the above analysis, the plugging depth is 20 m and the inner end of the plugging section of the plugging position is just at the peak stress point, which will not cause a blind area for drainage. When the plugging length is 13 m, which can prevent the natural fire of the coal seam and ensure the efficient gas drainage pick. Therefore, in order to increase the extraction efficiency and avoid the spontaneous combustion of the frac- tured coal around the borehole, comprehensive numerical simulation and field engineer- ing experiment analysis can determine the optimal sealing parameters of the coal seam at the 24,130 working face as 20-13. 3.3. New Prevention Technology of “Two Plugs, One Injection and One Row” The most commonly used downhole “two plugs and one injection” hole plugging device is used to observe the pressure with a grouting pressure gauge, which does not indicate that the borehole is filled with slurry. The gas in the borehole cannot be com- pletely discharged, and the slurry can easily gather around the borehole after solidifica- tion, causing the formation of gaps and gas segments. Figure 13. The entity of grouting. Gas drainage concentration/% Fire 2022, 5, x FOR PEER REVIEW 14 of 18 3.3.1. Development of “Two Plugs, One Injection and One Row” Device The “two plugs, one injection and one row” plugging device is composed of three main parts: inside and outside sealing pockets (two plugs), grouting pipe (one injection), and grouting pipe (one row); it is made with PVC-U special glue. After connecting the suction tube with glue, the outer sealing bag and the inner sealing bag were tied in a fixed position so that the suction pipe passes through the outer and inner sealing bags and the grouting pipe passes through the outer sealing bag. The bag and the inner sealing bag are parallel to the axis of the extraction pipe; the grouting pipe is located in the pipeline inside the outer sealing bag and the inner sealing bag. A one-way valve is provided in the pipe- line and the pipe wall. There is a grout outlet on the grouting pipe so that the grout flows into the outer and inner sealing bags after the grout passes through the one-way valve in the grouting pipe; when the upward drilling hole sealer is arranged, the adjustment row, the distance from the position of the slurry pipe to the inner sealing bag, is 5~10 cm and the position of the blasting valve to the mouth of the slurry pipe is 20~30 cm, as shown in Figure 12a; when the downward drilling and sealing device is arranged, the distance be- tween the position of the discharge pipe and the outer sealing bag to 5~10 cm were ad- justed. The position of the blasting valve to the mouth of the discharge pipe is 20~30 cm, as shown in Figure 12b. The grouting process operates as shown in Figure 13. 7 5 3 7 7 7 1 2 (a) Upward gas drainage borehole (b) Downward gas drainage borehole Figure 12. Schematic diagram of structures of hole packers for upward boreholes. 1 Extraction pipe; Fire 2022, 5, 65 14 of 18 2 grouting pipe; 3 grouting pipe; 4 mine drilling; 5 external blocking bag; 6 inner blocking bag; 7 metal clamp; 8 one-way valve; 9 burst valve. Fire 2022, 5, x FOR PEER REVIEW 15 of 18 3.3.2. Analysis of the Effect of the “Two Plugs, One Injection and One Row” Figure Figure 13. 13. The The entity entity of grouting. of grouting. Plugging Device 3.3.2. Analysis of the Effect of the “Two Plugs, One Injection and One Row” (1) Drilling and cutting analysis Plugging Device The simulated drilling adopts a seamless steel pipe (specification is 12 m/piece, 100 (1) Drilling and cutting analysis mm diameter), the extraction pipe adopts a polyvinyl chloride pipe (specification is 3 The simulated drilling adopts a seamless steel pipe (specification is 12 m/piece, m/piece, plug-in type, 50 mm bore diameter), and the hole sealer is designed to be 8 m 100 mm diameter), the extraction pipe adopts a polyvinyl chloride pipe (specification is long and 2 pcs. The length of the capsule band is 800 mm. Two sets of airtightness com- 3 m/piece, plug-in type, 50 mm bore diameter), and the hole sealer is designed to be 8 m long and 2 pcs. The length of the capsule band is 800 mm. Two sets of airtightness parison tests were designed, using “two plugs and one injection” for up- and downholes comparison tests were designed, using “two plugs and one injection” for up- and down- and “two plugs, one injection and one row” for up- and downholes. The cutting machine holes and “two plugs, one injection and one row” for up- and downholes. The cutting separately cut two simulated drill holes near the capsule belt near the exhaust pipe. machine separately cut two simulated drill holes near the capsule belt near the exhaust pipe. Whether the grouting is full is judged by whether the section is compact and complete, Whether the grouting is full is judged by whether the section is compact and complete, and and the cross-section the al vie cross-sectional ws are show views n iar n F e shown igures 1 in Figur 4 and es 15. 14 and 15. Outer capsule bag Inner capsule bag Figure 14. Borehole profile of boreholes for the “Two plugs and one injection” device. Figure 14. Borehole profile of boreholes for the “Two plugs and one injection” device. Inner capsule bag Outer capsule bag Figure 15. Borehole profile of boreholes for “Two plugs, one injection and one row”. Figure 14 shows the part of the traditional “two plugs and one injection” device that is not filled with grout. This part is in the shape of a three-quarter circle. The reason is partly because of cutting jitter and partly because the upward and downward drilling angles are 37°. Figure 15 shows a cross-sectional view of the “two plugs, one injection and one row” device. The bottom and upward borehole grouting part is dense, the grout is completely filled with the plugging section, and there is no air leakage channel, illustrat- ing that the grouting effect of the new process of “two plugs, one injection and one row” is obviously better than the traditional method of “two plugs and one bet”. (2) Analysis of engineering test results Fire 2022, 5, x FOR PEER REVIEW 15 of 18 3.3.2. Analysis of the Effect of the “Two Plugs, One Injection and One Row” Plugging Device (1) Drilling and cutting analysis The simulated drilling adopts a seamless steel pipe (specification is 12 m/piece, 100 mm diameter), the extraction pipe adopts a polyvinyl chloride pipe (specification is 3 m/piece, plug-in type, 50 mm bore diameter), and the hole sealer is designed to be 8 m long and 2 pcs. The length of the capsule band is 800 mm. Two sets of airtightness com- parison tests were designed, using “two plugs and one injection” for up- and downholes and “two plugs, one injection and one row” for up- and downholes. The cutting machine separately cut two simulated drill holes near the capsule belt near the exhaust pipe. Whether the grouting is full is judged by whether the section is compact and complete, and the cross-sectional views are shown in Figures 14 and 15. Outer capsule bag Inner capsule bag Fire 2022, 5, 65 15 of 18 Figure 14. Borehole profile of boreholes for the “Two plugs and one injection” device. Inner capsule bag Outer capsule bag Figure 15. Borehole profil Figure e of 15. boreholes Borehole profile for of “Two boreholes plug for s, “T on wo e injec plugs, tio one n and injection one row” and one.r ow”. Figure 14 shows the part of the traditional “two plugs and one injection” device that is Figure 14 shows the part of the traditional “two plugs and one injection” device that not filled with grout. This part is in the shape of a three-quarter circle. The reason is partly is not filled with grout. This part is in the shape of a three-quarter circle. The reason is because of cutting jitter and partly because the upward and downward drilling angles are 37 . Figure 15 shows a cross-sectional view of the “two plugs, one injection and one row” partly because of cutting jitter and partly because the upward and downward drilling device. The bottom and upward borehole grouting part is dense, the grout is completely angles are 37°. Figure 15 shows a cross-sectional view of the “two plugs, one injection and filled with the plugging section, and there is no air leakage channel, illustrating that the one row” device. The bottom and upward borehole grouting part is dense, the grout is grouting effect of the new process of “two plugs, one injection and one row” is obviously better than the traditional method of “two plugs and one bet”. completely filled with the plugging section, and there is no air leakage channel, illustrat- (2) Analysis of engineering test results ing that the grouting effect of the new process of “two plugs, one injection and one row” The lower side of 24,130 machine lane is constructed with 10 holes, the sealing depth is is obviously better than the traditional method of “two plugs and one bet”. 20 m, and the sealing length is 8 m. The downward hole was sealed along the layer and the (2) Analysis of engineering test results inclination angle was 25 with all adopting the technology of full screen mesh pipe. Hole 1–5# is grouted and sealed according to the new process of “two plugs, one injection and 0 0 one row” and hole 1 –5 # was sealed according to the existing two plugs and one injection. After the sealing was completed, 10 pairs of two boreholes were connected to the extraction pipe, and the gas concentration was measured. Figure 16 below shows the average gas concentration of two boreholes with different sealing methods in 90 days. Through the similar simulation cutting experiment of the uphole plugging device, it can be obtained that the traditional “two plugs and one injection” has a part that is not filled with grout. This part is in the shape of a three-quarter circle. The “two plugs, one injection and one row” borehole grouting is partly dense; the grouting effect of the new “two plugs, one injection and one row” process is significantly better than the traditional “two plugs and one injection” method”. Through underground engineering tests, it can be obtained that the average gas concentration of “two plugs, one injection and one row” within 90 days is 75%, and the gas concentration of traditional sealing technology is 55%, which is an average increase of 36.4% compared with traditional technology. Fire 2022, 5, x FOR PEER REVIEW 16 of 18 The lower side of 24,130 machine lane is constructed with 10 holes, the sealing depth is 20 m, and the sealing length is 8 m. The downward hole was sealed along the layer and the inclination angle was 25° with all adopting the technology of full screen mesh pipe. Hole 1–5# is grouted and sealed according to the new process of “two plugs, one injection and one row” and hole 1′–5′# was sealed according to the existing two plugs and one in- jection. After the sealing was completed, 10 pairs of two boreholes were connected to the extraction pipe, and the gas concentration was measured. Figure 16 below shows the av- Fire 2022, 5, 65 16 of 18 erage gas concentration of two boreholes with different sealing methods in 90 days. "Two plugs, one injection and one row" hole sealing "Two plugs and one injection" hole sealing 0 10 20 30 40 50 60 70 80 90 100 Gas drainage time/d Figure 16. Comparison of gas volume fraction of two sealing methods. Figure 16. Comparison of gas volume fraction of two sealing methods. 4. Conclusions Through the similar simulation cutting experiment of the uphole plugging device, it In view of the problem that it is difficult to control the spontaneous combustion of coal around the borehole induced by gas drainage, the judgment criteria for a dangerous can be obtained that the traditional “two plugs and one injection” has a part that is not area of spontaneous combustion of fractured coal around the borehole were theoretically filled with grout. This part is in the shape of a three-quarter circle. The “two plugs, one determined. The combined methods of experiment and simulation were presented in injection and one row” borehole grouting is partly dense; the grouting effect of the new this paper. The comprehensive prevention and control technologies of shotcrete, sealing “two plugs, one injection and one row” process is significantly better than the traditional parameters, and the “two plugs, one injection and one row” device were introduced, “t rwo esulting plug ins the anfollowing d one injec conclusions: tion” method”. Through underground engineering tests, it can (1) The critical oxygen concentration of coal spontaneous combustion was experimen- be obtained that the average gas concentration of “two plugs, one injection and one row” tally studied. A method of comprehensively dividing the spontaneous combustion risk within 90 days is 75%, and the gas concentration of traditional sealing technology is 55%, area of fractured coal around the borehole induced by gas drainage was proposed based which is an average increase of 36.4% compared with traditional technology. on the oxygen concentration and air leakage speed. The spontaneous combustion risk area was divided by the oxygen concentration of coal around the borehole at8% and the 4.air Co leakage nclusions speed of 0.004 m/s. Scientific division of “dangerous areas of drilling holes” can enhance the pertinence of fire-fighting measures, improve the effect of fire-fighting In view of the problem that it is difficult to control the spontaneous combustion of projects, and effectively prevent spontaneous fire accidents. Production has very important coal around the borehole induced by gas drainage, the judgment criteria for a dangerous practical significance. area o (2) f spon In view taneou of thes pr com oblem bus that tion it is of dif fra ficult ctuto recontr d coal a ol the round t spontaneous he bocombustion rehole weof re theoretically coal around the borehole induced by gas drainage, the optimal roadway shotcrete length determined. The combined methods of experiment and simulation were presented in this was determined to be 2–4 m using the numerical simulation method. The sealing hole depth paper. The comprehensive prevention and control technologies of shotcrete, sealing pa- of 20 m and length of 13 m were determined by combining theory and experiment, which rameters, and the “two plugs, one injection and one row” device were introduced, result- provides a scientific basis for preventing and controlling the spontaneous combustion of ing in the following conclusions: coal around the borehole. (3) On the basis of the “two plugs and one injection” process, the “two plugs, one (1) The critical oxygen concentration of coal spontaneous combustion was experi- injection and one row of hole sealers” method was developed. The simulated drilling mentally studied. A method of comprehensively dividing the spontaneous combustion section was cut. The new process has a compact structure in the pocket part. The traditional risk area of fractured coal around the borehole induced by gas drainage was proposed process has a three-quarter circular blank section. The gas concentration of the traditional based on the oxygen concentration and air leakage speed. The spontaneous combustion Gas drainage volume fraction/% Fire 2022, 5, 65 17 of 18 sealing process is 55% and the average gas concentration of the “two plugs, one injection and one row” method that accumulated within 90 days is 75%, which shows an average increase of 36.4% compared to the traditional process. The new “two plugs, one injection and one row” device is an effective method for improving gas drainage efficiency and preventing spontaneous combustion of gas drainage boreholes. Author Contributions: Conceptualization, J.L. and Y.Z.; Methodology, J.D.; software, J.D. and Y.Z.; validation, J.L., J.L. and J.D.; formal analysis, Y.Z.; investigation, Y.Z.; resources, Y.Z.; data curation, J.L.; writing—original draft preparation, J.L.; writing—review and editing, J.L. and Y.Z.; visualization, J.D. and Y.Z.; supervision, J.D. and Y.Z.; project administration, J.D., J.L. and Y.Z.; funding acquisition, J.L. and J.D. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the Fundamental Research Funds for the Central Universities of China (No.3142020020), China Coal Technology & Engineering Group Co., Ltd. (2019-2-ZD003), the National Natural Science Foundation Youth Project of China (grant number 51804161), the National Natural Science Foundation of China (grant number 52074156), the Langfang City Science and Technology Support Plan Project (grant number 2020011017), and the China Postdoctoral Science Foundation (2020M680490). Data Availability Statement: The data used to support the findings of this study are enclosed with the article. Acknowledgments: The authors would like to thank all the reviewers who participated in the review and Rodrigo Cabanero for linguistic assistance during the preparation of this manuscript. Conflicts of Interest: The authors declare no conflict of interest. References 1. Chen, L.; Chen, L.; Wu, S.; Guo, L.J. Study on the integrated planning of deep mining considering rock burst prediction. IOP Conf. Ser. Earth Environ. Sci. 2020, 570, 042047. [CrossRef] 2. Xie, H.P.; Zhou, H.W.; Xue, D.J.; Wang, H.W.; Zhang, R.; Gao, F. 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Journal

FireMultidisciplinary Digital Publishing Institute

Published: May 12, 2022

Keywords: coal spontaneous combustion; hazard zone; roadway shotcrete; optimization of sealing parameters; two plugs; one injection and one row

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