The Impact of the Coexistence of Methane Hazard and Rock-Bursts on the Safety of Works in Underground Hard Coal Mines

energies

Article The Impact of the Coexistence of Methane Hazard and Rock-Bursts on the Safety of Works in Underground Hard Coal Mines

Justyna Swolkie ´n* and Nikodem Szl ˛azak

Faculty of Mining and Geoengineering, AGH University of Science and Technology Kraków, 30-059 Kraków, Poland; [email protected] * Correspondence: [email protected]

Abstract: Several natural threats characterize hard coal mining in Poland. The coexistence of methane and rock-burst hazards lowers the safety level during exploration. The most dangerous are high- energy bumps, which might cause rock-burst. Additionally, created during exploitation, safety pillars, which protect openings, might be the reason for the formation of so-called gas traps. In this part, rock mass is usually not disturbed and methane in seams that form the safety pillars is not dangerous as long as they remain intact. Nevertheless, during a rock-burst, a sudden methane outflow can occur. Preventing the existing hazards increases mining costs, and employing inadequate measures threatens the employees’ lives and limbs. Using two longwalls as examples, the authors discuss the consequences of the two natural hazards’ coexistence. In the area of longwall H-4 in seam 409/4, a rock-burst caused a release of approximately 545,000 cubic meters of methane into the excavations, which tripled methane concentration compared to the values from the period preceding the burst. In the second longwall (IV in seam 703/1), a bump was followed by a rock-burst, which reduced the amount of air flowing through the excavation by 30 percent compared to the airflow before, and methane release rose by 60 percent. The analyses presented in this article justify that research is needed to create and implement innovative methods of methane drainage from coal seams to capture methane more effectively at the stage of mining.

Citation: Swolkień,J.; Szl ˛azak,N. The Keywords: methane hazard; rock-burst; the safety of exploration Impact of the Coexistence of Methane Hazard and Rock-Bursts on the Safety of Works in Underground Hard Coal Mines. Energies 2021, 14, 128. 1. Introduction https://doi.org/10.3390/en14010128 Extraction of coal in hard coal mines in Poland involves numerous natural and technical hazards [1,2]. When designing a new section’s exploitation, it is necessary to identify Received: 2 December 2020 the levels of natural hazards and apply the conclusions in the final design. The identi- Accepted: 24 December 2020 fication and prevention of natural hazards require state-of-the-art methods, technology, Published: 29 December 2020 instruments and machinery, as well as relevant expertise and know-how. Polish hard coal mines have to cope with adverse geological conditions and the Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional claims presence and coexistence of the following hazards [1,2]: in published maps and institutional • methane affiliations. • coal dust explosion • rock-bursts, cave-ins • fire • water Copyright: © 2020 by the authors. Li- • rock-and-gas outbursts censee MDPI, Basel, Switzerland. This • climate article is an open access article distributed under the terms and conditions of the Because mining in Poland reaches more profound levels every year, the degree of Creative Commons Attribution (CC BY) co-existing hazards rises considerably. At present, the average depth of exploitation is license (https://creativecommons.org/ 800 m below sea level, but in many mines it is even greater than 1000 m and probably will licenses/by/4.0/). increase in the years to come [2]. That means growth in methane release and higher virgin

Energies 2021, 14, 128. https://doi.org/10.3390/en14010128 https://www.mdpi.com/journal/energies Energies 2021, 14, 128 2 of 16

temperature of the rocks, which causes the climate conditions for deterioration and the endogenous fire hazard to rise. The presence of all these factors combined can lower the safety level of working underground. The considerable depth of exploitation of coal seams contributes to the increase in the scale of natural hazards. With increasing depth, the methane content in the coal seams increases. On the other hand, decreasing the rock-mass permeability with the rise of the stress state causes a reduction of methane drainage efficiency. The exploitation of coal seams at these depths increases the risk of bumps and rock-bursts. Improperly selected methods of preventing these threats may contribute to the sudden release of methane and the danger of miners’ death. Some of the most severe threats existing in Polish mines are methane and rock- burst hazards [1,2]. Using inadequate prevention methods can, as a consequence, lead to casualties. The present article highlights the implications of their coexistence and draws attention to the necessity of implementing appropriate preventive measures. In the beginning, the authors focus on describing the very nature of methane hazard and its prevention; in subsequent sections, they show the effects of the coexistence of the methane and rock-burst hazards, using longwalls in two Polish mines as examples. The description of the presented events was the part of the scientific report carried out under the supervision of Nikodem Szl ˛azak,co-author of the article, and commissioned by the Higher Mining Institute in Katowice and the Mining Commission for the Study of the Disaster Causes [3].

2. Sources of Methane Origin and Methods of Methane Hazard Prevention In the Upper Silesian Coal Basin’s coal measures—considering its area and hitherto identified layers—the presence of methane varies, and its distribution is very uneven [4,5]. In the northern and central parts of the Basin, methane is virtually non-existent or very scarce. On the other hand, its southern part has a very high methane content [4,5]. For example, this situation exists in Brzeszcze and Silesia mining plants suited in its eastern part, and in the western region, the Rybnik Coal Area is the most methane prone. The presence of methane does not correlate with specific stratigraphic layers [4,5]. It exists at all levels except for the Libi ˛az˙ layers. The same layers contain methane in some areas while being free from it elsewhere. There is no close correlation between the degree of carbonization and the methane content in the carboniferous [4,6]. In some mines, heavily metamorphosed coal contains minimal amounts of methane. On the other hand, much less metamorphosed coal in, e.g., the Silesia mining plant is characterized by high methane content. An essential factor for methane content in coal is the type of overburden’s thickness, but this is not the only condition of methane’s presence in larger amounts. The research conducted so far makes it possible to conclude that methane found in coal deposits exists in two primary forms [7]: • sorbed methane linked through its physical–chemical properties with coal substance; • free methane found in the pores and fractures in the barren rock and coal seams. Nevertheless, a close connection between methane content and tectonics exists as dislocations are clear boundaries separating blocks with different methane concentration degrees. Studies into the methane content of the rock mass, conducted in the prospecting holes in the south-western part of the Basin, confirmed the existence of a zone with high methane content in the roof of the carboniferous formation above the overburden [8]. Figure1 presents methane content changes, depending on the depth below the carboniferous roof [8]. The high-methane-content zone’s thickness is approximately 200 m, with the methane 3 3 content often exceeding 10 m CH4/Mg daf. Next, methane content sinks from 2 to 5 m 3 CH4/Mg daf, only to rise above 11 m CH4/Mg daf at a depth of 700–900 m. Below 1000 m, free nitrogen and helium occur. The first peak of methane content corresponds to the presence of the impermeable overburden at a depth of 150–200 m below the carboniferous roof; it is caused by a high degree of metamorphism in the carboniferous formation Energies 2021, 14, x FOR PEER REVIEW 3 of 16

Energies 2021, 14, 128 m, free nitrogen and helium occur. The first peak of methane content corresponds to3 the of 16 presence of the impermeable overburden at a depth of 150–200 m below the carboniferous roof; it is caused by a high degree of metamorphism in the carboniferous formation at a at a considerable depth, which had the following consequences: A greater degree of considerable depth, which had the following consequences: A greater degree of carboni- carboniﬁcation and a higher reduction in the amount of volatile matter, an increased amount fication and a higher reduction in the amount of volatile matter, an increased amount of of thermogenic methane in coal, reduced coal hardness and coal’s sorption capacity. These thermogenic methane in coal, reduced coal hardness and coal’s sorption capacity. These factors, combined with a large methane content and reduced coal hardness, contribute factors, combined with a large methane content and reduced coal hardness, contribute to to the emergence of a high-pressure gradient within the so-called gas trap. That causes the emergence of a high-pressure gradient within the so-called gas trap. That causes me- methane release into the excavations to grow, which may be the reason for methane and thane release into the excavations to grow, which may be the reason for methane and rock rock outbursts. outbursts.

Figure 1. The averaged changes in methane content g, moisture content Wc and volatile parts Vdaf Figure 1. The averaged changes in methane content g, moisture content W and volatile parts Vdaf in in the coal deposits of the Rybnik Coal Basin. Adapted from Tarnowski J. 1971c [8]. the coal deposits of the Rybnik Coal Basin. Adapted from Tarnowski J. 1971 [8].

FigureFigure 22 presents thethe distributiondistribution ofof methane methane content content according according to to depth depth in thein the D˛e- Dbieńskodepositębieńsko deposit [ 9[9]].. What Whatfollows follows isis thatthat coal extractionextraction from from seams seams with with high high methane methane concentrationconcentration calls calls for for implementing implementing accurate accurate measures measures to lower to lower methane methane release release into ex- into cavations.excavations. The Theproper proper assessment assessment of methane of methane hazards, hazards, forecasts, forecasts, monitoring, monitoring, hazard hazard con- trolcontrol and implemented and implemented preventive preventive measures measures is crucial is crucial for the for safety the safety of exploitation of exploitation in that in typethat of type seams. of seams. MethaneMethane hazard hazard prevention prevention involves involves both both procedures procedures for for identifying identifying and and controlling controlling methanemethane hazards hazards and and the the methods methods of of removing removing explosive explosive accumulations accumulations of of methane methane from from excavations.excavations. In In hard hard coal coal mines, mines, the the prev prevalentalent methods methods implemented implemented to to stop stop methane methane hazardhazard are are as as follows follows [1,10] [1,10:]: • • sufficientsufficient ventilation ventilation letslets avoidingavoiding the the emergence emergence of methane of methane ‘fuses’ ‘fuses’ or local or accumu- local accumulationslations of methane of methane in excavations in excavations ventilated ventilated by airflows by airflows generated generated by the main by the fans mainand infans workings and in workings ventilated ventilated using a separate using a ventilationseparate ventilation system; system; • • methanemethane drainage drainage fromfrom coalcoal seams seams through through drainage drainage boreholes, boreholes, drilled drilled from under-from undergroundground excavations excavations or the or surface; the surface; • • methanometricmethanometric systems systems to to control control methane methane co concentrationncentration in in mine mine air, air, using using sensors sensors deployeddeployed in in the the different different types types of of workings workings following following the the applicable applicable regulations; regulations; • • supplementarysupplementary ventilation ventilation devic deviceses installed installed in in places places with with limited limited ventilation ventilation and and locallocal accumulations accumulations of of methane. methane. Methane drainage from the rock mass is the most effective method of minimizing methane hazard, reducing methane release into working spaces and eliminating or minimizing the occurrence of such events as outflows, sudden outbursts of methane and coal [11–23]. The procedure that has proved to be the most efficient is draining methane from the rock mass and from waste areas secured with stoppings and transporting it to

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Methane drainage from the rock mass is the most effective method of minimizing methane hazard, reducing methane release into working spaces and eliminating or minimizing the occurrence of such events as outflows, sudden outbursts of methane and coal [11–23]. The procedure that has proved to be the most efficient is draining methane from Energies 2021, 14, 128 the rock mass and from waste areas secured with stoppings and transporting it to4 ofthe 16 surface, using the depression caused by pumping in a methane drainage station. This method helps maintain sufficient ventilation parameters and involves specific require- mentsthe surface, regarding using the the choice depression of methods caused for by pumpingdeveloping in acoal methane seams drainagecontaining station. methane. This Polishmethod coal helps mines maintain use pre-extraction sufficient ventilation methane parameters drainage only and sporadically involves specific (if at requirements all) because theregarding low permeability the choice of of coals methods means for that developing this method coal is seams inefficient containing [22,24–26] methane.. Polish coal mines use pre-extraction methane drainage only sporadically (if at all) because the low permeability of coals means that this method is inefficient [22,24–26].

Borehole Dębieńsko Głębokie 8 Borehole Szczygłowice 15 (b) (a)

Figure 2. The distribution of methane content according to depth within a selected area: (a) Description for borehole Szczygłowice borehole; (b) description for borehole D˛ebi´nskoGł˛ebokie8 [9]. Figure 2. The distribution of methane content according to depth within a selected area: (a) Description for borehole Szczygłowice borehole; (b) description for borehole Dębińsko Głębokie 8 [9]. 3. Examples of Overlapping of Methane and Outburst Hazard 3.3.1. Examples Research of Objectives Overlapping of Methane and Outburst Hazard 3.1. ResearchThe methane Objectives content in a seam decreases during extraction as methane is drained and releasedThe methane into workings, content where in a seam ventilation decreases air dilutes during it extraction to lower methaneas methane concentration. is drained andSpecially released constructed into workings, pillars where protect, ventilation active for anair extendeddilutes it period,to lower mining methane excavations concentra- in tion.seams Specially with a highconstructed methane pillars content. protect, In such active conditions, for an extended methane period, in seams mining that form excava- the tionssafety in pillars seams is with not dangerous a high methane as long content. as they remainIn such intact. conditions, A bump methane or a rock-burst in seams in that this formarea canthe releasesafety pillars a large is amount not dangerous of methane as long into as the they working, remain removing intact. A oxygenbump or from a rock- the burstmine in atmosphere this area can and release threatening a large the amount personnel of methane deployed into there. the Inworking, such cases, removing the lives oxy- of genthe minersfrom the working mine atmosphere in these excavations and threatening are at risk the not personnel only because deployed of a possible there. In sudden such energy discharge that deforms or destroys the working; another threat is the released cases, the lives of the miners working in these excavations are at risk not only because of methane, which displaces all oxygen from the excavation. a possible sudden energy discharge that deforms or destroys the working; another threat Here we describe the examples of such occurrences. They refer to actual accidents is the released methane, which displaces all oxygen from the excavation. recorded in longwall H-4 in seam 409/4 in mine X and longwall IV in seam 703/1 in mine Y, where bumps of energy 1.9·108 J and 9.8·107 J respectively, caused the rock-burst and sudden methane outﬂow in both of them.

3.2. The Methodology of the Research According to Polish mining regulations [27], all coal mines are equipped with the SMP- NT/A environmental parameters monitoring system with CMC-3MS telemetry centers Energies 2021, 14, 128 5 of 16

and an integrated CST security system with CST-40A telemetry exchanges. Automatic methane measurement protects all longwalls and faces supplied with electricity following the requirements of applicable regulations and as agreed by the Ventilation Department’s Head. The SMP-NT/A methane sensors with continuous recording protect all ongoing faces and longwalls and are powered directly from telemetry centers or, if possible, using in-house central units. The monitoring system is usually equipped with sensors to measure methane concentration, carbon monoxide, oxygen, air velocity, temperature, pressure and more. An intrinsically secure telecommunications network allows sending data to and from the sensors to the telemetry centers. Diagrams of the longwalls’ gasometric system, and the arrangement of CH4 and CO sensors, are presented in Figures3 and4. Additionally, ﬁgures show the analyzed sensors marked with a blue rectangle. Based on the analysis of the methane sensors indications for the longwalls mentioned earlier, it was possible to assess the consequences of the coexistence of methane and outburst hazard. The observations included sensors: 1. For the longwall H-4 in seam 409/4: Sensor MM187-RW-placed in the Drift H-2 to seam 409/3 in the distance of 10–15 m before the crosscut with the Transport Glade H-2 in seam 409/1 and 409/2 (Figure3); Sensor MM 123-RW placed in the Drift H-2 to seam 409/3 in the distance of 10–15 m before the crosscut with the Transport Glade H-2 in seam 409/1 and 409/2 (Figure3). 2. For the longwall IV in seam 703/1:

Sensor G of CH4 (1.5%) placed in the Researcher roadway 3a-E-E1 in seam 703/1 at the height of the longwall window (Figure4); Sensor M of CH4 (1.5%) placed in the Researcher roadway 3a-E-E1 in seam 703/1 5 m ahead of a transformer (Figure4). The description of the events that occurred in longwalls H-4 in seam 409/4 and IV in seam 703/1 was the part of the scientiﬁc report commissioned by the Higher Mining Institute in Katowice and the Mining Commission for the Study of the Disaster Causes [3]. Energies 2021Energies, 14, 128 2021, 14, x FOR PEER REVIEW 6 of 16 6 of 16

FigureFigure 3. Diagram3. Diagram of of the the gasometric gasometric system system and and ventilation ventilation in thein the area area of longwallof longwall H-4 H-4 in seamin seam 409/4. 409/4.

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FigureFigure 4. 4. TheThe arrangement arrangement of of the the sensors sensors of of the the meth methanometricanometric system system in in longwall longwall IV IV seam seam 703/. 703/.

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BeforeBefore the the event, event, in in the the H-4 H-4 longwall longwall area area in in seam seam 409/4, 409/4, the the coal coal mine mine carried carried out preparatoryout preparatory works works in three in longwall three longwall fronts; fronts; two in two the Tailgatein the Tailgate Road H-4 Road (two H-4 longwall (two faceslongwall led on faces compaction), led on compaction), and one in the and Tailgate one in Roadthe Tailgate H-2. Additionally, Road H-2. Additionally, in this area, the F-Hin Ventilation this area, the drift F-H was Ventilation carried out. drift Figure was 3carried presents out. a diagramFigure 3 presents of the H-4 a diagram longwall’s ventilationof the H-4 area longwall’s along with ventilation the placement area along of gasometric with the placement system sensors. of gasometric Additionally, sys- Figuretem5 sensors. shows the Additionally, map of the Figure developed 5 shows longwall the map H-4. of the developed longwall H-4.

FigureFigure 5. The5. The map map of of the the developed developed longwall longwall H-4 H-4 in seamin seam 409/4. 409/4.

4. Results and Discussion 4. Results and Discussion 4.1.4.1. The The Area Area of of Longwall Longwall H-4 H-4 in in Seam Seam 409/4 409/4 in in Mine Mine X X RealizationRealization of of the the preparatorypreparatory worksworks in the the deposit deposit H H required required air air from from the the Belt Belt drift 3 3 driftand and the theCircular Circular drift drift F, both F, both on the on thelevel level 900 900m, of m, 1570 of 1570 m /min m3/min and 1020 and 1020m /min, m3/min, respec- respectively.tively. After After airing airing the workings, the workings, the air the was air discharged was discharged via the via H-2 the Ventilation H-2 Ventilation ramp in 3 rampseam in 409/3-4, seam 409/3-4, to the toCircular the Circular and Belt and drift Belt driftF on Fthe on level the level 900 900m (1070 m (1070 m /min m3/min and and 1520 3 1520m /min, m3/min, respectively). respectively). TheThe part part H H in in the the seam seam 409/4 409/4 lays lays in in the the central central and and southern southern parts parts of of the the “Jastrz˛ebie “Jastrzębie GóGórnerne I” I” mining mining area, area, in thein the monoclonal monoclonal part part of the of deposit,the deposit, at a depth at a depth of ~815 of~815 m to ~990m to~990 m. Itm. was It accessiblewas accessible from from the souththe south through through the the Circular Circular and and Belt Belt drift drift F, bothF, both on on the the level level 900900 m m and and north north and and south south through through the the Transport Transport Glade Glade H-2 H-2 and and Drift Drift H-2, H-2, also also on 900 on m.900 m. There is an “eastern” fault h~15–45 m with the N-S course in the eastern border of the analyzedThere section, is an while“eastern” the westernfault h~15–45 edge ofm it with is a “Jastrz˛ebie”faultthe N-S course inh~25–30 the eastern m with border the of samethe analyzed course. Parallel section, faults while accompany the western these edge disturbances of it is a “Jastrz withę dischargesbie” fault h~25–30 up to h >m 5 with m. Athe standard same course. fault h~5–1 Parallel m with faults a path accompany similar tothes thee disturbances W-E direction with runs discharges through the up central to h > 5 partm. ofA thestandard section fault H and h~5–1 at a m distance with a of path ~30–75 similar m from to the the W-E “eastern” direction fault runs in the through eastern the region,central the part fault of h~0.5–2.5the section m H runs and parallel.at a distance In the of southern ~30–75 m part, from on the the “eastern” other hand, fault there in the areeastern assumed region, reverse the fault faults h~0.5–2.5 h~2.5–5 m.m runs In its parall borderel. parts,In the theresouthern are latitudinalpart, on the faults other from hand, h~6there m toare h~30 assumed m. reverse faults h~2.5–5 m. In its border parts, there are latitudinal faults fromBeforehand, h~6 m to h~30 the coalm. mine carried exploitation in three longwalls in the upper seam 409/3 transverselyBeforehand, tothe the coal direction mine carried of the deposit’sexploitati extent.on in three longwalls in the upper seam 409/3The transversely data from the to automaticthe direction methane of the deposit’s measurement extent. system’s sensors and the airﬂow balanceThe allowed data from calculating the automatic the section methane H area’s measurement absolute system’s methane sensors bearing and capacity the airflow [3]. 3 Beforebalance the allowed event, methane calculating emission the section was equal H area’s to 10.36 absolute m CH methane4/min, amongbearing which capacity faces [3]. Before the event, methane emission was equal to 10.36 m3 CH4/min, among which faces of

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3 3 of the Road H-4 and H-10 generated 6.00 m CH4/min. The remaining 4.36 m CH4/min came from the Tailgate Road H-2. The calculated values indicate that the discharge was much lower than it resulted from the absolute methane bearing capacity forecasts made for these workings. The area in which the incident took place showed previous seismic activity. From 4 April 2018 until a rock-burst on 5 May 2018, the total number of bumps in section H was 134. Most of them produced energy of approximately 102–103 J. Only six of them had energy approximating 104 J. Table1 shows the effect of the bumps on methane release into excavations during the period preceding the accident under analyzes.

Table 1. The effect of the bumps on methane release into excavations in section H [3].

A Bump in the Area in Section H with an Energy Equal to or Higher Excavation 1 × 104 J 25.04.2018 30.04.2018 01.05.2018 04.05.2018 Roadway H-10 seam none none none increase by 0.1% 409/3-409/4 increase by Tailgate Road H-4 none increase by 0.1% 0.1% increase by 0.1% increase by 1.0% (behind the dust increase by Maingate Road H-4 none collector) none 0.1–0.2% increase to 1.7% (the sensor in the face area) Ventilation drift F-H increase by none none increase by 0.5% level 900 0.1% Drift H-2 none none none none to the seam 409/3

The table data shows that section H’s bumps did not significantly increase the workings’ methane concentration. It did not exceed 0.5%, except for the H-4 Maingate Road on 1 May 2018; the methane concentration behind the dust collector increased by 1.0%, and the sensor in the face area showed 1.7% of methane. Taking the above into account, bumps with energy up to 104 J or higher do not pose a potential threat of rock-burst and a sudden outflow of methane. The low energy bumps do not cause sudden desorption of methane into the excavation. Therefore, one can conclude that the applied rock-burst prevention in such events was sufficient. On 5 May 2018 at 10:58:00, a bump with 1.9 × 108 J’s energy occurred, causing a rock-burst in section H, releasing a large amount of methane into the excavations [3]. The bump was so strong that people staying in the buildings located above the ground felt it. The burst epicenter’s localization was in the area of Roadway H-10 in seams 409/3 and 409/4, marked with a purple dot in Figure6. The consequence of the rock-burst was the destruction of the roadway. Energies 2021, 14, 128 10 of 16

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Figure 6. The map of seam 409/3 in the area of longwall H-4 with indicated spots where bumps Figure 6. The map of seam 409/3 in the area of longwall H-4 with indicated spots where bumps occurred with an energy of 106–108 J. occurred with an energy of 106–108 J.

FigureFigure7 presents 7 presents the the distribution distribution of methaneof methane content content in seamin seam 409/4 409/4 in sectionin section H, H, clearlyclearly showing showing that that methane methane content content in thein the excavation excavation of longwallof longwall H-4 H-4 was was significantly significantly lowerlower than than along along the the Roadway Roadway H-10 H-10 and and VentilationVentilation rampramp H-2H-2 of seam seam 409/3-409/4. 409/3-409/4. The Thelower lower methane methane content content measured measured in inseam seam 409/4 409/4 during during the thecreation creation of development of development work- workingsings for for longwall longwall H-4 H-4 resulted resulted from from the the degasification degasification of of seam seam 409/4 409/4 through through earlier earlier ex- extractiontraction inin seam seam 409/3. 409/3. InIn thethearea area of of Roadway Roadway H-10, H-10, the the methane methane content content approximated approximated 8 3m3 CH4/Mg daf, whereas, in the longwall H-4, it varied between 0.440 to 4.0073 m3 8 m CH4/Mg daf, whereas, in the longwall H-4, it varied between 0.440 to 4.007 m CH4/Mg daf. Creating a safety pillar protecting the openings into a section of the deposit CH4/Mg daf. Creating a safety pillar protecting the openings into a section of the deposit causedcaused the the high high methane methane content content in seamsin seams 409/3 409/3 and and 409/4 409/4 (Figure (Figure5). 5). In thisIn this part, part, rock rock massmass was was not not disturbed disturbed and and the the coal coal mine mine did did not not proceed proceed with with drainage. drainage. TheThe bumps bumps of highof high energy energy usually usually cause cause the the rock rock mass mass disturbance, disturbance, like like in longwallin longwall H-4H-4 and and a suddena sudden large large emission emission ofof methane. For For that that reason, reason, they they can can pose pose a severe a severe threat threatto the to theworking working crews’ crews’ health health and and safety. safety. Due Due to tothis, this, coal coal mines mines should should take take steps steps to to in- increasecrease rock-burstrock-burst prevention. prevention. AfterAfter a bump a bump occurred occurred on on 5 May 5 May 2018, 2018, sensors sensors MM-123 MM-123 and and MM-187 MM-187 recorded recorded high high methanemethane concentration concentration levels levels (Figure (Figure3). Of3). allOf theall the analyzed analyzed sensors sensors of the of the methanometric methanometric systemsystem in sectionin section H, H, those those two two survived. survived. They They were were installed installed in Driftin Drift H-2 H-2 leading leading to seamto seam 409/3,409/3, 10 10 to to 15 15 m m ahead ahead of aof crossing a crossing with with Transport Transport Glade Glade H-2 H-2 in seams in seams 409/1 409/1 and and 409/2 409/2 (Figure(Figure3). 3). AsAs a consequence a consequence of theof the bump bump described described above, above, multiple multiple accidents accidents occurred occurred in whichin which sevenseven employees employees suffered; suffered; five five of themof them lost lost their their lives, lives, while while the the other other two two suffered suffered minor minor injuries.injuries. Additionally, Additionally, methane methane release release of suchof such magnitude magnitude followed followed the the accident accident on on 5 May 5 May 20182018 delayed delayed the the rescue rescue operation. operation.

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FigureFigure 7. 7. Isolines Isolines of of methane methane content content in in the the areaar areaea of of longwall longwall H-4 H-4 inin in seamseam seam 409/4409/4 409/4 [3]. [[3].3].

FigureFigure8 8 8presents presents presents thethe the MM-123MM-123 MM-123 andand and MM-187MM-187 MM-187 sensors’sensors’ sensors’ concentrationsconcentrations concentrations after after after the the the accident accident accident (marked(marked in inin Figure FigureFigure 33) 3))[ [3] [3]3.]. .The The The results results results presented presented presented in in the inthe thefigure figure ﬁgure show show show that that thatit it increased increased it increased to to over over to over60%60% on 60%on the the on bump bump the bump day. day. Then Then day. it Thenit dropped dropped it dropped to to around around to around 10% 10% within within 10% withinthethe next next the two two next days. days. two On On days. 7, 7, 9 9 Onandand 7,12 12 9 May, May, and the 12the May,MM-187 MM-187 the sensor MM-187sensor showed showed sensor a a showedmethane methane aconcentration concentration methane concentration above above 30%. 30%. aboveConcentra- Concentra- 30%. Concentration on the MM-123 sensor rose to 22% on 7 May, and then fell below 10% and tiontion on on the the MM-123 MM-123 sensor sensor rose rose to to 22% 22% on on 7 7 May, May, and and then then fell fell below below 10% 10% and and remained remained remained at this level until 9 May. After that date, concentration on both sensors, except atat this this level level until until 9 9 May. May. After After that that date, date, conc concentrationentration on on both both sensors, sensors, except except the the 12 12 May May the 12 May (sensor MM-187), remained below 5%. (sensor(sensor MM-187), MM-187), remained remained below below 5%. 5%.

Figure 8. The values of CH4 concentration recorded by sensors MM-123 and MM-187 (5–15 May Figure 8. The values of CH4 concentration recorded by sensors MM-123 and MM-187 (5–15 May Figure 8. The values of CH concentration recorded by sensors MM-123 and MM-187 (5–15 May 2018)—both2018)—both sensors sensors marked marked4 with with rectangles rectangles in in Figure Figure 3. 3. 2018)—both sensors marked with rectangles in Figure3. Based on an analysis of the sensors’ values, it is possible to evaluate methane release BasedBased on on an an analysis analysis of of the the sensors’ sensors’ values, values, it it is is possible possible to to evaluate evaluate methane methane release release fromfrom section section H, H, in in which which the the bump bump and and the the resulting resulting rock-burst rock-burst occurred occurred on on 5 5 May May 2018 2018 from section H, in which the bump and the resulting rock-burst occurred3 on 5 May 2018 at at 10:58. The analysis reveals releasing approximately 545,000 m 3of methane into excava- 10:58.at 10:58. The The analysis analysis reveals reveals releasing releasing approximately approximately 545,000 545,000 m3 of m methane of methane into excavations.into excavations.tions. Thus, Thus, the the average average methane methane release release into into excavations excavations from from section section H H was was tripled tripled com- com- Thus,pared theto the average period methane preceding release the burst into excavations [3], which caused from section the accident H was tripleddiscussed compared here to topared the period to the precedingperiod preceding the burst the [3 ],burst which [3] caused, which thecaused accident the accident discussed discussed here to occur. here to occur.occur.

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Energies 2021, 14, x FOR PEER REVI 12 of 16 4.2. The Area of Longwall IV in Seam 703/1 in Coal Mine Y 4.2. The Area of Longwall IV in Seam 703/1 in Coal Mine Y The development of longwall IV in the seam 703/1 was being proceeded in the Fourth The development of longwall IV in the seam 703/1 was being proceeded in the Fourth Category conditions of methane and the Second Degree of rock-burst hazard. A bump Category conditions of methane and the Second Degree of rock-burst hazard. A bump occurred on 22 January 2019 at 23:35:41 with an energy of 9.8 × 107 J. It resulted in a occurred on 22 January 2019 at 23:35:41 with an energy of 9.8 × 107 J. It resulted in a rock- rock-burst in the area of crossing a longwall face with a ventilation heading. Figure9 burst in the area of crossing a longwall face with a ventilation heading. Figure 9 presents presents the map of the seam with the highlighted longwall IV with the greenish color. the map of the seam with the highlighted longwall IV with the greenish color. Figure 4, Figure4, on the other hand, shows the arrangement of the methanometric system’s sensors on the other hand, shows the arrangement of the methanometric system’s sensors in the in the workings at longwall IV. Additionally, the ﬁgure shows the analyzed sensors marked workings at longwall IV. Additionally, the figure shows the analyzed sensors marked with a blue rectangle. with a blue rectangle.

Figure 9. Seam 703/1 with longwall IV. Figure 9. Seam 703/1 with longwall IV.

TheThe datadata from thethe automatic methanemethane measurementmeasurement system’ssystem’s sensorssensors andand thethe airflowairflow balancebalance allowedallowed calculatingcalculating the the section section IV IV area’s area’s absolute absolute methane methane bearing bearing capacity. capacity. Before Be- fore the event, it was equal to 14.473 m3 CH4/min, among which the drainage system cap- the event, it was equal to 14.47 m CH4/min, among which the drainage system captured 3 3 4 3 3 4 3.9tured m 3.9CH m4/min. CH /min. The remainingThe remaining 10.57 10.57 m CH m4 /minCH /min was was released released to the to excavation.the excavation. TheThe burstburst causedcaused thethe longwall’slongwall’s finalfinal sectionsection andand ventilationventilation heading’sheading’s deformationdeformation and,and, inin thethe end,end, methanemethane releaserelease intointo thethe excavation.excavation. TheThe accidentaccident provedproved fatalfatal toto oneone employee,employee, working in thethe crossingcrossing areaarea ofof thethe longwalllongwall andand thethe roadway.roadway. TheThe airflowairflow throughthrough thethe longwall longwall excavation excavation was was reduced reduce tod 30%to 30% compared compared to the to one the before one before the burst. the burst.Changes in methane concentration on the sensor in the longwall and roadway pre- sentedChanges in Figure in 10methane clearly concentration show what happened on the sensor with in the the methane longwall concentration and roadway at pre- the momentsented in of Figure the accident. 10 clearly Sensor show G (Figureswhat ha4ppened and 10 )with recorded the methane a sharp increaseconcentration in methane at the concentrationmoment of the from accident. below Sensor 0.4% G to (Figures 5% at the 4 and incident. 10) recorded Next, the a sharp concentration increase in fluctuated methane betweenconcentration 2.2% andfrom 4.3%, below then 0.4% it beganto 5% toat drop,the incident. reaching Next, a value the belowconcentration 1.5% around fluctuated 2:00 a.m.between Between 2.2% 4:00and 4.3%, a.m. andthen 8:00 it began a.m. to on drop, 23 January, reaching methane a value concentration below 1.5% around fluctuated 2:00 wildly,a.m. Between from 1% 4:00 to 1.8%. a.m. Itand was 8: not00 a.m. until on 8:00 23 a.m. January, that concentration methane concentration dropped significantly fluctuated belowwildly, 1.5%. from 1% to 1.8%. It was not until 8:00 a.m. that concentration dropped significantly below 1.5%.

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Energies 2021, 14, x FOR PEER REVI 13 of 16 Energies 2021, 14, x FOR PEER REVI 13 of 16

Figure 10. A change in methane concentration after the burst recorded by a CH4 (G) sensor in- Figure 10. A change in methane concentration after the burst recorded by a CH4 (G) sensor installed Figurestalled 10. in Athe change Researcher in methane roadway concentration 3a-E-E1 in seamafter the703/1 burst at the recorded height byof thea CH longwall4 (G) sensor window inin the Researcher roadway 3a-E-E1 in seam 703/1 at the height of the longwall window (sensor G in stalled(sensor in G the in ResearcherFigure 4) [3]. roadway 3a-E-E1 in seam 703/1 at the height of the longwall window Figure(sensor4)[ G3]. in Figure 4) [3]. Additionally,Additionally, in in Figure Figure 11 11,, we we see see the the methane methane concentration concentration changes changes recorded recorded by by Additionally, in Figure 11, we see the methane concentration changes recorded by sensorsensor M M (Figures (Figures4 and 4 and 11 )11) in thein the Research Research roadway roadway 5 m 5 aheadm ahead of aof transformer. a transformer. The The sensor M (Figures 4 and 11) in the Research roadway 5 m ahead of a transformer. The highesthighest recorded recorded value value was was 7%. 7%. After a sharp sharp increase increase at at the the time time of of the the accident, accident, the the con- highest recorded value was 7%. After a sharp increase at the time of the accident, the con- concentrationcentration began began to to drop drop sharply, sharply, reaching reaching 2.6% 2.6% at at 4:00 4:00 a.m. a.m. From From now now on, on, it itstarted started de- centration began to drop sharply, reaching 2.6% at 4:00 a.m. From now on, it started de- decreasingcreasing more more slowly slowly to the value of 1.8% by 10:00 a.m. creasing more slowly to the value of 1.8% by 10:00 a.m.3 3 TheThe burst burst caused caused an an additional additional outﬂow outflow of of 4500 4500 m mCH CH4 within4 within 12 12 h, h, which which means means The burst caused an additional outflow of 4500 m3 CH4 within 12 h, which means thatthat methane methane release release increased increased by by 60% 60% compared compared to to the the period period before before [3]. [3]. that methane release increased by 60% compared to the period before [3].

Figure 11. A change in methane concentration after the burst recorded by a CH4 (M) sensor in- Figure 11. A change in methane concentration after the burst recorded by a CH4 (M) sensor in- Figurestalled 11. inA Researcher change in methaneroadway concentration 3a-E-E1 in seam after 703/1 theburst 5 m ahead recorded of a by transformer a CH4 (M) (sensor sensor installedM in Fig- stalled in Researcher roadway 3a-E-E1 in seam 703/1 5 m ahead of a transformer (sensor M in Fig- inure Researcher 4) [3]. roadway 3a-E-E1 in seam 703/1 5 m ahead of a transformer (sensor M in Figure4)[ 3]. ure 4) [3]. BothBoth accidents accidents analyzed analyzed here here involved involved a bumpa bump and and a resultinga resulting rock-burst, rock-burst, causing causing a a massivemassiveBoth outﬂow outflowaccidents of of methane analyzed methane into here into a involvedlongwalla longwall a excavation. bump excavation. and Thea resultingThe discharge, discharge, rock-burst, combined combined causing with with a massiveairflow reduction,outflow of ledmethane to the intoremoval a longwall of oxygen excavation. from mine The air. discharge, Of particular combined significance with airflow reduction, led to the removal of oxygen from mine air. Of particular significance

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airflow reduction, led to the removal of oxygen from mine air. Of particular significance is the rock-burst occurrence, which destroyed the excavation structure and changed airflow distribution. The resulting accidents caused seven casualties. To sum up: • A bump of high energy in section H caused the rock mass’s disturbance and a sudden large emission of approximately 545,000 m3 of methane into the excavations. • Such a large amount of methane was undoubtedly influenced by creating a safety pillar protecting the openings into a deposit section (Figure5). • In this part, rock mass was not disturbed and the coal mine did not implement any drainage methods. • The average methane release into excavations after the rock-burst was tripled compared to the period before the burst. • From 5 August 2018 to 15 August 2018, the sensors’ methane concentration changed from 60% to approximately 35% (Figure8). • The accident resulted in five casualties. • A bump in the longwall IV area in seam 703/1 occurred on 22 January 2019 and resulted in a rock-burst in the area of crossing a longwall face with a ventilation heading. • The amount of air flowing through the excavation decreased by 30% compared to the airflow before. 3 • The burst caused an additional outflow of 4500 m CH4 within 12 h, which means that methane release increased by 60% compared to the period before. • The accident proved fatal to one employee, working in the crossing area of the longwall and the heading. An analysis of these events provides strong evidence that coal extraction under the conditions of co-existing methane and rock-burst hazards requires implementing methods of forecasting and adequate preventing measures. The implemented measures must be able to prevent rock-burst.

5. Conclusions The natural hazards existing in mines often aggravate each other, increasing the risks to personnel working in excavations. Both accidents analyzed here involved a bump and a resulting rock-burst, causing a massive outflow of methane into a longwall excavation. Usually, bumps with energy up to 104 J or higher do not pose a potential threat of rock-burst and a sudden outflow of methane. Thus, the prevention methods implemented in coal mines regarding this particular hazard are well designed. Low energy bumps do not cause sudden desorption of methane into the excavation. The most dangerous are high-energy bumps like those which caused accidents in the aforementioned longwalls. Additionally, creating a safety pillar protecting the openings can cause high methane content in seams, which might be released during the rock-burst. In this part, rock mass is usually not disturbed, which causes creating so-called gas traps. The discharge of methane due to rock-burst, combined with airflow reduction, leads to the removal of oxygen from mine air. The analysis carried out in the article concludes that it is vital to research on designing and implementing innovative methods of draining methane from coal seams as a preventive method against sudden methane outflows. Undoubtedly it will increase the efficiency of methane capture during mining operations. It is necessary to develop methane drainage procedures to increase the amount of captured methane by drilling boreholes above the extracted seam as the longwall progresses. The implemented method should involve drilling long methane drainage boreholes fitted with pipes, combined with the currently used methane drainage system, particularly at longwalls mined using a U system of retreat longwall mining. Energies 2021, 14, 128 15 of 16

Regardless of the attempts to decrease the amount of methane in coal seams, it is necessary to monitor the rock-mass stresses and maintain ongoing prevention of rockbursts.

Author Contributions: Author Contributions: N.S. developed a concept and methodology for the presentation of research results. J.S. contributed analysis tools, analyzed data, and wrote the paper. All authors have read and agreed to the published version of the manuscript. Funding: The article was prepared as part of the Subsidy for the Maintenance and Development of Research Potential at Faculty of Mining and Geoengineering AGH University of Science and Technology No. 16.16.100.215. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Informed consent was obtained from all subjects involved in the study. Data Availability Statement: Upon authors’ request. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References 1. Szl ˛azak,N.; Zasadni, W. Wpływ Zagrozenia˙ T˛apaniami Na Dobór Profilaktyki Pozarowej˙ W Kopalniach W˛egla; Uczelniane Wydawnictwa Naukowo-Dydaktyczne AGH: Kraków, Poland, 2004. (In Polish) 2. Szl ˛azak,N.; Borowski, M.; Obracaj, D.; Swolkień,J.; Korzec, M. Selected Issues Related to Methane Hazard. In Hard Coal Mines; Uczelniane Wydawnictwa Naukowo-Dydaktyczne AGH: Kraków, Poland, 2004. 3. Szl ˛azak,N.; Tr˛eczek,S. Analysis of the Methane Hazard in the Context of a Rock Collapse with Effects in Workings Made in Seams 409/3 and 409/4, Which Occurred on 5 May 2018 at 1058 at JSW S.A. KWK “Borynia-Zofiówka-Jastrz˛ebie”Front “Zofiówka,” Taking into Account the Methods Used and Preventive Activities; Higher Mining Institute: Niger, Niamey, Unpublished document; 2018. (In Polish) 4. Kotarba, M. Geomechaniczne Kryteria Genezy Gazów Akumulowanych W Serii W˛eglono´snejGórnego Karbonu Niecki Wałbrzyskiej; Geologia No. 49; Zeszyty Naukowe AGH: Kraków, Poland, 1988. (In Polish) 5. Czapliński, A. W˛egielKamienny; Wydawnictwo AGH: Kraków, Poland, 1994. (In Polish) 6. Kotarba, M. Geneza Metanu Wyst˛epuj˛acegoW Serii W˛eglono´snejGZW IDZW; Wydawnictwo AGH: Kraków, Poland, 1994. 7. Zwietering, P.; Overeem, I.; Van Krevelen, W.D. Chemical structure and properties of coal 13. Activated diffusion of gases in coal. Fuel 1956, 35, 66–71. 8. Tarnowski, J. Wyst˛epowanieMetanu W Złozu˙ Południowej Cz˛e´sciRybnickiego Okr˛eguW˛eglowego; Komunikat nr 541: Katowice, Poland, 1971. (In Polish) 9. Geological Documentation of the “D˛ebieńsko1” Hard Coal Deposit in Categories B, C1, C2. Geological and Ecological Com- pany “Grafit”. 2007. Available online: https://docplayer.pl/56540786-Ii-interdyscyplinarna-sesja-wyjazdowa-doktorantow- politechniki-slaskiej.html (accessed on 15 September 2008). (In Polish). 10. Krzystolik, P.; Kobiela, Z. Ocena efektywno´scimetod podziemnego odmetanowania złóz˙ w˛eglakamiennego w Polsce na przykładzie kilku kopalń. Wiadomo´sciGórnicze 1999, 6, 265–270. (In Polish) 11. Brunner, D.J.; Schwoebel, J.J.; Brinton, J.S. Modern CMM Drainage Strategies. First Western States CMM Recovery and Use Workshop, April 19–20, EPA Coalmine Methane Outreach Program. J.S. 2005. Available online: www.epa.gov/cmop (accessed on 15 September 2008). 12. Coal Mine Staff (Ed.) C.R.—Collective Review: Brzeszcze Mine—Yesterday and today. In Brzeszcze; Review; Beskidzka Press Publisher: Brzeszcze, Poland, 2003. (In Polish) 13. Creedy, D.P.; Kershaw, S. Firedamp prediction—A pocket calculator solution. Min. Eng. 1988, 2, 377–379. 14. De Villiers, A.W. Methane drainage in longwall coal mining. J. South. Afr. Inst. Min. Metall. 1989, 89, 61–72. 15. Diamond, W.P. Methane Control. for Underground Coal Mines; Information Circular 1994; U.S. Bureau of Mines, IC 9395, United States Department of the Interior: Washington, DC, USA, 1994. 16. Fang, Z.; Li, X.; Wang, G.G.X. A gas mixture enhanced coalbed methane recovery technology applied to underground coal mines. J. Min. Sci. 2013, 49, 106–117. [CrossRef] 17. Karacan, C.Ö.; Ruiz, A.F.; Cotè, M.; Phipps, S. Coal mine methane: A review of capture and utilization practices with benefits to mining safety and to greenhouse gas reduction. Int. J. Coal Geol. 2011, 86, 121–156. [CrossRef] 18. Kissel, F.N. Handbook for Methane Control in Mining; National Institute for Occupational Safety and Health: Pittsburgh, PA, USA, 2006. 19. Krause, E.; Łukowicz, K. Methane drainage in Polish hard coal mines. Achievements and perspectives. In Proceedings of the 11th International Scientific and Technical Conference “Rockbursts 2004”, New Solutions in the Field of Preventing Rockbursts and Methane Hazards, Ustroń,Poland, 8–10 November 2004. (In Polish). 20. Noack, K. Control of gas emissions in underground coal mines. Int. J. Coal Geol. 1998, 35, 57–82. [CrossRef] 21. Szl ˛azak,N.; Kubaczka, C. Impact of coal output concentration on methane emission to longwall faces. Arch. Min. Sci. 2012, 57, 3–21. Energies 2021, 14, 128 16 of 16

22. Szl ˛azak,N.; Borowski, M.; Obracaj, D.; Swolkie´n,J.; Korzec, M. Effectiveness of coal mine methane drainage in Polish mines. In Proceedings of the Twenty-ninth Annual International Pittsburgh Coal Conference: Coal—Energy, Environment and Sustainable Development, Pittsburgh, PA, USA, 15–18 October 2012. 23. Whittles, D.N.; Lowndes, I.S.; Kingman, S.W.; Yates, C.; Jobling, S. The stability of methane capture boreholes around a log wall coal panel. Int. J. Coal Geol. 2007, 71, 313–328. [CrossRef] 24. Roszkowski, J.; Szl ˛azak, N. Selected Problems of Methane Drainage in a Coal Mine; Uczelniane Wydawnictwa Naukowo-Dydaktyczne: Kraków, Poland, 1999. (In Polish) 25. Ajruni, A.T. Prognozirovanie I Predotvrasenieˆ Gazodinamiˇceskih Âvlenij v Ugol’nych Šahtah; Nauka: Moskva, Russia, 1987. 26. Szl ˛azak,N.; Borowski, M.; Obracaj, D.; Swolkie´n,J.; Korzec, M. Odmetanowanie Górotworu W Kopalniach W˛eglaKamiennego; Wydawnictwa AGH: Kraków Poland, 2015. (In Polish) 27. Regulation of the Minister of Energy on Detailed Requirements for Operating Underground Mining Plants. 23 November, 2016 Journal of Laws 2017, Item 1118, Warsaw. Available online: https://dziennikustaw.gov.pl/DU/rok/2017/pozycja/1118 (accessed on 15 September 2008). (In Polish)