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: Swolkie´n,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 techni- cal 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 concentra- tion 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.