Improving Coal Mine Safety via Aerodynamic Resistance Control in Longwall Faces: A Quasi-Network Model Approach

Present day coal mines are technically and technologically complex production facilities. Their operation is associated with numerous risks, the management of which requires strict compliance with all the established safety standards. The legislative framework for ensuring safety in the coal industry includes both international standards and national legislation developed taking into account the characteristics and specifics of the industry.

To improve the safety of work in the mining industry, on June 22, 1995, the International Labor Organization adopted Convention No. 176 "On Safety and Health in Mines" in Geneva [1]. This convention proposes to prevent all the fatal accidents, injuries or diseases of workers that are caused by industrial activities in the mining industry. Article 7 of Convention No. 176 recommends that employers take all the necessary measures to eliminate and to minimize risks to life and health in mines.

Coal mines are considered hazardous production facilities. The most dangerous production factor is emission of methane from the coal massif in the course of stoping and preparatory work and formation of an explosive methane-air mixture in the mine workings. A methane explosion, including one involving coal dust, is the most dangerous emergency situation in a coal mine. Workers in the explosion zone are exposed to flame and high temperature, shock waves and toxic explosion products. Explosions are also accompanied by damage to the support of the mine workings, mechanisms and equipment located in them and cable networks.

Methane emissions and explosions in coal mines in various countries occur annually. Table 1 provides the information of emissions and explosions of methane-air mixtures that occurred in the period from 2014 to 2023 in the world [2-4].

Table 1. The information of emissions and explosions of methane-air mixtures that occurred within the period from 2014 to 2023 in the world

It can be seen from Table 1 that the consequences of methane outbursts and explosions are always tragic. All the tragic incidents that have occurred, point to a common problem in the coal industry related to the insufficient provision of safety measures during underground mining operations. All the accidents were caused by ineffective gas emission management during coal mining processes and unsatisfactory monitoring of the mine atmosphere, which led to the accumulation of methane with the formation of an explosive gas-air mixture. The use of obsolete and outdated equipment in mines also increases the risk of accidents and the occurrence of a source of ignition of the gas-air mixture. The accidents that have occurred, point to the need to improve gas emission management methods and the methane content monitoring, as well as to modernize mine equipment and to improve labor safety standards to prevent similar tragedies in the future.

For a methane-air mixture to explode, two conditions must be present simultaneously. The first condition is the methane-air mixture formation of a sufficient concentration for an explosion. For methane, this concentration is approximately 5–15% by volume. The second condition is the occurrence of an ignition source with a sufficient temperature or energy to initiate the methane combustion reaction. This can be a spark (electrical or mechanical), an open flame, or a high temperature of a heated surface. If two of these conditions are met simultaneously, an explosion occurs, releasing a large amount of heat, carbon monoxide, carbon dioxide, and water vapor. To reduce the risk of a methane-air mixture explosion, one of the above factors must be eliminated. Therefore, optimizing ventilation parameters is critical to preventing methane accumulation.

Mine workings contamination with gas can be the result of sudden coal and gas emissions, vent emissions, layered and local methane accumulations. All the cases of exceeding the established norms of methane concentration in mine workings constitute an emergency situation in the mine. Methane accumulations can also be caused by emergency stops of main and local ventilation fans, improper degassing of mine workings, incorrectly operating gas measuring equipment, as well as engineering errors in calculating the required amount of air and unsatisfactory control over ventilation of mine workings. Any contamination of mine workings in coal mines can form conditions for the formation of an explosive methane-air environment, which, in the presence of an ignition source, can lead to an explosion with significant human casualties and material losses.

Currently, coal mines use various special measures to combat methane to prevent dangerous methane accumulations and its ignition. However, the main measure to prevent the formation of dangerous concentrations of methane is effective ventilation, as a result of which the permissible methane content is maintained throughout the entire network of operating mine workings.

A significant number of scientific papers deal with the issues of ventilation of coal mines. The problem of increasing reliability and efficiency of ventilation of mine workings is of considerable interest and is the subject of studies in numerous scientific papers. This explains the growing attention and interest of scientists in the issues of ensuring safety during mining operations. This problem is studied especially intensely in China.

The analysis of mine ventilation in work [5] shows that in the period from 2010 to 2023, the studies in this area went through three stages: stable development, slow growth and rapid growth. The main areas of research are in the modeling of air flows, gas dynamics and the development of ventilation control systems. For example, in work [6], scientists built an experimental model of a three-dimensional ventilation network with an independent working face, developed a platform for monitoring ventilation parameters and fan control. Reference [7] discusses possible correlations between the characteristics of ventilation air and underground mining operations. In their research paper [8], Wang et al. present a method of air quantity distribution in the design of coal mine ventilation, and propose a method to eliminate air leakage through ventilation structures in the ventilation system. Zhou et al. [9] improved the ventilation network calibration method by applying sensor location correction factors to obtain a more accurate value of the average air velocity. In their paper, Pei et al. [6] reported the experimental results of the fact that the gas concentration in the working face has a power-law relationship with the operating frequency of the fan, and also confirm that increasing the air quantity and vacuum is an effective means to solve the problem of unbalanced gas emissions in the working face.

Numerous studies focused on the simplification of complex mine ventilation networks [10-13]. Network simplification involves reducing its size by reducing the number of nodes and branches. This method allows reducing the branching ratio of the ventilation network, significantly improving the simplicity of the ventilation network and has a higher applicability and greater flexibility than traditional methods. The use of network simplification technology significantly accelerates the analysis and reduces the time of developing a complex mine ventilation network. Simplifications of ventilation networks allow designing and building an experimental platform for monitoring and automatic regulation of air flow parameters in the face. For example, in work [12], the authors present an algorithm for equivalent simplification of complex ventilation networks based on the degree of entry and exit of nodes, as well as on the inflow and outflow nodes of branches, and also proposed an optimization strategy for a set of pairs of nodes requiring deep search in the network based on the principle of balance of flows in the incoming and outgoing branches of the subnetwork nodes. In work [13], scientists, using the method of equivalent transformation of the topological relationship of the network, proposed a method of solving the problem of constructing a Q-H graph for a network with unidirectional contours, and also developed a simplified mathematical model of the network based on the method of independent trajectories and the theory of adjacency matrices of nodes.

In modern research, innovative solutions such as remote ventilation control using sensors and the use of artificial intelligence for data analysis are recommended to effectively improve the optimal use of air supply. The growth of digitalization and intelligent technologies in the mining industry increased the research interest and importance of mine ventilation in recent years. The intelligent ventilation system using real-time monitoring data can immediately correct the inefficient distribution of air volume, realize automatic power adjustment, remote control and monitoring of the facility, and identify dangerous areas [14-16].

Despite the existence of a number of scientific studies in the field of mine ventilation, the application of innovative technologies in ventilation and improving the performance of modern fans, the problem of eliminating methane explosions in coal mines has not yet been solved. As the accident statistics presented in Table 1 show, explosions in mines with tragic consequences still occur.

The formation and diffusion processes of the gas-air mixture in the mined-out areas remain poorly understood during the ventilation of working sections using a direct-flow ventilation system with refreshed exhaust airflow. Previous studies [6-10] and the obtained patterns of air leakage through the mined-out space assume a constant value of its specific aerodynamic resistance. At the same time, according to other studies [17, 18], the specific aerodynamic resistance changes as the distance from the longwall face increases, which complicates the task of determining the filtration velocity and flow direction in the collapsed space.

The inaccessibility of the mined-out space for direct observations and measurements of air leakage and permeability has led to the use of a quasi-network model.

Assessing, managing, and reducing the risk of methane-air mixture explosion in working areas make an integral part of the industrial safety management system of mines and a relevant scientific problem. The authors of the article propose to manage gas emission in the working area by regulating air leaks through the mined-out space of the longwall to solve this scientific problem. In the study, the working area is considered as a quasi-network model, the structure of which is as close as possible to the real rock massif. The studies were carried out in working areas with a direct-flow ventilation scheme of the Karaganda coal basin mines.

This study aims to create a mathematical model of the resistance in the mined-out space to optimize ventilation parameters in the working area and evaluate the likelihood of explosive methane concentrations forming in the workings under a direct-flow ventilation scheme.

The study results enabled the identification of the optimal ratio between the airflow directed to the longwall and the additional airflow in the supported ventilation roadway, which prevents the formation of a gas accumulation zone at the working face and lowers the risk of a methane-air mixture explosion. Numerical experiments were conducted for three methods of gas emission control using the example of mines in the Karaganda coal basin: changing the air flow rates in the longwall and ventilation working; using insulation of the mined-out space; blasting the ventilation working.

The application of pillarless coal seam mining technology combined with a direct-flow ventilation system in the Karaganda coal basin has proven to be highly effective. It allows increasing the load on the working faces, reducing the volume of development workings, eliminating coal losses in the near-cut pillars, and forming favorable conditions for the isolated liquefaction and removal of methane at the sources of its intake. At the same time, a complicating factor in the widespread use of a direct-flow ventilation scheme for the working area is the need for reliable control of the aerogas dynamics of the mined-out space. Here the determining role is played by air leaks through the caving zones, which play an important role in the formation of the gas balance of the working area. On the one hand, due to leaks, a significant part of methane is removed directly to the ventilation working, bypassing the stoping working. On the other hand, leaks contribute to decreasing the volume of air used for ventilation of the face space and to a possible excess of permissible methane content standards in the working area of the longwall.

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Figure 1. Quasi-network representation of the mined-out space of the longwall

Considering that the working face in a direct-flow ventilation scheme is a diagonal element, i.e., it is located in a potentially unstable ventilation zone, it is important to study the process of filtering air leaks through the mined-out space. To investigate how the aerodynamic properties of the collapsed rock mass and the existing workings within the working area influence the distribution of airflow, a quasi-network model of the longwall's mined-out space, as shown in Figure 1 [19], is proposed for use.

In the conditions of an active stope, the origin of coordinates is the point located in the area of the stope line and the wall of the supported working junction on the side of the mined-out space. The OX axis is directed along the wall of the supported stope. The OY axis runs along the direction of the stope, while the OZ axis is oriented perpendicular to the coal seam plane. The number of branches connecting the nodes in the longwall and the ventilation working and simulating the directions of air movement through the collapsed space varies from 1 to n depending on the size of the mined-out space. Thus, this model represents a complex ventilation system system comprising 3n branches, 2n + 1 nodes, and n independent circuits.

The structure of the quasi-network model maximally reflects the real processes occurring with the rock massif in the conditions of an active stope. Therefore, this model allows simulating and solving complex ventilation problems.

The system of equations describing the studied circuit, for a given direction of bypassing the circuits, has a closed system with respect to the unknown air leakages q_i, $i=\overline{1, n}$ and has the form [20, 21]:

$\left\{\begin{array}{l}\sum_{i=1}^n R_{y, i}\left(Q_\pi-\sum_{j=1}^i q_j\right)^2+\sum_{i=1}^n R_{x, i}\left[\left(Q_\pi+Q_n\right)-\sum_{j=1}^i q_j\right]^2-r_1 q_1=0 ; \\ \sum_{i=2}^n R_{y, i}\left(Q_\pi-\sum_{j=1}^i q_j\right)^2+\sum_{i=2}^n R_{x, i}\left[\left(Q_\pi+Q_n\right)-\sum_{j=1}^i q_j\right]^2-r_2 q_2=0 ; \\ \sum_{i=3}^n R_{y, i}\left(Q_\pi-\sum_{j=1}^i q_j\right)^2+\sum_{i=3}^n R_{x, i}\left[\left(Q_\pi+Q_n\right)-\sum_{j=1}^i q_j\right]^2-r_3 q_3=0 ; \\ .\quad.\quad.\quad.\quad.\quad.\quad.\quad.\quad.\quad.\quad.\quad.\quad.\quad.\quad.\quad.\quad.\quad.\quad.\quad.\quad.\quad.\quad.\\ \sum_{i=n}^n R_{y, i}\left(Q_\pi-\sum_{j=1}^i q_j\right)^2+\sum_{i=n}^n R_{x, i}\left[\left(Q_\pi+Q_n\right)-\sum_{j=1}^i q_j\right]^2-r_n q_n=0 .\end{array}\right.$           (1)

This system of equations is nonlinear and is solved by the Newtonian linearization method [19]. The essence of this method is the linear approximation of nonlinear network laws. As a result of solving Eq. (1), the calculated values of air flow for each section of the longwall are determined. However, they do not always correspond to the actual data.

This is because, under a direct-flow ventilation scheme, the airflow changes along the length of the working area. Moreover, air leakage through the mined-out space has a significant impact on the turbulence level of airflow, as well as on parameters such as the Reynolds number and the aerodynamic resistance coefficient. As a result, the calculated airflow values need to be adjusted to account for air leakage through the mined-out space.

The most effective means of preventing local methane accumulations at the longwall face with the supported working junction is its effective ventilation. To determine the optimal ventilation parameters for the working area, air leaks through the mined-out space of the longwall face were simulated. The aerogas situation is controlled by aerodynamic methods, i.e. by changing the aerodynamic parameters of the branches that form the calculated ventilation scheme of the area. This primarily includes changing the ratio of the airflow volume between the longwall face and the ventilation roadway, as well as isolating the mined-out space from the side of the ventilation drift, blasting the ventilation drift in the areas with the greatest deformation of the working cross-section.

Numerical experiments were carried out to simulate the aerogas situation in the mine. This allowed a fairly thorough analysis of the nature of the air flow distribution in the areas under consideration under the action of various aerodynamic factors.

The initial data for calculating the air distribution in the section ventilation network were:

- S_x,i – cross-sectional area of the i-th segment of the supported roadway,

- S_y,i – cross-sectional area of the longwall face in the i-th section;

- R_B,i – aerodynamic resistance of the mined-out space in the i-th direction (according to formula 11).

Taking into account the practice of managing the aerogas situation at the extraction site, the following were selected as the main factors influencing air distribution:

- cross-section of the supported ventilation working S_x;

- aerodynamic resistance of the mined-out space of the longwall R_B;

- the volume of airflow delivered to the longwall Q₁;

- the volume of supplementary air delivered to the supported ventilation roadway Q₂.

The obtained results of numerical experiments with a change in the air flow rate in the longwall showed that with a change in the air supply to the longwall, the nature of ventilation of the unstable zone changes. Thus, with the Q₁/Q₂ ratio of 0.4, which corresponded to the supply of 6.96 m³/s of air to the longwall and 17.55 m³/s to the supported ventilation working, in the junction area between the longwall and the ventilation roadway in a section of 17 m, an overturning of the ventilation stream with the flow rate of 2.57 m³/s is observed. In this situation, the likelihood of methane accumulation in the working area of the longwall increases.

With increasing the volume of air entering the longwall and a decrease in its supply to the supported ventilation working, the normal ventilation mode of the unstable zone is restored. With the Q₁/Q₂ ratio of 0.67, the air flow rate at the exit from the longwall was 0.38 m3/s at a speed of 0.05 m/s. Further increasing the air supply to the longwall with simultaneous reducing its supply to the supported ventilation working ensures stabilization of the longwall ventilation mode, which meets industrial safety requirements. Thus, with Q₁/Q₂ equal to 1.45 and 2.52, which corresponded to a supply of 14 m³/s and 17.55 m³/s into the longwall face, with the supply of 10 m³/s and 6.96 m³/s into the supported ventilation workings, the air flow rates at the outlet from the longwall face were 5.04 m³/s (V=0.63 m/s) and 8.1 m³/s (V=1.01 m/s), respectively.

A similar trend in changing the value of air consumption and leaks through the collapsed massif was observed in the other longwalls of the Karaganda coal basin mines.

Thus, during the numerical experiment, the values of the (Q₁/Q₂) ratio were obtained. They allow estimating the probability of the explosive concentration of methane formation (Table 4).

Table 4. Criteria for assessing the probability of the formation of an explosive concentration of methane in the stope workings of the extraction area

One of the indicators that characterizes the reliability of a mine ventilation scheme is the stability of ventilation. which is assessed by the airflow rate and velocity in the workings of the mining area and in the zone of unstable ventilation of the longwall. The minimum permissible air velocity in the working faces of coal mines is regulated by safety requirements and must be at least 0.5 m/s.

Studies on the stability of ventilation were carried out under the conditions of four operating longwall faces of mines in the Karaganda coal basin as they were being mined. Below. Table 5 presents some actual indicators of air velocity measured in these longwall faces.

According to the data in Table 5, it can be seen that with the ratio of air supplied to the longwall and for auxiliary ventilation ranging from 0.27 to 0.67. the air velocity in the longwalls was between 0.8 m/s and 1.2 m/s. while methane concentrations in certain points of the longwall face were recorded at levels of 1.7–3.1%.

Table 5. Actual indicators of air velocity in the longwalls

With the ratio of air supplied to the longwall and for auxiliary ventilation ranging from 0.67 to 0.82. The air velocity in the longwalls increased and reached values from 1.2 m/s to 1.7 m/s. As a result. the stability of ventilation and the gas environment in the working face improved.

To reduce the risk of methane-air mixture explosion in the working areas, theoretical and experimental studies were carried out to assess the probability of formation of explosive concentrations of methane in the stope. During the studies, air leaks from the longwall and its inflows into the supported ventilation workings through the mined-out rock mass were studied. To examine how the aerodynamic properties of the collapsed rock mass and the active workings within the working area influence airflow distribution, a quasi-network model of the longwall's mined-out space was utilized. During the study, a mathematical model was obtained for calculating the value of aerodynamic resistance, taking into account the effect of the length of the workings of the column and the longwall. The resulting model allows determining air leakage through the caved-in rock mass under the conditions of the mines of the Karaganda coal basin.

The authors considered the methods of managing the aerogas situation in the working area by changing the ratio between the airflow directed to the longwall (Q₁) and the supplementary airflow supplied to the supported ventilation roadway (Q₂), isolating the mined space from the ventilation drift and blasting the ventilation working. In the course of the numerical experiment, the values of the (Q₁/Q₂) ratio were obtained. This allowed estimating the probability of formation of an explosive concentration of methane. The research results allow organizing stable, effective ventilation of the working area and ensuring safety of mining operations in the conditions of the mines of the Karaganda coal basin. Further research will be aimed at reducing the risk of methane-air mixture explosions during mining operations in coal mines. Particular attention will be paid to improving mine ventilation, including the development of more efficient ventilation schemes, determining and reducing the aerodynamic resistance coefficient of mine workings in the Karaganda coal basin. The research will cover issues of ventilation management to maintain optimal aerodynamic regimes, which will reduce methane concentration in mine workings and enhance the safety of miners.