Optimization of the Drawing Process of Low–Middle-Thickness Top Coal Based on a Simulation

Abstract

Based on a theoretical analysis and simulation, the automated drawing characteristics for low–middle-thickness top coal are investigated, and the drawing process of top coal is optimized. For top coal with a 3 m thickness, a comparison is made between traditional and automated drawing process of top coal, which is used to highlight the superiority of automated drawing process. Then, for the automated top coal drawing, the effects of the round number on the drawing results of the entire working face are investigated, and the advantages and disadvantages of the multi-round drawing process are compared. Finally, the coal drawing results combined with automated and traditional methods are discussed, and an ideal optimized coal drawing process is proposed. The research results indicate that, for top coal with a 3 m thickness, the recovery rate of top coal by automated one-round drawing is slightly higher than that of traditional drawing, the drawing amount and drawing time of various drawing openings are relatively uniform for automated drawing, while the traditional coal drawing process shows fluctuating characteristics. For automated multi-round drawing, the top coal recovery rate decreases with an increase in the round number; however, the rock mixed rate in the drawn top coal is lower, and the drawing results at various drawing openings become more uniform. The combined automated–traditional drawing can achieve layered drawing of the top coal, avoid gangue drawing, and maintain a relatively high recovery rate, which is the optimal drawing process for this research project.

1. Introduction

Regarding research on top coal drawing, the ellipsoid theory of metal ore extraction has become the primary guidance. This was mainly referenced during the initial research stage in China. Due to the complex and variable conditions of top coal caving, the ellipsoid theory has been continuously developed and improved. Wang et al. [1,2] believed that, during the coal caving process, the coal and rock above the working face are in a fragmented state, and their movement follows the laws of fragmentation–granular flow. The discrete element method (DEM) is suitable for simulating the transportation and drawing of loose fragmented top coal. For single hydraulic supports of low-position top coal caving, the drawn body axis and the gangue funnel formed by coal caving deviate towards the goaf at varying degrees. Using a granular model, Liu et al. [3] analyzed the coal-rock flow characteristics under different top coal and gangue fragmentations. Based on the Bergmark–Roos theory, Tao et al. [4] established a random media flow model from a mechanical perspective to study the formation mechanism of the caved top coal shape. Wei et al. [5] established a mechanical model to study the arching mechanism. To achieve a smooth drawing of the top coal, the gravity component in the caving direction must be greater than the frictional force; otherwise, it is prone to the formation of an arch structure. Li et al. [6] analyzed the influence of fragmented top coal properties on coal-rock caving. The arching mechanism of fragmented top coal was investigated, and it was shown that the equilibrium arch structure can be disrupted by the support tail beam and the changing of the drawing opening size and drawing round number, thus facilitating top coal drawing. Fan et al. [7] solved the problem of large coal blocks blocking the drawing opening by the coal caving of nearby supports. Through granular experiments, Liu et al. [8] introduced the turning point concept of the top coal caving rate. As the drawn gangue accounts for one-third of the drawn amount, stopping the caving can achieve a recovery rate of 94%. Jiang et al. [9] analyzed the influence of the drawing interval distance on the top coal drawing by a PFC simulation. Yang et al. [10] used the discrete element software PFC3D to study the top coal drawing at the end area under different caving methods, which indicates that roadway support drawing and transition support drawing can increase the top coal recovery by about 6%. Through UDEC and FLAC simulations, Wang et al. [11] pointed out that the coal-rock mass above the support is subjected to mining pressure. Above the support, the coal body in the coal wall side experiences shear sliding in a wedge shape, while tension failure occurs on the coal body near the goaf side as the roof subsides. Huo et al. [12] used a coupled numerical method of the finite difference method and discrete element method to establish a numerical model, and the evolution characteristics of the drawing body, loose body and top coal boundary were analyzed. Zhang et al. [13] used a 3D numerical simulation method of dynamic coupling of the FLAC3D block and PFC3D particles to study the progressive failure behavior of top coal along with the mining process. Overall, for the fully mechanized top coal caving, scholars have conducted relatively systematic studies on complex caving conditions. However, the above-mentioned studies mainly focus on traditional manual control methods for top coal caving.

Currently, technological innovation led by coal mine automation has become the driving force of the coal industry’s development. After 20 years of exploration and development, China’s coal mine automation has gone through three stages of development: following, running alongside, and taking the lead. A total of 494 automated working faces have been constructed, achieving research and engineering applications of automation mining technologies under different conditions. Automated once-mining-full-height technology has been tested and applied in various mining conditions [14,15,16,17]. Moreover, automated top coal caving technology has also been used in mines such as Ta Shan, Tong Xin, and Wang Jialing [18,19]. Compared to the fully mechanized once-mining-full-height technology, the mining working face used in top coal caving presents greater challenges for coordinated cutting and drawing and faces a more complex and unpredictable external environment. Moreover, coal-gangue identification technology for the automated control of coal drawing is still in the experimental testing stage. The current trend still emphasizes equipment automation control as the main approach and manual intervention control as a supplementary measure, which remains the main development direction.

For theoretical research on automated top coal caving, more studies are paying attention to the caving characteristics of ultra-thick top coal. Zhang et al. [20,21] proposed technology for automated top coal caving in ultra-thick coal seams, namely layered and multi-round drawing of top coal; moreover, a method for determining the automated control time of each drawing opening was also provided. Liu et al. [22,23] proposed the coordinated drawing concept of multiple drawing openings for the ultra-thick top coal caving and provided relevant theoretical formulations. Based on the coordinated drawing concept, Wang et al. [24] explored the results of the interval-coordinated drawing effect of multiple drawing openings in ultra-thick coal seams. Overall, the above studies have mainly focused on the automated drawing process of ultra-thick top coal. However, for top coal of low–middle thickness (1~5 m), the per-support discharge time in the per-round drawing is too short in the multi-round caving process. Coordinated drawing of multiple coal outlets is more likely to result in gangue drawing. Thus, for automated drawing of low–middle-thickness top coal, it is more difficult to reasonably determine the control parameters, which affects the drawing results of top coal across the entire working face. Given this, for top coal of low–middle thickness, a systematic study is needed to understand the automated drawing characteristics and develop appropriate drawing technologies, which differ from the drawing characteristics of ultra-thick top coal.

This study takes the 81,309 fully mechanized working faces of Baode Coal Mine as the engineering background. Through an on-site investigation, theoretical analysis, and simulation, the drawing laws and automated drawing process optimization of top coal are investigated in automated mining practices. The research results have a certain guiding significance for the automated mining operations under low–middle-thickness top coal conditions.

2. Research Background

This study is based on the 81,309 working faces used in fully mechanized top coal caving in the Baode Mine of Shendong Coal Group. Coal seam has a complex structure and is predominantly semi-bright to semi-dark coal. Coal seam is, on average, 7.0 m thick with three gangue layers. The predicted coal seam structure is 1.6 m (0.1 m), 1.6 m (0.4 m), 1.3 m (0.5 m), and 1.5 m, as shown in Figure 1. The coal type is gas coal, with a coal recoverability index of 1 and a variation coefficient of 27.5%. The coal seam and immediate roof have well-developed fractures. The thickness of the immediate roof strata ranges from 4.5 to 10.1 m, and the immediate floor thickness ranges from 0.3 to 2.0 m. The dip angle of coal seam ranges from 3° to 6°. The working face is arranged along the coal seam strike and mined along the 8# coal floor. The designed mining height is 3.7 m, the drawing height is 2.9 m, and the mining-to-drawing ratio is 1:0.78. The longwall retreat method is adopted, with the full caving method used to manage the roof.

In 2021, the State Energy Investment Group Co., Ltd. in china established a key project task focusing on research on intelligent and efficient production technologies for unmanned underground coal mines. The project selected a specific coal mine as an engineering demonstration pilot to research intelligent top coal drawing algorithms and technologies. This marked the beginning of automated mining at Baode Coal Mine. Currently, the coal mine has installed an intelligent coal drawing control system, which includes multi-source information acquisition, automatic drawing process decision-making, drawing control parameter calculation, overload shutdown prediction of the rear scraper conveyor, support shift determination, real-time monitoring and front-end display of coal drawing, parameter settings, and other functional modules. However, further research and determination are still needed regarding the establishment of a reasonable coal drawing model and the determination of highly efficient drawing technology.

3. Theory, Model, and Scheme Used in the Top Coal Drawing Investigation

3.1. Theoretical Analysis of Top Coal Drawing

The top coal masses are crushed into fragments and accumulated behind the support by the overburden pressure and then are drawn by the drawing opening. Because the structure of hydraulic supports does not significantly affect the drawing process on the cross-section parallel to the working face, for a theoretical model for automated drawing for a single support, which can refer to the Bergmark–Roos ellipsoid theory [2,20,21]. The movement trajectory of the fragmented top coal is shown in Figure 2a. The drawing body profile of top coal in the working face direction exhibits an approximately regular elliptical shape, as shown in Figure 2b.

From Figure 2, for a single drawing opening, top coal in an elliptical shape is drawn out, forming a drawing funnel. For multiple drawing openings of the working face, the theoretical models for traditional one-round drawing and automated multi-round coal drawing are shown in Figure 3.

The traditional drawing models of the working face are shown in Figure 3a. The termination condition of top coal drawing is “rocks appear, close the opening”. After the top coal in the first large elliptical area on the left of the model is drawn out, the formed drawing funnel is filled with gangue. To avoid drawing gangue, the adjacent supports only allow top coal to be drawn in the adjacent small elliptical areas. All drawing operations of top coal in entire working face are carried out sequentially. The traditional manual multi-round coal drawing process is complex; however, automated multi-round coal drawing is achieved automatically with time as a control variable. As shown in Figure 3b, for the automated multi-round drawing, the top coal in the small funnel-shaped layer zone is drawn out by each round. After the lower layer of top coal has been drawn out, the upper top coal is moved down in a layered manner. Due to the regularity and orderliness of drawing operations, it is relatively easy to achieve the stratified drawing of top coal under appropriate drawing time and round number.

3.2. Numerical Modeling and Parameters

The continuum–discontinuum element method (CDEM) is a numerical analysis method that enables coupled calculations of block finite elements, block discrete elements, particle discrete elements, and hydraulic supports. A particle element approach to the CDEM is used for the effective simulation of the top coal drawing process, coal-rock accumulation, and the drawing results of the overall working face [20,21,22,23,24,25]. To simulate top coal drawing and optimize the drawing technology, a coal-rock model is established based on the actual length of the mining working face, as shown in Figure 4. The top coal drawing of the working face is conducted using 100 hydraulic supports with an actual width of 2 m as the drawing opening size.

Under the assumption that the top coal has already been fragmented and accumulated above the drawing openings under mining pressure and disturbance. Therefore, the drawn coal-rock particles have no strength but only exhibit frication resistance of mutual movement. Thus, the density, Poisson’s ratio, and internal friction angle are needed as inputs for the simulation. To increase the critical time step allowed in the simulation, the elastic modulus of the coal-rock particles is appropriately reduced. The mechanical parameters of the simulation are listed in

Table 1. Mechanical parameters of coal and rock.

Rock Type	Density/(kg/m3)	Elastic Modulus/pa	Poisson’s Ratio	Cohesion/pa	Tensile Strength/pa	Internal Friction Angle/°
Coal	1373	2 × 108	0.29	0	0	39.3
Rock	2542	4 × 108	0.23	0	0	33.6

.

3.3. Simulation and Analysis Schemes

To further investigate the drawing characteristic of top coal, this study adopted a numerical model with 3 m thick coal and 100 drawing openings considering the actual conditions of the 81,309 fully mechanized top caving mining faces of Baode Coal Mine. A rigid wall element was used to simulate the scraper conveyor system, and coal bunkers were used for top coal recovery and storage, which can facilitate the visual analysis and statistical assessment of the top coal drawing of the entire working face. Firstly, the top coal recovery effects of the mining face under the automated one-round drawing process and the traditional one-round drawing process were compared. Subsequently, for the automated top coal drawing, the influence of the drawing round number on the top coal recovery results of the overall working face was studied. Finally, this study further compared and analyzed the top coal drawing results under two-round and three-round drawing technologies combined with automated and traditional methods.

In a simulation, for the traditional drawing of top coal, “rocks appear, close the opening” was the termination condition; for the automated drawing of top coal, this was achieved by presetting a specific time for each drawing opening. In simulations of this paper, the time step was set to one iteration step, which corresponded to 0.0001 s. Therefore, simulating the coal drawing process for 10,000 steps equated a drawing time of 1 s. It should be noted that the particles in the numerical model are used to represent fragmented top coal, which contains some differences; therefore, the simulated 1 s was only used to represent one unit of time of real coal drawing. For multiple drawing openings of the working face, coal drawing was sequentially conducted from the left to right of the model.

The recovery rate of top coal in production practice has a significant effect on the economic efficiency of the entire mine. The coal drawing time for each support affects the control parameter setting of different automated drawing technologies. For each drawing opening of the working face, the unevenness of the coal drawing amount affects the power consumption of the scraper conveyor system. Therefore, this study mainly focused on comparing and analyzing the statistical variations of the recovery rate, drawing time, and drawing amount of top coal under different conditions.

4. Research on the Optimization of Different Coal Drawing Technologies

4.1. Comparison between Automated and Traditional Drawing Technologies

In this section, the effects of the traditional and automated drawing methods on the top coal drawing results are investigated by simulations. A comparative analysis is carried out to assess the advantages and disadvantages of the two drawing methods. For traditional coal drawing, we adopt an approach of “rocks appear, close the opening”. The total top coal drawing time across the entire working face is approximately 96 s. As for the automated top coal drawing with a predetermined time, the time allocation for each drawing opening is based on the total drawing time obtained from the traditional approach, which is set to about 1 s.

For the coal-rock model with 3 m thick top coal, the simulation results of one-round coal drawing under the traditional and automated methods are shown in Figure 5.

Figure 5 shows that two technologies can draw out the majority of top coal with only a small amount of top coal remaining above drawing openings. For automated top coal drawing, the recovered top coal in 100 bunkers below the drawing openings exhibits an approximately horizontal layered distribution. However, for traditional coal drawing, the recovered top coal in 100 coal bunkers shows a fluctuating distribution with high and low levels. Because the traditional top coal drawing technology adopts a control approach of “rocks appear, close opening”, no gangue is drawn out by the drawing openings. However, for the automated one-round drawing technology, some rock particles are mixed into the coal bunkers below the drawing openings.

To quantitatively investigate the effects of traditional and automated drawing technologies on the drawing results, a statistical analysis of the drawing amount and drawing time under 100 drawing openings is carried out, as shown in Figure 6.

From Figure 6a, the automated coal drawing time of the 100 hydraulic supports remains constant without any fluctuations. However, for traditional top coal drawing with the gate closing after seeing gangue, the drawing time for each coal drawing opening exhibits a fluctuating variability with a mean square deviation of 0.22 s. After the coal drawing for the 100 supports, the traditional method draws out a total amount of 504.41 m² with a rock mixed rate of 0% in the drawn top coal. As observed from Figure 6b, for automated one-round top coal drawing, the total drawing amount of rock coal particles is 531.74 m². With a top coal amount of 517.78 m² and a gangue amount of 13.96 m², the rock mixed rate in the drawn top coal is 2.6%. For the automated one-round drawing process, the recovery rate of top coal across the entire working face is 78.5%, which is greater than the recovery rate of 76.4% achieved in the traditional one-round drawing process. Both drawing technologies exhibit variability in the drawing amount of each drawing opening. However, the standard deviation (1.197 m²) for the coal amount for each drawing opening in the traditional method is significantly higher than the standard deviation (0.322 m²) for automated one-round coal drawing.

4.2. Simulation Analysis of Automated Drawing with Different Rounds

To achieve a more uniform drawing of top coal from each support and reduce the gangue mixing ratio, the effects of the drawing rounds on the top coal drawing, rock mixed rate, and coal-rock interface morphology are investigated through a simulation.

The specific scheme is designed as follows: with a total coal drawing time of 100 units of time (s), for an automated two-round drawing process, each support will draw coal for 0.5 s in the first-round drawing. After the first-round drawing, the drawing openings for the second round will be opened from the starting position of the working face, and the top coal drawing time of each drawing opening is set as 0.5 s. For automated three-round drawing technology, the top coal drawing time of each drawing opening is set as 0.4 s during the first-round drawing, the drawing time of each drawing opening is set as 0.3 s during the second-round drawing, and the drawing time of each drawing opening is set as 0.3 s during the third-round drawing. The coal drawing results for different rounds are illustrated in Figure 7.

As shown in Figure 7, for a coal-rock model with 3 m thick top coal, under the automated technologies with different drawing rounds, the coal drawing amounts from 100 drawing openings are relatively uniform, and the drawn top coal in 100 coal bunkers shows a horizontal layered distribution after coal drawing in each round. Under the automated two-round drawing process, the majority of the top coal is drawn out, a small amount of top coal remains above the drawing openings, and a small number of gangue particles are mixed into the drawn top coal. However, a significant amount of top coal residue is observed above the drawing openings under the automated three-round drawing process, and there is a minimal mixing ratio of gangue into the drawn top coal.

To quantitatively investigate the effect of the drawing rounds on the top coal drawing, a statistical analysis on the drawing amount from 100 supports was conducted after each drawing round, as shown in Figure 8.

From Figure 8a, for the automated two-round drawing, the drawing amount from 100 drawing openings fluctuates around 2.13 m² during the first round. The average drawing amount in the second round is 2.33 m², with a higher variance of 0.224 m² compared to the variance of 0.173 m² in the first round. This indicates that the fluctuation of coal drawing during second-round drawing is greater than that during first-round drawing. As shown in Figure 8b, for the automated three-round drawing, the average drawing amount from 100 drawing openings is 1.62 m² during the first-round drawing, which is greater than the average drawing amount of 1.24 m² during the second-round drawing and is greater than the average drawing amount of 1.11 m² during the third-round drawing.

To further study the effect of the drawing rounds, a statistical analysis on the total drawing amount of each drawing opening under different rounds was conducted, as shown in Figure 9.

From Figure 9a, for a 3 m thick top coal and different drawing rounds, the total coal amount from 100 drawing openings under automated three-round drawing technology is generally less than that of automated two-round drawing which is, in turn, less than that of automated one-round drawing. However, under the three-round drawing process, the fluctuation degree of the total drawing amount of each drawing opening is also smaller than that of two-round drawing, which is smaller than that of three-round drawing. Specifically, from Figure 9b, the recovery rate of top coal is 78.5% under automated one-round drawing, which is higher than the recovery rate of 67.5% under automated two-round drawing, and higher than the recovery rate of 60.1% under automated three-round drawing. As shown in Figure 9c, the variance in the coal drawing amount from 100 drawing openings under automated one-round drawing is 0.322 m², which is higher than the variance of 0.282 m² under automated two-round drawing and higher than the variance of 0.275 m² under automated three-round drawing. Further, the rock mixed ratio in coal bunkers under automated one-round drawing is 2.6%, which is higher than the rock mixed ratio of 0.4% under automated two-round drawing and is higher than the rock mixed ratio of 0.026% under automated three-round drawing.

4.3. Analysis on the Combined Technology of Automated–Traditional Drawing

As shown in Section 4.1 and Section 4.2, the amount drawn by each support of the working face is uneven for the traditional drawing, but there is no mixture of gangue into the drawn top coal, and there is a higher recovery rate of top coal. On the other hand, for the automated drawing, the amount drawn by each support is relatively even, and the drawing operation is simpler. Moreover, the multi-round coal drawing process allows the layered–gradual–uniform release of top coal, reducing the disturbance to the entire coal-rock interface by adjacent drawing openings. As the number of coal drawing rounds increases, the variance in the coal amount from each drawing opening further decreases, which results in a more uniform release of top coal. However, the recovery rate of top coal continues to decrease with an increase in the drawing round number. To achieve a high recovery rate, a low rock mixed ratio and a relatively even coal drawing, this section conducts a simulation and analysis on the combined technologies of automated and traditional coal drawing.

4.3.1. Two-Round Drawing with Combined Automated and Traditional Technologies

In this section, a simulation is performed on the two-round drawing method with combined automated and traditional technologies. For the first-round drawing of top coal, the automated drawing time of each support is set as 0.5 s. After the first-round drawing has been completed, the drawing openings for the second round are opened from the starting drawing position of the working face, and the traditional control approach of “rocks appear, close the opening” is taken as the termination condition for coal drawing.

The coal drawing results produced from the combining of automation and traditional technologies are shown in Figure 10. From Figure 10a, after automated coal drawing in the first round, the top coal drawn from 100 drawing openings is relatively even, and the recovered top coal from 100 bunkers exhibits a horizontal layered distribution. As shown in Figure 10b, after the second round of traditional drawing, almost all of the top coal has been drawn, with only a small amount of residual top coal remaining above the drawing openings. The recovered top coal from 100 bunkers exhibits a fluctuating distribution, but the fluctuation degree is smaller than that of traditional one-round coal drawing.

To quantitatively explore the effect of the combined automated and traditional drawing methods, the drawn coal amounts from 100 drawing openings were compared and statistically analyzed during the first-round automated drawing and the second-round traditional drawing, as shown in Figure 11. From Figure 11a, compared to a constant drawing time (5 s) in the first round of automation, the drawing time for each support in the second round of traditional drawing exhibits fluctuating variations. The average drawing time of each support is 5.94 s with a standard deviation of 0.152 s. As shown in Figure 11b, after coal drawing from 100 supports, the average coal drawing amount in the first round of automation is 2.13 m², which is lower than the average coal drawing amount (2.86 m²) in the second round of traditional drawing. The variability in the coal drawing amount per support in the first round of automated drawing is still lower than that in the second round of traditional drawing. Specifically, the standard deviation of the first-round drawing amount is 0.173 m², and the standard deviation of the second-round drawing amount is 0.80 m².

4.3.2. Three-Round Drawing with Combined Automated and Traditional Technologies

In this section, the three-round drawing with combined automated and traditional technologies is simulated. The first-round automated drawing time of each support is set as 0.4 s, the second-round automated drawing time of each support is set as 0.3 s, and the drawing openings for the third round adopt the traditional control approach of “rocks appear, close the opening”.

The results for the three-round drawing with the combined automated and traditional methods are shown in Figure 12. After the first–second rounds of automated drawing, the drawn top coal in 100 coal bunkers below the drawing openings shows a horizontal layered distribution. After the third round of traditional drawing, the top coal morphology drawn from 100 drawing openings still exhibits a slightly fluctuating distribution.

To quantitatively investigate the effect of the three-round drawing with the combined automated and traditional methods, this section compares and analyzes the drawing results of each support in each round, as shown in Figure 13. From Figure 13a, after the automated first–second round drawing with a constant drawing time (0.7 s) for each support, the average drawing time per support for the third-round traditional method is still 0.489 s with a standard deviation of 0.127 s. As shown in Figure 13b, the average coal drawing amount per support for the third-round traditional method is 2.10 m², which is greater than the average drawing amounts (1.62 m² and 1.24 m²) for the first round and second round. Moreover, the drawing amount variance per support in the third round of the traditional method is 0.66 m², which is greater than the variances (0.16 m² and 0.14 m²) in the drawing amount in the first–second rounds of automated method.

To further study the combined technology, the total coal amount for each drawing opening was compared with those of the automated multiple-round technology, as shown in Figure 14.

From in Figure 14, for the coal-rock model with a 3 m top coal thickness, the drawing amount of the automated one-round technology is greater than that of the traditional one-round technology. The top coal recovery rate of the entire mining face using two-round automated technology is 67.5%, which is lower than the recovery rate of 75.6% for the two-round combination technology. Moreover, the recovery rate for the three-round automated technology is 60.1%, which is lower than the recovery rate of 75.2% for the three-round combination technology. Concretely, the total drawing amount for each drawing opening in two-round combination technology is an average of 4.99 m², which is higher than the total drawing amount (average 4.46 m²) of each drawing opening for the two-round automated technology. Moreover, the total drawing amount of each drawing opening for three-round combination technology is an average of 4.96 m², which is higher than the total drawing amount (average 3.97 m²) of each drawing opening for the three-round automated technology. Moreover, the two-round combination technology yields a drawing variance of 0.80 m², which is higher than the variance of 0.28 m² for the two-round automated technology. The three-round automation technology yields a drawing variance of 0.61 m², which is higher than the variance of 0.28 m² for the three-round automated technology.

5. Discussion on Drawing Process Optimization

Based on the theoretical analysis and simulation, for top coal of 3 m thickness, the traditional method with a control approach of “rocks appear, close the opening” tends to form a large drawing funnel, which is filled by gangue after the top coal is drawn. Under the effect of a rock funnel, there is early mixing of gangue during the coal drawing of adjacent supports, causing the early closure of the drawing opening. This results in a short and irregular coal drawing time for each support. Workers have to manually operate the drawing openings in an irregular and frequent manner, thereby increasing the labor intensity. However, the automated coal drawing method with a set time involves regular opening and closing of the drawing opening, making it easier to operate and significantly reducing the labor intensity for workers. In comparison, the automated coal drawing process is simpler, and the top coal drawing of each support is relatively uniform. This leads to smoother power transmission for the rear scraper conveyor, making it more suitable for automated mining and efficient transportation of the mechanical equipment. For automated multi-round drawing technology, the top coal can be gradually released in layers, thereby reducing the disturbance of the entire coal-rock interface caused by individual drawing openings. This results in a smoother sink of the coal-rock interface and a more standardized and orderly drawing of top coal.

On the other hand, the traditional drawing method can effectively avoid gangue drawing and achieve a higher recovery rate for top coal on the working face. In contrast, automated single-round drawing can release a larger amount of top coal, but some drawing openings may also release a certain quantity of gangue particles. Although automated multi-round drawing can further result in a smoother sink of the coal-rock interface, however, the top coal recovery rate decreases as the drawing round number increases. By combining automated drawing with the traditional method, we can not only obtain a higher recovery rate for top coal but can also achieve a relatively uniform distribution for drawn top coal in 100 coal bunkers, which results in smoother power transmission for the rear scraper conveyor. Moreover, the layered sinking of fractured coal-rock mass can effectively hinder the transportation of goaf gangue behind the support to the drawing openings, minimizing the gangue transported by the rear scraper conveyor.

Further, the recovery ratio, drawing time, and drawing amount of top coal were counted and analyzed to comprehensively optimize the technology combining the automated and traditional drawing methods, as shown in Figure 15. Overall, the top coal recovery rates of combined technologies involving the automated and traditional methods are greater than those of multiple-round automated technologies. Concretely, the top coal recovery rate of the one-round traditional drawing is 76.4%, which is higher than the recovery rate of 75.6% obtained using the two-round combination technology and the recovery rate of 75.2% obtained using the three-round combination technology, as shown in Figure 15a. However, the one-round traditional technology yields a coal drawing variance of 1.2 m², which is higher than the drawing variance (0.80 m²) obtained with the two-round combination technology and higher than the variance (0.61 m²) obtained with the three-round combination technology, although the coal drawing variance obtained with the multiple-round automated technology is smaller (0.28 m²), as shown in Figure 15b. Moreover, the drawing time variance for the one-round traditional technology is 0.22 s, which is greater than the drawing time variance (0.151 s) for the two-round combination technology and greater than the drawing time variance (0.126 s) for the three-round combination technology, although the coal drawing time is a constant value for the multiple-round automated technologies, as shown in Figure 15c.

Based on the data analysis of Figure 15, the recovery rate of top coal determines the economic efficiency of the mine, the variance in the drawing amount between drawing openings causes power instability in the scraper conveyor system, and the drawing time variance for each support influences the complexity of the control operation. For a 3 m top coal thickness, under different drawing technologies, two-round automation combined with “rocks appear, close the opening” produces a relatively optimal drawing effect by comprehensively considering the recovery rate, rock mixed rate, drawing time, and drawing amount of top coal in the entire working face.

6. Conclusions

In this study, the drawing characteristics for low–middle-thickness top coal were investigated by a theoretical analysis and simulation, the traditional coal drawing and automated coal drawing were compared, the effects of a round number on automated top coal drawing were analyzed, and a combined automated–traditional technology was discussed. The following conclusions were obtained:

(1) For a top coal thickness of 3 m, the traditional drawing method with “rocks appear, close opening” and automated one-round drawing can draw out most of the top coal. The top coal recovery rate is 78.5% for the automated one-round drawing, slightly higher than the recovery rate of 76.4% for the traditional drawing. The recovered top coal inside 100 coal bunkers beneath the drawing openings exhibits a layered distribution for the automated one-round drawing, while the recovered top coal morphology of traditional drawing shows a fluctuating distribution. For the variance in the drawing amount for each drawing opening, the traditional drawing method has a higher standard deviation of 1.197 m² compared to that of the automated one-round drawing with a standard deviation of 0.322 m². Compared to the constant drawing time of automated technology, the drawing time for each support in the traditional technology exhibits a fluctuating variation with a variance of 0.22 s.

(2) For the automated multi-round drawing of 3 m thick top coal, as the total round number of coal drawing increases, the top coal recovery of the working face gradually decreases, and the coal drawing amount from 100 drawing openings becomes more uniform; moreover, the rock mixed rate in the drawn top coal continuously decreases. Concretely, as the total round number increases from 1 to 3, the total coal drawing amount from 100 drawing openings continuously decreases. The recovery rate decreases from 78.5% to 60.1%, the average drawing amount from each drawing opening decreases from 5.32 m² to 3.97 m², the drawing amount variance decreases from 0.322 m² to 0.275 m², and the rock mixed rate decreases from 2.6% to 0.026%.

(3) For the combined automated–traditional drawing technology, the rock layer above the top coal sinks in a layered form, and the coal-rock interface gradually sinks, which promotes the relatively uniform and orderly drawing of fragmented top coal; moreover, the drawing openings can be closed in a timely manner by the “rocks appear, close opening” rule. This not only reduces the mix of rock into the drawn top coal but also obtains a higher recovery rate. For the engineering background of this study, the two-round automation combined with the “rocks appear, close the opening” rule has a relatively optimal drawing effect on the mining face.

In this study, an ideal optimized drawing process for 3 m thick top coal was proposed, which provides certain guidance for the automated mining operations under low–middle-thickness top coal conditions. However, the exact time required for each drawing opening in the combination technology still needs to be corrected by engineering practices, because the shapes, mechanical properties, and boundary conditions of coal-rock blocks and masses in the field are different from those of the simulation particles.