Top Coal Research Articles

Analysis of Coal Outlet Parameters by Simulation Modeling of Longwall Top Coal Caving

Analysis of Coal Outlet Parameters by Simulation Modeling of Longwall Top Coal Caving

Overburden Failure and Ground Pressure Behaviour of Longwall Top Coal Caving in Hard Multi-Layered Roof

Overburden Failure and Ground Pressure Behaviour of Longwall Top Coal Caving in Hard Multi-Layered Roof

Study on Roof Instability Effect and Bearing Characteristics of Hydraulic Support in Longwall Top Coal Caving

In longwall top coal caving (LTCC), due to the fracture and migration of top coal, the roof will break and collapse, which causes serious impact damage to hydraulic support. Therefore, we aimed to reveal the relationship between the roof instability effect and the bearing characteristics of hydraulic support in the LTCC face. Based on the occurrence conditions of the 08 mining area in the Shilawusu Coal Mine, the instability model of the upper immediate roof was established, and the working resistance of hydraulic support was derived. Secondly, the dynamic coupling model of roof-top coal-hydraulic support was established in LS-DYNA, and the crushing degree of top coal and the bearing characteristics of the hydraulic support in different roof instability fields were analyzed. The results show that the main factors affecting the working resistance of hydraulic support are the fracture position of the upper immediate roof, the acting force of the lower immediate roof, and the distribution of the gangue in the goaf. The rotary instability of the upper immediate roof at the coal wall brings serious impact effects, resulting in fractures in front of the coal wall and a large amount of crushed coal concentrated at the front end of the canopy. The crushing degree of top coal significantly impacts the canopy, especially the back end of the canopy and the hinged pin shaft, which is prone to bending fracture. The research results can provide references and experience for the stability control of roof strata and the structural optimization of hydraulic support.

Dynamic sublevel caving technology for thick seams with large dip angle in longwall top coal caving (LTCC)

Dynamic sublevel caving technology for thick seams with large dip angle in longwall top coal caving (LTCC)

Research on Coal Gangue Recognition Based on Multi-source Time-Frequency Domain Feature Fusion.

The over-exploitation of resources caused by the increasing coal demand has resulted in a sharp increase in solid waste emissions mainly gangue, which has made the burden on the environment, economy, resources, and society of our country heavier. In order to achieve a balance between energy consumption and solid waste emission in the process of top coal caving, this study carried out coal gangue recognition research based on multi-source time-frequency domain feature fusion (MS-TFDF-F). First, the process of coal gangue symbiosis and the harm of gangue in top coal caving are analyzed, and the fundamental method of comprehensive treatment of gangue is put forward, which is the accurate recognition of the coal gangue interface. Second, by building a top coal caving simulation test bed, the MS signals generated in the caving process of the coal gangue mixture with a gangue content of 0-100% are collected and the TFDFs are extracted. Third, the MS-TFDF-F-based coal gangue recognition model is established. Then, the recognition effect of the two TFDF-F sample sets was compared, and the results show that the time-frequency domain feature selection fusion method (TFDFS-FM) has higher accuracy. On this basis, this paper studies the variation law of the number of sensors on the coal gangue recognition accuracy of MS information fusion. Finally, the economic, social, environmental, and resource benefits of the model are qualitatively described. The final results show that the MS-TFDF-F-based coal gangue recognition model has the strongest recognition ability when fusing six sensor signals, and the recognition accuracy reaches 99% under the AdaBoost algorithm. The establishment of this model brings huge benefits to China's environment, economy, resources, and society, and it is helpful to realize the balance between loss reduction mining and solid waste emission reduction in the process of top coal caving.

Study on the Cause of Hypoxia in the Corner of Return Air of Shallow Buried Flammable Coal Seam Group Mining Face and the Coordinated Prevention and Control of Coal Spontaneous Combustion

Coal is the main energy source in China, and spontaneous combustion of coal is one of the main disasters in coal mines. Fully mechanized caving top coal mining technology is widely used in coal mines. Under the comprehensive action of multiple factors, gas generated by coal oxidation and low oxygen gas in goaf is easy to rush to working face, resulting in lower oxygen concentration in upper corner and exceeding harmful gas concentration. It seriously threatens the health of underground workers and the normal operation of production. If the oxygen concentration in the upper corner is changed, it is bound to cause the change of the oxygen concentration in the goaf and aggravate the threat of spontaneous combustion of coal in the goaf. In order to solve the double threat of low oxygen in upper corner and coal spontaneous combustion in goaf, based on Coulomb model, this paper uses command flow to conduct numerical simulation tests on the failure law of overlying strata, and obtains the influence of mining of the lower 1 coal seam and the lower 5 coal seam on the overlying strata subsidence and surface penetration. Based on the comprehensive consideration of atmospheric pressure, ore pressure activity, external air leakage and other factors, the mixed model of the source and emission of low oxygen gas in goaf was established, and the formation mechanism of low oxygen problem in the corner of return air of working face was defined. The use of pressure air belt to deal with low oxygen in return air corner is proposed creatively. The comprehensive fire prevention measures such as corner plugging, nitrogen injection and grouting are used to inhibit the oxidation of the left coal and the influx of low oxygen gas to the corner in the goaf, so as to achieve the goal of collaborative prevention and control of low oxygen in the corner of return air and coal spontaneous combustion. The research results have important reference significance for the mine with the risk of spontaneous combustion or the problem of low oxygen in the return air corner.

Numerical Investigation of the Influence of Roof-Cutting Parameters on the Stability of Top Coal Gob-Side Entry Retaining by Roof Pre-Fracturing in Ultra-Thick Coal Seam

Gob-side entry retaining by roof cutting, a pillarless mining technique, plays a critical role in maintaining continuous production, rapid connection, and enhancing the coal recovery rate in fully mechanized top coal caving working faces. This technique stands as a sustainable development method in coal mining. The present research, set against the backdrop of the Yitang Coal Mine 100602 top coal gob-side entry retaining by roof cutting, investigates the influence of roof-cutting borehole depth, borehole dip angle, mining height, and coal seam thickness on stability in an ultra-thick coal seam under 12 distinct mining conditions. A typical model of overburden structure post-roof pre-splitting was established to study the failure mechanism of the top coal roof. The results reveal that the dip angle and depth of the roof pre-fracturing borehole significantly impact the movement characteristics of the overlying strata. Optimal conditions are found when the dip angle and depth of the roof pre-fracturing borehole, the mining height, and the top coal thickness are 10°, 16 m, 4 m, and 4 m, respectively. Under these circumstances, the load transfer from the goaf to the gob-side entry can be effectively intercepted, mitigating the influence of roof fracture activities on the top coal gob-side entry. Field measurements confirm that suitable anchor-net support can stabilize the roof’s rock structure. This research underpins the significance of roof pre-fracturing for the promotion and application of top coal gob-side entry retaining by roof cutting in ultra-thick coal seams.

Theoretical investigation and key caving technology development at the end area of longwall top coal caving (LTCC) panels

Theoretical investigation and key caving technology development at the end area of longwall top coal caving (LTCC) panels

Cooperative mining technology and strata control of close coal seams and overlying coal pillars

Mining of close coal seams envisages reservation of a large number of section coal pillars of a total width ranging from 30 to 40 m. This results in an enormous waste of coal resources and risks of spontaneous combustion, instability, and damage to the remaining coal pillars. This study attempts to mitigate the above problems via the proposed cooperative mining technical method for close seams and overlying coal pillars. It expounds on the technical principles, the required mining equipment, technology, and the system layout, and analyzes the key influencing factors of strata control. Using the numerical simulation via FLAC3D and PFC2D commercial software packages, the effect of horizontal distance between the lower coal seam roadway and the overlying coal pillar (i.e., spacing) on the surrounding rock control and rational layout of the roadway is studied. The mechanism of the overlying coal pillar width effect on the ground pressure of the stope and coal pillar recovery rate is clarified. The engineering case study shows that when the horizontal spacing was increased from 0 to 15 m, the surrounding rock deformation, surrounding rock stress peak value, and plastic zone area of the roadway decreased significantly. When the coal pillar width in the overlying section was increased from 25 to 40 m, the peak values of lateral and advance bearing stresses in the stope dropped by 21 and 12 MPa, respectively. Besides, the ground pressure driving strength of the stope was significantly reduced, and the recovery rates of the overlying coal pillar and rock between the close coal seams were increased by 7.45 and 4.29%, respectively. Combined with the top coal pillar drawing pattern, optimization concepts and their realization methods were proposed.

Mechanism of the coal bursts in the working face during mining of steeply inclined and extra thick coal seam

Coal bursts caused by mining-induced seismicity occur in the working face in steeply inclined and extra thick coal seams (SIETCSs). This article presents a case study in SIETCS through discrete element simulation as well as field investigation. The evolution of the coal burst was studied from the perspective of stress distribution, crack propagation and coal fragment ejection. The results showed that dynamic failure mainly occurred in the coal near the roof. The deep bottom coal and the top coal near the roof are under high static stress due to the clamping effect of the roof and floor and the rotation of the deep roof. The coal burst in the working face was triggered by mining-induced seismicity. The coal burst occurred when the sum of the static stress and the dynamic stress caused by the mining-induced seismicity exceeds the sum of the ultimate strength of the coal and the support strength, which was called the ‘clamping-rotation-dynamic loads’ mechanism. During the coal burst, the cracks developed rapidly into the deep bottom coal and the top coal on the roof side. The results can serve as an important reference for the coal burst failure mechanism in the working face in SIETCSs.

Coal-gangue recognition via multi-branch convolutional neural network based on MFCC in noisy environment

Traditional coal-gangue recognition methods usually do not consider the impact of equipment noise, which severely limits its adaptability and recognition accuracy. This paper mainly studies the more accurate recognition of coal-gangue in the noise site environment with the operation of shearer, conveyor, transfer machine and other device in the process of top coal caving. Mel Frequency Cepstrum Coefficients (MFCC) smoothing method was introduced to express the intrinsic feature of sound pressure more clearly in the coal-gangue recognition site. Then, a multi-branch convolution neural network (MBCNN) model with three branches was developed, and the smoothed MFCC feature was incorporated into this model to realize the recognition of falling coal and gangue in noisy environment. The sound pressure signal datasets under the operation of different device were constructed through a great deal of laboratory and site data acquisition. Comparative experiments were carried out on noiseless dataset, single noise dataset and simulated site dataset, and the results show that our method can provide higher correct recognition accuracy and better robustness. The proposed coal-gangue recognition approach based on MBCNN and MFCC smoothing can not only recognize the state of falling coal or gangue, but also recognize the operational state of site device.

Impact Response of the Tail Beam Jack Based on Bidirectional Fluid-Structure Coupling Simulation.

In top coal caving mining, the coal rock collapse will cause an irregular impact on the tail beam jack of the caving control mechanism. The severe impact will lead to jack failure. The bidirectional fluid-structure coupling model is built on Fluent and Mechanical software to study the impact response of the tail beam jack. The dynamic flow velocity streamlines, hydraulic pressure distribution, stress field, and strain field of the jack under impact load are extracted. The response characteristics of the jack in the stationary state and motion state are analyzed. The conclusions are as follows: the stress and strain of the rodless cavity are much larger than those of the rod cavity, which is more likely to be damaged. The hydraulic pressure in the jack cavity is in vertical layered distribution. The flow velocity streamlines present spiral shapes. The response degree of the hydraulic pressure signal in the rodless cavity is stronger than that in the rod cavity, and the response degree of the flow velocity signal in the rod cavity is stronger than that in the rodless cavity. The impact response of the jack in the motion state is more sensitive and stronger than that in the stationary state. The coal rock collapse situation can be most effectively identified only by comprehensively analyzing the rodless cavity's pressure signal and the rod cavity's velocity signal. This paper innovatively visualizes the flow velocity streamlines and pressure distribution together. The bidirectional fluid-structure coupling method is innovatively applied to the tail beam jack. The findings of this study can help for better understanding of the tail beam jack's structural design and failure prevention. This study provides a certain research basis for the intelligent coal rock identification technology in mining coal based on jack vibration signals.

Numerical modelling of loose top coal and roof mass movement for a re-mined seam using the top coal caving method.

The re-mined face using top coal caving system is the most applicable method for recovering the remaining reserves of a previous partially-mined thick coal seam. However, this mining method may encounter the problems of low recovery and unpredicted geological conditions. A numerical model is developed using PFC2D for studying the movement of top coal mass and development of coal-rock mass interface at a longwall top coal caving re-mined face. The re-mined face advances in the lower seam below the upper solid coal pillar, previous entries and gob. A theoretical analysis according to the unsteady flow model is developed to calculate the proper time duration of caving operation. The results showed that the top coal to be recovered through the caving window before the initiation of caving operation is a partial spheroid-shaped geometry. The boundary between the coal and rock mass then develops to a funnel-shaped coal-roof interface as the caving operation continues. The top coal recovery is 98.1%, 77.1% and 70.5% for caving operations below the solid coal, entries and gob area in the upper seam, respectively. The proper timing of caving and interval of caving operation is important for obtaining a high coal recovery. Good agreement is achieved between the proposed model and the improved Boundary-Release model for short of B-R model. The study in this work may provide reference for the safety and efficiency of the extraction of the longwall top coal caving re-mined face.

Drawing mechanisms of granular top coal considering the structure of hydraulic support in longwall top coal caving

Drawing mechanisms of granular top coal considering the structure of hydraulic support in longwall top coal caving

Mechanical behavior of coal under true triaxial loading test connecting the effect of excavation- and mining-induced disturbances

In order to understand the mechanical behavior of a stope and analyse the stress conditions of surrounding rocks after ventilation shaft excavation and the abutment stresses of the top coal mining layout. In situ uniaxial stress monitoring tests were performed to understand the field abutment pressure evolution. Furthermore, the variation of the stress concentration factor was studied using the Tongxin Mine 8309 mining face of the TongMei Group, China, as a case study. A loading model of coal rock under excavation- and mining-induced disturbances was proposed, and the stress path and experimental method were designed to investigate the high excavation-damaged zone (HDZ), excavation-damaged zone (EDZ), and undisturbed zone (UZ). Moreover, the mechanical characteristics and failure characteristics of coal rock in different zones under site disturbance were obtained and the mesoscopic analysis is carried out by acoustic emission test. The results indicated that the strengths of the UZ, EDZ, and HDZ samples declined gradually. The volume of the UZ sample continuously expanded. However, the volumes of the EDZ and HDZ samples were compressed first and then expanded. Furthermore, the shear bands presented in all three samples resulted in failure. The cubic triaxial tests performed without considering mining-induced pressure produced higher strengths and less damage compared with the results of the true triaxial tests. The stress path used in the laboratory with the mining-induced stress throughout the stope could optimally reproduce the in situ mining process, which is significantly safe and efficient for the mining of deep resources.

Application of Gob-Side Entry Driving in Fully Mechanized Caving Mining: A Review of Theory and Technology

China has abundant coal resources, and the distribution of coal seams is complex. Thick coal seams account for more than 45% of all coal seams. Fully mechanized top coal caving mining has the advantages of large production, high efficiency, and low cost. In fully mechanized caving mining, especially in fully mechanized caving mining of extra-thick coal seams, the mining space is ample, the mine pressure is severe, and the roadway maintenance is complex. As a result, it is necessary to summarize and discuss the gob-side entry driving of fully mechanized caving in theory and technology, which will help to promote the further development of fully mechanized caving gob-side entry driving technology. First, in recent years, the research hotspots of gob-side entry driving have focused on the deformation mechanism and the control method of the roadway surrounding rock. Secondly, this paper discusses the theoretical models of the “triangle-block” and “beam” for the activity law of the overlying strata in gob-side entry driving, including the lateral breaking “large structure” model, compound key triangle block structure model in the middle and low position, the high and low right angle key block stability mechanics model, elastic foundation beam model, low-level combined cantilever beam + high-level multilayer masonry beam structure model, and the vertical triangular slip zone structure model. It introduces the “internal and external stress field theory” and the “stress limit equilibrium zone model”. Thirdly, it summarizes several numerical simulation analysis methods in different conditions or research focuses and selects appropriate constitutive models and simulation software. Finally, it introduces surrounding rock control technology, including two ribs, the roof, and under challenging conditions. It provides a method reference for support in similar projects.

Hybrid connected attentional lightweight network for gangue intelligent segmentation in top-coal caving face

The estimation of gangue content is the main basis for intelligent top coal caving mining by computer vision, and the automatic segmentation of gangue is crucial to computer vision analysis. However, it is still a great challenge due to the degradation of images and the limitation of computing resources. In this paper, a hybrid connected attentional lightweight network (HALNet) with high speed, few parameters and high accuracy is proposed for gangue intelligent segmentation on the conveyor in the top-coal caving face. Firstly, we propose a deep separable dilation convolution block (DSDC) combining deep separable convolution and dilation convolution, which can provide a larger receptive field to learn more information and reduce the size and computational cost of the model. Secondly, a bridging residual learning framework is designed as the basic unit of encoder and decoder to minimize the loss of semantic information in the process of feature extraction. An attention fusion block (AFB) with skip pathway is introduced to capture more representative and distinctive features through the fusion of high-level and low-level features. Finally, the proposed network is trained through the expanded dataset, and the gangue image segmentation results are obtained by pixel-by-pixel classification method. The experimental results show that the proposed HALNet reduces about 57 percentage parameters compared with U-Net, and achieves state-of-the art performance on dataset.

A novel non‐pillar coal mining technology in longwall top coal caving: A case study

AbstractIn longwall top coal caving (LTCC) mining methods, problems such as large deformations, loss of coal resources, and excavation relay out of balance are often encountered with the increased production in single mining panels. To address these problems, a novel non‐pillar coal mining technology in LTCC was used based on the equilibrium mining theory. This approach could cut off the stress transmission, eliminate the coal pillar, and retain the gob‐side entry. In this technology, the entry is first excavated along the roof of the coal, and then four techniques are applied to control the surrounding rock of the retained entry. The constant resistance and large deformation anchor cable are used to maintain the roof stability. The directional roof split blasting technique is adopted to separate the roof between the entry and gob. The blocking‐gangue support system (BGSS) is used to form the gangue rib lagging the mining panel. The temporary support system is applied to prevent the roadway from being affected by the dynamic pressure and prevented the BGSS from falling. Finally, the field monitoring results indicate that the surrounding rock of retained entry is stabilized at 200 m behind the working face. The deformation of retained entry is within a reasonable range at 180–190 m behind the mining panel. The results provide an important reference for the thick coal seam non‐pillar coal mining technology, specifically using this technology in LTCC panels.

Study on Coal Gangue Identification Based on Vibration and Its Adaptability to Coal-Caving Process Parameters

The broken top coal and gangue in the fully mechanized cave mining will be released from the rear of the hydraulic support in the form of bulk. In this process, the medium properties through the coal opening can be judged by monitoring the vibration signal on the tail beam. Based on the coupling method of the discrete element method (DEM) and the finite element method (FEM) in LS-DYNA, this article simulates the whole process of coal caving, establishes the finite element model of the tail beam of hydraulic support and the discrete element model of coal gangue particles, and analyzes the influence of coal caving step distance, mining and caving ratio, and pitching angle of the mining field on the application effect of the vibration identification method. The coal caving step distance has a great influence on the recognition effect, and some bottom coal should be appropriately sacrificed to reduce the gangue content. The application effect of the vibration identification method is good under different mining and caving ratios. Downward mining can cause gangue to be released in advance, making the recognizable signal lag behind the change of gangue content, while upward mining will lose a part of the coal, but the application of vibration recognition is better than downward mining.

Improving the top coal recovery ratio in longwall top coal caving mining using drawing balance analysis

ABSTRACT The recovery ratio of longwall top coal caving (LTCC) technology is an important measure of its effectiveness. However, the recovery ratio of single-opening sequential caving technology in thick and extra-thick coal seams needs improvement. To address this, an independent cluster-group caving technology is proposed in this study. Four numerical simulation experiments were conducted to compare the recovery ratio and drawing balance of four-opening independent cluster-group caving technology and single-opening sequential caving technology. Results show that the recovery ratio in four-opening independent cluster-group caving technology is approximately 6% higher than in single-opening sequential caving technology when the thickness of the broken gangue layer and the coal seam are the same. Additionally, a judgment formula for the broken immediate roof thickness is provided when the top coal recovery ratio is seriously affected. The independent cluster-group caving technology demonstrates stronger stability and better adaptability under different conditions, as its caving sequence can prevent larger thickness changes and gangue disturbances during the drawing process. Overall, this study highlights the potential of independent cluster-group caving technology to improve the recovery ratio of LTCC technology in thick and extra-thick coal seams.