Top Coal Recovery Research Articles

Study of automated top-coal caving in extra-thick coal seams using the continuum-discontinuum element method

In order to investigate top-coal drawing regularity and automated top-coal caving technology in extra-thick coal seams, an approach coupling the particle element and the block element is presented based on the continuum-discontinuum element method (CDEM). Then, the constitutive model of hydraulic support is introduced into the CDEM to simulate the top-coal drawing process. Meanwhile, the Bergmark–Roos model for coal drawing of single support is extended into the collaborative coal drawing of multiple supports to optimize the automated top-coal caving technology. Finally, CDEM models of hydraulic support, armored face conveyer and coal-rock strata are established according to the field conditions of the “Tongxin Coal Mine”. Then, the top-coal drawing mechanism is analyzed, and an automated top-coal caving technology is proposed. The results show that under the interaction between hydraulic support and surrounding rocks, the final drawing body of initial top-coal drawing appears as an irregular, deflected ellipsoid shape. During different support-moving cycles, the coal-rock structure affects the coal drawing amount and support resistance. A statistical comparison of the coal-rock drawing amount based on 100 drawing openings indicates that the standard deviation of the coal drawing amount in automated, one-round coal caving is 1.83 m2 less than that in traditional coal caving (8.23 m2), and the coal drawing amount from each drawing window is more uniform. For multi-round coal caving techniques, the average recovery rate remains at 79.4% and the rock mixed rate is less than 1%. Based on a comparison of several coal caving techniques, automated four-round coal caving technology is the optimal technology, in which the change rate of the standard deviation of the coal drawing amount as well as the top-coal recovery rate are both largest.

Effect of the size distribution of granular top coal on the drawing mechanism in LTCC

The size distribution of granular top coal holds great influence on the drawing mechanism in longwall top coal caving (LTCC) panel. In this paper, based on the data observed from the Ruilong mine in Shanxi province, China, the effect of the size distribution of granular top coal on the drawing mechanism was investigated using theoretical analysis, 3D physical simulation and discrete element numerical calculation. The results show that the volume of the drawing body increases linearly with increasing weighted average size of granular top coal, while the maximum width of the drawing body increases nonlinearly; with increasing length of the opening, the drawing volume also increases nonlinearly at a gradually decreasing rate. With the increasing weighted average size of granular top coal, the top coal recovery ratio of the panel increases first and then decreases, and when the weighted average size of granular top coal is in the range of 150–250 mm, the recovery ratio is higher. A situation in which the size distribution of granular top coal is relatively dispersed and the standard deviation is larger is more conducive to the granular top coal drawing in the working face. When the percent of small particle is larger, the top coal recovery ratio decreases with increasing length of the opening, and it is suggested to use a single opening; in contrast, when the percent of large size particles is larger, the top coal recovery ratio increases first and then remains at a certain level, and it is suggested to use a double opening. Suggested measurements are proposed to improve top coal recovery in LTCC panel based on the research results.

Caving mechanisms of loose top-coal in longwall top-coal caving mining based on stochastic medium theory

A good understanding of the caving mechanisms of loose top-coal is significant for longwall top-coal caving (LTCC) mining. In this paper, the equations of the particle motion, top-coal boundary, and drawing body were derived based on the stochastic medium theory. Then, the particles moving boundary and the shape of the top-coal loss were discussed, and the dynamic evolvement of the top-coal boundary and drawing body was analyzed. A theoretical model for calculating the top-coal recovery ratio was established, and the relationship between the top-coal recovery ratio and rock mixed ratio was studied and then the criterion for terminating the drawing cycle was proposed. Finally, model experiments were conducted to verify the theoretical analysis. It is concluded that the starting top-coal boundary keeps moving to the lower left with increasing drawing volume. The top-coal loss is zonal distribution and tilts toward the goaf and is composed of the top-coal surrounded by the floor, the starting top-coal boundary and the ending top-coal boundary between the drawing window and the lowest moving point in the starting top-coal boundary. The drawing body tilts toward the goaf and its deflection angle decreases with increasing drawing volume. The top-coal recovery ratio increases non-linearly with increasing rock mixed ratio, and the rock mixed ratio of 10–15% can be used as the criterion for terminating the drawing cycle. This work is the first attempt at demonstrating the caving mechanisms of the loose top-coal using the stochastic medium theory in LTCC mining.

Characterizing the Top Coal Cavability with Hard Stone Band(s): Insights from Laboratory Physical Modeling

One of the most challenging problems in longwall top coal caving is the top coal cavability. Stone bands, which are usually found in thick coal seams, have shown adverse effects on top coal fragmentation and cavability. Understanding the characteristics of the stone band caving process is a primary step in evaluating top coal cavability in such conditions. In this study, a series of laboratory tests was implemented to find an appropriate construction material that can truly display the whole caving process of top coal. On this basis, two large-scale physical models comprising sand, gypsum, and mica complying with relevant similarity rules were constructed to investigate top coal cavability with hard stone band(s) according to two real cases. Based on observations from physical models, it is concluded that hard stone band(s) embedded in top coal usually produce poor fragmentation and cavability of the top coal, which is also affected by the strength, thickness, and location of the stone band(s). According to its relation with the caved goaf and the top coal beneath it, the rock beam consisting of the stone band and the above top coal can produce three profiles before it caves, to which suitable fragmenting and caving characteristics were carefully discussed. Summarizing the stone band beam into different profiles enables the characterization of top coal cavability for various configurations of stone bands in top coal. The results also verified that the fragment size of the top coal can be used as an integrated parameter that reflects all potential factors that impact top coal recovery to evaluate its cavability.

Dynamic Random Arching in the Flow Field of Top-Coal Caving Mining

The large mining height fully mechanized top-coal caving mining technique has developed rapidly and become the most extensively used mining method for ultra-thick coal seams. The arching of coal/gangue and the drawing out of gangue are peculiar phenomena in the process of fully mechanized top-coal caving mining, which not only affects the recovery of top-coal, but also affects the quality of the coal. This paper studies the arching phenomenon in top-coal caving mining process of ultra-thick coal seam. A series of laboratory granular material simulation experiments were performing and a top-coal arching model in the framework of mechanics was established to explore the formation characteristic of arches and their effects to top-coal loss. Then the countermeasures against the arches and technology path of intelligent mining based on improving top-coal recovery were put forward and performed in practice. The results show that the recovery ratio of top-coal has increased nearly 6%, and increased the production efficiency at the same time. The research on arching mechanism and removing strategies of dynamic random arches effectively improves the efficiency of fully mechanized top-coal caving mining in ultra-thick coal seams, and provides the foundation for the realization of intelligent top-coal caving mining technology.

Experimental and numerical investigation on coal drawing from thick steep seam with longwall top coal caving mining

Steep coal seam mining activities will frequently occur during the next few decades in China. In this study, both experimental and numerical methods are employed to investigate the coal drawing from thick steep seam with longwall top coal caving mining. A series of analyses is performed to investigate the features of the drawing body, the distribution of top coal recovery ratio and the shape of the rock flow under steep conditions. The results indicate that the drawing body of top coal develops prior to upper side of the panel face obviously, and the top coal in the central part of the panel has a higher recovery ratio than that in the lower and upper parts in steep coal seam with caving mining method. The flow paths of the fragmented top coal are nearly straight lines moving towards the drawing window, and the fastest path maintains a constant angle with the plumb line. The spatial shape of the rock flow indicates “bidirectional asymmetry,” which results from the presence of the shield beam and dip angle of the coal seam; thus, this is the root cause of the appearance of the drawing body’s prior development towards the upper side of the panel. The field observation data indicates the same distribution of top coal recovery as that in the physical experiment and numerical simulation. Furthermore, suggested measurements are proposed to improve top coal recovery in steep seam mining based on the engineering practice of Dayuan coal mine.

Theoretical equation of initial top-coal boundary in longwall top-coal caving mining

The theoretical equation of initial top-coal boundary (ITCB) is one of fundamental problems of top-coal drawing law in longwall top-coal caving (LTCC) mining. The granular mechanical model of ITCB is established based on loose medium flow field theory. By analysing stressed top-coal on ITCB along section of coal seam thickness and face advancement directions, theoretical equation of ITCB is deduced respectively in two conditions of first gangue drawing out and excessive gangue drawing out. The theoretical equation is further modified by a correction coefficient k, which is given by a large number of experimental data. Meanwhile, it is found that ITCB at excessive gangue drawing out condition is in special shape that the top opening is elliptical and bottom opening is rectangle. The theoretical equation of ITCB can provide meaningful guidance to improvement of top-coal recovery ratio in LTCC.

Support-surrounding rock relationship and top-coal movement laws in large dip angle fully-mechanized caving face

When mining the fully-mechanized longwall caving face along strike, the unstable equipment, the low top-coal recovery ratio and the difficulty in controlling surrounding rock may occur due to large dip angle. Considering the effects of strike angle on support stability, the “support-surrounding rock” mechanical models of support topple and support slip were established in this paper. On the basis, the influencing factors of support stability were analyzed and the technical measures of controlling support and surrounding rock stability were put forward. Then the loose particles simulation experiment was conducted to analyze the impacts of caving directions and methods on the top-coal recovery in large dip angle fully-mechanized caving face. Finally, the “upward sequence and double-openings double-rounds” caving technology was determined. The research results are of great scientific significance and practical values to improve large dip thick seam mining technology.

A new research system for caving mechanism analysis and its application to sublevel top-coal caving mining

The study of top-coal drawing characteristics is key to improving the top-coal recovery ratio and to reducing the waste of coal resources in longwall top-coal caving (LTCC) mining. This paper introduces a Boundary-Body-Ratio (BBR) based research system established by the authors following a long-term detailed study of the top-coal drawing mechanism. For BBR system, a simplified parabolic model is proposed to describe the development process of the boundary of top-coal in normal top-coal drawing cycles, where the drawing body is a cut variant ellipsoid (CVE) that can be described by the modified Bergmark-Ross model. A method for improving the recovery ratio and reducing the rock mixed ratio of top-coal by selecting reasonable drawing technique parameters and controlling the shape of the boundary of the top-coal is given for the BBR system. Based on the BBR research system, the drawing mechanism for loose top-coal in sublevel top-coal caving (SLTCC) in steeply inclined coal seams is studied for different sublevel heights and drawing direction conditions using distinct element numerical calculations and loose top-coal drawing experiments. The results show that the boundary curve of top-coal can be fitted by a parabola and that the drawing body remains as a cut variation ellipsoid in SLTCC, as is the case for a flat coal seam. The convex point of the boundary of the top-coal moves toward the goaf with increasing sublevel height, which would result in the incomplete development of the top-coal drawing body. The drawing extent index decreases linearly with increasing sublevel height, and the top-coal recovery ratio decreases after an initial increasing phase. Approximately 90% of the residual coal in the current sublevel will not be extracted during the drawing process of the next sublevel. Drawing from the floor to the roof expands the overlap area between the drawing body and the boundary of the top-coal, improving the recovery of the top part of the top-coal. Such a drawing sequence decreases the coal loss near the floor by 38.2% compared to drawing from the roof to the floor and greatly increases the resource recovery ratio.

Arch structure effect of the coal gangue flow of the fully mechanized caving in special thick coal seam and its impact on the loss of top coal

Based on the characteristics of the top coal thickness of the fully mechanized caving in special thick coal seam, the long distance of coal gangue caving, as well as the different sizes of the coal gangue broken fragment dimension and spatial variation of drop flow, this paper uses laboratory dispersion simulation experiment and theoretical analysis to study the arch structure effect and its influence rule on the top coal loss in the process of coal gangue flow. Research shows that in the process of coal gangue flow, arch structure can be formed in three types: the lower arch structure, middle arch structure, and upper arch structure. Moreover, the arch structure has the characteristics of dynamic random arch, the formation probability of dynamic random arch with different layers is not the same, dynamic random arch caused the reduction of the top coal fluency; analyzing the dynamic random arch formation mechanism, influencing factors, and the conditions of instability; the formation probability of the lower arch structure is the highest, the whole coal arch and the coal gangue arch structure has the greatest impact on top coal loss. Therefore, to prevent or reduce the formation of lower whole coal arch structure, the lower coal gangue arch structure and the middle whole coal arch structure is the key to reduce the top coal loss. The research conclusion provides theoretical basis for the further improvement of the top coal recovery rate of the fully mechanized caving in extra thick coal seam.

Flow of top coal and roof rock and loss of top coal in fully mechanized top coal caving mining of extra thick coal seams

This study investigates the flow and caving characteristics of top coal and roof rock, as well as top coal loss pattern in the fully mechanized top coal caving mining of extra thick coal seams. The two dimensional discrete element numerical simulation software program, particle flow code (PFC), is used for the simulation of top coal caving and the inversion analysis. The original locations, distribution, and migration pattern of caved top coal and lost coal were obtained. The analysis shows that in the initial site of caving, the caved bodies are in the form of arc shaped strips in front of the working face. During the caving, caved bodies of different heights move towards the lower rear of the face at different speeds. The lost coal and caved roof rock are originally located at the interface between coal seam and roof, the lost coal is mainly distributed in the goaf on the floor. Behind the support, the caved top coal bodies originally are arc shaped strips, with the highest points located at the midline of the caving opening. The strips are more curved near the goaf than those near the support. During top coal caving, the strips successively cave, with the adjacent outer strip replacing the caved one. The variations of top coal loss and waste rock ratio with time reflect the different phases of top coal caving. In order to improve coal recovery and limit the amount of caved roof rock, the waste rock ratio should be controlled below 10 %. When the waste rock ratio reaches this value, the caving opening should be closed. This paper provides theoretical bases for the improvement of top coal recovery in the fully mechanized top coal caving mining of extra thick coal seams.

Caving mechanisms of loose top-coal in longwall top-coal caving mining method

The top-coal caving mining method, developed steadily in the past two decades, has become the main method for thick seam coal mining in China. In recent years, the core research areas in the top-coal caving mining are the mechanisms of top-coal caving and the improvement of top-coal recovery ratio. In this paper, the mechanisms of the top-coal caving are illustrated by a large number of physical simulation experiments and numerical modelings of actual cases. The marker method, which has sufficient accuracy to calculate the top-coal recovery ratio, is also described in detail.

Technological optimization of fully mechanized caving mining face with large mining heights

Fully mechanized cave mining with large mining height is a new mining method, due to its large mining thickness and lower roadway excavation, the technology has been widely used in China’s thick seam mining. In order to improve the top-coal recovery ratio of fully mechanized cave mining with large mining height, a study was conducted on optimizing the caving process, based on the mechanized caving face 1302N in Longgu Coal Mine. This was achieved by improving the PFC numerical calculation methods, and establishing a more accurate model system. On this basis, the recovery ratio of the top coal in different drawing intervals and technologies was investigated in order to achieve a reasonable caving process. The top-coal tracking system was used for practical surveying of the recovery ratio of top coal.

Technique of top coal caving with vibration

How to understand the top coal caving process and increase the top coal recovery ratio is one of the urgent problems to be resolved for the longwall top coal caving technology. The paper mainly studies a new theory and technique of top coal caving with vibration. The vibration device is mounted on the shield beam of the top coal caving support. The laboratory experiment and the numerical simulation were adopted in the study. The Distinct Element Method (DEM) numerical software (PFC2D) is used for the numerical simulation. Finally, the technique is applied to the 5209 top coal caving working face. Based on the experiment, the numerical simulation and the practice, the vibration can easily destroy the arch structure which is formed during the top coal caving process. The parameters about when the arch structure would be formed during the top coal caving process are obtained. The movement of the top coal caving process under the vibration condition makes a more stable layer uniform caving performance. The aim of the top coal caving with vibration technique, to increase the top coal recovery ratio and to lower the waste content rate, has been achieved.

Numerical simulation of the top coal caving process using the discrete element method

How to understand the top coal caving process and increase the top coal recovery ratio is one of the urgent problems to resolve for longwall top coal caving technology. This paper discusses a new theory and technology of top coal caving with vibration. The vibration device is mounted on the shield beam of the top coal caving support. The vibrator destroys the arch structure formed during the top coal caving process, which results in the smooth top coal caving process, and in this way the top coal recovery ratio is increased. The distinct element method (DEM) numerical software particle flow code in 2 dimensions (PFC2D) is used for the numerical simulation. It is proven that the vibration can easily destroy the arch structure which is formed during the top coal caving process. The parameters about when the arch structure would be formed during the top coal caving process have been obtained. The movement of the top coal caving process under the vibration condition makes a more stable layer uniform caving performance. The aim of the top coal caving with vibration technology, to increase the top coal recovery ratio and to lower the waste content rate, has been achieved.

Three-dimensional numerical analysis of deformation and failure during top coal caving

In this paper, the deformation and failure of the top coal in top coal caving was simulated to study the mining process with FLAC software. In order to improve the rate of hard top coal recovery, the top coal must be weakened. The variations of the deformation and failure in top coal before and after weakening are also discussed. Finally, the distribution of the abutment pressure was obtained. 11 refs., 9 figs., 1 tab.