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Coal and gas outburst is tightly related to the existence of tectonic coal. Because large blocks of tectonic coals are difficult to be collected, reconstituted coal specimens are often used to study the mechanical properties. In this study, reconstituted coal specimens were prepared under different external forces. Then, triaxial compression tests were conducted under different confining pressures. The deviatoric stress, volumetric strain, axial strain and dilatancy ratio were analyzed. From the laboratory tests, the cohesion of reconstituted coal is positively related to the applied external force. Bonds are developed between coal particles. It is found that the deviatoric stress mainly contributes to bond breakage and internal friction. Under low confining pressure testing condition, the specimen is in over-consolidation state. The failure process is similar to the failure of intact coal. The deviatoric stress reaches the peak value. Then a strain-softening effect is observed with the bond breakage. But for normal consolidation scenario, the bond-breakage stress is smaller than the maximum internal friction force. With the increase of deviatoric stress, the shear stress exceeds the threshold limit value of bond breakage first and then, it exceeds the maximum internal force. Before the new unstable fracture development, extensive bond breakage happens. When the shear stress exceeds the maximum friction force, internal slippage occurs with sharp increases of dilatancy ratio. From this study, the failure behaviors of reconstituted coal are closely related to the stress conditions. The understanding of tectonic coal failure process is enhanced.

The fragmentation of coal produces abundant coal gases and is presumed to be the defining characteristic of coal and gas outbursts. Knowing the mechanism of these catastrophic hazards is one of the most important breakthroughs in mining geology. In the outburst process, coal spallation represents a unique failure type and typically leaves behind a spallation area (with a series of fracture textures) in the coal seam. Revealing its features and formation mechanism is crucial in accurately interpreting the outburst process. In this study, we conducted a series of outburst experiments with different gases, including CO2, N2, and He. We establish that a spallation area can develop spontaneously during CO2 and N2 tests, whereas an outburst caused by He tests (even under stressed conditions) does not produce a spallation area. That is, the spallation area cannot be observed in non-absorbable gas outbursts. We, therefore, focus on the role of coal gas in spallation and propose a viable mechanism to explain its formation. During the outburst development stage, the influence of gas ad-/desorption is critical, as it controls the width of the spallation area and the spallation thickness. In contrast, stress is not a necessary condition. Whether a spallation area will be produced is particularly determined by the generation of a sufficient internal pressure gradient. Moreover, because of gas desorption, the total outburst energy can be increased by 1.84–5.30 times; and the mean outburst propagation velocity and the mean frequency of coal spallation ejected can be enhanced by 0.38–8.76% and 1.28–12.07%, respectively. Consequently, the destructiveness of outbursts depends on the contribution of desorbed gas.

This research reviewed the mechanics and gas desorption properties of intact coal, and tested the crushing work ratios of different intact coals, and then, studied the stress conditions for the failure or crushing of intact coal and the gas demand for the pulverization of intact coal particles. When a real-life outburst case is examined, the required minimum stress for intact coal outburst is estimated. The study concludes that the crushing work ratios of three intact coal samples vary from 294.3732 to 945.8048 J/m2. For the real-life case, more than 2300 MJ of transport work is needed, and 10062.09, 7046.57 and 5895.47 m3 of gas is required when the gas pressure is 1, 2 and 3 MPa, respectively. The crushing work exceeds the transport work and even reaches 13.96 times of the transport work. How to provide such an enormous crushing work is an energy-limiting factor for the outburst in intact coal. The strain energy is needed for the crushing work, and the required minimum stress is over 54.35 MPa, even reaching 300.44 MPa. These minimum stresses far exceed the in-situ vertical and horizontal stresses that can be provided at the 300–700 m mining depth range.

CO2 geo-sequestration in unminable coal seams is one promising method to mitigate greenhouse gas emissions and would be a means of cleaner production. At the same time, the injected CO2 can enhance coalbed methane recovery and offsets the economic cost for CO2 transportation and injection. However, this technology is still at its primary stage and so far, no large scale of field CO2 injection has been implemented. One of the main difficulties encountered in this process is the permeability reduction induced by CO2 injection and as a result, the injection rate is sharply dropped in the later period. In this paper, detailed laboratory experiments are conducted to investigate the permeability reduction effects during CO2 injections. The dynamic permeability reduction effects are measured with the continuous CO2 injection under different confining pressure conditions. Permeability reduction effects of 13 %–77 % are obtained with respect to the CO2 exposure time in different testings. In low permeability scenario, the permeability reduction effects are severer. Specifically, when the absolute permeability is 1mD, 0.42mD, 0.18mD and 0.03mD, the permeability reduction is 13 %, 36 %, 48 % and 77 %, respectively. Compared with the coal triaxial adsorption tests, it is suggested that the permeability equilibrium is not equal to the adsorption equilibrium. The triaxial adsorption equilibrium process takes much longer time than that of permeability equilibrium process. Based on the testing results, a novel dynamic permeability model is proposed to illustrate the permeability evolutions. The existence of weak internal surface influences the matrix strain and the permeability. This model is also validated by other’s experimental results and can be used for explaining coal seam permeability reduction effects of CO2 injection in coal seams.

Outburst energy is a major factor influencing coal and gas outbursts, albeit its estimation is difficult owing to the lack of amenable means for quantification of gas desorption. In the past decades, determining the mechanism of outbursts is one of the most challenging issues in rock mechanics. In this study, a triaxial coal and gas outburst simulation system was employed to perform simulated experiments using He (to rule out the influence from gas ad-desorption), N2, and CO2. This facilitated understanding of the energy principle underlying the said outbursts and evaluation of the effects of gas desorption on outburst development. Results of this study indicate that outburst energy and energy consumption are influenced by several factors, including outburst pressure, outburst intensity, ejection distance, and particle size of ejected coal. Among these, gas desorption demonstrates the greatest influence when performing controlled tests (using He). Considering the effects of gas desorption, the total outburst energy can be increased by 1.35–2.95 times, thereby causing an enormous increase in the destructive potential of outbursts. Additionally, values of the coal crushing and transport energies can be enhanced by the order of 118.9–206.6% and 157.8–406.6%, respectively, thereby resulting in a stronger conveying capacity of outburst coal–gas flow along with severe coal fragmentation. A further analysis of the energy distribution indicated that in the development stage, gas desorbed from coal acts as the force driving coal transport, whereas free gas energy is mainly consumed during coal crushing. Findings of this study highlight the importance of quantifying contributions of coal gas towards effective interpretation of outburst-causing mechanisms.

Deep mining and drilling activities typically trigger significant deformation of coal/rock roadway and even coal/rock dynamic hazards, which has attracted considerable attention. Hence, in this article, the mechanics and failure properties of intact coal are examined through triaxial compression tests and creep tests. First of all, triaxial compression experiments demonstrated that the complete stress–strain curves of intact coal are characterised by Class-I behaviour; and the post-peak modulus is negative. Second, with increasing confining pressure, peak strength and residual strength rise linearly, whereas elasticity modulus and peak strain increase as a power function form. Thirdly, compared with conventional triaxial load, tiered cyclic load can stiffen the strength of intact coal. Moreover, the data of creep tests are successfully fitted to the Burgers model. Maxwell elasticity modulus is significantly smaller than other creep parameters; Maxwell viscosity coefficient shows a positive correlation with deviatoric stress. Contrarily, Kelvin elasticity modulus and Kelvin viscosity coefficient drop with the rise in deviatoric stress. Additionally, with increasing deviatoric stress, single-step creep strain and average creep strain rate, proposed in this study, increase in a power function form. Finally, failure characteristics of intact coal under triaxial stress could be classified into four types (mainly shear failure and multiple shear fractures). The failure-plane angle generally linearly increases with the rise in confining pressure applied.

The intrinsic exothermic reaction of coal with oxygen during low-temperature oxidation is the main heat source responsible for the self-heating and spontaneous combustion of coal. Due to the comple...

It is imperative to have an in-depth understanding of CO overrun at the air return corner not only for pre-warning concentrations of safety and spontaneous combustion but also for providing scienti...

The catastrophic failure of coal under super-critical stresses and mining-induced disturbance is becoming one of major safety risks of underground mining. Research of the failure pattern o...

Coal mine fire often occurs in lean-oxygen environments caused by methane. In order to clarify the characteristics and mechanisms of coal-oxygen reaction under these conditions, the coal-oxygen reaction experiments were carried by using thermogravimetric analysis (TGA)-gas chromatograph (GC) combined technology in the range of 30-800°C. The thermogravimetric-subtraction/derivative thermogravimetric-subtraction (TG-S/DTG-S) curves which can more accurately reflect the coal-oxygen process were obtained by subtracting the TG/DTG curve of coal pyrolysis under the condition of pure N2. Through TG-S/DTG-S curves, the coal-oxygen reaction was divided into low temperature oxidation stage, accelerated oxidation stage and combustion stage. In the low temperature oxidation and accelerated oxidation stages, methane delayed the coal-oxygen reaction as an inert gas. Under the same oxygen concentration, the effect of methane on reducing CO, CO2 production and increasing ignition temperature (Ti) of coal was obvious than...

Coal and gas outbursts are small-scale geological disasters controlled by tectonic movement, and tectonic coal is widespread in outburst zones. In this study, we compare tectonic and intact coal specimens to examine the basic properties of tectonic coal. We estimate the different energies and limits of the crushing work ratio of coal from five typical outburst cases using on-site outburst data, and discuss the relationship between outbursts and tectonic coal. The results show that tectonic coal is a product of tectonic movement and its original primary structure is destroyed during the tectonic process. Compared with intact coal, tectonic coal shows low strength properties and a crushing work ratio of 22.11 J/m2. The specific surface area and total pore volume of the minipores, mesopores, and macropores of the coal strongly increase under conditions of intense tectonism, which indicates that tectonic coal has a very high capacity for rapid initial gas desorption. An adequate supply of gas is required to transport outburst coal, such that the existence of coal particles smaller than the critical diameter is important. Our calculations indicate that the crushing work ratio of coal from the five outburst case ranges from 22.19 to 78.67 J/m2. Only the crushing work ratio of tectonic coal satisfies the requirement for these cases. Therefore, the properties of the tectonic coal and crushing work ratio for the five cases indicate that the widespread occurrence of tectonic coal plays a crucial role in outbursts.

Spontaneous combustion of coal has long been a thermal dynamic hazard during coal mining, storage, and transport, posing a great threat to coal mine safety. Especially coal spontaneous combustion in longwall goaf poses a great threat to underground working crew. To investigate such a mine safety issue with more insights and “what if” scenarios, this study employs CFD technique to develop a proactive inertisation plan in a longwall goaf. Based on real on-site conditions of the longwall goaf, a three-dimensional transient non-equilibrium thermal CFD model was developed to study heating evolution and proactive inertisation plans of the longwall goaf. The theoretical model incorporated a set of governing equations including low temperature kinetics of coal oxidation, energy and mass conservation, momentum balance, and continuity equation. After the base model (1000 m) was validated and calibrated with field gas monitoring data, another model (500 m model) was studied to obtain an optimum inertisation plan. Both steady state and transient simulations were conducted to study the flow dynamics of air velocity, oxygen ingress, dispersion of gaseous products and heating evolution in the longwall goaf. Based on the flow dynamics field, proactive inertisation plans using nitrogen to suppress the onset and development of goaf heatings were then developed and studied.

Spontaneous combustion of coal in underground coal mine is a long-standing thermal dynamic hazard. The hazard is harmful in diverse aspects: causing loss of coal resource, raising safety concerns, and giving off noxious/greenhouse effect gases. Detection and trending analysis of a few particular gaseous products liberated during coal oxidation is the most fundamental spontaneous combustion forecasting technique in practice. This study mainly reviewed the mechanism and practical knowledge by using such technique to forecast spontaneous combustion. To give more insights in emerging order of fire gas, this study critically reviewed and analysed the detailed production sequences of these key gaseous products. It was indicated production of carbon oxide, hydrogen, methylene and some other hydrocarbon gases can be used to forecast early heating of coal. This study also summarised and discussed the interpretation of the index gases through absolute concentration of key gas indicators and composite ratios. Six common gas monitoring techniques (i.e. tube bundle system, telemetric system, portable system, gas chromatography, infrared spectroscopy, and tunable diode laser adsorption spectroscopy) are discussed in terms of their advantages and limitations in this study. Lastly, a practically demonstrated spontaneous combustion hazard management plan (i.e. TARP) is introduced. TARP uses different gas ratios in various locations to indicate escalating levels of severity of a heating event.

Dust management at longwall faces has always been a concern for mine operators. Recent respirable dust monitoring at a longwall top coal caving (LTCC) face indicated that extremely high dust exposure levels have been experienced at the face. Dust contamination from intake ventilation and the incorrect use of ventilation arrangements (i.e. curtains) at maingate (MG) area were identified as the main cause of this issue. Computational Fluid Dynamics (CFD) modelling studies were therefore conducted to investigate the impact of different ventilation arrangements on the dispersion behaviour of respirable dust at the LTCC face, with a special focus on the airflow patterns and the aerodynamics of fugitive dust at the intersection of MG and face. Field investigation was first carried out to observe the dust issue and obtain essential data for the development and validation of base model. Then parametric studies were conducted to evaluate the effectiveness of two different curtain configurations at MG considering the worst scenario of intake dust contamination to face (dust from travel road and beam stage loader (BSL) discharge point flows towards face) with the shearer cutting into the MG. Model results demonstrate that the occurrence of flow separation and incorrect use of curtains account for the main reasons of high dust exposure level at the intersection of MG and face, especially when the shearer is cutting into the MG. Ventilation arrangements at the MG and face entry are critical to minimise the impact of flow separation on the dust flow patterns at the intersection. Based on model results, new ventilation arrangements at the MG and face entry have been proposed and evaluated through which significant dust mitigation effect can be achieved at face entry, contributing to the overall reduction of dust exposure levels along the face.

Most of the coal seams in China are characterized by high in-situ stress, high gas pressure, high gas content and low permeability, which makes the gas extraction rather difficult. This infrastructure not only limits CBM production but also presents considerable challenges to the safety and efficiency of coal mining in China.

Adsorption is a type of interfacial phenomena, which includes physical adsorption, chemical adsorption and absorption. The gas adsorption in coal belongs to physical adsorption, which means that the gas molecules concentrate in the surface of coal pores, i.e. the gas molecule concentration at the interface between gas and coal is larger than elsewhere. For the physical adsorption, the adsorption force between the solid surface and the gas is the van der Waals force, and the adsorption volume depends mainly on the pressure, temperature and the size of coal surface area. Similar to the gas liquefaction and water vapor condensation, physical adsorption is also reversible. The heat released by gas adsorption is small, and generally, there is about 10–20 kJ of heat released when 1 mol of gas molecules are adsorbed, which is similar to that of gas liquefaction. For the chemical adsorption, chemical bonds are formed between the solid molecule and the gas molecule on the solid surface, i.e. there exists electron exchange between gas molecules and solid surface molecules. The main difference between chemical adsorption and physical adsorption is that the chemical adsorption is irreversible, and the generated heat of chemical adsorption is similar to the thermal effect of chemical reaction, which is generally ten times to several tens of times larger than the physical adsorption heat. A large number of experiments on coal adsorption show that the gas adsorption/desorption in coal is a reversible process and the volume of adsorbed gas is approximately the same as the desorbed gas. The gas adsorption heat measured by experiments ranges from 12.6 to 20.9 kJ/moL, which is approximately equal to the heat released by methane liquefaction.

In view of the problem where there is abnormal continuous CO exceedance in the tailgate corner without any sign of spontaneous combustion associated with the mining of low metamorphic coal seam, both desorption experiments and ambient temperature oxidation experiments were carried out, and the ambient temperature oxidation characteristics and its mechanism were studied. The results demonstrate that the original coal seam from which the experimental coal sample was taken, does not contain CO and CO2, and the coal sample can oxidize at ambient temperature. The O2 consumption rate, CO and CO2 production rate is not directly related to the particle size. Low O2 concentration can inhibit but not eliminate the ambient temperature oxidation. In addition to the active sites of coal itself, the ambient temperature oxidation reaction rate is also affected by the inhibitory substances produced by the reaction itself. R1(△CO/△O2), R2(△CO2/△O2) and R3(△CO/△CO2) indexes are mostly not affected by the changes of particle size, O2 concentration and oxidation rates, and the ambient temperature oxidation of coal is dominated by a single mechanism. The ambient temperature oxidation mechanism is also theoretically deduced. The research will have important significance for controlling the abnormal exceedance of CO.

Moisture, ash content, volatiles, density and hardness of coal. Pore, fracture structures of coal and characterization methods. Coal matrix and its scale features

Gas drainage technologies are the main means of lowering the gas content of coal seams and eliminating the gas hazard of coalmines. As China has a huge reservoir of coalbed methane (CBM) resources, the extraction and utilization of coal seam gas resources can realize the significant triple benefits of ensuring the safe mining of coal resources, promoting the clean and efficient use of coalmine gas, and protecting the environment.

Coal and gas outburst is the extreme instability caused by stress, gas, and coal. In this review article, dominant factors and inducing factors of outburst were summarized; geologic features of typical outburst cases and the effects of tectonic movement on outbursts were analyzed; the outburst stages with considerations to geologic factors were divided. It was found that inducing factors, including buried depth, tectonic movement, gas composition, coal seam conditions, overlying/underlying rock conditions, and mining mode, control the outburst by influencing the dominant factors (stress, gas, and coal). Among them, tectonic movement is the key of outburst. Influenced by tectonic movement, the primary structure of coals is damaged/pulverized due to the tectonic stress and unique tectonic mode, resulting in the formation of tectonic coals. When external dynamic factors are changed, tectonic coals are crucial to outburst control for its evolution of porous structure as well as the unique mechanical behaviors and gas flowing responses. Besides, the preparation stage of outburst includes the tectonic process and mining process. The former one refers to the restructuring process of the original coal-bearing strata by tectonic movement, while the mining process is the prerequisite of outburst and it refers to the disturbance of human mining activities to the initial coal seams. It is suggested that more work is required on geological factors of outburst, and a few research areas are proposed for future research.

Coal strength plays a significant role in various mining related engineering activities, including the evaluation of coal and gas outburst dangers, pillar design, hydraulic fracturing design (involving enhanced gas drainage), coalface support, coal seam CO2 sequestration, and other activities.

Coal formation, including coalification effect, peatization effect, diagenesis effect and metamorphism effect; Metamorphism types and maceral composition of coal; Generation of coal seam gas during coal metamorphism, basic composition of coal seam gas, and the existence forms of adsorbed gas and free gas. Controlling effect of magma intrusion on the coal metamorphism and coal seam gas occurrence.

The basic concept of force in coal mechanics consists of external force and stress. The external force is generated from the outside of coal seam and can be further divided into two types, surface force and body force. Surface force is applied to the coal surface, such as gas pressure, coal support force and the contact forces between different coal blocks. Body force, such as gravity and inertia force, is continuously distributed within the coal and is also called a mass force.

Based on the particle size evolution of tectonic coal in formation process, the differences between tectonic coal and intact coal in pore structure and initial gas desorption characteristics were studied. Then, the influences of particle size reduction on pore structure and initial gas desorption characteristics were investigated; and the initial gas energy release model was derived, which was used to calculate the gas expansion energy of tectonic coals with different particle sizes. This study concludes that the particle size of tectonic coal will be reduced under the pulverization effect during the formation process, and the particle sizes of these types of tectonic coal reach millimeter level or even smaller scale under pulverized state. The pore volume and specific surface area of minipores (pore size: 10–100 nm), mesopores (pore size: 100–1000 nm) and macropores (pore size: 1000–10000 nm) in tectonic coal are larger than those in intact coal by over 10 times. Tectonic coal’s average gas desorption rate in the first 10 s reaches up to 11.47 times of that of intact coal. Within the 5 particle size greater than 0.074 mm, the pulverization obviously transforms the minipores and mesopores in coals, but its effect on micropores (pore size: 0.9–10 nm) is limited. When the particles are pulverized to blow 0.074 mm, the micropores are obviously transformed. The initial gas energy release model for coal samples with different particle sizes is derived and verified through the experimental data. As calculated using the initial gas energy release model, the gas expansion energy will be reduced sharply with the particle size reduction when the particle size is smaller than 1 mm. Therefore, the particle size of tectonic coal decides its initial gas desorption capacity, which is of critical significance to the gas energy release in the outburst process. This study will be of great scientific significance for exploring the outburst mechanism and prevention.

Coal burst is the violent failure of overstressed coal, and it is often accompanied by sound, coal ejection and seismic events. It is subsequently recognized as a serious safety risk of Australia after double fatalities coal burst happened at Austar Coal Mine. Considering the increasing trend of coal burst severity and frequency with mining depth, it is an urgent task to develop the coal burst risk assessment methods for Australia underground coal mines. Coal burst propensity index method is a widely used method of burst risk evaluation of coal as it is summed up from the coal burst research and practice of many countries. This paper presents the experimental and theoretical research of coal burst propensity index method for coal burst risk assessment in Australia. The definition of four indexes including elastic strain energy index (WET), bursting energy index (KE), dynamic failure time (DT) and uniaxial compression strength (RC) is introduced in the first part. Then, the standard laboratory test process and test parameter of coal burst propensity index is presented. DT test is conducted with 0.3 mm/min displacement control loading rate while other test is with 0.5 mm/min. Besides, modified data processing and risk classification method of test are proposed. Differentiate analysis of stress-strain curve is adopted in the data processing of DT and KE index. A four level risk classification form of burst risk is recommended for Australian underground coal mines. Finally, two likely improvement methods of WET test, including volumetric strain indicator method and theoretical calculation method, are discussed. Keywords: Coal burst, Coal burst propensity, Risk evaluation, Underground mining

Spontaneous heating of coals continues to be a major environmental, safety and economic concern in coal storage, preparation and firing systems in power plants. Coal handling plants mitigate spontaneous heating in coal stockpiles by stacking in layers and compaction. This study was focused on investigating self-heating behaviour of compacted and loosely stacked stockpiles commonly used in a coal handling plant. Experimental stockpiles, each having 2500–3000 ton of coal, were built: compacted and loosely stacked. Temperature changes within the stockpiles were measured over a test period. Hot coals from loosely stacked stockpile were spread out to cool and re-stacked to form a third experimental stockpile.

The performance of pre-gas drainage in low permeable seams especially in CO2 abundant coal seams is not satisfactory. Due to the high affinity of coal to CO2, much lead time is needed for gas drainage boreholes to reduce the coal seam gas content below the threshold limit value (TLV), thus meeting the demand of the scheduled mining activities. In this study, simulations of N2 injection enhanced gas drainage were carried out for CO2-rich coal seams. A binary gas migration model was developed to illustrate gas migration in coal seam. The simulation results matched well with the laboratory test results, indicating that this model could be used to investigate the mechanism of N2 flushing coal seam gas. The parameters affecting the flushing performance were then studied using this model, including coal seam permeability, gas diffusion coefficient, N2 injection pressure andin-situ gas content. These results suggest that coal seam permeability and CO2 diffusion coefficient are the key parameters having significant impacts on the drainage performance whilst N2 diffusion coefficient only has negligible effect. A critical parameter theory was developed to describe the correlation between these parameters (especially coal seam permeability and the diffusion coefficient of coal seam gas). Specifically, in low permeable coal seams, permeability was the critical parameter and the effects of other parameters were insignificant. Similarly, if the diffusion coefficient of coal seam gas was very low, it could become a critical parameter.

Coalbed gas content measurement is important for both unconventional gas recovery and mining safety. This study experimentally discussed factors affecting coalbed gas content measurement. It is found that small coal particles demonstrated high desorption rate at the early stage of desorption tests. However, the difference in methane desorption rate reduces when the coal particle size is more than 1.0 mm, indicating that a threshold of coal particle size exists in methane desorption in coal. Also, the particle size distribution tends to be stable when coal sample mass is over 15 g, stabilizing at around 1.75 mm. Therefore, the coal sample over 15 g is recommended to use in desorption tests of coalbed gas content measurement. The temperature and gas pressure can enhance the desorption rate, especially at the early stage of desorption. This temperature effect, however, mainly occurred in the first 15 min of desorption test. After 30 min, the desorption rate tends to be stable, slightly changing from around 0.35 mol/kg/min to 0.75 mol/kg/min. Due to endothermic process, absolute isothermal condition does not exist in desorption test, but this temperature variation mainly occurred in the first 15 min. The results also show incorporating the desorption data from the time of exposure to 13 min of desorption tests is essential to typically reflect methane desorption kinetics in coal. The study also shows that the measurement difference in gas content between the case with sample being air-puffed and -flushed and the case with sample exposed to the air is small. Therefore, the air-puffing and flushing during the sample collection has an insignificant effect the gas contact measurement result. This study can help to advance the understanding of impact factors on coalbed gas content measurement method.

As the one of the most catastrophic hazards in underground mining, coal and gas outburst seriously threatens the safe mining of collieries. To understand the formation and transport mechanism of outburst coal-gas flow in roadway as well as evaluate the effects of gas desorption on its development, a new apparatus was developed to conduct simulated experiments with different gases of CO2 and N2. Results indicated that the outburst coal-gas flow was a high-speed (up to 41.02 m/s during tests) gas-solid two phases flow with extreme complexity, its transport/destructiveness characteristics were significantly influenced by a number of factors including the outburst pressure, coal sample composition, ejection distance and so on. Among these factors, the gas desorption showed the greatest impact when compared to the controlled tests which only considered the effect of free gas expansion. With the effect of gas desorption, especially the rapid gas desorption from powdered coal, the total outburst energy could be promoted by 1.30–2.43 times; the peak values of outburst shockwave could be enhanced by at least 13.67%–63.22%; the transport type of coal-gas flow could be changed from dynamic pressure pneumatic conveying to the static pressure conveying which providing higher capability for outburst coal/rock conveying; the motion of ejected coal flow could have higher speed, longer transport duration and could suffer secondary acceleration. As the result, the destructiveness of outburst coal-gas flow would be remarkably intensified. A further analysis for the energy consumption during the outburst coal-gas flow transport indicated that the free gas expansion energy was insufficient for the conveying of ejected coal (only accounting for less than half of the total energy), the difference of which was made up by the gas desorption, especially the rapid gas desorption from powdered coal (the average contribution ratio reached 56.0%, while the maximum reached 64.1%). Thus, it can be concluded that the rapid gas desorption from powdered coal played a decisive role on the promotion of outburst.

Spontaneous combustion of residual coal in longwall goaf is a long standing hazard. Airflow leakage into goaf is a major driver to the hazard and this issue deteriorates where longwalls are operating in multiple seams and shallow covers because mining-induced cracks are very likely to draw fresh airflow into goaf due to presence of pressure differential between longwall face and surface. To study the problem more critically, a ventilation simulation package “Ventsim” is used to conduct a case study from Bulianta colliery. It was found that isolating and pressurizing active longwall panel can mitigate the problem and the pressure differential can be adjusted by varying performance of auxiliary fan and resistance of ventilation regulator. A booster ventilation system can also mitigate the problem by adjusting fan duties. Ventilation simulation is a powerful tool to study spontaneous combustion control in underground coal mine. Keywords: Ventilation simulation, Spontaneous combustion, Longwall operation, Pressure differential, Ventsim

Seam gas pre-drainage, is widely used as an effective method to control gas and coal outburst in underground coal mines. However, in CO2 abundant low permeable seams, this technology seems to be less efficient due to the CO2 sorption characteristics and the lower safe mining threshold limit for CO2 applied in many outburst risk management plans. This paper presents the experimental investigations of the applicability of nitrogen (N2) injection to improve gas drainage in CO2 rich seams. Core specimens were obtained from Bulli coal seam, Sydney Basin and N2 flood tests were conducted under different permeability conditions (0.3 mD and 0.06 mD). A triaxial permeability test rig equipped with a back pressure regulator was used to conduct the test. The variation of gas composition, gas outlet flow rate, N2 flushing efficiency, and permeability variation were analyzed. In addition, a comparative study between N2 flushing coal seam CO2 and N2-ECBM was conducted and it was observed that CO2 was much more difficult to drain out and with longer time compared with CH4. Based on the test results, a two-stage flushing mechanism was obtained. In the first stage, the original free phase coal seam gas accounted for the large percentage and this stage last for a shorter time. In the second stage, desorption time governed the flushing efficiency and desorbed gas was the primary gas. It took much longer time than the first stage. The final recovery rate in 0.3 mD and 0.06 mD scenarios were 90.5% and 87.7%, and the N2 consumption/CO2 production ratios were 15 and 11.2, respectively. Through these laboratory experiments, we concluded that N2 injection can significantly decrease coal seam CO2 content level and increase gas drainage efficiency.

Carbon dioxide (CO2) enhanced coalbed methane (ECBM) is an effective method to improve methane (CH4) production and this technology has already been used to increase gas production in several field trials worldwide. One major problem is the injection drop in the later period due to permeability decrease caused by coal matrix swelling induced by CO2 injection. In order to quantify the swelling effect, in this work, coal samples were collected from the Bulli coal seam, Sydney Basin and adsorption tests with simultaneous matrix swelling measurement were conducted. The adsorption and swelling characteristics were analyzed by measuring the adsorption mass simultaneously with the strain measurement. Then experiments were conducted to replicate the ECBM process using the indirect gravity method to obtain the swelling strain change with CO2 injection. The results show that the coal adsorption capacity in CO2 is almost two times greater than that in CH4, and nitrogen adsorption is the least among these gases. A Langmuir-like model can be used to describe the strain with the gas pressure and the swelling strain induced by gas adsorption has a linear relationship with gas adsorption quantity. Moreover, swelling strain increase was observed when CO2 was injected into the sample cell and the swelling strain was almost the sum of the strains induced by different gases at corresponding partial gas pressure.

The relatively low strength and brittleness are believed to be two main factors restricting the widely use of coal reject concrete (i.e. CRC) in practical applications. This paper presents a concept of the novel hybrid concrete column incorporating fibre-reinforced polymer (i.e. FRP), termed FRP-confined CRC (i.e. FCCR) columns, to widen the use of CRC. The systematic compression tests on natural aggregates concrete (i.e NAC) and CRC with the large range replacement ratios of natural aggregates in volume (i.e. 0%, 25%, 50%, 75% and 100%) were firstly carried out, followed by the compressive tests on FCCR columns. Test variables covered the thickness of FRP jacket (i.e.0-ply, 2-ply and 4-ply), the replacement ratio of natural aggregates and the particle size of coal reject. In parallel, the FRP-confined NAC columns (i.e. FCNA) columns have been prepared and tested for comparison. Test results showed that the compressive strength of CRC decreases with the increased replacement ratio of natural aggregates. When the CRC is confined by the exterior FRP jacket, both the compressive strength and the ductility of which have been significantly enhanced. Furthermore, these FCCR columns with a thicker FRP jacket generally exhibited the superior compressive behaviour. The analysis-oriented model developed by Jiang and Teng for FCNA was adopted to predict the compressive behaviour of FCCR. The feasibility of Jiang and Teng’s model for predicting the behaviour of FCCR with the replacement of fine aggregate was verified.

Lean-oxygen environment caused by methane in high gassy coal mine goaf has a significant influence on the prediction index gases of residual coal spontaneous combustion. The coal sample from Xishan coalfield in China was used to conduct the low-temperature oxidation experiments under different lean-oxygen environments. The experimental results show that different lean-oxygen environments had a significant delay effect on the formation of CO, CO2, H2 and C2H6, and the lower the O2 concentration, the more obvious the delay effect, and the delay effect of methane was greater than that of N2. A lean-oxygen environment caused by methane also had a significant influence on the values of CO/ΔO2 ratio and CO2/ΔO2 ratio. However, when the temperature was T can be accurately expressed by exponential function. The absolute amount of CO, CO2 and C2H6 generated by 1 k g residual coal ( C CO , C CO 2 and C C 2 H 6 ), which can reduce the influence of air leakage and residual coal mass on prediction index gases, were introduced to predict the spontaneous combustion in high gassy coal mine goaf. Then, the distribution diagrams of C CO , C CO 2 and C C 2 H 6 with temperature and O2 concentration were obtained, so as to accurately predict the degree of spontaneous combustion in high gassy coal mine goaf. The research results are of great significance to the prevention of spontaneous combustion in goaf of high gassy mine.

The definition of “residual gas” can be found in different scenarios, such as the “fast” and “slow” desorption methods of measuring gas content and the sorption hysteresis test and gas management of coal mines, however, its meaning varies a lot in different contexts. The main aim of this paper is to discuss the existence of truly undesorbable residual gas in coal seam conditions and its impacts on sorption model and gas drainage efficiency. We believe the undesorbable residual gas does exist due to the observation of the extended slow desorption test and the sorption hysteresis test. The origin of undesorbable residual gas may be because of the inaccessible (closed or semi-closed) pores. Some gas molecules produced during coalification are stored in these inaccessible pores, since the coal is relatively intact in the coal seam condition, these gas molecules cannot escape during natural desorption and then create the undesorbable residual gas. Based on the existing adsorption models, we propose the improved desorption versions by taking into consideration the role of residual gas. By numerically simulating a gas drainage case, the gas contents after different drainage times are studied to understand the influence of residual gas content on gas drainage. The results indicate that the influence starts to be obvious even when the total gas content is at a high level, and the impact becomes more and more apparent with increasing drainage time. Our study shows that the existence of residual gas will impede the gas drainage and the total amount of recoverable coal seam methane may be less than expected. Keywords: Residual gas, Inaccessible pores, Desorption model, Gas drainage

Large scale open cut coal mining operations have significant impacts to groundwater in surrounding areas in both active and post-mining phases. The prediction of water inflows into a surface mine excavation is one of the many components involved in mine design phase. Groundwater performance also reacts to mining activities from the operational, economic and safety implications perspective. Under NSW planning legislation, as part of the comprehensive risk assessment, a groundwater impact assessment has to be conducted for a coal project to predict and mitigate the impacts in consideration of the government requirements. In this paper, the groundwater assessment modelling of mine pits was discussed in predicting of groundwater inflows and reviewing analytical and numerical approaches. A methodology of groundwater impact assessment for an open cut mine in NSW with a three-dimensional groundwater flow model Modflow Surfact demonstrated its functions in simulating the project’s impacts on the groundwater regime. The key findings with mitigations are discussed and recommended in the paper to reduce impacts on groundwater and fulfil regulation requirements in NSW. Keywords: Open cut coal mine, Groundwater impact assessment, Mining exposure area, Groundwater modelling methodology

Coal exhibits different creep behaviours when filled with different amounts of gas. Creep tests of coal filled with 0 and 0.5 MPa gas were performed, and strain under different axial stress was compared. The three creep constitutive models which were analysed using the method fitting experimental data for determining which creep model can reflect the creep process of the test best. The results show that the deformation of coal filled with 0.5 MPa gas is more higher than that of coal filled with 0 MPa gas under the same axial stress. Gas plays a positive effect on the deformation of coal process and will accelerate creep process. And gas will reduce coal intensity and change coal creep properties. Compared with Nishihara Model and Extensional Nishihara Model, Burgers Model can reflect the three stages of creep process of coal filled with gas better. The research results can contribute to reveal coal and gas outburst mechanism. Keywords: Coal, Gas, Creep, Constitutive model

Gas flow in coal seam consists of laminar flow through coal cleat and diffusion through pores of coal matrix. Previous studies on the prediction of CBM production mostly focused on the impact of permeability change while the gas exchange between matrix and cleat was assumed to obey unipore diffusion assumption with a single diffusion coefficient. However, numerous scholars have found that a single diffusion coefficient cannot reproduce the sorption kinetic data precisely for a lot of coals, while bidisperse diffusion with fast and slow diffusion coefficients can represent the diffusion process well. Until now, attempts on studying the impact of bidisperse diffusion on CBM production are very limited and mathematical model describing the gas flow with bidisperse diffusion is unavailable. In this study, we propose a fully coupled coal seam gas flow model with consideration of bidisperse diffusion and the interaction between bidisperse diffusion, adsorption strain and geomechanical response of coal. A series...

Pre-gas drainage plays a significant role in the prevention of coal and gas outburst in underground coal mines. However, difficulties of reducing the content of coal seam gas, especially localized coal seam CO 2 , to be below threshold values within a given drainage lead time have been encountered in numerous coal mines in Australia. As the proneness of nitrogen outburst is way smaller than CO 2 or methane outburst, nitrogen injection would be an attractive technology for the underground gas drainage management if it is really able to enhance the drainage efficiency. From this point of view, this paper aims at investigating the feasibility of implementing this technology in Bulli seam, Sydney Basin through numerical modelling. Four parts of work were conducted in this paper. Firstly, a binary gas transport model was developed through analysing the binary gas transport mechanism in both coal matrix and coal cleat, and also the matrix-cleat interaction due to binary gas sorption swelling. A real gas mixture viscosity was introduced and its change depends on the molar fractions of both components and their own viscosities. Secondly, the characteristics of the Bulli seam were studied to collect the parameters required by the binary gas transport model. Through laboratory experiment, field investigation and reviewing literature, the coal seam gas (methane, CO 2 ) and nitrogen adsorption and diffusion properties, in-situ seam properties such as permeability, porosity, cleat spacing and virgin pressure are either measured or estimated. Thirdly, the regular drainage effects in six scenarios with different initial permeability and coal seam gas type were examined and three of them are found to have drainage difficulties, i.e., the moderate permeability (0.5 mD) CO 2 case, the low permeability (0.005 mD) methane case and the low permeability (0.005 mD) CO 2 case. The effects of nitrogen injection on enhancing gas drainage efficiency, especially reducing the gas content, are studied for these scenarios. Results show that a satisfactory result is obtained for the moderate permeability CO 2 case, the CO 2 content can be reduced largely and an apparent increase of CO 2 production rate can be seen. However, the effect of nitrogen injection in low permeability methane scenario is even worse than the regular drainage using two boreholes, although it can increase the methane flow rate of single borehole. For low permeability CO 2 case, nitrogen injection can accelerate the reduction ratio of CO 2 content in part of the injection area and increase the total CO 2 flow rate, but the enhanced effect is limited and the CO 2 content is still high after 12 months of injection. Last but not least, the feature of the movement of coal seam gas content peak in the injection area is discussed and the reasons inducing the different injection effects between methane and CO 2 scenarios are analysed. A combination of lateral movement and drop of coal seam gas content peak is found, and permeability is believed to have great impacts on the movement speed. There is no sign indicating either diffusion or permeability change controls the different injection effects between methane and CO 2 scenarios, while the different competitive adsorption characteristics and the increase of mixture gas viscosities may have influences on this difference.

Self-heating or even spontaneous combustion of stockpiled coal, which is likely to outbreak under favourable circumstances during its transport, process, and storage, is a long-standing thermal dynamic hazard. This hazard is harmful in diverse aspects: causing loss of coal resource and caking property, raising safety concerns upon occurrence of open fire, and giving off noxious/greenhouse effect gases. Due to the complexity of involved physical process (e.g. heat and mass transport) and chemical process (e.g. coal oxidation), formulating an analytical solution to the problem with or even without a transient approach would be a daunting task and the problem is thus more often addressed numerically. So far many numerical models to self-heating of coal have been developed and to summarise these erratic findings, this work critically reviewed theses numerical solutions since the last four decades. Mechanism of self-heating on coal mass and low temperature coal oxidation especially kinetic modelling of coal oxidation is firstly investigated to clarify the involved physical and chemical processes. On basis of the mechanistic understanding, theoretical derivations and progressive advances on governing equations like energy, mass, and momentum conservation are reviewed and compiled in details. Through parametric studies or sensitivity check these models produced fruitful but slightly inconsistent findings. Therefore to provide industry more unbiased and comprehensive guides, the present work examined the influences of various contributors including wind flow, stockpile dimensions, coal particle size, moisture content, and packing porosity on the self-heating behaviour of stockpiled coal. Last not the least, major challenges and perspectives this subject may have are briefly discussed.

Gas pressure changes during the process of coal mine gas drainage and CBM recovery. It is of great importance to understand the influence of sorption pressure on gas diffusion, however, the topic remains controversial in past studies. In this study, four samples with different coal ranks were collected and diffusion experiments were conducted under different pressures through the adsorption and desorption processes. Three widely used models, i.e., the unipore diffusion (UD) model, the bidisperse diffusion (BD) model and the dispersive diffusion (DD) model, were adopted to compare the applicability and to calculate the diffusion coefficients. Results show that for all coal ranks, the BD model and DD model can match the experimental results better than the UD model. Concerning the fast diffusion coefficient Dae of the BD model, three samples display a decreasing trend with increasing gas pressure while the other sample shows a V-type trend. The slow diffusion coefficient Die of BD model increases with gas pressure for all samples, while the ratio &beta, is an intrinsic character of coal and remains constant. For the DD model, the characteristic rate parameter k&Phi, does not change sharply and the stretching parameter &alpha, increases with gas pressure. Both Dae and Die are in proportion to k&Phi, which reflect the diffusion rate of gas in the coal. The impacts of pore characteristic on gas diffusion were also analyzed. Although pore size distributions and specific surface areas are different in the four coal samples, correlations are not apparent between pore characteristic and diffusion coefficients.

This paper presents the results of an evaluation study of gas drainability in the Bulli seam in the Southern Coalfield of the Sydney Basin, NSW, Australia, where the coal seam gas (CSG) contains a high proportion of carbon dioxide (CO2). Historically the gas drainability in some particular areas of this coal seam was found to be particularly poor, which posed a significant challenge to gas predrainage. As a result, a large volume of greenhouse gases were released to the atmosphere during mining of the coal seam. Furthermore, the high gas content associated with the CO2-rich composition also increased the risks of coal and gas outburst incidents, affecting the safety of mining. After systematic literature review of evaluation factors affecting gas drainability, this evaluation study comprehensively analyzed the main critical factors, including the geology of the area, the coal cleat system, coal microstructure, coal permeability, coal sorption capacity, gas content, and gas composition. Field geology analysis showed geological variations that affected the variations of the coal cleat system and CO2 content in the coal seam. Scanning Electron Microscope (SEM) tests showed the tight and less-porous features in hard-to-drain coal samples. The colliery gas database analysis was carried out to assess the impact of gas content and gas composition on the drainability of the coal seam. Laboratory tests showed that the coal seam had a permeability of less than 1 mD and also showed that the coal seam was highly undersaturated, especially with high CO2 content.

Coal mine dust continues to be a health and safety issue in underground coal mines. Coal seam water infusion was developed and widely applied in European coal mines for dust control, and was also a common practice in most Chinese coal mines. This method typically involves the infusion of water into the coal seam to increase its moisture content, and therefore reduce dust generation during mining operations. With the availability of other dust control methods such as water spraying systems, the water infusion method has not been considered as a viable means for dust mitigation in modern mines. However, the increase in production output and the deployment of more powerful equipment for coal cutting and transport and intensive gas drainage practices mean that workers could be exposed to more dust contaminations. Whilst the mine operators are committed to suppress and dilute airborne dust particles using these passive measures, there is a need to critically examine and subsequently develop this proactive dust control technology for practical applications in Chinese coal mines. The paper provides a critical review of the water infusion technologies in view of its technological advances and practical application limitations. The methods of water infusion, mechanism of water flow in coal, the role of surfactants and the key parameters influencing the effect of water infusion on dust control are identified and discussed. Existing problems and prospects for water infusion are analysed.

This paper introduces a novel standing support incorporating fibre-reinforced polymer (FRP), termed FRP tubular standing support (FTSS). The exterior container of FTSS is the small rupture strain (SRS) GFRP tube wrapped with ultra-high molecular weight polyethylene (UHMWPE) FRP jacket with large rupture strain (LRS), while the infill is the coal rejects based backfill material (CBM). The CBM is made of industrial by-product (i.e. coal rejects) and high flowable cementitious grout material, leading to some environmental and economic benefits when it is used in underground mines. Test variables included the type of coal rejects (i.e. screened and unscreened), the thickness of SRS FRP tube (i.e. 1.5 mm and 3.0 mm) as well as the thickness of LRS FRP jacket (i.e. 2-ply, 3-ply and 4-ply). Test results showed that both the load carrying capacity and deformation ability of CBM in FTSS have been significantly enhanced attributed to the confinement provided by exterior FRP container. Existing stress–strain model for FRP-confined normal concrete can be used to reasonably predict the compressive behaviour of confined CBM in FTSS with large deformation ability. The ductile FTSS is suitable for underground spaces and infrastructures where the integrity of structure is the main concern.

Many researchers have concluded that the permeability should be measured after the adsorption equilibrium state is reached and in many permeability models, it is normally assumed that pm (pressure in the coal matrix) is equalized with pf (pressure in the fractures). However, when the dynamic balance between pm and pf is broken which exists in most coalbed methane engineering, the differences between the two pressures induced by gas diffusion and matrix shape factor will generate nonuniform matrix swelling which have not been well learned to date, thus influence the permeability evolution. It is of great importance to investigate this issue on account of it will play a more important role in CO2-enhanced coalbed methane and CO2 sequestration engineering as the coal adsorption capacity for CO2 is considerably higher than that for CH4. To achieve this target, a series of novel coal permeability tests and simultaneous strain measurements were conducted using both natural coal and reconstituted coal. In these experiments, the radial strain evolution characteristics and the permeability evolution characteristics as a function of the adsorption equilibrium time and confining pressure were obtained. Based on the experimental results, the effects of the differences between pm and pf on radial strain evolution and coal permeability evolution were discussed, and a new matrix swelling area propagation theory was achieved. In addition, the differences between natural coal and reconstituted coal in terms of their matrix swelling and stress sensitivity properties were obtained.

Spontaneous heating of coal stockpile has long been a thermal dynamic hazard during coal storage, processing, and transport. A transient non-equilibrium thermal CFD model has been developed to study the low-temperature self-heating behaviour of coal in multiple stockpiles under different prevailing wind conditions. Modelling results from the initial steady wind flow simulation indicate that a wake region can be induced on the leeward side of each coal stockpile. Pressure coefficient drops when the wind stream encounters or leaves a stockpile and the pressure coefficient profiles of multiple stockpiles tend to have more resemblance with a wider spacing. The first stockpile acts like a wind barrier to the adjacent stockpiles and the maximum temperature of it tends to be the first approaching the critical temperature. A ‘hot spot’ will develop and then migrate towards deep regions in each of these stockpiles that are loosely compacted under higher wind velocity conditions. Wind velocity and porosity of stockpile have significant influences on self-heating behaviour of the stockpiles and transport pattern of gaseous products liberated by coal oxidation. Compacting stockpiles from loosely packed scenario to slightly packed scenario might not be able to slow down the temperature rising rate at low-temperature range but could considerably minimize the volume of deteriorated coal. The highest temperature rising profiles of the stockpiles located in downwind side can approach to that of the first stockpile, particularly when they are more widely stacked. Stacking coal stockpiles as close as practically possible is recommended to maximise the “protection” of adjacent stockpiles but would cause undesirable accumulation of carbonic gases. Stockpiles in low height and gentle slope will have a prolonged safe storage period, especially for the first stockpile directly facing the wind direction. However, it may not slow down the self-oxidation process of the adjacent stockpile at very initial stage due to “weakened protection” of the first stockpile. This study has practical reference to coal industry especially where multiple coal stockpiles require to be constructed.

Coal seams that are prone to strong outbursts have low strength and cause heavy structural damage to the seam. Their outburst risk is highly related to the release of the adsorbed coalbed gas, which is controlled by the gas desorption and diffusion characteristics of coal. In the Haizi Coal Mine, China, an extremely high gas outburst risk was detected, and the coals from this area were found to have an unprecedented high degree of fragmentation and were present in the pulverized state. To explain the pulverization characteristics of the pulverized coal, the related physical parameters were investigated; the gas desorption and diffusion properties of the pulverized coal were analyzed and compared with those of the unpulverized coal. The results indicated that the pulverized coal could easily reach the required degree of fragmentation for a coal and gas outburst to occur. Furthermore, the pore volume and specific surface area of the pulverized coal differed according to the coal particle size. Compared with the unpulverized coal, the gas desorption and diffusion properties of the pulverized coal were largely varied, and the pore structure of the pulverized coal was much simpler. The formation of pulverized coal is believed to be closely related to complex geological conditions.

Several mines operating in the Bulli seam of the Sydney Basin in NSW, Australia are experiencing difficulties in reducing gas content within the available drainage lead time in various sections of the coal deposit. Increased density of drainage boreholes has proven to be ineffective, particularly in sections of the coal seam rich in CO2. Plus with the increasing worldwide concern on green house gas reduction and clean energy utilisation, significant attention is paid to develop a more practical and economical method of enhancing the gas recovery from coal seams. A technology based on N2 injection was proposed to flush the Coal Seam Gas (CSG) out of coal and enhance the gas drainage process. In this study, laboratory tests on CO2 and CH4 gas recovery from coal by N2 injection are described and results show that N2 flushing has a significant impact on the CO2 and CH4 desorption and removal from coal. During the flushing stage, it was found that N2 flushing plays a more effective role in reducing adsorbed CH4 than CO2. Comparatively, during the desorption stage, the study shows gas desorption after N2 flushing plays a more effective role in reducing adsorbed CO2 than CH4.

This paper focuses on the size distribution measurement of coal fragments by digital imaging processing. The fast and precise measurement of coal fragments, which is important to understand the crack propagation and energy dissipation process of coal failure, has not been achieved by previous research. In this paper, an image analysis method using MATLAB is proposed to measure fragment size distribution of coal fragments. The acquisition setup, analysis step and coding process for fragment size distribution measurement by digital imaging processing are introduced in detail. The statistical size distribution of coal fragments measured by image processing is compared with the theoretical distribution function and manual sieving results. This paper provides an innovative and efficient method for size distribution measurement in the study of coal failure process.