Coal Body Research Articles

Experimental Study on Spectral Response Characteristics and Mechanical State of Coal Based on Artificial Acoustic Signals

With the increase in coal mining depth, the stress and strain state of coal and rock mass affects the formation of dangerous zones of dynamic phenomena. In order to study the relationship between the frequency spectrum characteristics of artificial acoustic signals and the stress state of coal and gas pressure, a test device and system that can generate acoustic signals by mechanical vibration excitation are developed by using the design idea of the unit module. Firstly, the basic mechanical parameters of coal under uniaxial compression are analyzed. On this basis, we use the test device to study the qualitative and quantitative relationships between the relative stress coefficient K value of the coal body and the axial loading stress, whether it contains gas, and the mechanical vibration force. The test results show that when the gas-containing coal and the gas-free coal are subjected to the same external mechanical vibration knocking force to stimulate the artificial acoustic signal test, the relative stress coefficient K value increases first and then decreases with the increase in axial loading stress. The relationship between the relative stress coefficient K and the axial loading stress σ can be expressed in the form of exponential function K=e−Cσ. When the axial loading stress and the external mechanical vibration force are both fixed values, the relative stress coefficient K value of the coal body with gas is smaller than that without gas. When the axial loading stress and gas-bearing pressure of the coal body are both fixed values, the relative stress coefficient K value decreases with the increase in the impact force of the external mechanical vibration. This experimental study can provide a reference for the identification and prediction of dynamic disasters based on artificial acoustic signals.

Exploring the evolution of infrared radiation characteristics in coal bodies under water pressure during tunnel excavation

Exploring the evolution of infrared radiation characteristics in coal bodies under water pressure during tunnel excavation

Correlation between surface wettability and fluid physics characteristics of lignite under compound solution regulation

The specific composition and ratio of the composite wetting agent will affect its dust reduction effect. Therefore, in order to study the effect of the composite wetting agent on the wettability and physical structure of lignite, the anionic surfactant sodium dodecyl sulfate was selected and mixed with different concentrations of sodium chloride to obtain different concentrations of the composite wetting agent. The correlation between the fluid physical properties of these composite wetting agents and the wetting of lignite surfaces was comprehensively explored by contact angle measurement, surface tension analysis, and low-temperature liquid nitrogen adsorption experiments, and the potential effects on pore structure were investigated. The results showed that as the sodium chloride concentration increased, the surface tension, contact angle, adhesion work, and surface free energy of the composite solution decreased, while the diffusion coefficient increased. There was a high linear correlation between the pore diameter of coal and the contact angle (R2 = 0.955), which revealed the close relationship between the permeability behavior of the fluid in the coal pores and the physical structure of the coal body. In addition, composite wetting agents with high sodium chloride concentrations exhibited a specific pore expansion effect, which helped optimize the coal's pore distribution and thus promoted effective coal wetting. This study not only deepens our understanding of the role of fluid physics principles in dust removal technology but also provides an important theoretical basis for selecting suitable composite wetting agents and their optimal ratios.

Dynamic response characteristics of coal/rock during water injection and freezing process under gas atmosphere and its control effect on gas outburst

The freezing method compensates for the defect of sacrificing coal integrity to reduce gas content, which is the case with traditional methods, achieving the improvement of coal body strength while reducing coal seam gas energy storage, improving the safety of coal and gas outburst accidents in deep coal seams during the process of rock cross-cut coal uncovering. This study conducted water injection and low-temperature freezing experiments on coal/rock samples under the gas atmosphere, analyzing the effects of water and temperature on sample temperature, deformation, and gas adsorption and desorption characteristics. The results indicate that water can displace adsorbed gas in coal/rock samples, and the relationship between the gas displacement and the water content of the sample satisfies an improved exponential function. The center temperature Tm of low water content coal/rock samples decreases with time and gradually tends to stabilize, while the Tm of high water content samples experiences a short-term deceleration or stagnation due to the phase transition heat release of water when it drops to around 0 °C. The cooling rate of samples with low water content and no gas is higher and that of rocks is higher than that of coal samples. Coal/rock samples with high water content experience frost heave during the freezing process, but the overall deformation is still dominated by cold shrinkage, and the amount of deformation is negatively correlated with temperature and water. The gas adsorption capacity of coal decreases linearly with the temperature. At the same time, an increase in water content and a decrease in freezing temperature will significantly reduce the gas desorption capacity of coal samples, effectively reducing the gas expansion energy of coal samples, especially the desorption gas expansion energy. In engineering implementation of this method, the ice phase network can fill the coal pores and cracks and improve the mechanical properties of the coal/rock mass, and the gas pressure in the coal seam and stress concentration near the coal rock interface can be reduced by low temperature and cold shrinkage, thereby achieving safe exposure of the coal seam and preventing accidents from occurring.

Molecular dynamics simulation of water absorption and mechanical weakening in coal rocks based on Monte Carlo methods

Aiming at the problem of roadway destabilization in water encountered in the roadway of the back mining roadway of an extra-thick coal seam in Xinjiang Dabei Coal Mine, this paper used molecular dynamics simulation to study the interaction between coal and water and analyzed the adsorption characteristics of water molecules and the change of mechanical properties of coal. Firstly, the model of C181H138N2O24 bituminous coal was constructed by test, the coal-water adsorption was simulated based on the Monte Carlo method, and the coal-water adsorption configuration was analyzed. The results showed that: the saturation adsorption capacity was about 60 water molecules/cell, and the water molecule adsorption was concentrated in the vicinity of oxygen-containing groups and hydrogen atoms; the increase of water molecule content led to the decrease of heat of adsorption and diffusion capacity, and the heat of adsorption decreased by 9.1 %, and the diffusion coefficient of the early stage of adsorption was about three times of that of the final stage; the mechanical parameters of the coal body were significantly decreased, and the bulk modulus, Young's modulus, and shear modulus were respectively decreased by 21.90 %, 36.76 %, and 38.87 %, Poisson's ratio increased by 14.81 %, Poisson's ratio variability was low, volumetric modulus variability was medium, Young's modulus and shear modulus variability was high. The decrease in strength after coal water adsorption is due to the significant volume expansion of the coal body, the saturation expansion rate reaches 12.47 %, and at the same time, the total energy of the coal model decreases, where the weakening effect produced by the changes in the bonding and non-bonding energies results in the decrease in the mechanical strength of the coal molecules, and the weakening of the stability of the coal rock. The results of the study reveal the deformation and damage mechanism of the softening and deformation of the back-mining roadway in contact with water in Xinjiang Dabei Coal Mine, which provides a basis for the subsequent disaster prevention and control.

Influence of strength inhomogeneity on transboundary expansion characteristics of hydraulically fractured fractures in coal seams

Coal, a vital strategic resource, facilitates industrial development and socio-economic progress. Ensuring the high-quality development of the coal industry is crucial for national energy security and safety. Coal and gas outbursts are frequent hazards in coal mining processes. This research delves into the impact of heterogeneous coal seam strength on hydraulic fracturing propagation, utilizing both physical experimentation and the ABAQUS finite element approach. Experimental findings reveal distinct variations in water injection pressure profiles and fracturing fluid distribution patterns within coal seams of varying strengths, in contrast to those exhibiting uniform strength. When fractures propagate from a weaker to stronger coal seam region, a notable increase in pressure build-up effect is observed, leading to higher water injection pressure, wider fracture widths, augmented coal body displacement, and an elevated rate of rigidity reduction. In this study, the physical test results and numerical simulation results are verified with each other. After fractures propagate across the interface, zone with higher coal seam strength experience decreased fracture width, lower coal body displacement, and slower rates of rigidity decline compared to weaker seam zone. When fractures propagate from high to low coal seam areas, the fracture experiences instantaneous cross-boundary extension, resulting in a decrease in pore pressure, increased coal body displacement, and an elevated rate of rigidity decline.

Experimental study on creep characteristics of cemented coal gangue backfill under uniaxial compression

ABSTRACT The uniaxial compression creep test is used to study the cemented coal gangue backfill body specimens with different cement-sand ratios (1:4,1:6,1:8, respectively). Through the processing and analysis of the creep curve, we discussed the influence of different cement-sand ratios, stress load and loading time on the strain, creep rate and long-term strength of the backfill body. These studies have revealed the creep law of the backfill body and obtained accurate conclusions. The results show that: (1) Different proportions of cemented coal gangue backfill body under the action of the set step by step axial loading stress, the strain is positively correlated with the change of loading stress. (2) With the increase of axial stress, the instantaneous deformation of cemented coal gangue backfill with three kinds of cement–sand ratio increases with the decrease of cement–sand ratio under the same axial stress. (3) With the increase of the axial stress of the three kinds of cement–sand ratio cemented coal gangue backfill body, the strain rate of the deceleration creep speed and the constant creep stage increases, and the duration of the deceleration creep stage increases. The average creep strain rate of the three kinds of cement–sand ratio cemented coal gangue backfill materials before entering the accelerated creep stage is determined. (4) Using an improved steady-state creep rate inflection point method, the long-term strength of three kinds of cement–sand ratio of cemented coal gangue backfill is determined, and it is concluded that the long-term strength decreases with the decrease of cement–sand ratio, which is positively correlated.

Non-Pillar Coal Mining by Driving Roadway During Mining Period in High-Gas Top-Coal-Caving Working Face

To solve the problem of the inability to achieve Y-shaped ventilation in the boundary coal mining of high-gas mines and the problem of gas accumulation in the upper corner of a fully mechanized mining face, non-pillar coal mining technology is proposed by a driving roadway during the mining period. A high-gas working face requires Y-shaped ventilation to achieve upper corner gas control, but Y-shaped ventilation conditions are not available at the boundary coal body. In order to handle this challenge, studies have suggested non-pillar coal mining technology, which involves excavating roadways while mining in order to achieve non-pillar coal extraction and use recoverable wide coal pillars. During the simultaneous excavation of a working face and roadway, studies analyzed the distribution characteristics of the complicated stress environment. Following an evaluation of the impact of coal pillar width on the quality of an excavation roadway, this study’s development is in terms of an effective technique for retaining coal pillars as established. During the mining period of a working face, in the goaf of the working face, the research analyzed the distribution properties of the gas flow field, and findings from the study indicate that the width of the recovered coal pillar influences the distribution of gas. Finally, the width of the coal pillar was comprehensively determined, forming non-pillar coal mining technology by a driving roadway during the mining period. The on-site practice has shown that using a wide coal pillar with a width of 70 m to protect the roadway significantly reduces the deformation of the surrounding rock in the mining roadway, the gas concentration at the return airway is lower than the safety production standard, and by decreasing the mining succession time by 15 months, studies achieved improving the working face’s coal extraction rate by 12.6%.

Study on progressive damage and deformation law of coal body around borehole under different moisture states

Study on progressive damage and deformation law of coal body around borehole under different moisture states

Hydraulic horizontal slit-stress synergistic unloading fracturing in coal seams

Solving the problem of gas extraction from low-permeability coal seams is an urgent issue in the global coal industry, and exploring green and efficient mining methods has potential applications in this field. In this study, the No. 9 coal seam of the Guizhou Longfeng Coal Mine was selected as the research object. The method of using a hydraulic horizontal slit to create a compensation space and then synergizing it with the coal body’s own stress to decompress and increase the permeability of the coal body was studied through similarity simulation and numerical simulation experiments. This study showed that the slit can cause the overall development, expansion, and penetration of coal seam fissures. Thereby, a large number of fissures are produced in the coal seam and its upper rock layer, which form, as a whole, a “positive trapezoid” shape. The three stages of the slit produced 165, 487, and 1572 fissures, which unpressurized and increased the penetration of the coal and rock body. The slit causes the upper coal body to lose its support, and the original stress balance of the coal and rock is broken, forming a decompression zone. At the same time, the coal body below starts to expand and deform after losing the vertical stress. The decompression zone gradually expands up and down, and the stress on both ends of the compensation space becomes larger, forming a stress concentration area. After the overall displacement of the coal seam caused by the slit, a “positive triangle” area is formed. In the upper part of the area, six displacement dividing lines are created, which form five areas. The displacement shows the distribution characteristics of a large displacement in the middle and a small displacement at both ends. The results of the similarity simulation and numerical simulation experiments were verified. The study provides a new way of thinking for effectively preventing coal and gas protrusion and improving mining efficiency.

Initial Desorption Characteristics of Gas in Tectonic Coal Under Vibration and Its Impact on Coal and Gas Outbursts

The rapid desorption of gas in coal is an important cause of gas over-limit and outbursts. In order to explain the causes of coal and gas outbursts induced by vibration, this paper studies the gas desorption experiments of tectonic coal with different particle sizes and different adsorption equilibrium pressures under 0~50 Hz vibration. High-pressure mercury intrusion experiments were used to measure the changes in pore volume and specific surface area of tectonic coal before and after vibration, revealing the control of pore structure changes on the initial desorption capacity of gas. Additionally, from the perspective of energy transformation during coal and gas outbursts, the effect of vibration on the process of coal and gas outbursts in tectonic coal was analyzed. The results showed that tectonic coal has strong initial desorption capacity, desorbing 29.58% to 54.51% of the ultimate desorption volume within 10 min. Vibration with frequencies of 0~50 Hz increased both the gas desorption ratios and desorption volume as the frequency increased. The initial desorption rate also increased with the vibration frequency, and vibration can enhance the initial desorption capacity of tectonic coal and delay the attenuation of desorption rate. Vibration affected the changes in the initial gas desorption rate and desorption rate attenuation coefficient by increasing the pore volume and specific surface area, with the changes in macropores and mesopores primarily affecting the initial desorption rate and 0~10 min desorption ratios, while the changes in micropores and minipores mainly influenced the attenuation rate of the desorption rate. Vibration increased the free gas expansion energy of tectonic coal as the frequency increased. During the incubation and triggering processes of coal and gas outbursts, vibration has been observed to accelerate the fragmentation and destabilisation of the coal body, while simultaneously increasing the gas expansion energy to a point where it reaches the threshold energy necessary for coal transportation, thus inducing and triggering the coal and gas protrusion. The study results elucidate, from an energy perspective, the underlying mechanisms that facilitate the occurrence of coal and gas outbursts, providing theoretical guidance for coal and gas outburst prevention and mine safety production.

Practical research on coal pillar retention in deep mining roadways

To address the limitations of traditional coal pillar design methods in engineering practice for deep retreat mining roadways, this research focuses on the retreat mining face at the Pinggang Coal Mine in Heilongjiang Province. Combining mechanical experiments, theoretical calculations, numerical simulations, and field tests, this study calculates the ultimate equilibrium size of coal pillars. It also analyses the stress transfer and surrounding rock plastic evolution characteristics under different coal pillar sizes in deep retreat mining faces and carries out a field engineering application. After the factors mentioned above are analysed, the appropriate size for coal pillars that align with the current state of the surrounding rock in deep mines is established. The research findings indicate a correlation between the theoretically calculated ultimate stability range of coal pillars and the numerical simulation results on coal pillar size, suggesting a close consistency. The determined ultimate equilibrium coal pillar size falls within the range of 7.12–8.22 m. When the coal pillar size changes, the stress around the deep retreat mining roadway is transferred, and the plastic state of the surrounding rock also gradually changes. When the coal pillar width is 3–6 m, a stress concentration appears in the coal body, and the coal pillar has a lower bearing capacity and is prone to plastic failure. When the width is 7–8 m, the stress concentration gradually shifts to the sides of the coal pillar, enhancing its control over the roof, and a loosening zone forms in the surrounding rock. When the coal pillar width reaches 9–10 m, the stress concentration shifts completely from the coal body side to the coal pillar side, and the bearing performance of the coal pillar no longer improves. Plastic failure of the coal pillar disappears, but a greater degree of stress concentration occurs on the side of the coal pillar. The optimal size of the coal pillar is determined to be 7–8 m. Verified by the coal pillar retention engineering application at the Pinggang Coal Mine, when an 8 m coal pillar is retained, a peak stress of 27.54 MPa appears 12 m from the roadway, and the maximum deformations of both sides, the roof, and the coal pillar side are within the ideal range, indicating a relatively good overall state of the roadway. These research results provide new theoretical and practical guidance for minimizing coal loss and eliminating the safety hazards posed by unreasonable coal pillar retention at the Pinggang Coal Mine and offer a reference for the reasonable retention of coal pillars under similar conditions in the Northeast China mining area, thereby promoting the sustainable development of the coal industry.

Study on Damage Characteristics and Physical Field Characteristics of Roadway Surrounding Rock Under Multiple Disturbances

ABSTRACTDuring the mining process, repetitive stress disturbances induced by mining activities can lead to alterations in the physical properties of coal, potentially resulting in rockburst occurrences within tunnels. To investigate the propagation rule of physical field characteristics and characteristics of failure in roadway surrounding rock under multiple disturbance damage caused by dynamic load, a combined experimental and theoretical analysis is conducted to study the weakening effect of rock mass under various disturbance circumstances. A model of roadway surrounding rock loosening and failure under multiple disturbances was proposed. The degree of damage is quantified by defining the weakening coefficient Di, A “weakening variable method” is proposed to confirm the main parameters of the Holmquist‐John‐son‐Cook (HJC) model under different disturbance conditions. The reliability of these findings was validated through a microseismic event at the Tangshan coal mine's 0250 working face in 2020, followed by numerical simulation studies. The results indicate that damaged coal weakens the intensity of stress waves at the same source velocity, with the strongest effect observed at interfaces between different damage zones. Furthermore, damaged coal exhibits a stronger weakening effect on stress wave propagation speed compared to undamaged coal in non‐interface areas. The study on roadway stability reveals that severely damaged coal‐rock samples significantly weaken stress waves; however, they also exhibit lower minimum energy for dynamic failure in roadway surrounding rock, indicating that low‐stress waves cause greater damage under severe damage conditions. The study investigates the impact of coal rock mass degradation on the stability of surrounding roadways under various disturbance conditions, which holds significant implications for the timely identification of potential instability risks in damaged coal bodies, optimization of support strategies, and ensuring mining safety.

Preparation and performance study of low heat storage flame retardant polyurethane for coal mines

In order to meet the requirements for the use of polyurethane for reinforcement in mines, the synthesis temperature of polyurethane needs to be lowered, and the flame-retardant and mechanical properties of polyurethane should also be considered comprehensively. Factors affecting the synthesis temperature of polyurethanes are investigated, low heat storage polyurethanes that meet the temperature requirements for use in plugging crushed coal bodies in coal mines are formulated, and the synthesized polyurethanes are characterised for their flame retardant properties by FTIR, SEM, TG-FTIR and CONE experiments. It was found that in this paper, a new type of polyurethane for coal mines was formed by flame retardant modification of polyurethane by adding nano-system flame retardant and phosphorus-system flame retardant, and the synthetic reaction temperature of the material could reach as low as 51.2 °C, and the pyrolysis onset temperature was increased to 286 °C. The experimental results show that the flame retardant property of this synthetic polyurethane has been significantly improved, and the heat accumulation effect of the synthesis process on the surrounding coal body has been greatly reduced. The physical properties and thermal stability of this synthetic polyurethane in the grouting process meet the safety requirements for use in underground coal mines.

Study on instability mechanisms and control of coal pillar spalling and coal crumbs leakage during working face crossing faults

The stability of coal pillars in fault areas is crucial for ensuring the safe passage of working faces. Based on field observations, frequent coal pillar spalling and substantial tectonic coal crumbs leakage, as well as tilting of hydraulic supports, are observed when working faces transition from primary coal to tectonic coal. To analyze the instability mechanisms behind these phenomena, this paper establishes a mechanical model of coal pillars in fault areas and analyzes the distribution of tectonic stresses and factors affecting the stability of coal pillars. The results indicate that horizontal tectonic stress adheres to an exponential function dependent on the angle factor, where (k0) is a parameter associated with the friction angle of the coal body, the dip angle of the fault, and the friction angle of the fault plane. The stability of coal pillars is influenced by factors such as roof and floor pressures, coal pillar integrity, mining height, and shield support force, with coal pillar integrity being the most critical. To ensure the smooth passage of working faces through faults, this study proposes a combined control technique of “inclined mining” and “grouting,” including reducing mining heights, adjusting the slope of working face advancement, and pre-grouting of coal pillars. Industrial experiments conducted on-site have shown improved integrity of tectonic coal, enabling the working face to pass through faults smoothly and significantly increasing production efficiency.

Multi-scale reservoir response characteristics of coalbed methane development through stress relief via horizontal well cavity completion in tectonically deformed coals

China’s coalbed methane (CBM) resources hold significant potential. However, the widespread presence of soft and low-permeability tectonically deformed coal (TDC) presents a challenge to the efficient development of CBM. The development of CBM through stress relief via horizontal well cavity completion (HWCC) offers a promising innovative solution to this problem. To elucidate the multi-scale reservoir response characteristics of CBM development through stress relief via HWCC in TDCs, this article comprehensively employs particle discrete element and finite element numerical simulation methods, and carries out the numerical simulation studies of cyclic seepage at the specimen scale, HWCC at the coal seam scale, and CBM development at the coal seam group scale. The main findings are summarized as follows. Unlike the stress-compression specimens, which exhibited overall failure, the differential distribution of pressure gradients within the specimens during the gas seepage experiments resulted in the development of microcracks concentrated at the bottom of the specimens. With the escalation of coal body structural damage, the permeability stress compression coefficient during the elastic deformation stage shows an increasing trend. Upon entering the plastic deformation stage, the specimen’s permeability shifts from a decreasing to an increasing trend. Increasing the lower limit pressure ratio, upper limit pressure ratio, and cycle rate of gas injection pressure could accelerate the failure efficiency of the specimens. Increasing the lower limit pressure ratio has a more pronounced effect on the TDC specimens, while increasing the upper limit pressure ratio has a more significant impact on the primary undeformed and cataclastic coal specimens. In TDCs, the phenomenon of wellbore collapse and cavity expansion becomes more pronounced. The cavity volume tends to increase with the increase in the initial well diameter, in-situ stress, and alternating bottom-hole pressure differences. The HWCC could enhances reservoir permeability and production in both the cavity completed and adjacent coal seams. For two horizontal wells with cavity completions in the same layer, excessively close well spacing is unfavorable for fostering inter-well interference and reservoir pressure drop. When the HWCC is applied to thin coal seams, the enhancement of permeability and production is limited. Reasonable optimization of drainage rates could effectively mitigate the reservoir permeability velocity-sensitive damage. The research findings extend related theories and provide a robust reference for subsequent practical engineering applications.

Mathematical model of permeability evolution of liquid CO2 pressurized coal

To examine the permeability alterations in coal seams induced by liquid CO2 injection, we employed COMSOL simulations to recreate the flow, force, and temperature fields, along with the corresponding physical parameters of the geological formation. We established a mathematical model to track the evolution of the coal body’s permeability, enabling us to capture the phase distribution of CO2, the dynamics of fluid transportation, and the coal body’s mechanical responses during the liquid CO2 injection process. Our findings revealed that the coal seam undergoes significant changes due to low-temperature freezing and shrinkage, gas-liquid phase transitions, and expansion impacts from solid-liquid phase changes. These factors collectively modify the pore structure, subsequently influencing the coal seam’s permeability. Notably, the coal body’s permeability, influenced by the pore structure and the Klinkenberg effect, increases logarithmically with rising injection pressure and falling temperatures. The structural changes in the coal body are markedly impacted by seepage and phase change pressures, with the permeability enhancement from secondary phase changes being 2.28 times greater than that from primary phase changes, and five times greater than in scenarios without phase change.

Study on the mechanism and prevention system of creep induced instability of isolated coal pillars considering time effect

In response to the frequent occurrence of impact events in isolated coal pillars between the upper (lower) mountain areas of mining areas under conditions without obvious mining disturbance, this study takes multiple isolated coal pillar impact events that occurred in Zhaolou Coal Mine and Gengcun Coal Mine as examples. It conducts a mechanism study on the creep-induced impact ground pressure of isolated coal pillars considering the time effect. The occurrence of such impact ground pressure accidents often exhibits significant “lag” and “spontaneity”. Due to the unclear mechanism of occurrence and the difficulty in grasping the unloading timing, this type of impact ground pressure has become a typical hidden disaster, posing a serious threat to the deep mining of coal mines. Based on the analysis of the creep mechanical properties of coal and rock mass, the uniaxial compression acoustic emission creep test of coal body, and the FLAC3D numerical simulation, a creep instability impact mechanical model of coal and rock mass is established. This model reveals the mechanism of creep instability impact and the stress evolution law of coal and rock mass during creep. The mechanical and energy criteria for the occurrence of impact ground pressure in isolated coal pillars under the action of unstable creep are proposed. The study shows that the action of high ground stress is a necessary condition for the occurrence of "lag-type" impact ground pressure in isolated coal pillars. Long-term unstable creep will weaken the support strength of the entry. When the elastic energy accumulated in the isolated coal pillar exceeds its ultimate bearing capacity, impact instability will occur. Based on the mechanism of occurrence of such impact accidents, targeted deep hole blasting unloading measures are proposed, providing a good reference value for the prevention and control of "lag-type" impact ground pressure accidents in isolated coal pillars.

Study on the influence of aerosol atomization by ultrasonic atomization on coal spontaneous combustion

ABSTRACT Influenced by the complex environment and the nature of process materials in goaf, the prevention and control of spontaneous combustion of coal (CSC) in the deep part of goaf were facing many difficulties. To investigate the performance of ultrasonic atomized aerosols, a series of atomization experiments were conducted using a self-developed ultrasonic atomization device. The main conclusions are as follows: under the same atomization conditions, the atomized particle sizes of several inhibitors (except PAS-W) are concentrated in the <8 μm range, with a content of more than 88%, indicating that the aerosol particles produced by ultrasonic atomization are small in size and uniform in distribution. Compared with water and MgCl2 inhibitors, aerosol particles produced by PAS-W are less likely to pass through the voids of the coal body, and have poorer permeability and diffusivity. As the temperature increases, the viscosity and surface tension of the inhibitor decrease, and the percentage of aerosol particles with particle size <8 μm produced by ultrasonic atomization is more than 88%, indicating that the atomization effect is improved. After increasing the temperature, the contact angle between PAS-W inhibitor and coal reduces from 54.7° to 40.8°, and the wettability is better. Compared with raw coal, the final temperature of the PAS-W aerosol treated coal sample is delayed 21.86°C, and has significant retarding property. When the wind speed is 3 m/s, the effect of PAS-W aerosol particle generation, permeability, and particle size distribution is the best. The results of the study are of great significance in improving the ultrasonic atomized aerosol fire prevention technology and realizing large-scale CSC prevention in goaf.

Numerical Simulation of CBM Seepage Characteristics Based on Fracture Network Images

The natural fracture network within the coal body serves as the main pathway for gas migration, with its geometric characteristics significantly impacting coalbed methane flow. In order to enhance the numerical model for simulating coalbed methane flow based on fracture network images, we define porosity and permeability functions for these images and improve upon existing methods. By employing a pixel probability decomposition algorithm, we establish a geometric model of a rough discrete fracture network, which is imported into COMSOL Multiphysics to build a numerical model of gas flow. We analyze the impact of different fracture structures on coal seam permeability and find that gas primarily flows through interconnected fractures at much higher velocities compared to matrix pores. Furthermore, we observe that fracture network permeability increases with increasing porosity (0.0635–0.164), fractal dimension (1.571–1.755), maximum fracture branch length (0.0111–0.0249 m), and connectivity (0.808–2.789). Conversely, it decreases with an increasing fracture dip angle (1.61–88.39°) and tortuosity fractal dimension (1.0018–1.0195). Our simulation method based on fracture network imaging provides a simple yet feasible approach to simulate gas extraction while accurately capturing various stages in the extraction process, including the temporal and spatial evolution of gas velocity and pressure as well as differences between fractures and the coal matrix.