Mine Goaf Research Articles

An Experimental Study on the Heat and Mass Transfer of Liquid Nitrogen in a Loose Medium

Liquid nitrogen is a vital medium during the extinguishing and chilldown process of coal spontaneous combustion in coal mine goafs. In this paper, the heat and mass transfer of liquid nitrogen in a loose medium was investigated. A laboratory system including a temperature sensing system was designed and built to explore the effects of different nitrogen injection positions and angles on the chilldown effect. The results indicate that after liquid nitrogen injection, the temperatures in the liquid nitrogen flow area and the bottom of the model can be quickly reduced to −196 °C, which was the best chilldown effect zone. With the vaporization of the liquid nitrogen, the cryogenic nitrogen gradually diffused. At 20,000 s, the bottom temperature was about –63 °C, the middle was –30 °C, and the upper was 0 °C. When the model angle was 0°, the effective chilldown zone was the largest. As the angle increased, the effective chilldown volume and holding time decreased significantly. The model angle had a greater impact on the chilldown. For the position of nitrogen injection, the inlet was set slightly farther from the gas outlet’s position, leading to a larger coverage for the liquid nitrogen and cryogenic nitrogen chilldown. This study can provide a theoretical basis for the flow and chilldown of nitrogen injection in goafs.

Research on N2-inhibitor-water mist fire prevention and extinguishing technology and equipment in coal mine goaf.

In this study, a new type of N2-inhibitor-water mist (NIWM) technology was proposed to resolve the problem of fire prevention and extinguishing in the goaf of coal mine. The corresponding equipment was designed and manufactured. Under the condition that both gas pressure and liquid pressure were 0.5–2MPa, the NIWM equipment produced the water mist with Sauter mean diameter (SMD) range of 166–265μm. The experimental results of the operating parameters of NIWM equipment were in agreement with the theoretical derivation. The theory of two-phase flow atomisation can be used as theoretical guide for this technology. After that, on the basis of the NIWM equipment, the experiments of inhibiting low temperature (30–100°C) oxidation and extinguishing high temperature combustion of large dosage of coal sample were carried out. Water mist with SMD = 188μm had good diffusivity in the container. The inhibiting effect of N2-inhibitor-water mist on low temperature oxidation of coal was obviously greater than that of single material. N2-water mist extinguished the burning coal completely in 20 minutes. The addition of water mist solved the shortcoming of poor cooling effect of N2. In different stages of coal-oxygen reaction, N2, inhibitor and water mist play very different role in controlling the process of coal-oxygen reaction, which was not simple accumulation of the three. The combination of N2, inhibitor and water mist should be determined according to the state of the coal mine goaf fire. On the basis of the research conclusions, the onsite arrangement diagram of the NIWM fire prevention and extinguishing equipment in the goaf was designed. The research results proved the feasibility and effectiveness of this technology, and it is of great significance to the prevention and control of coal spontaneous combustion in goaf.

Field Study on Correlation between CO2 Concentration and Surface Soil CO2 Flux in Closed Coal Mine Goaf.

Self-heating of coal mine goaf or shallow coal seams can release an outbreak of unimaginable pollution disaster under suitable circumstances. As an indicator gas, CO2 is always used to determine the coal spontaneous combustion state during the self-heating process. Based on this, the paper investigated the influence of abandoned coal mine goaf CO2 on the surface environment by measuring the CO2 concentration in the borehole connected to the goaf and CO2 flux on the soil surface. Furthermore, rainfall and atmospheric temperature effects are discussed to illustrate the correlation between the CO2 concentration and surface soil CO2 flux in the closed mine goaf. Subsequently, the tracer gas experimental method is employed to analyze the effect of air leakage from an open-pit slope on CO2 flux. The experimental results demonstrated that the distribution of CO2 concentration in the borehole confirms the continuous diffusion of goaf CO2 onto the surface. The value of CO2 flux in the goaf is significantly higher than that of a normal area. Temperature is one of the primary factors that affect the CO2 flux on the field. Air leakage from the slope promotes the surface soil-overlying goaf CO2 diffusion. The study provides important reference data for the assessment of the mining area field environment and the determination of the spontaneous combustion risk of the residual coal in the goaf.

Mercury emission from underground coal fires in the mining goaf of the Wuda Coalfield, China

The Wuda Coalfield, Inner Mongolia suffers from serious coal fires for more than half a century. Fire-extinguishing projects have been carried out to suppress the coal fires since the last decade, but sporadic surface fires still occur and underground fires are more prevailing. Here, we used a real-time RA-915M Mercury Analyzer with modified inlet to monitor gaseous Hg concentrations in fumes emitted from boreholes that were designed to detect and control the underground coal fires. Meanwhile, offline methods were used to collect the fumes and analyze the contents of the gases including CO, CO2, CH4, C2H6, C2H4 and C2H2. The results showed that gaseous Hg concentrations in fumes from boreholes ranged from 6.42 ± 0.73 to 123.53 ± 34.66 ng m−3, with an average value of 49 ± 44 ng m−3. We suggest that the amounts of coal left for burning or smoldering mainly accounted for the large variation in fume Hg concentrations of underground coal fires. The gaseous Hg concentrations in near-surface air surrounding boreholes varied from 2.38 ± 0.28 to 13.10 ± 0.97 ng m−3, with a mean value of 6.68 ± 3.09 ng m−3. They were higher than the ambient air Hg concentrations measured at a background site near the Yellow River (<2 ng m−3), suggesting underground coal fires were one significant Hg pollution source. Importantly, we found that gaseous Hg concentrations in the boreholes had significantly positive correlations with temperatures and CO (a traditional coal-fire index gas) contents, implying that Hg has the potential to serve as an index gas to monitor the occurrences of underground coal fires in mining goafs.

Research in Optical Fiber Sensing Technology Fire Based on Goaf of Lao-Shidan Mine

In order to prevent the spontaneous combustion of the coal in the goaf of the 16401 working space of the Lao-Shidan coalmine, the optical flow sensing system, the oxygen concentration distribution and the temperature distribution of the goaf are detected in real time by using the optical fiber sensing system pre-buried in the channel. According to the preset mathematical model of CFD software, the velocity field cloud map, O2 concentration field cloud map and temperature field cloud map are constructed, and the consistency between the calculated value and the measured ones is established by the model to determine the accurate range of the “three belts” in the goaf of 16401 working space. The distance from 0 to 54m is the heat dissipation zone, 54m~88m is the oxidizing spontaneous combustion zone, and the suffocation zone is more than 88m. The measured data provides a solid theoretical basis for the feasibility of fire prevention measures in the goaf of the 16401 working space in the Lao-Shidan coalmine.

Goaf Locating Based on InSAR and Probability Integration Method

Mining goafs can cause many hazards, such as burst water, spontaneous combustion of coal seams, surface collapse, etc. In this paper, a feature-points-based method for the efficient location of mining goafs is proposed. Different interferometric synthetic aperture radar (DInSAR) is used to monitor the subsidence basin caused by mining. Using the principles of the probability integral method (PIM), the inflection points and the boundary points of the basin monitored by DInSAR are determined and used as feature points to locate the goaf. In this paper, the necessity of locating goafs and the traditional methods used for this task are discussed first. Then, the results of verifying the proposed method by both a simulation experiment and real data experiment are presented. Six RADARSAT-2 images from 13th October 2015 to 5th March 2016 were used to acquire the subsidence basin caused by the 15235 working faces of the Jiulong mining area. The average relative errors of the simulation experiment and real data experiment were about 6.43% and 12.59%, respectively. The average absolute errors of the simulation experiment and real data experiment were about 28 m and 38 m, respectively. In the final part of this paper, the error sources are discussed to illustrate the factors that can affect the location result.

Field Measurements of Compaction Seepage Characteristics in Longwall Mining Goaf

The spatiotemporal evolution of goaf caving zone compaction characteristics has important influences on surface subsidence, spontaneous combustion and gas and water migration characteristics in goaf. In this paper, the permeability of the caving zone during the advance of a longwall face is calculated based on the permeability calculation model and gas drainage data; additionally, the compaction stress and compaction time of goaf are measured by a borehole stress meter. The internal relations among compaction stress, compaction time and permeability at different positions in the caving zone are quantitatively analyzed. The results show that the farther away from the boundary of the caving zone, the lower the permeability, the greater the compaction stress and the longer the compaction time are. It is also confirmed by numerical simulation results. At the same time, the distribution characteristics of and quantitative relationships among compaction stress, compaction time and permeability are given. In addition, compaction time increases with the increase in the rate of advance of the longwall face, but the change is not large. Based on the above study, the space–time evolution of gob compaction characteristics is preliminarily grasped, which provides theoretical guidance for designing pressure relief gas drainage boreholes and the longwall face layouts.

Modeling temperature distribution upon liquid-nitrogen injection into a self heating coal mine goaf

Liquid N2 could be injected into goaf to decrease temperature and prevent spontaneous combustion during mining. A working face at Liangbaosi coal mine was adopted for study. Firstly, a mathematical model for calculating the temperature field in goaf was developed and field tests without injection of liquid N2 were conducted to validate the model. Comparable development trends and hot zones between the simulation and field measurements were found. The hot zones were located about 35–45 m behind the workface on the air-return and air-intake sides in the goaf. Then the model was employed to simulate the time development of temperature distribution in the goaf with injection of liquid N2 from different locations. It was observed that a low-temperature cooling zone (<300K) due to injection of liquid N2 gradually grew and became relatively stable 90 min after continuous injection. The size of the cooling zone depends on the injection location and flow rate. The cooling zone was smaller when the N2 was injected from the air-intake side than from the air-return side. The largest cooling zone was found when the N2 was injected 35m behind the workface from the air-return side. The cooling zone increases with increasing N2 perfusion rate. This study provides a quantitative assessment for preventing coal oxidation and spontaneous combustion using the liquid N2 technology.

Detection of Water-Filled Mining Goaf Using Mining Transient Electromagnetic Method

Water-filled mining goaves are extremely prone to water inrush accidents in coal mines, and the transient electromagnetic method (TEM) is a good geophysical method for detecting water-rich areas. Considering that conventional TEM was mainly carried out on the ground, to increase the detection resolution, the underground TEM was used to detect the water-filled goaves in this study. Based on the whole-space model, the data-processing method of the underground TEM was studied. The whole-space geoelectric model was established based on actual coal-measure strata data, and the whole-space TEM response of the water-filled goaves was modeled using the finite-difference time-domain method. The results showed that the low-resistance areas of the apparent resistivity contours can accurately reflect the water abundance of the mining goaves. The underground TEM was used to detect the water abundance of the mining goaf in a mine environment and its detection results were consistent with the actual results.

Estimation of hydrogen release and conversion in coal-bed methane from roof extraction in mine goaf

In order to effectively utilize the coal bed methane (CBM) resources extracted from underground coal mines, an evaluation method of hydrogen production capacity from CBM is established by means of experiments and theoretical models. The main components of CBM collected from coal working face are measured by gas chromatography, and the release law of trace hydrogen is obtained. Considering gas extraction technology in goaf roof, a computational fluid dynamics model for seepage problem in porous media is presented. The law of gas migration in goaf under extraction conditions is obtained by simulation, and the extraction rate of CBM and methane/hydrogen content are predicted. The technology of producing hydrogen from CBM extracted from goaf under low gas concentration is discussed, and the effect of methane content on auto-thermal conversion efficiency is analyzed. Under the condition of this study case, the theoretical estimation of pure hydrogen produced from goaf extraction can reach more than 10,000 m3 per day. The presented results could provide a new route for the utilization of CBM by underground extraction.

Study on Transient Electromagnetic Response of High Resistivity Goafs and Its Application

Based on the characteristics of mine goafs, the geo-electrical models are established. In order to study the response characteristics of goaf by transient electromagnetic method, we have performed three-dimensional numerical simulations in three cases, geo-electrical model of mine geology without goaf, geo-electrical model of mine geology with low resistivity goaf and geo-electrical model of mine geology with high resistivity goaf. Moreover, the physical experiments have also been performed for the above three cases. Furthermore, we compare numeral simulation results and physical experiment results. Finally, we obtain the conclusion that although the response of high resistivity goafs is weaker than that of low resistivity, its response characters still can indicate the existing of goaf. Hence, above work actually signifies that shallow high resistivity goafs can be detected in a reliable way by transient electromagnetic method.DOI: 10.5755/j01.eie.25.1.22733

A Numerical Study to Assist Assessment of the Stability of Shallow Coal Mine Goafs

Coal mine goafs are distributed widely across many parts of the world and their stability is a major cause for concern, particularly when designing new infrastructure. To reduce risk, the coal mine goafs are often stabilized using cement grout and such operations can be very expensive and difficult to verify. This paper uses the finite element method to examine the relative influence of a number of key factors controlling the stability and surface deformations of shallow, horizontal coal mine goafs overlain by sedimentary rock. Representative ranges in the stiffness and strength characteristics of coal and rock are examined and each material is assumed to satisfy the Hoek–Brown failure criterion. The analyses show that, for a typical maximum coal extraction rate of 40%, the critical goaf span varies linearly with the depth of the coal seam (for the maximum depth investigated of 45 m) and increases with the competency of the overlying rock. A relationship combining the rock quality and the ratio of the critical goaf span to the depth of the coal seam is proposed to enable assessment of abandoned coal mines with marginal stability. This relationship is shown to be consistent with observations made in two coal mining case histories.

Softening Damage Analysis of Gypsum Rock With Water Immersion Time Based on Laboratory Experiment

In view of the effect of water on the physical and mechanical parameters of natural gypsum rock, in this study, gypsum rock in the goaf of a gypsum mine was selected as the research object, and gypsum rock samples were prepared with different immersion times. In addition, uniaxially tests were performed separately on the gypsum rock samples. Compression and scanning electron microscopy experiments were carried out to analyze the effects of immersion time on the uniaxial compressive strength, elastic modulus, Poisson’s ratio, and microstructure of gypsum. The results show that the uniaxial compressive strength and elastic modulus of gypsum are inversely proportional to the water content. However, the Poisson’s ratio is direly proportional to the water content, and the failure mode is destroyed by the brittle fracture of a single crack and is transformed into the shear ductile failure of the Y-shaped crack. Microscopically, with increasing immersion time, the bonds in the crystal of the microporous cracks and microcrack tips are weakened by hydrolysis, and the macroscopic structure is complicated by the internal structure of gypsum, and the end of the crack is expanded by the compressive action of water. Based on the damage mechanics, the evolution equation of gypsum soaking softening damage based on time factor was derived. The relationship between the brittleness coefficient and softening damage variable is revealed, providing a theoretical basis for the determination of the softening degree of gypsum in the goaf.

Instability of Gypsum Mining Goaf Under the Influence of Typical Faults

Mine collapse is a common geological hazard associated with mining areas. In this paper, the goaf of a gypsum mine under the influence of typical faults is chosen as an example, and the stress on the center of the goaf roof is examined through theoretical analysis. The safety factors of the pillar at -127 m, -140 m, and -160 m are evaluated to be 1.471, 1.469, and 1.467, respectively, based on the Wilson theory. It is concluded that the structural parameters of the mining stope are reasonable without considering the influence of the fault. Furthermore, the numerical simulation by FLAC <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3D</sup> is used to verify the analytical results and to study the influence of typical faults on the stress and displacement of the goaf. The results showed that the load of the roof and the pillars at a distance from the fault was larger than the load near the fault. First, the roof at this location is pulled. Next, cracks appear and develop. Then, the middle of the stope roof is pulled and falls, and the overburden strata move. Finally, the unstable strata are transmitted to the surface, and the ground surface of the mining area subsides.

A Comparison of Surface-to-Coal Mine Roadway TEM and Surface TEM Responses to Water-Enriched Bodies

The surface transient electromagnetic method (TEM) is a geophysical technology normally used in detecting water-enriched zones. However, with an increase of mining depth, the accuracy and reliability of surface detection is gradually reduced, and surface TEM cannot meet the requirements of high-precision detection for coal mine production safety. In this study, surface-to-coal mine roadway TEM is proposed to detect water-enriched zones in coal mines. The resolution of this method in detecting targets, however, is still unknown. Based on a 1-D layered model, the surface-to-coal mine roadway TEM response is numerically simulated using the finite-difference time-domain method (FDTD). The results show that the surface-to-coal mine roadway TEM has higher resolution of the target above the underground receiving point and weaker resolution of the target below the receiving point. 3-D geo-electric models for typical water-enriched zones such as a water-filled mining goaf, a water-filled fault and a water-filled collapse column are established. The surface-to-coal mine roadway TEM responses are numerically simulated and compared with the surface TEM responses. The results show that for the goaf model, the surface-to-coal mine roadway TEM response is more sensitive than the surface TEM response. For fault and collapse column models, the surface-to-coal mine roadway response is more sensitive than surface response in the early delay stage but less sensitive in the late delay stage. The present study provides theoretical support for device selection and for data processing interpretation in actual work.

Study of Tunnel Damage Caused by Underground Mining Deformation: Calculation, Analysis, and Reinforcement

Bayueshan tunnel (BYS) is an important construction crossing over coal mine goaf. The underground mining subsidence has led the tunnel cracked seriously in three years after it was built. In order to evaluate the coal mine influence and future stability of the tunnels, probability integral method (PIM) was used to calculate the tunnel deformation. PIM is an experience function method based on random medium theory which is used widely in China. With the parameters analyzed, the tunnels’ subsidence was calculated. The results show that it can interpret the tunnel damage well, and the maximum normal strain positions fit the damaged tunnel positions well. It proved that PIM can be used to evaluate the tunnel’s radial deformation caused by underground coal excavation. In order to maintain tunnels to keep a long‐term stability, the future deformation was calculated in case the coal excavation continues. It shows that the tunnel would be cracked again if the excavation continued. Other reasons such as the old goaf deformation and water and vehicle dynamic load are also important reasons for the tunnels’ deformation. In order to keep traffic safety, it needs to reinforce the cracked foundation under the tunnel. Then, grouting injection was proposed to reduce the old goaf deformation under the tunnels. If the fracture zone under the tunnels disturbed by the dynamic traffic load, the old goaf will face a risk of sudden collapse. So, to ensure the grouting effect, the grouting depth should be deeper than the sum of traffic load influence depth and height of coal mine caved fissure zone. The grouting scope should keep all the crack rock area under the tunnel from being disturbed by the dynamic traffic load. This design can reduce the sudden collapse risk of the goaf and reduces the vehicles’ load disturbance on the cracked rock. The researched technology provides an engineering guidance to tunnel subsidence calculation, stability evaluation, and maintenance in complex geological and engineering situations.

Study on Catastrophe Instability of Support System in Gypsum Goaf Based on Energy Dissipation Theory

The stability of the goaf support system is the key to safe production in gypsum mines. Therefore, this study constructed a pillar-beam support system which contained pillar plastic zones. In this support system, the beam and pillar were taken as energy releaser and energy dissipater, respectively. Through establishing a cusp catastrophe model based on energy theory, the new criterion for instability was obtained which is related with geometric stiffness and system energy dissipation. The results indicate the instability of the support system is caused by the incompatibility of energy release, dissipation, and geometric deformation. When K &gt; 1, the energy released by the support system is compatible with geometric deformation. The support system experiences a quasistatic process from the static state in bottom page to the static state in top page along Path I. When K &lt; 1, the energy released by the support system cannot be in tune with geometric deformation. The support system experiences a catastrophe process along Path II. The evolution from the static state in bottom page to the static state in top page is not progressive, but catastrophic. The redundant energy released in this process leads to mechanical instability of the support system. This study provided theoretical foundation for the mining and treatment of mines. Based on actual engineering examples, the sensitivity of the geometric parameters of the support system was analyzed as well. These parameters are ranked by their sensitivity from high to low, as is shown below: beam thickness, plastic zone width, room span, pillar width, and pillar height. Then, the goaf was classified according to the geometric parameters. Energy catastrophe theory was applied to analyze the stability of the support system in different classes of goaf. The analysis results showed that Class D goaf should be labeled as the unstable zone, which was consistent with the result of field research. To conclude, energy catastrophe theory can be used to demonstrate the nonlinear mechanical mechanism of support system instability in room-pillar mining goaf.

Flow field analysis of Jiangou mine goaf

With the increase of mining depth, many coal mines have the co-occurrence of natural fire and gas disasters. According to the actual situation of alkali ditch coal mine, this paper established a mathematical model and geometric model of mined-out area by using COMSOL software to carry on the numerical simulation. Study of nitrogen injection and gas extraction of goaf flow field and the influence of oxidation temperature zone, provided the basis for mine fire and gas disaster prevention and control.

Gas analysis of jian-gou mine goaf

Jiangou coal mine is a fully mechanized working face, with a height of 23 meters, serious air leakage in the goaf, easy spontaneous combustion of the coal seam, easy spontaneous combustion of the residual coal in the goaf. In order to prevent and control the spontaneous combustion fire in the goaf, we analyzed the thermal weight of B3 + 6 coal seam, obtains five characteristic temperature points. We conducted the heating and oxidation experiment of coal, analyzes the variation rule of the generated gas and its concentration with the temperature, and obtain the CO concentration distribution map of goaf based on the numerical simulation and on-site monitoring data.

Disaster types and environmental engineering issues induced by coal mine goafs instability

The disaster and environmental engineering issues induced by coal mine goafs instability have become one of most important problems threatening safety production and the safty of cicizens. Based on the analysis of instablility modes of coal mine goafs, the disaster types and environmental engineering issues were researched. The results showed that, firstly, there are six types of instability modes, that is, the goafs instability under the influnce of single pillar, geological structures, goafs water, mining disturbance, static load and dynamic load. Secondly, There are two main types of disasters induced by coal mine goafs instability, and one refers to the damages of surrounding buildings and structures, mine earthquake and ecologically envirenmental damages caused by coal mine goafs instability and ground collapse; The other one refers to equipment damage and workers hazards caused by undermine roof impact. Thirdly, the environmental engineering issues involves three espects: the living environment, the ecological envrionment and the working envrionment. For these three environmental problems, this paper puts forward specific solutions and suggestions.