Coal Mine Goaf Research Articles

Characteristics of Heavy Metal Ion Adsorption by Silty Mudstones in Coal Mine Goafs

Coal mine goafs can be used to store and purify mine water; therefore, they are important for mitigating the environmental impact of mining on ecologically fragile areas in Western China. Coal gangue in goafs can reduce turbidity, suspended matter, total hardness, and chemical oxygen demand (COD) in mine wastewater. Because gangue in goaf areas is rich in clay minerals, they serve as ion exchange sites. We investigated the adsorption of heavy metal ions by silty mudstones in the Ulan Mulun mine. The adsorption kinetics and isothermal adsorption characteristics of four heavy metal ions, i.e., Pb(II), Cd(II), Cr(III), and Mn(II), were investigated using batch experiments. Under a pH of 6.75 and temperature of 20°C, the adsorption capacity of silty mudstone particles (<74 μm) was 6.3012 g, 1.5701, 1.2571, and 1.3729 mg/g for Pb(II), Cd(II), Cr(III), and Mn(II), respectively. Based on the experimental results, quantitative relationships between the adsorption quantity per unit of rock surface and the equilibrium concentration of heavy metals in solution were developed using Langmuir, Freundlich, and D–R equations. The relationships were used to obtain the equilibrium concentration of the solution. Finally, the ratio of the f value of the water-rock contact area in the goaf to the water volume was calculated and analyzed. The f value was inversely proportional to the average vertical opening of gaps between gangues in the goaf. The adsorption rate functions for Pb(II), Cd(II), Cr(III), and Mn(II) in the goaf were obtained, and the corresponding adsorption capacities were calculated. The adsorption capacities were controlled by the f value and adsorption properties of the gangue. These results can be used in future studies on mine wastewater treatment.

考虑煤矿采空区影响的建筑物稳定性分析

考虑煤矿采空区影响的建筑物稳定性分析

Feasibility prediction analysis of groundwater reservoir construction based on GMS and Monte Carlo analyses: a case study from the Dadougou Coal Mine, Shanxi Province, China

After years of mining, groundwater levels drop significantly. Reasonable use of coal mine goaf to build groundwater reservoirs has a large impact on solving the problem of water supplies around coal mines and is of great significance to regional water environmental protection. By investigating the hydrogeological conditions of the coal mines around Datong, Shanxi, the hydrogeological characteristics of the Dadougou mining area were analyzed, and the hydrogeological conceptual model and mathematical model were constructed and calibrated. Groundwater in the region was studied based on Groundwater Modeling System (GMS) simulation results combined with Monte Carlo analysis. The results show that the calibrated hydrogeological conceptual model can reflect the actual hydrogeological situation. It is mainly reflected in the fact that the simulated water level is consistent with the actual observed water level. From March to July, the water level continued to rise and reached its maximum in August, after which, the water level gradually fell. In the end, the water level in December was about 1.1 m higher than the water level in January this year, and the groundwater in the mining area was replenished. The water level change of the study area during the simulation period is greatly affected by the season, and the recharge conditions in the study area are conducive to the construction of an underground reservoir. According to the Monte Carlo analysis from the cumulative curve of calculated groundwater storage capacity, the cumulative frequency at more than 95% is greater than 1.15 × 106 m3, and the storage can thus be built underground.

Design Strategy of Water Circulation System Above and Below Ground in Western Mining Area

Coal mines in ecologically fragile areas in western China are seriously deficient in water. The contradiction between coal mining and water shortage is extremely sharp. Therefore, how to effectively protect and utilize water resources has always been an important problem that plagues western mining areas. Based on the analysis of the water environment characteristics of the western mining area, a coal mine waters circulation system is designed. It consists of ground sponge facilities and groundwater reservoirs in the coal mine goafs.

Preliminary feasibility analysis of a hybrid pumped-hydro energy storage system using abandoned coal mine goafs

Following the Paris climate agreement, a consensus has been made on the urgent need for increasing the use of clean energy and large-scale energy storage. This paper proposes a hybrid pumped-hydro energy storage system using goafs of abandoned coal mines. The performance of the energy storage system and the suitability potential of coal mine goafs serving as underground reservoirs were analyzed. Based on the designed conditions and meteorological data of a typical area (Inner Mongolia, China), the proposed system could have an average system efficiency of approx. 82.8% and a regulating-energy density of 1.06 kW·h/m3. Potential analysis for goafs serving as reservoirs show that a typical coal mine with a 3 × 5 km2 dimensions and 6 m coal thickness could have a usable capacity of 1.58 × 106 m3. To ensure the smooth exchange of water in goaf reservoirs, the permeability should be above 10−7 m2, corresponding to the usable coefficient (≥0.8) of goaf reservoirs. The minimum horizontal distance between two reservoirs with a typical geology has been set to 245 m so that the leakage rate could be smaller than 1%. A diameter of 1 m for vertical ventilation shafts is acceptable with respect to the air pressure loss (211 Pa). Based on the reckoning of the existing coal mine goaf space in China, it has been found that developing hybrid pumped-hydro energy storage plants using abandoned coal mine goafs for daily regulation is feasible in the short term.

Effects of particle sizes on compressive deformation and particle breakage of gangue used for coal mine goaf backfill

Broken gangue from the collapsing roof of a longwall coal panel can be used as the backfilling material for goaf backfilling. The particle size gradation has important influence on the compressive deformation of the gangue backfilling material and particle breakage. The compressive deformation of the gangue backfilling materials at 4 different particle size grades is simulated by PFC3D in this paper. The law of the compressive deformation of the gangue backfilling material, the particle cluster distribution and the variation of the gangue block shape are analyzed. The microscopic mechanism of the compressive deformation of the gangue backfilling material is investigated based on the force-chain distribution. The results indicate that when the gangue backfilling material has a small particle size, the specimen has low porosity and a few amounts of the broken particles can fill the pores of the material. When the particles have relatively large sizes, the specimen has relatively high porosity and the broken particles are unable to completely fill the pores. The loading conditions of the particles can only be changed by the frame structure formed by the large particles. If the proportion of the particle size grade is reasonable, a frame structure forms due to the large particles and the small particles with various particle sizes can fill the pores of the backfilling material. This enhances the deformability of the gangue and decreases the compressive deformation of the backfilling material. After backfilled into the goaf, the backfilling material bears the overburden pressure, which means the process of roof convergence is the process of compressive deformation of the backfill body. That is to say, the stiffer the backfilling material, the better the control effect. The importance of the reasonable proportion of the particle size grade is explained from the aspect of microscopic breakage of the particles, which provides a scientific basis for backfill mining.

Thermal characteristics and kinetic analysis of coal-oxygen reaction under the condition of inert gas

ABSTRACT To ascertain fire prevention and extinguishing mechanism of inert gas in coal mine goaf, TG-DTG-DSC experiments of coal-oxygen reaction at 30–850℃ under N2 and CO2 conditions were carried out, and the thermal characteristics and kinetic factors were analyzed. The process of coal-oxygen reaction was divided into five stages by the four characteristic temperatures of T1 (temperature at which water evaporates completely), T2 (temperature at which coal begins to decompose intensely), T3 (ignition temperature) and T4 (burnout temperature). With the decrease of O2 concentration, T2, T3, and T4 increased, the total mass loss and heat release decreased, and the decomposition degree increased. The injection of N2 and CO2 reduced the apparent activation energy and pre-exponential factor of T1–T2, T2–T3 and T3–T4 stages, and there was dynamic compensation effect between activation energy and pre-exponential factor. Under the same O2 concentration conditions, the effect of CO2 was more significant than that of N2. Reducing oxygen concentration by injecting N2 and CO2 could rapidly reduce the degree of coal-oxygen reaction rate. This study is of great significance for the prediction of coal spontaneous combustion and the using of N2 and CO2 coal fire prevention and extinguishing technology.

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.

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.

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.

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.

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.

Experimental study on adsorbing of flue gas and its application in preventing spontaneous combustion of coal

We propose an experimental adsorption device, imitating the environment of a coal-mine goaf and the composition of the flue gas in Tashan Mine Power Plant. The characteristics of the coal adsorbing flue gas were studied with the atmospheric volumetric method. The factors affecting the seal of CO2 were analyzed and the effect of power plant flue gas on fire prevention in the goaf was investigated at normal temperature and pressure. It can be inferred from the experiment that N2, SO2, and H2O can also reduce CO2 adsorption capacity. The increase or decrease in pH can increase the adsorption capacity of CO2, which is apparently larger when the pH is decreasing than when the pH is increasing. The O2 adsorption capacity can evidently be reduced when the power plant flue gas is injected into the goaf. The activation energy of coal burned in air is greater than that of coal burned in flue gas, indicating that the use of power plant flue gas, with N2 and CO2 as the main components, to replace the traditional inert gas can not only save N2 generation cost, but also reduce the emission of greenhouse gases, while the power plant flue gas can be adsorbed by coal.

GIS-Based Subsidence Prediction of Yuzhou Goaf Area at the Middle Route of the South-to-North Water Diversion Project

The phase I main canal of the Middle Route of the South-to-North Water Diversion Project crosses the coal mining area in Yuzhou city, and thus it is very important to predict the surface subsidence of the goaf. The GIS-based prediction system for Yuzhou goaf area subsidence has been developed by data collection, and the subsidence, horizontal movement, tilt and curvature are predicted for five mined out areas. The results show that the surface movement and deformation in the strike and dip directions are not more than the stability indexes specified in the coal mining regulation under buildings, water-bodies and railways, indicating that the residual deformation will not cause damage against the canal. The forecasting and monitoring values of Guo coal mine goaf and original Xinfeng goaf, located in Liang Town, Yuzhou City, are relatively large, thus the canal in this region may be affected and the safety monitoring should focus on this region.

Fibre optic sensors for coal mine hazard detection

A number of health and safety hazards present in underground coal mines, which include methane gas explosion, coal combustion, rock roof collapse, and flooding etc. Methane gas and coal combustion have been two major hazards, which resulted in most of the heavy casualties and economic losses. Conventional catalytic methane gas sensors suffer from poor accuracy and cumbersome maintenance, which is the bottleneck of methane hazard prevention. Coal mine combustion monitoring has been relying on gas tubing bundles system, which suffers from long time delay and poor reliability. Semiconductor laser diode methane gas sensors have been developed which has low power consumption, 0–100% full detection range and high accuracy, and no need of recalibration. Fibre optic Raman distributed sensors have been deployed in coal mine goaf and successfully detected combustion hazard in early phase. The FOS-based mine hazard detection system offers unique advantages of intrinsic safety, multi-location and multi-parameter monitoring. The application of FOS on monitoring of methane, coal combustion, micro-seismic and equipment condition are described, future trend of research in this area is also discussed.

Unique spatial methane distribution caused by spontaneous coal combustion in coal mine goafs: An experimental study

Abstract A new type of incident in the form of a methane explosion caused by spontaneous combustion of coal is receiving more and more attention due to the high ground temperature and high methane content in coal mines. To investigate the formation process of this type of incidents and advance the research on methane explosion risk caused by spontaneous coal combustion, a self-designed experimental platform was used to determine the influence of spontaneous coal combustion on methane migration in the goaf. Additionally, the effect of different ventilation velocities at the mining face on methane migration in the goaf was studied. The results reveal that air leakage plays a key role in methane migration in the goaf. The bigger the ventilation velocity, the larger the area affected by air leakage. In addition, the superposition of the air leakage and the horizontal and vertical movement of hot gases creates the entrainment effect in the spontaneous combustion area. Eventually, the methane concentration at the location of spontaneous combustion is the lowest and it gradually increases outside the spontaneous combustion area because of the superposition of air leakage, ‘fire pressure’ and the entrainment effect.

The effect of bituminous coal on methanogenic mixed cultures and pure cultures of Methanococcus and Methanosarcina

In numerous global coal–bearing basins, methane, possessing a biogenic stable isotope composition, is spatially and temporally associated with groundwater recharge. However, beyond groundwater inoculating the subsurface with microorganisms, the biological mechanisms that control the distribution of biogenic methane are poorly understood. In this study, we examined the interactions between bituminous coal and a) methanogenic microbial communities sourced from the goaf of a Bowen Basin underground coal mine and b) pure cultures of methanogens. When coal mine microbial consortium was amended with acetate and a low concentration of coal (1g in 25mL medium), methane production was stimulated compared to the addition of acetate alone, though the presence of coal did not affect the methane production from H2/CO2. To test whether methanogens benefited directly from the addition of coal, 1g of either quartz sand or bituminous coal was added to a pure culture of Methanococcus maripaludis, a hydrogenotrophic methanogen isolated from the coal mine and grown on H2/CO2, and to a pure culture of Methanosarcina barkeri, an acetoclastic methanogen grown on acetate. In this experiment coal was not included as an energy source for microbial growth but to test interactions between coal and methanogens. The presence of coal in the medium did not affect methane production by Ms. barkeri but slightly inhibited methane production by M. maripaludis at the start of the growth phase. Scanning electron microscopy revealed that M. maripaludis cells were attached to both sand grains and coal particles, with preferential attachment to rough surfaces, such as cracks within broken coal pieces and clay-rich areas of coal. When the experiment with M. maripaludis and Ms. barkeri was repeated with a 1:1 volumetric ratio of bituminous coal to medium, which more accurately reflects the environmental conditions of a coal seam, methane production by M. maripaludis and Ms. barkeri was completely inhibited. In addition, M. maripaludis cell numbers declined after inoculation. This suggests that at a high coal: fluid ratio, methanogenesis is inhibited by some component of the coal, e.g., bitumens in the coal or coal-sourced hydrocarbons dissolved into the medium. Based on these laboratory results, we propose that in coal seams, the dilution of inhibitory compounds may be one mechanism by which groundwater recharge promotes biogenic methane production.

Research on the Effectiveness of Electrical Detection

The detection of goaf is of great significance to the safe operation of power grid. In the traditional goaf detection technologies, electromagnetic detection method is more suitable for the detection of goaf under the transmission power line. There are many factors affect the effectiveness of the goaf detection, and it is very necessary to analyze these influence factors. In this paper, finite element analysis method has carried on the preliminary discussion to goaf detection effectiveness, and the influence of depth, size and water filling of coal mine goaf on the detection results are analyzed.