Coal Mine Gob Research Articles

Experimental study on CO2 sequestration capacity and mechanical characteristics evolution of solid wastes based carbon-negative backfill materials

The employment of solid waste, notably coal gangue, in carbon-negative backfill mining technology for mineralization and filling in coal mine gobs, is recognized as a productive method for carbon sequestration and waste management, with research primarily dedicated to engineering advanced mineralized backfill materials. Under this background, four variables, including the ratio of gangue to solids (RG-S), water-cement ratio (RW-C), ratio of fly ash to solid wastes providing calcium (Ca) (RA-Ca), and ratio of flue gas desulfurization (FGD) gypsum to carbide slag (RF-CS) were defined to formulate gangue-based mineralized backfill materials, with orthogonal experiments assessing their effects on CO2 sequestration and mechanical characteristics. The results indicate that RW-C consistently emerges as the primary factor influencing both the amount of sequestered CO2 and the mechanical properties of gangue-based solid waste mineralized backfill materials. The amount of sequestered CO2 demonstrates a direct proportionality to RW-C and mechanical strength demonstrates an inverse relationship with RW-C. After 45 minutes of stirring and carbon sequestration, samples in various groups exhibited an maximum sequestered CO2 amount of 18.96 g·kg−1, with a maximum sequestration rate of 0.20 g·kg−1·min−1. The process of CO2 sequestration enhances the compressive properties of mineralized backfill materials derived from gangue based solid wastes.

Study on the propagation characteristics of methane-air explosion under the promotion of crushed gangue

Methane-air explosion is one of the major disasters in industrial process. The explosion strength could be influenced by the crushed coal gangue, which is widely distributed in coal mine gob and roadway. To understand the influence of the coal gangue on gas explosion, an experimental system with a 0.2 × 0.2 × 3.0 m3 pipeline was designed and explosion experiments of coal gangue with 5 blockage length-diameter ratios (ratio of axial blockage length to pipeline equivalent diameter) were carried out. The results show that coal gangue can cause significant disturbances to the flame front, resulting in a violent acceleration of the explosion flame. The overpressure ratio presents a negative exponential function distribution with the blockage length-diameter ratio. The influence range increases with the blockage length-diameter ratio under the condition of rich fuel, and reaches the maximum when equivalent ratio is 1.237. The explosion intensity is more sensitive to the blockage length-diameter ratio for the equivalent ratio equals 1.0 and 1.237. The formation of high-intensity explosion should be avoided by controlling the accumulation state of the overlying rock in coal mining.

Development of an in-situ gel from CO2-captured complex solution and inhibiting coal spontaneous combustion: A case study in thermal engineering

The safety of underground coal mining is threatened by coal spontaneous combustion, making it essential to develop effective inhibitors to prevent such incidents. To address this, a new in-situ gel has been proposed, which is formed in coal pores from a fire-prevention aqueous solution (FPAS) that reacts with atmospheric CO2. To determine the appropriate concentrations of polyvinyl alcohol (PVA) and sodium-metasilicate nonahydrate (SN) for FPAS, 24 groups of wire-mesh basket tests were conducted on Baiyinhua coal. The gelling mechanism and performance on fire inhibition of FPAS were then investigated using temperature-programmed tests, scanning electron microscopy, energy dispersive spectrometry, fourier transform infrared tests, thermogravimetric differential thermal analysis tests, and fire-extinguishing tests. The results indicate an inhibition rate of up to 43.3% and effective protection of active groups from oxidation. The in-situ gel demonstrated excellent fire-extinguishing properties. Additionally, the self-ignition temperatures of residual coals with varying thicknesses were estimated using the Frank-Kamenetskii theory. The combined method of CO2 injection and FPAS spraying is recommended for achieving better fire-prevention effectiveness in coal mine gob. The findings are expected to provide guidance on the application of FPAS to address thermal hazards in coal mines.

Effects of CO2 injection factors on preventing spontaneous coal combustion in coal mine gobs: a simulation approach

The spontaneous combustion of residual coal in the gob seriously threatens the safety of coal mining. Injecting CO2 into the gob not only prevents the residual coal from spontaneous combustion but also realizes CO2 storage in the mined areas. Injection flux and burial depth of the port are crucial for CO2-preventing fire in coal mine gobs. In this study, the distribution of the oxidation zone in the Tanyaoping coal mine was field-measured, and the coal oxidation kinetic model was built by the adiabatic test. Then, a 3-D mathematical model was constructed based on the conditions of the 5011 working face by COMSOL Multiphysics. Furthermore, the coupled effects of the two factors on the distribution of the oxidation zone were investigated. Increases in both injection flux and burial depth result in a decrease in the oxidation zone volume. The reasonable ranges of the injection flux and burial depth are 540–720 m3 h-1 and 30–40 m, respectively. These results provide some guidelines on how to prevent the spontaneous combustion of residual coal in mine gobs.

Study on acceleration and suppression properties of coal gangue with different void fractions on gas explosion propagation

Gas explosion is one of the main disasters in coal mining. Plenty of coal gangue are generally distributed in the disaster areas in gob. Experiments were carried out to explore the propagation law of the gas explosion distributed by coal gangue. The variation characteristics of the overpressure, pressure rise rate, and flame shape with void fractions were analyzed. The results showed that the effect of the coal gangue on the explosion intensity changed from suppression to acceleration with the increase of void fraction, the flame front upstream blockage area changed from laminar state to turbulent divergent state, and a reverse flame was formed. When the void fraction of the coal gangue was 0.50–0.65, the maximum overpressure downstream of the blocked area were positively correlated with the void fraction and the critical suppression range was between 0.50 and 0.55. When the void fraction was lower than 0.50, the flame was quenched in the coal gangue, neither the flame nor the pressure could pass through the blocked area. It is helpful to guide the improvement of coal recovery process to avoid the expansion of the explosion impact in coalmine gob.

Parameters updating and calibrated double-yield model methods to simulate the compaction behaviour of waste rock backfill materials in coal mine gob

Coal mine waste rock can be filled into the gob to control strata movement and protect the mine environment. However, the compaction behaviour of the waste rock backfill materials (WRBM) must be considered when the they are filled into the coal mine gob. Based on the compaction properties of the WRBM tested by a purpose-made compaction device, Salamon's and Terzaghi's models are used to describe the stress–strain relationship of the WRBM, and the parameters updating (PU) and calibrated double-yield model (CDM) methods to simulate the WRBM are proposed in this paper. An engineering application in a backfill panel is used to validate the reliability of this method by numerical simulation and field measurement. Comparison of the vertical stress inside the WRBM obtained by the numerical simulation using the proposed method with those from field measurement indicates that the proposed method is reliable. The proposed method gives an opportunity to obtain the stress distribution accurately when the WRBM are filled into the coal mine gob.

Hole/pore-scale investigation of gas explosions in a coal-mine gob

The characteristics of a gas explosion were investigated with regard to the hole/pore scale in a coal-mine gob. A 3-D hole-pore single-layer network model (3-D HPSLNM) was developed, and the effects of the length-to-diameter ratio (L/D), ignition location, and gas concentration on the explosion flame and overpressure were determined with the FLACS flame acceleration simulator (Gexcon). The flame propagation speed was positively correlated with L/D and was lower in the hole than in the pores. The overpressure increased linearly with L/D. The flame propagation shape stretched and developed in different ways depending on the ignition location. The maximum flame speed and maximum overpressure for ‘end ignition’ were 56.5% and 25.2% higher than for ‘edge ignition’, respectively. The findings of this study will help further research into the hole/pore-scale effects of gas explosions in coal-mine gobs.

Piezoelectric Effect and Ignition Characteristics of Coal Mine Gob Roof Collapse.

Existing studies of coal self-ignition and impact frictional sparks do not provide valid support for the analysis of ignition sources in all cases of methane explosions in the gob. In this paper, the explosion in the gob of the Renlou coal mine is used to investigate the piezoelectric effect and ignition characteristics of roof collapse in identifying a new ignition source of gas explosion. Experimental and theoretical analyses conclude that the piezoelectric effect is produced by quartz, which is the main constituent of the roof sandstone. During the loading process, the piezoelectric effect and compressive strength are key factors in the gathering of free charges on rock tips. During rupture, the rock tip retains a large number of charges, forming a ″point-surface″ effect, which triggers an electron avalanche accompanied by an orange-yellow spark lasting over 22 ms, far exceeding the ionization energy and ignition induction period of methane–air mixtures. The piezoelectric effect and compressive strength of the rock cause the generation of electrical sparks, which is the ignition source of the explosion in the gob of Renlou mine II7322.

Recapitulation and Prospect of Research on Flow Field in Coal Mine Gob

Coal mine gob, mined-out areas in underground coal mines, often accumulates explosive methane-air mixtures that pose a deadly hazard to miners. A good understanding of the flow field in a sealed coal mine area is crucial in preventing and minimizing accidents associated with mine combustible gases and also for planning and implementing a mine rescue strategy. In recent years, the research on the flow field in the gob has changed from qualitative research in the past to quantitative research. This paper synthesizes the research results of flow field in gob in recent 40 years, covering the permeability of quarried areas, the airflow simulation in quarried areas, and the influence of ventilation parameters and geohydrological conditions on the flow field. Firstly, the overburden failure mechanism and fracture development characteristics of the mine gob, the distribution of porosity and permeability in the gob, and the relationship between them are introduced. Secondly, the development of research methods and numerical models used to study the flow field in mine gob is discussed. The distribution of the flow field in the gob under different conditions is expounded. Thirdly, the research on the prevention and control of fire and explosion risks in the gob is discussed. Finally, the problems to be solved in such research direction are addressed and suggestions are put forward.

Multifield Coupled Dynamic Simulation of Coal Oxidation and Self-Heating in Longwall Coal Mine Gob

The spontaneous combustion of residual coal in coal mine gob has long been a problem and poses a threat to the safe production of coal. Therefore, it is of great significance to conduct an in-depth study of the oxidation and self-heating progress of residual coal in the gob. Considering that the geometric dimensions and physical characteristics of the gob will change during the advance of the working face, the purpose of the present paper is to determine how the coal self-heating develops during and after coal mining. A fully coupled transient model including gas flow, gas species transport, and heat transfer in the gob and the butt entries, as well as heat transfer in the surrounding strata, is developed to quantify the evolution of coal self-heating in gob during and after mining. The model was solved by COMSOL Multiphysics package and then verified by comparing the field data with the simulated data. On this basis, parametric studies including the influences of the surrounding strata temperature, airflow temperature, coal-rock particle size, and advance rate of the working face on coal oxidation and self-heating in the gob were conducted. The results show that a tailing phenomenon of the high-temperature area is formed on the air inlet side of the gob during mining, and the temperature in the high-temperature zone decreases gradually due to the accumulation and compaction of the gob and heat dissipation to the surrounding strata. Also, although the temperature in gob increases gradually after the stopping of mining, the high-temperature area migrates towards the working face. Moreover, when the temperature of the surrounding strata is consistent, different ventilation temperatures have no obvious effect on the maximum temperature of the gob at the initial mining stage, whereas the higher ventilation temperature results in the higher self-heating temperature after several days of mining. Finally, the smaller average particle size or faster advance rate results in a lower maximum temperature of gob.

Discrete modeling of a longwall coal mine gob for CFD simulation

One area of concern in longwall coal mines is the active gob directly behind the longwall face, where high concentrations of methane are likely to accumulate and active roof caving occurs. Using computational fluid dynamics (CFD) to simulate gas flows in and through the gob, most researchers have represented the entire gob as a porous medium governed by Darcy’s law. However, Darcy-type porous flow may not be applicable for the highly porous and unconsolidated fringes of the gob. In addition, porous medium models do not allow for representative combustion modeling to simulate in-gob ignition and flame propagation. This study presents a hybrid approach to modeling the gob using CFD: the outer part of the gob is modeled as discrete objects that simulate coarse rock rubble, while the gob center is modeled as a porous medium.

Integrated experimentation and modeling of the formation processes underlying coal combustion-triggered methane explosions in a mined-out area

A methane explosion can take place when a methane concentration is within the explosive concentration range. Therefore, the methane migration around a coal combustion zone is important for assessing the disaster risk and revealing the disaster formation process. The thermal buoyancy effect has seldom been considered before, even though it could influence methane movement in coal mined-out areas. In this study, an integrated investigation using experimentation and modeling was conducted to explore the disaster formation process. For the experiment, a coal mine gob was heated locally to produce the buoyancy effect, and the consequent methane accumulation was observed. To explain this observation, a gas flow model reflecting the buoyancy effect was developed and verified against the experimental observations. Through this analysis, the formation processes for a methane explosion disaster in a coal mined-out area were revealed as follows: (1) coal combustion decreases the gas density to produce the buoyancy effect and create negative pressure in the combustion area; (2) this negative pressure leads to methane inflow, and the buoyancy effect causes upward methane movement; and (3) the local methane inflow and the local upward movement develop methane accumulation in the coal combustion area to form a methane explosion.

Effect of Pipe Diameter and Inlet Parameters on Liquid CO2 Flow in Transportation by Pipeline with Large Height Difference

Fire prevention and extinguishing and CO2 sequestration in coal mine gob require continuous transportation of liquid CO2 in pipelines with large height difference (from ground to underground). However, the temperature and pressure variation of liquid CO2 in pipelines with large height difference is still unclear, which hinders the design of a liquid CO2 pipeline transportation system. The influence of pipe diameter and inlet parameters (temperature and pressure) on the variation of temperature and pressure of liquid CO2 along the 1000 m vertical pipeline was studied in this paper. The study found that for each pipeline diameter considered there existed a range of flowrates where safe flow conditions could be ensured, at which no phase transition occurs throughout the length of the pipeline. When the transporting flow is larger than the maximum limit flow, phase transition occurs dramatically, which will lead to a sudden drop in temperature and pressure. When the transporting flow rate is lower than the minimum limit flow rate, phase transition of CO2 occurs slowly along the pipeline. According to the requirement of underground fire prevention and extinguishing for transporting flow rate and the economic cost of the pipeline system, the optimum diameter is 32 mm, and the corresponding safe transporting flow range is 507–13,826 kg/h. In addition, when the inlet pressure is constant, if the inlet temperature is too high, phase transition of CO2 occurs dramatically at the entrance. For a 1000 m vertical pipe with diameter of 32 mm, when the inlet pressure is 14 bar, 16 bar, 18 bar, 20 bar, 22 bar, 24 bar, the corresponding maximum allowable inlet temperatures are −30 °C, −26 °C, −23 °C, −19 °C, −16 °C and −13 °C, respectively. This research has significant guidance for safety transportation scheme of liquid CO2 from coal mine surface to underground.

Calculation and monitoring analysis of stress distribution in a coal mine gob filled with waste rock backfill materials

Crushed backfill materials are used to backfill the gob during solid backfill mining (SBM). Thus, the stress distribution in the coal mine gob is different from that found when using other methods including caving and paste backfill mining. The compaction properties of crushed waste rock backfill materials (WRBM) with different particle sizes were tested using a test machine and a self-made compaction device in order to analyze the stress distribution in the coal mine gob. Moreover, based on the stress-strain relationship for such WRBM, a method for calculating the stress distribution in a gob filled with backfill materials was proposed and a stress calculation model was established. In addition, taking engineering practice as a case study, stresses in the gob of an SBM-worked panel were calculated, measured on site, and analyzed. The calculated result is consistent with that measured on site, thus verifying the feasibility of the research. In addition, the influences of four primary factors including backfill material particle size, mining height, mining depth, and coal seam dip angle on the stress distribution in the gob were analyzed.

Study on the composition and structure of foamed gel for fire prevention and extinguishing in coal mines

The spontaneous combustion of coal in coal mine gob remains a serious threat to workers’ safety, and the present work examined the use of foamed gel to prevent coal ignition. A foamed gel is a polymeric substance consisting of foam agent, gelling agent, cross-linker and water. Gelation time, foaming multiple and water loss are key factors to evaluate the fire suppression properties of these materials. The experiments in this study demonstrated that the optimum volume fractions of the foaming, gelling and cross-linking agents were 0.5%, 2%, and 1% respectively. The average gelation time of the foamed gel was 174s, the foaming multiple was 5.4, and the water loss of the foamed gel over 10min was 11%, which was significantly less than the value of 91% for pure foam. 3D-X ray experiments showed that foamed gel has similar structure as pure foam, and gel skeleton could maintain the intact of foamed gel, enhance the stability, and increase heat capacity. Compared to pure foam, the foamed gel examined in this study exhibits superior fire prevention performance.

Influence of Stress on Void Ratios of Compacted Crushed Rock Masses in Coal Mine Gobs

The transfer and evolution of stress among rock blocks directly change the void ratios of crushed rock masses and affect the flow of methane in coal mine gobs. In this study, a Lagrange framework and a discrete element method, along with the soft-sphere model and EDEM numerical software, were used. The compaction processes of rock blocks with diameters of 0.6, 0.8, and 1.0 m were simulated with the degrees of compression set at 0%, 5%, 10%, 15%, 20%, and 25%. This study examines the influence of stress on void ratios of compacted crushed rock masses in coal mine gobs. The results showed that stress was mainly transmitted downward through strong force chains. As the degree of compression increased, the strong force chains extended downward, which resulted in the stress at the upper rock mass to become significantly higher than that at the lower rock mass. It was determined that under different degrees of compression, the rock mass of coal mine gobs could be divided, from the bottom to the top, into a lower insufficient compression zone (ICZ) and an upper sufficient compression zone (SCZ). From bottom to top, the void ratios in the ICZ sharply decreased and those in the SCZ slowly decreased. Void ratios in the ICZ were 1.2–1.7 times higher than those in the SCZ.

Risk assessment for long-distance gas pipelines in coal mine gobs based on structure entropy weight method and multi-step backward cloud transformation algorithm based on sampling with replacement

Destruction of pipelines because of coal mine gob disasters may result in enormous financial loss and significantly affect public safety. Hence, risk assessment of gob pipelines is of immense significance for field personnel. Given the lack of statistical data and the limitations of expert experience, a prior single risk value is often insufficient to reflect the actual situation and cannot meet the needs of the site. The backward cloud transformation (BCT) algorithm is a method that can fully mine the local information contained in expert experience to restore the overall information. However, the existing BCT algorithm has no solution under certain conditions, which considerably limits its application. This study proposes a comprehensive risk assessment method by combining the structural entropy method with a multi-step backward cloud transformation algorithm based on sampling with replacement (MBCT-SR). First, a simplified model for rapid identification is used to determine whether it is worth calculating risk values. Second, a fault tree that fits the actual situation is established, and the weights of the indexes are determined by the structural entropy weight method. Third, the interval scores of the indexes are transformed into numerical features of the cloud model, which are then logically operated using virtual cloud algorithms. Finally, the risk values of the pipeline can be obtained, the cloud droplet diagram of the risk values is clearly shown by the forward cloud transformation (FCT) algorithm, and the risk level can be obtained by the probability that the cloud droplet falls into each risk level interval. To validate the utility of the proposed method, a case study of a coal mine gob around a long-distance gas pipeline was investigated.

Distribution of gob empty space for methane drainage during the longwall mining: A case study

In China, there are a large number of coal mine gobs have been formed as a result of longwall mining processes. These mine gobs contain an enormous amount of methane storage spaces within the gob formations. The methane storage spaces as well as guide methane drainage via vertical boreholes require quantitative measurements. In this study, physical simulation experiments were conducted for the purpose of obtaining digital images of gobs at various working face advance distances. Then, a Matlab-based digital image processing and analysis program was developed to process the obtained high-definition images, extract the geometrical parameters, and calculate the areas of methane storage spaces within the gobs. The results showed that the heights of the methane storage spaces first increased, and then stabilized as the gob lengths increased. The area sizes of the gobs increased gradually with the lengths, while the methane storage space areas were observed to first increase steadily to a maximum, and then decrease sharply. The main methane storage spaces exhibited a decreasing trend stage as the gob lengths increased. Whereas, the secondary methane storage spaces showed the characteristics of increasing trends. These research findings provide a theoretical foundation for the accurate predictions of both the methane storage capacities and methane drainage efficiency of longwall mining gobs.

Surface-based radon detection to identify spontaneous combustion areas in small abandoned coal mine gobs: Case study of a small coal mine in China

Abstract In China, small coal mines refer to non-state-run coal mines with an annual production of less than 300,000 tons. The mining recovery rate of these small Chinese coal mines is low, and large amounts of coal remain in the gob. The mining seams of small coal mines are shallow, which induces extensive fissures between the gob and ground and provides easy pathways for air leakage. Consequently, long-term air leakage and oxygen supplies can result in spontaneous coal combustion in the gob, forming fire areas for many years. Determination of spontaneous combustion areas in the gob of small coal mines is difficult because fire sources are hidden underground and the geological data and roadway layout are poorly documented. Surface-based radon detection provides a fast, accurate, and low-cost method to identify spontaneous combustion areas in the gob of small coal mines. This study focuses on a small abandoned coal mine in Shanxi Province, China, using surface-based radon detection. Three abnormal temperature areas (A, B, C) and a potential abnormal temperature area (D) were identified. Drilling was subsequently performed to measure the temperature distribution in these areas. The results show that spontaneous combustion areas in small abandoned coal mine gobs can be successfully identified.

A conceptual model to characterize and model compaction behavior and permeability evolution of broken rock mass in coal mine gobs

This paper proposed a conceptual model of broken rock mass compaction based on elastic theory by simplifying the compression process. This conceptual model assumes that the contact connection between two adjacent rock particles is similar to a cubic mass. With this simplification, the stress-strain constitutive law is established. The change in the secant modulus in the mechanical model derived from the variation of the connection coefficient agrees well with the reported experimental results. The permeability of compacted rock mass evolution was modeled based on the cubic law. The mechanical compression model was coupled with the permeability evolution model. The modeled permeability evolution is consistent with reported simulation and experimental results. The modeled permeability results were validated using Karacan's data with broken shale rock properties. Compared to an intact rock mass, we found that the stress-strain curve of a compacted rock mass takes a longer compression path to reach linearity due to the void space compaction resulting from friction slipping and the re-arrangement of particles. It was also found that the particle elastic modulus does not contribute to the overall bulk compaction and permeability reduction at the initial compaction stage. However, the particle elastic modulus controls the permeability evolution for a fully compacted gob, where the gob can be treated as an intact rock mass. The proposed conceptual models will potentially lay the foundation for future permeability and caving behavior characterizations using numerical simulations for complex gob areas.