A method and system for determining the spontaneous combustion dangerous area in the goaf during the working face withdrawal stage

Abstract

The present invention belongs to the technical field of coal mine disaster prediction, and discloses a method and a system for determining the spontaneous combustion dangerous area in the goaf during the working face withdrawal stage. This method determines the influence of the development of air voids in the goaf, which forms an air leakage channel, on the spontaneous combustion dangerous area; verifies the influence of the distribution of residual coal on the spontaneous combustion dangerous area in the goaf; determines the influence of the void ratio and its distribution after the reduction of the coal caving amount in the working face on the distribution of the spontaneous combustion dangerous area in the goaf; during the withdrawal period, the air volume in the working face is reduced; after the later-stage end hydraulic support is withdrawn, the coal body collapses and blocks the return air channel, and the working face is switched from full negative pressure ventilation to local fan positive pressure ventilation; using the CFD-DEM simulation results, the formula for the change of void ratio and residual coal distribution is imported into the FLUENT numerical simulation software. The present invention provides a technical basis for determining the high-risk area of spontaneous combustion in the goaf from the end mining to the withdrawal of the working face, and helps the development of industry technology standardization and regularization; it avoids the generation of gases such as CO and CO2 caused by spontaneous combustion in the goaf.

Description

Technical Field

[0001] The present invention belongs to the technical field of coal mine disaster prediction, and particularly relates to a method and system for determining the spontaneous combustion dangerous area in the goaf during the working face withdrawal stage. Background Technique

[0002] During the coal mining process, the working face withdrawal stage is a critical period. At this time, the support system is removed, the roof management is complex, and the structure of the goaf changes significantly. These changes lead to complex gas flow and temperature distribution in the goaf, increasing the risk of spontaneous combustion. Traditional methods for determining the spontaneous combustion dangerous area are mostly based on experience or single-parameter analysis, such as only considering oxygen concentration or temperature, and do not fully consider the combined effect of ventilation mode, coal residue distribution, and void structure change on the oxygen distribution and temperature distribution in the goaf, resulting in insufficient accuracy of the determination results and difficulty in effectively guiding coal mine safety management.

[0003] Through the above analysis, the problems and defects of the existing technology are: the accuracy of determining the spontaneous combustion dangerous area in the goaf during the working face withdrawal stage under special technological conditions in the existing technology is poor. Summary of the Invention

[0004] To overcome the problems in the related technology, the disclosed embodiments of the present invention provide a method and system for determining the spontaneous combustion dangerous area in the goaf during the working face withdrawal stage, specifically relating to a method for determining the spontaneous combustion dangerous area in the goaf during the working face withdrawal stage considering special processes.

[0005] The technical solution is as follows: A method for determining the spontaneous combustion dangerous area in the goaf during the working face withdrawal stage includes the following steps:

[0006] S1, Determine the change law of the void structure behind the fully mechanized support during the working face withdrawal to determine the development of air leakage channels formed by the air leakage fissures in the goaf and their influence on the spontaneous combustion dangerous area.

[0007] S2, Determine the distribution law of the coal residue behind the fully mechanized support during the withdrawal period and extract the fitting formula of the coal residue data to verify the influence of the coal residue distribution on the spontaneous combustion dangerous area in the goaf.

[0008] S3, Derive the void ratio formulas at three positions in the middle of the goaf, the distance without caving the top coal behind the fully mechanized support, and the top of the fully mechanized support during the withdrawal period through theoretical models and numerical simulations to determine the influence of the void ratio and its distribution on the distribution of the spontaneous combustion dangerous area in the goaf after the reduction of the coal caving amount of the working face.

[0009] S4, During the goaf withdrawal stage, the air volume of the working face is reduced; when the support at the end working face starts to be withdrawn, the roof caving causes the full-pressure ventilation to be unable to be maintained, and it is switched to local fan ventilation, changing the full-pressure ventilation to positive-pressure ventilation and reducing the air volume.

[0010] S5. Write the data of the measured wind speed, remaining coal, void fraction, and oxygen consumption parameters during the withdrawal stage through UDF and import them into the FLUENT numerical simulation software;

[0011] S6. Obtain the spontaneous combustion dangerous area in the goaf under the special technology during the withdrawal stage through numerical simulation and on-site measurement results.

[0012] In step S1, determine the variation law of the void structure behind the fully mechanized support during the face withdrawal, including:

[0013] S1.1. Establish a caving model of overlying strata during coal seam mining during withdrawal using DEM numerical simulation software;

[0014] S1.2. According to the caving results of overlying strata during the advancement of the model, obtain the variation law of the void structure behind the fully mechanized support, the formation and development process of fractures, and the air leakage channels;

[0015] S1.3. Simulate the withdrawal stage of the face advancement, and do not release top coal at a certain distance from the stop line of coal mining, and obtain the variation law of the void structure with and without coal release.

[0016] In step S2, verify the influence of the remaining coal distribution on the spontaneous combustion dangerous area in the goaf, including:

[0017] S2.1. During the withdrawal stage, the accumulation state of coal and rock in the goaf is a mixed accumulation of coal and rock. There is less remaining coal in the middle of the goaf and more remaining coal behind the fully mechanized support;

[0018] S2.2. Extract the remaining coal in the middle of the goaf and the remaining coal without releasing top coal behind the fully mechanized support during the withdrawal stage respectively, and use MATLAB software to fit them into a mathematical formula to generate a fitting graph.

[0019] In step S2.2, use MATLAB software to fit into a mathematical formula to generate a fitting graph, including:

[0020] S2.2.1. Extract the remaining coal data using the internal code of PFC numerical simulation software;

[0021] S2.2.2. Smooth the data through origin software to generate two-dimensional plane data;

[0022] S2.2.3. Import the data into MATLAB software for fitting, and select the binomial to generate a fitting graph of the remaining coal height.

[0023] In step S3, determine the influence of the void fraction and its distribution on the distribution of the spontaneous combustion dangerous area in the goaf after the reduction of the face coal release amount, including:

[0024] S3.1. Use the measurement circle tool to extract the caving void structure in the goaf;

[0025] S3.2. Use a measurement circle to extract the remaining coal in the goaf in sub - regions. Among them, for the non - caving coal areas, the measurement circle is densified, and the extracted void ratio data is fitted into a formula and a void distribution map is generated.

[0026] S3.3. Fit the remaining coal data into a graph.

[0027] In step S4, switch to local fan ventilation, change the full - negative - pressure ventilation to full - positive - pressure ventilation and reduce the air volume, including:

[0028] S4.1. After the face supports start to be withdrawn, switch to local fan ventilation.

[0029] S4.2. Optimize the ventilation system for high - gas and easy - spontaneous - combustion coal seams.

[0030] S4.3. After the face stops mining and before the withdrawal fan is started, the air volume is not less than half of the original air volume.

[0031] In step S5, write through UDF and import it into the FLUENT numerical simulation software, including:

[0032] S5.1. Measure the inlet area and calculate the wind speed based on the air volume.

[0033] S5.2. Fit the measured remaining coal data into a formula, calculate the oxygen consumption rate and import it into FLUENT.

[0034] S5.3. Use the cftool tool of Matlab software to fit the void ratio data. The fitted void ratio formula is:

[0035] ;

[0036] In the formula, is the void ratio, are all constants, are the strike and dip of the goaf respectively, is the height of the goaf;

[0037] S5.4. Fit the obtained void ratio data to judge the distribution state of the data, fit the void ratio and generate a fitting graph.

[0038] S5.5. Evaluate the coincidence degree of the fitting result through the sum of squared errors and the correlation coefficient, and organize the data and import it into FLUENT.

[0039] In step S6, through numerical simulation and on - site measurement results, obtain the spontaneous - combustion dangerous areas in the goaf under special processes during the withdrawal stage, including:

[0040] S6.1. Establish a numerical simulation model of the goaf, incorporate the oxygen consumption void formula into FLUENT, and set various parameters for numerical simulation calculation;

[0041] S6.2. Delimit the spontaneous combustion danger area and scope according to the software calculation results in terms of the limiting oxygen concentration, minimum floating coal thickness, and limiting air leakage rate;

[0042] S6.3. The determination formula for the spontaneous combustion danger area in the goaf is:

[0043] ;

[0044] In the formula, is the floating coal thickness, is the oxygen concentration, is the air leakage rate, is the minimum floating coal thickness, is the limiting oxygen concentration, is the limiting air leakage rate;

[0045] S6.4. Obtain the spontaneous combustion danger area in the goaf through the verification of the numerical simulation results and the measured results.

[0046] In step S6.1, incorporate the oxygen consumption void formula into FLUENT and set various parameters for numerical simulation calculation, including:

[0047] S6.1.1. Draw a numerical simulation model of the goaf using CAD according to the actual situation;

[0048] S6.1.2. Write the code for the oxygen consumption and porosity formula using udf;

[0049] S6.1.3. Set the energy equation and turbulence model parameters in FLUENT.

[0050] Another object of the present invention is to provide a determination system for the spontaneous combustion danger area in the goaf during the face withdrawal stage. This system implements the determination method for the spontaneous combustion danger area in the goaf during the face withdrawal stage, and this system includes:

[0051] An influence determination module of the air leakage channel on the spontaneous combustion danger area, which is used to determine the change law of the void structure behind the fully mechanized support during the face withdrawal to determine the development of air voids in the goaf and the influence of the formed air leakage channel on the spontaneous combustion danger area;

[0052] A verification module of the influence of the remaining coal distribution on the spontaneous combustion danger area in the goaf, which is used to determine the remaining coal distribution law behind the fully mechanized support during the withdrawal and extract the fitting formula of the remaining coal data to verify the influence of the remaining coal distribution on the spontaneous combustion danger area in the goaf;

[0053] The void fraction distribution impact module on the spontaneous combustion dangerous area distribution in the gob area is used to derive the void fraction formulas at three positions, namely the middle of the gob area, the distance without caving coal behind the fully-mechanized support, and the top of the fully-mechanized support during the withdrawal period through theoretical models and numerical simulations, and to determine the impact of the void fraction and its distribution on the spontaneous combustion dangerous area distribution in the gob area after the reduction of the coal caving amount in the working face;

[0054] The local ventilator ventilation switching module is used to reduce the air volume in the working face during the gob area withdrawal stage; when the supports at the end working face start to be withdrawn, the roof caving causes the full-pressure ventilation to be unable to be maintained, and it is switched to local ventilator ventilation, changing the full-pressure ventilation to positive-pressure ventilation and reducing the air volume;

[0055] The parameter import module is used to write the data of the measured wind speed, remaining coal, void fraction and oxygen consumption parameters during the withdrawal stage through UDF and import them into the FLUENT numerical simulation software;

[0056] The gob area spontaneous combustion dangerous area identification module is used to obtain the gob area spontaneous combustion dangerous area under special processes during the withdrawal stage through numerical simulations and on-site measurement results.

[0057] Combining all the above technical solutions, the beneficial effects of the present invention are as follows: The present invention analyzes the dynamic change law of the void structure behind the fully-mechanized support in the gob area and the change law of parameters such as the remaining coal distribution under special process conditions during the working face withdrawal period, and combines the influence of different complex conditions such as exposure time, support withdrawal cycle and ventilation mode on the spontaneous combustion dangerous area distribution in the gob area. This method can scientifically, reasonably and accurately determine and identify the gob area spontaneous combustion dangerous area under complex conditions during the working face withdrawal stage, effectively improving the prevention ability of coal mine fires, which is of great significance to coal mine safety management. Brief Description of the Drawings

[0058] The drawings here are incorporated into the specification and form a part of this specification, showing the embodiments in line with the present disclosure, and are used together with the specification to explain the principles of the present disclosure;

[0059] Figure 1 It is a diagram of the method and system for determining the spontaneous combustion dangerous area in the gob area during the working face withdrawal stage provided by the embodiment of the present invention;

[0060] Figure 2 It is a diagram of the PFC overlying strata caving model simulating the final mining advancing stage of the working face provided by the embodiment of the present invention;

[0061] Figure 3 It is a diagram of the PFC overlying strata caving model simulating the working face withdrawal stage provided by the embodiment of the present invention;

[0062] Figure 4 It is a data effect diagram measured in the measurement circle of the present invention;

[0063] Figure 5 The present invention is Figure 4 The effect diagram is obtained through MATLAB linear fitting;

[0064] Figure 6 It is the coal fitting diagram of the present invention without placing top coal;

[0065] Figure 7 It is the fitting diagram of top coal caving of the present invention;

[0066] Figure 8 The application software of the present invention performs 3D modeling of the goaf;

[0067] Figure 9 It is a schematic diagram of the interior of the goaf model of the present invention;

[0068] Figure 10 It is the distribution map of the natural zone of top coal in the working face withdrawal stage of the present invention;

[0069] Figure 11 It is the top coal oxygen concentration distribution diagram during the working face withdrawal stage of the present invention;

[0070] Figure 12 It is a natural zone diagram of the final mining stage of the spontaneous combustion area of ​​the oxygen concentration in the goaf of the present invention;

[0071] Figure 13 It is a natural zone diagram of the withdrawal stage of the spontaneous combustion area of ​​the oxygen concentration in the goaf of the present invention. DETAILED DESCRIPTION

[0072] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific embodiments of the present invention are described in detail below in conjunction with the accompanying drawings. In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present invention, so the present invention is not limited by the specific implementation disclosed below.

[0073] The present invention solves the problem that the traditional three - zone in the goaf fails to consider the distribution of residual coal, the dynamic evolution of void structure, the change of ventilation mode, etc., resulting in the disadvantage of being unable to interpret the law and characteristics of residual coal spontaneous combustion during the withdrawal stage of fully - mechanized top - coal caving mining. The present invention first combines the fully - mechanized top - coal caving withdrawal process and proposes the idea of expanding the traditional spontaneous combustion three - zone area to a three - dimensional small - void - rate distribution range of residual coal in the fully - mechanized top - coal caving mining, namely, a three - zone spontaneous combustion area in a three - dimensional space. The innovation of the present invention lies in that the area near the working face stop - line is a high - incidence area of coal spontaneous combustion. During the stop - mining and support - withdrawal period, the special technological conditions lead to changes in the void structure, residual coal distribution and exposure time in the goaf, and the traditional spontaneous combustion three - zone theory cannot analyze the spontaneous combustion problem during this period. The present invention combines the variation law of key parameters caused by the special technological conditions of the fully - mechanized caving face, and focuses on analyzing the distribution law of the three - zone spontaneous combustion of the fully - mechanized support in three - dimensions and its influence on the spontaneous combustion of residual coal.

[0074] Example 1, as Figure 1 shown, the method for determining the spontaneous combustion dangerous area in the goaf during the working face withdrawal stage provided by the embodiment of the present invention includes:

[0075] S1. Determine the change law of the void structure at the rear of the fully - mechanized support during the working face withdrawal to determine the development of air - leakage channels formed by the void fissures in the goaf and its influence on the spontaneous combustion dangerous area;

[0076] S2. Determine the distribution law of the residual coal at the rear of the fully - mechanized support during the withdrawal period and extract the fitting formula of the residual coal data to verify the influence of the residual coal distribution on the spontaneous combustion dangerous area in the goaf;

[0077] S3. Derive the void - ratio formulas at three positions, namely, the middle of the goaf, the distance without top - coal caving at the rear of the fully - mechanized support, and the top of the fully - mechanized support during the withdrawal period through theoretical models and numerical simulations, and determine the influence of the void ratio and its distribution on the distribution of the spontaneous combustion dangerous area in the goaf after the reduction of the coal caving amount at the working face;

[0078] S4. During the goaf withdrawal stage, the air volume of the working face is reduced; when the support at the end - face working face starts to be withdrawn, the roof caving causes the full - negative - pressure ventilation to be unable to be maintained, and it is switched to local - ventilator ventilation, changing the full - negative - pressure ventilation to positive - pressure ventilation and reducing the air volume;

[0079] S5. Compile the data of the wind speed, residual coal, void ratio and oxygen - consumption parameters measured during the withdrawal stage through UDF and import them into the FLUENT numerical simulation software;

[0080] S6. Obtain the spontaneous combustion dangerous area in the goaf under the special technology during the withdrawal stage through numerical simulation and on - site measurement results.

[0081] Exemplarily, in step S1, the mining method is fully - mechanized top - coal caving, and the coal - seam occurrence conditions consider conditions such as coal - seam thickness, dip angle, and accumulation mode;

[0082] During the mining process, the PFC numerical simulation software is used to simulate the caving of overlying strata to determine the change law of the void structure during the withdrawal process.

[0083] In step S1, it specifically includes the following steps:

[0084] S1.1, Use the DEM numerical simulation software to establish a caving model of overlying strata during coal seam mining during withdrawal;

[0085] S1.2, According to the caving results of overlying strata during the advancement of the model, obtain the change law of the void structure behind the fully mechanized support, the formation and development process of fractures, and the air leakage channels;

[0086] During the withdrawal stage, a high void ratio area appears behind the fully mechanized support. As the stop - mining time extends, the void ratio area decreases, and multiple horizontal and vertical fractures appear.

[0087] S1.3, Simulate the withdrawal stage of the working face advancement, and do not release the top - coal at a certain distance from the stop - mining line to obtain the change law of the void structure with and without top - coal release.

[0088] Exemplarily, in step S2, it specifically includes the following steps:

[0089] S2.1, During the withdrawal stage, the coal - rock accumulation state in the goaf shows a mixed accumulation of coal and rock. The composition and structure of the coal - rock mixture will change due to roof caving and overlying strata fragmentation swelling. There is less remaining coal in the middle of the goaf and more remaining coal behind the fully mechanized support.

[0090] S2.2, Extract the remaining coal in the middle of the goaf and the remaining coal at a certain distance without releasing the top - coal behind the fully mechanized support during the withdrawal stage respectively, and use MATLAB software to fit them into mathematical formulas to generate fitting diagrams; specifically including:

[0091] S2.2.1, Use the internal code of the PFC numerical simulation software to extract the remaining coal data;

[0092] S2.2.2, Smooth the data through origin software to generate two - dimensional plane data;

[0093] S2.2.3, Import the data into MATLAB software for fitting, and finally select a binomial with a better fitting degree to generate a remaining coal height fitting diagram.

[0094] Exemplarily, in step S3, it specifically includes the following steps;

[0095] S3.1, Use the measurement circle tool to extract the caving void structure of the goaf; the number of measurement circles is less than the number of particles, the radius of the measurement circle is 0.5m, and it covers the model;

[0096] S3.2. Use the measurement circle to extract the remaining coal in the goaf area by region. For the area without top coal caving, the measurement circle is encrypted, and the extracted void ratio data is fitted into a formula and a void distribution map.

[0097] S3.3. Fit the remaining coal data into a graph.

[0098] Exemplarily, in step S4, it specifically includes the following steps;

[0099] S4.1. After the face support starts to be withdrawn, the caving of the roof may cause the full-pressure ventilation to be unable to be maintained. At this time, it is necessary to switch to local fan ventilation.

[0100] S4.2. Optimize the ventilation system for high-gas and easy-to-spontaneously-ignite coal seams; establish a coal mine ventilation system, reasonably optimize and design the ventilation network, and propose an optimization and transformation plan for the problems existing in the ventilation system to optimize the ventilation system.

[0101] S4.3. After the face stops mining and before the withdrawal fan is started, the air volume usually decreases, but it shall not be less than half of the original air volume. The adjustment should help reduce the air leakage in the goaf and ensure the ventilation requirements of the working area.

[0102] Exemplarily, in step S5, it specifically includes the following steps;

[0103] S5.1. Measure the inlet area and calculate the wind speed based on the air volume; the wind speed is calculated by the following formula:

[0104] ;

[0105] In the formula, is the wind speed, is the air volume, is the inlet and outlet area of the coal seam.

[0106] S5.2. Fit the measured remaining coal data into a formula to calculate the oxygen consumption rate and import it into FLUENT;

[0107] S5.2.1. Measure the remaining coal data: First, it is necessary to measure the remaining coal data in the coal seam, including the type of coal, humidity, temperature, etc.

[0108] S5.2.2. According to the remaining coal data, use an appropriate reaction equation to describe the oxidation process of coal and calculate the oxygen consumption rate. The formula is as follows:

[0109] ;

[0110] In the formula, is the oxygen consumption rate, is the coefficient related to the characteristics of the remaining coal, and the value is generally taken as 1, is the oxygen concentration.

[0111] S5.3. Use the cftool tool in Matlab software to fit the porosity data with a formula; the present invention innovatively proposes that the fitted porosity formula is:

[0112] ;

[0113] In the formula, is the porosity, are all constants, are the strike of the goaf and the dip of the goaf respectively, is the height of the goaf;

[0114] S5.4. Fit the obtained porosity data to judge the distribution state of the data, fit the porosity, and generate a fitting graph; perform the fitting through Origin software. The distribution state can be judged from the porosity surface in the fitting graph. The fitting graph is as shown in Figure 5 ;

[0115] S5.5. Evaluate the degree of agreement of the fitting result through the sum of squared errors and the root mean square error, and organize the data and import it into FLUENT.

[0116] S5.5.1. Represents the sum of the squares of the differences between the model prediction values and the actual observed values. The smaller the value, the more accurate the model prediction);

[0117] S5.5.2. The root mean square error is the square root of the sum of squared errors. It provides an error measure in the same unit as the original data. It measures the average difference between the model prediction values and the actual observed values. The smaller the RMSE, the higher the prediction accuracy of the model.

[0118] Exemplarily, in step S6, it specifically includes the following steps;

[0119] S6.1. Establish a numerical simulation model of the goaf, program formulas such as oxygen consumption and porosity into FLUENT, and set various parameters for numerical simulation calculation; specifically including:

[0120] S6.1.1. Draw a numerical simulation model of the goaf using CAD according to the actual situation;

[0121] S6.1.2. Write the code for the oxygen consumption and porosity formulas using udf;

[0122] S6.1.3. Set parameters such as the energy equation and the turbulence model in FLUENT.

[0123] S6.2. Delimit the spontaneous combustion danger zone and range according to the software calculation results in terms of the limiting oxygen concentration, the minimum floating coal thickness, and the limiting air leakage rate.

[0124] S6.2.1. According to the conditions of coal spontaneous combustion, the gob can be divided into three zones of coal spontaneous combustion, namely the heat dissipation zone, the oxidation zone, and the asphyxiation zone. Among them, the oxidation zone is the area with spontaneous combustion danger in the gob.

[0125] S6.2.2. During the actual division process, due to the very complex boundary conditions of the gob, the error in the range of the three zones divided is relatively large. Therefore, the principle of combining computer numerical simulation and on-site measurement can be adopted.

[0126] S6.3. The determination formula for the area with spontaneous combustion danger in the gob is:

[0127] ;

[0128] In the formula, is the thickness of floating coal, is the oxygen concentration, is the air leakage rate, is the minimum thickness of floating coal, is the limit oxygen concentration, is the limit air leakage rate;

[0129] S6.4. Through the verification of the numerical simulation results and the measured results, the area with spontaneous combustion danger in the gob is obtained.

[0130] Among them, the area with spontaneous combustion danger is divided by the oxygen concentration of 5% - 15%.

[0131] Example 2. The determination system for the area with spontaneous combustion danger in the gob during the face withdrawal stage provided by the embodiment of the present invention includes:

[0132] The module for determining the influence of the air leakage channel on the area with spontaneous combustion danger is used to determine the change law of the void structure behind the fully mechanized support during the face withdrawal period, so as to determine the development of air voids in the gob and the influence of the formed air leakage channel on the area with spontaneous combustion danger;

[0133] The module for verifying the influence of the distribution of residual coal on the area with spontaneous combustion danger in the gob is used to determine the distribution law of residual coal behind the fully mechanized support during the withdrawal period and extract the fitting formula of the residual coal data to verify the influence of the distribution of residual coal on the area with spontaneous combustion danger in the gob;

[0134] The module for determining the influence of the void ratio distribution on the distribution of the area with spontaneous combustion danger in the gob is used to derive the void ratio formulas at three positions, namely the middle of the gob, the distance without caving coal behind the fully mechanized support, and the top of the fully mechanized support during the withdrawal period through theoretical models and numerical simulations, and determine the influence of the void ratio and its distribution on the distribution of the area with spontaneous combustion danger in the gob after the reduction of the coal caving amount of the working face;

[0135] The local ventilator ventilation switching module is used to reduce the air volume at the working face during the gob retreat stage; when the supports at the end working face start to be withdrawn, the roof caving causes the full-pressure ventilation to be unable to be maintained, and it is switched to local ventilator ventilation, changing the full-pressure ventilation to positive-pressure ventilation and reducing the air volume.

[0136] The parameter import module is used to write the data of the measured wind speed, residual coal, porosity, and oxygen consumption parameters during the retreat stage through UDF and import them into the FLUENT numerical simulation software.

[0137] The gob spontaneous combustion hazard area identification module is used to obtain the gob spontaneous combustion hazard area under the special technology during the retreat stage through numerical simulation and on-site measurement results.

[0138] Example 3, as another example of the present invention, in a certain mining area, the fully mechanized top coal caving technology is adopted, the coal seam thickness is 7.5 m, the overlying rock caving law is relatively complex, and there is a high spontaneous combustion risk during the working face retreat stage. The traditional method only judges based on the oxygen concentration and temperature, and fails to comprehensively consider the influence of ventilation mode, residual coal distribution, and void structure change, resulting in insufficient accuracy of spontaneous combustion early warning.

[0139] The traditional method uses single-parameter analysis, mainly by monitoring the oxygen concentration and temperature in the gob to judge the spontaneous combustion hazard area. However, this method does not fully consider the comprehensive influence of ventilation mode change, residual coal distribution, and void structure change on oxygen distribution and temperature distribution, resulting in a large deviation between the judgment result and the actual risk area.

[0140] The present invention significantly improves the accuracy and scientificity of the judgment of the spontaneous combustion hazard area through the following improved technical points: comprehensively considering the influence of ventilation mode change on the gas flow and temperature distribution in the gob, introducing the local ventilator ventilation mode; analyzing the influence of residual coal distribution on the spontaneous combustion hazard area through numerical simulation and theoretical model, extracting residual coal data and fitting formulas; studying the law of void structure change, simulating the overlying rock caving process through the PFC numerical simulation software to determine the porosity distribution; integrating parameters such as wind speed, residual coal, porosity, and oxygen consumption into the FLUENT numerical simulation to establish a scientific judgment model for the gob spontaneous combustion hazard area.

[0141] Example 4, as another example of the present invention, applying the method of the present invention in a certain coal mine, through establishing a PFC model and a 3D numerical model, successfully predicted the spontaneous combustion hazard area in the gob during the working face retreat stage, guiding effective fire prevention and extinguishing measures.

[0142] The existing gob spontaneous combustion prediction methods mainly rely on empirical formulas and simple numerical simulations, and cannot fully consider the complex changes during the working face retreat stage.

[0143] Through the precise simulation by combining DEM and CFD software, the present invention takes into account the dynamic evolution of porosity and residual coal height, improving the prediction accuracy and adaptability. The method of the present invention can more accurately determine the spontaneous combustion hazard area in the goaf during the face withdrawal stage, providing strong technical support for the safety management of coal mines and significantly reducing the risk of spontaneous combustion.

[0144] To further illustrate the related effects of the embodiments of the present invention, the following experiments are carried out.

[0145] The method for determining the spontaneous combustion hazard area in the goaf during the face withdrawal stage provided by the embodiment of the present invention includes:

[0146] S101. Establish a PFC overlying strata caving model, including:

[0147] (1) The model size is length × height = 247 × 108 m, with 40 m mining boundaries left on both sides. The initial particle velocity, wall velocity, and acceleration are all 0;

[0148] (2) The upper half and the left half of the model are both taken as fixed boundaries by WAL1. The strata are added to the model above in the form of loads, and the load is taken as 8 MPa. Rigid walls are used on both sides and below the model;

[0149] (3) The working face is divided into 24 sections, each section is 7 m long, and the mining thickness is 7.45 m for mining simulation. The simulation results of the PFC overlying strata caving model are as Figure 2 、 Figure 3 shown.

[0150] S102. Extraction and compilation of the porosity during the face withdrawal.

[0151] This step includes:

[0152] 1) Utilize the characteristics of PFC2D software and adopt a specific measurement circle to track the change of the internal porosity of the established model;

[0153] 2) The measurement circle is always in a stable state, and the measurement circles at the four boundaries of up, down, left, and right are immovable. The vertical movement of particles will pass through the measurement circle;

[0154] 3) According to the porosity distribution and actual engineering conditions, select the data measured in the measurement circle and fit them linearly through MATLAB, as Figure 4 the data effect diagram measured in the measurement circle, as Figure 5 shown Figure 4 by the linear fitting through MATLAB;

[0155] 4) The fitted porosity formula is:

[0156] ;

[0157] S103. Based on the residual coal height data extracted during the normal advancement stage and the retreat stage of the working face during the retreat process, through formula fitting using MATLAB software, a sixth-order power function is obtained as Figure 6 The fitting graph of non-top-coal caving; Figure 7 As shown in the fitting graph of top-coal caving;

[0158] In this step, the formula for the residual coal height:

[0159] ;

[0160] Among them, p1 = -1.431e -9 , p2 = -7.7707e -06 , p3 = 9.951e -04 , p4 = -0.0002725, p5 = 0.07894, p6 = -0.1216, p7 = 2.302.

[0161] S104. According to the actual situation of the working face, the software is used to perform 3D modeling on the goaf as Figure 8 shown.

[0162] The internal schematic diagram of the goaf model in step S104 is as Figure 9 shown.

[0163] S105. According to the physical model, a three-dimensional numerical model composed of the intake side, the return air side, the working face, the local ventilator, and the goaf is established. The main parameter information is shown in Table 1 below.

[0164] Table 1 Main parameter information of the three-dimensional numerical model

[0165]

[0166] S106. Numerical simulation results and analysis.

[0167] In this step, (a) the top coal and the spontaneous combustion zone of the goaf are divided according to the oxygen concentration of 5% - 15%;

[0168] (b) The oxygen concentration distribution of the top coal body during the retreat stage of the working face is as Figure 11 , Figure 12 shown. It can be seen that the original support structure during the retreat stage of the working face is removed, resulting in the redistribution of stress in the remaining coal body and roof rock strata. This process will cause stress concentration in the weak areas of the working face, thereby causing the top coal to be fractured and generating an air leakage oxygen supply channel. Therefore, the range of the spontaneous combustion zone on the intake side is smaller than that on the return air side.

[0169] (c) The distribution of the oxygen concentration spontaneous combustion area in the goaf is as Figure 12 ,Figure 13 as shown

[0170] It can be seen from Figure 12 and Figure 13 that the oxygen concentration will decay rapidly with the increase of depth in the depth direction of the goaf. Specifically, the oxygen decay rate is faster in the area closer to the return air side than in the intake air side area. In addition, due to the relatively low air leakage intensity, the decrease in the oxygen concentration in the middle of the goaf is relatively slow. It should be noted that the position of the spontaneous combustion zone also shows a certain regularity, and the return air side is closer to the working face than the intake air side. During the working face withdrawal stage, as some fully mechanized mining supports near the return air side end are removed, the working face roof collapses. As a result, the original coal mining space is transformed into a goaf, which causes the ventilation path to change or the ventilation cross-section to decrease, making it difficult for the air current to pass through. At this time, a local ventilator is usually added, and the original ventilation method is adjusted, that is, from negative pressure ventilation to positive pressure ventilation.

[0171] The distribution range of the spontaneous combustion zone in the goaf is shown in Table 2.

[0172] Table 2 Distribution range of the spontaneous combustion zone in the goaf

[0173]

[0174] The present invention uses a method combining numerical simulation and on-site measurement to predict the spontaneous combustion dangerous area in the goaf based on the changes in the ventilation method, residual coal, and dynamic evolution law of porosity during the working face withdrawal stage. The prediction results have guiding significance for preventing the spontaneous combustion of residual coal in the goaf during the working face withdrawal stage.

[0175] The above is only a relatively preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any modification, equivalent replacement, and improvement made within the spirit and principle of the present invention by those skilled in the art within the technical scope disclosed by the present invention shall be covered by the protection scope of the present invention.

Claims

1. A method for determining the spontaneous combustion dangerous area in the goaf during the working face withdrawal stage, characterized in that, The method comprises the following steps: S1. Determine the variation law of the void structure at the rear of the fully-mechanized support during the face withdrawal to determine the development of air leakage channels formed by the void fissures in the goaf and their influence on the spontaneous combustion hazard area; S2. Determine the distribution law of the remaining coal at the rear of the fully-mechanized support during the withdrawal and extract the fitting formula of the remaining coal data to verify the influence of the remaining coal distribution on the spontaneous combustion hazard area in the goaf; S3. Derive the void ratio formulas at three positions, namely the middle of the goaf, the distance without caving coal at the rear of the fully-mechanized support, and the top of the fully-mechanized support during the withdrawal through theoretical models and numerical simulations, and determine the influence of the void ratio and its distribution on the distribution of the spontaneous combustion hazard area in the goaf after the reduction of the coal caving amount at the face; S4. During the goaf withdrawal stage, the air volume in the face is reduced; when the support at the end face of the working face starts to be withdrawn, the roof caving causes the full-pressure ventilation to be unable to be maintained, and it is switched to local fan ventilation, changing the full-pressure ventilation to positive-pressure ventilation and reducing the air volume; S5. Compile the data of the measured wind speed, remaining coal, void ratio, and oxygen consumption parameters during the withdrawal stage through UDF and import them into the FLUENT numerical simulation software; S6. Obtain the spontaneous combustion hazard area in the goaf under the special technology during the withdrawal stage through numerical simulation and on-site measurement results.

2. The method for determining the spontaneous combustion dangerous area in the goaf during the working face withdrawal stage according to claim 1, characterized in that In step S1, determining the variation law of the void structure at the rear of the fully-mechanized support during the face withdrawal includes: S1.

1. Establish a caving model of overlying strata during coal seam mining during the withdrawal using DEM numerical simulation software; S1.

2. Obtain the variation law of the void structure, the formation and development process of fissures, and the air leakage channels at the rear of the fully-mechanized support according to the caving results of the overlying strata during the model advancement; S1.

3. Simulate the withdrawal stage of the working face advancement, and do not caving coal at a certain distance from the stop line to obtain the variation law of the void structure with and without coal caving.

3. The method for determining the spontaneous combustion dangerous area in the goaf during the working face withdrawal stage according to claim 1, wherein, In step S2, verifying the influence of the remaining coal distribution on the spontaneous combustion hazard area in the goaf includes: S2.

1. During the withdrawal stage, the coal and rock accumulation state in the goaf is a mixed accumulation of coal and rock, with less remaining coal in the middle of the goaf and more remaining coal at the rear of the fully-mechanized support; S2.

2. Extract the remaining coal in the middle of the goaf and the remaining coal at the distance without caving coal at the rear of the fully-mechanized support during the withdrawal stage respectively, and use MATLAB software to fit them into a mathematical formula to generate a fitting graph.

4. The method for determining the spontaneous combustion dangerous area in the goaf during the working face withdrawal stage according to claim 3, characterized in that, In step S2.2, using MATLAB software to fit into a mathematical formula to generate a fitting graph includes: S2.2.

1. Extract the remaining coal data using the internal code of PFC numerical simulation software; S2.2.

2. Smooth the data through origin software to generate two-dimensional plane data; S2.2.

3. Import the data into MATLAB software for fitting, and select the binomial to generate the remaining coal height fitting graph.

5. The method for determining the spontaneous combustion dangerous area in the goaf during the working face withdrawal stage according to claim 1, characterized in that, In step S3, determining the influence of the void ratio and its distribution on the distribution of the spontaneous combustion hazard area in the goaf after the reduction of the coal caving amount at the face includes: S3.

1. Use the measurement circle tool to extract the caving void structure in the goaf; S3.

2. Use the measurement circle to extract the remaining coal in the goaf in different regions. Among them, the measurement circle is encrypted in the area without caving coal, and the extracted void ratio data is fitted into a formula and a void distribution graph; S3.

3. Fit the remaining coal data into a graph.

6. The method for determining the spontaneous combustion dangerous area in the goaf during the working face withdrawal stage according to claim 1, wherein, In step S4, switch to local ventilator ventilation, change the full negative pressure ventilation to full positive pressure and reduce the air volume, including: S4.1, after the face support starts to be withdrawn, switch to local ventilator ventilation; S4.2, optimize the ventilation system for high-gas and easy-to-spontaneously-ignite coal seams; S4.3, after the face stops mining and before the withdrawal fan is started, the air volume is not less than half of the original air volume.

7. The method for determining the spontaneous combustion dangerous area in the goaf during the working face withdrawal stage according to claim 1, characterized in that, In step S5, write through UDF and import it into the FLUENT numerical simulation software, including: S5.1, measure the inlet area and calculate the wind speed based on the air volume; S5.2, perform formula fitting on the measured residual coal data, calculate the oxygen consumption rate and import it into FLUENT; S5.3, use the cftool tool of Matlab software to perform formula fitting on the porosity data, and the fitted porosity formula is: In the formula, is the porosity, are all constants, are the strike and dip of the goaf respectively, is the height of the goaf; S5.4, perform fitting on the obtained porosity data to judge the distribution state of the data, perform fitting on the porosity, and generate a fitting graph; S5.5, evaluate the coincidence degree of the fitting result through the sum of squared errors and the correlation coefficient and organize the data, and import it into FLUENT.

8. The method for determining the spontaneous combustion dangerous area in the goaf during the working face withdrawal stage according to claim 1, characterized in that, In step S6, through numerical simulation and on-site measurement results, obtain the spontaneous combustion dangerous area in the goaf under special processes during the withdrawal stage, including: S6.1, establish a numerical simulation model of the goaf, incorporate the oxygen consumption-porosity formula into FLUENT, and set various parameters for numerical simulation calculation; S6.2, delimit the spontaneous combustion dangerous area and range according to the limit oxygen concentration, minimum floating coal thickness, and limit air leakage rate of the software calculation results; S6.3, the determination formula for the spontaneous combustion dangerous area in the goaf is: In the formula, is the thickness of floating coal, is the oxygen concentration, is the air leakage rate, is the minimum thickness of floating coal, is the limiting oxygen concentration, is the limiting air leakage rate; S6.4, through the verification of the numerical simulation results and the measured results, obtain the spontaneous combustion dangerous area in the goaf.

9. The method for determining the spontaneous combustion dangerous area in the goaf during the working face withdrawal stage according to claim 8, wherein In step S6.1, incorporate the oxygen consumption-porosity formula into FLUENT and set various parameters for numerical simulation calculation, including: S6.1.1, draw a numerical simulation model of the goaf using CAD according to the actual situation; S6.1.2, write the code of the oxygen consumption and porosity formula using udf; S6.1.3, set the energy equation and turbulence model parameters in FLUENT.

10. A determination system for the spontaneous combustion dangerous area in the goaf during the working face withdrawal stage, characterized in that, This system implements the method for determining the spontaneous combustion dangerous area in the goaf during the face withdrawal stage as described in any one of claims 1-9. This system includes: The module for determining the influence of the air leakage channel on the spontaneous combustion dangerous area is used to determine the change law of the void structure behind the fully-mechanized support during the face withdrawal, so as to determine the development of air voids in the goaf and form the influence of the air leakage channel on the spontaneous combustion dangerous area; The module for verifying the influence of the residual coal distribution on the spontaneous combustion dangerous area in the goaf is used to determine the distribution law of the residual coal behind the fully-mechanized support during the withdrawal and extract the fitting formula of the residual coal data to verify the influence of the residual coal distribution on the spontaneous combustion dangerous area in the goaf; The module for determining the influence of the porosity distribution on the distribution of the spontaneous combustion dangerous area in the goaf is used to deduce the porosity formulas at three positions in the middle of the goaf, behind the fully-mechanized support without caving coal, and at the top of the fully-mechanized support during the withdrawal through theoretical models and numerical simulations, and determine the influence of the porosity and its distribution on the distribution of the spontaneous combustion dangerous area in the goaf after the reduction of the coal caving amount in the face; The local ventilator ventilation switching module is used to reduce the air volume in the working face during the goaf withdrawal stage; when the support at the end working face starts to be withdrawn, the roof caving causes the full-pressure ventilation to be unable to be maintained, and it is switched to local ventilator ventilation, changing the full-pressure ventilation to positive-pressure ventilation and reducing the air volume. The parameter import module is used to write the data of the measured wind speed, residual coal, porosity, and oxygen consumption parameters during the withdrawal stage through UDF and import them into the FLUENT numerical simulation software. The goaf spontaneous combustion hazard area identification module is used to obtain the goaf spontaneous combustion hazard area under the special technology during the withdrawal stage through numerical simulation and on-site measurement results.

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