Method and system for judging spontaneous combustion dangerous area of goaf in working face withdrawing stage

Abstract

The invention belongs to the technical field of coal mine disaster prediction, and discloses a method and a system for judging a spontaneous combustion dangerous area of a goaf in a working face withdrawing stage. According to the method, influence of goaf air fracture development and air leakage channel formation on a spontaneous combustion dangerous area is determined; verifying the influence of residual coal distribution on the spontaneous combustion dangerous area of the goaf; determining the influence of void ratio and distribution on the spontaneous combustion dangerous area distribution of the goaf after the coal caving amount of the working face is reduced; the working face shrinks in the withdrawing period; after the end hydraulic support is withdrawn in the later period, the coal body collapses to block an air return channel, and full-negative-pressure ventilation of the working face is switched into positive-pressure ventilation of a local ventilator; and importing the void ratio and the residual coal distribution change formula into FLUENT numerical simulation software by utilizing a CFD-DEM simulation result. Technical basis is provided for judgment of the spontaneous combustion high-danger area of the working face from end mining to support removing goaf, and standardization and normalized development of the industry technology are assisted; and the generation of gases such as CO and CO2 caused by spontaneous combustion of the goaf is avoided.

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 the oxygen concentration or temperature, without fully considering the combined effect of ventilation mode, residual coal distribution and void structure change on the oxygen distribution and temperature distribution in the goaf, resulting in insufficient accuracy of the determination results and being difficult to effectively guide coal mine safety management.

[0003] Through the above analysis, the problems and defects of the existing technology are as follows: 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: S1. Determine the change law of the void structure behind the fully mechanized support during the working face withdrawal to determine the influence of the development of air leakage channels formed by the void cracks in the goaf on the spontaneous combustion dangerous area. S2. 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. S3. Derive the void ratio formulas at three positions, namely 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 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. 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. S5. Compile the data of the measured wind speed, residual 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 dangerous area in the goaf under special processes during the withdrawal stage through numerical simulation and on-site measurement results.

[0006] In step S1, determine the variation law of the void structure behind the fully mechanized caving support during the face withdrawal, including: S1.1. Use the DEM numerical simulation software to establish a caving model of overlying strata during coal seam mining during withdrawal. S1.2. According to the caving results of overlying strata during the model's advancement process, obtain the variation law of the void structure behind the fully mechanized caving support, the formation and development process of fissures, and the air leakage channels. S1.3. Simulate the withdrawal stage of the face advancement, and do not extract 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 extraction.

[0007] In step S2, verify the influence of the distribution of remaining coal on the spontaneous combustion dangerous area in the goaf, including: S2.1. During the withdrawal stage, the stacking state of coal and rock in the goaf is a mixed stacking of coal and rock. There is less remaining coal in the middle of the goaf and more remaining coal behind the fully mechanized caving support. S2.2. Extract the remaining coal in the middle of the goaf and the remaining coal at a certain distance without extracting top coal behind the fully mechanized caving support during the withdrawal stage respectively, and use MATLAB software to fit it into a mathematical formula to generate a fitting diagram.

[0008] In step S2.2, use MATLAB software to fit it into a mathematical formula to generate a fitting diagram, including: S2.2.1. Use the internal code of the PFC numerical simulation software to extract the remaining coal data. 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 a fitting diagram of the remaining coal height.

[0009] In step S3, 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 extraction volume in the face, including: 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 densified in the area without extracting top coal, and the extracted void ratio data is fitted into a formula and a void distribution diagram. S3.3. Fit the remaining coal data into a diagram.

[0010] In step S4, switch to local ventilator ventilation, change the fully negative pressure ventilation to fully positive pressure and reduce the air volume, including: S4.1. After the face support starts to withdraw, switch to local ventilator ventilation. S4.2. Optimize the ventilation system for high-gas and easily spontaneous combustion coal seams; S4.3. After the working face stops mining and before the withdrawal fan is started, the air volume shall not be less than half of the original air volume.

[0011] In step S5, write through UDF and import it into the FLUENT numerical simulation software, including: S5.1. Measure the air inlet area and calculate the wind speed based on the air volume; S5.2. Fit the measured data of residual coal with a formula, calculate the oxygen consumption rate and import it into FLUENT; S5.3. Use the cftool tool of Matlab software to fit the porosity data. 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. Fit the obtained porosity data to judge the distribution state of the data, fit the porosity and generate a fitting graph; S5.5. Evaluate the coincidence degree of the fitting result through the sum of squared errors and correlation coefficient, sort out the data and import it into FLUENT.

[0012] 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 scope according to the software calculation results in terms of the limiting oxygen concentration, minimum floating coal thickness, and limiting air leakage rate; S6.3. The formula for judging the spontaneous combustion dangerous area in the goaf is: ; 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; S6.4. Through the verification of the numerical simulation results and the measured results, obtain the spontaneous combustion dangerous area in the goaf.

[0013] In step S6.1, the oxygen consumption void formula is incorporated into FLUENT and various parameters are set for numerical simulation calculations, including: S6.1.1, draw a numerical simulation model of the gob area using CAD according to the actual situation; S6.1.2, write the code for the oxygen consumption and porosity formula using udf; S6.1.3, set the energy equation and turbulence model parameters in FLUENT.

[0014] Another object of the present invention is to provide a determination system for the spontaneous combustion dangerous area in the gob area during the face withdrawal stage. This system implements the determination method for the spontaneous combustion dangerous area in the gob area during the face withdrawal stage. This system includes: The influence determination module 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 caving support during the face withdrawal to determine the development of the air voids in the gob area and form the influence of the air leakage channel on the spontaneous combustion dangerous area; The influence verification module of the coal gangue distribution on the spontaneous combustion dangerous area in the gob area is used to determine the distribution law of the coal gangue behind the fully mechanized caving support during the withdrawal and extract the fitting formula of the coal gangue data to verify the influence of the coal gangue distribution on the spontaneous combustion dangerous area in the gob area; The influence module of the porosity distribution on the distribution of the spontaneous combustion dangerous area in the gob area is used to deduce the porosity formula at three positions in the middle of the gob area, the distance without caving coal behind the fully mechanized caving support, and the top of the fully mechanized caving 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 gob area 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 face during the gob area withdrawal stage; when the support at the end 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; The parameter import module is used to write the data of the measured wind speed, coal gangue, porosity, and oxygen consumption parameters during the withdrawal stage through UDF and import them into the FLUENT numerical simulation software; The identification module of the spontaneous combustion dangerous area in the gob area is used to obtain the spontaneous combustion dangerous area in the gob area under the special process during the withdrawal stage through numerical simulation and on-site measurement results.

[0015] 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 goaf under special technological conditions during the face withdrawal period and the change law of parameters such as the distribution of residual coal, and combines the influence of exposure time, support withdrawal cycle, ventilation mode and other different complex conditions on the distribution of spontaneous combustion dangerous areas in the goaf. This method can scientifically, reasonably and accurately determine and identify the spontaneous combustion dangerous areas in the goaf under complex conditions during the face withdrawal stage, effectively improving the prevention ability of coal mine fires, and is of great significance to coal mine safety management. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The accompanying drawings herein are incorporated into the specification and form a part of the specification, showing embodiments consistent with the present disclosure, and are used together with the specification to explain the principles of the present disclosure; Figure 1 It is a diagram of a method and system for determining the spontaneous combustion dangerous area in the goaf during the face withdrawal stage provided by an embodiment of the present invention; Figure 2 It is a diagram of the simulation of the final mining advancement stage of the working face by the PFC overlying strata caving model provided by an embodiment of the present invention; Figure 3 It is a diagram of the simulation of the face withdrawal stage by the PFC overlying strata caving model provided by an embodiment of the present invention; Figure 4 It is a data effect diagram measured in the measurement circle of the present invention; Figure 5 It is of the present invention Figure 4 The effect diagram linearly fitted by MATLAB; Figure 6 It is the fitting diagram of non-top coal caving of the present invention; Figure 7 It is the fitting diagram of top coal caving of the present invention; Figure 8 It is a 3D modeling diagram of the goaf by the application software of the present invention; Figure 9 It is an internal schematic diagram of the goaf model of the present invention; Figure 10 It is the distribution diagram of the spontaneous combustion zone of the top coal during the face withdrawal stage of the present invention; Figure 11 It is the distribution diagram of the oxygen concentration of the top coal during the face withdrawal stage of the present invention; Figure 12 It is the natural zone diagram of the final mining stage of the oxygen concentration spontaneous combustion area in the goaf; Figure 13 It is the natural zone diagram of the withdrawal stage of the oxygen concentration spontaneous combustion area in the goaf. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0017] To make the above objects, features, and advantages of the present invention more apparent and understandable, the following provides a detailed description of the specific embodiments of the present invention in conjunction with the accompanying drawings. Many specific details are set forth in the following description 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 departing from the connotation of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below.

[0018] The present invention solves the problems that the traditional three zones in the goaf do not consider the distribution of residual coal, the dynamic evolution of void structures, and the change of ventilation methods, resulting in the inability to interpret the laws 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 mining withdrawal process and proposes the idea of expanding the traditional spontaneous combustion three-zone area to a three-dimensional space small three-zone spontaneous combustion area range of the distribution range of small void ratios of residual coal in fully mechanized top coal caving mining. 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 and support withdrawal period, the special technological conditions cause 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 laws of key parameters caused by the special technological conditions in the fully mechanized caving face and focuses on analyzing the distribution law of the three-dimensional spontaneous combustion zone of the fully mechanized support and its influence on the spontaneous combustion of residual coal.

[0019] Example 1, as Figure 1 shown, the method for determining the spontaneous combustion dangerous area in the goaf during the withdrawal stage of the working face provided by the embodiment of the present invention includes: S1. Determine the change law of the void structure behind the fully mechanized support during the withdrawal of the working face to determine the influence of the development of air leakage channels formed by the air voids in the goaf on the spontaneous combustion dangerous area. S2. Determine the distribution law of the 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 residual coal distribution on the spontaneous combustion dangerous area in the goaf. S3. Derive the void ratio formulas at three positions, namely 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, 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 in the working face. S4. During the withdrawal stage of the goaf, 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 inability to maintain the full negative pressure ventilation, and it is switched to local fan ventilation, changing the full negative pressure ventilation to positive pressure ventilation and reducing the air volume. S5. Compile the data of the measured wind speed, residual coal, void ratio, and oxygen consumption parameters during the withdrawal stage through UDF and import them into the FLUENT numerical simulation software. S6. Through numerical simulation and on-site measurement results, obtain the spontaneous combustion dangerous area in the goaf under the special technology during the withdrawal stage.

[0020] 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; 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.

[0021] In step S1, it specifically includes the following steps: S1.1, use the DEM numerical simulation software to establish a caving model of overlying strata during coal seam mining during withdrawal; S1.2, according to the caving results of overlying strata during the advancing process 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; During the withdrawal stage, a high void ratio area appears behind the fully mechanized support. As the stop - mining time prolongs, the void ratio area decreases, and multiple horizontal and vertical fractures appear.

[0022] S1.3, simulate the withdrawal stage of the working face advancement, and do not release top coal at a certain distance from the stop - mining line to obtain the change law of the void structure with and without coal release.

[0023] Exemplarily, in step S2, it specifically includes the following steps: 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 swelling. There is less remaining coal in the middle of the goaf and more remaining coal behind the fully mechanized support.

[0024] S2.2, respectively extract the remaining coal in the middle of the goaf and the remaining coal at a non - top - coal - releasing distance behind the fully mechanized support during the withdrawal stage and use the MATLAB software to fit them into a mathematical formula to generate a fitting graph; specifically including: S2.2.1, use the internal code of the PFC numerical simulation software to extract the remaining coal data; S2.2.2, smooth the data through the origin software to generate two - dimensional plane data; S2.2.3, import the data into the MATLAB software for fitting, and finally select a binomial with a better fitting degree to generate a remaining coal height fitting graph.

[0025] Exemplarily, in step S3, it specifically includes the following steps; 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 0.5 m, and it covers the model; S3.2, use the measurement circle to extract the remaining coal in the goaf in different regions, and perform measurement circle densification processing on the non - top - coal - releasing area, and fit the extracted void ratio data into a formula and a void distribution graph; S3.3. Fit the gob coal data into a graph; Exemplarily, in step S4, the following steps are specifically included; 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; 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, propose an optimization and transformation plan for the existing problems in the ventilation system, and optimize the ventilation system.

[0026] 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 gob area while ensuring the ventilation requirements of the working area.

[0027] Exemplarily, in step S5, the following steps are specifically included; S5.1. Measure the inlet area and air volume to calculate the wind speed; the wind speed is calculated by the following formula: ; In the formula, is the wind speed, is the air volume, is the inlet and outlet area of the coal seam.

[0028] S5.2. Perform formula fitting on the measured gob coal data to calculate the oxygen consumption rate and import it into FLUENT; S5.2.1. Measure the gob coal data: First, it is necessary to measure the gob coal data in the coal seam, including the type of coal, humidity, temperature, etc.

[0029] S5.2.2. According to the gob 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: ; In the formula, is the oxygen consumption rate, is a coefficient related to the characteristics of gob coal, and the value is generally taken as 1, is the oxygen concentration.

[0030] S5.3. Use the cftool tool of Matlab software to perform formula fitting on the porosity data; the present invention innovatively proposes that the fitted porosity formula is: ; In the formula, is the porosity, are all constants, are the strike and dip of the gob area respectively, is the height of the gob area; S5.4. Fit the obtained void fraction data to judge the distribution state of the data. Fit the void fraction to generate a fitting graph. Use Origin software for fitting. The void fraction surface in the fitting graph can judge the distribution state. The fitting graph is as Figure 5 shown; 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.

[0031] S5.5.1. It 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 prediction of the model); 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.

[0032] Exemplarily, in step S6, it specifically includes the following steps; S6.1. Establish a numerical simulation model of the gob area, program formulas such as oxygen consumption and void fraction into FLUENT, and set various parameters for numerical simulation calculation; specifically including: S6.1.1. Draw a numerical simulation model of the gob area using CAD according to the actual situation; S6.1.2. Use udf to write the code for the oxygen consumption and void fraction formulas; S6.1.3. Set parameters such as the energy equation and turbulence model in FLUENT.

[0033] S6.2. Delimit the spontaneous combustion danger area and range according to the ultimate oxygen concentration, minimum floating coal thickness, and ultimate air leakage rate of the software calculation results.

[0034] S6.2.1. According to the conditions of coal spontaneous combustion, the gob area 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 spontaneous combustion danger area of the gob area; S6.2.2. In the actual delimitation process, due to the very complex boundary conditions of the gob area, the error of the delimited "three zones" range is relatively large. Therefore, the principle of combining computer numerical simulation and on-site measurement can be adopted; S6.3. The determination formula for the spontaneous combustion danger area of the gob area is: ; 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 ultimate oxygen concentration, is the limit air leakage rate; S6.4. Through the verification of the numerical simulation results and the measured results, the spontaneous combustion dangerous area in the goaf is obtained.

[0035] Among them, the spontaneous combustion dangerous area is delimited by an oxygen concentration of 5% - 15%.

[0036] Example 2. The system for determining the spontaneous combustion dangerous area in the goaf during the face withdrawal stage provided by the embodiment of the present invention 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 caving support during the face withdrawal period, so as to determine the development of air voids in the goaf and the influence of the formed air leakage channel on the spontaneous combustion dangerous area; The module for verifying the influence of the distribution of residual coal on the spontaneous combustion dangerous area in the goaf is used to determine the distribution law of residual coal behind the fully mechanized caving 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 spontaneous combustion dangerous area in the goaf; The module for determining the influence of the void ratio distribution on the distribution of the spontaneous combustion dangerous area in the goaf is used to deduce the void ratio formulas at three positions in the middle of the goaf, the distance without caving coal behind the fully mechanized caving support, and the top of the fully mechanized caving support during the withdrawal period through a theoretical model and numerical simulation, 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 of the working 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 face of the working face starts to be withdrawn, the caving of the roof 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; The parameter import module is used to write the data of the measured wind speed, residual coal, void ratio and oxygen consumption parameters during the withdrawal stage through UDF and import them into the FLUENT numerical simulation software; The module for identifying the spontaneous combustion dangerous area in the goaf is used to obtain the spontaneous combustion dangerous area in the goaf under the special process during the withdrawal stage through numerical simulation and on-site measured results.

[0037] Example 3. As another embodiment of the present invention, a certain mining area adopts the fully mechanized caving coal mining technology, the coal seam thickness is 7.5m, the caving law of the overlying strata is relatively complex, and there is a high risk of spontaneous combustion during the face withdrawal 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.

[0038] The traditional method uses single-parameter analysis, mainly by monitoring the oxygen concentration and temperature in the goaf to judge the spontaneous combustion dangerous area. However, this method does not fully consider the comprehensive influence of the change of ventilation mode, residual coal distribution and void structure on the oxygen distribution and temperature distribution, resulting in a large deviation between the judgment result and the actual risk area.

[0039] Through the following improved technical points, the present invention significantly improves the accuracy and scientificity of the determination of the spontaneous combustion dangerous area: comprehensively considering the influence of the change in the ventilation mode on the gas flow and temperature distribution in the goaf, introducing the local ventilator ventilation mode; analyzing the influence of the distribution of residual coal on the spontaneous combustion dangerous area through numerical simulation and theoretical models, extracting residual coal data and fitting formulas; studying the variation law of the void structure, simulating the caving process of overlying strata through the PFC numerical simulation software to determine the void ratio distribution; integrating parameters such as wind speed, residual coal, void ratio, and oxygen consumption into the FLUENT numerical simulation to establish a scientific determination model for the spontaneous combustion dangerous area in the goaf.

[0040] Example 4, as another embodiment of the present invention, applies the method of the present invention in a certain coal mine. By establishing a PFC model and a 3D numerical model, the spontaneous combustion dangerous area in the goaf during the face retreat stage is successfully predicted, guiding effective fire prevention and extinguishing measures.

[0041] The existing methods for predicting spontaneous combustion in the goaf mainly rely on empirical formulas and simple numerical simulations, and cannot fully consider the complex changes during the face retreat stage.

[0042] By combining the accurate simulations of DEM and CFD software, the present invention takes into account the dynamic evolution of the void ratio and the height of residual coal, improving the prediction accuracy and adaptability. The method of the present invention can more accurately determine the spontaneous combustion dangerous area in the goaf during the face retreat stage, providing strong technical support for the safety management of coal mines and significantly reducing the risk of spontaneous combustion.

[0043] To further illustrate the relevant effects of the embodiments of the present invention, the following experiments are carried out.

[0044] The method for determining the spontaneous combustion dangerous area in the goaf during the face retreat stage provided by the embodiment of the present invention includes: S101. Establish a PFC overlying strata caving model, including: (1) The model size is length × height = 247×108m, with a 40m mining boundary left on both sides. The initial particle velocity, wall velocity, and acceleration are all 0; (2) The upper half and the left half of the model are both set as fixed boundaries by WAL1. The rock strata are added to the upper part of the model in the form of load, and the load is taken as 8MPa. Rigid walls are used on both sides and below the model; (3) The working face is divided into 24 sections, each section is 7m long, and the mining thickness is 7.45m for mining simulation. The simulation results of the PFC overlying strata caving model are as Figure 2 、 Figure 3 shown.

[0045] S102. Extraction and compilation of the void ratio during the face retreat.

[0046] This step includes: 1) Utilize the characteristics of the PFC2D software and adopt a specific measurement circle to track the change of the void ratio inside the established model. 2) The measurement circle is always in a stable state, and the measurement circles at the four boundaries of top, bottom, left, and right are immovable. The particles move vertically through the measurement circle. 3) According to the void ratio distribution and actual engineering conditions, select the data measured in the measurement circle and fit it linearly through MATLAB, as shown in Figure 4 The data effect diagram measured in the measurement circle, as shown in Figure 5 For Figure 4 as shown by the linear fitting through MATLAB; 4) The fitted void ratio formula is: ; S103. Based on the gob coal height data extracted during the normal advancing stage and the retreating stage of the working face during the retreat, the present invention obtains a sixth-order power function through formula fitting with MATLAB software, such as Figure 6 The fitting diagram of non-top-coal caving, Figure 7 as shown in the fitting diagram of top-coal caving; In this step, the gob coal height formula: ; where p1 = -1.431e -9 , p2 = -7.7707e -06 , p3 = 9.951e -04 , p4 = -0.0002725, p5 = 0.07894, p6 = -0.1216, p7 = 2.302.

[0047] S104. According to the actual situation of the working face, the present invention uses software to perform 3D modeling of the gob area, as shown in Figure 8 shown.

[0048] The internal schematic diagram of the gob area model in step S104 is shown in Figure 9 shown.

[0049] S105. According to the physical model, establish a three-dimensional numerical model composed of the intake side, return air side, working face, local ventilator, and gob area. The main parameter information is shown in Table 1 below.

[0050] Table 1 Main parameter information of the three-dimensional numerical model

[0051] S106. Numerical simulation results and analysis.

[0052] In this step, (a) divide the top coal and the spontaneous combustion zone in the goaf according to an oxygen concentration of 5% - 15%; (b) The oxygen concentration distribution of the top coal body during the face withdrawal stage is as shown in Figure 11 , Figure 12 . It can be seen that the original support structure during the face withdrawal stage is removed, resulting in the redistribution of stress in the remaining coal body and roof strata. This process will cause stress concentration in the weak areas of the 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 side.

[0053] (c) The distribution of the spontaneous combustion area of the oxygen concentration in the goaf is as shown in Figure 12 , Figure 13 .

[0054] From Figure 12 , Figure 13 , it can be known that the oxygen concentration will decay rapidly with the increase of depth in the depth direction of the goaf. Specifically, the oxygen decay rate in the area near the return side is faster than that in the intake side area. In addition, due to the relatively low air leakage intensity, the reduction of 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 side is closer to the face than the intake side. During the face withdrawal stage, as some fully mechanized mining supports near the return side end are removed, the face roof collapses. Therefore, the original coal mining space is transformed into a goaf, resulting in a change in the ventilation path or a reduction in the ventilation cross-section, 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, changing from negative pressure ventilation to positive pressure ventilation.

[0055] The distribution range of the spontaneous combustion zone in the goaf is shown in Table 2.

[0056] Table 2 Distribution range of the spontaneous combustion zone in the goaf

[0057] 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 ventilation methods, remaining coal, and the dynamic evolution law of porosity during the face withdrawal stage. The prediction results have guiding significance for preventing the spontaneous combustion of the remaining coal in the goaf during the face withdrawal stage. As mentioned above, only the relatively optimal specific implementation manners of the present invention are described, but the protection scope of the present invention is not limited thereto. Any modification, equivalent replacement, and improvement made by any person skilled in the technical field within the technical scope disclosed by the present invention, as long as it is made within the spirit and principle of the present invention, shall be covered by the protection scope of the present invention.

Claims

1. A method for determining the spontaneous combustion hazard area of ​​the goaf during the working face withdrawal phase, characterized in that: The Method The following steps are involved: S1, determine the change law of the gap structure behind the fully mechanized mining support during the withdrawal of the working face, so as to determine the development of empty cracks in the goaf and the influence of the air leakage channel on the spontaneous combustion risk area; S2, determine the distribution law of the coal left behind the fully mechanized support during the withdrawal period and extract the fitting formula of the coal left behind data, and verify the influence of the distribution of coal left behind on the spontaneous combustion hazard area of ​​the goaf; S3, through theoretical model and numerical simulation, the void ratio formulas of the middle of the goaf, the distance of the rear of the fully mechanized support without top coal release and the top of the fully mechanized support were derived during the withdrawal period to determine the influence of the void ratio and distribution on the distribution of the spontaneous combustion hazard area in the goaf after the reduction of coal release in the working face; S4, during the withdrawal stage of the goaf, the working face shrinks; when the end working face support begins to withdraw, the roof collapses and the full negative pressure ventilation cannot be maintained, and the ventilation is switched to local fans, turning the full negative pressure ventilation into positive pressure ventilation and reducing the air volume; S5, the data of wind speed, coal residue, void ratio and oxygen consumption parameters measured in the withdrawal stage are compiled through UDF and imported into FLUENT numerical simulation software; S6, through numerical simulation and field measurement results, the spontaneous combustion hazard area of ​​the goaf under special technology in the withdrawal stage was obtained.

2. The method for determining the spontaneous combustion hazard zone of the goaf during the working face withdrawal phase according to claim 1 is characterized in that: In step S1, the change rule of the gap structure at the rear of the fully mechanized mining support during the withdrawal of the working face is determined, including: S1.1, DEM numerical simulation software is used to establish the overburden collapse model of coal seam mining during the withdrawal period; S1.2, according to the results of overburden collapse during the advancement of the model, the law of the change of the void structure at the rear of the fully mechanized support, the process of crack formation and development, and the air leakage channel are obtained; S1.3, simulate the retreat stage of the working face advancement, and do not release top coal for a certain distance from the stop mining line, and obtain the changing law of the gap structure between no coal release and coal release.

3. The method for determining the spontaneous combustion hazard zone of the goaf during the working face withdrawal phase according to claim 1 is characterized in that: In step S2, the influence of the distribution of the coal residue on the spontaneous combustion hazard area of ​​the goaf is verified, including: S2.1, in the withdrawal stage, the coal-rock accumulation state in the goaf is a mixed accumulation of coal and rock, with less coal left in the middle of the goaf and more coal left behind the fully mechanized mining support; S2.2, respectively extract the coal left in the middle of the goaf during the withdrawal phase and the coal left behind the fully mechanized mining support without placing the top coal, and use MATLAB software to fit them into mathematical formulas to generate fitting graphs.

4. The method for determining the spontaneous combustion hazard zone of the goaf during the working face withdrawal phase according to claim 3 is characterized in that: In step S2.2, the fitting graph is generated by fitting into a mathematical formula using MATLAB software, including: S2.2.1, extract the coal data using the internal code of PFC numerical simulation software; S2.2.2, smooth the data using origin software to generate two-dimensional plane data; S2.2.3, import the data into MATLAB software for fitting, and select binomial to generate the fitting graph of the residual coal height.

5. The method for determining the spontaneous combustion hazard zone of the goaf during the working face withdrawal phase according to claim 1 is characterized in that: In step S3, the influence of void ratio and distribution on the distribution of spontaneous combustion hazard areas in the goaf after the coal caving amount is reduced is determined, including: S3.1, use the measuring circle tool to extract the void structure of the goaf; S3.2, using the measuring circle to extract the coal in the goaf by region, wherein the measuring circle is encrypted in the area where the top coal is not placed, and the extracted void ratio data is fitted into a formula and a void distribution map; S3.3, fit the coal legacy data into a graph.

6. The method for determining the spontaneous combustion hazard zone of the goaf during the working face withdrawal phase according to claim 1 is characterized in that: In step S4, switching to local ventilator ventilation, changing full negative pressure ventilation to full positive pressure and reducing the air volume, including; S4.1, after the working face support begins to withdraw, switch to local ventilator ventilation; S4.2, optimize the ventilation system of coal seams with high gas content and prone to spontaneous combustion; S4.3, after the mining is stopped and before the withdrawal fan is turned on, the air volume shall not be less than half of the original air volume.

7. The method for determining the spontaneous combustion hazard zone of the goaf during the working face withdrawal phase according to claim 1 is characterized in that: In step S5, the UDF is used to write and import into the FLUENT numerical simulation software, including: S5.1, determine the air inlet area and air volume to calculate the wind speed; S5.2, fit the measured residual coal data with formula, calculate the oxygen consumption rate and import it into FLUENT; S5.3, use the cftool tool of Matlab software to fit the void fraction data. The fitted void fraction formula is: In the formula, is the void ratio, are constants, They are the direction of the goaf, the inclination of the goaf, is the height of the goaf; S5.4, fitting the obtained void ratio data to determine the distribution state of the data, fitting the void ratio, and generating a fitting graph; S5.5, evaluate the degree of fit of the fitting results by the square error and correlation coefficient, organize the data, and import them into FLUENT.

8. The method for determining the spontaneous combustion hazard zone of the goaf during the working face withdrawal phase according to claim 1 is characterized in that: In step S6, the spontaneous combustion hazard area of ​​the goaf under the special process in the withdrawal stage is obtained through numerical simulation and field measurement results, including: S6.1, by establishing a numerical simulation model of the goaf, the oxygen consumption void formula is compiled into FLUENT and various parameters are set for numerical simulation calculation; S6.

2. Delineate the spontaneous combustion hazard zone and range according to the software calculation results based on the limiting oxygen concentration, minimum floating coal thickness, and limiting air leakage rate; S6.

3. The formula for determining the spontaneous combustion hazard zone in the goaf is: 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 limit air leakage rate; S6.

4. By verifying the numerical simulation results with the actual measurement results, the spontaneous combustion hazard area of ​​the goaf is determined.

9. The method for determining the spontaneous combustion hazard zone of the goaf during the working face withdrawal phase according to claim 8 is characterized in that: In step S6.1, the oxygen consumption gap formula is compiled into FLUENT and various parameters are set for numerical simulation calculation, including: S6.1.1, use CAD to draw the numerical simulation model of the goaf according to the actual situation; S6.1.2, use udf to write the formula code for oxygen consumption and void fraction; S6.1.3, set the energy equation and turbulence model parameters in FLUENT.

10. A system for determining spontaneous combustion hazard areas in goaf areas during the working face withdrawal phase, characterized in that: The system implements the method for determining the spontaneous combustion hazard zone of the goaf during the working face withdrawal phase as described in any one of claims 1 to 9, and the system comprises: The module for determining the influence of air leakage channels on the spontaneous combustion hazardous area is used to determine the change law of the gap structure at the rear of the fully mechanized mining support during the withdrawal of the working face, so as to determine the development of empty cracks in the goaf and the influence of air leakage channels on the spontaneous combustion hazardous area; The module for verifying the impact of the distribution of residual coal on the spontaneous combustion hazard area in the goaf is used to determine the distribution law of the residual coal behind the fully-mechanized support during the withdrawal period and extract the fitting formula for the residual coal data to verify the impact of the distribution of residual coal on the spontaneous combustion hazard area in the goaf; The module on the influence of void ratio distribution on the distribution of spontaneous combustion hazardous areas in goaf is used to derive the void ratio formulas at three locations during the withdrawal period, namely, the middle of the goaf, the distance of the rear of the fully mechanized support without top coal release, and the top of the fully mechanized support through theoretical models and numerical simulations, and to determine the influence of void ratio and distribution on the distribution of spontaneous combustion hazardous areas in goaf after the amount of coal released on the working face is reduced; The local fan ventilation switching module is used to reduce the air flow at the working face during the withdrawal stage of the goaf. When the end working face support begins to withdraw, the roof collapses and the full negative pressure ventilation cannot be maintained, and the local fan ventilation is switched to change the full negative pressure ventilation into positive pressure ventilation and reduce the air volume. The parameter import module is used to compile and import the data of wind speed, coal residue, void ratio and oxygen consumption parameters measured in the withdrawal stage into the FLUENT numerical simulation software through UDF; The goaf spontaneous combustion hazard zone identification module is used to obtain the goaf spontaneous combustion hazard zone under special processes in the withdrawal stage through numerical simulation and field measurement results.

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