A method for early warning of spontaneous combustion of coal based on CO index at the return air corner

Abstract

This invention provides a method for early warning of coal spontaneous combustion based on the CO index at the return air corner, comprising: S1 determining characteristic temperature points at different oxygen concentrations through thermogravimetric analysis, determining the temperature at which the index gas appears through a programmed temperature rise experiment, and jointly calibrating the correlation between the coal spontaneous combustion stage and temperature; S2 constructing a predictive mathematical model for CO sources including the heat dissipation zone of the lower coal seam, the oxidation and heating zone, and the amount of secondary oxidation CO in the overlying goaf; S3 calculating multiple CO concentration early warning thresholds by substituting mine parameters; S4 establishing a multi-level graded early warning criterion by combining the stage-temperature relationship, the index gas, and the early warning thresholds; and S5 issuing an early warning based on the measured CO concentration at the return air corner. This invention, through oxygen concentration correction, coupling of CO leakage from the overlying goaf, and multi-threshold grading, solves the problems of large air leakage interference and inaccurate early warning in shallow, closely spaced coal seam groups, improving the accuracy of early warning and its engineering practicality.

Description

Technical Field

[0001] This invention relates to the field of early warning technology for spontaneous combustion of residual coal in coal mine goaf areas, specifically to an early warning method for spontaneous combustion of coal based on the CO index at the return air corner. Background Technology

[0002] Coal spontaneous combustion fires, one of the five major hazards in coal mines, have always seriously threatened the safe mining of coal and the lives and health of workers. As a major coal producer, China faces particularly severe coal spontaneous combustion hazards. With the shift of coal resource mining focus towards western mining areas, the safe mining of coal seams in these areas has received increasing attention. Coal seams in western mining areas are generally characterized by shallow burial, thin bedrock, and close spacing, and are often prone to spontaneous combustion. During the mining of shallow, closely spaced coal seam groups, complex underground, surface, and inter-sea air leakage exists. This leakage provides a continuous oxygen supply environment for the spontaneous combustion of residual coal in the goaf, accelerating the oxidation rate and increasing the risk of spontaneous combustion. Simultaneously, air leakage affects the generation and migration of CO gas in the goaf, easily leading to CO accumulation and continuous exceedances in the return air corners of the working face. This disrupts the correlation between CO gas concentration and the degree of spontaneous combustion, severely interfering with the prediction and early warning of spontaneous combustion. Summary of the Invention

[0003] To address the shortcomings of existing technologies, the purpose of this invention is to propose a method for early warning of spontaneous combustion of coal based on the CO index at the return air corner, thereby solving the problems mentioned in the background section.

[0004] This invention is achieved through the following technical solution: A method for early warning of spontaneous combustion of coal based on the CO index at the return air corner, the method comprising the following steps: S1: Under different oxygen concentration conditions, the characteristic temperature point of coal spontaneous combustion is determined by thermogravimetric analysis, and the temperature at which the index gas appears is determined by programmed temperature rise experiment. The two are used to calibrate the correspondence between the coal spontaneous combustion stage and temperature. S2: Construct a mathematical model for predicting CO gas concentration in the return air corner. The sources of CO in the prediction mathematical model include: the amount of CO generated by the oxidation of residual coal in the heat dissipation zone of the lower coal seam goaf, the amount of CO generated by the oxidation of residual coal in the oxidation and heating zone of the lower coal seam goaf, and the amount of CO generated by the secondary oxidation of residual coal in the overlying goaf. S3: Substitute the mine geological parameters, mining area ventilation parameters and coal oxidation kinetic parameters into the mathematical model to calculate multiple CO concentration warning thresholds corresponding to different stages of coal spontaneous combustion; S4: Based on the correspondence between the coal spontaneous combustion stage and temperature and the indicator gas calibrated in step S1, and combined with step S3, calculate multiple CO concentration warning thresholds corresponding to different stages of coal spontaneous combustion, and establish a graded warning criterion that includes multiple warning levels. S5: Based on the measured CO gas concentration at the return air corner, use the graded early warning criteria obtained in step S4 to issue an early warning for spontaneous combustion of residual coal in the goaf.

[0005] The beneficial effects of this invention are as follows: Firstly, step S1, through thermogravimetric analysis and programmed temperature rise experiments under different oxygen concentrations, calibrated the correspondence between the coal spontaneous combustion stage and temperature, and determined the temperature at which the indicator gas appeared. This step eliminated the interference of oxygen concentration changes on characteristic temperature points and gas characterization, enabling subsequent early warning criteria to more accurately reflect the actual degree of oxidation and spontaneous combustion of residual coal in the goaf, significantly reducing the false alarm and missed alarm rates caused by air leakage.

[0006] Secondly, the mathematical model for predicting CO concentration in the return air corner constructed in step S2 is the first to simultaneously incorporate the CO generated by the oxidation of residual coal in the heat dissipation zone of the lower coal seam goaf, the CO generated by the oxidation and heating zone of residual coal in the lower coal seam goaf, and the CO generated by the secondary oxidation of residual coal in the overlying goaf. This model overcomes the limitations of traditional prediction models that ignore CO leakage from the overlying goaf and inter-layer air leakage, providing a scientific basis for predicting CO concentration in the return air corner and calculating the safety threshold under shallow-buried, close-range coal seam mining conditions.

[0007] Third, step S3 calculates multiple CO concentration warning thresholds by substituting actual mine parameters. Step S4 combines the correspondence between coal spontaneous combustion stage, temperature, and gas and the temperature at which the indicator gas appears to establish a graded warning criterion that includes multiple warning levels.

[0008] In summary, this invention solves the problems of severe air leakage interference in shallow, close-range coal seam groups, disordered correlation between CO and coal temperature, crude early warning levels, and low model adaptability in the prior art, significantly improving the scientificity and effectiveness of coal spontaneous combustion early warning and ensuring safe production in mines. Attached Figure Description

[0009] Figure 1 This is a flowchart of a coal spontaneous combustion early warning method based on the CO index at the return air corner, according to the present invention.

[0010] Figure 2 This is a diagram showing the relationship between the spontaneous combustion stage of coal and its characteristic temperature.

[0011] Figure 3 This is a schematic diagram showing the gas concentration distribution on the return air side of the goaf and CO migration due to air leakage.

[0012] Figure 4 This is a diagram showing the relationship between temperature and gas during the spontaneous combustion stage of coal. Detailed Implementation

[0013] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. It should be noted that the description of these embodiments is for the purpose of aiding understanding the present invention, but does not constitute a limitation thereof. Furthermore, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0014] Reference Figure 1 As shown, a method for early warning of spontaneous combustion of coal based on the CO index at the return air corner is described, the method comprising the following steps: S1: Under different oxygen concentration conditions, the characteristic temperature point of coal spontaneous combustion is determined by thermogravimetric analysis, and the temperature at which the index gas appears is determined by programmed temperature rise experiment. The two are used to calibrate the correspondence between the coal spontaneous combustion stage and temperature. In step S1, the experimental coal samples are first pretreated: undisturbed coal samples are collected from fresh, exposed working faces underground. The oxide layer formed on the surface of the coal samples due to long-term contact with air is manually removed to avoid interference with experimental data from previous oxidation. A jaw crusher combined with a large agate mortar is used to crush and grind the coal, and two standardized coal samples of different sizes are sieved: coal samples with a particle size of 0.050~0.075mm are used for thermogravimetric analysis experiments, and coal samples with a particle size of 0.15mm are used for programmed temperature rise experiments. The sieved coal samples are then placed in glass bottles, sealed, labeled, and stored at room temperature away from light to prevent secondary oxidation.

[0015] Thermogravimetric analysis (TG-DTG) experiments were conducted: 5 mg of pretreated fine-particle coal was weighed for each experiment, and its oxidation characteristics were tested using a simultaneous thermal analyzer. Experimental boundary conditions were set as follows: temperature range 30–600℃, linear heating rate 5℃ / min, and ventilation flow rate 100 ml / min. Both air (O2 = 21 vol%) and low-oxygen (O2 = 5 vol%) environments were configured. TG-DTG curves were analyzed to obtain five characteristic temperature points, which served as references. Figure 2 As shown.

[0016] Five characteristic temperature points for low-temperature oxidation of coal spontaneous combustion were calibrated using TG-DTG characteristic curves: critical temperature T1, cracking temperature T2, activation temperature T3, acceleration temperature T4, and maximum weight gain temperature T5. Due to the coupling effect of oxygen concentration, each temperature point was characterized by a range: the temperature range of critical temperature T1 is 30~40℃, the temperature range of cracking temperature T2 is 70~80℃, the temperature range of activation temperature T3 is 120~130℃, the temperature range of acceleration temperature T4 is 150~160℃, and the temperature range of maximum weight gain temperature T5 is 230~240℃.

[0017] Reference Figure 2 As shown, the five characteristic temperature points used to illustrate the low-temperature oxidation of coal during spontaneous combustion are analyzed as follows: T1 is the critical temperature: the temperature corresponding to the first point of maximum weight loss rate on the TG-DTG characteristic curve. During this stage, the adsorbed gas inside the coal sample is desorbed in large quantities by heating. The intensity of gas desorption is greater than the oxygen absorption and weight gain effect of the active groups on the coal surface. Overall, the coal sample quality decreases rapidly, marking the coal sample officially entering the initial stage of spontaneous low-temperature oxidation. T2 is the drying temperature: the temperature corresponding to the first mass minimum on the TG-DTG characteristic curve. At this point, the rate of gas desorption inside the coal sample and the rate of oxygen absorption and weight gain on the surface reach a dynamic equilibrium, and the net mass change approaches zero. After exceeding this temperature node, the desorption of volatile gases basically ends, and the overall mass of the coal sample tends to be stable. It is an important dividing point for distinguishing between physical desorption and chemical oxidation. T3 is the activation temperature: it refers to the starting temperature at which the mass of the coal sample changes from decreasing to increasing again. At this temperature, a large number of active functional groups inside the coal are activated, the amount of chemically adsorbed oxygen increases sharply, effectively offsetting the mass loss caused by gas desorption. The rate of weight loss of the coal sample slows down significantly and begins to increase in weight again, representing that the coal-oxygen complex chemical reaction changes from weak to strong. T4 is the acceleration temperature: the temperature at which the coal-oxygen composite oxygen absorption and weight gain rate reaches its peak. At this stage, the coal sample's oxidation activity reaches a high level, the amount of oxygen adsorbed increases dramatically, the coal sample quality rises rapidly, the residual coal oxidation reaction enters an irreversible and rapid acceleration stage, and the risk of spontaneous combustion increases significantly. T5 is the maximum weight gain temperature: at this temperature point, the oxygen absorption mass of the coal sample reaches the maximum value of the entire process, and the oxygen adsorption of coal reaches its upper limit; after exceeding this temperature node, the organic matter in the coal undergoes violent pyrolysis and cracking reactions accompanied by a large amount of heat release, and the coal sample changes from oxygen absorption weight gain to combustion weight loss, officially entering the high-temperature combustion stage.

[0018] In thermogravimetric analysis experiments, this invention configured both an air environment (O2 = 21 vol%) and a low-oxygen environment (O2 = 5 vol%). Experiments verified that the ambient oxygen concentration directly affects the activation energy of coal molecules and the rate of coal-oxygen recombination reaction, thus causing shifts in characteristic temperature points. A fixed single-point temperature cannot adapt to the complex air leakage conditions with high oxygen concentrations underground. Therefore, this invention abandons fixed temperature values ​​and uses temperature ranges to characterize each characteristic temperature, thereby establishing a coal spontaneous combustion temperature benchmark database adapted to varying oxygen environments. Figure 2 As shown.

[0019] A temperature-programmed combustion experiment was conducted: the temperature-programmed combustion experimental apparatus was connected to a gas chromatograph. A 50g coal sample with a particle size of 0.15mm was placed in a coal sample container, which was then placed inside the temperature-programmed combustion chamber. Dry air was introduced at a rate of 100ml / min. The heating rate was set as follows: 0.5℃ / min for 30~100℃, 1.0℃ / min for 100~200℃, and 2.0℃ / min for 200~300℃. The gas composition and concentration were analyzed every 10℃ during the experiment. During the spontaneous combustion oxidation of coal, gases such as O2, CO, CO2, CH4, C2H4, C2H6, and C2H2 are all related to the coal temperature.

[0020] The following table (Table 1) can be established by conducting programmed temperature rise experiments:

[0021] Table 1 Table 1 shows that: this invention uses CO gas as the core main indicator for the entire process of coal spontaneous combustion, covering the latent stage to the combustion stage; C2H4 is used as a specific auxiliary marker gas for the rapid oxidation and pyrolysis fission stages of coal; and C2H2 gas is used as a characteristic gas for the early stage and the end stage of combustion. An additional dimensionless Graham coefficient I is introduced. CO The auxiliary differentiation determination formula is as follows: ; The Graham coefficient is unaffected by working face airflow and leakage intensity. It is specifically designed to accurately distinguish between easily confused slow oxidation stages and rapid oxidation stages. It complements CO gas, C2H4 gas, and C2H2 gas, achieving comprehensive joint judgment across all stages.

[0022] Preferably, this invention uses C2H4 and C2H2 as the indicator gases. These two gases are characteristic gases of the coal spontaneous combustion stage and are highly specific. C2H4's initial precipitation temperature is stable at 120-130℃, representing the critical stage of coal spontaneous combustion; C2H2's initial precipitation temperature is above 230℃, and it is only released during the combustion stage of coal spontaneous combustion. These gases serve as early warning signals for the combustion stage of coal spontaneous combustion, avoiding the risk of missed fire detection. Both gases are generated solely from the high-temperature pyrolysis reaction of coal, without secondary interference from operations, making them indispensable characteristic marker gases for graded early warning systems.

[0023] Reasons for excluding CH4 gas: CH4 is a native adsorbed gas in coal seams. Gas desorption is common in shallowly buried coal seam groups, and the connection of fractures in the upper and lower goaf areas can lead to the disorderly release of native CH4. At the same time, CH4 precipitation occurs across the entire temperature range, making it impossible to distinguish whether the gas source is native desorption or residual coal oxidation, and it is impossible to quantify the corresponding spontaneous combustion stage. This can easily lead to false alarms and is therefore unsuitable as an early warning indicator for this invention.

[0024] Reasons for discarding C2H6 gas: C2H6 gas has a wide temperature range for generation, and its concentration increases gradually. It does not have a significant inflection point as the coal temperature rises, and its temperature sensitivity is extremely low, making it impossible to define the switching point of the oxidation stage. In addition, this gas is easily affected by multiple factors such as underground ventilation dilution, gas cross-layering, and surrounding rock desorption, resulting in poor characteristic identification. It cannot meet the requirements for the use of a refined six-level spontaneous combustion classification early warning system, so it was discarded.

[0025] In summary, this invention obtains the characteristic temperature point of spontaneous combustion of the target coal sample through S11; S12 determines the baseline relationship between at least one spontaneous combustion indicator gas and coal temperature during the spontaneous combustion process, as well as the correspondence between the occurrence temperature of the indicator gas and the coal temperature; wherein, the spontaneous combustion indicator gas includes at least CO gas; S13, based on the baseline relationship between CO gas and coal temperature determined in step S12, establishes a correction relationship between CO gas and coal temperature under different oxygen concentration conditions, in order to correct the interference of oxygen concentration changes on the correspondence between CO gas and coal temperature. This invention effectively eliminates the interference of oxygen concentration fluctuations caused by air leakage in the goaf on the CO gas characterization of coal temperature, making the early warning criteria more accurately reflect the actual degree of oxidation and spontaneous combustion of residual coal in the goaf, and significantly reducing the false alarm and missed alarm rates caused by air leakage.

[0026] S3: Construct a mathematical model for predicting CO gas concentration in the return air corner. The sources of CO in the prediction mathematical model include: the amount of CO generated by the oxidation of residual coal in the heat dissipation zone of the lower coal seam goaf, the amount of CO generated by the oxidation of residual coal in the oxidation and heating zone of the lower coal seam goaf, and the amount of CO generated by the secondary oxidation of residual coal in the overlying goaf. Specifically, in step S3, refer to Figure 3 As shown, based on the working condition mechanism analysis, the CO gas in the return air corner of the working face contains four main sources: the first is CO gas generated by low-temperature oxidation of residual coal in the heat dissipation zone of the lower coal seam goaf; the second is CO gas generated by high-temperature oxidation of residual coal in the oxidation and heating zone of the lower coal seam goaf; the third is CO gas released into the lower working face through interlayer fissures from secondary oxidation of residual coal in the goaf of the overlying mined coal seam; and the fourth is anthropogenic instantaneous CO gas generated by coal cutting by the coal mining machine and trackless rubber-tired vehicle operations. Based on this, the present invention constructs a mathematical model for predicting CO concentration in the return air corner: ; In the formula: This indicates the uncorrected CO gas concentration at the return air corner, in ppm. This indicates the air leakage in the goaf of the target working face, in cubic meters (m³). 3 / min; This indicates the amount of CO gas produced by the oxidation of residual coal in the heat dissipation zone of the lower coal seam, expressed in cm³. 3 / min; This indicates the amount of CO gas produced by the oxidation of residual coal in the oxidation and heating zone of the lower coal seam goaf, expressed in cm³. 3 / min; This indicates the amount of CO gas produced by the secondary oxidation of residual coal in the goaf of the upper coal seam, in cm³. 3 / min; This indicates the amount of CO gas produced during the coal mining process and / or underground vehicle operations, such as coal cutting machines and rubber-wheeled vehicles, measured in cm³. 3 / min.

[0027] The mathematical model for predicting CO concentration in the return air corner constructed above is the first to simultaneously incorporate the CO generated by the oxidation of residual coal in the heat dissipation zone of the lower coal seam goaf, the CO generated by the oxidation and heating zone of residual coal in the lower coal seam goaf, and the CO generated by the secondary oxidation of residual coal in the overlying goaf. This model overcomes the limitations of traditional prediction models that ignore CO leakage from the overlying goaf and inter-layer air leakage, providing a scientific basis for predicting CO concentration in the return air corner and calculating the safety threshold under shallow-buried, close-range coal seam mining conditions.

[0028] Among them, the denominator in the uncorrected mathematical model (Air leakage in the goaf of the target working face) is a parameter that is difficult to measure directly on-site. Air leakage is affected by multiple factors, including the negative pressure of ventilation at the working face, the degree of development of surface fissures, and the condition of interlayer rock fractures, resulting in significant dynamic changes. Directly using measured air leakage would lead to large errors. This invention introduces the working face air leakage rate. The air leakage rate at the working face is indirectly calculated from the air supply volume Q. It can be obtained through regression analysis of mine ventilation resistance measurements or bundled tube monitoring data, offering better operability and stability. Furthermore, the mathematical model remains uncorrected. , , The model only represents CO production and does not establish a quantitative relationship with coal seam occurrence parameters (width, thickness, recovery rate) and coal oxidation kinetic parameters (CO production rate). This makes it impossible for the model to perform personalized calculations based on specific working face conditions. This invention introduces a correction coefficient. ,Will , , Parameterized as follows: ; ; ; In the formula: The oxidation correction factor represents the floating coal in the oxidation zone of the goaf of the lower coal seam, and the preferred value is 0.1 to 0.5; The oxidation correction coefficient represents the floating coal in the heat dissipation zone of the lower coal seam goaf, and is preferably taken as 0.5 to 1.0; The oxidation correction factor for floating coal in the spontaneous combustion hazard zone of the upper coal seam goaf is preferably taken as 0.1 to 0.3. This indicates the width of the oxidation zone in the goaf of the lower coal seam, in meters (m). Indicates the width of the heat dissipation zone in the goaf of the lower coal seam, in meters (m). This indicates the width of the spontaneous combustion hazard zone in the upper coal seam goaf, in meters (m). L This indicates the length of the lower coal seam working face, in meters (m). L ′ indicates the length of the upper coal seam working face, in meters; H Indicates the thickness of the coal seam being mined at the lower coal seam working face, in meters (m). H′ Indicates the thickness of the coal seam being mined at the upper coal seam working face, in meters (m). φ This indicates the recovery rate of the lower coal seam working face; φ′ This indicates the recovery rate of the upper coal seam working face; This indicates the rate of CO gas generation in the lower coal seam, expressed in cm³ / (min·m³). This indicates the rate of CO gas generation in the upper coal seam, expressed in cm³ / (min·m³). Furthermore, the air leakage in the goaf area , The preferred value for the air leakage rate at the working face is 0.06 to 0.1. Q represents the air supply volume at the lower coal seam working face, in m³ / min.

[0029] Combining the above two formulas, to facilitate the calculation of the CO safety index at the return air corner, the trace amounts of CO gas generated by coal body mechanical crushing and rubber-tired vehicles are considered to be much smaller than the amount of CO gas generated by coal body oxidation, and their generation is greatly affected by other parameters of the working face, making their generation unstable; therefore, they are ignored in the calculation. Simultaneously, the CO gas generation V from different sources is converted, and a mathematical model for predicting the CO gas concentration at the return air corner of the fully mechanized coal seam is established: To facilitate the calculation of CO safety indicators at the return air corner, the amount of CO gas generated during coal mining and / or underground vehicle operations is considered. The amount of CO gas produced is much smaller than that produced by coal oxidation, and its production is greatly affected by other parameters of the working face and is unstable, so it is ignored in the calculation. Item. Simultaneously, the amount of CO gas produced from different sources... , , The parameters are represented using correction coefficients, and the air leakage in the goaf in the denominator is included. Replacing ηQ with ηQ, the final mathematical model for predicting CO gas concentration at the return air corner is constructed, as shown in the following expression: ; This indicates the corrected CO gas concentration at the return air corner.

[0030] S5: Substitute the mine geological parameters, mining area ventilation parameters and coal oxidation kinetic parameters into the mathematical model to calculate multiple CO concentration warning thresholds corresponding to different stages of coal spontaneous combustion. In step S3, the multiple CO concentration warning thresholds include: the safe CO concentration value at the normal return air corner of the lower coal seam working face (i.e., the first threshold), the spontaneous combustion warning concentration value at the rapid oxidation temperature of coal spontaneous combustion (i.e., the second threshold), and the spontaneous combustion warning concentration value at the critical temperature (i.e., the third threshold). These are respectively denoted as the first threshold c1, the second threshold c2, and the third threshold c3.

[0031] S4: Based on the correspondence between the coal spontaneous combustion stage and temperature and the indicator gas calibrated in step S1, and combined with step S3, calculate multiple CO concentration warning thresholds corresponding to different stages of coal spontaneous combustion, and establish a graded warning criterion that includes multiple warning levels. In step S5, based on the relevant provisions of the "Coal Mine Safety Regulations" and the "Occupational Exposure Limits for Hazardous Factors in the Workplace", the harmful effects of CO gas on human health are comprehensively considered (see Table 2 for the correspondence between CO concentration and allowable working time in the work area). A table showing the classification and early warning method for CO concentration composite index at the return air corner of the lower coal seam working face is provided, as shown in Table 3.

[0032]

[0033] Table 2 In summary, by combining the relevant provisions of the "Coal Mine Safety Regulations" and the "Occupational Exposure Limits for Hazardous Factors in the Workplace," as well as the first threshold c1, the second threshold c2, and the third threshold c3, the early warning levels established in this invention are as follows (Table 3):

[0034] Table 3 Refer to Table 3 and Figure 4As shown, the relationship between the spontaneous combustion characteristic stage of the target coal sample and the warning level is as follows: The latent stage of spontaneous combustion of coal: the coal temperature is below T1. This latent stage is defined as the first warning level in the graded warning in step S4. Slow oxidation stage of coal spontaneous combustion: The coal temperature is between T1 and T2. This slow oxidation stage is defined as the second warning level in the graded warning in step S4. Rapid oxidation stage of spontaneous combustion of coal: The coal temperature is between T2 and T3. This rapid oxidation stage is defined as the third warning level in the graded warning in step S4. Critical stage of coal spontaneous combustion: The coal temperature is between T3 and T4. This rapid oxidation stage is defined as the fourth warning level in the graded warning of step S4. The pyrolysis and fission stage of spontaneous combustion of coal: the coal temperature is between T4 and T5. This pyrolysis and fission stage is defined as the fifth warning level in the graded warning of step S4. The combustion stage of spontaneous combustion of coal: when the coal temperature is greater than T5 and C2H2 appears, this combustion stage is defined as the sixth warning level in the graded warning in step S5.

[0035] S5: Based on the measured CO gas concentration at the return air corner, use the graded early warning criteria obtained in step S4 to issue an early warning for spontaneous combustion of residual coal in the goaf.

[0036] Specifically, the CO gas concentration in the return air corner of the lower coal seam working face is measured in real time or periodically using gas sensors or manual sampling and analysis. Simultaneously, the presence of C2H4 and C2H2 is monitored, and the duration of their concentrations is recorded. The measured data is compared with the graded early warning criteria established in step S4 (as shown in Table 3) to determine the current risk level of spontaneous combustion of residual coal in the goaf, and a corresponding early warning signal is issued to guide the implementation of appropriate disaster prevention measures on site.

[0037] In this embodiment, the key thresholds of this working face have been pre-calculated through the predictive mathematical model in step S4: First threshold: CO safe concentration value at the normal return air corner c1=81ppm; Second threshold: Warning concentration value at rapid oxidation temperature c2=163ppm; Third threshold: Warning concentration value at critical temperature c3=325ppm (the parameters are different for different mines and need to be calculated by themselves).

[0038] Based on the tiered early warning criteria in Table 3, the judgment rules are as follows: First warning level: If the measured CO concentration is <24ppm and both C2H4 and C2H2 are 0, the goaf is in a safe state and no warning is required.

[0039] Second warning level: If the measured CO concentration is between 24 ppm and c1 (81 ppm) and lasts for ≥30 minutes, and both C2H4 and C2H2 are 0, a gray warning is issued. This indicates that the residual coal in the goaf has begun to slowly oxidize (coal temperature approximately 30~70℃), and attention should be paid to the gas change trend, with strengthened daily monitoring.

[0040] Third warning level: If the measured CO concentration is between C1 and 160 ppm and lasts for ≥15 minutes, and both C2H4 and C2H2 are 0, a blue warning will be issued. This indicates that the residual coal has entered the rapid oxidation stage (coal temperature approximately 70~120℃), and measures should be taken to control air leakage, such as adjusting ventilation and sealing cracks.

[0041] Fourth warning level: If the measured CO concentration is between 160 ppm and C2 (163 ppm), and C2H4 is detected, a yellow warning will be issued. This indicates that the coal temperature has reached the critical temperature (120~150℃), and the oxidation of residual coal is intensifying. Fire prevention and extinguishing measures should be implemented immediately, such as grouting and nitrogen injection.

[0042] Fifth warning level: If the measured CO concentration is between C2 and C3 (325 ppm) and C2H4 continues to increase, an orange warning will be issued. This indicates that the coal temperature has risen to the pyrolysis and fission stage (150~230℃), the risk of fire is extremely high, and it is necessary to urgently evacuate personnel from the return air side and activate the full fire extinguishing plan.

[0043] Sixth Warning Level: If the measured CO concentration > c3 and C2H2 is detected, a red warning will be issued. This indicates that the coal temperature has exceeded 230℃, and there is a high probability of open flames in the goaf. The entire mine must be shut down and personnel evacuated immediately, and the mine fire emergency plan must be followed.

[0044] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0045] In the description of this invention, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined with "first" and "second" may explicitly or implicitly include one or more of that feature.

[0046] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the invention by those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the invention should be included within the scope of protection of the invention.

Claims

1. A method for early warning of spontaneous combustion of coal based on the CO index at the return air corner, characterized in that, The method includes the following steps: S1: Under different oxygen concentration conditions, the characteristic temperature point of coal spontaneous combustion is determined by thermogravimetric analysis, and the temperature at which the index gas appears is determined by programmed temperature rise experiment. The two are used to calibrate the correspondence between the coal spontaneous combustion stage and temperature. S2: Construct a mathematical model for predicting CO gas concentration in the return air corner. The sources of CO in the prediction mathematical model include: the amount of CO generated by the oxidation of residual coal in the heat dissipation zone of the lower coal seam goaf, the amount of CO generated by the oxidation of residual coal in the oxidation and heating zone of the lower coal seam goaf, and the amount of CO generated by the secondary oxidation of residual coal in the overlying goaf. S3: Substitute the mine geological parameters, mining area ventilation parameters and coal oxidation kinetic parameters into the mathematical model to calculate multiple CO concentration warning thresholds corresponding to different stages of coal spontaneous combustion; S4: Based on the correspondence between the coal spontaneous combustion stage and temperature and the indicator gas calibrated in step S1, and combined with step S3, calculate multiple CO concentration warning thresholds corresponding to different stages of coal spontaneous combustion, and establish a graded warning criterion that includes multiple warning levels. S5: Based on the measured CO gas concentration at the return air corner, use the graded early warning criteria obtained in step S4 to issue an early warning for spontaneous combustion of residual coal in the goaf.

2. The method for early warning of spontaneous combustion of coal based on the CO index at the return air corner as described in claim 1, characterized in that, The step S1 of obtaining the characteristic temperature points of spontaneous combustion of the target coal sample specifically includes: the critical temperature T1 of spontaneous combustion, the drying cracking temperature T2, the activation temperature T3, the acceleration temperature T4, and the maximum weight gain temperature T5.

3. The method for early warning of spontaneous combustion of coal based on the CO index at the return air corner as described in claim 2, characterized in that, In step S1, the coal spontaneous combustion indicator gases include C2H2 and C2H4.

4. The method for early warning of spontaneous combustion of coal based on the CO index at the return air corner as described in claim 3, characterized in that, The warning levels mentioned in step S4 include a first warning level, a second warning level, a third warning level, a fourth warning level, a fifth warning level, and a sixth warning level; The first warning level corresponds to the latent stage of coal spontaneous combustion, and the latent stage corresponds to a coal temperature below T1; The second warning level corresponds to the slow oxidation stage of coal spontaneous combustion, which corresponds to a coal temperature between T1 and T2. The third warning level corresponds to the rapid oxidation stage of coal spontaneous combustion, and the rapid oxidation stage corresponds to a coal temperature between T2 and T3. The fourth warning level corresponds to the critical stage of coal spontaneous combustion, and the critical stage corresponds to the coal temperature being between T3 and T4. The fifth warning level corresponds to the pyrolysis and fission stage of coal spontaneous combustion, and the pyrolysis and fission stage corresponds to a coal temperature between T4 and T5. The sixth warning level corresponds to the combustion stage of coal spontaneous combustion, and the combustion stage corresponds to a coal temperature greater than T5.

5. A method for early warning of spontaneous combustion of coal based on the CO index at the return air corner, as described in any one of claims 2 to 4, characterized in that, The critical temperature T1 has a temperature range of 30~40℃, the drying temperature T2 has a temperature range of 70~80℃, the activation temperature T3 has a temperature range of 120~130℃, the acceleration temperature T4 has a temperature range of 150~160℃, and the maximum weight gain temperature T5 has a temperature range of 230~240℃.

6. The method for early warning of spontaneous combustion of coal based on the CO index at the return air corner as described in claim 5, characterized in that, In step S3, the multiple CO concentration warning thresholds include: the safe CO concentration value at the normal return air corner of the lower coal seam working face, i.e., the first threshold; the spontaneous combustion warning concentration value at the rapid oxidation temperature of coal spontaneous combustion, i.e., the second threshold; and the spontaneous combustion warning concentration value at the critical temperature, i.e., the third threshold.

7. The method for early warning of spontaneous combustion of coal based on the CO index at the return air corner as described in claim 6, characterized in that, In step S5, the criteria for graded early warning based on the measured CO gas concentration at the return air corner are as follows: The conditions for the first warning level are: the measured CO gas concentration at the return air corner is less than 24 ppm, and C2H4 and C2H2 are not detected; The conditions for the second warning level are: the measured CO gas concentration in the return air corner is between 24 ppm and the first threshold for more than 30 minutes, and no C2H4 and C2H2 are detected; The conditions for the third warning level are: the measured CO gas concentration in the return air corner is between the first threshold and 160 ppm and lasts for more than 15 minutes, and no C2H4 and C2H2 are detected; The conditions for the fourth warning level are: the measured CO gas concentration at the return air corner is between 160 ppm and the second threshold, and C2H4 is detected; The condition for the fifth warning level is that the measured CO gas concentration at the return air corner is between the second and third thresholds. The conditions for the sixth warning level are: the measured CO gas concentration at the return air corner is greater than the third threshold, and C2H2 is detected; The first threshold, the second threshold, and the third threshold increase sequentially.

8. The method for early warning of spontaneous combustion of coal based on the CO index at the return air corner as described in claim 1, characterized in that, In step S3, the predictive mathematical model is expressed in its uncorrected form: ； In the formula: This indicates the uncorrected CO gas concentration at the return air corner. This indicates the air leakage in the goaf of the target working face; This indicates the amount of CO gas produced by the oxidation of residual coal in the heat dissipation zone of the lower coal seam. This indicates the amount of CO gas produced by the oxidation of residual coal in the oxidation and heating zone of the lower coal seam goaf. This indicates the amount of CO gas produced by the secondary oxidation of residual coal in the goaf of the upper coal seam.

9. A method for early warning of spontaneous combustion of coal based on the CO index at the return air corner as described in claim 8, characterized in that, The , , The corrected prediction mathematical model is obtained by parametrically representing the model using correction coefficients: ； ； ； In the formula: This represents the oxidation correction factor for floating coal in the oxidation zone of the lower coal seam goaf. This represents the oxidation correction factor for floating coal in the heat dissipation zone of the lower coal seam goaf; This represents the oxidation correction factor for floating coal in the spontaneous combustion hazard zone of the upper coal seam goaf. Indicates the width of the oxidation zone in the goaf of the lower coal seam: This indicates the width of the heat dissipation zone in the goaf of the lower coal seam; Indicates the width of the spontaneous combustion hazard zone in the goaf of the upper coal seam; L Indicates the length of the lower coal seam working face; L′ Indicates the length of the upper coal seam working face; H This indicates the thickness of the coal seam being mined at the lower coal seam working face; H′ This indicates the thickness of the coal seam being mined at the upper coal seam working face; φ This indicates the recovery rate of the lower coal seam working face; φ′ This indicates the recovery rate of the upper coal seam working face; This indicates the rate of CO gas production in the lower coal seam. This indicates the rate of CO gas production in the upper coal seam. Furthermore, the air leakage in the goaf area ,in Let Q be the air leakage rate of the working face, and Q be the air supply volume of the lower coal seam working face.

10. A method for early warning of spontaneous combustion of coal based on the CO index at the return air corner as described in claim 1, characterized in that, In step S1, the characteristic temperature point of spontaneous combustion of the target coal sample is obtained by thermogravimetric analysis, with oxygen concentration set at 21 vol% and 5 vol%, temperature range of 30~600℃, and heating rate of 5℃ / min.

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