Method for predicting spontaneous combustion degree of shallow coal seam goaf based on soil element change

Abstract

The invention discloses a method for predicting the spontaneous combustion degree of a shallow coal seam goaf based on soil element change, which comprises the following steps: collecting a surface soil sample and a coal sample overlying a shallow coal seam, and simulating the influence of a heat effect generated by coal oxidation and a gas product on different element contents of surface soil under different coal temperature conditions of coal spontaneous combustion by utilizing a temperature programming experiment. And calculating change rates of different element contents of the soil under different coal temperature conditions, performing function fitting on the change rates and the coal temperature, performing field verification and correction on the function, periodically monitoring the change rates of different elements of the overlying soil of the goaf, and judging the coal spontaneous combustion danger degree of the goaf of the shallow coal seam. According to the method, a corresponding function of the shallow coal seam goaf coal temperature and the change rate of different earth surface elements is established through a temperature programming experiment and field monitoring, and the change rate of different earth surface soil elements in the goaf is periodically monitored and analyzed, so that the coal spontaneous combustion danger degree of the shallow coal seam goaf is judged. And a basis is provided for coal spontaneous combustion early warning of the shallow coal seam goaf under a large-span time condition.

Description

Technical Field

[0001] The present invention belongs to the technical field of mine goaf fire prevention and extinguishing, and relates to a method for predicting the spontaneous combustion degree of goaf in shallow buried coal seams based on soil element changes. Background Technique

[0002] Coal spontaneous combustion is one of the five major disasters faced by coal mine safety production, and has always seriously threatened the safe mining of coal and the life and health of workers. The coal reserves in the western region of China are rich, with the characteristics of low metamorphic degree, shallow burial, easy spontaneous combustion, etc., and the mining area ecological environment is fragile. The goaf formed by high-intensity development of shallow buried coal seams is affected by the geological occurrence characteristics of western coal seams, and is easy to penetrate the surface to cause air leakage and oxygen supply, providing a continuous oxygen supply environment for the oxidation and spontaneous combustion of the remaining coal in the goaf, which is easy to cause the spontaneous combustion of the remaining coal in the goaf. The heat effect and gas products generated by the oxidation of the remaining coal will also affect the content of surface soil elements (nitrogen, phosphorus, potassium, etc.), posing a huge threat to coal safety mining and the ecological environment. Therefore, timely predicting the risk degree of spontaneous combustion of the remaining coal in the goaf of shallow buried coal seams and taking active prevention and control measures are of great significance for reducing casualties, reducing resource losses and protecting the regional ecological environment.

[0003] At present, the methods for predicting the risk of coal spontaneous combustion mainly include index gas analysis method, temperature measurement method, etc. These methods are used to predict the early stage of coal spontaneous combustion through the products and temperature rise characteristics of the coal oxidation process. However, after the shallow buried coal seam is mined, the remaining coal in the goaf remains underground for a long time, and there is still a risk of spontaneous combustion of the remaining coal under the influence of surface air leakage. However, due to the limited surface air leakage and oxygen supply, and the heat generated by coal oxidation dissipates through the heat conduction of the surrounding rock strata, it takes a long time for the remaining coal to develop from oxidation and heat accumulation to a violent combustion state, and the natural ignition time of coal can even reach several years to several hundred years. In the face of the prediction of the risk of coal spontaneous combustion in the goaf of shallow buried coal seams with a long time span, a method for easily detecting and identifying the risk degree of the remaining coal in the goaf of shallow buried coal seams in a long cycle and continuously is needed to provide a theoretical support for the safe development of shallow buried coal seams. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for predicting the spontaneous combustion degree of goaf in shallow buried coal seams based on soil element changes, which solves the problem that the existing methods for judging coal spontaneous combustion in goaf can only predict the early stage of coal spontaneous combustion and are difficult to be used for predicting the spontaneous combustion degree of coal in the goaf of shallow buried coal seams with a long time span.

[0005] The technical solution adopted by the present invention is a method for predicting the spontaneous combustion degree of goaf in shallow buried coal seams based on soil element changes, including the following steps: Step 1, collect surface soil samples and coal samples overlying the shallow buried coal seam; Step 2: Simulate the thermal effects generated by coal oxidation and the impacts of gas products on different elements in surface soil under different coal temperatures during coal spontaneous combustion through a programmed temperature rise experiment, and test the contents of different elements in the soil under different coal temperatures after the programmed temperature rise experiment using a soil fertilizer detector; Step 3: Calculate the change rates of the contents of different elements in the soil under different coal temperatures, perform a function fitting on them with the coal temperature, and conduct on-site verification and correction on this function; Step 4: Periodically monitor the change rates of the contents of different elements in the overlying soil of the gob area of the shallow buried coal seam to judge the degree of danger of coal spontaneous combustion in the gob area of the shallow buried coal seam.

[0006] In Step 1, for the collection of soil samples, use a manual drilling method to collect soil samples at a depth of 15 - 30 cm below the surface of the overlying strata of the shallow buried coal seam, then weigh them on-site and store them sealed.

[0007] In Step 1, for the collection of coal samples, collect coal samples from the fresh coal wall of the shallow buried coal seam working face. Before sampling, strip the surface layer of the coal wall with a thickness of 20 - 30 cm for sampling. After obtaining the coal samples, weigh them on-site and seal them immediately.

[0008] The specific process of Step 2 is as follows: Step 2.1: Weigh the same weight of coal samples and soil samples collected in Step 1 respectively; Step 2.2: Place the weighed coal samples and soil samples in a sealed container in layers. The coal samples and soil samples are placed on copper wire mesh partitions at different heights respectively, with the coal samples below the soil samples. The top of the sealed container is provided with an air outlet, and the bottom is provided with an air inlet; Step 2.3: Place the sealed container in a programmed temperature rise oven for heating. The heating temperature range is 30 - 200 °C, the air flow rate is 80 - 120 mL / min, the oxygen concentration at the inlet is 3 - 21%. After the coal samples are heated by 10 - 15 °C each time, maintain for 20 - 40 min, stop heating, and take out the soil samples after cooling to room temperature to obtain soil samples affected by different coal spontaneous combustion temperatures; Step 2.4: Select soil samples under different coal temperature conditions, grind them into powder, prepare a soil nutrient test solution to be measured, drop in the soil nutrient determination reagent and shake evenly, then send them into a soil fertilizer nutrient detector to group and detect the contents of nitrogen, phosphorus, and potassium in the soil, and record the detection results.

[0009] The specific process of Step 2.4 is as follows: Step 2.4.1: Prepare a soil nitrogen, phosphorus, and potassium content determination extractant solution with a concentration of 1% - 3% using a soil combined extractant; Step 2.4.2: Select 0.5 - 1 g of soil samples under different coal temperature conditions, grind them into powder, place them in different Erlenmeyer flasks, and add 15 - 20 mL of the soil extractant solution prepared in Step 2.4.1 to each Erlenmeyer flask, and shake well; Step 2.4.3: Add 1.0 g ± 0.1 g of soil decolorizer to each Erlenmeyer flask. After shaking for three minutes using an oscillator, filter through qualitative filter paper to obtain the test solutions for soil nitrogen, phosphorus, and potassium at different coal temperatures. Step 2.4.4: Take 1 mL - 5 mL of the test solutions for soil nitrogen, phosphorus, and potassium and add them to a test tube. Add 0.15 mL - 0.25 mL of ammonium nitrogen extraction solution, 0.15 mL - 0.25 mL of Nessler's reagent, and 0.15 mL - 0.25 mL of ethylenediaminetetraacetic acid masking agent to the test tube. After shaking well, place it in cuvette I. Step 2.4.5: Take 1 mL - 5 mL of the test solutions for soil nitrogen, phosphorus, and potassium and add them to a test tube. Add 0.05 mL - 0.10 mL of ammonium molybdate - sulfuric acid solution, 0.05 mL - 0.10 mL of ascorbic acid reducing agent, and 0.05 mL - 0.10 mL of citric acid buffer solution to the test tube. After shaking well, place it in cuvette II. Step 2.4.6: Take 1 mL - 5 mL of the test solutions for soil nitrogen, phosphorus, and potassium and add them to a test tube. Add 0.15 mL - 0.25 mL of sodium tetraphenylborate solution and 0.15 mL - 0.25 mL of ethylenediaminetetraacetic acid masking agent to the test tube. After shaking well, place it in cuvette III. Step 2.4.7: Use a soil fertilizer nutrient detector to detect the nitrogen content in cuvette I, the phosphorus content in cuvette II, and the potassium content in cuvette III, and record the test results. Step 2.4.8: Repeat steps 2.4.4 - 2.4.7 to complete the determination of the nitrogen, phosphorus, and potassium contents in the soil at each coal temperature, and record the test results.

[0010] The specific process of Step 3 is as follows: Step 3.1: According to the changes in different coal temperatures during the programmed temperature rise process and the changes in the nitrogen, phosphorus, and potassium contents in the soil at that coal temperature, calculate the change rates of the nitrogen, phosphorus, and potassium contents in the soil, and use the method of function fitting to obtain the relationship function between the coal temperature and the change rates of the nitrogen, phosphorus, and potassium contents in the soil. Step 3.2: Long - term monitor the relationship between the changes in the nitrogen, phosphorus, and potassium contents in the surface soil of the gob area of the shallow - buried coal seam and the temperature of the gob area, and compare it with the predicted results of the fitting function. If the error is within 5%, it is considered that the fitting function can accurately predict the coal temperature change in the gob area of the shallow - buried coal seam. If the error is greater than 5%, continue to correct the fitting function using on - site detection data and apply on - site verification again until the error is within 5%.

[0011] In Step 4, periodically monitor the change rates of different element contents in the overlying soil of the gob area of the shallow - buried coal seam over time, and at the same time monitor the change rates of different element contents in the surface soil without a gob area around the gob area. Compare and analyze the influence of coal spontaneous combustion on the change rates of different element contents in the overlying soil of the gob area, and combine with the fitting function in Step 3 to predict the temperature of the residual coal in the gob area.

[0012] In step 4, the risk degree of coal spontaneous combustion in the gob of shallow-buried coal seams is judged. According to the sectional characteristics during the coal oxidation and temperature-rising process, coal spontaneous combustion is divided into a latent period, a self-heating period, and a combustion period. Whether the residual coal in the gob is in the latent period of spontaneous combustion is judged by the predicted temperature of the residual coal in the gob. When it is in the latent period and there is no obvious upward development trend, it indicates that the risk degree of coal spontaneous combustion is relatively low, and monitoring should be continued. Otherwise, a coal fire warning is required, and coal spontaneous combustion control work should be carried out.

[0013] When the predicted temperature of the residual coal in the gob ≤ 70°C, the monthly change rate of the predicted coal temperature ≤ 5%, and the duration ≥ 3 months, it indicates that the residual coal in the gob is in the latent period of spontaneous combustion, and monitoring should be continued. When the predicted temperature of the residual coal in the gob > 70°C, the monthly change rate of the predicted coal temperature > 5%, and there is a continuous upward trend > 3 months, it indicates that the residual coal in the gob is in the self-heating period, and a coal fire warning is required.

[0014] The beneficial effects of the present invention are as follows: By establishing the corresponding function between the coal temperature in the gob of shallow-buried coal seams and the change rates of nitrogen, phosphorus, and potassium contents in the surface soil through programmed temperature-rising experiments and on-site monitoring, and by periodically monitoring and analyzing the change rates of nitrogen, phosphorus, and potassium contents in the surface soil of the gob of shallow-buried coal seams, the risk degree of coal spontaneous combustion in the gob of shallow-buried coal seams can be judged, providing a basis for predicting and warning coal spontaneous combustion in the gob of shallow-buried coal seams under large-span time conditions, improving the accuracy of preventing and controlling coal spontaneous combustion in the gob of shallow-buried coal seams. The method of the present invention is simple to operate, highly reliable, and highly practical, and has broad application prospects in the long-term warning of coal spontaneous combustion of residual coal in the gob after shallow-buried coal seam mining. Description of the Drawings

[0015] Figure 1 is a schematic flow chart of the method for predicting the spontaneous combustion degree of the gob of shallow-buried coal seams based on soil element changes according to the present invention; Figure 2 is a schematic structural diagram of the programmed temperature-rising experimental device used in the method for predicting the spontaneous combustion degree of the gob of shallow-buried coal seams based on soil element changes according to the present invention; Figure 3 is a schematic flow chart of the method for judging whether the residual coal in the gob is in the latent period of spontaneous combustion in the method for predicting the spontaneous combustion degree of the gob of shallow-buried coal seams based on soil element changes according to the present invention; Figure 4 is a function fitting graph of soil nitrogen content under different coal temperature conditions according to the present invention; Figure 5 is a function fitting graph of soil potassium content under different coal temperature conditions according to the present invention; Figure 6 is a function fitting graph of soil phosphorus content under different coal temperature conditions according to the present invention. Detailed Embodiments

[0016] The present invention will be described in detail below in conjunction with the drawings and specific embodiments.

[0017] Example 1 A method for predicting the spontaneous combustion degree of the gob area of shallow buried coal seams based on soil element changes, see Figure 1 , including the following steps: Step 1, collect surface soil samples and coal samples overlying the shallow buried coal seams; Step 2, use a programmed temperature rise experiment to simulate the thermal effects and gas products generated by coal oxidation under different coal temperature conditions of coal spontaneous combustion on different elements of the surface soil, and test the contents of different elements in the soil under different coal temperature conditions after the programmed temperature rise experiment through a soil sample fertilizer detector; Step 3, calculate the change rates of different elements in the soil under different coal temperature conditions, perform function fitting on them with the coal temperature, and conduct on-site verification and correction on this function; Step 4, periodically monitor the change rates of different elements in the soil overlying the gob area of the shallow buried coal seams to judge the risk degree of coal spontaneous combustion in the gob area of the shallow buried coal seams.

[0018] Example 2 A method for predicting the spontaneous combustion degree of the gob area of shallow buried coal seams based on soil element changes, including the following steps: Step 1, collect surface soil samples and coal samples overlying the shallow buried coal seams. For the collection of soil samples, use a manual drilling method to collect soil samples 15 cm below the surface overlying the shallow buried coal seams, then weigh them on-site and seal them for preservation; for the collection of coal samples, collect coal samples from the fresh coal wall of the shallow buried coal seam working face. Peel off the 20 cm surface layer of the coal wall before sampling, and immediately seal it after weighing the obtained coal samples on-site.

[0019] Step 2, use a programmed temperature rise experiment to simulate the thermal effects and gas products generated by coal oxidation under different coal temperature conditions of coal spontaneous combustion on different elements of the surface soil, and test the contents of different elements in the soil under different coal temperature conditions after the programmed temperature rise experiment through a soil sample fertilizer detector; Step 3, calculate the change rates of different elements in the soil under different coal temperature conditions, perform function fitting on them with the coal temperature, and conduct on-site verification and correction on this function; Step 4, periodically monitor the change rates of different elements in the soil overlying the gob area of the shallow buried coal seams to judge the risk degree of coal spontaneous combustion in the gob area of the shallow buried coal seams.

[0020] Example 3 A method for predicting the spontaneous combustion degree of the gob area of shallow buried coal seams based on soil element changes, including the following steps: Step 1: Collect surface soil samples and coal samples overlying the shallow buried coal seam. For the collection of soil samples, use manual drilling to collect soil samples 20 cm below the surface overlying the shallow buried coal seam, then weigh them on-site and store them sealed. For the collection of coal samples, collect coal samples from the fresh coal wall of the shallow buried coal seam working face. Before collection, strip 20 cm of the surface layer of the coal wall for sampling. After obtaining the coal samples, weigh them on-site and seal them immediately.

[0021] Step 2: Refer to Figure 2 , use a programmed temperature rise experiment to simulate the thermal effects and gas products generated by coal oxidation at different coal temperatures on different elements in the surface soil, and test the contents of different elements in the soil under different coal temperature conditions after the programmed temperature rise experiment using a soil fertilizer detector; The specific process of Step 2 is as follows: Step 2.1: Weigh the same weight of coal samples and soil samples collected in Step 1 respectively. Step 2.2: Place the weighed coal samples and soil samples in a stainless steel sealed container in layers. The coal samples and soil samples are placed on copper wire mesh partitions at different heights respectively. The coal samples are below the soil samples. The sealed container is provided with an air outlet at the top and an air inlet at the bottom. Step 2.3: Place the sealed container in a programmed temperature rise oven for heating. The heating temperature range is 30 - 200 °C, the air flow rate is 100 mL / min, the oxygen concentration at the inlet is 21%. After the coal samples are heated by 10 °C each time, keep them for 30 min, stop heating, and take out the soil samples after cooling to room temperature to obtain soil samples affected by different coal self-ignition temperatures. Step 2.4: Select soil samples under different coal temperature conditions, grind them into powder, prepare a soil nutrient test solution, drop in soil nutrient determination reagents and shake evenly, then send them into a soil fertilizer nutrient detector to group-detect the contents of nitrogen, phosphorus, and potassium in the soil, and record the detection results.

[0022] Step 3: Calculate the change rates of different element contents in the soil under different coal temperature conditions, perform function fitting with the coal temperature, and verify and correct this function on-site. Step 4: Periodically monitor the change rates of different elements in the soil overlying the gob area of the shallow buried coal seam to judge the degree of coal self-ignition danger in the gob area of the shallow buried coal seam.

[0023] Example 4 A method for predicting the self-ignition degree of the gob area of a shallow buried coal seam based on soil element changes, including the following steps: Step 1: Collect surface soil samples and coal samples overlying the shallow buried coal seam. For the collection of soil samples, use manual drilling to collect soil samples 15 - 30 cm below the surface overlying the shallow buried coal seam, then weigh them on-site and store them sealed. For the collection of coal samples, collect coal samples from the fresh coal wall of the shallow buried coal seam working face. Before collection, strip 20 - 30 cm of the surface layer of the coal wall for sampling. After obtaining the coal samples, weigh them on-site and seal them immediately.

[0024] Step 2: Use a programmed temperature experiment to simulate the thermal effects generated by coal oxidation and the effects of gas products on different elements in surface soil under different coal temperature conditions during coal spontaneous combustion, and test the contents of different elements in the soil under different coal temperature conditions after the programmed temperature experiment using a soil fertilizer detector; The specific process of Step 2 is as follows: Step 2.1: Weigh 500 g of the coal sample and 500 g of the soil sample collected in Step 1 respectively; Step 2.2: Place the weighed coal sample and soil sample in a sealed container in layers. The coal sample and soil sample are placed on copper wire mesh partitions at different heights respectively, with the coal sample below the soil sample. The top of the sealed container is provided with an air outlet, and the bottom is provided with an air inlet; Step 2.3: Place the sealed container in a programmed temperature chamber for heating. The heating temperature range is 30 - 200 °C, the air flow rate is 100 mL / min, the oxygen concentration at the inlet is 21%. After the coal sample is heated by 10 °C each time, it is maintained for 30 min, then the heating is stopped. After cooling to room temperature, take out the soil sample to obtain the soil sample affected by different temperatures of coal spontaneous combustion; Step 2.4: Select the soil samples under different coal temperature conditions, grind them into powder, prepare the soil nutrient test solution to be measured, drop in the soil nutrient determination reagent and shake evenly, then send it into the soil fertilizer nutrient detector, and group-detect the contents of nitrogen, phosphorus, and potassium in the soil, and record the detection results.

[0025] The specific process of Step 2.4 is as follows: Step 2.4.1: Prepare a soil nitrogen, phosphorus, and potassium content determination extractant solution with a mass concentration of 1% - 3% using a soil combined extractant. Pour the soil combined extractant powder into a measuring cup, add 200 mL of pure water and stir to dissolve, then transfer it to a 500 mL volumetric flask and make up the volume with pure water; Step 2.4.2: Select 0.5 g - 1 g of the soil sample under different coal temperature conditions, grind it into powder, place it in different Erlenmeyer flasks, and add 15 mL - 20 mL of the soil extractant solution prepared in Step 2.4.1 to each Erlenmeyer flask, and shake well; Step 2.4.3: Add 1.0 g ± 0.1 g of soil decolorizer to each Erlenmeyer flask, shake it with an oscillator for three minutes, and then filter it through qualitative filter paper to obtain the soil nitrogen, phosphorus, and potassium test solutions under different coal temperatures; Step 2.4.4: Take 1 mL - 5 mL of the soil nitrogen, phosphorus, and potassium test solution and add it to a test tube. Add 0.15 mL - 0.25 mL of ammonium nitrogen extractant, 0.15 mL - 0.25 mL of Nessler's reagent, and 0.15 mL - 0.25 mL of ethylenediaminetetraacetic acid masking agent to the test tube, shake well, and then put it into cuvette Ⅰ; Step 2.4.5: Take 1 mL to 5 mL of the soil nitrogen, phosphorus, and potassium test solution and add it to a test tube. Then add 0.05 mL to 0.10 mL of ammonium molybdate - sulfuric acid solution, 0.05 mL to 0.10 mL of ascorbic acid reducing agent, and 0.05 mL to 0.10 mL of citric acid buffer solution to the test tube. After shaking well, place it in colorimetric cell II. Step 2.4.6: Take 1 mL to 5 mL of the soil nitrogen, phosphorus, and potassium test solution and add it to a test tube. Then add 0.15 mL to 0.25 mL of sodium tetraphenylborate solution and 0.15 mL to 0.25 mL of ethylenediaminetetraacetic acid masking agent to the test tube. After shaking well, place it in colorimetric cell III. Step 2.4.7: Use a soil fertilizer nutrient detector to detect the nitrogen content in colorimetric cell I, the phosphorus content in colorimetric cell II, and the potassium content in colorimetric cell III respectively, and record the detection results.

[0026] Step 2.4.8: Repeat steps 2.4.4 to 2.4.7 to complete the determination of the nitrogen, phosphorus, and potassium contents in the soil under each coal temperature, and record the detection results. Step 3: Calculate the change rates of different soil elements under different coal temperature conditions, perform a function fitting with the coal temperature, and conduct on-site verification and calibration of this function. Step 4: Periodically monitor the change rates of different elements in the overlying soil of the gob area of the shallow - buried coal seam to judge the degree of coal spontaneous combustion danger in the gob area of the shallow - buried coal seam.

[0027] Example 5 A method for predicting the spontaneous combustion degree of the gob area of a shallow - buried coal seam based on soil element changes, comprising the following steps: Step 1: Collect surface soil samples and coal samples overlying the shallow - buried coal seam. For the collection of soil samples, use the method of manual drilling to collect soil samples 30 cm below the surface overlying the shallow - buried coal seam, then weigh them on - site and seal them for preservation. For the collection of coal samples, collect coal samples from the fresh coal wall of the shallow - buried coal seam working face. Before collection, strip 20 cm of the surface layer of the coal wall for sampling. After obtaining the coal samples, weigh them on - site and seal them immediately.

[0028] Step 2: Use a programmed - temperature experiment to simulate the thermal effects and gas products generated by coal oxidation on different soil elements on the surface under different coal temperature conditions during coal spontaneous combustion, and test the contents of different soil elements under different coal temperature conditions after the programmed - temperature experiment through a soil sample fertilizer detector. The specific process of Step 2 is as follows: Step 2.1: Weigh the same weight of coal samples and soil samples collected in Step 1 respectively. Step 2.2: Place the weighed coal samples and soil samples in a sealed container in layers. The coal samples and soil samples are placed on copper wire mesh partitions at different heights respectively, with the coal samples below the soil samples. The sealed container is provided with an air outlet at the top and an air inlet at the bottom. Step 2.3: Place the sealed container in a programmable temperature chamber for heating. The heating temperature ranges from 30 to 200 °C, the air flow rate is 100 mL / min, the oxygen concentration at the inlet is 21%. After the coal sample is heated by 10 °C, it is maintained for 30 min. Then, stop heating. After cooling to room temperature, take out the soil sample to obtain soil samples affected by coal spontaneous combustion at different temperatures. Step 2.4: Select soil samples under different coal temperature conditions, grind them into powder, prepare soil nutrient test solutions, drop in soil nutrient determination reagents and shake well, then send them into a soil fertilizer nutrient detector to group-detect the contents of nitrogen, phosphorus, and potassium in the soil, and record the test results.

[0029] The specific process of Step 2.4 is as follows: Step 2.4.1: Prepare an extraction agent solution for determining the contents of nitrogen, phosphorus, and potassium in the soil with a mass concentration of 3% using a soil combined extractant. Pour the soil combined extractant powder into a measuring cup, add 200 mL of pure water and stir to dissolve, then transfer it to a 500 mL volumetric flask and make up the volume with pure water. Step 2.4.2: Select 0.8 g of soil samples under different coal temperature conditions, grind them into powder, place them in different Erlenmeyer flasks, and add 15 mL of the soil extraction agent solution prepared in Step 2.4.1 to each Erlenmeyer flask, and shake well. Step 2.4.3: Add 1.1 g of soil decolorizer to each Erlenmeyer flask, shake for three minutes using an oscillator, then filter through qualitative filter paper to obtain soil nitrogen, phosphorus, and potassium test solutions under different coal temperatures. Step 2.4.4: Take 1 mL of the soil nitrogen, phosphorus, and potassium test solution and add it to a test tube. Add 0.15 mL of ammonium nitrogen extraction solution, 0.15 mL of Nessler's reagent, and 0.15 mL of ethylenediaminetetraacetic acid masking agent to the test tube, shake well, and then put it into cuvette Ⅰ. Step 2.4.5: Take 1 mL of the soil nitrogen, phosphorus, and potassium test solution and add it to a test tube. Add 0.05 mL of ammonium molybdate-sulfuric acid solution, 0.05 mL of ascorbic acid reducing agent, and 0.05 mL of citrate buffer solution to the test tube, shake well, and then put it into cuvette Ⅱ. Step 2.4.6: Take 1 mL of the soil nitrogen, phosphorus, and potassium test solution and add it to a test tube. Add 0.15 mL of sodium tetraphenylborate solution and 0.15 mL of ethylenediaminetetraacetic acid masking agent to the test tube, shake well, and then put it into cuvette Ⅲ. Step 2.4.7: Use a soil fertilizer nutrient detector to detect the nitrogen content in cuvette Ⅰ, the phosphorus content in cuvette Ⅱ, and the potassium content in cuvette Ⅲ respectively, and record the test results.

[0030] Step 2.4.8: Repeat Steps 2.4.4 to 2.4.7 to complete the determination of the contents of nitrogen, phosphorus, and potassium in the soil under each coal temperature, and record the test results. Step 3: Calculate the change rates of different soil element contents under different coal temperatures, perform function fitting on them with the coal temperature, and conduct on-site verification and calibration of the function; The specific process of Step 3 is as follows: Step 3.1: According to the changes in different coal temperatures during the programmed temperature rise process and the changes in the contents of nitrogen, phosphorus, and potassium in the soil at that coal temperature, calculate the change rates of the contents of nitrogen, phosphorus, and potassium in the soil, and use the method of function fitting to obtain the relationship function between the coal temperature and the change rates of the contents of nitrogen, phosphorus, and potassium in the soil; Step 3.2: Long-term monitor the relationship between the changes in the contents of nitrogen, phosphorus, and potassium in the surface soil of the gob area of the shallow-buried coal seam and the temperature of the gob area, and compare it with the prediction results of the fitting function. If the error is within 5%, it is considered that the fitting function can accurately predict the coal temperature change in the gob area of the shallow-buried coal seam. If the error is greater than 5%, continue to calibrate the fitting function using the on-site detection data and apply on-site verification again until the error is within 5%.

[0031] Step 4: Refer to Figure 3 , periodically monitor the change rates of different element contents in the overlying soil of the gob area of the shallow-buried coal seam over time, and at the same time monitor the change rates of different element contents in the surface soil without gob areas around the gob area. Compare and analyze the influence of coal spontaneous combustion on the change rates of different element contents in the overlying soil of the gob area, and combine the fitting function in Step 3 to predict the temperature of the residual coal in the gob area; The change rates of different element contents in the surface soil without gob areas around the gob area reflect the influence of natural factors (such as climate, vegetation absorption, microbial activities, weathering, etc.) on soil elements. Through comparison, the abnormal fluctuations in the element contents in the soil caused by coal spontaneous combustion can be separated to avoid misjudgment; Judge the degree of coal spontaneous combustion danger in the gob area of the shallow-buried coal seam. According to the segmented characteristics during the coal oxidation and temperature rise process, coal spontaneous combustion is divided into the latent period, self-heating period, and combustion period. That is, when the predicted temperature of the residual coal in the gob area ≤ 70°C, the predicted monthly change rate of the coal temperature ≤ 5%, and the duration ≥ 3 months, it indicates that the residual coal in the gob area is in the latent period of spontaneous combustion, and continue to monitor. When the predicted temperature of the residual coal in the gob area > 70°C, the predicted monthly change rate of the coal temperature > 5%, and it shows a continuous upward trend > 3 months, it indicates that the residual coal in the gob area is in the self-heating period. Judge whether the residual coal in the gob area is in the latent period of spontaneous combustion through the predicted temperature of the residual coal in the gob area. When it is in the latent period and there is no obvious upward development trend, it indicates that the degree of coal spontaneous combustion danger is relatively low, and continue to monitor. Otherwise, coal fire warning is required and coal spontaneous combustion control work is carried out.

[0032] Example 6 A method for predicting the spontaneous combustion degree of the gob area of a shallow-buried coal seam based on soil element changes, comprising the following steps: Step 1: According to the spatial coordinates of the gob area in the shallow-buried coal seam, select the surface area overlying the gob area. Use the method of manual drilling to collect soil samples 30 cm below the surface overlying the shallow-buried coal seam, then weigh them on-site and seal them for preservation. Take coal samples from the fresh coal wall of the shallow-buried coal seam working face. Before sampling, it is necessary to strip 25 cm of the surface layer of the coal wall. After collecting the coal samples, weigh them on-site and seal them immediately. Step 2: Conduct a simulation experiment on the impact of coal spontaneous combustion on the contents of nitrogen, phosphorus, and potassium in surface soil. Use the programmed temperature rise experiment to simulate the thermal effect and gas products generated by coal oxidation at different coal temperatures during coal spontaneous combustion and their impact on different elements in surface soil. And test the contents of different elements in the soil under different coal temperature conditions after the programmed temperature rise experiment through a soil fertilizer detector, and record each monitoring result as T n (temperature), N n (nitrogen), P n (phosphorus), K n (potassium).

[0033] The specific process of Step 2 is as follows: Step 2.1: Weigh 500 g of the coal samples and 500 g of the soil samples collected in Step 1 respectively. Step 2.2: Refer to Figure 3 , place the weighed coal samples and soil samples in a sealed container in layers. The diameter of this sealed container is 10 cm and the height is 22 cm. The coal samples and soil samples are placed on 100-mesh copper wire grid partitions at different heights respectively. The coal samples are below the soil samples. The top of the sealed container is provided with an air outlet, and the bottom is provided with an air inlet. Step 2.3: Place the sealed container in a programmed temperature rise oven for heating. The heating temperature range is 30~200 °C, the air flow rate is 100 mL / min, the oxygen concentration at the inlet is 21%. After the coal samples are heated by 10 °C each time, keep them for 30 min, stop heating, and take out the soil samples after cooling to room temperature to obtain soil samples affected by different temperatures of coal spontaneous combustion. Step 2.4: Select soil samples under different coal temperature conditions, grind them into powder, prepare soil nutrient test solutions, drop in soil nutrient determination reagents and shake evenly, then send them into a soil fertilizer nutrient detector to group-detect the contents of nitrogen, phosphorus, and potassium in the soil, and record the detection results.

[0034] The specific process of Step 2.4 is as follows: Step 2.4.1: Prepare a soil nitrogen, phosphorus, and potassium content determination extractant solution with a mass concentration of 2% using a soil combined extractant. That is, take one bag of soil combined extractant powder with 10 g / bag, pour it into a measuring cup, add 200 mL of pure water and stir to dissolve, then transfer it to a 500 mL volumetric flask and make up the volume with pure water. Step 2.4.2: Grind 1 g of soil sample into powder under different coal temperature conditions, place it in different Erlenmeyer flasks, add 20 mL of the soil extractant solution prepared in Step 2.4.1 to each Erlenmeyer flask, and shake well; Step 2.4.3: Add 1.0 g of soil decolorizer to each Erlenmeyer flask, shake for three minutes using an oscillator, and then filter through qualitative filter paper to obtain the soil nitrogen, phosphorus, and potassium test solutions under different coal temperatures; Step 2.4.4: Take 2 mL of the soil nitrogen, phosphorus, and potassium test solution and add it to a test tube. Add 0.2 mL of ammonium nitrogen extraction solution, 0.2 mL of Nessler's reagent, and 0.2 mL of ethylenediaminetetraacetic acid masking agent to the test tube. After shaking well, place it in cuvette I; Step 2.4.5: Take 2 mL of the soil nitrogen, phosphorus, and potassium test solution and add it to a test tube. Add 0.05 mL of ammonium molybdate - sulfuric acid solution, 0.05 mL of ascorbic acid reducing agent, and 0.05 mL of citrate buffer solution to the test tube. After shaking well, place it in cuvette II; Step 2.4.6: Take 2 mL of the soil nitrogen, phosphorus, and potassium test solution and add it to a test tube. Add 0.2 mL of sodium tetraphenylborate solution and 0.2 mL of ethylenediaminetetraacetic acid masking agent to the test tube. After shaking well, place it in cuvette III; Step 2.4.7: Use a soil fertilizer nutrient detector to detect the nitrogen content in cuvette I, the phosphorus content in cuvette II, and the potassium content in cuvette III respectively, and record the detection results; Step 2.4.8: Repeat Steps 2.4.4 to 2.4.7 to complete the determination of the nitrogen, phosphorus, and potassium contents in the soil under each coal temperature, record the detection results, and record each monitoring result as T n (temperature), N n (nitrogen), P n (phosphorus), K n (potassium), as shown in Table 1.

[0035] Table 1 Nitrogen, phosphorus, and potassium contents in soil under different coal temperature conditions

[0036] Step 3: Calculate the change rates of different element contents in the soil under different coal temperature conditions, perform function fitting on them with the coal temperature, and conduct on - site verification and calibration of this function; The specific process of Step 3 is as follows: Step 3.1: According to the different coal temperatures during the programmed temperature rise and the changes in the nitrogen, phosphorus, and potassium contents in the soil at this coal temperature, calculate the change rates of the nitrogen, phosphorus, and potassium contents in the soil, and use the method of function fitting to obtain the relationship function between the coal temperature and the change rates of the nitrogen, phosphorus, and potassium contents in the soil. The function fitting diagram is shown in Figures 4 - 6 , and the fitting function is as follows: (1) In the formula, y 1, y 2, y 3 are the contents of nitrogen, phosphorus, and potassium in the soil respectively, with the unit of mg / kg; x is the actual coal temperature, with the unit of °C, and the correlation coefficients of the fitting functions are 0.96871, 0.95975, and 0.95786 respectively.

[0037] To determine the change rates of the contents of elements such as nitrogen, phosphorus, and potassium in the soil under different coal temperature conditions, by taking the derivatives of the functions of the contents of nitrogen, phosphorus, and potassium in the soil under different coal temperature conditions, the relationships between the change rates of the contents of nitrogen, phosphorus, and potassium and the coal temperature are obtained, and a mathematical function of the coal temperature and the change rates of the contents of nitrogen, phosphorus, and potassium in the soil is constructed as follows: (2) In the formula, , , are the derivative results of the contents of nitrogen, phosphorus, and potassium in the soil respectively, x is the actual coal temperature, with the unit of °C.

[0038] Step 3.2, on-site verification: Based on the coal temperature - element change rate function model established by the programmed temperature rise experiment, multiple monitoring points are arranged in the goaf. Soil samples at 15 cm and 30 cm below the ground surface are collected periodically every month, the contents of nitrogen, phosphorus, and potassium are detected and the monthly change rates are calculated. At the same time, a distributed temperature sensor or a borehole temperature measurement is used to obtain the measured temperature in the goaf, and it is compared with the prediction results of the fitting function. If the error ≤ 5% for 6 consecutive months, it is considered that the coal temperature change in the shallow-buried coal seam goaf can be accurately predicted by this fitting function, and the model is determined to be effective; if it exceeds the limit, it is corrected, that is, the fitting function is continuously corrected using the on-site detection data, and the on-site verification is applied again until the error is within 5%; Subsequently, by periodically detecting the element change rate and substituting it into the verified formula, the temperature prediction of the goaf can be realized. At the same time, the model parameters are updated quarterly to adapt to the changes in geological conditions to ensure the accuracy of long-term monitoring.

[0039] Based on the change rates of the contents of nitrogen, phosphorus, and potassium in the surface soil of the shallow-buried coal seam goaf collected on-site over time and the coal temperature data, the function is corrected, and the relationship function between the coal temperature and the change rates of the contents of nitrogen, phosphorus, and potassium in the soil is obtained by means of function fitting as follows: (3) In the formula, m is the predicted coal temperature, °C; n 1, n 2, n 3 are the change rates of the concentrations of nitrogen, phosphorus, and potassium in the soil under different coal temperature conditions respectively. The change rates of nitrogen, phosphorus, and potassium in the soil under different coal temperature conditions are shown in Table 2.

[0040] Table 2 Variation rates of nitrogen, phosphorus and potassium in soil under different coal temperature conditions

[0041] Step 4: Periodically monitor the variation rates of different element contents in the overlying soil of the gob area of the shallow-buried coal seam with time, and at the same time monitor the variation rates of different elements in the surface soil without gob area around the gob area. Compare and analyze the variation rates of different elements in the overlying soil of the gob area, and combine with the fitting function in Step 3 to predict the temperature of the remaining coal in the gob area, and judge the degree of coal spontaneous combustion danger in the gob area of the shallow-buried coal seam, so as to provide a theoretical basis for the prediction, early warning and active prevention and control of coal spontaneous combustion in the gob area of the shallow-buried coal seam under large-span time conditions.

[0042] The variation rates of different element contents in the surface soil without gob area around the gob area reflect the influence of natural factors (such as climate, vegetation absorption, microbial activity, weathering, etc.) on soil elements. Through comparison, the abnormal fluctuations of element contents in the soil caused by coal spontaneous combustion can be separated to avoid misjudgment.

[0043] Step 4.1: According to the spatial coordinates of the gob area of the shallow-buried coal seam, respectively select the overlying surface area of the gob area and the surface area without gob area for soil sample collection. The measuring points can be appropriately increased or decreased according to the actual measured variation range of nitrogen, phosphorus and potassium elements in the surface soil and the surface environment. If there are multiple measuring points, the detection results of soil elements at the measuring points shall be based on the average value.

[0044] Step 4.2: After determining the collection area of the overlying surface soil of the gob area, monitor the contents of nitrogen, phosphorus and potassium in the soil at fixed surface positions at a certain period (monthly or annually). Calculate the variation trends of the contents of nitrogen, phosphorus and potassium according to the measured data, and judge the temperature of the gob area of the shallow-buried coal seam underground through formula (3), and analyze the variation trend of coal temperature with time; Step 4.3: According to the sectional characteristics in the process of coal oxidation and temperature rise during programmed temperature rise, coal spontaneous combustion is divided into the latent period, the self-heating period and the combustion period, that is, when the predicted temperature of the remaining coal in the gob area ≤ 70 °C, the monthly variation rate of the predicted coal temperature ≤ 5%, and the duration ≥ 3 months, it indicates that the spontaneous combustion of the remaining coal in the gob area is in the latent period, and continue to maintain monitoring. When the predicted temperature of the remaining coal in the gob area > 70 °C, the monthly variation rate of the predicted coal temperature > 5%, and it shows a continuous upward trend > 3 months, it indicates that the spontaneous combustion of the remaining coal in the gob area is in the self-heating period. Judge whether the spontaneous combustion of the remaining coal in the gob area is in the latent period through the predicted temperature of the remaining coal in the gob area. When it is in the latent period and there is no obvious upward development trend, it indicates that the degree of coal spontaneous combustion danger is low, and continue to maintain monitoring. Otherwise, coal fire early warning is required and coal spontaneous combustion control work is carried out.

Claims

1. A method for predicting the spontaneous combustion degree of a gob area in a shallow buried coal seam based on soil element changes, characterized in that, It includes the following steps: Step 1: Collect surface soil samples and coal samples overlying the shallow buried coal seam; Step 2: Use a programmed temperature rise experiment to simulate the thermal effects generated by coal oxidation and the effects of gas products on different elements in the surface soil under different coal temperature conditions during coal spontaneous combustion, and test the contents of different elements in the soil under different coal temperature conditions after the programmed temperature rise experiment using a soil fertilizer detector; Step 3: Calculate the change rates of the contents of different elements in the soil under different coal temperature conditions, perform a function fitting with the coal temperature, and verify and correct the function on-site; Step 4: Periodically monitor the change rates of the contents of different elements in the soil overlying the gob area of the shallow buried coal seam, and judge the degree of coal spontaneous combustion danger in the gob area of the shallow buried coal seam.

2. The method for predicting the spontaneous combustion degree of the gob area of shallow buried coal seams based on soil element changes according to claim 1, characterized in that In the above-mentioned Step 1, for the collection of soil samples, use the method of manual drilling to collect soil samples 15 - 30 cm below the surface overlying the shallow buried coal seam, then weigh them on-site and seal and store them.

3. The method for predicting the spontaneous combustion degree of a gob area in a shallow buried coal seam based on soil element changes according to claim 1, wherein In the above-mentioned Step 1, for the collection of coal samples, collect coal samples from the fresh coal wall of the shallow buried coal seam working face. Before sampling, strip 20 - 30 cm of the surface layer of the coal wall for sampling. After obtaining the coal samples, weigh them on-site and immediately seal them.

4. The method for predicting the spontaneous combustion degree of the gob area of shallow buried coal seams based on soil element changes according to claim 1, wherein The specific process of the above-mentioned Step 2 is as follows: Step 2.1: Weigh the same weight of coal samples and soil samples collected in Step 1 respectively; Step 2.2: Place the weighed coal samples and soil samples in a sealed container in layers. The coal samples and soil samples are placed on copper wire mesh partitions at different heights respectively. The coal samples are below the soil samples. The top of the sealed container is provided with an air outlet, and the bottom is provided with an air inlet; Step 2.3: Place the sealed container in a programmed temperature rise box for heating. The heating temperature range is 30~200°C, the air flow rate is 80 - 120 mL / min, the oxygen concentration at the inlet is 3 - 21%. After the coal samples are heated by 10 - 15°C each time, keep them for 20 - 40 min, stop heating, and take out the soil samples after cooling to room temperature to obtain soil samples affected by different temperatures of coal spontaneous combustion; Step 2.4: Select soil samples under different coal temperature conditions, grind them into powder, prepare soil nutrient test solutions to be measured, drop in soil nutrient determination reagents and shake evenly, send them into a soil fertilizer nutrient detector, and group-detect the contents of nitrogen, phosphorus, and potassium in the soil, and record the detection results.

5. The method for predicting the spontaneous combustion degree of the gob area of shallow buried coal seams based on soil element changes according to claim 4, characterized in that The specific process of the above-mentioned Step 2.4 is as follows: Step 2.4.1: Prepare a soil nitrogen, phosphorus, and potassium content determination extractant solution with a mass concentration of 1% - 3% using a soil combined extractant; Step 2.4.2: Select 0.5 g - 1 g of soil samples under different coal temperature conditions, grind them into powder, place them in different Erlenmeyer flasks, and add 15 mL - 20 mL of the soil extractant solution prepared in Step 2.4.1 to each Erlenmeyer flask, and shake well; Step 2.4.3: Add 1.0 g ± 0.1 g of soil decolorizing agent to each Erlenmeyer flask, shake for three minutes using an oscillator, and filter through qualitative filter paper to obtain soil nitrogen, phosphorus, and potassium test solutions to be measured at different coal temperatures; Step 2.4.4: Take 1 mL - 5 mL of the soil nitrogen, phosphorus, and potassium test solutions to be measured and add them to a test tube. Add 0.15 mL - 0.25 mL of ammonium nitrogen extractant, 0.15 mL - 0.25 mL of Nessler's reagent, and 0.15 mL - 0.25 mL of ethylenediaminetetraacetic acid masking agent to the test tube, shake well, and then put them into colorimetric cell Ⅰ; Step 2.4.5: Take 1 mL to 5 mL of the soil nitrogen, phosphorus, and potassium test solution and add it to a test tube. Then add 0.05 mL to 0.10 mL of ammonium molybdate-sulfuric acid solution, 0.05 mL to 0.10 mL of ascorbic acid reducing agent, and 0.05 mL to 0.10 mL of citric acid buffer solution to the test tube. After shaking well, put it into cuvette II. Step 2.4.6: Take 1 mL to 5 mL of the soil nitrogen, phosphorus, and potassium test solution and add it to a test tube. Then add 0.15 mL to 0.25 mL of sodium tetraphenylborate solution and 0.15 mL to 0.25 mL of ethylenediaminetetraacetic acid masking agent to the test tube. After shaking well, put it into cuvette III. Step 2.4.7: Use a soil fertilizer nutrient detector to detect the nitrogen content in cuvette I, the phosphorus content in cuvette II, and the potassium content in cuvette III respectively, and record the detection results. Step 2.4.8: Repeat Steps 2.4.4 to 2.4.7 to complete the determination of the nitrogen, phosphorus, and potassium contents in the soil at each coal temperature, and record the detection results.

6. The method for predicting the spontaneous combustion degree of a gob area in a shallow buried coal seam based on soil element changes according to claim 4, wherein The specific process of Step 3 is as follows: Step 3.1: According to the changes in different coal temperatures during the programmed heating process and the changes in the nitrogen, phosphorus, and potassium contents in the soil at that coal temperature, calculate the change rates of the nitrogen, phosphorus, and potassium contents in the soil, and use the method of function fitting to obtain the relationship function between the coal temperature and the change rates of the nitrogen, phosphorus, and potassium contents in the soil. Step 3.2: Long-term monitor the relationship between the changes in the nitrogen, phosphorus, and potassium contents in the surface soil of the gob area of the shallow-buried coal seam and the gob temperature, and compare it with the prediction results of the fitting function. If the error is within 5%, it is considered that the fitting function can accurately predict the change in the coal temperature in the gob area of the shallow-buried coal seam. If the error is greater than 5%, continue to correct the fitting function using the on-site detection data and apply on-site verification again until the error is within 5%.

7. The method for predicting the spontaneous combustion degree of a gob area in a shallow buried coal seam based on soil element changes according to claim 5, wherein, In Step 4, periodically monitor the change rates of the contents of different elements in the overlying soil of the gob area of the shallow-buried coal seam over time, and at the same time monitor the change rates of the contents of different elements in the surface soil without gob areas around the gob area. Compare and analyze the influence of coal spontaneous combustion on the change rates of the contents of different elements in the overlying soil of the gob area, and combine the fitting function in Step 3 to predict the temperature of the remaining coal in the gob area.

8. The method for predicting the spontaneous combustion degree of the gob area of shallow buried coal seams based on soil element changes according to claim 7, characterized in that In Step 4, judge the degree of coal spontaneous combustion hazard in the gob area of the shallow-buried coal seam. According to the sectional characteristics during the coal oxidation heating process, coal spontaneous combustion is divided into the latent period, the self-heating period, and the combustion period. Judge whether the remaining coal in the gob area is in the latent period of spontaneous combustion through the predicted temperature of the remaining coal in the gob area. When it is in the latent period and there is no obvious upward development trend, it indicates that the degree of coal spontaneous combustion hazard is relatively low, and continue to monitor. Otherwise, a coal fire warning is required and coal spontaneous combustion control work is carried out.

9. The method for predicting the spontaneous combustion degree of the gob area of shallow buried coal seams based on soil element changes according to claim 8, wherein When the predicted temperature of the remaining coal in the gob area ≤ 70 °C, the predicted monthly change rate of the coal temperature ≤ 5%, and the duration ≥ 3 months, it indicates that the remaining coal in the gob area is in the latent period of spontaneous combustion, and continue to monitor. When the predicted temperature of the remaining coal in the gob area > 70 °C, the predicted monthly change rate of the coal temperature > 5%, and it shows a continuous upward trend > 3 months, it indicates that the remaining coal in the gob area is in the self-heating period, and a coal fire warning is required.

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