Ignition control and waste heat assessment of spontaneous combustion gangue hill by gravity heat pipe group: a case study in Shanxi Province, China

Spontaneous combustion gangue hill has attracted great attention due to serious environmental pollution and terrible geological disasters. However, the rich thermal resources inside are often ignored. In order to control the spontaneous combustion of gangue hill and utilize the internal waste heat resources, this project studied the combined treatment effect of 821 gravity heat pipes, laid 47 sets of temperature monitoring devices, evaluated the storage of waste heat resources, and proposed different waste heat utilization methods. The results show that (1) the positions of spontaneous combustion are all located on the windward slope. The highest temperature is in the range of 6 ~ 12 m underground, exceeding 700 ℃. (2) The single-tube experiment of gravity heat pipe shows that the effective temperature control radius is 2 m. The cooling effect is obvious in the range of 3 ~ 5 m underground. However, the temperature rises at the depth of 1 m underground. (3) After 90 days of treatment of the gravity heat pipe group, the temperature at the depths of 3 m, 4 m, 5 m, and 6 m in the high-temperature zone dropped by 56 ℃, 66 ℃, 63 ℃, and 42 ℃, respectively. The maximum temperature drop exceeds 160 ℃. The average temperature drop in the middle- and low-temperature areas is between 9 and 21 °C. (4) The concentration of harmful gases (CO, SO2, and H2S) decreases by more than 90%. The hazard level is greatly reduced. (5) The amount of waste heat resources contained within 10 m of the spontaneous combustion gangue hill is 7.83E13J. Waste heat resources can be used for indoor heating and greenhouse cultivation. And, under the temperature difference of 50 °C, 100 °C, and 150 °C, the electric energy generated by the heat through the thermoelectric conversion device in the high-temperature zone of the gangue hill is 4056.8 kWh, 7468.2 kWh, and 10,603 kWh, respectively.


Introduction
Gangue hills accumulated in the open air for a long time are prone to oxidize and spontaneously combust. A large number of harmful gases (CO, SO 2 , H 2 S, etc.) and heavy metals (As, Se, Hg, etc.) are released, resulting in smog, acid rain, and underground drinking water pollution disasters (Deng et al. 2021;Wu et al. 2019). The production and life of residents are seriously threatened. However, the high-quality heat source properties of hundreds of thousands of degrees Celsius of spontaneous combustion coal gangue hills are often ignored (Wang et al. 2021a(Wang et al. , b, 2020a. In China, the coal gangue reserves are about 6 billion tons, and it is increasing at a rate of 300 to 700 million tons every year. In addition, more than 1600 gangue hills have spontaneously combusted (Wang et al. 2018;Li and Wang 2019). With the improvement of people's awareness of environmental pollution control and efficient utilization of resources, it is imperative to scientifically control spontaneous combustion gangue hills and efficiently utilize waste heat resources, which also endows this article with the purpose and value of the research.
In order to explain the spontaneous combustion of coal gangue hills, the theory of pyrite and coal oxidation was proposed (Carlson and Granoff 1991;Robertson et al. 1999). This theory believes that the spontaneous combustion process is divided into two stages: slow and rapid heating. In the first stage, pyrite with less content in coal gangue is easy to react with oxygen under wet and low temperature conditions. The released heat slowly increases the temperature of gangue hill (Wang et al. 2020a, b). The second stage is that the heat released by the reaction of residual coal in coal gangue with oxygen after reaching the critical temperature of spontaneous combustion prompts the temperature of gangue hill with poor heat dissipation conditions to rapidly increase to the ignition temperature of coal, resulting in spontaneous combustion (Wang et al. 2021a, b). In addition, many scholars have also carried out a series of indoor and field experiments to study the spontaneous combustion of coal gangue. Li and Yuanyuan obtained the thermodynamic parameters of coal gangue spontaneous combustion at different heating rates through indoor thermal gravity analysis experiments, which has guiding significance for the numerical simulation of coal gangue combustion (Li et al. 2020a, b;Yuan et al. 2015). Li established an indoor experimental model of a semi-open coal gangue pile and analyzed the temperature distribution of the coal gangue pile under four heat sources and the change of micropore surface of coal gangue under thermal damage (Li et al. 2021a, b). X. Querol investigated the environmental characteristics of coal gangue dumps in Shanxi Province, China, revealing that C, Cl, F, S, N, As, Cd, and Hg are continuously discharged into the surrounding environment during the spontaneous combustion of coal gangue hills (Querol et al. 2008). In order to accurately grasp the degree and depth of spontaneous combustion of gangue hills, Wang established a method to estimate the degree and depth of spontaneous combustion of gangue hills by taking temperature and radon concentration as test indexes and using the concentration of waste gas generated by spontaneous combustion as verification index of accuracy and reliability of the results (Wang et al. 2021a, b). Overall, large-scale, high-temperature, long-duration, and serious environmental pollution are relatively obvious characteristics of spontaneous combustion gangue hill. Although traditional treatment methods such as grouting , surface sealing (Zhai et al. 2017), and excavation and extinguishing method (Huang et al. 2019) have achieved good results, the potential thermal energy resources inside the coal gangue are wasted. Therefore, it is necessary to change the treatment ideas of spontaneous combustion gangue hills and put forward the technology of waste heat utilization.
In order to capture the spontaneous combustion thermal energy resources in the deep part of the gangue hill, the gravity heat pipe device with high thermal conductivity and good isothermal property is applied (Jouhara et al. 2017).
Due to convenience and high efficiency, heat pipes are commonly used in the recovery and utilization of thermal energy resources, such as automobile engines (Jang et al. 2015), energy storage (Liu et al. 2015), solar energy (Naghavi et al. 2017), and geothermal energy (Huang et al. 2022a, b). Different from the way of developing hot dry rock by hydraulic fracturing, Huang proposed to extract clean geothermal energy deep underground by using an ultra-long gravity heat pipe (Huang et al. 2022a, b). The simple structure of the gravity heat pipe makes it possible to exploit hot dry rock in a single well. Yu carried out long-term in situ experiments on the Qinghai-Tibet Plateau to study the cooling performance of gravity heat pipes (Yu et al. 2016). It provides a great reference value for the application of gravity heat pipe to ensure the stability of engineering structures in permafrost regions. Xiao studied the effects of different tilt angles and liquid filling rates on the heat extraction performance of gravity heat pipes during the low-temperature oxidation stage of coal piles (Xiao et al. 2021). Deng and Su studied the potential of gravity heat pipe combined with a thermoelectric conversion device to extract underground coal fire heat for power generation and heating through outdoor and indoor experiments (Deng et al. 2020;Su et al. 2017). The results show that a large amount of waste heat resources makes the scheme feasible even in the case of low thermoelectric conversion efficiency. In this article, the gravity heat pipe has two functions. The first is to control the spontaneous combustion of the gangue hill and reduce the internal temperature. The second is to extract the waste heat resources within the spontaneous combustion gangue hill for heating and power generation.
Although there have been many studies on the control of spontaneous combustion coal piles and gangue hills with gravity heat pipes, these studies have limitations such as indoor, short-term, and fixed heat sources (Li et al. 2021a, b;Zhai et al. 2017;Xiao et al. 2021). It can neither fully reproduce the real situation of spontaneous combustion gangue hill nor reflect the effect of combined treatment of gravity heat pipe group. Moreover, the rich spontaneous combustion heat energy inside the gangue hill is also ignored.
The spontaneous combustion of Danaoliang gangue dump in Yangquan City, Shanxi Province, China, is serious. Entrusted by Yangquan Coal Industry Co., Ltd., and in order to explore a more effective and scientific method to control spontaneous combustion gangue dump, our research group designed a dynamic cloud monitoring and early warning system for spontaneous combustion gangue dump and laid 821 gravity heat pipes, 47 temperature monitoring devices, and 2 gas detection devices on site. Through on-site monitoring, the distribution of the initial temperature field in the plane and vertical direction of the spontaneous combustion gangue hill and the current concentration of harmful gases were mastered. The cooling effect of gravity heat pipe 1 3 was studied by in situ experiment for 90 days. The storage capacity of waste heat resources in spontaneous combustion gangue dump was evaluated. Finally, we explored the utilization mode of heating and power generation by using the high-quality thermal energy contained in the spontaneous combustion gangue hill. The importance of this research is the change of method, that is, to maximize the recovery of waste heat while treating spontaneous combustion gangue hills. The research results of this paper can provide new ideas for the treatment and heat utilization of spontaneous combustion gangue hills.

Regional survey
Yangquan City, Shanxi Province, China, has a warm temperate continental climate with an annual average temperature of 10.7 °C. Northwest easterly winds prevail throughout the year, with an average wind speed of 1.7 m/s. The coal-bearing area in Yangquan City is 1051 km 2 . The coal geological reserves are 10.4 billion tons, and the annual production of raw coal is 35 million tons. The content of sulfur, pyrite, and coal in the local coal gangue is about 1%, above 6%, and within 15% to 30%, which leads to the serious problem of spontaneous combustion of coal gangue piles. The research object Danaoliang gangue hill is about 792 m in length, 531 m in width, and 140 m in height, covering an area of about 0.36 km 2 . At present, the gangue hill has stopped accumulating. As shown in Fig. 1, the Danaoliang gangue hill has three windward surfaces and large exposed slopes. Moreover, in the stacking process, the coal gangue dumped along the slope was not layered and rolled, resulting in loose accumulation and large porosity.
The on-site investigation found that the loess covered on the surface of the gangue hill was severely washed by rain, and the gangue in some areas was exposed. The northern and southwestern corners of the gangue hill have spontaneously combusted, resulting in the burning of surface vegetation and whitening or yellowing of the soil (Fig. 2). The air is full of choking smells, and researchers feel dizzy and nauseous at the site.

Systems and instruments
The coal gangue dynamic cloud monitoring and early warning system shown in Fig. 3 is composed of three parts: data monitoring and transmission, data acquisition and visualization, and an intelligent alarm device. A K-type high-temperature thermocouple (armored) was used to monitor and collect temperature data at different burial depths inside the spontaneous combustion gangue hill. The working principle of the gas collector is to convert the mass concentration of different gases into corresponding electrical signals. The monitored temperature data and gas content data are transmitted to the data collector wirelessly for long distance through the specific frequency signal emitted by the 4G LTE DTU data sensor. The adopted TN-TA wireless acquisition series IoT concentrator receives the radio signal from the data sensor once a day. The measured data is stored on the cloud platform. In addition, the harmful gas content value and the temperature value of each monitoring point can be viewed and downloaded at any time through the computer and mobile phone APP. It can also be post-processed into the temperature-time curve and the plane distribution of the temperature field at the same depth. The intelligent alarm device can automatically determine whether the content of stored harmful gases in the cloud platform reaches the preset alarm threshold and whether the alarm signal is issued.

Layout of monitoring points
The layout of the temperature measuring points is shown in Fig. 4. It can be divided into C series and S series according to the difference in monitoring depth and deployment time.
In order to investigate the initial temperature distribution of spontaneous combustion gangue hill, the C series measuring points with monitoring depths of 3 m and 6 m were set up. The quantity is 27 sets. Later, S series measuring points with a monitoring depth of 1 ~ 10 m were added. The quantity is 20 sets. In addition, a test experiment was set up at the S6 device to study the cooling radius and performance of a single gravity heat pipe, as shown in Fig. 5. The S6-1 temperature measuring device is buried close to the gravity heat pipe, and then three temperature measuring points are arranged at equal intervals of 1 m in the north-south and east-west directions, respectively.
Air collectors were placed at a height of 0.5 m from the surface on the west and east sides to monitor the content of carbon monoxide (CO), sulfur dioxide (SO 2 ), and hydrogen sulfide (H 2 S) in the air near the coal gangue mountain in real time. According to the hazard degree of harmful gases and the ambient air quality standard (GB3095-2012) issued by China, the degree of air pollution is divided into four levels: safety, mild, moderate, and severe. The threshold for grading is shown in Table 1.

Treatment steps for spontaneous combustion of gangue hills
The treatment process of Danaoliang spontaneous combustion gangue hill is divided into the following: (1) the surface of the hill covers the loess, which reduces the oxygen supply and slows down the oxidation reaction rate of pyrite and (2) Gravity heat pipes are arranged on the top of the hill to extract the heat generated by spontaneous combustion and destroy the heat storage environment inside the gangue hill. Then, grass and trees are planted on the hill surface to change the ecological appearance and improve air quality.

Covered loess
Studies have shown that measures such as covering the surface with loess can greatly reduce the probability of spontaneous combustion when coal gangue is stacked (Zhai et al. 2017). Therefore, the first step of the control measures is to cover the top and slope of the gangue hill with soil, which not only prevents air leakage but also provides a foundation for planting plants. For the convenience of construction and the uniform distribution of soil thickness, the top of the gangue hill is first flattened and the slope is repaired. Secondly, the top of the gangue is covered with soil in batches, that is, the loess is covered twice and compacted to a thickness of 0.5 m, and the compaction coefficient is not less than 0.85. The covered soil area is about 16,000 m 2 .

Arrangement of heat pipes
The second step of treatment measures is to arrange gravity heat pipes. The gravity heat pipe is a kind of sealing device, which relies on the phase change of the internal filling medium to transfer heat and has high unidirectional heat transfer efficiency (Chaudhry et al. 2012). In terms of axial structure, the gravity heat pipe is divided into three parts: evaporation section, adiabatic section, and condensation section. The working principle is shown in Fig. 6a. The working medium absorbs heat in the evaporation section and changes from liquid to gas. The water vapor then passes through the adiabatic section to the condensation section. The gaseous working medium releases heat and turns into a liquid state. Finally, under the action of its own gravity, the liquid flows  back to the evaporation section along the tube wall. Through such a periodic phase transition process, the gravitational heat pipe completes the continuous transfer of heat from the low position to the high position without external force. Therefore, the gravity heat pipe is used to efficiently extract the high-temperature heat inside the spontaneous combustion gangue hill. The physical image of the gravity heat pipe used in this test is shown in Fig. 6b. The outer diameter of the gravity heat pipe is 89 mm; the wall thickness is 6 mm, and the total length is 7 m. The lengths of the evaporation section, the adiabatic section, and the condensation section are 5 m, 0.1 m, and 1.9 m, respectively (the length of the fin section is 1.2 m, and the length of the light pipe section is 0.7 m). The fin radial height, axial thickness, and pitch are 25 mm, 1.5 mm, and 15 mm, respectively. The shell is made of high temperature and corrosion-resistant high thermal conductivity material. The internal working fluid is a water-based inorganic compound, and the working temperature is 50 ~ 800 ℃.
The layout of the gravity heat pipe group is shown in Fig. 7. A total of 821 heat pipes are arranged on the surface of the spontaneous combustion gangue hill. The layout spacing at the representative position is shown in Fig. 8. During construction, the evaporation section of the gravity heat pipe is all buried in the gangue hill, and the height above the ground is 2 m. The layout takes the triangle shape as the sub-unit. It can be seen from Fig. 10 that the temperature on the west side of the spontaneous combustion gangue hill is high, and the temperature on the east side is low. Correspondingly, the distribution density of gravity heat pipes is dense in the west (Fig. 8b, d) and sparse in the east (Fig. 8g,  h). And a row of heat pipes is encrypted in the middle (I-I) to prevent the high-temperature area from spreading to the lowtemperature area (the subunit is shown in e in Fig. 8). Two rows of gravity heat pipes are arranged on the slope of the gangue hill on the first windward side on the north side, with a spacing of 2 m on the west side (Fig. 8c) and a spacing of 2.5 m on the east side (Fig. 8f). The area numbered i is the  single-tube test site. The distance between the experimental heat pipe and the nearest gravity heat pipe on the east and south sides is not less than 7 m.

Chemical reaction theory
The oxygen infiltrating into the gangue hill through the pores is very scarce, and the combustible substances in the coal gangue cannot fully react with the oxygen. A small amount of rainwater can also seep into the inside of the gangue hill. Therefore, most of the inside of the gangue hill is an oxygendeficient and humid environment. Under this condition, the main reaction principle of combustible substances (coal and pyrite) in coal gangue is as follows.
The chemical reactions of pyrite with oxygen and gaseous water are Eqs. (3) and (4), respectively (Li et al. 2021a, b).
In addition, in order to compare the quantity relationship of harmful gases more intuitively, the mass concentration is converted into molar concentration. The calculation expression is as follows.
C n is the molar concentration, mol∕m 3 ; C m is the mass concentration, g∕m 3 ; and M is the molar mass of the gas, g∕mol.

Statistical theory of waste heat
The pore volume of gangue hill is very small and can be ignored. The heat energy contained in the pore air can be ignored. In addition, due to the low thermal conductivity of coal gangue, the heat released by spontaneous combustion is difficult to dissipate to the external environment, and most of the heat energy is used to increase its own temperature. It can be considered that the difference between the temperature at various points inside the gangue and the local average temperature is the available heat energy.
The calculation method of geothermal industry for thermal energy reserves of underground dry hot rock can be used for reference (Brown 2022;Fu et al. 2022). The calculation formula of waste heat resources in gangue hill is as follows.
Q is the waste heat resource of the gangue hill, J; ρ is the density, kg/m 3 ; C p is the specific heat capacity, J∕(kg ⋅ K) ; T(x, y, z) is the coal gangue temperature at different locations, K; and T 0 is the local annual average temperature, K.

Heat transfer theory of heat pipe
Based on the similarity theory, the heat transfer theory of the gravity heat pipe in Fig. 6a can be replaced by the thermal network shown in Fig. 9 (Deng et al. 2020). Each thermal resistance represents a sub-process of the gravity heat pipe heat transfer process. R 1 and R 9 represent the axial heat transfer thermal resistance of the gravity heat pipe wall and inner wall liquid film, respectively. R 2 and R 8 represent the thermal resistance of the heat pipe wall in the evaporation section and the condensation section, respectively. R 3 and R 7 represent the thermal resistance between the inner wall of the heat pipe in the evaporation section and the condensation section and the liquid circulating working medium, respectively. R 4 and R 6 represent the thermal resistance between the gaseous working medium and the liquid working medium in the evaporation section and the condensation section, respectively. R 5 L e and L c are the lengths of the evaporation section and the condensation section, respectively, m. w and f are the thermal conductivity of the tube wall and the liquid film, respectively. r o,e , r i,e , r o,c , and r i,c are the outer and inner diameters of the evaporation section and the condensation section, respectively, m. e and c are the heat transfer coefficients of the tube wall and liquid film of the evaporation section and the condensation section, respectively, W∕(m 2 ⋅ K) . R g is the gas constant, J∕(kg ⋅ K) . L is the characteristic length of the heat pipe, m. T v is the temperature of the gaseous working medium, °C. h he is the heat of vaporization of the working medium, J∕kg . P v is the pressure of the gaseous working medium, Pa . v is the density of the gaseous working medium, kg∕m 3 .
According to the heat transfer equivalent heat network model of the gravity heat pipe in Fig. 9, it can be known 2 L e w (9) R 3 = 1 2 r i,e L e e (10) that the equivalent thermal resistance R HP of the gravity heat pipe is:

Results and discussion
Initial temperature distribution of gangue hill Horizontal distribution Figure 10 is the measured temperature distribution in the horizontal direction of the spontaneous combustion gangue hill. According to the spontaneous combustion temperature (280 °C) and critical temperature (80 °C) of coal gangue, the spontaneous combustion gangue hill at the same depth can be divided into high-temperature zone, middle-temperature zone, and low-temperature zone from west to east (Yang et al. 2021). According to statistics, when the depth is 3 m, the area of each partition is 2256 m 2 (13.6%), 3221 m 2 (19.5%), and 11,088 m 2 (66.9%). When the depth is 6 m, the area of each partition is 4169 m 2 (24.2%), 3966 m 2 (22.9%), and 8931 m 2 (52.9%). As the depth increases from 3 to 6 m, the temperature inside the gangue hill rises sharply. The isolines of 280 °C and 80 °C spread eastward. The area of high-temperature and middle-high-temperature increased by 1913 m 2 (11.4%) and 2319 m 2 (14%), respectively. The coal gangue on the northwest side and southwest side of the gangue hill exceeds 280 ℃ and is in a state of complete combustion. This is mainly because the northwest side and the southwest side are the first and second windward sides, respectively. The northwest wind prevails in this area all the year round, which makes it very easy for oxygen to enter the inside of the gangue hill. Under the lowtemperature and humid environment, the pyrite inside the coal gangue is easily oxidized and exothermic, which facilitates the spontaneous combustion of the coal gangue (Jodeiri Shokri et al. 2016). In addition, the low thermal conductivity and heat transfer efficiency of coal gangue cause the heat accumulated in the gangue hill cannot be dissipated in time. Sufficient oxygen and combustibles, coupled with a better heat storage environment, lead to spontaneous combustion of coal gangue in this area (Wang et al. 2020a(Wang et al. , b, 2021a. Affected by the Mongolia-Siberian high-pressure area, the southeast wind in this region is basically absent, resulting in little oxygen leaking into the gangue hill through the third windward surface. Moreover, the oxygen infiltrated through the first and second windward side will be consumed sharply in the high temperature zone, and only a small amount of oxygen can reach the central region. Therefore, the eastern temperature is significantly lower than the western temperature. However, the coal gangue temperature in the central region still exceeds the critical temperature of spontaneous combustion. The rate of oxidation will increase dramatically, and the risk of spontaneous combustion is high. The coal gangue in the eastern region is also in the stage of slow oxidation, and there is also a risk of spontaneous combustion.

Vertical distribution
The temperature distribution in the vertical direction of the gangue hill is studied by using the S series measuring point data with a large monitoring depth. The monitoring temperature is shown in Tables 2 and 3. Blue, yellow, and red represent low temperature (T ≤ 80 ℃), medium temperature (80 ℃ < T < 80 ℃), and high temperature (T ≥ 280 ℃), respectively. With the increase of the measuring point number, the representative color gradually changes from yellow to blue at a depth of 1 ~ 3 m underground. At a depth of 4 ~ 10 m, the representative color changes from red to yellow and blue. It can be seen from Fig. 4 that from the west to the east of the gangue hill, the number of the S series increases gradually. Therefore, it can be concluded that the west side of the gangue hill has been completely burned, and the highest temperature that has been measured is 740 °C at the S07 measuring point. Moreover, the monitored temperatures at S02, S03, S04, and S06 are still increasing rapidly with the increase of depth. There is a high probability that the temperature will reach 800 °C below the depth of 10 m. The coal gangue below 4-m depth in the middle of the gangue hill (S11 ~ S17 measuring point) is also in the rapid oxidation stage, and it is only a matter of time before it reaches the combustion state. Although the temperature of S18 ~ S20 measuring points do not reach the critical temperature of spontaneous combustion, it gradually increases with the increase of depth. It can be boldly inferred that the maximum temperature of S01 ~ S17 measuring point data is in the range of 6 ~ 12 m underground. It can also be considered that this depth range is the most likely location for spontaneous combustion of naturally accumulated gangue hills, which is consistent with Yang Na's conclusion (Yang et al. 2021).
The main reason is that the internal pores of the gangue hills that are not layered and rolled during the accumulation process are well developed, forming air channels extending in all directions. The oxygen continuously infiltrating through the windward slope can easily enter the gangue hill to participate in the oxidation and exothermic reaction. Due to the chimney effect, the air moves upward ). At the same time, the heat carried in the air is transferred to the range of 6 ~ 12 m below the surface of the gangue hill by convection. The elevated temperature accelerates the rate of oxidation reactions in this region, resulting in spontaneous combustion.
It can be seen that most of the internal area of Danaoliang gangue hill has spontaneous combustion, which seriously endangers the surrounding air and soil environment. Moreover, if rainwater leaks through surface cracks, the evaporation of liquid water will sharply increase the internal pressure of gangue hill, which is prone to disasters such as hill explosion and slope sliding. It also poses a serious threat to the production and living of the surrounding residents. The implementation of governance measures is therefore urgent.

Effect analysis of group management
Due to the large number of monitoring points arranged on the test site and the large range of vertical monitoring, it is difficult and unnecessary to completely present the temperature data of all monitoring points. Considering that the gravity heat pipe group buried at a depth of 5 m mainly affects the temperature at a depth of 3 ~ 6 m underground, the monitoring temperature at a depth of 3 ~ 6 m is used to analyze the effect of the gravity heat pipe group on the treatment of Danaoliang gangue hill. The temperature difference at 3-m and 6-m depth of C series monitoring Table 2 Temperature data of S01 ~ S10 monitoring points Table 3 Temperature data of S11 ~ S20 monitoring points 1 3 points is shown in Fig. 11. The temperature difference at 3 ~ 6-m depth of S series monitoring points is shown in Figs. 12 and 13. The data on the top of the column in the figure represents the ratio of the absolute value of the temperature difference to the initial temperature. Although the gravity heat pipe with a depth of 5 m can only absorb heat above a depth of 5 m, the temperature at a depth of 6 m is also decreasing. This is because after the shallow temperature decreases, the heat in the deep layer propagates upward under the effect of the temperature gradient, which leads to the decrease of deep temperature . According to the distribution of temperature monitoring points in Fig. 4 and the range of high-temperature, medium-temperature, and low-temperature areas in Fig. 10, the C series and S series measuring points are divided into three categories, respectively. The classification results are shown in Table 4.
As can be seen from Fig. 11, except that the temperature at the depth of 6 m of C10 and the depth of 3 m of C19 has increased compared with the initial temperature, the temperature of other monitoring points has decreased to varying degrees. The drop range is between 1 and 135 °C. In the high-temperature area, the temperature drop at the depths of 3 m and 6 m at C03, C05, C06, and C08 measuring points is even more than 60 ℃. Among them, the temperature drop at the depth of 3 m at the C06 measuring point is the largest, reaching 135 ℃ (49%). The maximum temperature drop of the measuring points in the medium temperature area is 50 °C of C14, accounting for 20%. The minimum temperature drop is 3 °C of C15, accounting for Fig. 11 After 90 days of treatment, the temperature difference of C series monitoring points Fig. 12 After 90 days of treatment, the temperature difference of S01 ~ S10 monitoring points 2%. In the low-temperature area, even though the temperature of the C19 measuring point at a depth of 3 m rises, the temperature still does not exceed 50 °C. The temperature of the remaining monitoring points decreased by more than 8%. The possibility of spontaneous combustion in this area is greatly reduced.
It can be seen from Figs. 12 and 13 that the temperature of the S measuring point has a decreasing trend as a whole. Even the temperature at the depth of 4 ~ 6 m at S01 and 3 ~ 5 m at S04 decreased by more than 100 ℃. However, the temperature at the depth of 4 m and 5 m at S12 and S14 showed an increase. The same is true for temperatures at 6-m depth at S02 and S15. It is known that the temperature increases by 40 °C, 41 °C, 29 °C, and 15 °C at the depth of 7 ~ 10 m at S02, respectively. Therefore, it can be considered that the upward propagation of deep heat causes the temperature to be higher than the initial temperature. The S12, S14, and S15 measuring points in the middle-temperature region are also in the same situation. The unsatisfactory cooling effect near the S06 measuring point is caused by the large spacing of the heat pipes, because this area is to test the cooling performance of a single gravity heat pipe (Fig. 7i). Although the temperature of S10 decreased slightly, the temperature of deep region did not rise.
At the depth of 3 ~ 6 m, the temperature in the hightemperature zone decreased by 56 ℃, 66 ℃, 63 ℃, and 42 ℃ on average; the temperature in the middle-temperature zone decreased by 21 ℃, 13 ℃, 11 ℃, and 21 ℃ on average; the temperature in the low-temperature zone decreased by 12 °C, 12 °C, 9 °C, and 11 °C on average. The temperature drop of the measuring points in the high-temperature zone is larger than that in the medium-temperature zone and the low-temperature zone. Figure 14 shows the results of temperature measurement points at a depth of 1 ~ 7 m underground for a single gravity heat pipe. As shown in Fig. 14a, at a depth of 1 m, the temperature of S6-1 increased by 12 ℃ (24%). The temperature of S6-2 ~ S6-7 all showed a slight increase, between 2 and 6 ℃. As shown in Fig. 14b, although the temperature curve at the depth of 2 m has relatively severe fluctuations, there is no obvious change compared with the initial temperature. As shown in Fig. 14c, d, and e, at the depth of 3 ~ 5 m, the temperature of S6-1, S6-2, S6-3, S6-5, and S6-6, which are closer to the gravity heat pipe, all dropped significantly trend. However, the temperature of S6-4 and S6-7, which are far from the gravity heat pipe, both increased. As shown in Fig. 14f and g, the temperature of each measuring point located at 6 m and 7 m has a very obvious upward trend. During the test period, the temperature of the gravity heat pipe with good isothermal property near S6-1 at 1.5 m from the ground is basically between 60 and 80 ℃. This temperature is significantly higher than the initial monitoring temperature at the depth of 1 m and close to the initial temperature at the depth of 2 m. Therefore, the temperature at the depth of 1 m at the S6-1 measuring point rises slowly, while the temperature change at the depth of 2 m is very small. The reason for the temperature rises of S6-4 and S6-7 at the depth of 3 ~ 5 m is that the thermal conductivity of coal gangue is small (generally between 0.2 and 0.6 W/(m·K)), Fig. 13 After 90 days of treatment, the temperature difference of S11 ~ S20 monitoring points High-temperature zone C01 ~ C10 S01 ~ S10 Middle-temperature zone C11 ~ C16 S11 ~ S16 Low-temperature zone C17 ~ C27 S17 ~ S20

Analysis of single-tube cooling test
Fig. 14 Temperature change curve of each monitoring point at different depths which limits the cooling range of the gravity heat pipe (Li et al. 2020a, b). At depths of 6 m and 7 m, the temperature of S6-1 initially dropped slightly. It may be caused by the loss of heat in the depth range of 3 ~ 5 m, which promotes the upward transfer of deep heat. The temperature changes of other measuring points are basically not affected by the gravity heat pipe. The cooling radius of the gravity heat pipe is analyzed by plotting the difference between the temperature after 90 days of treatment and the initial temperature. The result is shown in Fig. 15. If the area where the temperature drop is greater than 5 °C is used as the heat transfer and cooling area of the gravity heat pipe, the effective temperature control radius is 2 m. In addition, within the radius of 1 m, the temperature drop can reach 15 °C. At depths of 3 m and 4 m, the temperature drop in the area adjacent to the gravity heat pipe even exceeded 20 °C. However, at a depth of 5 m, this was not the case. The reasons are as follows: (1) It can be seen from Fig. 14f and g that the temperature at the depths of 6 m and 7 m is much higher than that at the depth of 5 m. Under the action of the temperature gradient, a large amount of heat propagates upwards, resulting in a smaller drop in the temperature data at a depth of 5 m ). (2) During the downward flow of the liquid working medium inside the gravity heat pipe, it is heated and evaporated again from liquid to gas in the range of 2.5 ~ 4.5 m underground, and absorbs a lot of heat, which leads to a significant cooling effect in this area (Arat et al. 2021).

Changes in the concentration of harmful gases
The concentrations of harmful gases before and after treatment are shown in Table 5. According to Table 1, the gas pollution grades were divided. It can be seen that, whether on the west side or the east side, the three gases of CO, SO 2 , and H 2 S before treatment have reached the level of moderate or heavy pollution. During the on-site investigation, the air on the top of the gangue was very choking and nauseating, which seriously polluted the life and health of the surrounding residents. After treatment, the concentration of harmful gases decreased significantly. The decline has reached more than 90%. Both CO and H 2 S have reached the safety level. However, the SO 2 concentration on the west and east sides is still at the moderate and mild pollution level. Governance needs to be further improved.
According to the chemical reaction Eqs.
(1) and (2), the carbon conservation analysis is carried out. The reaction amount of the coal on the left side of the equal sign is equal to the carbon element content in CO on the right side of the equal sign. Similarly, according to the chemical reaction Eqs. (3) and (4), the sulfur conservation analysis is carried out. The amount of pyrite on the left side of the equal sign is equal to the sum of the amounts of SO 2 and H 2 S on the right side of the equal sign. Therefore, it can be considered that the molar concentration of carbon in CO can represent the chemical reaction amount of carbon, and the sum of molar concentrations of sulfur in SO 2 and H 2 S represents the chemical reaction amount of pyrite. Figure 16 shows the comparison of carbon and sulfur content (the molarity is calculated by Eq. (6)) in the harmful gases discharged from   . 16 Comparison of carbon and sulfur content in harmful gases emitted from east and west before treatment the east and west sides before treatment. The results show that the content of carbon element is basically twice that of sulfur element in the harmful gas emitted from the west side of the gangue hill. On the east side, the carbon content is basically the same as the sulfur content. Moreover, the coal gangue on the west side is in the high-temperature area, and the coal gangue on the east side is in the low-temperature area (Fig. 10). It shows that in the high-temperature region, the exothermic reaction of carbon dominates. In the lowtemperature region, the exothermic reaction rates of carbon and pyrite are approximately equal.

Evaluation and utilization of waste heat resources
As described in the "Initial temperature distribution of gangue hill" section, the abundant waste heat resources inside the gangue hill are often ignored. This paper calculates the storage of waste heat resources in Danaoliang spontaneous combustion gangue hill before treatment and analyzes the utilization potential of the heat energy from two aspects of heating and power generation.

Reserve evaluation
First, the geometric structure of the spontaneous combustion gangue hill in Danaoliang is established, and the temperature data of C series and S series measuring points are imported. Secondly, the interpolation function is used to realize the internal temperature assignment. Finally, Eq. (6) is used to calculate the storage capacity of waste heat resources inside the gangue hill. The parameter values are = 2500kg∕m 3 , C p = 1450W∕(kg ⋅ K) , and T 0 = 15 • C. After calculation, the amount of waste heat resources contained in the underground depth of 10 ms is 7.83E13J. If the calorific value of one ton of standard coal is 2.90E07J, it can be converted into 2700 tons of standard coal. And the total thermal energy value of spontaneous combustion gangue hill is 1.15E06 USD (the local coal price is 426.6USD/ton). In addition, the spontaneous combustion of gangue hills often lasts for more than 10 years or decades, during which heat will continue to be released. Therefore, while controlling the spontaneous combustion gangue hill, it is possible to utilize its abundant and lasting spontaneous combustion thermal energy resources. Figure 17 shows the schematic diagram and indoor temperature of indoor heating using the spontaneous combustion heat energy of the gangue hill. The steel water pipe is wrapped around the condensation section of the gravity heat pipe, and the outside is wrapped with thermal insulation cotton. The circulating water absorbs the latent heat released by the phase change of the working medium and transmits to the indoor radiator. After that, the cooled water returns to the condensation section of the gravity heat pipe to absorb heat under the action of the water pump (Fig. 17a). Figure 17b shows the temperature of the indoor radiator after the heating is stabilized, which is 58.8 °C. Figure 18 shows the use of coal gangue spontaneous combustion heat energy in a greenhouse. The greenhouse is built directly on the broad and flat gangue top (Fig. 18a). The heat dissipated in the condensation section is directly sent to the greenhouse to cultivate fruits and vegetables. It can be seen from Fig. 18b and c that the scheme of building a greenhouse to utilize the spontaneous combustion heat energy of the gangue hill is completely feasible.

Waste heat utilization
The waste heat utilization scheme of gravity heat pipe combined thermoelectric conversion unit has been applied in several in situ experiments (Deng et al. 2020;Su et al. 2018). The advantages of simple structure and high utilization rate of waste heat are its unique features. Taking gravity heat pipe single-tube test area as the research object, gravity heat pipe combined with thermoelectric conversion device was used to evaluate the potential of waste heat energy in spontaneous combustion gangue hill to produce electric energy. Figure 19 is the temperature nephogram drawn from the temperature data of S6-1 ~ S6-7. After calculation, the thermal energy contained in the cylindrical area with S6-1 as the center, radius of 3 m and depth of 10 m is 3.2E11J. According to Eqs. (7) to (16), the calculated thermal resistances of each heat transfer sub-process of the gravity heat pipe are shown in Table 6. During the 90-day test period, the average temperature of the evaporation section of the gravity heat pipe was 318 °C and the average temperature of the condensation section was 70 ℃. Therefore, the heat transfer power is (318-70 ℃)/(1.45E-02 K/W) = 17.1KW. The total heat extraction in 90 days was 1.33E11J. The theoretical conversion rates of the temperature difference at 50 °C, 100 °C, and 150 °C of the thermoelectric conversion device are 2.15%, 3.95%, and 5.62%, respectively. At the same time, the daily power generation of this type of gravity heat pipe under three temperature differences is 8.8 kWh, 16.2 kWh, and 23.0 kWh, respectively. In the high-temperature region, the daily total power generation of 461 gravity heat pipes of the same type under three temperature differences are about 461 × 8.8 = 4056.8 kWh, 461 × 16.2 = 7468.2 kWh, and 461 × 23 = 10,603 kWh, respectively. It can be seen that even in the case of low thermoelectric conversion power, the power generation potential of gravity heat pipe in the high-temperature zone of spontaneous combustion gangue hill is still considerable.

Conclusion
In this paper, we set up a monitoring device to master the temperature distribution and harmful gas concentration of gangue hill. The effect of using gravity heat pipe group to treat waste hill of spontaneous combustion is analyzed. The storage capacity of waste heat resources in gangue mountains of spontaneous combustion is evaluated and the utilization mode of waste heat of spontaneous combustion for heating and power generation is explored. The research results  can provide a new idea for the treatment and heat utilization of gangue hill in spontaneous combustion. According to the research results, the following conclusions are drawn: (1) Danaoliang spontaneous combustion gangue hill can be divided into high-temperature area, medium-temperature area, and low-temperature area from west to east according to the spontaneous combustion temperature (280 ℃) and critical temperature (80 ℃). The hightemperature areas are located at the first and second windward slopes. The depth range from 6 to 12 m underground is the area with the highest temperature of the accumulated spontaneous combustion gangue.
(2) After 90 days of gravity heat pipe group cooling test, the temperature inside the gangue hill shows a general trend of decrease within 3 ~ 6-m depth. The temperature in the hot zone dropped by an average of 56 ℃, 66 ℃, 63 ℃, and 42 ℃. Temperatures in the mesothermal zone dropped by an average of 21 ℃, 13 ℃, 11 ℃, and 21 ℃. The temperature in the low-temperature zone dropped by an average of 12 ℃, 12 ℃, 9 ℃, and 11 ℃.
(3) The gravity heat pipe single tube test shows that the cooling effect is more significant within 3 ~ 5 m underground, and the maximum temperature drop is 25 ℃ during the test. If the temperature drop of the gravity heat pipe is 5 ℃, the effective temperature control radius of the gravity heat pipe in the horizontal direction is 2 m. (4) Before the treatment, the three gases (CO, SO 2 , and H 2 S) have all reached the level of moderate or heavy pollution. After the treatment, the concentration of each harmful gas has been greatly reduced, and the decline rate has reached more than 90%. According to the element conservation analysis, in the high-temperature region, the exothermic reaction of carbon dominates. In the low-temperature region, the oxidation reaction rates of carbon and pyrite are basically equal. (5) The waste heat resource contained in the 10-m depth of Danaoliang spontaneous combustion gangue hill reaches 7.83E13J. The radiator temperature can reach 58.8 ℃ by the gravity heat pipe to heat the room. It is also feasible to cultivate fruits and vegetables by directly utilizing the heat lost in the condensation section of the gravity heat pipe through the greenhouse. In addition, the electric energy produced by gravity heat pipe in high-temperature area of spontaneous combustion gangue mountain through thermoelectric conversion device is also very considerable Although the treatment of gravity heat pipe can play a cooling effect to a certain extent, it can not completely control the spontaneous combustion of coal gangue. The main reason is that the deep part of gangue mountain is always in a state of combustion and continuously releases heat. Based on the principle of spontaneous combustion of coal gangue and the results of this test, the author believes that the steps of treating spontaneous combustion gangue mountain should be: (1) Grouting. At present, grouting is the most effective method to reduce deep high temperature rapidly. Its principle is to quickly extinguish the spontaneous combustion of coal gangue and fill the internal pores of gangue mountain to prevent oxygen leakage again, causing reignition. (2) Cover soil. One of the indispensable conditions for spontaneous combustion of coal gangue is the infiltration of oxygen. However, the overlying soil of gangue mountain can effectively reduce the infiltration of oxygen.
(3) Buried gravity heat pipe. In order to achieve spontaneous combustion, there must be a temperature accumulation stage. Gravity heat pipe can quickly and efficiently remove deep heat, prevent the temperature from reaching the critical temperature of spontaneous combustion, and thus avoid the occurrence of reignition.

Declarations
Ethical approval This research was approved by Institute of Geology and Mineral resources of Shandong Province.