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In this study, we aimed to improve the efficacy and longevity of single inhibitors. NaCl, CaCl2, MgCl2, and layered double hydroxides (LDHs) were selected as the inhibitory components for experimental study. The impact of the composite inhibitors on the spontaneous combustion characteristics of the coal was studied through temperature programmed experiment, thermal analysis experiments, thermogravimetric mass spectrometry and entropy weight method, and the optimal inhibitor was determined. The findings indicate that the apparent activation energy (Ea) of the coal sample, with the addition of inhibitors during the oxidation stage, was improved. Ea of MgCl2 +1.0 LDHs + Coal was the highest (127.28 kJ/mol), i.e. 35.99 kJ/mol higher than that of the original coal. The inclusion of the inhibitor increased the characteristic ignition temperatures of the coal samples compared with the original coal. The highest ignition temperature was observed for MgCl2 +0.5 LDHs + Coal, reaching (323.98°C), i.e. 11.42°C higher than that of the original coal. The addition of inhibitors significantly inhibited the production of CO. Among the three chloride salts, MgCl2 exhibited the strongest inhibitory effect on CO production. According to the entropy weight method, MgCl2 +0.5 LDHs showed the best inhibitory effect. The results show that the composite halogen inhibitor can effectively inhibit the spontaneous combustion of coal. [ABSTRACT FROM AUTHOR]

Understanding the characteristics of coal spontaneous combustion (CSC) in goaf under different porosities is crucial for comprehending the mechanism of CSC and its prevention and control. In this paper, a multi-field coupled model of CSC in the goaf, considering porosity variation, is developed to investigate the effect of porosity on the CSC characteristics in the goaf. The results indicate that, as the goaf depth increases, both porosity and permeability decrease. When the highest goaf porosity is 25%, the average airflow velocity is between 0.00134 and 0.00139 m/s. In contrast, the average airflow velocity in the goaf with a porosity of 40% is approximately six times greater than that of the goaf with a porosity of 25%. As the goaf porosity increases, the overall oxygen concentration, temperature, and oxidized zone area also rise. Moreover, the oxidation zone area can be quantified and visualized, thereby enabling more effective prediction of the CSC risk in the goaf. The findings of the study have a positive significance in guiding the prevention and control of coal fires. [ABSTRACT FROM AUTHOR]

The erosion of igneous rocks affects the structural and spontaneous combustion characteristics of coal. A series of tests were conducted, including programmed heating, thermogravimetric analysis, FT-IR spectroscopy, low-temperature nitrogen adsorption, and pressed mercury experiments on samples from primary coal and coal eroded by igneous rocks from the Tashan Mine and Xiaonan Mine within the same coal seam. Based on these experiments, we analyzed various properties of coal, such as the oxidation characteristics, spontaneous combustion limit, active functional group content, chemical structure, and pore structure, from both macroscopic and microscopic perspectives. The results indicated significant trends after the erosion of igneous rocks: (1) there were increases in the oxygen consumption rate, as well as the CO and CO2 release rates; (2) the upper limit of air leakage intensity increased, the minimum thickness of floating coal decreased, and the lower limit of oxygen volume fraction decreased; (3) there was a decrease in the activation energy required for coal ignition; (4) there was a decrease in the active functional group content while improving the structural stability; and (5) there were the alterations in the pore structure of coal. These promoted the oxidation reactions between oxygen and the active groups within the coal matrix, increasing the propensity for spontaneous combustion, particularly in the igneous rocks with low oxidation activity. [ABSTRACT FROM AUTHOR]

The thermal effect of coal oxidation has a deep connection with the coal microstructure. A long flame coal, prone to spontaneous combustion is selected for the Differential scanning calorimetry (DSC) experiment and Fourier transform infrared experiment (FTIR) to identify the multi-scale thermal effects and mechanism of oxidized coal at different pre-oxidation temperatures. A model of the relationship between macro thermal effect and microstructure during coal oxidation is established by adopting multiple linear regression. It indicated that, with the increase of pre-oxidation temperature, the oxidizing property and exothermic of oxidized coal first decrease and then increase. The FTIR illustrated that the intramolecular hydrogen bond and free and linked hydrogen bond in the hydroxyl group have multiple collinearities. The result showed that the coal oxidation process is divided into three stages. In the heat absorption stage, aliphatic hydrocarbons and C=C double bond are negatively correlated with the exothermic rate. In the slow heat release stage, the variance predicted by the regression model is 0.722, which characterizes and predicts the degree of coal spontaneous combustion. In the heat release stage, the key factors affecting the heat release rate are aliphatic hydrocarbons and carboxyl groups, and the amount of heat release increases exponentially. [ABSTRACT FROM AUTHOR]

To improve their whole-stage inhibition effects on the spontaneous combustion of coal, curcumin (CCM) and chitosan (CTS) were grafted, and the natural nanomaterial montmorillonite (MMT) was used for intercalation to prepare the composite gel inhibitor CTS-CCM/MMT. Its macro effect and micro mechanism of inhibition were studied. SEM analysis revealed a significant increase in the surface folds and pores of CTS-CCM, suggesting enhanced mechanical properties and thermal stability. XRD and DSC experiments demonstrated a reduction in the diffraction angle 2θ from 21.148° to 20.011° for CTS-CCM/MMT, along with a 21.3% increase in glass transition temperature. This supports the effective improvement of thermal stability through intercalation treatment. Thermogravimetric analysis indicated a higher characteristic temperature point for the inhibitor composite. Infrared diffuse reflection experiments showed deactivation of oxidation activity for the hydroxyl and aliphatic groups through combined inhibition treatment. These findings collectively suggest that the gel exhibits heightened stability and flame retardancy, and can effectively inhibit the entire spontaneous combustion reaction of coal. [ABSTRACT FROM AUTHOR]

According to the theory of coal–oxygen combination, oxygen has an important effect on the spontaneous combustion of coal. In this study, Hailar lignite is taken as the research object to prepare and analyze the coal sample. The variation trends of the free radical concentration, g factor value, line width and line shape are measured by electron spin resonance (ESR) technology, and the influence of different oxidation times and oxygen concentrations on the free radical parameters in coal are studied. The results show that the relationship between the free radical reaction and the oxidation time is not linear. The critical point of the free radical reaction is the oxidation time of 5 h. With an increase in the oxygen concentration, the free radical concentration, g factor value and line width also increase. In a nitrogen environment, the free radical reaction still exists, but the new free radicals are hydrocarbon free radicals or alkyne free radicals, which will not lead to spontaneous combustion of coal. Oxygen molecules, meanwhile, make the free radical reaction more active. Oxygen atoms combine with carbon chains to form oxygen-containing free radicals and peroxide free radicals, which further decompose and oxidize to produce CO and CO2, eventually triggering the spontaneous combustion of coal. [ABSTRACT FROM AUTHOR]

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To explore the spontaneous combustion characteristics and hazards of the low-temperature oxidation (LTO) stage in the process of spontaneous combustion of coal and mudstone, the pore structure, spontaneous combustion characteristic parameters, and exothermic characteristics of coal and mudstone were tested and studied, and the oxidation kinetic parameters were calculated. The results show that mudstone has a larger specific surface area and pore volume than coal. From the fractal characteristics, the pore structure of mudstone is more complex than that of coal. According to the comparison of theoretical and actual gas generation and oxygen consumption rate curves, it is found that there is an interaction between coal and mudstone in the LTO process. With the increase of mudstone mass ratio, gas production, and its oxygen consumption rate increase. Among them, CM-4 (Coal:Mudstone = 1:1) has the highest exothermic intensity and the exothermic factor (A) and fire coefficient (K) increase with the increase of mudstone content. The apparent activation energy of the mudstone sample is lower than that of the raw coal, indicating that the sample after adding mudstone is more likely to have spontaneous combustion in the LTO stage. [ABSTRACT FROM AUTHOR]

Accurate prediction of the coal spontaneous combustion hazard grades is of great significance to ensure the safe production of coal mines. However, traditional coal temperature prediction models have low accuracy and do not predict the coal spontaneous combustion hazard grades. In order to accurately predict coal spontaneous combustion hazard grades, a prediction model of coal spontaneous combustion based on principal component analysis (PCA), case-based reasoning (CBR), fuzzy clustering (FM), and the snake optimization (SO) algorithm was proposed in this manuscript. Firstly, based on the change rule of the concentration of signature gases in the process of coal warming, a new method of classifying the risk of spontaneous combustion of coal was established. Secondly, MeanRadius-SMOTE was adopted to balance the data structure. The weights of the prediction indicators were calculated through PCA to enhance the prediction precision of the CBR model. Then, by employing FM in the case base, the computational cost of CBR was reduced and its computational efficiency was improved. The SO algorithm was used to determine the hyperparameters in the PCA-FM-CBR model. In addition, multiple comparative experiments were conducted to verify the superiority of the model proposed in this manuscript. The results indicated that SO-PCA-FM-CBR possesses good prediction performance and also improves computational efficiency. Finally, the authors of this manuscript adopted the Random Balance Designs—Fourier Amplitude Sensitivity Test (RBD-FAST) to explain the output of the model and analyzed the global importance of input variables. The results demonstrated that CO is the most important variable affecting the coal spontaneous combustion hazard grades. [ABSTRACT FROM AUTHOR]

During the process of coal spontaneous combustion (CSC), a plethora of combustible gases alongside inert gases, such as CO2, are copiously generated. However, prior investigations have regrettably overlooked the pivotal influence of inert gas production on the propensity for methane explosions during CSC. To investigate the impact of the flue gas environment generated by CSC, containing both combustible and inert gases, on the risk of methane explosion, a high-temperature programmed heating test system for CSC was employed to analyze the generation pattern of flue gas. It was found that CO, CO2, and CH4 were continuously generated in large quantities during the process of CSC, which are the main components of CSC flue gas. The effect of the concentration and component ratio (CCO2/CCO) of the flue gas on the methane explosion limit was tested. It was found that the CSC flue gas led to a decrease in the methane explosion limit, and that the explosion limit range was facilitated at 0 < CCO2/CCO < 0.543 and suppressed at CCO2/CCO > 0.543. As the temperature of CSC increases, the risk of methane explosion is initially suppressed. When the coal temperature exceeds 330~410 °C, the explosion risk rapidly expands. [ABSTRACT FROM AUTHOR]

Coal spontaneous combustion (CSC) derives from the exothermic coal-oxygen reaction occurring at coal's surface. However, methane's influence on this process is still unclear in gob environments. To this end, non-isothermal experiments were performed to elucidate methane's impact on the exothermic effect, pore structure evolvement, and surface composite variation in CSC. Our findings indicated that methane inhibited the exothermic effect; however, low methane (50 %) methane concentrations, because coal lay in oxygen-rich and fuel-rich oxidation states, respectively. Methane's influence on exothermic reaction originates from competitive adsorption. Methane competes with oxygen and other gases for adsorption sites. This changes the surface properties and affects the competitive adsorption between methane and other gases. This study can provide valuable insights for mitigating CSC risk.

Relevance. The necessity to improve the accuracy of analysis and prediction of potential fire hazard of rock-coal massifs by using electromagnetic methods of endogenous fire detection. Taking into consideration that the increase in coal temperature changes a number of its parameters, such as dielectric permittivity and electrical resistivity, it is reasonable to use the method of electromagnetic reconnaissance in locating the focus of fire. Aim. To analyze the theoretical and practical knowledge about the anomalies formed in the area of spontaneous combustion and to evaluate the effectiveness of electromagnetic methods for locating the spontaneous combustion zones of a coal massif. Objects. Physical parameters of the ignition zone of the coal massif, such as dielectric permittivity and electrical resistivity, as well as the range of the GPR central frequency, allowing clearly defining the boundaries of the ignited zone. Method. Reviewing the proposed methods of determining the parameters serving for correct location of the fire zone. For GPR it is necessary to determine the center frequency of the standard antenna unit and resolution, then to estimate the rational frequency range of GPR. Results. Allow us to draw conclusions about the methods used to determine the rational range of GPR center frequency – it can be calculated by solving the system of equations of the signal attenuation function and energy potential of GPR, by the experimental value of the target function, including the depth and detail of sounding as a function of frequency, or on the basis of a complex parameter of GPR, including the radiated power of the antenna, the number of accumulations and the reflection coefficient from the boundaries. Taking into account such physical features of the fire zone as drying the rock-angle massif and the shape of the anomalous zone, the rational range of the center frequency for the GPR "OKO-2" was determined.

To investigate the mechanism of inhibition of spontaneous combustion of coal by degradation of white-rot fungus, the changes of macroscopic oxidizing characteristics and microstructure of the degraded coal were investigated by the programmed temperature-raising test and the in situ Fourier-transform infrared spectroscopy experiments. The difference in gas production between the degraded coal and the blank samples indicated that the degradation inhibited the oxidation process of coal, and the intensity of the inhibition effect was different at different oxidation stages. The strongest inhibition effect was found in the accelerated oxidation stage, i.e. 80–140 °C. To further analyze the micro-mechanism of the degradation of Phanerochaete chrysosporium affecting the spontaneous combustion of coal, the relative contents of active functional groups in coal samples were compared. The results showed that the degradation affected the relative contents of hydroxyl, aliphatic hydrocarbon, and oxygenated functional groups in coal. Among them, the most significant changes of –OH and –CO- were observed at 80–140 °C. Thus, it was analyzed that Phanerochaete chrysosporium could degrade the macromolecular structure of coal by generating biological enzymes to reduce the number of oxidizable functional groups such as –OH, –CH2, –CH3, –CO-, and so on, thus reducing the risk of spontaneous combustion of coal.

For reducing the incidence of mine fires, the limitations and shortcomings of the existing monitoring and early warning technologies for coal spontaneous combustion disaster are sorted out, and it is pointed out that the future development should focus on the application of advanced monitoring technology, the improvement of the accuracy of prediction technology, the development of early warning system and intelligent management. Aiming at the hidden, persistent, proliferation and difficult to control characteristics of coal spontaneous combustion disaster, the future development direction of coal spontaneous combustion disaster monitoring and early warning technology is proposed: improve the comprehensiveness of the method for determining the critical value of natural ignition of coal, and use emerging equipment and advanced technology to improve the method of determining the critical value of natural ignition of coal; for the complex coal spontaneous combustion environment of the mine, and the comprehensive and refined monitoring parameters of the spontaneous combustion of coal can be improved by studying new advanced technologies such as acoustic wave detection; promote the innovation of coal spontaneous combustion monitoring and early warning technology and the intelligence of decision-making, and use digital twin technology to provide accurate simulation and prediction capabilities, optimize prevention and control plans, provide training and emergency management, as well as the help in data analysis and decision support; focus on the intelligent integration of comprehensive perception, accurate prediction and prevention and control of coal spontaneous combustion disasters, and strengthen the research on technologies such as data analysis of coal spontaneous combustion disasters, active early warning before disasters, post-disaster accident management, and automatic closure of fire areas.

In order to explore the influence law of igneous rock erosion on coal spontaneous combustion characteristics and structure, taking primary coal and igneous rock eroded coal in the same coal face of Shanxi Mine and Liaoning Mine as the samples, the oxidation characteristics, the limit parameters of the spontaneous combustion, the content of the active functional group and pore structure characteristics of coal were analyzed from macro and micro perspectives by temperature programmed test, FT-IR experiment, low temperature nitrogen adsorption experiment and mercury injection experiment respectively. The results of temperature programmed test showed that igneous rock erosion increased the spontaneous combustion risk of coal, igneous rock erosion could enhance the gas circulation inside the coal body, which is conducive to the oxidation reaction between oxygen and active groups inside the coal body, and the coal body of the igneous erosion had higher oxidizing activity. In addition, igneous rock erosion altered the occurrence conditions of coal seams, increased the difficulty of coal seams mining and the intensity of wind leakage in the mining airspace, prolonged the oxidation time of the residual coal in the “oxidized heating zone” in goaf, thus increasing the risk of spontaneous combustion of coal from the level of external influencing factors. The erosion of igneous rock changes the structure of coal itself and the external environmental factors of coal spontaneous combustion, which intensifies the spontaneous combustion risk of the eroded coal seam.

The total amount and scope of mined-out areas caused by the coal mining activities in China are large. Effective identification and detection of the coal spontaneous combustion point in the mined-out zone is an important guarantee for the safe production of coal mines. From the process of coal spontaneous combustion in the mining area and the characteristics of the formation and transfer of high-temperature points, the characteristics of the generation and evolution of high-temperature points in the mining area are summarized, and the basic theoretical support is provided for the effective identification and detection of high-temperature points of coal spontaneous combustion in the mining area. Focusing on the basic principles, research progress and practical application effects of underground direct exploration technology, surface and airborne detection technology, the authors conduct multiple comparisons in terms of reliability and stability, and analyze the applicability of the existing detection technologies for identifying the high-temperature point of coal spontaneous combustion. Aiming at the difficulties in detection, the research status and application potential of emerging technologies such as the mine acoustic temperature information detection, the heat source detection based on quantum technology, and the millimeter radar wave detection technology are expanded and analyzed. The future development trend of coal spontaneous combustion hidden ignition source detection technology is envisioned through the comprehensive assessment of existing technologies and the prospective thinking of emerging technologies. Combined with the current development trend of physical exploration technology and multiple information fusion theory, the concept of intelligent visualization system for the dynamic transfer of high-temperature points in the mined-out zone is proposed. The significance of the construction of the platform for the whole life cycle of mining is explained in terms of the application of the integrated time-division and multi-dimensional detection mode of “space-sky-earth-hole” and the digital twin model of the step-by-step spatial-temporal evolution of “feature acquisition-scenario simulation planning-dynamic decision-making”. Through the gradual implementation of the concept, it provides a new decision-making idea for realizing the efficient identification and prediction of hidden fire sources in the mined-out area and the intelligent construction of mines.

Spontaneous combustion of coal increases the risk of fires during its storage and transportation, posing a challenge to the sustainable development of the coal industry. This study employs various similarity criteria, including geometric, time, kinematic, dynamical, Euler’s, Strohal’s, Vernold’s, and Reynolds’ criteria, to investigate the rules of the spread of loose coal body combustion under wind flow conditions using a self-built, open-vehicle coal transportation simulation experimental device. The device recreates the temperature rise process in loose coal bodies, the formation of high-temperature areas, and their subsequent spread. It also analyzes the internal high-temperature area spread and gas distribution rules under wind flow. During the experiment, the temperature of each measurement point changed as the constant temperature time increased, with the measurement points closer to the heat source showing a significant temperature increase compared with those farther away. The experiments were conducted for 70, 54, and 48 h under wind flows of 0, 1, and 2 m·s−1, respectively. The results reveal that the crosswind flow on the surface significantly accelerates the spread of the seed of coal sample combustion. Compared to wind flow at 0 m·s−1, the high-temperature area spread time is 0.3 times quicker (wind flow at 1 m·s−1) and 0.5 times quicker (wind flow at 2 m·s−1); moreover, the maximum temperature of the high-temperature area increases by 120 ± 20 ℃, eventually stabilizing at 510 ℃–560 ℃. The spreading path of the high-temperature area drifts toward the wind under the influence of crosswind flow on the surface. When the wind flow is 0 m·s−1 the difference between the time when the CO concentration peaks at the critical point of each layer and when the critical point reaches 300 ℃ (Δh1) gradually increases with the number of layers. However, under the effect of wind flow, Δh1 is maintained at 1.4 ± 1 h (wind flow at 1 m·s−1) and 0.9 h (wind flow at 2 m·s−1), thus intensifying the coal–oxygen reaction and causing the combustion to spread more rapidly. For each critical point of the oxygen volume fraction of the layers, the time elapsed during the rapid decline phase of the O2 volume fraction at the high-temperature point of combustion gradually increases with depth. When the wind flow is 0 m·s−1, the difference between the time to reach the limiting O2 volume fraction at the critical point and the time to reach 300 ℃ at the critical point (Δh2) increases with depth. However, under the effect of wind flow, Δh2 is maintained at 0.6 ± 0.3 h (wind flow at 1 m·s−1) and 1 ± 0.2 h (wind flow at 2 m·s−1), indicating that surface airflow can exacerbate the rate of O2 consumption during the downward spread of high temperatures. Therefore, controlling air leakage can delay the spread of coal fires. These findings offer valuable insights for developing guidelines to control coal fires during coal transportation and storage.

Air leakage is an important cause of coal spontaneous combustion in mines, which affect the normal production of the mine. In order to solve the problems of long gelling time and weak stacking ability of traditional inorganic solidified foam (TISF) in the process of filling and plugging, a rapid setting inorganic solidified foam (RISF) modified by liquid sodium silicate (LSS) was proposed. The influence of different dosages of LSS on the gelling time, stacking capacity and other properties of foam slurry was studied. By means of compressive strength, stability test and other characterization methods, the optimal addition amount of LSS was determined to be 5% (mass fraction). The 28 d compressive strength of RISF reached 2.49 MPa, 1.7 times that of TISF. The maximum density ratio R was 1.11, and the stability increased by about 20%; The test results of gelation time and stacking ability showed that compared with TISF, the setting time of RISF was significantly shortened (from 683 s to 52 s), and the stacking ability was improved (the maximum stacking height is increased by 1.9 times). Based on this, the rapid curing mechanism of RISF was explored by combining scanning electron microscope (SEM) and X-ray diffraction (XRD). The test results showed that RISF had more hydration products and different hydration products were closely intertwined. During the particle hydration process, LSS hydrolyzed to generate the orthosilicate and combined with calcium ions in the solution, which accelerated the hydration reaction and shortened the gelling time of foam slurry. However, it was also found that when the amount of LSS added exceeded 5%, the compressive strength and stability of RISF decreased. This is mainly because excessive LSS caused the calcium silicon ratio in the solution to be lower than 1.0, resulting in the formation of low strength hydration products. The results of air blockage and fire extinguishing tests showed that compared with TISF, RISF had higher air blockage efficiency and excellent fire prevention and extinguishing performance, with a maximum increase of 26.3% in blockage efficiency, and no reignition occurred when extinguishing coal fires. The on-site application of RISF in the gangue hill of Limin Coal Mine has shown that it has good cooling and fire extinguishing effects, effectively solving the high temperature problem of the gangue hill, and ensuring the normal production of the mine.

Spontaneous coal combustion adversely affects coal mine safety and restricts safe, efficient, and green coal mining. Inert gas fire prevention and extinguishing technology is a widely used fire prevention and extinguishing method in coal mines. CO2 is often used as the primary raw material for inert gas fire prevention and extinguishing, owing to its good inerting and cooling characteristics. However, the lack of data on the physical and chemical properties and fire extinguishing mechanism of CO2 by field personnel has limited the efficient application of CO2 in coal mine fire prevention and extinguishing. To explore the practical application effects of CO2 fire prevention and extinguishing technology on coal mine sites, this paper summarised and analysed the research and development status of CO2 fire prevention and extinguishing technology and expounded the physical and chemical properties, phase characteristics, and fire prevention and extinguishing mechanisms of CO2. The CO2 pipeline, CO2 pipeline intelligent monitoring and control system, CO2 inerting mechanism, and comprehensive gas fire prevention and extinguishing technology are summarised and discussed. This study provides a systematic theoretical basis for the field application of CO2 fire prevention and extinguishing technology.

Coal spontaneous combustion (CSC) is a multifaceted research domain that has been widely explored in the literature, ranging from analytical and numerical modeling to the development of fire suppression materials and methods. A comprehensive review of the literature has revealed several distinct research trajectories, or “roadmaps”, identified through criteria such as the volume of studies addressing each theme, the presence of review papers dedicated to a specific roadmap, and the explicit mention of coal spontaneous combustion in the title or keywords. This classification framework has outlined six primary roadmaps: (1) spread, quantification, and impact; (2) mechanisms, models, factors, and parameters; (3) experimental studies and models; (4) detection, monitoring, and prediction; (5) prevention and control; and (6) applications. While interconnections exist between these roadmaps, and all ultimately converge towards roadmap 5 (prevention and control), each roadmap constitutes a distinct research cluster. The focus of this review is on roadmap 4, specifically addressing the methods and technologies for detection, monitoring, and prediction of CSC events. This review encompasses studies published from 2010 to the present, providing a thorough examination of the various detection techniques employed, with particular emphasis on their limitations and the strategies proposed to overcome these challenges. A critical analysis highlights the key advantages and disadvantages of each category of techniques, offering insights into their practical applications and the potential for future advancements in this field. The present review aims to contribute to the refinement of detection and monitoring methods for CSC, with the goal of enhancing early detection capabilities and improving fire management strategies.

CO, H2, and other combustible gases will be produced during coal oxidation in coal mines, which will increase the risk of explosion when mixed with methane. Therefore, it is very important to understand the production characteristics of combustible gas during coal oxidation. In this paper, a programmed temperature gas test system is built to study the impact of lignite on the production of gases at different particle sizes and temperatures, and the release characteristics of gases are also analyzed. The result shows that the production of combustible gas is influenced by the coal particle size significantly when the temperature is above 200 °C, and it decreases as the particle size increases. CO is the main gas during the early stage of coal spontaneous combustion, and the release of CH4 and H2 increases after 300 °C. The fitted equations of gas generation and temperature are consistent with the experimental results. The research results are helpful in understanding the hazards of coal spontaneous combustion and have a certain guiding significance for coal mine monitoring and prevention of coal spontaneous combustion.

The oxidation characteristic parameters of residual coal in working face change with the advance of coal seam. To explore the influence of dynamic oxygen-consuming environments, we examined the influence of reducing the oxygen concentration on the formation characteristics and characteristic parameters of coal oxidation products, conducted with temperature-programmed experiments under staged oxygen consumption. The correlation between the characteristic oxidation parameters was determined, and the critical oxygen concentration that determined the gas yield was obtained. The results show that after staged oxygen-consumption, the oxidation of coal is stronger than that under constant low oxygen, the oxidation products are greatly affected, and the influence of pyrolysis gas is small. The oxidation characteristic parameters such as gas product volume fraction, production rate, and heat release intensity are positively correlated with the oxygen-consumed temperature. We found that the oxidation reaction heat of coal is 180~330 kJ·mol−1, and the maximum and minimum exothermic intensities are significantly linearly correlated with the oxygen consumption rate. Finally, the critical oxygen concentrations for gas production rate under oxygen-consumed conditions were 17, 9, and 5%. These results have practical significance for strengthening the prevention and control of spontaneous combustion disasters in goafs.

In order to comprehend the molecular composition of coal and better understand the process of coal combustion, this study involved the development of a molecular structure model for Heiyanquan coal in Xinjiang, as well as the optimization and annealing dynamics simulation of the model. Thermogravimetric analysis (TG), Fourier transform infrared spectroscopy (FTIR), and high-resolution transmission electron microscopy (HRTEM) were utilized to investigate the spontaneous combustion characteristics of coal at different temperatures (room temperature, 50–500 °C with 50 °C interval). The findings revealed that the coal primarily consists of aromatic carbon, with the aromatic structure mainly comprising naphthalene, anthracene, and phenanthrene, and the aliphatic carbon mainly consisting of CH2 and CH, along with a small quantity of minerals. The empirical molecular formula of Heiyanquan coal was determined to be C175H125O21N3. After the optimization, the total energy of the model was significantly reduced, and the aromatic layers tended to align in a regular parallel manner, with van der Waals energy playing a crucial role in maintaining structural stability. As the temperature increased, the activation energy of the three stages also increased, with the combustion stage exhibiting the highest activation energy. The presence of hydroxyl groups and oxygen-containing functional groups was found to mainly participate in the reaction, while the content of aromatic hydrocarbons remained relatively stable, C=C exhibited a decreasing trend, and C-O displayed an increasing trend. Moreover, it was observed that 1 × 1 and 2 × 2 were the predominant aromatic stripes in the coal samples, accounting for more than 90% of the total stripes. [ABSTRACT FROM AUTHOR]

Relative to conventional coal pillar retention mining technology (the 121 mining method), gob-side entry retaining by cutting roof (the 110 mining method), a non-pillar mining technique, efficiently addresses issues like poor coal resource recovery and significant rock burst damage. Nonetheless, the open-type goaf created by 110 mining techniques suffers from complex and significant air leaks, increasing the likelihood of coal spontaneous combustion (CSC) within the gob area. To address the CSC problem caused by complex air leakage within the goaf of gob-side entry retaining by roof cutting, this study takes the 17202 working face of Dongrong Second Coal Mine as the object of study. Field tests and simulation calculations are conducted to research the features of air leakage and the distribution of the oxidation zone within the goaf. Subsequently, plugging technology with varying plugging lengths is proposed and implemented. The tests and simulations reveal that the airflow migration within the goaf follows an L-shaped pattern, while air leakage primarily originates from gaps found in the gob-side entry retaining wall. The amount of air leaking into the gob-side entry retaining section is 171.59 m3/min, which represents 7.3% of the overall airflow. The maximum oxidation zone within the goaf ranges from 58.7 m to 151.8 m. After the air leakage is blocked, the airflow migration route within the goaf is transformed into a U-shaped distribution, and the maximum oxidation zone range changes from 42.8 m to 80.7 m. Engineering practice demonstrates that after air leakage plugging, the total air leakage volume within the gob-side entry retaining section significantly reduces to 20.59 m3/min, representing only 0.78% of the total airflow volume. This research provides reference on how to prevent the occurrence of CSC in similar mine goafs. [ABSTRACT FROM AUTHOR]

In order to study the effect of hydrated phase change materials on the suppression of spontaneous combustion in coal, a thermogravimetric experiment and a reaction activation energy analysis experiment were conducted to explore the changes in the combustion characteristic parameters, characteristic temperature, and activating energy of gas coal, long-flame coal, meagre coal, and lean coal before and after adding hydrated phase change materials. The research results indicated that hydrated phase change materials increased the characteristic temperature point of the coal samples and had effective inhibitory effects on different stages of the oxidation process. However, the effect was best at low temperatures, as hydrated phase change materials undergo phase change and absorb heat when heated at low temperatures, isolating coal from contact with oxygen. The activating energy increased by 1.138–23.048 KJ·mol−1 and the mass loss was reduced by 1.6%–9.3% after inhibition of the coal samples, indicating that the oxidation rate of the various coal samples was slowed down and, thus, spontaneous combustion can be suppressed through the use of hydrated phase change materials. At the same time, this material reduced the combustibility indices of meagre coal and lean coal, as well as the comprehensive combustion indices of long-flame coal and gas coal. [ABSTRACT FROM AUTHOR]

DL‐malic acid (DMA), as a recognized antioxidant inhibitor and chelating agent in the food and chemical industries, has good application prospects in coal fire prevention and control. This paper investigates the impact of a DMA solution on the surface properties and phase structure of coal through a range of analytical techniques, including electron microscope scanning, particle size analysis, and x‐ray diffraction. The thermal behavior characteristics of DMA‐treated coal were analyzed using thermogravimetric experiments. The results show that after adding malic acid, the particle size of coal sample is reduced, the sphericity and roundness are reduced, and the pore size is increased. The hydroxyapatite and calcite components in coal were dissolved and consumed, and the calcite components were gradually depleted with the increase of DMA concentration. The flocculent structure produced by the condensation of the crushing residue and broken small particles are attached to the internal pores of coal, which reduced the coal oxygen contact sites and increases the content of the sealed solution, resulting in the increase of the thermal characteristic temperature of the coal and the inhibition of the oxidation heat generation process. DMA has obvious inhibitory effect on fresh coal and oxidized coal, and the inhibitory effect is stronger on high‐temperature oxidized coal. Therefore, DMA can be effectively applied to the prevention and control of spontaneous combustion of residual coal in goaf. The research results provide a reference for the research of coal fire prevention technology and the preparation of environmentally friendly flame retardant materials. [ABSTRACT FROM AUTHOR]

[Objective] As the main energy source in China, coal has an important strategic position in the country's economic development. China's coal-based energy structure model will not change in the short term, but coal mine accidents occur frequently. Mine fire is a major disaster that threatens safe coal mine production, which incurs huge economic losses and threatens the personal safety of miners and coal mine production. However, coal spontaneous combustion primarily causes mine fires. The occurrence mechanism of coal spontaneous combustion is highly complex and is affected by the inherent combustion characteristics of coal and external environmental factors. Therefore, it is necessary to comprehensively analyze various phenomena and characteristics in the process of coal spontaneous combustion and apply quantitative indicators to express to predict the risk and ensure the safe production of the mine. [Methods] Oxygen concentration is an important factor influencing coal oxidation reaction. In this paper, the temperature-programmed experimental system is used to conduct the spontaneous combustion experiment of coal samples with three oxygen concentrations of 20.9%, 10%, and 7% regarding the heat dissipation, oxidation, and asphyxiation zones in the goaf. The gas products at different temperature stages are collected, the effects of temperature and oxygen concentration on the concentration of gas products are analyzed, and the index gases are classified and evaluated in detail. [Results] The results show the following: (1) The cross-point temperature (CPT) decreases with the increase in the oxygen concentration. The cross-point temperature under dry-air conditions is 144.6 °C. The error between the CPT value calculated by the BM empirical relationship and the experimental data is only 6.08%. The higher the oxygen concentration, the smaller the critical temperature, and the higher the gas concentration. (2) The content of oxidation products increases with the oxygen concentration. When heating commences, minimal CO and CH4 gases are generated, but with the increase in the temperature, C2H4 and C2H6 gases appear gradually. The oxidation reaction of coal samples is primarily affected by temperature and oxygen concentration. At a certain temperature, the higher the oxygen concentration, the more intense the coal-oxygen composite reaction, and more gas products appear. (3) For different oxygen concentration environments, the regularity of generating CO gas is highly significant, which can be used as an indicator gas to evaluate the spontaneous combustion of coal. In dry air, the temperature of C2H4 is earlier than that of C3H6, implying that the existence of C2H4 indicates that the coal oxidation process has entered an accelerated stage. The emergence of C3H6 gas marks the transformation of the coal oxidation into strong oxidation. Recently, the programmed temperature rise experiment has become an important research field. [Conclusions] Through indoor experiments, students can deeply understand the mechanism of coal spontaneous combustion and the process of establishing the prediction index system and introduce the experiment into the classroom. This practical teaching approach not only helps students to apply theoretical knowledge to practical problems but also broadens their horizons and improves their interest in scientific research, achieving better teaching results. [ABSTRACT FROM AUTHOR]

With the potential safety hazard of coal spontaneous combustion becomes more and more serious, in order to solve the problems of low water absorption rate and high cost of hydrogel inhibitor, in this study, a superabsorbent lignin-based composite inhibitor (SAP + SLS + Ben) was prepared. The water absorption/retention rate, micromorphology, inhibition capacity, air-leakage blocking efficiency and fire-extinguishing performance of SAP + SLS + Ben were studied. Results have revealed that: SAP + SLS + Ben had excellent water absorption/retention capacity; SEM images revealed that sodium lignosulfonate (SLS) and bentonite (Ben) provided the structural basis for the high water absorption/retention of SAP + SLS + Ben; Compared with SAP, SAP + SLS + Ben could effectively inhibit the oxidation of reactive functional groups in coal, and the amount of CO released from coal treated with SAP + SLS + Ben was significantly reduced at high temperature; The air-leakage blocking and fire-extinguishing experiments showed that SAP + SLS + Ben had good filling and blocking performance, and could effectively reduce the temperature of coal spontaneous combustion and prevent the re-ignition of coal. Hence, SAP + SLS + Ben had ideal physical and chemical inhibition properties, which could effectively prevent coal spontaneous combustion in goaf. [ABSTRACT FROM AUTHOR]

The oxidation characteristics and spontaneous combustion (SC) tendency of raw long-flame coal (RC), water-soaked 200-day coal (S200), pre-oxidized water-soaked coal at 200 °C (O200S200), and pre-oxidized soaked coal at 300 °C (O300S200) in an oxygen-poor environment were investigated using a programmed warming system. The results show that pre-oxidation water-soaked treatment (PWT) promotes the coal-oxygen complex reaction and increases the rate of coal oxygen consumption (OCR) and the rate of carbon and oxygen compound production. The rate of CO and CO2 production of the water-soaked (WS) coal increased by 0.329 mol·(cm3·s)−1 and 0.922 mol·(cm3·s)−1, respectively, compared with that of the original coal sample. PWT reduces the activation energy of coal in the low-temperature oxidation stage (the maximum difference can be up to 110.99 kJ/mol) and enhances the oxidizing and heat-releasing capacity. There was a synergistic effect between the pre-oxidation (PO) and WS treatment, and the lowest comprehensive determination index of the SC propensity of coal in O200S200 samples was 831.92 which was 4.72 lower than that of RC samples, presenting a more SC tendency. Low oxygen concentration has an inhibitory effect on the oxidation characteristic parameters of coal, and the apparent activation energy of the low-temperature oxidation stage of pre-oxidized water-soaked coal (PWC) increased to 206.418 kJ/mol at 3% oxygen concentration. The lower the oxygen concentration of the anoxic environment, the lower the risk of SC of the coal samples. The results of the study can provide theoretical guidance for the identification and prevention of SC disasters in coal seams with shallow burial and close spacing. [ABSTRACT FROM AUTHOR]

Aimed at the deprived research on physicochemical synergistic inhibition and the micromechanism of inhibition. The inhibitory effect and mechanism of MgCl2, TPPI and the combination of them were studied in this research. Low-temperature oxidation gas production experiment and synchronous thermal analyzer were used to analyze the inhibition and thermal behavior effects of different inhibitors, respectively. Fourier transform infrared spectrometer experiment was used to analyze the changes of the surface functional groups on coal samples before and after inhibition. Density functional theory was used to reveal the inhibitory mechanism of TPPI. In the low-temperature oxidation experiment, the CO release of Composite Inhibitor 5 (TPPI:MgCl2 = 1:1) treatment coal samples was the least. From the characteristic temperature points and heat release, it can be seen that the composite inhibitor reflects the physicochemical synergistic effect. The addition of MgCl2 will enhance the inhibitory effect at low-temperature stage (< 400°C) and weaken the inhibitory effect at high-temperature stage (> 400°C). Composite Inhibitor 3 (TPPI:MgCl2 = 3:7) showed strong inhibitory effect in both low and high-temperature phases, the comprehensive comparison showed the best inhibitory effect. The contents of aliphatic functional groups, hydroxyl groups and carboxyl groups decreased in the coal samples treated by TPPI and composite inhibitor, while the relative contents of stable ether bond and aromatic functional groups increased. The inhibitory mechanism of TPPI is to remove the free radical as R–COO·, R–CO· and R–C· by using itself and its hydrolytic products, to interrupt the chain reaction and finally to inhibit coal spontaneous combustion. The conclusions of this article provide theoretical support for the preparation of physicochemical synergistic inhibitors and the revelation of microscopic mechanism of TPPI inhibition on coal spontaneous combustion. [ABSTRACT FROM AUTHOR]

A hydrotalcite(layered double hydroxide, LDH) inhibitor which is suitable for the whole process of coal spontaneous combustion and a LDH inhibitor containing rare earth lanthanum elements were prepared. The inhibition effect and mechanism were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), thermal performance analysis, in-situ diffuse reflectance infrared spectroscopy and temperature-programmed experiment. The results have shown that the inhibitor containing lanthanum can play a good inhibitory role in every stage of coal oxidation. During the slow oxidation of coal samples, the inhibitor containing lanthanum ions can slow down the oxidation process of coal and increase the initial temperature of coal spontaneous combustion. At the same time, because the hydroxyl groups in LDHs are connected with -COO- groups on the coal surface through hydrogen bonds, the stability of coal is improved. With the increase of temperature, LDHs can remove interlayer water molecules and reduce the surface temperature of coal. CO release rate of coal samples decreases significantly after adding inhibitor containing lanthanum element, and the maximum inhibition rate of the inhibitor is 58.1%. [ABSTRACT FROM AUTHOR]

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Decomposition of oxygen-containing functional groups simultaneously produces CO and CO2, accompanied by the generation of active sites. Two different methods were applied in the study to speculate the kinetics of gas generation and the formation of active sites in coal pyrolysis. The evolution of CO and CO2 gas generation during isothermal pyrolysis under different temperatures was first analyzed, and the generation of CO and CO2 in the pyrolysis process was divided into three stages: the rapid decline stage, slow decline stage, and stabilization stage. In the early stage of the pyrolysis reaction, functional groups exhibited a rapid decomposition rate and a high conversion rate ($$a$$ a), which rapidly decreased the gas concentrations. With increasing pyrolysis time, the decomposition of functional groups declined with a low conversion rate ($$a$$ a) and showed a steady reaction rate in the later stage. On this basis, a method to calculate the activation energy during pyrolysis based on the data of gases generated in a steady period with a continuous cooling temperature was proposed. Activation energies of 70.12–148.39 kJ/mol for the CO formation and 64.47–127.38 kJ/mol for the CO2 formation were obtained from four different coals. In addition, a thermogravimetric analyzer was used to explore the apparent activation energies in the decomposition process via Coats-Redfern (C-R) method; activation energies of 39.33–126.65 kJ/mol and pre-exponential factors of 1.31–5.15E+08 s−1 were obtained for different coals. The results revealed that the CO had a greater generation activation energy than CO2, and a higher metamorphism degree implies a higher decomposition activation energy, which indicates that the decomposition of carboxyl groups and low-rank coals result in higher active sites. [ABSTRACT FROM AUTHOR]

To accomplish various and efficient inhibitions of coal spontaneous combustion (CSC) and solve the safety hazards of CSC encountered in the production and operation of coal mining enterprises, inhibitors play an important role in the prevention and management of CSC. Herein, we report quercetin, which is selection as an antioxidant to prepare a composite solid lipid inhibitor. The concentration of the composite solid lipid inhibitor with the best inhibition effect on the active groups is selected by Fourier pressure infrared experiments, the change pattern of gas content and characteristic temperature released from coal samples during an oxidation process warming process was tested by using a programmed warming test system and simultaneous thermal analysis experiments, and finally, the inhibitory rate and combustion characteristic index were calculated. The results of the research showed that 10 mass% of quercetin had the most impact on the various groups, the highest inhibitor rate of the composite solid lipid inhibitor is 80.67% in the coal low-temperature oxidation process, compared to the raw coal, the heat release of coal treated with the composite solid lipid inhibitor was decreased by 507.65 J g−1, the maximum amount of heat emitted was lowered by 60%, and the flammability index was reduced by 56.4. A composite solid lipid inhibitor containing quercetin developed in this paper indicated high effectiveness in inhibiting CSC, which is vital for coping with the safety management of coal mine production. [ABSTRACT FROM AUTHOR]

Cracks in coal goaf provide air leakage pathways and therefore increase the likelihood of coal spontaneous combustion, thus strongly affecting the safety of mining operations. Herein, an inorganic thixotropic gel (ITG) was proposed for inhibiting coal spontaneous combustion by blocking this leakage. The orthogonal experiments were designed to examine the effects of gel composition on gelation time and rheological/thixotropic properties. The three interval thixotropy test of the optimal-composition gel (4 wt% sodium bentonite, 3 wt% laponite, and 0.46 wt% silicate cement) revealed its good thixotropic recovery. The viscoelasticity test indicated that this gel retained elasticity-dominated behavior before being completely transformed into a sol and was not easy to break into pieces under pressure. By testing the characteristics of the ITG, it was proven that the ITG had an excellent water retention capacity, and high-temperature resistance and the apparent effects on blocking air leakage and extinguishing coal fire. By the thermogravimetric test, the influence of ITG on coal activation energy was calculated based on the ignition activation energy theory. FTIR was also introduced to analyze the effect of ITG on active groups of coal. In addition, structural characterization showed that ITG featured a tight surface and a stable overall structure, which further agreed with its prominent ability to block air leakage. [ABSTRACT FROM AUTHOR]

In order to study the characteristics of low-temperature oxidation of weakly viscous coal under different gas concentration conditions inside the mining area and the change law of kinetic parameters, this paper adopts the coal spontaneous combustion temperature programming experimental system to test and analyze the change law of gaseous products with temperature during the low-temperature oxidation of weakly viscous coal under different gas concentration conditions, and calculate the limit parameters and activation energy during the low-temperature oxidation of weakly viscous coal. The results show that: CO gas can be used as the main predictor gas for spontaneous combustion prevention and control of weakly viscous coal, and C2H4 gas and R2 can be used as auxiliary indicators for predicting the degree of spontaneous combustion of weakly viscous coal; the maximum limit parameters of methane samples with different gas concentration conditions are distributed in the temperature range of 60-85 ℃, which is similar to the critical temperature of spontaneous combustion of coal. With the methane volume fraction increasing from 0 to 4%, the apparent activation energy of weak sticky coal shows a gradual increasing trend, increasing by 35.061, 18.426 and 25.837 kJ/mol, respectively.

In order to study the distribution characteristics of the “three zones” of coal spontaneous combustion in the goaf of double cut roof retained roadway, taking the working face 4502 of Shaqu No.1 Mine as the engineering background, a combination of on-site monitoring and numerical simulation was adopted to study the air leakage law and the distribution characteristics of the “three zones” of coal spontaneous combustion in the goaf of the working face 4502 with double cut roof retained roadway. The results show that the main air leakage areas in the goaf of the double cut roof retaining roadway are the corners of the working face and the retaining roadways on both sides. The air flow mainly leaks into the goaf from the corners on both sides, and there is air exchange between the goaf and the retaining roadways on both sides. There is a slender oxidation zone on the side of the double retaining roadway, and the oxidation zone is distributed in a U-shaped throughout the goaf. At different stages of mining, there are significant differences in the distribution of the “three zones” of coal spontaneous combustion in the working face with double cut roof retaining roadway. In the initial stage of mining, it is necessary to strengthen the prevention and control of coal spontaneous combustion within a range of 5 m on the side of the double cut roof retaining roadway and 32-65 m in the middle of the goaf. In the middle and later stages of mining, it is necessary to strengthen the prevention and control of coal spontaneous combustion within a range of 2.5 m on the side of the transportation retaining roadway, within a range of 12 m after 38 m from the track retaining roadway, and within a range of 30-80 m in the middle of the goaf.

This study intends to explore the influence of temperature and molar ratio on the competitive adsorption of CH4/O2/N2 gas in coal. It probed into the competitive adsorption characteristics of CH4/O2 and N2/O2 binary mixed gas in Xiaolongtan lignite of Yunnan under different temperatures (303.15~383.15 K)and pressures (0~480 kPa)through grand canonical Monte Carlo (GCMC)and molecular simulation methods. Results show that : ① Within the range of test temperature and pressure, temperature will inhibit the adsorption of CH4, O2 and N2 by coal, and the adsorption capacity of coal to three gases is CH4>O2>N2. ② The selectivity coefficient of CH4/O2 adsorption is basically independent of the gas molar ratio, and shows decrease with the increase of temperature. The adsorption selectivity of N2/O2 on coal under different molar ratios was not significantly related to temperature and molar ratio. ③ With the increase of adsorption capacity, the isosteric adsorption heat of any component in the binary mixed gas shows linear increase. Under the same adsorption capacity, larger molar ratio of CH4/O2 and N2/O2 would lead to lower adsorption heat. However, when the adsorption capacity of CH4 is less than 0.029 mmol / g, the isosteric adsorption heat of CH4 gas shows little correlation with the molar ratio. The research reveals the influence mechanism of competitive adsorption behavior of CH4 and air on low temperature oxidation of coal.

To study the characteristics of coal with different particle sizes in the process of oxidation and spontaneous combustion, differential scanning calorimetry (DSC) was used to study the lignite from Chahasu Coal Mine, Inner Mongolia. The influence of coal particle size on the peak and critical characteristic temperature under different heating rates was studied, and the activation energy was quantitatively analyzed. The results show that the peak temperature and critical temperature of coal decrease with the decrease of particle size under the same heating rate. The peak temperature, critical temperature, and peak area of coal with the same particle size increase with the increase in heating rate. In the range of less than 0.50 mm granular coal sample, the larger the particle size, the higher the critical temperature corresponding to the larger heating rate; the Arrhenius formula method is used to solve the activation energy of coal samples. The change rate of heat absorption of coal samples with 0.15-0.25 mm particle size is the largest and the activation energy is the smallest. The coal in this particle size range needs the least energy to reach the activation state, and the possibility of spontaneous combustion of coal samples is the largest.

In view of the problems of low cross-linking strength, poor mechanical performance and easy damage of the structure of single network gel fire preventing and extinguishing materials, two interactions are introduced to construct a dual physical cross-linking network structure, and the Al3+-CMC/MMT-PAM dual network fire preventing and extinguishing gel is prepared, through the gelling time test of the gel under different ratios, the gel ratio suitable for coal spontaneous combustion fire prevention is selected, and the gelling time should be controlled at 4-7 min. The resistance performance of the gel was studied by program heating oxidation experiment, thermal analysis (TG-DSC), air plugging performance test and SEM, and the results show that: the weightlessness rate of gel-treated coal samples is reduced overall, the total calorific value is decreased significantly, and the activation energy is improved, which increases the difficulty of coal spontaneous combustion; after gel treatment, the release of CO and CO2 decreases, the resistance rate and temperature change are negatively related, and the intersection temperature increases; the coagulation process is controllable, and the coal gap can be filled within 5 min.

Abstract To test the effectiveness of N2 and CO2 in preventing coal from spontaneously combusting, researchers used an adiabatic oxidation apparatus to conduct an experiment with different temperature starting points. Non-adsorbed helium (He) was used as a reference gas, and coal and oxygen concentration temperature variations were analyzed after inerting. The results showed that He had the best cooling effect, N2 was second, and CO2 was the worst. At 70℃ and 110℃, the impact of different gases on reducing oxygen concentration and the cooling effect was the same. However, at the starting temperature of 150℃, CO2 was less effective in lowering oxygen concentration at the later stage than He and N2. N2 and CO2 can prolong the flame retardation time of inert gas and reduce oxygen displacement with an initial temperature increase. When the starting temperature is the same, N2 injection cools coal samples and replaces oxygen more effectively than CO2 injection. The flame retardancy of inert gas is the combined result of the cooling effect of inert gas and the replacement of oxygen. These findings are essential for using inert flame retardant technology in the goaf.

To explore the development of coal seam fractures around gas extraction borehole and the issue of self-ignition caused by air leakage, a combination of theoretical analysis, field testing, and laboratory experiments was used. The distribution characteristics of cracks in the crushed zone after borehole excavation were analyzed. An in-situ test using SF6 tracer gas was conducted to measure air leakage in the gas extraction borehole, revealing the law of air leakage around the gas extraction borehole during the process of gas pre-extraction. Using a program for temperature increase and gas chromatography combined experiment, the self-ignition characteristic parameters of coal under different air supply rates were studied as a research object of the 215 working face in a certain mine, and the functional relationship formula between oxygen consumption rate, CO generation rate, and coal temperature and air supply rate was obtained. The results showed that the main ways of air leakage around the gas extraction borehole were side leakage of roadway coal wall, drilling fracture leakage, and sealing section leakage. The minimum air leakage rate around the gas extraction borehole was quantitatively obtained between 0.19 m/min and 0.68 m/min, with an average air leakage rate of 0.41 m/min, and the range of air leakage in the borehole was greater than 4.35 m. The air supply rate had a promoting effect on the natural characteristics parameters of coal.

Mine fire has always been one of the disasters that restrict coal mining and endanger the life safety of underground workers in China, and will cause great waste of resources. In the face of the current serious mine fire accidents, the research and development of new fire prevention and extinguishing materials is undoubtedly an important topic to ensure the safety and efficient production of modern mines. This paper comprehensively discusses the inducements of the current mine fire, as well as the fire extinguishing mechanism and existing defects of conventional fire extinguishing materials such as inert gas injection, mud injection, and spray inhibitor, and new fire extinguishing materials such as gel foam, liquid carbon dioxide, and expandable graphite gel. The problems faced by some advanced fire prevention and extinguishing materials at home and abroad in the current practical application are proposed, and the development of new fire prevention and extinguishing materials in the future is prospected.

Under the influence of mining, the overlying strata of shallow coal seam fully mechanized top coal caving face collapses and forms cracks that connect with the surface. The vertical air leakage caused by these cracks has a significant impact on the spontaneous combustion of coal in the goaf. In order to study the vertical air leakage law of surface in goaf of shallow coal seam under different seasonal conditions, SF6 gas tracing method is used to measure the vertical air leakage on the 122108 working face of Shaanxi Coal Caojiatan Mining Co., Ltd. in four seasons: winter, spring, summer, and autumn. The temperature, atmospheric pressure, and air leakage rate changes on the surface and underground goaf during different seasons are analyzed. The results show the following points. ① In winter, the temperature difference between the surface of the working face and the underground goaf is relatively large, with a maximum temperature difference of 37.7 ℃. In summer, the temperature difference between the surface of the working face and the underground goaf is relatively small, with a minimum temperature difference of only 0.9 ℃. The maximum pressure difference between the surface of the working face and the underground goaf in winter is 40.37 hPa. The maximum pressure difference between the surface of the working face and the underground goaf in summer is 22.47 hPa. The temperature difference and pressure difference between the surface and underground goaf of the working face in spring and autumn are not significantly different. ② The winter air leakage rate is relatively high, with an average maximum air leakage rate of 8.364 m/min. The air leakage rate in summer is relatively small, with an average maximum air leakage rate of 6.918 m/min. The air leakage rates in spring and autumn are not significantly different. ③ When close to the working face, the air leakage rate is relatively high. According to the vertical air leakage law on the surface, measures such as underground pressure equalization, sealing of inlet and outlet air corners, and surface crack coverage can be taken to ensure the safe mining of 122108 working face.

Gas extraction is one of the commonly used methods for controlling the mine disasters. But during the extraction process, it will increase the amount of air leakage in the goaf, thereby increasing the risk of coal spontaneous combustion and directly affecting the distribution of coal spontaneous combustion dangerous areas in the goaf. Taking the 30503 working face of Tashan Coal Mine of China Coal Datong Energy Co., Ltd. as the research object, according to the actual situation of the goaf, a geometric model is established. The numerical simulation methods is used to analyze the impact of different buried pipe extraction positions and extraction negative pressure on the dangerous area of coal spontaneous combustion in the goaf. The results show the following points. ① With the deepening of the extraction position of the buried pipe on the inlet side, the oxygen volume fraction on the return side shows an increasing trend. The width of the oxidation zone does not change much. The overall oxygen volume fraction on the inlet side shows a decreasing trend, and the width of the oxidation zone first decreases and then increases. The oxidation zone area in the goaf first decreases and then increases. ② In the inlet side of the fixed buried pipe extraction position, the change of the negative pressure of extraction has a greater effect on the oxygen distribution on the inlet side of the extraction zone, while it has almost no effect on the return side. ③ As the negative pressure of extraction increases, the width of the oxidation zone on the inlet side first decreases and then increases, while the width of the oxidation zone on the return side remains almost unchanged. The area of the oxidation zone in the goaf first decreases and then increases. The relationship between the oxidation zone area and the extraction negative pressure is a quadratic function. ④ The optimal position for buried pipe extraction is at a distance of 20 m from the air inlet side of the goaf to the working face. The optimal negative pressure for extraction is 5 000 Pa. At this time, the oxidation zone area of the goaf is the smallest, that is, the coal spontaneous combustion danger area is the smallest.

Abstract The physical and chemical properties of the air-dried residual coal after soaking in the goaf will change, resulting in an increase in its spontaneous combustion tendency. This study aimed to look into the features and mechanism of soaked-dried coal's spontaneous combustion. Five samples of coal were dried to various degrees, and the weight loss features during thermal processing were examined. Based on this, the pore structure and chemical structure characteristics of the coal samples with the highest tendency to spontaneous combustion were quantitatively examined, and the mechanism by which soaking-drying affected the spontaneous combustion heating process of the remaining coal in goaf was investigated in turn. The results show that T1 decreases with the increase of drying time, T2–T6 shows a fluctuating change, and the ignition activation energy of 36-S-Coal is smaller than that of other coal samples. The pore type of 36-S-Coal changes from a one-end closed impermeable pore to an open pore, and the pore group area is large. During the 36 h drying process, the internal channels of the coal were dredged, and a large number of gravels and minerals were precipitated from the pores with the air flow. A large number of gravels were around the pores to form a surface structure that was easy to adsorb various gases. Furthermore, infrared spectroscopy was used to analyze the two coal samples. It was found that soaking and drying did not change the functional group types of coal samples, but the fatty chain degree of 36-S-Coal was reduced to 1.56. It shows that the aliphatic chain structure of coal is changed after 36 h of drying after 30 days of soaking, which leads to the continuous shedding of aliphatic chain branches of residual coal, and the skeleton of coal is looser, which makes the low-temperature oxidation reaction of 36-S-Coal easier. Based on the above results, the coal-oxygen composite mechanism of water-immersed-dried coal is obtained, and it is considered that the key to the spontaneous combustion oxidation process of coal is to provide oxygen atoms and accelerate the formation of peroxides.

To investigate the influence of moisture on the exothermic properties of coal oxidation, this study investigated the variation in thermal effect and radical parameters in the oxidation process of coal under different moisture content. This was achieved through thermogravimetric–differential thermal experiments and electron spin resonance experiments. Additionally, the study analyzed the impact of free radicals on the characteristics of the thermal effect of coal at different oxidation stages using the product–moment correlation method. The results indicate that the moisture content of 8% is a critical point for promoting or inhibiting the oxidation reaction of coal. Below this threshold, it promotes the oxidation reaction, while above it, it plays an inhibitory role. The most significant promotional effect was observed at 8% moisture content, which increased both the weight loss and heat release by 8.61% and 1567.04 J/g, respectively, while also significantly enhancing free radical species and complexity. Conversely, when the moisture content of coal reached 20%, there was a notable inhibition effect, with a reduced weight loss and exothermic capacity by 4.94% and 2705.03 J/g, respectively, along with decreased free radical species and complexity. The free radical species and heat effect parameters in coal showed a strong correlation at all stages of low-temperature oxidation, thus indicating that g-factor can characterize the coal oxidation process to some extent.

A thermal dynamic disaster in the goaf is one of the most serious coal mine disasters formed by coal spontaneous combustion and gas interweaving. However, the influence of the high-temperature hidden fire source formed in the goaf on the evolution law of thermal dynamic disasters is not clear, and effective prevention and control measures cannot be taken. Therefore, this paper uses the experimental platform of thermal dynamic disaster in the goaf to study the influence of different fire point positions on the development of thermal dynamic disaster in the goaf through a similar simulation experiment of thermal dynamic disaster evolution in the goaf and analyzes the corresponding relationship between temperature and CO concentration in the upper corner. The results show that under different locations of heat source, the high-temperature heat source of coal spontaneous combustion migrates to the air leakage side with sufficient oxygen supply, and an oxygen-poor circle is formed near the ignition point. Under the action of air leakage flow, CH4 accumulates in the deep part of the goaf on the return air side. Due to the increase in coal, part of CH4 is produced, which leads to the increase in concentration of CH4 at the ignition point. Under the action of different heat sources, the changing trend of concentration of CO and temperature in the return air corner is the same, but the temperature change in the return air corner shows a lag compared with the change in the concentration of CO, so concentration monitoring of CO can reflect the evolution process of the fire field in the goaf more quickly than temperature monitoring.

In order to improve the inhibition effect of gel on coal spontaneous combustion, a chitosan (CS)/polyacrylamide (PAM)/metal ion (Al3+) composite double-network gel was developed in this study. The optimum formula of the composite double-network gel was determined using orthogonal experimentation. The microstructure, water retention, compressibility, and anti-destruction properties of the composite double-network gel were analyzed. The inhibition effect of the composite double-network gel on coal spontaneous combustion was studied via infrared spectroscopy and a synchronous thermal analyzer from the micro and macro perspectives. The results show that the composite double-network gel has a denser interpenetrating double-network structure and a larger void ratio than the ordinary gel. The water retention rate was 55% after standing at 150 °C for 12 h. The deformation memory ratio of the composite double-network gel was 78%, which was 26.8% higher than that of the ordinary gel, and the compressive strength also increased by 59.96%. In addition, the critical temperature point and the maximum thermal weight-loss rate temperature point decreased by 7.01 °C and 39.62 °C, respectively, and the composite double-network gel effectively reduced active functional groups in the treated coal sample, such as hydroxyl and aliphatic hydrocarbons. In this study, a CS/PAM/Al3+ composite double-network gel was produced, which exhibited good gel performance and inhibition effects, with physical effects such as the covering, wetting, and cementation of coal.