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Coal spontaneous combustion can cause a series of problems in terms of wasted resources, casualties, and environmental pollution. Accurate detection of the fire source in loose coal is the key to preventing coal spontaneous combustion. Acoustic temperature measurement has significant advantages of strong stability, high accuracy, and wide measurement range, which can make up for the shortcomings of traditional fire source detection methods. Detecting the optimum source signal for loose coal temperature measurement is the basis and prerequisite for the realization of acoustic temperature measurement. Therefore, the anti-interference characteristics of three typical sound source signals were tested and analyzed through a self-designed sound wave propagation characteristic test system. The cross-correlations among maximum length sequence signal, pulse signal, and linear sweep signal were compared and analyzed. Compared to the other two signals, the main peak of the cross-correlation coefficient of the linear sweep signal was more prominent and its pseudo-peaks interfered less with its main peak. This signal had strong anti-interference ability, and it can be used as a basic acoustic source signal for temperature measurement of loose coal. To further screen out the optimal frequency band and length of the linear sweep signal, four bituminous coals were selected as the propagation medium. The main peak value and the difference between the main peak and the maximum pseudo-peak of the cross-correlation coefficient were proposed as the evaluation indicators. The optimum signal frequency bands of long-flame coal, non-caking coal, coking coal, and lean coal were 400–900, 400–900, 500–1200, and 400–900 Hz, and the optimum signal length of four coals was 0.1 s. The study results can provide theoretical support for the selection of acoustic temperature measurement signals for loose coal. [ABSTRACT FROM AUTHOR]

The non-uniform distribution of coal porosity has a considerable impact on coal spontaneous combustion (CSC) characteristics. In this study, a one-step global coal oxidation numerical model which took into account the thermal buoyancy and the generation and consumption of multi-component substances was established. The movement of the hot spot and the evolution characteristics of reactants during CSC were researched on. The results suggest that in the early stage of coal self-heating, the hot spots are generated in the high-porosity areas. As coal oxidation intensifies, the hot spots move to the air supply positions. Different coal porosity distributions result in different thermal runaway times, and the coal samples are prone to thermal runaway when the high-porosity areas are near the air supply positions. Resultantly, the movement path of the hot point becomes longer, leading to a larger reduction of average density. Meanwhile, the corresponding thermal runaway and the drastic change in oxygen mole fractions occur later, and the drastic change in thermal runaway is more likely to lag behind that in the oxygen mole fraction. Compared with the gradually changing porosity distribution, the abruptly changing porosity distribution tends to hinder the development of coal oxidation perpendicular to the sudden change cross-section. [ABSTRACT FROM AUTHOR]

The coupling disaster of coal spontaneous combustion and methane seriously threatens the safe production of coal mines. In order to study the low-temperature oxidation characteristics of coals oxidized at the methane-containing atmosphere, characteristics of free radicals and thermal effects of two coals oxidized at atmosphere containing different concentrations of CH4 were studied by employing calorimetric and electron spin resonance (ESR) spectrometer experiments. Furthermore, the oxidation characteristics of coals were related to its thermal effect based on the enthalpy changes of active groups' reactions calculated by Gaussian 16. It was found that CH4 in the oxidation atmosphere could reduce the amount of heat both absorbed and released during the process of low-temperature oxidation. And, the higher the concentration of CH4 in the atmosphere was, the smaller amount of heat absorbed and released. Experimental results of ESR suggested that the concentration, the g-factor, and the linewidth of free radicals were decreased as the concentration of CH4 in the oxidizing atmosphere increased. Analysis of elementary reactions and its corresponding enthalpy change indicated that the reduction of absorbed and released heat resulted by CH4 attributed to the fact that CH4 could block the generation of peroxides and depress the decomposition of the oxygen-containing free radicals. The changes of released carbon monoxide as the function of the concentration of CH4 were in agreement with this indication as well. This study will provide a scientific guidance for the prevention and control of the coupling disasters of coal spontaneous combustion and CH4 and benefit the development of the high-efficiency coal spontaneous combustion inhibitor. [ABSTRACT FROM AUTHOR]

• Coal spontaneous combustion can still occur at extremely low oxygen concentrations. • The hazard of coal spontaneous combustion is raised after undergoing high temperatures. • The hottest spot of coal moves against the airflow. • Local oxygen concentrations must be considered when judging the status of coal oxidation. Large-scale experiments could reflect the real status of coal spontaneous combustion and therefore were conducted to investigate the characteristics of coal oxidation during the development and decaying processes. Three coals with different metamorphic grades were collected and tested to acquire the information including the temperature profiles, movement of the hottest spots, gaseous products, limiting parameters and so forth. The results indicated that the trajectories of hottest spots did not overlap for the development and decaying processes. Meanwhile, compared to the development process, CO productions during the decaying process were higher in the air circumstance and lower in the nitrogen. Coupled with the changes of C 2 H 4 and oxygen consumptions, it could be concluded that both the oxygen concentrations and the stages (development or decaying process) must be taken into account when judging the status of coal spontaneous combustion. Additionally, the discussions about the limiting parameters of the coal spontaneous combustion showed that during the decaying process, the lower limit of coal thickness was smaller, the lower limit of oxygen concentration became less and the upper limit of air leakage turned larger. These results implied that the experience of high temperatures would raise the hazard of coal spontaneous combustion. [ABSTRACT FROM AUTHOR]

Highlights • Ultrasonic extraction technology was used to study the coal oxidations. • DMAC had the largest extraction rate for free radicals. • The largest extraction rate of hydroxyl was possessed by NMP. • Hydroxyl was the dominant functional group at the initial stage of coal oxidation. Abstract The reactions between oxygen molecules and functional groups are the internal cause leading to heat release and the corresponding coal spontaneous combustion. However, the difficult discrimination of the multitudinous functional groups tremendously limits the deep insight into the mechanism of coal-oxygen reactions. Ultrasonic extraction technology, in this case, was utilized to extract the different functional groups from coals. The extracted coal samples were further collected for tests of Fourier infrared (FTIR) spectrum, electron paramagnetic resonance (EPR) spectrum, and thermogravimetric (TG) analysis. The experimental results indicated that Dimethylacetamide (DMAC) had the largest extraction rate in terms of the changes of mass or free radicals. While Acetone exhibited great extraction ability for CH 3 and Cyclohexanone (CYC) could effectively extract C O C. The greatest extraction rate of OH was possessed by N-methylpyrrolidone (NMP). Both TG and differential scanning calorimeter (DSC) tests proved that OH played a dominant role in the oxidation stage of coal. Compared to OH, other functional groups like CH 2 , CH 3 and C O C had fewer effects on the mass change and heat release during coal spontaneous combustion. [ABSTRACT FROM AUTHOR]

The spontaneous combustion of coal, if not eradicated immediately, may lead to coal ignition and even a full-blown fire. A new method, DSC Inflection Point (DSCIP), was proposed to determine the coal auto-ignition temperature (CAIT). Heat fluxes and kinetic parameters before and after CAIT were comparatively investigated through TG/DSC analysis and mathematical model construction. Meanwhile, the impacts of temperature rise rate and oxygen concentration on CAIT were studied and two indexes representing the hazard and destructiveness of coal spontaneous combustion, respectively, were proposed. The results demonstrated that the heat flux curve of coal spontaneous combustion can be well fitted using Gaussian mixture model. Compared to the oxidation stage, the released heat during the combustion stage was greatly increased. Furthermore, the activation energy became larger and the reaction order decreased to zero when the temperature exceeded CAIT. The study also found that CAIT was enhanced with the increase of temperature rise rates or the decrease of oxygen concentrations. Changes of heat flux, free radicals, and the activation energy proved the rationality and feasibility of the DSCIP method in determining CAIT. Additionally, under the same environmental conditions, lignite had the largest hazard of coal spontaneous combustion and the anthracite had the biggest destructiveness. Both the hazard and the destructiveness of coal spontaneous combustion became stronger as oxygen concentrations increased. [ABSTRACT FROM AUTHOR]

Abstract: Laboratorial experiments of coal spontaneous combustion under non‐isothermal conditions were conducted using a self‐made instrument aimed at investigating the characteristics of carbon monoxide emissions and kinetic characteristics. The impacts of coal rank, particle size, and oxygen content on the CO production during the decaying process of spontaneous combustion were studied. The experimental results demonstrated that coal particle size had a significant influence on the spontaneous combustion at high temperatures and coals of a large particle size decreased slowly in terms of the total CO production. Meanwhile, it is also found that even if the oxygen content was as low as 3 %, the oxidation reaction of coal still existed, liberating CO and heat at the same time. Therefore, stricter standards were recommended when judging whether or not a spontaneous combustion had been extinguished in gobs. Additionally, the studies on oxidation kinetics of coal implied that the activation energy was dependent both on the oxidation temperatures and coal properties. Oxidation temperatures affect the magnitude of activation energy, while coal properties impact the response of activation energy to the oxygen contents. [ABSTRACT FROM AUTHOR]

Spontaneous combustion, if not eradicated immediately, may lead to coal ignition and even a full-blown fire. An understanding of the characteristics of coal oxidation under various environmental conditions is conducive to the reveal of the mechanism and the prevention of coal spontaneous combustion. TG/DSC–FTIR coupling technology was employed to investigate the evolutions of coal mass, heat and gaseous products during coal spontaneous combustion. The experiments conclude that the mass loss rate in Combustion Stage was much higher than other stages and the major of coal mass (over 80%) was consumed at this stage. As the oxygen concentration decreased, the combustion of coal was evidently postponed and the exothermic region shifted to a higher temperature. Correspondingly, the temperatures at which productions of CO and CO2 reached to the maximum were deferred as well. Experimental results also indicated that heating rates behaved differently before and after the ignition temperature. The mass loss rates of coal were independent of heating rates before ignition temperatures but obviously expanded and moved to higher temperatures after exceeding the ignition temperature. Additionally, with the increase of the heating rates, the exothermic region shifted to higher temperatures and the release of CO and CO2 were delayed and reached to the maximum in a longer time. [ABSTRACT FROM AUTHOR]

[Display omitted] • The co-spontaneous combustion process of coal and gangue is investigated. • The interaction of coal and gangue is affected by microstructure and reaction stage. • Promotion dominates the co-spontaneous combustion process of coal and gangue. • Co-combustion stage of mixtures of coal and gangue follows order reaction. • The C b is greatly affected by gangue content while the S c depends more on coal. The co-spontaneous combustion of coal and gangue widely exists in the world and threatens the safety of coal mines. In this paper, the microstructure of coal and gangue were tested and the thermal behavior during the co-spontaneous combustion process of coal and gangue with different mass ratios were investigated by using synchronous thermal analyzer, and the combustion characteristic indices and kinetic parameters were calculated. The results indicated the carbon content of gangue is much lower than that of coal, but the content of oxygen-containing functional groups and sulfides in gangue are far higher than that in coal. Moreover, gangue possesses more developed pores and specific surface area. With the increase of coal mass fraction, the combustion index of ignition of mixed samples increases linearly, while the combustion index of burnout and synthetical combustion grows exponentially. The mixtures of coal and gangue follows the interfacial reaction at the oxygen adsorption stage and the order reaction at the moisture evaporation and combustion stages. The practical apparent activation energy at the moisture evaporation and combustion stages is higher than the theoretical value. However, it shows the opposite result at the oxygen adsorption stage. Coal and gangue inhibit each other at the moisture evaporation and later combustion stages, but promote each other at the oxygen adsorption and earlier combustion stages. On the whole, the co-spontaneous combustion process of coal and gangue is dominated by promotion. As a result, the practical values of mass loss and heat release in the whole reaction process are greater than the theoretical ones. Therefore, the danger of spontaneous combustion is higher when coal and gangue coexist. [ABSTRACT FROM AUTHOR]

Owing to coal commonly existing in a fluctuating thermal environment, a two–step parallel reaction model for coal ignition was established to explore the development of coal ignition under fluctuating thermal convection, fluctuating thermal radiation, and convection–radiation coupling (CRC). To be specific, this included temperature evolution in a coal mass and fluctuating periodic characteristics. The nonlinear gain of coal spontaneous combustion caused by oscillatory heat flux conditions was researched. The results show that the thermal fluctuation at 0.1 Hz is conducive to coal ignition. The ignition delay time (IDT) exponentially decreases with the growth of both the thermal convection velocity and radiation heat flux. Under external thermal disturbance, the temperature, convection velocity, oxygen concentration, and depth of the sensitive area all vary periodically. Under different types of heat disturbance, the temperature and convection velocity fluctuate with an increasing trend, while the oxygen concentration fluctuates with a decreasing trend. There is a low phase difference between the temperature and the convection velocity while the phase difference between the temperature and oxygen concentration is about 90° Besides, the peak fluctuation of the oxidation rate of coal changes inversely with the depth of fluctuations of the sensitive area. [ABSTRACT FROM AUTHOR]

[Display omitted] • The relationship between thermal effects and groups during CSC is analyzed. • The key groups will change dynamically in the process of CSC. • The structural contribution of groups to the thermal effect of CSC is determined. • The mechanism of micro of preventing heat release during the CSC was expounded. This study is aimed at exploring the relationship between the variations of micro-groups and the thermal effect during the heating process of coal spontaneous combustion (CSC). To this end, the thermal effect and microstructure variation characteristics of CSC were analyzed by means of differential scanning calorimetry and in-situ infrared spectroscopy. The results reveal that the exothermic reaction of CSC falls into five stages: water evaporation, desorption-induced heat absorption, slow heat release, benzene ring pyrolysis and combustion. Aliphatic hydrocarbons in coal oxidize into carbonyl which further oxidizes into carboxyl. The dynamic variations of key groups during the different stages of CSC were determined through grey correlation analysis. In addition, structural contribution, a new parameter to characterize the contribution of microstructure to thermal effect of CSC, was defined. The results disclose that the correlations between the thermal effect and the microstructure variation in different stages differ, but the key groups follow the change process of hydroxyl → aliphatic hydrocarbons → oxygen-containing functional groups → aromatic hydrocarbons on the whole. The change of key groups is related to the thermal effect and microstructure reaction characteristics of CSC. The contribution of a microstructure to the thermal effect of CSC is determined by the comprehensive product of its initial content, variations in different stages and correlation. Different microstructures correspond to varying contributions in different stages of CSC. The key to preventing CSC is to hinder the oxidative consumption of –OH(f) and the oxidation of aliphatic hydrocarbons with the highest contents into -C = O and –COOH. [ABSTRACT FROM AUTHOR]

In order to accurately determine the ignition temperature (T i) of coal, Coal spontaneous combustion (CSC) process was tested by a simultaneous thermal analyzer. Next, the abrupt change point of the differential derivative thermogravimetric curve (DDTG) of coal after the high adsorption temperature was taken as the T i of CSC. Besides, variations of kinetics and thermodynamics during the CSC before and after the T i were calculated. It was found that the mass loss, heat releases and gaseous products of coal changed slowly before the T i and surged sharply after the T i. The coal at ignition temperature was in thermodynamic equilibrium with low activity. However, when the temperature was greater than T i , the aromatic hydrocarbons in coal begin to decompose, resulting in the increased of active sites and the release of volatiles, and the coal enters an irreversible combustion stage. At this time, upward trends were obtained for the apparent activation energy, pre-exponential factor, enthalpy change, and entropy change, while a gradual decrease was observed for the Gibbs free energy change. Moreover, reaction intensity between oxygen and coal would be increased due to increased of active sites in the coal and enhanced of volatiles release under a slower heating rate. [Display omitted] • The ignition temperature of coal is the abrupt change point of the DDTG. • The functional groups of coal reaction changed before and after ignition temperature. • The coal at ignition temperature is in the thermodynamic equilibrium state. • A small heating rate leads to the increase of active sites in coals. [ABSTRACT FROM AUTHOR]

[Display omitted] • A method to calculate the key parameters of coal oxidation reaction was proposed. • The course of coal oxidation reaction shows the characteristics of different stages. • Complexity of coal spontaneous combustion were enhanced by substance transfer and temperature-rise. • High-temperature gas and contact wall could heat the surrounding coal. Coal spontaneous combustion (CSC) is one of major disasters that threaten the safety and production of coal mines. A method to calculate the key parameters of coal oxidation reaction based on temperature-programmed tests was proposed. Moreover, a one-step global numerical model for exploring coal oxidation considering the effects of multi-component material was established. The evolution of the oxygen consumption rates (OCRs) in multiple stages of coal oxidation was investigated and the changes in temperature and reaction rate were assessed by changing thermal boundary conditions including gas temperature supplied and wall temperature. The results show that the coal oxidation can be simulated based on the obtained parameters; the process of coal oxidation can be partitioned into three stages (i.e. temperature-dominated, co-dominated by temperature and oxygen, and oxygen-dominated) as the reaction continues. Under thermal conditions involving high-temperature gas, the coal always exhibits a high temperature at the gas inlet and the dimensionless number Φ of the coal self-heating temperature presents three trends including slight variation, a gradual increase, and gradual reduction. Under conditions involving the high-temperature wall of coal-storage tanks in contact with coal (briefly called the contact wall hereinafter), the dimensionless number Φ of the coal self-heating temperature increases exponentially. [ABSTRACT FROM AUTHOR]

To understand the oxidation reaction of coal and predict and control generation and development of coal fires, a one-step global dynamic model of the chemical reaction between coal and oxygen considering multi-material actions was established. A volume-averaged oxygen consumption rate (OCR) to evaluate the characteristics of the oxidation reaction of coal was proposed. The results demonstrate that, in the reaction between coal and oxygen, the dominant effects of temperature and oxygen concentration on the reaction rate of coal oxidation also change. There is a certain time-lag between changes of the temperature during coal oxidation and combustion and the reaction rate: the changes of temperature parameters fail to reflect the instantaneous reaction rate in real time. By changing the volume fraction of oxygen and the programmed heating rate, it is found that increasing the volume fraction of oxygen and the heating rate both can promote the reaction between coal and oxygen and reduce the hysteretic effects of heating in the reaction under external heating. Therefore, controlling the temperature storage conditions is beneficial to preventing the oxidation reaction of coal in its early stage, while changing the oxygen supply conditions can control the development of coal spontaneous combustion in its later stage. Unlabelled Image • The dominant effects of temperature and oxygen concentration were changed in the process of coal oxidation. • There was a certain time-lag between changes of the temperature and the reaction rate. • A volume-averaged oxygen consumption rate was proposed. • The temperature storage and oxygen supply could prevent the oxidation reaction of coal in different stages. [ABSTRACT FROM AUTHOR]

In industries related to fossil energy, coal dust is often deposited at an angled position. A two–step parallel coal dust oxidation reaction mechanism is used to study the self–ignition and combustion characteristics of coal dust under inclination conditions. The consumption and transfer of multiple substances are considered. In addition, the gravity direction is used to simulate the influence of the inclination conditions on ignition characteristics of coal dust. Firstly, the reliability of the numerical model is verified, and the time series characteristics of the self–ignition and combustion of coal dust are studied. Then the characteristic parameters and evolution of self–ignition and combustion under different inclination are analyzed. The results show that under hot plate conditions, the ignition and combustion process of coal dust can be divided into five stages. The minimum ignition temperature of the coal dust layer (MITL) changes sinusoidally with the inclination angle. Coal dust deposited under the ceiling is easier to be ignited. As the temperature of the hot plate increases, the ignition delay time (IDT) is shortened. It is necessary to adjust the fire safety management according to the inclination conditions of the coal dust, especially in the area with an inclination angle of 150°–180°. [Display omitted] • Self-ignition and combustion characteristics of coal dust at different inclined angles are revealed. • MITLs change sinusoidally with the inclination angle. • Coal dust deposited under the ceiling is easier to be ignited. • Increasing of hot plate temperature leads to the advance of coal dust thermal runaway. • Hot spot moves from the inside to the surface, spreads on the surface, and moves from the surface to the inside. [ABSTRACT FROM AUTHOR]

• Combining theory with multi physical field models to eliminate scale effects. • Attenuation characteristics under sealed and nitrogen retarding measures. • Nitrogen quickly controls the local oxidation reaction and removes heat. • "Quasi spherical" reaction zone intensifies and evolves into a "crescent" Low temperature oxidation can occur in coal and coal piles, which can lead to severe fires in coal fields, storage rooms, and goaf areas. An important feature of these coal fire disasters is their large scale, providing a favorable environment for the inoculation of coal fires. A new method for studying coal oxidation reactions, which combines theoretical analysis with multi physical field numerical models, has been proposed to eliminate the influence of scale effects. Based on the theoretical analysis, the reliable numerical simulation conditions are obtained, and the results are compared with those of large coal spontaneous combustion experiment. The temperature, gas and reaction rate of coal oxidation have been analyzed and verified. The attenuation characteristics of coal oxidation reaction under sealed and nitrogen retarding measures have also been analyzed with emphasis. The research results indicate that the proposed new simulation method has been proven to be effective through multiple results validation. Sealing measures can weaken coal oxidation reactions, but heat elimination is slower. The measure of injecting nitrogen can quickly control local coal oxidation reactions and remove heat. The greater the flow rate of inert gas, the better the retarding effect on coal. The development process of coal oxidation will form "quasi spherical" and "crescent shaped", which are prominent characteristics of slow oxidation and rapid propagation, respectively. [ABSTRACT FROM AUTHOR]

In the hope of exploring the effect of coal gangue left in goaf on coal spontaneous combustion, the variations of thermal effects and free radical parameters during coexisting spontaneous combustion of coal and coal gangue were tested, and kinetic at different heating stages were calculated. The following beneficial findings were obtained. For the mixed samples, the heat flow curve, ignition temperature, and free radical concentration all decline with the increase of coal gangue content, and the free radical parameters exhibit periodic changes with increasing temperature. Both the experimental value of thermal effect and free radical concentration with coal and coal gangue mixture during the spontaneous combustion process are higher than the calculated value. The calculation results of kinetic show that at the endothermic stage, the activation energies of the mixed samples decline with the increase of coal gangue, and the experimental value of activation energy is greater than the calculate value; at the exothermic stage, the case is exactly the opposite. Furthermore, due to the higher reactivity of coal gangue groups and stronger thermal conductivity, the coexistence of coal and coal gangue spontaneous combustion reaction is promoted. Resultantly, coal spontaneous combustion is normally more threatening in gangue-containing goafs. • Free radical chain reaction during co-existence combustion of coal and coal gangue was studied. • Significant synergistic effects exist in the co-combustion heat release process of coal and coal gangue. • Coal gangue promotes the chain reaction of free radicals and heat release in coal. • Coal gangue has better thermal conductivity, which facilitates the spread of spontaneous coal combustion. [ABSTRACT FROM AUTHOR]

• The gas and temperature evolutions of the coal oxidation reaction process were time-varying. • The zone of low oxygen volume fraction moved towards the direction of air supply. • The multi-stages of high-temperature spots movement were distinguished. • Sufficient gas supply and higher thermal boundary temperature could promote the coal-oxygen reaction. To understand the development of coal oxidation reaction, the process of coal spontaneous combustion in a large-scale furnace was numerically simulated by employing finite element method and further validated through experiments. The evolutions of oxygen volume fraction and temperature distributions in the furnace were obtained. Furthermore, the influences of air supply volume and the thermal boundary temperature on carbon monoxide productions and maximum temperature changes were investigated. It was found that the released gaseous products and oxygen consumptions of the coal-oxygen reaction increased slowly before 60 °C, and rapidly after exceeding 100 °C. The low oxygen volume fraction region was generated away from the air inlet and moved towards the air inlet. The movement of high-temperature spots in the furnace could be divided into heat storage stage, thermal stability stage and rapid oxygen consumption stage. With the acceleration of coal-oxygen reaction, the region of high-temperature spot first moved away from the air inlet, and then migrated towards the air inlet. In addition, the higher volume of air supply and thermal boundary temperature could promote coal-oxygen reactions and cause thermal runaway when the critical values were reached. This study is helpful for the prevention of the coal spontaneous combustion. [ABSTRACT FROM AUTHOR]