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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]

Coal spontaneous combustion (CSC) is one of the main sources of greenhouse gases (GHGs) emission. In order to understand the GHGs emissions law of the heating and quenching processes in the coal low-temperature oxidation stage, three kinds of coal samples with different degrees of metamorphism were selected to investigate the influence of particle size and oxygen concentration on GHGs emissions. Compared with the heating process, the greenhouse effect of CSC under different inerting conditions was determined. The results showed that the CO2 and CH4 were the mainly GHGs in the initial stage of the coal oxidation, which were emitted from the oxidation and desorption of coal, while the particle size of coal displayed the opposite effect on GHGs. By comparison with the heating process, the average emission factor and total emission of CO2 of 1/3 JM, the average emission factor and total emission of CH4 of HM with 21% O2 were higher in the quenching process. Meanwhile, with the decrease in oxygen concentration, the emission rate, average emission factor and total emission of GHGs decreased, and the greenhouse effect of CSC became weaker. Among them, the highest inerting rates of GHGs of HM, 1/3 JM and WYM reached 81.5%, 64.8% and 77.7%, respectively. In addition, the oxygen concentration was linearly positively correlated with the total emissions of CO2 and CH4, and conformed to the quadratic polynomial function with the equivalent CO2 emission factor. However, due to the strong displacement effect of N2, the emission rate, average emission factor and total emission of CH4 of WYM and 1/3 JM with 0% O2 were higher than those with 7% O2, and it is shown that inert gases can weaken the coal oxidation, diminished GHGs emissions, reduced the greenhouse effect of CSC on the environment, and enhanced the desorption of gases in coal. The research results have important practical significance for improving the incomplete emissions of GHGs in the low-temperature oxidation process, and evaluating the emission reduction effect of CSC prevention. [ABSTRACT FROM AUTHOR]

Water and pyrite (FeS2) in coal have great synergistic effect on coal self-ignite. In order to explore the characteristics of spontaneous combustion of high humidity and high sulfur coal, this paper selected different concentrations of water and pyrite to carry out thermogravimetric experiment and oxidation reaction kinetics analysis. The changes of coal samples oxidation under low temperature in mass and characteristic temperatures were studied through thermogravimetric (TG) analysis. The results indicate that the promoting effect of water on coal spontaneous combustion turns to restrain when it reaches 15%, and water and pyrite have substantial synergistic effect on coal oxygen absorption and mass gain stage (T3–T5 in TG temperatures), besides there is a range of water/pyrite contents (10–15 mass% water and 2–4 mass% pyrite) that have the largest effect on coal oxidation reactions. Through the analysis of oxidation reaction kinetics, it was found that the change rule of the apparent activation energy (Ea) in coal oxygen absorption and mass gain stage is similar to that of the thermogravimetric change, and it was the lowest at 2–4 mass% pyrite and 10–15 mass% water content. The reaction process of coal oxidation by water and pyrite conforms to the equation n = 4 Avrami-Erofeev, therefore water and pyrite just can promote coal oxidation externally. This research provides support for further understanding and control the influence of water and associated pyrite on coal self-ignite. [ABSTRACT FROM AUTHOR]

Coal fires are widespread in the coal–major producing countries, which causes serious environmental issues due to the high temperature and hazardous gases during coal combustion. Under oxygen–depleted atmospheres, the thermal behaviors and kinetic characteristics during coal combustion at high temperatures were paid more attention by the method of TG–DSC synchronous thermal analysis. The results indicate that the focused weightlessness and heat release processes became scattered under oxygen–depleted atmospheres, and the influence of oxygen concentration obviously increased when it was below 13 vol.%. The mass presented a linear correlation to the quantity of heat release, and the relationship between the mass and quantity of heat release showed stage characteristics with oxygen concentration. Moreover, the kinetic modes transformed from random nucleation and subsequent growth to contractive sphere, and the critical oxygen concentrations are consistent to these of thermal behaviors. These findings are significant for better understanding the characteristics of coal spontaneous combustion at high-temperature and under low-oxygen concentrations. [ABSTRACT FROM AUTHOR]

Coal spontaneous combustion (CSC) characteristics were identified by their different types of coal. In this study, the samples of lignite, non-caking coal, lean coal, and anthracite were chosen to investigate the variations of gaseous products and main active groups during the low-temperature oxidation. The law of indicator gases was obtained by temperature-programmed experiment, and active groups of four coal samples had specific and different probabilities of spontaneous combustion via Fourier transform infrared spectrometer (FTIR). The relationship between the indicator gases at different stages and the main functional groups of coal were explored by Grey correlation analysis. The results indicated that the characteristic temperatures of water evaporation and gases desorption period of four types of coal showed the similar variation with those in thermal storage stage (Stage 1). Moreover, temperature ranges (TRs) in Stage 1 of lignite, non-caking coal, lean coal, and anthracite were 20–60, 20–60, 20–60, and 20–80°C, respectively; TRs in Stage 2 were 60–100, 60–140, 60–120, and 80–200°C, respectively; TRs in Stage 3 were 100–180, 140–180, 120–180, and 200–240°C, respectively. Based on the correlation analysis, the hydroxyl, carboxyl, and methylene had a sound temperature correlation with indicator gases emission. Eventually, the prediction system for CSC during the low-temperature oxidation was revealed. [ABSTRACT FROM AUTHOR]

• The spontaneous combustion tendency of coal was determined. • The evolution of functional groups was derived. • The addition of 2.0 mass% FeS 2 exerted a prominent promoting effect on coal oxidation. • The influence mechanism of FeS 2 on coal oxidation was determined. Spontaneous combustion of coal has become an important disaster that threatens the safety of coal mines. FeS 2 is the main component of pyrite, which is suspected to be a major contributor to coal spontaneous combustion (CSC). So, it has important significance to FeS 2 on the characteristics of coal oxidation for prevention and treatment. This study used coal samples mixed with different proportions of FeS 2 (2.0 mass%, 4.0 mass%, and 6.0 mass% mass percentage) were tested to investigate the characteristics of spontaneous combustion, as compared with the fresh sample. The CO and CO 2 production rates, critical temperature, and dry cracking temperature during oxidation were analyzed. The temperature-programmed experiments was conducted to simulate low-temperature oxidation processes realistically. In-situ infrared spectroscopy was used to appraise the evolution of low-temperature (< 200.0 °C) oxidation of aromatic hydrocarbons, aliphatic hydrocarbons, and oxygen-containing functional groups on the surfaces of the samples. Experimental results showed that FeS 2 exhibited a strong influence as the temperature exceeded the dry cracking temperature. Adding 2.0 mass% and 4.0 mass% FeS 2 showed a promoting effect on low-temperature oxidation; however, FeS 2 became an inhibitor which reached 6.0 mass%. Furthermore, adding 2.0 mass% FeS 2 showed the strongest promoting effect. From a microscale perspective, the promoting effect of FeS 2 on coal oxidation was due to thermal release from FeS 2 oxidation, enhancing the reaction between aliphatic hydrocarbons, such as methyl groups and methylene, and oxygen. Due to the reaction of FeS 2 , the acidic environment was conducive to the hydrolyzation of lipid structures into highly active alcohol or phenol structures, which in turn promoted the oxidation of the coal sample. These results are crucial for understanding the mechanism underlying the influence of FeS 2 on CSC and mine safety protocols. [ABSTRACT FROM AUTHOR]

Coal spontaneous combustion (CSC) is a global, long-standing thermal hazard in coal mining that causes serious environmental damage, particularly in developing countries. Ionic liquids (ILs) are environmentally friendly, green, economical, and a new type of fire-preventing material that inhibits coal oxidation and alleviates thermal production. In this study, 1-butyl-3-methyl imidazole tetrafluoroborate ([BMIM][BF4]) and 1-butyl-3-methyl imidazole ([BMIM][I]) at various mass ratios (0.0, 0.5, 2.0, 5.0, and 10.0 mass%) were used to treat three types of coal (lignite, 1/3 coking coal, and anthracite) for CSC. The combustion-mass loss and thermal release rate of ILs treated relative to untreated coal samples were investigated using a synchronous thermal analyzer, and the optimal addition mass ratio of ILs was predicted. Not only do ILs have the most efficient inhibiting effect, but they also consider the economic benefit. Per thermogravimetric-differential scanning calorimetry, for the content of 5.0 mass% [BMIM][BF4], the inhibiting effect is favorable for universal applications. In addition, coal samples treated with 5.0 mass% [BMIM][BF4] were subjected to programmed-temperature heating experiment testing. The oxygen consumption rate, inhibiting effect, as well as apparent activation energy, were determined. The inhibiting effect at 120.0–150.0 °C was the most obvious, which provided the basis of the application for CSC prevention. [ABSTRACT FROM AUTHOR]

To reduce the hazardous effect of coal fires, gas-heat dynamic characteristics were investigated in the closed fire zone. A temperature-programmed experiment and a C80 microcalorimeter were employed to evaluate the heating and decaying processes of coal spontaneous combustion, CO production and thermal oxidation were compared to analyze the gas-heat dynamic relationship and nonlinear dynamics. The results revealed that CO production and the heat flux exhibited obvious hysteresis in the decaying process, the oxygen concentration linearly increased with the total CO production and cumulative heat release. Moreover, the heat release in the heating process was lower than that in the decaying process under identical oxygen-poor conditions, this mechanism increasingly dominated with increasing oxygen concentration. Additionally, the total CO production exhibited a good linear relationship with the cumulative heat release during the heating process, the decaying process exhibited a nonlinear trend due to the hysteresis effect. Compared with that of the heating process, the apparent activation energy of the decaying process was lower, the coal spontaneous combustion risk was higher and reignition was more likely to occur. These research results are great practical significance for inhibiting coal fire hazards and guiding fire area unsealing in the closed fire zone. [Display omitted] • The heating and decaying processes of coal oxidation were simulated by experiments. • Correlation of CO and heat under various oxygen concentrations were analyzed. • Dynamic relationship of the heating and decaying processes was compared and analyzed. • The CO production and heat flux exhibited obvious hysteresis in the decaying process. • The decaying process of coal oxidation was more dangerous to reignite. [ABSTRACT FROM AUTHOR]

[Display omitted] • The results of two experiments in the heating and cooling processes were consistent. • A model for apparent activation energy was established with carbon monoxide. • A lower oxygen concentration promoted coal to capture oxygen actively. • A method of unit flow of oxygen consumption was proposed. • The monitoring of index gas in cooling process is more important. In order to explore the changes during the coal cooling process in a closed fire zone (CFZ), the oxidation characteristics of the heating and cooling processes were thoroughly examined and compared. The heating and cooling processes under various oxygen concentrations of three coals were simulated with temperature-programmed experiment and coal spontaneous combustion experiment. Furthermore, the hottest spot, oxygen consumption, index gases, and oxidation kinetics between the gaseous production rate (f) and coal temperature (T), air supply (Q), oxygen concentration (C O 2), and apparent activation energy (E a) were analysed. The results revealed that the movements of the hottest spots did not overlap in the heating and cooling processes. Moreover, the oxygen consumption during the cooling process was higher than that during the heating process, and a lower oxygen concentration was associated with higher oxygen consumption, a condition in which coal strongly captured oxygen actively. The index gases during the heating and cooling processes notably differed, which were related to coal types, and during the forced heating process, the CO 2 /CO ratios of 1/3 coking coal and anthracite were higher than that of the forced cooling process, even under the oxygen-lean conditions, the CH 4 /C 2 H 4 ratio of lignite during the forced cooling process was also notable. Moreover, the results in the cooling process of the temperature-programmed experiment in the carrier gases with 7% and 3% O 2 were consistent with that of the spontaneous combustion experiment. Finally, the apparent activation energy at different stages of the heating and cooling processes was calculated, exhibiting inconformity, and apparent activation energy is lower in the cooling process, which is related to its higher oxygen consumption, and showing stronger oxidisability in the cooling process. In addition, through the results of coal oxidation kinetics, it is found that coal temperature affected the magnitude of apparent activation energy, the oxygen concentration impacted the oxidation activity of coal. These research results are valuable for informing the process of fire extinguishment and guiding the fire area unsealing in CFZ. [ABSTRACT FROM AUTHOR]

[Display omitted] • The indicator gas analysis method was substantially improved. • The principle diagram of the low-temperature oxidation process of coal was drawn. • The actual exothermic strengths of the coal samples were compared. • The effect of the coal metamorphic degree on spontaneous combustion was determined. To investigate the effect of coal metamorphism on the probability of coal spontaneous combustion (CSC), five types of coal sample were subjected to a temperature-programmed testing system. The gas indicators and limiting parameters of CSC were analysed and the characteristics of different ranks of coal were observed during low-temperature oxidation. The CO production rate of each coal sample was positively correlated with temperature and the CO production rate of low-ranked coal was greater than that of high-ranked coal. The CO/O 2 and CO productivity trends were the same. Analysis of the change in the alkene ratio revealed that the oxidation reaction became more intense if the coal temperature exceeded the dry cracking temperature. The oxygen consumption rate of each coal sample was positively correlated with temperature, low-ranked coal samples exhibiting higher oxygen consumption rates. Graham's fire coefficient can be used as an auxiliary gas indicator to judge CSC tendency. Analysis of the limiting parameters of CSC revealed that the low limiting oxygen volume fraction and the minimal thickness of residual coal of high-rank samples were higher, whereas maximal air leakage intensity was lower. This paper further demonstrated differences in CSC tendency depending on the coal rank. These findings clarified certain factors affecting CSC, which may aid in preventing CSC in the future. [ABSTRACT FROM AUTHOR]

• Volatiles variations were inversely related with changes of fixed carbon. • A factor analysis method combining qualitative and quantitative was proposed. • Heat transfer mechanism of coal low temperature oxidation process was investigated. • Correlation between thermal properties and proximate analysis was approached. The low temperature oxidation of coal is a contradictory and unified dynamic process of coexisting mass and heat transfer. The thermophysical properties are crucial during coal spontaneous combustion. In the current paper, the variations of moisture, ash, volatiles, fixed carbon and thermophysical properties (thermal diffusivity, specific heat and thermal conductivity) of three coal samples from 30 °C to 300 °C were studied, and their grey correlation was analyzed. The results indicated that with the increase of temperature, the free moisture of Coals A and B decreased first but then increased, while the free moisture of Coal C kept decreasing without a later increase. The variation of surface moisture was consistent with that of free moisture. The trend of volatiles and fixed carbon was completely the opposite, showing a significant negative correlation. Ash was less affected by temperature. Along with the rise of temperature, the thermal diffusivity of three coal samples decreased first but later increased, and the specific heat was always in a state of increasing. The change in thermal conductivity was mainly affected by specific heat. By calculating the grey correlation degree, the major factors affecting the thermophysical properties were obtained. [ABSTRACT FROM AUTHOR]

• Mechanisms causing mass variation of combusting coal were separated and evaluated. • Contributions of different mechanisms to mass variation of coal were researched. • Transformation of the kinetic mode in spontaneous coal combustion was explored. • Kinetic compensation effect of variations in E a and A was investigated. Coal spontaneous combustion (CSC) is a severe threat to coal mining operations. Using thermogravimetric experiments, the contributions of different mechanisms to the mass variation during CSC were analysed. Furthermore, the kinetic mode and kinetic compensation processes were investigated. Among the different mechanisms of coal–oxygen reaction, solid-phase combustion contributes least to mass loss; for coals of high rank, gas-phase combustion makes the greatest contribution. Oxidative decomposition contributes most for coals of low rank. Furthermore, the kinetic mode governing CSC is three-dimensional diffusion in the mass gain stage and changes to random nucleation and subsequent growth in the mass loss stage. The apparent activation energy reaches its maximum near the ignition temperature and indicates a kinetic compensation effect with the pre-exponential factor in each stage. These findings are crucial for understanding the processes involved in CSC. [ABSTRACT FROM AUTHOR]

• Variation in sound absorption is studied based on the structural properties of coal. • Sound waves mainly propagate along the voids in loose coal. • Loose coal belongs to the high absorption medium of sound waves. • The obtained peak frequency provides basis for the selection of the acoustic signal. With the advantages of high applicability and reliability, the acoustic detection method has been widely used in the field of temperature measurement, positioning, and flaw detection. To apply acoustic temperature measurement techniques to the detection of high temperature points of loose coal spontaneous combustion, the basic acoustic properties of loose coal need to be clarified. Four bituminous coals with different degrees of metamorphosis were selected for the study. The stacking structural characteristics of loose coal were studied by industrial computerized tomography. The sound absorption coefficient of loose coal was tested based on the sound absorption test system. The results revealed that with the increase of particle size, the voids of loose coal gradually became larger and the connectivity became better. The distribution characteristics of voids in loose coal were intrinsic to the variation in sound absorption coefficients. The sound absorption coefficient was more influenced by the particle size and stack thickness than by the degree of deterioration. Except for the particle size of less than 0.9 mm and the stacked thickness of 2 cm coal samples, the absorption coefficients of the coal samples increased gradually with decreasing particle size at the same acoustic frequency, and the peak absorption coefficients gradually shifted towards lower frequencies with increasing thickness. The peak frequency range of the absorption coefficient was calculated to provide a basis for the selection of the frequency band of the acoustic temperature measurement signal for loose coal. [ABSTRACT FROM AUTHOR]