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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]

Among coal-producing countries, China has witnessed the highest frequency and severity of coal fires. During mining in the re-mining face and the lower part of the thick coal seam (or coal seam group), second or multiple oxidations of the residual coal can readily occur in mined-out areas, severely increasing the risk of coal fires. The study of multiple oxidations of coal aids in the early detection of spontaneous combustion of remaining coal in the mined-out areas. The characteristics of coal fires are demonstrated using fresh coal samples (first-oxidized coal) and oxidized coal samples (second-oxidized coal) from four different ranks of coals. Proximate and elemental analysis, Fourier transform spectroscopy, and thermogravimetry-differential scanning calorimetry were adopted to investigate the quality, microcharacteristics, characteristic temperatures, and apparent activation energy of second-oxidized coal. In addition, the effect of spontaneous combustion at heating rates of 5.0, 10.0, and 15.0 °C min was studied. Compared with the four samples of first-oxidized coal, those of second-oxidized coals cause fire easily under the same conditions. [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]

• Correlation between gas products and active groups is determined for CDDM. • Relationship between active groups and exothermic is characterized for CDDM. • Evolution difference of active groups is explored for CDDM. • Oxidizing ability of active groups determine macroscopic characteristics of coal. The exothermic kinetic properties of the oxidation of key active groups are closely related to the coal with different degrees of metamorphism (CDDM) in the low-temperature oxidation of coal. To study the dynamic characteristics of oxidative exotherm of the key active groups for the CDDM, and reveal the macroscopic and microscopic interrelationships, the CDDM were selected and analyzed through temperature-programmed testing system, microcalorimetry experimental system, and in-situ Fourier transform infrared spectrometer experimental system. The results indicated that the carbonyl groups were oxidized to produce CO and CO 2 at an increased rate in the accelerated oxidation stage. In the vigorous oxidation stage, the accumulation of heat accelerated the conversion of methyl and methylene groups to carbonyl groups, and promoted the oxidation reaction. At the same temperature conditions, the total production of CO and CO 2 released from a coal with a high degree of metamorphism is lower. The key preventing the low-temperature oxidation is obstructing the reaction progression of the accelerated oxidation stage and the carbonyl groups, which are the key stage and active groups respectively. [ABSTRACT FROM AUTHOR]