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To study the metal ion chelating agent (DL‐malic acid) inhibiting oxidation characteristics of residual coal in goaf, the microscopic thermal transformation characteristics of the treated coal samples were obtained using apparent morphology, infrared spectroscopy, free radical testing, and thermal analysis. The results demonstrate that after the addition of malic acid, the sphericity and roundness of the coal samples decrease, and the small fractured particles accumulate in the pores of the oxidized coal samples and, thereby, reduce the number of coal–oxygen contact sites. The addition of 10% malic acid promotes the conversion of functional groups (FG) and free radicals in pre‐oxidized coal, inhibits the catalytic effect of metal ions, generates more active sites, and increases the total heat release during the combustion process. There are strong endothermic points in the malic acid‐treated coal samples, and the free water evaporation blocked by small particles in the holes delays the initial heat release temperature of the coal samples. Pre‐oxidation can promote changes in the valence state of metal ions in coal, increase the adhesion between broken pre‐oxidized coal particles via malic acid chelation, and lose the catalytic effect of metal ions; this inhibits the process of oxidative re‐ignition of coal samples. [ABSTRACT FROM AUTHOR]

The water immersion residual coal tend to happen spontaneous combustion accidents after mining the coal seam. For exploring the oxidative re-ignition properties of high-temperature oxidized water-impregnated long-flame coal, the thermal evolution law toward each pre-oxidized coal soaked in water under varying heating rates (Rh) (5, 10, 15°C/min) has been analyzed by thermogravimetric differential scanning calorimetry (TG-DSC). The findings are as follows: the DTG at the critical temperature (T1) and maximum weight loss temperature (T5) increases, the activation energy (E) decreases, and the heat output increases (maximum 6391J/g) with the rising Rh. The violent oxidated reaction process of coal samples has been delayed as the Rh was 10°C/min. The water-immersed coal sample (I200) contains more free water on the surface, and the maximum (0.45 mW/mg) and total (178.8 J/g) heat absorption of are higher. Oxidation at 200°C and soaking in water (O200I200) reduced the ignition point temperature (minimum 399°C) and E (minimum 18.5 kJ/mol), and increased the total heat release (maximum 323 J/g) making it easier to oxidize and re-ignite. Under the same Rh, the oxidation and combustion traits of each pre-oxidized water-immersed coal (PWIC) remained unchanged. Reducing the Rh can effectively prevent spontaneous combustion of high-temperature oxidized water-soaked coal in shallow-buried and short-distance coal seams in northwest of China. [ABSTRACT FROM AUTHOR]

Residual coal is easily submerged and damaged by groundwater. In the course of mining underlying coal seam, coal belonging to the overlying gob area tends to spontaneously combust during draining water. To explore the effect of micro-functional group transformation in immersed coal on the coal spontaneous combustion (CSC) traits during the low-temperature oxidation process (LTO), the migrant law of special functional groups under varying oxygen concentration (Co) of immersed coal was discussed by infrared spectroscopy. The endothermal and exothermal properties of the function group inside of coal were studied via thermal analysis. It is shown that the higher Co, the more oxygenic function groups on the soaked, and by contrasted with the Co of 18% the 14% has a small increase and the chain length is shortened. The temperature increment together with the Co content determines the functional group content, the pyrolysis process produces some function group. The lower the Co, endothermic coefficient decreases and endothermic temperature increases, the higher the ignition temperature of coal oxy-combustion, the lower the total heat release. And the speed of the oxygen-absorbing weight gain and the conversion of the functional group would be improved followed by the burnout temperature rising. Below 14% Co, the oxidative combustion process will be greatly inhibited. Those findings complement a theoretical basis for preventing long-term water-soaked coal oxidative spontaneous combusting in shallow buried coal seams. [ABSTRACT FROM AUTHOR]

[Display omitted] • The characteristics of low temperature oxidation kinetics of water-soaked coal samples were analyzed. • Find out the migration law of functional groups in the oxygen-lean combustion process of water-soaked coal. • The thermal evolution process of water-soaked coal samples in an oxygen-lean environment is proposed. • A micro-reaction mechanism of coal-oxygen-water molecular chemisorption was constructed. Oxidative spontaneous combustion is likely to occur after long-term water immersion in the gob area of coal mines. Mostly, the oxidation process is slowly carried out under different air leakage conditions. This paper summarizes the four stages of the low-temperature oxidation of water-soaked coal samples through temperature-programmed experiments. Using infrared spectroscopy and thermal analysis experiments, the migration laws of the functional groups and the thermal reaction characteristics of the four stages were investigated. The results show that as the oxygen concentration increases, the gas product concentration increases, and the activation energy of the deep oxidation stage decreases. With the progress of the heating stage, when the oxygen concentration is higher than 10%, the change in each functional group is larger, and the conversion of the functional group and the intermediate complex is relatively closer at an oxygen concentration of 21%. The low-temperature oxidation reaction is a parallel process of the pyrolysis and micro-oxygen absorption, and the DTG size is not affected by the oxygen concentration at the critical temperature. The higher the oxygen concentration is, the lower the initial exothermic temperature is, which promotes the exothermic process in the deep oxidation and oxidation-limited stages. The microreaction mechanism among coal, oxygen and water molecular chemisorption is constructed. By controlling the coal oxidation reaction stages under different air leakage conditions, the spontaneous combustion of water-soaked coal in the goaf is avoided. [ABSTRACT FROM AUTHOR]

[Display omitted] • Analyzed the characteristics of lean oxygen re-ignition of high-temperature oxidized coal soaked in water. • Find out the change of microscopic pore structure of high-temperature oxidized water-immersed coal sample. • The effect of coal oxygen binding sites on the re-ignition of coal samples is proposed. After coal seam mining is over, the coal left in the goaf is prone to spontaneous combustion accidents due to air leakage. To explore the oxygen-lean re-ignition characteristics of long flame coal after high-temperature oxidation and water immersion. Using scanning electron microscopy (SEM), nitrogen adsorption experiment method, characterize the high-temperature oxidized water-immersed coal sample. Using programmed temperature rise and synchronous thermal analysis experimental methods, the oxidation and re-ignition characteristics of coal samples under oxygen lean conditions were studied. The results show that compared with the untreated coal sample, the surface of the water-immersed coal sample is rougher, with more pores below 10 nm. The high-temperature oxidized water-immersed coal sample has the phenomenon of pore-to-pore, and the proportion of medium-to-large pores increases. The average pore diameter of the oxidized 200℃ water immersion (O 200 I 200) coal sample is the largest, which increases the number of coal oxygen reaction adsorption sites. The crossover temperature of the O 200 I 200 coal samples is lower at various oxygen concentrations, and the 2–10 nm pores affect the maximum exothermic temperature and initial exothermic temperature changes of the low-temperature oxidation process. Mesopores and macropores mainly affect the change in total heat release during high-temperature oxidation. A high oxygen concentration (above 10%) will promote the oxidation and re-ignition process of the pre-oxidized immersed coal sample. This result provides a theoretical basis for preventing the spontaneous combustion of coal in water immersed by high-temperature oxidation in shallow coal seams. [ABSTRACT FROM AUTHOR]

To investigate the characteristics of adiabatic oxidation phase and build the prediction model of coal spontaneous combustion critical point, the adiabatic oxidation experiment data of eight coal samples are given in this paper, based on the adiabatic oxidation experiment, element analysis experiment and industrial analysis experiment. The phase heating rates of adiabatic oxidation of these samples are also calculated. Then using the thermodynamic parameter calculation method, Tr0.05 and TΔmax are defined and obtained. Furthermore, based on the Levenberg-Marquardt method, the relationship between the coal element content, volatility, the phase heating rate of adiabatic oxidation and Tr0.05 are established. The results show that the content of N or S correlates positively with Tr0.05 Osand that O, C, or H content correlates negatively with Tr0.05. However, the link between these elements and stage-heating rate is the opposite. The law also suggests that stage-heating rate will be much faster and coal spontaneous combustion will be easier when Tr0.05 becomes much lower. The more volatile coal is, the lower Tr0.05, and the more likely it is to spontaneously combust. [ABSTRACT FROM AUTHOR]