Study on Spontaneous Combustion Characteristics and Microstructure of Bituminous Coal under Water Immersion.

In complex geological mining conditions, residual coal often collapses into the goaf, where it becomes saturated with water and undergoes air drying. This process ultimately leads to the formation of water-immersed coal. Coal that has been immersed in water shows a much greater tendency for spontaneous combustion than untreated coal, posing a significant safety hazard in mining operations. This study seeks to investigate how water immersion affects the heating and oxidation processes of bituminous coal along with the changes in key chemical groups during these stages. Long-flame coal and fat coal were selected as the research materials, and water-immersed coal samples were prepared with water to coal mass ratios of 1:2, 1:1, and 2:1. Experiments using scanning electron microscopy, low-temperature nitrogen adsorption, programmed temperature gas chromatography, and in situ Fourier transform infrared spectroscopy were conducted to examine the alterations in the microscopic physical structure, oxidation behavior, and active functional groups of coal samples before and after water immersion. Pearson correlation analysis was utilized to determine the primary active groups in coal samples throughout each phase of heating and oxidation. The research results indicate that (1) as the duration of water immersion increased, both the pore and fracture structures of long-flame coal and fat coal exhibited a progressive enlargement. The average pore diameter of the raw coal increased from 4.16 and 7.33 nm to 5.12 and 9.09 nm in the C2:1 and F2:1 coal samples, respectively. The proportions of mesopores and macropores increased to 21.87, 19.64, and 78.16, 73.24%, respectively. (2) In the early stages of coal spontaneous combustion and oxidation, water immersion acts to hinder the oxidation process of bituminous coal. However, as the temperature rises, the moisture inside the coal pores evaporates, causing the water immersion to reversely promote the oxidation of bituminous coal. During the rapid oxidation stage, the highest oxygen consumption for C1:2 and F1:1 coal samples was 9.94 and 10.93%, respectively. Their oxygen consumption rates were 1.43 and 1.21 times that of raw coal, respectively. During the intense oxidation stage, the highest CO production for C1:2 and F1:1 coal samples was 23,157 and 25,699 ppm, respectively. Compared to raw coal, this represents an increase of 1.83 and 1.48 times, respectively. (3) Water immersion results in a higher concentration of hydroxyl and oxygen-containing functional groups in the coal, while simultaneously reducing the proportion of aliphatic and aromatic hydrocarbon groups. Hydroxyl groups are the key functional groups in the slow oxidation stage, exhibiting correlation coefficients of -0.955 and -0.941 with untreated long-flame coal and bituminous coal, respectively. Aliphatic hydrocarbons also serve as critical functional groups during the slow oxidation stage, with correlation coefficients of -0.876 and -0.892 for untreated long-flame coal and bituminous coal, respectively. In the intense oxidation stage, oxygen-containing functional groups are pivotal, where untreated long-flame coal and fat coal show correlation coefficients of 0.934 and 0.980 with carbonyl (C=O) groups and 0.859 and 0.913 with carboxyl (-COOH) groups, respectively.

The spontaneous combustion of coal is affected by many factors, among which the influence of water is significant and complicated. To explore the influence of water on the spontaneous combustion characteristics of goaf residual coal, coal samples with similar particle size distributions to those of goaf residual coal were prepared. After the coal samples were immersed in water for 7–21 days and the external flowing water was drained, spontaneous combustion experiments were carried out using a temperature-programmed method. The results showed that soaking in water could promote and inhibit the spontaneous oxidative combustion of large coal particles in different temperature ranges. When the coal temperature was below 50 °C, water immersion had a significant inhibition effect on coal oxidation and spontaneous combustion. When the temperature of coal was 50–110 °C, soaking in water for 7 days could promote the oxidation and spontaneous combustion of coal. However, soaking for 14 and 21 days had a significant inhibition effect in this temperature range. When the coal temperature was higher than 110 °C, water immersion had a significant inhibition effect on the coal. Moreover, a prolonged immersion time significantly enhanced the inhibition effect. When the immersion time was less than 21 days, the spontaneous combustion of large coal particles by short-term soaking was mainly inhibited.

The drying process of immersed lignite has a significant influence on the characteristics and progress of spontaneous combustion. To reveal the influence mechanism of the drying process of immersed coal on the spontaneous combustion characteristics and the change rule of the spontaneous combustion process, in this research, we measured the mass change during coal oxidation with thermogravimetry, and the change of the functional groups with Fourier transform infrared spectroscopy. The influence of the drying process on the coal was analyzed by comparing activation energy, functional group of immersed coal with different drying degrees and raw coal. The results showed that, compared with raw coal, the content of Ar–C–O– and antisymmetric stretching vibration of the carboxylate group (–COO–) as well as the stretching vibration in the quinone group (C=O) and the –OH group increased. For the content of delta s.{text{RCH}}_{{3}},delta as.{text{R}}_{{2}} {text{CH}}_{2}delta as.{text{RCH}}_{{3}}, the value of Asym.CH2/Asym.CH3 decreased. The content of various functional groups changed to be favorable for oxidation and heat release. At different reaction stages, the activation energy was differently affected by the degree of drying. Average values of activation energies at different reaction stages are shown raw coal had the lowest activation energy. After soaking in water and drying, the activation energy of coal is increased to varying degrees, the reactivity is reduced, and the risk of spontaneous combustion is reduced. After soaking in water and drying, the activation energy of coal is increased to varying degrees, the reactivity is reduced, and the risk of spontaneous combustion is reduced. The activation energy of the coal samples dried for 24 h after soaking in water is the lowest among the coal samples dried for different times after soaking in water, and the moisture content is 10.5%.

Spontaneous combustion of coal in re-mining coal mine is a serious threat to the safety recovery of coal resources. In this paper, six coal samples, including two raw bituminous coal samples from the new mining working face and four oxidized bituminous coal samples from the re-mining working face, are selected. The pore structures, thermogravimetric properties, and spontaneous combustion characteristics of raw coal and oxidized coal are studied. The pore structures show that the surface of oxidized coals has more cracks and higher macropore volume and porosity as compared with raw coal. The thermogravimetric experiments demonstrate that the oxidized coal has a higher combustion rate and turns to the combustion reaction zone in advance. Moreover, the oxidized coals perform a better combustion property by showing a comprehensive combustion index of 6.7 × 10−9 min−2·°C−3 and 8.9 × 10−9 min−2·°C−3, higher than the raw coal of 4.0 × 10−9 min−2·°C−3. The spontaneous combustion characteristics show that the oxidized coal performs a greater rate of oxygen consumption and more oxidation products released than raw coal. The CPT of raw coal is higher than 157°C, while the CPT of oxidized coal is between 132°C and 140°C. The research results can provide a risk assessment method for coal spontaneous combustion in the re-mining coal mine, and provide guidance for avoiding the occurrence of coal fires.

Effect of water soaking and air drying on the thermal effect and heat transfer characteristics of coal oxidation at the low-temperature oxidation stage

The effect of hydrothermal coupling on the risk of spontaneous combustion of coal (SCC) soaked at different water temperatures was studied. The results showed that the porosity and permeability of coal increased significantly after immersion in water at different temperatures. The increase at a water temperature of 30 °C was the largest, the porosity was 22.77% higher, and the permeability was 3.44 times that of raw coal. The relative content of oxygen-containing functional groups, aliphatic functional groups, and hydroxyl functional groups reached 1.8-4.5, 1.1-4.4, and 0.7-1.48 times that of raw coal after soaking in water at different temperatures. The increase ratio of functional groups is the largest under the water temperature of 30 °C. The activation energy of coal samples soaked at water temperatures of 30 and 45 °C decreased by 9.4 and 2.9%, respectively. Under the condition that the coal sample temperature was lower than 110 °C, the highest oxygen consumption rate and heat release rate were coal samples soaked at water temperatures of 30 °C, and the lowest was raw coal. When the coal temperature was higher than 170 °C, all coal samples soaked in different water temperatures are higher than that of raw coal, and the water temperature of 30 °C is the highest. The risk of spontaneous combustion of coal is highest after long-term soaking at 30 °C water.

Thermal effects and active group differentiation of low-rank coal during low-temperature oxidation under vacuum drying after water immersion

After a coal seam is mined, the coal remaining in the goaf is prone to flooding and spontaneous combustion accidents. To explore the reignition (secondary oxidation) characteristics of long-flame coal after oxidation and water immersion, the experimental methods of thermogravimetric analysis and infrared spectroscopy were used to analyze coal samples of oxidation first and then water immersion (FO) and samples of water immersion first and then oxidization (FI) at different pre-oxidation temperatures. The results showed that the content of main oxygen-containing functional groups (hydroxyl, carbonyl, and carboxyl groups) of the FO120 (oxidation 120 °C first and then water immersion) coal sample increased, and the FI 90 (water immersion first and then oxidization 90 °C) coal sample decreased. Pre-oxidation at 120 °C will slow down the decrease in the extent of low-temperature secondary oxidation TG, as the pre-oxidation temperature increases, the total heat release of the FO coal samples first increase and then decrease, and the heat released is high at 120 °C. The FI coal samples transfer active sites during the water immersion process, and the high pre-oxidation temperature leads to the rapid increase of the speed of the primary active site, which leads to the transformation between the secondary active site and the oxygen-containing group, resulting in the cleavage of the oxygen-containing group and increasing the heat production. Water immersion pre-oxidation performed under different conditions has the dual effects of promoting and inhibiting spontaneous coal combustion. This result provides a theoretical basis for preventing spontaneous combustion in coal-mined areas in shallow coal seams after soaking in water.

The current reaction kinetics of coal–oxygen compound in water-soaked coal has not been accurately explained. Therefore, thermal behavior and microscopic characteristics during spontaneous combustion of water-soaked coal samples were analyzed. Five kinds of coal samples were selected with moisture contents of 5%, 10%, 15%, and 20% of the raw coal. Fourier transform infrared (FTIR), thermogravimetric (TG), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were adopted. The results showed that the hydroxyl (–OH) group is greatly affected by the moisture content of coal samples. The area of hydroxyl absorption peaks in coal samples with 5% and 10% moisture content increased by 1.76% and 1.12%, respectively. The critical temperature T1 decreased the most at 10% moisture content compared with the raw coal, which was about 6.5 °C. This indicated that the heat storage process was accelerated and the activity of the coal samples was increased. According to the thermodynamic analysis, the activation energy of the five kinds of water-soaked coal samples are 148.078 kJ/mol, 147.674 kJ/mol, 146.197 kJ/mol, 145.019 kJ/mol, and 145.934 kJ/mol, respectively. This proves that the activation energy of coal samples is generally reduced after immersion in water, and coal samples with 15% moisture content are most susceptible to spontaneous combustion. As the temperature was raised, aliphatic hydrocarbons (–CH3, – CH–) and C = O increased rapidly. The above results indicate that water immersion accelerates the spontaneous combustion and oxidation of coal to a certain extent.

When mining deep coal seams, the overlying mined-out area must be pumped and drained, and the remnant coal in the mining void area is vulnerable to spontaneous combustion via water immersion and air drying. To investigate the effect of soaking time on the spontaneous combustion characteristics of lignite, low-temperature N2 adsorption experiments and Fourier transform infrared spectroscopy (FTIR) experiments were used in this study to investigate the changes in pore structure and functional group content of coal samples by increasing the soaking time. Based on programmed warming experiments, macroscopic characteristic indicators of coal spontaneous combustion (CSC) oxidizability, such as the oxygen consumption rate and index gas, were derived. Results showed that the effect of soaking caused the total pore volume and specific surface area of lignite to increase to different degrees, which enhanced the adsorption capacity of coal to oxygen and improved the effective contact of oxygen with the active sites on the coal surface. However, there was no consistency in the effect of different soaking times on the content of reactive functional groups of the coal, with higher methyl and methine contents after 60 days of soaking (S60) and 120 days of soaking (S120) than in the raw coal (RC), and less in 90 days of soaking (S90). The propensity of coal to spontaneously combust is determined by both the pore structure and reactive functional groups, which shows the propensity of spontaneous combustion S60>S120>RC>S90. Results from this study are important in understanding the mechanism of CSC of water-soaked lignite in mined-out areas and can provide theoretical guidance to prevent CSC in mining areas.

Coal spontaneous combustion (CSC) severely affects the development of the coal industry and threatens the safety of coal miners. This study investigated how immersing coal in aqueous solution affects the CSC risk and evaluated the effect of the immersion duration. Fourier transform infrared spectrometry and thermogravimetric−differential scanning calorimetry analysis were performed. Coal samples were immersed in water from an underground coalmine or in distilled water for 30 or 60 days. Changes in microscopic functional groups and thermokinetic behaviors were determined. Immersion in distilled water for 60 days led to 4.41%, 2.26%, and 6.25% smaller masses of C=O, –CH3, and free hydroxyl groups, respectively, relative to their levels in raw coal. Additionally, the characteristic temperatures T 4 , T 5 , and T 6 were 8.8%, 9.0%, and 9.2% lower, respectively, and the average apparent activation energy was 15.67% lower. Immersion in water from an underground coalmine for 60 days led to 5.68%, 2.58%, and 6.54% higher masses of C=O, –CH3, and free hydroxyl groups, respectively, and 12.8%, 13.0%, and 13.5% higher characteristic temperature, respectively; the average apparent activation energy was 20.43% lower. These findings suggest that the coal immersed in water from an underground coalmine was the most susceptible to CSC.

To reveal the influential laws and mechanism of water immersion on the spontaneous combustion traits of lignite in the low-temperature oxidation stage. Raw coal (HL), water-immersed for 30 days (HL-S30), 90 days (HL-S90) and 150 days (HL-S150), were taken as the research objects. The pore evolution characteristics and functional group migration law of coal were explored by Scanning Electron Microscopy (SEM), Low-temperature Nitrogen Adsorption Test (LNAT) and Fourier Transform Infrared Spectroscopy (FTIR). The low-temperature oxidation procedure was simulated by Programmed Heating Test (PHT), and the gas release characteristics, exothermicity and ultimate spontaneous combustion parameters of the relic coal were compared. These findings demonstrated that the discriminant indexes of spontaneous combustion tendency of four specimens change periodically with temperature, but generally the spontaneous combustion intensity of HL-S30 and HL-S150 is greater than HL, and HL-S90 is the least. The micro reasons for increasing oxidation activity of HL-S30 and HL-S150 are as follows: the number of micropores of water-immersed coal is almost 3 ~ 4 times that of HL, and the pores expanded with immersion which helps to enhance the adsorption of oxygen; the content of broken aromatic hydrocarbon -C = C- reached 50%, the total amount of hydroxy -OH decreased to 2/3 of HL, while the content of carboxyl -COOH and ether-oxygen bond -C-O- of active group doubled which are beneficial to accelerate the process of free radical chain reaction, and promote the gas release and the temperature of residual coal to reach the ignition point. The research findings enrich the basic theory of spontaneous combustion traits of water-impregnated coal, which help reduce the fire accidents and hidden dangers in actual production.

To study the macro and micro characteristics of the spontaneous combustion of pre-oxidation coals (POC) with different pre-oxidation temperatures (80°C, 120°C, 160°C, 200°C), we used programmed temperature experiments, BET, and in situ FTIR. The results show that the spontaneous combustion characteristic parameters (oxygen consumption rate, gas release rate (CO and CO2), and exothermic intensity) of POC show disparities. Especially after the functional group content of coals reaches the extreme value at 160°C, the differences are further enlarged. Different primary oxidation processes change the specific surface area (SSA) and functional group content to varying degrees. The 80°C pre-oxidation process increases SSA and functional group content. 80°C POC reacts more violently than Y (raw coal), which can produce more reducing groups (-OH, -CH3, and -CH2-) and lower oxidizing groups (C = O and -COO-), showing a stronger ability to combine with oxygen. Moreover, it consumes more oxygen and releases more CO, CO2, and heat. The macro and micro spontaneous combustion characteristics of 200°C POC are the weakest. It can be judged that the coal under a pre-oxidation temperature of 80°C has a stronger tendency to spontaneous combustion, and the pre-oxidation process at 200°C reduces the tendency of coal to spontaneous combustion.

To study the influence of water immersion on the evolution of the groups and spontaneous combustion characteristics of coal samples with different sizes, raw coal from the Fengshuigou Coal Mine operated by Pingzhuang Coal Company in Inner Mongolia was studied. The infrared structural parameters, combustion characteristic parameters, and oxidation reaction kinetics parameters of D1-D5 water immersion coal samples were tested, and the mechanism of spontaneous combustion during the oxidation of submerged crushed coal was investigated. The results were as follows. The water immersion process promoted the re-development of coal pore structure, and the micropore volume and average pore diameter were 1.87-2.58 and 1.02-1.13 times those of raw coal, respectively. The smaller the coal sample sizes, the more significant the change. At the same time, the water immersion process increased the contact point between the active group and oxygen in the coal, and the C=O, C-O, and -CH3/-CH2- groups in coal were further promoted to react with oxygen to generate -OH functional groups and improve the reactivity of coal. The characteristic temperature of water immersion coal was affected by the temperature rise rate, coal sample size, coal voidage, and other factors. Compared with the raw coal, the average activation energy of the water immersion coal with different sizes decreased by 12.4-19.7%, and the apparent activation energy of the coal sample with a size of 60-120 mesh was the lowest on the whole. In addition, the apparent activation energy in the low-temperature oxidation stage was significantly different.

To explore the influence of the oxidation and spontaneous combustion process of fractured coal at different burial depths under uniaxial stress, the spontaneous combustion characteristics of coal under loading was studied within the testing device of coal spontaneous combustion and loading. Bituminous coal from the Liuhuanggou mining area in Xinjiang was selected and oxidized in the oxygen-lean environment loaded at the range of 0–8 MPa. Based on the relationship between the gas generated in the experiment and the temperature, we calculated the apparent activation energy and oxygen consumption rate of coal samples under uniaxial stress. We combined the oxidation kinetics and pyrolysis parameters of spontaneous coal combustion to describe the nonlinear development of coal from slow to rapid oxidation under uniaxial stress. Based on catastrophe theory, the catastrophic temperature and critical temperature of bituminous coal oxidation-combustion process under test conditions were calculated, and four characteristic parameters were determined: catastrophic temperature \begin{document}$ {T}_{\mathrm{C}\mathrm{O}} $\end{document} (characterization of CO) and \begin{document}$ {T}_{\mathrm{H}\mathrm{Y}} $\end{document} (characterization of oxygen consumption rate), and critical temperature \begin{document}$ {T}_{\mathrm{C}\mathrm{O}}^{'} $\end{document} (characterization of CO) and \begin{document}$ {T}_{\mathrm{H}\mathrm{Y}}^{'} $\end{document} (characterization of oxygen consumption rate), and analyzed the variation of different characteristic parameters with uniaxial stress. The analysis results show that the pyrolysis gas concentration, apparent activation energy, and oxygen consumption rate follow a cubic function law that first increases, then decreases, and then increases with increases in the uniaxial stress (the critical axial pressures at 1.8 and 5.5 MPa). At 1.8 MPa, the apparent activation energy and various parameter values are lowest, the oxygen reaction rate of coal is fastest, and the oxygen consumption rate is the highest. When the uniaxial stress is 5.5 MPa, the oxygen consumption rate is the highest, the greatest number of new cracks is created, and the characteristic \begin{document}$ {T}_{\mathrm{C}\mathrm{O}} $\end{document} parameters have the greatest impact. The temperature index of spontaneous coal combustion slowly transitions to rapid oxidation, and the catastrophic temperature \begin{document}$ {T}_{\mathrm{C}\mathrm{O}} $\end{document} characterized by the CO concentration is the most accurate. The research results have important theoretical guiding significance for the early warning and prevention and control of spontaneous combustion of coal at different buried depths.