Reducing spontaneous combustion propensity of lignite through functional group regulation by microbial flame retardant.

ABSTRACTThe distribution of oxygen-containing functional groups, aromatic hydrocarbons, and aliphatic groups in coal with different spontaneous combustion propensity were studied. Adiabatic method was used to determine the spontaneous combustion propensity of six different coal samples. FTIR was used to collect the spectrum of the coal, and the peak areas of the functional groups were analyzed. The proportion of C = C, CH3/CH2, OH, C = O, COOH, C-O, and C-O-O obtained based on the peak areas was calculated, and the spontaneous combustion propensity was characterized by the distribution of the functional groups. It was found that there is a good corresponding relationship between the content of aromatic functional groups and oxygen-containing functional groups with the self-heating time of the coal samples. The coal with shorter self-heating time has less aromatic groups and more oxygen-containing functional groups. With the increase of the spontaneous combustion propensity of coal, there are less aliphatic hydrocarbons functional groups generally. But among different coal samples, the contents of aliphatic hydrocarbons functional groups differ not great.

In this article, a series of experiments have been carried out to study the spontaneous combustion and oxidation mechanism of coal after water immersion and investigate its tendency to spontaneous combustion, analyze the difficulty of spontaneous combustion of coal samples under different water immersion conditions, and establish a kinetic model of water immersion coal oxidation (taking the Bulianta 12# coal as a case study). They rely on physical oxidation adsorption, scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), thermogravimetry, and oil bath heating. SEM has been used to analyze the characteristics of coal pore structure under different water immersion conditions (water-saturated coal samples under different water loss conditions until the coal samples are completely dried); FTIR served to investigate the characteristics of the molecular chemical structure of the coal surface before and after the coal is immersed in water. Through programmed temperature oxidation experiments combined with FTIR analyses and gas chromatographic (GC) analysis of gaseous products, it has been possible to study the changes of molecular structure and gas products on the surface of coal samples at different temperatures and water immersion conditions. The oxidation reaction rate of the 12# coal samples of Shendong Mine’s Bulianta Mine under different water content conditions during the spontaneous combustion process has been quantitatively studied. The difficulty of spontaneous combustion of coal samples has been correspondingly addressed. A kinetic model from the perspective of oxygen consumption has been proposed. Thermogravimetry-differential scanning calorimetry (TG-DSC) has been used to analyze and study the exothermal oxidation process before and after coal immersion. From the perspective of the exothermic intensity of the coal-oxygen reaction, an oxidation kinetic model for immersed coal samples has been developed to qualitatively determine its spontaneous combustion tendency. Results have shown that the increase in the specific surface area increases the risk of spontaneous combustion, and coal samples after soaking and drying have a stronger tendency to spontaneous combustion than raw coal. The moisture content of the coal sample leading to the easiest ignition conditions is 16.05%. Regardless of the moisture content, the critical temperature is maintained at 65–75°C, and the temperature of the left coal in the goaf should be prevented from exceeding this critical value.

Micro-characteristics of low-temperature coal oxidation in CO2/O2 and N2/O2 atmospheres

Changes in stacking structure and the functional groups of three non-coking coals during pyrolysis have been carried out using X-ray diffraction (XRD) and Fourier-Transform Infrared Spectroscopy (FTIR). The interlayer spacing of the stacking structure changed from 3.44 to 3.66 Å, 3.49 to 3.68 Å, and 3.60 to 3.72 Å and aromaticity changed from 0.64 to 0.70, 0.63 to 0.74, and 0.68 to 0.78 due to pyrolysis of the samples AMD, BMD and JMD respectively. The rank parameters also change from 1.80 to 2.29, 1.72 to 2.36, and 1.61 to 3.49 for the same coals in similar order. It is observed that the interlayer spacing, aromaticity and rank of coal increase with the increase in temperature. The FTIR results show that the functional group associated with minerals increases while the functional groups associated with coal macerals like methyl group, C=C aromatic, and oxygen-containing functional groups decrease due to pyrolysis. The FTIR structural parameters such as the ratio of aliphatic to the total atomic hydrogen, and the ratio of carbonyl to aromatic groups decrease with the increase in temperature while aromaticity, degree of condensation of aromatic rings, and the ratio of aliphatic to aromatic carbon increase with the increase in temperature up to 600°C. The sudden changes in FTIR structural parameters of coals are observed at 800°C. The present study shows that with increasing temperature, the aromatization and degree of condensation of aromatic rings of coal increase with the removal of aliphatic side chains and reduction in oxygen-containing functional groups.

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.

When residual coalin high-temperature mine goafs collapses, itis subjected to prolonged exposure to hot airflow before reachingspontaneous combustion conditions. This process significantly affectsthe coal’s oxidation activity and spontaneous combustion risk.To determine the influence patterns of high-temperature environmentand high-temperature airflow on the functional groups in coal, tests,and analysis of the functional groups of coal under different temperatureswithout airflow and different airflow rates were conducted. The researchfindings indicate that the influence patterns of environmental temperatureand airflow rate on different functional groups in coal moleculesvary. The high-temperature environments of 35 and 65 °C withoutairflow result in an increase in the relative contents of most oxygen-containingfunctional groups, aliphatic units, and hydroxyl functional groups.Under the long-term erosion of different temperatures and airflowrates, most functional groups increase or decrease. Notably, oxygen-containingfunctional groups exhibited a significant increase under low-flow65 °C conditions. In comparison with raw coal, under the circumstancesof 35 and 65 °C, the aromatic hydrogen ratio (AHR) of the coalsamples exhibited varying degrees of increase resulting from erosionin both windless and low wind flow conditions. With the exceptionof the coal sample eroded by a 200 mL/min 35 °C hot airflow overan extended period, where the aromatic ring condensation degree (AOC)decreased, the AOC of other coal samples rose to different extents.Whether subjected to high-temperature alone or high-temperature airflow,the length of the aliphatic chain of the coal samples all manifesteda decreasing tendency to varying degrees. Nevertheless, the aliphaticchain (CL) value of the coal samples eroded by hot airflow for a prolongedtime decreased relatively significantly, generating more oxygen-containingfunctional groups such as hydroxyl and carboxyl. Save for the coalsamples eroded by 200 and 100 mL/min 35 °C hot airflow over anextended period, where the CO to CC ratio (C) of the raw coal decreased, the C valueof the coal molecules of other coal samples increased markedly. Theexperimental results of functional groups and related indicators suggestthat long-term erosion in a high-temperature windless environmentor by low-flow high-temperature airflow might cause strong oxidationreactivity of coal, facilitating oxidation and increasing the propensityfor spontaneous combustion.

This study investigates the influence of long-term water immersion on the spontaneous combustion behavior of bituminous coal across four particle size ranges (0.5–1, 3–5, 6–9, and 9–12 cm). To simulate underground goaf conditions, samples were immersed in water for three years. Combustion characteristics were assessed using Crossing Point Temperature (CPT), Thermogravimetric Analysis (TGA), and Scanning Electron Microscopy (SEM). Raw coal showed the highest susceptibility, with a CPT of 145°C and an IFCC index of 11.85 min−1. Water immersion increased ignition thresholds, especially in coarse samples: CPT values rose to 168°C for 6–9 cm and 166°C for 9–12 cm, placing them in the low-risk category. Activation energy values supported this trend, increasing from 85.32 kJ/mol in raw coal to 110.3 kJ/mol in fine immersed coal and up to 119.0 kJ/mol in larger particles, indicating enhanced thermal stability. SEM results showed that fine particles retained a porous structure that facilitates oxygen uptake, whereas coarse samples developed smoother, agglomerated surfaces that restricted diffusion. Overall, long-term immersion increased reactivity in fine coal but significantly reduced the spontaneous combustion propensity of coarse particles. These findings highlight the importance of particle size management in reducing combustion hazards in submerged or collapsed coal environments.

Among the fossil fuels, coal is the most widely used one all over the world for different purposes and is a stable source of energy. Coal mining has serious hazards such as spontaneous combustion. Many factors can influence the tendency for occurrence of this phenomenon in coal mines and coal storages, one of which is the petrographic characteristics (maceral and pyrite contents). The petrographic characteristics are directly affected by the growth of coalification and origin of coal forming. In this research work, firstly, the maceral and pyrite contents in coal samples were identified in two various mines in Iran (Tabas Parvadeh and Eastern Alborz coal mines). Then the spontaneous coal combustion propensity of each sample was measured using the Crossing Point Temperature (CPT) test method. The aim for carrying out this research work was to measure the effect of the maceral and pyrite contents on the grade of spontaneous coal combustion propensity. It was found that the pyrite contents and the vitrinite, liptinite, and inertinite mean levels in the Tabas Parvadeh and Eastern Alborz coal mines were (2.08%, 53.30%, 9.91%, 34.71%) and (1.14%, 53.06%, 6.39%, 39.41%), respectively. The mean CPT value was 148.43ºC in the Tabas Parvadeh coal mines and 175.86ºC in the Eastern Alborz coal mines. By examining the results of the experiments and comparing the CPT values, it was found that with an increase in the liptinite and pyrite contents and a decrease in the inertinite content, the Tabas Parvadeh coal mines tend to have more tendency for spontaneous coal combustion in comparison with the Eastern Alborz coal mines.

Effect of microwave irradiation on the propensity for spontaneous combustion of Inner Mongolia lignite

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.

ABSTRACTAs coal reservoirs containing high methane content are more prone to spontaneous combustion, disasters caused by the combined effects of methane and coal spontaneous combustion have been increasingly prominent. Given that air flows in different zones in a goaf contain methane with different contents, the mechanism governing the changes of micropores and functional groups of left-over coal subjected to spontaneous combustion in a goaf with air flows containing methane were investigated by sufficiently considering the diluted influence of methane. The change of microscopic functional groups and variation of gaseous products from coal oxidized at low-temperature (i.e., 70°C< it < 230°C) under different oxidizing atmospheres were separately acquired by employing Fourier transform infrared spectroscopy (FTIR) and gas chromatography. By using the Brunauer, Emmett and Teller (BET) method, a low-temperature nitrogen adsorption experiment was carried out on coal samples taken from the Shigang coal mine in Shanxi province, China under different methane-diluted oxidizing atmospheres. The results showed that the content of aliphatic hydrocarbons (methyl and methylene) of the coal oxidized at low temperature decreased with rising oxidizing temperature and reached a maximum at the condition of a 25% methane concentration. Meanwhile, other oxygen-containing functional groups and corresponding gaseous products were generated. The aforementioned results indicated that the initial temperature for the generation of CO and the amount generated both showed delayed effects in oxidizing atmospheres with different methane concentrations. By carrying out a low-temperature nitrogen adsorption experiment, it was determined that the average sizes of the pores in coal decreased in an oxidizing atmosphere (i.e., < 15% methane concentrations at a high temperature). However, the specific surface area (SSA) and accumulative total internal surface area of the pores increased. Additionally, the pore structure in the coal tended to be microscopic and complex in the aforementioned oxidizing atmosphere, as determined by using the fractal dimension D. The pore structure in the coal was increasingly complex after being oxidized in an oxidizing atmosphere containing methane, which increased the likelihood of the coal oxidation reaction and increased the occurrence probability of coal spontaneous combustion.

Characteristics and mechanism of Glutathione in inhibiting coal spontaneous combustion

Spontaneous combustion behaviour and physicochemical characteristics of Ximeng lignite dewatered by hydrothermal dewatering (HTD) were investigated. In addition, effect of upgrading temperature, as well as the mechanism for evolution of spontaneous combustion propensity was discussed. The results showed that after HTD, fixed carbon content of lignite increased, whereas equilibrium moisture content and volatile content decreased; aromatic carbons increased relatively at the expense of oxygen‐containing functional groups and aliphatic hydrocarbons. Pore structures of lignite were developed by different extents after HTD. Spontaneous combustion propensity of upgraded lignite was evaluated using the crossing‐point temperature method. The results indicated that the spontaneous combustion propensity of HTD‐upgraded lignite significantly depended on upgrading temperature. When upgrading temperature was lower than 230 °C, spontaneous combustion propensity was raised due to development in pore structure. However, when upgrading temperature exceeded 230 °C, spontaneous combustion propensity was suppressed because active sites involved in coal oxidation were extensively removed. In conclusion, a higher upgrading temperature is recommended to extend the upgrading effect and to further suppress the spontaneous combustion propensity of lignite.

Two Australian thermal coals were treated with four different ionic liquids (ILs) at temperatures as low as 100 °C. The ILs used were 1-butylpyridinium chloride ([Bpyd][Cl]), 1-ethyl-3-methylimidazolium dicyanamide ([Emim][DCM]), 1-butyl-3-methylimidazolium chloride ([Bmim][Cl]), and 1-butyl-3-methylimidazolium tricyanomethanide ([Bmim][TCM]). Visual comparisons were made between the raw and IL-treated coals via optical microscopy. Changes in thermal behavior of these treated coals were compared against raw coals via pyrolysis experiments in a thermogravimetric analyzer (TGA). Changes in functional group composition in the treated coals were probed via Fourier transform infrared (FTIR) spectroscopy. The recovered ILs were also analyzed via FTIR and nuclear magnetic resonance (NMR) spectroscopies to observe any changes after recovery. Low-temperature IL treatment of each of the coals resulted in fragmentation and fracturing, reducing the average particle size. An increase in mass loss in the treated coals was also observed when compared to each raw coal, indicating an increase in lower molecular weight fragments after treatment. This was corroborated by a large increase in aliphatic hydrocarbons being observed in the treated coals, along with a decrease in oxygenated functional groups and mineral matter in one coal. The recovered ILs were shown to be unchanged by this treatment process, indicating their potential recyclability. These results indicate the potential for ILs to be implemented as solvent treatments for coal conversion processes.

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