An experimental investigation of early warning index for coal spontaneous combustion with consideration of particle size: a case study

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.

Study on the influence of key active groups on gas products in spontaneous combustion of coal

Comprehensive evaluation of low-temperature oxidation characteristics of low-rank bituminous coal and oil shale

The substantial accumulation of oxidized coal in goaf areas poses significant coal spontaneous combustion (CSC) risks, threatening coal mine safety and ecological preservation. To systematically investigate the influence of pre-oxidation temperature (POT) on the oxidation characteristics and CSC limit parameters of non-caking coal, temperature-programmed experiments were conducted under varying pre-oxidation conditions. The evolution of CO emissions and oxygen consumption behavior was analyzed, three oxidation stages were delineated, and key oxidation kinetic parameters alongside CSC limit parameters were determined. The experimental results revealed that in the slow and rapid oxidation stages, oxidized coal samples exhibit higher oxygen consumption rates and heat release intensities than raw coal, with these values decreasing as POT increases, while in the deep oxidation stage, coal samples with high pre-oxidation degrees (YH160, YH200) show significantly greater reactivity than other samples. Apparent activation energy displays distinct stage-dependent responses to POT: it decreases with increasing POT in the slow oxidation stage, increases with rising POT in the rapid oxidation stage, and first decreases then increases as POT increases in the deep oxidation stage. This also confirmed that the “promotion-inhibition-promotion” dual effect of POT on the secondary oxidation of coal. Additionally, POT exerts a non-negligible influence on spontaneous combustion limit parameters. As POT increased, the lower limit of coal thickness(hmin) and the lower limit of oxygen concentration (cm) decreased, whereas the upper limit of air leakage intensity (Qmax) increased, collectively amplifying combustion risks of spontaneous. The research findings can provide a theoretical reference for the prediction of re-ignition in coal mine goafs and the development of fire prevention and extinguishing technologies.

It is imperative to have an in-depth understanding of the effect of extraneous moisture on the spontaneous combustion of coal not only for the control and prevention of coal spontaneous combustion in the coal mining industry, but also for the optimization design and application of the technological process. In this study, the type of moisture in a coal body has been redefined for the first time from the perspective of disaster prevention and control, i.e., original occurrence of moisture in the coal matrix and the extraneous moisture from the technological process. A suit of coal bodies with different extraneous moisture was prepared by soaking long-flame coal with a low water content. Using a temperature-programmed oxidation test, the effects of extraneous moisture on the temperature increase rate of coal bodies and the emission characteristics of gaseous products during coal spontaneous combustion were studied. Moreover, combined with the characterization of thermal analysis and of pore structure test, the action the mechanism of extraneous moisture on the coal spontaneous combustion process was also explored. The experimental results indicated that the effect of the extraneous moisture content varied with the development of coal spontaneous combustion. In the slow oxidation stage, extraneous moisture played a physical inhibition role in the coal oxidation. In the accelerated oxidation stage, extraneous moisture exhibited a catalytic effect on the coal–oxygen reaction or directly participated in the reaction. After entering the rapid oxidation stage, a delayed effect appeared. When the coal temperature exceeded 180 °C, the spontaneous combustion characteristics of coals with different initial moisture contents gradually tended to achieved balance.

It is imperative to have an in-depth understanding of the intrinsic reaction between coal and oxygen during low-temperature oxidation, as the reaction is the main source responsible for the self-heating and spontaneous combustion of coal. As low-temperature oxidation of coal involves a series of physical and chemical process and many parallel reactions, it is difficult to directly investigate the intrinsic reaction between coal and oxygen by conventional analytical method. Thermogravimetric analysis (TGA) was used to investigate the intrinsic reaction between coal and oxygen based on the mass change. By means of the subtraction analysis method of TGA, the TG-subtraction curves were obtained by subtracting the TG-N2 curves from the TG-air curves. The results indicate that a TG-subtraction curve can better reflect the intrinsic reaction of coal oxidation than a TG-air curve by eliminating the influence of evaporation of water and thermal decomposition of inherent oxygen-containing groups. In terms of the rate of mass increase, the intrinsic reactions can be divided into three stages: slow oxidation stage, advanced oxidation stage and rapid oxidation stage. The activation energy at each of the stages, obtained by Coats and Redfern’s model, can be used to as a technical parameter to evaluate the proneness of coal spontaneous combustion. The optimum experiment conditions were also developed to study low-temperature coal oxidation with the subtraction method of TGA.

Much of the residual coal in a goaf was immersed in water for extended periods of time, which leads to complicated coal spontaneous combustion (CSC) characteristics that are difficult to predict. In this work, five kinds of coal samples with different moisture content (MC) of 4%, 11%, 18%, 25% and 32% were prepared by soaking, and a temperature programmed test was carried out. The gas produced by the oxidation of coal samples with different moisture contents and its concentration variation law were analyzed during temperature rise, and the index gas suitable for predicting CSC was selected. The concentration of gas generated by coal sample heating increased with increasing temperature. We also found that CO, C2H6, C2H4 and C3H8 can be used as the single auxiliary index gas for CSC prediction, and the composite gases CO/CO2 (ω) and C2H4/C2H6 (ξ) can be used to predict the CSC oxidation stage comprehensively and reliably under different degrees of water immersion. The process of coal oxidation was divided into three stages: oxygen absorption and thermal storage (ω ≤ 0.133), slow oxidation stage (0.758 ≤ ξ ≤ 0.919) and accelerated oxidation stage (ω ≥ 0.401 or ξ ≥ 0.919). Combined with the actual conditions at different coal mines, timely parameter index correction can improve the CSC prediction index and provide guidance for preventing and controlling coal spontaneous combustion and safety management of coal mines.

Air humidity has a significant effect on the characteristic parameters and heat changes of the coal spontaneous combustion process. In this study, a self-built precision humidity-generating device with adjustable humidity was used to investigate the effect of different humidity conditions on the spontaneous combustion process of coal. The release of indicator gases was determined by a humidity-generating unit in conjunction with a temperature-programmed device for coal oxidation, and the effect of air humidity on coal spontaneous combustion was studied from a macroscopic perspective. A quantitative study of the effect of air humidity on the heat released during the spontaneous combustion of coal was carried out by obtaining data for exothermic heat release intensity by conducting thermal analysis. Meanwhile, the numerical simulation method was used to study the influence of air humidity on the heat released during the coal spontaneous combustion process. The experimental results indicated that the introduction of moist air could significantly increase the oxygen consumption rate of the coal samples and promote the production of CO and CO2. In the slow oxidation stage, the air humidity has a greater effect on the heat release than that in the accelerated and fast oxidation stages. The simulated results of heat release in the slow oxidation phase are consistent with the experimental data, verifying that the moisture in the air can increase the amount of heat release and promote the coal-oxygen reaction. When the moisture in the air first came into contact with the coal, it released a large amount of adsorption heat in a short time, which increased the coal temperature.

Kinetics characteristics of coal low-temperature oxidation in oxygen-depleted air

The intrinsic exothermic reaction of coal with oxygen during low-temperature oxidation is the main heat source responsible for the self-heating and spontaneous combustion of coal. Due to the complexity of low-temperature oxidation of coal with many parallel reactions, it is difficult to directly investigate the heat evolution due to the reaction of coal with oxygen by conventional analytical method. Differential scanning calorimeter (DSC) was introduced in this study to study the intrinsic exothermic reaction of coal with oxygen based on the heat evolution. Using the subtraction analysis method of DSC, the DSC-subtraction curves can be obtained by subtracting the DSC–N2 curves from the DSC-air curves. Experimental results indicate that the DSC-subtraction curve can better reflect the intrinsic exothermic reaction of coal oxidation than a DSC-air curve by eliminating the influence of evaporation of water and thermal decomposition of inherent functional groups. The experiment factors affecting the intrinsic reaction were also investigated with the subtraction method of DSC. In terms of the kinetics characteristics of heat evolution, the intrinsic reaction of coal with oxygen is divided into three stages: slow oxidation stage, advanced oxidation stage, and rapid oxidation stage. The activation energies and pre-exponent factors at each of the stages were also calculated by oxidative pyrolysis kinetics. The subtraction method applied in this study is a quite promising method to determine the susceptibility of coal to self-heating and spontaneous combustion.

As a key parameter, the particle size of residual coal contributes greatly to its oxidation characteristics, so it is a significant and far-reaching topic to explore the role of different particle sizes in coal spontaneous combustion disaster. In this work, temperature-programmed system (TPS) was applied to analyze the oxygen consumption rate and CO and C2H4 production rules of six groups of coal samples with different particle sizes in the process of oxidation heating. The critical temperature (CT) and xerochasy temperature (XT) of different coal samples were obtained, and the coal oxidation process was divided into three stages (S1, slow oxidation stage; S2, fast oxidation stage; and S3, combustion stage). Then, the apparent activation energy (E) and pre-exponential factor (A) in three stages were regressed combined with Arrhenius formula. The results show that the smaller the coal particle size is, the larger the specific surface area is, the stronger the adsorption capacity of coal molecules and oxygen molecules is, resulting in the larger oxygen consumption rate. The values of CT and XT with particle size of 0.125-0.18mm and 2-4mm are the smallest and largest. For coal samples with the same particle size, the maximum values of E and A occur in stage S3 and the minimum values appear in stage S1. This is mainly due to the higher temperature of stage S3, which allows the activation of functional groups with higher apparent activation energy, stronger collisions between activated molecules, and more intense oxidation reactions.

Due to high stress, high ground temperature, high moisture, and other factors in deep mines, the risk of coal spontaneous combustion (CSC) is enhanced, seriously affecting the safety of coal mining. To achieve early prediction of spontaneous combustion in the No. 3 coal seam at the Juye coalfield in the deep mine, this paper employs a temperature-programmed device to analyze the changing pattern of single-index gases and composite gas indices with temperature derived from the gas produced during csc. It also optimizes the index gas of coal sample spontaneous combustion. Simultaneously, the characteristics of coal temperature and a four-level warning indicator system for CSC are determined based on the analysis of indicator gas growth rate method, carbon-to-oxygen ratio, and the characteristics of the indicator gas. The composite index gases of the No. 3 coal seam in Juye coalfield are selected in the initial oxidation stage (Rco), accelerated oxidation stage (R1, G1), intense oxidation stage (R2, G1, G3), and oxidative decomposition stage (G3). This leads to the construction of a six-level warning system consisting of initial warning value, blue, yellow, orange, red, and black levels. Meanwhile, warning thresholds are also established.

Liquid carbon dioxide has an excellent ability of endothermic cooling and inhibition on coal fire, which was an effective coal spontaneous combustion prevention technology. To analyze the oxidation characteristics and variation of apparent activation energies, a carboniferous–permian coal sample was investigated in O2/N2 and O2/CO2 atmospheres by the coal spontaneous combustion oxidation and the Fourier transform infrared spectroscopy experiments. The results indicated that with temperature, carbon monoxide (CO) concentration and oxygen (O2) consumption rate increased. While O2 concentration decreased, CO concentration and oxygen consumption rate reduced. At the same O2 concentration, the oxygen consumption rate and CO concentration on the O2/CO2 atmospheres were less than on the O2/N2 atmospheres. Therefore, O2 concentration reduced, or added CO2 significantly inhibited coal oxidation. As the temperature elevated, the apparent activation energy gradually increased. Furthermore, the apparent activation energy increased when the oxygen concentration reduced in the physical–chemical adsorption stage and the slow oxidation stage. In the rapid oxidation stage, the apparent activation energy lessened with increase in oxygen concentration. Through correlation analysis, the key functional groups in the physical–chemical adsorption stage were hydroxyl, C–O, –COO–, and aliphatic hydrocarbons. During the slow oxidation, the key functional groups were –COO– and aliphatic hydrocarbons. The key functional groups in the rapid oxidation stage were hydroxyl and C–O.

One of the most catastrophic events that can occur during coal mining is a fire triggered by spontaneous combustion of the coal. Due to complex geological conditions, some coal mines in China have the characteristics of shallow burial and small spacing between coal seams. With the increase of mining depth, there will be many cracks in the overlying goaf. Surface water and groundwater flow into the goaf along these fissures. After long-term water immersion, the increase of ground temperature will lead to the spontaneous combustion of coal. A thorough analysis of the impact of water immersion on the spontaneous combustion of coal necessitates a review of the most recent findings in this field. The review will promote the further development of the theory and prevention of spontaneous combustion of water-immersed coal and provide theoretical help for monitoring and preventing spontaneous combustion of water-immersed coal. The creation of water-immersed coal and the reason for spontaneous combustion is investigated, as are the characteristics of the water accumulation in the goaf and their origins. The research status of spontaneous combustion of water-immersed coal is presented and illustrated domestically and internationally. On this basis, the theoretical and experimental research directions, problems, and prospects of spontaneous combustion of water-immersed coal are put forward. This study can provide a theoretical basis for monitoring, preventing, and controlling the spontaneous combustion of water-immersed coal, and provide a reference for reducing energy waste.

Conventional single gas alarm method and coal spontaneous combustion three-stage alarm method have become increasingly inadequate to meet complex underground conditions. To address this issue, this study focuses on the coal in the goaf of the Z109 working face in the Donggucheng Mine as the research object. Through program-controlled heating experiments, the production of carbon monoxide and hydrocarbon gases during the coal oxidation process was determined, and the variation characteristics of gas ratios with temperature were further analyzed. The coal spontaneous combustion process was subdivided into seven small stages, and a quantitative composite parameter coal spontaneous combustion grading warning system was formulated. Based on its characteristics, measures to be taken under different warning levels were proposed, and it was determined that 120, 140, and 160 °C are the key temperatures for coal spontaneous combustion prevention and control in the Z109 working face of Donggucheng Mine. By using numerical simulation, the optimal nitrogen injection position for the working face was determined and the on-site fire prevention and extinguishing measures were optimized, providing insights into the establishment of a coal mine spontaneous combustion warning system.