Kinetic Characteristics and Mechanism of CO Generation in Coal Spontaneous Combustion Under the Influence of Ventilation Rate

Substituent positions and types for the inhibitory effects of phenolic inhibitors in coal spontaneous combustion

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.

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.

Spontaneous combustion of coal is a phenomenon that exists in all coal mining countries. The characteristics of CO production during the spontaneous combustion of coal are the focus of attention of researchers. A novel programmed oxidation heating simulation system with a small-sample size was developed to study the combustion characteristics. The device was used to study the characteristics of CO generation during low-temperature coal oxidation under different conditions (three coal ranks, six particle sizes, four air volumes, and four oxygen concentrations). The microscopic characteristics of the coal structure were tested through Fourier-transform infrared spectroscopy (FTIR), and three types of active functional groups were observed. Ten factors affecting gas production in the low-temperature oxidation stage were analyzed by gray correlation. This study found that moisture in coal had the greatest effect on CO generation, followed by O-containing functional groups. Aliphatic hydrocarbons had the least correlation with CO generation. Further, the CO concentration increased exponentially during the heating process in low-temperature oxidation. In the low-temperature stage, carboxyl, carbonyl, and side-chain C–O bonds of the benzene ring were the main sources of CO generation.

The spontaneous combustion of coal caused by oxidation often leads to catastrophic fires. However, the understanding of oxidized carbon gas as a predictor of coal’s spontaneous combustion is still in its infancy. To better study the characteristics of CO2 and CO generation during low-temperature coal oxidation, the chemical reactions and activation energies during the formation of oxidized carbon gases within coal molecules were investigated using the molecular simulation method, and the reaction characteristics at different temperatures were determined. In addition, TG was used to experimentally analyze the variations in coal weight, exothermic conditions, and gas generation patterns. The results show that the low-temperature oxidation process consists of four different phases, each of which is characterized by unique CO and CO2 generation. The results of this study are important for the prevention and prediction of the spontaneous combustion of coal.

Study on evolution characteristics of thermal contribution functional groups in low temperature oxidation process of bituminous coal

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.

ABSTRACTTo explore the effects of methane on the characteristics of coal spontaneous combustion, coal low-temperature oxidation and Fourier transform infrared spectroscopy experiments were conducted. The tested airflows had an oxygen–nitrogen ratio of 0.266, and methane concentrations of 0−3% by volume. Moreover, the coal oxidation characteristics and the various types and quantities of functional groups were investigated. The results show that CO/CO2 production and oxygen consumption rates were higher for coal samples under airflow atmospheres containing methane than for airflow atmospheres without methane. However, the apparent activation energy of coal samples in airflows containing methane was lower than for airflow atmosphere without methane. It is speculated that airflows with 1−3% methane concentrations have a positive impact on coal spontaneous combustion. This study used OMNIC and PeakFit v4.12 programs to analyze the effect of airflows containing methane on the functional groups of coal spontaneous combustion. The Pearson correlation coefficient method was introduced to analyze the relationship between the key functional groups and apparent activation energies of coal samples. The results indicated that the key functional groups of the coal samples below the critical temperature were methyl (−CH3) and methylene (−CH2). Above the critical temperature, aromatic C−H, methyl (−CH3), and methylene (−CH2) were the key functional groups. After reaching the cracking temperature, aromatic C−H and alkyl ether C−O−C became the key functional groups. With increasing methane concentration, aromatic C–H makes a negative contribution to coal spontaneous combustion, while methyl (−CH3) and methylene (−CH2) make positive contributions. The inhibition of alkyl ether C−O−C to coal spontaneous combustion is weakened, and the inhibition of aromatic ring C=C to coal spontaneous combustion is enhanced.

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

Variation characteristics of active groups and macroscopic gas products during low-temperature oxidation of coal under the action of inert gases N2 and CO2

Spontaneous coal combustion is one of the most common disasters in coal mine production. In order to explore the mechanism of coal spontaneous combustion more deeply, coal samples from the Yangdong wellfield of Jizhong Energy were selected for oxidative heat energy analysis experiments. A temperature-programmed experiment was selected to study the changes in characteristic parameters during the low-temperature oxidation of coal under different air supply conditions. TG-DSC experiments were conducted to study the characteristic temperature changes and thermodynamic characteristics of coal combustion processes at different heating rates. The study results show that the coal is most easily oxidised in the low-temperature oxidation stage when the air supply is 120 ml/min. The oxygen consumption rate, CO generation rate, and maximum and minimum heat release intensity are all greater at this airflow than under other conditions. The process of spontaneous combustion of coal has six characteristic temperature points and is divided into five stages. The characteristic temperature of the coal sample increased with the increase of the heating rate, and the TG/DTG curve showed a hysteresis phenomenon. DSC temperature curve shifts toward the high temperature with the increase of the heating rate, and the exothermic region is expanded. Isokinetic analysis (F-W-O and V-W) and Coats-Redfern model for calculating thermodynamic parameters. The activation energy of the samples decreased with the increase of the heating rate in the range of 2∼20°C·min−1 and showed a decreasing trend with the increase of the conversion rate.

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

Low temperature oxidation of coal is the nature of oxidation of coal with oxygen molecules at temperatures below its oxidation critical point or ignition point, usually from ambient temperature to about 200°C. Air leakage and uneven oxygen distribution significantly impact coal spontaneous combustion in goaf. Understanding the changes in free radicals and functional groups during low-temperature oxidation of coal under different oxygen concentrations is crucial for preventing and controlling residual coal self-ignition. This study employs Fourier transform infrared spectroscopy to analyze the effects of varying oxygen concentrations (3%, 5%, 9%, 15%, 21%) on functional groups and structural parameters during coal’s low-temperature oxidation. Results indicate that increasing oxygen concentration promotes the consumption of oxygen-containing functional groups, reducing their content from 71.9% to 52%. Concurrently, the activity and content of aromatic hydrocarbon functional groups rise rapidly from 4.8% to 52.6%. The aliphatic hydrocarbon structure shows a downward trend, indicating its minimal presence. Aromatic ring condensation increases coal sample aromaticity, suggesting higher maturity and weaker hydrocarbon generation potential, with minimal effect from oxygen concentration. Early-stage reactions show little change in coal’s basic structure, and coal-oxygen reactions remain low in low oxygen environments. This study provides an important basis for understanding the mechanism of coal spontaneous combustion in goaf and formulating effective prevention measures for coal spontaneous combustion.

The intrinsic reaction of coal with oxygen in the process of low-temperature oxidation is the main reaction path leading to self-heating and spontaneous combustion of coal. Most of the existing studies regard the coal oxidation as an overall reaction, ignoring the multi-path characteristics of coal low-temperature oxidation, and it is difficult to accurately explore the intrinsic reaction characteristics between coal with oxygen. Therefore, the low-temperature oxidation process of coal was studied by using a C80 microcalorimeter and in situ FTIR technology from the macro and micro levels. The "profile subtraction method" was used to study the coal-oxygen intrinsic reaction process, and the reaction heat effect and the change characteristics of key functional groups in the process were analyzed. Furthermore, the gray correlation analysis method was used to study the relevant characteristic parameters in the reaction process and grasp the essential structure-activity relationship. The experimental results show that, compared with the overall reaction process in air atmosphere, the change in the heat release of the coal-oxygen intrinsic reaction path has changed to different degrees, and the change in the slow oxidation stage is the most significant (the heat absorption decreases by 70.1-90.9%). In addition, the characteristic temperature points show different degrees of advance, of which the initial exothermic temperature point is the largest (about 21-46 °C), which directly leads to a significant shortening of the slow oxidation stage (30.1-47.4%). The changes of functional groups in the intrinsic reaction path are more regular. With the increase of temperature, the oxygen-containing functional groups -C=O and the aliphatic hydrocarbon functional groups -CH2- and -CH3 showed a fluctuating trend of increasing and decreasing, respectively. The oxidation heat-contributing functional groups of coal are mainly related to the degree of metamorphism and the functional group reaction characteristics during the reaction. With the deepening of coalification degree, the main heat-contributing functional groups as a whole showed the change rule of oxygen-containing functional groups → aliphatic hydrocarbon functional groups → aromatic hydrocarbon functional groups. In addition, the change of -OH content in the three coal samples has a high correlation with the change of the total heat release of coal.

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.