Low-temperature Oxidation Of Coal Research Articles

Effect of Water in Coal Matrix on the Physicochemical Adsorption Process During Low-Temperature Oxidation of Coal

ABSTRACT This study investigates the impact of water on oxygen adsorption in coal using quantum simulations, focusing on three typical oxygen-containing functional groups in lignite. Key factors, including equilibrium configurations, electrostatic potential, and reduced density gradients of water and oxygen molecules adsorbed on the coal surface, were analyzed. Thermal analysis and quantum chemical calculations were also performed to determine enthalpy changes and activation energies in oxidation reactions. The results indicate that water influences oxygen adsorption on coal surfaces through both physical and chemical mechanisms. Physically, water molecules form hydrogen bonds with the oxygen-containing groups in coal, altering the electronic density distribution and enhancing oxygen adsorption. Chemically, water reduces activation energy, modifies reaction barriers, and facilitates spontaneous combustion at lower temperatures, while also increasing enthalpy changes of oxidation reactions. Overall, water plays a dual role in regulating coal oxidation: it enhances adsorption through hydrogen bonding while also altering reaction pathways and lowering energy barriers, thereby promoting coal oxidation.

Thermodynamic Studies on the Pathway and Mechanism of Coal-Oxygen-Water Coupling Reaction of Low-Temperature Oxidation

ABSTRACT It is imperative to have an in-depth understanding of the pathway and mechanism of coal-oxygen-water coupling reaction during low-temperature oxidation of coal, not only for preventing fires in the coal industry but also for reducing emissions of hazardous gases. In this study, in-situ Infrared spectroscopy characterization and quantum chemical calculations were combined to investigate in detail the chemical interactions between coal, moisture, and oxygen. In-situ infrared oxidation experiments ascertained the types and amounts of hydroxyl, aliphatic hydrocarbon, and carbonyl functional groups in lignite coal samples with varied moisture content during LTO. Subsequently, three typical molecular structures of functional groups mainly involved in the coal-oxygen reaction were chosen as the research representatives, and their reactivity was analyzed using density functional theory (DFT). Thus, the main reaction pathways of these functional groups during the LTO of coal were studied using quantum chemical methods. The results indicated that the moisture in coal significantly affected the coal–oxygen reaction by altering the chemisorption processes in the coal–oxygen combination and the active sites in coal molecules. Quantitative calculation of thermodynamic parameters such as the enthalpy and activation energy showed that moisture had a promoting effect on some reaction steps while hindering others. Moreover, the mechanism of coal-oxygen-water coupling reaction was also explored based on the reaction sequence of intermediate complexes.

Study on Microstructure Evolution Characteristics of Coal in Low Temperature Oxidation Stage in Goaf of Longfeng Coal Mine

ABSTRACT 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.

Study on heat transfer law of low temperature oxidation of coal under convection condition

Coal oxidation in the goaf generates heat accumulation influenced by airflow, leading to heat transfer within the porous coal structure. This study experimentally investigates the heat transfer characteristics of coal under convective conditions. A temperature migration rate measurement device was developed, and a formula for calculating the temperature migration rate was derived using the steady-state heat conduction differential equation and experimental data. The study examines the effects of temperature and airflow on the temperature migration rate and heat transfer characteristics. The experimental results indicate that the heat transfer effect of coal at low temperatures is minimal and volatile. As the temperature increases, efficiency improves, and heat transfer stabilizes. Airflow facilitates coal's heat transfer, causing the temperature generated by coal oxidation to concentrate on the downwind side. Additionally, airflow inhibits coal oxidation at low temperatures, while high temperatures promote it. Furthermore, the temperature migration rate of coal decreases with increasing temperature, initially decreases and then increases with rising airflow at low temperatures, whereas the opposite trend is observed at high temperatures.

Mutual replacement mechanism of inert gas with oxygen in coal and its effect on the low-temperature oxidation process of metamorphosed coal

The ability of inert gas to inhibit the low-temperature oxidation of coal is crucial for preventing spontaneous combustion of coal. In this paper, the replacement process of inert gas and O2 in coal and the low-temperature oxidation process of inert metamorphosed coal samples were studied. The results show that when significant O2 is displaced by the inert gas, the displacement process is inhibited and some of the O2 is reabsorbed into the coal, resulting in a decrease in O2 displacement. This decrease in the amount of O2 displacement is called the amount of O2 reverse displacement (Vi). One-time and continuous replacement experiments were designed to elucidate the mutual replacement process between inert gas and O2 in coal. The Vi can be calculated as the difference between the continuous replacement amount (VC) and the one-time replacement amount (VO). In the continuous displacement process, when CO2 is used for replacement, O2 is replaced in large quantities over a short period, and the amount of O2 is replaced following the ExpDec2 function. When N2 is used for replacement, O2 is consistently replaced over a long period. This trend follows the Boltzmann function. In the one-time replacement process for 40–100 mesh coal samples, the replacement amount of O2 reaches its maximum at 90 min. When replaced by N2, the reverse replacement process of O2 is obvious, and the maximum Vi reaches 60.53 %, while 17.69 % by CO2. When replaced by mixed gas, VO increases with the increasing CO2 content in the mixed gas. After inserting, the amount of indicator gases such as CO, C2H4, and C2H6 produced during the low-temperature oxidation of coal decreases significantly, and the activation energy increases significantly. As the inerting time increases from 0 to 150 min, the inhibition first increases and then decreases, but is always present. At 90 min, the indicator gas production reached the lowest value, and the activation energy reached the peak (E1 of the metamorphic coal after N2 and CO2 insertion increased by 54.67 % and 218.78 %, respectively, in contrast to the raw coal). The activation energy of the CO2-inerted metamorphic coal sample was 202.23 % of that of the N2-inerted metamorphic coal sample.

Thermal index parameter investigation of coal–oxygen adsorption during low-temperature oxidation of coal

Oxidative spontaneous combustion is an inherent property of coal. In which, coal oxygen adsorption heat is the decisive speed step of coal spontaneous combustion self-acceleration process, and coal oxygen adsorption heat is closely related to coal pore structure. By utilising an automatic physicochemical adsorption instrument, it was found that the form of pore existence in coal was dominated by mesopores, that the pore structure, oxygen uptake capacity and temperature varied non-linearly, and that pore size was positively correlated with pore volume; Using a self–built air adsorption calorimetric experimental system, it was found that when oxygen was involved in the adsorption reaction, the presence of oxygen would lead to a significant time lag in the adsorption equilibrium. Additionally, the transition temperature of coal adsorption mode has been determined to be 40 °C, shedding light on the mechanism behind the thermal self-accelerating reaction of coal oxygen adsorption; Using the correlation method, it was obtained that the index affecting the physical adsorption heat release is the percentage of mesopore volume in the coal, while the index affecting the chemical adsorption heat release is the pore size. The research findings are anticipated to offer theoretical backing for advancing flame-resistant materials.

Inhibitory Kinetics and Mechanism of the Low Temperature Oxidation of Coal by Polyethylene Glycol-Citric Acid

ABSTRACT To address the issues of low inhibition rate, high cost, and insufficient research on physicochemical synergistic inhibition mechanisms, this study examined the inhibitory effect and mechanism of Polyethylene Glycol-Citric Acid (PEG-CA) on low-temperature oxidation of coal. Differential scanning calorimetry (DSC) was used to analyze the inhibition and thermal behavior at various inhibitor ratios. Fourier transform infrared spectroscopy was used to experimentally assess changes in the functional groups of the coal samples pre- and post-inhibition. Principal component analysis identified the key mechanism of PEG-CA in inhibiting the low-temperature oxidation of coal. It showed that PEG-CA increased the heat absorption of the coal and delayed the average thermal equilibrium temperature of the coal oxidation process by more than 70°C. The addition of PEG-CA increased the maximum heat absorption rate of the coal sample by 2.4 times and the activation energy of coal oxidation by 1.6–3.9 times. The primary inhibition mechanism is that PEG-CA-forming chelates with the metal ions of coal, preventing C-H bond activation and reducing the content of this functional group. Additionally, PEG-CA etherifies with the hydroxyl groups of the coal, creating relatively stable ether bonds. This study offers a cost-effective and eco-friendly solution for the coal mining industry and provides a new theoretical foundation for developing composite corrosion inhibitors and controlling spontaneous coal combustion.

Investigating how oxygen levels and particle size impact the thermodynamics of low temperature coal oxidation: A case study using weakly caking coal

Oxygen concentration and particle size play critical roles in the combustion of coal. Notably, the low temperature oxidation stage is considered pivotal for coal spontaneous combustion (CSC). In this study, we examined the exothermic oxidation of low-temperature weakly caking coal. We used a C80 microcalorimeter to assess the impact of different oxygen concentrations and particle sizes. For weakly caking coal during low-temperature oxidation, the heat flow (HF) curve decreases as oxygen concentration drops. The characteristic temperature increases, and the HF curve shows a noticeable lag. Under identical external conditions, decreasing the particle size of coal results in a reduced minimum floating coal thickness, lowers the oxygen concentration needed for CSC, increases the upper-limit of air leakage intensity, and makes the coal more susceptible to CSC. Reducing the particle size significantly increases the exothermic heat release during low-temperature oxidation, leading to a higher risk of CSC. These research results not only deepen the understanding of the law of coal spontaneous combustion, but also provide a scientific basis for improving the technical level of coal spontaneous combustion fire prevention and control.

Mechanism and In Situ Prevention of Oxidation in Coal Gangue Piles: A Review Aiming to Reduce Acid Pollution

The acid pollution produced from coal gangue piles is a global environmental problem. Terminal technologies, such as neutralization, precipitation, adsorption, ion exchange, membrane technology, biological treatment, and electrochemistry, have been developed for acid mine drainage (AMD) treatment. These technologies for treating pollutants with low concentrations over a long period of time in coal gangue piles appear to be costly and unsustainable. Conversely, in situ remediation appears to be more cost-effective and material-efficient, but it is a challenge that coal producing countries need to solve urgently. The primary prerequisite for preventing acidic pollutants is to clarify the oxidation mechanisms of coal gangue, which can be summarized as four aspects: pyrite oxidation, microbial action, low-temperature oxidation of coal, and free radical action. The two key factors of oxidation are pyrite and coal, and the four necessary conditions are water, oxygen, microorganisms, and free radicals. The current in situ remediation technologies mainly focus on one or more of the four necessary conditions, forming mixed co-disposal, coverage barriers, passivation coatings, bactericides, coal oxidation inhibitors, microorganisms, plants, and so on. It is necessary to scientifically and systematically carry out in situ remediation coupled with various technologies based on oxidation mechanisms when carrying out large-scale restoration and treatment of acidic coal gangue piles.

Research on the effect and mechanism of composite phase change materials inhibiting low-temperature oxidation of coal

Research on the effect and mechanism of composite phase change materials inhibiting low-temperature oxidation of coal

Analysis of stage parameters of low-temperature oxidation of water-soaked coal based on kinetic principles

Mine fires caused by spontaneous coal combustion are major disasters in coal mines. The staged oxidation kinetic parameters of various coal samples at oxygen concentrations of 21 %, 15 %, 10 %, 5 %, and 3 % were analyzed using a programmed temperature testing system. Herein, the temperature increase rate of coal, the temperature difference between the furnace and coal, and the oxygen consumption characteristics were obtained. Based on the amount of CO produced and the temperature sensitivity coefficient, three characteristic temperatures and four stages of low-temperature oxidation (LTO) were identified. The results showed that at a critical temperature (TC), the amount of CO gas released from the coal samples increased with increasing oxygen concentration, and the difference in the oxygen consumption rate increased. After the limit temperature (Tu), the amount of CO gas increased steadily, and the increase in the oxygen consumption rate stagnated. CO production, the maximum heating rate, and the maximum heat release rate were positively correlated with the oxygen concentration. As the oxygen concentration increased, the activation energy during the oxygen absorption stage gradually decreased. The average reaction enthalpy (ΔH) of pre-oxidized water-immersed coal was 19.37 kJ/kg greater than that of raw coal. The equation for the conservation of energy of the coal oxidation warming process was normalized. The theoretical values of the awakening stage and the stable stage were τν and τν (1-B), respectively. When B was >1, pre-oxidized water-immersed coal at a low oxygen concentration was prone to crossover points during the oxygen absorption stage, which increased the risk of coal spontaneous combustion (CSC). The research results could provide a theoretical basis for the staged control of the spontaneous combustion of water-immersed coal in goaf areas.

Online CO Monitoring During Low Temperature Oxidation of Zonguldak Coals

ABSTRACT During low temperature oxidation of coals, due to half reaction of carbon with oxygen, concentration of CO is readily being increased and it is monitored by the field operators via gas detectors. In this study, low temperature oxidation tests were carried out for 26 samples, which were collected from Zonguldak hardcoal basin. In order to have observation CO concentrations levels during low temperature oxidation, a complete experimental set up was established. The experimental set up abovementioned includes two gas chromatographs (GC HP 5890 and GC HP 6890), one of which (HP 5890) is employed for oven purposes. As in the case of low temperature oxidation testing procedure, a starting temperature and the ramp is defined as 30°C and 0.5 °C/min, respectively. Every 15 minutes, (7.5°C increase) gas sample produced from coal via the process of low temperature oxidation was taken and analyzed by the gas chromatograph. Based on the results obtained, the amount of CO generated at low temperature (at 37.5 °C) was found to be a significant measure for the coal spontaneous combustion liability. As for the indicator gas for the spontaneous combustion of coal, results implied the fact that increase in the amount of CO generated during spontaneous combustion was observed. Due to the abovementioned fact in terms of indicator gas for spontaneous coal combustion, it was found out the fact that the increase in the amount is significant. CO (ppm) result at early temperature was found out to be in very strong relationship with corresponding spontaneous combustion liability test results. The correlation between CPT and CO (ppm) at 37.5°C for 26 coal samples was found out to have a correlation coefficient of 0.82, i.e. R2 = 0.8224. Increase in CO (ppm) was observed with the increase in the environment temperature for each of 26 coal samples studied. Based on the FCC index values (coal spontaneous combustion liability index) calculated for each of 26 coal samples, the correlation between CO (ppm) and the corresponding FCC index values yielded a correlation coefficient of 0.86, (R2 = 0.8585).

Research on the mechanism of coal adsorption of CO2 hindering oxygen

This paper investigates the inhibitory effect of CO2 on oxygen adsorption during the low-temperature oxidation of coal. Taking the example of Tingnan coal, the physical adsorption barriers of CO2 during coal oxidation at different temperatures were analyzed in terms of competitive adsorption curves, adsorption capacity, adsorption selectivity and diffusion coefficients using a combination of indoor experiments and molecular simulations. It is proposed that the competitive adsorption advantage of CO2 is significantly greater than that of O2, and it is negatively correlated with temperature. Additionally, the variations of aliphatic gas-phase products are evident during the pre-oxidation temperature stages. When the pre-oxidation temperature exceeds 70 °C, the amounts of CH4 and C2H4 released increase with the pre-oxidation temperature, and the inhibitory effect of injected CO2 deteriorates. The simulation results validate the experimental conclusions and are in line with the actual situation, providing auxiliary optimization and guidance for coal mine fire prevention work.

Hydrogen abstraction reaction mechanism of oil-rich coal spontaneous combustion

Oil-rich coal is a type of coal with a hydrogen-rich structure, making it susceptible to spontaneous combustion at low temperatures. To investigate the process of hydrogen production through the low-temperature oxidation of oil-rich coal, nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR) experiments were conducted, along with quantum chemical simulations, to identify the dynamic changes in functional groups during low-temperature oxidation. The experimental results indicate that oil-rich coal primarily comprises hydrogen-containing structures, such as aromatic hydrocarbons and aldehyde groups. Based on the observed patterns of the functional group changes, three stages of hydrogen generation during the low-temperature oxidation of oil-rich coal were deduced. Furthermore, quantum chemical simulations were employed to model the target functional groups' structural changes during oil-rich coal's low-temperature oxidation. Combining the results of the experiments and simulations revealed the primary mechanism behind the low-temperature oxidation of oil-rich coal, leading to hydrogen production. This study identified the reaction of methylmethylene side chains forming aldehyde groups as a precursor to hydrogen. The simultaneous oxidation and structural interactions eventually result in the formation of hydrogen. These findings provide a more profound insight into the mechanism behind the spontaneous combustion of coal oxidation and provide a theoretical foundation for developing effective prevention and management technologies for the spontaneous combustion of oil-rich coal.

Study on the kinetic characteristics and control steps of gas production in coal spontaneous combustion under the oxidation path

The low-temperature oxidation of coal involves multiple gas production paths including desorption, pyrolysis and oxidation. These paths interfere with each other, resulting in unclear kinetic characteristics of CO and CO2 production in coal oxidation and incomplete mechanism of gas production. Therefore, this paper proposed a method that can effectively exclude the effects of desorption and pyrolysis on gas production and adopted this method to explore the kinetic characteristics of CO and CO2 production under different paths. Besides, with the aid of microscopic characterization methods, the kinetic steps controlling gas production and the mechanism of gas production were analyzed. The experimental results demonstrate that the proposed method can effectively eliminate the effects of desorption and pyrolysis, and can obtain the kinetic characteristics of gas production under the oxidation path that can reflect the intrinsic characteristics of low-temperature oxidation of coal. Based on the difference method, the activation energies of CO and CO2 produced under the oxidation path are 59.49 kJ/mol and 60.09 kJ/mol, which activation energies are almost the same, suggesting that CO and CO2 are likely to be produced from the same precursor. The gas production in low-temperature oxidation is a process in which methylene groups in coal react with oxygen to generate oxygen-containing intermediates which then decompose to produce CO and CO2. The reaction of methylene groups with oxygen is the control step of gas production kinetics in coal spontaneous combustion, resulting in the same apparent activation energies of CO and CO2 production in the stable phase under the oxidation path.

Experimental investigation on microstructure fractal characteristics of low-temperature oxidation of gas-bearing coal

Experimental investigation on microstructure fractal characteristics of low-temperature oxidation of gas-bearing coal

Study on the evolution characteristics of molecular surface active sites of low-rank coal in low-temperature oxidation stage

Compared with other coal types of coal, low - metamorphic coal contains numerous active sites, making it susceptible to spontaneous combustion and posing significant risks during production, transportation, and storage. To further reveal the evolution characteristics and laws of low-rank coal's active sites during the process of low-temperature oxidation of coal (LTOC), X-ray photoelectron spectroscopy (XPS), 13C nuclear magnetic resonance (13C NMR), and Fourier transform infrared (FTIR) experiments were used to determine the basic structural parameters such as the valence state of non-carbon features, types of carbon elements in coal macromolecules, and structure and quantity of functional groups. Combined with molecular modelling software (GaussView 6.0) and macromolecular calculation software (Grimme-XTB), macromolecular models of coal at different temperature points were constructed, and the surface energy field distribution was calculated. The number and energy change information of chemical and physical adsorption sites of low-rank coal during heating were obtained. The results show that during the LTOC, the chemical adsorption sites such as aliphatic hydrocarbons (-CH3/-CH2) have higher reactivity and can be converted into oxygen-containing functional groups (-CHO, –OH, –COOH); the number of physical adsorption sites increased first and then decreased, and reached the maximum at 50 °C. Coal exhibited a higher affinity for O2 at this temperature. The peroxide (-C-O-O-H) undergoes thermal decomposition, leading to H2O and weight loss of coal. The –CO and –COO structures undergo decarboxylation to generate CO, CO2, and other gases. This study comprehensively revealed the evolution of active sites through two key coal oxygen processes, physical and chemical adsorption, based on the gradual temperature elevation of low-rank coal. This holds immense significance for improving and enriching the understanding of the mechanism of coal spontaneous combustion(CSC), preparing more targeted CSC inhibition, and formulating efficient fire prevention and extinguishing measures.

Microscopic mechanism of carbon oxides formation during long-flame coal oxidation at molecular scale

Precursors of carbon oxides in coals are considered to be related to aldehyde group(–CHO) and carboxyl group (–COOH), but the way of decomposition of precursors to produce carbon oxides is not clear. Through theoretical analysis, experimental research and quantum chemical calculation at molecular scale, this paper explores the decomposition mode and products of carbon oxide precursors during coal spontaneous combustion (CSC). A programmed heating device was used to investigate the change rule of gas product concentration during the constant temperature oxidation of coals, and the microstructure of the coals was characterized by low-temperature nitrogen adsorption experiments (BET), X-ray Diffraction (XRD) and X-ray photoelectron spectrometer (XPS). The results of the study show that during coal oxidation, the inhomogeneous energy exchange between the coals and the external heat source results in the change of gas product concentration with time following two rules. BET experiments reveal that oxidation leads to the evolution of fine mesopores to macropores and an increase in the number of pores with diameters larger than 20 nm in the coals, which facilitates the transport of oxygen molecules through the pore structure. Within 200 °C, the microcrystalline structure of coals is basically unchanged, and the active structure C–C/C–H on the side chain is converted to –CHO and –COOH and then decomposed to produce carbon oxides. Quantum chemical calculation shows that the reaction energy barrier of carbonyl group and carboxyl group decomposed step by step to form carbon oxides is much smaller than that of direct decomposition, and carbon oxides and alkyl radicals can be produced even at room temperature by step decomposition. The research results provide theoretical guidance for the formation mechanism of carbon oxides during low temperature oxidation of coals.

Molecular simulation study of microstructural evolution during low-temperature oxidation of coal

In this study, industrial analysis, 13C NMR spectroscopy, X-ray photoelectron spectroscopy (XPS), and in-situ infrared spectroscopy were employed to determine the elemental composition and structure of Tingnan coal. The peak area, pore volume, and specific surface area were used as the main parameters to investigate the structural changes of coal molecules and pore evolution during low-temperature coal oxidation, using an automated surface area and pore size analyzer. The analysis revealed that with increasing oxidation degree, the CH groups in the aromatic structure gradually decreased, while the total content of oxygen-containing functional groups such as alcohols, phenols, carboxyls, and carbonyls remained relatively constant. However, the relative content of hydroxyl groups and ether linkages increased with increasing oxidation temperature. Additionally, coals with different oxidation degrees exhibited well-developed micropores, and the specific surface area gradually decreased while the pore size increased with temperature. Molecular models of Tingnan coal in its original state and oxidized at temperatures of 50 °C, 70 °C, and 110 °C were constructed using Materials Studio software. Based on these models, pores with diameters of 2.79 nm, 3.07 nm, 3.07 nm, and 3.11 nm and a length of 4.5 nm were constructed using the coal molecular structure as the pore walls.

Mechanisms of CO and CO2 Production during the Low-Temperature Oxidation of Coal: Molecular Simulations and Experimental Research

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.