Low-temperature Oxidation Of Coal Research Articles

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.

Study on the influence of different rock/coal mixture ratio on thermal storage and low temperature oxidation of coal

Coal mines in the goaf are formed by the collapse and accumulation of coal rock, and the process of oxidizing and heat storage of singular coal does not reflect the real environmental conditions within the airspace. In this study, coal and rock mixing on coal spontaneous combustion (CSC) is experimentally investigated, and the bituminous coals and typical siltstones, mudstones and shales in the overlying rock layers of the goaf are selected. The different rock/coal blending ratios and real low oxygen conditions in the goaf are considered. The results show that in addition to coal, shale also contains some substances such as graphite and fixed carbon, which would participate in oxidization and heat storage processes. The thermal conductivity and thermal diffusivity of the rock/coal mixtures increased with the increase of the rock blending ratio. The heat storage capacity of the three rocks is shale > mudstone > siltstone. After blending rocks in bituminous coal, the contents of functional groups –OH, –CH3, –CH2, CC, etc. is decreased, and the degree of decrease is greater with the increase of blending ratio, and the degree of decrease of shale/coal mixture is relatively small compared with the others. As the proportion of rock in the rock/coal mixture increases, all oxidized combustion stages are delayed to varying degrees. Under the same oxygen concentration, the activation energy of rock/coal mixtures is higher than bituminous coal, and the activation energy of siltstone/coal, mudstone/coal, and shale/coal increased by 25.28 %, 20.12 %, and 14.84 %, respectively. The degree of CSC inhibition by the three rock/coal mixtures is siltstone/coal > mudstone/coal > shale/coal. The research results are of great significance to the prevention and control of CSC in the real environment of goaf.

Study on Highly Stable Biomass Gel Foam with Ultra-Strong Film-Forming Performance Based on the Double Network Structure to Inhibit Coal Spontaneous Combustion

ABSTRACT To further enhance gel foam stability, an ultra-strong film-forming stabilized biomass gel foam (PE/SA-Ca) with the double network structure was achieved by using pectin (PE), sodium alginate (SA), calcium L-lactate (Ca-L) and biomass foaming systems (BS). The results showed that the double network gel foam had ultra-strong film-forming and stability performances than the single network gel foam. Specifically, when the PE:SA ratio of the double network gel matrix was 5:5, the film thickness of PE/SA-Ca increased from 1 to 4.25 mm and the half-life increased from 3 to 22 days compared to the single network gel foam. In addition, PE/SA-Ca could maintain film integrity at high temperature up to 120°C. The water retention rate of PE/SA-Ca was increased by 79.5% compared to pure foam. The inhibition rate of PE/SA-Ca in the coal low-temperature oxidation experiment was as high as 78.06% at 180°C. Fire extinguishing experiment showed that PE/SA-Ca could rapidly reduce the coal fire temperature from 960 to 68.9°C within 20 min and effectively prevent prevention of coal re-ignition. This research provides a technical method for the development of highly stable, ultra-strong film-forming and eco-friendly gel foam.

Molecular simulation and experimentation studies on the low-temperature oxidation of water-containing coal in the goaf atmosphere

ABSTRACT The moisture content of coal tends to have a significant impact on the occurrence of spontaneous combustion. In the present study, both molecular simulation and experimental investigation are performed to examine the interaction among water-containing coal and O2 in the goaf atmosphere, with the following conclusions drawn. Firstly, the presence of water and methane impedes the physical adsorption of coal and oxygen at a temperature of 298K. Methane mainly hinders the contact between coal and oxygen through the reduced concentration of oxygen and competitive adsorption. The increase in water content reduces the adsorption density of each gas but makes no difference to their relative proportions in the amounts of adsorption. At the initial stage of coal oxidation (30°C-80°C), the rate of oxygen consumption declines with a rise in water content. However, it is observed immediately after entry into the rapid oxidation stage (80°C-230°C) that the coal sample with a water content of 9.05% is the highest in the intensity of reaction. A higher concentration of methane in the initial atmosphere produces an inhibitory effect on both oxygen adsorption and chemical reaction, which is effective in reducing spontaneous combustion.

Exploring the factors influencing the low temperature CO gas production characteristics of low-metamorphic coal

CO as an important indicator gas for predicting early spontaneous combustion of coal mainly comes from the oxidation of residual coal in goaf. Low-metamorphic coal has a stronger tendency for spontaneous combustion and is more prone to oxidation to produce CO gas at low temperatures. For example, there are no signs of natural combustion in certain goaf areas of mines, but the CO concentration in the working face continues to exceed the limit, which misleads early natural combustion prediction and warning work. Therefore, taking low metamorphic coal as the main research object, the CO gas production characteristics of different coal samples at low temperatures were determined through temperature-programmed experiments. It was found that the CO gas production rate and concentration of CYM at low temperatures were much higher than those of the other two metamorphic coal samples (approximately 2 times RNM and 3–10 times QM); Determination of content and proportion changes of hydroxyl, aliphatic, Aromatic hydrocarbon and oxygen-containing functional groups in different coal samples before and after heating by Fourier transform infrared spectroscopy. Based on the analysis of CO gas production characteristics of different coal samples, it was found that the intermolecular hydrogen bonds in the hydroxyl groups of CYM and the C-O bonds in the oxygen-containing functional groups decreased significantly. The strongest correlation between intermolecular hydrogen bonds in hydroxyl groups and C-O bonds in oxygen-containing functional groups and the low-temperature CO gas production characteristics of coal is demonstrated; Finally, scanning electron microscopy was used to study the differences in apparent characteristics of different coal samples before and after heating, to determine more complex apparent structures that increase oxygen adsorption sites and promote low-temperature oxidation and CO production of coal during the low-temperature stage. Further guidance will be given to the research on the suppression of intermolecular hydrogen bonds and C-O bonds, as well as the sealing of coal micro pores and crack structures, to improve the targeted application and fire prevention efficiency of fire prevention materials.

Stage changes in the oxidizing properties of long-term water-soaked coal and analysis of key reactive groups

To elucidate the impact of prolonged water immersion on the evolution of key active functional groups during the low-temperature oxidation of coal, the programmed temperature ramping method, in-situ Fourier transform infrared spectroscopy (in-situ FTIR), microcalorimetry (C80) experiments, thermodynamic analysis, and gray and Pearson correlation analysis were performed. The analysis involved coal samples exposed to different immersion durations (3, 6, 9, and 12 months). The results reveal that long-term water immersion exerts a stage-wise influence on the oxidation characteristics of coal, with the critical time point being 6 months. Before this period, the gradual accumulation of active sites progressively enhances the oxidation characteristics. However, after 6 months, the generation of active sites is suppressed, resulting in a gradual weakening of the coal's oxidation behavior. Correlation analysis revealed that the correlation between free hydroxyl groups and gaseous products is strongest in coal immersed for 6 months (A6), with a correlation coefficient of 0.91. During the accelerated oxidation stage, the heat released from the soaked coal is significantly greater than that in other stages, which promotes the formation of C=O and –COOH functional groups and further reduces the apparent activation energy. According to the Coats–Redfern method, A6 exhibits the smallest apparent activation energy—only 30 % of the original value of coal. The correlation between free hydroxyl groups and apparent activation energy is strongest at this stage, with a correlation coefficient of 0.92. This indicates that free hydroxyl groups and the accelerated oxidation stage are, respectively, the key active functional groups and critical phases in the coal oxidation process.

Quantitative calculation of gases generation during low-temperature oxidation of coal.

The gases evolution during the low-temperature oxidation of coal is an essential parameter used to assess the state of coal oxidation and to estimate the gaseous pollutants. However, the current semi-quantitative method, which employs gas concentration as the measurement standard, is flawed. This paper presents a quantitative calculation method for gas products during coal oxidation. N2 is used as the tracer gas in the experiment, because nitrogen is an inert gas that will not participate in the reaction, and the amount of matter will not change in the reaction. According to the formula [Formula: see text], the corresponding mass flow rates of each gases component were calculated, and the gas yields during the reaction period were determined by comprehensive calculation. To this end, experiments were conducted on the low-temperature oxidation of coal using a flow reactor. After undergoing quantitative calculations, the main gas products' mass flow rates, yields, and energies, including CO, CO2, CH4, C2H4, C2H6, C2H2, and C3H8 between 30 and 180 °C were obtained. The findings showed that CO2 > CO > CH was generated in all the coal samples. The amount of gases produced in the low-temperature oxidation of coal is proportional to the level of oxygen concentration. When the oxygen concentration ranges from 0 to 21%, the gaseous production of MTH coal ranges from 381.44g/ton to 8562.80g/ton. The results of gaseous energy calculations showed that the energy loss for low temperature oxidation of the four coal samples ranged from 4334.14~26,772.73 kJ/ton under air atmosphere. Energy loss is also significantly affected by the oxygen concentration, and the energy loss of MTH coal increases significantly from 520.52 kJ/ton at 0% oxygen concentration to 26,772.73 kJ/ton at 21% oxygen concentration, an increase of about 50 times. Significantly, this method not only reflects the real gas evolution during low-temperature oxidation of coal but also computes the gas emission and energy loss, which is crucial for studying the mechanism of coal spontaneous combustion and assessing gases pollutants.

Reaction heat effect and change characteristics of key groups in coal-oxygen intrinsic reaction path.

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.

Study on the Coal-Oxygen Intrinsic Reaction at Low Temperature by the Subtraction Method.

Coal oxidation involves complex physical and chemical processes as well as many reaction pathways. The subtraction method was used to investigate the law of the coal-oxygen intrinsic reaction at low temperatures. The subtraction method is to subtract the characteristic curve of the coal reaction in N2 from the characteristic curve of coal oxidation in air, which can remove the influence of pyrolysis reaction, water evaporation, and gas desorption on coal low-temperature oxidation, and ultimately obtain the coal-oxygen intrinsic reaction curve under the pure oxidation pathway. To reveal the internal relationship of the coal-oxygen intrinsic reaction between the release of gas products and the conversion of active functional groups, a temperature-programmed system was used to analyze the law of index gas products. Furthermore, Fourier infrared spectroscopy was used to analyze the conversion of active functional groups in the low-temperature reaction process of coal. The findings revealed that the coal-oxygen intrinsic reaction obtained through subtraction played a significant role in the release of gas and the conversion of aliphatic hydrocarbons in the overall process of the coal low-temperature reaction. However, the alterations in the N2 atmosphere were relatively weak. The gas release of CO and CO2 gradually increased with increasing temperature, as did the concentration of oxygen-containing functional groups, and there was a positive correlation between them.

Microstructure evolution and higher-molecular-weight gas emission during the low temperature oxidation of coal

The higher-molecular-weight gases (HMWGs) released from coal oxidation carry a wealth of information on the mechanism of coal spontaneous combustion. To explore the formation process of HMWGs, the emission of HMWGs and the evolution of active functional groups during coal oxidation were tested using gas chromatography and in-situ FTIR, and the relationship between them was further analyzed using grey relational analysis. It was found that the concentration of most HMWGs changed regularly with the increase in temperature and showed a significant correlation with oxygen concentration. As the temperature increased, the content of alkyl groups changed little at first and then decreased by more than 40%. The content of oxygen-containing functional groups increased gradually by 2–3 times. The production of straight-chain alkanes correlated most strongly with alkyl groups compared to other active groups. While branched alkanes, benzene, aldehydes and ketones showed a higher correlation with the increase of oxygen-containing functional groups. It indicated that the decrease of alkyl groups and the formation of aldehyde groups contributed to the emission of hydrocarbon HMWG. The formation pathways of benzene, aldehydes and ketone during coal oxidation need to be further investigated and verified.

The auto-oxidation characteristic of coal at different stages of the low-temperature oxidation process

The spontaneous combustion in the mining gob are often caused by the low-temperature oxidation of coal, and the low-temperature oxidation process has a long process leading to different characteristics in each stage, which brings great difficulties to the prevention and control of coal spontaneous combustion in the underground mining gob. Therefore, this article investigated the oxidation characteristics of coal at different stages of the low-temperature oxidation, mainly including: analyzing the changes in radical concentration and g factor based on electron spin resonance (ESR) experiments, studying the variations in coal temperature by using the programmed temperature controller and analyzing the characteristics of gas products and oxygen consumption by using the gas chromatograph. And the degree of auto-oxidation of coal was characterized by the difference between dry air and nitrogen conditions during the constant temperature period. The results showed that when the constant temperature reached 130 ℃, the coal began to exhibit auto-oxidation characteristics, while when the constant temperature reached 180 ℃, the auto-oxidation phenomenon of the coal significantly intensified, the types and content of radicals rapidly grew, the coal temperature significantly rose, and the oxygen consumption rapidly increased, while releasing a large amount of CO and CO2. And the relationship between the spontaneous oxidation ability of coal radicals and the temperature change of coal during constant temperature was obtained. The conclusions obtained provide theoretical guidance for the safety and clean production of coal resources in mines, as well as the prevention and control of coal spontaneous combustion fires.

Correlation between oxygen concentration and reaction rate of low-temperature coal oxidation: A case study of long-flame coal

Correlation between oxygen concentration and reaction rate of low-temperature coal oxidation: A case study of long-flame coal

Kinetic Analysis of Low Temperature Oxidation of Coals with Pre-Heating Histories: A DSC Study

Kinetic Analysis of Low Temperature Oxidation of Coals with Pre-Heating Histories: A DSC Study