Raw Coal Research Articles

Preparation and study of a sodium alginate film for preventing spontaneous combustion of water-soaked coal in goaf

Considering the high risk of spontaneous combustion of soaked coal compared to raw coal and the limitations of existing technology in preventing spontaneous combustion of coal in goaf water, a sodium alginate film has been developed which can effectively solve the problem of preventing spontaneous combustion of soaked coal by oxygen insulation, heat absorption and inhibition. The optimum composition ratio and the properties of the film are evaluated by film formation and thermogravimetric experiments. The results show that the film properties are best when the ratio of sodium alginate to gelatin is 8:2, the sodium tripolyphosphate concentration is 0.5 %, the ammonium polyphosphate concentration is 0.2 %, and the concentration of CaCl2 for the first and second cross-linking is 0.01 % and 0.2 %, respectively. The combination of sodium alginate and CaCl2 forms a stable and compact spatial network structure that can reduce the temperature of the covered coal by approximately 36 °C. Throughout the field trials, the temperature change of the goaf treated with sodium alginate film is opposite to that of the untreated area, showing a downward trend, with a total decrease of 0.4 °C in 20 days. This study provides a new idea for preventing spontaneous combustion of soaked coal.

PURIFICAÇÃO ÁCIDO-BASE EXPERIMENTAL DE CASCAS DE ARROZ CARBONIZADAS: EXPLORANDO SUA EFICÁCIA NA ADSORÇÃO DE RESÍDUOS NÃO VALORIZADOS DA INDÚSTRIA DE ARROZ

The burning of rice husks produces natural silica and generates charcoal with excellent properties for the adsorption of different pollutants. The objective of this study was to enhance the adsorption properties of this industrial residue from burning rice husk through acid and base purifications. The raw coal (RC) was initially modified with HCl(aq.) and then with NaOH(aq.), and subsequently renamed MC. Efficiency was verified by the adsorption of methylene blue (MB) dye, considering a concentration of 10 mg L-1, dosage of 0.05 g L-1, pH 6.7, and 120 min. MB removal was 78% for RC and 97% for MC, with experimental capacities of 144.78 mg g-1 and 180.78 mg g-1, respectively. The kinetics were better adjusted by Avrami (R2 0.99, error <2.8%). The properties of charcoal improved with the modification, presenting a more porous structure, greater surface area and functional groups that contributed to adsorption, proving that simple purification methods, such as acid-base purification, can be crucial for waste reuse.

Effect on the oxidation characteristic at lignite of two types of endogenous microorganisms from coal

In recent years, coal biotechnology has gained considerable popularity due to its economic, eco-friendly, and sustainable advantages, which offer a novel perspective and pathway for the development of green technologies aimed at preventing and controlling coal spontaneous combustion (CSC). Two endogenous microorganisms from lignite were succeeded in extracting in this work, namely Aspergillus and Bacillus velezensis. Subsequently, the effect of these microorganisms on the microscopic structure and macroscopic oxidation characteristics of coal was investigated by X-ray diffraction, Fourier-transform infrared spectroscopy, and thermogravimetric analysis. The results revealed that both Aspergillus and Bacillus velezensis treatments altered the microcrystalline structure of the coal, disrupting the aliphatic hydrocarbon structure and oxygen-containing functional groups of the coal molecules. Particularly, the OH − O hydrogen bond of two kinds of microbial treatment coal samples content respectively decreased significantly by 17.29 % and 11.06 % compared to the raw coal sample. However, the effects of different microorganisms on coal varied. Thermodynamic behavior demonstrated that the critical temperature of the two treated coal samples was delayed by 29.44 °C and 26.62 °C, and the ignition temperature was delayed by 11.31 °C and 5.18 °C. Overall, all characteristic temperature points of the Aspergillus group were delayed, but the maximum weight loss rate temperature and the burn-out temperature of the Bacillus velezensis group exhibited slight advancement. The thermal mass loss characteristics also changed during the oxidation combustion stage. In addition, the average activation energies of oxidized combustion stage calculated by the Kissinger-Akahira-Sunose method were 74.26 kJ/mol and 78.61 kJ/mol, respectively, and 82.41 kJ/mol and 86.15 kJ/mol for the Friedman method, which were both increased compared to the raw coal. The results indicate that both Aspergillus and Bacillus velezensis have effects with varying degrees on the coal oxidation characteristics. It can be observed that the effect of treatment with Aspergillus is better than Bacillus velezensis from the overall changes in characteristic temperature points. This work lays the foundation for investigating the effect of microbial action on the oxidative properties of CSC.

Effects of circulating ash and dolomite on mercury transition during coal step conversion of pyrolysis and combustion

In aim to study the influence of circulating ash used as heat carrier and dolomite used as catalyst on mercury transition during the step conversion of coal pyrolysis and combustion, five experiments with different operating conditions were conducted in this study, including four pyrolysis experiments of raw coal, coal/ash, coal/dolomite, coal and ash/dolomite, and combustion experiment of pyrolysis solid product. By changing the experimental conditions including final pyrolysis temperature, heating rate, coal-ash ratio and dolomite type, the transition path and final mercury distribution in the step conversion process were comprehensively analyzed. Experiments results indicated that mercury completely released out from coal when the final pyrolysis temperature was 600 °C. The individual addition of ash could promote the mercury release from coal at lower temperature. Dolomite could catalyze the oxidation of Hg0 in gas phase, and the modified dolomite had better catalytic capacity for Hg0 oxidation than natural dolomite. The addition of circulating ash and natural dolomite both had a certain catalytic effect on Hg0 emission peaks during the coal pyrolysis. Increasing the coal-ash ratio was conducive to the oxidation of Hg0 in pyrolysis gas. During the direct combustion of coal, the content of Hg0 released into the gas phase could reach to 69.97%. By adding ash, ash/natural dolomite and ash/modified dolomite to the coal pyrolysis process, the content of Hg0 in pyrolysis gas decreased to 58.99%, 34.77% and 31.30%, respectively. The research is able to offer the theoretical foundation for mercury contaminants’ removal and process optimization of the step conversion technology.

Low-carbon development in China's transportation sector: Multidimensional characteristics and policy implications

Although low-carbon development of transportation sector is crucial for global deep decarbonization, the multidimensional characteristics (e.g., inequality, complex network and efficiency-related drivers) carbon intensity (CI) in the sector at sub-national level is still lacking in-depth evaluation for an emerging economy in particular. Here, using social network analysis, inequality decomposition and production-theoretical decomposition analysis with a focus on regional heterogeneity, the study comprehensively investigated the complex network, inequality and novel drivers related to efficiency and technological change of CI for transportation sectors across 30 provinces in China over 1997–2019. We found that first, the strict spatial correlation structure of CI in the transportation sector was gradually broken and the inter-provincial correlation were enhanced. Second, the inequality of CI declined where gasoline, diesel oil, kerosene and raw coal were the main contributors. Third, the potential carbon factor mainly promoted the reduction of CI while the impact of potential energy intensity exhibited an "inverted U″ trend over the period. Forth, the group technical change effects of energy usage and CO2 emission first promoted the decline of CI and then inhibited it. The study highlights the importance of the multidimensional characteristics in the transportation sector when formulating CI-related policies according to the local conditions.

Life cycle assessment of coal mines of diverse scales over time in China

Coal mining has important detrimental effects on the environment and human health. By the end of 2022, China mined more than 4 billion tons of raw coal, and coal mining contributed to adverse environmental impacts. The objective of this work is to evaluate the environmental impacts emanated from coal mines in different periods (construction period, production period and closing period) and to find the relationship between coal mine scale and ecological impacts. This study uses coal mines that produce 0.45 Mt/a (considered a medium sized mine), 3 Mt/a and 8 Mt/a (both classified as large mines in this study) and a 12 Mt/a extra-large coal mine. Based on the time dimension, the mine life cycle was classified into construction, production and closing period, and the life cycle assessment method was used to conduct environmental assessment. The main influencing substances and key processes were tracked. The results indicated that mining engineering and gangue are the main factors affecting the construction and production periods of coal mines. Freshwater ecotoxicity, marine ecotoxicity, and human toxicity are the main environmental effects of coal produce, and they are mostly brought up by the release of hazardous elements like copper, chromium, zinc, nickel, and copper. Furan, formaldehyde, and chromium emissions during mine closure can be effectively reduced through environmental compensation, however coal mines' environmental compensation during mine closure is minimal. The environmental impact of coal mines producing 3 Mt and 8 Mt annually is minimal. The environmental impact of 0.45 Mt/a and 3 Mt/a coal mines is more prominent in the construction period. The pollutant discharge throughout the production phase, particularly the metal leaching discharge from gangue, needs to receive more attention from the 8 Mt/a and 12 Mt/a coal mines. Additionally, the larger the scale of coal mine production, the greater the proportion of the total environmental impact in the production stage.

Study on the catalysis of copper-based compounds on coal spontaneous combustion and its mechanism

ABSTRACTTo study the effect of the copper element in coal on the spontaneous combustion process of coal, corresponding copper-containing compounds were added to the coal-based on their occurrence state, and thermodynamic experiments and in-situ diffuse reflectance infrared spectroscopy experiments were conducted. Thermodynamic oxidation experiments have shown that the catalytic effects of Cu (NO3)2, CuCl2, CuCl, and Cu2O on three types of coal with different degrees of metamorphism are mainly reflected in the accelerated oxidation stage. After adding copper-containing compounds, the ignition temperature of coal decreased by 22.1°C to 134.4°C, the heat release increased by 1.33 to 3.19 times, and the CO emission and oxygen consumption rate also increased. CuCl has the greatest impact on the ignition point temperature of coal and increases the oxygen consumption rates of DNH and TS by 2.32 and 1.51 times that of raw coal. CuCl2 has the strongest catalytic effect on coal oxidation heat release. Through in-situ diffuse reflectance infrared spectroscopy experiments, it was found that the reaction rate of fatty hydrocarbons in the oxidation process of three types of coal with the addition of copper-containing compounds increased to 1.3–3 times that of raw coal, and the oxidation rate of oxygen-containing functional groups also significantly increased, while the cracking temperature of aromatic structures significantly decreased. This is because copper ions, as strong free radicals, induce free radical chain reactions in coal and exhibit a strong promoting effect on the coal oxygen reaction process.

Effect of alkyl glycoside surfactant on the explosion characteristics of bituminous coal: Experimental and theoretical discussion

To reduce the risk of secondary dust formation and explosions of coal dust captured by spray with surfactant in coal mine working faces, the influence of alkyl polyglucoside (APG) on the explosion characteristics of bituminous coal was studied. The thermokinetic behavior of the APG-coal compound system was analyzed, and the chemical structural evolutions that occurred in coal powder before and after APG treatment were determined. Finally, the mechanism of effect by which APG affects bituminous coal explosion was discussed. The results showed that the carbonyl structure content of samples increased after APG soaking treatment, and the oxidation process of coal powder changed. Coal dust soaked in APG solutions reached a maximum explosion pressure earlier than the raw coal, resulting in a decrease of the boundary concentration. The maximum explosion pressure slightly decreased and the explosion induction time was extended. Additionally, the thermal decomposition rate of the APG-coal mixture system decreased and the maximum heat flow increased. The mechanisms by which APG affects bituminous coal dust explosion were summarized to be a barrier effect and an equivalent combustion effect. The results of this research serve as a reference for coal dust explosions control and for the preparation of new coal dust suppressants were provided.

Coal and Gangue Classification Based on Laser-Induced Breakdown Spectroscopy and Deep Learning.

During the extraction and processing of coal, a large amount of solid waste, collectively known as gangue, is produced. This gangue has a low carbon content but a high ash content, accounting for approximately 15 to 20% of the total coal yield. Before coal is used, coal and gangue must be effectively separated to reduce the gangue content in the raw coal and improve the efficiency of coal utilization. This study introduces a classification method for coal and gangue based on a combination of laser-induced breakdown spectroscopy (LIBS) and deep learning. The method employs Gramian angular summation fields (GASF) to convert 1D spectral data into 2D time-series data, visualizing them as 2D images, before employing a novel deep learning model-GASF-CNN-for coal and gangue classification. GASF-CNN enhances model focus on critical features by introducing the SimAM attention mechanism, and additionally, the fusion of various levels of spectral features is achieved through the introduction of residual connectivity. GASF-CNN was trained and tested using a spectral data set containing coal and gangue. Comparative experimental results demonstrate that GASF-CNN outperforms other machine learning and deep learning models across four evaluation metrics. Specifically, it achieves 98.33, 97.06, 100, and 98.51% in the accuracy, recall, precision, and F1 score metrics, respectively, thereby achieving an accurate classification of coal and gangue.

Experimental study on the desorption law and diffusion kinetic characteristics of gas in raw coal and tectonic coal

To better reveal the nature of gas flow in raw coal and tectonic coal composite seams during gas extraction, experiments on the gas desorption and diffusion kinetic characteristics of raw coal and tectonic coal under different gas adsorption equilibrium pressures were carried out. Meanwhile, the applicability of Airey empirical formula was analysed, and the unipore model and bidisperse model were used to fit the experimental results and calculate the diffusion coefficient. The results show that the initial gas desorption rate and gas desorption volume of raw coal are much smaller than those of tectonic coal at the initial stage of gas desorption; at the later stage, the gas desorption rate of tectonic coal decays rapidly and the ultimate gas desorption volume tends to a stable value, however, the gas desorption rate of raw coal decays slowly and the ultimate gas desorption volume continues to increase. Airey empirical formula has some limitations in characterizing the gas desorption law in coal and is more suitable for fracture-rich “block” coal, but for the tectonic coal, the fitted ultimate gas desorption volume has a significant error from the actual one. The unipore model is suitable for describing some specific periods in the initial stage of the gas desorption process of raw coal, and the bidisperse model makes up for the defects of the unipore model and can accurately describe the gas diffusion law of tectonic coal. The study provides a theoretical basis for the scientific planning of coal and coalbed methane mining.

Advanced Equilibrium Studies for the Synergetic Impact of Polyaniline on the Adsorption of Rhodamine B Dye by Polyaniline/Coal Composite.

The synergetic improvement effect of the polyaniline (PANI) hybridization process on the adsorption of rhodamine B dye (RB) by PANI/coal hybrid material (PANI/C) has been evaluated using both traditional equilibrium modeling and advanced isotherm investigations. The composite was prepared by polymerizing polyaniline in the presence of coal fractions with a surface area of 27.7 m2/g. The PANI/C hybrid has an improved capacity to adsorb RB dye (423.5 mg/g) in comparison to coal particles (254.3 mg/g). The maintained increase in the elimination properties of PANI/C has been illustrated using the steric characteristics of active site density (Nm) as well as the total number of adsorbed RB on a single active site (n). However, the incorporation of PANI did not yield any substantial impact on the existing active sites' quantity, but the hybridization processes greatly influenced the selectivity and affinity of each active site, in addition to the aggregation characteristics of the dye as it interacts with the composite's surface. Whereas raw coal can only adsorb three molecules of RB, each active site throughout the PANI/C surface can adsorb approximately eight RB molecules. This is also evidence of RB dye adsorption in a vertical arrangement, which involves multimolecular processes. The Gaussian energy (4.01-5.59 kJ/mol) and adsorption energy (-4.34-4.68 kJ/mol) revealed the controllable impact of physical mechanisms. These mechanisms may include van der Waals forces, dipole-dipole interactions, and hydrogen bonds (<30 kJ/mol). The thermodynamic functions, such as enthalpy, internal energy, and entropy, that have been assessed provide evidence supporting the exothermic and spontaneous nature of the RB uptake processes by PANI/C.

Changes in physical and chemical structure and Full-stage oxidation characteristics of coal caused by igneous intrusion

The influence of igneous intrusion on entire coal oxidation and its mechanism remains unclear, impacting mining operations. To investigate spontaneous combustion characteristics, this study analyzed the composition and physicochemical structure of igneous intrusion coal (II coal) using various experimental methods: low-temperature oxidation, low-pressure N2 adsorption, XPS, FTIR, and TGA-DSC. Results show that II coal has developed pores favourable for coal-oxygen interaction, but increased moisture and ash relative content, and decreased volatile and reactive functional groups, implying reduced low-temperature oxidation and combustion reactivity. The decrease in CO and CO2 production and the increase in XPT and apparent activation energy confirm this result. Notably, II coal releases more CO and CO2 below 400 K and the gases are lower than raw coal after 400 K, suggesting native CO and CO2 exist in coal and abundant native active sites which are rapidly consumed during this stage. These findings provide insights into predicting spontaneous combustion in mines extracting coal seams affected by igneous intrusion.

Experimental Study on the Microstructural Characterization of Retardation Capacity of Microbial Inhibitors to Spontaneous Lignite Combustion

Mine fires are one of the common major disasters in underground mining. In addition to the external fire sources generated by mining equipment and mechanical and electrical equipment during operations, coal is exposed to air during mining, and spontaneous combustion is also the main cause of mine fires. In order to reduce the hidden danger of coal mines caused by spontaneous coal combustion during lignite mining, the microbial inhibition of coal spontaneous combustion is proposed in this paper. Via SEM, pore size analysis, and NMR and FT-IR experiments, the mechanism of coal spontaneous combustion is discussed and revealed. The modification of lignite before and after the addition of retardants is analyzed from the perspective of microstructure, and the change in flame retardancy of the lignite treated with two retardants compared with raw coal is explored. The results show that, compared with raw coal, a large number of calcium carbonate particles are attached to the surface of the coal sample after bioinhibition treatment, and the total pore volume and specific surface area of the coal sample after bioinhibition treatment are decreased by 68.49% and 74.01%, respectively, indicating that bioinhibition can effectively plug the primary pores. The results of NMR and Fourier infrared spectroscopy show that the chemical structure of the coal sample is mainly composed of aromatic carbon, followed by fatty carbon and carbonyl carbon. In addition, the contents of active groups (hydroxyl, carboxyl, and methyl/methylene) in lignite after bioretardation are lower than those in raw coal, and methyl/methylene content is decreased by 96.5%. The comparison shows that the flame-retardant performance of biological retardants is better than that of chemical retardants, which provides an effective solution for the efficient prevention and control of spontaneous combustion disasters in coal mines.

A comparative study of molecular structure and combustion behavior of coal and its separated vitrains

A systematic study has been conducted on two coking coals and their separated vitrains to understand the macromolecular structure and their thermal behavior. The structural parameters have been determined by using XRD (X-ray diffraction), Raman spectroscopy, FTIR, Solid-state 13C NMR and TG/DTG–DSC. The proximate and ultimate analyses results indicate that vitrinite is the major organic component of coal and it is type III kerogen having H/C ratio in the range of 0.58–0.72 and O/C ratio in the range of 0.09 to 0.16. The inter-layer spacings, crystallite size, the average number of the aromatic layers, aromaticity, and rank of demineralized coal and vitrain as obtained from XRD studies vary in the range of 3.49–3.70 Å, 13.24–16.72 Å, 3.60–4.74, 0.72–0.80 and 2.62–3.98 respectively. The values of aromaticity, rank and inter-layer spacing of vitrain are higher however the crystallite size and the average number of the aromatic layer are lower than that of respective demineralized coal. Raman spectra results show that vitrain is more graphitic and stable in nature compared to their bulk coal and the structure of coal gets disordered during demineralization. The FTIR spectrum shows that carboxylic (–COOH), carbon–Carbon double bonds (–CC–), carbonyl/carboxyl (CO), methyl and methylene (–CH3, –CH2–), and a hydroxyl group (-OH) are the major functional groups. The structural parameters as obtained from FTIR studies such as aromaticity (fa)FTIR, degree of condensation of aromatic rings and aliphatic to aromatic ratio are higher in vitrain than their respective coals, while the aliphatic hydrogen to total hydrogen ratio (Hal/H) of demineralized coal is higher than the raw coal and vitrain. The 13C NMR spectra show the fraction of unsaturated carbon is higher in vitrain than the respective raw coal. The thermal analysis result shows the reactivity of the samples increases in the order of demineralized coal > vitrain > raw coal and the activation energy increases in the order of raw coal > demineralized coal > vitrain.

Evaluating the efficiency of the enhanced ultrasonic–assisted hydrogen peroxide degradation of low–rank coal biogenic gas

ABSTRACT The efficiency of biogas formation from low-rank coal was improved through ultrasonic-assisted hydrogen peroxide degradation. We investigated various aspects of this process, including the mass of the small-molecule organic matter, changes in the composition of biogas components in three systems, and alterations in the microscopic morphology of coal before and after biogas formation. Our analytical methods included UV-vis spectrophotometry, drainage gas collection method, gas chromatography, infrared spectroscopy, and scanning electron microscopy. Optimal pretreatment conditions were identified with an ultrasonic power of 400 W, a reaction time of 90 min, and a reaction temperature of 60°C. The biogenic methanogenic cycle was delineated into three stages, with the raw coal and filtrate systems exhibiting fast and slow gas production stages during the first and second stages, respectively. In contrast, the residual coal system displayed slow and fast gas production stages during these stages. Methane yields were measured at 270.72 μmol g−1 (raw coal), 303.50 μmol g−1 (residual coal), and 293.43 μmol g−1 (filtrate). Pretreated coal exhibited biomethane yields (residual coal and filtrate) approximately 120% higher than untreated raw coal. Notably, the gas-forming potential of the filtrate should not be underestimated. Ultrasonic-assisted hydrogen peroxide pretreatment disrupted the macromolecular reticulation structure within the raw coal, resulting in the opening of a portion of the aromatic ring structure. This innovative approach offers a novel strategy for augmenting biomethane production from low-rank coal and furnishes a scientific foundation for advancing industrial applications.

Evolution and oxidation properties of the functional groups of coals after water immersion and air drying

The evolution of functional groups, the gas generation laws in oxidation and temperature increase, and the oxidation kinetics parameters of the coal samples immersed in water for 30 days, 90 days, and 150 days (hereinafter referred to as 30 d coal, 90 d coal, and 150 d coal, respectively) were analyzed through Fourier transform infrared spectrometer, programmed temperature increase test system, and thermal gravimetric analyzer to disclose spontaneous combustion and the influencing mechanism of long-flame coals immersed in water for a long period. Results show that the aliphatic hydrocarbon and the hydroxyl contents of the coal samples increase after water immersion and air drying. In particular, the aliphatic hydrocarbon contents in 30 d coal, 90 d coal, and 150 d coal increase from 12.61 % for raw coal to 15.84 %, 14.08 %, and 13.90 %, respectively, whereas the hydroxyl content increases from 48.75 % for raw coal to 50.77 %, 50.13 %, and 49.23 %, respectively. In the low-temperature oxidation process, the CO output increases, and the apparent activation energy declines. When the temperature is 200 °C, the CO outputs of 30 d coal, 90 d coal, and 150 d coal are 28088, 25421, and 22112 ppm, which are 37.4 %, 24.4 %, and 8.2 % higher than the CO output of raw coal (20437 ppm), respectively. The apparent activation energy decreases from 65.7 kJ mol−1 for raw coal to 61.12 kJ mol−1, 63.08 kJ mol−1, and 64.5 kJ mol−1. According to the comparison of characteristic temperature points, the temperature of the mass extreme (T3) and the temperature of ignition point (T4) of the immersed coal samples both decrease. The T3 values of 30 d coal, 90 d coal, and 150 d coal decrease from 282.71 °C for raw coal to 274.87, 278.33, and 280.50 °C, whereas their T4 values decrease from 424.24 °C for raw coal to 419.23, 420.14, and 423.42 °C, respectively. This study proves that water immersion increases the oxidative activity of coals, thereby simplifying spontaneous combustion.

Social and Economic Impact Assessment of Coal Power Phase-Down at the Provincial Level: An Entropy-Based TOPSIS Approach

With the rollout of the carbon peak and neutrality targets, conventional coal-fired power plants will gradually be phased down in China’s power system in an orderly manner. The economic and social impact of the energy transition is a vital topic that requires scientific measurements and evaluation. In this paper, we establish a comprehensive approach to assess the impact of provincial coal power phase-down with 11 indicators covering dimensions of economy, society, and industry. An entropy-based Technique for Order of Preference by Similarity to the Ideal Solution (TOPSIS) approach is adopted to calculate entropy weight, relative closeness, and other evaluation benchmark data. Then, the influence degree in 30 provinces is ranked based on the assessment. The results show that there is a significant regional imbalance in the process of coal power phase-down, among which Shanxi, Inner Mongolia, and other coal base provinces are the most vulnerable regions bound by their huge raw coal production and coal industry employment. Although the coal power industry is less affected than the coal industry, it will face pressure from the optimization of coal power units, followed by the dual impact of taxation and employment issues. Finally, the potential impacts of coal power phase-down and policy implications are proposed.

Apparent activation energy of mineral in open pit mine based upon the evolution of active functional groups

This study aimed to investigate the mechanism of mineral spontaneous combustion in an open pit. On the study of coal and mineral mixture in open pit mines, as well as through the specific surface area and Search Engine Marketing (SEM) experiments, the specific surface area and aperture characteristics of distribution of open pit coal sample and pit mineral mixture samples were analyzed. Thermal analysis experiments were used to divide the oxidation process was divided into three stages, and the thermal behavior characteristics of experimental samples were characterized. On the basis of the stage division, we explored the transfer law of the key active functional groups of the experimental samples. The apparent activation energy calculation of the key active groups, performed by combining the Achar differential method with the Coats–Redfern integral method, microstructural and oxidation kinetic properties were revealed. The resulted showed that the mixed sample had high ash, the fixed carbon content was reduced, the specific surface area was far lower than the raw coal, the large aperture distribution was slightly higher than the medium hole, the micropore was exceptionally low, the gas adsorption capacity was weaker than the raw coal, the pit coal sample had the exceedingly more active functional groups, easy to react with oxygen, more likely to occur naturally, and its harm was relatively large. The mixed sample contained the highest C–O–C functional group absorbance. The functional groups were mainly influenced by the self-OH content, alkyl side chain, and fatty hydrocarbon in the sample. The main functional groups of the four-like mixture had the highest apparent activation energy, and the two reactions were higher in the low-temperature oxidation phase.

Effect of water soaking and air drying on the thermal effect and heat transfer characteristics of coal oxidation at the low-temperature oxidation stage

The microstructure, thermal behaviour (30 °C-300 °C) and thermophysical properties (30 °C-300 °C) of raw and water-soaked air-dried coal samples were analysed through scanning electron microscopy, differential scanning calorimetry and laser thermal conductivity meter to study the effect of water soaking and air drying on the thermal effect and heat transfer characteristics of coal at the low-temperature oxidation stage. Experimental results show that the action of water soaking and air drying changes the apparent morphology of coal, increasing the density of surface pore distribution and number of all pore types to different degrees. Under the same temperature conditions, water-soaked air-dried coal has lower heat absorption and heat release than raw coal. Moreover, the time required by water-soaked air-dried coal to reach thermal equilibrium is 32.4 s faster than that needed by raw coal. The activation energy of water-soaked air-dried coal calculated by using the KAS and FWO methods is reduced to about 83% of that of raw coal. This result indicates that water soaking and air drying increase the tendency of coal to self-ignite. The thermal diffusivity of water-soaked air-dried coal is higher than that of raw coal and shows a maximum difference of 0.005 mm2/g at 60 °C. The specific heat capacity and thermal conductivity of water-soaked air-dried coal are lower than those of raw coal and show the maximum differences of 0.19 J g/K and 0.18 W/m·K at 210 °C and 300 °C, respectively. Although the spontaneous combustion of water-soaked air-dried coal spreads at a faster rate than that of raw coal, the hazards following the spontaneous combustion is less than that of raw coal. This research contributes to understanding the effect of water soaking and air drying behaviour on coal spontaneous combustion characteristics.

Study on the Difference in Oxidation Characteristics of Coal and Gangue from the Same Coal Seam

ABSTRACT The gangue hills and underground backfill mining gob where crushed coal and coal gangue coexist face severe risks of spontaneous combustion. Studying the differences in oxidation characteristics between coal and gangue from the same coal seam is beneficial for spontaneous combustion. This paper tested the changes in radicals, functional groups, and gas products during the oxidation of raw coal, raw gangue, and washed gangue and analyzed their differences. Finally, gray correlation analysis was used to calculate the correlation between active groups and gas. The results showed that raw coal contained a more significant initial radical concentration and g factor value than raw and washed gangue. Its aliphatic hydrocarbons and oxygen-containing functional groups content was also significantly higher. The growth rate of radical concentration in the early oxidation stage of washed gangue was larger than that of raw coal and raw gangue due to the high sulfur content. At the same time, the temperature at which the g factor started to increase rapidly and the temperature at which the aliphatic hydrocarbons decreased rapidly was lower than that of raw coal and raw gangue. Gray correlation analysis showed that the active groups closely related to oxygen consumption were CH3 and CH2, while the groups closely associated with CO generation were C=O, COOH, Ng, and g factor.