Functional Groups In Coal Research Articles

Research on the Optimization for Acidification Modification Scheme Considering Coal's Wettability Based on the AHP-TOPSIS Method.

Acidification technology is an important measure for enhancing the extraction of coalbed methane from seams with low permeability and abundant minerals, and the acidification scheme is the key to the success of acidification treatment. To determine the optimal acidification modification scheme, an improved AHP-TOPSIS method is proposed to decide on the optimal conditions for wettability modification. This method constructs an evaluation index system, taking the wettability of coal as the target layer and the pro/hydrophobic functional groups in coal as the index layer. Meanwhile, it innovatively takes the adsorption energy of each functional group when absorbing a single water molecule as the basis for assigning weights to the evaluation indexes. Then, nine acidification modification schemes are evaluated and selected by the improved AHP-TOPSIS method based on the test results of different schemes to get the optimal one. The optimal scheme selected by the AHP-TOPSIS method is validated by water adsorption tests and isothermal adsorption tests. The results showed that the significance of each evaluation index is ranked as follows: aromatic structures > hydroxyl groups > aliphatic functional groups > oxygen-containing functional groups. The optimal acidification modification scheme is selected by the AHP-TOPSIS method with a HF concentration of 4% and a reaction time of 6 h. The ranking of acidification modification schemes obtained by the AHP-TOPSIS method is in high agreement with the ranking of water adsorption tests. When compared with raw coal, the coal samples treated with the optimal scheme have lower adsorption capacity for gas, which indicates that the aforementioned method could be used to evaluate and select the optimal acidification modification scheme, and the selected optimal scheme has the potential to increase the output of coalbed methane.

Study on the influence of different-voltage plasma breakdowns on functional group structures in coal

Promoting gas desorption is the key to improving gas extraction efficiency, physical discharge-based plasma carries massive joule heat, and inevitably affects the structures of functional groups in coal, which has a certain impact on the adsorption and desorption of gas, but existing literature lacks in-depth research on this. This study explores the changes of functional groups on GHS high metamorphic anthracite and HL low metamorphic bituminous coal samples under different breakdown voltages (BVs), from 15.3 kV to 17.3 kV, which serves as the key to determining the amount of energy injected into plasma channels. This study also explains the influence of BV on functional groups in coal from the perspective of electrical parameters such as voltage, current waveform and deposited energy. For two kinds of coal samples, the results reveal the following: (1) With the breakdown voltage increases, the coal is more likely to be polarized and the pre-breakdown period is shorter; accordingly, the coal can be broken down more easily. (2) The peak current increases linearly with the rise of BV, and the energy conversion efficiency jumps first and then tends to stabilize; (3) The plasma-induced oxidation reaction degree of the coal surface gradually strengthens as the BV rises, because more energy is used for reactions on the coal surface. Specifically, the higher the BV is, the faster the contents of hydroxyl and oxygen-containing functional groups decline, and the faster the aliphatic hydrocarbons in the coal decompose.

The Effects of Solvent Extraction on the Functional Group Structure of Long-Flame Coal

The functional group structures of coal molecules are one of the most important factors affecting spontaneous combustion. However, it is difficult to determine the exact effects of such structures. Extraction technology is able to modify the functional groups in coal as a means of inhibiting spontaneous combustion reactions. The present work extracted coal from the Caojiatan mine in northern Shaanxi, China, with various solvents. The extraction effectiveness of these solvents was found to decrease in the order of dioctyl sulfosuccinate (AOT) > water > n-hexane > cyclohexane + AOT + ethanol > cyclohexane > ethanol > methanol. With the exception of the AOT, the concentration of functional groups in the extracted coal was decreased compared with that in a control specimen extracted using only water. Ethanol, n-hexane, and methanol provided the optimal extraction efficiencies in terms of capturing coal molecules with aromatic structures, aliphatic structures, and oxygen-containing groups, respectively. The results of this work are expected to assist in future research concerning the extraction of coal molecules with specific functional groups. This work also suggests new approaches to the active prevention and control of spontaneous combustion during the mining, storage, and transportation of coal.

Effect of organic carboxylates in coal on the production of active sites and coal self-ignition behavior

The heat release of coal oxidation lead to the thermal decomposition of carboxylic groups, which include carboxylic acid and carboxylate. However, few researchers have studied the effect of the decomposition of intrinsic carboxylate structure on coal self-ignition process. In this work, different coal samples were selected and treated with alkali solutions to form organic carboxylate structures, and the thermal decomposition of the raw and alkali-treated coals followed by the room temperature oxidation process were analyzed. The effect of the carboxylate structures on the thermal behavior, elemental distribution, and functional groups of coal was also studied by means of TGA, FTIR, XPS and EPR. The results showed that the carboxylic groups in coal were transformed into carboxylate structures by ion exchange. In addition, highly reactive free radical active sites were produced and accumulated along with the decomposition of carboxylates, thus accelerating the self-heating of coal. In the room-temperature oxidation experiment, the temperature of alkali-treated Wulan coal was spontaneously raised by 13.9℃, while that of raw coal was 4.3 ℃. The results showed that the thermal decomposition of organic carboxylate played an important role in promoting the spontaneous combustion of coal, and acid pretreatment could be a potential method to effectively inhibit the coal self-ignition process.

Experimental investigation on the characteristics of XG/GG/HPAM gel foam and prevention of coal spontaneous combustion

Polymer gel foam shows excellent development potential and application prospects in the prevention of coal spontaneous combustion. To reveal the relationship between the characteristics of gel foam and the prevention of coal spontaneous combustion, XG/GG/HPAM gel foam consisting of polymer xanthan gum (XG), guar gum (GG), and polyacrylamide (HPAM) is prepared. The characteristics of gel foam are investigated in terms of microstructure, viscosity, thermal stability, and film-forming. The effects of gel foam on gas products, characteristic temperature, combustion characteristics parameters, and functional groups during coal oxidation are studied. The results show that polymer delays the foam coarsening and improves the foam stability. XG/GG/HPAM gel foam exhibits a slow foam coarsening process, concentrated bubble diameter distribution, shear-thinning behavior, excellent thermal stability, and film-forming properties. Moreover, the inhibition rate of XG/GG/HPAM gel foam on coal is 43.63% higher than that of ordinary foam. XG/GG/HPAM gel foam effectively improves the apparent activation energy of coal, the maximum weight loss rate of the coal is decreased by 7.55%, and the combustibility, combustion stability, and comprehensive combustion index are diminished by 8.50%, 7.64%, and 10.17%, respectively. Simultaneously, XG/GG/HPAM gel foam reduces the oxidation activity of aliphatic hydrocarbons and oxygen-containing functional groups in coal, blocks the chain reaction, and thus inhibits coal spontaneous combustion. The results provide support for achieving efficient fire prevention performance of gel foam.

Study on the effect of inorganic and organic sodium on coal spontaneous combustion

In order to investigate the effect of sodium in coal spontaneous combustion, different types of inorganic and organic sodium were added to the raw coal to analyze the changes of spontaneous combustion characteristics of coal samples. The combustion characteristics parameters of each group of coal samples were calculated by simulating the spontaneous combustion reaction process of coal through programmed warming thermogravimetric experiments, and the results showed that the combustibility index of coal samples loaded with various types of sodium-containing reagents increased by 8.32 %–45.37 %, and the combustion characteristics index decreased by 13.82 %–39.15 %. It indicates that inorganic sodium salts and organic sodium salts promote the combustion of coal in the low temperature stage and inhibit it in the high temperature stage. Subsequently, the changes of characteristic functional groups in coal during combustion after loading sodium were analyzed by using in-situ infrared spectroscopy experiments, and it was concluded that sodium in organic salts would combine with peroxyl radicals and thus inhibit the oxidation process of oxygen-containing functional groups in coal; sodium in inorganic salts would replace hydrogen in carboxyl and hydroxyl groups and thus promote the oxidation reaction of coal. The results of the study can provide some theoretical basis for the development of new coal spontaneous combustion inhibitors with specific functions.

The effect of functional ionic liquids on the microstructure and wettability of coals with different metamorphic degrees

As the degree of coal mine mechanization increases, the dust concentration underground increases significantly, and the effective reduction of dust concentration can guarantee the safe production in the mine. Wet dust removal method is the main means to suppress coal dust, and wettability is its theoretical basis. To study the effect of functional ionic liquids on the microstructure of coal and the effect of coal microstructure on wettability, [AMIm][BF4], [HOEtMIm][BF4], and [HOEtMIm][NTf2] were selected to treat lignite and anthracite with different metamorphic degrees. Firstly, the effects of ionic liquids on the wettability of coal surface were investigated by surface tension and contact angle experiments. Secondly, the carbon structure skeleton of coal was analyzed by nuclear magnetic resonance (NMR) experiments. Finally, the correlation between the structural parameters obtained by NMR and the contact angle was analyzed by origin software. The results show that [HOEtMIm][NTf2] has the most obvious wetting effect on coal, and different ions have different effects. The ionic liquid treatment will partially dissolve the functional groups in coal. As faH(protonated aromatic carbon) decreases, faC(carbonyl carbon) and falO(oxygen-bonded aliphatic carbon) increase, the hydrophilicity of coal increases, and as fa′(aromatic ring carbon) and fal∗(methyl carbon) decrease, falH(methylene carbon and quaternary carbon) increases, the hydrophobicity of coal decreases. The results of this paper can provide basic theoretical support for improving the hydrophilicity of coal surface to improve the efficiency of mine dust reduction.

Characteristics and mechanism of Glutathione in inhibiting coal spontaneous combustion

Inhibitors play an important role in preventing and controlling coal spontaneous combustion (CSC). In this study, the inhibitory effects of antioxidant-reduced glutathione (G-SH) on CSC were examined. Thermogravimetric-differential scanning calorimetry (TG-DSC), cross-point temperature (CPT), and oxidation product tests were used to examine the effect of G-SH on the tendency of CSC and compare the discrepancy in the inhibition effect of G-SH and the halogen inhibitor. The changes in the free radical type, concentration, and surface functional groups in coal before and after the addition of G-SH were analyzed using electron spin resonance spectroscopy (ESR) and Fourier transform infrared spectroscopy (FT-IR). Six common active radical models were constructed, and the thermodynamic parameters of the reaction were calculated using density functional theory (DFT). The results revealed that G-SH inhibits the spontaneous combustion of coal. The inhibitory effect of G-SH was significantly stronger than that of the halogen salt inhibitors CaCl2 and MgCl2, and the higher the concentration of G-SH, the stronger is the inhibitory effect. The characteristic temperature points in the thermogravimetric experiment of coal samples with G-SH shifted to a higher temperature zone, and the CO concentration and oxygen consumption were lower than those of raw coal. After G-SH treatment, the type and relative concentration of free radicals in coal were lower than those in raw coal, the content of aliphatic hydrocarbon functional groups and oxygen-containing functional groups decreased, and the content of sulfur-containing functional groups increased. According to the calculation of Gibbs free energy and reaction enthalpy, G-SH can effectively neutralize the active free radicals in the process of coal oxidation, and the reaction activity with the free radicals in the process of coal oxidation is ·OOH > ·OH > ·CH3 > RO· > R· > ROO·. According to the experimental results, it is inferred that G-SH continuously supplies a large number of hydrogen donors (·H) for the reaction during the oxidation process and simultaneously neutralizes carbon free radicals and carbon oxygen free radicals, thereby reducing the concentration and reactivity of free radicals in the whole chain reaction process and blocking the occurrence of coal-oxygen composite reaction.

Spontaneous Combustion Characteristics and Inhibition of Air-Dried Coal with Varying Water Soaking Times

ABSTRACT Coal left in the goaf is prone to oxidation reactions when soaked in water and air-dried, which can cause a temperature increase and potentially lead to a mine fire. The spontaneous combustion characteristics of residual coal soaked in the goaf differ significantly from those of dry coal. Analyzing the variation rules of functional groups in coals with different water soaking times (WSTs) during low-temperature oxidation and exploring the inhibitory effect of an inhibitor on coals with different WSTs are vital in preventing and controlling fires in soaked and air-dried coal seams in the goaf. This paper analyzes the variation rules of functional groups in raw coal samples and samples soaked for one, three, and five months, as well as the corresponding samples under the influence of the inhibitor, MgCl2, using an experimental system of coal low-temperature oxidation and a Fourier transform infrared spectrometer. The following findings were obtained: as the WST increases, the aliphatic hydrocarbon content increases, the low-temperature oxidation activity becomes higher, and the inhibition effect of the inhibitor weakens gradually. As the WST increases, the -CH2, -CH3, -OH, and -COOH contents fall gradually, while the C=O content rises. At the beginning of the oxidation temperature rise (30–70°C), the inhibitor, MgCl2, slows down the consumption of oxygen-containing functional groups and aliphatic hydrocarbon functional groups. However, when the temperature exceeds 130°C, its inhibitory effect on coal oxidation weakens gradually. In this case, a growing number of functional groups are involved in coal oxidation, and the inhibitor fails to effectively prevent coal oxidation. Therefore, it is necessary to strengthen temperature monitoring in the goaf and regularly replenish inhibitors in soaked and air-dried coal seams to enhance the inhibitory effect and prevent the SC of such seams.

Effect of active functional groups in coal on the release behavior of small molecule gases during low-temperature oxidation

Effect of active functional groups in coal on the release behavior of small molecule gases during low-temperature oxidation

Effect of Iron Component on the Structural Evolution of Carbon Bonds in Hydrochloric Acid-Demineralized Lignite During Pyrolysis.

The pyrolysis characteristics of hydrochloric acid-demineralized Shengli lignite (SL+) and iron-added lignite (SL+-Fe) were investigated using a fixed-bed reactor. The primary gaseous products (CO2, CO, H2, and CH4) were detected via gas chromatography. Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy techniques were used to study the carbon bonding structures of the lignite and char samples. In situ diffuse reflectance infrared Fourier transform spectroscopy was used to better understand the effect of the iron component on the transformation of the carbon bonding structure of lignite. The results showed that CO2 was released first during pyrolysis, followed by CO, H2, and CH4, and this order was unaffected by the addition of the iron component. However, the iron component promoted the generation of CO2, CO (<340 °C), and H2 (<580 °C) at lower temperatures and inhibited the formation of CO and H2 at higher temperatures while also inhibiting the release of CH4 throughout the pyrolysis process. The iron component may form an active complex with C=O and a stable complex with C-O, which can promote the fracture of carboxyl functional groups and inhibit the decomposition of ether bonds, phenolic hydroxyl groups, methoxy groups, and other functional groups, thus promoting the decomposition of aromatic structures. At low temperatures, it promotes the decomposition of aliphatic functional groups and finally the bonding and fracture of functional groups in coal, leading to the change of the carbon skeleton, resulting in the change of gas products. However, it did not significantly affect the evolution of -OH, C=O, C=C, and C-H functional groups. According to the above results, a developing reaction mechanism model of Fe-catalyzed lignite pyrolysis was established. Therefore, it is worth doing this work.

Effects of Functional Groups in Coal with Different Vitrinite/Inertinite Ratios on Pyrolysis Products.

Maceral composition is one of the key factors affecting the liquefaction and gasification of coal, which has attracted extensive attention of researchers working on coal chemical industry. To elucidate the impact of vitrinite and inertinite in coal on pyrolysis products, vitrinite and inertinite were extracted from a single coal sample and mixed to create six samples with varying vitrinite/inertinite ratios. The samples were subjected to thermogravimetry coupled online with mass spectrometry (TG-MS) experiments, and the Fourier transform infrared spectrometry (FITR) experiment was used to determine the macromolecular structures before and after the TG-MS experiments. The results show that the maximum mass loss rate is proportional to the vitrinite content and inversely proportional to the inertinite content, and increased vitrinite content accelerates the pyrolysis process and shifts the pyrolysis peak to low temperatures. Based on FTIR experiments, the sample's CH2/CH3 content, representing the length of aliphatic side chains, decreases significantly after pyrolysis, and the greater the loss of CH2/CH3, the greater the intensity of organic molecule production, indicating that aliphatic side chains are likely to yield organic molecule products. The aromatic degree (I) of samples rises sharply and steadily with increasing inertinite content. After high-temperature pyrolysis, the polycondensation degree of aromatic rings (DOC) and relative abundance of aromatic and aliphatic hydrogen (Har/Hal) within the sample increased significantly, indicating the thermal degradation rate of aromatic hydrogen content is much lower than that of aliphatic hydrogen. When the pyrolysis temperature is lower than 400 °C, the higher the inertinite content, the easier it is to produce CO2, whereas an increase in vitrinite leads to an increase in CO production. At this stage, the "-C-O-" functional group is pyrolyzed to produce CO and CO2. When the temperature exceeds 400 °C, the CO2 output intensity of vitrinite-rich samples is much higher than that of inertinite-rich samples, while the CO output intensity of vitrinite-rich samples is lower, and the higher the vitrinite content, the higher the peak temperature of CO gas production of samples, indicating that when the temperature exceeds 400 °C, the increase of vitrinite inhibits CO production and promotes CO2 production. At this stage, the reduction of each sample's "-C-O-" functional group after pyrolysis is positively correlated with the maximum CO gas production intensity, and the reduction of each sample's "-C=O" functional group after pyrolysis is positively correlated with the maximum CO2 gas production intensity. As a result, the "-C-O-" functional group is more likely to produce CO, whereas the "-C=O" functional group is more likely to be pyrolyzed to CO2. Hydrogen is primarily produced during the polycondensation and aromatization processes, and its production is proportional to the dynamic DOC values after pyrolysis. The higher the I value after pyrolysis, the lower the maximum gas production peak intensity of CH4 and C2H6, which indicates that increasing the aromatic proportion is detrimental to CH4 and C2H6 production. This work is expected to provide theoretical support of the liquefaction and gasification of coal with different vitrinite/inertinite ratios.

Infrared Spectrum Characteristics and Quantification of OH Groups in Coal.

The KBr pellet press method for detecting the infrared spectrum of coal is one of the commonly used methods for analyzing the types and content of functional groups in coal. However, KBr crystalline water or moisture has a significant impact on the peak position, peak shape, and peak area of the organic O-H based stretching vibration wave in coal. In this paper, the theoretical characteristics of infrared spectra of phenols and alcohols have been simulated and analyzed using the Gaussian 16 series of programs. Four infrared spectral analysis techniques, in situ infrared, KBr pellet press, dry KBr pellet press, and paste methods, have been used to detect the infrared spectra of coal. The results show that the stretching vibration peaks of free O-H radicals without hydrogen bonding are located between 3700 and 3600 cm-1. After the O-H form hydrogen bonds with each other, the O-H stretching vibration frequency moves toward the low frequency direction, and the lower the wavenumber, the more O-H content. The conventional KBr gasket manufacturing process will absorb moisture in the air to interfere with the hydroxyl absorption peak of coal, and the experimental process requires absolute drying. The relative content of hydroxyl in coal can be compared and analyzed based on the peak position, peak shape, and peak area of the hydroxyl stretching vibration wave. Quantitative analysis of hydroxyl groups in coal also requires combination of elemental analysis and X-ray photoelectron spectroscopy.

Study on the Occurrence Difference of Functional Groups in Coals with Different Metamorphic Degrees.

In order to quantitatively study the difference in occurrence content of functional groups in coals with different metamorphic degrees, the samples of long flame coal, coking coal, and anthracite of three different coal ranks were characterized by FTIR and the relative content of various functional groups in different coal ranks was obtained. The semi-quantitative structural parameters were calculated, and the evolution law of the chemical structure of the coal body was given. The results show that with the increase in the metamorphic degree, the substitution degree of hydrogen atoms on the benzene ring in the aromatic group increases with the increase in the vitrinite reflectance. With the increase in coal rank, the content of phenolic hydroxyl, carboxyl, carbonyl, and other active oxygen-containing groups gradually decreased, and the content of ether bonds gradually increased. Methyl content increased rapidly first and then increased slowly, methylene content increased slowly first and then decreased rapidly, and methylene content decreased first and then increased. With the increase in vitrinite reflectance, the OH-π hydrogen bond gradually increases, the content of hydroxyl self-association hydrogen bond first increases and then decreases, the oxygen-hydrogen bond of hydroxyl ether gradually increases, and the ring hydrogen bond first significantly decreases and then slowly increases. The content of the OH-N hydrogen bond is in direct proportion to the content of nitrogen in coal molecules. It can be seen from the semi-quantitative structural parameters that with the increase in coal rank, the aromatic carbon ratio fa, aromatic degree AR and condensation degree DOC increase gradually. With the increase in coal rank, A(CH2)/A(CH3) first decreases and then increases, hydrocarbon generation potential 'A' first increases and then decreases, maturity 'C' first decreases rapidly and then decreases slowly, and factor D gradually decreases. This paper is valuable for analyzing the occurrence form of functional groups in different coal ranks and clarifying the evolution process of structure in China.

Study on the Variations of Key Groups and Thermal Characteristic Parameters during Coal Secondary Spontaneous Combustion.

To investigate the effect of preoxidation on the secondary spontaneous combustion of coal, the changes in the key groups and thermal characteristic parameters in coal after preoxidation were investigated through Fourier transform infrared spectroscopy (FTIR), laser thermal conductivity, and thermogravimetric experiments. Results showed that the aromatic hydrocarbons in coal gradually decrease with the rise in the preoxidation temperature, the aliphatic hydrocarbons increase and then decrease, the -C-O- group gradually decreases, and the -C=O and -COO- group content slowly increases. Preoxidation promotes the breakdown of aromatic hydrocarbons and the oxidation of oxygen-containing functional groups in coal. Meanwhile, the thermal diffusivity of coal decreases after preoxidation, while the specific heat capacity and thermal conductivity increase and then decrease. The results of the thermogravimetric analysis indicate that preoxidation changes the characteristic temperature, but it does not change the process of spontaneous combustion. The spontaneous combustion process of raw and preoxidized coals can be divided into three stages: water evaporation, oxygen adsorption, and combustion. Further, the apparent activation energy increases and then decreases with a rise in the preoxidation temperature during the moisture evaporation stage, increases during the oxygen adsorption stage, and decreases during the combustion stage.

Towards understanding of internal mechanism of coal reactivity enhancement after thermal decomposition at low temperature

Coal after thermal decomposition has high reactivity and can easily self-heating and self-ignite. The spontaneous combustion problem of thermally decomposed coal has been widely concerned, especially in the unsealing of closed fire area, the magmatic erosion area of coal and the storage of low-rank coal after drying. Related studies mainly explain the phenomenon from the perspective of physical pore and moisture, the effect of the chemical decomposition process on the subsequent spontaneous combustion process of coal is worthy of further study. In this paper, the room temperature oxidation of thermal decomposed coal was first carried out. The physical and chemical parameters of coal before and after the decomposition process were than investigated by modern analytical instruments such as scanning electron microscope (SEM), nitrogen adsorption instrument, X-ray diffraction (XRD), infrared spectrometer (FTIR), X-ray photoelectron spectroscopy (XPS) and Electronic paramagnetic resonance (EPR). Results show that a large number of active sites are generated and accumulated in the thermally decomposed coal, and these sites can rapidly oxidize at room temperature, leading to an increase in coal temperature. Changes in physical structures indicate that thermal damage at high temperatures results in the development of cracks and pores. Variations of chemical structures manifest that the coal decomposition process leads to the decomposition of oxygen-containing functional groups and the increase of active alkyl radicals. It is revealed that the thermal decomposition of oxygen-containing functional groups in coal produces a large number of reactive alkyl radicals, and the oxidation of these radicals is the internal reason for the high reactivity and self-heating of thermally decomposed coal. The development of physical structures such as pores and cracks provide a wider oxygen transport channel for the oxidation of the active sites. This paper is of great significance in revealing the spontaneous combustion mechanism of thermally decomposed coal and helping the development of high-efficiency chemical retardant.

Adsorption of CH4/CO2/N2 by different functional groups in coal

To explore the microscopic mechanism of gas adsorption by different functional groups in coal and provide theoretical support for the prevention and control of coal mine gas disasters, a slit-pore model and fragment model modified by different functional groups were built using Materials Studio software, based on density functional theory and the Monte Carlo method. The adsorption characteristics of seven different functional group models on the gas were analyzed, and the microscopic mechanism of adsorption performance of different functional groups on CH4/CO2/N2 was revealed. The results showed that the order of the electrophilic ability of each functional group fragment followed benzene ring > –OH > –CO > –O–CH3 > –COOH > –CH3 > –CH2–CH3, and the order of nucleophilic ability was benzene ring > –CH2–CH3 > –CH3 > –COOH > –OH > –CO > –O–CH3. After adding different functional groups to the surface of the complete activated carbon, the electrophilic ability and nucleophilic ability decreased. The adsorption capacity was affected by both gas properties and functional group structure. The adsorption heat was not only affected by the adsorption amount but also by the non-uniformity of the adsorbent surface. The oxygen-containing functional groups were beneficial to the adsorption of CO2 gas, and the aliphatic functional groups were committed to the adsorption of CH4 and N2.

Experimental investigation on using antioxidants to control spontaneous combustion of lignite coal

To investigate the effects of antioxidants on the microstructure and characteristic parameters of coal during the oxidation process of coal, three antioxidants (tea polyphenols, resveratrol, and β-carotene) at different mass ratios (1 wt%, 3 wt%, 5 wt%, and 7 wt%) were selected to treat raw coal samples. Fourier transform infrared spectroscopy was conducted to explore the changes in functional group content before and after antioxidant treatment of coal samples. The results show that the content of functional groups in coal had a trend of decreasing and then increasing with a rising mass fraction of antioxidants, and the optimum mass ratio of antioxidants was predicted. On this basis, the effects of antioxidants on characteristic temperature points and gas products of coal samples were determined by the thermogravimetric-infrared experiment. The test results show that the critical temperature and dry cracking temperature of the antioxidant-treated coal samples increased to different degrees compared to the raw coal during the low-temperature oxidation stage. Meanwhile, the concentration and yield of CO, CO2, and CH4 of the antioxidant-treated coal samples showed a decreasing trend compared to raw coal. The chemical kinetics method was used to determine the apparent activation energy in the process of coal oxidation. The apparent activation energies of TP, Res, and β-Car inhibited coal samples in the stage of water evaporation and gas desorption increased by 39 %, 30 %, and 26 %, respectively, compared with the raw coal, while the apparent activation energies in the oxygen absorption and weight gain stage were respectively increased 19.4 %, 12.9 %, and 14.3 % higher than raw coal. The ability of antioxidants to inhibit the spontaneous combustion of coal is TP > Res > β-Car. The results enrich the research on antioxidant inhibition of coal spontaneous combustion and provide a theoretical basis for preventing coal spontaneous combustion.

Inhibition effect and mechanism of nano-aluminum hydroxide foam on coal spontaneous combustion

The inhibition performances of nano-aluminum hydroxide (nano-ATH) foam were evaluated by thermal gravimetric analyzer and cone calorimeter. Results demonstrate that the dry cracking temperature point and ignition temperature point of coal samples increase after nano-ATH foam processing, while the percentage of weight loss decreases. In particular, the dry cracking temperature point increases form 93.2 °C to 164.6 °C, the ignition temperature point increases from 406.9 °C to 443.6 °C, and the weight loss percentage decreases from 34% to 20%. According to the combustion experiment, the peak heat release rate of the coal samples processed by nano-ATH foam decrease from 123.36 kW/m2 of the raw coal to 49.849 kW/m2. Meanwhile, the peak smoke release rate, and the total smoke releasing volume decrease from 0.093 m2/s to 0.006 m2/s, and from 1.027 m2 to 0.574 m2, respectively. Finally, infrared spectral analysis of the coal samples was carried out to investigate variations in functional groups in coals under different characteristic temperatures.

Influence of Organic Sulfur on Low-Temperature Oxidation of Coal and its Transition Characteristics.

The low-temperature oxidation spontaneous combustion of coal was caused by the active groups in its structure. The oxidation mechanism of carbon and oxygen functional groups in coal had been extensively studied, but there were few reports on the study of sulfur functional groups initiating the coal spontaneous combustion. To investigate the influence of organic sulfur functional groups on the spontaneous combustion of high-sulfur coal and explore its transformation characteristics, the low-temperature oxidation experimental system was used to study the spontaneous combustion tendency of coal with similar metamorphic degrees and different organic sulfur contents. The variations of element forms and organic sulfur functional groups were analyzed by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy during low-temperature oxidation of coal and model compounds. The results showed that the forms of organic sulfur in coal mainly included mercaptans, thioethers, thiophenes, sulfones (sulfoxides), and sulfates, and the low-temperature oxidation of coal was not only related to the content of organic sulfur but also related to its type. The coal samples, which possess a low content of total sulfur and a small proportion of active organic sulfur groups such as mercaptans and thioethers, had a lower concentration of indicator gas and a smaller tendency to coal spontaneous combustion. After low-temperature oxidation, the content of mercaptan, thioether, thiophene, methyl(methylene), and pyridine in coal decreased, and the content of oxygen-containing groups such as sulfone (sulfoxide), sulfate, carboxyl, and nitrogen oxide increased. The elements of S, C, and N all changed to a high-valent state. In the oxidation reaction of model compounds, mercaptans were more reactive than thiophenes in the low-temperature region, and the oxidation of thiophene could direct form sulfone (sulfoxide), while the oxidation of mercaptan formed disulfide first. It is speculated that low-valence sulfur migrated to a high-valent state by providing sulfhydryl radicals (•SH) and sulfur radicals (C-S•) combined with active oxygen atoms. After the low-temperature oxidation reaction of model compounds, some organic sulfur existed in the form of aromatic sulfur or sulfur oxides and a small part of sulfur escaped as SO2 and H2S gases in the solid oxidation product.