Oxidation Of Coal Research Articles

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.

Development of potentiometric titration method for assessing coals’ oxidation degree

Oxidation of coal leads to some unwanted consequences, such as losses of products quality and increase in proneness to spontaneous combustion. One of the indicators of coals’ oxidation is changing in amount and composition of active functional groups in organic matter. On using a wide number of samples and adjusting the experimental procedure, a method was developed for coals’ oxidation degree evaluation by potentiometric titration based on evaluation of contents of total acidic groups (a sum of phenol and carboxyl groups). It was shown that potentiometric titration is not applicable for determination of phenol and carboxyl groups in coals separately. A phenomenon of appearance of two waves at titration curve was connected with absorption of CO2 from air into the alcohol-alkaline solution. For coals with vitrinite reflectance varying in range of 0.5-0.72 %., total acidic groups amount decrease with rank growth, whereas for higher rank coals such an alteration is less pronounced. It is shown that it is expedient to use the developed method of potentiometric titration together with the petrographic method for determining the oxidation degree. On the basis of the proposed method of coals’ oxidation degree determination by potentiometric titration, a National standard of the Russian Federation GOST R 59012-2020 has been introduced. According to it, the oxidation degree is determined by the increase of the total amount of acidic groups – phenolic and carboxyl hydroxyls in coal as compared to the content of total acidic groups in the control sample, which is used as a seam sample of coal taken outside the oxidation zone, or coal taken when laying the pile, or coal received for benefication, sorting or moving. The research has been carried out in the framework of strategic academic leadership program “Priority 2030”. The paper was written with the participation of E. L. Kossovich, Senior Researcher, Candidate of Physico-mathematical Sciences.

Heat Release Characteristics of L-Malic Acid Inhibited-Coal Oxidation

ABSTRACT Transition metal ions in coal can catalyze the oxidation of coal, reduce the activation energy of the coal-oxygen reaction, and promote spontaneous combustion. Adding a metal chelating agent to coal can form chelates with metal ions so that the metal ions lose their catalysis. A green metal chelating agent, L-malic acid (L-MA), was proposed in this study. A C600 microcalorimeter was used to study the inhibitory effect of L-MA on spontaneous coal combustion. The results show that under the same temperature-programmed conditions, the heat absorption capacity of coal treated with L-MA solution is higher than that of raw coal, but the heat release capacity of treated coal is lower. With increase in the concentration of the L-MA solution, the inhibitory effect first increased and then decreased. The maximum heat flow of raw coal is 292 mW, and the maximum heat flow of coal samples treated with malic acid is 246 mW, 237 mW, 232 mW, 240 mW and 255 mW, respectively. The maximum heat flow of 15 wt% coal sample is 20.55% lower than that of raw coal. Based on the thermal analysis, kinetic theory, and experimental data, the activation energy of each coal sample can be calculated. The activation energy of raw coal is the lowest, and the activation energy of each group first increases and then decreases with increasing L-MA concentration, which is in agreement with the law of heat flow data. Combined with the microscopic process of coal oxidation at different temperature stages, the mechanism of L-MA inhibiting spontaneous coal combustion was explored. In summary, L-MA can effectively reduce the concentration of related free radicals in the chain reaction of spontaneous coal combustion and inhibit spontaneous coal combustion.

Non-linear soft sensing method for temperature of coal spontaneous combustion

The accurate measurement of the temperature of coal spontaneous combustion is a key technology for coal spontaneous combustion and fire prevention and is important to realising safe coal mining and reducing energy losses. A soft sensor model based on a hybrid-kernel-function support vector machine with an improved genetic algorithm is proposed for the nonlinear relationship between the concentration of a characteristic gas of coal spontaneous combustion and the temperature of the coal body. The natural oxidation process of the coal body is simulated using a coal-spontaneous-combustion oxidation warming experiment bench. The gas concentration and maximum temperature of the coal body at different moments in an oxidation warming experiment are taken as the model input data. Various soft sensor models are automatically optimised by a genetic algorithm in the training process, and the performances of the models are compared and evaluated in the field. The results show that the evaluation indexes of the hybrid-kernel-function model are better than those of the single-kernel function. In the training process, the evaluation indexes of the model with hyperparameters automatically optimised by the genetic algorithm are better than those other models, further verifying the nonlinear relationship between the gas concentration and coal temperature in the process of coal spontaneous combustion and oxidation. The use of the improved support vector machine model basically results in no deviation between the soft sensor results and actual temperature data in the field test, indicating that the soft sensor model has high accuracy, generalisation ability and applicability. The improved support vector machine model has a mean absolute percentage error of ≤ 1.616%, mean absolute error of ≤ 0.533 ℃, and root-mean-square error of ≤ 0.608 ℃. These results indicate that the soft sensor problem of coal spontaneous combustion can be considered a typical small-sample regression method. The proposed method can be further applied to similar problems in the energy industry.

Study the effect of drying on the oxidation thermogravimetric and functional group composition characteristics of immersed lignite

The drying process of immersed lignite has a significant influence on the characteristics and progress of spontaneous combustion. To reveal the influence mechanism of the drying process of immersed coal on the spontaneous combustion characteristics and the change rule of the spontaneous combustion process, in this research, we measured the mass change during coal oxidation with thermogravimetry, and the change of the functional groups with Fourier transform infrared spectroscopy. The influence of the drying process on the coal was analyzed by comparing activation energy, functional group of immersed coal with different drying degrees and raw coal. The results showed that, compared with raw coal, the content of Ar–C–O– and antisymmetric stretching vibration of the carboxylate group (–COO–) as well as the stretching vibration in the quinone group (C=O) and the –OH group increased. For the content of delta s.{text{RCH}}_{{3}},delta as.{text{R}}_{{2}} {text{CH}}_{2}delta as.{text{RCH}}_{{3}}, the value of Asym.CH2/Asym.CH3 decreased. The content of various functional groups changed to be favorable for oxidation and heat release. At different reaction stages, the activation energy was differently affected by the degree of drying. Average values of activation energies at different reaction stages are shown raw coal had the lowest activation energy. After soaking in water and drying, the activation energy of coal is increased to varying degrees, the reactivity is reduced, and the risk of spontaneous combustion is reduced. After soaking in water and drying, the activation energy of coal is increased to varying degrees, the reactivity is reduced, and the risk of spontaneous combustion is reduced. The activation energy of the coal samples dried for 24 h after soaking in water is the lowest among the coal samples dried for different times after soaking in water, and the moisture content is 10.5%.

Effects of inert gas CO2/N2 injection on coal low-temperature oxidation characteristic: Experiments and simulations

To deeply understand the mechanism of inert gases in inhibiting coal spontaneous combustion, the effects of dry air, CO2, and N2 on coal spontaneous combustion were analyzed experimentally. To this end, bituminous coal prepared from Dongrong No. 2 Coal Mine was considered the research object. Based on the adsorption configuration of the oxygen-containing coal, the displacement behavior of O2 by CO2 /N2 was studied using the grand canonical Monte-Carlo (GCMC) and molecular dynamics (MD) methods. The obtained results show that the injection of CO2 and N2 reduces the ability of spontaneous combustion of coal. It is found that among the studied gases, CO2 has a stronger inhibition effect on coal spontaneous combustion, which increases the temperature of CO occurrence by 5℃, decreases the concentration of CO by 29.91%, and inhibits low-temperature oxidation of coal. From the microscopic point of view, CO2 /N2 gases can effectively displace O2 by diffusion and occupying adsorption sites. It is found that after the injection of CO2, the concentration of O2 molecules increases significantly in the vacuum layer. Compared with N2, injection of CO2 increases the diffusion activation of O2 by 5.89%. This indicates that the injection of an inert gas significantly reduces the oxygen absorption capacity of coal, thereby decreasing the coal-oxygen combination reaction and preventing the spontaneous combustion of coal. The performed analyses demonstrate that CO2 outperforms N2 in restraining the spontaneous combustion of coal.

Experimental study and thermodynamic analysis of coal spontaneous combustion characteristics

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

Crystalline structure development of coal during low-temperature oxidation under lean-oxygen environments caused by methane emission in gob

The residual coal in gobs is usually oxidized under a lean-oxygen condition due to methane emissions, thus affecting the crystalline structure of coal that is closely associated with heat transfer during low-temperature oxidation. This study set out to investigate the crystalline structure development of coal during low-temperature oxidation under this practical condition. For this purpose, non-isothermal oxidation and X-ray diffraction (XRD) experiments were employed to conduct lean-oxygen oxidation of coal and crystalline structure analysis, respectively. The results indicate that crystalline structure was closely associated with methane at different temperatures. Methane in gobs reduced the oxidation degree of coal, making the crystalline structure became less orderly; it restrained the condensation of crystalline structure, resulting in a less compact aromatic structure compared with that under aerial conditions. The condensation occurs between aromatic layers and along the plane of the aromatic layer (methane < 20.0 %), while it mainly happens between aromatic layers when methane was higher(>20.0 %). During self-heating, the rising methane increased the interlayer spacing of aromatic layers and reduces the crystallite height, the graphitization degree, and the number of effectively stacked aromatic layers. Methane’s influence on crystallite diameter was found to be sensitive to oxidation temperature. Crystallite diameter consistently grew with oxidation temperature regardless of methane, but the growth rate became apparent only when the oxidation temperature exceeded 150 ℃ and methane concentration was lower than 20.0 %. The findings of this research will promote our understanding of crystalline structure development under oxygen-lean conditions caused by methane emission in gobs.

Optimized TGA-based experimental method for studying intrinsic kinetics of coal char oxidation under moderate or intense low-oxygen dilution oxy-fuel conditions

Accurate assessment of coal char oxidation kinetics is essential for the design of efficient combustion systems; nonetheless currently, there is little research on this aspect under moderate or intense low-oxygen dilution (MILD) oxy-fuel (MO) conditions. In this study, experimental and modeling studies were conducted for analyzing the intrinsic kinetic properties of coal char oxidation under MILD air-combustion (MA) and MO conditions based on thermogravimetric analysis (TGA). The experimental conditions were improved to minimize various diffusion resistances in the crucible during the TGA experiments. Under the optimal conditions of TGA experiments, the reactivity differences of coal char under the MA and MO atmospheres were small. Both nonlinear model-fitting and linear model-free analysis methods were used to analyze kinetic parameters of coal char oxidation. The results showed that the random pore model correlated the best with the experimental results. The activation energies of coal char under the optimal conditions in MA and MO atmospheres (i.e., 104.4 kJ/mol and 98.2 kJ/mol, respectively) were much higher than those obtained by the conventional method. The reaction orders of coal char oxidation in MA and MO atmospheres were close to 1.0, and the specific values were affected by the diffusion effect and background atmosphere.

Preparation of New Eco-Friendly Gel Foam Based on Biomass Pectin Material for Fire Prevention of Coal

ABSTRACT The traditional gel foam materials have achieved certain effects in preventing coal fire. However, the traditional gel foam materials commonly use chemical compounds which cause harm to the environment. Therefore, based on biomass materials such as low methoxyl pectin (LMP), calcium L-lactate (Ca-L), and biomass compound foaming agent (BF), a new eco-friendly gel foam (LMP-Ca) was prepared. The foaming performance, gelling time, stability, coal oxidation inhibition and fire extinguishing performances of LMP-Ca were systematically studied. The study shows that the gelling time of LMP-Ca with 1.2 wt% LMP, 0.075 wt% Ca-L and 0.03 wt% BF is 10 min and the half-life is up to 216 h. Moreover, the LMP-Ca has excellent film-forming performance, and the isolation film with thickness up to 2.5 mm can be formed on the surface. The coal oxidation inhibition experiment shows that LMP-Ca can effectively delay the oxidation rate of coal with the temperature rising, and the inhibition rate of CO is 72.1% at 180°C. The fire extinguishing experiment shows that the temperature of the coal treated by LMP-Ca decreases rapidly from 960 to 60°C in 20 min and to 30°C in the following 110 min without the reignition phenomenon. The result indicates that LMP-Ca has excellent fire extinguishing performance. This study is helpful to develop eco-friendly coal fire prevention and control technology, and overcome the shortcomings of traditional gel foam on environmental pollutions.

Study on Oxidation Dynamic Characteristics of Pressure Bearing Broken Coal with Different Water Content

ABSTRACT Gob is one of the most serious areas of coal spontaneous combustion. With the continuous increase of mining depth, the residual coal in gob bears increased axial stress, the rise of ground temperature, and the degree of fragmentation of mining coal and rock continues to increase, the external water in the mining process changes the water content of broken residual coal, under the influence of multiple factors, the prevention and control situation of coal spontaneous combustion in gob is becoming more serious. Based on the actual accumulation environment of residual coal in gob, this paper carried out the oxidation and temperature rise experiment of broken coal under the condition of water bearing and pressure. The variation laws of oxidation characteristic parameters such as outlet oxygen concentration, oxidation derived gas, oxygen consumption rate and heat release intensity were obtained. It is found that with the increase of water content of coal sample, the water content first promotes and then inhibits the oxidation process of coal. It is found that after water molecules form a relatively stable hydrogen bond with oxygen-containing functional groups, the total energy of the two-phase materials decreases, and the reason for the decrease of wetting heat release is analyzed. The influence mechanism of axial stress loading on coal spontaneous combustion oxidation process is studied. It is found that the increase of axial stress first promotes and then inhibits the process of coal spontaneous combustion.

Study on the Law and Risk of Spontaneous Combustion of Residual Coal during Water Drainage in Goaf

Water–gas displacement occurring during the drainage of water-soaked goafs facilitates the oxidation of water-soaked coal. The characteristics of oxygen migration and the oxidation and spontaneous combustion (SC) of soaked residual coal during goaf drainage were explored through laboratory research, water drainage simulation and on-site measurement. The results reveal that compared with raw coal samples, the amount and rate of gas products of water-soaked coal samples are higher in the heating oxidation process, demonstrating a strengthened spontaneous combustion (SC) propensity. Its cross-point temperature falls and the apparent activation energy decreases by 1.43–8.75%, that is, the soaked coal sample is easier to spontaneously combust during the drainage of water-soaked goafs. Through simulation, it is found that after water is drained, air leakage in the goaf is significantly intensified, and the pressure difference inside and outside the goaf reaches 498 Pa. By taking the air inlet roadway as the air leakage point for fitting, it is found that the oxygen concentration in the air leakage range increases to 18% during water drainage. The simulation results are basically consistent with the on-site measurement. The on-site monitoring result shows that during water drainage of 7225 goaf in Qinan Coal Mine, water-immersed coal is more prone to spontaneous combustion, and air leakage leads to low-temperature oxidation of water-immersed coal, which increases the on-site temperature rapidly and increases the risk of spontaneous combustion in the goaf. With respect to water drainage in the goaf, an optimization measure of fixed-point and quantitative nitrogen injection during water drainage was put forward on site.

Kinetic study in selective reduction of limonitic and saprolitic nickel ore

ABSTRACT Saprolite and limonite have different behaviour in the carbothermic reduction process due to the differences in their atomic structures. This work investigates the kinetic study of these two different kinds of lateritic nickel ores. Nickel ore, coal, sodium sulfate, and calcium oxide were mixed, then pelletised into 10–15 mm diameters. The pellets were reduced with a non-isothermal condition using a muffle furnace at 950–1150 °C for 5–120 min. The reduced pellets were crushed into less than 74 µm prior to the wet magnetic separation process to obtain concentrates (ferronickel) and tailings (impurities). The result showed that the reduction rate for both lateritic nickel ores increased with the increase of reduction temperature and holding time. The Ginstling-Brouhnstein model was appropriate to describe the diffusion mechanism of the reduction process of limonite and saprolite. The activation energy in the reduction of limonite was lower than in saprolite. It indicated that the iron and nickel in magnesium silicate in saprolite have lower reducibility than the oxide structure in limonite. Modifying basicity in nickel laterite with CaO addition could also reduce the activation energy.

Heat Recovery Potential and Hydrochemistry of Mine Water Discharges From Scotland’s Coalfields

Prospective and operational mine water geothermal projects worldwide have faced challenges created by mine water chemistry (e.g., iron scaling, corrosion) and high expenditure costs (e.g., drilling or pumping costs) among others. Gravity fed or actively pumped drainages can be cheaper sources of low-carbon mine water heating when coupled with adequately sized heat exchanger and heat pump hardware. They also provide valuable chemical data to indicate mine water quality of associated coalfields. Field collection of temperature and flow rate data from mine water discharges across the Midland Valley of Scotland, combined with existing data for Coal Authority treatment schemes suggest that mine water heat pumps could provide a total of up to 48 MW of heat energy. Chemical characterisation of mine waters across the research area has created a valuable hydrochemical database for project stakeholders investigating mine water geothermal systems using boreholes or mine water discharges for heating or cooling purposes. Hydrochemical analytical assessment of untreated gravity discharges found that most are circumneutral, non-saline waters with an interquartile range for total iron of 2.0–11.6 mg/L. Stable isotope analysis indicates that the discharges are dominated by recent meteoric waters, but the origin of sulphate in mine waters is not as simple as coal pyrite oxidation, rather a more complex, mixed origin. Untreated gravity discharges contribute 595 kg/day of iron to Scottish watercourses; thus, it is recommended that when treatment schemes for mine water discharges are constructed, they are co-designed with mine water geothermal heat networks.

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.

Understanding mechanisms of pyridine oxidation with ozone addition via reactive force field molecular dynamics simulations

Ozone assisted combustion is a promising method to control combustion, ignition and pollutant formation. In this study, we investigated the ozone behaviours in fuel-NOx control through reactive force field (ReaxFF) molecular dynamics (MD) simulations of pyridine (a main nitrogen-containing compound in coal) oxidation under different ozone concentrations. The results show that ozone enhances the pyridine combustion process and facilitates the conversion of CO to CO2 and NO to NO2. Ozone participates in the reactions with intermediates and promotes the generation of active particles like OH, HO2, HO3 and H2O2. This research reveals mechanisms, at the atomic level, for the effects of main products formation during pyridine oxidation under different levels of ozone addition. The present study provides the scientific base for the control of NOx emissions through ozone assisted combustion technology.

Study on the Influence of the Mechanical Crushing Process on Low-Temperature Oxidation Characteristics of Coal Samples

ABSTRACT In order to study the influence of the mechanical crushing process on the low-temperature oxidation characteristics of coal, the programmed heating experiments of coal samples with different particle sizes were carried out using a programmed heating device. By comparing and analyzing the changes in CO and CO2 emission, cross-point temperature, heat release intensity, and apparent activation energy of coal samples with different particle sizes during temperature heating, the influence of particle sizes on the spontaneous combustion characteristics of coal samples was obtained. On the basis of the infrared spectrum experiment and ESR (electron paramagnetic resonance) experiment of coal samples with different particle sizes, the changes of functional groups and free radicals were further compared, and the deep-seated reasons that the mechanical crushing process affected the spontaneous combustion characteristics of coal were explored. The results showed that with the decrease in particle size, more alkyl side chains are dehydrogenated to produce alkyl radicals during mechanical crushing, and the alkyl radicals are oxidized with oxygen to form more oxygen-containing functional groups. These oxygen-containing functional groups can accelerate the low-temperature oxidation process of coal in the subsequent secondary low-temperature oxidation process, release more heat, and increase the heat release intensity. In the process of mechanical crushing, the coal sample not only changes in physical structure but also changes in chemical structure. With the decrease in particle size of coal samples, more CO and CO2 are produced in the process of low-temperature oxidation of coal, and the time to reach the cross-point temperature is advanced. However, the cross-point temperature and the apparent activation energy decreases, and the spontaneous combustion characteristics of coal are enhanced.

Study on Diffusion Behavior of Inorganic Materials and Performance of Extinguishing Coal Fire

ABSTRACT The majority of a coal mine’s goaf is semi-closed complex porous medium space, which is a critical place for coal spontaneous combustion disasters. To investigate the flow diffusion pattern of inorganic compounds in fractured coal in the goaf, as well as the capacity to put out coal fires. It examine the viscosity properties of an inorganic material slurry as a function of water/material ratio and time. By building a three-dimensional grouting test platform in the lab, simulating the broken coal body with sand and gravel, designing orthogonal tests to run multiple groups of grouting tests, and analyzing the effects of factors besides grouting pressure, water/material ratio, and air space ratio on the slurry diffusion radius. The effect of grouting pressure on the pattern of slurry diffusion radius of inorganic material and the water/material ratio under the action of grouting pressure is studied. The test of inorganic materials to extinguish coal pile fire is done, and the fire extinguishing and cooling ability is assessed in comparison to mud of the same grade, based on a prior research of inorganic materials to suppress CO formation in coal oxidation warming. According on the findings, the three-dimensional grouting test platform can pass the grouting test of loose sand and gravel bodies at various grouting pressures. The effects of grouting pressure, water/material ratio, and air space ratio on slurry dispersion are discovered to be multivariate power function relationships rather than linear correlations. The related slurry diffusion radius equation is established, and the variables impacting the parameters of the slurry diffusion range are evaluated, with the grouting pressure and water/material ratio having a substantial impact on slurry diffusion. Inorganic materials, as opposed to mud, require less time to extinguish coal pile fires and are less prone to scatter. It can form an oxygen barrier through continually covering the surface of coal. The study’s findings might give a solid foundation for the avoidance of slurry and fire suppression of inorganic materials in the goaf.

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.

Evolution and mechanism for the terahertz dielectric spectrum of coal during oxidation

In this paper, the dielectric spectrums of lignite during oxidation and its corresponding dielectric response mechanisms were investigated by means of THz-TDS and synchronous thermal analyzer, combined with the Clausius-Mossotti equation and chemical reaction pathway. Results indicate that dipole polarization is the dominant polarization mechanism during coal oxidation in the THz field. There are four dielectric response mechanisms, which gradually participate in the competition of polarity changes with the increase of oxidation temperature. The dielectric response of lignite shows significant temperature dependence. The dielectric constant first decreases until 150 °C due to the dewatering. From 150 °C to 275 °C, the dielectric constant tends to increase. Before 225 °C, the increase is due to the formation of more oxygen-containing bonds. Beyond 225 °C, the coupling effect of the formation of oxygen-containing bonds and the twisting deformation of the molecular chain skeleton become the main reason. Then, due to the fracture of polar bonds, the dielectric constant drops significantly until 400 °C. Beyond 400 °C, the dielectric constant increases, possibly due to enhanced electron migration. The outcomings of the current study help to identify the oxidation stage of coal and enrich the application basis of THz technology in the fields of coal fire identification and mineral processing.