Long-flame Coal Research Articles

Modification of carbon sorbents by ozonation

The paper presents the results of a study of changes in the textural and physicochemical properties of the carbon sorbent surface after oxidative modification with an ozone-oxygen mixture. For the study, we used a carbon sorbent sample obtained on the basis of long-flame coal. It is shown that oxidative modification with the mixture leads to an increase in the surface area of the studied sorbent, mainly due to the growth of micro-and mesopores. In this case, according to IR spectroscopy data, an increase in the content of oxygen-containing groups on its surface is observed.

Magnetic sorbents based on Kuzbass long-flame coal

The paper investigates the textural characteristics of carbon sorbents obtained from Kuzbass coal of the LF grade. The sorbents were obtained by activation using ferric chloride, pre-impregnating the coal with an iron III chloride solution at a FeCl3/coal ratio of 1:7 g/g. Powder X-ray diffraction in sorbents proved the presence of the Fe3O4 phase of magnetite. The amount of magnetite in the sorbents varies within 7%. It is shown that sorbents obtained from coal activated with iron chloride have a sufficiently developed porous structure and significant microporosity. The measured values of the specific surface area of sorbents at an activation temperature of 800 °C reach 250 m2/g, and at 700 °C they are an order of magnitude lower. The total pore volume does not exceed 0.1 cm3/g with a micropore fraction of more than 70%.

Study for the Effect of Temperature on Methane Desorption Based on Thermodynamics and Kinetics.

Desorption hysteresis is important for primary gas production. Temperature may cause serious changes in the methane adsorption/desorption behaviors. In order to study the mechanism of methane desorption and desorption hysteresis, three sets of samples of long-flame coal, coking coal, and anthracite were collected, and experiments such as microscopic composition determination, liquid nitrogen adsorption, and isothermal adsorption/desorption were performed. From the perspectives of desorption kinetics, desorption thermodynamics, and methane occurrence state, the differences in methane and methane desorption characteristics and the desorption hysteresis mechanism are discussed. The results show that at the same temperature, anthracite (SH3#) has the largest saturated adsorption capacity and residual adsorption capacity, followed by coking coal (SGZ11#), and long-flame coal (DFS4#) has the smallest. As the temperature increases, the theoretical desorption rate and residual adsorption capacity of anthracite (SH3#) and coking coal (SGZ11#) will increase first and then decrease. Temperature and methane desorption do have positive effects, but temperature may have a threshold for promoting methane desorption. It is necessary to comprehensively consider the influence of temperature on the activation of gas molecules and the pore structure of coal. Under the premise of a certain temperature, as the pressure increases, the desorption hysteresis rate changes in a logarithmic downward trend, the methane desorption hysteresis rate in the low-pressure stage (P < 4 MPa) is large, and the methane desorption hysteresis rate in the high-pressure stage (P > 4 MPa) is lower; during the isobaric adsorption process, the adsorption capacity of anthracite (SH3#) increases the fastest, followed by SGZ11#, and that of DFS4# is the smallest. In the low-pressure stage (P < 4 MPa), the adsorption capacity increases significantly with the increase in pressure, but in the high-pressure stage (P > 4 MPa), the adsorption capacity does not change significantly with pressure, instead gradually stabilizes. Under the same pressure, the molecular free path of methane increases with temperature. Under the premise of constant temperature, in the low-pressure stage (0 < P < 4 MPa), when the pressure continues to decrease, the free path of methane molecules increases significantly, resulting in a decrease in diffusion capacity. In the high-pressure stage (4 < P < 8 MPa), when the pressure continues to decrease, the free path of methane molecules does not change significantly; the sample desorption process of three sets of samples DFS4#, SGZ11#, and SH3# occurs, and the intermediate adsorption heat is greater than the isometric adsorption heat during the adsorption process, indicating that the desorption process needs to continuously absorb heat from outside the system. The energy difference produced in the process of adsorption and desorption causes the desorption hysteresis effect. The greater the difference in the isometric heat value of adsorption, the more significant the hysteresis is.

Synergistic Inhibitory Effect of Free Radical Scavenger/Inorganic Salt Compound Inhibitor on Spontaneous Combustion of Coal

ABSTRACT From the aspects of isolating oxygen, lowering temperature and terminating free radical chain reaction, an inorganic salt inhibitor and a radical scavenger were combined to form a compound inhibitor for preventing of coal spontaneous combustion. A long-flame coal with easy self-ignition was chosen to study the inhibitory performance of different inhibitors using inhibition performance tests and by characterizing reactive functional groups and performing thermal analysis. The experimental results show that the compound inhibitor has a coordinated inhibitory effect throughout the whole process of coal spontaneous combustion, with physical and chemical double inhibition effect. Compared with the raw coal, the amount of CO released from the coal sample treated by the compound inhibitor was reduced by 88.7% at 128°C, and the inhibition rate reached 82.5% at 100°C. The inhibition ratio is higher than 80% during the initial weight loss stage and the oxygen absorption and weight increase stage. The compound inhibitor plays an inhibitory role by reducing the contents of hydroxyl, methyl, methylene groups and active components containing carbonyl groups in the coal, wherein, the relative content of a carbonyl compounds decreased obviously. During the initial weight loss stage, the water retention due to inorganic salts physically isolated oxygen and cooled the coal. During the oxygen adsorption and mass gain stage, free radicals were released from the compound inhibitor and captured both oxygen free radicals and carbon free radicals via chemical inhibition. N,N-Dibenzylhydroxylamine (DBHA) contains multiple absorption sites that can simultaneously capture multiple free radicals in coal. Butylated hydroxyl toluene (BHT) reacts with free oxygen-containing functional groups to form stable compounds, which interrupts the formation paths for ketones and aldehydes. Moreover, the inhibition mechanism of the compound inhibitor on coal spontaneous combustion was also explored.

The graded warning method of coal spontaneous combustion in Tangjiahui Mine

The accurate prediction of the degree of coal spontaneous combustion is critical for the prevention and control of spontaneous combustion in coal mines. However, the qualitative three-stage classification method is difficult to meet the demand for accurate early warning of coal spontaneous combustion. Therefore, with long-flame coal in the Tangjiahui Mine as the study object, this paper conducted the temperature-programed coal spontaneous combustion experiment and the coal spontaneous combustion experiment. The parameters that were measured were O2 concentration and gas generation volume during the process of coal spontaneous combustion under different oxygen supply conditions. Furthermore, this study analyzed the characteristics of gas and gas ratio variation with coal temperature, screened out the characteristic gas indexes of coal spontaneous combustion in Tangjiahui Mine, and defined six characteristic temperatures and their threshold values. Accordingly, the coal spontaneous combustion process was divided into seven stages, including the latent stage, recombination stage, self-heating stage, activation stage, thermal decomposition stage, fission stage, and combustion stage. We provide the early warning criteria and the threshold values of characteristic gas parameters at each stage and define the names of graded warnings, such as gray, blue, yellow, orange, red, and black. With this warning method, we successfully forecasted the degree of coal spontaneous combustion in the gob of the 61,102 working face, guaranteeing the successful withdrawal of this working face.

Steel-Industry Applications of Kazakhstan’s Long-Flame Coal and Special Cokes

Uses of Kazakhstan coal and coke in the steel industry are proposed: long-flame coal in the converter; carbonizing agents based on special coke; and coke in the carburization of steel. Technology for preheating of scrap in converters using sorted Shubarkol long-flame coal (Kazakhstan) is developed and introduced. Coal of the 13–25 mm size class gives the best results. This technology is recommended for converter shops at steel plants. The quality of special coke produced at AO Shubarkol Komir produced from low-ash, low-sulfur, and low-nitrogen long-flame coal is investigated. On that basis, it is proposed as a carbonizing agent. When using different classes of coke, improvement in the production process and in steel quality is predicted.

Structures and fractal characteristics of pores in long-flame coal after cyclical supercritical CO2 treatment

Injecting CO2 in deep coal seams for enhanced coalbed methane recovery (ECBM) also benefits the environment through simultaneous CO2. Most studies have concentrated on the changes of pore structures that result from sustained supercritical CO2 (ScCO2) treatment. Conversely, the effects of cyclic ScCO2 treatment have been rarely investigated. In this study, a high-pressure reactor system was used to perform cyclic ScCO2 treatment of long-flame coal. Nuclear magnetic resonance techniques and low-pressure nitrogen gas adsorption were employed to determine the fractal dimensions of coal samples and quantitatively characterize their pore structures before and after cyclic treatment. The results demonstrated that the porosity and proportion of macropores increased significantly after treatment. Therefore, new pores were formed, and some small pores might have been converted into macropores. As the treatment duration increased, the daily average porosity rate exhibited an increasing trend that was presumably caused by the effects of coal matrix fatigue. Further, a decreasing tendency was captured in both the pore-surface fractal dimensions of adsorption pores and the pore-volume fractal dimensions of seepage pores, while increasing the cyclic treatment. This phenomenon after treatment indicated that pore roughness and complexity were decreased. A conceptual model was proposed to explain the mechanisms underlying the evolution of coal-pore structures during the treatments, and the effects of the cyclic injection on the CO2-ECBM field were also analyzed. Therefore, our findings have important guiding significance for selecting suitable CO2 injection methods for CO2-ECBM projects.

Wettability effect of ethoxylated nonyl phenol with different ethylene oxide chain length on Shendong long-flame coal surface

Low-rank coal is coal with a low degree of metamorphism, it has high moisture content and strong hydrophilicity. It is important for low rank coal to improve its hydrophobicity. In this paper, the wettability effects of ethoxylated nonylphenol (NPEO) with different ethylene oxide chain length on Shendong long-flame coal surface were explored. Contact angle experiments proved that long-flame coal adsorbing NPEO significantly enhances its surface hydrophobicity, and it is the most effective when the equilibrium concentration is close to its critical micelle concentration. Moreover, NPEO-8 is the best for improving the hydrophobicity of long-flame coal. Compared with long-flame coal without NPEO, the surface energy and adhesion work of coal are lower after adsorbing NPEO, in addition, NPEO-8 is the most effective of three NPEO with 8, 10 and 12 ethylene oxide. NPEO aggregate at the coal/water interface and the hydrophobic carbon chains intertwine to form a hemispherical aggregate structure, which reduce the interaction energy between water and coal. Therefore, the hydrophobicity of long-flame coal was improved after adsorbing NPEO, and NPEO-8 is the best, followed by NPEO-10, NPEO-12 is the last.

Ignition of coal‐water fuel droplets with addition of isopropyl alcohol

Coal-water fuels have been actively studied in the last decade due to the fact that when they are burned in the furnaces of steam and hot-water boilers, much less anthropogenic oxides of sulfur, nitrogen, and carbon are formed in comparison with coal. The main problem that hinders the widespread introduction of such fuels in the energy sector is long (up to 30 seconds) ignition delay times. Significant (10%-50%) reduction in the ignition delay time is possible when a small (up to 10%) amount of available and inexpensive liquid combustible organic compounds are added to the suspension. The purpose of the work is to substantiate (from the results of experimental studies) the possibility of significant reduction of the ignition delay times of coal-water fuels by introducing a third component into the suspension—isopropyl alcohol in relatively small (from 3% to 8%) concentrations. According to the results of experimental studies on the ignition process of multicomponent coal-water fuel droplets, based on long-flame coal, with addition of isopropyl alcohol in oxidizer (still air) heated to high temperatures (from 873 to 1273 K), with recording the processes of heating, ignition, and combustion by a high-speed video camera (speed of shooting up to 105 frames per second), the mechanism, characteristics, and conditions of ignition of the researched compositions of quite promising fuels were established. Experiments have shown that the delay time of stable ignition of the studied suspensions based on long-flame coal decreases by 12% to 47% with the increase (from 3% to 8%) of the mass concentration of isopropyl alcohol in the composition of coal-water fuel at the initial characteristic size of droplets from 1.5 to 3.5 mm. Each studied composition was compared with conventional coal-water fuel burned under similar conditions. The influence of mass concentration of isopropyl alcohol in the composition of the coal-water fuel on the characteristics of the initiation of ignition and combustion processes, established in the experiments, proves the possibility of effective an application of such three-component suspensions in thermal power engineering. The possibility of formation of a significantly larger quantity (about 80%) of small droplets (250-400 μm) in the flow during atomization of coal-water fuel with addition of isopropyl alcohol in comparison with the usual coal-water suspension was experimentally proved. The results are of significant practical importance because they illustrate the possibility of achieving optimum conditions for ignition and combustion of promising multi-component coal-water slurries with the addition of alcohol in the furnaces of boiler plants of large- and small-scale power engineering.

Electrothermal Production of Silicon Using Carbon-Containing New-Generation Reducing Agent – Rexil

To reduce the consumption of unprocurable and expensive charcoal or eliminate it from the silicon production process, alternative reducing agents and a silicon smelting technique for reduction furnaces operation are much needed. This article considers the possibility of using a new-generation solid carboncontaining reducing agent, rexil, obtained from non-caking Shubarkol long-flame coal, for the electrothermal production of metal silicon. Using rexil reduces the amount of charcoal in the charge and can also expand the raw material base, as well as preserve the country’s forested areas.

Effect of Water-immersion and Air-drying Period on Spontaneous Combustion Characteristics for Long-flame Coal

ABSTRACT During the mining of coal seam groups, the physical and chemical properties of the residual coal in goaf will be changed after undergoing water leaching and air-drying, affecting the oxidation characteristics of the coal. To examine the effect of the water immersion and air-drying times on the oxidative combustion characteristics of long-flame coal, this paper uses thermal gravimetric (TG) analysis and infrared spectral analysis to analyze coal samples with different immersion and air-drying times. The change in activation energy (AE), which is associated with the change in the functional groups, was analyzed for the coal-oxygen chain reaction process under coal immersion and air-drying conditions. The results show that coal samples soaked in air-dried for 10 days after water for 20 days have a lower AE (39.14 kJ/mol) than coal samples that underwent other treatment times and have a higher oxygen absorption rate and a higher tendency to undergo spontaneous combustion. According to the TG-DSC test, a range of weight loss and exothermic strengths are observed during the longest immersion and air-drying times. Additionally, during the longest immersion and air-drying times, the variations in the different functional groups are more obvious, the aliphatic hydrocarbon chain breaks and the coal rank is lower. The period of immersion and air-drying can change the spontaneous combustion oxidation characteristics of coal. After being soaked in water for 20 days and air-dried for 10 days, the coal sample was looser and the chain reaction process was accelerated, which promoted the oxidation and spontaneous combustion of coal.

Substantiation of laboratory facilities set to study physico-mechanical properties of coal-gas-moisture composites

Intensive development of coal deposits along with introduction of advanced high-performance machinery leads to higher number of fire- and explosion-hazardous events induced by gas dynamic phenomena at mines. Mass media regularly inform on tragic events concerning the combustible gas explosions and loss of lives at collieries in Russia and abroad. The main subject is development of a composite on the basis of solid coal and rock particles, gasses (methane, hydrogen) and moisture, prone to penetrate into mechanisms, initiating catastrophic dynamic processes in course of mining operations. The present research objective is foundation of a laboratory base to develop coal-gas-moisture composites in the form of coal cores, experimental investigation into their physico-mechanical properties. The special stand for development and investigation into coal-gas-moisture composites was designed. The relationships of variations in physico-mechanical properties of cores versus pressure were obtained experimentally on various materials (coal, limestone, graphite, etc) at the new-designed stand. The long-flame coal of different coarseness were used to establish specific variations in temperature, deformation of briquetted mass, its density under increase in pressure up to 200 MPa. It is demonstrated that temperature of a coal briquette increases by maximum 14 °C under atmosphere pressure and 10 °C in vacuum. The test stand allows physical modeling of coal and rocks with account for their material composition. The scientific novelty involves development of composites simulating mineral coal and rocks under prescribed physical and mechanical conditions.

Ignition of bio-water-coal fuel drops

The researchers from many countries are currently developing new eco-efficient fuel compositions. Such promising fuels include water-coal suspensions (WCF). But the ignition delay times of WCF particles are sometimes tens of seconds (up to 30–40). One of the possible methods to increase the reactivity of WCF is to add additives “accelerating” the ignition process, for example biomass, to its structure. This is already a complex composite bio-water-coal fuel (Bio-WCF). The physical theory and mathematical base of the processes of ignition and combustion of Bio-WCF drops have not been developed yet. The article presents the results of the theoretical and experimental studies of the ignition of substantially inhomogeneous drops of bio-water-coal fuel based on long-flame coal and woodworking waste. We have found that the addition of wood to the composition of water-coal composites leads to a significant reduction (up to 50%) of the entire induction period (ignition delay time — tign). According to the results of the experiments, the influence of the type of wood on the characteristics and conditions of ignition of the bio-WCF drops has been analyzed. It has been shown that pine-based bio-water-coal fuels ignite faster than larch and birch bio-WCF.We have developed a mathematical model that describes the combined course of the main processes of heat and mass transfer under conditions of the intensive phase (water evaporation) and thermochemical transformations (thermal decomposition of organic coal and wood). The corresponding non-trivial problem has been solved. Based on the results of the numerical simulation, we have established the theoretical values of tign. A comparative analysis of the theoretical and experimental values of tign has shown their good agreement. According to the results of the mathematical modeling, the temperature regimes of ignition of the bio-WCF particles have been established, which significantly differ from the ignition regimes of drops of water-coal fuel. It has been shown that the dynamics of the ignition of the bio-water-coal fuel particles is significantly affected by the type of wood fuel component. Based on the results of the mathematical modeling, the effect of the composition of volatiles on the characteristics and conditions of ignition has been analyzed. It has been shown that the content of hydrogen and methane in coal has a significant effect on the dynamics of ignition. As a result of the mathematical modeling, the limiting (in terms of ambient temperature) ignition modes of the fuel have been established.

Nonstationary Steam–Oxygen Gasification of Solid Fuel in a Fixed Bed Under Pressure

A two-dimensional model of nonstationary steam–oxygen gasification of solid fuel in a fixed bed has been constructed, which took into account the interphase convective heat transfer, radiative-conductive heat transfer of the solid phase, radiative and conductive heat transfer of the bed with the reactor wall, heterogeneous and homogeneous chemical reactions, gravity force, and the aerodynamic drag force. The influence of the process parameters on the thermochemical treatment of the Shubarkol’ bituminous long-flame coal under a pressure of 3 MPa at ratios of the mass fractions of the steam–oxygen mixture components H2O/O2 = 45/55, 40/60, and 72/28 was studied.

Investigation on Ash-Fusion Characteristics of Livestock Manure and Low-Rank Coals

Large amounts of livestock manure are produced annually, and cogasification offers a promising method for large-scale and clean use of manure. To investigate the ash-fusion characteristics (AFC) of livestock manure and coal, and their variation mechanisms, the AFCs of two coals (Yima long-flame coal and Husheng lignite), two manures (cattle manure (CM), and swine manure (SM)), and their mixtures were explored. An increase in the ash mass ratio of the CM or SM increased the sintering temperature (Ts) of two mixed coal ashes (the SM enhanced the Ts more than that of the CM); however, their ash-fusion temperature (AFT) decreased first and then increased (the turning points for the Yima long-flame coal ash mixture were CM: ∼60% and SM: ∼40%, and those for the Husheng lignite were SM: ∼70% and SM: ∼50%). The Ts was induced by partial particle surface fusion, and was related to the formation of a low melting eutectic (e.g., binary systems for K2SiO3–Na2Si2O5 and Na2S–FeS) and their contents. The AFT indicated that the bulk melting property was determined mainly by ash composition, especially for a basic/acid ratio. The disappearance of mullite and the formation of high melting-point phosphates (e.g., KCaPO4, K2CaP2O7, and Ca2.71Mg0.27(PO4)2) resulted in AFT fluctuations of the mixtures.

Study on the Effect of Extraneous Moisture on the Spontaneous Combustion of Coal and Its Mechanism of Action

It is imperative to have an in-depth understanding of the effect of extraneous moisture on the spontaneous combustion of coal not only for the control and prevention of coal spontaneous combustion in the coal mining industry, but also for the optimization design and application of the technological process. In this study, the type of moisture in a coal body has been redefined for the first time from the perspective of disaster prevention and control, i.e., original occurrence of moisture in the coal matrix and the extraneous moisture from the technological process. A suit of coal bodies with different extraneous moisture was prepared by soaking long-flame coal with a low water content. Using a temperature-programmed oxidation test, the effects of extraneous moisture on the temperature increase rate of coal bodies and the emission characteristics of gaseous products during coal spontaneous combustion were studied. Moreover, combined with the characterization of thermal analysis and of pore structure test, the action the mechanism of extraneous moisture on the coal spontaneous combustion process was also explored. The experimental results indicated that the effect of the extraneous moisture content varied with the development of coal spontaneous combustion. In the slow oxidation stage, extraneous moisture played a physical inhibition role in the coal oxidation. In the accelerated oxidation stage, extraneous moisture exhibited a catalytic effect on the coal–oxygen reaction or directly participated in the reaction. After entering the rapid oxidation stage, a delayed effect appeared. When the coal temperature exceeded 180 °C, the spontaneous combustion characteristics of coals with different initial moisture contents gradually tended to achieved balance.

Inhibiting effects of layered double hydroxides containing the rare-earth element lanthanum on coal spontaneous combustion

Five layered double hydroxides (LDHs) containing different ratios of the rare-earth element La3+ were prepared, and their suitability was investigated to prevent spontaneous combustion of long-flame coal (LFC). LDHs with different ratios of the rare-earth element La3+ were prepared and characterised through X-ray diffraction (XRD), scanning-electron microscopy (SEM), and EDX analysis. The five LDHs were mixed uniformly with LFC and called LFC-ZnMgAl-LDH, LFC-LaZnMgAl-LDH-1, LFC-LaZnMgAl-LDH-2, LFC-LaZnMgAl-LDH-3, and LFC-LaZnMgAl-LDH-4. Thermogravimetry-differential scanning calorimetry and in situ diffuse-reflectance infrared Fourier transform spectroscopy revealed that with an increase in the La3+ concentration, the characteristic temperatures of complexes increased roughly. Concurrently, active functional groups in complexes decreased gradually, and LDHs exhibited a stronger inhibition effect in the following order: ZnMgAl-LDH < LaZnMgAl-LDH-1 < LaZnMgAl-LDH-2 < LaZnMgAl-LDH-3 < LaZnMgAl-LDH-4. The inhibition effect of LaZnMgAl-LDH-4 on coal spontaneous combustion was found to be the most suitable. These findings provide a reference for the application of new CSC inhibitors.

The pyrolysis of vitrinite and inertinite by a combination of quantum chemistry calculation and thermogravimetry-mass spectrometry

In this study, pyrolysis behaviour of vitrinite and inertinite from Heidaigou long-flame coal was studied via quantum chemical calculation and TG-MS. Quantum chemical calculation showed that vitrinite contains bonds with long bond length and small bond order, whereas inertinite revealed the opposite. And vitrinite and inertinite presented the same order of breaking bonds, but the numbers of breaking bonds showed differences. Before CO2 release, bond breakages and initial breakdown of macromolecular structure were observed. TG-MS analysis showed that the emissions with the same number of carbon, whether from vitrinite pyrolysis or inertinite pyrolysis, showed the same variation trend. Emissions from both vitrinite and inertinite can be divided into following four types. The first includes CO2, which was detected at 300 °C. The second consists of C1, C2, C3 and C4 compounds, which were observed at 420 °C and showed a fast release rate. The third comprises C6 and C7 compounds and CO, which were also detected at 420 °C with a slow release rate, and the fourth includes H2, which was identified at 500 °C. The escapes of the same substances during pyrolysis of vitrinite and inertinite were compared. Almost all emissions from vitrinite were higher than those from inertinite, but the temperature of maximum CO2 release from vitrinite was lower than that of inertinite.

Wettability modification and its influence on methane adsorption/desorption: A case study in the Ordos Basin, China

AbstractWettability modification of coal rock may cause serious change in the methane adsorption/desorption behaviors. Thus, the contact angle measurement and isothermal adsorption/desorption experiment of raw coal and samples treated with surfactants (STS) were conducted to investigate the wetting properties and methane adsorption/desorption characteristics. The results indicated that the long‐flame coal (LFC) has the good hydrophilicity itself, and there was no evident sign of saturated adsorption for different samples at relatively low‐pressure (&lt;8 MPa) stage. The sample treated with the wettability reversal agent (G502) had the larger methane adsorption capacity than the wetted samples (LAS, JFC, and 6501), with the latter facilitating the methane desorption. Additionally, the adsorption capacity of the air‐dried samples was stronger than that of the water‐containing samples, including the moisture‐equilibrated, LAS, JFC, and 6501, reflecting that the better the hydrophilicity, the weaker the methane adsorption. Besides, the desorption hysteresis rate of the same sample was mainly controlled by pressure, i.e. the desorption hysteresis indicated a logarithmic decrease with the increasing pressure, but the STS clearly showed the desorption hysteresis to varying degree due to the wettability differences. The pressure drop in the low‐pressure stage was difficult to drive the adsorbed methane to immediately desorb from the matrix surface, whereas even caused further adsorption or re‐adsorption, meaning that the coal required a greater depressurization to enter the effective desorption stage. Finally, the positive implications of wettability modification including enhancing CBM recovery, as well as coal and methane outburst prevention and coal dust control in the mining industry were discussed.

Factors influencing dielectric properties of coal of different ranks

We measured the relative dielectric constant and dielectric loss of three coal samples under conditions of 0–80 °C and 500–1000 MHz using the ultra-wideband dielectric spectrum. We analyzed the influence of ultra-wideband frequency and test temperature on the dielectric properties of coals with different degrees of metamorphism and their variations. The results show that the relative dielectric constant of the coals initially decreases with increasing frequency to a minimum and then increases, followed by a subsequent decrease. The relative dielectric constant increases with increasing temperature. The dielectric loss increases with increasing test frequency, and differs significantly for the different coals. At the same frequency and temperature, the dielectric loss of lean coal is greater than that of long-flame coal, which is greater than lignite. The dielectric loss of the coals increases with temperature above 60 °C. The relative dielectric constant and dielectric loss of the samples are more apparent at 1000 MHz and 25 °C. These values can be used as the target frequency and temperature for ultra-wideband radar.