Coal Wettability Research Articles

Research of the coal surface wettability by filtering the liquid through a porous layer

Many technological processes occurred upon the extraction and primary processing of coal (dust suppression, grouting, hydraulic fracturing, wet enrichment, etc.) depend on the wettability of the coal surface, determined by the physicochemical properties of the interacting media. The filtration properties of a fractured-porous coal massif significantly depend on the wettability of the surface. We present the results of studying the wettability of the coal surface with water and its filtration through a layer of coal powder. It is shown that the increased humidity of coal contributes to an increase in the wetting and filtration properties of the coal layer in relation to water due to the creation of a hydrate shell at the contact surface. It is revealed that the method of coal sample preparation significantly affects the functional composition and hydrophilicity of the outer surface of coal particles. Grinding coal in the presence of oxygen in air contributes to the formation of polar oxygen groups (hydroxyl, carboxyl) on the surface of coal particles, which leads to an increase in the hydrophilicity and filtration properties of coal. The results obtained can be used to predict the wettability of coals with process fluids, to improve technologies for mining, enrichment and processing of coals.

Study on wetting mechanism of nonionic silicone surfactant on coal dust

Coal dust disasters are serious in coal mining. The use of nonionic surfactants can effectively improve the wettability of coal dust and reduce the content of suspended coal dust in the air. For the problem of low wettability of ordinary surfactants, this paper selects silicone surfactants with high surface activity and low surface tension to improve the wetting ability of coal dust. To explore the wettability of nonionic silicone surfactants on coal dust, the effects of six nonionic silicone surfactants on the wettability of coal dust surfaces were studied by experiments. The test objects were four kinds of coal samples with different metamorphic degrees. The surface tension, wetting time, and contact angle experiments were carried out, and the critical micelle concentration and the expansion coefficient of the coal surface were calculated. The wetting time of the compound solution was measured to verify the synergistic effect of the compound solution. The results show that: 6 # has the best wetting effect on coal dust, followed by 4 # and 2 #; The order of surface tension is: 1 # < 3 # < 4 # < 6 # < 5 # < 2 #, the surface tension of 1 # is the lowest (19.962 mN/m); 1 # and 4 # are easier to spread on the surface of coal dust, the spreading coefficient of coking coal is the largest and the contact angle is the smallest, which is 18.8°. The 4 # and 6 # with a mass ratio of 8:2 were compounded. The compound surfactant solution had a significant synergistic effect. Compared with the monomer surfactant solution, the wettability of long-flame coal and coking coal increased by 15.14% and 10.00%, respectively. The results of this study can provide reference and experimental support for the development of high-efficiency dust suppressants based on silicone surfactants.

Characteristics of the methanotroph used in coalbed methane emission reduction: Methane oxidation efficiency and coal wettability

In the process of coal mining, a large amount of methane is emitted into the atmosphere every year, which enhances the greenhouse effect of the atmosphere. Microbial methane oxidation technology provides a new idea for methane emission reduction and methane control in coal mines. To improve the environmental adaptability of microorganisms, mixed methanotrophic consortia were selected from the natural environment. During the growth process, microorganisms grew most vigorously in the 4th–10th days and peaked on the 7th day. Affected by microbial metabolism, the pH (potential of hydrogen) of the solution increased, and the ORP (oxidation–reduction potential) decreased, both of which were negatively correlated. The selected mixed methanotrophic consortia have extremely strong methane-oxidizing ability, the volume of methane gas decreased by 71.19 %. It is particularly important that the microorganisms still have a certain methane oxidation capacity, after which the oxygen in the device is exhausted. The results showed that the mixed methanotrophic consortia adjusted the population structure to adapt to the changes in the surrounding environment and continued to exert methane oxidation ability. The contact angle between the bacterial solution and coal sample surface was far lower than that of pure water and culture medium. Microorganisms adsorbed onto the surface of the coal sample through self-movement, greatly improving the contact ability between the bacterial solution and the coal sample. The retention of microorganisms in the coal sample was affected by the flow velocity and the particle size of the coal sample. A low flow velocity and large contact area can improve the contact efficiency of microorganisms on the coal surface. Mixed methanotrophic consortia have high application value and feasibility in coal mine methane emission reduction and underground methane treatment.

Experimental study on the influence of acid fracturing fluid on coal wettability

With the continuous improvement of mechanized mining intensity, dust has seriously threatened the safety of mine production and the health of underground workers. Coal seam water injection is one of the effective means to moisten the primary coal dust in the coal body. Through testing by the surface tension tester, contact angle meter, FTIR, low-temperature liquid nitrogen adsorption, and SEM, the impact of acid fracturing fluid which can be used for coal seam water injection on the wettability and pore structure of coal was studyed. The results showed that HCl and CTAB had good synergistic acidification effect, which can effectively reduce the surface tension of CTAB by 10 ∼ 30 mN/m. Also the contact angle between the solution and the coal sample is significantly affected by the acidity of the solution. However, the impact of the acid fracturing fluid varies for coal with differing levels of metamorphism and mineral content. For low rank bituminous coal, the contact angle decreased by 39.55° after contact with the sample surface for 1 s, and the solution was completely immersed in the coal body after 5 s. The absorbance of hydrophilic functional groups of coal becomes stronger after treatment with acid fracturing fluid, which is conducive to upgrading the wettability of coal. Meanwhile, the acid fracturing fluid increases coal’s connectivity and average pore diameter, which in turn influences the wettability of coal. Exploring the influence of acid-based fracturing fluid on coal can provide guidance for improving the wettability of coal, reducing dust concentration and optimizing the underground working environment.

Study on coupling chelating agent and surfactant to enhance coal wettability: Experimental and theoretical discussion

To further enhance the impact of water injection and dust reduction in the deep mining process of the coal mine, an efficient dust suppressant for water injection was synthesized by coupling a chelating agent and a surfactant. The optimal composition solution was determined by surface tension, pre-treatment settling time and contact angle. The experimental results display that the composite solution (0.20 wt%) of Tetrasodium iminodisuccinate (IDS) and Sodium dodecyl sulfate surfactant (SDS) has the best wetting influence on the coal. Under this condition, the surface tension, contact angle and settling time are 23.69 mN/m, 25.72° and 48 s, respectively. FTIR tests exhibit that the proportion of hydrophilic functional groups in the coal sample treated by IDS and SDS composite solution is the highest, reaching 94.23 %. The results of particle size distribution tests show that IDS and SDS composite solution effectively reduces the dust production capacity of the coal body. In addition, the synergistic mechanism of IDS and SDS molecules is discussed from two aspects: Enhancing the pore fluidity and improving the hydrophilicity of coal. In this study, the wetting effect of the dust suppressant is better than traditional surfactants, and the results have important reference significance for improving the water injection effect.

Experimental and molecular dynamics investigations of the effects of ionic surfactants on the wettability of low-rank coal

The wetting properties of ALES (Ammonium lauryl ether sulfate), SLES (sodium lauryl ether sulfate), TD (dodecyl triethanolamine salfate) and SDS (sodium dodecyl sulfate) solutions and their adsorption capacities on coal dust surface were evaluated by surface tension, contact angle, sink time, wetting rate, and changes in the micromorphology and functional groups on coal surface. The results showed that the four surfactants share similar adhesion and spreading wettability, while they differ notably in immersion wetting. A water-surfactant-low rank coal (LRC) system was established using Materials Studio software and the Wender coal model; then quantum chemical calculations and molecular dynamics (MD) simulations were conducted. The results showed that ALES is of the widest relative concentration distribution range (25.25–60 Å), the largest overlap range (25 Å) and the largest diffusion coefficient (D = 0.318); NH4+ can easily penetrate the surfactant layer, which proves the strong modification ability of ALES to LRC. ALES/LRC/H2O has the lowest interaction energy and the most H-bonds, indicating that ALES is of a strong adsorption capacity. Based on the experimental data and simulation results, the integrated wettabilities of the four surfactants follow ALES＞SLES＞TD＞SDS, partially contributed by the hydrolytic cations and EO groups.

Synthesis and properties of wettability-increasing agent with multi-layer composite network structure for coal seam water injection

To improve the efficiency of water injection and reduce the amount of dust from the source. In this paper, a wettability-increasing agent with a multilayer composite network structure was prepared. The modified sodium carboxymethyl cellulose (CMC) and linear micelles were rearranged and combined by using the optimal ratio experiment. Through proton nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, and X-ray diffraction analysis, the functional-group structure and crystallinity of the modified CMC were investigated. The elemental compositions and surfaces of C1 and C2 were examined via energy dispersive spectrometer, optical microscopy, and transmission electron microscopy. A performance test revealed that C2 had excellent temperature shear resistance, and its wettability was good, which was conducive to reduce the spreading of pulverized coal. By observing the coal in dry and wet states, the reason why C2 was suitable for dust reduction was determined. The changes in the functional groups of the soaked coal were analyzed, and the soaking effect of the C2 on the coal was determined. Finally, molecular dynamics simulation was used to verify that C2 played an effective role in promoting moisture wetting of coal.

Influence of Surface Oxidation on the Wettability and Filtration Properties of Coal

Influence of Surface Oxidation on the Wettability and Filtration Properties of Coal

Effect of oxygen-containing functional groups on the wettability of coal through DFT and MD simulation

To investigate the wettability of different oxygen-containing functional group (OFG) surfaces, graphite substrates were used as a model for coal adsorbents. The substrates were modified with COOH, OH, CO, and OCH3. The adsorption-diffusion behavior of H2O molecules/water droplets on different OFG surfaces was investigated using molecular dynamics (MD) simulations with frontier orbital energy difference as a metric for different surface wettability degrees in quantum chemical analysis. The results indicated that the frontier orbital energy difference of the H2O molecule was 3.480, 3.491, 3.631, and 3.680 eV for PhCOOH, PhOH, PhCO, and PhOCH3, respectively. In addition, the equilibrium contact angle, interaction energy, and number of hydrogen bonds after the adsorption equilibrium of water droplets for COOH, OH, CO, and OCH3 surfaces were 22.34°, −5.03 kcal/mol, and 652; –23.72°, −4.19 kcal/mol, and 450; 68.01°, −0.79 kcal/ mol, and 61; 90.51°, −0.50 kcal/mol, and 28, respectively. The smaller the energy difference between the frontier orbitals of the H2O molecule and the OFG, the smaller the equilibrium contact angle between the water droplet and the OFG surface, the more hydrogen bonds were formed, and the larger the absolute value of the interaction energy, the better the wettability of the surface of the OFG. The order of wettability of the different OFG surfaces was COOH > OH > CO > OCH3, which is consistent with the radial distribution function and the analysis results for the extended area, etc. The results of density functional theory (DFT) calculations and MD simulations exhibited identical patterns, indicating the reasonableness of the simulations. This study may serve as a reference for the suppression of hydrophilicity in low-order coal and the enhancement of the flotation effect.

Cracking and improved wettability of coal through liquid CO2 cyclic cold soaking for dust prevention

To solve the problem of water injection in low–porosity and low–permeability coal seams, a new idea of liquid CO2 cyclic cold soaking to improve the water absorption and wetting efficiency of coal is proposed. The evolution of the pore structure and mechanical properties of coal were studied using low–field nuclear magnetic resonance and uniaxial compression tests. The wettability and water absorption of coal before and after the experiment were determined using contact angle and water absorption tests. The results show that as the number of cycles increases, the effective porosity of coal increases significantly, the mechanical strength is weakened, and the compressive strength and elastic modulus decrease. The wettability and water absorption of coal samples were significantly improved by liquid CO2 treatment, and the changes in porosity, mechanical strength, and water absorption wettability of coal samples in the saturated group were more obvious compared to dry coal. This paper lays a theoretical and experimental foundation for the development of a new technology for coal seam water injection dust prevention.

Novel Discrete Fracture Network Model for Multiphase Flow in Coal

Multiphase flow intensely affects the movement and accumulation of other fluids in coal and plays an essential role in predicting the permeability of coal during coalbed methane (CBM) production. Fractures have played a decisive role in the transport of CBM after hydraulic fracturing has occurred. In this study, a multiscale pore network model (PNM) was constructed on the basis of focused ion beam scanning electron microscopy (FIB-SEM) image results. Additionally, a novel discrete fracture network model, fracture–pore network model (F–PNM), was proposed to investigate the effect of fracture density, fracture developing direction, and wettability on multiphase flow. The results reveal that the permeability of F–PNM increases with the increase of the fracture density, which could be the result of the predominance of snap off. The permeability decreases as the angle between the fracture and flow direction increases; initially, the permeability decreases steeply and then it tends to remain stable; and for angles between 0° and 15°, the permeability decreased by as much as 61.8%. Moreover, the wettability of coal has limited impact on its water relative permeability; however, it has a measurable effect on gas relative permeability, which could be owing to water accumulation on the coal surface under different wettability conditions. A good wetting performance would have a negative effect on the CBM production and reduce flowback efficiency.

Effects of surface roughness on wettability and surface energy of coal

Surface roughness has an important effect on the wettability and surface energy of coal. Although the predecessors had studied the effect of surface roughness on the wettability of coal, there were few researches on the effect of surface energy of coal, and the relationship between wettability and surface energy had not been thoroughly studied. This paper aimed to study the effect of surface roughness on the wettability and surface energy of coal, and revealed the relationship between the wettability and surface energy of coal. Based on the surface roughness of coal, this paper selected Hami lignite, Anyang coking coal and Zhaogu anthracite as the research objects, used the experimental methods to measure the surface roughness and experimental contact angle of coal under different conditions to study the effect of surface roughness on the wettability, and then applied the OWRK method to calculate the surface energy of coal to study the effect of surface roughness on the surface energy. The experimental results showed that the wettability of hydrophilic coal became better and the surface energy increased with the increase of surface roughness, while the results of hydrophobic coal were opposite. There was a positive correlation between the wettability and surface energy of coal, and the wettability of coal could be analyzed from the perspective of surface energy. Surfactant could change the wettability of coal, but unchanged the law of wettability with surface roughness.

Enhancement of the wettability of a coal seam during water injection: effect and mechanism of surfactant concentrations above the CMC.

This paper determines the optimal surfactant concentration for enhancing coal's wettability and explores the wetting mechanism at surfactant concentrations above the critical micelle concentration (CMC) during coal seam water injection. In this study, laboratory experiments and field tests were used to investigate the influence of monomeric surfactants and compound surfactants at various concentrations on coal's wettability. The results showed that when the surfactant solution concentration was greater than the CMC, the coal's wettability was significantly enhanced as the surfactant concentration increased. However, the coal's wettability did not monotonically increase with the concentration, and the maximum value was reached in the range of 0.5-3 wt.%. Increasing the surfactant adsorption density and changing the adsorption state on the coal surface were the essential reasons surfactants continued improving the coal's wettability at concentrations above the CMC. The Marangoni flow effect and changes in the viscosity of the surfactant solution with concentration were also important factors that affected the coal's wettability.

Effect of SDS modified coal microstructure on wettability and methane adsorption

Adding ionic solution can improve the microstructure of coal, inhibit the gas absorption capacity and effectively reduce the outburst risk. In this paper, the high-quality anthracite in Jincheng, Shanxi Province is selected as the research object, and sodium dodecyl sulfate (SDS) ion solutions with different concentrations are configured. Combined with infrared spectroscopy, mercury intrusion test, the change features of coal microstructure after soak by SDS solutions with different concentrations are studied, and the variations of the contact angle and adsorption constant of samples before and after modification are tested. The paper further discusses the influence of changing coal microstructure on the wettability and gas adsorption. The experimental results show that SDS can modify the microstructure of coal to some extent: –OH, -CO, and COC increase obviously, macropores and mesopores increase, micropores and ascospores decrease. After SDS modification, the contact angle and the maximum gas adsorption a decrease linearly as oxygen groups and vtotal increase. In addition, the hydrophilic groups in the coal body increase significantly, which makes it easier to attract water molecules, and strengthen the pore connectivity and coal wettability. The polar oxygen-containing functional group molecules of coal also increase, occupying the adsorption site of non-polar molecular methane, and the adsorption site (micropore) decreases. The conclusion of this paper establishes theoretical foundation for the exploitation of coal seam gas resources. Adding SDS can directionally ameliorate coal microstructure, and then regulate the coal wettability and its ability to adsorb methane, to utilize coal resources safely and efficiently.

Effect of active composite ionic liquids treatment on chemical structure and wettability of coal

To investigate the effect of different ionic liquid composite solutions on the wettability and chemical structure of coal dust, 14 ionic liquid compound solutions with various composite ratios were selected to act on long flame coal. Through the contact angle measuring instrument, 14 ionic liquid compound solutions were optimized, and 4 ionic liquid compound solutions with the same anion/cation were selected. The wetting agent with good water retention performance in the 4 kinds of composite solutions was selected by the water retention experiment. FTIR was used to research the change of functional group content in the coal treated by the 4 kinds of composite solutions. The results displayed that the contact angle of [Bmim][BF4]/[Emim][BF4] solution with the mixture ratio of 1:5 on the coal surface reached the min value of 34.483°. The [Bmim][BF4]/[Bmim][Cl] solution with a compounding proportion of 1:5 had the best water retention performance. The hydrophilic and hydrophobic functional groups of long flame coal treated by ionic liquid composite solutions had changed significantly. The [Bmim][BF4]/[Emim][BF4] composite solutions with the compounding proportion of 2:4 and 1:5 and the [Bmim][BF4]/[Bmim][Cl] composite solutions with the compounding proportion of 2:4 and 1:5 can not only increase the content of hydrophilic functional groups OH⋯O, Si–O–Si, COC, CO, and CO, but also decrease the content of hydrophobic functional groups CH2 and 2H. By analyzing the relationship between coal wettability and functional groups, it was discovered that the hydrophilic functional group CO was the primary functional group affecting the contact angle, and –OH and COC were the primary functional groups affecting the moisture content of coal samples. In this essay, the novel composite wetting agent type was prepared by compounding ionic liquid solutions with the same anion/cation in pairs, which has certain guiding significance for further using the novel composite moistening agent to improve the wettability of coal and preventing and control mine dust.

Study of the Micromechanism of the Effect of Fatty Alcohol Poly(oxyethylene) Ether-9 on the Wettability of Jincheng Anthracite

The influence mechanism of the adsorption of fatty alcohol poly(oxyethylene) ether (AEO9) on the wettability of anthracite coal was studied by means of experiments and simulations. First, the contact angle and surface tension were measured. When the AEO9 concentration was 0.5 wt %, the contact angle and surface tension were the smallest, which were 10.28° and 25.39 mN m-1, respectively. X-ray photoelectron spectroscopy (XPS) indicated that the content of C-O functional groups on the anthracite surface increased by 20.76% after adsorption of AEO9. The molecular orbital energy and electrostatic potential of AEO9 and anthracite were calculated by density functional theory (DFT). There are two modes of electron transfer between the two orbitals: highest occupied molecular orbital (HOMO) transfer of AEO9 to lowest unoccupied molecular orbital (LUMO) transfer of anthracite and HOMO transfer of anthracite to LUMO transfer of AEO9. The dynamics simulation results show that the addition of AEO9 increases the migration rate of water molecules, promotes the movement of a large number of water molecules toward the surface of anthracite, and enhances the thickness of the water molecular layer on the surface of anthracite. The analysis of the relative concentration shows that AEO9 is distributed at the anthracite/water interface. AEO9 molecules are intertwined and connected by hydrophobic chains to form a network structure, which covers the anthracite surface horizontally, thus promoting the strength of the anthracite/water interaction.

Molecular mechanism study on the effect of nonionic surfactants with different degrees of ethoxylation on the wettability of anthracite

A serious risk to the production safety of coal mines is coal dust. The wettability of coal may be successfully changed by adding surfactants to water. However, the creation of very effective dust suppressants is constrained by the lack of knowledge about the microscopic interaction mechanism between coal dust and surfactants. In this investigation, we explained macroscopic experimental phenomena from a molecular perspective. The lauryl polyoxyethylene ethers (C12 (EO)n, n = 7,15,23) were selected. The macromolecular model of anthracite with 55 different components was constructed. Surface tension experiments and hydrophilic lipophilic balance (HLB) calculations showed that the ability of surface hydrophilicization followed the order of C12 (EO)7<C12 (EO)15<C12 (EO)23. Contact angle experiment, XPS and FTIR experiments proved that after the surfactants were adsorbed on the surface of anthracite, the content of carbon element decreased and the content of oxygen element increased, indicating the enhanced surface hydrophilicity. The simulation results showed that with the degree of ethoxylation increases, the adsorption strength of surfactants becomes stronger, and the hydrophilic head group of surfactant on anthracite surface is more uniformly distributed. The greater the degree of ethoxylation, the more powerfully the modified coal surface can bind to water molecules.

Study on the effect of SDBS and SDS on deep coal seam water injection

Coal seam water injection, as an important disaster prevention means in the process of coal mining, can effectively suppress coal dust, add water injection additives, can effectively improve the wettability of coal body, improve the permeability of coal body, so as to achieve the prevention of rock burst. To improve the wettability of coal in coal seam water injection, the surfactant is often added to water, but sodium dodecyl benzene sulfonate (SDBS) and sodium dodecyl sulfate (SDS) have limitations in improving wettability of deep coal seam by injecting water. Therefore, it is very important to determine the influencing factors of SDBS and SDS to improve the wettability of coal. In this paper, the effects of oxygen-containing functional groups and minerals in coal on the wettability of coal are revealed, and the wettability mechanism of SDBS and SDS is expounded from the microscopic point of view. SEM was used to characterize the interaction between coal surface and surfactant, and the contact angle experiment was used to verify the influence of minerals in coal on wettability. Inductively coupled plasma atomic emission spectroscopy (ICP) and dynamic light scattering (DLS) tests were used to characterize the interaction of SDBS, SDS with minerals and the size of precipitation generated by the interaction of SDBS, SDS and mineral ions. The results showed that SDBS and SDS interact with Ca2+ to produce precipitation and block the flow of water in coal, which is not conducive to improving the wettability of deep coal seam to a certain extent. The significant chelating effect of chelating agent and Ca2+ provides a feasible solution to this problem.

Effect of glycerol microemulsion on coal seam wetting and moisturizing performance

In coal mining, coal seam water injection is effective for rock burst prevention and dust suppression, and the microemulsion as a water injection agent can greatly improve the wettability of coal seams. In order to further study the effect of glycerol in microemulsion as a phase substance on the wettability of coal body after water injection in coal seam, and its role in promoting coal seam moisturizing performance, on the basis of changing the phase material, the influence of different microemulsion components on the wettability of coal is explored through the contact angle experiment and Fourier infrared spectroscopy experiment, and the effect of glycerol on the moisturizing performance of coal is analyzed through the wettability control experiment in this paper. The experimental results show that the microemulsion as a wetting agent exhibits good wettability, and the composition of the microemulsion phase material affects the formation and dissociation of the functional groups on the coal surface by the microemulsion to a certain extent. The dissociation of the functional groups on the coal surface by the surfactant components in the microemulsion dominates the wetting mechanism of coal, and the effect of glycerol molecules on the functional groups shows a certain degree of wetting promotion. In addition, glycerol molecules easily react with other molecules such as water and sulfur dioxide molecules. After being injected into the coal seam, the glycerol microemulsion has a good moisturizing effect and a certain effect of coalbed methane purification. Therefore, it has practical significance for reducing coal dust concentration on coal mining face and increasing coal seam permeability.

Study on the wetting mechanism of SDBS with coals of different ranks based on surface free energy theory

Surfactants to improve the wetting ability of water sprays are considered an effective method of suppressing coal dust. Sodium dodecyl benzene sulfonate (SDBS) was used to understand the wetting mechanisms of an anionic surfactant on coals of different ranks. Atomic force microscopy (AFM) experiments and molecular dynamics simulations were used to investigate the surface free energy and hydration film. The analysis showed that the hydration film thicknesses obtained by both the experiment and simulation increase and then decrease with increasing SDBS concentration, and the high-rank coal had a thicker hydration film than the low-rank coal. The electrostatic potential distribution, hydrogen bonding, Mulliken charge, and interface formation energy were analyzed to illustrate the wetting mechanism. It was confirmed that coal's wettability is mostly determined by the electrophilicity of hydrogen atoms in oxygen-containing groups, and for high-rank coal, van der Waals forces gradually become the dominant forces in the wetting of the coal by SDBS. In conclusion, surface free energy theory by MD simulation proved to be a reasonable way to reveal the wetting mechanism of surfactants on coal, and the enhancement effect of SDBS on the wettability of high-rank coal is stronger than that of low-rank coal, which contributes to the development of surfactants for practical engineering applications.