Wettability Of Coal Surface Research Articles

Interfacial effects of SHMP/polycarboxylate synergistic reduction on coal surface wettability: Atomic force microscopy and density functional theory

The objective of this study was to elucidate the cooperative modification mechanism of anionic composite pharmaceuticals on coal substrates. Composite pharmaceuticals were synthesized using sodium hexametaphosphate (SHMP) and sodium polycarboxylate to reduce coal hygroscopicity. The mesoscopic alteration process between the coal surfaces and the compound pharmaceuticals was examined via atomic force microscopy (AFM). AFM showed that SHMP was multilayer-adsorbed on coal via electrostatic and hydrogen bonding, while sodium polycarboxylate was single layer-adsorbed at the SHMP layer via electrostatic interactions. Composite pharmaceutical adhesion to coal was lower than that for individual pharmaceuticals, indicating synergistic effects. The mechanism underpinning the microscopic synergistic effect of complex pharmaceuticals on the coal surface was further examined using density functional theory (DFT) at the molecular scale. DFT calculations showed that the adsorption energy of SHMP binding to a sodium polycarboxylate molecule was greater than that of SHMP binding to two identical pharmaceutical molecules. The SHMP portion generally interacted with the aromatic coal nucleus via its ring structure, exposing the sodium polycarboxylate carbon chain to maximize the adsorption energy. Combining DFT and AFM provided insight into the composite pharmaceutical function, and a theoretical framework for coal flotation separation was established.

Molecular simulation of the effect of anionic surfactant on methane diffusion in coal

Coal seam water injection is one of the important means of coal mine disaster prevention and control, which helps to reduce gas concentration. Anionic surfactants can change the hydrophilicity of coal to enhance the wetting effect of coal seam water injection and inhibit the diffusion of methane. However, the research on the effect of surfactants on the diffusion process of methane is still relatively weak. In order to explore the influence mechanism of anionic surfactant on coal surface wettability and methane diffusion from a microscopic point of view. Using wiser coal model and two commonly used anionic surfactants, sodium dodecyl benzene sulfonate (SDBS) and sodium dodecyl sulfate (SDS), the surfactant/water interface configuration, water/coal contact angle configuration and coal seam methane diffusion configuration were constructed. The interfacial stability of surfactant and water was discussed, and the wetting modification effect of surfactant on coal surface and the law of gas diffusion were analyzed. Finally, by studying the microscopic properties of each molecule in the simulation system, the adsorption mode of surfactants, the hydrophilicity of polar groups, the number of hydrogen bonds and the change of the pore volume in the coal seam system were analyzed, and the wetting modification mechanism of surfactants on coal surface and the inhibition effect of methane diffusion were discussed. The results show that with the increase of surfactant concentration, the interface formation energy is constantly enhanced, and the absolute value of electrostatic energy gradually increases, making the effect between surfactant and water stronger. The contact angle of coal surface under the action of SDS surfactant is the smallest, which is 36.230°, indicating that SDS surfactant has the best effect on the wettability of coal surface. The amount of methane diffusion in the surfactant action system decreased significantly compared with the pure water system, and the distribution of methane in the coal seam increased, indicating that the interaction between methane and coal is enhanced under the action of surfactant. With the increase of surfactant concentration, surfactant molecules are easier to spread on the surface of coal seam, and the wettability of coal surface increases. The research results can provide some research ideas and reference significance for anionic surfactants in the prevention and control of coal mine gas disasters.

Variations in the pore structure and fluid mobility under anionic surfactant assisted matrix acidification of coal based on nuclear magnetic resonance T1-T2 spectra

Matrix acidification is a potential method for increasing the permeability of coalbed methane reservoirs. However, current research on the acid corrosion process is still unclear, and acid solutions can cause water blocking damage to reservoirs. Therefore, this article obtained a T1-T2 joint spectrum through nuclear magnetic resonance (NMR) test and conduct contact angle (CA) test to analyze the pore structure and fluid mobility evolution of bituminous and anthracite coal under sole acidification and Sodium secondary alkyl sulfate (SAS) surfactant assisted acidification. The results show that the acidification process exhibits two distinct stages. Stage 1: acidification mainly expand the pore space of the free fluid, while stage 2: acidification significantly increases both the bound fluid and free fluid space. After sole acidising, the porosity of bituminous and anthracite coal increased by 4.44% and 31.31%, while increased by 18.28% and 38.35% under SAS assisted acidification, respectively. The adding of SAS resulted in better connectivity of coal samples, with pore throat (>0.1 μm) of bituminous and anthracite increasing by 56.02% and 351%, respectively. Under SAS assisted acidification, the maximum decrease of T2cutoff value of bituminous and anthracite coal is 62.79% and 20.28% respectively. Surfactants can effectively alleviate the negative impact of acidification on coal surface wettability. The results can provide references for field application of matrix acidification in coalbed methane reservoir.

Study on low-rank coal surface wettability effect and recyclability of different anionic magnetic ionic liquids

Three types of magnetic ionic liquids (MIL) ([C12mim]FeCl4, [C12mim]2CoCl4, and [C12mim]2NiCl4) were chosen to study the enhancement of the quality and utilization of low-rank coal (LRC). The mechanism of coal wettability modification was explained from the macrocosm and microcosm points of view by contact angle, adhesion work, and adsorption studies, and the recovery of MIL was realized by salting out the separation-strong magnetic recovery (SMR). The contact angle and adhesion work results showed that [C12mim]FeCl4 had the greatest effect on coal wettability. The adsorption on the LRC surface was as follows: [C12mim]FeCl4 > [C12mim]2CoCl4 > [C12mim]2NiCl4. The adsorption thermodynamics indicated that MIL conformed to the Langmuir isotherm model, which is a spontaneous reaction of exothermic entropy minus physical adsorption. The adsorption kinetic process belong to the pseudo-second-order kinetic model and was influenced by the combined action of intraparticle and liquid film diffusion. From the contact angles and adsorption thermodynamics, it can be inferred that [C12mim]FeCl4 has the highest activity and can better adsorb onto the LRC surface, covering the oxygen-containing functional groups and reducing the wettability to achieve efficient quality improvement and utilization of LRC. Through the research of salting-out separation-SMR, the most suitable salting-out reagent was found to be NaCl, and the highest recovery was obtained when the concentration reached the solubility level. As the magnetic field strength increases, the recovery rate gradually increases. The recovery of [C12mim]FeCl4 was more than 90% when it reaches 1.5 T, and it could be recovered by magnetic separation.

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

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

Influence of Air Contact on the Surface Wettability of Coal and Its Filtration and Flotation Properties

Influence of Air Contact on the Surface Wettability of Coal and Its Filtration and Flotation Properties

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 the surface wetting mechanism of bituminous coal based on the microscopic molecular structure.

The chemical wet dust removal method is one of the hot methods for coal dust control, and the key to its success lies in whether the surface of coal dust can be well wetted or not. Nowadays, the wetting mechanism of the coal dust surface is understudied, limiting the further application of chemical wet dust removal. Thus, the exploration of the wetting mechanism based on the microscopic molecular structure characteristics of the coal surface provides a new solution to improve the wet dust removal efficiency. Herein, the bituminous coal collected from 3 groups of coal seams in the Pingdingshan mining area was used as the object of study to reveal the microscopic wetting mechanism. Proximate analysis, nuclear magnetic resonance carbon spectroscopy (13C NMR) and X-ray photoelectron spectroscopy (XPS) can well distinguish the microstructural information of the coal surface, enabling building the molecular structure models of three groups of coal. Joint contact angle experiments were conducted to explore the influencing factors between the molecular structure of coal dust and its wettability. Molecular simulation techniques, combined with indoor experiments, were used to explore the essential causes of the differences in the wetting mechanisms of bituminous coal dust. The results showed that the composition and structure of carbon and oxygen elements on the coal surface has a significant effect on the wettability of coal dust. The higher the relative content of aromatic carbon and oxygen elements, the better the wettability of the coal surface. An opposite trend occurred for the aliphatic carbon. The difference in wettability of coal surfaces mainly stems from the ability of hydrophilic functional groups on coal surfaces to form hydrogen bonds with water molecules. The aromatic hydrocarbon structure has a much greater ability to adsorb water molecules than the aliphatic hydrocarbon structure. The research results can provide scientific guidance for the design of efficient and environmentally friendly dust suppressants to realize clean coal production in mines.

Interfacial porosity model and modification mechanism of broken coal grouting: A theoretical and experimental study

Grouting is the most effective approach for underground broken coal treatment. Traditional cement (TC) has difficulty achieving an ideal grouting anti–seepage and reinforcement effect due to the hydrophobic property of coal. This work developed an interfacial porosity model given Gibbs free energy changes during the droplet spreading on coal surface. And the key factors that affect the interface bonding behaviors were analyzed by means of model calculation. Accordingly, four types of wetting agents were selected to modify the coal hydrophobic property via Fourier Transform Infrared Reflection, pulverized coal settling, and contact angle tests. Results showed that the anionic wetting agent WA–2 exhibited the greatest improvement on the coal wettability, and its addition resulted in a fine promotion effect at 0.5%. Furthermore, grouting simulation experiments on broken coal were conducted to compare the actual grouting effects of the modified and traditional cement. Nuclear Magnetic Resonance results reveal that the porosity of the grouted coal specimens by modified cement (MC) was less than that of TC. Moreover, the quantities of mesopore and macropore were remarkably decreased by 85.76% and 96.69%, respectively. Scanning Electron Microscopy results also presented a superior bonding performance between the MC and the injected coal, which indicates that the surface wettability of coal can be improved by the MC, thereby weakening the coal hydrophobic effect and promoting ion diffusion during the cement hydration process. In effect, resulting in the enhancement of the occlusion force and bonding strength of the grout–coal interface. The finding demonstrates that the wettability modification of cement grout contributes to improving the microstructure of the grout–coal interface, substantially ameliorating the grouting quality and prolonging the stability of broken coal on the macro–level.

Experimental study on preparation of nanoparticle-surfactant nanofluids and their effects on coal surface wettability

To improve the efficiency of coal seam water injection, the influence of nanofluids on coal surface wettability was studied based on the nano drag reduction and injection enhancement technology in the field of tertiary oil recovery. The composition optimization and performance evaluation of nanofluids with nano-silica and sodium lauryl sulfate as the main components were carried out, and the effects of the nanofluid with the optimal ratio on coal wettability were studied through spontaneous upward imbibition experiments. The results show that the composite nanofluid has a lower surface tension, and the lowest value of the interfacial tension is 15.79 mN/m. Therefore, the composite nanofluid can enhance the wettability of coal. However, its effects on coal samples with different metamorphic degrees is different, that is, low rank coal is the largest, middle rank coal is the second, and high rank coal is the least. In addition, a functional relationship between time and imbibition height is found for pulverized coal with different particle sizes. When the particle size of pulverized coal is 60–80 mesh, the wettability of nanofluid to coal is best. The findings in this paper provide a new perspective for improving the water injection efficiency for coal seams with low permeability.

Effect of anionic/nonionic surfactants on the wettability of coal surface

To study the effect of different surfactants on the wettability of coal surface, the adsorption simulation of different coal-surfactant-water model systems was carried out by using materials Studio 8.0 molecular simulation software. Combined with the contact angle experiments between different surfactants and coal samples, the effect law of different surfactants on the wettability of three coal samples was obtained. The adsorption simulation results were analyzed from the aspects of adsorption spacing, adsorption sites and “reverse adsorption” phenomenon, and the mechanism of the influence of different surfactants on the wettability of coal surface was revealed from a microscopic point of view. The research results show that: the higher the degree of coal metamorphism, the lower the water absorption and the worse the wettability of coal. Anionic surfactant SDBS has better effect on improving the wettability of Hami (HM) coal with low metamorphic degree, while nonionic surfactant AEO9 has better effect on improving the wettability of Yuwu (YW) and Tunliu (TL) coal with high metamorphic degree. By comparing intermolecular distances, it is shown that nonionic surfactants are more likely to aggregate on the surface of coal molecules and have a greater effect on the wettability of the coal surface. The middle position of the benzene ring structure of coal is the best adsorption site for the hydrophobic tail of the surfactant. By calculating the adsorption energy of different simulation systems, it is confirmed that the phenomenon of “reverse adsorption” between nonionic surfactants and low metamorphic coal is caused by hydrogen bonding. The results of the study provide a theoretical basis for the effective application of different surfactants in coal dust control.

Electrochemical Modification on CH4 and H2O Wettability of Qinshui Anthracite Coal: A Combined Experimental and Molecular Dynamics Simulation Study

The wettability of gas and liquid on the coal surface is one of the fundamental factors that affect the depressurization process during the coalbed methane (CBM) extraction. The wettability of coal surface changed after electrochemical modification, leading to the change in methane adsorption/desorption and water movement in coal reservoirs. Thus, the CH4 adsorption amount, desorption ratio, and coal–water contact angle of raw and modified anthracite samples were measured and simulated. The mechanism of electrochemical modification was analyzed by functional groups, surface free energy, pore characteristics, interaction energies, and coal swelling. The experimental results showed that the saturated adsorption amount of methane decreased from 41.49 to 34.72 mL/g, and the simulation results showed that the saturated adsorption amount of methane decreased from 2.01 to 1.83 mmol/g. The coal–water contact angle also decreased from 81.9 to 68.6°. Electrochemical modification mainly affects the wettability of CH4 and H2O by changing the functional groups and pore structures of anthracite, and the influence on functional groups of coal surface is greater. This work provided a basis for enhancing CBM extraction by electrochemical modification.

Dependence of Coal Surface Wettability on the Petrographic Composition

Based on a comparison of elemental analysis data and IR- and 13C NMR-spectroscopic data, principal differences in the chemical compositions of lithotypes isolated from Kuznetsk Basin coals of different ranks based on characteristic properties were established. The influence of the chemical composition and physical properties (porosity and surface topography) of the lithotypes on the wettability of their surfaces with water was found. It was shown that coals with high concentrations of fusainized components have increased surface hydration due to developed porosity, pronounced surface texture, coal matter mineralization, and the presence of aromatic fragments with substituted oxygen heteroatoms, which have greater hydrophilicity than that of hydrocarbon structures, in the organic matter of coal (OMC).

New insight into surface wetting of coal with varying coalification degree: An experimental and molecular dynamics simulation study

The surface wettability is an essential factor in coal flotation and varies with the degree of coalification. Basic understanding of the wetting mechanism of different coal types is a prerequisite for proposing a new enhanced coal flotation method. In this study, the surface wettability of coal with varying coalification degrees, namely low-rank coal (LRC), bituminous coal (BC), and anthracite coal (AC), was investigated using contact angle measurements, X-ray photoelectron spectroscopy (XPS), and molecular dynamics (MD) simulations. The contact angle tests demonstrated that the water contact angle decreased in the following order: BC, AC, and LRC. This was due to the increased oxygen-containing function groups, as confirmed by the XPS analysis; the variation trend of the contact angle was further verified via MD. In the simulations, the variation of the contact angle of the simulated calculation agreed with the experimental results, suggesting that the set of parameters chosen for the simulation were appropriate. The spread of water molecules on the coal surface was highly related to the hydrogen bonds formed between water and coal, as directly confirmed by the statistical analysis of the number of hydrogen bonds. LRC and BC had the highest and lowest number of observed hydrogen bonds, respectively.

Effect of vacancy defects on electronic properties and wettability of coal surface

The water wettability of coal is a major factor affecting its flotation performance, and thus it is imperative to explore the wetting mechanism. Previous studies on wettability mainly focused on the contact angle and interfacial tension, and there are only a few reports on the electronic properties that affect the adsorption behavior of H2O. In this study, the electronic properties of perfect and defective surfaces of coal, the adsorption behavior of H2O on the surface, and wettability were tested using molecular simulations. X–ray diffraction (XRD) analysis of anthracite was performed, and two–layered graphite was used as an anthracite model. The results show that the aromatic structure of anthracite is very similar to the crystal structure of graphite; the difference lies only in the crystallite size: anthracite crystallites are smaller than graphite crystallites. Therefore, the modified graphite structure can be used to simulate the local structure of coal. The bandgap of defective surfaces is lower, their electronic density of states near Fermi energy is greater, and their electronic activities are better than those of perfect surfaces. Defects decrease the energy gap of frontier orbitals (ΔELUMO–HOMO) between H2O and the surface, and strengthen the interaction between H2O and the surface·H2O adsorbs physically on the coal surface with two hydrogen atoms facing the surface. Defects increase the wettability of the coal surface, and the order of wettability improvement by defects is: double vacancy defect > single vacancy defect > Stone–Wales defect.

Experimental study of influence of natural surfactant soybean phospholipid on wettability of high-rank coal

Changing wettability of coal rock surface for enhanced gas recovery, surfactant is widely used in coalbed methane exploitation. Recently, soybean phospholipid as non-toxic and degradable natural surfactant has gained increasing importance. However, effect of soybean phospholipid on wettability of high-rank coal is still unclear. In this work, typical high-rank coals were selected from the Qinshui Basin to analyze this effect mechanism. Experiments including surface tension, surface contact angle, coal fines imbibition, dispersion effect as well as gas-based permeability were carried out. Moreover, water locking mechanism of fracturing fluid in microfractures of coal reservoir was analyzed in detail. Results show that surface of high-rank coal is negatively charged in solution and have weak hydrophilicity. Soybean phospholipid can significantly reduce the surface tension of gas-liquid, and increase wettability of coal surface. Thus, soybean phospholipid contributes coal fines to disperse observed from filter cake using scanning electron microscope. Additionally, many conventional surfactants e.g. SDBS, CTMAB, OP-10 as well as soybean phospholipid were experimentally compared in performance in reducing capillary pressure. Results indicate that single soybean phospholipid do not show perfect performance. Specially, the imbibition rate of single soybean phospholipid solution along the pipe filled with coal fines was lower than that of conventional anionic and cationic surfactants, but still higher than that of stilled water and non-ionic surfactant. To further improve the capability of soybean phospholipid solution, composite surfactant was prepared with soybean phospholipid and OP-10. We found that composite surfactant at the mass ratio of 1:3 has a lowest capillary pressure with low surface tension and large contact angle. Finally, effect of three typical surfactants on permeability of coal was investigated. Results show that this suggested compound surfactant may be conducive to discharge of fracturing fluid from coal reservoir microfractures as well as improve gas-phase relative permeability in coal engineering practice.

Role of surface roughness in the attachment time between air bubble and flat ultra-low-ash coal surface

Surface roughness plays an important role in the floatability and wettability of mineral and coal particles. However, the role of surface roughness in the floatability of coal particles has not been sufficiently investigated because coal consists of organic and inorganic materials. This paper is to obtain some insights into the role of surface roughness in the coal surface wettability. In this investigation, Taixi ultra-low-ash coal particles, with ash content of 1.55%, were polished and the polished flat coal surfaces with various roughness were obtained. The attachment time between air bubble and flat coal surface were tested at the water solution without flotation reagents. Throughout this paper, it was found that the attachment time increased with the increase of roughness of coal surface. The attachment area between bubble and flat coal surface was decreased with the increase of surface roughness. The attachment between bubble and flat rough coal surface was unstable, compared to the attachment between bubble and smooth coal surface. The rougher coal surface makes the detachment between bubble and coal surface easier than the smoother coal surface. The higher the roughness of coal surface is, the more pores and grooves the coal surface has. The pores, gaps and grooves are filled up with the water in the slurry and have negative effects on the attachment between bubble and flat coal surface.

Dependence of the water wettability of the surfaces of fossil coals on their structure and properties

The dependence of the water wettability of coal surface on the structure and properties of coals of different stages of diagenesis and metamorphism was studied based on the evaluation of the angles of contact. It was found that the caking coals of Zh and K ranks exhibited maximum hydrophobic properties. Hydrophilic properties were most pronounced in brown coals and low-rank black coals with the highest concentrations of hydroxyl and carboxyl groups and a developed porous structure.

About determining the wettability of coal surface

A new method of measuring the wettability of coal powders based on determining the mass fraction of wetted powder particles at their interaction with water droplets during the process of gravitational sedimentation in a transparent cell has been proposed. Wettability parameter is calculated from the results of measuring the dependence of spectral transmittance of laser emission in the cell on time of powder sedimentation.

Micro-scale simulation of bubble-water flow in coal seams by the lattice Boltzmann method

The production of coal seam gas initially requires pumping and removing significant amounts of water to sufficiently reduce the hydrostatic pressure in the subsurface, so that methane can desorb from the matrix and diffuse into the cleat systems; majority of the methane molecules gather into nucleation or bubbles. During the depression, the flow pattern of gas in cleats changes from bubble flow to slug flow, and finally forms circular flow. The significance of the bubble flow process—during which the liquid phase is continuous while the gas phase exists as small bubbles randomly distributed within the liquid—has not been emphasised because of its complexity. In this study, a free energy based two-phase lattice Boltzmann model is used to simulate the gas bubble/water flow behaviour in micro-cleats of a coal seam gas reservoir. The model was validated by comparison with analytical results based on dimensionless numbers, and good agreement was found in general. The influences of bubble shape, bubble size, and coal surface wettability on gas water two-phase flow in micro-cleats are discussed. The simulation results indicate that the bubble size and wettability of gas have significant impacts on the flow capacity of both gas and water. A decrease of the water flow rate is observed when large bubbles occur, and the gas flow rate decreases when the gas wettability becomes stronger. The bubble flow process significantly influences the drainage of water and the further gas production.