Experimental Study On The Modification Of Anthracite And Its Effect On Adsorption And Wettability

The characteristics of strong adsorption,low porosity and difficult wetting of coal seams have brought great difficulties to coalbed methane/coal mine gas drainage and dust control in China.From the perspectives of safety,energy and environment,it is of great practical significance to strengthen the research on coalbed methane extraction.The adsorption of gases on coal is mainly affected by coal stress,pore-fracture structure and coal surface properties.However,previous work has mainly focused on the pressure relief.There are few reports on the change of pore-fracture structure and coal surface properties,especially for high metamorphic anthracite.Therefore,the experimental study on physical?temperature?,chemical and electrochemical modification of anthracite samples from Sihe Coal Mine of Jincheng Anthracite Coal Group was carried out.Through the comparative analysis of modification methods and coal characteristics before and after modification,the main results are as follows:?1?A method based on mechanical tests to determine the surface energy of heterogeneous coal and rock materials under variable temperature conditions was proposed.The surface energy of semi-bright coal,semi-dark coal and carbonaceous mudstone in anthracite under different temperature conditions was tested and analyzed.The results show that the surface energy of the coal sample decreases linearly with increasing temperature,and the coefficient of variability of the test results increases exponentially with increasing temperature;The surface energy is affected by the macroscopic coal type,the coal rock composition and the surface roughness of the sample.The surface energy of the carbonaceous mudstone sample is the largest,followed by the semi-dark coal and semi-bright coal,and the surface energy decreases with a negative exponential law as the surface roughness of the sample increases;the mechanism of temperature affecting surface energy is the result of thermal motion and thermal expansion of coal-rock molecules.The difference in temperature response of different coal-rock components leads to the exponential increase of the surface energy dispersion coefficient with the increase of temperature;the method proposed in this paper to determine the surface energy of solids based on mechanical tests makes up for the static contact angle and surface energy of the test points obtained by optical methods.It is difficult to accurately evaluate the overall contact angle and surface energy of the bulk samples of heterogeneous solid materials.It makes the test results have large deviations,and it is difficult to adjust and maintain the temperature defects during the test,which provides a simple and accurate method for the determination and evaluation of the surface energy of heterogeneous porous solid materials.The surface energy of coal obtained in this paper and its variation with temperature and coal composition and the mechanism revealed provide a theoretical basis for temperature enhancement and enhanced coalbed methane extraction.?2?Based on the mechanism research of the additive solution on coal surface,the anthracite samples were chemically modified using sodium sulfate and sodium dodecyl sulfate?SDS?.The results show that the surface morphology of the coal samples had changed by the chemical action;the micropore diameter and the porosity of the coal sample were increased;the l group on the coal surface has changed.The Si-OH bond of coal had increased,the C=C and aliphatic hydrocarbon content had reduced in coal by sodium sulfate solution and SDS solution modified.The bridge bonds and aromatic hydrocarbons in coal structure were broken and formed new oxygen-containing functional groups on the coal surface.This result was enhanced hydrophilicity of the coal surface.The chemical modification had changed the zeta potential,dynamic contact angle and surface energy of the coal surface.The zeta potential of the modified coal samples surface increased by the exponential law,the dynamic contact angles were reduced followed a linear law and the surface energy were increased first and then decreased with the law of quadratic polynomial with the increased of temperature.As the concentration increases,the sodium sulfate solution increased the zeta potential of the coal surface followed the exponential law of positive direction and the SDS solution caused the Zeta potential on the coal surface to decreased followed the exponential law of negative direction,as well as the dynamic contact angles decreased and the surface energy increased by the exponential law.The chemical modification had changed the settling velocity of coal fines in water.Sodium sulphate solution was promoted sedimentation of coal fines in water and the test results show that with the increased of the modified solution concentration,the settling velocity of coal fines increased by the exponential law.SDS solution was prevented sedimentation of coal fines in water and the test results show that with the increased of the modified solution concentration,the settling velocity of coal fines decreased by the exponential law.The chemical modification had changed the swelling degree of coal fines in water.Sodium sulphate solution was promoted swelling degree of coal fines in water and the test results show that with the increased of the modified solution concentration,the swelling degree of coal fines increased followed the exponential law.SDS solution was prevented swelling degree of coal fines in water and the test results show that with the increased of the modified solution concentration,the swelling degree of coal fines decreased by the negative exponential law.In addition,the swelling degree of coal were increased followed a linear law with the temperature of the two modified solutions increased.The mechanism of chemical modification was discussed.The results show that the interaction between the molecules of the modified solution and the coal molecules was greater than the interaction between the molecules of the modified solution.The heat of wetting had changed the morphology of the tiny particles on the surface of the coal sample.Moreover,the water-swelling of clay minerals in coal caused the surface morphology of coal to change.The modification of SDS solution was enhanced the negative charge of the surface of coal sample,which reduced the zeta potential,enhanced the stability of the system,and reduced the settling velocity of coal fines.Sodium sulfate solution had the opposite effect,The modification of sodium sulfate solution was enhanced the positive charge of the surface of coal sample,which increased the zeta potential,enhanced the aggregation of the system,and increased the settling velocity of coal fines.After modification with sodium sulfate,the surface of the coal sample contains Na⁺andSO₄^2-,these ions will not be polarized.Therefore,result would have a strong charge effect on coal and promoted swelling of coal.Coal sample modified by SDS solution was coated with SDS molecules,and the hydrophilic group is tightly integrated with water molecules;the hydrophobic group on the coal surface gradually discharged the water molecules.Under the combined action of gravity and pressure,the gap between coal particles was gradually reduced,resulting in a decreased in swelling degree.?3?Based on the principle of electrochemical redox,electroosmotic and electrophoresis,the experimental device for electrochemically modified coal samples was built,and the cylindrical,block and powder coal samples were modified.The results show that the electrochemical effect changes the surface morphology of the coal sample,and the effect of the cathode region is better than that of the anode region.The effect of the SDS electrolyte is better than that of the sodium sulfate solution.The pore structure of the coal is changed,and the SDS electrolyte is superior to the sodium sulfate electrolyte.The average pore diameter and porosity of the pores having a pore diameter of 2.9?m can be increased by 54.93%and 119.4%,respectively.Electrochemical changes the surface groups of coal,reducing the content of aromatic hydrocarbons and aliphatic hydrocarbons on the surface of coal,increasing the number of oxygen-containing functional groups,and with the increase of potential gradient and electrolyte concentration,the oxidation of coal surface is strengthened,which will produce new oxygen-containing functional groups.The zeta potential of coal surface was changed.With the increase of electrolyte concentration,the zeta potential of coal surface in the anode and cathode of sodium sulfate electrolyte increased exponentially.The zeta potential of coal surface in the anode and anode regions of SDS electrolyte was exponential law decreases negatively.With the increase of the potential gradient,the zeta potential of the coal surface of the two electrolytes increases linearly in the positive direction,and the zeta potential of the coal surface in the cathode region decreases linearly in the negative direction.The dynamic contact angle of coal-water and the surface energy of coal were also changed.With the increase of potential gradient and electrolyte concentration,the dynamic contact angle of coal/water increased exponentially,and the surface energy of coal increased exponentially with the increase of potential gradient.When the concentration of electrolyte increases,the surface energy of coal decreases exponentially,and the modification effect of SDS is better than that of sodium sulfate.In the process of electrochemical action,the oxidation reaction of the anode generates H⁺,and the electrolyte in the anode region is acidic;the reduction reaction of the cathode generates OH^-,and the electrolyte in the cathode region is alkaline.Acidification causes dissolution of clay minerals in coal pores and increases the porosity of coal.Alkalinization leads to the dispersion of clay mineral aggregates in coal.It moves away from the coal body to the anode under electrophoresis,and the electrolyte is enriched to the cathode.Electrode reaction with charged solid particles,thereby increasing the number of pores in the coal;in addition,the sodium sulfate electrolyte has a higher electrical conductivity,electrically heating the coal matrix,and the SDS electrolyte has a lower surface tension.The wettability of coal is strong,and the two solutions have different modification effects on coal in different ways.?4?The gas adsorption,desorption,water wetting and impact particle size distribution of the modified coal samples were experimentally determined.The results show that the electrochemical modification reduces the adsorption capacity of coal samples.The maximum adsorption capacity of coal samples in the anode and cathode areas of SDS electrolyte is reduced by 36.15%and22.31%,respectively.The adsorption amount decreased by 48.63%,while the coal sample in the sodium sulfate cathode area did not change significantly;electrochemical action effectively improves the final desorption rate of gas in the absorption equilibrium;The SDS solution has the effect of improving the water absorption of the coal sample.The saturated water absorption growth rates of the soaked modified coal sample,the electrochemical anode coal sample and the cathode coal sample were 15.43%,20.11%,and 23.01%,respectively;the sodium sulfate solution has the effect of reducing the water absorption of the coal,The reduction rates of saturated water content of sodium sulfate soaked coal samples,sulfur electrochemical anode coal samples and cathode coal samples were 16.81%,14.05%,and 11.97%,respectively.The particle size-mass distribution of natural coal samples and modified coal samples subjected to impact crushing and dust production conforms to the fractal law.After the modification,the fractal dimension of the dried coal sample increases,and the fractal dimension of the saturated coal sample decreases,which changes the particle size distribution of the coal powder.