Study on Microstructural Characteristics of Coal Surface and the Mechanism of Electromagnetic Radiation

Author:Liu Xian Feng

Supervisor:he xue qiu nie bai sheng


Degree Year:2018





The research work of this dissertation is based on the State Key Research Development Program of China(2016YFC0801408)and the State Key Program of National Natural Science Foundation of China(Grant No.51634001).Coal or rock dynamic disaster,such as rockburst,coal and gas outburst etc.remains severe in China,and its reliable monitoring and early warning is of great importance for effective prevention.Electromagnetic radiation technique can continuously monitor the deformation and fracture of coal/rock without touch,which has been widely used underground coal mine.However,given the fact that the mechanism of electromagnetic radiation(EMR)is still unclear,the application reliability of EMR technique is needed to be improved.Based on this fact,this dissertation concentrates on the microstructural characteristics of coal surface and the EMR mechanism.Starting with the surface physical and chemical structure of coal,experimental test,theoretical analysis and combined with model research were adopted to investigate the surface weak electrical property and its influencing factors of coal with different metamorphism degree.The polarity characteristic and potential charging of coal surface functional groups were analyzed,and the microstructural basis of EMR was determined.The quantitative relationship between the microstructure and surface electrical property of coal was also established for in depth exploring the generation mechanism of coal surface charge.In the dissertation,the EMR model of coal or rock in the process of deformation and failure was established to reveal the EMR mechanism,aiming to provide the guidance for the prevention and monitoring and early warning of coal or rock dynamic disasters.The main conclusions in this dissertation are shown as follows:(1)The surface physical microstructural characteristic of different coal samples was studied by experiment,finding that the pore parameters vary a lot among different coal samples,and these parameters are closely related to coal metamorphism degree.As coal rank increases,the variation trend of pore/fracture on coal surface is as follows:macro-crack—small fracture+pore clusters—micro-crack+micropore.The complexity level of coal surface structure can be effectively described by fractal theory.The value of fractal dimension generally displays a U-shape curve with increasing coalification,indicating that the surface physical microstructure of coal experiences the change from complexity to relative simpleness and then to complexity.(2)The coal surface chemical microstructural characteristic and its distribution law were analyzed.The statistical analysis method of coal surface microstructure was obtained.The results indicate that the aromatic structural parameters of different coal samples vary a lot.With the increase of vitrinite reflectance(Ro),a nonlinear change of the ratio of aromatic carbon atom(fa)is observed,displaying a periodical evolutionary characteristic,but the general variation trend is monotone increasing.Coalification plays an important role in aliphatic structural parameters.As coal rank increases,A(CH3)/A(CH2)first declines,then rises,which means that the methyl side-chains are gradually reduced,but the methylene side-chains are increased with increasing coalification.At around Ro=1.3%,the content of methyl groups reaches the lowest bottom,but it tends to increase with further coalification.Combined the Fourier Transform Infrared Spectroscopy(FTIR)and Raman spectroscopy of coal,the surface chemical microstructural parameters and functional groups in coal with different ranks exhibit a periodical distribution.At the low rank coal stage(0.5%<Ro<1.3%),coal molecules contain a large number of oxygen-containing functional groups,and the structural defects are great.At the medium rank coal stage(1.3%<Ro<2.0%),the oxygen-containing functional groups in coal are lost and decreased,leading to the relative increase of aromatic carbon atom ratio.In the meanwhile,both the aromatic rings and peak position difference(G-D1)are enhanced with the sharp increase of aromatic C=C groups.As for the high rank coal(2.0%<Ro<4.0%),the aromatic structural systems are more ordered formed in coal,due to the intensive condensation of coal aromatic rings.(3)Based on atomic force microscopy,the estimation method of coal surface electric charge density was proposed through simplified calculation.The coal surface electrical property was studied,finding that the surface potential of all the coal samples demonstrates a standard normal distribution overall.The mean value of surface potential is on the magnitude of 10μV,and the coal surface electric charge density ranges from 58.98μC/m2312.97μC/m2.As coal rank increases,the surface electric charge density first reduces then increases,exhibiting a nonlinear variation.At the nano scale,coal surface contains both positive and negative potential areas,displaying certain electrical characteristic.However,at the micrometer scale(5μm×5μm),the coal surface is generally electronegative,and the electrical property is weak or neutral.It can be inferred that at the meso or macro statistic scale level,coal surface shows weak electrical property or even electric neutrality,but it exhibits electrical features at nano scale.(4)The electric charge intensity of coal surface can be reflected by the phase shift.Viewing from the distribution of coal surface phase,it is found that the electrical property of coal surface is rather weak,even electrically neutral,without the influence of external electric field.When voltage is applied in tip,the electrical property of coal surface is changed under the impact of external electric field.On one hand,the free electric charge(electron)within the coal sample and the bound charge(ion)immigrate to coal surface,consequently,the electrical property of coal surface is enhanced.On the other hand,partial functional groups on coal surface are easily polarized by external electric field,leading to the misalignment of positive and negative charge center for these functional groups.Thus,the electrical property of coal surface appears.(5)The influencing factors of weak electrical property of coal surface were investigated.The research results show that both the mean value of surface potential and electric charge density tend to increase with increasing micropore specific surface area,and they are also positively correlated with mesopore specific surface area.Macropore can weaken the coal surface electrical property.As for the effect of chemical microstructure,the electronegativity of coal surface displays a positively linear relationship with the content of C=C groups,but it declines with the increase of hydroxyl groups(OH)and C-O groups.The C=O groups are also beneficial for the electronegativity of coal surface.In the process of coalification,the aromatic lamellas within the coal macromolecular enlarge,as a consequence,the motion space for free electron becomes large.This enables electron migration to happen,leading to the increase of coal electric conductivity and surface electrical property.(6)The electrical property of coal surface was investigated by molecular simulation,and the electrostatic potential of coal surface was also calculated.The simulation result shows that the distribution of electrostatic potential of coal surface is symmetrical to the zero electrical potential,namely normal distribution.This indicates that electrical features are presented at nano scale,but coal surface shows weak electrical property or even electric neutrality at the meso or macro statistic scale level,which confirms the experimental test results with regard to the coal surface electrical property in the third chapter.In the process of coalification,the ratio of aromatic carbon atom(fa)continuously increases,and the defects decline.The coal macromolecular structure tends to be ordered.During this process,the aromatic rings in coal enlarge,correspondingly enabling the increase of aromatic sheets within the coal molecular.Theπpathways are connected within coal,enlarging the motion range of free electric charge of coal surface.These free electrons are able to immigrate,resulting in the enhancement of coal electric conductivity and surface electrical property.Especially during the transition stage from bituminous coal to anthracite,the aromatic structure and electric conductivity of coal sharply enlarges with the dramatic decrease of electrical resistivity.The number of free electric charge is increased to a large degree,and the electric conductivity of free electron is also improved,thus,the explored augmentation of coal surface electrical parameters is greater.(7)The main surface polar groups contained in coal are hydroxyl groups(-OH),C-O groups and carbanyl groups(C=O),etc.These groups are not electrically neutral due to the misalignment of positive and negative charge center,acting as electric dipoles included in coal surface.The generation mechanism of coal surface electric charge was also explored in depth.It is considered that the electronegativity of coal surface groups,the existence of double electrode layer,the potential of dislocations/grain boundary and the charge effect in the coal surface space are mainly responsible for the generation of coal surface electric charge.The quantitative relationship between the microstructure and surface electrical property of coal was established.It is found that the increase in the electronegativity of groups enables to significantly enhance the surface negative potential of coal.(8)The electric field intensity and instantaneous EMR energy of coal surface were studied at different pore scales.It is estimated that the maximum values of electric field intensity in micropore,mesopore and macropore surfaces are on the magnitude of 107V/m、106 V/m、105 V/m,respectively.The instantaneous EMR energy varies in the range of 0.48×10-12 J48.7×10-12 J.The EMR charge source originates from two aspects.The one is the original net charge distributed on coal surface.The other is the free charge generated in the process of loading and failure.The EMR model of coal or rock failure was established,and the EMR mechanism was revealed.It is believed that the vibration of charged fracture walls and the deformation of electric dipoles induced by crack propagation account for the EMR generation during coal or rock failure.The research results are of theoretical and practical significance for revealing the EMR mechanism and improving the reliability of EMR monitoring and early warning of coal or rock dynamic disaster.