Macromolecular Structure Model Construction And HRTEM Analysis Of Caocun No.2 Coal

Coal is a heterogeneous substance.It is of great significance to identify or even quantify coal heterogeneity for a comprehensive understanding of coal structure.The result show that this heterogeneity is also reflected the large difference in lattice fringes from different regions of coal by HRTEM image analysis of Caocun No.2 coal.Based on this,this paper takes Caocun No.2 coal as the research object and analyzes the molecules in coal from the two levels including molecular scale and aggregate state scale comprehensively.We are trying to investigate connections and differences between different levels in coal structure,then rise to the mechanism of coal heterogeneity.In this paper,we used FTIR,Raman,XPS,¹³C-NMR and other methods to analyze Caocun No.2 coal on the molecular scale.The macromolecular structure model of Caocun No.2 coal was constructed using ACD,GNMR and other software.After molecular mechanics and molecular dynamics optimized through Material Studio simulation calculation software,the 3D optimized stable structure model of Caocun No.2 coal was obtained.In addition,XRD and HRTEM were used to characterize the crystallite structure parameters of Caocun No.2 coal in the aggregate state scale,including the layer spacing(d₀₀₂),the layer diameter?La?,the stacking height?Lc?,the number of aromatic piles(N_ave),et al.On the basis of better understanding the different scale structures of coal,the different types of aggregates in coal are classified and quantitatively characterized,including the length and orientation of lattice fringes,and curvature characteristics?segment angle,cumulative angle and tortuosity?.In addition,the pores of different aggregated structural regions were analyzed;the formation mechanism of coal aggregates was discussed combining the perspective of coal molecules by FTIR and Raman.The main conclusions are as follows:1.The molecular structure information of Caocun No.2 coal was obtained by FTIR,Raman,XPS and ¹³C-NMR.?1?The content of methylene group was higher than methyl,and the ratio of CH₂ to CH₃ was 1.55,indicating that there are more shorter branches in the coal.The Raman parameter ID1/IG ratio is 2,reflecting the overall disordered arrangement of carbon atoms in coal.?2?The carbon in coal consists mainly of aromatic carbon and aliphatic carbon,of which aromatic carbon accounts for 68.1%,aliphatic carbon accounts for 27.45%,carbonyl and carboxyl carbon accounts for 4.45%,and the structure is aromatic.The ratio of aromatic bridge carbon to aromatic peripheral carbon is 0.201.?3?The heteroatoms in coal are mainly composed of N and O.N mainly exists in the form of pyridine and pyrrole,and the ratio in the macromolecular structure of coal is 1:1;O is mainly in the form of ether,phenol,carbonyl,carboxyl,and the ratio is 3:6:5:3.2.The molecular structure of Caocun No.2 coal was analyzed.The molecular formula of Caocun No.2 coal was C₁₇₆H₁₃₂O₂₀N₂ with a molecular weight 2595.The 3D space optimization of the model yields the lowest energy structural model.The 3D model visually shows that the distribution of aliphatic carbon atoms in coal is disordered and it reflects a disordering distribution characteristics,while the distribution of the aromatic carbon ring is in more order,which is represented by the distribution characteristics of the stack.3.From the aggregation scale state we obtains:?1?The microcrystal structure parameters of coal were characterized by XRD.The result shows that d₀₀₂02 is 0.355nm,La is 1.109nm,Lc is 1.607nm,and N_aveve is 4.53,which is basically consistent with the results obtained by HRTEM image processing analysis.?2?For the lattice fringes of Caocun No.2 coal,the short fringes are distributed more and dispersed,and the long fringes are less;the middle fringes are concentrated,showing good orientation.The lattice fringe length is about0.3-1.14nm,which accounts for about 90%of the total number of fringes,mainly including short naphthalene fringes;it is consistent with the aromatic structure of Caocun No.2 coal molecular model.From the orientation point of view,the overall orientation of the lattice fringes is not aligned,and some regions have good orientation characteristics,which are expressed in the ordered regions;while the fringe orientation is disordered in most other regions.4.Based on HRTEM image processing technology,the classification and quantitative characterization of different aggregated nanoscale fringes in Caocun No.2 coal were carried out:?1?Three different fringe region types of aggregated structural regions were defined:good orientation,similar to a ring-shaped,and disordered.?2?The fringe lengths in the three regions have a relatively uniform distribution,mainly dominated by 1x1,2x2,and 3x3 type.The single lattice fringe exhibits large curvature,which is characterized by a large segmentation angle and tortuosity,in a fringe region with good orientation.For the fringe region with similar to a ring-shaped,the rose pattern exhibits a"cross-shaped"distribution in terms of orientation.In the disordered fringe region,the linearity of the single fringe is better,which is characterized by smaller segmentation angle and tortuosity.Studies have shown that short fringes are the main part of the curved fringes,and there may be exsist more non-six-membered rings or lattice defects.?3?The pore size distribution of three regions was analyzed,and in all three regions developed micropores.The pore size were concentrated at0.4-0.5 nm in a fringe region with good orientation,while it was slightly larger in the disordered fringe region.5.Combining the molecular structure parameters of FTIR and Raman,as well as the lattice fringe characteristics and pore size distribution of different regions in coal,the formation mechanism of aggregate structure in coal was discussed.The aggregate state in coal is the result of the interaction of different aromatic structures,macromolecular group formed between aliphatic structure and other molecules,defects or pores inside the molecular group.